Theoretical Investigation of TI-Doped Zirconia Surfaces

We use density functional theory calculations with Hubbard corrections (DFT+U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti$^{4+} \quad \to $ Ti$^{2+}$ reduction takes place, while if the vacancy is created in the vicinity of a pair of dopants, each Ti atom adopts a 3+ oxidation state, with additional decrease in the vacancy formation energy. We investigate in detail the relevant distribution of dopants and vacancies in the system, and discuss the implications of our results for some applications of zirconia-based ceramics.