Shear‐Catalyzed Vacancy Engineering Enabling Rapid Synthesis of High‐Crystallinity Magnetite via Microfluidic Ferrous Oxidation

Iron oxides are widely used in environmental and energy applications, with magnetite particularly favored for its excellent crystallinity and strong magnetic properties. However, synthesizing well-crystalline magnetite typically requires long reaction times due to the slow phase transition kinetics of goethite. Herein, a shear-catalyzed strategy in microchannels is proposed for the rapid and continuous synthesis of magnetite with excellent crystallinity. The preparation process is completed in just 30 min, a 292-fold enhancement over batch reactors, which achieve 68.6 wt.% magnetite after 100 h. Additionally, the resulting magnetite shows a significantly higher saturation magnetization of 59.5 emu g^-1, compared to 48.4 emu g^-1 from conventional batch reactors. The results demonstrate that Fe vacancies can be induced on goethite by shear stress in microchannels during magnetite nucleation, which not only creates more active sites for Fe²⁺ adsorption but also significantly lowers Fe²⁺ diffusion barrier, thereby facilitating the transformation from goethite to magnetite by reducing the energy barrier for Fe²⁺ migration. Additionally, the applicability is confirmed by the successful rapid synthesis of other oxide nanomaterials (SiO₂, MnO₂, and ZnO), demonstrating its versatility.