Illuminating the earliest stages of the soot formation by photoemission and Raman spectroscopy

In spite of advances in recent years, the current understanding of the nucleation and initial growth of soot in flames is still rather limited. The chemico-physical properties of just-nucleated particles and particles at the early stage of growth are here investigated by X-ray and ultraviolet photoemission spectroscopy,UV-visible and Raman spectroscopy. Both classes of particles are produced and selectively analyzed by opportunely selecting both flame and sampling conditions. The complementarity of the methods and the consistency of the results allowed us to evidence the differences in the carbon structure of the two particles. Both kind of particles are predominantly composed by sp(2) carbon and contain a few percent of oxygen mainly as ether-like bonds. A few percent of spa carbon has been observed in grown particles, and it coexists with a more developed graphitic structure with moderately larger aromatic islands, a lower band gap and higher density of states. The results of this work help to clarify the physicochemical transformation experienced by the just nucleated particles with sizes of 2-3 nm during the early steps of the growth process, which is fundamental for the improvement of current models of soot inception and growth. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.