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[求助] 求助：经典Hummers法合成氧化石墨，最后加双氧水，怎么不变黄色？

经典Hummers法低、中、高温反应各两小时，合成氧化石墨，最后加双氧水，一直是棕色，怎么变不成想要的黄色，出错在什么地方？

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The Hummers method with minor modification was used for the preparation of GOs from graphite. Briefly, to the concentrated H2SO4(10 mL) in a flask at 80 °C were added (NH4)2S2O8 (0.9 g) and P2O5(0.9 g), and the mixture was stirred until the reagents were completely dissolved. Graphite (1 g) was added, and the resulting mixture was allowed to react at 80 °C for 4.5 h. Upon being cooled to room temperature, the reaction mixture was diluted with water (250 mL) and kept for ∼12 h. It was then filtered and washed repeatedly with water, followed by drying in a vacuum oven. The solid sample was added to concentrated H2SO4 (40 mL) in a flask cooled in an ice bath. To the mixture in the flask was added slowly KMnO4(5 g over 40 min), during which the temperature was kept at <10 °C. The resulting mixture, with a change in color from black to greenish-brown, was heated at 35 °C for 2 h, followed by dilution with water (85 mL; Caution!the temperature must be kept at <35 °C throughout!) and further stirring for 2 h. The reaction mixture was poured into a large
beaker, to which water (250 mL) and then aqueous H2O2
(30%,10 mL) were added. Bubbles from the aqueous mixture along with a color change to brilliant yellow were observed. The mixture was allowed to settle for ∼12 h. The clear supernatant was decanted, and the sediment was washed repeatedly with an aqueous H2SO4 (5 wt %), H2O2(0.5 wt %), and HCl (10 wt%) solution, followed by washing repeatedly with water until no layers were observed in the centrifugation. The sample was then dialyzed (MWCO ∼ 3500) against water for 7 days to yield a clean aqueous dispersion of GOs