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Water electrolysis using proton exchange membrane technology offers an ideal process for green hydrogen production, but widespread deployment is inhibited by insufficient catalyst activity, stability and affordability. Iridium-based oxides provide the best overall performance for acidic water oxidation, the limiting reaction for this process, but further improvements are impeded by poor understanding of the restructured active catalyst surface that forms under reaction conditions. Here we present a combination of X-ray and electron scattering data that reveals direct evidence for three paracrystalline structural motifs at the restructured surfaces of highly active catalysts (including rutile IrO2 and perovskite SrIrO3) that have previously been described as amorphous. These insights enable the design of a paracrystalline IrOx catalyst that is independent of the bulk crystalline support and maintains higher activity, longer stability and more effective use of iridium to promote the production of green hydrogen. 

Multimetal oxyhalide intergrowths show promise for photocatalytic water splitting. However, the relationships between intergrowth stoichiometry and their electronic and nanoscale structures are yet to be identified. This study investigates Bi4TaO8Cl–Bi2GdO4Cl intergrowths and demonstrates that stoichiometry controls the tilting of [TaO6] octahedra, influencing the bandgap of the photocatalyst and its valence and conduction band positions. To determine how the [TaO6] octahedral tilting in the intergrowths manifests as a function of intergrowth stoichiometry, we investigated changes in crystal symmetry by analyzing features arising at the higher order Laue zone (HOLZ) of convergent-beam electron diffraction patterns. Higher Ta content intergrowths displayed a more intense outer HOLZ ring compared to lower Ta content intergrowths, indicating transformation from P21cn (orthorhombic) to P4/mmm (tetragonal). This finding suggests that more distortion occurs along the ⟨001⟩ directions of the crystal than the ⟨100⟩ and ⟨010⟩ directions. This variation directly impacts the electronic structure, affecting both conduction and valence band energy levels. By combining ultraviolet photoelectron spectroscopy, UV-visible diffuse reflectance spectroscopy, and electron energy loss spectroscopy, the absolute band positions of the intergrowths were determined. Agreement between the bandgaps obtained via ensemble and nanoscale measurements indicates nanoscale homogeneity of the electronic structure. Overall, the integrated approach establishes that the bandgap energy increases with increasing Ta content, which is correlated with the crystal symmetry and [TaO6] octahedral tilting. Broadly, the modular nature of intergrowths provides building block layers to tune octahedral tilting within perovskite layers for manipulation of optoelectronic properties.