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

Abstract

Stable blue suspensions of 2 nm diameter iridium oxide (IrO x · n H 2 O) nanoparticles were obtained by hydrolyzing IrCl 6 2− in base at 90 °C to produce [Ir(OH) 6 ] 2− and then treating with HNO 3 at 0 °C. UV−visible spectra show that acid condensation of [Ir(OH) 6 ] 2− results in quantitative conversion to stable, ligand-free IrO x · n H 2 O nanoparticles, which have an extinction coefficient of 630 ± 50 M −1 cm −1 at 580 nm. In contrast, alkaline hydrolysis alone converts only 30% of the sample to IrO x · n H 2 O 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 IrO x · n H 2 O electrode films were grown anodically from pH 1 solutions, and were found to be highly active for water oxidation between pH 1 and 13.