Preparation And Properties Of NaYF₄ And Y₂O₃ Luminescent Materials Doped With Rare Earth Ions Via Hydrothermal Synthesis

In this experiment, two kinds of phosphors, NaYF₄, Yb³⁺, Ln³⁺(Ln = Er³⁺, Tm³⁺) and Y₂O₃: Eu³⁺, were prepared via hydrothermal synthesis. The effect of reaction temperature, reaction time, pH value and dopant content of the rare earth ions on the luminescent property was systematically focused.The hydrothermal synthesis route is applied to prepare NaYF₄:Yb³⁺, Er³⁺ phosphors. With improving reaction temperature and prolonging reaction time, more hexagonal-phase of NaYF₄:Yb³⁺, Er³⁺ phosphors were obtained and stronger light was observed. Optimal condition for the phosphors was obtained when the concentration of Er³⁺ ions was 2mol% and pH value was 4-5. An amount of F^- ions can be supplied by adding NH₄HF₂ and NaF in the hydrothermal synthesis system. The sufficient F^-ions act as mineralization to obtain more hexagonal-phase phosphors and increase the saturation of the phosphors in the water. The downconversion luminescent spectrum of the phosphors obtained was also measured. The maximal excitation wavelength and emission wavelength is 488nm and 540nm, respectively. The luminescence intensity of the samples obtained by hydrothermal method can be remarkably improved by means of annealing at 500℃for 30min. When the ligand of EDTA or sodium citrate was applied, corresponding morphology of the NaYF₄:Yb³⁺, Er³⁺ phosphors was prism or hexagonal flake. Effect of the hydrothermal conditions on luminescence properties is discussed and optimal condition is obtained by the orthogonal experiment.NaYF₄:Yb³⁺, Tm³⁺ upconversion phosphors were also obtained via hydrothermal route. The strongest blue light of the samples was observed when the concentration of Tm³⁺ ions was 0.2mol%, the concentration of Yb³⁺ ions was 40mol%, and pH value was 4. The downconversion luminescent spectra of the NaYF₄:Yb³⁺, Tm³⁺ phosphors obtained by hydrothermal method was also measured. The maximum excitation wavelength and emission wavelength was 447nm and 541nm, respectively. When the ligand of EDTA or sodium citrate was applied, morphology of the NaYF₄:Yb³⁺, Tm³⁺ phosphors was spheroid or prism. To optimize the experimental parameters of hydrothermal synthesis on the intensity of blue light, the data from the orthogonal experiment were treated with range analysis and variance analysis. The result reveals the concentration of Yb³⁺ ions is the important and notable factor for the intensity of blue light.Y₂O₃:Eu³⁺ phosphor particles were synthesized by heat-treating the precursor obtained by hydrothermal method. The optimal conditions for the precursor was synthesized at 200℃for 12h via hydrothermal route, when pH value was 10 and the concentration of Eu³⁺ ions was 7mol%. After the heat-treatment, the final product was cubic-phase Y₂O₃. The morphology of the phosphors was uniform stick and its length was almost 2μm when heat-treated at 1100℃for 3h. The maximum excitation wavelength and emission wavelength of Y₂O₃:Eu³⁺ phosphors is 254nm and 609nm, respectively. The luminescence intensity of Y₂O₃:Eu³⁺ phosphors can be improved by the addition of Li₂CO₃ as flux, and the luminescence intensity was obviously enhanced when the weight ratio of Li₂CO₃ was 2%.Based on the above results, grain growth mechanism of NaYF₄ crystals via the hydrothermal synthesis was also discussed at the end of this thesis.