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Two methods are mainly used to prepare CNF from chemical precursors. One is catalytic thermal chemical vapor deposition (CVD) growth, and the other is electrospinning followed by heat treatment; whereas CNF is usually prepared mostly by CVD from biological precursors, it could be either with or without catalyst. To fabricate CNFs using the catalytic CVD growth method, some metals and alloys (Fe, Co, Ni, Cr, Mo and V) are chosen as the catalysts. Catalyst plays a very important role in deciding the morphology of CNF. Shapes of catalytic nano-sized metal particles decide the structures of the CNF. In the electrospinning process, the polymer types and the carbonization process play the most important roles in the type and quality of the CNFs. Vapor-grown carbon fibers (VGCFs) or vapor-grown carbon nanofibers (VGCNFs) are cylindrical nanostructures with graphene layers arranged as stacked cones, cups or plates. Whereas carbon with graphene layers wrapped into perfect cylinders are called carbon nanotubes.

Chemical precursors that have been used for the synthesis of CNF are methane, carbon monoxide, synthesis gas (H₂/CO), and liquid organic waste from petrochemical industries, which include acetylene (C₂H₂), ethylene (C₂H₄) (Figure 2.12), and pyrene (C₁₆H₁₀); and various carbon- containing gases, such as carbon monoxide (CO), methane (CH₄), etc., are also used as the carbon source. Che et al. (1998) have shown that solid carbon nanofibers are formed by increasing the deposition time at 900 °C in a CVD unit when ethylene and pyrene were used as precursor [53]. The morphologies of the CNF synthesized from chemical precursors could be coiled or straight CNFs.

Figure 2.12 CNF synthesized from chemical precursors: (a) Ethanol and (b) Acetylene using CVD method in our lab.

Figure 2.13 Schematic illustration of CNF and CNT.