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  • [ September 19, 2011]

    Small body, big role

  • Nanotubes can clean up oil leakage, remove heavy metals from water, or even be woven into soldiers’ clothes to provide electricity.

    Nano-scientist Anyuan Cao, who is working in nanotechnology and nanomaterial in the Department of Materials Science and Engineering, is exploring carbon nanotubes’ potential in new energy and environmental areas.

    Carbon nanotubes are very small in diameter. The diameter of a human hair is equivalent to that of about 100,000 nanotubes; it can only be seen by an electrical microscope. However, nanotubes have excellent mechanical, optical and electrical properties.

    “It is robust and elastic, with very high electrical conductivity,” said Anyuan Cao.

    In one of his research programs, Cao and his research fellows create nanotube sponges which can selectively and quickly absorb oil membranes and a wide range of solvents in preference to water, giving nanotubes great potential as environmental materials. Made up of a large number of interconnected CNT skeletons, the nanotube sponge has very low density and 99 percent porosity, so it will not sink as a polyurethane sponge for kitchen use does when placed in water. Experiments in Cao’s lab have shown that nanotube sponge has excellent absorption capacity of up to 180 times its own weight and adimension area of up to 800 times that of the sponge, making it an excellent choice for oil cleanup.

    A sponge pellet cleaning up a diesel oil film spreading on water and swelling to a rectangular shape after absorbing all the oil

    Carbon nanotubes research offers other promising uses in the environment.

    “One is to use it as a filter to remove contaminants such as heavy metals from water. The other is to detoxify vehicle exhaust or decompose water pollutants by putting catalysts in the porous sponge,” Cao said.

     Many companies in the industry have already shown interest in them.

    Carbon nanotubes also function well as a good conductor to transfer sunlight’s power into electricity. In another program, Cao and his colleagues try to replace silicon-based solar cells with carbon nanotubes as a cheap alternative. They use carbon nanotubes to make transparent conductive films and coat them on silicon cells to form hybrid CNT-Si solar cells. Compared to conventional all-silicon cells, the fabrication process is simplified and cheaper. It does not involve a high temperature diffusion process or special instruments.

    However, the disadvantage lies in its inefficiency, as hybrid cells convert a meager 7 percent of sunlight’s power into electricity, which is below the economic efficiency barrier (10 percent). Nevertheless, the group’s recent work has improved the efficiency to 13.8 percent by combining carbon nanotubes and Si and doping with dilute nitric acid, for when acid infiltrates nanotube networks it reduces the internal resistance, providing more paths for charge carriers to exit the cell and produce external current.

    This work has been published on Nano Lett. 2011, 11, 1901-1905 and selected as one of the research highlights by Nature, May 2011, Vol 473, 127.

    Besides the CNT-Si approach, Cao’s group is also working on all nanomaterial-based solar cells, and using carbon nanotubes and nanowires to make fiber-shaped photovoltaics. Compared with conventional planar solar cells, thin fiber-shaped cells have advantages such as lightweight and device flexibility and can be integrated into textiles in various forms.

    “Our goal is to select appropriate materials with good properties and develop good device structures,” said Cao.

    Enlarged view of CNT-covered TiO2 nanotube solar cells

    In his lab,Lab for Nano Devices and Applications, Cao and his researchers create carbon nanotubes and nanowires, and test solar device output power with a machine that can stimulate the sunlight. Other equipment tests the nanotube’s mechanical properties.

    Cao’s research group currently includes seven graduate and three undergraduate students.The group also has close ties with scientists and chemists from Tsinghua University, Stanford University, Rice University and also the College of Chemistry at Peking University.

    “Nanomaterial studies is interdisciplinary,” said Cao. “You have to cooperate with people from other aspects.”

    Anyuan Cao joined the College of Engineering  in January 2009 after a seven year stint in the U.S. including a three years  as an assistant professor at the University of Hawaii. His current research is supported by the National Natural Science Foundation of China and the Beijing Natural Science Foundation.