Semiconductor-metal-solution system

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. 

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). 

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

Metals, semiconductors, electrolyte solutions, and molten salts have in common the fact that they contain given or variable densities of mobile charge carriers. These carriers move to screen externally imposed or internal electrostatic fields, thus substantially affecting the physics and chemistry of such systems. The Debye-Huckel theory of screening of an ionic charge in an electrolyte solution is an example familiar to many readers. 

Several other attempts have been made by various authors to avoid anodic corrosion at n-type electrodes and surface recombination at p-type electrodes, by modifying the surface or by depositing a metal film on the electrode in order to catalyse a reaction. It has been frequently overlooked that the latter procedure leads to a semiconductor-metal junction (Schottky junction) which by itself is a photovoltaic cell (see Section 2.2) [14, 27]. In the extreme case, then only the metal is contacting the redox solution. We have then a pure solid state photovoltaic system which is contacting the solution via a metal. Accordingly, catalysis at the semiconductor electrode plays a minor role under these circumstances. 

Multiple reports on the presence of this noise in such a diverse group of systems as carbon resistors, semiconductors, metallic thin films, and aqueous ionic solutions led researchers to believe in the existence of some profound... 

It is apparent from Figs. 2.3 and 2.4 that there is a close correlation between the behavior of the density and that of the conductivity. As we discuss in later chapters, the density variation is the single most influential factor governing the MNM transition in mercury and the alkali metals. In this important respect the MNM trcmsition in fluid metals is similar to the continuous MNM transitions observed in other systems where the density or concentration of a metallic component can be varied. These include heavily doped semiconductors, metal-ammonia solutions, metal-nonmetal aggregates like mercury-xenon, molten metal-salt solutions, etc. (see, e.g., Edwards and Rao, 1985). 

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