*3.2. Influence of Hydrothermal Temperature*

The effect of hydrothermal temperature on the synthesis of BaTiO3 nanoparticles was investigated by changing the hydrothermal temperature from 150 to 210 ◦C under the conditions: *R*Ba/Ti = 2.0, *t* = 8 h and [NaOH] = 2.0 mol L<sup>−</sup>1, and Figure 4 shows their characterization results of XRD and SEM.

**Figure 4.** XRD patterns (**a**,**b**), particle sizes and yields (**c**,**d**), and SEM images (**e**–**h**) of the BaTiO3 nanocrystals obtained with *R*Ba/Ti = 2.0 under hydrothermal conditions at 150–210 ◦C for 8 h ([NaOH] <sup>=</sup> 2.0 mol L<sup>−</sup>1, pH <sup>≈</sup> 13.6).

Figure 4a,b shows the typical XRD patterns of the BaTiO3 samples obtained at different hydrothermal temperatures. From Figure 4a, one can see that all the BaTiO3 samples show similar XRD patterns, all peaks of which can be attributed to the cubic BaTiO3 phase (JCPDS card no. 31-0174), and no impure XRD peaks are found, indicating that the formation of pure BaTiO3 crystals. The partially enlarged XRD patterns located in 2θ = 44–46◦ (Figure 4b) show the XRD peaks become wider and wider with the increase of hydrothermal temperature from 150 to 210 ◦C, and can be sub-divided to two peaks at 44.9◦ and 45.3◦, attributable to the (200) and (002) reflections of the tetragonal BaTiO3 phase.

Figure 4c shows the particle-size distribution plot versus hydrothermal temperature (*T*). When *T* = 150 ◦C, the particle sizes of the as-obtained BaTiO3 nanocrystals are 85 ± 15 nm. When *T* = 165 ◦C, the particle size of the as-obtained BaTiO3 is about 74 ± 13 nm, seeming to become smaller, but their uniformity is low. When the temperature increases to 180 ◦C, the particle size of the as-obtained BaTiO3 is 88 ± 10 nm, and the morphology of the BaTiO3 particles becomes relatively uniform. When *T* = 210 ◦C, the particle size of the as-obtained the BaTiO3 sample is 91 ± 14 nm, just a slight increase. As Figure 4c shows, the particle sizes of the BaTiO3 samples obtained at various hydrothermal temperatures are kept almost constant at about 80–90 nm.

Figure 4d shows the plot of the yield of the BaTiO3 sample versus the hydrothermal temperature. One can see that during the hydrothermal temperature of 150–180 ◦C, the yield is close to 100%; when the hydrothermal temperature is 210 ◦C, the yield slightly decreases because of the complete dehydration reaction in the elevated temperature.

Figure 4e–h shows the typical SEM images of the BaTiO3 samples synthesized under various hydrothermal temperatures for 8 h ([NaOH] = 2.0 mol L−1, *R*Ba/Ti = 2.0): (e) 150 ◦C, (f) 165 ◦C, (g) 180 ◦C, and (h) 210 ◦C. One can see that all the BaTiO3 samples consist of spherical nanoparticles. With

the increase of hydrothermal temperature, the as-obtained BaTiO3 samples exhibit a higher degree of crystallinity indicated by the clean-cut crystal planes.

According to the XRD patterns (Figure 4a,b) and SEM images (Figure 4e–h), we find that a higher hydrothermal temperature is helpful to form tetragonal BaTiO3 nanocrystals with more uniform spherical morphology. For safety's sake, the hydrothermal temperature is chosen as 210 ◦C for the synthesis of BaTiO3 nanocrystals in the following investigation. Cautions: the working temperature limit of a PTFE hydrothermal reactor is usually about 220 ◦C, and a too high temperature will cause explosion.
