Core-shell nanostructures combining zinc oxide (ZnO) and silicon dioxide (SiO2) present a promising approach to enhance the durability of UV-blocking materials. ZnO is widely recognized for its strong absorption of ultraviolet (UV) radiation, particularly in the UVA (320–400 nm) and UVB (280–320 nm) ranges. However, its susceptibility to photocorrosion under prolonged UV exposure limits its long-term performance in applications such as sunscreens, coatings, and textiles. Encapsulating ZnO nanoparticles within a protective SiO2 shell mitigates this degradation while preserving UV absorption efficiency.

The synthesis of ZnO/SiO2 core-shell nanostructures typically involves a two-step process: the preparation of ZnO nanoparticles followed by the deposition of a SiO2 shell. The Stöber method is a widely employed sol-gel technique for SiO2 coating due to its ability to produce uniform and controllable shell thicknesses. In this process, ZnO nanoparticles are dispersed in an alcohol-water mixture, and a silica precursor, such as tetraethyl orthosilicate (TEOS), is introduced under alkaline conditions. The hydrolysis and condensation of TEOS result in the formation of an amorphous SiO2 layer around the ZnO core. The thickness of the SiO2 shell can be precisely tuned by adjusting parameters such as reaction time, TEOS concentration, and ammonia catalyst amount. Studies indicate that shell thicknesses between 5–20 nm provide optimal protection without significantly compromising UV absorption.

Shell thickness optimization is critical to balancing protection and performance. A SiO2 layer that is too thin may fail to prevent photocorrosion, while an excessively thick shell can scatter incident UV light, reducing ZnO’s absorption efficiency. Experimental data show that a 10 nm SiO2 shell offers a suitable compromise, reducing ZnO degradation by over 80% after extended UV exposure while maintaining 90% of the core’s UV-blocking capacity. The SiO2 shell acts as a physical barrier, preventing direct contact between ZnO and reactive species such as water and oxygen, which are responsible for photocorrosion. Additionally, SiO2 is chemically inert and transparent to UV light, ensuring that the underlying ZnO remains functionally active.

The mechanism by which SiO2 prevents ZnO degradation involves several key factors. First, the SiO2 shell isolates the ZnO core from environmental moisture, inhibiting the formation of reactive hydroxyl radicals under UV irradiation. Second, the shell minimizes surface defects on ZnO nanoparticles, which are common initiation sites for photocorrosion. Third, the amorphous nature of SiO2 ensures minimal lattice mismatch with ZnO, reducing interfacial strain that could otherwise compromise structural integrity. Spectroscopic analyses confirm that SiO2-coated ZnO nanoparticles exhibit negligible changes in crystallinity and optical properties after accelerated UV aging tests, unlike uncoated ZnO, which shows significant deterioration.

Beyond UV-blocking durability, the SiO2 shell enhances the dispersibility and compatibility of ZnO nanoparticles in various matrices. The hydrophilic surface of SiO2 facilitates integration into aqueous formulations, while surface modification with silane coupling agents can improve compatibility with organic polymers. This versatility makes ZnO/SiO2 core-shell nanostructures suitable for diverse applications, including transparent UV-protective films, functional textiles, and cosmetic formulations.

In summary, the design of ZnO/SiO2 core-shell nanostructures addresses the limitations of bare ZnO in UV-blocking applications by combining the strong UV absorption of ZnO with the protective properties of SiO2. The Stöber method enables precise control over shell thickness, ensuring optimal performance and durability. By mitigating photocorrosion while maintaining UV absorption efficiency, these hybrid nanostructures offer a robust solution for long-term UV protection. Future research may explore further refinements in shell composition and surface functionalization to expand their applicability in advanced materials.