This magnified image shows nanoscale mesocrystals (inset) starting to assemble and form an ordered supracrystal structure, seen in green. Image: Inna Soroka.
This magnified image shows nanoscale mesocrystals (inset) starting to assemble and form an ordered supracrystal structure, seen in green. Image: Inna Soroka.

A team of researchers from the KTH Royal Institute of Technology in Sweden and the Max Planck Institute of Colloids and Interfaces in Germany has found the key to controlled fabrication of cerium oxide mesocrystals. Reported in a paper in Angewandte Chemie International Edition, this research represents an advance in tuning these nanomaterials, which can serve a wide range of uses – including solar cells, fuel catalysts and even medicine.

Mesocrystals are nanoparticles with identical size, shape and crystallographic orientation, and they can be used as building blocks to create artificial nanostructures with customized optical, magnetic or electronic properties. In nature, these three-dimensional structures are found in coral, sea urchins and calcite desert rose, for example. Artificially produced cerium oxide (CeO2) mesocrystals – or nanoceria – are well-known as catalysts, with antioxidant properties that could be useful in pharmaceutical development.

“To be able to fabricate CeO2 mesocrystals in a controlled way, one needs to understand the formation mechanism of these materials,” says Inna Soroka, a researcher in applied physical chemistry at KTH. Using radiation chemistry, she and her team have now determined this formation mechanism for the first time.

Because of the complexity of mesocrystals, it turns out they don’t follow the same formation path as ordinary crystals – a process called Ostwald Ripening, in which smaller particles in solution dissolve and deposit on larger particles. Instead, the researchers found that a gel-like, amorphous phase forms a matrix in which primary particles, about 3nm in size, first align with each other and then self-assemble into mesocrystals with a diameter of 30nm.

“If the mesocrystal was a house, this amorphous phase plays the role of the cement that connects the aligned bricks in the walls,” Soroka explains.

The researchers also found that the mesocrystals can further self-organize to form supracrystals, which are visible to the naked eye. “Just as an architect may design not a single house but a whole neighborhood with the houses oriented in a certain way to serve the needs of their inhabitants.”

According to Soroka, this multi-level hierarchical architecture of supracrystals is an interesting concept for future materials design. “People are fascinated by the variety of structures and complex forms that are found in nature, such as sea urchins and corals. And scientists are interested in how the crystallation processes work. Our work is a contribution to this understanding.”

This story is adapted from material from the KTH Royal Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.