Shaping Light-Activated Nanocatalysts

A comparative study from Rice University researchers in the college’s Laboratory for Nanophotonics (LANP) demonstrated how the shape of nanoparticles affects their ability to drive light-activated reactions. The work is part of LANP’s ongoing green chemistry initiative to develop commercially viable, light-activated nanocatalysts that can insert energy into chemical reactions with surgical precision.

The researchers studied differently shaped, but otherwise identical, aluminum nanoparticles, the most rounded of which had 14 sides and 24 blunt points. The second was cube shaped with six sides and eight 90° corners. The last, which the team dubbed “octopod,” also had six sides, but each of its eight corners ended in a sharper-pointed tip.

A study of aluminum nanocatalysts by researchers in Rice University’s Laboratory for Nanophotonics found that octopods (left, six-sided particles with sharply pointed corners) had a reaction rate 5× higher than nanocubes (center) and 10× higher than 14-sided nanocrystals. Courtesy of Lin Yuan, Rice University.

Each variety had the ability to capture energy from light and release it periodically in the form of energetic hot electrons that can speed catalytic reactions. Lin Yuan, a graduate student and chemist in the research group of LANP Director Naomi Halas, experimented to see how each of the particles performed as photocatalysts for a hydrogen dissociation reaction. Tests demonstrated that the octopod shape had a reaction rate 10× higher than the 14-sided nanocrystal, and 5× higher than the nanocubes. Octopods also had a lower apparent activation energy — about 45% lower than nanocubes, and 49% lower than nanocrystals.

“The experiments demonstrated that sharper corners increased efficiencies,” Yuan said, co-lead author of the study. “For the octopods, the angle of the corners is about 60°, compared to 90° for the cubes and more rounded points on the nanocrystals. So the smaller the angle, the greater the increase in reaction efficiencies. But how small the angle can be is limited by chemical synthesis.”

“This study shows that photocatalyst shape is another design element engineers can use to create photocatalysts with the higher reaction rates and lower activation barriers,” Halas said.

LANP researchers previously demonstrated catalysts for ethylene and syngas production, the splitting of ammonia to produce hydrogen fuel, and for breaking apart what are known as “forever chemicals.”

The results were verified by Minhan Lou, a physicist and co-lead author in the research group of LANP’s Peter Nordlander, using a theoretical model of the hot electron energy transfer process between the light-activated aluminum nanoparticles and hydrogen molecules.

“We input the wavelength of light and particle shape,” Lou said. “Using these two aspects, we can accurately predict which shape will produce the best catalyst.”

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.0c05383).