In a past life I was deputy editor on the RSC journal Chemical Communications and recall the excitement and tension around the time we published Sir Harry Kroto’s pioneering fullerene paper. At the time, there were all kinds of imaginative plans emerging for what might be done with these odd all-carbon soccerball shaped molecules.
However, it was not the spherical, nor the spheroidal, fullerenes that became the darlings of materials scientists and nanotechnologists the world over. Instead it was their stretched cousins – the carbon nanotubes. Resembling a sub-microscopic roll of chicken wire, these long, hollow molecules have been touted as potential components for the future of microelectronics, as conducting connectors for nano devices, as catalysts and even as smart drug delivery agents.
Chemical scientists have developed various methods for synthesising nanotubes that are just a single atom thick, others that have a double wall, like two layers of rolled up chickenwire, all just a few nanometres in cross-sectional diameter. However, according to researchers writing in the International Journal of Nanotechnology (2007, 4, 618-633) there are no widely accepted techniques for producing useful quantities of short nanotubes of a specific length.
Simon Smart, G.Q. Lu, and D.J. Martin of the Australian Institute for Bioengineering and Nanotechnology, University of Queensland, Australia, and W.C. Ren and H.M. Cheng of Shenyang National Laboratory for Materials Science, of the Chinese Academy of Sciences, have now devised a production method for making shortened double-walled carbon nanotubes using by high-energy ball milling.
On the everyday scale of things, a ball mill is type of cylindrical grinder within which are loose balls (ceramic, steel, or flint pebbles, commonly) and to which is added the material to be milled. The ideal technique for shortening nanotubes has to have three characteristics, say the researchers. First, from a practical point of view, it should be able to produce gram quantities of individual samples. Secondly, it has to be able to shorten the nanotubes without significantly impacting on their purity of destroying the nanotubes entirely. Finally, the method has to be controllable so that it can shorten the nanotubes reproducibly and accurately.
Other researchers have attempted to shorten nanotubes using ultrasonic agitation, chemical cutting techniques, and ball milling. Ultrasound is not particularly controllable in terms of producing large quantities of nanotubes of a similar length while chemical methods are convoluted and can damage the nanotube walls. So, Smart and colleagues have focused on ball milling. This technique requires no chemical additives and can have a high throughput.
The team tested the shortened nanotubes using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS).
The researchers now plan to use their ball-milled carbon nanotubes in novel polymer nanocomposite materials and to carry out toxicological studies. It should be possible to disperse these shorter nanotubes much more effectively in composite materials, explain the researchers, because they do not form bundles so readily as long nanotubes but would still endow the nanocomposite with novel strength and flexibility properties. “In fact, early results are showing that in polyurethane elastomers, the shorter nanotubes out perform the longer ones,” Smart told Sciencebase. He adds that, “These materials are most suited towards polymer nanocomposite materials. The presence of
carbonyl functional groups on the sidewalls does lend itself towards further chemistry and possible applications in drug delivery or sensing applications. However, at this stage of research, the ball milling process induces too many defects for use in applications that utilize the nanotubes
unique electronic properties.”