Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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TiO2 nanoparticles versus TiO2–SiO2 nanocomposites: A comparativestudy of photo catalysis on acid red 88

http://dx.doi.org/10.1016/j.saa.2014.02.1271386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +91 4252 223062.E-mail address: rvenckat@gmail.com (R. Venckatesh).

K. Balachandran a, Rajendran Venckatesh b,⇑, Rajeshwari Sivaraj c, P. Rajiv c

a Department of Chemistry, Vivekanandha College of Engineering for women, Namakkal, Tamil Nadu, Indiab Department of Chemistry, Government Arts College, Udumalpet 642 126, Tamil Nadu, Indiac Department of Biotechnology, School of Life Sciences, Karpagam University, Eachanari Post, Coimbatore 641 021, Tamil Nadu, India

h i g h l i g h t s

� Nano TiO2 and TiO2–SiO2 compositeswere successfully synthesized by sol–gel method.� Acid red 88 was degraded by Nano

TiO2 and TiO2–SiO2 composites usingsolar energy.� TiO2 nanoparticles and TiO2–SiO2

nanocomposites are good photocatalysts.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 October 2013Received in revised form 1 February 2014Accepted 19 February 2014Available online 12 March 2014

Keywords:Acid red 88DecompositionNanocompositesPhoto catalysisTiO2–SiO2

a b s t r a c t

A novel, simple, less time-consuming and cost-effective wet chemical technique was used to synthesisTiO2 nanoparticles and TiO2–SiO2 nanocomposites using Titanium tetra isopropoxide (TTIP) as a precur-sor relatively at low temperature in acidic pH. Titania sol was prepared by hydrolysis of TTIP and wasmixed with silicic acid and tetrahydrofuran mixture. The reaction was carried out under vigorous stirringfor 6 h and dried at room temperature. The resulting powders were characterized by UV–Visible spectros-copy, Fourier transform infrared (FT-IR), X-ray diffraction, scanning electron microscope (SEM) and trans-mission electron microscope (TEM). The grain size of the particles was calculated by X-ray diffraction,surface morphology and chemical composition was determined from scanning electron microscopy–energy dispersive spectroscopy, metal oxide stretching was confirmed from FT-IR spectroscopy, bandgap was calculated using UV–Visible spectroscopy. Surface area of the composite as calculated by BETanalyzer and it was found to be 65 and 75 m2/g for TiO2 and TiO2–SiO2 respectively. The photocatalyticexperiments were performed with aqueous solution of acid red 88 with TiO2 and TiO2–SiO2 batch studiesfor 4 h irradiation, direct photolysis of TiO2 and TiO2–SiO2 contributed 94.2% and 96.5% decomposition insolar radiation for the optimized concentration of acid red 88.

� 2014 Elsevier B.V. All rights reserved.

Introduction photo catalysts are ultra-small semiconductor particles which are

Now-a-days nano crystalline photo catalysts attract theresearchers for its environmental applications. Nano crystalline

a few nanometers in size. During the past decade, the photochem-istry of nano semiconductor particles has been one of the fastestgrowing research areas in physical chemistry. The interest in thesesmall semiconductor particles originates from their unique photophysical and photocatalytic properties [1].

K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474 469

Titanium dioxide has a special feature that it can use natural UVto appropriate energetic separation between its valence and con-duction bands which can be surpassed by the energy content bya solar photon. Therefore, degradation of the organic pollutantspresent in wastewater using irradiated TiO2 suspension is the mostpromising process [2]. Titanium dioxide has strong resistance tochemical and photo corrosion and its safety, low cost andbiological harmless issues, limits the choice of convenient alterna-tive [3]. Moreover, TiO2 is more stable than other photo catalysts inambient conditions and can be recycled [4]. Among varioussemiconductor materials tested under similar conditions for thedegradation of organic compounds, titanium dioxide (TiO2) hasbeen demonstrated as the most active photo catalyst [4].

TiO2 photo catalyst has the advantages of high chemical stabil-ity, high photocatalytic activity to oxidize pollutants in air andwater, relative low-price and nontoxicity [5]. TiO2 thin film photocatalysts have been used for environmental applications (air andwastewater treatment and deodorizers), because of its large spe-cific surface area, high photocatalytic activity, strong oxidizingpowder, self-cleaning function, bactericidal and detoxificationactivities [6–8]. To date, various investigations have been con-ducted and reported that the mixed oxide (TiO2/SiO2) is moreeffective than that of Titanium dioxide [9]. Moradi et al. [10] inves-tigated that TiO2/SiO2/Co nanocomposites with Hydroxyl PropylCellulose had the best photo-catalytic activity. The photocatalyticdegradation of organic compounds of olive mill wastewater wasinvestigated by using core–shell–shell Fe3O4/SiO2/TiO2 nanoparti-cles as catalyst [11]. Balachandran et al. stated that Isolan blackdye was decolorized by TiO2 and SiO2 doped on TiO2 [12].

Daneshvar et al. [13] reported that zinc oxide is a suitable alter-native to TiO2 since its photo degradation mechanism have beenproven to be similar to that of TiO2. The textile dyes produce largequantity of highly colored effluents, which are generally toxic andresistant to destruction by biological treatment methods for colorremoval [13,14]. Even though many physical and biological meth-ods are available, the direct use of clean and renewable solar lightand active photo catalysts for decolourization and degradation ofazo dyes have attracted great interest in recent years [15,16].

Furthermore to improve the efficiency of the catalytic activitymore emphasis is placed on mixing TiO2 with SiO2 [17]. The addi-tion of SiO2 helps to create new catalytic active sites due to inter-action between TiO2 and SiO2 [17–20]. TiO2–SiO2 mixed oxidesshow higher thermal stability, adsorption capability and good re-dox properties [21].

In the present work we synthesized TiO2 nanoparticles andTiO2–SiO2 nanocomposites and characterized them by UV spec-troscopy, FTIR, XRD, SEM and TEM. The photocatalytic propertiesof synthesized TiO2 nanoparticles and TiO2–SiO2 nanocompositeswere studied using acid red-88 dye under solar radiation.

Materials and methods

Materials

All reagents used were of analytical grade purity and were pro-cured from Merck Chemical Reagent Co., Ltd., India.

Synthesis of TiO2 nanoparticles

Titanium tetra isopropoxide (TTIP) was used as a precursor,Hydrochloric acid (HCl) as peptizing agent and ethanol was usedas solvent medium. HCl was mixed with ethanol and was stirredfor few minutes. To this mixture TTIP was added in the ratio of1:4:2 and the stirring were continued for 1 h at room temperature.Then 50 ml of distilled water was added, the temperature was

raised to 50 �C and stirred for 3 h until the solution changed intocolorless gel. The high viscous gel was dried at room temperatureto fine powder. The resulting powder was heated at 100 �C for 1 hin a hot air oven. Finally the colorless powder was calcined at400 �C.

Synthesis of TiO2–SiO2 nanocomposites

SiO2 sol was prepared by mixing silicic acid with THF (Tetrahy-drofuran) in the ratio of 1:2. SiO2 sol was added drop-wise to theTiO2 sol, which resulted in yellowish brown solution. The mixturewas stirred for 3 h at room temperature, then the temperature wasraised to 80 �C, and stirring was continued for an hour. Finally yel-lowish brown changes to yellow, and then the solution was driedat room temperature. Yellow powder was obtained and it washeated in hot air oven at about 100 �C for 1 h. Finally all the com-posites were calcined at 400 �C.

Characterization of synthesized TiO2 nanoparticles and TiO2–SiO2

nanocomposites

Synthesized TiO2 nanoparticles and TiO2–SiO2 nanocompositesoptical properties were characterized by UV–Visible spectroscopy(Carry 5000 UV–Vis-NIR spectrophotometer, Varian, USA). Thecrystalline structure of synthesized TiO2 nanoparticles andTiO2–SiO2 nanocomposites were analysed using D8 AdvanceX-ray diffraction meter (Bruker AXS, Germany) at roomtemperature, operating at 30 kV and 30 mA, using Cu Ka radiation(k = 0.15406 nm) and crystal size was calculated by Scherrer’s for-mula. Surface morphology of synthesized TiO2 nanoparticles andTiO2–SiO2 nanocomposites were characterized using scanningelectron microscope (SEM) (Model JSM 6390LV, JOEL, USA). Thenanoparticles and composites were viewed through transmissionelectron microscope (TEM) (JEOL-TEM 2100) at high magnificationand exact particle size was predicated. TiO2 nanoparticles andTiO2–SiO2 nanocomposites functional groups were described usingFourier trans-form infrared spectroscopy (FTIR) spectra (ThermoNicolet, USA) with a wave number range of 4000–400 cm�1.

Photocatalytic decomposition of acid red 88

Measurement of photocatalytic activityExperiments were performed with aqueous solution of acid red

88 with TiO2 nanoparticles and TiO2–SiO2 nanocomposites undersunlight radiation. All the reactions were carried out under thepH 9. TiO2/acid red 88 and TiO2–SiO2/acid red 88 suspensions wereprepared for 10–40 mg L�1 of acid red 88 solutions. Prior to sun-light radiation (4 h irradiation) the suspensions were stirred for15 min to allow for dye adsorption onto the nanoparticle surface.10 mL of the sample was collected and centrifuged to removenanoparticles and the clear solution was carefully transferred intoa quartz cuvette and the absorption was evaluated by UV–Visspectrometer (kmax). The dye adsorbed after equilibrium timewas separated by centrifugation and the quantity of dye adsorbedwas determined by employing a UV–Visible spectrophotometer,(Model U3210 Hitachi) in respective kmax 505 nm of the dye. Thepercentage of dye adsorption was calculated from the absorbancevalue before and after treatment.

Effect of dye concentrationIn the typical textile effluent, dye concentration ranges from

0.15 to 0.2 g L�1. By varying the initial concentration from 10 to40 mg L�1 of acid red 88 solutions at constant catalyst loading(0.2 g L�1), its effect on the decomposition rate was calculated.

470 K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474

Effect of photo catalyst concentrationThe effect of photo catalyst concentration on the photo degra-

dation rate of the dye was investigated by employing different con-centrations of TiO2 and TiO2–SiO2 varying from 0.02 to 0.12 mg/100 mL for the dye used and decomposition rate was calculated.

Fig. 2. XRD patterns of TiO2 and TiO2–SiO2 nanocomposites.

Results and discussion

Characterizations of TiO2 nanoparticles and TiO2–SiO2 nanocomposites

Fig. 1 represents UV–Visible spectra of TiO2 and TiO2–SiO2. It isshown that TiO2 is an oxide semiconductor and its anatase formhas an optical absorbance range around 384 nm, band gap 3.2 eV.In the current measurement the onset of absorption peak of max-imum absorbance occurred at 372 nm for TiO2 and 352 nm forTiO2–SiO2 nanocomposites. The blue shift in absorbance corre-sponds to smaller particle size. The blue shift observed in theabsorbance spectra indicated the Quantum confinement effect[22]. The optical band gap of the material was calculated by effec-tive mass approximation and it was found to be 3.3 for TiO2 and3.54 for TiO2–SiO2 nanocomposites. Increase in the band gap ofthe prepared composites allowed for possible applications likephoto luminescence, photocatalytic and optoelectronic device fab-rications. Since the results derived from the study reveal that en-hanced band gap could be a better photo catalyst light below itswave length holds sufficient energy to excite the electrons andhence absorbed by TiO2 based composites. Therefore all the com-posites are excellent UV absorbers [23].

Fig. 2 shows the XRD patterns of TiO2 and TiO2–SiO2 nanocom-posites. The crystallite types of the TiO2–SiO2 nanocompositeswere pure anatase. The most intense reflection at 2h = 25.3� is as-signed to anatase (d101). Not much difference has been detected be-tween patterns of TiO2 and TiO2–SiO2. The powders showed thecrystalline pattern and the observed d-lines match the reportedvalues for the anatase phase. The intensity of reflections appearedto be decreased for TiO2–SiO2 as compared to TiO2 due to inclusionof amorphous SiO2. The average crystallite size was determined bycarrying slow scan of the powders in the range 24–27� with thestep of 0.01� min�1 from the Scherrer’s equation using the (101)reflections of the anatase phase assuming spherical particles. Anestimate of the grain size (G) from the broadening of the main(101) anatase peak can be done by using the Scherrer formula.The nanocrystallite sizes were found to be 15–20 nm for TiO2 while

300 400 500 600 700

ba

Abso

rban

ce

Wavelength

Fig. 1. UV–Visible spectra of (a) TiO2 and (b) TiO2–SiO2 nanocomposites.

7–10 nm for TiO2–SiO2 powders. The weakening and broadening ofthe XRD peaks may be attributed to the decrease of the samplegrain size and the increase of the SiO2 content. The introductionof SiO2 can effectively suppress the grain growth of anatase com-pared with pure TiO2. Moreover, the suppression is more remark-able with the introduction of higher silica content, which isconsistent with the literature [24]. The major differences werethe narrower first maximum and the broader second maximum.

Supplementary information A1 shows the SEM and TEM imagesalong with the particle size distribution of the pure TiO2 nanopar-ticles and TiO2–SiO2 nanocomposites. The pure TiO2 particlesexhibited irregular morphology due to the agglomeration of pri-mary particles and with an average diameter of 15–20 nm. Onthe other hand, TiO2–SiO2 nanocomposites exhibited regular mor-phology, since the TiO2 cores were coated by SiO2 particles. Theaverage particle size of the colloidal TiO2–SiO2 nanocompositeswas measured to be 7–10 nm.

Fig. 3 represents the FT-IR spectra of TiO2 particles andTiO2–SiO2 composites. The peaks at 3400 and 1650 cm�1 in thespectra are due to the stretching and bending vibration of the–OH group. In the spectrum of pure TiO2, the peaks at 550 cm�1

shows stretching vibration of Ti–O and peaks at 1450 cm�1 showsstretching vibrations of Ti–O–Ti. The spectrum of TiO2–SiO2 showthe peaks at 1400 cm�1, 450–550 cm�1 exhibiting stretchingmodes of Ti–O–Ti, 1100 cm�1 shows Si–O–Si bending vibrationsand peak at 950 cm�1 shows Si–O–Ti vibration modes which isdue to the overlapping from vibrations of Si–OH and Si–O–Tibonds. This result indicates that the TiO2–SiO2 nanocompositeswere prepared by a combination of TiO2 with SiO2 nanoparticles[25].

500 1000 1500 2000 2500 3000 3500

%T

Wavenumbers (cm-1)

T iO2

T iO2-S io

2

Fig. 3. FTIR spectra of TiO2 and TiO2–SiO2 nanocomposites.

K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474 471

Photocatalytic decomposition studies

Specific surface areaThe BET specific surface areas of the samples calcined at 400 �C

were 65 and 75 m2/g for TiO2 and TiO2–SiO2, respectively. Themean pore size calculated from BET isotherms was 10 and15 nm, respectively. The surface area and pore volume of theTiO2 decreases as a result of surface modification with SiO2 or acombination of other materials.

Batch studiesExperiments were performed with aqueous solution of acid red

88 with TiO2 and TiO2–SiO2 under batch studies for 4 h irradiation,direct photolysis of TiO2 and TiO2–SiO2 contributed 96.5% and94.2% decomposition in solar irradiation for the optimized concen-tration of dye.

Effect of TiO2 loadingFig. 4 shows the effect of TiO2 and TiO2–SiO2 loading on percent-

age decomposition of the dye solution from 10 to 40 mg L�1. Thepercentage decomposition increased rapidly with increase in theamount of TiO2 and TiO2–SiO2 from 0.02 to 0.12 mg per 100 mL�1

for the dye used. The minimum percentage decomposition at lowerTiO2 loading can be attributed to the fact that more light is trans-mitted which is not utilized in the photocatalytic reaction [17].Dye decomposition rate was significantly high in TiO2–SiO2 andTiO2 dose 0.02–0.12 g L�1. Beyond 0.12 mg of TiO2, and TiO2–SiO2

show slightly increase the percentage decomposition. This phe-nomenon may be due to the aggregation of TiO2 particles at highconcentrations (0.12 g L-1) causing a decrease in the number of sur-face active sites. Similar trends are reported with photocatalyticreactions over TiO2–SiO2 and TiO2 catalyst [26]. This may be dueto availability of active sites on TiO2 and TiO2–SiO2 surface andthe light penetration of photo activating light into the suspension.The availability of active sites increases with the suspension of cat-alyst loading, but the light penetration and, hence, the photo acti-vated volume of the suspension reduces. It clearly reveals that,decomposition efficiency of TiO2–SiO2 composites is greater thanTiO2 which may be due to increase in surface area and active sites.

Effect of concentration and agitation time on dye removalThe effect of initial concentration of dyes on the percentage

decomposition was studied by varying the initial concentration

Fig. 4. Effect of dosage on% removal of

from 10 to 40 mg L�1 in the case of acid red 88 dyes with optimumcatalyst loading. It is seen from Supplementary information A2.That percentage decomposition decreased with increasing initialconcentration of the dye. It is also found that the decompositionof acid red 88 was higher for TiO2–SiO2 than TiO2. This may bedue to the fact that as the initial concentration of the dye increasedthe path length of photons entering the solution decreases and atlow concentration reverse effect is observed, thereby increasingthe number of photon absorption by the catalyst at lowers concen-trations [27]. This is in agreement with the earlier results observed[28]. This suggests that as the initial concentration of the dye in-creases, the requirement of catalyst surface needed for the decom-position also increases. Since illumination time and amount ofcatalyst are constant, the –OH radical formed on the surface ofTiO2 is also constant and hence the relative number of free radicalsattacking the dye molecules decreases with increasing amount ofthe catalyst [29]. Hence, at higher concentration, decompositiondecreases at sufficiently longer distances from the light source orreaction zone due to the retardation of penetration of light. Thus,the rate of decomposition decreases with increase in concentrationof dyes.

Sobana et al. [30] reported on the absorption spectral changes ofDirect Brown 53 on the TiO2 catalyst as a function of the irradiationtime. The results of photodegradation and decolourisation of DirectBrown 53 using pure TiO2 and silver doped TiO2 along with the rateconstants and the initial rates of degradation and decolourisationby TiO2 and doped catalysts. Muruganandham [31] suggested thatTiO2 nanoparticles, in presence of sunlight showed a 73.55% of dyedecolourisation at a time period of 80 min.

Effect of pHThe effect of pH on the decomposition of the dye by TiO2 and

TiO2–SiO2 is shown in Fig. 5. At optimum concentration of dyesin both acidic and alkaline pH, it seems to decrease the percentagedecomposition of the dye. The increased effect seems to be morepronounced in the alkaline pH 9. The pH affects not only the sur-face properties of TiO2, but also the dissociation of dyes and forma-tion of hydroxyl radicals. The importance of each one depends onthe substrate nature and pH [32]. Protonation or deprotonationof the dye can change its adsorption characteristics and redoxactivity. In an aqueous system, TiO2 is amphoteric [33]. The TiO2

surface is predominantly negatively charged when the pH is higherthan the TiO2 isoelectric point. As the pH decreases, the functional

acid red 88 (a) TiO2 (b) TiO2–SiO2.

Fig. 5. Effect of pH on% removal of acid red 88 (a) TiO2 (b) TiO2–SiO2.

Fig. 6. Pseudo second order plots for acid red 88 (a) TiO2 (b) TiO2–SiO2.

472 K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474

groups are protonated, and the proportion of the positivelycharged surface increases. Thus, the electrical property of theTiO2 surface varies with the pH of the dispersion. The surface ofTiO2 would be negatively charged and adsorb cationic species eas-ily under pH > pHzpc (zero point charge) conditions while in the re-verse condition it would adsorb anionic ones. However, theadsorption of the substrate onto the TiO2 surface directly affectsthe occurrence of electron transfer between the excited dye andTiO2 and further influences the decomposition rate. The surface be-comes positively charged, and the number of adsorption sites maydecrease above the isoelectric point of TiO2. A similar effect of thepH on the adsorption and photocatalytic reaction has been re-ported [23].

Jain and Sikarwar [34] revealed that higher decolourization ofcongo red was achieved at pH 6.2. They also reported that at lowerpH the rate of decolourization was slow and remained constantfrom pH 3 to 4 and at higher pH level, the photodegradation ofthe dye increased. They also suggested that at a pH of 6.2, the sur-face will be negatively charged and thereby the higher rate of deg-radation occurs. It has been already proved, that at high pH values,

cations are favourably adsorbed due to the negatively charged sur-face sites [35].

Adsorption kinetics

Adsorption rate constantsMany attempts have been made to formulate a general expres-

sion describing the kinetics of sorption on solid surfaces for liquid–solid phase sorption systems. The pseudo-first order equation wasfirst represented by Lagergren [36] for the adsorption of oxalic acidand malonic acid onto charcoal. The Lagergren kinetic model hasbeen used to investigate the mechanism of sorption and potentialrate controlling steps such as mass transport and chemical reactionprocesses.

Pseudo-first order equationThe pseudo-first order equation of Lagergren is generally ex-

pressed as follows:

dq=dt ¼ Kadðqe � qtÞ ð1Þ

Fig. 7. Elovich plots for acid red 88 (a) TiO2 (b) TiO2–SiO2.

K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474 473

where qe and qt are the sorption capacity at equilibrium and at time,‘t’ and Kad is the rate constant of pseudo-first order sorption (1/min).After integration and applying boundary condition t = 0 to t = t andq = 0 to q = qt, the integrated form of Eq. (1) becomes:

logðqe � qtÞ ¼ log qe � ðKad=2:303� t ð2Þ

where qe is the Amount of dye adsorbed (mg/g) at equilibrium; qt

the Amount of dye adsorbed (mg/g) at time, (t); Kad the rate con-stant of adsorption, (1/min) and t – Agitation time (min.)

Linear plots of log(qe�qt) vs t show the applicability of theabove model. The Kad values have been calculated from the slopeof the linear plots and are represented in Fig. 8 for different con-centrations of the dyes studied. The rate constants observed arecomparable with the values reported earlier [37].

Diffusion film transport through the boundary layer is referredto as external or film diffusion. Diffusive transport through theinternal pores of the nanocomposites and/or along the pore–wallsurface (intraparticle diffusion) adsorption or attachment of thesolute particle at a suitable site on the nanoparticle surface oneor more of the above steps may be the rate controlling factor(Supplementary information A3).

Fig. 8. Fractional power plots for aci

Adsorption isotherm studiesLangmuir equation. The Langmuir isotherm is valid for monolayeradsorption onto a surface containing a finite number of identicalsites. The model assumes uniform energies of adsorption ontothe surface and no transmigration of adsorbate in the plane ofthe surface.

Based upon these assumptions, Langmuir represented the fol-lowing equation:

qe ¼ fðQ 0 � b� CeÞ=ð1þ ðb� CeÞg ð3Þ

where qe is the equal to the quantity of dye adsorbed in mg G�1 ofthe adsorbent; Q0 The maximum quantity of dye adsorbed inmg G�1 of the adsorbent, b The constant of Langmuir adsorption,Ce The dye concentration at equilibrium in mg L�1

Langmuir adsorption parameters are determined by transform-ing the Langmuir equation, which is in linear form. The isothermcan be made linear in four different possible methods. Dependingon the linearization, different estimates are obtained for the valuesof the Langmuir parameters. The linear plot of Ce/qe vs Ce showedthat the adsorption followed Langmuir isotherm model as shownin Supplementary information A4. The values of monolayer

d red 88 (a) TiO2 (b) TiO2–SiO2.

474 K. Balachandran et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 468–474

capacity ‘Q0’ and Langmuir constant ‘b’ had been evaluated from theintercept and slope of these plots by using graphical techniques.

Effect of isotherm shape. The effect of isotherm shape has been ta-ken into consideration with a view to predict whether the studiedadsorption system is favorable or unfavorable. The essential fea-tures of the Langmuir isotherm may be expressed in terms of equi-librium parameter RL, which is a dimensionless constant referredto as separation factor or equilibrium parameter [38].

RL ¼ 1=ð1þ bC0Þ

where C0 is the initial concentration and ‘b’ is the constant relatedto the energy of adsorption (Langmuir Constant). The values of RL

indicate the nature of the isotherm.In the present investigation, the value of RL was less than one

which showed that the adsorption process was favorable. The re-sults of present investigation were compared with the earlier re-port [39], which were carried out as adsorption studies by usingdifferent types of dyes and on low cost adsorbent. The RL valuesfrom the studies lie between 0.02481 and 0.09925 for TiO2 andTiO2–SiO2 composites at different concentrations of 10, 20, 30and 40 mg L�1 and indicate the favorable adsorption of dye onthe nanomaterials.

Pseudo-second-order model. The pseudo-second-order model [40]is represented by the following differential equation:

dqt=dt ¼ k2ðqe � qtÞ2

where qe = Amount of dye adsorbed (mg/g at equilibrium),qt = Amount of dye adsorbed (mg/g at time t) k2 is the equilibriumrate constant of pseudo-second order (g/mg/min) adsorption.Integrating the above equation for the boundary condition t = 0 tot and qt = 0 to q, gives:

t=q ¼ 1=k2q2e þ 1=qe � t

The slope and intercept of plot of t/q versus t were used to calculatethe second-order rate constant k2 (Fig. 6). The correlation coeffi-cients of all examined data were found very high which showedthat the model can be applied for the entire adsorption process.

Elovich equation. The simple Elovich model equation is generallyexpressed by the following equation

qt ¼ aþ b ln t

The slope and intercept of plot of qt vs. ln t were used to calculatethe values of the constants ‘a’ and ‘b’ as shown in (Fig. 7).

Harkin’s–Jura adsorption isotherm. This can be expressed as

1=q2e ¼ ðB=AÞ � ð1=AÞ log Ce

where B and A are the isotherm constants. The Harkins–Jura adsorp-tion isotherm accounts to multilayer adsorption and can be ex-plained with the existence of a heterogeneous pore distribution.1/qe2 was plotted vs. logCe (Supplementary information A5).

Fractional power model. The fractional power model is a modifiedform of the Freundlich equation and can be expressed as:

ln qt ¼ ln aþ b ln t

where qt the amount of the dyes sorbed by the adsorbent at time,‘t’and ‘a’ and ‘b’ are constants with b < 1. The function ‘a’ ‘b’ is also aconstant, being the specific sorption rate at unit time, i.e., whent = 1. The plot of ln t vs lnqt showed the linear relationship andthe computed constants ‘a’ and ‘b’ from the intercepts and slopesof the plots are presented in Fig. 8.

Conclusion

Anatase phase TiO2 nanoparticles and TiO2–SiO2 nanocompos-ites were successfully synthesized by wet chemical technique.The introduction of SiO2 can effectively suppress the grain growthof anatase compared with pure TiO2. The nanocomposites wereused for the removal of the acid red 88 dye. Batch mode adsorptionprocess reported that the adsorption process was dependent on theinitial dye concentration and TiO2, TiO2–SiO2 loading. The study re-ports the TiO2, and TiO2–SiO2 nanoparticles to be efficient and eco-nomical in harvesting solar energy and the degradation of dyescompared to conventional treatment methods.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2014.02.127.

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