Abstract:

The rutile TiO₂ nanoparticles were encapsulated with silicon dioxide （SiO₂） film by sol-gel method. The preparation conditions were optimized by investigating the dosage of polyvinylpyrrolidone （PVP）, the molar ratio of silicon to titanium, and the dosage of aqueous ammonia. The optimized preparation conditions are as follows: the mass of TiO₂ is 0.3 g, PVP dosage is 0.4 g, the molar ratio of silicon to titanium is 2∶1, and the dosage of aqueous ammonia is 5 mL. The SiO₂-TiO₂ composite nanoparticles with small size, thin SiO₂ coatings and good monodispersity were thus obtained. The modified nanoparticles were characterized by FT-IR, XRD, SEM, TEM and UV-Vis. The results show that, the vibration absorption peak of Si-O-Ti bond appears at 963 cm^-1 in IR spectrum of SiO₂-TiO₂ composite nanoparticles, which confirms that the surface of TiO₂ nanoparticles are coated by SiO₂ though covalent binding between Si-OH and Ti-OH to form Si-O-Ti. XRD analysis indicates that, several diffraction peaks attributed to rutile TiO₂ still exist after the superficial modification of TiO₂ nanoparticles by SiO₂, and no new diffraction peaks exist. The TiO₂ nanoparticles before and after modification are both rutile type. SEM and TEM show that the modified composite nanoparticles are more regular, and the particle size is approximately 50 nm. Moreover, the agglomeration phenomenon of composite nanoparticles is weakened, and the SiO₂ film of thickness of approximately 10 nm is formed on the surface. Therefore, the surface of TiO₂ nanoparticles has been successfully modified by SiO₂ coatings. The photocatalytic activity of TiO₂ nanoparticles and SiO₂-TiO₂ composite nanoparticles was tested by photodegradation of methylene blue as a probe reaction. The results show that, after surface modification, the photocatalytic activity of composite nanoparticles is effectively inhibited, and the safety of SiO₂-TiO₂ composite nanoparticles as a sunscreen is greatly improved. The results of UV reflectance show that, after surface modification, the UV transmittance of SiO₂-TiO₂ composite nanoparticles has been decreased, i.e., the UV blocking ability is enhanced. After introducing the composite nanoparticles into a basic formula of sunscreen, the SPF value of SiO₂-TiO₂ composite nanoparticles reaches 43.76, which is increased by 40% than that by adding TiO₂ nanoparticles into the basic formula of sunscreen, and the sun protection performance is significantly improved.

Key words: titanium dioxide nanoparticle, silica, sunscreen performance, composite nanoparticle, surface modification