The new materials are precipitated from uranium and neptunium peroxide solutions at room temperature. They consist of groups of 24, 28 or 32 identical polyhedra that are linked into clusters measuring about two nanometers – billionths of a meter – in diameter.

Researchers discovered the materials in the course of their work within the Environment Molecular Science Institute (EMSI). Argonne and Notre Dame are partners in this joint Department of Energy/National Science Foundation institute that is funded to explore the basic science of molecular interactions involved in the transport of nuclear materials in the environment.

Scientists are studying the chemistry of actinides – the radioactive elements that constitute the bottom row of the Periodic Table. “Since there are no historic examples,” chemist Lynda Soderholm said “there is a huge void in understanding, so we are investigating almost any situation we think could be found in nature related to nuclear materials interacting with the environment.” Soderholm is a senior scientist and leader of the Heavy Elements Chemistry and Separations Science Group in Argonne's Chemistry Division.

These actinyl-peroxide nanospheres may form in alkaline mixtures of nuclear waste, such as the high-level nuclear waste tanks found at the Hanford, Wash., site, according to the researchers. Hanford's nine nuclear reactors produced plutonium for four decades, leaving more than 50 million gallons of high-level liquid waste in 177 storage tanks and billions of gallons of contaminated groundwater.

“No one has ever seen anything like these,” said Peter Burns, chair of the Civil Engineering and Geological Sciences Department at Notre Dame. “These very small nanoscale aggregates of actinides in solution could play an important role in actinide transport in the environment.”

Nanoparticles are believed to be important in environmental systems, as they often form at low temperatures, can impact the transport of heavy metals and radionuclides in geologic fluids, and are small enough that their properties can vary with their size.

When materials are created from particles just a few molecules across and measured in the billionths of meters, they have enhanced properties when compared to materials created from bulk.

“In retrospect,” Soderholm said, “I think this material has been seen before, but the structures are so complicated that it took the right combination of X-ray diffraction facilities and expertise to unravel them.”

“In papers published in the 1960s,” Burns said, “Russian scientists working in these chemical systems described crystals that could have been these materials. They had crystals with similar colors and shapes. We strongly suspect they had some of the same materials but there was no way you could begin to analyze crystal structures of this complexity in the 1960s or even the early 1990s.

“These things are in an unusual size range,” Burns said, “that provides an opportunity to understand well-defined nanospheres. The clusters exist in solution and build up into molecular crystals much like atoms grow into molecules.

“They are not dissolved," he said, "in the normal sense of what we think of a cation being surrounded by water, but they are not big enough to be a solid in suspension. They are in an intermediate range.”

The scientists theorized that the clusters form spontaneously in solution by self assembly. “We used the Advanced Photon Source at Argonne to probe the solution and verify that the clusters exist as formed nanospheres in solution,” Soderholm said. The Advanced Photon Source is this hemisphere's most brilliant source of research X-rays. “Now we want to look at the material's electronic properties and see if there will be any interesting interactions within the clusters.

“Since the materials are formed in solution,” she said, “it is easier to study their catalytic properties.”

“We want to know everything,” Burns said. “How they assemble, are they stable in solution, what external factors will modify them, do they form near nuclear wastes and if so, how far might they be transported in the environment?”

The chemists plan to focus on the self-assembling aspect of these materials. Reproducible, self-assembling nanostructures are the current “Holy Grail” in the nanotechnology world. When they can be manufactured, industry hopes to use them as catalysts, computer chips, solar cells, flexible batteries and data storage devices.

“This family of self-assembling structures,” said Soderholm, “will provide new insights about the influence of the nanoscale on electronic, magnetic and structural properties and should provide novel materials.”

Research with the uranium structures began at Notre Dame, but moved to Argonne because the Chemistry Division has hot labs allowing the research on neptunium to be performed safely.

A post-doctoral appointee and several graduate and undergraduate students are playing key roles in the ongoing research. “We are training the next generation of environmental chemists and geologists,” said Soderholm.

“It's really exciting,” Burns said, “to see the students catching the research bug.”

The research is being published this March in Angewandte Chemie International. — Evelyn Brown

For more information, please contact Catherine Foster (630/252-5580 or cfoster@anl.gov) at Argonne