Measurement of the driving force dependence of interfacial charge-transfer rate constants in response to pH changes at n-ZnO/H₂O interfaces

Changes in pH have been used to shift the band-edge positions of n-type ZnO electrodes relative to solution-based electron acceptors having pH-independent redox potentials. Differential capacitance vs. potential and current density vs. potential measurements using [Co(bpy)₃]^3+/2+ and [Ru(bpy)₂(MeIm)₂]^3+/2+ (where bpy = 2,2′-bipyridyl and MeIm = 1-methyl-imidazole) allowed investigation of the pH-induced driving-force dependence of the interfacial electron-transfer rate in the normal and inverted regions of electron transfer, respectively. All rate processes were observed to be kinetically first-order in the concentration of electrons at the ZnO surface and first-order in the concentration of dissolved redox acceptors. Measurements using [Co(bpy)₃]^3+/2+, which has a low driving force and a high reorganization energy in contact with ZnO electrodes, and measurements of [Ru(bpy)₂(MeIm)₂]^3+/2+, which has a high driving force and a low reorganization energy in contact with ZnO electrodes, allowed for the evaluation of both the normal and inverted regions of interfacial electron-transfer processes, respectively. The rate constant at optimum exoergicity was observed to be approximately 5 × 10^-17 cm⁴ s^-1. The rate constant vs. driving-force dependence at n-type ZnO electrodes exhibited both normal and inverted regions, and the data were well-fitted by parabolas generated using classical electron-transfer theory.