Insight into the synthesis of carbon quantum dots by gas-liquid discharges: The role of precursors

Carbon quantum dots (CQDs) have been widely used in many fields due to their excellent optoelectronic properties. However, the impact of precursor on the structure, properties, and formation mechanism of CQDs under gas-liquid discharge process is poorly understood, which invites more detailed studies toward structure design and fluorescence regulation. In this study, the CQDs of blue fluorescence are successfully synthesized using only low-cost organic solvent such as ethanol, 1-butanol, and 1-hexanol as the precursors by gas-liquid discharges within 5 min. Entering argon gas into the liquid reactant greatly lowers the difficulty of discharges in the liquids and improves the discharge stability, and thereby produce a more uniform particle of the synthesized CQDs. The average size of CQDs synthesized by alcoholic organics is 2–3 nm, and the surfaces of the products are functionalized by various oxygen-containing groups. The CQDs present bright blue fluorescence with an excitation wavelength of 365 nm and exhibit an excitation-dependent property. In situ observations of optical emission spectrometry (OES) during discharges indicate that the precursors are break down into free radicals such as C₂, CH and H, and then radicals recombine and grow continuously into CQDs. During the formation of CQDs, molecular fluorophores are produced concomitantly. In addition, changing the functional groups of the precursors significantly influences the fluorescence properties of the synthesized CQDs. For CQDs prepared from solvents containing heterogeneous elements such as nitrogen and sulfur show green fluorescence properties, i.e., fluorescence red-shift. This work provides a pathway for fabricating diversified CQDs by modulating the structure and functional groups of the selected precursors.