TiO2-Seeded Hydrothermal Growth of Spherical BaTiO3 Nanocrystals for Capacitor Energy-Storage Application

Simple but robust growth of spherical BaTiO3 nanoparticles with uniform nanoscale sizes is of great significance for the miniaturization of BaTiO3-based electron devices. This paper reports a TiO2-seeded hydrothermal process to synthesize spherical BaTiO3 nanoparticles with a size range of 90–100 nm using TiO2 (Degussa) and Ba(NO3)2 as the starting materials under an alkaline (NaOH) condition. Under the optimum conditions ([NaOH] = 2.0 mol L−1, RBa/Ti = 2.0, T = 210 ◦C and t = 8 h), the spherical BaTiO3 nanoparticles obtained exhibit a narrow size range of 91 ± 14 nm, and the corresponding BaTiO3/polymer/Al film is of a high dielectric constant of 59, a high break strength of 102 kV mm−1, and a low dielectric loss of 0.008. The TiO2-seeded hydrothermal growth has been proved to be an efficient process to synthesize spherical BaTiO3 nanoparticles for potential capacitor energy-storage applications.

The miniaturization of electronic components and nanotechnology makes it necessary to synthesize nanometer-scale BaTiO 3 materials, including nanowires [20] and nanoparticles [21], with scientific appeal and technical urgency. Device miniaturization and high dielectric constant can be achieved by controlling their microstructures and compositions, which are strongly dependent on the phase, uniformity, surface area, and size of the BaTiO 3 materials [22][23][24]. For the applications in MLCC, BaTiO 3 powders are usually used as dielectric fillers and blended with a polymer to a fabricate composite film with a compact and flexible surface. In order to manufacture a reliable BaTiO 3 -based MLCC, high-quality BaTiO 3 powders with high purity, high crystallinity, high dispersibility, and uniform small size are the precondition. The BaTiO 3 fillers with a narrow particle-size distribution and suitable phases are in favor of obtaining a compact composite film with a lower content of pores, and the dense and homogeneous BaTiO 3 phase in polymer matrix can lead to higher dielectric properties of the composite films [25]. R.K.Goyal et al. found that the dielectric constants of the composite films filled with tetragonal BaTiO 3 powders are higher than those of the films with cubic BaTiO 3 fillers; whereas the effect of crystal phase on the dielectric losses presents an opposite trend that the composite filled with a cubic BaTiO 3 filler shows a lower dielectric loss than that of the tetragonal BaTiO 3 composite film [26]. Therefore, a high-quality BaTiO 3 filler is important for high performance composite dielectric films, and a recent investigation on the synthesis of BaTiO 3 nanocrystals via various processes has become one of the hot topics.
There have been a number of methods developed to prepare high-quality BaTiO 3 powders [27]. As mentioned above, the conventional route used to prepare BaTiO 3 powders is via a solid-state reaction between BaCO 3 and TiO 2 at a high temperature of 850-1400 • C [28]. This solid-state method is easy in operation and allows for mass production, but there are a number of serious drawbacks in the control of particle-size (morphology) and compositional purity. Ball-milling is usually used to mix BaCO 3 and TiO 2 . It is not only time-consuming and labor-intensive but also easy to introduce impurities [29]. As an alternative to the solid-state process, various "wet chemical" methods, including sol-gel process [30,31], hydrothermal method [32], micro-emulsions [33], and oxalate process [34] have been developed to synthesize BaTiO 3 powders. These methods can produce high-purity, uniform, ultrafine BaTiO 3 powders. Because of the complexity of operation, multi-stage, and relatively high cost, most of these methods are mainly used at the laboratory level. It should be noted that the hydrothermal process is a promising method to synthesize BaTiO 3 powders with controllable morphology and chemical uniformity.
The hydrothermal method can use various processing conditions in the synthesis of BaTiO 3 powders including the sources of barium and titanium in an aqueous medium under crystallization or amorphous state, the hydrothermal temperature and time, and morphology-controlled agents. Because of the diversity of the factors that affect the synthesis of BaTiO 3 nanoparticles, hydrothermal methods are full of opportunities to improve their quality in phase composition, dimensions, and morphology. Li et al. [35] reported the synthesis of tetragonal BaTiO 3 nanocrystals using TiCl 4 (or TiO 2 ) as the source of titanium, BaCl 2 as the source of barium, and polymer(vinylpyrrolidone) (PVP) as the surfactant. Grendal et al. [36] used two titanium sources of amorphous titanium dioxide and a Ti-citrate complex solution to synthesize BaTiO 3 nanoparticles with a size range of 10-15 nm at different hydrothermal temperatures and times. Zhao et al. [37] used cetyltrimethylammonium bromide (CTAB), Ba(OH) 2 ·8H 2 O, and tetrabutyl titanate as the precursors to synthesize BaTiO 3 nanocrystals via a self-assembly process. Ozen et al. [38] reported the hydrothermal synthesis of tetragonal BaTiO 3 nanocrystals from a single-source amorphous barium titanate precursor in a high concentration sodium hydroxide solution via a homogeneous dissolution-precipitation reaction. From the above cases, one can see that different hydrothermal parameters and growth mechanisms can effectively adjust the formation of BaTiO 3 nanocrystals. In addition, a single cubic phase of BaTiO 3 can be formed at a low alkalinity, and a tetragonal phase of BaTiO 3 is easily formed under a strong alkaline condition [39].
With the motivation of preparing cubic/tetragonal BaTiO 3 nanocrystals with a spherical morphology, this paper herein develops a TiO 2 -seeded hydrothermal process to grow BaTiO 3 nanocrystals using Ba(NO 3 ) 2 and TiO 2 (P25) as the barium and titanium sources, respectively. This synthesis is conducted under a strong alkaline NaOH aqueous solution (pH = 13.6), and the factors that affect the formation of BaTiO 3 nanocrystals are systematically investigated. The major influencing factors involve molar Ba/Ti ratios, hydrothermal temperature, and hydrothermal time, and their effects on the morphology, particle size, and phase composition of the BaTiO 3 nanoparticles are investigated. The possible growth mechanisms are discussed. The BaTiO 3 /polymer/Al films containing the BaTiO 3 nanoparticles obtained under the optimum conditions are of a high dielectric constant of 59, a high break strength of 102 kV mm −1 and a low dielectric loss of 0.008. This work achieves this aim to seek optimum methods to synthesize spherical BaTiO 3 nanoparticles with potential applications in capacitor energy-storage and other electric devices.

Growth of Spherical BaTiO 3 Nanoparticles
BaTiO 3 samples were synthesized via a hydrothermal process using TiO 2 (P25) nanoparticles as the Ti source and seeds. The synthetic process of the BaTiO 3 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 TiO 2 nanoparticles were first added into 75 mL of distilled water under magnet stirring; then a given amount of Ba(NO 3 ) 2 was added to the above suspension containing TiO 2 nanoparticles and NaOH under magnetic stirring. In the final suspensions, the molar ratios of Ba(NO 3 ) 2 to TiO 2 (R Ba/Ti ) were kept at 1.6-2.0, and the molar concentration of NaOH was about 2 mol L −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 BaTiO 3 solids were ground into powders using an agate mortar. These white powders, i.e., BaTiO 3 nanocrystals, were collected and used for characterization. The detailed processing parameters for the synthesis of BaTiO 3 nanocrystals are listed in Table 1. It was assumed that TiO 2 added was completely converted into BaTiO 3 , and the theoretical mass could be calculated. The yield of BaTiO 3 was the ratio of the actual mass of the BaTiO 3 sample to their corresponding theoretical mass.  To determine the possibility of the as-obtained BaTiO3 nanocrystals to form a uniform film for capacitor energy-storage application, we chose sample S8 (in Table 1) as an example to prepare BaTiO3/polymer/Al films (BPA films, Figure 2) using the similar method reported in our previous work [25]. Typically, the BaTiO3 nanocrystals (S8) were mixed with a silicon-containing heat-resistant resin (CYN-01), and then some silane coupling agent (KH550) was added into the above mixture. Dimethylacetamide (DMAc, Guangzhou Jinhuada Chemical Reagent Co., Ltd., Guangzhou, China)) was used as the solvent. The mass ratio of MBaTiO3:MDMAc:MPolymer:MKH550 was kept at 100:45:25:4. The as-prepared mixture was ultrasonically treated for 30 min for a uniform slurry. The above slurry was coated on an Al foil by a bar coater (T-300CA) and a coating rod (D10-OSP010-L0400) from Shijiazhuang Ospchina Machinery Technology Co., Ltd (Shijiazhuang, China)). The as-formed films were then dried in an oven at 220 °C for 10 min and finally used for the test of dielectric properties.

Preparation of BaTiO 3 /Polymer/Al (BPA) Films
To determine the possibility of the as-obtained BaTiO 3 nanocrystals to form a uniform film for capacitor energy-storage application, we chose sample S8 (in Table 1) as an example to prepare BaTiO 3 /polymer/Al films (BPA films, Figure 2) using the similar method reported in our previous work [25]. Typically, the BaTiO 3 nanocrystals (S8) were mixed with a silicon-containing heat-resistant resin (CYN-01), and then some silane coupling agent (KH550) was added into the above mixture. Dimethylacetamide (DMAc, Guangzhou Jinhuada Chemical Reagent Co., Ltd., Guangzhou, China)) was used as the solvent. The mass ratio of M BaTiO3 :M DMAc :M Polymer :M KH550 was kept at 100:45:25:4. The as-prepared mixture was ultrasonically treated for 30 min for a uniform slurry. The above slurry was coated on an Al foil by a bar coater (T-300CA) and a coating rod (D10-OSP010-L0400) from Shijiazhuang Ospchina Machinery Technology Co., Ltd (Shijiazhuang, China)). The as-formed films were then dried in an oven at 220 • C for 10 min and finally used for the test of dielectric properties. The X-ray diffraction (XRD) patterns of the BPA composite films and BaTiO3 powders were recorded by a DX-2700BH X-ray diffractometer (Dandong, China) using Cu Kα irradiation. The morphologies and particle sizes of the BaTiO3 samples were measured using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The particle-size distribution was statistically analyzed according to the SEM images. The pH values of the suspensions were measured using a pH meter (PHS-2C). The yields of the BaTiO3 samples were calculated according to the ratios of experimental BaTiO3 mass to its theoretical mass on the basis of Ba conservation. Fourier-transform infrared (FT-IR) spectra were recorded on a Bruker-Equinox 55 spectrometer in a wavenumber range of 4000-400 cm −1 using the KBr technique. The dielectric constant (ε) and loss (tanδ) of the BPA films were measured using a high-precision high-voltage capacitor bridge (QS89, Shanghai Yanggao Capacitor Co., Ltd., Shanghai, China), and the frequency during dielectric performance test was kept at 10 Hz. The breakdown strengths of the BPA films were measured using a withstand voltage tester (GY2670A, Guangzhou Zhizhibao Electronic Instrument Co., Ltd., Guangzhou, China).

Results and Discussion
The TiO2-seeded growth process of BaTiO3 nanocrystals is shown in Figure 1. The commercially available TiO2 (P25) nanoparticles, with a mixed phase of anatase and rutile and a size range of 20-25 nm, are used as the Ti source and seeds in the synthesis of BaTiO3 nanocrystals via a conventional hydrothermal process in a strongly basic aqueous solution. In this synthesis, TiO2 nanoparticles can first react with NaOH and form insoluble titanate species (e.g., Na2TiO3), which then act like the crystal nucleus to form BaTiO3 nanocrystals by reacting with Ba 2+ ions under the hydrothermal conditions. We systematically investigated the effects of molar ratios of Ba/Ti (RBa/Ti), hydrothermal temperature (T/°C) and time (t/h) on the phase, morphology and particle size of the BaTiO3 nanocrystals.

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 RBa/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 −1 (pH = 13.6), T = 200 °C, and t = 8 h. The typical results of these samples are shown in Figure 3.

Characterization of BaTiO 3 Nanocrystals and BPA Films
The X-ray diffraction (XRD) patterns of the BPA composite films and BaTiO 3 powders were recorded by a DX-2700BH X-ray diffractometer (Dandong, China) using Cu Kα irradiation. The morphologies and particle sizes of the BaTiO 3 samples were measured using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The particle-size distribution was statistically analyzed according to the SEM images. The pH values of the suspensions were measured using a pH meter (PHS-2C). The yields of the BaTiO 3 samples were calculated according to the ratios of experimental BaTiO 3 mass to its theoretical mass on the basis of Ba conservation. Fourier-transform infrared (FT-IR) spectra were recorded on a Bruker-Equinox 55 spectrometer in a wavenumber range of 4000-400 cm −1 using the KBr technique. The dielectric constant (ε) and loss (tanδ) of the BPA films were measured using a high-precision high-voltage capacitor bridge (QS89, Shanghai Yanggao Capacitor Co., Ltd., Shanghai, China), and the frequency during dielectric performance test was kept at 10 Hz. The breakdown strengths of the BPA films were measured using a withstand voltage tester (GY2670A, Guangzhou Zhizhibao Electronic Instrument Co., Ltd., Guangzhou, China).

Results and Discussion
The TiO 2 -seeded growth process of BaTiO 3 nanocrystals is shown in Figure 1. The commercially available TiO 2 (P25) nanoparticles, with a mixed phase of anatase and rutile and a size range of 20-25 nm, are used as the Ti source and seeds in the synthesis of BaTiO 3 nanocrystals via a conventional hydrothermal process in a strongly basic aqueous solution. In this synthesis, TiO 2 nanoparticles can first react with NaOH and form insoluble titanate species (e.g., Na 2 TiO 3 ), which then act like the crystal nucleus to form BaTiO 3 nanocrystals by reacting with Ba 2+ ions under the hydrothermal conditions. We systematically investigated the effects of molar ratios of Ba/Ti (R Ba/Ti ), hydrothermal temperature (T/ • C) and time (t/h) on the phase, morphology and particle size of the BaTiO 3 nanocrystals.

Influence of Molar Ba/Ti Ratio
In order to verify the effect of the molar Ba/Ti ratio on the formation of BaTiO 3 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 −1 (pH = 13.6), T = 200 • C, and t = 8 h. The typical results of these samples are shown in Figure 3. Crystals 2020, 10, x FOR PEER REVIEW 6 of 15   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 RBa/Ti value from 1.6 to 2.5, the peaks near 45° become wider and wider, suggesting that a higher RBa/Ti value is favorable in forming a tetragonal BaTiO3 phase. Figure 3c shows the plots of particle size dependent on the RBa/Ti values. When RBa/Ti = 1.6-1.8, the particle sizes are 90-100 nm (97 ± 15 nm for RBa/Ti = 1.6 and 93 ± 24 nm for RBa/Ti = 1.8), but the uniform degree is not high. Figure 3d shows the yields of BaTiO3 samples synthesized with various RBa/Ti values after hydrothermally treating at 200 °C for 8 h ([NaOH] = 2.0 mol L −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 3g), the particle size of the BaTiO3 sample is 91 ± 22 nm, and it shows a more uniform solid spherical particle morphology. When RBa/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.   [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 BaTiO 3 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 BaTiO 3 samples synthesized with various R Ba/Ti values after hydrothermally treating at 200 • C for 8 h ([NaOH] = 2.0 mol L −1 ). One can see that the yields of all the samples are close to 100%, indicating the complete conversion of TiO 2 to BaTiO 3 nanocrystals. The formation of a small amount of crystal water may make the BaTiO 3 yield a little larger than 100% according to the TiO 2 amount [42].  Figure 3g), the particle size of the BaTiO 3 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 BaTiO 3 sample is 98 ± 26 nm, and one can see that it shows obviously clean-cut crystal faces for the BaTiO 3 particles, suggesting a higher degree of crystallinity and favorable formation of the tetragonal BaTiO 3 phase.

Influence of Hydrothermal Temperature
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 BaTiO 3 nanocrystals with a more uniform size.

Influence of Hydrothermal Temperature
The effect of hydrothermal temperature on the synthesis of BaTiO 3 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 −1 , and Figure 4 shows their characterization results of XRD and SEM.    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 4c shows the particle-size distribution plot versus hydrothermal temperature (T). When T = 150 • C, the particle sizes of the as-obtained BaTiO 3 nanocrystals are 85 ± 15 nm. When T = 165 • C, the particle size of the as-obtained BaTiO 3 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 BaTiO 3 is 88 ± 10 nm, and the morphology of the BaTiO 3 particles becomes relatively uniform. When T = 210 • C, the particle size of the as-obtained the BaTiO 3 sample is 91 ± 14 nm, just a slight increase. As Figure 4c shows, the particle sizes of the BaTiO 3 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 BaTiO 3 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. 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 BaTiO 3 nanocrystals with more uniform spherical morphology. For safety's sake, the hydrothermal temperature is chosen as 210 • C for the synthesis of BaTiO 3 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.

Influence of Hydrothermal Time
The effect of hydrothermal time on the formation of BaTiO 3 nanocrystals ( Figure 5 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 BaTiO 3 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 BaTiO 3 sample obtained with a longer hydrothermal time has more tetragonal BaTiO 3 species. Crystals 2020, 10, x FOR PEER REVIEW 8 of 15 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.

Influence of Hydrothermal Time
The effect of hydrothermal time on the formation of BaTiO3 nanocrystals ( Figure 5 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 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-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 (RBa/Ti = 2.0, T = 210 °C, [NaOH] = 2.0 mol L −1 ). The bands at 3431 and 1568 cm −1 can be attributed    Figure 5e-g, exhibit a spherical shape; when the hydrothermal time increases to 12-16 h, as Figure 5h,i shows, the as-obtained BaTiO 3 samples take on a planar polyhedral morphology. It is interesting that the particle sizes of the BaTiO 3 samples are close to 100 nm and not changed obviously with the prolonging of hydrothermal time to 16 h. In addition, as Crystals 2020, 10, 202 9 of 15 Figure 5i shows, the BaTiO 3 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 BaTiO 3 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 −1 can be attributed to the stretching mode of the adsorbed water molecules and O-H groups, indicating that the surfaces of the BaTiO 3 nanocrystals contain some adsorbed water and -OH groups. The weak band at 1400 cm −1 can be attributed to the stretching mode of the C-O groups because of the incorporation of CO 2 into the basic solution. The broad and strong absorption bands at 562 cm −1 is attributed to the normal vibration of Ti-O I stretching, and the weaker and sharper absorption bands near 438 cm −1 can be attributed to the normal vibration of Ti-O II bending. When the hydrothermal time is extended from 2 h to 16 h, the bands at 562 and 438 cm −1 become stronger and sharper, indicating that the BaTiO 3 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 BaTiO 3 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 BaTiO 3 nanocrystals can be R Ba/Ti ≥ 2, T ≥ 200 • C, t ≥ 8 h. The as-obtained BaTiO 3 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 BaTiO 3 nanocrystals show a high performance in ceramic capacitor for energy-storage applications.

Understanding of Growth Mechanism
In the hydrothermal synthesis of BaTiO3 nanocrystals, TiO2 (P25) nanoparticles are used as the solid-state Ti source and seeds for crystal growth. The possible growth mechanism of the BaTiO3 nanocrystals by the hydrothermal process is shown in Figure 7. TiO2 nanoparticles first react with OH − ions in a strong alkaline solution to form a soluble titanium hydroxide complex, which can form a negatively charged Ti-O chain. These negatively charged Ti-O chains attract positively charged Ba 2+ or BaOH + ions to form BaTiO3 nuclei, on which the excess Ba 2+ species continue to grow in the strong alkaline solution under the hydrothermal conditions for a long time. The possible reactions for the growth of BaTiO3 nanocrystals can be described as follows:

Understanding of Growth Mechanism
In the hydrothermal synthesis of BaTiO 3 nanocrystals, TiO 2 (P25) nanoparticles are used as the solid-state Ti source and seeds for crystal growth. The possible growth mechanism of the BaTiO 3 nanocrystals by the hydrothermal process is shown in Figure 7. TiO 2 nanoparticles first react with OH − ions in a strong alkaline solution to form a soluble titanium hydroxide complex, which can form a negatively charged Ti-O chain. These negatively charged Ti-O chains attract positively charged Ba 2+ or BaOH + ions to form BaTiO 3 nuclei, on which the excess Ba 2+ species continue to grow in the strong alkaline solution under the hydrothermal conditions for a long time. The possible reactions for the growth of BaTiO 3 nanocrystals can be described as follows: 2 (1) Ti(OH) 6 Ti(OH)6 2− + Ba + → BaTiO3 + H2O Using TiO2 (P25) nanoparticles as the seeds and Ti source for the synthesis of BaTiO3 nanocrystals, the negatively charged Ti-O chains are first formed on the surface of TiO2 (P25) particles in the strong alkaline solution, and the whole TiO2 (P25) nanoparticles are then gradually transformed to the [Ti(OH)x] 4−x species. The negatively charged Ti-O chains (i.e., [Ti(OH)6] 2− ) react with Ba 2+ ions to form BaTiO3 nanocrystals under hydrothermal conditions. The large spherical particles in situ formed on the TiO2 (P25) nuclei may overcome the agglomeration because of their weak attraction to each other. The small particles can be self-regulated by the interaction of van der Waals torque (Casimir Torque) under high-temperature Brownian motion via the orientation attachment mechanism [43]. During the long hydrothermal reaction, smaller crystals dissolve and re-deposit on larger particles for orientation attachment and crystal extension via the Ostwald ripening process. Therefore, the growth mechanism for the formation of BaTiO3 nanoparticles may involve the following steps:

Dielectric Properties of the BPA Film with BaTiO3 Nanoparticles
The spherical BaTiO3 nanoparticles with a size range of 91 ± 14 nm (S8 in Table 1) obtained under the optimum conditions ([NaOH] = 2.0 mol L −1 , RBa/Ti = 2.0, T = 210 °C and t = 8 h) were used to prepare BaTiO3/polymer/Al (BPA) composite films to verify the feasibility of the BaTiO3 sample in capacitor energy-storage applications.
The typical XRD patterns, SEM image and dielectric properties of the typical BPA films with the BaTiO3 sample (S8) are shown in Figure 8. Figure 8a shows the XRD patterns of the BaTiO3 sample, polymer/Al foil, and BPA film. According to the JCPDS card (No. 99-0005), the diffraction peaks at 2 θ = 38.47°, 44.72°, and 65.09° correspond to the (111), (200), and (220) of the Al foil, respectively. The XRD pattern of the BPA film is a superposition of the BaTiO3 sample and Al foil, and no other impurities are found in the BPA film. Figure 8b shows a typical SEM image of the BPA film. The film exhibits a uniform distribution of BaTiO3 nanoparticles. Figure 8c gives the dielectric properties of the BPA films with spherical BaTiO3 nanoparticles. As the statistical results show, the average dielectric constant of the BPA films reaches 59, the average dielectric loss reaches 0.008, and the Using TiO 2 (P25) nanoparticles as the seeds and Ti source for the synthesis of BaTiO 3 nanocrystals, the negatively charged Ti-O chains are first formed on the surface of TiO 2 (P25) particles in the strong alkaline solution, and the whole TiO 2 (P25) nanoparticles are then gradually transformed to the [Ti(OH) x ] 4−x species. The negatively charged Ti-O chains (i.e., [Ti(OH) 6 ] 2− ) react with Ba 2+ ions to form BaTiO 3 nanocrystals under hydrothermal conditions. The large spherical particles in situ formed on the TiO 2 (P25) nuclei may overcome the agglomeration because of their weak attraction to each other. The small particles can be self-regulated by the interaction of van der Waals torque (Casimir Torque) under high-temperature Brownian motion via the orientation attachment mechanism [43]. During the long hydrothermal reaction, smaller crystals dissolve and re-deposit on larger particles for orientation attachment and crystal extension via the Ostwald ripening process. Therefore, the growth mechanism for the formation of BaTiO 3 nanoparticles may involve the following steps: (1) TiO 2 (P25) nanoparticles are transformed to [Ti(OH) x ] 4−x species in the strong alkaline solution; (2) Ba 2+ ions reacts with [Ti(OH) x ] 4−x species to form BaTiO 3 nanocrystals; (3) small BaTiO 3 nanocrystals grows to large ones via the Ostwald ripening process and the orientation attachment mechanism.

Dielectric Properties of the BPA Film with BaTiO 3 Nanoparticles
The spherical BaTiO 3 nanoparticles with a size range of 91 ± 14 nm (S8 in Table 1) obtained under the optimum conditions ([NaOH] = 2.0 mol L −1 , R Ba/Ti = 2.0, T = 210 • C and t = 8 h) were used to prepare BaTiO 3 /polymer/Al (BPA) composite films to verify the feasibility of the BaTiO 3 sample in capacitor energy-storage applications.
The typical XRD patterns, SEM image and dielectric properties of the typical BPA films with the BaTiO 3 sample (S8) are shown in Figure 8. Figure 8a shows the XRD patterns of the BaTiO 3 sample, polymer/Al foil, and BPA film. According to the JCPDS card (No. 99-0005), the diffraction peaks at 2θ = 38.47 • , 44.72 • , and 65.09 • correspond to the (111), (200), and (220) of the Al foil, respectively. The XRD pattern of the BPA film is a superposition of the BaTiO 3 sample and Al foil, and no other impurities are found in the BPA film. Figure 8b shows a typical SEM image of the BPA film. The film exhibits a uniform distribution of BaTiO 3 nanoparticles. Figure 8c gives the dielectric properties of the BPA films with spherical BaTiO 3 nanoparticles. As the statistical results show, the average dielectric constant of the BPA films reaches 59, the average dielectric loss reaches 0.008, and the average breakdown strength reaches 102 kV mm −1 . These electrical properties are much higher than those of the previous reports [44][45][46][47][48][49]. The TiO 2 -seeded hydrothermal process is an efficient process to synthesize spherical BaTiO 3 nanoparticles for potential capacitor energy-storage applications.
Crystals 2020, 10, x FOR PEER REVIEW 11 of 15 average breakdown strength reaches 102 kV mm −1 . These electrical properties are much higher than those of the previous reports [44][45][46][47][48][49]. The TiO2-seeded hydrothermal process is an efficient process to synthesize spherical BaTiO3 nanoparticles for potential capacitor energy-storage applications. We compared the dielectric constant, dielectric loss, and breakdown strength of the BPA films with those of the literature reports [25,31,44,46,49,50], and the results are shown in Table 2. One can find that the BPA films with the TiO2-seeded BaTiO3 nanocrystals exhibit an excellent balanced dielectric performance.  We compared the dielectric constant, dielectric loss, and breakdown strength of the BPA films with those of the literature reports [25,31,44,46,49,50], and the results are shown in Table 2. One can find that the BPA films with the TiO 2 -seeded BaTiO 3 nanocrystals exhibit an excellent balanced dielectric performance.

Conclusions
TiO 2 (P25) nanoparticle assisted hydrothermal process has been developed to synthesize BaTiO 3 nanocrystals in a strong alkaline solution (pH = 13.6) using TiO 2 (P25) and Ba(NO 3 ) 2 as the starting materials and NaOH as the mineralizer. The particle sizes, morphologies, and phases of the BaTiO 3 nanocrystals have been controlled by changing the molar Ba/Ti ratio, the hydrothermal temperature, and time. The XRD and SEM results indicate that a high Ba/Ti ratio (≥2.0), a high hydrothermal temperature (≥200 • C), and a long hydrothermal time (≥8 h) are favorable in forming a mixture of cubic/tetragonal BaTiO 3 nanocrystals with a uniform, well-dispersed spherical particulate morphology (90-100 nm). Under the optimum conditions ([NaOH] = 2.0 mol L −1 , R Ba/Ti = 2.0, T = 210 • C and t = 8 h), the as-obtained spherical BaTiO 3 nanoparticles have a narrow particle size range of 91 ± 14 nm. It should be emphasized that the particle size and morphology of the BaTiO 3 nanocrystals are kept relatively stable when the hydrothermal conditions change in a proper range, suggestive of a robust and efficient process toward spherical BaTiO 3 nanocrystals. The growth mechanism of the TiO 2 -assisted hydrothermal process for the synthesis of BaTiO 3 nanocrystals has been attributed to the dissolution-crystallization, Oswald ripening, and oriented attachment process. The BaTiO 3 /polymer/Al films containing the above BaTiO 3 nanoparticles are of a high dielectric constant of 59, a high break strength of 102 kV mm −1 , and a low dielectric loss of 0.008. The TiO 2 -seeded hydrothermal process developed here is an efficient process to synthesize spherical BaTiO 3 nanoparticles for potential capacitor energy-storage applications.