Carbon nanotubes are made every time you light a match. The problem is that there are so few of them and they are mixed in with other forms of carbon so they just aren’t useful. The focus of carbon nanotube synthesis is in making a considerable quantity of pure nanotubes with minimal contamination. It takes a great deal of effort with highly specialized equipment to ensure that the tubes are the same width and length. As a result, most high volume synthesis methods do not provide much control over tube diameter or length.
This technique involves biasing a sharp carbon rod to many thousand volts. The voltage is then discharged from the rod, rapidly heating the rod and vaporizing some of the carbon. This carbon vapor is allowed to cool, and as it does CNTs are produced. The arc discharge method generally involves the use of a vacuum chamber and an inert gas supply. Even single-walled nanotubes can be produced if the proper metal ions are introduced. When optimized, this method can turn roughly 30% of the carbon into carbon nanotubes.
Arc Discharge schematic for CNT synthesis
Laser ablation works through the same mechanism as arc discharge, but instead of producing heat through electrical discharge a laser is used. A focused and powerful laser is used to rapidly heat carbon and vaporize it. The additional benefit of this is that the temperature and pulse times can be precisely controlled. Because of these strict controls the parameters of the CNTs can be finely tuned and up to 70% of the carbon can be turned into CNTs.
Laser ablation schematic for CNT synthesis
Chemical Vapor Deposition
The most popular and simplest way to grow carbon nanotubes in the laboratory is to use chemical vapor deposition (CVD). A CVD system for CNT growth injects a vaporized hydrocarbon compound (methane or ethane are common) into a high temperature zone in a furnace. The hot zone contains a substrate on which has been pre-deposited a thin film of iron, nickel or cobalt that has either separated or been pre-patterned into nanoscale islands of the metal. These nanoscale islands catalyze the growth of the carbon nanotubes. The catalyst is the key to the whole process and careful attention must be given to its deposition. Both single and multi-walled CNTs can be produced via CVD.
Chemical Vapor Deposition (CVD) schematic for CNT synthesis
A variant of CVD developed by scientists at the NASA Glenn Research Center, and embodied by the Nanotech Innovations SSP354 reactor involves injecting a liquid precursor that already contains both the carbon and catalyst atoms in it into a two temperature zone reactor tube. The first zone vaporizes the liquid and the iron and carbon atoms are swept into a second, higher temperature zone where the growth occurs. The iron atoms from the vapor congregate into nanoparticle islands and the CNT growth takes place from those. This allows for the entire inside of the hot zone of the reactor tube to be used as a growth surface, both simplifying the process and greatly increasing yield. Since this technique eliminates the need for pre-patterned catalyst deposition prior to carbon atom injection by using a single compound that contains both atoms it is often called a “single source precursor” method. It saves both time and money when making multi-walled nanotubes. At present the single source precursor method is used to produce only multi-walled tubes. Work is underway to extend the technique to include growth of single-walled nanotubes.