Fluorescent carbon dots synthesis in premixed flames: Influence of the equivalence ratio

Flame synthesis could represent a controllable method for production of carbon dots (CDs) with tunable emission properties, alternative to process involving oxidation agents and wet chemistry processing. To control final properties of CDs, a thorough analysis of different reactor parameters has to be conducted. Here we investigate the effect of combustion operative conditions, namely equivalence ratio, for CD production in a premixed flame reactor. CDs with specific optical properties have been synthesized at different equivalence ratios, burning both pure ethylene and ethylene/benzene mixtures. Carbon particulate matter was thermophoretically collected at the exhaust of the flame reactor. Successively, after dichloromethane washing, CDs were simply obtained by N-methyl-pyrrolidone extraction and filtration (<20 nm). In ethylene flames, CDs with fluorescence ranging from green to yellow, as equivalence ratio increases, were synthesized. In ethylene/benzene flames, produced CDs emit fluorescence from blue to green and equivalence ratio seems to play even a more marked role, inducing significant change in the emission spectra with small changes of equivalence ratio. Globally, their quantum yield ranges between 2 and 11%. CDs have been found generally stable for mild change of temperature (up to 80 °C), mild basic pH (pH = 8-10) and over long time at room temperature (up to 240 days). Overall, equivalence ratio demonstrates to be a powerful and simple tool to produce tunable CDs by flame synthesis on large scale.