Wavelength-dependent effects of carbon quantum dots on the photocatalytic activity of g-C3N4 enabled by LEDs

Although the photocatalytic activity of g-C3N4 can be evidently enhanced by implanting 0-dimensional (0D) carbon quantum dots (CQDs), the enhancement mechanism, especially the relationship between the role of CQDs and photo-excitation wavelength, remained largely unclear. Herein, 0D/2D CQDs/g-C3N4 heterostructures were fabricated for sulfamethazine (SMZ) photodegradation under both polychromatic (simulated daylight, 380 <= lambda <= 780 nm) and monochromatic light (395, 410, 425, 435, 455, 475 and 500 nm) emitting from wavelength-tunable LEDs. We confirmed the promotional role of CQDs as a donor-sensitizer-acceptor to facilitate the charge separation and enhance light absorption. Further investigation showed that CQDs even exerted opposite effects in g-C3N4 photocatalytic activity under different excitation wavelengths: i) inhibition (380 <= lambda <= 410 nm), ii) transition (410 < lambda <= 425 nm), and iii) promotion (425 < lambda <= 780 nm). Such oscillating behavior was attributed to the competition between synergistic effect and down-conversion fluorescent property of CQDs. The inherently versatile role of CQDs was among the first revealed, which was largely ignored in previous studies. Moreover, water quality parameters (pH, HCO3-, Br-,Cl-, NO3-, and SO42-) showed complicated impacts, and potentially improved the photocatalytic performance in actual water matrices. This study is of referential significance for developing LED-based photocatalysis technology for water decontamination, especially when CQDs are incorporated to improve the performance.