With nanoscale cage-like structures featuring exceedingly large internal surface areas, MOFs can be used to capture and store molecules, giving them a wide range of potential applications from gas storage and capturing CO2 to microelectronics, drug encapsulation and use in sensors. In order to turn potential into reality, it’s necessary to understand their physical structure at a fundamental level, and how this determines their properties on the macroscopic scale. The Multifunctional Materials & Composites (MMC) research group, led by Prof J.C. Tan at Oxford University has been using ISIS and Diamond Light Source to study a group of MOFs and how their properties can be tuned. Their research has recently been published in Physical Review Letters.
The group studied zeolitic imidazolate frameworks (ZIFs), a topical subset of MOFs. These materials have a nanoporous structure similar to zeolites, which are currently widely used in industry. However zeolites have their flaws – they are rigid and difficult to control. ZIFs have similar chemical stability to inorganic zeolites, but mechanically they are more flexible, and importantly their properties can be controlled by design. But to fully exploit the properties of these new materials, it is necessary to understand their structure and how this changes with temperature and pressure.
The group used a combination of inelastic neutron scattering and synchrotron radiation far-infrared absorption spectroscopy, in conjunction with density functional theory (DFT), to examine low-frequency terahertz (THz) vibrations in three prototype ZIF materials.
Using this combination of techniques, the team discovered that these vibrations are intrinsically linked to observed physical phenomena in the ZIFs. For example pore breathing, where the cage structure opens and closes, and gate opening, where the material undergoes a step change in the amount of molecules it is able to capture, can be linked back to THz vibrational modes.
First author of the study, Matthew Ryder, a DPhil student from the University of Oxford says, “What we have shown is that ab initio calculations, confirmed by a combination of high-resolution neutron and synchrotron spectroscopy, can be used to explain crucial mechanisms that are intrinsic to the understanding of various physical phenomenon. This include not only gate opening and pore breathing phenomena, but also mechanisms that explain framework elasticity, instability and collapse. Our methodology provides an important starting point to being able to understand the detailed physics underpinning complex mechanisms in framework materials. This in turn allows us to predict and fine tune the properties central to a wide range of emerging technological applications.”
Members of the ISIS Molecular Spectroscopy Group, Dr Svemir Rudic and Prof Felix Fernandez-Alonso who co-authored the paper say “Once again the capability of neutrons and TOSCA spectrometer to probe the vibrational dynamics of solid materials with excellent resolution across the wide energy range has been confirmed”.
Research date: December 2014
For more information on the Multifunctional Materials & Composites (MMC) research group
MR Ryder et al, Phys. Rev. Lett. 113 (2014) 215505.