Study: Pointed tips on aluminum octopods increase catalytic reactivity.
Points are important when developing nanoparticles that control important chemical reactions using the power of light.
Researchers at Rice University’s Laboratory of Nanophotonics (LANP) have long known that the shape of a nanoparticle affects how it interacts with light, and their latest study shows how shape affects a particle’s ability to use light to catalyze important chemical reactions use.
In a comparative study, LANP alumni Lin Yuan and Minhan Lou and their colleagues examined aluminum nanoparticles with identical optical properties but different shapes. The most rounded had 14 sides and 24 blunt points. Another was cube-shaped with six sides and eight 90-degree corners. The third, which the team called the “octopod,” also had six sides, but each of its eight corners ended with a sharp point.
All three types have the ability to capture energy from light and periodically release it in the form of over-energetic hot electrons that can accelerate catalytic reactions. Yuan, a chemist in LANP Director Naomi Halas’ research group, conducted experiments to determine how well each of the particles worked as photocatalysts for the hydrogen dissociation reaction. The tests showed that octopods had a reaction rate 10 times faster than the 14-sided nanocrystals and five times faster than the nanocubes. Octopods also had lower apparent activation energies, about 45% lower than nanocubes and 49% lower than nanocrystals.
“The experiments showed that sharper corners increase efficiency,” said Yuan, co-lead author of the study, which is published in the Journal of the American Chemical Society ACS Nano. “With the octopods, the angle of the corners is about 60 degrees, compared to 90 degrees with the cubes and rounded points on the nanocrystals. The smaller the angle, the greater the increase in reaction efficiency. How small the angle can be, however, is limited by chemical synthesis. These are single crystals that prefer certain structures. You can’t make it infinitely more sharp. “
Lou, physicist and co-lead author of the study in Peter Nordlander’s research group at LANP, verified the results of the catalytic experiments by developing a theoretical model of the energy transfer process of hot electrons between the light-activated aluminum nanoparticles and hydrogen molecules.
“We enter the wavelength of the light and the shape of the particle,” Lou said. “With these two aspects, we can predict exactly which shape will produce the best catalyst.”
The work is part of LANP’s ongoing efforts to develop environmentally friendly chemistry, commercially viable light-activated nanocatalysts that can inject energy into chemical reactions with surgical precision. LANP has already demonstrated catalysts for ethylene and synthesis gas production, the breakdown of ammonia to make hydrogen fuel, and the breakdown of “forever chemicals”.
“This study shows that the shape of photocatalysts is another design element that engineers can use to create photocatalysts with higher reaction rates and lower activation barriers,” said Halas, Stanley C. Moore Professor of Electrical and Computer Engineering at Rice, director of Smalley-Curl -Rice Institute and Professor of Chemistry, Bioengineering, Physics and Astronomy, and Materials Science and Nanotechnology.
Reference: “Morphology-dependent reactivity of a plasmonic photocatalyst” by Lin Yuan, Minhan Lou, Benjamin D. Clark, Minghe Lou, Linan Zhou, Shu Tian, Christian R. Jacobson, Peter Nordlander and Naomi J. Halas, August 12, 2020, ACS Nano.
DOI: 10.1021 / acsnano.0c05383
Nordlander is a Wiess chair and professor of physics and astronomy as well as professor of electrical and computer technology as well as materials science and nanotechnology.
Other co-authors on the study include Benjamin Clark, Minghe Lou, Linan Zhou, Shu Tian, and Christian Jacobson, all from Rice. The research was supported by the Welch Foundation (C-1220, C-1222), the Air Force Scientific Research Agency (FA9550-15-1-0022), and the Defense Threat Reduction Agency (HDTRA 1-16-1-0042). and the Department of Defense Graduate Fellowship Program.