Red Emissive Upconversion Carbon Quantum Dots Modifying TiO₂ for Enhanced Photocatalytic Performance

Carbon quantum dot (CQD) decorated semiconductor materials have become promising photocatalysts in photocatalysis, with a chance to solve energy and environmental crises. Red emissive carbon quantum dots (R-CQDs) have excellent photoluminescence (PL), upconversion, narrowed band structure, electron-accepting, and transmission properties compared to other colors emitting CQDs. In this work, we use the as-synthesized R-CQDs to decorate P25 (R-CQDs/P25) for highly enhanced visible-light photocatalysis. The results show that the photocatalytic performance of the R-CQDs/P25 composites is considerably enhanced with the increasing amount of R-CQDs in the visible-light region. The red emissive upconversion R-CQDs play dual roles as a cocatalyst to separate the photogenerated carriers and an upconversion converter from lower energetic photons to high energetic photons. Various measurements reveal that the R-CQD/P25 photocatalysts have a narrowed band gap, improved charge transfer efficiency, and lower recombination rate, synergistically endowing the R-CQDs/P25 composites with excellent photocatalytic performance. The R-CQD/P25–2.5 composite exhibits the highest photocatalytic activity, which is ∼10 times higher for hydrogen evolution and ∼20 times higher for the photodegradation of methylene blue (MB) compared to pure P25, and its apparent quantum yield (AQY) is 4.94% under 450 nm monochromatic light. This work opens the door for rational design of different fluorescence CQDs and their potential application for solar hydrogen production in a low-cost noble-metal-free system.