Electron transfer at semiconductor electrodes

Koval CA, Howard JN (1992) Electron transfer at semiconductor electrode-liquid electrolyte interfaces. Chem Rev 92 411 33... 

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. 

We consider a simple redox electron transfer of hydrated redox particles (an outer-sphere electron transfer) of Eqn. -1 at semiconductor electrodes. The kinetics of electron transfer reactions is the same in principal at both metal and semiconductor electrodes but the rate of electron transfer at semiconductor electrodes differs considerably from that at metal electrodes because the electron occupation in the electron energy bands differs distinctly with metals and semiconductors. 

It is important to note that the description of electron transfer kinetics is different in the case of semiconductor electrodes. For an n-type semiconductor electrode in the dark, the rate of electron transfer depends not only on the concentration of redox species in the solution but also on the potential dependent density of electrons in the semiconductor. Under depletion conditions, most of the potential drop is located in the solid, so that to a good approximation the activation energy for electron transfer is independent of potential. Electron transfer at semiconductor electrodes is therefore characterised in terms of a second order heterogeneous rate constant with units cm4 s-1. 

Koval, C. A., Howard, J. N., Electron transfer at Semiconductor Electrode Liquid Electrolyte Interfaces, Chem. Rev. 1992, 92, 411 433. 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

A proper treatment of electron transfer at semiconductor electrodes is beyond the scope of this chapter, but further details can be found, for example, in books by Morrison [16] and Myamlin Pleskov [28]. 

Electron transfer at semiconductors can be contrasted with that at metal electrode surfaces. In the latter case, as shown in Fig. 41(a), electron transfer normally takes place to or from energy levels within a few kT of the Fermi... 

The simple models of electron transfer at semiconductor interfaces, which have been used until recently, are now being extended and improved, and Wilson has provided an authoritative review of the theory, which includes some discussion of solar photoelectrochemical cells. Albery et have explored the transport and kinetics of minority carriers at illuminated semiconductor electrodes. The exact analytical solution of the problem is obtained in terms of confluent... 

The diagram shown in Fig. 3.10 is now widely used to describe electron transfer processes at electrodes, and it has the merit that it can be extended readily to the discussion of electron transfer at semiconductor and insulator electrodes [16]. The theoretical basis for the diagram is to be found in the fluctuating energy level model of electron transfer which has been discussed by Marcus [12,13], Gerischer [17-19], Levich [20], and Dogonadze [21]. 

A further difference between metal and semiconductor electrodes is that electron transfer at semiconductors can involve minority carriers generated by light, and while in the dark only small currents flow when a depletion layer is formed at the semiconductor solution interface, illumination of the electrode gives rise to much larger photocurrents. The current-voltage curves for semiconductor electrodes are therefore not only dependent on whether the semiconductor is n- or p-type but also on whether the electrode is in the dark or illuminated with light of sufficient energy to promote electrons from the valence band to the conduction band. Instead of the symmetrical Butler-Volmer plots obtained for metal electrodes, essentially diode-like behaviour is expected for extrinsic semiconductor electrodes, as shown in Fig. 3.18. 

SEMICONDUCTOR ELECTRODES 9.4.1 Electron transfer at semiconductor-electrolyte interface... 

Horrocks, B. R., Mirkin, M. V., Bard, A. J. Scanning electrochemical microscopy. 25. Application to investigation of the kinetics of heterogeneous electron transfer at semiconductor (WSej and Si) electrodes, J. Phys. Chem. 1994, 98, 9106-9114. 

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

The distribution of the exchange transfer current of redox electrons o(e), which corresponds to the state density curves shown in Fig. 8-11, is illustrated for both metal and semiconductor electrodes in Fig. 8-12 (See also Fig. 8-4.). Since the state density of semiconductor electrons available for electron transfer exists only in the conduction and valence bands fairly away from the Fermi level nsc), and since the state density of redox electrons available for transfer decreases remarkably with increasing deviation of the electron level (with increasing polarization) from the Fermi level CFciiEDax) of the redox electrons, the exchange transfer current of redox electrons is fairly small at semiconductor electrodes compared with that at metal electrodes as shown in Fig. 8-12. 

Fig. 8-16. Electron state density for a redox electron transfer reaction of h3rdrated redox particles at semiconductor electrodes (a) in the state of band edge level pinning and (b) in the state of Fermi level pinning dashed curve = band edge levels in reaction equilibrium solid curve = band edge levels in anodic polarization e p,sq = Fermi level of electrode in anodic polarization e v and c c = band edge levels in anodic polarization.

From these illustrations it follows, in general, that Ihe transfer reaction of redox electrons at semiconductor electrodes occurs via the conduction band mechanism if its equilibrium potential is relatively low (high in the Fermi level of redox electrons) whereas, the transfer reaction of redox electrons proceeds via the valence band mechanism if the equilibriiun redox potential is high (low in the Fermi level of redox electrons). 

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