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.