Synthesis And Characterization Of Core@ Shell Trichromatic Phosphors Using Submicron SiO₂ Particle As Core

In nowdays,rare-earth luminescence material is still one of the most important optoelectronic functional materials.It is important to develop core@shell rare-earth phosphors with low cost and high performance so that to improve the utlization efficency of rare-earth resource in our nation.In this thesis,the principle and development of rare-earth phosphors were reviewed,and the design idea of core@shell structure was proposed to develop rare-earth phosphors including the SiO₂@SrTiO₃:Pr,blue SiO₂@CaWO₄ and green SiO₂@LaF₃:Ce,Tb trichromatic luminescent materials.Above mentioned core-shell structures phosphors were synthesized by Pechini sol-gel method using mono-dispersive SiO₂ microspheres as cores.The particle sizes,morphologies, component of shells and phases of as-prepared phosphors were investigated with XRD,FE-SEM,EDS,TEM,FT-IR,PL measurement,etc.The luminescent properties of rare reath ions(Pr³⁺,Tb³⁺)and energy transfer properties from Ce³⁺to Tb³⁺in core-shell structure luminescent materials were analyzed.The relationships among preparation techniques,composition and structure of shells,structures of shells and cores,and luminescent properties of these phosphors were discussed.The optimal synthetic conditions of core-shell phosphors were obtained according to the relations of luminescent intensities,heat-treatment temperature,coating times and the PEG concentrations in the precursor.Spherical SiO₂ particles were coated with SrTiO₃:Pr phospher as shell layer by Pechini sol-gel method.The FE-SEM and TEM results indicated that SrTiO₃:Pr can be coated on SiO₂ surfaces using this technique,and the core-shell structured phosphor particles are uniform and maintain the mono-dispersive morphology of initial SiO₂ core.The thickness of shell layer increased with increased coating cycles. The PL results showed that the core-shell phosphor emits red light originated from the ¹D₂→³H₄ transition of Pr³⁺ion with a maximum peak at 615 nm.Core-shell structured SiO₂@CaWO₄ phosphor was synthesized by coating mono-dispersive SiO₂ microspheres with CaWO₄ phosphor as shell layer by Pechini sol-gel method.The FE-SEM and TEM results showed that particles with core-shell structure have smooth surface,high dispersion state,spherical shape and narrow size distribution with an average size of about 300 nm.PL results showed that the phosphor emits a strong blue emission band that was suggested to be originated from the characterestic luminescence of WO₄^2-as independent luminescent centers.LaF3:Ce³⁺,Tb³⁺phosphor layer has been deposited successfully on mono-dispersed and spherical SiO₂ microspheres by Pechini sol-gel method.The FE-SEM results showed the core-shell structured phosphor maintains the morphology of SiO₂ with spherical shape and smooth surface with an average particle size of about 310 nm,nevertheless obvious aggregation occurs in this system.The XRD and EDS results indicated that SiO₂@LaF₃:Ce,Tb phosphors with core-shell structure were successfully synthesized.PL measurements indicated that under the excitation of 330 nm,four emission peaks occurred at 492 nm,544 nm,586 nm and 612 nm for SiO₂@LaF₃:Ce,Tb products,where these peaks were ascribed to the Tb³⁺⁵D₄→⁷F_J (J=6,5,4,3)transitions.The emission peak with greatest intensity is at 544 nm, which is characteristic of green light emission in the core-shell particles.Therefore,red emitted SiO₂@SrTiO₃:Pr,blue emitted SiO₂@CaWO₄ and green emitted SiO₂@LaF₃:Ce,Tb trichromatic luminescent phosphors with spherical shape, whose sizes could be tuned according to the size of core-shell particles,could be successfully synthesized using low-cost SiO₂ as core materials.It was also found that the photo-luminescent intensity of phosphors prepared by coating methods was not as good as that of phosphors prepared by solid state method,and it may be improved by controlling the core-shell particle sizes and synthetic techniques,etc.