Comparison of Surface-Bound and Free-Standing Variations of HKUST-1 MOFs: Effect of Activation and Ammonia Exposure on Morphology, Crystallinity, and Composition.

Metal-organic frameworks (MOFs) are extremely porous, crystalline materials with high surface area for potential use in gas storage, sequestration, and separations. Toward incorporation into structures for these applications, this study compares three variations of surface-bound and free-standing HKUST-1 MOF structures: surface-anchored MOF (surMOF) thin film, drop-cast film, and bulk powder. Herein, effects of HKUST-1 ammonia interaction and framework activation, which is removal of guest molecules via heat, are investigated. Impact on morphology and crystal structure as a function of surface confinement and size variance are examined. Scanning probe microscopy, scanning electron microscopy, powder X-ray diffraction, Fourier-transform infrared spectroscopy, and energy dispersive X-ray spectroscopy monitor changes in morphology and crystal structure, track ammonia uptake, and examine elemental composition. After fabrication, ammonia uptake is observed for all MOF variations, but reveals dramatic morphological and crystal structure changes. However, activation of the framework was found to stabilize morphology. For activated surMOF films, findings demonstrate consistent morphology throughout uptake, removal, and recycling of ammonia over multiple exposures. To understand morphological effects, additional ammonia exposure experiments with controlled post-synthetic solvent adsorbates were conducted utilizing a HKUST-1 standard powder. These findings are foundational for determining the capabilities and limitation of MOF films and powders.

Tailoring the nanoscale morphology of HKUST-1 thin films via codeposition and seeded growth.

Integration of surface-anchored metal-organic frameworks (surMOFs) within hierarchical architectures is necessary for potential sensing, electronic, optical, or separation applications. It is important to understand the fundamentals of film formation for these surMOFs in order to develop strategies for their incorporation with nanoscale control over lateral and vertical dimensions. This research identified processing parameters to control the film morphology for surMOFs of HKUST-1 fabricated by codeposition and seeded deposition. Time and temperature were investigated to observe film formation, to control film thickness, and to tune morphology. Film thickness was investigated by ellipsometry, while film structure and film roughness were characterized by atomic force microscopy. Films formed via codeposition resulted in nanocrystallites anchored to the gold substrate. A dynamic process at the interface was observed with a low density of large particulates (above 100 nm) initially forming on the substrate; and over time these particulates were slowly replaced by the prevalence of smaller crystallites (ca. 10 nm) covering the substrate at a high density. Elevated temperature was found to expedite the growth process to obtain the full range of surface morphologies with reasonable processing times. Seed crystals formed by the codeposition method were stable and nucleated growth throughout a subsequent layer-by-layer deposition process. These seed crystals templated the final film structure and tailor the features in lateral and vertical directions. Using codeposition and seeded growth, different surface morphologies with controllable nanoscale dimensions can be designed and fabricated for integration of MOF systems directly into device architectures and sensor platforms.