Synthesis of NaYF4 microcrystals with different morphologies and enhanced up-conversion luminescence properties

Abstract

In this paper, NaYF₄ microcrystals with a variety of morphologies, such as microsphere, hexagonal prism, microtube and octahedron shapes, have been synthesized via a hydrothermal method using sodium poly(4-styrenesulfonate) (PSS) as a template. The structural and kinetic factors that govern the phase and shape evolution of NaYF₄ microcrystals have been systematically studied. And the influence of reaction time, pH values, fluoride sources and reaction temperature on the phase and shape of the as-synthesized NaYF₄ was systematically investigated and discussed. It is found that PSS can be employed as a shape modifier, which is responsible for the shape evolution. The possible formation mechanism for the products with various architectures has been presented. The up-conversion (UC) properties of Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺ or Yb³⁺/Tm³⁺ codoped NaYF₄ samples were studied. In particular, the luminescence intensity can be markedly enhanced by Li⁺ doping, the related mechanism has been discussed. Furthermore, a systematic study on the UC emissions of Yb³⁺/Tm³⁺ codoped NaYF₄ samples with microsphere, hexagonal prism, microtube and octahedron shapes has shown that the optical properties of these phosphors are strongly dependent on their morphologies and sizes. This study would be suggestive for the precisely controlled growth of inorganic crystals, especially for those rare earth fluoride compounds.