C.M. Sims, R.A. Maier, A.C. Johnston-Peck, J.M. Gorham, V.A. Hackley, B.C. Nelson
National Institute of Standards and Technology,
Keywords: cerium oxide nanoparticles, oxidation state, orthogonal analysis, method development, XPS, EELS, STEM
Summary:Of the many engineered nanomaterials being incorporated into our society, cerium oxide (ceria, CeO2-x) based nanomaterials (NMs) are receiving increased attention due to their unique chemical properties and vast number of current and potential applications (e.g., automotive catalysts, UV filters, agricultural treatment agents, antioxidant therapeutics.). Previous research designed to understand the potential environmental and toxicological effects of ceria NMs have yielded conflicting results, with ceria NMs found to be both toxic and non-toxic to cells and organisms. As the overall environmental and toxicological outcomes of ceria NMs are not yet fully understood, it is imperative to develop a comprehensive understanding of their physicochemical properties since these properties will influence the interactions of ceria NMs with biological and environmental systems. Here, we describe the development of an analytical procedure designed to measure the cerium oxidation state in ceria NMs using orthogonal approaches. Preparation of materials for control measurements and methods for optimizing data acquisition and processing were developed to efficiently analyze and objectively interpret the distribution of Ce3+ vs. Ce4+ oxides. Our results demonstrate a high degree of agreement between the utilized techniques (electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and annular dark field scanning transmission electron microscopy (ADF-STEM)) when stable control materials are employed (Figure 1). The methodology is applied to thoroughly characterize a suite of commercial ceria NMs. In agreement with previous research, our results suggest the primary particle size has a large influence on the oxidation state of the ceria particles, with smaller particles having increased Ce3+/Ce4+ ratios compared to their larger counterparts.