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Green preparation of reduced graphene oxide for sensing and energy storage
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Article in Scientific Reports · April 2014

DOI: 10.1038/srep04684 · Source: PubMed

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 OPEN Green preparation of reduced graphene
SUBJECT AREAS:
oxide for sensing and energy storage
ELECTRONIC PROPERTIES
AND DEVICES applications
NANOSENSORS
Zheng Bo1, Xiaorui Shuai1, Shun Mao2, Huachao Yang1, Jiajing Qian1, Junhong Chen2, Jianhua Yan1
& Kefa Cen1
Received
24 January 2014
1
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Department of Energy Engineering,
Accepted Zhejiang University, Hangzhou, Zhejiang Province, 310027, China, 2Department of Mechanical Engineering, University of
21 March 2014 Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, WI 53211, USA.

Published
15 April 2014 Preparation of graphene from chemical reduction of graphene oxide (GO) is recognized as one of the most
promising methods for large-scale and low-cost production of graphene-based materials. This study reports
a new, green, and efficient reducing agent (caffeic acid/CA) for GO reduction. The CA-reduced GO
Correspondence and (CA-rGO) shows a high C/O ratio (7.15) that is among the best rGOs prepared with green reducing reagents.
requests for materials Electronic gas sensors and supercapacitors have been fabricated with the CA-rGO and show good
performance, which demonstrates the potential of CA-rGO for sensing and energy storage applications.
should be addressed to
S.M. (shunmao@uwm.

G
edu) or J.H.C. raphene, a two-dimensional (2D) carbon material, has shown great promise in various applications due
(jhchen@uwm.edu) to its unique structure and properties1,2. To promote the practical applications of graphene-based mate-
rials, a priority should be given to the exploration for large-scale preparation of high-quality graphene
with easy processing route and low cost. Up to now, diverse strategies have been applied for the production of
graphene, mainly including mechanical or ultrasonic exfoliation3, chemical vapor deposition (CVD)/plasma-
enhanced CVD (PECVD)4,5, epitaxial growth6, electric arc discharge7, chemical intercalation8, thermal/chemical
reduction of graphene oxide (GO)9–11. Among these methods, chemical reduction of GO is recognized as a
versatile and suitable method for the preparation of graphene in bulk quantities at a low cost. Unfortunately, a
large number of widely used reducing agents are toxic and/or explosive, such as the commonly-used hydrazine
hydrate (HH)12 and sodium borohydride13. As a consequence, continuous endeavors have been directed towards
the development and optimization of eco-friendly reducing agents for GO reduction.
Recent studies revealed that some natural materials/chemicals are promising substitutes for toxic/explosive
reducing agents for GO reduction, such as metals (e.g., iron, zinc, and aluminum)14–16, alkaline solutions (e.g.,
sodium hydroxide and potassium hydroxide)17, phenols (e.g., gallic acid, Tannin acid, dopamine, and tea poly-
phenol)18–21, alcohols (e.g., methyl alcohol, ethyl alcohol, and isopropyl alcohol)22, sugars (e.g., glucose, fructose,
sucrose, and natural cellulose)23,24, microbes (e.g., Escherichia coli and baker’s yeast)25,26, and other substances
(e.g., glycine, vitamin C, sodium citrate, and protein bovine serumalbumin)27–30. Generally, with eco-friendly
reducing agents, the reduction of GO was successfully demonstrated to alleviate the environmental issues.
However, challenges/problems still exist with the above green reduction processes. For example, inevitable
impurities may remain in the products when using metals as the reductants14–16 and fairly low deoxygenation
of GO can be obtained due to the poor reducing ability of gallic acid (C/O ratio of 3.89–5.28)19, tea polyphenol (C/
O ratio of 3.1)20, and methanol (C/O ratio of 4.0)22. In addition, for some reductants, a rigid/harsh reduction
condition was required, e.g., alkaline environment is needed for sugars, dopamine, protein bovine serumalbu-
min21,24,29, and ionic liquids are needed for natural cellulose23. To this end, there is still a strong need to further
explore novel green reducing agents for clean and effective reduction of GO.
Caffeic acid (CA, 3,4-dihydroxycinnamic acid, C9H8O4) is one of the most predominant hydroxycinnamic
acids in the species of phenolic compounds, a group of substances recognized as excellent antioxidant. The
chemical structure of CA can be described as two adjacent hydroxyl groups on an aromatic ring attached to
the highly conjugated propenoic side chain. Previous studies have shown that the high antioxidant capacities of
CA were attributed to the presence of hydroxyls at positions 3 and 4; and the two hydroxyl groups in CA
contributed to a further increase in antioxidant potential by the donation of the hydrogen atom31. As a

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consequence, CA has been widely used as an effective antioxidant for rGO using CA as the reducing agent. Compared with previously
applications ranging from the storage of soybean biodiesel31, the reported reducing agents, CA shows advantages in terms of high
prevention of cardiovascular disease and cancer for human32, to reduction efficiency, low level of residual impurities, mild reaction
the reduction of Cr(VI) for soil-plant system33. Since CA has excel- conditions, and most importantly, environmentally friendly fabrica-
lent antioxidant activity and is widely available in plants and food34,35, tion procedure. It is believed that this approach has great potentials
it is reasonable to consider CA as a green, effective, and low-cost for low cost and large-scale production of graphene-based materials
deoxygenation agent for GO reduction. from graphite.
Inspired by the above facts, we herein, for the first time, propose a
green and facile method for the chemical reduction of GO using CA Results
as the reducing agent. The reduction of GO was successfully per- The prepared rGO samples were characterized to understand the
formed with a simple procedure, while the reduction level of the rGO structure and the reduction efficiency of CA. Graphene and
reduced GO (rGO) is among the best for green reducing reagents rGO are 2D nanosheets and usually bear transparent and wrinkled
with a high C/O ratio of 7.15 (Supplementary Information, Table S1). features under a microscope. Figs. 1a–c show the transmission elec-
The restoration of the electrical properties of graphene, e.g., the high tron microscopy (TEM) and high-resolution TEM (HRTEM) images
conductivity, through reduction is critical for applications of the of the rGO reduced with CA for 24 hours (24h-CA-rGO). After CA
rGO. Therefore, CA-reduced rGO (CA-rGO)-based electronic gas reduction, the intrinsic features of rGO such as large (few-micron
sensors and supercapacitors were demonstrated. The gas sensors size), transparent, and thin nanosheets with typical wrinkled and
with CA-rGO as the sensing material exhibited fast responses and scrolled structure were observed. The HRTEM images show that
high sensitivities to low-concentration NO2 (100 ppm) and NH3 the prepared rGO samples are few-layer (around 6–8 layers)
(1%). For the supercapacitor application, electric double-layer capa- nanosheets. Previous studies have shown that the thickness of the
citors (EDLCs) were fabricated using CA-rGO as the active materials. GO sheet may decrease after reduction due to the removal of the
The capacitors showed substantially higher specific capacitances oxygen groups in the GO carbon plane9. To reveal the thickness
than those of GO due to the enhancement in active material conduc- change in the GO sheet, atomic force microscopy (AFM) imaging
tivity, confirming the high reduction efficiency of CA. The demon- and thickness measurements on GO and rGO sheets were carried
strated device applications confirm the successful preparation of out. Fig. 1f shows the AFM images and height profiles of GO and

Figure 1 | Structure characterizations of GO and CA-rGO. (a) TEM and (b, c) HRTEM images of 24h-CA-rGO. Inset: SAED patterns. (d) Digital
photographs of aqueous dispersions of GO before and after reduction by CA for different reaction time. (e) Water droplet on the surface of GO and CA-
rGO sheets. The error in the contact angle measurements is on the order of 0.1% of the measured values. (f) Tapping-mode AFM images and the
corresponding height profiles of GO and 24h-CA-rGO dispersed on a mica substrate. About twenty 24h-CA-rGO sheets were characterized by AFM
(Supplementary Information, Fig. S1). The height and size distributions of the 24h-CA-rGO sheets were obtained from the data shown in Fig. S1.

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Figure 2 | XRD and Raman data of GO and CA-rGO. (a) XRD patterns of pristine graphite, GO, and 24h-CA-rGO. (b) Raman spectra of GO before and
after CA reduction for different reduction time.

24h-CA-rGO. The thickness of GO measured from AFM data was epoxide groups on the carbon basal plane38. The G peak positions of
1.116 nm, in accordance with the thickness values of single-layer three samples were in the order of graphite (,1,575 cm21) , 24h-
GOs reported in previous literature20,23. The thickness of the sin- CA-rGO (1,586 cm21) , 12h-CA-rGO (1,590 cm21) , 2h-CA-rGO
gle-layer 24h-CA-rGO was around 0.846 nm, which is obviously (1,596 cm21) , GO (1,599 cm21). The intensity ratio of D to G peak
smaller than that of the single-layer GO sheet, suggesting the effective ID/IG was in the order of GO (0.86) , 2h-CA-rGO (0.92) , 12h-CA-
reduction of GO with CA. To obtain reliable height and size informa- rGO (1.03) , 24h-CA-rGO (1.15). The increase of ID/IG ratio after
tion of the 24h-CA-rGO, AFM images were taken for around twenty reduction is commonly found in GO chemical reduction stud-
24h-CA-rGO sheets (Supplementary Information, Fig. S1). The ies12,27,39–42. It can be attributed to a decrease in the average size of
height and size distributions of 24h-CA-rGO obtained from the the sp2 domains upon reduction of the GO, in which new graphitic
AFM data are shown in Fig. 1f. Results indicate that the as-obtained domains were created that have smaller sizes than the ones present in
24h-CA-rGO is a mixture of single-layer and multilayer sheets and GO before reduction, but are larger in quantities. Therefore,
the size of the sheets is in the range of 0.07 to 0.66 mm2. although there are more defect-free sp2 carbons after reduction, these
Fig. 1d shows the color change of GO suspension over the reaction carbons form smaller domains than those in the GO, which leads to
time from 2 to 24 hours. The yellow brown GO suspension changed large quantities of structural defects12,42. Another possible reason is
its color to black, indicating the reduction of GO25. Completely black the increased fraction of graphene edges, which could also contribute
homogeneous suspension was obtained for 24h-CA-rGO, suggesting to the increase in the ID/IG ratio27. To better understand the structure
the restoration of aromatic graphene structure36. As shown in Fig. 1e, of CA-rGO, Fourier transform infrared (FTIR) and ultraviolet–vis-
GO exhibited highly hydrophilic nature as a result of sufficient oxida- ible (UV-vis) absorption spectra (Supplementary Information, Fig.
tion. The contact angle of water on GO surface was 36.6u/38.5u, S2) were included and the results further confirm that the GO was
confirming the high wettability of GO in aqueous solutions. In con- successfully reduced by CA.
trast, the 2h-CA-rGO, 12h-CA-rGO, and 24h-CA-rGO samples The GO reduction level was also investigated by X-ray photoelec-
showed increased hydrophobicity with a contact angle in the range tron spectroscopy (XPS) measurements. Fig. 3a shows the XPS sur-
of 54.8u–93.3u, which is in agreement with the previous work37. The vey spectra of GO and rGO samples (CA5GO 5 5051). As the
increase in contact angle can be ascribed to the removal of oxygen reduction time prolonged, the C/O ratio increased from 2.46 (GO),
functional groups in GO sheets, and the increased contact angle with to 3.17 (2h-CA-rGO), 5.23 (12h-CA-rGO), and 7.15 (24h-CA-rGO).
different reaction times suggests that a higher reduction level of CA- The results indicate the removal of oxygen-containing groups in the
rGO was achieved with a longer reaction duration. CA-rGOs and the reduction level increased with the reaction time.
To understand the atomic structures and interlayer spacings of the The C/O ratio of 24h-CA-rGO sample was close to that using hydra-
GO and rGO samples, X-ray diffraction (XRD) was carried out and zine monohydrate (C/O of 10.3)12, and much higher than those using
the results are shown in Fig. 2a. The graphite exhibits a basal tannin acid (C/O of 2.44)18, tea solution (C/O of 3.10)20, natural
reflection (002) with a strong and sharp peak at 26.6u (corresponding cellulose (C/O of 5.47)23, baker’s yeast (C/O of 5.90)25, L-Ascorbic
to a d-spacing of 0.335 nm). Due to the oxidation of pristine graph- acid (C/O of 5.70)43, and gallic acid (C/O of 5.28)19. To investigate the
ite, the diffraction peak of GO shifts to a lower angle of 10.02u impact of the CA5GO ratio on the reduction level, experiments with
(corresponding to a d-spacing of 0.880 nm). As for the 24h-CA- different ratios of CA to GO (1051, 3051, 5051, and 7051) for a fixed
rGO, the reflection peak at 10.02u disappeared while a broad peak reduction time (24 hours) were carried out. The XPS results (Fig. 3b)
centering at 24.79u (corresponding to a d-spacing of 0.359 nm) was show that the C/O ratio of CA-rGO increased with the increasing
observed. The relatively larger d-spacing of GO than that of pristine CA5GO ratio. For instance, the C/O ratio increased from 4.59 to 5.62
graphite is due to the intercalation of water molecules and the forma- and 7.15 when the CA5GO ratio increased from 1051 (pH 5 5.4) to
tion of oxygen-containing functional groups between the layers of 3051 (pH 5 5.0) and 5051 (pH 5 4.7), respectively. However, the C/
graphite5. However, after reduction, the d-spacing of rGO was greatly O ratio showed a very small change (from 7.15 to 6.91) when the
decreased, indicating the removal of oxygen-containing functional CA5GO ratio increased to 7051 (pH 5 4.5). Therefore, a CA5GO
groups18. ratio of 5051 (pH 5 4.7) and a reaction time of 24 hours are deter-
The structure configurations of GO and rGO samples were further mined to be optimum for the CA reduction of GO. Figs. 3c and d
investigated by Raman spectroscopy, as shown in Fig. 2b. Typically, show the Gaussian line fitted C1s spectra of GO and 24h-CA-rGO. In
two main bands exist in the spectra of graphite and graphene-based the spectra, four peaks centering at 284.6 eV (C5C/C–C), 286.5 eV
materials, i.e., the G band assigned to the first-order scattering of the (C–OH), 287.6 eV (C5O), and 289.1 eV (O5C–OH) are found,
E2g phonon from sp2 carbon (graphite lattice), and the D band result- corresponding to different functional groups. The C–OH, C5O,
ing from the structural imperfections created by the hydroxyl and and O5C–H peaks indicate the existence of oxygen-containing

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Figure 3 | XPS data of GO and CA-rGO. (a) XPS survey spectra of the as-prepared GO and CA-rGOs (CA5GO 5 5051) for different reaction times.
(b) XPS survey spectra of the as-prepared GO and CA-rGOs with different CA to GO ratios (reaction time: 24 hours). Gaussian line fitted C1s spectra of
(c) GO and (d) 24h-CA-rGO.

groups in the GO, e.g., hydroxyl, epoxide, and carbonyl. After reduc- sensing channel and the working principle of the gas sensor is based
tion (Fig. 3d), the intensities of C–OH, C5O, and O5C–H peaks on the charge/electron transfer between the adsorbed gas molecules
greatly decreased, accompanied by an increase of the sp2 carbon and the rGO sheet45. In general, by measuring the resistance/conduc-
peak, revealing that a large number of oxygen-containing groups tivity change of the rGO sheet in different gases, the presence and the
were removed and the majority of the sp2 carbon networks were concentration of the gas could be determined. Before the sensor was
restored. Based on the above XPS results, the reduction of GO was tested in different gases, the resistance of the 24h-CA-rGO device
confirmed by the significant decrease in the oxygen contents in GO. was measured with direct-current (dc) measurements, as shown in
Thermogravimetric analysis (TGA) (Supplementary Information, Fig. 4c. Based on dc measurement results, the 24h-CA-rGO sensor
Fig. S3) was also carried out to study the amount of oxygen groups shows a resistance of 104 to 105 V, which is much smaller than that of
in rGOs and the results show that the 24h-CA-rGO has a much GO (1010 V)44, with a linear I–V curve, indicating the reduction of
smaller mass loss at elevated temperatures than GO, further proving GO. To study the transistor properties of rGO, field-effect transistor
that GO was reduced with CA. The reduction mechanism of CA was (FET) measurements were carried out in air. The gate potential (Vg)
widely studied as a four-electron release process33, in which the dependence of the drain current (Id) of the 24h-CA-rGO sensor
release of four electrons can be expressed by the formation of semi- shows that the rGO was a p-type semiconductor (Fig. 4d), and the
quinonic radicals easily oxidizable to quinonic groups, whose further Id decreased when Vg ramping from negative to positive. The FET
oxidation leads to the formation of carboxylic groups. results are in accordance with previous studies of rGOs9,46 and the
on-off current ratio of the rGO sensor is relatively low compared with
Discussion semiconducting carbon nanotubes and nanowires, which is because
Most applications of graphene rely on its high electrical conductivity the rGO has a small bandgap and the 24h-CA-rGO has a multiple-
and unique structure. However, GO is non-conductive because of the layer structure.
extensive presence of saturated sp3 bonds, the high density of elec- Figs. 4e and f show the dynamic responses (Id vs. time) of the 24h-
tronegative oxygen atoms bonded to carbon, and other ‘‘defects’’44. CA-rGO sensor exposed to 100 ppm NO2 and 1% NH3 diluted in air.
Therefore, restoration of the high conductivity of graphene sheet is The sensor was first exposed to a clean air flow for 10 minutes to
critical for its applications. In this study, two CA-rGO-based devices, record a base resistance; then a target gas flow was injected into the
i.e., gas sensor and supercapacitor, have been developed and demon- sensing chamber for 5 minutes to register a sensing signal; and finally
strated for environment and energy applications. The results from a clean air flow was injected for 15 minutes to recover the sensor.
the demonstrated device applications show that the rGO produced From the sensing results, the sensor showed fast responses to both
with CA has a high conductivity, and the reported method may serve gases and the sensor resistance decreased with NO2 exposure and
as a simple and efficient method for green production of graphene. increased when exposed to NH3. The difference in the sensor res-
Figs. 4a and b show a schematic of the rGO-based gas sensor and ponse is because the NO2 and NH3 work as electron acceptor or
the scanning electron microscopy (SEM) image of the rGO sheets on donor in the sensing reaction and extracts electrons from rGO
the sensor electrodes. In this type of gas sensor, the rGO works as the or injects electrons into rGO, respectively, thereby increasing or

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Figure 4 | Gas sensor application of CA-rGO. (a) Schematic diagram of the rGO-based gas sensor device. (b) SEM image of 24h-CA-rGO sheets bridging
a pair of gold sensor electrodes. (c) Direct current measurement results of 24h-CA-rGO with drain-source potential ramping from -1.0 to 11.0 V.
(d) FET results (Vd 5 0.5 V) of the 24h-CA-rGO. Dynamic gas sensing results of the 24h-CA-rGO gas sensors for (e) 100 ppm NO2 and (f) 1% NH3 tested
under room temperature.

decreasing the rGO conductivity. The 24h-CA-rGO sensor has a TEABF4/AN: 6.0 F/g). Compared with the quite distorted shape of
sensitivity of ,1.33 (ratio of device resistance in air to that in target the CV curves of GO, those of 24h-CA-rGO were much closer to the
gas) to 100 ppm NO2 and ,1.35 (ratio of device resistance in target quasirectangular shape, indicating the faster charging and dischar-
gas to that in air) to 1% NH3, which are similar to our previous ging responses to the applied potential due to the significantly
reports44,46. The results from the sensor demonstration show that improved material conductivity after CA reduction. According to
the CA-rGO can be readily used in sensor applications without the Nyquist plots obtained from the electrochemical impedance
any additional treatment and the performance of the sensor could spectroscopy (EIS) tests (Supplemental Information, Fig. S5), the
be further improved through surface functionalization of the rGO imaginary component presents a sharp increase with a near-vertical
sheet. line at low frequencies, confirming the predominant EDLCs.
Graphene-based structures have been recognized as quite The charge/discharge curves of the capacitors obtained at different
promising active materials for supercapacitors (i.e., EDLCs) due to current densities (1, 5, and 10 A/g) can be found in the Supple-
graphene’s huge specific surface area and high electrical conduc- mentary Information (Fig. S6). At a current density of 1 A/g, the
tivity47,48. Figs. 5a and b show a schematic of the graphene-based specific capacitances of 24h-CA-rGO were calculated as 136 and
EDLCs and the digital picture of an LED light powered by a super- 92 F/g for aqueous and organic electrolytes, respectively. The specific
capacitor cell. EDLCs store charges electrostatically via reversible ion capacitance of 24h-CA-rGO was comparable to the ones using rGO
adsorption at the electrode/electrolyte interface, where the Ohmic prepared by toxic HH and significantly higher than those using non-
resistance of the active materials will obviously influence the charge toxic alcohol-reduced rGOs22,49. This observation could be somehow
transport during the charge/discharge processes. To this respect, the related to the difference in the specific surface areas of various active
CA-rGO sheet with an obviously improved conductivity than that of materials. The Brunauer–Emmett–Teller (BET) specific surface area
the parent GO, due to the restoration of p–p conjugated structure in of 24h-CA-rGO was measured as 122 m2/g (see N2 adsorption/
graphene sheets, is therefore expected to show attractive EDLC desorption analysis in the Supplemental Information, Fig. S4), which
properties. is higher than that of the rGO reduced by alcohols (5.8–35.9 m2/g)22
Figs. 5c and d show the cyclic voltammetry (CV) curves of GO and while lower than that of rGO reduced by HH (normally 400–700 m2/
24h-CA-rGO based working electrodes employing KCl and tetra- g)12,50. It could be attributed to the easy formation of restacked rGO
ethylammonium tetrafluoroborate in acetonitrile solvent (TEABF4/ nanosheets during the reduction process with a relatively long reduc-
AN) as the aqueous and organic electrolytes, respectively. The CV tion time due to the poorer reducing capabilities of alcohols and CA
curves of 24h-CA-rGO (scan rate: 100 mV/s) show obviously larger than that of HH22. The CV curves of 24h-CA-rGO supercapacitors at
CV areas, and correspondingly, higher specific capacitances (KCl: different scan rates (10, 20, 50, 100, and 200 mV/s) are presented in
96 F/g; TEABF4/AN: 74 F/g) than those of the GO (KCl: 3.7 F/g; the Supplementary Information (Fig. S7). With an increasing scan

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Figure 5 | Supercapacitor application of CA-rGO. (a) Schematic diagram of the rGO-based double-layer supercapacitor. (b) Digital photographs
of an LED light powered by a supercapacitor cell. Inset: digital photograph of a bare Ni foam before and after being coated with 24h-CA-rGO sheets.
CV curves of the supercapacitors using GO and 24h-CA-rGO working electrodes in (c) 1.0 M KCl and (d) 1.0 M TEABF4/AN electrolytes tested at a scan
rate of 100 mV/s.

rate from 20 to 200 mV/s, the capacitance retention of 24h-CA-rGO corresponding products with different reduction time of 2, 12, and 24 hours were
labeled as 2h-CA-rGO, 12h-CA-rGO, and 24h-CA-rGO, respectively. The resulting
in aqueous electrolyte was 68% (from 126 to 86 F/g), close to that of suspension was collected by vacuum filtration and washed with deionized water and
HH (78%)49, indicating a good rate performance. The above results ethanol for 10 times. Finally, rGO was collected after drying under vacuum condition.
demonstrate the good electrochemical properties of CA-rGO and its For comparison, reduction of GO with HH was also conducted. Briefly, 100 mL of
high potential for energy storage applications. Further improve- GO dispersion (concentration: 1 mg/mL) was mixed with 1 mL hydrazine hydrate
ments on the capacitive behavior of CA-rGO appear likely through (98% from Sigma Aldrich), which was then kept in a 95uC oil bath and stirred for
24 hours. The as-obtained sample was labeled as HH-rGO.
the optimization of material preparation and supercapacitor
assembly. Material characterizations. TEM images and selected area electron diffraction
In summary, rGO was successfully prepared with CA as the redu- (SAED) were obtained with a Technai G2 F30 S-Twin TEM (Philips-FEI). A Hitachi
cing agent. The rGO has been proved to have high a C/O ratio and a S-4800 SEM was used for SEM characterization at an acceleration voltage of 10 kV.
low oxygen content. The demonstrated electronic gas sensors with UV-vis spectra were recorded on a Shimadzu UV-2550 spectrophotometer
(Kyoutofu, Japan). FTIR spectra were carried out on a Nicolet 5700 FTIR
rGO as the sensing materials show fast and large responses to differ- spectrometer. XRD patterns were recorded with a XRD-6000 Diffractometer using
ent gases under room temperature. The rGO has also been used as the Cu Ka Radiation (l 5 0.15425 nm, Shimadzu). XPS measurement was performed on
active materials in EDLCs and the capacitors show comparable spe- a VG Escalab Mark II system employing a monochromatic Mg Ka X-Ray source (hm
cific capacitance with that of hydrazine-reduced GO. We believe the 5 1,253.6 eV, West Sussex). The Raman spectra were taken with a DXR 532 Raman
spectrometer (Thermo Fisher Scientific) in an excitation wavelength of 532 nm at
rGO prepared with CA could be used in many environmental and room temperature. AFM images were taken on a MultiMode AutoProbe CP/MT
energy applications, e.g., sensors, supercapacitors, batteries, and Scanning Probe Microscope (Veeco Instruments, Woodbury, NY) operating in the
catalysis. And this green reduction method could be attractive for tapping mode. TGA was performed using a Thermogravimetric Analyzer (Perkin
facile large-scale manufacturing of graphene materials with a low Elmer, USA) under argon atmosphere with a flow rate of 100 mL/min. N2
adsorption-desorption measurements were carried out at 77.4 K using a
cost. Quantachrome Autosorb gas-sorption system (AUTOSORB-IQ-MP). Electrical
conductivity measurement was carried out on a HALL5500 digital four-point probe
Methods system (Bio-Rad Co., USA). The rGO films for electrical conductivity measurement
Synthesis of GO. GO was synthesized following a modified Hummer’s method51. In a were prepared by filtration of rGO suspensions prepared in Dimethylformamide
typical procedure, 1 g natural graphite powder (XFNANO Materials Tech) was (DMF). The contact angles of the samples were measured by using a DropMeterTM
dispersed in 25 mL concentrated sulfuric acid at room temperature and the mixture Professional A-200 digital goniometer.
was cooled down to 0uC in an ice bath. Subsequently, 3.5 g potassium permanganate
(Sinopharm Chemical Reagent) was slowly added, and a 2 h stirring was conducted in Gas sensor tests. The details of the gas sensor fabrication were reported in our
a 35uC water bath followed by adding 100 mL deionized water. Then, 8 mL hydrogen previous studies44,46. In a typical sensor, 0.5 mL 24h-CA-rGO suspension (0.1 mg/
peroxide solution (30 wt.% aqueous solution) was added until the color of the reaction mL) was pipetted on the sensor electrode and dried under room temperature in air.
mixture turned to bright yellow. Dilute hydrochloric acid solution (10% by volume) To study the conductivity and FET characteristics of the rGO sensor, direct current
and deionized water were used to wash and remove the excess manganese salt and and FET measurements were carried out using a Keithley 2602 source meter. The
acids in the product. The product powder was obtained from centrifugation direct current measurement was performed by recording the drain current when
(8,000 r.m.p., 10 min) after repeating the washing process for four times. Finally, the ramping the drain-source voltage Vd from 21.0 to 11.0 V (with a step of 0.1 V);
product GO powder was dried at 35uC under vacuum. while the FET measurement was performed by recording the drain current when
ramping the gate voltage Vg from 240 to 140 V (with a step of 0.1 V). The gas
Reduction of GO with CA. The as-dried GO powder (100 mg) was dispersed in sensing performance of as-fabricated rGO was characterized against low-
1000 mL deionized water, followed by an ultrasonication for 1.5 h (FB15150, 300w, concentration NO2 (100 ppm) and NH3 (1%) diluted in dry air. Variations in the
Fisher, Scientific). Different amounts of CA powder (Huilin Bio Tech) was added into electrical conductance of rGO were monitored by simultaneously applying a constant
the GO aqueous solution (concentration: 0.1 mg/mL) at room temperature. The pH dc voltage and recording the change in current passing through rGO sheets, which
of the mixture was measured after 10 minutes of mixing. The mixture was then heated were exposed periodically to clean air, target gas, and clean air (flow rate: 2 lpm for all
to 95uC in an oil bath with the assistance of magnetic stirring for reaction. The gases).

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Supercapacitor tests. Supercapacitors with GO and rGO as the active materials were 22. Dreyer, D. R., Murali, S., Zhu, Y. W., Ruoff, R. S. & Bielawski, C. W. Reduction of
assembled into a two-electrode system with nickel foam as the current collector. The graphite oxide using alcohols. J. Mater. Chem. 21, 3443–3447 (2011).
test coin cell consisted of a metal cap, a metal case with polymer seal, a spring, two 23. Peng, H. D., Meng, L. J., Niu, L. Y. & Lu, Q. H. Simultaneous Reduction and
stainless steel spacers, two current collectors coated with active materials, and a Surface Functionalization of Graphene Oxide by Natural Cellulose with the
membrane separator. 1.0 M KCl (Sigma Aldrich) and 1.0 M TEABF4/AN (Sigma Assistance of the Ionic Liquid. J. Phys. Chem. C 116, 16294–16299 (2012).
Aldrich) were used as the aqueous and organic electrolytes, respectively. For 24. Zhu, C. Z., Guo, S. J., Fang, Y. X. & Dong, S. J. Reducing Sugar: New Functional
supercapacitors with organic electrolyte, coin cells were assembled in the vacuum Molecules for the Green Synthesis of Graphene Nanosheets. ACS Nano 4,
glove box with argon atmosphere to avoid oxygen and moisture52. To prepare film- 2429–2437 (2010).
like rGO-based active materials, the rGO suspension was treated by a 1-h vacuum 25. Khanra, P. et al. Simultaneous bio-functionalization and reduction of graphene
filtration through a membrane filter of 0.22 mm in pore size, followed by a 12-h oxide by baker’s yeast. Chem. Eng. J. 183, 526–533 (2012).
freeze-drying process which could benefit the formation of an rGO film with a 26. Wang, G. M., Qian, F., Saltikov, C., Jiao, Y. Q. & Li, Y. Microbial reduction of
relatively large size. The GO film was fabricated by 12-h vacuum filtration of the GO graphene oxide by Shewanella. Nano Res. 4, 563–570 (2011).
dispersion through the same membrane filter. No binder or conductive agent was 27. Bose, S., Kuila, T., Mishra, A. K., Kim, N. H. & Lee, J. H. Dual role of glycine as a
applied, and the rGO and GO films peeled off the membrane were used for chemical functionalizer and a reducing agent in the preparation of graphene: an
supercapacitor test cell assembly. The mass of the rGO and GO films in a single environmentally friendly method. J. Mater. Chem. 22, 9696–9703 (2012).
electrode was identical (,1 mg). The capacitive behavior of supercapacitors was 28. Fernandez-Merino, M. J. et al. Vitamin C Is an Ideal Substitute for Hydrazine in
tested by CV, galvanostatic charge/discharge, and EIS on an electrochemical the Reduction of Graphene Oxide Suspensions. J. Phys. Chem. C 114, 6426–6432
workstation (PGSTAT302N, Metrohm Autolab B.V.) at room temperature. Based on (2010).
the CV curves, the specific capacitance of a single electrode (Ccv, unit: F/g) was 29. Liu, J. B., Fu, S. H., Yuan, B., Li, Y. L. & Deng, Z. X. Toward a Universal "Adhesive
calculated as: Nanosheet" for the Assembly of Multiple Nanoparticles Based on a Protein-
ð Induced Reduction/Decoration of Graphene Oxide. J. Am. Chem. Soc. 132,
Ccv ~ IdV=(v|DV|m), ð1Þ 7279–7281 (2010).
30. Wan, W. B. et al. "Green" reduction of graphene oxide to graphene by sodium
where I is the response current (unit: A), v is the potential scan rate (unit: V/s), V is the citrate. New Carbon Mater. 26, 16–20 (2011).
potential window (unit: V), and m is the mass of active materials on the single 31. Santos, N. A. et al. Caffeic Acid: An Efficient Antioxidant for Soybean Biodiesel
electrode, respectively. With the galvanostatic charge/discharge plots, the specific Contaminated with Metals. Energ. Fuel. 25, 4190–4194 (2011).
capacitance of a single electrode (Cg, unit: F/g) was calculated as: 32. Olthof, M. R., Hollman, P. C. H. & Katan, M. B. Chlorogenic acid and caffeic acid
are absorbed in humans. J. Nutr. 131, 66–71 (2001).
Cg ~2iDt=(DU|m), ð2Þ 33. Deiana, S., Premoli, A. & Senette, C. Reduction of Cr(VI) by caffeic acid.
Chemosphere 67, 1919–1926 (2007).
where i is the constant discharge current (unit: A), t is the discharge time (unit: s), and 34. Chen, J. H. & Ho, C. T. Antioxidant activities of caffeic acid and its related
U is the voltage drop upon discharging (unit: V). hydroxycinnamic acid compounds. J. Agric. Food Chem. 45, 2374–2378 (1997).
35. Gulcin, I. Antioxidant activity of caffeic acid (3,4-dihydroxycinnamic acid).
Toxicology 217, 213–220 (2006).
1. Geim, A. K. Graphene: Status and Prospects. Science 324, 1530–1534 (2009). 36. Song, P., Zhang, X. Y., Sun, M. X., Cui, X. L. & Lin, Y. H. Synthesis of graphene
2. Geim, A. K. & Novoselov, K. S. The rise of graphene. Nat. Mater. 6, 183–191 (2007). nanosheets via oxalic acid-induced chemical reduction of exfoliated graphite
3. Hernandez, Y. et al. High-yield production of graphene by liquid-phase oxide. RSC Adv. 2, 1168–1173 (2012).
exfoliation of graphite. Nat. Nanotechnol. 3, 563–568 (2008). 37. Wan, D. Y. et al. Low-Temperature Aluminum Reduction of Graphene Oxide,
4. Bo, Z. et al. Understanding growth of carbon nanowalls at atmospheric pressure Electrical Properties, Surface Wettability, and Energy Storage Applications. ACS
using normal glow discharge plasma-enhanced chemical vapor deposition. Nano 6, 9068–9078 (2012).
Carbon 49, 1849–1858 (2011). 38. Yang, D. et al. Chemical analysis of graphene oxide films after heat and chemical
5. Reina, A. et al. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 47,
Chemical Vapor Deposition. Nano Lett. 9, 30–35 (2009). 145–152 (2009).
6. Sutter, P. W., Flege, J. I. & Sutter, E. A. Epitaxial graphene on ruthenium. Nat. 39. Chen, Y., Zhang, X., Zhang, D., Yu, P. & Ma, Y. High performance supercapacitors
Mater. 7, 406–411 (2008). based on reduced graphene oxide in aqueous and ionic liquid electrolytes. Carbon
7. Sun, Z. Z. et al. Growth of graphene from solid carbon sources. Nature 468, 49, 573–580 (2011).
549–552 (2010). 40. Chua, C. K., Ambrosi, A. & Pumera, M. Graphene oxide reduction by standard
8. Malik, S. et al. High purity graphenes prepared by a chemical intercalation industrial reducing agent: thiourea dioxide. J. Mater. Chem. 22, 11054–11061
method. Nanoscale 2, 2139–2143 (2010). (2012).
9. Mao, S., Pu, H. H. & Chen, J. H. Graphene oxide and its reduction: modeling and 41. Moon, I. K., Lee, J., Ruoff, R. S. & Lee, H. Reduced graphene oxide by chemical
experimental progress. RSC Adv. 2, 2643–2662 (2012). graphitization. Nat. Commun. 1, (2010).
10. Hong, Y., Wang, Z. & Jin, X. Sulfuric Acid Intercalated Graphite Oxide for 42. Wang, H., Robinson, J. T., Li, X. & Dai, H. Solvothermal Reduction of Chemically
Graphene Preparation. Sci. Rep. 3, 3439 (2013). Exfoliated Graphene Sheets. J. Am. Chem. Soc. 131, 9910–9911 (2009).
11. Hu, C. et al. Spontaneous Reduction and Assembly of Graphene oxide into Three- 43. Zhang, J. L. et al. Reduction of graphene oxide via L-ascorbic acid. Chem.
Dimensional Graphene Network on Arbitrary Conductive Substrates. Sci. Rep. 3, Commun. 46, 1112–1114 (2010).
2065 (2013). 44. Mao, S. et al. A new reducing agent to prepare single-layer, high-quality reduced
12. Stankovich, S. et al. Synthesis of graphene-based nanosheets via chemical graphene oxide for device applications. Nanoscale 3, 2849–2853 (2011).
reduction of exfoliated graphite oxide. Carbon 45, 1558–1565 (2007). 45. Mao, S., Lu, G. & Chen, J. Nanocarbon-based gas sensors: progress and challenges.
13. Shin, H. J. et al. Efficient Reduction of Graphite Oxide by Sodium Borohydrilde J. Mater. Chem. A 2, 5573–5579 (2014).
and Its Effect on Electrical Conductance. Adv. Funct. Mater. 19, 1987–1992 46. Mao, S. et al. Tuning gas-sensing properties of reduced graphene oxide using tin
(2009). oxide nanocrystals. J. Mater. Chem. 22, 11009–11013 (2012).
14. Fan, Z. J. et al. Facile Synthesis of Graphene Nanosheets via Fe Reduction of 47. Tao, Y. et al. Towards ultrahigh volumetric capacitance: graphene derived highly
Exfoliated Graphite Oxide. ACS Nano 5, 191–198 (2011). dense but porous carbons for supercapacitors. Sci. Rep. 3, 2975 (2013).
15. Fan, Z. J. et al. An environmentally friendly and efficient route for the reduction of 48. Zhang, L. et al. Porous 3D graphene-based bulk materials with exceptional high
graphene oxide by aluminum powder. Carbon 48, 1686–1689 (2010). surface area and excellent conductivity for supercapacitors. Sci. Rep. 3, 1408
16. Yang, S. et al. A facile green strategy for rapid reduction of graphene oxide by (2013).
metallic zinc. RSC Adv. 2, 8827–8832 (2012). 49. Bo, Z. et al. Vertically oriented graphene bridging active-layer/current-collector
17. Fan, X. B. et al. Deoxygenation of Exfoliated Graphite Oxide under Alkaline interface for ultrahigh rate supercapacitors. Adv. Mater. 25, 5799–5806 (2013).
Conditions: A Green Route to Graphene Preparation. Adv. Mater. 20, 4490–4493 50. Stoller, M. D., Park, S. J., Zhu, Y. W., An, J. H. & Ruoff, R. S. Graphene-Based
(2008). Ultracapacitors. Nano Lett. 8, 3498–3502 (2008).
18. Lei, Y. D., Tang, Z. H., Liao, R. J. & Guo, B. C. Hydrolysable tannin as 51. Hummers, W. S. & Offeman, R. E. PREPARATION OF GRAPHITIC OXIDE.
environmentally friendly reducer and stabilizer for graphene oxide. Green Chem. J. Am. Chem. Soc. 80, 1339–1339 (1958).
13, 1655–1658 (2011). 52. Bo, Z. et al. One-step fabrication and capacitive behavior of electrochemical
19. Li, J., Xiao, G. Y., Chen, C. B., Li, R. & Yan, D. Y. Superior dispersions of reduced double layer capacitor electrodes using vertically-oriented graphene directly
graphene oxide synthesized by using gallic acid as a reductant and stabilizer. grown on metal. Carbon 50, 4379–4387 (2012).
J. Mater. Chem. A 1, 1481–1487 (2013).
20. Wang, Y., Shi, Z. X. & Yin, J. Facile Synthesis of Soluble Graphene via a Green
Reduction of Graphene Oxide in Tea Solution and Its Biocomposites. ACS Appl.
Mater. Interfaces 3, 1127–1133 (2011). Acknowledgments
21. Xu, L. Q., Yang, W. J., Neoh, K. G., Kang, E. T. & Fu, G. D. Dopamine-Induced Financial support for this work was provided by the National Natural Science Foundation of
Reduction and Functionalization of Graphene Oxide Nanosheets. China (No. 51306159), the Zhejiang Provincial Natural Science Foundation of China
Macromolecules 43, 8336–8339 (2010). (No. LY13E020004), the Foundation of National Excellent Doctoral Dissertation of China

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(No. 201238), and the U.S. National Science Foundation (IIP-1128158). The authors thank Competing financial interests: The authors declare no competing financial interests.
Dr. H.A. Owen for technical support with SEM analyses and the SEM imaging was
How to cite this article: Bo, Z. et al. Green preparation of reduced graphene oxide for
conducted at the Electron Microscope Laboratory of UWM.
sensing and energy storage applications. Sci. Rep. 4, 4684; DOI:10.1038/srep04684 (2014).

Author contributions This work is licensed under a Creative Commons Attribution 3.0 Unported License.
Z.B., S.M. and K.C. designed this research; X.S. synthesized the GO and rGO materials; S.M. The images in this article are included in the article’s Creative Commons license,
conducted the gas sensing tests; H.Y. and J.Q. carried out the supercapacitor tests; X.S. and unless indicated otherwise in the image credit; if the image is not included under
J.Y. analyzed the data; Z.B., S.M. and J.C. drafted the manuscript; and all authors the Creative Commons license, users will need to obtain permission from the license
commented on the final manuscript. holder in order to reproduce the image. To view a copy of this license, visit
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