There has been tremendous interest in metal–organic frameworks (MOFs) in the past decade, particularly for separation applications, owing to their unprecedented porosities, uniform and tuneable pore sizes and chemical modularity. Nevertheless, the scalable preparation of these materials, especially with a high degree of orientation, remains a huge challenge. To date, current methods for achieving orientation of MOFs rely on either layer by layer (LBL) deposition or controlled epitaxial growth of surface anchored MOFs (SURMOFs) on a functional support. These can however be lengthy and tedious processes, and the buildup of successive layers can compromise crystal orientation. In addition, activation of the films to free up their pores following synthesis and detachment from the underlying support is required either through vacuum heating or freeze drying. This can nevertheless lead to incomplete activation due to collapse of the pores, as evidenced by the discrepancies between the theoretical and the experimental surface areas of the MOFs. Other techniques rely on exchanging the higher boiling point solvents trapped within the pores with lower boiling point ones. However, in some cases solvent exchange strategies still fails to yield the expected surface areas.

Surface acoustic waves (SAWs), discovered in 1885 by Lord Rayleigh, are nanometer amplitude surface waves that can be generated along the surface of a piezoelectric material. When in contact with an overlying liquid, these SAWs leak their energy to generate sound waves in the liquid whose attenuation generates acoustic streaming flows that can be harnessed to drive a wide range of microscale fluid actuation processes for different microfluidic applications.  

By breaking the symmetry of these planar SAWs with respect to the liquid they leak into, it is possible to drive an internal microcentrifugal flow, which has been previously demonstrated for extremely efficient micromixing and particle concentration. In our work, we show that exposure of the precursors to such microcentrifugation at different acoustic intensities over several minutes gives rise to nucleation and subsequent crystallisation of MOFs. We demonstrate this with two MOF systems, HKUST-1 and MIL88-B. Interestingly, we observed that the MOFs that crystallise in this technique exhibited a high degree of out-of-plane orientation parallel to the vertical {222} crystal plane, especially at higher voltages.  

We have attempted to speculate a mechanism for this peculiar observation. In particular, we noticed that when we altered the nature of the SAW such that it comprised horizontally-polarised (in the plane of the substrate) surface shear waves (i.e., shear-horizontal SAWs) as opposed to the original transversely-polarised (out-of-plane of the substrate) surface compressional waves (i.e., Rayleigh SAWs), the crystals instead possessed an in-plane orientation parallel to the {200} crystal plane. This therefore hints at the role of the evanescent electric field of the electromechanical SAW wave in the vertical dipole stacking of the solute molecules along its gradient. This is aided by fast turbulent convection associated with the SAW microcentrifugation flow that rapidly transports the solute molecules to the substrate surface, so as to promote the vertical stacking of the molecular layers into a large, ordered three-dimensional superlattice structures.  

We find that the MOFs synthesized in this manner are not only free-standing, not requiring detachment from the piezoelectric substrate, but also simultaneously activated, therefore removing the need for solvent exchange processes, which can have a large environmental impact. The surface area of the synthesized HKUST-1 crystals are observed to increase with the input voltage up to approximately 1600 m^2/g, which is close to that reported for activated commercial HKUST-1 crystals and more than double the surface area for HKUST-1 crystals produced in the absence of SAW.  

With this new platform, we envisage rapid and versatile one-step synthesis and activation of other types of MOFs. Moreover, the miniature dimension of the SAW chips, which can also be fabricated at low costs (about US$1 per device) allows them to be easily scaled in large numbers, therefore making efficient, large-scale, environmentally-friendly production of this exciting class of materials one step closer to realisation.  

To read more about this, please check out our article “Acoustomicrofluidic Assembly of Oriented and Simultaneously Activated Metal–Organic Frameworks” in Nature Communications DOI 10.1038/s41467-019-10173-5.