Fullerenic carbon in combustion-generated soot

Abstract

Soot samples collected as bulk solids and by thermophoretic sampling at different residence times in a fullerene-forming premixed benzene/oxygen flat flame (C/O=0.96, P=5.34 kPa, 10% argon, v=25 cm/s) were analyzed by high resolution electron microscopy. The samples contained soot particles that were composed to some extent of amorphous and fullerenic carbon (e.g., curved layers and fullerene-molecule-sized closed-shell structures). Qualitative and quantitative analyses of residence time-resolved samples showed that the length of curved layers increases and their radius of curvature decreases with increasing residence time in the flame. The number of closed-shell structures in the soot as well as the concentration of fullerene molecules in the gas phase increase with increasing residence time, consistent with fullerenes concentration increasing with residence time and with the consumption of fullerenes by reaction with soot. The data suggest that the formation of amorphous and fullerenic carbon occurs in milliseconds, with the fullerenic carbon becoming more curved as a soot particle traverses the length of the flame. Conversely, the formation of highly ordered carbon nanostructures, such as tubes and onions, appears to require much longer residence times, perhaps seconds or minutes depending on the temperature, in the flame environment.