m a te ri a l wo rl d
Carbon nanotubes take new shape
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arbon nanotubes in planar membrane sheets have become quite common. But experts are now excited about a novel arrangement of carbon nanotubes: a macroscopic, hollow cylinder (Nat. Mater. 2004, 3, 610–614). The new geometry, which was created by Pulickel M. Ajayan’s group at the Rensselaer Polytechnic Institute and Onkar N. Srivastava’s group at the Banaras Hindu University (India), could lead to more applications for carbon nanotubes, including their use in separations. Ajayan, Srivastava, and colleagues fabricated the cylinder by spraying a combination of benzene and a ferrocene-derived iron catalyst into the bore of a capillary tube. Hydrocarbons produced by the breakdown of benzene formed carbon nanotubes that grew along the radial axis of the capillary tube’s inner walls. Growth of the nanotubes inside the capillary occurred at a sufficiently high density that the attractive forces between the nanotubes were strong enough to form a continuous surface. Additional supporting polymers or other types of matrices were not necessary. The final result was a cylinder with a diameter over a centimeter and a length of several centimeters. The thickness of the cylinder wall ranged from 0.3 to 0.5 mm. Srivastava says that the investigators’ objective behind the growth of bulk cylinders of carbon nanotubes “was to find new applications [for nanotubes], which is not possible without definite shape.” The well-defined pore size of the cylinder is an improvement over current polymer membrane filters that contain a range of pore sizes. Charles Martin of the University of Florida says, “The geometry is a hollow fiber. That is extraordinarily important because a hollow-fiber geometry allows
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A scanning electron micrograph of a cylinder made out of carbon nanotubes. (Adapted with permission. Copyright 2004 Macmillan Publishers Ltd.)
you to pack more surface area into a given volume.” Bruce Hinds at the University of Kentucky points out another attractive feature: The fabrication is a chemical vapor deposition process that is easy to scale up. Ajayan, Srivastava, and colleagues showed that the cylinder was mechanically and thermally robust. The investigators measured a Young’s modulus— a measure of a material’s stiffness—of ~50 MPa for the cylinder, an indication that it could tolerate sufficient mechanical stress and pressure. The cylinders could also be heated to 400 °C. In comparison, conventional polymer membrane filters can only tolerate temperatures of ~50 °C. The cylinder was found to be chemically inert and therefore could be used for filtration applications. Hinds says, “It is nice to have aligned nanotubes in a cylinder geometry, because this way you can do cross-flow separations easily.” Cross-flow filtration occurs when a fluid flows parallel to the membrane sur-
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face and chemicals in the fluid diffuse through the membrane. An advantage of cross-flow filtration is that the membrane doesn’t get clogged with particles. The researchers used the new cylinder to separate petroleum into several of its constituent hydrocarbons. They also used it to remove microbes, including ~500-nm E. coli bacteria and ~25-nm poliovirus, from contaminated water. Once the separations were completed, the cylinder could be cleaned by sonication and autoclaving and re-used. Martin raises a single concern about the work. “There’s no evidence that the molecules are passing through the nanotubes themselves. The powerful feature of using carbon nanotubes for separations is you would like to think that the carbon nanotube diameter can be controlled at will, and that the inside tube walls could be chemically functionalized to give you the transport selectivity you want. But unfortunately, in this case, there’s no evidence that any of this is relevant.” On this point, Ajayan admits that the mechanism of filtration is not exactly known. He says, “The solvents could pass through the interstitial spaces or inside the nanotubes. Now, the problem is we can’t distinguish between the two because the spaces are pretty similar [in size]. We think the inside space of the nanotubes are plugged by [iron] particles, so the chances, I think, are [that] the filtration is occurring between the nanotubes.” The investigators are now interested in building other types of complex geometries out of carbon nanotubes. In addition, they hope to functionalize cylinders with different properties. “The real impact would be if you could modify the surfaces chemically so you make the filtration much more specific,” says Ajayan. a —Rajendrani Mukhopadhyay © 2004 AMERICAN CHEMICAL SOCIETY
