Understanding Graphene – Part 2
Graphene can be made by separating the stacked layers in graphite or by self-assembly of the structure atom by atom. Separating the layers is called exfoliation.
That’s the short version, dear Investorintel reader, for more detail please read on…
Graphene is the new wonder material, which promises to revolutionise everything from batteries to bulletproof vests. It is a superb conductor of electricity and heat, 200 times stronger than steel and can form a transparent and flexible layer just 1 atom thick. Small wonder then that this has attracted a lot of attention from investors.
A Google search shows 327,000 hits for graphene manufacturers. There is a lot of content out there associated with making this material. At Investorintel, we have been analysing the global graphene supply market and the report will be available soon. In creating this report we have looked at who is making graphene by what method to understand the current state of the art. This column contains some of the background information.
So, how is this material made?
There are two basic methods to make graphene, they are classed as the top down and bottom up approach. This column will focus on the top down approach, and we will look in detail at the bottom up approach in the next column.
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The Top down approach to make graphene:
The starting material is graphite, which is made of layers of graphene rather like a pack of playing cards as described in part 1of this series.
The scotch tape method:
Place a thin sample of graphite on to sticky tape then place another piece of sticky tape on top of the exposed part of the graphite sample. Press the sticky tape together and peel the tape apart. This will cleave the layers. Take one of the pieces of sticky tape and repeat the process. This will eventually leave a single layer of true graphene stuck to the tape. At this point stick the tape to a suitable surface such as silicon glass. Solvents can then be used to remove the adhesive and tape leaving the graphene layer transferred to the substrate of your choice.
This hand-prepared method can produce very high quality pieces of true graphene. This is the prime source universities use for research quality samples. The leading producers of this type of product are based in the UK at the University of Manchester where the Nobel prizewinning work was originally performed.
The size of the graphene monolayer is determined by the flake size in the original sample. Typical flake sizes are around 2000 micrometres (µm or micron) the largest graphene monolayer flake size found so far is 1,500,000 square micrometres. If that sounds big then consider this character in the brackets:
This is about the size of that largest piece of graphene found in graphite so far.
This artisan graphene is not cheap; prices are typically 0.5 GBP per micrometre. So expect to pay at least $750.00 USD for a piece of flake monolayer graphene transferred to SiO2 glass.
Industrial exfoliation methods
It will not surprise you to know that people have been looking for cheaper ways to make graphene in industrial quantities. The main alternative method is to tear apart the layers in graphite using a shear mixer.
The basic science has been worked out and graphite can be sheared down to create a suspension of individual flakes of graphene in various liquids. A 1mg/L suspension of monolayer graphene in ethanol will cost approximately $1000.00 USD per litre.
Organisations are competing against one another to develop patent protected methods for creating exfoliated graphene from graphite. These include electrochemical methods where graphite is put in suspension in water with a conducting chemical additive such as acrylonitrile.
Other industrial processes use a stabilizing solvent such as N-Methyl-2-pyrrolidinone (NMP). This works because the surface energy of the solvent matches that of graphene. So once the graphene platelets are broken apart the solvent keeps them dispersed. The solvent can then be evaporated to create a graphene powder that contains pieces of graphene up to 2 micrometres in size.
Another common route to making graphene is to subject graphite to chemical oxidation creating graphene oxide. The most common route is Hummer’s method. This is then reduced back to graphene again using an oxygen scavenger such as hydrazine hydrate. Graphene Oxide is a fascinating material in its own right and has quite different properties to graphene which we will explore in a future column.
The market overview
The market for graphene suspensions and powders has become rather crowded and the market shakeout had started. Our first survey found 55 organisations making graphene by the top down method. The latest update has found 48 manufacturers around the world.
All these industrial methods produce tiny particles of graphene. If anyone knows of a process that can reconnect these small pieces into large sheets of continuous defect free graphene, please let me know as I have not found any credible evidence for this yet.
The size of the sheet of graphene is important. To fully realise the amazing properties of graphene in the promised new products requires the use of continuous defect free sheets of graphene of the sort of scale where you could take a single sheet and hold it between your hands.
No matter how graphene is made from graphite it is worth noting that all these methods produce very small pieces of graphene, some call these graphene sheets. Look at the full stop (period) at the end of this sentence; it is 100,000 times bigger than the graphene sheets produced by these industrial exfoliation methods.
Using the term sheets to describe graphene gives the casual observer the impression we are dealing with something of the scale of copier paper when in fact these ‘sheets’ are a hundred thousand times smaller than a small dot. This is not to say the exfoliated graphene products are no good, they do have uses, which we will look at in another column. This tells you more about the power of marketing hype surrounding the current state of graphene manufacturing.
So, making graphene by the top down route from graphite can produce industrial scale quantities of the wonder material, just in very, very tiny pieces. Larger sheets of graphene are required to realise its full potential. We need to look at other ways of making the stuff and that, dear Investorintel reader, is the subject of the next column…
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>