The synthesis of GR-MWCNTs composites consisted of the following three steps according to previous method (Woo et al., 2012): first, we prepared graphite oxide, which was prepared by oxidizing natural graphite powder based on a modified Hummers method as shown in our previous article (Lian et al., 2012). Then, the synthesized graphene oxide was suspended in water (0.05 wt%) to give a brown dispersion. Exfoliation of graphene oxide was achieved by ultrasonication for 3 h and subsequent dialysis for 6 h to remove residual salts and acids. Then the obtained solution was subjected to 20 min of centrifugation at 3000 rpm to remove unexfoliated graphene oxide and then dried at 50 1C for 24 h.