Abstract
We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (CoO). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O to O states hybridized with the unoccupied states of cobalt atoms. Also, since CoO contains two types of Co atoms, i.e., Co and Co, we find that contribution of Co ions to the pre-edge peak is solely due to single spin-polarized orbitals (, , and ) while that of Co ions is due to spin-up and spin-down polarized orbitals ( and ). Furthermore, we deduce the magnetic moments on the Co and Co to be zero and 3.00 respectively. This is consistent with an earlier experimental study which found that the magnetic structure of CoO consists of antiferromagnetically ordered Co spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.
Highlights
For transition metal atoms that serve as active sites in many catalytic reactions, L23 edge X-ray absorption spectra (XAS) can be used to probe 3d valence orbitals via the dipole-allowed 2p − 3d transitions
The oxygen K-edge spectra show a strong preedge peak which can be ascribed to dipole transitions from O 1s to O 2p states which are hybridized with the unoccupied 3d states of cobalt atoms
X-ray absorption near-edge spectroscopy (XANES) and electronic structure analysis, we deduce the magnetic moments on the two types of Co atoms that are found in the Co3 O4, i.e., Co3+ and Co2+
Summary
Co3 O4 nanoparticles are potential candidates to support many chemical reactions in heterogeneous catalysis [1–4]. From a theoretical point of view, obtaining a clear picture of the oxidation states from the L23 -edge spectrum as calculated by density functional theory (DFT)-based single-electron approaches is not straightforward. With respect to the Co3 O4 nanoparticles in particular, in addition to Co ions, it is important to probe the electronic structure of O ion for completeness Such a holistic approach is a major motivation in our research group, in our study of electronic, magnetic as well as catalytic applications of nanostructured Co3 O4 surfaces. In the present work and as a step towards the understanding of effects of nanostructuring on the physical and chemical properties of the surfaces, including the influence of operando conditions on the local environment of Co active sites, we have performed combined experimental and theoretical studies of X-ray diffraction (XRD) and X-ray near-edge spectroscopy (XANES).
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