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.