Ionic liquid assisted microwave synthesis route towards color-tunable luminescence of lanthanide- doped BiPO₄

Ln³⁺-doped (Ln=Sm, Eu, Tb, Dy) nanoparticles of BiPO₄ with a particle size below 10 nm were synthesized in a straightforward manner from the appropriate mixture of the respective metal acetates and the task-specific ionic liquids choline or butylammonium dihydrogen-phosphate by conversion in a laboratory microwave (120 °C, 10 min). The ionic liquid acts not only as a solvent and microwave susceptor, but also as the reaction partner and nanoparticle stabilizer. The materials were thoroughly characterized not only with respect to their optical properties but also by PXRD, FT-IR, TEM techniques. Depending on the lanthanide, the nanomaterial shows intense luminescence of different colors such as: orange (Sm³⁺), red (Eu³⁺), green (Tb³⁺) or even white (Dy³⁺). - Highlights: • For the first time a synthesis of BiPO₄ based on ionic liquid is reported, in which the ionic liquid acts as the phosphate group donor, the reaction medium and as an in-situ stabilizer. • In the preparation route two ionic liquids were used - new synthetized– butylammonium dihydrogen phosphate and commercially available choline dihydrogen phosphate ionic liquid to compare their role as morphology control agents. • The forming particles are relatively small (below 10 nm) and not agglomerated but it has been shown that the IL cation had little influence on the particle morphology and size. • The low temperature route (reaction at 120 °C) leads to monazite phase and the obtained material is water-free, not requiring post reaction heat treatment. • Lanthanide doped BiPO₄ showed intense luminescence of different colors such as: orange (Sm³⁺), red (Eu³⁺), green (Tb³⁺) or white (Dy³⁺).