Site-selective X-ray absorption spectroscopy of cobalt nanoparticles

An extensive study of fluorescence-detected X-ray absorption near-edge structure (XANES) spectroscopy, with the purpose of realizing site-selectivity, applied to smoothly oxidized cobalt nanoparticles is presented. For this, resonant inelastic X-ray scattering (RIXS) planes of the nanoparticles were recorded using high-resolution detection of K beta emission. The K beta line represents a superposition of emission lines that correspond to different homovalent compounds inherent in the nanoparticle and that are energetically different. Therefore Co K-edge XANES spectra, extracted from distinct emission energies of the RIXS planes, show partial valence-selectivity, which by assuming a simple core-shell model for our nanoparticle, turns over to partial site-selectivity. The pure site-specific XANES spectra, for the core and shell respectively, have been obtained by means of a numerical procedure. The influences of the lifetime broadening related to site-selectivity and XANES were considered and have been accounted for in the final solutions. A metallic Co core exhibiting the crystallographic beta-manganese and hexagonal-close-packed phases as well as a Co-O/C shell of valence 2 showing a wurtzite-type contribution to the standard rocksalt structure are recognized as the final site-specific candidates for our system. The separate determination of the physical properties of atoms on different sites in nanoparticles provides for the first time some information about the interaction between the nanoparticle and the corresponding coating that determines at least partly the properties of the particle. This offers new opportunities for tailoring the properties of nanoparticles using suitable surfactants or coatings. Copyright (C) 2011 John Wiley & Sons, Ltd.