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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 (average
flake 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 for
large-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphite
flakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highly
exfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH
2.3. This reduced GO exhibited a high electrical conductance of 24.6 μS attributed to its higher C/O ratio (E 8:2) 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 years
because of global warming attributed to several types of
pollution induced by fossil fuel consumption. Consuming fossil
fuels emits different compounds, such as sulfur dioxide, nitrogen
oxides, ground-level ozone, particulate matter, carbon monoxide, carbon dioxide (CO2), and volatile organic compounds that
include benzene, certain heavy metals, and a number of other
pollutants. These side products from fossil fuel combustion
behave as an insulation layer that hinders heat from dissipating
the surface of the Earth. According to the annual temperature
anomaly simulated by the Providing Regional Climates for
Impacts Studies, temperature is predicted to increase in future
years; Table 1 summarizes the results of the annual temperature
anomaly [1]. Therefore, green and renewable energy should be
developed to address this potential problem.
Dye-sensitized solar cells (DSSCs) have emerged to global
attention because of their low fabrication cost, high energy
n
Correspondence to: Level 3, Block A, Institute of Postgraduates Studies,
University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: þ603 7967 6960;
fax: þ603 7967 6556.
E-mail address: [email protected] (C.W. Lai).
conversion efficiency, and environmental friendliness. Nevertheless, the use of graphene in DSSCs could enhance their
performance in energy conversion because of its excellent
optical and electrical characteristics [2]. Certain researchers
have 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 be
synthesized in bulk for large-scale industrial production.
Graphene, which is a versatile two-dimensional (2D) material
with sp2 honeycomb lattice-structured C atoms, has attracted
enormous global attention because of its unique characteristics.
Several scientists and researchers have shown keen interest in the
one C atom-thick graphene, and various solar-based studies have
been performed because of the high conductivity and transparency of graphene. This material allows light to penetrate, and a
universal 2.3% linear optical adsorption can be achieved by
pristine graphene. Novel graphene sheets have significantly
affected areas in modern chemistry, physics, material science,
and engineering. Numerous efforts to obtain highly pure and
highly conductive graphene have been embarked from various
perspectives. Some of the typical methods used to synthesize
graphene include chemical vapor deposition (CVD) [5–7],
micromechanical graphite exfoliation [8,9], epitaxial growth on
electrically insulated surface [10,11], and production of colloidal
http://dx.doi.org/10.1016/j.ceramint.2015.04.008
0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphene oxide using a natural reducing agent, Ceramics International (2015), http:
//dx.doi.org/10.1016/j.ceramint.2015.04.008
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2
suspensions [12,13]. Decomposition of alcohol on a Cu surface is
an example of CVD graphene synthesis [14]. Meanwhile,
micromechanical exfoliation [15] is performed by peeling off
using a Scotch tape, and epitaxial growth [16] involves growing
graphene on electrically insulating surfaces, such as SiC.
Colloidal suspensions [13] can be produced by dispersing GO
in aqueous and various organic solvents. Table 2 shows the
comparison of the aforementioned techniques.
However, industries demand the most economical and simple
but most effective way of large-scale graphene synthesis. Thus,
this work synthesized graphene by chemical exfoliation through a
simplified Hummer's method [17]. Bulk graphite powder is
oxidized using strong oxidizing agents, such as potassium
permanganate (KMnO4), to introduce oxygenated functional
groups into the graphite structure. Oxygenated functional groups
are introduced into the graphite structure to weaken the interlayer
van der Waals forces. Reduction is then performed. The
simplified Hummer's method does not use phosphoric acid and
sodium nitrate; these compounds release harmful gas [18]. Lemon
juice, vinegar, and N2H4 were used as potential reducing agents,
Table 1
Annual temperature anomaly simulated by the Providing Regional Climates for
Impacts Studies [1].
Peninsular Malaysia
Year
Temperature anomaly (1C)
2000
2007
2014
2021
2028
2035
2042
2049
2056
2063
2070
2077
2084
2091
2098
0.5
0.6
0.8
0.9
1.0
1.2
1.4
1.9
1.9
2.3
2.6
3.0
3.2
3.4
3.6
and their effects were compared to select the compound that
produced high-quality graphene. Both GO and reduced GO
(rGO) were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), Raman spectroscopy,
energy dispersive X-ray spectroscopy (EDX), tabletop scanning
electron microscopy (SEM), and field emission-scanning electron
microscopy (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 ice
bath. Exactly 9 g of KMnO4 (Chemolab) was slowly added into
the mixture, which was then stirred at a constant speed. The
temperature of the suspension was maintained below 20 1C to
avoid possible explosion. The temperature was then raised to
35 1C and stirred for 30 min after KMnO4 was completely added
in the mixture. Approximately 150 mL of deionized (DI) water
was then added, and the temperature was raised to 95 1C.
Approximately 500 mL of water and 15 mL of 30% hydrogen
peroxide (Chemolab) were added to the suspension to terminate
the reaction. The suspension was then washed with 10 mL of 1 M
hydrochloric acid (Chemolab) and centrifuged at 7000 rpm for
15 min. The supernatant was decanted; the sediment was washed
with DI water and centrifuged again. Washing was repeated
twice; it was performed to remove the metal ions [18]. Reduction
was then conducted. Lemon juice and vinegar were used
as reducing agents to compare the effects of these environment-friendly reducing agents with N2H4. Fig. 1 shows the
experimental setup.
2.2. Characterization
The surface morphologies of GO and rGO were observed
using an FEI Quanta 200F Environmental SEM at 5.0 kV with
10 mm working distance and a TM3030 tabletop SEM at
Table 2
Comparison of graphene synthesis techniques.
Synthesis technique Processing steps
CVD
Micromechanical
exfoliation
Epitaxial growth
Advantages
Disadvantages
Year
Refs.
Requires precise parameter
control
Can achieve single to few layers Low productivity, difficult to achieve single-layer
2010, [2–4]
of graphene
graphene, costly machine, lack of homogeneity on large 2013
areas
Simple tool (scotch tape)
Able to obtain single-layer
Hard to transfer and scale
2011, [5,6]
graphene
2013
Involves many machineries and Fairly good quality
Not transferable
2012 [7,8]
tools, complicated procedures
Colloidal
Simple procedures
suspension
Reduction of
Simple procedures
exfoliated graphene
oxide
Wide variety of organic solvents High impurity
can be used
Can be produced in bulk, simple Quality not as good as CVD-produced graphene
procedure, transferable, scalable
2009, [9,10]
2010
2013 [14,15]
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Thermometer
Retort Stand
AutoLab
PGSTAT 204
Ice Bath
Graphite + H2SO4 + KMnO4
Heat
Stirring
Magnetic Hot Plate
Stirrer
Fig. 1. Experimental setup of simplified Hummer's method.
FTO glass
Power
Supply
Distilled
water +
Acetone
Reference
Electrode
(Ag/AgCl)
Counter
Electrode (Ti
Probe)
FTO glass
coated with
rGO
Electrolyte
Fig. 3. Electrical characterization setup using the AutoLab PGSTAT204
system.
60 V for 5 min. The FTO glasses were then coated with rGO,
and the resulting samples were connected to an AutoLab
PGSTAT204 instrument to determine their electrical conductance, as shown in Fig. 3. Linear sweep voltammetry was
performed at a voltage sweep from 0.5 V to 0.5 V. For the
solar cell characterization, solar cells were prepared by
compositing TiO2 with the graphene samples and then
depositing on FTO glasses. The counter electrodes were
prepared using 2B pencil on the FTO glass. The prepared
solar cells were then connected to the AutoLab PGSTAT204
instrument to determine their IV characteristics. The efficiency,
fill factor, short-circuit current, and open-circuit potential were
calculated based on the IV characteristics, and the results were
tabulated in Table 6.
3. Results and discussions
Hotplate
Stirrer
Fig. 2. Experimental setup of electrolysis deposition method.
15.0 kV. The samples were prepared by adhering GO and rGO
powder onto carbon conductive tape. The morphologies of GO
and rGO were further confirmed using a JEM 2100-F high
resolution-transmission electron microscopy (HR-TEM) system at 200 kV accelerating voltage. Approximately 0.01 g of
GO and rGO samples were dispersed in ethanol, and a droplet
of each sample was loaded on a copper grid.
The changes in functional groups were then determined
using FTIR (Bruker-IFS 66/S). Approximately 3 g of GO and
rGO powder were pressed into pellet form for the FTIR
analysis. Elemental analysis of GO and rGO were performed
using an EDX equipped with FE-SEM. Phase determination of
GO and rGO was carried out using a D8 Advance X-ray
diffractometer-Bruker AXS at a scanning rate of 0.0331 s 1
and 2θ from 21 to 901 with CuKα radiation (λ ¼ 1.5418 Å).
The vibrational and rotational modes and crystallinity of the
samples 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 by
electrolysis. The FTO glasses were immersed in a mixture of
100 mL of DI water, 1 mL of acetonitrile, and 0.01 g of rGO
sample, as shown in Fig. 2. This process was performed at
A color change in the graphite mixture before and after
oxidation for 24 h was observed. The color of the mixture
before oxidation was dark green, and the mixture became dark
brown after oxidation. The dark green color of the graphite
mixture before oxidation was caused by Mn2O7. Mn2O7
appeared as dark red oil at room temperature; however, its
color changed to dark green when it came in contact with
H2SO4. Initially, the reaction produced permanganic acid
(HMnO4 or HOMnO3), which was then dehydrated by
H2SO4 to form its anhydride, Mn2O7 as shown in Eq. (1).
2KMnO4 þ 2H2SO4-Mn2O7 þ H2O þ 2KHSO4
(1)
SEM is used to produce images of samples by scanning with a
focused electron beam. The sample images are produced by
detecting the secondary electrons emitted by the atoms excited
by the electron beam. The graphite powder and rGO images
were captured at 10k magnification. Fig. 4 shows the comparison of the optical inspection performed between graphite
powder and rGO. Fig. 4(b) shows the image of the rGO by
lemon, whereas Fig. 4(c) displays the image of the rGO by
vinegar. 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
Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphene oxide using a natural reducing agent, Ceramics International (2015), http:
//dx.doi.org/10.1016/j.ceramint.2015.04.008
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147nm
26.4nm
10µm
10µ
10µ
10µm
29.3nm
26.1nm
10µm
10µ
10µm
10µ
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 importantly, the SEM images and measurements of the sheet thickness
confirmed that lemon juice effectively reduced GO and can be
used as a substitute for N2H4.
Similar with the working principle of SEM, FE-SEM
employs an intensive and monochromatic electronic beam,
which produces good resolution. Fig. 5 shows the FE-SEM
images of the graphite powder and the rGO by lemon, vinegar,
and N2H4. Both N2H4 and lemon juice produced highly
exfoliated graphene sheets compared with graphite, which
appeared as a group of thick sheets stacked together. The
individual ultra-thin sheet image shown in Fig. 5(b) further
confirmed that lemon juice effectively exfoliated GO. However, the rGO sample reduced by vinegar accumulated charges
and 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 were
extremely thin; thus, the beam passed through the sample. The
obtained graphene sheets were not agglomerated after reduction.
An infrared spectrum represents the fingerprint of a sample, in
which absorption peaks correspond to the frequencies of vibrations between the bonds of the atoms of a material. Each material
exhibits its own unique combination of atoms; thus, no two
compounds produce the same infrared spectrum. Infrared spectroscopy 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 oxidation
time to produce high-yield GO sheets. The sample that produced
the highest yield (Table 3) was used in the second part of the
study, wherein the best reducing agent was determined. This
reducing agent should form thin monolayer of rGO by eliminating 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 FTIR
spectra of GO and rGO. The graphs show intense peaks at 3450
(hydroxyl group), 2350 (carbon dioxide), 1620 (alkene group
CQC), and 1060 cm 1 (alcohol group C–O) [19]. Fig. 7(i)
shows high-intensity peaks (e.g., hydroxyl, carbon dioxide, and
alcohol groups), which indicate that large amounts of O functional
groups were introduced into the graphite powder after oxidation.
The highest intensity of hydroxyl group was obtained after
oxidizing GO for 72 h. This result was mainly attributed to several
O molecules that were able to diffuse into the graphite flakes
during the reaction with KMnO4. The oxidation time of 72 h was
the maximum oxidation time used in the experiment because of
evaporation. The peak at 1620 cm 1 corresponded to the adsorption 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 in
the presence of a strong acid [20].
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5
2µm
2µ
2µm
2µ
2µm
2µ
2µm
2µ
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.
KMnO4 þ 3H2SO4-K þ þ MnO3þ þ H3O þ þ 3HSO4
(2)
MnO3þ þ MnO4 -Mn2O7
(3)
Tromel and Russ demonstrated the ability of Mn2O7 to
selectively oxidize unsaturated aliphatic double bonds over
aromatic double bonds; this process may be related to the
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reaction that occurs during oxidation [21]. Graphite flakes
were the source of C in the experiment; these flakes contain
numerous localized defects in their π structure, and these
defects can bind with oxygenated groups during oxidation.
However, the complexity of graphite flakes hindered the
elucidation of the exact oxidation mechanism.
The GO that was oxidized for 24 h was selected for the
reduction because it yielded the highest mass (3.1926 g)
among the samples. Fig. 7(ii) shows that reduction occurred
with 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 both
compounds are natural anti-oxidizing agents. Fig. 7(ii) shows
that the hydroxyl group, CO2, and CQC groups were
significantly reduced in the GO reduced by N2H4. Lemon
juice also reduced GO; however, N2H4 achieved the best
reduction 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 after
both compounds were oxidized, and Fig. 8(ii) demonstrates
GO after it was reduced. Fig. 8(i) shows that pristine graphite
exhibited a sharp and high-intensity diffraction peak at
2θ ¼ 26.71. This result shows that a highly organized layer
structure with an interlayer distance (d spacing) of 0.34 nm
along the (002) orientation was produced [8]. The (002) peak
Table 3
Product weight after oxidation.
Oxidation duration (h)
Product weight (g)
1
12
24
48
72
2.6412
2.9667
3.1926
2.5090
2.9114
shifted to 2θ ¼ 10.91 after oxidation; thus, the d-spacing
increased from 0.34 nm to 0.81 nm. This increase in dspacing was caused by the intercalation of O functional groups
and H2O molecules into the graphite interlayers [9,10].
Meanwhile, Fig. 8(ii) shows that the peak of reduced GO at
2θ¼ 10.91 significantly disappeared after the compound was
treated with lemon juice (b), vinegar (c), and N2H4 (d). The
decrease in peak intensity clearly indicates that the O-containing
groups of GO were efficiently removed [11]. Fig. 8(ii) also shows
that lemon juice exhibited a reduction effect comparable with
N2H4. Gao et al. suggested particular possible routes to remove
an O group from GO [23]; they proposed three routes of
reduction mechanism. In Route 1, epoxides from GO are attacked
by N2H4 from the back side of the epoxide ring. In Route 2,
N2H4 attacks the sp2 C nearest the epoxide group from the front
side of the epoxide ring. In Route 3, N2H4 first attacks the sp2 C
located 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 inelastic
scattering [24]. The phonon energy shift of the down laser
energy caused by laser excitation produced two main peaks
from the rGO samples. These two peaks were the primary inplane vibrational mode at G (1600 cm 1) and 2D (2700 cm 1),
as well as the second-order overtone of a different in-plane
vibration at D (1380 cm 1) [25]. The 514 nm-excitation laser
was occupied. The spectrum changed from a single-layer
graphene (2D peak) into increasing number of modes as the
number of graphene layers increased; these modes can combine
to produce a wide, short, and high-frequency peak, and this
phenomenon occurred because of an added force caused by
interactions between layers of stacked graphene [26]. Fig. 9
shows that rGO exhibited low-intensity peaks compared with a
CVD-synthesized graphene sheet, which showed a highintensity peak at 2700 cm 1. However, the synthesized rGO
sheets were highly exfoliated. These low-intensity peaks might
be caused by the stacking of graphene platelets, one on top of
Fig. 7. (i) FTIR spectra of (a) graphite, (b) GO after 1 h of oxidation, (c) GO after 12 h of oxidation, (d) GO after 24 h of oxidation, and (e) GO after 72 h of
oxidation; (ii) FTIR spectra of (a) GO after 24 h of oxidation, (b) rGO by lemon, (c) rGO by vinegar, and (d) rGO by hydrazine.
Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphene oxide using a natural reducing agent, Ceramics International (2015), http:
//dx.doi.org/10.1016/j.ceramint.2015.04.008
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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.
Table 4
EDX analysis of the elements present in the rGO by hydrazine, lemon juice,
and vinegar.
Materials Carbon Oxygen Sulfur Chlorine Sodium Potassium Total
(%)
(%)
(%)
(%)
(%)
(%)
(%)
rGO by 70.9
hydrazine
rGO by 79.4
lemon
juice
rGO by 68.27
vinegar
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.
the other, during sample preparation. The G peak also exhibited
an extremely small red shift because of the increased number of
layers. 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 is
not visible in pristine graphene because of crystal symmetries
[27]. A charge carrier should be excited and inelastically scattered
by a phonon, and a second elastic scattering by defect or zone
boundary must occur to produce recombination and cause the
appearance of a D peak [28]. A Lorentzian peak for the 2D band
of the monolayer graphene platelets was observed at 2700 cm 1.
This peak did not shift after reduction occurred, indicating that
the graphene platelets did not exhibit restacking [19].
28.76
0.17
0.10
–
–
100
19.83
0.09
0.26
0.42
–
100
31.48
–
0.12
–
0.13
100
Table 4 shows the EDX analysis of the elements present in
the rGO by N2O4, vinegar, and lemon. The rGO by N2H4
contained 70.9% C and 28.76% O, the rGO by lemon juice
contained 79.4% C and 19.83% O, and the rGO by vinegar
contained 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 further
confirm that lemon juice produced a competitive quality of
graphene sheets. The surface area of the FTO glass was 2 cm2.
The experiment was performed using an AutoLab system at a
voltage sweep from 0.5 V to 0.5 V. Fig. 10 illustrates the
calculated conductance of the rGO by N2H4, lemon juice, and
vinegar, and the results are tabulated in Table 5. The
calculations were based on Eqs. (4) and (5):
Resistance; R ¼
V
I
Conductance; G ¼
ð4Þ
1
I
¼
R
V
ð5Þ
C atoms contain six electrons, where two electrons are present
in 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 C
atoms on the 2D plane; thus, only one free electron is available
for electronic conduction [29]. These high-mobility free electrons
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Table 6
VOC, ISC, FF, and η calculated from Fig. 11.
PV cell
fabricated by
using
Short-circuit
Open-circuit
voltage, VOC (V) current, ISC (A)
rGO by
0.28
hydrazine
rGO by lemon 0.28
rGO by vinegar 0
Fig. 10. Current––voltage curves generated by AutoLab system for (a) rGO by
vinegar, (b) rGO by hydrazine, and (c) rGO by lemon juice.
FF ¼
Materials
η¼
rGO by
0.5
hydrazine
rGO by vinegar 0.5
rGO by lemon 0.5
Current
(μA)
2.5
1.0
12.3
Resistance
(Ω)
2,00,000
5,00,000
40,650.4
Conductance
(μΩ 1)
5
2
24.6
Efficiency,η
(%)
0.000003
0.119
6.66 10 8
0.0000034
0
0.105
0
6.66 10 8
0
used to construct photovoltaic (PV) cells to study its efficiency
toward light conversion into electricity. The dimension of the
PV cells was fixed at 1 cm2, and the light source used had a
power of 150 W. The experiment was performed using a
forward-bias connection. The fill factor (FF) and power
conversion efficiency (η) were calculated from Fig. 11 using
Eqs. (6) and (7), respectively, and the results were tabulated in
Table 6.
Table 5
Resistance and conductance values of the rGO calculated at voltage V=0.5 V.
Voltage
(V)
Fill
factor,
FF
I MAX V MAX
I SC V OC
I MAX V MAX
FF I SC V OC
¼
PLIGHT
PLIGHT
ð6Þ
ð7Þ
The efficiency of the PV cells constructed using rGO was low
because of the absence of electrolyte and dye to aid the
electron transfer in the PV cell. However, the main purpose of
this part of the study was to confirm that the effectiveness of
the rGO by lemon juice was as good as that of the rGO by
hydrazine. By comparing the calculated efficiency, the effectiveness of the rGO by lemon juice was found to be same as
that of the rGO by hydrazine. Therefore, lemon juice is a
potential candidate as substitute for hydrazine in the reduction
process.
4. Conclusion
Fig. 11. (a) Current–voltage characteristics of PV cell without light illumination and of PV cells constructed with rGO by (b) hydrazine, (c) lemon juice,
and (d) vinegar under light illumination.
located above and below the graphene sheets, which are called pi
(π) electrons, are responsible for the calculated value of conductance. Table 5 shows that the conductance of the rGO by
lemon juice was significantly higher than those of the rGO by
N2H4 and vinegar. This phenomenon may be attributed to the
lower O content in the rGO by lemon juice than in the other
samples. Thus, more electrons were able to travel within the rGO
by lemon juice, and a high current output was observed.
To further confirm our results showing that lemon juice can
produce a competitive quality of graphene sheets, the rGO was
This study demonstrated that lemon juice (pH 2.3) behaved
as an efficient reducing agent to reduce GO to rGO with a high
C/O ratio of E 8:2. The resultant graphene film showed higher
conductance (24.6 μS) compared with the other samples. This
result was mainly attributed to the low O content in rGO after
it was reduced by lemon juice. From the comparison of the
efficiency of the solar cells, the solar cell prepared using the
rGO by lemon juice and hydrazine shared the same efficiency.
Thus, lemon juice, which is a natural reducing agent, was as
effective as N2H4 in reducing GO. This environment-friendly
graphene synthesis method used one-pot preparation process;
thus, it was low cost. This method can be potentially used to
produce bulk quantity of GO.
Acknowledgments
The authors thank the University of Malaya for funding this
research work under the High Impact Research Chancellory
Grant UM.C/625/1/HIR/228 (J55001-73873).
Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphene oxide using a natural reducing agent, Ceramics International (2015), http:
//dx.doi.org/10.1016/j.ceramint.2015.04.008
S.W. Chong et al. / Ceramics International ] (]]]]) ]]]–]]]
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Please cite this article as: S.W. Chong, et al., Green preparation of reduced graphene oxide using a natural reducing agent, Ceramics International (2015), http:
//dx.doi.org/10.1016/j.ceramint.2015.04.008