Controlling the morphology of nanocrystals is important because many of their physical and chemical properties are highly shape-dependent. Most research so far has focused on heterostructures made on chalcogenide interfaces, yet controlling the shape of individual binary metallic nanoparticles has been little studied.

Yang and co-workers used highly faceted cubic platinum nanocrystals as nucleation seeds to direct the overgrowth of a lattice-matched palladium-based compound in solution. The platinum cubes measured around 13 nm along each face and have only (100) surface planes. The Pd undergoes conformal growth on the surface of these seeds to produce Pt/Pd bimetallic core-shell structures.

Depending on the reaction environment, which contains nitrogen dioxide, the team obtained three different shapes – cubes, cuboctahedra and octahedra – with different types of surfaces. These were (100) surface planes for cubes, (111) surfaces for octahedra and mixed surface for cuboctahedra. The researchers were able to observe these structures using electron microscopy (see figure). They say that the amount of nitrogen dioxide in the reaction system is responsible for the final shape of the nanocrystals because it regulates the growth rate of the Pd along its <100> and <111> directions.

In contrast, when no seeds were used no shape control was seen, just a mixture of large particles including non-polyhedral particles, rods and various faceted particles. According to the team, this means that the cubic Pt seeds serve two purposes: providing a well-defined surface for the overgrowth of the secondary metal and dictating the final shape of the core-shell heterostructures.

"Such shape control enables the production of nanocrystals with specific shapes and surface types," Yang told nanotechweb.org. "It could therefore allow catalytic reactions with great selectivity."

The researchers say that their technique could be applied to other material systems, for example ruthenium, iron-platinum and cobalt-platinum. These novel heterostructures could be used for catalysis, as well as optical and magnetic applications, where shape control plays a crucial role.

The work was published in Nature Materials.