Synthesis,Phase Composition And Luminescence Property Of Rare Earth Molybdate Red Phosphors

Red phosphors are key materials for the improvement of color rendering index of LED lighting, and play an important role in white LED energy-efficient lighting devices. Among them, rare earth doped molybdate red phosphors have attracted widespread attention due to their high stability and high luminescence efficiency. However, the phase composition of molybdate is changeable with synthesis conditions, which leads to a significant impact on their luminescent properties. The aim of this study was focused on the exploitation of red phosphors with high luminescence efficiency by tuning the phase composition through the synthesis condition optimizing.Firstly, a series of zinc europium molybdate red phosphors have been prepared using a simple chemical co-precipitation method, and their structure, photo luminescent (PL), particle size and morphology were characterized. It was found that the excitation spectra of as-synthesized phosphors are attributed to the charge transfer absorption band of molybdate and the f-f characteristic emission of Eu3+at395nm and466nm, and the emission spectra of Eu3+display the transition of5D0-7Fj with main peak at613nm. The as-synthesized samples are composed of two phases, one is the fine crystalline Zn5Mo2O11.5H2O, another is an unknown phase with poor crystal. Strengthening stirring or prolonging aging time can reduce the phase content Zn5Mo2O11.5H2O and decrease the fluorescence intensity as well as the particle size, indicating that Zn5Mo2O11.5H2O has more significant contribution on the fluorescence intensity and can transfer to the unknown phase during the aging process. The optimal synthesis conditions regarding to the fluorescence intensity were determined to be pH value7.0-8.0, Eu3+doping concentration25%, reaction temperature35℃. Charge compensation can effectively improve the luminous intensity of phosphors and Li+is found to be the best one. SEM results showed that the morphology of samples prepared by co-precipitation is irregular shape, however, the particles prepared under the existing of PVP is hexagonal sheet with particle size ranging from2to4μm, illustrating that the addition of dispersants can influence on the morphology of phosphors.Zn0.5MoO4:Eu0.25Li0.25red phosphor synthesized by high temperature solid state can be effective excited by395nm near-ultraviolet and466nm blue-ray, and emit the pure red color at615nm. However, it is difficult to obtain phosphor with pure phase and the doped concentration of europium is high. So a series of Bi3+, Sm3+, Y3+and Gd3+co-doped phosphor were synthesized by high temperature solid-phase method to investigate the influence of partial replacement of Eu3+by Bi3+, Sm3+, Y3+or Gd3+and charge compensation replacement of2Zn2+with Gd3++Li+on the phase composition, luminescence intensity. The results demonstrated that partial replacement of Eu3+by Bi3+, Y3+or Gd3+does not lower the fluorescence intensity, which is beneficial for the synthesis of phosphors with high efficiency and low Eu3+doping concentration. The optimal percentage of Bi3+for replacing Eu3+is8%. At the same time, a remarkable red shift was observed in the emission spectrum of sample with co-doping of Y3+. On contract, co-doping with Sm3+resulted in the fluorescence intensity decreasing. With the replacement percentage of2Zn2+with Gd3++Li+increasing, the fluorescence intensity of zinc molybdate was increased first and then decreased, correspondingly, the phase composition varied from multiphase to single phase. This fact illustrated that the existing of few of zinc molybdate phase shows contribution to the luminescence emission, because the emission of samples with pure calcium molybdate phase is lower than that with multiphase. Therefore, it is an effective method for exploiting high efficiency luminescence materials and optimizing synthesis condition through the tuning of phase composition.