Annealing TiO2(110) in vacuum at high temperature (above about 800K) generated oxygen vacancy sites that were associated with reduced surface cations. Numerous studies had shown that these sites could be oxidized by exposure to molecular oxygen, but the mechanism and temperature dependence of this oxidation process were not well understood. Results were presented which suggested that low temperature (<600K) O2 exposure oxidized oxygen vacancies but also left oxygen-containing species on the surface that were proposed to be oxygen adatoms. The presence of these oxygen adatoms was evident in the temperature-programmed desorption spectrum of water. Oxidizing the vacuum annealed surface at 700K produces a fully oxidized TiO2(110) surface that gives a single monolayer temperature-programmed desorption state for water at 270K. Exposing the vacuum annealed surface to O2 at temperatures between 90 and 600K followed by water adsorption at 90K resulted in a new water temperature-programmed desorption state 25K higher in temperature. Similar results were obtained using ammonia instead of water. Isotopic labelling experiments, in which the vacuum annealed surface was dosed with 18O2 at 135K followed by H2 16O at 135K, indicated that the new water temperature-programmed desorption state resulted from recombinative desorption, whereas no such effect was observed for the surface exposed to 18O2 at 700K. The effect on water was also absent in temperature-programmed desorption if the low temperature O2 treated surface was heated to 600K prior to water adsorption at 90K, suggesting that the oxygen adatoms desorb from the surface or diffuse into the bulk. It was proposed that, at low temperatures, O2 dissociated at oxygen vacancies, filling each defect site with one O atom and depositing a second O adatom at a five-coordinate Ti4+ site or that O2 interacted with surface hydroxyl groups resulting in O2 dissociation and the presence of the O adatom. The new dissociative water chemistry resulted from the interaction of water molecules with these oxygen adatoms. After high temperature (>600K) O2 exposure, no dissociative water chemistry was observed, suggesting that these oxygen adatoms were not present on the surface. The presence of surface O adatoms may explain inconsistencies in the literature regarding the reactivity of water, and potentially other species, on TiO2(110). These results also detail the importance of sample preparation techniques on the chemistry which could occur at a solid surface.

Evidence for Oxygen Adatoms on TiO2(110) Resulting from O2 Dissociation at Vacancy Sites. Epling, W.S., Peden, C.H.F., Henderson, M.A., Diebold, U.: Surface Science, 1998, 412-413, 333-43