SWNTs [59]. Colomer et al. [60] obtained SWNTs in high yield by the decomposition of methane over transition metal supported MgO substrates. Flahaut et al. [61]
have synthesized SWNTs by passing a H 2 aCH 4 mixture over transition metal containing oxide spinels, obtained by the combustion route. The quality of SWNTs has
been characterized on the basis of adsorption measurements. Zeolites containing one-dimensional channels have also been investigated for the synthesis of
monosized SWNTs [62]. As-prepared SWNTs contain several contaminants such as
amorphous carbon and nanometer-size catalyst particles coated with carbon. The
amorphous carbon can be burnt away by heating the nanotubes in air at around
300
C. Bandow et al. [63] use microfiltration to clean SWNTs of the other contaminants. Size exclusion chromatography of the surfactant-stabilized raw SWNTs
is also employed to purify the nanotubes from amorphous carbon, metal particles
etc [64]. In addition to purification, size separation has also been achieved. Since
SWNTs occur as large bundles with lengths of the order of microns, it is desirable
to break them from the bundles, for purposes of further manipulation. Liu et al.
[65] have employed chemical processing based on ultrasound treatment, wherein
the SWNTs in an acidic medium were subjected to sonication so that the bundles
break up into open-ended small fragments of 100–300 nm length. The smaller
fragments were functionalized.
8.2.2
Structure and Characterization
Transmission electron microscope observations show that the nanotubes prepared
by the arcing process generally consist of multi-layered, concentric cylinders of
single graphitic (graphene) sheets. The diameter of the inner tubes is of the order
of a few nanometers. The outermost tubes could be as large as 10–30 nm as shown
in Figure 8.1(a). During the curling of a graphene sheet into a cylinder, helicity is
introduced. Electron diffraction studies establish the presence of helicity, suggesting that the growth of nanotubes occurs as in the spiral growth of crystals. The
separation between concentric cylinders in MWNTs is about 3.45 A ˚ , which is close
to the separation between the (002) planes of graphite. These are the lowest energy
surfaces of graphite with no dangling bonds, so that the nanotubes are in fact the
expected structures. In the electron microscope images, one typically observes
nanotubes along their lengths, with the electron beam falling perpendicular to the
axis of the nanotube. In high-resolution images, it is possible to see spots due to
the lattice planes running along the length of the nanotubes. Iijima [66] has published such an image for the (110) planes separated by 2.1 A ˚ . Ring-like patterns are
found due to individual tubes comprising cylindrical graphitic sheets which are
independently oriented (with no registry between the sheets) with helical symmetry for the arrangement of the hexagons. Graphitic cylinders would have dangling
bonds at the tips, but the carbon nanotubes are capped by dome-shaped hemispherical fullerene-type units. The capping units consist of pentagons to provide
the curvature necessary for closure. Ajayan et al. [8a] studied the distribution of
pentagons at the caps of carbon nanotubes, finding that the caps need not be perfectly conical or hemispherical, but can form skewed structures. The simplest
8.2 Carbon Nanotubes 217
have synthesized SWNTs by passing a H 2 aCH 4 mixture over transition metal containing oxide spinels, obtained by the combustion route. The quality of SWNTs has
been characterized on the basis of adsorption measurements. Zeolites containing one-dimensional channels have also been investigated for the synthesis of
monosized SWNTs [62]. As-prepared SWNTs contain several contaminants such as
amorphous carbon and nanometer-size catalyst particles coated with carbon. The
amorphous carbon can be burnt away by heating the nanotubes in air at around
300
C. Bandow et al. [63] use microfiltration to clean SWNTs of the other contaminants. Size exclusion chromatography of the surfactant-stabilized raw SWNTs
is also employed to purify the nanotubes from amorphous carbon, metal particles
etc [64]. In addition to purification, size separation has also been achieved. Since
SWNTs occur as large bundles with lengths of the order of microns, it is desirable
to break them from the bundles, for purposes of further manipulation. Liu et al.
[65] have employed chemical processing based on ultrasound treatment, wherein
the SWNTs in an acidic medium were subjected to sonication so that the bundles
break up into open-ended small fragments of 100–300 nm length. The smaller
fragments were functionalized.
8.2.2
Structure and Characterization
Transmission electron microscope observations show that the nanotubes prepared
by the arcing process generally consist of multi-layered, concentric cylinders of
single graphitic (graphene) sheets. The diameter of the inner tubes is of the order
of a few nanometers. The outermost tubes could be as large as 10–30 nm as shown
in Figure 8.1(a). During the curling of a graphene sheet into a cylinder, helicity is
introduced. Electron diffraction studies establish the presence of helicity, suggesting that the growth of nanotubes occurs as in the spiral growth of crystals. The
separation between concentric cylinders in MWNTs is about 3.45 A ˚ , which is close
to the separation between the (002) planes of graphite. These are the lowest energy
surfaces of graphite with no dangling bonds, so that the nanotubes are in fact the
expected structures. In the electron microscope images, one typically observes
nanotubes along their lengths, with the electron beam falling perpendicular to the
axis of the nanotube. In high-resolution images, it is possible to see spots due to
the lattice planes running along the length of the nanotubes. Iijima [66] has published such an image for the (110) planes separated by 2.1 A ˚ . Ring-like patterns are
found due to individual tubes comprising cylindrical graphitic sheets which are
independently oriented (with no registry between the sheets) with helical symmetry for the arrangement of the hexagons. Graphitic cylinders would have dangling
bonds at the tips, but the carbon nanotubes are capped by dome-shaped hemispherical fullerene-type units. The capping units consist of pentagons to provide
the curvature necessary for closure. Ajayan et al. [8a] studied the distribution of
pentagons at the caps of carbon nanotubes, finding that the caps need not be perfectly conical or hemispherical, but can form skewed structures. The simplest
8.2 Carbon Nanotubes 217