Cage structures made with nanoparticles could be a way to create organized nanostructures with mixed materials, according to researchers at the University of Michigan, who showed how to do it using computer simulations.
The discovery could open the way for photonic materials to manipulate light in ways that natural crystals cannot. It also showed an unusual effect known as entropy compartmentalization, which the team named.
The structure takes advantage of an unusual physics phenomenon and could allow engineers to manipulate light in new ways.
Entropy is often explained as a disorder in a system. However, it reflects the system’s tendency to more correctly maximize its possible states.
Sharon Glotzer, Anthony C. Lembke’s Department Chair of Chemical Engineering, who led the study, said: “We are developing new ways to structure matter across scales, discovering the possibilities and what forces we can use. Entropic forces can stabilize even more complex crystals than we thought.”
While entropy is often explained as a disorder in a system, it more accurately reflects a tendency to maximize its possible states. This often leads to disorder in the informal sense. Oxygen molecules don’t huddle together in a corner, but spread out to fill a room. However, if you place them in the right size box, they spontaneously organize into a recognizable structure.
On the left, the blue bipyramid particles fan out around the red bipyramid particles, which resemble blue daisy petals with red centers peeking out of the page.
Red host bipyramids are nestled between and within the spherical blue-gray cages in the layer of the structure depicted with the cages on the right.
Nanoparticles do the same function. Glotzer’s team previously showed that if bipyramid particles (two short, three-sided pyramids hooked together at their base) are placed in a sufficiently compact envelope, they will build structures like fire ice. Fire ice is formed from water molecules that form cages around methane and can both burn and melt.
This substance, abundant under the ocean floor, is an example of a clathrate. Clathrate structures are being studied for various applications, including the capture and removal of carbon dioxide from the environment.
Unlike water clathrates, previous nanoparticle clathrate formations lacked gaps that could be filled with other materials to provide new and exciting possibilities for altering the features of the structure. The team wanted to change that.
“This time we investigated what happens when we change the shape of the particle. We reasoned that if we shortened the particle a bit, space would be created in the cage of the bipyramid particles.” said Sangmin Lee, a recent doctorate in chemical engineering and the paper’s first author.
He took out the three center corners of each bipyramid and found the sweet spot where cavities formed in the structure. However, the sides of the pyramids remained so intact that they did not rearrange.
When they were the only particle in the system, the cavities were filled with more truncated bipyramids. When a second form was introduced, it became the captured guest particle.
Glotzer has ideas for creating selectively sticky sides that allow different materials to act as cage and host particles. In this case, however, there was no glue holding the bipyramids together. Instead, entropy stabilized the structure.
Glozer said, “The fascinating thing about the simulations is that the host network is almost completely frozen. The host particles move, but they all move together as a single, rigid object, which is exactly what happens with water clathrates. But the host particles are spinning like crazy – as if the system has dumped all the entropy into the host particles.”
Oxygen molecules do not creep into corners. However, if placed in the correct size box, they will spontaneously organize into a recognizable shape. Clathrate structures are being studied for a variety of applications, including capturing and removing carbon dioxide from the environment.
On the other hand, previous nanoparticle clathrate structures lacked gaps to fill with different materials that could open up new and fascinating possibilities to alter the properties of the structure.
Sangmin Lee, the paper’s first author and recent PhD in chemical engineering, took out the three central corners of each bipyramid and found the sweet spot where spaces appeared in the structure. However, the sides of the pyramids remained so intact that they did not organize themselves differently.
When a second shape was added, that shape became the captured guest particle. Glotzer has ideas for creating selectively sticky sides that allow different materials to act as cages and host particles. In this case, however, no glue held the bipyramids together.
The blue bipyramids of the cages form a network dotted with red guest bipyramids. The blue bipyramids build cages around the red host particles in a 3D representation of the structure.
The essential information in this section is that the host network of water clathrates is almost frozen, with the host particles moving together as a single, rigid entity while the host particles spin frantically.
This system had the most degrees of freedom that the truncated bipyramids could build in a limited space. However, the host particles controlled pretty much everything. After the host particles disappeared, the structure dumped bipyramids that were part of the networked cage structure into the cage interiors.
This study was funded by the Department of Energy and the Office of Naval Research.
Magazine reference:
- Lee, S., Vo, T. & Glotzer, et al. Entropy compartmentalization stabilizes open host-guest colloidal clathrates. Nature chemistry. DOI: 10.1038/s41557-023-01200-6
