Structural analysis of char by Raman spectroscopy: Improving band assignments through computational calculations from first principles

The complex heterogeneous nature of chars has confounded the complete analysis of the Raman spectra of these materials. The additional shoulders observed on the defect (D)-band and high intensity valley between the D and graphitic (G)-bands represent the primary regions of uncertainty. In this paper the effects of various vacancy and substitution defects in a coronene parent molecule have been systematically analyzed using density functional theory (DFT). The impacts of these defects are best understood in terms of a reduced symmetry as compared to a “parent” coronene molecule. Based on simulation results, a total of ten potential bands have been assigned between 1000 cm−1 and 1800 cm−1. These bands have been used to deconvolute a thermoseries of cellulose chars produced by pyrolysis at 300-700 °C. The shoulder observed in chars near 1200 cm−1 has been assigned to the symmetric breathing mode of various small polyaromatic hydrocarbons (PAH) as well as rings containing seven or more carbons. Intensity between 1400 cm−1 and 1550 cm−1 results from a range of coupled vibrational modes from various defect structures. The deconvolution of cellulose derived chars shows consistent growth of PAH clusters, loss of oxygen, and development of non-hexoganal ring systems as pyrolysis temperature increased.