Interfacial electron-transfer rates dependence

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

C. Characteristic micrometer size dependence of the interfacial electron transfer rate 191... 

C. Characteristic Micrometer Size Dependence of the Interfacial Electron Transfer Rate... 

Alternatively, the dynamics of trapping and detrapping of electrons localized in intra-bandgap states can control the overall reaction kinetics, which would not depend upon the sole interfacial electron transfer rate. 

FIGURE 12.13 Gerischer-type diagram for interfacial electron transfer. The rate constants for interfacial electron transfer are dependent on the overlap of the sensitizer and the semiconductor density of states. Note that the density of states of the semiconductor is not a singular parameter and can shift with a change in environment, that is, pH, ionic strength, solvent, and so on. 

This case is shown in Fig. 10.6c and d where through absorption of light a photohole in the vb and a photoelectron in the cb are formed. The probability that interfacial electron transfer takes place, i.e. that a thermodynamically suitable electron donor is oxidized by the photohole of the vb depends (i) on the rate constant of the interfacial electron transfer, kET, (ii) on the concentration of the adsorbed electron donor, [Rads]. and (iii) on the rate constants of recombination of the electron-hole pair via radiative and radiationless transitions,Ykj. At steady-state of the electronically excited state, the quantum yield, Ox, ofinterfacial electron-transfer can be expressed in terms of rate constants ... 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Gerischer s distribution curves can be interpreted as representing the energy dependence of the electron transfer rate constants involving the reduced and oxidized species. Only a few electrochemical studies have attempted to evaluate the model and quantify the distributions and reorganizational parameters [17, 18]. Nevertheless, it has become common practice to draw a pictorial representation of the distributions when discussing interfacial electron transfer kinetics relevant to dye sensitization. 

The theory of interfacial electron transfer, and the dependence of electron-transfer rate on interfacial energetics, are addressed in Chapter 4 of this book, and the... 

The activity of an adsorbed electron donor for the interfacial electron transfer depends on both its ability to be oxidized by the photogenerated hole (Fig. 1 ) j and its adsorption properties. The first is a thermodynamic, the second a kinetic requirement. Kinetic experiments performed at various concentrations of the electron donor can give information on its adsorption properties. Different authors have reported, that the rate of the photocatalytic oxidation of an electron donor at the Ti02 surface varied as a function of the dissolved concentration of the electron donor according to a Langmuir-type isotherm (l ig. 

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