Semiconducting metal oxide

Active gold catalysts are advantageous in that water usually enhances the catalytic activity [39]. Reducible or semiconductive metal oxide supports do not need moisture for room temperature catalytic activity, while non-reducible metal oxides such as AI2O3 and Si02 do [39] (Figure 10). 

Various other semiconductor materials, such as CdSe, MoSe, WSe, and InP were also used in electrochemistry, mainly as n-type photoanodes. Stability against photoanodic corrosion is, naturally, much higher with semiconducting oxides (Ti02, ZnO, SrTi03, BaTi03, W03, etc.). For this reason, they are the most important n-type semiconductors for photoanodes. The semiconducting metal oxide electrodes are discussed in more detail below. 

Other inorganic reactions shown to be photo-induced at colloidal semiconducting metal oxide surfaces include the synthesis of ammonia from water and nitrogen (19) and the oxidation of halide ions 1 ,... 

For mixed metal oxides obtained from their hydroxide or carbonate precursors after calcination, it is generally difficult to determine whether the as-prepared precursor is a single-phase or multiphase solid solution [35]. Non-aqueous solvents appear superior for achieving two dissimilar metal oxides such as MM Oz or MM 04 precipitates such reactions cannot be carried out simultaneously in aqueous solution due to the large variations in pH necessary to induce precipitations [41,42]. Table 6.1 summarizes some of the nanoparticulate semiconducting metal oxides and mixed metal oxides prepared via co-precipitation techniques. The general procedure of achieving metal loaded nanoparticles on an oxide support is shown in Fig.6.5. 

The mode of action of semiconducting metal oxides can be illustrated with reference to catalysis of the oxidation of carbon monoxide ... 

In order to use semiconductive metal oxides as an oxygen sensor, both thermal stability at elevated temperatures and atmospheric stability under reductive environments are required for reproducibility and accuracy of the sensor. 

Figure 10. Dependence of the electrical conductivity of semiconductive metal oxides on oxygen partial pressure.

The sensing action by metal oxides depends on several factors such as grain size (available surface area) and surface states as well as the efficiency with which the test gas molecules adsorb on the surface [25-27]. The sensing mechanism of n-type semiconducting metal oxides involves... 

An undoped or pure semiconductor material is called an intrinsic semiconductor. Dopants (or impurities) are often added at very low concentration to modify the type and/or number of charge carriers in the material (i.e., adjust the Fermi level). There are two types of semiconductor materials, n-type in which the majority charge carriers are (negative) electrons, andp-fypein which the majority charge carriers are (positive) holes or electron deficiencies. Most (but not all) semiconducting metal oxides are of the n-type. 

In the case of dissolution of a number of semiconducting metal oxides in acid aqueous solutions, it can be calculated from the rate of solution, that a phase boundary reaction must be rate determining. Moreover, it can be shown, that this reaction is electrochemical in nature, for its rate depends on the potential of the dissolving oxide. As an example, Fig. 1 shows the influence of the electrode potential on the rate of dis -solution of FeQ g Oin sulfuric acid and hydrochloric acid. The rate of dissolution is expressed in terms of a dissolution current. A linear relation exists between the electrode potential... 

Figure 5. A methodological approach for simultanemis spectrtscopic and electrical (phenoiwnological) characterization of gas sensors based on semiconducting metal oxides [15].

Heterogeneous layers based on semiconducting metal oxides are known to show gas-sensing features different as compared to single oxides [1,2], The mentioned synergetic phenomenon can be provided by two effects i) the presence of active centers with diverse adsorption and catalytic behavior ii) the efficient separation of the sensor functions, receptor and transducer, between different oxide phases. 

Inorganic Gas response of polycrystalline semiconducting metal oxides Thermo-mechanical and thermo-chemical effects on percolation-dependent electron transport 41... 

FIGURE 22.38 Electronic energy diagrams for a metal electrode in contact with a p-type semiconducting metal oxide (a) prior to contact, (b) posterior to contact, and (c) with photoexcitation neF = quasi-Fermi level for photoexcited electrons in oxide, eM = Fermi level of metal dissolution reaction, and eHY = Fermi level of hydrogen reaction. 

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