Formation of O adatom pairs and charge transfer upon O-2 dissociation on reduced TiO2(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.