The dumbbell- and flower-like Au−Fe3O4 heterostructures by thermal decomposition of the iron−oleate complex in the presence of Au nanoparticles (NPs) have been successfully fabricated using different sizes of Au NPs as the seeds for magnetically recyclable catalysis of p-nitrophenol and 2,4-dinitrophenol reduction. The heterostructures exhibit bifunctional properties with high magnetization and excellent catalytic activity toward nitrophenol reduction. The epitaxial linkages in dumbbell- and flower-like heterostructures are different, leading to the change in magnetic and catalytic properties of the heterostructured nanocatalysts. The pseudo-first-order rate constants for nitrophenol reduction are 0.63−0.72 min−1 and 0.38−0.46 min−1 for dumbbell- and flower-like Au−Fe3O4 heterostructures, respectively. In addition, the heterostructured nanocatalysts show good separation ability and reusability which can be repeatedly applied for nearly complete reduction of nitrophenols for at least six successive cycles. The reaction mechanism for nitrophenol reduction by Au−Fe3O4 nanocatalysts is also proposed and confirmed by XPS and FTIR analyses. These unique properties make Au−Fe3O4 heterostructures an ideal platform to study various heterogeneous catalytic processes which can be potentially applied in a wide variety of fields in purification, catalysis, sensing devices, and green chemistry.

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Au@Fe3O4 yolk–shell nanocatalysts based on the thermal decomposition of iron pentacarbonyl in the presence of 2.5–10-nm Au core nanoparticles were successfully fabricated for the catalytic reduction of nitroarenes. The particle sizes of the Au@Fe3O4 nanostructures were in the range 8–15 nm, with Fe3O4 shell layer thicknesses of 2.0–2.4 nm. The Fe3O4 layer not only can form a magnetic shell for recovery but also enables the protection of the catalytic activity of the Au core nanoparticles toward the reduction of nitrobenzene derivatives including 2-nitrophenol, 4-nitrophenol, 4-nitrotoluene, and 1-chloro-4-nitrobenzene in the presence of NaBH4. The catalytic performance of Au@Fe3O4 is highly dependent on the particle size of the Au core materials and the substituent groups of the nitroarenes. The reduction rates of nitroarenes with electron-withdrawing groups were found to be 2.3–2.6 times higher than those of nitroarenes with electron-donating groups. In addition, the reduction of nitroarenes by the Au@Fe3O4 yolk–shell nanocatalysts was found to be a surface-mediated reaction, and the relationship between the reduction rate and the initial NaBH4 concentration was found to follow Langmuir–Hinshelwood kinetics. Moreover, the yolk–shell nanoparticles showed good separation ability and reusability, as they could be repeatedly applied for the nearly complete reductions of 4-nitrophenol and 1-chloro-4-nitrobenzene for at least five successive cycles. These unique properties make Au@Fe3O4 nanocatalysts an ideal platform for tailoring yolk–shell nanoreactors with various active materials and also for studying various heterogeneous catalytic processes.

Monodisperse and size-tunable magnetic iron oxide nanoparticles (NPs) have been synthesized by thermal decomposition of an iron oleate complex at 310 °C in the presence of oleylamine and oleic acid. The diameters of the as-synthesized iron oxide NPs decrease with increasing concentrations of iron oleate complex and oleic acid/oleylamine. In addition, the size-dependent crystallinity and magnetic properties of iron oxide NPs are presented. It is found that larger iron oxide NPs have a higher degree of crystallinity and saturation magnetization. More importantly, various M-iron oxide heterostructures (M = Au, Ag, Pt, Pd) have been successfully fabricated by using the same synthesis procedure. The iron oxide NPs are grown over the pre-made metal seeds through a seed-mediated growth process. The physicochemical properties of Au-Fe 3 O 4 heterostructures have been characterized by X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry and UV-vis spectroscopy. The as-synthesized Au-Fe 3 O 4 heterostructures show a red-shift in surface plasmon resonance peak compared with Au NPs and similar magnetic properties to Fe 3 O 4 NPs. The heterojunction effects present in such nanostructures offer the opportunity to tune the irphysicochemical properties. Therefore, this synthesis process can be regarded as an efficient way to fabricate a series of heterostructures for a variety of applications.