Mössbauer and Raman spectroscopic characterization of iron and carbon in iron-loaded Japanese cypress charcoal

Japanese cypress (Chamaecyparis obtusa) wood powder impregnated with Fe3+ ions was carbonized using one-step and two-step procedures, and the iron-loaded charcoal was examined using Mössbauer and Raman spectroscopy to elucidate the physicochemical states of iron and carbon after carbonization at various temperatures. In the one-step procedure, the Mössbauer spectra confirmed that charcoal carbonized at 750 °C and 800 °C contained Fe3C, suggesting that the reduction of Fe3+ started at lower than 750 °C. The content of Fe3C increased substantially in charcoal carbonized at 850 °C and 900 °C, and metallic iron (γ-Fe and/or α-Fe) was also detected in these charcoal samples. Raman spectral line-shapes of the charcoal carbonized at 750 °C and 800 °C were notably different from those of charcoal carbonized at 850 °C and 900 °C, which indicated that crystallization of carbon progressed rapidly in the temperature range 800–850 °C. The changes in Raman spectra probably corresponded to those in the Mössbauer spectra, namely, the graphitization was closely related to the reduction of Fe3+. The degree of graphitization in the charcoal carbonized at 850 °C using the two-step procedure was higher than that using the one-step procedure. Mössbauer spectra suggested that the two-step carbonization enhances the yield of ferromagnetic iron species in iron-loaded charcoal.