Tailoring the Oxygen Content of Graphite and Reduced Graphene Oxide for Specific Applications

Graphene oxide (GO) is widely recognized as a promising material in a variety of fields, but its structure and composition has yet to be fully controlled. We have developed general strategies to control the oxidation degree of graphene-like materials via two methods: oxidation of graphite by KMnO4 in H2SO4 (oGO), and reduction of highly oxidized GO by hydrazine (rGO). Even though the oxygen content may be the same, oGO and rGO have different properties, for example the adsorption ability, oxidation ability, and electron conductivity. These differences in property arise from the difference in the underlying graphitic structure and the type of defect present. Our results can be used as a guideline for the production of tailor-made graphitic carbons. As an example, we show that rGO with 23.1 wt% oxygen showed the best performance as an electrode of an electric double-layer capacitor.


Preparation of several oxidation degree of GO by the oxidation of graphite (oGO).
Graphite (3.00 g) was stirred in 95% H2S O4 (75.0 mL). The required amount of KMnO4 (0.75,1.50,2.25,3.00,3.75,4.50,5.25,6.00, 7.50 and 9.00 g) was gradually added to the solution keeping the temperature <10 °C. The mixture was then stirred at 35 °C for 2 h. The resulting mixture was diluted by water (75.0 mL) under vigorous stirring. The suspension was further treated by adding 30% H2O2 solution (7.50 mL).
The resulting graphite oxide suspension was purified by centrifugation with water. Several oGO were analyzed by CHN elemental analysis to evaluate the oxygen content.
The result of elemental analysis is shown in Table S1.

Preparation of several oxidation degree of GO by the reduction of GO (rGO).
GO (2.00 g, O: 58.8 w%) which was synthesized by an above mentioned method using 6.0 g of KMnO4 was dispersed in water (200 mL), then the required amount of hydrazine monohydrate (62.5, 125, 187.5, 250, 312.5, 375, 500, 750, 1000, 1500 and 2000 L) was added before heating at 90 °C for 2 h. After cooling, the product was purified washed with water. Several rGO were analyzed by CHN elemental analysis to evaluate the oxygen content. The result of elemental analysis is shown in Table S2. Table S2. Elemental compositions of rGO.

XRD measurement of intermediate.
To confirm the homogeneous formation of graphite intercalated compound (GIC), XRD spectra were measured before treating with water.
Graphite (1.00 g) was stirred in 95% H2SO4 (25 mL). Small amount of KMnO4 (0.10 g) was gradually added to the solution keeping the temperature <10 °C. The mixture was then stirred at 30 °C for 1 h. The mixture was centrifuged and XRD analysis was performed for the resulting precipitant. (c)

UV-vis spectroscopy.
Each sample was diluted to GO content of ca. 0.1 w% by ion exchanged water, then sonicated for 10 min before measurement.

Electrical conductivity measurement
Each sample was pelletized before the measurement using four-point probe. To investigate the electrical conductivity of GO, the average resistance was measured at 3 sampling points. he specific resistance was calculated according to  = RFL ,where  is the specific resistance (cm), R is the pellet thickness (cm), F is the correction coefficient which was determined from the distance between the probes attached with the instruments and L is the measured pellet resistance (). The electrical conductivity (Scm -1 ) was calculated reciprocal number of specific resistance.

Capacitance measurement
For the active electrode preparation, GO (30.0 mg), acetylene black (5.6 mg) and PVDf (2.0 mg) were mixed together in a mass ratio of 80 : 15 : 5, then N-methyl-2-pyrrolidine (13.1 mg) was added to the mixture. PVDf and acetylene black were used as the binder and conductive agent, respectively. Then the mixture was bound with Ni mesh and dried at 50 °C overnight in a vacuum oven. Active material (10.0 mg) was bound in the electrode. Each sample was directly used as electrodes in a three-electrode test cell using 1.0 M KOH aqueous solution as an electrolyte.
Measurements were performed at the scan rate 20 mVs -1 . The gravimetric capacitance of the electrodes was calculated according to C = ʃidV/mvV ,where C is the specific capacitance (Fg -1 ), i is current (A), V is the potential (V), v is the scan rate (Vs -1 ), and m is the mass of the active material (g). Calculation for ʃidV was made using cyclic voltammograms of the second cycle.

Galvanostatic charge-discharge measurement
The potential range was chosen by taking into account the CV curves. The current density was fixed at 0.2 Ag -1 for direct performance comparison between the individual samples.  Figure S6. Galvano tatic charge/discharge curves of GO and rGO at 0.2 A g -1 .

Oxidation of benzylalcohol by using GO
To the solution of 1,2-dichloroethane (0.50 mL) was added GO(20.0 mg) in the presence of benzylalcohol (30.9 L, 0.30 mmol) under Ar atomosphere and the mixture was stirred at 60 °C for 12 h. After the reaction, the reaction mixture was analyzed by gas chromatography using dodecane as an internal standard.