Graphene synthesis, characterization and its applications
1. Introduction
To understand the graphene history, some terms such as carbon, graphite, and graphene are important to be discussed. Carbon, the most abundantly element on earth, takes its name from the Latin word carbo meaning charcoal [1], [2], [3]. Carbon forms many allotropes with many potential applications. The major ones known since antiquity are graphite and diamond [4]. Until the mid-1980, carbon was believed to exist in two only physical forms which are diamond and graphite, and it were in 1985 that fullerene cluster, a new form of allotropes of carbon was discovered [5]. It was believed that graphite was a lead ore and was called “plumbago”. In 1779, Scheele demonstrated that plumbago is actually carbon, not lead since people used it to write marks on their sheep, Verner (1789) named it graphite (a Greek word for “writing”) [6]. During the Middle Ages, the weak dispersion forces in between the adjacent sheets and layers were utilized to produce marking instruments, just in the similar way that we make pencils today from graphite [7].
Ubbelohde and Lewis (1960) isolated a mono-atom plane of graphite and pointed out that graphite consists of layers, which are a network of hexagonal rings of carbon atoms [6]. Therefore, the single carbon atomic layer of graphite may compactly arrange into a two dimensional (2D) honeycomb lattice known as graphene. That name was introduced by Boehm et al. in 1994 [8], however, according to Mouras et al. (1987), the term “graphene” first appeared in 1987 to describe single sheet of graphite, that is the main building block of graphitic materials such as graphite, fullerene, and carbon nanotubes [6], [9]. Even though graphene came into existence in 1859 by Benjamin Collins Brodie, it has been studied theoretically for many years by Wallace [10] and was originally observed in an electron microscope in 1962 [11].
However , it is in 2004, when Novoselov and Geim successfully isolated and studied a single-atom-thick crystallite (graphene) from bulk graphite and transfer them onto thin silicon dioxide on a silicon wafer by a famous Scotch Tape Technique [6], [8] that the properties of graphene material were achieved [12]. It is noted that, the first graphene was extracted from graphite using a technique called micromechanical cleavage [13] and the Nobel Prize in 2010 was awarded jointly to Konstatntin Novoselov and Andre Geim in Physics “for groundbreaking experiments on the two-dimensional material graphene” [1], [14], [15]. Currently, graphene is a trending nanomaterial which is replacing silicon in different applications across various research fields.
This is due to their nano-scale physical, thermal, mechanical, and chemical characteristics. Considering graphene’s promising properties, it is attracting sponsors and large grants [16]. According to the Chemical Abstracts Service (CAS) database and the National Science Foundation (NSF), the number of publications on graphene has increased significantly and millions of dollars are spent to fund graphene research.
2. Structure and properties of graphene
Graphene is a carbon nanomaterial made of two-dimensional layers of a single atom thick planar sheet of sp2-bonded carbon atoms packed tightly in a honeycomb lattice crystal [13], [17]. Graphene’s structure is similar to lots of benzene rings jointed where hydrogen atoms are replaced by the carbon atoms Fig. 1a and is considered as hydrophobic because of the absence of oxygen groups [10]. Furthermore, it is a carbon allotrope in the structure of a plane of sp2 bonded atoms with a molecule bond length of 0.142 nm. Layers of graphene are piled together to form graphite, with an inter-planar spacing of 0.335 nm [11].
Fig. 1This stacking makes a three-dimensional structure of graphite (Fig. 1b), while graphene is a two-dimensional, one-atom-thick material [6]. Graphene has a hybridized sp2 bonding. It displays a single π orbital and three σ bonds perpendicular to the plane (Fig. 1c). The strong in-plane σ bonds act as the hexagonal rigid backbone structure, while the out-of plane π bonds control the interaction between different graphene layers. In any case, the changes in the structure of graphene are mainly due to the absence of one or more sp2 carbon atoms or the presence of one or more different atoms with sp3 hybridization [20]. Over the previous decades, single sp2 bonded carbon atom allotropes from 0 to 3 dimensionalities such as nanotube, fullerene, graphene, and graphite have been integrated into diverse polymer matrices due to their exceptional thermal, mechanical, electrical properties and lightweight [21].
With the carbon atoms bonded covalently within the plane, forming σ-bonds with three neighboring carbon atoms and one out-of-plane Ï€-bond, this network of sp2 carbon atoms gives graphene its unique properties [12]. Graphene’s tensile strength is around 125 GPa and its elastic modulus is 1.1 TPa, this makes the strength of graphene to be 100 times more than that of steel [22]. The thermal conductivity of graphene is around (5 × 103 W/mK), henceforth 10 times higher than that of copper (401 W/mK). Graphene’s electron mobility has a conductivity of 106 S/m, and resistance of 31 Ω/sq. which accounts for ultra-high mobility of (2 × 105 cm2/V.s) that is 140 more than that of silicon. The reason for this ultra-high mobility is sp2 hybridization which donates an extra electron to Ï€ bond, these pi electrons are delocalized at room temperature yielding high conductivity [23].
Besides, graphene is naturally a semimetal and its benzene ring-like electronic structure comprises of six π-orbitals with three occupied bonding and three unoccupied antibonding orbitals, separated by a bandgap. Fusing of these benzene rings leads to a little overlap between the valence and conduction bands thus the electrons from the top valence band can flow to the bottom conduction band without stimulating any heat (Fig. 2) [24], [25]. Aggregation of the layers of graphene determines the specific surface area. The modification of pristine graphene with other compounds decreases the aggregation and increases the effective surface area [26]. In recent years, graphene has fascinated much attention of researchers, owing to its extraordinary electronic, optical, magnetic, thermal, and mechanical properties as well as large surface area [27].
Fig. 23. Growth mechanism of graphene
Recently, there are several methods used to synthesize graphene. This synthesis process is known as extracting graphene depending on the purity and the desired product [28]. After the 2004 graphene’s discovery, different techniques were developed to produce layers and thin films of graphene [29]. As shown in Fig. 3, Chemical vapor deposition (CVD) [30], chemical exfoliation [31], chemical synthesis [32], and mechanical cleaving [33] are some of the commonly used methods of graphene synthesis.
Fig. 3Top-down and bottom-up synthesis methods are determined by the number of layers, thickness, the nature and average size of the graphene materials. In top-down growth mechanism process, graphene sheets are produced by exfoliation/separation of graphite and its derivatives including graphite oxide (GO). Table 1 summarizes both top-down and bottom-up synthesis methods as well as their advantages and disadvantages.
Table 1. A summary of top-down and bottom-up methods of graphene’s growth mechanism [34].
| Method | Thickness | Lateral | Advantage | Disadvantage |
|---|---|---|---|---|
| Micromechanical exfoliation | Few layers | µm to cm | Unmodified and large size graphene sheets | Very small scale production |
| Electrochemical exfoliation | Single to few layers | 500–700 nm | High electrical conductivity of the functionalized graphene | High cost of ionic liquids |
| Direct sonication of graphene | Both single and multiple layers | µm | Inexpensive and unmodified graphene | Low yield |
| Reduction of carbon monoxide (CO) | Multiple layers | Sub-µm | Un-oxidized sheets | Contamination with α-Al2S and α-Al2O3 |
| Epitaxial growth on SiC | Few layers | Up to cm size | Very large area of pure graphene | Very small scale |
| Unzipping of carbon nanotubes | Multiple layers | few µm long nano ribbons | Size depend on the starting nanotubes | Expensive and oxidized graphene |
| CVD | Few layer | Very large (cm) | Large size; high quality | Small production scale |
3.1. Micromechanical exfoliation/cleavage
Mechanical exfoliation is also known as Scotch tape or peel-off method. It was the first method to be used by Novoselov and Geim for the production of graphene with the help of an adhesive tape to force the graphene layers apart (Fig. 4) [28], [35], [36]. In this method, multiple layers of graphene remain on the tape after peeling off, but with recurrent peeling, it splits open into a handful of graphene flakes. For detachment, the tape is attached to a certain substrate (acetone) and a final peeling by using a fresh tape is carried out to obtain flakes different in both size and thickness which can be observed under a light microscope on SiO2/Si substrates [37]. This process is slow and imprecise, hence the material produced is most often used to study the properties of graphene rather than actually using it commercially [35]. This method can also be performed by using different agents such as electric field [38], epoxy resin [39], and by transfer printing technique [40].
Fig. 43.2. Electrochemical exfoliation
Graphite exfoliation by electrochemical techniques has become a simple but yet high yielding method in recent times for the mass production of graphene [41]. This method involves the use of various forms of graphite such as graphite foils, plates, rods and graphite powders as electrodes in an aqueous or non-aqueous electrolyte and electric current to bring about the expansion of electrodes. The electrodes could be of two types based on the power applied, that is, Cathodic (Negative) and Anodic (Positive) electrodes [42], [43]. Wang et al. [44] in their study used Pure Graphite as electrodes and PSS (Polysodium-4-styrenesulfonate) solubilized in deionized water to incarnate the electrolyte. They accommodated the graphite rods in an electrochemical cell which was filled with the electrolyte. A persistent current of 5 V was applied. Few minutes of electrolysis resulted in the accumulation of a black material at the anode. The exfoliation process was carried on for 4 h to isolate the product from the cell. It was then centrifuged at 1000 rpm and then the product was gradually poured out. The dispersion obtained was found to be extremely stable. Powdered dry graphene was achieved by washing the dispersion with deionized water and alcohol followed by vacuum drying. Then the yield was calculated by weighing the dry powder and sediment [8].
3.3. Pyrolysis
The word pyrolysis originated from the Greek-derived element pyro and lysis. Pyro means fire and lysis stand for separating. Synthesized carbon atom on a metal surface is a simple procedure used to fabricate few-layer graphene [45]. One of the common techniques of graphene synthesis is the thermal decomposition of silicon carbide (SiC). At high temperature, Si is desorbed leaving behind C atom which forms few graphene layers as shown in [46]. This technique has received a significant improvement through the continuous production of graphene films in mm scale at a temperature of 750 ℃ on a thin film of nickel coated on SiC substrate [47]. The advantage of this method is the continuous production of graphene films over the entire SiC coated surface. However, this method cannot be used in the synthesis of graphene in large scale. A similar approach is applied in the thermal decomposition of ethylene at 1000 k. The advantage of this synthesis method is the production of high purity graphene mono-layer [48].
3.4. Chemical vapor deposition
Chemical vapor deposition (CVD) is a bottom-up synthesis technique used for production of high-quality graphene on a large-scale basis [49]. This method involves combining a gas molecule with a surface substrate inside a reaction chamber under temperature, pressure, and gas flow rate conditions [50]. A typical CVD instrument includes a quartz reaction chamber, a mass flow controller, a pump, thermocouples for temperature measurement, a gas delivery system, a vacuum system, an energy system and a computer for auto-control (Fig. 5) [51], [52]. Different substrates are used in CVD for graphene film growth, they include Nickel (Ni) [53], Copper (Cu) [54], Iron (Fe) [55], and Stainless steel [56]. Methane (CH4) and acetylene (C2H2) are normally used as a carbon source. Two CVD processes are used to activate the carbon source and include; thermal CVD and plasma-enhanced CVD (PECVD) [57].
Fig. 5Thermal CVD consists of a vacuum tube, high-temperature heating furnace, vacuum pump, vacuum gauge to control pressure and a mass flow controller to regulate carbon and career gas used in the synthesis of graphene. In PECVD, plasma leads to the decomposition of the gas source and then reacts with metal substrate resulting in the growth of graphene films [58]. Power sources such as direct current (DC) [59], microwave [60], and radio frequency (RF) [61] have been used as a plasma source. Advantage of PECVD over thermal CVD is that graphene growth mechanism can occur under low pressure and temperatures [62]. Carbon source decomposes at high temperature into hydrogen and carbon atoms [63].
- (i)
CH4(g) ---- greater than 2H2 (g) + C(S) (ii) C2H2(g)--- --> H2(g) + 2C(s)
In this CVD method, hydrogen and argon gas are used as carrier gases to remove and clean unwanted oxides on the metal catalyst surface.
Transition metal substrates such as Cu and Ni have typically been utilized for the CVD growth of graphene [64]. This growth process follows two main steps: i) Formation of carbon by pyrolysis of the gas precursor and ii) the formation of the carbon structure of graphene using the segregated carbon on the surface of the metal catalyst [65]. For instance, the production of graphene by using polycrystalline Ni is achieved by first annealing Ni in an H2 atmosphere at the desired temperature of 900–1000 ℃ with grain size [66]. The substrate is exposed to H2/CH4 gas mixture, here CH4 is used as the source of carbon. The decomposition of hydrocarbon makes the carbon atom to dissolve in the Ni film where the solid solution is formed. Ni has high solubility at high temperature and the solid solution of Ni is cooled down in argon gas to form Ni-C precipitate which etches graphene (Fig. 6) [67]. Although Ni is a good substrate for the production of graphene, the quality of Ni film can affect the percentage and size of the monolayer graphene. The cooling rate affects the thickness and quality of graphene, and the microstructure of Ni also can affect the formation of graphene morphology [68].
Fig. 64. Characterization of graphene
Graphene’s characterization is an important aspect of the study and research of graphene. Characterizations involve the investigation of graphene morphology, properties, defects, and layers based on spectroscopic and microscopic measurements[69], [70]. Raman Spectroscopy [71], Scanning electron microscope (SEM), Transmission electron microscope (TEM), X-ray diffraction (XRD), ultraviolet–visible spectroscopy (UV–Vis), and atomic force microscopy (AFM) are used characterization techniques [72].
By Raman Spectroscopy Graphene layers and the structural quality can be studied by Raman spectroscopy [73]. Monochromatic radiation of Raman spectroscopy interacts with the molecular vibration of graphene resulting in a shift in radiation due to scattering [74]. Three main peaks are observed in graphene and include; D, G and 2D peaks. D peak is observed at 1350 cm−1 indicating a disorder in sp2 hybridization [75]. G peak is located at 1580 cm−1 representing lattice vibration and 2D is located at 2700 cm−1 originating from Raman scattering second order at Dirac point (Fig. 7a) [76]. An increase in graphene disorder increases the ratio ID/IG because of the elastic scattering due to the higher defect intensity. However, when the carbon structure becomes more amorphous, the ID/IG ratio decreases [77]. Jorio et al. [78] conducted a Raman study on the ion-induced defects in N- layer graphene. Raman spectrum of graphene also known as the G band, exhibits G mode due to the stretching of the C–C bond [79]. It is marked by a strong peaks at 1580 cm−1 which is the first-order of the Raman spectrum-allowed feature originating from the zone centre (Photon wave vector q = 0).
Fig. 7Transmission Electron Microscope (TEM) is the most used technique in the study of graphene’s structural quality and the number of layers. TEM images are formed when the electron beam interacts with the material under study [81]. Fig. 7b displays TEM image of graphene’s number of layers in dimethyl sulfoxide. 3, 4, 5, and 6 dark fringes can be observed which represent the layers of graphene. Graphene layers are parallel to the beam of the electron.
SEM is used to study the morphology of graphene. SEM imaging involves advantages such as detection of impurities, graphene folds, and discontinuities during the synthesis process. However, it’s limited in the resolution of ultrathin layers of graphene. Fig. 7c represents the structure of graphene sheets which are encapsulated by polyethylene terephthalate (PET) at 2 nm. Due to the interaction of PET and graphene, the thickness of the graphene sheets increased from 1.57 nm to 50 nm. Large surface area of graphene nanosheets increased the interface area between PET and graphene thus providing many tunneling sites for electron transport [82].
X-Ray Diffraction technique is mainly used to identify the phase of the material based on cell dimension units [81]. Fig. 7d exhibits the XRD of graphene, graphite oxide and graphite. A sharp and high diffraction peak of graphite occurs at 26.6 degrees. The peak then shifts to 13.3 degrees indicating the presence of oxygen molecule. After fabrication, there is no peak hence indicating that graphene was synthesized [80].
UV–visible spectroscopy can be used to characterize the layers and types of graphene (graphene oxide and pristine graphene). UV–visible absorbance results from transition of electrons at carbon pi bonds (Ï€-Ï€* transition) exhibits an absorption peak of monolayer graphene oxide at around 230 nm, whereas pristine graphene exhibit its absorption peak between 250 and 270 nm [83]. The number of layers and graphene thickness can be studied by ultraviolet transmittance. Atomic force microscopy (AFM) is utilized in determining the surface structure and thickness of graphene [84]. An AFM generates images by scanning a small cantilever over the surface of a sample. The sharp nanoscale tip at the end of the cantilever contacts the surface, thus bending the cantilever and changes the amount of laser light reflected into the photodiode. The cantilever height is then adjusted to restore the response signal arising from the measured cantilever height tracing the surface.
5. Applications of graphene
Due to the exemplary properties of graphene such as lightweight, electrical conductivity, strong mechanical, and thermal strength, graphene is widely involved in different applications such as electrochemiluminescence (ECL) sensor [85], transistors, water filtration, energy storage, biosensors and solar cells [18].
5.1. Energy storage
Graphene is applied in energy storage devices such as batteries and supercapacitors because of its high surface area [86]. In Li-ion batteries, graphene is widely used as anode and has a capacity of about 1000 mAh g−1 which is three times higher than that of graphite electrode. Graphene also offers longer-lasting batteries and faster recharge time in seconds. Also, due to its flexibility, graphene is used as solid-state supercapacitor printed device in textiles for wearable electronics (Fig. 8a) [87]. The theoretical specific energy density increases with increase in graphene content in the supercapacitor as illustrated in Fig. 8b. Wang et al. [88], synthesized pillared graphene using a chemical vapor deposition method using hydrogen gas and ethylene as carbon source on 20 μm copper foil at 750 degrees. They used the synthesized graphene as the anode in Li-ion battery and demonstrated exemplary cycling stability of more than 250 cycles.
Fig. 85.2. Water filtration technology
Graphene nanoporous membranes can ideally be used for water desalination and filtration with an efficiency of 33 % to 100 % depending on the pore size and the applied pressure. Tanugi and Grossman [89], first studied the classical molecular dynamics and reported that water could flow through graphene membranes at a range of up to 100L cm−2 day -1 which is higher in magnitude than RO diffusive membranes [89]. The desalination capacity of salt water depends on the pore size of the membrane, quantity of chemical composition, and the applied pressure. Graphene membranes have the capacity to reject 97 % of NaCl from seawater. Abraham et al. [90] showed that the decrease in the interlayer of graphene decreased the permeation rate, but the transportation of water was least affected.
5.3. Transistors
At room temperature, the carrier mobility of graphene is very high, this property makes graphene to be applied in transistors [91]. Lemme et al. [92], initiated the graphene’s field-effect transistors. In their experiment, the top gate modulated the drain current and one layer graphene had better field-effect characteristics compared to silicon transistors. Wu et al [93], illustrated how the fabrication of radio frequency top-gate graphene transistors exhibited cut-off frequencies of more than 155 GHz with reduced gate length to 40 nm. Vicarelli et al. [94], used antenna-coupled field-effect graphene transistors to detect radiation of terahertz and from then field-effect graphene transistors have been applied in sensing proteins, biomolecules, cells, gas and DNA.
5.4. Solar cells
Recently, solar cells have played a vital role in the production of electrical power. Solar cells efficiency is an important factor in converting light energy into electricity [95]. Graphene, because of their exemplary optical, mechanical, and electrical properties, have been incorporated in dye-sensitized solar cells (DSSC) and used as electrodes to boost the efficiency of photovoltaic cells [96], [97]. They were for the first time used as transparent electrodes in 2008 to replace fluorine-doped tin oxide (FTO) at the photo anode [98]. CdS/graphene nanocomposites exhibits superior photocatalytic activities. Introduction of graphene in CdS semiconductor improves photostability, increases the number of reaction sites, and enhances light absorption capacity in solar cells [99], [100], [101], [102]. Shin et al. [103], exfoliated porous silicon heterojunction photovoltaic cells by using graphene doped with Ag nanowires. The exfoliated solar cells had an efficiency of 4.03%. Shi et al. [104], improved the photovoltaic cells efficiency by adding carbon nanotubes to graphene. During the absorption of light, a minor hole transfer along carbon nanotubes impacted to increased efficiency to 15.2%.
5.5. Biomedicine
Graphene has a vast number of applications in biomedicine. Recently nano-graphene has been developed for potential applications in imaging, and photothermal therapy [105]. The sensing capacity of graphene-based electrodes can be tuned by modifying the surface chemistry of graphitic materials, resulting in biosensing applications such as detection of organic molecules, microbial cells and biomolecules (Fig. 9). Nano graphene expresses ultra-high surface area convenient for effective binding to biomolecules and used as carriers for gene and drug delivery [106].
Fig. 95.5.1. Graphene based biosensors
Graphene sheets are altered by using electrodeposition and electronic doping methods to construct biosensors [108]. Its excellent properties including optical properties, electrical thermal properties and large surface-to-volume ratio leads to develop more accurate biosensors [109]. Graphene on Polyethylenimine-functionalized ionic liquid (PFIL) is used to develop glucose biosensors. It is an excellent candidate to develop affordable biosensors which have good stability and sensitivity towards enzymes [110]. Graphene electrodes are good electrocatalysis to O2 and H2O2 and induces the kinetics of electron transfer of the reaction. Liberated H2O2 can be used to quantify the amount of glucose which recognize enzymatic hydrolysis of negatively charged glucose oxidase [111]. Basically, graphene acts as a matrix to the enzyme.
Another example is real-time label-free optical biosensors which are constructed by using graphene-based materials [112]. Surface plasmon resonance biosensor (SPR) is an example for optical biosensors which are used in medical diagnostics, environmental monitoring, etc. Graphene improves the sensitivity of SPR biosensors [113] and acts as a good adsorbing layer for biomolecules due to graphene’s hexagonal and carbon ring structure. Biomolecules are adsorbed to graphene by Ï€-Ï€ interactions and embedding gold metals enhances the sensitivity of SPR biosensors [114]. Graphene materials are also used to develop protein and DNA biosensors. Sensitivitys of the biosensors depend on interaction between DNA and carbon nanomaterials [115].
5.5.2. Multi model imaging and photothermal therapy
In vivo imaging and photothermal therapy depend on the photosensitivity of the material [116]. Graphene nanoplatelets have been used in imaging techniques due to its strong near infrared (NIR) absorbance, low toxicity, and efficient tumor targeting properties [117]. Pegylated (polyethylene glycol) nano-graphene oxide (NGO) is used for cellular imaging. NGO can act as a NIR fluorophore for specific biological samples and imaging. How nano-graphene sheets with polyethylene glycol (NGS-PEG) were injected into the mice and observed for the tumor growth. There was no effect with only PEG-NGS or only laser. However, NGS-PEG and NIR laser treatment together were able to eliminate the tumor completely.
5.5.3. Drug delivery system and tissue engineering
Graphene has successfully been applied in drug delivery systems because of its unique properties such as conductivity, mechanical stability, and biocompatibility [118]. The combination of graphene oxide and polymers like polyethylene glycol (PEG), poly (vinyl alcohol) (PVA) are used as adsorbents for hydrophobic molecules [119]. Hydrogels are used in drug delivery systems in biomedical applications. An example is multiwalled carbon nanotubes (pMWNTs) used to enhance the electrical response in hydrogels of methacrylic acid [120].
6. Conclusion
In this review, a brief history, the structure, properties, growth mechanisms, characterizations, and applications of graphene are discussed. Graphene is considered as one of the most attractive functional nanomaterials in the world due to its excellent unique properties such as high tensile strength, high electrical conductivity, high carrier mobility, high elasticity, high thermal conductivity and optical transparency of about 97%. Since its discovery in 2004, graphene has enticed a wide array of applications in different fields like transistors, solar cells, water purification technologies, sensors, batteries and supercapacitors, which have incredibly attracted a lot of attention among scientists and engineers around the globe. Up to date, CVD is the synthesis technique of high-quality graphene on large-scale basis. However, to explore the full potential of graphene economically, some critical points such as toxicity, long-term stability, and environmental effects need to be addressed.
7. Prospects
Commercialization progress of graphene-based nanomaterials is very promising. There are a lot ongoing innovative researches in research institutes around the world to modify the current growth mechanisms of graphene to produce fine products at large scale. Further development focuses on the application of graphene in electronic devices due to the high demand. Future research directive is on the graphene-based application in the production of ultra-thin flexible displays, smartphones with transparent touchscreens, biosensors in medical science, nanorobots, and super light cables in aircraft and satellites.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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