Nina Fabienne Suremann: Brick by Brick: Assembly and Exploration of Metal–Organic Framework Thin Films

Abstract

Catalysis is essential for many industrial processes, from fertilizer and medicine production to renewable fuel generation. As the demand for clean energy grows, better catalysts for solar fuel production are needed. Metal–organic frameworks (MOFs) have emerged as promising candidates, where metal-based nodes and organic linkers form stable, customizable 3D structures. Embedding molecular catalysts into MOFs helps prevent unwanted interactions that often reduce their activity. For electrocatalytic applications, electrodes are modified with electrochemically active MOF thin films. However, controlling their thickness, which is crucial for performance, remains a challenge. This thesis investigates two synthesis techniques that allow thickness control: vapor-assisted conversion (VAC) for UiO-type MOFs (UiO = Universitetet i Oslo) and atomic layer deposition (ALD) combined with pseudomorphic replication (PMR) for porphyrinic MOFs.

First, VAC was studied for the synthesis of crystalline UiO-67 thin films. While considered a straightforward method, reproducibility issues arose. Literature suggests that water plays a crucial role in forming zirconium clusters, but this study reveals that excess water, absorbed from atmospheric moisture, can harm MOF crystallinity. Fortunately, this effect can be countered by increasing the concentration of the modulator acetic acid, which competes with the linker for coordination sites at the nodes, slowing down crystallization.

Next, the ALD/PMR approach was explored for the porphyrinic MOF family Al2(OH)2MTCPP (M = Co, Cu, Fe, Zn, H2; H2TCPP refers to the free-base porphyrin). ALD, a gas-phase technique, enables thickness-controlled deposition of inorganic coatings, such as metal oxides, while PMR transforms one material into another. By combining these techniques, various substrates (silicon, indium tin oxide-coated glass, gas diffusion electrodes, fluorine-doped tin oxide-coated glass, and copper) were coated with porphyrinic MOF layers using conventional or microwave-assisted heating. Here, the metal oxide serves as a node precursor, which, upon the introduction of a metalloporphyrin linker, forms the MOF layer.

Beyond synthesis, the ALD/PMR strategy was explored for surface patterning. Using a mask during ALD creates a metal oxide template, which can be converted into a MOF pattern with sub-micrometer resolution. The synthesized Al2(OH)2CoTCPP@silicon composite was applied as a photocathode for solar fuel production, selectively reducing protons to hydrogen with 100% faradaic efficiency. Demonstrating the system’s photovoltage, the reduction onset potential of the photoelectrode was observed at more positive potentials compared to that of a dark cathode.

The provided insights into synthesis parameters, surface patterning, and the application of MOF-modified electrodes for solar fuel production, highlight both the importance of controlled synthesis and the versatility of MOF materials.