Several different charge transfer and Ce3+ localization scenarios for Rh–CeO2(111)

Abstract

We present DFT+U based electronic structure calculations in a p(3 × 3) slab supercell, for low coverages of atomically dispersed Rh interacting with the CeO₂(111) surface, comparing Rh as an adatom, and as a dopant substituted into the surface layer. We find that, energetically, a Rh atom approaching a ceria(111) surface with both sparse O and Ce vacancies present strongly prefers to heal the Ce vacancies, but next it prefers to adsorb on a stoichiometric region rather than healing an O vacancy. In the adatom system, Rh is oxidized by electron transfer to a 4f orbital on one Ce ion in the surface layer, which is then nominally converted from Ce⁴⁺ → Ce³⁺ (i.e. Rh adatoms are single donors). We show that there are a number of different local minima, with Ce³⁺ localization at 1^st, 2^nd or 3^rd nearest neighbour Ce sites. The second neighbour is the most stable, but all are close in energy. In the Rh-doped system (Rh replaces Ce), Rh is oxidized by charge transfer to neighbouring O atoms, and Rh doping leads to deep acceptor and donor states. Rh is not stable in the O sublattice. Moreover, based on vacancy formation energies, we find that oxygen vacancy formation is strongly enhanced in the vicinity of Rh dopants, but slightly suppressed in the vicinity of Rh adatoms.