Researchers determined for the first time what controls formation of two different nanoscale crystalline structures in the metal cobalt. Using high-energy x-ray beams, scientists at the Georgia Institute of Technology watched a high-pressure, high-temperature chemical reaction, which allowed them to study cobalt nanoparticles as they grew from clusters including tens of atoms to crystals as large as five nanometers.
The research provides the proof-of-principle for a new technique to study crystal formation in real-time, with potential applications for other materials, including alloys and oxides. Data from the study produced “nanometric phase diagrams” showing the conditions that control the structure of cobalt nanocrystals as they form.
The research, reported in the Journal of the American Chemical Society, was sponsored by the National Science Foundation, and used U.S. DOE-supported synchrotron x-ray beam lines at Brookhaven National Laboratory and Argonne National Laboratory.
“We found that we could indeed control formation of the two different crystalline structures, and that the tuning factor was the pH of the solution,” said Hailong Chen, an assistant professor in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “Tuning the crystalline structure allowed us to control the functionality and properties of these materials. We believe this methodology could also be applied to alloys and oxides.”
In bulk cobalt, crystal formation favors the hexagonal close-pack (HCP) structure because it minimizes energy to create a stable structure. 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, Chen said.
Working with researchers from the two national laboratories and the Department of Materials Science at the University of Maryland, Chen and graduate research assistant Xuetian Ma examined the polymorphic structures using theoretical, experimental and computational modeling techniques.
The x-ray diffraction results confirmed the theoretical predictions and computational modeling done by Yifei Mo, an assistant professor in the A. James Clark School of Engineering at the University of Maryland. Mo and colleagues Adelaide Nolan and Shuo Zhang used density functional theory to describe how the crystal would nucleate under differing conditions.
The success with cobalt suggests the methodology could be used to produce nanometric phase diagrams for other materials, including more complex alloys and oxides, Chen said.
“Our goal was to build a model and a systematic understanding about the formation of crystalline materials at the nanoscale,” he said. “Until now, researchers had been relying on empirical design to control growth of the materials. Now we can offer a theoretical model that would allow systematic prediction of what kinds of properties are possible under different conditions.”
As a next step, the Georgia Tech researchers plan to study alloys, to further improve the theoretical model and experimental approach.
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Image – Hailong Chen, an assistant professor in the George W. Woodruff School of Mechanical Engineering, and Xuetian Ma, a graduate research assistant, are shown in their laboratory. Courtesy of Georgia Tech/Allison Carter.
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