Synthesis And Photoluminescence Study Of Bismuth Silicate Rear-earth Red-emitting Materials

The ubiquitous items such as family lighting,LED display,traffic lights and car lights in the daily life mark the pivotal social status of LED lights.With the continuous improvement of people’s needs,the white LED which is closer to natural light has become one of the hot spots of LED research today.Due to the lack of reconciliation of red light,the current three main methods of synthetic white LED have the problem of high color temperature and low color rendering index.Therefore,to develop an efficient and highly stable red phosphor is one of the key factors to promote the development of white LED.Bismuth silicate has been widely concerned by the majority of scholars,because of its good thermal stability and chemical stability,it has good fluorescence properties when doped with rare earth and is particularly suitable for LEDs.However,it is necessary to find a method and process which is most suitable for the preparation of bismuth silicate phosphor with excellent properties.It is the key to the application of LED and the focus of this paper.In this paper,Bi₄Si₃O₁₂:Sm³⁺phosphors were synthesized respectively by solution combustion and chemical solution decomposition（CSD）,the effects of several factors on the synthesis of phosphors were discussed and the synthesis processes were optimized,such as the effects of preparation temperature,heat preservation time,bismuth silicon ratio and so on.In addition,it is expected that the fluorescence intensity will be further increased by co-doping Pr³⁺and Dy³⁺ions.The synthesized samples were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,infrared spectroscopy（FTIR）and fluorescence spectroscopy.The effects of phase composition,fluorescence property,energy transfer process and ions co-doping on the luminescent properties of the samples were discussed in detail.In the end,the advantages and disadvantages of the different methods to prepare Bi₄Si₃O₁₂ phosphors were compared and summarized briefly.（1）Two kinds of phosphors Bi₄Si₃O₁₂:Sm³⁺and Bi₄Si₃O₁₂:Sm³⁺,Pr³⁺were prepared by solution combustion method.The optimum calcination temperature was900℃and the holding time was 1 hour.The optimum ratio of citric acid was Bi₄Si₃O₁₂:C₆H₈O₇=1,and the amount of nitric acid only needs a small range of more than the theoretical value.In addition,with the gradually increasing of doping ion concentration,and the gradually increasing of luminescence intensity of the phosphor,when the concentration of the Sm³⁺ions were x=4mol%,the luminescence intensity was the strongest.The strongest peak was at 607nm,due to the transition of ⁴G_5/2 to⁶H_7/2 of Sm³⁺,when more than 4mol%fluorescence intensity gradually decreased,and led to the phenomenon of concentration quenching.（2）Bi₄Si₃O₁₂:Sm³⁺,Pr³⁺phosphors were prepared by co-doping Pr³⁺ions.By doping different concentrations of Pr³⁺ions,it was found that when the Sm³⁺ions concentration were 4mol%and the Pr³⁺ions concentration were 0.1mol%,the luminescence intensity was the highest.The strongest excitation peak at 403nm belongs to the ⁶H_5/2 to ⁴F_7/2 transition of Sm³⁺,which is suitable for white light LED excited by near ultraviolet or blue light LED chips.and the strongest emission peak at607 nm due to the ⁴G_5/2 to ⁶H_7/2 transition of Sm³⁺ions.In addition,it was found that there was energy transfer between Sm³⁺and Pr³⁺ions in the direction of Pr³⁺ions to Sm³⁺ions.In the end,due to the co-doping of Pr³⁺ions,the color coordinates of the Bi₄Si₃O₁₂:Sm³⁺phosphors were shifted toward the red region.（3）Bi₄Si₃O₁₂:Sm³⁺and Bi₄Si₃O₁₂:Sm³⁺,Pr³⁺phosphors were prepared by chemical solution decomposition.the pure Bi₄Si₃O₁₂ phase could be successfully prepared when the Bi³⁺:Si⁴⁺=1:2,the calcination temperature was 800℃and the holding time was 1 hour.In addition,when the concentration of the Sm³⁺were 4mol%,the emission intensity was the strongest and the strongest peak was 606nm due to the4G_5/2 to 6H_7/2 transition of Sm³⁺ions,which was not very different from the phosphor produced by solution combustion method.（4）Bi₄Si₃O₁₂:Sm³⁺,Dy³⁺phosphor was prepared by co-doping Dy³⁺ions.the phase and morphology of Bi₄Si₃O₁₂:Sm³⁺,Dy³⁺phosphor samples did not change significantly.When the Sm³⁺ions concentration were 4mol%and the Dy³⁺ions concentration were 1mol%,the emission intensity was the highest than other variables.the strongest emission peak was 606nm,which was attributed to the ⁴G_5/2 to ⁶H_7/2transition of Sm³⁺ions.In addition,the conclusion showed that there was energy transfer between Sm³⁺and Dy³⁺ions in the direction of Dy³⁺ions to Sm³⁺ions.Finally,due to the co-doping Dy³⁺ions,the color coordinates of the Bi₄Si₃O₁₂:Sm³⁺phosphor slightly moved toward the yellow region,but remained in the orange-red region.（5）The solution combustion process is relatively simple,the synthesis time is about 3⁴ hours,it is found that the sample particles is about 15μm.The whole process takes 10¹1 hours for the whole process of CSD,the morphology is single columnar particles.Under the same doping concentration,the sample prepared by the CSD has a higher luminescence intensity than the solution combustion method,and the particle size of the sample is about 3-6μm,which is closer to the commercial high-quality phosphor.Therefore,considering the respective advantages and disadvantages of the above methods,chemical solution decomposition has overcome the defects of fluorescence intensity and particle size of samples prepared by solution combustion method,and has potential application in mass production.