Scientists in New Zealand, the US and Germany have developed a way of using tiny pores in a graphene sheet to separate different isotopes of helium. By creating nanoscale holes in the material, the researchers calculated that it should be possible to alter the permeability of graphene to allow helium-3 isotopes to tunnel through, while heavier helium-4 isotopes cannot. This approach has potential applications in the production of helium-3 for scientific research as well as for the separation of gases in other scientific and industrial contexts.
Helium-3 is present in the Earth's atmosphere in a ratio of 1.4 parts of helium-3 to a million parts of helium-4. It also exists as a primordial nuclide in the Earth's mantle, created by nucleosynthesis during the Big Bang. It is used extensively in fusion research and low temperature chemistry, though most helium-3 used by researchers and industry comes from the radioactive decay of tritium. However, demand for helium-3 has risen considerably in recent years and supply has been unable to keep pace with demand. Harvesting helium-3 from terrestrial sources of helium would help to plug this gap.
Using quantum chemistry calculations and potential energy simulations, the researchers found that removing rings from a perfect graphene sheet reduced the energy barrier of the material, which determines its permeability. However, this approach was not sufficiently sensitive to achieve the specific potential required to permit tunnelling of helium-3 while excluding helium-4. Functionalising the nanopores with nitrogen enabled the researchers to fine tune the tunnelling barrier, which could one day allow the efficient separation of the two isotopes at an industrially acceptable gas flux1,2.
In addition to potentially providing a new source of helium-3, this approach could provide a novel way of separating other gases. 'Graphene is a relatively inert material. If it is possible to produce controlled nanoholes in graphene sheets, it can be used for gas separation,' says Peter Schwerdtfeger from the New Zealand Institute for Advanced study, who led the research team. 'Helium-3 and helium-4 was used just to see if quantum mechanical tunnelling effects can be used for isotope separation.'
De-en Jiang at the US Oak Ridge National Laboratory, a member of the first group to develop a computational proof of concept for the use of a nanoporous graphene membrane in gas separation3, welcomes these recent findings and looks forward to experimental confirmation. 'All the work so far has remained at the stage of computational proof of concept,' he tells Chemistry World. 'It'd be far more exciting for someone to confirm it experimentally. To my knowledge, several groups are working hard on it.'
There are, however, a number of technical barriers that researchers will first need to overcome. It is difficult to create large sheets of graphene, for example, and the process needs to take place at very low temperatures of around 10K. But the practical realisation of this approach could open up a broad range of applications, says Schwerdtfeger, including the separation of methane, one of the most potent greenhouse gases, from other gases in the atmosphere.
- 1 A Hauser, J Schrier and P Schwerdtfeger, J. Phys. Chem. C, 2012, DOI: 10.1021/jp302498d
- 2 A Hauser and P Schwerdtfeger, J. Phys. Chem. Lett., 2012, 3, 209 (DOI: 10.1021/jz201504k)
- 3 D Jiang, V R Cooper and S Dai, Nano. Lett., 2009, 9, 4019 (DOI: 10.1021/nl9021946)