BaTiO3 nanoparticles (average parti

BaTiO3 nanoparticles (average particle size: 100 nm, BET surface area = 10.07 m2g-1) synthesized by typical oxalate method were used in this study (obtained from Kyocera, Kagoshima, Japan). The as-received BaTiO3 powder was first calcined at 700 °C for 1 h to completely remove organic residues. The calcined BaTiO3 powder (4.0 g) was mixed with a 1M acetic acid aqueous solution (20 mL) at 80 °C for 1 h for further BaTiO3 surface preparation, which facilitates the densification22 probably due to removal of the BaCO3 impurity26 and/or the formation of a nanometer amorphous layer,27 which could be dissolved more easily than the crystalline surface. After the treatment, the powder was collected by filtration (EZFlow PES membrane, 0.22 μm), washed with deionized water, and dried at 120 °C for 24 h.