Study On Preparation And Characterization Of Lanthanide Doped Compound Oxide Luminescent Nanomaterials

Nanoscaled luminescent materials have been the hot research because of their peculiar characteristics compared to bulk counterparts.Alkaline earth aluminates and zirconates have excellent thermal and chemical stability.They are an important kind of photoactived host materials.At present,solid state reaction method has been used to prepare alkaline earth aluminates and zirconates,while wet chemistry method is less reported.In this dissertation,alkaline earth aluminates and zirconates luminescent nanomaterials prepared by citrate sol-gel combustion method have been reported,and their photoluminescent properties were investigated systemically,some new luminescent phenomena were detected.In addition,mesoporous rare-earth(RE) ions-doped BaAl₂O₄ and CaAl₂O₄ nanoparticles were prepared and characterized.In Chapter 1,we briefly introduced the theory of luminescence,the luminescent mechanism,preparation methods,characterization means and properties of lanthanide luminescent materials.The research progress in the field of alkaline earth aluminates and zirconates luminescenct materials was also summarized.In Chapter 2,Eu²⁺,R³⁺ co-doped alkaline earth aluminates MAl₂O₄:Eu²⁺,R³⁺(M = Sr,Ba and Ca;R = Dy,Nd and La) nanoparticles with high brightness and long afterglow have been prepared by solution-combustion synthesis at 600℃without a post-annealing process for the first time.We studied luminescent properties of the samples and influence of host on luminescent properties.The samples crystallized in monoclinic phase structure of SrAl₂O₄,the hexagonal phase structure of BaAl₂O₄ and the orthorhombic phase structure of CaAl₂O₄ respectively.The little amount of doped rare earth ions has nearly no effect on the phase structures.It's observed that from SrAl₂O₄:Eu²⁺,Dy³⁺ to BaAl₂O₄:Eu²⁺,Nd³⁺ and to CaAl₂O₄:Eu²⁺,La³⁺,the main peaks of emission spectra of the luminescent nanoparticles shift to shorter wavelength(from 516nm to 500nm and then to 440nm),and correspondingly,the luminescence changes from yellow-green to blue-green and then to blue-purple.This phenomenon is derived from the changing of the matrix crystal structure.The existence of trap level should answer for the properties of long-persistent luminescence.The appropriate trap level in different host was formed by different co-dopant ions for Eu²⁺.The lifetime of afterglow is related with the energy store in the trap level and the number of electrons which stay in the trap level.The intensity of afterglow is related with the velocity,including the velocity of electrons escape from the trap level and the velocity of energy transfer.The afterglow of phosphor nanoparticles,which allowed the time to be visually recognized (≥0.32mcd/m²),lasted for over 7h after the excited source was cut off.In fact,the afterglow time of the samples is shorter than that of the powders prepared by the solid-state reaction method.Mesoporous CaAl₂O₄:Eu²⁺,La³⁺ long lifetime luminescence nanoparticles have been successfully prepared without using any templates or surfactants by sonication-combustion synthesis.The porous structure was confirmed by the hysteresis loop in the N₂ adsorption-desorption isotherm.The Brunauer-Emmett-Teller(BET) surface area is 20.30m²/g.The uniform sol-granules were formed in the sonication progress and the accumulating of nanometer sized sol-granules created a network.The channels were filled with water.The pore formation is attributed to the loss of water molecules during the combustion process.Mesoporous SrAl₂O₄:Eu²⁺,Dy³⁺ long lifetime luminescence nanoparticles have been successfully prepared without using any templates or surfactants by sonication-combustion and refiuxing-combustion synthesises.The Brunauer-Emmett-Teller(BET) surface area is 13.45m²/g.The mechanism of mesoporous SrAl₂O₄:Eu²⁺,Dy³⁺ made by sonication-combustion and refiuxing-combustion synthesises is similar to the one of CaAl₂O₄:Eu²⁺,La³⁺.In Chapter 3,we studied the preparation and luminescent properties of Li_0.9Y_(0.9-x-y)Zr_0.1O₂:Eu_x³⁺,Ry³⁺(R=Ce,Bi,Dy) nanocrystals.Low temperature citrate sol-gel combustion method has been used to prepare the samples,and the combustion method has been confirmed to be an efficient method of preparing zirconates nanocrystals.A considerable loss of the lithium is induced at high temperature due to the evaporation,the amount of lithium loss being proportion to the firing temperature. So,amount of excess Li component was needed to obtain the composition close to the nominal one.In our experiments,the precursors contained excess 10 mol%amount of Li according to the stoichiometric amount.It can be seen that combustion the metal urea-citrate precursor at 800℃is sufficient for the formation of pure crystal phase from the data of XRD.Nevertheless,such a low reaction temperature is surprising because the product was formed at 1200℃when employed solid-state reaction. Li_0.9Y_(0.9-x)Zr_0.1O₂:Eu_x³⁺ nanopowders exhibit a strong red emission under 395 nm excited.The lower symmetry of the Eu³⁺ sites can increase the emission intensity and the extent of peak splitting of Eu³⁺.The emissions of Li_0.9Y_(0.9-x)Zr_0.1O₂:Eu_x³⁺ from the ⁵D₀→⁷F_1,2,3,4 transitions of the Eu³⁺ ions were detected,including three ⁵D₀→⁷F₁ transitions and two ⁵D₀→⁷F₂ transitions.Ce and Bi were introduced into the system as co-activator as well as sensitizer of luminescence and the luminescence intensity of Li_0.9Y_(0.9-x-y)Zr_0.1O₂:Eu_x³⁺ were increased in 900%and 600%respectively.This observation unequivocally shows that the energy transfer from Bi³⁺ to Eu³⁺ is very efficient.Photoluminescence spectra of Li_0.9Y_(0.9-x-y)Zr_0.1O₂:Eu_x³⁺,Dy_y³⁺ samples indicated the dominant red emissions(613nm) at a single-wavelength ultraviolet excitation(395nm) due to ⁵D₀→⁷F₂ transition.When the concentration of Eu³⁺ and Dy³⁺ exceeded a critical amount(5mol%and 4mol%respectively),the red emission became lower coincidence with the appearance of blue and green emission which attribution to ⁴F_9/2→⁶H_13/2 transition of Dy³⁺ and Eu²⁺ transition.Pure and Eu-doped Ba₂YZrO_5.5 nanoparticles have been prepared by sol-gel combustion method and post-annealing process in this work.Photoluminescence properties and structure character were studied clearly.From XRD datas we could see that the samples have lower crystallinity after the combustion process at 800℃;and post-annealing process at 1100℃was necessary.The samples have the cubic perovskite structure with about double cell of ideal perovskite. Three emission bands centered at 468 nm,514 nm and 637 nm can be detected,and were ascribed to the metal-to-ligand charge-transfer transition(MLCT) in [ZrO₅-V(o|¨)-YO₆]groups,electrons transition between oxygen vacancies and the emission from surface state defects,respectively.Ba₂YZrO_5.5:Eu indicated the dominant red emissions(615nm) at a single-wavelength ultraviolet excitation(395nm) due to ⁵D₀→⁷F₂ transition.Ba₂LaZrO_5.5:Ce full-color-emitting phosphor has been prepared by sol-gel combustion method at 800℃.The structure character is similar to Ba₂YZrO_5.5. Three emission bands centered at 467 nm,535 nm and 623 nm can be detected,and were ascribed to the metal-to-ligand charge-transfer transition(MLCT) in [ZrO₅-V(o|¨)-LaO₆]groups,the 5d-4f transition of Ce³⁺ and the redshift of Ce³⁺ emission. The chromaticity coordinate of Ba₂LaZrO_5.5:Ce³⁺0.03 was(0.298,0.317).In chapter 4,a concise summary of the contents was given.