**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 [1–5]. 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 [12–14], 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 [22–24]. 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−<sup>1</sup> 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.
