Voltammetry of adsorbed monolayers: computer simulation and experiment

Abstract

Facile and accurate determination of both kinetic and thermodynamic electrochemical parameters from simple cyclic voltammograms of adsorbed species has been achieved using a new computer based theoretical model. This removes, for the most part, a major deficiency in cyclic voltammetric analysis. This model incorporates an integrated approach to simulation in cyclic voltammetry involving a combination of Marcus electron transfer theory and a simplex fitting algorithm. The model uses a modem electron transfer theory in a unique way and is demonstrated to be an important and useful diagnostic tool for the electrochemist. With this model, the effect of the electrode material on the electrochemical response of adsorbed osmium complexes has been investigated and it has been found that the non-adiabatic rate of heterogeneous electron transfer does not depend simply on the density o f states within the electrode. In contrast, it is found that the non-adiabatic rate of heterogeneous electron transfer depends on the density of states modulated by the square of the coupling. Studies on the effect o f bond conjugation within bridging ligands in adsorbed monolayers using the electron transfer model have been carried out. Surprisingly, the presence of bond conjugation gives a lower rate constant. Analogous experiments were carried out on complexes in solution. The model has been comprehensively tested using both theoretical and experimental data and has proven to be highly sensitive to the heterogeneous electron transfer rate constant, k°, and to a lesser extent, the reorganisation energy, X. This electron transfer model will enable future studies of adsorbed monolayers where distributions of formal potentials and / or distributions of heterogeneous electron transfer rate constants exist