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Posted: Dec 12, 2011
Promising method for the synthesis of palladium nanosprings
(Nanowerk News) In order to exploit the particular material properties that appear at the nanoscale, it is first necessary to fabricate materials with nanoscale structures in a controlled and repeatable fashion. Reliable methods for the fabrication of simple shapes such as nanorods, nanocubes and nanotubes are now available, but more complex shapes still pose a challenge. Sungho Park and co-workers from Sungkyunkwan University in Korea have now reported a promising method for the synthesis of palladium nanosprings ("Wet-Chemical Synthesis of Palladium Nanosprings").
A scanning electron microscopy image of palladium nanosprings
"We wanted to be able to make more complex nanostructures," explains Park. "Structures such as our nanosprings could be used as functional parts in nanomachines, sensors or photonic metamaterials, but the fabrication methods are expensive, requiring high-purity chemicals and expensive equipment."
Nanofabrication techniques can be broadly divided into top-down approaches, involving the gradual removal of material to etch out a nanostructure, and bottom-up methods, which as the name suggests involves assembling the structure from scratch using various molecules as building blocks.
The new nanospring fabrication described by Park and his colleagues has elements of both approaches. They first cast an acidic solution containing palladium and copper salts inside a nanochannel template made of aluminum oxide. The application of a voltage then caused hydrogen to be produced on the nanochannel surface, reducing the palladium salt to form a crystal structure but leaving the copper salts unmodified. In the center of the channel, both salts are reduced due to the distribution of electric potential.
The result is a nanorod with a copper core and palladium shell. After removing the template and etching away the copper using nitric acid, the researchers found that the palladium had formed a coiled structure resembling a nanospring (see image). "Our initial investigations indicate that this coil structure is formed by screw dislocation — a common type of defect that occurs in crystal growth," says Park.
By controlling the electrical charge transported across the template, the researchers were able to prepare nanosprings with a variety of lengths, and they anticipated that springs with a range of diameters could be prepared in this way using different templates. "We expect that the methodology will be applicable to the synthesis of other interesting nanostructures," says Park.