Special Features of Reactions at Semiconductor Electrodes

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. 

Depending on the nature of the electrode and reaction, the carriers involved in an electrochemical reaction at a semiconductor electrode can be electrons from the conduction band (in the following to be called simply electrons), electrons from the valence band (holes), or both. The concentration of the minority carriers in semiconductors (electrons in p-type, and holes in n-type semiconductors) is always much 

A typical featnre of semicondnctor electrodes is the space charge present in a relatively thick surface layer (see Section 10.6), which canses a potential drop across this layer (i.e., the appearance of a snrface potential %). This potential drop affects the rate of an electrochemical charge-transfer reaction in exactly the same way as the potential drop across the diffnse EDL part (the / -potential) hrst, through a change in carrier concentration in the snrface layer, and second, throngh a change in the effect of potential on the reaction s activation energy. 

As an example, consider a simple anodic redox reaction involving electrons of the valence band (i.e., holes). The reaction eqnation can be written as 

The form of the kinetic equation depends on the way in which the surface potential X varies with electrode potential E. When the surface potential is practically constant, the first factor in Eq. (14.24) will also be constant, and the potential dependence of the reaction rate is governed by the second factor alone. The slope b of the polarization curve will be RT/ F (i.e., has the same value as that found when the same reaction occurs at a metal electrode). When in another case a change in electrode potential E produces an equally large change in surface potential (i.e., E = x + const), while there is practically no change in interfacial potential. Then Eq. (14.24) changes into 

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