Controllable synthesis of water-soluble NaYF₄: Yb, Er upconversion nano-particles and their luminescent properties

NaYF₄: Yb, Er upconversion nanoparticles with the size of around 30 nm were fabricated through solvothermal method. The appliation of citric acid (CA) enables the good water solubility. The results indicate that the dosage of NaOH or CA determines the concentration of -COO- group, which is acted as an effective coordinating group of rare earth ion. It further tune cubic to hexagonal phase transformation and realize the morphology control and luminescent properties tuning. With an increased amount of NaOH, the product change from 30 nm sized α-NaYF₄: Yb, Er nanoparticles to 300 nm sized β-NaYF₄: Yb, Er submicrospheres. An increased amount of CA promotes the formation of β-NaYF₄: Yb, Er as well. Moreover, the up-conversion luminescent properties of these products with different morphologies are studied.