Preparation And Fluorescence Properties Of Rare Earth Ion-doped Lutetium Oxysulfate/oxysulfide

Rare earth oxysulfate and rare earth oxysulfide are used as efficient phosphor matrix materials with high luminescence efficiency,high chemical stability,high melting point and high homogeneity.Lutetium,as the last element in the lanthanide family,has a 4f electron layer full of electrons,its physicochemical properties are stable and with a high density,and it has great potential and wide applications in X-ray sensitizing screens,scintillator materials,X-ray computed tomography（X-CT）,etc.Therefore,it is important to investigate the preparation method and luminescence properties of Lu₂O₂SO₄:Eu³⁺/Lu₂O₂S:Eu³⁺luminescent materials.The main research of this thesis is summarized as follows.Lutetium oxide,nitric acid,europium nitrate,urea and ammonium sulfate were used as starting materials,where urea was used as a precipitant to provide hydroxyl and carbonate functional groups and ammonium sulfate provided the sulfur source.The pompon-like spherical Lu₂O₂SO₄:Eu³⁺phosphor with a particle size of about 800 nm was successfully prepared by a simple homogeneous precipitation method.The synthesis mechanism and photoluminescence properties of Lu₂O₂SO₄:Eu³⁺were investigated,and the formation mechanism of the pompon-like structure was elaborated from the perspective of dissolution-recrystallization and self-assembly.The result shows that the optimal ratio between Lu³⁺,（NH₄）₂SO₄ and CO（NH₂）₂ is 1:25:50,the optimal calcination temperature is 800°C,and Lu₂O₂SO₄:x%Eu³⁺shows the strongest red luminescence at 620 nm under 396 nm UV excitation,which is attributed to the ~5D₀-~7F₂ transition of Eu³⁺,the optimal doping concentration is 6.0%,and its CCT（color correlation temperature）is 1915K.In addition,the fluorescence decay curve of the phosphor can be well fitted with a double exponential function,and the average fluorescence lifetimeτis 39.558μs.A simple co-precipitation method was proposed to prepare“universal”precursors for the synthesis of Lu₂O₂SO₄ and Lu₂O₂S,respectively,using lutetium oxide,europium nitrate,nitric acid,ammonium sulfate,ammonia,sodium carbonate and sublimed sulfur as the main raw materials,and on this basis,the quasi-spherical like Lu₂O₂SO₄:Eu³⁺and Lu₂O₂S:Eu³⁺with a particle size of about 1μm were synthesized,respectively.Among them,Lu₂O₂SO₄ phosphor was synthesized by calcining the precursor at 800℃in air,while Lu₂O₂S was synthesized by mixing the precursor,sodium carbonate,sublimed sulfur and then transferring them to a closed crucible for calcination at 500℃.The result shows that the optimal doping ratio of Lu³⁺to（NH₄）₂SO₄ during the co-precipitation process is 2:50,and the optimal addition of 1.5 M NH₃·H₂O is 22 m L.The obtained precursors are amorphous structures containing Lu³⁺,OH^-,SO₄^2-and some crystalline water.The PL results showed that the optimal Eu³⁺doping concentrations of Lu₂O₂SO₄:x%Eu³⁺phosphor and Lu₂O₂S:y%Eu³⁺phosphor were 3.0%and6.0%,respectively.In addition,the Lu₂O₂S:6.0%Eu³⁺phosphor with mixed rod-like and near-spherical morphology has stronger red luminescence intensity,shorter afterglow lifetime（12.650μs）and CCT value（1738 K）than the Lu₂O₂SO₄:3.0%Eu³⁺phosphor with bulk-like morphology.Based on the above two synthesis methods,a modified co-precipitation method and an improved homogeneous precipitation method were proposed to synthesize the“universal"precursors of Lu₂O₂SO₄ and Lu₂O₂S respectively,using lutetium oxide,sulfuric acid,europium nitrate,ammonia,urea,sodium carbonate and sublimed sulfur as the main raw materials,and the experimental steps were simplified.Based on this method,Lu₂O₂SO₄:Eu³⁺and Lu₂O₂S:Eu³⁺phosphors were synthesized and their photoluminescence properties,formation mechanism were investigated.The result shows that the optimal calcination temperatures of Lu₂O₂SO₄:x%Eu³⁺phosphor and Lu₂O₂S:x%Eu³⁺phosphor are consistent with the previous results,where Lu₂O₂SO₄:x%Eu³⁺was obtained by calcination at 800°C for2 h under air atmosphere and Lu₂O₂S:x%Eu³⁺was obtained by calcination at500°C for 2 h under a confined sulphuration atmosphere created by the solid-state method.In addition,its PL luminescence exhibited the typical red-light emission of Eu³⁺with the optimal doping concentration of 7.5%.At the same doping concentration,the similarly morphology Lu₂O₂SO₄:7.5%Eu³⁺phosphor have higher luminescence intensity,afterglow lifetime（1.664 ms）and CCT value（2669 K）compared with Lu₂O₂S:7.5%Eu³⁺phosphor.Their fluorescence decay curves are well fitted with single exponential functions.