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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
2D Materials: Chemistry and Applications offers a concise exploration of the revolutionary 2D materials synthesis, their properties, and diverse applications. It presents information about graphene and other 2D materials like germanene and stanene, emphasizing their synthesis, functionalization, and technological use. The book chapters in part 1 cover the foundational aspects of graphene's structure and production techniques, highlighting their potential in areas like energy storage, drug delivery, and nanoelectronics. The book also explains the versatile applications of graphene-based nanocomposites, highlighting their multifunctional capabilities. Chapters also demonstrate the impact of functionalization on applications like biomedical imaging, microbial control, and environmental sustainability. The challenges and solutions concerning the toxicity of graphene-related materials are also highlighted. This book is a foundational resource for researchers, academics, and industry professionals in materials science, nanotechnology, chemistry, and environmental engineering on 2D materials. Readership Researchers, academics, and industry professionals in the field of materials sciences and applied chemistry.
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Seitenzahl: 484
Veröffentlichungsjahr: 2024
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PREFACE
In the rapidly evolving field of materials science, the exploration and utilization of two-dimensional (2D) materials have marked a revolutionary shift in how scientists and engineers approach the empirical evidence in developing new technologies. Among these materials, graphene stands out due to its exceptional electrical, thermal, and mechanical properties, which have led to its exploration across various domains—from energy storage and environmental engineering to biomedicine and electronics. This book, "2D Materials: Chemistry and Applications," aims to capture the holistic view and essence of recent research and advancements by evidence based assessment of advancements in its fundamental chemistry and mushrooming usage in copious applications.
The opening chapter of this book provides a foundational understanding of graphene and its structure at the molecular level, various types of defects, synthesis methods, and functionalization techniques, with the inclusion of recent advancements. This groundwork is crucial for both seasoned researchers and newcomers to the field, helping them in understanding the differentiating properties of graphene along with strategies that allow surface revamping and exploration of this material in various applications.
As the chapters progress, the focus shifts to more complex constructs such as hybrid materials combining graphene with various nanoparticles. These hybrids exhibit unique resonance of individual properties of graphene and the nanoparticles that can be harnessed for several established and emerging applications, including sensors, catalysis, and energy conversion technologies. This discussion leads naturally into a detailed examination of multifunctional graphene-based nanocomposites, which are positioned at the cutting edge of materials research with their ability to perform multiple functions simultaneously.
Further, the book investigates the life sciences applications of graphene, illustrating its potential in gene and drug delivery systems where it promises to increase efficiency and targeting accuracy. The subsequent chapters explore the role of graphene in biomedical imaging and its emerging significance in microbial control, where its properties can be leveraged to offer new solutions in fighting infections and enhancing hygiene.
One of the most compelling areas of graphene application covered in this book is in cancer therapy. Graphene-based materials have been identified as potent tools in the diagnosis and treatment of cancer, reflecting a major theme of contemporary research efforts. Additionally, the application of graphene in tissue engineering is discussed, providing insights into its use in constructing scaffolds that mimic the extracellular matrix for tissue regeneration.
The final chapter looks at the broader implications of graphene derivatives in biotechnology, underscoring the vast potential of these materials to influence future developments in science and engineering.
"2D Materials: Chemistry and Applications" is designed to serve as a comprehensive resource for scientists, engineers, and students who are engaged in or entering the field of graphene research. It aims to inspire further innovations and applications of this remarkable material, paving the way for new solutions to some of the world's most pressing challenges. Through this book, we invite readers to explore the multifaceted world of graphene and envision the future it is capable of creating.
List of Contributors
Graphene: Understanding its Structure, Synthesis, and Functionalization
Vinay Deep Punetha1,*,Anton Kuzmin2,3,Golnaz Taghavi Pourian Azar4Abstract
Material science has gone through several evolutionary stages; especially the discovery of graphene has added one of the most defining chapters in this journey. Owing to the enormous potential of this material in various applications, a tremendous pace can be seen in the development of graphene-derived materials and technologies. The 2D revolution in material science can be marked by the shift from the bulk form of materials to their intelligent and efficient two-dimensional (2D) analogs and their use in developing innovative contrivances. Various forms of 2D graphene have recently evolved, including mainly monolayer graphene, bilayer graphene, graphene oxide, graphene nanoribbons, and graphene quantum dots. These materials have shown great potential to revolutionize various aspects of human life, from electronics and actuation to healthcare and energy.
Its exceptional properties make it an ideal candidate for various applications. Continuing explorations and epistemological pieces of evidence will likely reveal even more prospective applications. The book chapter deals with a concise overview of the structural aspects of graphene, the presence of defects, methods of synthesis, and functionalization. The chapter will help develop an essential understanding of the critical aspects of science and recent developments around it. This chapter aims to provide a quick and easily understandable summary of various complex aspects of it by reducing irrelevant or extraneous information.
INTRODUCTION
Two-dimensional (2D) materials are characterized by their nano-dimensions, which fall between 0.3 and 100 nanometers. They consist of mostly a single layer of atoms in two dimensions with atomic thickness. However, a few layers of atoms can also be seen if left for a period due to stacking or agglomeration. The 2D materials include Xenes (2D materials of 14 group elements), transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), boron nitride (BN), and black phosphorus (BP) [1], The scientific community is particularly interested in the 14 group 2D materials due to their chemical and optoelectronic properties. Recent efforts have concentrated on exploring and realizing the vast potential of these materials. The critical elements of this class include 2D structures of carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) with graphene, silicene, germanene, tinene, and plumbene as their respective 2D analogs. Graphene is the most studied material with concrete experimental evidence. Primarily theoretical investigations using density functional theory (DFT) have been performed so far with heavier 2D analogs such as tinene and plumbene. The reduced dimension and high surface-to-volume ratio of these materials offer exceptional physics with significant applications in optoelectronics, mechanical, thermal, and biological applications [2]. Graphene, in particular, and its derivatives have found astounding applications in various fields due to nonpareil properties, such as its ability to transmit electric current and the potential to bear mechanical forces or stresses such as tension, compression, bending, or torsion. In addition, graphene allows tuning various matrices as reinforcing material with opportunities to develop the next generations of composite materials [2]. As a reinforcement material, graphene offers excellent strength, stiffness, and several other desirable properties. These composites have shown incredible potential in developing numerous applications such as remote actuation, soft robotics, disease diagnosis, cancer therapy, targeted drug delivery, bio-imaging, and many more and quite elaborately discussed in the review article by Punthea et al. [3]. Several forms of graphene have been studied extensively in recent times. A summary of these forms is given in Table 1. Monolayer graphene, graphene oxide, and functionalized graphene oxide are the most investigated forms of graphene in various applications (Fig. 1). However, graphene structures such as nanoribbons, quantum dots, and 3D graphene are relatively new in their exploration journey [4]. Nanoribbons are thin strips of nanomaterials that are only a few nanometers wide and can be made from a variety of materials, quantum dots are tiny semiconductor particles that are only a few nanometers in size and 3D graphene, on the other hand, is a three-dimensional form of graphene that has a complex, porous structure. All three materials are currently being studied and developed for a wide range of applications, and their unique properties make them promising candidates for future technologies.
One of the most sophisticated tasks is to synthesize pristine graphene; especially the commercial production of monolayer graphene has witnessed many obstacles [5]. Various methods have been proposed recently for synthesizing graphene and graphene derivatives. These methods can be categorized into top-down and bottom-up methods [6]. In the top-down method, the bulk precursor is broken down to the required size by applying mechanical forces. Simultaneously, the bottom-up approach deals with the synthesis of graphene structure by using atomic precursors. The top-down approach is good when it comes to the commercial production of these materials. At the same time, the bottom-up method generally deals with producing consistent 2D graphene-like skeletons with high quality [6, 7]. The chapter, in detail, discusses these approaches with extended discussions of their added pros and cons.
The functionalization of graphene is another crucial aspect, which has been discussed in detail in this chapter. Functionalization refers to the surface revamping of graphene to modify its properties by structural manipulations on its atomic organization. Functionalization plays a pivotal role in significantly altering the characteristics of graphene, opening up a multitude of possibilities for its widespread utilization across various applications (Fig. 2). In popular science, there are mainly two methods to modify the structure of graphene by introducing new moieties on its surface. These are, (i) Covalent functionalization and (ii) Non-covalent functionalization [3, 6]. This categorization is based on the interaction between the graphene surface (host) and the newly introduced moiety (guest). Primarily, the guest moieties can be categorized into four major types; (i) Oxygen-containing hydroxyl, carboxyl, ethereal groups, (ii) Nitrogen-containing amine and pyridine groups, (iii) Polymer units such as polyvinyl alcohol (PVA), polyethylene glycol (PEG) and (iv) Metal-based functional moieties [3]. The functionalization dramatically affects the physical properties of graphene, such as solubility, thermal and electrical properties, and many more. In addition, it prevents the agglomeration of the graphene sheets and opens its significant application in composite fabrications [3, 6, 8]. Functionalization changes graphene by introducing various functional groups or molecules to its surface, which alters its physical, chemical, and electrical properties.
Fig. (1)) Illustration of different forms of graphene. Fig. (2)) Illustration summarizing applications of graphene.The purpose of this chapter is to offer readers a concise introduction to the subject matter. It is divided into three main sections that cover different aspects related to graphene and its properties. The first part focuses on explaining the detailed structure of graphene and exploring its various morphological appearances. The aim is to provide a comprehensive understanding of graphene's structural characteristics and nuances. Moving on to the second part of the chapter, emphasis is placed on innovative approaches to synthesizing graphene. The chapter highlights the latest ideas and techniques used in the synthesis process, showcasing advancements in this field. Lastly, the chapter concludes with a summary of the surface modification of graphene-like structures. It presents a concise account of how the structure of graphene can be modified and discusses significant findings in this area. Overall, this book chapter offers a concise yet detailed exploration of the classification of graphene structures, different synthesis methods, and the ways in which the surface of graphene can be modified. It aims to provide readers with a critical understanding of these topics and their significance in the field.
STRUCTURE OF GRAPHENE, VARIOUS REGIONS, AND DEFECTS
The flat, two-dimensional structure of graphene can be understood and explained by considering the inherent properties of its constituent atom carbon. The electronic configuration of C, having four electrons in the valance shell and a desirable atomic radius (0.077 nm) for the effective lateral overlapping of the orbitals, results in the consistent sp2 hybridized skeleton of the graphene [9]. The orbital electronic configuration suggests four unpaired electrons in its excited state, resulting in sp2 hybridized orbitals of similar shape and energy and one signally occupied ‘p’ orbital. The three sp2 hybridized orbitals form sigma bonds with neighboring carbon atoms and signally occupied 'p' orbital participate in forming pi- bond by the lateral overlap of the orbitals [10]. The participating sp2 hybridized orbitals rearrange themselves in 3D space, forming a geometry with the most negligible electronic repulsion and a planer geometry where hybridized orbitals lie at an angle of 120 degrees from each other, resulting in the honeycomb lattice [1, 5]. The exceptional stability in graphene arises due to the extended conjugation and resonance of the electrons [11].
A two-dimensional structure like graphene or germanene becomes more attractive than bulkier analogs due to slight variations arising from various structural reasons, such as buckling in the 2D structure of germanene changes the band structure and alters optoelectronic properties [12-16]. Similarly, various edge types also affect the photoelectric response of graphene. Graphene possesses various edge architectures (Fig. 3), which are fundamentally different from each other and possess different characteristic properties [17-19].
Fig. (3)) Various edge structures in graphene.In principle, there exist five different types of edge structures: zigzag, armchair, cove, gulf, and fjord [17-19]. Each edge type affects the overall band structure, opportunities for orbital coupling, and various other parameters, thereby regulating the optoelectronic performance of the graphene sheets. A summary of edge types and their characteristic properties has been given in Table 2. Xu et al. explored the magnetic properties of zigzag graphene and observed that the ferromagnetic state is energetically favorable over the antiferromagnetic states. In addition, his theoretical analysis suggests a stark dependence of energy difference between these states on the width of graphene structures [20]. The armchair edges of graphene have shown great importance in developing direct semiconductors [21-23].
Researchers studied the electronic and adsorption properties of the cove graphene and suggested using these materials as potential topological insulators. Yao et al. [28] synthesized fjord graphene by regioselective cyclo dehydrogenation, the fjord periphery of such architectures is found to be chiral and capable of simulating chiroptical responses. In most cases, the properties are greatly dependent on the width of the graphene sheets, making the graphene synthesis method more crucial and significant. The structural aspects of graphene also affect the chemical reactivity of these sub-architectures in graphene. The 2D graphene sheet does not exhibit uniform reactivity in given reaction conditions. The chemical reactivity of basal planes in graphene is almost half that of the edges. The STM analysis demystifies the phenomenon of inconsistent reactivity in terms of the electron density in these regions. The results of STM analysis show that edges retain higher electron density of states (DOS) near the Fermi level than the basal planes [25, 28].
The resulting transformation in hybridization can also explain the higher reactivity of the edges compared to that of the basal plane. When the graphene surface reacts due to the cleavage of the sp2 hybridized bond, the newly formed structure transitions from sp2 to sp3 hybridization; subsequently, the geometry of the reaction center also changes from planer to tetrahedral. The carbon located at the basal plane shows stark reluctance to this transition, as it will distort the basic structure creating more strain due to popping out of the carbon atoms from the plane to attain tetrahedral geometry. The terminal carbon atoms are free from one side and more susceptible to sp2-sp3 transition. Moreover, defects on the graphene surface play a significant role in the reactivity. Introducing defects on the graphene surface can enhance the reactivity and is often regarded as a proven strategy to functionalize the graphene sheets. However, creating defects on the graphene surface has certain drawbacks as it restricts its wide use in electronics, the development of electrodes, and spintronic devices. Transmission electron microscopy (TEM) and scanning tunneling microscopy (SEM) have been critical in studying these defects. In general, the defects in graphene can be categorized into two parts (i) Intrinsic defects, and (ii) Extrinsic defects. A summarized comparison of these defects has been given (Table 3.) The structural aspects of these defects can be understood by illustrations given in Fig. (4).
Fig. (4)) Various forms of defects in graphene.“TOP-DOWN” AND “BOTTOM-UP” METHODS OF SYNTHESIS OF PRISTINE GRAPHENE
Since the discovery of graphene in 2004, the research community involved in the synthesis of graphene has made significant progress. Several methods have been developed over the period and are characterized by uniqueness in their approach. In principle, these methods are categorized into two groups; the first “top-down” and second “bottom-up” methods [32]. The top-down method includes bulk precursors having structural similarities with the graphene and breaks them down into nano architectures. These methods' main operational components include exfoliation and chemical oxidation-reduction elements. The other class includes methods in which atomic precursors fabricate a hexagonal pattern in two-dimension resulting in graphene. Chemical vapor deposition (CVD) and epitaxial growth methods are included in this category.
CVD Method
Recently, the CVD method has evolved as one of the most reliable methods to synthesize graphene. The hydrocarbons are used as the precursor for this method by creating conditions of hydrocarbon decomposition. The decomposition furnishes carbon atoms catalyzed further to form a 2D monolayer. Condition of high vacuum and temperature is used for the rapid decomposition of the precursor. The heavy metal atoms of ‘d’ block, such as Cu, Ni, and Pt, are the CVD method's most frequently used catalytic material [33-36]. The results obtained by copper as the substrate are exceptional as the reaction includes a self-limiting feature, which helps synthesize monolayers. Once the monolayer is prepared, it can be etched out of the metallic surface and utilized in further applications. The in-depth discussion on CVD is beyond the scope of this book chapter; hence we are limiting this discourse to the description of two main CVD methods: thermal CVD and microwave-assisted CVD [37-39].
Thermal CVD Method
The high temperature and vacuum are the key reaction-enforcing parameters of this method. Chemically this method can be termed a gas phase reaction assisted with the help of metal catalysts. The thermal CVD method is a gas phase reaction that mechanistically can be divided into three significant steps. The first step is thermal activation, characterized by high temperature and collisional frequency between the precursor gaseous atoms. The second step includes thermal decomposition, in which gaseous precursor moieties break into individual atoms or smaller gaseous substances [33]. The third and the last step is a thermal combination of individual atoms on the metal surface. The thermal decomposition method suffers from minor drawbacks, as sometimes high temperature evaporates a few atoms from the metal surface, resulting in the graphene with single vacancy defects. The first precursor ever used for synthesizing graphene was camphor on the catalytic surface of Ni substrate in 2006. The process includes the camphor's evaporation and pyrolysis at high temperatures. Argon was used as the carrier gas [37-39]. The most significant revelation on the CVD method came back in 2009 when monolayer growth was observed on the Cu surface. It was observed that Cu acts as the best substrate by offering a self-limiting feature that prevents the multilayer growth of the graphene sheet [34]. This method is widely used to produce pristine monolayer graphene with minor methodological optimization and improvements. The most widely accepted method uses methane and hydrogen gas as a precursor on the Cu surface. However, this method is a slow process but commits to pristine graphene of high quality. In recent advancements, the dependence on the quality of graphene is reduced on the catalytic surface by applying plasma enhanced CVD method [35]. This method has evolved as one of the most promising methods as the quality parameters of graphene can be controlled easily by changing the parameters such as pressure and applied power [36]. Moreover, this method was found to help control the defects on the graphene surface with great potential to use defect-maneuvered graphene [37] for the development of sensors and other electrochemical applications [38, 39].
Microwave Plasma-enhanced CVD
Traditional CVD methods are often recorded with the slow synthesis of graphene and the high-temperature requirement [40, 41]; these two concomitant features of this process make it a slow and cost in-effective graphene synthesis method [42, 43]. The microwave plasma-enhanced CVD method is an ameliorated form of CVD, which is very impactful in dealing with the above-mentioned drawbacks. The plasma-enhanced CVD is characterized by microwave frequency to create plasma characterized by charged particles and neutral atoms having random kinetic motions and collisions inside the reactor [44]. Externally applied fields can regulate the plasma state inside the reactor. Since microwave frequency is used in the decomposition and ionization of the precursors, the temperature required for plasma-enhanced CVD ranges from 300 to 700°C depending upon the type and nature of the catalytic surface [45-47]. For instance, the synthesis of graphene on the Ni surface requires a temperature in the range of 300 °C to 400 °C, while robust synthesis is observed only over 600 °C in case of the Cu substrate. However, this method has certain drawbacks, especially related to the hexagonal consistency in the 1D monolayer. As the plasma phase and the substrate are very close to each other in this method, this monolayer is often reported with the presence of defects.
The minimized interaction between the plasma phase and the substrate prevents diffusion of the plasma phase into the growing substrate and consequently perturbs the growing monolayer to the least, known as protective shield (Fig. 5). In addition to plasma perturbation into the growing monolayer, a tiny fraction of metal atoms get doped into the monolayer in due process. In the case where pristine graphene is required, unintentional doping of the graphene sheet becomes a problem [46, 47]. Zhang et al. developed a more reliable method to prepare pristine doping-free consistent hexagonal array graphene by employing nonmetallic growth of a hexagonal array of graphene films on different non-conducting substrates. Gaseous precursors used in his method were C2H2, NH3, and H2 treated on the nonmetallic surface of Al2O3 and SiO2. It has been established in his study that the optimum temperature required in both processes was different. Due to the low activation energy barrier of Al2O3, low-temperature defect-free synthesis of graphene can be achieved [48].
Epitaxial Growth
Epitaxial growth on diverse substrates is an alternative bottom-up approach for the production of large-area and few-layer graphene. The synthesis of graphene using epitaxial growth is highly sensitive to various thermodynamic, kinetic, and growth parameters; hence precise control over these parameters extends the scope of regulating the shape and size of the graphene layer [49]. Silicon Carbide (SiC) is used most frequently as the substrate, while other substrates such as hexagonal boron nitride (hBN), Platinum (Pt), Copper (Cu), and Nickel (Ni) can also be used [49]. The synthesis of graphene is studied mainly on the SiC surface as it offers several unique advantages. The unique crystal structure of 6H-SiC exhibits two kinds of faces; Si-terminated (0001) face on one hand and a C-terminated (0001) face on the other. The deposition of carbon atoms on these faces results in two different kinds of graphene [50]. The growth on the Si-terminated face is found to be more suitable for consistent hexagonal layer formation with fewer defects [51]. One of the most significant advantages of using SiC is the precise control over the thickness of graphene. Regulation of growth temperature is crucial to controlling and optimizing the layers in the graphene. Gao et al. reported epitaxy graphene using the Pt(111) surface [52]. His research suggests that the structural parameters of graphene can be controlled by regulating the temperature. In addition, regulating ethylene exposure can improve the quality of graphene.
Fig. (5)) Plasma-assisted synthesis of graphene.Moreover, the interaction between synthesized graphene and Pt surface was negligible, which offers the opportunity for easy transfer of the monolayer from the substrate. Yang et al. studied the epitaxial growth of graphene onto the hBN surface with fixed stacking orientation [53]. Gao et al., and Wessep et al., produced theoretical conclusions on the nucleation of C atoms on the Ni and Cu surfaces. The results of the first principle theory exhibited stark differences in the nucleation process while using Ni and Cu surfaces. The nucleation on the Ni surface is initiated at the step edges; contrary to this, the nucleation on the Cu surface begins simultaneously all over the surface [52-54]. Various methods of graphene synthesis have been summarized in Fig. (6).
Fig. (6)) Various methods of graphene synthesis (i) Epitaxial growth (ii) Liquid exfoliation (iii) Wet chemical synthesis.Exfoliation Methods
Mechanical Exfoliation
One of the most cost-effective methods to synthesize graphene is the exfoliation of graphene precursors. Mechanical exfoliation is one of the most suitable methods to commercially synthesize graphene for its more comprehensive application. The most suitable precursors for this method are the graphite family materials, which include highly oriented pyrolytic graphite (HOPG), single-crystal graphite, and natural graphite. The different layers of precursors are attached with the help of various intermolecular forces, predominantly π-π stacking. Applying mechanical forces on these stacked layers helps overcome the stacking forces and helps peel off mono or few-layer graphene from the graphitic bulk. Mechanical exfoliation can be conducted using various methods, such as ball milling graphitic precursors and blenders as simple as scotch tapes. One of the most critical aspects of using these methods is the involvement of environment-friendly tools and techniques compared to other methods where high temperatures and various ranges of chemicals are required. In principle, mechanical exfoliation can be categorized into the following three major categories: (i) Micromechanical cleavage, (ii) Continuous mechanical cleavage (iii) Shear exfoliation [55, 56].
The micromechanical method is the simplest among all the mechanical exfoliations; however, it cannot be stretched to commercial graphene production. This method separates graphene layers from the graphitic bulk using adhesive tape. Tape is used to peel the layers off from the bulk; the layers get stacked on the adhesive and can be separated using suitable solvents in which the adhesive shows considerable solubility [57]. Continuous mechanical exfoliation is an extended method of micromechanical cleavage in which a three-roll-mill machine and polyvinyl chloride (PVC) adhesive are used. In principle, this method is not cost-effective as the polymer adhesive increases the cost of production; in addition, the separation of adhesive from the graphene sheets involves various sophisticated procedures. Another method to exfoliate graphene involves the use of shear forces. When shear forces are applied on the graphitic surfaces, they try to overcome the force of attraction between layers. As the shear stress increases and becomes more than stacking forces, layers are separated [58]. This method is suitable as it can produce defect-free graphene sheets; however, at the same time, this low pristine yield is a limitation of this method which prevents its wide applications in graphene synthesis [59, 60].
Chemical Exfoliation
The chemical exfoliation technique uses different kinds of chemical moieties that can insert into the layers of the graphitic precursors and can separate them. This method generally requires a liquid suspension of graphitic precursor and substances that can furnish intercalating moiety. Small alkali metals are primarily used to separate the stacked layers due to potential ionization differences with precursors. These methods include various precursors, media for suspensions, and intercalating species. The most beneficial aspect of chemical exfoliation is a relatively greater yield and low operating temperature. However, in most cases, the exfoliation of monolayers from the bulk is reversible, and significant agglomeration of sheets can be seen [61]. Chemical exfoliation can be categorized into the liquid phase and supercritical fluid exfoliation [62, 63]. Sonication and cavitation are the two crucial aspects of liquid-phase exfoliation. The precursors are dispersed in suitable solvents, and the mixture is sonicated. The sonication induces the growth and collapse of solvent bubbles. The bubbles inside the graphitic stacks grow and reduce the van der Wall forces. This triggers the exfoliation of layers. As discussed previously, this exfoliation suffers reversibility, and to prevent this, additional surfactants are used to exfoliate the stacks. Supercritical fluid exfoliation works on the same principle; however, supercritical fluids are used to exfoliate the stacks. CO2 is the most abundant supercritical fluid that expands after entering the stacks at defined temperature and pressure parameters. The process involves immersing the precursors into the fluid; sonication assists in the penetration of stacks by the sonication. The sonication is followed by rapid depressurization, which results in a significant enhancement in the volume of the CO2 moieties resulting in the exfoliation of stacks to the monolayer to few-layer graphene Fig. (7) [64] .
Fig. (7)) Illustration of exfoliation of graphene sheets with the help of supercritical fluid.Electrochemical Exfoliation
Electrochemical exfoliation, in principle usage, applies voltage to trigger the penetration of charged species inside the graphitic stacks. The anions form gaseous species inside the precursors and assist in exfoliation, known as electrochemical exfoliation. There are various electrochemical assembly components, including an electrode of graphite, which works as a precursor. The other components include the counter electrode, which helps circulate electric current. Both electrodes are separated with the help of an electrolyte solution. The direct current ranging from 5V to 30 V is used as the power supply source.
In most cases, graphite is used to develop the anodic electrodes for the process [65]. The anodic electrode invites anionic species to intercalate. In most cases, salts of sulfates (SO42-) are used as an electrolyte. The intercalation of anions separates the sheets due to their bulky size. The electrodes include graphite foil, rod, expanded graphite, graphite flakes, HOPG, and graphite sheet as a precursor. A wide range of electrolytes is used as a source of anionic intercalating species, which include different concentrations of K2SO4, (NH4)2SO4, H2SO4, Na2SO4, and several other electrolytes. In addition, organic salts such as C6H5COOH, C6H5COONa, and others are also used as electrolytes [66, 67].
Reduction
Reduction of the graphene oxide surface is a popular method to transform it into graphene. GO contains a wide range of oxygen-containing functional groups such as hydroxyl (−OH), epoxide (C–O–C), carbonyl (C═O), carboxyl (−COOH), and epoxy [68]. The reduction of these groups results in the formation of reduced graphene with less oxygen-containing functional groups. There are two popular methods to reduce GO, viz. (i) Chemical reduction and (ii) Electrochemical reduction. In chemical reduction, the oxidized surface is immersed into chemical reducing agents of varying strength at optimized temperature range and reaction conditions [69]. The transformation can be identified with a change in color of brown graphene oxide to black graphene [3]. In addition, the transformation can be seen in various properties of the graphene oxide, which includes a decrease in the hydrophilicity of the reduced surface due to a significant lowering in hydrogen bonds with water molecules, enhanced electrical conductivity due to the restored π electron conjugation and many more. Various reducing agents are used for reduction, including natural and artificial agents such as vitamin C, hydrazine hydrate, resveratrol, chitosan, polyethylene amine, sodium borohydride, and others [70]. The electrochemical reduction of graphene oxide sheets can be achieved by using various electrochemical methods such as cyclic voltammetry, linear sweep voltammetry, or at a constant potential mode in a standard three-electrode electrochemical system at room temperature. The mechanistic detailing of the process reveals that the transfer of electrons from the electrode to the graphene sheets causes a reduction of the oxygen-containing groups. Using this method allows the collection of reduced graphene oxide from the electrode. The reduced graphene obtained via this method exhibits significantly different properties than graphene oxide and pristine monolayer graphene due to residual oxygen-containing functional groups. The aqueous solubility of the reduced graphene oxide is more than that of the pristine graphene but significantly lower than graphene oxide. In addition, these structures' electrical and thermal conductivity is significantly higher than graphene oxide but lower than pristine graphene monolayers [71, 72].
Functionalization of Graphene
Functionalization is an essential operation that introduces various functional groups, polymer segments, biomolecules, and nanoparticles onto the graphene surface. Functionalization is a structural remedy to prevent drawbacks intrinsically associated with the graphene surface. It addresses one of the most common problems of restacking the graphene monolayers resulting in the formation of graphene stacks. In addition, functionalization has allowed the stretching of the application of graphene from biomedical to electronics and energy to aerospace. The attachment of these moieties on the graphene surface could be permanent or temporary, depending upon the mode of functionalization. Fundamentally, the methods of functionalization can be categorized into two broader categories (i) Covalent Functionalization and (ii) Non-covalent functionalization [3]. This chapter segment deals with a detailed discussion of the details of functionalization.
Covalent Functionalization
The covalent functionalization on graphene or graphene derivatives can be initiated either through the π-skeleton of the hexagonal array or by taking advantage of the functional groups present on the surface of graphene. In general, the functionalization of graphene takes place through established mechanisms of organic chemistry such as addition, substitution, and rearrangement [3, 61, 66]. The π skeleton of graphene and oxidized graphene exhibit two different reaction preferences. Most of the reactions proceeding through the involvement of sp2 hybridized carbon are [2+1] cycloaddition of carbenes, [2+1] cycloaddition of nitrenes, addition of radicals, [2+1] cycloaddition of malonates, 1,3-Dipolar cycloaddition, [4+2] cycloaddition of dienes, [4+2] cycloaddition of dienophiles, [2+2] cycloaddition of arynes [3, 73], Friedal-Crafts acylation, Kolbe-Schmitt type oxidation. These surface revamping modes help introduce various functional groups and polymeric chains via nitrenes or carbenes cycloaddition on the graphene surface. The structurally revamped graphene sheets exhibit improved solubility in aqueous media and stabilities concerning various chemicals and heat [73]. A brief of these modes of reaction has been summarized in Table (4).
However, these reactions are kinetically unfavorable on oxidized graphene surfaces due to bulky groups. In the case of the oxidized graphene analogs, highly reactive epoxide groups on the surface invite attack of nucleophiles and can also undergo substitution reactions such as chlorination and bromination via hydroxyl group on the surface. The –COOH group on the surface exhibits an esterification reaction with alcohols and forms peptide linkages with amines [3]. As the carboxyl, epoxy, and hydroxyl groups are not evenly distributed on the graphene surface, reactions concerning the above-mentioned functional group significantly alter the physics of the oxidized graphene with a substantial change in the band gap, electrical and thermal conductivity, solubility, etc. Various chemical moieties, such as chromophores, polymers, and functional groups, can be attached to the graphene sheet via these methods [62, 66, 67].
Covalent heteroatom doping onto the graphene surface has also been studied extensively recently. Doping graphene is a useful technique to modify its electronic and structural properties. The choice of dopant and doping method depends on the desired properties and applications of the doped graphene. However, it is essential to carefully control the doping level and the type of defects introduced to avoid the degradation of graphene's properties. Most studies of hetero atom doped graphene include; N-doped graphene, S dope graphene, P-doped graphene, etc. In addition, halogen atoms (F, Cl, Br) have been prepared and evaluated for their potential in various applications [75, 76]. The doping of graphene with nitrogen atoms has exhibited some remarkable properties with significant applications in electronics and developing energy devices. Pyrolyzing carbon-nitrogen precursor does the N-doped graphene synthesis at 800–1200 °C. The doping dramatically affects the energy bands in graphene and is widely used in manipulating the band gap. In addition, N-doping provides promising roots to alter the graphene surface [77, 78].
Contrary to N doping, phosphorus doping is done with the help of ball milling. The graphene is ball-milled in red phosphorus; the milling process creates active sites on the graphene surface, which rapidly reacts with the phosphorus atom. The chemical exfoliation of graphite fluoride can prepare the F-doped graphene. Fluorine doping significantly changes the chemical reactivity by acting as a nucleophilic center [79]. The chlorine doping can be done on the hexagonal lattice by treating it with Cl2 gas in the presence of ultraviolet light [80]. The chlorine-doped graphene participated in the reaction with the Grignard reagent. This method is frequently used to insert alkyl chains into the basal plane of graphene.
Non-covalent Functionalization
Non-covalent functionalization of graphene monolayer includes the involvement of π- skeleton for stimulating interaction with the π-skeleton of the host species through π-π interactions [3]. As these interactions are weak, such temporary revamping of the surface is considered reversible. As direct covalent fond fission or formation is not involved in the non-covalent functionalization, the graphene surface's properties remain intact, making this method a better option in many applications. The non-covalent interactions with graphene can mainly be categorized into (i) π-π stacking and, (ii) van der Waals (iii) ionic Interactions and, (iv) hydrogen Bonding [80]. The π-π stacking takes place when a π-electron cloud of the host species interacts with the π-electron cloud of the graphene surface. The π-π interactions are weak due to the van der Wall type interactions and assist in the smooth delivery of various biomolecules such as genes, bio-molecules, amino acids, and targeted delivery of many therapeutic agents [81]. The strength of π-π interaction between the guest molecule and graphene surface can be studied with the help of single-molecule force spectroscopy.
In addition, several aromatic entities containing polymers are also found to interact with the graphene surface [82]. Polymers such as Kevlar, having hydrogen-bonded aromatic rings show tremendous interaction with the graphene oxide surface, resulting in robust mechanical performance and enhanced electrical conductivity and heat transport [83]. In addition, interaction dramatically improves the solubility of the monolayers of graphene. For example, the sulfonated derivative of polyaniline exhibits π-π interaction with the Graphene oxide surface through π stacking and prevents the restacking in the water media [84]. Zhang et al. used a series of aromatic compound functionalized polyethylene glycols (f-PEG) to facilitate the pi-interaction and enhance the resulting composites' mechanical performance. In other ways of interactions, guest moieties and GO surfaces can also interact due to larger surfaces (van der Waals interaction) and the presence of oppositely charged ions and hydrogen bonding [85]. Li et al. showed that the interaction of tetradecyl-trimethylammonium bromide, a quaternary ammonium salt with oxidized and reduced graphene surface, is purely electrostatic. The cationic part of the quaternary ammonium salt is found to be oriented toward the electron-rich graphene surface [86].
Moreover, the functionalization of heteroatoms alters the optoelectronic properties of graphene. Perumal et al. developed a single-step synthesis of non-covalent functionalization of a monolayer surface of graphene with poly-N-vinyl-2-pyrrolidone (PNVP). The functionalized graphene oxide exhibited enhanced electrochemical parameters [87]. A summary of a few significant studies has been tabulated in Table 5.
In addition to this, non-covalent doping also plays an important role in manipulating several properties of graphene. Noncovalent doping of graphene refers to the introduction of foreign atoms or molecules into graphene without forming covalent bonds with the carbon atoms of graphene [103]. This type of doping technique is advantageous because it preserves the structural integrity and electrical conductivity of graphene. Noncovalent doping of graphene can be achieved by two main methods: adsorption and intercalation [75-80]. In adsorption, foreign atoms or molecules are adsorbed on the surface of graphene through van der Waals forces. In intercalation, foreign atoms or molecules are inserted between the layers of graphene. One of the most commonly used noncovalent dopants for graphene is graphene oxide (GO) [103]. GO is a partially oxidized form of graphene, which contains oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups. GO can be used as a dopant for graphene by dispersing it in a solvent and mixing it with graphene. The oxygen-containing functional groups of GO can interact with the surface of graphene through van der Waals forces and can alter its electronic properties. Another commonly used noncovalent dopant for graphene is metal ions, such as Au, Ag, Pt, and Pd [103]. Metal ions can be adsorbed on the surface of graphene through electrostatic interactions or coordination bonds. Metal ions can induce charge transfer to graphene and can alter its electrical conductivity. In conclusion, the functionalization of graphene has emerged as a promising approach to enhance its properties and expand its applications. By introducing functional groups or molecules onto the graphene surface, the material can be tailored for specific purposes, such as sensing, catalysis, energy storage, and biomedical applications. Functionalization can also improve the dispersion of graphene in solvents or matrices, leading to better processing and integration into various devices. However, the functionalization process can also introduce defects and alter the electronic properties of graphene, which may affect its performance in certain applications. Therefore, careful control of the functionalization conditions and characterization of the modified graphene are crucial for achieving the desired properties and avoiding unintended effects. Undoubtedly, functionalization of graphene is a promising avenue for further research and development especially when cost-effective development of nanomaterials has taken a giant leap [104-108].
