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April 2018

Form Damages Function and Magnetism Suffers

Researchers discover mechanism that decreases the magnetism of metallic core particles with a metal-organic framework shell, opening doors to new material designs

Artistic representation of dropping magnetism of MOF wrapped magnetic particles
The magnetic strength (left) drops when magnetic iron microspheres (black) are wrapped up in a metal-organic framework shell (blue). Image courtesy of Nathan Johnson, Pacific Northwest National Laboratory Enlarge Image.

Surface mining for rare earth elements used in smartphones and wind turbines is difficult and rarely done in the United States. Scientists wanted to know if they could pull the metals, present at trace levels, from geothermal brines using magnetic particles. The particles, wrapped in a molecular framework shell known as a metal-organic framework (MOF), should easily trap the metals and let the rest flow past. However, the team led by Dr. Pete McGrail at Pacific Northwest National Laboratory found the magnetic strength dropped by 70 percent after the MOF shell was formed.

Why It Matters: The use of MOFs may allow for the separation of yttrium, scandium, and other elements from saline water from geothermal sources, produced waters from oil and gas fields, or wastes such as fly ash. "These elements have a lot of applications -- petroleum refining, computer monitors, magnets in wind turbines," said Dr. Praveen Thallapally, the materials design lead on the study. "Right now, 99 percent of these rare earths are imported to the U.S."

The fundamental knowledge gained from this research shows why this MOF affected the magnetic strength so much and offers insights into methods to avoid these problems.

Summary: Scientists began with an MOF called Fe3O4@MIL-101-SO3. It contains chromium ions connected by organic ligands. The synthesis process forms the MOF shell by a molecular self-assembly process with the MOF building up a layer around the magnetite core particles. Researchers expected the shell to have little impact on the magnetic strength of the particles but found it dropped by 70 percent.

"We wanted to figure out why," said Thallapally. Theories abounded, but nobody had brought together the materials, expertise, and instrumentation to definitively prove what was happening.

They used imaging capabilities at DOE's Environmental Molecular Sciences Laboratory, an Office of Science user facility located at PNNL. Specifically, they used scanning electron and transmission electron microscopy to study the MOF shell. They found that the particles increased in size as expected. This meant the problem wasn't the magnetite particles dissolving in the liquids used during synthesis, a common theory.

Next, they also used 57Fe-Mössbauer spectroscopy to study the oxidation state of the metal core. They found a larger amount of oxidized ferric iron than expected. Digging in further with atom probe tomography, the team found that chromium had crept inside the iron cores. They obtained more details on the chromium oxidation state using X-ray absorption fine structure spectroscopy at the Advanced Light Source, a DOE Office of Science user facility at Lawrence Berkeley National Laboratory.

PowerPoint slide summarizing research
Cleared slide summarizing research conducted for the DOE Geothermal Technologies Office and the Office of Science. Download PowerPoint slide.

In the end, the team showed that the chromium penetrated into the pores in the iron particles and was reduced by capturing an electron from the iron thus oxidizing it. The magnetic strength of magnetite is strongly determined by the amount of ferrous versus ferric (oxidized) iron in the material. The iron oxidation thus degraded the magnetic properties. These fundamental insights will allow materials science researchers to adjust the MOF chemistry to prevent the unwanted oxidation-reduction reactions and better retain the core-shell material's magnetic properties.

Acknowledgments:

Sponsors: Department of Energy, Geothermal Technologies Office; Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering (synthesis of the material)

User Facilities: Mössbauer, atom probe, scanning electron microscopy, and transmission electron microscopy, and magnetometry experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy (DOE) Office of Science's Office of Biological and Environmental Research; X-ray absorption fine structure spectroscopy was performed in the Advanced Light Source, supported by the Director, Office of Science, Office of Basic Energy Sciences, DOE

Research Team: Sameh K. Elsaidi, Pacific Northwest National Laboratory and Alexandria University; Michael A. Sinnwell, Debasis Banerjee, Arun Devaraj, Ravi K. Kukkadapu, Timothy C. Droubay, Zimin Nie, Libor Kovarik, Murugesan Vijayakumar, Manjula Nandasiri, B. Peter McGrail, and Praveen K. Thallapally, Pacific Northwest National Laboratory; Sandeep Manandhar, Pacific Northwest National Laboratory and University of Texas at El Paso

Reference: SK Elsaidi, MA Sinnwell, D Banerjee, A Devaraj, RK Kukkadapu, TC Droubay, Z Nie, L Kovarik, M Vijayakumar, S Manandhar, M Nandasiri, BP McGrail, and PK Thallapally. 2017. "Reduced Magnetism in Core-Shell Magnetite@MOF Composites." Nano Letters 17(11):6968-6973. DOI: 10.1021/acs.nanolett.7b03451


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