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Energy Storage of TiO2-WO3 Photocatalysis Systems in the Gas Phase Tetsu Tatsuma,*,† Shuichi Saitoh,‡ Pailin Ngaotrakanwiwat,† Yoshihisa Ohko,§ and Akira Fujishima*,§ Institute of Industrial Science, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan, Koyo Engineering Co., Ltd., 2-880 Takaragimachi, Utsunomiya, Tochigi 320-0061, Japan, and Department of Applied Chemistry, School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received May 30, 2002. In Final Form: July 25, 2002 Reductive energy generated at a TiO2 photocatalyst under UV light can be stored in WO3 by coupling them together, and the stored energy can be used after dark. However, the reduction of WO3 requires cation intercalation for charge neutralization. Thus, behavior of the TiO2-WO3 composite on an ITO electrode was examined in nonelectrolytic media. When the TiO2 and WO3 were close to each other (less than 1 mm), WO3 could be reduced even in pure water or humid air (relative humidity g25%), by irradiating the composite with UV light. In dry air, WO3 was not reduced efficiently, even if the TiO2 and WO3 nanoparticles were mixed well. These results suggest that protons generated at the TiO2 surface as a result of photocatalytic oxidation of water are intercalated into WO3, and therefore ionic conductivity of the medium or the composite film surface is important. The composite film charged in air exhibited almost the same electrode potential as that of the film charged in aqueous NaCl.
Introduction Although photoresponsive semiconductors including TiO21,2 are promising materials for the conversion of light energy to chemical or electrical energy, they function only under light illumination. As a solution for this limitation, we have recently proposed the energy storage photocatalyst, in which a photocatalyst (e.g., TiO2) is combined with an energy storage material (e.g., WO3).3 Reductive energy generated by UV-irradiated TiO2 can be stored in WO3 in an aqueous NaCl solution, and the WO3 retains the reductive energy for a certain period even after the light is turned off (Figure 1a). It is known that TiO2 coated on copper4 or type 304 stainless steel5-7 protects these metals from corrosion under UV light illumination by the photogenerated reductive energy. Our TiO2-WO3 combined system protected type 304 stainless steel from corrosion for 5 h after 1 h of UV irradiation.3 Since the reduced WO3 (i.e., WO3 charged with reductive energy) is gradually oxidized by oxygen in air, H2O2 may be generated at the reduced WO3 surface. If so, some bactericidal effect could also be expected. In addition, the color of WO3 is changed from pale yellow to blue by the reduction. Thus, the TiO2-WO3 composite film can be used as a photochromic film. †
Institute of Industrial Science, University of Tokyo. Koyo Engineering Co. § Department of Applied Chemistry, School of Engineering, University of Tokyo. ‡
(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735-758. (3) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Chem. Mater. 2001, 13, 2838-2842. (4) Yuan, J.; Tsujikawa, S. J. Electrochem. Soc. 1995, 142, 34443450. (5) Imokawa, T.; Fujisawa, R.; Suda, A.; Tsujikawa, S. Zairyo-toKankyo 1994, 43, 482-486 (in Japanese). (6) Akashi, M.; Iso-o, H.; Hirano, K.; Kubota, N.; Fukuda, T.; Ayabe, M. Int. Symp. Environ. Degrad. Mater. Nucl. Power Syst.sWater React., 7th 1995, 1, 621-628. (7) Ohko, Y.; Saitoh, S.; Tatsuma, T.; Fujishima, A. J. Electrochem. Soc. 2001, 148, B24-B28.
Figure 1. (a) Mechanism of reductive energy storage of TiO2WO3 combined system. (b) Proposed models of electron and ion transfer in the charging and self-discharging processes of the TiO2-WO3 composite film in humid air.
Although there have been reported some TiO2-WO3 combined systems, these are for photochromic effects8-10 or sensitization of TiO2 hydrophilization,11 but not for energy storage. A similar TiO2-Ni(OH)2 system has also (8) Ohtani, B.; Atsumi, T.; Nishimoto, S.; Kagiya, T. Chem. Lett. 1988, 295-298. (9) Tennakone, K.; Ileperuma, O. A.; Bandara, J. M. S.; Kiridena, W. C. B. Semicond. Sci. Technol. 1992, 7, 423-424. (10) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608-610. (11) Miyauchi, M.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 2000, 12, 1923-1927.
10.1021/la026011i CCC: $22.00 © 2002 American Chemical Society Published on Web 09/13/2002
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Langmuir, Vol. 18, No. 21, 2002
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been studied as a photochromic system.12 We have also combined WO3 with SrTiO3, so that energy storage under UV irradiation was accelerated.13 However, it has been unknown whether the energy storage is possible in pure water without electrolytes or in the gas phase. When WO3 is reduced, cations are intercalated into WO3 for charge neutralization. In the case of our TiO2-WO3 system examined in a 3 wt % NaCl solution,3 Na+ was chiefly intercalated. In a nonelectrolytic medium, supply of a cation must be the most important key for the energy storage. In the present work, we demonstrate that the energy storage is possible with the TiO2-WO3 system even in pure water or air. This is important for the application of the energy-storage photocatalyst not only for the protection from corrosion but also for the possible bactericidal effect and photochromism. Experimental Section Preparation of the Coatings. An indium tin oxide (ITO)coated glass plate was used as a substrate (2.5 × 4.0 cm). The TiO2 coating was prepared by a spray-pyrolysis technique at 300 °C from a 0.05 M ethanol solution of bis(2,4-pentanedionato)titanium oxide (Tokyo Kasei). The average thickness of the resulting TiO2 coating was ca. 1.2 µm (measured by scanning electron microscopy, SEM). For the preparation of WO3 coatings, crystalline WO3 nanoparticles (Japan New Metals; diameter measured by SEM, ca. 10-500 nm) were suspended in an alkoxysilane solution (NDC100A, Nippon Soda) (WO3 content, 58 g L-1; Si content, 3.5 wt %). The suspension was coated on a substrate by spin-coating at 1500 rpm. The substrate was annealed at 200 °C for 30 min. Silica forms in the film and functions as a binder, but electrical contact between WO3 and the electrode is retained.3 TiO2-WO3 composite coatings were prepared by a spraypyrolysis technique at 300 °C from a 0.05 M ethanol solution of bis(2,4-pentanedionato)titanium oxide containing the WO3 nanoparticles (2.3 g L-1). The thickness of the composite coating was controlled with the amount of the suspension used (thick film, 100 mL; thin film, 50 mL). Measurements. The coatings were irradiated with UV light by a 200-W Hg-Xe lamp (Luminar Ace 210, Hayashi Tokei) together with a 365-nm band-pass filter. The light intensity was about 22 or 10 mW cm-2 at the sample surface. Reflectance of WO3 coatings was measured by using a reflectance spectrophotometer (“Handy-Spec”, GY-Gardner). Potential of a coated ITO electrode was measured in an air-saturated 3 wt % NaCl aqueous solution, pH 5. As-prepared films were used, unless otherwise noted.
Results and Discussion Photochromic Behavior in Water. First we studied photochromic behavior of an ITO electrode coated with a TiO2 film and a WO3 film (area, 50:50, Figure 2a, type a) in water. We have reported that, in this system, the WO3 film was charged in 3 wt % NaCl under UV light irradiation, and the color of WO3 was varied from pale yellow to blue. However, such behavior was not observed in pure water (data not shown). Only WO3 in the vicinity of the TiO2/WO3 borderline (within 1 mm) was colored slightly in blue by UV irradiation for 1 h. There are two possible reasons for this: lack of cation that can be intercalated into WO3 and low ionic conductivity of the liquid. In the present system, an excited electron and a corresponding hole are generated on TiO2 under UV light UV
TiO2 98 TiO2*(e- + h+)
(1)
(12) Kostecki, R.; Richardson, T.; McLarnon, F. J. Electrochem. Soc. 1998, 145, 2380-2385.
Figure 2. (a) Schematic models for TiO2-WO3 separated films (type a) and TiO2-WO3 composite film (type b). (b) Changes in the reflectance of the TiO2-WO3 composite film (type b, thick film) after UV irradiation (22 mW cm-2, 20 min) at a relative humidity of about 0, 25, 50, and 90%.
The electron may be transported through TiO2 (n-type semiconductor) and ITO (conductor) to WO3, and if so, a cation should be intercalated into WO3. In pure water, only H+ is available as a cation.
WO3 + xe- + xH+ f HxWO3
(2)
On the other hand, the hole should also be consumed at the TiO2 surface by H2O to generate chiefly O2 and H+.
2H2O + 4h+ f O2 + 4H+
(3)
Thus, H+ is generated by TiO2 (eq 3) and is consumed by WO3 (eq 2). However, pure water is not ionically conductive enough. This should be the reason why the reduction of WO3 was possible only in the vicinity of TiO2. Although surface hydroxyl groups on TiO2 and WO3 may contribute to the ionic conduction by dissociation into -O- and H+, its ionic conductivity may not be enough. The above result also indicates that the present WO3 film does not show photochromism by itself, probably because we used crystalline WO3 nanoparticles. Although some groups have reported photochromic behavior of WO3 colloids14,15 and amorphous WO3,16,17 crystalline WO3 is known to exhibit no photochromic behavior.16 Next we examined an ITO electrode coated with the TiO2-WO3 composite film (Figure 2a, type b, thick film) in water. By irradiation with UV light for 1 h, the composite film was uniformly colored in blue. This was observed because most WO3 particles are in the vicinity of TiO2. Photochromic Behavior in Air. The ITO electrode coated with the TiO2-WO3 composite film (type b, thick (13) Ohko, Y.; Saitoh, S.; Tatsuma, T.; Fujishima, A. Electrochemistry 2002, 70, 460-462. (14) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 11064-11070. (15) Hotchandani, S.; Bedja, I.; Fessenden, R. W.; Kamat, P. V. Langmuir 1994, 10, 17-22. (16) Kikuchi, E.; Iida, K.; Fujishima, A. J. Electroanal. Chem. 1993, 351, 105-114. (17) Yao, J. N.; Chen, P.; Fujishima, A. J. Electroanal. Chem. 1996, 406, 223-226.
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Figure 3. Changes in the potential of an ITO electrode coated with the TiO2-WO3 composite film (type b, thin film) after irradiated with UV light (10 mW cm-2, 1 h) in a 3 wt % aqueous solution (pH 5) or air (relative humidity, 50%).
film) was also subjected to the 1-h UV irradiation in air. As shown in the Figure 2b, the behavior strongly depended on the relative humidity of the atmosphere. The film was well-colored in a humid atmosphere (relative humidity g25%), while it was not colored well in a dry atmosphere. This can be explained in terms of ionic conductivity of the film surface. In the humid atmosphere, an adsorbed water layer should form on the film surface. This layer and the surface hydroxyl groups, of which dissociation should be facilitated by this water layer, should contribute to the ionic conduction, which is necessary for the photoelectrochemical reduction of WO3 (Figure 1b). To the contrary, in the dry atmosphere, the adsorbed water layer is almost absent, so that the ionic conductivity should not be sufficient for the WO3 reduction. Electrochemical Characterization of the System Charged in Air. On the basis of the photochromic behavior of the TiO2-WO3 composite film, we can conclude that WO3 can be reduced under UV light in a humid atmosphere, if the TiO2 and WO3 are close to each other. This was further verified by electrochemical measurements. The ITO electrode coated with the composite film (type b, thin film) was irradiated with UV light for 1 h at relative humidity of 50%, the blue-colored electrode was transferred into a 3 wt % NaCl aqueous solution (pH 5), and the potential was measured. The potential was retained at around -0.2 V vs Ag|AgCl for about 4 h, and the potential gradually shifted to the positive direction toward its original rest potential before irradiation (ca. +0.15 V vs Ag|AgCl) (Figure 3). This behavior was similar to that of a sample irradiated with UV light in a 3 wt % NaCl solution (Figure 3). At -0.2 V, reduced WO3 is gradually reoxidized to WO3 by dissolved oxygen (selfdischarging).3 After the film is almost reoxidized, the potential gradually reverts.
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Figure 4. Changes in ∆reflectance (reflectance before UV irradiation - reflectance after UV irradiation) at 600 nm for the TiO2-WO3 composite film (type b, thick film). The film was irradiated with UV light (22 mW cm-2, 20 min) in air (relative humidity, 80%).
The film reduced in air exhibited a longer reoxidation period than did the film reduced in the solution. Since the present sample could not be fully charged within 1 h both in air and the solution, the observation reflects faster reduction in air or slower reoxidation of the sample reduced in air. Studies of further details of the reduction and reoxidation kinetics are currently underway. Cyclability. Charge and discharge processes were repeated with the composite film (type b, thick film) in air to examine the cyclability. Figure 4 shows the changes in the ∆reflectance (reflectance before UV irradiation reflectance after UV irradiation) at 600 nm, which corresponds to the amount of charge stored. After the irradiation for 20 min, the film was left in the same atmosphere until it was self-discharged. The ∆reflectance value increased gradually in the initial eight cycles, and after that it decreased slightly. Although an initial increase (one-nine cycles) and a following decrease in the amount of charge stored were also observed when the chargedischarge cycles were examined in 3 wt % aqueous NaCl,3 these changes of the charge amount observed in the solution were much larger, by an order of magnitude. The more stable behavior observed in air may be because the ion intercalated in air (H+) is smaller than that intercalated in the solution (Na+); a smaller ion should generate weaker stress in the WO3 particles. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (Area No. 417, Research No. 14050028 for T.T.) from the Ministry of Education, Science, Sports and Culture of Japan, Kanto Bureau of Economy, Trade and Industry (for T.T.), and Tokuyama Science Foundation (for T.T.). LA026011I
