Grain size engineering and growth mechanism in hydrothermal synthesis of Bi_0.5Na_0.5TiO₃ thin films on Nb-doped SrTiO₃ substrates

Considering the full utilization of energy and pursuing thin-film capacitors with high energy-storage density, the grain size engineering is used to adjust domain size, in order to enhance the energy-storage efficiency and energy-storage density of thin-film capacitors. Therefore, in this work, lead-free Bi_0.5Na_0.5TiO₃ (BNT) films with designed grain size were grown on Nb-doped SrTiO₃ (Nb:STO) (001) single-crystalline substrates by modulating the mineralizer concentrations via hydrothermal synthesis. The nature of epitaxial growth near the single-crystalline substrates was proved by transmission electron microscopy (TEM). In addition, the phenomenon of the decrease of grain size and the increase of [100] in-plane orientation with the decline of mineralizer concentrations were clarified by grazing-angle incidence X-ray diffraction (GIXRD) and field emission scanning electron microscope (FESEM). By reducing the grain size, an ultrahigh energy-storage efficiency (η) of 75.56% and superior the recoverable energy-storage density (W_rec) of 16.47 J/cm³ were acquired in pure Bi_0.5Na_0.5TiO₃ films. Furthermore, the fine-grained film exhibits weak dependence on the frequency and has excellent anti-fatigue property. Therefore, hydrothermal synthesis is an efficient, inexpensive, and easy method, which is the most promising way to adjust grain size and energy storage of the film capacitor. [Figure not available: see fulltext.]