Transmission electron microscopy (TEM, Thermo Fisher, Themis Z) samples were prepared via drop casting as follows. The mixture of Cr2O3/rGO in ethanol was further sonicated by probe ultra-sonication at 750 W 20% amplitude for 15 min. Then, 4 µl supernatant was dropped onto lacey carbon TEM grids. High-angle annular dark-field (HAADF 200kV) STEM and selected area electron diffraction (SAED) were applied.