Formation of O adatom pairs and charge transfer upon O(2) dissociation on reduced TiO(2)(110).

Scanning tunneling microscopy and density functional theory have been used to investigate the details of O(2) dissociation leading to the formation of oxygen adatom (O(a)) pairs at terminal Ti sites. An intermediate, metastable O(a)-O(a) configuration with two nearest-neighbor O atoms is observed after O(2) dissociation at 300 K. The nearest-neighbor O(a) pairs are destabilized by Coulomb repulsion of charged O(a)'s and separate further along the Ti row into energetically more favorable second-nearest neighbor configuration. The potential energy profile calculated for O(2) dissociation on Ti rows and following O(a)'s separation strongly supports the experimental observations. Furthermore, our results suggest that the itinerant electrons associated with the O vacancies (V(O)) are being utilized in the O(2) dissociation process at the Ti row. Experimentally this is supported by the observation that not all V(O)'s can be healed by O(2) exposure at 300 K, as some V(O)'s becoming less reactive due to supplying certain charge to O(a)'s. Further, theoretical results show that at least two oxygen vacancies per O(2) molecule are required in order for the O(2) dissociation at the Ti row to become viable.