Semiconductors electrochemical redox reactions

It follows from the Franck-Condon principle that in electrochemical redox reactions at metal electrodes, practically only the electrons residing at the highest occupied level of the metal s valence band are involved (i.e., the electrons at the Fermi level). At semiconductor electrodes, the electrons from the bottom of the condnc-tion band or holes from the top of the valence band are involved in the reactions. Under equilibrium conditions, the electrochemical potential of these carriers is eqnal to the electrochemical potential of the electrons in the solution. Hence, mntnal exchange of electrons (an exchange cnrrent) is realized between levels having the same energies. 

The band bending at the semiconductor/liquid (electrolyte solution) interface can be understood by considering the potential distribution at this interface. In a case where the electrolyte solution contains a redox couple (R/Ox), which causes an electrochemical redox reaction,... 

Pourbaix diagrams for the aqueous Cd-S, Cd-Te, Cd-Se, Cu-In-Se, and Sb-S systems have been compiled and discussed by Savadogo [26] in his review regarding chemically and electrochemically deposited thin Aims for solar energy materials. Dremlyuzhenko et al. [27] analyzed theoretically the mechanisms of redox reactions in the Cdi xMn , Te and Cdi- , Zn i Te aqueous systems and evaluated the physicochemical properties of the semiconductor surfaces as a function of pH. 

The net result of a photochemical redox reaction often gives very little information on the quantum yield of the primary electron transfer reaction since this is in many cases compensated by reverse electron transfer between the primary reaction products. This is equally so in homogeneous as well as in heterogeneous reactions. While the reverse process in homogeneous reactions can only by suppressed by consecutive irreversible chemical steps, one has a chance of preventing the reverse reaction in heterogeneous electron transfer processes by applying suitable electric fields. We shall see that this can best be done with semiconductor or insulator electrodes and that there it is possible to study photochemical primary processes with the help of such electrochemical techniques 5-G>7>. 

The symbols in parentheses denote the electrochemical potential levels for the corresponding reaction (the level Fdec is a particular case of the level Fred0J for a redox reaction, in which the electrode material is destructed). Once Fdec is calculated (from tabular values of thermodynamic characteristics of substances involved see, for example, Latimer, 1952), the equilibrium potentials of the reactions of anodic [Pg.286]

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

Thus, the electron flows from the reductant to the oxidant through photoexcited semiconductor particles. Therefore, the particle suspended in a solution of reductant and oxidant can be regarded as a very small electrochemical cell in which the irradiated light energy is used as free energy change and/or activation energy of the redox reaction of the reductant and the oxidant (Fig. 11.2). 

N. S. Lewis, C. M. Gronet, G. W. Cogan, J. E. Gibbons, and G. M. Moddel,./. Electrochem. Soc. 131 2873 (1984). Nonaqueous solution study of redox reactions at light activated semiconductors confirming applicability of Schottky-type theory. 

The mechanism of the fourth category of bimolecular surface steps is peculiar to redox reactions catalysed by metals and semiconductors. Here both reactants sit on the surface, not necessarily on adjacent sites, and the electrons are transferred from the reducing to the oxidising species through the solid catalyst. The rate therefore depends not only on the concentrations at the surface but also on the potential taken up by the catalyst, and this potential in turn is a function of the concentrations of the electroactive species present. Equations (28) and (29) fail to represent the kinetics in these cases because khel is no longer independent of concentration. These kinetics must accordingly be treated by an electrochemical method of analysis and this is done in Sect. 4.1. 

One particularly appealing route for effecting controlled redox reactions involves an array of surface-mediated reactions initiated by ultraviolet irradiation of suspended semiconductor particles [3-13]. Such reactions involve band-gap excitation of the semiconductor, interfacial electron transfer, and secondary dark chemical reactions of singly oxidized and reduced adsorbates. Because the semiconductor surface is restored to its original structure and oxidation level after these transformations, these photoreactions are often called photocatalytic, leaving the light-responsive photocatalyst ready to act as initiator for another cycle. The use of such photocatalysts also obviates the need to acquire expensive electrochemical equipment. 

Figure 2.30 Energy-level scheme and cell half-cell reactions for a semiconductor I redox system I metal electrochemical photovoltaic cell.

In the non-adiabatic limit, the total ET rate can be expressed as the sum of ET rates to all possible accepting states in the semiconductor (Marcus, 1965 Gao et al, 2000 Gao and Marcus, 2000 Gosavi and Marcus, 2000). For electron injection from an adsorbate excited state with electrochemical redox potential of U°(S /S ) to a semiconductor k state at e = E- Ecb) above the band edge (with flatband potential of U°cb), the reaction can be written as... 

In some cases it is of interest to determine products formed at semiconductor electrodes. If redox reactions are involved this can be done by using a rotating ring disc electrode assembly (RRDE), which has proved to be a powerful tool for investigating electrochemical reactions at metal electrodes. The technique and corresponding results as obtained with metal electrodes have been reviewed by Bruckenstein and Miller [6] and by Pleskov et al. [7]. 

Indeed, based on experience gained in pigmented BLM research in artificial photosynthesis, a novel type of photoelectric cell, termed a semiconductor septum electrochemical photovoltaic (SC-SEP) cell, has been developed. In a SC-SEP cell, a semiconductor septum (e.g., CdSe) is used in place of a pigmented BLM to separate two aqueous solutions. When light shines on the semiconductor membrane, photoinduced redox reactions are observed (82-84). Operation under short-circuit conditions allows the cell to be used for photolysis of water using solar energy (85). 

Surface states and crystal imperfections have been found to play an important role in charge-transfer and redox reactions at the semiconductor-electrolyte interface (see Refs. 161-173). Mathematical and conceptual relationships have been developed which describe electrochemical reactions at the semiconductor-electrolyte interface in terms of surface states and potentials (see, e.g., Refs. 17, 71, and 174-182). Electrochemical reaction via surface states has been included within an analytic model,183 but this model is still limited by the restrictions described above. 

Whereas the addition of a donor or acceptor molecule to the polymer is called doping, the reaction that takes place is actually a redox reaction and is unhke the doping of Si or Ge in semiconductor technology, where there is substitution of an atom in the lattice. The terminology in common use will be retained here, but it should be remembered that the doping of conductive polymers involves the formation of a polymer salt, and this can be effective either by immersing the polymer in a solution of the reagent or by electrochemical methods. 

Electron transfer, a fundamental chemical process underlying all redox reactions, has been under experimental and theoretical study for many years [1-6]. Theoretical studies of such processes seek to understand the ways in which their rate depends on donor and acceptor properties, on the solvent, and on the electronic coupling between the states involved. The different roles played by these factors and the way they affect qualitative and quantitative aspects of the electron transfer process have been thoroughly discussed in the past half-century. This kind of processes, which dominate electron transitions in molecular systems, is to be contrasted with electron transport in the solid state, that is, in metals and semiconductors. Electrochemical reactions that involve both molecular and solid-state donor/acceptor systems, bridge the gap between these phenomena [6]. Here, electron transfer takes place between quasi-free electronic states on one side, and bound molecular electronic states on the other. 

Co 7Co02) and metal ion/metal systems (Ag7Ag) can be used [4]. In the former system, metal ions are oxidized to colored metal oxides by holes generated in the valence band of the photo-irradiated semiconductor and bleached by electrochemical reduction of the oxide. In the latter, metal ions are reduced to colored metal particles by photoexcited electrons in the conduction band. A system with Prussian white/ Prussian blue is also used. Prussian white on Ti02 is oxidized to Prussian blue by UV irradiation [5]. Redox reactions of conducting polymers such as... 

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