State of the art
Despite crystal defects can be classified as structural imperfections, structural disorder is a fascinating area in crystal chemistry and materials science, as defects can strongly affect the physical and chemical properties of the material. Defects have implication in mechanical and thermal stability, photo stimulation, transport and storage performance, chemical reactivity and porosity, and thus are strongly related with the material function. For example, defect engineering – the control and manipulation of a type of defect, its concentration and spatial distribution within a crystalline material – is now used as fine-tune technology for mediated metal doping in semiconductors such as TiO2 or ZnO.
Metal organic frameworks (MOFs)  are a new generation of crystalline hybrid materials composed of metal ions or clusters coordinated by multidentate organic bridging ligands. Owing to MOFs’ attractive properties, such as high porosity and unlimited chemical and structural diversity for a rich landscape of functions, these porous solids have aroused tremendous interest within the scientific community during the last 15 years, with promising application in selective gas capture or storage,catalysis, water treatment and drug delivery. Therefore, defect engineering of MOFs has recently received attention as a chemical tool to fine-tune their properties and enhance their performance in different contexts. Defects can be introduced to MOFs during synthesis (by coordination modulation) and postsynthetically (by acid/ base/temperature treatments and activation/harvesting procedures) and defects can be generally classified as truncated and missing linkers and missing clusters, which are a consequence of the spatial concentration and critical distribution of missing linkers. Despite ca. 85,000 MOF structures being estimated by the Cambridge Structural Data base in 2019, defect engineering of MOFs is still limited in the vast majority to Zr-based MOFs (stands for University of Oslo) with the UiO topology, which incorporate high concentration of defets whilist maintaining their stabillity.Â
Current limitations
Understanding the formation of defects at a molecular level and establishing direct structure-function correlation is imperative for the application of defected MOFs. Elucidation of the type of defects, their concentration and spatial distribution within the framework at a molecular level is still a challenge for conventional characterization techniques – periodic arrangement of the crystal lattice is necessary to observe changes in the Bragg’s reflection peaks – and the handful of studies available are still limited to UiO-66 derivatives. Despite defect engineering being a versatile tool to modify MOFs’ properties towards different applications, the synthetic control of defected MOFs is still a challenging goal and defect engineering of MOFs is mostly limited to Zr6-based MOFs of the UiO topology. Limitations often reside in the fact that, in general, MOFs structural integrity is not always maintained upon structural disorder. Titanium-based MOFs – which are photoactive and have superior structural and chemical stability compared to other MOFs, including Zr6–based MOFs – are emerging in the literature, but their defect chemistry remains unexplored. Titanium has at the same time low cytotoxicity, a lower density than zirconium, and it is abundant. Importantly, TiO2 has been recently classified as a possible carcinogenic to humans (Class B2) by the International Agency for Research on Cancer, and its replacement in diverse applications might be achieved by the design of biocompatible Ti-MOFs.
What DefTiMOFs aims to do
 The ultimate aim of `Defective Titanium Metal-Organic-Frameworks(DefTiMOFs)’ is to develop novel high-throughput (HT) synthetic methodologies for the control of not only defect chemistry of Ti-MOFs, but also of their particle size and inner surface (pore functionalisation) towards the controllable modification of their properties for desired applications of environmental relevance.Â
Inspired by the high demand for clean and renewable energy sources including efficient and affordable water delivery systems in places with limited access to drinkable water, DefTiMOFs aims to correlate defect chemistry of Ti-MOFs with their performance towards environmentally friendly applications.Â
What DefTiMOFs is doing
HT synthesis is being convined with a set of novel characterisation techniques (mainly synchrotron-based) for atomic and molecular level of characterisation of defects, aiming to correlate synthetic conditions with defect formation (defect type, density and spatial distribution within the framework) in order to provide the base of knowledge to anticipate their properties based on the synthetic conditions.
- The materials are stable in water at different pHs and their water adsorption properties are currently being investigated.Â
- Promising defective materials are currently being tested towards their catalytical appplication.
- Nanomaterials are currently being tested for biocompatibility at Dr Victoria del Pozo´s Team at Fundación Jimenez Diaz.
