The voltammetric response of nanometer-sized carbon electrodes

Carbon electrodes of nanometer size have been fabricated by electrochemical etching of carbon fibers followed by deposition of electrophoretic paint. These electrodes have been characterized with steady-state voltammetry using a range of redox probes. These redox probes were chosen because they show a range of formal potentials and kinetic rate constants (0) and are charged to different extents. The voltammetric response of these electrodes in both the presence and absence of supporting electrolyte has been investigated. In the presence of supporting electrolyte, well-defined steady-state voltammograms of sigmoidal shape have been obtained on electrodes with effective radii as small as 1 nm. In the absence of supporting electrolyte, however, the voltammetric behavior varies with the redox system. Deviation from the expected migration-diffusion response is observed when the electrode is smaller than 20 nm for the reduction of the multicharged cationic species hexammineruthenium(III) (Ru(NH3)(6)(3+)). Deviation from the ideal behavior of migration-coupled diffusion is exhibited at electrodes even of micrometer size during the reduction of the multicharged hexacyanoferrate(III) anion (Fe(CN)(6)(3-)), which has a median reduction potential and relatively low k(0). Such an effect is also observed for the oxidation of the hexacyanoferrate(II) anion (Fe(CN)(6)(4-)). In comparison, the hexachloroiridate(IV) anion (IrCl62-), with a higher k(0) and more positive reduction potential, shows deviations similar to those seen for hexaammineruthenium(III); i.e., no deviation from expected behavior is seen until the electrode size is less than ca. 20 nm. It is argued that the dynamic diffuse double-layer effect rather than the Frumkin effect is the major source of the observed nonideal behavior, The results indicate that the dynamic diffuse double-layer effect can function even when the electrode reaction is reversible or quasi-reversible and becomes more pronounced at very small electrodes. The nature of size effects on the voltammetric response at nanometer size electrodes are discussed.