Phase-selective sol–gel auto-combustion synthesis of γ-Fe2O3 and Fe3O4 nanoparticles

Abstract: This study presents a roadmap for the controlled sol–gel auto-combustion synthesis of magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles, with a focus on the influence of atmospheric conditions during the synthesis process. Combustion in ambient air results in a mixture of hematite (α-Fe2O3) and spinel-type iron oxides (Fe3O4/γ-Fe2O3), as confirmed by x-ray diffraction and magnetic measurements. In contrast, combustion performed in a tubular oven under air predominantly yields the γ-Fe2O3 phase. When the process is conducted under an argon atmosphere, nearly pure Fe3O4 nanoparticles are obtained, exhibiting high saturation magnetization (~74 Am2kg−1 at 300 K) and a clear Verwey transition at ~117 K. Additionally, Mössbauer spectrometry study confirmed formation of distinct iron oxide phases by different hyperfine parameters. The scalability and reproducibility of the argon-based synthesis were demonstrated across 20 independent batches, all displaying consistent structural and magnetic characteristics. Post-synthesis annealing in air further elucidates the thermal phase transformation from Fe3O4/γ-Fe2O3 to α-Fe2O3 at elevated temperatures. These findings underscore the pivotal role of atmospheric control in tailoring the phase composition and magnetic properties of iron oxide nanoparticles.