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.