Controlled Hydrothermal Synthesis And Luminescence Properties Of Sr₅(PO₄)₃F:RE Micro-/Nano-phosphors

Controlled synthesis of micro-/nano-sized particles for RE3+-doped strontium fluorophosphate phosphors(Sr5(PO4)3F:RE,RE=Eu,Tb,Dy,Yb/Ho,Yb/Er,Yb/Tm)were attained via hydrothermal reaction,with an in-depth investigation of the effect of EDTA chelate on phase structure of the product and luminescence properties of the Sr5(PO4)3F:RE(RE=Eu,Tb,Dy)down-conversion phosphors.It was found that EDTA is beneficial to the formation of phase-pure Sr5(PO4)3F:RE and the initial pH of the reaction system,R=EDTA/(Sr2++RE3+)molar ratio,hydrothermal temperature and RE3+ content(x)have important influences on particle morphology of the product.The as-prepared samples were characterized by XRD,FE-SEM,TEM,STEM,FT-IR and PL-PLE,and the main conclusions of this work are summarized as follows.Without EDTA chelate,phase-pure Sr5(PO4)3F can be synthesized but doping of the compound with RE ions was failed.With the chelating effect of EDTA toward Sr2+and RE3+,phase-pure Sr5(PO4)3F:RE powders were successfully produced.It was found that the product obtained under the hydrothermal conditions ofR=0.5,pH=3 and 120? is SrF2,which can be transformed into phase-pure Sr5(PO4)3F:RE by raising solution pH or reaction temperature.With increasing R from 0.5,the product tends to change from nanoparticles to self-assembled polycrystalline micro spheres.Solution pH significantly affects nucleation rate,and the grain and particle sizes of the Sr5(PO4)3F:RE product gradually decreased at a higher pH.Rising temperature of hydrothermal reaction was found to promote not only nucleation rate but also grain growth of Sr5(PO4)3F:RE.The down-conversion luminescence properties of Sr5(PO4)3F:RE(RE=Eu,Tb,Dy)were systematically investigated.Upon 393 nm excitation,the Sr5(PO4)3F:xEu(x=1-5 at%)micro-phosphors exhibited the characteristic emissions of Eu3+,and the 613 nm red emission,arising from 5D0?7F2 forced electric-dipole transition,is dominant since the Eu3+ ions are located at non-centrosymmetric lattice sites.Moreover,the luminescence of Er3+ shows a steadily higher intensity at a higher Eu3+ content and has color coordinates of(0.64,0.36).Both the micro-and nano-phosphors of Sr5(PO4)3F:xTb(x=5-20 at%)exhibited the characteristic 5D4?7FJ(J=3-6)emissions of Tb3+ under 217 nm excitation,with the 546 nm green emission arising from 5D4?7F5 transition being dominant.Luminescence intensity of the micro-phosphor did not change but that of the nano-phosphor decreased with the increasing Tb3+ content,and the former has an intensity about 1.5 times that of the latter.The Sr5(PO4)3F:xTb micro-and nano-phosphors have color coordinates of(0.29,0.55)and(0.28,0.49)for their luminescence,respectively;Under 350 nm excitation,the Sr5(PO4)3F:xDy(x=0.3-1.2 at%)micro-phosphors exhibited the 4F9/2? 6H15/2(480 nm,blue)and 4F9/2?6H13/2(575 nm,yellow)transitions of Dy3+ and a broad emission band(peaked at?423 nm)of the Sr5(PO4)3F host,with the latter being dominant and the luminescence of Dy3+shows a steadily higher intensity at a higher Dy3+content.The color coordinates of Sr5(PO4)3F and Sr5(PO4)3F:xDy(x=0.3-1.2 at%)were analyzed to be(0.17,0.14)and(0.19,0.16),respectively.