Synthesis of praseodymium-ion-doped perovskite nanophosphor in supercritical water

We report the synthesis of praseodymium-doped calcium strontium titanate nanoparticles, (Ca_0.6Sr_0.4)_0.997Pr_0.002TiO₃ (PCSTO), using hydrothermal synthesis under supercritical water conditions and the production of red luminescence. Starting solutions were prepared by dissolving calcium nitrate, strontium nitrate, titanium hydroxide sols, and praseodymium nitrate in distilled water. We investigated the effect of the reaction temperature, concentration, and pH of the starting solution on the luminescence properties. Synthesis was conducted at temperatures of 200 °C–400 °C, a reaction pressure of 30 MPa, and for reaction times of 4–20 s. The Pr concentration was set to 0.2 mol% relative to the (Ca_0.6Sr_0.4) ions. We also investigated the effect of high temperature annealing on the luminescence properties of the PCSTO nanoparticles. Particle characteristics were evaluated using x-ray diffraction, a scanning transmission electron microscope (STEM) equipped with an energy-dispersive x-ray spectrometer, and a fluorometer. Single-phase perovskite particles were obtained at hydrothermal reaction temperatures of over 300 °C even for a reaction time of several seconds. STEM images showed that the particles had cubic-like shapes with diameters of 8–13 nm and that they were chemically homogeneous. The PCSTO nanoparticles exhibited sharp red luminescence at 612 nm corresponding to the f–f transition of Pr³⁺ ions. Moreover, annealing at 1000 °C led to particle growth, achieving diameters of 40 nm and an increase in the quantum efficiency to around 12.0%. (paper)