Researchers discover a new way to form complex crystals

When material reaches extremely small scales, strange things begin to happen. One of the phenomena is the formation of mesocrystals.

Despite being separate crystals, mesocrystals come together to form a larger, fused structure that acts as a pure, single crystal. However, these processes occur on scales that are far too small to see the human eye, and their creation is extremely challenging to observe.

Because of these challenges, scientists could not confirm exactly how mesocrystals form.

Now, new research by the Pacific Northwest National Laboratory (PNNL) team has used advanced transmission electron microscopy (TEM) techniques to see mesocrystals form in real-time. What they have seen is contrary to conventional wisdom and their insights may one day help scientists design materials for energy storage and understand how minerals form in the soil.

Instead of individual crystals being nucleating, the step that initiates crystal formation, and then randomly merges into mesocrystals in two unrelated steps, the researchers observed that nucleation and attachment are closely linked to the formation of these highly uniform structures. The researchers reported their work in the February 18, 2021 issue Nature.

“Our findings identify an important new way of crystallization by particle attachment and solve the key questions about mesocrystalline formation,” said PNNL and Guomin Zhu, a materials scientist at the University of Washington. He was part of the research team led by Jim De Yoreo, materials scientist at PNNL and co-director of the Northwestern Institute of Materials Physics, Chemistry and Technology. “We suspect this is a widespread phenomenon with significant implications for the synthesis of designed nanomaterials and for the understanding of natural mineralization,” Zhu added.

See crystallization in real time

The project took years to execute and required significant problem solving. For the microscopy experiments, the scientific team selected a model system containing hematite, an iron compound commonly found in the earth’s crust, and oxalate, a natural mixture in the soil.

They visualized the process using in situ TEM, which gives researchers the ability to see crystallization on the nanometer scale as it happens. They combine this real-time method with ‘freeze-and-look’ TEM that has enabled them to track an individual crystal at different points during growth. Theoretical calculations helped complete the picture so that the PNNL team could compile how the mesocrystals grew.

Researchers usually perform most TEM experiments in situ at room temperature to simplify the experimental setup and reduce the potential for damage to the sensitive instrument, but the formation of mesocrystals is fast enough to observe. about 80 ° C.

“The extra equipment used to heat the samples made the experiments very challenging, but we knew that the data would be the key to understanding how the mesocrystals form,” Zhu said.

Once heated, the new hematite nanocrystals make it easy to quickly assemble, resulting in averaging final mesocrystals of approximately the same size and shape.

Mesocrystals in nature

The chemical key to this fast, reliable adhesion is the oxalate molecules present in the solution. After the first few small crystals have formed, the oxalate additives help to create a chemical gradient at the interface between the liquid and the growing crystal. More chemical components needed for particle nucleation depend on the crystals, which dramatically increases the probability of new particles near existing forms.

Although this crystal growth has been observed on a very small scale in controlled conditions, according to the researchers it probably also occurs in natural systems. Some mineral deposits, including an Australian hematite deposit, contain mesocrystals. Given the natural abundance of oxalate and the observation by the PNNL team that hematite can become mesocrystals at temperatures as low as 40 ° C, it seems likely that this formation pathway occurs in nature.

Since mesocrystals occur throughout nature, the findings can be applied to the understanding of nutrient cycle in the environment, among others. The route to create almost uniformly complex structures also requires an understanding of how methods for forming such materials work and how to control them. This work, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Department of Chemical Sciences, Earth Sciences, and Biosciences, therefore offers new possibilities for compiling mesocrystals or mesocrystalline materials.