carbon source, form metallic carbides easily and enable diffusion or dissolution of carbon through bulk or through the surface of the metal, leading to precipitation of carbon nanomaterial. The phase diagram of carbon and the transition metals, namely iron, cobalt and nickel, indicate solubility. The growth of the CNM starts at the catalytic surface; hence, it is imperative that the size of the catalyst should be in the nanometer scale range, i.e., 1–100 nm. Koziol et al. [9] have proposed two mechanisms for catalyst-fueled growth of CNM by CVD method: (a) Tip Growth Method, which states that after decomposition of the carbon species, the carbon dissolves in the catalyst and diffuses through it and finally precipitates at the end to form carbon nanotubes. The catalyst is predicted to be always present at the top of the growing nanotube. (b) Base Growth Model, which involves bottom carbon diffusion through catalytic particle in which the catalyst particle is proposed to be present at the base of the growing nanotube.2 (ii) The dissolution of the carbon at one part of the catalyst and subsequent precipitation at another part is aided by a temperature gradient which might develop due to the exothermic nature of the decomposition of hydrocarbons. The carbon diffusion parameter depends on the dimensions of the catalyst particles, nature of metal catalyst, temperature and hydrocarbons and gases involved in the process. It has also been proposed that when the substrate catalyst interaction is strong, CNT grows up with the catalyst particle rooted at its base. Conversely, when the substrate catalyst interaction is weak, the catalyst particle is lifted up by the growing nanotube. The driving force for carbon diffusion and subsequent CNM formation is the difference in solubility of carbon at the gas-catalyst interface and the catalyst-CNM interface, determined by the affinity for carbon formation in the gas phase and the thermodynamic properties of the CNM respectively. Qi et al. (2016) [10] have highlighted the effect of not only catalyst but their method of preparation, presence of catalyst support and catalyst promoters also.Synthesis of CNM without any external catalyst has also been successfully done during biogenic synthesis of CNF because some precursor contains certain elements (K, Mg, Ca, Si, O, Fe, S, P) which aid in the formation of CNMs and the presence of an external catalyst is not required. These minerals aid in the growth of the nanoforms by adjusting pore size and modifying the pyrolytic process. Chen et al. (2009) [10] synthesized CNFs using activated carbon produced from agricultural waste by CVD. They reported that the activated carbons containing iron can be used directly to synthesize CNFs. This eliminates the need for the preparation of iron catalyst.
3 (iii) Carrier Gas: Most common carrier gases used during the CVD method and pyrolysis are argon, nitrogen and hydrogen. The most important function of a carrier gas is to remove all traces of oxygen, which might oxidize the CNM produced, and to provide an inert atmosphere for the production of CNMs. Khorrami and Lotfi (2016) [12] conducted a study on the impact of carrier gas flow rates on growth of CNTs over copper catalyst, in which a higher gas flow rate showed a decrease in the growth of CNTs from ethanol precursor. This study involved the deposition of a copper nanolayer on mirror polished Si wafer by sputtering technique. The results led to the conclusion that equilibrium is established between the flowing gas molecules and the adsorbed gas molecules on the catalyst nanolayer. With an increase in carrier gas flow rates most of the catalyst sites are occupied by the carrier gas molecules, hence unavailable for the precursor molecules. This leads to a drop in production of CNTs at higher carrier gas flow rates. Also, it has been observed that a better yield is obtained with hydrogen gas since it helps in the reduction of hydrocarbons present in the precursor.
2.2.1 Plant Parts
Plant materials that are rich in plant fibers have been the material of choice of many carbon nanotechnologists; for example, Paul (2012) [13] has worked on a number of plant species from Northeast India and others have tried sugar cane [14] (Romanovicz, 2013), cotton fiber [15] (Sharon et al., 2011), rice straw [16] (Viswanathan et al., 2014), corncob [17] (Shukla et al., 2012), bamboo [18] (Zhu et al., 2012), etc., as raw materials with promising results. The aquatic algae Euglena [19] has also been tried. The nature of precursor to a large extent determines the type of CNMs obtained. The advantage of using plant fibers is that its inherent anatomy helps in producing the unique morphology of CNF with channel-like structures which otherwise will be difficult to fabricate by other methods.
2.2.1.1 Fibrous Plant Material Used for Synthesizing CNF
Plant stems, leaves and even seeds have fibrous tissues. There are three major plant fibers made up of tissues having thickened walls and complex permanent dead tissue. They are:
1 (i) Mature xylem tissues composed of tracheids, tracheae, fibers (Figure 2.1), which translocates water and mineral in plants.Walls of tracheids, tracheae and plant fibers of all types are composed of cellulose and lignin. Cellulose is a linear polysaccharide polymer composed of many β-glucose monosaccharide units, i.e., in cellulose the acetal linkage is mainly beta. Alpha glucose monomer unit is also found in the cellulose; Lignin is formed by the removal of hydroxyl groups from sugars, creating phenolic compounds and short-chain alcohol ligands. Lignin polymers are heavily crosslinked. The basic monomer of lignin is 4-alkylcatechol.Figure 2.1 Schematic diagram of Tracheid, Tracheae and Xylem fiber.
2 (ii) Sclerenchyma are long narrow tapering tissues with thick side walls. It has great tensile strength and yet are elastic and gives mechanical support to plants. There are two types of sclerenchyma fibers and scelereids (Figure 2.2).
3 (iii) Phloem Fiber or Bast Fiber: Tissues are composed of long cells with lignified cell walls. Flax and hemp are phloem fibers (Figure 2.3).
A look at the SEM images of plant fibers [20], sclerenchyma and xylem tissues (tracheids, tracheae) and CNF fabricated from them gives an existing correlation between the two (Figure 2.4).
Figure 2.2 Schematic diagram of Sclerenchymatous fiber (left) in L.S. view and right in T.S. view.
Figure 2.3 Schematic diagram of Phloem fiber.
Figure 2.4 (a) Maize fiber and (b) maize fiber after pyrolysis as CNF.
2.2.1.2 Characterization of CNF Obtained by Pyrolysis of Plant Seeds
SEM Images: Morphological characterization of various plant materials (Figure 2.5) by many scientists have shown that CNFs derived from plants have channel-like structure, offering many sites for its use in adsorption/ absorption of organic and inorganic materials, gases and liquids.
XRD Characterizations: Characteristic planes for each carbon material obtained by XRD analysis
