Semiconductor-electrolyte interfaces, electron

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

Between 0.20 and 0.30 V, a decay of the initial photocurrent and a negative overshoot after interrupting the illumination are developed. This behavior resembles the responses observed at semiconductor-electrolyte interfaces in the presence of surface recombination of photoinduced charges [133-135] but at a longer time scale. These features are in fact related to the back-electron-transfer processes within the interfacial ion pair schematically depicted in Fig. 11. 

Basic properties of semiconductors and phenomena occurring at the semiconductor/electrolyte interface in the dark have already been discussed in Sections 2.4.1 and 4.5.1. The crucial effect after immersing the semiconductor electrode into an electrolyte solution is the equilibration of electrochemical potentials of electrons in both phases. In order to quantify the dark- and photoeffects at the semiconductor/electrolyte interface, a common reference level of electron energies in both phases has to be defined. 

For a more detailed description of the semiconductor/electrolyte interface, it is convenient to define the quasi-Fermi levels of electrons, eFyC and holes, p p,... 

The thermodynamic feasibility of redox reactions at the semiconductor-electrolyte interface can be assessed from thermodynamic considerations. Since typical redox potentials for many redox couples encountered in electrolytes of natural or technical systems often lie between the band potentials of typical semiconductors, many electron transfer reactions are (thermodynamically) feasible (Pichat and Fox, 1988). With the right choice of semiconductor material and pH the redox potential of the cb can be varied from 0.5 to 1.5 V and that of the vb from 1 to more than 3.5 V (see Fig. 10.4). 

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

Since the metal can be treated as a nearly perfect conductor, C is high compared with C, and cannot influence the value of the measured doublelayer capacitance. The role of the metal in the double layer structure was discussed by Rice, who suggested that the distribution of electrons inside the metal decides the properties of the double-layer. This concept was later used to describe double-layer properties at the semiconductor/electrolyte interface. As shown later, the electron density on the metal side of the interface can be changed under the influence of charged solution species (dipoles, ions). ... 

Electrons, generated near the semiconductor-electrolyte interface are unable to stay in this region because of the electric field there which drives them into the bulk of the TiOz crystal, out through the metallic contact, the external circuit (where the photo-current may be measured) and into the catalytically active metal. At the interface of this metal with the electrolyte solution, reaction occurs ... 

In this type of cell both electrodes are immersed in the same constant pH solution. An illustrative cell is [27,28] n-SrTiOs photoanode 9.5-10 M NaOH electrolyte Pt cathode. The underlying principle of this cell is production of an internal electric field at the semiconductor-electrolyte interface sufficient to efficiently separate the photogenerated electron-hole pairs. Subsequently holes and electrons are readily available for water oxidation and reduction, respectively, at the anode and cathode. The anode and cathode are commonly physically separated [31-34], but can be combined into a monolithic structure called a photochemical diode [35]. 

In contrast to metal electrodes, for a semiconductor-electrolyte interface most of the potential drop is located in the semiconductor making it difficult to study interfacial processes using potential perturbation techniques [11,20,55,58,60-65,75-78]. H. Gerischer [76] proposed a model in which electrons and holes are considered as individual interfacial reactants. Distinct and preferential electron transfer reactions involve either the conduction band or valence band as dependent on the nature of the redox reactants of the electrolyte, with specific properties dependent upon the energy state location. 

As shown in Fig. 3.13(b) and 3.13(c) when ratio n/nsfl is less than or greater than 1 the system is in non-equilibrium resulting in a net current, with the electron transfer kinetics at the semiconductor-electrolyte interface largely determined by changes in the electron surface concentration and the application of a bias potential. Under reverse bias voltage, Vei > 0 and ns,o > ns as illustrated in Fig. 3.13(b), anodic current will flow across the interface enabling oxidized species to convert to reduced species (reduction process). Similarly, under forward bias, Ve2 < 0 and ns > ns,o as illustrated in Fig. 3.13(c), a net cathodic current will flow. 

Fig. 3.13 Semiconductor-electrolyte interface (a) at equilibrium, (b) under reverse bias (c) under forward bias. Arrows denote direction of current flow [reduction reaction ox + e red], (d) Electron transfer mediated through surface states.

Boroda YG, Voth GA (1996) A theory of adiabatic electron transfer processes across the semiconductor-electrolyte interface. J Chem Phys 106 6168-6183... 

Figure 4.12 is an illustration of the potential distribution for n-type semiconductor particles at the semiconductor-electrolyte interface. There are two limiting cases of equation (4.8.11) for photo-induced electron transfer in semiconductors. For large particles the potential drop within the semiconductor is defined by ... 

Thus, there are three capacitors in series at a semiconductor/electrolyte interface rather than two capacitors as at a metal/solution interface. What is observed largely depends upon the electron concentration in the semiconductor (how low it is) and the ionic concentration in solution. 

The theoretical developments in the above areas were influenced, to a considerable extent, by concepts borrowed from semiconductor physics and the physics of surfaces. Other fields of photoelectrochemistry of semiconductors were affected to a greater degree by progress achieved in the study of metal electrodes. Here we mean photoemission of electrons from semiconductors into solutions and electroreflection at a semiconductor-electrolyte interface. 

An electrical double layer is usually formed at a semiconductor-electrolyte interface, as well as at the boundary between two solids. This layer consists of plates carrying opposite charges, each being located in one of the phases in contact. In the semiconductor the charge in the region near the surface is formed due to redistribution of electrons and holes in the electrolyte solution, due to redistribution of ions, which form the ionic plate of the double layer. 

Fig. 6. Electron-hole currents at a semiconductor-electrolyte interface (a) and their dependence on the change of potential drop in the semiconductor (b) 1—i , 2—i , 3—ip,...

First, the level F, whose position determines the thermodynamic work function wx, is located in the case of semiconductors in the forbidden band. The energy characteristics of a semiconductor-electrolyte interface under photoemission are presented in Fig. 31, which shows, in particular, that the threshold frequency is given by the relation tiw0 = Eg + where % is the difference between the potential energy level of a delocalized electron outside... 

Fig. 31. Energy diagram of a semiconductor-electrolyte interface under photoemission. The potential energy of the delocalized electron in the solution dtloc is taken as the origin.

We can infer that the band positions of the irradiated semiconductor are greatly influential in controlling the observed redox chemistry and that formation of radical ions produced by photocatalyzed single electron transfer across the semiconductor-electrolyte interface should be a primary mechanistic step in most such photocatalyzed reactions. Whether oxygenation, rearrangement, isomerization, or other consequences follow the initial electron transfer seem to be controlled, however, by surface effects. 

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