Preparation And Luminescence Properties Of Rare Earth Ions Doped NaGd（WO₄）₂ Phosphors

Rare earth tungstate materials are widely used in the field of photoluminescence due to their unique structure,excellent chemical stability,high ultraviolet absorption efficiency and self-activated luminescence properties.In recent years,the synthesis of tungstate luminescent materials with regular morphology,uniform particle size and good dispersion has become a research hotspot due to the close interrelation between the chemical and physical properties of materials and their composition,structure,shape,size and dimension.Rare earth tungstate phosphors with excellent luminescent properties can be obtained through morphology control synthesis.In this paper,a series of rare earth doped NaGd（WO₄）₂ luminescent materials were synthesized by mild hydrothermal method.The samples were analyzed and characterized by X-ray powder diffraction,scanning electron microscopy and fluorescence spectrophotometer.The effects of different synthesis condition,rare earth ion doping concentration and sensitizer on the phase structure,morphology and luminescent properties of phosphors were investigated.Moreover,the morphology formation mechanism and energy transfer mechanism were discussed in detail.The main research contents and results are as follows:（1）A series of morphology-controlled NaGd（WO₄）₂:Eu³⁺phosphors were prepared by a facile hydrothermal method in the absence of any surfactants and templates.It was found that the pH value of the precursor solution had a remarkable impact on the phase purity,structure,morphology and luminescent properties of the products.The possible formation mechanism for square plate structure was proposed based on the time-dependent experiments.Moreover,the effects of the initial pH value,reaction time and Eu³⁺concentration on the luminescent properties of NaGd（WO₄）₂:Eu³⁺phosphors were investigated in detail.The results revealed that the square plate-like microcrystals synthesized at pH=8 for 20 h showed excellent red emission under near-ultraviolet（NUV）excitation.In addition,no concentration quenching occurred when the Eu³⁺concentration was as high as 0.6.The intense red emission and high red color purity showed that NaGd（WO₄）₂:Eu³⁺microplates had promising applications as red component in NUV-excited WLEDs.（2）A series of red phosphors NaGd_0.95-x（WO₄）₂:0.05Eu³⁺,xBi³⁺（x=0,0.02,0.04,0.06,0.08）and NaGd_0.95-y（WO₄）₂:0.05Eu³⁺,ySm³⁺（y=0,0.01,0.02,0.03,0.04）were synthesized via hydrothermal process.The results indicate that the crystal structure of NaGd（WO₄）₂ is almost not changed by doping a small amount of ions,and all samples are tetragonal scheelite-type pure phase.The particles are square plate in shape,relatively uniform and well dispersive.With the introduction of Bi³⁺or Sm³⁺,the particle size increases from the original 4μm to 5μm and 6μm,respectively.The series of phosphors can be effectively excited under NUV（at394 nm）,and the strongest emission peak is located at 614 nm which corresponds to the⁵D₀→⁷F₂ transition of Eu³⁺.In addition,doping with an appropriate amount of Bi³⁺or Sm³⁺can improve the luminescence intensity and red-light purity of NaGd_0.95（WO₄）₂:0.05Eu³⁺phosphors,especially for Sm³⁺.（3）A series of single-phase color-tunable NaGd（WO₄）₂:Tm³⁺,Dy³⁺phosphors were prepared by a facile hydrothermal method.The square plate-like samples obtained at pH=8for 20 h are NaGd（WO₄）₂ pure phase with good crystallinity,uniform morphology and high luminescence intensity.Under the excitation of UV light,NGW:Tm³⁺and NGW:Dy³⁺exhibit the characteristic emissions of Tm³⁺（¹D₂→³F₄）and Dy³⁺(⁴F_9/2→⁶H_13/2),respectively.White light can be generated by appropriately combining blue emission of Tm³⁺and yellow emission of Dy³⁺.In Tm³⁺,Dy³⁺co-doped system,there is energy transfer from Tm³⁺to Dy³⁺and the main mechanism is electric dipole-dipole interaction.The luminescent color of NaGd（WO₄）₂:0.01Tm³⁺,xDy³⁺phosphors can be tuned from blue to white and then to yellow by changing Dy³⁺ions concentration.Among them,the color coordinates（0.3279,0.3277）of NaGd（WO₄）₂:0.01Tm³⁺,0.04Dy³⁺phosphor are the closest to the standard white light（0.33,0.33）.