Reduction semiconduction

At high oxygen pressures, oxide phases show defect electron (hole) conduction (oxidation semiconduction) and at low oxygen pressures excess electron conduction (reduction semiconduction). The transport number of excess electrons in Zro.ssCao.isOi.ss as a function of the oxygen partial pressure could be determined by measurements with a Ca,CaO/air cell [79]. The hole conduction of zirconia-based solid electrolytes was noticed for the first time when cells with Ni,NiO reference electrodes for gas potentiometry [44,91] were tested in air. The harmful oxygen permeability was measured potentiometrically in 1965 [92]. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Chandra S, Sahu SNJ (1984) Electrodeposited semiconducting molybdenum selenide films. 1. Preparatory technique and stractural characterization. J Phys D Appl Phys 17 2115-2123 Dukstiene N, Kazancev K, Prosicevas 1, Guobiene A (2004) Electrodeposition of Mo-Se thin films from a sulfamatic electrolyte. J SoUd State Electrochem 8 330-336 Ponomarev EA, Neumann-SpaUart M, Hodes G, Levy-Qement C (1996) Electrochemical deposition of M0S2 thin films by reduction of tetrathiomolybdate. Thin SoUd EUms 280 86-89... 

Schneemeyer LF, Wrighton MS (1979) Flat-band potential of n-type semiconducting molybdenum disulfide by cyclic voltammetry of two-electron reductants Interface energetics and the sustained photooxidation of chloride. J Am Chem Soc 101 6496-6500... 

Solorza-Eeria O, EUmer K, Giersig M, Alonso-Vante N (1994) Novel low-temperature synthesis of semiconducting transition metal chalcogenide electrocatalyst for multielectron charge transfer Molecular oxygen reduction. Electrochim Acta 39 1647-1653... 

Alonso-Vante N, Tributsch H, Solorza-Eeria O (1995) Kinetics studies of oxygen reduction in acid medium on novel semiconducting transition metal chalcogenides. Electrochim Acta 40 567-576... 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

Some insulating oxides become semiconducting by doping. This can be achieved either by inserting certain heteroatoms into the crystal lattice of the oxide, or more simply by its partial sub-stoichiometric reduction or oxidation, accompanied with a corresponding removal or addition of some oxygen anions from/into the crystal lattice. (Many metal oxides are, naturally, produced in these mixed-valence forms by common preparative techniques.) For instance, an oxide with partly reduced metal cations behaves as a n-doped semiconductor a typical example is Ti02. 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

Let us initially look at a semiconducting binary oxide Ai gO in a chemical gradient an oxygen potential gradient. Reduction takes place on the low oxygen... 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

Recently a new hybrid power source has been reported that couples oxidation at a dye-photosensitized nanocrystalline semiconducting SnOz photoanode with the enzyme-catalyzed reduction of O2. Although miniaturization has not yet been reported for this new hybrid, the developments already achieved to miniaturize biofuel cells coupled to those being developed for charge-insertion oxides should be technically transferable to this system. 

Although the pseudohalide Au" seems a likely candidate in the white solid, the decomposition seems unusual considering the high stabilities of KAu and the semiconducting CsAu (discussed In 31), unless the decomposition reaction actually Involves reduction of the ligand. 

Sometimes, semiconductivity depends on the type of a structural phase that arises from synthesis. Thus, in the case of (TCNQ) Cu the semiconducting phase is thermodynamically disfavored. To prepare this semiconductor, Harris et al. (2005) proposed to perform the reduction of TCNQ in acetonitrile at glass-carbon, gold, or platinum electrode in the presence of Cu. This allows the electrocrystallization of sparingly soluble TCNQCu semiconducting phase to occur by a nucleation... 

Gautron J, Marucco JE, Lemasson P (1981) Reduction and doping of semiconducting rutile. Mater Res Bull 16 575-580... 

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