Hydrothermal synthesis of Eu3+ doped yttria nanoparticles using a supercritical flow reaction system

Highly crystalline Eu3+ doped yttria nanoparticle was synthesized by hydrothermal reaction in supercritical water using a continuous flow reaction system (FHT). The reactants of Y(NO3)3/Eu(NO3)3 mixed solution and KOH solution were used as starting materials and that was heated quickly up to 350–450 °C under the pressure of 30 MPa for 0.1–15 s as reaction time. The XRD results revealed that the crystal phase of as-prepared particles was YOOH and converted into cubic-phase Y2O3 after annealing above 550 °C. Primarily particle size of the YOOH was as small as less than 50 nm, keeping after annealing at 800 °C. Effects of reaction time, annealing temperature and Eu doping amount on photoluminescence were examined. The as-prepared particles exhibited red emission without annealing at high temperatures whereas photoluminescent intensity at 612 nm was increased with an increase in the annealing temperature. Photoluminescent intensity was increased with an increase in the Eu doping amount until 4 mol % and saturated at 8 mol %. The photoluminescent property was compared with reference samples via conventional co-precipitation (CP) and batchwise hydrothermal (BHT) methods. The photoluminescent intensity for annealed samples increased in the order: FHT < BHT < CP owing to the increased particle size.