The oxidized
graphite powder (20 g) was put into cold (0 °C) concentrated
H2SO4 (460 mL). KMnO4 (60 g) was added gradually with
stirring and cooling, so that the temperature of the mixture
was not allowed to reach 20 °C. The mixture was then stirred
at 35 °C for 2 h, and distilled water (920 mL) was added. In
15 min, the reaction was terminated by the addition of a large
amount of distilled water (2.8 L) and 30% H2O2 solution (50
mL), after which the color of the mixture changed to bright
yellow. The mixture was filtered and washed with 1:10 HCl
solution (5 L) in order to remove metal ions. The GO product
was suspended in distilled water to give a viscous, brown, 2%
dispersion, which was subjected to dialysis to completely
remove metal ions and acids. The resulting 0.5% w/v GO
dispersion, which is stable for a period of years, was used to
prepare exfoliated GO