Structural engineering through modulator incorporation in UiO-66-NH₂ metal-organic frameworks for piezoresponse regulation

Metal-organic frameworks (MOFs) are generally considered non-piezoelectric due to their highly symmetric structures. Some sub-species such as the UiO-66 series are found to have a piezoelectric response, and display various degrees of piezoresponse depending on the ligands. Novel methods to regulate the piezoresponse of piezoelectric MOFs are desired to expand their potential applications. Here, we demonstrate the successful piezoresponse modulation of UiO-66-NH₂ (Hf, Zr) by adjusting their structure through a simple acetic acid (AA) modulator modification, achieving an ascending piezoresponse trend as its amount increases. During the synthesis, single carboxylates were incorporated into the created missing-linker defects, which significantly altered the structure. The modulator incorporation breaks the original high symmetry of the structure, leading to an improvement of piezoelectricity. This work opens a new way to regulate the piezoresponse of existing MOFs through structural modulation for further research on piezoelectric MOFs.