Semiconductor powders sulfide

For the influence of the specific surface area of the semiconductor powder on the rate of product formation, two opposite effects are of major importance [81]. One is concerned with the rate of electron-hole recombination which increases linearly with surface area, and accordingly the reaction rate should decrease. The other is a linear increase in the reaction rate of the reactive electron-hole pair with the adsorbed substrates, which should increase product formation. It is therefore expected that, depending on the nature of semiconductor and substrates, the reaction rate, or increasing surface area. This is nicely reflected by the CdS/Pt-catalyzed photoreduction of water by a mixture of sodium sulfide and sulfite. The highest p values are observed with small surface areas and are constant up to 2 m g". From there a linear decrease to almost zero at a specific surface area of 6 m g" takes place. Upon further increase to 100 m g" this low quantum yield stays constant [82]. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

To estimate the thermodynamic feasibility of interfacial redox reactions at a hitherto unknown semiconductor surface, it is therefore of basic importance to know the position of the quasi-Fermi level. The quasi-Fermi level of electrons of powders or thin films of modified titania photocatalysts can be easily obtained by the suspension method developed by Bard et al. (9,10) and modified by Roy et al. (11) for titania and cadmium sulfide. It is based on the pH-dependence of the flat-band potential of Ti02-... 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Nanoscale materials are those with dimensions less than 100 nm. Most of the nanomaterials used, such as oxides, sulfides, nitrides, and others are well known, in many cases since the beginning of civilization. In recent decades, it has been observed that specific properties of these materials, useful in biomedical, electromagnetic, mechanical, and catalytic areas," can be enhanced by reducing particle size to nanoscale dimensions. Many synthetic strategies have been developed in order to obtain nanometric materials with specific properties. Thin films of powders, in particular, have been the subject of current investigations. Studies of new synthetic approaches for nanometric films are intimately connected with the development of the chemical vapor deposition technique, which has widespread acceptance and is used for the production of important supplies for semiconductor electronic applications. ... 

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