title: Nontrivial redox behavior of nanosized cobalt: new insights from ambient pressure X-ray photoelectron and absorption spectroscopies.
abstract: The reduction and oxidation of carbon-supported cobalt nanoparticles (3.50±0.22 nm) and a Co (0001) single crystal was investigated by ambient pressure X-ray photoelectron (APPES) and X-ray absorption (XAS) spectroscopies, applied in situ under 0.2 mbar hydrogen or oxygen atmospheres and at temperatures up to 620 K. It was found that cobalt nanoparticles are readily oxidized to a distinct CoO phase, which is significantly more stable to further oxidation or reduction compared to the thick oxide films formed on the Co(0001) crystal. The nontrivial size-dependence of redox behavior is followed by a difference in the electronic structure as suggested by theoretical simulations of the Co L-edge absorption spectra. In particular, contrary to the stable rocksalt and spinel phases that exist in the bulk oxides, cobalt nanoparticles contain a significant portion of metastable wurtzite-type CoO.
authors.affiliation: Laboratoire LMSPC UMR7515, CNRS, University of Strasbourg, F-67087, Strasbourg cedex 2, France.
authors.forename: Vasiliki
authors.initials: V
authors.lastname: Papaefthimiou
authors.forename: Thierry
authors.initials: T
authors.lastname: Dintzer
authors.forename: Véronique
authors.initials: V
authors.lastname: Dupuis
authors.forename: Alexandre
authors.initials: A
authors.lastname: Tamion
authors.forename: Florent
authors.initials: F
authors.lastname: Tournus
authors.forename: Arnaud
authors.initials: A
authors.lastname: Hillion
authors.forename: Detre
authors.initials: D
authors.lastname: Teschner
authors.forename: Michael
authors.initials: M
authors.lastname: Hävecker
authors.forename: Axel
authors.initials: A
authors.lastname: Knop-Gericke
authors.forename: Robert
authors.initials: R
authors.lastname: Schlögl
authors.forename: Spyridon
authors.initials: S
authors.lastname: Zafeiratos
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D046911:Macromolecular Substances / Q000737:chemistry; D008422:Materials Testing; D008968:Molecular Conformation; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D036103:Nanotechnology / Q000379:methods*; D010084:Oxidation-Reduction; D010316:Particle Size; D011312:Pressure; D013057:Spectrum Analysis / Q000379:methods; D013499:Surface Properties
chemical_list: D046911:Macromolecular Substances; D003035:Cobalt