Electron Transfer Rate Constants for Majority Electrons at GaAs and GaInP₂ Semiconductor−Liquid Interfaces

Abstract

Current−voltage and impedance measurements have been conducted at different n-type III−V semiconductor electrodes in acetonitrile as a function of the concentration of cobaltocenium−cobaltocene (Co(Cp)2+/0), an outer-sphere and fast redox couple. The measurements reveal that the interfacial chemistry, especially as affected by etchants, has a very strong influence on the kinetics of the charge transfer reaction. According to our impedance analysis, which covered a frequency range between 10 Hz and 400 kHz, movement of the energy bands in the onset region 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 transfer reaction across the semiconductor−electrolyte interface. The combined analysis of the steady-state current/potential curves and the impedance spectra show that for the semiconductor electrode−electrolyte systems we have studied, the electron transfer rate constant ranges from about mid-10-10 cm4 s-1 to 10-17 cm4 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.