Carbon nanotubes
are cylindrical structured wrapped graphene sheets. They may be single walled
carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). SWCNTs
are cylinders of single graphene sheets and usually there are only 10 atoms in
the circumference of single walled carbon nanotubes.A nanotube can be
composed of millions of atoms and is considered as single molecule. Due to very
high length to diameter ratio (~1000) nanotubes are considered as
one-dimensional molecule.MWCNTs
are the cylinders of multiple graphene sheets concentric about a common axis
having separation almost 0.34 nm with increasing diameters. The name
multiwalled carbon nanotube is specifically used for nanotubes having diameter
less than 15 nm. Multi walled carbon nanotubes which have diameter 1-2 nm are
usually curved instead of straight structure (Fig. 1.1a, c) (Balasubramanian et
al., 2005). Double wall carbon nanotube (DWCNT) also exists, but in
recent times among all types of carbon nanotubes the MWCNT received the great
attention due their excellent promising features.
TEM image of SWCNTs TEM image of MWCNTs
Schematic Molecular Representation of SWCNTs and MWCNTs
Carbon nanotubes synthesis techniques
There are
different techniques have been introduced for the synthesis of MWCNTs and
SWCNTs at laboratory levelwith
different morphology and structures. But three methods have been widely used
for synthesis of CNTs: chemical vapor deposition (CVD)(Su at al., 2000), arc
discharge (Journetat al., 1997)and laser ablation (Thess at al., 1996). Sufficient
amount of energy, source of carbon and catalyst are basic elements which are
required for the synthesis of CNTs. Similarity among these three methods
is to provide energy to the source of carbon which results in the formation of
fragments (single carbon atom or groups carbon atoms). Recombination of these
fragments results in the formation CNTs. The source of energy may be
high-intensity light from a laser (laser ablation), electricity for arc
discharge and heat from a furnace (∼900∘C) for CVD.
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