Types of CNTs

Carbon nanotubes are of two types :

Single wall carbon nano tube (SWNT)
Multiple wall carbon nano tube (MWNT)

Single wall nanotube (SWNT ) consist of one cylinder. It is made of single graphene sheet rolled up into cylinder closed by two caps (semi fullerenes). 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 SWNTs are microporous and the specific surface area is in the range of 1300 m²/g (outer surface). SWCNTs are commonly arranged in bundles. SWNTs have less topological defects and have better mechanical and electro physical properties. Electronic properties of SWNTs are governed by two factors, tube diameters and helicity, which further depend on the way graphene layer is rolled up, arm chair or chiral. Armchair SWNTs shows conductivity as similar to metal whereas zigzag SWNTs behave as semiconductors. In catalysis CNTs have high application as support. 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 with stronger interaction giving 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.

Multiwall (MWNT) nano tubes consist of many nested concentric SWNTs cylinders with increasing successive radii. The concentric walls are spaced regularly at 0.34 nm similar to inter graphene distance. MWNTs have outer diameter in range of 2 – 100 nm depending on number of coaxial tubes present. MWNTs are usually mesoporous in nature and specific area depends on the number of walls. The length of MWNTs can range from few to hundreds μm. The advantage of MWNT over SWNT is that the multi-shell structures of MWNTs are stiffer than single wall hence stability is higher. Also large scale synthesis of MWNT is possible by various methods. The most common characterization techniques of these materials are electron microscopy, Raman spectroscopy, TGA , IR and UV-Vis.