Highly Porous Nanocrystalline UiO-66 Thin Films via Coordination Modulation Controlled Step-by-Step Liquid-Phase Growth

Metal-organic frameworks (MOFs) possess exciting properties, which can be tailored by rational material design approaches. Integration of MOFs in functional nano- and mesoscale systems require selective crystallite positioning and thin-film growth techniques. Stepwise layer-by-layer liquid-phase epitaxy (LPE) emerged as one of the methods of choice to fabricate MOF@substrate systems. The layer-by-layer approach of LPE allows a precise control over the film thickness and crystallite orientation. However, these advantages were mostly observed in cases of tetra-connected dinuclear paddle-wheel MOFs and Hoffmann-type MOFs. Higher connected MOFs (consisting of nodes with 8-12 binding sites), such as the Zr-oxo cluster based families, are notoriously hard to deposit in an acceptable quality by the stepwise liquid-phase process. Herein, we report the use of coordination modulation (CM) to assist and enhance the LPE growth of UiO-66, Zr ₆ O ₄ (OH) ₄ (bdc) ₆ (bdc ^2- = 1,4-benzene-dicarboxylate) films. Highly porous and crystalline thin films were obtained with good control of the crystallite domain size and film thickness in the nanoscale regime. The crystallinity (by grazing incidence X-ray diffraction), morphology (by scanning electron microscopy, atomic form microscopy), elemental composition (by X-ray photoelectron spectroscopy), binding properties (by infrared spectroscopy), and adsorption capacity (by quartz crystal microbalance adsorption experiments) for volatile organic compounds (e.g. CH ₃ OH) of the fabricated thin films were investigated. These results substantiate a proof-of-concept of CM-LPE of MOFs and could be the gateway to facilitate in general the deposition of chemically very robust and higher-connected MOF thin films with automatic process-controlled LPE techniques under mild synthetic conditions.