Quantitative Analysis of the Oxidation State of Cobalt Oxides by Resonant Photoemission Spectroscopy

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co³⁺ and Co²⁺ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co₃O₄(111) film to calibrate the RER with respect to the atomic Co³⁺/Co²⁺ ratio. The method was applied to monitor the reduction of a well-ordered Co₃O₄(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.