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 sem iconductor/liquid junction have been modified to include the effects o f adiabaticity and the existence of a Helmholtz layer at the solid/liq uid interface. These expressions have yielded an estimate of the maxim um interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorb ing, outer-sphere redox species. An experimental upper bound on this i nterfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semi conductor electrodes in contact with outer-sphere redox species. For t his 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-pot ential and differential capacitance-potential methods. The upper limit s for the interfacial charge-transfer rate constant at these semicondu ctor/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 a ssess the validity of prior kinetic measurements on these interfaces.