FREE-ENERGY DEPENDENCE OF ELECTRON-TRANSFER RATE CONSTANTS AT SI LIQUID INTERFACES/

Differential capacitance vs potential and current density vs potential measurements have been used to characterize the interfacial energetic s and kinetics, respectively, of n-type Si electrodes in contact with a series of one-electron, outer-sphere redox couples. The differential capacitance data yielded values for the electron concentration at the surface of the semiconductor as well as values for the driving force of the interfacial electron-transfer event at Si/CH3OH-viologen(2+/+)? junctions. The differential capacitance vs potential measurements wer e essentially independent of the ac frequency imposed on the interface , with linear Bode plots (log\impedance\vs log frequency, at a fixed p otential) between approximate to 10(3) and approximate to 10(5) Hz, wi th slopes typically between -0.99 and -1.00. The slopes of C-2-E (Mott -Schottky) plots were in excellent agreement with theory, and little f requency dispersion was observed in the x-intercepts of such plots. Th e conduction band edge of the n-type Si anodes was invariant to within +/-40 mV in response to a variation in the redox potential of the sol ution of greater than 400 mV, indicating ''ideal'' interfacial energet ic behavior of this system with no evidence for Fermi level pinning.,. From these measurements, the surface-state density of the Si/CH3OH co ntact can be estimated as <10(11) cm(2)?, i.e., less than 1 defect for 10(4) surface atoms. The current density vs potential plots exhibited a first-order kinetic dependence on the concentration of electrons at the semiconductor surface and a first-order kinetic dependence on the concentration of accepters in the solution. Rate constants for transf er of charge from the semiconductor to the acceptor were determined as a function of the driving force for the interfacial charge-transfer e vent. The rate constants varied from 4 x 10(-18) cm(4) s(-1) to 6 x 10 (-17) cm(4) s(-1) and were well fit to Marcus-type behavior, with a re organization energy of 0.7 eV and a maximum rate constant at optimal e xoergicity of 6 x 10(-17) cm(4) s(-1). This maximum rate constant valu e is in excellent agreement with theoretical expectations for transfer of charge from a delocalized carrier in a semiconductor to a one-elec tron, outer-sphere redox acceptor dissolved in the electrolyte solutio n.