Carbon nanostructure and reactivity of soot particles from non-intrusive methods based on UV-VIS spectroscopy and time-resolved laser-induced incandescence

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O-2 from easily measurable optical properties. First, ex situ experiments utilizing thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) serve to evaluate the kinetics of soot oxidation with O-2 and relate reactivity to particle morphology and nanostructure. Second, ultraviolet-visible (UV-VIS) absorption spectra provide wavelength-dependent absorption cross sections and refractive-index functions E((m) over tilde, lambda). From these, optical band gap energies, E-OG, and coefficients xi* for single parameter functions describing the wavelength-dependency of E(me, l) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E((m) over tilde, lambda(i))/E((m) over tilde, lambda(j)) at three excitation wavelengths and primary particle size distributions are acquired. The ex-situ experiments show that the size of the graphene layers predominantly determines soot reactivity against oxidation. Graphene layer size and, therefore, soot reactivity are reflected in the UV-VIS absorption spectra and E((m) over tilde, lambda), EOG, and xi*, respectively. Similarly, scattering-corrected ratios E((m) over tilde, lambda(i))/E((m) over tilde, lambda(j)) from TR-LII also reflect graphene layer size and, hence, soot reactivity. The established strong correlations between the optical properties, nanostructural characteristics and reactivity against oxidation make UV-VIS spectroscopy as well as TR-LII useful fast in-situ diagnostic methods for soot reactivity. (C) 2021 The Authors. Published by Elsevier Ltd.