Optimisation of accurate rutile TiO2 (110), (100), (101) and (001) surface models from periodic DFT calculations

In this paper, geometric bulk parameters, bulk moduli, energy gaps and relative stabilities of the TiO₂ anatase and rutile phases were determined from periodic DFT calculations. Then, for the rutile phase, structures, relaxations and surface energies of the (110), (100), (101) and (001) faces were computed. The calculated surface energies are consistent with the natural rutile powder composition, even if a dependence on the number of layers of the slab used to model the surface was identified. Internal constraints, consisting in freezing some internal layers of the slab to atomic bulk positions, were thus added to mimic the bulk hardness in order to stabilise the computed surface energies for thinner systems. In parallel, the influence of pseudopotentials was studied and it appears that four valence electrons for titanium atoms are sufficient. The aim of this study was to optimise accurate rutile TiO₂ surface models that will be used in further calculations to investigate water and uranyl ion sorption mechanisms.