A6.3 Carbon Nanotubes
These are cylindrical structures containing only carbon atoms, arranged in hexagons with a structure similar to that in one layer of graphite (with
$\textsf{sp}{^\textsf{2}}$
hybridised carbon atoms) rolled into a cylinder.
They can be either single-walled or, double-walled, with one tube inside another.
They have very low density (around
$\textsf{1.4 g }\textsf{cm}{^\textsf{-3}}$
) and they are the strongest and stiffest materials discovered to date.
A single fibre with the same cross section area of a human hair would support over 200kg with its specific strength being over 300 times that of high carbon steel.
The strength arises from their
$\textsf{sp}{^\textsf{2}}$
pi-bonding.
Added in small amounts, they find applications as additives to give strength to the materials used in making car parts and sports goods (e.g. baseball bats, golf clubs) Several methods are used to synthesis carbon nanotubes including:

## 1. Arc discharge

Carbon nanotubes can be made by using an electric arc discharge between two graphite electrodes, suspended in an inert gas, like helium. An inert gas is used, and oxygen is excluded, to prevent oxidation of the carbon atoms. The carbon in the negative electrode (cathode) sublimes and carbon nanotubes are formed on the positive electrode (anode). If the anode contains a metal catalyst, and a pure graphite cathode is used, single walled carbon nanotubes are formed. Alternatively an electric arc can be formed between metal electrodes suspended in a hydrocarbon solvent. The solvent is decomposed and the carbon atoms form carbon nanotubes form on the anode.

## 2. Chemical vapour disposition (CVD)

CVD is widely used to produce carbon nanotubes. This method involves a chemical reaction between a suitable surface and a gas. The reaction occurs on an inert surface on which a layer of metal catalyst nanoparticles have been deposited. The metal is most commonly nickel, cobalt, or iron. After heated to approximately 700°C gases are introduced to the reactor including a carbon-containing gas like methane or ethane. As the carbon-containing gas is broken apart, carbon nanotubes grow on the metal catalyst’s surface.

## 3. High pressure carbon monoxide (HIPCO)

Carbon monoxide (
$\textsf{CO}$
) and iron pentacarbonyl,
$\textsf{Fe(CO)}_\textsf5$
, are heated together at about 1000°C and at a very high level of pressure. The
$\textsf{Fe(CO)}_\textsf5$
breaks apart leaving free iron atoms and the carbon monoxide reacts to release carbon atoms which assemble on the iron atoms as nanotubes. The reaction which releases free carbon atoms is
$\textsf{2CO} \rightarrow \textsf{CO}_\textsf2 \textsf{ + C}$
.