Microstructure analysis of graphene oxide synthesized from corn cobs waste

Graphene oxide is one carbon-based material decorated by oxygen functional group that has been applied in various field. Active oxygen functional group allows graphene oxide served as precursor for many applications including DSSC, Field Effect Transistor, LED, and Transparent Conductive Film. Unfortunately, high cost of pure raw material inhibits the wide application of graphene oxide. Corn cobs is one of highly produced waste from agricultural activity that become an unsolved environmental problem. Meanwhile, high content of carbon material in corn cob allows it to be a potential carbon source for graphene oxide synthesis. In this report the synthesis of graphene oxide from corn cobs using modified hummers method is discussed. As synthesized sample was analysed using FTIR, XRD, and SEM to confirm the successful of graphene oxide form corn cobs.


Introduction
Graphene has attracted most attention in the few years because of its unique properties and potency [1].The excellent properties of graphene allow it to be applied in many field and device.Electrical, mechanical, thermal, and optical property of graphene have become the appeal for scientists to explore more about graphene and its application in various fields of technology [2]- [5].Graphene has applied DSSCs, LEDs, transistors, batteries [6]- [9].Graphene can be made using various methods and the cheapest method is the chemical method through the synthesis of graphene oxide [10] Graphene oxide is resulted from oxygen intercalation from strong acids into the graphitic layers which is then followed by mechanical exfoliation.As the result of acid attack, graphene oxide was decorated by the oxygen functional groups existing in its basal plane.The functional groups play main role in the hydrophilic property of graphene oxide that allows it to be used as a precursor in deposition or chemical reduction process.The application of hydrophilic nature of graphene oxide was found int production of graphene oxide-based thin films [11].By applying reduction step after graphene oxide production, the reduced graphene oxide could be obtained which the properties are similar to graphene.
Usually graphene oxide was synthesized from pure graphite as carbon source which went through a complicated purification process resulting expensive price of purchasing.Meanwhile, carbon is able to be obtained from various natural sources that its existence is abundance.Carbon composes the majority of the tissues of living things and vegetations.
Natural wastes are one of the carbon sources that have not been used.As the side product of agricultural or commercial process, these materials are being the problem for the environment and one rapid solution for rubbish-like material is burning.As a result, they contribute to the increasing of the carbon dioxide emissions in the air and escalate the climate problems.Several studies have begun to utilize natural waste as a carbon source for activated carbon that applied in absorption.Carbon gathered from biomass waste was extracted and activated with acid or base and gain the ability to absorb impurities in the water.The natural carbon source that have been utilized are coconut shells, rice husks, corn cobs, oil palm empty bunches, and bagasse [12]- [15].The high concentration of carbon compared to other constituents makes these side products are suitable for carbon-based materials processing.
Corn plantation leaves side material in the form of corn cobs that currently are not utilized properly.Meanwhile, corn cobs consist majority of carbon materials in form of cellulose.Utilization of corn cobs as a source of active carbon material has been carried out and can be used as a water absorbent material [13].Based on this finding, the use of corn cobs as a carbon source material for the synthesis of graphene oxide becomes possible.Utilization of corn cobs as a carbon source material for the synthesis of graphene oxide will increase the value of this agricultural waste into a material that has high value in addition to reducing environmental problems.This will shoot two birds with one bullet.

Materials
Corn cobs were gathered from local source.Sodium hydroxide, potassium permanganate, hydrogen peroxide, sodium nitrate, and sulfuric acid were purchased from EMD Millipone Corporation, Germany.Ethanol was obtained from PT. Brataco, Indonesia.DI water was purchased form local suppliers.

Raw material preparation
Corn cob waste gathered from local source was cleaned up to remove impurities and chopped into small pieces.An aluminum foil vessel was prepared as the placement of corn cobs in the oven to desiccate it in the temperature of 105C for 2 hours.Afterwards, raw material was prepared for carbonization process by using furnace heating in various temperature of 250C, 300C, 350 C 400C and 450C for 30 minutes for each temperature treatment.Water contain existing in the sample was removed along the heating process that resulted the black charcoal.The charcoal was then refined into smaller size by grinding and filtering process with 170 mesh sieves.Silica extraction was carried out to ensure that it`s contain was below 10%.The expected dominant element in the charcoal was carbon as the precursor of graphene oxide synthesis.XRF analysis was performed to examine the elements in the charcoal.Carbon activation was performed by mixing 8 grams of charcoal and 100 ml of Natrium Hydroxide in a covered flask and stored for 24 hours.The compound then separated from the solvent by using filter paper and washed with DI water until normal pH was reached.The precipitate then dried using oven with the temperature of 105C for 30 minutes.

Synthesis of graphene oxide
Modified Hummer`s method was applied to synthesis graphene oxide.It was started by preparing 1 gram of activated charcoal of corn cobs, 0.5 gram of sodium nitrate and 25 ml of sulfuric acid in an Erlenmeyer.The mixture was then stirred using magnetic stirrer for 30 minutes in the ice bath.The temperature of the mixture dropped into around 4 C and the potassium permanganate was added slowly.High amount of potassium permanganate that poured into the mixture in an instant time would trigger the explosion.Afterwards, the ice bath was removed and stirring process was continued for 3 hours until the mixture turned into dark green colour.The solution was diluted by adding 150 ml DI water gradually to prevent the temperature rose suddenly and followed by stirring process for 2 hours.The reaction was terminated by adding hydrogen peroxide and the mixture was left for 30 minutes.Sonication process was carried out to peel the oxidized carbon into the layers by using commercial sonication (Elmasonic S30H) for 2 hours.Sonicated mixture was then washed using DI water until the pH was normal.Centrifuge was used to separate the solution and the precipitate was then dried in the oven for 1 hour with temperature of 105 C.

Characterization
X-ray diffraction (PANalitycal MPD PW/3040/60) was used to analyse crystal structure and interlayer spacing of the sample.Fourier the functional group exists in the sample was examine by using Transform Infra-Red Spectra (Shimadzu IRPESTIGE 21).The morphology of the samples was observed using SEM (HITACHI SU-3400 M.)

Result and discussion
As synthesized corn cobs waste-based graphene oxide samples were analysed using optical camera to anatomize the appearance of the sample.The samples showed dark black colour in powder phase as presented by Figure1 .Fourier Transform Infrared analysis was performed to examine oxygen functional groups in the samples.Sample that synthesized from 350 C corn cob heating was choosen to be measured and the result was presented in Figure 2. The transmittance peak arose at wave number of 3353 cm -1 corresponds to the hydroxyl (O-H) bonds.Carbonyl (C-O) was found in the sample that signed by peak appeared at 1227 cm -1 .Building of graphene oxide that represented by C=C aromatic bonds was found in wave number of 1603 cm -1 .Peak arose at wavenumber of 1702 cm -1 related to the carboxyl (C=O) boding.These findings were correlated to that of previous report about graphene oxide [16], [17].This suggest that graphene oxide synthesis by using corn cobs waste as starting material was successful.
Further analyse was carried out by using X-Ray Diffraction to study to crystal properties and crystal size of the graphene oxide samples.The XRD spectra of graphene oxide that synthesized from corn cobs with 350 C carbonization (GO 350) was presented in Figure 3 Figure 2 FTIR Spectra of Graphene oxide sample from corn cobs with 350 C heating The major peak was found at angle of 2θ=26.5 degree and the minors are at 20,7 and 42,4 degrees that correspond to the carbon pattern with average size of crystal is 44.9 nm.This patterns also reported by [18].The Interlayer spacing of the samples was varied from 0,1-2 nm that is still smaller than that of a graphene oxide both theoretical and other experiments [19].This result might be caused by standing alone commercial sonication while used in exfoliation process.The combination with ultra-turax and higher power and frequency of sonication were assumed to produce higher interlayer spacing of the graphene oxide.The morphological properties of the graphene oxide samples were analysed by using SEM.The SEM image of graphene oxide samples was presented in Figure 4 and Figure 5. SEM images of graphene oxide with magnification of 1.000 (Figure 4) depicted the rolled structure of graphene oxide layers.The figures was convinced by magnification of 10.000 times that several structure were in sheet form and others were in rolled structure.This result was driven by inadequate power in exfoliation process.Typically, exfoliation in chemical synthesis was performed using ultraturrax-assisted sonication process in favour of more intense exfoliation rate.Sample heated by 300 C carbonization produces the particle average diameter of 30.28 nm.Meanwhile other variation of heating processes result the sample diameter of 32.57nm, 39.79 nm, 35.23 nm for 350 C, 400 C, and 450 C carbonization process respectively.The average diameter of the particles generally is the same for all variations.

Conclusion
The synthesis of graphene oxide from corn cobs wastes had been successful.This success is supported by FTIR, XRD and SEM measurement.FTIR data indicate the presence of oxygen functional groups present in the same sample as those of graphene oxide.The XRD data show the carbon diffraction pattern which is the basal plane of graphene oxide.The SEM data show the morphology of the graphene oxide particles.Further research on the synthesis of graphene oxide from corn cobs is still required, especially to produce products that have better properties.

Figure 1 :
Figure 1 : As synthesized corn cob waste-based graphene oxide sample

Figure 3 XRD
Figure 3 XRD Spectra of Graphene oxide sample from corn cobs with 350 C heating