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Right-click to download interview with Mario Hofmann (13.2 MB MP3)

“Graphene is basically a metal so it’s boring,” says Mario Hofmann a researcher at National Cheng Kung University in Taiwan. “But if you start adding defects you start getting interesting effects.” Key to exploiting defects in graphene is control. As is often the case, Mario Hofmann and his colleagues at National Cheng Kung University and National Chung Cheng University in Taiwan found the solution to cost-effective tuned graphene production was not a new technique.

“Electrochemistry has been around a long time but nanotechnology can really take advantage of it because it’s so finely tuneable,” Hofmann tells Electrochemical synthesis involves applying avoltage to graphite in a solvent until it exfoliates sheets of graphene. Hofmann and his colleagues showed that by varying the voltage setting during electrochemical synthesis they could determine the thickness, flake area and the number of defects in the resulting graphene – all of which affect the properties of the material. “Basically you can just replace the whole chemistry cabinet with a dial on your potentiostat.”

Too slow, too pure, just right

Hofmann describes how in 2008 the only way to make graphene was by using sticky tape and rubbing graphite until you sheared off a single layer. Half an hour in a clean room working with this approach was enough to persuade him and his colleagues that there must be another way.

The researchers then showed that they could make high-quality graphene by chemical vapour deposition, but the process was very expensive. In addition the quality of the resulting graphene was in some ways too high. “In other materials you are looking to get rid of defects but in graphene we are looking for ways to controllably introduce defects,” says Hofmann. In terms of cost and tuneability their electrochemical approach proved just right.

The team’s experiments confirmed that there were two stages to the exfoliation mechanism in electrochemical graphene synthesis, and that each stage had different voltage requirements, providing a high degree of control over the product. First the water molecules ”intercalate” between the layers in the graphene, which requires a voltage of 1-2.5 V. Then the water expands, exfoliating sheets of graphene; this stage requires much higher voltages - around 10 V.

“In the beginning of the 1990s people came up with the mechanism but nobody proved it,” says Hofmann. “That model is surprisingly accurate and we’re the first ones to have proved the mechanism really is what people were hypothesizing before.”

A new look for electrochemistry

Hofmann and his team enjoy tinkering with electronics, as he tells, which led them to set up their own electrochemical system using pulsed instead of continuous voltages. “This pulsed voltage allows us to enter the time evolution of graphene exfoliation, so we can check if graphene is exfoliating and even how much is exfoliating over time,” says Hofmann.

The researchers also developed their own approach for tracking graphene synthesis. Initially they had been monitoring the current in the system to determine graphene production. However Hofmann’s student Wan-Yu Chiang pointed out that the current increase associated with graphene synthesis occurred before it was exfoliated, which she could witness directly as it turned the solution black. Hofmann stresses the advantages of observant students who “don’t blindly agree with what their supervisor is saying”. Using a photodiode and light source the researchers could use the change in the solution’s transparency to quantitatively monitor graphene production.

Next steps

Surprisingly, another parameter played an important role in electrochemical graphene exfoliation - the sign of the voltage. “Electrochemists would throw their hands up in the air and say nothing will happen,” says Hofmann. In fact he and his colleagues found that the expansion, and consequent exfoliation, happens at positive or negative voltages but the intercalation will not take place under a negative voltage. This finding suggests that water is responsible for the exfoliation but charged ions can help the water to intercalate between the graphite layers.

Next the team plans to study the effects of pulse duration and whether the optimum settings for the electrochemical process parameters evolve over the course of the exfoliating process.

More details are reported in the full article at Nanotechnology 26 335607, which features in Nanotechnology Select.