Formation of hollow upconversion rare-earth fluoride nanospheres: Nanoscale Kirkendall effect during ion exchange

In this work, we report a facile solution-phase synthesis of hollow cubic phase α-NaYF₄
nanoparticles by a controlled ion exchange process from cubic phase Y₂O₃ nanospheres. We demonstrate that hollow nanoparticles with controlled size are formed owing to the nanoscale Kirkendall effect. The formation mechanism was studied with the XRD, STEM, and EDS line scanning characterization. The crystal structure similarity between the parent and the final product is essential for framework and morphology preservation. Although the sphere particles are polycrystalline and composed of the nanocrystals, the cubic structure of α-NaYF₄ nanocrystals displays a noticeable structure similarity with Y₂O₃, which we believe facilitates ion exchange between the primary nanocrystals and preservation of the secondary sphere morphology. The multicolor upconversion (UC) fluorescence (PL) was successfully realized in the Yb³⁺/Er³⁺(Tm³⁺) codoped α-NaYF₄ hollow nanospheres by excitation in the near-infrared (NIR) region. The various UC emission ratios of the samples were investigated as a function of hydrothermal reaction time to research the UC properties of the products and to further demonstrate the thermodynamic driving solution ion-exchange process.