Following Hummers’ method42, concen

Following Hummers’ method42, concentrated H2SO4 (70 mL) was added to a mixture of graphite flakes (2.0 g) and NaNO3 (1.2 g). Then the mixture was cooled to 0 °C with stirring for 1 h. Afterwards, KMnO4 (6.0 g) was added slowly in portions, keeping the reaction temperature below 10 °C for 1 h. The reaction was stirred for another 4 hour in an ice8bath (below 10 °C). When the reaction was finished, the mixed solution was warmed to 35 °C and stirred for 30 min. After water (300 mL) was added slowly, external heating was introduced to maintain the reaction temperature at 98 °C for 15 min. Then the hot solution was poured into 400 mL of water (60 °C). An additional 30% H2O2 (3 mL) was added and the mixture stood for 2 days. After multiple washings, centrifugations, and ultrasonic dispersion, the suspension ofGO platelets was obtained with the concentration of ~5.0 mgmL−1.