X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co3O4: Experimental and Theoretical Studies

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 (Co3O4). 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 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2−y2 and dz2). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 μB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.

Probing the local environment of active sites during 2-propanol oxidation to acetone on the Co3O4 (001) surface: insights from first principles O K-edge XANES spectroscopy

“…This is similar to what we observed in our recent work on bulk Co 3 O 4 , where Co +3 active sites were found in the octahedral symmetry. 10 In the present work, the adsorption of 2-propanol tends to reconstruct the octahedral symmetry environment of Co atom. Thus, according to the ligand field theory 33,34,35,36 for an octahedral coordination, the splitting of the d-shell…”

“…28 A Lorentzian convolution with a variable broadening parameter γ has been applied in the continued fraction as we deed previously, to reproduce the experimental spectrum in the case of bulk Co 3 O 4 , where we used γ = 0.3 eV for incident photon energy up to 1.5 eV and γ = 0.8 eV from 10 eV of photon energy, with a linear variation in the intermediate photon range. 10…”

“…Indeed, the increase in the covalency of metal-oxygen bond implies a better overlap between the O 2p orbitals and the Co 3d orbitals, and hence a higher intensity of the 3d contribution in the O K-edge spectrum. 10,12,37 It worth noticing that from the 2-propanol to 2propoxide, there is the formation of a metal-alkoxide bond with a stronger covalent character than in the parent alcohol. 38 Such a change could explain the increase of the intensity of the pre-edge peak while the formal oxidation state does not change.…”

“…In the case of the free 2-propoxide, the O K-edge spectrum of the molecule (red curve in Figure 2a The parameters usually invoked to explain the increase of pre-edge peak intensity of the ligand K-edge spectrum are the increase of metal oxidation state as well as the hybridization of ligand p and metal 3d empty states. 10,11,12 Indeed, the stronger the pre-edge, the emptier are the 3d states, which may suggest higher metal oxidation state. In order to verify this presumption, we examined more closely the electronic states of oxygen and cobalt atoms.…”

“…We recently used theoretical XANES spectroscopy to provide insight into the above-mentioned informations for bulk Co 3 O 4 combining experiment and theory . 10 Theoretically, as also successfully done on other relevant oxides, 11,12 we achieved it by tuning the incident photon energy to the K edge energy of O atom in the material and used the ligand K-edge to scan unoccupied 3d Co states, 13 instead of the L 23 -edge of Co atoms themselves as done in experiment. This stems from shortcomings of DFT-based single-electron methods that prohibit correct reproduction of experimental L 23 -edge spectra.…”