Hydrothermal synthesis and site symmetry tuning of polycrystalline YVO4:Eu nanoparticles via a continuous-flow microreactor.

Yttrium orthovanadate (YVO4) has been widely used as a host material for low- and medium-power diode-pumped solid-state lasers due to its excellent thermal, mechanical, and optical properties. This work demonstrates the synthesis and site symmetry tunning of polycrystalline YVO4:Eu nanoparticles with uniform size and shape using a continuous-flow microreactor at high pressures. High-quality YVO4:Eu nanoparticles were created using a residence time of fewer than 20 s. Carefully controlling the heat flux and flow rate can produce the YVO4:Eu nanoparticles showing different crystallinity, crystal morphologies, site symmetry around Eu3+, and therefore optical emission. The site symmetry of YVO4:Eu is adjusted without any stoichiometric modification of the precursors by simply varying the flow rate and heat flux of the microreactor. The site symmetries of the as-synthesized YVO4:Eu nanoparticles are studied by investigating their photoluminescent emission spectra and computational model of first-principle density functional theory (DFT). The DFT model indicates that the oxygen vacancy influenced the V-O association and the overlap between Eu 4f and V 3d states which can contribute to different optical transitions and, therefore, distinct emission spectrum. The use of a continuous flow microreactor at high pressure provides better understandings of the hydrothermal syntheses of functional nanoparticles and enables scalable manufacturing concurrently.