Ultra-small iridium oxide nanoparticles: from one-pot hydrothermal synthesis to oxygen evolution reaction catalyst

Abstract

Iridium oxide nanoparticles are efficient catalysts for the acidic oxygen evolution reaction (OER). We present a straightforward one-pot hydrothermal synthesis method to produce sub-1 nm Ir oxide nanoparticles in a single step, with size control achieved through post-synthesis annealing. By combining X-ray total scattering and pair distribution function (PDF) analysis with small-angle X-ray scattering (SAXS), we find that the sub-nanometer-sized oxide has an increased number of edge-sharing [IrO6]-octahedra compared to the thermodynamically favorable rutile structure. PDF modelling using various cluster motifs reveals that a sheet-like cluster, derived from rutile and comprising seven [IrO6]-octahedra with a (110) exposed surface, can describe the increased Ir-Ir edge-sharing connectivity. We further find that cluster growth leads to a decrease in the number of edge-sharing motifs, going towards the bulk rutile structure upon annealing. Operando X-ray total scattering and PDF analysis during OER reveal high structural stability of the ultra-small (<3 nm) Ir oxides.