Figure 2. Scanning electron micrographs of the catalytically-grown
multi-walled carbon nanotube material used as a filler.
The scanning electron micrographs are illustrated in Figure 3 for the
graphitic carbon nanofibers that represent a class of nanostructured
carbon fibers having atomic structures uniquely different from that of
carbon nanotubes. Interest in developing carbon nanostructures
appropriately surface-derivatized for diverse applications remains high
[47, 48]. Considerable progress has been made in controlling the
dispersibility and wettability properties of single-walled or
multi-walled carbon nanotubes through either covalent or non-covalent
surface derivatization [49, 50]. Herringbone graphitic carbon
nanofibers possess canted graphene sheets, also described as
geodesic-like conical graphene sheets, stacked in a nested fashion along
the long fiber axis. Graphitic carbon nanofibers of this type can be
prepared having average diameters from 25 nm-200 nm and lengths on the
micron scale. The graphitic atomic structure of herringbone graphitic
carbon nanofibers gives a carbon nanofiber long-axis surface comprised
of carbon edge sites, usually passivated by hydrogen atoms. The
surface-functionalization of herringbone graphitic carbon nanofibers
with reactive linker molecules using surface oxidation and carboxyl
group coupling chemistry occurs without degradation of the structural
integrity of the graphitic carbon nanofiber backbone and affords
surface-derivatized graphitic carbon nanofibers having a high surface
density of functional groups. Covalent binding of such linker molecules
to either polymer resins or ceramic condensation oligomers gives
graphitic carbon nanofiber-polymer or graphitic carbon
nanofiber-ceramics hybrid materials. A particularly interesting class of
carbon nanotubes are the vertically aligned carbon nanofibers.
Vertically aligned carbon nanofibers are multi-walled carbon nanotubes
that are typically grown in a direct current plasma, yielding nanofibers
that are aligned vertically from the surface. The resulting nanofiber
forests have interesting properties because in addition to providing
edge planes along the nanofiber walls, the interstices between the
fibers are straight and relatively large, providing a high degree of
accessibility to analytes. The presence of well-defined interstices is
important because very small pores cannot support electrical
double-layers and diffusion limitations can reduce the effective surface
area. Vertically aligned carbon nanofibers are a promising high surface
area, nanoscale carbon material. Vertically aligned carbon nanofibers
have similar electrochemical and mechanical properties as other
nanoscale carbon materials. The advantage of vertically aligned carbon
nanofibers is the ability to control their physical dimensions allowing
for large, accessible surface areas. Thus, vertically aligned carbon
nanofibers are an ideal platform for modifications leading to increased
surface area, such as covalent functionalization with molecular layers
and decoration with metal coatings. Although aligned carbon nanotube
arrays have drawn significant research interest, there exists a
potential drawback for their implementation in applications.
Conventionally-prepared carbon nanotube arrays are prepared by
depositing a thin, insulating oxide layer upon a substrate, followed by
deposition of a catalyst layer upon the oxide layer. The oxide layer
supports the catalyst, maintains its activity and promotes the growth of
carbon nanotubes. The general requirement of an intervening oxide layer
between the substrate and the catalyst prevents the carbon nanotubes
from becoming bonded directly to the substrate. Although simple
techniques have been developed to transfer aligned carbon nanotubes from
a growth substrate to a desired substrate, direct growth on a desired
substrate would be a far more efficient process. Furthermore, the oxide
layer is not compatible with a number of substrates, so there is a
process limitation on the types of substrates upon which carbon
nanotubes can be grown. As a result, for a number of interesting
substrates, direct carbon nanotube growth is not possible by
conventional growth processes. For example, direct growth of dense
arrays of carbon nanotubes on a carbon surface or a conducting surface
such as, for example, a metal is not possible by conventional growth
methods. Carbon fibers are a substrate of particular interest due to
their well-established use in the aerospace and polymer composite
industries. Direct growth of carbon nanotubes on carbon surfaces
according to conventional growth methods typically results in low carbon
nanotube yields, sparse growth and potential damage to the carbon
surface by the catalyst. Alignment generally enhances the electrical,
thermal and mechanical properties of the carbon nanotubes relative to
their unaligned counterparts. Aligned carbon nanotube arrays are
conventionally prepared by coating a growth substrate with a thin
insulating layer of alumina, followed by coating of the alumina layer
with a thin iron catalyst layer. Upon reduction of the iron catalyst
layer, the iron typically breaks apart to form point sources of
high-density nucleation sites operable for forming carbon nanotubes in
the presence of a feedstock gas. In this well-established growth
technique, the catalyst is supported by the alumina layer and resides at
the base of the growing carbon nanotube array next to the substrate. The
catalyst particles absorb carbon from the gas phase and provide it at
the particle interface to form the growing carbon nanotubes. As a
result, bonding to the substrate occurs through the metal catalyst
particle. This type of carbon-metal bond is air sensitive and breaks
upon exposure to air. Therefore, conventionally-prepared carbon nanotube
arrays are easily detached from their growth substrate.