Researchers 'redesigning' platinum

Researchers have developed a way of changing the properties of platinum by manipulating the metal at the nanoscale. The method mimics the action of photosynthetic proteins.

As a result, the future may see the development of smaller and/or more sensitive catalysts, sensors, and other devices. Investigations of the new technique are continuing at Sandia and the University of New Mexico.

"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 of these metal nanostructures," says Sandia's John Shelnutt.

He adds that while research groups have reported a few platinum nanostructures -- including nanoparticles, nanowires, nanosheets, and others -- the addition of new types of nanostructures is "highly desirable and potentially technologically important."

Like photosynthesis

The idea for the technique is similar to photosynthesis, a process in which plants use the energy from sunlight to produce sugar. But instead of manufacturing sugar, the new method changes a platinum ion to neutral metal atoms.

Certain molecules mimic these photosynthetic proteins, repeatedly converting metal ions each time light is absorbed and depositing the metal atoms as desired at the nanoscale.

The method involves putting porphyrin molecules -- the active part of photosynthetic proteins -- along with the platinum salt in an aqueous solution of ascorbic acid at room temperature. When light is shined on the solution, the porphyrins excite, becoming catalysts for platinum reduction and deposition. As this occurs, the metal grows onto the surfaces of the surfactant structures as a thin sheet or in other ways.

For the metals platinum and palladium that form these nanostructures, it is enough for the porphyrin molecule to grow only a small metal "seed" particle composed of about 500 atoms. When it reaches this size, the seed starts to catalyze its own rapid growth (by oxidation of ascorbic acid), budding off arms in all directions and creating the "Koosh^TM"-ball-like nanostructures.

The platinum nanostructures take on a different form when they are prepared under different conditions -- for example they may look like three-dimensional Koosh^TM balls or lace-like sheets. Under some solution conditions, growth can give platinum foam-like materials and foam nanoballs.

Since the porphyrin remains attached to the platinum nanostructure and active in the presence of light, it can also perform other functions besides growing itself. For example when illuminated with light, the platinum nanostructure evolves hydrogen from water. This reaction is similar to one of interest to car manufacturers looking for new ways to build automobiles powered by hydrogen fuel cells.

Technical Contact: John Shelnutt jasheln@sandia.gov, 505-272-7160

Media Contact: Chris Burroughs coburro@sandia.gov, 505-844-0948