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Green preparation of reduced graphene oxide using a natural reducing agent

Soon Weng Chong, Chin Wei Lain, Sharifah Bee Abdul Hamid

Nanotechnology & Catalysis Research Centre (NANOCAT), Level 3, IPS Building, University of Malaya (UM), 50603 Kuala Lumpur, Malaysia

Received 19 October 2014; received in revised form 2 December 2014; accepted 1 April 2015

Abstract

A simple and efficient method was introduced for the high-conversion preparation of graphene oxide (GO) from large graphite flakes (averageflake size¼100 μm) using a simplified Hummer's method. Natural reducing agents such as lemon juice and vinegar were compared with hydrazine(N2H4) as potential reducing agents. Graphene was prepared by chemical reduction of GO because this method was low cost and could be used forlarge-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphiteflakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highlyexfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH2.3. This reduced GO exhibited a high electrical conductance of 24.6 μS attributed to its higher C/O ratio (E8:2) compared with other samples.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Natural reducing agent; Chemical reduction; One-pot preparation

1. Introduction

Malaysia weather is currently hotter than in previous yearsbecause of global warming attributed to several types ofpollution induced by fossil fuel consumption. Consuming fossilfuels emits different compounds, such as sulfur dioxide, nitrogenoxides, ground-level ozone, particulate matter, carbon monox-ide, carbon dioxide (CO2), and volatile organic compounds thatinclude benzene, certain heavy metals, and a number of otherpollutants. These side products from fossil fuel combustionbehave as an insulation layer that hinders heat from dissipatingthe surface of the Earth. According to the annual temperatureanomaly simulated by the Providing Regional Climates forImpacts Studies, temperature is predicted to increase in futureyears; Table 1 summarizes the results of the annual temperatureanomaly [1]. Therefore, green and renewable energy should bedeveloped to address this potential problem.

Dye-sensitized solar cells (DSSCs) have emerged to globalattention because of their low fabrication cost, high energy

10.1016/j.ceramint.2015.04.00815 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

nce to: Level 3, Block A, Institute of Postgraduates Studies,laya, 50603 Kuala Lumpur, Malaysia. Tel.: þ603 7967 6960;6556.ss: [email protected] (C.W. Lai).

article as: S.W. Chong, et al., Green preparation of reduced graphe.1016/j.ceramint.2015.04.008

conversion efficiency, and environmental friendliness. Never-theless, the use of graphene in DSSCs could enhance theirperformance in energy conversion because of its excellentoptical and electrical characteristics [2]. Certain researchershave reported a photoelectrical conversion efficiency of 7.02%by using a TiO2 and graphene composite photo-electrode [3,4].Thus, highly pure and highly conductive graphene should besynthesized in bulk for large-scale industrial production.Graphene, which is a versatile two-dimensional (2D) material

with sp2 honeycomb lattice-structured C atoms, has attractedenormous global attention because of its unique characteristics.Several scientists and researchers have shown keen interest in theone C atom-thick graphene, and various solar-based studies havebeen performed because of the high conductivity and transpar-ency of graphene. This material allows light to penetrate, and auniversal 2.3% linear optical adsorption can be achieved bypristine graphene. Novel graphene sheets have significantlyaffected areas in modern chemistry, physics, material science,and engineering. Numerous efforts to obtain highly pure andhighly conductive graphene have been embarked from variousperspectives. Some of the typical methods used to synthesizegraphene include chemical vapor deposition (CVD) [5–7],micromechanical graphite exfoliation [8,9], epitaxial growth onelectrically insulated surface [10,11], and production of colloidal

ne oxide using a natural reducing agent, Ceramics International (2015), http:

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suspensions [12,13]. Decomposition of alcohol on a Cu surface isan example of CVD graphene synthesis [14]. Meanwhile,micromechanical exfoliation [15] is performed by peeling offusing a Scotch tape, and epitaxial growth [16] involves growinggraphene on electrically insulating surfaces, such as SiC.Colloidal suspensions [13] can be produced by dispersing GOin aqueous and various organic solvents. Table 2 shows thecomparison of the aforementioned techniques.

However, industries demand the most economical and simplebut most effective way of large-scale graphene synthesis. Thus,this work synthesized graphene by chemical exfoliation through asimplified Hummer's method [17]. Bulk graphite powder isoxidized using strong oxidizing agents, such as potassiumpermanganate (KMnO4), to introduce oxygenated functionalgroups into the graphite structure. Oxygenated functional groupsare introduced into the graphite structure to weaken the interlayervan der Waals forces. Reduction is then performed. Thesimplified Hummer's method does not use phosphoric acid andsodium nitrate; these compounds release harmful gas [18]. Lemonjuice, vinegar, and N2H4 were used as potential reducing agents,

Table 1Annual temperature anomaly simulated by the Providing Regional Climates forImpacts Studies [1].

Peninsular Malaysia

Year Temperature anomaly (1C)

2000 0.52007 0.62014 0.82021 0.92028 1.02035 1.22042 1.42049 1.92056 1.92063 2.32070 2.62077 3.02084 3.22091 3.42098 3.6

Table 2Comparison of graphene synthesis techniques.

Synthesis technique Processing steps Advantages

CVD Requires precise parametercontrol

Can achieve single to few laof graphene

Micromechanicalexfoliation

Simple tool (scotch tape) Able to obtain single-layergraphene

Epitaxial growth Involves many machineries andtools, complicated procedures

Fairly good quality

Colloidalsuspension

Simple procedures Wide variety of organic solvcan be used

Reduction ofexfoliated grapheneoxide

Simple procedures Can be produced in bulk, simprocedure, transferable, scala

Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphe//dx.doi.org/10.1016/j.ceramint.2015.04.008

and their effects were compared to select the compound thatproduced high-quality graphene. Both GO and reduced GO(rGO) were characterized by Fourier transform infrared spectro-scopy (FTIR), X-ray diffractometry (XRD), Raman spectroscopy,energy dispersive X-ray spectroscopy (EDX), tabletop scanningelectron microscopy (SEM), and field emission-scanning electronmicroscopy (FE-SEM).

2. Methodology

2.1. GO synthesis

GO was synthesized via simplified Hummer's method.Approximately 3 g of graphite (graphite flakes, Sigma-Aldrich)was mixed with 70 mL of 0.5 M H2SO4 (Chemolab) in an icebath. Exactly 9 g of KMnO4 (Chemolab) was slowly added intothe mixture, which was then stirred at a constant speed. Thetemperature of the suspension was maintained below 20 1C toavoid possible explosion. The temperature was then raised to35 1C and stirred for 30 min after KMnO4 was completely addedin the mixture. Approximately 150 mL of deionized (DI) waterwas then added, and the temperature was raised to 95 1C.Approximately 500 mL of water and 15 mL of 30% hydrogenperoxide (Chemolab) were added to the suspension to terminatethe reaction. The suspension was then washed with 10 mL of 1 Mhydrochloric acid (Chemolab) and centrifuged at 7000 rpm for15 min. The supernatant was decanted; the sediment was washedwith DI water and centrifuged again. Washing was repeatedtwice; it was performed to remove the metal ions [18]. Reductionwas then conducted. Lemon juice and vinegar were usedas reducing agents to compare the effects of these envir-onment-friendly reducing agents with N2H4. Fig. 1 shows theexperimental setup.

2.2. Characterization

The surface morphologies of GO and rGO were observedusing an FEI Quanta 200F Environmental SEM at 5.0 kV with10 mm working distance and a TM3030 tabletop SEM at

Disadvantages Year Refs.

yers Low productivity, difficult to achieve single-layergraphene, costly machine, lack of homogeneity on largeareas

2010,2013

[2–4]

Hard to transfer and scale 2011,2013

[5,6]

Not transferable 2012 [7,8]

ents High impurity 2009,2010

[9,10]

pleble

Quality not as good as CVD-produced graphene 2013 [14,15]






Thermometer

Heat Stirring

Ice Bath

Graphite + H2SO4 + KMnO4

Magnetic Hot Plate Stirrer

Retort Stand

Fig. 1. Experimental setup of simplified Hummer's method.

Power Supply

Hotplate Stirrer

FTO glass

Distilled water + Acetone

Fig. 2. Experimental setup of electrolysis deposition method.

AutoLab PGSTAT 204

Counter Electrode (Ti Probe)

Reference Electrode (Ag/AgCl)

FTO glass coated with rGO Electrolyte

Fig. 3. Electrical characterization setup using the AutoLab PGSTAT204system.

S.W. Chong et al. / Ceramics International ] (]]]]) ]]]–]]] 3

15.0 kV. The samples were prepared by adhering GO and rGOpowder onto carbon conductive tape. The morphologies of GOand rGO were further confirmed using a JEM 2100-F highresolution-transmission electron microscopy (HR-TEM) sys-tem at 200 kV accelerating voltage. Approximately 0.01 g ofGO and rGO samples were dispersed in ethanol, and a dropletof each sample was loaded on a copper grid.

The changes in functional groups were then determinedusing FTIR (Bruker-IFS 66/S). Approximately 3 g of GO andrGO powder were pressed into pellet form for the FTIRanalysis. Elemental analysis of GO and rGO were performedusing an EDX equipped with FE-SEM. Phase determination ofGO and rGO was carried out using a D8 Advance X-raydiffractometer-Bruker AXS at a scanning rate of 0.0331 s�1

and 2θ from 21 to 901 with CuKα radiation (λ¼1.5418 Å).The vibrational and rotational modes and crystallinity of thesamples were investigated using a Raman spectroscopy(Renishaw inVia Microscope, HeCd laser) system.

Meanwhile, fluorine-doped tin oxide-coated glass slide(FTO; Sigma-Aldrich) was coated with rGO samples byelectrolysis. The FTO glasses were immersed in a mixture of100 mL of DI water, 1 mL of acetonitrile, and 0.01 g of rGOsample, as shown in Fig. 2. This process was performed at


60 V for 5 min. The FTO glasses were then coated with rGO,and the resulting samples were connected to an AutoLabPGSTAT204 instrument to determine their electrical conduc-tance, as shown in Fig. 3. Linear sweep voltammetry wasperformed at a voltage sweep from �0.5 V to 0.5 V. For thesolar cell characterization, solar cells were prepared bycompositing TiO2 with the graphene samples and thendepositing on FTO glasses. The counter electrodes wereprepared using 2B pencil on the FTO glass. The preparedsolar cells were then connected to the AutoLab PGSTAT204instrument to determine their IV characteristics. The efficiency,fill factor, short-circuit current, and open-circuit potential werecalculated based on the IV characteristics, and the results weretabulated in Table 6.

3. Results and discussions

A color change in the graphite mixture before and afteroxidation for 24 h was observed. The color of the mixturebefore oxidation was dark green, and the mixture became darkbrown after oxidation. The dark green color of the graphitemixture before oxidation was caused by Mn2O7. Mn2O7

appeared as dark red oil at room temperature; however, itscolor changed to dark green when it came in contact withH2SO4. Initially, the reaction produced permanganic acid(HMnO4 or HOMnO3), which was then dehydrated byH2SO4 to form its anhydride, Mn2O7 as shown in Eq. (1).

2KMnO4þ2H2SO4-Mn2O7þH2Oþ2KHSO4 (1)

SEM is used to produce images of samples by scanning with afocused electron beam. The sample images are produced bydetecting the secondary electrons emitted by the atoms excitedby the electron beam. The graphite powder and rGO imageswere captured at 10k magnification. Fig. 4 shows the compar-ison of the optical inspection performed between graphitepowder and rGO. Fig. 4(b) shows the image of the rGO bylemon, whereas Fig. 4(c) displays the image of the rGO byvinegar. Fig. 4(d) illustrates the image of the rGO by N2H4,whereas Fig. 4(a) shows the image of graphite powder.Fig. 4(a) demonstrates that the thickness of the graphite layer

was 147 nm. This obtained value was due to multilayer graphene,which was bound by the van der Waals forces of graphite.The sheet thicknesses of the rGO by lemon, vinegar, and N2H4

were 26.4, 29.3, and 26.1 nm, respectively. Therefore, GO was






29.3nm

26.4nm

147nm

26.1nm

10µ10µm

10µ10µm10µ10µm

10µ10µm

Fig. 4. SEM images at 10k magnification of (a) graphite powder, (b) rGO by lemon, (c) rGO by vinegar, and (d) rGO by hydrazine.


significantly exfoliated in the proposed approach. More impor-tantly, the SEM images and measurements of the sheet thicknessconfirmed that lemon juice effectively reduced GO and can beused as a substitute for N2H4.

Similar with the working principle of SEM, FE-SEMemploys an intensive and monochromatic electronic beam,which produces good resolution. Fig. 5 shows the FE-SEMimages of the graphite powder and the rGO by lemon, vinegar,and N2H4. Both N2H4 and lemon juice produced highlyexfoliated graphene sheets compared with graphite, whichappeared as a group of thick sheets stacked together. Theindividual ultra-thin sheet image shown in Fig. 5(b) furtherconfirmed that lemon juice effectively exfoliated GO. How-ever, the rGO sample reduced by vinegar accumulated chargesand caused the sheets to agglomerate.

Fig. 6 shows the HR-TEM images of the rGO by N2H4 at(a) 2k and (b) 35k magnifications. The graphene sheets wereextremely thin; thus, the beam passed through the sample. Theobtained graphene sheets were not agglomerated after reduction.

An infrared spectrum represents the fingerprint of a sample, inwhich absorption peaks correspond to the frequencies of vibra-tions between the bonds of the atoms of a material. Each materialexhibits its own unique combination of atoms; thus, no twocompounds produce the same infrared spectrum. Infrared spectro-scopy can identify (i.e., qualitative analysis) all kinds of materials.This paper discusses the effect of oxidation time up to 72 h on


GO formation. The aim was to determine the optimum oxidationtime to produce high-yield GO sheets. The sample that producedthe highest yield (Table 3) was used in the second part of thestudy, wherein the best reducing agent was determined. Thisreducing agent should form thin monolayer of rGO by eliminat-ing a hydroxyl group from the C basal plane.The size of the peaks in the FTIR spectrum indicates the amount

of material present in the sample. Fig. 7(a and b) shows the FTIRspectra of GO and rGO. The graphs show intense peaks at 3450(hydroxyl group), 2350 (carbon dioxide), 1620 (alkene groupCQC), and 1060 cm�1 (alcohol group C–O) [19]. Fig. 7(i)shows high-intensity peaks (e.g., hydroxyl, carbon dioxide, andalcohol groups), which indicate that large amounts of O functionalgroups were introduced into the graphite powder after oxidation.The highest intensity of hydroxyl group was obtained afteroxidizing GO for 72 h. This result was mainly attributed to severalO molecules that were able to diffuse into the graphite flakesduring the reaction with KMnO4. The oxidation time of 72 h wasthe maximum oxidation time used in the experiment because ofevaporation. The peak at 1620 cm�1 corresponded to the adsorp-tion of water molecules.The simplified Hummer's method used a combination of

KMnO4 and H2SO4. An active species of dimanganese heptoxide(Mn2O7) was produced when KMnO4 and H2SO4 were mixed.Eqs. (2) and (3) show the formation of Mn2O7 from KMnO4 inthe presence of a strong acid [20].






2µ2µm2µ2µm

2µ2µm2µ2µm

Fig. 5. FE-SEM images of (a) graphite powder, (b) rGO by lemon, (c) rGO by vinegar, and (d) rGO by hydrazine.

Fig. 6. HRTEM images of rGO by hydrazine at (a) 2k and (b) 35k magnifications.


KMnO4þ3H2SO4-KþþMnOþ3 þH3O

þþ3HSO�4 (2)

MnOþ3 þMnO�

4 -Mn2O7 (3)


Tromel and Russ demonstrated the ability of Mn2O7 toselectively oxidize unsaturated aliphatic double bonds overaromatic double bonds; this process may be related to the







reaction that occurs during oxidation [21]. Graphite flakeswere the source of C in the experiment; these flakes containnumerous localized defects in their π structure, and thesedefects can bind with oxygenated groups during oxidation.However, the complexity of graphite flakes hindered theelucidation of the exact oxidation mechanism.

The GO that was oxidized for 24 h was selected for thereduction because it yielded the highest mass (3.1926 g)among the samples. Fig. 7(ii) shows that reduction occurredwith various reducing agents, namely, lemon juice, vinegar,and N2H4. N2H4 is a commonly used reducing agent; however,this compound is highly toxic and potentially explosive [22].Therefore, lemon juice and vinegar were chosen because bothcompounds are natural anti-oxidizing agents. Fig. 7(ii) showsthat the hydroxyl group, CO2, and CQC groups weresignificantly reduced in the GO reduced by N2H4. Lemonjuice also reduced GO; however, N2H4 achieved the bestreduction among the samples tested.

After that, XRD was used to determine the crystal structure,orientation, and interlayer distance between GO and rGO.Fig. 8(i) shows the XRD patterns of graphite and GO afterboth compounds were oxidized, and Fig. 8(ii) demonstratesGO after it was reduced. Fig. 8(i) shows that pristine graphiteexhibited a sharp and high-intensity diffraction peak at2θ¼26.71. This result shows that a highly organized layerstructure with an interlayer distance (d spacing) of 0.34 nmalong the (002) orientation was produced [8]. The (002) peak

Table 3Product weight after oxidation.

Oxidation duration (h) Product weight (g)

1 2.641212 2.966724 3.192648 2.509072 2.9114

Fig. 7. (i) FTIR spectra of (a) graphite, (b) GO after 1 h of oxidation, (c) GO afteoxidation; (ii) FTIR spectra of (a) GO after 24 h of oxidation, (b) rGO by lemon,


shifted to 2θ¼10.91 after oxidation; thus, the d-spacingincreased from 0.34 nm to 0.81 nm. This increase in d-spacing was caused by the intercalation of O functional groupsand H2O molecules into the graphite interlayers [9,10].Meanwhile, Fig. 8(ii) shows that the peak of reduced GO at

2θ¼10.91 significantly disappeared after the compound wastreated with lemon juice (b), vinegar (c), and N2H4 (d). Thedecrease in peak intensity clearly indicates that the O-containinggroups of GO were efficiently removed [11]. Fig. 8(ii) also showsthat lemon juice exhibited a reduction effect comparable withN2H4. Gao et al. suggested particular possible routes to removean O group from GO [23]; they proposed three routes ofreduction mechanism. In Route 1, epoxides from GO are attackedby N2H4 from the back side of the epoxide ring. In Route 2,N2H4 attacks the sp

2 C nearest the epoxide group from the frontside of the epoxide ring. In Route 3, N2H4 first attacks the sp

2 Clocated at the meta position of epoxide.Raman spectroscopy uses a monochromatic laser to interact

with the molecular vibrational modes and phonons in a sample;thus, laser energy is shifted up or down through inelasticscattering [24]. The phonon energy shift of the down laserenergy caused by laser excitation produced two main peaksfrom the rGO samples. These two peaks were the primary in-plane vibrational mode at G (1600 cm�1) and 2D (2700 cm�1),as well as the second-order overtone of a different in-planevibration at D (1380 cm�1) [25]. The 514 nm-excitation laserwas occupied. The spectrum changed from a single-layergraphene (2D peak) into increasing number of modes as thenumber of graphene layers increased; these modes can combineto produce a wide, short, and high-frequency peak, and thisphenomenon occurred because of an added force caused byinteractions between layers of stacked graphene [26]. Fig. 9shows that rGO exhibited low-intensity peaks compared with aCVD-synthesized graphene sheet, which showed a high-intensity peak at 2700 cm�1. However, the synthesized rGOsheets were highly exfoliated. These low-intensity peaks mightbe caused by the stacking of graphene platelets, one on top of

r 12 h of oxidation, (d) GO after 24 h of oxidation, and (e) GO after 72 h of(c) rGO by vinegar, and (d) rGO by hydrazine.






Fig. 8. (i) XRD spectra of (a) GO after 72 h of oxidation, (b) GO after 24 h of oxidation, (c) GO after 12 h of oxidation, (d) GO after 1 h of oxidation, and(e) graphite; (ii) XRD spectra of (a) GO after 24 h of oxidation, (b) rGO by lemon, (c) rGO by vinegar, and (d) rGO by hydrazine.

Fig. 9. Raman spectra of (a) GO after 24 h of oxidation, (b) rGO by lemon, (c)rGO by vinegar, and (d) rGO by hydrazine.

Table 4EDX analysis of the elements present in the rGO by hydrazine, lemon juice,and vinegar.

Materials Carbon(%)

Oxygen(%)

Sulfur(%)

Chlorine(%)

Sodium(%)

Potassium(%)

Total(%)

rGO byhydrazine

70.9 28.76 0.17 0.10 – – 100

rGO bylemonjuice

79.4 19.83 0.09 0.26 0.42 – 100

rGO byvinegar

68.27 31.48 – 0.12 – 0.13 100


the other, during sample preparation. The G peak also exhibitedan extremely small red shift because of the increased number oflayers. The G-vibration mode, which was found at 1600 cm�1,showed first-order scattering of E2g phonons by sp2 C atoms,and the D-vibration mode of the κ-point photons of A1g

symmetry were observed at 1380 cm�1.Fig. 9 shows that the D band was lower than the G band

because the graphene sample was ground into powder; thus,platelets, instead of a thin film layer, formed. First-order D peak isnot visible in pristine graphene because of crystal symmetries[27]. A charge carrier should be excited and inelastically scatteredby a phonon, and a second elastic scattering by defect or zoneboundary must occur to produce recombination and cause theappearance of a D peak [28]. A Lorentzian peak for the 2D bandof the monolayer graphene platelets was observed at 2700 cm�1.This peak did not shift after reduction occurred, indicating thatthe graphene platelets did not exhibit restacking [19].


Table 4 shows the EDX analysis of the elements present inthe rGO by N2O4, vinegar, and lemon. The rGO by N2H4

contained 70.9% C and 28.76% O, the rGO by lemon juicecontained 79.4% C and 19.83% O, and the rGO by vinegarcontained 68.27% C and 31.48% O.The rGO by lemon juice was placed as a layer of thin film

on an FTO glass to investigate its conductance and furtherconfirm that lemon juice produced a competitive quality ofgraphene sheets. The surface area of the FTO glass was 2 cm2.The experiment was performed using an AutoLab system at avoltage sweep from �0.5 V to 0.5 V. Fig. 10 illustrates thecalculated conductance of the rGO by N2H4, lemon juice, andvinegar, and the results are tabulated in Table 5. Thecalculations were based on Eqs. (4) and (5):

Resistance; R ¼ V

Ið4Þ

Conductance; G ¼ 1R¼ I

Vð5Þ

C atoms contain six electrons, where two electrons are presentin the inner shell and four electrons are found in the outer shell.The four outer-shell electrons are available for chemical bonding.However, each atom in graphene is connected to three other Catoms on the 2D plane; thus, only one free electron is availablefor electronic conduction [29]. These high-mobility free electrons






Fig. 10. Current––voltage curves generated by AutoLab system for (a) rGO byvinegar, (b) rGO by hydrazine, and (c) rGO by lemon juice.

Table 5Resistance and conductance values of the rGO calculated at voltage V=0.5 V.

Materials Voltage(V)

Current(μA)

Resistance(Ω)

Conductance(μΩ�1)

rGO byhydrazine

0.5 2.5 2,00,000 5

rGO by vinegar 0.5 1.0 5,00,000 2rGO by lemon 0.5 12.3 40,650.4 24.6

Fig. 11. (a) Current–voltage characteristics of PV cell without light illumina-tion and of PV cells constructed with rGO by (b) hydrazine, (c) lemon juice,and (d) vinegar under light illumination.

Table 6VOC, ISC, FF, and η calculated from Fig. 11.

PV cellfabricated byusing

Open-circuitvoltage, VOC (V)

Short-circuitcurrent, ISC (A)

Fillfactor,FF

Efficiency,η(%)

rGO byhydrazine

0.28 0.000003 0.119 6.66� 10�8

rGO by lemon 0.28 0.0000034 0.105 6.66� 10�8

rGO by vinegar 0 0 0 0


located above and below the graphene sheets, which are called pi(π) electrons, are responsible for the calculated value of con-ductance. Table 5 shows that the conductance of the rGO bylemon juice was significantly higher than those of the rGO byN2H4 and vinegar. This phenomenon may be attributed to thelower O content in the rGO by lemon juice than in the othersamples. Thus, more electrons were able to travel within the rGOby lemon juice, and a high current output was observed.

To further confirm our results showing that lemon juice canproduce a competitive quality of graphene sheets, the rGO was


used to construct photovoltaic (PV) cells to study its efficiencytoward light conversion into electricity. The dimension of thePV cells was fixed at 1 cm2, and the light source used had apower of 150 W. The experiment was performed using aforward-bias connection. The fill factor (FF) and powerconversion efficiency (η) were calculated from Fig. 11 usingEqs. (6) and (7), respectively, and the results were tabulated inTable 6.

FF ¼ IMAX � VMAX

ISC � VOCð6Þ

η¼ IMAX � VMAX

PLIGHT¼ FF � ISC � VOC

PLIGHTð7Þ

The efficiency of the PV cells constructed using rGO was lowbecause of the absence of electrolyte and dye to aid theelectron transfer in the PV cell. However, the main purpose ofthis part of the study was to confirm that the effectiveness ofthe rGO by lemon juice was as good as that of the rGO byhydrazine. By comparing the calculated efficiency, the effec-tiveness of the rGO by lemon juice was found to be same asthat of the rGO by hydrazine. Therefore, lemon juice is apotential candidate as substitute for hydrazine in the reductionprocess.

4. Conclusion

This study demonstrated that lemon juice (pH 2.3) behavedas an efficient reducing agent to reduce GO to rGO with a highC/O ratio of E8:2. The resultant graphene film showed higherconductance (24.6 μS) compared with the other samples. Thisresult was mainly attributed to the low O content in rGO afterit was reduced by lemon juice. From the comparison of theefficiency of the solar cells, the solar cell prepared using therGO by lemon juice and hydrazine shared the same efficiency.Thus, lemon juice, which is a natural reducing agent, was aseffective as N2H4 in reducing GO. This environment-friendlygraphene synthesis method used one-pot preparation process;thus, it was low cost. This method can be potentially used toproduce bulk quantity of GO.

Acknowledgments

The authors thank the University of Malaya for funding thisresearch work under the High Impact Research ChancelloryGrant UM.C/625/1/HIR/228 (J55001-73873).







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