Ever since its isolation in 2004, graphene and graphene-like materials (e.g., graphene oxide, graphene nanoplatelets) have attracted enormous attention both in academia and industry due to exceptional properties such as superb mechanical strength, outstanding electrical and thermal properties, and excellent gas barrier properties.
In 2019, the global graphene market size reached nearly USD 80 million and is expected to exceed USD 1 billion by 2027 as a result of the demand for efficient, lightweight, and durable materials in the electronics, composites, and energy sectors. Consequently, this drives the development of cost-effective and high-output production methods of good-quality graphene and graphene-like products.
Conventional methods of obtaining graphene:
The simplest method for obtaining defect-free graphene is by using Scotch tape and a graphitic source. Since graphite consists of numerous loosely stacked graphene sheets, the adhesive force is sufficient to unpeel a few-layered graphene sheets, which then can be transferred onto a substrate. Naturally, such a process is labor intensive and unsuited for large-scale production.
Other, somewhat higher yield but expensive and multi-staged methods are chemical vapor deposition (CVD), epitaxial growth, and arc discharge. Here, graphene is obtained through a series of physical and chemical changes using high temperatures and carbon precursor gas (e.g., methane, ethane) and metal (e.g., copper, nickel) substrates; silicon carbide; or gas (e.g., hydrogen, helium) and graphite sources, respectively.
- 2D Carbon Tech and CealTech successfully produce good-quality graphene on a large scale utilizing CVD
- Graphene & Nanotechnologies and Graphensic use the epitaxial growth method.
On the other hand, liquid and electrochemical exfoliation methods offer more commercially viable and truly industrial outputs. In the former case, relatively concentrated, stable, and adequate quality graphene dispersions are obtained through the vigorous mixing of graphite in a solvent with or without a surface-active agent. The high shear force readily separates graphene sheets, and the affinity to the solvent (in some cases together with a surface-active agent) prevents the graphene sheets from restacking.
A study published in 2014 demonstrated that a regular kitchen blender is enough to yield stable liquid dispersions not only of graphene but also of other two-dimensional materials, such as boron nitride, molybdenum disulphide, too.
- The most notable example of a company employing this method is NanoXplore, the largest Canadian graphene manufacturer.
The chemical exfoliation method is among the most attractive for large-scale and commercially viable production and entails treating graphite with strong oxidizing agents and acids. The graphite oxide obtained is dispersed in a solvent, usually water, by means of sonication and then converted back to graphene, albeit not without permanent damage to its lattice structure.
- Examples of companies offering chemically-derived graphene products include but are not limited to Abalonyx and GOgraphene.
Lastly, electrochemical exfoliation lends itself to low-cost and efficient manufacturing too, as evidenced by First Graphene’s large-scale manufacturing capability. Here, a graphitic working electrode is immersed in an electrolyte solution such as ammonium sulphate, wherein the solute molecules insert themselves into the empty space between graphene sheets in the graphitic electrode. Upon the application of an electrical potential, the evolved gas expands graphite, which then readily exfoliates in a solvent and thus yields a graphene dispersion for further use.
A summary of the discussed and other main methods is presented in the figure below.
Miscellaneous sources for graphene production
In an attempt to drive down the product cost, and in some cases address environmental concerns like the use of hazardous chemicals, a significant research effort has been put into alternative sources for graphene production. The methods, sources, principal characteristics, and possible applications are summarized in Table 1.
Table 1. Methods, principal characteristics, and potential applications of graphene derived from miscellaneous sources; adapted from Kwon, S.‐J., et al. /Adv. Sustainable Syst. 3 (2019), 1800016; Bazaka, K. et al./Chem. Rev. 1 (2016), p. 163-214
As indicated, graphene and its products are obtained from a range of diverse, even exotic (e.g., cookies, grass, honey), sources via three different methods. For instance, graphene can be produced from plants through thermal decomposition at high temperatures in an inert atmosphere in the process of pyrolysis. If a sugar and sand mixture or asphalt are chosen as the precursors, the pyrolysis process yields graphene-sand composites that are suitable for water purification, as demonstrated by the ability to remove colorants from rhodamine 6G solution and Coca-Cola. Though the quality of graphene obtained this way is good, it is nevertheless insufficient for high-end optoelectronic applications.
A range of food products, polymers, raw coal, plastic, organic, and industrial waste can be converted into graphene via the CVD method, wherein the source decomposes to gases that then react with a catalytic metal substrate at a high temperature and under a reductive gas atmosphere. CVD-produced graphene exhibits excellent optical and electronic properties and thus is suitable for optoelectronic applications. Overall, despite the high demand for energy and somewhat complicated production processes, both pyrolysis and CVD have shown a great potential for environmentally-friendly and low-cost graphene manufacturing.
By contrast, the reactive plasma technique allows for the production of tailored graphene structures, such as vertical graphene (VG) sheets and metal-doped graphene, in a matter of minutes from a range of natural precursors (e.g., milk, honey, butter, sugar) without the need of a catalyst and at lower temperatures. VG in particular is of special interest because its long reactive edges available for further functionalization are advantageous for emerging energy, environmental, and catalysis applications.
Summary
As the graphene market continues to grow, so does the need for cost-effective, high-output, high-quality graphene production methods. The conventional manufacturing methods use graphite or carbon-rich gas and are fairly well commercialized. The alternative production methods, on the other hand, are still in the development stage but are capable of converting a variety of low or negatively valued carbon sources into good-quality graphene suited for emerging nanotechnologies and optoelectronic applications.
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