Interface semiconductor-electrolyte solution

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

Electrical double layers are also characteristic of the semiconductor-electrolyte solution, solid electrolyte or insulator-electrolyte solution interface and for the interface between two immiscible electrolyte solutions (ITIES) (Section 4.5). 

The situation of the electric double layer at a semiconductor/electrolyte solution interface affected by light radiation will be dealt with in Section 5.10. 

Figure 4, Semiconductor-electrolyte solution interface in the dark. An n-type semiconductor with a depletion layer at the surface is illustrated. E is electron energy, Ej and pEj . are the equal Fermi leveh for electrons and holes at equilibrium, other symbols as in Figure 3 (13).

Figure 5. n-Type semiconductor—electrolyte solution interface with a surface depletion layer, in the dark and with two intensities of illumination. Symbols as in Figure 3 and 4 with Ec and E the band edges of the conduction and valence bands, respectively, under illumination, and Ef(H2) Ef(Om) abbreviations for Ef(H20/h2) and Ep(02/H20)y respectively. The quasi-Fermi levels Ei> and pEp are at different positions in the surface region than in the bulk as a result of the limited penetration of light into the interior. Fermi levels in solution as in Figures 3 and 4(13). 

The photoelectrochemistry of semiconductors studies processes of various nature that occur at a semiconductor-electrolyte solution interface under the action of electromagnetic radiation (mainly in the visible, UV and IR regions). These processes include ... 

The semiconductor-electrolyte solution interface is a contact of two conducting media, so that some of its properties are similar to those of contacts between a semiconductor and a metal or between two semiconductors. At the same time, the interface considered is a contact of two media with essentially different types of conductivity—electronic and ionic moreover, these media are in different states—solid and liquid. Therefore, such an interface possesses a number of unique features. 

R. Memming, Charge transfer processes at semiconductor electrodes in Electroanalytical Chemistry. A.J. Bard. Ed., Vol. 11 (1979), p. 1. IVIarcel Dekker, New York. (Charge and potential distribution and charge transfer at and through semiconductor-electrolyte solution interfaces.)... 

Figure 19. The electronic structure of an n-type semiconductor/electrolyte solution interface under conditions of free electron depletion at the surface. Shown are the conduction and valence band edges as a function of the distance from the surface. The interfacial potential drop is distributed over a region in the solid (depletion region, width 4c) and the molecular Helmholtz layer at the liquid side (not shown). The interfacial capacitance is represented by a series connection of the capacitance of the depletion layer (Csc) and the Helmholtz layer (Csoi).

Usually, when thinking of electrochemical reactions, reactions are considered at a metal/electrolyte or semiconductor/electrolyte interface, but rarely about the interface between electrolyte solutions or, more recently, the electrolyte-ionic liquid interface or even the interface between two immiscible ionic liquids. 

Semiconductor Electrode, Fig. 2 Energy diagrams at an n-type semiconductor-electrolyte solution interface (a) at equilibrium, (b) under forward bias, and (c) under reverse bias. Ec (, or or is required). Energy of the... 

After band structures of metal, insulator, and semiconductors are described and historical back-grotmd of semiconductor electrochemistry is presented, electronic structure of semiconductor/ electrolyte solution interface is discussed in relation to the unique electrochemical behavior of semiconductor electrode. Finally, effect of illumination as well as the surface modification on the electrochemical behavior of semiconductor electrode are described. Fundamental knowledge of semiconductor electrode presented here should be very important for the future development of photoelectrochemical and photocatalytic energy... 

Figure Bl.28.9. Energetic sitiration for an n-type semiconductor (a) before and (b) after contact with an electrolyte solution. The electrochemical potentials of the two systems reach equilibrium by electron exchange at the interface. Transfer of electrons from the semiconductor to the electrolyte leads to a positive space charge layer, W. is the potential drop in the space-charge layer.

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

This chapter will include equilibria at non-polarizable interfaces for a metal or semiconductor phase-electrolyte system (a galvanic cell in the broadest sense) and for two electrolytes (the solid electrolyte-electrolyte solution interface, or that between two immiscible electrolyte solutions). 

Fig. 4.12 Dependence of concentrations of negative charge carriers (ne) and positive charge carriers (np) on distance from the interface between the semiconductor (sc) and the electrolyte solution (1) in an w-type semiconductor. These concentration distributions markedly differ if the semiconductor/electrolyte potential difference A cp is (A) smaller than the flat-band potential AF

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

The term photovoltaic effect is further used to denote non-electrochemical photoprocesses in solid-state metal/semiconductor interfaces (Schottky barrier contacts) and semiconductor/semiconductor pin) junctions. Analogously, the term photogalvanic effect is used more generally to denote any photoexcitation of the d.c. current in a material (e.g. in solid ferroelectrics). Although confusion is not usual, electrochemical reactions initiated by light absorption in electrolyte solutions should be termed electrochemical photogalvanic effect , and reactions at photoexcited semiconductor electodes electrochemical photovoltaic effect . 

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. 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

In such devices the light-absorbing semiconductor electrode immersed in an electrolyte solution comprises a photosensitive interface where thermodynamically uphill redox processes can be driven with optical energy. Depending on the nature of the photoelectrode, either a reduction or an oxidation half-reaction can be light-driven with the counterelectrode being the site of the accompanying half-reaction. N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, and... 

Scheme II. Representation of the interface energetics for an ideal n-type semiconductor in contact with electrolyte solutions of differing Eredox.

When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 7.3). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. 

Figure 7.4 Band bending at the interface between a semiconductor and an electrolyte solution (a)-(c) n-type semiconductor (a) enrichment layer, (b) depletion layer, (c) inversion layer (d)-(f) p-t.ype semiconductor (d) enrichment layer, (e) depletion layer, (f) inversion layer.

This book is divided into three parts the first part covers the fundamental aspects, which should form the backbone of any course. As is evident from the title I consider electrochemistry to be a science of interfaces - the definition is given in the introduction -, so I have treated the interfaces between a metal or a semiconductor and an electrolyte solution, and liquid-liquid interfaces. I have not considered solid... 

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