Concentration-driven structural evolution and enhanced photoluminescence dynamics in Ho³⁺-activated SrMoO₄...

Rare-earth-activated molybdate phosphors have attracted considerable interest because of their tunable structural and luminescence properties for photonic applications. In this work, Ho³⁺-doped SrMoO₄ nanoparticles were synthesized through a controlled hydrothermal method employing Chikku (Manilkara zapota) leaf extract as a plant-derived processing additive. Structural characterization revealed the formation of a phase-pure tetragonal scheelite SrMoO₄ structure for all investigated Ho³⁺ concentrations, with no detectable secondary crystalline phases. The observed structural changes are consistent with Ho³⁺ incorporation into the SrMoO₄ host lattice, although direct confirmation of the substitutional site requires complementary characterization. The average crystallite size decreased from approximately 34 to 25 nm with increasing Ho³⁺ concentration from 1 to 7 mol%. UV-vis diffuse reflectance measurements showed a gradual modification of the absorption-edge profile, with the estimated optical band gap increasing from 4.10 to 4.17 eV. Under 451 nm excitation, the samples exhibited characteristic visible Ho³⁺ emissions, with prominent green and red emission bands near 545 and 658 nm, respectively. The relative emission intensity increased with Ho³⁺ concentration, reaching the highest value for the 7 mol% sample. Time-resolved photoluminescence measurements revealed a progressive increase in the emission lifetime from 148.62 µs for pristine SrMoO₄ to 326.95 µs for the 7 mol% Ho³⁺-doped sample. No reduction in lifetime was observed within the investigated concentration range, indicating that significant concentration quenching is not evident up to 7 mol% Ho³⁺. Overall, the results demonstrate a concentration-dependent relationship between the structural characteristics, morphology, visible photoluminescence, and emission decay dynamics of Ho³⁺ activated SrMoO₄ nanoparticles prepared through a plant-assisted hydrothermal route.