Optical properties of carbon materials formed by pyrolysis of novolac-resin/biomass composites

The optical properties of carbon materials formed by pyrolysis of novolac-resin/biomass composites were studied by Raman, photoluminescence (PL) and (modulated) photoreflectance (PR) spectroscopy in dependence of the pyrolysis temperature and the heating rate. In the Raman spectra, the frequency, width and intensity ratio of the disordered graphitic D-band to the graphitic G-band, ID/IG, indicate an increase in ordered sp2-bonds in composites pyrolyzed at 1000 °C in comparison with those at lower pyrolysis temperatures. Moreover, the intense PL, observed at 2.2 eV for pyrolysis temperatures in the range 400–600 °C, is significantly decreased at temperatures ≥800 °C, which implies that the materials pyrolyzed at higher temperatures have less defects. According to the results of Raman-studies and complementary FTIR-studies, the material pyrolyzed at 1000 °C exhibits spectral characteristics similar to those of activated charcoal. A weak PR-band observed at 2 eV in the spectra of activated charcoal gives an indication of the (optical) band gap of charcoal and related composite materials. Materials pyrolyzed at high temperatures (≥1000 °C) are expected to be suitable for electrochemical applications.