First-principles density-functional theory calculations in the generalized gradient approximation were carried out to study the relative stabilities of oxygen vacancies at surface and sub-surface sites of anatase TiO2(101) and TiO2(001), and, for comparison, of the prototypical rutile TiO2(110) surface. These results indicated that these defects were significantly more stable at sub-surface than at surface sites in the case of anatase surfaces, whereas bridging oxygen sites were favored for O vacancies at rutile TiO2(110). Also, calculations of O-vacancy diffusion pathways at anatase TiO2(101) showed that the energy barrier to diffuse from surface-to-subsurface sites was sufficiently low to ensure a rapid equilibration of the vacancy distribution at typical surface annealing temperatures. These results could explain why, experimentally, anatase surfaces were found to have a significantly lower defect concentration and/or to be more difficult to reduce than those of rutile.

Surface and Subsurface Oxygen Vacancies in Anatase TiO2 and Differences with Rutile. Cheng, H., Selloni, A.: Physical Review B, 2009, 79[9], 092101