*3.3. Influence of Hydrothermal Time*

The effect of hydrothermal time on the formation of BaTiO3 nanocrystals (Figure 5) are investigated under the conditions: *R*Ba/Ti = 2.0, *T* = 210 ◦C, [NaOH] = 2.0 mol L<sup>−</sup>1, and *t* = 2–16 h. Figure 5a,b shows their XRD patterns. As Figure 5a shows, the XRD peaks of all the samples can be assignable to the cubic/tetragonal BaTiO3 phase with no other identifiable impurity peaks. The partially enlarged XRD patterns in Figure 5b shows the details that the XRD peaks at around 45◦ become wider and wider as the hydrothermal time increases from 2 h to 16 h, indicating that the BaTiO3 sample obtained with a longer hydrothermal time has more tetragonal BaTiO3 species.

**Figure 5.** XRD patterns (**a**,**b**), particle sizes and yields (**c**,**d**), and SEM images (**e**–**i**) of the BaTiO3 nanocrystals obtained with *<sup>R</sup>*Ba/Ti <sup>=</sup> 2.0([NaOH]=2.0 mol L<sup>−</sup>1, pH <sup>≈</sup> 13.6) by hydrothermally treating at 210 ◦C for various times (*t* = 2–16 h).

Figure 5c shows the BaTiO3 sample gradually changes from small nanoparticles (~70 nm) to large ones (~100 nm) as the hydrothermal time is prolonged from 2 h to 16 h. Figure 5d shows the yield plot of the BaTiO3 nanocrystals versus hydrothermal time. With a short hydrothermal time of 2 h, the BaTiO3 yield is about 92% because of the incomplete reaction. When the hydrothermal time increases to 4–16 h, the yields of the BaTiO3 samples is close to 98%.

Figure 5e–h shows the SEM images of the BaTiO3 samples obtained with various hydrothermal times (*R*Ba/Ti = 2.0, *T* = 210 ◦C, [NaOH] = 2.0 mol L−1). The BaTiO3 samples obtained with short hydrothermal times of 2–8 h, as shown in Figure 5e–g, exhibit a spherical shape; when the hydrothermal time increases to 12–16 h, as Figure 5h,i shows, the as-obtained BaTiO3 samples take on a planar polyhedral morphology. It is interesting that the particle sizes of the BaTiO3 samples are close to 100 nm and not changed obviously with the prolonging of hydrothermal time to 16 h. In addition, as Figure 5i shows, the BaTiO3 nanoparticles obtained by hydrothermal treating at 210 ◦C for 16 h are uniform in particle size and well dispersed.

Figure 6 shows the FT-IR spectra of the BaTiO3 samples synthesized with different hydrothermal times (*R*Ba/Ti = 2.0, *T* = 210 ◦C, [NaOH] = 2.0 mol L−1). The bands at 3431 and 1568 cm−<sup>1</sup> can be attributed to the stretching mode of the adsorbed water molecules and O–H groups, indicating that the surfaces of the BaTiO3 nanocrystals contain some adsorbed water and –OH groups. The weak band at 1400 cm−<sup>1</sup> can be attributed to the stretching mode of the C–O groups because of the incorporation of CO2 into the basic solution. The broad and strong absorption bands at 562 cm−<sup>1</sup> is attributed to the normal vibration of Ti–OI stretching, and the weaker and sharper absorption bands near 438 cm−<sup>1</sup> can be attributed to the normal vibration of Ti–OII bending. When the hydrothermal time is extended from 2 h to 16 h, the bands at 562 and 438 cm−<sup>1</sup> become stronger and sharper, indicating that the BaTiO3 nanocrystals with a high degree of crystallinity are formed. According to the XRD patterns (Figure 5a,b), SEM images (Figure 5e–i) and FT-IR spectra (Figure 6), the BaTiO3 nanocrystals obtained by hydrothermal treating at 210 ◦C for more than 8 h are of uniform spherical morphologies with a size range of 95–100 nm and high degree of crystallinity. Therefore, the optimum hydrothermal parameters for the synthesis of BaTiO3 nanocrystals can be *R*Ba/Ti ≥ 2, *T* ≥ 200 ◦C, *t* ≥ 8 h. The as-obtained BaTiO3 nanocrystals are of a mixture of cubic and tetragonal phases and exhibit a uniform spherical particulate morphology with a size range of 90–100 nm. The as-obtained spherical BaTiO3 nanocrystals show a high performance in ceramic capacitor for energy-storage applications.

**Figure 6.** Typical FT-IR spectra of the BaTiO3 nanocrystals obtained by hydrothermally treating at 210 ◦C for various times (2–16 h) with *R*Ba/Ti = 2.0 and [NaOH] = 2.0 mol L<sup>−</sup>1.
