Oxygen vacancies and CDs synergistically mediated CD/TiO2 composite materials: investigation of electronic energy band structure modulation, photocatalytic performances, and mechanisms

Abstract

Visible light-driven carbon dot/defect titanium dioxide (CDT) nanocomposites were synthesized utilizing a straightforward solvothermal in conjunction with a wet chemical method. The CDT demonstrated a markedly enhanced capability for dye degradation under visible light, achieving a catalytic activity of up to 0.2485 min⁻¹ for a 20 mg L⁻¹ rhodamine B (RhB) solution, which is 21.6 times greater than that of commercial TiO₂ (P25). The observed enhancement in visible light photocatalytic activity is attributed to the synergistic effects of oxygen vacancies and carbon dots (CDs), which synergistically improved the CDT's response to visible light and mitigated charge carrier recombination. The defects induced by oxygen vacancies function as electron capture centers, optimizing the bandgap structure and effectively facilitating the separation of electrons and holes. Meanwhile, the loading of CDs effectively modulated the concentration of oxygen vacancies, significantly enhancing the carrier separation efficiency and migration rate of the composites. Moreover, under various environmental conditions and across multiple cycles, the material consistently exhibited commendable degradation efficiency, high catalytic activity, cycling stability, and robust resistance to environmental interference in the degradation of RhB. Notably, free radical capture experiments indicated that the primary active species in the photodegradation process transitioned from superoxide radicals (˙O₂⁻) and hydroxyl radicals (˙OH) to superoxide radicals (˙O₂⁻) and holes (h⁺), thereby confirming the existence of a type II heterojunction at the interface between the CDs and TiO₂, and proposing a plausible photocatalytic mechanism. This study presents an effective, environmentally friendly, and convenient methodology for the preparation of efficient photocatalytic composites characterized by defects and heterojunctions, thereby significantly advancing the research and application of catalysts for the removal of recalcitrant organic pollutants under visible light.