Improvement of photocatalytic activity of TiO2-WO3

Colloids and Surfaces A: Physicochem. Eng. Aspects 506 (2016) 804–811

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Improvement of photocatalytic activity of TiO2 -WO3 nanocomposite by the anionically substituted N and S P.N. Gaikwad a , T.M. Wandre a , K.M. Garadkar a , P.P. Hankare a , Jagannath b , R. Sasikala c,∗ a b c

Department of Chemistry, Shivaji University, Kolhapur, MH 416 004, India Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• N and S co-doped TiO2 -WO3 exhibit • • • •

enhanced visible light photocatalytic activity. Anionically doped N and S modify the band structure of the composite. Decreased bandgap and improved visible light absorption occurs in the composite. Better separation of photogenerated charge carriers occurs in the composite system. This composite is stable for repeated use.

a r t i c l e

i n f o

Article history: Received 15 March 2016 Received in revised form 30 May 2016 Accepted 6 July 2016 Available online 19 July 2016 Keywords: TiO2 WO3 N and S co-doped Photocatalyst Methyl orange

a b s t r a c t A nanocomposite of N and S co-doped TiO2 -WO3 exhibit enhanced visible light photocatalytic activity for the degradation of methyl orange compared to undoped TiO2 -WO3 composite, WO3 and TiO2 . Though, the TiO2 -WO3 composite has been reported earlier, the effect of anionic doping of TiO2 -WO3 with both N and S on the photocatalytic activity is not reported so far. A significant increase in visible light absorption is observed after N and S co-doping due to the decrease of the bandgap energy as a result of doping. In the composite, TiO2 exists as anatase phase and WO3 exists as monoclinic phase. This nanocomposite has a particle size of 20–30 nm. XPS results indicate that in the doped samples, anionic substitution of both N and S occurs and substitute for oxygen in the lattice. The nanocomposite catalyst is found to be stable for repeated use. The enhanced photocatalytic activity is attributed to the increased visible light absorption and better separation of photogenerated charge carriers in the doped composite catalyst. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Sasikala). http://dx.doi.org/10.1016/j.colsurfa.2016.07.015 0927-7757/© 2016 Elsevier B.V. All rights reserved.

Industrial effluents contain a lot of toxic organic waste, which has to be treated before it is sent to environment. Photocatalytic degradation of organic compounds using solar radiation in the presence of semiconductors is a potential method for the effluent

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treatment. For this process to be effective, suitable semiconductors, whose bandgap is < 3 eV is needed so that the solar energy is utilized efficiently. Though, TiO2 is a well known photocatalyst used for this purpose, its wide bandgap (3.2 eV) makes it a poor absorber of solar radiation. Several modifications of TiO2 including cationic [1–5] and anionic doping [6–10] have been tried to overcome this problem. In most of the cases, the dopants decrease the bandgap of TiO2 and makes it visible light active. Other than doping, sensitization of TiO2 by dyes [11] or by carbon quantum dots has been employed to improve the visible light absorption of TiO2 and to enhance the photocatalytic/photoelectrochemcial activity [12,13]. Incorporation of gold nanoparticles on TiO2 brings about visible light photoelectrochemcial water splitting through surface plasmon resonance (SPR) of gold nanoparticles [14]. TiO2 containing gold nanoparticles show strong SPR responses when irradiated with light having a wavelength of 532 nm. The ‘hot’ electrons thus produced in gold nanoparticles are injected into the conduction band of TiO2 resulting in enhanced photoelectrochemical water splitting efficiency. Another problem encountered in the photocatalytic process is the fast recombination of photogenerated electrons and holes, which in most cases is less than 1 ns [15]. Because of the short lifetime, the charge carriers cannot reach the surface and react with the adsorbed organic molecules to initiate the degradation process. To minimize the recombination of charge carriers and to facilitate the separation of charge carriers, several measures have been taken in the past. One method is to use a noble metal co-catalyst like Pt, Pd etc. along with the semiconductor (SC) co-catalyst, which can effectively separate the photogenerated electrons from the SC and improve the photocatalytic activity [15–18]. Another method is to couple the SC catalyst with another having suitable conduction and valence band potentials so that the charge transfer from one SC to the other can occur through the heterojunctions [19–23]. Thus, many semi conductors like SnO2 [24,25], In2 O3 [26–28], WO3 [29–31], Fe2 O3 [32–34] and ZnO [35,36] have been coupled with TiO2 to enhance the charge separation. In all cases, increased photocatalytic activity is obtained as a result of improved charge separation. In the present study, we have employed a novel form of TiO2 based composite catalyst to increase the photocatalytic activity. Two strategies have been employed to improve the efficiency of TiO2 . The first is to couple TiO2 with WO3 , so that the photogenerated charge carriers are separated and the recombination is minimized. The aim of choosing WO3 along with TiO2 is that the conduction and valence band potentials of TiO2 and WO3 are suitable for the transfer of photogenerated electrons from NS-TiO2 to NS-WO3 and photogenerated holes from NS-WO3 to NS-TiO2 so that the lifetime of the charge carriers can be increased. The second is to co-dope the composite with N and S so that the absorption of visible light is extended to longer wavelength region. Though, the TiO2 -WO3 composite has been reported earlier, to the best of our knowledge, N and S anionic co-doped TiO2 -WO3 composite is not reported so far. This composite is used for the photocatalytic degradation of methyl orange as a model pollutant and the observed activity is explained on the basis of the improved physico-chemical properties.

2. Experimental 2.1. Synthesis of the WO3 -TiO2 nanocomposites Titanium tetraisopropoxide (Ti (OC3 H8 )4 , Spectrochem Chem., >98%) was used as a precursor of titania. In a typical synthesis, 3 ml of titanium tetraisopropoxide was mixed with100 ml of isopropyl alcohol. This solution was stirred for 2 h followed by the addition of

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50 ml of double distilled water under vigorous stirring for 5 h. The obtained gel was separated, washed 3 times with double distilled water and acetone and dried in air at 80 ◦ C. The resultant white powder was calcined at 400 ◦ C for 2 h in air. Different compositions of TiO2 − WO3 (WO3 is loaded by weight% of TiO2 ) nanocomposites were prepared through wet-impregnation method by using sodium tungstate as the precursor for tungsten. For this purpose, TiO2 nanoparticles were dispersed in required amount of sodium tungstate dihydrate (Na2 WO4 ·2H2 O, Thomas Baker, 99%) aqueous solution. To this, 3 M hydrochloric acid was added at 80 ◦ C drop by drop under constant stirring until the formation of slight yellow precipitate (pH ∼1). The precipitate thus obtained was washed with double distilled water and acetone 3 times, dried in air at 80 ◦ C and calcined at 500 ◦ C in air for 2 h. Three compositions, 20% (T2W), 40% (T4W) and 80% (T8W) of WO3 on TiO2 were synthesized. WO3 was prepared by the method mentioned above for the synthesis of composite without using TiO2 powder. N, S co-doped TiO2 , WO3 and N,S co-doped T4W (T4W was used for doping as it showed the highest photocatalytic activity) nanocomposites were synthesized by grinding the as synthesized TiO2 , WO3 or TiO2 -WO3 composite with thiourea (1:4) and calcining the mixture at 350 ◦ C for 3 h. N doped samples were synthesized by the same method mentioned above using urea. 2.2. Characterization of the samples X-ray diffraction (XRD) patterns of all samples were recorded on Brukar D2 diffractometer using Cu k␣ radiation (␭ = 0.1540 nm). The UV–vis Diffuse Reflectance spectra (UV-visDRS) were recorded on UV–vis spectrophotometer (Shimadzu-3600) equipped with an integrating sphere assembly, using BaSO4 as a 100% reflecting sample. The transmission electron microscopy(TEM) images of the samples and SAED patterns were obtained from a Philips-200 TEM microscope. X-ray photoelectron spectroscopic (XPS) studies were carried out in a VG Microtech electron spectrometer using Mg-K␣ X-rays (h␯ = 1253.6 eV) as the primary source of radiation. Chamber pressure was maintained at 1 × 10−9 torr. Appropriate correction for charging effect was made with the help of C 1s signal appearing at 284.5 eV from adventitious carbon. The spectra were analyzed using XPSPEAK41 soft-ware. After calibration, the background from each spectrum was subtracted using a Shirley-type background. 2.3. Photocatalytic experiment Photocatalytic activity of the nanocomposite was examined by using an aqueous solution of methyl orange (10 ppm) under UV and visible light irradiation separately. The source of UV light was low pressure Hg lamp (SAIC 125W) in the wavelength range of 254–356 nm. The source of visible light was medium pressure Hg lamp (SAIC 400W). The short wavelength components (␭ < 420 nm) of the 400 W lamp were cut off by using a cut off filter. 0.1 g of nanocomposite sample was dispersed in a beaker having 100 ml of 10 ppm methyl orange aqueous solution. This suspension was magnetically stirred in dark for 30 min to set up adsorption/desorption equilibrium at room temperature. During irradiation, stirring was continued to keep the mixture in suspension. 3 ml of solution was taken out at regular intervals and filtered to obtain the clear solution. Concentration of the solution was monitored using UV–vis-NIR spectrophotometer (Shimadzu-3600) in the 200–800 nm wavelength range. The reaction rate (k) was calculated from the slope of straight line obtained by plotting Ln C/C0 versus illumination time, (t) as Ln

C = - kt C0

(1)

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Fig. 1. Powder XRD patterns of TiO2 , WO3 , T2W, T4W and T8W.

Fig. 3. A) UV-Vis absorption spectra of TiO2 , WO3 , T4W, NS-TiO2 , NS-WO3 and NST4W. B) Transformed Kubelka-Munk function plotted against photon energy for TiO2 , WO3 , T4W, NS-TiO2 , NS-WO3 and NS-T4W.

Where t is the irradiation time, k is the first-order rate constant of the reaction, C and C0 are concentration of dye at time t and 0 respectively. 3. Results and discussion 3.1. Characterization XRD patterns of TiO2 -WO3 nanocomposites along with TiO2 and ◦ WO3 calcined at 500 C for 2 h are shown in Fig. 1. The XRD pattern of TiO2 suggests that it exists as anatase phase (JCPDS card No.211272) and that of WO3 shows that it has monoclinic structure (JCPDS No. 72-1465). The pattern of TiO2 –WO3 composite show the formation of anatase phase of titania and monoclinic phase of tungsten oxide. There are no extra peaks in this diffractogram corresponding to any impurity phase. TEM images and SAED patterns of TiO2 , WO3 and T4W are shown in Fig. S1, S2 and S3 of Supplementary data. It can be seen from Fig. S1 that TiO2 is nanosized and the size is in the range of 10–15 nm.

SAED pattern of the sample (inset of Fig. S1) shows continuous rings indicating that it is polycrystalline in nature and can be indexed as anatase phase of TiO2 . TEM image of WO3 shows that its particle size is relatively higher compared to TiO2 and is in the range of 60–80 nm. SAED (inset of Fig. S2) reveals a dot pattern, which can be indexed as monoclinic phase of WO3 . Fig. S3 shows that the particle size of the T4W composite is 10–15 nm. The SAED pattern indicates the presence of both TiO2 and WO3 and can be indexed as anatase phase of TiO2 and monoclinic phase of WO3 . TEM, HRTEM images and SAED pattern of NS-T4W is shown in Fig. 2. It can be seen that the particle size is in the range of 10–20 nm. The SAED pattern (inset of Fig. 2) shows both dot and ring patterns and can be indexed as TiO2 and WO3 . HRTEM images show lattice fringes with ‘d’ spacings corresponding to both TiO2 and WO3 . This observation is in conformity with the XRD results.

Fig. 2. TEM HRTEM images and SAED pattern of NS co-doped T4W catalyst.

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Fig. 4. N1s and S2p XPS of NS-TiO2 , NS-WO3 and NS-T4W.

Fig. 5. Ti 2p, W4f and O 1s XPS of TiO2 , WO3 , NS-TiO2 , NS-WO3 and NS-T4W.

3.2. Optical property The UV–vis DRS of TiO2 , WO3, T4W, NS-TiO2 , NS-WO3 and NST4W are shown in Fig. 3A. A significant red shift in the absorption edge is seen for all samples compared to single phase TiO2 . Besides, it may be noted that N and S co-doping in TiO2 , WO3 and T4W results in a red shift of the absorption edge compared to their respective undoped samples. The transformed KM function plotted against photon energy is shown in Fig. 3B. The bandgap energy values obtained from this figure for TiO2 , WO3 , T4W, NS-TiO2 , NS-WO3 and NS-T4W are 3.21, 2.94, 3.13, 3.0, 2.87 and 2.94 eV respectively.

The decrease of bandgap due to N and S co-doping is due to the introduction of additional levels within the bandgap of TiO2 /WO3 resulting in the modification of the band structure.

3.3. Surface characterization by XPS Fig. 4 shows the N 1s XPS of NS-TiO2 , NS-WO3 and NS-T4W. The fitted peak positions of the N 1s peak for NS-T4W, NS-WO3 and NSTiO2 are at 398.2, 398.1 and 397.9 eV respectively, which can be assigned to N-Ti-O type bonding in the doped sample. The assignment of N1s binding energy (BE) to N-Ti-O type bonding differs in

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the literature reports. The reason for this is cited as the method of preparation of N doped TiO2 [37]. In some reports, the peak seen at 400 eV is assigned to the chemisorbed N species whereas the one at 396 eV is assigned to the Ti—N bond in TiO2 [37–41]. Lower BE value (396 eV) was reported for N in N-doped TiO2 when it was synthesized by calcination at high temperature in nitrogen flow. Higher BE value for N (around 399 eV) has been reported for samples synthesized through soft chemical route. In our work, as the N doped sample is synthesized at low temperature, the peak seen at 398 eV can be assigned to the N substituted for O in TiO2 . S 2p XPS of NS doped samples are shown in Fig. 4. The BE of the S 2p peak for NS-TiO2 , NS-WO3 and NS-T4 W are 163.3, 162.9 and 162.6 eV respectively. S 2p peak in the BE region of 160–163 eV is assigned to the Ti-S bond, which is formed by the substitution of one oxygen by an S atom [42]. Hence, S2p XPS suggests that S has got substituted anionically in TiO2 , WO3 and T4 W samples. It may also be mentioned that no peak is seen in the region of 167–170 eV indicating that surface sulfate or cationically substituted S6+ is absent in our samples. The Ti 2p, W4f and O1s spectra of all samples are shown in Fig. 5. Ti 2p spectra of all samples show two peaks with BE around 458 (2p3/2 ) and 463 eV (2p1/2 ), which are close to the value reported for Ti4+ oxidation state in TiO2 [43]. The spectrum of WO3 is fitted as three peaks having BE of 31.7, 35.8 and 37.9 eV. The peaks at 35.8 and 37.9 eV can be assigned to the doublet, W 4f 7/2 and W 4f 5/2 of W6+ state [44]. The peak seen at 31.7 eV can be attributed to a Wx+ state, which is an intermediate oxidation state between W4+ and W0 [44]. NS-WO3 is fitted as four peaks with peak positions at 33.5, 35.5, 35.8 and 37.7 eV. The peaks at BE of 33.5 and 35.8 can be assigned to the W 4f 7/2 and W 4f 5/2 doublet of W4+ state. The peaks at 35.5 and 37.7 eV can be assigned to the W 4f 7/2 and W 4f 5/2 components of W6+ oxidation state. NS-T4 W too shows the doublet of W4+ oxidation state at 33.5 eV (4f 7/2 ) and 35.8 eV (4f 5/2 ) and the doublet corresponding to W6+ state at 35.5 eV (4f 7/2 ) and 37.8 eV (4f 5/2 ). The O 1s XPS of all samples show a peak around 530 eV, which can be assigned to the metal-oxygen bond [43]. The additional higher binding energy peak seen in all spectra can be assigned to the OH group present on the surface of these oxides [45–47]. In summary, the XPS results indicate that N and S get substituted anionically in WO3 and TiO2 by substituting oxygen from the lattice. The concentration of N and S in these samples obtained by the analysis of XPS peaks is ∼1.0% and 0.6% (atom%) respectively.

Fig. 6. Relative concentration of MO as a function of time when irradiated under UV and visible light using TiO2 , WO3 , T2W, T4W and T8W photocatalysts. ‘Dark’ indicates the experiment conducted under dark conditions using T4W catalyst. ‘Blank’ is the experiment carried out without catalyst.

3.4. Photocatalytic activity studies To study the photocatalytic activity of TiO2 –WO3 nanostructures, the photocatalytic degradation of MO (10 ppm) under UV as well as visible light irradiation was evaluated. The photocatalytic activity of TiO2 –WO3 composites of different compositions along with TiO2 and WO3 are shown in Fig. 6. It can be seen that the composites of all compositions show improved photocatalytic activity compared to single phase TiO2 or WO3 . Among the composites, T4W shows the highest degradation activity and it is found to be the optimum concentration. It may be noted that when no catalyst is used (blank) slow degradation occurs under UV irradiation whereas under dark conditions (T4W suspended in 10 ppm MO solution) no significant degradation is seen. It may be seen that when irradiated with visible light, a similar trend is seen and T4W shows the highest activity (Fig. 6). Almost no degradation of MO is observed when irradiated with visible light without a catalyst. Fig. 7 shows the photocatalytic activity of N and S co-doped T4W along with NS-TiO2 , NS-WO3 , TiO2 and WO3 under both UV and visible light irradiation. Photocatalytic activity of N doped T4W and S doped T4W was also studied and the results are shown in Fig. S4 and S5 of Supplementary data. A significant increase in photocatalytic activity is exhibited by NS-T4W composite compared to NS-TiO2 ,

Fig. 7. Relative concentration of MO as a function of time when irradiated under UV and visible light using TiO2 , WO3 , T4W, NS-TO2 , NS-WO3 and NS-T4W photocatalysts.

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Fig. 8. A Schematic illustration of the modification of the band gap of TiO2 by N and S co-doping. B) Schematic illustration of the possible electron transfer process occurring in NS-T4W nanocomposite.

Table 1 Apparent rate constant values for MO degradation reaction under UV and visible light irradiation. Sr. No

Sample

k(apparent) min−1 (for initial 30 min)UV light

k(apparent) min−1 (for initial 90 min) visible light

12345678

TiO2 WO3 T4W T2W T8W NS-TiO2 NS-WO3 NS-T4W

1.6 × 10−2 1.3 × 10−2 6.4 × 10−2 2.9 × 10−2 4.9 × 10−2 2.3 × 10−2 3.6 × 10−2 1.6 × 10−1

2.0 × 10−4 5.0 × 10−4 1.9 × 10−2 7.7 × 10−3 1.1 × 10−2 3.5 × 10−3 3.9 × 10−3 2.3 × 10−2

NS-WO3 , S-T4W and N-T4W when irradiated with UV radiation. Complete degradation of MO occurs in 30 min when NS-T4W is used as catalyst. Under visible light irradiation also, NS-T4W shows the highest activity and almost complete degradation occurs within 150 min under these conditions. All doped catalysts, NS-TiO2 , NSWO3 , N-T4W and S-T4W, show enhanced activity compared to undoped samples. The apparent rate constant for the degradation reaction for different samples are given in Table 1. It is seen that N and S co-doped T4W has the highest rate constant value for MO degradation under both UV and visible light irradiation. Reproducibility of photocatalytic activity of NS-T4W under visible light irradiation was tested using three samples. The mean and standard deviation values obtained for the repetitive experiments are provided in Table S6 of Supplementary data. The results indicate that the activity is reproducible and almost complete degradation occurred within 120 min for all samples and gave a standard deviation of ∼1.1%. The enhanced photocatalytic activity of the N and S co-doped composite can be explained on the basis of improved visible light absorption and better separation of charge carriers occurring in this system. Increased visible light absorption occurs as a result of decreased bandgap and hence the absorption is extended to longer wavelengths. It is reported that N and S dopants can introduce additional levels within the bandgap of TiO2 /WO3 . The dopants N and S are known to create additional levels near the valence band (VB) [48,49]. There are different opinions regarding the position of the energy levels created by N and S dopants in TiO2 . According to some authors, discrete energy levels are created above the valence band [48], whereas some others suggest that these new levels get mixed with the VB of TiO2 [50]. In both cases, the bandgap is decreased and results in visible light absorption. For the NS-T4W system, it is proposed that the levels created by these dopants are getting mixed with the VB based on the UV–vis absorption spectra of these doped samples. In the absorption spectra, it is seen that absorption edge is red shifted and only one absorption edge is seen. Thus, N and S modify the VB of both TiO2 and WO3 and decrease the bandgap of TiO2 /WO3 effectively as shown in Fig. 8A and extend the absorption to longer wavelengths of visible light. Another reason for the increased activity of the TiO2 -WO3 and the NS-doped composite is due to the better separation of photogenerated charge carriers. The conduction band (CB) potential of

TiO2 is more negative (∼−0.2 V) than that of WO3 ∼+0.3 V with respect to NHE at pH = 0 [51]. When the N and S doped T4W system is irradiated, the photogenerated electrons from the CB of TiO2 can migrate to the CB of WO3 . As the VB potential of WO3 is slightly more positive (+3.1 V) than that of TiO2 (+3.0 V), photogenerated holes from WO3 can get transferred to TiO2 . This process minimizes the recombination of charge carriers and can enhance the photocatalytic activity. Thus more and more e− and h+ can reach the surface of the catalyst and initiate the degradation reaction. The schematic of the modified band structure of TiO2 and WO3 by different dopants and the possible electron transfer process occurring in this system is shown in Fig. 8B. The possible reactions occurring on the surface of the composite can be as follows. TiO2 + hv (UV) → TiO2 (eCB − +hVB + ) TiO2 (hVB + ) + H2 O → TiO2 + H+ + OH• TiO2 (hVB + ) + OH− → TiO2 + OH• TiO2 (eCB − ) + O2 → TiO2 + O2 O2

•−

•−

+H+ → HO2 •

WO3 + hv → WO3 (eCB − + hVB + ) WO3 (hVB + ) + H2 O → WO3 + H+ + OH• WO3 (hVB + ) + OH− → WO3 + OH• Dye + OH• → degradationproducts Dye + hVB + → oxidationproducts Dye +hVB + → Dye Dye

•+

•+

+ HO2 • → Degradationproducts

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.07. 015.

References

Fig. 9. Photocatalytic degradation of 10 ppm of MO using NS-T4W catalyst for different cycles.

3.5. Reusability study The recyclability of NS-T4W was tested for repeated cycles of degradation study under visible light and the results are shown in Fig. 9. It can be seen from Fig. 9 that the catalyst is stable and no significant decrease in the activity is observed after four cycles of experiment. A small decrease in the efficiency observed after each successive cycle can be due to the loss of the catalyst during repeated filtration and separation from the solution. XRD pattern of the used NS-T4W sample along with that of the fresh one is shown in Fig. S7 of Supplementary data. It can be seen that there is no significant change in the phases of the composite after use suggesting that the system is stable after prolonged use.

4. Conclusions A nanocomposite of TiO2 -WO3 exhibit enhanced photocatalytic activity for the degradation of methyl orange compared to single phase TiO2 and WO3 . The improved activity of the composite is due to the better separation of photogenerated charge carriers occurring in this system. N and S co-doping of the composite further enhances the activity by decreasing the band gap and extending the absorption of light to longer wavelengths. In the doped samples, anionic doping occurs by replacing oxygen atoms of TiO2 /WO3 with N and S. These composites are nanosized and both TiO2 and WO3 exist in a dispersed state. This nanocomposite catalyst is found to be stable and no significant decrease in the photocatalytic activity occurs during repeated cycles of degradation reaction. Though the present study shows enhanced photocatalytic activity and improved visible light absorption for TiO2 -WO3 by co-doping with N and S, efficient utilization of solar radiation still cannot take place because the bandgap of the modified system is 2.94 eV. For efficient utilization of solar radiation, the bandgap of the material has to be around 2 eV and hence more efforts are to be made in this direction in future.

Acknowledgements One of the authors, P. P. Hankare, is thankful to BRNS, Trombay, Mumbai and UGC for financial assistance through Major research project No. 2012/37C/58/BRNS/1431 and UGC-BSR faculty fellowship No. 18-1(46)/2013.

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