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Metal-Based Nanostructures

Through the study of nanoporous Pd films described in Chap. 4 (Vol. 1), it was demonstrated that the H2 detection limit and response time could be improved in nanoporous structures with increased surface area and decreased distance for bulk diffusion. Taking into account mentioned above, one can conclude that films from metal nanoparticles and metal nanowires would be ideal structures for fast detection of low gas concentrations. Experiment has shown that this assumption is valid and noble metal nanoparticles can be successfully incorporated into gas sensors. The selection of noble metals such as Au and Pt for gas sensor fabrication is based on their chemical inertness (Dovgolevsky et al. 2009). It was established that, compared to conventional metal oxide chemiresistors, MNP-based devices have the advantage that they can be operated at room temperature or slightly above, which enables easy device integration and low-power operation (Joseph et al. 2008; Saha et al. 2012).

4.1 4.1.1

Metal Nanoparticles Properties

Metal nanoparticles (MNPs) are discrete clusters of a finite number of atoms, generally in the range of 1–100 nm in size. For example, an Au NP 5.2 nm in diameter consists of 2,951 atoms. It is known that the surface area of nanocrystals increases markedly with the decrease in size (Rao et al. 2002). Thus, a small metal nanocrystal 1 nm in diameter will have 100 % of its atoms on the surface. A nanocrystal 10 nm in diameter, on the other hand, would have about 15 % of its atoms on the surface. A small nanocrystal with a higher surface area would therefore be expected to be more reactive (Rao et al. 2005). Furthermore, the qualitative change in the electronic structure arising due to quantum confinement in small nanocrystals will also bestow unusual adsorption and catalytic properties on these particles, totally different from those of the bulk metal (Daniel and Astruc 2004). For example, a low-temperature study of the interaction of elemental O2 with Ag nanocrystals of various sizes has revealed the ability of smaller nanocrystals to dissociate dioxygen to atomic oxygen species (Rao et al. 1992). On bulk Ag, the adsorbed oxygen species at 80 K is predominantly in the form of O2−. The ability of Cu, Pd, Pt, and Ni nanoparticles to absorb CO at increased temperatures has been thoroughly investigated as well (Matolin et al. 1990). Carbon monoxide from a bulk Cu surface desorbs above 250 K. Small Cu particles, however, retain CO up to much higher temperatures (Santra et al. 1994). A similar observation has been made in the case of Pd particles (Gillet et al. 1986). The results obtained with Ni particles are more interesting. In addition to showing a trend similar to the above, G. Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, DOI 10.1007/978-1-4614-7388-6_4, © Springer Science+Business Media New York 2014

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Metal-Based Nanostructures

Fig. 4.1 (a) Total number of catalytically produced CO2 molecules as a function of cluster size. (b) Total