Interaction Between Y3+ and Oleate Ions for the Cubic-to-Hexagonal Phase Transformation of NaYF4 Nanocrystals

Understanding the phase transformation of NaYF4 nanocrystals from cubic to hexagonal is of great importance for both scientific interests and applications. Herein, based on the density functional theory, we found that the dominant difference between cubic and hexagonal phase NaYF4 is the location of Y3+, and oleate ions are particularly favorable to bonding with Y3+. The results indicated that oleate salts as ligand possibly induce the orderly arrangement of Y3+ and lower the energy barrier for the formation of hexagonal phase. The experiments of using different amount of oleate ions as ligand were designed to elucidate their influence on the phase transformation. We found that the approach with relatively high amount of sodium oleate results in dramatic shortening of reaction time (down to 5 min) on cubic-to-hexagonal phase transition for preparing ultrasmall (∼13 nm) hexagonal phase NaYF4 nanocrystals. To exclude the influence of Na+ increasing in using sodium oleate, potassium oleate was also used as ligand in the synthesis, and similar results in phase transformation were observed. Our results suggest that the interaction between oleate ions and Y3+ efficiently promotes the phase transformation of NaYF4 nanocrystals and provide new insight into how the ligand affects the phase transformation of nanocrystals.