Structural Characteristics and Eutaxy in the Photo-Deposited Amorphous Iron Oxide Oxygen Evolution Catalyst

A central challenge for hydrogen generation via electrolytic water splitting is the identification of efficient oxygen evolution reaction (OER) catalysts; a key aspect of the challenge hinges on an ability to relate atomic-scale structure to observed activities. Amorphous iron-based oxide(hydroxides) prepared by photochemical metal organic decomposition (PMOD) are proven OER catalysts, but their atomistic structures have been elusive. Here, a combination of powder diffraction and pair distribution function (PDF) analyses enables the formulation of a set of structural characteristics that capture the salient features of amorphous iron oxide(hydroxide) ( a -FeO x ), a model compound for this class of materials. a -FeO x contains only octahedrally coordinated iron atoms, which form clusters of both edge- and corner-sharing octahedra. A degree of “eutaxy” with predominantly ABC-type anion stacking persists at length scales beyond the dimensions of cluster domains–consistent with thermally induced crystallization into the defect spinel γ-Fe 2 O 3 . Evidence for considerable octahedral irregularities suggests the presence of a large number of bridging and terminal hydroxyl or water ligands, which would provide a high concentration of potential active sites. The structural features of a -FeO x are reminiscent of other first-row transition metal oxyhydroxide OER catalysts that comprise layers of edge-sharing octahedral ions capable of electron transfer and ligand association/dissociation. In keeping with ABC anion stacking, however, the title compound more closely resembles a highly defective spinel lattice rather than a layered hydroxide.