VOLTAMMETRY OF REDOX-ACTIVE GROUPS IRREVERSIBLY ADSORBED ONTO ELECTRODES - TREATMENT USING THE MARCUS RELATION BETWEEN RATE AND OVERPOTENTIAL

A treatment of linear sweep voltammetry for redox-active groups irreve rsibly immobilized on electrodes is presented with use of the Marcus t heory of electrode kinetics to relate rate constants to overpotential. The present treatment extends an earlier treatment of the same proble m (Laviron, E. J. Electroanal. Chem. 1979, 101, 19) that used the Butl er-Volmer theory to relate rate constants to overpotential. The behavi or predicted in the present treatment matches that of the earlier trea tment for very high reorganization energies; however, for reorganizati on energies below about 2.0 eV, voltammograms are predicted to be broa der and peak potentials are in most cases predicted to shift further f rom E degrees, than in the earlier treatment. These effects are most p ronounced at high overpotentials and at high sweep rates. Voltammetric data acquired over a wide range of sweep rates for ferrocene oxidatio n/reduction in self-assembled monolayers of N-(15-mercaptopentadecyl)f errocenecarboxamide coadsorbed with 16-mercaptohexadecanol onto gold w ere analyzed using the present treatment. Predictions of broader volta mmograms and greater shifts in peak potential with increasing sweep ra te were fully realized in the experimental data. A protocol based on f itting peak potential vs log (sweep rate) data to predictions from the ory is suggested as the preferred means of analyzing voltammetric data . Fitting was accomplished using a simplex algorithm with the heteroge neous electron-transfer self-exchange rate constant, k(0), and the reo rganization energy, lambda as fitting parameters. Values of k(0) = 7.0 s(-1) and lambda = 0.87 eV, and k(0) = 6.6 s(-1) and lambda = 0.87 eV , were obtained from fits to two independent voltammetric data sets fo r two separate ferrocene-containing monolayers. These k(0) and lambda values are in excellent agreement with those in a recent report (Chids ey, C. E. D. Science 1991, 251, 919) on a related system studied by po tential-step amperometry.