Approaches for the quantitative analysis of oxidation state in cerium oxide nanomaterials

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.