ADSORBED OMEGA-HYDROXY THIOL MONOLAYERS ON GOLD ELECTRODES - EVIDENCE FOR ELECTRON-TUNNELING TO REDOX SPECIES IN SOLUTION

Au electrodes are derivatized by self-assembled organic monolayers of omega-hydroxy thiols from aqueous solutions. The capacitance, ellipsometric, XPS, heterogeneous electron-transfer properties, and stability of these derivatized electrodes are consistent with the formation of a pinhole-free hydroxy thiol monolayer. In particular, the logarithmic dependence of the heterogeneous electron-transfer rate constant of Fe(CN)6(3-) and Fe3+ in 0.1 M KCl on the thickness of the monolayer film and the lack of a temperature dependence at high overpotentials are consistent with electron tunneling through the full thickness of the self-assembled film. A tunneling constant beta of 0.9 per methylene group is measured. Tafel plots for Fe(CN)6(4-/3-) and Fe2+/3+ couples display curvature at high overpotentials, which is consistent with Marcus electron-transfer theory.