IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Monday Sessions
       Session SS2-MoA

Paper SS2-MoA2
Evidence for a Radical-Radical Reaction between O@sub 2@ and OH on TiO@sub 2@(110)

Monday, October 29, 2001, 2:20 pm, Room 121

Session: Molecular Interactions with Oxide Surfaces
Presenter: M.A. Henderson, Pacific Northwest National Laboratory
Authors: M.A. Henderson, Pacific Northwest National Laboratory
C.L. Perkins, Pacific Northwest National Laboratory
Correspondent: Click to Email
Water readily dissociates at oxygen vacancy sites on TiO@sub 2@(110) to form bridging OH groups.@footnote 1-3@ These OH groups exhibit properties that are more consistent with what one would expect from OH radicals rather than from OH@super -@ ions. A surface consisting solely of these OH groups is easily prepared by adsorption of a multilayer water exposure at 130 K followed by heating to 300 K to desorb all molecular water. Recombination of the bridging OH groups occurs in a TPD peak at about 500 K. ELS spectra of the 10% vacancy-covered surface before and after exposure to water both possess the 0.8 eV loss feature attributable to Ti@super 3+@ cations. The inability of water/OH to oxidize Ti@super 3+@ cations at vacancy sites is consistent with earlier photoemission studies.@footnote 4@ In contrast, this ELS feature is absent after O@sub 2@ exposure at RT due to oxidation of the vacancies. Surprisingly, exposure of O@sub 2@ to bridging OH groups results in replacement of the 500 K recombinative desorption state of water with a sharp H@sub 2@O TPD state at 300 K. Concurrent with this change, the Ti@super 3+@ cations are oxidized and O adatoms are deposited on the surface. We speculate that the most likely explanation for this behavior is a radical-radical reaction between O@sub 2@ and OH. Such a reaction does not occur between OH groups and diamagnetic molecules like CO@sub 2@.@footnote 5@ These findings suggest that bridging OH groups on TiO@sub 2@(110) formed from the dissociative adsorption of H@sub 2@O at vacancy sites might be useful in exploring the thermal chemistry OH radicals believed to be formed on TiO@sub 2@ photocatalysts. @FootnoteText@ @footnote 1@. M.B. Hugenschmidt, et al., Surf. Sci. 302 (1994) 329. @footnote 2@. M.A. Henderson, Langmuir 12 (1996) 5093. @footnote 3@. W.S. Epling, et al., Surf. Sci. 412/413 (1998) 333. @footnote 4@. R.L. Kurtz, et al., Surf. Sci. 218 (1989) 178. @footnote 5@. M.A. Henderson, Surf. Sci. 400 (1998) 203. R.