Semiconducting properties dissolution

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

For photoelectrochemical functions the semiconductors should have the following properties, in addition to their semiconducting properties (1) the energy of the band gap should correspond to the energy of the visible part of the solar radiation, which represents a significant portion of the solar spectrum, and (2) they should be resistant against corrosion or dissolution caused by electrolyte solutions in processes under illumination as well as in darkness. 

Another essential requirement for the photocatalyst is its resistance to reactions at the solid/liquid interface that may result in a degradation of its properties. These reactions include electrochemical corrosion, photocorrosion, and dissolution (Morrison, 1980). A large group of photocatalysts with suitable semiconducting properties for solar energy conversion (CdS, GaP, etc.) are not stable in the water-oxidation reaction because the anions of these materials are more susceptible to oxidation than water, causing their degradation by oxidation of the material (Ellis et al., 1977 Williams, 1960). 

Figure 5.30 shows how many parameters have to be faken into account, the metal fractions Xa and Xg at the metal surface, fhe cationic and anionic fractions within the film, and fhe dissolution rates of A and B af fhe film surface. Furthermore, the different transfer rates of fhe cations may cause a gradient in the layer composition. Finally, the cations A and B + may be further oxidized at sufficiently positive potentials causing a distribution of lower and higher valent species within the film. This in turn requires the knowledge of the semiconducting properties that are involved in the oxidation of cations as well as the reactions of redox systems at the film surface, which require electron conduction across the layer. All these details show that the semiconductor properties and the chemical composition and structure have to be studied with appropriate tools. The complexity of these systems requires the application of surface analytical methods in order to understand the properties of these films and their influence on the corrosion properties of alloys. 

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