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Explain CVD method for preparation of carbon nanotubes.
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Chemical vapour deposition (CVD) is a versatile process in which gas phase molecules are decomposed to reactive species, leading to film or particle growth.

CVD appears as a promising technique for production high quality nanotubes at large scales using methane as carbon source, reaction temperature in the range of 850 to $1000^0C$.

Procedure:

1. In a typical process, the synthesis of CNTs is carried out in a ceramic tube reactor placed in a horizontal furnace.
2. The reactant gases employed are of high purity methane and hydrogen and also ferrocene is used as catalyst.
3. A quartz substrate (1cm X 1cm) is placed inside the ceramic tube.
4. At first, argon flow is passed through the ceramic tube to remove air inside the chamber, whereas the furnace is heated to reach $950^0$C.
5. Then argon is pumped out of the chamber and mixture of methane and $H_2$ are introduced to the reactor after the sublimation of ferrocene.

The following reaction occurs with addition of ferrocene –

$CH_4 (g) + H_2 600 - 12000$ $\xrightarrow[\text{ferrocene}]{\text{600-$1200^0$}}$ $C (s) + 3H_2 (g)$

The figure below shows schematic of the synthesis