Surface Functionalizing of Carbon-Based Gas-Sensing Materials
Surface functionalizing of carbon nanotubes and other carbon-based nanomaterials is discussed in present chapter. In particular, in chapter one can find a description of approaches and modifiers typically used for surface functionalizing of carbon-based n
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Surface Functionalizing of Carbon-Based Gas-Sensing Materials
25.1
Surface Functionalizing of Carbon Nanotubes and Other Carbon-Based Nanomaterials
As indicated in Chap. 1 (Vol. 2), in general, CNTs do not have sensing response to all gases and vapors but only those with high adsorption energy or that can interact with them. Pristine CNT-based gas sensors are currently limited to sensing gases such as NH3, NO2, SO2, O2, and NO. Gas molecules such as toxic gases (CO) and water, however, cannot be detected since they do not react with (adsorb on) the surface of pure carbon SWCNTs (Terrones et al. 2004). For H2 detection, bare CNT does not exhibit appreciable sensitivity as well. Experimental and theoretical studies established that the sensitivity of CNT sensors can be improved by doping the CNTs (Zhang and Zhang 2009). It was found that if the surface of the tube is doped with a donor or an acceptor, drastic changes in the electronic properties are observed. The N-doped nanotubes, for example, exhibit a higher reactivity toward reactants when compared to un-doped tubes due to the introduction of nitrogen species and the structural irregularity of carbon hexagonal rings (see Fig. 25.1). The N substitution reactions are also able to create radicals over nanotube surfaces, which can react with suitable reactants. As a result of the binding of the molecules to the doped locations—because of the presence of holes (B-doped tubes) or donors (N-doped tubes), the surfaces became more reactive and sensitive to surrounding gas. Using first-principle calculations, Peng and Cho (2003) demonstrated that B- or N-doped SWCNT-based sensors can detect CO and water molecules, and, more importantly, the response of these sensors can be controlled by adjusting the doping level of heteroatoms in a nanotube. It is also important to point out that, in spite of high sensitivity, CNT-based sensors are nonselective, which limits their use for sensing purposes in real samples. This means that mechanisms need to be developed to increase the selectivity of the detectors. As shown before, surface functionalizing is the most effective methods suitable for these purposes. At present there are two main approaches for the surface functionalizing of CNTs: covalent functionalization and non-covalent functionalization, depending on the types of linkages of the functional entities onto the nanotubes (Zhang et al. 2008). Several mechanisms of covalent and non-covalent modification of CNTs are illustrated in Fig. 25.2. It should be noted that altering the nanotube surface, besides influencing gas-sensing properties, strongly affects solubility properties, which can affect the ease of fabrication of CNT sensors. The reviews by Hirsh (2002), Balasubramanian and Burghard (2005), Shen et al. (2008), and Zhang and Zhang (2009) discuss many of the functionalizations that have commonly been used. Currently, most covalently functionalized CNTs are based on esterification or amidation of carboxylic acid groups that are introduced on defect sites of the CNTs duri
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