Hydrothermo Synthesis And Mesoscale Regulation Of Novel Rare Earth Upconversion Materials

Upconversion material has great potential in various fields due to its unique advantages such as sharp emission bandwidth,high light resistance and low background spontaneous fluorescence.Synthesis of rare earth doped upconversion materials with high-efficiency upconversion fluorescence by facile,energy-saving and environment-friendly methods,and analysis of the evolution law and growth mechanism of upconversion crystals from the perspective of mesoscale are the most concerned research topics at present.This subject is mainly divided into three parts:Part?:The green and efficient hydrothermal synthesis method has been used,DDBAC as surfactant,EDTA as morphology control agent,to obtain spherical and hexagonal UCs.The influence of different hydrothermal conditions on the morphological properties of synthesized?-NaYF₄:Er³⁺/Yb³⁺crystals is investigated:The higher the solution concentration of precursor,the larger the aspect ratio of hexagon prism.Under acidic conditions,the smaller the pH value of solution,the larger the aspect ratio of hexagon prism.With the prolongation of reaction time,the UCs underwent several phases of transformation,including sphericity,sphericity mixing with hexagonal prism and pure hexagonal prism.The reaction additive GA in the transformation on the crystal growth can serve as"blasting agent".The dodecyl dimethyl ammonium bromide DDBAC serves as cationic surfactants and has played an important role in stabilizing surface and reducing the reaction speed.EDTA is the key factor for the crystal transformation,which is not only helpful for crystals to change from isotropic nanoclusters to anisotropic hexagonal prisms,but also necessary raw materials and conditions for the formation of chirality in UCs.Part?:We reveal NaYF₄:Er³⁺/Yb³⁺crystal growth internal evolution law and growth mechanism through different characterization means,and explain the effect of hydrothermal conditions on upconversion crystal growth from the perspective of mesoscale.The three-stage evolution mechanism of formation and growth of?-NaYF₄:Er³⁺/Yb³⁺crystals is proposed:?1?Formation of hexagon-rings base;?2?Filiform growth and spiral winding;?3?Sidewall flat,inward filling and end capping.In the first stage,according to the lattice arrangement between adjacent particles,we give three principles of assembling spherical particles into hexagon rings:180-degree type,120-degree type and mixed type.In the stage?,we assume that chirality forms in the filamentous growth and spiral winding,which has been proved by Circular dichroic spectrometer and polarimeter,and clarify the important role of EDTA in chiral filamentous spiral growth.In the stage?,we believe that a great process of particle filling,smooth outer wall and capping need a proper compatibility between the shell growth speed and particle filling speed,otherwise it's easy to get the ends of the product uneven.Part?:We explore the relationship between the morphology of UCs and its fluorescence intensity.We find that the fluorescence intensity of hexagonal prisms with the aspect ratio close to 1 is much higher than that of slender prism or flat prism,and the effect of chiral crystals on the enhancement of upconversion fluorescence is pointed out.YF₃:Er³⁺/Yb³⁺crystals have been synthesized by the same hydrothermal conditions and methods,and the reaction conditions,action mechanism and growth and evolution rules influencing the synthesis morphology have been preliminarily explored.The fluorescence–enhanced NaYF₄:Er³⁺/Yb³⁺crystals has been simply applied to 3D fluorescence display,a transparent model is made and great fluorescence quality is obtained under excitation of980nm,demonstrating its potential and advantages as a fluorescent material.