(Sub)surface mobility of oxygen vacancies at the TiO$_2$ anatase (101) surface

TL;DR

This study investigates the mobility of oxygen vacancies at the anatase TiO₂ (101) surface, revealing temperature-dependent migration behavior and estimating activation energies through experimental STM and DFT calculations.

Contribution

It provides the first detailed analysis of oxygen vacancy mobility at the anatase TiO₂ surface, combining experimental and theoretical approaches.

Findings

V_O vacancies migrate to subsurface sites at temperatures ≥200 K

Activation energies for migration are estimated between 0.6 and 1.2 eV

DFT predicts a barrier of approximately 0.75 eV

Abstract

Anatase is a metastable polymorph of TiO$_2$. In contrast to the more widely-studied TiO$_2$ rutile, O vacancies (V$_\mathrm O$'s) are not stable at the anatase (101) surface. Low-temperature STM shows that surface V$_\mathrm O$'s, created by electron bombardment at 105 K, start migrating to subsurface sites at temperatures $\geq$ 200 K. After an initial decrease of the V$_\mathrm O$ density, a temperature-dependent dynamic equilibrium is established where V$_\mathrm O$'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 1.2 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.