Proton Conducting Self-Assembled Metal–Organic Framework/Polyelectrolyte Hollow Hybrid Nanostructures
journal contributionposted on 19.08.2016, 00:00 by Unal Sen, Mustafa Erkartal, Chung-Wei Kung, Vijay Ramani, Joseph T. Hupp, Omar K. Farha
Herein, a room temperature chemical process to synthesize functional, hollow nanostructures from zeolitic imidazolate framework-8 (ZIF-8) and poly(vinylphosphonic acid) (PVPA) is reported. Syntheses are initiated by physically blending the componentsa process that is accompanied first by encapsulation of ZIF-8 crystallites by PVPA and then by fragmentation of the crystallites. The fragmentation process is driven by partial displacement of the methyl-imidazolate ligands of Zn(II) in ZIF-8 by phosphonate groups on PVPA. Differences in rates of diffusion for the components of the reactive mixture yield a Kirkendall-like effect that is expressed as a hollow-particle morphology. The obtained hollow nanostructures feature hybrid shells containing PVPA, ZIF-8, and their cross-reacted products. The hybrid structures display substantial proton conductivities that increase with increasing temperature, even under the anhydrous conditions prevailing at temperatures above the boiling point of water. For example, at T = 413 K the proton conductivity of ZIF-8@PVPA reaches 3.2 (±0.12) × 10–3 S cm–1, a value comparatively higher than that for PVPA (or ZIF-8) in isolation. The high value may reflect the availability in the hybrid structures of free (and partially free), amphoteric imidazole species, and their hydrogen-bonding interactions with phosphonate and/or phosphonic acid units. The persistence of ample conductivity at high temperature reflects the elimination of phosphonic acid group dehydration and dimerizationan effect that strikingly degrades the conductivity of pure PVPA under anhydrous conditions.
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phosphonic acid group dehydration413 KZIF -8 crystallitesZIF -8Kirkendall-like effectmethyl-imidazolate ligandsroom temperature chemical processstructures displayreactive mixturenanostructures featurefragmentation processzeolitic imidazolate framework -8phosphonate groupsproton conductivitiescross-reacted productshydrogen-bonding interactionsPVPAproton conductivityhollow-particle morphology