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Synthesis of immobilized metal-organic frameworks for integrated applications

Publication date: 2016-05


Stassen, Ivo


Metal-organic frameworks (MOFs) are crystalline nanoporous materials that consist of inorganic metal-containing nodes connected by polytopic organic linkers. MOFs are promising materials for novel applications in various industries because of their high specific surface area, synthetic flexibility and tunable structure-property relationships. For many application areas, such as catalysis, molecular separation and gas storage, MOFs can be utilized as bulk materials. For fabrication of solid-state integrated MOF devices, by contrast, immobilization of MOFs directly on substrates is required, i.e. material synthesis by thin film deposition. Examples of promising applications of this type are chemical sensors, microelectronic chips, supercapacitors or other (opto)electronics. Research on the latter applications is recently emerging and gives rise to a growing need for suitable MOF integration routes. In this Ph. D. research project, novel integration routes for MOFs are explored in order to increase the industrial feasibility of MOF integration into applications such as solid-state devices. The main text of the manuscript is a compilation of five peer reviewed papers and their supporting information in the as-published form. The articles are complemented by an introduction and a general conclusion providing background information on the position of the project in the overarching research fields. The first part of the manuscript focuses on integration of zirconium (IV) carboxylate MOFs. These materials are interesting for applications because of their robustness and flexible chemistry. However, their deposition as films on substrates is challenging due to difficult-to-control crystallization kinetics. Electrochemical deposition is demonstrated as a novel route for their integration, permitting stimulated deposition on conductive surfaces via two distinct mechanisms. Moreover, a synthesis modulation approach can be utilized to regulate the morphology and adhesion of the films. As a proof-of-concept of the beneficial integration of these materials, application in a sorbent trap for analytical sampling of volatile organics is demonstrated. In a subsequent phase, the extraordinary potential of zirconium (IV) carboxylate covered electrodes for field effect gas sensing devices is evidenced by Kelvin probe sensing experiments. Parts-per-billion trace levels of volatile phosphonates are reproducibly detected on a background of dry or humid air, owing to strong physisorption of these analytes at lattice defects in the confined cages of the MOF. Furthermore, this study demonstrates that there is general potential in judicious design of MOFs for high-sensitivity detection of specific analytes. The second part of the manuscript discusses the transformation of deposited metal oxides into MOF films. Metal oxides are promising sacrificial precursor materials for MOF integration, owing to their controlled deposition on substrates by well-established techniques. Whereas transformation of metal oxides into MOFs can be conducted using solution methods, eliminating the role of the solvent during film deposition is interesting for numerous reasons (e.g. sustainability, industrial compatibility, avoidance of dissolution issues). Therefore, solvent-free routes are explored for the conversion of zinc oxide to zeolitic imidazolate framework (ZIF) films. In a first approach, zinc oxide films are reacted with a melted imidazole-based linker for formation of ZIF films. Precise replication of patterns and mesoscopic architectures is achieved, demonstrating spatial localization of the crystallization process by the solid zinc ion source. In a second approach, deposition of nanoscale metal-organic framework films is achieved through heterogeneous vapor-solid reaction between vaporized organic linkers and ultrathin sacrificial metal oxide films. The solid-vapor reaction method, MOF-CVD, is the first demonstrated vapor-phase deposition method for crystalline nanoporous network solids. Moreover, conformal deposition on high-aspect-ratio features and microscopic patterning by additive photolithography is accomplished based on the unique properties of this method. MOF-CVD shows potential for extension to various different MOF classes and will become an important new direction in research on fabrication of MOFs for integrated applications.