The graphene flakes were first prepared through electrochemical exfoliation. Briefly, 0.1 M of H2SO4 solution was used as the electrolyte, with the graphite tape (anode, Fig. ) and a steel mesh (cathode) placed vertically in the reaction cell, as shown in Fig. . A constant voltage (10 V) was applied to the electrodes for 10 min. After the exfoliation, the EEG flakes (Fig. ) were filtered and washed by deionized (DI) water until neutral (pH=7), followed by vacuum drying (50 °C) overnight.