A textbook process known as “Ostwald ripening,” named after Nobel Prize-winning chemist Wilhelm Ostwald, has guided the design of new materials (How are nanoparticles grown) for decades, including nanoparticles — tiny material so small that the eye can’t see it.
According to this theory, small particles melt and redeposit onto larger particles, and the larger particles continue to grow until all the smaller particles have dissolved.
But now, new video footage captured by Berkeley Lab scientists shows that the growth of nanoparticles is not driven by size differences, but by defects.
The researchers recently reported their findings in the journal Nature Communications.
“This is a big milestone. We’re writing a chemistry textbook, and it’s very exciting,” said lead author Haimei Zheng, senior scientist in Berkeley Lab‘s materials science department and associate professor of materials science and engineering at UC Berkeley.
For the study, the researchers suspended a solution of cadmium sulfide (CdS) nanoparticles with cadmium chloride (CdCl2) and hydrogen chloride (HCl) in a customized liquid sample holder. The researchers exposed the solution to an electron beam to create Cd-CdCl2 core-shell nanoparticles (CSNPs) – which look like flat hexagonal disks – where cadmium atoms are the core and cadmium chloride is the shell.
Using a technique called high-resolution liquid cell transmission electron microscopy (LC-TEM) at the Molecular Foundry, the researchers captured LC-TEM videos of Cd-CdCl2 CSNPs maturing in solution in real time at the atomic scale. In a key experiment, an LC-TEM video shows a small Cd-CdCl2 core-shell nanoparticle fusing with a large Cd-CdCl2 CSNP to form a larger Cd-CdCl2 CSNP.
However, the growth direction is not guided by the size difference, but by the crack defect in the shell of the first larger CSNP. “The finding was unexpected, but we are very satisfied with the results,” said Qiubo Zhang, first author and postdoctoral researcher in the Division of Materials Sciences.
The researchers say their work is the highest resolution LC-TEM video ever recorded. The breakthrough – watching nanoparticles mature in solution in real time – was made possible by a custom-made ultra-thin “liquid cell” that creates a small amount of liquid between two carbon film membranes on a copper grid.
The researchers observed the liquid sample through ThemIS, a special electron microscope at the Molecular Foundry capable of recording atomic-scale changes in liquids at 40-400 frames per second. The microscope’s high-vacuum environment keeps the sample liquid.
“Our study fills a gap for nanomaterial transformations that cannot be predicted by traditional theory.” Said Zheng, who pioneered LC-TEM at Berkeley Lab in 2009 and is a leading expert in the field. “I hope our work will inspire others to think about new rules for designing functional nanomaterials for new applications.”
