Luminescence Studies and Judd–Ofelt Analysis on SiO2@LaPO4:Eu@SiO2 Submicro-spheres with Different Size of Intermediate Shells

The novel submicro-spheres SiO 2 @LaPO 4 :Eu@SiO 2 with core-shell-shell structures were prepared by connecting the SiO 2 submicro-spheres and the rare earth ions through an organosilane HOOCC 6 H 4 N(CONH(CH 2 ) 3 Si(OCH 2 CH 3 ) 3 (MABA-Si). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (IR). It is found that the intermediate shell of the submicro-spheres was composed by LaPO 4 :Eu nanoparticles with the size of about 4, 5–7, or 15–34 nm. A possible formation mechanism for the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres has been proposed. The dependence of the photoluminescence intensity on the size of the LaPO 4 :Eu nanoparticles has been investigated. The intensity ratios of electrical dipole transition 5 D 0  →  7 F 2 to magnetic dipole transition 5 D 0  →  7 F 1 of Eu 3+ ions were increased with decreasing the size of LaPO 4 :Eu nanoparticles. According to the Judd-Ofelt (J-O) theory, when the size of LaPO 4 :Eu nanoparticles was about 4, 5–7 and 15–34 nm, the calculated J-O parameter Ω 2 (optical transition intensity parameter) was 2.30 × 10 −20 , 1.80 × 10 −20 and 1.20 × 10 −20 , respectively. The increase of Ω 2 indicates that the symmetry of Eu 3+ in the LaPO 4 lattice was gradually reduced. The photoluminescence intensity of the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres was unquenched in aqueous solution even after 15 days.