A strong, mouldable graphene

Materials World magazine
,
27 Feb 2019

Graphene could one day become a common engineering as researchers develop it in a soft, kneadable dough form. Katherine Williams found out more.

Graphene oxide (GO) has been turned into a soft, mouldable and kneadable dough that can be shaped and reshaped into free-standing, three-dimensional structures, by a team at Northwestern University, USA. Their material is known as GO dough and the work is reported in Nature Communications.

University Professor of Materials Science and Engineering, Jiaxing Huang, explained the origins of the material to Materials World, ‘My group has been fascinated by dough-like materials. One of the authors, Alane Lim, when taking one of my classes during her first year in graduate school, demonstrated a conductive playdough based on a mixture of gluten and conducting polymers. Her experience became the inspiration for several other students in the group to make dough-like materials with carbon nanotubes and graphene.

‘Normally, people have to add soft additives to make materials into the dough state. We thought that, since graphene oxide sheets have very high aspect ratios (length/thickness) and are quite soft, if we make them into a densely packed state where they crumple and entangle, we create a dough without any additives.’

The team faced the technical challenge of how to densely pack graphene oxide sheets and make them entangle. Usually the sheets are packed layer by layer, forming a lamellar microstructure.

First author Che-Ning Yeh discovered one day that when a form of GO is hydrated by being left in wet air, it collapses into a solid chunk that can be kneaded without leaving stains. This experience helped the team discover that the key is to add very small amounts of water to hydrate GO.

Beautiful solution

When considering how to controllably hydrate GO, the team arrived at a beautifully simple solution. ‘A handheld ultrasonic aerosol generator – a nano spray sold as a beauty product works out really well. It allows us to add very small amount of water in the form of mist, so that we can wet a piece of GO uniformly,’ explained Huang.

The dough can be re-shaped by cutting, pinching and kneading, just like playdough. However, the team was amazed by how deformable it was, as a small piece of GO dough can be easily rolled into a strip that is over 100 times longer.

Another surprise was that the GO dough, after moulding and shaping, can still be re-dispersed in water to give single layer dispersion. Second author, Haiyue Huang, demonstrated that this works very well for manufacturing (see image).

GO is normally stored and sold in the form of dispersions. Some vendors do have a paste-like GO as a compact form, but those pastes do not re-disperse well in water to form nice single layer dispersions. The Northwestern work demonstrates that this can be done with a much more compact form factor without degrading the quality of the material.

In practice

After being shaped into structures, the dough can be converted into dense solids that are electrically conductive, chemically stable and mechanically hard. The dough can also be processed further to make bulk graphene oxide and graphene materials of different forms with tunable microstructures.

‘In this work,’ Huang says, ‘we showed that such graphene solids already exhibit higher hardness than isotropic graphite, which is made under very high temperatures, well over 2,000°C, and high pressure. And I think this work will lead to more projects to turn graphene into a common engineering material like steel, glass and plastic.

‘Dense solids made of disorderly packed sheets exhibit higher hardness than those with lamellar microstructures, for both GO and the converted graphene product. We drew inspiration from bulk metallic glass, where disorder arrangement of metal atoms leads to a wide array of exciting properties – higher hardness, corrosion resistance. So we thought that we have made a graphene version of that, which we called “bulk graphenic glass”, where disorderly stacked sheets give new properties.’

The next step is to expand the study of the properties of such glassy state GO or graphene solids, again drawing inspirations from bulk metallic glass, Huang notes. ‘We are also exploring their electrochemical properties,’ he said, ‘and finally we want to improve the mechanical properties further, by improving the dough making process, and the conversion chemistry to graphene.’

Huang believes that water should be able to make other wires, fibres and sheet-like nanomaterials with high aspect ratios into similar dough states.