Metal Oxide Micro-/Nanostructures Doped With Rare Earth Ions: Solution-phase Synthesis, Characterization And Fluorescent Properties

Metal oxide micro-/nanomaterials doped with rare earth ions have enormous potential applications in photoelectric devices, nanodevices, sensors, catalysis and magnetism due to its special physical and chemical properties. Preparation of micro/nanomaterials with well-defined size, morphology dimensionality and diversity through novel synthesis approaches and investigation of their formation mechanism should be a key precodition to reveal the relation between the structures and properties, fabricate a novel micro/nano-structure system for metal oxides doped by rare earth ions and pave the way to real applications. New solution-based manipulated synthetic strategies and fluorescent properties of CeO2, Ga2O3, LaVO4 and YBO3 micro/nanomaterials doped by Eu3+, Dy3+ and Sm3+, respectively, have been detailedly investigated in this dissertation. The valuable explorations have been carried out on the relationship between fluorescence and size/morphology of the prepared micro/nanomaterials. Some new and interesting results were achieved and listed as follows.A new hydrothermal-decomposition method was developed to prepare straw-like CeO2:Sm3+ microcrystals in water solution assisted by macromolecule surfactant (PVP). Herein, the morphologies could be well-controlled through the changes of original concentration, reaction time and temperature as well as the content of PVP. Straw-like CeO2:Sm3+(5.0 at.%) microcrystals prepared exhibited the best fluoresence intensity in comparation with other shapes, and high annealing temperature was helpful to enhance intensities of yellow and blue emission. Besides, a simple solovthermal route was proposed to synthesize monodispersed nanocrystals in ethanol-water mixed solution. Employing Ce(NO3)3, Eu(NO3)3 and NH3·H2O as reaction materials, cubic fluorite CeO2:Eu3+ with single crystal structure, the diameter of which was shorter than 100 nm, could be prepared in the solution with the pH value of 9 at 180℃for 12 h. CeO2:Eu3+ nanocrystal possessed a good ultraviolet absorption and effectively transmited the absorbed energy to Eu3+, resulting in high strength emissions at 593,612, and 632 nm, respectively. The hydrothermal/ solvothermal methods are availed to controlled synthesize polymorphological CeO2:Ln3+ micro-/nanocrystals.When the content of CTAB was 0.6 g, and the pH value of solution was 9, as well as the reaction was carried out at 160℃for 24 h, monodispersed squarelike Ga2O3:Dy3+ particles with well-crystallization could easily synthesized via hydrothermal route assisted by CTAB. The effects of pH, the content of CTAB, reaction time and temperature on the morphology and structure of the sample were detailedly discussed. The formation mechanism of squarelike Ga2O3:Dy3+ nanocrystals was proposed. It was found that the doping of Dy3+ enhanced the efficiency of energy transfer between Ga2O3 and Dy3+ and improved its luminescence, and squarelike Ga2O3:Dy3+ nanocrystal with doped Dy3+of 6 at.% showed the highest blue emisssion. With Ga2O3, EU2O3, HCl and NaOH as original materials, a simple hydrothermal method was applied to prepare rodlike GaOOH in water solution with the pH value of 6 at 140℃for 10 h. The as-prepared GaOOH could be decomposed thermally into Ga2O3:Eu3+ nanorod. It was found that the aspect ratios and morphologies of the samples could be well-controlled by adjusting pH values of the solutions. Both a-Ga2O3:Eu3+and p-Ga2O3:Eu3+ could absorb ultraviolet radiation (≤275 nm), and the energy bands of which were 4.40 and 4.30eV, respectively. The morphologies and crystal structures of the obtained samples showed significant influences on color purity, fluorescene intensity of the samples. Rodlikeβ-Ga2O3:Eu3+ had better luminous intensity and a larger ratio of red emission to orange emission than rodlikeα-Ga2O3:Eu3+.A simple hydrothermal method was adopted to synthesize a novel polyhedron LaVO4:Dy3+ nanocrystal. For LaVO4:Dy3+polyhedrons, the crystal size, crystal structure, shapes and chemistry constituent of the samples could be controlled by adjusting the parameters of pH, reaction time and temperature, which have been synthesized in the solution with the pH value of 9 at 160℃for more than 24 h. It was found that the as-prepared polyhedron showed higher color purity and stronger yellow emission. Higher annealing temperatures for the samples enhanced the intensities of blue and yellow emission, and the increasing rate of intensity from blue emisssion was more larger than yellow emission. Furthermore, a novel complexing-hydrothermal method assisted by EDTA was also proposed to synthesize LaVO4:Sm3+ nanorods. The effects of concentration of EDTA, pH values, reaction time and temerature on crystal structure, size and morphology of the samples were discussed. It was found that double reactions of mating reaction and induction behavior from EDTA played important roles for LaVO4:Sm3+ to accomplete the transformation from monoclinic (m-) structure to tetragonal (t-) phase and to facilely form nanorods. Rodlike t-LaVO4:Sm3+ showed higher red emisssion than m-LaVO4:Sm3+ nanorod. The content of Sm3+ of nanorod was more than the sample with other shapes, taking on three stronger characteristic emisssions. Increasing rate of emission intensity from 4G5/2→6H5/2 was higher than that of emission from 4G5/2→6H9/2. Besides,4G5/2→6G7/2 transition of t-LaVO4:Sm3+ nanorod could be used as red laser output.A simple and environmentally friendly solvothermal strategy was designed to fabricate a novel fishbonelike LaVO4:Eu3+micro-/nanocrystals in mixed ethanol-water solution. The formation mechanism for fishbonelike LaVO4:Eu3+ crystal has been investigated in detail after the effects of experimental parameters on the structure and morphology of the samples were disscussed. A great deal of fishbonelike LaVO4:Eu3+ crystals with stronger red emission could be facilitatedly synthesized when the content of ethanol was 20 ml, the concentration of Eu3+ was 4.0 at.% and the pH value of solution was 9, as well as the reaction was carried out at 160℃for 24h. The delay curve from 5D1-7F2 transition of Eu3+ in the as-prepared crystals accorded to exponential delay rule. The fluorescence lifetime of 5D1-7F2 transition of Eu3+ for fishbonelike LaVO4:Eu3+ crystal was a little longer than rod-like sample. As a red phosphor, fishbone-like LaVO4:Eu3+did possess the undoubted advantages.YBO3:Eu3+ micro/nanocrystals with novel morphologies of petal sector and sandwich cake were synthesized by surfactant-assisted hydrothermal/solvothermal methods. The studies on the crystal growth revealed the relationship among the crystal Structures, the morphology and fluorscence.The experimental results from ultraviolet light absorbancy and fluorescence properties presented by YBO3:Eu3+ crystals with various morphologies indicated that petal sector-like YBO3:Eu3+ possessed some red emission advantages than the others, becoming a promising candidate of PDP red phosphor.