Rate Constants for Charge Transfer Across the Semiconductor/Liquid Interface

Author: Fajardo, Arnel Manasan

Year: 1999

Degree: Dissertation (Ph.D.)

Advisor: Lewis, Nathan Saul

Committee Members: Bercaw, John E.; Lewis, Nathan Saul; Marcus, Rudolph A.; Okumura, Mitchio

Option: Chemistry

DOI: 10.7907/8m2k-4x31

Abstract

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 surf ace 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 electron-transfer rate constants as a function of the free-energy change at the n-Si/bipyridinium2+/+ 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 electron-transfer rate constants at n-InP/methanol contacts. Nonideal electrical behavior, likely orginating from surf ace electrical traps, prevented any kinetic characterization of the p-InP/ methanol system.

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