Nanocrystalline Y₂O₃:Eu Phosphors Prepared by Alkalide Reduction

(Y 0.95 Eu 0.05 ) 2 O 3 nanoparticles have been synthesized by subambient homogeneous reduction using alkalide solutions and subsequent oxidation. As synthesized, the material consists of free-flowing agglomerates of ill-defined, amorphous, or subnanocrystalline nanoparticles. Samples annealed at 500 °C or greater are crystalline, consisting of agglomerated nanocrystals. As the annealing temperature is raised from 500 to 900 °C, the nanocrystals grow from an average of 10.9 to 14.2 nm, the agglomerates decrease in size, and the surface area increases from 49 to 59 m 2 /g. The crystallite size determined by X-ray diffraction line-broadening correlates well with TEM observations, indicating that the particles are individual nanocrystals. The increase in the surface area is consistent with the uncovering of the surfaces of the nanocrystals as a result of the reduction of the size of the agglomerates, counteracting the loss of surface area due to crystallite growth. Annealing at 1000 °C results in growth of the nanocrystals to an average size of 25.1 nm and a decrease in the surface area to 35 m 2 /g. Photoluminescence spectra do not show any fluorescence from samples until they have been annealed at 500 °C or higher. Visual inspection found fluorescence from a small fraction of samples annealed at temperatures as low as 200 °C. The photoluminescence intensity of the 5 D 0 → 7 F 2 transition increases with annealing temperature from 500 to 700 °C, levels-off between 700 and 900 °C, then dramatically increases after annealing at 1000 °C. This trend mirrors that seen for the cell parameter for this cubic system, which decreases from 10.639 to 10.615 Å, larger than the 10.604 Å expected for bulk material. The expanded cell volume may play an important role in the low quantum yields found for these nanocrystalline samples (<8%).