The semiconductor-electrolyte interface

2 The Semiconductor-Electrolyte Interface Band bending at the interface 

Helmholtz [71] first described the interfacial behavior of a metal and electrolyte as a capacitor, or so-called electrical double layer, with the excess surface charge on the metallic electrode remaining separated from the ionic counter charge in the electrolyte by the thickness of the solvation shell. Gouy and Chapmen subsequently 

For p-type semiconductors, an accumulation layer forms when excess positive charge (holes) accumulate at the interface, which is compensated by negative ions of an electrolyte. Fig. 3.7(d). Similarly, a depletion layer forms when the region containing negative charge is depleted of holes, and thus positive counter ions 

Equilibrium between the two phases at a semiconductor-electrolyte interface, solid and liquid, can only be achieved if their electrochemical potential is the same, that is  

The electrochemical potential of the solution and semiconductor, see Fig. 3.6, are determined hy the standard redox potential of the electrolyte solution (or its equivalent the standard redox Fermi level, Ep,redo, and the semiconductor Fermi energy level. If these two levels do not lie at the same energy then movement of charge across the semiconductor - solution interface continues until the two phases equilibrate with a corresponding energy band bending, see Fig. 3.8. 

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Figure Bl.28.10. Schematic representation of an illuminated (a) n-type and (b) p-type semiconductor in the presence of a depletion layer fonned at the semiconductor-electrolyte interface.

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... 

Jaegermann, W. The Semiconductor/Electrolyte Interface A Surface Science Approach 30... 

Boddy PJ (1965) The structure of the semiconductor-electrolyte interface. J Electroanal Chem 10 199-244... 

The diffuse charge distribution in the semiconductor s surface layer leads to a drastically lower cell capacitance at the semiconductor-electrolyte interface. Typical... 

The basic difference between metal-electrolyte and semiconductor-electrolyte interfaces lies primarily in the fact that the concentration of charge carriers is very low in semiconductors (see Section 2.4.1). For this reason and also because the permittivity of a semiconductor is limited, the semiconductor part of the electrical double layer at the semiconductor-electrolyte interface has a marked diffuse character with Debye lengths of the order of 10 4-10 6cm. This layer is termed the space charge region in solid-state physics. 

Green, M., Electrochemistry of the semiconductor-electrolyte interface, MAEy 2, 343 (1959). 

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. 

Fig. 5.60 The semiconductor/electrolyte interface (a) before equilibration with the electrolyte, (b) after equilibration with the electrolyte in the dark, and (c) after illumination. The upper part depicts the n-semiconductor and the lower the p-semiconductor...

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,... 

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

Light-Induced Redox Processes at the Semiconductor-Electrolyte Interface... 

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... 

Semiconductor - Electrolyte Interlace The electric field in the space charge region that may develop at the semiconductor electrolyte interface can help to separate photogenerated e /h 1 couples, effectively suppressing recombination. When a semiconductor is brought into contact with an electrolyte, the electrochemical potential of the semiconductor (corresponding to the Fermi level, Ey of the solid [50]) and of the redox couple (A/A ) in solution equilibrate. When an n-type semiconductor is considered, before contact the Ey of the solid is in the band gap, near the conduction band edge. After contact and equilibration the Ey will... 

Fig. 10.1 Equivalent circuits used to represent the semiconductor-electrolyte interface, (a) A more complete approach taking into account the series resistance (Ry), the depletion layer (Csc, Rsc), an oxide surface film...

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