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 (α)→β 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 deg. C, NaF generation was the limiting step and α-UCNPs were formed via a 'burst of nucleation'. In stage II, above 310 deg. 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.