Pure uranium: a Raman first

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  • Published: Jun 1, 2011
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Pure uranium: a Raman first

Superficial access to uranium

Scientists at the Indira Gandhi Centre for Atomic Research in Tamil Nadu have carried out the first study of "pure" uranium using Raman spectroscopy. The fundamental research offers new insights into this radioactive metal and may even have implication for developments in nuclear energy.

T. R. Ravindran and A. K. Arora point out that, while powerful, Raman spectroscopy usually requires a sample that can give strong scattering. Technologically important metals, such as the actinides uranium and plutonium, have not been subjected to Raman investigations because they produce such poor signal intensities. However, a thin gold layer with an enhancing geometry on the surface of a sample of uranium can cause surface-enhanced Raman scattering and so allow even this metal to be studied.

Few elemental metals have succumbed to Raman investigation and only those with two atoms per crystallographic unit cell - beryllium, gallium, zirconium and one form of iron at very high pressure in a diamond anvil cell. Uranium metal has an orthorhombic crystal structure with two atoms per unit cell and calculations suggest that it should have three Raman modes. The team reports Raman spectra detected from the pits and scratches of the sample rather than its smooth, polished surface; polishing removes interfering surface oxides. The team points out that room-temperature laser excitations at 514.5 or 785 nm gives rise to Raman spectra whereas 325 nm excitation does not; which is consistent with SERS enhancement from gold nanorods, Ravindran told SpectroscopyNOW. The results at room temperature and in the range 80 to 300 K were consistent with inelastic neutron scattering, the team says.

Please SERS

"The phenomenon of SERS that greatly enhances Raman signal intensity of weakly scattering molecules adsorbed on roughened gold or silver surfaces was discovered in 1977," the team explains. They point out that there are no SERS studies, as far as they know, that investigate the underlying metal as observed in this work. The approach to studying the actinides by giving them a thin SERS-active metal coating to facilitate enhancement in this way might be extended to other metals.

To explain the phenomenon, the team has considered the thin uranium oxide layer that forms rapidly and naturally on the polished surface even before the gold coating can be deposited. This oxide might, they suggest, localize the surface plasmons, it being akin to the nanometre-thick shells of silica or alumina used to coat 55 nm gold particles in the recently reported technique of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique . They add that the, "sharp edges of gold in the scratch lines act like nanorods with a range of aspect ratios. When these edges are excited by the 514- or 785-nm laser, the SERS effect enhances the Raman signal from uranium."

"There is currently a lack of experimental work on actinide metals due to their toxicity, radioactivity and scarcity," Ravindran explains. This situation has limited our understanding of these technologically important materials. He asserts that the approach taken in this work opens up the possibility of enabling such studies using the versatile laboratory technique of Raman spectroscopy. Previously, studies have only been possible at large experimental facilities such as nuclear reactors and synchrotrons.

While the research is novel it might, for the time being, be regarded as basic research. "The novel SERS geometry used in the overlayer on the material to be studied might be employed to study other metals and materials with low Raman scattering cross section. Further studies need to be done to exploit the full potential of this sub-SERS technique," Ravindran told us.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Scientists at the Indira Gandhi Centre for Atomic Research in Tamil Nadu have carried out the first study of
Uranium succumbs to Raman

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