prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/2484/1/2565S.pdfDeclaration This is to...

Doped Titanium Dioxide Nanostructures for Environmental Applications

Islamabad

A dissertation submitted to the Department of Chemistry, Quaid-i-Azam University, Islamabad, in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

in

Physical Chemistry

by

Asima Siddiqa

Department of Chemistry Quaid-i-Azam University

Islamabad 2014

Declaration This is to certify that this dissertation submitted by Ms. Asima Siddiqa is accepted

in its present form by the Department of Chemistry, Quaid-i-Azam University Islamabad,

Pakistan, as satisfying the dissertation requirements for the degree of Doctor of

philosophy in Physical Chemistry.

Supervisor _____________________

Prof. Dr. M. Siddiq Department of Chemistry Quaid-i-Azam University Islamabad.

Chairman _____________________

Prof. Dr. Amin Badshah Department of Chemistry Quaid-i-Azam University Islamabad.

Co-supervisor _____________________

Prof. Dr. Syed Tajammul Hussain (Late) National Centre for Physics

Quaid-i-Azam University Islamabad.

Dedicated To

My Parents, Ijlal Ahmed

And

My Co-Supervisor

Prof. Dr. Syed Tajammul Hussain (Late)

i

ACKNOWLEDGEMENTS

I fully realize the blessings upon me by the most gracious and divine force of all forces that

enabled me, and gave me sense and insight to accomplish this scientific assignment objectively

and successfully. Allah blessed me with this vision and insight to acquire knowledge without

which I can never satisfy and justify my existence in this world.

First and foremost, I owe my profound thanks to my supervisor Prof. Dr. Muhammad Siddiq,

whose personal scientific interest, provoking guidance, valuable suggestions and discussions

enabled me to complete this tedious work. I feel great pleasure in expressing my ineffable thanks

to my co-supervisor Prof. Dr. Syed Tajammul Hussain (late) for his constant support and

guidance which helped me to manage my career along with my research. Heartfelt gratitude to

Chairman Department of Chemistry Prof. Dr. Amin Badshah for providing necessary research

facilities. I would not forget to owe my regards to Prof. Dr. Kazuhiro Takanabe,

Photocatalysis Lab, King Catalysis Centre, King Abdullah University of Science and

Technology, for his kind and sincere help for providing me lab facilities during my project. I

would like to thanks to all faculty members of QAU, especially to Dr. Saqib Ali for moral and

encouraging behavior.

I extend my deepest gratitude to Dr. Delavar Anjum at KAUST for helping me to carry out

TEM analysis. I am also great full to Dr. Willayat Hussain, Dr. Dilshad Masih, Dr. Ayesha

Kausar, Dr. Hamid Saleem, Dr. Bushra, Ela, Angel, Ajmal, Luqman, Rashid, Zafar, Mohib,

Roman, Dr. Sara Qaisar, Dr. Tariq Mehmood, Zulfiqar Ali, Nisar Ahmad, Niaz Ahmad, Rafaqat

Hussain, Yaqoob Khan, Anila Iqbal, Ikhtiyar, my all friends, lab fellows and my all colleagues

for their invaluable guidance, encouragement and co-operation which enabled me to complete

my PhD research and thesis. I also pay my special gratitude to Mr. Sharif Chohan and all

supporting technical staff of Chemistry Department, QAU. I also highly acknowledge financial

support provided during research work by Higher Education Commission, Pakistan,

Finally, I am indebted to my family members, especially my parents, sibling and Ijlal

Ahmed, whose constant care and support made this journey successful.

ASIMA SIDDIQA

ii

ABSTRACT

Titanium dioxide (TiO2) nanostructures have enormous application in various fields such as

sensors, water splitting, super capacitors and, photovoltaic devices, etc. And they are extensively

exploited for number of other energy and environmental applications now-a-days. As, the rapid

urbanization and industrialization is polluting both water, air and existing developed

technologies do not have adequate potential to overwhelm this environmental dilemma.

Photocatalysis based on TiO2 nanostructures have procured significant attention in current era for

the complete decomposition of hazardous compounds from water and purification of air due to

low cost, thermal stability, chemical stability, huge surface area, non toxicity. This thesis is

mainly focused on the use of doped nanostructure titanium dioxide as photocatalyst for

environmental application specifically mineralization of dyes (alizarin red S, procion blue MXR,

malachite green) and phenol, and photoreduction of carbon dioxide using un-doped and co-

doped titanium dioxide nanostructures with anion i.e., sulfur (1 wt%) and transition metal ions

(copper, cobalt, ruthenium, iron and chromium with varying the concentration from 1-5 wt %)

having excellent chemical and photostability, good crystallite size, homogenous distribution,

superior structural properties and excellent surface area and pore volume were synthesized by

singe-step sol-gel reaction. The structural and morphological properties of prepared

nanostructures were exploited by X-Ray Diffraction (XRD), Diffuse Reflectance Spectroscopy

(DRS), Scanning Electron Microscopy coupled with Energy Dispersive Spectroscope (FESEM-

EDX), High Resolution Transmission Electron Microscopy (HRTEM), Raman Spectroscopy,

Thermal Analysis (TGA/DSC), Brunauer–Emmett–Teller (BET) surface analysis, Rutherford

Back Scattering (RBS) and Fourier Transform Infra Red Spectroscopy (FTIR). The synergetic

effect of anion and metal ion doping on titanium dioxide tailored the morphological and bulk

superficial properties of the samples. Doping induced structural changes, enhancement of the

visible light absorption capability, surface area, stability and photocatalytic activity. However,

5% metal ion co-doped titanium dioxide nanostructures demonstrated efficient band gap, thermal

stability, good particle size, higher surface area and remarkably higher photocatalytic activity for

photodegradation of dyes and phenol and CO2 photoreduction as compared to un-doped, S-doped

and co-doped TiO2 with lower amount of metal ion. The parameters that affect the photocatalytic

activity of TiO2 nanostructure powders for degradation of pollutants, namely concentration of

iii

dyes, catalyst loading, pH, irradiation source, and recyclability were optimized. The CO2

reduction and recycling of CO2 into value added products such as methane, methanol, ethanol,

etc. was carried out under both UV and visible range and hydrogen was obtained from water in-

situ. TiO2 nanostructures were found to be feasible and attractive for CO2 environment

management and waste water treatment due to rapidness, cost effectiveness, catalyst inert nature,

photostability and competent reusability. Hence, the activity of titanium dioxide nanostructure in

visible range suggested that solar energy can be an alternative cost effective light source to

resolve the environmental problems in future and this single step process can useful for

industries.

iv

LIST OF FIGURES

Figure Title Page

Figure 1.1 Water withdrawal by sector in the regions of the world (2000). 3

Figure 1.2 Conventional methods for waste water treatment. 10

Figure 1.3 Air stripping method for waste water treatment. 11

Figure 1.4 Ozonation method for waste water treatment. 12

Figure 1.5 Waste water treatment by ion exchange method. 13

Figure 1.6 Chlorination process for waste water treatment. 13

Figure 1.7 World total primary energy consumption by region, reference

case 1990-2030. 16

Figure 1.8 Major green house gases and their composition. 17

Figure 1.9 Carbon dioxide during the last 400,000 years and the rapid rise

since the industrial revolution. 18

Figure 1.10 Global mean surface temperatures 1856 to 2018. 19

Figure 1.11 Sources of CO2 emission from fossil fuel combustion. 19

Figure 1.12 Green house effect caused by increase in temperature. 20

Figure 1.13 Chart illustrating volume of research work carried out each

year in the field of photocatalysis. 23

Figure 1.14 Schematic illustration of steps involve in a photocatalytic

process. 24

Figure 1.15

Band gap positions various semiconductors. The energy scale

is indicated in electron volts using either the vacuum level

(left) or the normal hydrogen electrode (NHE) (right) as a

reference.

26

Figure 1.16 Elemental unit cells of anatase (a), rutile (b) and brookite (c). 26

Figure 1.17 Application fields of TiO2 photocatalysis. 28

Figure 1.18 Solar irradiance and UV-Vis range of solar spectrum. 29

Figure 1.19 (a) Dye sensitization of titanium dioxide (b) TiO2/CdS coupled

semiconductor (c) TiO2 deposited with metal. 31

Figure 1.20 Un-doped, anion doped and anion-metal ion co-doped TiO2. 31

v

Figure 1.21 Photocatalytic process followed during photocatalytic

degradation of dyes and pollutants. 34

Figure 1.22 Photocatalytic process followed during photocatalytic

reduction of carbon dioxide. 36

Figure 2.1 Experimental set up for the synthesis of titania nanotubes 53

Figure 2.2 Setup for (a) photolysis and photocatalytic degradation of dyes

and phenol (b) photo reduction of carbon dioxide. 55

Figure 3.1 Effect of annealing temperature on the phase of un-doped TiO2. 67

Figure 3.2 Effect of sulfur doping on the phase of un-doped TiO2. 67

Figure 3.3

XRD patterns of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST (e)

3Co-ST; (f) 4Co-ST; (g) 5Co-ST; (h) 6Co-ST calcined at 500

0C.

68

Figure 3.4 Band gap plots of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e)

3Co-ST; (f) 4Co-ST; (g) 5Co-ST. 70

Figure 3.5 SEM images of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e)


Figure 3.6 TEM images (a) TEM at low magnification; (b) FFT Pattern;

(c, d) HRTEM of pure TiO2 74


(c, d) HRTEM of 1%S doped TiO2. 75


(c, d) HRTEM of 5%Co-S co-doped TiO2. 76

Figure 3.9 RBS spectra of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e)


Figure 3.10 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Co-ST; (d)

2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST. 80

Figure 3.11 FTIR spectra of calcined (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-

ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST. 80

Figure 3.12 Raman spectra of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e)


Figure 3.13 TGA profile of un-calcined (a) PT; (b) ST; (c) 1Co-ST; (d) 83

vi

2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

Figure 3.14

UV spectra of photolysis of (a) ARS; (b) CV; (c) MG; (d) PB-

MXR; (e) PH; (f) Percent degradation of dyes and phenol

under UV irradiation, time; 6h, pH ~7, conc.; 20 ppm.

85

Figure 3.15

UV spectra of photolysis of (a) ARS; (b) CV; (c) MG; (d) PB-

MXR; (e) PH; (f) Percent degradation of dyes and phenol

under vis. irradiation, time; 6h, pH ~7, conc.; 20ppm.

86

Figure 3.16

Percent adsorption of dyes and phenol using (a) TiO2 (pH

~4.5); (b) TiO2 (pH ~7); (c) TiO2 (pH ~9.5); (d) ST (pH ~4.5);

(e) ST (pH ~7); (f) ST (pH ~9.5); (g) 5Co-ST (pH ~4.5); (h)

5Co-ST (pH ~7); (i) 5Co-ST (pH ~9.5) under dark, time; 6h,

conc.; 20ppm, catalyst; 50mg.

88

Figure 3.17

Percent degradation of various concentrations of (a) ARS; (b)

PB-MXR; (c) MG; (d) CV; (e) PH as function of time under

vis. irradiation using TiO2, pH ~7, catalyst; 50 mg.

91

Figure 3.18



Vis. irradiation using S-doped TiO2, pH ~7, catalyst; 50 mg.

92

Figure 3.19



vis. irradiation using 5Co-ST, pH ~7, catalyst; 50 mg.

93

Figure 3.20

Percent degradation of dyes and phenol as function of catalyst

dose using (a) PT; (b) ST; (c) 5Co-ST under visible

irradiations, time; 50 min, conc.; 20 ppm, pH ~7.

95

Figure 3.21

Percent degradation of dyes and phenol as function of pH

using (a) PT; (b) ST; (c) 5Co-ST under visible irradiations,

time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

97

Figure 3.22

Percent degradation of dyes and phenol as function of dopant

content under visible irradiations, time; 50 min, catalyst; 50

mg, conc.; 20 ppm.

99

vii

Figure 3.23 Comparison of percent degradation of dyes and phenol under

UV and visible irradiation with 5% Co-S co-doped TiO2. 100

Figure 3.24

Plots of recyclability of dyes and phenol using 5% Co-S co-

doped TiO2 under visible irradiations, time; 50 min, catalyst;

50 mg, conc.; 20 ppm.

101

Figure 3.25 Product formation by photocatalytic reduction of carbon

dioxide as function of time using (a) PT; (b) ST; (c) 5Co-ST. 103

Figure 3.26

XRD patterns of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST (e)

3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST; (h) 6Cu-ST calcined at 500

0C.

106

Figure 3.27 Band gap plots of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e)

3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST. 108

Figure 3.28 SEM images of (a) 1Cu-ST; (b) 2Cu-ST; (c) 3Cu-ST; (d) 4Cu-

ST; (e) 5Cu-ST. 109


(c, d) HRTEM of 5Cu-ST. 111

Figure 3.30 RBS spectra of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e)


Figure 3.31 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cu-ST; (d)

2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST. 115

Figure 3.32 FTIR spectra of calcined (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-

ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST. 115

Figure 3.33 Raman spectra of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e)


Figure 3.34 TGA profile of un-calcined (a) PT; (b) ST; (c) 1Cu-ST; (d)

2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST. 118

Figure 3.35

Percent adsorption of dyes and phenol using Cu-S co-doped

TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time;

6h, conc.; 20 ppm, catalyst; 50 mg.

119

Figure 3.36 Percent degradation of various concentrations of (a) ARS; (b)

PB-MXR; (c) MG; (d) CV; (e) PH as function of time under 122

viii

vis. irradiation using 5Cu-ST, Ph ~7, catalyst; 50 mg.

Figure 3.37


dose using 5Cu-ST under visible irradiations, time; 50 min,

conc.; 20 ppm, pH ~7.

123

Figure 3.38


using 5Cu-ST under visible irradiations, time; 50 min, catalyst;


124

Figure 3.39



mg, conc.; 20 ppm.

126


UV and visible irradiation with 5% Cu-S co-doped TiO2. 127

Figure 3.41

Plots of recyclability of dyes and phenol using 5 % Cu-S co-



128

Figure 3.42 Methanol production by photoreduction of CO2 as function of

time using 5% Cu-S co-doped TiO2. 129

Figure 3.43

XRD patterns of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST (e)

3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST; (h) 6Ru-ST calcined at 500

0C.

131

Figure 3.44 Band gap plots of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e)

3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST 133

Figure 3.45 SEM images of (a) 1Ru-ST; (b) 2Ru-ST; (c) 3Ru-ST; (d) 4Ru-

ST; (e) 5Ru-ST. 134


(c, d) HRTEM of 5Ru-ST. 135

Figure 3.47 RBS spectra of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e)

3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST. 136

Figure 3.48 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Ru-ST; (d)

2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST. 139

Figure 3.49 FTIR spectra of calcined (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru- 139

ix

ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

Figure 3.50 Raman spectra of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e)

3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST. 140

Figure 3.51 TGA profile of un-calcined (a) PT; (b) ST; (c) 1Ru-ST; (d)

2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST. 141

Figure 3.52

Percent adsorption of dyes and phenol using Ru-S co-doped

TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time; 6

h, conc.; 20 ppm, catalyst; 50 mg.

143

Figure 3.53



Vis. irradiation using 5Ru-ST, pH ~7, catalyst; 50 mg.

145

Figure 3.54


dose using 5Ru-ST under visible irradiations, time; 50 min,


147

Figure 3.55


using 5Ru-ST under visible irradiations, time; 50 min, catalyst;


147

Figure 3.56



mg, conc.; 20 ppm.

149


UV and Visible irradiation with 5% Ru-S co-doped TiO2. 150

Figure 3.58

Plots of recyclability of dyes and phenol using 5 % Ru-S co-



150

Figure 3.59 Methane production by photoreduction of CO2 as function of

time using 5% Ru-S co-doped TiO2 151

Figure 3.60

XRD patterns of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST (e)

3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST; (h) 6Fe-ST calcined at 500

0C.

154

Figure 3.61 Band gap plots of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 156

x

3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

Figure 3.62 SEM images of (a) 1Fe-ST; (b) 2Fe-ST; (c) 3Fe-ST; (d) 4Fe-

ST; (e) 5Fe-ST. 157


(c, d) HRTEM of 5Fe-ST. 159

Figure 3.64 RBS spectra of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e)

3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST. 160

Figure 3.65 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Fe-ST; (d)

2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST. 163

Figure 3.66 FTIR spectra of calcined (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-

ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST. 163

Figure 3.67 Raman spectra of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e)

3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST. 164

Figure 3.68 TGA profile of un-calcined (a) PT; (b) ST; (c) 1Fe-ST; (d)

2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST. 165

Figure 3.69

Percent adsorption of dyes and phenol using Fe-S co-doped



167

Figure 3.70



Vis. irradiation using 5Fe-ST, pH ~7, catalyst; 50 mg.

169

Figure 3.71


dose using 5Fe-ST under visible irradiations, time; 50 min,

conc.; 20 ppm.

171

Figure 3.72


using 5Fe-ST under visible irradiations, time; 50 min, catalyst;


171

Figure 3.73



mg, conc.; 20 ppm.

173

Figure 3.74 Comparison of percent degradation of dyes and phenol under 174

xi

UV and Visible irradiation with 5% Fe-S co-doped TiO2.

Figure 3.75

Plots of recyclability of dyes and phenol using 5 % Fe-S co-



175

Figure 3.76 Ethanol production by photoreduction of CO2 as function of

time using 5% Fe-S co-doped TiO2. 176

Figure 3.77

XRD patterns of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST (e)

3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST; (h) 6Cr-ST calcined at 500

0C.

178

Figure 3.78 Band gap plots of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e)

3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST. 180

Figure 3.79 SEM images of (a) 1Cr-ST; (b) 2Cr-ST; (c) 3Cr-ST; (d) 4Cr-

ST; (e) 5Cr-ST. 181


(c, d) HRTEM of 5Cr-ST. 183

Figure 3.81 RBS spectra of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e)


Figure 3.82 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d)

2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST. 187

Figure 3.83 FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d)


Figure 3.84 Raman spectra of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e)


Figure 3.85 TGA profile of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d)


Figure 3.86

Percent adsorption of dyes and phenol using Cr-S co-doped



191

Figure 3.87



vis. irradiation using 5Cr-ST, pH ~7, catalyst; 50 mg.

193

xii

Figure 3.88


dose using 5Cr-ST under visible irradiations, time; 50 min,


195

Figure 3.89


using 5Cr-ST under visible irradiations, time; 50 min, catalyst;


195

Figure 3.90



mg, conc.; 20 ppm.

197


UV and visible irradiation with 5% Cr-S co-doped TiO2. 198

Figure 3.92

Plots of recyclability of dyes and phenol using 5 % Cr-S co-



198

Figure 3.93

Comparative percent degradation plots of dyes and phenol

using 5% Co-S co-doped TiO2, 5% Cu-S co-doped TiO2, 5%

Ru-S co-doped TiO2, 5% Fe-S co-doped TiO2, 5% Cr-S co-

doped TiO2 under visible irradiations and optimized conditions

199

Figure 3.94 Ethanol production by photoreduction of CO2 as function of

time using 5% Cr-S co-doped TiO2. 200

Figure 3.95 XRD patterns of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-

TNT; (e) FCS-TNT; calcined at 500 0C.

203

Figure 3.96 Band gap plots of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-

TNT; (e) FCS-TNT. 204

Figure 3.97 SEM images of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-

TNT; (e) FCS-TNT. 205

Figure 3.98 TEM of (a) FC-TNT and (b) FCS-TNT. 206

Figure 3.99 Percent degradation of phenol as function of time using (a)

TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-TNT; (e) FCS-TNT. 209

Figure 3.100 Ethanol production by (a) FC-TNT and (b) FCS-TNT. 210

xiii

LIST OF TABLES

Table Title Page

Table 1.1 Organic pollutants and their hazardous aspects. 4

Table 1.2 Industrial source of dyes causing the water pollution. 5

Table 1.3 Nature and composition of textile effluent from different

processing stages.

6

Table 1.4 Composite textile industry waste water characteristics. 6

Table 1.5 Classes of synthetic dyes according to chemical structure. 7

Table 1.6 Classification of dyes according to application. 8

Table 1.7 Percentage of dye lost to effluent. 9

Table 1.8 Advantages and disadvantages of current conventional methods. 15

Table 1.9 Properties of different form of titanium dioxide. 27

Table 2.1 List of reagents used and their percentage purities. 47

Table 2.2 Chemical structures of the dyes and phenol with their codes 48

Table 2.3 Sample codes of all prepared sample with their composition 51

Table 3.1 Calculated structural parameters and band gap data of un-doped,

S-doped and Co-S co-doped TiO2.

70

Table 3.2 Elemental analysis of the un-doped, S-doped and Co-S co-doped

TiO2.

78

Table 3.3 BET surface area and pore volume data of un-doped, S-doped and

Co-S co-doped TiO2.

78

Table 3.4 Weight loss data of the un-doped, S-doped and Co-S co-doped

TiO2.

82

Table 3.5 Percent degradation data of dyes and phenol obtained by

photolysis.

87

Table 3.6 Percent adsorption data of dyes and phenol under dark at various

pH.

89

Table 3.7 Percent degradation data of dyes and phenol at different

concentration.

94

xiv

Table 3.8 Percent degradation data of dyes and phenol with different doped

sample.

99


S-doped and Cu-S co-doped TiO2.

108

Table 3.10

Elemental analysis of the un-doped, S-doped and Cu-S co-doped

TiO2.

112


Cu-S co-doped TiO2.

113

Table 3.12 Weight loss data of the un-doped, S-doped and Cu-S co-doped

TiO2.

117


pH.

120


concentration.

123


sample.

126


S-doped and Ru-S co-doped TiO2.

133

Table 3.17 Elemental analysis of the un-doped, S-doped and Ru-S co-doped

TiO2.

137


Ru-S co-doped TiO2.

137

Table 3.19 Weight loss data of the un-doped, S-doped and Ru-S co-doped

TiO2.

141


pH.

143


concentration.

146


sample.

148

xv


S-doped and Fe-S co-doped TiO2.

156

Table 3.24 Elemental analysis of the un-doped, S-doped and Fe-S co-doped

TiO2.

160


Fe-S co-doped TiO2.

161

Table 3.26 Weight loss data of the un-doped, S-doped and Fe-S co-doped

TiO2.

165


pH.

167


concentration.

170


sample.

173


S-doped and Cr-S co-doped TiO2.

180

Table 3.31 Elemental analysis of the un-doped, S-doped and Cr-S co-doped

TiO2.

184


Cr-S co-doped TiO2.

185

Table 3.33 Weight loss data of the un-doped, S-doped and Cr-S co-doped

TiO2.

189


pH.

191


concentration.

194


sample.

197

Table 3.37 Band gap data of the un-doped and doped TiO2 nanotubes. 204

Table 3.38 Surface area and EDX analysis of doped and un-doped TNTs. 207

xvi

LIST OF SCHEMES

Table Title Page

Scheme 1.1 Photocatalytic steps involved during degradation of pollutants 33

Scheme 1.2 Photocatalytic steps involved during carbon dioxide reduction 35

Scheme 1.3 Schematic diagram showing synthesis and characterization of

prepared samples

45

Scheme 1.4 Schematic diagram showing photocatalytic application of the TiO2

nanostructures 46

Scheme 1.5 Schematic diagram showing synthesis and applications of

prepared nanotubes 46

Scheme 2.1 Synthesis of nanoparticles using sol-gel method 51

TABLE OF CONTENTS Page

Acknowledgements i

Abstract ii-iii

List of Figures iv-xii

List of Tables xiii-xv

List of Scheme xvi

Chapter-1 Introduction 1-46

1.1 Water- The Vehicle of Nature and Present Scenario 2

1.2 Textile Waste Water 5

1.2.1 Classification of Dyes According to their Chemical Structure 7

1.2.2 Classification of Dyes According to their Applications 7

1.2.3 Discharge Statistics of Dyes 9

1.2 Waste Water Discharge Treatment 10

1.3.1 Waste Water Treatment by Physiochemical Methods 11

1.3.1.1 Air Stripping Processes 11

1.3.1.2 Adsorption Process 11

1.3.1.3 Membrane Filtration Method 12

1.3.1.4 Ozonation 12

1.3.1.5 Ion Exchange 12

1.3.1.6 Chlorination 13

1.3.1.7 Electrochemical Processes 14

1.3.1.8 Waste Water Treatment by Biological Methods 14

1.3.2 Pros and Cons of Current Conventional Methods 14

1.4 Carbon Dioxide Increase: An Environmental Problem 16

1.4.1 Global Warming 18

1.4.2 Carbon Dioxide Reduction Management 20

1.4.2.1 Bio Chemical Conversion of CO2 21

1.4.2.2 Chemical Reduction of Carbon Dioxide 21

1.4.2.3 Thermo-chemical Conversion of Carbon Dioxide 21

1.4.2.4 Electrochemical Reduction of CO2 22

1.4.3 Limitations of Current Carbon Dioxide Strategies 22

1.5 Nanoscience and Nanotechnology Approaches 22

1.5.1 History of Photocatalysis 23

1.5.2 Photocatalyst, Photocatalysis and Principle of Photocatalysis 23

1.5.3 Photocatalyst Materials 25

1.6 TiO2 Nanostructures 26

1.6.1 TiO2 Nanostructures as Photocatalyst 27

1.6.2 TiO2 as Photocatalyst: Key Limitations 28

1.6.3 Approaches to Improve TiO2 Photocatalytic Performance 29

1.6.3.1 Surface Sensitization 30

1.6.3.2 Dual Semiconductor Systems 30

1.6.3.3 Metal Deposition 30

1.6.3.4 Band Gap Engineering by Doping 31

1.7 Environmental Applications of TiO2 Nanostructures as

Photocatalyst

32

1.7.1 TiO2 Photocatalyst for Water Treatment 32

1.7.1.1 Mechanism for Pollutant Degradation 33

1.7.2 TiO2 Photocatalyst for Carbon Dioxide Reduction 34

1.7.2.1 Mechanism for Carbon Dioxide Reduction 35

1.8 Literature Survey 37

1.9 Present Work 44

Chapter-2 Experimental 47-65

2.1 Chemicals Used 47

2.2 Synthesis of Nanoparticles 49

2.2.1 Synthesis of TiO2 Nanoparticles 49

2.2.2 Synthesis of Sulfur doped TiO2 Nanoparticles 49

2.2.3 Synthesis of Cobalt-Sulfur co-doped TiO2 Nanoparticles 49

2.2.4 Synthesis of Copper-Sulfur co-doped TiO2 Nanoparticles 49

2.2.5 Synthesis of Ruthenium-Sulfur co-doped TiO2 Nanoparticles 50

2.2.6 Synthesis of Iron-Sulfur co-doped TiO2 Nanoparticles 50

2.2.7 Synthesis of Chromium-Sulfur co-doped TiO2 Nanoparticles 50

2.3 Synthesis of Nanotubes 52

2.3.1 Synthesis of Iron doped and Iron-S co-doped Titania Nanotubes 52

2.3.2 Preparation of Fe-Cr co-doped Titania Nanotubes 52

2.3.3 Preparation of Fe-Cr/S co-doped Titania Nanotubes 52

2.4 Adsorption Studies of Dyes and Phenol 53

2.5 Photolysis Experiments 53

2.6 Photocatalytic Degradation of Dyes and Phenol 54

2.7 Photocatalytic Reduction of Carbon Dioxide 54

2.8 Characterization Techniques 56

2.8.1 X-Ray Diffraction (XRD) 56

2.8.2 UV-Vis Diffuse Reflectance Spectroscopy (DRS) 57

2.8.3 Scanning Electron Microscopy (SEM) Coupled with Energy

Dispersive Spectroscopy (EDX)

58

2.8.4 Transmission Electron Microscopy (TEM) 59

2.8.5 Rutherford Back Scattering (RBS) 60

2.8.6 Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda

(BJH)

60

2.8.7 Fourier Transform Infra Red (FTIR) Spectroscopy 61

2.8.8 Raman Spectroscopy 62

2.8.9 Thermo-gravimetric Analysis (TGA) 64

2.8.10 UV-Vis Spectroscopy 64

2.8.11 Gas Chromatography Mass Spectroscopy 65

Chapter-3 Results and Discussion 66-212

3.1 Optimization of Temperature and Dopant 66

3.1.1 Effect of Annealing Temperature on the Phase of the TiO2 66

3.1.2 Effect of Sulfur Contents on the Phase of the TiO2 66

3.2 Characterization and Photocatalytic Applications Co-S co-doped

TiO2 Nanostructures 68

3.2.1 XRD Analysis 68

3.2.2 Band Gap Studies 69

3.2.3 Morphological Studies 71

3.2.3.1 SEM Analysis 71

3.2.3.2 TEM Analysis 73

3.2.4 Elemental Analysis 77

3.2.4.1 EDX (Energy Dispersive X-rays) Studies 77

3.1.4.2 RBS (Rutherford Back Scattering) Studies 77

3.2.5 BET Surface Area Studies 78

3.2.6 FTIR Studies of Un-calcined and Calcined Samples 79

3.2.7 Raman Studies 79

3.2.8 TGA Analysis 82

3.3 Applications of Prepared Co-S co-doped Titanium Dioxide

Nanostructure 83

3.3.1 Photolysis of Dyes and Phenol 83

3.3.2 Adsorption Studies of Dyes and Phenol under Dark 84

3.3.3 Photocatalysis of Dyes and Phenol 89

3.3.3.1 Effect of Initial Concentration of Dyes and Phenol 90

3.3.3.2 Effect of Catalyst Dose 94

3.3.3.3 Effect of pH 96

3.3.3.4 Effect of Dopant Content 98

3.3.3.5 Comparison of Photocatalytic Activity under UV and Visible

Irradiation 100

3.3.3.6 Re-use of the Photocatalyst 101

3.3.4 Photocatalytic Reduction of Carbon Dioxide 102

3.3.4.1 Photocatalytic Reduction of Carbon Dioxide using PT, ST and

5Co-ST 102

3.4 Conclusions 104

3.5 Characterization and Photocatalytic Applications Cu-S co-doped














3.6 Applications of Prepared Cu-S co-doped Titanium Dioxide

Nanostructures 118








Irradiation 125


3.6.3 Photocatalytic Reduction of Carbon Dioxide using 5Cu-ST 128

3.7 Conclusions 129

3.8 Characterization and Photocatalytic Applications Ru-S co-doped 131

TiO2 Nanostructures













3.9 Applications of Prepared Ru-S co-doped Titanium Dioxide

Nanostructure 142








Irradiation 149


3.9.3 Photocatalytic Reduction of Carbon Dioxide using 5Ru-ST 151

3.10.3 Conclusion 152

3.11 Characterization and Photocatalytic Applications Fe-S co-doped












3.11.7 Raman Analysis of the Samples 162


3.12 Applications of Prepared Fe-S co-doped Titanium Dioxide

Nanostructure 166








Irradiation 172


3.12.3 Photocatalytic Reduction of Carbon Dioxide using 5Fe-ST 175

3.13 Conclusions 177

3.14 Characterization and Photocatalytic Applications Cr-S co-doped






3.14.3.2 TEM Studies 182


3.14.4.1 EDX Studies (Energy Dispersive X-rays) Studies 182

3.14.4.2 RBS Studies RBS (Rutherford Back Scattering) Studies 182


3.14.6 FTIR Studies on Un-calcined and Calcined Samples 185



3.15 Applications of Prepared Cr-S co-doped Titanium Dioxide

Nanostructure 190








Irradiation 196

3.15.2.6 Reuse of the Photocatalyst 196

3.15.2.7 Comparison of Percent Degradaton of Dyes and Phenol by Un-

doped and TiO2 co-Doped With Different Elements 199

3.15.3 Photocatalytic Reduction of Carbon Dioxide using 5Cr-ST 200

3.16 Conclusions 201

3.17 Characterization and Photocatalytic Applications of TiO2

Nanotubes 202

3.17.1 XRD Studies 202

3.17.2 UV-Vis Diffuse Reflectance Spectroscopy 203

3.17.3 Scanning Electron Microscopy and EDX Analysis 203

3.17.4 Transmission Electron Microscopy 206

3.17.5 Elemental Composition 207

3.17.6 Surface Area Analysis 207

3.18 Photocatalytic Applications of Prepared Nanotubes 207

3.18.1 Photocatalytic Degradation of Phenol 208

3.18.2 Conversion of CO2 under Visible Irradiation 209

3.19.5 Conclusions 210

3.20 In General Inferences and Future Prospects 212

Chapter 4 References 214

Chapter 5 List of Publications 223

1

1. INTRODUCTION

Earth is recently facing complex problems related to food, energy, water, scarcity,

population and pollution for many years. Both energy and environment have been a concern

since past few years and increasingly becoming a serious issue. The main concern is the

environmental issue which gained much attention over the past decades and it is needed to

converse its main causes. One obvious cause is based on the fact that basically human life‘s

quality is dependent on the environmental quality. It is the issue that not only troubles us

economically and physically but also affects our lives [1,2]. Unfortunately, the world’s

population is rapidly increasing resulting in the increased discharge of hazardous chemicals into

the environment. Particularly in developed countries the industries are expanding so rapidly

which has imparted serious environmental problems to both natural air and water qualities

resulting in intense water and air pollution. Consequently, pollution is caused by the excessive

use of natural resources and the rapid growth of anthropogenic activities [3].

An instant growth in world’s population and rapid industrialization resulted in the

world’s natural resources obscurity in keeping up with human being demands. Currently, energy

demands are fulfilled by burning the fossil fuels such as coal, natural gas, oil or by deforestation

[4]. According to the International Energy Outlook 2010, the total energy consumption by the

world increased by 49 % from 495 quadrillion Btu (2007) to 739 quadrillion Btu (2035).

Moreover, rapid development of civilization and industrialization have brought not only new

inventions, technology, high living standard, expediency to humanity but also resulted in

pollution [5]. However, inevitable consequence of burning of fuel sources and emission from

factories, chemical plants, industry, and vehicles are exhausting greenhouse gases (methane,

carbon dioxide SOx and NOx, etc.) to the atmosphere. Therefore, green house effect is chiefly

brought about by increasing level of carbon dioxide concentration in the atmosphere and causing

earth’s global warming effect [5,6].

Besides air pollution, the more critical issues we are facing today is contamination of

water. Most countries do not have enough water to fulfill their needs due to shortage of fresh and

clean water and it is recognized as one of the most severe political and social issues. A large

quantity of toxic organic materials is introduced into the marine environment from diverse

sources such as industrial effluents, chemical spills and agricultural technology [7]. The toxicity,

2

stability and persistence of these chemicals in the environment are the major concern of

developing societies and regulation authorities around the world. The causes, impact, protection

and correction of environmental problem are major questions and are needed to be discussed for

an effectual improvement quality and sustainability of life [5-7].

1.1 Water- The Vehicle of Nature and Present Scenario

The most pressing issues which the world is facing today is the availability of fresh and

abundant clean water for the use of human beings which is necessary for the survival of their life

on earth. This is a crucial issue because it is not only a vital element of metabolic processes

occurring in the body but also known as universal solvent [8]. The evolution of all civilization

has always emerged in the region where water existed such as the Nile was the sustenance of the

initial Egyptian Civilization and the Indus Valley Civilization which prospered on the Indus

River’s topography. Thus water can be considered as one of the clear signs of strength,

prosperity, peace, splendor, creativity and many other vital features which makes it to be known

as “the vehicle of nature” [7,9].

Water is available in the universe and abundantly used for multiple activities including

domestic purposes, various industries, agriculture and most importantly for energy production

(Figure 1.1). In past few decades, with increase in the human population and the growth of

agricultural and development of the industries led to incessant increase in the water demands.

The key environmental issue of the 21st century is the preservation of natural water resources

[10]. Universally, 15% of the total water is consumed for domestic use only, while the demand

for industrial activities and agriculture is 25% and 60%, respectively [9,11]. One of the most

incessant problems allied to each of these activities is inadequate access to fresh, clean and safe

drinking water which imparts adverse effect on the human being throughout the world.

According to UN survey report “Sick Water”, worldwide more than four billion human beings

lack an access towards clean water, and things are getting worst [12]. About 2.2 million people

die by drinking of contaminated water and it is estimated that average supply of clean water per

person will fall down by one third in the next two decades which will possibly cause premature

death of millions of people [11,12].

Out of 122 nations, Pakistan’s water quality ranks on 80th number and according to the

Pakistan Council of Research and Water Resources (PCRWR), 50% of urban water supply is

insufficient for drinking and individual use. About, 25.61 % of Pakistan’s populations have

3

approach to innocuous water available for drinking [13]. All industrial activities, domestic use

and commercial processes produce waste products which are detrimental for both flora and

fauna. There is a strong relationship between the waste contaminants generated and living

standards of society. More or less, a big portion of world’s population (23%) lives in developed

countries which utilize 78% of the resources and engender 82% of the waste compounds [14].

Besides, it has to be noticed that the level of residual waste is increasing exceptionally with the

rise of the industrialization [14,15]. Several industries such as petrochemical, mining, chemical,

semiconductor, pharmaceutical, and microelectronics are established worldwide with the fast

expansion of science and technology.

Figure 1.1: Water withdrawal by sector in the regions of the world (2000) [14].

Currently, more than five million known substances are being registered which are

widely used in various industries worldwide and 1,000 of new chemical substances are being

added to the list every year. A large quantity of water required these industries is used for various

processing. These industries discharge water which is contaminated with dangerous organic

pollutants which prompted the hydrosphere to be polluted with organic and inorganic

contaminates with increased rate [16]. The tolerable exculpated levels have been greatly

surpassed resulting in the alteration of characteristics of natural resources. This variation in the

chemical, physical and biological condition of water refers to the water pollution which disrupts

the natural balance of the ecosystem. Water pollution is generally measured by the effects of

4

pollutants on the water bodies. A variety of industrial processes and domestic activities generate

highly toxic effluents and are directly disposed of into the environment without any treatment.

These toxic substances include dyes, herbicides, pesticides, surfactants, phenols, detergents,

fertilizers, and other different chemical products, as demonstrated in Table 1.1 [17, 18]. The

existence of these types of pollutants in water body is highly problematic as the residual waste

cannot be stored for long time. Moreover, a little volume of contaminated water is capable of

contaminating huge amount of water. Water pollution can be categorized according to the; (i)

nature of pollutants, (ii) the sources from where they are discharged and (iii) the water bodies

into which they are released [17, 18].

Table 1.1: Organic pollutants and their hazardous aspects [17-18].

Organic pollutants Hazardous aspects Oils spills Oil forms a thick coating covering the water surface prohibiting

the light to reach marine plants and reducing the photosynthesis, poison the exposed organisms, damage the wild life and their habitats and harm the surface resources.

Petrochemicals These are formed from natural gas, petrol, oil and other fossilized hydrocarbons and are lethal to marine and plant life, damage the coral reefs, cause unhealthy effects to humans causing variety of diseases including cancer.

Phenols and chlorophenols, polyphenols, etc.

Phenol based compounds are discharged from various industries including pulp mills, refinery, coal mines, wood preservation plants, and other chemical industries. Its exposure to humans cause serious effects including lung cancer, heart disease, loss of coordination, muscle tremors, loss of immune system and can cause death in some cases.

Synthetic dyes Dyes are discharged from various industries including food, textile, paper and chemical industry. High dose of dye is toxic, mutagenic, carcinogenic and retards the photosynthetic activity in the marine life.

Pesticides and herbicides Contaminated water by pesticides and herbicide is lethal to the aquatic life resulting in killing fishes and aquatic plants on which aquatic life is dependant especially zooplankton.

A large amount of synthetic dyes are widely used in numerous fields of industrial

technology, e.g., textile industry, paper production, leather industry, agricultural research, food

technology, light-harvesting arrays and photo electrochemical cell etc [19]. Effluents discharge

from these industrial technologies contain significant amount of synthetic organic dyes and their

release is serious challenge to environmental scientists. As shown from Table 1.2, it can be seen

5

that huge amount of synthetic dyes are discharged from textile industry, polluting the marine

environment.

Table 1.2: Industrial source of dyes causing the water pollution [19].

1.2 Textile Waste Water

As discussed generally earlier, water pollution is caused by various kinds of pollutants

and waste water being discharged from the textile industry has been considered in this regard.

Textile industry is one of the major consumers of water which is ultimately causing intense water

pollution. The extensive utilization of various kinds of chemicals by textile industry results in

production of huge amount of polluted waste water [20]. According to the U.S. EPA report,

annually 109 kg and 10,000 of various kinds of synthetic dyes, chemicals and pigments are

produced annually. They are widely used in various dye and printing industries in all over the

world. It is approximated that ~20% of these chemicals and dyestuffs are lost in waste water

generated from textile industry because all of these are not restrained in final manufactured

items, become wasted and disposed off. The waste water is produced in various processing steps

such as sizing, desizing, scouring, bleaching, washing, dyeing and finishing processes [21, 22],

shown in Table 1.3. Table 1.4 represents the typical characteristics of textile industry waste

water and constituents of effluent associated with polluted water discharged at each step of

processing [24]. Table 1.3 depicts that textile waste water is strongly colored; having high

chemical oxygen demand (COD) and biological oxygen demand (BOD) values because of fiber

residues and large quantity suspended solids. It contains high dose of organic components, high

pH, hazardous substances, non-biodegradable compounds, detergents, surfactants, leveling

agents, acids, sulfide and alkalis, oil and grease, high suspended solids, high salt concentration.

Large quantity of reactive dyes, heavy metals (chromium, copper, zinc and mercury) and other

soluble substances are also included in waste water [16, 23].

S. No. Type of industry Percentage, % 1 Palm Oil 11.6 2 Raw natural rubber 8.6 3 Leather industry 14.1 4 Textile industry 40.5 5 Paper 4.4 6 Chemical 11.8 7 Total 100

6

Table 1.3: Nature and composition of textile effluent from different processing stages [23].

Table 1.4: Composite textile industry waste water characteristics [24].

Process Composition of effluent Nature of effluent Sizing Starch, carboxymethyl cellulose (CMC),

polyvinyl alcohol (PVA), grease, wetting agents and waxes

High COD and BOD,

Desizing Starch, carboxymethyl cellulose (CMC), waxes, pectin, polyvinyl alcohol (PVA) and fats etc.

High COD, BOD, high suspended solids and dissolved solids

Bleaching Sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), chlorine, acids, sodium hydroxide (NaOH), surfactants and sodium phosphate, etc

High total dissolved solids (TSS), high alkalinity

Mercerizing sodium hydroxide (NaOH), cotton wax High pH, low biological oxygen demand (BOD), and total dissolved solids (TDS)

Dyenig Dyestuff urea, acetic acid, reducing agents, oxidizing agents, wetting agents, alkali, detergents, etc.

Strongly colored, heavy metals, high biological oxygen demand (BOD), total dissolved solids (TDS), low total dissolved solids (TSS).

Printing Pastes, binders, cross-linkers, gums, oils, acid, urea, starches, alkali, thickners and reducing agents

Strongly colored, low TSS, slightly alkaline, oily appearance, low biological oxygen demand (BOD), etc

Finishing Toxic chemicals and inorganic compounds

Slightly alkaline, low BOD

S. No. Characterizing parameters (mg/L) Average values 1 Biochemical Oxygen Demand 150 – 12,000 2 Chemical Oxygen Demand 80 – 6,000 3 Total Suspended Solids 15 – 8,000 4 Total Dissolved Solids 2,900 – 3,100 5 Chloride 1000 – 1600 6 Total Kjeldahl Nitrogen 70 – 80 7 Color 50 – 2500 8 pH 5-9

7

Out of these all chemicals, it has been documented that the residual color constitutes

major portion of the waste water which is highly toxic, having low biodegradability and is

carcinogenic in nature. In addition, phenols are also referred as toxic chemical along dyes which

constitutes 0.5 µg/mL [21, 22].

1.2.1 Classification of Dyes According to their Chemical Structure The variation in chemical structure of dyes makes their classification difficult into

different distinct groups. The “Society of Dyers and Colourists” and “American Association of

Textile Chemists and Colorists” published the Colour Index (C.I.) since 1924 in which a C.I.

generic name is given to each dye according to its characteristics, application and colour [23].

The dyes are classified as azo, sulphur, indigoid, anthraquinone and phthalocyanine derivatives

according to their chemical structures. 20-30 various classes of dyes can be distinguished

according to their chemical structure or presence of distinct chromospheres, a few examples are

given in Table 1.5 [24, 25].

Table 1.5: Classes of synthetic dyes according to chemical structure [25].

1.2.2 Classification of Dyes According to their Applications

Synthetic dyes display substantial industrial applications based on their structural

diversity and chemically classified more frequently on their industrial scale applications. Table

1.6 contains a representative classification of dyes according to their applications in the industry

[25, 26]. Their classification according to the method of application depends on the nature of the

fiber (silk, wool, fur and leather) to which it is applied. Dyes are classified as acid dyes, basic

dyes, direct dyes, disperse dyes, sulfur, reactive, vat, mordant and other pigments according to

their application and type of the fabric to which they are applied (Table 1.6).

Code Chemical Class Code Chemical Class Code Chemical Class 10000 Nitroso 42000 Triarylmethane 53000 Sulphur 10300 Nitro 45000 Xanthene 55000 Lactone 11000 Monoazo 46000 Acridine 56000 Aminoketone 20000 Diazo 47000 Quinoline 57000 Hydroxyketone 30000 Triazo 48000 Methine 58000 Anthraquinone 35000 Polyazo 49000 Thiazole 73000 Indigoid 40000 Stilbene 49400 Indamine/Indophenols 75000 Natural 40800 Carotenoid 50000 Azine 76000 Oxidation Base 41000 Diphenylmethane 51000 Oxazine 77000 Inorganic

8

Table 1.6: Classification of dyes according to application [25, 26].

Dyes Characteristics Fiber Chemical class Interaction

Acid Anionic, highly

water soluble,

poor wet fastness

Polyamide,

wool, silk and

modified acryl

azo, anthraquinone

or triarylmethane,

azine, xanthene,

nitro, nitros

Ionic bond

Basic Cationic, highly

Soluble

Synthetic fiber

(modified

polyacryl)

Diarymethane,

triarylmethane,

anthraquinone or azo

Ionic bond

Direct Anionic, highly

water soluble,

poor wet fastness

fibers (acryl,

polyester, poly-

amide, cellulose)

Diazo, triazo dyes or

phthalocyanine,

stilbene or oxazine

Vander Waal

Forces

Disperse Colloidal low

solubility, good

wet fastness

Polyester, nylon,

acrylic cellulose

Acetate

small azo/nitro,

anthraquinones

metal azo compound

Colloidal

impregnation

and adsorption

Metal complex

(Cr, Co, Cu)

Anionic, low

water solubility,

good wet fastness

Wool, nylon Azo Ionic bond

Reactive Anionic, highly

water soluble,

good wet fastness

Cotton,

viscose, wool

Azo, metal complex

azo, anthraquinone,

phthalocyanine

Covalent bond

Sulfur Colloidal after

reaction in fiber,

insoluble

Cellulose

(cotton, viscose)

Polymeric aromatics

with heterocyclic

containing rings

Dye

precipitated in

situ in fiber

Vat

Colloidal after

reaction in fiber,

insoluble

Cellulose

(cotton, viscose)

Anthraquinones or

Indigoids

Dye impregnate

the fabric by

oxidation

Mordant dyes Colloidal in fiber,

Insoluble

wool, leather,

silk, paper

azo, oxazine or

triarylmethane

in situ in fiber

9

Apart from these dyes mentioned above, other pigment dyes are making a small group of

colorants which are non-ionic in nature and are insoluble compounds. Mostly pigment dyes are

azo compounds, anthraquinone and quinacridone or metal complex phthalocyanine and a

dispersing agent is required to obtain a dispersed aqueous solution [28].

1.2.3 Discharge Statistics of Dyes

Acidic and basic dyes represent the biggest group of colorants and their share among

reactive, azo, and direct dyes is considerably high. It is estimated that they constitute the

immense portion of discharged dyes from textile industries. The percentage of dye being lost to

the effluent is presented in Table 1.7 for all class of dyes [29].

Table 1.7: Percentage of dye lost to effluent [29].

On a global scale, million tons of synthetic dyes are synthesized and discharged directly into

the environment. Waste water containing dyes make the environmental challenge for textile

industry pose to main risk to surrounding environment, marine ecological system and the human

health [28, 29]. The serious impacts of waste water on natural water bodies and surrounding land

area include the obstruction of photosynthesis process which in turn causes alteration of habitat.

Besides, the serious impacts caused to the health of human beings include skin diseases

(chemical burns, irritation, etc), ulcers, lungs infection, and diarrhea. Adequate treatment of

contaminated waters is of primary concern in order to preserve the natural ecosystem [29, 30].

Therefore proper handling of the hazardous chemical, dyes, pigments is challenging today

otherwise they will pose adverse effects to all species on the earth. Industrial sectors are strictly

forced by water scarcity, environmental regulations and sustainable future approach to

implement the recycling of treated waste water. Therefore recovery of dyes is not an option but a

Dye class Loss to effluent (%) Acid 10-40 Basic 15-30 Direct 5-30 Disperse 0-10 Metal-complex (Cr, Co, Cu) 2-10 Reactive 5-20 Sulfur 10-40 Vat 5-20 Azoic/Ingrain 2-3

10

suitable treatment method must lead to final decomposition or removal of these contaminants

[31].

1.3 Waste Water Discharge Treatment

Since environmental safety regulations is becoming stringent, research and development

is required by textile plants for the advancement of waste water treatment before discharging it to

the environment. The purpose is to dispose of industrial wastes in a way without harming the

human health or damaging the natural ecosystem. The industrial effluents and household waste

water have been main cause of residual dye pollutants which are not readily biodegradable and

are disposed into the environment improperly. The water treatment process mainly depends on

the, nature of industry, type of the contaminants, and on its concentration level in the water [8, 9,

30]. The major motivation of study of water treatment is the lower the concentration of pollutant

in the discharge stream in order to fulfill the environmental regulation requirement so that water

can be reused in textile, semiconductor, rubber, microelectronic, pharmaceutical and various

other industries [11, 23]. Moreover, the choice of a particular, cost effectiveness, novel, efficient

and suitable process plays an important role for the water treatment to assure that overall water

quality is maintained after removing the pollutants, making it suitable for reuse or discharging it

to the environment [18].

Over past few decades, a variety of treatment methods have been flourished for the

removal waste from water (Figure 1.2) and much effort are paid for the elimination of colour

from waste waters contaminated with dye in order to meet the requirements of environmental

regulations and also economic constraints [20, 22].

Figure 1.2: Conventional methods for waste water treatment [20, 22].

Physiochemical Methods

1)Air stripping 2)Adsorption on activated carbon3) Membrane filtration4) Ozonation5) Cholorination6) Ion Exchange7)Electrochemical processes

Biological Methods

1) Bacteria (aerobic)2) Bacteria (anaerobic)3) Fungus

11

These conventional developed technologies for the degradation of dye or pollutants from

contaminated water include; i) physical methods such as adsorptive separation (using carbon,

ash, coal, mud, etc), electrocoagulation, coagulation, osmosis, filtration, ii) Chemical

processing such chlorination, ozonization, etc; and iii) biological treatment strategies (including

aerobic and anaerobic biological methods) [20, 21]. However the complete description of all

conventionally followed methods for degradation of these intractable organic pollutants in waste

water is given below in detail.

1.3.1 Waste Water Treatment by Physiochemical Methods

1.3.1.1 Air Stripping Processes

Air stripping is conventional method in which contaminated water is brought intimately

in contact with a gas (air) for the removal of undesirable compounds present in the liquid phase

which are carried away by the gas (Figure 1.3). It is mainly employed for the removal of odor,

color, oxidizing contaminants and volatile organic compounds [22, 23].

Figure 1.3: Air stripping method for waste water treatment [22].

1.3.1.2 Adsorption Process

Removal of dye and volatile organic pollutants from aqueous media is commonly carried

out by commonly used method of adsorption and is efficient for adsorbing mordant, direct, vat,

and acid dyes but less effectively for the removal of reactive dyes, cationic, dispersed and

pigments. The effectiveness of the methods mainly depends on the kind of granular activated

carbon (GAC) to be used and the type of the waste water to be treated [18]. Adsorption process

is generally a surface phenomenon in which organic and inorganic pollutants contact with a

12

highly porous solid surface. Intermolecular forces of attraction between liquid–solid cause some

of the contaminated molecules to saturate or deposited at the surface. This method usually

exhibit slow removal rates which can be improved to some extend by using high doses of

adsorbate [16, 17]. It is well suited for specific waste removal and is inefficient for removal of

others and effectiveness of dye elimination becomes erratic and reliant on substantial amount of

carbon. The other disadvantage of this technique is that activated carbon is costly and it produces

spent carbon as a waste product.

1.3.1.3 Membrane Filtration Method

Membrane filtration method has been extensively applied since 1960 for the separation of

dye molecules from textile effluents. Membrane filtration method is appropriate for textile

effluent if it contains little amount of dyes but it is not suitable for elimination of high dose of

dye and the dissolved solid contents which make the re-usability water difficult [23]. The

remaining residual concentration of dye after separation causes again disposal problems and

membrane replacements [14, 24].

1.3.1.4 Ozonation

Ozone waste water treatment gained popularity due to is its most potent and effective

germicide activity. During ozonation, ozone is generated by dissociation of oxygen (O2)

molecules into oxygen atoms by imposing a high voltage which subsequently collides with

oxygen molecule to form highly reactive (O3) which finally sanitize waste water (Figure 1.4).

Ozone is a very strong oxidant molecule but its life time is short for complete removal of color

and process offers high cost [22].

Figure 1.4: Ozonation method for waste water treatment [22].

1.3.1.5 Ion Exchange

Since, the innovation of ion exchange, ion exchange method is used for purification of

textile water. Usually, two types of ion exchange systems are being employed i.e. anion and

13

cation exchange resins. Waste water containing colored pigments is passed slowly through the

ion exchange resin so that cation and anion dyes are separated, resulting in the saturation of

active sites of ion exchanger by dye molecules [17, 28]. A process diagram is shown in Figure

1.5. Due to limitation of removal of certain dyes this method has not been extensively utilized for

the remedy of dye containing effluents. On the other hand resins are highly sensitive to the

organic matter present in water which cause the contamination of the resins which needs a

separate treatment section for the removal of suspended solids and organics [15, 27, 28].

Figure 1.5: Waste water treatment by ion exchange method [27, 28].

1.3.1.6 Chlorination

Chlorination is common method for waste water treatment, disinfection of municipal and

decolourization of textile dyes (Figure 1.6). Chlorine degrades the colour from textile water

having concentration in the range of 10-30 ppm when are added to effluent as gaseous, solid or

liquid form. However the discharge of effluent containing chlorinated organics is becoming

undesirable rapidly [22, 29].

Figure 1.6: Chlorination process for waste water treatment [29].

14

1.3.1.7 Electrochemical Processes

Electrochemical treatment involves the treatment of effluent by oxidation of dyes at

anode surface in electrolytic cell which consist of anode (Ti/Pt), cathode (stainless steel) and

electrolyte (sodium chloride). The dye destruction usually occurs by the active species which are

generated during the electrochemical process. The advantage of the current method is that there

is no formation of sludge and no utilization of chemicals but it is useful only for removal of

sulfur and vat dyes which puts restriction for its wide application [26, 29, 30].

1.3.1.8 Waste Water Treatment by Biological Methods

The biological processes basically involve aerobic biological treatment, anaerobic

biological treatment or treatment by utilizing the fungus. Biological treatment, coupled with

physiochemical method has been used to decolorize the dye effluent more economically now-a-

days [26, 31]. Aerobic microorganisms (usually bacteria) metabolize the organic matter present

in the water resulting in the production of inorganic by-produce especially CO2, NH3, and H2O.

Dyes (azo dyes) which are not effectively removed by aerobic process can be effectively

decomposed by anaerobic treatment process [28]. The by-products formed as a result of azo dye

degradation are usually toxic and colorless aromatic amines which are carcinogenic and

mutagenic and their complete biodegradation require a secondary aerobic treatment step. The

extend of treatment of both aerobic and anaerobic process depends on the degree of

contamination and yield huge volume of sludge equal to the volume of treated water and its

recycling is essential. Moreover, sedimentation process is carried out to separate the

microorganisms and to produce clarified secondary effluent [31].

1.3.2 Pros and Cons of Current Conventional Methods

Apart from the classification of dyes and their treatment methods mentioned above,

during degradation or decomposition of the chromophoric system of the dyes, dye intermediates

(aromatic compounds) with low biodegradability are formed. These intermediates or byproducts

are non toxic in nature and cause various health hazards when introduced to the aquatic system

[28, 32]. A variety of dyes intermediates are bromo or chloro derivatives of hydroxy, amino or

nitro aromatic compounds, naphthalene or sulfonated which are found in the effluents and are

difficult to remove by existing discoloration method [26, 30]. Some physical methods (air

stripping and activated charcoal adsorption) just transfer the pollutants from one form to another

15

form which are finally disposed into the environment and cause the air pollution. These

secondary products discharged to landfills and energy-intensive process is required for their

treatment. Moreover, economic reasons, dependency on the effluent characteristics and their

affinity to produce harmful by-products, make these processes of limited use [28, 31]. The

advantages and disadvantages of current conventional methods of dye removal from industrial

effluents are given in Table 1.8 [32, 33].

Table 1.8: Advantages and disadvantages of current conventional methods [32, 33].

The inefficiency of conventional waste water treatment methods to eliminate many

colored pollutants effectively urges the textile industries to explore efficient treatment methods

which give complete destruction and mineralization of these dyes effectively [17, 31]. New

methods should fulfill the constraint of recycling the decolorized water, with lower toxicity level,

capable of mineralizing the toxic organic components, no intermediate or harmful end/by

products should be formed during and after degradation process [33]. Currently, photocatatalytic

degradation of dyes received much attention due to its cost effectiveness, complete

mineralization of all dyes and organics, with no secondary products and excellent efficiency

[34]. This method degrades a number of recalcitrant pollutants into biodegradable compounds by

Methods Advantages Disadvantages Air stripping processes

• Effective for decolouration of soluble and insoluble dyes

• It can only take out chemicals that can evaporate

• massive dyes cannot be taken out Adsorption • Efficient for extensive removal of

variety of dyes • Very costly

Membrane filtration method

• Removal all types of dyes occur • Concentrated sludge generation • Contamination of membrane

Ozonation • Decomposes a wide range of dyes • Complete sterilization • on demand production &treatment

• Short life time of ozone • Toxic reaction product • Very expensive

Chlorination • Disinfection of waste waste • Decolourization of mostly dyes

• Only 10-30 ppm can be degraded • Chlorinated organic are formed

Ion exchange • No loss of adsorbent occur due to regeneration

• Not efficient for destruction of all dyes

Electrochemical processes

• Efficient decolonization • Products are non-toxic

• Production of sludge • High consumption of electricity

Biological methods

• Efficient degradation of dyes • Sludge production • Require high land area

16

the formation of highly reactive chemical species and emerged one of the promising technologies

in current era [33, 34].

1.4 Carbon Dioxide Increase: An Environmental Problem

With the growth of population, the basic demand of man towards energy increased

therefore human being developed different ways to utilize sources of energy [1, 3]. They

developed the skills to find better resources of energy such as fossil fuel, biomass, wind, fire, and

water. In the early 20th century, due to rapid increase in population and fast development in

industry raised difficulties for the world’s natural resources to sustain the demands. By and large,

the carbon based fuels such as fossil fuels (coal, oil, petroleum or natural gas) are burned in

power plants and in industry and this is resulting in the rapid depletion of the natural energy

sources [4, 5]. A major threat is rapid depletion of fossil hydrocarbon resources due to their

world wide consumption [6, 7]. Coal, natural gas and petroleum are the most abundant natural

resources which have been accumulated since tens of millions of years and have been consumed

rapidly since last 80 years, as shown in Figure 1.7 [35]. Out of these natural sources, Petroleum

and natural gas provide 85% of the energy requirements of the world and in late 20th century

with the development of fuel combustion raised their annual production rate. Worldwide,

consumption of petroleum, coal, and natural gas increased to 22%, 27%, and 71%, respectively

from 1980 to 2001 [34].

Figure 1.7: World total primary energy consumption by region, reference case 1990-2030 [35].

17

This fast production and utilization of energy is impacting the environment, the more

energy we utilize, the more pollutants we discharge in the environment [5, 7]. The

industrialization and civilization not only accrued technology, advance life expediency to

humanity but also introduced pollution due to emission from power plants, vehicles, and

factories [35, 36, 37]. However, various energy sources alternative to fossil fuels including wind,

solar, geothermal systems hydroelectricity, wind-powered electric turbines and nuclear power

but these sources cannot be long-term substitutes of fossil fuel because environmental and social

costs put limitation on their viability [38]. Utilization of each fuel produces different kind and

magnitude of environmental consequences. The consumption of fossil fuel resulted in the

emanation of green house gases (GHGs) which would extensively affect the global climate [36,

37]. In addition the drawbacks associated with alternative energy sources are dramatic

alternation of ecosystem and terrestrial habitat is caused by hydroelectric power; waste disposal

by fissile nuclear power; installation of basic infrastructure of wind-powered electric turbines

cause the death of migratory birds; emission of CO2 and hydrogen sulfide by geothermal plants

[36, 37]. These GHGs discharged from coal and petroleum combustion into the atmosphere

include the carbon dioxide, methane, SOx, NOx, and VOCs (Figure 1.8). Acid rain, global

warming, smog, and nitrogen loading are the innumerable environmental problems, caused by

these green house gases [38].

Figure 1.8: Major green house gases and their composition [37].

18

The major distress of these GHGs is enduring chronic health impacts related to large

concentrations of carbon dioxide which causes health damage. Their harmful effects on aquatic

flora and fauna and on foliage and soils are believed to be caused by sulfur or nitrogen oxides.

The major concern regarding the air pollution is carbon dioxide which contributes 50% of the

greenhouse gas effect because it has long life time in environment and high infra red absorption

capacity [38]. The concentration of carbon dioxide on a global scale is increasing and

concentration of CO2 into atmosphere increased from 300 ppm to 390 ppm from 1960 to 2009

and it is still accelerating further (Figure 1.9) [38, 39]. This rapid increase in concentration of

carbon dioxide is leading towards the climate change which is one of the biggest threats of

current century [39].

Figure 1.9: Carbon dioxide during the last 400,000 years and the rapid rise since the industrial revolution [38, 39].

1.4.1 Global Warming

Global warming refers to the increase in the average temperature of the earth in recent

few decades and in the 20th century it raised to 0.6 ± 0.2 °C (Figure 1.10) [40]. This effect is

observed predominately in the last 50 years which have been due to sudden increase in the

concentration of green house gas which is largely contributed by the burning of fossil fuels and

anthopogenic activities [36]. The global warming effect is mainly associated with the green

house gases (CO2, N2O, CH4, HFCs, PFCs, etc) present in our environment and CO2 is major

contributor which is coming from the fossil fuel consumption. CO2 is an odorless and colorless

19

gas with concentration of 0.039 % (389 ppmv), occurring in nature and is a major source of

carbon for photosynthesis of plants [5-7, 34]. Various industries (fossil fuel burning) along

human activities such as tropical deforestation, cement production caused CO2 production with

its increase in the atmosphere. Worldwide, the fossil fuel combustion contributes approximately

30 Gt per of total CO2 emissions, 37 billion tons (37 Gt) coming from human activity [34, 37]. 1

ton burning of carbon in fossil fuels produces 3.5 ton of carbon dioxide. Figure 1.11 presents the

carbon dioxide emission from various sources such as public electricity, power generation, fossil

fuel combustion. Other energy sectors cause the emission of carbon dioxide include manufacture

of solid fuels, coal mining, oil refineries, oil, gas extraction, and others [41].

Figure 1.10: Global mean surface temperatures 1856 to 2018 [40].

Figure 1.11: Sources of CO2 emission from fossil fuel combustion [40].

20

The increased CO2 caused global warming i.e. increase in global temperature which

resulted in the melting of ice near the poles, rise in sea level due to thermal expansion of the

ocean (Figure 1.12) [42]. The variation in temperature considerably affects the extent, regularity,

and intensity of the weather events (as storm, floods, famines, volcano eruptions, heat waves,

and cyclones). Other effects may include the variation in agricultural yields, species extinctions,

reduced summer stream flows and raise in the ranges of disease vectors [43]. An estimated

atmospheric CO2 has life time of 50 to 200 years and warming is expected to continue with its

long lasting effects on both human life and environment and it is believed by scientist that along

with variation in weather events and environmental destruction, the global warming may be

cause of disease and deaths across the world [44].

Figure 1.12: Green house effect caused by increase in temperature [42].

1.4.2 Carbon Dioxide Reduction Management

Carbon dioxide is an inert and stable gas and its reduction is highly challenging and its

control is one of the most important areas of GHGs control and this idea emerged from Kyoto

Protocol. Concerning the global challenges faced due GHGs there is need of the long-term

solution with inventive thoughts and innovatory approaches [40, 41]. There are five broadly

defined technical approaches for carbon dioxide management and these include energy

efficiency, energy choices, carbon dioxide sequestration, carbon dioxide capture, and carbon

dioxide utilization [45]. The choices including capturing of carbon dioxide, separation

techniques, purification methods, and its transportation are costly processes and huge amount of

energy is needed while dealing with them [44, 45]. CO2 can be captured from various sources by

21

physical, chemical adsorption, membrane separation and cryogenic processes which can further

be stored and used for various industrial applications including methanol production, for freezing

the items etc [46]. The advanced solution is the conversion of carbon dioxide which can be

converted into other industrially useful chemical products along other co-reactants at significant

scale [47]. The conversion and utilization strategies should be based on the distinctive the

chemical and physical proactive approach to the renewable augmentation which could preserve

carbon resources [45, 46]. Recently, few strategic approaches for carbon dioxide conversion and

utilization are listed below are for developing technologies. These strategies include (i) thermo

chemical, (ii) biochemical, (iii) electrochemical, (iv) radiochemical and (v) photochemical

methods [47].

1.4.2.1 Bio Chemical Conversion of CO2

Use of microorganism such as bacteria emerges as the most promising way of carbon

dioxide conversion into value added products. Theses bacteria include Methanobacterium

thermoautotrophicum [43, 44].

CO2+4H2 → CH4 + 2H2O (1) Bacteria

1.4.2.2 Chemical Reduction of Carbon Dioxide

Chemical reduction of carbon dioxide by the use of metals occurs at relatively high

temperature. Usually Mg, Na, Sn, K are the metals used for carrying the chemical reduction [46].

2Mg + CO2 → 2MgO + C (2)

Sn + 2CO2 → 2SnO2 + 2CO (3)

2Na + 2CO2 → Na2C2O4 (4)

1.4.2.3 Thermo-chemical Conversion of Carbon Dioxide

For thermo-chemical conversion an external input of energy is required and CO2

conversion into hydrocarbons or oxygenated hydrocarbons occurs at elevated temperatures and

pressures. The one step thermo chemical process which causes the direct decomposition of CO2

to CO and oxygen usually requires the free energy change of 257 kJ/mol and 3075 °C is needed

for 100% conversion of CO2 and the CO yield can be as high as 30% at 2400 °C, but the yield is

very low due to due to back reactions [44, 46].

CO2 + energy→ CO + 1/2O2 (5)

22

1.4.2.4 Electrochemical Reduction of CO2

The electrochemical methods for reduction of carbon dioxide have been widely studied

because these methods have noteworthy effect and it provided a good solution for both the

environment and energy related issues since past few decades [47]. In electrochemical reduction

of carbon dioxide an external power is supplied in the form of electricity generated by renewable

energy sources such as solar, tides, nuclear, wind, wave, hydro, and geothermal for electron

transport [48].

CO2+ xe− + xH+ →CO, HCOOH, (COOH) 2 (6)

1.4.3 Limitations of Current Carbon Dioxide Strategies

Biochemical conversion requires long period of time for CO2 reduction and obtained

efficiency is very low [40, 41]. While thermo-chemical and electrochemical method work at

elevated temperature or energy is provided by an external voltage bias to carry the reaction and it

also provides the low efficiency [44]. CO2 reduction is challenging and reported methods have

their own advantages but the disadvantages associated with them make the process tedious and

cost effective [41, 42]. In recent era, the photocatalytic reduction of CO2 into value added

compounds received much attention because it not only helps in removal CO2 from effluent

gases but also plays an important role in CO2 conversion into other chemical compounds

(methane, ethane, methanol, ethanol and formaldehyde) [49]. In contrast to conventional

methods, this process utilizes less energy-consuming source and occurs at low temperature and

pressure [50].

1.5 Nanoscience and Nanotechnology Approaches

As discussed previously, industrialization and urbanization resulted in the emission of

toxic chemical into the environment causing both water and air pollution and various treatment

technologies have been developed with the passage of time to meet these issues [29, 40, 42]. But

these remediation methods do not provide effective, stable, safe, economic and long term

solution [43]. Currently, attentions have been paid for the development of nanostructures for the

removal of toxic contaminates from water and air in order to facilitate the safe environment [33,

34, 50]. Over the past few years, photocatalysis based on nanomaterials appeared as promising

solution for the remediation of environmental related problems. The significant feature of

23

photocatalysis in the realm of environmental remediation is that it offers stable, cost-effective

and efficient solution to meet the environmental problems [51].

1.5.1 History of Photocatalysis

Photocatalysis is an important area of the nanotechnology which is growing promptly and

made significant development in scientific research. The beginning of photocatalysis appeared in

1972 by by Fujishima and Honda who reported the photocatalytic splitting of water and scientific

interest in this field increased with huge number of publications towards the practical and

theoretical applications of the photocatalysis [52]. Figure 1.13 demonstrates the current status of

scientific research made by photocatalyst materials in various fields. More recently, scientific

research in the field of photocatalysis is focused on its promising applications towards

environment and this includes waste water treatment and cleaning of air [53].

Figure 1.13: Chart illustrating volume of research work carried out each year in the field of photocatalysis [53].

1.5.2 Photocatalyst, Photocatalysis and Principle of Photocatalysis

By definition photocatalysis mean “catalysis or change in chemical reaction under light

radiation in the presence of a catalyst called photocatalysts which accelerates the chemical

process without being modifying itself” [51]. The term photocatalysis is broadly divided into two

classes i.e., homogenous photocatalysis (reaction occurs in homogeneous phase) and

heterogeneous photocatalysis (reaction occurs at the boundary between two different phases i.e.,

solid-gas, solid-liquid and liquid-gas [53].

24

A photocatalyst is semiconductor materials which is characterized by its characteristic

electronic structure and band gap i.e., energy gap of difference (ΔEg) between the valence and

the conduction band. Valence band (VB) is characterized as the highest energy band which

consist of occupied energy levels (by electrons) while conduction band (CB) is the lowest

occupied energy band (without electrons) [54]. According to theory of the band gap, when the

photocatalyst is illuminated with light (photons) having energy equal to or higher than the band

gap (ΔEg), the excitation of electron (e-) from valence to the conduction band occurs, leaving the

positive hole (h+) in the valence band and formation electron-holes pairs (e--h+), demonstrated in

Figure 1.14 [53, 54].

If recombination does not occur, these electrons and holes migrate to the surface of

semiconductor or photocatalyst thus inducing the redox reactions and participate in various

reduction oxidation processes with adsorbates (oxygen, water, and other organic, inorganic

species) having suitable values of redox potentials [55]. The process of electron-hole pair

recombination is dominant in the absence of suitable adsorbates which occur with the emission

of light or heat. Thermodynamically, oxidation of adsorbates occurs if valence band has

sufficiently more positive redox potential than adsorbates and reduction occurs if conduction

band posses negative redox potential much higher than the adsorbates. This oxidation and

reduction processes occur simultaneously on the photocatalyst surface which form the basis of

photocatalytic water and air remediation and various other processes [56].

Figure 1.14: Schematic illustration of steps involve in a photocatalytic process [53, 54].

25

1.5.3 Photocatalyst Materials

A variety of semiconductors oxide or semiconductor sulphides are particularly employed

as the photocatalysts for carrying the photocatalytic processes [57]. These materials usually have

the favorable combinations of properties such as suitable electronic structure, good light

absorption capacity, well defined band gap, electron-hole pair generation properties, capacity for

separation and transport of charges and good surface energy for carrying out the chemical

reactions [53, 54]. The band gap of semiconductors usually fall in the range of ΔEg ~ 350 to

1100 nm [55]. A large number of semiconductors (metal oxides and metal sulfides) have been

examined as photocatalysts which are available commercially and are reported in literature for

carrying various photocatalytic processes. These metal oxides and metal sulfides based

semiconductors include ZrO2, ZnO, ZnS, CeO2, TiO2, CdS, Fe2O3, WO3, MoS2 etc which have

the ability to promote chemical reactions by the absorption of light [58, 59]. The properties of an

ideal semiconductor to be an efficient photocatalyst include that it must be photoactive i.e., able

to cause excitation of electron in the presence of UV or visible light, photostable, chemically

inert, biologically inert, inexpensive and non-toxic [54]. Furthermore, the redox potential of the

conduction and valence band should be more negative and more positive, respectively, to reduce

the adsorbed species [55, 56].

Figure 1.15 depicts the values of redox potential and band gap energies of the commonly

used semiconductors. Some other photocatalyst does not meet the full criteria of an efficient

photocatalyst such as CdS, ZnS, CdSe and PbS are reposted as inadequately stable for

photocatalyst in aqueous media and are highly susceptible to the photo corrosion [59]. Some of

them (α-Fe2O3) exhibit visible light absorption but lower photocatalytic activity as compared to

the TiO2 or ZnO [60]. Similarly, GaP are degraded itself during the photocatalytic process and

cannot be used for several cycles and show toxicity [61]. Among different semiconductor

materials mentioned in Figure 1.15, titanium dioxide (TiO2) and zinc oxide (ZnO) is

demonstrated suitable for various environment related applications because of similar band gap

values i.e., (3.2 eV). Like other photocatalyst ZnO is readily converted to Zn(OH)2 when

disperse in the aqueous solutions which cause the deactivation of the catalyst [61]. However,

Titanium dioxide’s exhibits strong chemical, biological as well as photo stability, recyclability

and its low cost make it an alternative photocatalyst [62].

26

Figure 1.15: Band gap positions various semiconductors. The energy scale is indicated in electron volts using either the vacuum level (left) or the normal hydrogen electrode (NHE)

(right) as a reference [59-62]. 1.6 TiO2 Nanostructures

Titanium metal which is abundabtly available in the world and in oxide forms it exists in

three polymorphs forms anatase, rutile and brookite (Figure 1.16) [62]. Among these mentioned

classes, a anatase and rutile are of important due to their exceptional properties and can be

employed as photocatalysts. Both anatase and rutile structures possess tetrahedral geometry with

slightly distorted octahedra which constitute the basic building block of TiO2 [60]. The basic unit

cell in both cases consists of a titanium atom which is surrounded by six oxygen atoms in regular

octahedron. Both anatase and rutile structures show little structural differences but significant

electronically variations [63], basic characteristics are tabulated in Table 1.9.

Figure 1.16: Elemental unit cells of anatase (a), rutile (b) and brookite (c) [62].

(a) (b) (c)

27

Table 1.9: Properties of different form of titanium dioxide [63].

Figure 1.16 illustrates the three phases of TiO2 i.e. anatase, rutile, and brookite. In TiO2,

the valence and conduction bands are formed by O2p states and Ti3d states, respectively. Anatase

and rutile have band gap of about ~3.2 eV and ~3.0 eV, respectively, but exhibit different

catalytic activity due to difference in the position of these bands with respect to redox potential

of water makes them to and higher photocatalytic activity is shown by anatase [63].

1.6.1 TiO2 Nanostructures as Photocatalyst

Among all semiconductor sulphides and transition-metal oxides, TiO2 is demonstrated as

one of the most efficient compound in the field of material science. In the beginning of 20th

century, TiO2 is applied for commercial applications and its demand rose rapidly with the

passage of time [62]. TiO2 has wide band gap energy ~ 3 eV, and electrons having energy in the

UV range can excite electrons from valence to conduction band. Immense efforts have been

stanched after the work of Fujishima and Honda to the research on TiO2 material [50]. The better

performance of TiO2 as compared to other phtocatalyst is highly attributed to oxidizing holes and

hydroxyl radicals which are formed after photoexcitation of electrons [63]. These hydroxyl

radicals in term of oxidation potential possess high oxidation powerful than those available

oxidants. TiO2 is most well known candidate among several investigated photocatalyst because it

acquired inimitable physical and chemical properties such as [64, 65];

• Huge surface area and surface active sites

• Non-toxicity

• Low cost and available abundantly

• High photocatalytic activity as compared to other phtocatalyst

• Extraordinary optical, electronic and thermal properties

• Excellent biological, thermal, physical and chemical stability

Property Anatase Rutile Brookite Meting point (oC) ~1825 ~1825 ~1825 Boilng point (oC) 2500~3000 2500~3000 2500~3000 Crystallographic structure Tetragonal Tetragonal Orthorhombic Lattice parameters (nm) a=0.3784

c=0.9515 a=0.4594 c=0.2959

a=0.5442 c=0.9168

Unit cell volume [10-3 nm3] 34.172 31.216 32.567 Density 298 K (g/cm3) 4.230 4.250 4.133 Band gap (eV) 3.2 3.03 2.99

28

• Insoluble in water & exhibit good recyclability

All these mentioned properties make TiO2 better candidate for its applications towards

energy and environmental in contrast to other photocatalysts materials [65]. It is utilized various

fields in our everyday life such as white pigments, cosmetics, cements, ceramics, papers, paints,

beauty creams, coatings, food additives, killing bacteria and more recently its application as

photocatalyst include anti-fogging, solar cells, solar fuels, gas sensors, anti-microbial water

splitting, decontamination of contaminants from water, air purification [65, 66]. Figure 1.17

demonstrates the wide range of applications of TiO2 as photocatalyst in different domain of

environment [66].

Figure 1.17: Application fields of TiO2 photocatalysis [66].

1.6.2 TiO2 as Photocatalyst: Key Limitations As mentioned previously, the fundamental steps which direct the photocatalytic process

includes the absorption of light, formation of electron and holes, trapping of charges carries

species in the lattice [51, 52]. For a photocatalyst to exhibit excellent efficiency, these all

parameters must b fulfilled. As demonstrated earlier, that there are many semiconductors which

do not execute these requirements and does not fall in the category of an efficient photocatalyst

[59, 60]. Although, TiO2 is found to be a potential candidate which is dominant over other

semiconductors due to its stout characteristic but its ability to absorb in UV region limits its

29

efficiency [64]. Our solar spectrum contains limited fraction of UV portion and constitute 5%

and visible spectrum consist of 50-52%, this UV region activity enhance the charge

recombination thus lowering the catalytic activity [66, 67]. Moreover, a significant fraction of

the solar radiation is therefore unavailable for reactions [67].

Our earth receives huge amount of energy from the sun which is used from various

purpose, including heating the earth, carrying the photosynthesis, maintaining the temperature of

the atmosphere [67]. Approximately, 1.7x1014 kW (1.5x1018 kWh per year) energy density

(power density) is received from the sun by earth and its value may vary from sunny to cloudy

day as shown in Figure 1.18 [67]. Therefore, the purpose is the development of visible light

photocatalyst which can cover wide spectrum of visible range and can be used under natural

sunlight for carrying the phtocatalysis of the pollutants and to exhibit maximum activity [65].

Figure 1.18: Solar irradiance and UV-Vis range of solar spectrum [67].

1.6.3 Approaches to Improve TiO2 Photocatalytic Performance

Recently, many research efforts have been made for the development of TiO2 with high

activity and high efficiency in the visible region or under natural sunlight.. The photocatalytic

activity of titanium dioxide mainly depends on the particle size, surface area, surface active sites,

porosity and band gap [64, 65]. Being UV absorber candidate, TiO2 show low photonic yield for

degradation of pollutants which can be enhanced by following approaches [68, 69];

(i) Band gap engineering or photosensitization which extends the band gap to visible region.

30

(ii) Lowering the electron hole recombination or enhancement of charge transport to the

surface.

(iii) Deposition of noble metals on the surface.

(iv) Doping of titania matrix with cation or anion which will shift the position of conduction

or valence band and low energy photons will cause the photoexcitation.

These aforementioned approaches have been widely adopted by the scientific community

because they result in the improvement of the photocatalytic performance of titanium dioxide

[70].

1.6.3.1 Surface Sensitization

On way of modification of titanium dioxide is the sensitization with a dye molecule

which extends the absorption capacity toward visible region because dyes exhibit usually visible

response due to conjugated system [Figure 1.19 a]. Dyes used for sensitization include the

thionine, rhodamine, ruthenium (II) trisbipyridine and phtahlocyanines. Upon illumination, dye

molecule is excited which then transfers photoexcited electron to the conduction band of TiO2

thus improving the separation of electron and hole [71].

1.6.3.2 Dual Semiconductor Systems

Coupling of titanium dioxide with other semiconductor (composite photocatalyst) having

different position of valence and conduction band provides another tool for charge separation,

increase in the life time of separated charges, increase in visible response and charges transfer

from the semiconductor surface to the adsorbed specie [72]. One of the photocatalyst in coupled

semiconductor has low band gap while other posses wide band gap with suitable potential

values. In general coupled semiconductor includes TiO2/SnO2, TiO2/CdS, TiO2/ZrO2, TiO2/ZnO,

TiO2/Fe2O3, and TiO2/WO3 [71, 73]. For example in CdS/TiO2 system (Figure 1.19 b), the

photogenerated electrons are transferred from CdS to TiO2 particles while holes remain in CdS

resulting in accumulation of holes in valence band of one photocatalyst while electron in the

conduction band of other photocatalyst [73].

1.6.3.3 Metal Deposition

The photocatalytic processes can be altered by the deposition of the metals on the

semiconductor surface, the deposited metal usually alter the surface properties [67]. During the

excitation of electron in TiO2 system, the excited electrons are trapped by the deposited metal

31

which leads to the decrease recombination and improved separation of electro-hole pair (Figure

1.19 c). TiO2 surfaces are usually deposited by Pd, Pt, Ni, Ag, Cr, Cu, etc [72, 73].

Figure 1.19: (a) Dye sensitization of titanium dioxide (b) TiO2/CdS coupled semiconductor (c) TiO2 deposited with metal [70-73].

1.6.3.4 Band Gap Engineering by Doping

Band gap engineering by doping is the process in which the band gap of the

semiconductors is altered by varying its electronic properties [74]. Doping of semiconductor

with impurities introduces additional energy levels within the band of the photocatalyst, resulting

in the shrinkage of the band gap [75, 76]. The purpose of modification by impurities is to

enhance absorption properties, reduce rate of recombination, extend visible respoce, and increase

surface properties [76]. Semiconductors can be doped by anion/non-metal (C, N, S, B) or

transition metal ion (V, Cr, Mn, Co, Ru, Ni, Cu, etc) or co-doped with anion and metal ions [68,

69, 74, 76, 77]. Interestingly, co-doped metal ions lower the band gap and introduce visible

response; transition metal ion not only reduces the band gap in co-doped sample. It also lowers

the chances of excited electron to recombine with the photogenerated hole (Figure 1.20) [78].

Figure 1.20: Un-doped, anion doped and anion-metal ion co-doped TiO2 [78].

(a) (b) (c)

32

Modification by doping is an efficient way to enhance the photocatalytic activity because

doping effects largely electronic properties, structural properties, nanocrystallite size, chemical

environment surface area and porosity and these factors determine the catalytic efficiency of the

photocatalyst [72, 73]. Theoretically, the concentration of defects in the crystal system increases

with increase in the impurity concentration and this can cause the narrowing of the band gap,

improvement of charge separation and visible response of the photocatalyst [75].

1.7 Environmental Applications of TiO2 Nanostructures as Photocatalyst

TiO2 nanostructures attained broad applications in the field of photocatalysis such as

treatment of waste water and carbon dioxide reduction in the presence of UV, visible or solar

light because of the unique properties of these materials such as low cost, thermal stability,

chemical stability, etc [62, 63]. The photocatalytic treatment of water and photoreduction of

carbon dioxide (i) usually occurs at ambient temperature and pressure, (ii) the radicals generated

as result of photocatalysis are enough strong which cause the complete mineralization of the

pollutants, (iii) no extra input of energy in supplied externally, iv) the photocatalysis process for

water treatment and CO2 reduction are less expensive, stable, non-hazardous and show

recyclability [72, 77].

1.7.1 TiO2 Photocatalyst for Water Treatment

The photocatalytic treatment of waste water especially for the decomposition of organic

pollutants (dyes, phenol, chlorophenol etc) drawn much attention in past few years because it

provides a new and an efficient way of complete decomposition of the pollutants [55, 62, 63].

Usually, photocatalysis using semiconductor materials such as titanium dioxide is employed

when the pollutants are not effectively treated by the conventional treatment methods such as

adsorption, air stripping, coagulation/flocculation, membrane filtration etc [14, 24].

photocatalysis decompose or mineralize the pollutant into harmless or biodegradable products in

the presence of UV or visible or light [57, 58]. The advantages of the photocatalysis over

previously used treatment technologies include [72, 73];

1. The pollutant which are resistant to other oxidizing agent can be easily oxidized by the

hydroxyl radical generated during photocatalytic process by the breakdown of water.

2. This process yields harmless products and thus this method is known as green technology

3. Atmospheric oxygen and water are used as oxidant and reductant, respectively, and there is no

need of addition of extra oxidant.

33

4. The photocatalysts cause the fast treatment of water.

1.7.1.1 Mechanism for Pollutant Degradation

When titanium dioxide absorbs UV radiation from sunlight or illuminated light source, it

will produce pair of electrons and holes [53, 54]. The electrons of valence band are excited when

illuminated by light. The excess energy of this excited electro promoted the electron to the

conduction band of titanium dioxide therefore producing the negative electron (e-) and positive

hole (h+) pair within the catalyst [55, 56]. A portion of this photoexcited electron-hole pairs

diffuse to the surface of the catalytic particle (electron hole pairs are trapped at the surface) and

take part in the chemical reaction with the adsorbed donor (D) or acceptor (A) molecules [49, 50]

starting a redox reaction. The positive holes can oxidize donor molecules i.e., break apart water

molecules to form hydrogen cation (H+) and hydroxyl radicals (OH●) [62, 63]. Whereas the

negative electrons can reduce appropriate electron acceptor molecules i.e., react with oxygen

molecules to form super oxide radical anion (Figure 1.21) which participate in a series of

oxidation reactions and generate hydroxyl radicals (OH●) [64]. Thus illuminated TiO2

photocatalysts is capable for the generation of hydroxyl free radical through photogenerated

holes and electrons which can finally decompose and mineralize organic compounds by leading

to dye decomposition to harmless substance that can be released to the environment [78]. This

cycle continue when the light is available and the relevant reaction is shown in Scheme 1.1 and

Figure 1.21.

TiO2 + hʋ→ e─+ h+ (1)

H2O + h+ → OH● + H+ (2)

O2+ e- → O2● ─ (3)

O2● ─ + H+ → HO2

● (4)

HO2● + HO2

● → H2O2 + O2 (5)

H2O2 + e- → OH● + OH─ (6)

OH● + dye → Products (7)

Scheme 1.1: Photocatalytic steps involved during degradation of pollutants [78].

34

Figure 1.21: Photocatalytic process followed during photocatalytic degradation of dyes and pollutants [78].

1.7.2 TiO2 Photocatalyst for Carbon Dioxide Reduction

Recently, the reduction of carbon dioxide or its recycling to fuel under visible irradiation

or solar light not only offers to meet the environmental problems but offers a new breakthrough

for a sustainable energy future [49, 50]. By the photocatalysis process the carbon dioxide can be

converted into various industrial and energy bearing compounds such as methane, ethanol,

methanol, formaldehyde, etc [62, 63]. The wavelength of light and position of valence and

conduction band or their potential values directs the effective carbon dioxide reduction [58, 59].

Semiconductors with large band gap such as TiO2 provides photoreduction of carbon dioxide

because of its sufficient negative and positive redox potentials of conduction and valence band,

respectively [63, 64]. Carbon dioxide is stable molecule and huge amount of energy is required

for the breakage of bond. Solar energy can be renewable source of energy for fixation of

economical CO2. However, difficulty is faced during reduction or splitting of water to give

hydrogen [78]. The breakage of water molecule and reduction of carbon dioxide occur

simultaneously to form hydrocarbons. The band gap or position of conduction and valence band

puts restriction to fulfill the thermodynamic requirements of carrying these processes

simultaneously [65]. This can be overcome by doping which shifts the position of the bands, thus

providing minimum energy needed for splitting of both CO2 and water [77, 78].

35

1.7.2.1 Mechanism for Carbon Dioxide Reduction

Photochemical conversion of CO2 and H2O into products follows the similar mechanism

as discussed previously in section 1.7.1 [63]. Upon illumination of TiO2 with UV irradiation

electron-hole pairs are generated which initiate a photocatalytic reaction. The photoexcited

electron (e-) and positive hole (h+) are formed in the catalyst lattice are separated and trapped by

appropriate sites TiO2 to evade recombination [55, 56]. The hole (h+) reacts with adsorbed H2O

on the surface of the catalyst giving rise to OH- ions and H+. The H+ ions interact with excited

electrons, resulting in the formation of H● radical, the OH- ions reacts with hole, donating

electron to hole and form OH● radical. Meanwhile, the adsorbed CO2 react with excited electron

and form CO2● ─. These highly active radical H● and CO2

● ─ react with each other to produce CO

and OH─. The OH─ is readily converted into OH● radical by reacting with hole as mentioned

previously, whereas CO passes through consecutive reactions and form ●CH3 which ultimately

react with H● to produce CH4 [79]. While ●CH3 may react with OH● radical to form methanol or

ethanol is formed by reaction of ●CH3 with different radicals, generating different intermediate

which ultimately combine OH● to give up ethanol [80]. The use of water here is as reductant

which is source of hydrogen. Two important species which play vital role in the photoreduction

of CO2 are H● and CO2● ─ radicals which are formed by the transfer of electron from conduction

band [78-80]. This process is referred to as heterogeneous photocatalysis or more specifically,

photocatalytic carbon dioxide reduction. In the CO2+H2O conversion to alcohol (methanol,

ethanol) and methane is depicted in Scheme 1.2 and Figure 1.22 [80].

TiO2 + hʋ→ e─+ h+ (1) H2O + h+ → OH● + H+ (2) H++ e- → H● (3) OH─ + h+ → OH● (4) CO2 + e- → CO2

● ─ (5) CO2

● ─+ H●→ CO + OH─ (6) CO + e-→ CO● ─ (7) CO● ─+ H ● → ●C+OH ─ (8) ●C+ H+ + 2e- → ●CH2 + H+ + 2e- → ●CH3 (9) ●CH3 + H+ + 2e- → CH4 (10) ●CH3 + OH● → CH3OH (11) ●CH3 + ●CH2 + e- →● CH3─CH2 + OH●→ CH3─CH2─OH (12) Scheme 1.2: Photocatalytic steps involved during carbon dioxide reduction [80].

36

Figure 1.22: Photocatalytic process followed during photocatalytic reduction of carbon dioxide [78].

37

1.8 Literature Survey

In the last few decades, titania as a photocatalytic material has been investigated extensively

for environmental remediation, solar energy utilization and hydrogen production due to its low

cost, non-toxicity, chemical stability and high oxidative power of generated holes [63, 64].

Nanophased TiO2 which has a large surface area that can facilitate a fast rate of surface

reactions, is a widely used wide band gap semiconductor which has attained considerable

attention due to its distinctive applications as photocatalysts, gas sensors, solar cells and

electrochemical devices [65, 66]. Recent research on Titanium dioxide (TiO2) is mainly focused

on the understanding of its strong photocatalytic activity which is useful in environmental

pollution remediation, such as air purification, hazardous waste remediation and water

purification [66]. The presence of pollutants CO2, NO2 in air, organic dyes and many other

organic hydrocarbons (phenol, formaldehyde and gasoline), produced as a result of many

industrial processes (manufacture of dyes, food processing, pesticides, polymers) and in

industrial wastes have caused severe environmental problems [3, 4]. However, there are some

limitations regarding its relatively large (3 to 3.2eV) band gap and high electron hole

recombination. Less than 5% of the whole radiant solar energy can be captured by pure titania [4,

64].

To address these drawbacks various modifications have been made since Fujishma and

Honda’s pioneer work in 1972 [5]. There are however, a number of attempts physical and

chemical processes which were devoted to design and develop a second generation of visible-

light sensitive photocatalysts of titanium dioxide [68, 69]. Until now, several strategies involving

noble metal deposition, transition-metal ions doping, coupled semiconductor systems [70, 71]

has been reported. Doping with anions not only modifies the conductivity and optical properties

but also introduce new surface states that may lie close to the conduction or valence band of

TiO2 [72]. Recently an innovative technique involves the doping of TiO2 with anions such as

carbon, nitrogen, phosphorous, sulfur and fluorine reported as a good tool in a desired band gap

narrowing and an enhancement in the photodegradation efficiency under visible light [72,73].

Among the anionic doping into titania, Asahi et al and others [74, 75] reported that nitrogen is

the most suitable dopant due to its comparable size and electronegativity to that of oxygen which

consequently reduces the band gap width. Extending the TiO2 spectral response and improving

its photoreactivity by doping with transition metals such as Cr, V and Fe [72, 73] have also been

38

explored recently. It was believed that transition metals could act as shallow traps in the lattice of

TiO2, which benefit to suppress the recombination of photoinduced electron-hole pairs when

migrating from inside of the photocatalyst to the surface [76, 77].

These reports clearly indicated that modification of titania by codoping proved to be an

effective tool for enhancing its photocatalytic activity [73, 75, 76]. Silver can scavenge

photogenerated electrons and improves the quantum efficiency by Schottky barrier formation

without having considerable effect on the band gap width of titania [78, 79]. Doping with metals

and non metals were the most feasible methods for improving the photocatalytic activity of

titania [79]. Khan et al. [80] reported that photocatalytic degradation of toluene by using boron

and carbon co-doped titania nanoparticles. Zhao et al. [81] reported that the photocatalytic

response of titania could be enhanced by using B–Ni co-doped photocatalyst, modified sol–gel

method. They proposed that boron incorporated into TiO2 extends the spectral response to the

visible region while Ni doping could greatly enhance the photocatalytic activity [82]. Balek et al.

[82] reported that N and F co-doped titania photocatalyst are good candidate for for acetaldehyde

decomposition. The effect of N-F co-doping on photocatalytic activity was investigated in detail

and the reasons for exhibiting the photocatalytic activity under visible light were also elucidated.

Yin et al. [83] synthesized F and Zn co-doped TiO2 nanopowders for improving the

photocatalytic performance. Wei et al. [84] reported the preparation of boron and cerium co-

doped TiO2 for extending spectral response to the visible light region and pointed out that the

photocatalytic activity of Boron-Ce codoped titania was much higher than that of P25. LI Fa-

tang et al., reported the photocatalytic decomposition of methylene blue (MO) in aqueous media

under UV irradiation using nano-F−/Fe3+/TiO2 particles [85]. Kewei Li e t. al., fabricated rutile-

anatase phase mixed Fe + N codoped TiO2 nanowires by a two-step anodic oxidation method for

the photocatalytic degradation of methylene blue [86]. Yang et al. [87] and Liu et al. [88]

designed bismuth/tin oxide and nitrogen/cerium co-doped TiO2 photocatalyst, respectively with

enhanced photocatalytic activity for the degradation of MB under visible light irradiation. Ye

Cong et. al., studied the photocatalytic activity of Rhodamine B in the visible-light range [89].

Hussain and his coworkers developed an ultra efficient cobalt tailored silver and nitrogen co-

doped titania (TiON/Ag2O/Co) visible nanophotocatalyst using modified reverse micelle

processing. The cobalt tailored nanophotocatalyst showed remarkable activity against

Eriochrome Black T (EBT) [90].

39

In another study, Hussain et al., prepared an ultra efficient visible nanophotocatalyst

based on transition metal (silver, ruthenium and copper) and nitrogen doped titania (TiON/Ag,

TiON/Cu, TiON/Ru) having homogeneous metal distribution, narrow band gap and high surface

area by co precipitation method. The visible light catalytic activity of samples was investigated

for the photodegradation of auramine O, congo red and methyl yellow as model water pollutants.

The synergetic effect of transition metal and nitrogen boosts up the visible light activity of

modified titania nanoparticles. While, the comparative degradation behavior shows that

TiON/Ru responded with extraordinary photocatalytic activity against auramine O, congo red

and methyl yellow which can be predominately attributed to enhanced surface area, low band

gap energy and efficient distribution of metal in TiO2 network [91]. Erin M. Rockfellow

prepared sulfur doped titania which resulted in increases the absorbance toward visible light by

introducing orbitals (S3p orbitals) within the band gap. This visible light photocatalyst was then

used for efficient degradation of phenol, quinoline and other aromatic organic molecules (4-

methoxyresorcinol 1-(p-anisyl)neopentanol) [92]. Highly efficient sulfur doped doped titania

photocatalyst was also reported by Yuping Wang et al., for photodegradation of L-acid under

visible light [93]. Minghua Zhou demonstrated that C,N, and S tridoped titanium dioxide

exhibited enhanced degradation in UV-Vis light for decomposition of formaldehyde as

compared to undoped TiO2 powder [94].

S doped Titania nanophotocatalysts was successfully fabricated through sol gel technique

by Hamadanian et al., [95] using sulfur source i.e thiourea to attain elevated photoactivity in the

visible range. He scrutinized the photoactivity by decomposition of methyl orange as model

pollutant. Anatase TiO2 co-doped with 1 wt% of Mg (ll) and Cu (ll) was fabricate by T. A.

Segne, [96] which exhibited visible light response and highest efficiency for degradation of

methylene blue. A.Dobosz and A. Sobczynski, [97] studied phenol degradation by the Ag

deposited titania and 30% increase for titania activity after 1.5% Ag deposition was obtained.

The Pd additive influences the activity of TiO2 for phenol oxidation, demonstrated by Papp et al,

[98]. Phenol degradation was also carried out by Crittenden et al, [99] by the surface

modification of TiO2 by Pt, Ag and Fe2O3. Kyoung et al, [100] observed the phenol

decomposition by Cu deposition on titania surface which resulted in increase in the activity for

phenol oxidation.This titania based nanomaterial have also been extensively explored as a

catalyst for water splitting, the production of solar hydrogen and for the conversion of green

40

house gases into energy producing products for methane and methanol [49, 50]. Recycling

carbon dioxide into valuable products has been carried out since long ago and Halmann

summarized utilization of various semiconductors for carbon dioxide reduction in aqueous media

[101, 102]. The study on the photo catalytic reduction of CO2 in aqueous solutions was carried

out by Inoue et al. in 1979 utilized various semiconductors (such as WO3, TiO2, ZnO, CdS, GaP,

and SiC) and also determined photoreduction of carbon dioxide into various products including a

mixture of formaldehyde, methanol, formic acid and methane by using both mercury and xenon

lamp as source of irradiation [103].

Anpo and Yamashita compiled studies focusing on heterogeneous photocatalytic systems

using different titanium oxide catalysts for the photocatalytic reduction of carbon dioxide in the

aqueous solution [104]. In 1987, investigations related to the photo-methanation of CO2 were

investigated by Thampi et al. in the gas phase over Ru-loaded TiO2. Catalyst, 1 mL CO2 and 12

mL H2 at STP were filled in pyrex cell and methane formation rate was monitored which was

0.17 mmol/h, .18 mmol/h and 10.5 mmol/h at 25oC, 46oC and 90oC, respectively, under a solar

simulator [105]. Since few decades, titanium dioxide emerged as an ideal catalyst for

photocatalytic reduction of carbon dioxide [104, 105]. In various studies, authors demonstrated

the improved performance of titanium dioxide by the addition of cocatalyst such as Pt, Au, Ni,

Fe Cu, Pd, Rh, and Ru with TiO2. An enhanced methanol and formaldehyde yield was obtained

by photocatalytic reduction CO2 by copper deposited TiO2 suspension in aqueous phase as

reported by Hirano et al [106]. Cu/TiO2 and Ag/TiO2 were synthesized by a sol-gel method by

Tseng et al., which showed an active production of methanol when CO2 was passed through an

aqueous solution of NaOH. 2 wt% Cu loaded TiO2 produced 16.7 mmol/g-cat/h of methanol

under irradiation of 254 nm source of light. XPS and EXAFS studies proved that this catalyst has

isolated Cu(I) on the surface which act as active site for photo reduction. However, as compared

to Cu/TiO2 the Ag/TiO2 gave lower methanol production rate i.e., 14.3 mmol/g-cat/h which may

be due to the size of Ag clusters which cause reduction in yield [107]. Yamashita et al prepared

TiO2 dispersed zeolites by anchoring and ion exchange method and predicted high photocatalytic

CO2 reduction into CO over Ti-ZSM-5 while higher selectivity to CH4 and CH3OH was obtained

by Ti-Y-zeolite and with Ti dispersed on porous Vycor glass (PVG). The photoreaction

experiments were performed with mixture of CO2 and H2O vapors at 0-50 oC using high pressure

mercury lamp. Highly dispersed titanium oxide was detected by XANES and EXAFS spectra

41

which act as active site for photocatalytic CO2 reduction. 24 mmol CO2 and 120 mmol H2O were

detected with observable quantity of methane and methanol i.e., 3.6 and 1.4 mmol/g-TiO2/h,

respectively, after illumination at 55oC [108].

The effect of Pt as co-dopant on TiO2 was also investigated and it was found that Pt

encourages the selectivity of CH4 as compared to CH3OH and it levels off at 13.3 mmol/g-TiO2/h

for CH4 and 0.2 mmol/g-TiO2/h for CH3OH. Presence of Pt acts as electron reservoir which

transfer electron to the reactants and results in methane formation [109]. Tan et al used TiO2

pellets for carrying the photo catalytic reduction of carbon dioxide and achieved significant yield

due to increase surface area of the pellet which enhance the contact areas and ultimately

adsorption capacity thus resulting in high yield [110]. The effect of particle size of TiO2 was

studies in terms of photo catalytic reduction of carbon dioxide and products selectivity by Koci

et al and it and high yield of the products (methanol and methane) was obtained with TiO2 nano

particles having small particle size under illumination of light [111]. Multi-walled carbon

nanotube supported TiO2 were prepared by Xia et al using sol gel and hydrothermal method and

were used for the photo-reduction of CO2 with H2O and different products were obtained by this

composite. The main product was C2H5OH which was obtained by the composite catalysts

prepared by the sol–gel while HCOOH was investigated as major product with good yield by the

sample synthesized by the hydrothermal method [112].

NiO/Ca2Fe2O5 nanocatalyst were prepared by Y.Wang et al and were used for the

Photocatalytic production of H2 in the presence of carbon dioxide. It is investigated that carbon

dioxide may react with water to give HCO-3 and CO32- intermediates which further prop up the

hole scavenging process by OH radicals resulting in enhancement in the photocatalytic activity

and production of formic acid [113]. In another study, the CO2 reduction was determined on

Cu/Ce co-doped titanium dioxide by Luo et al. The material was prepared by wetness

impregnation method and the maximum production of methanol reached upto 180.3 μmol/g-cat

rapidly this co-doped titanium dioxide. It was believed that Ce atoms act as site which causes the

activation of H2O and CO2 moieties, while Cu atoms act as photoelectrons reservoir thus

averting the electrons and holes recombination process [114]. In the past decades other literature

reports on photocatalytic CO2 reduction and degradation of dyes show that much progress in the

field of TiO2 nanotubes also. After the innovative study of Kasuga et al. [115], titanium dioxide

nanotubes (TNTs) have gained much attention of researchers because of their specific

42

microstructures, phases, and excellent optical properties. They have been widely explored for

applications in photovoltaic cells, batteries, oil-refining, desalination, sensing, photocatalysis, etc

[116]. The unique characteristics of these nanotubes are largely due to the presence of defects

and local distortions in the metal–oxygen layers. Similarly, the possibility of changing the degree

of oxidation of the central atom in the metal–oxygen polyhedrons of the nanotubulene layers also

results in the alteration of their properties. A large surface area, high aspect ratio and pore

volume play a key role in determining the electronic structure and the physicochemical

characteristics of nanotubulenes [116, 117, 118]. A number of synthesis methods have been used

to synthesize nano size TiO2 tubes: via: anodic oxidation, photo electrochemical etching, sol–gel

processing, hydrothermal synthesis, template synthesis, etc [119, 120, 121]. Among these

processes, the hydrothermal synthesis is a relatively simple method to synthesize porous and

tubular structures. TiO2 nanotubes fabricated by this method are highly ordered and crystalline in

nature, and they have high aspect ratios. The mechanism of formation and the morphology

(diameter and length, wall thickness, size distribution and size of nanotube agglomerates) of

nanotubes are still an issue of debate [122, 123].

Hussain and Siddiqa developed a facile and a new synthesis procedure for the production

of Fe/Cr doped and undoped TiO2 nanotubes via hydrothermal synthesis from TiO2 nanoparticles

and its application for the degradation of organic waste, like phenol and carbon dioxide

conversion to alcohol using UV and visible radiation was reported. The effect of dopant on

degradation of phenol, conversion of carbon dioxide to alcohols and on its optical properties

were also studied, the results compared with other studies described above. The data suggest the

improvement not only in the synthesis, but also in its industrial applications [124]. In another

studies, Hussain and his coworkers demonstrated relatively low-temperature/pressure and the

time-efficient synthesis of TNTs doped with different concentrations of Fe using

cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method. And determined that

the significance of current method lies mainly in its simplicity, flexibility, short reaction

conditions, high purity, high yield, and the control of material morphology which determines the

magnetic properties. The prepared nanotubes exhibited extraordinary visible response and good

photocatalytic activity for the photo-reduction of carbon dioxide in visible range [125]. Liang

and Zhong prepared titanium dioxide nanotubes by anodization method and evaluated the

photocatalytic degradation of 2,3-dichlorophenol by the UV light [126]. The photocatalytic

43

oxidation of acetone and photocatalytic degradation of methyl orange was carried out by

mesoporous titania nanorod/titanate nanotube composites in aqueous phase and 2.5 times higher

photocatalytic oxidation of acetone was obtained with nanotubes as compared to P25 commercial

titania. This high efficiency of nanotubes could be ascribed to the larger specific surface area,

good band gap and a higher pore volume [127]. Ma et al., investigated the activity of titania

nanotubes and Fe doped titania nanotubes for photodegradation of direct blue, methyl blue, and

reactive black, effect of dye concentration, reaction time, dopant content was also assessed under

UV irradiation. The maximum degradation efficiency was obtained with 10 ppm direct b lack 22

which was below 40% with 0.04 gL−1 titania nanotubes [128].

Kar et al., successfully fabricated the titanium dioxide (TiO2) nanotubes by anodization

method over titanium wires and titanium foil and revealed that TNTs grown on titanium wire

exhibited a significant photocatalytic activity as compared to those grown on Ti foils. The

degradation efficiency was determined in terms of decomposition of textile dye, methyl orange

and increase in efficiency was observed from 19% over a foil to 40% over wires [129]. Varghese

and his coworkers achieved high-rate solar photocatalytic conversion of carbon dioxide and

water vapor to fuel products i.e., hydrocarbons using nitrogen-doped titania nanotube. The

hydrocarbon production rate was 111 ppm cm-2.h-1 when nitrogen doped titanium dioxide

nanotubes were co-doped with Pt and Pd due to its visible light activity [130]. These literature

reports on degradation of dyes and photocatalytic CO2 reduction demonstrate that

semiconductors especially titania exhibited much progress in the field of photocatalysis due to its

tunable band gap, photostability, non-toxicity, easy preparation and good chemical stability.

Modification of titania nanoparticles and titania nanotubes with doped and co-doped with anion

and cation can bring a breakthrough in its photocatalytic efficiency and photocatalytic activity in

terms of carbon dioxide reduction and degradation of photostable dyes [129, 130].

44

1.9 Present Work

The present work intends to develop the common strategy to synthesize the titanium

dioxide nanoparticles co-doped with anion and transition metal ions to tailor the band gap

towards visible range to achieve controllable size, phase and morphology.

I. In current study, we report the single step, size controlled fabrication of titanium dioxide

nanoparticles, co-doped with anions and transition metal ions through sol-gel method.

Sol-gel is a useful method in order to control the particle size and various parameters

such as geometry, morphology, homogeneity, band gap and surface area of the obtained

product.

II. Titanium dioxide nanoparticles (TiO2), co-doped with anion i.e., sulfur (1 wt %) and

transition metals i.e. copper, cobalt, iron, ruthenium and chromium with different weight

percent ranging from 1 wt % to 5 wt % (Scheme 1.3).

III. The prepared samples were characterized using X-ray diffraction (XRD), Diffuse

reflectance spectroscopy (DRS), Scanning electron microscopy coupled with energy

dispersive spectroscope (FESEM-EDS), high resolution transmission electron

microscopy (HRTEM), Raman spectroscopy, thermal analysis (TGA/DSC), Brunauer–

Emmett–Teller (BET) surface analysis, Rutherford Back Scattering (RBS), Fourier

transform infra red spectroscopy (FTIR) to evaluate the morphology, phase and surface

properties (Scheme 1.3).

IV. The photocatalytic applications of the prepared samples towards the photoreduction of

carbon dioxide and photocatalytic degradation of organic waste like phenol, crystal

violet, procion blue MXR, alizarin Red S and Malachite green under UV and visible

irradiations (Scheme 1.4). Furthermore, the effects of dye concentration, catalyst amount,

pH and doping concentration on the phase transformation of titania, optical properties,

surface area and photocatalytic activity were also studied for comparison.

V. The prepared samples are also used for the photocatalytic reduction of carbon dioxide

using water to fuels/chemicals (methane, ethanol, methanol, etc.,) under visible light UV

light for comparison (Scheme 1.4).

VI. Finally, pure titanate nanotubes (TNT). titanate nanotubes doped with Fe and S and Cr

co-doped titania nanotues, Fr-Cr-S co-doped titania nanotubes were fabricated by the

hydrothermal treatment. The morphology, crysralline phase, composition were

45

Characterization

XRD DRS SEM TEM EDX RBS BET FTIR Raman TGA

characterized by powdered X-ray diffraction (XRD), scanning electron microscopy

(SEM), transmission electron microscopy (TEM), Barrett–Joyner–Halenda methods

(BET). The band gap of the TiO2 nanotubes was determined using transformed diffuse

reflectance spectroscopy according to the Kubelka-Munk theory.

VII. The photocatalytic activity of doped nanotubes were evaluated in terms of degradation of

phenol and photoreduction of CO2 under UV and visible irradiation at optimized

condition (Scheme 1.5).

Scheme 1.3: Schematic diagram showing synthesis and characterization of prepared samples.

Cu-co-doped

•1 %Cu-1%S•2% Cu-1%S•3% Cu-1%S•4% Cu-1%S•5% Cu-1%S

Co-S co-doped

•1 %Co-1%S•2% Co-1%S•3% Co-1%S•4% Co-1%S•5% Co-1%S

Fe-S co-doped

•1 %Fe-1%S•2% Fe-1%S•3% Fe-1%S•4% Fe-1%S•5% Fe-1%S

Ru-S co-doped

•1 %Ru-1%S•2% Ru-1%S•3% Ru-1%S•4% Ru-1%S•5% Ru-1%S

Cr-S co-doped

•1 %Cr-1%S•2% Cr-1%S•3% Cr-1%S•4% Cr-1%S•5% Cr-1%S

Synthesis of Nanostructures

1% S doped TiO2

Co-doping of 1% S doped TiO2 with transition metals

Titanium dioxide (TiO2)

46

Photoreduction of CO2

UV irradiation Visible Irradiations

Photocatalytic Degradation of Dyes

Phenol Crystal violet Malachite green Alizarin red S Procion blue MXR

UV and Visible Irradiations

Photocatalytic Applications

PhenolDegradation

CO2

Reduction

Characterization

XRD

DRS

SEM

TEM

EDX

BET

Scheme 1.4: Schematic diagram showing photocatalytic application of the TiO2 nanostructures.

Scheme 1.5: Schematic diagram showing synthesis and applications of prepared nanotubes.

Photocatalytic Applications

1) Effect of dye concentration 2) Effect of catalyst loadings 3) pH effect 4) Effect of dopant content 5) Effect of irradiation source (UV& Vis) 6) Recyclability of the photocatalyst

Synthesis of Nanotubes

As prepared TNTs

Fe doped TNTs

Fe-S co-doped TNTs

Fe-Cr co-doped TNTs

Fe-Cr-S co-doped TNTs

47

2. EXPERIMENTAL

This chapter provides the detail of methodologies used for the synthesis of titanium

dioxide nanostructures, photocatalytic experiments and characterization techniques.

2.1 Chemicals Used

The chemicals used in present study with their percentage purity are presented in Table

2.1. All reagents used were of analytical grade and were used without any further purification.

The chemical structures of the dyes and phenol is presented in Table 2.2.

Table 2.1: List of reagents used and their percentage purities.

Compounds Chemical formula Molar mass % Purity Supplier

Titanium isopropoxide Ti{OCH(CH3)2}4 937.00 97 Aldrich TritonX100 C14H22O(C2H4O)n(n=

9-10) 625 98 Aldrich

Copper (II) nitrate Cu(NO3)2 187.56 98.5 Merck Cobalt nitrate hexahydrated Co(NO3)2.6H2O 291.03 99.9 Aldrich Chromium (III) nitrate nonahydrate

CrN3O9.9H2O 400.15 99.9 Aldrich

Ruthenium chloride RuCl3·xH2O 207.43 98.9 Aldrich Iron(III) nitrate Fe(NO3)3 241.86 98.9 Aldrich Thiourea CH4N2S 76.12 98 Riedal Ethyl alcohol C2H5OH 46.0 99.8 Merck Dimethyl sulfoxide C2H6OS 78.13 99.5 Merck Phenol C6H6O 94.11 99 Aldrich Crystal violet C25N3H30Cl 407.979 dye content

≥35% Aldrich

Malachite green C6H5C(C6H4N(CH3)2

)2]C 364.91 dye content

≥90% Merck

Alizarin red S C14H7NaO7S 342.26 dye content ≥90%

Aldrich

Procion blue MXR C23H14Cl2N6O8S2 637.42 95 Aldrich Acetic acid CH3CO3H 60.08 98 Aldrich Sodium hydroxide NaOH 40 99 Aldrich Cetyltrimethylamonium bromide

((C16H33)N(CH3)3Br 364.25 98.5 Aldrich

48

Table 2.2: Chemical structures of the dyes and phenol with their codes.

Code Name of compound Chemical structure

PH Phenol

CV Crystal violet

MG Malachite green

ARS Alizarin red S

PB-MRX Procion blue MXR

49

2.2 Synthesis of Nanoparticles

2.2.1 Synthesis of TiO2 Nanoparticles

Titania nanoparticles were synthesized using sol-gel method. In a typical synthesis

procedure, 0.1 mol of titanium isopropoxide was added in 100 mL of absolute ethanol to form

solution 1. Solution 2 was prepared by mixing 7:3:1 of ethanol: acetic acid: water. Then, the

solution 1 was added into the solution 2 drop-wise under continuous stirring at room temperature

until sol is formed, followed by vigorous stirring for 6 h and aged for overnight at room

temperature to prepare the gel. The resulting gel was dried at 100 oC for 10 h and calcined at 500 oC for 4 h to eliminate the remaining organic impurities and to obtain anatase TiO2.

2.2.2 Synthesis of Sulfur doped TiO2 Nanoparticles

1% sulfur-doped titania photocatalyst were fabricated by a similar sol-gel process as

discussed above. Solution 1 containing ethanol: acetic acid: water was prepared in the molar

ratio of 7:3:1 with subsequent addition of 0.789 g of thiourea. Required amount of titanium (IV)

isopropoxide in 100 mL of absolute ethanol (solution 2) was added drop wise to above solution

under continuous stirring for 6 h, followed by the formation of sol. The prepared sol was kept for

aging overnight in the dark for 24 h to allow further nucleation processes. The gel was then dried

at 100 ˚C, crushed into fine powder and subsequently calcined at 500 ˚C for 4 h in a muffle

furnace.

2.2.3 Synthesis of Cobalt-Sulfur co-doped TiO2 Nanoparticles

Synthesis of titanium dioxide nanoparticles co-doped with cobalt (1%, 2%, 3%, 4% and 5

wt% of Co) and sulfur (1 wt%) was carried out by a conventional sol-gel method as stated above.

First of all, the desire liquid medium containing by mixing 7:3:1 molar ratio of ethanol, acetic

acid and water, respectively was prepared. Measured amounts of thiourea and cobalt nitrated

were added in above solution mixture, followed by stirring for 15 min. TiO2 sol was slowly

formed drop-by-drop addition of TTIP solution in above mixture which was stirred magnetically

for 6h until homogenous sol is formed. The sol was further aged overnight, dried at 100 ˚C to

eliminate water and ethanol, grounded into fine powder and finally calcined at 500 ˚C for 4 h to

achieve desire anatase phase of TiO2.

2.2.4 Synthesis of Copper-Sulfur co-doped TiO2 Nanoparticles

Copper and sulfur co-modified titania nanoparticles were synthesized using similar sol

gel procedure. During synthesis, required concentration of TTIP were reacted in ethanol

50

(solution-A). In the second step, modified solution (solution-B) was synthesized by mixing

desire amount of copper nitrate and thiourea in to ethanol: acetic acid: water (with molar ratio

7:3:1). In the third step of synthesis the solution A was added drop wise in solution B, stirred for

6 h, aged overnight, dried at 100 oC, finally calcined at 500 oC. Similar methodology is adopted

for the synthesis of all copper and sulfur co-doped samples with constant S content (1%) and

varying copper contents (1 wt% to 5 wt%).

2.2.5 Synthesis of Ruthenium-Sulfur co-doped TiO2 Nanoparticles

The same procedure was adopted for the preparation of Ruthenium and sulfur co-doped

titanium dioxide nanoparticles by the addition of stoichiometric amounts of ruthenium (III)

chloride and thiourea in solution 1 (mentioned above). 100 ml solution mixed with ethanol and

required amount of Titanium (IV) isopropoxide was dropped in the above solution until the

formation of sol, followed by the formation of gel. The precipitation was dried at 100 oC,

crushed and then calcined for 4 h at 500 oC to remove organic substituent. The doping

concentrations of the ruthenium on titania was varied from 1 to 5 wt % of ruthenium with

constant sulfur content (1 wt %).

2.2.6 Synthesis of Iron-Sulfur co-doped TiO2 Nanoparticles

Anatase TiO2 co-doped with iron and sulfur was prepared by sol gel procedure by the

hydrolysis of titanium (IV) isopropoxide as mentioned earlier. A stoichiometric amount of the

thiourea and iron nitrate was added to the solvent mixture of ethanol: acetic acid: water (7:3:1

molar ratio) to get the dopant concentration of 1 wt% S and iron (1%, 2%, 3%, 4% and 5 wt%

Fe). Another solution containing TTIP in ethanol was added in above mixture drop wise with

continuous stirring until a homogenous sol is formed. The sol was aged in air for 24h to allow

further hydrolysis and gelation, dried at 100 oC, crushed into fine powder and finally calcined for

5h at 500 oC in air to attain the desired nanoparticle.

2.2.7 Synthesis of Chromium-Sulfur co-doped TiO2 Nanoparticles

Similar methodology is adopted for the synthesis of titanium dioxide nanoparticles co-

doped with different concentration of chromium (1%, 2%, 3%, 4% and 5 wt% of Cr) and sulfur

(1 wt%) as discussed previously, using thiourea and chromium nitrate as precursor.

A general schematic diagram showing the synthesis procedure is shown below (Scheme 2.1) and

codes are tabulated in Table 2.3.

51

Scheme 2.1: Synthesis of nanoparticles using sol-gel method [120].

Table 2.3: Sample codes of all prepared sample with their composition.

S.No Sample codes Composition S.No Sample codes Composition

1. PT Plane TiO2 15. 3Ru-ST 3 % Ru doped ST

2. ST 1 % S doped TiO2 16. 4Ru-ST 4 % Ru doped ST

3. 1Cu-ST 1 % Cu doped ST 17. 5Ru-ST 5 % Ru doped ST

4. 2Cu-ST 2 % Cu doped ST 18. 1Cr-ST 1 % Cr doped ST

5. 3Cu-ST 3 % Cu doped ST 19. 2Cr–ST 2 % Cr doped ST



8. 1Co-ST 1 % Co doped ST 22. 5Cr–ST 5 % Cr doped ST

9. 2Co-ST 2 % Co doped ST 23. 1Fe-ST 1 % Fe doped ST

10. 3Co-ST 3 % Co doped ST 24. 2Fe –ST 2 % Fe doped ST



13. 1Ru-ST 1 % Ru doped ST 27. 5Fe –ST 5 % Fe doped ST

14. 2Ru-ST 2 % Ru doped ST

Sonication

Titanium Isopropoxide/Ethanol

Urea+Salt Soln.

R.T

Ethanol: Acetic Acid: Water

Sonication

Aging Overnight

Drying

Calcination

12 h 100 °C

500 °C

52

2.3 Synthesis of Nanotubes

2.3.1 Synthesis of Iron doped and Iron-S co-doped Titania Nanotubes

5 wt% iron doped and 5wt% Fe-1wt% S co-doped titania naotubes were prepared by

following method. For iron doped titania nanotubes, slurry was prepared by dissolving 20 ml of

TiCl4 in 200 mL of H2SO4 which was then titrated slowly with appropriate amount of 5M NaOH

with constant stirring till a pH of slurry reached ~7. 0.5 M solution of CTAB and an aqueous

solution of iron nitrate with required amount were then added to the above mixture. The reaction

mixture was stirred for some time and then the reactants were transferred into a teflon-lined

stainless steel autoclave which was sealed and maintained at 110 ○C for 8 h under a H2 pressure

of 2.5 x 106 Pa. The synthesis layout is shown in Figure 2.1. After the hydrothermal treatment,

the precipitates were separated by filtration and washed with dilute HCl thrice and then with

distilled water repeatedly until the pH of the resulting solution reached ~7. The final product was

obtained through centrifugation and then air dried at 110 ○C overnight. Similar procedure is

adopted for the synthesis of 5 wt% Fe – 1 wt% S co-doped titania nanotubes, by addition of

required amount of thiourea in the reaction mixture (mentioned above).

2.3.2 Preparation of Fe-Cr co-doped Titania Nanotubes

Iron chromium co-doped titania nanotubes (2%Cr-3%Fe) were produced using

hydrothermal method similar to that described above. In typical nanotube preparation, for iron

and chromium co-doped titania nanotubes, slurry was prepared by the same method as described

above. A 0.5 M solution of CTAB, an aqueous solution of iron and chromium nitrate with

required amount were then added to the above mixture followed by hydrothermal treatment in a

Teflon-lined autoclave high-pressure stainless steel and maintained under same condition as

mentioned before. After hydrothermal treatment the precipitates were separated through filtration

and washed with 0.1M HCl and distilled water numerous times until the pH of the filtrate turned

to ~7. The resulting sample was dried in oven at 100 ◦C overnight to get co-doped titania

nanotubes.

2.3.3 Preparation of Fe-Cr/S co-doped Titania Nanotubes

Fe-Cr-S co-doped titania nanotubes (2% Cr-3% Fe-1%S) were prepared by hydrothermal

method as mentioned above with the addition of required amount of thiourea in the reaction

mixture.

53

Figure 2.1: Experimental set up for the synthesis of titania nanotubes [121].

2.4 Adsorption Studies of Dyes and Phenol

The adsorption studies of model pollutants i.e. crystal violet, phenol, procion blue MXR,

alizarin red S, malachite green were carried out using TiO2, ST, 5Cu-ST, 5Co-ST, 5Ru-ST, 5Ag-

ST, 5Fe-ST under dark conditions in order to determine the adsorption of dye on the

photocatalyst surface under neutral (pH ~7), acidic (pH ~4.5) and basic (pH ~9.5) media. The pH

was adjusted by adding 1N HNO3 or 1N NaOH. These adsorption experiments were conducted

with 100 mL aqueous solution of each dye (conc. ~20 ppm) at different pH using 50 mg of

photocatalyst under dark at 25 oC. For adsorption experiments, the photocatalyst is placed in 100

ml of each dye solution, stirred for 6 h. To determine the residue concentration of dye, the

solution was filtered using 0.45 μm filter after each hour and its absorbance was measured using

spectrophotometer.

2.5 Photolysis Experiments

Degradation of all dyes (crystal violet, procion blue MXR, alizarin red S and malachite

green) was investigated under both UV (with cutoff filter λ<380 nm) and visible irradiation (with

cutoff filter λ > 420 nm) using 500-W Xenon lamp (Ushio, model UI-502Q). Figure 2.2 (a)

depicts the photolysis setup used for the decomposition of dyes and phenol at neutral pH and

room temperature. The degradation rate was recorded in terms of change in intensity of

characteristic peak of each sample was determined by spectrophotometer.

54

2.6 Photocatalytic Degradation of Dyes and Phenol

Photocatalytic degradation experiments were conducted in 500 ml Pyrex beaker placed

on a magnetic stirrer and irradiated with 500-W Xenon lamp (Ushio, model UI-502Q). Figure

2.2 (a) shows the experimental setup used for photocatalytic experiments. Stock solution of each

dye was prepared with distilled water, a required amount of catalyst was added into it and was

left for 30 min in the dark in order to get the maximum adsorption desorption of the dye onto the

catalyst surface, under vigorous stirring. The suspension was then irradiated by a 500-W Xenon

lamp (Ushio, model UI-502Q) equipped with a cutoff filter to completely eliminate UV and

visible radiations in order to obtain required range for study. The degradation reaction was

carried out under both UV (with cutoff filter λ<380 nm) and visible irradiation (with cutoff filter

λ > 420 nm) and the reaction temperature was maintained at 25 oC. The sampling was performed

at regular interval of time during reaction and the residue concentration of sample was

determined by measuring its absorbance using UV–Visible Spectrophotometer. The

photocatalytic degradation of all dyes and phenol was carried out by varying the initial

concentration of the dye from 20 to 100 ppm at pH~7 under visible. In second step, the

experiments were performed by varying catalyst concentration from 10 mg/100 mL to 70

mg/100 mL at 20 ppm, pH~7 and visible irradiation. In the third step, the role of pH on

degradation of compounds was determined at pH ~4.5, pH ~7 and pH ~9.5, keeping the

concentration; 20ppm/100mL and catalyst~50 mg/ml. The effect of dye concentration, catalyst

loading and pH on photodegration of all compounds was evaluated using TiO2, ST, 5Cu-ST,

5Co-ST, 5Ru-ST, 5Ag-ST, 5Fe-ST. In the 4th step, under optimized condition of the dye

concentration, catalyst loading and pH, the experiments were carried out with different doped

catalyst. Finally, comparative studies under UV region and recyclability of the catalyst were also

made.

2.7 Photocatalytic Reduction of Carbon Dioxide

The photocatalytic reduction of carbon dioxide was carried out with a modified reaction

cell of 100 ml capacity having quartz window, with an inlet for introduction of carbon dioxide

and an outlet for collection of the products, an external irradiation source, a magnetic stirrer plate

(see Figure 2.2 (b)). For the CO2 photoreduction experiments, 50 mg of catalyst powder was

dispersed in 50 mL of 0.1N NaOH aqueous solution (pH ~9.5). The whole suspension was

55

transferred in catalytic reactor, sealed and evacuated to a base pressure of ~ 10-7 torr in order to

eliminate the dissolved oxygen. Ultra pure CO2 (~99.99%) was introduced into the reactor for 10

min, maintained at atmospheric pressure (1barr). The suspension was stirred under dark for 30

min prior to the illumination, to obtain the sorption equilibrium. The reactor was then irradiated

with 500-W Xenon lamp (Ushio, model UI-502Q) equipped with a cutoff filter (λ>420 nm) to

remove UV light and (λ<380 nm) to remove visible light, from the top of the catalytic reactor for

12h. The distance between the quartz window of the reactor and lamp house exit was set to 20

mm. After the photocatalytic experiment, the temperature of the system was raised and kept at 80 ○C for 30 min and gaseous products were collected by gastight syringe (500 µl) and measured by

a gas-chromatography (GC). The GC was equipped with a porapak Q column and a thermal

conductivity detector (TCD)/Flame ionization detector (FID) and helium gas is used as the

carrier gas. Blank sample of CO2 was also run on GC as reference. The mass spectrum of the

reactants and products were obtained by the mass spectroscopy equipped with the GC setup to

confirm the products. Water is being used as reducing agent and aqueous NaOH solution (0.1N)

is used as sacrificial agent. The photoreduction experiments were carried out under both UV and

visible light using TiO2, ST, 5Cu-ST, 5Co-ST, 5Ru-ST, 5Cr-ST, 5Fe-ST for 12h at room

temperature and atmospheric pressure (1 bar).

(a)

(b)

Figure 2.2: Setup for (a) photolysis and photocatalytic degradation of dyes and phenol (b) photo reduction of carbon dioxide.

56

2.8 Characterization Techniques

The properties, morphology and structural features of prepared materials were estimated

by the techniques as described below. A brief summary of the working principle and the practical

characteristic of each technique utilized for the current study are described below.

2.8.1 X-Ray Diffraction (XRD)

It is the most widely and commonly used simple and non destructive technique employed

to obtain the information regarding crystal structure, composition, lattice parameters, particle

size and growth orientation etc. The powder diffraction data of more than 150,000 unique

organic, inorganic organometallic and mineral samples have been assembled into a single

database named as JCPDS (joint committee on powder diffraction standards). In crystalline

solids, the powder patterns are determined by two distinct features (1) size and shape of unit cell

and (2) atomic number and the position of certain atoms in the unit lattice. Therefore, two

materials having same crystal structure thus exhibit quite different powder patterns because each

crystal solid possess its own distinctive X-ray diffraction pattern which can be used as finger

print for identification. The powder pattern also exhibits two typical characteristic (i) the d-

spacing of the lines or specific interplanar distance and (ii) intensity of the lines [131].

X-rays are high energy electromagnetic radiation with the wavelength ranging about 0.5 - 2.5

Å. When X-rays beam intrudes on the atom or material, two processes occur (1) the beam may b

absorbed by the atom followed by the ejection of electron or it may be scattered in various

directions. The phenomena of diffraction occurs under the Bragg’s law:

nλ = 2d sinθ (1)

Where, n= 1,2,3……, λ = is the X-ray wavelength, d = inter planer distance between the atomic

layers in the unit cell and θ is angle of diffraction i.e., the angle at which the X-rays are

diffracted at various angles and these diffracted rays results in the specific diffraction pattern.

The labels used for describing these planes are known as miller indices and are given the

descriptions h k and l. These x-ray reflections occur from a series of parallel set of equally

spaced planes inside the crystal. These planes are usually described in terms of miller indices and

are labeled as h k and l values which define the orientation and inter planar distances of these

planes. While h, k, l value is the integers which may have positive, negative or zero values [132].

This diffraction pattern gives specific information like crystallite size, stress and strain values,

57

unit cell symmetry, d-spacing values, intensity of the peaks and specific growth orientation. The

average crystallite size is estimated by Debye Scherrer equation:

𝐷𝐷 = 𝑘𝑘𝑘𝑘𝛽𝛽𝛽𝛽𝛽𝛽𝛽𝛽𝛽𝛽

(2)

Where, β = full width half maxima of the diffraction peak, K= constant having value in the range

of 0.9-1 depending on the particle morphology, λ = X-ray wavelength and is equal to 1.542 Ao.

While Lattice parameter “a” was calculated using the following formula, shown in equation (3):

1𝑑𝑑2 = ℎ2+k2

𝑎𝑎2 = 𝑙𝑙2

𝛽𝛽2 (3)

Where d is d-spacing values of lines appearing in XRD pattern, hkl correspond to miller indices,

while “a” and “c” corresponds to the lattice parameters, respectively and for tetragonal system

like TiO2, a=c always. The crystallinity, structure and phase of samples were determined by

XRD (Phillips PW 3040/60 X Pert Pro) powder diffractometer with Cu Kα radiation of

wavelength of 1.5406 A0. The XRD profiles were collected between 15o to 75o (2θ) with a step

size of 0.050 and step time of 3 s.

2.8.2 UV-Vis Diffuse Reflectance Spectroscopy (DRS)

UV-Vis diffuse reflectance spectroscopy was used to probe the band structure and energy

levels in the crystal. When beam of electromagnetic radiation in the UV/Visible range fall on a

sample, four types of interactions may occur: the radiation is absorbed, reflected, transmitted or

scattered. Normally UV/Vis spectrometers are used to measure the transmittance or absorbance

of a transparent material and can also be used to measure the reflected and scattered energy from

a sample when equipped with the proper accessories. Specular and/or diffuse reflectance is

measured by coupling the integrating sphere with the Lambda 35 spectrometer. The integrating

sphere is housed in the sample beam of the spectrometer and is used in place of the sample beam

detector. It can be used over the wavelength range of 250-1100 nm for either diffuse reflectance

or diffuse transmittance The instrumental detector, external to the sphere, detect the reference

beam when light enters the sphere and bounces around used at the detector in diffuse reflectance

mode. Diffuse reflection is usually the reflection of light when incident rays are reflected from a

surface at several angles rather than single angle as in the case of specular reflection [133].

The band gap is determined by Kubelka-Munk function F(R) which is related to the diffuse

reflectance, R, of the sample and given by the following relation [134].

58

F(R) =(1-R)2/2R (4)

Where, R is the absolute value of reflectance. The energy band gap of the samples were

determined from their diffuse reflectance spectra by plotting the square of the Kubelka-Munk

function (F(R)*hv)2 vs. Energy in electron volts. The direct energy band gap was obtained by

linear part of the curve by extrapolating to F(R)2 = 0. The optical band gap energies of all

samples can be obtained by using the above method. A (Lambda -950 Perkin-Elmer) UV-Vis

spectrophotometer equipped with an integrated sphere was used to record the diffuse reflectance

spectra (DRS) of the samples. The reflectance spectra of the samples were analyzed under

ambient condition in the wavelength range of 200-1000 nm.

2.8.3 Scanning Electron Microscopy (SEM) Coupled with Energy Dispersive

Spectroscopy (EDX)

Scanning electron microscope is useful techniques which are used to determine the

information regarding morphology, overall appearance, phase distribution, crystal orientation

and topographical characteristics of the material. As compared to transmission electron

microscopy, the electron beam is focused on the sample in the SEM which is scanned over the

sample surface. The acceleration voltage of electron used is usually in the range of 5-20 kV

[135].

In typical SEM process, a beam of electrons is produced by an electron gun is focused

toward the sample, followed by a vertical path through the microscope under vacuum passing

through the electromagnetic fields and lenses. When this electron beam hits the sample, electrons

and X-rays are emitted from the sample. Usually, two types of electrons are emitted by the

sample: 1) backscattered electrons (BSE) having energy greater than the 50 eV, which are

emitted as result of elastic interactions between the incident electrons and positively charged the

sample nuclei and scattered back out of the sample without slowing down. 2) Secondary

electrons (SE) having energy lower than 50 eV than the incident electrons which are generated as

results of the excitation of electrons sample during the ionization atoms. These secondary

electrons are readily attracted to a detector due to low energy and produce the image of sample

with good resolution. These secondary electrons (SE) are also used to determine topographic

contrast i.e., for the visualization of surface characteristics and roughness in the SEM. The

topographical image is majorly dependent on the number of secondary electrons hitting the

59

detector. Elements having high atomic numbers are more positively charged nucleus which result

in more backscattered electrons are backscattered giving high backscattered signal [136]

The characteristics X-rays emitted as a result of the interaction of the primary electron

beam with the sample are detected by the EDS detector. These X-rays photons give information

regarding the elemental composition of the sample and are used as micro analytical technique in

the SEM. These X-rays photons have characteristic energy corresponding to characteristic

element and are generated by an excited atom when it returns to ground state [136]. Scanning

Electron Microscope S-3500 N with Absorbed Electron Detector S-6542 (Hitachi Science

System, Ltd), EDX (Inca Energy, Oxford Instruments Microanalysis, Ltd) under specific

conditions of 20 keV, 25 mm working distance and magnification varying from x 300-60000

were used to examine the surface comparison of the samples.

2.8.4 Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy is widely used to determine the structural

characteristics such as high magnification image and diffraction patterns of the sample. TEMs is

capable of producing image at a considerably higher resolution even up to 2 Ao i.e., smaller than

the interatomic distance. TEM yields information regarding the sample in two types of modes; 1)

Diffraction mode in which diffraction pattern is achieved from the sample area when illuminated

by the beam of electrons. These diffraction patterns originated from samples are equivalent to

XRD pattern i.e., single crystal generates a spot pattern while ring pattern is obtained by poly

crystalline material while amorphous material gives diffuse pattern 2) image mode in which

object image is obtained by the of elastic scattering of electrons when beam of electrons having

energy equivalent or greater to 100 kV impinges through the samples [137].

Fast-Fourier transforms (FFTs) of HRTEM micrographs were calculated to find the

spatial frequencies (lattice d-spacings) in the samples and hence to allow the determinations of

crystal structure of samples [138]. TEM analysis of samples was performed in a microscope

from FEI Company (FEI Company, Hillsboro, OR) of model Titan G2 80–300 by operating it at

primary beam energy of 300 keV and its point-to point resolution was recorded to be 0.235 nm at

that beam energy. The size of nanoparticles present in samples and their structure were carried

out by setting the instrument in TEM mode to acquire high resolution TEM (HRTEM)

micrographs.

60

2.8.5 Rutherford Back Scattering (RBS)

Rutherford Backscattering Spectrometry (RBS) is a extensively employed nuclear

method for the surface analysis of solids. During Rutherford Backscattering Spectrometry

(RBS), a target sample is bombarded with ions having energy typically in the range of 0.5–4

MeV, and the energy of the backscattered projectiles is determined with energy sensitive detector

(solid state detector). RBS is quantitative technique which helps in the determination of

determination of the material’s composition and depth profiling of individual elements present in

the sample. RBS is non-destructive quantitative techniques, with a good depth resolution (order

of several nm), good sensitivity for heavy metals (order of ppm) and does not require reference

samples. The drawback of RBS for low sensitivity of detecting light elements can be overcome

by which combination of other nuclear based methods like nuclear reaction analysis (NRA) or

elastic recoil detection analysis (ERDA) [139]. During RBS analysis, a collimated 2.0-MeV

He2+ beam 20nA and 20μC of 5mm diameter produced by Tandem Pelletron Accelerator

(5UDH-2, National Electrostatic Corporation, USA) was used for RBS measurements. The

samples were mounted on a high precision (0.01○) five-axis goniometer in a vacuum chamber, so

that the orientation of samples relative to the He2+ beam could be precisely controlled. The

backscattered particles were accepted by Si Surface Barrier (SSB) detector. The detection angle

was 170◦ and its energy resolution was about 25 keV. The detector signals were amplified by a

pre-amplifier and then sharpen and amplified by a spectroscopy amplifier (Ortec 572/673)

directly coupled to a multichannel analyzer (Canberr-88MCA). The RBS spectra, recorded after

calibration, were analyzed using the computer Code Software “SIMNRA” (version 6.02).

2.8.6 Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH)

Brunauer–Emmett–Teller (BET) theory basically focuses at the physical adsorption of

gas molecules on a solid substance surface and is used as an important analytic tool for the

determination of the specific surface area of a substance. The BET equation is used basically to

measure the surface area by physisorption of a gas (N2 or CO2) on a material surface. Nitrogen is

usually used in BET surface area analysis due to its availability in high purity and strong

interaction with most materials [140]. Relatively weak forces (van der Waals forces) exist

between adsorbate gas molecules and the surface which is usually weak, the surface is cooled

using liquid N2 to attain detectable amounts of adsorption. Known quantity of N2 gas is then

61

released stepwise into the sample cell until the saturation point is obtained and further no more

adsorption occurs with any further increase in pressure [140]. After the formation of adsorption

layer, the sample is taken out from the nitrogen environment and heated to release the adsorbed

nitrogen from the sample and quantified. The data is obtained in the form of a BET isotherm,

which plots the quantity of gas adsorbed on the surface as a function of the relative pressure in

the monomolecular layer formed on the surface by a volumetric or continuous flow procedure.

BET analysis is an efficient tool which provides accurate information for assessment of the

specific surface area of materials by measuring the nitrogen multilayer. The technique yield

important information regarding evaluation of external area and pore area to determine the total

specific surface area in m2/g and helps in studying the effects of surface porosity and particle size

also. Pore area, pore size distribution and specific pore volume is evaluated by BJH analysis

using adsorption and desorption techniques. The BET instrument is an efficient tool which

provides single sample surface area and pore size distribution analysis [141]. BET surface areas

and pore volume were estimated by Quntachrome NovaWin2 apparatus using N2-

adsorptiondesorption isotherms at 77.4K.

2.8.7 Fourier Transform Infra Red (FTIR) Spectroscopy

Fourier Transform Infra Red (FTIR) Spectroscopy is technique used for the

determination of qualitative and quantitative features material which are of IR-active molecules

and is commonly employed for organic materials, inorganic solid, liquid and even gas samples. It

is a comparatively fast and relatively inexpensive technique also used for the evaluation of solids

that are crystalline, amorphous, or films [142].

When IR radiation interacts to the material, some of the radiations are absorbed by the

material while others are transmitted. Only those radiations are absorbed whose vibrational

frequency matches with frequency of the bonds. The vibrational frequency is affected by the

bond strength and masses of the bonded atoms. The simplest type of vibrational modes in

molecules is the IR active modes i.e., stretching and bending modes which give rise to

absorptions. However, the stretching and bending are also complex type of vibrational modes

which are also IR active. Vibrations are also classified as symmetric and asymmetric vibrations

depending upon how the symmetry of molecules is affected by different modes of vibrations.

When IR radiations coming from the source fall on the sample, they interact with the oscillating

bond of the atoms [143]. Only those vibrations are IR active which show net dipole change

62

during the vibration and this change could be perpendicular or parallel to the bond axis. An

infrared spectrometer essentially measures the spectrum as plot of intensity versus time. Since

every type of molecule has a different natural frequency, and two of the same type of bond in

two different compounds are in two slightly different environments, no two different structure

have the same spectrum. Thus, infrared spectrum can be used for the molecules much as

fingerprint can be used for the human beings [142]. IR spectra were recorded on a Nicolet

NEXUS 670.

2.8.8 Raman Spectroscopy

Raman spectroscopy is usually based o the Raman Effect and gives molecular

information similar to the Infrared spectroscopy. When intense beam of monochromatic visible

light usually laser light is irradiated on molecule, some of the light is transmitted while some is

scattered back. Radiations scattered with frequency lower than the incident beam is called Stoke

radiations while that at higher frequency is known as anti-Stokes. Stoke radiation is generally

more intense than the anti-Stoke. While some of the radiations are scattered with unchanged

frequency and is called Rayleigh line. The Stoke and anti-stoke lines appear usually

symmetrically on both sides of the parent line and this phenomena is called Raman Effect [144].

These both lines have intensity much lower than the Rayleigh line while stoke lines are relatively

stronger than the anti-stoke lines. Generally, the there is a single Stoke’s line in diatomic

molecules. In poly atomic molecules, several Stoke’s lines are observed as result of excitations

of several fundamental stretching and bending vibrational modes. The difference between the

frequency of parent line i.e., Rayleigh line and the Raman line i.e, Stoke’s line is known as

Raman shift. The Raman shift value is characteristic of the molecule irradiated and equivalent to

the vibrational frequencies of that certain molecule encountered in the IR spectroscopy. Raman

spectrum is usually the plot of intensities of the Raman lines against the Raman frequency shift

and is similar to the corresponding Raman spectrum. The intensities of the band in IR spectrum

is directly related to the change in the dipole moment of the bond during vibration while the

intensity of Raman band is dependent on the change in polarizability of the bond in the same

vibration [145]. Generally, the molecules having centre of symmetry are IR inactive when the

vibrations are symmetrical about the centre of symmetry. On the other hand, the vibrations which

are not centrosymmetrical are Raman inactive while IR active. The major advantage of Raman

spectroscopy on the IR spectroscopy is; 1) the vibrations which are generally weak or not appear

63

in IR region or the vibrations of symmetrical molecules usually give strong Raman Lines, 2) the

vibrations of low frequency region i.e., below 600 cm-1 can also be easily studied [145]. Raman

analysis of samples was carried out on FT Raman Spectrophotometer in the range of 100- 800

cm-1.

2.8.9 Thermo-gravimetric Analysis (TGA)

Thermal analysis may be defined as the measurement of physical and chemical

characteristics of sample as a function of temperature in a particular atmosphere. Generally,

thermal analysis covers specific properties only including heat capacity, mass and thermal

expansion coefficient. The two major thermal analysis techniques include the thermogravimetric

analysis and differential thermal analysis. Of the two, the thermogravimetric analysis (TGA) is

the most important one in which the change in weight of a substance is recorded as a function of

temperature or time. Usually, a few milligrams of the samples is heated at continuous rate,

typically in the range of 1 to 20 oC min-1 till it starts to decompose [146].

TGA setup consists of a thermo balance, an electronic microbalance, furnace and

temperature programmer software. Usually, thermo balance is normally placed in a glass or

metal systems to control the pressure and the atmosphere inside and it measures the changes in

mass with temperature. The gases are flowed at regular rate over the sample in order to remove

the evolved gases during the chemical decomposition of the sample carefully so that they do not

disturb the balance. The heating rate is determined from temperature controller which is attached

to the instrument and consists of either platinum resistance thermometers or thermocouples

[147]. Thermogravimetric analysis provides the information regarding thermal stability, phase

properties, electrical or magnetic properties as a function of temperature.

Other characteristics of the materials that can be obtained from thermal analysis

techniques include the temperatures at which phase transition occur, heat capacity, glass

transition temperature and purity. It is also used to evaluate the groups attached to the surface of

the material and moisture contents of the sample [147]. . Thermal gravimetric analysis (TGA)

spectrum was recorded with Diamond Series TGA/DT Perkin-Elmer, USA.

2.8.10 UV-Vis Spectroscopy

When continuous beam of light passes through the transparent sample, a portion of light

is absorbed, yielding a spectrum with gaps in it and called absorption spectrum. The

64

electromagnetic radiations that are absorbed has energy equivalent to the energy difference

between the excited state and the ground state. In case of UV and visible spectroscopy, the

transition between the electronic levels results in the absorption of electromagnetic radiations in

UV and Visible region and occur between the highest occupied molecular level (HOMO) and

lowest unoccupied molecular level (LUMO) [148].

For most of the molecules, the lowest energy occupied molecular orbital correspond to

the σ-bond, the π-bond are present somehow at higher energy levels while the bonds holding the

unshared pair of electrons are (non-bonding orbitals) located at even higher energies. The

unoccupied or antibonding orbitals called π* and σ* levels, respectively are of highest energy

orbital [149].

In UV-Vis spectroscopy a band appears due to overlapping of so many transitions which

occur between closely spaced energy levels and spectrophotometer is unable to resolve them.

UV-Vis spectroscopy is technique used to evaluate quantitative characteristics of substances and

most importantly for the identification of substances through spectrum emitted or absorbed,

measurement substance concentration using the Beer Lambert Law. The Beer Lambert Law

describes that the amount of light absorbed is proportional to the concentration of the absorbing

substances through which the light is passing and given by the expression [149].

A = Log Io/I =ε c l (5)

Shimadzu UV-1700 Spectrophotometer in the range 200-800 nm was used to determine

the absorbance of the samples.

2.8.11 Gas Chromatography Mass Spectroscopy

Gas chromatography–mass spectrometry (GC-MS) is an important analytical tool having

the features of gas-liquid chromatography and mass spectrometry coupled to identify different

kind of substances contained in a test sample. The GC-MS consist of two major parts: the gas

chromatograph and the mass spectrometer. The gas chromatograph exploits a capillary column

which depends on the phase properties and dimensions of column i.e., length, diameter, film

thickness. After injecting the sample into GC column, it passes through column and the

separation of molecules occurs depending upon the difference in the chemical properties of

different molecules in a sample mixture and their relative affinity to stationary phase of the

column. The molecules come out from the column at different time known as retention time and

65

are captured by mass spectrometer where they are ionized, deflect and finally detected by

detector, separately [150].

Usually, the mass spectrometer performs three actions; 1) It subjects the molecules to

bombardment by stream of high energy electrons converting them to the ions which are then

accelerated by an electric field. 2) These ions are separated according to mass to charge ratio in a

magnetic or electric field. 3) Finally these ions with particular mass to charge ratio are detected

by device which counts the number of ions striking on detector. The output of detector is

amplified and fed to the recorder which records the mass spectrum-a graph of the number of

particles detected as function of mass to charge ratio [151]. The products were analyzed with

reaction time by Gas chromatography Perkin Elmer Gas Chromatography – equipped with

column: Poroplot Q, length: 30 m, Thickness: 0.25 µm, diameter: 0.25 mm, detector: FID/TCD,

injector temperature: 150 °C, column temperature: 50 °C isothermal, detector temperature: 150

°C and carrier gas: Helium, 3 mL/min.

66

3. RESULT AND DISCUSSION

This chapter provides insight into the structural properties of the synthesized nanostructures

(TiO2, S-doped TiO2, Co-S co-doped TiO2, Cu-S co-doped TiO2, Ru-S co-doped TiO2, Fe-S co-

doped TiO2, Cr-S co-doped TiO2 and titania nanotubes) and their photocatalytic applications

such as degradation of dyes, phenol and photocatalytic reduction of carbon dioxide.

3.1 Optimization of Temperature and Dopant

The effect of different calcinations temperature (300 ○C, 400 ○C, 500 ○C and 600 ○C) and

sulfur content (1 wt%, 2 wt%, 3 wt%) is evaluated to obtain the optimum calcination temperature

and S content.

3.1.1 Effect of Annealing Temperature on the Phase of the TiO2

The as synthesized TiO2 powder was calcined at different temperature ranging from 300-600 oC for 5h and crystallite phase was determined by XRD (Figure 3.1). It is obvious from the XRD

result that by increasing the temperature, the materials is changed from amorphous to crystalline

anatase phase [152, 78]. The material shows amorphous nature till 400 oC, whereas at 500 oC

crystalline anatase TiO2 is obtained. The increase in calcinations temperature above 500 oC,

rutile phase begins to appear and mixed rutile, anatase TiO2 phases exist at 600 oC (Figure 3.1).

Since rutile is not of our concern for present study therefore to focus only on anatase TiO2, 500 oC is selected as optimal temperature of calcination for all samples.

3.1.2 Effect of Sulfur Contents on the Phase of the TiO2

Figure 3.2 demonstrates the XRD patterns of plane TiO2 and S-doped TiO2 with varying

sulfur (S) contents from 1-3 wt %. It is clear from these spectra that 1% S containing TiO2

exhibits pure anatase phase. The anatase to rutile phase transformation can be seen above 1 wt%

S containing samples. 2 wt% and 3 wt% S-doped TiO2 exhibited predominant rutile phase in

mixed anatase-rutile phases. Therefore, the level of S doping is maintained up to 1% in all

samples because higher level of S (above 1%) introduced the structural distortion and phase

transformation from anatase to rutile [153, 81, 82]. For further studies, the sulfur was kept

constant i.e., 1 wt % while varying the transition metal concentration from 1-5 wt%.

67

10 20 30 40 50 60 70 80

R RR

R

R

R

R

AA

AA AA AAAA

R

600 oC

500 oC

400 oC

300 oC

2θ (Degree)

Inte

nsity

(a.u

.)

As-prepared TiO2

A

Figure 3.1: Effect of annealing temperature on the phase of un-doped TiO2.

10 20 30 40 50 60 70 80

3%S-TiO2

2%S-TiO2

Plane TiO2

AR R

R

RR

R

R

AA

A RAA AAA

R

2θ (Degree)

Inte

nsity

(a.u

.)

1% S-TiO2

A

Figure 3.2: Effect of sulfur doping on the phase of un-doped TiO2.

68

3.2 Characterization and Photocatalytic Applications Co-S co-doped TiO2 Nanostructures

3.2.1 XRD Analysis

The XRD spectra for all prepared samples are shown in Figure 3.3 (a-h) and the obtained

data is summarized in Table 3.1. X-ray diffraction analysis of TiO2, S-doped TiO2, Co-S co-

doped TiO2 (with Co contents varying from 1-6 wt %) confirmed to the typical peaks for anatase

TiO2 (JCPDS standard files #21-1272). The samples with cobalt amount ~6 wt% exhibited the

anatase-rutile phase mixture as seen in Figure 3.3 (h), implying that cobalt doping above certain

level may possibly induce lattice distortion and bond weakening, thus leading to instability and

phase transformation. No evidence of any extra phase of impurity may be seen in all samples

(Figure 3.3 (a-g)), indicating that dopants are well dispersed in all these samples and may occupy

either interstitial position or substitutional sites in TiO2 crystal structure [88, 89]. In contrast to

un-doped TiO2 nanocrystal, a slight shift in the peak position and broadening of the peaks

occurred with the doping of S and Co, with subsequent in decrease in particle size. The average

crystallite sizes were calculated using Scherrer´s formula and demonstrated in Table 3.1.

10 20 30 40 50 60 70 80

R

R

R(h)

215

220

116

204

211

105202

11200

410

3101

(g)

(f)(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

R RR

R

RR

R

Figure 3.3: XRD patterns of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST; (h) 6Co-ST calcined at 500 ○C.

69

A distinct change in the lattice parameters and d-spacing value is also observed after

doping the titania with sulfur and cobalt. This may be due to the fact that cobalt and sulfur can

introduce little distortion after being incorporated into TiO2 lattice up to certain [99]. However, it

can be argued that cobalt doping is only effective at lower concentration level; otherwise it can

have disruption effect on crystal structure, resulting in anatase-rutile transformation Figure 3.3

(h) [92]. Since, rutile phase is not of our interest for future application, so all further studies are

carried out with the samples having cobalt 1-5 wt%.

3.2.2 Band Gap Studies

The optical properties of TiO2, 1% S doped TiO2, sulfur-cobalt (1, 2, 3, 4, 5 wt % of cobalt)

co-doped TiO2 nanoparticles are evaluated by measuring the diffuse reflectance spectroscopy

(DRS) in term of Kubelka–Munk function (discussed in section 2.8.2).The band gap energies of

all materials is derived from Tauc plot [72, 73], obtained by plotting (F(R)*hv)0.5 versus energy

(eV) and are tabulated in Table 3.1.

Figure 3.4 (a-g) illustrates the optical band gap measurements of the un-doped and doped

TiO2 nanoparticles in terms of Taudc plot. In Co-S co-doped nanoparticles, a characteristic shift

towards lower energy is observed initially with addition of sulfur dopant. It was reported

previously that doping of an anion i.e., sulfur leads to the mixing of S3p orbitals with the O2p

orbitals resulting in the mid gap levels and shifting the absorption towards the visible light region

[101]. It is obvious from Figure 3.4(b) that S doped TiO2 has a band gap at 2.89 eV as compared

to bandgap (3.17 eV) of pure TiO2, which is consistent with its visible-light absorbance ability.

A distinctive band gap narrowing is also observed continuously with increasing cobalt

contents on S doped TiO2. The 5% Co-S co-doped TiO2 powder exhibited much higher visible-

light absorption capacity as compared to others as shown in Table 3.1. An increase in the cobalt

ion concentration resulted in the increase in visible-light absorbance, suggesting that the cobalt

addition promotes visible-light absorption in S doped TiO2 nanoparticle by introducing its energy

levels near conduction band [78]. A decrease in band gap from 2.89 eV to 1.77 eV is obtained

with increase in cobalt content from 1% to 5%, suggesting that cobalt may be incorporated to

TiO2 [98].

70

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

gfed c b

a

(F (R

).hv)

0.5

Energy (eV)

Figure 3.4: Band gap plots of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

Table 3.1: Calculated structural parameters and band gap data of un-doped, S-doped and Co-S co-doped TiO2.

Catalyst Particle size

(nm) d-spacing

(Å) a (Å) c (Å) Band gap

(eV)

TiO2 20.05 3.5113 3.7810 9.5410 3.17

ST 19.18 3.5179 3.7816 9.5511 2.89

1Co-ST 18.33 3.5247 3.7813 9.5562 2.45

2Co-ST 16.38 3.5398 3.7811 9.5598 2.26

3Co-ST 15.63 3.5454 3.7812 9.5630 2.15

4Co-ST 14.19 3.5580 3.7811 9.5699 2.02

5Co-ST 13.35 3.5601 3.7813 9.5701 1.75

71

3.2.3 Morphological Studies

3.2.3.1 SEM Analysis

Morphologies of pure, S-doped TiO2 and sulfur-cobalt co-doped, revealed by SEM

micrographs are presented in Figure 3.5 (a-g). All samples appeared as agglomerates of smaller

particles with fine and homogenous size distribution. Particle size of the as-synthesized powder

and S-doped titanium dioxide nanomaterials (Figure 3.5 a&b) is in the range of 17-22 nm.

A marked difference in particle size with gradual reduction can be seen with the increase in

cobalt contents (Figure 3.5 (c-g)). It is obvious from the images that particle growth of the

powder is strongly dependent on the concentration of cobalt [75]. This illustrates that the cobalt

addition may hinder the growth of TiO2 nanoparticles without effecting crystallinity, this is

consistent with the XRD analysis. SEM micrographs of all Co-S co doped TiO2 powder exhibits

the particle in the range of 12-15 nm which is similar to that obtained from XRD analysis.

0.1 µm 11 20 SE I20 kV X 50,000

(b)

0.1 µm 09 27 SE I20 kV X 50,000

(a)

72

Figure 3.5: SEM images of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

0.1 µm 22 15 SE I20 kV X 60,000

(g)

0.1 µm 21 02 SE I20 kV X 60,000

(e)

0.1 µm 09 47 SE I20 kV X 60,000

(d)

0.1 µm 08 10 SE I20 kV X 60,000

(c)

73

3.2.3.2 TEM Analysis

The characteristic TEM micrograph, FFT patterns and HRTEM image of pure TiO2 are

shown in Figure 3.6 (a), Figure 3.6 (b) and Fig. 3.6 (c&d), respectively. It is obvious from these

images of TiO2 that the particles exhibit narrow size distribution and their morphology is

qausispherical with average particle size of around 18-20 nm. Average crystallite size calculated

from TEM evidently suggests that these nanoparticles consist of nanoscale single crystals. FFT

diffractograms is obtained by capturing the area of interest from the HRTEM micrograph of the

TiO2 nanoparticles, shown in Figure. 3.6 (b). It is consist of five resolved concentric rings which

give the value of the inter-planar d-spacings whose Miller indices are (hkl) [82]. The d-spacing

value obtained from the diffraction pattern of TiO2 is 3.51 A° which closely resemble with the d-

spacing values measured from the X-ray diffraction data.

Figure 3.7 (a-d) indicates the TEM, FFT and HRTEM of 1 % S-doped TiO2 nanoparticles.

TEM micrograph as shown in Figure 3.7 (a) indicates that particles are agglomerated to some

extend and show homogenous size distribution with distinguishable grain boundaries (Figure 3.7

(a&c)). Moreover the average crystallite size calculated from HRTEM show that it is in range of

15-17 nm after sulfur doping whereas the particles become more refine after doping and matched

well with the XRD results. Lattice image of the S-doped particles can be clearly viewed from

HRTEM, depicting the irregular, high crystallinity and sharp edges of the particles. Figure 3.7

(c&d) shows a high-resolution (HR) TEM image of S-doped TiO2 and its corresponding fast

Fourier Transform (FFT) given in (Figure 3.7 (b)). Interplanar spacing’s d is determined by

HRTEM and Fast Fourier Transform (FFT) which consist of six concentric well defined rings.

The space between the lattice plane is 3.52 Ao which corresponds well to the d value of the (101)

plane for anatase [83], in agreement with the theoretical data (XRD).

Similarly, The particle size, surface morphology and interplaner spacing of the 5% Co-S co-

doped titania nanoparticles is shown in Figure 3.8 (a-d). Figure 3.8 (a) shows that particles are

homogenously distributed and appear as small agglomerates. The corresponding HRTEM

(Figure 3.8 (d)) depicts clear lattice fringes with well defined surface. The d spacing value

calculated from the fringes is 3.56 Ao which can b indexed to 110 plane of TiO2. The

corresponding fast fourier transform (FFT) pattern exhibit an ordered array of spots which can

confirm the single cryatallinity (Figure 3.8 (b)) [85]. The average particle size calculated is in the

range of 9-12 nm which matches well with the XRD results.

74

Figure 3.6: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of pure TiO2.

75

Figure 3.7: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of 1% S doped TiO2.

76

Figure 3.8: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of 5% Co-S co-doped TiO2.

77

3.2.4 Elemental Analysis

3.2.4.1 EDX (Energy Dispersive X-rays) Studies

The chemical composition of undoped, S- doped and S, Co co-doped samples is

established from the EDX analysis and presented in Table 3.2. The chemical composition of all

powder samples observed from EDX analysis confirms that targeted composition of S, cobalt is

obtained in the all powder samples.

3.1.4.2 RBS (Rutherford Back Scattering) Studies

RBS analysis is carried out for detailed composition and determination of dopant’s

distribution in the synthesized materials. Figure 3.9 (a-g) shows the RBS spectra for TiO2, S-

doped TiO2 and Co-S co-doped TiO2. RBS data also enabled the quantitative determination of

the amount of dopants present in the titania matrix. For stoichiometry of the nanophotocatalysts,

the experimental data was simulated and resultant simulated curve showed a good agreement

with that of experimental observations. The composition profile analysis of the nanoparticles is

summarized in Table 3.2, which tells that all dopants are well and practically homogeneously

distributed into titania and this corroborate the EDX results.

400 600 800 1000 1200 1400 1600

Nor

mal

ized

Yei

ld

Channel

(a)(b)(c)(d)(e)(f)(g)

OS

Ti

Co

Figure 3.9: RBS spectra of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

78

Table 3.2: Elemental analysis of the un-doped, S-doped and Co-S co-doped TiO2.

Catalyst EDX elemental composition (wt%) RBS elemental composition (wt%)

Ti O S Co Ti O S Co PT 59.94 40.06 - - 58.71 41.29 - - ST 59.09 39.94 0.97 - 59.11 39.90 0.99 - 1Co-ST 59.14 39.89 0.97 0.96 58.16 39.87 0.98 0.99 2Co-ST 57.75 39.40 0.96 1.89 57.74 39.39 0.95 1.92 3Co-ST 57.20 38.98 0.94 2.88 57.15 38.94 0.96 2.95 4Co-ST 57.03 38.16 0.92 3.89 57.03 38.12 0.94 3.91 5Co-ST 56.51 37.71 0.91 4.87 56.46 37.70 0.93 4.91

3.2.5 BET Surface Area Studies

BET surface areas and average pore volume of all samples, calculated from the linear

parts of the BET plots and BJH methods, respectively and is summarized in Table 3.3. The

surface area obtained for S-doped TiO2 is 78.62 m2/g which is larger than bare TiO2 i.e. 68.50

m2/g. Additionally, increasing cobalt doping concentration, an enhancement effect on surface

area and average pore volume is observed. These results are consistent to XRD data because the

surface area increased by increasing doping level, due to reduction in particle size. This endorses

that cobalt doping is effective at higher concentrations which results in decrease particle size

with subsequent increase in porosity and BET surface area [155]. The enhanced surface area of

the 5% Co-S co-doped TiO2 (111.25 m2/g) as compared to 1% Co-S co-doped TiO2 (81.12 m2/g)

and TiO2 (68.50 m2/g) which renders it to be more effective from applications point of view.

Table 3.3: BET surface area and pore volume data of un-doped, S-doped and Co-S co-doped TiO2.

Composition BET S.A. (m2/g) Pore volume (cc/g)

TiO2 68.50 0.33

ST 78.65 0.53

1Co-ST 81.12 0.98

2Co-ST 91.32 1.03

3Co-ST 97.12 1.10

4Co-ST 105.56 1.18

5Co-ST 111.25 1.20

79

3.2.6 FTIR Studies of Un-calcined and Calcined Samples

FTIR spectra of un-calcined and calcined samples of pure TiO2, S-doped TiO2 and Co-S

co-doped TiO2 are shown in Figure 3.10 (a-g) and Figure 3.11 (a-g), respectively. FTIR pattern

of un-calcined samples (Figure 3.10 (a-g)) demonstrates similar pattern for un-doped and co-

doped samples. In the region below 1000 cm-1 [101], broad region around 500-700 cm−1 is

assigned to symmetric stretching vibrations of M-O (Ti-O & Co-O). The weak bands appearing

at 1030-1040 cm-1 may due to bending vibrations of Ti-O-C bonds, which may result from the

interaction between the Ti–O network and the carbon formed during the decomposition of the

organic precursors [90]. FT-IR spectra contain peaks corresponding to bending vibrations and

stretching vibrations of the O–H of the adsorbed water molecules about 1600–1745 cm-1 and

3000–3800 cm-1, respectively [91].

The weak peaks appearing at 1337-1340 cm-1 correspond to bending vibrations of C-H

group indicating the formation of some hydrocarbons residues originated from combustion of the

precursors which may bond to the TiO2 surface [85]. The bands present at 1410- 1430 cm-1 are

due to organic groups (acetate) bonded to titanium in pure TiO2 or may be due to nitrates group

in doped samples [91, 92]. For the samples calcined at 500 °C, the FTIR spectra show (Figure

3.11 (a-g)) that the band above 1100 cm-1 disappeared after calcination indicating the removal of

organic groups and bonded water while the band centered at 500-700 cm-1 may be due to metal-

oxygen bonds [93].

3.2.7 Raman Studies

Raman spectroscopy provides useful information related to phase composition, oxygen

vacancies, defect concentration and crystallanity [146]. Figure 3.12 (a-g) exhibits the Raman

spectra of TiO2, S-doped TiO2 and the catalyst co-doped S and cobalt with different amount of

cobalt. Raman spectra with well resolved peaks appearing at 144.3 cm-1, 196 cm-1, 397 cm-1,

514.33 cm-1 and 640 cm-1 may depict the well defined repetitive structure of tetragonal anatase

TiO2 [99]. The strongest Eg mode appearing at 144.3 cm-1 basically originates from the extension

vibrations of the anatase crystal structure [100]. One may observe that Raman bands shift

towards higher wave number along broadening upon doping of sulfur, demonstrating that S may

be incorporated into TiO2 lattice [70].

80

4000 3500 3000 2500 2000 1500 1000 50030

40

50

60

70

80

90

100

110

120

W avenum ber (cm -1)

(g)(f)(e)(d)(c)(b)(a)

3756.63 3065.42

700.10500

1030.10

1340.10

1622.051420.63

Figure 3.10: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

4 0

6 0

8 0

1 0 0

W a ve n u m b e r (cm -1 )

(g )(f)(e )

(d )

(c )

(b )

(a )

6 7 0 .0 35 0 0

Figure 3.11: FTIR spectra of calcined (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

81

Moreover, it has been observed that cobalt ion inclusion resulted in increase in band shift,

broadening and decrease in peak intensity. This suggests that the inclusion of cobalt ions into the

perfect crystal of TiO2 distorts the perfect crystal structure inducing the defects and reducing the

crystal growth of TiO2 which ultimately leads to the change in intensity, bandwidth and position

of the Raman bands [156].

Another factor contributing to the Raman shift can be the oxygen vacancies, created as a

result of doping which cause the lattice distortion leading to the variation in vibrations [101,

105]. Moreover, no new Raman bands appeared suggesting that TiO2 retained its anatase phase

and no new phase is formed after doping [156].

200 300 400 500 600 700 800Raman Shifts (cm-1)

Inte

nsity

(a.u

)

Eg

Eg

B1g A1g

Eg

(a)

(b)

(c)

(d)

(e)(f)

(g)

Figure 3.12: Raman spectra of (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f) 4Co-ST; (g) 5Co-ST.

82

3.2.8 TGA Analysis

Thermal analysis of un-clacined samples (un-doped TiO2, S-doped TiO2 and Co-S co-

doped) is carried out to evaluate the; phase transformation from amorphous to anatase TiO2,

effect of dopant level on the thermal stability and mass loss behavior of TiO2. Two distinct

weight losses appear in the TGA curve of as prepared TiO2, S-doped TiO2 and S-Co co-doped

TiO2, as presented in Figure 3.13 (a-g). The first weight loss occurring from 25 OC to 110 OC in

all samples can be attributed to desorption of adsorbed water and elimination of residual organic

solvents like ethanol, acetic acid from the pores of dried gel [91, 96]. A significant second

weight loss begins at 130 OC and ends up at 400 OC in un-doped, doped and co-doped samples.

This represents that water of crystallization is removed or un-hydrolyzed TTIP is being

decomposed. This can also be due to elimination of nitrates or other organic moieties giving the

slow decomposition process in doped samples (Figure 3.13 (b-g)). The corresponding weight

loss at both steps and char values (determined at 600 ○C) associated with each sample is

summarized in Table 3.4.

It can be seen that increasing the dopant level in S-Co co-dopaed samples, the total

weight loss reduced, indicating that material with high concentration of dopants provides less

space for the water or other organic contents to reside in the crystal lattice. A constant stability

can be seen above 400 OC, depicting that sample is thermally stable, free of residual impurities.

The residual yield is ~70-75 % in all samples. These results are consistent to the FTIR data of the

calcined sample which show absence of vibration peaks of organic species after calcinations

(section 3.27). Moreover, this also represents the crystallization of material from amorphous to

anatase phase above 400 OC, in agreement with XRD data [157].

Table 3.4: Weight loss data of the un-doped, S-doped and Co-S co-doped TiO2.

Sample Code

% Weight loss Char yield At 110 ºC At 400 ºC At 600 ºC

TiO2 8.82 % 27.31 72.69 ST 8.31 24.21 75.79

1Co-ST 6.81 23.62 76.40 2Co-ST 6.79 22.69 77.31 3Co-ST 6.71 21.75 78.25 4Co-ST 5.86 20.56 79.44 5Co-ST 5.76 19.07 80.93

83

100 200 300 400 500 600 700 80070

75

80

85

90

95

100

Wei

ght (

%)

Temperature (oC)

(a) (b) (c) (d) (e) (f) (g)

Figure 3.13: TGA profile of un-calcined (a) PT; (b) ST; (c) 1Co-ST; (d) 2Co-ST; (e) 3Co-ST; (f)

4Co-ST; (g) 5Co-ST.

3.3 Applications of Prepared Co-S co-doped Titanium Dioxide Nanostructure

The prepared nanomaterials are used for the photocataytic degradation of organic

pollutants (crystal violet, procion blue MXR, alizarin red S and malachite green) and for

photocatalytic reduction of carbon dioxide. On the basis of above results it was found Co-S co-

doped exhibited a much smaller band gap, good stability, high surface area depicting better

visible-light activity as compared to TiO2, S-doped TiO2 and co-doped TiO2 having low

concentration of cobalt.

On the basis of these findings, 5% Co-S co-doped samples is selected for carrying the

photocatalytic degradation of organic pollutants (dyes and phenol) and photoreduction of carbon

dioxide. Before photocatalytic studies, the photolysis of all dyes under both UV and visible

irradiation, and adsorption studies at different pH values under dark, are also carried out.

3.3.1 Photolysis of Dyes and Phenol

The direct photodegradation of all dyes including phenol is observed using 20 ppm of

each compound’s solution (100 ml) at pH ~7 for 6h under both UV and visible irradiation. The

84

dye and phenol concentration after each hour is measured spectrophotometrically. As depicted in

Figure 3.14 (a-e) and Figure 3.15 (a-e), no appreciable degradation is found under both UV and

visible regions for all compounds. The degradation observed is in the range of 0 to 1% (Table

3.5) for all dyes and phenol.

3.3.2 Adsorption Studies of Dyes and Phenol under Dark

The adsorption studies of dyes and phenol using TiO2, ST, and 5% Co-S co-doped TiO2

under dark conditions are carried out in order to determine the adsorption of dye on the

photocatalyst surface under neutral (pH ~7), acidic (pH ~4.5) and basic (pH ~9.5) media.

The industrial waste water usually has a wide range of pH value therefore pH value is

crucial parameter in determining the activity because it not only imparts role on the

characteristics of waste waters but also influence the surface charge properties of TiO2

photocatalyst which alternatively affects the adsorption of the molecules on the catalyst surface

[158]. Hence, the results of UV spectra exhibit that cationic dye (crystal violet and malachite

green) exhibit more adsorption in basic media as compared to acidic and neutral pH. While

anionic dyes (procion blue MXR and alizarin red S) and phenol showed more percent adsorption

values under acidic condition (pH ~4.5) (Figure 3.16 (a-i)).

However, overall adsorption observed is negligible i.e. less than 5% (Table 3.6). The

results of percent adsorption of all dyes and phenol at different interval of times and different pH

are demonstrated in Figure 3.16 (a-i) and data is tabulated in Table 3.6. Hence TiO2 is

amphoteric in nature when pH is lower, TiO2 surface is positively charged and favors adsorption

of anionic molecules while under basic condition, TiO2 surface is negatively charged and

enhances the adsorption of positively charged molecules [110, 158]. This is why the cationic

dyes exhibited more adsorption rate under basic conditions while anionic dyes and phenol

showed considerable good adsorption under higher pH value [111].

85

Figure 3.14: UV spectra of photolysis of (a) ARS; (b) CV; (c) MG; (d) PB-MXR; (e) PH; (f) Percent degradation of dyes and phenol under UV irradiation, time; 6h, pH ~7, conc.; 20 ppm.

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength (nm)

Stock solution 1h 2h 3h 4h 5h 6h

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

Perc

ent D

egra

datio

n

Time (h)

Malachite green Procion blue MXR Alizarine red S Crystal violet Phenol

(b)(a)

(c) (d)

(e) (f)

86

Figure 3.15: UV spectra of photolysis of (a) ARS; (b) CV; (c) MG; (d) PB-MXR; (e) PH; (f) Percent degradation of dyes and phenol under Vis. irradiation, time; 6h, pH ~7, conc.; 20 ppm.

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Abs

orba

nce(

a.u)

Wavelength(nm)


300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


240 280 320 360 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abs

orba

nce(

a.u)

Wavelength(nm)


0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

Per

cent

Deg

rada

tion

Time (h)

Malachite green Alizarine red S Procion blue MXR Crystal violet Phenol

(b)(a)

(c) (d)

(e) (f)

87

Table 3.5: Percent degradation data of dyes and phenol obtained by photolysis.

Compound Percent degradation (%) under UV irradiation

Percent degradation (%) under Visible irradiation

Alizarin red S 0.63 0.90

Procion blue MXR 0.61 0.81

Malachite green 0.62 0.85

Crystal violet 0.51 0.83

Phenol 0.42 0.77

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

1.2

Per

cent

Ads

orpt

ion

Time (h)

ARS PB-MXR MG CV PH

0 1 2 3 4 5 60.0

0.1

0.2

0.3

0.4

0.5 ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60.0

0.1

0.2

0.3

0.4

0.5

0.6


Per

cent

Ads

orpt

ion

Time (h)0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

1.2


Perc

ent A

dsor

ptio

n

Time (h)

(b)(a)

(c) (d)

88

Figure 3.16: Percent adsorption of dyes and phenol using (a) TiO2 (pH ~4.5); (b) TiO2 (pH ~7); (c) TiO2 (pH ~9.5); (d) ST (pH ~4.5); (e) ST (pH ~7); (f) ST (pH ~9.5); (g) 5Co-ST (pH ~4.5);

(h) 5Co-ST (pH ~7); (i) 5Co-ST (pH ~9.5) under dark, time; 6h, conc.; 20 ppm, catalyst; 50 mg.

0 1 2 3 4 5 60.0

0.1

0.2

0.3

0.4

0.5

0.6


Per

cent

Ads

orpt

ion

Time (h)0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0


Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3

4

5

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)0 1 2 3 4 5 6

0.0

0.4

0.8

1.2

1.6

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3

4

5 ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)

(e) (f)

(g) (h)

(i)

89

Table 3.6: Percent adsorption data of dyes and phenol under dark at various pH.

Compound

Percent adsorption (%)

PT at pH ST at pH 5Co-ST at pH

4.5 7 9.5 4.5 7 9.5 4.5 7 9.5

Alizarin red S 0.92 0.41 0.19 1.12 0.55 0.10 3.63 1.17 0.88

Procion blue MXR 1.01 0.23 0.18 0.99 0.63 0.18 3.98 1.14 0.59

Malachite green 0.22 0.46 0.60 0.20 0.48 1.20 1.79 1.26 3.61

Crystal violet 0.35 0.31 0.61 0.20 0.51 1.10 1.07 1.32 4.26

Phenol 0.97 0.32 0.21 1.27 0.33 0.23 3.82 1.37 0.33

3.3.3 Photocatalysis of Dyes and Phenol

The photocatalytic degradation of dyes and phenol solution is investigated under both UV

(with cutoff filter λ<380 nm) and visible irradiation (with cutoff filter λ > 420 nm) source using

500-W Xenon lamp (Ushio, model UI-502Q) irradiation using TiO2, S doped TiO2 and 5% Co-S

co-doped TiO2. Initially blank experiments were performed under UV and visible irradiation

without catalyst and negligible degradation was observed after 6 hours (as discussed previously

in section 3.2.1). Thereafter, the adsorption of the dye and phenol was investigated with these

catalysts at different pH values and negligible adsorption was observed (discussed previously in

section 3.2.2). Therefore photocatalytic experiments are conducted using all catalysts under

different experimental conditions to evaluate maximum degradation efficiency. During the

photocatalytic degradation process in waste waters, following parameters greatly affect the

photocatalytic process: concentration of dye solution, catalyst dose, pH of the solution to be

degraded, dopant content and irradiation source [115, 157, 158]. During this photocatalytic

discussion section, these parameters, their effect on percent degradation will be considered one

after the other. Finally, photo degradation of dyes and phenol is made under UV and visible

irradiation under optimized condition for comparison. The extent of the degradation dyes with

reaction time in each case is estimated by using following equation [159]:

Percent degradation= (Co-C)/Co× 100 (1)

90

Where C, Co and X is initial dye concentration, concentration of dye solution at time “t” and

percent degradation, respectively.

3.3.3.1 Effect of Initial Concentration of Dyes and Phenol

The effect of initial dye and phenol concentration on the degradation is studied by

varying concentrations such as 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm, keeping the

constant amount of catalyst (50 mg/100) and constant pH (pH ~7) under visible irradiation using

5% Co-S co-doped TiO2. The effect of initial concentration of dyes and phenol is also studied on

S-doped TiO2 and plane TiO2 (PT) for comparison. Figure 3.17 (a-e), 3.18 (a-e) and 3.19 (a-e)

represents the observed percent degradation change with irradiation time at various concentration

of dyes and phenol using TiO2, S doped TiO2 and 5% Co-S co-doped TiO2, respectively. The

obtained percent degradation data obtained at various concentrations is depicted in Table 3.7.

These results demonstrate that rate decreased with increase in dye concentration and maximum

degradation is achieved at 20 ppm concentration of solution with all catalysts.

The effect of decrease in degradation rate with increase in concentration can be

rationalized on the basis that as the concentration is increased, more molecules adsorb on the

surface, causing the screening of catalyst surface. This screening effect of surface of catalyst

with dye or phenol results in less number of photon to reach the catalyst surface, resulting in less ●OH generation hence inhibition of degradation [160]. Table 3.7 explains the effect of dye and

phenol concentration on percent degradation using TiO2, S-doped TiO2 and 5% Co-S co-doped

TiO2.

From Table 3.7, it can be seen that 5% Co-S co-doped exhibited more degradation rate as

compared to TiO2 and S-doped TiO2 because of its band gap which lies in visible range as

compared to other catalyst, making it more efficient under visible irradiation. However, the

maximum degradation of all dyes and phenol obtained with 5% Co-S co-doped lies in the range

of 60-70%, showing that 100% degradation is not obtained. Therefore the effect of catalyst dose,

pH and dopant content will be studied further at 20 ppm of concentration to obtain maximum

degradation rate.

91

Figure 3.17: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using TiO2, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

2

4

6

8

10P

erce

nt D

egra

datio

n

Time (min)

20ppm 40ppm 60ppm 80ppm 100ppm

0 5 10 15 20 25 30 35 40 45 500

3

6

9

12

15

18 20ppm 40ppm 60ppm 80ppm 100ppm

Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

2

4

6

8

10


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

2

4

6

8

10


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

3

6

9

12

15


Per

cent

Deg

rada

tion

Time (min)

(b)(a)

(c) (d)

(e)

92

Figure 3.18: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using S-doped TiO2, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30

35

40

45P

erce

nt D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30

35


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30

35

40


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

5

10

15

20

25

30

Per

cent

Deg

rada

tion

Time (min)


0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30


Per

cent

Deg

rada

tion

Time (min)

(b)(a)

(c) (d)

(e)

93

Figure 3.19: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using 5Co-ST, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60P

erce

nt D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60

70

Perc

ent D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)

(b)(a)

(c) (d)

(e)

94

Table 3.7: Percent degradation data of dyes and phenol at different concentration.

Catalyst

Conc. (ppm)

Percent degradation (%) Alizarin

red S Procion blue

MXR Malachite

green Crystal violet Phenol

TiO2

20 9.25 15.90 10.21 10.25 13.76 40 9.23 14.81 6.75 7.93 13.06 60 7.35 8.25 5.38 6.38 11.32 80 5.35 4.45 3.41 3.58 9.31 100 5.11 4.25 2.56 2.85 7.70

ST

20 35.60 34.29 37.33 27.97 32.64 40 31.19 38.82 27.47 20.66 31.19 60 30.03 27.64 27.25 19.78 30.03 80 24.07 21.75 25.38 18.51 24.07 100 15.45 16.11 14.08 15.77 15.45

5Co-ST

20 63.78 68.85 60.22 66.8 65.21 40 40.28 42.37 45.61 40.08 40.83 60 33.44 31.30 41.38 34.58 31.24 80 24.65 23.20 33.32 31.59 27.25 100 17.40 12.05 20.01 25.69 21.24

3.3.3.2 Effect of Catalyst Dose

The amount of catalyst is one of the crucial factors which affect the degradation

efficiency. The effect of catalyst amount on the degradation of dyes and phenol is studied at

constant concentration i.e. 20 ppm in aqueous solution (pH ~7) under visible irradiation. In order

to acquire the optimized amount of photocatalyst for efficient degradation of dyes and phenol,

the amount of catalyst is varied from 10-70 mg in each case. The effect of catalyst amount (TiO2,

S doped TiO2 and 5% Co-S co-doped TiO2) on percent degradation is presented in Figure 3.20

(a-c). The straight line relationship is observed initially for the plot of percent degradation and

amount of the catalyst as indicated in Figure 3.20 (a-c) and after 50 mg of catalyst, the

degradation rate deteriorates (Figure 3.20 (a-c)). At 50 mg of catalytic dose, the dye and phenol

is found to be degraded maximum on irradiation of visible irradiation for 50 min. However,

when amount is increased above 50 mg, the photocatalytic degradation efficiency tended to

decrease drastically for all catalysts. The photocatalytic degradation efficiencies at this optimum

photocatalyst amount of 50 mg of 5% Co-S co-doped TiO2 is 63.78% of alizarine red S, 68.85%

of procion blue MXR, 60.22% of malachite green, 66.8% of crystal violet and 62.1% of phenol.

As shown from Figure 3.20 (a-c), the photocatalytic efficiency trend is in order of 5Co-ST > ST

> PT.

95

0 10 20 30 40 50 60 700

2

4

6

8

10

12

14

16

18

Per

cent

Deg

rada

tion

Catalyst Amount (mg)

MG PH CV ARS PB-MXR

0 10 20 30 40 50 60 700

5

10

15

20

25

30

35

40

45

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

0 10 20 30 40 50 60 7010

20

30

40

50

60

70

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.20: Percent degradation of dyes and phenol as function of catalyst dose using (a) PT; (b) ST; (c) 5Co-ST under visible irradiations, time; 50 min, conc.; 20 ppm, pH ~7.

(a)

(b)

(c)

96

Regarding the optimum catalyst amount, the results can be rationalized on the basis of

availability of the active site on photocatalyst surface, the light absorption capacity of the

photocatalyst, and the light penetration path-length into the reaction suspension [158, 159]. With

increasing photocatalyst dosage up to the optimum value of 50 mg, active catalytic sites on

catalyst increases and absorption ability for photons by the catalyst increases correspondingly.

This large amount of the active sites on the photocatalystic system enhances the production of

hydroxyl and superoxide radicals on the catalyst surface hence resulting in increase degradation

efficiency [171].

At higher dose of the photocatalyst i.e., above 50 mg, the photocatalyst suspended has a

greater probability to get agglomerated. This makes some part of the catalyst surface to be

unavailable for photon absorption, resulting in lower number of photogenerated active species

for the subsequent dye degradation reactions [160, 161]. Therefore, an optimum dose of

photocatalyst is needed to ensure maximum absorption of solar light for efficient photocatalytic

efficiency [161]. On the basis of present investigation 50mg/100ml is chosen as optimum amount

of catalyst for all other experiments.

3.3.3.3 Effect of pH

As discussed in section 3.3.2, the most important factor that influences the photocatalytic

degradation is the pH of solution. It is worth mentioning that the waste water discharged from

the textile industry has a wide range of pH values [111]. In general, the pH value is considered as

significant factor in controlling the photocatalytic processes because it influences the surface

charge properties of TiO2 photocatalyst which affect the adsorption of species to be degraded

[158]. As in the present study two types of compounds i.e. cationic and anionic are selected to

determine the effect of acidic, neutral and basic pH (~4.5, ~7, ~9.5) on the degradation. And

mentioned in our previous studies that the degradation of the dyes and phenol was not much

appreciable at neutral pH with different catalyst loadings and concentration of solution, therefore

the photocatalysis is carried out under different pH values in order to check the maximum

degradation rate.

The effect of pH on the photocatalytic degradation of dyes and phenol using TiO2, S-

doped TiO2 and 5% Co-S co-doped TiO2 as a function of pH using 20 ppm of solution and 50

mg of catalyst is conducted for 50 min, as shown in Figure 3.21 (a-c).

97

0

20

40

60

80

100

4.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

9.57

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

Figure 3.21: Percent degradation of dyes and phenol as function of pH using (a) PT; (b) ST; (c)

5Co-ST under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

(a)

(b)

(c)

98

As shown from the Figure 3.21 (a-c), variation in pH resulted in drastic change in the

degradation activity. For 5% Co-S co-doped TiO2, the complete degradation of cationic species

(99.49% of malachite green and 100% of crystal violet) is found under basic media while anionic

species (99.9% of alizarin red S, 99.76 % of procion blue MXR and , 99.85% of phenol) are

degraded in acidic medium. For TiO2 and S-doped TiO2, the change in activity with changing the

pH is not as much significant (Figure 3.21 (a&b)). This behavior of complete removal of anionic

and cationic species in their respective pH can be explained on the basis that the variation in pH

of solution varies the surface charge properties of the TiO2 nanoparticles as a result the

adsorption of dyes on the surface changes resulting a change in the reaction rate. Under acidic or

basic media the surface of titania can be protonated or deprotonated, respectively, according to

the following relation [161, 162].

TiOH + H+ → TiOH2+ (2)

TiOH + OH- → TiO- + H2O (3)

The titania surface acts as positively charged under acidic medium and negatively

charged under alkaline medium [160, 161]. Therefore the activity of cationic species is

pronounced under basic media because these species are positively charged. While

photodegradation of anionic dyes is appreciable in acidic solution as compared to neutral and

basic pH. At high pH, the negative charges are developed on TiO2 surface which repel the

anionic molecules giving decrease in efficiency of photodegradation of anionic dye in basic

media. Similarly, low value of pH has same effect on the degradation efficiency of cationic dyes

and phenol due to charge repulsion between catalyst and dyes.

3.3.3.4 Effect of Dopant Content

The effect of dye concentration, catalyst dose and pH was studied previously with 5%

Co-S co-doped TiO2 under visible irradiation. The effect of cobalt contents from 1 to 5 wt% is

evaluated in terms of degradation of dyes and phenol for comparison under optimized condition

of concentration, pH and catalyst dose. The effect of cobalt contents on the photocatalytic

degradation of dyes and phenol is demonstrated in Figure 3.22, the photodegradation data of

TiO2 and S-doped TiO2 is also included for comparison. As seen from Figure 3.22 and Table 3.8,

the S-doped TiO2 exhibited higher activity for all dyes and phenol under visible irradiation as

compared to as un-doped sample.

99

PT ST 1Co-ST 2Co-ST 3Co-ST 4Co-ST 5Co-ST10

20

30

40

50

60

70

80

90

100

Perc

ent D

egra

datio

n

Dopant Concentration (%)

MG PH CV ARS PB-MXR

Figure 3.22: Percent degradation of dyes and phenol as function of dopant content under visible

irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

Table 3.8: Percent degradation data of dyes and phenol with different doped sample.

Catalyst


red S Procion blue

MXR Malachite

green Crystal violet

Phenol

PT 13.50 17.20 12.20 11.54 14.50 ST 41.26 39.94 41.51 40.01 42.04 1Co-ST 45.63 47.56 48.55 46.21 46.14 2Co-ST 52.89 51.87 55.54 56.52 57.13 3Co-ST 65.56 63.91 68.23 62.64 62.90 4Co-ST 76.78 72.84 74.22 75.84 73.21 5Co-ST 99.90 99.76 99.49 100 99.85

100

This is because of the band gap of S-TiO2 which lies in visible region, making it to

exhibit good efficiency in visible range. On the other hand as the amount of cobalt is increased

from 1 wt% to 5 wt% in Co-S co-doped samples, the photodegradation enhanced. These results

can be rationalized on the basis of band gap and surface area, because the increase in cobalt

caused narrowing of band gap (discussed earlier), enhancement of surface area along increase in

surface active sites and ultimately good catalytic activity [155].

3.3.3.5 Comparison of Photocatalytic Activity under UV and Visible Irradiation

The photocatalytic degradation of dyes and phenol (conc.; 20 ppm) using 20 mg of

photocatalyst under UV irradiation source in comparison with visible irradiation are also carried

out and the results are presented in Figure 3.23. It can be seen that considerably low degradation

is achieved within 6h under UV irradiation, whereas, complete degradation is obtained within 50

min in visible irradiation for all dyes and phenol under optimized conditions. The difference in

the rate of degradation in UV and visible region can be attributed to narrow band gap of this

catalyst making it enable to give maximum degradation efficiency under visible light. Therefore,

Co-S co-doped sample can be effectively used for photocatalytic degradation of textile pollutants

in waste water effectively in tropical countries using natural sunlight as source of energy.

UV(6h) Visible(50min)0

20

40

60

80

100

Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH

Figure 3.23: Comparison of percent degradation of dyes and phenol under UV and visible irradiation with 5% Co-S co-doped TiO2.

101

3.3.3.6 Re-use of the Photocatalyst

One of the major focuses of industrial waste water treatment strategies is to develop such

green technologies and management practice which have beneficial environmental applications.

Therefore, catalyst recycling after a photocatalytic process can be expected as good criteria for

sustainable waste water treatment. 5% Co-S co-doped TiO2 is selected for the determination of

the reusability of the catalyst as shown in Figure 3.24. For the recyclability of the catalyst, the

catalyst is separated though the reaction mixture after the degradation of the dye, washed

thoroughly with distilled water, dried and reused again for degradation of fresh dye solution. It

can seen from figure that on second and third time, comparable efficiency is observed and further

use of catalyst for 4th and 5th cycles, lesser efficiency (~94-96%) and (~92-94%), respectively, is

found. This may be due to deposition of organic species on the catalyst active sites which can be

easily removed by simple calcinations process [162, 163]. For the 6th cycle, the catalysts was

separated, dried and calcined at 500 oC to get rid of impurities which were saturated on the

catalyst surface and then catalyst was reused under same conditions (see Figure 3.24). Almost ~

99-100% efficiency is obtained after 6th cycle which is comparable to that of first cycle.

1 2 3 4 5 60

20

40

60

80

100After Calcination

Per

cent

Deg

rada

tion

Cycles (n)

ARS MG CV PB-MXR PH

Figure 3.24: Plots of recyclability of dyes and phenol using 5% Co-S co-doped TiO2 under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

102

To summarize, the Co-S co-doped TiO2 proved to be an efficient photocatalyst due to its

low band gap, high surface area, recyclability, high photo activity, excellent chemical and

photochemical stability and can be commercially used for the degradation of the textile effluents.

3.3.4 Photocatalytic Reduction of Carbon Dioxide

The photocatalytic reduction of CO2 is considered as an important focus of the recent

research because it not only solves problems related to the environmental pollution but also helps

in maintaining the carbon sources and offers low cost, clean and environmental friendly

production of the fuel [129, 130, 163]. In current study, the photocatalytic reduction of CO2 in

the presence of H2O as reductant is investigated under both UV (with cutoff filter λ<380 nm) and

visible irradiation (with cutoff filter λ > 420 nm) using TiO2, S-doped TiO2 and 5% Co-S co-

doped TiO2, in the current section while the photoreduction of carbon dioxide being carried out

by 5% Cu-S co-doped TiO2, 5% Fe-S co-doped TiO2, 5% Ru-S co-doped TiO2, 5% Cr-S co-

doped TiO2 is described further. Sodium hydroxide is used as hole scavenger and to enhance

solubility of carbon dioxide in water. The OH- ions are formed by ionization of NaOH in aqueous

solution which scavenges the photogenerated holes thereby reducing the recombination on

electron hole pair thus providing more electrons at the surface [98, 99].

3.3.4.1 Photocatalytic Reduction of Carbon Dioxide using PT, ST and 5Co-ST

Figure 3.25 (a-c) describes the photoreduction of CO2 in aqueous media using TiO2, S

doped TiO2 and 5% Co-S co-doped TiO2 in both and visible irradiation. Figure 3.25 (a-c) shows

the concentration of products as function of time for all catalysts. It can be seen that methanol is

the only product formed in case of TiO2 and S-doped TiO2 catalyst as a result of photoreduction

of the carbon dioxide under both irradiation sources (Figure 3.25 a&b) while significant amount

of ethanol is observed with Co-S co-doped photocatalyst under visible range with negligible

production is determined under UV irradiation.

The methanol production with TiO2 is 0.03 µmol and 0.002 µmol under visible and UV

irradiation, respectively, after 12h of irradiation. The enhanced production of methanol under

visible irradiation and negligible production under UV irradiation is observed with S-doped TiO2

(Figure 3.25 b). Approximately, 1.25 µmol of methanol is obtained after 12h of the reaction time

under visible irradiation with S-doped TiO2.

103

0 2 4 6 8 10 120.00

0.01

0.02

0.03

0.04

Time (h)

Met

hano

l Pro

duct

ion

(µm

ol/h

-cat

al.)

Vis Irradiation UV Irradiation

0 2 4 6 8 10 120.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time (h)

Met

hano

l Pro

duct

ion

(µm

ol/h

-cat

al.)


0 2 4 6 8 10 12

0

2

4

6

8

10

12

14

Time (h)

Eth

anol

Pro

duct

ion

(µm

ol/h

-cat

al.)


Figure 3.25: Product formation by photocatalytic reduction of carbon dioxide as function of time using (a) PT; (b) ST; (c) 5Co-ST.

(a)

(b)

(c)

104

The Co-S co-doped sample exhibited visible light activity for CO2 photoreduction to ethanol

and the concentration of ethanol increased linearly with the reaction time and leveled off after 10

h. Maximum amount of ethanol obtained after 12h of irradiation time is 13.38 µmol. The

significant activity of Co-S co-doped sample under visible irradiation is likely due to synergetic

effects of slight increased surface area, enhanced visible light absorption ability due to Co and S

co-doping and improved charge separation [123]. Thus high activity obtained with Co-S co-

doped sample under visible irradiation compared to the literature data makes it potentially cost

effective catalyst.

3.4 Conclusions

• In summary, ultra efficient Co-S co-doped nanophotocatalyst with varying dopant level

from 1 wt% to 5 wt% was successfully synthesized using sol-gel method.

• The cobalt concentration was found to have significant effect on the structure, optical,

and photocatalytic properties of TiO2 nanostructures.

• Our characterization data suggested that all dopants are well and homogeneously

dispersed into the titania matrix.

• Synergistic effect between S dopant and Co doping is responsible for the enhancement of

the visible light absorption capability of the nanoparticles hence good photocatalytic

activity.

• Excellent optical properties enhanced surface area and high quantum efficiency of Co-S

co-doped nanophotocatalyst resulted from adopted methodology and cobalt modification.

However, 5%Co-S co-doped TiO2 nanoparticles demonstrated efficient band gap, thermal

stability, good particle size and remarkably higher surface area in comparison to un-

doped, S-doped and Co-S co-doped TiO2 with lower amount of cobalt.

• In this work, adsorption and photocatalytic degradation of dyes (alizarin red S, crystal

violet, melachite green, procion blue MXR), phenol using un-doped, S-doped and Co-S

co-doped TiO2 were also shown under UV and visible irradiation. Optimum degradation

parameters (catalyst load, pH, dye concentration, dopant amount, effect of UV and visible

irradiation etc.) were studied to achieve high degradation rate.

• A remarkable degradation activity was observed with 5 wt% cobalt modified co-doped

titanium dioxide. As prepared TiO2 particles did not show any visible light absorption

105

hence no photocatalytic activity for degradation of dyes and phenol. The better

performance and higher photocatalytic efficiency of 5% Co-S co-doped titanium dioxide

can be accounted to its band gap and surface area properties.

• Recyclability experiments strongly propose that catalyst can be regenerated after each

photcatalysis process, suggesting that it can be reused for several cycles thus making it

economically feasible.

• 5% Co-S co-doped TiO2 also showed pronounced effect on photoreduction of carbon

dioxide in visible irradiation as compared to UV irradiation. The potential by products

formed by the reduction of CO2 in presence of water was ethanol.

• On the basis of results obtained, it can be concluded that Co-S co-doped catalyst could be

recommended for the practical application to the CO2 treatment and waste water

treatment due to its excellent photocatalytic and optical properties.

106

3.5 Characterization and Photocatalytic Applications Cu-S co-doped TiO2 Nanostructures

3.5.1 XRD Analysis

Figure 3.26 (a-h) shows the XRD patterns of prepared TiO2, S-doped TiO2 and Cu-S co-

doped TiO2 with varying Cu contents from 1-6 wt%. A closer look at Figure 3.26 (a-h) reveals

that only diffraction peaks of anatase can be seen from the XRD pattern of TiO2, S-doped TiO2,

Cu-S co-doped TiO2 having Cu contents from 1-5 wt% (Figure 3.26 (a-g)). However, the

diffraction peaks of rutile are observed with 6 wt% Cu containing sample, exhibiting the

presence of mixed anatase-rutile phase. These results indicate that doping of Cu after certain

level accelerates the anatase-rutile phase transformation due to distortion of TiO2 crystal

structure. The XRD patterns of all samples can be assigned to pure anatase phase with reflection

peaks in (101), (103), (004), (112), (202), (105), (211), (204), (116), (220) and (215) crystal

planes (JCPDS standard files #21-1272). However, little shift in peak, broadening of FWHM and

decrease in peak intensity of (101) plane is observed with S and Cu addition on TiO2 [121, 122.

156]. The average crystallite sizes, lattice parameters of all samples are estimated by Scherrer’s

equation and Bragg’s equation, respectively and tabulated in Table 3.9. Compared to pure TiO2,

a marked effect on particle size and lattice parameter (along c-axis) can be seen with the doping

of S and co-doping of Cu-S on TiO2.

10 20 30 40 50 60 70 80

R (h)

215

220

116

204

211

105202

11200

410

3

101

(g)

(f)

(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

R RR

R

RR

R

R

Figure 3.26: XRD patterns of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST (e) 3Cu-ST; (f) 4Cu-ST;

(g) 5Cu-ST; (h) 6Cu-ST calcined at 500 0C [156].

107

It can be observed that the crystallite size gradually reduced and the c-axis parameter and

d-spacing values also changed significantly, demonstrating that S and Cu may be incorporated

into TiO2 structure [96]. On the basis of above results, Cu-S co-doped anatase TiO2 with doping

level of Cu from 1-5 wt% is preferred for further studies, to avoid the anatase-rutile mixed phase.


UV-Vis diffuse reflectance spectroscopy is used to probe the band structure and energy

levels in the crystal. The reflectance spectra of all samples are taken in the range of 200-800 nm

at room temperature. The energy band gap of the samples are determined from their Tauc plot by

plotting (F(R)*hv)0.5 versus energy (eV) [72, 73]. The direct energy band gap are obtained by

linear part by extrapolating (F(R)*hv)0.5 of the curve to energy axis. Figure 3.27 shows the band

gap calculation of all samples determined from K-Munk function in the form of Tauc plot. As it

is clear from Figure 3.27 and Table 3.9 that energy band gap reduced with addition of sulfur (1

wt%), whereas, further addition of copper to S-doped TiO2 resulted in gradual decrease in band

gap. The band gap energies determined from the intercept of the tangents to the plots are 3.17 eV

for TiO2 and 2.89 eV for 1% S-doped TiO2. While the band gap shifted from 2.89 eV to 1.80 eV

with the increase in copper contents from 1-5 wt %. The narrowing of the band gap with increase

in Cu percentage can be ascribed to that Cu atom may be incorporated into the lattice of titania,

thus resulting in the change in crystallinity and electronic structures [82, 83]. In addition, this

change in band gap energy is directly reflected in the photocatalytic activity [156].



The surface morphology of all Cu-S co-doped titanium dioxide is studied using scanning

electron microscopy, as illustrated in Figure 3.28 (a-e). It is obvious from the images that the

morphology, size and shape of the nanoparticles are greatly influenced by the doping of copper

contents. Close observation reveals that nanoparticles are agglomerated to some extent and the

incorporation of copper resulted in the reduction in the average particle size of co-doped sample

as depicted from Figure 3.28 (a-e). This may be due to reason that that presence of Cu+2 may

hinder the growth of TiO2 [98, 99]. A homogeneous nanograin structure with average particle

size of 10-14 nm is obtained when copper doping is increased up to 5%. It is obvious that the

average particle size of the samples is in well agreement with that obtained from the XRD

pattern.

108

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

g

f ed

cb a

(F (R

).hv)

0.5

Energy (eV)

Figure 3.27: Band gap plots of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

Table 3.9: Calculated structural parameters and band gap data of un-doped, S-doped and Cu-S co-doped TiO2 [156].


(nm) d-spacing


(eV)

TiO2 20.05 3.5113 3.7810 9.5410 3.17

ST 19.18 3.5179 3.7816 9.5511 2.89

1Cu-ST 18.52 3.5184 3.7819 9.5632 2.74

2Cu-ST 17.30 3.5235 3.7821 9.5689 2.52

3Cu-ST 16.29 3.5313 3.7822 9.5781 2.35

4Cu-ST 15.17 3.5460 3.7825 9.5810 2.24

5Cu-ST 13.05 3.5491 3.7828 9.5880 1.80

109

Figure 3.28: SEM images of (a) 1Cu-ST; (b) 2Cu-ST; (c) 3Cu-ST; (d) 4Cu-ST; (e) 5Cu-ST.

0.1 µm 12 02 SE I20 kV X 60,000

(e)

0.1 µm 12 02 SE I20 kV X 60,000

(d)

0.1 µm 12 02 SE I20 kV X 60,000

(c)

0.1 µm 12 02 SE I20 kV X 60,000

(b)

0.1 µm 12 02 SE I20 kV X 60,000

(a)

110


Figure 3.29 shows TEM image in (a), high-resolution TEM images (HRTEM in (c), (d))

and FFT pattern (b) of 5% Cu-S co-doped TiO2 sample. Both types of images i.e., TEM (a) and

HRTEM (c) show agglomerates of TiO2 nanocrystals with crystallite size is in the range of 10-12

nm, which is in good agreement with the average size calculated from the XRD. Moreover, a

clear lattice fringes and single-crystalline structure with well defined edges can be viewed from

HRTEM (d) with distances of the 2D crystal lattices of 3.55 Å, which can be indexed to (101)

plane of anatase TiO2.

The corresponding Fast Fourier Transform (FFT) pattern (Figure 3.29 b) taken by

selecting a specific area from HRTEM shows an ordered array of spots, which can confirm the

single crystallinity of nanoparticles [100].



The EDX is a chemical microanalysis technique used together with SEM to determine the

chemical composition of the materials. The S and Cu contents determined from EDX

measurements are tabulated in Table 3.10. It can be clear from the values obtained from EDX

analysis that S and Fe contents on the nanoparticles are in good agreement with theoretical

values. It is noteworthy that mostly all nanoparticles are consist of Ti and O with S and Cu

contents.


Figure 3.30 shows the combined RBS experimental spectra of the un-doped, doped and

Cu-S co-doped. The composition profile analysis of the all nanomaterials is summarized in Table

3.10. RBS depth profile data of all samples clearly depicts the presence of homogenous single

layer which strongly suggests that dopants are practically well and homogenously distributed

into TiO2 lattice [102]. Moreover, these elemental compositions determined by RBS are

consistent to those determined by EDX and summarized in Table 3.10.

111

Figure 3.29: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of 5Cu-ST.

112

400 600 800 1000 1200 1400 1600

Nor

mal

ized

Yei

ld

Channel

(a)(b)(c)(d)(e)

(f)(g)

OS

TiCu

Figure 3.30: RBS spectra of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

Table 3.10: Elemental analysis of the un-doped, S-doped and Cu-S co-doped TiO2.


Ti O S Cu Ti O S Cu

PT 59.94 40.06 - - 58.71 41.29 - -

ST 59.09 39.94 0.97 - 59.11 39.90 0.99 -

1Cu-ST 58.26 39.90 0.95 0.89 58.21 39.85 0.97 0.97

2Cu-ST 57.41 39.75 0.97 1.87 57.45 39.68 0.97 1.90

3Cu-ST 57.29 38.88 0.93 2.90 57.17 38.92 0.95 2.96

4Cu-ST 56.42 38.80 0.92 3.86 57.32 38.78 0.96 3.94

5Cu-ST 55.55 38.68 0.92 4.85 55.53 38.65 0.90 4.92

113


Measurements of specific surface area, pore volume of all Cu, S co-doped TiO2 samples

are presented in Table 3.11. The surface area of the as synthesized TiO2 is also included for

comparison. It is known that BET surface area has no effect on the phase of the TiO2 nano

particles but they do have an effect on the photcatalytic reactions [95, 96, 156]. Larger surface

area mean that large active sites are exposed, consequently resulting in good photocatalytic

activity. It can be seen in Table 3.11 that sulfur doping significantly resulted in the increase in

surface area from 68.50 m2/g and 78.62 m2/g which may be due to decrease in particle size upon

incorporation of sulfur into TiO2 lattice, similar trend is observed in pore volume. Moreover, an

obvious increase in BET surface area from 89.02 m2/g to 120.11 m2/g is observed with

increasing the copper amount from 1 wt% to 5 wt% while keeping the sulfur contents constant (1

wt%). This higher surface area of 5% Cu-S co-doped TiO2 as compared to other doped and un-

doped can be attributed to its small crystallite size, shown in Table 3.11. However, pore volume

which is strongly correlated with BET surface area is increased; depicting that porosity of 5%

Cu-S co-doped sample is significantly higher than other samples. The higher surface area and

high porosity strongly determines the higher photocatalytic activity.

Table 3.11: BET surface area and pore volume data of un-doped, S-doped and Cu-S co-doped TiO2.


TiO2 68.50 0.33

ST 78.65 0.53

1Cu-ST 89.02 0.72

2Cu-ST 90.35 0.99

3Cu-ST 98.79 1.08

4Cu-ST 101.65 1.15

5Cu-ST 120.11 1.23

114


FTIR spectrum of the gel dried at 100 °C and calcined at 500 °C is shown in Figure 3.31

(a-g) and Figure 3.32 (a-g), respectively. The FTIR spectra of un-doped, doped and co-doped

TiO2 samples dried at 100 oC exhibit quit similar pattern of the peaks. In Figure 3.31 (a-g) the

wide peaks in region of 500 cm−1-800 cm−1 correspond to M–O stretching vibrations mainly due

to Ti-O or Cu-O bonds [90]. The band at 1030-1040 cm-1 and 1337-1340 cm-1 correspond to

bending vibrations of Ti-O-C and C-H bonds in all samples which mainly come from the

residual organic species interaction to TiO2 surface, respectively [91]. The bands appearing at

1370-1430 cm-1 indicate the presence of organic groups (acetate) bonded to TiO2. The broad

bands at 3000-3800 cm-1 and 1630 cm−1 are ascribed, respectively, to the O-H stretching and

bending vibrations of hydroxyl group (–OH) or the adsorbed water on the titanium dioxide

surface [93]. In addition, doping seems to have marked effect on the quantity and properties of

surface groups resulting in the decrease in the peak intensity with increase in doping contents.

Moreover, the samples calcined at 500 oC (Figure 3.32 (a-g)) show only metal oxide peaks of Ti-

O or Cu-O symmetry stretching vibration at 500-700 cm-1. The absence of other bands at higher

wave number is indication of removal of adsorbed water and organic contents from TiO2 matrix

after calcination [107].

3.5.7 Raman Studies

Figure 3.33 shows the Raman scattering spectra of un-doped, S-doped and Cu-S co-

doped TiO2 (with 1% S and 1, 2, 3, 4, 5 wt % Cu). The tetragonal anatase structure possess two

TiO2 molecules per unit cell exhibiting the Raman bands at 144.3 cm-1, 196 cm-1, 397 cm-1,

514.33 cm-1 and 640 cm-1 which are assigned to Eg, Eg, B1g, A1g, and Eg modes of vibrations,

respectively [99]. Figure 3.33 (a-g) shows that peak appearing at 144 cm-1 (Eg) confirm the

anatase phase of TiO2 and is effected by the S doping to TiO2, depicting the shift and broadening

in the band width. This may be attributed to the reduction in particle size, introduction of phonon

confinement into TiO2 after S doping which cause the distortion of TiO2 crystal geometry [115].

With the addition of copper to the S-doped TiO2 a gradual shift in Eg mode of vibration to high

wave number and broadening in all Raman bands is observed resulting in the reduction in band

intensity. The observed shift may be ascribed to the decrease in particle size with doping, as the

particle size reduces to the nanometer scale range, the vibrational properties of the materials are

effected significantly [115-117].

115

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 03 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

W a ve n u m b e r (cm -1 )

(a )

(b )(c )(d )(e )(f)

(g )

3 7 6 4 .0 2 3 0 8 1 .7 5

7 5 6 .9 65 0 0

1 0 3 6 .4 8

1 3 3 9 .0 2

1 6 2 4 .6 81 4 2 2 .6 3

Figure 3.31: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

4000 3500 3000 2500 2000 1500 1000 50040

50

60

70

80

90

100

110

W avenumber (cm-1)

(g)(f)

(e)(d)(c)(b)(a)

680.25500

Figure 3.32: FTIR spectra of calcined (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

116

One of the major factors which may be observed at nanometer scale is the volume

contraction arising from size induced radial pressure upon doping. This volume contraction leads

to decrease in interatomic distance resulting in the increase in force constant and hence Raman

bands shift toward higher ῡ. In vibrational transitions the force constant is related to the wave

number according to the following relationship ῡ=k1/2 [118]. The other factor contributing to the

band broadening and Raman shift is the phonon confinement which arises at small particle scale

and discussed previously [120]. The Raman results along the XRD data strongly suggest that the

copper ions may be substituted in the anatase structure. Moreover, by increase in doping amount,

the disorder increases, and ideal symmetry would be destroyed resulting in increase in FWHM of

the Raman modes as observed in copper doped TiO2 nanoparticles [88, 96].

200 300 400 500 600 700 800

Raman Shifts (cm-1)

Inte

nsity

(a.u

)

Eg

Eg

B1g A1g

Eg

(a)

(b)

(c)

(d)(e)

(f)

(g)

Figure 3.33: Raman spectra of (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

117

3.5.8 TGA Analysis

The thermal stability of titania is one of the significant constraints for its use as catalyst.

To investigate the effect of dopant on thermal stability of TiO2, TGA analysis was conducted up

to 800 oC with an interval of 10 °C as shown in Figure 3.34. All samples exhibited somehow

similar pattern of thermal behavior with two step weight loss. Initial weight loss appeared in the

range of 25-120 0C for all samples (Figure 3.34 (a-g)) which can be attributed to the evaporation

of physically adsorbed water or thermal decomposition of the organic solvents i.e., ethanol,

acetic acid [88, 89]. The second major weight loss begins at 120 oC and continues up to 400 oC

which corresponds to dehadrayion and slow decomposition of organic moieties [96]. The step

weight loss and char yield of TiO2, S doped TiO2, Cu-S co-doped TiO2 is summarized in Table

3.12. It can be seen that the overall weight loss decreased with the S doping and increasing the

copper contents in co-doped sample. At first, inclusion of S into TiO2 latticed resulted reduction

in weight loss from 27.31%-24.21%, depicting the incorporation of S into TiO2 framework.

Further, the weight loss decreased from 27.31% to 15.85% and residual yield increased from

77.58-84.15% (at 600 oC) with increase in Cu contents from 1-5 wt% depicting increase in

thermal stability. This behavior may be due to the structural changes occurring in TiO2

framework, induced by the copper at high concentration [112]. Notably, above 400 ºC constant

weight is observed for all samples depicting that the samples are stable and free of organic

components. It is worth mentioning here that, on the basis of these results the calcination

temperature was selected as 500 oC in order to obtain the stable anatase TiO2.

Table 3.12: Weight loss data of the un-doped, S-doped and Cu-S co-doped TiO2.

Sample Code

% Weight loss % Char yield At 120 ºC At 400 ºC At 600 ºC

TiO2 8.82 27.31 72.69 ST 8.31 24.21 75.79 1Cu-ST 7.50 22.42 77.58 2Cu-ST 3.70 21.63 79.37 3Cu-ST 3.60 18.21 81.79 4Cu-ST 3.60 16.82 83.18 5Cu-ST 2.77 15.85 84.15

118

100 200 300 400 500 600 700 80070

75

80

85

90

95

100

Wei

ght (

%)

Temperature (oC)

(a) (b) (c) (d) (e) (f) (g)

Figure 3.34: TGA profile of un-calcined (a) PT; (b) ST; (c) 1Cu-ST; (d) 2Cu-ST; (e) 3Cu-ST; (f) 4Cu-ST; (g) 5Cu-ST.

3.6 Applications of Prepared Cu-S co-doped Titanium Dioxide Nanostructures

As mentioned above the co-doping of 5% Cu-S doped TiO2 resulted in low band gap,

good stability and enhanced surface area of TiO2 which makes it efficient photocatalyst as

compared to TiO2, ST and Cu-S co-doped catalyst with lower concentration of Cu. The

photocataytic activity is determined in terms of photo degradation of organic pollutants (phenol,

crystal violet, procion blue MXR, alizarin red S and malachite green) and photocatalytic

reduction of carbon dioxide under both visible and UV irradiation. Before photocatalytic

degradation of dyes and phenol, the adsorption studies are carried out under dark conditions to

establish the adsorption efficiency.


The adsorption studies of dyes and phenol are carried out at different pH values i.e. pH

~7, pH ~4.5 and pH ~9.5. As discussed previously, pH value is crucial factor which affect the

adsorption capacity of the dyes and phenol on TiO2 surface because changing the pH value

changes the surface properties of the TiO2 which ultimately affects the adsorption capability. The

119

results of adsorption studies of dye and phenol solution with 5% Cu-S co-doped TiO2 in the dark

are shown in Figure 3.35 (a-c) and percent adsorption data is tabulated in Table 3.13.

In all cases, 5% Cu-S co-doped TiO2 provided the greater adsorption of cationic dyes

under basic pH while for anionic dyes and phenol under acidic conditions. As seen from the data

only 0-5 % adsorption of the all dyes and phenol is seen at 5% Cu-S co-doped TiO2 which is not

appreciable.

Figure 3.35: Percent adsorption of dyes and phenol using Cu-S co-doped TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time; 6h, conc.; 20 ppm, catalyst; 50 mg.

0 1 2 3 4 5 60

1

2

3

4

5

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)0 1 2 3 4 5 6

0.0

0.3

0.6

0.9

1.2


Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3

4

5


Per

cent

Ads

orpt

ion

Time (h)

(b)(a)

(c)

120


Compound


pH ~ 4.5 pH ~7 pH ~9.5

Alizarin red S 3.96 1.28 0.22

Procion blue MXR 3.41 1.11 0.25

Malachite green 0.10 1.28 4.65

Crystal violet 0.30 0.95 5.25

Phenol 3.61 1.27 0.67


The results of photodegradation of dyes and phenol without photocatalyst (5% Cu-S co-doped) is

discussed previously (section 3.3.1). The dye and phenol did not show degradation by direct

photolysis under UV-Vis irradiation and by catalyst in dark. Negligible degradation is observed

by direct photolysis and adsorption process (section 3.6.1). This reveals that the dye and phenol

can be degraded only in the presence of photocatalyst and irradiation source.

5% Cu-S co-doped TiO2 is used to determine the photocatalytic degradation of dyes and

phenol under both UV (with cutoff filter λ<380 nm) and visible irradiation (with cutoff filter λ >

420 nm). The effect of dye concentration, catalyst dose, pH of the solution, dopant content and

different irradiation source (UV and visible) was also seen on the rate of degradation because

these parameters greatly affect the photocatalytic process in waste water treatment [158]. In this

study the photocatalytic activity of photocatalysts is evaluated in terms of degradation of dye and

phenol employing UV–visible spectrometry. The phenol and dye conversion with reaction time

is estimated using the following formula [159, 160].

Percent degradation= (Co-C)/Co× 100 (1)

Where C and Co is initial dye concentration and concentration of dye solution at time “t,

respectively.

121


During the evaluation of photocatalytic activity of 5% Cu-S co-doped TiO2, the effect of

initial dye and phenol concentration on the degradation is investigated by varying the

concentration from 20-100 ppm (see Figure 3.36 (a-e)) while keeping the catalyst amount 50 mg

and pH ~7. The result of percent degradation of all dyes and phenol with reaction time at various

concentrations is shown in Figure 3.36 (a-e) and given in Table 3.14. It can be seen from Figure

3.36 (a-e) and Table 3.14 that increase in the dye and phenol concentration from 20 ppm to 100

ppm decreased the percent removal of alizarin red S from 57% to 26%, procion blue MXR from

64% to 15%, malachite green from 62% to 30%, crystal violet from 67% to 22% and phenol

from 65% to 21%. Higher amount of reactant molecules decrease the path length of photon

entering into the dye solution and prohibiting them to reach at the catalyst. This reduces the

generation of electron-hole pair and ultimately catalytic efficiency. To obtain the maximum

degradation rate, the photocatalysis is carried out at different catalyst dose and pH values further.


As mentioned previously, 20 ppm of the dye and phenol exhibited maximum degradation

therefore; 20 ppm is selected as an optimum amount of the solution for further studying the

effect of catalyst dose. The effect of different amount of catalyst (5% Cu-S co-doped TiO2) i.e.

10 mg 20 mg, 30 mg, 40 mg, 50 mg, 60 mg, 70 mg is studied on the degradation of 20 ppm

aqueous solution at neutral pH under visible irradiation. The percent degradation results with the

catalyst dose are shown in Figure 3.37. As seen from the Figure 3.37, by increase in amount of

the photocatalyst from 10 mg to 50 mg, the rate of degradation increased. Above 50 mg, the

catalytic efficiency reduced drastically for all dyes and phenol. This effect of catalyst dose on the

activity can be explained on the basis that increase in the amount of catalyst results in increase in

surface active site which are available for more absorption of light. This causes the increase in

production of radical which actively participate in the catalytic reaction [158, 159]. Beyond the

certain level of catalyst amount, degradation efficiency tends to decrease because of

agglomeration of catalyst which results in decrease in the light penetration. Consequently the

photo activated volume of the catalyst shrinks resulting in less availability of active sites [165,

166]. From these results, the optimum concentration of the photocatalyst for efficient

degradation of dyes and phenol is found to be 50 mg/100 ml.

122

Figure 3.36: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using 5Cu-ST, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70P

erce

nt D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60

70


Perc

ent D

egra

datio

n

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70


Perc

ent D

egra

datio

n

Time (min)

(b)(a)

(c) (d)

(e)

123


Catalyst

Conc.

(ppm)

Percent degradation (%)

Alizarin

red S

Procion blue

MXR

Malachite

green

Crystal

violet Phenol

5Cu-ST

20 57.81 64.10 62.10 67.64 65.20

40 53.55 51.32 47.35 43.30 50.83

60 43.45 31.25 40.55 34.65 40.31

80 32.32 26.23 38.15 29.51 28.02

100 26.60 15.50 30.32 22.45 21.53

0 10 20 30 40 50 60 7010

20

30

40

50

60

70

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.37: Percent degradation of dyes and phenol as function of catalyst dose using 5Cu-ST under visible irradiations, time; 50 min, conc.; 20 ppm, pH ~7.

124


The pH of textile effluent varies to a great extent therefore the effect of pH on the dye

and phenol degradation is carried out at various pH (~4.5, ~7, ~9.5), using 50 mg of the catalyst

and 20 ppm of aqueous solution under visible irradiation. As seen in previous studies, complete

degradation was not observed at different concentration and catalyst dose, therefore

photodegradation is carried out in different media having different pH in order to obtain the

complete degradation of the dyes and phenol. The pH of the solution is an important parameter

which dictates the charge on the particle surface and photocatalytic processes occurring on it. As

seen from the Figure 3.38, the degradation of dyes varies with the change in pH values, at pH

~4.5 procion blue MXR, alizarin red S and phenol exhibited maximum degradation whereas

malachite green and crystal violet are degraded completely at pH ~9.5. In addition, cationic dyes

show less degradation in acidic media due to repulsion of similar charges on the catalyst surface

and dye. Similarly, deterioration in degradation of anionic species is observed in basic media due

to negative-negative charge repulsion between catalyst surface and the dye [156, 112]. On the

basis of these findings further studies are carried out at pH ~4.5 for procion blue MXR, alizarin

red S and phenol whereas at pH ~9 for crystal violet and malachite green.

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

Figure 3.38: Percent degradation of dyes and phenol as function of pH using 5Cu-ST under

visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

125


The dopant content is considered as another important parameter which has strongly

influence on the degradation kinetics of dye and phenol solution [66, 67]. As discussed earlier, S

is doped on TiO2 lattice, the band gap shifted towards visible region. At the same time, by co-

doping of Cu (1 wt% to 5 wt%) on Cu-S co-doped samples, the band gap not only significantly

decreased but surface area also enhanced which ultimately boosts the catalyst active sites [115].

The effect of dopant on percent degradation of 20 ppm dyes and phenol solution using 50 mg of

catalyst under visible irradiation is shown in Figure 3.39 and demonstrated in Tables 3.15. It is

obvious from results that the photocatalytic activity increased with increasing the dopant

contents and trend of photocatalytic degradation of dyes and phenol with catalyst is found to be

in the order of 5Cu-ST > 4Cu-ST > 3Cu-ST > 2Cu-ST > 1Cu-ST > ST > PT. The higher

photocatalytic activity of 5Cu-ST as compared to other catalyst can be attributed to its higher

surface area and lower band gap as compared to other catalysts.


The effect of UV and visible light on the degradation of dyes and phenol by

photocatalytic process are also investigated. The comparative study are carried out for the

degradation of dye and phenol solution under optimized conditions (as discussed) for 50 min

under visible light and for 6h under UV light. Figure 3.40 demonstrate the degradation rate as a

function of irradiation source. The rate of degradation of dyes and phenol is found to be

significantly higher in the visible light in comparison to UV light. After 50 minutes of reaction

time, the percentage degradation for all samples is in the range of ~98-99 % in visible light while

~11-16 % efficiency is achieved after 6h in UV light. This is due to ability of the catalyst to

absorb part of the visible light because of its narrow band gap.

The percentage degradation obtained under visible light clearly indicates that solar light

can be efficiently used for the photocatalytic degradation of textile effluents from waste water.

This can make it cost effectively process employed for the degradation of real textile effluent

under solar light in future.

126

PT ST 1Cu-ST 2Cu-ST 3Cu-ST 4Cu-ST 5Cu-ST10

20

30

40

50

60

70

80

90

100

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.39: Percent degradation of dyes and phenol as function of dopant content under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.


Catalyst


red S Procion blue

MXR Malachite

green Crystal violet

Phenol

PT 13.5 17.2 12.2 11.5 14.4 ST 41.2 39.9 41.5 40.5 42.7 1Cu-ST 43.3 45.2 48.1 40.5 41.7 2Cu-ST 50.5 58.4 55.5 54.2 55.7 3Cu-ST 65.7 64.0 62.1 67.6 65.1 4Cu-ST 72.4 73.4 77.0 76.5 72.1 5Cu-ST 99.8 99.6 98.9 99.8 99.2

127

UV (6h) Visible (50min)0

20

40

60

80

100

Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH

Figure 3.40: Comparison of percent degradation of dyes and phenol under UV and visible

irradiation with 5% Cu-S co-doped TiO2.


In order to ensure the reuse-ability of the photocatalyst, dye and phenol degradation

experiments are carried out with 5% Cu-S co-doped TiO2 under visible light irradiation using the

optimized concentration, catalyst amount and pH. It is found that up to 4th cycle, the activity

remained almost same and for 5th it reduced to some extend for all dyes (sees Figure 3.41). The

observed decrease on the degradation rate may be due to agglomeration and sedimentation of the

reactant molecules on catalyst surface after each cycle of photocatalytic degradation [158, 167].

This results in unavailability of the new parts of the catalyst surface for dye and photon

absorption thus reducing the efficiency of the catalytic reaction.

Therefore, for the 6th cycle catalyst was separated, dried and calcined at 500 oC to remove

the adsorb organics from the catalyst surface and reused for the photodegradation (Figure 3.41)

and maximum degradation is observed, comparable to the first cycle. Thus it can be concluded

from these studies that photocatalyst is photostable during the photocatalytic degradation dye and

phenol and can be reused for several times, which makes it very important for practical

applications.

128

1 2 3 4 5 60

20

40

60

80


Per

cent

Deg

rada

tion

Cycle (n)

ARS MG CV PB-MXR PH

Figure 3.41: Plots of recyclability of dyes and phenol using 5 % Cu-S co-doped TiO2 under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

3.6.3 Photocatalytic Reduction of Carbon Dioxide using 5Cu-ST

Photocatalytic reduction of carbon dioxide is carried out at ambient temperature and

pressure in aqueous media containing NaOH as hole scavenger using 5% Cu-S co-doped TiO2

under UV and visible irradiation. 5% Cu-S co-doped TiO2 exhibited high surface area, visible

light absorption ability and good photocatalytic activity for degradation of dyes as compared to

other catalyst (discussed already). Methanol is the only product formed as result of reduction of

carbon dioxide by 5% Cu-S co-doped TiO2. Figure 3.42 compares the production of methanol

under UV and visible irradiation. In UV irradiation, no significant methanol is found, whereas,

maximum production of methanol observed under visible irradiation is ~10.71 µmol at 10h after

that it remained constant. Cu and S addition in TiO2 not only lowers the band gap but also

improve the charge separation of photo generated electron-hole pairs thus enhancing the overall

photocatalytic activity under visible illumination [121, 123].

129

0 2 4 6 8 10 12

0

2

4

6

8

10

12


Time (h)

Met

hano

l Pro

duct

ion

(µm

ol/h

-cat

.)

Figure 3.42: Methanol production by photoreduction of CO2 as function of time using 5% Cu-S co-doped TiO2 [156].

The deposition of Cu enhances the efficiency of photoreduction of CO2 because Cu

species in TiO2 lattice traps and promotes the transfer of photoexcited electrons thus suppressing

the recombination rate of electron-hole pairs [156, 166]. The main effect of Cu is to conduct the

photoelectron and activate the CO2 more significantly and yield the CH3OH.

3.7 Conclusions

• Cu-S co-doped titanium dioxide nanoparticles with various copper loadings (1-5 wt%)

are synthesized by a single step using sol-gel method.

• From the characterization tools, it can be implied that S and Cu-S co-doping has

significant impact on the crystal structure of TiO2. Copper, S co-doped TiO2

photocatalyst showed high specific area, small crystallite size, narrow band gap which

contributed to their high photocatalytic activity.

• The characterization of the nanomaterials was done from DRS, XRD, SEM, TEM/EDX,

RBS, TGA, Raman and BET.

• The effect of S doping on the absorption ability of was evidently seen in the shifting of

band gap to higher wavelength.

130

• With increase in Cu doping from 1 wt% to 5 wt%, the catalyst properties varied

significantly. Compared to un-doped, S-doped and all Cu-S co-doped TiO2, 5% Cu-S co-

doped TiO2 exhibited smaller crystallite size, high surface area, good thermal stability

and narrow band gap.

• Photocatalytic degradation of dye and phenol was carried out using Cu-S co-doped TiO2

photocatalyst under UV and visible irradiation.

• The degradation studies were done under various reaction conditions to observe the

effects of dye concentration, catalyst dose, pH, dopant ions, nature of light (UV and

Visible) on degradation rate. The degradation of cationic species was favored greatly by

the pH and complete removal was for cationic species at pH ~ 9 and for anionic at pH

~4.5.

• The observations of photocatalytic degradation under UV and visible irradiation

demonstrate the importance of choosing the visible light to obtain high degradation rate,

which is vital for any practical application of photocatalytic degradation. The compounds

were found to degrade more rapidly in the presence visible light as compared UV light.

• The catalyst showed good recyclability many times along excellent degradation

efficiency.

• Efficient conversion of CO2 and water vapor to methanol is achieved using Cu-S co-

doped TiO2 nanoparticles, giving good yield of methanol under visible light.

• The copper-S co-doped -TiO2 is found to be feasible and attractive to be used for further

investigation of industrial and environment management studies especially for waste

water treatment and carbon dioxide reduction strategies.

131

3.8 Characterization and Photocatalytic Applications Ru-S co-doped TiO2 Nanostructures

3.8.1 XRD Analysis

Figure 3.43 (a-h) depicts the typical XRD patterns of calcined TiO2, S-doped TiO2 and

Ru-S co-doped TiO2 with 1-6% ruthenium. XRD pattern of each spectra can be indexed as

anatase phase of TiO2 [JCPDS No. 21-1272] while traces of secondary phase (rutile) can be seen

with 6% Ru-S co-doped sample.

A closer look at Figure 3.43 (a-h) reveals that S and Ru-doping resulted in slight shift in

2θ values along broadening of peak. This demonstrates that S and ruthenium dopants are

probably incorporated at interstitial or subsitutional sites in TiO2 crystal structure. However, no

extra phase of impurity appeared, suggesting that no significant segregation of dopant is

produced on sample [150, 151]. The crystallite sizes estimated from XRD pattern (by Scherrer

formula) for pure and doped samples are shown in Table 3.16. According to the data, the particle

size reduced with addition of S and Ru which may be due to the delay in the particle growth in

presence of Ru and S ions leading to the reduction of particle size [122, 123]. The calculated

structural parameters (Table 3.16) show little expansion of the lattice along c-axis and increase in

d-spacing value with increase ruthenium contents.

10 20 30 40 50 60 70 80

R (h)21

5

220

116

204

211

105202

11200

410

3

101

(g)

(f)

(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

R RR

R

R

RR

R

Figure 3.43: XRD patterns of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST; (h) 6Ru-ST calcined at 500 0C.

132

It is evident from these results that little distortion may be brought by the addition of S

and Ru [130]. Compared to the un-doped TiO2, a bigger portion of anatase is transformed

(Figure 3.43 (h)) to rutile phase when doping exceeded to 6 wt%, probably due to the instability

of the crystalline lattice induced by high concentration of Ru ions. Therefore, the samples with

Ru up to 5 wt% are selected for further studies.


The band gap spectra of pure TiO2, S-doped TiO2, ruthenium-sulfur co-doped TiO2 is

shown in Figure 3.44 (a-g). The band gap energies of the prepared nanomaterials are derived

using Tauc plot by plotting (F(R)*hv)0.5 versus energy (eV) (mentioned in section 2.8.2). The

comparative band gap energy values calculated for all samples are tabulated in Table 3.16. The

spectra shown in Figure 3.44 (a-g) exhibit similar characteristics to those of cobalt and copper

co-doped nano TiO2. The shift towards lower energy is observed on S-doped TiO2 which can be

ascribed to the introduction of energy levels anion atom above the valence band of TiO2,

resulting in narrowing of the band gap [168]. The band gap of TiO2 nanoparticles is reduced

further by co-doping of Ru-S which could be primarily attributed to the substitution Ru ions in

TiO2 lattice which introduces new electron states below the conduction band of TiO2[117, 118].

Moreover, ruthenium ions also act as efficient electron scavenger to trap the conduction band

electrons and reducing the electro hole recombination in titania [113, 116]. The shift towards the

visible region is more pronounced with increasing the ruthenium doping level and maximum

reduction in band gap is achieved from 2.89 eV to 2.07 eV with 5% Ru-S co-doped TiO2.



Figure 3.45 (a-e) shows SEM images of Ru-S co-doped nanoparticles with different

weight percent of ruthenium ranging from 1% to 5%. The SEM images depict that particle are

slightly agglomerated, well disperse and show well defined crystalline nature. Whereas a slight

decrease in the particle size can be seen with increasing the ruthenium contents from 1-5 wt%.

This decrease in particle size with increase in ruthenium contents may suggest that high amount

of ruthenium inhibits the growth of TiO2 crystal [67, 88] (as shown in Figure 3.45 a-e). The

average particle size calculated for Ru-S co-doped samples lies in the range of 14 nm to 16 nm,

which is comparable to the particle size obtained from the XRD analysis.

133

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

g

f

edc b

a

(F (R

).hv)

0.5

Energy (eV)

Figure 3.44: Band gap plots of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

Table 3.16: Calculated structural parameters and band gap data of un-doped, S-doped and Ru-S

co-doped TiO2.

Catalyst Particle size (nm)

d-spacing (Å)

a (Å) c (Å) Band gap (eV)

TiO2 20.05 3.5113 3.7810 9.5410 3.17

ST 19.18 3.5179 3.7816 9.5511 2.89

1Ru-ST 17.11 3.5199 3.7817 9.5556 2.55

2Ru-ST 15.07 3.5240 3.7817 9.5612 2.41

3Ru-ST 14.67 3.5310 3.7818 9.5699 2.27

4Ru-ST 13.11 3.5428 3.7818 9.5710 2.19

5Ru-ST 12.06 3.5592 3.7818 9.5782 2.07

134

Figure 3.45: SEM images of (a) 1Ru-ST; (b) 2Ru-ST; (c) 3Ru-ST; (d) 4Ru-ST; (e) 5Ru-ST.

(a) (b)

(c) (d)

(e)

135


Figure 3.46 (a-d) illustrates the representative TEM, HRTEM and FFT images of Ru-S

co-doped TiO2 nanoparticles. TEM image shows long-range order and confirms particle are

agglomerated and distributed homogenously. High-resolution TEM as shown in Figure 3.46 (c &

d) of individual nanocrystals demonstrates the well-resolved crystalline planes with a spacing of

3.56 Ao, which is similar to that of the (101) plane in anatase [99, 103], confirmed by XRD. The

particle size distribution is rather homogeneous with an average size of 9-12 nm. Figure 3.46 (b)

is an image of Fast Fourier Transform (FFT) of the selected regular region from HRTEM.

Fourier Transform (FFT) demonstrates that HRTEM is taken from [101] plane of anatase TiO2.

Figure 3.46: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of

5Ru-ST.

136



The elemental composition of the all samples was determined by EDX analysis and is

tabulated in Table 3.17 and elemental compostion of S, Ru is found to be consistent with

calculated values.


The elemental composition profiles of the all samples are measured by Rutherford

Backscattering Spectroscopy (RBS) using He2+ ion beam energy of 3.6 MeV. A typical RBS

spectrum of a TiO2, S-TiO2 and Ru-S co-doped TiO2 is shown in Figure 3.47 while

stoichiometric composition of all elements i.e., S, O, Ti, Ru present in all samples is given in

Table 3.17. It is obvious from the depth profile data that dopants are well distributed in TiO2

lattice and determined composition is in accordance with that obtained from EDX data.

400 600 800 1000 1200 1400 1600

Nor

mal

ized

Yei

ld

Channel

(a)(b)(c)(d)(e)(f)

(g)

OS

Ti

Ru

Figure 3.47: RBS spectra of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

137

Table 3.17: Elemental analysis of the un-doped, S-doped and Ru-S co-doped TiO2.


Ti O S Ru Ti O S Ru

PT 59.94 40.06 - - 58.71 41.29 - -

ST 59.09 39.94 0.97 - 59.11 39.90 0.99 -

1Ru-ST 58.25 39.91 0.94 0.90 58.23 39.89 0.93 0.95

2Ru-ST 57.42 39.80 0.93 1.85 57.32 39.77 0.97 1.94

3Ru-ST 56.75 39.40 0.95 2.90 56.65 39.45 0.93 2.97

4Ru-ST 55.78 39.32 0.96 3.94 57.83 39.30 0.95 3.92

5Ru-ST 55.12 39.10 0.90 4.88 55.08 39.01 0.96 4.95


BET (brunauer-Emmett-Teller) and BJH (Baretter-Joyner-Halenda) methods are

employed to investigate surface areas and average pore volume. The effect of ruthenium and S

co-doping on the BET surface area and pore volume of TiO2 is analyzed and tabulated in Table

3.18. It can be seen that co-doped sample exhibits more surface area and pore volume as

compared to un-doped and S-doped TiO2 nanoparticles. However, the inclusion of ruthenium to

S-doped catalyst resulted in enhancement in surface area and pore volume and its value reached

to 117.98 m2/g and 1.29 (cc/g), respectively, for 5% Ru-S co-doped sample. This increase in

surface area may be due to decrease in particle size of 5% Ru-S co-doped sample as compared to

TiO2 and S-TiO2 [56].

Table 3.18: BET surface area and pore volume data of un-doped, S-doped and Ru-S co-doped TiO2.


TiO2 68.50 0.33

ST 78.65 0.53

1Ru-ST 87.36 0.78

2Ru-ST 96.23 0.89

3Ru-ST 104.36 1.07

4Ru-ST 111.11 1.18

5Ru-ST 117.98 1.29

138


The FTIR spectra of all prepared samples dried at 100 oC and calcined at 500 oC were

recorded in the mid IR region, i.e. 400-4000 cm-1 and are depicted in Figure 3.48 (a-g) and

Figure 3.49 (a-g), respectively. In Figure 3.48 (a-g), the characteristic vibration band in the

range of 500-760 cm-1 observed for all samples is related to the stretching vibrations of Ti-O

bond [89]. In addition all sample show the broad band in the region of 3080-3764 cm-1 and

narrow band at 1630 cm-1 which correspond to the presence of surface adsorbed water molecules

[99, 100]. The narrow band centered at 1030-1040 cm-1 is observed which may be due to Ti-O-C

bond formation [67, 89]. This contributes to the several kind of hydrocarbon adsorbed at TiO2

surface. The bands observed at 1330-1340 cm-1 and 1340-1425 cm-1 are related to the C-H

bending mode of vibration arising from organic contents and presence of organic group (acetate

etc.) bonded to TiO2, respectively [76, 78]. In addition the all absorption bands except that in the

range of 500-660 cm-1 diminished after calcination, showing the complete removal of organic

residue and physiosorbed water molecules after calcinations (Figure 3.49 (a-g)) [92].

3.8.8 Raman Studies

Figure 3.50 (a-g) presents the Raman spectra for pristine TiO2, S-doped and Ru-S co-

doped TiO2. The Raman spectra of all samples, shown in Figure 3.50 (a-g) are typical of the

anatase TiO2 phase which matches well with the XRD data. According to group theory,

tetragonal anatase structure exhibits six Raman active modes with two formula units per unit cell

[78, 109]. These Raman peaks appearing at 145, 196, 397, 516 and 640 cm-1 for all samples are

labeled as the Eg, Eg, B1g, A1g and Eg modes of anatase phase, respectively [56]. The intense

Raman band at 145 cm-1 indicates that TiO2 possess a certain degree of long-range order,

whereas the weak broader peak occurring in the high-frequency region depicts the presence of

short-range order in the anatase TiO2 [155, 167].

A dominant broadening in peak width and shift in all Raman bands of TiO2 is observed

upon doping with ruthenium ions which enhanced with increasing the doping amount. This may

be attributed to the surface pressure in the TiO2 nanocrystals induced by the dopants or

breakdown of long-range order in crystal symmetry caused by the incorporation of dopant [155,

156]. These variations in Raman bands are strongly dependant on the grain size of the TiO2

nanocrystals which decreased with doping.

139

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

W a ve n u m b e r (c m -1 )

(g )( f )(e )(d )(c )(b )(a )

3 7 6 4 .0 2 3 0 8 7 .5 8

7 4 9 .5 85 0 0

1 0 3 7 .1 0

1 3 3 7 .9 0

1 6 1 9 .9 0

1 4 2 3 .5 6

Figure 3.48: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

6 0

9 0

W a v e n u m b e r (c m -1 )

5 0 0(g )( f )(e )(d )(c )(b )(a )

6 6 0 .3 5

Figure 3.49: FTIR spectra of calcined (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f)

4Ru-ST; (g) 5Ru-ST.

140

100 200 300 400 500 600 700 800

Raman Shifts (cm-1)

Inte

nsity

Eg

Eg

B1g A1g

Eg

(a)

(b)(c)

(d)

(e)

(f)(g)

Figure 3.50: Raman spectra of (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

3.8.9 TGA Analysis

To determine the thermal behavior associated with the un-doped, S-doped and Ru-S co-

doped TiO2 sample, TGA analysis is carried out. A clear difference can be seen in the thermal

behaviors of TiO2, S-doped TiO2 and Ru-S co-doped TiO2 (Figure 3.51 (a-g)). The initial weight

loss observed at 120 oC in all samples represents the expulsion of unbound water or dissipation

of organic solvents from the TiO2 surface [109, 127]. A broad weight loss observed in the range

of 130- 400 oC, may probably be due to the thermal decomposition of residual organic species

present in the as prepared samples [113]. However, No thermal events observed after 400 oC in

TGA curve of the all materials, confirming the crystallization and phase transition of TiO2 from

amorphous to stable anatase form [156, 165]. The weight loss associated at different steps

(Figure 3.51 (a-g)) in different samples is given in Table 3.19 along with their char yield

determined at 600 oC which depicts the remaining weight of the sample. From the TGA profile

of all samples, it is observed the dramatic decrease in weight loss from 23.62 % to 15.66 %

occurred in the Ru-S co-doped sample with increasing the Ru ions from 1% to 5% (Figure 3.51

(c-g)).

141

100 200 300 400 500 600 70070

75

80

85

90

95

100

Wei

ght (

%)

Temperature (oC)

(a) (b) (c) (d) (e) (f) (g)

Figure 3.51: TGA profile of un-calcined (a) PT; (b) ST; (c) 1Ru-ST; (d) 2Ru-ST; (e) 3Ru-ST; (f) 4Ru-ST; (g) 5Ru-ST.

Table 3.19: Weight loss data of the un-doped, S-doped and Ru-S co-doped TiO2.

Sample Code % Weight loss % Char yield

At 120 ºC At 400 ºC At 600 ºC

TiO2 8.82 27.31 72.69

ST 8.31 24.21 75.79

1Ru-ST 6.81 23.62 77.35

2Ru -ST 6.25 19.74 80.26

3Ru -ST 5.02 18.55 81.45

4Ru -ST 4.39 16.69 83.31

5Ru -ST 4.35 15.66 84.34

142

This phenomenon can be explained on the basis of loss of more water contents and

organic moieties residing in the structure of TiO2 dried gel with lesser dopant content hence

allowing the more loss [156]. However by increasing the dopant ion resulted in the decrease in

the weight loss due to the fact that structural spaces are being occupied by the dopant ion in-spite

of organic impurities thus inducing the more stability.

The TGA curve also reveals that after 400 oC, organics are removed from the sample,

which is consistent with the FTIR and XRD data [134]. On the basis of these results, a

calcinations temperature above 400 oC (i.e. below the phase transition temperature) is chosen in

order to obtain the crystalline phase of anatase TiO2, as demonstrated from XRD data.

3.9 Applications of Prepared Ru-S co-doped Titanium Dioxide Nanostructure

The catalytic properties of 5% Ru-S co-doped TiO2 are evaluated in terms of

photocatalytic degradation of dyes and phenol and photocatalytic reduction of carbon dioxide

under visible and UV irradiation. The narrow band gap and huge surface area of 5% Ru-S co-

doped TiO2 makes it an efficient photocatalyst. Prior to photocatalytic studies, the adsorption

studies of dyes and phenol are carried out under dark conditions to evaluate the adsorption

efficiency of the 5% Ru-S co-doped TiO2.


The adsorption studies of all dyes and phenol are carried out using 5% Ru-S co-doped

TiO2 at different pH values i.e., pH ~4.5, ~7, and ~9.5 under dark for 6h. The percent adsorption

data for dyes and phenol is shown in Figure 3.52 (a-c) and given in Table 3.20. It is obvious from

the results that the changing the pH value, the degradation efficiency changed considerably. This

is because of different surface properties of TiO2 under different pH values which ultimately

affect the adsorption of dyes at TiO2 surface [67, 75]. As seen from Table 3.20, adsorption is not

significant in all cases and about 0.5-5 % adsorption is observed for all dyes and phenol at all pH

values under dark conditions.

143

Figure 3.52: Percent adsorption of dyes and phenol using Ru-S co-doped TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time; 6h, conc.; 20 ppm, catalyst; 50 mg.


Compound


pH ~ 4.5 pH ~7 pH ~9.5





Phenol 4.68 1.17 1.07

0 1 2 3 4 5 60

1

2

3

4


Perc

ent A

dsor

ptio

n

Time (h)0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

1.2


Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3


Perc

ent A

dsor

ptio

n

Time (h)

(a) (b)

(c)

144


The photocatalytic experiments are conducted to evaluate the photocatalytic degradation

of dye and phenol employing 5%Ru-S co-doped TiO2 under both UV (with cutoff filter λ<380

nm) and visible irradiation (with cutoff filter λ > 420 nm) using 500-W Xenon lamp (Ushio,

model UI-502Q). It was seen from previous results that 5% Ru-S co-doped TiO2 shown best

results as compared to other catalyst with low concentration of dopant due to its low band gap

and large surface area which enhances its light absorption capacity.

Various parameters which affect the degradation efficiency such as initial concentration,

catalyst loading, pH of solution, dopant content and light source (UV, visible) are also studied

[103, 108]. The degradation rate is computed by the change in dye and phenol concentration

employing UV–visible spectrometry as a function of irradiation time (discussed in section 3.3.3).


The effect of initial concentration of dyes on the percentage degradation is studied by

varying the initial concentration from 20 ppm to 100 ppm, using 50 mg of catalyst at pH ~7

under visible irradiation. The plots of percent degradation with reaction time are shown in Figure

3.53 (a-e) and are tabulated in the Table 3.21. As seen from the Figure 3.53 (a-e), the degradation

rate increased with increase in reaction time at all concentrations and become constant after

certain time due to saturation of the catalyst surface by the dye or phenol molecules [81, 83]. On

the other hand the degradation rate is found to be higher at low concentration of all dyes and

phenol i.e. 20 ppm and considerable low degradation is observed at higher concentration i.e. 100

ppm.

The presumed reason of this trend is that at a higher concentration, the generation of •OH

radicals on the surface of catalyst will be reduced due to the blocking of active sites by dye or

phenol molecules [125]. This may be due to limited amount of photons which reach at the

catalyst surface because of the screening of catalyst surface by greater number the reactant

molecules [127, 128]. Therefore, 20 ppm is preferred as optimum concentration for further

studies while dopant contents, pH values, and catalyst amount is varied to obtain the maximum

degradation rate and will be discussed further.

145

Figure 3.53: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using 5Ru-ST, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70P

erce

nt D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Perc

ent D

egra

datio

n

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60

Perc

ent D

egra

datio

n

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

Per

cent

Deg

rada

tion

Time (min)


(a) (b)

(c) (d)

(e)

146


Catalyst

Conc. (ppm)


Alizarin Red S

Procion blue MXR

Malachite green

Crystal violet Phenol

5Ru-ST

20 58.5 60.3 57.1 61.7 62.4 40 49.5 40.5 48.8 40.6 38.2 60 41.6 32.6 38.4 34.5 30.2 80 35.5 25.8 25.5 28.5 26.3 100 26.0 23.5 21.7 25.7 21.8


After optimizing the concentration of dyes and phenol, the catalyst dose is considered as

another main factor which has strong influence on the degradation process. To establish the

effect of catalyst loading, the amount of catalyst is varied from 10 to 70 mg at 20 ppm

concentration of solution and at pH ~7 under visible irradiation. The results of dependency of

percent degradation on the catalyst dose are depicted in Figure 3.54. It can be seen that the initial

reaction rate for the photodegradation process increased drastically by increasing the catalyst

loading from 10mg to 50 mg which may be due to increase in actives sites of catalyst surface,

available for carrying the reaction [77, 89]. Maximum degradation is observed with 50 mg of

catalyst while further increase of catalyst amount up to 70 mg, a decrease in the reaction rate of

photodegradation is observed. A possible reason of this behavior can be low absorption of light

by the catalyst surface due to decrease number active sites caused by the agglomeration of

catalyst [118]. On the basis of above findings 20 ppm of dyes and phenol solution and 50 mg of

the catalyst is selected for conducting further studies.


After optimizing the concentration and catalyst dose, the further studies are conducted at

different pH values using 50 mg of the catalyst under visible irradiation and the results are shown

in Figure 3.55. pH is considered a crucial parameter which influences the photodegradation

processes [155]. When pH is lower than ~7, the TiO2 surface is positively charged resulting in

degradation of anionic species due to more interaction between positive charged surface of

catalyst and negatively charged dye.

147

0 10 20 30 40 50 60 7010

20

30

40

50

60

70

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.54: Percent degradation of dyes and phenol as function of catalyst dose using 5Ru-ST under visible irradiations, time; 50 min, conc.; 20 ppm, pH ~7.

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

Figure 3.55: Percent degradation of dyes and phenol as function of pH using 5Ru-ST under


148

In basic solution the surface of TiO2 is negatively charged which gives more degradation

of cationic species because of interaction between opposite charges of catalyst and dye [121,

129]. As shown from the Figure 3.55 at pH ~9, 99.8% of malachite green and 99.76% of crystal

violet is removed whereas at pH ~4.5, 99.3% of Alizarin red S, 99.8% of procion blue MXR and

and 99.5% of phenol is degraded. However, the cationic species exhibited low degradation in

acidic media and anionic compounds showed considerably less degradation in basic media due to

repulsion of similar charges on the dyes and catalyst surface. On the basis of results of percent

degradation, pH ~4.5 is preferred for anionic species and pH ~9 is preferred for cationic species

for further experiments.


As mentioned previously, various parameters which affect the degradation efficiency

were assessed to ensure the maximum degradation efficiency. Furthermore, the effect of dopant

content (1%, 2%, 3%, 4%, 5% of Ru on Ru-S co-doped TiO2) on the photocatalytic degradation

anionic dyes and phenol and cationic dyes is investigated under pH ~4.5 and pH ~9, respectively,

at optimized value of concentration and catalyst amount. It is to note that the degradation

efficiency of dyes and phenol is comparatively very low with TiO2 (Figure 3.56, Table 3.22)

while the degradation activity enhanced with S-doped TiO2 because of its low band gap and

good surface area as compared to un-doped sample. The results shown in Figure 3.56 indicate

that significant increase in the photocatalytic degradation efficiency is observed with co-doped

samples and it reached maximum when the photocatalyst having 5% Ru content is used. These

results are consistent to the low band gap and high surface area of 5% Ru-S co-doped TiO2

nanoparticles as compared to un-doped , doped and co-doped sample with low Ru concentration.


Catalyst


red S Procion blue

MXR Malachite

green Crystal violet Phenol

PT 13.5 17.2 12.2 11.5 14.0 ST 41.2 39.9 41.5 40.2 42.1 1Ru-ST 45.1 49.5 43.0 47.8 48.9 2Ru-ST 45.6 55.6 55.2 58.9 59.5 3Ru-ST 57.5 66.5 63.7 64.4 67.4 4Ru-ST 69.9 72.5 70.5 71.8 73.7 5Ru-ST 99.3 99.8 99.8 99.7 99.5

149

PT ST 1Ru-ST 2Ru-ST 3Ru-ST 4Ru-ST 5Ru-ST10

20

30

40

50

60

70

80

90

100

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR



The results of comparative study of photocatalytic degradation of dyes and phenol under

UV (with cutoff filter λ<380 nm) and visible irradiation (with cutoff filter λ > 420 nm) are

carried out for 50 min and 6h, respectively, are shown in Figure. 3.57. After 50 min, removal

efficiency obtained is ~99% under visible irradiations for all compounds while 12-16%

efficiency is achieved under UV irradiation. The higher efficiency of photocatalytic process

under visible radiation is due to its band gap which lies in visible range.


To evaluate reused photocatalyst efficiency, experiments are performed by using 0.5 g/L

of photocatalyst under visible irradiations at optimized conditions. As seen in Figure 3.58,

although photocatalytic efficiency remained almost same after three cycles and deteriorated on

repeated use in 4th and 5th cycle but it still remained sufficiently high in terms of degradation.

The reduction in efficiency can be explained on the basis of the deposition of photo-insensitive

hydroxides on the catalyst surface which blocks its active sites [119, 120].

150

UV (6h) Visible(50min)0

20

40

60

80

100

Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH

Figure 3.57: Comparison of percent degradation of dyes and phenol under UV and Visible irradiation with 5% Ru-S co-doped TiO2.

1 2 3 4 5 60

20

40

60

80


Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH

Figure 3.58: Plots of recyclability of dyes and phenol using 5 % Ru-S co-doped TiO2 under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

151

The regeneration of the catalyst is done simply separating the catalyst, drying at 110 ○C

for 24h and calcining the catalyst at 500 oC which recovers the active sites by destruction of

organics deposited on the surface. The refreshed catalyst is then the applied for the degradation

of dyes and phenol for the sixth cycle as shown in Figure 3.58.

It can be seen from the results that the activity of catalyst is resumed after calcinations

and comparable degradation efficiency to that of first cycle is observed. Therefore, these results

confirm the effective use of catalyst for the several times for photocatalytic degradation process.

Therefore, the reuse ability of the catalyst may form the basis of important techniques for

environmental treatment. The photocatalytic process can also be applied as a new methodology

for reduction of other organic chemicals in aqueous solution.

3.9.3 Photocatalytic Reduction of Carbon Dioxide using 5Ru-ST

The photocatalytic reduction of CO2 with H2O is investigated with 5% Ru-S co-doped

TiO2 nanoparticles under UV (with cutoff filter λ<380 nm) and visible irradiation (with cutoff

filter λ > 420 nm) and process led to the formation of methane (CH4), detected by GC and

confirmed by mass spectrum. The time profile of production of CH4 under UV and visible

irradiation is demonstrated in Figure 3.59.

0 2 4 6 8 10 120

2

4

6

8

10

12

14

16

18

20

CH

4 Pr

oduc

tion

(µm

ol/h

-cat

al.)

Time (h)


Figure 3.59: Methane production by photoreduction of CO2 as function of time using 5% Ru-S

co-doped TiO2.

152

CO2 photoreduction experiments being carried out in UV irradiation showed no

production of CH4 while significant CH4 is formed under visible irradiation, providing that CO2

reduction only occurs in the visible region due to visible absorption property of the catalyst [103,

105]. It is indicated from Figure 3.59 that methane yield increased steadily up to 12h and CH4

production of approximately 17.82 µmol is achieved. The presence of S and Ru imparts an

important role in the reduction of CO2 into methane under visible irradiation. The contact

between sulfur to TiO2 lowers the band gap of TiO2 and substitution of ruthenium not only alters

the band gap in the visible region but minimizes the recombination probability of electron-hole

pairs by trapping the photo excited electron and increasing the photoreduction of CO2 into

methane. These results prove that modified S-TiO2 with Ru can improve the efficiency of

producing CH4.

3.10.3 Conclusion

• Ru-S co-doped TiO2 nanoparticles with varying Ru ratio (1-5 wt%) have been

fabricated successfully by an effective and facile sol-gel method. The implication of

present synthesis method relies in its simplicity, high yield with high porosity, surface

area and low band gap energy.

• The structural analysis of the materials revealed that S-doped and all Ru-S co-doped

TiO2 materials prepared herein exhibited absorption spectra, extended into visible

beyond 400 nm which may be due to the synergetic affect or Ru and S.

• Among all nanoparticles, Ru-S co-doped TiO2 with 5% Ru contents exhibited low

band gap energy, high pore volume, and good structural properties.

• The photocatalytic performance of un-doped, doped and co-doped was demonstrated

for photocataytic degradation of dyes and phenol and photoreduction of carbon dioxide

under visible irradiation with different interval of time.

• Diverse operational factors affected the effectiveness or photo catalytic activities of

Ru-S co-doped TiO2 photocatalysts, particularly catalyst dose, concentration of

solution, pH and type of illumination source.

• Some dyes are completely removed at higher pH, while others show excellent

degradation at lower pH.

153

• Though the visible irradiation, better efficiency in the degradation of compounds is

attained and catalyst recyclability manifest that Ru-S co-doped catalyst may be a

viable for treatment of waste water several times.

• It can be concluded from the findings that Ru-S co-doped catalyst depict profound

potential for photocatalytic degradation of environmental hazardous substances from

contaminated water. Moreover, due to its tremendous photocatalytic characteristic it

can be proficiently exploited for water treatment under sun irradiations which could

ensure more cost-effective solution for waste water management.

• The photocatalytic applications of the prepared Ru-S co-doped TiO2 catalysts were

also evaluated by photocatalytic reduction of carbon dioxide under visible and UV

irradiation. The catalyst demonstrated the photocatalytic conversion of CO2 to

methane in the presence of water using only visible light excitation.

• The CO2 reduction results strongly suggest the recycling of CO2 into a fuel using

sunlight, further attempt is mandatory for increasing sunlight-to-fuel photoconversion

efficiencies.

154

3.11 Characterization and Photocatalytic Applications Fe-S co-doped TiO2 Nanostructures

3.11.1 XRD Analysis

The XRD analysis is performed to determine the effect of S-doping and Fe-S co-doping

on the crystallinity and structure of TiO2. XRD patterns of plane TiO2 and S- doped TiO2 and Fe-

S co-doped TiO2 (with varying Fe concentration from 1 wt % to 6 wt %) is shown in Figure 3.60

(a-h). XRD spectra of all samples corresponds to the anatase TiO2 [JCPDS standard files #21-

1272]. The absence of any extra peaks in the XRD pattern of all co-doped samples (Figure 3.60

(a-g)) suggest that all these samples possess single phase whereas Fe-S co-doped TiO2

containing 6 wt% Fe exhibits mixed anatase-rutile phase. The lattice parameters, d-spacing

values, crystallite size are also calculated and tabulated in Table 3.23. Increase in Fe

concentration from 1 wt% to 5 wt% in co-doped samples resulted in lowering of peak intensity

and broadening of peak (Figure 3.60 (a-g)) with subsequent decrease in particle size. This

reduction in crystallite size with increase in dopant concentration was also noticed in our

previous results. According to the lattice parameter calculations, a significant change in the d-

spacing value and c-axis is also found with increase in Fe concentration.

10 20 30 40 50 60 70 80

R (h)21

5

220

116204

211

105202

11200

410

3

101

(g)

(f)

(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

RRR

Figure 3.60: XRD patterns of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST (e) 3Fe-ST; (f) 4Fe-ST; (g)

5Fe-ST; (h) 6Fe-ST calcined at 500 0C.

155

This fact may be attributed to the deformation of the unit cell along c-axis which

manifests the possibility of incorporation of Fe ions along S ion either at substitutional or

interstitial sites [82, 82]. The deformation is relatively high in sample containing 6 wt% Fe

contents which resulted in phase transformation of TiO2 into rutile phase ((Figure 3.60 (h)),

therefore further studies are carried out with samples having Fe contents below 6 wt%.


The band gap energies of un-doped and co-doped TiO2 nanophotocatalyst are calculated

from their diffuse-reflectance spectra by Tauc Plot which is obtained by plotting (F(R)*hv)n

versus energy in electron volts, where n=0.5 for direct band gap [71, 75]. The linear part of the

curve was extrapolated to energy axis to obtain the direct energy band gap as shown in Figure

3.61 (a-g) and given in Table 3.23. The comparative band gap energies depict that doping of

anion i.e., sulfur (1 wt%) into titanium dioxide tuned the optical band gap in visible range in

comparison TiO2, thus narrowing the band gap from 3.17 to 2.89 eV. The shifting of S-doped

TiO2 can be attributed to the mixing of the S3p and O2p states with the near the valence band of

TiO2 which increases the width of VB thus leading to the narrowing of band gap [55]. The band

gap is reduced from 2.55 eV to 1.97 eV further with doping of Fe from 1 wt % to 5 wt% in co-

doped TiO2. It is obvious from the results that increase in iron contents induced a larger shift

toward the lower energy thus changing the crystalline and electronic structures [161, 162]. This

phenomenon suggests that the Fe3+ may be incorporated into the lattice of TiO2 [57, 68]. The

synergetic effect of iron and sulfur shifted the band edge of TiO2 significantly towards the

visible-light region.



SEM is carried out to determine the information regarding structure and morphology of

the Fe, S co-doped TiO2 nanoparticles, respectively (Figure 3.62 (a-e)). The SEM images of co-

doped samples reveals that particles are crystalline, monodisperse and roughly having random

shape. These co-doped samples appear as aggregates composed of 10-15 nm in size. The

presence of iron (III) ions in the S-doped TiO2 seems to have pronounced effect on their

crystallization and the particle size. From the micrographs it is evident that the growth of particle

is suppressed by Fe, S doping to as depicted in Figure 3.62 (a-e).

156

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

g

f

e

d

cb

a

(F (R

).hv)

0.5

Energy (eV)

Figure 3.61: Band gap plots of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

Table 3.23: Calculated structural parameters and band gap data of un-doped, S-doped and Fe-S co-doped TiO2.


(nm) d-spacing


(eV)

TiO2 20.05 3.5113 3.7810 9.5410 3.17

ST 19.18 3.5179 3.7816 9.5511 2.89

1Fe-ST 17.88 3.5184 3.7811 9.5597 2.55

2Fe-ST 15.97 3.5285 3.7814 9.5605 2.41

3Fe-ST 14.55 3.5373 3.7808 9.5655 2.25

4Fe-ST 12.34 3.5494 3.7812 9.5720 2.10

5Fe-ST 10.55 3.5460 3.7816 9.5790 1.97

157

Figure 3.62: SEM images of (a) 1Fe-ST; (b) 2Fe-ST; (c) 3Fe-ST; (d) 4Fe-ST; (e) 5Fe-ST.

0.1 µm 09 08 SE I20 kV X 60,000

(e)

0.1 µm 11 16 SE I20 kV X 60,000

(d)

0.1 µm 09 08 SE I20 kV X 60,000

(c)

0.1 µm 10 11 SE I20 kV X 60,000

(b)

0.1 µm 12 10 SE I20 kV X 60,000

(a)

158


The microstructure of Fe-S co-doped TiO2 nanoparticles is studied by TEM and HRTEM.

Figure 3.63 (a) shows the TEM image, (c, d) shows HRTEM, (b) illustrates FFT pattern of the

Fe-S co-doped TiO2. It can be observed from Figure 3.63 (a, c) that the nanocrystallite appear as

agglomerates, randomly oriented and possessing a different morphology i.e., semi-spherical,

heaxagonal. Figure 3.63 (d) shows the corresponding HRTEM image of the prepared sample

depicting high crystallinity, well-defined boundaries and the clear-resolved lattice fringes, which

is used for the identification of crystallographic spacing. The lattice fringe spacing of ca. 3.55 Ao

corresponds to that of the (101) crystallographic plane of TiO2 anatase [78, 98], consistent with

XRD data. The average particle size of the particles estimated from the HRTEM image is nearly

uniform and is about 8-10 nm, which is in good agreement with the value calculated from XRD

data. The corresponding Fast Fourier Transform (FFT) pattern (Figure 3.63 (b)) shows an

ordered array of spots, consisting of seven concentric rings and confirm the single crystallinity of

the nanoparticles.



EDX spectroscopy which is specific quantitative technique is used to determine the

distribution of sulfur and iron in the titania matrix. A representative EDX composition of all

samples is presented in Table 3.24, indicating that experimental composition of Fe, S, Ti and O

is in good agreement with theoretical calculation.


Figure 3.64 (a-g) shows the combined RBS experimental spectra of doped, un-doped and

Fe-S co-doped TiO2 nanoparticles. The composition profile analysis of all materials is

summarized in Table 3.24. RBS depth profile data of all samples clearly depicts the presence of

homogenous single layer which strongly suggests that dopants are practically well and

homogenously distributed in framework of titania particle [115, 116]. Moreover, the elemental

compositions determined by RBS are consistent to those determined by EDX and summarized in

Table 3.24.

159

Figure 3.63: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of

5Fe-ST.

160

400 600 800 1000 1200 1400 1600

Nor

mal

ized

Yei

ld

Channel

(a)(b)(c)(d)(e)(f)(g)

OS

Ti

Fe

Figure 3.64: RBS spectra of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g)

5Fe-ST.

Table 3.24: Elemental analysis of the un-doped, S-doped and Fe-S co-doped TiO2.


Ti O S Fe Ti O S Fe

PT 59.94 40.06 - - 58.71 41.29 - -

ST 59.09 39.94 0.97 - 59.11 39.90 0.99 -

1Fe-ST 58.30 39.85 0.95 0.90 58.36 39.82 0.93 0.89

2Fe-ST 57.47 39.70 0.96 1.87 57.46 39.64 0.95 1.95

3Fe-ST 56.67 39.55 0.93 2.85 56.71 39.47 0.90 2.92

4Fe-ST 55.86 39.30 0.94 3.90 55.84 39.35 0.92 3.89

5Fe-ST 55.05 39.18 0.89 4.88 54.99 39.12 0.96 4.93

161


The surface area and pore volume of the TiO2, S doped TiO2 and Fe-S co-doped samples

(with different amount of Fe varying from 1 wt% to 5 wt%) are determined by using the BET

(brunauer-Emmett-Teller) method and BJH (Baretter-Joyner-Halenda), respectively and are

summarized in Table 3.25. As seen from results, the surface area value of TiO2 enhanced with S

addition and remarkable increase in surface area values are observed from 88.56-123.10 m2/g

with increase in the Fe amount from 1 wt % to 5 wt%.

The increase in surface area with increase Fe doping is agreeable with reduction in the

particles size, as seen in XRD and TEM. On the other hand, Fe-S co-doped samples exhibited

high pore volume i.e., from 0.99 (cc/g) to 1.35 (cc/g) with increasing the Fe contents. As it can

be seen from Table 3.25, that 5% Fe-S co-doped sample possesses high surface area hence high

porosity and these results are consistent with XRD analysis and photocatalytic activity.

Table 3.25: BET surface area and pore volume data of un-doped, S-doped and Fe-S co-doped TiO2.


TiO2 68.50 0.33

ST 78.65 0.53

1Fe-ST 88.56 0.99

2Fe-ST 96.35 1.09

3Fe-ST 105.99 1.18

4Fe-ST 114.66 1.25

5Fe-ST 123.10 1.35

162


Figure 3.65 (a-g) and Figure 3.66 (a-g) display the FTIR spectra of as-prepared TiO2 and

TiO2 heat-treated at 500 ○C, respectively. In the typical FTIR spectra of un-calcined sample as

shown in Figure 3.65 (a-g), there are some main characteristic peaks which can be attributed to

following modes of vibration: Peaks located at around 1610.1 cm−1-1670 cm−1 are ascribed to the

deformation vibration of H-O-H bonds of the water adsorbed in TiO2 framework while the broad

peaks appearing in the range of 3080 cm−1- 3760 cm−1 are assigned to surface Ti-OH bonds [93,

94]. These results strongly suggest the existence of hydroxyl ions in the structure of the co-doped

and un-doped samples. The band below 1000 cm-1, appearing at 500-576 cm-1 is due to

stretching vibrations of M-O (Ti-O & Fe-O) moiety [81, 83]. The peaks at 1020 cm-1-1060 cm-1

arise from the Ti-O-C linkage which may result from the interaction between the Ti–O network

and the carbon from the residual organic precursors, deposited on the surface [88]. The

significant peaks at about 1360-1450 cm−1 can be assigned to the bending C-H vibration from the

remaining organic-moieties in the crystal structure [145, 148]. Interestingly, as samples are

treated at 500 oC (Figure 3.66 (a-g)), the characteristic vibration peaks of O―H, C-H, Ti-O-C

disappeared due to complete decomposition of organic residue and removal of adsorbed water

from the TiO2 framework [97]. Whereas all sample exhibit strong absorption bands of stretching

vibration of Ti―O―Ti bonds in the range of 500 cm-1-600 cm-1 after calcinations.

3.11.7 Raman Analysis of the Samples

Raman spectroscopy is applied to study the doping effect and to determine the surface

crystal and morphology of the TiO2 nanoparticles. Figure 3.67 (a-g) shows the Raman spectra of

the un-doped and S–doped TiO2 and Fe-S co-doped TiO2 nanoparticles with Fe concentration

varying from 1-5 wt%. According to the symmetry group analysis, all samples exhibit six Raman

active modes (A1g+2B1g+3Eg), which appear at 144.3 (Eg), 196 (Eg), 397 (B1g), 514.33 (A1g), and

640 (Eg) cm-1. The intensities and position of five Raman active modes confirm the anatase phase

of TiO2 which matches well with the literature [120, 121]. It is depicted from Raman spectra that

the shifting towards higher wave number and peak broadening is observed in Fe-S co-doped

TiO2 sample. The first Eg mode is influenced more exhibiting the red shifts in frequency and

broadening of the bands with increasing the iron amount.

163

4000 3500 3000 2500 2000 1500 1000 50030

40

50

60

70

80

90

100

110

W avenum ber (cm -1 )

(g )(f)(e )(d )(c)

(b )(a )

3764 .02 3081 .75

756 .96500

1036.48

1339 .02

1624 .68

1422 .63

Figure 3.65: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0


(g )

( f )(e )(d )(c )(b )(a )

6 6 2 .3 0

5 0 0

Figure 3.66: FTIR spectra of calcined (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

164

200 300 400 500 600 700 800Raman Shifts (cm-1)

Inte

nsity

Eg

Eg

B1gA1g

Eg

(a)(b)

(c)

(d)

(e)

(f)(g)

Figure 3.67: Raman spectra of (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

This behavior can be ascribed to various factors including increased disorder in the TiO2

lattice due inclusion of iron ion into the titania framework or due to decrease in crystallite size

caused by the presence of iron impurities at the grain boundaries. Moreover, these findings also

confirm that TiO2 structure is retained after doping with transition metal and sulfur and are in

good agreement with XRD studies.

3.11.8 TGA Analysis

Figure 3.68 (a-g) and Table 3.26 demonstrates the TGA curve and weight loss data

associated at each step, respectively, for TiO2, S-doped TiO2, Fe-S co-doped TiO2. The thermal

analysis is carried out to evaluate the effect of dopant on the thermal behavior of Fe-S co-doped

samples. The thermograms of all samples (Figure 3.68 (a-g)) exhibit a sharp decrease in weight

loss between at 25-110 oC which may be due to elimination of physically adsorbed water or

evaporation of embedded organic solvent, retained in the TiO2 structure [151, 161].

165

100 200 300 400 500 600 70070

75

80

85

90

95

100

(a) (b) (c) (d) (e) (f) (g)

Wei

ght (

%)

Temperature (oC)

Figure 3.68: TGA profile of un-calcined (a) PT; (b) ST; (c) 1Fe-ST; (d) 2Fe-ST; (e) 3Fe-ST; (f) 4Fe-ST; (g) 5Fe-ST.

Table 3.26: Weight loss data of the un-doped, S-doped and Fe-S co-doped TiO2.


At 120 ºC At 400 ºC At 600 ºC TiO2 8.82 27.31 72.69

ST 8.31 24.21 75.79

1Fe-ST 6.82 20.74 79.26

2Fe-ST 5.54 16.19 83.71

3Fe-ST 3.79 15.39 84.61

4Fe-ST 2.47 10.66 89.34

5Fe-ST 2.43 9.82 90.18

166

The second step, extending up to 400 oC, is related to dehydration, burning of un-

hydrolyzed TTIP or decomposition of organic fractions present in doped samples [170]. Above

400 oC, no thermal event associated with the weight is observed in all samples, representing the

thermal stability of material. This also signifies the crystallization of the material from

amorphous to anatase form, as confirmed from XRD results also. Moreover, it may also attribute

that all materials are free of organic contents above 400 oC, as illustrated from FTIR data

(section 3.11.7). A comparative study of TGA profile shows the decrease in weight loss from

20.74 to 9.82% with corresponding increase in residual or char yield (79.26 to 90.18%) with

increasing Fe contents in Fe-S co-doped samples (Table 3.26). This behavior corresponds to the

more structural stability instigated in TiO2 framework at higher dopant level, allowing the more

structural spaces being occupied by dopant as compared to organic fractions [163].

3.12 Applications of Prepared Fe-S co-doped Titanium Dioxide Nanostructure

The photocataytic activity of 5% Fe-S co-doped TiO2 is determined in terms of photo

degradation of dyes and phenol and photocatalytic reduction of carbon dioxide under both visible

and UV irradiation. The adsorption studies of dyes and phenol are carried out under dark

conditions prior to photocatalytic activity to evaluate the adsorption efficiency of the catalyst.


The adsorption studies of crystal violet, phenol, procion blue MXR, alizarin red S,

malachite green on 5% Fe-S co-doped TiO2 are carried out at three different pH i.e. pH ~4.5, ~7,

and ~9.5 under dark for 6h. The percent adsorption of the dyes and phenol with different

irradiation time is shown in Figure 3.69 (a-c) and percent degradation data is given in Table 3.27.

It is obvious from the results that the changing the pH value, the degradation efficiency is alerted

significantly. This is because of different behavior of TiO2 surface under different pH values

hence affecting the adsorption capacity [89, 121]. However, overall negligible adsorption i.e. 0-

6 % is observed for all dyes and phenol at all pH values under dark conditions.

.

167

Figure 3.69: Percent adsorption of dyes and phenol using Fe-S co-doped TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time; 6h, conc.; 20 ppm, catalyst; 50 mg.


Compound


pH ~ 4.5 pH ~7 pH ~9.5

Alizarin red S 4.83 1.21 0.24 Procion blue MXR 4.68 1.58 0.10 Malachite green 0.44 1.44 4.76 Crystal violet 0.32 1.30 4.84 Phenol 5.28 1.08 0.69

0 1 2 3 4 5 60

1

2

3

4


Perc

ent A

dsor

ptio

n

Time (h)0 1 2 3 4 5 6

0.0

0.4

0.8

1.2

1.6

2.0

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3

4


Per

cent

Ads

orpt

ion

Time (h)

(a) (b)

(c)

168


As discussed previously, control experiments are conducted to evaluate the degradation

of dye and phenol without photocatalysis. And no considerable change in the concentration of

dye and phenol was observed either in light (UV and visible) without catalyst and in presence of

catalyst under dark conditions. On the basis of these findings, the effect of the degradation

process of dyes and phenol is carried out in the combination of catalyst under light to confirm

that the degradation process only occurs through photocatalysis. The effect of concentration,

catalyst dose, pH value, dopant content and different irradiation source (UV and visible) is also

studied to obtain the maximum degradation rate under optimized conditions. The degradation

rate is computed by the change in concentration of solution employing UV–visible spectrometry

as a function of irradiation time [110, 118].

3.12.2.1Effect of Initial Concentration of Dyes and Phenol

The effect of initial concentration on photocatalytic degradation of dyes and phenol is

studied by using constant amount of catalyst (50 mg), pH ~7 and varying the concentration of

solution from 20 ppm to100 ppm. The initial concentration effect is evaluated using 5% Fe-S co-

doped TiO2 under visible irradiation as shown in Figure 3.70 (a-e) shows and Table 3.28. From

the Figure 3.70 (a-e), it can be seen that at higher concentration, the degradation is found to be

less and maximum degradation of all dyes and phenol is obtained at 20 ppm. However, with

reaction time the rate of reaction increased and observed to be constant after certain time. The

effect of decrease in degradation rate with increase in concentration can be rationalized on the

basis that increase concentration results more reactants to be adsorb on catalyst surface, making

the surface to be unavailable for photons to reach the catalyst surface [78, 79]. This results in the

reduction of concentration of radical ions generated by the catalyst and thus resulting in decrease

in the efficiency of the catalytic reaction [151]. The maximum degradation obtained by 5% Fe-S

co-doped TiO2 after 50 min is 59% of alizarin red S, 64.18% of procion blue MXR, 64.8% of

malachite green, 61.8% of crystal violet and 66.47% of phenol. Therefore, further studies are

carried out under different experimental conditions i.e. at different catalyst dose, different pH to

determine the optimum conditions for maximum degradation rate.

169

Figure 3.70: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under Vis. irradiation using 5Fe-ST, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Perc

ent D

egra

datio

n

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70

80

Per

cent

Deg

rada

tion

Time (min)


(b)(a)

(c) (d)

(e)

170


Catalyst

Conc. (ppm)


Alizarin red S

Procion blue MXR

Malachite green

Crystal violet

Phenol

5Fe-ST

20 59.48 64.18 64.8 61.86 66.47 40 40.66 48.18 49.35 42.73 53.00 60 32.72 38.22 40.50 31.65 43.55 80 25.21 25.10 28.02 23.64 32.69 100 18.71 21.10 21.51 12.15 26.67


The effects of catalyst loading on photocatalytic activity of dyes and phenol in waste

waters have been studied previously, showing the dependency of the catalytic reaction on the

degradation rate. The degradation of dyes and phenol (initial concentration; 20 ppm) using 5%

Fe-S co-doped TiO2 is studied using different catalyst dose (20 to 70 mg) at pH ~7 under visible

irradiation. Figure 3.71 demonstrates the effect of different dose of catalyst on the percent

degradation of dye and phenol solution at natural pH. It can be seen from the trend that initially

the degradation activity increased greatly with increase in catalyst amount from 20 to 50 mg

thereafter the rate of degradation decreased with further increase in the catalyst dose. This

dependency of catalytic reaction on catalyst dose can be explained on the basis that optimum

catalyst loading is found to be effective on degradation rate because increasing the catalyst

dosage the total active surface area increases. Hence, the availability of active sites available for

degradation of dye and phenol on catalyst surface also increases [162, 163]. At the same time,

high dose of photocatalyst (above 50 mg) results in agglomeration leading to the decrease in

penetration of light and photo activated volume of suspension thus reducing the activity [101].

Thus optimum concentration of the catalyst for efficient degradation of dye is found to be 50 mg.


The efficiency of the photocatalytic processes strongly depends on the pH of the dye

solution. The degradation of cationic species (malachite green and crystal violet) and anionic

species (procion blue MXR, alizarin red S and phenol) is carried out at various pH (~ 4.5, 7, 9)

using 50 mg of the catalyst and 20 ppm of aqueous solution under visible irradiation. The

degradation of the dyes and phenol with different pH is shown in Figure 3.72.

171

0 10 20 30 40 50 60 7010

20

30

40

50

60

70

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.71: Percent degradation of dyes and phenol as function of catalyst dose using 5Fe-ST under visible irradiations, time; 50 min, conc.; 20 ppm.

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

Figure 3.72: Percent degradation of dyes and phenol as function of pH using 5Fe-ST under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

172

The surface charge property of TiO2 changes with change in pH of the solution thus

changing the photocatalytic activity [56, 56]. It is obvious from these results that increase in pH

value above ~7 resulted in increase degradation of the malachite green and crystal violet and

complete degradation (~ 100%) is attained at pH ~9.5. Whereas procion blue MXR, alizarin red

S and phenol exhibited less degradation at pH ~9.5 because the negative charges on TiO2 are

expected to repel the anionic dyes. Low pH value i.e. below ~7, resulted in enhanced reduction

of anionic species showing complete degradation of procion blue MXR and alizarin red S and

phenol. Similarly, a decrease in the efficiency of photodegradation procion blue MXR and

alizarin red S and phenol with increasing pH is expected. This may be attributed to the repulsion

of same charges on catalyst and the dye resulting in less adsorption and degradation [78, 87]. On

the basis of these results further experiments are carried out in respective pH of each dye.


The dopant concentration is very important parameter which influences the phocatalytic

degradation process, because changing the dopant content, the structural properties such as band

gap and surface area changes which impart crucial role on the catalytic activity. The effect of Fe

contents on the degradation kinetics is carried out under visible light under optimized conditions

of concentration, catalyst doses and pH. The photodegradation of anionic dyes and cationic dyes

is carried in aqueous solution of pH ~4.5 and ~9, respectively, and shown in Figure 3.73 and

demonstrated in Tables 3.29. As seen from the data, the photocatalytic activity boosted with

increasing the Fe contents in Fe-S co-doped samples from 1wt% to 5wt% and maximum

degradation efficiency is achieved with 5% Fe-S co-doped TiO2 as compared to TiO2, S doped

TiO2 and other Fe-S co-doped samples having lower dopant concentration. This can be attributed

to higher surface area and lower band gap of 5% Fe-S co-doped TiO2 as compared to other

catalysts which enables it efficient visible light photocatalyst.


The comparison of percentage degradation of dyes phenol under UV and visible light is

shown in Figure 3.74. It is observed that complete decomposition of samples is achieved after 50

min under visible irradiation while only little efficiency is obtained UV irradiation after 6h.

173

PT ST 1Fe-ST 2Fe-ST 3Fe-ST 4Fe-ST 5Fe-ST10

20

30

40

50

60

70

80

90

100

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR



Catalyst


Alizarin

red S

Procion blue

MXR

Malachite

green

Crystal

violet Phenol

PT 13.5 17.2 12.2 11.5 14.8

ST 41.2 39.9 41.5 40.5 42.4

1Fe-ST 45.0 45.8 48.5 46.4 48.8

2Fe-ST 55.5 58.7 56.5 59.8 61.5

3Fe-ST 61.1 68.9 65.8 67.4 67.9

4Fe-ST 69.4 75.1 74.8 76.8 76.4

5Fe-ST 99.7 100 99.6 100 99.5

174


20

40

60

80

100

Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH


irradiation with 5% Fe-S co-doped TiO2.

Thus the faster degradation rate under visible light as compared to UV light is an

agreement with previous results and directly can be correlated to the visible band gap of the

catalyst. Thus it can be concluded from above observations that natural solar radiation could be

used as an alternative of the artificial visible light which can efficiently remove the textile dyes

and other organic pollutants such as phenol from waste water.


The purpose of reusing the photocatalyst is to determine the cost effectiveness of the

method. Therefore; 5% Fe-S co-doped TiO2 catalyst is selected and recycled for consecutive

reuse on the dyes and phenol degradation. The reusability process is repeated up to six times

using 0.5 g·L−1 (or 50 mg/100 ml) of the catalyst under visible irradiation at optimized values of

pH and concentration. The photodegradation efficiency is assessed and compared between the

reused cycles, as demonstrated in Figure 3.75. The first, second and third cycle, comparable

degradation efficiency after 50 min of irradiation is seen. For the fourth and fifth cycle

degradation efficiency reduced to some extent. But the degradation rate is still significant after

five times of TiO2 reuse.

175

1 2 3 4 5 60

20

40

60

80


Per

cent

Deg

rada

tion

Cycle (n)

ARS MG CV PB-MXR PH

Figure 3.75: Plots of recyclability of dyes and phenol using 5 % Fe-S co-doped TiO2 under


The reduction in degradation efficiency can be regarded to accumulation of the dye

molecules on the surface of TiO2 particles which can be removed by calcining the catalyst after

recovery [168]. Therefore for carrying the reuse ability of the catalyst for the 6th cycle, the

catalyst is calcined at 500 oC which may probably remove the deposited impurities and then it is

subjected for degradation process. Figure 3.75 demonstrates that photocatalystic activity

enhanced after 6th cycle and that the removal ratio kept above ~99% after 6th cycle. These reuse

ability experiments revealed that catalyst reuse is effective and it exhibited good stability after

recovery cycles. This capability of the photocatalyst to be reused goes towards green chemistry

key principles.

3.12.3 Photocatalytic Reduction of Carbon Dioxide using 5Fe-ST

The photocatalytic reduction of CO2 is carried out in aqueous media under optimized

conditions (discussed in previous section) using 5% Fe-S co-doped TiO2 under both UV (with

cutoff filter λ<380 nm) and visible irradiation (with cutoff filter λ > 420 nm). As demonstrated

earlier, the combinational effect of Fe and S not only enhances the visible light absorption

176

property but also significantly enhances the surface area. Since maximum activity obtained for

degradation of dye was with 5% Fe-S co-doped TiO2 as compared to TiO2, S-TiO2 and Fe-S co-

doped catalysts with lower dopant level, therefore CO2 reduction is being carried out using this

catalyst due to aforementioned properties. The effect of irradiation time on the photocatalytic

reduction and the formation of the product is evaluated over a period of 0-12 h and shown in

Figure 3.76. It is evident from the GC data that only ethanol is observed and no other products

were be detected. As seen from the Figure 3.76 negligible yield of product is seen under UV

region for period of 12h and significant ethanol production is determined under visible

illumination because of its visible light absorption property. Initially, the product yield under

visible light is considerably low and a substantial increase is observed after 1 hr of illumination

and production of ethanol increased linearly with time, reaching a maximum of 14.25 µmol after

12h of irradiation time. Generally, the photocatalytic activity of Fe-S co-doped sample can be

attributed to the two factors (1) band gap and (2) surface area [66, 81, 89]. The highest surface

area of Fe-S co-doped as compared to other doped samples provides more active sites for the

reaction to takes place while narrow band gap makes it be active under visible light.

0 2 4 6 8 10 120

2

4

6

8

10

12

14

16


Time (h)

Eth

anol

Pro

duct

ion

(µm

ol/h

-cat

al.)

Figure 3.76: Ethanol production by photoreduction of CO2 as function of time using 5% Fe-S co-doped TiO2.

177

3.13 Conclusions

• Bare and doped TiO2 nanoparticles were prepared by sol-gel method which facilitates the

synthesis of nanometer sized, crystalline TiO2 nanoparticles with level of high purity at

relatively low temperature.

• The characterization analysis explored that doping of Fe and S induced structural changes

resulting in good visible light absorption capacity, extraordinary high surface area,

unique stability and homogeneous size distribution with crystalline anatase phase.

• Out of all sample 5% Fe-S co-doped sample exhibited profound structural characteristics

and seemed to have potential for carrying the photocatalysis of dyes, phenol and CO2

more proficiently as compared to other doped and undoped nanoparticles.

• Effectiveness of photocatalytic process (degradation of dyes and phenol) depends on

diverse parameters, chiefly on the type and surface status of TiO2 which is controlled by

pH, the concentration, catalyst amount and irradiation source.

• The extraordinary and rapid photocatalytic processes mediated by Fe-S co-doped TiO2

for degradation of organic dyes and phenol is found to be an effective method for waste

water remediation due to rapidness, cost effectiveness, catalyst inert nature, photostability

and competent reusability.

• The photoreduction of CO2 by Fe-S co-doped photocatalyst ascertained one of the most

promising processes, since CO2 can be reduced to valuable products (ethanol) by

irradiating it with visible light.

• The results imply that solar energy can be an alternative cost effective light source due it

excessive availability and non-hazardous nature and can resolve the problems related to

water contamination and air pollution.

178

3.14 Characterization and Photocatalytic Applications Cr-S co-doped TiO2 Nanostructures

3.14.1 XRD Analysis

XRD measurements are accomplished in order to assess the structural changes induced

by S doping and Cr-S co-doping. Figure 3.77 (a-h) shows the XRD patterns of TiO2, S-doped

TiO2 and TiO2 co-doped with S (1 wt %), chromium (1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6

wt%). All XRD peaks of TiO2, S-TiO2 and Cr-S co-doped correspond to anatase phase [JCPDS

standard files #21-1272], hence no rutile phase is observed in the samples with chromium

concentration up to 5%. Above 5% chromium, the phase transformation from anatase to rutile

occurred due to dopant-induced lattice distortion. It is obvious from Figure 3.77 (a-g), by

increase in chromium ion concentration (Figure 3.77 (g)), the peak intensity decreased slightly

and FWHM increased with subsequent decrease in crystallite size along variation in lattice

parameters i.e., d-spacing and c-axis value (see Table 3.30). The crystallite size of all samples is

obtained from the strongest XRD peak (at 2θ=25o) by Scherrer’s formula while lattice constants

are determined by Bragg’s equation and tabulated in Table 3.30.

10 20 30 40 50 60 70 80

R

R

R

R (h)

215

220

116204

211

105202

11200

410

3

101

(g)

(f)

(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

R RR

R

R

R

Figure 3.77: XRD patterns of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST; (h) 6Cr-ST calcined at 500 0C.

179

The phenomena of decreases in the peak intensity, variation in lattice parameters and

reduction in the crystallite size with the addition of dopant suggest that Cr-S ions may substituted

or incorporated into TiO2 lattice [61, 62]. On the basis of these findings it can be suggested that

addition of Cr up to 5% stabilizes anatase TiO2 and beyond 5% Cr, it transforms anatase TiO2

into rutile TiO2, therefore the doping above 5% is restricted in current work.


Figure 3.78 (a-g) depicts the Tauc plot i.e. (F(R)*hv)0.5 vs. energy for the determination of band

gap energies, constructed by utilizing the Kubulka-Munk function [77, 86]. The bans gap values

determined by the linear Tauc region by extrapolation of line to the photon energy axis and given

in Table 3.30. It is clear from the Figure 3.78 (a-g) that energy shoulder for the S, Cr co-doped

shifted to lower energy, suggesting the incorporation of chromuim and sulfur in TiO2 lattice. S

doping is found to be effective in reducing the band gap of TiO2 from 3.17 eV to 2.89 eV [56,

58]. The comparative band gap displayed in Figure 3.78 (a-g) show that band gap value

decreased gradually from 2.89 eV to 2.02 eV with the co-doping Cr, S on TiO2 and maximum

reduction is observed with 5% co-doping of chromium, depicting that chromium improves the

visible light absorption properties.



SEM is used to examine effect of doping of the morphology of the nanostructures. SEM

images in Figure 3.79 (a-e) show that most of the particles are with unevenly distributed,

crystalline and agglomerates of TiO2 nanocrystals. A prominent effect on the surface

morphology with the addition of chromium can be seen. Addition of chromium from 1-5 wt% in

Cr-S co-doped TiO2 show the reduction in particle size. The overall size of the co-doped

particles appears to be smaller and average size of al sample ranges between 10-15 nm. Such a

difference in size arises due to increase in chromium ion contents which may tend to inhibit the

growth of titania nanocrystal. However, the aggregation in the samples may be caused due to the

higher surface energy related with the smaller-sized particles [78, 85].

180

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

gf

e

d

c

b

a

Energy (eV)

(F (R

).hv)

0.5

Figure 3.78: Band gap plots of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

Table 3.30: Calculated structural parameters and band gap data of un-doped, S-doped and Cr-S co-doped TiO2.


(nm) d-spacing


(eV)

TiO2 20.05 3.5113 3.7810 9.5410 3.17

ST 19.18 3.5179 3.7816 9.5511 2.89

1Cr-ST 18.20 3.5263 3.7813 9.5587 2.71

2Cr-ST 16.70 3.5339 3.7811 9.5610 2.62

3Cr-ST 15.61 3.5540 3.7812 9.5688 2.43

4Cr-ST 14.20 3.5590 8.7814 9.5702 2.40

5Cr-ST 12.19 3.5662 3.7813 9.5756 2.02

181

Figure 3.79: SEM images of (a) 1Cr-ST; (b) 2Cr-ST; (c) 3Cr-ST; (d) 4Cr-ST; (e) 5Cr-ST.

0.2 µm 09 29 SE I20 kV X 60,000

(e)

0.2 µm 10 47 SE I20 kV X 60,000

(d)

0.1 µm 10 27 SE I20 kV X 60,000

(c)

0.1 µm 09 15 SE I20 kV X 60,000

(b)

0.1 µm 12 20 SE I20 kV X 60,000

(a)

182

3.14.3.2 TEM Studies

TEM microscopy is employed to examine the structural characteristics of Cr-S co-doped

TiO2 nanoparticles. The TEM image (Figure 3.80 (a)) of nanoparticles shows a relatively narrow

size distribution and they consist of small aggregates. HRTEM (Figure 3.80 (c,d)) demonstrate a

well-faceted crystal pattern with a mean particle diameter of 10-13 nm, which is in good

agreement with the XRD results. The FFT pattern of the titania is determined from the inset of

HRTEM (Figure 3.80 (c)) which shows semi circle like diffuse spots consisting of five circles.

This pattern shows single (110) plane reflections. The d spacing value is calculated from

utilizing the HRTEM (Figure 3.80 (c)) which is 3.56 Ao and corresponds to the d-spacing values

observed in XRD.


3.14.4.1 EDX Studies (Energy Dispersive X-rays) Studies

The elemental composition of Cr and S is evaluated from EDX analysis and tabulated in

Table 3.31. Table 3.31 clearly depicts that Cr and S contents determined from EDX analysis

match well with those calculated theoretically.

3.14.4.2 RBS Studies RBS (Rutherford Back Scattering) Studies

Rutherford back scattering spectroscopy (RBS) is employed to evaluate the elemental

composition of in TiO2 lattice as a function of depth profile. Since, a secondary phase related to

chromium is not seen in the XRD spectra and RBS also ascertained the successful intermixing of

the chromium dopant with the titania. RBS depth profile data of all materials is shown in Figure

3.81 (a-g). This RBS technique is mostly useful due to its direct analysis of the nuclei in the

material because the RBS probes to large depth in the sample, analyzing the sample as a whole,

rather than simply the surface and gives the relative concentration of the dopant. The elemental

composition obtained from RBS is tabulated in Table 3.31 which matches well with the EDX

values.

183

Figure 3.80: TEM images (a) TEM at low magnification; (b) FFT Pattern; (c, d) HRTEM of 5Cr-ST.

184

300 600 900 1200 1500

Nor

mal

ized

Yei

ld

Channel

(a)(b)(c)(d)(e)(f)(g)

OS

TiCr

Figure 3.81: RBS spectra of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

Table 3.31: Elemental analysis of the un-doped, S-doped and Cr-S co-doped TiO2.


Ti O S Cr Ti O S Cr

PT 59.94 40.06 - - 58.71 41.29 - -

ST 59.09 39.94 0.97 - 59.11 39.90 0.99 -

1Cr-ST 58.22 39.90 0.95 0.93 58.19 39.87 0.98 0.96

2Cr-ST 57.35 39.78 0.93 1.94 57.44 39.69 0.97 1.90

3Cr-ST 56.53 39.67 0.95 2.85 57.50 39.62 0.95 2.93

4Cr-ST 55.60 39.55 0.93 3.92 57.60 39.50 0.96 3.94

5Cr-ST 54.85 39.30 0.90 4.95 54.74 39.39 0.95 4.92

185


Effect of Cr- S co-doping is also determined in terms of surface area and pore volume of

utilizing the BET plots and BJH methods, respectively and are given in Table 3.32. The surface

area and pore volume of TiO2 increased to 78.65 m2/g and 0.53 (cc/g) with the addition of 1%

sulfur to TiO2 lattice. Furthermore, increasing Cr doping concentration from 1 wt% to 5 wt%,

resulted in remarkable enhancement in surface area and average pore volume. Large surface area

(110.21 m2/g) and pore volume (1.22 cc/g) is obtained for 5% Cr-S co-doped sample. These

results are in good agreement to XRD data which shows reduction in particle size upon addition

of chromium. This reduction in particle size results in increase in surface area and porosity and

hence enhancement in catalytic ability due to availability of more active sites on the surface

[159].

Table 3.32: BET surface area and pore volume data of un-doped, S-doped and Cr-S co-doped TiO2.


TiO2 68.50 0.33

ST 78.65 0.53

1Cr-ST 82.32 0.97

2Cr-ST 90.23 1.08

3Cr-ST 98.10 1.12

4Cr-ST 104.44 1.19

5Cr-ST 110.21 1.22

3.14.6 FTIR Studies on Un-calcined and Calcined Samples

FT–IR spectra provide significant information about surface functional groups and phase

purity of the samples. Comparative infrared spectra of the un-calcined TiO2, S-doped TiO2, Cr-S

co-doped TiO2 is shown in Figure 3.82 (a-g). A broad band in the range 400–900 cm-1 is

attributed to Ti–O stretching and Ti–O–Ti bridging stretching modes. The sample exhibited

number of peaks at 1029 cm-1, 1340 cm-1, 1420 cm-1 attributed to Ti-O-C stretching vibrations,

C-H bending vibrations and CH2 bends [75, 77]. The broad peak appearing at 3072-3760 cm-1

186

and narrow band at 1620 cm-1 are related to hydroxyls vibration and the vibrations of the surface-

adsorbed H2O or Ti–OH bonds, respectively [85]. The variation in peak intensities is mainly due

to the doping of chromium contents into TiO2 lattice resulting in decrease in peak intensity. The

FTIR spectra of all samples calcined at 500 oC (Figure 3.83 (a-g)) show only dominant band at

500-600 cm-1 indicating that materials are free of impurities [99, 110]. This confirms that organic

residue and adsorbed water are removed from TiO2 lattice after heat treatment [89].

3.14.7 Raman Studies

Raman spectroscopy yields important information regarding the nature of solid

(crystallinity etc…) on a scale of few nanometer, depicts the correlation between the change in

vibrational characteristics, structural properties and morphological variations occurring in

nanostructure materials [111, 141, 151]. The inclusion of dopant into the TiO2 lattice not only

results in the creation of extra oxygen vacancies, distortion of perfect crystal structure but also

cause a decrease in particle size [149]. All these factors lead to the shift, lowering of the band

intensity and asymmetrical broadening of the Raman modes in Raman spectra of nanoparticles

[75, 78]. In the spectra of all TiO2 samples, the Raman modes can be labeled to the Raman active

modes of the anatase phase: 144.3 (Eg), 196 (Eg), 397 (B1g), 514.33 (A1g), and 640 (Eg) cm-1, as

shown in Figure 3.84.

it can be seen from figure that the main features of S doped and Cr-S co-doped samples

are very similar to that of reference TiO2 depicting that anatase phase of all sample holds a

certain degree of long range order. However, the Raman intensity, line width and position of

Raman modes vary little with the co-doping of S and Cr (1, 2, 3, 4, 5 wt%). Several factors

including particle size induces volume contraction. phonon confinement, stress, oxygen

vacancies, defects concentration can contribute to the these variations in Raman mode upon

doping of TiO2 nanoparticles [165, 167].

187

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 03 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0


(a )

(b )(c )(d )(e )( f )(g )

3 7 5 6 .6 3 3 0 7 2 .8 0

1 6 2 0 .2 91 4 2 0 .6 3

1 3 4 0 .1 0

7 2 7 .8 05 0 0

1 0 2 9

Figure 3.82: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

000 3500 3000 2500 2000 1500 1000 500

0

0

0

0

W avenumber (cm -1)

500(g)(f)(e)(d)

(c)

(b)

(a)

672.68

Figure 3.83: FTIR spectra of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

188

200 300 400 500 600 700 800

A1gB1g

Eg

Eg

Eg

(g)

(f)

(e)(d)

(c)

(b)

(a)

Inte

nsity

Raman Shifts (cm-1)

Figure 3.84: Raman spectra of (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

3.14.8 TGA Analysis

To explore the process of phase transformation of titanium (IV)-oxide-hydroxide to

titanium oxide and the effect of doping and co-doping content on the weight loss is conducted by

TGA and illustrated in Figure 3.85. As shown in Fig. 3.85 (a-g), in TGA curve of all samples,

two substantial weight losses are observed in the temperature ranges between 25 and 120°C and

between 130 and 400°C, respectively. A sharp weight loss appearing at 120 °C is assumed to be

due to the release of absorbed substances such as water, ethanol and acetic acid from the dried

gel [112, 113]. The second stage is from 130 to 400 °C, corresponds to the combustion of

residual organic species, including the dehydroxylation of the gel, decomposition of un-

hydrolyzed TTIP (for un-doped sample) and decomposition other organic moieties such as

nitrate ions, thiourea (for doped and co-doped samples) [107, 109]. The corresponding weight

loss of all samples at both temperature range, calculated from TGA is tabulated in Table 3.33.

189

100 200 300 400 500 600 700 80070

75

80

85

90

95

100

Wei

ght (

%)

Temperature (oC)

(a) (b) (c) (d) (e) (f) (g)

Figure 3.85: TGA profile of un-calcined (a) PT; (b) ST; (c) 1Cr-ST; (d) 2Cr-ST; (e) 3Cr-ST; (f) 4Cr-ST; (g) 5Cr-ST.

Table 3.33: Weight loss data of the un-doped, S-doped and Cr-S co-doped TiO2.


At 120 ºC At 400 ºC At 600 ºC

TiO2 8.82 27.31 72.69

ST 8.31 24.21 75.79

1Cr-ST 6.02 19.50 80.54

2Cr-ST 4.94 19.50 80.54

3Cr-ST 4.35 17.43 82.58

4Cr-ST 2.57 14.87 85.58

5Cr-ST 2.56 11.55 88.27

190

The period from about 400 oC to 800 oC, the weight loss is not very obvious, possibly

depicting the TiO2 phase transformation from amorphous to anatase and the stability of the TiO2.

However, the char yield corresponding to the remaining weight of un-doped, doped and co-

doped samples is calculated at 600 oC and summarized in Table 3.33. From the TGA plots, it is

obvious that the doping of S and co-doping of S-Cr significantly affect the thermal properties of

TiO2. In case of S-Cr co-doped samples, weight loss in the range of 130-400 oC is slower as

compared to other samples, due slow decomposition of organic matters which may be hindered

by Cr ions [94, 98]. The second distinct phenomena observed is the decrease in the weight loss

of S-Cr doped and increase in char yield with increase in Cr contents (Table 3.30). This may be

due to the reason that stability of TiO2 is effectively encouraged by doping of Cr ions. It should

be pointed that the above TGA results were well in agreement to the XRD data.

3.15 Applications of Prepared Cr-S co-doped Titanium Dioxide Nanostructure

The prepared nanomaterials are used for the photocatalytic degradation of dyes and

phenol and photocatalytic reduction of carbon dioxide under visible and UV irradiations. Similar

to the previous studies, the adsorption studies of dyes and phenol is conducted under dark to

determine the adsorption efficiency.


The adsorption studies of all dyes and phenol is carried out using 5% Cr-S co-doped TiO2

at different pH values i.e. pH ~4.5, ~7, and ~9.5 under dark for 6h. The percent adsorption values

for dyes and phenol are shown in Figure 3.86 (a-c) and tabulated in Table 3.31. The result clearly

demonstrates that adsorption value changes with the changes in pH value (discussed previously)

and is not significant in all cases. As seen in Table 3.34, about 0-5 % adsorption is obtained for

all dyes and phenol at all pH values under dark conditions on 5Cr-ST.


The influence of UV and visible light on the degradation of dye and phenol is

investigated previously depicting stability of dye and phenol under UV and visible irradiation

(section 3.4.1). The effect of adsorption on catalyst under dark was also observed demonstrating

poor adsorption kinetics (section 3.11.1). Therefore, photocatalysis is carried out in presence of

visible and UV light to achieve high activity for the degradation of phenol and dye.

191

Figure 3.86: Percent adsorption of dyes and phenol using Cr-S co-doped TiO2 at (a) pH ~4.5; (b) pH ~7; (c) pH ~9.5 under dark, time; 6h, conc.; 20 ppm, catalyst; 50 mg.


Compound


pH ~ 4.5 pH ~7 pH ~9.5





Phenol 2.96 1.11 0.87

0 1 2 3 4 5 60

1

2

3

4

5

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)0 1 2 3 4 5 6

0.0

0.5

1.0

1.5

2.0

ARS PB-MXR MG CV PH

Per

cent

Ads

orpt

ion

Time (h)

0 1 2 3 4 5 60

1

2

3

4


Per

cent

Ads

orpt

ion

Time (h)

(b)(a)

(c)

192

The effect of various parameters such as initial concentration, catalyst loading, pH, dopant

content and irradiation source (UV, visible) is also assessed. The rate of degradation in terms of

percent degradation is computed by the change in concentration using UV–visible spectrometry

as a function of irradiation time [115].


The effect of initial concentration of dyes and phenol on the percentage degradation is

studied by varying the initial concentration from 20-100 ppm at constant catalyst amount (50

mg) and at pH ~7 under visible irradiation. The results of percent degradation with various

concentrations at different reaction times are demonstrated in the Figure 3.87 and data is

tabulated in Table 3.35. As shown Figure 3.87 (a-e) that percentage degradation of all dyes and

phenol is found to increase with the reaction time and after certain time it became constant.

Meanwhile, the degradation rate decreased with increase in initial concentration of the solution

and maximum degradation is obtained with 20 ppm of concentration in all cases. The possible

explanation for decrease in the degradation rate with increase in the dye and phenol

concentration may be due to decrease in the path length of photons entering the solution and

reaching the catalyst surface at high concentration [81, 97].

Another possible reason for this trend may be the screening of the catalyst surface by the

dye itself at high concentration, resulting in suppression of generation of free radicals i.e., •OH

and O2•− which are required to carry the photocatalytic reduction [102, 107]. The maximum

degradation obtained for all dyes at 20 ppm is in the range of 60-70% which is not appreciable,

therefore further studies such as optimization of catalyst amount, pH and dopant content are

required to obtain the maximum efficiency. On the basis of above results 20 ppm is selected as

optimum concentration for carrying the further studies.

193

Figure 3.87: Percent degradation of various concentrations of (a) ARS; (b) PB-MXR; (c) MG; (d) CV; (e) PH as function of time under visible irradiation using 5Cr-ST, pH ~7, catalyst; 50 mg.

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70

Per

cent

Deg

rada

tion

Time (min)


0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60


Per

cent

Deg

rada

tion

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50


Per

cent

Deg

rada

tion

Time (min)0 5 10 15 20 25 30 35 40 45 50

0

10

20

30

40

50

60

70


Perc

ent D

egra

datio

n

Time (min)

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70


Per

cent

Deg

rada

tion

Time (min)

(b)(a)

(c) (d)

(e)

194


Catalyst

Conc. (ppm)


Alizarin red S

Procion blue MXR

Malachite green

Crystal violet

Phenol

5Cr-ST

20 61.34 65.29 57.24 63.51 69.83 40 41.90 50.29 40.24 45.14 46.10 60 31.50 34.81 35.55 37.08 34.36 80 24.01 25.31 27.44 30.32 25.07 100 22.91 20.23 22.56 27.25 18.44


The effect of catalyst dose on the photocatalytic degradation of dyes and phenol is made

by varying the amount from 10-70 mg, as shown in Figure 3.88. A significant increase in

degradation rate can be observed with increasing the catalyst loadings up to 50 mg. This may be

due to more availability of active sites exposed to the light at low dose amount. On the other

hand, the deterioration in the catalytic activity above 50 mg can be seen which may be attributed

to decrease in active sites caused by the agglomeration of catalyst at high concentration [107,

111, 162]. This agglomeration not only reduces the active sites on the surface but also decreases

the penetration of light due to which production of radicals reduces which are vital role for

catalytic processes [164, 165]. Therefore In all other experiments, a catalyst concentration of 50

mg is used for studying the effect of pH and dopant contents.


As discussed above, for all dyes and phenol the complete degradation was not observed

in entire range of the catalyst and at different concentration of solution under visible irradiation.

Therefore, to obtain maximum degradation, experiments are performed at three different pH i.e.

~ 4.5, 7 and 9, using 50 mg of the catalyst and 20 ppm of solution under visible irradiation and

shown in Figure 3.89. From the experimental results shown in Figure 3.89, it can be seen that the

optimum pH for photocatalytic degradation of anionic and cationic compounds is found to be at

~4.5 and ~9, respectively, exhibiting the maximum degradation efficiency i.e. ~100%. For

further experimental studies, the degradation of anionic dyes is conducted in acidic media and

for cationic dye in basic medium.

195

0 10 20 30 40 50 60 7010

20

30

40

50

60

70

80

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR

Figure 3.88: Percent degradation of dyes and phenol as function of catalyst dose using 5Cr-ST

under visible irradiations, time; 50 min, conc.; 20 ppm, pH ~7.

0

20

40

60

80

100

9.574.5

Per

cent

Deg

rada

tion

pH

ARS MG CV PB-MXR PH

Figure 3.89: Percent degradation of dyes and phenol as function of pH using 5Cr-ST under visible irradiations, time; 50 min, catalyst; 50 mg, conc.; 20 ppm.

196


The effect of dopant content (1 wt %, 2 wt %, 3 wt %, 4 wt % and 5 wt % Cr co-doped

with 1%S TiO2) is evaluated in terms of the photocatalytic degradation of the dyes and phenol

under optimized condition of concentration, catalyst dose and pH. Figure 3.90 and Table 3.36

demonstrates the effect of S doping, Cr-S co-doping on the photodegradation of dyes and phenol

after 50 min under visible irradiation. As the doping level is increased from 1 wt% to 5 wt %, the

activity enhanced and ~100% degradation is observed with 5% Cr-S co-doped photocatalyst.

This may be due to its high surface area, narrow ban gap as compared to other photocatalysts,

which make it an efficient visible light photocatalyst.


The photocatalytic degradation of the dyes using Cr-S co-doped TiO2 as photocatalyst

and visible and UV irradiation as light source is also carried out and the results are presented in

Figure 3.91. From Figure 3.91, it can be seen that complete degradation of dyes and phenol is

achieved within 50 min of visible light irradiation while considerably low degradation is

obtained for 6h in UV irradiation. The difference in the rate of degradation under UV and visible

light may be attributed to band gap and surface area of the nanomaterials. The band gap energy

of Cr-S co-doped sample corresponds to the visible region, making it to be active under visible

irradiation [94, 169].

3.15.2.6 Reuse of the Photocatalyst

It is vital to for the photocatalyst to be reusable without significant loss of activity and

with minimal need for regeneration [170]. In order to determine the reusability of the catalyst,

experiments are conducted under similar conditions (as used for Fe-S co-doped TiO2) for six

cycles using 5% Cr-S co-doped TiO2 and shown in Figure 3.92. It can be seen from Figure 3.92

that after 3rd cycle, efficiency remained almost same but it decreased after that in fourth and fifth

cycle. For sixth cycle, the catalyst is regenerated by calcination at 500 oC, after separation. It is

obvious from the results shown for the sixth cycle in Figure 3.92, the activity of catalyst

significantly enhanced, reaching to the maximum. The results obtained strongly suggest Cr-S co-

doped TiO2 nanoparticles can be novel photoactive material for removal of textile dyes and

phenol from the contaminated aqueous media and can be reused effectively.

197

PT ST 1Cr-ST 2Cr-ST 3Cr-ST 4Cr-ST 5Cr-ST10

20

30

40

50

60

70

80

90

100

Per

cent

Deg

rada

tion


MG PH CV ARS PB-MXR



Catalyst


Alizarin

red S

Procion blue

MXR

Malachite

green

Crystal

violet Phenol

PT 13.5 17.2 12.2 11.5 14.5

ST 41.1 39.9 41.5 40.1 42.1

1Cr-ST 52.5 48.5 49.6 50.6 47.4

2Cr-ST 60.6 58.6 62.5 61.5 63.5

3Cr-ST 70.3 69.6 72.5 68.9 71.5

4Cr-ST 75.6 76.0 76.5 7212 78.3

5Cr-ST 100 99.6 99.9 100 99.8

198


20

40

60

80

100

Per

cent

Deg

rada

tion

Irradiation Source

ARS MG CV PB-MXR PH


irradiation with 5% Cr-S co-doped TiO2.

1 2 3 4 5 60

20

40

60

80


Per

cent

Deg

rada

tion

Cycle (n)

ARS MG CV PB-MXR PH

Figure 3.92: Plots of recyclability of dyes and phenol using 5 % Cr-S co-doped TiO2 under


199

3.15.2.7 Comparison of Percent Degradaton of Dyes and Phenol by Un-doped and TiO2

co-Doped With Different Elements

The comparison of photocatalytic degradation of the dyes and phenol using S and metal ion co-

doped TiO2 as photocatalyst under visible irradiation is presented in Figure 3.93. From Figure

3.93, it can be seen that negligible degradation can be seen with TiO2 while significantly higher

degradation was obtained with S doped TiO2 which may be due to visible light activity. A

complete degradation of dyes and phenol is achieved under optimized conditions and visible

light irradiation with 5% Co-S co-doped TiO2, 5% Cu-S co-doped TiO2, 5% Ru-S co-doped

TiO2, 5% Fe-S co-doped TiO2, 5% Cr-S co-doped TiO2. This suggest that TiO2 co-doped with

different metal ions have similar tendency to decompose organic pollutants and suitable for

water purification for future applications.

PT ST 5Co-ST 5Cu-ST 5Ru-ST 5Fe-ST 5Cr-ST0

20

40

60

80

100

120

Per

cent

Deg

rada

tion

Catalyst

ARS MG CV PB-MXR PH

Figure 3.93: Comparative percent degradation plots of dyes and phenol using 5% Co-S co-doped TiO2, 5% Cu-S co-doped TiO2, 5% Ru-S co-doped TiO2, 5% Fe-S co-doped TiO2, 5% Cr-

S co-doped TiO2 under visible irradiations and optimized conditions.

200

3.15.3 Photocatalytic Reduction of Carbon Dioxide using 5Cr-ST

The photocatalytic reduction of in aqueous media is carried out with 5% Cr-S co-doped

catalyst in both UV (with cutoff filter λ<380 nm) and visible irradiation (with cutoff filter λ >

420 nm) and demonstrated in Figure 3.94. The products of photoreduction of CO2 are analyzed

by GC and it can be seen that there is no product formation under UV irradiation whereas a

significant yield of ethanol is determined under visible irradiation. The curve of ethanol

production with reaction time follows the same pattern as described previously for other

catalysts. Ethanol production increased with reaction time and reaches a peak value of 13.28

µmol at around 10h and after that it remained constant. It should be emphasize that the increased

production rate of ethanol may be due to the synergetic effect of Cr and S. Cr species which not

only suppress the recombination of photo induced electron and holes by capturing the electron

but also increases the visible absorption capacity of the catalyst, thereby increase the catalytic

activity. The higher amount of photogenerated electron is available under visible irradiation,

participating in the photoreduction of CO2 [71, 81]. These findings reported here may indicate

the potential for carrying the photoreduction of CO2 under natural sunlight thus exploring the

potential environmental application.

0 2 4 6 8 10 120

2

4

6

8

10

12

14

16 Vis Irradiation UV Irradiation

Time (h)

Eth

anol

Pro

duct

ion

(µm

ol/h

-cat

al.)

Figure 3.94: Ethanol production by photoreduction of CO2 as function of time using 5% Cr-S

co-doped TiO2.

201

3.16 Conclusions

• The Cr-S co-doped nanoparticles with good crystallite size, tunable band gap properties,

good structural and photostability and excellent pore volume were synthesized by singe

step sole-gel reaction.

• The major focus of doping Cr and S was on the development of new photocatalysts and

approach to improve the activity of photocatalytic dyes and phenol removal from

contaminated water and CO2 reduction into value added products.

• The S and Cr-S co-doping extended the absorption spectra of TiO2 to the visible light

region and improves charge separation.

• Significant enhancement of dyes and phenol degradation and CO2 photoreduction to

methanol was observed for 5% Cr-S co-doped TiO2 compared with bare TiO2 under both

visible and UV irradiations.

• This significantly enhanced dyes degradation and CO2 photoreduction rates could be

allied to the synergistic combination of narrow band gap and high surface area.

• In conclusion, this research investigation has opened up new opportunity for exploration

on waste textile water treatment and CO2 photo reduction. Choice of catalyst synthesis

methods, crystal structure, and crystallite size, band energy uniqueness of the catalyst and

the wavelength of the irradiation source are the major feature that directs the activity and

product selectivity thus making it a potentially cost-effective photocatalyst.

202

3.17 Characterization and Photocatalytic Applications TiO2 Nanotubes

An ultra efficient Fe, Fe-S co-doped (5%Fe-1%S), Fe-Cr co-doped (3%Fe-2%Cr) and Fe-

Cr-S co-doped (3%Fe-2%Cr-1%S) titania nanotubes are successfully synthesized using modified

hydrothermal.

Composition, phase, distribution of dopants, morphology of synthesized materials are

investigated by means of X-ray diffraction XRD, SEM and EDX based techniques. The prepared

nanotubes are labeled as TNT (as prepared TiO2 nanotubes), F-TNT (5% Fe doped TiO2

nanotubes), FS-TNT (5%Fe-1%S co-doped TiO2 nanotubes), FC-TNT (3%Fe-2%Cr co-doped

TiO2 nanotubes), FCS-TNT (3%Fe-2%Cr-1%S co-doped TiO2 nanotubes) [124, 125].

3.17.1 XRD Studies

Figure 3.95 shows the XRD patterns of the F-TNT (5% Fe doped TiO2 nanotubes), FS-

TNT (5%Fe-1%S co-doped TiO2 nanotubes), FC-TNT (3%Fe-2%Cr co-doped TiO2 nanotubes),

FCS-TNT (3%Fe-2%Cr-1%S co-doped TiO2 nanotubes). The XRD patterns of un-doped and

doped TiO2 nanotubes exhibit strong and broad diffraction peaks at 2θ = 25.60, 37.86, 48.13,

53.85, 55.01, 62.73, 68.77. 71.03 and 74.85 which corresponds to diffractions planes of the

(101), (004), (202), (105), (211), (204). (116), (220) and (215) anatase type TiO2. From XRD

spectra it can be seen that XRD peaks become broader on the account of doping indicating that

the modified TiO2 nanotubes possess higher long range order and small grain size [167].

However no new diffraction peaks appeared which indicates that geometry of the

synthesized materials is modified without change in its phase and structure. This can be

attributed to the new synthesis procedure along with precipitation and reducing agent adopted in

this study. The XRD data indicates that Fe, S and Cr has strong bonding with the crystalline

lattices of TiO2 nanotubes and is not piled up around the surface, hence, confirming the

modification of TNTs at a molecular level [124, 125, 168, 169].

203

10 20 30 40 50 60 70 80

215

220

11620

4211

10520

2

004

101

(e)

(d)

(b)

(c)

2θ (Degree)

Inte

nsity

(a.u

.)

(a)

Figure 3.95: XRD patterns of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-TNT; (e) FCS-TNT; calcined at 500 0C.

3.17.2 UV-Vis Diffuse Reflectance Spectroscopy

The band gap of the pure and Fe, Fe-S, Fe-Cr, Fe-Cr-S doped TNTs are shown in Figure

3.96 (a-e) and Table 3.37. It is apparent that the diffuse reflectance spectra of all Fe doped TNTs

exhibit lower energy shift. It could be concluded that Cr, Fe and S ions occupy substitutional or

interstitial positions and form a geometry which results in the decrease in the band gap [117,

170]. The onset of the energy edge for pure TNTs is at 3.25 eV, which is consistent with the

intrinsic band-gap absorption of pure anatase TiO2 (3.2 eV).

3.17.3 Scanning Electron Microscopy and EDX Analysis

SEM photographs of the un-doped and Fe, Fe-S, Fe-Cr, Fe-Cr-S co-doped TiO2 nano

tubes are shown in Figures 3.97 (a-e). From the investigation of SEM, it can be observed that the

diameter of pure TiO2 nano tubes is 8-10 nm with length around 400-460 nm. The diameter of

the TiO2 nano tubes becomes smaller with the increase in dopant content [108, 169, 124, 125].

204

1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.60

5

10

15

20

e

d

c

b

a

(F (R

).hv)

0.5

Energy (eV)

Figure 3.96: Band gap plots of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-TNT; (e) FCS-TNT.

Table 3.37: Band gap data of the un-doped and doped TiO2 nanotubes.

S. NO. Catalyst Band gap (eV)

1. TNT 3.25

2. F-TNT 2.81

3. FS-TNT 2.65

4. FC-TNT 1.85

5. FCS-TNT 1.78

205

Figure 3.97: SEM images of (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-TNT; (e) FCS-TNT.

(e)

(d)(c)

(b)(a)

3.17.4 T

T

3.98 (a&

open end

nanotube

walled. T

in length

conversio

Transmission

TEM analysi

&b) which ve

ds having n

es and Fe-Cr

TiO2 nanotub

h (Figure 3.9

on of nanopa

Fig

(a)

(b)

n Electron M

is Fe-Cr dop

erify that tub

needle shape

r-S co-doped

bes have an

98 (a&b)). N

articles into

gure 3.98: T

)

)

Microscopy

ped and Fe-

bes are hollo

e structures.

d TiO2 nano

average dia

No TiO2 nan

nanotubes u

TEM images

y

-Cr-S co-dop

ow, exhibit t

. Furthermo

otubes show

ameter of 8-1

noparticles e

under employ

s of (a) FC-T

ped TiO2 na

tubular and u

ore, TEM pa

w that these t

12 nm and a

exist around

yed experim

TNT and (b)

anotubes is

uniform mor

attern of Fe

tubes are un

are up to sev

the nanotub

mental condit

FCS-TNT.

shown in F

rphology wi

e-Cr doped

niform and s

veral hundre

bes, proving

ions [124, 12

206

Figure

ith an

TiO2

single

d nm

high

25].

207

3.17.5 Elemental Composition

The chemical composition and respective percentage of each atom present in the samples,

is analyzed by EDX. The EDX analysis reported in Table 3.38 indicates that the experimental

concentrations of Fe, Cr, S and TiO2 are close to the nominal concentrations within the limit of

experimental errors. Table 3.38 presents the percentages of O, Ti, Fe, Cr elements confirming the

formation of doped and un-doped TNTs.

3.17.6 Surface Area Analysis

BET specific surface area of doped and un-doped TNTs is much higher to the TiO2

nanoparticles. BET surface area has no effect on the phase of the TNTs but has an effect on the

photocatalytic degradation reaction [122, 134]. Doping of titania nanotubes with metals resulted

in increase in BET surface area. The trend of surface area is found to be TNT < F-TNTS < FS-

TNT < FC-TNT < FCS-TNT. The highest surface area is observed for FCS-TNT i.e. 410 m2g-

1.Larger surface area show increased adsorption-desorption capacity, consequently good

photocatalytic activity. Table 3.38 presents the measured surface area of the prepared titania

nanotubes, the surface area of the commercial TiO2 is also included for comparison.

Table 3.38: Surface area and EDX analysis of doped and un-doped TNTs.

Catalyst EDX elemental composition

(wt%)

BET Surface area

(m2g-1)

Ti O S Cr Fe

TNT 59.40 40.60 - - - 110

F-TNT 59.09 39.94 - - 0.97 180

FS-TNT 58.18 39.89 0.97 0.96 190

FC-TNT 55.85 39.20 - 1.98 2.97 400

FCS-TNT 55.17 38.96 0.94 1.92 3.01 410

3.18 Photocatalytic Applications of Prepared Nanotubes

The Cr-Fe co-doped titania nanotubes and Fe-Cr-S co-doped titania nanotubes responded

with extraordinary band gap and surface area properties as compared to Fe, Fe-S co-doped and

un-doped titania nanotubes. Therefore the photocatalytic activity of Cr-Fe co-doped titania

208

nanotubes and Fe-Cr-S co-doped titania nanotubes is determined for the degradation of phenol

(as model pollutant) and photo-reduction of CO2 in visible irradiation at optimized conditions (as

used for nanoparticles).

3.18.1 Photocatalytic Degradation of Phenol

In this study photocatalytic activity of the titania nanotube (Fe-Cr codoped and Fe-Cr-S co-

doped TiO2 nanotubes) is evaluated for photocatalytic degradation of phenol (0.5 g/L)

degradation under visible light illumination using 10 mg of catalyst under optimized condition

(used for nanoparticles), the results are shown in Figure 3.99. The degradation kinetics is

computed by the change in phenol concentration employing UV–visible spectrometry as a

function of irradiation time [125]. The phenol conversion is estimated using the following

formulation [126, 127];

Phenol conversion (%) = [Phenol]0 - [Phenol]t / [Phenol]0 x100 (1)

The study of Figure 3.99 indicates that Fe-Cr doped nanotubes and Fe-Cr-S co-doped

nanotubes decompose phenol more rapidly than does titania nanotubes alone under visible light.

The photocatalytic decomposition of phenol is remarkably accelerated by the doped-TNTs

photocatalysts prepared in the present work. This may be due to the inhibition of a spontaneous

recombination between the hole and electron occurring on the surface of excited TNTs by the

dopant doped in the TNTs surface [107, 109]. The photodegradation efficiency of Fe-Cr-S co-

doped titanate nanotubes is 99.9% in 30 min, Fe-Cr showed 99.99% efficiency in 50 min under

visible irradiation while significant degradation is not observed in 80 min for TNT, Fe-doped

TNT and Fe-S co-doped TNT.

Metal ion doping influence the photoactivity of TiO2 by electron or hole traps whereas

the trap causes the formation of some active species that benefit degradation of phenol [75, 85].

Here the Sulfur plays a vital role in reducing the band gap while electron scavenger effect of

Cr3+/Fe3+ prevent the recombination of electron and hole pairs thus resulting in increase of the

efficiency of photodegradation process [127, 169]. Electrons are either directly trapped at Ti(IV)

surface sites (form Ti3+) or in deeper Fe(III)/ Cr(III) sites (form Cr2+/Fe2+). In this case, the

trapped electron can be easily transferred from Cr2+/Fe2+ to a neighboring surface Ti4+ because of

the proximity of the energy levels [155, 167].

209

0 10 20 30 40 50 60 70 800

20

40

60

80

100

Per

cent

Deg

rada

tion

Time (min)

(a) (b) (c) (d) (c)

Figure 3.99: Percent degradation of phenol as function of time using (a) TNT; (b) F-TNTS; (c) FS-TNT; (d) FC-TNT; (e) FCS-TNT.

Since Fe-Cr co-doped and Fe-Cr-S co-doped TiO2 nanotubes exhibited remarkably higher

surface area and visible absorption properties as compared to Fe-doped and Fe-S co-doped TiO2

nanotubes therefore remarkable activity was observed with these nanotubes. This fast

degradation rate of these nanotubes can be correlated to the (a) lowest band gap i.e. 1.85 eV and

1.78 eV for Fe-Cr codoped and Fe-Cr-S co-doped TiO2 nanotubes, respectively as compared to

other photocatalyst and (b) remarkably higher surface area gap i.e. 400 m2g-1 and 410 m2g-1 for

Fe-Cr co-doped and Fe-Cr-S co-doped TiO2 nanotube, respectively, as compared to other

photocatalyst.

3.18.2 Conversion of CO2 under Visible Irradiation

Due to low band gap and huge surface area the synthesized Fe-Cr co-doped and Fe-Cr-S co-

doped TiO2 nanotube are used to evaluate conversion of CO2 into alcohol (ethanol) and

presented in Figure 3.100 (a&b). The co-doping effect of Fe-Cr and Fe-Cr-S makes it more

visible response catalyst as compare to other photocatalyst and also results in higher surface

area, providing more exposure of active sites for the CO2 reduction in visible irradiation [109.

121, 170]. Therefore enhanced efficiency for production of ethanol is observed with these

210

photocatalysts. The comparative study of Figure 3.100 (a&b) reveals that the both doped TNTs

show considerably higher conversion of CO2 in comparison with the doped nanoparticles

(discussed before). The ethanol production of 25.3 µmol and 19.2 µmol is observed with Fe-Cr

co-doped and Fe-Cr-S co-doped TiO2 nanotube in 8h of reaction time, respectively.

0 1 2 3 4 5 6 7 80

3

6

9

12

15

18

21

24

27

30

(b)

Time (h)

Eth

anol

Pro

duct

ion

(µm

ol/h

-cat

al.)

(a)

Figure 3.100: Ethanol production by (a) FC-TNT and (b) FCS-TNT.

3.19.5 Conclusions

• The doped and un-doped titanium dioxide nanotubes (TNTs) were synthesized using

hydrometallurgical process. The significance of current method lies mainly in its

simplicity, flexibility, short reaction conditions, high purity, high yield and the control of

material morphology which determine the physical properties. Direct hydrothermal

synthesis method is easy and efficient to synthesize pure and titanate nanotubes doped

with transition metals.

• The XRD results indicate that TNTs are crystalline in nature and exist in anatase form

High aspect ratio of TiO2 un-doped and doped TiO2 nanotubes of homogenous length are

synthesized. XRD result also suggests that cryatallanity enhanced while the band gap and

BET surface area increased after doping.

211

• The SEM results show that nanotubes are tubular in nature and average diameter of the

TNTs is 12-14 nm and length 400-460 nm.

• Fe-Cr doped TNTs and Fe-Cr-S co-doped TNTs revealed a larger specific surface area

and a good visible light absorption properties than that of other TNTs and nanoparticles,

leading to an obvious enhancement of the photocatalytic activity for the photocatalytic

degradation of pollutants (CO2 and phenol) under visible irradiation.

• Doping of metals, such as iron and chromium and anions i.e. S codoped TNTs resulted in

enhanced rate of the photocatalytic decomposition of organic compounds in aqueous

solutions.

• In conclusion, TiO2 nanotubes are found to be feasible and attractive for use in further

investigation of CO2 reduction for CO2 environment management and waste water

treatment.

212

3.20 In General Inferences and Future Prospects

1. TiO2 nanostructures with good crystallite size, homogenous distribution and good

structural properties were synthesized successfully.

2. Synergistic effect between S dopant and transition metal ion doping induced structural

changes i.e., enhancement of the visible light absorption capability, surface area, stability

and photocatalytic activity.

3. Photocatalytic degradation of dyes and CO2 reduction been shown under UV and visible

irradiation at room temperature and atmospheric pressure.

4. Optimum degradation parameters were studies to achieve high degradation rate of dyes

and phenol.

5. High stability of the catalyst under UV and Visible irradiation at various pH is found and

possibility of recycling it during pollutants degradation.

6. The CO2 reduction results strongly suggest the recycling of CO2 into a value added

products (methane, methanol, ethanol) and hydrogen is obtained from water in-situ.

7. TiO2 nanostructures are found to be feasible and attractive for CO2 environment

management and waste water treatment due to rapidness, cost effectiveness, catalyst inert

nature, photostability and competent reusability.

8. Solar energy can be an alternative cost effective light source to resolve the environmental

problems in future.

9. The process is single step process and can be useful for industries.

213

The future plans of current study include:

• Theoretical calculations and in situ studies of dye degradation and CO2 photo reduction

to determine the mechanism and intermediates or products formed during reaction.

• Effect of different additives (salt, H2O2, O2), hole scavengers, temperature, pressure and

natural solar radiations on the photocatalytic kinetic.

• Determine the photocatalysis of industrial effluents discharged from the textile, rubber

and other industries.

• Determine the kinetics and thermodynamics parameters of the reaction.

• Modification of the photocatalyst with different metal and non metals and development

of nanotubes of the prepared samples similar to Fe-S co-doped and investigated their

photocatalytic properties.

• Investigate effect of photocatalysis on antibacterial, antifungal, COD and BOD properties

of water; and their applications towards gas sensing can be studied.

214

4. REFRENCES

[1] Qamar M, Gondal MA and Yamani ZH Catal Commun 2009; 10: 1980-1984.

[2] Laws EA and Berning JL Bioresour Technol. 1991; 37: 25-30.

[3] Maginn EJ. Journal of Physical Chemistry Letters 2010; 1: 3478–3479.

[4] Jiang D, Xu Y, Wu D and Sun Y. J Solid State Chem 2008; 181: 593-602.

[5] Usubharatana P, McMartin D, Veawab A and Tontiwachwuthikul P. Eng. Chem. Res

2006; 45: 2558-2568.

[6] Tan SS, Zou L and Hu E. Res J Chem Environ 2005; 9: 66-75.

[7] Alam MZ, Ahmad S, Malik A and Ahmad M. Ecotoxicology and Environmental Safety

2010; 73:1620–1628.

[8] Montgomery MA and Elimelech M. Environ Sci Technol 2007; 41: 17–24.

[9] Singh P and Bengtson L. J Hydrol 2005; 300: 140–154.

[10] Richardson S. Anal Chem 2003; 75: 2831–2857.

[11] Kyung H, Lee J and Choi W. Environ Sci Technol 2005; 39: 2376–2382.

[12] Li X. J Phys Chem C 2007; 111: 13109–13116.

[13] Weiyang D, Chul WL, Xinchun L, Yaojun S, Weiming H, Guoshun Z, Shicheng Z,

Jianmin C, Huiqi H and Dongyuan Z. Appl Catal B Environ 2010; 95: 197–200.

[14] Weber EJ and Adams RL. Environ Sci Technol 1995; 29: 1163-1165.

[15] Jung KY and Park SB. J Chem Eng 2001; 18: 879–88.

[16] Benkli YE, Can MF, Turan M and Celik MS. Water Res. 2005; 39: 487–493.

[17] Selcuk H. Dyes Pig 2005; 64: 217– 222.

[18] Ghoreishi SM and Haghighi R. Chem Eng J 2003; 95: 163–169.

[19] Neelavannan MG, Revathi M and Basha CA. J Hazard Mater 2007; 149: 371–378.

[20] Lalov IG, Guerginov II, Krysteva MA and Fartsov K. Water Res 2000; 34: 1503–1506.

[21] Gogate R and Pandit AB. Adv Environ Res 2004; 8: 501–551.

[22] Chen D and Ray AK. Chem Eng Sci 2001; 56: 1561–1570.

[23] Zhao C, Chen Y, Wang W, Ma J, Zhao T and Rajh L. Environ Sci Technol 2008; 42:

308–314.

[24] Habibi A and Hassan SM. J Photochem Photobiol A Chem 2005; 172: 89–96

[25] Rao NN, Bose G, Khare P and Kaul SN. J Environ Engg 2006; 132: 367-376.

215

[26] Saria V, Parra S, Adler N, Peringer P, Benitez N and Pulgarin C. Catalysis Today 2002;

76; 301-315.

[27] Scott JP and Ollis DF. Environ Prog 2006; 14: 88-103.

[28] Shu HY. J Haz Mat 2006; 33: 92-98.

[29] Solmaz SKA, Birgul A, Ustun GE and Yonar T. Coloration Technology 2006; 122:102-

109.

[30] Spiewak I, Benmair R, Messalem R and Radchenko O. Proceedings of the Indian

Academy of Sciences: Chemical Sciences 1998; 110: 229-238.

[31] Suslick KS. Annu Rev Mater Sci 1999; 29: 295-326.

[32] Swaminathan K, Sandhya S, Carmalin SA, Pachhade K and Subrahmanyam YV.

Chemosphere 2003; 50: 619-625.

[33] Vajnhandl S and Marechal AML. Dyes and Pigments 2005; 65: 89-101.

[34] Alkemade UG and Schumann B. Sol Stat Ion 2010; 177: 51-59.

[35] Chiron M. Stud Surf Sci Catal 1987; 30:1–10.

[36] Mauzerall DL, Sultan B, Kim N and Bradford DF. Atmos Environ 2005; 39: 2851–2866.

[37] Agarwal AK. Prog Energ Comb 2007; 33: 233–271.

[38] Dey GR. Journal of Natural Gas Chemistry 2011; 16: 217–226

[39] Demeestere K, Dewulf J and Langenhove HV. Critical Reviews in Environmental

Science and Technology 2007; 37: 489-538.

[40] Centi G, Ciambelli P, Perathoner S and Russo P. Catal Tod 2002; 75: 3–15.

[41] Azuma K, Hashimoto M, Hiramoto M and Watanabe TS. Journal of the

Electrochemical Society 1999; 137: 1772-1778.

[42] Gattrell M, Gupta N and Co A. Journal of Electroanalytical Chemistry 2006; 594: 1-19.

[43] Hoffmann MR, Martin ST, Choi W and Bahnemann DW Chemical Reviews 1995; 95:

69- 96.

[44] Hashimoto K, Irie H and Fujishima A. Japanese Journal of Applied Physics 2005; 44:

8269–8285

[45] Roy SC, Varghese OK, Paulose M and Grimes CA. American Chemical Society Nano

letters 2010; 4: 1259–1278

[46] Tsuneoka H, Teramura K, Shishido T and Tanaka T. Journal of Physical Chemistry C,

114, 8892–8898.

216

[47] Teramura K, Okuokab S-i, Tsuneokab H, Shishidob T and Tanaka T. Applied Catalysis

B: Environmental 2010; 96: 565–568.

[48] Tan SS, Zou L and Hu E. Catalysis Today 2006; 115: 269–273.

[49] Chen YS, Crittenden JC, Hackney S, Sutter L and Hand DW. Environ Sci Tech 2005;

39: 1201-1208.

[50] Khan MA, Jung HT and Yang OB. J Phys Chem B 2010; 110: 6626-6630.

[51] Mohamed MM and Khairou KS. Microporous Mesoporous Mater 2011; 142: 130-138.

[52] Fujishima A and Honda K. Nature 1972; 238: 37–38.

[53] Oaxaca RA and Becerril JJ. J Mex Chem Soc 2010; 54: 164-168.

[54] Ma XG, Jiang JJ and Liang P. Acta Phys Sin 2008; 57: 3120-3125.

[55] Hwang KS, JH Jeong, Ahn JH and Kim BH. Ceram Int 2006; 32: 935–937.

[56] Carp O, Huisman CL and Reller A. Prog Solid State Chem 2009; 32: 33–177.

[57] Carlier F, Benrezzak S, Cahuzac P, Kebali N, Masson A, Srivastava AK, Colliex C and

Brechignac C. Nano Lett 2013; 6: 1875-1879.

[58] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R, Yang P.

Science 2001; 292: 1897.

[59] Bahadur H, Srivastava AK, Sharma RK, Chandra S. Nanoscale Res Lett 2007; 469: 2-7.

[60] Deepa M, Srivastava AK, Agnihotry SA. Acta Mater 2006: 55; 6095-6099.

[61] Mor GK, Shankar K, Paulose M, Varghese OK and Grimes CA. Nano Lett 2010; 6: 215-

220.

[62] Rigby SJ, Obaidi AHRA, Lee SK, McStay D and Robertson PKJ. Appl Surf Sci 2008;

252: 7948-7954.

[63] Khan MA, Jung HT and Yang OB. J Phys Chem B 2006; 110: 6626-6630.

[64] Samarghandi MR, Nouri J, Mesdaghinia AR, Mahvi AH, Nasseri S and Vaezi F. Int J

Environ Sci Technol 2007; 4: 19-26.

[65] Yu JC, Zhang L, Zheng Z and Zhao J. J Chem Mater 2003; 15: 2280-2286.

[66] Chatterjee D and Mahata A. J Appl Catal B 2009; 33: 119-125.

[67] Masakazu A. Pure Appl Chem 2000; 72: 1787-1792.

[68] Bouras P, Stathatos E and Lianos P. Appl Catal B 2007; 73: 51-59.

[69] Chen X. Chin J Catal 2009; 30: 839-851.

217

[70] Belessi VC, Bakas TV, Costa CN, Efstathian AM and Pomonis PJ. Appl. Catal. B:

Envir. 2000; 28: 13–28.

[71] Yamashita H, Harada M and Misaka J. J Synchrotron Radiat 2001; 8: 569-571.

[72] Zhu H, Tao J and Xiang D. J Phys Chem C Nanomater Interface 2010; 114: 2873-2879.

[73] Wenfang Z, Qingju L, Zhongqi Z and Ji Z. J Phys D Appl Phys 2010; 43: 035301-

035305.

[74] Asahi R, Morikawa T, Ohwaki T, Aoki K and Taga Y. Science 2001; 293: 269-271.

[75] Divalentin C, Finazzi E, Pacchioni G, Selloni A, Livraghi S, Paganini M and Giamello

E. J Chem Phys 2007; 339: 44-56.

[76] Cam L, Luu1 Q, Tuan N and Si T. J Adv Nat Sci Nanosci Nanotechnol 2010; 1:

015008-015010.

[77] Yu T, Tan X, Zhao L, Yin Y, Chen P and Wei J Chem Eng Technol 2010; 157: 86-92.

[78] Seery MK George R, Floris P and Pillai SC. J Photochem Photobiol A 2007; 189: 258-

263.

[79] Wu P, Xie R, Imlay K and Shang JK. Environ Sci Technol 2010; 44: 6992–6997.

[80] Khan R, Sun WK, Tae-Jeong K and Chang N. Mater Chem Phys 2008; 112: 167-172.

[81] Zhao W, Wanhong M, Chuncheng C, Jincai Z and Zhigang S. J Am Chem Soc 2004;

126: 4782- 4783.

[82] Balek V, Li D, Subrt J, Veernikova E, Hishita S, Mitsuhashi T and Haneda H. J Phys

Chem Solids 2007; 68: 770-774.

[83] Yin Y and Xin W. J Adv Mater Res 2009; 79: 329-332.

[84] Wei CH, Tang XH, Liang JR and Tan SY. J Environ Sci 2007; 19: 90-96.

[85] Li FT, Liu RH, Zhao, DS, Sun ZM, Qu, ZM. Nonferrous Met Soc China 2007; 17: 713-

S715.

[86] Kewei L, Haiying W, Chunxu P, Jianhong W, Rui X and Jing S. International Journal

of Photoenergy 2012; 10: 1-8.

[87] Yang Y and Jingfei L. Molecules 2012; 17: 2752-2772.

[88] Liu Y, Wang X, Yang F and Yang X. Microporous Mesoporous Mater 2010; 114: 431-

439.

[89] Ye C, Peng Q, Xuan KL, Zhi JD and Guan MY. Advanced Materials Research 2011;

192: 179-180.

218

[90] Hussain ST, Rashid M, Anjum D, Siddiqa A and Badshah A. Materials Research

Bulletin 2013; 48: 705–714.

[91] Hussain, ST, Siddiqa A, Ilyas H, Bakhtiar M and Siddique M. International Review of

Chemical Engineering 2012; 4: 547-553.

[92] Erin RM, Stewart LK and Jenks WS. J catal B environment 2009; 91: 554–562.

[93] Wang Y, Li J, Peng P, Lu T and Wang L. J Applied Surface Science 2008; 254: 5276–

5280.

[94] Minghua ZM and Yu J. J Hazard Mater 2008; 3: 1229-1236.

[95] Hamadanian M, Reisi-Vanani A and Majedi A. J Materials Chem Phys 2009; 116: 376–

382.

[96] Segne TA, Tirukkovalluri SR and Challapalli S. International Research Journal of Pure

& Applied Chemistry 2011; 3: 84-103.

[97] Dobosz A and Sobczyński A. Water Research 2003; 37: 1489–1496.

[98] Papp J, Shen HS and Kershaw R. J Chem Mater 2008; 5: 284-289.

[99] Crittenden JC, Liu J and Hand D. Water Res 2009; 31: 429-439.

[100] Kyoung HK and Ihm SK. J Hazard Mater 2007; 146: 610-615.

[101] Halmann MM. Chemical Fixation of Carbon dioxide: Methods for Recycling CO2 into

Useful Products, CRC Press, New York, 1993, p. 172.

[102]Halmann M and Gra¨tzel M (Eds.). Energy Resources Through Photochemistry and

Catalysis, Academic Press, New York, 1983, p. 507.

[103] Inoue T, Fujishima A, Konishi S and Honda K. Nature 1979; 277: 637–638.

[104] Anpo M, Yamashita H and Schiavello M. (Eds.), Heterogeneous Photocatalysis,

Wiley, Chichester, New York, 1997, p. 133.

[105] Thampi KR, Kiwi J and Gratzel M. Nature 1998; 327: 506-508.

[106] Hirano K, Inoue K and Yatsu T. J Photochem Photobiol A 1992; 64: 255-258.

[107] Tseng I-H, Chang WC and Wu JCS. Applied Catalysis B Environmental 2002; 37: 37–

48.

[108] Yamashita H, Shiga A, Kawasaki S, Ichihashi Y. Ehara S and Anpo M, Energy

Convers Manage 1995; 36: 617-620.

[109] Hwang JS, Chang JS, Park SE, Ikeue K and Anpo M. Top Catal 2005; 35: 311-319.

[110] Tan S, Zou SL and Hu E, Catalysis Today 2006; 115: 269–273.

219

[111] Koci K, Mateju K, Obalova L, Krejcikova S, Lacny Z, Placha D, Capek L,

Hospodkova A and Solcova O. Applied Catalysis B: Environmental 2010; 96: 239–244.

[112] Xia X-H, Z-J. Jia, Yu Y, Liang Y, Wang Z and Ma L-L. Carbon, 2007; 45: 717–721.

[113] Wang Y, Wang Y and Gao Y. Reaction Kinetics Mechanics Catalysis 2010; 99: 485–

491.

[114] Luo D, Bi Y, Kan W, Zhang N and Hong S. Journal of Molecular Structure, 2011; 15:

20-30.

[115] Kasuga T, Hiramatsu M, Hoson A, Sekino T and Niihara K. Adv Mater 2011; 11:

1307–1311.

[116] Yang H, Zhang D and Wang L. Mater Lett 2007; 57: 674–678.

[117] Zhang Y. Chem Mater 2003; 15: 4028–4033.

[118] Zhang S, Chen Y, Yu Y, Wu H, Wang S, Zhu B, Huang W and Wu S, J Nanopart Res

2008; 10: 871–875.

[119] Sekino T, Okamoto T, Kasuga T, Kusunose T, Nakayama T and Niihara K. Key Eng

Mater 2006; 317: 251–254.

[120] Sahoo TR, Manoharan SS, Kurian S and Gajbhiye NS. Hyperfine Interact 2007; 188:

43–49.

[121] Suzdalev IP. Nanotechnol Russ 2010; 5: 223–226.

[122] Song XC, Zheng YF and Cao GS. Acta Phys-chim Sin 2005; 21: 1076–1081.

[123] Wang W, Zhang J, Huang H, Wu Z and Zhang Z. Colloid Surf A 2008; 317: 270–276.

[124] Hussain ST and Siddiqa A. Int J Environ Sci Tech 2011; 8: 351-362.

[125] Hussain ST, Siddiqa A, Siddiq M and Ali S. J Nanopart Res 2011; 13: 6517–6525.

[126] Liang H and Li HZ. Journal of Hazardous Materials 2009; 162: 1415–1422.

[127] Huogen Y, Jiaguo Y, Bei C and Jun L. Journal of Hazardous Materials 2007; 147:

581–587.

[128] Ma CM, Lin JY, Shiue RW and TC Chang. International Journal of Photoenergy,

2013; 107: 1-13.

[129] Kar A, York R. and Vaidyanathan S. Environ Sci Technol 2009; 43: 3260–3265.

[130] Varghese OK. Maggie P, Thomas J, La T and Craig AG Nano Lett 2012; 9: 731-737.

[131] Cullity BD. Elements of X-ray diffraction, Addison-Wesley publishing Company, Inc,

1956, p. 170.

220

[132] West AR. Solid State Chemistry and its Applications, John Wiley & Sons: Singapore,

2003, p. 157.

[133] WM M, Wendlandt HG. Reflectance Spectroscopy, Interscience Publishers/John Wiley

1966, p. 200.

[134] Ali M and Mohammad AB. Photochem Photobiol 2011; 87: 795–801

[135] Kaufmann, EN. Characterization of Materials, Jhon Wiley & Sons: New Jersey, 2003 p

50.

[136] Goldstein GI, Newbury DE, Echlin P, Joy DC, Fiori C and Lifshin E. Scanning

electron microscopy and x-ray microanalysis, New York: Plenum Press,2009 p. 320.

137] David B, Williams C and Barry Carter, Transmission Electron Microscopy;A Textbook

for Materials Science, Springer Science, Business Media, LLC, 2009 p. 978.

[138] Brydson, RMD, Sauer H and Engel W. Probing Materials Chemistry using ELNES in

Transmission Electron Energy Loss Spectrometry in Materials Science and the EELS Atlas

CC Ahn Wiley-VCH Weinheim Germany, 2004, p. 150.

[139] Chu WK, Mayer JW and Nicolet MA. Backscattering Spectrometry, Academic

Press, New York, 1978, p. 580.

[140] Brunauer S., Emmett PH and Teller E. J Am Chem Soc 1983; 5: 309-319.

[141] Lowell, Shields, Thomas and Thommes. Characterization of Porous Materials and

Powders: Surface Area, Pore Size and Density, Springer, 2004, p. 140.

[142] Nishikida K, Nishio E and Hannah RW. Selected applications of FT-IR techniques. Gordon and Breach. 1995, p. 240.

[143] Griffiths P and de Hasseth JA. Fourier Transform Infrared Spectrometry, Wiley-

Blackwell, 2007, 450.

[144] Harris and Bertolucci. Symmetry and Spectroscopy. Dover Publications, 1989, p. 560.

[145] Gardiner DJ. Practical Raman spectroscopy. Springer-Verlag, 1989, p. 978.

[146] ) Michael EB. Brown Introduction to Thermal Analysis: Techniques and Applications,

Kluwer Academic Publisher, 2002, p. 280.

[147] Zielenkiewicz EM. Theory of Calorimetry, Kluwer Academic Publisher, 2002,p. 353.

[148] Skoog; et al. Principles of Instrumental Analysis, Belmont, CA: Thomson Brooks/

Cole. 2007, p. 169.

221

[149] Misra, Prabhakar, Dubinskii, Mark (eds.). Ultraviolet Spectroscopy and UV Lasers.

New York: Marcel Dekker, 2002, p. 240.

[150] Hübschmann HJ. Handbook of GC/MS: Fundamentals and Applications, Completely

Revised and Updated Edition, John Wiley & Sons, 2008, p. 736.

[151] Hübschmann HJ. Handbook of GC/MS: Fundamentals and Applications, John Wiley &

Sons, 2009, p. 719.

[152] Einaga H, Ibusuki T and Futamura S. Environ Sci Technol 2004; 38: 285−289.

[153] Gao H, Lu B, Liu F, Liu Y. and Zhao X. International Journal of Photoenergy 2012;

9:12-15.

[154] Zhu W, Qiu X and Iancu V. Physical Review Letters 2009; 103: 22-25.

[155] Kansal SK, Ali AH and Kapoor S. Desalination 2010; 259:147-155.

[156] Hussain ST, Mazhar M, Siddiqa A, Javaid H and Siddiq M. The Open Catalysis

Journal 2012; 5: 21-30.

[157] Yang J, Dai J and Li J. Applied Surface Science 2011; 257: 21-26.

[158] Hamal DB and Klabunde KJ. J Colloid Interface Sci 2007; 311: 514-522.

[159] Rungnapa T and Kongthip S. J Sci Technol 2008; 30: 201-206.

[160] Colon G, Maicu M, Hidalgo MC and Navio JA. J Appl Catal B 2010; 67: 41-45.

[161] Hussain ST, Khan K and Hussain R. J Nat Gas Chem 2009; 18: 383-391.

[162] Ohno T, Akiyoshi M, Umebayashi T, Asai K, Mitsui T and Matsumura M. J Appl

Catal A Gen 2004; 265: 115-121.

[163] Samokhvalov A, Nair S, Duin EC and Tatarchuk BJ. Appl Surf Sci 2010; 256: 3647-

3652.

[164] Moon J. Catal Today 2003; 87: 77-86.

[165] Iliev V, Tomova D and Rakovsky S. Desalination 2010; 260: 101-106.

[166] Liga MV, Bryant EL, Colvin VL and Li Q. Water Res. 2011; 45: 535-544.

[167] Jin S and Shiraishi F. Chem Engineer J 2004; 97: 203-211.

[168] Syoufian A, Oktaviano HS and Nakashima K. Catal Commun 2007; 8: 759-765.

[169] Xu MW, Bao SJ and Zhang XG, Mater Lett 2010; 59: 2194-2198.

[170] Liu SM, Gan LM, Liu LH, Zhang WD and Zeng HC, Chem Mater 2002; 14: 1391-

1397.

[171] Hou LR, Yuan CZ and Peng Y. J Hazard Mater B 2009; 139: 310-315.

222

[172] Zhang M, Jin ZS, Yung JJ and Zhang ZJ. J Molec Catal A: Chem 2012; 217: 203-210.

[173] Kubo T, Yamasaki Y, Honma T, Umesaki N and Nakahira A. J Mat Onlin 2006; 2: 1-

6.

[174] Tongpool R, Chaleawlert S and Yoriya S. J Nat Sci 2011; 6: 355-360.

[175] Quan X, Yang SG, Ruan XL and Zhao HM. Environ Sic Technol 2005; 39: 3770-3775.

[176] Mohapatra SK, Misra M, Mahajan VK and Raja KS. J Catal 2009; 246: 362--369.

[177] Macak JM, Tsuchiya H, Ghicov A, Yasuda K, Hahn R, Bauer S and Schmuki P. Curr

Opin Mat Sci 2010; 11: 3-18.

[178] Cho JM, Seo JM, Lee JK, Zhang H and Lamb R. Phys B: Cond Matt 2009; 404: 127-

130.

[179] Sun X and Li Y. Chem A; Eur J 2009; 9: 2229–2238.

[180] Yoon-Chae N, Indhumati P and Patrik S. ChemPhysChem 2010; 11: 2698–2713.

223

5. LIST OF PUBLICATIONS

INTERNATIONAL PUBLICATIONS FROM CURRENT STUDY

1. Iron-doped titanium dioxide nanostructures: a study of electrical, optical, and magnetic

properties, ST Hussain, Asima Siddiqa, M.Siddiq, Salamat Ali, J Nanopart Res., 2011,

13, 6517–6525.

2. Cu loaded S-TiO2 for reduction of phenol and Carbon dioxide under visible irradiations,

Syed Tajammul Hussain, Asima Siddiqa, M.Siddiq Hina Javed, Open Journal of

Catalysis. 2012, 5, 21-30.

3. Iron and chromium doped titanium dioxide nanotubes for the degradation of

environmental and industrial pollutants, S.T Hussain, Asima Siddiqa, “ Int. J. Environ.

Sci. Tech., 2011, 8 , 351-362.

4. Co-S co-doped titanium dioxide nanostructures for reduction of carbon dioxide into value

added products, Asima Siddiqa, Muhammad Siddiq, International Journal of Science and

Technology (Submitted).

5. Ru-S co-doped based novel Nanomaterial for Reduction of Greenhouse Gas and Toxic

Chemical Into Value Added Products Muhammad Siddiq, Asima Siddiqa, Materials

Chemistry (Submitted).

OTHER INTERNATIONAL PUBLICATIONS

1. The effect of carbidic/graphitic carbon on the surface reactivity of the alumina supported

nano bimetallic catalyst. Syed Tajammul Hussain, Asima Siddiqa, Der Pharma Chemica,

2010, 4, 264-270.

2. XPS, SSIMS, TGA, FTIR, SEM studies of catalytic pyrolysis of methane over Ni:Cu/Al

and Ni:Cu:K/Al modified supported nano catalysts, Syed Tajammul Hussain, Asima

Siddiqa, 2011, 333-344.

224

3. Spectrophotometric analysis of flavonoid–DNA binding interactions at physiological

conditions, Naveed Kausar Janjua, Asima Siddiqa, Romana Qureshi, Spectrochimica

Acta Part A: Molecular and Biomolecular Spectroscopy, 2011, 74, 1135-1137.

4. Syed Tajammul Hussain, Asima Siddiqa, Voltammetric and Viscometric Studies of

Flavonoids Interactions with DNA at Physiological Conditions, European Journal of

Chemistry,2011, 2, 109-112.

5. Syed Tajammul Hussain, Naeem Shahzad, Asima Siddiqa, and Muhammad Anwar Baig,

Sulphur Reduction Using the S-Doped TiO2 nanaoparticles and TiO2 nanotubes, J.

Nanosci. Nanotechnol., 2012, 12, 5061–5065.

6. Catalytic Conversion of Diesel Exhaust, S. T. Hussain, Asima Siddiqa, Almas Hamid,

Nida Zahir, International Review of Chemical Engineering, 2012, 4, 280-288.

7. Synthesis, Characterization and Enhanced photocatalytic Activity of ZnO based

Nanocomposites, S. T. Hussain, Asima Siddiqa, Almas Hamid, Maria Aftab, Munir

Aslam, accepted, International Review of Chemical Engineering, 2012, 4, 430-435.

8. Cu, Ag, Ru loaded N-TiO2 for reduction of organic dyes under visible irradiations Syed

Tajammul Hussain, Asima Siddiqa, Hira Qureshi, Bukhtiyar Muhammad, Muhammad

Siddique , International Review of Chemical Engineering, 2012, 4, 550-553.

9. Synthesis of visible light driven cobalt tailored Ag2O/TiON nanophotocatalyst by

reverse micelle processing for degradation of Eriochrome Black T, S.T Hussain, Rashid,

Dalaver Anjum, Asima Siddiqa, Amin Badshah, Materials Research Bulletin,2013, 48,

705–714.

10. SiO2-TiO2 nanocomposites for degradation of organic dyes under visible irradiations

Asima Siddiqa, Sumbal Sabir, Syed Tajammul Hussain, European Journal of Chemistry,

2013, 4, 388⁷ 395.

11. Synthesis and Characterization of Doped Nano Heat Transfer Fluid for Concentrated

Solar Power Plant, Asima Siddiqa, Huma Naeed, Shahid Naseer, Rohama Gill, Energy

Efficiency (Submitted).

prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/2484/1/2565S.pdfDeclaration This is to...

Documents

Transcript of prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/2484/1/2565S.pdfDeclaration This is to...