| This dissertation focuses on the controllable synthesis of rare-earth orthophosphate(REPO4)crystals,with in-depth investigation into the mechanisms and kinetics of crystal structure/morphology evolutioin and also up-/down-conversion(UC/DC)photoluminescence of the products.Monoclinic(m-)and hexagonal(h-)structured REPO4(RE=La-Dy)crystals,which present strong one-dimensional(ID)growth preference,have been controllably synthesized via hydro-/solvothermal reaction in the presence of tartrate(Tar2-)or ethylene glycol(EG),and highly[001]oriented m-(La0.95Eu0.05)PO4 nanowires and quasi-equiaxed m-/h-REPO4 crystals were successfully obtained.Through applying Tar2-malate(Mal2-),ethylenediamine tetraacetic acid(EDTA)and citric acid(CA)as shape/structure modifiers in hydro-/solvothermal reactions,monodispersed and micro-/nanosized t-/h-YPO4 crystals with multiple morphologies were successfully fabricated,and the effects of Tar2-on the inverse phase transformation from thermodynamically stable tetragonal(t-)to metastable h-YP04 were clarified.Taking advantages of the selective and rapid heating of microwave and the charge balance effect of SO42-,micro-sized and monodispersed spheres(monospheres)of REPO4(RE=La-Lu,and Y)have been originally synthesized by homogeneous precipitation.REPO4 phosphors with favorable UC/DC luminescence were obtained through doping sensitizer/activator ions and after appropriate calcination,and the effects of phase structure and crystal morphology on luminescence properties were investigated in detail.The main achievements are summarized as follows:(1)Tar2-as a shape modifier can preferentially cap the {120} facets of m-(La0.95Eu0.05)P04 under the hydrothermal conditions of 200℃ and pH=1 and thus led to nano wires of high[001]orientation.The aspect ratio of m-(La0.95Eu0.05)P04 was drastically increased from-5-27 to-30-75 by increasing the n(Tar2-):n((La0.95Eu0.05)3+)molar ratio from 0 to 0.6.An alkaline hydrothermal condition inhibits the 1D growth of h-GdPO4 via enhancing nucleation.Under the solvothermal conditions of 200℃ and pH=12,EG can further inhibit the 1D growth of h-GdPO4 by selectively capping the {001} facets and meanwhile affects nucleation density,leading to the formation of quasi-equiaxed h-GdPO4 crystals at V(EG)>30 mL(total volume of solvent:70 mL).Quasi-equiaxed REPO4 crystals(RE=La-Dy)were all produced at V(EG)=50 mL,and owing to lanthanide contraction,those of RE=La-Sm and Gd-Dy crystallized with the monoclinic and hexagonal structures,respectively.(2)Tar2-exhibits a strong ability to coordinate with Y3+at 150℃ and pH=6,which greatly raises the surface energy of t-YPO4 nanocrystals and,thus,led to phase transformation to metastable h-YPO4.Both an alkaline condition and superfluous PO43-can weaken the ability of Tar2-to promote the formation of t-YPO4.The hexagonal prism-like microcrystals of h-YPO4 and spindle,plate and cubic-like nanocrystals of t-YPO4 were obtained via precisely modulating the pH and n(PO43-):n(Tar2-):n(Y3+)molar ratio of the reaction system.Under 200℃ and pH=12,h-YPO4 nano wires with aspect ratios of up to～32 have been directly synthesized via hydrothermal reaction at n(Mal2-):n(PO43-):n(Y3+)=6:3:1.It was found that Mal2-and superfluous PO43-both favor crystallization of metastable h-YPO4,and the latter promotes the development of nanorod morphologies.In the presence of EDTA/CA,h-YPO4 crystals with the multiform morphologies of microprisms,quasi-ellipsoidal nanoparticles,and sphere-like nanoparticles have been successfully synthesized via solvothermal reaction at 200℃,pH=12,and V(EG)=40 mL.A series of precisely controlled experiments indicated that EG can significantly amplify the abilities of EDTA and CA in modifying the growth regimes of YPO4 through prohibiting the diffusion and dissociation of configuration ions.Increasing the EDTA/CA dosage enhanced the nucleation of h-YPO4 and surface adsorption,and therefore allowed size control of h-YPO4 in the range of～500±229 to 97±39 nm.(3)REPO4 monospheres(RE=La-Lu,and Y)have been originally synthesized through microwave-assisted homogeneous precipitation.Owing to lanthanide contraction,the REPO4 of RE=La-Tb were crystallized in the hexagonal structure while those of RE=Dy-Lu and Y in the quasi-tetragonal structure.Urea molecules were rapidly hydrolyzed under microwave irradiation,which is beneficial to rapid dissociation of H3PO4 and burst nucleation of REPO4.SO42-was proposed to counteract the repulsion resulted from surface absorbed H+,leading to sufficient aggregation of the nuclei/subunits for the formation of monospheres.The aggregating role of SO42-was found to be strongly affected by n(SO42-):n(PO43-):n(Y3+)molar ratio and solution pH,and the average diameter of the spheres can be varied in the range of～1.8-2.7 μm via varying n(SO42-):n(Y3+).Solubility product(Ksp)and 1D growth preference show great influences on nucleation density and nuclei aggregation,which further affected the morphology of REPO4 products.(4)Thermal behaviors and high-temperature phase/morphology evolution were investigated for the representative m-(La0.95Eu0.05)PO4,h-LaPO4,t-YPO4,h-YPO4,and quasi-equiaxed t-YPO4 products.It was found that the elimination of the hydrogen bonding water of m-(La0.95Eu0.05)PO4,t-YPO4,and quasi-equiaxed t-YPO4 were accomplished at～700-750℃,while the loss of the hydration water of h-LaPO4 and h-YPO4 occurred at the higher temperature of～900℃.Either larger crystal size or smaller lattice parameters tend to strengthen the binding force of h-REPO4 towards hydration water,which endowed higher temperature of h-→t-phase transformation of micro-sized h-YPO4(>700℃)than nano-sized h-YPO4(～600℃)and higher initial temperature of h-GdPO4(～700℃)than final temperature of h-LaPO4(～600℃)for the h-→t-phase transformation.Dehydration led to quasi-topotactic phase transformation from h-to m-LaPO4,which mainly involves displacement of the LaOs polyhedra along the a/b-axis.(5)The effects of aspect ratio,extent of hydration,and phase structure on DC luminescence were investigated with m-(La0.95-xBixEu0.05)PO4(x=0-0.01)nano wires,h-(Gdo.95Dy0.05)PO4 quasi-equiaxed crystals,and h-/t-(Y0.95Eu0.05)PO4 micro-/nanoparticles as representatives.The DC luminescence of various RE3+activators(RE=Ce,Pr,Sm,Eu,Tb,Dy,Er and Tm)in t-YPO4 monospheres was comprehensively elaborated.The thermal quenching of t-(Y0.99Eu0.01)PO4 monosphere luminescence was investigated in detail,and the activation energy of which was determined to be～0.275 eV.Through an in-depth study of the DC luminescence of h-(Y0.99-xTb0.01Smx)PO4(x=0-0.06)nanowires and m-(La0.96-xCe0.04Tbx)PO4(x=0-0.12)monospheres,the Tb3+→Sm3+and Ce3+→Tb3+energy transfer in the two phosphors were found to occur via electric dipole-dipole and electric dipole-quadrupole interactions,with the highest efficiencies of～31.2%and 44.48%,respectively.Upon laser excitation at 978 nm,the t-(Yb0.98RE0.02)PO4 binary and t-(Y0.90Yb0.08RE0.02)PO4 ternary monospheres(RE=Ho,Er,and Tm)exhibited favorable UC emissions,and the 2 or 3-photon UC processes were expatiated. |
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