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. H.Cheng, A.Selloni: Physical Review B, 2009, 79[9], 092101