Fig. 1: Transmission electron microscopy image of carbon nanotubes synthesized using the ECS method.

The structure of carbon nanotubes (CNTs)—diameter , single or multi-walled and so on—affects their physical and chemical properties. Thus development of methods for synthesizing CNTs exhibiting the desired structures is important for device applications. Jeung Ku Kang and colleagues1 from Korea Advanced Institute of Science and Technology have developed a method enabling control of both diameter and number of walls during the growth of CNTs. The researchers used an exposed-core/shell (ECS) method, which also allowed flexible manipulation of the alignment and patterning of the nanotubes on the substrate.

Kang and his group started with a solution of polymer micelles. Then, iron precursors were added to this solution, which accumulated in the micelle cores. Next, by simply spin-coating the resulting solution onto a substrate and plasma treatment, the researchers created hexagonal patterns of iron nanoparticles. A second plasma treatment—using nitrogen—created iron nitride in the cores of the nanoparticles.

Importantly, unlike the outer pure iron, the iron nitride is inactive as a catalyst in the chemical vapour deposition technique that the researchers used to synthesize the nanotubes. Chemical etching of the nanoparticles exposed this inner core, leaving only a narrow ring around the edge of catalytically active particles.

The researchers compared their so-called ECS method with direct synthesis using untreated iron nanoparticles. The nanotube diameter (7–8nm) was unaffected by the iron nitride core, however, the number of walls varied from six or seven for pure iron particles to only two or three for the ECS method (Fig. 1). Both types of catalysts produced vertical nanotubes with average lengths of 48μm.

“Our method enabled us to control the number of walls in carbon nanotubes based on the observation that they always grow along the active catalyst.” says Kang.

Highly efficient storage of gas is just one of many potential applications of these nanotubes. “By optimizing the diameter and number of walls in carbon nanotubes for gas adsorption, “says Kang. “batteries and biosensors are also an exciting field in which this method can be applied because the diameter, number of walls and even chirality can be controlled with this method.”