The study of nanocrystalline cerium oxide by X-ray absorption spectroscopy

X-ray absorption spectroscopy has been used to study the structural and ele ctronic properties of cerium atoms in nanocrystalline cerium oxide, These n anocrystalline cerium oxides were prepared by precipitation followed by the aging process, To increase the particle size, the as-prepared cerium oxide s were calcined at various temperatures. The nanocrystalline phase was reta ined, even when the samples mere calcined at 600 degrees C, The analyses of X-ray absorption near edge structures (XANES) for cerium oxides show that the increase in the relative intensity of the transition to 2 (p) under bar 4f(1)5d*(L) under bar final state and its transition energy shifts toward higher energy are due to the increase in covalence between cerium and oxide ligands with increasing particle size. The extended X-ray absorption fine structure (EXAFS) results for cerium oxide show that the third Ce-O shell i s degraded and the coordination number around cerium is decreased, resultin g in a decrease in the bond distances of RCe-O and RCe-Ce for particle size <15 nm, However, when the particle size is increased, the third Ce-O shell started growing and the coordination number around cerium attained normal coordination. The present results provide evidence for the formation of nan ocrystalline cerium oxide prepared by precipitation followed by the aging p rocess. (C) 2000 Academic Press.