Measurement of the Dependence of Interfacial Charge-Transfer Rate Constants on the Reorganization Energy of Redox Species at n-ZnO/H_2O Interfaces

Abstract

The interfacial energetic and kinetics behavior of n-ZnO/H_2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)_3^(3+/2+)), ruthenium pentaamine pyridine (Ru(NH_3)_5py^(3+/2+)), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)_2^(3+/2+)), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me_2bpy)_2(Im)_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 × 10^(-19) to 6 × 10^(-17) cm^4 s^(-1). The interfacial electron-transfer rate constant decreased as the reorganization energy, λ, of the acceptor species increased, and a plot of the logarithm of the electron-transfer rate constant vs (λ + ΔG°')^2/4λk_BT (where ΔG°' is the driving force for interfacial charge transfer) was linear with a slope of ∼ −1. The rate constant at optimal exoergicity was found to be ∼5 × 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.

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