Carbon nanotubes (CNT)

CARBON NANOTUBES (CNT)
Carbon nanotubes are allotropes of carbon with a nanostructure having a length-to-diameter ratio greater than 1,000,000. When graphite sheets rolled into a cylinder their edges joined and form carbon nanotubes. Carbon nanotubes are extended tubes of rolled graphite sheets.
Nanotubes naturally align into ropes and held together by Vander Waals forces. But each carbon atoms in the carbon nanotubes are linked by the covalent bond.

Structure (or) types of carbon nanotubes
Carbon nanotubes are lattice of carbon atoms, in which each carbon is bonded
to three other carbon atoms.
Depending upon the way in which graphite sheets are rolled two types of CNTs are formed.

1. Single - walled nanotubes (SWNTs)
2. Multi – walled nanotubes (MWNTs)

Single – walled nanotubes (SWNTs)
SWNTs consist of one layer of graphite. It is one-atom thick having a diameter of 2 nm and length of 100 um. SWNTs are very important because they exhibit important electrical properties. It is an excellent conductor.
Three kinds of nanotubes are resulted based on the orientation hexagon lattice.

(i) Armchair structures : The lines of hexagons are parallel to the axis of the nanotube.
(ii) Zig-zag structures: The lines of carbon bonds are down the centre.
(iii) Chiral nanotubes: It exhibits twist or spiral around the nanotubes.
It has been confirmed that armchair carbon nanotubes are metallic while zig- zag and chiral nanotubes are semiconducting.

2. Multi-walled nanotubes (MWNTs)
MWNTs (nested nanotubes) consist of multiple layers of graphite rolled on themselves to from a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.

Synthesis of carbon nanotubes
Carbon nanotubes can be synthesized by any one of the following methods.

1. Pyrolysis of hydrocarbons.
2. Laser evaporation.
3. Carbon arc method.
4. Chemical vapour deposition

Pyrolysis
Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 700 deg C in the presence of Fe-silica or Fe-graphite under inert conditions.

Laser evaporation
It involves vapourization of graphite target containing small amount of cobalt and nickel by exposing it to an intense pulsed laser beam at higher temperature (1200 deg C) in a quarz tube reactor . An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector nanotubes.

Carbon arc method
It is carried out by applying direct current (60-100 A and 20-25 V ) arc between graphite electrode of 10-20 um diameter.

Chemical vapour deposition
It involves decomposition of vapour of hydrocarbons such as methane acetylene ethylene, etc. at high temperatures (100 deg C) in presence of metal nanoparticle catalysts like nickel cobalt iron supported on MgO or Al2O3.