Insulator-solution interface

ABSTRACT The chemical and electrical implications of charge transfer are discussed. The basic differences between chemical and electrochemical reactions are highlighted. Electrochemical kinetics and its various aspects are treated in detail. Tunneling, electronic, and surface states are discussed in the context of interfaces. A current potential relation at semiconductor-solution interfaces receives attention, as do insulator-solution interfaces. 

The presentation we have made hitherto is then background for the real thing in respect to this book because here we are interested virtually in the insulator-solution interface. 

The double layer at the insulator-solution interface is related to the semiconductor solution interface. The difference between a semiconductor and insulator is rather conditional. Like the semiconductor, the insulator has a forbidden energy gap between the valence band and the conduction band, but the gap is much larger compared to that in the semiconductor. But unlike the semiconductor, the insulator is characterized by a small conductivity in comparison with semiconductors such as Si, Ge, etc. 

A drop of potential at the insulator-solution interface is similar to that at semiconductor-solution interface. The potential drop takes place in three regions (1) in the region confined by the space charge in the insulator (2) in a dense part of a double layer containing no free charges and (3) in the diffuse part of the double layer in the electrolyte. On the basis of these considerations one can develop a model of the double layer shown in Figures 20a-20c. 

The Current-Potential Relation at the Insulator-Solution Interface... 

The mathematical formulation of the current-potential relation at the insulator-solution interface is similar to that at the semiconductor-solution interface. The main difference is that the range of potential is much higher in the case of the insulator electrode compared to that at the semiconductor electrode. The applied potential usually ranges from 10 to 10 V at the insulator, but at the semiconductor electrode it ranges from +0 to 2V. 

Studies of the semiconductor-solution interface (68) are quite rare and the ideas of the structure of so-called insulator-solution interfaces (69) are just beginning. 

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 adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

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. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

With metals, semiconductors, and insulators as possible electrode materials, and solutions, molten salts, and solid electrolytes as ionic conductors, there is a fair number of different classes of electrochemical interfaces. However, not all of these are equally important The majority of contemporary electrochemical investigations is carried out at metal-solution or at semiconductor-solution interfaces. We shall focus on these two cases, and consider some of the others briefly. 

Because they are electrical insulators, the net potential difference across a ceramic powder/water interface /o cannot be directly measured. On the other hand, the other two important interface variables (ao and /from other experimentally observable quantities for a complete characterization of interfacial properties. In this section we first introduce the so-called Nemst approximation for the surface potential /o. The limitation of the Nemst equation is then discussed, and the modified Nemst equation for /o of a ceramic powder/aqueous solution interface is subsequently introduced. 

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