Hydrothermal Synthesis and Upconversion Properties of α-NaYF4:Yb3+, Er3+ Nanocrystals Using Citric Acid as Chelating Ligand and NaNO3 as Mineralizer.

Yb3+ and Er3+ ions co-doped α-NaYF4:Yb3+, Er3+ nanocrystals were successfully synthesized via a facile hydrothermal method using citric acid (CA) as chelating ligand and NaNO3 as mineralizer. The influences of the CA/RE (RE = Y+Yb+ Er) molar ratio, reaction time, reaction temperature and the NaNO3/RE molar ratio on the crystal phases and shapes of as-prepared products have been well investigated. The possible formation mechanism has been discussed in detail and an aggregation growth theory has been proposed. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and photoluminescence (PL) spectra were used to characterize the products. Under 980 nm excitation, the emission intensities of the as-prepared nanocrystals were much stronger than that of the nanocrystals modified by ethylenediaminetetraacetic acid (EDTA). Additionally, the as-prepared products maintained spherical shape and high degree of monodispersion, which is expected to be suggestive for the preparation of other complex rare earth fluoride compounds.