Electrochemical and electrochemically modulated reflectance AC voltammetry studies of electron transfer kinetics between attached redox centers and a mirror gold electrode

Small amplitude electrochemical ac voltammetry (ACV) is employed to investigate kinetics of electron transfer between Ru redox centers attached to the electrode surface via C-10 alkanethiols and gold minor electrodes. The equations for faradaic admittance of strongly adsorbed electroactive species in the case of a Langmuir isotherm are applied to determine the total coverage of redox centers (theta(total)) and kinetic parameters: the standard rate constant (k(s)) and transfer coefficient, k(s) appears to increase and theta(total) appears to decrease as the perturbation frequency increases. In a separate experiment, large amplitude ACV is performed simultaneously with electrochemically modulated reflectance ac voltammetry (EMR ACV) on the same electroactive monolayer/electrode system. The electromodulation reflectance coefficient (X) is defined in the frequency domain as a ratio of ac electroreflectance to both de electroreflectance and the interfacial ac potential (E-inter). X is shown to be a more useful quantity than the ratio of ac electroreflectance to de electroreflectance for representation of the electroreflectance data. X is found to be exactly out-of-phase with the faradaic admittance at the wavelength region (410-440 nm) corresponding to the absorption band of the reduced form of the Ru complex. Therefore, the ac electroreflectance signal is dominated by the modulation of the electrode coverage of a given redox state with E-inter. Electrochromic effects are negligible. Accurate calculation of k(s) from EMR ACV data is complicated because of the nonlinear relationship between the faradaic ac current and large amplitude E-inter. Thus, under our experimental conditions, ACV is the preferred method to determine k(s). (C) 2000 The Electrochemical Society. S0013-4651(00)02-116-9. All rights reserved.