Thin films of metal–organic frameworks

The fabrication of thin film coatings of metal-organic frameworks (MOFs) on various substrates is discussed in this critical review. Interestingly, the relatively few studies on MOF films that have appeared in the literature are limited to the following cases: [Zn4O(bdc)3] (MOF-5; bdc=1,4-benzenedicarboxylate), [Cu3(btc)2] (HKUST-1; btc=1,3,5-benzenetricarboxylate), [Zn2(bdc)2(dabco)] (dabco=1,4-diazabicyclo[2.2.2]octane), [Mn(HCOO)], [Cu2(pzdc)2(pyz)] (CPL-1; pzdc=pyrazine-2,3-dicarboxylate, pyz=pyrazine), [Fe(OH)(bdc)] (MIL-53(Fe)) and [Fe3O(bdc)3(Ac)] (MIL-88B; Ac=CH3COO-). Various substrates and support materials have been used, including silica, porous alumina, graphite and organic surfaces, i.e. self-assembled monolayers (SAMs) on gold, as well as silica surfaces. Most of the MOF films were grown by immersion of the selected substrates into specifically pre-treated solvothermal mother liquors of the particular MOF material. This results in more or less densely packed films of intergrown primary crystallites of sizes ranging up to several microm, leading to corresponding film thicknesses. Alternatively, almost atomically flat and very homogenous films, with thicknesses of up to ca. 100 nm, were grown in a novel stepwise layer-by-layer method. The individual growth steps are separated by removing unreacted components via rinsing the substrate with the solvent. The layer-by-layer method offers the possibility to study the kinetics of film formation in more detail using surface plasmon resonance. In some cases, particularly on SAM-modified substrates, a highly oriented growth was observed, and in the case of the MIL-53/MIL-88B system, a phase selective deposition of MIL-88B, rather than MIL-53(Fe), was reported. The growth of MOF thin films is important for smart membranes, catalytic coatings, chemical sensors and related nanodevices (63 references).