synthesis of wo3tio2 nanocomposites via sol–gel method

Journal of Alloys and Compounds 398 (2005) 200–202 Synthesis of WO 3 /TiO 2 nanocomposites via sol–gel method Huaming Yang a,∗ , Rongrong Shi a , Ke Zhang a , Yuehua Hu a , Aidong Tang b , Xianwei Li c a Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China b Institute of Functional Materials, School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China c Institute of Resources and Environmental Engineering, Technology Centre, Baoshan Iron and Steel Co. Ltd., Shanghai 201900, China Received 23 December 2004; received in revised form 31 January 2005; accepted 1 February 2005 Available online 4 March 2005 Abstract Synthesis of WO 3 /TiO 2 nanocomposites by a sol–gel method was investigated using differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Thermal treatment of the precursor at 400 ◦ C in air resulted in the formation of WO 3 /TiO 2 nanocomposites with a particle size of about 60nm. The c-axis parameter of TiO 2 in the WO 3 /TiO 2 nanocomposites, lower than that of pure TiO 2 , increased with increasing calcination time. Doping TiO 2 with WO 3 can lower its band gap and shift its optical response to the visible region. This nanocomposite should be effective as a visible-light-driven photocatalyst. © 2005 Elsevier B.V. All rights reserved. Keywords: WO 3 /TiO 2 nanocomposites; Photocatalysis; Sol–gel method; Lattice parameters 1. Introduction It is well known that nanosized TiO 2 powder is one of the suitable semiconductors for photocatalyst and has been widely applied in various photocatalytic fields, such as en- vironmental purification, decomposition of organic contam- inants and water photosplitting into H 2 and O 2 [1–5].How- ever, its properties, not only the photo-efficiency or activity but also the photoresponse, are not sufficient [6]. The vital snag of TiO 2 semiconductor is that it only absorbs a small portion of solar spectrum in the ultraviolet (UV) region (band gap energy of pure TiO 2 is 3.2 eV). Hence, in order to absorb maximum solar energy, it is necessary to shift the absorption threshold towards the visible region. The high recombina- tion ratio of photo-induced hole–electron pairs also reduces its catalytic efficiency. Recently, various modifications have been performed on nanosized TiO 2 to extend its optical ab- sorption edge into the visible light region and to improve its photocatalytic activity, including surface modification, metal depositing, transition metal and transition metal oxide com- plexes [7–13]. Coupling TiO 2 with WO 3 , which is a semi- ∗ Corresponding author. Tel.: +86 731 8830 549; fax: +86 731 8710 804. E-mail address: hmyang@mail.csu.edu.cn (H. Yang). conductor used as photocatalyst (E g = 2.8 eV), can achieve an efficient charge separation. It is reported that WO 3 /TiO 2 nanocomposites have higher photocatalytic activity [14,15]. In this paper, synthesis of WO 3 /TiO 2 nanocomposites via the sol–gel method was attempted. 2. Experimental details The starting materials were AR-grade Ti(OBu 4 ), am- monium tungstate and anhydrous alcohol. Ten millilitres Ti(OBu 4 ) was dissolved in 10 ml anhydrous alcohol, and ul- trasonicallydispersedtoformamixture.Five millilitres water was slowly dripped into the mixture, which wasstirred for1 h at room temperature. Then different additions of ammonium tungstate solution were dripped into the mixture according to the required amount of WO 3 in the WO 3 /TiO 2 nanocompos- ites. The pH value of the solution was kept to be 10. The so- lution was aged for 12h at ambient temperature, followed by filtering, washing for several times with deionized water and anhydrous alcohol, drying at 80 ◦ C for 12h to produce a pre- cursor. Subsequent calcination of the precursor at 400 ◦ C for different hours in air resulted in the formation of WO 3 /TiO 2 nanocomposites. 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.002 H. Yang et al. / Journal of Alloys and Compounds 398 (2005) 200–202 201 Differential thermal analysis (DTA) of the precursor was carried out using an SDT2960 thermal analyzer at a heat- ing rate of 10 ◦ /min. The structure of the sample was ex- amined using a D/max-␥A diffractometer (Cu K␣ radiation, λ = 0.154056nm). The morphology of the nanocomposites was observed using a JEM-200CX transmission electron mi- croscope (TEM). The UV–vis absorption spectra of the sam- ples were recorded on a Shimadzu UV-3101PC spectropho- tometer. 3. Results and discussion TheprecursorwassubjectedtoDTAanalysis.Thepurpose was to determine thetemperatures of possible decomposition and phase changes of the precursor during the thermal treat- ment. Fig. 1shows the typicalDTAcurves forthe as-prepared precursor. The exothermic peak round 280 ◦ C is associated with the decomposition of residual OH groups and the con- densationofnonbondedoxygen.Anexothermicpeakatabout 430 ◦ Cwasclearlyobserved,whichpossiblycanbeattributed to thecrystallization alterationof theanatase TiO 2 phase. But there still exists a little difference in exothermic peak around 430 ◦ C for the nanoparticle precursor with different amount of WO 3 . Fig. 2 shows the X-ray diffraction (XRD) patterns of 10 wt% and 20 wt% WO 3 /TiO 2 nanocomposites after cal- cination at 400 ◦ C for 12h. As can be seen, all the peaks can be assigned to anatase TiO 2 and no crystalline WO 3 was de- tected. Ma et al. [14] also reported the same result in their Fig. 1. DTA curves of the precursor with different amount of WO 3 (a) 10 wt%, (b) 20wt% and (c) 40 wt%. Fig. 2. XRD patterns of the WO 3 /TiO 2 nanocomposites with different amounts of WO 3 (a) 10 wt% and (b) 20wt%, respectively. Fig. 3. XRD patterns of 40 wt% WO 3 /TiO 2 nanocomposites after calcined at 400 ◦ C for different hours (a) 4h, (b) 8 h and (c) 12 h, respectively. studies, they thought that amorphous tungsten oxide phase covered the TiO 2 surface. Fig. 3 shows the XRD patterns of 40wt% WO 3 /TiO 2 nanocomposites after calcination at 400 ◦ C for (a) 4 h, (b) 8h and (c) 12h, respectively. As shown in Fig. 3, the WO 3 /TiO 2 nanocomposites prepared by the sol–gel method was ob- servedtohave theanatasestructureandtungstenoxide,show- ing the presence of a sharp peak at 25.3 ◦ of 2θ which is the major peak for the anatase TiO 2 . Due to the increase in the calcination time, the intensities of the peaks associated with WO 3 increased to some extent. Fig. 4 shows a TEM micro- graph of the 40 wt% WO 3 /TiO 2 nanocomposites after heat treatmentat400 ◦ Cfor12 h. The particle size of the nanocom- posites observed in the TEM image was about 60nm indiam- eter, while monodispersive particles with uniform size were present. The lattice parameters (a and c) of pure TiO 2 and WO 3 /TiO 2 nanocomposites after calcination were calculated using the formula: 1 d 2 = h 2 + k 2 a 2 + l 2 c 2 (1) where d is the interplane spacing, h, k and l are all Miller’s indices. The values were listed in Table 1. The lattice param- eters of pure anatase TiO 2 obtained by the sol–gel method are a = b= 3.7523 ˚ A, c = 10.0664 ˚ A. It is clear that the lattice parameters increase along the a- and b-axes while the c-axis parameter decreases as tungsten oxide was doped. Longer Fig. 4. TEM image of the 40 wt% WO 3 /TiO 2 nanocomposites. 202 H. Yang et al. / Journal of Alloys and Compounds 398 (2005) 200–202 Table 1 The lattice parameters of TiO 2 in WO 3 /TiO 2 nanocomposites a Samples Calcination time (h) a (=b)( ˚ A) c ( ˚ A) Pure TiO 2 12 3.7523 10.0664 10 wt% WO 3 /TiO 2 12 3.7961 9.3540 20 wt% WO 3 /TiO 2 12 3.7836 9.6761 40 wt% WO 3 /TiO 2 12 3.7903 9.5307 40 wt% WO 3 /TiO 2 8 3.7908 9.5172 40 wt% WO 3 /TiO 2 4 3.7914 9.5100 a Calcination temperature was 400 ◦ C. Fig. 5. UV–vis absorption spectra of the sample. calcination time resulted in an increase in c-axis parameter of the 40 wt% WO 3 /TiO 2 nanocomposites. The decrease in the lattice parameters of the WO 3 /TiO 2 nanocomposites in comparison to pure TiO 2 may be attributed to the decrease in the cation size in the octahedral site. The W 6+ ions have a lower ionic radius (41 pm) than Ti 4+ (53 pm) in the octa- hedral site of TiO 2 . This result also indicates that a doping effect exists in the WO 3 /TiO 2 composite nanocrystallites. Fig. 5 shows the absorption spectrum of the WO 3 /TiO 2 nanocomposites. The spectrum shows that the onset of ab- sorption appears at about 475 nm. The onset of the optical absorption of WO 3 /TiO 2 particles relative to the bulk anatase TiO 2 (λ E = 387 nm) implies a red shift. The band gap energy of the nanocomposites can be determined to be 2.67 eV from the transformed Kubelka–Munk function, while the band gap energy of pure TiO 2 is about 3.2eV, accordingly, this absorp- tion feature suggests that the WO 3 /TiO 2 photocatalyst can possibly be activated by the visible light, which can absorb the maximum solar energy. 4. Conclusions In summary, WO 3 /TiO 2 nanocomposites have been suc- cessfully prepared by a sol–gel method. The intensities of the XRD peaks associated with TiO 2 increased gradually with increasing calcination time. Addition of WO 3 resulted in a decrease in the c-axis parameter of the TiO 2 , which also in- creasedwithincreasingcalcinationtime.Thisnanocomposite is promising for high-performance visible-light-driven pho- tocatalysts. A detailed study on the photocatalytic activity of the nanocomposite is in progress. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 50304014, 50474046). References [1] H.S. Bae, M.K. Lee, Colloids Surf. A 220 (2003) 169. [2] L. Znaidi, R. S ´ eraphimova, Mater. Res. Bull. 36 (2001) 811. [3] Q. Zhang, L. Gao, J. Guo, Appl. Catal. B 26 (2000) 207. [4] S. Goeringer, C.R. Chenthamarakshan, K. Rajeshwar, Electrochem. Commun. 3 (2001) 290. [5] Z. Wang, U. Helmersson, P O. Kall, Thin Solid Films 405 (2002) 50. [6] T. Kawai, T. Sakata, J. Chem. Soc. Chem. Commun. 15 (1980) 694. [7] T. Ohno, S. Saito, K. Fujihara, M. Matsumura, Bull. Chem. Soc. Jpn. 69 (1996) 3059. [8] B. Ohtani, K. Iwai, S. Nishimoto, S. Sato, J. 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