Synthesis of ordered mesoporous Fe₃O₄ and γ-Fe₂O₃ with crystalline walls using post-template reduction/oxidation

Ordered mesoporous Fe₃O₄ with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous α-Fe ₂O₃ (corundum structure) to Fe₃O₄ spinel then to γ-Fe₂O₃ by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of α-Fe₂O₃ (corundum structure) to the cubic close packed subarray of Fe₃O₄ spinel and γ-Fe ₂O₃. Preliminary magnetic measurements reveal that the spins in both Fe₃O₄ and γ-Fe₂O ₃ are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (>8 nm).