AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS-ThA

Paper SS-ThA8
The Origin of Photo-induced Hydrophilicity on TiO@sub2@ Surfaces: Photo-generation of Surface Oxygen Vacancies or Photo-oxidation of Adsorbed Organics?

Thursday, November 6, 2003, 4:20 pm, Room 328

Session: Oxide Reactions and Catalysis
Presenter: J. Szanyi, Pacific Northwest National Laboratory
Authors: J. Szanyi, Pacific Northwest National Laboratory
M.A. Henderson, Pacific Northwest National Laboratory
J.M. White, University of Texas, Austin
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Since the discovery of the photocatalytic nature of TiO@sub2@ in the 1970s, significant efforts have been made to understand both the properties of the oxide material itself and the catalytic processes that occur on its surfaces. In recent years the photo-induced hydrophilic nature of TiO@sub2@ has been discovered and studied. The hydrophilic behavior continues after removal of the light, but gradually degrades to the pre-irradiation conditions. Most of the original work from Japan suggests that hydrophilicity and photo-oxidation of surface organic contaminants are unrelated properties of TiO@sub2@. They propose UV generated oxygen defect sites on the TiO@sub2@ surface as the origin of hydrophilicity. In turn, the oxidation of oxygen vacancies by O@sub2@ in the air has been proposed as the mechanism for the slow degradation of hydrophilicity of a TiO@sub2@ surface. In order to understand the origin of photo-induced hydrophilicity of TiO@sub2@, we have studied the adsorption of water on a clean and trimethyl acetic acid covered TiO@sub2@(110) surface under carefully controlled experimental conditions. TPD of H@sub2@O suggests that irrespective of the presence or absence of vacancies, water wets the TiO@sub2@ surface as evidenced by the sequential filling of monolayer, second layer and multilayer states as a function of increasing coverage. On the other hand, the monolayer water TPD state is absent for water adsorbed on a TiO@sub2@(110) surface pre-saturated with trimethyl acetate (TMA) species, formed from decomposition of the acid. Exposure of the TMA adlayer to UV light in 5x10@super-6@ torr O@sub2@ results in nearly complete photo-oxidation of the organic and restoration of the ability of the surface to directly bind water. In this presentation, based on these molecular-level UHV studies, we propose that the origin of photo-induced hydrophilicity on TiO@sub2@ surfaces is likely due to photo-oxidation of adsorbed organics that create a hydrophobic monolayer.