Highly conducting reduced graphene synthesis via low temperature chemically assisted exfoliation and energy storage application

We report a facile approach towards mass production of few layered reduced graphene by low-temperature (160 °C) exfoliation of graphite oxide under ambient atmospheric condition with the aid of formic acid in a short duration (~14 min). The obtained reduced graphene showed very high bulk electrical conductivity (1.6×10 S/cm) at room temperature due to restoration of extended conjugation of sp network during rapid exfoliation. Protonation followed by hydride transfer mechanism has been proposed for the restoration of extended conjugation and high electrical conductivity. BET surface areas of 789 and 1130 m/g with narrow mesopores distribution (1.9-2.5 nm) were obtained for two different samples prepared by modification in synthetic methodology. Reduced graphenes were tested as supercapacitors and specific capacitances of 152 and 157 F/g with excellent cyclic stability were observed for two samples in aqueous electrolyte. Page 1 of 21 Journal of Materials Chemistry A Jo ur na lo fM at er ia ls C he m is tr y A A cc ep te d M an us cr ip t


Introduction
Graphene is a two dimensional single atom thick high surface area material which exhibits excellent electrical, mechanical, thermal properties having many potential applications in the field of electronics, optoelectronics, and energy devices.High intrinsic mobility (2 x 10 5 cm 2 /V.s), good thermal conductivity (5,000 W/m.K) and excellent electrical conductivity (10 6 S/cm) of graphenes facilitates electron or hole transfer along its twodimensional surface have triggered their extensive investigation in the field of electronics and thermal energy management.2][3][4][5] Supercapacitors, also called electrochemical double layer or ultracapacitors, provide a huge amount of energy in a short period of time, which makes them suitable for surge power delivery.Different synthesis methods such as mechanical exfoliation, chemical vapour deposition, chemical reduction and exfoliation have been investigated for the production of high quality graphene. 5As per large scale synthetic requirement for energy applications, chemical exfoliation 6 methods are exclusively on duty but produced few layered graphene having low electrical conductivities and surface areas with several defects compared to other methods. 4Cheng and co-workers achieved reduced graphene (RG) with high electrical conductivity of 2×10 3 S/cm by hydrogen arc discharge exfoliation 7 of graphite oxide (GO) at high temperature (1050 °C), a synthetic methodology of high cost. 8,9 esearchers also reported low temperature exfoliation of GO with the chemical aid like HCOOH, 10 HCl, 11 and H 2 SO 4 , 12 but the produced RG suffered with low electrical conductivity.Above discussion indicates that modulation of chemical and thermal exfoliation may lead to large scale production of superior quality graphene.
Herein, we report an example of chemically assisted low-temperature exfoliation route for mass production of graphene.In this work, judiciously formic acid has been chosen, which governs exfoliation of GO at low temperature (160 °C) within a short duration of 14 min with outburst.Produced graphene exhibits excellent bulk electrical conductivity (1.6×10 3 S/cm), high surface area (1130 m 2 /g), and good electrochemical energy storage capacity (~ 157 F/g).Moreover, we propose plausible mechanisms based on fundamental organic chemistry reactions for excellent electrical conductivity of RG and also suggest the usage of as prepared RG as electrode material for supercapacitor.This approach will lead to new facile ways for the mass production of quality graphene under ambient conditions which can be used as energy devices.

Synthesis of RG
GO was prepared using Hummer's method. 13Graphite (particle size 150 micron) (2.5 gm) was first sonicated in acetone for 30 minutes.Thereafter, graphite was filtered through a 46 mL concentrated sulfuric acid in an ice bath (0°C) in a 250 mL round bottom flask equipped with a magnetic stir bar.Separately, synthesis was also performed by suspending graphite (2 gm) and NaNO 3 (1 gm) in concentrated sulfuric acid for GO synthesis.6 gm of KMnO 4 was then added cautiously covering a time span of approximately 20 min so that the temperature of the solution did not exceed 20 °C.The solution was then stirred at 35 °C using a reflux condenser for 3 h.After that, 92 mL of distilled water was added and stirring continued for an additional period of 30 min.Then the content was poured into 280 mL of distilled water and 10 mL of 30% hydrogen peroxide was added to destroy the excess KMnO 4 .The complete removal of KMnO 4 was indicated by a color change from dark to yellow.GO was then isolated by a sintered glass filter and washed with copious amount of water (500 ml).Washing was continued until sulphate was no longer detected in the filtrate which was confirmed by addition of barium chloride solution in the filtrate.Thereafter, filtrate was washed with formic acid solution (20 %) to intercalate formic acid in the inter-lamellar region of GO and dried in an oven at 60 °C for 24 h.Prepared GOs were named as GO(150) and GO(150+N) (NaNO 3 was used during synthesis).The numeric 150 indicates the particle size of parent graphite precursor purchased from Sigma-Aldrich.After drying, GO samples were stored in vials in ambient conditions.The bold sentence indicates the synthetic modification of this study from our previous study. 14For GO reduction, 0.5 gm of prepared GOs were poured in 500 ml conical flask.Thereafter conical flask was kept inside the preheated hot air oven at 160 °C for 14 min to complete exfoliation process.One of the aims of this work was to perform the exfoliation process at a lowest possible temperature to make the graphene synthesis cost effective.By performing several experiments, it has been found that 160 °C is the minimum temperature required for exfoliation to achieve high quality graphene.Digital images were taken before and after exfoliation and are shown in ESI (Fig. S1).Two different reduced graphene (RG) materials were named as RG(150) and RG(150+N).

Characterization
The powder X-ray diffraction (PXRD) patterns were recorded by Bruker AXS D8 Advance with Cu Kα radiation (1.54 Å) with a step size of 0.02° in a 2θ range of 0-60°.Thermal gravimetric analysis (TGA) was performed by Perkin Elmer TGA 4000 instrument in a temperature range from 30 -900 °C at a scan rate of 5 °C/min with a N 2 flow rate of 10 mL/min.For morphological studies, dried samples were spread over carbon tape and gold coated for 120 s, and scanning electron microscopy (SEM) studies were performed using Carl ZEISS (ultraplus) FE-SEM at 20 kV.Energy dispersive X-ray (EDS) experiments were performed at a working voltage of 20 kV and was standardized with the Co element by using a spectrometer (Oxford Instruments X-Max N ) attached to SEM.Transmission electron microscopy (TEM, JEOL 2100F) experiments were performed under an accelerating voltage of 200 kV.TEM samples were prepared by drop casting the suspension of the sample (0.5 mg in 15 mL isopropanol) onto a carbon-coated copper grid and solvent was evaporated under ambient conditions overnight.The Raman spectroscopy experiments were performed by Lab RAM HR 800 (HORIBA) with exciting wavelength of 632.8 nm.Nitrogen adsorption/desorption experiments were performed on Quantachrome Autosorb QUA211011 equipment for surface area measurements.Samples were degassed under high vacuum at 100 °C for 24 h.FT-IR spectra were collected using Perkin Elmer spectrum BX spectrophotometer using pallets of samples with KBr.Electrical conductivity measurements were carried on physical property measurement system (PPMS, Model 6000) Quantum Design, USA.Electrochemical experiments were carried out using CH Instruments, Austin, TX bipotentiostat (Model CHI 760D) and potentiostat (Model CHI 620E).Capacitance electrochemical impedance spectroscopy (EIS) techniques with regular three electrodes cell.
Electrode preparation details and capacitance calculation methodologies are discussed in ESI ‡.Elemental analysis experiment was carried out on Elementar Analysen systeme Gnbh, Germany.Samples were dried at 60 °C for 12 h before performing the experiment.

Results and discussions
RGs were synthesized by reducing GO at low temperature (160 °C) with the chemical aid of formic acid as described in the experimental section.Graphite oxide (GO) contains oxygen functionalities such as epoxy, hydroxyl, carboxyl etc. Due to the presence of these functional groups, GO decomposes to produce gases such as CO 2 and H 2 O during thermal treatment at low temperatures, but cannot generate enough pressure to overcome van der Waals forces at low temperatures.In order to make the exfoliation process efficacious, the pressure generated from evolved gases should surpass the van der Waals forces holding GO sheets which can be achieved by external chemical aids.We accomplished this process by introducing formic acid (HCOOH) as hydride donor 15 and volatile substance into the inter-lamellar region of GO during filtration process.The reducing nature of formic acid can be understood by its propensity to lose both hydrogen and generate CO 2 . 15During heating process, CO 2 was released from formic acid after hydride donation to electrophilic centers and supplied extra pressure to accelerate the expansion of GO, leading to fast exfoliation under ambient conditions (schematically represented in Fig. 1   in number of layers. 14Synthesized RGs had around 5 layers of graphene sheets, calculated using Scherrer's formula and Voigt function 16 for peaks fitting, as given in ESI ‡.As an example, utilizing a low temperature chemically assisted exfoliation method, Zheng and coworkers obtained 5-6 layers of graphene sheets. 11Weak peaks were also observed around 2θ maxima 42.5° for (100) plane which were fitted using Voigt function and crystalline lengths along a-axis (L a ) were calculated using Scherrer's formula.L a values of RGs were found to be ~7.5 nm.Table 1 shows the decrease in crystalline length (25 times decrease along c-axis i.e.L c and 11 times decrease along a-axis i.e.L a ) for reduced graphenes compared to parent graphite.leading to retain the hexagonal symmetry in RG (150).Further, qualities of synthesized RGs were evaluated by TGA and the results are shown in ESI (Fig. S2).RG (150) and RG (150+N) showed weight loss of 8 and 11%, respectively up to 250 °C, which can be attributed to the loss of water and labile oxygen functionalities.Furthermore, 7.5 and 11% weight loss were observed for RG (150) and RG (150+N), respectively, in the temperature range 250-510 °C, that can be attributed to the loss of residual functional groups.The higher weight loss for RG (150+N) indicates presence of more functional groups in RG (150+N).In a previous report by our group, 14 we concluded that the addition of NaNO 3 during GO synthesis favors strong basal plane oxidation on graphene sheets which generate more functional groups on basal planes and hence slightly more functional group may remain in graphene sheets for RG (150+N) compared to RG (150).This conclusion can be further supported by lower conductivity of RG (150+N) compared to RG (150) (vide infra).As a strong tool to characterize sp 2 carbon domains, Raman spectroscopy was performed for synthesized RG samples and spectra are shown in ESI (Fig. S3).RGs exhibit D band around 1330 and G band around 1589 cm -1 .D band corresponds to disorder while G band indicates the in-plane vibrations of carbon atoms.A large shift in G band of RGs compared to parent graphite was observed, indicating few layered structures of RGs. 17 Intensity ratio of D band to G band (I D /I G ) directly measure the degree of disorder. 18Decreased I D /I G ratios for RGs compared to that of GOs indicate the reduced disorder due to thermal exfoliation as described by Kudin and co-workers. 19T adsorption/desorption profiles for N 2 have been shown in Fig. 4a which exhibits characteristic of type-III and IV isotherm with hysteresis loop of H2 type (according to IUPAC classification) in the range (0.45-1.0) for P/P 0 , implying the mesoporous nature of synthesized RGs.BET surface areas of 789 and 1130 m 2 /g with pore volume 5.5 and 11.84 cm 3 /g were observed for RG (150) and RG (150+N), respectively.RGs showed narrow pore size distribution in the range of 1.9-2.5 nm, calculated using Barrett-Joyner-Halenda (BJH) thermal exfoliation at 130 °C, Aksay and co-workers 9 reported 650 m 2 /g at 1050 °C, and Ruoff and co-workers 21 reported surface area 705 m 2 /g for chemically exfoliated graphene sheets.Surface areas for RGs achieved by present method, are higher than other thermal and chemical exfoliation methods.FT-IR spectra of GOs and RGs were compared in Fig. 4b.Significant decrease in the intensity of stretching vibrations of C=O (1654 cm -1 ), C-O (1080 cm -1 ) and increase in the stretching vibration of C=C (1466 cm -1 ) were observed from GOs to RGs.These results indicate the reducing nature of formic acid towards selective oxygen functionalities.
Restoration of π conjugation in synthesized RGs were evident by electrical conductivity measurements.High bulk electrical conductivities of (1.6±0.09)×10 3 and (1.4±0.07)×10 3 S/cm were observed for RG (150) and RG (150+N) respectively, measured by four probe method.Bulk electrical conductivity achieved in the present work is found to be superior than most of the reports as summarized in Table 2.The excellent electrical conductivity of synthesized RGs can be attributed to the significant decrease in the C=O stretching (Fig. 4b), as described in other reports. 7,22 e 11 of 21 Journal of Materials Chemistry A

Present study
The plausible mechanisms for chemically assisted thermal exfoliation have been shown in Scheme 1. Basal planes of graphene sheets in GOs contain functional groups such as hydroxyl and epoxy attached with sp 3 hybridized carbons and sheet edges contains carbonyl functionality attached with sp 2 hybridized carbons.Protonation followed by hydride (H -) transfer from formic acid (HCOOH) and thereafter release of gases such as H 2 , CO 2 and H 2 O will lead to removal of these functional groups from graphene sheets. 15,23 eleased gases created extra pressure for the expansion of graphene sheets at low temperature.The most important feature of formic acid assisted exfoliation is that not only the functional groups were removed but also the driving forces for these reactions were to change the hybridization of carbon atoms from sp 3 to sp 2 for restoration of π networks.Hence, the mechanism also explains the superior conductivity of RGs.Inspired by these observations, electrochemical experiments were performed to test these RGs as electrochemical supercapacitor using regular three electrodes cell by depositing the materials on platinum (Pt) electrode with poly(vinylidene fluoride) binder in 2 M H 2 SO 4 as electrolyte.Nearly, rectangular cyclic voltammograms (CV) were obtained (Fig. 5a) even at a faster scan rate of 100 mV/s indicating quality charge propagation. 21The CVs at slower scan rates are shown in ESI (Fig. S6).Specific capacitances of 157 and 152 F/g were observed for RG(150) and RG(150+N), respectively at voltage sweep rate of 5 mV/s.Retentions of nearly 86 and 78% for specific capacitances of RG(150) and RG(150+N), respectively were observed at the high voltage scan rate of 100 mV/s (Fig. 5b).The drop in specific capacitances with increasing scan rate can be attributed to the pseudocapacitance (redox) contribution from the remaining oxygen functionality.This redox contributions in RGs arise due to the presence of hydroxyl functionalities on the sheet edges of graphene sheets 24,25 and according to Scheme 1, formic acid cannot remove these functionalities from graphene sheets.TGA results suggest that the functionalities present in RG(150+N) was RG(150) was higher than RG(150+N).Apparently, these results may seem to contradict with each other but can be understood utilizing the observations of our previous studies 14 where we reported that GO synthesis in absence of NaNO 3 results higher sheet edge oxidation whereas presence of NaNO 3 during GO synthesis results higher basal plane oxidation on graphene sheets.Hence, RG(150) may have more hydroxyl functionalities on edges of graphene sheets compared to RG(150+N) which is responsible for enhanced redox response of RG(150).It is also important to mention that specific capacitances of RGs in the present study were comparable or greater than most of the other reports for graphene, as shown in Table 3. High surface area and good electrical conductivity of as synthesized RGs result in good capacitance performance.Specific capacitances of RGs were also calculated using galvanostatic charge/discharge measurements (Fig. 5c).Calculated specific capacitances with both methods indicate strong agreement between two electrochemical techniques (Table 4).Nyquist plots were analyzed to understand the rate of mass transfer in RGs (Fig. 5d).In case of RG (150), diffusion of electrolyte started at 215 Hz corresponding to 0.9 Ω resistance and completed at 1.8 Hz corresponding to 1.5 Ω resistance and thereafter complete capacitor behavior was achieved. 26 case of RG (150+N) diffusion of electrolyte started at 175 Hz corresponding to 0.9 Ω resistance and completed at 1.2 Hz corresponding to 1.7 Ω resistance and thereafter capacitor behavior was completely achieved.These results indicate excellent mass transfer inside the RGs.The explanation for excellent mass transport was due to narrow mesopore distribution in the range (1.9-2.5 nm) (vide supra) in RGs which can decrease the ion-diffusion length, hence minimizes inner-pore resistance and also provide large accessible surface area for ion transport/charge storage. 27,28 he cyclic performance (Fig. 5e) indicates nearly 100% stability for both the samples after 1000 cycles, indicating the robustness of material and excellent ion behaviour of produced RGs (Fig. 5f).
Higher capacitance and low cost production of prepared RGs compared to other reports 29- 31 clearly indicate the suitability of prepared RGs for supercapacitor electrodes.Narrow mesoporous texture of RGs reduces the inner pore resistance and facilitates high mass transfer even at higher frequencies.Additionally, higher electrical conductivity of RGs can reduce the ohmic loss and cut down the internal resistance for better charge propagation, as observed in near square shape of CV curves at high scan rates (Fig. 5a).with and without addition of NaNO 3 .The results presented here show that reduced graphene prepared from GO without the addition of NaNO 3 had better properties such as higher conductivity, higher specific capacitance due to the presence of lesser functional groups.
Lastly, it is worth mentioning that well known fundamental organic chemistry reactions between oxygen functional groups of GO and formic acid were the key behind the success of this work.
and digital images of the reaction are shown in Fig. S1 of ESI).The detailed mechanistic understanding of synthetic methodology has been discussed as well (vide infra).

Fig. 1 :
Fig. 1: Schematic representation of chemically assisted low temperature exfoliation of GO with aid of formic acid.

Scheme 1 :
Scheme 1: Mechanisms for the reactions between different functionalities of GO and formic acid.(i) hydroxyl (ii) epoxy, and (iii) carbonyl.

Fig. 5 :
Fig. 5: (a) Overlap plot of CVs at a scan rate of 100 mV/s.(b) Variation of specific capacitances at different scan rates of 100, 50, 20, 10 and 5 mV/s in cyclic voltammetry.(c) Galvanostatic charge/discharge measurements at current densities of 1, 2, 5, and 10 A/g from right to left respectively.(d) EIS collected at 0 V versus standard calomel electrode (SCE) with inset zoomed view of high frequency region with mass transfer frequency.(e) Cyclic stability performance at a scan rate of 20 mV/s of cyclic voltammetry.(f) Phase angle versus frequency in EIS experiment.

Table 2 :
Comparison of electrical conductivity value of present study with reported values.