Investigating Complex Structures of Blended Metal-Organic Framework Glasses

The chemical and structural complexity of crystalline metal-organic frameworks (MOFs), which are network architectures formed from linked inorganic and organic components, presents great challenges for their characterization. Recently, one family of MOFs have been shown to melt (without decomposition), and form glasses upon quenching of the resultant liquids. The arrangements of atoms in these liquids and glasses are, by their nature, highly disordered and lack a unit cell. They are thus even more demanding to characterize than their crystalline counterparts.
Published in Chemistry

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In previous work, the liquid and glass states of a ZIF of composition Zn(Im)2, (Im – imidazolate C3H3N2-) have been investigated by techniques including 13C solid state nuclear magnetic resonance spectroscopy. Perhaps the most informative however has been the application of Total Scattering techniques, and acquisition of pair distribution function (PDF) data. These data, which are effectively a histogram of atom-atom correlations in a material (crystalline or amorphous), are then modelled using the reverse Monte Carlo technique, to yield atomistic configurations.

In this work, we were interested in the application of traditional glass and organic polymer processing techniques, to the liquid and glass MOF states. Two species were blended together using the high temperature liquid state of ZIF-62 [Zn(Im)1.75(bIm)0.25] (bim – benzimidazolate C7H5N2), and on quenching formed a single glass species. However, if the characterization of a single amorphous MOF species is challenging, then building a picture of a material containing two complex amorphous MOF structures is somewhat intimidating.

To surmount this challenge, we turned to electron microscopy. Electron microscopy is a workhorse technique for understanding the nanoscale organization of predominantly crystalline materials like metals, semiconductors, and ceramics. The capabilities for imaging chemical information in a sample, using spectroscopic techniques in the electron microscope, is what really drew us to electron microscopy for understanding these intertwined amorphous MOFs. By forming a blend starting from MOFs with two different metal centres (Zn, Co) we created chemical tags, analogous to fluorescent labels in light microscopy, to be able to track the two MOFs in the blend.

On top of that, we were able to use electron microscopy to record a 3D image of the metal-centre distributions, using the electron microscope like a CT scanner. Making sure the MOFs didn’t fall apart under the damaging electron beam irradiation required us to be cautious with electron dose while obtaining the sufficient signal for a usable 3D image quality. We were able to strike a good balance by combining this 3D imaging technique with advanced computational techniques to recover high quality 3D information from limited data. Using these approaches, we were able to determine that the two MOF phases form interlocked structures but without much mixing at the boundaries of the two MOFs. Understanding these microscopic details in turn helped us interpret the mechanical behaviour of these new glass blends.

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