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Controlling chemistry improves potential of carbon nanotubes

Nanotechnology breakthrough could lead to better batteries, more sensitive biosensors

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Rebecca Copeland
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Left: The Billups-Birch alkylcarboxylation reaction allows functional groups to propagate down the CNT from points of pre-existing defects. Right: Electron microscopy shows "banded" CNTs with distinct functionalized and intact regions along their lengths. Photo credits: Nature Communications. Click here for high-resoulution version of this illustration.

A team of University of Maryland nanotechnology researchers has solved one of the most vexing challenges hindering the use of carbon nanomaterials in increasing electrical energy storage efficiency in batteries or enhancing the fluorescence sensing capabilities of biosensors. The findings are published in the July 12 issue of Nature Communications.

The breakthrough research was led by Chemistry Assistant Professor YuHuang Wang and conducted in the Nanostructures for Electrical Energy Storage center (an Energy Frontier Research Center of the Department of Energy), Northwestern University, and the Maryland NanoCenter.

Carbon nanotubes (CNTs) have enormous potential. They are some of the most conductive structures ever made—highly efficient electrodes with enormous surface area. To take full advantage of these properties, however, CNTs must be soluble—that is, have the ability to be dispersed in a liquid environment or to evenly coat a solid composite material. Unfortunately, in their raw state CNTs are insoluble; they clump together rather than disperse.

For more than a decade, researchers have been developing new chemical processes to address this challenge. One idea has been to create permanent defects on the surfaces of CNTs and "functionalize" them so they are soluble. Unfortunately, this also has the undesired side effect of quickly destroying the CNTs' electrical and optical properties.

Wang and his team have developed a new functionalization process for CNTs that delivers solubility and preserves electrical and optical properties. They purposefully functionalize defects on the tubes in useful--not random--places, creating strategic "functional groups." These carefully placed molecular groups allow CNTs to readily disperse while retaining their optical properties and ability to conduct electric current in large regions along the tube.

The challenge has been to control the chemical reactions that produce the functional groups on the CNTs. By using a chemical process called Billups-Birch reductive alkylcarboxylation, Wang's team found they could progressively add new functional groups to the CNT wall in a controlled way without introducing unintended new defects.

When the CNTs are immersed in a chemical solution for a specific length of time, the functionalized groups on the nanotubes lengthen by a predictable amount. Each time the process is repeated, or as the time in the solution increases, the sections grow longer. When the CNTs are viewed under a special, high magnification electron microscope, it is evident that the functionalization has progressed lengthwise along the tube.

The propagation can initiate from either naturally occurring or intentionally introduced defects. Because the propagation mechanism confines the reaction and strategically controls where the functional groups grow, Wang’s team can produce clustered functional groups at a controlled, constant propagation rate. It is the first clearly established wet chemistry process that does so.

The breakthrough makes it possible to create new functional structures such as "banded" nanotubes with alternating segments of functionalized and intact regions. The functionalized regions keep the CNTs from clumping, making them among the most water-soluble CNTs known. At the same time, the bands of intact, non-functionalized regions of the CNTs allow electrical and optical properties to be retained.

"This is important for the future use of these materials in batteries and solar cells where efficient charge collection and transport are sought," Wang explains. "These CNTs also could be used as highly sensitive biochemical sensors because of their sharp optical absorption and long-lived fluorescence in the near infrared regions where tissues are nearly optically transparent."

"This is a major step towards building the controlled nanostructures needed to understand electrochemical science and its value for energy solutions," says University of Maryland NanoCenter Director, Professor Gary Rubloff, a collaborator on the project.

The research team also includes theoretical chemist Professor George Schatz of Northwestern University, postdoctoral associates and graduate students Shunliu Deng, Yin Zhang, and Alexandra Brozena, who are equal contribution first authors, as well as Maricris Mayes, Parag Banerjee and Maryland NanoCenter staff member Wen-An Chiou.

More Information: "Confined propagation of covalent chemical reactions on single-walled carbon nanotubes"

YuHuang Wang's research group

About the Nanostructures for Electrical Energy Storage center, a DOE Energy Frontier Research Center
The Nanostructures for Electrical Energy Storage Energy Frontier Research Center (NEES-EFRC) is one of 46 EFRCs across the country established by the U.S. Department of Energy in 2009. The center develops highly ordered nanostructures that someday could store electrical energy more efficiently, offering greener solutions in smaller, lighter packages. Consisting of six institutions led by the University of Maryland, the Center has 17 senior investigators and approximately 35 students and postdoctoral fellows who study carbon, silicon, and ceramic materials and nanoscale geometries. More information

About the Maryland NanoCenter
The Maryland NanoCenter is a partnership between two University of Maryland colleges: The A. James Clark School of Engineering and the College of Computer, Mathematical, and Natural Sciences, with sustaining support from the University of Maryland. The Maryland NanoCenter promotes major nanotechnology research and education initiatives, provides one-stop shopping for those seeking expertise and/or partnerships at Maryland, and supplies infrastructure to facilitate nanotechnology activities at Maryland through equipment, staff support, and informational and administrative functions. More information

About the College of Computer, Mathematical and Natural Sciences
The College of Computer, Mathematical and Natural Sciences (CMNS) is nationally recognized for education and research. Many major programs are ranked among the top 10 public research universities in the nation. Students have the opportunity of working closely with first-class faculty in state-of-the-art labs both on and off campus on some of the most exciting problems of modern science, reflecting the evolving nature of our disciplines to the rapidly changing world of science and mathematics. CMNS includes the following departments: Astronomy, Atmospheric and Oceanic Science, Biology, Cell Biology and Molecular Genetics, Chemistry and Biochemistry, Computer Science, Entomology, Geology, Mathematics, and Physics. It also houses six research institutes. More information