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Reducing coal to several graphene quantum dots

Posted: 10 Dec 2013     Print Version  Bookmark and Share

Keywords:Rice University  graphene quantum dot  coal  medical imaging 

A team of researchers from Rice University has developed a method for mass producing graphene quantum dots (GQDs) from coal. GQDs are microscopic discs of atom-thick graphene oxide that could be used in medical imaging as well as sensing, electronic and photovoltaic applications.

Band gaps determine how a semiconducting material carries an electric current. In quantum dots, band gaps are responsible for their fluorescence and can be tuned by changing the dots' size. The process developed by the scientists allows a measure of control over their size, generally from 2-20nm, depending on the source of the coal.

Reducing coal to several graphene quantum dots

Figure 1: Vials of graphene quantum dots drawn from bituminous coal, anthracite and coke glow under a fluorescent lamp. The highly soluble dots were extracted in bulk from coal in a process developed at Rice University.

There are many ways to make GQDs now, but most are expensive and produce very small quantities, James Tour said, chemist at Rice U. Though another Rice lab found a way last year to make GQDs from relatively cheap carbon fibre, coal promises greater quantities of GQDs made even cheaper in one chemical step, he added.

"We wanted to see what's there in coal that might be interesting, so we put it through a very simple oxidation procedure," Tour explained. That 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.

Tour depended on the lab of Rice chemist and co-author Angel Marti to help characterise the product. It turned out different types of coal produced different types of dots. GQDs were derived from bituminous coal, anthracite and coke, a by-product of oil refining.

The coals were each sonicated in nitric and sulphuric acids and heated for 24 hours. Bituminous coal produced GQDs between 2-4nm wide. Coke produced GQDs between 4-8nm, and anthracite made stacked structures from 18-40nm, with small round layers atop larger, thinner layers. (Just to see what would happen, the researchers treated graphite flakes with the same process and got mostly smaller graphite flakes.)

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