- Email: [email protected]
Preparation of nanocrystalline Sn–TiO2−X via a rapid and simple stannous chemical reducing route
Applied Surface Science 255 (2009) 5896–5901
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of nanocrystalline Sn–TiO2X via a rapid and simple stannous chemical reducing route Baifu Xin a,b,*, Dandan Ding a, Yina Gao a, Xinghai Jin a, Honggang Fu a, Peng Wang b a Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China b School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 November 2008 Received in revised form 5 January 2009 Accepted 9 January 2009 Available online 20 January 2009
The Sn–TiO2X nanoparticles have been prepared via a rapid and simple stannous chemical reducing method. The as-prepared Sn–TiO2X nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), XPS, and DRS technology as well as photocatalytic degradation of RhB were studied under illumination. The experiment results revealed that the reduction of the TiO2 particles raised their Fermi level, which can enhance the driven force of photoinduced electrons transferring from TiO2 to adsorbed O2 and SnO2 on the surface of TiO2. On the other hand, the amount of oxygen vacancies of the Sn–TiO2X increased after the stannous chemical reduction. The oxygen vacancies can also effectively inhibit the recombination of photoinduced electrons and holes pairs. These factors are favorable to the photocatalytic reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: TiO2 Stannous Chemical reduction Photocatalysis
1. Introduction TiO2, a wide band gap semiconductor, has been widely employed in the photocatalytic decontamination treatment of polluted water and air purification [1–5]. However, the efficiency of photocatalytic reactions is limited by the high recombination rate of photoinduced electron–hole pairs formed in photocatalytic processes and by the absorption capability for visible light of photocatalysts. Many studies have been devoted to the improvement of photocatalytic efficiency of TiO2, such as depositing noble metals [6–14] and doping metal or nonmetal ions [15–20]. Also, the photocatalytic activity can be increased using coupled semiconductor particles, especially SnO2/TiO2 system. Lin and coworkers [21,22] have found that Sn(IV) substitution for Ti(IV) in rutile TiO2 increases the photocatalytic activity of titanium oxide powders up to 15 times for the degradation of acetone. Zheng et al. [23] were able to increase the TiO2 photocatalytic activity in the degradation of rhodamine B dye by the implantation of tin ions into TiO2 films. Also the results of Cao et al. show an improvement of the photocatalytic activity of titanium oxide by doping the anatase lattice with Sn4+ ions in thin films prepared by plasma enhanced chemical vapor deposition [24]. There is a suppressing of the recombination process by coupling two different semiconduc-
* Corresponding author at: School of Chemistry and Materials Science, Heilongjiang University, XueFu Road No. 74, Harbin 150080, China. Tel.: +86 451 86608715; fax: +86 451 86608781. E-mail address: [email protected] (B. Xin). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.027
tors with dissimilar Fermi levels, i.e. the introduction of SnO2 to the TiO2 produces a more efficient separation of the photogenerated pairs [21,22,24]. In addition, the thermal hydrogen (H2) treatment of TiO2 was also found to be capable to accelerate the e–h separation [25,26]. Heller et al. reported the enhanced photocatalytic activity of the TiO2 catalyst by a thermal reduction treatment using hydrogen gas at 550 8C [26]. Liu et al. reported that conventional TiO2 powder was heated in hydrogen gas at a high temperature [27]. Their experimental results proved the presence of oxygen vacancy and Ti3+ in the lattice of the H2-treated TiO2 and indicated that both were contributed to the enhancement of photocatalytic activity. However, there were many insufficiencies in the process of the thermal hydrogen (H2) treatment of TiO2, such as, there are the factors of inflammable and explosive, the technologies route is complicated, furthermore, this method is too time-consuming and expensive for this process. These factors limited its practical application. To overcome these difficulties and disadvantages, in this paper, a rapid and simple method, the so-called stannous chemical reducing method was developed to prepare the nanocrystalline Sn–TiO2X. Firstly, nanocrystalline TiO2 was synthesized by sol–gel method, and then it was immerged into the fresh aqueous solution of SnCl2. Based on the following electrode potential of TiO2 and SnO2: 2TiO2 þ 2Hþ þ 2e ! Ti2 O3 þ H2 O
E0 ¼ 0:556 V
(1)
SnO2 þ 4Hþ þ 2e ! Sn2þ þ 2H2 O
E0 ¼ 0:770 V
(2)
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5897
Sn2+ ion reduced TiO2 to Ti2O3 and formed SnO2 on the surface of TiO2. Such, the nanocrystalline Sn–TiO2X was prepared. This method has many advantages over the conventional thermal hydrogen (H2) treatment processes, such as, simple technologies and facilities, easy operation, safety, low cost, so, which has more significant practical value. The photocatalyst prepared by the stannous chemical reducing method have efficacies of both the thermal hydrogen (H2) treatment and Sn4+ doping modification. This work aimed at studying the TiO2 photocatalytic oxidation affected by the stannous chemical reduction, in which RhB was used as a model chemical. 2. Experimental 2.1. The preparation of Sn–TiO2X nanoparticles Specimen of nanocrystalline TiO2 was prepared by sol–gel method. 7.6 ml of Ti(OBut)4 was dissolved in 5.5 ml of anhydrous ethanol under vigorous stirring. In order to control the hydrolyzation, 3 ml of CH3COOH was added to the solution before suitable amount of deionized water (2 ml) was added to it. The resulting transparent colloidal liquid was continuously stirred till the gel was formed. The gel was dried at 100 8C for 6 h, calcined at 400 8C for 2 h in air and ground to obtain the TiO2 nanoparticles. An amount of TiO2 nanoparticles was immerged into 20 ml different concentration of the fresh aqueous solution of SnCl2, and retained at 10, 15, 30, and 60 min, respectively. Then, centrifuged, washed three times with deionized water, dried at 100 8C for 2 h and ground to obtain the nanocrystalline Sn–TiO2X. They were named as Sn–TiO2X-A-B, respectively. Here, A refers to impregnation time; B refers to the concentration of the SnCl2 aqueous solution.
Fig. 1. The XRD patterns of pure TiO2 and the Sn–TiO2X-15-1.
dark for 20 min to establish an adsorption/desorption equilibrium condition prior to the photoreaction. Then, the light was turned on to initiate the reaction. The collected samples after filtration and centrifugation were measured by a Shimadzu UV-2550 UV–vis spectrometer to determine the concentration of RhB solution before and after photocatalytic degradation.
3. Results and discussion
2.2. Characterization of samples
3.1. XRD and EDS measurement
XRD analysis of TiO2 and Sn–TiO2X powders was carried out on a Rigaku D/MAX-rA X-ray diffractometer, employing Cu Ka (Ni filtered) radiation l = 0.15418 nm. The patterns were recorded in a range of 10–658 (2u). XPS spectra was recorded with an Escalab MK II (VG Company, UK). All binding energies (BE) were calibrated by the BE (284.6 eV) of C 1s, which gave BE values within an accuracy of 0.1 eV. Energy dispersive X-ray spectroscopy (EDS) was recorded on a KYKY2000 SEM. The diffuse reflection spectra of the powders were obtained in the wavelength range 300–700 nm using a Shimadzu UV-2550 UV–vis spectrometer equipped with the integrating sphere accessory. A specimen thin film was placed in the sample holder on integrated sphere for the reflectance measurements. The BaSO4 was used as the reference material. The SPS instrument was assembled at Jilin University, monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF-XQ500W, China) through a double-prism monochromator (SBP300, China). The slit widths of entrance and exit were 2 and 1 mm, respectively. A lock-in amplifier (SR830, USA), synchronized with a light chopper (SR540, USA), was employed to amplify the photovoltage signal. The powder sample is sandwiched between two ITO glass electrodes.
XRD was usually used for identification of the crystal phase and estimation of crystallite size and degree of crystallinity. The XRD peaks at 2u = 25.258 and 48.08 in the spectrum of TiO2 are easily identified as the crystal of anatase form. The crystallite size can also be determined from the broadening of corresponding X-ray spectral peaks by Scherrer formula [28]: L ¼ K l=ðbcosuÞ, where L is the crystallite size, l is the wavelength of the X-ray radiation (Cu Ka = 0.15418 nm), K is usually taken as 0.89, and b is the line width at half-maximum height. Fig. 1 shows the XRD patterns of pure TiO2 and the Sn–TiO2X15-1 nanoparticles, respectively. The XRD results suggest that there have been no change in the crystal phase and crystal sizes of samples after the stannous chemical reduction. There are no peaks that indicate the presence of SnO2 and other Sn species within the limit of detection after the stannous chemical reduction. However, the EDS spectra of the Sn–TiO2X-15-1 sample (Fig. 2) shows that there is a little stannum element in the sample.
2.3. Evaluation of photocatalytic activity of Sn–TiO2X The photocatalytic degradation of RhB over Sn–TiO2X was carried out in a home-built reactor. A 160 W high-pressure mercury lamp was used as light source, whose intensity was 10,000 lx. In each run 0.10 g TiO2 catalyst was added into 20 ml RhB solution of 10 mg/l. To eliminate the adsorption effect, the RhB solution after adding TiO2 powder was magnetically stirred in the
3.2. XPS characterization The XPS spectra of O 1s of TiO2 (A) and the Sn–TiO2X-15-1 (B) are shown in Fig. 3. They were fitted with the non-linear least square fit program using Gauss–Lorentzian peak shapes and three O 1s peaks appear after deconvolution, which are attributed to lattice oxygen (OL, 530.1 eV), surface hydroxyl oxygen (O–OH, 531.6 eV) and adsorbed oxygen (OS, 533.0 eV) in TiO2 [29]. According to the principle method and handbook of the XPS instrument, the relatively quantitative analysis can be performed by utilizing the XPS peak area of different elements and their own sensitivity factor according to the following equation: n(E1)/ n(E2) = [A(E1)/S(E1)]/[A(E2)/S(E2)]; where n is the atomic number, E is a element, S is elemental sensitivity factor. Thus, some
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5898
Fig. 2. EDS spectra of the Sn–TiO2X-15-1.
Fig. 4. XPS spectra of Cl 2p of the Sn–TiO2X-15-1 nanoparticles.
important XPS data could be obtained. The approximate formulas for calculating the percentage of oxygen vacancies are as follows: ððThe atomic number ratio of Ti and SnÞ 4Þ ðThe atomic number ratio ofOL 2Þ 100; V O¨ % ¼ 2 where VO¨ is the oxygen vacancy. Table 1 shows the XPS data of O and Ti elements on the surface of TiO2 and the Sn–TiO2X-15-1. Based on the data of Fig. 3 and Table 1, it can be found that the amount of surface hydroxyl oxygen and oxygen vacancies of the Sn–TiO2X-15-1 increased but the amount of crystal lattice oxygen and adsorbed oxygen decreased. The oxygen vacancies are a very active group, which can easily combine with other atoms or groups to become stable, and it accounts for an increase in the amount of
the surface hydroxyl oxygen [30,31]. An increase in the surface hydroxyl is favorable to photocatalytic reactions. But the percentage of adsorbed oxygen decrease in the Sn–TiO2X-15-1 may be unfavorable to photocatalytic reactions. The reason for the amount of adsorbed oxygen decreased is because of competitive adsorption of the Cl ion on the surface of TiO2 after treating by the SnCl2 aqueous solution, this can be verified by the XPS spectra of Cl 2p of the Sn–TiO2X-15-1 (Fig. 4). The Sn 3d XPS spectrum of the Sn–TiO2X-15-1 as shown in Fig. 5 demonstrates the existence of stannum species on the surface of TiO2. From Fig. 5, it can be seen that the Sn 3d5/2-binding energy (486.2 eV) of the Sn–TiO2X-15-1 is less than the reference data of Sn 3d5/2-binding energy (486.6 eV) reported by Zeller et al. [32]. Moreover, the Ti 2p binding energy of the Sn–TiO2X-15-1 sample is increased comparing with that of pure TiO2 as shown in
Fig. 3. XPS spectra of O 1s on the surface of TiO2 (A) and the Sn–TiO2X-15-1 (B) nanoparticles, respectively.
Table 1 XPS data of different chemical states of O and Ti elements on the surface of TiO2 and the Sn–TiO2X-15-1. Samples
TiO2 Sn–TiO2X-15-1
Binding energy (eV) O 1s (OL)
O 1s (O-OH)
O 1s (OS)
Ti 2p3/2
530.1 530.0
531.6 531.3
533.0 532.5
457.6 458.2
Atomic number ratio of OL and Ti and Sn
Percentage of OL
Percentage of O–OH
Percentage of OS
Evaluated percentage of oxygen vacancies
100:58.3 100:59.3:2.5
77.0 71.6
15.5 28.8
7.5 0.91
17 23.6
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
Fig. 5. XPS spectrum of Sn 3d on the surface of the Sn–TiO2X-15-1.
5899
Fig. 7. The DRS spectra of pure TiO2 and Sn–TiO2X.
ions on the surface of TiO2 nanoparticles were reduced to Ti3+ ions after treating by the SnCl2 aqueous solution. 3.3. DRS analysis Fig. 7 shows DRS spectra of the pure TiO2 and the Sn–TiO2X-151. The absorption edge can be calculated by inflection point of UV spectra. From Fig. 7, a novel blue shift of TiO2 absorption edge from 381.5 to 375.5 nm can be found after treating by the SnCl2 aqueous solution. This is because of that the reduction of the TiO2 particles raised their Fermi level and increased the Eg of TiO2. The experimental results consist with that of Heller reported [26]. 3.4. SPS analysis
Fig. 6. XPS spectra of Ti 2p of TiO2 and the Sn–TiO2X-15-1 nanoparticles.
Fig. 6. This is because that the Fermi levels of SnO2 is lower than that of TiO2 so that the electrons of TiO2 may transfer to the SnO2 dispersed on the surface of TiO2, which results in changes in the outer electron cloud density of Ti and Sn ions. So, the Ti 2p binding energy increases and Sn 3d binding energy decrease. This fact suggests that there is an intense interaction between TiO2 and stannum species. On the other hand, we can see two new response peaks at about 455 and 460 eV as shown in Fig. 6, they are attributed to the Ti 2p1/2 and Ti 2p2/3 binding energies of Ti3+species, respectively [33]. This fact implies that a few Ti4+
The surface photovoltage (SPV) method is a well-established contactless technique for the characterization of semiconductors. It can offer important information about semiconductor surface, interface, and bulk properties, mainly reflecting the carrier separation and transfer behavior with the aid of light [34]. The SPS principle can be explained as shown in Fig. 8. The absorbed photons induce the formation of free carriers by creating electron–hole pairs via band-to-band transitions. The photoinduced electrons may be transferred from the surface to the bulk and the photogenerated holes can be moved to the surface under the built-in electric field. Thus, the surface potential across the surface SCR (space charge region) change from V1s to V1s0 . It is the signal of SPS that the variational value of surface potential before and after illumination during the test, i.e. DV1.
Fig. 8. The effect of the stannous chemical reduction on the surface photovoltage of TiO2.
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5900
Fig. 9. SPS spectra of the pure TiO2 and the Sn–TiO2X-15-1.
While the TiO2 nanoparticles were treated via the stannous chemical reduction route, it can be found that the SPS spectra of pure TiO2 is different from that of the Sn–TiO2X, The SPS intensity of the Sn–TiO2X is decreased obviously as shown in Fig. 9. This is because that (1) the reduction of the TiO2 particles raised their Fermi level and Ec, which make the absolute value of surface potential decrease from V1s to V2s. (2) Due to the Fermi levels of SnO2 is lower than that of TiO2, which leads to the photoinduced electrons transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2, not to the bulk of TiO2. Meanwhile, the raised Fermi level makes the driven force of the photoinduced electrons from TiO2 to SnO2 increase. These factors result in that the SPS intensity of Sn–TiO2X, i.e. DV2 is much less than that of pure TiO2 during the illumination. Thus, the recombination of photoinduced electron–hole pairs is inhibited, which is of benefit to the photocatalytic reaction. It is obvious that descend of SPS intensity of Sn–TiO2X derives from the common effects of SnO2 and Ti3+ ions. 3.5. Evaluation of photocatalytic activity Figs. 10 and 11 show the photocatalytic decoloration curves of RhB over TiO2 and Sn–TiO2X photocatalysts synthesized by immerging different time in different concentration of SnCl2 solution. The experimental results reveal that the photocatalytic activity of Sn–TiO2X is the highest over Sn–TiO2X-15-1, this is because of that the larger the concentration of SnCl2 solution, the higher the redox potential of SnCl2. So, the decoloration ratio of
Fig. 11. The photocatalytic decoloration ratio curve of the Sn–TiO2X-A-1 samples.
RhB is increased with increasing of the concentration of SnCl2 solution from 0 up to 1 mol/l. While at the higher concentration of SnCl2 solution and for a longer impregnation time, excessive SnO2 can cover the surface of TiO2, leading to decrease in the photocatalytic activity of photocatalyst. It can be found that the photocatalytic activity of Sn–TiO2X is better than that of pure TiO2, which could be attributed to the following: (1) The reduction of the TiO2 particles raised their Fermi level and Ec, which make the reductive potential of photoinduced electrons on the conductive band of TiO2 increase; meanwhile, the driven force of photoinduced electrons transferring from TiO2 to adsorbed O2 also increase, which improves the separation of photoinduced electrons and holes. Based on the following photochemistry reaction process, the amount of hydroxyl in reaction system is increased. Furthermore, the amount of surface hydroxyl oxygen of Sn–TiO2X is more than that of pure TiO2 according to XPS analysis. ecb þ O2 ! O2
(1)
H2 O þ O2 ! HOO þ OH
(2)
2HOO ! H2 O2 þ O2
(3)
H2 O2 þ ecb ! OH þ OH
(4)
(2) The amount of oxygen vacancies of the Sn–TiO2X increased, which can effectively capture the photoinduced electrons and inhibit the recombination of photoinduced electrons and holes. (3) The Fermi levels of SnO2 is lower than that of TiO2, which leads to the photoinduced electrons transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2, meanwhile, the raised Fermi level makes the driven force of the photoinduced electrons from TiO2 to SnO2 increase, these factors can also increase the separation ratio of photoinduced electrons and holes. It is obvious that the increase in the photocatalytic activity of Sn–TiO2X can be attributed to the common effects of SnO2 and Ti3+ ions. 4. Conclusions
Fig. 10. The photocatalytic decoloration ratio curve of the Sn–TiO2X-15-B samples.
The Sn–TiO2X nanoparticles have been prepared via a rapid and simple method, the so-called stannous chemical reducing method. The TiO2 photocatalytic oxidation affected by the stannous chemical reduction and also the correlation between the photocatalytic activity and the change of TiO2 structure were investigated by means of XPS, SPS and DRS technology. The experimental results suggest that: (1) the reduction of the TiO2 particles raised their Fermi level; (2) the amount of surface hydroxyl and oxygen vacancies of the Sn–TiO2X increased; (3) the
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
photoinduced electrons can transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2. These factors can effectively inhibit the recombination of photoinduced electrons and holes pairs. So, the photocatalytic activity of TiO2 was markedly improved. It is obvious that the increase in the photocatalytic activity of Sn–TiO2X can be attributed to the common effects of SnO2 and Ti3+ ions. Acknowledgments
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
This project is supported from the Key Program Projects of National Natural Science Foundation of China (No. 20431030), the Foundation for Excellent Youth of Heilongjiang University (2005), the Foundation for Youth Science and Technology Innovation Talents of Harbin of China (No. 2006RFQXS096), the Key Item Projects of Educational Department of Heilongjiang Province of China (No. 1152Z002), for which the authors are very grateful. References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1–21. [2] S. Brosillon, L. Lhomme, C. Vallet, A. Bouzaza, D. Wolbert, Appl. Catal. B 78 (2008) 232–241. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [4] G. Cappelletti, C.L. Bianchi, S. Ardizzone, Appl. Catal. B 78 (2008) 193–201. [5] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Environ. Sci. Technol. 38 (2004) 1600–1604. [6] M. Bowker, D. James, P. Stone, R. Bennett, N. Perkins, L. Millard, J. Greaves, A. Dickinson, J. Catal. 217 (2003) 427–433. [7] C. Young, T.M. Lim, K. Chiang, J. Scott, R. Amal, Appl. Catal. B 78 (1–2) (2008) 1–10.
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
5901
J. Sa´, J.A. Anderson, Appl. Catal. B 77 (3–4) (2008) 409–417. T.G. Schaaff, D.A. Blom, Nano Lett. 2 (2002) 507–511. E. Stathatos, P. Lianos, P. Falaras, A. Siokou, Langmuir 16 (2000) 2398–2400. C.B. Zhang, H. He, K. Tanaka, Appl. Catal. B 65 (1–2) (2006) 37–43. H. Einaga, T. Ibusuki, S. Futamura, Environ. Sci. Technol. 38 (2004) 285–289. B. Sun, A.V. Vorontsov, P.G. Smirniotis, Langmuir 19 (2003) 3151–3156. S. Kim, W. Choi, J. Phys. Chem. B 106 (2002) 13311–13317. S. Jeon, P.V. Braun, Chem. Mater. 15 (2003) 1256–1263. O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates Jr., J. Phys. Chem. B 108 (2004) 6004–6008. M. Mrowetz, W. Balcerski, A.J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 108 (2004) 17269–17273. A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650– 3655. J. Premkumar, Chem. Mater. 16 (2004) 3980–3981. J.D. Bryan, S.M. Heald, S.A. Chambers, D.R. Gamelin, J. Am. Chem. Soc. 126 (2004) 11640–11647. Y.R. Do, W. Lee, K. Dwight, A. Wold, J. Solid State Chem. 108 (1994) 198. J. Lin, J.C. Yu, J. Lo, S.K. Lan, J. Catal. 183 (1999) 368. S.K. Zheng, T.M. Wang, W.C. Hao, R. Shen, Vacuum 65 (2002) 155. Y. Cao, W. Yang, W. Zhang, G. Liu, P. Yue, New J. Chem. 2 (2004) 218. L.A. Harris, R. Schumacher, J. Electrochem. Soc. 127 (5) (1980) 1186–1188. A. Heller, Y. Degani, D.W. Johnson, P.K. Gallagher, J. Phys. Chem. 91 (23) (1987) 5987–5991. H. Liu, H.T. Ma, X.Z. Li, W.Z. Li, M. Wu, X.H. Bao, Chemosphere 50 (2003) 39–46. Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B 26 (2000) 207. S.K. Poznysk, V.I. Pergushov, A.I. Kokorin, A.I. Kulak, C.W. Schlpafer, J. Phys. Chem. B 103 (1999) 1308–1315. B.F. Xin, L.Q. Jing, Z.Y. Ren, B.Q. Wang, H.G. Fu, J. Phys. Chem. B 109 (2005) 2805– 2809. E.L. Bullock, L. Patthey, S.G. Steinemann, Surf. Sci. 352–354 (1996) 511–517. M.V. Zeller, P.A. Grutsch, T.P. Fehlner, Inorg. Chem. 12 (6) (1973) 1431–1433. Ju.F. Huravlev, M.V. Kuznetsov, V.A. Gubanov, J. Electron. Spectrosc. Relat. Phenom. 38 (1992) 169–176. L.Q. Jing, X.J. Sun, J. Shang, W.M. Cai, Z.L. Xu, Y.G. Du, H.G. Fu, Sol. Energ. Mater. Sol. C 79 (2003) 133–151.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of nanocrystalline Sn–TiO2X via a rapid and simple stannous chemical reducing route Baifu Xin a,b,*, Dandan Ding a, Yina Gao a, Xinghai Jin a, Honggang Fu a, Peng Wang b a Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China b School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 November 2008 Received in revised form 5 January 2009 Accepted 9 January 2009 Available online 20 January 2009
The Sn–TiO2X nanoparticles have been prepared via a rapid and simple stannous chemical reducing method. The as-prepared Sn–TiO2X nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), XPS, and DRS technology as well as photocatalytic degradation of RhB were studied under illumination. The experiment results revealed that the reduction of the TiO2 particles raised their Fermi level, which can enhance the driven force of photoinduced electrons transferring from TiO2 to adsorbed O2 and SnO2 on the surface of TiO2. On the other hand, the amount of oxygen vacancies of the Sn–TiO2X increased after the stannous chemical reduction. The oxygen vacancies can also effectively inhibit the recombination of photoinduced electrons and holes pairs. These factors are favorable to the photocatalytic reaction. ß 2009 Elsevier B.V. All rights reserved.
Keywords: TiO2 Stannous Chemical reduction Photocatalysis
1. Introduction TiO2, a wide band gap semiconductor, has been widely employed in the photocatalytic decontamination treatment of polluted water and air purification [1–5]. However, the efficiency of photocatalytic reactions is limited by the high recombination rate of photoinduced electron–hole pairs formed in photocatalytic processes and by the absorption capability for visible light of photocatalysts. Many studies have been devoted to the improvement of photocatalytic efficiency of TiO2, such as depositing noble metals [6–14] and doping metal or nonmetal ions [15–20]. Also, the photocatalytic activity can be increased using coupled semiconductor particles, especially SnO2/TiO2 system. Lin and coworkers [21,22] have found that Sn(IV) substitution for Ti(IV) in rutile TiO2 increases the photocatalytic activity of titanium oxide powders up to 15 times for the degradation of acetone. Zheng et al. [23] were able to increase the TiO2 photocatalytic activity in the degradation of rhodamine B dye by the implantation of tin ions into TiO2 films. Also the results of Cao et al. show an improvement of the photocatalytic activity of titanium oxide by doping the anatase lattice with Sn4+ ions in thin films prepared by plasma enhanced chemical vapor deposition [24]. There is a suppressing of the recombination process by coupling two different semiconduc-
* Corresponding author at: School of Chemistry and Materials Science, Heilongjiang University, XueFu Road No. 74, Harbin 150080, China. Tel.: +86 451 86608715; fax: +86 451 86608781. E-mail address: [email protected] (B. Xin). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.027
tors with dissimilar Fermi levels, i.e. the introduction of SnO2 to the TiO2 produces a more efficient separation of the photogenerated pairs [21,22,24]. In addition, the thermal hydrogen (H2) treatment of TiO2 was also found to be capable to accelerate the e–h separation [25,26]. Heller et al. reported the enhanced photocatalytic activity of the TiO2 catalyst by a thermal reduction treatment using hydrogen gas at 550 8C [26]. Liu et al. reported that conventional TiO2 powder was heated in hydrogen gas at a high temperature [27]. Their experimental results proved the presence of oxygen vacancy and Ti3+ in the lattice of the H2-treated TiO2 and indicated that both were contributed to the enhancement of photocatalytic activity. However, there were many insufficiencies in the process of the thermal hydrogen (H2) treatment of TiO2, such as, there are the factors of inflammable and explosive, the technologies route is complicated, furthermore, this method is too time-consuming and expensive for this process. These factors limited its practical application. To overcome these difficulties and disadvantages, in this paper, a rapid and simple method, the so-called stannous chemical reducing method was developed to prepare the nanocrystalline Sn–TiO2X. Firstly, nanocrystalline TiO2 was synthesized by sol–gel method, and then it was immerged into the fresh aqueous solution of SnCl2. Based on the following electrode potential of TiO2 and SnO2: 2TiO2 þ 2Hþ þ 2e ! Ti2 O3 þ H2 O
E0 ¼ 0:556 V
(1)
SnO2 þ 4Hþ þ 2e ! Sn2þ þ 2H2 O
E0 ¼ 0:770 V
(2)
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5897
Sn2+ ion reduced TiO2 to Ti2O3 and formed SnO2 on the surface of TiO2. Such, the nanocrystalline Sn–TiO2X was prepared. This method has many advantages over the conventional thermal hydrogen (H2) treatment processes, such as, simple technologies and facilities, easy operation, safety, low cost, so, which has more significant practical value. The photocatalyst prepared by the stannous chemical reducing method have efficacies of both the thermal hydrogen (H2) treatment and Sn4+ doping modification. This work aimed at studying the TiO2 photocatalytic oxidation affected by the stannous chemical reduction, in which RhB was used as a model chemical. 2. Experimental 2.1. The preparation of Sn–TiO2X nanoparticles Specimen of nanocrystalline TiO2 was prepared by sol–gel method. 7.6 ml of Ti(OBut)4 was dissolved in 5.5 ml of anhydrous ethanol under vigorous stirring. In order to control the hydrolyzation, 3 ml of CH3COOH was added to the solution before suitable amount of deionized water (2 ml) was added to it. The resulting transparent colloidal liquid was continuously stirred till the gel was formed. The gel was dried at 100 8C for 6 h, calcined at 400 8C for 2 h in air and ground to obtain the TiO2 nanoparticles. An amount of TiO2 nanoparticles was immerged into 20 ml different concentration of the fresh aqueous solution of SnCl2, and retained at 10, 15, 30, and 60 min, respectively. Then, centrifuged, washed three times with deionized water, dried at 100 8C for 2 h and ground to obtain the nanocrystalline Sn–TiO2X. They were named as Sn–TiO2X-A-B, respectively. Here, A refers to impregnation time; B refers to the concentration of the SnCl2 aqueous solution.
Fig. 1. The XRD patterns of pure TiO2 and the Sn–TiO2X-15-1.
dark for 20 min to establish an adsorption/desorption equilibrium condition prior to the photoreaction. Then, the light was turned on to initiate the reaction. The collected samples after filtration and centrifugation were measured by a Shimadzu UV-2550 UV–vis spectrometer to determine the concentration of RhB solution before and after photocatalytic degradation.
3. Results and discussion
2.2. Characterization of samples
3.1. XRD and EDS measurement
XRD analysis of TiO2 and Sn–TiO2X powders was carried out on a Rigaku D/MAX-rA X-ray diffractometer, employing Cu Ka (Ni filtered) radiation l = 0.15418 nm. The patterns were recorded in a range of 10–658 (2u). XPS spectra was recorded with an Escalab MK II (VG Company, UK). All binding energies (BE) were calibrated by the BE (284.6 eV) of C 1s, which gave BE values within an accuracy of 0.1 eV. Energy dispersive X-ray spectroscopy (EDS) was recorded on a KYKY2000 SEM. The diffuse reflection spectra of the powders were obtained in the wavelength range 300–700 nm using a Shimadzu UV-2550 UV–vis spectrometer equipped with the integrating sphere accessory. A specimen thin film was placed in the sample holder on integrated sphere for the reflectance measurements. The BaSO4 was used as the reference material. The SPS instrument was assembled at Jilin University, monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF-XQ500W, China) through a double-prism monochromator (SBP300, China). The slit widths of entrance and exit were 2 and 1 mm, respectively. A lock-in amplifier (SR830, USA), synchronized with a light chopper (SR540, USA), was employed to amplify the photovoltage signal. The powder sample is sandwiched between two ITO glass electrodes.
XRD was usually used for identification of the crystal phase and estimation of crystallite size and degree of crystallinity. The XRD peaks at 2u = 25.258 and 48.08 in the spectrum of TiO2 are easily identified as the crystal of anatase form. The crystallite size can also be determined from the broadening of corresponding X-ray spectral peaks by Scherrer formula [28]: L ¼ K l=ðbcosuÞ, where L is the crystallite size, l is the wavelength of the X-ray radiation (Cu Ka = 0.15418 nm), K is usually taken as 0.89, and b is the line width at half-maximum height. Fig. 1 shows the XRD patterns of pure TiO2 and the Sn–TiO2X15-1 nanoparticles, respectively. The XRD results suggest that there have been no change in the crystal phase and crystal sizes of samples after the stannous chemical reduction. There are no peaks that indicate the presence of SnO2 and other Sn species within the limit of detection after the stannous chemical reduction. However, the EDS spectra of the Sn–TiO2X-15-1 sample (Fig. 2) shows that there is a little stannum element in the sample.
2.3. Evaluation of photocatalytic activity of Sn–TiO2X The photocatalytic degradation of RhB over Sn–TiO2X was carried out in a home-built reactor. A 160 W high-pressure mercury lamp was used as light source, whose intensity was 10,000 lx. In each run 0.10 g TiO2 catalyst was added into 20 ml RhB solution of 10 mg/l. To eliminate the adsorption effect, the RhB solution after adding TiO2 powder was magnetically stirred in the
3.2. XPS characterization The XPS spectra of O 1s of TiO2 (A) and the Sn–TiO2X-15-1 (B) are shown in Fig. 3. They were fitted with the non-linear least square fit program using Gauss–Lorentzian peak shapes and three O 1s peaks appear after deconvolution, which are attributed to lattice oxygen (OL, 530.1 eV), surface hydroxyl oxygen (O–OH, 531.6 eV) and adsorbed oxygen (OS, 533.0 eV) in TiO2 [29]. According to the principle method and handbook of the XPS instrument, the relatively quantitative analysis can be performed by utilizing the XPS peak area of different elements and their own sensitivity factor according to the following equation: n(E1)/ n(E2) = [A(E1)/S(E1)]/[A(E2)/S(E2)]; where n is the atomic number, E is a element, S is elemental sensitivity factor. Thus, some
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5898
Fig. 2. EDS spectra of the Sn–TiO2X-15-1.
Fig. 4. XPS spectra of Cl 2p of the Sn–TiO2X-15-1 nanoparticles.
important XPS data could be obtained. The approximate formulas for calculating the percentage of oxygen vacancies are as follows: ððThe atomic number ratio of Ti and SnÞ 4Þ ðThe atomic number ratio ofOL 2Þ 100; V O¨ % ¼ 2 where VO¨ is the oxygen vacancy. Table 1 shows the XPS data of O and Ti elements on the surface of TiO2 and the Sn–TiO2X-15-1. Based on the data of Fig. 3 and Table 1, it can be found that the amount of surface hydroxyl oxygen and oxygen vacancies of the Sn–TiO2X-15-1 increased but the amount of crystal lattice oxygen and adsorbed oxygen decreased. The oxygen vacancies are a very active group, which can easily combine with other atoms or groups to become stable, and it accounts for an increase in the amount of
the surface hydroxyl oxygen [30,31]. An increase in the surface hydroxyl is favorable to photocatalytic reactions. But the percentage of adsorbed oxygen decrease in the Sn–TiO2X-15-1 may be unfavorable to photocatalytic reactions. The reason for the amount of adsorbed oxygen decreased is because of competitive adsorption of the Cl ion on the surface of TiO2 after treating by the SnCl2 aqueous solution, this can be verified by the XPS spectra of Cl 2p of the Sn–TiO2X-15-1 (Fig. 4). The Sn 3d XPS spectrum of the Sn–TiO2X-15-1 as shown in Fig. 5 demonstrates the existence of stannum species on the surface of TiO2. From Fig. 5, it can be seen that the Sn 3d5/2-binding energy (486.2 eV) of the Sn–TiO2X-15-1 is less than the reference data of Sn 3d5/2-binding energy (486.6 eV) reported by Zeller et al. [32]. Moreover, the Ti 2p binding energy of the Sn–TiO2X-15-1 sample is increased comparing with that of pure TiO2 as shown in
Fig. 3. XPS spectra of O 1s on the surface of TiO2 (A) and the Sn–TiO2X-15-1 (B) nanoparticles, respectively.
Table 1 XPS data of different chemical states of O and Ti elements on the surface of TiO2 and the Sn–TiO2X-15-1. Samples
TiO2 Sn–TiO2X-15-1
Binding energy (eV) O 1s (OL)
O 1s (O-OH)
O 1s (OS)
Ti 2p3/2
530.1 530.0
531.6 531.3
533.0 532.5
457.6 458.2
Atomic number ratio of OL and Ti and Sn
Percentage of OL
Percentage of O–OH
Percentage of OS
Evaluated percentage of oxygen vacancies
100:58.3 100:59.3:2.5
77.0 71.6
15.5 28.8
7.5 0.91
17 23.6
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
Fig. 5. XPS spectrum of Sn 3d on the surface of the Sn–TiO2X-15-1.
5899
Fig. 7. The DRS spectra of pure TiO2 and Sn–TiO2X.
ions on the surface of TiO2 nanoparticles were reduced to Ti3+ ions after treating by the SnCl2 aqueous solution. 3.3. DRS analysis Fig. 7 shows DRS spectra of the pure TiO2 and the Sn–TiO2X-151. The absorption edge can be calculated by inflection point of UV spectra. From Fig. 7, a novel blue shift of TiO2 absorption edge from 381.5 to 375.5 nm can be found after treating by the SnCl2 aqueous solution. This is because of that the reduction of the TiO2 particles raised their Fermi level and increased the Eg of TiO2. The experimental results consist with that of Heller reported [26]. 3.4. SPS analysis
Fig. 6. XPS spectra of Ti 2p of TiO2 and the Sn–TiO2X-15-1 nanoparticles.
Fig. 6. This is because that the Fermi levels of SnO2 is lower than that of TiO2 so that the electrons of TiO2 may transfer to the SnO2 dispersed on the surface of TiO2, which results in changes in the outer electron cloud density of Ti and Sn ions. So, the Ti 2p binding energy increases and Sn 3d binding energy decrease. This fact suggests that there is an intense interaction between TiO2 and stannum species. On the other hand, we can see two new response peaks at about 455 and 460 eV as shown in Fig. 6, they are attributed to the Ti 2p1/2 and Ti 2p2/3 binding energies of Ti3+species, respectively [33]. This fact implies that a few Ti4+
The surface photovoltage (SPV) method is a well-established contactless technique for the characterization of semiconductors. It can offer important information about semiconductor surface, interface, and bulk properties, mainly reflecting the carrier separation and transfer behavior with the aid of light [34]. The SPS principle can be explained as shown in Fig. 8. The absorbed photons induce the formation of free carriers by creating electron–hole pairs via band-to-band transitions. The photoinduced electrons may be transferred from the surface to the bulk and the photogenerated holes can be moved to the surface under the built-in electric field. Thus, the surface potential across the surface SCR (space charge region) change from V1s to V1s0 . It is the signal of SPS that the variational value of surface potential before and after illumination during the test, i.e. DV1.
Fig. 8. The effect of the stannous chemical reduction on the surface photovoltage of TiO2.
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
5900
Fig. 9. SPS spectra of the pure TiO2 and the Sn–TiO2X-15-1.
While the TiO2 nanoparticles were treated via the stannous chemical reduction route, it can be found that the SPS spectra of pure TiO2 is different from that of the Sn–TiO2X, The SPS intensity of the Sn–TiO2X is decreased obviously as shown in Fig. 9. This is because that (1) the reduction of the TiO2 particles raised their Fermi level and Ec, which make the absolute value of surface potential decrease from V1s to V2s. (2) Due to the Fermi levels of SnO2 is lower than that of TiO2, which leads to the photoinduced electrons transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2, not to the bulk of TiO2. Meanwhile, the raised Fermi level makes the driven force of the photoinduced electrons from TiO2 to SnO2 increase. These factors result in that the SPS intensity of Sn–TiO2X, i.e. DV2 is much less than that of pure TiO2 during the illumination. Thus, the recombination of photoinduced electron–hole pairs is inhibited, which is of benefit to the photocatalytic reaction. It is obvious that descend of SPS intensity of Sn–TiO2X derives from the common effects of SnO2 and Ti3+ ions. 3.5. Evaluation of photocatalytic activity Figs. 10 and 11 show the photocatalytic decoloration curves of RhB over TiO2 and Sn–TiO2X photocatalysts synthesized by immerging different time in different concentration of SnCl2 solution. The experimental results reveal that the photocatalytic activity of Sn–TiO2X is the highest over Sn–TiO2X-15-1, this is because of that the larger the concentration of SnCl2 solution, the higher the redox potential of SnCl2. So, the decoloration ratio of
Fig. 11. The photocatalytic decoloration ratio curve of the Sn–TiO2X-A-1 samples.
RhB is increased with increasing of the concentration of SnCl2 solution from 0 up to 1 mol/l. While at the higher concentration of SnCl2 solution and for a longer impregnation time, excessive SnO2 can cover the surface of TiO2, leading to decrease in the photocatalytic activity of photocatalyst. It can be found that the photocatalytic activity of Sn–TiO2X is better than that of pure TiO2, which could be attributed to the following: (1) The reduction of the TiO2 particles raised their Fermi level and Ec, which make the reductive potential of photoinduced electrons on the conductive band of TiO2 increase; meanwhile, the driven force of photoinduced electrons transferring from TiO2 to adsorbed O2 also increase, which improves the separation of photoinduced electrons and holes. Based on the following photochemistry reaction process, the amount of hydroxyl in reaction system is increased. Furthermore, the amount of surface hydroxyl oxygen of Sn–TiO2X is more than that of pure TiO2 according to XPS analysis. ecb þ O2 ! O2
(1)
H2 O þ O2 ! HOO þ OH
(2)
2HOO ! H2 O2 þ O2
(3)
H2 O2 þ ecb ! OH þ OH
(4)
(2) The amount of oxygen vacancies of the Sn–TiO2X increased, which can effectively capture the photoinduced electrons and inhibit the recombination of photoinduced electrons and holes. (3) The Fermi levels of SnO2 is lower than that of TiO2, which leads to the photoinduced electrons transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2, meanwhile, the raised Fermi level makes the driven force of the photoinduced electrons from TiO2 to SnO2 increase, these factors can also increase the separation ratio of photoinduced electrons and holes. It is obvious that the increase in the photocatalytic activity of Sn–TiO2X can be attributed to the common effects of SnO2 and Ti3+ ions. 4. Conclusions
Fig. 10. The photocatalytic decoloration ratio curve of the Sn–TiO2X-15-B samples.
The Sn–TiO2X nanoparticles have been prepared via a rapid and simple method, the so-called stannous chemical reducing method. The TiO2 photocatalytic oxidation affected by the stannous chemical reduction and also the correlation between the photocatalytic activity and the change of TiO2 structure were investigated by means of XPS, SPS and DRS technology. The experimental results suggest that: (1) the reduction of the TiO2 particles raised their Fermi level; (2) the amount of surface hydroxyl and oxygen vacancies of the Sn–TiO2X increased; (3) the
B. Xin et al. / Applied Surface Science 255 (2009) 5896–5901
photoinduced electrons can transfer from the conductive band of TiO2 to that of SnO2 on the surface of TiO2. These factors can effectively inhibit the recombination of photoinduced electrons and holes pairs. So, the photocatalytic activity of TiO2 was markedly improved. It is obvious that the increase in the photocatalytic activity of Sn–TiO2X can be attributed to the common effects of SnO2 and Ti3+ ions. Acknowledgments
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
This project is supported from the Key Program Projects of National Natural Science Foundation of China (No. 20431030), the Foundation for Excellent Youth of Heilongjiang University (2005), the Foundation for Youth Science and Technology Innovation Talents of Harbin of China (No. 2006RFQXS096), the Key Item Projects of Educational Department of Heilongjiang Province of China (No. 1152Z002), for which the authors are very grateful. References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1–21. [2] S. Brosillon, L. Lhomme, C. Vallet, A. Bouzaza, D. Wolbert, Appl. Catal. B 78 (2008) 232–241. [3] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [4] G. Cappelletti, C.L. Bianchi, S. Ardizzone, Appl. Catal. B 78 (2008) 193–201. [5] K. Nagaveni, G. Sivalingam, M.S. Hegde, G. Madras, Environ. Sci. Technol. 38 (2004) 1600–1604. [6] M. Bowker, D. James, P. Stone, R. Bennett, N. Perkins, L. Millard, J. Greaves, A. Dickinson, J. Catal. 217 (2003) 427–433. [7] C. Young, T.M. Lim, K. Chiang, J. Scott, R. Amal, Appl. Catal. B 78 (1–2) (2008) 1–10.
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
5901
J. Sa´, J.A. Anderson, Appl. Catal. B 77 (3–4) (2008) 409–417. T.G. Schaaff, D.A. Blom, Nano Lett. 2 (2002) 507–511. E. Stathatos, P. Lianos, P. Falaras, A. Siokou, Langmuir 16 (2000) 2398–2400. C.B. Zhang, H. He, K. Tanaka, Appl. Catal. B 65 (1–2) (2006) 37–43. H. Einaga, T. Ibusuki, S. Futamura, Environ. Sci. Technol. 38 (2004) 285–289. B. Sun, A.V. Vorontsov, P.G. Smirniotis, Langmuir 19 (2003) 3151–3156. S. Kim, W. Choi, J. Phys. Chem. B 106 (2002) 13311–13317. S. Jeon, P.V. Braun, Chem. Mater. 15 (2003) 1256–1263. O. Diwald, T.L. Thompson, T. Zubkov, E.G. Goralski, S.D. Walck, J.T. Yates Jr., J. Phys. Chem. B 108 (2004) 6004–6008. M. Mrowetz, W. Balcerski, A.J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 108 (2004) 17269–17273. A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650– 3655. J. Premkumar, Chem. Mater. 16 (2004) 3980–3981. J.D. Bryan, S.M. Heald, S.A. Chambers, D.R. Gamelin, J. Am. Chem. Soc. 126 (2004) 11640–11647. Y.R. Do, W. Lee, K. Dwight, A. Wold, J. Solid State Chem. 108 (1994) 198. J. Lin, J.C. Yu, J. Lo, S.K. Lan, J. Catal. 183 (1999) 368. S.K. Zheng, T.M. Wang, W.C. Hao, R. Shen, Vacuum 65 (2002) 155. Y. Cao, W. Yang, W. Zhang, G. Liu, P. Yue, New J. Chem. 2 (2004) 218. L.A. Harris, R. Schumacher, J. Electrochem. Soc. 127 (5) (1980) 1186–1188. A. Heller, Y. Degani, D.W. Johnson, P.K. Gallagher, J. Phys. Chem. 91 (23) (1987) 5987–5991. H. Liu, H.T. Ma, X.Z. Li, W.Z. Li, M. Wu, X.H. Bao, Chemosphere 50 (2003) 39–46. Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B 26 (2000) 207. S.K. Poznysk, V.I. Pergushov, A.I. Kokorin, A.I. Kulak, C.W. Schlpafer, J. Phys. Chem. B 103 (1999) 1308–1315. B.F. Xin, L.Q. Jing, Z.Y. Ren, B.Q. Wang, H.G. Fu, J. Phys. Chem. B 109 (2005) 2805– 2809. E.L. Bullock, L. Patthey, S.G. Steinemann, Surf. Sci. 352–354 (1996) 511–517. M.V. Zeller, P.A. Grutsch, T.P. Fehlner, Inorg. Chem. 12 (6) (1973) 1431–1433. Ju.F. Huravlev, M.V. Kuznetsov, V.A. Gubanov, J. Electron. Spectrosc. Relat. Phenom. 38 (1992) 169–176. L.Q. Jing, X.J. Sun, J. Shang, W.M. Cai, Z.L. Xu, Y.G. Du, H.G. Fu, Sol. Energ. Mater. Sol. C 79 (2003) 133–151.