The exceptional electronic and mechanical properties of few-walled carbon nanotubes (FWCNTs) promise to revolutionize nanoelectronics and materials sciences, but their tendency to aggregate into ropes and bundles negatively affects their performance. One way to prevent this detrimental clustering, and thus optimize interactions with the surrounding environment, is to modify the FWCNT outer wall surface with various chemical groups. Using benzoic acid derivatives as starting materials, Jong-Beom Baek and coworkers from the Ulsan National Institute of Science and Technology in Korea and the Georgia Institute of Technology in the USA have successfully introduced chemical groups onto FWCNTs under mild conditions1.

Reactions that add functional chemical units to FWCNTs typically need to be carried out under harsh acidic conditions, which also have a detrimental effect on nanotube properties. “Functionalization in strong acids such as sulfuric acid and nitric acid transforms carbon nanotubes into amorphous carbon materials,” says Baek. “Our group has developed a non-destructive functionalization method for these materials.” The researchers relied on a mixture of milder reagents — poly(phosphoric acid) and phosporous pentoxide — to attach the benzoic acid derivatives to the nanotubes and generate FWCNTs with a range of surface wettability.

Fig. 1: Snapshot of carbon nanotubes modified with polar nitrogen-containing groups hydrated with 800 water molecules. The FWCNTs are laterally interconnected through hydrogen bonding.© 2010 J.-B. Baek

The researchers evaluated the ability of nanotubes modified with either ‘polar’ nitrogen-containing functional groups or less-polar carbon-based groups to absorb water. Unexpectedly, the nanotubes bearing non-polar groups absorbed substantially more water than their polar counterparts. Computer simulations revealed that this surprising sponge-like behavior occurred only when the nanotubes were surrounded by a large number of water molecules. According to Baek, the polar FWCNTs form stronger hydrogen-bonding interactions with neighboring polar nanotubes than with water molecules (Fig. 1). As a result, they form a network from which water molecules get squeezed out.

Furthermore, the team found that the surface properties of the modified nanotubes could determine their electrochemical activity and affinity toward ions. Specifically, they discovered that, unlike the amino-ending groups, the ethyl-based groups covered the nanotube surfaces uniformly, hampering the inclusion and exclusion of ions.

Baek believes that their polar FWCNTs can be heat-treated to produce nitrogen-doped nanotubes, which are useful as oxygen reduction catalysts. The team is also planning to extend their chemistry to the mass production of high-quality graphene — another technologically important carbon-based material.