No, all graphene is not the same.
It is critical for end-users, researchers and investors to understand the differences between the commercially available materials that are called “graphene” and most importantly the limitations on those products in certain uses. Choosing the right product that will give the desired performance for their application, however, can be challenging for the non-expert. The potential applications for graphene are heavily influenced by the method in which it was made so a basic understanding of production techniques is important.
There are many methods for producing graphene. Nanoplatelets can be produced by peeling layers from graphite, a form of carbon found in pencils and lubricants. Other methods use small reactive pre-cursor chemical molecules and gases deposited on a solid substrate that catalyse the crystal growth over a large area. Within both of these broad approaches lie several processes that give graphene of varying thickness, size and carbon purity. Thickness (number of graphene layers), lateral size and the presence of defects are the three key properties that will determine the potential application areas .
The key concept for graphene commercialization is this: there is no universal material that is suitable for every potential product based on graphene. Furthermore, no single production method that can make the most suitable type of graphene for every application.
In this post, methods that use liquids to assist the production process are discussed. The first and perhaps most common, is chemically modified graphene, usually prepared through first oxidizing graphite using harsh acid treatments to create defects (ie lowers purity). The defects assist in the separation of the now so called graphene oxide sheets. A very high proportion of this single layer graphene like material can be generated using this method. Many of the defects in graphene oxide can then be repaired, if required, using sometimes highly toxic reactants such as hydrazine to help regenerate the graphene structure. This process, however, is not perfect and some defects persist. There are some less toxic reducing agents however they are not as effective as hydrazine. The presence of defects can significantly reduce the electrical and thermal conductivity of the graphene nanoplatelets, two principal properties of graphene that have captured the imagination of researchers and industry alike. This process though does introduce a chemical “handle” for further reactions to modify the sheets and give them other new and interesting properties.
Graphene can also be exfoliated from graphite using other special solvents such as n-methylpyrrolidone (NMP) or ionic liquids (ILs). Relatively low concentrations of single and few layer graphene are produced by suspending graphite in NMP followed by sonication or another type of agitation. The graphene sheets are relatively pristine, or defect free, however the solvent is difficult to remove and was recently listed as a substance of very high concern by the EU regulatory body REACH. Ionic liquids (ILs) are also another class of solvent options that allow the production of high quality graphene materials. ILs have very low environmental footprint because they evaporate incredibly slowly but this also presents a challenge in removing this solvent on drying of the graphene product or mixing with other liquids. It is also important to recover the ILs as the costs of these solvents are very high.
Graphene nanoplatelets can also be produced in an environmentally friendly process with water as a solvent resulting in large area, defect free single and few layer graphene with the high aspect ratio and high conductivity resulting from large lateral area and low thickness. Using water also has the advantage of lower costs and easier post processing. In order for the graphene sheets to remain well separated, a surfactant or detergent type molecule is often added.
Practically speaking, none of these liquid phase processes described produce 100% defect free single layer graphene according to the strict IUPAC definition however some methods produce material that is closer than others. There is currently a strong effort to standardise “graphene” materials using a limited number of parameters such as number of graphene layers, lateral dimensions and carbon to oxygen ratio. This will assist researchers and consumers to understand which of the myriad available materials is right for their applications as well as assist investors in determining the potential addressable markets for products enhanced with a specific type of graphene.
One non-trivial challenge for many producers remains: scaling production in a cost efficient manner whilst maintaining consistent properties of the graphene produced. This is of tremendous significance particularly for uses of graphene in high volume applications such as composites, coatings and inks where they will be competing with established products that are more or less commoditised.