Hydrothermal synthesis of controllable size, morphology and optical properties of β-NaGdF4: Eu3+ microcrystals

Eu3+-doped β-NaGdF4 (hexagonal NaGdF4) microcrystals with various sizes and morphologies have been prepared via a facile hydrothermal route with and without trisodium citrate (Na3Cit) under different Ln3+/NaF (Ln=Gd, Eu) molar ratios and pH values. The microstructures and luminescence properties of the as-synthesized microcrystals were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) spectroscopy. Moreover, the photoluminescent properties of Eu3+-doped β-NaGdF4 samples synthesized at different pH values were systematically discussed. As a result, monodisperse and uniform spherical and rod-like crystals were successfully synthesized by varying Ln3+/NaF molar ratio, Ln3+/Na3Cit molar ratio and pH value, and the obtained feasible range of pH value is 8-11, the optimal molar ratios of Ln3+/NaF and Ln3+/Na3Cit are 1:6 and 1:3, respectively. Our results show that there is an energy transfer process between the host and activator ions, emissions from high energy configurations to ground state are gradually quenched in the β-NaGdF4: Eu3+ phosphors obtained in the strong alkaline solution. In addition, pure orange-red photoluminescent color can be obtained from β-NaGdF4: 2 mol% Eu3+ phosphors synthesized under the obtained processing conditions.