Studies on atomic layer deposition of MOF-5 thin films

• Continuous MOF-5 thin films were deposited using atomic layer deposition (ALD) and a two-step crystallization procedure.
• For the first time MOF films were synthesized using a gas phase method.
• The advantages of ALD such as excellent conformality can be applied to MOF deposition.

Deposition of MOF-5 thin films from vapor phase by atomic layer deposition (ALD) was studied at 225–350 °C. Zinc acetate (ZnAc2) and 1,4-benzenedicarboxylic acid (1,4-BDC) were used as the precursors. The resulting films were characterized by UV–Vis spectrophotometry, Fourier transform infrared spectroscopy (FTIR), optical microscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), time-of-flight elastic recoil detection analysis (TOF-ERDA), isopropanol adsorption tests, and nanoindentation. It was found out that the as-deposited films were amorphous but crystallized in humid conditions at room temperature. The crystalline films had an unidentified structure with a large unit cell similar to MOF-5. High temperature XRD (HTXRD) of the films showed structural changes at 200 and 300 °C. A complete decomposition occurred at 400 °C. Adsorption tests showed no porosity in the films crystallized in the moist air. These films were recrystallized into the MOF-5 phase in an autoclave with dimethylformamide (DMF) at 150 °C which was confirmed by XRD. Two main uptake regions were seen in the isopropanol adsorption tests corresponding to micro- and macroporosity.

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