Metal Oxide-Based Nanocomposites

It should be noted that metal oxide-based colorimetric gas sensors are also designed mainly on the basis of composites because, in addition to metal oxides, they include noble metals as catalysts. Typical metal oxide-based composites used in optical sensors are listed in Table 15.7. 

Features of the optical gas-sensing characteristics of the Au-transition metal oxide composites are summarized in Table 15.8. It was found that the combination of small Au particles with NiO film (Kobayashi et al. 1993 Ando et al. 1994,1996) was effective in enhancing the optical CO sensitivity in the resulting Au-NiO composite film (type 1). In the case of the Au-COjO film (Ando et al. 

Source Reprinted with permission from Ando et al. (2003), Copyright 2003 Elsevier 

1997b), the enhancement (type 1) in the optical sensitivity for CO and was found and, furthermore, the changes of plasmon absorption of small Au particles (type 2) appeared for but not for CO. This selectivity created a function to recognize CO and in the Au-COjO film. The Au-WOj composite film (Ando et al. 2001) showed H -sensitive plasmon absorption change (type 2). CO- and Hj-sensitive plasmon absorption change was also observed for Au-CuO composite film (Ando et al. 1997a, 2003) (type 2). 

Comparison of the CO-sensing performance of Au-transition metal oxide composite films carried out by Ando et al. (2003) has shown that the Au-CuO composite film shows better sensitivity and resolution than the Au-NiO (Kobayashi et al. 1993 Ando et al. 1994,1996) and the Au-COjO (Ando 

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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. 

In addition, we have to take into account that metal oxide-based nanocomposites have specific structure. Research on such a two-phase system, in which the concentration of the second oxide phase is much less than the concentration of the base oxide, has shown that the second phase, as a rule, is finely dispersed on the surface of the base oxide grains (Szezuko et al. 2001 Pagnier et al. 2000 Carreno et al. 2002). Possible versions of segregation layers of foreign cations on the surface of SnOj grains are shown in Fig. 14.4 (Varela et al. 1999 Carreno et al. 2002). It has been established... 

Table 14.3 presents effects which can be achieved in metal oxide-based composites. It is seen that the use of nanocomposites in gas sensors really can produce great improvements in sensor parameters. 

Nanocomposites based on other nanofillers like metal oxides, hydroxides, and carbonates... 

Recently, several metal oxides apart from silica have been investigated and reported for mbber-based nanocomposites. Some important and commercially meaningful oxides used in rubber are zinc oxide (ZnO), magnesium hydroxide (MH), calcium carbonate, zirconate, iron oxide, etc. 

The wide assortment of polymer systans (polypropylene, poly(methyl methacrylate) [PMMA], polyepoxide, polystyrol, PC, etc.) is used as a polymeric matrix for nanocomposites production (Ray and Okamoto 2003). The most well-known fillers of polymeric matrix are nanoparticles (silica, metal, and other organic and inorganic particles), layered materials (graphite, layered aluminosilicates, and other layered minerals), and fibrous materials (nanofibers and nanotubes) (Thostenson et al. 2005). Nanocomposite polymer materials containing metal or metal oxide particles attract growing interest due to their specific combination of physical and electric properties (Rozenberg and Tenne 2008, Zezin et al. 2010). Nanocomposites on the base of layered materials... 

It is the aim of this chapter to give an overview on both chemical and electrochemical techniques for producing metallic-particle-based CP nanocomposite materials and to outline the progress made in this field. The various synthetic approaches are organized in such a way as to present first those involving metal particle deposition in the course of polymerization, and subsequently post-polymerisation procedures that involve chemical, electrochemical, or adsorption processes (Figure 7.1). Well-established approaches, along with some newly developed techniques will be discussed, with special emphasis on those that are still underdeveloped. Synthesis of metal oxide particle-based CP composites (see e.g. [8]), as well as modification of CPs with transition-metal complexes (see e.g. [5]) remain outside the scope of this chapter. 

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