OH- ions-controlled synthesis and upconversion luminescence properties of NaYF4:Yb3+,Er3+ nanocrystals via oleic acid-assisted hydrothermal process

Yb3+ and Er3+ ions co-doped NaYF4 nanocrystals were synthesized with different amounts of NaOH via oleic acid (OA)-assisted hydrothermal process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectra were used to characterize the products. The result indicated that the introduction of NaOH into the initial reaction solution effectively promoted the cubic (alpha-) to hexagonal (beta-) phase transition of NaYF4:Yb3+,Er3+ nanocrystals, while excessive amount of NaOH favored the formation of alpha-NaYF4:Yb3+,Er3+ nanocrystals. Besides, with the increase of NaOH amount, the morphologies of beta-NaYF4:Yb3+,Er3+ nanocrystals varied from irregular nanobranch to uniform nanorod. Further investigation revealed that the addition of NaOH could facilitate the deprotonation of OA, leading to the formation of oleate (OA-), and meanwhile increased the concentration of OH-ions, inducing consequently the phase transition and morphology evolution of NaYF4:Yb3+,Er3+ nanocrystals. Moreover, the upconversion luminescence properties of NaYF4:Yb3+,Er3+ nanocrystals were systematically investigated. It was found that the upconversion emissions not only depended on the phase and morphology but also were influenced by the surface groups.