Morphology and phase evolvement of Yb3+/Er3+ co-doped NaYF4 microtubes prepared with YF3 submicrospindles as precursor

Hexagonal phase NaYF4 microtubes co-doped with Yb3+ and Er3+ were synthesized through a hydrothermal process with YF3 submicrospindles as precursor. The X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were utilized to characterize the structure and morphology of the as-prepared products. XRD results show that pure cubic NaYF4 crystals can be obtained when reaction time is 2 h. While the product is mixture of cubic and hexagonal phase NaYF4 when reaction time is from 7 to 20 h. Continuing to increase the reaction time to 24 h, the pure hexagonal NaYF4 crystals were formed. The FE-SEM and TEM results show that the morphology of pure cubic NaYF4 is spherical clusters composed of spherical nanoparticles with average diameter of about 100 nm and the pure hexagonal NaYF4 crystals have tubular structure with out diameter of about 0.3–0.5 μm, inner diameter of about 0.5–1 μm and length ranging from 3 to 12 μm. The luminescence properties of Yb3+/Er3+ co-doped cubic and hexagonal phase NaYF4 microcrystals were also studied. Under 980-nm excitation, the upconversion luminescence (UCL) intensity of hexagonal phase NaYF4 microtubes is much stronger than that of cubic phase clusters. Moreover, both red and green upconversion are ascribed to the two-photon process. Therefore, hexagonal phase NaYF4 microtubes with high UCL efficiency may have a potential application in photonic device.

► Yb3+/Er3+ co-doped NaYF4 microtubes were prepared through a hydrothermal process with YF3 submicrospindles as precursor.
► Hydrothermal treatment time plays an important role in the control of phase and morphology of the product.
► The UCL efficiency of Yb3+/Er3+co-doped NaYF4 microtubes is much higher than that of cubic phase Yb3+/Er3+ co-doped NaYF4 clusters.
► Yb3+/Er3+ co-doped NaYF4 microtubes may have a potential application in photonic device.