Study of the Voltammetric Spike Response of Heptyl Viologen at a HOPG Electrode Horizontally Touched to a Gas/Heptyl Viologen Aqueous Solution Interface

Abstract

We investigated the electrochemistry of a basal plane highly oriented pyrolytic graphite electrode horizontally touched to a gas/heptyl viologen aqueous solution (G/S) interface using cyclic voltammetry, electroreflectance (ER) spectroscopy, potential step chronoamperometry, chronoreflectometry, and chronocoulometry. The spike response on the voltammogram, which was previously analyzed by Tokuda et al. (see: J. Electroanal. Chem. 1999, 473, 138.), was found to be observable only in restricted conditions. The experimental ER spectrum at the spike potential was in good agreement with the simulated one, indicating that the redox reaction of heptyl viologen takes place at the spike potential and the radical cation adsorbs on the electrode surface in a nonupright orientation. The voltammogram and potential step response suggested that the kinetics of the spike response can be explained in terms of a nucleation−growth−collision (NGC) mechanism. It was revealed that the transient of reflectance in response to the potential step synchronizes with the NGC current component. The plot of the charge as a function of the final potential in the potential step coulometry measurement exhibited a transition point at which the interconversion of the oxidation state amounts to more than half of the adsorbed viologen. The spike response was assigned as the first-order faradaic phase transition, presumably between a Gibbs monolayer and a Langmuir monolayer. The presence of the Gibbs monolayer of heptyl viologen at the G/S interface was confirmed by surface tension measurements.