The Hydrothermal Synthesis And Luminescent Properties Of La₂O₃:Eu³⁺ And La₂O₂S:Eu³⁺ Red Emitting Micro-/nano- Phosphors

This dissertation mainly focuses on the synthesis and the properties of Eu3+ doped La2O3, La2O2S and La2O2SO4 red phosphors. One-dimensional nanowires of (La,Eu)(OH)3 and (La,Eu)2(OH)4SO4-nH2O layered hydroxyl sulfate with two distinctive morphologies of nanoplates and microprisms were synthesized via hydrothermal reaction from the mixed solutions of component nitrates. Calcining nanowires precursors at≥800℃ yielded (La,Eu)2O3 red phosphors. (La,Eu)2O2SO4 and (La,Eu)2O2S phosphors were obtained by calcining the (La,Eu)2(OH)4SO4-nH2O at 300-1000 ℃ in air and at≥700 ℃ in hydrogen atmosphere. Detailed characterizations of the precursors and the obtained phosphors were achieved by combined techniques of FT-IR, TGA/DSC, FE-SEM, TEM, XRD and PLE/PL.The results indicated that the obtained (La,Eu)2O3 phosphor with a partical size of 400-500 nm exhibited the strongest red emission at 630 nm (the 5D0â†'7F2 transition of Eu3+) upon UV excitation into the charge transfer band at 285 nm. An enhanced luminescent intensity was found for the phosphor calcined at a higher temperature, which shows a significant calcination-temperature dependence. The uniformly (La,Eu)2O2SO4 particles with a good dispersion emitted an excellent red emission at 620 nm, and the luminesecent intensity significantly denpends on the morphology and the calcination temperature. The luminescent intensity of nanoplates is stronger than that of microprisms owing to its good crystallinity, and an increased luminescent intensity was observed at a higher calcination temperature. The lifetimes of nanoplates and microprisms are 1.56 ms ± 0.02 and 1.17 ± 0.02 ms respectively. The resultant (La,Eu)2O2S phosphors calcined at≥700 ℃ in hydrogen atmosphere well retained the original morphologies of the precursors, whose emission intensity also exhibited morphology and calcination-temperature dependencies. As the covalent of oxysulfide is tronger than that of oxide, a narrow band gap and a red shiftting of exitation spectrum were observed for the (La,Eu)2O2S phosphors, which thus showed a stronger emission intensity. phosphors with various morphologies showed the strongest red emission at 627 nm (5Doâ†'7F2 transition of Eu3+ ions), which exhibits a single exponential decay behavior. The lifetimes of nanoplates and microprisms (La,Eu)2O2S phosphors are 0.52 ± 0.02 ms and 0.49 ± 0.01 ms respectively...