Phase transitions of carbon-encapsulated iron oxide nanoparticles during the carbonization of cellulose at various pyrolysis temperatures

Crystallographic structures of carbon-encapsulated iron oxide nanoparticles, derived from the carbonization of cellulose were investigated. The phase transitions of carbon-encapsulated iron oxide nanoparticles were characterized and analyzed after pyrolysis treatment at four temperatures: 500 °C, 800 °C, 1000 °C and 1600 °C, respectively. Transmission Electronic Microscope (TEM) and X-ray diffraction (XRD) techniques were employed for the analysis. Samples treated at 500 °C spontaneously combusted immediately after being exposed to the air indicating the existence of α-Fe2O3 particles. Core-shell structures consisting of dark grains and a light matrix with graphitic structure appeared in samples treated at 800 °C. XRD and TEM selected area electron diffraction demonstrated that between 800 °C and 1600 °C, catalytic graphitization occurred, and the iron carbide/iron crystalline phase was developed for the cores. It was found that the phase transition from Fe3C to Fe occurred at 1600 °C.