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 stirre