Electron Transfer Dynamics of Iridium Oxide Nanoparticles Attached to Electrodes by Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of carboxylated alkanethiolates (−S­(CH₂)_<i>n</i>−1CO₂^–) on flat gold electrode surfaces are used to tether small (ca. 2 nm d.) iridium­(IV) oxide nanoparticles (Ir^IVO_X NPs) to the electrode. Peak potential separations in cyclic voltammetry (CV) of the nanoparticle Ir^IV/III wave, in pH 13 aqueous base, increase with <i>n</i>, showing that the Ir^IV/III apparent electron transfer kinetics of metal oxide sites in the nanoparticles respond to the imposed SAM electron transfer tunneling barrier. Estimated apparent electron transfer rate constants (<i>k</i>_app⁰) for <i>n</i> = 12 and 16 are 9.8 and 0.12 s^–1. Owing to uncompensated solution resistance, <i>k</i>_app⁰ for <i>n</i> = 8 was too large to measure in the potential sweep experiment. For the cathodic scans, coulometric charges under the Ir^IV/III voltammetric waves were independent of potential scan rate, suggesting participation of all of the iridium oxide redox sites (ca. 130 per NP) in the NPs. These experiments show that it is possible to control and study electron transfer dynamics of electroactive nanoparticles including, as shown by preliminary experiments, that of the electrocatalysis of water oxidation by iridium oxide nanoparticles