Fig. 1: Graphene paper

Graphene — single-atomic-layer sheets of carbon, which form graphite when stacked together — has excellent electronic, thermal and mechanical properties, and therefore has great potential for applications in diverse fields such as sensors, electronic circuits, solar cells, batteries and even medical bionic devices.

However, because of its large specific surface area, in water, graphene tends to aggregate irreversibly into large clusters and even restacks to form graphite. This behaviour makes it very difficult to process graphene on a large scale, thus significantly limiting industrial applications. Attempts to prevent this aggregation have mainly focused on attaching polymers or surfactants to the surface of graphene-sheets to hold them apart. But this approach has its own problem — the attached molecules can adversely affect the unique and desirable properties of graphene.

Now, a team led by Dan Li and Gordon Wallace at the University of Wollongong, Australia has developed a method for producing stable aqueous dispersions of graphene without attaching additional molecules.1 Zeta potential measurements led the group to conclude that the reason graphite oxide forms stable dispersions is because of the electrostatic repulsion from the negatively charged carboxylic acid groups on the surface of the sheets; the researchers thought the same could be true for the reduced form, graphene.

They followed a well known chemical-conversion process of using hydrazine to reduce graphite oxide to graphene, but also added ammonia to increase the pH to about 10, so as to maximize the charge density on the resulting graphene sheets. In this way the electrostatic repulsion, and therefore the separation between the sheets was maximized.

Li and colleagues demonstrated the possibility of using their process for large scale processing by preparation of ‘graphene paper’ by vacuum filtration of the dispersion, and peeling the film off the filter membrane. The as-formed paper exhibited electrical high conductivity—comparable to that of carbon-nanotube paper—and was strong and thermally stable.

“We are continuing to prepare new graphene-based materials and exploring their applications for energy conversion and storage, transparent electrodes, and biomedical devices,” says Li.