A
study of aluminum nanocatalysts by Rice University's Laboratory for
Nanophotonics found that octopods (left), six-sided particles with sharply
pointed corners, had a reaction rate five times higher than nanocubes (center)
and 10 times higher than 14-sided nanocrystals.
Courtesy: Lin Yuan/Rice
University.
Researchers
at Rice University's Laboratory for Nanophotonics (LANP) have long known that a
nanoparticle's shape 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.
In a
comparative study, LANP graduate students Lin Yuan and Minhan Lou and their
colleagues studied 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 dubbed "octopod," also had six sides, but each of its eight
corners ended in a pointed tip.
All three
varieties have the ability to capture energy from light and release it
periodically in the form of super-energetic hot electrons that can speed up
catalytic reactions. Yuan, a chemist in the research group of LANP director
Naomi Halas, conducted experiments to see how well each of the particles
performed as photocatalysts for hydrogen dissociation reaction. The tests
showed octopods had a 10 times higher reaction rate than the 14-sided
nanocrystals and five times 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,"
said Yuan, co-lead author of the study, which is published in the American
Chemical Society journal ACS Nano. "For the octopods, the angle of the
corners is about 60 degrees, compared to 90 degrees 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. These are single crystals that prefer certain structures.
You cannot make infinitely more sharpness.
Lou, a
physicist and study co-lead author in the research group of LANP's Peter
Nordlander, verified the results of the catalytic experiments by developing 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 work
is part of an ongoing green chemistry effort by LANP to develop commercially
viable light-activated nanocatalysts that can insert energy into chemical
reactions with surgical precision. LANP has previously demonstrated catalysts
for ethylene and syngas production, the splitting of ammonia to produce
hydrogen fuel and for breaking apart "forever chemicals.
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, said Halas, Rice's Stanley C. Moore Professor of
Electrical and Computer Engineering, director of Rice's Smalley-Curl Institute
and a professor of chemistry, bioengineering, physics and astronomy, and
materials science and nanoengineering.