Contraction, cation oxidation state and size effects in cerium oxide nanoparticles.

An accurate description of the structural and chemical modifications of cerium oxide nanoparticles (NPs) is mandatory for understanding their functionality in applications. In this work we investigate the relation between local atomic structure, oxidation state, defectivity and size in cerium oxide NPs with variable diameter below 10 nm, using x-ray absorption fine structure analysis in the near and extended energy range. The NPs are prepared by physical methods under controlled conditions and analyzed in morphology and crystalline quality by high resolution transmission electron microscopy. We resolve here an important question on the local structure of cerium oxide NPs: we demonstrate a progressive contraction in the Ce-O interatomic distance with decreasing NP diameter and we relate the observed effect to the reduced dimensionality. The contraction is not significantly modified by inducing a 4%-6% higher Ce concentration through thermal annealing in high vacuum. The consequences of the observed average cation-anion distance contraction on the properties of the NPs are discussed.