Microstructures And Photoluminescence Of Rare Earth Ion Doped Phosphors By Self-assembly Of Nano Pseudoboehmite Composite Sol

A two-step of Spraying and Sintering of Pseudoboehmite Composite Sol (SSPCS) technique was developed to fabricate several hierarchical sphere-like phosphors for optoeletronic field. Stable pseudoboehmite composite sol was obtained using pseudoboehmite sol as dispersant, with the addition of SiO₂, Sr(NO₃) ₂, Eu(NO₃) ₃ and Dy(NO₃) ₃．The self-assembly mechanisms of ions, nanocrystals and colloidal nanoparticles consisted in the composite sol were analyzed on multi-scales of atom, nanometer and micrometer. Phosphors of Al₂O₃:Eu³⁺, SrAl₂O₄:Eu²⁺,Dy³⁺, Sr₂Al₂SiO₇:Eu³⁺ and Sr₂Al₂SiO₇:Eu²⁺ were prepared by the SSPCS technique. The granularity and morphology of the phosphors can be controlled by changing the composite sol concentration and the spraying parameters. The influences of microstructures on the photoluminescence characteristics of the phosphors were analyzed. The main results as following:1．The microstructures of pseudoboehmite colloidal particles were investigated. XRD and Field Emission Transmission Electron Microscope (FETEM) results show that pseudoboehmite powder is composed of AlOOH nanocrystals. Stable pseudoboehmite sol was obtained by adding HNO₃ as peptizing agent and adjusting pH=2．0．The peptizing mechanism was presented. HNO₃ react with AlOOH at the interfaces of AlOOH nanocrystals. The AlOOH nanocrystals were electropositive when pH=2．0 adjusted by HNO₃．Negative NO₃^-ions were then adsorbed on the AlOOH nanocrystal surface and formed electronic double layer. TEM result shows that the AlOOH colloidal particle size is～30nm.2．The crystallization mechanism for pseudoboehmite sol was different from that for nitrate solution during the spray-drying process. The crystallization of nitrate solution during spray-drying process is presumed fitting homogeneous nucleation mechanism. However, as for pseudoboehmite sol, colloidal AlOOH nanoparticles act as crystallization centres. The crystallization of pseudoboehmite sol agrees with a heterogeneous nucleation mechanism. Colloidal AlOOH nanoparticles agglomerated under van der waals and capillary forces and formed hierarchical sphere dry AlOOH gel particles in the spray-drying process.3．The interaction of Eu³⁺ ions and colloidal AlOOH nanoparticle was investigated. The self-assembly of Eu³⁺ ions on the surface of AlOOH nanocrystal was attributed to the chemical adsorption caused by electrostatic attraction. Electropositive colloidal nucleus AlOOH/Eu³⁺ were consequently formed. The photoluminescence characters of Eu in AlOOH/Eu(NO₃) ₃ composite sol system show that the adsorption density of Eu³⁺ ions on colloidal AlOOH nucleus is determined by the mol ratio of Eu³⁺ to AlOOH. AlOOH/Eu(NO₃) ₃ colloidal particles were composed of AlOOH/Eu³⁺ nucleus and NO₃^-electronic double layer. Three-dimensional hierarchical sphere dry AlOOH/Eu(NO₃) ₃ gel particles were prepared by spray-drying technique, using AlOOH/Eu(NO₃) ₃ composite sol as precursor.Hierarchical Al₂O₃:Eu³⁺ phosphor particles were obtained by sintering dry AlOOH/Eu(NO₃) ₃ gel particle at a temperature range of 600～1000℃. The effect of dopant Eu³⁺ ion on the phase transformation of AlOOH was investigated. XRD and DSC results indicated that the phase transformation temperatures ofγ-Al₂O₃→θ-Al₂O₃ andθ-Al₂O₃→α-Al₂O₃ were increased from 882℃to 1054℃and 1224℃to 1237℃, respectively, by doping 2%(mol) Eu³⁺ ion. The calculated lattice constants results indicate that Eu³⁺ ions partial substitute Al³⁺ ions inγ-Al₂O₃ andθ-Al₂O₃ lattices. The crystal growth ofγ-Al₂O₃ andθ-Al₂O₃ phases is hindered by the Eu³⁺ ions. A newly formed compound EuAl₁₂ O₁₉ was detected by XRD. The compound EuAl₁₂ O₁₉ possibly resultes in the increase of theθ-Al₂O₃→α-Al₂O₃ phase transformation temperature.The photoluminescence spectra of Al₂O₃:Eu³⁺ phosphor was detected. The emission of 578nm, 588nm(593nm) and 616nm are attributed to the transition from ⁵D₀ to ⁷F₀, ⁷F₁ and ⁷F₂ in Eu³⁺ ions of Al₂O₃:Eu³⁺ phosphor, respectively. The emission of ⁵D₀→⁷F₂ transition lies at 617 nm, which is same to the emission of Eu³⁺ ion inα-Al₂O₃:Eu³⁺ nanoparticle and has a red shift of 5nm compared with the emission of Eu³⁺ ion in bulk Eu₂O₃． 4．Spheric SrAl₂O₄:Eu²⁺_0．02 ,Dy³⁺_0．04 afterglow phosphor was prepared by the SSPCS technique, with addition of a small amount of H₃PO₄ as flux. The SrAl₂O₄:Eu²⁺_0．02 ,Dy³⁺_0．04 phosphor particles were unnecessary of further ball-grinding and with a mean particle size of～5μm. The spectra study indicated that the emission spectra of the SrAl₂O₄:Eu²⁺_0．02 ,Dy³⁺_0．04 phosphor was a broad band spectra peaking at 516 nm. The trap depth of the SrAl₂O₄:Eu²⁺_0．02 ,Dy³⁺_0．04 phosphor was calculated to be 0．589eV.5．Substrate Sr₂Al₂SiO₇ and phosphors Sr₂Al₂SiO₇:Eu³⁺ and Sr₂Al₂SiO₇: Eu²⁺ were prepared by the SSPCS technique. The reaction mechanism for synthesization of substrate Sr₂Al₂SiO₇ was investigated. The dry gel particles were consisted of AlOOH, SiO₂ and Sr(NO₃) ₂．On heating, the components AlOOH and Sr(NO₃) ₂ decomposed and reacted with each other. Two compounds SrAl₂O₄ and Sr₂SiO₄ were synthesized when the sintering temperature was below 1000℃. XRD results showed that pure Sr₂Al₂SiO₇ compound was composed when the sintering temperature was above 1200℃.6．The photoluminescence characteristics and mechanisms of the Sr₂Al₂SiO₇:Eu³⁺ phosphors obtained with different sintering temperatures were examined. The emission of 578nm, 593nm and 616nm are attributed to the transition from ⁵D₀ to ⁷F₀, ⁷F₁ and ⁷F₂ in Eu³⁺ ions of Sr₂Al₂SiO₇:Eu³⁺ phosphor, respectively. The ratio of intensity of 613nm peak to that of 578nm peak was defined as k. The value of k increased with the increasing of sintering temperature from 1000℃to 1400℃. It indicates that the crystal symmetry of substrate Sr₂Al₂SiO₇ enhances with the increasing of sintering temperature.7．The photoluminescence characteristic and mechanism of the Sr₂Al₂SiO₇:Eu²⁺ phosphor was detected. XRD result and the calculated lattice constant indicate that the Eu³⁺ ions partial substitute on Sr²⁺ sites in Sr₂Al₂SiO₇ lattice. The emission peak centered at 500nm observed in the Sr₂Al₂SiO₇: Eu²⁺ phosphor was considered arise from the transition of 4f⁶ 5d¹â†’4f⁷ in Eu²⁺ cation.The substitution mechanisms of Mg²⁺ ,Ca²⁺ and Ba²⁺ ions for Sr²⁺ in Sr₂Al₂SiO₇ lattice were respectively discussed. The effects of the substitutions on the microstructures and the photoluminescence characteristics of Sr₂Al₂SiO₇:Eu²⁺phosphors were researched. It indicates that the crystal field intensity of Sr₂Al₂SiO₇ decreases when the Sr²⁺ site is substituted with Mg²⁺ or Ca²⁺ ion. which size are smaller than that of Sr²⁺ ion. The split of 5d energy level of Eu²⁺ decreases and the emission spectra of peak Sr₂Al₂SiO₇:Eu²⁺ phosphor blue shifts to 470nm or 496nm when substitute Sr²⁺ with Mg²⁺ or Ca²⁺ ions。On the contrary, for larger ion, the substitution of Ba²⁺ for Sr²⁺ results in increasing of the crystal field intensity of Sr₂Al₂SiO₇．The split of 5d energy level of Eu²⁺ increases. The emission spectra of Sr₂Al₂SiO₇:Eu²⁺ phosphor shift to 520nm.