Natural and waste hydrocarbon precursors for the synthesis of carbon based nanomaterials: Graphene and CNTs

Renewable and Sustainable Energy Reviews 58 (2016) 976–1006

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Natural and waste hydrocarbon precursors for the synthesis of carbon based nanomaterials: Graphene and CNTs Rajesh Kumar a,n, Rajesh Kumar Singh b,n, Dinesh Pratap Singh c a

Centre for Semiconductor Components, State University of Campinas (UNICAMP), Campinas, Sao Paulo 13083-870, Brazil Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India c Departamento de Física, Universidad de Santiago de Chile, Avenida Ecuador 3493, Estación Central, Santiago 9170124, Chile b

art ic l e i nf o

a b s t r a c t

Article history: Received 8 August 2015 Received in revised form 11 December 2015 Accepted 17 December 2015

Carbon nanomaterials have huge potential in the field of energy and environmental applications. However, a wide range of greener and environment friendly synthesis methods utilizing natural, renewable, cheaper waste materials has to be developed. This will lead to the reduction of green house gases, exploitation of toxic materials and helps in the development of sustainable technologies. In this review, the details progress made in the last ten years concerning the synthesis of new one dimensional (carbon nanotubes CNT, carbon nanofiber) and two dimensional (graphene) carbon based materials using natural precursors and waste materials is summarized. The aim of this review paper is to provide a comprehensive scientific progress of synthesis of graphene and carbon nanotubes using natural precursor and waste materials for the future perspective. This paper also concludes with a brief discussion on the impact of natural precursor for the graphene and CNTs for environment, its toxicological effects and its future prospects in this rapidly emerging field. Natural precursors and waste carbon containing products are emerging as a new class of materials that have efficiency to produce graphene and CNTs. The various synthesis processes of graphene, CNTs and carbon dots has been reported using several natural hydrocarbon precursors (turpentine oil, eucalyptus oil, palm oil, neem oil, sunflower oil, castor oil, biodiesel, tea-tree extract, honey, milk, sugar, butter, egg etc.). Also, some research groups have used foods wastes (cookie and chocolate), vegetation wastes (woods, leaf, grass, fruit wastes), animal/bird/ insect wastes (bone and cow dung, dog feces, chicken feather) and agro waste (sugarcane bagasse) for the synthesis of graphene and CNTs. Research on natural hydrocarbon precursors and wastage materials has increased in recent years as they promise to produce better and high quality of graphene and CNTs in large quantities. The fascinating aspect of this research area is that it guides the use of natural hydrocarbons to explore the possibilities of improving graphene stability and robustness suitable for different type of applications. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Graphene Carbon nanotubes Natural precursors Waste products

Contents 1. 2. 3.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 977 Carbon based fossil hydrocarbon for synthesis of graphene and CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 978 Carbon based natural hydrocarbon precursors, waste materials and food products for the synthesis of graphene and CNTs (SWCNTs/MWCMTs) 978 3.1. Solid natural hydrocarbon precursor: soild camphor (C10H16O) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 979 3.2. Liquid natural hydrocarbon precursors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989 3.2.1. Turpentine oil (C10H16) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989 3.2.2. Eucalyptus (C10 H18O) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 991 3.2.3. Palm oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 991 3.2.4. Neem oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992 3.2.5. Sunflower oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992 3.2.6. Jatropha-derived bio-diesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992

Corresponding authors. E-mail addresses: [email protected] (R. Kumar), [email protected] (R.K. Singh).

http://dx.doi.org/10.1016/j.rser.2015.12.120 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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3.2.7. Castor oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 3.2.8. Sesame oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 3.2.9. Camphor oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 3.2.10. Tea tree extract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 3.3. Eatable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995 3.3.1. Honey, sugar, butter, milk and cheese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995 3.3.2. Chicken eggs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 3.4. Solid natural waste and industrial carbonaceous products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997 3.4.1. Food, insect and other natural waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997 3.4.2. Natural and industrial carbonaceous waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 3.4.3. Camphor dead leaves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 3.4.4. Agro sugarcane bagasse waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 3.4.5. Solid plastic waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 3.4.6. Chicken feather waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004

1. Introduction Carbon based various confined nanostructures like fullerene, carbon nanotubes (CNTs) and recently discovered graphene have attracted considerable worldwide attention and are studied extensively [1–6]. Carbon based one dimensional and two dimensional materials such as CNTs and graphene, respectively, has emerged as an exotic material of the recent technology time, and received world-wide attention due to its exceptional and unique charge transport, thermal, optical, and mechanical properties. Graphene and its derivatives are being studied in nearly every field of medical, science and engineering. Recent progress has shown that the CNTs and graphene-based hybrids materials can have a profound impact on chemical sensors, nanocomposites, electronic, optoelectronic devices, and energy storage. The Graphite confined in two dimensions named as graphene, and in one dimension as CNTs have demonstrated remarkable properties, including high aspect ratio, high specific surface area, high chemical stability, high mechanical strength, and so on [7–11]. The structures of graphene and CNTs are composed of hexagonally ordered sp2-hybridized carbon atoms. Besides recently achieved single-atom thick perfect two dimensional structures: Graphene, comprised of a monolayer of hexagonally arranged carbon atoms has received a great interest, because of its high electron mobility, thermal conductivity, elasticity, and stiffness [3,12–14]. These properties make graphene attractive for application in nanoelectronic devices, sensors, functional composites, and energy storage [15–19]. Specially, recent advancements in graphene research have opened up a wide range of applications in electrochemical devices, energy storage, catalysis, cell imaging, photochemotherapy, drug delivery, biosensors, contamination purification, and extraction devices for chemical, biological, and environmental samples [20–24]. The synthesis of new generation nanocarbon materials, including smart materials exhibiting high performance and multifunctionality have become a topic of interest. Precursors used for growing carbon based materials as graphene and CNTs have a critical role in deciding the viability of a technology. To date, several carbon precursors such as methane, acetylene, benzene, xylene, toluene, etc. have been used as a carbon feedstock to synthesize CNTs [25–29]. These carbon precursors are related to fossil fuels and there may be a crisis for these precursors in the near future. Therefore in order to develop a more competitive carbon material, it is necessary to consider developing carbonbased materials from the natural resources. The main motivation of utilizing these sources is to provide “green” alternatives and

cheap raw materials for industrial-scale production of clean carbon based structures. Natural precursors as source of hydrocarbons, which are renewable and cheap, have the potential to be the green alternative for industrial scale production of graphene and CNTs. The advantages of using natural precursors as a carbon feedstock for synthesizing CNTs is that they are inexpensive and have no chance of shortage in the near future. According to the principle of green chemistry, the feed stock of any industrial process must be renewable, rather than depleting a natural resource. Moreover, the process must be designed to achieve maximum incorporation of the constituent atoms (of the feed stock) into the final product. The use of natural precursors for synthesis of functional carbon nanomaterials has recently attracted significant attention as a safe and environmentally-benign route towards a diverse range of applications. The hydrocarbon based natural precursors and waste materials are already available in abundance on earth to prepare large amounts of carbon based materials as CNTs and graphene, so the subjects of synthesis precursors are available in sufficient amounts. With pyrolysis, plantderived precursors and wastes such as seeds, fibers, oils, and bagasse have yielded different forms of carbon. The study of ultrastructures of plant material and carbon produced by them has revealed that plant-derived precursors have some basic hydrocarbon compound containing skeletal structures. The market prices of natural precursors are very low and cheap as comparison to fossils gaseous and liquid hydrocarbons. The starting raw natural hydrocarbon materials transform into 1D and 2D carbon materials. These natural precursors supplies an environmentally friendly approach for large-scale production of CNTS and graphene for different application. Hence, it is the time's prime demand to explore regenerative materials for CNTs and graphene synthesis with high efficiency. It is clear that scientists and engineers working in many different directions and finding their inspiration from materials, chemical, and physical sources, are all contributing greatly to the field of carbon based new materials. Researchers have prepared high quality graphene, single-walled CNTs (SWCNTs), multi-walled CNTs (MWCNTs) and carbon dots by thermal decomposition of turpentine oil, sesame oil, neem oil (Azadirachta indica), eucalyptus oil, palm oil, jatropha oil, camphor, tea-tree extract, waste food, insects, agro waste, food products etc. [30–42]. Being a green-plant product, these oils are eco-friendly source and can be easily cultivated in as much quantity as required. Also natural and industrial waste products are available cheaply which can be used for hydrocarbon precursors for the synthesis of graphene and CNTs and their use has beneficial environmental impact. Unlike any

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fossil/petroleum hydrocarbon source, there is no fear of its ultimate shortage because of extensive exploitation as they are regenerative sources. These sources are readily available, cheap and also environmental-friendly for chemical vapor deposition (CVD) due to its volatile and non-toxic nature. As larger quantities of graphene and CNT materials reaches the consumer market, it will also be necessary to establish disposal and/or reuse procedures. As one of the most important branches of current nanotechnology, the fabrication of graphene and CNTs are necessary employing natural and waste products present in environment. Their advantageous applications have triggered detailed studies in theoretical as well as applied research. In this review we have explored in detail the synthesis of 2D graphene and 1D-CNTs (MWCNTs/SWCNTs) using natural hydrocarbon, food and waste products precursors as a carbon source. The various facile and rapid approaches for the clean and efficient synthesis of graphene and CNTs have been described in detail. The advantages of natural and waste hydrocarbon precursors are compared and summarized according to the available literature till date. In addition, new trends in materials development have also been proposed. It is obvious that the importance and number of natural hydrocarbon/waste materials approaches will grow dramatically in the coming years with the rise of carbon based nanotechnology applications. The bifurcation of these natural and waste products for the synthesis of CNTs and graphene is shown briefly in Fig. 1.

2. Carbon based fossil hydrocarbon for synthesis of graphene and CNTs Graphene as well as CNTs has been synthesized using fossil hydrocarbons which are costly and not easily available. The graphene and CNTs have been generally synthesized as products of the action of a catalyst on the fossil gaseous species originating from the thermal decomposition of hydrocarbons. Different types of CNTs, carbon nanofibers (CNFs), vapor grown carbon fiber and other types of carbon nanostructure materials has been produced by various routes. The most common techniques used for the

Fig. 2. Synthesis of graphene and CNTs using different liquid and gaseous hydrocarbon fossil precursors.

synthesis of carbon based materials are arc discharge, laser ablation and CVD etc. Heterogeneous CVD processes simply involve passing a gaseous or liquid flow containing a given proportion of a hydrocarbon mainly benzene (C6H6), xylene (C8H10), toluene (C7H8), hexane (C6H14) ethanol (CH3OH), methanol (C2H5OH), methane (CH4), acetylene (C2H2), ethylene (C2H4) and carbon monoxide (CO) etc. (usually as a mixture with either H2 or an inert gas such as Ar) over small size metal particles (Cu, Fe, Co, Ni) in a furnace [43–46]. Solid carbon source as polyimide, poly methyl methacrylate (PMMA), and coal has been also used for the synthesis of graphene and CNTs [47–54]. Most of the gaseous and liquid fossil hydrocarbons are toxic or explosive and are not good for atmosphere as well as human health. Several review articles are published using fossil hydrocarbon as carbon source for the synthesis of graphene and CNTs [6,55–60], but only few reports shows the synthesis of graphene and CNTs by naturally occurring precursors and waste products [61–63]. Fig. 2 shows the different gaseous and liquid hydrocarbon fossils frequently used for the synthesis of CNTs and graphene employing different metal catalysts at higher temperature.

3. Carbon based natural hydrocarbon precursors, waste materials and food products for the synthesis of graphene and CNTs (SWCNTs/MWCMTs)

Fig. 1. Natural and waste liquid and solid hydrocarbon precursors used for the synthesis of graphene and CNTs.

Natural botanical hydrocarbon precursors and waste natural products are the source of hydrocarbon which is renewable, cheap and easily available for the synthesis of different carbon nanostructures. These natural waste and precursors have potential applications for synthesis of different carbon nanostructure and can be used for the industrial scale production of graphene and CNTs. As one of the most important branches of current nanotechnology, the fabrication of graphene and CNTs is significant by using natural precursor present in environment. There are several advantages of using natural precursors and waste products as a carbon feedstock for synthesizing high quality CNT and graphene. Use of these precursors produces scalable amount, are environmentally benign and cost-effective. The transformation of natural precursors and waste products into high-quality CNTs and graphene structures, which could be promising for a variety of advanced electronic and energy applications are beneficial. Fig. 3 shows the different type of natural precursors, eatable food and waste materials used for synthesis of various new novel carbon nanostructures.

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Fig. 3. Illustration of the current natural precursors, eatable food and waste hydrocarbons used for synthesis of various carbon related new materials.

Fig. 4. TEM images of the (a) single, (b) bi and (c) few-layer graphene using camphor [30].

3.1. Solid natural hydrocarbon precursor: soild camphor (C10H16O) Large area, single or few layer, high-quality graphene and CNTs has been produced recently using different natural precursors and waste hydrocarbons by CVD and spray pyrolysis which makes this one of the most promising materials for industrial-scale fabrication of graphene. Solid plants based natural botanical hydrocarbon camphor (C10H16O) is obtained from the latex of cinnamomum camphora tree of lauraceae family. It is a white crystalline solid that sublimates easily at room temperature and melt at 180 °C. It has long been valued for its great medicinal uses in the East but remained less known in Europe and America. It is commonly used in homes as an insect repellent and also in sweets to keep them disinfected from germs. Some research group has synthesized few layers graphene as well as CNTs using camphor as a carbon source with the help of some metal catalyst [30,64–68]. Camphor materials have been extensively used for the synthesis of graphene and CNTs. Sharma et al. and his group produced domain size of single layer and bi-layer and few-layer graphene on nickel foil using solid camphor as a carbon source by the CVD process at 800 °C in different (H2 and Ar) atmosphere [30]. The synthesized graphene structure shows the significance difference in domain structure with different gas atmosphere. The few layers graphene domain

grown with the carrier gas Ar/H2 mixture shows larger size than those grown with pure Ar carrier gas. Fig. 4a–c shows high resolution transmission electron microscopy (HRTEM) images of the graphene film using camphor precursor in Ar þH2 atmosphere and this shows bi-layer and few layer graphene on the edges. Graphene sheets having different number of layers were observed in the TEM studies. The TEM study shows that the deposited graphene film contains single and bi-layer graphene along with the few-layer graphene. The graphene film synthesized in the presence of H2 and Ar gas, and shows large size (30–50 μm) graphene domains. The formation of graphene domains appears due to segregation and precipitation of high amount of carbon from the source material in the grain boundaries of the Ni surface. Without H2 gas there is no formation of graphene domains. Fig. 5 shows the optical and scanning electron microscopy (SEM) image of graphene domains at different mixed gas atmosphere. Fig. 5c shows the SEM image of the graphene film deposited in Ar atmosphere, presenting formation of graphene domains of few micron size. The graphene film synthesized in ArþH2 atmosphere shows completely different domain size structure as shown in Fig. 5d. The significant difference in contrast of the graphene domain appears, which presents formation of domains consisting different number of layers with respect to grain and grain

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Fig. 5. Optical images of the graphene film deposited in (a) Ar and (b) Ar þ H2 atmosphere. SEM images of the graphene film deposited in (c) Ar and (d) Ar þH2 atmosphere [30].

boundaries of the Ni foil. The increase in domain size can be attributed to the enchantment of Ni grain during annealing and better coordination of segregated carbon to enhance the lateral growth of graphene in presence of H2. Some other groups have also synthesized graphene by the pyrolysis of camphor as a solid hydrocarbon precursor on Cu and Ni catalyst substrate [64,65]. Kalita et al. [64] reported the synthesis of large sized graphene sheets by controlled pyrolysis in CVD furnace at 800 °C on polycrystalline Ni using camphor as shown in Fig. 6. The synthesized graphene sheets using camphor was used for the fabrication of transparent electrodes. They reported that this technique to fabricate few layer of graphene as transparent electrode from camphor is viable as well as scalable for potential large area optoelectronic applications. They have synthesized highly ordered graphene sheet with minimum defects for thinner (4 layers) and thicker (13 layers) graphene sheets. Iodine-doped graphene has also been successfully fabricated through a simple and economical approach. In a later report, Kalita et al. [65] demonstrated a simple and controllable synthesis process of iodine-doped graphene film using camphor. Camphor mixed with iodine precursor are simultaneously evaporated for CVD pyrolysis at 800 °C with argon as carrier gas on a Ni substrate. Fig. 7a shows the TEM image of few-layers graphene sheet synthesized from camphor pyrolysis. Fig. 7b presents the HRTEM image at the edge of the graphene sheet showing few layer of graphene. Polycrystalline Ni has micron size grain domains. The TEM studies also showed that the structure of graphene films did not changes with iodine doping and are similar to that of a pristine graphene. The growth of controlled single and few-layer graphene through CVD process is remarkable for different device application. In 2011, Kalita et al. [66] synthesized monolayer and few layer graphene using camphor by CVD process at 1020 °C with H2 as carrier gas on Cu foil. HRTEM images of pristine graphene sheets derived from camphor are shown in Fig. 8. Fig. 8a–c presents monolayer, bilayer and trilayer graphene sheet.

The authors also described the decomposition of camphor at higher temperatures and formation mechanism of graphene on Ni and Cu substrate. Fig. 9a presents molecular structure of camphor, which is composed of hexagonal and pentagonal ring with methyl carbon [66]. The carbon solubility and surface catalytic activity jointly effect the graphene growth through surface diffusion of atomic carbon and low carbon solubility in Cu plays the important role for mono layer graphene. The carbon rings from camphor can directly diffuse on Cu surface to form large area graphene film. The molecular composition of camphor influences the formation of graphene. The abundance of hydrogen and presence of oxygen in camphor allows good coordination between the substrates and helps in the graphene formation. Monolayer and few-layer graphene formation processes from camphor on Ni and Cu foil, respectively, are presented in Fig. 9b. It has been observed that camphor pyrolysis on Ni substrates always form few-layers graphene films rather than mono layer graphene. This is due to the fact that, Ni has high carbon solubility and has small grain size and the multilayer graphene forms in the grain boundaries. These results indicate that the suppression of the carbon dissolution and substrate preferential diffusion is a key factor for single-layer graphene growth from camphor. The CVD process for the production of continuous-high qualitygraphene layers on polycrystalline Ni foil at 850 °C and Ar as an inert carrier gas has also been reported by Ravani et al. [67]. Catalytic decomposition of camphor molecules leads to a continuous coverage of graphene films on the polycrystalline Ni surface. SEM images in Fig. 10 shows that the produced graphene films are wrapped as carpets on the polished Ni surface and there sizes varies from 1 to 5 μm as shown in Fig. 10a–c. The grain boundaries are visible under neath the graphene carpets. The wrinkled feature of the carpets is due to the difference of thermal coefficient between the Ni substrate and graphene. The ultrathin transparent graphene films with size up to 0.7 μm are illustrated in Fig. 10d while in the background low height darkened ridged features are formed. Furthermore, well-shaped hexagonal domains reflecting the hexagonal graphitic structure are clearly presented in Fig. 10e;

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Fig. 6. Presentation of TEM image for (a) thinner and (b) thicker graphene layers synthesized from camphor pyrolysis. HRTEM image at the edge showing (c) 4 layers of graphene for thinner graphene sheet and (d) 13 layers of graphene for thicker graphene sheet (inset intensity pattern of graphene layers) [64].

Fig. 7. (a,b) TEM image of a graphene sheet deposited from camphor iodine mixture. (b) The edge of the graphene sheet shows presence of few layers of graphene [65].

the size of the hexagons is estimated to be  1.3 μm. The AFM image shown in Fig. 10f shows the buckled graphene surface topology. The measured height of the formed wrinkles is about 30 nm in accordance with the STM data. Some other research groups have also used camphor as carbon source for synthesis of synthesis of SWCNTs and MWCNTs using

camphor as botanical hydrocarbon precursor [69–82]. The SWCNTs and MWCNTs are grown from pyrolysis of camphor with ferrocene in the temperature range 800–1050 °C in argon atmosphere [69]. High yield and pure MWCNTs have been prepared by catalytic decomposition of camphor, an unconventional precursor. Crystallinity of these nanotubes is fairly good; formation of

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Fig. 8. Presentation of HRTEM images of graphene sheet at the folded edge showing (a) monolayer (b) bi-layer and (c) tri-layer graphene sheet synthesized from botanical precursor camphor [66].

Fig. 9. Representation of (a) molecular structure of camphor and (b) monolayer graphene formation mechanism on copper foil is presented. In a camphor molecule there are hexagonal and pentagonal carbon rings along with methyl carbon. During pyrolysis and rapid cooling process atomic carbon from camphor can be segregated on low carbon soluble copper to form mono layer graphene [66].

amorphous carbon is extremely low, and presence of catalyst particles in as-grown CNTs is almost negligible. Hence, no postdeposition purification is required. Fig. 11a and b shows TEM image of as-grown MWCNTs illustrating a high yield formation with uniform diameter of approximately 30–40 nm. Also SWCNTs formed have uniform diameter 1.2–1.3 nm (Fig. 11b and c). In addition to MWNTs, SWNTs can also be produced in single experiment by varying the temperature and camphor. The SWCNTs and MWCNTs of controlled diameter distribution were grown by thermal decomposition of camphor, on a high-silica zeolite support impregnated with Fe–Co catalyst as reported by Kumar et al. [70]. Effect of growth temperature was investigated in a range of 500–1000 °C in steps of 25 °C. Fig. 12 shows typical TEM images of MWNTs grown at different temperatures. At 550 °C, very short-length tubes emerges from the zeolite pores suggest that the catalyst activity and hence the CNTs growth rate is quite low. On the other hand, 600 °C samples clearly show a profound CNTs

growth from all the pores around the zeolite. TEM investigation of a large number of samples prepared at several temperatures reveals that the tube diameter increases with the increasing temperature. In the temperature range of 575–700 °C, the diameter distribution is almost constant (4–14 nm), however, the tube density increases gradually. The occurrence of SWNTs dramatically increases in the samples at 850 °C. Also upward and thick bundles of SWNTs were formed at 900 °C (Fig. 13). The MWCNTs has been also synthesized by Antunes et al. [71] using pyrolysis of camphor mixed with 16% of ferrocene, at 850 °C at atmospheric pressure. The postvacuum treatments give MWCNTs with a purity degree higher than 99% and improve the graphitic ordering. Fig. 14 shows TEM images of MWCNTs before and after thermal annealing at 1800 °C under vacuum for 2 h. After vacuum annealing at high temperature, most of the iron nanoparticles get removed from the surface and inner core of MWCNTs. Andrews et al. [74] also synthesized MWCNTs from camphor and investigated the effect of camphor’s molecular structure on the MWCNTs growth and quality. The alignment technique that can create uniform and thick CNT films on a large area is still required to take full advantage of anisotropic charge-transport and enhanced performance as highstrength composite materials. Therefore, scalable and aligned CNTs (ACNT) arrays are studied vigorously as a material to produce high strength and lightweight structures. Kumar et al. [72] synthesizes the sub-gram quantities of high purity ACNTs using camphor and ferrocene in Ar atmosphere employing CVD method at 900 °C. The vertically aligned MWCNTs have diameter 20–40 nm and densely packed vertical columns with length of 200 mm. Fig. 15a shows the SEM image of CNTs grown from camphor on a flat quartz substrate. An edge-on view of the same sample (Fig. 15b) clearly depicts the aligned and perpendicularly grown CNTs on the sidesurfaces of the substrate. As well as, each vertical column consists of innumerous nanotubes self-organized into long ropes, as seen in the magnified SEM image (Fig. 15c). Further from high magnification SEM images, CNTs density was estimated to be the order of  1010 nanotubes/cm2. High quantity of MWCNTs was obtained by pyrolyzing only 0.2 g camphor per run and nearly 25% of the feedstock gets converted into desired product. The authors have also suggested that it is possible to increase this product/feed ratio

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Fig. 10. SEM and AFM micrographs of the produced graphene films using camphor. (a) An extended bouncing of wrinkles. Ni boundaries are marked with dashed arrows. (b)–(d) Free standing few layers of graphene. (e) Well shaped hexagonal domains. (f) AFM image of flat and wrinkled graphene carpets on Ni surface [67].

Fig. 11. (a, b)MWCNTs and (c, d) SWCNTs images using camphor as hydrocarbon source [69].

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Fig. 12. TEM images of MWCNTs grown at (a) 550 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C, (f) 950 °C temperature [70].

Fig. 13. HRTEM image of SWNTs grown at 900 °C [70].

by constraining the out-flowing pyrolyzed camphor vapor to the reaction zone of the furnace. The sizes and the distribution of vertically ACNTs play important roles especially in the field emission application. Vertically ACNTs, grown on various substrates by thermal decomposition of camphor, were investigated for field emission application by Kumar et al. [73]. Fig. 16 shows typical SEM images of nanotubes grown in CVD reactor at 850 °C on a quartz and n-type silicon

substrate with camphor (200 mg) and 1 wt% ferrocene. Low magnification image (Fig. 16a) illustrates the uniform growth of vertical aligned dense nanotubes of as long as 120 mm. Magnified image (Fig. 16b) reveals that the individual CNTs are not perfectly parallel but considerably curled and entangled securing a good electrical contact all across. Similar but relatively short-heighted growth was observed on silicon and cobalt-coated silicon (Co/Si) substrates. CNTs grown from simple pyrolysis of camphor have shown appreciable field emission properties, such as low turn-on voltage (2–3 V/mm), high current density (14 mA/cm2 at an applied field of 7–8 V/mm), moderate vacuum requirement (10  5– 10  6 Torr), low degradation and good reproducibility of results. The synthesis of ACNTs of high quality and in large quantity remains a considerable challenge for researchers. Kumar et al. [83] used camphor to produce ACNTs in large quantities; like a nanotube garden containing SWCNTs and MWCNTs. These SWCNTs and MWCNTs grow as a result of thermal decomposition of camphor and ferrocene through CVD at 875 °C in Ar atmosphere. SWCNTs are found in low quantity, whereas the yield of MWCNTs is as high as 90%. Moreover, vertically aligned nanotubes grow on large-area quartz substrates inserted in the reaction zone. The as grown CNTs show good crystallinity, with extremely low unwanted catalyst particles. The length of aligned CNTs is  100 μm as shown in Fig. 17. The dense and uniform CNTs growth on the entire

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Fig. 14. TEM images of the iron located inside the nanotubes and into their walls: (a) as-grown MWCNTs powder and (b) after purification at 1800 °C in vacuum [71].

Fig. 15. SEM images of as-grown aligned nanotubes on a flat quartz substrate: (a) top view, (b) side view, and (c) magnified view [72].

substrate indicates the uniform dispersion of uniform-size catalyst particles on the substrate. Somani et al. [84] synthesized vertically aligned carbon nanofibers (CNF) and MWCNTs by catalytic thermal CVD method (700– 800 °C) with Ar as carrier gas on Co and Co/Fe thin films (for CNF) and on silicon substrates using a mixture of camphor and ferrocene (for MWCNTs). Fig. 18a and b shows the SEM images of the CNF film deposited on quartz–Co and quartz–Co/Fe thin films. The diameters of as grown CNF are different for Co (150 nm) and Co/Fe (35 nm) thin films. Also the density of the CNF grown on Co film is higher as comparison to Co/Fe films. Fig. 18c shows the FE-SEM image of the vertically aligned MWCNTs grown from camphor on silicon substrates. HRTEM image of the MWCNTs (Fig. 18d) shows the diameter ranging from 35 to 45 nm. Authors have mentioned that the camphor derived MWCNTs show the good performance for field electron emission properties because of its highest aspect ratio, good graphitization and good density. Italian carbon research group investigated different aspects of camphor for the growth of aligned ACNTs and published a series of reports. Musso et al. [78] achieved low-temperature (650 °C)

synthesis of vertically ACNTs by CVD on various substrates using camphor and ferrocene for catalyst and nitrogen as carrier gas. Fig. 19 shows the SEM images of vertically ACNTs and well packed to form a layer of uniform thickness over the silicon substrate. Fig. 19a shows a delaminated fragment of ACNTs layer and it appears as CNTs carpet with 4 μm thickness. Fig. 19b shows the side view of the ACNTs carpet, whose is surface free from Fe nanoparticle and also from amorphous carbonaceous materials. Porro et al. [79,85] synthesized ACNTs using camphor and ferrocene by CVD method at 850 °C on silicon and quartz substrates in nitrogen atmosphere. The authors have performed the hydrogen storage application of as synthesized ACNTs and they have found an adsorption capacity of about 1.7 wt%, at 14 bar and 190 °C. The SEM images of as grown ACNTs shown in Fig. 20 and it appears as carpet of well aligned CNTs normal to the substrate surface and the average diameter of 80 nm, and a length that varies up to few hundreds of micrometers. Musso et al. [80,81] reports the growth of ACNTs using camphor and ferrocene (20:1) by catalytical CVD method at 850 °C in nitrogen atmosphere on bare silicon substrate [80]. The as grown

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Fig. 16. Typical SEM images of ANTs grown on a quartz substrate at low (a) and high (b) magnifications [73].

Fig. 17. SEM images of CNTs grown like a garden on a quartz substrate [83].

ACNTs are MWCNTs mats and covers large area with millimetersthick sheets on silicon substrate. The as grown millimeters-thick large area self-standing blocks of ACNTs show outstanding

mechanical properties and are highly hydrophobic. Fig. 21 shows the SEM images of camphor derived aligned MWCNTs. Fig. 21a shows that the MWCNT mats is thick, aligned and self standing

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Fig. 18. SEM of CNF grown on (a) quartz–Co film and (b) quartz–Co–Fe film (c) FE-SEM of vertically aligned MWCNTs films on Si substrate (d) TEM image of the MWCNTs (Inset shows the intensity pattern along the line marked in the figure and yields the diameter of MWCNTs) [84].

Fig. 19. SEM images of samples grown on silicon substrates at 650 °C (a) a delaminated CNTs carpet made from camphor–ferrocene (b) magnified view of ACNTs carpet [78].

Fig. 20. SEM images of ACNTs synthesized from camphor–ferrocene [79].

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Fig. 21. Images of the MWCNT mats. (a) SEM image of a CNT-coated substrate edge (b) SEM image showing the macroscopic vertical alignment of the MWCNT and (c,d) SEM images with increasing magnification, showing the diameter distribution and the entanglement of nanotubes [80].

Fig. 22. SEM images of vertically ACNTs with different temperature and nitrogen flux (a) thick (850 °C, 400 sccm) and (b) thin layer (700 °C, 1500 sccm) [82].

with 0.5 g/cm3 volume density. Fig. 21d shows the higher magnification micrograph which confirms that the maximum MWCNTs are aligned but some of them are not fully aligned. Later, the same group studied and published the report on fluid-dynamic analysis of the carrier-gas (nitrogen) flow for camphor-CVD system [81]. In this work authors have investigated the correlation between growth rate of the carbon material and the development of secondary transversal vortex flows caused by the effects of temperature gradient inside the deposition system. They have found carpet of vertically oriented nanotubes, with spare signs of other forms of carbon (650–900 °C), nano-graphite layer and carbon fibers (950–1070 °C). Pavese et al. [82] worked on the synthesis of camphor derived (with ferrocene) two different CNTs structures by the varying the synthesis temperature (700 and 850 °C) and nitrogen gas flux (400 and 1500 sccm) by CVD method on silicon substrate. First structure was synthesized at 850 °C temperature at a nitrogen flux of 400 sccm with a camphor/ferrocene mass ratio of 20. The structure obtained at this condition comprised of thick carpets (2 mm thick) of vertically ACNTs (height mill metric range and diameter 30–

80 nm) (Fig. 22). The second structure was synthesized at different camphor–ferrocene mass ratio (camphor/ferrocene mass ratio 30) at lower temperature (700 °C) and higher nitrogen flux (1500 sccm) and the structure obtained at this condition was a thin layer (5–20 mm) of entangled CNTs (diameter10–40 nm) (Fig. 22a). The authors have measured the contact angle and hydrophobic behavior, depending on the nanotubes orientation inside the layered structure. In the vertically ACNTs the contact angle was exceptionally high (176°), while over the horizontally aligned entangled CNTs has an angle of 144°. It confirms that vertically ACNTs have more hydrophobic nature as compared to aligned entangled CNTs. The intentional introduction of impurities (dopants) into CNTs via the substitution of carbon atoms with nitrogen atoms has been shown to modify the chemical, electrical and structural properties of the CNTs [56,86,87]. Ghosh et al. [88] used conventional CVD furnace at 800 °C and atmospheric pressure, to grow N-doped CNTs from camphor, using dimethylformamide (DMF) as a nitrogen source. Camphor– DMF-grown CNTs (CD-CNTs) have corrugated outer walls and rounded tips. Fig. 23a and b shows the typical SEM and TEM images of CD-

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Fig. 23. (a) SEM image of CD-CNTs and (b) TEM image of CD-CNTs (inset HRTEM) [88].

Fig. 24. High resolution TEM images of as-grown MWCNTs at (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C and (e) 900 °C [31].

CNTs. This image clearly indicates that CD-CNTs have curled hair-like array and an average length of 200 mm. TEM image shows that the inner channel is blocked due to inter nal cappings giving rise to the bamboo-like structure. Most probably the pentagonal carbon fragments of pyrolized camphor are responsible for the outer surface corrugation of the CD-CNTs. The average diameter of CD-CNTs in the body region is  50 nm whereas at the tip is 25 nm, indicating the body-to-tip diameter ratio of 2. The microstructure of N-doped CNTs is governed by the nitrogen concentration up to certain extent, but greatly on the type of the nitrogen moieties incorporated in the CNTbody and tip region. 3.2. Liquid natural hydrocarbon precursors 3.2.1. Turpentine oil (C10H16) The use of turpentine oil for the synthesis of carbon nanomaterials is described in several reports. Turpentine oil is a plant based precursor and used as acarbon source in different field after decomposition at high temperature. Some groups reported the synthesis of CNTs using turpentine oil as precursors. Chatterjee et al. [89] reported the synthesis of CNTs by CVD method, decomposing turpentine oil over finely dispersed Co metal catalyst at 675 °C and shows its application as electrochemical double layer capacitor. Afre et al. [31] investigated the growth of MWCNTs by spray pyrolysis of turpentine oil. MWCNTs were synthesized at different temperatures ranging from 500 to 900 °C with an interval of 100 °C. The bimetallic catalyst Co and Fe supported on silica gel particles improves the quality, quantity and uniformity of CNTs. Fig. 24 shows the HRTEM analysis of the MWCNTs clearly showing the distinct graphene walls of the nanotube. Fig. 24a shows the CNTs grown at low temperature (500 °C), at this temperature nm range thick amorphous carbon layers are formed on the walls of MWCNT. Fig. 24b–d shows the HRTEM micrographs of MWCNTs at

600, 700 and 800 °C, the amorphous carbon gets reduced with 15– 18 graphene walls and an inner diameter of 10 nm. But at higher temperature (900 °C), the amorphous carbon is nearly absent with 40–45 graphene walls and inner diameter of 10 nm. At lower temperatures (500 and 600 °C), the MWCNTs yield are very less because at these temperatures carbon source decomposes partially leading to the formation of amorphous carbon. At 700 °C, carbon source sufficiently pyrolyse and interact with catalysts, and catalyst-carbon source interaction makes it easy to form fairly uniform diameter nanotubes. At higher temperatures (800 and 900 °C) thick MWCNTs were formed mostly with different structures i.e.. coiled nanotubes, fibers, etc. and CNTs yield also gets decreased to 21% and 15% respectively. In general higher temperatures ignite the pyrolysis of carbon source completely results in the formation of other nano-materials comprised of structural defects. Afre et al. [90] in another work show the growth of aligned ACNTs by spray pyrolysis of turpentine oil and ferrocene mixture at 700 °C. They have grown vertically ACNTs of 300 μm length and diameter in the range of 50–100 nm on Si(100) substrate. Here, Ferrocene act as an in-situ Fe catalyst precursor forming the nanosize iron particles for formation of ACNTs on Si and quartz substrates. SEM of the samples are shown in Fig. 25. Typical top view images of vertical ACNTs grown on bottom side of Si and quartz are shown in Fig. 25a–d, respectively. Alignment of CNTs is more on silicon substrate than on quartz substrate. Fig. 25b shows a block of ACNTs peeled off from the Si substrate. Each vertical column consists of innumerous nanotubes self-organized into rope like structure. It is interesting to observe that very small growth is observed on the top surface of the Si substrate. Some research group focused their research on zeolites powder as catalyst supporting material for SWNTs production using CVD method. Ghosh et al. [91] synthesized SWCNTs using turpentine

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Fig. 25. The SEM images of ACNTs grown on (a, b) silicon and (c, d) quartz by spray pyrolysis of turpentine oil and ferrocene at 700 °C [90].

Fig. 26. SEM images of as-grown CNTs inside the zeolite matrix at (a) 800 °C and (b) 850 °C [91].

oil as hydrocarbon source using CVD process. SWNTs were prepared by catalytic decomposition of turpentine oil over welldispersed metal particles supported on high silica Y-type zeolite. SWNTs formed selectively at 850 °C while mixture of SWNTs and MWNTs were formed at 800 °C. Fig. 26 shows the SEM of the as grown products synthesized over Fe–Co bimetallic catalyst, impregnated in zeolite at 800 and 850 °C under the flow of nitrogen by simple spray pyrolysis method. SEM images clearly reveal that the CNTs grew nicely on all the surface of the zeolite particles. In all cases the catalyst surfaces are completely covered with CNTs. In another study, aligned nitrogen-doped CNTs were synthesized by Ghosh et al. from the pyrolysis of a mixture of turpentine oil, 4-tert-butylpyridine and ferrocene on two different substrates in nitrogen atmosphere at 700 °C [92]. These studies show that two different types of N atoms are present in these materials. The morphology of the products is shown in Fig. 27. Fig. 27a–d depicts the SEM images of N-doped CNTs grown on silicon and quartz

substrate. It shows a large no of well-aligned CNTs perpendicular to the surface of the substrate. SEM images clearly reveal that the nanotubes are densely packed due to Van der Waals interactions between the neighboring nanotubes. From SEM images it can be observed that the lengths of the as-grown CNTs are 12 μm and 9 μm on silicon and quartz substrate, respectively. Large scale synthesis of bundles of ACNTs has also been reported using turpentine oil. Awasthi et al. [93] shows that spray pyrolysis of the ferrocene and turpentine oil solution at 800 °C leads to the formation of ACNTs. The bundles of ACNTs are grown directly inside the quartz tube. Fig. 28 shows the SEM image which reveals that the CNTs exist in the form of bundles made up of ACNTs. As can be seen, the as-grown ACNTs bundles are clean and free from carbonaceous and impurities materials. The length of the bundles varies from 70 to 130 mm. The magnified view of CNTs bundles reveals a dense, self-aligned growth of CNTs bundles. The as-grown MWCNTs have an outer diameter of 15–40 nm.

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Fig. 27. SEM images of aligned N-CNTs grown on silicon substrates (a) and (b) and on quartz substrates (c) and (d) at 700 °C [92].

3.2.2. Eucalyptus (C10 H18O) Eucalyptus oil has also been found to be a good promising natural precursor for CNTs synthesis. This oil is cheap and readily obtained, making it an ideal green biosource for the synthesis of CNTs. The molecular formula of the main component of eucalyptus oil is C10H18O which contains carbon, hydrogen and small amount of oxygen. Ghosh et al. [32] demonstrated a very simple and inexpensive technique for the production of SWNTs at atmospheric pressure using eucalyptus oil and Fe/Co impregnated on zeolite as catalyst support at 850 °C. This synthesis technique does not require any pretreatment of the catalyst precursor to generate active catalyst precursor. Fig. 29 shows the SEM and HRTEM images of the as-grown SWCNTs, demonstrated that the CNTs grew densely on all the surface of the zeolite particles. The HRTEM observation of SWCNTs confirms that the SWNTs formed are in bundles.

Fig. 28. (a) SEM images bundles of ACNTs and (b) the magnified SEM image of the rectangular marked region [93].

3.2.3. Palm oil Palm oil is an inexpensive natural precursor for the growth of carbon nanomaterials.. Surani et al. [34] reported that the vertically ACNTs can be synthesized on silicon substrates in a thermal catalytic chemical vapor reactor using natural palm oil as the carbon source. The furnace containing the palm oil source is heated up to 450 °C essentially decomposes the ferrocene to Fe particles. At the same time, palm oil molecules decompose into a hydrocarbons containing C, H and O molecules. The hydrocarbon vapor mixed with nanosized Fe catalysts settled on the Si substrate in the synthesis region where the temperature was kept at 750 °C.

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Fig. 29. SEM and HRTEM image of SWNTs grown inside the zeolite matrix at 850 °C [32].

3.2.4. Neem oil Neem oil (also named margosa oil, extracted from the seeds of the neem-Azadirachta indica) being a natural source has the potential to be the green alternative for industrial scale production of CNTs. Kumar et al. [33] reported the synthesis of aligned CNTs bundles using neem oil as the carbon source employing the spray pyrolysis technique. Spray pyrolysis of ferrocene with neem oil solution leads to the formation of large amount of CNTs. The bundles of ACNTs have been successfully prepared in large scale at 825 °C under Ar atmosphere. Dense ACNT bundles with length in the range of 20–50 μm were formed directly inside the quartz tube. The as-grown MWCNTs show outer diameter in the range of 15–30 nm (Fig. 31). The present technique gives higher yield and high density of CNTs bundles. Graphitization of these CNTs is fairly good, and the presence of catalyst particles in the as-grown CNTs is almost negligible. 3.2.5. Sunflower oil Synthesis of aligned Bamboo-Shaped nitrogen doped CNTs (NCNTs) have also been synthesized by Kumar et al. [94] using sunflower oil. Fig. 32 shows the SEM micrographs of CNTs using sunflower oil with ferrrocene and NH3 ratio of 10:3 at 825 °C. This represents the cross-sectional view of aligned N-CNTs nanotubes. The nanotubes are normal to the walls of the quartz tube having uniform length of about 25 μm. The bamboo-shaped N-CNTs were made up of several compartments of almost equal numbers of layers with decreasing compartment spacing. The N-CNTs have been capped with Fe catalyst particles and the hollow compartments of the bamboo structures are repeated by  7 graphitic layers.

Fig. 30. SEM images of VACNT grown on a Si substrate, (a) peeled off bundle of dense CNT, (b) side view of the aligned CNTs [34].

The catalytic decomposition of the hydrocarbon occurred resulting in the dissolution of carbon into the Fe particles and the release of hydrogen and oxygen. This technique was capable of producing CNTs of about 90% purity. SEM images in Fig. 30 shows the densely packed VACNTs grown at 750 °C for 30 min deposition time. The length of the nanotubes was estimated to be about 110 μm, giving an approximate growth rate of 4 μm/min. The side view of the aligned bundle of CNTs (Fig. 30b) shows that it consists of wriggly individual CNTs. A closer inspection reveals that the bundle consisted of entwined individual CNTs.

3.2.6. Jatropha-derived bio-diesel Jatropha derived biodiesel is also a good carbon precursor for the synthesis of CNTs. The spray pyrolysis of biodiesel–ferrocene – acetonitrile solution at 850 °C under Ar gas flow leads to the formation of bamboo shaped CNTs. Kumar et al. prepared CNTs and N-CNTs using biodiesel as carbon precursor [95]. MWCNTs were prepared with high yield by spray pyrolysis of biodiesel–ferrocene solution at 850 °C under Ar atmosphere. The N-CNTs were synthesized by modifying the biodiesel–ferrocene precursor with acetonitrile at 800 °C. SEM images as elucidated in Fig. 33a and b show the formation of dense CNTs configuration. As can be seen, the as-grown CNTs are clean and free from the other carbonaceous materials. The average length of CNTs is 10 μm. The TEM investigation confirms that the as-grown CNTs are MWCNTs. Typical TEM

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Fig. 31. TEM micrographs of the as-grown ACNTs at 825 °C. (c, d) TEM images of the as-grown CNTs. (e) HRTEM of CNTs and (f) SAED pattern [33].

Fig. 32. Micrographs of N-CNTs synthesized at 825 °C from sunflower oil. (a) SEM image and (b) TEM image of N-CNTs [94].

image of the as-grown CNTs is shown in Fig. 33c. The diameter of nanotubes has been found to vary between 20 and 50 nm. The size distribution of Fe catalyst particles is responsible for the variation in nanotubes diameter. HRTEM image (Fig. 33d) clearly shows well graphitized layers of typical MWCNTs. The side wall of CNTs was found to consist of  32 graphitic layers 3.2.7. Castor oil Castor oil obtained by pressing the seeds of the castor oil plant and it is the source of ricinoleic acid (85–95%), a monounsaturated, 18-carbon fatty acid, which has long chain of carbon atoms attached with hydrogen. Kumar et al. [96] have used castor oil as hydrocarbon precursors for the synthesis of CNTs and N-doped CNTs employing spray pyrolysis method. Castor oil–ferrocene solution was sprayed at  850 °C in Ar atmosphere in quartz tube. Fig. 34 shows the SEM morphology of the as-grown CNTs and the

maximum length of CNTs is 10 μm with varying diameters ranging from  20 to 60 nm. 3.2.8. Sesame oil Sesame oil is an edible vegetable oil derived from sesame seeds. Its composition has different fatty acids i.e. linoleic acid, oleic acid, palmitic acid, stearic acid and others in small amounts. Large scale with clean morphology using spray pyrolysis of ferrocene with sesame oil has been used for the synthesis of ACNT and GNSs by Kumar et al. [38]. Only temperature gradient provides two different carbon nano-structures, rectangular ACNTs bundles and GNSs. These carbon nanostructures are grown on SiO2 substrate and Ar gas was used as the carrier gas. This new green natural precursor gives higher yield and high density of aligned CNTs bundles. Fig. 35 shows the TEM micrographs of ACNTs and GNSs. These MWCNTs have an empty and uniform central core and

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Fig. 33. SEM micrographs (a) and (b) of as-grown CNTs obtained by spray pyrolysis of biodiesel–ferrocene solution at 850 °C. TEM micrographs (c) of as-grown CNTs and HRTEM micrograph (d) of a typical side wall of CNTs [95].

Fig. 34. (a) SEM and (b) TEM micrographs of the as-grown CNTs obtained by spray pyrolysis of castor oil–ferrocene solution at 850 °C [96].

Fe nanoparticles filling in MWCNTs was found to be nearly absent. The as synthesized GNSs show the formation of few layers GNSs and ACNTs with an inner diameter of 50–60 nm. TEM micrograph of GNSs has corrugations and scrolling on the edge of the graphene sheets. An inherent sheet-like structure displaying an intricate long-range array of folds is evident. 3.2.9. Camphor oil Yousefi et al. [97] inspite of camphor used camphor oil as carbon source, ferrocene as source of Fe catalyst and porous silicon as substrate for the growth of aligned MWCNTs via CVD processes (750–850 °C) in Ar gas flow. The authors have shown the effect of carrier gas on the variation of diameter of aligned CNTs. Fig. 36a and b shows the FESEM images of as synthesized well-aligned CNTs with varying diameters at different Ar flow rates. They found that the diameter depends upon the Ar flow rate. As the Ar flow

rate is decreased, the CNTs diameter gradually increases. It shows that at 600, 400 and 200 sccm Ar flow the average diameter of aligned CNTs varies as 30.6 nm, 71.2 nm and 0.16 mm, respectively. 3.2.10. Tea tree extract The tea tree extract is hydrocarbon-rich and an abundant commercial resource obtained by distillation from leaves of Melaleuca alternifolia, commonly known as tea-tree. Studies by Jacob et al. [35] from Queensland University have shown that high quality graphene films on silicon substrates can be grown using extract from tea tree plant (M. alternifolia). They used a fast, sustainable, scalable, and potentially low cost simple plasma assisted CVD process for synthesis without any catalyst. Fig. 37a–d shows the morphology of synthesized graphene at different time duration. The SEM images show that no significant changes were observed beyond 4 min. The growth of graphene is along the

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Fig. 35. TEM micrographs of (a), (b) ACNTs and (b), (c) GNSs [38].

Fig. 36. Camphor oil derived MWCNTs images (a) low magnification and (b) high magnification FESEM images [97].

vertical direction on the substrate as the deposition time increases. This vertically oriented graphene have high surface area and long edges provide unique functionalities. TEM image in Fig. 37e shows that the graphene films of three to four layers are obtained for 2 min deposition duration. Fig. 37f shows the glancing angle X-ray diffraction (GAXRD) pattern indicating the crystalline nature of as synthesized multilayered graphene for 2 min deposition.

3.3. Eatable products Apart from the above different natural hydrocarbon precursors (solid, liquid and extract) mentioned in Section 3.2, graphene and CNTs have also been synthesized from many other eatable products like honey, sugar, butter, milk, cheese etc, especially for fabrication of graphene. Seo et al. from University of Sydney and CSIRO Materials Science and Engineering, Australia have synthesized vertical graphene nanosheets (VGNS) and CNTs from several eatable products and applied them as super capacitor electrode [36,98–100] .

3.3.1. Honey, sugar, butter, milk and cheese Impressive graphene synthesis results were obtained by Seo et al. [36] and they reported a novel method for the growth of VGNS using five different kinds of eatable products i.e. honey, sugar, butter, cheese and milk in different physical states by plasma technique. This low-temperature (450 °C), plasma-enabled process does not require any catalyst and hazardous precursor nor external heating, and employs renewable natural resources. The major benefit of this unifying approach is that the same process works for diverse precursors such as honey, sugar, butter and milk. Fig. 34 shows how different precursors in different states and their characterization. The optical images shown in Fig. 38a honey, (b) table sugar, (c) butter and (d) condensed milk can be transformed into VGs under treatment by (f) argon and hydrogen plasma, without any catalyst or external heating. The SEM images in Fig. 38f–i, show the morphology of VGs, for all natural precursors. Fig. 38j–m shows the corresponding Raman spectra of all VGs produced. The ration of ID and IG peaks are 0.45, 0.7, 0.6, and 0.51 for honey, table sugar, butter and condensed milk, respectively. The above ratio confirms that VGs grown by honey have

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Fig. 37. SEM images of samples fabricated on silicon substrates (1 cm  1 cm) for (a) 1 s (b) 1 min, (c) 2 min and (d) 4 min. (e) TEM image of a 2 min graphene sample and (f) GAXRD pattern obtained from a 2 min sample [35].

minimum defect (ID/IG ¼0.45) and the VGs grown by table sugar have maximum defect (ID/IG ¼ 0.7) in all natural precursors. In another work, Seo et al. [98] demonstrated the plasma-assisted CVD process (pressure¼2.5 Pa, RF power¼1 kW) to produce VGNS on porous Ni foam (porosity  95%) using butter as natural precursor in the presence of Ar/H2 mixture gas for 9 min. The substrate temperature was estimated to be 400–450 °C due to the plasma-heating effects. Fig. 39a–c, represent the optical image of before and after plasma treated butter precursor loaded on Ni foam. Fig. 39f shows the SEM images of VGNS grown with 80% H2, where dense and uniform structure of VGNS was observed to cover the whole Ni foam. They found that along with the increase in H2 concentration, the graphitic ordering in the VGNS increased and the thickness of edge planes decreases. In application, the as synthesized VGNS gives rise to high super capacitance and excellent cycle stability of the electrodes. As discussed in above reports, there were only formations of VGNS on different substrates. Later in 2014, Seo et al. [99] reported 3D VGNS/CNTs hybrid (combination of 1D and 2D materials) structure formed by the direct growth of CNTs onto VGNS in two steps. In first step, VGNS was synthesized by radio frequency inductively coupled plasma CVD chamber (pressure ¼ 2.5 Pa, RF power¼ 1 kW) with help of commercially available butter spread on flexible graphite paper substrate and Ar/H2 mixture gas. In second step, the direct growth of CNTs on VGNS was performed in a thermal CVD system. Catalysts for the growth of CNTs were prepared by sputtering Mo and Co sequentially onto the pristine VGNS. After that, catalyst-loaded VGNS were treated at 750 °C in the presence of Ar/H2 and C2H4 for the direct growth of CNTs on VGNS. The as-grown VGNS/CNTs hybrid structure on a flexible graphite substrate is shown in Fig. 40b. The SEM images of pure VGNS and the VGNS/CNTs hybrid obtained after the direct growth process are shown in Fig. 40c and d, respectively. The as synthesized 3D VGNS/CNTs hybrid material worked as a highperformance supercapacitor electrodes Again recently Seo et al. [100] demonstrated the fabrication of vertical graphene nanosheets (VGNS) from commercially available

cheese precursors via green, low-temperature, plasma-based reforming processes. The transformation of cheese precursors (processed cheese and crème cheese) into VGNS was carried out in a RF inductively coupled plasma CVD system. First, the Ni substrates were uniformly coated with the cheese at 80 °C. A mixture of H2/Ar gas was fed into the reactor, where the plasma was ignited (pressure¼2 Pa, RF power¼ 1 kW). To reduce the amount of amorphous carbon formed in the growth process, the samples were treated in air at 300 °C. Fig. 41a shows the schematic of the single-step, plasma-enabled reforming of cheese into VGNS. Ni foam was chosen as the substrate for VGNS as it provides a porous 3D scaffold for the growth of VGNS. Fig. 41b and c shows the highresolution SEM images of VGNS derived from processed and cream cheeses, respectively. Both images revealed a uniform, dense, 3-D network of thin VGNS. Good performance was also demonstrated using VGNS as super capacitor electrode application. 3.3.2. Chicken eggs A simple and attractive method was developed for the synthesis of highly crystalline carbon dots (CDs) through plasmainduced method. Wang et al. [37] develop a strategy to synthesized carbon dot (CDs) by plasma-induced method using cheap and natural chicken eggs as the precursor and this method can be further used to produce CDs from a wide range of carbon sources. The authors have extended the application of these CDs as fluorescent inks for multicolor patter nings using inkjet and silk-screen printing. Fig. 42 shows the fabrication and characterization of eggderived fluorescent CDs and their application as “inks” for luminescent patterns using inkjet or silk-screen printing. As shown in Fig. 42a, that a glass dish filled with egg white or yolk was placed between two quartz slides of the plasma generator (50 V, 2.4 A). The intense and uniform plasma beams generated from the upper electrode irradiated the egg samples for 3 min to yield dark black products, referred to as CDpew and CDpey for the plasma-treated egg white and yolk, respectively. The solutions of CDpew and CDpey display bright blue fluorescence under UV light. Fig. 42b–e shows

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Fig. 38. Fabrication of uniform vertical graphenes using natural precursor by plasma based method and characterization. (a–d) Photographs of natural precursors in different states, pasted on the clean SiO2 substrates and (e) the plasma used to produce vertical graphenes. Scanning electron microscope images and Raman spectra of vertical graphenes produced using (f, j) honey, (g, k) sugar, (h, l) butter and (i,m) milk [36].

3.4. Solid natural waste and industrial carbonaceous products

There are few reports on the synthesis of graphene using natural and industrial carbonaceous wastes [62,63]. Some groups have synthesized graphene using chocolate, grass, plastics, roaches, and dog faeces [101]. Also, high-quality graphene oxide and reduced graphene oxide sheets from various natural and industrial carbonaceous wastes such as vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), a semiindustrial waste (newspaper), and industrial waste (soot powders produced in exhaust of diesel vehicles) has been synthesized [102]. Agro wastage of sugarcane has been also used for the synthesis of graphene oxide by directly oxidizing the waste materials [103].

Beside the use of natural hydrocarbon precursor and eatable products, the wastage carbon containing materials has also been used as carbon precursors for the synthesis of CNTs and graphene.

3.4.1. Food, insect and other natural waste According to Tour et al. [101], less expensive carbon sources, such as food, insects, and waste can be used without purification

HRTEM images of the CDs. CDpey had uniform dispersion without apparent aggregation and a mean particle diameter of 2.15 nm. Detectable rings in the selected-area electron-diffraction (SAED) pattern revealed the nanocrystalline structure of CDpey (Fig. 42b inset). On the other hand, CDpew have well distributed with particle diameter of 3.39 nm and appeared amorphous in nature. The difference in the morphologies of CDpey and CDpew might be explained by the higher lipid content in egg yolk (33%) than in egg white (0.01%).

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Fig. 39. Growth of VGNS using natural precursor butter on the flexible, porous Ni foam. (a–c) Illustration of how butter is transformed into VGNS structure using the plasmaenabled process: (a) photograph of butter pasted on porous Ni foam, (b) radio-frequency (RF) inductively-coupled plasma in operation with Ar/H2 gas mixture, and (c) the grown VGNS. (d) Photograph of bent VGNS on Ni foam, demonstrating the flexibility of the material. SEM images of (e) pristine porous Ni foam and (f) VGNS grown with 80% H2 , showing that dense graphene nanosheets uniformly covered on the top of Ni foam after the plasma-assisted growth process [98].

Fig. 40. (a) Schematic for the direct growth of CNTs onto VGNS. (b) Photograph of the as-grown VGNS/CNTs hybrid on a flexible graphite substrate. (c) SEM micrograph of pristine VGNS prior to CNT growth. (d) SEM micrograph of the final hybrid VGNS/CNTs nanoarchitecture in which the graphene nanosheets were decorated with a high density of CNTs [99].

to grow high-quality monolayer graphene directly on the backside of Cu foils under the H2/Ar flow using CVD method. They applied a general method to grow high-quality graphene from various waste raw carbon materials at 1050 °C under vacuum and H2/Ar flow. The carbon sources were foods (cookie and chocolate), waste

(grass, plastic, dog feces) and insect-derived. With this technique, many kinds of solid materials that contain carbon are used without purification as the feed stocks to produce high quality graphene. Fig. 43a shows the schematic diagram of synthesis process. Fig. 43b shows the growth of graphene from cockroach legs. The

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Fig. 41. (a) Schematics of the experimental process that transforms the cheese precursor into VGNS. SEM images of VGNS produced from (b) processed cheese (c) and cream cheese [100].

Fig. 42. (a) Digital photographs of plasma-induced fabrication of egg derived CDs and their application as fluorescent carbon inks. Egg white or yolk, after a few minutes of plasma treatment under ambient conditions, were transformed into well-defined CDs with bright blue emission under UV light. The CD solutions can also be used as inks for making luminescent patterns by inkjet or silk-screen printing. HRTEM images of CDpey (b, c) and CDpew (d, e) in aqueous solution, with an individual carbon dot at higher magnification (c, e). Inset: SAED patterns of the CDs [37].

high-quality pristine graphene with few defects and  97% transparency was grown on the backside of the Cu foil, as confirmed by Raman and UV visible spectroscopy. 3.4.2. Natural and industrial carbonaceous waste Akhavan et al. [102] synthesized high-quality graphene oxide and reduced graphene oxide sheets from various natural and industrial carbonaceous wastes such as vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), a semi-industrial waste (newspaper), and an industrial waste (soot powders produced in exhaust of diesel vehicles). These above mentioned wastes were initially carbonized at 400–500 °C

through imperfect burning. Then, the residue charcoal materials and an industrial waste (exhaust soot of diesel vehicles) were used for chemical exfoliation of the graphitized materials employing Hummers' method. The surface morphology, chemical state, carbonaceous structure, and electrical properties of the sheets synthesized by the various feedstocks were found to be nearly same and are also comparable to those of the graphene sheets obtained by high purity graphite. This elucidate that many kinds of solid carbonaceous wastes having the capability of graphitization can be used in the production of high quality graphene sheets. AFM images shown in Fig. 44 represent the as-synthesized GO sheets by using various types of waste materials. Moreover, this method

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Fig. 43. (A) Schematic representation of the experimental apparatus for the growth of graphene from food, insects, or waste in a tube furnace. (B) Optical image and growth of graphene from a cockroach leg. (a) One roach leg on top of the Cu foil. (b) Roach leg under vacuum. (c) Residue from the roach leg after annealing at 1050 °C for 15 min. The pristine graphene grew on the bottom side of the Cu film [101].

Fig. 44. (A) AFM images and (B) height profile distributions of GO sheets produced from (a) highly pure graphite, (b) wood, (c) leaf, (d) bagasse, (e) fruit, (f) newspaper, (g) bone, (h) cow dung, and (i) soot [102].

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Fig. 45. (a) HRTEM images of few layer graphene synthesized from camphor leaves (b) AFM image of few layer graphene showing layer thickness (2.37 nm) [104].

Fig. 46. (a) Schematic representation of graphene oxide synthesis from agro waste and (b, c, d and e) SEM images of graphene oxide obtained from sugarcane bagasse [103].

provides successful recycling of low-value and sometimes pollutant and/or hazardous wastes into valuable and high-quality graphene nanomaterials. 3.4.3. Camphor dead leaves More recently, Shams et al. [104] reported synthesis of few layer graphene by dead camphor leaves using one-step pyrolysis under nitrogen atmosphere at 1200 °C. After the pyrolysis of the camphor leaves, the sample was treated with different chemical

process and after sonication it produces few layer graphene suspensions. TEM image in Fig. 45a confirms the formation of few layer graphene sheets from the dead camphor leaves. Fig. 45b shows the AFM image estimate the thickness 2.37 nm which shows the few layer graphene contains approximately 7 layers. 3.4.4. Agro sugarcane bagasse waste Graphene oxide has also been synthesized by directly oxidizing agro waste sugarcane bagasse under muffled atmosphere.

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Fig. 47. (a) Optical microscopic images of individual graphene crystals after low temperature oxidation of Cu foil. (b) Color difference of grain and twin boundaries of Cu foil and hexagonal graphene can be visualized with Cu surface oxidation. (c) Merging of neighboring graphene crystals. (d) Individual crystal almost merged together to form a continuous graphene film [105].

Fig. 48. Chicken feathers conversion into N-CNTs after different process [106].

Somanathan et al. [103] reported a simple and rapid method to convert solid sugarcane bagasse waste into value-added graphene oxide materials. This method is environment friendly as it avoids the emission of toxic gas during the oxidation and graphene oxide synthesis process. Fine powder of sugarcane bagasse mixed with

ferrocene was placed in a crucible and placed directly into a muffle furnace at 300 °C for 10 min under atmospheric condition. The asproduced black solid powder shows the characteristic properties of graphene oxide. The SEM images (Fig. 46) of the graphene oxide reveals its sheet-like structure.

Table 1 Natural precursors and waste hydrocarbons products used for the synthesis of graphene and CNTs. Type of graphene/CNTs formed

Temperature and catalyst

Yield/purity

Properties (diameter/length/size/surface area)

Refs.

Camphor (C10H16O)

Single, bi-layer and few layer graphene Few layers grapheme (4 and 13 layers) Iodine-doped few layers graphene Monolayer and bi-layer graphene Few layer graphene flakes SWCNTs and MWCNTs SWCNTs and MWCNTs

800 °C, Ni foils

–

2–3 μm

[30]

800 °C, polycrystalline Ni

–

2 cm  2 cm

[64]

– – Scalable 90% High yield 30% 90% – More than 50% in weight –

ACNTs

850 °C, ferrocene

–

ACNTs MWCNTs and SWCNTs CNFs and MWCNTs

700 and 850 °C, ferrocene 875 °C, ferrocene 700-800 °C, ferrocene

– 90%

N-doped CNTs MWCNTs

– Maximum at 700 °C (33%) Less 57% purity

– 2 cm  2 cm 1–5 μm Diameter: 1.2–1.3 nm Diameter SWCNTs: 0.86–1.23 nm, diameter MWNTs: 10 nm Length:  100 μm, diameter: 15–50 nm Diameter: 20–40 nm length: 120 μm Thickness of the ACNTs carpet: 4 μm Length: few μm to hundreds μm diameter: 80 nm 2 mm thick MWCNT mat diameter 10– 100 nm Diameter 10–40 nm and 30–80 nm Length: 100 μm 25–50 nm CNF diameter: 35 nm and 150 nm MWCNTs diameter: 35–45 nm Length: 200 mm diameter: 50 ( 7 10) nm diameter: 20 nm (500–700 °C) 40 nm (800–900 °C) Diameter: 50–100 nm Diameter: 1.37–0.82 nm

[65] [66] [67] [69] [70]

MWCNTs MWCNTs MWCNTs ACNTs ACNTs

800 °C, Ni foil 1020 °C, Cu foil 850 °C, Ni substrate 800–1050 °C, ferrocene 550–1000 °C silica-zeolite support impregnated with Fe/Co catalyst 850 °C, ferrocene 900 °C, ferrocene 850 °C, cobalt-coated silicon plates 650 °C, ferrocene 850 °C, ferrocene

–

Diameter: 3–25 nm

[92]

high yield dense

Diameter: 15–30 nm Diameter: 0.79–1.71 nm

[93] [32]

Diameter: 0.6 nm–1.2 nm Length: 20–50 μm, diameter: 15–30 nm

[34] [33]

Diameter: 20-40 nm Length: 20 μm, diameter: 20–50 nm, (Ndoped diameter: 30–60 nm) Length: 5–10 μm, diameter: 20–60 nm Diameter: 50–60 nm

[94] [95]

30.6 nm, 71.2 nm and 0.16 mm 1 cm  1 cm silicon substrates Average edge length  182 nm

[97] [35] [36]

Amorphous (CDpew) and crystalline (CDpey) nature High quality

Specific surface area 448 m2/g CDs particle diameter 2.15 nm (CDpey) and 3.39 nm (CDpew) 1 cm  1 cm

[100] [37]

High-quality

–

[102]

– – High-quality –

– – 90–100 μm –

[104] [103] [105] [106]

MWCNTs SWCNTs

Palm oil (C67H127O8) Neem oil

SWCNTs MWCNTs

Sunflower Bio-diesel

N-doped MWCNTs MWCNTs and N-doped CNTs

825 °C, NH3 and ferrocene 850 °C (800 °C), acetonitrile and ferrocene

90% High density ACNTs – High yield

Castor oil Sesame oil

850 °C, ferrocene 800 °C, SiO2 substrate

– Higher yield and high density

Camphor oil Tea-tree extract Honey, sugar, butter and milk

MWCNTs MWCNTs and graphene nanosheets MWCNTs Graphene VGNS

750–850 °C, porous silicon substrate 0.5 kW , 800 °C Pressure ¼2.5 Pa, RF power¼ 1 kW, 450 °C

– Ultra-long edges and enhanced surface Ultra-long reactive edges, edge density of more than 1.0 km cm  2

Cheese Chicken eggs

VGNS Carbon dots

Pressure ¼2 Pa, RF power¼ 1 kW, 400 °C Plasma (voltage¼ 50 V, current ¼2.4 A)

Foods (cookie and chocolate), waste (grass, plastic, dog feces) and insect-derived Vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung) Camphor leave Agro sugarcane bagasse waste Solid plastic waste Chicken feather waste

Monolayer graphene

1050 °C, Cu foil

Graphene oxide and reduced graphene oxide sheets Few layer graphene Graphene oxide Single crystal graphene N-doped CNTs

Carbonized at 400–500 °C and Hummer’s method 1200 °C 300 °C, ferrocene 1020 °C 650 °C

MWCNTs SWCNTs N-doped CNTs

[80] [82] [83] [84] [88] [31] [90] [91]

[96] [38]

[101]

1003

Eucalyptus oil (C10 H18O)

800, octadecylamine with 4 wt% ferrocene 500–900 °C cobalt and iron supported on silica gel particles 700 °C, ferrocene 850 °C Iron nitrate and cobalt nitrate with silica zeolite 700 °C, 4-tert-butylpyridine (C9H13N) and ferrocene 800 °C 850 °C, silica–zeolite support impregnated with Fe/Co catalyst 750 °C 825 °C, ferrocene

Turpentine oil (C10H16)

[71] [72] [73] [78] [79]

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Natural precursor/waste materials

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3.4.5. Solid plastic waste Sharma et al. [105] demonstrated the synthesis of high quality single crystal graphene using solid waste plastic as carbon source in ambient pressure (AP) utilizing CVD process. The graphene was synthesized on Cu foil as substrate using waste materials in the atmosphere Ar/H2 gas at the temperature of 1020 °C. The growth of individual crystals is strongly influenced by the injection rate of decomposed polymeric components generated during pyrolysis of waste plastic. Fig. 47a shows the optical image of hexagonal graphene crystals, distributed over the Cu substrate with a size of 100 μm. Fig. 47b and c shows growth of graphene on Cu grain, grain boundaries as well as on twin boundaries. Fig. 47d shows the uniform distribution of graphene all over the Cu foil. With continuous increase in growth time, individual graphene crystal increases in size and merge together to form continuous film in order of cm scale. Structural and morphological analysis confirm the hexagonal shaped graphene growth with a lower pyrolysis rate of waste plastic The growth of large round shaped graphene crystals on increasing the pyrolysis rate of waste plastic feedstock depends on the growth velocity of the individual graphene in an isotropic growth process. The bilayer or few-layer graphene crystals were nucleated with a higher injection rate of polymeric components. The evolution of hexagonal graphene crystals synthesized by AP-CVD process from waste plastic can be significant to obtain large single crystal graphene and stacked bi-layer structure.

of these products as a precursor of carbon source will not only reduces the cost and consumption of limited fossil fuels but will also help us to treat our environment in a more benign way. In the present work, we have summarized the synthesis of novel carbon nanostructures using low cost natural hydrocarbons, eatable products and carbon containing waste products. Natural precursors like turpentine oil, eucalyptus oil, palm oil, neem oil, sunflower oil, castor oil, biodiesel etc can be easily grown in an environmental benign process. Besides these huge amount of large carbon containing waste products are spread around that are either unutilized or polluting the environment. These products can be efficiently utilized for the synthesis of various nanostructures. Using natural waste products for graphene and CNTs, ranging from surface chemistry to large-scale manufacturing, will contribute to the frontier of nanotechnology and related commercial products for many more years to come.

3.4.6. Chicken feather waste Gao et al. [106] described a strategy for the catalytic conversion of chicken feather waste to Ni3S2–carbon coaxial nanofibers (Ni3S2@C) which can be further converted to nitrogen doped carbon nanotubes (N-CNTs) as shown in Fig. 48. The authors have mentioned that for completely pyrolyzation of chicken feathers, 650 °C temperature was appropriate. The outer CNTs are highly doped by nitrogen with a content of 6.43%. About ca. 86.6% S atom in the chicken feathers has been converted into Ni3S2, which is beneficial to reduce the emissions of harmful gases. This strategy provides a feasible way for the comprehensive utilization of chicken feathers and can convert waste materials chicken feathers into two highly valuable materials, which opens up a new application direction for N-CNTs. The products formed yield, purity and synthesis condition of the as synthesized graphene and CNTs using natural precursors, eatable products and waste hydrocarbon precursors has been summarized in Table 1.

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4. Conclusions Rapidly growing research interest and widespread applications of carbon based nanostructures (carbon particles, carbon nanotubes and graphene) in various emerging areas like supercapacitors, sensors, gas storage, and bio-medical applications demand the use of organic chemicals and heavy hydrocarbons to fulfill the requirement. CNTs, graphene and its derivatives, emerge as a novel nanomaterial platform for different applications in multiple disciplines including chemistry, physics, biology and medicine. Rapid innovations in graphene and CNTs synthesis and characterization such as CVD methods and spray pyrolysis techniques have leveraged findings from carbon base nanostructure research. Carbon precursors based on fossil fuels such as methane acetylene, benzene, xylene etc that are widely used for the synthesis of carbon based nanostructures, have their very limited store create pollution and will not be available in near future. In this situation, the natural precursors, eatable products and waste carbon containing products may be proved as a milestone. The use

Acknowledgments D.P Singh acknowledges with gratitude the financial support from Fondecyt Regular 1151527 Conicyt Chile.

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