Size-Controlled Growth of β-NaGdF4 and β-NaGdF4:Yb,Er Nanocrystals: The Influence of the Surface Area of NaF on the Nucleation of the β-Phase

Nearly monodisperse beta-NaGdF4 and beta-NaGdF4:Yb,Er nanocrystals in the size range of 3-14 nm and 6-48 nm, respectively, were prepared by the reaction of solid NaF powder with a solution of the lanthanide oleates in oleic acid/octadecene. Surprisingly, the surface area of the NaF powder has a strong influence on the nucleation of beta-phase seeds and, hence, the final size of the particles: NaF powder with high surface area prepared by shock-frosting and freeze-drying an aqueous solution of NaF yields much smaller particles than the same amount of microcrystalline NaF powder. In both cases, 2-3 nm particles are formed as the first product, but the ratio of alpha-phase to beta-phase particles in this first product decreases with increasing surface area of the NaF powder. Subsequent heating of the small particles in oleic acid/octadecene results in dissolution of the alpha-phase particles and growth of the beta-phase particles to their final size, which depends on the initial ratio of alpha-phase to beta-phase particles. A similar effect of the surface area of NaF is observed for group III nanocrystals like NaYF4, but the final particle sizes are much larger because of the much lower number of beta-phase seeds formed. The hitherto unknown influence of the surface area of NaF may be the origin of problems with the reproducibility of the synthesis of NaREF4 particles. It is also the reason why the particle size can be controlled by simply varying the amount of NaF powder in the synthesis.