Study On Synthesis, Structure And Properties Of Low-dimensional Vanadates Nanomaterials

In the dissertation the low-dimensioned orthovanadate nanomaterials such as Lanthanum orthovanadate (LaVO₄) nanorods, Cerium orthovanadate (CeVO₄) nanorods, Yttrium orthovanadate (YVO₄) nanorods and one-dimensional ammonium vanadate nanomaterials were chosen as the objects of study. Modern testing methods were used to study the preparation, structure and properties of low-dimensioned orthovanadate nanomaterials. The obtained main results are as follows:(1) Synthesis of one-dimensional rare earth orthovanadate nanorods:â‘ LaVO₄ nanorods with diameter of 80～150nm and length of 1μm were synthesized in a hydrothermal method using La (NO₃)₃ and Na₃VO₄ as the precursor and EDTA as the template when the pH of growth solution was 9.0.â‘¡CeVO₄ nanorods with length of about 600nm and diameter of about 80nm were synthesized in a hydrothermal method using Ce(NO₃)₃ and Na₃VO₄ as the precursor and Na₂EDTA as the template when the pH of growth solution was 10.0.â‘¢YVO₄ nanorods with diameter of 20～50nm and length of 200～400nm were synthesized in a hydrothermal method using Y(NO₃)₃ and Na₃VO₄ as the precursor and Na₂EDTA as the template when the pH of growth solution was 8.5.â‘£Based on these techniques, one-dimensional rare earth orthovanadate nanorods doping with Eu³⁺ ions were synthesized by adding 2～5% mol Eu(NO₃)₃ respectively.(2) Structures and growth mechanism of one-dimensional rare earth orthovanadate nanorods:â‘ All one-dimensional rare earth orthovanadate nanorods were the tetragonal zircon-type single crystal phase structures with high crystallization. And they all were anisotropic, structurally uniform and free from defects and dislocation. Their direction of growth was along the c-axis. The growth of CeVO₄ nanorods was controlled the pH of growth solution and the template. The pH affected the aggregation of VO₄^3-. And Na₂EDTA played the double roles as chelating ligand and capping reagent.â‘¡Due to the effect of the lanthanide contraction, the ionic radius of Eu³⁺ is smaller than that of La³⁺. So the unit cell parameters of LaVO₄:Eu nanorods decrease. However, the ionic radius of Eu³⁺ is almost the same as that of Ce³⁺ and Y³⁺. As a result, the unit cell parameters of CeVO₄:Eu nanorods and YVO₄:Eu nanorods changed very little.â‘¢The crystalline phase and morphology of LaVO₄ nanorods were determined by the vanadium source, the time of hydrothermal process, the pH of growth solution and the template. Generally the larger La³⁺ ion prefers the monazite type due to its higher oxygen coordination number of 9 as compared to 8 of the zircon type. EDTA has appropriately chelating and capping ability, but also can inhibit La³⁺ ion' higher oxygen coordination number of 9.â‘£In the synthesis process of CeVO₄ nanorods and YVO₄ nanorods, Na₂EDTA was used the template to avoid the emulous chelation of NH₄⁺, which could restrain the chelating and capping ability of EDTA. The value of pH was decreased to 8.5 because of the weak chelating ability between Y³⁺ and Na₂EDTA.(3) Properties of low-dimensional rare earth orthovanadate nanomaterials:â‘ Compared with nanoparticles, the photoluminescence performance of low-dimensional rare earth orthovanadate nanorods become better due to the anisotropy of one -dimensional nanorods.â‘¡The Eu³⁺ in LnVO₄ nanorods leads the noticeable improvement of luminescent properties. So the zircon-type LnVO4: Eu nanorods may serve as good red phosphor candidate. Compared with YVO₄ nanoparticles, the luminescent properties of YVO₄ nanorods become better because of the enhanced one dimensional anisotropy.â‘¢LaVO₄ and YVO₄ nanocrystals exhibit the antiferromagnetic behavior at low temperature due to the peculiar 3d and 4f electron configuration of lanthanum and yttrium. Moreover, this strong 1D anisotropy and the Eu³⁺ ions doping can lead to the greaterχ_M.â‘£CeVO₄ nanocrystals exhibit the noticeable superparamagnetic property at low temperature because of the size effects of CeVO₄ nanocrystals and 4f electron of cerium. And this strong 1D anisotropy and the Eu³⁺ ions doping can improve the magnetic performances remarkably.(4) 1D ammonium vanadate nanomaterials:â‘ 1D ammonium vanadate nanocrystals were synthesized in a hydrothermal method using NH₄VO₃ as the precursor. The final products were NH₄V₄O₁₀ nanobelts, (NH₄)₂V₆O₁₆·1.5H₂O nanowires and (NH₄)₆V₁₀O₂₈·6H₂O nanobundles respectively when the pH of the growth solution varied from 2.5, to 1.5, then 0.5.â‘¡Due to the difference of pH of the growth solution, leads to the difference of the hydrogen bonding interaction and the surface free energies that determining the phase-composition and morphology of the ammonium vanadates.â‘¢Because of the different NH₄⁺ content of crystals structures, the conductivity of 1D ammonium vanadates were different. The conductivity of (NH₄)₆V₁₀O₂₈·6H₂O nanobundles is maximal, and the conductivity of NH₄V₄O₁₀ nanobelts is minimal.