Carbon nanotube (CNT):
Carbon nanotube is an allotrope of carbon. It is like hollow tube of about 1-3 nm diameter and a much longer length. Carbon nanotubes are sheet of graphite (called graphiene) rolled into cylinder (called graphene). They are many ways to roll up graphene. The way graphene rolled up drastically changes physical properties.
How was CNT discovered?
Roger bacon first produced images of carbon nanotubes as far back as 1959. In 1990, Richard shared an idea in which he said that if the buckyballs get too big, they become carbon cylinders. But in 1991, scientist Sumio Iijima who was working as researchers in the Fundamental Research Lab of NEC took pictures of carbon nanotubes. Generally he was tested a carbon buckyballs with an electron microscope. In this, he did some photographs study but he got some needle shaped structure and got his attention. Rather what exactly is Nanotubes and described it to the world and gave him the name “carbon Nanotube”.
Basically these needle shapes were actually cylinders of carbon atoms that were formed at the same time that the buckyballs were formed. Like buckyballs, these cylinders (called carbon nanotubes) are each a lattice of carbon atoms with each atom covalently bonded to three other carbon atoms.
Synthesis of carbon nanotubes:
For production of carbon nanotubes mainly three methods are used. These methods are arc discharged methods, laser ablation of graphite, and chemical vapor deposition (CVD). These all three methods required metal catalyst such as iron, nickel, cobalt. CVD process is the most reliable method of these two processes to production of carbon nanotubes.
Chemical vapor deposition (CVD) process: This method is widely used in production of carbon nanotubes and currently it holds the greatest promise. This process allow to large production of carbon nanotubes at lower cost with more easy controllable conditions. In this process, need of a metal catalyst (such as iron) with carbon-containing reaction gases (such as hydrogen or carbon monoxide). These are form carbon nanotubes on catalyst at inside very high temperature furnace (500˚C -1300˚C). More recently, some new metal catalyst are introduced such as Au, Ag, Cu. Catalyst act as nucleation sites and also promotes pyrolysis of hydrocarbons.
Purification: Even if we synthesize CNT’s with the purity, but some impurities are present in them. These foreign nanoparticles as well as structural defects that occurred during synthesis, due to which the physicochemical properties of CNTs can vary. That’s why carbon nanotubes needed purification with the help of process such as acid treatment or ultrasound at the end of the production process.
Types of CNT: Carbon nanotubes are two types such as:
- Single wall carbon nanotubes (SWNT)
- Multiple wall carbon nanotubes (MWNT)
Single wall carbon nanotubes (SWNT): Single-walled nanotubes (SWNT) has diameter approximately to 1 nanometer, and length of the tube approx about 50-150 micrometer. Single-wall nanotubes have the ability to exhibit important electronic properties, while the double-walled nanotube does not have these electronic properties. SWNT have less topological defects. It has better mechanical and electro physical properties. Electronic properties of SWNT are governed by two factors, tube diameters and helicity, which further depend on the way graphene layer, is rolled up. Currently Single wall nanotubes are the most commonly used in electronics field.
Multiple wall carbon nanotubes (MWNT): Multi-walled nanotubes (MWNT) are made of multiple rolled layers or concentric tubes of graphite. The concentric walls are separated regularly at 0.34 nm similar to inter graphene distance. MWNT have outer diameter in range of 2 – 100 nm depending on number of coaxial tubes present. Generally MWNT are mesoporous in nature and specific area depends on the number of nanotube walls. MWNT has multi-shell structures that’s why MWNT is stiffer than single wall hence stability is higher. Many techniques for characterization of MWNT such as electron microscopy, Raman spectroscopy, TGA, IR etc.
Read continue about the properties of CNT and its applications……
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