Combustion-Synthesized BaAl2O4: Eu2+, Nd3+, Pr3+ Triple-Co-Doped Long-Afterglow Phosphors: Luminescence and Anti-Counterfeiting Applications

Solution combustion-synthesized BaAl₂O₄: Eu²⁺, Nd³⁺, and Pr³⁺ blue-green long-afterglow phosphors are prepared and systematically investigated. First, XRD confirms the BaAl₂O₄ host and screens for trace residual features. SEM reveals the agglomerated granular morphology typical of combustion products. XPS verifies the valence states (Eu²⁺, Nd³⁺, Pr³⁺) and the chemical environment of the host lattice. UV-Vis diffuse reflectance spectra, transformed via the Kubelka-Munk function and analyzed using Tauc plots (indirect-allowed), indicate a wide band gap of the BaAl₂O₄ host with small, systematic shifts upon Nd³⁺/Pr³⁺ co-doping. PL measurements show Eu²⁺ 4f-5d emission and co-dopant-assisted excitation/defect pathways without altering the Eu²⁺ emission band shape. Afterglow lifetime and decay analyses correlate trap depth/distribution with the extended persistence. Finally, we demonstrate anti-counterfeiting by (i) snowflake printing and (ii) a binary 3 × 3 grid printed with two afterglow inks of different lifetimes to realize multi-level authentication. The sequential evidence links structure, chemistry, optical absorption, carrier trapping, and practical readout, providing a coherent basis for performance enhancement and application.