The solution was subjected to magnetic stirrer for stirring

The modified Hummers method was used for preparing graphene oxide from
natural flake graphite. In this method, first the graphite sheets are extended
and converted into graphite oxide in the presence of high concentrations of
oxidizing agents. This process is accompanied by an increase in interlayer
spacing, which, of course, depends on the amount of oxidation and sintering. Briefly,
the flake graphite was added to solution of sulfuric acid in ice bath. The
proportion of acid to graphite should be 1:46; however, in this research for
0.5 gr graphite, 32 ml sulfuric acid was used. The obtained solution was
subjected to magnetic stirrer for stirring in 400 rpm in 1 hour. In this step
the pore structure of the 3d graphene was determined. If the stirring time
continues, the graphite sheets will fractionalize more. Consequently, the synthesized
3d graphene with 3 hour stirring GO has more porosity than the 3d graphene
which obtains from 1 hour stirring graphene oxide. While the mixture got colder,
0.5 gr NaNO3 was introduced to the solution. In the next step,
required oxygen for better reduction was provided by KMnO4 in the
proportion of 1:6 to graphite (3 gr). The KMnO4 should be added
slowly under the stirring condition and after 15 minute the solution color
changed to dark green. After reaching to the room temperature, the solution was
heated up to 35 °C for 1 hour under stirring condition. This heating up helped the reduction
process to place the carboxylate group between the graphene sheets.

In the next step, 40 cc DI water was gradually introduced to the solution
and then heated up to 90 °C under stirring in 30 min, which the mixture color changed to dark brown.
The quality of graphene sheets is closely related to the way which droplets of
DI water was added to the mixture. Thus, the more gradually droplets were added,
the higher quality graphene sheets will be synthesized. Afterwards, 100 cc DI
water was added to the mixture for two reasons; first, diluted the solution,
second, to assured the reduction completion. The end point of the reduction
happened when the color of mixture changed to light orange. To reduce the remain
KMnO4 for terminating the reduction, 5 cc H2O2
was introduced to the solution. For purification the filtered GO was washed by
10% HCl solution to remove salts and furthermore, washed again by 100 cc DI
water. The PH of GO should be increased for enhancing the stability; hence, the
filtered GO was rinsed in more than three days. The filtered graphene oxide was
carved from filter paper when PH of the solution reached to 5. Using ultrasonic
bath for scattering the graphene sheets it is also required.

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In this study, the 3d graphene was synthesized by mild chemical reduction.