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Graphene (G) is a 2D material of sp2 hybridized carbon atoms, discovered by Geim and Novoselov in 2004. The material presents a wide range of peculiar electronics and thermal, optical, mechanical, structural, and surface properties, which have attracted considerable interest from researchers and technologists. The conevntional techniques for graphenization have presented some drawbacks including low yield, costliness, high chances of contamination, and their time-consuming nature. These limitations have led to proliferation of research, which has led to the discovery of more advanced techniques for G synthesis over the years. At the moment, trending approaches to G production include chemical vapor disposition (CVD), epitaxial growth on silicon carbide (SiC), G oxide reduction, chemical synthesis, electrochemical synthesis, and laser-induced graphenization. There is a growing demand to produce G in large quantities and good quality. Nonetheless, because the conventional techniques have presented significant difficulties and imperfections in large-scale G production, various investigations have been conducted to identify new techniques for manufacturing cost-effective, large-scale, and high-quality G with novel applications such as energy storage, sensors, drug delivery, and biomedical devices. Each technique can be used for certain applications and has its own advantages. This chapter deals with the two approaches (top-down and bottom-up) for the synthesis of G and their procedure, limitations, and applications.
Faculty of Pharmacy, Shri Ram Murti Smarak College of Engineering and Technology, Bareilly, Uttar Pradesh, India
Priyanka Verma
Faculty of Pharmacy, Shri Ram Murti Smarak College of Engineering and Technology, Bareilly, Uttar Pradesh, India
Haris Saeed*
Keck School of Medicine, University of Southern California, Los Angeles, USA
*Address all correspondence to: haris6229@gmail.com
1. Introduction
In 1960, researchers Ubbelohde and Lewis successfully isolated a monolayer of graphite and determined that graphite is composed of layers, consisting of interconnected hexagonal carbon atom rings [1]. However, as stated by Mouras et al. in 1987, the term “graphene” was initially coined to refer to a solitary sheet of graphite during that time. Graphite serves as the foundational element in graphitic materials such as graphite itself, fullerene, and carbon nanotubes [2]. Although G was discovered to be in existance by Benjamin Collins Brodie in 1859 [3], since its first discovery in 1962 [4] which was observed in an electron microscope, Wallace has spent many years studying it theoretically [5, 6]. However, the properties of the graphenic material were only attained in 2004 when Novoselov and Geim successfully isolated and studied a single-atom-thick crystallite (G) from bulk graphite and transferred them onto thin silicon dioxide on a silicon wafer using the well-known scotch tape technique. Notably, the pioneering method of producing the first G involved micromechanical cleavage of graphite [7]. Konstantin Novoselov and Andre Geim shared the 2010 Nobel Prize in Physics for their ground-breaking work on this two-dimensional material as a result of this feat [8, 9]. The popular nanomaterial G is currently taking the place of silicon in a variety of scientific disciplines. This is because of their nanoscale mechanical, chemical, thermal, and physical characteristics, and due to its potential, G is attracting sponsors and significant donations [10].
The exploration of various commercial applications for these materials is currently underway with a particular emphasis on a range of fields, which include sensors, energy generation, and energy storage devices, which represent some of the fastest growing domains of technology [11, 12, 13]. Additionally, the realm of 2D-layered materials exhibits an extensive array of crystalline structures, corresponding to a diverse spectrum of physical properties [1, 14].
The atoms of these bulk layered materials are weakly connected by inter-layer van der Waals forces, but they are strongly bound to one another within the same plane [14]. The creation of 2D-layered nanoscale materials from these bound sheets through chemical or physical interactions opens up interesting possibilities for novel devices that are distinct from those made from conventional bulk materials [15, 16]. Plus, due to the wide range of shapes and sizes that a single graphitic layer can take, G is sometimes referred to as the “mother” of all graphitic-based nanostructures. It can be folded into one-dimensional (1D) carbon nanotubes (CNTs), wrapped into a zero-dimensional (0D) “buckyball” structure, or layered into multiple-layer 3D G sheets [17, 18, 19, 20, 21, 22].
There are numerous ways by which G is produced. The conventional techniques include (1) mechanical exfoliation, (2) chemical vapor deposition, and (3) epitaxial growth. At the moment, the trending G production techniques are electrochemical exfoliation, laser-induced graphenization, hydrothermal synthesis, and microwave-assisted synthesis. This chapter provides a summary of conventional and trending techniques that are used for the synthesis of G. Nonotheless, G needs to be produced at a price that is comparable to or lower than that of existing materials in order to be successfully applied in industry. The development of G synthesis processes with the qualities of high product yield, high product quality, cost-effectiveness, and scalable production is, however, difficult. As a result, these properties are the main subject of the discussion in the subsequent sections but G’s full production and characterization mechanisms have been documented elsewhere in the literature [22].
G must be produced at rates that are compatible with existing materials in order to be utilized by various industries [23, 24]. The development of the production processes that deliver high yields and high-quality products while being cost-effective, reliable, and scalable remains to be a big challenge [25]. As a result, these attributes are the main focus of the following discussion on techniques of G synthesis [26]. Depending on the desired outcome and purity, different methods have been developed to create layers and thin films of G. The schematic representation of the bottom-up (construction) and top-down (destruction) mechanism is depicted in Figure 1.
Figure 1.
Synthesis of G from top-down and bottom-up approaches.
Thus, a bottom-up process is the one in which the initial raw materials are found in a smaller entity and are then transferred to a larger entity or sustrate. Chemical vapor deposition (CVD) is an example of a bottom-up strategy for synthesizing G. Because it is more likely to result in fewer flaws, a more homogeneous chemical composition, and better short- and long-range ordering of G, the bottom-up strategy is preferable to the top-down approach. On the other hand, a top-down strategy begins with large or small-scale structures and reduces the size using a variety of techniques. Furthermore, it is believed that the planes or layers (the building blocks) necessary to create the desired nanostructure are taken out of the substrate during the synthesis process. For better control of G synthesis, the top-down method is typically used. The graphite exfoliation in the top-down techniques can be achieved by both mechanically and chemically in the top-down approach [27, 28].
The conventional top-down and bottom-up techniques used for G synthesis are displayed in Figure 2.
Figure 2.
Conventional G synthesis approaches.
2.1 Top-down approaches
The top-down approach is defined as a tactic that concentrates on the attack of powdered raw graphite. Eventually, the attack will split its layers and create G sheets. Chemical synthesis and mechanical or chemical exfoliation are some of the commonly used top-down approaches for G synthesis [29] and are discussed below.
2.1.1 Mechanical exfoliation
Multiple single-atomic layers of G make up the structure of graphite. Weak van der Waals forces accumulate and hold everything together. The interlayer spacing between each layer is 3.3 Å (0.33 nm), and the interbond energy is 2 eV/nm2 (eVnm−2). Mechanical exfoliation is the rarest and earliest recognized technique for extracting G flakes from a graphitic substrat. This is a top-down approach in nanotechnology in which longitudinal or transverse stress is applied to the graphite surface using a cheap adhesive tape (scotch tape) or AFM (atomic force microscopy) tip. Mechanically cleaving graphitic materials such as highly oriented pyrolytic graphite (HOPG), single-crystal graphite, or natural graphite, can produce single-layer graphene (SLG) to few-layer graphene (FLG) by slicing down the layers [30, 31]. A single G layer may be separated from graphite using this technique by applying an external strain of approximately 300 nN/μm2 [32]. A variety of methods such as scotch tape, use of an electric field [33], and ultrasonication [34] or even by transfer printing techniques [35, 36] can be used to exfoliate G from graphite. To increase the yield of SLG and FLG flakes, the HOPG has occasionally been bonded to the substrate using a common adhesives like epoxy resin [34, 37]. A recent study has demonstrated how gold films can be used to transfer print macroscopic G designs from patterned HOPG [38]. It is undoubtedly the least expensive way to make high-quality G. G is exfoliated from a graphite crystal by adhesive tape in this micromechanical process. Multiple-layer graphene that is still on the tape after being removed from the graphite is separated into numerous flakes of few-layer G through repeated peeling. In order to detach the tape, it is then bonded to the acetone substrate, and the last peeling is done with an unused tape. The resulting flakes have varied sizes and thickness, with diameters of a single-layer G based on wafer ranging from nanometers to many tens of micrometers. Due to interference effects, single-layer G on SiO2/Si can still be seen under a light microscope despite having an absorption rate of 2% [39]. Optical microscopy, Raman spectroscopy, and AFM are typically used to characterize G flakes produced by mechanical exfoliation techniques.
Despite the absence of sustainable flakes, it is actually difficult to produce high quantity of G with this exfoliation technique. Although this process has very little difficulty, it requires a lot of labor to locate the G flakes on the substrate surface. The prepared G has nearly no defects and a very high quality, which is suited for the manufacture of FET (field effect transistor) devices. However, for large-scale, defect-free, high-purity G for mass manufacturing in the field of nanotechnology, the mechanical exfoliation method still needs to be improved.
2.1.2 Chemical exfoliation
One of the finest methods for producing G is chemical synthesis. The first chemical production of G oxide occurred in 1860 using the Brodie method [40]. This was followed by the development of the Hummers [41] and Staudenmaier [42] processes. G is modified from graphite and a graphite intercalation compound using a chemical processs that produces colloidal suspension. Chemical exfoliation involves two steps; it increases the interlayer separation by first reducing the van der Waals forces between the graphite layers by creating G intercalated compounds (GICs) [43, 44]. Then, fast heating or sonication are used to exfoliate single to few layers of G. Ultrasonification is used to produce single-layer G oxide (SGO) [2, 42, 45, 46, 47, 48], while density gradient ultracentrifugation is used to create different layer thicknesses [49, 50]. By using the Hummers process, which involves oxidizing graphite using potent oxidizing agents like KMnO4 and NaNO3 in H2SO4/H3PO4, it is simple to produce G oxide (GO) [42, 51]. SLG was produced via ultrasonication in a DMF/water (9:1) combination. Interlayer spacing therefore increases from 3.7 to 9.5 A. High density of functional groups make oxidation necessary, and reduction is required to get G-like characteristics. Chemical reduction with hydrazine monohydrate is used to scatter single-layer G sheets [2, 47].
Elsewhere, G has been produced using polycyclic aromatic hydrocarbons (PAHs) [51, 52, 53] by utilizing a precursor to the dendrict that has undergone cyclodehydrogenation and planarization [54] to create tiny G domains. According to these authors, small domains of G can be produced by using this approach, but larger flakes are produced by the precursor of poly-dispersed hyper-branched polyphenylene. The first one was synthesized via oxidative cyclodehydrogenation with FeCl3. Orthodichlorobenzene [55], perfluorinated aromatic solvents [56], and even low-boiling solvents like chloroform and isopropanol [57, 58] are employed to disperse G. G on SiO2/Si substrates exhibits electrostatic interaction between HOPG and the Si substrate [59]. HOPG has also been subjected to pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser exfoliation in order to create FG [60, 61]. Good-quality G, also known as reduced G oxide (rGO), is produced via reduction of graphite oxide (GO). Fabrication of energy storage materials [62], polymer composites [63], transparent conductive electrodes [64], and several types of paper [40], among others, have already employed the chemical technique to produce G.
2.1.3 Oxidative exfoliation reduction
The majority of GO is created through the reduction of reduced G oxide (rGO) or from G sheets after oxidative graphite exfoliation. Staudenmaier, Brodie, Hummers, and Hofmann are the most common methods for producing GO [27].
2.1.3.1 Brodie’s method
The first person to use KClO3 in severely fuming HNO3 to produce a novel compound containing carbon, oxygen, and hydrogen was Benjamin Collins Brodie, at the University of Oxford, London. The quality of flake G has improved as a result of this innovative approach. The batch was washed to remove any salts that had formed throughout the process, dehydrated at 100°C, and then put back into the oxidizing atmosphere. A substance with a “light yellow color” that did not change with additional oxidation treatment was produced after three successive attempts of that method. He provided the final molecular composition of the oxidized graphite as C11H4O5, in accordance with the elemental analysis of his product. In addition, he discovered that the chemical precipitated in acidic surroundings but dispersed in basic or pure water, after which he came up with the name “graphic acid” for the newly synthesized substance. The shift in the material’s C:H:O composition to 80.13:0.58:19.29 was brought about by the reduction in carbonic acid and carbonic oxide after heating at 220°C. However, despite the fact that this method can oxidize graphite, its application is constrained by the lengthy response time and dangerous toxic gas emissions [41, 65].
2.1.3.2 Staudenmaier’s method
Staudenmaier from Freising university, München, Germany, used enormous quantities of sulfuric acid and an excess of oxidizing chemicals to enhance Brodie’s method. Numerous aliquots of concentrated sulfuric acid (H2SO4) were added, and throughout the procedure, potassium chlorate solution was added to the reaction mixture in order to improve Brodie’s KClO3-fuming HNO3 formulation’s ability to oxidize. These modifications made it possible to produce a highly oxidized GO product in a single reaction vessel, greatly simplifying the GO synthesis process. Although Staudenmaeir’s approach was improved, Brodie’s method’s shortcomings remained, making the oxidation process time-consuming and dangerous. When potassium chlorate is introduced, it can last for more than a week, and when chlorine dioxide is removed from the inert gas, deadly gas explosions can occur continuously [43, 65].
2.1.3.3 Hofmann’s method
To improve the work of both Brodie and Staudenmaier, Hofmann et al. used concentrated sulfuric acid and concentrated nitric acid and KClO3 to synthesize GO in place of fuming nitric acid because fuming HNO3 is exceedingly deadly and dangerous. In addition to acting as an in situ source of dioxygen in acid solutions, KClO3 demonstrated a significant degree of oxidation capacity as the principal oxidant. So far, numerous research teams have used this technique to effectively manufacture GO [65].
2.1.3.4 Hummer’s and modified Hummer’s method
So, a different method for producing GO was developed by chemists Hummers and Offeman at the Mellon Institution of Industrial Research around 60 years after Staudenmaier’s approach. A concentrated sulfuric acid, sodium nitrate, and potassium permanganate solution was prepared and held at temperatures under 45°C to oxidize graphite. First, 100 g of graphite powder and 50 g of sodium nitrate were added to 2.3 L of sulfuric acid to chill it to 0°C in an ice bath. After then, the suspension received 300 g of potassium permanganate gradually. The suspension’s internal temperature was raised to 35° when the ice bath was removed, and it was maintained there for 30 min. The combination turned brownish-gray and pasty, and barely any gas had emerged after 20 minutes. After 30 minutes, 4.6 L of water was gradually added to the paste, causing it to fizz vigorously and increase in temperature to 98°C. When this reaction was allowed to continue for 15 minutes at this temperature, the diluted sample turned brown. In order to convert the leftover permanganate and manganese dioxide into colorless soluble manganese sulfate, 3% H2O2 was added after the operation. To remove the soluble salt of mellitic acid, the diluted solution was filtered and constantly washed with warm water. Centrifugation was used initially to obtain the dry form of GO and then dehydration at 40°C over phosphorous pentoxide under vacuum.
2.1.3.5 Recent advances in G and G oxide synthesis
Figure 3 illustrates the response patterns of these low-temperature operation G production approaches; normally conducted at lowest possible reaction temperatures to keep production costs down. However, most of these methods except Hummers method result in the production of toxic gases such as nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) [29] raising public health and environmental safety concerns. The Hummers technique is presently extensively employed for the synthesis of GO due to its notable efficiency and safety. Additionally, it does not produce dangerous gases like ClO2 (chlorine dioxide) or acidic fog as it uses sodium nitrate (NaNO3) and potassium permanganate (KMnO4) rather than nitric acid (HNO3) and potassium perchlorate (KClO4). Overtime, improvements to the Hummers method have led to a more environment-friendly method of producing GO.
Figure 3.
Various routes of G oxidation.
However, the G research “gold-rush” started in 2004, and GO also became a prominent material. Numerous articles on its preparation, reduction, and structure have been written. In 2010, Marcano et al. revealed an improved method for making GO. They reduced the amount of NaNO3 and raised the amount of KMnO4 in this new procedure. They also introduced H3PO4 to the reaction container in the place of the original acid. This improved method prevents a significant exotherm and produces no toxic gas. GO produced using this approach has a higher yield and degree of oxidation than GO produced using Hummers’ process. They also found that the new method affects the graphite’s basal plane less than Hummers’ approach [42, 65].
The addition of graphite intercalation components in the oxidation process of graphite leads to a notable enhancement in the separation between graphite layers. Consequently, this enhancement may facilitate the formation of well-dispersed single-, bi-, and few-layer G oxide (GO) structures in suitable solvents. Tetrahydrofuran (THF), water, and N-methyl-2 pyrrolidone (NMP) are some of the fluids in which GO can disperse due to oxygen-containing functional groups including carboxylic, hydroxyl, and carbonyl groups. Electrochemical, thermal, and chemical reduction is then employed to eliminate the functional groups and reconstruct the honeycomb lattice of GO as its sp2 bonding is broken. In the chemical reduction techniques, hydrazine (N2H4) is typically used to lower the oxygen content of GO. However, N2H4 use is restricted due to its high cost and dangerous nature. As a result, a safer alternative to the dangerous N2H4 used in GO reduction was developed. Among the reducing agents used were proteins, microorganisms, plant extracts, amino acids, metal-alkaline, metal-acid, reagents (nitrogen, sulfur, oxygen), hydrohalic acid, aluminum hydrides, borohydrides, and hormones.
Nonetheless, even though numerous GO reduction techniques are mentioned in the literature [66] including electrochemical reducation, plasma therapy, sonochemical, photocatalytic, laser, photothermal, and microwave technology, the electrochemical GO reduction has gained more prominence because of its economic viability, quick reduction, and simplicity of implementation. More importantly, it uses safer reductants than chemical reduction does.
In general, a reasonable cost and good yield are associated with the production of GO through the oxidative exfoliation of graphite and subsequent reduction to rGO. However, it is found that the product from this approach has a small surface area, lower solubility, and weaker electrical conductivity because of van der Waals attraction, and it is liable to irreversible sheet restacking [67, 68].
Furthermore, it is still not possible to fully reduce GO to produce pure G [69]. Nonetheless, although it has some imperfections and different sizes from pure G, the resulting rGO is pretty comparable to it. Greater rGO quality is produced via high deoxygenation, which is caused by a high C:O ratio.
2.1.4 Liquid phase exfoliation
Liquid phase exfoliation or LPE has traditionally involved two basic methods for exfoliating graphite: (1) cavitation in sonication, and (2) shear forces in high-shear mixers [68]. Recently, it has been discovered that a microfluidizer may effectively exfoliate graphite in appropriate aqueous solutions at high shear rates [70]. LPE is an easy-to-use high-shear mixing or sonication instrument that is widely accessible. Additionally, LPE does not need a vacuum or high-temperature systems to operate because its working conditions are mild. However, the low amount of G and the significant energy consumption during the fabrication process has limited the widespread use of sonication-assisted LPE as shown in Figure 4. The microfluidizer or high-shear mixing is a developing LPE technique, which successfully exfoliates graphite using fluid dynamics powered by a high-shear mixer [71, 72].
Figure 4.
Liquid exfoliation.
2.1.4.1 Sonication
At high concentrations, sonication is an effective exfoliation method that has the potential to produce monolayer- or few-layer G. By creating cavitation bubbles, sonication is frequently used to effect physical or chemical changes in a variety of systems [73]. Compressions and rarefactions impose high and low pressure, pushing and pulling molecules, as ultrasonic waves move across the medium. With each cycle of rarefaction, microbubbles get larger until they reach an unstable state and burst, producing enormous shockwaves [74].
Sonication is classified into two types, bath sonication (BS) and tip sonication (TS), which have been used together or separately to generate monolayer- or few-layer G sheets by exfoliating G [75]. Sonication-assisted LPE typically consists of three phases [76]: (1) production of graphite dispersion in a particular solvent; (2) exfoliation of graphite dispersion using sonication; and (3) G purification. The cavitation-induced pressure pulsations are responsible for the formation and collapse of microbubbles in liquids during sonication. The cavitation action produces high-speed microjets and shockwaves, which create normal and shear stresses on graphite [77], which are important in the exfoliation of graphite to produce G [75]. The exfoliation impact is generally affected by the sonication power, the liquid medium, and the centrifugation rate utilized to disperse the G nanosheets [78]. The appropriate liquid media are chosen to establish an environment that allows for stable G dispersions during sonication, and centrifugation is used in removing big and unevenly distributed graphite particles or aggregates.
2.1.4.2 High-shear mixing
Until recently, the exfoliation of graphite by high-shear mixing has not been well researched; in particular, few investigations have focused on high-shear mixing in aqueous systems utilizing ionic- or non-ionic surfactants. Nonetheless, it has been demonstrated that G may be exfoliated from graphite using shear force in a suitable liquid [79]. Furthermore, strong shear pressures applied by high-shear mixers are considered to be scalable techniques of graphite exfoliation [80]. Shear exfoliation is analogous to sonication exfoliation in that aqueous liquids may be utilized to aid graphite exfoliation and generate stable G dispersions, hence removing the need for use of toxic organic liquids. It is critical to develop industrially scalable ways for producing high-quality G using novel exfoliation processes such as sonication. Coleman and co-workers achieved substantial success in G production by shear exfoliation in 2014 [80], which fueled the great growth of the shear exfoliation process. They proved that high-shear mixing of graphite in suitable solvents might produce high-concentrated G nanosheet dispersions. The resulting G flakes had not been oxidized and had no basal-plane flaws. Notably, when the local shear rate surpassed 104 s−1, graphite was exfoliated in both laminar and turbulent zones. When comparing shear exfoliation to sonication, shear exfoliation is more efficient, and when a liquid volume of 10 m3 is reached, their scaling rule allowed for a rate of up to 100 gh−1. Recently, the modified Hummers’ approach was used to generate G oxide using high-shear mixing [81]. Table 1 displays the concentration of G dispersion produced by high-shear mixing with various solvents, with 10 mgmL−1 being the greatest concentration.
G concentrations obtained via high-shear mixing in different solvents.
2.1.4.3 Microfluidization
High pressure is applied to the fluid during the high-pressure homogenization process known as microfluidization, which drives the fluid through a microchannel with a diameter of d < 100 μm [83, 84]. It produces mild exfoliation conditions, which can assist to reduce the creation of flaws. In general, microfluidization is used in nanoemulsification [85], the food sector [86], cell disintegration [87], carbon nanotube dispersion [85], and fluidizing active medicinal components [88]. Recently, G quantum dots [89] and G-based conductive inks were produced using a microfluidizer. The fundamental advantage of mircrofluidization over sonication and high-shear mixing is that a high shear rate (less than 106 cm − 1) exists throughout the fluid area.
2.1.5 Unzipping carbon nanotubes (CNTs)
In this process, cylindrical carbon nanotubes (CNT) are sliced into flat G sheets with one, two, or a few layers in either the axial or longitudinal orientation. Starting materials can be single-wall carbon nanotubes (SWCNT) or multi-wall carbon nanotubes (MWCNT) [90] The unzipping of CNT can be accomplished using a number of techniques, including plasma etching [91], chemical unzipping [92], intercalation and exfoliation [93], and metal catalyzed cutting [94]. The generation of nanoribbons is caused by the longitudinal unzipping of CNT.
In chemical unzipping, H2SO4 treatment and oxidation with KMnO4 results in longitudinal cutting of CNTs. After that, chemical reduction of the oxidized G will take place utilizing NH4OH and hydrazine monohydrate (N2H4H2O) solution [92]. Because the precursor is destroyed during oxidation and G loses some of its electrical properties due to the presence of oxygen defect sites, this approach is regarded as being of low significance [95]. When MWCNT is intercalated in oxalic acid, as a chemical medium, before the chemical unzipping, G yield is increased. The suitable size of oxalic acid, which intercalates nicely between the interlayers of the MWCNT [96], resulting in an increase in G yield is 0.34 nm. Polymer films like poly-methyl methacrylate are utilized in plasma etching. This technique involves embedding CNT into the film, followed by the separation of the film and CNT combination in a KOH solution. After that, CNT is exposed to plasma made of argon, and the CNT’s longitudinal C▬C link is dismantled to provide G with clean edges [91]. Another way to unzip CNT is through intercalation and exfoliation, where CNTs are exposed to a reaction between lithium and ammonia. The separation of the G layers occurs as a result of a significant amount of stress being applied between the CNT layers during this procedure [93]. Researchers looked at other metal nanoparticles like nickel, cobalt, and copper and discovered that these nanoparticles broke the C▬C bonds and hydrogen bonds in MWCNT [97], but the techniques employed expensive and dangerous chemicals. Using an electric field from a tungsten electrode, MWCNT can be unzipped [98] producing highly pure, defect-free G nanoribbons. Therefore, due to its accuracy, viability, and environmental friendliness, the electric field approach is preferred.
2.1.6 Ball milling
An innovative technique for producing high-quality G by dissolving layered graphite into G is ball-milling method, which is said to have begun some 150 years ago, when it was used to produce talc powder, communicate ore size, and for a number of other purposes, but now, the method has recently been suggested for the producing nanoparticles at room temperature [99, 100, 101]. It is an easy and highly successful solid-state approach for reducing a variety of materials to fine powders, synthesizing nanocomposites, and foxides, making it a potential method for producing G in large quantities at a reasonable cost [102]. Large graphite sheets undergo shear stresses during milling, whereas normal force is used to crush graphite flakes into nano-sized materials and expose cracks in the basal plane. Both wet and dry conditions can be used to perform the milling procedure [103]. Lv et al. employed Na2SO4 salt to make G nanosheets with ripple-like corrugations in hundreds of square nanometer range [103, 104]. Elsewhere, ball milling was carried out in a planetary ball-mill machine with graphite, dry ice, and stainless-steel balls as shown in Figure 5 [105]. It is found that the type of media utilized affects the size and quality of the materials produced. By using wet ball milling to exfoliate graphite into G flakes in a liquid media, Zhao et al. provided a fresh perspective on the ball milling process. The measured thickness was found to be between 0.8 and 1.8 nm, which is equivalent to discrete monolayer and few-layer G (up to three layers). Because of this, there has been a lot of recent research activities in ball milling [106]. Additionally, Caicedo et al. reported on the oxidation of graphite using the ball-milling process in order to exfoliate G from graphite using KClO4 and filtered water. It was shown that, as the milling time was increased, the degree of oxidation also did. Furthermore, the effects of oxidation were investigated using different ball milling time intervals (6, 12, 18, 24, and 30 h) in this manner, and the results were compared to those of the samples that were obtained form the Hummers methods. Thus, after 18 h of milling, the sample displayed improved dispersion and a darker hue as a result of the elimination of functional groups including carboxyl, hydroxyl, and epoxy [99, 107]. Here, the sample obtained after 16 h of milling was the best sample in terms of the level of oxidation, length, and energy utilization parameters assessed. The advantage of the ball milling approach is its ability to produce high-quality and low-cost G. It is a useful technique for exfoliating and functionalizing G. However, the prolonged processing times have resulted in a significant decrease in synthesis of G [99].
Figure 5.
Ball milling method.
2.2 Bottom-up approaches
In the bottom-up techniques of synthesis, G sheets are generated directly from organic precursors such as methane and other hydrocarbon sources as illustrated in Figure 2. These methods transform molecular source materials into G-based substances [108]. Various examples of these approaches include chemical vapor deposition (CVD) [109], epitaxial growth [110], substrate-free gas-phase synthesis (SFGP) [111], template routes [112], and comprehensive organic synthesis [113]. While bottom-up strategies yield G products characterized by large surface areas and minimal defects, they often entail elevated production expenses.
2.2.1 Chemical vapor disposition
Methane (CH4), acetylene (C2H2), ethylene (C2H4), and hexane (C6H14) are a few of the hydrocarbon gases that are broken down during CVD in order to develop G sheets on metallic catalysts (such as Cu and Ni films) at high temperatures (650–1000°C) [27]. The carbon precursor separates into free carbon and hydrogen atoms when it comes into contact with a metal catalyst’s heated surface. Once the carbon atom attains the carbon solubility threshold, it undergoes diffusion throughout the surface and the bulk of the metal catalyst, ultimately resulting in the formation of a G layer on the metal substrate. Substrates such as glass, quartz, silicon, silicon oxide, boron nitride, and sapphire have been utilized for G production in an effort to optimize CVD techniques described in the work of Chen et al. [114].
Chemical vapor deposition (CVD) enables the production of high-quality G characterized by, a densely interconnected structure, substantial surface area, and minimal structural defects. Nevertheless, it presents drawbacks such as elevated production expenses, modest output, the necessity for additional purification to eliminate catalyst remnants, and the challenge of transferring G to alternative substrates [115]. Furthermore, CVD alone falls short of meeting commercialization needs due to the demand for substantial improvements in manufacturing cost and yield. To surmount these challenges without compromising G quality, researchers have directed their efforts toward refining synthesis conditions, targeting lower temperatures and ambient pressure [116]. For example, by employing surface wave plasma-enhanced CVD (PECVD), Kalita et al. achieved G synthesis at 450°C. This innovation considerably enhanced the overall process by significantly reducing growth temperature and deposition time (5 min) [113, 117]. The conventional CVD setup can be adapted to accommodate PECVD. To be practical on a commercial scale, the development of CVD must continue in conjunction with other technologies like thermal-based and plasma-based CVD.
2.2.2 Epitaxial growth on silicon carbide
G can also be synthesized by thermally decomposing silicon carbide (SiC) hexagonal substrate at temperatures between 120o and 1600°C in an inert or vacuum environment, as shown in Figure 6. Because silicon (Si) melts at a high temperature (1100°C), too many C atoms remain behind and create a sp2 hybridized network, which promotes the formation of G [117]. The epitaxial growth of G on SiC is the term used to describe this process [118]. The G created using this process, however, is not homogeneous [119]. It has been documented that SiC and polytetrafluoroethylene (PTFE) undergo a unique exothermic reaction [120]. Due to the energy-intensiveness of the technique and constrained size of commercial SiC substrate, epitaxial development of G would be expensive under current synthesis conditions. Additionally, the epitaxial development might result in polar faces like Si-face or C-face that degrade the quality of the finished G product as shown in Figure 6 [113, 121]. This technique makes it simple to modify the amount of G layers that are dependent on the heating temperature [122].
Figure 6.
Epitaxial growth on SiC showing Si (yellow spheres) and C (gray spheres) atoms (under elevated temperatures, Si atoms evaporate (arrow), creating a carbon-rich surface that gives rise to the formation of G sheets).
The most noticeable benefit of G on the Si-face is that we can easily regulate the G thickness at the wafer size on the semi-insulating SiC substrate. By adjusting the growth temperature optimally, this control is accomplished. Images, captured by a high-resolution transmission electron microscope (HRTEM), of the G produced is monolayer, bilayer, trilayer, and eight-layer configurations at various temperatures. The number of G layers can be calculated directly from HRTEM images. Monolayer G is used to layer the buffer layer [123]. Like graphite, bilayer G is AB-stacked (Bernal-stacked) [124]. Further HRTEM observation revealed an ABC-stacking (rhombohedral stacking) of more than three layers [124]. This is a striking contrast to the Bernal bulk stacking graphite. An electric field-induced bandgap is present in the ABC-stacked trilayer G. The ABC-stacked trilayer G demonstrates a ferrimagnetic spin arrangement, an anomalous quantum Hall effect, and an electric field-driven bandgap [125, 126, 127, 128]. By using HRTEM observation, the aforementioned atomic-scale growth mechanism was also studied.
2.2.3 Pyrolysis
Using the solvothermal approach, G can be formed chemically using the bottom-up method throughout the pyrolysis technique. For example, during the thermal reaction, the molar ratio of sodium and ethanol is determined 1:1 in the reaction vessel. This process involves heating a 1:1 molar reaction between 2 g sodium and 5 mL ethanol in a sealed reactor vessel at 220°C for 72 hours to produce the G precursor, also known as the solid solvo thermal product. The leftover precursor is then quickly pyrolyzed and washed with 100 mL deionized water. After that, the suspended solid is vacuum-filtered and dried for 24 hours at 100°C. This process yields 0.5 g of G per reaction, or 0.1 g of G every milliliter of ethanol [123]. Another example is the sonication-based pyrolization of sodium ethoxide. A PTFE (Polytetrafluoroethylene) melting pot was placed in an inert atmosphere and filled with 5 ml of ethanol and 2 g of sodium. To produce sodium ethoxide, the tank was firmly sealed and heated to 180°C for 24 hours. This was placed in an ignition dish and ignited in air and left to burn. Keep in mind that handling high pressure, sodium, or open flames requires the utmost caution. After being gathered, the carbonized product was broken up using a pestle, combined with deionized water, and sonated for a number of hours. After that, it was cleaned in deionized water using the method in [123]. The procedure might easily improve the detachment of G sheets. As a result, the produced G sheets have a thickness of 10 μm. To further investigate the graphitic properties, crystalline structure, band structure, and various layers of materials is executed by using Raman spectroscopy, selected area (electron) diffraction, and transmission electron microscopy [129].
2.2.4 Substrate-free gas-phase method
The substrate-free gas-phase (SFGP) approach is a relatively recent technique for producing G materials through gas-phase reactions by eliminating the need for substrates [130]. This process involves introducing liquid ethanol and Ar gas into microwave-generated plasma under atmospheric conditions. As ethanol droplets vaporize and dissociate within the plasma region over a time span of approximately 1 second, G is synthesized. Notably, this method reportedly yielded 2 mgmin−1 of G from an input of 164 mgmin−1 of ethanol [131]. Dato and Frenklach [132], on the other hand, explored the potential of different carbon precursors using this technique. Isopropyl alcohol and dimethyl ether were identified as possible precursors for generating G nanosheets.
2.2.5 Total organic synthesis
Total organic synthesis involves using polycyclic aromatic hydrocarbons (PAHs) with specific characteristics to create G. PAHs, often described as 2D G segments, composed entirely of sp2 carbons due to their structural resemblance, can be readily modified with aliphatic chains to tailor the solubility of the final product [133]. Crucial to this method is the selection of appropriate precursors that produce high yield quality G through a straight forward reaction pathway. The process results in 2D G nanoribbons (GNRs) up to 12 nm in length, as demonstrated by Yan et al. [134, 135]. However, the relatively narrow size distribution of PAHs might affect the quality of G due to reduced solubility and potential side reactions stemming from their higher molecular weight. Subsequent improvements by Yan et al. addressed these limitations [135].
2.2.6 Template route
The template route employs one-dimensional templates, such as metals, oxides, or polymers, to synthesize G derivatives with high throughput, quality, and well-defined structures [135]. Wei et al. initiated this approach [136]. Through physical vapor deposition (PVD), ZnS ribbons were created as templates for G formation using CH4 as the carbon source. Subsequent etching with HCl removed residual ZnS nanobelts. Another template-based strategy involves G synthesis through the self-assembly of G on a meso-structured silica template formed from a pyrrole moiety-containing surfactant [137].
2.3 Trending techniques of synthesis of G
2.3.1 Microwave-assisted synthesis
Graphite or GO that has undergone a modified version of Hummer’s process is typically used to produce G nanosheets (GNS) [60, 138, 139, 140]. Chemical procedures are typically used to rGO in the presence of several hazardous reduction agents, including hydrazine and NaBH4. Thermal treatment, in contrast, uses no toxic reduction chemicals, making it a greener option. The environmentally friendly microwave-assisted method has gained increasing attention as a substitute for the traditional preparation of G in traditional heating systems (furnace or oil bath). In this method, GO [141, 142] or natural graphite [143] can be treated using the microwave-assisted solvothermal/hydrothermal methods in a microwave oven or microwave plasma-enhanced chemical vapor deposition (MPCVD) system.
In this method, nontoxic solvents are used to exfoliate GO within a short reaction period of 1 to 15 min at a relatively low temperature range of 180 to 300°C [138]. According to a study, an easy microwave-assisted solvothermal technique may produce a stable G suspension from a GO suspension in an alkaline medium (pH 10) or in polar solvents such as N,N-dimethylformamide, ethanol, 1-butanol, and water. Additionally, the water-soluble polymer-grafted G sheets have been produced from GO by preparing them for 4 min at 450 W in a standard household microwave [138]. A residential microwave was used to create a three-dimensional (3D) nanostructure of a “G nano-cup” anchored on a few layers of G substrate [138]. Two stages were reported: the one-pot synthesis of G coated metal nanoparticles anchored on the G sheets and the subsequent etching of metals. This was done under the microwave irradiation in a home microwave oven [138]. More importantly, highly hydrogenated G could be created from GO by a one-step microwave irradiation process in hydrogen plasma, in which the deoxidation and concurrent hydrogenation were both accomplished. Giant G sheets could also be obtained by double microwave-assisted exfoliation of expandable graphite. High local temperatures and pressure are provided by microwave irradiation, and energy is sent directly into the inside of the GO. The polar link of oxygen-containing functional groups on the surface and edge of GO sheets interacts with radiation to produce heat [144]. Additionally, a key element in determining the regularity of deposits is the interaction between polar solvents and the surface oxides on GO sheets. Furthermore, the reduction degree of G sheets is further enhanced, and the functional groups on the surface of GO are successfully lowered.
There are several distinct advantages of utilizing microwave technology to create G. First of all, the microwave-assisted technique is quick and does not require a difficult synthesis process. Second, compared to more traditional methods, this technique is very cost-effective because it uses less chemicals. Third, compared to G made using the traditional heating approach, those produced using microwave-assisted technology may have an average size that is ten times larger. Finally, high-quality G with regulated structure and residual functional groups is produced using microwave-assisted technique [138]. Microwave sources enable localized high temperatures, rapid energy transfer, and efficient precursor disintegration. These attributes lead to homogenous nucleation environments, rapid crystallization, controlled particle size distribution, and precise morphology regulation [145, 146]. This technique facilitates the creation of G-based nanocomposites with adjustable sizes and shapes, enhancing applications like particle/crystal-on-sheet, nanorod/nanofiber-on-sheet, and nanosheet-on-sheet [147].
2.3.2 Electrochemical exfoliation
Electrochemical exfoliation of graphite has become a popular way to make G compounds in recent years. Graphite can be utilized as working electrodes in liquid electrolytes in a range of geometries, such as powders, foils, rods, flakes, or plates [148]. There are two types of exfoliation techniques: cathodic, which applies a negative bias to graphite electrodes, and anodic, which uses a positive bias. In cathodic exfoliation, positively charged electrolyte ions such Li+ would be drawn to graphite electrodes. In anodic exfoliation, negatively charged ions, such as SO4−2, may be drawn to electrodes. The van der Waals forces have been hypothesized to be broken by electrochemical reactions, which causes graphite’s structural expansion [149]. Additionally, during electrochemical exfoliation, chemical interactions with functionalizing agents can occur simultaneously in order to perform in situ chemical doping (functionalization) of G materials to create a variety of G-based composite materials [150, 151].
2.3.3 Hydrothermal synthesis
Hydrothermal treatment is a thermo-chemical conversion technique that results in efficient hydrolysis, pyrolysis, dehydration, polymerization, and aromatization of organic precursors, as well as high oxygenated functional group content and condensed aromatic structures [152]. The hydrothermal treatment approach, which primarily involves the carbonization and reduction process, can convert biomass and GO to solid char and G, respectively [153]. During the hydrothermal treatment of biomass, small-molecules soluble byproducts such as aromatic compounds, polysaccharides, aldehydic, ketonic, and furan derivatives are also created, from which solid products are produced by further polymerization [154]. Based on pyrolysis, self-assembly, and dehydration, water-soluble and low-molecular weight fulvic acid (FA) can be transformed into G quantum dots. To our knowledge, however, no parallel investigations on water-insoluble, high-molecular-weight humic acid and the associated reaction mechanism have yet been published.
2.3.4 Laser-induced graphenization
Laser-induced graphitization (LIG) is an innovative technique that transforms carbon-rich sources into conductive carbon using lasers [155]. This approach holds promise for creating highly conductive graphenic materials suitable for energy storage devices, sensors, biomedical applications, and hydrogen evolution [156]. LIG has been applied to a variety of bio-based carbon sources, such as paper and wood, to achieve significant graphitization, potentially contributing to their use in various applications.
Due to its large surface area, thermal, electrical, and physical properties, the carbon material known as “graphene” has become important in the fields of micromanufacturing, nanomaterials, biomedical, and composite materials. However, due to its vast range of applications, G must be produced in large quantities, which has recently received a lot of attention from researchers and technologists. Therefore, improving the production process is essential for increasing the yield. Producing high-quality G is also necessary, but the most critical step is to use an easy, affordable, and environmentally friendly method. Future research should therefore focus on the yield measurement, biocompatibility as a result of the use of nontoxic chemicals, high energy, pressure, and poor transfer processes in chemical approaches, all of which have significantly contributed to high production costs, poor yield, and imperfections in the obtained G. The development of new techniques and environmentally friendly materials is crucial for the synthesis of G materials in electronics, nanomaterials, and biomaterials, all other factors being held constant. The size, shape, and optical properties of the resulting material are significantly influenced by the manufacturing processes and the carbon precursor sources used. G is suitable for use in electronics, energy storage, sensors, composites, drug delivery, and biomedical devices, because of its high transparency, thermal and electrical conductivity, and mechanical strength, in addition to its specific area. The improvements in G synthesis have opened up new opportunities for exploiting its extraordinary qualities and developing cutting-edge technology for a variety of industries. In summary, despite extensive study into producing G since its discovery, no method has been found to satisfactorily produce G on an industrial scale. This overview covered prospective uses for G as well as a comparative research and potential approaches.
We would like to express our sincere gratitude to Dr. Arshad Jamal Ansari, Postdoctoral Scholar, at University of Southern California and institutions for his valuable suggestions and editing to the completion of this research endeavor.
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Written By
Aafreen, Priyanka Verma and Haris Saeed
Submitted: 30 August 2023Reviewed: 06 February 2024Published: 18 April 2024
Open access peer-reviewed chapter - ONLINE FIRST
