Studies of Heterogeneous Electron Transfer

11 Tip steady-state voltammograms for the oxidation of 5.8 mM ferrocene in 0.52 M TBAPF, in acetonitrile at a 2.16 /xm diameter Pt tip. Solid lines calculated from Eq. (10). Tip-substrate separation decreases from line 1 (infinity) to line 5 (0.108 /xm). (From Ref. 72. Copyright 1993 American Chemical Society.) 

The alternative approach to examine heterogeneous electron transfer kinetics on the substrate is to hold the tip at a potential where the reaction is mass transfer controlled and study the approach curve as a function of substrate potential. For example, one can generate iron(II) by reduction of iron(III) at the tip and study the oxidation of iron(II) at the substrate. One 

12 Schematic representation of the TLC formed inside the Hg pool. C60 is reduced at the Pt tip to produce C o, which is reoxidized at the Hg anode. The solution layer is shown greatly enlarged for clarity the actual thickness is smaller than the electrode radius a. (From Ref. 74. Copyright 1993 American Chemical Society.) 

An easily assessable and well-researched model system—the glassy carbon electrode—is frequently used for studies of heterogeneous electron transfer with quasi-reversible and irreversible kinetics (34). Moreover, carbon particles can be spray-coated on electrode surfaces to modify its properties, and carbon fibers have been used as microelectrodes of defined diameter. 

Wipf and Bard used glassy carbon to study the redox behavior of Fe2+/3+ in 1 M H2S04. While the carbon fiber tip enabled a diffusion-limiting reduction of Fe3+, the C substrate oxidized Fe2+ at a finite rate (35). This system provided a direct method of varying the driving force for the sample 

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Detailed studies of ligand and solvent effects on electrochemical reductions of Cr111 amino-polycarboxylates have been performed.741,742 The observed differences in electrochemical behavior between cis- and trans-N204 Cr111 complexes have been explained by different Jahn-Teller distortions (JTD) of their Cr11 analogs (Section 4.6.6.8).741 Electrochemical reduction of Crm-edta complexes has been used as a model reaction in studies of heterogeneous electron-transfer processes.743... 

The recent discoveries of superconductivity in materials lacking rare earth elements (1-S) have added new excitement to the already active field of high temperature superconductivity. Studies of heterogeneous electron transfers at interfaces between these... 

Nickel tetra-aminophthalocyanine polymer was ingeniously tested for its electronic conductivity via kinetics studies of heterogeneous electron transfer taking place at the interface between them and the liquid phase ... 

Study of heterogeneous electron transfer on the graphene/self-assembled monolayer modified gold electrode by electrochemical approaches. J. Phys. Chem. C, 114, 14243-14250. 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

The validity of an electroanalytical measurement is enhanced if it can be simulated mathematically within a reasonable model , that is, one comprising all of the necessary elements, both kinetic and thermodynamic, needed to describe the system studied. Within the chosen model, the simulation is performed by first deciding which of the possible parameters are indeed variables. Then, a series of mathematical equations are formulated in terms of time, current and potential, thereby allowing the other implicit variables (rate constants of heterogeneous electron-transfer or homogeneous reactions in solution) to be obtained. 

By the use of various transient methods, electrochemistry has found extensive new applications for the study of chemical reactions and adsorption phenomena. Thus a combination of thermodynamic and kinetic measurements can be utilized to characterize the chemistry of heterogeneous electron-transfer reactions. Furthermore, heterogeneous adsorption processes (liquid-solid) have been the subject of intense investigations. The mechanisms of metal ion com-plexation reactions also have been ascertained through the use of various electrochemical impulse techniques. 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

Activation volume — As in case of homogeneous chemical reactions, also the rate of heterogeneous electron transfer reactions at electrode interfaces can depend on pressure. The activation volume AVZ involved in electrochemical reactions can be determined by studying the pressure dependence of the heterogeneous -> standard rate constant ks AVa = -RT j (p is the molar - gas constant, T absolute temperature, and P the pressure inside the electrochemical cell). If AI4 is smaller than zero, i.e., when the volume of the activated complex is smaller than the volume of the reactant molecule, an increase of pressure will enhance the reaction rate and the opposite holds true when A14 is larger than zero. Refs. [i] Swaddle TW, Tregloan PA (1999) Coord Chem Rev 187 255 [ii] Dolidze TD, Khoshtariya DE, Waldeck DH, Macyk J, van Eldik R (2003) JPhys Chem B 107 7172... 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

The heterogeneous rate constant [236] of electrochemical reduction of [Co "(bpy)3] + to [Co"(bpy)3] + is relatively slow, about 0.1 cm s in CH3CN or CH2CI2. Detailed studies of solvent and pressure effects [236, 237] have revealed that the rate of heterogeneous electron transfer is controlled by solvent dynamics. This implies that the electron transfer is adiabatic. 

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

It has been shown very recently that l,3,5-tris(4-(ALphenyl-AL3-methylphenyl)phenyl) benzene (56), when studied in DMF and dichloromethane solutions, forms two and three reversible one-electron anodic waves, respectively92. The kinetics of heterogeneous electron transfers has recently been studied using the high speed microband channel electrode in solutions containing 0.1 M TBAP as electrolyte it was possible to find standard potentials E°, transfer coefficients a and, finally, standard rate constants k°. The obtained data, labeled by subscripts 1, 2 and 3 for the first, second and third electron transfer, respectively, are collected in Table 2. 

Dick, L.A., Haes, A.J., and Van Duyne, R.P. (2000) Distance and orientation dependence of heterogeneous electron transfer a surface-enhanced resonance Raman scattering study of cytochrome c bond to carboxylic acid terminated alkanethiols adsorbed on silver electrodes. Journal of Physical Chemistry B, 104, 11752-11762. 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

One can see from Eq. (36) that at L 1, mo D/a (as for a microdisk electrode alone), but at L 1, mo D/d, i.e., a thin-layer cell (TLC)-type behavior. This suggests that the SECM should be useful for studying rapid heterogeneous electron transfer kinetics. By decreasing the tip/substrate distance, the mass-transport rate can be increased sufficiently for quantitative characterization of the electron-transfer kinetics, preserving the advantages of steady-state methods, i.e., the absence of problems associated with ohmic drop, adsorption, and charging current. 

This chapter reviews in detail the principles and applications of heterogeneous electron transfer reaction analysis at tip and sample electrodes. The first section summarizes the basic principles and concepts. It is followed by sections dedicated to one class of sample material glassy carbon, metals and semiconductors, thin layers, ion-conducting polymers, and electrically conducting polymers. A separate section is devoted to practical applications, in essence the study of heterogeneous catalysis and in situ characterization of sensors. The final section deals with the experiments defining the state of the art in this field and the outlook for some future activities. Aspects of heterogeneous electron transfer reactions in more complex systems, such as... 

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