Evidence for oxygen adatoms on TiO2(110) resulting from O2 dissociation at vacancy sites

Annealing TiO2(110) in vacuum at high temperature (above about 800 K)generates oxygen vacancy sites that are associated with reduced surface cations. Numerous studies have shown that these sites can be oxidized by exposure to molecular oxygen, but the mechanism and temperature dependence of this oxidation process are not well understood. We present results that suggest low temperature (1600 K) O-2 exposure oxidizes oxygen vacancies but also leaves oxygen-containing species on the surface that we propose are oxygen adatoms, The presence of these oxygen adatoms is evident in the temperature-programmed desorption (TPD) spectrum of water. Oxidizing the vacuum annealed surface at 700 K produces a fully oxidized TiO2(110) surface that gives a single monolayer TPD state for water at 270 K, Exposing the vacuum annealed surface to O-2 at temperatures between 90 and 600 K followed by water adsorption at 90 K results in a new water TPD state 25 K higher in temperature. Similar results were obtained using ammonia instead of water. Isotopic labeling experiments, in which the vacuum annealed surface was dosed with O-18(2) at 135 K followed by (H2O)-O-16 at 135 K, indicate that the new water TPD state results from recombinative desorption, whereas no such effect is observed for the surface exposed to O-18(2) at 700 K. The effect on water is also absent in TPD if the low temperature O-2 treated surface is heated to 600 K prior to water adsorption at 90 K, suggesting that the oxygen adatoms desorb from the surface or diffuse into the bulk. We propose that at low temperatures, O-2 dissociates 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 O-2 interacts with surface hydroxyl groups resulting in O-2 dissociation and the presence of the O adatom. The new dissociative water chemistry results from the interaction of water molecules with these oxygen adatoms. After high temperature (1600 K) O-2 exposure, no dissociative water chemistry is observed, suggesting that these oxygen adatoms are 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 can occur at a solid surface. (C) 1998 Elsevier Science B.V. All rights reserved.