Reactions at Semiconductor Electrodes

There are many organic molecules such as Ru(II)(bipy)3, which can be oxidized as well as reduced. If, in this case, only molecules in their original oxidation state are present in the solution, then both redox potentials, Fp.redoxCM/lVr ) and predox(M/M ), should determine the equilibrium. Since the concentrations of M and M are usually very small, the two redox potentials merge to one value as given by 

redox occurs just in the middle of the two standard potentials. At equilibium Fpredox must be equal to p of the semiconductor. Since the concentrations of and M are very low under these conditions, slight changes of the or concentration leads to dramatic variations in p,redox stid the equilibrium conditions. 

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Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

Chapter 10 deals with photoelectrode reactions at semiconductor electrodes in which the concentration of minority carriers is increased by photoexcitation, thereby enabling the transfer of electrons to occur that can not proceed in the dark. The concept of quasi-Fermi level is introduced to account for photoenergy gain in semiconductor electrodes. Chapter 11 discusses the coupled electrode. mixed electrode) at which anodic and cathodic reactions occur at the same rate on a single electrode this concept is illustrated by corroding metal electrodes in aqueous solutions. 

The flat band potential in electrochemistry of semiconductors is equivalent to the zero-charge potential in electrochemistry of metals (more exactly, to the potential of the zero free charge—see Frumkin, 1979). As with the zero-charge potential in the electrochemistry of metals, the flat band potential is rather important in the kinetics of both dark and photoelectrochemical reactions at semiconductor electrodes. Several methods, including photoelectrochemical ones, have been developed to determine q>tb (see Section 7). 

The Influence of Surface Orientation and Crystal Imperfections on Photoelectrochemical Reactions at Semiconductor Electrodes... 

Frese, J.K.W. and Canfield, D. (1984) Reduction of C02 to Methanol and Methane by Photo and Dark Reactions at Semiconductor Electrodes, Gas Research Institute Annual Report. 

Willig, F. Eichberger, R. Sundaresan, N. S. Parkinson, B. A. Experimental time scale of Gerischer s distribution curves for electron-transfer reactions at semiconductor electrodes, J. Am. Chem. Soc. 1990, 112, 2702. 

ANODIC DISSOLUTION REACTIONS at semiconductor electrodes require electron holes. 

Japanese workers have published a paper regarding the study of photoelectro-chemical reactions at a n-type polycrystalline zinc oxide electrode using photoacoustic detection [125], They monitored in situ photoelectrochemical reactions at semiconductor electrodes using photoacoustic techniques. 

Gerischer H. (1973), Charge-transfer reactions at semiconductor electrodes , J. Chim. Phys. Phys.-Chim. Biol. 70, 584-585. 

Gerischer H. (1987), Catalysis of electrochemical and photoelectrochemical reactions at semiconductor electrodes . React. Kinet. Catal. Lett. 35, 459-468. 

Gao Y. Q., Georgievskii Y. and Marcus R. A. (2000), On the theory of electron transfer reactions at semiconductor electrode/liquid interfaces , J. Chem. Phys. 112, 3358-3369. 

Peter L. M. (1990b), Kinetics and mechanisms of photoelectrochemical reactions at semiconductor electrodes , Croat. Chem. Acta 63, 401 15. 

The Fermi level concept is very useful in the quantitative description of reactions at semiconductor electrodes, as described in Section 7.4. Other energy states of a redox system besides the Fermi level can also be defined. This problem is discussed in detail in Chapter 5. 

It should emphasized again that this relation is usually not valid for reactions at semiconductor electrodes because the reorganization energy may be larger than 0.5A (homogeneous solutions). 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

The mechanism of an electrochemical reaction at semiconductor electrodes depends upon the position of the redox Fermi level in solution with respect to the position of the bandedges of the semiconductor. In this study we investigated the reduction of copper ions on Si surfaces in HF solutions and we examined the effect of adding HCI to the HF solutions. 

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