| China has the world's largest rare earth resources, and cerium accounts for 28% of total rare earth, so it is very benificial to develop CeO2-based functional materials. Nanoparticles of ceria (CeO2) has unique characteristics which make it be used as broad-spectrum ultraviolet shielding agent, that's because the 4f electron of cerium makes it sensitive to ultraviolet absorption. However, because of its high catalytic ability in the oxidation of organic materials, it has been rarely large-scale used in ultraviolet shielding field. In this work, the co-doption of Ca2+ or Sr2+ ions which have larger ionic size than that of Ce4+ ions, and F- ions which have smaller ionic size than that of O2- anions into ceria modifies ceria's crystal lattice structure, increase the radius ratio of cation/anion, and makes the ionic size ratio nearer to 0.732(the ideal r(Mn+)/r(O2-) ionic size ratio in eight coordinations oxide:MO8), consequently stabilizing the crystal lattice structure of ceria, and greatly decreasing the oxidation catalytic activity of ceria. Furthermore the change of the lattice parameters, the ultraviolet shielding properties, the transparent to visible light, and the oxidation catalytic activities, resulting from crystal modification, is also researched. The principal results are as follows:1. F-/Ca2+ co-doped ceria and F-/Sr2+ co-doped ceria nanoparticles were synthesized by coprecipitation. X-ray diffraction revealed that the two type co-doped ceria samples all have the cubic fluorite structure, doping of ions increased the lattice parameters, and the increased parameters depends on the amount of doped ions. The solubility limit of F-/Ca2+ co-doped ceria has been estimated to be 14%Ca+28%F(mol%), and its lattice parameter is 5.4190A. The solubility limit of F-/Sr2+ co-doped ceria is 9%Sr+18%F(mol%) and the lattice parameter is 5.4231 A. SEM results showed that two types co-doped cerias are 50～100nm particals, and F-/Sr2+ co-doped ceria is spherical particals, while the morphology of F-/Ca2+ co-doped ceria change with the doping amount of fluorine and calcium.2. The ultraviolet shielding properties of F-/Ca2+ co-doped ceria and F-/Sr2+ co-doped ceria were better than undoped ceria. Doped ions in ceria reduced the transition energy of electron from forbidden band to conduction band, which narrows the forbidden band and causes red-shift in UV absorption edge. The values of band gap energy were reduced to 2.94eV for 14%Ca+28%F(mol%) co-ceria and 2.78eV for 9%Sr+18%F(mol%) co-ceria.The visible light transmittance of F-/Ca2+ co-doped ceria decreased slightly, but that of F-/Sr2+ co-doped ceria increased slightly.3. The oxidation catalytic activities of F-/Ca2+ co-doped ceria and F-/Sr2+ co-doped ceria were lower than that of undoped ceria, that's because co-doping of cations and anions reduced the transition tendency of Ce4+ to Ce3+ in ceria, and thus the fluorite structure of ceria was stabilized, thereby reducing the oxidation catalytic activities. The oxidation catalytic activities of F-/Ca2+ co-doped ceria reduced in a larger extent than that of F-/Sr2+ co-doped ceria.4. Codoping of F-/Ca2+ or F-/Sr2+ can inhance the ultraviolet shielding properties, reduce the oxidation catalytic activities, which can extentd the application of ceria in ultraviolet shielding materials.
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