Formation Mechanism and Hydrothermal Synthesis of Highly Active Ir_1–<i>x</i>Ru_<i>x</i>O₂ Nanoparticles for the Oxygen Evolution Reaction

Iridium dioxide (IrO₂), ruthenium dioxide (RuO₂), and their solid solutions (Ir_1-<i>x</i>Ru_<i>x</i>O₂) are very active electrocatalysts for the oxygen evolution reaction (OER). Efficient and facile synthesis of nanosized crystallites of these materials is of high significance for electrocatalytic applications for converting green energy to fuels (power-to-X). Here, we use <i>in situ</i> X-ray scattering to examine reaction conditions for different Ir and Ru precursors resulting in the development of a simple hydrothermal synthesis route using IrCl₃ and KRuO₄ to obtain homogeneous phase-pure Ir_1-<i>x</i>Ru_<i>x</i>O₂ nanocrystals. The solid solution nanocrystals can be obtained with a tunable composition of 0.2 < <i>x</i> < 1.0 and with ultra-small coherently scattering crystalline domains estimated from 1.3 to 2.6 nm in diameter based on PDF refinements. The <i>in situ</i> X-ray scattering data reveal a two-step formation mechanism, which involves the initial loss of chloride ligands followed by the formation of metal-oxygen octahedra clusters containing both Ir and Ru. These octahedra assemble with time resulting in long-range order resembling the rutile structure. The mixing of the metals on the atomic scale during the crystal formation presumably allows the formation of the solid solution rather than heterogeneous mixtures. The size of the final nanocrystals can be controlled by tuning the synthesis temperature. The facile hydrothermal synthesis route provides ultra-small nanoparticles with activity toward the OER in acidic electrolytes comparable to the best in the literature, and the optimal material composition very favorably combines low overpotential, high mass activity, and increased stability.