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.
Netravathi et al. (Wed,) studied this question.