Morphology-controllable synthesis and enhanced luminescence properties of β-NaLuF₄:Ln (Ln = Eu, Tb and Ce/Tb) microcrystals by solvothermal process

Controlled synthesis of β-NaLuF₄ with uniform morphology, dimension and considerable monodispersity was designed via a gentle solvothermal process by using ethylenediamine (EDA) and ethylene glycol (EG) as the mixed solvent and NaNO₃ as a mineralizer. X-ray diffraction (XRD), field scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectra (EDS) and down-conversion (DC) photoluminescence spectra were used to characterize the samples. The results indicate that the as-prepared β-NaLuF₄ microcrystals can be rationally modified in phase, size and morphology by tuning the solvent constitution, NaNO₃ content, and reaction time. Moreover, the crystal growth process was thoroughly discussed through a series of time-dependent experiments and a possible formation mechanism was proposed. Furthermore, the DC luminescence properties as well as the emission mechanisms of β-NaLuF₄:Ln³⁺ (Ln = Eu, Tb and Ce/Tb) microcrystals were systematically investigated. It is found that the DC luminescence properties can be improved observably by introducing Ce ³⁺ into the β-NaLuF₄ microcrystals and the doping concentration of the Ce³⁺ is optimized under a fixed concentration of Tb³⁺. It is expected that the synthesis strategy can be applied to prepare many other types of micro- and nano-crystals as well.