Liquid phase heteroepitaxial growth of moisture-tolerant MOF-5 isotype thin films and assessment of the sorption properties by quartz crystal microbalance

The moisture-tolerant metal-organic frameworks (MOFs) of formula [Zn ₄O(L)₃]_n (L = di-substituted carboxypyrazolate derivatives) are fabricated as thin films on self-assembled monolayer (SAM) functionalized gold substrates by employing the step-by-step liquid phase epitaxial (LPE) deposition method in a continuous operation mode. The in situ monitoring of the deposition by quartz crystal microbalance (QCM) and grazing incidence X-ray diffraction reveal different growth regimes and crystallinities of the obtained thin films in dependence of the chosen alkyl side chain functionality at the carboxypyrazolate linkers, L. To overcome the relatively poor crystallinity and low porosity of a particular homostructured metal-organic framework type B film, the step-by-step heteroepitaxial growth of this MOF B on top of the crystallite surfaces of a well-grown and lattice-matched MOF type A is applied. This approach enables the fabrication of oriented, core-shell-like MOF B@A surface mounted heterocrystals as an intergrown homogeneous coating for the selective adsorption of volatile organic compounds. The accessible pore volumes of the individual components and the heterostructured films are characterized by performing adsorption measurements of different organic probe molecules using an environmentally controlled QCM instrument. The results show good adsorption capacity, excellent size exclusion selectivity for alcohols, and a high degree of moisture-tolerance of the heteroepitaxial MOF films. The step-by-step heteroepitaxial growth method allows the fabrication of high-performance metal-organic framework (MOF)-on-MOF films. The MOF with sophisticated substituents that never achieves high crystallinity as a homostructured film can be epitaxially grown on another lattice-matched crystalline MOF. The hybrid MOF film shows the excellent size selectivity of alcohol adsorption by the pore opening window of the MOF shell component.