CADMIUM SULPHIDE CHAPTER 3...
Transcript of CADMIUM SULPHIDE CHAPTER 3...
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CHAPTER 3
CADMIUM SULPHIDE NANOPARTICLES …………………………………………………………………………………………
3.1. INTRODUCTION:
Cadmium sulphide (CdS) is a II-VI semiconductor which is insoluble in water, but
soluble in dilute mineral acids. It exhibits intrinsic n-type of conductivity caused by
sulphur vacancies due to excess cadmium atoms1. CdS in bulk has band gap energy of
2.42eV at 300K with absorption maxima at 515nm2,3. It can attain three types of
crystal structures namely wurtzite, zinc blend and high pressure rock-salt phase
(Figure 3.1). Among these, wurtzite is the most stable of the three phases and can be
easily synthesized. Wurtzite phase have been observed in both the bulk and
nanocrystalline CdS while cubic and rock-salt phases are observed only in
nanocrystalline CdS4,5. The wurtzite form comprises of hexagonal close packing (hcp)
in which the stacking sequence of the atoms is ABABAB…, while, the zincblende
and rock salt structure have the stacking sequence of the atoms as ABCABCA…, i.e.,
called cubic close packing (ccp). In hexagonal wurtzite and cubic zinc blend, each
atom is coordinated to four other atoms in tetrahedral fashion such that each atom has
four neighboring atoms of the opposite type1, whereas in rock-salt each atom is
coordinated to six other atoms in octahedral fashion such that each atom has six
neighboring atoms of the opposite kind.
Figure 3.1: A representative diagram for the unit cell for crystal structure of CdS,
showing (a) wurtzite (hcp), (b) zinc blend (ccp) and (c) rock salt (ccp) phases.
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The nanoparticles of CdS show unique physical, chemical and structural properties
from the bulk. The melting point, electronic absorption spectra, band gap energy
crystal structure, and other properties of cadmium sulphide nanoparticles (CdS-NP)
are affected by size6-8. Thus, CdS on the whole is an attractive system for practicing
synthetic chemistry for nanocrystals and for understanding the chemistry, growth
history of nanomaterials and also for various technical applications9,10. Colloidal
dispersions of CdS semiconductor nanoparticles can display spectacular color changes
of their fluorescence depending on the size of the particle7. The CdS nanoparticles
shows quantum size effect, due to which the size of the cadmium sulphide particles is
directly related to the absorption wavelength9,11. The structure of the nanocrystalline
CdS can play an important role in determining the electronic properties. It can
crystallize in different structures upon size reduction, depending upon the reaction
conditions4.
Due to high stability, excellent physical, chemical and structural properties,
availability, ease of preparation and handling, CdS nanomaterials can be exploited in
various fields of life. Owing to quantum size effects and surface effects, CdS
nanoparticles can display novel optical, electronic, magnetic, chemical and structural
properties that might find many important technological applications. In addition to
size/volume ratio, the distribution of atoms over the surface is found to be a key
component of CdS semiconductor electrodes12. CdS-NPs are also used as pigment in
paints and in engineered plastic due to their good thermal stability1,13. CdS have large
band gap energy of 2.42eV at room temperature that enables its nanoparticles to be
remarkable in optoelectronics, photonics, photovoltaics and photocatalysis. Due to
photon-induced conduction, 1D CdS nanoparticles can be used in optoelectronics for
making photocells, light emitting diode (LED)14, lasers15, field-effect transistors
(FETs)16 and address decoders17. In photonics, due to its photoconducting and
electrical properties can be used in sensors, photodetectors, optical filters, and all
optical switches4,18-21. It exhibits high photosensitivity and its band gap appears in the
visible spectrum22, enabling it to be useful for many commercial and potential
applications in photovoltaics, as hetero-junction solar cells and thin film solar
cells4,21,23,24. In photocatalysis25, owing to its photochemical and catalytic properties,
CdS nanoparticles can be used for water splitting (hydrogen production)26,27 as well as
for water and air purification13,28. CdS NP can be used for the diagnosis and treatment
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of cancer due to its high optical and fluorescence properties. Diagnosis or imaging of
cancer cells can be done by accumulating CdS nanoparticles inside cancer cells,
which then can be easily visualized by irradiated with ultra violet radiation. For
treatment of cancer, photo activation of fluorescent CdS nanoparticles (photodynamic
cancer therapy) accumulated within cancer cell with radio sensitizing agents could
induce cell death29,30. In ophthalmology the CdS nanoparticles can be used for the
purposes of visualization as well as for drug delivery to the tissues of the eye,
including retina and cornea31.
Due to wide range of applications in different fields of life, CdS nanomaterials have
been synthesized extensively. Various techniques have been applied to fabricate CdS
in the form of thin films or powder, such as RF-magnetron sputtering technique32,
physical evaporation33, thermal evaporation34, hydrothermal synthesis35,36, electron
beam vacuum evaporation technique37, electrodeposition38, physical vapor deposition
(PVD)39, pulsed laser deposition40, laser ablation method41, spin-coating technique14,
solvothermal method42, template synthesis43, chemical bath deposition (CBD)44,
chemical precipitation method45, chemical vapor deposition (CVD)46, simulating
biomineralization technique13, biological synthesis using bacteria, fungi, yeast etc30.
Out of these synthetic methods, Chemical precipitation method is considered to be the
most appropriate due to its ease and simplicity. The chemical method usually required
simple lab equipments, ambient environmental conditions and the experiment usually
complete within hours, whereas other methods often required sophisticated
equipments, extreme environmental conditions (temperature, pressure etc.) and large
time interval. The particle sizes and stability are controlled either by restricting the
reaction space within matrices viz., zeolites, glasses, silica, polymers, reverse
micelles, vesicles and LB films32,42,47, or by using stabilizers and capping agents, like
thiols, phosphates, phosphine oxides, mercaptoacetic acid, long chain alkyl xanthates,
thiourea, and thioglycerol10,27. Another factor affecting the particle size of the CdS
nanoparticle is the solvent. Solvents are known to affect the kinetics and equilibria of
synthesis reactions, the spectroscopic properties of solutes and even the facets present
on crystals47-50.
Synthetic textile dyes and other industrial dyestuffs are one of the largest groups of
water pollutants in the world because of their displeasing, noxious, mutagenic,
consistent nature13,51-53. The photocatalytic degradation is one of the most efficient
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and economical method for non-destructive physical water treatment processes as
most of the conventional physicochemical and biological treatment methods being
inadequate for their effective removal51,54-58. In the last decade, photocatalytic
degradation using semiconductors have been shown to be effective for degradation of
pollutants in water. Several semiconductors such as ZnO, ZnS, TiO2 and Fe2O3 have
been used for heterogeneous photocatalytic degradation of organic wastes in
water25,58,59. CdS is a significant visible-light-sensitive semiconductor, which makes it
possible to utilize solar energy efficiently. CdS have been extensively used for
photocatalytic splitting of water for hydrogen production60 and for photodegradation
of organic or inorganic pollutants in air and water61 under visible light (VL).
Specifically, these materials have a relatively narrow band gap with the conduction
band edge sufficiently more negative than the reduction potential of protons, and thus
can efficiently absorb visible light62,63. However, the reported quantum efficiency of
the photocatalytic reactions by CdS is fairly poor due to the fast recombination of
photo-generated charge carriers. Various attempts to improve the efficiency of the
photocatalytic activity of CdS include changing the surface structure of CdS
nanoparticles by controlling morphology (size and structure)64, depositing CdS to
Nafion membranes or polymers to get homogeneously distributed quantum sized CdS
nanoparticles13,51,65, doping of transition metal ions into CdS28, and coupling of two
semiconductors66-68. Recently, CNTs (Carbon Nano tubes) decorated with CdS
nanoparticles and nanowires have been reported69,70.
In the present work, CdS nanoparticles were synthesized by six different
combinations of chemical precursors using Hydrogen Sulphide (H2S), Sodium
Sulphide (Na2S) and Ammonium Sulphide ((NH4)2S) as source of (Sulphide) S2- ions.
CdS nanoparticles were grown by simple chemical precipitation reactions in aqueous
medium at room temperature. The effect of stabilizers on the stability and size of CdS
nanoparticles was studied. The effect of different S2- ion sources ((NH4)2S, H2S and
Na2S) on the size of nanoparticles, respective band gaps and crystalline structure were
studied. Finally, the series of synthesized nanoparticles were exploited for the
degradation of Acid Blue-29 (AB-29), under visible light. The photocatalytic
efficiency of the synthesized nanoparticles, using different reactant combinations,
were compared and optimized.
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3.2. MATERIALS AND METHODS:
3.2.1. Synthesis:
CdS nanoparticles were grown by simple chemical precipitation reactions in aqueous
medium at room temperature and pressure. All chemicals were of analytical grade and
used as received without further purification. Six different reaction combinations were
set to synthesize CdS nanoparticles. A stock solution of Cadmium Nitrate (Cd(NO3)2)
(0.085M) was prepared and six different syntheses reactions were performed using
(NH4)2S, H2S and Na2S as S2- ion sources in presence and absence of stabilizing
agents. The reactions are summarized as below:
Reaction 1 (R1): Synthesis of CdS nanoparticles using Ammonium sulphide:
100mL aqueous solution of Cd(NO3)2 (0.085M) was added drop wise to 100 mL
aqueous solution of (NH4)2S (0.1M) with vigorous stirring. Stirring was continued for
5 hours. The dark yellow precipitates of CdS nanoparticles were obtained. This
reaction was similar to the reaction performed by P.P. Favero et al. 200612.
Reaction 2 (R2): Synthesis of CdS nanoparticles using Ammonium sulphide and
1-thioglycerol:
100mL aqueous solution of Cd(NO3)2 (0.085M) solution was stirred vigorously for 10
minutes. 1.6mL of 1-thio glycerol (98%) (0.18M) was added drop wise into the
solution with continuous stirring for 30 minutes. 20mL of (NH4)2S (20%) (0.5M) was
added to the solution under ambient conditions and the stirring was continued for
additional 5 hours, which yielded a dark yellow solution.
Reaction 3 (R3): Synthesis of CdS nanoparticles using Hydrogen Sulphide:
100mL Cd(NO3)2 (0.085M) solution was kept in H2S atmosphere for 1 minute with
vigorous stirring for additional 5 hours upon which the solution turned transparent to
yellow.
Reaction 4 (R4): Synthesis of CdS nanoparticles using Hydrogen Sulphide and
Methanol:
50mL Methanol (CH3OH) (24.44M) was added to a 100mL Cd(NO3)2 (0.085M) drop
wise with continuous stirring. The reaction was then carried out in H2S atmosphere
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for 1 minute with vigorous stirring continued for additional 5 hours upon which the
solution turned transparent to light yellow.
Reaction 5 (R5): Synthesis of CdS nanoparticles using Sodium Sulphide:
100mL aqueous solution of Na2S (0.1M) was added drop wise to a 100mL Cd(NO3)2
(0.085M) with continuous stirring for additional 5 hours. As the formation of
nanoparticles started the reaction system gradually changed from transparent to light
yellow. This reaction is similar to the reaction reported by V. Singh et al. 200971.
Reaction 6 (R6): Synthesis of CdS nanoparticles using Sodium Sulphide, Sodium
Hydroxide and Methanol:
100mL aqueous solution of Sodium Hydroxide (NaOH) (0.1M) and 50mL Methanol
(MeOH) (24.44M) was added slowly to 100mL aqueous solution of Cd(NO3)2
(0.085M) with continuous stirring continued for 1/2 hour. To this a 100mL aqueous
solution of Na2S (0.1M) was added drop-wise with vigorous stirring continued for
additional 5 hours, and a green-yellow solution was obtained. This reaction is similar
to the reaction reported earlier72. The precipitates thus obtained from the above
reactions were washed 3-4 times with water and acetone (used as non solvent) and
were air dried.
3.2.2. Characterization:
The functional and elemental analyses were carried out by Fourier Transform Infrared
Spectroscopy (FTIR) Spectroscopy and Energy Dispersive X-ray Spectroscopy
(EDS). The structural and morphological properties were studied by X-Ray
Diffraction (XRD) Spectroscopy, Scanning Electron Microscopy (SEM) and
Transmission Electron Microscopy (TEM). Thermal properties were determined by
Thermal Gravimetric Analysis (TGA), Differential Thermogravimetry (DTG) and
Differential Thermal Analysis (DTA) while the Optical properties were determined by
employing UV-Visible Spectroscopy.
3.2.3. Photocatalytic Experiment:
The photocatalytic activity of the CdS nanoparticles was studied by studying the
decolorization of a derivative Acid Blue-29 (AB-29) in presence of visible light.
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The photocatalytic experiments were performed in an immersion well photoreactor
made of Pyrex glass consisting of inner and outer jacket and equipped with a
magnetic bar, a water circulating jacket and an opening for molecular oxygen.
Irradiation was carried out using 500W halogen liner lamp (9500Lumens). The
catalyst dosage was optimized by irradiating the aqueous solution of the dye with
different strengths of CdS catalyst. 180mL of the dye solution of desired
concentration (0.06mM) containing the appropriate quantity of the catalyst (1gL-1)
was magnetically stirred in dark, in presence of atmospheric oxygen for at least 20
minutes to attain adsorption–desorption equilibrium between dye and catalyst surface.
A 5mL blank sample (0 minute) was taken out prior starting the irradiation. Other
samples (5mL) were collected at regular intervals during the irradiation and analyzed
after centrifugation. The suspensions were continuously purged with molecular
oxygen throughout each experiment and a constant temperature (20±0.3°C) was
maintained using refrigerated circulating liquid bath. The decolorization of AB-29
was monitored by the change in absorption spectroscopy using UV-vis. spectroscopic
analysis technique (Shimadzu UV-Vis 1601). The concentration of dye was calculated
by standard calibration curve obtained from the absorbance of the dye at different
known concentrations. For the purpose of practical implementation, it is essential to
evaluate the stability and reuse of the catalyst. The photocatalytic performances of the
nanomaterials were studied for five consecutive cycles using the same portion of
catalyst nanomaterials and a fresh solution of dye sample every time under similar
conditions.
3.3. RESULTS AND DISCUSSION:
The CdS nanoparticles obtained showed color variation from dark yellow to green-
yellow. This change in color from higher wavelength to shorter wavelength (blue
shift) might be due to the decrease in particle size7. Figure 3.2 shows the synthesized
CdS nanoparticles obtained as such in suspension form, and Figure 3.3 shows the
CdS NPs obtained after washing and drying. The effect of different S2- ions source
and the presence or absence of stabilizers are summarized in Table 3.1.
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Figure 3.2: The synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6)
obtained as such in suspension form.
Figure 3.3: The synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6)
obtained after washing and drying.
Table 3.1: The effect of different sulphide ion sources on synthesized CdS
nanoparticles (R1, R2, R3, R4, R5 and R6).
S.no. S2-
ion source Stabilizing agent Agglomeration Average Particle size
R1 (NH4)2S Absent Present 10.0nm
R2 (NH4)2S Present Absent 9.0nm
R3 H2S Absent Present 6.5nm
R4 H2S Present Absent 6.0nm
R5 Na2S Absent Present 5.0nm
R6 Na2S Present Absent 4.5nm
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3.3.1. ELEMENTAL ANALYSES:
The elemental analyses and determination of composition of synthesized CdS NPs
were carried out by Fourier Transform Infrared Spectroscopy (FTIR) Spectroscopy
and Energy Dispersive X-ray Spectroscopy (EDS).
3.3.1.1. Fourier Transform Infrared (FTIR) Spectroscopy:
FTIR is used to study the purity and composition of the synthesized products. It is
used to determine the functional groups and types of bonds present in the system. The
dried CdS nanoparticles mixed with KBr were characterized with FTIR. The FTIR
spectra could be explained by various peaks (Figure 3.4) obtained by the sample.
Table 3.2 contains the explanation of the peaks obtained by all the synthesized CdS
nanoparticles73,74. The absorption peak in the range of 3600-3100cm-1 could be
attributed to the –OH group of water adsorbed by the samples. The weak absorption
band at 1635cm-1 was assigned to CO2 adsorbed on the surface of the particles. In
fact, adsorption of water and CO2 are common for all powdered samples exposed to
atmosphere and are even more pronounced in case of nanosized particles with high
surface area9. Small peak near 400-470cm-1 indicated the formation of CdS
nanoparticles as this region was assigned to metal-sulphur (M-S) bond73-75. The peak
at 405cm-1 corresponded to the characteristic peak of CdS76-78.
Table 3.2: Interpretation of the peaks obtained by the FTIR spectra of the synthesized
CdS nanoparticles (R1, R2, R3, R4, R5 and R6).
Peak Region Intensity Significance
A 400-410 Small and weak Cd-S bond (CdS nanoparticles)
B 570-620 Small and weak S-S bond (crystal S-S bond)
C 820-850 Sharp S-S-S bending or C-H stretching
D 1060-1120 Sharp C-O or S-O (acetone or sulphate)
E 1380-1420 Sharp or Broad C-H bending of CH3 (Acetone)
F 2340-2360 Small and weak S-H bond (Free H2S)
G 3140-3470 Broad Intermolecular H-bonds (Lattice water)
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Figure 3.4: The FTIR spectra of
R5 and R6).
3.3.1.2. Energy Dispersive X-ray
Figure 3.5 reveals the EDS spec
and R6), the presence of Cd and
no other elemental impurity. T
55.5:44.5 for R1, 54.0:46.0 for R
R5 and 56.5:43.5 for R6. Other
and silicate, were due to sputter
not considered in elemental analy
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of the synthesized CdS nanoparticles (R1, R2, R3
ay Spectroscopy (EDS):
ectra of the synthesized CdS NP (R1, R2, R3, R
nd S peaks confirmed the formation of pure CdS
The average atomic percentage ratio of Cd:S
R2, 52.5:47.5 for R3, 55.0:45.0 for R4, 51.5:48
er peaks in this figure corresponded to carbon, ox
er coating of glass substrate on the EDS stage and
alysis of Cd and S.
R3, R4,
R4, R5
S with
:S were
48.5 for
oxygen
nd were
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Figure 3.5: The EDS spectra of synthesized CdS nanoparticles (R1, R2, R3, R4, R5
and R6).
3.3.2. STRUCTURAL ANALYSES:
The structural and morphological properties were determined by diffraction studies
(using X-Ray Diffraction (XRD) Spectroscopy) and microscopic studies using
Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy
(TEM).
3.3.2.1. Diffraction Studies:
The Diffraction Studies were carried out using X-Ray Diffraction (XRD)
Spectrometer.
X-Ray Diffraction (XRD) Spectrometer:
The XRD data revealed the formation of hexagonal-wurtzite type and cubic-zinc
blend type structured CdS NP (R1, R2, R3, R4, R5 and R6). The XRD pattern
displayed in Figure 3.6 showed that the crystal structure changed from hexagonal to
cubic with the decrease in particle size. The XRD pattern for CdS NP R1 (Figure 3.6)
can be consistently indexed on the basis of the hexagonal, W-type structure1,45,71 in
which the six prominent lines correspond to the reflections at 2�=25.182˚ (100),
26.816˚ (002), 28.465˚ (101), 37.372˚ (102), 47.206˚ (103) and 51.534˚ (112). The
weak 43.938˚ (110) peak was also observed. The peaks at 2��37˚ and 47˚ are
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characteristic for hexagonal W-type structure33,42,71. The estimated crystallite size for
R1 was ~9nm based on the FWHM of the (101) peak.
Similarly the XRD spectrum from R2 exhibited peaks at 2�=24.563˚ (100), 26.359˚
(002), 28.668˚ (101), 37.077˚ (102), 43.879˚ (110) and 51.063˚ (112) corresponding to
hexagonal, W-type structure. The estimated X-ray size for R2 (based on the FWHM
of the (101) peak) was ~7nm.
The XRD spectrum from R3 and R4 apparently exhibited only three broad peaks,
centered at 2��27˚, 43˚ and 51˚. The main broad peak at 27˚ on close observation was
found to be an overlap of multiple peaks, comprising of shoulders on both the sides at
2�� 24˚ and 28˚, respectively, resulting from the overlap of (100), (002) and (101)
peaks of hexagonal W-type structure. The increase in overlap in R3 and R4 was
clearly a result of line broadening due to the smaller particle size in these samples as
compared to R1 and R2. However, the three most prominent peaks for cubic CdS
with Z-type structure also occur at 27˚ (111), 43˚ (220) and 51˚ (311). Thus, the
presence of cubic CdS could not be ruled out based on XRD data. Therefore, it can be
concluded that R3 and R4 exhibited pronounced features of both phases and had a
distorted structure resulting due to the partial contents of both the phases6,79. The
estimation of the mean crystallite size was not possible for R3 and R4 due to the peak
overlap as well as the possible presence of a mixture of cubic and hexagonal phases.
The XRD pattern for R5, also exhibited three broad peaks centered at 2��27˚, 43˚ and
51˚ (Figure 3.6), but there were two main differences between the diffraction patterns
of R5 and R3/R4. In R5, the width of the diffraction peak at 27˚ was significantly
smaller, and it was much more symmetric. These observations indicated that the 27˚
peak in R5 is a single peak and not an overlap of multiple peaks. Thus, the XRD
peaks for R5 can be identified as 2�=26.864˚ (111), 43.281˚ (220), 51.403˚ (311)
peaks for cubic Z-type structure9,27,45,51. The mean crystallite size of R5, calculated
from the FWHM of the peak 26.864˚ (111) was ~5nm.
The nature of XRD pattern for R6 was similar to R5, but the widths of all the three
peaks were significantly larger. The mean particles size for R6 was ~4nm. Table 3.3
lists the crystal structures and mean particle size derived from the X-ray diffraction
data, for the different samples of CdS nanoparticles.
Figure 3.6: XRD spectra
R6).
Table 3.3: The crystalline
CdS
Nanoparticles
R1
R2
R3 Hex
R4 Hex
R5
R6
3.3.2.2. Microscopic Stu
Microscopic Studies we
Transmission Electron M
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tra of synthesized CdS nanoparticles (R1, R2, R3
ine phase and average crystallite size obtained by
Crystalline
Phase
FWHM (in
degree)
Crys
Hexagonal 0.9037
Hexagonal 1.12
exagonal +cubic -
exagonal +cubic -
Cubic 1.74
Cubic 2.0145
tudies:
ere done using Scanning Electron Microscope
Microscope (TEM).
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3, R4, R5 and
y XRD data.
ystallite size
(nm)
9.068
7.320
-
-
4.693
4.053
pe (SEM) and
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Scanning Electron Microscopy (SEM):
The SEM micrographs showed amorphous mass with very fine particle structure. A
decrease in porosity was observed indicating reduction of particle size. Definite
particle shape was not visible due to more of a fine amorphous powder. Figure 3.7
shows the SEM images of the synthesized CdS NP at 5000 times magnification (5kx).
Figure 3.7: SEM images of the CdS nanoparticles (R1, R2, R3, R4, R5 and R6) at
5000x magnification.
Transmission Electron Microscopy (TEM):
Figure 3.8, displays the TEM images of the CdS-NP showing spherical quantum dots
with particle size less than 10nm. TEM images revealed that smaller sized NP were
formed when H2S (R3 and R4) was used as a source of sulphide ions instead of
(NH4)2S (R1 and R2), while Na2S (R5 and R6) gave the smallest sized NP. The
reason could be that, (NH4)2S is less active source of S2- ions than H2S and liberates
S2- ions less readily, and H2S liberates S2- ions even with lesser ease than Na2S47. This
means that the rate of precipitation has direct effect on the particle size. Hence, in
case of Na2S, the particle sizes were smallest of all. Also, the presence of stabilizers
showed a reduction in the aggregation and coagulation of NP (R2, R4 and R6). The
particle sizes obtained from the TEM images are listed in Table 3.4.
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Figure 3.8: TEM images of synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and
R6).
Table 3.4: Particle size of synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and
R6) obtained by TEM.
Sample R1 R2 R3 R4 R5 R6
Particle Size 10.0nm 9.0nm 6.0nm 6.5nm 5.0nm 4.5nm
3.3.3. THERMAL ANALYSES:
The thermal studies were done by using Thermal Gravimetric Analysis (TGA),
Differential Thermogravimetry (DTG) and Differential Thermal Analysis (DTA). The
TGA, DTA and DTG curves revealed high thermal stability of the synthesized
nanoparticles (R1, R2, R3, R4, R5 and R6), with high melting point and absence of
any impurity or intermediate complex. The synthesized CdS nanoparticles were found
to be thermally stable upto temperature as high as 700°C. Thereafter a gradual weight
loss was observed.
Thermal Gravimetric Analysis (TGA):
Figure 3.9 shows the TGA thermograph of synthesized CdS nanoparticles. CdS
nanoparticles showed good thermal stability upto 700°C and thus, can be used as
pigment in paints and in engineered plastic.
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Figure 3.9: The TGA spectra of
and R6) showing thermal stabilit
Differential Thermogravimetry (
Figure 3.10 shows the DTG cur
with time against the temperature
compounds were found to be ab
700°C to 900°C due to phase cha
Figure 3.10: The DTG curve for
R5 and R6).
Differential Thermal Analysis (D
The DTA curve (Figure 3.11) sh
to thermal decomposition.
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of synthesized CdS nanoparticles (R1, R2, R3, R
lity upto 700°C temperature.
y (DTG):
urve, obtained by plotting the rate of change of w
ure and the result revealed that the melting points
above 1000°C with a slight weight loss in the r
hange.
for the synthesized CdS nanoparticles (R1, R2, R3
(DTA):
showed endothermic arrest leading to weight los
R4, R5
f weight
ts of the
e region
R3, R4,
loss due
Figure 3.11: The DTA cu
R5 and R6) showing endo
3.3.4. OPTICAL ANALY
Optical properties were d
UV-Visible (UV-Vis.) Spe
The UV-vis. spectroscop
range 200 to 800nm of s
515nm while in prepared
wavelengths smaller than
This blue shift was in go
to lower wavelength (blu
the synthesized CdS nan
(Figure 3.12). Figure 3
nanoparticles in the ran
nanoparticles R2 showed
bulk CdS (515nm), this
stabilizer preventing furt
(465nm and 445nm respe
reason that H2S is a mo
absorption edge (445nm)
methanol which acted as
blue shift (445nm) than R
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Nida Qutub, Ph.D. Thesis 2013, A.M.U., Ind
curve for the synthesized CdS nanoparticles (R1
dothermic peaks.
YSIS:
determined by employing UV-Visible Spectrosco
pectroscopy:
opy gave the absorption spectra of the nanoma
f solar spectrum. The absorption edge in bulk Cd
red CdS nanoparticles the absorption edges were
an the bulk which indicated a blue shift in abs
good agreement with the results reported before4,
lue shift) in absorption spectra than the bulk was
anoparticles as decrease in the particle sizes w
3.13, displays the absorption spectra of synt
range 400nm to 600nm of the visible spectr
ed greater blue shift (470nm) than R1 (475nm) as
is was due to the presence of thio-glycerol whic
urther crystal growth. R3 and R4 showed great
spectively) in the absorption edge than R1 and R
ore active source of S2- ion than (NH4)2S. R4
m) than R3 (465nm). This might be due to the
as a stabilizing agent. Similarly, R5 (425nm) sh
R4, as Na2S is more active source of S2- ion tha
India���� 76
1, R2, R3, R4,
scopy.
aterials in the
CdS is found at
ere observed at
bsorption edge. ,11. Also, shift
as observed in
were observed
nthesized CdS
ctra. The CdS
as compared to
hich acted as a
eater blue shift
R2 due to the
R4 had smaller
the presence of
showed greater
han H2S47. And
Chapter 3
77 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�
�
finally, R6 (415nm) showed maximum blue shift due to the presence of stabilizing
agent (NaOH and MeOH) and Na2S as a source of S2- ions.
Figure 3.12: UV-visible absorption spectrum of synthesized CdS nanoparticles (R1,
R2, R3, R4, R5 and R6) showing blue shift in absorption edge.
Figure 3.13: UV-visible absorption spectrum of synthesized CdS nanoparticles (R1,
R2, R3, R4, R5 and R6) showing absorption edge.
Based on Tauc relation1,80, Figure 3.14, shows the plot of (�h�)2 versus h�, whose
intercept on energy axis gave the band gap energy (Eg) of the nanoparticles. The plot
showed a shift towards higher band gap. Figure 3.15 displays the obtained band gap
energy of the nanoparticles which are listed in Table 3.5.
Figure 3.14: The band g
towards higher band gap e
Figure 3.15: The band ga
R4, R5 and R6).
The particle radius (R) o
Brus Equation81-83 keepin
for CdS11,71, and Eg=(as o
Brus equation and band g
observed that the values
Chapter 3
Nida Qutub, Ph.D. Thesis 2013, A.M.U., Ind
d gap energy curve based on Tauc relation show
p energy of CdS NPs (R1, R2, R3, R4, R5 and R
gap energy of the synthesized CdS nanoparticles
of the synthesized CdS nanoparticles were cal
ping Eg0=2.42eV (bulk CdS), me=1.73x10-19, mh
observed, Table 3.5). The particle sizes (2R) cal
d gap energy are listed in Table 3.5. Form this ta
es of the band gap of synthesized CdS nanop
India���� 78
owing the shift
R6).
es (R1, R2, R3,
calculated from
h=7.29 x10-19,
calculated using
table it can be
oparticles were
Chapter 3
79 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�
�
higher than the bulk band gap of CdS (2.42eV)2,3 and the band gap energy became
larger with the decrease in particle size. Thus, it can be concluded that the synthesized
CdS nanoparticles followed the quantum confinement effect84,85.
Table 3.5: The band gap energy and particle size (2R) of synthesized CdS
nanoparticles (R1, R2, R3, R4, R5 and R6) obtained by absorption spectra.
CdS nanoparticles R1 R2 R3 R4 R5 R6
Absorption wavelength (nm) 475 470 465 445 425 415
Band gap (eV) 2.61 2.64 2.70 2.8 2.93 3.0
Particle size (nm) 10.18 8.06 7.04 5.16 4.26 3.96
3.3.5. PHOTOCATALYTIC PROPERTIES:
The photocatalytic activity of the synthesized CdS nanoparticles (R1, R2, R3, R4, R5
and R6) was studied by photo-degradation experiment of a dye derivative AB-29 in
presence of visible light. The blank experiments were also separately carried out by
irradiating the aqueous solution of the dye derivative in absence of the photocatalyst
and in presence of the photocatalyst under dark condition. Analysis of the samples in
both cases did not show any appreciable loss of the dye (Figure 3.16). In order to
attain the superior photocatalytic activities of CdS nanoparticles, the activity of all the
six samples were compared. The effect of catalyst, synthesized by six different
modes, on the removal of dye was studied and the results are presented in Figure
3.16.
Figure 3.16 shows the relative change in the concentration (C/C0) of AB-29 in the
presence and absence of different photocatalysts (R1, R2, R3, R4, R5 and R6) as a
function of time. The kinetic results revealed that R6 had the highest activity and
almost completely decolorized the solution in a period of only 90 minutes. The
percentage decolorization of the dye followed the order; R1 (54%) < R2 (66%) <R3
(69%) <R4 (72%) <R5 (76%) <R6 (79%). R6 had smallest size so highest
surface/volume ratio and largest band gap, thus, highest photocatalytic activity. On
the other hand, in the absence of photocatalyst, no observable decrease in the dye
concentration could be seen.
Figure 3.16: Change in
absence of synthesized Cd
The highest photocatalyt
factors. The mechanism
nanoparticles can be expl
the absorption of a photo
nanoparticles, producing
leaving behind a positiv
aqueous solutions on sem
holes generated by pho
positive holes, the photog
holes in order to improv
methods is to increase t
decrease the recombinatio
Many researchers have
follow Langmuir-Hinshel
as:
���
��� �
��
��
Where k is the reaction ra
reactant (mM-1), t is the
Chapter 3
Nida Qutub, Ph.D. Thesis 2013, A.M.U., Ind
in concentration of AB-29 with time in the p
CdS nanoparticles (R1, R2, R3, R4, R5 and R6).
lytic activity of sample R6 could be the resul
m behind the enhancement of photocatalytic act
plained as follows. The photocatalysis over CdS i
ton with energy equal or comparable to the band
g photoexcited electrons (e-) in VB which migra
tive vacancy known as hole (h+). Photocatalytic
semiconductor particles is effected by electrons
hotoexcitation. Since the electrons tend to rec
togenerated electrons should be separated effectiv
rove the photocatalytic efficiency. One of the m
the band gap of the semiconductor catalyst w
tion of electron and hole86.
e reported that photocatalytic decolourization o
helwood kinetic model87-89 which can be genera
rate constant (mMmin-1), K is the adsorption coef
he reaction time and C is the dye concentration
India���� 80
e presence and
.
ult of multiple
activity in CdS
S is initiated by
and gap of CdS
grate to the CB
tic reaction, in
ns and positive
ecombine with
tively from the
most effective
which thereby
of most dyes
rally expressed
[3.1]
oefficient of the
n (mM). If the
81 ��Nida Qutub, Ph.D. Thesis 2
concentration C is very small,
Equation 3.1 could be simplified
���
��� �� � �����
However, the degradation curve
exponential decay curve suggest
the rate constant was calcula
concentration as a function of irra
On the basis of the following equ
���
�� ������
Where kapp is the apparent pseu
time (min), C0 is the initial conce
at time t (mM).
For our experimental conditions
pseudo first-order reaction as al
time as shown in Figure 3.17.
Figure 3.17: Change in conce
absence of synthesized CdS nano
Chapter 3
s 2013, A.M.U., India�
l, KC will be negligible with respect to unity so
ied to an apparent pseudo-first-order kinetics13,88.
[3.2]
ve (Figure 3.16) could be fitted reasonably well
esting pseudo first order kinetics. For each experi
lated from the plot of natural logarithm of
irradiation time13,52,88.
quation:
[3.3]
eudo-first-order rate constant (min-1), t is the rea
ncentration of dye (mM) and C is the dye concent
ns, data (Figure 3.16) were in good agreement
also depicted by plotting ln (C0/C) versus irrad
centration of AB-29 with time in the presence
noparticles (R1, R2, R3, R4, R5 and R6).
so that
ll by an
eriment,
of dye
reaction
ntration
ent with
adiation
nce and
Chapter 3
Nida Qutub, Ph.D. Thesis 2013, A.M.U., India���� 82
�
The slope of the plot of ln (C0/C) vs time gave the rate constant (kapp, apparent
pseudo-first-order rate constant) of the catalytic reaction, which was used to calculate
the decolorization rate. The correlation constant (R2) for the fitted lines was calculated
to be about 0.99 for all the experiments.
The decolorization rate of the dye was calculated using the formula given below87:
�d�C�
dt� k�C�n [3.4]
k=rate constant (molL-1min-1), C=concentration of the dye (mM), n=order of reaction.
The decolorization rate Figure 3.18 for the decomposition of AB-29 in the presence
of different photocatalysts (R1, R2, R3, R4, R5 and R6) revealed that the
decolorization of AB-29 proceeded faster as the sizes of the CdS nanoparticles
decreased. The decolorization rate followed the order R1 (3.7 10-4) < R2 (4.4 10-4) <
R3 (4.5 10-4) < R4 (4.8 10-4) < R5 (5.0 10-4) < R6 (5.2 10-4molL-1min-1).
Figure 3.18: The decolorization rate of AB-29 in the presence of different
synthesized CdS nanoparticles (R1, R2, R3, R4, R5 and R6).
An increase in decolorization rate was observed on the decrease in particle size, which
can be attributed to the increase in the catalyst surface area90. Also, due to decrease in
particle size, band gap energy increases, which diminished the recombination of
charge carriers. It is generally accepted that a larger band gap corresponds to more
powerful redox ability91,92. Hence, R6 possessed largest surface area, increased band
Chapter 3
83 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�
�
gap and high redox capability with small photocorrosion which was responsible for its
highest photocatalytic activity.
The mechanism involving photogenerated electrons and holes can be given as; after
excitation of CdS the electrons in the VB jumped into the nearby CB, leaving behind
a hole in VB (Equation 3.5). Consequently, the as generated photoexcited electrons
and holes acted as redox centers and induced reduction and oxidation reactions
respectively on the catalyst surface. The electrons were scavenged by molecular
oxygen (O2) to yield the superoxide radical anion O2•− (Equation 3.6) and hydrogen
peroxide H2O2 (Equation 3.7) in oxygen-equilibrated media. These new formed
intermediates then interacted to produce hydroxyl radical •OH (Equation 3.8). It is
well known that the •OH radical is a powerful oxidizing agent capable of degrading
most pollutants (Equation 3.9)10 thus would lead to the degradation of dye into the
final products. However, the photo-generated holes in CdS nanocrystals cannot
oxidize hydroxyl groups to hydroxyl radicals due to its valence band potential. This
could result in the photocorrosion of CdS, forming cadmium cations10,93. Figure 3.19
gives the schematic representation of the mechanism involved in photocatalysis by
CdS NPs.
Figure 3.19: Schematic representation of the mechanism involved in photocatalysis
by CdS NPs (R1, R2, R3, R4, R5 and R6).
Chapter 3
Nida Qutub, Ph.D. Thesis 2013, A.M.U., India���� 84
�
The reactions of the mechanism involved in the photoreaction by CdS NPs can be
given as:
CdSh�� CdS��h� �e� [3.5]
e �O2 �� �O2� [3.6]
e� �O2 �2H�� ��H2O2 [3.7]
H2O2 �O2� �� ��OH OH O2 [3.8]
�OH dye� � degradation�products [3.9]
Even though, the photo-generated hole formed in the VB of CdS cannot oxidize
hydroxyl groups (OH¯ ) and water (H2O) molecule to produce hydroxyl radicals, they
possibly oxidized dye molecules to reactive intermediates, and further to final
products to some extent (Equation 3.10)94.
CdS e h�� � Dye � CdS�e� Dye�� � CdS�e� degradation�products[3.10]
For the purpose of practical implementation, it is essential to evaluate the stability and
reuse of the catalyst. Figure 3.20 shows the repetitive photodegradation of AB-29
during five consecutive cycles with the same 1gL−1 catalyst at 0.06mM dye
concentration. After each cycle, the nanocomposite catalyst was washed with double
distilled water and a fresh solution of AB-29 was added before each photocatalytic
run. The relative decolorization using CdS nanocatalysts (R1, R2, R3, R4, R5 and
R6) for the 5 cycling reuse after 90 minutes of reaction time are given in the Table
3.6.
The results showed that the catalytic activity of CdS nanocatalysts (R1, R2, R3, R4,
R5 and R6) decreased after first cycles. Among them R1 showed relatively maximum
stability as compared to other CdS nanocatalysts this might be due to the reason that
hexagonal CdS are more stable than cubic CdS. The decrease in the stability of CdS
during photocatalytic degradation reactions might be the result of photocorrosion of
CdS, forming cadmium cations as already mentioned before. CdS leaching out is a
serious concern. Firstly, because catalytic activity is lowered as the amount of
photocatalyst decreased. Secondly, the Cd2+ ions are hazardous to health93. Therefore,
the use of naked CdS as photocatalyst for water purification is questionable. An
Chapter 3
85 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�
�
approach is to be pursued to make CdS photocatalyst stable and applicable for water
purification.
Figure 3.20: The relative decolorization using synthesized CdS nanomaterials (R1,
R2, R3, R4, R5 and R6) for the 5 cycling reuse after 90 minutes of reaction time
under visible light irradiation.
Table 3.6: The relative decolorization using CdS nanocatalysts (R1, R2, R3, R4, R5
and R6) for consecutive 5 cycling reuse after 90 minutes of reaction time.
Cycle R1 R2 R3 R4 R5 R6
I 53.8% 66.0% 68.6% 72.1% 76.5% 78.9%
II 51.0% 58.8% 62.2% 67.8% 72.9% 76.2%
III 49.7% 55.9% 60.1% 64.2% 68.4% 71.5%
IV 46.7% 52.1% 56.2% 60.8% 64.2% 67.0%
V 45.1% 49.8% 52.3% 56.1% 61.0% 63.6%
Chapter 3
Nida Qutub, Ph.D. Thesis 2013, A.M.U., India���� 86
�
3.4. CONCLUSION:
Cadmium sulphide nanoparticles were synthesized via chemical precipitation method,
using different sulphide ion sources ((NH4)2S, H2S, Na2S) and in presence and
absence of stabilizing agents. The S2- ion source affected the nanoparticles size, the
more active source lead to smaller sized CdS-NPs. The presence of stabilizing agent
prevented the agglomeration of the nanoparticles. The CdS-NPs showed good thermal
stability and fine elemental purity. They exhibited quantum size effect and showed an
increase in band gap with the decrease in particle size. All the synthesized NPs
showed considerable blue shift in absorption edge with respect to the bulk CdS.
Crystalline nature of the CdS-NPs was confirmed by the presence of hexagonal and
cubic type phases. The average particles sizes obtained by TEM, XRD, UV-visible
spectroscopy were found to be in good agreement with each other and were in the
range 4-10nm. The synthesized cadmium sulphide nanoparticles were exploited for
the successful photodegradation of an azo dye, Acid Blue-29, in aqueous medium.
The rate of degradation was found to increase with the decrease in particle sizes. But,
the stability of the CdS NPs decreased with time after consecutive degradation cycles.
This might be due to the photocorrosion of CdS. Thus, some effort has to be taken in
order to utilize the useful photocatalytic properties of CdS, by making it less
photocorrosive and more stable.
Chapter 3
87 ��Nida Qutub, Ph.D. Thesis 2013, A.M.U., India�
�
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