Smart materials made in minutes using eco-friendly acoustic technique

Materials World magazine
,
1 Jul 2019

An eco-friendly technique that can produce metal-organic frameworks in minutes has been demonstrated by using sound waves to manipulate atoms. Shardell Joseph finds out more.

A method that can quickly produce metal-organic frameworks (MOFs) using high-frequency sound waves has shown to be a more environmentally stable approach to traditional processes. Researchers from Royal Melbourne Institute of Technology (RMIT) University, Australia, have developed the smart MOF materials using a process that avoids high temperatures or chemical solvents.

The versatile and super porous nanomaterial has increased in popularity among material scientists, due to having a large range of uses for a wide variety of applications. MOFs can be used to store, separate, release or protect almost anything. The common process for creating them, however, is environmentally unsound and can take a long time.

‘Traditional methods of producing MOFs involve the use of harmful solvents which are trapped within the pores of the MOFs,’ RMIT Researcher and lead author of the study published in Nature Communications, Dr Heba Ahmed, told Materials World. ‘Currently, to remove these solvents scientists either use high temperature and vacuum, a process that releases even more greenhouse gases, or they use other harmful solvents.

‘This combination of vacuum and high temperatures or harmful chemical solvents, a process called activation, makes the traditional process for creating them environmentally unsustainable and can take several hours or even days.’

Demonstrating a method that can produce a customised MOF in minutes, the researchers have harnessed the precision power of high-frequency sound waves, removing the need for chemical solvents making the technique clean.

‘At our labs, we were able to harness a type of high-frequency sound wave – non-audible megahertz waves – called surface acoustic waves, to precisely tailor these MOFs and cut the lengthy synthesis time to few minutes,’ said Ahmed. ‘The sound waves were able to flush out the solvents from the pores during synthesis, leaving the pores vacant, eliminating the need for activation.’

Benefiting from MOFs

Predicted to be the defining material of the 21st Century, MOFs have become a prevalent smart material, ideal for the sensing and trapping of substances at minute concentrations, purifying water or air, or for holding large amounts of energy, which can improve batteries and energy storage devices.

‘MOFs are self-assembled combinations of metals and inorganic ligands that result in a relatively young class of highly ordered, porous materials,’ said Ahmed.

‘Because of the number of structural and chemical possibilities, high surface area, controlled pore volume, and thermal properties, they can be efficiently tailored and adapted for number of applications.’

Due to the great flexibility to tailor MOFs for virtually any application, scientists have designed more than 88,000 customised MOFs – for applications from agriculture to pharmaceuticals. Conventional use of chemical solvents, processes that contribute to carbon emissions, and length of time to create, however, led the team to incorporate acoustic waves into the process.

Introducing sound

Using sound waves to manipulate atoms within a material is an innovation for MOFs, but is not a new phenomenon. Discovered in 1885 by Lord Rayleigh, surface acoustic waves (SAWs) are nanometre scale surface waves that travel across a piezoelectric material. These types of sound waves have been commonly used in telecommunications and to manipulate – or precisely, to move – fluids and particles in drops through pumping them.

‘In our lab, we thought to utilise this phenomenon for making intricate structures of crystalline materials, said Ahmed. ‘We tried this approach in the past on simple table salt – sodium chloride – in a previously published study in Advanced Materials, and we were able to make new types of the common table salt with better solubility and enhanced taste. On trying the acoustic approach on MOFs, we were surprised that they exhibited large surface area after the synthesis without any activation.’

Ahmed explained how the sounds waves can precisely orient and move the atoms to form intricate highly organised porous MOF networks with preferred directions, which can help with specific capture of the target molecules. In addition, the inherently vacant pores from the acoustic synthesis process will allow for more surface area available for maximum capture.

‘This acoustic technique can be extended to virtually any MOF, and can even be used to tailor a completely different generation of new materials. This indeed can open an exciting new era in the discovery of materials of the future.’

Looking ahead, the team is confident in the scalability of its acoustic approach. ‘Although this is a chip scale technique, it can be scaled out through using a number of these chips,’ said Ahmed. ‘These acoustic chips are very cheap – US$1 – and robust. With the technique able to be expanded to other MOFs and scaled out for efficient green production of these smart materials, we feel others will start to adopt this approach.’