Advanced Carbon Materials and Technology

Preface

The expansion of carbon materials is the focal point of materials research and technology which is mostly related to physics, chemistry, biology, applied sciences and engineering. Research on carbon materials has mainly focused on the aspects of fundamental physics that have unique electrical, thermal and mechanical properties applicable for a range of applications. The electrons in graphene and other derived carbon materials behave as dirac fermions due to their interaction with the ions of the lattice. This direction has led to the discovery of new phenomena such as Klein tunneling in carbon-based solid state systems, and the so-called half-integer quantum Hall effect due to a special type of Berry phase. In pursuit of the same goal, Advanced Carbon Materials and Technology offers detailed, up-to-date chapters on the processing, properties and technological developments of graphene, carbon nanotubes, carbon fibers, carbon particles and other carbon-based structures, including multifunctional graphene sheets, graphene quantum dots, bulky balls, carbon balls, and their polymer composites.

Nanoscaled materials have properties which make them useful for enhancing surface-to-volume ratio, reactivity, strength and durability. The chapter entitled, Synthesis, Characterization and Functionalization of Carbon Nanotubes and Graphene: A Glimpse of Their Application, encompasses the principles of nanotubes and graphene production, new routes of preparation and numerous methods of modification essential for various potential applications. The chapter on, Surface Modification of Graphene, covers a range of covalent and non-covalent approaches. In the chapter, Graphene and Carbon Nanotube-Based Electrochemical Biosensors for Environmental Monitoring, the use of carbon nanotubes and numerous graphene-based affinity electrodes for the development of novel tools for monitoring environmental pollution are described. The chapter on, Catalytic Application of Carbon-Based Nanostructured Materials on Hydrogen Sorption Behavior of Light Metal Hydrides, describes the state-of-the-art of carbon nanotubes, carbon nanofibers and graphene as a catalyst for the aforesaid hydrogen storage materials. An informal presentation about recent progress in the advances in synthetic techniques for large-scale production of carbon nanotubes, their purification and chemical modification, and the emerging technologies they enable are presented in the chapter, Carbon Nanotubes and Their Applications. Moreover, a chapter dedicated to the, Bioimpact of Carbon Nanomaterials, discusses graphene, nanotubes and fullerenes, along with their nanotoxicity, nanoecotoxicity, and various biomedical applications.

Carbon nano-objects including fullerenes, carbon nanotubes, carbon quantum dots, shungites and graphenes, show unique photorefractive characteristics. The chapter on, Advanced Optical Materials Modified with Carbon Nano-Objects, illustrates the spectral, photoconductive, photorefractive and dynamic properties of the optical carbon objects-based nanomaterials. Covalent and Non-Covalent Functionalization of Carbon Nanotube: Applications, deals with the photocatalytic nature of carbon nanotube-based composites. Illustrated in, Metal Matrix Nanocomposites Reinforced with Carbon Nanotubes, are the preparation and properties of nanocomposites based on aluminium, copper, magnesium, nickel and titanium with reinforced matrix of nanofiller carbon materials (e.g., nanoplatelets, nanoparticles, nanofibers and carbon nanotubes) using various processing techniques. The chapter also discusses reinforcement using carbon nanotubes, interfacial bonding, thermal, mechanical, and tribological properties and tne challenges related to the synthesis of composites.

Fly ash, a waste by-product of coal thermal power plants, is a carbon-based lightweight material. Fly ash is generally inexpensive and is considered to be an environmental hazard, thus utilization of fly ash in composites proves to be both economically and environmentally beneficial. In this way, use of fly ash in developing advanced composites is very encouraging for the next generation of advanced lightweight composites. The discussion in, Aluminum/Fly Ash Syntactic Foams: Synthesis, Microstructure and Properties, is focused on the methods of synthesis for fly ashfilled aluminum matrix composites along with their microstructure and mechanical properties, and the tribological properties of Al/fly ash syntactic foams. The chapter entitled, Engineering Behavior of Ash Fills, covers the extensive characterization, hardening, bearing capacity and settlement of ash fill technology. The chapter on, Carbon-Doped Cryogel Thin Films Derived from Resorcinol Formaldehyde, presents results of the structural and optical properties of carbon-doped cryogel thin films derived from resorcinol formaldehyde.

This book is written for a large readership, including university students and researchers from diverse backgrounds such as chemistry, materials science, physics, pharmacology, medical science and engineering, with specializations in the civil, environmental and biomedical fields. It can be used not only as a textbook for both undergraduate and graduate students, but also as a review and reference book for researchers in materials science, bioengineering, medicine, pharmacology, biotechnology and nanotechnology. We hope that the chapters of this book will provide the readers with valuable insight into state-of-the-art advanced and functional carbon materials and cutting-edge technologies.

Editors

Ashutosh Tiwari, PhD

S.K. Shukla, PhD

Managing Editors

Swapneel Despande

Sudheesh K. Shukla

Part 1

GRAPHENE, CARBON NANOTUBES AND FULLERENES

Chapter 1

Synthesis, Characterization and Functionalization of Carbon Nanotubes and Graphene: A Glimpse of Their Application

Mahe Talat and O.N. Srivastava*

Nanoscience and Nanotechnology Unit, Department of Physics, Banaras Hindu University, Varanasi, India

*Corresponding author: [email protected]

Abstract

Since the discovery of nanomaterials, carbon nanotubes structures have attracted great interest in most areas of science and engineering due to their unique physical and chemical properties and are supposed to be a key component of nanotechnology. The most recent addition to the family of carbon nanostructures is graphene. Graphene is a one-atom-thick material consisting of sp²-bonded carbon with a honeycomb structure. It resembles a large polyaromatic molecule of semi-infinite size. In the past five years, graphene-based nanomaterials have been the focus of not only material scientists but also engineers and medical scientists. The interesting and exciting properties of single-layer graphene sheets have excited the scientific community especially in the areas of materials, physics, chemistry and medical science. The state-of-the-art CNT production encompasses numerous methods and new routes are continuously being developed. The most common synthesis techniques are arc discharge, laser ablation, high pressure carbon monoxide (HiPCO) and chemical vapor deposition (CVD) with many variants. Most of these processes take place in vacuum or with process gases. By choosing appropriate experimental parameters, large quantities of nanotubes can be synthesized by these methods. It is possible to control some properties of the final product, such as type of CNTs synthesized (MWNTs vs. SWNTs), the quality of the nanotubes, the amount and type of impurities, and some structural CNT features. In this chapter we discuss some of the methods employed in our lab for the synthesis and characterization of the CNTs and graphene. For application in biomedical and targeted drug delivery, the major limitation of these nanomaterials is their poor solubility, agglomeration and processibility. Functionalization of CNTs and GS is, therefore, necessary to attach any desired compounds including drug and also to enhance the solubility and biocompatibility of these nanomaterials. Two types of functionalization methods, i.e., covalent and non-covalent methods are generally being adopted. We deliberate these two procedures of functionalization of CNTs and GS. The merits of these two modes of functionalization will also be discussed.

Keywords: Synthesis, characterization, application, CNT, graphene

1.1 Introduction

Carbon is the base element of all organic materials, one of the most abundant elements on earth and also the only element of the periodic table that occurs in allotropic forms from 0 dimensions to 3 dimensions due to its different hybridization capabilities. It has the unique ability to form allotropes, which can be described by valence bond hybridization, spn. The well-known forms are diamond, where n = 3, and graphite, having n = 2. It was shown that all other 1fullerenes added a new dimension to the knowledge of carbon science, which led to the Nobel Prize being awarded to R.F. Curl, H. Kroto and R.E. Smalley in 1996. Stimulated by fullerene discoveries, the Japanese scientist Ijima [1] discovered another new form of carbon-graphitic tubules and the CNT was born. He also suggested the possible structure of these tubes, which was later proven right. The subsequent discovery of carbon nanotubes (CNTs) added a new dimension to the science and technology of new carbons.

The most recent addition to the family of carbon nanostructures is graphene. Graphene is a one-atom-thick material consisting of sp²-bonded carbon with a honeycomb structure. It resembles a large polyaromatic molecule of semi-infinite size. In the past five years, graphene-based nanomaterials have been the focus of not only material scientists but also engineers and medical scientists. The interesting and exciting properties of single-layer graphene sheets, such as high mechanical strength, high elasticity and thermal conductivity, demonstration of the room-temperature quantum Hall effect, very high room temperature electron mobility, tunable optical properties, and a tunable band gap have excited the scientific community especially in the areas of materials, physics, chemistry and medical science. Following the series of Nobel Prizes awarded after the discovery of fullerenes, another nano star—graphene—received the 2010 Nobel Prize in Physics for groundbreaking experiments regarding the two-dimensional material graphene performed by Andre Geim and Konstantin Novoselov [2].

Therefore, the discovery and subsequent applications of these carbon nanomaterials have allowed the development of an entire branch of nanotechnology based on these versatile materials.

1.2 Synthesis and Characterization of Carbon Nanotubes

The first experimental evidence of carbon nanotubes (CNTs) came in 1991 [3] in the form of multi-wall nanotubes (MWNT), which motivated a sudden increase in nanotubes synthesis research. In 1993, the first experimental evidence of single-wall nanotubes (SWNT) was introduced [4]. Since then, the synthesis methods for CNTs have been developed tremendously. Production methods for carbon nanotubes (CNTs) can be broadly divided into two categories, chemical and physical, depending upon the process used to extract atomic carbon from the carbon-carrying precursor. Chemical methods rely upon the extraction of carbon solely through catalytic decomposition of precursors on the transition metal nanoparticles, whereas physical methods also use high energy sources, such as plasma or laser ablation to extract the atomic carbon. However, the most common synthesis techniques are arc discharge, laser ablation, high pressure carbon monoxide (HiPCO) and chemical vapor deposition (CVD) with many variants [5]. Most of these processes take place in vacuum or with process gases. By choosing appropriate experimental parameters, large quantities of nanotubes can be synthesized by these methods. It is possible to control some properties of the final product, such as type of CNTs synthesized (MWNTs vs SWNTs), the quality of the nanotubes, the amount and type of impurities, and some structural CNT features [6]. Other reported methods include plastic pyrolysis [7], diffusion flame synthesis and electrolysis using graphite electrodes immersed in molten ionic salts [8] and ball-milling of graphite [9].

In this chapter, we will discuss the method of synthesis of CNTs employed in our lab such as spray pyrolysis, arc discharge method and synthesis of CNTs by low pressure chemical vapor deposition (LPCVD) method, and catalytic decomposition of hydrocarbon gases, e.g., methane and ethylene, onto the Ferritin/Fe- SiO2- Si substrates.

Synthesis of CNTs was carried out using spray pyrolysis-assisted CVD method, where ferrocene (C10H10Fe) was used as a source of iron (Fe) which acts as a catalyst for the growth of CNTs. Castor oil was used as the carbon source; castor oil contains carbon, hydrogen and lower amount of oxygen. The spray pyrolysis setup consisted of a nozzle (inner diameter ~ 0.5 mm) attached to a ferrocene-castor oil supply used for releasing the solution into a quartz tube (700 mm long and inner diameter 25 mm), which was mounted inside a reaction furnace [10].

Spray pyrolysis of castor oil-ferrocene solution at ~ 850°C in Ar atmosphere leads to a uniform thick black deposition on the inner wall of the quartz tube at the reaction hot zone (~ 850°C). Figure 1.1(a) shows the SEM morphology of the as-grown CNTs. The length of CNTs was ~ 5–10 μm. Structural details of the as-grown CNTs sample were further investigated by TEM. Typical TEM image of the as-grown CNTs is shown in Figure 1.1(b). The TEM investigation of the as-grown CNTs confirms that the CNTs are multi-walled in nature. These nanotubes have varying diameters ranging from ~ 20–60 nm. In the spray pyrolysis reaction, the castor oil-ferrocene solution was atomized via spray nozzle and sprayed through carrier gas (Ar). The Fe particles (liberated by the decomposition of ferrocene) were deposited on the inner walls of the quartz tube. The carbon species released from decomposition of castor oil and also from ferrocene got adsorbed on the Fe particles and diffused rapidly along the axial direction leading to the formation of CNTs. A study was also done using castor oil-ferrocene with ammonia solution so as to develop CNTs containing nitrogen, i.e., C-N nanotubes. This was done keeping in view the fact that nitrogen-doped CNTs are considered as one of the important ingredients of CNT-based electronics. Figure 1.2(a) shows a typical SEM micrograph of as-grown C-N nanotubes, which reveals the wavy morphology of nanotubes. These wavy nanotubes are most likely due to pentagonal and heptagonal defects that are introduced in the hexagonal sheets. TEM images of the as-grown C-N nanotubes are shown in Figure 1.2(b). These CNTs have bamboo-shaped structures. The TEM image in Figure 1.2(b) shows that the nanotubes have a range of diameters varying from ~ 50–80 nm. It is suggested that the bamboo-shaped morphologies arise from the incorporation of pyridine-like N atoms within the carbon framework [11]. Also, no encapsulated metal particle was found inside the nanotubes.

Figure 1.1 SEM (a) and TEM (b) micrographs of the as-grown CNTs obtained by spray pyrolysis of castor oil-ferrocene solution at ~ 850°C [11].

Figure 1.2 SEM (a) and TEM (b) micrographs of the as-grown C-N nanotubes obtained by spray pyrolysis of castor oil-ferrocene with ammonia solution at ~ 850°C [11].

Another method used is arc discharge method where SWNTs webs have been synthesized using Fe as catalysts by this method in argon atmosphere. SWNTs has been synthesized by using electric arc discharge of graphite cathode (3cmx1cmx1cm) and Fe as well as Ni-Y filled anode (5cmx0.8cmx0.8) at 200 torr pressure of argon gas. The SWNTs webs have been synthesized using Fe as catalysts by arc discharge method in argon atmosphere. Figure 1.3 shows the low magnification SEM micrographs of as-synthesized SWNTs web. The inset of Figure 1.3(a) shows the optical photograph of the as-deposited web on the chamber walls. The length of these webs is around 4 to 6 cm. The webs are ~75 to 100 μm thick, which abundantly contains the SWNTs, as is clear from Figure 1.3(b). Figure 1.3(c) shows the SEM image from the inner region between the two webs, dominantly containing SWNTs. A few SWNTs are also visible in Figure 1.3(d), which are coming out from these bundles.

Figure 1.3 (a–d) SEM micrographs of as synthesized SWNTs webs, Inset of Fig. 1.3(a) shows the optical photograph of SWNTs web, some SWNTs which are coming out of the bundle.

Figure 1.4 shows the transmission electron micrographs of SWNTs bundles. Figure 1.4(a) reveals the bundles containing large amount of SWNTs along with the catalyst particle which have been used to synthesize these SWNTs. Figure 1.4(b) shows the HRTEM image of as synthesized SWNTs as shown in Figure 1.4(a). Figure 1.4(c,d) are from the different regions of the samples containing SWNTs bundles and also catalysts particles and some amorphous carbon which have been coated onto the SWNTs bundles. The diameter of individual SWNTs is found to be approximately 1.6 nm. (Synthesis of SWNTs and graphene by electric arc discharge method in Ar atmosphere, S. Awasthi, K. Awasthi and O.N. Srivastava, J Nanosci Nanotech [under submission].)

Figure 1.4 (a–d) HRTEM micrographs of SWNTs web from different regions.

Raman analysis of as-synthesized SWNTs webs was also done. In Figure 1.5, the presence of RBM at 132 cm−1 confirms the formation of SWNTs. The diameter of SWNTs as calculated from RBM is found to be 1.91 nm, which is in good agreement with the HRTEM results. Since G peak is related to the sp² bonding and is common to all the graphitic materials, and D peak is the defect-induced peak, showing the mixed state of sp²-sp3 bonding, the intensity ratio of the D peak to the G peak can be used to quantify the quality of the graphitic samples. In the present case, this ratio was found to be ~0.08, which confirms that the as-synthesized nanotubes are highly crystalline and defect free in nature.

Figure 1.5 Raman spectrum of as-synthesized SWNTs webs.

Through synthesis of CNTs by low pressure chemical vapor deposition (LPCVD) method we have grown CNTs (single-walled) by LPCVD method on Ferritin-based Fe catalyst. The SWNTs were grown using LPCVD unit (Automate, USA) (Fig.1.6). The substrate, i.e., ferritin-SiO2-Si (supplied by Automate USA), was placed into the center of a 50 mm quartz tube reactor. First, the iron oxide particles were reduced under a H2 pressure of 200 mbar at 800°C for 10 min. CNTs growth was then performed on the substrate at 800°C for 15 and 30 min under flows of 1400 sccm of methane, 100 sccm of ethylene and 500 sccm of H2. After CVD reaction, the furnace was switched off and allowed to cool down to room temperature under Ar gas flow of 500 sccm. (Synthesis of SWNTs and graphene by electric arc discharge method in Ar atmosphere, S. Awasthi, K. Awasthi and O.N. Srivastava, J Nanosci Nanotech [under submission].)

Figure 1.6 SEM images of as-grown CNTs using ferritin-SiO2-Si substrate at different temperatures.

1.3 Synthesis and Characterization of Graphene

Graphite is stacked layers of many graphene sheets, bonded together by week van der Waals force. Thus, in principle, it is possible to produce graphene from a high purity graphite sheet, if these bonds can be broken. Exfoliation and cleavage use mechanical and chemical energy, respectively, for breaking these weak bonds and separate out individual graphene sheets. To scale up the production, various synthetic methods are being developed. Some of the methods listed are presented below.

1.3.1 Micromechanical Cleavage of Highly Oriented Pyrolytic Graphite

The remarkably simple yet efficient method developed by Novoselov and Geim consists in using common adhesive tape to repeat the stick and peel process a dozen times which statistically brings a 1mm-thick graphite flake to a monolayer thin sample. The first piece of graphene sheet was obtained via manual mechanical cleavage of graphite with a Scotch tape [12], which seems to break the rule that no 2D crystals can exist under ambient conditions and shows us many unusual properties [12]. The exfoliated graphene manifests a unique structure and superior properties, although this production method is not applicable on a large scale. Inspired by this pioneering work, several alternative techniques have been developed for fabricating graphene materials.

Graphene has been made by four different methods. This approach, which is also known as the Scotch tape or peel-off method, was based on earlier work on micromechanical exfoliation from patterned graphite [13], and the fourth was the creation of colloidal suspensions. Micromechanical exfoliation has yielded small samples of graphene that are useful for fundamental study, although large-area graphene films (up to ~1cm²) of single- to few-layer graphene have been generated by CVD growth on metal.

1.3.2 Chemical Vapor Deposition Growth of Graphene either as Stand Alone or on Substrate

Another feasible method is by chemical vapor deposition (CVD) and epitaxial growth, such as decomposition of ethylene on nickel surfaces [14]. These early efforts (which started in 1970) were followed by a large body of work by the surface-science community on monolayer graphite [15]. Epitaxial growth on electrically insulating surfaces such as SiC has also been used for the growth of graphene [16, 17]. One of the highly popular techniques of graphene growth is thermal decomposition of Si on the (0001) surface plane of single crystal of 6H-SiC [18]. Graphene sheets are found to be formed when H2-etched surface of 6H-SiC was heated to temperatures of 1250 to 14500°C for a short time (1–20 minutes). Graphene epitaxially grown on this surface typically has 1 to 3 graphene layers; the number of layers being dependent on the decomposition temperature. In a similar process, Rollings et al. have produced graphene films as low as one atom thick [19]. The first report on planar few-layer graphene (PFLG) synthesized by CVD was in 2006 [20]. In this work, a natural, eco-friendly, low-cost precursor, camphor, was used to synthesize graphene on Ni foils. Camphor was first evaporated at 1800°C and then pyrolyzed in another chamber of the CVD furnace at 700 to 850°C using argon as the carrier gas. Large-area, high quality graphene can also be grown by thermal CVD on catalytic transition metal surfaces such as nickel and copper [21, 22]. Reina et al. prepared single- to few-layer graphene on polycrystalline Ni film of 1–2 cm² [23]. The Ni film (500 nm thick) was evaporated on a SiO2/Si substrate and was annealed in Ar+H2 atmosphere at 900 to 10000°C, for 10 to 20 minutes. This annealing step created Ni grains of 5 to 20 μm in size. After CVD at 900 to 10000°C for 5 to 10 minutes, using 5 to 25 sccm CH4 and 1500 sccm H2, graphene was found to form on the Ni—the size of each graphene being restricted by the Ni grain size. For Ni, mixed mono-and bi-layer graphene coverage of 87% has been reported [24, 25], while for Cu foils, an average of 95% of surfaces were covered by mono-layer graphene [26]. The graphene was later transferred to any substrate, keeping its electrical properties unchanged, thus making them suitable for various electronic applications. Typical CVD graphene growth uses gaseous hydrocarbons at elevated temperatures as the carbon source, such as methane, ethylene [27-29] and acetylene [3]. Single-layer graphene were synthesized from ethanol on Ni foils in an Ar atmosphere under atmospheric pressure by flash cooling after CVD, but a wide variation in graphene layer number was observed over the metal surface [31]. Single-and few-layer graphene films were grown employing a vacuum-assisted CVD technique on Cu foils using n-hexane as a liquid precursor [32]. Copper appears to have a small affinity for oxygen that allows for graphene growth even if the source of carbon is a solid, such as the sugar as reported recently [33]. Graphene, thus synthesized and transferred onto a glass substrate, has shown 90% optical transmittance [34].

1.3.3 Chemical and Thermal Exfoliation of Graphite Oxide

Some recent success in regards to graphene includes chemical exfoliation through the formation of derivatized graphene sheets such as GO [35, 36], r-GO [37], or halogenated graphene, solvent-assisted ultrasonic exfoliation [38]. Graphite oxide was first prepared in the nineteenth century [39], and since then it has been mainly produced by the following methods pronounced by Brodie, Staudenmaier [40] and Hummers [41]. All three methods involve oxidation of graphite in the presence of strong acids and oxidants. The level of the oxidation can be varied on the basis of the method, the reaction conditions and the precursor graphite used. Although extensive research has been done to reveal the chemical structure of graphite oxide, several models are still being worked out. Graphite oxide consists of a layered structure of graphene oxide sheets that are strongly hydrophilic such that intercalation of water molecules between the layers readily occurs [42]. The interlayer distance between the graphene oxide sheets increases reversibly from 6 to 12 Å with increasing relative humidity [43]. Notably, graphite oxide can be completely exfoliated to produce aqueous colloidal suspensions of graphene oxide sheets by simple sonication [44] and by stirring the water/graphite oxide mixture for a long enough time [45]. The second approach is the oxidation-exfoliation-reduction of graphite powder [46]. Severe oxidation treatment converts graphite to hydrophilic graphite oxide which can be exfoliated into single-layer graphite oxide (graphene oxide) via stirring or mild sonication in water. Graphene oxide can be regarded as a functionalized graphene containing hydroxyl, epoxy and carboxylic groups, providing reaction sites for chemical modifications [47]. Reducing graphene oxide can partly restore its graphitic structure as well as conductivity [48]. Although reduced graphene oxide (r-GO), (also called chemically modified graphene [CMG], chemically converted graphene [CCG] or graphene), has considerable defects, it is one of the most widely used graphene-based renewable energy materials due to its low cost, facile preparation process, large productivity, and potential for functionalization [49].

1.3.4 Arc-Discharge Method

The arc-discharge method has also been used to prepare graphene sheets. Rao et al. reported for the first time that the arc-discharge method can also be used for the synthesis of graphene sheets [50]. By using graphite rods as electrodes, they have synthesized pure graphene with mainly 2–4 layers in the inner wall region of the arc chamber under relatively high pressure of hydrogen without any catalyst. Moreover, through this method, nitrogen-doped and boron-doped graphene sheets can also be easily synthesized with boron sources (B2H6) or nitrogen sources (pyridine) mixed into the hydrogen gas. However, the size and thickness of the pure graphene sheets still have room to improve, and the properties of the N- and B-doped graphene sheets synthesized also need further studies.

1.4 Methods Used in Our Lab: CVD, Thermal Exfoliation, Arc Discharge and Chemical Reduction

To scale up the production, various synthetic methods are being developed. One of the methods is micromechanical cleavage of highly oriented pyrolytic graphite, however, the yield is very low. Another feasible method is by chemical vapor deposition (CVD) and epitaxial growth, chemical and thermal exfoliation of graphite oxide. Graphite oxide was first prepared in the nineteenth century [51], and since then it has been mainly produced by the following methods pronounced by Brodie, Staudenmaier and Hummers. All three methods involve oxidation of graphite in the presence of strong acids and oxidants. Graphite oxide consists of graphene sheets decorated mostly with epoxide and hydroxyl groups. Graphite oxide resulting from the deployment of these methods when subjected to thermal exfoliation leads to the formation of graphene oxide, and that method is called thermal exfoliation. Here the dried graphite oxide powder (~ 200 m) was placed in a quartz tube (diameter ~ 25 mm and length ~ 1.3 m). The sample was flushed with Ar for 15 min and the quartz tube was quickly inserted into a furnace preheated to 1050°C and held in the furnace for 30 s. The as-prepared GO was a brownish powder while the exfoliated version was of light consistency and shiny black. The XRD pattern of graphite, graphite oxide and graphene are shown in Figure 1.7. The XRD pattern of graphite shows an intense peak 2θ = 26.4°. This peak corresponds to 002 plane of graphite with interlayer spacing of 0.34 nm. In the XRD pattern of graphite oxide a new peak appears at 2θ = 13.2°, corresponding to the 002 plane of graphite oxide [52]. The interlayer spacing of GO is ~ 0.75 nm, which is significantly larger than that of graphite, due to intercalating oxide functional groups. The mechanism of exfoliation is mainly the expansion of CO2 evolved into the interstices between the graphene sheets during rapid heating. The disappearance of native graphite XRD peaks in the XRD pattern of as-prepared graphene sample supports the formation of graphene sheets.

Figure 1.7 The XRD patterns of graphite, graphite oxide and graphene samples.

Few-layer graphene (FLG) has been synthesized by using electric-arc discharge of graphite electrodes in argon ambience at different pressure. The arc was maintained by continuously translating the anode to keep a constant distance of ~1 mm from the cathode. The deposits collected from the inner walls of the arc chamber have been characterized and Figure 1.8(a,b) shows the TEM images of as-synthesized FLG at 350 torr pressure of argon. Distinct features of the micrographs are the large-area graphene sheet-like structures as can clearly be seen in the Figure 1.8. Few-layer graphene nanosheets are clearly visible in the images. The Figure 1.8(b) shows the HRTEM image of the large-area graphene with minimum number of layers equal to four. The width of these graphene nanosheets is ~100–200 nm.

Figure 1.8 TEM micrographs of as-synthesized graphene nanosheets at 350 torr argon pressure.

1.4.1 Raman Spectra

It is worth mentioning that an extensive analysis of graphene has been reported by Ferrari et al. [53], who have demonstrated that the second order Raman peak centered at 2700 cm–1 (2D peak) is the characteristic graphene feature and can be very useful in identifying the number of layers in a few-layer graphene sample. The number of layers in a graphene film can be estimated from the intensity, shape and position of the G and 2D bands. While the 2D band changes its shape, width and position with an increasing number of layers, the G-band peak position shows a down-shift with the number of layers. From XRD and TEM analysis it has been found that at 350 torr the minimum numbers of graphene layer are formed. By monitoring the width and position of this 2D peak one can deduce the number of layers from graphene samples. In Figure 1.9 at 350 torr, the 2D peak is at 2687cm−1, confirming the formation of FLG.

Figure 1.9 Raman Spectra at 350 torr. (Synthesis of SWNTs and graphene by electric arc discharge method in Ar atmosphere, S. Awasthi, K. Awasthi and O.N. Srivastava, J Nanosci Nanotech [under submission].)

Synthesis of large-area, high-quality and uniform graphene films on metal substrate by chemical vapor deposition (CVD) is shown in Figure 1.10. Chemical vapor deposition (CVD) is one of the main interesting synthetic procedures because it employs hydrocarbon decomposition over substrates, where metal nanoparticles have been placed. A one meter long quartz tube is used in the CVD system in our laboratory. The CVD system has a two zone furnace and the diameter of the quartz tube is 2 inches. The quartz tube is adjusted in the two zone CVD furnace. The solid carbon precursor (sugar) is placed in an alumina boat and placed on the middle portion in the first zone furnace. Alloy pieces are placed in a second alumina boat and it is placed in the second zone furnace. The distance between substrate and precursor is nearly 22 cm. In the first few (15) minutes only high purity gas flows. After that, the heating starts in the second zone (substrate) furnace set at 850°C in the presence of 200 sccm flow rate of argon gas. In the metal substrate we used copper and gold foil, and copper-gold alloy (Cu3Au2) as a substrate. Next, CH4 was introduced at a flow rate of 30 sccm and H2 at 50 sccm for annealing. After this the solid carbon precursor was vaporized and deposited on the substrate in the form of graphene.

Figure 1.10 CVD set-up.

In Figure 1.11(a) given below, Raman spectra of graphene was deposited on Cu3Au2 substrate. The appearance of a well-pronounced 2D peak at 2685cm−1 reveals the formation of high quality (probably single-layer) graphene. While in Figure 1.11(b), the growth of a large graphene island on Cu3Au2 substrate is visible in the image captured by Raman spectrophotometer.

Figure 1.11 (a) Raman spectra of graphene deposited on Cu3Au2 substrate; (b) image captured by Raman spectrophotometer.

1.4.2 Electrochemical Exfoliation

The electrochemical methods of preparing GN flakes involve the application of cathodic or anodic potentials or currents in either aqueous (acidic or other media) or non-aqueous electrolytes. One of the most important parameters for consideration of scaling-up the electrochemical technology is the yield of GN flakes. Single- or multi-layered GN flakes can easily be produced in short periods of time. It is a simple and fast method to exfoliate graphite into thin graphene sheets, mainly AB-stacked bilayered graphene with large lateral size (several to several tens of micrometers). In this method we take a chemical mixed with deionized water; graphite is used as an anode and platinum is used as cathode, and voltage is applied. Within a few minutes whole graphite foil is exfoliated into thin graphene sheets and dispersed into whole electrolyte (Figure 1.12). The electrical properties of these exfoliated sheets are radially superior to commonly used reduced graphene oxide, whose preparation typically requires many steps including oxidation of graphite and high temperature reduction.

Figure 1.12 A set up in our lab showing (a) before exfoliation; (b) after exfoliation.

1.5 Functionalization of Carbon Nanotubes and Graphene

Beyond synthesis by different techniques, it appears immediately clear that CNTs and graphene need processing after their synthesis. To address this issue, purification methods and, above all, functionalization approaches, are essential to allow manipulation and further application of this material. Usually, metal nanoparticles and amorphous carbon are present as a synthetic residue. In general, these carbon nanomaterials are a fluffy powder difficult to manage, while chemical functionalization contributes to the preparation of more homogenous and soluble material. SWNTs are highly polarizable smooth-sided carbon compounds with attractive interaction of 0.5 eV per nanometer of inter-tube contact. This extreme cohesive force makes it difficult to disperse SWNTs into individual state. Pristine SWNTs tend to agglomerate in the polymer matrix and form bundles. Similarly, pristine graphene is also hydrophobic, so producing stable suspension of graphene in water or organic solvents is an important issue for the fabrication of many graphene-based devices [54, 55]. Prevention of aggregation was of particular importance for graphene sheets because most of their unique properties were only associated with individual sheets and keeping them well separated was required. Strategies for functionalizing these carbon nanomaterials are important for the pursuit of these applications so these materials should be dispersed uniformly and form stable suspension. In recent years, new functionalization methods have been developed to disperse these carbon nanomaterials which allow their application [56, 57] A wide variety of functionalizations have been reported in the literature [58–60], the most important of which are summarized in Scheme 1.1. We can categorize mainly covalent and non-covalent approaches.

Scheme 1.1 A wide variety of functionalizations adapted from ref. [61].

1.5.1 Covalent Functionalization

In this method CNTs are functionalized by nonreversible attachment of appendage on the sidewalls and/or on the tips. Also in this case, many different approaches are reported [13]. Briefly, reactions can be performed at the sidewall site (sidewall functionalization) or at the defect sites (defect functionalization), usually localized on the tips. In the first case, fluorination with elemental fluorine at high temperature (400–600°C) has been explored, accomplishing further substitutions with alkyl groups. Furthermore, radical addition via diazonium salt has been proposed by Tour’s group [62]. On the other hand, cycloadditions have found wide interest. Cycloadditions have been reported, such as carbene [2+1] cycloadditions or Diels-Alder via microwave (MW) irradiation cycloaddition. Side defects functionalization occurs via amidation or esterification reactions of carboxylic residues obtained on CNTs. Moreover, it is feasible that in general caps (i.e., tips when they are not cut) are more reactive than sidewalls because of their mixed pentagonal-hexagonal structure. The f-MWCNTs are not modified in their electronic structure and new properties can be added by means of functionalization. Instead, electronic properties of SWCNTs are perturbed by covalent functionalization and double bonds are irreversibly lost. This may affect conductive property, preventing further CNT applications.

1.5.2 Non-Covalent Functionalization

Recently, non-covalent functionalization has been preferred as it has several advantages over covalent functionalization. Noncovalent functionalization preserves the structural and electrical properties of CNTs and graphene which may be advantageous for future application. Noncovalent functionalization of these carbon nanonmaterials is basically through van der Waals, electrostatic and π-stacking interaction, etc. [63]. CNTs wrapping by polymers, including DNA, have been studied [64]; also, proteins are able to non-convalently interact with CNTs and these are often used for biosensor applications [65]. Furthermore, these procedures are usually quite simple and quick, and involve simple steps like ultrasonication, filteration, magnetic stirring and centrifugation, etc., and also do not perturb the electronic structure of CNTs, graphene and SWCNTs in particular.

These functionalization methods generally involve the conjugation of these carbon nanomaterials with the biological species like protein, carbohydrates and nucleic acid, etc.

In our lab we have functionalized CNT and graphene both by covalent and non-covalent methods. Covalent functionalization with amine groups of CNTs was achieved after such steps as carboxylation, acylation and amidation [66]. These CNTs were treated with a concentrated H2SO4/HNO3 mixture to form a stable aqueous suspension containing individual oxidized CNTs with carboxyl groups (Figure 1.13). Then carboxylated CNTs were treated with ethylenediamine [NH2(CH2)2NH2], forming an active amine group on the nanotubes surface. The optical image of as-synthesized CNTs and amino-functionalized CNTs is also shown in Figure 1.14(a).

Figure 1.13 Schematic of the reaction scheme to form carbon nanotube (CNT) with amino functionalization [66].

Figure 1.14 (a) As synthesized CNTs; (b) Amino functionalized CNTs.

Several reports are available for functionalizing graphene and CNTs by biomolecules. The biological molecules which possess hydrophobic and hydrophilic moieties provide a more efficient means to solubilize the nanomaterial in water than the use of surfactants and polymers. Molecules containing aromatic groups or electron-rich environments have also been reported to modify nanotubes/nanosheets via -π stacking. The non-covalent approaches are based on interactions of the hydrophobic part of the adsorbed molecules with nanotube sidewalls through van der Waals, -π, CH-, and other interactions, and aqueous solubility is provided by the hydrophilic part of the molecules. If any, the charging of the nanotube surface by adsorbed ionic molecules additionally prevents nanotube aggregation by the coulombic repulsion forces between modified CNTs. In the last few years, the non-covalent treatment of CNTs with surfactants and polymers has been widely used in the preparation. On the basis of these observations, we explored the use of a novel amino-acid-based non-covalent functionalization of graphene and CNTs using amino acid L-cysteine.

Purified CNTs and graphene were dispersed in double distilled water and sonicated for one hour at room temperature for CNTs and one and a half hours for graphene to obtain homogeneous solution. Then, 0.1 M of L-cysteine was added and sonicated for 30 min followed by 2 h for CNTs and 3 h for graphene of constant stirring to each solution. The CNTs and graphene so obtained were thoroughly washed with double-distilled water in centrifuge at 10,000 rpm for 10 min and the solution phase was discarded. This washing was repeated five times in order to remove any unbound L-cysteine.

1.5.3 FTIR Analysis of CNTs and FCNTs

The presence of functional groups after the functionalization of CNTs and functionalized CNTs by FTIR spectroscopy are shown in Figure 1.15. Figure 1.15(a) shows the FTIR spectra of purified CNTs, the band at 3450cm−1 is attributed to the presence of –OH group on the surface of CNTs and is believed to be due to the oxidation during the purification of nanotubes. The peak present at 1585 cm−1 corresponds to the C=C stretching vibration of CNTs. In the FTIR spectra of amino FCNTs, Figure 1.15(b), the peak at 3430 cm−1 is due to the NH2 stretch of the amine group overlapped with –OH stretching vibration. The presence of peaks at 1472 and 1385cm−1 correspond to the N-H and C-N bond stretching of amine group, respectively. The peaks at 2955 and 2835 cm−1 are due to the –CH stretching of CH2 group.

Figure 1.15 FTIR spectra of (a) CNTs and (b) FCNTs FTIR analysis of CNTs and FCNTs.

The TEM image of GS reveals a wrinkled paper-like structure (Figure 1.16a). The inset in Figure 1.16(a) is the selected area electron diffraction pattern (SAED) of GS, showing a clear diffraction spot. The diffraction spots were indexed to hexagonal graphite crystal structure, whereas the TEM images of FGS shows a smoothened surface which is due to the functionalization by L-cysteine Figure 1.16(b).

Figure 1.16 TEM images of (a) GS and (b) FGS.

1.6 Applications

Nanomaterials have become the focus of scientific research in the past few years because of their unique electronic, chemical, optical, thermal and mechanical properties. Among the numerous active nanomaterials, carbon nanomaterials are at the forefront of research in a variety of physical and chemical disciplines. Both multi- and single-walled carbon nanotubes and graphene sheets (GS) show great promise for advancing the fields of biology [67], medicine [68], electronics [69], composite material [70], energy technology [71], etc. Currently there is a flurry of activity amongst scientists to exploit the unique properties of these carbon nanomaterials for potential application. We have also used these carbon nanomaterials for various applications such as drug delivery, biosensing, protein immobilization, protein and DNA immobilization, hydrogen storage, etc. Some of the applications used in our lab are discussed briefly below.

a) Targeted killing of Leishmania donovani in vivo and in vitro with amphotericin B attached to functionalized car bon nanotubes and graphene

The development of new drug delivery systems is attractive as it allows optimization of the pharmacological profile and the therapeutic properties of existing drugs. Within the family of nanomaterials, carbon nanotubes and graphene have emerged as a new and efficient tool for transporting therapeutic molecules, due to their unique physical and chemical properties. By employing the drug delivery system of f-CNTs to the known antileishmanial drug AmB, we found that antileishmanial efficiency is significantly increased in both in vivo and in vitro settings. This, together with low cytotoxicity of f-CNT–AmB, means that it is a viable compound for further drug development [72]. The synthesis of f-CNT–AmB was performed on a large scale and it was stored at room temperature for at least six months without any loss of efficacy.

Encouraging in vitro/in vivo results have prompted us to carry out further research on graphene-based drug delivery for Leishmaniasis treatment. The results obtained by targeted drug delivery were more promising as compared to CNT-based drug delivery. The probable reason could be due to the large surface area of graphene enabled to load more drug, thereby improving the bioavailability of drug to the cells. Figure 1.17 below shows the drug-attached graphene and Figure 1.18 shows