NASA’s Graphene – Making holes within graphene nanoplatelets

NASA got in touch with me. “Would I like to attend a briefing on a new graphene technology they had been working on?” The first thing I did was check up to make sure this was real. Yes it was the NASA Langley Research Center. So I joined a few others and listened attentively to what their scientists had to say about something they call holey graphene. Read on to find out more…

Dear InvestorIntel readers, you are sophisticated. So I’ll spare the usual graphene introductions and assume you know all about graphene’s amazing physical properties. I will draw your attention to one aspect of this material though. It is impermeable, even gases cannot pass through.

This impermeability is seen as an advantage because there are few other materials with this property. However there are situations where putting holes in graphene could be an advantage, such as making filters or electrodes for batteries and supercapacitors.

Graphene porosity currently means gaps between nanoplatelets

Regular readers will know about graphene filters, for example Manchester University has created a filter that can remove the salt from seawater.

They achieved this feat by making the filter from graphene nanoplatelets that were pressed together. Think of the nanoplatelets as stacks of playing cards viewed edge on. Water can be forced through the gaps between the layers, however salt molecules cannot pass through these gaps and this creates the separation effect.

Making holes within graphene nanoplatelets

Creating holes in graphene nanoplatelets makes graphene permeable. Other teams have done this. For example punching holes with an electron beam, or etching with oxygen plasma. Both of these methods are effective on the small scale but complicated and expensive to scale up.

The NASA process

What NASA has done is to develop a simpler way of making the holes. In essence they dust graphene with a solid powder of silver nanoparticles. This dust clings to defects on the nanoplatelets surfaces. Then they introduce oxygen. The metal nanoparticles act as a catalyst completely oxidising the surrounding graphene to carbon dioxide and this creates the holes. Because the silver nanoparticles act as a catalyst they are unaffected by the oxidation and remain. So they dissolve the metal with acid. Then wash the acid away.

Graphic showing NASA’s manufacturing process for holey graphene

NASA has already shown that this increases the surface area and can catalyse the breakdown of hydrogen peroxide into hydrogen and oxygen. Other materials can do this so that is not unique.

So What?

The technology also relies on the graphene not being perfect. It needs to exploit defects in the graphene nanoplatelets. Regular readers will recall that near perfect sheets of graphene have been made by researchers in China. This single crystal graphene will not contain defects and so this process may not work as effectively on these new larger scale sheets that will emerge in the future.

Much was made of the applications of these holey platelets but I didn’t see any evidence for them actually making a measurable difference to filtration or energy uses yet. This means the technology is yet to be proved as a leap forward over standard graphene nanoplatelets. However, the NASA process for making holey graphene is straightforward and should not add much cost to conventional graphene nanoplatelets. It should be scalable too, which is good from a commercial point of view.

Single Crystal Graphene: The next manufacturing frontier

We don’t often think about crystals being capable of bending without breaking, but a single crystal of graphene will be rather flexible. In the world of graphene language is important and it will help you dear InvestorIntel reader to find out why this is the case…

The current state of the art for making graphene

Commercially available graphene is available in tonne quantities. These are manufactured as platelets, nano platelets or nano sheets. They are usually sold as powders dispersions and pastes that can be used as additives in products such as rubber tyres, carbon composites and paints.

Larger sheets of graphene can be made at square centimetre scale by the chemical vapour deposition (CVD) method grown on surfaces such as solid copper and silicon. These surfaces are not completely smooth and this creates imperfections in the resulting graphene layer making it polycrystalline. This CVD graphene is essentially a batch process and not currently suitable for making the material on anything other than sample scales. Graphene sheet made by this method has been found to be suitable for flexible touchscreens, but does not have the strength needed for other uses.

So at the moment graphene can be made in very tiny pieces or as imperfect samples.

On perfection…

The size and perfection of the sheet is necessary to realise the amazing properties of graphene. Think about the metaphor of a chain. We all know that a chain is only as strong as its weakest link. CVD graphene could not realise the promised strength of the perfect material because the defects introduce weak links across the sheet. Now imagine a chain where all the links are separate. It doesn’t matter what are the properties of the individual links, it cannot be used as a chain unless the links are embedded in some glue like material. This is one of the reasons nanoplatelets are used as additives in composite materials.

Close to perfection

Perfection in graphene terms is a sheet of carbon where all the atoms are connected by bonds to form hexagons. This pattern is uninterrupted and continuous through the entire sheet, be it on the nano-scale, metre-scale or kilometre-scale. This is what scientists mean by a single crystal. In essence there is a continuous two-dimensional chain of bonds throughout the whole material.

If you think perfection is impossible, you would have been right until a few weeks ago. Researchers at Peking University in China have made single crystal graphene on copper foil that they claim is almost perfect at 99.9% alignment.

State of the art of manufacturing graphene 2017

However, the graphene must be removed from the solid growth surface and this separation damages the sheet, and makes graphene by a batch process. The continuous manufacture of single crystal graphene is not quite there yet. The next step will probably be when the researchers start to perfect the growth of graphene on liquid surfaces as we outlined in a previous column entry.

So What…

Well, if you could make single crystal graphene by a continuous process a myriad of amazing industrial products would result.

Just one example: Architects would be freed to create a whole new range of super tall skyscrapers. Elevator technology is one of the main constraints placed on the height of skyscrapers. At the time of writing the tallest building is 828m (0.8Km) high.

A layered graphene elevator tether ribbon would allow people to be carried right to the top of tall buildings in one smooth action without having to change elevators which would bring in a new age of kilometre and mile high buildings. All this is foreseeable now the manufacturing frontier has been extended into single crystal graphene technology.