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
“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.
We report a methodology for synthesis of palladium (Pd) nanospring structures using an anodic aluminum oxide (AAO) membrane template and facile electrochemical deposition. The hydroxyl-terminated surfaces of alumina nanochannels and localized hydrogen evolution contribute to the growth of Pd atoms at peripheral positions of the alumina nanochannels in the presence of an effectual electric potential and a plating solution consisting of PdCl2, CuCl2, and HCl. Structural characterization including EDS line analysis and element mapping revealed Pd nanodomains curling up on the Cu nanorods. A clear Pd nanospring shape was observed after selectively removing Cu. The lengths of the nanosprings were dictated by the charges transported through electrodeposition, and the diameters of the nanosprings were tunable by altering the diameter of the alumina nanochannels. Screw dislocation is the most probable crystallographic defect responsible for the formation of coiled Pd nanostructures. Pd nanosprings have potential applications in nanomachines, nanosensors, nanoinductors, and metamaterials. We anticipate that our synthesis method will motivate and inform the synthesis of more advanced nanomaterials.