Carbon Nanomaterials: Carbon Nanotubes, Graphene, and Carbon Nanofibers

Introduction

Materials have always had an important place in the improvement of societies since the beginning. The prehistoric ages were named according to the commonly used material such as; stone, bronze and iron ages which reflects the importance of the materials in the development of civilizations. When we look at the today’s technological developments, composite materials gain a huge impact due to their unique and combined properties in the same product. As represented in Figure 1 our century is called the Nano age due to the developments on the engineered nano materials. Not only their neat forms but also their cooperation with the previously improved polymeric matrixes take attention and have a pivotal role in the material science.

Figure 1.

The materials time-line with a nonlinear scale

When Richard Feynman emphasized the importance of nanoparticles by saying “There’s plenty of room at the bottom.” nanomaterials start to draw attention. However, characterization tools had not been powerful enough to examine the submicron systems, thus; technological developments needed to wait until they let the examine nano sized materials. Nano materials have distinctively different properties than their bulk forms in terms of surface area and dominance of quantum effects, while increased surface area play a role on the improved chemical reactivity of the material, quantum effects are resultant with the change a material’s optical, magnetic or electrical properties (Britain, 2004). Thus, polymer composites with nanoparticle addition display different role than the bulk size of polymeric composites. Total investment in the nanotechnology is getting rise day by day due to the proliferation product range (Navarro et al., 2008).

Carbon is not the most abundant element in the world however, its role is fundamental and Earth’s 0.2% of mass consists of carbon (Delhaes, 2013).

Figure 2.

(A) Graphene, (B) CNT, (C)Fullerene, (D)Carbon Nanofiber

Different carbon atom bonding causes to obtain allotropes of carbon which are comprised of unique properties. Most well-known carbon allotropes are; diamond and graphite (Figure 2). Besides, graphene, fullerenes, carbon nanotubes are the new generation of carbon allotropes (Krüger, 2010; Delhaes, 2013). The ideal crystal structure of graphite (see Figure) consists of layers in which the carbon atoms are arranged in an open honeycomb network containing two atoms per unit cell in each layer, a carbon nanotube is a graphene sheet appropriately rolled into a cylinder of nanometer size diameter. Therefore, it is expected to the planar sp² bonding is characteristic of graphite to play a significant role in carbon nanotubes (Ajayan, 1999). Each carbon atom has six electrons which occupy 1s², 2s², and 2p² atomic orbitals. Three possible hybridizations occur in carbon: sp, sp², and sp³ while other group IV elements such as Si and Ge exhibit primarily sp³ hybridization (Dresselhaus & Avouris, 2001). The discovery more than two decades ago of the allotrope of carbon referred to Buckminsterfullerene which was a pivotal event at the beginning of nanotechnology. This key discovery showed that; how intense energy sources may be used to vaporize ordinary carbon and convert it into the nanomaterials such as fullerenes and carbon nanotubes (Weisman & Subramoney, 2006). Multi-walled carbon nanotube variants were synthesized initially in 1991 (Iijima, 1991), while graphene is the unwrapped form of the carbon nanotubes.