Soot structure by high resolution electron transmission microscopy (HR-TEM) image analysis: current potentialities and perspectives

Direct molecular imaging of carbon structures is allowed by recent progresses in high resolution electron transmission microscopy (HR-TEM). Advanced procedures of image analysis have been set up in order to extract quantitative structural information describing the shape, size and orientation of graphene sheets and their distribution inside the carbon particle. The HR-TEM data gathered on soot formed in flames fueled by different hydrocarbons (ethylene, benzene) are critically reviewed in the present paper putting in evidence the potentialities and the drawbacks of current procedures of HR-TEM imaging analysis. Some intriguing differences in coherent domains organization (Basic Structural Units, BSU) of soot were found in dependence on the fuel, particle aging and temperature. The primary particles early formed at soot inception (young soot) in ethylene flames were found to consist mostly of non stacked short graphitic segments mixed with small nuclei of amorphous/disordered carbon material. These structures transformed during soot aging with the enhancement of the stacked units. Soot from aromatic fuels presented, already from the beginning, a larger presence of BSU and negligible amorphous carbon. Moreover, benzene soot nanostructure showed a significant radial variation and the presence of shell almost and/or completely closed of few nanometers. In spite of these visual observations, small differences in quantitative parameters featuring the BSU of different soot were observed. Indeed, only a slight increase of the layer length was observed whereas the spacing of the BSU layers appeared to be quite constant (0.38 nm) independently on the fuel, aging and temperature. A more evident decrease of the percentage of non-stacked layers (nsl) and the increase of fractional coverage were observed with soot aging in particular for aromatic fuels and higher flame temperatures. In the present paper the perspectives of new imaging analysis methods are analyzed for a more robust statistic of BSU parameters evaluation (as for example the effective spaces between fringes) taking into consideration the addition of two key parameters as tortuosity and curvature radius which account for the deviations from the planar shape of the graphene layers. These improvements open the way to more efficient insights on soot nanostructure analysis by HR-TEM.