fig2

Figure 2. Hard XAS/XES for electrocatalysts characterization. (A and C) representative energy level diagrams depict the origin of the spectral features. The XAS involves the excitation of core-level electron to the unoccupied states or into the continuum, serving as a probe of the unoccupied orbitals of chemical species and local atomic structure. In contrast, the XES monitors the decay of core holes (fluorescent X-rays associated with electron from higher-lying orbitals decay to fill the core hole) and, therefore, probes the occupied valence states in an atom-specific projection; (B) in-situ Co K-edge XANES study on CoNC and CoNOC during oxygen reduction reaction operation. Comparative study reveals the stable and rigid in-plane embedded Co₁N_x moiety and the flexible edge-hosted Co₁N₂ sites. The observed changes in the XANES spectra are primarily dictated by potential-driven structure evolution, whereas the adsorption of oxygenated intermediates exerts only a negligible effect. These figures are quoted with permission from Hu et al^[27]; (D) comparison of in-situ Kβ mainline XES spectra recorded on DW21 catalyst in N₂-saturated 0.5 M H₂SO₄ at OCV and 0.2 V vs. RHE, along with corresponding fit results. The inset shows a magnified view of the Kβ’ region; (E) the time course of applied bias in the in situ XES measurements, whereby each potential hold lasted 10 min; (F) Average spin states at each corresponding potential. These figures in (D-F) are quoted with permission from Saveleva et al.^[31]; (G) valence-to-core XES of various chromium-based compounds exhibiting sensitivity of the Kβ’’ feature to the light atom identity (upper); Intensity and energy of Valence-to-core XES correlates to protonation state of ligands in manganese dimers (below). These figures are quoted with permission from Cutsail et al.^[33]; (H) valence-to-core X-ray emission spectra of Pt foil (left) and Pt₃Ni (right) collected at the Pt L₃-edge. The energy difference between the resonant inelastic scattering (RIXS, blue dash line) and the elastic scatting (ES, green dash line) is labeled for each sample, with the full width at half maximum of the inelastic peaks summarized in (I). These figures are quoted with permission from Chen et al.^[34]. XAFS: X-ray absorption fine structure; XANES: X-ray absorption near-edge structure; XES: X-ray emission spectroscopy; OCV: open circuit; RHE: reversible hydrogen electrode; XAS: X-ray absorption.