Anal. Chem. 1994,66, 4525-4531
This Research Contribution is in Commemoration of the Life and Science of 1. M. Kolthoff (1894- 1993).
Kinetics of Reduction of Dimethylferrocenium Ion in Acetonitrile at Nearly Ideal Regions of n-Tungsten Diselenide Electrodes Jason N. Howard* and Carl A. Koval* Depattment of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
An electrochemical minicell was used to investigate the
interfacial properties and redox kinetics at nonilluminated regions (2-3 m m in diameter) of n-WSez crystals in acetonitrile solutions. Interfacial capacitance measurements at high and low frequencies together with the shape of current potential curves were used to identi@regions of n-WSez crystals with properties that approached what is expected for an ideal semiconductor-solution interface. Kinetic currents measured for reduction of dimethylferrocenium ion at these nearly ideal regions were independent of concentrationbetween 0.05 and 250 mM. Since the reduction potential of the dimethylferroceniumdimethylferrocene couple is 0.65 V positive of the conduction band edge of n-WSe2, the rate constant for outersphere electron transfer between conductionband electrons and dimethylferrocenium ions should be near its maximum value. However, the capacitance- and kinetic current-potential data indicate that this rate constant at n-WSez must be less than lo-’’ cm4 s-l. The concentration independent currents can be explained by postulating the existence of a low level of surface states, N, = 1 x 10l2 cm-2, with energies several hundred millivolts positive of the conduction band. Understanding electron transfer processes at nonilluminated, semiconductor-electrolyte interfaces (SEIs) is important for both practical and theoretical reasons. For example, dark redox processes are an important component limiting the efficiency of photoelectrochemical cells in which photons produce minority carriers that subsequently cause uphill redox chemistry in the electrolyte. Injection of majority carriers across the same interface promotes the reverse reaction, reducing overall conversion efficiency. In the 1960s, H. Gerischer first formulated a model for electron transfer (m? at semiconductor electrodes based on thermionic emission from the semiconductor and fluctuating energy levels of solution redox species.’ In the interim, several modifications and refinements have been added, but the fundamental aspects of the model remain unchanged. Since this model is applied to virtually all SEI redox processes, experimentally demonstrating the validity of this model is essential.
Lewis recently discussed the various theoretical equations associated with heterogeneous ET at the SEI by showing the relationship of these equations to the better known rate expressions for homogeneous reactions and ET at metal-electrolyte interfaces? Lewis also examined the experimental evidence that allows quantitative comparisons with the Gerischer model and concluded that relatively few experimental/theoretical comparisons are possible. Similar conclusions were also reached in a recent review on ET at the SEI by the author^.^ One of the most direct experimental tests of the Gerischer model involves determining the heterogeneous ET rate constant, ket, at a nonilluminated SEI. One of Gerischer’s recent papers concludes with the statement: “In order to check this theoretical analysis, careful experiments are needed where the rate of simple redox reactions in the forward direction is studied at various concentrations of the redox species and the band bending can be well enough determined. Very few data exist in the literature.” This situation is largely due to experimental daculties associated with studies of many SEIs. In order to interpret a measured value of ket properly, several issues must be considered. The interfacial energetics (IE) associated with a particular SEI describe the relationship between the energy states of the semiconductor and the solution. In many studies of kinetically controlled currents at SEIs, the IEs have not been determined. Deviations from the predicted diode-like current-potential behavior are often observed,usually indicating adsorption, tunneling, or corrosion. Finally, the simplest forms of the model assume that the only ET mechanism is isoenergetic exchange with the majority carrier band edge. Competing mechanisms such as surface state mediated transfer or minority band processes are assumed to be negligible. WSez has been employed in fundamental studies of the SEI due to its excellent characteristics as a semiconductorelectrode. Previous work has shown this material to have extremely stable IEs that are virtually insensitive to solution species in a~etonitrile.~-~ Furthermore, WSez has been shown to be highly stable, free of surface oxide layers, generally
Current address: Energy Products Division, Motorola, Inc., 8000 W. Sunrise Blvd., Fort Lauderdale, FL 33322. (1) Gerischer. Adv. Electrochem. Electrochem. Engl. 1961, 1, 139.
(2) Lewis, N. S. Annu. Rev. Phys. Chem. 1 9 9 1 , 4 2 , 543. (3) Koval, C. A; Howard, J. N. Chem. Rev. 1992, 92,411. (4) Gerischer, H. 1.Phys. Chem. 1991, 95,1356. (5) Koval, C. A; Olson, J. B. 1.Elecroanal. Chem. 1 9 8 7 , 2 3 4 , 133. (6) Koval, C. A; Olson, J. B.1.Phys. Chem. 1988, 92, 6726. (7) Koval, C. A; Olson, J. B.; Parkinson, B. A. In Electrochemical Surfoce Science: Molecular Phenomena at Electrode Suffaces;Soriaga, M. P., Ed.; ACS Symposium Series 378;American Chemical Society: Washington, DC, 1988;p 438.
0003-2700/94/0366-4525$04.50/0 0 1994 American Chemical Society
Analytical Chemistfy, Vol. 66, No. 24, December 15, 1994 4525
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immune to corrosion in nonaqueous solvents, and relatively free of surface states.5-1° The nature of the redox solution is equally important in obtaining interpretable measurements of ket at the SEI. Past studies in aqueous solvents have often been conducted with couples such as Fe(CN)63-/4-,Fe3+I2+,13-/Iz,and Ce4+/Ce3+.11J2 Electrochemists are well acquainted with the complications of these couples at metal electrodes, including adsorption and ionpairing effects. Furthermore, semiconductor electrodes are prone to undesirable chemical interactions with water. For many semiconducting materials, pH effects are known to play a major role in the IE, and corrosion and oxidation of the surface is often more pronounced in aqueous so1vents.l1J2The use of metallocene redox couples in nonaqueous solvents avoids many of these problems. Metallocenes often exhibit chemically reversible, singleelectron, outer-sphere redox chemistry. The reduction potential can be varied by interchanging the central metal ion @e2+vs Co2+)or the cyclopentadienyl ring substituents (H, CH3, COCH3). Changes in the substituents have little effect on the reorganization energy of the couple.13J4 Prior studies of fundamental ET using WSedmetallocene redox solutions yielded conflicting result^.^-'^ The superior stability of the IE was clearly demonstrated, yet the ET results were not in accord with theoretical predictions. For an interface with a relatively small junction barrier (J~cB - E"'
