27 July 2005: Carbon nanotubes receive support for metallisation

Carbon nanotubes have been fixed to metal surfaces to increase the range of metals that can be coated by electroless deposition  (deposition without electrodes)  .

The functionalisation of carbon nanotubes with metals is key to improving their physical and chemical characteristics, especially their electrochemical properties. Electroless deposition by displacement reaction is an attractive option because of its inherent simplicity and the potential for large-scale assembly. Metal ions with a redox potential higher than that of the nanotubes are deposited from solution onto the nanotubes.

The addition of a reducing agent that spontaneously reduces the metal is one adaptation that has increased the range of metals that can be deposited. Now, researchers at the University of Dayton, US, have devised a simple alternative, which they call substrate-enhanced electroless deposition (SEED).

Nanotubes are supported on the surface of a metal with a redox potential lower than that of the metal to be deposited. Metal nanoparticles are formed on the outer walls of the nanotubes without the use of reducing agent. The researchers prepared nanotubes in an alumina membrane which was subsequently fixed to copper foil with sticky tape. On immersion in a solution of tetrachloroauric acid, gold nanoparticles were formed spontaneously on the nanotube walls within 10 seconds.

Platinum and palladium were also deposited and, by replacing the copper foil with zinc, which has a lower redox potential than copper, the range of deposited metals was extended further to include copper. Preliminary research showed that metal nanoparticles can also be deposited on the inner wall surfaces and that both types of coated nanotubes are electrochemically active.

’The SEED process is simple but highly effective for electroless deposition of various nanoparticles onto many conducting/semiconducting materials, including carbon nanotubes,’ team leader Liming Dai told Chemistry World. ’In principle, there is no size limit to SEED.’

Milo Shaffer, researcher in nanomaterials at Imperial College, London, UK is intrigued, but notes potential drawbacks. ’The nature of the technique is likely to limit its scalability, and the presence of the sacrificial metal may not always be convenient,’ said Schaffer. ’However, extending the range of metals that can be deposited, apparently with ease, may lead to applications in (electro)catalysis, supercapacitors, or gas sensors.’ Steve Down