Synthesis of Highly Reduced Graphene Oxide for Supercapacitor

A facile method to synthesize highly reduced graphene oxide in solid phase was developed. The reduced graphene oxide was scarcely prepared in solid phase. Solid substances act as spacers and pillaring agents. Sheets can not be close to each other in reduction process, and sheets agglomeration might not form. After reduction reaction is complete, the spacers and pillaring agents are removed. The average interlayer spacing and surface area of product are bigger than those of reduced graphene oxide. The product has few-layered sheet, and the ratio of carbon to oxygen is high, which might imply that the product is more similar to graphene compared to reduced graphene oxide.The specific capacitance of product is almost three times higher than that of reduced graphene oxide at the same current density.


Introduction
Graphene has created a revolution in the field of nanotechnology [1,2].It shows excellent electrical conductivity and catalysis and cocatalysis and large specific surface area and good thermal conductivity [3][4][5].Because of these unique properties, it is expected that graphene can be used as supercapacitor and electrode material.The preparation and characterization of graphene are of great interest to chemists.It can be prepared by many techniques including chemical vapour deposition, epitaxial growth on electrically insulating surfaces, micromechanical exfoliation of graphite using Scotch tape, chemical intercalation-exfoliation, and electrochemical exfoliation techniques [6,7].Among these reported methods, the chemical oxidation-reduction method is the most facile, for its cost-effectiveness and potential for scale-up of production.However, the agglomeration of graphene sheets during reduction is a great concern for facile applications as all the excellent properties are associated with the monolayer graphene structure [8,9].Surface modification of graphene sheets through the insertion of some organic or nonorganic spacers is a solution to prevent agglomeration [10,11].This approach might partially regain the high surface area of monolayer graphene, but these spacers might be impurity for graphene.
Graphene oxide (GO) begins in monolayer form with high area; the extent of surface area losses by restacking during reduction process is very large.In simple reduction process, the surface area of reduced graphene oxide (RGO) decreases quickly as the number of sheets per stack increases [12].Here, we report the preparation of reduced graphene oxide by pillared method in solid phase.The "pillars" are removed after RGO formed.The mixture of GO and solvent form suspension. GO sheets are reduced; many oxygenated functional groups are lost.With the aid of stirring and Brownian movement, the sheets can freely move, and they can be close to each other.If there are no spacers between monolayer sheets, agglomeration is easily formed, which often lead to layer stacking and even form graphite.GO sheets were dispersed in solid phase, the sheets can not move, and then agglomeration might not form.After reduction reaction is complete, the spacers and pillaring agents are removed, and highly reduced graphene oxide (HRGO) is formed.The reduction process might be like plastic deformation, and sheets might not restack to form graphite.

Synthesis of HRGO and RGO.
A mixture of GO (0.50 g) and water (50 mL) was sonicated for 1 h.Water bath sonication was performed using JL-60 DTH sonicator (100 W).Ammonium chloride (30 g, 0.56 mol) was added to the GO dispersion at 50 ∘ C stirring for 2 h, and then it was dried by natural drying method.Then the mixture was calcined at 320 ∘ C for 1 h, the mixture was cooled down to ambient temperature, and the mixture was washed with water (20 mL × 3).The HRGO was dried at 50 ∘ C for 12 h to yield a black fluffy powder (0.36 g).Other samples were calcined at 240, 280, and 360 ∘ C.
The RGO was prepared according to that of a previous literature [13].

Capacitance Experiments.
The electrode was made by coating 85 wt% of HRGO and 15 wt% of acetylene black onto a working electrode.The working electrode was dried at 100 ∘ C for 10 h.The CV and CD of HRGO were measured with CH660D electrochemical work station using a threeelectrode configuration.Glassy carbon was used as the working electrode, Pt wire served as the counter electrode, and saturated AgCl/Ag served as the reference electrode [14,15].All electrochemical measurements were carried out in 0.1 M Na 2 SO 4 solution as the electrolyte.).The graphene sheets are rather wrinkled, indicating the single-layer and/or few-layer graphene sheets [16].The monolayer sheets dispersed in solvent and they easily formed wrinkles and twists.These sheets were fixed by solid, and then thermal reduction proceeded.The sheets can not freely stretch.The sheets with wrinkles and twists can not easily form multilayer graphene or graphite.The corresponding EDX spectrum revealed the presence of C and O (Figure 1(a)).The peak of oxygen is weak, which implies that GO is reduced.

Results and Discussion
The FT-IR of RGO and HRGO are shown in Figure 2(a).HRGO shows the stretching vibrations of -OH groups at 3420 cm −1 , skeletal vibration of unoxidized graphitic domains at 1560 cm −1 , and alkoxy C-O at 1180 cm −1 .The peak of skeletal vibration of unoxidized graphitic is evident, implying that the HRGO is reduced.RGO shows the stretching vibrations of -OH groups at 3410 cm −1 , skeletal vibration of unoxidized graphitic domains at 1560 cm −1 , and alkoxy C-O at 1040 cm −1 .
The Raman spectra of HRGO present broadened G band at 1572 cm −1 because of the formation of a conjugated system, in contrast to the G band (1565 cm −1 ) of RGO (Figure 2(b)).The intensity of the D band at 1352 cm −1 of HRGO increases substantially, indicating the decrease in size of the in-plane sp 2 domains, possibly due to edges, defects, and disordered carbons [17].Another intense peak is at ∼2672 cm −1 , called the 2D band.The peak is always exhibited by raw graphite, representing the second-order zone boundary phonons.The 2D band is assigned to the band structure of graphene layers.The 2D band of graphene is also very useful for the identification of the number of layers as reported by some literatures [18][19][20][21].Upon careful investigation and comparison with the above literatures, the 2D band of HRGO suggests the formation of few-layer graphene.
The XRD patterns of HRGO and RGO are shown in Figure 3(a).HRGO exhibits the intensity and broadness of the peak at 2 = 27.1 ∘ , and the average interlayer spacing of HRGO is about 0.38 nm.The characteristic peak of RGO is at 27.6 ∘ (0.36 nm) [22].The graphene is single-layer or few-layer sheet by micromechanical exfoliation of graphite using Scotch tape; it is in metastable state.If this graphene is squeezed in right direction, it might partly form multilayer graphene or graphite.Some modified graphene types have wider average interlayer space compared to RGO [23][24][25].Graphene sheets, with surface modification or functional group, can be squeezed; they can withstand external forces.Though HRGO have few spacers or oxygenated functional groups, the average interlayer spacing of HRGO is wider than that of RGO.
The XPS is also used to analyze the samples of HRGO.The core-level XPS signals of C1s are shown in Figure 3   The peak centered at approximately 284.6 eV originates from the graphitic sp 2 carbon atoms; the peak is strong.The peaks of C-O and C=O are at 286.4 and 292.5 eV, respectively.Compared with the peak of graphitic sp 2 carbon atoms, the peaks of C-O and C=O are weak.In some similar reports about RGO [13,26], the ratio of C to O is about 5. The ratio of C to O in HRGO is 9 in Table 1, also indicating that HRGO is highly reduced.The surface area of HRGO is 468.6 m 2 g −1 .The common technique to determine the graphene stack mean layer number is the scaling law: where  is number of sheets in a stack, 2600 is the theoretical surface area of graphene in m 2 g −1 , and A is the measured BET surface area of the sample in m 2 g −1 .According to (1) [12],  is about 6, which might also imply the HRGO is fewlayered sheets.Nitrogen adsorption-desorption isotherm is close to a characteristic IUPAC type IV curve, indicating the presence of pores in the fabricated material, but it is not a typical characteristic IUPAC type IV curve in Figure 4(a).The curve is not closed, and the reason might be that the pores of HRGO are not stable in desorption process.The porous property of HRGO can be further confirmed by pore size distribution analysis determined by the Barret-Joyner-Halenda method in Figure 4(b), from which we can see that there is a distributed peak of micropores centered at 1.9 nm.

Electrochemical Properties of HRGO.
The HRGO degree of reduction with different temperature and HRGO specific capacitances are shown in Table 2.The ratio of carbon to oxygen range between 7.6 and 9.4 was used in the present study.The values of specific capacitance decreased with the increasing ratio of carbon to oxygen from 7.6 to 9. The high specific capacitances were observed at ratio of carbon to oxygen of 7.6-8.2.As for highly reduced graphene oxide (HRGO), the ratio of carbon to oxygen should be high.The temperature is 360 ∘ C, while the yield is low.The temperature = 320 ∘ C is selected in experiments.
To investigate the capacitance behavior of HRGO, the cyclic voltammetry (CV) and the galvanostatic chargedischarge (CD) were carried out as shown in Figures 5(a current density under the same potential compared to RGO at high scan rate of 500 mV s −1 (Figure 5(a)), which implied electron-transporting is easier and faster in the HRGO electrode.So HRGO has excellent electrochemical behavior.Further, as shown in Figure 5(b), HRGO have similar CV curves when the scan rate increased from 20 mV s −1 to 500 mV s −1 and the current density increased with the increase of scan rate, which indicted good rate capability of HRGO electrode.As can be seen from the CV curve, the shape is approximate rectangle, meaning electrical doublelayer capacitance mechanism for HRGO based supercapacitors.Compared to physical methods, chemical reduction methods provide graphene sheets with lots of oxygen groups, thus decrease their electrical conductivity, and result in large leakage potential [27][28][29].Therefore, the effect of leakage potential was obviously observed in the two ends of the curve of HRGO in Figures 5(a) and 5(b).As described by Du et al. [30], polymer grafted graphene containing lots of oxygen groups have high leakage potential due to the low conductivity.Although HRGO was reduced to high level compared to general reduction methods, oxygen groups still existed in the graphene structure confirmed by the XPS curve.And, also, the CD curve shows the specific capacitance of HRGO is higher than that of RGO at the same current density (1 A g −1 ), implying the HRGO have stronger interaction with electrolyte (Figure 5(c)).From the CD curve, the leakage potential was also observed during the discharge process where the potential fell down from 1.0 V to 0.7 V, also indicating the low conductivity of HRGO with oxygen containing groups.Figure 5(d) shows the specific capacitances of HRGO and RGO at different current densities.The specific capacitance of both HRGO and RGO decreased with the increase of current density, resulting from the weak electrical doublelayer.It is well-known the specific capacitance is relative to the electrical double-layer mechanism.When the current density increased, the direction of electric field switched so fast that the charge-carrying easily happened on the surface of the electrode and not deeply into the electrode, weakening electrical double-layer.It can be found that the specific capacitance of HRGO is almost three times higher than that of RGO at the same current density.The specific capacitance of RGO is 41 F g −1 while the value is 128 F g −1 for HRGO at the current density of 1 A g −1 .The specific capacitance of HRGO is 72 F g −1 , higher than that of RGO with 23 F g −1 when applied to the current density of 5 A g −1 .And even the specific capacitance of HRGO (64 F g −1 ) at 7 A g −1 is higher than that of RGO at 1 A g −1 .Therefore, HRGO retains higher specific capacitance and has a higher rate capability than RGO at high current density.The Nyquist plot of HRGO hybrid electrode shows a straight line in the low frequency region and an unconspicuous arc in the high frequency region in Figure 5(e).This high frequency loop is related to the electronic resistance inside the electrode materials.At low frequency, compared with the RGO impedance curve, the slope of the HRGO impedance curve is closer to 45 ∘ , indicating that the HRGO based supercapacitor is closer to the electrical double-layer capacitor characteristic.Despite high resistances of HRGO, their specific capacitance is high compared to RGO which may be attributed to high surface area and porous structure, and therefore they help to achieve the good contact between electrolyte and HRGO and improve charge mobility [31,32].

Conclusions
In all, highly reduced graphene oxide was prepared in practical solid method.The product has few-layered sheet, and the ratio of carbon to oxygen is high.It might be more similar to graphene compared to RGO.The product has higher specific capacitance and has a higher rate capability compared to RGO at high current density.This work shows that HRGO presents unique characteristics, including facile synthesis and full reduction, which render it suitable for high-quality electronic devices, sensors, and supercapacitors, as well as fundamental studies about the chemistry and structure of graphene.

Figure 4 :
Figure 4: Typical nitrogen adsorption and desorption of HRGO (a) and pore size distribution curves of HRGO (b).

Figure 5 :
Figure 5: (a) CV curves of RGO and HRGO at 500 mV s −1 ; (b) CV curves of HRGO at different scanning rates; (c) the galvanostatic charge/discharge behavior of RGO and HRGO at 1 A g −1 ; (d) the specific capacitance of RGO and HRGO based supercapacitors at different current densities; and (e) Nyquist plots of RGO and HRGO electrodes using a sinusoidal signal of 25 mV with the frequency range from 10 mHz to 10 kHz.

Table 2 :
The HRGO degree of reduction with different temperature and yield and their specific capacitances.