Freeze-drying assisted liquid exfoliation of BiFeO3 for pressure sensing

Breaking up bulk crystals of functional materials into nanoscale thinner layers can lead to interesting properties and enhanced functionalities due to the size and interface effects. However, unlike the van der Waals layered crystals, many materials cannot be exfoliated into thin layers by liquid exfoliation. BiFeO₃ is a piezoelectric ceramic material, which is commonly synthesized as bulk crystals, limiting its wider applications. In this contribution, a freeze-drying assisted liquid exfoliation method was adopted to fabricate thin-layered BiFeO₃ nanoplates with lateral sizes of up to 500 nm and thicknesses of 10–20 nm. The freeze-drying process showed a vital role in the preparation process by imposing stress on the dispersed BiFeO₃ crystals during the liquid-to-solid-to-gas transition of the solvent. Such stress resulted in lattice strains in the freeze-dried BiFeO₃ crystals, which enabled their further exfoliation under subsequent ultrasonication. Considering the intrinsic piezoelectric effect of BiFeO₃, pressure sensors based on bulk and thin-layer BiFeO₃ were also fabricated. The pressure sensor based on BiFeO₃ nanoplates exhibited a largely enhanced sensitivity with a wider working range than the bulk counterpart, because of the stronger piezoelectric effect induced and the extra electrical charges at abundant interlayer interfaces. We suggest that the freeze-drying assisted liquid exfoliation method can be applied to other non-van der Waals crystals to bring about more functional material systems.