grapheneGraphite oxide was synthesized by a modified Hummers mhod [44,45]. Concentrated H2SO4(12 mL) was added into a50 mL beaker and heated in an 80◦C water bath. Then 2.5 g K2S2O8and 2.5 g P2O5were added into the beaker. Next, 3 g Graphite pow-der was accurately weighed and added into the beaker. The mixturewas stirred and kept at 80◦C for 4.5 h. Then, it was diluted with0.5 L of water and left overnight. After that, the mixture was fil-tered through a 0.22 m membrane and washed with 1 L of water.The product was dried at 60◦C. This pre-oxidized graphite wasadded into 120 mL concentrated H2SO4in an ice bath. After that,15 g KMnO4was slowly (0.5 g min−1) added to the mixture understirring. The temperature must be kept below 20◦C during this pro-cess. Then the ice bath was removed and the mixture was stirredat 35◦C for 2 h. After that, 250 mL of water was added to the mix-ture in an ice bath to keep the temperature below 50◦C and stirredfor another 2 h. Then the mixture was diluted with 0.7 L of wateragain. After the addition of water, 20 mL of H2O2(30%, v/v) wasadded, causing the color turning into yellow along with bubbling.When no bubble was produced, the mixture was centrifuged for10 min at 5000 rpm, and washed with 1 L of HCl (1:10, v/v) and1 L of water until the pH was 7.0. The product was dark yellowwhen it was washed with hydrochloric acid and it turned into darkbrown when it was washed with water. In the mean time, the vis-cosity increased with the increase of the number of washing. Afterwashing, the product was dried at 50◦C.Hydrazine was used to reduce graphene oxide to synthesizegraphene. Graphite oxide (0.5 g) was dispersed in 500 mL of water,and ultrasonicated for 1 h to exfoliate graphite oxide to grapheneoxide. Then 12 mL of hydrazine hydrate (50%) was added to thedispersion. The mixture was refluxed and stirred for 24 h in an oilbath at 95◦C. The final product was filtered, washed with water,and dried at 50◦C.