A textbook process called Ostwald ripening, named after the Nobel Prize-winning chemist Wilhelm Ostwald, has guided the design of new materials such as nanoparticles for decades. These are small materials that appear so small that they are not visible to the naked eye.
New videos captured by Berkeley Lab scientists reveal for the first time that nanoparticle growth is driven not by size difference, but by defects. Image credit: Haimei Zheng/Berkeley Lab. Courtesy of Nature Communications.
According to this theory, small particles dissolve and then redeposit on the surface of huge particles, and large particles continue to grow until all the small particles are dissolved.
But now, new videos captured by researchers at Berkeley Lab reveal that the growth of the nanoparticles was driven not by differences in size, but by defects.
The scientists recently reported their findings in the journal Nature Communications.
This is a huge milestone. We are rewriting the chemistry textbook and that is very exciting.
Haimei Zheng, senior study author and assistant professor, Materials Science and Engineering, University of California, Berkeley
Zheng is also a senior scientist in the materials science department at Berkeley Lab.
For the study, the scientists suspended a solution of cadmium sulfide (CdS) nanoparticles with hydrogen chloride (HCl) and cadmium chloride (CdCl2) in a custom liquid sample holder.
The scientists discovered the electron-beam solution to generate Cd-CdCl2 core-shell nanoparticles (CSNPs)—which looked like a flat, hexagonal disk, where cadmium chloride forms the shell and cadmium atoms develop the core.
Using a new method known as high-resolution liquid cell transmission electron microscopy (LC-TEM) at the Molecular Foundry, scientists captured real-time and atomic-scale LC-TEM videos of Cd-CdCl2 CSNPs maturing in solution.
In one of the experiments, an LC-TEM video shows a small Cd-CdCl2 core-shell nanoparticle mixing with a giant Cd-CdCl2 CSNP to produce a larger Cd-CdCl2 CSNP. But the growth direction was driven not by a size change but by a crack defect in the shell of the initially larger CSNP.
The discovery was very unexpected, but we are very pleased with the results.
Qiubo Zhang, first author of the study and postdoctoral researcher, Department of Materials Science, University of California, Berkeley
The scientists claim their work is the highest resolution LC-TEM video ever recorded. The advance – tracking how nanoparticles mature in solution in real time – was made possible by a custom-made and ultra-thin “liquid cell” that holds a small amount of liquid between two carbon film membranes on a copper grid.
The scientists observed the liquid sample using ThemIS, a specific electron microscope at the Molecular Foundry that has the potential to record atomic-scale variations in liquids at 40 to 400 frames per second. The high vacuum environment of the microscope helps keep the liquid sample intact.
Our research fills the gap for nanomaterial transformations that cannot be predicted by traditional theory. I hope our work will inspire others to think about new rules for designing functional nanomaterials for new applications.
Haimei Zheng, senior study author and assistant professor, Materials Science and Engineering, University of California, Berkeley
Zheng pioneered LC-TEM at Berkeley Lab in 2009 and is also a leading expert in the field.
Journal reference:
Zhang, Q., et al. (2022) Defect-Mediated Maturation of Core-Shell Nanostructures. Nature Communications. doi.org/10.1038/s41467-022-29847-8.
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