Design of the Hybrid Metal-Organic Frameworks as Potential Supramolecular Piezo-/Ferroelectrics

Metal-organic framework (MOF) UiO-66 nanocrystals were previously believed to be piezo-/ferroelectrically inactive because of their centrosymmetric lattice symmetries [Fm (3) over barm (225)] revealed by Powder X-ray diffraction. However, via the nanoscale probing studies through the delicate dual AC resonance tracking piezoresponse force microscopy and piezoresponse force spectroscopy characterizations, our work for the first time demonstrates that UiO-66 nanocrystals show certain piezo-/ferroelectric responses. The compelling experimental and theoretical analyses disclose that the structure of UiO-66 may not be the highly centrosymmetric Fm (3) over barm (225) but a reduced symmetry form. In addition, UiO-66(Hf)-type MOF nanocrystals possess stronger piezoresponses and better ferroelectric switching behaviors than their counterparts UiO-66(Zr)-type nanocrystals. Our study can further enrich the structural understanding of UiO-66 MOF and also suggests possible design of promising electronic properties of the MOFs by judicious selection of metal ions and functional ligands.