An examination, using first-principles density functional theory, with inclusion of on-site electronic correlations (DFT+U), was made of the geometry and electronic structure of (111), (110) and (100) ceria surfaces that included O vacancies. It was found, for all of the surfaces, that the surface (atomistic) structure was strongly perturbed and that the extraction of an O vacancy was associated with a reduction of 2 neighboring Ce(IV) species to Ce(III) rather than partial reduction of all Ce ions in the simulation cell. In the electronic density of states, a new gap state appeared between the top of the valence band and the bottom of the unoccupied Ce 4f states. Localization of charge due to the gap state and excess spin density on Ce3+ sites neighboring the vacancy was observed for all 3 surfaces. These DFT+U results were validated by recent experimental results regarding the electronic structure of reduced ceria surfaces, in contrast to previous density functional theory results. An interesting result was that the vacancy formation energies did not have the same order as the stabilities of the pure surfaces, as measured by the surface energy. Thus, the (110) surface had the lowest vacancy formation energy.

The Electronic Structure of Oxygen Vacancy Defects at the Low Index Surfaces of Ceria. M.Nolan, S.C.Parker, G.W.Watson: Surface Science, 2005, 595[1-3], 223-32