Page 105 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 91

are difficult to purify, manipulate, and assemble for building nanotube-
device architectures for practical applications.
To overcome some of the difficulties of these high-energy processes,
Smalley and coworkers developed a chemical catalysis method. In 1998
Smalley and coworkers reported the use of hydrocarbons as a carbon
feedstock for single-walled tube growth [181]. Here molybdenum and
iron/molybdenum catalysts were heated in a tube furnace to 850 C
under 1.2 atm of ethylene. Previous reports utilizing a gas-phase growth
reaction had produced multiwalled tubes or single-walled tubes in very
low yield [182, 183]. The use of CO as a feedstock led to the development
by Smalley and coworkers of the high-pressure carbon monoxide (HiPco)
procedure [184]. By this method, it was possible to produce gram
quantities of SWNTs. The process involves injecting Fe(CO) into a gas-
5
phase reactor operating between 800–1200 C and 1–10 atm carbon
monoxide. The HiPco method was better than previously reported gas-
phase growth methods because it did not use a hydrocarbon as a feed-
stock. The key to the HiPco process is the formation of metal catalyst
particles in the vapor phase, and it is thought that these are responsi-
ble for the SWNT growth, based upon an analogy with the growth by a
catalyst on substrate.
The growth of SWNTs from chemical processes was first reported in
1992, and can be likened to the vapor-liquid-solid (VLS) growth of SiC
wiskers [185, 186]. During VLS growth a preformed catalyst particle
(most commonly nickel, cobalt, iron, or a combination thereof) is placed
on a substrate. The diameters of the nanotubes that are to be grown has
been proposed to be related to the size of the metal particles; however,
recent work has shown that this is not necessarily true [187].
VLS growth apparatuses are usually constructed of a tube furnace that
is set up so gases can flow through the tube while it is being heated to
high temperatures. As with other growth systems, multiwalled tubes
were the first type of tubes grown [188]. In 1999, Dai and coworkers
reported the large-scale VLS growth of SWNTs using iron-impregnated
silicon nanoparticles and methane [189]. Methane was chosen as the
feedstock because of its high thermal stability and its ability to retard
the formation of amorphous carbon in the reactor. Since these reports,
a wide range of precursors have been used for the catalyzed VLS growth
of SWNTs. Recent approaches have involved the use of well-defined
nanoparticle or molecular precursors [187, 190]. Many different tran-
sition metals have been employed, but iron, nickel, and cobalt remain
the focus of most research. SWNTs grow at the sites of the metal cata-
lyst; the carbon-containing gas is broken apart at the surface of the
catalyst particle, and the carbon is transported to the edges of the par-
ticle, where it forms the SWNTs. The catalyst particles generally stay
at the tip of the growing SWNT during the growth process, although in