To realize how to produce graphene in 5 min.

Introduction of the production for graphene

As a rising material star followed by the fullerenes in 1980s and carbon nanotubes in 1990s, graphene, with one layered or several layered structure, is a well-known carbon allotrope. Since Dr. Andre Geim and Konstantin Novoselov discovered in 2004 that single-layer graphene can stably exist in nature, and then won the Nobel Prize for this important achievement in 2010, more researches and applications for graphene are in full swing. However, the key to the commercialization of graphene are not only whether the production scale or relative technology could meet the demand from the market, in fact, due to the production methods are strongly related to the characteristics of application fields or their products, graphene materials which are produced in a suitable method in accordance with the requirement from end-application will be a key in the last mile of graphene commercialization.

The classification for the production methods of graphene

Zong et al. (Zong et al., Scalable production of graphene via wet chemistry: progress and challenges) has classify the most common 9 methods for producing graphene, including micromechanical cleavage, anodic bonding, Photoexfoliation, liquid phase exfoliation, Growth on SiC, Precipitation from metal, Chemical vapor deposition, Molecular beam epitaxy, and chemical synthesis, as shown in the figure in below.

The figure is shown the 9 common methods for producing graphene classified by Zong et. al. (Zong et al., Scalable production of graphene via wet chemistry: progress and challenges)

On the other hand, defined by the morphology of graphene material, the production methods could be divide into two different kinds of production methods are “bottome-up” and “top-down” routes, represented to two different type of final products for graphene film and graphene powders. Furthermore, the production methods for graphene powders also could be classified into dry state process for graphene powders and wet state process for graphene suspension.

Figure of the production routes of bottom-up process and top-down process.

The following describes the short details of bottom-up process and top-down process:

- The processing routes for bottom-up process

The concepts for bottom-up process is using a series of carbon hexagon rings bonded by pyrolytic carbon atoms to grow a continuous and certain size of graphene film. Two representative methods for chemical vapor deposition and  epitaxial crystal growth method are illustrated in below. In general, due to their higher reaction temperature, the graphene film could have relative higher crystallinity and less defects for better conducting behaviors, it would be easy to apply into transparent conducting film, thin film transistor, bio-sensor or light emission electrode.

• Chemical vapor deposition：

Chemical vapor deposition is a common process for producing a well-structured and few layers graphene with lower defects on a metal substrate. Using a carbonaceous atmosphere as reactive material, the pyrolytic carbon atoms could be dissolved into metal and be precipitated on the surface of the metal substrate to form graphene film during cooling down. In general, nickel with higher carbon solubility could be used to grow multi-layered graphene, and single- or bi-layered graphene could be grown on copper substrate.

• Epitaxial crystal growth：

The concept for epitaxial crystal growth method is that graphene could be grown on a substrate with a same or similar lattice constant. Because the graphite/graphene has a same hexagonal crystal structure to silicon carbide (H series), and the small difference in lattice constant to silicon carbide, silicon atoms will be removed by breaking silicon-carbon bonds in high temperature treatment, and remaining carbon atoms will be reconstructed to form graphene film under an approximate crystal structure.

After the growth of graphene film, it is usually that the as-grown graphene film needs to be transferred to the target substrate for following application. The crucial key for the graphene film manufacture is how the transfer a large area graphene film to the target substrate without generating the defects on the graphene film. For the chemical vapor deposition, the transfer process usually is carried out with an acid etching process to remove the metal substrate, and graphene film could be easily transfer to target substrate using their hydrophobic behavior. But, the crystal structure of graphene film would be easily to damage due the acid etching, and further reduce the characteristic of graphene film. The researcher from China developed a fast exfoliate method by electrolysis, it will remove the graphene film from the substrate without any damage (or with minor damages), and further maintain the quality of graphene film.

Up to now, there are many options of production instruments for the development for bottom-up process in market, the representative production instrument is from LG, Samsung and their relative venders, South Korea. The figure in below shows a R2R production instrument from Graphene Square, South Korea. Despite the issues for the lower production yield, over high reaction temperature, and unstable characteristics affected by transfer process, many research institute, start-up, and companies still presume the development of graphene film, because the products from this kind of process are much close to the “ideal” single-layered graphene. In general, the production process still need 3-5 years to meet the expectation of the cost, stability and reliability from end market.

Scheme of preparation of graphene film from graphene Square, South Korea.

- Top Down route for the production of graphene

The concept of top down route is based on the exfoliation of graphite materials with a proper shear force in a certain medium. The common methods are including the direct exfoliation or to form an oxidized graphene precursor (or as called as graphene oxide) first and then transform to a graphene through reduction. Two different methods could be defined as dry process and wet process depending on the type of medium. Dry process, which means to produce the graphene in a dry state, use air, nitrogen, or carbon dioxide as medium under a proper reaction temperature in atmosphere or vacuum. Three major methods for dry process are shear milling, dry-state milling, and super-fluid method. In the other hand, liquid medium such as water, acid/base solution, or organic solvents would usually be utilized as medium for wet process. The reaction condition would require a certain temperature for chemical reaction with some external shear force and electrical field to enhance the exfoliation. Three major methods for wet process are high powered super sonification, liquid exfoliation, and chemical oxidation.

Technology map for top-down route for producing graphene.

• Dry state milling process

Dry state milling process is a simple way to produce graphene. The mechanism is to utilize the continuous collision generated from waterfall flow to exfoliate graphite under a certain rotation. The exfoliation would be affected by the materials of milling ball, size of milling balls, and packing density for milling balls and graphite.

Scheme of preparation of graphene for ball milling process

• Shear milling process

The mechanism for shear milling process is to use a shear force generated from the gap between the stator and rotor to exfoliate the graphite under a constant air flow. The factors for the generation of shear force will come from the mechanical design, and the speed of material charging and air flow.

• Supercritical fluid method

The way for supercritical fluid process is utilize the penetration ability of supercritical fluid to form a graphite/graphene intercalation compound with a intercalated structure, the intercalated supercritical fluid will transform to become a gas/liquid and expend their volume to exfoliate the graphite and to form graphene.

Scheme of preparation of graphene for supercritical fluid method.

• Chemical oxidation method

Chemical oxidation method for the preparation of graphene is similar with the process to synthesize expended graphite, it is why chemical oxidation method is considered as a promise process in large scaled production. In chemical oxidation method, graphite as the raw material will be mixed with strong sulfuric acid with strong oxidized agent, a intense oxidation will carried out at the surface of the graphite. It will generate a lots of oxygenated functional group and decrease the van der waals force of interlayer of graphite, and form a graphite oxide with a massive functional groups. With the following rapid heat treatment, the as-formed oxidized functional groups will vaporized to become carbon oxide/dioxide and leads to a rapid volumetric expansion inside the interlayer graphite oxide, and finally form the graphene oxide with single-layered or multi-layered structure. Because of the massive oxidized functional groups on the surface of graphene oxide such as epoxyl-, hydroxyl-, and carbonxyl- groups and the damaged crystal structure generated during the reaction, it will usually have a large reduction in the characteristics of graphene oxide. Typically, the oxidized functional groups could be removed with a following reduction process. In general, the graphene oxide products after a reduction is called as “reduced graphene oxide”. The reduction process usually is carried out in a chemical reduction with a sort of reduction agents, or by a high temperature reduction with hydrogen atmosphere. The damaged crystal structure of graphene will be partially healed through the high temperature treatment, which means that the characteristics of reduced graphene oxide will not be as good as the graphene prepared from mechanical exfoliation method.

Scheme of preparation of graphene for chemical oxidation method

• Liquid exfoliation method

Liquid exfoliation method is usually carried out with a high shear exfoliation in a certain organic solvent with a same or similar surface tension. In general, a well-dispersed graphene suspension will be the typical product using liquid exfoliation method. The advantages for liquid exfoliation method are not only to minimize the impact from the acid waste solution in chemical oxidation process, also to prepare a suitable dispersion for downstream customer.

Scheme of preparation of graphene for liquid exfoliation method.

• High powered super sonification method

Like the way for liquid exfoliation process, high powered super sonification method also uses a liquid as the medium, but shear force is generated by high powered sonicator. In general, the tip of high powered sonicator will directly immerse into a graphite mixed solution. A strong shocking waves, which comes from a lot of micro-bubbles generated by the sonicator, will break the inter-layered bonding to exfoliate graphene from graphite rapidly.

Scheme of preparation of graphene for high powered super sonification method

- Summary

After Dr. Andre Geim and Konstantin Novoselov awarded the Nobel prize in almost one decade, the production of graphene has approached the large -scale production. For the production process of graphene powder, several feasible processes have been applied into large-scale production. With the similar process and input materials, chemical oxidation process has been massively applied into production by over 60% of graphene manufactures in the world. But, when the pollution issue of wastewater in chemical oxidation process has been gradually considered as an important issue, several environment-friendly processes, like liquid exfoliation or supercritical fluid method, have been proposed and applied into feasible production process. In the other hand, due to the characteristics of graphene film would be close to the “ideal” single-layered graphene, many research institute, start-up, and companies still presume the development of production process, most of them start from the development of feasible instrument. But In general, the production process still needs 3-5 years to meet the expectation of the cost, stability, and reliability from end market.