Two-Dimensional β-NaLuF₄ Hexagonal Microplates

A simple hydrothermal method has been developed to synthesize monodisperse β-NaLuF₄ microplates in a large scale. The microcrystals have a perfect hexagonal shape with a diameter of about 5.2 µm and a thickness of 300 nm. Trisodium citrate (Cit^3-), which is introduced into the reaction mixture and acts as the chelating agent and shape modifier, plays a key role in fine-tuning the microstructures. The dominant adsorption of Cit^3- onto the {0001} facets lowers the surface energy of these facets. Consequently, the typical growth in the [0001] direction is prohibited, and the growth of nuclei is driven along the six symmetric directions ±[101̅0], ±[11̅00], and ±[011̅0], which directly results in the formation of β-NaLuF₄ microplates. The microstructure and growth mechanism of β-NaLuF₄ microplates are discussed in detail. Additionally, we investigated the photoluminescence properties of β-NaLuF₄:Ln³⁺ (Ln = Eu, Tb, and Yb/Er). This powerfully demonstrates that β-NaLuF₄ is an excellent host lattice for down-conversion and up-conversion luminescence of various optically active lanthanide ions.