Co Polyoxometalates and a Co₃O₄ Thin Film Investigated by L-Edge X-ray Absorption Spectroscopy

Abstract

We have performed cobalt Ledge X-ray absorption spectroscopy (XAS) on important materials for photoactive catalysis, namely nanoscale cobalt polyoxometalates (Co POM) and a Co 3 O 4 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 Co 3 O 4 thin film, thereby characterizing atomic multiplet and ligand field effects, including the ligand field parameter, structural distortions, and electron−electron interactions for Co 2+ and Co 3+ ions in both O h and T d 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.

Nitrogen‐Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

“…However, it does not show up in XAS spectra of our catalysts and precursors, indicating that these Co atoms are in different local environments. As XAS provides fingerprints of local geometric and electronic structures of the studied element, comparisons of our measured spectra with known spectra suggest Co atoms in our catalysts and precursor are tetrahedrally coordinated (T d ) with a coordination number of 4: the peak at ≈777.6 eV is assigned to T d Co 3+ , and other features match T d Co 2+ . The different shapes of these XAS spectra are resulted from the variation of ligand field parameter value, 10 Dq .…”

“…As XAS provides fingerprints of local geometric and electronic structures of the studied element, comparisons of our measured spectra with known spectra suggest Co atoms in our catalysts and precursor are tetrahedrally coordinated (T d ) with a coordination number of 4: the peak at ≈777.6 eV is assigned to T d Co 3+ , and other features match T d Co 2+ . The different shapes of these XAS spectra are resulted from the variation of ligand field parameter value, 10 Dq . The mixed Co oxidation states are consistent with hard XAS measurements, and the local symmetry strongly supports Co K‐edge EXAFS analyses.…”

“…Further splitting at L 3 peak is due to the multiplet structure from electron–electron interactions that can imply the local symmetry. The small peak at 776.4 eV is assigned to ocatehedrally coordinated (O h ) Co atoms, which is the basic building block of CoO and LaCoO 3 . However, it does not show up in XAS spectra of our catalysts and precursors, indicating that these Co atoms are in different local environments.…”

Direct synthesis and characterization of mixed-valent Li0.5−δCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4

“…All spectra were normalized to the incoming ux and the energy scale was calibrated to match that of ref. 24, followed by a background subtraction to a line, and a nal area normalization for comparison. In order to produce the most stable and reproducible ts, the tting was limited to the region just around the L 3 edge (774-784 eV), in which the normalization was further restricted through another line subtraction and area normalization.…”

Synthesis and characterization of metastable, 20 nm-sized Pna 2 1 -LiCoPO 4 nanospheres

“…Co3O4 and CoO powders were used as references and all spectra are on a common energy scale as in reference 18 . The spectra were normalized to the beam current and the background subtracted.…”