Electrical double-layer effects on mass transfer and electron transfer at nanometer-scale electrodes

This dissertation describes the investigation of the influence of the electrical double-layer on the mass- and electron-transfer rates at Pt nanometer-size electrodes. The electrical double-layer is the thin region adjacent to the metal/solution interface where a large electric field exists due to the difference of the metal and solution work functions. The thickness of the double-layer is a function of the electrolyte concentration and typically a few nanometers. Nanometer-size electrodes are used because the electrode radius determines the volume that is probed during an experiment. Hemispherical Pt electrodes were fabricated with voltammetrically measured radii approaching 1 nm. Transmission electron microscopy was used to image an electrode with a radius of 70 nm. These electrodes were further characterized by high-speed cyclic voltammetry of an adsorbed redox active monolayer to measure their electroactive areas. Voltammetric peaks corresponding to the oxidation of ∼7,000 molecules are reported, allowing measurement of electroactive areas as small as 10 -10 cm2.; Utilizing the fast mass-transfer rates inherent to nanometer-size electrodes, the heterogeneous electron-transfer rates for the oxidation of the ferrocenylmethyltrimethylammonium cation, (FcTMA+) and hexachloroiridate (III) anion (IrCl63-) were measured assuming Butler-Volmer kinetics. In the absence of supporting electrolyte, when the electrical double-layer is thickest, the electron transfer rate constant for IrCl6 3- decreased by an order of magnitude from 2.9 +/- 0.2 cm/s to 0.65 +/- 0.07 cm/s. The mass-transfer rate to nanometer-size electrodes for FcTMA+ increased while the rate for IrCl 63- decreased from the theoretical values calculated neglecting double-layer effects in the absence of supporting electrolyte. These phenomena are attributed to the influence of the electrical double layer.; The homogeneous kinetics of ion-pair formation were also studied using nanometer-size electrodes. The voltammetric position for the oxidation of the IrCl63- anion exhibits a dependence on the supporting electrolyte cation identity and concentration. The kinetics of ion-pair formation of IrCl63- with K +, Ca2+ and tetraethylammonium (TEA+) were studied at Pt disk and nanometer-size hemispherical shaped electrodes. Numerical simulations utilizing a general square-scheme mechanism for the voltammetric response suggest that the dissociation rate constant of the reactant ion pair can be determined from the voltammetric wave shape and very small electrodes.