*2.2. Synthesis of Reduced Graphene Oxide (RGO)*

For the synthesis, a mixture of 50 mL concentrated H2SO4 and 5.5 mL of H3PO4 was taken in a beaker. Then, 1.0 g of graphite powder and KMnO4 (4.0 g) were added to the mixture after maintaining 10 ◦C under magnetic stirring in ice bath. The mixture was heated to 35 ◦C governed by a water bath. After 2 h of stirring, the reaction mixture was sonicated 10 times with the help of ultrasonicator. To stop the reaction, deionized water (200 mL) and 1 M NaOH solution were added dropwise to maintain the pH = 6 of solution mixture. The reaction process was then followed with the addition of H2O2 (15 mL) which led to a change in suspension color to yellow. The mixture was kept overnight. Thereafter, the solution of ascorbic acid (0.227 mol L−1) was added dropwise under magnetic stirring at 95 ◦C. In this step, the color of the solution changes slowly from greenish-yellow to black. Finally, the mixture was allowed to settle down for 1 h. Black precipitates were formed which were then centrifuged and washed several times using ethanol.
