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 (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) 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-Fe 2 O 3 ) and spinel-type iron oxides (Fe 3 O 4 /γ-Fe 2 O 3 ), as confirmed by x-ray diffraction and magnetic measurements. In contrast, combustion performed in a tubular oven under air predominantly yields the γ-Fe 2 O 3 phase. When the process is conducted under an argon atmosphere, nearly pure Fe 3 O 4 nanoparticles are obtained, exhibiting high saturation magnetization (~74 Am 2 kg −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 Fe 3 O 4 /γ-Fe 2 O 3 to α-Fe 2 O 3 at elevated temperatures. These findings underscore the pivotal role of atmospheric control in tailoring the phase composition and magnetic properties of iron oxide nanoparticles. Graphical abstract Impact statement This work advances the synthesis of iron oxide nanoparticles by introducing a simple and scalable sol–gel auto-combustion (SGAC) that can selectively yield either magnetite (Fe 3 O 4 ) or maghemite (γ-Fe 2 O 3 ). Spinel iron oxides are central to multiple fields, ranging from geoscience and archaeology to biomedical imaging, catalysis, and data storage. However, controlling which phase is obtained remains a long-standing challenge because of their structural similarity and overlapping magnetic responses. Our study demonstrates that controlling the synthesis atmosphere in SGAC governs phase formation, enabling the reproducible preparation of high-purity magnetite or maghemite. Importantly, magnetite samples exhibit a clear Verwey transition — a key electronic ordering phenomenon — providing a reliable fingerprint of phase purity and stoichiometry. The ability to systematically compare the two phases synthesized under identical conditions provides new insights into oxidation-driven transformations in spinel iron oxides. Beyond the fundamental interest, this work offers a practical pathway toward producing well-defined iron oxide nanomaterials at scale, without relying on expensive precursors or complex processing. By bridging fundamental mechanisms with reproducible synthesis strategies, our results are relevant not only to materials chemists but also to researchers developing technologies in energy, environment, medicine, and information storage.