Cerium oxidation state in ceria nanoparticles studied with X-ray photoelectron spectroscopy and absorption near edge spectroscopy

X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy experiments are used to investigate the oxidation state of cerium ions in ceria nanoparticles. A comparison of results shows that XPS yields a higher concentration of Ce3+ ions, even after analysis with a core-shell model. Three factors are proposed for the discrepancy between results: surface reduction of ceria in the XPS vacuum chamber enhanced by X-ray radiation, fast reduction dynamics associated with ceria nanoparticles, and a diffuse depth profile of the Ce3+ concentration inside ceria particles. Our results suggest that the high-vacuum XPS studies of ceria have overestimated the Ce3+ concentration in ceria nanoparticles under ambient condition. More importantly, we have demonstrated the importance of using complimentary surface analysis techniques to investigate the valence state of ceria nanoparticles. (C) 2004 Elsevier B.V. All rights reserved.