Synthesis and characterisation of conductive metal-organic framework materials

Thesis

Electrically conductive metal-organic frameworks (MOFs) have emerged as a promising new class of materials for use in applications ranging from organic electronics to chemical sensing. However, integration of MOFs into functional devices remains limited in part because of a variety of synthesis and processing challenges. For example, the growth of large oriented MOF crystals is difficult, and there are substantial challenges in creating MOF nanoparticle suspensions. In this thesis novel synthesis strategies are reported that enable control of the morphology, orientation, and processability of conductive MOFs. The application of these materials as chemiresistive gas sensors is investigated, and their paramagnetic spin activity is explored. These findings contribute to the broad goal of advancing the fundamental understanding of these new electrochemically-grown materials with a view to integrating them into novel electronic devices.

An electrochemical deposition strategy was developed to fabricate highly oriented Cu3(HHTP)2 MOF films on ITO glass using a dual-working-electrode approach. A systematic investigation into the electrochemical synthesis mechanism reveals how ligand concentration and synthesis conditions influence film orientation. This method enables precise control over crystal orientation, which directly impacts charge transport behaviour. The anisotropic electrical conductivity of these films was analysed.

Electrochemically-grown oriented Cu3(HHTP)2 films were used as the active material in chemiresistive gas sensors. By selecting the film orientation the sensors were optimised for ammonia detection with a limit of detection in the parts-per-billion (ppb) range. A direct correlation between MOF film structure and sensing response is shown. Structural and spectroscopic analyses (XRD, XPS, and FT-IR) before and after gas exposure provide insights into gas adsorption and charge transport mechanisms.

To improve the processability of MOFs for scalable device fabrication, two solution-based synthesis methods were developed: a modulator-assisted and a surfactant-based approach. Highly dispersible Cu3(HHTP)2 nanocrystals with morphology control were created. These solution-processable conductive MOFs were successfully deposited onto interdigitated electrodes via spray-coating, yielding good gas sensing behaviour.

Finally, the thesis describes experiments on redox-state tuning in MOFs. A 3D conductive MOF (Cu-DBC) was synthesized, where electrochemical oxidation-state modulation induces the formation of organic radicals. Pulsed electron paramagnetic resonance (EPR) measurements reveal spin characteristics with long spin-lattice relaxation times.

Authors: Wang, Y

• DOI: 10.5287/ora-xobmj5jod
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: chemiresistive gas sensor; electrochemistry; conductive metal-organic framework
• Subjects: chemiresistive gas sensor; 2D MOF