Metal conductometric sensor

Chapter 10 deals with composite films synthesized by the physical vapor deposition method. These films consist of dielectric matrix containing metal or semiconductor (M/SC) nanoparticles. The film structure is considered and discussed in relation to the mechanism of their formation. Some models of nucleation and growth of M/SC nanoparticles in dielectric matrix are presented. The properties of films including dark and photo-induced conductivity, conductometric sensor properties, dielectric characteristics, and catalytic activity as well as their dependence on film structure are discussed. There is special focus on the physical and chemical effects caused by the interaction of M/SC nanoparticles with the environment and charge transfer between nanoparticles in the matrix. 

From numerous results achieved using combinatorial and high-throughput methods, the most successful have been in the areas of molecular imprinting, polymeric compositions, catalytic metals for field-effect devices, and metal oxides for conductometric sensors. In those materials, the desired selectivity and sensitivity have been achieved by the exploration of multidimensional chemical composition and process parameters space at a previously unavailable level of detail at a fraction of time required for conventional one-at-a-time experiments. These new tools provided the opportunity for the more challenging, yet more rewarding explorations that previously were too time consuming to pursue. 

Electrical sensors, operating due to a surface interaction with target gas, cover a large group of gas sensors polymer, metal, metal oxide, or saniconductor conductometric sensors capacitance sensors and work-function-type and Schottky barrier-, MOS-, and FET-based sensors (Korotcenkov 2011). 

Metal oxides fonn the class of materials which has seen the widest application in gas sensors (Park and Akbar 2003 Korotcenkov 2007a, b). As can be seen in Table 2.1 and Fig. 2.1, they can be used in every type of gas sensor. For example, in conductometric sensors, semiconducting metal oxides are typically used as gas-sensing materials that change their electrical resistance upon exposure to oxidizing or reducing gases. 

Experiment has shown that complex metal oxides can be successfully applied only for design of gas sensors intended for the detection of specific gases such as H S (Tamaki et al. 1998 Jianping et al. 2000) or CO. In particular, Chapelle et al. (2010) have shown that Cu0-Cu Fe3 04 films can be used for design conductometric sensors for CO detection. It was found that as a rule film conductance changes in CO -sensitive materials upon interaction with CO are achieved via the rapid and reversible formation of carbonates. We have to note that conventional binary metal oxides due to high... 

Table 2.14 Comparison of responses of metal oxide-based conductometric sensors to 2,000 ppm CO, in...

Figure 38.1 Structure of a typical metal oxide conductometric sensor.

An interesting P(ANi)-based sensor for the detection of HCN gas was described by Langmaier and Janata [805] Into a P(ANi) film electropolymerized on a Pt substrate were incorporated Hg and Ag salts, which formed P(ANi)-Hg and P(ANi)-Ag clusters due to the spontaneous reduction of the metals. These transition metal/CP clusters then adsorbed HCN, causing a change in the open circuit potential of the P(ANi) film. The response time was however very slow, of the order of minutes. Fig. 17-17 shows some results from this work. As with the conductometric sensor work cited earlier, this work appeared to show that CP sensors are not particularly amenable to detection of gases. 

A good example for a microanalytical device is the gas sensor array. The conductometric approach for gas sensing was favored during the last years using metal oxides or conductive polymers. Unfortunately such sensors are quite unspecific and therefore sensor arrays with modified sensing layers have to be used. The selectivity derives from a sophisticated data processing using neural networks. Complete gas analysis systems with microfluidic and data acquisition are now under development. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

Metal oxides are among the most used active materials for conductometric chemical sensors. They have a wide variety of electrical properties spanning from insulator to quasi metallic behavior. The discovery of their sensing properties was made more than five decades ago, thereafter the interest of researchers was focused on nanostructured materials. These materials may give a greater modulation of the electrical properties for the interaction with the surrounding atmosphere thanks to the higher surface to volume ratio. 

Oxides are normally stable at the operating temperatures necessary to enhance the interaction between their surface and the gas phase, much more stable compared to organic materials. They are normally operated between 500 and 800 K where the conduction is electronic and oxygen vacancies are doubly ionized. Different oxides have been proposed for conductometric chemical sensors, the most studied is by far tin dioxide that has also been commercialized in form of thick film sensors. Other oxides studied are titanium oxide, tungsten oxide, zinc oxide, indium oxide and iron oxide, first in form of thick and then in form of thin films. Furthermore, the use of mixed oxides, as well as the addition of noble metals, has been studied to improve not only selectivity but also stability. 

Surface Modifiers for Metal Oxides in Conductometric Gas Sensors. 273... 

Table 2.3 Characteristics of several metal oxide nanofiber-based conductometric gas sensors...

Table 3.1 Table of conductometric gas sensors based on ID nanostructiures of metal oxide classified by sensing oxide... 

It should also be noted that, as a rule, the response of NT-based sensors, especially of conductometric types, is very low in comparison with conventional metal oxide-based gas sensors (Drake et al. 2007). In particular. Fig. 4.10 shows that appreciable response is observed at concentrations of VOCs exceeding 10,000 ppm. 

Daniel M-C, Astruc D (2004) Gold nanoparticles assembly, suprrunolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 104 293-346 De Jongh LJ (ed) (1994) Physics and chemistry of metal cluster compounds. Kluwer, Dordrecht Di Francia G, Alfano B, La Ferrara V (2009) Conductometric gas nanosensors. J Sensors 659275(18)... 

Metal Oxide-Based Nanocomposites for Conductometric Gas Sensors... 

Metal oxide-metal oxide-based nanocomposites, Me 0-Me 0, are also interesting for gas sensor design (Yamazoe et al. 1983 Yamazoe 1991 Ferroni et al. 1999 Yamaura et al. 2000 Comini et al. 2002 Korotcenkov 2007 Gas kov and Rumyantseva 2009). It was established that one of the ways for improving selectivity and stability of metal oxide conductometric gas sensors is the modification of metal oxide, Me O by the introduction of catalytic or structure modifiers, Me 0, in the nanostruc-tured metal oxide matrix and, thereby, the development of nonhomogeneous complex materials, i.e., nanocomposites Me 0-Me 0. It was also expected that other highly sophisticated surface-related properties important for gas sensor applications such as optical, electronic, catalytic, mechanical, and chemical can also be obtained in complex metal oxides and composites. 

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