Graphene-fullerene hybrids

Carbon nanotubes (CNTs) constitute a nanostructured carbon material that consists of rolled up layers of sp2 hybridized carbon atoms forming a honeycomb lattice. After diamond, graphite and fullerenes, the one-dimensional tubular structure of CNTs is considered the 4th allotrope of carbon (graphene is the 5th). 

The structure of CNTs can be understood as sheets of graphene (i.e. monolayers of sp2 hybridized carbon, see Chapter 2) rolled-up into concentric cylinders. This results in the saturation of part of the dangling bonds of graphene and thus in a decrease of potential energy, which counterbalances strain energy induced by curvature and thus stabilizes the CNTs. Further stabilization can be achieved by saturating the dangling bonds at the tips of the tubes so that in most cases CNTs are terminated by fullerene caps. Consequently, the smallest stable fullerene, i.e. C60, which is - 0.7 nm in diameter, thus determines the diameter of the smallest CNT. The fullerene caps can be opened by chemical and heat treatment, as described in Section 1.5. 

Although there have been great advances in covalent functionalization of fullerenes to obtain surface-modified fullerene derivatives or fullerene polymers, the application of these compounds in composites still remains unexplored, basically because of the low availability of these compounds [132]. However, until now, modified fullerene derivatives have been used to prepare composites with different polymers, including acrylic [133,134] or vinyl polymers [135], polystyrene [136], polyethylene [137], and polyimide [138,139], amongst others. These composite materials have found applications especially in the field of optoelectronics [140] in which the most important applications of the fullerene-polymer composites have been in the field of photovoltaic and optical-limiting materials [141]. The methods to covalently functionalize fullerenes and their application for composites or hybrid materials are very well established and they have set the foundations that later were applied to the covalent functionalization of other carbon nanostructures including CNTs and graphene. 

CNTs (Oberlin and Endo, 1976 Wiles and Abrahamson, 1978 lijima, 1991 Kavan et al., 2001 Yang et al., 2005) and carbon nanoribbons (CNRs) (Novoselov et al., 2004) are graphene-type nanostructured materials having in common a fiber-type structure (Figure 7.4). In CNTs, s T-hybridized carbon atoms are arranged in graphite-type sheets building up seamless hollow tubes capped by fullerene-type (yide infrd) hemispheres. 

Carbon exists in different allotropic forms, viz. Fullerene (OD, a zerodimensional carbon), carbon nanotube (ID, a one-dimensional carbon), graphene (2D, a two-dimensional carbon), graphite (3D, a three-dimensional carbon) and diamond (3D, a three-dimensional carbon). All the above forms except diamond (which bears sp hybridized carbons) have a structural similarity having sp hybridized carbon. 

Carbon nanostructures (Fig. 6.2) comprise a variety of materials [42] based mainly on carbon atoms with sp hybridization. The most widespread in the field of electroanalysis are CNTs [1,25,29, 30,43,44] and graphene [1, 7,9,12,21,22, 45 9], although applications involving fullerene [50] and carbon black [51] have also been reported. Both CNTs and graphene are graphite derivatives, consisting of rolled or planar sheets, respectively. Both single- and multi-wall CNTs have been... 

Fig. 6.13 Tentative classification of bi-component hybrids most commonly reported in electro-analytical applications. A alloy and core shell metal structures, B nanoparticles (NPs) and carbon nanotubes (CNTs) encapsulated by a thin polymeric layer, C NPs grafted on the surface of CNTs and graphene, D mixture of NPs, E fullerenes included in polymeric matrices, F NPs and CNTs in polymeric matrices (Reproduced fi om Ref [169] with the permission of Springer)...

Some of the most important routes of graphene surface functionalizing, which are not based on surface-bound oxygen and carbonyl moieties, are depicted in Fig. 25.7. It is important that most sp -hybridized carbon scaffolds (including carbon nanotubes, fullerenes, and graphitic carbon shells) are amenable to these reactions. 

As fibers, CNTs can be considered one-dimensional molecules. Their radius virtually reduces to zero, whereas their lengths are typically on the order of micrometers. A useful simplification describes CNTs as hybrids of two other low-dimensional molecules graphene sheets (2D) are rolled up into tubes and capped at either end with fullerene (OD) hemispheres. It is worth considering some of the properties of these materials in detail to better understand certain properties of CNTs. 

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