Semiconductors solution

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... 

Rajeshwar K (2007) Hydrogen generation at irradiated oxide semiconductor-solution interfaces. J Appl Electrochem 37 765-787... 

In semiconductors, which have a bandgap, recombination of the excited carriers— return of the electrons from the conduction band to vacancies in the valence band—is greatly delayed, and the lifetime of the excited state is much longer than in metals. Moreover, in n-type semiconductors with band edges bent upward, excess electrons in the conduction band will be driven away from the surface into the semiconductor by the electrostatic held, while positive holes in the valence band will be pushed against the solution boundary (Fig. 29.3). The electrons and holes in the pairs produced are thus separated in space. This leads to an additional stabihzation of the excited state, to the creation of some steady concentration of excess electrons in the conduction band inside the semiconductor, and to the creation of excess holes in the valence band at the semiconductor-solution interface. 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

On the other hand, Switzer et al. proposed a different model for the oscillation. They attributed the oscillation to repetitive build-up and breakdown of a thin CU2O layer, which is a p-type semiconductor and acts as a thin rectifying (passivating) layer [24]. Disappearance of the oscillation under irradiated condition supports this model. Light will generate electron-hole pairs in the CU2O and lower the rectifying barrier at the semiconductor/solution interface. 

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. 

Figure 7.3 Variation of the potential at the semiconductor-solution interface...

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

The authors propose that a major difficulty in interpreting kinetic current flow at the semiconductor-solution interface lies in the inability of experimentalists to prepare interfaces with ideal and measurable properties. In support of this hypothesis, the importance of ideal interfacial properties to metal electrode kinetic studies is briefly reviewed and a set of criteria for ideality of semiconductor-solution interfaces is developed. Finally, the use of semiconducting metal dichalcogenide electrodes as ideal interfaces for subsequent kinetic studies is explored. 

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... 

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. 

Figure 7.5 Schematic representation of an n-type semiconductor-solution electrolyte junction showing the formation of depletion layer, band bending and Helmholtz layer (a) before immersion and (b) after immersion in solution.

The theory of the electrochemistry of insulators derives from the theory of the semiconductor/solution... 

At a semiconductor/solution interface, an n-type semiconductor (carrier density of 10 electrons cm A is in contact with a nonaqueous system using a redox system, i.e., no surface states. The capacity of this interface is 4 pF cm-2. Evaluate the potential differences within the semiconductor. (Bockris)... 

The n-p Junction. Before beginning a discussion of electron transfer at interfaces between H-type semiconductor/solution interlaces, it is helpful to describe something of the theory of the famous n-p junction. This is not a part of electrode-process chemistry (which deals with electron-transfer reactions between electronically and ionically conducting phases), but it is the basis of so much modem technology (e.g., the transistor in computers) that an elementary version of events at the junction should be understood. Further, knowing about the n-p junction makes it easier to understand electrochemical interfaces involving semiconductors. 

Fig. 7.25. Changes in the potential energy of the electrons in a p-type semiconductor at the semiconductor/solution interface when there are no surface states at the semiconductor electrode surface.

Thus, in a region in which the current density at a driven semiconductor/solution interface is low enough that the electrons in the semiconductor are in equilibrium between surface and bulk (i.e., not rate-determined by charge carrier transport—... 

M. Green, Electrochemistry of the Semiconductor Solution Interface, in Modem Aspects of Electrochemistry, J. O M. Bockris, ed., Vol. n, Butterworths, London (1959). First kinetic treatment of electron transfer at semiconductor/solution interfaces. 

K. Uosaki and M. Koinuma, STM and Semiconductor-Solution Interfaces, Faraday Disc. 94 361 (1992). 

Fig. 7.51. Equivalent circuits for a semiconductor/solution interlace, (a) Conventional equivalent circuit (b) Equivalent circuit used in this work.

Tafel s law applies also in current density ranges well below that of the limiting current at semiconductor/solution interfaces and to photoelectrochemical reactions. Its application to liquid-liquid interfacial electron transfer is also good [see Fig. 9.25(d)] (Schmickler 1995). In hydrogen evolution, it has been followed down to the picoam-pere region and up to 100 A cm-2. 

At the same time, it is the position of the Fredox level that determines the thermodynamic properties of a semiconductor-solution interface. In particular, proceeding from the equilibrium condition F = Fredox, one may write the condition of an electrochemical reaction in the following form (Gerischer, 1977c) ... 

Photo-Induced Electron Transfer Reactivity at Nanoscale Semiconductor-Solution Interfaces Case Studies with Dye-Sensitized Sn02-Water Interfaces... 

The second-order kinetics problem for semiconductor-solution interfaces has been considered in some detail by Lewis and co-workers [41,42]. For molecules immobilized on a semiconductor surface and assuming that electrons are transferred from the conduction band, not surface states, their rate law can be written as... 

Perhaps the simplest explanation for the pH independence of back-ET rates at metal oxide semiconductor-solution interfaces is that the formal potential of the dye moves in registry with the conduction-band edge. The energy difference, Ecb — Ef (dye), is then unchanged with respect to pH and the back-ET reaction experiences a pH-independent driving force. To amplify briefly, the idea is that... 

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