A single-step synthesis and the kinetic mechanism for monodisperse and hexagonal-phase NaYF4:Yb, Er upconversion nanophosphors

A single-step synthesis for monodisperse and hexagonal-phase (β) NaYF4:Yb, Er upconversion nanophosphors (UCNPs) with a consistent hexagonal prism shape in the size range from 18 to 200 nm was achieved. The kinetic mechanisms for the particle phase transition and growth were examined. The β-UCNPs were obtained via co-thermolysis of trifluoroacetate precursors in octadecene (ODE) with combined ligands of oleic acid (OA) and trioctylphosphine (TOP). The experimental results showed that the combined OA–TOP ligand was crucial for changing the surface energy and controlling the particle shape over a broad size range. It was found that the particle sizes could be controlled by varying the molar ratios of Na(CF3COO)/Re(CF3COO)3 (Re = Y, Yb, and Er). A high Na/Re ratio accelerated the cubic-phase transition and promoted the growth of smaller β-UCNPs. The formation of β-UCNPs was classified into kinetic and diffusion controlled stages, depending on the reaction temperature and the dominant crystalline phases formed in each stage. In stage I, 250–310 °C, NaF generation was the limiting step and α-UCNPs were formed via a 'burst of nucleation'. In stage II, above 310 °C, the α-UCNPs formed were re-dissolved and the growth of β-UCNPs was a diffusion controlled process governed by the Gibbs–Thompson effect. A quasi-steady-state species assumption for NaF and a chemical potential equilibrium in the solution were introduced to explain the particle size dependence on Na/Re ratios. The study of UC luminescence showed that the UC intensity was proportional to the sizes of the β-UCNPs.