Laplace-Pressure-Stabilized Rutile Solid-Solution Catalysts for Acidic Water Oxidation: Enabling DSA-Inspired...

An early phase diagram study in 1967 revealed surprisingly wide immiscible gaps in the rutile-type IrO₂─TiO₂ and IrO₂─SnO₂ systems, even though the three oxides share the same crystal structure and nearly identical ionic radii. This finding indicates that the formation of IrO₂-based solid solutions, especially with oxides of different structures, is thermodynamically challenging. Using more than 20 foreign metal (M) cations, we demonstrate that single-phase rutile (Ir, M)O₂ and (Sn, M)O₂ solid solutions containing up to 30 at.% M can be obtained when the crystal size is below 10 nm, whereas larger, submicron-scale crystals exhibit immiscible behavior. In particular, complete solid solutions are achieved across the entire composition range in the nanoscale IrO₂─SnO₂ system. This enhancement in miscibility under high Laplace pressure is not restricted to specific foreign cations but represents a general phenomenon. These results are exploited to synthesize rutile-phase nanocrystals incorporating DSA-inspired quaternary cations (Sn, Nb, Ru, and Ir), which show high corrosion resistance and catalytic activity under anodic potentials. Quantitative electrochemical analyses further show that the catalytic durability of these quaternary nanocrystals, despite the significantly reduced Ir content, is comparable to that of IrO₂ during the oxygen evolution reaction in acidic water electrolysis.