**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, *R*Ba/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.
