Continuous-flow synthesis of MIL-53(Cr) with a polar linker: probing the nanoscale piezoelectric effect

Abstract

Polar metal–organic frameworks (MOFs) have potential applications due to the fact that their organic linkers can be conveniently modified, which endows them with different properties such as non-linear optical, piezoelectric, pyroelectric, and ferroelectric performance. In the present study, an MIL-53(Cr) MOF material was prepared with a polar organic ligand including a fluorine group. In addition, a polar MOF material can be produced under continuous flow conditions with a space time yield (STY) of 7188 kg m⁻³ day⁻¹. Furthermore, a response surface methodology (RSM) was used to optimize the process conditions with the Box–Behnken design. Moreover, the piezoelectric properties of MIL-53(Cr)-F nanocrystals were probed using dual AC resonance tracking piezo-response force microscopy (PFM) and piezoresponse force spectroscopy (PFS). A PFM amplitude of 300–400 pm and a maximum amplitude of 1051 pm in a butterfly loop were observed indicating its piezoelectricity and ferroelectricity. The MIL-53(Cr)-F nanocrystals display second harmonic generation activity, indicating their polar organization. Therefore, MIL-53(Cr) with a polar linker has the potential to be used as a promising piezoelectric material, which can be produced at a large scale by continuous-flow chemistry.