Ligand Effect On The Growth Process Of NaYF₄Crystals And Self-assembly Research

Hexagnol NaYF4(γ-NaYF4) crystals are one of the best host materials of Up/Down-conversion for their low lattice energy, transparency in visualble region, etc. Because of the so-called phase dependent properties of nano/mciromaterials, morphology controlled synthesis ofβ-NaYF4has moved into focus in recent years. What’s more, more and more effort has been payed to research the self-assembly of uniform β-NaYF4nano/microcrystals with different shape and size for the potential property following self-assembly process. In this thesis, ligand effect on the morphology, growth process of NaYF4crystals as well as sefl-assembly phenomenon of the as-prepared uniform NaYF4microcrystals have been systematically researched.Hydrothermal method has been chosen due to its mild reaction conditions and wild applicability of ligands. In oleic acid system, when the amount of NaOH was increased from0.30g to0.80g, the aspect ratio of β-NaYF4microcrystals decreased and then increased. With F/Y ratio increasing from4to6, the aspect ratio of the as-prepared samples significantly increased. However, further increasing the F/Y ratio scarely influenced the shape and size of β-NaYF4microcrystals. In addition, the higher the boiling point of the alcohol, the larger the crystal size as well as the slower the phase transition process.In citric acid system, in a proper amount range of water form40mL to50mL, the size of the as-prepared NaYF4samples changed little. However, the charging sequence affected the crystal size a lot, that is, when adding NaF firstly and then Y(NO3)3solution, micrometer scale crystals could be formed.Here, we introduced the concept of ligand dynamic effecet into the controlled synthesis of β-NaYF4microcrystals for the first time. A series of fatty acids with only one anchoring group and a hydrocarbon chain were selected. Under given synthesis conditions (F/Y3+=4,200℃,7h), microrods, microprisms with pointed centers, disk-like submicroplates with different aspect ratios were successfully synthesized. To the best of our knowledge, microprisms with star-like centers, microdisks with sunflower-like top/bottom surfaces and microdisks with spire-like centers haven’t been reported before. Then, a systematical research of time-dependent experiments was undertaken to study the effect of different ligands on the crystal growth and phase transition processes. It has been found that the crystal growth process varies with different chain-length ligands and can be clearly classified into three categories. In addition, the chain length of ligands significantly influences the αâ†'β phase transition process, which has not been reported ever before.Uniform NaYF4submicrodisks coated by Myristic acid can be well prepared by controlling the synthesis conditions and the size of the crystals can be continuously tuned by doping with various amount of Gd3+ions.The self-assembly phenomenon of NaYF4cystals has been studied by a series of methods, such as dip-coating, solvent evaporation, interface self-assembly. It has been found that air/water interface self-assembly method is more advanced than others in preparing large scale self-assembly firms.