Approaches for the Quantitative Analysis of Oxidation State in Cerium Oxide Nanomaterials
Christopher M. Sims, Russell A. Maier, Aaron C. Johnston-Peck, Justin M. Gorham, Vincent A. Hackley, Bryant C. Nelson
Cerium oxide nanomaterials (nanoceria, CNMs) are receiving increased attention from the research community due to their unique chemical properties, most prominent of which is their ability to alternate between the Ce3+ and Ce4+ oxidation states. While many analytical techniques and methods have been employed to characterize the amounts of Ce3+ and Ce4+ present (Ce3+/Ce4+ ratio) within nanoceria materials, to-date no studies have used multiple complementary analytical tools (orthogonal analysis) with technique-independent oxidation state controls for quantitative determinations of the Ce3+/Ce4+ ratio. Here, we describe the development of analytical methods measuring the oxidation states of nanoceria analytes using technique-independent Ce3+ (CeAlO3:Ge) and Ce4+ (CeO2) control materials, with a particular focus on X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) approaches. The developed methods were demonstrated in characterizing a suite of commercial nanoceria products, where the two techniques (XPS and EELS) were found to be in good agreement with respect to Ce3+/Ce4+ ratio. Potential sources of artifacts and discrepancies in the measurement results were also identified and discussed, alongside suggestions for interpreting oxidation state results using the different analytical techniques. The results should be applicable towards producing more consistent and reproducible oxidation state analyses of nanoceria materials.