Radha Devi Chekuri,Siva Rao Tirukkovalluri*

Department of Inorganic and Analytical Chemistry,School of Chemistry,Andhra University,Visakhapatnam,India

1.Introduction

15%of the total dye production has been reported from the textile industrialwaste and are rated as the mostpolluting among allindustrial sectors[1].Effluents released from these waste waters contain large amount of azo dyes,which are non-biodegradable,toxic and potential carcinogenic and persistent in nature[2]causing serious problem threatening the environment.Hence more attention has been focused for the degradation of these dye pollutants.For this purpose Advanced Oxidation Processes(AOPs);were preferred.But AOPs oxidize quickly and non-selectively broad range of organic pollutants.Hence some of the methods like sonolysis Fenton Oxidation,and Electrochemicaltreatment,Photochemical treatment method have been preferred[3-5].But they could succeed only to some extent,due to some limitations like generation of hardly oxidizable carboxylic acids,cost effective etc.whereas Heterogeneous Photocatalysis is found to be the most advanced oxidation process suitable and efficient method for the removal of such dyes from industrial waste waters.For this process titania is selected as the best photocatalyst since it is biologically and chemically inert,stable with respect to photo chemical corrosion,insoluble under any drastic conditions and nontoxic[6].However,due to high band gap energy TiO2(Ebg＜3.2 eV),it can be excited only by UV light(λ=387 nm)and the high rate of electron-hole recombination,within nanoseconds,at TiO2particles which results in less photocatalytic activity in the visible light.Therefore it is more advantageous to modify titania,for it to exhibit high reactivity under visible light(λ ＞ 400 nm),where visible light constitutes about 40-50%of the solar energy,hence doping of titania with metals[7]and nonmetals[8-15]were preferred.A systematic study of metal ion doping in quantum sized TiO2for 21 metalions was performed by Choi etal.,[16].The doping of various transition metal ions into TiO2could shift its optical absorption edge from UVto visible lightrange,buta prominentchange in pure titania was notobserved[17].Umebayashi etal.,[18-20]have succeeded to synthesize S-doped TiO2,which was used as an efficient visible light induced photocatalyst for visible-light catalytic degradation of methylene blue.In order to synthesize a nano-catalyst with higher photo-catalytic activity,restriction of the particle size during its synthesis plays a prominent role and substantially affects the properties of synthesized titania nanomaterials.Minimizing the particle size leads to increase in surface area which results in higher photocatalytic activity.In order to attain this,recently investigations were carried regarding the synthesis of metal doped titania by Surfactant Template method.Though surfactant medium plays a vital role in the synthesis of variety of nanoparticles[21],it involves complicated procedure where the parameters are to be carefully controlled.The photocatalytic efficiency of such titania samples depends on the template selected.Hence in order to synthesize a new nano-photocatalyst with decrease in the particle size and for attainment oflarge surface area,ourprime focus aims atstudy regarding introduction ofsulfate ions i.e.deposition ofsulfate ions on copperdoped titania and to study the corresponding decrease in particle size and presumably with larger active surface area and to investigate its applications towards enhancementin photocatalytic activity.Doping ofcopperinto titania matrix was proposed,for decreasing the band gap energy and to modify the surface properties of titania and to observe the shift of absorption band towards the visible light range.Hence,we preferred synthesizing copper doped sulfated titania and to investigate its photocatalytic efficiency.It is a single step process employing sol-gel method.

2.Experimental Procedure

2.1.Materials required

Titanium tetra-n-butoxide(Ti(O-Bu)4)and copper sulfate were obtained from E.Merck(Germany).Super-dry purified ethanol has been used.All the other chemicals and reagents are of Merck(India)analytical grade.Furnace with maximum temperature limit of 1200°C of V.B.Ceramics India Ltd,have been used for the calcination purpose.

2.2.Synthesis of copper doped sulfated TiO2

Titanium tetra-butoxide and copper sulfate were considered as the precursors for titanium and copper respectively.We have taken 40.0 ml of absolute ethanol(100%),7.1 ml of water and 1.1 g of copper sulfate with required percentages 2.0 wt.%-10.0 wt.%were prepared(solution I).Solution II was prepared by taking 20.0 ml of titanium tetra-butoxide in 40.0 ml of absolute alcohol with 3.0 ml of nitric acid was added drop wise under continuous stirring for 30 min.Both the solutions were mixed and vigorously stirred at room temperature until the transparent sol was obtained.Later,the gel was dried at 80°C in an oven for36 h.The catalystpowder was calcined at450°C in a furnace for4 h.The synthesis pattern ofthe catalysthas been shown in Fig.1.Required wt.%ranging from(2.0,5.0 and 10.0)Cu2+-SO42-/TiO2was prepared.The powdered samples were finely grinded.A similar procedure was adopted for the preparation of undoped TiO2.The synthesized catalyst samples were characterized by XRD analysis,UV-Vis DRS,XPS,SEM,TEMand FT-IR.Later,degradation ofMOdye in the presence of visible light region has been carried out.

Fig.1.Schematic representation of the catalyst preparation.

2.3.Characterization of photocatalysts

The XRD patterns were recorded with a PAN Analytical diffractometer at room temperature with a copper(Kα)anode material of wavelength(λ)0.15418 nm,and carbon monochromator were used.The accelerating voltage of 35 kV and emission current of 30 mA were employed.The UV-Visible absorption spectroscopic analysis of the undoped TiO2 and doped TiO2 samples were done by using a UV-Visible spectrophotometer (Hitachi,U-3210).X-ray photo electron spectroscopy(XPS)of the prepared undoped and doped TiO2solid samples were recorded with the PHI quantum ESCA microprobe system,using the AlKα line of a 250 W X-ray tube as a radiation source with the energy of1253.6 eV,16 mA×12.5 kVand a working pressure lowerthan 1×10-8·Nm-2.Scanning electron micrographs(SEM)of the samples were recorded using Phillips XL 30 model.The infrared spectra of the synthesized samples were recorded on Thermo Nicolet Nexus 670 Spectrometer,with resolution of4 cm-1in KBrpellets.For understanding the particle size of doped and un-doped TiO2,TEM measurements were carried out using Tecnai SE20,operated at 120 kV as accelerating voltage.

2.4.Photocatalytic activity of the catalyst—Degradation of MO

The photocatalytic degradation of MO dye was carried out in the presence of visible light in the photocatalytic reactor.The required amount of catalyst was suspended in 100 ml of standard aqueous solution in a 150 ml Pyrex glass vessel.The mixture was stirred for 45 min in the dark to reach adsorption equilibrium.Later,the suspensions were then irradiated under visible light(wavelength range 400-800 nm)using a UV filtered Osram high pressure mercury vapour lamp with power 400 W and 35,000 lm.The distance between the light and the reaction vessel was 20 cm.The experiments were performed at room temperature and at regular intervals,5 ml of the aliquots was taken by 0.45 μm millipore syringe filter and for the measurement of absorbance and were analyzed,the quantitative determination of MO was performed by measuring the absorption of solution at a wave length of 464 nm with a Milton Roy Spectronic 1201,UV-Vis spectrophotometer.The extent of MO photo catalytic degradation was calculated using a calibrated relationship between the measured absorbance and its concentration.MO cannot be photodegraded in the absence of the catalyst under the same irradiation conditions.

Degradation=（A0-At）/A0×100%

where

A0initial absorbance of dye solution

Atabsorbance of dye solution at time t.

3.Results and Discussion

3.1.X-ray diffraction studies

Fig.2.XRDpatterns of(a)undoped TiO2(b)2.0,(c)5.0,and(d)10.0 wt.%ofCu2+-SO42-/TiO2,and precursor gel powder heat treated at 450°C for 4 h.

The XRDpatterns for both doped and undoped TiO2powder samples were represented in Fig.2(a)to 2(d).All the samples were reported to be in the anatase phase.(JCPDS File number:21-1272).From this we can conclude that the Cu2+ions and SO42-ions in TiO2did not in fluence the crystal patterns of TiO2particle.The d spacing and hkl values at different 2θ were recorded.The ionic radius of Cu2+(0.073 nm)is closerto thatofTi4+(0.068 nm);so itiseasierfor Cu2+ionsto be incorporated into the matrix as substituents of TiO2without causing much crystalline distortion whereas ionic radius of sulfate ions is much larger in size and hence cannot be substituted by replacing oxygen ofTiO2and hence deposition of sulfate ions on the titania surface is the only favorable condition.It was found that the crystallinity could be improved by increasing the percentage doping of Cu2+.The(101)plane of anatase peaks of the Cu2+-SO42-/TiO2sample slightly shifts to higher values of 2θ in comparison with that of undoped TiO2.The average crystallite size of prepared catalyst was calculated from the broadening of the Full Width at Half Maximum(FWHM)peak by using Scherrer's equation ranging from 7 to 12 nm.The tendency for the formation of smaller TiO2crystalline nanoparticles has been confirmed.

3.2.UV-visible diffusion reflectance spectroscopy

Fig.3.UV-Visible absorption spectra of(a)undoped TiO2(b)2.0 wt.%ofCu2+-SO42-/TiO2 and(c)5.0 wt.%of Cu2+-SO42-/TiO2.

Fig.3 represents the UV-Visible absorption spectra of 2.0 wt.%,5.0 wt.%Cu2+-SO42-/TiO2and undoped TiO2.From the spectrum of pure TiO2there is a broad intense absorption band observed at 380 nm which is due to charge-transfer from the valence band(mainly formed by 2p orbital of the oxide anions)to the conduction band(mainly formed by 3d t2gorbitalofthe Ti4+cations)[22].Thus undoped TiO2doesn't respond to visible light whereas in the copper doped sulfated TiO2there is a shift noticed towards the higher wavelength region(red shift)i.e.visible region ataround 500 to 700 nm.From this we can conclude that there is a contact between TiO2and Cu2+grains and this leads to the enhancement in the photocatalytic activity of catalysts in visible light and it is confirmed from the later investigations.The absorption peak for 5.0 wt.%of the prepared catalyst,is noticed at greater shift than that of 2.0 wt.%of Cu2+-SO42-/TiO2,i.e.at 600 nm from which we can say that,with increase in percentage of doping of copper in the TiO2lattice,there is increase in absorbance towards higher visible region.Due to absorption towards visible region there is an increase in the number of photo generated electrons and holes which leads to the enhancement in the photocatalytic activity of the TiO2.Lettmann et al.[23]reported that there is good relation between lightabsorption properties and the photocatalytic activities.

3.3.X-ray photoelectron spectroscopic study

From High resolution XPS analysis we can find out the detailed chemical state information of Cu 2p,S 2p,O 1s,and Ti 2p.Fig.4(a)shows the high resolution spectra of Ti 2p,O 1s of the undoped TiO2.From Fig.4(b)and(c)Cu 2p3/2and Cu 2p1/2peaks were located at binding energies of 934.599 eV and 954.781 eV respectively,and they belong to the compound of CuO[24,25]and shake-up peaks are also observed which are characteristic is for Cu2+that bonds with oxygen atoms.The binding energies of Ti 2p3/2and Ti 2p1/2were found to be 459.029 eV and 464.616 eV these binding energies belongs to Ti4+[26].Fig.4(c)shows the high resolution XPS Spectra of the S 2p region where peaks around 169.843 eV can be attributed to SO42-,where sulfur atoms are present in the state of+6 oxidation state.From Fig.4(a),the Ti 2p and O 1s peaks of copper doped sulfated TiO2samples are slightly shifted toward lower binding energy,which is due to incorporation of Cu2+ions into TiO2lattice.From the high resolution XPS spectra of the S 2p region a peak around 170 ev is observed[27].From this we can say that sulfur atoms are in the state of S6+,the absence of the peaksataround 160-163 eV,indicate thatthere isno Ti-S bond.Anionic sulfate doping cannotbe carried outsince SO42-has a significantly largerionic radius compared to thatofO2-(0.122nm).Hence doping ofsulfate ions is not proved to be successful.The most favorable condition is surface deposition of Ti4+by SO42-ions.So from the XPS we can finally conclude that the existence of sulfuris+6 oxidation state and copper is in+2 oxidation state.This may be the reason for the activation of the synthesized nano photo catalyst in the presence of visible light.The XPS observations were consistent with EDS.

3.4.Scanning electron microscopic study

Fig.5(a)and(b)depicts the SEM micrographs of pure TiO2powder and 5.0 wt.%Cu2+-SO42-/TiO2.The SEM-image of the pure TiO2appears as large blocks of the coarse material.In a catalyst of 5.0 wt.%,the nanoparticles are found to be uniform and spherical particles,with reduction in the particle size in the range of 7-12 nm.SEM indicates the change in the morphology of the nanoparticles,where the particles are of rough nature with increase in surface area as shown in Table 1.This is possible due to the restriction in the further growth of particles size,where there is no agglomeration of the particles;may be due to the sulfate deposition on the surface of copper doped titania which is confirmed from XRD.

3.5.Energy dispersive spectrometry

It is used to identify the elements that are present in the prepared catalyst.It is done by taking a selective portion of SEM image in the form of peaks of spectrum.Fig.6 indicates,the spectrum of 5.0 wt.%Cu2+-SO42-/TiO2sample.The presence of Cu,S,Ti and O from the synthesized catalyst,were confirmed from EDS.

Fig.4.(a):XPS of undoped TiO2.(b):XPS of 5.0 wt.%Cu2+-SO42-/TiO2.(c):XPS of 5.0 wt.%Cu2+-SO42-/TiO2.

3.6.Fourier transform infrared spectroscopy

Fig.5.(a):SEM images of undoped TiO2.(b):5.0 wt.%Cu2+-SO42-/TiO2.

Table 1 BET surface area of the prepared samples

Fig.7(a),(b)indicates the spectra of pure TiO25.0 wt.%,Cu2+-SO42-/TiO2respectively.From Fig.7(a)peaks are observed at 3373.01,2922.25 and 1623.98 cm-1which correspond to the stretching vibrations of the O-H and bending vibrations of the adsorbed water molecules.Similarly from Fig.7(b)bands were noticed at 3383.86 and 1627.52 cm-1.This confirms the presence of hydroxyl ions in the Cu2+-SO42-/TiO2.Below 1000 cm-1region the Ti-O-Ti stretching peak appears at 585 cm-1.The S=O stretching frequencies of SO42-/TiO2are found in 1384.36 cm-1,1123.31 cm-1,and 1066.98 cm-1.This is compared with reported stretching frequencies[28-31]of the three bands at 1385.50 cm-1,1143.39 cm-1and 1055.33 cm-1which show small deviation,this may be due to Cu2+doped substitutionally in the TiO2matrix.The free sulfate ion has tetrahedral symmetry.There is no band at 1105 cm-1,which indicates that no free SO42-exists in the prepared catalyst.All these bands indicate the existence of sulfate ions in the prepared catalyst and these are due to the copper doped sulfated titania,characteristic of chelating bidentate SO42-and uni-dentate SO42-.There are absorption peaks observed at 1384.36 and 1123.31 cm-1and these are related to asymmetric and symmetric stretch of S=O.In SO42-/TiO2there is a peak observed at 1046 cm-1,this is attributed to asymmetric S-O bond[32]In the Cu2+-SO42-/TiO2sample there is a peak observed at 1066.98 cm-1and this may be due to doping of copper and presence of sulfate ions on the surface of TiO2.The structure of SO42-/TiO2is expressed in Fig.8.Due to surface deposition of SO42-,it limits the further growth of the crystallite size and forms nanocrystallites which was confirmed with the XRD data.The anionic sulfate doping i.e.replacementofoxide ion with the sulfate ion is notfavorable,due to larger ionic radius of sulfate ions hence it is surface deposited on the TiO2,and this limits the further growth of crystalline size of TiO2,which forms nanocrystalline particles.

Fig.6.EDS spectrum of 5.0 wt.%of Cu2+-SO42-/TiO2.

Fig.7.FT-IR spectra of(a)undoped TiO2,(b)5.0 wt.%Cu2+-SO42-/TiO2.

3.7.Transmission electron microscopy

Fig.8.Structure of sulfated TiO2.

Fig.9(a)and(b)represents the TEM images of un-doped and 5.0 wt.%Cu2+-SO42-doped TiO2respectively.The particle size was found to be 7-12 nm,respectively.From TEM analysis also we can conclude the decrease in the particle size,which was consistent from XRD analysis.This signifies the effect of sulfate deposition on the synthesis of copper doped titania nanophotocatalyst.Similarly sulfate ions tend to form the complex with the surface oxygen of TiO2and this suppresses the further growth of TiO2crystalline particles.Hence higher rate of photocatalytic activity was observed from the later studies.

3.8.Evaluation of photocatalytic activity of the catalyst

Experiments were carried out to find the optimum conditions of various parameters like effect of dosage,dopant concentration,pH of the solution,and concentration of the dye were studied in detail.

3.8.1.Effect of dopant concentration

A set of experiments were carried out using 2.0 wt.%,5.0 wt.%,and 10.0 wt.%of Cu2+-SO42-/TiO2by maintaining other conditions like pHand dye concentration constant.Itis compared with thatofundoped TiO2for the degradation of MO,under visible light irradiation.It was found that 5.0 wt.%of copper doped sulfated titania has shown maximum enhancementin the photocatalytic activity.Thus 5.0 wt.%appears to be an optimal dopant concentration.

Fig.9.(a):TEM image of undoped TiO2.(b):TEM image of 5.0 wt.%of Cu2+-SO42-/TiO2 sample.

The results of the experiments were presented in Fig.10.The results from this figure indicated that rate of degradation of MO increased at 5.0 wt.%of copper doped sulfated Titania.It is observed that in the case of 10.0 wt.%of copper doped TiO2,the photo reactivity decreases.With increase in copper wt.%,copper is deposited on TiO2,instead of doping.The observed rate constant for effect of dopant concentration is 11.55×10-2·min-1and further doping became detrimental.

Fig.10.Effect of dopant concentration on the degradation of MO.

3.8.2.Effect of pH

To understand the optimum pH of the solution experiments were carried out by varying the pH from 1 to 3 and the results are presented in Fig.11.Results from this figure reveal that at pH 1.5,the rate of degradation of MO is high,the rate increases at high acidic pH due to the amphoteric behavior of titanium dioxide and the change of the surface charge properties of TiO2with the changes of pH values[33]around its point of zero charge pH(pzc)according to the following reactions:

Fig.11.Effect of pH on the degradation of MO.

MO being an anionic dye,having electronegative centers(S and O),hence maximum adsorption on the catalyst surface takes place at a lower pH.This is an important step for the photo-oxidation to take place.The observed rate at a pH value of 1.5 is 7.6×10-2min-1.

3.8.3.Effect of catalyst dosage

Experiments were performed by varying the concentrations of Cu2+-SO42-/TiO2from 0.1 to 0.3 g in 100 ml aqueous solution of MO dye at pH of 1.5.Fig.12 represents the effect of catalyst dosage.It is observed that the rate of degradation increases linearly with increase in the amount of catalyst up to 0.5 g,and then decreases(leveling off).With the increase of catalyst amount,the number of photons and the number of MO molecules adsorbed are increased.With increase in the number of catalyst particles,it leads to increase in photocatalytic efficiency.

Fig.12.Effect of catalyst dosage on the degradation of MO.

From Fig.12,beyond 0.5 g,there is a decrease in the photo catalytic process,i.e.the rate levels off.This may be due to increased concentrations of catalyst,although more areas are available for constant MO molecules to absorb the number of substrate molecules present in the solution remains the same,but the solution turbidity increases and it interferes the penetration of light and also cause scattering of radiation.Hence at a certain level presence of catalyst in excess amount may not involve in catalysis and thus the rate levels off.The observed rate constant at catalyst dosage 0.2 g is 13.0×10-2min-1.

3.8.4.Effect of initial dye concentration

A set of experiments were conducted varying the concentration of MO from 1.0 to 10.0 mg·L-1at a constant Cu2+-SO42-/TiO2loading of 0.2 g and a solution pH of 1.5.The results from the graphical representation are given in Fig.13,which indicates that the degradation rate increases with increase in dye concentration to an extent of 10.0 mg·L-1and a further increase leads to decrease in the rate of degradation,this is due to the screening effect at concentrations higher than 10 mg·L-1.The observed rate at initial dye concentration of 5 ppm is 15.33×10-2·min-1.With the increase in the concentration of the dye the probability of the reaction between the dye and the oxidizing species increases,there is enhancement in the photocatalytic degradation process.Furthermore,the generation of·OH radicals is constant for a given quantity of the catalyst and hence the available·OH radicals are insufficient for MO degradation at higher concentrations.High concentration dyes start covering the surface of the photocatalyst as a blanket from light intensity thus 5×10-6mg·L-1of the MO dye concentration has been found to be the optimum condition.

Fig.13.Effect of MO dye concentration on the degradation of MO.

3.8.5.Photocatalytic mechanism

Fromthe experimentalresults the following mechanism is proposed for the photocatalytic reactions of Cu2+-SO42-/TiO2.

These trapped electrons can be subsequently scavenged by molecular oxygen,which is adsorbed on the TiO2surface,to generate the superoxide radical,and this in turn produce hydrogen peroxide(H2O2),hydroperoxy(HO2·)and hydroxyl(·OH)radicals[34,35].

(1)The positive holes in the valence band act as good oxidizing agents available for degradation of pollutants in the solution

Where “Red”is the pollutant an electron donor(reductant).

Thus the dye pollutant MO is attacked by the hydroxyl radicals formed both by trapped electrons and hole in the VB as given in the above equations,to generate organic radicals or other intermediates.

4.Conclusions

In our present investigation,we have been successfully synthesized the copper doped sulfated titania as nanophotocatalyst with different 2.0 wt.%-10.0 wt.%,and theirphotocatalytic activity wasstudied by degradation ofMO,a modelazo dye pollutantundervisible lightirradiation.It was found that the transition metal copper ions were doped into the TiO2lattice which reduced the band gap energy ofTiO2.By the introduction of nonmetal sulfate ions on the surface of the TiO2limited the further growth of the particle size and thus forming nanocrystalline titania powders which was confirmed with the XRD and TEM data.From XRD data it is observed that all the samples were shown to be in anatase phase.Sulfation(i.e.deposition of sulfate ions,on the surface of TiO2)has hindered further growth of crystalline size of the synthesized catalyst.This is possible due to the surface deposition of sulfate ions,where there is restriction of the particle size with large surface area has been observed.The agglomeration of the particles has been minimized due to repulsion of similarly charged sulphated species which are deposited on copper doped titanianano particles.This finally leads to enhancement in the photocatalitic activity.The SEM analysis has shown the change in morphology of the titania particles which is due to copper doping of sulfated titania.It was found that the prepared sample showed increased photocatalytic activity in the degradation of MO,in the presence of visible light.

The observed rate constant at optimum concentrations of various parameters such as effect of dopant concentration(5.0 wt.%),effect of pH(pH of 1.5),catalyst dosage(0.2 g)and initial dye concentration(5 mg·L-1)is 15.33×10-2min-1are the optimal conditions for better degradation of MO dye.Finally from this study we can conclude that by the introduction of transition metal like copper(copper ions)being substituted into the TiO2matrix and sulfation of the copper doped sulfated nano titania catalyst which can effectively shift the absorption ofTiO2from UVto Visible region and controlthe selective crystallization of anatase phase of TiO2;which minimizes band gap energy and facilitating for high degradation of dye pollutant in the visible light.

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(Please refer to the online version for the color figures)