Persistent Homology and Bond Orientational Order in Ir-Cu Solid-Solution Alloy Nanoparticles: Implications for Electrocatalysts

Solid-solution alloy nanoparticles (NPs) comprising a noble metal and a transition metal, which are almost immiscible in the bulk phase, are promising cost-effective electrocatalysts for oxygen-evolution and oxygen-reduction reactions. However, the crystal structures of these NPs cannot be determined by the usual crystallographic techniques. Instead, high-energy X-ray diffraction structure factors were fitted by using the reverse Monte Carlo (RMC) method. Three-dimensional (3D) configuration models of IrxCu1-x solid-solution alloy NPs indicated spherical, highly disordered atomic configurations, with random mixing of Cu and Ir atoms. 3D local structures of the NPs were investigated via persistence diagrams (PDs) and bond orientational orders (BOOs). PDs of RMC NP models exhibited a broad distribution with very small "death" values along the diagonal and no vertical profile, which would indicate network formation. PDs of IrxCu1-x NPs also confirmed the absence of cavities or voids. The q(4) and q(6) BOO parameters for Ir and Cu in the RMC NP models had a broad distribution, unlike that for ideal NP models. The BOO parameter P-BOO provided compelling evidence for the contribution of the local structure and the lower coordination numbers. This enabled the availability of active sites on the IrxCu1-x NP surface, enhancing the oxygen-evolution reaction.