|
| Type | Method | Process | Final product and applications | Advantage | Disadvantage | Reference |
|
| Top-down methods | Mechanical cleavage | A 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 graphene | Pristine monolayer graphene: used in graphene analysis for labs, primarily due to high quality | Pristine single layer graphene sheets. Control on graphene thickness | Size 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 exfoliation | Graphene 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 coats | High-quality few-layer graphene dispersion. Scalable process. Better thermal and electrical conduction than reduced graphene oxide | Edge defects. Separation of graphene from solution required for certain applications usually through centrifuge | [20, 23, 24, 27, 28, 32, 33, 78, 79] |
| Solid exfoliation | Graphene 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 groups | Few-layer graphene nanosheets: used in paints, composites, and inks | Allows mixing of graphene with other materials for composites. Graphene functionalization | Nonuniform flake size and thickness with edge defects and impurities (from metal chipped off balls and high extensive impacts) |
| Intercalation exfoliation | Graphite 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 solution | Few-layer graphene nanosheets: used in anticorrosive coats, battery components, composites, and rubbers | Scalable process. Enables functionalization of graphene. Graphene flakes have been noted to have minimal defects | Expensive ionic liquids. Time-intensive process |
| Oxidation exfoliation reduction | Graphite 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 borohydride | Few-layer reduced graphene oxide (rGO): used in anticorrosive coats, battery components, composites, and rubbers | Scalable process. Enables functionalization of graphene | Use 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 oxidation |
| Arc discharge | Graphite 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 nanoflakes | Graphene nanoplatelets used in lubricants, composites, inks, and coats | Catalyst not required. Highly suited for producing graphene at a rate of decagrams per hour | Low yield. Amorphous carbon impurities. Required annealing which increases processing time | [53–55, 80] |
|
| Bottom-up methods | Chemical vapor deposition | Carbonaceous 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 films | Monolayer or few-layer graphene films used in optoelectronics, sensors, and touch panels | High-quality large graphene films. Potential for upscaling process | Process is expensive (high energy required). Removal of catalyst and transferring of graphene onto suited substrate | [58, 59, 61] |
| Epitaxial graphene | Layers of graphene can be grown on silicon carbide (SiC) crystal by thermal decomposition at temperatures greater than 1000°C under an ultrahigh vacuum | Few-layer graphene wafer: used as conductive films and transistors | High-quality large-area graphene wafer. SiC crystal on which graphene is grown on is a highly suited substrate for applications in electronics | Energy intensive process. SiC is expensive | [56, 57, 81] |
| Unzipping carbon nanotube | Carbon 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 plasma | Graphene nanoribbons: used in composites, electronics, and spintronics | Size of graphene nanoribbons can be controlled by selection of nanotube | Oxidation and defects evident along edge where splitting occurs. High cost of chemicals and precursors | [62–67] |
| Laser-induced graphene | CO2 laser is used to irradiate polyimide substrate in ambient conditions and natural resources in either an inert atmosphere or ambient conditions to produce porous graphene | Porous graphene: used in electronic devices such as sensors and capacitators | Process allows for control of morphology and patterning of graphene | Laser irradiation of certain materials at ambient conditions results in ablation and thermal damage | [68] |
| Rapid thermal annealing | A 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 atmosphere | Monolayer graphene films: flexible displays, thin film for photovoltaics, and printable electronics | Shorter processing time. Allows for modification and improvement of graphene-based materials and films | Impurities 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 graphene | A carbonaceous fluid such as ethanol is subjected to an argon plasma generated by microwaves at atmospheric pressure | Graphene nanoplatelets: used in lubricants, composites, inks, and coats | Substrate not required. Process performed at atmospheric conditions | In-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] |
|
