Co polyoxometalates and a Co3O4 thin film investigated by L-edge X-ray absorption spectroscopy

Abstract

We have performed cobalt L-edge X-ray absorption spectroscopy (XAS) on important materials for photoactive catalysis, namely nanoscale cobalt polyoxometalates (Co POM) and a Co3O4 thin film. A set of Co POM analogues were studied that vary according to the position and number of cobalts within the POM structure, metal valence state, oxygen ligand coordination geometry and heteroatom identity. Ligand field multiplet calculations simulate experimental XAS spectra in well-defined model systems provided by the Co POMs and extended to a Co3O4 thin film, thereby characterizing atomic multiplet and ligand field effects, including the ligand field parameter, structural distortions, and electron-electron interactions for Co2+ and Co3+ ions in both Oh and Td environments. The ligand field parameter, 10Dq, is determined to within an accuracy of ±0.1 eV, the spectra are sensitive to small structural distortions that further split d-levels (0.16 eV), and the strength of electron-electron interactions is found to within ±5% of the atomic value. We also find that the electronic structure parameters and the XAS spectra do not vary among POMs with pronounced differences in catalytic activity, and therefore X-ray spectroscopies even more sensitive to the 3d electronic structure (such as resonant inelastic X-ray scattering (RIXS)) should be used to differentiate the more active catalysts.