X-ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) measures the absorption of X-rays as their energy is scanned through a core-level edge of a chosen element, so it reports on that element alone, whether in a crystalline, nanoscale or amorphous phase. The region near the edge, X-ray absorption near edge structure (XANES), is sensitive to oxidation state (the edge shifts with valence) and to coordination symmetry (pre-edge features). The oscillations above the edge, extended X-ray absorption fine structure (EXAFS), are Fourier transformed to give the distances and coordination of neighboring atom shells. Because hard X-rays penetrate liquids, XAS can be run in situ in an electrochemical cell under reaction conditions, but it averages over all atoms of the element, so surface changes can be hidden when most atoms are in the bulk. Machine learning approaches that invert XANES spectra to structure are discussed in Inverse problems.
Fe and Ni K-edges of Ni-Fe oxide OER catalysts (2012)
Landon and coworkers used Fe K-edge XANES on a 10 mol % Fe mixed Ni-Fe oxide, compared with FeO, Fe3O4 and α-Fe2O3 references: the main edge matched Fe3+ in α-Fe2O3, while a pre-edge peak at 7115 eV indicated some tetrahedrally coordinated Fe, consistent with a NiFe2O4 spinel. In situ EXAFS at the Ni and Fe K-edges, in air, at open circuit in alkaline electrolyte and at 300 mV overpotential for oxygen evolution, showed almost no change at the Ni edge (possibly because Ni is mostly in bulk NiO) but an increased Fe-O peak and altered Fe-cation shells at the Fe edge, interpreted as higher Fe coordination under reaction conditions and evidence that Fe takes part in the oxygen evolution chemistry (Landon et al. 2012). Summary: landon2012; see also Oxygen evolution reaction.