In many instances, platinum works more efficiently if it is in a form that maximizes its surface area.
Until now, platinum crystals had only five, six, or eight sides. But researchers at Georgia Institute of Technology have made 24-sided platinum crystals with up to four times the catalytic activity per unit area as current platinum particles. The increased activity is due to what chemists call dangling bonds and atomic steps that dot the crystals' high-energy surface.
The crystals are terahexahedrals made electrochemically from polycrystalline platinum spheres (750-nm diameter) on a substrate of glassy carbon, which is also called amorphous carbon. Carbon's role in this transformation is not yet fully understood, but it is thought to keep the resulting crystals nearly all the same size and shape. Altering the voltage applied to the platinum spheres controls the crystal's size, and scientists can create batches of crystals that vary in size by no more than 4.5%.
The crystals remain stable at temperatures up to 800°C, but that could climb higher as research continues. This stability means the crystals could be recycled and reused.
The problem is that the crystals are too large, 20 times the size of current platinum catalysts, so the total surface area from a given amount of platinum is reduced. "We need to find a way to make these nanocrystals smaller while preserving the shape," says Zhong Lin Wang, a professor at Georgia Tech. "If we can reduce the size by better controlling the processing, we will have a catalytic system that produces hydrogen more efficiently."
Platinum crystals with 24 facets represent a new form of that element. The small crystals also have highly energetic surfaces, making them well suited to serving as catalysts.