Oxide as semiconductor

Shimizu Y, Hyodo T, Egashira M (2004) Meso- to macro-porous oxides as semiconductor gas sensors. Catal Surveys from Asia 8 127-135... 

Metal oxides usually consist of bulk oxides. As semiconductors, metal oxides catalyze the same kind of reactions as metals but in processes requiring higher temperatures. Often a mixture of various oxides is applied to increase the catalytic activity. For example, transition metals, such as M0O3 and Cr203, are good catalysts for polymerization of olefins a mixture of copper and chromium oxides, named copper chromite, is used for hydrogenation and a mixture ofiron and molybdenum oxide (ie, iron molybdate), is used for formaldehyde formation from methanol. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

As of now such semiconductor oxides as ZnO, Sn02 and Ti02 are most widely used as operational sensor elements. This is initially explained by the vast amount of experimental data gathered for above compounds and on the other hand by the importance of their being used as catalysts in various reactions. Finally, this can be explained by the fact that they are most suitable from the stand-point of requirements... 

Metal oxides. Noble metals are covered with a surface oxide film in a broad range of potentials. This is still more accentuated for common metals, and other materials of interest for electrode preparation, such as semiconductors and carbon. Since the electrochemical charge transfer reactions mostly occur at the surface oxide rather than at the pure surface, the study of electrical and electrochemical properties of oxides deserves special attention. 

High electrical conductivity is also attained in oxides with very narrow, partially filled conduction bands the best known example is Ru02. This material has a conductivity of about 2-3 104S/cm at the room temperature, and metal-like variations with the temperature. Some authors consider Ru02 and similar oxides as true metallic conductors, but others describe them rather as n-type semiconductors. 

Lira-Cantu, M. Norrman, K. Andreasen, J. W. Casan-Pastor, N. Krebs, F. C. 2007. Semiconductor oxides as electron acceptors in hybrid organic-inorganic solar cells. ECS Trans. 3 1-9. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Polyoxometalates (POMs) have been the object of growing interest as photocatalysts for the oxidation or reduction of organic or inorganic compounds [225-230], Many of these POM systems share the same general photochemical characteristics as semiconductor photocatalysts. For example,... 

Some of the passive films have been characterized as semiconductors in this case, corrosion of these oxides may imply transfer of holes (h+) from the valence band to the reductant and of electrons (e ) from the conduction band, in the case of iron(III) oxides as Fe(II). 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

Uses. The unalloyed metal cannot be directly used owing to its bad mechanical properties and its high oxidability. Several thallium alloys are used as semiconductors or ceramic compounds it may be used as additive to gold, silver or copper contacts in the electronic industries. Thallium is dangerously toxic. 

A convenient application uses an inorganic salt (e.g., a polyoxometallate)" or a semiconductor oxide as the sensitizer. Such materials are often chemically more stable with respect to organic molecules, and again can be conveniently used for the generation of a radical from unconventional precursors. 

It should be mentioned that, of the other first-row transition metal oxides crystallizing with the NaCl structure, none has been found to superconduct down to 2.5 K. Some of these oxides undergo magnetic ordering at low temperature and most behave as semiconductors at all temperatures. These would include MnO, FeO, CoO, and NiO. Studies performed on CuO, which has a different crystalline structure, showed only semiconducting behavior to very low temperatures (1.9 K). 

An important feature of photoanodes is that the photogenerated holes, which are normally very strongly oxidizing, may oxidize the semiconductor instead of, or as well as, the electrolyte species. This phenomenon is known as photocorrosion. For the purposes of the limited explanation of PECs given here, it is enough to note that continuous photocorrosion will destroy the photoelectrode. (This need not necessarily occur if corrosion is confined to the semiconductor surface.)... 

Before any further discussion of the implications of these adsorption characteristics, some pertinent properties of zinc oxide as a semiconductor will be reviewed. 

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