| Long afterglow phosphor is an important optical functional material that can absorb sunshine and then emit visible light in darkness for several hours. Many kinds of materials have widely used in night identification and safety indicators in emergency cases. However, the mainly colors of the afterglow phosphors were blue and green. The red afterglow phosphor with outstanding performance is difficult to synthesize.In this paper, the surface of Y2O2S:Eu3+ phosphor was coated with SiO2, which was formed by TEOS hydrolysis techniques. The coating was supposed to enhance the stability of PL intensity in high temperature and seawater immersion environment. Field emission scanning electron microscope (FESEM), energy dispersive spectroscopy (EDS), fluorospectro photometer (FP) and video-based, contact angle measuring device were used to characterize the changes from uncoated phosphors to coated phosphors. The results showed that the SiO2 coating had formed in a convenient way. After immersed in seawater for two months, the luminescence intensity of uncoated phosphors decreased to 50% of the initial value, while the coated samples only decreased to 80%. The PL intensity of the SiO2-coated phosphors at 200℃still remained 60% of the initial value, while the uncoated value was only 25%. The contact angle of the phosphor also increased from 33.4°to 126.5°after surface modification.Sm3+-activated La2O2S phosphors with good crystallinity were prepared by the solid-state reaction. The influences of temperature and Sm3+ doping concentrations of phosphors to crystallinity were characterized by X-ray diffraciton (XRD). According to the UV-vis absorption spectra results, the La2O2S:Sm3+ phosphors showed stronger optical absorption in UV-vis region than the Y2O2S:Eu3+ phosphors. The emission peaks locate at 570,606 and 659 nm, and the highest intensity at 606 nm can be attributed to the 4G5/2→6H7/2 transition of Sm3+. The highest excitation peak at 413 nm was due to the H5/2→4L13/2 transition of Sm3+. The strongest PL emission of La2O2S:Sm3+ was obtained when the concentration of Sm3+ was 5 mol% and decreased due to concentration quenching. The long afterglow test showed that the phosphors had better long-lasting properties when the Sm3+ concentration was 0.3% than the samples with Sm3+ concentration of 3% and 5%.In order to seek more excellent red phosphors, the La2O2S:Sm3+phosphors were doped with different ions. La2O2S:Sm3+, Eu3+,La2O2S:Sm3+, Tb3+ and La2O2S:Sm3+, Tm3+ afterglow phosphors were synthesized by a solid state reaction method. XRD spectra showed a well-defined crystalline structure without impurity phases for doped samples, except that the cell parameters were changed slightly. The PL analysis showed that the emission intensity of the La2O2S:Sm3+ phosphors that doped with Tb3+ was increased while that of the samples doped with Eu3+ and Tm3+ were decreased. The La2O2S:Sm3+, Eu3+ afterglow phosphor showed emission at 538.6,586.8 and 625.2 nm, which can be attributed to the transition of O2-to Eu3+. We also doped and co-doped La2O2S:Sm3+ phosphors with Mg2+ and Ti4+ and found co-doping with certain proportion of Mg2+ and Ti4+ would improve the emission intensity. Chromaticity coordinate test showed doped ions were conducive to obtaining higher color purity. The afterglow properties of all phosphors were summarized in the end. |
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