*2.1.2 Procedure for synthesis of CNTs by thermal CVD method*

CNTs are synthesized by thermal CVD method by using hydrocarbon gas as carbon source. In this method, a quartz tube is placed inside a furnace maintained at high temperature (500–900°C) heated by RF heater. A crucible containing the substrate coated with catalyst nanoparticles is placed inside quartz tube filled with inert gas

**Figure 1.** *CVD method.*

such as argon gas. The hydrocarbon gas (carbon source) is pumped into the quartz tube which undergoes pyrolysis reaction and forms vapor carbon atoms. These carbon atoms bind to the substrate and join to eachother by Vanderwaal force of attraction and grow as multi-walled carbon nanotubes (MWCNTs) on the substrate [23]. To synthesize single-walled carbon nanotubes catalyst nanoparticles of Fe, Co, Ni are used. The obtained CNTs are further purified to get the pure form of CNTs.
