SURFACE-CHEMISTRY AND CHARGE-TRANSFER KINETICS AT SEMICONDUCTOR-LIQUID INTERFACES

Various factors influencing the kinetics of charge transfer processes at semiconductor electrodes are described. The investigations were res tricted to the transfer of majority carriers because the corresponding currents are potential dependent which gives valuable information on the rate determining steps. Current-potential curves as obtained by va rious authors are analyzed in terms of theoretical models for charge t ransfer processes at semiconductor electrodes. Only in few cases do th e current potential curves exhibit a slope of 60 mV/decade predicted t heoretically. Examples are selected redox reactions at ZnO, GaAs and W Se2 electrodes. The origin of the deviations from an ideal behaviour i s still not clear. Various authors explain the deviations by assuming an electron transfer via surface states. Crystalline steps at the surf ace do not play an essential role. In the case of an ideal behaviour o f the current potential curve a second order rate constants in the ord er of 10(-17) cm(4) s(-1) and higher have been determined. Interesting ly, extremely high rates of k > 10(-12) cm(4) s(-1) have been found fo r the reduction of protons at n-GaAs electrodes. In this case the over all rate was not determined by the surface kinetics but by the transpo rt of electrons through the space charge region toward the surface.