Measurement of the dependence of interfacial charge-transfer rate constants on the reorganization energy of redox species at n-ZnO/H2O interfaces

The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)(3)(3+/2+)), ruthenium pentaamine pyridine (Ru(NH3)(5)py(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)(2)(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me(2)bpy)(2)(lm)(2)(3+/2+)), which have similar formal reduction potentials yet which have reorganization energies that span approximately 1 eV. Differential capacitance vs potential and current density vs potential measurements were used to measure the interfacial electron-transfer rate constants for this series of one-electron outer-sphere redox couples. Each interface displayed a first-order dependence on the concentration of redox acceptor species and a first-order dependence on the concentration of electrons in the conduction band at the semiconductor surface, in accord with expectations for the ideal model of a semiconductor/liquid contact. Rate constants varied from 1 x 10(-19) to 6 x 10(-17) CM4 s(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, A, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (lambda + Delta G degrees')(2)/4 lambda k(B)T (where Delta G degrees' is the driving force for interfacial charge transfer) was linear with a slope of similar to -1. The rate constant at optimal exoergicity was found to be similar to 5 x 10(-17) cm(4) s(-1) for this system. These results show that interfacial electron-transfer rate constants at semiconductor electrodes are in good agreement with the predictions of a Marcus-type model of interfacial electron-transfer reactions.