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128 Smart Nanomaterials for Sensor Application Zhu et al.
fullerenes [1]. As shown in the Fig. 2a, the chamber consists of one carbon stick at the cathode and the
other at the anode, where inert gases are helium or argon. The gap between the anode and cathode can be
reduced to less than 1 mm by movement of the anode, and creates plasma by 100 A current passing through
the electrode. This process typically causes the temperature of the plasma to be as high as 4000 K, and thus
carbon on the anode is vaporized and deposited on the cathode or the chamber wall.
MWNT synthesis by the arc discharge technique is straightforward if two graphite electrodes are
introduced, however, a great amount of side products, like fullerenes, amorphous carbon and graphite
sheets, are simultaneously formed, which cause difficulty and increase cost to meet the need for
purification. SWNTs can be produced by incorporating a metal catalyst on the cathode or anode [2, 16].
The metal concentration, type and pressure of inert gas, current etc. will greatly affect the quality of tubes.
The arc-discharge is the simplest and most common method to synthesize CNTs, especially for MWNTs.
However, the purification of tubes is costly and time-consuming and may damage the CNTs.
(a) (b)
Anode
Cathode
Plasma
Figure 2. (a) Schematic diagram for Arc-discharge setup, two graphite electrodes are used to produce a dc electric arcdischarge
in inert gas atmosphere. (b) Schematic diagram for laser ablation apparatus. Reprinted with permission from
[15].
1.1.2. Laser-Ablation
This was first introduced by Guo et al. [17] when direct laser vaporization of transition-metal/graphite
composite rods produced Single-Walled Carbon Nanotubes (SWNTs) in the condensing vapour in a heated
flow tube. An oven laser-vaporization apparatus is illustrated in Fig. 2b. A pulsed or continuous laser beam
was introduced into a 1200 o C furnace to vaporize a target, which is made of graphite and metal catalysts
(cobalt or nickel), in the presence of helium or argon gas. The growing mechanism was suggested by Scott
and co-workers in 2001 [18]. A very hot vapour plume is formed that expands and cools rapidly. As the
vaporized species cool, small carbon molecules and atoms quickly condense to form larger clusters,
possibly including fullerenes. The catalysts also begin to condense, but more slowly at first, and attach to
carbon clusters and prevent them closing into cage structures. Catalysts may also open cage structures when
they attach to them. From these initial clusters, tubular molecules grow into single-wall carbon nanotubes
until the catalyst particles become too large, or until conditions have cooled sufficiently that carbon no
longer can diffuse through or on the catalyst particles. It is also possible that the particles coated with a
carbon layer can no longer absorb more carbon and nanotube growth ceases.
The merit of the laser-ablation method can be summarized as following: i) relatively high purity SWNTs
can be synthesized, ii) a lower temperature furnace can be used with a CO2 infrared laser system, iii) the
quality of CNTs is tunable by adjusting the nature of gas and its pressure. On the other hand, it only
suitable for the growth of SWNTs, and is limited to laboratory scale system. Compared with the arcdischarge
method, both techniques produce reasonable yields (ca. 70%) of CNTs with impurities in the
form of amorphous carbon and catalyst particles because of the high temperature of the heated source. The
CNTs obtained from both these techniques are tangled and thus post growth purification is essential.