(Sub)Surface mobility of oxygen vacancies at the TiO ₂ anatase (101) surface

Anatase is a metastable polymorph of TiO ₂. In contrast to the more widely studied TiO ₂ rutile, O vacancies (V _O's) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V _O's, created by electron bombardment at 105K, start migrating to subsurface sites at temperatures 200K. After an initial decrease of the V _O density, a temperature-dependent dynamic equilibrium is established where V _O's move to subsurface sites and back again, as seen in time-lapse STM images. We estimate that activation energies for subsurface migration lie between 0.6 and 1.2eV; in comparison, density functional theory calculations predict a barrier of ca. 0.75eV. The wide scatter of the experimental values might be attributed to inhomogeneously distributed subsurface defects in the reduced sample.