Reduction and oxidation of Au adatoms on the CeO₂(111) surface - DFT+U versus hybrid functionals

Recently we showed that Au atoms may titrate Ce³⁺ ions in near-surface layers of reduced CeO₂(111). This surface contained oxygen vacancies in subsurface position within the topmost O-Ce-O trilayer [Pan et al., Phys. Rev. Lett., 2013, 111, 206101.]. The present work builds upon these findings and discusses additional results obtained using PBE+U and hybrid functionals. These approaches do not predict the same relative stabilities for the various adsorption sites of a single Au adatom at an O-defect concentration of a ¼ ML or 1.984 nm^-2. We attribute this discrepancy to a different alignment within the O 2p-Ce 4f gap, i.e. a different order by energy of partially occupied Ce 4f and Au 6s orbitals. The energy offset of these orbitals matters, because the adsorption of Au⁰(6s¹) atop Ce³⁺(4f¹) or atop a subsurface oxygen atom in the first coordination shell of a Ce³⁺(4f¹) involves creation of Au^-(6s²) and Ce⁴⁺(4f⁰) ions. The electron transfer to Au is coupled to stabilizing ionic relaxation in the lattice, commonly known as polaronic distortion, reinforcing the Au-Ce bond. The order of 4f and 6s orbitals depends on the density functional approximation and is also strongly influenced by the oxygen defect concentration.