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Article

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

by
Ming Li
1,2,
Lulu Gu
1,
Tao Li
2,
Shiji Hao
2,
Furui Tan
2,
Deliang Chen
2,3,*,
Deliang Zhu
1,*,
Yongjun Xu
2,
Chenghua Sun
2 and
Zhenyu Yang
2,*
1
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China
2
School of Materials Science and Engineering, Institute of Science & Technology Innovation, Dongguan University of Technology, Dongguan 523808, China
3
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
*
Authors to whom correspondence should be addressed.
Crystals 2020, 10(3), 202; https://doi.org/10.3390/cryst10030202
Submission received: 17 February 2020 / Revised: 29 February 2020 / Accepted: 2 March 2020 / Published: 14 March 2020
(This article belongs to the Special Issue Advances in Industrial Crystallization)
Browse Figures
Versions Notes

Abstract

:
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.

1. Introduction

Barium titanate (BaTiO3) has been an important material in the manufacture of electronic components for many years due to its unique properties of high dielectric constant, high ferroelectricity, and piezoelectricity [1,2,3,4,5]. BaTiO3-based ceramics are of a wide range of potential applications in ferroelectric random access memory (FRAM) [6], photoelectric humidity sensors [7], solid oxide fuel cells [8], superconductors [9], ferromagnets [10,11], high capacitance capacitors [12,13,14], pyroelectric detectors [15], and magneto resistors [16]. Especially, tetragonal BaTiO3 ceramics are widely used in multi-layer ceramic capacitors (MLCC) [17], thermistors [18], and piezoelectric sensors [19]. Conventional methods used to prepare BaTiO3 ceramics are solid-state reaction processes using TiO2 and BaCO3 as the raw materials at an elevated temperature of more than 1200 °C. The large size and low purity of the BaTiO3 ceramics obtained by the solid-state reaction have limited their applications in nanotechnological fields.
The miniaturization of electronic components and nanotechnology makes it necessary to synthesize nanometer-scale BaTiO3 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 BaTiO3 materials [22,23,24]. For the applications in MLCC, BaTiO3 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 BaTiO3-based MLCC, high-quality BaTiO3 powders with high purity, high crystallinity, high dispersibility, and uniform small size are the precondition. The BaTiO3 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 BaTiO3 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 BaTiO3 powders are higher than those of the films with cubic BaTiO3 fillers; whereas the effect of crystal phase on the dielectric losses presents an opposite trend that the composite filled with a cubic BaTiO3 filler shows a lower dielectric loss than that of the tetragonal BaTiO3 composite film [26]. Therefore, a high-quality BaTiO3 filler is important for high performance composite dielectric films, and a recent investigation on the synthesis of BaTiO3 nanocrystals via various processes has become one of the hot topics.
There have been a number of methods developed to prepare high-quality BaTiO3 powders [27]. As mentioned above, the conventional route used to prepare BaTiO3 powders is via a solid-state reaction between BaCO3 and TiO2 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 BaCO3 and TiO2. 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 BaTiO3 powders. These methods can produce high-purity, uniform, ultrafine BaTiO3 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 BaTiO3 powders with controllable morphology and chemical uniformity.
The hydrothermal method can use various processing conditions in the synthesis of BaTiO3 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 BaTiO3 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 BaTiO3 nanocrystals using TiCl4 (or TiO2) as the source of titanium, BaCl2 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 BaTiO3 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∙8H2O, and tetrabutyl titanate as the precursors to synthesize BaTiO3 nanocrystals via a self-assembly process. Ozen et al. [38] reported the hydrothermal synthesis of tetragonal BaTiO3 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 BaTiO3 nanocrystals. In addition, a single cubic phase of BaTiO3 can be formed at a low alkalinity, and a tetragonal phase of BaTiO3 is easily formed under a strong alkaline condition [39].
With the motivation of preparing cubic/tetragonal BaTiO3 nanocrystals with a spherical morphology, this paper herein develops a TiO2-seeded hydrothermal process to grow BaTiO3 nanocrystals using Ba(NO3)2 and TiO2(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 BaTiO3 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 BaTiO3 nanoparticles are investigated. The possible growth mechanisms are discussed. The BaTiO3/polymer/Al films containing the BaTiO3 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 BaTiO3 nanoparticles with potential applications in capacitor energy-storage and other electric devices.

2. Materials and Methods

2.1. Chemicals and Settings

Barium nitrate (Ba(NO3)2, analytical grade) was purchased from Tianjin Shengao Chemical Reagent Co., Ltd (Tianjin, China). Titanium dioxide (TiO2, P25, chemically pure) was purchased from Degussa. Sodium hydroxide (NaOH, analytical grade) was purchased from Tianjin Komi Chemical Reagent Co., Ltd (Tianjin, China). Ethanol (analytical grade) was purchased from Tianjin Kaitong Chemical Reagent Co., Ltd (Tianjin, China). Distilled water was used in all the experiments. The drying oven (XMTD-8222) was purchased from Shanghai Jinghong Experimental Equipment Co., Ltd (Shanghai, China). The desktop high-speed centrifuge (H1850) was purchased from Hunan Xiangyi Centrifuge Co., Ltd (Xiangtan, China). The polymer (ceramic glue), a silicon-containing heat-resistant resin, was purchased from the IPINRU Chen Yu Technology Co., Ltd (Chengdu, China, Product No. CYN-01 with a curing temperature of ~220 °C). A silane coupling agent (KH550, NH2CH2CH2CH2Si(OC2H5)3) was purchased from Guangzhou Yuantai Synthetic Material Co., Ltd (Guangzhou, China). Al foils (thickness = ~12 μm, tensile strength ≥ 180 MPa, ductility ≥ 15%) were purchased from Shenzhen Kejing Star Technology Co., Ltd (Shenzhen, China).

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 (RBa/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 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.

2.3. Preparation of BaTiO3/Polymer/Al (BPA) Films

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.

2.4. Characterization of BaTiO3 Nanocrystals and BPA Films

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).

3. 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 Ba2+ 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.

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 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.
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 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 3e–h shows the typical SEM images of the BaTiO3 samples obtained with various RBa/Ti values ([NaOH] = 2.0 mol L−1, T = 200 °C, t = 8 h). According to the SEM observations, when RBa/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 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.

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: RBa/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 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, RBa/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.

3.3. Influence of Hydrothermal Time

The effect of hydrothermal time on the formation of BaTiO3 nanocrystals (Figure 5) are investigated under the conditions: RBa/Ti = 2.0, T = 210 °C, [NaOH] = 2.0 mol L−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 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 (RBa/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 (RBa/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 BaTiO3 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 CO2 into the basic solution. The broad and strong absorption bands at 562 cm−1 is attributed to the normal vibration of Ti–OI stretching, and the weaker and sharper absorption bands near 438 cm−1 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−1 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 RBa/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.

3.4. 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 Ba2+ or BaOH+ ions to form BaTiO3 nuclei, on which the excess Ba2+ 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:
TiO2(P25) + OH− → TiO(OH)2
TiO(OH)2 + OH− + H2O → Ti(OH)62−
Ti(OH)62− + 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 Ba2+ 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: (1) TiO2 (P25) nanoparticles are transformed to [Ti(OH)x]4−x species in the strong alkaline solution; (2) Ba2+ ions reacts with [Ti(OH)x]4−x species to form BaTiO3 nanocrystals; (3) small BaTiO3 nanocrystals grows to large ones via the Ostwald ripening process and the orientation attachment mechanism.

3.5. 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 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 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 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.

4. Conclusions

TiO2 (P25) nanoparticle assisted hydrothermal process has been developed to synthesize BaTiO3 nanocrystals in a strong alkaline solution (pH = 13.6) using TiO2 (P25) and Ba(NO3)2 as the starting materials and NaOH as the mineralizer. The particle sizes, morphologies, and phases of the BaTiO3 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 BaTiO3 nanocrystals with a uniform, well-dispersed spherical particulate morphology (90–100 nm). Under the optimum conditions ([NaOH] = 2.0 mol L−1, RBa/Ti = 2.0, T = 210 °C and t = 8 h), the as-obtained spherical BaTiO3 nanoparticles have a narrow particle size range of 91 ± 14 nm. It should be emphasized that the particle size and morphology of the BaTiO3 nanocrystals are kept relatively stable when the hydrothermal conditions change in a proper range, suggestive of a robust and efficient process toward spherical BaTiO3 nanocrystals. The growth mechanism of the TiO2-assisted hydrothermal process for the synthesis of BaTiO3 nanocrystals has been attributed to the dissolution-crystallization, Oswald ripening, and oriented attachment process. The BaTiO3/polymer/Al films containing the above BaTiO3 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 TiO2-seeded hydrothermal process developed here is an efficient process to synthesize spherical BaTiO3 nanoparticles for potential capacitor energy-storage applications.

Author Contributions

Conceptualization, D.C.; data curation, T.L. and F.T.; funding acquisition, D.C., C.S., and Z.Y.; investigation, D.Z. and Y.X.; methodology, F.T.; Software, S.H.; writing–original draft, M.L. and L.G.; writing–review and editing, M.L., L.G., T.L., and D.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partly supported by the National Natural Science Foundation of China (Grant No. 51574205), the Natural Science Foundation of Guangdong Province (Grant No. 2018B030311022), Guangdong Innovation Research Team for Higher Education (Grant No. 2017KCXTD030), the Engineering Research Center of None-Food Biomass Efficient Pyrolysis & Utilization Technology of Guangdong Higher Education Institutes (Grant No.2016GCZX009), High-level Talents Project of Dongguan University of Technology (Grant No. KCYKYQD2017017), and Program from Dongguan University of Technology (Grant No. G200906-17).

Conflicts of Interest

The authors declare no conflict of interest.

References

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Crystals 10 00202 g001 550
Figure 1. Schematic of the hydrothermal synthesis of BaTiO3 nanocrystals using TiO2 (P25) nanoparticles as the seeds and Ti source.
Figure 1. Schematic of the hydrothermal synthesis of BaTiO3 nanocrystals using TiO2 (P25) nanoparticles as the seeds and Ti source.
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Crystals 10 00202 g002 550
Figure 2. A schematic diagram of the BaTiO3/polymer/Al film for capacitor cells.
Figure 2. A schematic diagram of the BaTiO3/polymer/Al film for capacitor cells.
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Crystals 10 00202 g003 550
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 (RBa/Ti = 1.6–2.5) under hydrothermal conditions at 200 °C for 8 h ([NaOH] = 2.0 mol L−1, pH ≈ 13.6).
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 (RBa/Ti = 1.6–2.5) under hydrothermal conditions at 200 °C for 8 h ([NaOH] = 2.0 mol L−1, pH ≈ 13.6).
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Crystals 10 00202 g004 550
Figure 4. XRD patterns (a,b), particle sizes and yields (c,d), and SEM images (e–h) of the BaTiO3 nanocrystals obtained with RBa/Ti = 2.0 under hydrothermal conditions at 150–210 °C for 8 h ([NaOH] = 2.0 mol L−1, pH ≈ 13.6).
Figure 4. XRD patterns (a,b), particle sizes and yields (c,d), and SEM images (e–h) of the BaTiO3 nanocrystals obtained with RBa/Ti = 2.0 under hydrothermal conditions at 150–210 °C for 8 h ([NaOH] = 2.0 mol L−1, pH ≈ 13.6).
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Figure 5. XRD patterns (a,b), particle sizes and yields (c,d), and SEM images (e–i) of the BaTiO3 nanocrystals obtained with RBa/Ti = 2.0([NaOH]=2.0 mol L−1, pH ≈ 13.6) by hydrothermally treating at 210 °C for various times (t = 2–16 h).
Figure 5. XRD patterns (a,b), particle sizes and yields (c,d), and SEM images (e–i) of the BaTiO3 nanocrystals obtained with RBa/Ti = 2.0([NaOH]=2.0 mol L−1, pH ≈ 13.6) by hydrothermally treating at 210 °C for various times (t = 2–16 h).
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Figure 6. Typical FT-IR spectra of the BaTiO3 nanocrystals obtained by hydrothermally treating at 210 °C for various times (2–16 h) with RBa/Ti = 2.0 and [NaOH] = 2.0 mol L−1.
Figure 6. Typical FT-IR spectra of the BaTiO3 nanocrystals obtained by hydrothermally treating at 210 °C for various times (2–16 h) with RBa/Ti = 2.0 and [NaOH] = 2.0 mol L−1.
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Figure 7. Possible growth mechanism for the synthesis of BaTiO3 nanocrystals under the hydrothermal conditions using TiO2 nanoparticles (P25) as the seeds and Ti source.
Figure 7. Possible growth mechanism for the synthesis of BaTiO3 nanocrystals under the hydrothermal conditions using TiO2 nanoparticles (P25) as the seeds and Ti source.
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Figure 8. XRD, SEM, and dielectric properties of the typical BaTiO3/polymer/Al films (BPA) films with BaTiO3 nanocrystals (S8): (a) XRD patterns (A-BaTiO3 nanocrystals, B-Polymer/Al film and (C)-BaTiO3/polymer/Al (BPA) film); (b) the typical SEM image of the BPA film; (c) typical electric properties of the BPA films.
Figure 8. XRD, SEM, and dielectric properties of the typical BaTiO3/polymer/Al films (BPA) films with BaTiO3 nanocrystals (S8): (a) XRD patterns (A-BaTiO3 nanocrystals, B-Polymer/Al film and (C)-BaTiO3/polymer/Al (BPA) film); (b) the typical SEM image of the BPA film; (c) typical electric properties of the BPA films.
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Table
Table 1. A summary of experimental conditions for hydrothermal synthesis of BaTiO3 nanoparticles.
Table 1. A summary of experimental conditions for hydrothermal synthesis of BaTiO3 nanoparticles.
Sample[NaOH] (mol L−1)RBa/TiHydrothermal Temperature (°C)Hydrothermal Duration (h)Particle Size (nm)
S121.6200897 ± 15
S221.8200893 ± 24
S322.0200891 ± 22
S422.5200898 ± 26
S522.0150885 ± 15
S622.0165874 ± 13
S722.0180888 ± 10
S822.0210891 ± 14
S922.0210276 ± 17
S1022.0210490 ± 15
S1122.021012100 ± 20
S1222.021016103 ± 20
Table
Table 2. Comparisons of dielectric constant, dielectric loss and breakdown strength of the composites containing BT particles.
Table 2. Comparisons of dielectric constant, dielectric loss and breakdown strength of the composites containing BT particles.
FillersPolymer MatrixDielectric ConstantBreak StrengthDielectric LossReference
BT microparticlesResin3220.8 V/μm0.014[25]
BT microparticlesResorcinol and formaldehyde16.6/0.019[31]
PDA coated BT nanoparticles (100 nm)/BN nanosheetsPoly(vinylidene fluoride-chlorotrifluoroethylene)11.7425 MV/m0.10[44]
Sphere-like TiO2 nanowire clustersPoly(vinylidene fluoride-co-hexafluoropylene)11.9160 kV/mm0.048[46]
CaCu3Ti4O12@TiO2 nanofibersIn suit prepared polyimide5.85236 kV/mm0.025[49]
PVP coated BT nanoparticles (100 nm)Poly(vinylidene fluoride)80.4240 kV/mm0.085[50]
BT microparticlesResin59102 kV/mm0.008This work

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MDPI and ACS Style

Li, M.; Gu, L.; Li, T.; Hao, S.; Tan, F.; Chen, D.; Zhu, D.; Xu, Y.; Sun, C.; Yang, Z. TiO2-Seeded Hydrothermal Growth of Spherical BaTiO3 Nanocrystals for Capacitor Energy-Storage Application. Crystals 2020, 10, 202. https://doi.org/10.3390/cryst10030202

AMA Style

Li M, Gu L, Li T, Hao S, Tan F, Chen D, Zhu D, Xu Y, Sun C, Yang Z. TiO2-Seeded Hydrothermal Growth of Spherical BaTiO3 Nanocrystals for Capacitor Energy-Storage Application. Crystals. 2020; 10(3):202. https://doi.org/10.3390/cryst10030202

Chicago/Turabian Style

Li, Ming, Lulu Gu, Tao Li, Shiji Hao, Furui Tan, Deliang Chen, Deliang Zhu, Yongjun Xu, Chenghua Sun, and Zhenyu Yang. 2020. "TiO2-Seeded Hydrothermal Growth of Spherical BaTiO3 Nanocrystals for Capacitor Energy-Storage Application" Crystals 10, no. 3: 202. https://doi.org/10.3390/cryst10030202

APA Style

Li, M., Gu, L., Li, T., Hao, S., Tan, F., Chen, D., Zhu, D., Xu, Y., Sun, C., & Yang, Z. (2020). TiO2-Seeded Hydrothermal Growth of Spherical BaTiO3 Nanocrystals for Capacitor Energy-Storage Application. Crystals, 10(3), 202. https://doi.org/10.3390/cryst10030202

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