Fundamental Formation of Three-Dimensional Fe3O4 Microcrystals and Practical Application in Anchoring Au as Recoverable Catalyst for Effective Reduction of 4-Nitrophenol

In this study, the three-dimensional (3D) flowerlike porous Fe3O4 microcrystals were prepared by a self-assembly approach with the assistance of ethylene glycol (EG). The generation mechanism of the 3D flowerlike Fe3O4 microcrystals was revealed through controlling the parameters of the hydrothermal reaction time, the molar mass of the urea, and the calcination temperature. The proposed 3D flowerlike Fe3O4 microcrystals exhibited superparamagnetic behaviors with high saturation magnetization (i.e., up to 73.1 emu·g–1) at room temperature. The Fe3O4–Au magnetic composites (MCs) were further prepared by a seed deposition process, and surface features were revealed by TEM, XRD, XPS, UV–vis, and SQUID techniques. Compared with the Fe3O4 microcrystals themselves, the Au (∼20 nm) covered Fe3O4 microcrystals provided efficient and recyclable catalytic performance (e.g., unprecedented high turnover frequency of 2.874 min–1) for 4-nitrophenol (4-NP). More importantly, the proposed Fe3O4–Au MCs could be used to reduce 4-NP for more than six cycles, elaborating that Fe3O4–Au MCs are promising catalysts in the field of environmental purification.