X-ray photoelectron spectroscopy of metal oxide nanoparticles: chemical composition, oxidation state and functional group content

Surface chemistry drives the interaction of a material with its surroundings, therefore it can be used to understand and influence the fate of nanomaterials when used as functional materials or when released to the environment. Here we have used X-ray photoelectron spectroscopy (XPS) to probe the chemical composition, oxidation state and functional group content of the near surface region of four families of commercially available metal oxide nanoparticles from several different suppliers. The analyzed nanoparticles varied in size and surface functionalization (unfunctionalized vs. amine, stearic acid, and PVP-coated samples). Survey and high-resolution scans have provided information on the atomic composition of the samples, including an estimate of the stoichiometry of the metal oxide, the presence of functional groups and the identification and quantification of any impurities on the surface. The presence of significant impurities for some samples and the variation from the expected oxidation state in other cases are relevant to studies of the environmental and health impacts of these materials as well as their use in applications. The functional group content measured by XPS shows a similar trend to earlier quantitative nuclear magnetic resonance (qNMR) data for aminated samples. This indicates that XPS can be a complementary probe of surface functional group content in cases where the functional group contains a unique element not otherwise present on the nanoparticles.