α‑Fe₂O₃ versus\nβ‑Fe₂O₃: Controlling the Phase\nof the Transformation Product of ε‑Fe₂O₃ in the Fe₂O₃/SiO₂ System

Abstract

The influence of added sucrose on\nthe phase composition of iron\noxide nanoparticles was studied in the Fe₂O₃/SiO₂ nanocomposites. The nanocomposites were prepared\nby impregnation of mesoporous silica SBA-15 with iron nitrate containing\nvarious amounts of sucrose and their subsequent annealing at 1100\nand 1200 °C. The phase composition of the iron oxide nanoparticles\nin the prepared nanocomposites was analyzed by X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy on polycrystalline samples.\nProfile analysis of the X-ray diffraction maxima was used to determine\nthe apparent crystallite size of the different polymorphs. The content\nof β-Fe₂O₃ in the samples increased with\nan increasing amount of sucrose, and no β-Fe₂O₃ was observed in the samples without sucrose. These samples\ncontained only ε-Fe₂O₃ and hematite. The\napparent crystallite size was found to be similar for both hematite\nand β-Fe₂O₃. Therefore, sucrose was identified\nas the primary variable, which directs the ε-Fe₂O₃ transformation to either hematite or β-Fe₂O₃. Our findings shift the way we understand the β-Fe₂O₃ formation from ε-Fe₂O₃ in the Fe₂O₃/SiO₂ system from the\nsize-directed to the starting material-directed transformation.