Tunable core–shell MOF nanoparticles

We are delighted that Kieran’s paper based on his MChem Part II research project in Oxford has now been published in Chemical Science!

The article describes how, by shortening the length of reaction, Zn/Cd-based ZIF-8 nanoparticles form with a Cd-rich core and Zn-rich shell. We collaborated with Sean Collins, whose beautiful scanning transmission electron microscopy showed us the core–shell structures, which we then used as the basis for a new model, first suggested by Andrew Goodwin to fit high-resolution X-ray diffraction data. This model allowed Kieran to quantify for numerous bulk samples the amount of Cd-rich material and Zn-rich material in the particles, as well as where the core–shell interface lay and how diffuse it was. He performed 99 syntheses at a range of temperatures and Zn/Cd ratios to map out how the nanoparticles’ internal interface and structure varied as a function of reaction conditions. Finally, we showed using in situ X-ray diffraction that the particles form first with a Cd-rich core followed by Zn-rich shell and the interface becomes increasingly diffuse the longer the reaction goes on.

By developing this simple synthesis and powerful new analysis method, and understanding the underlying formation mechanism, we have shown that it is indeed possible to control the spatial distribution of different components in metal–organic frameworks (MOFs) such as ZIF-8, which is really important to enable researchers to tap into their enormous potential as gas storage, separations and catalysis materials.

See the citation and all our publications here.

This work could not have been performed without several amazing co-authors: thank you Sean Collins for the STEM–EDS, Andrew Goodwin for co-supervision, Emily Reynolds (now at ISIS), Frank Nightingale, Hanna Boström (now at the Max Planck Institute for Solid State Research, Germany) and Simon Cassidy in the Goodwin group for help with all aspects of the XRD, Daniel Dawson and Sharon Ashbrook for NMR insights, Oxana Magdysyuk at Diamond beamline I12 for help with the in-situ beamtime, and Paul Midgley at Cambridge for support with the microscopy – Well done and thank you!

Monitoring MOFs

Ever wanted to monitor your MOF synthesis on the cheap? Look no further, because Felicity’s Open Access paper describing how MOF scale-up can be improved using an open source, multi-channel monitor – all built for less than $100 – is now out in Scientific Reports! She used simultaneous temperature, turbidity, pH, and visible light absorbance to track the formation of STA-16(Ni), observing the reaction critical processes that guided the development of a faster and more efficient synthesis route to material with comparable porosity.

The work was performed while Felicity was a Part II student in Oxford during her project at Johnson Matthey, co-supervised by Tim Johnson, Stephen Poulson and Stephen Bennett.

Sensing the strain, quickly

When metal–organic frameworks (MOFs) adsorb small molecules their structures often change, sometimes really noticeably, sometimes in such small ways that it’s hard to see. In a collaboration with Kota Shiba, Genki Yoshikawa and Kosuke Minami at the National Institute for Materials Science, Japan, we put MOF nanoparticles on a unique sensor device, the membrane-type surface stress sensor (MSS), by inkjet printing and spray-coating, and found that the MSS can detect these changes really well. The volatile organic compounds (VOCs) that we tested can be detected even at parts-per-million levels (one hundred times more dilute that CO2). The response is different for different VOCs and different MOFs, which enables them to be easily discriminated. What’s more, because of the high external surface area of the MOF nanoparticles, the response of the MOF-MSS sensor is really quick–– it takes just seconds to get a reading. This could make such technology really useful for real-time monitoring of chemical processes or biomarkers in healthcare.

The paper, “Strain-based chemical sensing using metal–organic framework nanoparticles” is published in the Journal of Materials Chemistry A. See our publications for more details.

Squeezing conductivity from a molecular crystal

Most molecular materials don’t conduct electricity well because the electrons in them can’t travel around easily. Materials that do conduct have pathways for their electrons, formed by overlapping atomic orbitals–the space where electrons usually stay–between molecules. In order to increase the conductivity of a molecular crystal, Nickel 5,6-dihydro-1,4-dithiin-2,3-dithiolate, we used high pressure to squeeze the molecules together.

This work was an international collaboration with some really talented scientists in the RIKEN institute, Japan, Kumamoto University, Japan, and Diamond Light Source synchrotron, UK.

Hengbo and Reizo at RIKEN made the molecular crystals and measured their conductivity under high pressure. They did this by attaching tiny wires to the crystal inside a Diamond Anvil Cell (DAC)–a device that generates pressure larger than found at the bottom of the ocean. They found that, whilst normally the crystal didn’t conduct electricity at all, when it was squeezed its conductivity increased dramatically.

In order to understand why, we used X-ray diffraction to work out the crystal structure–where the atoms are and how they are bonded together. Aided by Chloe and Mark, we fired intense X-ray beams produced at Diamond Light Source through our crystal in the DAC and measure how they scattered. From the scattering, we could show for the first time that the molecules got closer and closer together under pressure.

The final piece in the puzzle came from Takao at Kumamoto, who used theoretical calculations based on the X-ray crystal structures to work out the changes in orbital interactions. And the calculations showed that the orbitals between the molecules indeed overlapped more and more with pressure, explaining why the crystals’ conductivity got higher.

A link to the paper can be found here. It’s Open Access, meaning anyone can read it for free!

Thanks and well done to Hengbo, Takao, Chloe, Reizo and Mark!

Pre-equilibrium species in MOF crystallization

We’re very pleased to announce our paper on the crystallization of ZIF-8 has just been accepted! It’s been a challenging piece of work, not least because it all began when we made the surprising observation that crystallization got SLOWER when we increased the concentration of our reactants…

There is an increasingly large amount of interest in metal-organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. Their exciting properties arise from their modular architectures, which self-assemble from different combinations of metal-based and organic building units. However, the exact mechanisms by which they crystallize remain poorly understood, thus limiting any realisation of real “structure by design”. We report important new insight into MOF formation, gained using in situ X-ray diffraction, pH and turbidity measurements to uncover for the first time the evolution of metastable intermediate species in the canonical zeolitic imidazolate framework system, ZIF-8. We reveal that the intermediate species exist in a dynamic pre-equilibrium prior to network assembly and, depending on the reactant concentrations and the progress of reaction, the pre-equilibrium can be made to favour under- or over-coordinated species, thus accelerating or inhibiting crystallization, respectively. We thereby find that concentration can be effectively used as a synthetic handle to control particle size, with great implications for industrial scale-up and gas sorption applications. This finding enables us to rationalise the apparent contradictions between previous studies and, importantly, opens up new opportunities for the control of crystallization of network solids more generally, from the design of local structure to assembly of particles with precise dimensions.

The paper is published with Angewandte Chemie, International Edition and can be found here. A previous version can also be downloaded for free on ChemRxiv.

Many thanks to all co-authors, Diamond for beamtime, SCG Innovation for funding and everyone else that helped out along the way!