Carbon Nanotubes Synthesis: The Four Methods To Achieve It

Carbon nanotube synthesis is a vital process. It is significant, as carbon nanotubes (CNTs) have also attracted a wide interest worldwide. Carbon nanotube synthesis can be done using various methods, of which the most commonly employed ones are:

  • Arc Discharge method
  • Laser Ablation method
  • Catalytic Chemical Vapour Deposition method (CCVD)
  • High Pressure Carbon Monoxide method (HiPco)

Majority of these carbon nanotube synthesis processes take place with the help of process gases, or inside a vacuum. Furthermore, large quantities of carbon nanotubes can also be synthesised using these techniques.

The synthesis gains prominence, as nanotubes are known to have the simplest chemical composition with atomic bonding configuration. However, they exhibit extreme diversity and richness among all nano-materials in terms of structures and structure - property relations.

Let's take a look at the various ways of achieving carbon nanotube synthesis with effectiveness and good results.

Carbon nanotube synthesis using Arc Discharge method

This is the same method of carbon nanotube synthesis that was employed by Sumio Iijima, for production of multi-walled carbon nanotubes, back in 1991. In the Arc Discharge method, a DC arc plasma is ignited between the two graphite electrodes, under an inert atmosphere (created using some inert gas like argon or helium), at a low pressure ranging between 50 to 700 mbar.

The anode is brought closer to the cathode (almost 1mm) until an arc appears, which takes around a minute. A soft fibrous deposit consisting of multi walled carbon nanotubes apart from other carbon particles gets formed on the cathode.

Single-wall carbon nanotubes can also be obtained by using an appropriate catalyst like Ni-Y, Co-Y or Ni-Co on the electrodes. The carbon nanotubes produced using the Arc Discharge method are normally tangled and are of different diameters and lengths. But they're of good quality and less defects.

Carbon nanotube synthesis using the laser ablation method

This method of carbon nanotube synthesis was first employed for production of fullerenes. In the laser ablation method, a block of graphite is vapourised in a flowing inert atmosphere (using argon gas) through the laser irradiation. The process happens at extremely high temperature of around 1200°C. Such vaporisation leads to the creation of carbon species that get swept away by the gas and get accumulated on a water-cooled collector.

The graphite target can be doped with small quantities of transition metals like Co and Ni, leading to creation of single-wall carbon nanotubes. The SWNTs produced are highly uniform in terms of their diameter.

Carbon nanotube synthesis through CCVD

CCVD or catalytic chemical vapour deposition has been in use for production of carbon fibres and filaments since the 60s.

This carbon nanotubes synthesis method involves decomposition of a carbon-containing gas (through chemical breakdown of a hydrocarbon) on a substrate, at high temperatures of around 600 o C or more, with the help of catalyst metal particles.

The comparatively lower temperatures employed in CVD bring down the production costs, and the catalysts' deposition on the substrate facilitates the formation of new structures. However, the carbon nanotubes produced using this method usually have plenty of defects.

Carbon nanotube synthesis using HiPco method

HiPco or high-pressure conversion of carbon monoxide method is used specifically for the production of single-wall carbon nanotubes.

In this carbon nanotubes synthesis method, the catalyst precursor iron pentacarbonyl (Fe(CO)5)and carbon monoxide (CO) gas (referred to as the carbon feedstock) are passed through a well-heated reactor at a temperature of around 800°C 1200°C. The produced single-wall carbon nanotubes are easily the thinnest as they have an average diameter of around 1.1 nm. Their quality is also pretty high, with hardly any structural defects and high intrinsic selectivity.

Talking about their purity, the synthesis product yield is around 97% and the purification yield around 90%.

Regardless of the progress made by the scientific community, carbon nanotube synthesis at low cost, high purity and uniformity, and all that at a large scale continues to be a major challenge for the industry even today. But these methods assure us that in the future, more methods cost-friendly methods will be introduced to make the process more industry-conducive.