Formation of Fe3O4@MnO2 ball-in-ball hollow spheres as a high performance catalyst with enhanced catalytic performances

While the synthesis of heterogeneous catalysts is well established, it is extremely challenging to fabricate complex hollow structures with mixed transition metal oxides. Herein, we report a facile in situ growth process of SiO2@Fe3O4MnO2, followed by an etching method to synthesize a hierarchical hollow structure, namely Fe3O4@MnO2 ball-in-ball hollow spheres (Fe3O4@MnO2 BBHs). The as-prepared Fe3O4@MnO2 BBHs were applied to degrade methylene blue (MB) by catalytic generation of active radicals from peroxymonosulfate (PMS), exhibiting the merits of excellent catalytic performance, easy separation, good stability and recyclabiLity. In this architecture, the degradation process can be divided into three Layers. The outer hierarchical MnO2 nanosheets could accumulate and transport the pollutants by electrostatic interactions and catalyze the generation of active radicals for degradation. Both the inner MnO2 nanosheets and the outer Fe3O4 hollows could produce active radicals to accelerate the pollutant degradation. The active catalytic sites also existed in the inner Fe3O4 hollows, which could further degrade the highly concentrated pollutants in the hollows. This work provides new strategies for the controllable synthesis of complex hollow structures and their application in environmental remediation.