Kmno4

To synthesize GO, we used the well-known modified Hummers method [22 (link)]. First, 0.5 g of graphite flakes (graphite flake, Alfa Aesar, Haverhill, USA, ~325 mesh) were mixed with 20 mL of concentrated sulfuric acid (H₂SO₄, Daejung Chemicals, Siheung-si, Republic of Korea) in an Erlenmeyer flask. Then, 1.65 g of potassium permanganate (KMnO₄, Daejung Chemicals, Siheung-si, Republic of Korea) was slowly added into these mixtures while stirring at 300 rpm for 10 min. The temperature of the mixture was maintained at 35 °C for 2 h. After stirring for 2 h, the temperature was let to drop below 10 °C, and 300 mL of DI water was added to the flask extremely slowly. Finally, hydrogen peroxide (H₂O₂, Daejung Chemicals, Siheung-si, Republic of Korea) was added to the flask until the solution turned yellow. The synthesized GO solution was washed with 10% hydrochloric acid and 50 mL of DI water. The filtered GO was dried in a convection oven at 60 °C for 1 day.