Rate constants for charge transfer across the semiconductor/liquid interface

摘要

The interface between a semiconductor and a solution is relevant not only to various technological applications but also to the study of fundamental heterogeneous charge-transfer processes. The rate constants for charge transfer from a semiconductor electrode to an outer-sphere redox couple in solution were examined using standard electrochemical methods. Differential capacitance-potential measurements provided information on the interfacial electric field, which in turn quantified the concentration of relevant charge carriers at the surface of the semiconductor. Current density-potential measurements characterized the kinetic flux of charge carriers across the interface. The first section of this thesis presents results on the upper limits for electron-transfer rate constants at the n-Si/methanol-dimethylferrocene+/0 and n-GaAs/acetonitrile-ferrocene+/0 contacts. Magnitudes of the empirical upper limits were fully consistent with the theoretical maximum rate constants. The second section describes the measurement of electrontransfer rate constants as a function of the free-energy change at the n-Si/bipyridinium 2+/+ interface and compares these kinetic results to a theoretical treatment based on Marcus theory. Excellent agreement was found between the experimental values and the predictions of several charge-transfer theories. The third section outlines an attempt to compare the hole-transfer rate constants at p-InP/methanol junctions with the electrontransfer rate constants at n-InP/methanol contacts. Nonideal electrical behavior, likely originating from surface electrical traps, prevented any kinetic characterization of the p-InP/methanol system.