Ultrasonic exfoliation of hydrophobic and hydrophilic metal-organic frameworks to form nanosheets

A new collaboration between Dr Jona Foster and Professor Anthony Meijer's research groups has provided new insights into the synthesis of functionalised metal-organic nanosheets.

The new study, which was led by PhD student Dave Ashworth and included work by undergraduate student Adam Cooper, has been published in Chemistry – A European Journal. Below, we explain the findings.


Imagine a deck of playing cards thrown into the air. Now, imagine that again but shrunk by a factor of a million – now you're thinking nanosheets.

Metal-organic nanosheets (MONs) are a new class of programmable 2D material made of metal nodes linked by organic linkers, extending "infinitely" in two-dimensions. Their programmable nature has already made them useful in a variety of fields, ranging from heterogeneous catalysis and chemosensing, to functional electronic devices and molecular separation membranes.

In this collaborative study between the groups of Dr Jona Foster and Professor Anthony Meijer, MONs were functionalised with hydrophilic or hydrophobic "arms" intended to bestow functionality to the MON surfaces. Using experimental techniques including X-ray powder diffraction, infra-red spectroscopy and thermal gravimetric analyses, the Foster group was able to determine the solid state structure of the nanosheet layers. The functionalised arms allowed the nanosheets to be tailored to production in solvents with differing polarities.

Nanosheet

However, the observed differences between the materials in different solvents prompted the question: what do these MONs look like in suspension? Theoretical density functional theory calculations overseen by Professor Meijer, showed how each of the functionalised arms interact with solvent to aid the production of MONs in particular solvents.

PhD student Dave Ashworth, the lead author on the paper and principal investigator in the study, said: "It is great to see published work generated in part from the hard work of three different undergraduate level students, which goes to show that a passion for science can be rewarding early on in a scientific career." Dave completed our MChem Chemistry with Study in Industry course, spending a year working with LGC Ltd before beginning his PhD with Dr Foster in the Department of Chemistry in 2016.

The nanosheets were synthesised through "exfoliation", the removal of layers, from their layered parent frameworks using ultrasound. This process was explored in great detail by undergraduate MChem student Adam Cooper, in order to understand and optimise the procedure to obtain ultrathin nanosheets. Atomic force microscopy revealed this technique could produce MONs down to just two nanometers thick – only two layers of material! These systems were then demonstrated to function as chemical sensors towards small aromatic heterocycles.

The work carried out in this paper has stimulated further research into functionalised nanosheets within the Foster group, carried out by both undergraduates and postgraduates alike.