Atomistic origin of high-concentration Ce³⁺ in {100}-faceted Cr-substituted CeO₂ nanocrystals

Improving the potential of promising CeO₂-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and the formation of oxygen vacancies (V_Os). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO₂ nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cerium valence states are investigated layer by layer across the ultrafine Cr-substituted CeO₂ NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce³⁺ cations is present in Cr-substituted CeO₂ NCs, particularly in the internal atomic layers, compared to the pristine CeO₂ NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of V_Os and increases the mobility of oxygen atoms for the nano-sized CeO₂. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of V_Os in the doped CeO₂ NCs.