Utilizing rare-earth doping and photoluminescence: La3+ co-doped SrMnO3 hetero structure nanocomposites for enhanced photocatalytic dye degradation

In this study, La co-doped SrMnO₃ (LSM) perovskite nanomaterials with varying La contents (0.0-2.0 wt%) were synthesized via a solid-state method and evaluated for visible-light-driven photocatalytic degradation of methylene blue (MB). Structural characterization using X-ray diffraction (XRD) confirmed the formation of a single-phase perovskite structure, while transmission electron microscopy (TEM) revealed nanoscale morphology with reduced particle size upon optimal La incorporation. Surface functional groups were analyzed using Fourier-transform infrared spectroscopy (FTIR). Optical investigations by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) demonstrated a slight band gap narrowing from 2.38 to 2.24 eV, enhancing visible-light absorption, while photoluminescence (PL) analysis confirmed significant suppression of electron-hole recombination, particularly for the LSM3 sample (1.0 wt% La). The LSM3 photocatalyst exhibited the highest degradation efficiency (~ 99.03%) and rate constant (0.01648 min⁻¹), significantly outperforming pristine SrMnO₃ (~ 85.61%). Band edge analysis revealed that the valence band is sufficiently positive for hydroxyl radical (•OH) generation, whereas the conduction band is not favorable for direct superoxide formation. Accordingly, the photocatalytic process is governed by a defect-mediated charge transfer mechanism, in which La-induced oxygen vacancies and localized electronic states act as electron traps, enabling stepwise reduction of oxygen to generate reactive oxygen species. Radical scavenger experiments confirmed that •OH and •O₂⁻ are the dominant active species. The enhanced photocatalytic activity is attributed to the synergistic effects of defect-induced charge separation, reduced crystallite size (~ 9 nm), and internal electronic redistribution within the single-phase lattice. This work highlights an effective defect-engineering strategy for developing efficient visible-light photocatalysts for wastewater treatment applications.