A Mini-Review on Graphene: Exploration of Synthesis Methods and Multifaceted Properties ^†

Abstract

Graphene, a single layer of carbon atoms arranged in a two-dimensional lattice, has emerged as a material of immense scientific and technological interest. This review article provides a comprehensive overview of the various synthesis techniques for graphene, including chemical vapor deposition (CVD), epitaxial growth on SiC, mechanical cleavage, and exfoliation of graphite oxide. The article further delves into the distinctive electronic, mechanical, optical, and thermal properties of graphene that make it a promising material for numerous applications. From high electrical conductivity to remarkable strength and unique optical characteristics, graphene’s attributes are explored in detail. The thermal stability of graphene, its interaction with different substrates, and potential applications in electronic devices are also discussed. The review concludes with a summary of the current state of research and prospects for future exploration, emphasizing graphene’s potential to revolutionize various industrial sectors.

1. Introduction

Graphene is a two-dimensional material from the carbon family, consisting of a single layer of graphite with a hexagonal arrangement of sp2-bonded carbon atoms. The structure of graphene is shown in Figure 1. It was discovered through the mechanical exfoliation of graphite in 2004 by Geim and Novoselov [1,2]. This breakthrough has attracted interest across various scientific disciplines, including physics, chemistry, engineering, and biology [3,4,5]. The growing attention towards graphene can be attributed to its remarkable properties. It has a large theoretical specific surface area (2630 m² g⁻¹) [6], good flexibility, high breaking strength (42 N m⁻¹), exceptional optical transparency (97.7%) [7], efficient thermal conductivity (3000–5000 Wm⁻¹ K⁻¹) [8], a high transition rate at room temperature (10,000 cm² V⁻¹ s⁻¹) [9], and high mechanical hardness (Young’s modulus of ≈1.0 TPa) [10]. These unique properties make graphene a promising candidate for various applications, including electronic devices [11], biosensors [12], energy storage (supercapacitors) [13], fuel cells [14], transparent conductive films [15], solar cells [16], catalysts [17], batteries [18], drug delivery [19], composite materials [20], heavy metal removal [21], and dye removal [22]. While pure graphene is hydrophobic and not suitable for water treatment, the modification methods that add functional groups, such as carboxyl, epoxy, ketone, and alcohol, to graphene oxide and reduced graphene oxide have substantially increased its applicability [23,24].

2. Synthesis of Graphene

Since the discovery of graphene in 2004, extensive efforts have been directed towards producing high-quality graphene. The synthesis of graphene can be classified into two main approaches: bottom-up and top-down, in line with the general classification of nanomaterial synthesis methods [25].

2.1. First Approach: Bottom-Up Methods

Bottom-up methods involve the direct synthesis of graphene from a carbon source, acting as the initiator of the reaction. These include:

• Chemical vapor deposition (CVD) [26]
• Epitaxial growth on SiC
• Solvothermal reaction
• Organic synthesis [27]

These methods require large amounts of carbon to produce high-quality graphene, and, thus, necessitate high temperatures. Their main disadvantages are the associated high costs and limited scalability over a broader range [28].

2.2. Second Approach: Top-Down Methods

Top-down methods involve synthesizing graphene from a graphite source as the reaction initiator. These include:

• Mechanical cleavage (adhesive tape) [29]
• Liquid-phase exfoliation [30]
• Electrochemical exfoliation of graphite [31]
• Solvothermal synthesis combined with pyrolysis [32]
• Exfoliation of graphite intercalation compounds (GICs) [33]
• Chemical reduction of GO [34]
• Photothermal reduction of GO [35]
• Thermal exfoliation of graphite oxide [36]
• Electrochemical method to reduce GO [37]

These methods can overcome the agglomerated layers associated with weaker van der Waals forces. Despite their ease of operation, cost-effectiveness, and scalability, the quality of the graphene layer obtained through these methods needs improvement [28].

The following sections will discuss some methods from each approach, beginning with bottom-up approaches (CVD and epitaxial growth) and ending with top-down ones (mechanical cleavage, exfoliation of graphite oxide), as illustrated in Figure 2.

3. Chemical Vapor Deposition (CVD)

Graphene was successfully synthesized using the CVD technique for the first time in 2006 by Prakash and coworkers, employing camphor as the precursor on Ni foils [38,39]. The basis of this technique lies in the thermal decomposition of hydrocarbons on transition metals. As a promising, cost-effective, and straightforward method, CVD is responsible for producing high-quality graphene sheets, using low-mass hydrocarbons, such as methane, as carbon feedstock.

Notably, CVD is superior for the production of single-layer graphene, as the starting material assumes a specific form, a feature not found in the mechanical exfoliation of graphite. The quality of the resultant graphene sheet largely depends on the choice of metal, as it influences the sheet’s characteristics. The growth of graphene using CVD involves three key steps: The metal is annealed at high temperatures (such as 1000 °C) in an Ar/H₂ atmosphere to increase particle size, followed by exposure to an H₂/CH₄ gas mixture. The hydrocarbons are analyzed and the carbon atoms dissolve on the nickel film, forming a solid solution. Finally, the mixture is cooled under argon gas. Different metals, such as Cu [40], Ir [41], Pt [42], Co [43], Pd [44], Re [45], and Ru [46], can be employed. Cu appears to be the best alternative to replace Ni due to its low cost, ability to control graphene growth, and ease of transfer to metal substrates. The copper growth mechanism on graphene film includes heating the copper sheet in a furnace at 1000 °C in the presence of hydrogen gas, placing the mixture in an H₂/CH₄ gas environment to initiate the growth of graphene, and cooling the system to room temperature after the formation of the graphene layer above the copper substrate.

4. Epitaxial Growth on SiC

The process of forming a graphene sheet on the silicon carbide (SiC) surface is achieved through the preferential sublimation of silicon and carbon atoms on the SiC surface. Epitaxial graphene holds a broad range of potential applications in future electronics. Producing graphene using this technique is quite complex due to the requirement of the heating conditions that reach temperatures greater than 1000 °C and the ultra-high vacuum conditions (${10}^{-9}$ Torr) in argon atmospheres. These conditions alter the sublimation rate of silicon, permitting higher temperatures, which result in high-quality graphene. Hexagonal silicon carbide (6H-SiC) is widely utilized in the synthesis of graphene, where it takes appropriate structures for both the silicon-rich and carbon-rich SiC faces of hexagonal SiC [47,48]. However, there is a constant barrier between graphene and silicon carbide in both cases, leading to noticeable differences in graphene growth on SiC for the two different sides. The growth on the Si-rich side is more intriguing to researchers. This growth is unidirectional (rotated 30°), resulting in high-quality graphene with a large similarity between layers. Conversely, the growth on the carbon-rich silicon carbide results in stacked layers. Each form yields a suitable structure. The variation between the two faces is attributed to the rotation that causes the separation of the graphene layers, ultimately rendering a significant similarity between the properties of monolayer and multilayer graphene [49,50].

5. Mechanical Cleavage

The mechanical cleavage method, commonly known as the ’scotch tape’ method, involves the exfoliation of graphite using adhesive tape to separate the layers from each other. This method has been employed to isolate graphene experimentally and was first implemented in 2004 using graphite films [51]. The technique encompasses various mechanical peeling procedures, including micro-mechanical cleavage, sonication, ball milling, fluid dynamics, supercritical fluid, and more [52,53]. The method operates by overcoming the van der Waals attraction forces between the graphene layers. Mechanical cleavage employs two types of forces: normal and lateral. Normal forces are used to overcome the van der Waals force and separate the graphene layers, as observed in the mechanical division using adhesive tape. Lateral forces, on the other hand, can enhance the relative movement between the layers of the graphene.

Controlling the mechanical peeling forces is vital for obtaining high-quality and high-efficiency graphene. Since the tape method is free from impurities, it enables researchers to analyze the electrical and mechanical properties of graphene. An illustration of the mechanical exfoliation of graphene using the adhesive tape method is depicted in Figure 3 [54].

6. Exfoliation of Graphite Oxide

The method of peeling and reducing graphite oxide has gained significant attention from researchers in recent years. Graphite oxide is prepared through graphite oxidation using concentrated acids and potent oxidants, relying on methods such as those of Staudenmaier [55], Brodie, or Hummers [56]. The Hummer method is particularly prevalent globally, as it clearly describes the composition of graphite oxide through various models, including the widely used Lerf–Klinowski model [57]. The resultant graphite oxide is characterized by layers containing OH- and epoxy groups at the center, and carboxyl groups at the edges. These oxygen-containing groups confer a hydrophilic character to graphite oxide and create active groups between the layers, resulting in an internal space ranging from 6 to $12\AA$, depending on the amount of added water [58]. Pure graphene can be achieved through the exfoliation of graphite oxide into graphene oxide, followed by the reduction of graphene oxide, culminating in graphene production. This product is often referred to as reduced graphene oxide, rather than graphene, as the reduction process is not entirely completed. The exfoliation process of graphite oxide is considerably faster than that of graphite, and can be accomplished using thermal treatments or sonication in water. A schematic of the mechanism for the exfoliation and functionalization of graphite oxide is depicted in Figure 4 [59].

7. Properties of Graphene

7.1. Electronic Properties

The primary feature of graphene that has drawn the attention of researchers is its distinctive electronic properties. The calculated electrical conductivity of graphene, at $64\mathrm{mS}{\mathrm{cm}}^{-1}$, is significantly higher than that of single-walled carbon nanotubes (SWCNTs) [60]. Graphene exhibits semiconductor characteristics, possessing a zero-bandgap and high dispersion energy, with a slight overlap between the valence and conductance bands. Moreover, the charge carriers’ movement in graphene is subject to dispersion, depending on the surrounding conditions [61]. One of the most intriguing aspects of graphene is the highly unusual nature of its charge carriers, which behave as massless relativistic particles known as Dirac fermions. The behavior of Dirac fermions is markedly abnormal compared to ordinary electrons, particularly when subjected to magnetic fields. For example, an anomalous integer quantum Hall effect (QHE) has been observed [62]. This effect has even been detected at room temperature, where the influence of a strong electric field becomes apparent. The concentration of charge carriers reaches up to ${10}^{13}{\mathrm{cm}}^{-2}$, with room temperature mobilities of approximately $10,000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ [9].

7.2. Mechanical Properties

The mechanical properties of single-layer graphene membranes have been explored through nanoindentation using atomic force microscopy (AFM). The force required to break graphene is $42\mathrm{N}{\mathrm{m}}^{-1}$, which is over 100 times greater than that required for a hypothetical steel film of the same thickness. Furthermore, the Young’s modulus of graphene has been measured to be $1.0\mathrm{TPa}$, confirming its remarkable strength [39,43]. Calculations of the photon spectra of graphene, as a function of surface tension and based on density functional perturbation theory, have confirmed that the surface tensile value reaches up to $\sigma =130\mathrm{GPa}$, and the Poisson’s ratio is $\nu =0.186$ [63]. These properties underscore the robust nature of graphene. Moreover, external mechanical loading on graphene can induce changes in electronic properties, such as field emission performance [64]. Researchers have suggested that this unique property might find applications in sensor membranes, responding to minute changes in pressure, or in selective boundaries that can filter gases and provide a physical barrier between different phases of matter [65].

7.3. Optical Properties

Graphene exhibits unique optical characteristics, making it one of the most intriguing materials in this regard. The absorbance of white light in graphene has been determined to be $2.3\%$, with a negligible reflectance of less than $0.1\%$. The absorbance has been observed to increase with the number of graphene layers, ranging from 1 to 5 [7]. For monolayer graphene on the SiC substrate, optical reflectivity and transmission of photon energy values were measured to fall between $0.2$ and $1.2$ eV [66]. The intriguing optical properties of graphene are attributable to its structure and electronic spectrum, particularly when the photon energy greatly exceeds temperature, and Fermi’s energy remains frequency-independent. Investigations using ultrafast optical pump-probe spectroscopy have revealed carrier relaxation times for the growth of graphene layers on SiC substrate. The values for an initial fast relaxation transient ranged from 70 to 120 fs, followed by a slower relaxation process in the $0.4$ to $1.7$ ps range [67]. These impressive optical properties, combined with graphene’s excellent electronic characteristics, have attracted significant research interest. Applications extend across various domains, including electronic devices, photodetectors, touchscreens, light-emitting devices, photovoltaics, transparent conductors, terahertz devices, and optical limiters [68].

7.4. Thermal Properties

Monolayer suspended graphene exhibits remarkable thermal stability, owing to the presence of robust covalent bonds between its carbon atoms. Thermal conductivity measurements at room temperature have revealed values ranging from 3000 to $5000\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, depending on the size of the graphene layer. However, when placed on an amorphous silicon substrate, the thermal conductivity of graphene diminishes significantly to approximately $600\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. This reduction in thermal stability can be attributed to the interaction of photons between the graphene–silica interfaces, along with a pronounced internal dispersion. Nevertheless, the thermal properties of graphene remain substantial, being about 2 and 50 times greater than those of copper and silica, respectively. Such distinctive thermal attributes position graphene as a valuable material for wide applications in electronic devices [69].

8. Conclusions

Graphene, a two-dimensional structure composed of carbon atoms, has garnered significant interest for its exceptional electronic, mechanical, optical, and thermal properties. This review has offered an in-depth examination of these attributes and various methods of synthesis. The article began with an overview of diverse techniques for graphene synthesis, including chemical vapor deposition (CVD), epitaxial growth on SiC, mechanical cleavage, and exfoliation of graphite oxide. Each method was explored in detail, providing insights into their underlying principles, advantages, and potential applications. The exploration of graphene’s electronic properties highlighted its high electrical conductivity, unusual charge carrier behavior, and promising potential in quantum Hall effect applications. The mechanical properties section revealed graphene’s extraordinary strength and versatility, even surpassing conventional materials like steel. The investigation of optical attributes emphasized its unique absorbance and reflectance characteristics, opening doors for its usage in various optical devices and systems. Lastly, the thermal properties section illustrated graphene’s remarkable thermal stability and wide-ranging applicability in electronic devices. The rapid advancements in graphene research and technology, as presented in this review, accentuate its importance in shaping the future of materials science and engineering. Graphene’s unique combination of properties continues to inspire novel applications, from transparent conductors to terahertz devices, potentially revolutionizing various industrial sectors. Thus, in a nutshell, it can be said that this review not only consolidates current understanding and knowledge of graphene but also paves the way for further research and exploration. With continuous innovations and discoveries, graphene is poised to play a pivotal role in the advancement of science and technology, underpinning new horizons in diverse fields.