Characterization of sp2- and sp3-bonded carbon in wood charcoal

Japanese cedar (Cryptomeria japonica) preheated at 700 degrees C was subsequently heated to 1800 degrees C and characterized by electron microscopy, X-ray diffraction, and micro-Raman spectroscopy. The degree of disorder of carbon crystallites and the amount of amorphous phase decreased considerably with an increase in heat treatment temperature to 1400 degrees C, while carbon crystallites clearly developed above this temperature, showing that the microstructure of carbonized wood undergoes drastic changes around 1400 degrees C. Besides showing the bands for sp(2-)bonded carbon, the Raman spectra showed a shoulder near 1100 cm(-1) assigned to sp3-bonded carbon. With an increase of heat treatment temperature, the peak position of the Raman sp(3) band shifted to a lower frequency from 1190 to 1120 cm(-1), which is due to the transformation of sp3-bonded carbon from an amorphous phase to a nanocrystalline phase. These data showed that the microstructure of carbonized wood from 700 degrees to 1800 degrees C consisted of the combination of sp(2-) and sp(3-)bonded carbon, which is probably due to the disordered microstructure of carbonized wood. It is suggested that the sp(3-)bonded carbon is transformed from an amorphous structure to a nanocrystalline structure with the growth of polyaromatic stacks at temperatures above 1400 degrees C.