The Study On Design,Preparation And Tuning Spectra Of Zirconium Silicate Based Fluorescent Materials Under Ultraviolet Excitation

Ultraviolet?UV?excited fluorescent materials are required to convert invisible UV light into visible light and even white light in the fields of UV excited white LED?Light Emitting Diode?,mercury free fluorescent lamp,plant growth lighting,UV detection,and UV excited fluorescent anticounterfeiting technique.Sunlight is a precious gift from nature,and simulating the spectrum and variation of sunlight is an ideal state for artificial light source.If the emission spectra of fluorescent materials can be adjusted as needed,they can be used in various fields.Silicate materials suitable for luminescent hosts have advantages of low cost,abundant raw materials,moisture protection,and high chemical and thermal stability.The O^2--Zr⁴⁺charge transfer transition usually can absorb UV light in zirconium silicate materials.Monoclinic Ca₃ZrSi₂O₉ and hexagonal BaZrSi₃O₉ all have suitable layer structure,and,theoretically,they can accommodate more activated ions.The band gap energy?E_g?is 4.41 and 4.63 eV for Ca₃ZrSi₂O₉ and BaZrSi₃O₉,respectively,and the upper part of valence band and the lower part of conduction band is mainly dominated by O-2p and Zr-4d,respectively,so the O^2--Zr⁴⁺charge transfer transition may happen to effectively absorb UV light in Ca₃ZrSi₂O₉ and BaZrSi₃O₉.Therefore,Ca₃ZrSi₂O₉ and BaZrSi₃O₉ are suitable to act as hosts for UV excited fluorescent materials.The present work focuses on the studies about UV excited monoclinic Ca₃ZrSi₂O₉ and hexagonal BaZrSi₃O₉ zirconium silicate based phosphors.?1?Ca₃ZrSi₂O₉ based UV excited fluorescent materialsBased on trichromatic principle,it is quite promising to obtain UV excited fluorescent materials with tunable emission spectra,and even UV excited white light emitting fluorescent materials,by introducing red,green,blue luminescent activators into Ca₃ZrSi₂O₉ host.Firstly,UV excited blue light emitting phosphors Ca_3?1-x?ZrSi₂O₉:3xCe³⁺were studied.Samples Ca_3?1-x?ZrSi₂O₉:3xCe³⁺were prepared by solid state reaction.Combining crystal field splitting theory and crystal structure of Ca₃ZrSi₂O₉ host,it is concluded that two sets of individual photoluminescence spectra observed in Ca_3?1-x?ZrSi₂O₉:3xCe³⁺originate from Ce?1,3?³⁺and Ce?2?³⁺ions occupying three different Ca²⁺sites.The emission spectra of Ca_3?1-x?ZrSi₂O₉:3xCe³⁺red shift with the red shift of excitation wavelength or the increase of doping concentration of Ce³⁺ions,because Ca_3?1-x?ZrSi₂O₉:3xCe³⁺containing two sets of individual photoluminescence spectra,and enhancement of crystal filed and energy transfer.However,the emission intensity of Ca_3?1-x?ZrSi₂O₉:3xCe³⁺is weak,and it is difficult to meet the practical application.Secondly,Bi³⁺doped phosphors Ca_2.85ZrSi₂O₉:0.15Bi³⁺were further studied.Under the excitation of UV light,Ca_2.85ZrSi₂O₉:0.15Bi³⁺phosphors emit broad band blue light originated from ³P₁-¹S₀ transition of Bi³⁺ions.The emission intensity of Ca_2.85ZrSi₂O₉:0.15Bi³⁺is high,and the fluorescent quantum efficiency is 44.5%.Introducing red light emitting Eu³⁺ions at the same time,and the scheme of codoping Eu³⁺and Bi³⁺ions was designed.The blue light from Bi³⁺ions?³P₁-¹S₀ transition?and red light from Eu³⁺ions?⁵D₀-⁷F_J transition?are simultaneously observed in Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺under the excitation of UV light.The codoping of Bi³⁺ions significantly enhances the emission intensity of Eu³⁺ions.The energy transfer mechanism is analyzed,and the photoluminescence spectra and decay time all indicate energy transfer existing from Bi³⁺to Eu³⁺ions.The codoping of Eu³⁺and Bi³⁺ions can effectively adjust the emission spectra,and the chromaticity coordinates of Ca_2.83-yZrSi₂O₉:0.17Eu³⁺,yBi³⁺are tuned in pink orange region,nearly located on the line between the chromaticity coordinates of Eu³⁺singly doped Ca_2.83ZrSi₂O₉:0.17Eu³⁺and Bi³⁺singly doped Ca_2.85ZrSi₂O₉:0.15Bi³⁺.Finally,further introducing green light emitting Tb³⁺ions,the broad band blue light from ³P₁-¹S₀ transition of Bi³⁺ions,the green light belong to ⁵D₄-⁷F_J'transitions of Tb³⁺ions,and the red light attributed to ⁵D₀-⁷F_J transitions of Eu³⁺ions all appear in the Eu³⁺,Bi³⁺,Tb³⁺tri-doped phosphors Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺under the excitation of UV light.The energy transfer mechanism is analyzed,and the photoluminescence spectra and decay time all indicate energy transfer existing from Bi³⁺to Tb³⁺ions.The further introducing of Tb³⁺ions effectively adjusts the emission spectra,and the chromaticity coordinates of Ca_2.74-zZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,zTb³⁺?z=0.03-0.50?move gradually to top left and finally directly into white light region from pink orange region,with the increase of Tb³⁺ions'doping concentration z.The thermal quenching mechanism is analyzed,and the fluorescent thermal stability of Ca_2.24ZrSi₂O₉:0.17Eu³⁺,0.09Bi³⁺,0.50Tb³⁺does not look bad.The total emission intensity at 100 and 150°C can remain about 83%and 63%of the initial value at room temperature,respectively,and the chromaticity coordinates are always in white light region from 25°C to 275°C.?2?BaZrSi₃O₉ based UV excited fluorescent materialsIt has been reported that Eu ions can exist in the form of Eu²⁺emitting blue-green light,and Eu ions can also exist in the form of Eu³⁺emitting typical red light in BaZrSi₃O₉ host,therefore,Eu²⁺and Eu³⁺ions may coexist in BaZrSi₃O₉ host.It is possible to adjust the emission spectra of BaZrSi₃O₉ host by singly doping rare earth Eu ions,and it is expected that even directly white light emitting phosphors can be obtained.Samples Ba_1-xZrSi₃O₉:xEu were prepared by solid state reaction.The coexistence of Eu²⁺and Eu³⁺ions is realized,and three luminescent activators(O^2--Ti⁴⁺charge transfer state coming from impurities Ti⁴⁺difficultly separated from commercial ZrO₂raw materials,doped Eu²⁺and Eu³⁺ions)are observed in samples Ba_1-xZrSi₃O₉:xEu.On the one hand,the doping of Ti⁴⁺ions significantly enhances the emission intensity of sample BaZrSi₃O₉ host.On the other hand,first principle calculation results indicate that the doping of Ti⁴⁺ions introduces impurity energy levels coming from Ti-3d in the band gap of BaZrSi₃O₉:Ti.Therefore,the broad band blue light originates from O^2--Ti⁴⁺charge transfer state in sample BaZrSi₃O₉ host.The doped Eu ions(Eu²⁺or Eu³⁺)substitute the Ba²⁺sites in samples Ba_1-xZrSi₃O₉:xEu,and Ba²⁺site is structural stable because there is no distortion in BaO₆ octahedron.The coexistence of Eu²⁺and Eu³⁺ions is explained by charge compensation mechanism,and partial Eu³⁺ions are reduced to Eu²⁺ions during the preparation of samples.Multiple luminescent centers(O^2--Ti⁴⁺charge transfer state,Eu²⁺?4f⁶5d¹-4f⁷?and Eu³⁺?4f-4f?ions)coexist in Ba_1-xZrSi₃O₉:xEu,and there are energy transfers between them.Therefore,under UV excitation at 260 nm,the luminescence from O^2--Ti⁴⁺charge transfer state and Eu³⁺ions are simultaneously observed.Under longer near UV excitation at 392 nm,the luminescence from Eu²⁺and Eu³⁺ions are simultaneously observed.Under shorter vacuum UV excitation at 180 nm,the luminescence from O^2--Ti⁴⁺charge transfer state,Eu²⁺and Eu³⁺ions are simultaneously observed.The fluorescent quantum efficiency of Eu²⁺and Eu³⁺ions is comparatively high in Ba_1-xZrSi₃O₉:xEu,and the fluorescent quantum efficiency is65.8%and 94.8%under UV excitation at 355 and 392 nm,respectively.The chromaticity coordinates of Ba_1-xZrSi₃O₉:xEu are tuned in blue,pink and white region,and Ba_1-xZrSi₃O₉:xEu?x=0.15?can directly emit white light under multi-wavelength UV light excitation(?_EX=392,260,180nm).The fluorescent thermal stability of Ba_1-xZrSi₃O₉:xEu?x=0.15?is good,especially under UV excitation at 355 and 392nm,and the emission intensity at 150°C can remain about 93%and 96%of the initial value at room temperature,respectively.