The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)33+/2+), ruthenium pentaamine pyridine (Ru(NH 3)5py3+/2+), cobalt bis-1,4,7- trithiacyclononane (Co(TTCN)23+/2+), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me2bpy)2(lm) 23+/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 cm4 s-1. The interfacial electron-transfer rate constant decreased as the reorganization energy, λ, of the acceptor species increased, an a plot of the logarithm of the electron-transfer rate constant vs (λ + ΔG°′)2/4λkBT (where AG°′ 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 cm4 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry