Materials Chemistry and Physics
104 (2007) 454–459
Sol–gel preparation and characterization of nanosize
TiO2:Its photocatalytic performance
N.Venkatachalam,M.Palanichamy,V.Murugesan∗
Department of Chemistry,Anna University,Chennai600025,India
Received14October2006;received in revid form29March2007;accepted5April2007
Abstract
Nanocrystalline TiO2photocatalysts of different anata to rutile ratios with high surface area have been prepared by sol–gel technique using titanium(IV)isopropoxide as the precursor.The prepared materials were characterized by XRD,BET,UV–vis,FT-IR,SEM and TEM techniques. Reaction parameters,such as hydrolyzing agent,molar ratio,aging time and calcination temperature were varied during the synthesis in order to get uniform nanosize TiO2particles.XRD,SEM and TEM obrvations confirmed the nanocrystalline nature of TiO2.The band gap value and surface area were higher,whereas particle size of nano TiO2was lower than the commercial TiO2(Degussa P-25).The photocatalytic activity of nano TiO2was evaluated in the phtocatalytic oxidation of bisphenol-A as a model reaction.Under the optimum conditions,nano TiO2showed higher photocatalytic activity for the degradation of bisphenol-A than commercial TiO2(Degussa P-25).
© 2007 Elvier B.V. All rights rerved.
Keywords:Nano TiO2;Sol–gel process;Anata pha;Bisphenol-A;Mineralization
1.Introduction
Semiconductor-mediated photocatalytic oxidation has been accepted as a promising method for the removal of organic contaminants from wastewater.Among the miconductors employed,TiO2is known to be a good photocatalyst becau of its high photonsitivity,non-toxicity,easy availability,strong oxidizing power and long-term stability[1,2].Existing bulk miconducting materials posss low surface area,less adsorp-tion property and fast electron–hole recombination.In order to circumvent such problems rearchers are interested in recent days in the synthesis of nanomaterials for environmental applica-tions[3,4].Nanocrystalline materials exhibit unique properties, such as quantum size effect,high surface area,short inter-face migration distance and visible light active,all of which achieve enhanced photocatalytic performance.Particle size is another important parameter for photocatalysis since it directly impacts the specific surface area of a catalyst.With a small par-ticle size,the number of active surface sites increas and so ∗Corresponding author.Tel.:+914422203144;
fax:+914422200660/22350397.
E-mail address:v (V.Murugesan).does the surface charge carrier transfer rate in photocatalysis [5].
Recently,sol–gel process is a novel technique for the prepa-ration of nanocrystalline TiO2.It has been demonstrated that through sol–gel process,the physico-chemical and electrochem-ical properties of TiO2can be modified to improve its efficiency. It provides a simple and easy means of synthesizing nanopar-ticles at ambient temperature under atmospheric pressure and this technique does not require complicated t-up.Since this method is a solution process,it has all the advantages over other preparation techniques in terms of purity,homogeneity,felicity andflexibility in introducing dopants in large concentrations, stoichiometry control,ea of processing and composition con-trol.Through sol–gel process,the growth of TiO2colloids in sub micrometer range can be effectively controlled by hydroly-sis and condensation of titanium alkoxides in aqueous medium [6].
Nanosize TiO2ud so far in photocatalytic applications has been prepared by hydrolysis of titanium precursors fol-lowed by annealing,flame synthesis,hydrothermal and sol–gel process[7,8].In most studies,attempts have been made to enhance the photocatalytic activity of TiO2by only varying the c
alcination temperature and in a few cas aging period and drying conditions.The prent work deals the influence of the
0254-0584/$–e front matter© 2007 Elvier B.V. All rights rerved. doi:10.1016/j.matchemphys.2007.04.003河北教师网
N.Venkatachalam et al./Materials Chemistry and Physics 104 (2007) 454–459455
most important preparation conditions,such as pH,hydrolyzing agent,aging time,aging temperature and calcination conditions on the physico-chemical properties of nano TiO2and con-quently its photocatalytic activity.Bisphenol-A(BPA)has been taken as a model pollutant for photocatalytic studies as it is frequently encountered in wastewater.It has estrogenic activ-ity and rves as an environmental endocrine disruptor[9,10]. Hence,it is esntial to develop treatment methodology for BPA degradation.
2.Experimental
2.1.Materials and methods
All the chemicals were obtained from Merck(India)and ud as such with-out further purification.The
commercially available TiO2(Degussa P-25)was obtained from Degussa Chemical,Germany.The typical synthesis procedure for nano TiO2is as follows:titanium(IV)isopropoxide,glacial acetic acid and water were maintained in a molar ratio1:10:350.Titanium(IV)isopropoxide(18.6ml) was hydrolyzed using35.8ml glacial acetic acid at0◦C.To this solution water (395ml)was added drop wi under vigorous stirring for1h,subquently the solution was ultrasonicated for30min and continued the stirring for further5h until a clear solution of TiO2nanocrystals was formed.After the period,the solution was placed in an oven maintained at a temperature of70◦C for a period of12h for aging process.The gel was then dried at100◦C and subquently the catalyst was crushed intofine powder and calcined in a muffle furnace at500◦C for5h.This process was adopted as an optimized method for the preparation of nano TiO2.
2.2.Catalyst characterization and analytical method
The XRD patterns were recorded on a PANAalytical X’pert PRO X-ray diffractometer using Cu K␣radiation as the X-ray source.The average crystallite size of anata and rutile phas was determined according to the Scherrer equation using the full width at half maximum(FWHM)data of each pha after correcting the instrumental broadening.The specific surface area (BET method),specific pore volume and average pore diameter(BJH method) of the samples were determi
ned by nitrogen adsorption–desorption isotherms using Quantochrome Autosorb1sorption analyzer.The calcined samples were outgasd at250◦C under vacuum(10−5mbar)for3h prior to adsorption experiments.The particle size and morphology of nano TiO2were obrved using transmission electron microscope(TEM)(JEOL3010)and scanning electron microscope(SEM)(Stereo scan LEO440).UV–vis absorption spec-tra of the samples were recorded using UV–vis spectrophotometer(Shimadzu 2601).FT-IR spectra of the samples were recorded on a FT-IR spectrometer (Nicolet Avatar360).The extent of BPA degradation was monitored using UV–vis spectrophotometer(Shimadzu1601)and high performance liquid chro-matograph(HPLC)(Shimadzu LC10ATVP ries equipped with UV-Visible detector).The intermediates were identified using gas chromatograph coupled with mass spectrometer(GC–MS)(Perkin-Elmer Clarus500).The extent of mineralization was determined using a total organic carbon analyzer(TOC) (Shimadzu V CPN).
2.3.Photocatalytic studies
Photocatalytic degradation was carried out in a slurry photocatalytic reac-tor.The design description of slurry photocatalytic reactor was reported in our earlier study[11].The experimental conditions were:initial concentra-tion of BPA=250mg l−1,volume of BPA=100ml,solution pH9and catalyst dosage=200mg.The experiments were performed at room temperature.Prior to UV irradiation(low-pr
essure mercury lamps with64W light intensity emitting 365nm radiation),the slurry was aerated for30min to reach adsorption equilib-rium.The adequate aliquot of sample was withdrawn after periodic interval of irradiation and analyzed after centrifugation for degradation and
mineralization.Fig.1.XRD patterns of nano TiO2prepared from(a)Ti(IV)isopropoxide and
(b)Ti(IV)chloride calcined at500◦C.
3.Results and discussion
3.1.Physico-chemical characterization
X-ray diffraction pattern of nano TiO2(obtained from the hydrolysis of titanium(IV)isopropoxide)is illustrated in Fig.1a. The patterns apparently revealed the effect of calcination tem-perature on the pha change of nano TiO2.The nano TiO2 samples distinct the amorphous structure calcined at500◦C.It is obrved that increa of calcination temperature from300to 600◦C,the peak intensity of anata increas and the width of (101)plane and diffraction peak of anata(2θ=25.3◦)become narrow.The increa of calcination temperature forced conden-sation of free OH groups on the surface of nano TiO2particles. It incread the crystallinity and so also the intensity of diffrac-tion peaks of anata pha.The rutile pha started appearing at500◦C.When nano TiO2was calcined at800◦C,the pattern exhibited a complete rutile TiO2structure indicating complete pha transformation from anata to rutile at this temperature. The average particle size was calculated by applying the Scher-rer formula on the anata(101)and rutile(110)diffraction peaks(the highest int
ensity peak for each pure pha).
D=
Kλ
βcosθ
where D is the crystal size of the catalyst,λthe X-ray wave-length(1.54˚A),βthe full width at half maximum(FWHM)of the catalyst,K a coefficient(0.89)andθis the diffraction angle. An average crystal size of around10–15nm was obtained for nano TiO2samples.The nanocrystalline anata structure was confirmed by(101),(004),(200),(105)and(211)diffrac-tion peaks[12].Further,the XRD patterns of TiO2(Fig.1b) derived from the hydrolysis of titanium tetrachloride showed broad peaks,less crystalline and less intensity compared to the pattern obtained from the hydrolysis of titanium(IV)iso-propoxide.Bad on this result,it is suggested that the TiO2 particles obtained from titanium tetrachloride are highly defec-tive in nature and this may be attributed to rapid agglomeration in comparison to crystallization.
456N.Venkatachalam et al./Materials Chemistry and Physics 104 (2007) 454–459
Table1
Physico-chemical properties of nano TiO2catalysts
Catalyst BET surface area(m2g−1)Anata crystallite size(nm)Anata to rutile ratio Band gap energy(eV) Hydrolyzing agent
Methanol(nano01)691769:31 3.19
Isopropyl alcohol(nano02)8412.674:26 3.21
Glacial acetic acid(nano03)1078.382:18 3.27
Source:solvent:water molar ratio蜂蜜的用处
1:10:150861273:27 3.19
1:10:2509410.678:22 3.26
1:10:3501108.183:17 3.28
1:10:450918.674:26 3.21
Aging temperature(◦C)
508710.279:21 3.26
60989.678:22 3.24
701058.483:17 3.28
809711.376:24 3.23
Aging time(h)
69510.677:23 3.27
宁做鸡头不做凤尾121088.382:18 3.29
249310.273:27 3.28
Calcination temperature(◦C)
400125 6.289:11 3.29
5001068.282:18 3.28
6008614.374:26 3.21
8003621.1100%rutile 3.14
Table1prents the textural properties of nano TiO2pre-
pared by sol–gel process under different conditions.It can be速度与激情主题曲
en that the specific surface area shifted towards lower val-
ues at higher calcination temperatures.Fig.2shows the pore
size distribution curve calculated from the desorption branch of
nitrogen isotherm by the BJH method and the corresponding
nitrogen adsorption–desorption isotherms of nano TiO2(nano
03)powder calcined at500◦C for5h.The sharp decline in the
desorption curve indicates the hysteresis(type H2)between the
two curves,demonstrating that there is a diffusion bottle neck
possibly caud by the non-uniform pore size of TiO2[13].The
pore size distribution graph revealed the pore size of nano TiO
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Fig.2.Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curve calculated from desorption branch of the nitrogen isotherm(int)of nano TiO2(nano03)calcined at500◦C.in the range of3–8nm.The pores are mainly resulted from a variety of accumulated pore voids between the particles,and par-ticles with different calcination temperatures.The pores may allow rapid diffusion of BPA molecules during photocatalytic reactions.However,the surface area and pore volume decrea with increa of calcination temperature.Meanwhile,the pore diameter increas due to increa in the crystal size of TiO2. Fig.3shows the SEM picture of nano TiO2calcined at500◦C for5h.It is obrved that nano TiO2particles are spherical in shape with an average grain size of10–12nm,which are con-sistent with XRD results.Fig.4illustrates the TEM histogram (particle size distribution)of nano TiO2.Both are proven the nanocrystalline nature of TiO2obtained from the hydrolysis of titanium(IV)
isopropoxide.
Fig.3.SEM picture of nano TiO2(nano03)calcined at500◦C.
射手座男人N.Venkatachalam et al./Materials Chemistry and Physics 104 (2007) 454–459
457
Fig.4.Particle size distribution histogram of nano TiO 2(nano 03)calcined at 500◦C.
FT-IR spectra of nano TiO 2powder show peaks correspond-ing to the stretching vibrations of O–H and bending vibrations of adsorbed water molecules around 3350–3450cm −1and 1620–1635cm −1,respectively.The intensity of the peaks became low with increa of calcination temperature,indicat-ing the removal of large portion of adsorbed water from TiO 2.UV–vis absorption spectra of nano TiO 2and Degussa P-25TiO 2are shown in Fig.5.The absorption spectrum of nano TiO 2con-sists of a single and broad inten absorption around 400nm due to charge-transfer from the valence band (mainly formed by 2p orbitals of the oxide anions)to the conduction band (mainly formed by 3d t 2g orbitals of Ti 4+cations)[14].Nano TiO 2exhib-ited absorption in the shorter wavelength region than Degussa P-25TiO 2and there was sufficient decrea in the particle size,and conquently incread the band gap values of TiO 2which minimized the electron–hole recombination during the photocat-alytic degradation of BPA.The formation of nano TiO 2particles in the sol–gel process is also evident by the difference in the height of absorbance of nano TiO 2and Degussa P-25TiO 2
.
Fig.5.UV–vis absorption spectra of TiO 2calcined at 500◦C.
3.2.Optimization of preparation conditions
Different nanocrystalline TiO 2powders were prepared from the hydrolysis of Ti(IV)isopropoxide by the addition of a hydrolyzing agent.In order to study the influence of preparation conditions on the photocatalytic activity of the final TiO 2,the most important experimental parameters,such as pH,calcination temperature,hydrolyzing agent,molar ratio of water and aging time were varied.The characterization studies of nano TiO 2powders prepared by various conditions demonstrate different physico-chemical properties.Different preparation conditions led to titania samples with very different crystalline pha com-positions,band gap energies and surface areas.The correlation between physico-chemical properties and photocatalytic activity is discusd below.
3.2.1.Influence of the hydrolyzing agents and pH
Hydrolysis of Ti(IV)isopropoxide in the prence of iso-propanol in the mole ratio,1:10:350yielded nanosize TiO 2particles (nano 02).When the same process was repeated with acetic acid instead of isopropanol similar nanosize TiO 2parti-cles (nano 03)were obtained,but the particle size was less than tho obtained with isopropanol.This is due to slow hydrolysis of Ti(IV)isopropoxide in the pre
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nce of isopropanol in which partially hydrolyzed monomeric Ti(IV)isopropoxide can very well combined to form dimeric species and partially hydrolyzed trimeric species and so on.Hence,hydrolysis in the prence of isopropanol yielded TiO 2nanoparticles of larger size than tho obtained in the prence of acetic acid.Since hydrolysis in the prence of acetic acid is rapid,the formation of oligomeric tita-nium alkoxide species will be largely suppresd.Hence,this process yielded TiO 2nanoparticles of much smaller size than tho obtained in the prence of isopropanol.Rapid forma-tion of titanium hydroxide and their condensation to form TiO 2nanoparticles were therefore very well favored in the prence of acetic acid.Hence,acetic acid functions as a catalyst whereas isopropanol does not.As the preparation was carried out in the prence of acetic acid who p K a is clo to three;there is every chance for protonation of TiO 2nanoparticles which could sup-press further crystallization.In addition,the excess acetate anion adsorbed on the surface of TiO 2could also suppress the growth of TiO 2.This type of complexation of acetate anion on the sur-face of anata TiO 2might be responsible for decrea in the crystallite size of TiO 2during sol–gel synthesis.The addition of acetic acid does not cau residual impurities on the surface of TiO 2after calcination.
Preparation of TiO 2from titanium tetrachloride always yielded bulk TiO 2particles.Since the medium
is more acidic as a result of hydrolysis of titanium tetrachloride,the formation of titanium hydroxide and its condensation to form TiO 2parti-cles completed before dispersion of added titanium tetrachloride into water.In order to understand the influence of hydrolyzing pH during the preparation of nano TiO 2,catalysts were prepared following the above said procedure,changing the pH from 3to 9by the addition of sodium hydroxide.Samples prepared at pH 9showed very low surface area.The reason for the large decrea in surface area at higher pH could be rapid hydroxylation of
458N.Venkatachalam et al./Materials Chemistry and Physics 104 (2007) 454–459
titanium precursor,which could lead to agglomeration of TiO2 particles[15].
3.2.2.Influence of water
During sol–gel synthesis of nano TiO2,high water ratio was kept to enhance the nucleophilic attack of water on tita-nium(IV)isopropoxide and to suppress the fast condensation of titanium(IV)isopropoxide species to yield TiO2nanocrystals.In addition,the prence of residual alkoxy groups can significantly reduce the rate of crystallization of TiO2which favored the for-mation of less den anata pha exclusively.The hydrolysis rates are low for less amount of water,and excess titanium alkox-ide in the solvent favors the development of Ti–O–Ti chains through alcoxolati
on.Since each titanium is coordinated with four oxygen atoms,the development of Ti–O–Ti chains results in three-dimensional polymeric skeletons with tight packing favors the formation of high ratio of rutile pha[16,17].
3.2.3.Effect of aging time and temperature
The aging time is one of the preparation parameters that can influence the formation of nano TiO2.In order to study this influ-ence,nano TiO2catalysts were prepared by changing the aging time from6to24h.A decrea in the degree of crystallinity of the samples was obrved especially when the aging time was extended from6to24h.With longer aging time,precipitate of high polymeric structure order was obtained.During the period of aging,process of dissolution and reprecipitation of the gel were produced,driven by differences in solubility between sur-faces with different radii of curvature.This led to structures with lower amount of defects and vacant sites,and due to that,in sam-ples prepared with longer aging times,the diffusion pathways for nucleation and crystal growth during the calcination step would be prevented considerably,leading tofinal materials with lower crystallite sizes.During the variation of aging temperature from 50to80◦C,there are no significant physico-chemical changes in nano TiO2.The samples aged at70◦C for12h showed high surface area and band gap energy.This may be due to com-plete polymerization of the titanium precursor leading
to better dispersion of TiO2nanoparticles[18,19].
3.3.Photocatalytic degradation of BPA
The extent of degradation and mineralization of BPA was followed by UV–vis spectroscopy,HPLC and TOC analyzer. The initial hydroxylation of aromatic rings by hydroxyl radi-cals plays an anticipating role in a quential ring cleavage in the ca of nano TiO2promoted photocatalysis of BPA[20]. The electron-donating hydroxyl groups normally increa the electron density of the aromatic ring and subquently elec-trophilic attack by hydroxyl radicals.Thus,the photocatalytic degradation of BPA might be initiated by the attack of elec-trophilic hydroxyl radicals.The formation of hydroquinone, p-hydroxybenzoic acid and p-hydroxybenzaldehyde as interme-diates during photocatalytic degradation of BPA was identified by GC–MS.The results of the intermediates give convincing evidence for the hydroxylation and aromatic ring opening of BPA.The linear decrea of TOC against irradiation time in
the Fig.6.Comparison of photocatalytic mineralization of BPA over nano TiO2and bulk TiO2(experime
ntal conditions:initial concentration of BPA=250mg l−1, volume of BPA=100ml,solution pH9and catalyst dosage=200mg). degradation of BPA using nano TiO2and TiO2(Degussa P-25)is depicted in Fig.6.The TOC removal efficiency of nano TiO2is higher than that of TiO2(Degussa P-25).Under identical exper-imental conditions,the mineralization of BPA required480and 660min with nano TiO2(nano03)and TiO2(Degussa P-25), respectively.
4.Conclusions
The results of the investigation conclude that optimization of preparation conditions are esntial for obtaining nanocrys-talline TiO2materials with notably higher activity than Degussa P-25TiO2.The optimum molar ratio of alkoxide,acetic acid and water is1:10:350.The preparation conditions clearly affect the physico-chemical properties of nano TiO2which in turn exhibit a crucial influence on its phtocatalytic activity.The sol–gel pro-cess coupled with ultrasonic treatment not only enhances the formation of uniform distribution of nanocrystalline anata TiO2particles but also enhances the hydrolysis of titanium alkoxide for the formation of less den anata nano TiO2 particles.The photocatalytic degradation of the model pollu-tant,BPA over nanosize TiO2revealed higher activity than bulk TiO2(Degussa P-25).The enhanced adsorption of BPA over nano TiO2surface posssing high surface area and small par-ticle size is suggested to be the cau for higher activity of nano TiO
2.
Acknowledgements
The authors gratefully acknowledge thefinancial support from the University Grants Commission(UGC),New Delhi, through Centre with Potential for Excellence in Environmen-tal Science in our University.The authors like to place on record thefinancial support from the Department of Science and Technology(DST),New Delhi,under FIST programme for the sophisticated equipment facility in the Department.One of the authors,N.Venkatachalam is thankful to the CSIR,New Delhi, India for the award of Senior Rearch Fellowship.
