High temperature oxidation of iron-iron oxide core-shell nanowires composed of iron nanoparticles

This work describes an oxidation process of iron-iron oxide core-shell nanowires at temperatures between 100 degrees C and 800 degrees C. The studied nanomaterial was synthesized through a simple chemical reduction of iron trichloride in an external magnetic field under a constant flow of argon. The electron microscopy investigations allowed determining that the as-prepared nanowires were composed of self-assembled iron nanoparticles which were covered by a 3 nm thick oxide shell and separated from each other by a thin interface layer. Both these layers exhibited an amorphous or highly-disordered character which was traced by means of transmission electron microscopy and Mossbauer spectroscopy. The thermal oxidation was carried out under a constant flow of argon which contained the traces of oxygen. The first stage of process was related to slow transformations of amorphous Fe and amorphous iron oxides into crystalline phases and disappearance of interfaces between iron nanoparticles forming the studied nanomaterial (range: 25-300 degrees C). After that, the crystalline iron core and iron oxide shell became oxidized and signals for different compositions of iron oxide sheath were observed (range: 300-800 degrees C) using X-ray diffraction, Raman spectroscopy and Mossbauer spectroscopy. According to the thermal gravimetric analysis, the nanowires heated up to 800 degrees C under argon atmosphere gained 37% of mass with respect to their initial weight. The structure of the studied nanomaterial oxidized at 800 degrees C was mainly composed of alpha-Fe2O3 (similar to 93%). Moreover, iron nanowires treated above 600 degrees C lost their wire-like shape due to their shrinkage and collapse caused by the void coalescence.