Investigations Of The Preparations And Luminescence Properties Of The Eu³⁺ Doped Y₂O₃ Phosphors

Rare earth luminescent materials are representative functional materials in which rare earth elements act as activators or host components.Rare earth luminescent materials have the advantages of strong absorption coefficient,high energy conversion efficiency and efficient luminescence emissions from the ultraviolet,passing through the whole visible,to the mid-infrared light region.Rare earth luminescent materials are attributed to be a promising phosphor for applications in the fields of displays,lightings,bioimaging,sensors,photocatalysts.In order to improve the luminescence intensity and efficiency of materials,a lot of work devoted to the preparation of many kinds of micro/nano structure and the research on enhancing the luminescence intensity.The rare earth oxides such as Y₂O₃ and Lu₂O₃,have been research hotspots because their stable chemistry and physics properties,non-toxic,low cost and easy preparation.The visible and infrared lights can be achieved by doping different rare earth elements when yttrium oxide is the host.Y₂O₃:Eu³⁺ phosphor is a typical kind of red phosphors,which has been widely applied in the field of fluorescents and cathode ray tube imaging.In this work,wet chemical methods were used to synthesized many kinds of Y₂O₃:Eu³⁺ micro/nanostructures and the effect of micro/nanostructures,surface defects and morphologies on the photoluminescence and electroluminescence properties were studied.The main results of the dissertation are as follows:1.The mesoporous Y₂O₃:Eu³⁺ powder was prepared by the penchini sol-gel method combined with template method.The XRD results indicate that the final product after calcination is cubic yttrium oxide phase.The experimental results of N2 adsorption–desorption isotherms confirm the existence of mesoporous structure and the pore size is about 4 nm.The mesoporous Y₂O₃:Eu³⁺ particles show stronger photoluminescence intensity than that of particles without special structure due to the advantage of surface area and pores.The mesoporous structure not only possesses large surface area for absorbing excitation light but also enables multiple light reflection and scattering between the pores.Both of them contribute to improving light emission and enhancing photoluminescence efficiency.Improving the sintering temperature can increase the crystallinity of Y₂O₃ which results in the enhancement of photoluminescence intensity.2.The size adjustable Y?OH?3:Eu³⁺ nanorods were synthesized by a facile hydrothermal method.The Y?OH?3:Eu³⁺ nanorods were decomposed into Y₂O₃:Eu³⁺ phosphors with the shape preserved during the high temperature sintering.The effects of urea and NaOH concentration on crystal growth,photoluminescence and electroluminescence performance were studied systematically.The crystal structure,composition and morphology of one dimension Y₂O₃:Eu³⁺ nanorods were characterized by XRD,STA and SEM.The growth mechanism of Y₂O₃:Eu³⁺ nanorods and the effect of structure-directing agent urea are investigated in deep.The Y?OH?₃ nanorods exhibit a highly anisotropic structure along the c-axis,which drives the anisotropic growth of RE hydroxide.When increasing the NaOH concentration,the growth rate of crystals increases and the size of the nanorods increase too.The urea plays an important role in improving the nanorods uniformity and reducing the nanorods size.The photoluminescence intensities increase and then decrease when the urea content increases.The highest emission intensity is obtained from the sample prepared with 7.5 mmol of urea.Both the photoluminescence and electroluminescence intensities increase when adding urea.3.The monoclinic Y₄O?OH?₉NO₃ with nanosheets,submicrospheres,and microrods have been successfully prepared by a nonaqueous solvothermal method without any assistance of the templates and surfactant.In the reaction process,the solvent is benzene methanol,the octylamine acting as the alkaline reagent and yttrium nitrate acting as the yttrium source.The precursors Y₄O?OH?₉NO₃ decompose into cubic Y₂O₃:Eu³⁺ phosphors after sintering.The morphologies and sizes of Y₂O₃:Eu³⁺ particles can be easily controlled by changing the growth rates of various crystallographic facets during solvothermal process.With the increment of the rare-earth nitrates concentration,the morphologies transform from 2D nanosheets to sub-microspheres,and eventually to microrods.For the best of our known,it is the first time that the Y₂O₃:Eu³⁺ nanosheets,sub-microspheres and microrods can be synthesized at the same time just by changing the concentrations.When increasing the benzyl alcohol/1-octylamine volume ratio,the length of Y₄O?OH?₉NO₃ increases obviously but the diameter of Y₄O?OH?₉NO₃ has no significant changes.The benzyl alcohol could absorb on certain crystal face inducing oriented growth.The dependence of Y₂O₃:Eu³⁺ photoluminescence performances on nanosheets,sub-microspheres and microrods are discussed.When adjusting the RE³⁺ concentrations,the samples with larger size emitting stronger red light indicate that the PL intensity may be associated with the size of the samples.The reduced periodicity outside the plane direction of two dimensional nanosheets causes the quenching through the non-radiative relaxation.4.Unknown compound with a rhombic dodecahedral morphology has been prepared via a facile hydrothermal method with the assistance of NaOH.Through detailed and in-depth characterization analysis,the unknown compound should be hydrolysis-resistant yttrium alkoxide.The hydrolysis resistance of yttrium alkoxide is due to the associated copolymer from the reaction between ethylene glycol and Y³⁺ ions.The reaction mechanism of yttrium alkoxide was investigated systematically by changing NaOH concentrations and EG/H₂ O volume ratios.In the reaction,NaOH as a strong base not only affords OH-to form Y?OH?₃ but also catalyses the reaction for the synthesis of yttrium alkoxide because of its strong ability to capture protons from the hydroxyl of EG.The size of yttrium alkoxide rhombic dodecahedrons can be precisely adjusted from 1 ?m to 15 ?m by changing NaOH concentration.Excessive NaOH or H₂O caused the hydrolysis of yttrium alkoxide rhombic dodecahedrons to form Y?OH?₃ microrods.We traced the morphology evolution with the change in hydrothermal time and temperature in order to understand the growth process of the yttrium alkoxide rhombic dodecahedrons.The growth mechanism of rhombic dodecahedrons follows dissolution–recrystallization.Although the mean size of Y₂O₃:Eu³⁺ is smaller than that of yttrium alkoxide,the shape of rhombic dodecahedra with uniform size is still preserved.There is only slight shrinkage and rupture on the surface of some particles.The excitation peak location of Y₂O₃:Eu³⁺ transformed from Y?OH?3:Eu³⁺ microrods shifted to the long wavelength compared with that of Y₂O₃:Eu³⁺ transformed from yttrium alkoxide.The photoluminescence intensity of Y₂O₃:Eu³⁺ rhombic dodecahedrons increased with the increase of particle size.