Scanning electrochemical microscopy of redox-mediated hydrogen evolution catalyzed by two-dimensional assemblies of palladium nanoparticles

The kinetics of the hydrogen evolution reaction (HER) catalyzed by two-dimensional assemblies of 13 nm diameter palladium (Pd) nanoparticles on mica substrates was investigated by scanning electrochemical microscopy.(SECM). The assemblies were prepared by electrostatic adsorption of citrate-stabilized Pd nanoparticles on poly-L-lysine treated mica. Atomic force microscopy (AFM) studies of the adsorption process provided information on the nanoparticle number density as a function of the adsorption time. The HER kinetics was determined by examining SECM feedback approach curves, employing the methyl viologen ((MV2(+/+.))) couple as the redox probe. With this configuration, the potential of the Pd nanoparticles is effectively determined by the local concentration ratio of the redox probe. The overpotential for proton reduction can be finely tuned by the concentration of the redox species, the size of the ultramicroelectrode (UME) tip, and the distance between the UME and the nanoparticle assembly. The SECM analysis allowed the mean exchange current density per Pd nanoparticle [j(0(pH = 3)) = (1.19 +/- 0.08) x 10(-6) A cm(-2)] to be evaluated. Significantly, the SECM methodology described is highly sensitive to the transfer coefficient of the HER, with a value of 0.5 providing the most satisfactory fit in the overpotential range investigated. It is concluded that the reactivity of the Pd nanoparticles is comparable to the bulk metal, which is consistent with the bulk-like electronic structure of Pd clusters of this dimension.