Tech Briefs Magazine - November 2023 - 45

Materials & Coatings
Making the Structure of 'Fire Ice' with Nanoparticles
Cage structures made with nanoparticles could be a step toward making organized
nanostructures with mixed materials.
University of Michigan, Ann Arbor, MI
C
age structures made with nanoparticles
could be a step toward making
organized nanostructures with mixed materials,
and researchers at the University
of Michigan have shown how to achieve
this through computer simulations.
The finding could open new avenues
for photonic materials that manipulate
light in ways that natural crystals can't.
The work, published in Nature Chemistry,
also showcased an unusual effect
that the team is calling entropy compartmentalization.
" We
are developing new ways to structure
matter across scales, discovering the
possibilities and what forces we can use, "
said study lead Sharon Glotzer, the Anthony
C. Lembke Department Chair of
Chemical Engineering. " Entropic forces
can stabilize even more complex crystals
than we thought. "
While entropy is often explained as
disorder in a system, it more accurately
reflects the system's tendency to maximize
its possible states. Often, this ends
up as disorder in the colloquial sense.
Oxygen molecules don't huddle together
in a corner, they spread out to fill a
room. But if you put them in the right
size box, they will naturally order themselves
into a recognizable structure.
Nanoparticles do the same thing.
Previously, Glotzer's team had shown
that bipyramid particles will form structures
resembling that of fire ice if you
put them into a sufficiently small box.
Fire ice is made of water molecules that
form cages around methane, and it can
burn and melt at the same time. This
substance is found in abundance under
the ocean floor and is an example of a
clathrate. Clathrate structures are under
investigation for a range of applications,
such as trapping and removing carbon
dioxide from the atmosphere.
Unlike water clathrates, earlier
nanoparticle clathrate structures had
no gaps to fill with other materials that
might provide new and interesting possibilities
for altering the structure's properties.
The team wanted to change that.
Tech Briefs, November 2023
The cages of the host network of bipyramid particles are shown in blue on the left side, becoming
increasingly transparent toward the right. The red bipyramid particles are guest particles, trapped
in the cages of the clathrate structure. (Image: Sangmin Lee, Glotzer Group)
" This time, we investigated what happens
if we change the shape of the particle.
We reasoned that if we truncate the
particle a little, it would create space in
the cage made by the bipyramid particles, "
said first author Sangmin Lee.
He took the three central corners off
each bipyramid and discovered the sweet
spot where spaces appeared in the structure,
but the sides of the pyramids were
still intact enough that they didn't start
organizing in a different way. The spaces
filled in with more truncated bipyramids
when they were the only particle in the
system. When a second shape was added
it became the trapped guest particle.
" What's really fascinating, looking at
the simulations, is that the host network
is almost frozen. The host particles
move, but they all move together like
a single, rigid object, which is exactly
what happens with water clathrates, "
Glotzer said. " But the guest particles
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are spinning around like crazy - like
the system dumped all the entropy into
the guest particles. "
This was the system with the most degrees
of freedom that the truncated bipyramids
could build in a limited space,
but nearly all the freedom belonged to
the guest particles. Methane in water
clathrates rotates too, the team said.
What's more, when the researchers removed
the guest particles, the structure
threw bipyramids that had been part of
the networked cage structure into the
cage interiors - it was more important
to have spinning particles available to
maximize the entropy than to have complete
cages.
" Entropy compartmentalization. Isn't
that cool? I bet that happens in other
systems too - not just clathrates, " Glotzer
said.
For more information, contact
umichnews@umich.edu.
45

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Tech Briefs Magazine - November 2023

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