Nanoscale Carbon Materials for Contaminant Separation - Nanotechnologies for Water Environment Applications - page 270

Figure 10.1 Structure of a C 60 molecule with 20 hexagons and 12 pentagons.
Although discovered in 1985, fullerene was produced in bulk quantities in 1990
(Kratschmer et al., 1990). A simple carbon arc method was used to vaporize graphite in
a helium atmosphere, and then the soot that settled on the walls of the vessel was
collected. Based on the well-soluble property of fullerene in aromatic solvents (e.g.,
benzene and toluene), C 60 , C 70 … molecules could be separated from the other forms of
carbon in the soot. The products obtained after the solvent evaporated were found to be
90% C 60 crystals and 10% C 70 crystals. Besides the carbon-arc method, the other
methods using a hydrocarbon flame or field-induced hot carbon plasma have been
applied to fullerene synthesis (Loutfy et al., 2002). In particular, the use of field-induced
high-intensity plasma is considered an attractive alternative to the arc process, as it uses
an inexpensive powder, non-conductive carbon, or hydrocarbon gas as the starting
material.
10.1.2 Carbon Nanotubes
It is interesting that by using the same experiment as Kratschmer et al. (1990),
Iijima (1991) discovered long hollow fibers in the deposit formed on the graphite
cathode after arc-evaporation. These novel structures were named graphitic carbon
needles (by Iijima) and had an outer diameter ranging from 4-30 nm and a length of up
to 1 μm (Iijima, 1991); later these structures became known as carbon nanotubes. There
are two forms of carbon nanotubes: multiwalled nanotubes (MWNTs) and single-walled
nanotubes (SWNTs). Multiwalled nanotubes, discovered in 1991 by Iijima, are made of
concentric cylinders (see Figure 10.2), with a spacing between the adjacent layers of
about 3.4 A o - close to the interlayer separation of graphite (3.354 A o ) (Ajayan, 1999).
Virtually all of the tubes are closed at each end with caps that are pieces of fullerenes.
Ideally, the caps are semi-fullerene, but in practice a variety of cap morphologies have
been observed (Iijima et al., 1992). Single-walled nanotubes have a novel structure that
was independently reported by both Iijima and Bethune (Bethune et al., 1993; Iijima and