Anatase is a metastable polymorph of TiO2. In contrast to the more widely-studied TiO2 rutile, O vacancies (VO's) are not stable at the anatase (101) surface. Low-temperature STM shows that surface VO's, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures ≥ 200K. After an initial decrease of the VO density, a temperature-dependent dynamic equilibrium is established where VO'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 1.2 eV; in comparison, DFT calculations predict a barrier of ca. 0.75 eV. The wide scatter of the experimental values might be attributed to inhomogeneously-distributed subsurface defects in the reduced sample.
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Corresponding author: Ulrike Diebold. (diebold).