10.1038/srep45030
Role of oxygen functional groups in reduced graphene oxide for lubrication
 The sample was prepared by Hummer's method44,45. Graphite flakes (10 g) and NaNO3 (10 g) were mixed in 500 mL of H2SO4 (98%) at 5 degC with continuous stirring for 2 h, and KMnO4 (30 g) was gradually added to the suspension at a controlled reaction temperature lower than 15 degC. The ice bath was then removed, and the mixture was stirred at 35 degC until it became pasty brownish; stirring continued for two days, after which the mixture was diluted with a slow addition of 300 ml of water. The reaction temperature was rapidly increased to 98 degC, and the color changed to dark brown. This solution was further diluted by adding an additional 400 ml of water and stirring continuously. The solution was finally treated with 150 ml of H2O2 to terminate the reaction, indicated by the appearance of a yellow color. For purification, the mixture was washed by rinsing and centrifugation with 10% HCl and then with deionized water (DI) several times. After filtration and drying under a vacuum at room temperature, the GO1 was obtained as powder. Reduction was performed to reduce the functional group density, towards decreasing the inter-plane spacing and the flake dimensions and improving the friction properties. Reduction was done via treating 100 mg of GO1 in 30 mL of water with 3 mL of hydrazine and refluxing at 95 degC for 24 h, followed by filtering, washing and drying at 60 degC under vacuum to obtain the rGO1 powder. 
Graphite flakes (10 g) and NaNO3 (10 g) were mixed in 600 mL of H2SO4 (98%) at 5 degC with continuous stirring for 6 h, and KMnO4 (60 g) was gradually added to the suspension while maintaining the temperature at 15 degC. The mixture was diluted with a slow addition of 400 mL of water and with stirring maintained for 2 h. The ice bath was then removed, and the mixture was stirred at 35 degC for 2 h and then refluxed at 98 degC for 30 min. The solution was finally treated with 200 ml of H2O2 and the color changed to bright yellow. The resulting mixture was washed by centrifugation with 10% HCl and then with DI water several times until it became gel-like. After centrifugation, the gel was vacuum dried at 80 degC for 8 h to obtain the GO2 powder. Reduction was performed similarly to rGO1.
Graphite flakes
mixed
10 g
g
NaNO3
mixed
10 g
g
H2SO4
500 mL
mL
H2SO4
98%
%
Graphite flakes (10 g) and NaNO3
mixed
5 degC
degC
Graphite flakes (10 g) and NaNO3 (10 g)
mixed
2 h
h
KMnO4
added
30 g
g
KMnO4 (30 g) was gradually added
controlled reaction temperature
lower than 15 degC
degC
mixture
stirred
35 degC
degC
water
addition
300 ml
ml
mixture
stirring
two days
days
reaction temperature
rapidly increased
98 degC
degC
water
adding
400 ml
ml
H2O2
150 ml
ml
HCl
10%
%
water
30 mL
mL
water
GO1
100 mg
mg
hydrazine
3 mL
mL
100 mg of GO1 in 30 mL of water with 3 mL of hydrazine
refluxing
95 degC
degC
100 mg of GO1 in 30 mL of water with 3 mL of hydrazine
refluxing
24 h
h
100 mg of GO1 in 30 mL of water with 3 mL of hydrazine
drying
60 degC
degC
Graphite flakes (10 g) and NaNO3 (10 g) were mixed in 600 mL of H2SO4 (98%) at 5 degC with continuous stirring for 6 h, and KMnO4 (60 g) was gradually added to the suspension
maintaining the temperature
15 degC
degC
Graphite flakes (10 g) and NaNO3 (10 g) were mixed in 600 mL of H2SO4 (98%)
continuous stirring
6 h
h
Graphite flakes
mixed
10 g
g
NaNO3
mixed
10 g
g
H2SO4
600 mL
mL
H2SO4
98%
%
Graphite flakes (10 g) and NaNO3 (10 g)
mixed
5 degC
degC
KMnO4
added
60 g
g
water
slow addition
400 mL
mL
mixture
stirring maintained
2 h
h
mixture
stirred
35 degC
degC
mixture
stirred
2 h
h
mixture
refluxed
98 degC
degC
mixture
refluxed
30 min
min
H2O2
200 ml
ml
HCl
10%
%
gel
vacuum dried
80 degC
degC
gel
vacuum dried
8 h
h