CARBON NANO-TUBESBY: ILAM.Sc. II YEAR CARBON NANO-TUBESBY: ILA M.SC. II year
Derived from the Latin word Carbo for Coal, Carbon is the 6th most abundant element on earth.Two most important properties of Carbon which make it very unique are Catenation & Allotropy.Due to catenation or self-linkage of carbon , We have got separate branch of Chemistry, Organic Chemistry.Different allotropes of Carbon are
Carbon Nano-tubesA nano material is something with any one of its dimension as a nano-meter or one billionth of a meter.Nano-tubes are tube/cylinder shaped materials with diameter in the nano meter range.Different types of nano-tubes are Carbon nano-tubes, metal oxide nano-tubes, membrane nano-tubes etc.Carbon based nano-structures are known as fullerenes..Two kinds of fullerenes are Buckminster fullerenes and Carbon nano-tubes.
Discovery & HistoryIn 1952 a paper describing hollow graphitic carbon fibers appeared in the SovietJournal of Physical Chemistry. The authors were Radushkevich and Lukyanovich.Since then many scientists discussed similar growths of carbon fibers.But it came in limelight in the year 1991 when A report on Multi-walled Carbon nano-tubes was published by S. Ijima in Nature. (S. Iijima, Helical microtubules of graphite carbon , Nature 354, 56 (1991)).S. Ijima
Again S. Ijima and T. Ichihashi published report on Single walled Carbon nano-tubes. (S. Ijima and T. Ichihashi, Nature 363, 603 (1993))In the same year German scientists published report on Cobalt catelysed growth of Single walled CNTs. (D. S. Bethune, C. H. Klang, M. S. De Vries, G. Gorman, R. Savoy, J. Vazquez, and R. Beyers, Nature 363, 605 (1993))
TYPES OF CNTsSingle walled CNTs (SWCNTs)Multi walled CNTs(MWCNTs)Single walled CNTs have single cylindrical wall(Diameter 1-3nm) while Multi walled CNTs have wall within wall(Diameter 5-100nm).
MWCNTs are comparatively cheaper and easier to be produced than SWCNTs.
GEOMETRY OF CNTsCNTs exist in three kinds of geometry.The three different geometries are also referred to as flavours. The three flavours are Armchair zig-zag Chiral
HOW THE THREE DIFFERENT GEOMETRIES ARE ACHEIVED?
The different geometries are achieved by rolling up the graphene sheet in 3 different ways.Arm-chairZig-zag
Methods of PreparationThere are three main methods of Synthesis of carbon nano-tubes.Arc-DischargeLaser AblationChemical Vapour Deposition
Nanotubes were observed in 1991 in the carbon soot of graphiteelectrodes during an arc discharge, by using a current of 100amps. it has been the most widely used method of nanotube synthesis.The yield for this method is up to 30% by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects.
LASER ABLATIONIn the laser ablation process, apulsed laservaporizes a graphite target in a high-temperature reactor while aninert 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 reactiontemperature. However, it is more expensive than either arc discharge or chemical vapour deposition.
CHEMICAL VAPOUR DEPOSITION(CVD)During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt,iron, or a combination.
The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700C.
To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such asammonia,nitrogenorhydrogen) and a carbon-containing gas (such asacetylene,ethylene,ethanol ormethane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes.
TEM images of MWCNTs grown at low temperatures and atmospheric pressure: (a) 550 C, (b) 600 C, (c) 900 C, (d) SWCNTs at 900 C.
PROPERTIESHigh length to diameter ratioHigh tensile strengthVery lightElectrical propertiesBecause of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, ifn=m, the nanotube is metallic; ifnmis a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderatesemiconductor. Thus all armchair (n=m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting
Optical PropertiesAct as LED and photo-detector Carbon nanotubes as a black bodyAn idealblack bodyshould have emissivityorabsorbanceof 1.0, which is difficult to attain in practice, especially in a widespectral range. Vertically aligned "forests" of single-wall carbon nanotubes can have absorbances of 0.980.99 from thefar-ultraviolet(200nm) tofar-infrared(200 m) wavelengths.
If We join two CNTs with different electrical properties, it can act as Diode.Carbon nano-tubes can be a very good tip for AFM due to their high Aspect ratio.Carbon Nanotubes are good candidates to act asconducting wires because- their length can be of the order of nanometer.- they can act as ideal one dimensional conductors.- they are among the strongest materials in nature.If we imagine building a space elevator then also they are an ideal material.
As Sensors:- If We modify a glassy Carbon electrode and apply it to determination of Folic Acid with Voltammetry.- Modifying the Surface of the electrode with CNT greatly increased the reduction peak current of Folic acid so that even a trace amount of it can be determined.CNTs can extract and retain genomic materials (RNA & DNA) of bacteria and pathogens which can be useful in diagnosing diseases.As a potential candidate for making Artificial muscle:- If we apply electrical voltage to A CNT fibre, it shows a motion similar to natural muscle.
CNTs can now be woven to make threads and super-strong ropes . By spraying or printing a superconducting powder on the threads while they being twisted we can create a pair of separated yarn battery electrodes about the size of a human hair that could be woven into high tech clothing to store energy.MIT is working on combat jackets that use CNT fibres to stop bullets and to monitor the condition of the wearer. . plus there are many more possible applications as electrodes, ultra-capacitors etc.
To sum Up..CNTs have very promising future due to their novel properties . But There are some toxicology related concerns also.The needle fibre shape of CNTs have been compared with Asbestos and similar health effects have been found.