The electronic structure of oxygen vacancy defects at the low index surfaces of ceria

Ceria, CeO2, plays an important role in catalysis, participating directly in the conversion of environmentally sensitive molecules. This arises from the ability of ceria to store and release oxygen depending upon the conditions present in the exhaust gas. Obtaining a basic understanding of oxygen vacancy defects in ceria and the interaction of defective structures with such molecules is central to our understanding of the role of ceria in catalysis. In this work we examine using first principles density functional theory (DFT), with the inclusion of on site electronic correlations (DFT + U), the geometry and electronic structure of (1 1 1), (1 1 0) and (1 0 0) ceria surfaces that include oxygen vacancies. We find for all surfaces that the surface (atomistic) structure is strongly perturbed and the extraction of an oxygen vacancy is associated with a reduction of two neighbouring 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 appears between the top of the valence band and the bottom of the unoccupied Ce 4f states. Localisation of charge due to the gap state and excess spin density on Ce3+ sites neighbouring the vacancy is observed for all three surfaces. These DFT + U results are validated by recent experimental results regarding the electronic structure of reduced ceria surfaces, in contrast to previous DFT results. We observe an interesting result that the vacancy formation energies do not follow the same order as the stabilities of the pure surfaces, as measured by the surface energy; thus, the (1 1 0) surface has the lowest vacancy formation energy. The impact of this for the study of catalytic reactions on ceria surfaces is discussed.