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

Herein, NaOH is innovatively introduced into the solvothermal synthesis of β-NaYF₄:Yb,Er with environmental benign solvent of ethanol/water solution. It is found that well-crystallized β-NaYF₄ nanoparticles with small sizes around 20–40 nm can be obtained under 240 °C for only 2 h in moderate pH conditions, reducing the typical reaction temperature (300 °C) and time (10 h), and in contrast to the typical acidic condition for solvothermal synthesis of β-NaYF₄. To further decrease reaction temperature, relatively high alkalinity is needed to overcome the transformation energy barrier from α-NaYF₄ to β-NaYF₄. Mechanistic investigation reveals that OH^- promotes the α- to β-phase transition by releasing free F^- ions and reducing the surface energy of β-phase. An optimal Na/F:Ln³⁺ ratio of 5:1 ensures phase purity. Excited at 980 nm, the best β-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.