Chemistry Reference
In-Depth Information
Fig. 3 Schematic
representation of an arc
discharge chamber
Iijima found that the use of a small amount of metal catalyst was critical for
SWCNTs production. By drilling a hole in the anode and filling it with a small piece
of iron he observed formation of SWCNTs with a broad diameter distribution
between 0.7 and 1.65 nm. SWCNTs synthesis was also reported by Bethune,
utilizing Co as catalyst instead of Fe [
4
,
5
]. Actually several metal catalyst
compositions have been found to produce SWCNTs, but the most widely used is
a Y:Ni mixture able to yield up to 90% SWCNTs.
In general, MWCNTs obtained with this method present an inner diameter
from 1 to 3 nm, a maximum length of 1
m, and closed tips, while SWCNTs are
smaller in diameter (maximum 1.4 nm) and longer (several microns), with closed
tips as well.
Possible variations to the final product could be introduced by tailoring the
Ar:He ratio (higher Ar content for smaller diameters) or generally by playing
with the chemical and physical factors influencing the nucleation and growth of
the nanotubes, such as the overall gas pressure, presence of hydrogen, or carbon
vapor dispersion [
17
,
18
].
One of the most important drawbacks of this technique lies in the extensive tube
purification required before use. On the other hand, the method can produce large
quantities of product at relatively low cost.
μ
2.2 Laser Ablation
Arc discharge and laser ablation are similar methods, since they both exploit
a graphitic source which is consumed at high temperatures in order to generate a
plasma plume of vaporized carbon.
