Unravelling the origin of piezo/ferro-electric properties of metal-organic frameworks (MOFs) nanocrystals

TL;DR

This study reveals that UiO-66 MOF nanocrystals exhibit piezo/ferroelectric properties contrary to previous beliefs, due to a lower symmetry structure, with implications for tuning electronic properties via metal ion selection.

Contribution

First nanoscale demonstration of piezo/ferroelectric response in UiO-66 nanocrystals, challenging prior assumptions about their centrosymmetric structure.

Findings

UiO-66 nanocrystals show piezo/ferroelectric response

UiO-66 has a lower symmetry structure than previously thought

Hf-based MOFs exhibit stronger piezoresponse and ferroelectric switching

Abstract

Metal-organic framework (MOF) UiO-66 nanocrystals were previously believed to be piezo/ferro-electrically inactive because of their centrosymmetric lattice symmetries (Fm-3m (225)) revealed by Powder X-ray diffraction. However, via delicate dual AC resonance tracking piezoresponse force microscopy and piezoresponse force spectroscopy characterizations, our nanoscale probing for the first time demonstrate that UiO-66 nanocrystals show piezo/ferro-electric response. Our compelling experimental and theoretically analyses disclose that the structure of UiO-66 should not be the highly centrosymmetric Fm-3m (225) but a reduced symmetry form instead. UiO-66(Hf)-type MOFs possess stronger piezoresponse and better ferroelectric switching behaviours than their counterparts UiO-66 (Zr)-type MOFs. Our study not only enriches the structural understanding of UiO-66 MOF, but also suggests possible modification of electronic property of the MOFs by judicious selection of metal ions and functional ligands.