Electronic Structure and Redox Properties of the Ti-Doped Zirconia (111) Surface

We have applied density functional theory calculations with Hubbard corrections (DFT+U) to investigate the structural, electronic, and redox properties of Ti-substituted zirconia (111) surfaces. The calculations show that titanium dopants are likely to accumulate at the oxide surface, where an isolated dopant is 0.25 eV more stable than in the bulk. We have investigated in detail the relative distribution of dopants and oxygen vacancies at the surface and report the most stable configurations for each composition. It is found that the formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. The analysis of the electronic structure of the doped and reduced surfaces reveals that, when an O vacancy is created around an isolated Ti dopant, a Ti4+ -> Ti2+ reduction takes place, with the reduced cation in a high-spin configuration. However, if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a +3 oxidation state with an additional decrease in the vacancy formation energy.