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Langmuir 2001, 17, 2298-2300
In Situ Observation of the Photoenhanced Adsorption of Water on TiO2 Films by Surface-Enhanced IR Absorption Spectroscopy Ryuhei Nakamura, Kazuhiro Ueda, and Shinri Sato* Catalysis Research Center and Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan Received November 20, 2000. In Final Form: February 13, 2001 Surface-enhanced IR absorption spectroscopy (SEIRAS) is so sensitive as to detect surface species at submonolayer coverages and available even in the presence of gas- or liquid-phase medium that absorb IR intensively. In the present study, we report in situ SEIRAS observation of the photoenhanced adsorption of water on thin TiO2 films coated with an island Au film. The O-H stretching of photoadsorbed water appeared at lower frequencies than that of physisorbed water, indicating stronger interactions among the photoadsorbed species. The addition of oxygen suppresses significantly the photoadsorption of water and reduces the amount of photoadsorbed water in the dark.
1. Introduction Although IR spectroscopy is a powerful tool for investigation of surface reactions, an IR transmission method is hardly applied to studies on heterogeneous reactions in the presence of gas- or liquid-phase medium that absorbs IR intensively. Surface-enhanced IR absorption has been observed for molecules adsorbed on a thin island film of a free-electron metal such as Au or Ag1-10 and applied as IR spectroscopy (SEIRAS) to the studies of surface species in electrode reactions5,8 and surface photochemical reactions.9 When SEIRAS is carried out in attenuated-totalreflection configurations, it has a great advantage that IR measurements are not disturbed by the strong IR absorption of water even in an aqueous solution.4-8 Since the metal films used in SEIRAS have discontinuous island structure, SEIRAS may detect chemical species which exist not only on the metal islands but also in the vicinity of the metal islands, though a detectable range is limited to ca. 10 nm.9 We have already demonstrated that SEIRAS is able to detect CO molecules adsorbed on Pt particles deposited on an Au island film.11 In the present paper we report in situ SEIRAS observation of the photoenhanced adsorption of water on TiO2 films coated with an Au island film in the presence of gas-phase water. We also show that the water photoadsorption is significantly suppressed in the presence of O2 and photoadsorbed water is removed by O2 in the dark. These results explain well recent findings of the photoenhanced hydrophilicity of TiO2 surfaces.12-15 * To whom correspondence may be addressed. Fax: +81-11709-4748. E-mail:
[email protected]. (1) Hartstein, A.; Kirtly, J. R.; Tsang, C. T. Phys. Rev. Lett. 1980, 45, 201. (2) Hatta, A.; Suzuki, Y.; Sue¨taka, W. Appl. Phys. 1984, A35, 135. (3) Kamata, T.; Kato, A.; Umemura, J.; Takenaka, T. Langmuir 1987, 3, 1150. (4) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (5) Osawa, M.; Ataka, K.; Yoshiil, K.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 371. (6) Osawa, M.; Ataka, K.; Yoshiil, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (7) Nishikawa, Y.; Fujiwara, K.; Ataka, K.; Osawa, M. Anal. Chem. 1993, 65, 556. (8) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (9) Sato, S.; Suzuki, T. Appl. Spectrosc. 1997, 51, 1170. (10) Zhang, J.; Zhao, J. Langmuir 1998, 14, 5521. (11) Sato, S.; Kamada, K.; Osawa, M. Chem. Lett. 1999, 15.
Figure 1. Experimental setup for SEIRAS measurements by the use of an Au/TiO2/CaF2 disk.
2. Experimental Section A transparent TiO2 film was coated on a CaF2 disk (30 mm in diameter, 2 mm thick) by standard procedures of a sol-gel method: The disk was dipped into a TiO2 sol prepared by the hydrolysis of Ti isopropoxide in 2-propanol, followed by drying in N2 atmosphere for 5 h and calcination in air at 500 °C for 3 h. This process was repeated three times. Au deposition on the TiO2 films was carried out in an ultrahigh vacuum chamber by evaporating Au grains in a W basket under a vacuum of
