The PDDA-graphene (Gp) was synthesized from GO reduction by PDDA. The dispersion of GO (60 mg GO in20 mL water) was sonicated for 10 min. 800 mLof35wt%PDDA was added into the dispersion and kept stirring for10 min. Then the dispersion was kept for 5 h at 90 8Cwhile the color turned from yellow-brown to black. The resulting dispersion was filtered and washed with ultra-pure water for more than 3 times.