Page 106 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

92 Principles and Methods

(a) (b)
Catalyst Growth direction
Catalyst

Catalyst support
Figure 3.46 Schematic representation of supported catalyst
SWNT growth in which the SWNT grows parallel to the
surface (a) or out from the surface (b).

some cases they remain at the SWNT base, depending on the adhesion
between the catalyst particle and the substrate.
The length of the SWNTs grown in surface-supported catalyst VLS sys-
tems appears to be dependent on the orientation of the growing tube with
the surface. Within particular catalyst samples there are often two
classes of tubes grown: short, straight SWNTs and long, curved ones. It
has been proposed that the straight SWNTs are a result of growth along
the surface (Figure 3.46a) while the longer SWNTs are formed by growth
out of the plane of the surface (Figure 3.46b). The growth rate of the
former will be limited due to SWNT/surface interactions, while the later
has unrestricted growth away from the surface [187]. Once the reaction
run is complete (and the gas flow is removed) the SWNTs grown out of
the surface will fall over. In the absence of additional factors, the rate of
SWNT growth parallel to the surface is controlled by the frictional forces
between the SWNT and the surface. By properly adjusting the surface
concentration and aggregation of the catalyst particles, it is possible to
synthesize vertically aligned carbon nanotubes—that is, as a carpet per-
pendicular to the substrate (Figure 3.47).
Of the various means for nanotube synthesis, the chemical processes
show the most promise for industrial scale deposition in terms of its
price/unit ratio. There are additional advantages to the VLS growth of
SWNTs. Unlike the above methods, VLS is capable of growing SWNTs
directly on a desired substrate, whereas the SWNTs must be collected
in the other growth techniques. The growth sites are controllable by
careful deposition of the catalyst. Additionally, no other growth meth-
ods have been developed to produce vertically aligned SWNTs.

Chemical functionalization of carbon nanotubes. The limitation on using
carbon nanotubes in any practical applications has been their solubil-
ity; SWNTs have little to no solubility in most solvent due to the aggre-
gation of the tubes. Aggregation is a result of the highly polarizable,