Multiple ZnO Core Nanoparticles Embedded in TiO₂ Nanoparticles as Agents for Acid Resistance and UV Protection

Zinc oxide (ZnO) nanoparticles have long been recognized as ultraviolet (UV) scatterers because of their ability to protect the human skin from UV radiation. However, their durability against acids is low, resulting in their degradation on the surface of the human skin. In this study, we demonstrated the effectiveness of titanium oxide (TiO₂) nanoparticles composited with ZnO nanoparticles to improve the acid resistance. Spherical ZnO/TiO₂ composite particles with the TiO₂ weight percentages of 0–30% were successfully prepared using a flame-assisted aerosol process. The Ti and Zn atoms were uniformly distributed in the obtained particles, and TiO₂ nanoparticles were found to form a nanocrystalline structure. The unique structures that multiple ZnO core nanoparticles embedded in TiO₂ as agents were confirmed by a detailed cross-sectional analysis. The acid resistance evaluation test using sulfuric acid drops showed that the dissolution of Zn²⁺ was suppressed by compositing with TiO₂ nanoparticles, and its acid resistance was enhanced. Furthermore, the acid resistance tended to be improved by increasing the amount of TiO₂. The synthesized TiO₂/ZnO particles exhibited not only absorption of UVA (320–400 nm) by ZnO but also absorption of UVB (290–320 nm) by TiO₂, as well as high visible-light transmittance.