A challenging part of many experiments is the growth of carbon nanotubes. At present there is no possibility to control the structure of nanotubes and all fabrication methods yield mixtures of nanotubes with different lengths, helicities and diameters. In addition, nanotubes can be further modified during post-growth treatments such as purification or annealing.

 

Arc discharge

The arc discharge was the first available method for the production of both MWNTs and SWNTs . It is worth noting that this technique has been in use for a long time for the production of carbon fibers and that it is very probable that nanotubes were observed before 1991 but not recognized as such (follow, e.g., this link) .

MWNTs can be produced in a carbon arc apparatus similar to the one depicted below using the method described by Ebbesen and Ajayan. An arc is struck between two graphite electrodes in a gas atmosphere. MWNTs produced by arc discharge are long and straight tubes closed at both ends with graphitic walls running parallel to the tube axis.

Iijima et al. and Bethune et al. reported in 1993 that an arc discharge with a cathode containing metal catalysts (such as cobalt, iron or nickel) mixed to graphite powder results in a deposit containing SWNTs. SWNTs are usually assembled in ropes but some single tubes can also be found in the deposits.

Schematics of an arc-discharge apparatus, along with electron microscopy pictures of the products obtained with doped and pure anodes.

 

Laser ablation

Another method to grow SWNTs using laser ablation was demonstrated in 1996 by Smalley's group and has prompted a lot of interest. Thess et al. showed that the synthesis could be carried out in a horizontal flow tube under a flow of inert gas at controlled pressure. In this set-up the flow tube is heated to ~1200°C by a tube furnace. Laser pulses enter the tube and strike a target consisting of a mixture of graphite and a metal catalyst such as Co or Ni. SWNTs condense from the laser vaporization plume and are deposited on a collector outside the furnace zone.

Schematics of a laser ablation set-up, reproduced from 
B. I. Yakobson and R.E. Smalley, American Scientist 85, 324 (1997).

 Catalytic growth

An alternative to the arc discharge and laser ablation methods is the catalytic growth of nanotubes. This method is based on the decomposition of a hydrocarbon gas over a transition metal to grow nanotubes in a chemical vapor deposition (CVD) reactor such as the one shown below.

Schematics of a CVD deposition oven.

Carbon filaments and fibers have been produced by thermal decomposition of hydrocarbons since the 1960s. Usually, a catalyst is necessary to promote the growth. A similar approach was used for the first time in 1993 by Yacaman et al. to grow MWNTs from the decomposition of acetylene over iron particles.

For the production of MWNTs acetylene is usually used as source of carbon atoms at temperatures typically between 600 – 800°C. To grow SWNTs the temperature has to be significantly higher (900 – 1200°C) due to the fact that they have a higher energy of formation. In this case carbon monoxide or methane must be used because of their increased stability at higher temperatures as compared to acetylene.