Surface perspective on self-diffusion in rutile TiO2

Although there has been considerable debate in the solid state literature regarding oxygen vacancy versus titanium interstitial models for point defect diffusion in rutile TiO2, ultrahigh vacuum (UHV) surface scientists have all but unanimously adopted the oxygen vacancy model in explaining mass transport phenomena occurring between the bulk and surface. This unanimity, however, has not arisen from detailed mechanistic studies, but from a general misunderstanding that surface oxygen and titanium concentration changes measured by techniques like X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) can be interpreted mechanistically. This thinking is especially pervasive in investigations of the bulk-assisted reoxidation of ion sputtered TiO2 surfaces. It is well known that ion sputtering TiO2 preferentially depletes the surface of oxygen until a steady-state level of surface reduction has been established. The surface stoichiometry of the sputtered TiO2 surface can be restored when annealing in UHV through transport of sputter damage to the bulk. In this paper, the mechanism of the bulk-assisted reoxidation of ion sputtered TiO2 is addressed by monitoring the diffusion of oxygen and titanium in the O-18 and Ti-46 isotopically enriched surfaces of TiO2(110) using static secondary ion mass spectrometry (SSIMS). The sputtered TiO2(110) surface remains heavily reduced <400 K, but the O-to-Ti ratio increases slowly above ca 400 K and rapidly above ca 700 K. No diffusion is detected between the surface and bulk <400 K, however diffusion of both oxygen and titanium are detected between ca 400 and 700 K but without a significant change in the stoichiometry of the surface. This is interpreted to mixing within the sputtered film since angle-resolved photoemission studies in the literature have shown that sputtered TiO2 surfaces possess a concentration gradient of reduced titanium cations normal to the surface. Above 700 K, only titanium diffusion from the surface to the bulk is detected. These data suggest that the major diffusive species responsible for the bulk-assisted reoxidation of ion sputtered TiO2 surfaces is titanium cations and not oxygen anions/vacancies. Published by Elsevier Science B.V.