Formation of O adatom pairs and charge transfer upon O₂ dissociation on reduced TiO₂(110)

Scanning tunneling microscopy and density functional theory have been used to investigate the details of O₂ 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 O₂ 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 O₂ 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 O₂ dissociation process at the Ti row. Experimentally this is supported by the observation that not all V_O‘s can be healed by O₂ 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 O₂ molecule are required in order for the O₂ dissociation at the Ti row to become viable.