The graphene oxide (GO) was prepared according to the
reported method [43]. Briefly, 1 g of graphite was added
into 23 mL of 18 M H2SO4, and the mixture was kept
below 20 8C by ice bath. 3 g of KMnO4 was slowly added
into the mixture under continuous stirring. After it was
heated to 35 8C by oil bath, the mixture was stirred for
another 30 min. After that, ultra-pure water was added
into above mixture and the temperature was increased to
1008C, and the mixture was maintained at 98 8C for
15 min. Then, 140 mL of ultra-pure water and 10 mL of
30% H2O2 were added into the mixture. The solid products
were centrifuged at 5000 rpm, subsequently washed
with 5% HCl until could not appear SO4
2 react with
BaCl2. Finally, the products were washed with acetone for
three times, and dried all night at 658C.