obtain aligned carbon nanotubes. Pyrolysis of Fe(II)phthalocyanine also yields
aligned nanotubes [46b]. Hexagonally ordered arrays of nanotubes are produced by
using alumina templates with ordered pores [47a]. By employing catalytic chemical
vapor deposition (CCVD), Mukhopadhyay et al. [47b] have obtained quasi-aligned
carbon nanotubes using metal impregnated zeolite templates. The advantage of
the precursor method is that the aligned bundles are produced in one step, at a
relatively low cost, without prior preparation of substrates. The precursor route to
carbon nanotubes has been discussed recently by Rao and Govindaraj [48a].
TEM observations of aligned nanotubes produced by ferrocene þ hydrocarbon
pyrolysis show the presence of iron nanorods encapsulated inside the carbon
nanotubes, the proportion of the nanorods depending on the proportion of ferrocene. Typical TEM images of such nanorods are shown in Figure 8.5(a), (b) and (c).
The inset in Figure 8.5(b) shows the selected area electron diffraction (SAED)
pattern of the nanorods showing spots due to the (010) and (011) planes of a-Fe.
The high-resolution electron microscope (HREM) image of the iron nanorod
shows well-resolved (011) planes of a-Fe in single-crystalline form. X-ray diffraction
studies also show the presence of a-Fe with a small portion of Fe 3 C as the minor
phase. In addition to the nanorods, there are iron nanoparticles (20–40 nm diameter) encapsulated inside the graphite layers. Both iron nanorods and nanoparticles
are well protected against oxidation by the graphitic layers. The iron nanorods also
exhibit a complex behavior with respect to magnetization reversal, showing Barkhausen jumps [48b]. Iron-filled carbon nanotubes could be useful as probes in
magnetic force microscopy.
8.2.1.3 Single-Walled Carbon Nanotubes
The nanotubes generally obtained by the arc method or hydrocarbon pyrolysis are
multi-walled, having several graphitic sheets or layers (Figure 8.1). Single-walled
nanotubes (SWNTs) were first prepared by metal-catalyzed dc arcing of graphite
rods [4, 5] in a He atmosphere. The graphite anode was filled with metal powders
(Fe, Co or Ni) and the cathode was made of pure graphite. SWNTs generally occur
in the web-like material deposited behind the cathode. Various metal catalysts have
been used to make SWNTs by this route. Dai et al. [49] prepared SWNTs by the
disproportionation of CO at 1200
C over Mo particles of a few nanometers diameter dispersed in a fumed alumina matrix. Saito et al. [50] compared SWNTs produced by using different catalysts and found that a Co or a Fe/Ni bimetallic catalyst
gives rise to tubes forming a highway-junction pattern. SWNTs are also prepared
by using various oxides Y 2 O 3 , La 2 O 3 , CeO 2 as catalysts [51]. The arc discharge
technique, though cheap and easy to implement, gives low yields of SWNTs. Journet et al. [52] obtained @80% yield of SWNTs in the arc, by using a mixture of
1 at.% Y and 4.2 at.% Ni as catalyst. Arc evaporation of graphite rods filled with Ni
and Y 2 O 3 in a He atmosphere (660 torr) gives rise to web-like deposits on the
chamber walls near the cathode, consisting of SWNT bundles [45]. HREM images
show bundles consisting of 10–50 SWNTs forming highway junctions (Figure 8.6).
The average diameter of the SWNTs was around 1.4 nm and the length extended
upto 10 mm. SWNTs have been produced in more than 70% yield by the conden8 Nanotubes and Nanowires
214
aligned nanotubes [46b]. Hexagonally ordered arrays of nanotubes are produced by
using alumina templates with ordered pores [47a]. By employing catalytic chemical
vapor deposition (CCVD), Mukhopadhyay et al. [47b] have obtained quasi-aligned
carbon nanotubes using metal impregnated zeolite templates. The advantage of
the precursor method is that the aligned bundles are produced in one step, at a
relatively low cost, without prior preparation of substrates. The precursor route to
carbon nanotubes has been discussed recently by Rao and Govindaraj [48a].
TEM observations of aligned nanotubes produced by ferrocene þ hydrocarbon
pyrolysis show the presence of iron nanorods encapsulated inside the carbon
nanotubes, the proportion of the nanorods depending on the proportion of ferrocene. Typical TEM images of such nanorods are shown in Figure 8.5(a), (b) and (c).
The inset in Figure 8.5(b) shows the selected area electron diffraction (SAED)
pattern of the nanorods showing spots due to the (010) and (011) planes of a-Fe.
The high-resolution electron microscope (HREM) image of the iron nanorod
shows well-resolved (011) planes of a-Fe in single-crystalline form. X-ray diffraction
studies also show the presence of a-Fe with a small portion of Fe 3 C as the minor
phase. In addition to the nanorods, there are iron nanoparticles (20–40 nm diameter) encapsulated inside the graphite layers. Both iron nanorods and nanoparticles
are well protected against oxidation by the graphitic layers. The iron nanorods also
exhibit a complex behavior with respect to magnetization reversal, showing Barkhausen jumps [48b]. Iron-filled carbon nanotubes could be useful as probes in
magnetic force microscopy.
8.2.1.3 Single-Walled Carbon Nanotubes
The nanotubes generally obtained by the arc method or hydrocarbon pyrolysis are
multi-walled, having several graphitic sheets or layers (Figure 8.1). Single-walled
nanotubes (SWNTs) were first prepared by metal-catalyzed dc arcing of graphite
rods [4, 5] in a He atmosphere. The graphite anode was filled with metal powders
(Fe, Co or Ni) and the cathode was made of pure graphite. SWNTs generally occur
in the web-like material deposited behind the cathode. Various metal catalysts have
been used to make SWNTs by this route. Dai et al. [49] prepared SWNTs by the
disproportionation of CO at 1200
C over Mo particles of a few nanometers diameter dispersed in a fumed alumina matrix. Saito et al. [50] compared SWNTs produced by using different catalysts and found that a Co or a Fe/Ni bimetallic catalyst
gives rise to tubes forming a highway-junction pattern. SWNTs are also prepared
by using various oxides Y 2 O 3 , La 2 O 3 , CeO 2 as catalysts [51]. The arc discharge
technique, though cheap and easy to implement, gives low yields of SWNTs. Journet et al. [52] obtained @80% yield of SWNTs in the arc, by using a mixture of
1 at.% Y and 4.2 at.% Ni as catalyst. Arc evaporation of graphite rods filled with Ni
and Y 2 O 3 in a He atmosphere (660 torr) gives rise to web-like deposits on the
chamber walls near the cathode, consisting of SWNT bundles [45]. HREM images
show bundles consisting of 10–50 SWNTs forming highway junctions (Figure 8.6).
The average diameter of the SWNTs was around 1.4 nm and the length extended
upto 10 mm. SWNTs have been produced in more than 70% yield by the conden8 Nanotubes and Nanowires
214