Theoretical and experimental upper bounds on interfacial charge-transfer rate constants between semiconducting solids and outer-sphere redox couples

Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH₃OH-dimethylferrocenium-dimethylferrocene, n-GaAs/CH₃CN-ferrocenium-ferrocene, and p-InP/CH₃CN-cobaltocenium-cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer rate constant at these semiconductor/liquid contacts were found to be consistent with the upper limits predicted by theory. The current density-potential behavior of n-InP and p-InP/Fe(CN)₆ ^3-/4-(aq) junctions was also examined in order to assess the validity of prior kinetic measurements on these interfaces.