Development of metal-organic frameworks for photocatalytic applications
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In this research photocatalytic water splitting by metal-organic frameworks (MOFs) has been studied to replace fossil fuels by renewable energy. It has been found that MOFs can be immobilized on F:SnO2 (FTO) as a conductive substrate. When using these photoelectrodes in photoelectrochemical tests it was observed that the materials are electrochemically unstable and that they possess no observable photoelectrochemical activity towards hydrogen generation. The metal nodes studied are based on Zr, Cu, Co and Ti, the linkers are 2-aminoterephthalic acid and 2,5-dihydroxyterepthalic acid. None of these MOFs were found to be capable of photoelectrochemical proton reduction to generate hydrogen. The MOFs were also studied as photocatalyst particles for photocatalytic activity towards proton reduction. No hydrogen evolution was observed with gas chromatography during photocatalytic tests. This agreed with the lack of photoelectrochemical activity for the photoelectrodes. Finally to study the charge dynamics transient absorption spectroscopy was used. It was found that the charge separation in the MOFs after light absorption is mostly related to the organic linker. For MOFs with 2,5-dihydroxyterephthalic acid linkers a transient state was observed with lifetimes up to 5 ns. MOFs with 2-aminoterephthalic acid linkers show two transients, one with lifetimes up to 15 ns and another with lifetimes extending to 600 ns. The chemical transition from protons to hydrogen takes place on a microsecond scale, meaning that the competing recombination of charge carriers is at least one order of magnitude faster than the desired reaction. This research has been conducted as part of the NIOK Solar Fuels graduate program. It was a joined research between Utrecht University and the Technical University of Eindhoven, supervised by Dr. Monica Barroso (UU), Dr. Jan Philipp Hofmann (TU/e) and Prof. dr. ir. Bert Weckhuysen (UU).