Compound Semiconductor Electrodes

It is interesting to note that the flatband potential may depend on the crystal faces. For instance, n-type GaAs electrodes with (111) surfaces were prepared. The electrodes where the (111) Ga face contacted the electrolyte, the flatband potential was generally found to be 0.25 V more positive than for (111) As surfaces. Such effects can be understood in terms of differences in the Helmholtz layer. The same results were obtained with corresponding p-type GaAs electrodes [63]. 

The potential distribution at the interface has been studied for many compound semiconductors. Usually a straight line has been obtained when plotting 1/Csc vs. according to Eq. (5.27), as shown, for example, in Fig. 5.15 for an n-type CdS electrode in an aqueous electrolyte [28]. In addition, the flatband potential = 0), has 

The flatband potential of many semiconductor electrodes depends on the pH, as already described for germanium electrodes. This indicates that many semiconductor surfaces exhibit a strong interaction with water. Some semiconductors, such as layered transition metal compounds and especially their basal planes, show a weaker inter- 

16 Schematic presentation of a step at the surface of a layered compound such as WSe2, 

In the case of GaAs a change of the potential across the Helmholtz layer was observed upon anodic and cathodic prepolarization, which was interpreted in terms of hydroxyl and hydride surface layers, as for Ge (see Section 5.3.1) A linear Mott-Schottky dependence for an n-GaAs electrode was only found at sufficiently high scan rates after anodic or cathodic prepolarization as shown in Fig. 5.17 [40], It is worth mentioning that all reliable capacity measurements could be interpreted in terms of space charge capacities, i.e. additional capacities due to surface states were not found. 

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Fig. 5-65. Flat band potential for several compound semiconductor electrodes in aqueous solutions ZnO with surface lattice planes of (1010), (OOOl) and (0001). [From Geiischer, 1990.]...

Fig. 5-64. Band edge levels of compound semiconductor electrodes in aqueous solutions at different pH values hydrated redox partides and their standard redox potentials are on the right hand side. [From Gleria-Memming, 1975.]...

Figure 5-64 shows the band edge potential for compound semiconductor electrodes in aqueous solutions, in which the standard redox potentials (the Fermi levels) of some hydrated redox particles are also shown on the right hand side. In studying reaction kinetics of redox electron transfer at semiconductor electrodes, it is important to find the relationship between the band edge level (the band edge potential) and the Fermi level of redox electrons (the redox potential) as is described in Chap. 8. 

FIGURE 22.11 Energy diagrams for anodic and cathodic dissolution of compound semiconductor electrodes (a) anodic dissolution impossible, (b) anodic dissolution possible, (c) cathodic dissolution impossible, and (d) cathodic dissolution possible [16] P(redox) = Fermi level of anodic and cathodic semiconductor dissolution reactions. 

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Fig. 9-13. Reaction rate of simultaneous dissolution of surface cations and anions from a semiconductor electrode of ionic compound as a iimction of potential of a compact layer 4 )=potmitial of acorn-...

Semiconductor electrodes of ionic compounds can also dissolve with the oxidation of surface anions or with the reduction of surface cations as shown schematically in Fig. 9-15. 

Fig. 9-15. Oxidative and reductive dissolution reactions of semiconductor electrodes of ionic compounds (a) cation dissolution coupled with anodic hole oxidation of surface anions, (b) anion dissolution coupled with cathodic electron reduction of surface cations.

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

Fig. 9-17. Thermodynamic stability of electrodes of compound semiconductors relative to oxidative and reductive dissolution in the state of band edge level pinning (a) oxidative dissolution is thermodynamically impossible (eFXp.< Cb) oxidative dissolution may occur (er(p.dK)> Ev)> (c) reductive dissolution is thermodynamically impossible (cnn.M> E ), (d) reductive dissolution may occur < Cc) pip. sk) (cpbi. d i) = equivalent Fermi...

Fig. 9-18. Band edge levels and equivalent Fermi levels of oxidative and reductive dissolution reactions of compound semiconductors in aqueous sohitions at pH 7 en ) - F(a

For compound semiconductors, the adsorbed proton level differs with different constituents in the semiconductor thus, the distinction between the acidic and basic proton levels, pKi and pKt, is greater than in the case of elemental semiconductors. For example, on metal oxide electrodes, the acidic proton dissoci-... 

Fujishima A, Honda K (1972) Electrochemical photolysis of water at a semiconductor electrode. Nature 238 37-38 Yamashita H, Harada M, Misaka J, Takeuchi M, Neppolian B, Anpo M (2003) Photocatalytic degradation of organic compounds diluted in water using visible light-responsive metal ion-implanted Ti02 catalysts Fe ion-implanted Ti02. Catal Today 84 191-196... 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

Photosensitive substances adsorbed on the semiconductor surface are especially efficient in sensitization reactions. Thus, sensitizing effect can be enhanced if a sensitizer is attached to the semiconductor surface by a chemical bond. For this purpose one has to create either the ether bond -O-between the semiconductor and reactant, using natural OH groups, which exist on the surface of, for example, oxide semiconductors (Ti02, ZnO) or oxidized materials (Ge, GaAs, etc.) in aqueous solutions, or the amide bond -NH- in the latter case a monolayer of silane compounds with amido-groups is preliminarily deposited on the semiconductor surface (see, for instance, Osa and Fujihira, 1976). With such chemically modified electrodes the photocurrent is much higher than with ordinary (naked) semiconductor electrodes. 

The second topic of this chapter is the role of coordination compounds in advancing electrochemical objectives, particularly in the sphere of chemically modified electrodes. This involves the modification of the surface of a metallic or semiconductor electrode, sometimes by chemical reaction with surface groups and sometimes by adsorption. The attached substrate may be able to ligate, or it may be able to accept by exchange some electroactive species. Possibly some poetic licence will be allowed in defining such species since many interesting data have been obtained with ferrocene derivatives thus these organometallic compounds will be considered coordination compounds for the purpose of this chapter. 

A high-resistance compound semiconductor film 13 is formed on a silicon substrate 3 at openings in an insulating film 6. A detector element 2 is composed of an n-type and a p-type HgCdTe layer 14 and 15 formed on the film 13. The detector element is connected to an electrode 24 and to a MOS transistor in the silicon substrate by a connector 21 A. 

Electromagnetic radiation, besides being a probe of surface structure, can excite electrons in the species in solution (especially in organic compounds) or in the electrode itself (especially in semiconductor electrodes). This photon excitation can lead to electron transfer between electrode and solution. The study of these phenomena is photoelectrochemistry and can be very important in conversion of solar energy into electricity in order to convert substances (photoelectrolysis). 

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