In the absence of GO, pure MnO2 exh

In the absence of GO, pure MnO2 exhibits an interconnected spherelike structure (average diameter of ca. 1 μm, see Fig. 2a) consisting ofthe densely-aggregated petals with an average thickness of ca. 8 nm(Fig. 2b). Due to the presence of oxygen functional groups, the negatively-charged GO sheets act as 2D scaffolds to chelate Mn2+ by electrostatic forces. These adsorbed Mn2+ ions on the surface of 2D GOthen provide abundant active sites for the nucleation of MnO2 by redoxreaction between BrO3− and Mn2+. With the reaction proceeds, petallike MnO2 arrays were in-situ grown on the both sides of GO sheets. Asshown in the SEM images, compared to the uncovered GO sheets (seethe inserted TEM image in Fig. 2c), the approximately-vertical MnO2petals were well coated on the surface of graphene (Fig. 2c and d) afterthermal reduction of GO. Furthermore, TEM images (Fig. 2e and f) alsoshow that graphene sheets are uniformly, completely covered by petallike MnO2 arrays, forming the sandwiched composite flakes. Such unique architectures ensure the spatial isolation and full utilization ofgraphene and MnO2 petals to the electrolyte ions, thus promising excellent supercapacitor performance [15]. The sonochemical methodmay provide an unusual and energy-efficient route for mass productionof homogeneous metal oxide/graphene composites at ambient conditions in a short period of reaction times.