Faulty Goods? the exciting consequences of introducing defects in MOF frameworks with water

Summary by Francesca Firth

Reading the abstracts from conferences on fields ranging from crystallography through energy storage to the many branches of materials chemistry, it is a challenge to avoid spotting one on the subject of metal-organic frameworks (MOFs). With their incredible tunability and versatility, arising from the wide range of metal nodes and organic linkers available from which to build a multi-dimensional porous network, MOFs are the source of great excitement in the search for materials for all kinds of applications. However, changing the identity of the metal or linker is not the only method of altering the properties of the framework – the formation of defects, such as missing linkers, missing nodes, or stacking faults, in a structure can also significantly change the properties of a material (often with less disastrous consequences than a misplaced screw in a build-at-home chair…). UiO family MOFs are ideal for studying defects, as their strong metal-linker coordination and high connectivity allow them to incorporate linker and metal-cluster vacancies to a high degree.

Although most defects in MOFs are random, in our previous work. we found that control over the synthesis conditions of UiO-66(Hf) and UiO-67(Hf) caused the formation of correlated defects, leading to (in the former case) nano-regions of a different topology (reo) inside the main (fcu) matrix, via the introduction of metal cluster vacancies, and (in the latter case) an entirely new bulk crystal phase (hcp), formed when linker vacancies order within a lattice plane and are compensated by metal-cluster condensation. Both of these phenomena were caused by the presence of formic acid in the synthesis, as the formate anions can “cap off” the sites where linkers would attach to the clusters. Rather than causing the framework to collapse (like our unfortunate DIY chair!), the deliberate introduction of these “faults” allowed access to previously-unexplored structures with new and interesting properties.

This led us to investigate other species in the synthesis of UiO frameworks which might also act as “capping” ligands and introduce defects. Among these is water – while it might seem innocuous, and is used as an environmentally-attractive “green” solvent, any potential effects of water in the synthesis on the formation of defects have not been well-explored. Therefore, in this paper, we systematically explore the effect of including water in the reaction mixture on the introduction of defect structures in UiO frameworks.

We found that varying the amount of water in the synthesis allowed us to tune the crystal phase of the UiO MOF which formed. Due to the symmetry differences between the phases, we were able to use powder X-ray diffraction as an easy tool to distinguish between them; different phases (in these cases) also have different appearances, which can be seen via electron microscopy.

For UiO-67(Hf), increasing the amount of water in the hcp synthesis (also containing formic acid) causes the formation of a new phase, hns, which forms as hexagon-shaped nanosheets. Unlike the nanosheets formed by sonicating the layered hxl phase we described in our previous paper, the hns phase is obtained directly through synthesis, giving a fast and straightforward route to obtain highly crystalline nanosheets. Furthermore, both the hcp and the hns phase synthesised using water were stable for at least 19 months, an improvement over the week-long stability we previously reported for the hcp.

We then moved on to other members of the UiO family, showing that for UiO-66 with a fluorinated linker (F₄BDC) water again determines the phase formed. This demonstrates that the selectivity between the classic fcu structure (no water) and the hcp/hns enabled by water is general to more than one UiO framework.

We then moved on to the archetypal UiO-66; since its discovery in 2008, it has been the subject of much research by the MOF community, and is particularly well-known for stability even when containing defects, meaning that using defect engineering to obtain a new phase of UiO-66 would be particularly exciting. Indeed, by varying the amount of water and formic acid in the synthesis, we were able to obtain the hcp phase. Moreover, the inclusion of water suppressed the formation of other phases such as the nano-reo we previously described, or a hafnium-formate phase, allowing a high degree of phase selectivity and purity. Higher concentrations of water and acid in the reaction mixture also allowed us to tune the amount of missing-linker defects present in the hcp. By further exploration of the reaction conditions, we might be able to obtain the hns nanosheet phase of this archetypal framework.

This study shows that water plays a key role in the synthesis of defect phases of UiO family MOFs; by controlling the manner in which the defects are formed, we are able to control which crystal phase is obtained, and therefore the properties of the UiO framework; the creation of defects does indeed have exciting results! Looking towards future work, these defects may also have an effect on the acidity or catalytic ability of the framework, and the creation of novel stable MOF-based nanosheets in this way represents a potential strategy for synthesising MOF membranes, potentially for use as separator films in energy storage devices.