Titration of Ce3⁺ ions in the CeO₂(111) surface by Au adatoms

The role of surface and subsurface O vacancies for gold adsorption on crystalline CeO₂(111) films has been investigated by scanning tunneling microscopy and density functional theory. Whereas surface vacancies serve as deep traps for the Au atoms, subsurface defects promote the formation of characteristic Au pairs with a mean atom distance of two ceria lattice constants (7.6 Å). Hybrid density functional theory calculations reveal that the pair formation arises from a titration of the two Ce3⁺ ions generated by a single O vacancy. The Au-Ce3⁺ bond forms due to a strain effect, as the associated charge transfer from the spacious Ce3 ⁺ into the adgold enables a substantial relaxation of the ceria lattice. Also the experimentally determined Au-pair length is reproduced in the calculations, as we find a Ce3⁺-Ce3⁺ spacing of two ceria lattice parameters to be energetically preferred. Single Au atoms can thus be taken as position markers for Ce3⁺ ion pairs in the surface, providing unique information on electron-localization phenomena in reduced ceria.