A Rice University chemist has found simple methods to reduce three types of coal into graphene quantum dots.
Graphene quantum dots are microscopic discs of atom-thick graphene oxide. Applications they could be used for include medical imaging sensing, electronic and photovoltaic technologies.
Band gaps establish how semiconducting materials carry an electric current. In quantum dots, the band gaps are responsible for their fluorescence and can be tuned by changing the dots’ size.
The process developed by Rice’s James Tour and his team allows a measure of control over their size, generally from 2 to 20 nanometers, depending on the source of the coal.
Cheaper and More Efficient
Many ways currently exist to make graphene quantum dots (GQDs), most being expensive producing very small quantities, says Tour. Coal promises greater quantities of GQDs made cheaply in one chemical step, he said.
“We wanted to see what’s there in coal that might be interesting, so we put it through a very simple oxidation procedure,” Tour said. It involved crushing the coal and bathing it in acid solutions to break the bonds that hold the tiny graphene domains together.
“You can’t just take a piece of graphene and easily chop it up this small,” he said.
To help characterize the product, Tour turned to the lab of Rice chemist and co-author Angel Martí. It turns out different types of coal produce different types of dots.
Three Types of Coal
GQDs were derived from bituminous coal, anthracite and coke, a byproduct of oil refining. Each coal was sonicated in nitric and sulfuric acids and heated for 24 hours.
(Illustration shows the nanostructure of bituminous coal before separation into graphene quantum dots. Credit: Tour Group/Rice University)
Bituminous coal produced GQDs between 2 and 4 nanometers wide.
Coke produced GQDs between 4 and 8 nanometers, and anthracite made stacked structures from 18 to 40 nanometers, with small round layers atop larger, thinner layers.
The researchers also treated graphite flakes with the same process and got mostly smaller graphite flakes, just to see what would happen.
The dots are water-soluble, Tour said. Early tests have shown them to be nontoxic. That offers the promise that GQDs may serve as effective antioxidants, he said.
Medical imaging could also benefit greatly, as the dots show robust performance 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,” Martí said. “They’re still there, but they have been photo-bleached. They don’t fluoresce anymore.”
Testing indicates GQDs resist bleaching.
After hours of excitation, Martí said, the photoluminescent response of the coal-sourced GQDs was barely affected. This may make them suitable for use in living organisms. “Because they’re so stable, they could theoretically make imaging more efficient,” he said.
A small change in the size of a quantum dot – as little as a fraction of a nanometer – changes its fluorescent wavelengths by a measurable factor, and that proved true for the coal-sourced GQDs, Martí said.
Low cost will also be a benefit, Tour said. “Graphite is $2,000 a ton for the best there is, from the U.K.,” 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. It’s going to show the world that inside of coal are these very interesting structures that have real value.”
Coal as an abundant source of graphene quantum dots Ruquan Ye, Changsheng Xiang, Jian Lin, Zhiwei Peng, Kewei Huang, Zheng Yan, Nathan P. Cook, Errol L.G. Samuel, Chih-Chau Hwang, Gedeng Ruan, Gabriel Ceriotti, Abdul-Rahman O. Raji, Angel A. Martí & James M. Tour Nature Communications 4, Article number: 2943 doi:10.1038/ncomms3943