Materials tend to behave differently under very small environments, such as in the micro and nano scales. In some substances, it includes the formation of mesocrystals - and researchers have observed their formation for the first time.
Mesocrystals refer to material structures where separate individual crystals are arranged in a regular pattern, forming a larger structure similar to pure crystals.
However, in these structures, the individual crystals are aligned yet remain spatially separated from each other. The processes that lead to the creation of these materials are too minute, making observations extremely difficult.
Using TEM Scans to Watch Mesocrystal Formation
In a new study led by members from the Pacific Northwest National Laboratory (PNNL), headquartered in Richland, Washington, they detail the use of advanced transmission electron microscopy (TEM) methods to observe how mesocrystals form in real time.
Interestingly, their findings are contrary to the previous understanding of these processes occur and offer insights on mineral formation in soil.
Instead of the individual crystals nucleating - seen as the start of the formation of the larger crystal - and then randomly aggregating together to form mesocrystals in two separate processes, researchers found out that nucleation and aggregation were two interconnected processes that result in the creation of these highly uniform structures. Details of their work appear in the February 18 issue of the journal Nature.
"Our findings identify an important new pathway of crystallization by particle attachment and resolve key questions about mesocrystal formation," said materials scientist Guomin Zhu, member of the research team from PNNL and University of Washington. "We suspect this is a widespread phenomenon with significant implications both for the synthesis of designed nanomaterials and for understanding natural mineralization."
The entire process was visualized using in situ TEM setups, allowing researchers to observe mesocrystals to form at the nanometer scale in real-time. It actually took years to successfully execute and required them to solve problems before they were able to image the process.
For example, researchers used a model system to contains hematite - an iron compound found in the Earth's crust - and oxalate, an organic material abundant in soil.
Additionally, most of their in situ TEM scans are conducted under room temperature environments to keep it simple and minimize harm to the scanning equipment. However, they note that mesocrystals form at observable speeds requires a temperature of at least 80 degrees Celsius.
Forming Natural Mesocrystals
Based on their observation, the oxalate in the solution serves as the catalyst for the process, creating reliable attachments between the individual crystals. After the first crystals nucleate, oxalate serves to create a chemical gradient between the liquid and the forming crystal.
Although the mesocrystals formed and observed in the study were under controlled conditions, researchers noted that this is also likely to happen in nature. This is further supported in some mineral deposits, including an Australian hematite deposit that contains mesocrystals.
They also suggested that with the natural abundance of oxalate, plus the possibility of forming mesocrystals at temperatures as low as 40 degrees Celsius, these material structures could actually occur naturally.
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