Morphology-controlled Synthesis Of Dy³⁺/Eu³⁺ Doped Nano/micro Structure Tungstate And Molybdate And Photoluminescence Mechanism Study

Due to high melting point,high refractive index,long afterglow time and other characteristics,scheelite structured tungsten and molybdate were used in solid scintillator,microwave,optical fiber,catalysis,medicine,high energy physics and other applications.Especially in photoluminescence,as their wide intrinsic excitation spectra,stable emission spectra,rare earth doped tungsten and molybdate could effectively absorb NUV?near ultraviolet?light emitted from semiconductor chips transferring energy to the rare earth ions and generate efficient luminescence.Therefore,rare earth doped tungsten and molybdate are attracting more and more scientific researchers' interest.In this study,tetragonal scheelite structured CaWO₄,CaMoO₄,BaWO₄ and BaMoO₄ were selected as matrixes of the phosphors.Functional nano/micro-structured rare earth Dy³⁺ and Eu³⁺ ions doped tungsten and molybdate were fabricated.First principles study was used to calculate the band structure and density of states?DOS?of these four tungsten or molybdate.The results showed that the bands gaps are around 5.0 eV and the tungstate has larger band gap than the molybdate.The band structure o f CaWO₄ and CaMoO₄ are similar,with flat top of conduction band and bottom of valence band where the electrons and holes are hard to transit with large effective mass.O n the contrary,the band structure o f BaWO₄ and BaMoO₄ are similar with fluctuant top of conduction band and bottom of valence band where the electrons and holes with small effective mass are easy to transit.Different arrangement of outer-shell electrons leads to diversity of optical properties as the calculated static optical dielectric constant of CaWO₄ and CaMoO₄ are 3.10 and 3.42 greater than the ones of BaWO₄ and BaMoO₄ 2.78 and 2.98,respectively.Different morphologies of tungstate and molybdate were prepared by precipitation or hydrothermal method.Systematic researches have been taken with variation of the reactants concentration,the reaction temperature and p H value of the solution,and intrinsic excitation and emission performance of tungsten and molybdate were taken.Based on the analys of the kinetics and thermodynamics condition of the reaction,the tungsten and molybdate self-assembly growth models were established.Under the orientation attachment driven,the nanoparticles nucleated in precipitation arranged orderly to form needle-like structures?second particle?which arranged parallel forming rod-like structures.Driven by the crystallization cooperation force,the surfaces of rod-like structures with high free energy were fused together to form a dumbbell-like structure.These structures grew crossly to form flower-like structures.The smaller flower-like structures gradually disintegrated into discrete rod-like particles or nanoparticles because of Ostwald ripening mechanism,while large flower-like structures adsorbed nanoparticles to form twinned hemisphere structures.These structures grew gradually fusing the dividing line between the hemispheres slowly and finally grew into spherical nano/micro-structures.Without any surfactant or other organic additive,heterogenous core-shell structured CaWO₄@CaWO₄:Dy³⁺ and CaWO₄:Eu³⁺@CaWO₄:Dy³⁺ microspheres were synthesized via a facile hydrothermal method for the first time.The growth mechanism model of the core-shell structure was proposed.Under the high pressure and temperature of hydrothermal condition,the hollow microspheres broke into small crystals while the solid ones maintained its spherical morphology.The atoms in the surface of these small crystals were inclined to leave the surface and redissolve in the solution under the Ostwald ripening mechanism.Consequently,new surface atoms would expose and dissolved which resulted in the whole small crystal melted.The ion concentration of the solution would raise and reach saturation.Then the ions congealed on the surface of the solid microspheres and self-assembled the heterogeneous shell structure in order to reduce the surface energy of the whole system.The whole process is dissolution-recrystallization dominated by Ostwald ripening mechanism.In the heterogeneous CaWO₄@CaWO₄:Dy³⁺ core/shell structure,the distance between the luminescent lanthanide ions is increased and the surface quenchers are decreased,thus reducing the nonradiative pathways and suppressing the energy quenching in the energy transfer process.The emission intensity of core-shell structured phosphor was increased by 273% compared with the directed doped one with same doping concentration while the doping concentration of Dy³⁺ ions was increased from 5 mol% to 10 mol%.The emission spectrum of core-shell structured CaWO₄:Eu³⁺@CaWO₄:Dy³⁺ phosphor exhibited characteristic energy transition of Dy³⁺ and Eu³⁺ ions.The chromaticity and emission strength can be adjusted by doping different rare earth ions separately in the core and shell region.Anisotropic structured Ba WO₄ and BaMoO₄ crystals are prone to forming at high reaction temperature and acidic solution conditions as shuttle-like structures are found in our work.The difference among crystal orientations would be weakened under alkaline solution condition,which gives rise to the formation of octahedral-like structure.Uniform and well-dispersed BaMoO₄ microspheres can be got by hydrothermal method with the addition of citric acid as surfactant agent.In short,tungsten and molybdate are excellent matrix materials for rare earth doped phosphor.The heterogeneous core-shell structure can not only effectively reduce the usage amount of rare earth,but the luminescence intensity of the material has also been significantly improved.Heterogeneous core-shell structure of Dy³⁺ doped tungsten and molybdate has great application potentials as white LED phosphor.