Cr2O3 has multiple aligned grains. For better understanding, we show a unit cell of Cr2O3 (11 � 0) with corresponding crystal planes ( ) and (006) in Figure . High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) images of Figure and indicate that these small Cr2O3 grains are uniformly distributed along the basal plane of the rGO. Figure shows a magnified TEM image of the Cr2O3/rGO-1.8 V-15 min and Figure . 3c is the enlarged TEM image of the marked area (white box in Figure ). In two areas with different contrasts, the high contrast area shows a crystalline structure (Figure , S22c-e). The EDS results in Figure verify that the high contrast areas contain high amounts of Cr and O. Whereas, C-signal is uniformly distributed resembling the basal plane of rGO flakes. These results confirm that small grains of crystalline Cr2O3 aligned along the basal plane of the rGO are synthesized within the small interlayer space of the rGO.