Transport Properties in the CeO2-x(111) Surface: From Charge Distribution to Ion-Electron Collaborative Migration

Charge distribution and ion and electron migration have been theoretically studied in the reduced CeO2(111) surface by means of density functional calculations including on-site localization corrections (DFT+U). The analysis of the charge distribution shows that nearest-neighbor and next-nearest-neighbor configurations of Ce3+ and oxygen vacancies are the most stable arrangements for both surface and subsurface oxygen vacancies. Electron transfer between Ce3+ and Ce4+ centers corresponds to a polaron hopping involving the exchange of a 4f electron across the surface, with activation energies of about 0.3 eV. Activation barriers for oxygen atom migration on the surface strongly depend on the charge of the Ce ions surrounding the atom that actually moves. Namely, the migration is particularly facile when the migration occurs crossing the line through Ce4+ ions. The analysis also shows some coupling between polaron hopping and oxygen diffusion, suggesting that, for specific arrangements, the polaron hopping in some way assists the vacancy migration at the surface. The present results afford a quantitative illustration of the low barrier for the oxygen diffusion across the cerium oxide surface.