for Research in Nanotechnology and Bionanotechnology, WCAS, Solapur, Maharashtra, India
My relationship to plants becomes closer and closer.
They make me quiet; I like to be in their company.
Peter Zumthor
2.1 Introduction
With the advent of nanotechnology, the first question that arose was how to synthesize the desired nanoparticles, resulting in the evolution of many physical and chemical methods [1] to synthesize various nanomaterials. The curious nature of human beings has always directed them to look to nature for solutions to many questions. Primitive man exploited his gray matter to unlock the mystery of plants and the possibilities stored in them, be it for food, shelter, weapons, medicine, source of energy (fire), clothes, etc. With the increase in their intellectual capacities they used plants for developing vehicles, furniture, pulleys and the list goes on. The scenario is no different today; nanotechnologists have turned their attention to plants for solving puzzles to synthesize nanoparticles. Scientists have realized the perfection of nanosized particles (DNA) apparent in their mega impact. Biological materials are highly organized from the molecular to the nanoscale, microscale and macroscale, often in a hierarchical manner with intricate nano-architecture that ultimately makes up a myriad of different functional elements. Nature uses commonly found materials. Properties of the materials surfaces result from a complex interplay between the surface structure and the morphology and physical and chemical properties. Many materials, surfaces and devices provide materials and fibers with high mechanical strength, biological self-assembly, antireflection properties, structural coloration, thermal insulation, self-healing and sensory aid mechanisms, which are just some of the examples found in nature that are of interest in fabricating nanoparticles. Carbon nanomaterials (CNMs) are on the verge of becoming an important material for various industrial applications. One of the hurdles in the production of CNMs is the cost of the raw material. There are two components in producing the CNM, the precursors and the technique of the synthesis. It is almost certain that chemical vapor deposition (CVD) technique would be most suitable for the production of large quantities of CNM. As a result, the CVD technique is being explored by many research groups for the synthesis of CNM. This book encompasses the various aspects of carbon nanofibers (CNF), one of them being their synthesis, and plants have provided a unique platform that offers their metabolites as well as organs as a base or precursor to fabricate CNF.
No doubt there are many chemical precursors, mostly hydrocarbons, that have been used for synthesis of CNF [2]; but most of them are chemicals derived from fossil fuels. Fossil fuels are destined for depletion in the very near future. Hence, as an alternative to fossil fuel-derived materials Sharon’s group started looking at plant-derived materials as a precursor of carbon nanomaterials including CNF, because carbon is known to be a vital constituent of all living organisms.
Sharon’s group was the first to begin developing carbon nanomaterials from both plant parts and plant metabolites over more than the last two decades from many plants [1–5]. The advantages of using plant materials are that they can be cultivated when desired, hence are regenerative material, and are low-cost material. Moreover, plant tissues are composed of oils, lipids, carbohydrates, proteins, cellulose, lignin, etc., which are a rich source of carbon.
Both CNTs and CNFs appear almost the same under SEM analysis. Both are like a hollow cylinder. The outer layer of CNT is made of unbroken graphene sheet whereas CNFs are made of broken graphene sheet. Biogenic CNF has been mostly synthesized by high temperature pyrolysis (Figure 2.13). The parameters that affect the synthesis process are the same as for the standard CVD process of CNM synthesis, i.e., temperature, duration of pyrolysis, carrier gas, catalyst and precursor (plant part or plant metabolite).
2.2 Plants as Source of Precursor for CNF Synthesis
Most of the plant parts that have been used as precursor for the synthesis of carbon nanomaterial have yielded CNF. Whereas, plant-derived products or metabolites have produced different forms of carbon nanomaterials (CNM) such as single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon nanobeads (CNB), etc. Almost all parts of plants (stem, roots, leaves, seeds, etc.), and many plant-derived products (camphor, juice, latex, oil pinene, resins, etc.) have been used as precursors of CNMs. Both plant parts and plant metabolites have wide availability, making it a suitable choice. Moreover, they are composed of many different hydrocarbons, a rich source of carbon. Synthesis of CNF from plant parts as well as plant derivatives is mostly done by the process of chemical vapor deposition (CVD) under pyrolytic conditions. The setup used for the CVD process is explained in detail in Chapter 1 of this book. The CVD method involves thermal decomposition of the carbon-containing material into carbon vapors. The carbon vapors are deposited on the catalyst, usually transition metals, leading to formation of various forms of carbon nanomaterials.
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
