IMAGE: Researchers used advanced microscopy techniques to look at the shape of mesocrystals in real time. view more
Credit: (Complex Image by Mike Perkins | Pacific Northwest National Laboratory)
When blade materials reach a very small size, strange things start to happen. One of these onions is the formation of mesocrystals.
Despite being made up of individual crystals, mesocrystals come together to form a larger, cooled structure that behaves like a pure crystal. However, these processes take place at blades far too small for the human eye to see and it is extremely challenging to create them.
Because of these challenges, scientists have not been able to definitively determine how mesocrystals form.
New research by the Pacific Northwest National Laboratory (PNNL) team has now used advanced electrocardiogram (TEM) microscopy techniques to see mesocrystals in solution in real time. What they saw was contrary to conventional wisdom and their ideas could one day help scientists design materials for energy storage and gain an understanding of how minerals form in earth.
Instead of circulating individual crystals, the step that crystal formation begins, and then randomly merging into mesocrystals in two unrelated steps, the researchers found that nucleation and bonding were closely linked. in the creation of these very uniform structures. The researchers reported their work in a February 18, 2021 issue of Nature.
“Our findings mark an important new path of grain-linked crystals and solve key questions about mesocrystal formation,” said PNNL and Washington University materials scientist Guomin Zhu. He was part of the research team led by Jim De Yoreo, a PNNL materials scientist and co-director of the Northwest Institute for Materials Physics, Chemistry and Technology. “We suspect that this is a widespread phenomenon with a major impact both on the synthesis of designed nanomaterials and for the understanding of natural mineralization,” Zhu said.
Seeing a crystal in real time
The project took years to implement and required problem solving. For the microscope experiments, the scientific team selected a model system that included hematite, iron fertilizers commonly found in the Earth’s crust, and oxalate, a fertilizer that is naturally abundant in soil.
They saw the process using TEM in situ, which allows researchers to see crystallization at the nanometer scale as it unfolds. They combined this real-time method with a “freeze-and-look” TEM that allowed individual crystals to follow at different stages during growth. Theoretical calculations helped complete the picture, allowing the PNNL team to synthesize how the mesocrystals evolved.
Researchers typically run most TEM tests in situ at room temperature to simplify the experimental setting and reduce the chances of damage to the sensory instrument, but mesocrystal formation is fast enough to be observed to occur at about 80 ° C.
“The additional equipment used to heat the samples made the experiments very challenging, but we knew the data would be basic to understand how the mesocrystals formed,” Zhu said.
Once heated, the new hematite nanocrystals make it easy for them to bond rapidly together, leading, on average, to final mesocrystals of the same size and shape.
Mesocrystals in nature
The chemical key to this fast, reliable bonding is the oxalate molecules present in the solution. After the first few small crystals, the oxalate additions help to create a chemical gradient at the interface of the liquid and the growing crystal. More chemical components are necessary for the germination of grains near the crystals, which greatly increases the likelihood of new particles approaching the existing ones.
While this crystal growth pathway has been observed in controlled conditions at very small scales, it also appears to occur in natural systems, according to the researchers. Some mineral deposits, including Australian hematite deposits, contain mesocrystals. Given the natural abundance of oxalate and the PNNL team’s observation that hematite can form mesocrystals at temperatures as low as 40 ° C, it seems plausible that this formation pathway occurs in nature. .
Because mesocrystals are found throughout nature, the conclusions can be applied to understanding the nutrition cycle in the environment, among other applications. In addition, the path to the creation of complex structures close to uniform requires an understanding of how methods for 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, Geosciences, and Biology, therefore opens up new opportunities to create mesocrystals-like or mesocrystal-like materials. .
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The high-resolution images and simulations were performed in EMSL, Environmental and Molecular Sciences Laboratory, DOE Science Office User Facility located at PNNL. In addition to Zhu and De Yoreo, this work features contributions from PNNL researchers Maria Sushko, John Loring, Benjamin Legg, Miao Song, Jennifer Soltis, Xiaopeng Huang, and Kevin Rosso.
The Pacific Northwest National Laboratory draws on signaling capabilities in chemistry, Earth sciences, and data analysis to advance scientific discovery and create solutions to the country’s toughest challenges to energy sustainability and national security. Founded in 1965, PNNL is run by Battelle for the U.S. Department of Energy’s Office of Science. The DOE Office of Science is the single largest supporter of fundamental research in the physical sciences in the United States and is working to address some of the most important challenges of our time. For more information, visit the PNNL News Center. Follow us on Facebook, LinkedIn, Twitter, and Instagram.
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