An environmentally friendly route to upconverting micro- and nano-particles: low temperature ligand-free ambient pressure aqueous synthesis

Rare-earth doped upconverting nanomaterials (UCNPs) have attracted considerable interest due to their unique ability to convert near-infrared (NIR) light into visible emission, making them useful for a variety of applications including bioimaging, sensing, anti-counterfeiting, and photonic devices. Among them, β-NaYF₄:Yb/Er is the most studied for upconversion applications. However, conventional synthesis methods typically require high temperatures, organic solvents, or sealed high-pressure systems, which limit scalability and environmental compatibility. In this work, we demonstrate a low-temperature, ligand-free, and ambient-pressure synthesis of hexagonal NaYF₄ using only water as the solvent. By optimizing precursor ratios and reaction temperature, we achieved: (i) formation of the hexagonal phase at room temperature; (ii) phase-pure hexagonal NaYF₄ at 95 °C; and (iii) controlled cubic-to-hexagonal conversion of ∼50 nm particles via 400 °C annealing without sintering or size growth. No surfactants, organic solvents, or pH-adjusting agents were used at any stage. The synthesized Yb/Er-doped NaYF₄ displays strong upconversion emission under 980 nm excitation. We further show that the method is also applicable to Tm³⁺ and Ho³⁺ dopants. This versatile and environmentally friendly strategy provides a new route for scalable synthesis of upconversion nanomaterials.