Interfacial band engineering and enhanced visible-light photocatalytic activity of carbon quantum dot modified Mn3O4/TiO2 thin films

The uncontrolled release of synthetic dyes into water bodies presents severe environmental and health concerns due to their high stability and toxicity. In this work, carbon quantum dot (CQD)-modified Mn 3 O 4 /TiO 2 composite thin films were fabricated via a hydrothermal synthesis route to enhance photocatalytic performance driven by visible light. Structural and spectroscopic analysis confirms the successful integration of CQDs and Mn 3 O 4 with TiO 2 , resulting in a reduced optical band gap from 2.20 eV (CQD/TiO 2 ) to 2.01 eV (CQD-Mn 3 O 4 /TiO 2 ) and a lower Urbach energy, indicating suppressed band-tail states. Time-resolved photoluminescence analysis reveals a decrease in average carrier lifetime from 15.2 ns to 6.9 ns, confirming rapid interfacial charge transfer in the ternary heterostructure. Electrochemical impedance spectroscopy further demonstrates reduced charge-transfer resistance upon Mn 3 O 4 incorporation. Under natural sunlight irradiation, the CQD-Mn 3 O 4 /TiO 2 thin films achieve complete (100%) degradation of methylene blue (MB), as supported by chemical oxygen demand (COD) analysis and near-complete degradation of rhodamine B (Rh B), following pseudo-first-order kinetics with an apparent rate constant of 0.0466 min⁻¹ for methylene blue and 0.0276 min⁻¹ for Rhodamine B, significantly outperforming CQD/TiO 2 . Reactive species trapping experiments and Electron Paramagnetic Resonance (EPR) studies identified hydroxyl and superoxide radicals as the dominant reactive oxidizing species. Excellent photostability, with over 95% degradation efficiency retained after 10 cycles, highlights the robustness of the thin-film system. This work demonstrates that interfacial band engineering using CQDs and Mn 3 O 4 is an effective strategy for designing durable, sunlight-active photocatalysts for sustainable wastewater treatment.