One-Step One Chemical Synthesis Process of Graphene from Rice Husk for Energy Storage Applications

Few layer graphene was synthesized using rice husk ash (RHA) and potassium hydroxide (KOH). This methodology demonstrates the utility of RHA as carbon source for graphene synthesis and as a protective barrier against oxidation of parent rice husk and KOH mixture. Oxidation may occur during synthesis process due to high temperature annealing of RHA and KOH mixture. Electrochemical characterization showed decent capacitance value 86 F∙g at 500 mV∙s. XRD and Raman spectroscopy analysis confirmed the presence of graphitic structure. Transmission electron microscopy visually confirmed presence of few layer graphene. Novelty of this synthesis technique can be described as one-pot, one chemical synthesis technique. Use of natural precursor makes this technique highly cost effective for large scale production.


Introduction
Graphene is a 2D carbon allotrope with honeycomb lattice structure.Individual carbon atoms are bonded by sp 2 hybridization [1].Graphene has outstanding properties like: excellent electrical and thermal conductivity, flexibility, optical transparency, high specific surface area and much more.These properties make it suitable for various applications, like energy storage and harvesting application (super capacitors, battery, solar cell and so on) [2]- [7], fabrication of transistors [8] [9], biomedical application [10] [11], designing mechanically robust materials [12] [13] [14] [15] etc. Synthesis of single layer of graphite has been reported as early as in 1975 by B.
Lang [16].After some scattered attempts by various scientist, finally Novoselov et al. have gained the credit for discovery of graphene in 2004 [17].They introduced a reproducible technique of graphene synthesis by mechanical exfoliation but this technique is not suitable for large scale production.There are some other well established methods also available for synthesis of graphene such as chemical vapour deposition, chemical reduction of graphene oxide and epitaxial growth on silicon carbide.
Currently, many researchers are trying to develop green synthesis methods for graphene production [18] [19] [20].The purpose of green synthesis process is to use less toxic chemicals and natural precursor.Muramatsu et al. have reported synthesis of graphene from rice husk using KOH and carbon black [20].
In this research work, we have successfully replaced carbon black by Rice husk itself, in this way, this approach become even more cost effective synthesis methodology.In our synthesis process we have used RHA as carbon source for graphene synthesis and KOH as activation agent.The obtained results have revealed successful synthesis of few layer graphene.Cyclic voltammetry has been performed to evaluate electrochemical performance for energy storage application.

Materials and Methods
The synthesis process is shown in Figure 1, in typical synthesis process, analytical grade reagent KOH and rice husk were purchased from local market.In typical synthesis process, rice husk collected from local rice mill, was washed several times to remove silica and other contamination as much as possible.After washing, the RHA was prepared by the combustion of rice husk into air.Furthermore, 3 gm of rice husk ash was mixed with 15 gm of KOH and followed by grinding process for 15 min.The mixture of rice husk and KOH was compacted into porcelain crucible.This crucible was covered with ceramic wool and fixed into a larger graphite crucible.The top of the graphite crucible was covered

Results and Discussion
The FESEM images of successfully synthesized Graphene by activation treatment of RHA using KOH are shown in Figure 2. From this morphological analysis, flakes of graphene sheets with silica nanoparticles can be observed clearly.Herein, the dual function of KOH is; removal of amorphous carbon and separation of individual graphene sheet by intercalation of potassium atoms.
TEM images of RHA derived graphene by KOH activation are shown in above Figure 3.The micro graphical images have confirmed the synthesis of few layer graphene.From these images, we can observe the few layer graphene and agglomeration of silica particles.In the inset, selected area electron diffraction pattern (SAED) has shown, the individual spots have merged into the rings.This shows the characteristic of polycrystalline sample [21] and suggests the stacking of graphene sheets and aggregation of silica particles with random arrangement.FTIR spectrum of rice husk derived graphene-silica composite is shown in Figure 4. From FTIR spectra, the adsorption band at 1040 cm −1 and 660 cm −1 which reflect asymmetric and symmetric stretches of Si-O-Si, respectively [22].
Moreover, the absorption band at 1420 cm −1 has been observed due to the Si-O-C=O bonding [23] and the peak at 1600 cm −1 in the spectrum confirms the presence of aromatic -C=C-bond [24].The infrared band at 3000-3500 cm −1 appears due to the asymmetric stretching and bending vibration of surface hydroxyls and adsorbed water [25].
XRD pattern of prepared graphene by the activating of RHA using KOH at 900˚C is shown in Figure 5. From XRD analysis, the diffraction peaks around 20.96˚, 45.66˚ have been assigned with lattice plane (111) of cristobalite silica (JCPDS card No.: 89-3435) and (100) corresponds to graphitic structure of carbon, the weak intense peaks (100) of graphene specifies the non-appearance of a repeatedly stacked graphitic structure [20].etc. in the sample.The G' band also known as 2D band induced due to phonon scattering process.In our case, the intensity of G band is observed much higher greater than D band.The I D /I G ratio from Raman spectra has been found 0.55.This result attributes to more graphitic structure present in RHA derived graphene.Das et al. [25] proved that, the I D/ I 2G intensity ratio was found ~0.24 for single layer graphene.Moreover, Das et al. also suggest that, I D/ I 2G intensity ratio linearly increases up to 6-8 layers.In present analysis, the obtained I D/ I 2G intensity ratio is found to be ~0.67,which indicates the synthesized sample contains few layer graphene.This conclusion is in best consent to our TEM images as it can be observed that synthesized graphene is no single layer.The thermal behaviour of synthesized rice husk derived graphene is probed using thermogravimetric analysis (TGA) as shown in Figure 7. From TGA curve, it was found to ~17.80% of weight loss up to 400˚C, which is due to evaporation of physically adsorbed water content and may be attached functional group like hydroxyl from the synthesized sample.Thereafter, a weight loss of ~47.60% observed from 400˚C -730˚C, which is attributed to decomposition of carbon in the synthesized simple.Haung et al. [26] have been shown the thermal stability of bare silica after 700˚C.Therefore, from TGA analysis, we can Figure 7. TGA analysis of RHA derived graphene.conclude that, the Silica (SiO 2 ) content is 33.9 (100 − 64.1 = 34.6)% in the prepared sample.
We finally studied the electrochemical analysis of our sample.The working electrode for electrochemical analysis was prepared using 80 wt% active material, 10 wt% graphite powder as a conductive material, and 10 wt% PVDF as a binder.These materials were mixed using DMF as a solvent to prepare the slurry.The slurry was deposited on stainless steel sheet (1 cm × 1 cm) and dried in oven at 100˚C for 12 hours to prepare working electrode.Finally, the electrode was assembled in three electrode configuration with standard Ag/AgCl electrode as the reference and platinum wire as a counter electrode.1 M Na 2 SO 4 solution was used as the electrolyte.The specific capacitance (F•g −1 ) of prepared sample was calculated from CV curve using the equation ( 1) where i is the average current (A), ʋ is the scan rate (V•s −1 ) and M is the mass of working electrode (g) [27].The specific capacitance of RHA derived graphene was calculated according to the Equation (1) and has been found 86 F•g −1 at 500 mV•s −1 .Moreover distortion in CV curve as seen from Figure 8 is suggested due to slow electron transfer process because of silica content in the sample [28].A Brief comparison of various graphene synthesis method is shown in Table 1.

Conclusion
We have demonstrated very simple, one-pot, one chemical approach to synthesize graphene from rice husk.Transmission electron microscopy revealed the synthesis of few layer graphene with nanosized silica content.XRD analysis has also confirmed the synthesis of few layers graphene.From Raman spectroscopy it was found that I D /I G predicted less defects in graphene sheets.Capacitance value may not be up to the mark, but this approach can be used to synthesize cheaper graphene.Here we want to mention that, although silica was not desirable in the synthesized graphene but presence of Nano sized silica may be benefi-

Figure 1 .
Figure 1.Experimental set-up for synthesis of RHA derived graphene.

Figure 3 .
Figure 3. TEM images of RHA derived graphene at different magnification.

Table 1 .
[35]synthesis of graphene with different approaches and their capacitive performance analysis.Li-ion battery based energy storage devices.It has been well documented that introduction of silica increases capacity, cyclic stability, and efficiency of electrode[32] [33][34][35].Another added advantage is that, presence of silica can improve thermal stability of the electrode.