Self-Assembled β-NaGdF4 Microcrystals: Hydrothermal Synthesis, Morphology Evolution, and Luminescence Properties

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Abstract
A self-assembly process has been designed for the controlled synthesis of β-NaGdF4 with uniform morphology, dimension, and considerable monodispersity under a gentle hydrothermal condition using sodium citrate as the chelating agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrum (EDS), and up-conversion (UC) photoluminescence spectra were used to characterize the samples. The results indicate that the NaGdF4 microcrystal can be rationally modified in phase, size, and morphology through tuning the pH value, sodium citrate content, and reaction time. Moreover, the hybrid process of the crystal growth and the self-assembly were thoroughly discussed, and a possible formation mechanism was proposed. Furthermore, the UC luminescence properties as well as the emission mechanisms of β-NaGdF4:17%Yb3+/3%Ln3+ (Ln = Er, Tm, Ho) microcrystals were systematically investigated. It is found that under 980 nm excitation, only limited emission bands were discovered which can be attributed to the energy gap and migration function of the Gd3+ ions in the β-NaGdF4 microcrystals. It is expected that the synthetic strategy can be applied to prepare many other types of micro- and nanocrystals as well.

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