Progress in Graphene Synthesis and its Application: History, Challenge and the Future Outlook for Research and Industry

One of the main concerns for today is the availability of renewable and clean energy. In response, scientists have made great efforts on seeking and designing materials that have the right properties for energy storage technology. Energy storage technology is found in solar cell, fuel cell, batteries, and supercapacitors. Useful properties of graphene such as high mechanical flexibility, high specific surface area, ultra-thinness, good electrical conductivity, and high theoretical capacitance can be used for energy storage technology.^38–40 Graphene have been used and thoroughly researched for lithium ion batteries, flexible or micro-supercapacitors, lithium air batteries, lithium-sulfur batteries, electrode for fuel cell and solar cell. On this basis, ultra-high specific surface area of graphene is needed for large ion storage in electric double layer capacitors, whereas functionalized graphene is needed for anchoring other active species in batteries. Highly flexible and conductive graphene-based membranes may also be used as either interlayers or current collectors in lithium-sulfur batteries. Graphene with a macro porous structure is substantial for catalytic growth/decomposition and accommodation of lithium batteries.^40–44

Environmental protection has also become another major issue during this decade. Environmental issues should receive more attention in order to maintain the sustainability of the planet Earth. Strategies for pollution treatments have received more interest to be researched. Zhao et al., (2012) reported that graphene is a good sorbent, as well as being able to be recycled.³⁸ It was proven that graphene can adsorb liquids up to 600 times heavier than its own weight. Graphene can also perform exceptionally well when adsorbing gasoline until it reaches an adsorption of 2.77 × 10² gg⁻¹. Other substances that graphene could adsorb include ethanol, olive oil, nitrobenzene, acetone, and dimethyl sulfoxide. Graphene was also investigated for desalination technology.^45–49 It should be noted that an essential property of graphene when it comes to environmental treatment and technology is its surface area.

Academicians and scientists have been investigating the feasibility of implementing graphene in biomedical industry.⁵⁰ It was reported that several outstanding properties of graphene, such as its high opacity, high chemical reaction, and unparalleled thermal conductivity, are suitable for biomedical purposes. The great functional groups of graphene, such as graphene oxide and N-graphene, are being preferred for biomedical application. These functional groups produce high and effective results.⁵⁰ As presented in Fig. 1, graphene is already used in several biomedical applications, such as diagnostic technology,^51–55 drug delivery,^56–60 therapy technology,^57,61,62 and scaffolding materials.^63–67

A strong reason for using graphene in biomedical purposes is due to its consistency and its ability to develop a uniform structure. The suitability of graphene when it comes to biomedical application depends on its shape, size, morphology, thickness and degree of oxidation.⁵⁰ Another suitable property of graphene is its low toxicity, proven by its ability to remain stable for a period of long time in metabolic pathways and during cellular intake. However, further observations are needed on the in vivo process with graphene, especially for drug delivery application.^58,68,69 For commercialization purposes, biomedical industries should give more attention towards the aforementioned points. Hence for this reason, prior to commercialization, research is needed to be executed which may latterly be used as a basis and evidence to show the benefits of graphene towards major biomedical industries.

Graphene is widely used as materials engineering due to its highly appealing properties since its first appearance in 2004.⁷⁰ This material can be used as one of active material for Li-ion battery (LIB) anode and electrochemical double-layer capacitor (EDLC) electrode due to its presents a Li⁺ storage capacity of 744 mAh.g⁻¹ and electric double-layer capacitance of 550 F.g⁻¹.⁷¹ Han et al. in 2020 also reported the polymer composite with vertically graphene architecture.⁷² This material is a promising candidate for thermal interface materials due to its thermal conductivity reached 2.18 W·m⁻¹·K⁻¹.

Graphene and its derivatives also can be used as sensor and bio-sensor materials. Bai et al. made a new sensor using reduced graphene oxide (rGO) with combination by polyoxometalates-doped Au nanoparticles for sensing uric acid in urine.⁵² The sensor has a low detection limit for uric acid determination, i.e. 8.0 × 10⁻⁸ M. Graphene-based bio-sensor also developed for early detection of Zika virus infection.⁵³ The bio-sensor response for Zika virus is excellent in buffer condition at concentrations as low as 450 pM. Potential diagnostic applications were applied by measuring the Zika virus in a human serum. Another sensor based on graphene also fabricated by Khalifa et al. in 2020.⁷³ They made a smart paper from graphene coated cellulose for high-performance humidity and piezoresistive force sensor. The sensor has high piezoresistive response i.e. between 125%–250%. They stated that cellulose paper with low cost, lightweight and biocompatibility combined with graphene could be a promising material for smart, wearable electronic devices. In the other side, graphene is also good material for semiconductor^74,75 and electronic device.^76,77 Graphene also can be formed as flexible and transparent materials^77,78 for various application. The material easy to change become an ink with high conductivity^79–83 for injecting printing purposes. Based on described examples, the presence of graphene in the recent times could serve as a future material for various fields especially as sensor, bio-sensor, and heat-sink.

Exfoliation is a simple and common technique that can be used for graphene synthesis from graphite or other carbon sources. There are several types of exfoliation such as mechanical exfoliation, chemical exfoliation and/or electrochemical exfoliation. Mechanical exfoliation or more commonly known as the Scotch tape method, is a traditional method that has been applied for decades.^1,21,89 This method is made famous by K. Novoselov and Andre Geim since they both won the Nobel Prize in Physics due to the results obtained by implementing this method. Mechanical exfoliation (Fig. 5a) is the first method to obtain one layer of graphene.^21,89,127 Examples of mechanical peels are micromechanical peels as shown in Table II. Dasari et al. in 2017 showed a representation of micromechanical stripping of graphene sheets using Scotch tape method.⁴¹ Graphite was placed on the substrate and repeatedly peeled using adhesive tape until a monolayer sheet is obtained. Although this process is simple but the main challenges with this method is that the product that obtained is small and contains some structural defects.⁴¹

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Chemical peel is an effective way to produce large amounts of graphene. However, this method has several disadvantages, such as it involves complex chemical processes and it also generally produces sheet-shaped graphene that has low conductivity.^147–149 Liquid phase exfoliation (LPE) is a new top-down method that only involves flaking natural graphite through high-shear mixing or sonication.^21,128,150 Illustration of liquid phase exfoliation is shown in Fig. 5b. Until now, there are two different graphite peeling techniques using LPE, and these are cavitation in sonication and high-shear mixing. In practice, the LPE operating conditions are very mild and do not need a vacuum or high temperature system. For large scale applications, the high-shear mixing method or microfluidizer is more recommended than the sonification assisted LPE method.^100,151 This is due to the low graphene product and high energy consumption for process with LPE sonication method. Meanwhile, high-shear mixing or microfluidizer method can exfoliate graphite better than the LPE sonication method.^100,151,152

The electrochemical method is carried out using a minimum of four main components, and these are anode, cathode, electrolyte, and power supply (Fig. 6a). Anode, being a source of carbon, will be oxidized and exfoliated to produce graphene. The cathode can be varied by using either a graphite (Figs. 6b and 6c) or a platinum (Fig. 6d). The experiment is usually arranged as shown in Fig. 6. The anode and cathode are immersed into the electrolyte at a certain distance. Positive or negative voltage is applied to the anode depending on the desired peeling mechanism.^129,153,154 The choice of anode, cathode and electrolyte solution used is a crucial factor for the electrochemical process as it can affect the graphene product obtained.

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Liu et al. used a pencil graphite as a source of graphite, which subsequently used it as anode and cathode (Fig. 6b).¹⁵⁵ The anode and cathode are immersed in 1 M H₃PO₄ with an applied potential between +7 V and −7 V. The product obtained is not homogeneous, and the thickness and size distribution were quite wide. Parvez et al. in 2013 used graphite as anode and platinum (Pt) as cathode.¹⁵⁶ The electrodes were immersed in sulfuric acid (electrolyte solution) with potential of +10 V for 10 min. The yield of this process was about 60% and the product obtained has 1–3 layers.

Based on the results of the above research, sulfuric acid is an electrolyte suitable for electrochemical intercalation and graphite exfoliation process. The size of the sulfate ions (0.46 nm) which is similar to the graphite interlayer distance (0.335 nm) may contribute to the intercalation process. In addition, electrolysis of sulfate ions and co-intercalated water causes the formation of gases such as SO₂, O₂ and H₂.^157,158

The use of acid electrolyte caused graphite to be flaking during the intercalation process. This process produce graphene with several layers while the expected product was one layer.^{15,130,159,160,161} Parvez et al., (2014) proposed a general concept for electrochemical stripping of graphite in inorganic salts as shown in Fig. 7.¹⁶² Graphite electrodes were immersed in ammonium sulfate solution [(NH₄)₂SO₄, H₂O] and applied potential was set to +10 V (Fig. 7a). At this potential, water reduction occurs at the cathode and produces hydroxyl (OH–) ions as strong nucleophiles. Nucleophiles attack the edges and boundaries of the graphite grains, resulting in oxidation reactions and intercalation of sulfate ions in the graphite layer (Fig. 7b). Oxidation reactions induce the depolarization and expansion of the graphite layer (Fig. 7c). During this process, the water molecules act as a co-intercalation with SO₄²⁻ ions. Oxidized water molecules and SO₄²⁻ ion reduction produce gas species (SO₂, O₂, etc.). Gas species have enough energy to separate the graphite layer and produce layered graphene (Fig. 7d). The electrolyte concentration at the applied potential also affects the graphite exfoliation process. Parvez et al., reported that the lowest concentration of electrolyte ((NH₄)₂SO₂) for the graphite intercalation process was less than 0.01 M to produce 5 wt% graphene.¹⁵⁶ When the concentration was increased in the range of 0.01 to 1.0 M, graphene products will increase to more than 75% by weight. This phenomenon is in accordance with the general mechanism of electrochemical exfoliation of graphite in inorganic salts.^156,162

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Graphene can also be synthesized using electrochemical methods in combination with a sonication method.^163–167 This combination method can eliminate the use of high temperatures, high pressures, and tiring work steps in the synthesis process.^{14,129,167–169} Bai et al., (2017) studied the effect of sonication on disperse graphene stability.⁵⁶ Three types of sonication were used, which are bath (200 W, 35 kHz), horn (50 W, 20 kHz) and high-power micro tip (1000 W, 20 kHz). The result shows that horn sonication give the most stable dispersion of the graphene product which is can last up to two years. Meanwhile, the micro-tip sonication requires a higher power intensity that exceeds the optimal energy required, hence this type of sonication fails to produce stable graphene dispersion. Several studies indicate that all sonication techniques are able to produce reduced graphene oxide (RGO).^{56,165,166,170,171}

Bakhshandeh and Shafiekhani, (2018) reported the effects of ultrasonic waves on the graphene properties synthesized by using electrochemical methods.¹⁶⁹ It was found that the ultrasonic waves had a direct effect on graphene production. Ultrasonic waves have the ability to homogenize electrolyte solutions and reduce oxygen groups in graphene structures. The ratio of oxygen groups reduction is proportional to the decrease in defects of the graphene structure. The effect of temperature (25 °C–95 °C) in combination of electrochemical-sonication method for producing graphene also investigated by Hossain and Wang.³³ Characterization using Raman spectroscopy shows the smallest defects in graphene products with increasing temperature. According to the results, in order to obtain the minimum defects, scientists and industries use a high temperature and controlled ultrasonic wave during the synthesis process by electrochemical-sonication combination method.

Laser ablation is a new method to synthesis nanomaterial, especially graphene. This method has promising advantages, including being environmentally friendly, easy experimental settings (which do not require extreme conditions), long-lasting nanoparticle stability, a free unwanted contaminant in nanoparticle products and avoiding the use of harmful synthesis reactants.^172–174 Cappeli et al. in 2015 used the laser ablation to synthesis graphene. Their research was done on silicon (Si) substrates with variations in temperature (from room temperature to 900 °C) using laser Nd:YAG laser operating in the near IR (λ = 523 nm, repetition rate (ν) = 10 Hz, pulse width (τ) = 7 ns, fluence (φ) ∼ 7 J cm⁻², deposition time = 15 min).¹⁷⁵ After that, the developing method is carried out with a variety of conditions in order to obtain a high-quality graphene.^35–37,176

Laser ablation techniques require solid carbon/graphite as a carbon source in order for the laser source to erode the carbon surface and produce graphene.^103,177,178 Several laser parameters must be controlled during the synthesis process of graphene.¹⁰³ This parameter can be adjusted according to the laser ablation system which can affect the product quality. The first parameter that must be controlled is the laser itself, such as fluence laser, wavelength, repetition rate, and pulse duration. The second parameter controlled is the gas background, pressure background, the distance of substrate, substrate temperature. The selection of substrate also influences the product. Koh et al. conducted a feasibility test of various metals as a substrate for graphene synthesis by using the laser ablation method. The metals studied were nickel (Ni), copper (Cu), cobalt (Co) and iron (Fe).¹⁷⁷ The results showed that graphene products have a lattice constant of 0.357; 0.352; 0.361; 0.251 and 0.287 nm when using Ni, Cu, Co and Fe substrates respectively.¹⁷⁹ Hermani et al. obtained a high-quality bilayer graphene using Ni/SiO₂ substrate.¹⁷⁸ This data proved by Raman spectroscopy which shows an improvement of 60% in the results compared to when other substrates are used. The effect of pulses on various substrates has been investigated by Pechlivani et al. in 2017.¹⁸⁰ Ultra-short pulse lasers were used with picosecond lasers for the corresponding wavelengths and pulse energy. The results of this study proved that ultra-short pulse laser technology can be the next generation technology in promoting micro graphene as a valuable material in manufacturing sector. Ultra-short laser ablation was also applied by De Bonis et al. and produced a good graphene product.¹⁸¹ Generally, the mechanism of graphene synthesis using laser ablation is described as Fig. 8. The laser source directly hits the carbon/graphite solid and breaks down its structure so that it is released as graphene.

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CVD is one of the most useful methods for synthesizing structural mono or few layer graphene.^9,182 A graphene with a large area can be obtained by using CVD by exposing the precursors at a high temperature. The general CVD instrument consists of tube furnace, gas flow, tail gas treatment and substrate as shown in Fig. 9a. Substrate is a significant material in order for the graphene side to be able to be synthesized. The commonly used substrate is either nickel (Ni) or copper (Cu). Mechanism of the graphene synthesis using CVD method depends on the substrate used (Fig. 9b).^{26,41,183,184} When using Ni substrate, carbon is used as a raw material to produce graphene as it dissolves in Ni at high temperature. The dissolved carbon is separated and precipitated to obtain graphene during the appropriate cooling rate. In contrast, the mechanism of graphene synthesis in Cu substrate by CVD will occur when carbon deposits directly on the surface of Cu substrate. The C/H ratio, substrate quality, temperature, pressure, and oxygen on the substrate surface also affects the process of graphene synthesis when using the CVD method. The optimization process of the size and quality of graphene product by CVD method is usually very difficult due to the numerous interdependent parameters in this method. Papon et al. in 2017 applied a method namely "designs of experiments" to estimate the relative importance and value of the parameters.¹⁸⁵ This design explains interaction of several independent parameters in producing graphene on Cu substrate. The result showed that temperature, time, rate of heating and pre-annealing time of the substrate influence significantly in quality of graphene product. Particularly, there are two main factors for the size of graphene product, i.e., graphene growth time and the increasing rate of the carbon source temperature. It is mean that the researcher just needs modify those factors to change the size dramatically. Furthermore, Liu and Liu in 2017 also stated that a good controlling in synthesis parameters can produce large-area, high quality graphene and large-sized graphene single crystal with different shapes and layers.¹⁸⁶

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Overall, the CVD method is still one of the successful methods for producing large-scale graphene. The graphene produced using the CVD method is 1.5 times better compared to other methods such as Scotch tape, thermal decomposition, graphene oxide reduction, liquid exfoliation, and even bottom-up method. It is because the graphene single crystal can be produced within 2.5 h when using the CVD method. The graphene single crystal product shows Hall's mobility of 10,000–20,000 cm² V ¹ S¹ at room temperature.^41,104 This property is compatible to be applied in semiconductor technology, solar cells, and transparent conductive films (TCFs).¹⁸⁶

Arc discharge is a relatively cost-effective and environmentally friendly method for graphene synthesis.^{86,123,147,143} Graphene can be produced using the arc discharge method under hydrogen (H₂), helium (He), or nitrogen (N₂) conditions.^187,188 Wu et al. (2010) have developed the arc discharge method to produce few-layered graphene under conditions of He and carbon dioxide (CO₂) mixture.¹⁴⁷ The result shows the obtained graphene has fewer defects than graphene produced by chemical methods. The obtained graphene is also able to disperse easily in organic solvents for further applications. The combination of He and CO₂ conditions during the arc discharge process is capable of producing a good graphene for making electrodes in various devices. Another advantage of the graphene produced by using such method is it makes it as a suitable choice for an electric charger used for conducting composite materials.

Kim et al. in 2016 reported a controllable and scalable aqueous arc discharge process that produces high quality bi- and trilayers of graphene.¹⁴⁴ However, they still found by products when using this method, hence a separation method needs to be developed. Development of the arc discharge method to produce graphene was also studied by Cheng et al. in 2018,²³ where they combined a vacuum arc discharge by using CVD method. Graphene was synthesized in a copper foil by using a furnace at a high temperature embedded in a vacuum arc discharge. This merging method can produce a single layer graphene at a high temperature.

Wu et al. explained the mechanism of the arc discharge method to obtain graphene sheets in different atmospheres for large-scale graphene production (Fig. 10).¹⁸⁹ Graphene sheets were synthesized using activated carbon as an anode and cathode by arc discharge method under a mixed gases conditions where in this case, nitrogen (N₂) and hydrogen (H₂) gases were used. The alternating current in the process causes both electrodes to react and evaporate simultaneously, thus eliminating the formation of deposits at the cathode.^143,187,189 This process increases the temperature, which is needed to increase the diffusion rate of carbon atoms and clusters. The increasing of diffusion rate allows all carbon species and gas molecules to collide between each other. Graphene product can easily be obtained only if hydrogen gas is used since hydrogen gas has a very high cooling rate. To obtain such conditions, Wu et al. combined the hydrogen gas with inert gas such as N_2, which has low thermal conductivity, in order to generate a graphene product with satisfactory quality.¹⁸⁹

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According to the explanation above, the advantages and disadvantages of all methods mentioned above (top-down and bottom-up) are summarized in Table II.