Synthesis Of Rare Earth Luminescent Materials-Lanthanum Compounds And Investigation On Their Optical Properties

In our study, the La₂O₃:Eu phosphors with a rather strong f-f transition excitation in near the UV region and the nanorod-shaped La₂O₂S:Eu phosphors with effective photoluminescence were successfully synthesized by citric acid (CA) and poly (ethylene glycol) (PEG) precursor route and microemulsion method with high temperature reduction, respectively. Then the novel lanthanum phosphors (La₂O₂CO₃) were prepared through a simple and direct solid-state reaction. In the process, their structures and optical properties were characterized by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), UV-vis spectroscopy, Transmission Electron Microscope (TEM), FT-IR spectrum, Scanning Electron Microscope (SEM), and photoluminescence (PL) spectra, respectively. We systematically researched the growth process of the lanthanum compounds and discussed the influence in their morphology and photoluminescence, which were caused by changing the reaction conditions. Some useful explanation and interesting results were gained. The detailed contents are listed as following:In this paper, La₂O₃:Eu phosphors were fabricated via CA and PEG precursor route, and their structure and photoluminescence properties were thoroughly investigated. The FI-IR spectrum indicated the reactions between CA and PEG, contributing to a good dispersion to the metal ions. Compared with the solid state reaction method, the CA precursor route, especially PEG added in the sol-gel process, greatly enhanced the PL intensity, in which the significant changes were observed from the enhanced CT band and Eu³⁺ f-f transition excitation. The emission intensity of phosphors was intensified with the increase of the Eu3+ concentration, reaching a maximum value when the Eu³⁺concentration is 8 %, and then weakened due to the increasing doping concentration of Eu3+. In this system, PL intensity ratio of the (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) showed a decrease in site symmetry and an increase in the purity of the red color with increasing Eu3+ doping concentration. Since the La₂O₃:Eu samples offer a rather strong f-f transition excitation in near the UV region, the as-prepared phosphors are probable to be used as potential red phosphors for white LEDs.In this paper, we have adopted two-step method, w/o microemuleion and high temperate reduction to fabricate La₂O₂S:Eu nanorods. At first, La₂(SO₄)₃ nanorods were successfully prepared by microemuleion using La(NO₃)₃ and H₂SO₄ solutions. There are two key points: the concentration of reactant, the value of wo, which are responsible for the control the morphology of the products. In our system, the uniform La2(SO4)3 nanorods in morphology prepared under the reaction conditions of the best wo value=5, and the best reactant concentration [La(NO3)3] = 0.3 mol/L and [H2SO4] = 0.45 mol/L. The specific surface of even nanorods was large, which could be used as catalyst in organic synthesis. In such optimization condition, we synthesized uniform La2(SO4)3:Eu nanorods. And then the nanorod-shaped La₂O₂S:Eu products deoxidized from La2(SO4)3:Eu nano-rods in reduced atmosphere in high temperature. Compared with the samples prepared by conventional solid state method, the reaction temperature was reduced significantly. The thought of two-step method can also offer route for how to synthesizing La₂O₂S:Eu nano-material.A novel rare earth green phosphor La₂O₂CO₃ has been synthesized successfully by sintering the mixture of La2(CO₃)3 and alkali metal salts. The XRD results reveal the structure and component of La₂O₂CO₃, in which the additive alkali metal salts not only affect the crystal form and the degree of grain, but also greatly promote its luminescence. Compared with the morphology of raw materials, the SEM images show that the as-obtained phosphors were composed of many small grains of nanoparticles. The morphology and PL intensity of the samples were varied from the amount of various alkali metal salts and the annealing time. The as-prepared material shows a strong green emission band centered at 470 nm under the excitation of 254 nm. We synthesized another luminescent material by sintering the mixture of La2(CO₃)₃, alkali metal salts and sulfur finely dispersed. The phosphors reveal a yellow emission band centered at 570 nm under the excitation of 320 nm. According to our primary analysis, these emissions also can be related to the vacancies in lattice. For this aim, we provided the quotable experience for a new field of rare earth luminescent material, especially for lanthanum compound.