2.1.1. Synthesis of NaNbO3

The NaNbO3 powder was prepared by the hydrothermal method. Briefly, 1 g Nb2O5 was added into 30 mL sodium hydroxide (NaOH, 10 M) aqueous solution and stirred for 2 h. Then, transferred above solution into a 50 mL Teflon-lined autoclave and reacted at 180 ◦C for 48 h. The precipitate was washed thoroughly and dried at 80 ◦C.
