ELECTRON-TRANSFER RATE CONSTANTS FOR MAJORITY ELECTRONS AT GAAS AND GAINP2 SEMICONDUCTOR-LIQUID INTERFACES

Current-voltage and impedance measurements have been conducted at diff erent n-type III-V semiconductor electrodes in acetonitrile as a funct ion of the concentration of cobaltocenium-cobaltocene (Co(Cp)(2)(+10)) , an outer-sphere and fast redox couple. The measurements reveal that the interfacial chemistry, especially as affected by etchants, has a v ery strong influence on the kinetics of the charge transfer reaction. According to our impedance analysis, which covered a frequency range b etween 10 Hz and 400 kHz, movement of the energy bands in the onset re gion of the forward current can be excluded for this system. For this reason a comparatively simple model can be used for the evaluation of the second-order rate constants for the electrochemical charge transfe r reaction across the semiconductor-electrolyte interface. The combine d analysis of the steady-state current/potential curves and the impeda nce spectra show that for the semiconductor electrode-electrolyte syst ems we have studied, the electron transfer rate constant ranges from a bout mid-10(-10) cm(4) s(-1) to 10(-17) cm(4) s(-1). The former value is unusually large and is over 6 orders of magnitude greater than the maximum possible rate constant predicted and reported by other workers ; possible explanations are discussed.