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Deactivation of the TiO2 Photocatalyst by Coupling with WO3 and the Electrochemically Assisted High Photocatalytic Activity of WO3 Hiroaki Tada,*,† Akio Kokubu,‡ Mitsunobu Iwasaki,‡ and Seisihro Ito†,‡ Molecular Engineering Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Received November 8, 2003. In Final Form: March 9, 2004 Patterned TiO2 stripes were formed on a sol-gel crystalline WO3 film by using a chemically modified sol-gel method (pat-TiO2/WO3), and the coupling effect on the photocatalytic activity was studied. Although the photoinduced electron transfer from TiO2 to WO3 was confirmed by labeling and visualization of the reduction sites with Ag particles, the photocatalytic activities of TiO2 for both the gas-phase oxidation of CH3CHO and the liquid-phase oxidation of 2-naphthol decreased significantly with the coupling. This finding was rationalized in terms of the decrease in the rate of the electron transfer from the semiconductor(s) to O2 with the coupling, which was estimated from the kinetic analysis of the photopotential relaxation. When the excited electrons were removed by a SnO2 underlayer, the WO3 film exhibited a high photocatalytic activity exceeding that of TiO2 for the oxidation of 2-naphthol.
Introduction The TiO2/WO3 hybrid system is a system of semiconductor-semiconductor coupled materials attracting much attention in the field of photoelectrochemistry. The composite material has recently been reported to increase the photocurrent as compared to that of each component1 and to improve the performance of solar cells, that is, increases in open-circuit voltage and short-circuit current.2 Also, the application to a new type of photoelectronic devices including a photochromic device,3 a photoelectrochromic device,4 and a light-energy-storage device,5 all of which use the TiO2/WO3 composite as a key material, was proposed. In addition, the increase in photocatalytic activity of TiO2 was confirmed by several other research groups as a result of coupling with WO3 for the decomposition of dichlorobenzene,6,7 2-propanol,7 benzene,7 methylene blue,7 and butyl acetate.8 On the contrary, Miyauchi et al. reported that the coupling of TiO2 and WO3 does not affect the photocatalytic oxidation process, whereas it enhances the photoinduced hydrophilicity when WO3 has an amorphous structure.9 * To whom correspondence should be addressed. Phone: +816-721-2332. Fax: +81-6-721-3384. E-mail:
[email protected]. kindai.ac.jp. † Molecular Engineering Institute. ‡ Department of Applied Chemistry. (1) (a) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1998, 145, 3981. (b) de Tacconi, N. R.; Chenthamarakshan, C. R.; Rajeshwar, K.; Pauporte, T.; Lincot, D. Electrochem. Commun. 2003, 5, 220. (2) Kang, T. S.; Moon, S. H.; Kim, K. J. J. Electrochem. Soc. 2002, 149, E155. (3) He, T.; Ma, Y.; Cao, Y.; Hu, X.; Liu, H.; Zhang, G.; Yang, W.; Yao, J. J. Phys. Chem. B 2002, 106, 12670. (4) (a) Haush, A.; Georg, S.; Baumgartner, S.; Opara Krasovec, U.; Orel, B. Electrochim. Acta 2001, 46, 2131. (b) Haush, A.; Georg, S.; Opara Krasovec, U.; Orel, B. J. Electrochem. Soc. 2002, 149, H159. (5) Tatsuma, T.; Saitoh, S.; Ngaotrakanwiwat, P.; Ohko, Y.; Fujishima, A. Langmuir 2002, 18, 7777. (6) Do, Y. R.; Lee, W.; Dwight, K.; Wold, A. J. Solid State Chem. 1994, 108, 198. (7) Kwon, Y.-Y.; Song, K.-Y.; Lee, W.-I.; Choi, G.-J.; Do, Y.-R. J. Catal. 2000, 191, 192. (8) Keller, V.; Bernhardt, P.; Garin, F. J. Catal. 2003, 215, 129. (9) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. (Weinheim, Ger.) 2000, 24, 1923.
In this study, patterned TiO2 films were formed on solgel WO3-film-coated glass by using a chemically modified sol-gel method. Also, the photocatalytic activity was evaluated to examine the coupling effect of TiO2 and WO3, which is not understood systematically, as described above. Further, the electrochemically assisted photocatalysis of the WO3 film prepared on a SnO2 electrode was performed. Experimental Section Preparation of WO3 Film and pat-TiO2/WO3. H2WO4 (1.25 g, 5.0 × 10-3 mol) was dissolved into a 10 M H2O2 solution (25 mL) by stirring for 24 h. To the solution, a 1.8 wt % aqueous solution of hydroxypropylcellulose (25 mL) was added and stirred for 0.5 h. This solution was used for the coating of WO3 films. Coating was carried out in a dippingwithdrawing manner in an ambient atmosphere (with drawing speed ) 2.0 mm s-1). After the sample had been dried at 100 °C for 10 min, it was heated in air at 350 °C for 1 h and then at 500 °C for 1 h. Quartz, SiO2-film-coated glass, and SnO2-film-coated glass (1.6 × 103 S cm-1, Nippon Sheet Glass Co.) were used as substrates. If necessary, this procedure was repeated to control the film thickness. Further, regularly spaced, 1 mm wide stripes of TiO2 film were formed on the WO3 film by using a chemically modified sol-gel method previously reported.10 Characterization of WO3 Film. The film thickness (d) was determined using a surface profilometer (Dektak 3 Surface Profiler). Electronic absorption spectra were measured using a spectrophotometer (Hitachi U-4000) mounted with an integral sphere in the wavelength range between 200 and 700 nm. X-ray diffraction (XRD) measurements were performed on a Rigaku Mini Flex X-ray diffractometer operating at 40 kV and 80 mA. The scans were collected in the range from 5 to 105° (2θ) by the use of Cu KR radiation (λ ) 1.545 Å). The surface morphologies of the films before and after Ag photodeposition were observed by scanning electron microscopy (SEM, Hitachi S-800) at an acceleration voltage of 15 kV. Photopotential Measurements. The photoelectrochemical cell (PEC) was designed using a semiconductor photoelectrode (WO3, TiO2, or pat-TiO2/WO3), a Pt counter electrode, and a (10) Tada, H.; Hattori, A.; Tokihisa, Y.; Imai, K.; Tohge, N.; Ito, S. J. Phys. Chem. B 2000, 104, 4585.
10.1021/la036104f CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004
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Figure 1. XRD patterns of the tungsten oxide films formed on SiO2-film-coated glass: d(n ) 1) ) 55 ( 15 nm and d(n ) 2) ) 60 ( 10 nm. Ag/AgCl reference electrode (in sat. KCl). All the measurements were carried out in a 0.02 M HCl electrolyte solution. After a constant potential had been reached by bubbling argon for 30 min in the dark, irradiation (λex > 300 nm, the light intensity integrated from 320 to 400 nm (I320-400) ) 5 mW cm-2) was started by using a 500 W Xe lamp as a light source (Wacom HX-500). Electrochemical response with the irradiation was followed for the PEC connected with a potentio/galvanostat (Hokuto Denko, HSV-100). Photocatalytic Reaction. The photocatalytic activity for the gas-phase oxidation of CH3CHO was evaluated in the same manner as described in the previous paper.10 Also, the activity of the semiconductor thin films for the electrochemically assisted photocatalytic decomposition of 2-naphthol (2-NAP) was examined in the following manner. The PEC cell consisted of a WO3 (or TiO2)/SnO2 working electrode, a Pt counter electrode, and a Ag/AgCl reference electrode. The concentration of 2-NAP in a 0.02 M HCl electrolyte solution was 2 × 10-5 M. After 30 min of argon bubbling, irradiation (λex > 320 nm, I320-400 ) 3 mW cm-2) was carried out by using the Xe lamp. Two sheets of SnO2film-coated glass, which were placed between the electrode and the light source, served as a cutoff filter to restrict the selfphotodecomposition of 2-NAP. The concentration of 2-NAP was determined from the absorbance of the peak maximum at 224 nm (max ) 6.80 × 104 M-1 cm-1) on a Hitachi U-4000 spectrophotometer. Further, the liquid-phase oxidation of 2-NAP was carried out under the same conditions except there was no application of external voltage.
Results and Discussion Figure 1 shows XRD patterns of the tungsten oxide film formed on SiO2-film-coated glass. In the XRD patterns, sharp diffraction peaks are observed at 2θ ) 24.2, 34.2, 49.8, and 55.8°, which are in agreement with the diffraction from the (100), (110), (200), and (210) planes of monoclinic WO3 (c-WO3), respectively,11 whereas the WO3 films prepared so far by thermal oxidation,12 chemical vapor deposition,13 and sputtering14 have a triclinic structure and an electrodeposition method produces a hexagonal structure.15 Figure 2 shows (A) electronic absorption spectra of the c-WO3 films coated on quartz substrate and (B) the plots of (RE)1/2 versus E, where R and E are the absorption coefficient and the photon energy, respectively. In spectrum A, the absorption due to the interband transition of c-WO3 is seen at λ ) 420 nm. Each plot is linear, and the intercept of the abscissa gives the indirect band gap of ∼2.8 eV indepen(11) JCPDS No. 71-0305. (12) Reichman, B.; Bard, A. J. J. Electrochem. Soc. 1979, 126, 583. (13) Maruyama, T.; Arai, S. J. Electrochem. Soc. 1994, 141, 1021. (14) Wang, X. G.; Jang, N. G.; Yang, N. G.; Wang, Y. M.; Yuan, L.; Pang, S. Sol. Energy Mater. Sol. Cells 1986, 13, 153. (15) Shiyanovskaya, I.; Hepel, M. Tewksburry, E. J. New Mater. Electrochem. Syst. 2000, 3, 241.
Figure 2. (A) Electronic absorption spectra of the c-WO3 films coated on quartz substrate. (B) Plots of (RE)1/2 vs E1/2, where R and E are the absorption coefficient and the photon energy, respectively.
dently of film thickness. The literature values range from 2.6 to 2.8 eV.16 The gas-phase oxidation of CH3CHO can be used for a photocatalytic test reaction.10,17-19 For the reaction in air, a mechanism, where adsorbed O2 acts as an electron acceptor, was previously presented.19b Figure 3 shows time courses for the photocatalyzed decomposition of CH3CHO. Only irradiation (λex > 300 nm) without photocatalysts led to no decomposition of CH3CHO. In the TiO2 system (b), the concentration of CH3CHO decreases with increasing irradiation time. On the other hand, the decomposition is very slow in the c-WO3 system (a) regardless of its larger apparent surface area (σ(c-WO3) ) 10 cm2, σ(TiO2) ) 2 cm2). Noticeably, the photocatalytic activity of TiO2 is reduced significantly as a result of coupling with c-WO3 (c). In every system, the decomposition of CH3CHO apparently followed the first-order rate law, and the rate constants (k) were k(c-WO3) ) 0.010 ( 0.004, k(TiO2) ) 0.16 ( 0.03, and k(pat-TiO2/c-WO3) ) 0.029 ( 0.008 min-1. In contrast to this deactivation of TiO2 by coupling with c-WO3, the coupling of TiO2 and SnO2 remarkably enhances the photocatalytic activity of TiO2 for the same (16) Bamwencla, G. R.; Sayama, K.; Arakawa, H. J. Photochem. Photobiol., A 1999, 122, 175. (17) Ohko, Y.; Tryk, D.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1998, 102, 2699. (18) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2811. (19) (a) Sopyan, I.; Murasawa, S.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1994, 723. (b) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1996, 98, 79.
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Figure 4. SEM photographs of (A) pat-TiO2/c-WO3 before irradiation, (B) TiO2 after irradiation, and the surfaces of (C) TiO2 and (D) c-WO3 of pat-TiO2/c-WO3 after irradiation. Figure 3. (A) Time courses for the gas-phase photocatalyzed decomposition of CH3CHO: (a) c-WO3 (d ) 60 ( 10 nm, σ ) 10 cm2); (b) TiO2 (d ) 60 ( 10 nm, σ ) 2 cm2); (c) pat-TiO2 (d ) 60 ( 10 nm, σ ) 2 cm2)/c-WO3 (d ) 60 ( 10 nm, σ ) 8 cm2). All the substrates are SiO2-film-coated glass plates. (B) Time profiles for the liquid-phase photocatalyzed decomposition of 2-NAP: (a) self-decomposition; (b) c-WO3 (d ) 60 ( 10 nm, σ ) 10 cm2); (c) TiO2 (d ) 60 ( 10 nm, σ ) 2 cm2); (d) pat-TiO2 (d ) 60 ( 10 nm, σ ) 2 cm2)/c-WO3 (d ) 60 ( 10 nm, σ ) 8 cm2). All the substrates are SiO2-film-coated glass plates.
reaction.10 The oxidation of 2-NAP was also examined as a liquid-phase test reaction. Previously, the formation of phthalic acid was confirmed in the TiO2 photocatalytic reaction of 2-NAP, although the yield is low.20 Figure 3B shows time profiles for the photocatalyzed decomposition of 2-NAP. Under the conditions (λex > 320 nm), the selfdecomposition of 2-NAP is suppressed below 5.4% at t e 3 h. A trend similar to that of the CH3CHO oxidation is apparent also in this liquid-phase reaction; that is, the activity of TiO2 remarkably decreases when it is coupled with c-WO3. The difference in the activity between c-WO3 and pat-TiO2/c-WO3 is much smaller than the difference in the amount of self-decomposition and does not deserve further consideration. To clarify the reason, Ag particles were photodeposited from AgNO3 aqueous solutions to visualize the reduction sites of pat-TiO2/c-WO3, and the amounts (x g cm-2) were determined by induced coupled plasma spectroscopy. Figure 4 shows SEM photographs of (A) pat-TiO2/c-WO3 before irradiation, (B) TiO2 after irradiation, and the surfaces of (C) TiO2 and (D) c-WO3 of pat-TiO2/c-WO3 after irradiation. The comparison of part A with the others (parts (20) Tada, H.; Matsui, H.; Shiota, F.; Nomura, M.; Ito, S.; Yoshihara, M.; Esumi, K. Chem. Commun. 2002, 1678.
B-D) indicates that Ag particles deposit on the surfaces of TiO2 and c-WO3 with irradiation. The amount of Ag deposited on the TiO2 surface of pat-TiO2/c-WO3 (C, x ) 0.14 mg cm-2) is smaller than that on the TiO2 single film (B, x ) 0.19 mg cm-2). In pat-TiO2/c-WO3 after irradiation, Ag particles deposit preferentially on c-WO3 (D, x ) 0.22 mg cm-2) over TiO2 (C), whereas the amount of Ag photodeposited on the c-WO3 single film is very small (x ) 7.9 µg cm-2). These results demonstrate that a part of the photoexcited electrons transfers from TiO2 to c-WO3. Similar interfacial electron transfer between coupled semiconductors was confirmed to take place also in the TiO2/SnO2 10 and TiO2(anatase)/TiO2(rutile) systems.18 Figure 5 shows the photoinduced potential change for (a) TiO2/SnO2, (b) c-WO3/SnO2, and (c) pat-TiO2/c-WO3/ SnO2 (A, left) without and (B, left) with argon bubbling: the pH of the electrolyte solution is 1.8. Upon irradiation, the potential of the SnO2 electrode shifts cathodically in every system. Electron-hole pairs are generated by photoexcitation of TiO2 and/or c-WO3 (eq 1). The holes in the valence band (vb) oxidize chemisorbed and/or physisorbed water, and the electrons left in the conduction band (cb) are thought to cause the potential shift (eq 2). When irradiation is stopped, the potential is relaxed toward the equilibrium value in the dark. Since the coupling of the •OH radicals to yield H2O2 is very fast (eq 3),21 the relaxation would mainly result from the trapping of e-(cb) by H2O2 (eq 4). A close comparison of parts A and B indicates that the rate of the potential relaxation decreases by bubbling with argon in every system. This (21) The rate constant of the reaction in eq 3 was reported to be 5 × 109 M-1 s-1 in the following literature: Saito, I.; Matsuura, T. Kikan Kagaku Sosetsu 1990, 7, 14. Thus, even at [•OH] ) 10-6 M, the lifetime of the •OH radicals is as short as 200 µs.
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Figure 5. Photoinduced potential change for (a) TiO2/SnO2, (b) c-WO3/SnO2, and (c) pat-TiO2/c-WO3/SnO2 (A) without and (B) with deaeration.
fact suggests that this relaxation process involves the electron transfer from the cb of the semiconductor to O2 (eq 5).
TiO2 (or WO3) + hν f e-(cb)‚ ‚ ‚h+(vb)
(1)
e-(cb)‚ ‚ ‚h+(vb) + OHsurface (and/or H2Oad) f •OH + e-(cb) (2) •OH f 1/2H2O2
(3)
e-(cb) + 1/2H2O2 + H+ f H2O (under acidic conditions) (4) e-(cb) + O2 f O2-
(5)
Thus, if both the potential shift and the rate of the electron transfer to O2 are assumed to be proportional to the amount of the electrons accumulated in the cb,22 eq 6 is obtained
ln ∆U0/∆U(t) ) kt
(6)
where ∆U0 and ∆U(t) are the potential shifts from the equilibrium value in the dark at t ) 0 and t ) t, respectively. (22) Since the initial relaxation of the photoinduced potential shift corresponds to an infinitesimal change in Fermi energy, the density of energy states in the energy region can be thought to be constant. Thus, as far as the initial relaxation is concerned, this assumption is valid.
The plots of ln ∆U0/∆U(t) versus t in the (A, right) presence and (B, right) absence of O2 are also shown in Figure 5. From the slopes of the initial tangents in the plots, the rate constants for the systems with O2 (ko) and without O2 (kn) were obtained. Further, the apparent rate constants of the electron transfer (ket′ ) ket[O2ad]) to O2 were calculated from the difference (eq 7) to be (1.7 ( 0.2) × 10-2 s-1 for TiO2/SnO2,
