Electronic and geometric factors affecting oxygen vacancy formation on CeO_2(111) surfaces: A first-principles study from trivalent metal doping cases

摘要

Oxygen vacancies (O-v) on CeO2 surfaces create polarons, involving both electron localization on Ce ions and local geometry distortions. The relative positions of reduced Ce(III) and O-v affect the energies. We use trivalent doping to study the factors affecting the O-v formation energy on the CeO2(111) surfaces. We find that it is easier to form an O-v adjacent to the dopant with a smaller radius for Al, Ga, In, Tl, Sc, and Y doped cases, and on the second nearest neighbor O to the dopant with a larger radius (La and Ac). It is ascribed to that the smaller dopant could give more space to relax when the Ce(III) is sharing neighboring O to it, while the larger ones not. The Ce (III) could also be considered as a case of trivalent doping, following the same trend varying with dopant radius. Compared with the other trivalent dopants, the reduced Ce(III) produces a new occupied 4f state in the band gap, increases the O-v formation energy, and would be more easily oxidized and favorable for the redox cycle. This study gives a theoretical view of the O-v formation process and takes us closer to the physical nature of the doped ceria system.