Helical carbon nanotubes (CNTs) are expected to play a pivotal role in the nanoscale engineering of high-performance materials due to their ability to switch between metal and semiconductor behavior depending on the degree of twisting. They can also induce the formation of a magnetic field when subjected to an electric current — a highly desired property for electromagnetic nanodevices. However, the synthesis of CNTs into a double-helical structure that resembles that of DNA, which could broaden the range of potential applications, remains difficult.

Now, a research team led by Fei Wei from Tsinghua University in Beijing, China, has developed a method that generates double-helical arrays of CNTs using synthetic nanostructured anionic clays called layered double hydroxides (LDHs).1

Fig. 1: Schematic representation showing carbon nanotubes growing from a clay flake and coiling into double-helical arrays.© 2010 F. Wei

The researchers dispersed active iron catalyst nanoparticles on both faces of the ultralight LDH flakes, allowing the nanotubes to grow on both surfaces of the clay (Fig. 1). “Most aligned CNTs grow on only one side of a flat substrate, such as a wafer or quartz,” says Fei. “If they can be grown on both sides of an ultralight substrate, they easily self-organize into three-dimensional nanoarchitectures.”

Wei and his team prepared the clay flakes by heating a solution containing magnesium, aluminum and iron salts in the presence of urea to form a precipitate. They then sprayed the flakes uniformly over a quartz plate and placed the sample in a reactor heated to 750 °C in an argon atmosphere. Finally, they successively introduced ethylene and hydrogen gases into the hot reactor to produce the CNT arrays.

According to Wei, the CNT arrays grow freely from the LDH flakes in a way that minimizes stress associated with their development. “When the CNT array tips meet space resistance, they grow continuously from the LDH flake by coiling themselves into double helices,” he says.

The researchers found that introducing cobalt into the flakes altered the number of walls forming the shells of the nanotubes without affecting the double-helical structure. In situ current–voltage measurements showed that the arrays exhibited high current, which is needed for application in electromagnetic devices. Soaking the arrays in ethanol resulted in thinner but denser double helices with higher conductivity.

The team plans to tune the structure and helicity of the arrays for applications in electronics, electromagnetics, composites and energy conversion and storage. “Patterned three-dimensions nanoarchitectures based on CNT double helices will also be explored,” says Wei.