# State-of-the-Art Graphene Synthesis Methods and Environmental Concerns

Authors: Kaamil Edward; Kabir Mamun; Sumesh Narayan; Mansour Assaf; David Rohindra; Upaka Rathnayake
Journal: Applied and Environmental Soil Science
 (2023)

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## Abstract

Graphene, a 2D sp2 hybridized carbon sheet consisting of a honeycomb network, is the building block of graphite. Since its discovery in 2004, graphene’s exceptional electronic and mechanical properties have peaked interest in various applications. However, the inability to mass produce high-quality graphene affordably currently limits the practical application of the material. Researchers are continuously working on advancing graphene synthesis methods to alleviate these limitations. Therefore, this review looks at the overview of established graphene synthesis methods and characterization techniques, and then highlights an in-depth review of graphene production through flash joule heating. The environmental concerns related to graphene synthesis are also presented in this review paper.


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## Body

## 1. Introduction

Since graphene was first isolated in 2004, research on production methods and applications have increasingly progressed due to its unique properties which suggest the imminent transformation in the materials used in electronics, composites, energy, and anticorrosive coats. With demand for graphene and graphene-based materials growing (an estimated USD 170 million in global markets by 2022), there is a need to synthesize it at an industrial capacity. Existing manufacturing methods are not fully capable of producing reliable and cost-effective graphene to meet industrial applications; hence, the implementation of graphene has been limited [1]. However, the waste can be converted into high-quality turbostratic graphene using the flash joule heating (FJH) process. The FJH method shows the immense capability of being a scalable and cost-effective synthesis process because of the compatibility of the process with various carbon precursors without the need for pretreating with chemicals, buffer gases, substrates, or washing of graphene.Joule heating approach has been previously investigated for the synthesis of carbon materials and alloy nanoparticles.(i)
A chemical vapor deposition system that uses Joule heating to heat a nickel film on a SiO2/Si substrate to 900 to 1000°C (using current and voltage range was 15–30 A and 8–12 V) followed by annealing in an argon and hydrogen mixture at a flow rate of 50 sccm. The substrate is then exposed to methane at 50 sccm for 20 to 60 s. 20 s of exposure results in the formation of graphene with a resistance of 600 Ω/square with a 93% transmittance [2].(ii)
A carbothermal shock process uses carbonized silk fabric (CSF) loaded with a solution of transition metal salts and ethanol (carbon precursor) as a substrate which is treated using joule heating with a pulse voltage of 40 V for 50 ms. The ethanol pyrolyzes to enable the growth of carbon nanotubes on the CSF as it heats up to a temperature of 1700 K in the presence of the metal nanoparticle catalysts [3] (carbothermal shock enabled the facile and fast growth of carbon nanotubes in a second).(iii)
A two-step carbothermal shock that involves rapid heating to a temperature of 2000 K for a shock duration of 55 ms followed by cooling is used to create high-entropy-alloy nanoparticles from metal precursors (several different metal salts) on carbon nanofibers [4] (carbothermal shock synthesis of high-entropy-alloy nanoparticles).Graphene was first isolated using Scotch tape from bulk graphite in 2004 [5]. The technique used involved peeling graphite layers repeatedly with adhesive tape until a monolayer of pristine graphene stays. The simplicity of the Scotch tape method and its capability of producing pristine graphene flakes has enabled immense research on the properties (refer to Table 1) and consequent applications of graphene. However, the technique is limited for laboratory-scale production due to the tedious process of finding the number of graphene layers under a microscope after peeling and minimal yield [6].Table 1
Properties of graphene.
PropertyQuantityMaterial comparisonReferenceElectron mobility2 × 105 cm2 V−1 s−1140 times greater than that of silicon[11]Young’s modulus1 TPa—[12]Ultimate tensile strength130 GPa100 times greater than that of steel[12]Thermal conductivity4840–5300 W/mK10 times greater than that of copper[13]Optical absorption2.3%Ideal substitute for ITO and FTO films[14]Reduction of graphene oxide (GO) has the potential to rapidly produced inexpensive graphene-based materials at an industrial scale. Utilizing oxidation and reduction reactions, graphite (and other carbon precursors) is converted sequentially into the following derivatives [7]:(i)
Graphene oxide (GO): It was synthesized through oxidizing graphite using an oxidizing agent and an acid, the oxidation leads to segregation of the graphite into monoatomic thick layers while also causing hindrance on the sp2 bonds [9]. With the sp3 hybridized carbon bonds out of the original plane and covalent bonding of oxygen-containing functional groups as shown in Figure 1, GO layers are thicker than graphene [8]. The primary prospect of graphene oxide is in nanocomposite fabrication where use of graphene is not applicable. Pristine graphene tends to restack because of its π-π interactions and hydrophobic nature which leads to phase separation within fabricated nanocomposites. The oxygen-containing functional groups in graphene oxide permit better distribution and consolidation. However, the oxygen-containing functional groups require removal to improve electrical conductivity [10].(ii)
Reduced graphene oxide: It has structural similarity to graphene; however, it contains defects, additional carbon ring domains, and a few oxygen-containing groups as shown in Figure1 [8].With most research on graphene synthesis processes being applied at the lab scale, there are notable challenges with upscaling to industrial mass production of graphene regarding cost, safety, repeatability, workability, and quality (refer to Table2). Also, commercialization depends on the viability of graphene to supersede other materials in current products at a competitive cost [1, 15].Figure 1
Structure of graphene, graphene oxide (GO), and reduced graphene oxide (rGO) [8].Table 2
Challenges in industrial-scale graphene synthesis [1].
FactorChallengesQualityCommercially available graphene flakes lack uniformity in both size and number of layers with varying degrees of impurity and defectsCostHigh energy consumption, specialized equipment, thermal annealing, and filtration accumulate the price of producing graphene on a large scaleRepeatabilityQuality and properties of graphene varies depending on the source material (existing structural defects) and synthesis conditions (process parameters and equipment)WorkabilityCommercially available graphene products are only 50% actual graphene which hinders potential applications. Poor repeatability has also restricted applications to use in composites where consistency is not a prioritySafetyUse of chemicals (oxidizing agents and surfactants) in certain graphene synthesis processes are hazardous and/or cause pollutionCommercial success was first noted with anticorrosion coatings and graphene-reinforced tennis rackets [16]. To expand on this success, it is imperative that [1](i)
Precise control is ensured over processing techniques, equipment, and parameters for transitioning lab-scale processes to industrial scale.(ii)
Graphene synthesis techniques and products are standardized with respect to quality, purity, and performance parameters.Even with advances in graphene manufacturing, an economically possible and environmentally friendly synthesis method for mass production of uniform high-quality graphene has not been established, limiting potential industrial applications of graphene. Therefore, this review paper discusses graphene synthesis methods, environmental issues related to graphene production, and the importance of the flash joule heating method of graphene synthesis. When discussing synthesis methods, this paper reviews newly developed graphene manufacturing processes such as layer-engineered exfoliation (LEE), laser-induced graphene (LIG), and flash joule heating. Then, the paper highlights key environmental concerns about mass graphene production followed by the importance of the flash joule heating method for large-scale graphene manufacturing.Table 3
Summary of graphene synthesis methods.
TypeMethodProcessFinal product and applicationsAdvantageDisadvantageReferenceTop-down methodsMechanical cleavageA film of gold is deposited on graphite surface through thermal evaporation. PMMA poly (methyl methacrylate) and thermal release tape is used to lever the gold film. Due to the binding energy of the gold and graphite being greater than the interlayer binding energy in graphite, the tensile stress induced from peeling off the film causes spalling of the top layers of graphene. Graphite is first set in epoxy and then a diamond wedge mounted on an ultrasonic oscillation system is used to cleavage off required thickness of graphenePristine monolayer graphene: used in graphene analysis for labs, primarily due to high qualityPristine single layer graphene sheets. Control on graphene thicknessSize of graphene is limited by surface size of graphite. Ideal metal depositions (as known as stressor film) are rare and/or precious metals (gold, cobalt)[17–19]Liquid phase exfoliationGraphene is sheared of using external forces (using shearing rotors, ball milling, or sonication) in a surfactant solution (primarily N-methyl-2-pyrrolidone (NMP))Pristine few-layers graphene nanoplatelets: used as composite filler inks and conductive coatsHigh-quality few-layer graphene dispersion. Scalable process. Better thermal and electrical conduction than reduced graphene oxideEdge defects. Separation of graphene from solution required for certain applications usually through centrifuge[20, 23, 24, 27, 28, 32, 33, 78, 79]Solid exfoliationGraphene layers are sheared off from graphite either through use of ball milling or rollers with oxalic acid (reducing agent). The resultant graphene sheets are subjected to thermal annealing in buffer gases (argon, nitrogen, and hydrogen) to restore graphitic lattice structure and reduce oxygen groupsFew-layer graphene nanosheets: used in paints, composites, and inksAllows mixing of graphene with other materials for composites. Graphene functionalizationNonuniform flake size and thickness with edge defects and impurities (from metal chipped off balls and high extensive impacts)Intercalation exfoliationGraphite is added to an intercalation solution and/or electrolytic solution (eutectic salt, Li/PC, (NH4)2SO4, H2SO4, H2O2, ionic liquids) in conjunction with electrochemistry or heat. As the intercalation solution reacts, gases produced causes the graphite to expand. The expanded graphite is sonicated in water, NMP, or DMF to produce graphene nanosheets in a solutionFew-layer graphene nanosheets: used in anticorrosive coats, battery components, composites, and rubbersScalable process. Enables functionalization of graphene. Graphene flakes have been noted to have minimal defectsExpensive ionic liquids. Time-intensive processOxidation exfoliation reductionGraphite is oxidized (using oxidizing agents such as KMnO4, NaNO3, H2SO, and HNO3) to introduce oxygen groups which increases the space between the layers of graphene. With the interlayer van der Waals forces reduced; the graphite oxide is exfoliated into few layers of graphene oxide (GO). Exfoliation is achieved through sonication or stirring in a surfactant solution (NMP, H2O, N, and N-dimethylformamide (DMF)). Graphene oxide nanosheets undergo reduction to produce reduced graphene oxide (rGO) through: thermal annealing in buffer gas or vacuum. Hydrothermal reduction: low concentration of GO aqueous solution is subjected to high pressure and temperature. Chemical reduction using either hydrazine, sodium hydroxide, potassium hydroxide, or sodium borohydrideFew-layer reduced graphene oxide (rGO): used in anticorrosive coats, battery components, composites, and rubbersScalable process. Enables functionalization of grapheneUse of hazardous chemical. Time-intensive process. Produced graphene requires washing and treatment to remove chemicals. Process has poor repeatability. Prominent defects in rGO due to oxidationArc dischargeGraphite rods set a few millimeters apart are exposed to buffer gases (hydrogen, helium, nitrogen, argon, or a mixture of those gases) in a stainless-steel vessel. Direct current is passed through the rods which produces an arc plasma. The rods vaporize from the high temperatures from the plasma and cool form graphene nanoflakesGraphene nanoplatelets used in lubricants, composites, inks, and coatsCatalyst not required. Highly suited for producing graphene at a rate of decagrams per hourLow yield. Amorphous carbon impurities. Required annealing which increases processing time[53–55, 80]Bottom-up methodsChemical vapor depositionCarbonaceous substance (commonly methane gas) is pyrolyzed at high temperatures (with a mixture of argon and hydrogen gases) to produce carbon which deposits on a metallic substrate (copper or nickel) to form graphene filmsMonolayer or few-layer graphene films used in optoelectronics, sensors, and touch panelsHigh-quality large graphene films. Potential for upscaling processProcess is expensive (high energy required). Removal of catalyst and transferring of graphene onto suited substrate[58, 59, 61]Epitaxial grapheneLayers of graphene can be grown on silicon carbide (SiC) crystal by thermal decomposition at temperatures greater than 1000°C under an ultrahigh vacuumFew-layer graphene wafer: used as conductive films and transistorsHigh-quality large-area graphene wafer. SiC crystal on which graphene is grown on is a highly suited substrate for applications in electronicsEnergy intensive process. SiC is expensive[56, 57, 81]Unzipping carbon nanotubeCarbon nanotubes are split open using acids and oxidizing agents (sulfuric acid and potassium permanganate), intercalation compounds (ammonium and lithium), metal nanoparticle catalyst, electric current, and argon plasmaGraphene nanoribbons: used in composites, electronics, and spintronicsSize of graphene nanoribbons can be controlled by selection of nanotubeOxidation and defects evident along edge where splitting occurs. High cost of chemicals and precursors[62–67]Laser-induced grapheneCO2 laser is used to irradiate polyimide substrate in ambient conditions and natural resources in either an inert atmosphere or ambient conditions to produce porous graphenePorous graphene: used in electronic devices such as sensors and capacitatorsProcess allows for control of morphology and patterning of grapheneLaser irradiation of certain materials at ambient conditions results in ablation and thermal damage[68]Rapid thermal annealingA carbon source and metal catalyst are deposited (either chemically or electrochemically) on a pretreated substrate (Si, SiC, SiO2, or Cu) which undergoes rapid thermal annealing at high temperatures either under vacuum or in an inert atmosphereMonolayer graphene films: flexible displays, thin film for photovoltaics, and printable electronicsShorter processing time. Allows for modification and improvement of graphene-based materials and filmsImpurities on substrate can affect the quality of graphene. Use of metal catalyst or substrate requires a strong acid to separate it from the graphene. This can result in defects and impurities[69, 82]Gas-phase-synthesized grapheneA carbonaceous fluid such as ethanol is subjected to an argon plasma generated by microwaves at atmospheric pressureGraphene nanoplatelets: used in lubricants, composites, inks, and coatsSubstrate not required. Process performed at atmospheric conditionsIn-depth statistical analysis of dependency of production variables (plasma gas flow rate, microwave forward power, carbon precursor material composition, and flow rate) has not been investigated through design of experiments[72–75]


## 2. Graphene Synthesis

Graphene synthesis methods are characterized into the following two categories that are distinguished from the starting material used:(1)
Top-down methods: graphite is the parent material. The primary concept of these methods (summarized in Table3) is overcoming the van der Waals forces between the graphene layers to produce few-layer or monolayer flakes.(2)
Bottom-up methods: a range of carbonaceous precursors have been used over various bottom-down methods (summarized in Table3) as the parent material. These synthesis methods use high energy to decompose the carbonaceous material and graphitize the carbon produced to form graphene.


### 2.1. Top-Down Methods

#### 2.1.1. Mechanical Cleavage

Adhesive tape is used to peel off graphene sheets from graphite. This method enables the production of high-quality sheets but at a low quantity and rate. The number of graphene layers reduces as the sample is repeatedly peeled using the tape [17].Methods of graphite mechanical cleavage have been explored further from the Scotch tape method to obtain pristine graphene sheets in an industrial-scale capacity.(i)
Oscillating diamond wedge: a sharp diamond wedge is used to cleave off layers of graphene from graphite set in epoxy (refer to Figure2) [18].(ii)
Layer-engineered exfoliation: metal stressor film is deposited on bulk graphite. The stressor film is then layered with thermal release tape and PMMA to aid the handling of cleavage of the top layer of graphene (refer to Figure3) [19].


#### 2.1.2. Liquid-Phase Exfoliation

Graphite is added to an aqueous solution or an organic solvent (N-methyl-2-pyrrolidone (NMP)) before it is subjected to an external force (shear mixing, sonication, or ball milling) to overcome the van der Waals forces between the graphene layers resulting in the production of graphene flakes. With ball milling, solid graphite is ground at high speed in a sealed rotating jar having steel balls. The resultant graphene flakes are then dispersed into a liquid. However, shear mixing and sonication are carried out on graphite in a solution. The process can be improved by intercalating the layers using chemicals such as ferric chloride, sulfuric acid, cesium, or potassium [17].Figure 2
Diamond wedge method [18].Figure 3
Layer-engineered exfoliation process [19].The limitations to this process are that the flakes vary in thickness, i.e., there are variances in the number of layers which results in a low total yield per solvent volume, and extended periods of exfoliation, specifically through sonication, lead to fragmenting and oxidation of graphene [20].Liquid exfoliation shearing methods are as follows:(1)
Sonication: sound energy is used to produce cavitation [21]. Either using a sonication probe or bath, the liquid solution is agitated to form cavitation bubbles which generate shockwaves upon collapsing [22] (refer to Figure 4). These shockwaves transmit through the graphite which produces tensile stress. Due to multiple cavities, the resultant tensile stress peels off layers of graphene from the graphite [23]. The efficiency of the process and graphene quantity is highly dependent on the geometry of the vessel exposed to the sound energy and the dispersion volume [24]. The graphene produced through sonication has defects due to high pressures and local temperatures generated by the cavitation [25].(2)
High-shear mixing: a rotor-stator or rotating blade is used to move the solution with the graphite at high speed within a region to create shear stress to peel layers of graphene off. For rotor-stator mixing (refer to Figure5), the production rate (PR) is influenced by the volume, V, of the solution as well as the solution compound [27];(i)
N-methyl-2-pyrrolidone (NMP): production rate is proportional to the volume of NMP solution raised by a factor of 1.1; PRα V1.1.(ii)
Surfactant (such as NaC): production rate is proportional to the volume of surfactant solution raised by a factor of 1.6; PRα V1.6.


#### 2.1.3. Solid Exfoliation

(1)
Ball milling: graphite is placed in a rotating vessel along with the balls. The balls collide with the graphene when the vessel is rotating which results in the layers of graphene exfoliating or fragmenting off the graphite (refer to Figure6) [26].(2)
Three-roll milling: a mixture of an adhesive (dioctyl phthalate and polyvinyl chloride) and graphite is fed through rollers. The rollers force the graphite to exfoliate into graphene and disperse in the adhesive. The final mixture is added to alcohol and heated to 500°C to remove the adhesive (refer to Figure7) [29].


#### 2.1.4. Electrochemical Exfoliation

The graphite rod is submerged in an electrolyte (ammonium sulphate ((NH₄)₂SO₄)) along with a counter electrode. When a current is passed through the electrodes, the electrolyte intercalates the layers within the graphite rod, causing effective exfoliation of graphene flakes as illustrated in Figure8. The flakes are then collected through vacuum filtration and dispersed in solvents such as NMP [30]. TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl) can be added to the electrolyte to prevent hydroxyl radicals (produced via electrolysis of water) from damaging graphene flakes. The flakes have been noted to have minimal defects and high production quantity [17].Figure 4
Working principle of sonication [26].Figure 5
Rotor-stator mixing [28].Figure 6
Ball milling mechanism [26].Figure 7
Three-roll milling [29].Figure 8
Electrochemical exfoliation mechanism and equipment setup for electrochemical exfoliation [31].


#### 2.1.5. Oxidation Exfoliation-Reduction

Graphite is oxidized through a process known as Hummers’ method which involves the use of sulfuric acid, sodium nitrate, and potassium permanganate, which produces graphite oxide. Other methods, illustrated in Figure9, are not rapid and produce graphene oxide with a lower carbon to oxygen ratio. The graphite oxide is exfoliated to produce graphene oxide (GO) flakes which are then treated to produce reduced graphene oxide (rGO). rGO can be produced in massive quantities; however, the inferior quality and use of multiple oxidizing agents and acids require further improvement in this process [32]. When compared to pristine graphene, graphene oxide is a poor electrical conductor because of the presence of oxides and reduced graphene oxide contains structural defects after the reduction process that in turn degrades structural and electrical properties [20]. The oxidation exfoliation method is applicable to a range of carbon precursors.(i)
Carbon nanofibers: the nanofibers are subjected to Hummer’s method [33].(ii)
Charcoal: carbonaceous waste material such as leaves, wood, bagasse, bones, and animal waste are subjected to approximately 500°C while wrapped in aluminium foil for 5 days. The resulting charcoal ground up, rewrapped in foil, and heated for 24 hours at 450°C. Then, a gram of the carbonized material is added to 100 ml of distilled water with 0.5 grams of iron (III) chloride hexahydrate with the addition of hydrochloric acid to maintain a pH of 2. The solution is mixed for 5 hours at 60°C followed by evaporation of water at room temperature over 5 days. Lastly, the remnants are dried for 5 hours at 100°C resulting in graphitized solid material. The material is then subjected to a modified Hummer’s method to produce graphene [34].Figure 9
Methods of oxidizing graphene [35].(1) Methods of graphene oxide reduction [10](1)
Hydrazine hydrate:Graphene oxide is reduced using hydrazine monohydrate (N2H4⋅H2O) to produce reduced graphene oxide with structural and electrical properties near equivalent to pristine graphene. With the temperature of the reduction reaction being influential, a study showed that reducing graphene oxide at 95°Celsius for 3 hours resulted in reduced graphene oxide with a carbon to oxygen ratio of 15 : 1 and an electrical conductivity of 5 S cm−1 [36].(2)
Catalysts:Zinc and aluminium functions as a reduction catalyst in hydrochloric acid. The metal catalyst reacts with the acid to form hydrogen gas and metal chloride. The hydrogen gas exothermically reacts with the oxygen in the graphene oxide to produce reduced graphene oxide. The resulting reduced graphene oxide shows an electrical conductivity of 2.1 × 103 Sm−1 [37–39].(3)
Organic solvents:Graphene oxide dispersions made with organic solvents such as DMF are reduced at 463 K for an hour. The DMF breaks down to carbon monoxide and dimethyl amine. The resulting carbon monoxide is a strong reducing agent which reacts with the oxygen from the graphene oxide to form a dispersion of reduced graphene oxide [40].Graphene oxide prepared from graphite using the Hummer’s method can be heated in situ with oxidizing agents at 393 K for 2 hours to produce reduced graphene oxide dispersion. The resulting reduced graphene oxide was highly electrochemical stable and 1000 times more conductive than graphene oxide [41].(4)
Sodium compounds:A multitude of sodium based reductants have been utilized to produce reduced graphene oxide. Sodium borohydride has been used to reduced graphene oxide dispersions in ambient conditions in the presence of silver nano particles. The sodium borohydride hydrolyses to form borohydride which oxidizes on the surface of the silver nano particles which leads to the transfer of electrons on said silver nanoparticles. The electrons are then transferred to graphene oxide and initiates reduction resulting in reduced graphene oxide. When sodium borohydride reacts in an alkaline condition, that is, in the presence of sodium hydroxide, a stable dispersion of reduced graphene oxide which can be formed into films through vacuum filtration with an electrical conductivity range from 10 to 1500 S cm−1 [42].(5)
Laser:A laser with a spot diameter of 1064 nanometres and power of 50 microwatts most efficiently reduced graphene oxide at a scan speed of 30 mm/s under nitrogen along with potassium hydroxide. Theoretically, the laser is capable of inducing ocalized annealing temperatures of up to 1273 K for a few nanoseconds which is the causation oxygen-containing functional group decomposition on graphene oxide, hence producing reduced graphene oxide [43].(6)
Plasma:A scanning atmospheric plasma at 300 Watts for an exposure time of 90 seconds has multiple reduction methods through the generation of charged ions. The resulting charged ions inelastically collide with the graphene oxide which ruptures the oxygen-containing groups to produce oxygen ions that react with surrounding oxygen species to form oxygen. Another mechanism involves interaction of free electrons from the plasma with oxygen in the surrounding atmosphere to produce oxygen absorbates. The absorbates transfers its free electrons to the graphene oxide surface thus producing reduced graphene oxide with improved doping. Finally, nitric oxide forms in the hot plasma zone from atmospheric nitrogen and oxygen. The highly reactive nitric oxide with the oxygen-containing groups on graphene oxide surface to form reduced graphene oxide and nitrogen dioxide [44].(7)
Photocatalysis:Photocatalytic graphene oxide reduction has two major approaches:(i)
The use a photoinitiator that is activated using ultraviolet: When exposed to ultraviolet irradiation at 100 mW cm−2 (with a spot diameter of 395 nm), the photo-initiator, such as phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, produces free radicals which reacts with the oxygen functional groups to produce reduced graphene oxide with an electrical conductivity of ∼3.7 × 103 S m−1. The reaction is conducted with an oxygen inhibitor (monoethanolamine) to improve photoreduction reaction through quenching of molecular oxygen [45].(ii)
The use of photoactivatable inorganic nanoparticles such as silver nanoparticles: The silver nanoparticles are the photocatalyst that reduces graphene oxide in the presence of surface plasmon resonance induced visible light and dimethylformamide (DMF) as an electron donor. A 400 Watt Xe lamp at wavelength greater than 420 nm is exposed to a mixture of silver nanoparticles, graphene oxide, and DMF for 40 minutes to produce silver-reduced graphene oxide aggregates with a sheet resistance of 9000 Ω/sq. [46].(8)
Thermal:Utilizing thermal annealing to reduce graphene oxide produces reduced graphene oxide with varying electrical and structural properties. Determination of reduction temperature is the end application of the reduced graphene as annealing temperature affects reduction extent and structural decomposition of the graphene plane. At 573 K, graphene oxide releases water, carbon dioxide, and carbon monoxide due to graphene oxide basal plane decomposition leading to carbon bond cleavage [47].Annealing at a temperature of 3000 K results in activation of bridged ether bonds and carbon to carbon bonds situated near hydroxyl groups which produces large carbon rings. With a continuous increase in temperature, the carbon backbone breaks which leads to an amorphous state. This is prominent at temperatures of 5000 K where there is an increase in sp2- hybridized carbon atoms [48].(9)
Microwave:Microwave inducted reduction of graphene oxide starts off with removal of carbonyl groups at less than 600 K followed by the hydroxyl group at 1500 K and then the carboxyl groups at near 2200 K. Finally, the epoxy groups are removed at 2800 K. The process is limited in its reducing ability as the oxygen-containing functional groups located on the edges of the sheet could not be removed due to structural decomposition of reduced graphene oxide that would result from required temperatures. The final product contains minimal to no oxygen-containing functional groups and is achieved in a brief time with the convenience of having control over reduction temperature [49].(10)
Hydrothermal:Hydrothermal reduction occurs through hydroxyl group dehydration using an acid catalyst (with water being the source of positively charge hydrogen ions). In acidic conditions, the carboxyl groups and hydroxyl groups undergo protonation resulting in reduced graphene oxide sheets to aggregate through H-bonding orπ-π interactions. The process is conducted at 453 K for 6 hours [50].(11)
Electrochemical reduction (ECR):There are primarily two methods of electrochemical reduction of graphene oxide:(i)
The one-step process involves in situ electrochemical reduction of colloidally dispersed graphene oxide in an electrolyte. This forms thin films of reduced graphene oxide on an electrode. The electrolyte conductivity (ideally between 4–25 S m−1), pH (less than 10), and applied potential (∼0 to −1.5 V) govern the efficiency of reduction process.(ii)
The two-step process which requires a coating of graphene oxide on the electrode surface first, and then, it is reduced using electrochemical process that utilizes a three electrode system in an electrolyte.The properties of the resulting reduced graphene oxide from the first method are dependent on the applied potential while in the second method, reduced graphene oxide films of uniform shape, size, and thickness can be produced by controlling initial depositing conditions of graphene oxide [51, 52].


#### 2.1.6. Arc Discharge Method

The arc discharge method consists of graphite rod anode and cathode that are exposed to buffer gases in a vacuum vessel, inert gases, or air. A constant current is passed through the electrodes from a DC power supply resulting in plasma formation (which reaches a temperature from 3727–5727°C [53]) between the cathode and anode (refer to Figure 10). The graphite anode vaporizes into the plasma forming graphene [54].Figure 10
Arc discharge method [55].


### 2.2. Bottom-Up Synthesis

#### 2.2.1. Silicon Carbide Sublimation (Epitaxial Growth)

Layers of graphene can be grown on silicon carbide (SiC) crystal by thermal decomposition at temperatures greater than 1000°C under an ultrahigh vacuum. As the SiC crystal is heated, it decomposes to its constituents silicon and carbon. The silicon vaporizes off from the crystal surface while the carbon atoms form graphene layers. The graphene grown on the Si-terminated surface of SiC are of high quality, ordered crystalline films, with minimal wrinkles [1]. The major drawbacks of this method are the elevated temperatures required to decompose SiC and the limited size of graphene films due to the excessive cost of SiC wafers. This has limited the use of epitaxially grown graphene for selected electronics such as high-frequency transistors [56].Hexogonal SiC crystals used for graphene synthesis have are made of Si-terminated (0001) surface and C-terminated (0001¯) surface as shown in Figure 11. The process begins with the sublimation of silicon atoms along the step edge. This is because the silicon atoms at the step edge have more dangling bonds compared to the silicon atoms found at other surface zones or within the crystal. The carbon atoms that stay on the edge form a graphene nucleus continues to grow laterally towards the upper surface of the step edge (Figures 12(b)–12(d)). The same graphene growth from the lower surface joins with the graphene produced at the step (Figures 12(e) and 12(f)). The first carbon layer is a buffer layer which is then followed by a layer of graphene (Figures 12(h) and 12(g)) [57].Figure 11
SiC crystal with reference to (0001) surface and (0001¯) surface [57].Figure 12
Graphene growth on silicon carbide (0001) surface [57].The difference between the graphene growth on the silicon-face (0001) and the carbon-face (0001¯) is that the graphene layers formed on the (0001¯) surface are stacked in a disorderly manner (turbostratic), while on the (0001) surface, the graphene showed ABC stacking. Graphene from carbon-face (0001¯) SiC also has high charge-carrier mobility, weaker coupling to the SiC surface and greater defects (Figure 13(f)) than graphene from silicon-face (0001) [57].Figure 13
Graphene growth on silicon carbide (0001¯) surface [57].


#### 2.2.2. Chemical Vapor Deposition

A carbon-rich source (hydrocarbons) is thermally decomposed at elevated temperatures and the carbon atoms graphitize on a substrate which is usually transition metals such as copper or nickel. To produce monolayer graphene, copper is used due to low carbon solubility and moderate catalytic activity. The graphene film is then transferred onto a transferring substrate (usually silicone or silicon dioxide) with the use of support polymers (primarily polymethylmethacrylate (PMMA)). Using ferric chloride (FeCl3), the copper substrate is etched to allow the PMMA and the graphene to be placed on a target substrate. Finally, the PMMA supports are removed using acetone [17].The process involves the formation of few-layer graphene from a carbon-rich source material over a catalytic metal substrate at elevated temperatures. The carbon-based source decomposes to release the carbon which is then either absorbed by the metal substrate (such as nickel and cobalt) and then precipitated as graphene or undergoes nucleation and grows graphene on the surface of the metal substrate (such as copper). The governing parameters that affect the graphene produced are as follows [59]:(i)
Concentration and exposure of carbon over the substrate(ii)
Flow rate of the carbon-rich source material(iii)
Cooling rate(iv)
Type of carbon-rich source materialTaking copper as the substrate and methane (CH4) as the carbon-rich source material, CVD synthesis process involves [60]:(i)
Methane decomposes to form CXHY species on the surface of the copper substrate which is in a methane/hydrogen atmosphere (Figures 14(a) and 14(d)).(ii)
As the copper is supersaturated with CXHY, nuclei begin to form (Figures 14(b) and 14(c)).(iii)
These nuclei form zones of graphene on the copper surface (Figure14(e)).(iv)
As those zones grow and join with each other, the copper surface gets covered by graphene while the undecomposed methane and by-products are removed from the chamber (Figure14(f)).An example of the processing parameters and conditions are as follows:(i)
Preannealing for 1 to 2 hours, the copper substrate in a furnace is set at 1000°C that has been backfilled with hydrogen gas (or a mixture of hydrogen and argon) while keeping a hydrogen flow at 50 sccm.(ii)
Methane at a flow of 35 sccm is introduced for a growth time of 5–20 min.(iii)
Then, the furnace is allowed to slowly cool to room temperature with the hydrogen atmosphere [60].Figure 14
Graphene growth mechanism in CVD [58].CVD can produce high-quality graphene films. However, the process is expensive and not readily scalable. There is also a need to improve manufacturing and product parameters which are as follows [61]:(i)
Achieving graphene growth at lower temperatures rapidly.(ii)
Attaining a high degree of uniform crystallinity with better oriented crystalline.(iii)
Developing scalable graphene transfer techniques.In terms of upscaling the graphene film production, two processes have been designed [17]:(i)
Roll to roll (R2R) produces a continuous graphene film (refer to Figure15).(ii)
Batch to batch (B2B) produces graphene wafers.


#### 2.2.3. Unzipping Carbon Nanotubes

Carbon nanotubes (CNTs) are split open using various methods (Figure16) to produce graphene nanoribbons:(a)
Intercalation and exfoliation: multiwalled carbon nanotubes (MWCNT) are added with lithium and liquid ammonia. The Li-NH3 complex with dilute hydrochloric acid and heat intercalates and splits the MWCNT walls [62].(b)
Chemical: MWCNT is suspended on concentrated sulfuric acid and treated with potassium permanganate (500 wt.%) for an hour at room temperature followed by another hour at 55–70°C [63].(c)
Catalytic: CNT catalytically unzipped with lead nanoparticles in an oxygen liquid medium under microwave irradiation [64].(d)
Electrical: an MWCNT is contacted by a moving electrode with a current passing through it which ruptures the outer layer of CNT [65].(e)
Physiochemical: MWCNT split using hydrogen plasma reaction at 300°C [66].Figure 15
Roll to roll CVD graphene production [16].Figure 16
Methods of unzipping carbon nanotubes [67].


#### 2.2.4. Laser-Induced Graphene (LIG)

Laser irradiation transforms carbonaceous materials such as polyimide (PI) into graphene under ambient conditions. A 4.8 W laser produces graphene of the highest crystallinity, fewer defects, and the largest domain size on a PI. The minimum required energy to start graphene formation on PI was noted to be approximately 5.5 J/cm2. LIG morphology can be controlled through pulses per inch (PPI), lines per inch (LPI), and laser spot size.(i)
1000 PPI × 1000 LPI with 100μm laser spot size: LIG has an in-plane porous structure.(ii)
500 PPI × 500 LPI with 60μm laser spot size: LIG forms as out-of-plane fibers.Figure17 shows the set up used when producing LIG from wood where it is lasered in an inert atmosphere to prevent ablation [68].Figure 17
LIG synthesis on wood [68].


#### 2.2.5. Rapid Thermal Annealing

A metal catalyst (nickel, copper, Ni-Cu alloy, or cobalt) is deposited onto a substrate (silicon, silicon carbide, silicon dioxide wafers, or copper films) along with a carbon source which is annealed at elevated temperatures (500–1000°C) under low vacuum as illustrated in Figure18 [69].(1)
Thermal evaporation: nickel is deposited on a SiO2 substrate in a vacuum (10–6 mbar) through evaporation.(2)
Pulsed laser deposition: a laser ablates a carbon source to deposit amorphous carbon on the substrate in a vacuum chamber (10–7 mbar).(3)
Rapid thermal heating: the substrate is heated to an elevated temperature (900°C) for 420 seconds at a 10–2 mbar vacuum to induce graphene growth.(4)
Nickel etching: FeCl3 is used to remove nickel to produce transfer-free graphene.Figure 18
Rapid thermal annealing synthesis route [69].


#### 2.2.6. Gas-Phase-Synthesized Graphene (GSG)

A carbonaceous gas (ethanol) is subjected to a microwave inducted argon plasma at atmospheric pressure [70, 71]. The ethanol and argon mixture enter the section of the quartz tube where the plasma electrons absorb the microwaves which transfer the energy to heavier particles through collisions. This results in an electron dense environment (greater than 1013 cm−3) where gas temperatures reach 3000 K [71–74]. The ethanol decomposes into reactive fragments that flow out of the quartz tube to plasma afterglow where it reacts to form GSG (Figure 19) [74].Figure 19
Microwave plasma reactor for GSG production [75].Top-down and bottom-up graphene synthesis methods discussed in Section2 are summarized in the following table (Table 3) for a detailed understanding of the readership.


## 2.1. Top-Down Methods

### 2.1.1. Mechanical Cleavage

Adhesive tape is used to peel off graphene sheets from graphite. This method enables the production of high-quality sheets but at a low quantity and rate. The number of graphene layers reduces as the sample is repeatedly peeled using the tape [17].Methods of graphite mechanical cleavage have been explored further from the Scotch tape method to obtain pristine graphene sheets in an industrial-scale capacity.(i)
Oscillating diamond wedge: a sharp diamond wedge is used to cleave off layers of graphene from graphite set in epoxy (refer to Figure2) [18].(ii)
Layer-engineered exfoliation: metal stressor film is deposited on bulk graphite. The stressor film is then layered with thermal release tape and PMMA to aid the handling of cleavage of the top layer of graphene (refer to Figure3) [19].


### 2.1.2. Liquid-Phase Exfoliation

Graphite is added to an aqueous solution or an organic solvent (N-methyl-2-pyrrolidone (NMP)) before it is subjected to an external force (shear mixing, sonication, or ball milling) to overcome the van der Waals forces between the graphene layers resulting in the production of graphene flakes. With ball milling, solid graphite is ground at high speed in a sealed rotating jar having steel balls. The resultant graphene flakes are then dispersed into a liquid. However, shear mixing and sonication are carried out on graphite in a solution. The process can be improved by intercalating the layers using chemicals such as ferric chloride, sulfuric acid, cesium, or potassium [17].Figure 2
Diamond wedge method [18].Figure 3
Layer-engineered exfoliation process [19].The limitations to this process are that the flakes vary in thickness, i.e., there are variances in the number of layers which results in a low total yield per solvent volume, and extended periods of exfoliation, specifically through sonication, lead to fragmenting and oxidation of graphene [20].Liquid exfoliation shearing methods are as follows:(1)
Sonication: sound energy is used to produce cavitation [21]. Either using a sonication probe or bath, the liquid solution is agitated to form cavitation bubbles which generate shockwaves upon collapsing [22] (refer to Figure 4). These shockwaves transmit through the graphite which produces tensile stress. Due to multiple cavities, the resultant tensile stress peels off layers of graphene from the graphite [23]. The efficiency of the process and graphene quantity is highly dependent on the geometry of the vessel exposed to the sound energy and the dispersion volume [24]. The graphene produced through sonication has defects due to high pressures and local temperatures generated by the cavitation [25].(2)
High-shear mixing: a rotor-stator or rotating blade is used to move the solution with the graphite at high speed within a region to create shear stress to peel layers of graphene off. For rotor-stator mixing (refer to Figure5), the production rate (PR) is influenced by the volume, V, of the solution as well as the solution compound [27];(i)
N-methyl-2-pyrrolidone (NMP): production rate is proportional to the volume of NMP solution raised by a factor of 1.1; PRα V1.1.(ii)
Surfactant (such as NaC): production rate is proportional to the volume of surfactant solution raised by a factor of 1.6; PRα V1.6.


### 2.1.3. Solid Exfoliation

(1)
Ball milling: graphite is placed in a rotating vessel along with the balls. The balls collide with the graphene when the vessel is rotating which results in the layers of graphene exfoliating or fragmenting off the graphite (refer to Figure6) [26].(2)
Three-roll milling: a mixture of an adhesive (dioctyl phthalate and polyvinyl chloride) and graphite is fed through rollers. The rollers force the graphite to exfoliate into graphene and disperse in the adhesive. The final mixture is added to alcohol and heated to 500°C to remove the adhesive (refer to Figure7) [29].


### 2.1.4. Electrochemical Exfoliation

The graphite rod is submerged in an electrolyte (ammonium sulphate ((NH₄)₂SO₄)) along with a counter electrode. When a current is passed through the electrodes, the electrolyte intercalates the layers within the graphite rod, causing effective exfoliation of graphene flakes as illustrated in Figure8. The flakes are then collected through vacuum filtration and dispersed in solvents such as NMP [30]. TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl) can be added to the electrolyte to prevent hydroxyl radicals (produced via electrolysis of water) from damaging graphene flakes. The flakes have been noted to have minimal defects and high production quantity [17].Figure 4
Working principle of sonication [26].Figure 5
Rotor-stator mixing [28].Figure 6
Ball milling mechanism [26].Figure 7
Three-roll milling [29].Figure 8
Electrochemical exfoliation mechanism and equipment setup for electrochemical exfoliation [31].


### 2.1.5. Oxidation Exfoliation-Reduction

Graphite is oxidized through a process known as Hummers’ method which involves the use of sulfuric acid, sodium nitrate, and potassium permanganate, which produces graphite oxide. Other methods, illustrated in Figure9, are not rapid and produce graphene oxide with a lower carbon to oxygen ratio. The graphite oxide is exfoliated to produce graphene oxide (GO) flakes which are then treated to produce reduced graphene oxide (rGO). rGO can be produced in massive quantities; however, the inferior quality and use of multiple oxidizing agents and acids require further improvement in this process [32]. When compared to pristine graphene, graphene oxide is a poor electrical conductor because of the presence of oxides and reduced graphene oxide contains structural defects after the reduction process that in turn degrades structural and electrical properties [20]. The oxidation exfoliation method is applicable to a range of carbon precursors.(i)
Carbon nanofibers: the nanofibers are subjected to Hummer’s method [33].(ii)
Charcoal: carbonaceous waste material such as leaves, wood, bagasse, bones, and animal waste are subjected to approximately 500°C while wrapped in aluminium foil for 5 days. The resulting charcoal ground up, rewrapped in foil, and heated for 24 hours at 450°C. Then, a gram of the carbonized material is added to 100 ml of distilled water with 0.5 grams of iron (III) chloride hexahydrate with the addition of hydrochloric acid to maintain a pH of 2. The solution is mixed for 5 hours at 60°C followed by evaporation of water at room temperature over 5 days. Lastly, the remnants are dried for 5 hours at 100°C resulting in graphitized solid material. The material is then subjected to a modified Hummer’s method to produce graphene [34].Figure 9
Methods of oxidizing graphene [35].(1) Methods of graphene oxide reduction [10](1)
Hydrazine hydrate:Graphene oxide is reduced using hydrazine monohydrate (N2H4⋅H2O) to produce reduced graphene oxide with structural and electrical properties near equivalent to pristine graphene. With the temperature of the reduction reaction being influential, a study showed that reducing graphene oxide at 95°Celsius for 3 hours resulted in reduced graphene oxide with a carbon to oxygen ratio of 15 : 1 and an electrical conductivity of 5 S cm−1 [36].(2)
Catalysts:Zinc and aluminium functions as a reduction catalyst in hydrochloric acid. The metal catalyst reacts with the acid to form hydrogen gas and metal chloride. The hydrogen gas exothermically reacts with the oxygen in the graphene oxide to produce reduced graphene oxide. The resulting reduced graphene oxide shows an electrical conductivity of 2.1 × 103 Sm−1 [37–39].(3)
Organic solvents:Graphene oxide dispersions made with organic solvents such as DMF are reduced at 463 K for an hour. The DMF breaks down to carbon monoxide and dimethyl amine. The resulting carbon monoxide is a strong reducing agent which reacts with the oxygen from the graphene oxide to form a dispersion of reduced graphene oxide [40].Graphene oxide prepared from graphite using the Hummer’s method can be heated in situ with oxidizing agents at 393 K for 2 hours to produce reduced graphene oxide dispersion. The resulting reduced graphene oxide was highly electrochemical stable and 1000 times more conductive than graphene oxide [41].(4)
Sodium compounds:A multitude of sodium based reductants have been utilized to produce reduced graphene oxide. Sodium borohydride has been used to reduced graphene oxide dispersions in ambient conditions in the presence of silver nano particles. The sodium borohydride hydrolyses to form borohydride which oxidizes on the surface of the silver nano particles which leads to the transfer of electrons on said silver nanoparticles. The electrons are then transferred to graphene oxide and initiates reduction resulting in reduced graphene oxide. When sodium borohydride reacts in an alkaline condition, that is, in the presence of sodium hydroxide, a stable dispersion of reduced graphene oxide which can be formed into films through vacuum filtration with an electrical conductivity range from 10 to 1500 S cm−1 [42].(5)
Laser:A laser with a spot diameter of 1064 nanometres and power of 50 microwatts most efficiently reduced graphene oxide at a scan speed of 30 mm/s under nitrogen along with potassium hydroxide. Theoretically, the laser is capable of inducing ocalized annealing temperatures of up to 1273 K for a few nanoseconds which is the causation oxygen-containing functional group decomposition on graphene oxide, hence producing reduced graphene oxide [43].(6)
Plasma:A scanning atmospheric plasma at 300 Watts for an exposure time of 90 seconds has multiple reduction methods through the generation of charged ions. The resulting charged ions inelastically collide with the graphene oxide which ruptures the oxygen-containing groups to produce oxygen ions that react with surrounding oxygen species to form oxygen. Another mechanism involves interaction of free electrons from the plasma with oxygen in the surrounding atmosphere to produce oxygen absorbates. The absorbates transfers its free electrons to the graphene oxide surface thus producing reduced graphene oxide with improved doping. Finally, nitric oxide forms in the hot plasma zone from atmospheric nitrogen and oxygen. The highly reactive nitric oxide with the oxygen-containing groups on graphene oxide surface to form reduced graphene oxide and nitrogen dioxide [44].(7)
Photocatalysis:Photocatalytic graphene oxide reduction has two major approaches:(i)
The use a photoinitiator that is activated using ultraviolet: When exposed to ultraviolet irradiation at 100 mW cm−2 (with a spot diameter of 395 nm), the photo-initiator, such as phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, produces free radicals which reacts with the oxygen functional groups to produce reduced graphene oxide with an electrical conductivity of ∼3.7 × 103 S m−1. The reaction is conducted with an oxygen inhibitor (monoethanolamine) to improve photoreduction reaction through quenching of molecular oxygen [45].(ii)
The use of photoactivatable inorganic nanoparticles such as silver nanoparticles: The silver nanoparticles are the photocatalyst that reduces graphene oxide in the presence of surface plasmon resonance induced visible light and dimethylformamide (DMF) as an electron donor. A 400 Watt Xe lamp at wavelength greater than 420 nm is exposed to a mixture of silver nanoparticles, graphene oxide, and DMF for 40 minutes to produce silver-reduced graphene oxide aggregates with a sheet resistance of 9000 Ω/sq. [46].(8)
Thermal:Utilizing thermal annealing to reduce graphene oxide produces reduced graphene oxide with varying electrical and structural properties. Determination of reduction temperature is the end application of the reduced graphene as annealing temperature affects reduction extent and structural decomposition of the graphene plane. At 573 K, graphene oxide releases water, carbon dioxide, and carbon monoxide due to graphene oxide basal plane decomposition leading to carbon bond cleavage [47].Annealing at a temperature of 3000 K results in activation of bridged ether bonds and carbon to carbon bonds situated near hydroxyl groups which produces large carbon rings. With a continuous increase in temperature, the carbon backbone breaks which leads to an amorphous state. This is prominent at temperatures of 5000 K where there is an increase in sp2- hybridized carbon atoms [48].(9)
Microwave:Microwave inducted reduction of graphene oxide starts off with removal of carbonyl groups at less than 600 K followed by the hydroxyl group at 1500 K and then the carboxyl groups at near 2200 K. Finally, the epoxy groups are removed at 2800 K. The process is limited in its reducing ability as the oxygen-containing functional groups located on the edges of the sheet could not be removed due to structural decomposition of reduced graphene oxide that would result from required temperatures. The final product contains minimal to no oxygen-containing functional groups and is achieved in a brief time with the convenience of having control over reduction temperature [49].(10)
Hydrothermal:Hydrothermal reduction occurs through hydroxyl group dehydration using an acid catalyst (with water being the source of positively charge hydrogen ions). In acidic conditions, the carboxyl groups and hydroxyl groups undergo protonation resulting in reduced graphene oxide sheets to aggregate through H-bonding orπ-π interactions. The process is conducted at 453 K for 6 hours [50].(11)
Electrochemical reduction (ECR):There are primarily two methods of electrochemical reduction of graphene oxide:(i)
The one-step process involves in situ electrochemical reduction of colloidally dispersed graphene oxide in an electrolyte. This forms thin films of reduced graphene oxide on an electrode. The electrolyte conductivity (ideally between 4–25 S m−1), pH (less than 10), and applied potential (∼0 to −1.5 V) govern the efficiency of reduction process.(ii)
The two-step process which requires a coating of graphene oxide on the electrode surface first, and then, it is reduced using electrochemical process that utilizes a three electrode system in an electrolyte.The properties of the resulting reduced graphene oxide from the first method are dependent on the applied potential while in the second method, reduced graphene oxide films of uniform shape, size, and thickness can be produced by controlling initial depositing conditions of graphene oxide [51, 52].


### 2.1.6. Arc Discharge Method

The arc discharge method consists of graphite rod anode and cathode that are exposed to buffer gases in a vacuum vessel, inert gases, or air. A constant current is passed through the electrodes from a DC power supply resulting in plasma formation (which reaches a temperature from 3727–5727°C [53]) between the cathode and anode (refer to Figure 10). The graphite anode vaporizes into the plasma forming graphene [54].Figure 10
Arc discharge method [55].


## 2.1.1. Mechanical Cleavage

Adhesive tape is used to peel off graphene sheets from graphite. This method enables the production of high-quality sheets but at a low quantity and rate. The number of graphene layers reduces as the sample is repeatedly peeled using the tape [17].Methods of graphite mechanical cleavage have been explored further from the Scotch tape method to obtain pristine graphene sheets in an industrial-scale capacity.(i)
Oscillating diamond wedge: a sharp diamond wedge is used to cleave off layers of graphene from graphite set in epoxy (refer to Figure2) [18].(ii)
Layer-engineered exfoliation: metal stressor film is deposited on bulk graphite. The stressor film is then layered with thermal release tape and PMMA to aid the handling of cleavage of the top layer of graphene (refer to Figure3) [19].


## 2.1.2. Liquid-Phase Exfoliation

Graphite is added to an aqueous solution or an organic solvent (N-methyl-2-pyrrolidone (NMP)) before it is subjected to an external force (shear mixing, sonication, or ball milling) to overcome the van der Waals forces between the graphene layers resulting in the production of graphene flakes. With ball milling, solid graphite is ground at high speed in a sealed rotating jar having steel balls. The resultant graphene flakes are then dispersed into a liquid. However, shear mixing and sonication are carried out on graphite in a solution. The process can be improved by intercalating the layers using chemicals such as ferric chloride, sulfuric acid, cesium, or potassium [17].Figure 2
Diamond wedge method [18].Figure 3
Layer-engineered exfoliation process [19].The limitations to this process are that the flakes vary in thickness, i.e., there are variances in the number of layers which results in a low total yield per solvent volume, and extended periods of exfoliation, specifically through sonication, lead to fragmenting and oxidation of graphene [20].Liquid exfoliation shearing methods are as follows:(1)
Sonication: sound energy is used to produce cavitation [21]. Either using a sonication probe or bath, the liquid solution is agitated to form cavitation bubbles which generate shockwaves upon collapsing [22] (refer to Figure 4). These shockwaves transmit through the graphite which produces tensile stress. Due to multiple cavities, the resultant tensile stress peels off layers of graphene from the graphite [23]. The efficiency of the process and graphene quantity is highly dependent on the geometry of the vessel exposed to the sound energy and the dispersion volume [24]. The graphene produced through sonication has defects due to high pressures and local temperatures generated by the cavitation [25].(2)
High-shear mixing: a rotor-stator or rotating blade is used to move the solution with the graphite at high speed within a region to create shear stress to peel layers of graphene off. For rotor-stator mixing (refer to Figure5), the production rate (PR) is influenced by the volume, V, of the solution as well as the solution compound [27];(i)
N-methyl-2-pyrrolidone (NMP): production rate is proportional to the volume of NMP solution raised by a factor of 1.1; PRα V1.1.(ii)
Surfactant (such as NaC): production rate is proportional to the volume of surfactant solution raised by a factor of 1.6; PRα V1.6.


## 2.1.3. Solid Exfoliation

(1)
Ball milling: graphite is placed in a rotating vessel along with the balls. The balls collide with the graphene when the vessel is rotating which results in the layers of graphene exfoliating or fragmenting off the graphite (refer to Figure6) [26].(2)
Three-roll milling: a mixture of an adhesive (dioctyl phthalate and polyvinyl chloride) and graphite is fed through rollers. The rollers force the graphite to exfoliate into graphene and disperse in the adhesive. The final mixture is added to alcohol and heated to 500°C to remove the adhesive (refer to Figure7) [29].


## 2.1.4. Electrochemical Exfoliation

The graphite rod is submerged in an electrolyte (ammonium sulphate ((NH₄)₂SO₄)) along with a counter electrode. When a current is passed through the electrodes, the electrolyte intercalates the layers within the graphite rod, causing effective exfoliation of graphene flakes as illustrated in Figure8. The flakes are then collected through vacuum filtration and dispersed in solvents such as NMP [30]. TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl) can be added to the electrolyte to prevent hydroxyl radicals (produced via electrolysis of water) from damaging graphene flakes. The flakes have been noted to have minimal defects and high production quantity [17].Figure 4
Working principle of sonication [26].Figure 5
Rotor-stator mixing [28].Figure 6
Ball milling mechanism [26].Figure 7
Three-roll milling [29].Figure 8
Electrochemical exfoliation mechanism and equipment setup for electrochemical exfoliation [31].


## 2.1.5. Oxidation Exfoliation-Reduction

Graphite is oxidized through a process known as Hummers’ method which involves the use of sulfuric acid, sodium nitrate, and potassium permanganate, which produces graphite oxide. Other methods, illustrated in Figure9, are not rapid and produce graphene oxide with a lower carbon to oxygen ratio. The graphite oxide is exfoliated to produce graphene oxide (GO) flakes which are then treated to produce reduced graphene oxide (rGO). rGO can be produced in massive quantities; however, the inferior quality and use of multiple oxidizing agents and acids require further improvement in this process [32]. When compared to pristine graphene, graphene oxide is a poor electrical conductor because of the presence of oxides and reduced graphene oxide contains structural defects after the reduction process that in turn degrades structural and electrical properties [20]. The oxidation exfoliation method is applicable to a range of carbon precursors.(i)
Carbon nanofibers: the nanofibers are subjected to Hummer’s method [33].(ii)
Charcoal: carbonaceous waste material such as leaves, wood, bagasse, bones, and animal waste are subjected to approximately 500°C while wrapped in aluminium foil for 5 days. The resulting charcoal ground up, rewrapped in foil, and heated for 24 hours at 450°C. Then, a gram of the carbonized material is added to 100 ml of distilled water with 0.5 grams of iron (III) chloride hexahydrate with the addition of hydrochloric acid to maintain a pH of 2. The solution is mixed for 5 hours at 60°C followed by evaporation of water at room temperature over 5 days. Lastly, the remnants are dried for 5 hours at 100°C resulting in graphitized solid material. The material is then subjected to a modified Hummer’s method to produce graphene [34].Figure 9
Methods of oxidizing graphene [35].(1) Methods of graphene oxide reduction [10](1)
Hydrazine hydrate:Graphene oxide is reduced using hydrazine monohydrate (N2H4⋅H2O) to produce reduced graphene oxide with structural and electrical properties near equivalent to pristine graphene. With the temperature of the reduction reaction being influential, a study showed that reducing graphene oxide at 95°Celsius for 3 hours resulted in reduced graphene oxide with a carbon to oxygen ratio of 15 : 1 and an electrical conductivity of 5 S cm−1 [36].(2)
Catalysts:Zinc and aluminium functions as a reduction catalyst in hydrochloric acid. The metal catalyst reacts with the acid to form hydrogen gas and metal chloride. The hydrogen gas exothermically reacts with the oxygen in the graphene oxide to produce reduced graphene oxide. The resulting reduced graphene oxide shows an electrical conductivity of 2.1 × 103 Sm−1 [37–39].(3)
Organic solvents:Graphene oxide dispersions made with organic solvents such as DMF are reduced at 463 K for an hour. The DMF breaks down to carbon monoxide and dimethyl amine. The resulting carbon monoxide is a strong reducing agent which reacts with the oxygen from the graphene oxide to form a dispersion of reduced graphene oxide [40].Graphene oxide prepared from graphite using the Hummer’s method can be heated in situ with oxidizing agents at 393 K for 2 hours to produce reduced graphene oxide dispersion. The resulting reduced graphene oxide was highly electrochemical stable and 1000 times more conductive than graphene oxide [41].(4)
Sodium compounds:A multitude of sodium based reductants have been utilized to produce reduced graphene oxide. Sodium borohydride has been used to reduced graphene oxide dispersions in ambient conditions in the presence of silver nano particles. The sodium borohydride hydrolyses to form borohydride which oxidizes on the surface of the silver nano particles which leads to the transfer of electrons on said silver nanoparticles. The electrons are then transferred to graphene oxide and initiates reduction resulting in reduced graphene oxide. When sodium borohydride reacts in an alkaline condition, that is, in the presence of sodium hydroxide, a stable dispersion of reduced graphene oxide which can be formed into films through vacuum filtration with an electrical conductivity range from 10 to 1500 S cm−1 [42].(5)
Laser:A laser with a spot diameter of 1064 nanometres and power of 50 microwatts most efficiently reduced graphene oxide at a scan speed of 30 mm/s under nitrogen along with potassium hydroxide. Theoretically, the laser is capable of inducing ocalized annealing temperatures of up to 1273 K for a few nanoseconds which is the causation oxygen-containing functional group decomposition on graphene oxide, hence producing reduced graphene oxide [43].(6)
Plasma:A scanning atmospheric plasma at 300 Watts for an exposure time of 90 seconds has multiple reduction methods through the generation of charged ions. The resulting charged ions inelastically collide with the graphene oxide which ruptures the oxygen-containing groups to produce oxygen ions that react with surrounding oxygen species to form oxygen. Another mechanism involves interaction of free electrons from the plasma with oxygen in the surrounding atmosphere to produce oxygen absorbates. The absorbates transfers its free electrons to the graphene oxide surface thus producing reduced graphene oxide with improved doping. Finally, nitric oxide forms in the hot plasma zone from atmospheric nitrogen and oxygen. The highly reactive nitric oxide with the oxygen-containing groups on graphene oxide surface to form reduced graphene oxide and nitrogen dioxide [44].(7)
Photocatalysis:Photocatalytic graphene oxide reduction has two major approaches:(i)
The use a photoinitiator that is activated using ultraviolet: When exposed to ultraviolet irradiation at 100 mW cm−2 (with a spot diameter of 395 nm), the photo-initiator, such as phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide, produces free radicals which reacts with the oxygen functional groups to produce reduced graphene oxide with an electrical conductivity of ∼3.7 × 103 S m−1. The reaction is conducted with an oxygen inhibitor (monoethanolamine) to improve photoreduction reaction through quenching of molecular oxygen [45].(ii)
The use of photoactivatable inorganic nanoparticles such as silver nanoparticles: The silver nanoparticles are the photocatalyst that reduces graphene oxide in the presence of surface plasmon resonance induced visible light and dimethylformamide (DMF) as an electron donor. A 400 Watt Xe lamp at wavelength greater than 420 nm is exposed to a mixture of silver nanoparticles, graphene oxide, and DMF for 40 minutes to produce silver-reduced graphene oxide aggregates with a sheet resistance of 9000 Ω/sq. [46].(8)
Thermal:Utilizing thermal annealing to reduce graphene oxide produces reduced graphene oxide with varying electrical and structural properties. Determination of reduction temperature is the end application of the reduced graphene as annealing temperature affects reduction extent and structural decomposition of the graphene plane. At 573 K, graphene oxide releases water, carbon dioxide, and carbon monoxide due to graphene oxide basal plane decomposition leading to carbon bond cleavage [47].Annealing at a temperature of 3000 K results in activation of bridged ether bonds and carbon to carbon bonds situated near hydroxyl groups which produces large carbon rings. With a continuous increase in temperature, the carbon backbone breaks which leads to an amorphous state. This is prominent at temperatures of 5000 K where there is an increase in sp2- hybridized carbon atoms [48].(9)
Microwave:Microwave inducted reduction of graphene oxide starts off with removal of carbonyl groups at less than 600 K followed by the hydroxyl group at 1500 K and then the carboxyl groups at near 2200 K. Finally, the epoxy groups are removed at 2800 K. The process is limited in its reducing ability as the oxygen-containing functional groups located on the edges of the sheet could not be removed due to structural decomposition of reduced graphene oxide that would result from required temperatures. The final product contains minimal to no oxygen-containing functional groups and is achieved in a brief time with the convenience of having control over reduction temperature [49].(10)
Hydrothermal:Hydrothermal reduction occurs through hydroxyl group dehydration using an acid catalyst (with water being the source of positively charge hydrogen ions). In acidic conditions, the carboxyl groups and hydroxyl groups undergo protonation resulting in reduced graphene oxide sheets to aggregate through H-bonding orπ-π interactions. The process is conducted at 453 K for 6 hours [50].(11)
Electrochemical reduction (ECR):There are primarily two methods of electrochemical reduction of graphene oxide:(i)
The one-step process involves in situ electrochemical reduction of colloidally dispersed graphene oxide in an electrolyte. This forms thin films of reduced graphene oxide on an electrode. The electrolyte conductivity (ideally between 4–25 S m−1), pH (less than 10), and applied potential (∼0 to −1.5 V) govern the efficiency of reduction process.(ii)
The two-step process which requires a coating of graphene oxide on the electrode surface first, and then, it is reduced using electrochemical process that utilizes a three electrode system in an electrolyte.The properties of the resulting reduced graphene oxide from the first method are dependent on the applied potential while in the second method, reduced graphene oxide films of uniform shape, size, and thickness can be produced by controlling initial depositing conditions of graphene oxide [51, 52].


## 2.1.6. Arc Discharge Method

The arc discharge method consists of graphite rod anode and cathode that are exposed to buffer gases in a vacuum vessel, inert gases, or air. A constant current is passed through the electrodes from a DC power supply resulting in plasma formation (which reaches a temperature from 3727–5727°C [53]) between the cathode and anode (refer to Figure 10). The graphite anode vaporizes into the plasma forming graphene [54].Figure 10
Arc discharge method [55].


## 2.2. Bottom-Up Synthesis

### 2.2.1. Silicon Carbide Sublimation (Epitaxial Growth)

Layers of graphene can be grown on silicon carbide (SiC) crystal by thermal decomposition at temperatures greater than 1000°C under an ultrahigh vacuum. As the SiC crystal is heated, it decomposes to its constituents silicon and carbon. The silicon vaporizes off from the crystal surface while the carbon atoms form graphene layers. The graphene grown on the Si-terminated surface of SiC are of high quality, ordered crystalline films, with minimal wrinkles [1]. The major drawbacks of this method are the elevated temperatures required to decompose SiC and the limited size of graphene films due to the excessive cost of SiC wafers. This has limited the use of epitaxially grown graphene for selected electronics such as high-frequency transistors [56].Hexogonal SiC crystals used for graphene synthesis have are made of Si-terminated (0001) surface and C-terminated (0001¯) surface as shown in Figure 11. The process begins with the sublimation of silicon atoms along the step edge. This is because the silicon atoms at the step edge have more dangling bonds compared to the silicon atoms found at other surface zones or within the crystal. The carbon atoms that stay on the edge form a graphene nucleus continues to grow laterally towards the upper surface of the step edge (Figures 12(b)–12(d)). The same graphene growth from the lower surface joins with the graphene produced at the step (Figures 12(e) and 12(f)). The first carbon layer is a buffer layer which is then followed by a layer of graphene (Figures 12(h) and 12(g)) [57].Figure 11
SiC crystal with reference to (0001) surface and (0001¯) surface [57].Figure 12
Graphene growth on silicon carbide (0001) surface [57].The difference between the graphene growth on the silicon-face (0001) and the carbon-face (0001¯) is that the graphene layers formed on the (0001¯) surface are stacked in a disorderly manner (turbostratic), while on the (0001) surface, the graphene showed ABC stacking. Graphene from carbon-face (0001¯) SiC also has high charge-carrier mobility, weaker coupling to the SiC surface and greater defects (Figure 13(f)) than graphene from silicon-face (0001) [57].Figure 13
Graphene growth on silicon carbide (0001¯) surface [57].


### 2.2.2. Chemical Vapor Deposition

A carbon-rich source (hydrocarbons) is thermally decomposed at elevated temperatures and the carbon atoms graphitize on a substrate which is usually transition metals such as copper or nickel. To produce monolayer graphene, copper is used due to low carbon solubility and moderate catalytic activity. The graphene film is then transferred onto a transferring substrate (usually silicone or silicon dioxide) with the use of support polymers (primarily polymethylmethacrylate (PMMA)). Using ferric chloride (FeCl3), the copper substrate is etched to allow the PMMA and the graphene to be placed on a target substrate. Finally, the PMMA supports are removed using acetone [17].The process involves the formation of few-layer graphene from a carbon-rich source material over a catalytic metal substrate at elevated temperatures. The carbon-based source decomposes to release the carbon which is then either absorbed by the metal substrate (such as nickel and cobalt) and then precipitated as graphene or undergoes nucleation and grows graphene on the surface of the metal substrate (such as copper). The governing parameters that affect the graphene produced are as follows [59]:(i)
Concentration and exposure of carbon over the substrate(ii)
Flow rate of the carbon-rich source material(iii)
Cooling rate(iv)
Type of carbon-rich source materialTaking copper as the substrate and methane (CH4) as the carbon-rich source material, CVD synthesis process involves [60]:(i)
Methane decomposes to form CXHY species on the surface of the copper substrate which is in a methane/hydrogen atmosphere (Figures 14(a) and 14(d)).(ii)
As the copper is supersaturated with CXHY, nuclei begin to form (Figures 14(b) and 14(c)).(iii)
These nuclei form zones of graphene on the copper surface (Figure14(e)).(iv)
As those zones grow and join with each other, the copper surface gets covered by graphene while the undecomposed methane and by-products are removed from the chamber (Figure14(f)).An example of the processing parameters and conditions are as follows:(i)
Preannealing for 1 to 2 hours, the copper substrate in a furnace is set at 1000°C that has been backfilled with hydrogen gas (or a mixture of hydrogen and argon) while keeping a hydrogen flow at 50 sccm.(ii)
Methane at a flow of 35 sccm is introduced for a growth time of 5–20 min.(iii)
Then, the furnace is allowed to slowly cool to room temperature with the hydrogen atmosphere [60].Figure 14
Graphene growth mechanism in CVD [58].CVD can produce high-quality graphene films. However, the process is expensive and not readily scalable. There is also a need to improve manufacturing and product parameters which are as follows [61]:(i)
Achieving graphene growth at lower temperatures rapidly.(ii)
Attaining a high degree of uniform crystallinity with better oriented crystalline.(iii)
Developing scalable graphene transfer techniques.In terms of upscaling the graphene film production, two processes have been designed [17]:(i)
Roll to roll (R2R) produces a continuous graphene film (refer to Figure15).(ii)
Batch to batch (B2B) produces graphene wafers.


### 2.2.3. Unzipping Carbon Nanotubes

Carbon nanotubes (CNTs) are split open using various methods (Figure16) to produce graphene nanoribbons:(a)
Intercalation and exfoliation: multiwalled carbon nanotubes (MWCNT) are added with lithium and liquid ammonia. The Li-NH3 complex with dilute hydrochloric acid and heat intercalates and splits the MWCNT walls [62].(b)
Chemical: MWCNT is suspended on concentrated sulfuric acid and treated with potassium permanganate (500 wt.%) for an hour at room temperature followed by another hour at 55–70°C [63].(c)
Catalytic: CNT catalytically unzipped with lead nanoparticles in an oxygen liquid medium under microwave irradiation [64].(d)
Electrical: an MWCNT is contacted by a moving electrode with a current passing through it which ruptures the outer layer of CNT [65].(e)
Physiochemical: MWCNT split using hydrogen plasma reaction at 300°C [66].Figure 15
Roll to roll CVD graphene production [16].Figure 16
Methods of unzipping carbon nanotubes [67].


### 2.2.4. Laser-Induced Graphene (LIG)

Laser irradiation transforms carbonaceous materials such as polyimide (PI) into graphene under ambient conditions. A 4.8 W laser produces graphene of the highest crystallinity, fewer defects, and the largest domain size on a PI. The minimum required energy to start graphene formation on PI was noted to be approximately 5.5 J/cm2. LIG morphology can be controlled through pulses per inch (PPI), lines per inch (LPI), and laser spot size.(i)
1000 PPI × 1000 LPI with 100μm laser spot size: LIG has an in-plane porous structure.(ii)
500 PPI × 500 LPI with 60μm laser spot size: LIG forms as out-of-plane fibers.Figure17 shows the set up used when producing LIG from wood where it is lasered in an inert atmosphere to prevent ablation [68].Figure 17
LIG synthesis on wood [68].


### 2.2.5. Rapid Thermal Annealing

A metal catalyst (nickel, copper, Ni-Cu alloy, or cobalt) is deposited onto a substrate (silicon, silicon carbide, silicon dioxide wafers, or copper films) along with a carbon source which is annealed at elevated temperatures (500–1000°C) under low vacuum as illustrated in Figure18 [69].(1)
Thermal evaporation: nickel is deposited on a SiO2 substrate in a vacuum (10–6 mbar) through evaporation.(2)
Pulsed laser deposition: a laser ablates a carbon source to deposit amorphous carbon on the substrate in a vacuum chamber (10–7 mbar).(3)
Rapid thermal heating: the substrate is heated to an elevated temperature (900°C) for 420 seconds at a 10–2 mbar vacuum to induce graphene growth.(4)
Nickel etching: FeCl3 is used to remove nickel to produce transfer-free graphene.Figure 18
Rapid thermal annealing synthesis route [69].


### 2.2.6. Gas-Phase-Synthesized Graphene (GSG)

A carbonaceous gas (ethanol) is subjected to a microwave inducted argon plasma at atmospheric pressure [70, 71]. The ethanol and argon mixture enter the section of the quartz tube where the plasma electrons absorb the microwaves which transfer the energy to heavier particles through collisions. This results in an electron dense environment (greater than 1013 cm−3) where gas temperatures reach 3000 K [71–74]. The ethanol decomposes into reactive fragments that flow out of the quartz tube to plasma afterglow where it reacts to form GSG (Figure 19) [74].Figure 19
Microwave plasma reactor for GSG production [75].Top-down and bottom-up graphene synthesis methods discussed in Section2 are summarized in the following table (Table 3) for a detailed understanding of the readership.


## 2.2.1. Silicon Carbide Sublimation (Epitaxial Growth)

Layers of graphene can be grown on silicon carbide (SiC) crystal by thermal decomposition at temperatures greater than 1000°C under an ultrahigh vacuum. As the SiC crystal is heated, it decomposes to its constituents silicon and carbon. The silicon vaporizes off from the crystal surface while the carbon atoms form graphene layers. The graphene grown on the Si-terminated surface of SiC are of high quality, ordered crystalline films, with minimal wrinkles [1]. The major drawbacks of this method are the elevated temperatures required to decompose SiC and the limited size of graphene films due to the excessive cost of SiC wafers. This has limited the use of epitaxially grown graphene for selected electronics such as high-frequency transistors [56].Hexogonal SiC crystals used for graphene synthesis have are made of Si-terminated (0001) surface and C-terminated (0001¯) surface as shown in Figure 11. The process begins with the sublimation of silicon atoms along the step edge. This is because the silicon atoms at the step edge have more dangling bonds compared to the silicon atoms found at other surface zones or within the crystal. The carbon atoms that stay on the edge form a graphene nucleus continues to grow laterally towards the upper surface of the step edge (Figures 12(b)–12(d)). The same graphene growth from the lower surface joins with the graphene produced at the step (Figures 12(e) and 12(f)). The first carbon layer is a buffer layer which is then followed by a layer of graphene (Figures 12(h) and 12(g)) [57].Figure 11
SiC crystal with reference to (0001) surface and (0001¯) surface [57].Figure 12
Graphene growth on silicon carbide (0001) surface [57].The difference between the graphene growth on the silicon-face (0001) and the carbon-face (0001¯) is that the graphene layers formed on the (0001¯) surface are stacked in a disorderly manner (turbostratic), while on the (0001) surface, the graphene showed ABC stacking. Graphene from carbon-face (0001¯) SiC also has high charge-carrier mobility, weaker coupling to the SiC surface and greater defects (Figure 13(f)) than graphene from silicon-face (0001) [57].Figure 13
Graphene growth on silicon carbide (0001¯) surface [57].


## 2.2.2. Chemical Vapor Deposition

A carbon-rich source (hydrocarbons) is thermally decomposed at elevated temperatures and the carbon atoms graphitize on a substrate which is usually transition metals such as copper or nickel. To produce monolayer graphene, copper is used due to low carbon solubility and moderate catalytic activity. The graphene film is then transferred onto a transferring substrate (usually silicone or silicon dioxide) with the use of support polymers (primarily polymethylmethacrylate (PMMA)). Using ferric chloride (FeCl3), the copper substrate is etched to allow the PMMA and the graphene to be placed on a target substrate. Finally, the PMMA supports are removed using acetone [17].The process involves the formation of few-layer graphene from a carbon-rich source material over a catalytic metal substrate at elevated temperatures. The carbon-based source decomposes to release the carbon which is then either absorbed by the metal substrate (such as nickel and cobalt) and then precipitated as graphene or undergoes nucleation and grows graphene on the surface of the metal substrate (such as copper). The governing parameters that affect the graphene produced are as follows [59]:(i)
Concentration and exposure of carbon over the substrate(ii)
Flow rate of the carbon-rich source material(iii)
Cooling rate(iv)
Type of carbon-rich source materialTaking copper as the substrate and methane (CH4) as the carbon-rich source material, CVD synthesis process involves [60]:(i)
Methane decomposes to form CXHY species on the surface of the copper substrate which is in a methane/hydrogen atmosphere (Figures 14(a) and 14(d)).(ii)
As the copper is supersaturated with CXHY, nuclei begin to form (Figures 14(b) and 14(c)).(iii)
These nuclei form zones of graphene on the copper surface (Figure14(e)).(iv)
As those zones grow and join with each other, the copper surface gets covered by graphene while the undecomposed methane and by-products are removed from the chamber (Figure14(f)).An example of the processing parameters and conditions are as follows:(i)
Preannealing for 1 to 2 hours, the copper substrate in a furnace is set at 1000°C that has been backfilled with hydrogen gas (or a mixture of hydrogen and argon) while keeping a hydrogen flow at 50 sccm.(ii)
Methane at a flow of 35 sccm is introduced for a growth time of 5–20 min.(iii)
Then, the furnace is allowed to slowly cool to room temperature with the hydrogen atmosphere [60].Figure 14
Graphene growth mechanism in CVD [58].CVD can produce high-quality graphene films. However, the process is expensive and not readily scalable. There is also a need to improve manufacturing and product parameters which are as follows [61]:(i)
Achieving graphene growth at lower temperatures rapidly.(ii)
Attaining a high degree of uniform crystallinity with better oriented crystalline.(iii)
Developing scalable graphene transfer techniques.In terms of upscaling the graphene film production, two processes have been designed [17]:(i)
Roll to roll (R2R) produces a continuous graphene film (refer to Figure15).(ii)
Batch to batch (B2B) produces graphene wafers.


## 2.2.3. Unzipping Carbon Nanotubes

Carbon nanotubes (CNTs) are split open using various methods (Figure16) to produce graphene nanoribbons:(a)
Intercalation and exfoliation: multiwalled carbon nanotubes (MWCNT) are added with lithium and liquid ammonia. The Li-NH3 complex with dilute hydrochloric acid and heat intercalates and splits the MWCNT walls [62].(b)
Chemical: MWCNT is suspended on concentrated sulfuric acid and treated with potassium permanganate (500 wt.%) for an hour at room temperature followed by another hour at 55–70°C [63].(c)
Catalytic: CNT catalytically unzipped with lead nanoparticles in an oxygen liquid medium under microwave irradiation [64].(d)
Electrical: an MWCNT is contacted by a moving electrode with a current passing through it which ruptures the outer layer of CNT [65].(e)
Physiochemical: MWCNT split using hydrogen plasma reaction at 300°C [66].Figure 15
Roll to roll CVD graphene production [16].Figure 16
Methods of unzipping carbon nanotubes [67].


## 2.2.4. Laser-Induced Graphene (LIG)

Laser irradiation transforms carbonaceous materials such as polyimide (PI) into graphene under ambient conditions. A 4.8 W laser produces graphene of the highest crystallinity, fewer defects, and the largest domain size on a PI. The minimum required energy to start graphene formation on PI was noted to be approximately 5.5 J/cm2. LIG morphology can be controlled through pulses per inch (PPI), lines per inch (LPI), and laser spot size.(i)
1000 PPI × 1000 LPI with 100μm laser spot size: LIG has an in-plane porous structure.(ii)
500 PPI × 500 LPI with 60μm laser spot size: LIG forms as out-of-plane fibers.Figure17 shows the set up used when producing LIG from wood where it is lasered in an inert atmosphere to prevent ablation [68].Figure 17
LIG synthesis on wood [68].


## 2.2.5. Rapid Thermal Annealing

A metal catalyst (nickel, copper, Ni-Cu alloy, or cobalt) is deposited onto a substrate (silicon, silicon carbide, silicon dioxide wafers, or copper films) along with a carbon source which is annealed at elevated temperatures (500–1000°C) under low vacuum as illustrated in Figure18 [69].(1)
Thermal evaporation: nickel is deposited on a SiO2 substrate in a vacuum (10–6 mbar) through evaporation.(2)
Pulsed laser deposition: a laser ablates a carbon source to deposit amorphous carbon on the substrate in a vacuum chamber (10–7 mbar).(3)
Rapid thermal heating: the substrate is heated to an elevated temperature (900°C) for 420 seconds at a 10–2 mbar vacuum to induce graphene growth.(4)
Nickel etching: FeCl3 is used to remove nickel to produce transfer-free graphene.Figure 18
Rapid thermal annealing synthesis route [69].


## 2.2.6. Gas-Phase-Synthesized Graphene (GSG)

A carbonaceous gas (ethanol) is subjected to a microwave inducted argon plasma at atmospheric pressure [70, 71]. The ethanol and argon mixture enter the section of the quartz tube where the plasma electrons absorb the microwaves which transfer the energy to heavier particles through collisions. This results in an electron dense environment (greater than 1013 cm−3) where gas temperatures reach 3000 K [71–74]. The ethanol decomposes into reactive fragments that flow out of the quartz tube to plasma afterglow where it reacts to form GSG (Figure 19) [74].Figure 19
Microwave plasma reactor for GSG production [75].Top-down and bottom-up graphene synthesis methods discussed in Section2 are summarized in the following table (Table 3) for a detailed understanding of the readership.


## 3. Turbostratic Graphene Produced through Flash Joule Heating (FJH)

This bottom-up approach converts carbonaceous material to turbostratic graphene using high current electric pulse discharges using a capacitor bank. The carbonaceous material is compacted in a quartz tube between two electrodes which are connected to a capacitor bank circuit as shown in Figure20.Figure 20
Electrical circuitry for FJH graphene [76].An AC-DC converter supplies a DC voltage to charge the capacitor bank. The lamp connected parallel to the capacitor bank shows when the capacitors are fully charged. An insulated-gate bipolar transistor (stated as a power switch in Figure19) with a custom LabView program is used to control the time of discharge. The flash chamber is where the carbonaceous material is compressed between electrodes in a desiccator under a slight vacuum to aid in the expulsion of heteroatoms and hydrogen through the clearance between the electrodes and the quartz tube [76].Amorphous carbon black powder is compressed between electrodes in a quartz tube and subjected to a slight vacuum of −10 mm Hg to aid in the sublimation of heteroatoms and hydrogen (however, the process is possible at atmosphere pressure). While in flash joule heating, the content in the quartz tube must reach a minimum temperature of 3000 K to ensure effective conversion of the carbon black into high-quality turbostratic graphene. This was achieved through utilizing a 60-mF capacitor bank discharging 110–120 V for a flash duration of 30–100 ms (with a discharge time of 50 ms) when working with 0.03 g of carbon black. The quality of the turbostratic graphene is clear in the Raman spectra in Figure20 [76, 77]:(i)
At capacitor voltages less than 90 V, the flash graphene produced had significant structural defects shown by the high D peaks in the Raman spectra.(ii)
Optimal capacitor voltage is shown to be 110 V with a higher I2D/G and low ID/G (indicative of low defect concentration).(iii)
The graph shows the ideal temperature and flash duration, 3100 K and 10 ms, respectively, when the 2D peak is the highest with f and g, indicative of the fact from the Raman spectra (Figure21).When the compression of the carbon source is increased between the electrodes, its conductivity increases, resulting in resistance variance of carbon source. Figure21(e) shows the effect that the resistance of starting material of the sample has on the quality of FJH graphene.Figure 21
(a) Raman spectra of carbon black after being processed using FJH at the different initial voltage at which the capacitor bank is discharged. (b) Ratio of 2D and D peaks to G peak. Ideally, FJH graphene should have low D/G ratio (shows low defect concentration) and high 2D/G ratio (indicates turbostratic stacking). (c and f) Temperature vs time graph of FJH graphene production with Raman spectra. (d and g) Temperature vs flash duration with cooling rates and the Raman spectrum of the resulting graphene. (e) Raman spectra of FJH graphene obtained from carbon black under varying compressions [76, 77].
(a)(b)(c)(d)(e)(f)(g)


## 4. Importance of Flash Joule Heating Method

Implementing the flash joule heating process to convert solid waste to flash graphene on an industrial scale has the potential to reduce the need for landfills. Compared to current solid waste treatment processes where the product is disposed of (as with ash and char from pyrolysis), flash graphene from solid waste can be used in the construction of concrete buildings and strengthening composites. Carbon residue from gasification and pyrolysis can also be subjected to flash joule heating (refer to Table4). The FJH process may also help with managing hazardous biomedical waste and replace existing methods of treatment, such as biochemical conversion, which are time intensive and requires a large ecological footprint, or incineration, which results in the production of carcinogenic compounds and cause pollution. Similarly, agricultural waste and used plastics can be upcycled into flash graphene without the need for the prewashing process [83]. Urban mining and removal of heavy metals from electronic waste have also been achieved using the flash joule heating process [84].Table 4
Summary of flash joule heating parameters for high-quality turbostratic graphene [76, 77, 87–89].
Starting materialStarting material preparationMass (grams)Resistance (Ω)Atmospheric conditionCapacitance (mF)PretreatmentFinal discharge voltage (VDC)Pulse duration (ms)Carbon black—0.031.5Atmospheric pressure or in a slight vacuum (10 mm Hg)60N/A11050Rubber tireRubber tire is shredded and mixed with carbon black (95% and 5%, respectively)0.5Less than one hundred60—140–150500Tire is pyrolyzed to produce carbon black—PlasticPolypropylenePyrolysis at 450°C0.41560The pyrolysis ash is subjected to 50 V, 70 V and 90 V for 450 ms each160450Mixture of plastic wastePlastic waste is grinded and mixed with 5% carbon black0.1160The plastic waste mixture is subjected to 120 VAC, 60 Hz for 8 seconds in a vacuum desiccator110500The FJH process can also be used to produce alternate fluorinated carbon phases (fluorinated amorphous carbon, fluorinated graphene, fluorinated nanodiamonds, and fluorinated concentric carbon moieties) [85] as well as 2D transition metals dichalcogenides, MoS2, and WS2, which have application in solar cells, transistors, photocatalysts, and spintronics [86].


## 5. Environmental Concerns due to Graphene Synthesis

Mass-produced graphene dispersions are commonly produced via chemical exfoliation, either using Hummer’s method or a modification of it since it is cost-effective for bulk graphene synthesis [90]. However, the process requires the use of harsh ecotoxic oxidants and acids. The use of these acids and oxidants for graphite oxidizing makes wastewater treatment a challenge [91].When comparing water consumption between oxidation-reduction exfoliation and ultrasonic exfoliation, chemical reduction uses approximately 170% more water with 70% of water use coming from H2SO4 production and 14% needed for cleaning the reduced graphene oxide for total usage of approximately 13000 liters per kilogram of graphene. The major water consuming processes in the ultrasonic exfoliation are the production of ethylene (57%) and diethyl ether (41%) for total usage of approximately 4800 liters per kilogram of graphene [92].Furthermore, the ecotoxic potential of the two wet exfoliation synthesis methods is similar. The total ecotoxic potential in chemical reduction is 1.7 CTUe/kg with the major contributor being vanadium emissions when hydrazine is produced (95%). The ultrasonic exfoliation has a total of 1.9 CTUe/kg with ethylene production being the major contributor because of copper constituent emissions (74%) [92].Moreover, certain oxidation-reduction exfoliation methods produce hazardous gases, namely, dinitrogen tetroxide and nitrogen dioxide [93, 94]. Also, there is a production of explosive gases, namely, acid fog and chlorine dioxide due to the use of nitric acid [95]. Additionally, the use of toxic chemicals such as aluminium hydride, hydrazine, and borohydrides as reducing agents indicates that cost and safety considerations must be accounted for when moving toward industrial-scale synthesis using wet exfoliation techniques [33].Obtaining graphite for top-down synthesis methods adds to environmental devastation. Graphite extraction requires large-scale milling, crushing, and flotation to segregate flakes from rocks. The graphite flakes are then subjected to acid leaching to remove minerals lodged in the layers. For graphite required for battery-grade anodes, it is further processed using hydrofluoric acid and sodium hydroxide. The culmination of acid leaching and processing graphite for battery-grade anodes involves chemicals that are harmful to the environment [96].Bottom-up synthesis methods, on the other hand, require greater energy input to enable the synthesis of graphene from various carbon precursors. While the chemical and thermal reduction methods consume approximately 0.008 and 0.0011 MJ per gram, chemical vapour deposition requires approximately 257 MJ per gram. CVD has also been studied to be the most environmentally impactful method of synthesis at a commercial scale compared to chemical and thermal reduction methods. When CVD is compared to the chemical reduction of oxidized graphite, CVD has [97](i)
7 × 106% greater global warming potential.(ii)
5.5 × 106% greater photochemical ozone formation.(iii)
35.2 × 106% greater ecotoxicity potential.As with CVD, epitaxial growth of graphene on SiC substrate requires a copious amount of energy. While CVD energy requirements range from 7 kJ/cm2 to 160 kJ/cm2, epitaxial graphene requires a minimum of 2 MJ/cm2 [98]. Additionally, certain modified versions of epitaxial growth, such as an exothermic reaction of PTFE with SiC for high-quality graphene with an oxygen content of less than 1 wt.% has environmental concerns regarding gases produced from the reaction, specifically tetrafluoroethylene [99].


## 6. Conclusions

The challenges in the mass production of graphene hinder the implementation of its physicochemical properties in various applications. Chemical vapor deposition, liquid-phase exfoliation, and chemical exfoliation have shown potential for large-scale production, however, the time intensiveness, environmental risks, and inconsistency in the final products need to be overcome to successfully commercialize graphene and graphene-based products.Regarding flash graphene, the FJH process presents a graphene synthesis route that is faster than other synthesis methods as well as economically lucrative for upscaling. Focusing research on optimizing the process parameters for obtaining flash graphene from various carbonaceous materials and reliably upscaling FJH increases the potential environmental benefits with the use of waste plastics, wood, and oil as starting materials.Furthermore, as discussed, the environmental cost must be carefully analyzed and then remedies should be presented and applied to conserve the environment. Therefore, this review paper will be summarizing the detailed approach of graphene extraction techniques, recent advances in technologies, and the environmental cost.



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*Source: 8475504-2023-02-02.xml*