Diffusional mediation of surface electron transfer on TiO2

The results of a kinetic study are reported for cross-surface electron tran sfer in the reversible oxidation of [Os-II(bpy)(2)(4,4'-(CO2H)(2)bpy)](PF6) (2) to Os-III surface adsorbed to nanocrystalline TiO2 films on optically t ransparent ITO (tin-doped indium oxide) electrodes. The kinetics are sensit ive to the extent of surface loading. From monolayers adsorbed from CH3CN, a percolation threshold of similar to 60% was measured. Adsorption isotherm s for [Os-II(bpy)(2)(4,4'-(CO2H)(2)bpy)](PF6)(2) in EtOH and CH3CN show tha t surface coverage from EtOH is similar to 60% of that from CH3CN at simila r adsorbate concentrations. Apparent charge-transfer diffusion coefficients in 0.1 M [N(n-C4H9)(4)](PF6) in CH3CN measured by chronoabsorptometry are D-app = 1.4 x 10(-9) cm(2)/s (Gamma = 1.1 x 10(-10) mol/cm(2), CH3CN) and 1 .4 x 10(-11) cm(2)/s (Gamma = 7 x 10(-11) mol/cm(2), EtOH). Electron-transf er mediation of the oxidative component of the Os-III/II surface couple occ urs in the presence of added Ru(bpy)(3)(2+) by stepping the potential past the Ru(bpy)(3)(3+/2+) wave. Mediation of both oxidative and reductive compo nents occurs with added Os(bpy)(3)(2+). The apparent diffusion coefficient for oxidation of the Os-III/II surface couple in the presence of RU(bpy)(3) (2+) increases Linearly with the concentration of added mediator.