A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity

Stable blue suspensions of 2 nm diameter iridium oxide (IrOx·nH2O) nanoparticles were obtained by hydrolyzing IrCl62− in base at 90 °C to produce [Ir(OH)6]2− and then treating with HNO3 at 0 °C. UV−visible spectra show that acid condensation of [Ir(OH)6]2− results in quantitative conversion to stable, ligand-free IrOx·nH2O nanoparticles, which have an extinction coefficient of 630 ± 50 M−1cm−1 at 580 nm. In contrast, alkaline hydrolysis alone converts only 30% of the sample to IrOx·nH2O at 2 mM concentration. The acidified nanoparticles are stable for at least one month at 2 °C and can be used to make colloidal solutions between pH 1 and 13. At pH 7 and above, some hydrolysis to form [Ir(OH)6]2− occurs. Uniform IrOx·nH2O electrode films were grown anodically from pH 1 solutions, and were found to be highly active for water oxidation between pH 1 and 13.