Synthesis and UV-shielding properties of calcia-doped ceria nanoparticles coated with amorphous silica

Nanoparticles of calcia-doped ceria were prepared by adding CeCl3–CaCl2 mixed solution and NaOH solution simultaneously in distilled water followed by oxidation with hydrogen peroxide solution at 40 °C and pH 12. After that, amorphous silica was coated by dispersing them in Na2SiO3 aqueous solution at pH 10, followed by decreasing the solution pH to 5. Doping with calcia resulted in a considerable decrease in particle size to form nanoparticles of ceria (2–4 nm in diameter) possessing excellent UV ray (λ<400 nm) absorption ability and high transparency in visible light (λ>400 nm). Coating calcia-doped ceria nanoparticles with amorphous silica was effective in depressing particle agglomeration. The catalytic activity for the air oxidation of castor oil substantially decreased by both doping with calcia and coating with amorphous silica. The photocatalytic activity of ceria was much smaller than that of titania and zinc oxide. Although as-prepared ceria was yellow, the powders calcined above 700 °C were white. These findings revealed that nanoparticles of calcia-doped ceria coated with amorphous silica can be safely used to provide high UV-shielding ability while maintaining a natural appearance when they are applied to the skin of humans.