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John O | January 2019

High-energy X-ray beams reveals secrets of reactions that lead to nanoscale crystal formation

By Josh Perry, Editor


Scientists at the Georgia Institute of Technology in Atlanta used high-energy X-ray beams to visualize high-pressure (1,800 pounds per square inch), high-temperature (more than 200°C) chemical reactions to determine what factors into the creation of different nanoscale crystalline structures in cobalt.


Georgia Tech graduate research assistant Xuetian Ma holds a reaction vessel similar to those used in the research on nanoscale crystalline growth. (Allison Carter/Georgia Tech)


The researchers studied the growth of cobalt nanoparticles from clusters of 10 atoms all the way up to five nanometers, according to a report from the school. Researchers were able to produce nanometric phase diagrams that show the condition of the cobalt crystals during formation.


“In bulk cobalt, crystal formation favors the hexagonal close-pack (HCP) structure because it minimizes energy to create a stable structure,” the article explained. “At the nanoscale, however, cobalt also forms the face-centered cubic (FCC) phase, which has a higher energy. That can be stable because the high surface energy of small nanoclusters affects the total crystalline energy.”


Researchers, working with teams from Brookhaven National Laboratory, Argonne National Laboratory, and the University of Maryland, reduced cobalt hydroxide in a solution of ethylene glycol and adjusted the solution’s pH level with potassium hydroxide.


The tests took place in a high-strength quartz tube that was small enough (a millimeter in diameter) to allow X-ray transmission but able to withstand the pressure requirements. A small heater was used and a thermocouple to measure the temperature.


“Data obtained by varying the pH of the reaction produced a nanometric phase diagram showing where different combinations produced the two structures,” the article added. The experimental results were confirmed by theoretical and computational simulations.


The research was recently published in the Journal of the American Chemical Society. The abstract stated:


“Conventionally, phase diagrams serve as road maps for the design and synthesis of materials. However, bulk phase diagrams are often not as predictive for the synthesis of nanometric materials, mainly due to the increased significance of surface energy. The change of surface energy can drastically alter the total energy of the nanocrystals and thus yields a polymorph or metastable phase different from the stable phase in bulk, providing a means for controlling the synthesis of metastable phases.


“To achieve a theoretical and systematical understanding on the polymorphism of nanomaterials, metallic cobalt was chosen as a model system, where the two polymorphs, fcc and hcp phases, can be tuned with 100% selectivity in a solvothermal reaction. Advanced in situ synchrotron X-ray diffraction (XRD) technique and density functional theory (DFT) calculations were complementarily employed to reveal the size- and surface-dependent polymorphism at nanometer scale.


“The nanometric phase diagram provides a general predictive approach to guide the synthesis of metastable materials.”

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