Abstract
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, Zr6O4(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. CH3OH) 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.