Topochemical Synthesis of High-Aspect-Ratio Lead-Free (K, Na)NbO3 Plate-Like Structures

Single crystalline lead-free (K, Na)NbO3 (KNN) nanostructures have attracted much attention for the high piezoelectricity and environmental compatibility. Especially, the plate-like KNN structures are supposed to be utilized in the synthesis of textured KNN-based ceramics. The KNN plate-like templates were prepared by a two-step molten salt reaction. The layered-perovskite Bi2.5Na3.5Nb5O18 (BNN5) was first prepared by using Bi2O3, Nb2O5, Na2CO3, NaCl as raw materials. The BNN5 precursors possess high aspect ratio with an average size of 15 - 20 μm in width and 0.5 - 1 μm in thickness. Then Bi element in the BNN5 was replaced by Na/K through topochemical reaction, and KNN plate-like products which have anisotropic crystal structure were fabricated. It should be emphasized that piezoresponse force microscopy (PFM) was used to confirm three-dimensional (3-D) morphology, while piezoelectric properties of single crystalline KNN platelets were acquired simultaneously. The as-synthesized KNN structures are expected to play an important role in the study of textured KNN ceramics. Meanwhile, the methodology investigated in this paper can also be applied for further fundamental studies on KNN system as well as other lead-free piezoelectric materials.