Process design and the influence of key aspects in the synthesis of core-shell structured ZnO@Bi₂O₃ nanoparticles

Bi₂O₃ is a critical additive in ZnO varistor formulations and requires uniform distribution to optimize electrical performance. Surface coating technology enables the synthesis of ZnO@Bi₂O₃ core-shell particles, effectively mitigating the uneven distribution of Bi₂O₃ within the particles. Among the currently reported synthesis methods of ZnO@Bi₂O₃, there are significant variations in the selection of core material (ZnO source), and the coating effects obtained from different coating processes are highly inconsistent. Additionally, the characterization of coating structures remains scarce in the literature. This study proposes a novel synthesis route for ZnO@Bi₂O₃ particles, utilizing commercial nano-ZnO powder as the core material to ensure industrial quality standards for ZnO in production. Bismuth ions (Bi³⁺) are deposited via chemical precipitation to form the shell layer. Hydrothermal treatment strengthens the core-shell binding, facilitates phase transformation, and final calcination purifies the product to achieve the Bi₂O₃ phase. The effects of pH value and Bi³ ⁺/Zn²⁺ molar ratio were systematically investigated. Prepared ZnO@Bi₂O₃ core-shell structures were thoroughly characterized and analyzed using X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the optimal coating performance is achieved when pH= 10.5 and the Bi³ ⁺/Zn²⁺ molar ratio is near 0.05, whereby the Bi₂O₃ phase forms uniform belt-like and spherical shells on the ZnO surface. This research integrates the common industry powder preparation techniques, including chemical precipitation, hydrothermal reaction, and calcination, to establish a foundational basis for the industrialization synthesis of ZnO@Bi₂O₃.