Metal oxide, semiconductive

In this entry, we focus on the discussion of the platform technology for electrochemical sensors, metal oxide semiconductive (MOS) sensors, and piezoelectric based quartz crystal microbalance (QCM) sensors. There are other types of chemical sensors, such as optical sensors, Schottky diode based sensors, calorimetric sensors, field-effect transistor (FET) based sensors, surface acoustic wave sensors, etc. Information of these specific sensors can be found elsewhere and in current journals on sensor technologies. Because of the increasing importance of microfabricated sensors, a brief discussion of microsensors is also given. 

Metal oxide semiconductive sensors are an important class of chemical sensors particularly for gaseous sensing. Among the metal oxide semiconductive materials. 

Metal oxide semiconductive materials exhibit a relatively low conductivity at ambient temperature. Thus, it will be very difficult to observe a small conductivity change because of its reaction with a reducing gas. Therefore, it is common to operate a MOS based sensor at elevated temperature. At higher temperature, the conductivity of the MOS increases substantially, and a change caused by the reaction with reducing gas is now observable. 

Metal oxide semiconductive sensors are not limited to tin oxide only. Many other metal oxides, such as zinc oxide, tungsten oxide, and others can also be used for chemical and gas sensing. It is understandable that an incorporation of a selective catalyst or a dopant may enhance the selectivity of the MOS sensors. Palladium, platinum, and others have been used as catalytic dopants for these sensors. The processes... 

Chapters 7-11 continue the theme and explore different types of chemical sensors. Chapter 7 describes the application of metal oxide semiconducting resistive sensors, and then Chapters 8-11 cover mainly recent developments of electrochemical sensors. 

Metal oxide semiconducting Very high sensitivity, limited High-temperature operation, high power... 

A t5 ical p-channel enhancement-type metal oxide semiconducting EET (MOSFET) is shown in Figure 22.7. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

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. 

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. 

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. 

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