Facile synthesis of highly water-dispersible and monodispersed Fe3O4 hollow microspheres and their application in water treatment

Highly water-dispersible and monodispersed Fe3O4 hollow microspheres have been synthesized by a simple one-pot hydrothermal method, in which sodium polyacrylate (PAAS) was used as a dispersant and size-controlling reagent. The results showed that the obtained Fe3O4 hollow microspheres were composed of nanoparticles less than 15 nm in diameter and exhibited superparamagnetic properties with relatively high saturation magnetization at room temperature. Meanwhile, the negatively-charged carboxyl groups from PAAS were closely attached to the surface of the Fe3O4 hollow microspheres, which made them highly water-dispersible. The reaction process was systematically investigated and we found that the Fe3O4 hollow microspheres in our experiment were formed through a self-template process, where the solid alpha-Fe2O3 microspheres generated at the initial stage transformed into Fe3O4 hollow microspheres through the synergetic effect of Ostwald ripening and reduction. Additionally, the PAAS, by regulating the viscosity of the reaction solution through varying the concentration, could indirectly adjust the size of the Fe3O4 hollow microspheres ranging from 200 nm to 340 nm. Finally, the obtained Fe3O4 hollow microspheres were demonstrated to be an excellent adsorbent of heavy metal ions like Pb2+ due to their abundant carboxyl groups and magnetic separability.