Carbon Nanotubes (Buckytubes): Types and Synthesis |ChemFam #07|

In my previous post, I was talking about nanomaterials; their classes and approach for synthesis. Today, I am going to talk about one of the widely used carbon based nanomaterial i.e. carbon nanotubes(CNTs).

As we have already discussed that carbon nanotubes are one dimensional class of nanomaterials made up of nano sized channels. The attention that carbon nanotubes received has been significant in recent years. Their role in catalytic activities has been enormous. In heterogeneous catalysis CNTs are being investigated as nano reactors, supports, adsorbents and active components. These are actually elongated form of fullerenes and were identified in 1991 by Sumio Iijima of Japan. In simple language we can say, carbon nanotubes are cylinders of one or more layers of graphene (lattice). They shows a unique combination of stiffness, strength and tenacity compared to other fibre materials. The electrical and thermal conductivity of CNTs are also high as compared to other conductive materials.

Image Source

Fullerenes were discovered by Harry Kroto, Robert Curl and Richard Smalley in 1985. They were named so due to their high resemblance to the highly symmetric domes designed by the architect Richard Buckminster Fuller. They consist of a family of spheroidal or cylindrical molecules with all the carbon atoms sp² hybridized. C60 was the first fullerene to be discovered. It was called the bucky ball, it has the shape of a soccer ball(icosahedral) with 60 carbon atoms bonded together having 12 pentagons and 20 hexagons arranged spherically. The tubular form of these fullerenes are basically carbon nanotubes and it is the reason why they are called buckytubes. Thus in general carbon nanotube structures consist of graphene cylinders closed at either end with caps containing pentagonal rings. C70 is smallest nanotube. Nanotubes are formed by rolling up a graphene sheet into cylinder and capping each end with half of a fullerene molecule. Different wrapping results different structures with different electronic properties.

Image Source

Types of Carbon Nanotubes

Carbon nantubes are of two types- single wall carbon nanotubes (SWNT) and multiple wall carbon nanotubes (MWNT). Let’s closely discuss them one by one.

Single Wall Nanotubes
Single wall nanotube (SWNT ) consist of one cylinder. These are tubes of graphite that are normally capped at the ends. The SWNTs have diameter in the range of 0.5 -2.0 nm. The length is in the range of 50-150 μm length. The structure of a SWNT can be visualized as a layer of graphite (called graphene) which is rolled into a seamless cylinder. The SWNTs are microporous and the specific surface area is in the range of 1300 m²/g (outer surface). They are commonly arranged in bundles. The topological defects are less within them and they have better mechanical and electro physical properties. Furthermore the electronic peoperties of SWNTs are governed by two core factors. They are- tube diameter and helicity. These are further depend on the way graphene layer is rolled up which can be arm chair or chiral. A few C-C bonds lies perpendicular to tube axis in armchair tubes whereas chiral tubes have intermediate orientation. Armchair SWNTs shows conductivity as similar to metal whereas zigzag SWNTs behave as semiconductors. In catalysis CNTs have high application as support. The extent of electrical conductivity, surface curvature and presence of inner cavity in CNTs make the metal – support interaction different compared to that in activated carbon or graphite support. Mechanically bent SWNTs present kink sites that are chemically more active. Metal nanoparticles size depends strongly on metal-CNT interactions. The stronger interaction gives rise to smaller nanoparticles. Studies have shown that convex surface of CNTs are more reactive than concave surface and the difference in reactivity increases when the tube diameter decreases.

Multi Wall Nanotubes
Multi wall carbon nanotubes have similar properties to single wall nanotubes. They consist of many nested concentric SWNTs cylinders with increasing successive radii. The concentric walls are spaced regularly at an interval of 0.34 nm similar to inter graphene distance. They have outer diameter in the range of 2 to 100 nm depending on number of coaxial tubes present. Thus they are usually mesoporous in nature and and the specific area depends on number of walls. The length of MWNTs can range from few to hundreds μm. The notable advantage of MWNT over SWNT is that the multi-shell structures of MWNTs are stiffer than single wall hence stability is higher. A comparatively higher tensile strength is been found in multi walled nanotubes than single wall nanotubes. Also they can be prepared in a large scale by various methods. The most common characterization techniques of these materials are electron microscopy, Raman spectroscopy, TGA , IR and UV-Visible. MWNTs are easier to produce than SWNT. However, the structure of MWNT is less well understood because of its greater complexity and variety. Regions of structural imperfection may diminish its desirable material properties.

Synthesis of Carbon Nanotubes

There are several techniques for producing both single wall and multi wall carbon nanotubes and all of them have advantages and disadvantages. Generally, they can be classified into two types depending on whether they require high or medium temperatures. They are -

Sublimation of graphite with subsequent desublimation
This method involves condensation of carbon atoms generated from evaporation of solid carbon sources of graphite. The sublimation of the solid can be done using electric arc or laser ablation where the temperature reaches to 2500 - 3500°C. Electric arc discharge is the most common method used to fabricate carbon nanotubes. Typically, about 60 to 70 wt% of the arc-synthesized soot is CNT. The rest of the soot comprises of fullerenes, amorphous carbon and catalyst nanoparticles. An electric arc is an electrical breakdown of a gas which produces a plasma discharge. The arc occurs in a gas-filled space between two conductive electrodes and it results in a very high temperature, capable of melting or vaporizing almost anything. The anode is drilled and filled with catalysts. The metal oxides (Ni, Co, Fe) are used as catalyst. The catalyst/graphite composite is used as electrode in some cases. The synthesis is performed in cooled chamber in presence of gases like helium, argon or methane. During the arcing process, the catalyst/graphite anode is evaporated and consumed with simultaneous carbon deposition around the cathode. The quality of CNT samples depends upon the stability of arc, current density and cooling rate of the cathode. Recently, arc discharge in liquid media has been developed to synthesize several types of nano carbon structures such as carbon onions, carbon nanohorns and carbon nanotubes. This is a low cost technique as it does not require expensive apparatus. In laser ablation method the graphite target is subjected to beam of laser and sublimated carbon is recollected. Inert gas atmosphere is maintained within the chamber.

Image Source

Decomposition if carbon containing compounds
The most used method to prepare CNT is pyrolysis of hydrocarbon gases or vapors such as propane, butane, hexane, benzene, toluene etc. The method is also known as chemical vapor deposition (CVD) process. It is a versatile method that can produce bulk amounts of defect or impurity free CNTs at relatively low temperatures. In CVD process, the thermal decomposition of a hydrocarbon vapour is achieved in the presence of a metal catalyst. By this method, CNTs can be prepared in a large quantities. The temperature of the process may vary from 500-1300°C. It includes hydrocarbon precursors such as methane, ethyne, benzene, alcohol etc. In CVD method initially there is dissociation of hydrocarbons followed by dissolution and saturation of C atoms in metal nanoparticles. Thereafter precipitation of carbon occurs. Vapor-grown CNTs generally use metal catalyst particles. Mostly Fe, Co and Ni catalysts are used for the catalytic growth of CNTs. Recently, CNTs have also been grown from metal such as Au, Ag and Cu. Catalyst serves as nucleation sites and also promotes pyrolysis of hydrocarbons. If a solid hydrocarbon is used as the CNT precursor, it can be directly kept in the low-temperature zone of the reaction vessel. Volatile materials such as camphor, naphthalene etc directly turn from solid to vapour, and perform CVD while passing over the catalyst kept in the high-temperature zone.