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

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 mid10^-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.