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Na-RGO transparent network on glass.

Thermal reduced RGO network on glass slides, coated 1 to 6 times.

Researchers from the University of Maryland’s Department of Materials Science and Engineering (MSE) are the first to report that intercalating (embedding) sodium ions in a reduced graphene oxide (RGO) network printed with graphene oxide (GO) ink can significantly improve its performance as a transparent conductor in displays, solar cells and electronic devices. A paper describing the new technology, developed by MSE assistant professor Liangbing Hu’s group at the University of Maryland Energy Research Center, was recently published in Nano Letters.

The work builds on the group’s previous development of nearly transparent, highly conductive ultrathin graphite sheets that can be used to create more efficient solar cells and highly sensitive touchscreens. Hu and his colleagues were the first to accomplish this by intercalating (embedding) lithium between flakes of graphene.

MSE graduate student and Hu group member Jiayu Wan, the paper’s first author, explains that while the sheets were a success, they would be difficult to produce at an affordable, commercial scale. He and his team decided to explore how they could achieve similar results with more cost-effective materials and production techniques. They swapped graphene flakes for graphene oxide ink and expensive lithium for sodium, a far more abundant natural resource.

The result, he says, is a highly scalable printed electronics system that produces cheaper and more stable conductors than the group’s lithium intercalated graphene sheets. The group theorizes the increased stability is due to the natural oxidation of sodium along the edges of the printed networks – something like a crust on a piece of bread – which forms a barrier that prevents ion loss. Networks printed with the ink exhibit up to 79 percent optical transmittance and 311 Ohms per square of sheet resistance. 

The process begins with the creation of specially formulated ink containing graphene oxide (GO), which is applied evenly to a thin glass slide, called a substrate, using a tiny roller. The coated slide is heated and dried to evaporate the ink solution, leaving a thin, semitransparent layer of GO behind. It is then subjected to a second heat treatment at 300 degrees Celsius, which converts the GO to reduced graphene oxide (RGO). The process is repeated up to six times. Wan then uses a tiny blade to cut and scrape away excess RGO, leaving behind a layered network. After adding thin film electrodes, he moves the device into a glovebox – a chamber with an inert atmosphere – where he is able to add electrolytes and intercalate the sodium ions without the risk of unwanted chemical reactions. As the ions are inserted into the space between each layer of RGO, the network becomes more transparent and more conductive. Finally, after sealing the device, Wan is able to remove it from the glovebox to characterize its optical and conductive properties. 

“This technology is a few steps closer to commercial application than our lithium-graphene product,” says Wan. “Large-scale production of solution-based, printed transparent conductors are potentially much lower in cost and can be used to create a range of electronic devices including solar cells, displays, and organic light-emitting diodes.”

Wan’s co-first authors on the paper are MSE graduate student Feng Gu and postdoctoral research associate Dr. Wenzhong Bao, who are also members of the Hu’s research group.

For More Information:

Jiayu Wan, Feng Gu, Wenzhong Bao, Jiaqi Dai, Fei Shen, Wei Luo, Xiaogang Han, Daniel Urban, and Liangbing Hu. "Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks." Nano Letters, Article ASAP. 1 May 2015. DOI: 10.1021/acs.nanolett.5b00300. Abstract »

Related Articles:
Welcome to Graphene Beach!
Nilsson Wins Wylie Fellowship
Adding Lithium Boosts Transparency and Conductivity of Graphite

June 5, 2015

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