*3.1. Influence of Molar Ba*/*Ti Ratio*

In order to verify the effect of the molar Ba/Ti ratio on the formation of BaTiO3 nanoparticles, we synthesized a series of samples with various *R*Ba/Ti values from 1.6 to 2.5, and the other hydrothermal conditions were kept as the same: sodium hydroxide concentration [NaOH] = 2.0 mol L−<sup>1</sup> (pH = 13.6), *T* = 200 ◦C, and *t* = 8 h. The typical results of these samples are shown in Figure 3.

**Figure 3.** X-ray diffraction (XRD) patterns (**a**,**b**), particle sizes and yields (**c**,**d**), and scanning electron microscope (SEM) images (**e**–**h**) of the BaTiO3 samples obtained with various molar Ba/Ti ratios (*R*Ba/Ti <sup>=</sup> 1.6–2.5) under hydrothermal conditions at 200 ◦C for 8 h ([NaOH] <sup>=</sup> 2.0 mol L<sup>−</sup>1, pH <sup>≈</sup> 13.6).

Figure 3a,b shows the XRD patterns of the BaTiO3 samples synthesized at different molar Ba/Ti ratios. From Figure 3a, one can find that all the samples show seven distinct peaks at around 21.98, 31.36, 38.64, 44.92, 50.58, 55.86 and 65.44◦, corresponding to the (100), (110), (111), (200), (210), (211) and (220) reflections of the cubic BaTiO3 phase, respectively, according to the JCPDS card no. 31-0174 [40]. No peaks belonging to other identifiable impurities can be found in all the samples obtained, indicating the as-obtained BaTiO3 samples are pure. As Figure 3b shows, the peak at about 45◦ can be divided into two diffraction sub-peaks at 44.9 and 45.3◦, attributable to the (200) and (002) reflections of the tetragonal BaTiO3 species, respectively [41]. With the increase of the *R*Ba/Ti value from 1.6 to 2.5, the peaks near 45◦ become wider and wider, suggesting that a higher *R*Ba/Ti value is favorable in forming a tetragonal BaTiO3 phase.

Figure 3c shows the plots of particle size dependent on the *R*Ba/Ti values. When *R*Ba/Ti = 1.6–1.8, the particle sizes are 90–100 nm (97 ± 15 nm for *R*Ba/Ti = 1.6 and 93 ± 24 nm for *R*Ba/Ti = 1.8), but the uniform degree is not high. Figure 3d shows the yields of BaTiO3 samples synthesized with various *R*Ba/Ti values after hydrothermally treating at 200 ◦C for 8 h ([NaOH] = 2.0 mol L<sup>−</sup>1). One can see that the yields of all the samples are close to 100%, indicating the complete conversion of TiO2 to BaTiO3 nanocrystals. The formation of a small amount of crystal water may make the BaTiO3 yield a little larger than 100% according to the TiO2 amount [42].

Figure 3e–h shows the typical SEM images of the BaTiO3 samples obtained with various *R*Ba/Ti values ([NaOH] = 2.0 mol L−1, *T* = 200 ◦C, *t* = 8 h). According to the SEM observations, when *R*Ba/Ti = 2.0 (Figure 3g), the particle size of the BaTiO3 sample is 91 ± 22 nm, and it shows a more uniform solid spherical particle morphology. When *R*Ba/Ti = 2.5 (Figure 3h), the particle size of the BaTiO3 sample is 98 ± 26 nm, and one can see that it shows obviously clean-cut crystal faces for the BaTiO3 particles, suggesting a higher degree of crystallinity and favorable formation of the tetragonal BaTiO3 phase.

Taking the results of XRD and particle-size distribution into account, we can tentatively conclude that a higher Ba/Ti ratio is more favorable in forming tetragonal BaTiO3 nanocrystals with a more uniform size.
