“We see the possibility of manipulating the nanoscale structure of platinum so that we can have control over the size, porosity, composition, surface species, solubility, stability and other functional properties,” said John Shelnutt of Sandia. “Such control means that the redesigned platinum could be used in many new applications, including catalysis, sensors and optoelectronic and magnetic devices.”

Shelnutt and colleagues used a tin-porphyrin photocatalyst to kick-start the reduction of platinum salts by ascorbic acid in the presence of surfactant and light. This created a large number of seed crystals, each containing around 500 platinum atoms. Once the seed crystals had formed, the reaction became autocatalytic and platinum nanodendrites grew onto the seeds. The researchers tailored how the platinum grew by confining the porphyrin photocatalyst to particular locations - either attached to micelles or liposomes; carrying out the reaction without surfactant produced macroscopic pieces of platinum.

“To our knowledge, metallic platinum nanodendrites have not been made previously by autocatalytic growth or any similar approach,” said the researchers, in a paper in the Journal of the American Chemical Society.

When the porphyrin catalyst was linked to micelles, the platinum formed a ball-type dendritic nanostructure around the micelle. The scientists found they could control the size of the nanostructures by changing the porphyrin concentration, the concentration of platinum in solution, the amount of light illuminating the reaction and the length of time the light was on. They produced nanostructures with diameters of 6 to 200 nm.

In contrast, in the presence of porphyrins contained inside liposomes, the platinum grew in lace-like circular sheets around 2 nm thick on the surface of the liposome. And if conditions were such that the liposomes aggregated, platinum formed along the interfaces between the liposomes, creating foam-like materials.

The researchers also found that the platinum nanostructures, which remained attached to the porphyrin photocatalyst, were able to catalyze the evolution of hydrogen from water in the presence of light. Such reactions could be of interest to car manufacturers looking to build automobiles powered by hydrogen fuel cells.

The researchers reported their work in the Journal of the American Chemical Society.