Understanding photocatalytic processes in carbon dots with nitrogen and sulfur heteroatoms: Energy or electron transfer?

This study explores the impact of surface chemistry and heteroatom doping on the photocatalytic activity of carbon dot (CD), with a focus on their roles in energy and electron transfer mechanisms for reactive oxygen species generation. A series of N- and S-doped CDs were synthesized and thoroughly purified. Photocatalytic singlet oxygen production was detected via endoperoxide formation, while hydroxyl radicals were monitored through peroxide-assisted hydroxylation of terephthalic acid. The findings reveal that specific CDs primarily govern either energy or electron transfer mechanisms. Fluorescence lifetime analysis suggested that long-lived photon emission generally correlates with increased singlet oxygen production, though the effects vary depending on the type of nitrogen and sulfur doping. This also resulted in an alteration from energy transfer to electron transfer processes. Doping also influences the shift from energy to electron transfer pathways. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis suggested that the partial crystallinity and the presence and arrangement of surface oxygen-containing groups, such as S--O, -OH, and -COOH, may drive this transition between the two mechanisms.