Abstract
AbstractThe landscape of possible polymorphs for some metal–organic frameworks (MOFs) can pose a challenge for controlling the outcome of their syntheses. Demonstrated here is the use of a template to control in the vapor‐assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. Introducing a small amount of either ethanol or dimethylformamide vapor during the reaction between ZnO and 4,5‐dichloroimidazole vapor results in the formation of the porous ZIF‐71 phase, whereas other conditions lead to the formation of the dense ZIF‐72 phase or amorphous materials. Time‐resolved in situ small‐angle X‐ray scattering reveals that the porous phase is metastable and can be transformed into its dense polymorph. This transformation is avoided through the introduction of template vapor. The porosity of the resulting ZIF powders and films was studied by N2 and Kr physisorption, as well as positron annihilation lifetime spectroscopy. The templating principle was demonstrated for other members of the ZIF family as well, including the ZIF‐7 series, ZIF‐8_Cl, and ZIF‐8_Br.
Highlights
We previously reported chemical vapor deposition (CVD) of metal–organic frameworks (MOFs), which enables the deposition of smooth and defect-free thin films on a fullwafer scale.[9, 10]
This MOF-CVD method relies on the conversion of a precursor layer (e.g., ZnO, CoO, CuO) by vapors of the organic linker
The MOF-CVD method has been extended to a number of other MOFs (e.g., zeolitic imidazolate framework (ZIF)-67, MAF-6, CuBDC).[13, 16,17,18]
Summary
When DMF or EtOH vapors are present instead, ZIF-71 formed at 120 8C, and the corresponding diffraction peaks grew continuously in intensity, while the formation of ZIF-72 was postponed significantly (> 40 min) (Figure 2 c–f) These results suggest that all three vapors facilitate the oxide-to-ZIF conversion, as evidenced by the reaction proceeding at lower temperatures. ZIF-8_Cl and ZIF-8_Br are isostructural to ZIF-8, and form through the coordination of ZnII to 2-chloroimidazole and 2-bromoimidazole, respectively.[44] The presence of DMF vapor during the conversion of ZnO at 110 8C induced the formation of the desired MOF phase, while no crystallization took place below For these linkers, temperatures above 130 8C could not be tested because of thermal decomposition. These findings will be of considerable value in reinforcing the current understanding of the MOF-CVD synthetic approach
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