The intrinsically large surface area to volume ratio of carbon nanotubes makes them attractive for storing hydrogen, electrodes for batteries and chemical catalysis. But if similar structures could be made from materials other than carbon then this potential could be even greater. Although several techniques for doing so have been reported, most rely on complex chemical processes that are applicable to only a narrow range of materials. More importantly, such processes can introduce unwanted impurities that can have adverse effects on the properties of the resulting nanotubes.

Now, M. Venkata Kamalakar and Arup K. Raychaudhuri of the S. N. Bose National Centre for Basic Sciences, India describe a simple approach—employing porous alumina templates with arrays of cylindrical pores—for growing metal nanotubes that could avoid such problems.1 Similar templates have been used to grow solid metal nanowires, but their use to grow tubular metal structures has only been possible by identify suitable chemical reactions to induce selective deposition of metals on the pore walls. But here, instead of relying to chemistry to achieve such selectivity, Kamalakar and Raychaudhuri used a rotating electric field.

Fig. 1: Electric field assisted growth of copper nanotubes. When a porous alumina template immersed in a solution of copper sulphate is subjected to a rotating electric field perpendicular to its pores, the resulting helical motion of copper ions cauReproduced with permission. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The field was generated by applying two sinusoidally varying electric fields to two sets of electrode plates aligned perpendicular to each other and to the pores in an alumina template. Next, when the template was immersed in a solution of copper sulphate, the electric field caused copper ions to travel in circles within its pores. At sufficiently large fields, the diameters of the rotating ion were larger than those of the pores, which caused the ions to collide and adhere to the pore walls (grazing the wall surface in course of their motion), thus forming cylindrical copper films. Subsequent chemical etching of the template yielded aligned arrays of copper nanotubes (see Fig. 1).

The versatility of this procedure was demonstrated by the growth of 160–230 nm diameter copper nanotubes with walls that were a mere 20 nm thick. Electron energy dispersive analysis and X-ray diffraction showed them to be both single crystalline and chemically pure—important properties for practical applications. Most significantly, the authors state that their procedure in principle could be used to grow nanotubes made from almost any material that can be electrodeposited, including many metals and even some semiconductors.