Page 57 - Carbon Nanotubes

48 C.-H. KIANG et al.
tubes growing radially (urchin style) for fcc-Ni or anode[4,5]. These parameters are found to change the
NiC3 particles in the rubbery collar that forms around yield of nanotubes, but not the tube characteristics
the cathode have also been found[16,17]. Second de- such as the diameter distribution. In contrast, the pres-
spite the fact that copper reportedly does not catalyze ence of certain additional elements, although they do
single-layer tube growth in the gas phase[ 1 I], Lin et ai. not catalyze nanotube growth when used alone, can
found that numerous short, single-layer tube struc- greatly modify both the amount of nanotube produc-
tures form on the cathode tip when a Cu-containing tion and the characteristics of the nanotubes. For ex-
anode is used[lS]. Finally, the growth of single-layer ample, sulfur[5], bismuth, and lead[6] all increase the
tubules on a graphite substrate by pyrolyzing a hydro- yield and produce single-layer nanotubes with diam-
gedbenzene mixture in a gas-phase flow-reactor at eters as large as 6 nm, much larger than those formed
1000°C was recently reported[l9]. That experimental with Co catalyst alone. Sulfur also appears to promote
result is unique among those described here, in that the encapsulation of Co-containing crystallites into
no metal atoms are involved. An overview of some of graphitic polyhedra. Lambert et al. recently reported
the experimental results on single-layer nanotubes is that a platinum/cobalt 1: 1 mixture also significantly
presented in Table 1. increased the yield of nanotubes[ 111, even though Pt
In the arc-production of nanotubes, experiments to alone also has not produced nanotubes[9,11].
date have been carried out in generally similar fash- Different product morphologies have been found
ion. An arc is typically run with a supply voltage of in different regions of the arc-reactor chamber. On
20-30 V and a DC current of 50-200 A (depending on the cold walls, a primary soot is deposited. In normal
the electrode diameters, which range from 5-20 mm). fullerene production, this soot has a crumbly, floccu-
Usually He buffer gas is used, at a pressure in the lent character. However, under conditions that lead
range 50-600 Torr and flowing at 0-15 ml/min. The to abundant nanotube growth, the density of tubes in
anode is hollowed out and packed with a mixture of this soot can be high enough to give it a rubbery char-
a metal and powdered graphite. In addition to pure acter, allowing it to be peeled off the chamber wall in
iron group metals, mixtures of these rnetals[7,S,lO] sheets. This rubbery character may be caused by either
and metal compounds (oxides, carbides, and sulfides) chemical or physical cross-linking between the nano-
[5] have been successfully used as source materials for tubes and the soot. We note that fullerenes in amounts
the catalytic metals in nanotube synthesis. The ratio comparable to those obtained without a metal present
of metal to graphite is set to achieve the desired metal can be extracted from the rubbery soots using the nor-
concentration, typically a few atomic percent. mal solvents. Second, a hard slag is deposited on the
Parameter studies have shown that single-layer cathode tip. This cathode tip contains high densities of
nanotubes can be produced by the arc method under multilayer nanotubes and polyhedral particles[20,21].
a wide range of conditions, with large variations in The fact that the transition-metal-catalyzed single-
variables such as the buffer gas pressure (100-500 layer nanotubes are distributed throughout the soot
Torr), gas flow rate, and metal concentration in the and rarely in the slag deposit leads to the conclusion

Table 1. Results on single-layer nanotubes*

Fe 0.7-1.6 0.80, 1.05 Fe,C
Fe 0.6-1.3 0.7-0.8 -
co 0.9-2.4 1.3, 1.5 fcc-co
co 1-2 1.2-1.3 Co wrapped with graphene layers
co 0.6-1.8 -
Ni 1.2-1.5 - fcc-Ni in polyhedra in cathode deposit
Ni 0.6-1.3 0.7-0.8
Fe+Ni 0.9-3.1 1.7
Fe+Ni >0.6 1.3-1.8
Co+Ni 10.6 1.2-1.3 -
co+s 1.0-6.0 1.3, 1.5 Co(C) in polyhedra and fcc-Co
Co+Bi 0.8-5 1.2, 1.5
Co+Pb 0.7-4 - -
Co+Pt =2 CoPt
Y 1.1-1.7 YC2 in polyhedra
cu 1-4 Cu in polyhedra t181 g
no metal >2 graphite substrate 1191 h
*Unless specified, samples were from soot deposited on the chamber wall and the buffer gas was helium. Elements are
those incorporated in the graphite anode, D is the nanotube diameter range, D, is the most abundant nanotube diame-
ter, and Crystallites refers to metal-containing particles generated by the arc process and found in the soot.
‘Statistics from 60 tubes; bfrom 40 tubes; ‘from over 100 tubes; dfrom 70 tubes; ‘from over 300 tubes; ‘Nanotubes grew
radially out of YC, crystals, 15-100 nm long; gNanotubes found in the cathode deposit, 3-40 nm long; hNanotubes formed
by C6Hs pyrolysis on graphite substrate.