10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Interpretation of UV-Visible Absorption Measurements of Flame-synthesized Carbon Nanoparticles by Molecular Modeling

DONGPING CHEN, Hai Wang, Beijing Institute of Technology

     Abstract Number: 64
     Working Group: Combustion

Abstract
Recent UV-Visible absorption spectral analyses of a series of flame-synthesized carbon nanoparticles show that these particles exhibit the quantum confinement behavior. The measured, apparent optical bandgaps increase with a decrease in particle size; they lie in the range of 0.7 to 1.8 eV for particles with median mobility diameter ranging from 3 to 13 nm. To interpret these results, we carried out molecular dynamics/electronic structure calculations for homogeneous clusters of polycyclic aromatic hydrocarbons (PAHs) including ovalene, coronene, pyrene and naphthalene. PAH clusters were built using molecular dynamics method followed by a search of minimum HOMO-LUMO gap over sampled configurations. The HOMO-LUMO gap was computed from the difference in energies between the HOMO and LUMO Kohn–Sham orbitals at the B3LYP/6-31G(d) level of theory. It was found that the morphology of PAH clusters (< 2 nm) have a large impact on the HOMO-LUMO gap; a T-shaped configuration always leads to a lower gap value. Similarly to experimental observations, the HOMO-LUMO gap of PAH clusters is found to be a function of the particle size. By extrapolating the correlation between HOMO-LUMO gap and particle size, the difference in the bandgap of a single PAH molecule and the bulk PAH is 1.4+/–0.2 eV. From the measured optical bandgap values, the largest PAH within a particle of a given size may be estimated. For a particle 3 nm in diameter with a measured bandgap of 1.8 eV, the largest PAH is around the size of ovalene. For a particle 10 nm in diameter with a bandgap value of 0.7 eV, the largest PAH corresponds to 40 carbon atoms. These estimates were made with the assumption that the carbon nanoparticles do not contain any oxygenates. If oxygenates are present, the estimated, largest PAH size is expected to decrease. Overall, the results suggest that molecular modeling coupled with UV-vis absorption spectroscopy is a valuable approach to estimating the largest PAH component in carbon nanoparticles.