Indian researchers have made thin films of carbon called graphene using an electrochemical method. The technique could make it easier for scientists to prepared and study the material, which is already finding uses in sensors, solar cells and electronic devices.

Graphene — a sheet of carbon just a single atom thick — is extremely strong, heat resistant, and conducts electricity well. Yet while researchers are excited by finding potential applications for graphene, the conventional method for making graphene is extremely time-consuming, involving scraping flakes of material from graphite crystals. Scientists have been searching for better ways to make graphene, such as using chemical methods to remove oxygen atoms from sheets of graphene oxide, which can be prepared easily from graphite.

Fig. 1: Graphene oxide can be stripped of its oxygen atoms electrochemically to afford a thin, reduced graphene-oxide film having many of the properties of pure graphene.

Ganganahalli Ramesha and Srinivasan Sampath of the Indian Institute of Science1 in Bangalore, India, have now made the same transformation using electricity. They first coated a gold electrode with a sulphur- and nitrogen-containing molecule called cystamine. They then dipped the electrode into an acidic mixture of graphene oxide flakes in water. The acid imparts the cystamine surface with positive charge, which attracts graphene-oxide flakes to form a layer roughly 1.2 nanometers thick. Each of the flakes covers several square micrometers.

The researchers used a technique called Raman spectroscopy, which probes the scattering of laser light from the material surface, to study how the structure of the graphene-oxide layer changed as they increased the voltage across the electrode up to 1 volt. Variations in the wavelength of peaks in the Raman spectra revealed that an increasing amount of oxygen was lost from the layer with increasing voltage, leaving behind a 'reduced' or oxygen-deficient form of graphene oxide. Although the new material still contained a small amount of oxygen, and had some defects in its crystal structure, it shared many of the properties of pure graphene. For example, the reduced graphene-oxide layer conducted thousands of times more current than the original graphene oxide from which it was prepared, consistent with what would be expected for graphene.

The scientists now hope to create larger sheets of reduced graphene oxide that could be used to make biosensors or field-effect transistors.