First principles and classical modeling of the oxidized titanium (0001) surface

The formation of a native oxide layer on the Ti(0001) surface is studied by means of FPMD simulations. In agreement with experimental findings, at low temperature we observe quick saturation of the surface reactivity, whereas incorporation of further O2 molecules in the oxide network takes place after thermal annealing at ∼ 800 K. At an O coverage of 2 ML the oxide layer presents an amorphous structure, an approximate TiO stoichiometry, and a broad distribution of Ti oxidation states from + 1 to + 4. We find consistency between the computed Bader atomic charges on Ti atoms and the correspondent charges computed classically by simple electrostatic minimization methods. On this basis we develop an analytic potential to simulate Ti/TiOx interfaces, including only Coulomb interaction and short-range atomic repulsion terms. Two different parameter sets are proposed and their transferability among TiO2 allomorphs and thin-layer oxide structures is tested both in static relaxations and in room-temperature MD simulations. These show only negligible changes in the topology of the oxide network after annealing and relaxation of the reference quantum model at the classical level. Moreover, we show that superficial oxide layers can be successfully generated purely classically by truncation of a large-scale Ti/amorphous-TiO2 system. This may enable large-scale applications of our potential to tribology and biomolecular adsorption phenomena.