Synthesis of CNTs by HiPco and LASER Ablation

What are Carbon Nanotubes? Carbon nanotubes are hexagonally shaped arrangements of carbon atoms that have been rolled into tubes. These tiny straw-like cylinders of pure carbon have useful electrical properties. They have already been used to make tiny transistors and one-dimensional copper wire. 2

Nanotubes have been constructed with length- to-diameter ratio of up to 132,000,000:1,[1] which is significantly larger than any other material. These cylindricalcarbon molecules have novel properties that make them potentially useful in many applications. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors.

Synthesis Techniques have been developed to produce nanotubes in sizeable quantities,some of them are:- Arc discharge Laser ablation Chemical vapor deposition (CVD) HIPCO

High Pressure Carbon Monoxide (HiPco) Developed at Rice University in 1999 Unlike other methods in which the metal catalysts are deposited or embedded on the substrate before the deposition of the carbon begins, in this method catalyst is introduced in gas phase.

Both the catalyst and the hydrocarbon gas are fed into a furnace, followed by catalytic reaction in the gas phase. This method is suitable for large-scale synthesis, because the nanotubes are free from catalytic sup-ports and the reaction can be operated continuously. Usually CO gas is used as hydrocarbon gas which reacts with iron pentacarbonyl, Fe(CO)5 to form SWNT

This process is called HiPco process. SWNT have also been synthesized in a variant of HiPco process in which a mixture of benzene and ferrocene, Fe(C5H5)2 reacts in a hydrogen gas flow to form SWNT . In both methods, catalyst nanoparticles are formed through thermal de-composition of organometallic compounds, such as iron pentacarbonyl and ferrocene.

It produces continuously single-walled carbon nanotubes at a rate of 0 It produces continuously single-walled carbon nanotubes at a rate of 0.45 g/h. The purity of the created product or the synthesis product yield is as high as 97% and the purification yield is 90%.

HiPco process

Advantages It is a continuous process. HiPco SWNTs are extremely clean as- grown. All of the other techniques lack one or both of these qualities making them much less economical as a large-scale process.

Thinnest single-walled carbon nanotubes of very high quality. Few structural defects. High intrinsic selectivity were achieved.

Laser ablation In the laser ablation process, a pulsed laser vaporizes a graphite target in a high- temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses.

A water-cooled surface may be included in the system to collect the nanotubes. The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, it is more expensive than either arc discharge or chemical vapor deposition

Tubes grow in this method on catalysts atoms and continue to grow until to many catalyst atoms aggregate at the end of the tube. The tubes produced by this method are in the form of mat of ropes 10 - 20 nm in diameter and up to 100 micron or more in length. .

By varying temperature, catalyst composition and other process parameters average diameter and length of carbon nanotube could be varied

Advanatages Simplicity of the procedure. Sharp well defined patterns. Absence of chemical reagents in solution. It has appeared to be the most flexible and promising technique because of its ability to ablate almost all kinds of material.

Disadvantages 1) This is energy extensive methodsa large amount of energy is needed to produce laser used for ablation processes. Such a huge amount of energy is not only impossible but also uneconomical for large scale production. This method requires solid carbon/graphite as target which has to be evaporated to get nanotubes

It is difficult to get such large graphite to be used as target in industrial process which limits its exploitation as large scale process. This proess grow nanotubes in highly tangled form, mixed with unwanted form of carbon or catlysts. Thus CNTs produced by this process requires purifi-cation to get purified and assembled forms.

The designing of such refining process is difficult and expensive. All the above mentioned factors severely limit the use of laser ablation as large scale processes for production of carbon nanotubes.