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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - chem-fs-MOF (Chemical Engineering of Functional Stable Metal-Organic Frameworks: Porous Crystals and Thin Film Devices)

Teaser

Summary

Regarding chemically demanding processes, MOFs still need to fulfil critical requirements like time-on-stream behaviour and leaching stability. Though high-temperature heterogeneous catalysis is likely to be dominated by inorganic solids like zeolites, MOFs display adequate thermal stability for low-temperature processes and unique chemical/structural control for an unlimited range of materials. However, except for particular cases, MOFs generally lack the chemical stability and structural robustness required due to to weak coordination bonds that hydrolyse with moisture or chemical decomposition in presence of acid or base. This is arguably limiting the incorporation of MOFs into a broader range of practical applications nowadays. Besides chemical stability, the insulating character of MOFs is another key limitation. Provided combination of high surface areas with good charge mobility, MOFs could become a game changer and make significant contributions to applications of environmental relevance as photovoltaics, photocatalysis, electrocatalysis, supercapacitors or sensing (transduction of electrical signal), to cite a few. Assembly of 2D nanosheets with atomic thickness of layered MOFs also represents an interesting technological avenue. This would enable fabrication of solid-supported devices in which function would be reduced to a single/few layers whilst retaining most of the properties of the bulk alongside integration into a useful platform (direct contact with another materials).

Work performed

1. New photoactive titanium-organic frameworks and water-stability in MOFs: We have reported a new family of heterometallic titanium-organic frameworks that enlarges the limited number of crystalline, porous materials available for this metal. They are chemically robust and can be prepared as single crystals at multi-gram scale from multiple precursors. Compared to other methodologies based on the post-synthetic metallation of MOFs, our approach is well fitted for controlling the positioning of dopants at an atomic level to gain more precise control over the band gap and electronic properties of the porous solid. Changes in the band gap are also rationalized with computational modelling and experimentally confirmed by photocatalytic H2 production that outperforms the intrinsic activity of other Ti-MOFs available. We have also reported the synthesis of a mesoporous Ti-MOF displaying a MIL-100 topology. MIL-100(Ti) combines excellent chemical stability and mesoporosity. We are confident the addition of a mesoporous solid to the scarce family of crystalline, porous titanium-frameworks available will be a valuable asset to accelerate the development of new porous photocatalysts without the pore size limitations currently imposed by the microporous materials available. We have also succeeded in demonstrating the use of non-conventional linkers for the assembly of MOF type architectures based on unexplored metal-ligand joints of the siderophore-type. As for improving the hydrolytical stability of poorly stable MOFs, we have reported the ability of HKUST to induce the formation of polydopamine-like coatings by direct reaction with synthetic catechols in anaerobic conditions without base addition. This approach provides control over the function of the protective shell as it can be engineered by suitable choice of the synthetic catechols. By using alkyl fluorinated derivatives, we have demonstrated that these coatings are gas permeable and show efficient hydrophobic protection for HKUST against water degradation to retain its original sorption capacity intact.

2. Extrinsic electrical conductivity in MOF crystals by incorporation of conducting polymers and snapshots on polymerization mechanism. We have shown the possibilities offered by these crystalline, porous nanoreactors to capture highly-reactive intermediates for a better understanding of the mechanism of polymerization reactions. By using a cyclodextrin framework we are capable of restricting the polymerization of pyrrole to a low-degree, capturing the formation of terpyrrole cationic intermediates. Single-crystal X-Ray diffraction was used to provide definite information on the supramolecular interactions that induce the formation and stabilization of a conductive array of cationic complexes within the porous framework that displays an increase in the electrical conductivity close to 3 orders of magnitude over the pristine MOF.

3. MOF ultrathin films and single-layers for solid supported photocatalysis: towards device integration. We have described the use of Self-Assembled Monolayer (SAM) substrate modification and bottom-up techniques to produce prefrentially oriented, ultrathin, conductive films of a conductive and porous MEOF: Cu-CAT-1. Our approach permits to fabricate and study the electrical response of MOF-based devices incorporating the thinnest MOF film reported thus far (10 nm thick). Comparison between multiple families of devices was used to understand the effect of the fabrication method, film thickness and channel length over their electrical response, which will be of fundamental value for the integration of MOFs in electronic and photocatalytic devices. One of the main interest of conductive Metal-Organic Frameworks is the development of chemiresistive sensors capable of transducing the presence of specific guests into an electrical response with good selectivity and sensitivity. By combining experimental data with computational modelling, we have recently de

Final results

Most developments in the chemistry and applications of Metal-Organic Frameworks (MOFs) have been made possible thanks to the value of reticular chemistry in guiding the unlimited combination of organic connectors and secondary building units (SBUs) into targeted architectures. However, the development of new titanium-frameworks remains limited by the difficulties in controlling the formation of persistent Ti-SBUs with predetermined directionality amenable to the isoreticular approach. We have implemented new synthetic methodologies (high-throughput exploration oft he chemical space) to accelerate the discovery of new materials and optimize their synthesis at high-scale regardless the precursor of choice. Our approach has also included exploration of unexplored metal binders rather than more conventional carboxylate or azolate linkers.
We havo also established new methodologies to access MOF ultrathin films of sufficient quality for application in photocatalytic devices. Besides their processing, we have developed the techniques required for evaluating charge transport in nanometric thick films

For the second half of the project we aim to:
1. Produce new heterometallic titanium-organic frameworks by new methodologies that enlarge the scope of materials currently available. We identify this family of mixed-metal MOFs as an excellent platform to combine chemical stability with unprecedented chemical reactivity.
2. Develop multicomponent MOF hybrids for performance enhancement from synergetic interaction by: i) chemical engineering for controlled sensitization and ii) integration of light sensitizers with homogeneous catalysts.
3. After optimizing the methodology for fabricating MOF ultrathin films, we aim to study their ability to transform light into current in phototransistors now. Overall, this activity shall pave the way for the fabrication of photoelectrodes for sustainable, direct solar water splitting with MOF-based photoelectrochemical cells, still unprecedented.