Surface chemistry and charge transfer kinetics at semiconductor-liquid interfaces

Abstract

Various factors influencing the kinetics of charge transfer processes at semiconductor electrodes are described. The investigations were restricted 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 various authors are analyzed in terms of theoretical models for charge transfer processes at semiconductor electrodes. Only in few cases do the current potential curves exhibit a slope of 60 mV/decade predicted theoretically. Examples are selected redox reactions at ZnO, GaAs and WSe 2 electrodes. The origin of the deviations from an ideal behaviour is still not clear. Various authors explain the deviations by assuming an electron transfer via surface states. Crystalline steps at the surface do not play an essential role. In the case of an ideal behaviour of the current potential curve a second order rate constants in the order of 10 −17cm 4 s −1 and higher have been determined. Interestingly, extremely high rates of k > 10 −12 cm 4 s −1 have been found for the reduction of protons at n-GaAs electrodes. In this case the overall rate was not determined by the surface kinetics but by the transport of electrons through the space charge region toward the surface.