Metal-organic frameworks and porous composites for photocatalytic and sensing applications
The sustainability and environmental issues associated with using fossil fuels threaten human health and the climate, making a move towards renewable energy critical. Metal-organic frameworks (MOFs) promise a solution. MOFs are a class of porous and crystalline materials comprised of inorganic building units, such as metals/metal oxide clusters, linked through organic ligands. As a way of producing green hydrogen, MOFs can be employed as photocatalysts which can absorb light energy, leading to the reduction of water to hydrogen with no carbon emissions. The current research used densified, monolithic MOFs (monoMOF) in contrast to conventional and widely reported powder MOFs. Such monoliths (prepared without binders or under high pressure) are mechanically robust and porous materials with the ability to host catalytically active nanoparticles (NPs). This project aimed to assess the performance of monoNH2-UiO-66 MOF as a photocatalytic agent for green hydrogen evolution from water, making it the first use of a densified, monolithic MOF as a photocatalyst for water-splitting. The MOF was sensitized with organic dyes to promote harvesting of the visible region of the solar spectrum and loaded with platinum (Pt) NP co-catalysts for proton reduction. Electron transfer and charge transport drive the photocatalytic process of evolving hydrogen via water-splitting. However, most reported MOFs utilized for this purpose are essentially insulating and their lack of conducting ability limits their performance in this application. Semiconductive and porous MOFs are an interesting class of material and, owing to their intrinsic or induced charge carrying properties, these MOFs are undoubtedly excellent candidates for photocatalytic applications. Interestingly, there is only limited evidence of this class of MOFs being used to drive the hydrogen evolution process and there is need to investigate this. Moreover, inherently conductive MOFs also offer suitability in the field of chemiresistive gas sensing. For this reason, the assessment of MOFs with charge carrying properties was conducted by collaborators who computationally screened published structures. Comprehensive high through-put screening yielded 1,692 MOFs which underwent DFT calculations on their electronic and conductive properties. Experimental validations of these DFT (low-level and high-level) calculated MOFs were attempted by synthesis of iv computationally shortlisted MOFs. The experimental validation processes assessed the conductivity, surface area and chemical stability of the short-listed MOFs and the correlation of their measured bandgaps with calculated values. Results obtained from experimental work suggested significant shortcomings in current standards of DFT prediction.