Effect of the vapor-deposited Au nanoparticles on the rate of the Fe(CN)6-3/Fe(CN)6-4 redox reaction at the highly oriented pyrolytic graphite electrode

Effect of the vapor-deposited Au nanoparticles on the rate of the Fe(CN)6-3/Fe(CN)6-4 redox reaction at the highly oriented pyrolytic graphite (HOPG) electrode is investigated by cyclic and convolution potential sweep voltammetry. Surface morphology and electronic structure of Au deposits is studied by AFM and XP spectra measurements of Au 4f core electrons and of valence electrons, respectively. Au/HOPG surfaces are shown to have the morphology, which is characterized by the preferential deposition of Au nanoparticles along step edges and the formation of larger Au islands on HOPG planes. Au deposits after annealing have the electronic structure close to that of the bulk metal, they remain stable during voltammetric measurements of the Fe(CN)6-3/Fe(CN)6-4 electron transfer, but they dissolve upon the surface oxide formation at far positive potentials. Dissolution does not allow using the usual electrochemical pretreatment, i.e., cycling the electrode potential between limits at which gold surface is oxidized and reduced, prior to kinetic measurements. The rate of the Fe(CN)6-3/Fe(CN)6-4 electron transfer on the Au/HOPG electrodes increases with the increasing Au coverage. Apparent standard rate constant related the Au coverage attains a maximum of approximately 10% of the value for a polycrystalline Au electrode. It is proposed that slower kinetics is related to the presence of the adsorbed impurities on the Au/HOPG surface, or to a double layer effect. Linear diffusion and the Marcus kinetic model provide a plausible description of the electrode process at both the HOPG and Au/HOPG electrodes.