Preparation of Magnetic Hybrid Copolymer–Cobalt Hierarchical Hollow Spheres by Localized Ostwald Ripening

We report a one-step solvothermal method to prepare magnetic hybrid copolymer–Co hierarchical hollow spheres by localized Ostwald ripening. The initial stage is the aggregation of primary inorganic nanoparticles whose surfaces are partially adsorbed by copolymer PEG–PPG–PEG to form solid spheres followed by hollowing of the core and the progressive matter redistribution from the interior to the exterior. Meanwhile, amorphous inorganic particles grow to cubic crystallites. Finally, a hollow spherical architecture with an extensively roughened surface is obtained by this self-transformation. The effect of NaOH concentration (served as a catalyst and reagent) on the sphere morphology is also checked by time-dependent experiments. When decreasing the mass of NaOH from 7.5 to 2.5 mmol, a morphology transformation from porous hollow spheres to double-wall spheres and then to single-wall spheres with platelet-like building units aligned disorderly on the outer surfaces is observed, and this morphology transformation is related to Ostwald ripening. Our results suggest that these nanoparticle-based hollow hierarchical spheres collected by the self-transformation of precursor objects could be of potential applicability in the fields of material science and biomedical technology.