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

Abstract

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O2 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 O2 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~,λ). From these, optical band gap energies, EOG, and coefficients ξ∗ for single parameter functions describing the wavelength-dependency of E(m~,λ) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E(m~,λi)/E(m~,λj) 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~,λ), EOG, and ξ∗, respectively. Similarly, scattering-corrected ratios E(m~,λi)/E(m~,λj) 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.