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!

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!

Compositional inhomogeneity in mixed-metal ZIF-8 analogues

Adam’s work on mixed-metal ZIF-8 analogues is out in Chemical Communications!

We study the structural and thermomechanical effects of cation substitution in the compositional family of metal–organic frameworks Zn1−xCdx(mIm)2 (HmIm = 2-methylimidazole). We find complete miscibility for all compositions x, with evidence of inhomogeneous distributions of Cd and Zn that in turn affect framework aperture characteristics. Using variable-temperature X-ray powder diffraction measurements, we show that Cd substitution drives a threefold reduction in the magnitude of thermal expansion behaviour. We interpret this effect in terms of an increased density of negative thermal expansion modes in the more flexible Cd-rich frameworks.

The paper can be found here and the preprint is available for free on ChemRxiv.

Congratulations Adam!