By using a simple, inexpensive, quick process researchers from the University of Madras have been able to produce a monolayer or a few layers thick graphene nanosheets from graphite material without using any strong oxidising or reducing agents. The results were published in the journal Materials Letters.
Among several techniques adopted to synthesize graphene from graphite, the liquid phase exfoliation methods have been found to be facile and cost-effective. Yet, the use of strong solvents and relatively low yield have turned out to be major drawbacks limiting its utilisation in the large-scale production of graphene.
“Commercialization of high quality graphene sheets calls for the exploration of a simple, effective, and inexpensive method for its engineering and environmental applications,” says Dr. S. Balakumar, Director of the National Centre for Nanosciences and Nanotechnology, University of Madras.
So he and one of his students M. Balasubramaniam used ultrasound along with water, glacial acetic acid (CH3COOH), which is a mild solvent, and ethanol (C2H6O) to exfoliate graphite into graphene sheets.
The graphene nanosheets had over 94 per cent ability to retain their original charges even after 50 cycles of charging and discharging.While electrostatic bonds between the cleavage planes do not allow the acid to get in, supplying additional mechanical energy in the form ultrasonic waves enables the organic acid to penetrate deeper into the graphitic flakes and exfoliate the individual layers of graphene.
“Probe sonication is an effective method for the exfoliation of molybdenum sulphide, another material similar to graphite. It is likely that in presence of molecules like acetic acid, exfoliation of graphite is effective,” says Prof. T. Pradeep from the Department of Chemistry, IIT Madras, who wasn’t involved in the research.
Breaking the bonds takes time but once broken the graphene attracts hydroxide (OH) from ethanol and -COOH from glacial acetic acid onto the open sites. “The hydroxide and -COOH form functional groups on the surface of graphene and the individual exfoliated graphene nanosheets are prevented from restacking,” says Dr. Balakumar.
The –COOH is hydrophilic in nature and so allow the graphene nanosheets to remain dispersed in water. And the similarity in the structures of ethanol and acetic acid facilitate the long term suspension stability says Mr. Balasubramaniam.
Due to minimised restacking and long term stability, the capacitor characteristics become better. Compared with graphene-metal oxide nanocomposites, the graphene nanosheets exhibited a much greater capacity for storing charge. The graphene nanosheets had over 94 per cent ability to retain their original charges even after 50 cycles of charging and discharging.
“The graphene nanosheets are free of damages and are of superior quality. The yield of nanosheets has also been manifold. Though it’s a proof-of-concept work, our novel approach can certainly play a vital role in some specific applications such as supercapacitors and other engineering related applications and environmental remediation,” says Dr. Balakumar.