| In this dissertation,monodispersed YPO4:Ln3+(Ln= Gd,Dy,Tb,Eu,Ho,Tm,Ce)spheres with a diameter of 0.7pm-1.5μm and a tetragonal crystal structure have been successfully synthesized via urea-based microwave-assisted homogeneous precipitation from mixed solutions of the component nitrates in the presence of ethylene glycol(EG)and cetyltrimethyl ammonium bromide(CTAB).A systematic study was carried out on the formation mechanism of phosphate spherical particles.The effects of particle morphology,calcination temperature,doping content,coordination symmetry,water molecules in structure and energy transfer on various aspects of luminescence of the resultant phosphors have been discussed in detail,and the following conclusions have been reached:The monodispersed YPO4:Ln3+ spheres are polycrystalline and their growth followed an aggregation process.Ethylene glycol addition was beneficial to the formation of spherical particles.Excessive doping of Gd3+ or Eu3+ leads to the appearance of hexagonal rods on particle surface.The phosphate hydrates converted to anhydrous tetragonal ones via calcination,and the spherical shape,phase and excellent dispersion of the original particles can be well retained until the temperature above 1000℃.In the crystal structure,the water molecule are hydrated,and its loss leads to more PO43-shifting from symmetric stretching vibration to anti-symmetric one,along with lattice expansion.Elevating the calcination temperature higher than 1000℃ induced merging of spherical particles with larger crystals and sintering phenomenon.Upon UV excitation at 274nm,(Y0.99-xGdxDy0.01)PO4 spheres exhibited the characteristic blue emission at-483nm and the yellow one at-575nm,and their CIE chromaticity coordinates are around at the white light area(0.33,0.33).Enhanced emission intensities and similar fluorescence lifetimes were observed with the increasing the Gd3+ content.However,stronger quenching effect of water on Eu3+ transition than that on Tb3+ one,especially for the near-infrared region,is significantly affected by the hydrate water.Hydrate water missing leads to a gradual blue shift in(Y0.95Eu0.05)PO4·nH20 PLE spectra.Similarly for(Y0.98-xTb0.02Eux)PO4 system,a long fluorescence lifetime can be obtained and the emission color moved faster from green to red region by hydrate water loss,although the efficiency of Tb3+→Eu3+energy transfer was kept at a constant of～29%.The thermal stability of three typical samples are in the order:(Y0.88Tb0.02Eu0.10)2PO4<(Y0.98Tb0.02)PO4<(Y0.95Eu0.95)PO4.For(Y0.88Tb0.02Eu0.10)2PO4 sample,two splits of 541-nm peak(5D’4→7F5 transition)and 557-nm peak(5D"4→7F5 transition)appeared instead of the original one at 546nm(5D4→7F5 transition)by heating processing.Upon UV excitation,the colorful fluorescence of YPO4:Ln3+ emission were achieved,which are the characteristic emissions of Ln3+(Ln=Eu,Tb,Dy,Ho,Tm,and Ce)and give red,green,near white,green,blue,and blue-green emissions,respectively.(Y0.95Eu0.05)PO4·nH2O/PVA and(Y0.95Eu0.05)PO4/PVA transparent films were successfully fabricated via solution casting with monodispersed phosphate spheres as dispersed particles,which exhibited high transmittances of ≥83%in the 400-850nm region.The hybrids films can be coated on paper money for anti-fake label application. |
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