ON THE OCCURRENCE OF FLUORESCENT CARBON IN TARRY CARBON PRODUCED IN FUEL-RICH FLAMES

Carbon dots emitting visible fluorescence have been recently added to the big family of carbon materials, after their discovery in 2004 in arc discharge soot [1]. Research is currently very active in studying alternative production sources of carbon nanodots to laborious processes as laser ablation plasma, chemical vapor deposition processes involving also laboratory treatment with oxidation agents. Combustion in fuel-rich, i.e. pyrolytic, conditions is a known source of strongly blue-violet fluorescing molecules as small polycyclic aromatic hydrocarbons (PAH) with carbon number<24 [2]. Here are presented insights in the spectroscopic (absorption and fluorescence) properties of blue- to green-fluorescing carbon we discovered in the tarry/polymeric carbon accompanying soot formation in fuel-rich flames. Flame-formed carbon particulate matter was sampled in fuel-rich flames and extracted with two solvents of increasing polar character and viscosity, separating blue and green fluorescent carbon from solid carbon (soot) particles. Blue fluorescent carbon featured by a subnanometric size and a high quantum efficiency (20%) was found to be mainly composed of large PAH (C>24). The green fluorescence is instead own of species successively broadly absorbing until to the visible and presenting a lower quantum efficiency (5%). On the basis of the spectroscopic results, green fluorescing carbon is suggested to be composed of aromatic domains (10-20 aromatic rings), bonded and/or turbostratically stacked each other. Different flame conditions in terms of fuel type and mixture richness were investigated to verify the possibility of exploiting fuel-rich combustion as source of carbon nanodots.