Stable oxide semiconductors

The valence band top of d° or d10 stable oxide semiconductor with good photo-catalytic activity consists of an O 2p orbital. Commonly, the electrochemical potential of this VB is 2.5-3.5 V vs. NHE, which is considerably more positive than the redox potential of the O2/H2O redox couple. Strategies to narrow the band gap of these oxides and better match the redox potentials are ... 

On the other hand, oxide semiconductor materials such as ZnO and 2 have good stabilities under irradiation in solution. However, such stable oxide semiconductors cannot absorb visible light because of their wide band-gap character. Sensitization of wide-band-gap oxide semiconductor materials by photosensitizers, such as organic dyes which can absorb visible light, has been extensively studied in relation to the development of photography technology since the middle of the nineteenth century. In the sensitization process, dyes adsorbed onto the semiconductor surface absorb visible light and excited electrons of dyes are injected into the conduction band of the semiconductor. Dye-sensitized oxide semiconductor photoelectrodes have been used for PECs. 

IPCE and APCE can have values close to 100%. As discussed before, the maximum attainable photoconversion efficiency in a single bandgap photoelectrolysis cell is 30.7%. Although stable, the photoconversion efficiencies of most oxide semiconductors are low (<2% except the case of 8.35% reported for carbon modified titania [121]) due to their large band gap. 

A few works have also been published reporting unusual valence states of metal ions in lattices of such oxide semiconductors as ln203, ZnO, Sn02 [44, 55-68]. These compounds attract researchers attention because they are very perspective materials like thin films and ceramics for constructing new chemical sensors [55], as well as highly conductive thermo- and chemically stable n-type conductors (ln203). 

There are several, relatively simple reasons for the dominance of silicon The band gap of 1.1 eV is just suitable for devices operating at temperatures encountered in the most applications (-40 to +50 °C). From the chemical point of view, silicon is the only semiconductor which forms a stable oxide with excellent dielectric properties. In spite of the incommensurability of their crystal lattices, an almost perfect interface with a low defect density of < 10 cm can be routinely prepared between silicon and... 

The oxidation of acetate by O2 is a downhill reaction, which is catalysed by Ti02 in the presence of light absorbed by the semiconductor. Corresponding reactions are performed with stable oxides, primarily with Ti02 particles. The basic processes are illustrated in Fig. 2.34 for dispersed small semiconductor particles as typically used in this application. The holes produced by light excitation are used for the oxidation of the acetate, whereas electrons are transferred to O2. 

It is clear from the cell design that only semiconductor electrodes which are initially stable can be used. There are several oxide semiconductors available which show suffi-... 

Thermoelectric power measurements indicated that all the compounds are p-type semiconductors and thermo - emf increases with an increase in Zn2+ content (Table 1). In this system Zn + ions with stable oxidation state occupy A sites. With the introduction of Zn + in the lattice the concentration of Cu occupying octahedral site decreases, however the concentration of Mn ions at the octahedral sites of the series Cuj. Zn MnCr04 remains same. Therefore, with the increase in value of Zn2+, the number of ion pairs of Mn3+ - Mn + start decreasing thereby increasing the resistivity of Zn rich compositions. Seebeck coefficient ( a ) varied between +26 to +66 pV/K which shows with increase in zinc contents of the spinel, the Seebeck coefficient increases. 

In Section 2C-3 (Figure 2-19). we described the metal oxide semiconductor licld-effeci transistor (MOSFhT). which is widely used in computers and other electronic circuits as a switch to control current in circuits. One of the problems in using this type of device in electronic circuits has been its pronounced sensitivity to ionic surface impurities, and a great deal of money and effort has been expended by ihe electronics industry in minimizing or eliminating this sensitivity to produce stable transistors. 

To provide a stable attachment of a sensitizer to a wide-bandgap oxide semiconductor and to allow for a fast electron injection from the excited state of a sensitizer molecule to the conduction band of the oxide, covalent links are preferable as opposed to simple coordination or van der Waals bonds [22]. On the other hand, very acidic sensitizer molecules favor corrosion of ZnO and thereby lead to inefficient charge transfer at the interface and to a low cell stabihty [94]. Carboxylic or... 

We have shown that the measurement of ammonia gas behind a gas permeable membrane can be an interesting alternative to the use of a pH electrode in solution. Ammonia sensors based on metal-oxide-semiconductor structures appear to be useful in this context they are reasonably fast, stable and sensitive. It is furthermore demonstrated that these devices can be used in combination with dry reagent chemistry to provide simple assay methods. The determination of substrates which can be made to release ammonia in the presence of the... 

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