Interfacial Electron-Transfer Reactions at Semiconductor Electrodes

Abstract

Differential capacitance versus potential and current density vs. potential measurements were used to determine the energetics and kinetics, respectively, of the interfacial electron-transfer processes of n-type ZnO electrodes in contact with aqueous solutions. The electron-transfer rate constant, ket, vs. driving force was investigated employing a series of non-adsorbing, one-electron, outer-sphere redox couples with formal reduction potentials spanning approximately 900 mV in the band-gap region. The data were well-fit by a parabola generated using classical Marcus theory with a reorganization energy, λ, of 0.67 eV. The dependence of ket on λ was determined using a series of compounds with similar formal reduction potentials, but reorganization energies that span approximately 1 eV. The interfacial electron-transfer rate constant decreases as the reorganization energy of the acceptor species increases and a plot of the logarithm of the electron-transfer rate constant vs. (λ+ΔG0'2/4λKBT is linear with a slope of ≈-1. Changes in solution pH were used to shift the band-edge positions of ZnO electrodes relative to solution-based electron acceptors having pH-independent redox potentials. This strategy allowed investigation of the pH-induced driving-force dependence of ket in the normal and inverted regions. It was further found that introduction of the tert-butyl functionality on osmium tris-bipyridyl decreased the self-exchange rate constant, determined from NMR line-broadening measurements, by a factor of 50 and the interfacial electron-transfer rate constant by 100, compared to that of the analogous methyl-substituted complex. The results indicate that the tert-butyl group can act as a spacer on an outer-sphere redox couple to significantly decrease the electronic coupling of the electron-transfer reaction both in self-exchange and interfacial electron-transfer processes. Methyl-terminated, n-type, (111)-oriented Si surfaces in contact with an electron acceptor having a pH-independent redox potential were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. These results, taken together, provide strong evidence that interfacial electron-transfer rate constants at semiconductor electrodes are in excellent agreement with the predictions of a Marcus-type model of interfacial electron-transfer reactions.