NaOH Promoted Synthesis of β-NaYF4:Yb,Er Nanoparticles with Enhanced Upconversion Luminescence

Herein, NaOH is innovatively introduced into the solvothermal synthesis of beta-NaYF4:Yb,Er with environmental benign solvent of ethanol/water solution. It is found that well-crystallized beta-NaYF4 nanoparticles with small sizes around 20-40 nm can be obtained under 240 degrees C for only 2 h in moderate pH conditions, reducing the typical reaction temperature (300 degrees C) and time (10 h), and in contrast to the typical acidic condition for solvothermal synthesis of beta-NaYF4. To further decrease reaction temperature, relatively high alkalinity is needed to overcome the transformation energy barrier from alpha-NaYF4 to beta-NaYF4. Mechanistic investigation reveals that OH- promotes the alpha- to beta-phase transition by releasing free F- ions and reducing the surface energy of beta-phase. An optimal Na/F:Ln3+ ratio of 5:1 ensures phase purity. Excited at 980 nm, the best beta-NaYF4:Yb,Er sample exhibits upconversion efficiency of 0.21% peaked at 538 and 654 nm with red to green ratio = 0.75, which is expected to be useful in solar cell applications as a light upconversion strategy to enhance light harvesting.