with the temperature, and carbon atoms then get into the internal areas of the catalysts to form CNF [46]. The temperature requirement depends on the precursor used; e.g., methane requires higher temperature than propane due to its higher decomposition energy to carbon and hydrogen (37.8 and 26.0 kJ mol–1 H2 for methane and propane, respectively) [47]. Increased reaction temperatures favor the migration of catalyst particles on the support surface, which results in their aggregation, leading to the growth of thicker CNFs [38]. The diameter and the structure of grown CNFs are strongly dependent on the temperature of the CCVD process. The synthesis temperature has an indirect influence on the CNF structure through various diffusion rates of carbon into the catalyst particles and different orientations of the graphene layers precipitated on the metal nanoparticle [38].
Growth or Reaction Time: The growth time plays a major role in tailoring the morphology of nanomaterials in terms of diameter and density of growth [48]. In the initial stage of growth, a part of the metal catalyst (nickel) surface is covered by carbon, leading to formation of relatively catalyst-rich carbon area. In the next stage, dissolved carbon is released to form CNF with the lift-off of the former catalyst-rich carbon particles out of the substrate surface. Generally, the reaction duration is 60 min, while a total time of 3 h is needed for the synthesis of CNF in the decomposition of ethylene over an unsupported Ni-Cu alloy [49].
Decomposition of Hydrocarbon: This is the final stage, where decomposition of hydrocarbon on the new catalyst initiates new deposition of carbons from the same particle, covering the host surface completely by a dense layer of CNF.
Size of Catalyst: It is well known that the diameter of nano fibers is controlled by the size of the catalyst particle responsible for their growth [50]. The parallel orientation of the graphene layers and the lower diameter arose from the changes to the catalyst and the carbon precursor. The use of an iron catalyst promotes the growth of CNTs, whereas nickel favors the formation of herringbone CNFs [51].
3.6.3 Self-Propagating High-Temperature Synthesis (SHS) or Combustion Synthesis (CS)
This is a well-known reaction used to fabricate new compounds and structures. Once started, the reaction continues with the help of the energy provided by itself rather than from outside the system. CNF is synthesized by this method, where the exothermic reaction between aluminum and iron oxide is used for preparing the heat required for SHS. In addition, the iron produced from the reaction can play the role of catalyst for growing the CNFs from activated carbon. Once the process starts, the reaction continues with the help of the energy provided by itself rather than from outside the system. Rapid solidification of products in CS creates nanostructures that would be otherwise difficult to make. Carboniferous materials (soda, limestone, and Teflon) are used along with reducers (magnesium, lithium, and sodium) with the addition of a nickel or iron as a catalyst. The yield of CNTs has shown to be weakly sensitive to the amount of catalyst. They have also found that the carbon source is an important factor: carbonaceous materials which were used required energy to decompose and to take part in the formation of nanostructures, which implies that there are no diluents in the reaction. These conditions cause agglomeration and condensation of the materials, preventing the formation of carbon nanostructures. A simple reaction called a thermite reaction is used to produce CNTs and CNFs in just one step.
3.6.4 Floating Catalyst Method
This is a novel method used to synthesize herringbone-stacked carbon nanofibers in high selectivity using cobaltocene as the catalytic precursor. Thiophene is essential for CNF growth while hydrogen is used as carrier gas at 1100 °C. The conversion rate of the CNFs collected in the cold trap is approximately 1.5 wt% of the initial precursor. The effects of the catalytic precursor temperature, thiophene and acetylene are the parameters that decide the diameter and selectivity of CNF.
3.6.5 Electrospinning Method
Electrospinning is a process that uses an electric potential to overcome the surface tension of a solution to produce an ultrafine jet, which elongates, thins and solidifies as it travels through the electric field to a collector (Figure 3.8). Despite being a relatively simple procedure to undertake in a laboratory, as it requires minimal equipment, the physics behind the process are complex. To gain an understanding of all the variables and interactions involved in electrospinning, consideration must be given to polymer chemistry, electric field interactions, fluid mechanics, environmental conditions and kinetics. CNFs are prepared from acrylonitrile, pitch, polyimide, poly(vinylidene fluoride) and phenol via electrospinning followed by carbonization. The properties of the final CNFs are decided by the types of polymer solution and the processing parameters. Once the polymer nanofibers have been successfully prepared, a heat treatment is applied to carbonize the polymer nanofibers to form CNFs. The morphology, purity, crystallinity, diameters and porosity are governed by the parameters of the heat treatment process such as atmosphere and temperature. The carbonization process is followed by heating the polymer nanofiber up to 1000 °C in a specific environment. Generally, volume and weight change occur during the carbonization process, which results in the decrease of the diameter of the CNFs. In most cases, the CNFs prepared by the electrospinning method are prone to form web or mat structures, which are used as electrode materials for batteries, supercapacitors, electrochemical capacitors, anode for lithium-ion rechargeable batteries, catalyst support and composite.
Figure 3.8 Schematic diagram of the electrospinning setup used for CNF fabrication. (Source: Feng et al. 2014) [57].
There are two major approaches of CNF production based on the precursor’s state of matter: solid-phase synthesis and vapor-phase synthesis. The precursor materials used for CNF production by electrospinning are in their solid state at the initial stage of fabrication. Precursors are aligned in fibers through different spinning techniques, followed by stabilization and carbonization at temperatures up to about 1300 °C. Recently, exploitation of a gaseous state of precursors has also gained in popularity. Vapor-grown carbon fibers (VGCF) are prepared by thermal decomposition or chemical vapor decomposition (CVD) of organic precursors, where stabilization is not necessary. While fibers’ solid-phase precursors are obtained as a continuous fiber with different morphologies (monofibers, strands of fibers, woven fabric with diverse woven modes, chopped fibers and also nonwoven mats), the vapor-grown carbon fibers are relatively short [52]. Some of the common solid phase CNF precursors given below.
3.6.5.1 Polyacrylonitrile (PAN)
Carbon fibers derived from PAN have high tensile strength and modulus. The carbon content of acrylonitrile (CH2=CHCN) is very high. In order to be suitable as precursor the acrylic fiber should contain at least 85% of acrylonitrile monomer to provide as high a carbon yield and it should preserve the fibrous structure of the polymer precursor throughout the carbonization stage [53].
3.6.5.2 Pitch
Pitch-based fibers are produced from low-cost by-products of the destructive distillation of coal, crude oil, or asphalt, and can be separated into two groups: (i) Isotropic Pitch Fibers, which have low mechanical properties (Young’s modulus of 35–70 GPa) and are relatively cheap, and (ii) Mesophase Pitch Fibers, which have very high modulus (above 230 GPa) but are more expensive. The carbon yield for the pitch-based fibers is even higher than for PAN-based ones — it can exceed 60% [52].
3.6.5.3
