Understanding ceria nanoparticles from first-principles calculations

Octahedral clusters (CeO2-x)(n) (n <= 85) have been studied using a density functional method (DFT+U) adapted to model nanocrystalline ceria. The binding energies of the clusters Ce19O32, Ce44O80, and Ce85O160 are shown to converge to the bulk limit almost linearly with respect to the average coordination number of Ce. The experimentally detected anomalous lattice expansion for nanoscale (CeO2-x)(n) particles of decreasing size is explicitly assigned to the presence of oxygen vacancies. Partially reduced Ce3+ cations are found to occupy more open edge and corner sites of the nanoparticles, whereas most oxidized Ce4+ centers are located in highly coordinated positions. This finding is crucial for the understanding of ceria reactivity at the nanoscale.