Diffusion of Ti through the TiO 2 (110) rutile surface plays a key role in the growth and reactivity of TiO 2 . To understand the fundamental aspects of this important process, we present an analysis of the diffusion of Ti ad-species at the stoichiometric TiO 2 (110) surface using complementary computational methodologies of density functional theory corrected for on-site Coulomb interactions (DFT + U) and a charge equilibration (QEq) atomistic potential to identify minimum energy pathways. We find that diffusion of Ti from the surface to subsurface (and vice versa) follows an interstitialcy exchange mechanism, involving exchange of surface Ti with the 6-fold coordinated Ti below the bridging oxygen rows. Diffusion in the subsurface between layers also follows an interstitialcy mechanism. The diffusion of Ti is discussed in light of continued attempts to understand the re-oxidation of non-stoichiometric TiO 2 (110) surfaces.

We present calculated surface and interstitial transition barriers for Ti, O, O-2, TiO, and TiO2 atoms and clusters at the rutile (110) surface. Defect structures involving these small clusters, including adcluster and interstitial binding sites, were calculated by energy minimization using density-functional theory (DFT). Transition energies between these defect sites were calculated using the NEB method. Additionally, a modified SMB-Q charge equilibration empirical potential and a fixed-charge empirical potential were used for a comparison of the transition energy barriers. Barriers of 1.2-3.5 eV were found for all studied small cluster transitions upon the surface except for transitions involving O-2. By contrast, the O-2 diffusion barriers along the [001] direction upon the surface are only 0.13 eV. The QEq charge equilibration model gave mixed agreement with the DFT calculations, with the barriers ranging between 0.8 and 5.8 eV

We present calculations for Ti adatoms and interstitials at the (110) surface of rutile TiO 2 , where these species are known to play a crucial role in surface chemistry. We review structural calculations performed using the DFT þ 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.