Proton\nConducting Self-Assembled Metal–Organic Framework/Polyelectrolyte\nHollow Hybrid Nanostructures

Herein,\na room temperature chemical process to synthesize functional, hollow\nnanostructures from zeolitic imidazolate framework-8 (ZIF-8) and poly­(vinylphosphonic\nacid) (PVPA) is reported. Syntheses are initiated by physically blending\nthe componentsa process that is accompanied first by encapsulation\nof ZIF-8 crystallites by PVPA and then by fragmentation of the crystallites.\nThe fragmentation process is driven by partial displacement of the\nmethyl-imidazolate ligands of Zn­(II) in ZIF-8 by phosphonate groups\non PVPA. Differences in rates of diffusion for the components of the\nreactive mixture yield a Kirkendall-like effect that is expressed\nas a hollow-particle morphology. The obtained hollow nanostructures\nfeature hybrid shells containing PVPA, ZIF-8, and their cross-reacted\nproducts. The hybrid structures display substantial proton conductivities\nthat increase with increasing temperature, even under the anhydrous\nconditions prevailing at temperatures above the boiling point of water.\nFor example, at <i>T</i> = 413 K the proton conductivity\nof ZIF-8@PVPA reaches 3.2 (±0.12) × 10^–3 S cm^–1, a value comparatively higher than that\nfor PVPA (or ZIF-8) in isolation. The high value may reflect the availability\nin the hybrid structures of free (and partially free), amphoteric\nimidazole species, and their hydrogen-bonding interactions with phosphonate\nand/or phosphonic acid units. The persistence of ample conductivity\nat high temperature reflects the elimination of phosphonic acid group\ndehydration and dimerizationan effect that strikingly degrades\nthe conductivity of pure PVPA under anhydrous conditions.