Nano Lett. http://doi.org/qhp (2013)

The electrochemical activity of carbon nanotubes is not well understood. Some researchers have reported high conversion efficiencies, which suggest that the nanotubes could be useful in sensing and energy applications, but others are more sceptical and are not ready to discard subtle side effects. To shed light on the situation, Patrick Unwin and colleagues at the University of Warwick have now developed a microscopy technique that can map the electrochemical response of individual carbon nanotubes.

The microscope uses a tip at the bottom of which a tiny meniscus of redox-active solution is formed. The tip is scanned across a carbon nanotube while a potential is applied to the nanotube. The electrochemical activity of the carbon nanotube is then recorded as a function of tip position by measuring the current generated by the redox reaction that occurs in the meniscus. The resolution of the technique is around 6 nm, which means that the role of defects can be accounted for because they are more widely spaced than this.

With the approach, the researchers show that, under the right amount of bias, the sidewalls of both metallic and semiconducting nanotubes are electrochemically active towards simple, one-electron transformations, a result that has previously been disputed.