Synthesis and UV-shielding properties of silica-coated calcia-doped ceria nanoparticles via soft solution processes

White nanoparticles of calcia-doped ceria were prepared from the precipitate by reacting CeCl₃-CaCl₂mixed solution with NaOH solution at pH 12 and the oxidation with hydrogen peroxide solution at 40°C, followed by the calcination at 700°C for 1 h. The sample before calcination contained significant amount of OH^- in the lattice and was yellow, but the powders calcined above 700°C were white, indicating that cation defect formed by replacing O^{2 -} with OH^- played as the color center. It is confirmed that calcia-doped ceria showed much lower photocatalytic activity as well as lower generation of singlet oxygen under UV light irradiation than those with titania and zinc oxide. Calcia-doped ceria particles were coated with amorphous silica by means of sol-gel reaction technique using hydrolysis of tetraethylorthosilicate (TEOS) or acid hydrolysis of sodium silicate. The silica coating by sol-gel reaction with TEOS was much more efficient for the reduction of catalytic activity of ceria for the oxidation of organic materials without loss of UV-shielding ability than that by acid hydrolysis of sodium silicate.