Facile Synthesis of Substantially Magnetic Hollow Nanospheres of Maghemite (γ-Fe₂O₃) Originated from Magnetite (Fe₃O₄) via Solvothermal Method

In the present study, we report on the successful synthesis of hollow nanospheres of maghemite (γ-Fe₂O₃). The γ-Fe₂O₃ hollow nanospheres were synthesized by annealing magnetite (Fe₃O₄) hexagonal nanoparticles produced by solvothermal method. Structural analysis by slow-scan X-ray diffraction (XRD) marks a small peak shift towards high angle confirming the change in the iron oxide phase from Fe₃O₄ to γ-Fe₂O₃. Further, FT-IR spectroscopy reveals for Fe₃O₄ a band at 571 cm⁻¹, which shows the Fe-O stretching mode of tetrahedral and octahedral sites. Formation of γ-Fe₂O₃ upon annealing was again confirmed from the broadening and splitting of band 571 cm⁻¹ into two bands, viz. 556 cm⁻¹ and 637 cm⁻¹. Magnetic properties of the synthesized samples were studied using a vibrating sample magnetometer (VSM) at 300 K. The value of saturation magnetization (M_S) of Fe₃O₄ nanoparticles and γ-Fe₂O₃ hollow nanospheres was 56 emu/g and 43 emu/g, respectively. For Fe₃O₄, infrequent Verwey transition appeared when variation in magnetization with temperature was studied. Morphological analysis confirms the hexagonal shape of Fe₃O₄ nanoparticles with an average particle size of 57 nm. The same study interestingly epitomizes a hollow nanospherical shape with an average particle size of 12 nm for γ-Fe₂O₃. The emergence of the Verwey transition in Fe₃O₄ hexagonal nanoparticles and the existence of significant magnetization of 43 emu/g in hollow nanospheres of γ-Fe₂O₃ in this study are contentious and fascinating for possible applications.