Ideal polarizable semiconductor-solution

KOVAL ETAL. Ideal Polarizable Semiconductor—Solution Interfaces... 

Criteria for the Ideal Polarizable Semiconductor-Solution Interface... 

Based on the discussion above, it seems evident that a detailed understanding of kinetic processes occurring at semiconductor electrodes requires the determination of the interfacial energetics. Electrostatic models are available that allow calculation of the spatial distributions of potential and charged species from interfacial capacitance vs. applied potential data (23.24). Like metal electrodes, these models can only be applied at ideal polarizable semiconductor-solution interfaces (25)- In accordance with the behavior of the mercury-solution interface, a set of criteria for ideal interfaces is f. The electrode surface is clean or can be readily renewed within the timescale of... 

Herein, criteria are developed for ideal polarizable semiconductor electrode-solution interfaces. A variety of experimental studies involving metal dichalcogenide-solution interfaces are discussed within the context of these criteria. These interfaces approach ideality in most respects and are well suited for fundamental studies involving electron transfer to solution species or adsorbed dyes. 

Let us consider in more detail, using the above concepts, how a photocorrosion process occurs under the illumination of a semiconductor. Suppose that electron transitions at the interface between the semiconductor and solution do not take place in darkness in a certain potential range (the semiconductor behaves like an ideally polarizable electrode). This range is confined to the potentials of decomposition of the semiconductor and/or solution. The steady state potential of a semiconductor is usually determined in this case by chemisorption processes (e.g., of oxygen) or, which is the same in the language of the physics of semiconductor surface, by charging of slow surface states. It is these processes that determine the steady state band bending. 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

The attainment of equilibrium between the semiconductor and solution, however, is somewhat hindered. Many semiconductor electrodes behave over a broad range of potentials as ideally polarizable electrodes. 

Although this description is frequently given of the semiconductor-solution junction, in fact, such reversible behavior of a semiconductor electrode is rarely found, especially for aqueous solutions. This lack of equilibration can be ascribed to corrosion of the semiconductor, to surface film (e.g., oxide) formation, or to inherently slow electron transfer across the interface. Under such conditions, the behavior of the semiconductor electrode approaches ideal polarizability (see Section 1.2). 

Most of the events in electrochemistry take place at an interface, and that is why interfacial electrochemistry constitutes the major part of electrochemical science. Relevant interfaces here are the metal-liquid electrolyte (LE), metal-solid electrolyte (SE), semiconductor-electrolyte, and the interface between two immiscible electrolyte solutions (ITIES). These interfaces are chargeable, that is, when the external potential is applied, charge separation of positive and negative charges on the two sides of the contact occurs. Such an interface can accumulate energy and be characterized by electric capacitance, within the range of ideal polarizability beyond which Faraday processes turn on. 

The principal object of electrochemical interest is given by another type of electrified interface, contacts of an electronic (liquid or solid metal, semiconductor) and an ionic (liquid solution, SEs, membranes, etc) conductor. For numerous contacts of this kind, one can ensure such ionic composition of the latter that there is practically no dc current across the interface within a certain interval of the externally apphed potential. Within this potential interval the system is close to the model of an ideally polarizable interface, the change of the potential is accompanied by the relaxation current across the external circuit and the bulk media that vanishes after a certain period. For sufficiently small potential changes, d , the ratio of the integrated relaxation current, dQ, to dE is independent of the amplitude and it determines the principal electrochemical characteristics of the interface, its differential capacitance per unit surface area, C ... 

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