pH Mediated Control Synthesis of Lanthanide-Doped YPO4 Upconversion Nano/Microcrystals

A pH-mediated rational control over the phase transformation, morphology and size of lanthanide doped YPO₄ nano/microcrystals has been accomplished using a hydrothermal route. The morphology of the resulting particles evolves from the spheres, to the nanoflakes and finally to the nanorods when increasing the pH value of the reaction solution from 1 to 9, while the induction of crystal phase transformation from a tetragonal structure to a hexagonal structure takes place at pH = 5. When excited at 980 nm, the hexagonal phase YPO₄•0.8H₂O: 18%Yb³⁺, 2%Er³⁺ nanoflakes prepared at pH = 7 exhibit the strongest UC emission intensity among investigated samples of varying size, phase and morphologies. This work provides a paradigm on the controlled synthesis of lanthanide doped upconverting orthophosphates, which hold great promise for various photonic and biophotonic applications.