December 6, 2013
Scientists: Coal Is Better, Cheaper For Making Graphene Quantum Dots
Brett Smith for redOrbit.com – Your Universe Online
A team of scientists from Rice University has found a way to use coal as a source of graphene quantum dots (GQD), tiny fluorescent particles that could be used in medical imaging and photovoltaic applications – according to a new report in Nature Communications.
The use of coal to make GQDs would be much cheaper than conventional means, which are expensive and only produce small quantities.
“We wanted to see what’s there in coal that might be interesting, so we put it through a very simple oxidation procedure,” said study author James Tour, a chemist at Rice University.
In the study, the team crushed coal samples and bathed them in acid solutions to fracture specific bonds that hold together graphene, a two-dimensional, crystalline form of carbon found in coal.
“You can’t just take a piece of graphene and easily chop it up this small,” Tour said.
The team discovered that different types of coal created different types of dots. The Rice team extracted GQDs from bituminous coal, anthracite and coke – an industrial byproduct. Bituminous coal resulted in GQDs between two and four nanometers wide. Coke resulted in GQDs between four and eight nanometers, and anthracite produced piled structures from 18 to 40 nanometers, with small round layers sitting on larger, thinner layers.
[ Watch the Video: Coal Yields Production of Graphene Quantum Dots ]
The resulting dots are water-soluble and preliminary tests have shown they are nontoxic. According to Tour, the study results suggested GQDs may serve as effectual antioxidants. The dots also performed well as fluorescent agents.
“One of the problems with standard probes in fluorescent spectroscopy is that when you load them into a cell and hit them with high-powered lasers, you see them for a fraction of a second to upwards of a few seconds, and that’s it,” said co-author Angel Martí, another chemist at Rice. “They’re still there, but they have been photo-bleached. They don’t fluoresce anymore.”
The study team also found that GQDs are resistant to bleaching and resilient during use. After hours of excitation, Martí said, the luminescence of the coal-sourced GQDs was barely degraded, which indicates they could be suitable to use in living organisms.
“Because they’re so stable, they could theoretically make imaging more efficient,” he said.
In conventional GQDs, a tiny change to the size of a dot – just a fraction of a nanometer – shifts its fluorescent wavelengths by a significant factor. The same size change to coal-sourced GQDs produced the same shift in wavelengths, Martí said.
According to Tour, the low cost of coal is what makes the Rice team’s discovery so valuable.
“Graphite is $2,000 a ton for the best there is, from the UK,” he said. “Cheaper graphite is $800 a ton from China. And coal is $10 to $60 a ton.”
“Coal is the cheapest material you can get for producing GQDs, and we found we can get a 20 percent yield. So this discovery can really change the quantum dot industry,” he added. “It’s going to show the world that inside of coal are these very interesting structures that have real value.”