Facile synthesis of superparamagnetic Fe3O4@noble metal core–shell nanoparticles by thermal decomposition and hydrothermal methods: comparative study and catalytic applications

Herein, we report on developing a facile synthetic route for reusable nanocatalysts based on a combination of the supermagnetic properties of magnetite with the unique optical and catalytic properties of noble metal hybrid nanomaterials. We compare two different synthetic methods, to find out which is best from synthetic and application points of view, for the synthesis of Fe3O4 and Fe3O4@M (M = Ag or Au) core–shell hybrid nanostructures. The two different single-step synthetic methods are: thermal decomposition and hydrothermal. The structural, morphological and magnetic properties of the obtained Fe3O4 and Fe3O4@M nanoparticles were characterized by various techniques. XRD of the Fe3O4 nanoparticles exhibited sharp and strong diffraction peaks, confirming the highly crystalline structure of the Fe3O4 particles synthesized by the hydrothermal method, while broad and weak peaks were observed on using the thermal decomposition method. Both Fe3O4@Ag and Fe3O4@Au core–shells obtained by the hydrothermal method showed the reflection planes of Fe3O4 and additional planes of Ag or Au. But on the formation of Fe3O4@Ag/Au by the thermal decomposition method the peak for Fe3O4 disappeared and only the diffraction peaks of Ag or Au appeared. According to TEM analysis there was a broad particle-size distribution, random near-spherical shapes and slight particle agglomeration for Fe3O4 synthesized by the thermal decomposition method. However, there was a moderate size distribution, spherical shapes and well-dispersed particles without large aggregations for the hydrothermal method. TEM images of the synthesized nanoparticles by the two methods used showed a pronounced difference in both size and morphological shape. The catalytic performance of the synthesized nanoparticles was examined for the reduction of Congo red dye in the presence of NaBH4. The Fe3O4 nanocatalyst maintained its catalytic activity for only one cycle. In the cases of Fe3O4@Au and Fe3O4@Ag, the catalytic activity was conserved for four and ten successive cycles, respectively. Based on the obtained results, it was concluded that the hydrothermal synthesis of Fe3O4, Fe3O4@Ag and Fe3O4@Au nanostructures is highly recommended due to their selectivity and merits.