Synthesis of carbon nanotubes
Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.
[1] Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes.
[4] Arc-discharge technique uses higher temperatures (above 1,700 °C) for CNT synthesis which typically causes the expansion of CNTs with fewer structural defects in comparison with other methods.
[6] Later that year the team used a composite of graphite and metal catalyst particles (the best yield was from a cobalt and nickel mixture) to synthesize single-walled carbon nanotubes.
[7] The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature.
[4] Single-walled carbon nanotubes can also be synthesized by a thermal plasma method, first invented in 2000 at INRS (Institut national de la recherche scientifique) in Varennes, Canada, by Olivier Smiljanic.
In this method, the aim is to reproduce the conditions prevailing in the arc discharge and laser ablation approaches, but a carbon-containing gas is used instead of graphite vapors to supply the necessary carbon.
[10] The method is similar to arc discharge in that both use ionized gas to reach the high temperature necessary to vaporize carbon-containing substances and the metal catalysts necessary for the ensuing nanotube growth.
[14] During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt,[15] iron, or a combination.
[18] Thermal catalytic decomposition of hydrocarbon has become an active area of research and can be a promising route for the bulk production of CNTs.
One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes.
Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.
[26] Researchers at Rice University, until recently led by the late Richard Smalley, have concentrated on finding methods to produce large, pure amounts of particular types of nanotubes.
[27] Super-growth CVD (water-assisted chemical vapor deposition) was developed by Kenji Hata, Sumio Iijima and co-workers at AIST, Japan.
For comparison, the as-grown HiPco CNTs contain about 5–35%[31] of metal impurities; it is therefore purified through dispersion and centrifugation that damages the nanotubes.
The zipping effect is caused by the surface tension of the solvent and the van der Waals forces between the carbon nanotubes.
It aligns the nanotubes into a dense material, which can be formed in various shapes, such as sheets and bars, by applying weak compression during the process.
Thus the net reaction is In other words, the reactant is only greenhouse gas of carbon dioxide, while the product is high valued CNTs.
[38] Additionally, by changing electrolysis conditions such as electrolyte, electrode, temperature, and/or current density, a wide range of carbon nanotubes can be grown through this process including: helical; thin; thick; doped with either nitrogen, boron, sulfur, or phosphorus; bulbous; and more with multiple macrostructures being produced, some quite porous with potential uses as sponge or electrodes.
[48] Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames,[49] produced by burning methane,[50] ethylene,[51] and benzene,[52] and they have been found in soot from both indoor and outdoor air.
[54][55][56][57] Such methods have promise for large-scale, low-cost nanotube synthesis based on theoretical models,[58] though they must compete with rapidly developing large scale CVD production.
[62] While unencapsulated catalyst metals may be readily removable by acid washing, encapsulated ones require oxidative treatment for opening their carbon shell.
[66] Many electronic applications of carbon nanotubes crucially rely on techniques of selectively producing either semiconducting or metallic CNTs, preferably of a certain chirality.
[71] SWNT diameter separation has been achieved by density-gradient ultracentrifugation (DGU)[72] using surfactant-dispersed SWNTs and by ion-exchange chromatography (IEC) for DNA-SWNT.
This can be achieved by CVD that involves a combination of ethanol and methanol gases on a quartz substrate, resulting in horizontally aligned arrays of 95–98% semiconducting nanotubes.
