Page 256 - Adsorbents fundamentals and applications

CARBON NANOTUBES 241

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in the range between 450–750 C. The fibers have dimensions of 30–500 ˚ A 2
in cross-sectional area and 10–100 µm in length. They consist of platelets of
graphite layers similar to the MWNTs. However, the layers are not parallel to
the axial direction of the fiber, but instead form an angle with it. The structure
has been described as “herring-bone.” Thus, many graphitic edges are exposed
2
on the surface of the fiber. These edges have unsaturated sp bonds, or free sp 2
electrons. Hence much interesting chemistry can be exploited with the edges of
GNFs. Of particular interest is the possibility of hydrogen storage (Rodriguez
and Baker, 1997).

9.1.2. Arc Discharge and Laser Vaporization
Since the 1950’s (Bacon, 1960), hollow graphite filaments have been found in
deposits of high-temperature furnaces containing hydrocarbons or CO, with or
without catalysts. The first substantive study of graphite fibers formed by arc
discharge was performed by Bacon (1960). He used a dc arc generated by a mod-
erate 75–80 volts and 70–75 amperes between two vertically aligned graphite
electrodes. The arc provided the high temperature to vaporize the electrodes.
The graphite vapor condensed on the cathode where graphite filaments were
deposited. At the time, it was thought that the filaments would be formed at near
the triple point of graphite, that is, near 100 atm and 3900 K. Thus, Ar at 92 atm
was used as the inert atmosphere. A TEM with a resolution no better than 5 ˚ A
was employed. Selected-area electron diffraction/TEM analysis confirmed that
the walls of the filaments were formed by cylindrical layers of perfect graphite
sheets. Unfortunately, without high resolution, a “scroll” structure, rather than
concentric tubes, was proposed. There is little doubt that what is now called
MWNTs were formed in Bacon’s samples.
Interest in the arc discharge technique was later revived by the discovery
of the fullerenes in 1985 (Kroto et al., 1985). A detailed review on fullerenes
(including their formation) is available in Dresselhaus et al. (1996). Kroto et al.
(1985) identified the C 60 molecule found in the soot, which was condensed from
the carbon vapor generated by laser heating. Carbon vapor can be generated by
a number of means, for example, arc discharge, laser ablation, resistive heat-
ing, or combustion (with deficient oxygen). It turned out that both MWNTs and
fullerenes are formed in the condensed carbon. An interesting finding was that
the formation of these materials depends on the pressure of the inert atmosphere.
Also, a minimum pressure is required (Dresselhaus et al., 1996).
Table 9.2 is a summary of the main findings for the syntheses of carbon
nanotubes and fullerenes by using the route of carbon vaporization/condensation.
SWNT has never been found without the use of a catalyst. Fe, Co, and Ni are
the main catalysts used in forming SWNTs.
The smallest SWNTs that have been grown are 4 ˚ A in diameter. They have
been grown by pyrolyzing tripropylamine filled in the 7.3 ˚ A channels of AlPO 4 -5
molecular sieve (Wang et al., 2000). An inner wall diameter of 4 ˚ AofaMWNT
formed by arc discharge has also been reported (Qin et al., 2000). Syntheses of
5 ˚ A SWNTs have been reported as well (Peng et al., 2000; Sun et al., 2000).