Controlled synthesis of β-NaYF₄:Yb³⁺/Er³⁺ microstructures with morphology- and size-dependent upconversion luminescence

Hexagonal NaYF₄ microcrystals with controllable morphologies and tunable sizes have been successfully synthesized by a facile hydrothermal method. The influences of the molar ratios of trisodium citrate (Cit^3-) to Y³⁺ and pH values on the phase and morphology evolution were investigated and discussed in detail. It is found that trisodium citrate and pH value as two significant external factors play key roles in the formation of the anisotropic architectures of β-NaYF₄ microcrystals. Taking hexagonal β-NaYF₄ microprisms as a candidate, the possible formation mechanism has been proposed on the basis of time-dependent experiments. In addition, the upconversion (UC) luminescence properties of β-NaYF₄:Yb³⁺/Er³⁺ samples were systematically studied and discussed. The results reveal that the UC luminescence properties are strongly dependent on their morphologies and sizes. This study not only provides important information for shape-controlled synthesis of other complex rare earth fluoride compounds, but also gives a reference for exploration of morphology- and size-dependent UC luminescence properties.