Figure 2.7 (a) Raman spectra of amorphous carbon film showing an overlapping with a broad G-band and D-band [22, 23]. (b) Diamond showing a sharp D-band [24] and (c) showing intense G-band and small hump of D-band obtained for CNT and intense peak of G-band for HOPG [25].
When seeds were pyrolyzed to get CNF, the fibrous tissues present in various seeds yield channel-like fibrous structures which show graphitic nature (Figure 2.8).
2.2.2 Plant Metabolites
It has been observed that oils from seeds of plants which are high in concentration of fatty acids (hydrocarbon) serve as excellent precursors for the synthesis of CNMs. Sharon’s group has worked with many plant metabolites such as oil, camphor, latex of Calotropis and cellulose [6]. There are several other studies about the plant-based oils used as precursors [26–29].
Figure 2.8 Characterization of CNF obtained by pyrolysis of plants seeds (Courtesy of Sunil Bhardwaj).
Figure 2.9 Characterization of CNF obtained by pyrolysis of plant oils (Courtesy of Suman Tripathu).
These hydrocarbon metabolites mostly yield CNTs (both SWCNT and MWCNT) and CNB. However, CNF has also been successfully synthesized from plant metabolites. Results of the pyrolysis of some of the oils are presented in Figure 2.9.
2.2.2.1 Characterization of CNF Obtained by Pyrolysis of Plant Metabolites
Plants [30] are a major source of CNM, which can catalytically decompose to liberate carbon atoms and constitute carbon materials of different sizes and shapes.
XRD analysis of all the three oils shows a diffraction peak at 26° {002}, which according to [31] is designated to graphitic carbon. Moreover, peak at 44° is also seen in all three graphs that are known to be associated with carbon {111}. A small peak present at 78° of XRD of castor oil depicts the presence of the Silica, which could be due to the boat in which the catalyst was placed.
Raman Spectroscopic analysis of all the three oils shows D-band as well as G-band, thus confirming its graphitic nature (Figure 2.9).
Plant parts like straw, stem, etc., are not only composed of pure hydrocarbons but also other chemicals; therefore, they produced plant anatomybased CNF. Whereas plant-derived oils that are pure hydrocarbon produced defined CNF.
Cellulose is another plant metabolite which is also secreted by many bacteria and extracted from various marine filamentous algae. Cellulose nanofibers are considered as efficient replacements for conventional polymers due to their nano size, ease of preparation, low cost, tunable surface properties and enhanced mechanical properties. CNF obtained from algal sources is less compared to plants and bacterial sources. CNF finds a wide variety of applications such as drug carriers, tissue regenerating scaffolds, water purification, etc. Mulyadi et al. (2017) [32] have used doped carbon electrocatalyst from cellulose nanofibrils for metal-free oxygen reduction and hydrogen evolution.
2.3 CNF Derived from Parts of Different Plants and Their Applications
The CNF synthesized from different plant-based precursors has been used for various applications. Moreover, CNF has been doped or decorated with metals and conjugated with polymers to enhance its applicability for different purposes.
2.3.1 Hydrogen Storage in CNF
Sharon and Sharon (2012) [5] have noted that plant fibers, being rich sources of carbon and having different inherent plant structures, have yielded carbons with different morphologies, pore sizes, surface areas and densities. Depending on these properties, they have shown differences in their capacity to adsorb hydrogen gas. These findings on hydrogen adsorption capacity, and an analysis of the morphology of the carbon material synthesized from different plant fibers, open a new possibility of correlating the finer structure of the synthesized carbon and its various applications. Variation in structures will help in the future in synthesizing desired types of carbon material from waste plant material for different applications. Carbon materials possessing lesser density, larger surface area, and are more graphitic with less sp3 carbon contribution along with having pore sizes around 10 μm favor hydrogen adsorption. Carbon materials synthesized from bagasse meet these requirements most effectively, followed by cotton fiber, which was more effective than the carbon synthesized from the other plant fibers.
Hollow CNF synthesized by pyrolysis of cotton at 750 °C in argon atmosphere for 3 h, then treated with Ni(NO3)2 and thermally treated in argon for 3 h at 850 °C has exhibited a maximum of 8.75 wt.% hydrogen adsorption [6].
2.3.2 Removal of Heavy Metals by CNF
Arsenic is a naturally occurring dissolved element in ground and surface waters throughout the world [33]. It exists in a different oxidation state in organic and inorganic forms in many environmental matrices such as natural water and soil. The predominant oxidation states of arsenic are As(III), i.e., arsenite, and As(V), i.e., arsenate ions [34], which can bind to give organic materials commonly present in the environment. Arsenic is a ubiquitous trace element, classified as semi-metal or metalloid. The toxicity, availability and environmental mobility of arsenic are very much dependent on the chemical forms in which it exists [35, 36]. The problem with arsenic is that it is frequently found at higher than acceptable concentration due to anthropogenic contributions, including pesticides, herbicides, industrial waste and the burning of fossil fuels [37]. The health concerns associated with arsenic are well known and generally the word “arsenic” is readily associated with “poison” Among its various forms, inorganic arsenic species are known to be more toxic than the organic ones and As(III) is more toxic than As(V). Some of the technologies used for removing this notoriously poisonous material are oxidation, sedimentation, coagulation/ co-precipitation, filtration, ion exchange, membrane/reverse osmosis, biological and adsorption modification. Use of nanoforms of carbon is based on the adsorption principle.
For absorbing arsenic, our group has used CNF synthesized from calyx, hair and maize stem by spectrophotometric method using Safranin-O. The method is based on the reaction of As(III) with Potassium Iodate in acidic medium to liberate iodine. The liberated iodine bleaches the pinkish-red color of Safranin-O. The decrease in absorbance at 532 nm is directly proportional to the As(III) concentration and obeys Beer’s Law [38]. As can be seen from Table 2.1, the surface area of CNF is one of the important parameters in adsorbing arsenic from the water.
Sharon’s group [39] have also analyzed the adsorption of arsenic over CNMs prepared from different oils using Toluidine blue dye. They have shown that surface area is one of the important concerns in adsorbing arsenic from the water and have proposed arsenic removal as another application of CNFs. The adsorption was attributed to the presence of dangling bonds on