Syntheses And Photoluminescence Properties Of Rare Earth Doped Tricolor Phosphors

Phosphor, a substance that exhibits photoluminescence（PL） phenomena, is an important class of material that has been widely used in cathode-ray tubes（CRT）, plasma display video screens, sensors, white light-emitting diodes and so on. Nowadays, rare earth（RE） doped tricolor phosphors have attracted considerable attention due to their many advantages such as energy-saving, environment-friendly, high efficiency, etc. Owing to their excellent physical and chemistry properties, Y₂O₃:Eu³⁺ and YVO₄:Eu³⁺, GdNbO₄:Tb³⁺, YPO₄:Yb³⁺ have been respectively selected as red-emitting phosphor, green-emitting phosphor, blue-emitting phosphor and the synthesis routs and PL properties were systematically studied in this dissertation. The major synthesis strategies and significant results achieved in this paper can be briefly introduced as followed:（1） Novel Y₂O₃ coated Y₂O₃:Eu³⁺ nanotubes with different coating ratios were synthesized successfully by a facile two-step process, including hydrothermal synthesis of Y（OH）₃ coated Y（OH）₃:Eu³⁺ as precursors and then calcination of them at 1000 °C for 2 hours. X-ray diffraction（XRD） patterns and field emission scanning electron microscope（SEM） images indicated these Y₂O₃ coated Y₂O₃:Eu³⁺ phosphors possess tubular nanostructures. The PL properties of Y₂O₃ coated Y₂O₃:Eu³⁺ were systematically investigated by PL spectra, and PL enhancement was observed after proper coating. In other words, the coating ratio played a crucial role in PL efficiency. When it was 1/9, the PL intensity of 5D0→7F2 emission（about 613 nm） was 60% higher than that of Y₂O₃: Eu³⁺ phosphors under 255 nm excitation. Therefore, surface coating may be an alternative route for enhanced PL properties of the Y₂O₃:Eu³⁺ red-emitting phosphor.（2） The Eu³⁺ doped YVO₄ phosphors with high-intensity PL were synthesized by a facile hydrothermal method assisted with several organic additives. The organic additives used in this paper were sodium dodecyl sulfate（SDS）, polyvinylpyrrolidon（PVP） and disodium ethylenediaminetetraacetic acid（Na₂EDTA）. Moreover, the effect of different organic additives on the microstructures, morphologies and PL properties of YVO₄:Eu³⁺ phosphors were systematically investigated by XRD, SEM and PL. By the analysis of XRD patterns, pure YVO₄:Eu³⁺ crystals could be obtained and the organic additives did not influence the crystallographic structure significantly. However, the results of SEM images demonstrated that organic additives played a key role in the formation of YVO₄:Eu³⁺ phosphors with different morphologies, including ball-like and cube-shaped morphologies. It can be concluded from PL spectra that the PL intensity has been greatly enhanced by tuning the morphology via adding different organic additives, which provides a great opportunity for systematically evaluating their PL properties, as well as fully exploring their applications in many types of color fields.（3） The GdNbO₄:xTb³⁺ phosphor with high green emission（543 nm） intensity under ultraviolet excitation（270 nm） was synthesized by the traditional high temperature solid-state reaction method with different fluxes at 1200 °C for 2 hours. Based on the analysis of PL spectra, the green emission intensity increased with increasing the content of Tb³⁺ until x = 20%, and then reduced rapidly when the content of Tb³⁺ exceed 20%, so the optimum concentration of Tb³⁺ is 20%. To further enhance the green emission intensity, fluxes（Na₂CO₃, Li₂CO₃, H₃BO₃, KCl, NaF, Na₂SO₄） were used during the preparation of the GdNbO₄:20%Tb³⁺ phosphor. The results of SEM images demonstrated that the fluxes played an important role in controlling the morphologies. When Na₂SO₄ was chosen as flux, large-scale cobblestone-shaped micro particles ranged from 1 to 5 μ m with excellent monodispersion was obtained. The emission intensity of green light was greatly influenced by the morphologies and sizes, and the highest green emission intensity was observed in the sample calcinated with Na₂SO₄, which was almost 2.3 times higher than those samples without any fluxes.（4） Blue-emitting phosphors of YPO₄ doped with Yb³⁺ were prepared by a simple hydrothermal method. All the products were characterized by XRD and transmission electron microscopy（TEM）, which revealed that they were zircon structure with leaf-like morphology. According to the analysis of PL spectra, upon ultraviolet（275 nm） excitation, the Yb³⁺ doped YPO₄ phosphor showed an intense blue emission composed of two main bands at 420 nm and 620 nm assigned to charge transfer state（CRS） → 2F5/2 and CRS → 2F7/2, respectively. Moreover, the optimum doping concentration of Yb³⁺ in YPO₄ phosphor was 1%, which exhibited the maximum emission intensity. The possible physical mechanism of concentration quenching was discussed, and the critical transfer distance determined to be 23.889 ?. In particular, the color purity of the as-synthesized Yb³⁺ doped YPO₄ phosphor was as high as 83%, which made it an excellent candidate of blue-emitting materials for RE doped tricolor phosphors.In conclusion, the red phosphors of Y₂O₃:Eu³⁺@Y₂O₃ core/shell structures and YVO₄:Eu³⁺ with ball-shaped micro particles, green phosphors of GdNbO₄:Tb³⁺ with cobblestone-shaped micro particles, and the blue phosphor of YPO₄:Yb³⁺ with high color purity were prepared in this thesis and the synthesis strategy here may provide a new way to researchers for enhancing the PL intensity of RE doped tricolor phosphors.