Two-step ion-exchange synthetic strategy for obtaining monodisperse NaYF4:Ln3+ nanostructures with multicolor luminescence properties

Abstract

Monodisperse β-NaYF₄:Ln³⁺ (Ln = Eu, Tb, Yb/Er, or Yb/Tm) nanocrystals with multiform novel morphologies have been synthesized for the first time by a novel two-step ion-exchange route expressed as Y₂(OH)₅NO₃·nH₂O (LYH) → Y(OH)_xF_3−x → β-NaYF₄. In step I (LYH → Y(OH)_xF_3−x), the Y(OH)_xF_3−x is fabricated using a one-pot method from the starting point of a unit layer at low temperature through a novel style of ion-exchange from LYH based on the special crystal structural similarity between the LYH unit layer and the Y(OH)_xF_3−x {0001} plane. Of particular interest is that the ion-exchange process in step I can be completed within the initial few minutes, and the unique two-dimensional layered structure of LYH greatly favors the ion-exchange reaction by exfoliating into atomically thick monolayers. Meanwhile, the identical structural framework between Y(OH)_xF_3−x and β-NaYF₄ ensures the in situ ion-exchange reaction in step II. The composition of the intermediate precursor Y(OH)_xF_3−x can be tuned in a wide range, which gives the synthetic parameters high maneuverability and greatly simplifies the experimental variables. In addition, the luminescence properties showed that the as-prepared β-NaYF₄:Ln³⁺ nanocrystals exhibited strong characteristic multicolor emissions through upconversion and downconversion.