Nitrogen-Containing Carbon Nanotubes. A Theoretical Approach

Abstract

Carbon nanotubes are important materials for a variety of scientific and technological applications due to their unique properties (Frank et al., 1998; Ganji, 2008; Hone et al., 2000; Marulanda, 2010; Yu et al., 2000). Of course, their properties depend on their structure. For instance, nanotube conductivity depends on chirality, diameter, and length (Alam & Ray, 2007; Hamada et al., 1992; Saito et al., 1992; S.H. Yang et al, 2008). The purity of the nanotubes and the presence of defects also affect their conductivity. Chirality or helicity refers to the way the nanostructure arises by the folding of a graphene sheet. Nanotube chirality is usually characterised by two integers, n and m, known as Hamada indices, defining three classes of nanotubes. For instance, armchair (n,n) nanotubes exhibit metallic behaviour, zigzag (n,0) nanotubes are semiconductors, and chiral (n,m) nanotubes exhibit metallic behaviour if the difference (n m) is a multiple of 3 and semiconductor behaviour otherwise (Charlier, 2002). For instance, a (7,1) chiral nanotube is a conductor but the chiral (7,3) nanotube is not. Within the numerous potential applications imagined for carbon nanotubes, hydrogen storage represents the most promising application capable of making a safe, efficient and “green” contribution to fuel cells with hydrogen management in the solid state. The principal hydrogen-adsorption mechanisms associated with nanotube hydrogen uptake are the physisorption and chemisorption of hydrogen. During physisorption, hydrogen interacts with selected sites of a carbon nanotube or substrate. The interaction energy increases as the substrate polarisability increases. Densityfunctional theory calculations indicate that nanotube-hydrogen interactions are weak and that hydrogen diffusion from the nanotube is facilitated by slightly increasing temperature (Mpourmpakis et al., 2006). The hydrogen-binding energies, calculated using densityfunctional theory are small and similar for metallic and semiconducting nanotubes, indicating that substantial adsorption is only possible at very low temperatures (Cabria et al., 2006). The same conclusion is reached by studying hydrogen adsorption in carbonnanotube arrays through molecular dynamic simulation (Kovalev et al., 2011), in which a second adsorption layer is detected at 80 oK. This second layer of hydrogen is not detected at room temperature. Through chemisorption, hydrogen is covalently bonded to carbon atoms in such a way that a change of sp2 to sp3 carbon hybridisation occurs, which is manifested in the C—C bond length values. A typical (sp3)C—C(sp3) bond length is 1.54 A. The calculated C—C bond