The re-oxidation of Ar+-sputtered TiO2 surfaces by thermal annealing in vacuum was shown to result primarily from the diffusion of titanium cations, presumably in reduced oxidation states, from the damaged surface into the bulk, and not from the diffusion of bulk oxygen anions to the surface. Temperature programmed static secondary ion mass spectrometry measurements indicate that the ratios of O to Ti in the Ar+ sputtered surfaces of TiO2(110) and TiO2(100) increased significantly as the temperature exceeded 700K. With oxygen-18 incorporated into the Ar+ sputtered surfaces, the ratios of 16O to 18O remained constant during this re-oxidation process. However, for both surfaces, the ratios of 16O to Ti and 18O to Ti increased during the re-oxidation process. These ratios increased primarily as a result of a decrease in the surface Ti concentration which was attributed to the diffusion of reduced Ti cations into the bulk of the oxide.

Mechanism for the Bulk-Assisted Reoxidation of Ion Sputtered TiO2 Surfaces: Diffusion of Oxygen to the Surface or Titanium to the Bulk? Henderson, M.A.: Surface Science, 1995, 343[1-2], L1156-60