TEM images of graphene oxide (GO).
TEM images of graphene oxide (GO).

A simple and ‘green’ chemical method of preparing sheets of graphene – a single layer of hexagonally bonded carbon atoms – could pave the way for large-scale production in a new generation of electronic and biomedical devices, say researchers from Australia.

Currently, the most common route of producing graphene nanosheets cheaply is the chemical exfoliation of naturally occurring graphite using oxidizing agents and acid. The process reduces graphene oxide (GO) with hydrazine and metal hydrides that strip surface oxygen atoms from GO and produces graphene as similar as possible to the pristine material formed by mechanical exfoliation. But the approach relies on highly toxic and environmentally unfriendly chemicals, making the process unsuitable for large-scale production.

Efforts to find greener alternatives have pinpointed vitamin C (ascorbic acid) and, more recently, amino acids as possible replacements for hydrazine. Now researchers from The University of Adelaide in Australia have demonstrated that the common amino acid L-aspartic acid (L-Asp) can also be used as a reductant to produce high-quality reduced GO (rGO) [D. N. H. Tran, et al., Carbon 76 (2014) 193-202 (DOI: 10.1016/j.carbon.2014.04.067)

“This method eliminates the use of toxic and harmful chemicals to humans and the environment, which makes it compatible with the large-scale production of graphene using natural graphite as the raw material,” Dusan Losic told Materials Today.

Previous studies on amino acids for GO reduction have focused on species with electron-rich aromatic groups and thiol-based amino acids. But Losic and his team demonstrate that L-Asp can produce fully exfoliated graphene nanosheets from a GO dispersion in about 3 hours in a simple, bench-top process. The resulting graphene/L-Asp mixture is very stable and can be left under ambient conditions for months. In its favor, L-Asp is nontoxic, biocompatible, and available on an industrial scale. Other amino acids such as valine could also work just as well, the researchers show.

But what the researchers do not fully understand yet is exactly how the complex process works. Losic and his team hypothesize that the amine group on L-Asp acts as a nucleophile, attacking the electrophilic carbon atom in the C-O bond and breaking it. The oxygen atom is removed, ultimately forming water molecules, while the L-Asp is polymerized into cyclic polysuccinimide (PSI).

“We found that by controlling the time of the reduction process it is possible to control precisely the density of oxygen groups on the basal plane of the graphene nanosheets,” explains Losic. “To have graphene with controllable oxygen groups and controllable size of graphene nanosheets is important for broad biomedical applications.”

The new approach provides a simple and scalable means of producing graphene nanosheets from raw graphite without toxic chemicals or expensive processes.

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