*2.2. Growth of Spherical BaTiO3 Nanoparticles*

BaTiO3 samples were synthesized via a hydrothermal process using TiO2 (P25) nanoparticles as the Ti source and seeds. The synthetic process of the BaTiO3 nanocrystals is shown in Figure 1. Teflon-lined autoclaves with a volume of 100 mL were used as the reaction vessel. Typically, 6.0 g of NaOH and 1.5 g of TiO2 nanoparticles were first added into 75 mL of distilled water under magnet stirring; then a given amount of Ba(NO3)2 was added to the above suspension containing TiO2 nanoparticles and NaOH under magnetic stirring. In the final suspensions, the molar ratios of Ba(NO3)2 to TiO2 (*R*Ba/Ti) were kept at 1.6–2.0, and the molar concentration of NaOH was about 2 mol L<sup>−</sup>1. The pH values of the as-obtained suspensions before hydrothermal treatment were about 13.6. The prepared suspensions were then transferred into the Teflon-lined steel autoclaves. After carefully sealing, the autoclaves were heated in an oven at 150–210 ◦C for 2–16 h. After the hydrothermal reaction, the autoclaves were cooled naturally, and the solid samples were collected using a centrifugal machine (5000 rpm, 5 min), followed by washing with water for more than three times and drying at 120 ◦C for 24 h. The as-obtained BaTiO3 solids were ground into powders using an agate mortar. These white powders, i.e., BaTiO3 nanocrystals, were collected and used for characterization. The detailed processing parameters for the synthesis of BaTiO3 nanocrystals are listed in Table 1. It was assumed that TiO2 added was completely converted into BaTiO3, and the theoretical mass could be calculated. The yield of BaTiO3 was the ratio of the actual mass of the BaTiO3 sample to their corresponding theoretical mass.

**Figure 1.** Schematic of the hydrothermal synthesis of BaTiO3 nanocrystals using TiO2 (P25) nanoparticles as the seeds and Ti source.


**Table 1.** A summary of experimental conditions for hydrothermal synthesis of BaTiO3 nanoparticles.
