In a first-of-its-kind study, researchers used an ingenious experimental setup and high-energy X-ray beams to observe a high-temperature, high-pressure chemical reaction to establish how the formation of two varied nanoscale crystalline structures in the metal cobalt is controlled.

The method enabled continuous analysis of cobalt nanoparticles as they formed from clusters that include tens of atoms to large crystals measuring 5 nm.

The study offers the proof-of-principle for a novel method to analyze the real-time formation of crystals, with prospective applications for other materials, such as oxides and alloys. The study data created “nanometric phase diagrams” that revealed the conditions controlling the structure of cobalt nanocrystals as they grew.
Published in the Journal of the American Chemical Society on November 13th, 2018, the study applied the U.S. Department of Energy-supported synchrotron X-ray beam lines at Argonne National Laboratory and Brookhaven National Laboratory. It was sponsored by the National Science Foundation.

Crystal formation in bulk cobalt prefers the hexagonal close-pack, or HCP, structure since it reduces energy to produce a stable structure. However, at the nanoscale, cobalt also tends to form the face-centered cubic, or FCC, phase, which possesses a higher energy. That can be stable because the total crystalline energy is affected by the high surface energy of tiny nanoclusters, Chen informed.
“When the clusters are small, we have more tuning effects, which is controlled by the surface energy of the OH minus group or other ligands,” he added. “We can tune the concentration of the OH minus group in the solution so we can tune the surface energy and therefore the overall energy of the cluster.”
In association with scientists from the Department of Materials Science at the University of Maryland and the two national laboratories, Chen and Xuetian Ma, a graduate research assistant, studied the polymorphic structures using experimental, theoretical, and computational modeling methods.