Graphene oxide was synthesized using a modified Hummers
method [26,27]. In detail, graphite (2 g, 500 meshes) was put into a
250 mL flask, and 50 mL concentrated sulfuric acid was added. After
several minutes’ vigorous stirring, 2.5 g of NaNO3 was added. The
mixture was then cooled to 0 ◦C using an ice-water bath and kept
stirring for 2 h. Subsequently, 7.3 g of KMnO4 was added in small
portions to preventthe rapid temperature rise,followed by increasing
the temperature to 35 ◦C. After keeping stirring for another 2 h,
the suspension was treated further by adding the mixture of H2O2
(7 mL, 30%) and ice water (55 mL). The graphene oxide was separated
from the reaction mixture by filtration, and washed with
plenty of aqueous HCl (3%). Further washing with water was carried
out until the chloride test with AgNO3 was negative, and the
resultant products were dried at 40 ◦C for 24 h in the vacuum oven.