Chemical trickery allows separation by electrostatic repulsion alone
Researchers in Australia have developed a new way to make graphene - the atom-thin sheets that stack together to make the graphite found in pencil ’lead’. The discovery could speed the use of graphene in flexible electronics - such as thin LCD displays.
Graphene sheets have an extremely large surface area and non-bonding interactions can cause the sheets to stack together into graphite. Current ways of producing them involve carefully peeling away individual sheets from graphite - a process that is impractical on a large scale.
One alternative is to use chemicals to break down graphite oxide into graphene - but this has previously required surfactants and polymers to keep the individual sheets apart, preventing the graphene from being easily integrated into materials or devices.
Now a team at the University of Wollongong, New South Wales, have shown that this process can be modified, allowing sheets to be kept apart in aqueous solution by electrostatic repulsion alone - without the need for chemical stabilizers.
Dan Li, the lead author on the project, explains: ’We discovered that chemically prepared graphene is negatively charged in water. By controlling experimental conditions, we managed to maximize these charges. This means that we can make graphene sheets that are inherently repelled from each other via electrostatic repulsion.’
The team optimised the existing process by adding ammonia solution to keep the mixture at pH 10, ensuring the graphene sheets form with maximum charge density. Another important aspect was the complete removal of any residual metal salts and acids that could have neutralised the charge on the sheets.
The aqueous dispersion of graphene can be filtered to yield sheets or sprayed into thin films or coatings. The approach could be scaled up to make graphene more cheaply for a wide range of applications.
’The method proposed in this paper should allow easier production of high quality graphene,’ Kostya Novoselov of the University of Manchester’s mesoscopic physics group in the UK, told Chemistry World. ’There are many possibilities for this, such as making transparent electrodes for LCD displays. At the moment we can only make small displays with graphene, but using this method we could potentially make full-scale displays.’
The team have filed a patent on their new process and are continuing to study the fundamental properties of graphene and investigating its potential in energy conversion and storage.
Meanwhile, another study published earlier this month reports a new chemical technique to make strips of graphene or ’carbon nanoribbons’. Hongjie Dai and colleagues at Stanford University, US, first loosened layers of graphene from graphite by heating it to 1000?C for a minute in 3 per cent hydrogen in argon gas. The team then broke up the graphene into strips using ultrasound. Nanoribbons made in this way have much ’smoother’ edges than those produced by traditional lithographic methods, the researchers say.
1 D Li et alNature Nanotech.2 X Li et alScience