Making the structure of 'fire ice' with nanoparticles
The structure harnesses a strange physical phenomenon and could enable engineers to manipulate light in new ways.

Date:
May 25, 2023
Source:
University of Michigan
Summary:
Cage structures made with nanoparticles could be a route
toward making organized nanostructures with mixed materials,
and researchers have shown how to achieve this through computer
simulations.


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FULL STORY ==========================================================================
Cage structures made with nanoparticles could be a route 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. It 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 Sharon Glotzer,
the Anthony C.

Lembke Department Chair of Chemical Engineering, who led the study
published today in Nature Chemistry. "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 -- like two short, three-sided pyramids stuck
together at their bases -- 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.

"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 Sangmin Lee, a recent doctoral graduate in chemical engineering
and first author of the paper.

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,
that shape became the trapped guest particle.

Glotzer has ideas for how to create selectively sticky sides that would
enable different materials to act as cage and guest particles, but in
this case, there was no glue holding the bipyramids together. Instead,
the structure was completely stabilized by entropy.

"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 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 researchers say. What's more, when
they 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.

Thi Vo, a former postdoctoral researcher in chemical engineering at U-M
and now an assistant professor of chemical and biomolecular engineering
at the Johns Hopkins University, contributed to the study.

This study was funded by the Department of Energy and Office of Naval
Research, with computing resources provided by the National Science Foundation's Extreme Science and Engineering Discovery Environment and
the University of Michigan.

Glotzer is also the John Werner Cahn Distinguished University Professor
of Engineering, the Stuart W. Churchill Collegiate Professor of Chemical Engineering, and a professor of materials science and engineering, macromolecular science and engineering, and physics.

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========================================================================== Story Source: Materials provided by University_of_Michigan. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* Cage_network_structure_illustration ========================================================================== Journal Reference:
1. Sangmin Lee, Thi Vo, Sharon C. Glotzer. Entropy compartmentalization
stabilizes open host-guest colloidal clathrates. Nature Chemistry,
2023; DOI: 10.1038/s41557-023-01200-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230525141429.htm

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