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Scherb, Camilla (2009): Controlling the Surface Growth of Metal-Organic Frameworks. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

This work is focused on the investigation of synthesis conditions for the direct growth of metal-organic frameworks (MOFs) on self-assembled monolayers (SAMs) used for surface functionalization. The characterization of the surface-grown crystals is a central part of this thesis, in order to learn more about the mechanism of MOF crystallization and the interesting properties of this class of materials leading to future applications. The tuneable oriented growth of thin films of the porous MOF HKUST-1 (Cu3(C9H3O6)2(H2O)3•xH2O) on different functionalized SAMs is demonstrated. Films grown on carboxylate-terminated SAMs are highly oriented along the [100] direction, while alcohol-terminated surfaces induce a completely different orientation along the [111] direction. Homogeneous but less oriented thin films are also obtained on methyl-terminated SAMs. A combination of X-ray diffraction and scanning electron microscopy was used to study the film growth, including the morphological evolution of the crystals on the SAMs. In situ atomic force microscopy was used to directly investigate the growth processes of the oriented MOF HKUST-1 grown on SAMs on gold substrates. This approach provides direct evidence for a layer-by-layer mechanism of the constituent 1.5 nm d111 crystal spacing step, its form during growth, and the influence of the step vertex on the rate of growth. The orientation and structure of porous MOFs based on iron terephthalate can be controlled by heterogeneous nucleation on self-assembled monolayers of mercaptohexadecanoic acid (MHDA). The framework MIL-53 is the product of homogeneous nucleation, whereas in the same crystallisation solution, oriented MIL-88B grows on the functionalized gold surface. A new sample cell was developed to allow for the investigation of the structural changes of flexible, porous materials during adsorption and desorption of guest molecules. Crystals of the flexible MOF Fe-MIL-88B were investigated as bulk material as well as surface-grown, oriented crystals. We were able to follow the structural changes of the Fe-MIL-88B crystals upon ad- and desorption of water. Due to the orientation of the crystals on the gold substrates, structural changes in [001] direction could be observed. For the randomly oriented bulk crystals the structural changes in all crystallographic directions are observable and the changes of the lattice constants a and c and the cell volume could be determined quantitatively by indexing of the complete diffraction patterns. The sorption isotherms recorded with the help of the sorption@XRD method show two distinct steps of structural changes. During adsorption of water the lattice parameter c is slightly but still detectably decreased, whereas the lattice parameter a shows a prominent two step-increase resulting in an increase of the cell volume about 40 %. The conventional volumetric water sorption measurement also shows two distinct steps of the amount of adsorbed volume that can be correlated to the structural changes observed in X-ray diffraction. We have demonstrated that advantages of functionalization can also be realized in oriented film synthesis of MOFs: We have shown for the first time that functionalized MOFs can be grown on self-assembled monolayers and that a preferred orientation of the crystals is achieved. This was demonstrated for NH2-MIL-101 and to a lesser extent for NH-MIL-88B. Importantly for potential storage applications, it was also shown that the adsorption capacity of NH2-MIL-101 was not significantly reduced by the amino functionalization.