For preparing CNF from plant sources, the traditional CVD process is slightly modified. However, the various parameters which play a definitive role in the type of CNM being synthesized by the standard CVD process remain the same. The parameters are:
1 (i) Precursor: Impact of precursor can be explained by the pyrolysis of bamboo, which forms CNM only in the range of 1200 to 1400 °C. This is because bamboo contains CaSiO3 which melts in this range. But at temperatures greater than or equal to 1500 °C, there is no CNT formation as the nature of CaSiO3 changes. There have been efforts to thermally (calcining) or chemically [6] pretreat plants prior to pyrolysis.
2 (ii) Pyrolysis Temperature: Controls the yield and nature of CNMs; yield of CNM at higher temperature is significantly higher than at lower temperatures. This is because at higher temperature carbonaceous compounds are completely cracked into carbon. Moreover, reformation of graphene-like honeycomb structure of carbon arrangement into different forms on the CNM also depends on the CVD temperature. Moreover, when the algae Euglena was used as precursor [7] for CNM he found that the diameter distribution of MWCNTs increases with increasing temperature. The lower the temperature, the narrower the diameter distribution. At higher temperature the diffusion of carbon particles in the catalyst or vice versa is more facile.
Not only plant parts but also plant metabolites exhibit the cardinal role of temperature on the formation of CNM; for example, oil being pure hydrocarbons is accepted as suitable precursor for synthesis of CNM, also shows the impact of temperature during pyrolysis in the formation of CNM. While reaction at lower temperature does not lead to complete decomposition of oil into graphitic carbon synthesis, at higher temperature it dictates higher yield because the stability of the graphitic carbon is affected at higher temperatures, which narrows the temperature range at which CNTs are obtained in scalable amounts. Here precursor also plays an important role. The most dramatic effect of temperature on the quantity and quality of CNMs is shown in the study of Karthikeyan and Mahalingam [8], who have synthesized CNMs using pine oil, methyl ester of Jatropha curcas oil and methyl ester of Pongamia pinnata oil at comparatively lower temperatures of 550°, 650° and 750 °C. Their study shows a relatively low yield of CNTs at lower temperature of 550 °C, while at 650 °C the formation of well-defined MWCNTs is high as the decomposition of vapors into carbon over catalyst is more effective. Conversely, the studies show a rise in yield of amorphous carbon upon raising the temperature to 750°C. Hence, the intermediate temperature of 650 °C is found to be optimum for producing graphitic carbon. A similar impact can be exhibited during formation of CNF.
1 (i) Catalyst: Transition metals serve as excellent catalysts for synthesis of CNMs as they enable catalytic decomposition of 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 (