Chemistry Reference
In-Depth Information
Fig. 4 Illustration of the
components forming a laser
ablation apparatus
The method, discovered by Guo et al. in 1995 and developed for the large-scale
production of nanotubes by Smalley et al. in 1996, was conceived with the aim to
improve the synthesis of SWCNTs in terms of quantity and purity [
18
,
19
].
The laser ablation technique utilizes a laser beam focused onto a graphite target
with small amounts of Ni and Co, which is located in a 1,200
C furnace with an
inert atmosphere of Ar or He. The carbonaceous products formed after target
vaporization are collected in a water-cooled chamber outside the furnace (Fig.
4
).
Like the arc discharge method, this technique allows one to obtain both
SWCNTs and MWCNTs depending on the composition of the graphitic source.
The greatest strength of this technique is to produce SWCNTs with higher yields
(
>
70%) and purity (
>
90%) with respect to the arc discharge process.
2.3 Chemical Vapor Deposition
In 1993 a new candidate for nanotube synthesis emerged with the promise
to achieve large-scale production and controlled direction of growth on a
substrate [
20
].
The method is based on the catalytic decomposition of a gaseous carbon
feedstock (usually a mixture of hydrocarbon gas and nitrogen) at high temperatures
(550-1,200
C) from which precipitation of carbon follows after cooling to room
temperature. The catalysis is mediated by transition metals (most frequently Fe, Co,
and Ni) that can be introduced into the gas furnace in the form of free nanoparticles
or deposited on a substrate, allowing the formation of forests of vertical aligned
nanotubes for use in electronics [
21
] (Fig.
5
).
The choice of catalyst nanoparticle size and substrate strongly influences the
forest development, affecting respectively the diameter of the tubes and the speed
