Adatoms and interstitials in the rutile TiO2 (110) surface: structure and dynamics

We present calculations for Ti adatoms and interstitials at the (110) surface of rutile TiO2, where these species are known to play a crucial role in surface chemistry. We review structural calculations performed using the DFT+U methodology, which have been benchmarked using controlled self-doping experiments on thin rutile films. The ab initio results have further been used to assess the ability of empirical charge equalisation (QEq) potentials to correctly predict the energetics of these structures. A simple modification to the potential, whereby the oxygen charge is fixed while allowing charge redistribution between Ti ions, has been shown to greatly improve its performance in terms of the energy landscape of the Ti adatoms and interstitials. In this paper, we extend the QEq calculations to consider the diffusion pathways and barriers in the surface using nudged elastic band calculations. We find that key barriers involved in the diffusion of Ti interstitials to adatom sites are much lower with the modified potential, implying that the diffusion is active on experimental time scales at temperatures where the regrowth of reduced rutile crystals exposed to oxygen has been observed.