Let's not stick together

Graphene can be made water soluble by adding sulphonic acid groups

Credit: © 2008 ACS

The physical properties and potential applications of graphene have intrigued scientists ever since it was shown that single layers of carbon atoms could be peeled off from pyrolytic graphite. However, it is still difficult to produce large quantities of this novel material, partly because graphene sheets have a strong tendency to stick together. Chemical approaches — such as oxidation in strong acids — can prevent such aggregation in aqueous solution, but they also adversely affect the electronic properties of the graphene.

Now, Yongchao Si and Edward Samulski of the University of North Carolina at Chapel Hill have demonstrated a simple and scalable procedure for producing aqueous solutions of isolated sheets of graphene¹. They start by using sodium borohydride to reduce graphene oxide, and then introduce a small number of p-phenyl-SO₃H groups to sulphonate the sample before completing the reduction with hydrazine. Repulsion between the negatively charged -SO₃ units prevents aggregation in solution and results in highly dispersed graphene in solution.

Thin evaporated films of the sparingly sulphonated graphene exhibit electrical conductivities of 1,250 S m^-1, compared with 6,120 S m^-1 for similarly prepared graphite, which implies that the bonding structure responsible for the electronic properties of graphene is present in the water-soluble samples.