Redox couples, energy level discussion

The Gaussian distribution is most often used in photoelectrochemistry to describe the energy levels of redox couples and to discuss qualitatively the possibility of the reaction. The major difficulty of this model is that it cannot explain the Tafel behavior... 

Let us now consider the formation of the semiconductor/solution interface. The Fermi level in the solution phase, can be identified as Ji by (18.2.4) and is calculated in terms of values by the procedures described in Section 2.2. For most electrochemical purposes, it is convenient to refer values to the NHE (or other reference electrodes), but in this case it is more instructive to estimate them with respect to the vacuum level. This can be accomplished, as discussed in Section 2.2.5, by theoretical and experimental means with relaxation of thermodynamic rigor, so that one obtains an energy level value for the NHE at about —4.5 0.1 eV on the absolute scale (45) (Figure 18.2.5a). Consider the formation of the junction between an n-type semiconductor and a solution containing a redox couple 0/R, as shown in Figure 18.2.5. When the semiconductor and the solution are brought into contact, if electrostatic equilibrium is attained, in both phases must become equal (or equivalently the Fermi levels must become equal), and this can occur by... 

A central concept in describing the interaction of a semiconductor with an electrolyte is the equilibration of the Fermi level of the solution and of the semiconductor. Although the concept of Fermi level has been initially introduced for an electronically conducting phase, as a level for which the probability of electron occupation of a given state is 1/2, in our discussion the Fermi level can be conveniently identified with the electrochemical potential of electrons in the solid and of a given redox couple (0,R) in solution, where the usual free energy relationships hold. Obviously, the solution phase does not contain free electrons, but contains available electronic states [in the form of oxidized (O) and reduced species (R)] which can equilibrate with free electrons in the solid. As a consequence, = (1 + KT n Co/C ), where fl are the electrochemical... 

The accepted mechanism of electron transfer across the redox sites in the polymer is redox electron hopping [19-26], which is discussed in Section IV.A. The electronic donor and acceptor states that are involved may have identical energy levels (disregarding the thermal fluctuations). Typically, these are polymers with bound redox couples, such as Oslbipy). -". Such polymers are termed redox polymers. In other systems, the energies of the states are spread over a range of levels. This... 

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