Water Vapor-Mediated Thermal Decomposition of FeCO₃ to Macroporous Fe₃O₄ Microspheres: Cooling-Stage Redox Control for Phase Purity

Abstract

 Macroporous magnetite (Fe₃O₄) microspheres with strong magnetic response and pore diameters exceeding 50 nm are promising for nanoparticle capture and environmental purification. Thermal decomposition of iron carbonate (FeCO₃) microspheres in H₂ containing water vapor can produce macroporous Fe₃O₄ as the primary phase, yet undesirable iron oxide by-products (e.g., FeO and α-Fe₂O₃) remain a persistent challenge. Here, we show that controlling redox reactions, particularly during cooling, enables singlephase Fe₃O₄ formation. By switching the atmosphere from wet H₂ to dry H₂ within an optimized temperature window during cooling, thereby extending the duration of water vapor introduction, we suppress secondary oxidation/reduction pathways and stabilize the Fe₃O₄ phase. Our results establish water vapor-mediated redox control as a general strategy to access phase-pure macroporous Fe₃O₄ and highlight the importance of cooling-stage kinetics in tailoring the phase stability of iron oxides.