Abstract
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.
Highlights
Previous research has suggested that the ratios of these two oxidation states within nanoceria determines the extent of their beneficial properties, such as the ability to function as an oxygen donor for more efficient fuel combustion[9, 10] or reactive oxygen species (ROS) scavenger for antioxidative therapies,[11] and their potential detrimental effects on biological[12] and environmental systems.[13]
We describe the development of analysis methods for X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) using techniqueindependent Ce3+ (CeAlO3:Ge) and Ce4+ (CeO2) control materials towards quantitatively measuring the oxidation states of nanoceria materials
Based on our findings and supplemented by those found in the literature, we propose the following steps when pursing oxidation state studies of cerium oxide materials, especially in the context of comparing new findings with previous literature reports: 1) control materials should be obtained and calibrated to the analytical technique to be utilized; 2) if possible, the same control material should be used for each analytical technique to be utilized; 3) technique-specific variables should be identified and considered to limit artifacts that might impact the determination of oxidation state; 4) sample-specific variables should be identified and accounted for when measuring oxidation state
Summary
Cerium oxide (CeO2-x, ceria) nanomaterials (nanoceria, CNMs) are receiving increased attention due to their current and potential use in a vast number of applications, such as chemical mechanical polishing/planarization (CMP) processes, industrial and automotive catalysis, electrochemical devices, agricultural products and medicinal treatments.[1,2,3,4] While the performance of nanoceria in these applications depends on many physicochemical properties (e.g. size, shape, surface chemistry), the ability of nanoceria to exist in and cycle between the Ce3+ and Ce4+ oxidation states has been proposed as the key feature behind their unique activity.[5,6,7,8] Previous research has suggested that the ratios of these two oxidation states (which is dependent on several factors including their intrinsic physicochemical properties, extrinsic defects or impurities, and surrounding environment) within nanoceria determines the extent of their beneficial properties, such as the ability to function as an oxygen donor for more efficient fuel combustion[9, 10] or reactive oxygen species (ROS) scavenger for antioxidative therapies,[11] and their potential detrimental effects on biological[12] and environmental systems.[13]. Several analytical techniques have been used to gain insight into the Ce3+/Ce4+ ratios of nanoceria materials, including X-ray photoelectron spectroscopy (XPS),[14,15,16,17,18] electron energy loss spectroscopy (EELS),[19,20,21,22,23] X-ray absorption spectroscopy (XAS),[24,25,26] Raman spectroscopy,[27,28,29] and Ultraviolet/Visible/Infrared (UV/Vis/IR) spectroscopy;[30,31,32,33] of these, XPS and EELS are perhaps the most widely used characterization tools.[34,35,36,37] each of these techniques operates under different fundamental principles[37] with inherent variances and can produce different results on the Ce3+/Ce4+ ratios of nanoceria analytes These technique-sensitive variances and the lack of technique-independent controls, complicates the ability to compare analytical data generated by different techniques, even in the few cases where multiple techniques were employed in a single study.[37, 38] New approaches in determining the oxidation states of nanoceria, using multiple analytical techniques and technique-independent controls, could help generate more accurate and reproducible measurements of the Ce3+/Ce4+ ratio, facilitating a greater understanding of nanoceria properties and interactions.[13, 36, 37]
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