Controlling defects in metal-organic frameworks

Metal-organic frameworks (MOFs) are versatile materials with many applications in areas such as catalysis and hydrogen storage. Recently it was found that certain MOFs contain defects, which could lead to new functionality if the locations of the defects are better understood so that they can be exploited. Correlated disorder and defects are abundant in exotic materials such as ferroelectrics and superconductors, where the defects are not randomly distributed but interact such that their positions are dependent on other defects. Previous research has indicated that one of the canonical MOFs, UiO-66(Hf), contains ligand-absence defects. The Extreme Conditions beamline (I15) at Diamond Light Source has the capacity to measure anomalous scattering from hafnium (Hf), and so was able to confirm that UiO-66(Hf) can contain not only ligand-absence defects, but also correlated Hf cluster absences.

Beamline I15 Scientific Highlight

A combination of diffuse scattering, Electron Microscopy (EM) and anomalous X-ray scattering techniques was used to determine if hafnium (Hf) absences were responsible for defect correlations. Data showed subtle changes in peak intensities which implied that Hf absences caused the observed correlations. Structural models were produced that could account for all the experimental data collected, establishing that the Hf-defects in UiO-66(Hf) occur in a regular manner: the position of one vacancy affects nearby vacancies. The nanoscale defect structures that emerge are analogues of vacancies found in transition metal oxides, which could have implications for storage, transport, optical, and mechanical properties in metal-organic frameworks (MOFs). This is the first example of correlated defects in a MOF though the results are unlikely to be unique to this particular example. The findings therefore open up the possibility of exploiting correlated defects in a wide range of MOF materials.

Figure 1: The structure of non-defective fcu UiO-66(Hf). A single Hf6(OH)4O4(O2C)12 cluster is shown in full in the top left of unit cell, with the rest of the unit cell represented as polyhedra. The Hf clusters are represented by blue truncated octahedra and the bdc2- ligands by pink rectangles. The atomic colour scheme is as follows: Hf, dark blue; O, light blue and C, pink. H atoms are omitted for clarity.

Point defects, and the local fields they cause, are crucial for many materials’ properties, as they allow for the creation of correlated states with unusual properties, for example relaxor ferroelectrics1, colossal magnetoresistance2, and the correlated motion of ions through superionic conductors3. Despite the importance of defects in conventional materials, it is only recently that the role that defects could play in the chemistry of MOFs has begun to be appreciated. Recent work has now established that it is possible to dope MOFs with large numbers of ligand-absence defects. These defective MOFs have been shown to have significantly enhanced both guest capacities4 and catalytic activity5. For some MOFs, including the canonical MOF UiO-66, these ligand-absences are present even under standard reaction conditions. UiO-66 is assembled from of MIV 6(OH)4O4 clusters (M = Zr,Hf) linked by benzene-1,4-dicarboxylate (H2bdc) linkers into a face centred cubic structure (Fig. 1), and its combination of high connectivity and strong metal-ligand bonding means that it is one of the most stable MOFs. The concentration of defects in UiO-66 can be regulated by altering the concentration of ‘modulators’ – monotopic acids, typically acetic, benzoic or trifluoroacetic acids – added to the reaction mixture. In all these reported cases of ligand-absence defects, the presence of correlations was not observed.

Figure 2: (a) The low angle region powder X-ray diffraction patterns of UiO-66(Hf), for samples containing different defect orderings, from top to bottom, nano-reo UiO-66(Hf), experimental and calculated from a supercell model, experimental and calculated nondefective fcu UiO-66(Hf) and defective ordered reo UiO-66(Hf); (b) Measurement of experimental powder diffraction patterns at the Hf K-edge (65.3 keV) and away from it (55 keV) revealed a subtle change in the relative intensities of the broad superlattice reflections, indicating that scattering from Hf is contributing to the diffuse scattering.

However, when formic acid was used as a modulator to promote defects, X-ray powder diffraction measurements revealed the presence of broad superlattice reflections in systematically-forbidden primitive positions (Fig. 2a). This structured diffuse scattering is a signature of correlated disorder. The analysis of structured diffuse scattering still presents substantial challenges, as the inherently short-range nature of correlated disorder prevents the application of the many powerful techniques of conventional crystallography. The nature of UiO-66(Hf) also presented difficulties of its own, as its chemistry makes the synthesis of single crystals extremely challenging and its sensitivity to radiation damage precludes the use of high-resolution EM. Thus the determination of the origin of the diffuse scattering was essentially reliant on X-ray powder diffraction techniques. This diffuse scattering had been previously noted and attributed to partially ordered solvent. This explanation was able to be eliminated on the basis of high-temperature diffraction measurements that confirmed the presence of diffuse scattering even in the desolvated material. Additionally, modelling suggested that ligand-absence defects would not, on their own, be able to account for the experimentally observed pattern of diffuse scattering. This suggested that in addition to ligand-absence defects, metal clusterabsences must be contributing to the observed ordering.

Figure 3: The structure of defective nano-reo UiO-66(Hf). (a) single unit cell of the defective reo topology UiO-66(Hf). The colour scheme is the same as for Fig. 1; (b) section of a supercell of the model of nano-reo UiO-66(Hf); (c) structuring of nanodomains in nano-reo UiO-66(Hf). Hf6O8 clusters are represented as spheres, with absent clusters shown in a colour that indicates which of the four different reo domains they belong to, and present clusters shown in grey.
To confirm this, it was necessary to ascertain whether hafnium was contributing to the observed diffuse scattering. This was established by carrying out an anomalous diffraction experiment at the Hf K-edge at beamline I15. The large unit cell of UiO-66(Hf) and long range of these correlations, in combination with the high energy of the Hf K-edge (65.3 keV) made this a challenging experiment as it meant the diffuse scattering occurs at very low scattering angles. Nevertheless, comparison of the diffraction patterns measured at the Hf K-edge and far from it (55 keV) revealed a small, but significant, alteration in the relative intensity of the two strongest diffuse peaks (Fig. 2b), confirming that hafnium was contributing to the diffuse scattering and hence that Hf clusters are implicated in the correlated disorder. In-house total scattering measurements further demonstrated that local-coordination environment of the clusters was unchanged. The origin of the structured diffuse scattering could therefore be identified as correlated Hf cluster absences.
Using this information it was therefore possible to build structural models able to account for the experimental data. The primitive structure of the diffuse scattering along with the observed cubic symmetry permits only one chemically reasonable defect ordering: metal cluster absences correlated along the <100> directions. This corresponds to a a short-range ordered version of the eight-connected reo topology (Fig. 3a). Quantum mechanical calculations demonstrated both the stability of this reo topology UiO-66(Hf) and the close match in unit cell dimensions between the defective and non-defective phases – the difference is less than 0.05%. A model was therefore built consisting of nanodomains of defective, reo topology UiO-66(Hf) within a nondefective UiO-66(Hf) matrix (Fig. 3b,c), which, by varying the concentration and size of the nanodomains was able to reproduce the observed diffraction data (Fig. 2a).
This nano-reo UiO-66(Hf) is the first reported example of defect correlations in a MOF, but the increasing numbers of defective MOFs that have been synthesised suggest that it is unlikely that UiO-66 will be the only MOF to display this phenomenon. The presence of defects and their interactions will have important consequences for mechanical properties, due to the altered network rigidity; gas sorption, due to the changed pore structure; and could even facilitate complex electronic, magnetic or optical properties, as correlated defect states are often essential for creating the necessary careful balance between interactions. In this way, the possibility of control over nanoscale disorder in MOFs opens up new opportunities for creating functional materials.

Source publications:
Cliffe, M. J., Wan, W., Zou, X., Chater, P. A., Kleppe, A. K., Tucker, M. G., Wilhelm, H., Funnell, N. P., Coudert, F. –X. & Goodwin, A. L. Correlated defect nanoregions in a metal-organic framework. Nature Communications 5, doi: 10.1038/ncomms5176 (2014).

References:
1. Xu, G. Y., Zhong, Z., Bing, Y., Ye, Z. G. & Shirane, G. Electric-field-induced redistribution of polar nano-regions in a relaxor ferroelectric. Nature Materials 5, 134-140, doi:10.1038/nmat1560 (2006).

2. Adams, C. P., Lynn, J. W., Mukovskii, Y. M., Arsenov, A. A. & Shulyatev, D. A. Charge ordering and polaron formation in the magnetoresistive oxide La0.7Ca0.3MnO3. Physical Review Letters 85, 3954-3957, doi:10.1103/ PhysRevLett.85.3954 (2000).

3. Miller, T. A. et al. The mechanism of ultrafast structural switching in superionic copper (I) sulphide nanocrystals. Nature Communications 4, doi:10.1038/ncomms2385 (2013).

4. Park, J., Wang, Z. U., Sun, L.-B., Chen, Y.-P. & Zhou, H.-C. Introduction of Functionalized Mesopores to Metal-Organic Frameworks via Metal- Ligand-Fragment Coassembly. Journal of the American Chemical Society 134, 20110-20116, doi:10.1021/ja3085884 (2012).

5. Vermoortele, F. et al. Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal-Organic Frameworks: The Unique Case of UiO-66(Zr). Journal of the American Chemical Society 135, 11465-11468, doi:10.1021/ja405078u (2013).

Funding acknowledgements:
M.J.C. and A.L.G. acknowledge financial support from the E.P.S.R.C. (EP/G004528/2) and the E.R.C. (Grant Ref: 279705. X.Z. and W.W. acknowledge financial support from the Swedish Research Council (VR), the Swedish Governmental Agency for Innovation Systems (VINNOVA) and the Knut and Alice Wallenberg (KAW) Foundation through the project grant 3DEM-NATUR. The EM facility is supported by the KAW Foundation. This work was performed using HPC resources from GENCI-IDRIS (grant i2014087069).

Corresponding author:
Matthew Cliffe, University of Oxford, matthew.cliffe@chem.ox.ac.uk

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