Solvothermal Synthesis of Lanthanide-doped NaYF4 Upconversion Crystals with Size and Shape Control: Particle Properties and Growth Mechanism

Lanthanide-doped NaYF4 upconversion nano- and microcrystals were synthesized via a facile solvothermal approach. Thereby, the influence of volume ratios of ethylene glycol (EG)/H2O, molar ratios of NH4F/RE3+ (RE3+ represents the total amount of Y3+ and rare-earth dopant ions), Gd3+ ion contents, types of activator dopant ions, and different organic co-solvents on the crystal phase, size, and morphology of the resulting particles were studied systematically. A possible formation mechanism for the growth of crystals of different morphology is discussed. Our results show that the transition from the alpha- to the beta-phase mainly depends on the volume ratio of EG/H2O and the molar ratio of NH4F/RE3+, while the morphology and size could be controlled by the type of organic co-solvent and Gd3+ dopant ions. Furthermore, the reaction time has to be long enough to convert alpha-NaYF4 into beta-NaYF4 during the growth process to optimize the upconversion luminescence. The formation of larger beta-NaYF4 crystals, which possess a higher upconversion luminescence than smaller particles, proceeds via intermediates of smaller crystals of cubic structure. In summary, our synthetic approach presents a facile route to tailor the size, crystal phase, morphology, and luminescence features of upconversion materials.

Automated summary

Lanthanide-doped NaYF4 upconversion nano- and microcrystals were synthesized via a facile solvothermal approach, and the effects of various factors on the crystal phase, size, and morphology were systematically studied. The transition from alpha- to beta-phase depended on the volume ratio of EG/H2O and the molar ratio of NH4F/RE3+, while the morphology and size could be controlled by the type of organic co-solvent and Gd3+ dopant ions. The formation of larger beta-NaYF4 crystals with higher upconversion luminescence proceeded via intermediates of smaller crystals of cubic structure.