Metal-Organic Frameworks (MOFs)-Based Materials for Room Temperature Gas Sensing
Creators: name: Khan, Muhammad Waqas
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 401605
subject: Functional materials

description: Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size have been considered a promising candidate for developing room temperature highly-sensitive gas sensors. The partial replacement of the metal in metal nodes of MOFs or consolidation of the functional materials with MOFs to develop the composite leads to creating peculiar and unique surface properties to enhance the gas sensing performances. This PhD thesis explores the synthesis of the MOFs-based materials and studies their potential for toxic NO2 gas sensing. Overall, this thesis covers three key phases. In the first phase of PhD work, the partial replacement of metal within the metal nodes of MOFs is explored. The hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. During the study, Fe atoms in the Fe3 PCN-250 MOFs are partially replaced by transition metal Co, Mn, and Zn through a facile hydrothermal approach, leading to the formation of hetero-metallic MOFs (Fe2IIIMII, M=Co, Mn, and Zn). While the PCN-250 framework is maintained, the morphological and electronic band structural properties are manipulated upon the partial metal replacement of Fe. More importantly, the room temperature NO2 sensing performances are significantly manipulated, in which Fe2Mn PCN-250 demonstrates the largest response magnitude for ppb-level NO2 gas compared to those of pure Fe3 PCN-250 and other hetero-metallic MOF structures mainly attributed to the highest binding energy of NO2 gas. Second, a plasmonic MOF nanocomposite is developed to mitigate the individual disadvantages of MOFs by combining the functional material to get new features and improved functional properties. MOF nanocomposites consisting of the ultra-thin degenerated doped molybdenum oxide core and the flexible iron MOF (FeMOF) shell through the hydrothermal growth is developed, featuring the economical cost, facile synthesis, and non-toxicity. More importantly, the incorporation of plasmonic oxides in the highly porous MOF structure enhances the visible light absorbability, demonstrating improved photobleaching performances of various azo and non-azo dyes without the incorporation of oxidative agents compared to that of pure FeMOF. Furthermore, the nanocomposite exhibits enhanced sensitivity and selectivity towards gaseous NO2 at room temperature in the oxygen-minimum condition, attributed to the electron-rich surface of plasmonic oxides. Finally, 2D heterostructure (HS) formed by the combination of two different components is investigated. 2D HS presents charge transfer, surface reconstruction, stability, band alignment and conductivity ascribed to the development of the heterointerface. 2D/2D ZIF-67/Fe2O3 vertical heterostructures (HSs) are constructed via facile liquid liquid-interfacial growth at room temperature to get the enhanced and new functionalities through synergistic effect and heterointerface. The two dissimilar sheets meet at the basal plane with each other to form the vertical HS. The heterointerface of the ZIF-67/Fe2O3 HS allows the rapid transfer of charges with high accessible active sites and ultrathin-thickness. Interestingly, ZIF-67/Fe2O3 HSs demonstrate the improved sensitivity and selectivity towards the sub ppm-levelled NO2 gas at room temperature, attributed to unique chemistry resulting from heterointerface allowing higher adsorption of NO2.Overall, the author of this PhD thesis considers that this presented work will enable the exploration of the cost-effective synthesis techniques for MOFs-based materials, including MOFs and MOF composites, to contribute towards the efficient and suitable chemical sensing materials. It will also demonstrate the strong potential of the carefully engineered novel MOFs-related materials for power-saving and high-performance gas sensing devices and broaden the scope of these multifunctional materials for various applications.