First-principles density-functional theory calculations in the generalized gradient approximation were carried out in order to study the relative stabilities of oxygen vacancies at the surface and sub-surface sites of anatase (101), TiO2(001) and prototypical rutile (110) surfaces. The 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 favoured for O vacancies at rutile (110) surfaces. Calculations of O-vacancy diffusion pathways at anatase (101) surfaces showed that the energy barrier to diffusion from surface to sub-surface sites was sufficiently low to ensure rapid equilibration of the vacancy distribution at typical surface annealing temperatures. These results could explain why anatase surfaces were found experimentally 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