As one of the lead-free piezoelectric systems, potassium sodium niobate [(K, Na)NbO₃, abbreviated as KNN] has been a research interest due to its high piezoelectric property, high Curie temperature as well as the tailored phase structures. However, there are still some problems in the study of one dimensional (1D) KNN structures. For instance, the orientation growth of crystals is difficult to be controlled, the piezoelectric properties of the product should be further enhanced. In this paper, the hydrothermal synthesis method was applied to fabricate the 1D KNN structures on the Nb-doped SrTiO₃ (NSTO) substrate. The hydrothermal temperature, Nb₂O₅ concentration and the hydrothermal time were studied, respectively, and the oriented KNN nanorod arrays (NRAs) with high quality were achieved. The results show that the KNN NRAs with single orientation, good crystallinity and high piezoelectric response can be obtained when the hydrothermal temperature is 190 ℃, the Nb₂O₅ concentration is 40 mL/g, and the reaction time is 18 h, respectively.

Two-dimensional (2D) lead-free (K, Na)NbO₃ (KNN) micro/nano structures with controllable K/Na ratio were successfully fabricated via a two-step molten salt synthesis (MSS). In this work, the reaction factors, including the proportion of molten salts, the types of carbonates, the sintering temperature, and the sintering time, were discussed in detail and the optimized condition was identified. The microstructure of KNN was confirmed by confocal Raman spectroscopy, while piezoresponse force microscopy (PFM) was applied to measure three-dimensional (3D) morphology and piezoelectric properties of KNN particles. The as-synthesized KNN platelets apparently possess anisotropic morphology and uniform structure, the size of which reaches 5–20 µm in length/width and 0.5–1 µm in thickness. It should be noted that the K/Na ratios of the KNN crystals are basically consistent while the proportion of salts changes within a certain range. The enrichment of Na element in the products is also observed, which owes to the smaller ionic radius of Na⁺ comparing to that of K⁺. This result provides a reference for the further preparation of textured ceramics and flexible piezoelectric generators.