Raman Spectroscopy of Iron-Oxide Core-Corona Nanoparticles: Effect of Synthesis Temperature

Abstract

The morphology and composition of metal–oxide coronaparticles after the thermal oxidation of iron particles within the temperature range of 25–600°C under atmospheric conditions were investigated by in situ Raman-scattering spectroscopy. Thermal treatment at 100–300°C results in the predominant formation of a Fe3O4 and γ-Fe2O3 mixture, with a small amount of α-Fe2O3, on the surface of the iron particles, while annealing at higher temperatures results in further oxidation of iron, as well as phase transformation of Fe3O4 and γ-Fe2O3 phase transformation to the most thermodynamically stable form of iron oxide α-Fe2O3. At thermal treatment at 300°C and higher, localization of the reaction of hematite formation is observed: one-dimensional nanofilaments of the hematite-forming corona start to grow radially from spherical iron microparticles that are accompanied by depassivation and acceleration of growth of the oxide layer. With an increase of temperature and time of oxidation, the microparticles transform into multilayer metal–oxide coronaparticles consisting of a metal–oxide core and corona of the elongated crystalline α-Fe2O3 nanowhiskers growing transversely to the surface of the particles. Growth of the hematite nanowhiskers lasts until complete oxidation of the metal core has taken place. It has been hypothesized that the reason for the growth of hematite nanowhiskers at iron oxidation is associated with autolocalization of the topochemical reaction of maghemite formation in the magnetite–hematite interface phase boundary, as well as acceleration of transport of metal into the growing hematite whiskers by the effect of internal stresses in the growing oxide layer.