One-Step Continuous Fabrication of Solid, Porous and Hollow YAG:Eu3+ Microspheres by Ultrasonic Spray Pyrolysis: A Rare-Earth-Saving Strategy for Red-Emitting Phosphors

Reducing the consumption of scarce and costly rare-earth elements without sacrificing emission performance is a recognised challenge for the sustainable production of phosphors. Hollowing the phosphor particle is an attractive route to this goal, because in micro- and sub-micron phosphors the rare-earth ions that contribute most to the emission reside mainly in the surface shell. However, existing hollow-phosphor routes (template, hydrothermal and sol–gel) are multi-step, batch-type and difficult to operate continuously, and they have rarely been applied to yttrium aluminium garnet (Y3Al5O12, YAG), a technologically important but hard-to-crystallise host that conventionally requires solid-state firing above 1600 °C. Here we report a one-step, continuous ultrasonic spray pyrolysis (USP) route that fabricates solid, porous and hollow YAG:Eu3+ microspheres from the same general process at only 900 °C, using citric acid as a pore-forming agent and sucrose as a film-forming agent. Single-phase cubic YAG was obtained for all three morphologies without any post-annealing, and the surface porosity and shell thickness varied with the additive concentration. On the same particle population, the hollow microspheres (H3) retained 96.1% of the relative emission intensity of their solid counterparts under 250 nm excitation while consuming only ~57% of the rare-earth-bearing raw material—a ~43% saving geometrically estimated with a spherical-shell volume model at the measured mean outer diameter and shell thickness—corresponding to a geometrically estimated ~1.68-fold gain in emission per unit of rare-earth input. Absolute quantum-yield measurements indicate that the hollow shell preserves the luminescence efficiency of the solid particle (27.97% versus 29.37% under 250 nm excitation), so the retention of almost the full emission with only ~57% of the material is consistent with the removed core being largely optically inactive rather than any efficiency penalty. A shell-volume model further projects that the saving would increase to ~77–80% for larger thin-walled spheres, although the hollow powders obtained here have mean diameters below 1 μm. The route is simple and compatible with continuous processing, and offers a potentially general, material-saving strategy for the production of rare-earth phosphors and other functional oxides.