Synthesis, Characterization Of Rare-earth Fluorides And Borides Nanomaterials And Their Properities

In this dissertation,solution and solid-based routes were developed to realize the chemical synthesis and simultaneous assembly of one-dimensional(1D) nanostructures.This dissertation aims at searching for new rare-earth luminescent nanomaterial labels and mild conditions synthesis of rare-earth borides field-emission nanomaterials by adjusting the reaction conditions to control the morphology of the products.The as-prepared products are applied in different fields to check their performance,and it is expected that the products possess some novel physical and chemical properties.The details are summarized as follows:1.A new example of novel spindle-like Ln³⁺-doped YF₃ luminescent nanomaterial has been prepared through a facile method by low-temperature hydrothermal treatment of YCl₃,NaF and EDTA at 140℃for 12h.High quality, large scale,and uniform nanospindles with a mean length of 560 nm and a mean width of 240 nm can be easily obtained.The size of the products can be controlled by varying reaction conditions.The effect of the molar ratio of EDTA to Y³⁺,reaction temperatures and reaction time on the nanospindle growth has been investigated in detail.The possible growth mechanism of nanospindles has also been discussed.The results of the photoluminescence spectroscopy measurements reveal that the as-prepared Ln³⁺-doped YF₃ nanospindles show strong red and green emission.It is found that the morphology and size of the products have great influence on their emission intensity.Since the products exhibit excellent luminescence and can be expected to become a good candidate for research in optical and optoelectronic devices.2.Bifunctional nanocomposites with superparamagnetic and NIR luminescent properties were synthesized by a layer-by-layer and a modified St(?)ber method.Fe₃O₄ nanoparticles(NPs)as the core were coated with NaYF₄:Ln³⁺(Ln=Nd,Er,Pr,or Ho) to form the first layer.Then,the second layer was coated with silica to improve the chemical stability and photostability.The X-ray diffraction(XRD)patterns showed that a cubic spinel structure of Fe₃O₄ and coexistence of cubic and hexagonal structure of NaYF₄ were obtained.Energy dispersive X-ray(EDX)spectroscopy analysis confirmed the core/shell structure of Fe₃O₄@NaYF₄:Ln³⁺.Transmission electron microscope(TEM)images revealed that the bifunctional nanocomposites consisted of crystalline Fe₃O₄@NaYF₄:Ln³⁺cores and amorphous SiO₂ shells,in a spherical shape with a narrow size distribution.Magnetic measurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior.Emission spectra indicated that the bifunctional nanocomposites possessed a high NIR luminescent intensity.Moreover,the hexagonal phase NaYF₄:Ln³⁺showed NIR emission 10～15 times stronger than the cubic phase.3.A family of rare-earth hexaborides(RB₆,R=La,Ce,Pr,Nd)nanocrystals was prepared by solid-state reactions in an autoclave.Single-crystalline RB₆ nanocubes with mean size of～200 nm were prepared at 500℃starting from B₂O₃,RCl₃·6H₂O and metallic magnesium powder.If using NaBH₄ instead of B₂O₃,RB₆ nanoparticles with mean particle size of 30 nm could be obtained around 400℃.The X-ray diffraction(XRD)patterns of the samples could be indexed as cubic RB₆ with the lattice constants in good agreement with the literature values.An atomic ratio of B to R very close to 6:1 was determined from energy dispersive X-ray spectroscopy(EDS) of the samples.Field-emission scanning electron microscope(FESEM)and transmission electron microscope(TEM)were used to characterize the morphologies of the samples.Raman spectra of the samples were investigated.This novel and efficient method using cheap and simple inorganic salts as reactants could be extended further to the preparation of other rare-earth metals hexaborides.4.The above-mentioned method was extended to prepare rare-earth hexaborides (RB₆,R=Sm,Eu,Gd,Tb)nanocubes.Single-crystalline RB₆ nanocubes with mean size of～200 nm were prepared at 500℃for 12h starting from B₂O₃,RCl₃·6H₂O and metallic magnesium powder.By prolonging reaction time to 24h,however, single-crystalline RB₆ nanorods with dimeter of about 200 nm and length of about 3μm were obtained.Controlled synthesis of one dimension rare-earth borides nanostructure was realized by adjusting expenment parameters.