One-step chemical reduction of graphene oxide with oligothiophene for improved electrocatalytic oxygen reduction reactions

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One-step chemical reduction of graphene oxide with oligothiophene for improved electrocatalytic oxygen reduction reactions Mohammad Shamsuddin Ahmed, Hyoung Soon Han, Seungwon Jeon

*

Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Oligothiophene (nTP, n = 1, 2, 3) has been used as the reductant for the first time in the prep-

Received 17 January 2013

aration of graphene by the reduction of graphene oxide (GO). A simple single-step chemical

Accepted 28 April 2013

approach has been developed to reduce and/or functionalize GO with nTP. The reaction

Available online 2 May 2013

takes place at room temperature under stirring of a suspension of GO and nTP in MeCN. The nTP has been grafted onto the surface of GO by reacting epoxy groups together with the reduced graphene oxide (rGO). It was observed that increasing the thiophene ring (hereafter, thiophene is referred to as TP; 2,2 0 bithiophene as 2TP; and 2,2 0 :5 0 ,200 terthiophene as 3TP) can enhance the reduction reaction. All instrumental experiments have confirmed that nTP not only covalently bonded to the GO but also partly restored the conjugate structure of GO, as a reducing agent. The resultant rGO with 3TP (rGO3TP) has been demonstrated to show remarkable electrocatalytic activity toward oxygen reduction reaction (ORR) compared to typical rGO. The observed ORR electrocatalytic activity induced by the intermolecular charge-transfer provides a general approach to various carbon-based metal-free ORR catalysts. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene consists of single-atom-thick sheets of carbon organized in a two-dimensional honeycomb lattice [1], and is the thinnest material [2]. Graphene has attracted a great deal of attention in recent years because of its unique properties [3–6], such as thermal conductivity (up to 5300 W m1 K1) [7], mechanical strength (E = 1.0 TPa) [8], and specific surface area (about 2600 m2 g1) [9]. The exfoliated sheets are distorted carbon networks carrying carboxylate, hydroxyl, and other oxygen-containing functional groups commonly referred to as graphene oxide (GO). GO is a water-dispersible platelet material, resulting from the treatment of graphite with powerful oxidizing agents [10,11]. During oxidation, the graphene sheets of which the bulk graphite is composed be-

come functionalized with hydroxyl and epoxide groups on their basal plane, while the edges are decorated with carbonyl and carboxyl groups [12,13]. While these functional groups enable the suspension of GO in polar solvents including water, their sp2 network is partially destroyed. As a consequence, GO displays electrically insulating characteristics. Many methods, such as mechanical exfoliation [14], chemical vapor deposition [15], and reduction of GO [16–19] have been reported for the preparation of high-quality graphene. It has been well demonstrated that GO is an excellent precursor to prepare graphene by thermal [20,21], annealing [22] electrochemical [23,24], and chemical reduction methods [25–32]. Most chemical reductions are complicated and subject to heat treatment from 70 °C to 1100 °C [16–22,27–32]. Although the reduction mechanism remains ambiguous, chemical

* Corresponding author: Fax: +82 62 530 3389. E-mail address: [email protected] (S. Jeon). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.04.080

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reduction of GO is one of the established methods to make graphene in large volume. At present, the chemical reduction of GO is usually fulfilled by using hydrazine [17,19,32], dimethylhydrazine [10], hydroquinone [33] or NaBH4 [34,26] as the reducing agent. Unfortunately, the use of these poisonous and explosive reducing agents requires great care. Harmless and easy approaches for mass production of high-quality graphene are still necessary. Oligothiophenes (nTP, n = 1, 2, 3) with well-defined chemical structure are among the most investigated conjugated oligomers. They have many interesting technological applications [35]. The reasons for the spectacular applications of these compounds are related to their good environmental stability, in both neutral and doped states, and their structural versatility, which has provided opportunities for improving its electrical properties [36]. The use of nTP as a reductant has been reported [37], and their reaction typically proceeds through the positions a to sulfur on the thiophene ring [38]. In order to reduce the usage of precious Pt metal and the cost of fuel cells (FCs), the development of nonprecious metal or metal-free oxygen reduction reaction (ORR) electrocatalysts has generated a great deal of interest [39–44]. Because of high cost, limited supply, poor durability and CO poisoning [40] involved with Pt, the large-scale application of FCs has been hindered. Numerous efforts have been made to reduce or replace the Pt-based catalysts in FCs [45,46]. In particular, the search for new nonprecious-metal catalysts (NPMCs) with high activity and practical durability for ORR has been one of the most active fields in chemistry [46]. Very recent studies have confirmed that functionalized graphene [47,48] or various doped graphenes [21,49] are promising candidates for replacing Pt-based catalysts for FCs, because they not only exhibit high catalytic activity, long term stability, and excellent methanol tolerance in alkaline media, but also possess the advantages of low cost and environmental friendliness. In this paper, we describe the efficient synthesis and functionalization/adsorption of graphene by a simple chemical reduction reaction between GO and nTP at room temperature, as well as the use of nTP as the reductant in the preparation of graphene for the first time. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and UV–vis spectroscopy experiments confirm that nTP acts as a reducing agent and covalently bonds to GO. As a result, reduced and functionalized/adsorbed GO can be achieved. Better reduction occurred with 2,2 0 :5 0 ,200 -terthiophene (3TP) than with thiophene (TP) or 2,2 0 bithiophene (2TP). Moreover, we demonstrate that the best resulting 3TPreduced GO (rGO3TP) can serve as a metal-free electrocatalyst for the ORR, showing comparable electrocatalytic activity to other typical rGO.

2.

Experimental

2.1.

Synthesis of rGOnTP, rGO;N2H4 and rGO;NaBH4

GO was synthesized from graphite powder according to a modified Hummers method [32,50,51]. 25 mL of 1 mM nTP (TP or 2TP or 3TP) (in MeCN) was loaded into three separate 50-mL round bottom flasks, followed by the addition of

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12 mg of GO to produce an inhomogeneous yellow-brown dispersion. This dispersion was sonicated until the coagulate particles become free, and was kept under stirring for 6 h at room temperature. Thereafter, the final product, rGOnTP (rGOTP or rGO2TP or rGO3TP), was collected through a 0.45 lm polyethersulfone filter for three times repetition with 10 min sonication in MeCN, and dried for 24 h at 40 °C in a vacuum oven. The procedure to obtain the rGO;N2H4 and rGO;NaBH4 is described elsewhere [32,51].

2.2.

Characterizations of rGOnTP

FTIR measurements were performed on an FTIR spectroscope (PerkinElmer). XPS measurements were performed on a MultiLab 2000 (Thermo Electron Corporation, England) with a 14.9-keV Al Ka X-ray source. Raman spectra were collected with a LabRam HR800 UV Raman microscope (Horiba Jobin– Yvon, France), using an Ar+ ion laser with excitation wavelength of 514 nm. Thermogravimetric analysis (TGA) was performed in nitrogen atmosphere with a Perkin-Elmer TGA 2050 instrument at a heating rate of 20 °C/min. The scanning electron microscope (SEM) images were obtained on a JSM-7500F field emission scanning electron microanalyzer (JEOL).

2.3.

Electrochemical measurements

For the electrode preparation, rGO3TP, rGO;N2H4, and rGO;NaBH4 suspensions in ethanol (1 mg/mL) were prepared by introducing a predetermined amount of the corresponding sample under sonication. 10-lL portions of the rGO3TP, rGO;N2H4, or rGO;NaBH4 suspensions were then dropped onto the surface of a glassy carbon electrode (GCE, 0.5 cm in diameter) that was prepolished with 0.05-lm alumina suspension on a polishing cloth (BAS, USA). All voltammetric measurements were taken using a three-electrode potentiostat [CHI 700C electrochemical workstation (USA)] in a grounded Faraday cage. Platinum wire was used as an auxiliary electrode. A calibrated Ag/AgCl electrode from Bioanalytical Systems Inc. was used as a reference electrode. All electrochemical measurements, including cyclic voltammograms (CVs), rotating-disk electrode (RDE) voltammograms, and rotating ringdisk electrode (RRDE) voltammograms were performed at room temperature in 0.1 M KOH solutions, which were purged with high-purity oxygen for at least 30 min prior to each measurement.

3.

Results and discussions

3.1.

Reaction monitoring

GO can be contained several oxygen-containing functional groups, such as epoxide, hydroxyl, carbonyl, carboxyl, lactone, and quinone at the plane [22]. The functionalization/ adsorption of GO with nTP by the reaction between a positions of H to sulfur of nTP and epoxide groups of GO is shown in Fig. 1. A first indication of the reduction of GO is the color change of the GO and nTP dispersion, from brownish yellow to black, after 6 h stirring at room temperature. This suggests the restoration of electronic conjugation and the removal of

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Fig. 1 – Schematic structure of rGO produced by a simple chemical reduction of GO by nTP under stirring for 6 h, and the corresponding photographs in solution; insets: the oxidized form of nTP.

oxygen containing groups from the graphene sheet plane. As can be seen in the rGOnTP photograph, the rGO3TP is more black than rGO2TP; and the rGOTP has a brownish yellow color darker than the color of GO. This indicates that the 3TP acts as the strongest reductant for the preparation of graphene compared to 2TP and TP. The reduction progress of rGO3TP was first monitored by time-dependent UV–vis spectroscopy, as shown in Fig. 2. As can be seen the absorption spectra of the mixture of GO and 3TP is increasing, and the color of the solution gradually turns to black with increasing reaction time (Fig. 2 inset, (a)). This indicates that the increasing of resulting rGO3TP subsequently with partial restoration of p bonds within the carbon structure (Fig. 2 inset, (b)) via covalent bonding. The same behavior was observed by Wang’s group [52]. Moreover, a new peak forms between 243 nm and 247 nm with higher absorption (Fig. 2 inset, (c)), indicating a chemical change between GO and 3TP.

3.2.

Characterization of rGOnTP

The reduction of GO and concomitant functionalization/ adsorption of the resultant rGOnTP were further followed by FTIR spectroscopic measurements (Fig. 3). As shown in Fig. 3, the FTIR spectrum of GO shows a strong peak at around 1631 cm1 attributable to aromatic C@C, along with peaks characteristic of C@O stretching (1730 cm1), carboxyl, CAOAH (1417 cm1), and epoxy CAOAC (1224 cm1 and 985 cm1) [25,28,53] Although, GO contents very small amount of sulfur due to Hummer’s process [54] and a tiny signal at 1033 cm1 attributable to CAS. The reduction of GO to rGOnTP by nTP is evidenced by the dramatic decrease in peak intensity at 1730 cm1, 1417 cm1, 1226 cm1 and 985 cm1. Functionalization/ adsorption of rGOnTP with nTP is reflected by the appearance of new peaks at around 1033 cm1, which are most likely attributable to the CAS bond in the adsorbed nTP. After close observation, it can be distinguished that the intensity of C@O at 1630 cm1 and epoxy at 985 cm1 gradually decreased, and the intensity of the CAS bond gradually increased according to the degree of reduction, indicating the best reduction reaction by the strongest reducing agent 3TP. The rGO3TP was more successfully reduced than that of rGO2TP and rGOTP.

1417

1730 1631

1033 1224 985

C–OH

Fig. 2 – The UV–vis spectra of the GO dispersed in 3TP/MeCN solution as a function of reaction time; insets: corresponding time dependent photographs (a) and magnified UV–vis spectra at the respective wavelength (b) and (c).

C–S epoxy epoxy

500

1000

C=O C=C

1500

2000

2500

3000

3500

4000

Wavenumber, cm-1 Fig. 3 – FT-IR spectra of GO, rGOTP, rGO2TP, and rGO3TP.

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Details of the chemical changes during the reduction of GO by nTP were further elucidated by XPS measurements. Fig. 4(a) shows XPS survey spectra for GO, rGOTP, rGO2TP, and rGO3TP. As can be seen, the C/O atomic ratio significantly increased upon the reduction reaction. The C/O atomic ratio of GO, rGOTP, rGO2TP, and rGO3TP are calculated as 1.72, 1.83, 4.19 and 6.12, respectively, and the value of rGO3TP is comparable to other reports [20,25,26,55]. For instance, the C/O atomic ratios of rGO2TP and rGO3TP are 2.5 and 3.5 magnitudes higher than that of GO. C 1s and O 1s peaks were accompanied by the appearance of S 2p and S 1s peaks located around 162.5 eV and 228.2 eV, respectively (Fig. 4(a)). High-resolution C 1s XPS spectra were used to characterize the removal of oxygen groups and the formation of chemical bonds on the surface of GO before and after its functionalization/adsorption with nTP (Fig. 4(b)). As for GO, four different peaks centered at 285.1 eV, 286.3 eV, 287.0 eV, and 289.4 eV were observed, corresponding to CAC/C@C in unoxidized graphite carbon, CAOAH in hydroxyl group, CAOAC in epoxide group, and O@CAO in carboxyl group, respectively [25,27,53]. Very recently Whitby et al. also shows that upon reduction of GO the carboxyl group can be significantly re-

duced. However, a small number of carboxyl group detected that might be the part of peripheral acid anhydride groups [56,57]. After the reaction with TP, the peaks of oxygen containing groups have no significant reduction. After the reaction with 2TP, the peak reduction of oxygen containing groups was comparable to GO and rGOTP. A tiny new peak can be seen at 286.2 eV corresponding to CAS bond. Accordingly, in the C 1s spectra of rGO3TP, the peaks corresponding to the oxygen-containing groups are significantly weakened, especially the peak of CAOAC in epoxy group. The increase of the CAS bond intensity compared to rGO2TP indicated that most of the oxygen-containing groups were removed after the reduction. Moreover, a new characteristic peak corresponding to the CAS group located at 286.1 eV appeared, suggesting that 3TP is grafted onto the GO surface. The corresponding S 2p high-resolution XPS spectra can be seen in Fig. 4(c). As can be seen, the GO and rGOTP have no visible peak intensity for S 2p due to very small sulfur content. Nevertheless, the S 2p peak intensity increased little for rGO2TP with at.% 1.56 S. In S 2p high-resolution XPS spectra of rGO3TP, the peak intensity increased significantly with at.% 3.66 S.

rGO2TP

rGOTP GO

GO

rGOTP

167

rGO2TP

Fig. 4 – The XPS survey spectra (a), and high-resolution spectra of C 1s (b), S 2p (c) for GO, rGOTP, rGO2TP and rGO3TP, respectively.

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Raman spectroscopy was used to characterize the GO before and after reduction by nTP (Fig. 5(a)). The typical features in the Raman spectra are a G band at 1600 cm1 and a D band at 1355 cm1 for GO and rGOTP. The G band is usually assigned to the E2g phonon of C sp2 atoms, while the D band is a breathing mode of j-point phonons of A1g symmetry. A prominent D band is an indication of disorder in the Raman spectrum of the GO, originating from defects associated with vacancies [28,58]. The intensity ratio (ID/IG) of the D band to the G band of the GO is about 1.19. The chemical treatment with T results in no significant decrease in ID/IG (1.18), but after the chemical treatment with 2TP and 3TP, ID/IG significantly decreased to 1.11 and 1.02, respectively (Fig. 5(a)). This suggests that the chemical reducing reaction with nTP and GO is able to recover the aromatic structures by repairing defects. This ID/IG ratio is consistent with chemical reduction reports [21]. Therefore, we can conclude that the chemical reduction route is a more effective process in repairing the sp2 network of GO [48]. Moreover, the hypsochromic shifts of the G band were also registered for rGO2TP and rGO3TP as 1595 cm1 in previous work, due to the reduction of GO by 2TP and 3TP [28]. Furthermore, two different peaks at 1050 cm1 and 1454 cm1 have been observed in the rGO2TP and rGO3TP Ra-

man spectra, corresponding to CAH bond (in plane) and C@C (symmetric stretching) [58], whereas, those peaks were not found in rGOTP, indicating that GO was reduced, and that the 2TP and 3TP were successfully grafted via covalent bonding. The variations of the corresponding 2D band can be seen in the inset of Fig. 5(a). Fig. 5(b) shows typical TG curves for the GO and reduced rGOnTP. Generally for GO, there are two major steps in mass loss upon the increase of temperature. The loss of mass at around 100 °C can be ascribed to the removal of absorbed water, and at around 200 °C, it can be attributed to the decomposition of labile oxygen functional groups [59]. The reduction of GO with nTP improves the thermal stability of GO (Fig. 5(b)). For GO, there is a small mass loss (14.0 wt.%) around 100 °C, and a main mass loss of 88 wt.% around 200 °C owing to the decomposition of the labile oxygen functional groups, yielding CO2, CO and vapor. The final mass loss is 90 wt.% at 400 °C because of the removal of more stable oxygen functionalities. However, the final mass losses at 400 °C are 51 wt.% and 43 wt.% for rGO2TP and rGO3TP, respectively. The rGOTP has no significant variation from the GO curve. This indicates that the addition of 2TP and 3TP is favorable for improving the deoxygenation of GO. These results clearly

Fig. 5 – Raman spectra (a), inset: 2D band; and TGA curves (b) of GO, rGOTP, rGO2TP and rGO3TP.

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suggest that rGO2TP and rGO3TP materials possess higher reduction with successful restoration of graphitic structures and thermal stability than GO. Finally, the morphology of GO and rGO3TP were characterized by SEM and TEM (Supporting information Fig. SI1), which are usually employed to observe the layer-by-layer structure and lateral size of graphene. The 2-D GO exhibits a typically wrinkled and sheet-like structure (Fig. 6(a)). The layer-bylayer assembled 3-D like feature can be seen at rGO3TP image(Fig. 6(b)). This is supposed to be due to the aH to sulfur atom on the thiophene ring in 3TP (or 2TP, TP) that covalently linked in between to rGO sheets. As a result, an interlinked and layer-by-layer stacking rGO sheet structure can be appeared. The SEM images of GO and rGO3TP have a good agreement with the Fig. 1.

3.3.

The ORR on the rGO3TP and other typical rGO

For the electrocatalytic ORR investigations, we exploited the possibility of using rGO3TP (as the leading material of the rGOnTPs studied) as a novel metal-free catalyst for electrochemical reduction of O2. The CVs for oxygen reduction on the GO3TP and N2H4-reduced rGO (rGO;N2H4) and NaBH4-reduced rGO (rGO;NaBH4) electrodes are shown in Fig. 7(a), for a constant active mass loading (0.01 mg) in an aqueous O2saturated 0.1 M KOH solution. As can be seen, the onset potentials of ORR for the rGO;N2H4 and rGO;NaBH4 electrodes are at 0.2 V and 0.19 V (vs. Ag/AgCl), with cathodic reduction peaks around 0.4 V and 0.36 V (vs. Ag/AgCl), respectively. Upon reduction/adsorption of GO by 3TP, both the onset potential and the ORR reduction peak potential shifted positively to around 0.05 V and 0.25 V (vs. Ag/AgCl), respectively, accompanied by a concomitant increase in the peak current density (Fig. 7(a)). These results clearly demonstrated a significant enhancement in the ORR electrocatalytic activity for the rGO3TP with respect to other typical rGO electrodes. To further investigate the ORR performance, we carried out the linear sweep voltammetric (LSV) measurements on a RDE with rGO;N2H4, rGO;NaBH4 and rGO3TP in the same electrolyte solution. As shown in Fig. 7(b), the ORR at the rGO;N2H4, and rGO;NaBH4 electrodes commenced around 0.2 V (onset potential) [47,48], whereas the ORR onset potential at the rGO3TP electrode significantly shifted positively to 0.01 V with the higher current density. The time dependent synthesized rGO3TP has also employed for the ORR catalysis while the onset potential was improved according to the reduction time increasing (Supporting information Fig. SI2). It is, however,

Fig. 6 – Typical SEM images of GO (a) and rGO3TP (b) at 200 nm bar scale.

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the electrocatalytic activity of rGO3TP toward ORR is significantly better than that of the conventional rGO in terms of the onset potential and current density.

3.4.

The ORR kinetics on the rGO3TP and other typical rGO

To quantify the ORR electron transfer, we employed a RRDE technique, with which the amount of H2O2 generated at the disk electrode could be accurately determined [48]. The disk potential was scanned from 0.2 V to 0.12 V at a rotation speed of 900 rpm while holding the ring potential constant at 1.2 V, with a scan rate of 10 mV s1. This data is shown in Fig. 8 for rGO3TP (a), rGO;N2H4 (b), and rGO;NaBH4 (c). All three electrodes generated ring currents at the onset potential for the ORR. The number of electrons (n) transferred per O2 molecule was calculated from the KouteckyLevich equation [48], as shown in Fig. 8(d), in which the n value was found to be dependent on the potential for both the three electrodes. In particular, the n value increased with decrease in the negative potential. The n value for ORR at the rGO3TP electrode is always higher than that on the rGO;N2H4 and rGO;NaBH4 electrodes over the potential range covered in this study, within the range of the electron transfer number from 3.2 to 4, indicating that rGO3TP is a more efficient ORR electrocatalyst than rGO;N2H4 and rGO;NaBH4. All three electrodes generated ring currents at the onset potential for the ORR. The formation of H2O2 intermediate can be calculated from the ring and disk currents, and the equation used to calculate the percentage of H2O2 is given as follows: [60] % of H2 O2 ¼

2ir  100 ðjid j  N þ ir Þ

ð1Þ

where ir is the current at the ring electrode, id is the current at the disk electrode, and N (0.18) is the RRDE collection efficiency. Notably, both rGO;N2H4 and rGO;NaBH4 exhibit higher ring currents than rGO3TP (Fig. 8(a)–(c)). Specifically, the measured H2O2 yield for rGO3TP was 12% at a potential of 0.9 V, while those for rGO;N2H4 and rGO;NaBH4 were 39% and 29%, respectively (Fig. 8(e)). This is consistent with the relatively high calculated kinetic current density, jk [48] for ORR at the rGO3TP electrode with respect to the rGO;N2H4 and rGO;NaBH4 electrodes (Fig. 8(f)). Nevertheless, Ohsaka et al. [61] suggest that when ir is close to 0 and/or 1, the transferred electron numbers could be 4 and 2, respectively. Fig. 8(g) exhibits the variation of the ratio ir/Nid vs. the disk electrode potentials. According to Fig. 8(g), the ratio of the ir/Nid value for rGO3TP is very close to 0 than that of the value for rGO;N2H4 and rGO;NaBH4. This evaluation also suggests that the maximum amount of electron was transferred with rGO3TP among the three modified electrodes. Further, to understand the kinetics of the fuel cell reaction, a Tafel plot has been plotted (Fig. 8(h)). The Tafel slope is a good indicator of the mechanism of the electrode reaction, which is related to the change in the nature of adsorbed oxygen species and their coverage variation with the potential [62]. Generally, the Tafel slope of ca. 120 mV attributes the rate-determining process to the transport of oxygen to the electrocatalyst [63]. As can be calculated from Fig. 8(h), the Tafel slopes were ca. 120 mV, 210 mV and 123 mV for rGO3TP, rGO;N2H4 and rGO;NaBH4, respectively.

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Fig. 7 – The CV curves of oxygen reduction on the rGO3TP, rGO;N2H4 and rGO;NaBH4 electrodes at a scan rate of 50 mV s1 (a), and LSV curves for oxygen reduction on the rGO3TP, rGO;N2H4 and rGO;NaBH4 electrodes at a rotation rate of 900 rpm and at a scan rate of 10 mV s1 (b) in an O2-saturated 0.1 M KOH solution.

Fig. 8 – RRDE voltammograms for oxygen reduction on the rGO3TP (a), rGO;N2H4 (b) rGO;NaBH4 (c) modified electrodes at rotation speed of 900 rpm and at scan rate of 10 mV s1; the transferred electron number (d), % of H2O2 synthesis (e), current density (f), ir/Nid ratio (g), and Tafel plots (h) on rGO3TP, rGO;N2H4, and rGO;NaBH4 electrodes.

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Conclusions

A simple single-step method for the synthesis of rGO by chemical reduction of GO with nTP at room temperature has been described. The merit of this method lies in its low cost, non-use of toxic agent, efficient removal of oxygen-containing functional groups in GO, and easy applicability. It was also found that the 2TP and 3TP are grafted via covalent bonding on rGO. The final product rGO3TP as the material with maximum reduction performance is highly active towards ORR compared to rGO;N2H4 and rGO;NaBH4. The excellent effective reduction of GO with 3TP opens an exciting opportunity for the production of rGO on a large scale. This scalable process may enable important commercial applications for graphene materials. As an example, we have demonstrated the use of this material for electrocatalytic oxygen reduction.

Acknowledgements This research has supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010–0007864).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.carbon.2013.04.080.

R E F E R E N C E S

[1] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183–91. [2] Geim AK, MacDonald AH. Graphene: exploring carbon flatland. Phys Today 2007;60:35–41. [3] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless dirac fermions in graphene. Nature 2005;438:197–200. [4] Zhang YB, Tan YW, Stormer HL, Kim P. Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 2005;438:201–4. [5] Meyer JC, Geim AK, Katsnelson MI, Novoselov MI, Booth TJ, Roth S. The structure of suspended graphene sheets. Nature 2007;446:60–3. [6] Zhao X, Zhang QH, Chen DJ, Lu P. Enhanced mechanical properties of graphene-based poly(vinyl alcohol) composites. Macromolecules 2010;43:2357–63. [7] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [8] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385–8. [9] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [10] Kovtyukhova NI, Ollivier PJ, Martin BR, Mallouk TE, Chizhik SA, Buzaneva EV, et al. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem Mater 1999;11:771–8.

171

[11] Brodie BC. On the atomic weight of graphite. Philos Trans R Soc Lond 1859;149:249–59. [12] Cai W, Piner RD, Stadermann FJ, Park S, Shaibat MA, Ishii Y, et al. Synthesis and solid-state NMR structural characterization of 13C-labeled graphite oxide. Science 2008;321:1815–7. [13] He H, Riedl T, Lerf A, Klinowski J. Solid-state NMR studies of the structure of graphite oxide. J Phys Chem 1996;100:19954–8. [14] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [15] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Largescale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009;457:706–10. [16] Tung VC, Allen MJ, Yang Y, Kaner RB. High-throughput solution processing of large-scale graphene. Nat Nanotechnol 2009;4:25–9. [17] Zhu CZ, Guo SJ, Fang YX, Dong SJ. Reducing sugar: new functional molecules for the green synthesis of graphene nanosheets. ACS Nano 2010;4:2429–37. [18] Kim D, Ahmed MS, Jeon S. Different length linkages of graphene modified with metal nanoparticles for oxygen reduction in acidic media. J Mater Chem 2012;22:16353–60. [19] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [20] Dubin S, Gilje S, Wang K, Tung VC, Cha K, Hall AS, et al. A one-step, solvothermal reduction method for producing reduced graphene oxide dispersions in organic solvents. ACS Nano 2010;4:3845–52. [21] Yang S, Zhi L, Tang K, Feng X, Maier J, Mullen K. Efficient synthesis of heteroatom (N or S)-doped graphene based on ultrathin graphene oxide-porous silica sheets for oxygen reduction reactions. Adv Funct Mater 2012;22:3634–40. [22] Tanaka H, Obata S, Saiki K. Reduction of graphene oxide at the interface between a Ni layer and a SiO2 substrate. Carbon 2013;59:472–8. [23] Ramesha GK, Sampath S. Electrochemical reduction of oriented graphene oxide films: an in situ Raman spectroelectrochemical study. J Phys Chem C 2009;113:7985–9. [24] An SJ, Zhu Y, Lee SH, Stoller MD, Emilsson T, Park S, et al. Thin film fabrication and simultaneous anodic reduction of deposited graphene oxide platelets by electrophoretic deposition. J Phys Chem Lett 2010;1:1259–63. [25] Kaminska I, Das MR, Coffinier Y, Niedziolka-Jonsson J, Sobczak J, Woisel P, et al. Reduction and functionalization of graphene oxide sheets using biomimetic dopamine derivatives in one step. ACS Appl Mater Interfaces 2012;4:1016–20. [26] Shin HJ, Kim KK, Benayad A, Yoon SM, Park HK, Jung IS, et al. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv Funct Mater 2009;19:1987–92. [27] Wang Y, Shi Z, Yin J. Facile synthesis of soluble graphene via a green reduction of graphene oxide in tea solution and its biocomposites. ACS Appl Mater Interfaces 2011;3:1127–33. [28] Chen W, Yan L, Bangal PR. Chemical reduction of graphene oxide to graphene by sulfur-containing compounds. J Phys Chem C 2010;114:19885–90. [29] Xu LQ, Yang WJ, Neoh K-G, Kang E-T, Fu GD. Dopamineinduced reduction and functionalization of graphene oxide nanosheets. Macromolecules 2010;43:8336–9. [30] Fan Z-J, Kai W, Yan J, Wei T, Zhi L-J, Feng J, et al. Facile synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide. ACS Nano 2011;5:191–8.

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[31] Zhu Y, Stoller MD, Cai W, Velamakanni A, Piner RD, Chen D, et al. Exfoliation of graphite oxide in propylene carbonate and thermal reduction of the resulting graphene oxide platelets. ACS Nano 2010;4:1227–33. [32] Long D, Li W, Ling L, Miyawaki J, Mochida I, Yoon S-H. Preparation of nitrogen-doped graphene sheets by a combined chemical and hydrothermal reduction of graphene oxide. Langmuir 2010;26:16096–102. [33] Wang GX, Yang J, Park J, Gou XL, Wang B, Liu H, et al. Facile synthesis and characterization of graphene nanosheets. J Phys Chem C 2008;112:8192–5. [34] Si Y, Samulski ET. Synthesis of water soluble graphene. Nano Lett 2008;8:1679–82. [35] Rubio M, Orti E, Pou-Amerigo R, Merchan M. Electronic spectra of 2,2 0 -bithiophene and 2,2 0 :5 0 ,200 -terthiophene radical cations: a theoretical analysis. J Phys Chem A 2001;105:9788–94. [36] Fichou D, editor. Handbook of oligo and polythiophenes. Weinheim, Germany: Wiley-VCH; 1999. [37] Kumar SS, Kumar CS, Mathiyarasu J, Phani KL. Stabilized gold nanoparticles by reduction using 3,4ethylenedioxythiophene-polystyrenesulfonate in aqueous solutions: nanocomposite formation, stability, and application in catalysis. Langmuir 2007;23:3401–8. [38] Roncali J, Garnier F, Lemaire M, Garreau R. Poly mono-, biand trithiophene: effect of oligomer chain length on the polymer properties. Synth Met 1986;15:323–31. [39] Ahmed MS, Jeon S. The nanostructure of nitrogen atom linked carbon nanotubes with platinum employed to the electrocatalytic oxygen reduction. J Nanosci Nanotechnol 2013;13:306–14. [40] Zhang JD, Vukmirovic MR, Xu Y, Mavrikakis M, Adzic RR. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates. Angew Chem Int Ed 2005;44:2170–3. [41] Wu G, More KL, Johnston CM, Zelenay P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011;332:443–7. [42] Yang W, Wang Y, Li J, Yang X. Polymer wrapping technique: an effective route to prepare Pt nanoflower/carbon nanotube hybrids and application in oxygen reduction. Energy Environ Sci 2010;3:144–9. [43] Wang S, Jiang SP, White TJ, Wang X. Synthesis of Pt and Pd nanosheaths on multi-walled carbon nanotubes as potential electrocatalysts of low temperature fuel cells. Electrochim Acta 2010;55:7652–8. [44] Wang S, Yang F, Jiang SP, Chen S, Wang X. Tuning the electrocatalytic activity of Pt nanoparticles on carbon nanotubes via surface functionalization. Electrochem Commun 2010;12:1646–9. [45] Gong KP, Du F, Xia ZH, Durstock M, Dai LM. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009;323:760–4. [46] Serov A, Kwak C. Review of non-platinum anode catalysts for DMFC and PEMFC application. Appl Catal B: Environ 2009;90:313–20. [47] Wang S, Yu D, Dai L, Chang DW, Baek J-B. Polyelectrolytefunctionalized graphene as metal-free electrocatalysts for oxygen reduction. ACS Nano 2011;5:6202–60209.

[48] Ahmed MS, Jeon S. New functionalized graphene sheets for enhanced oxygen reduction as metal-free cathode electrocatalysts. J Power Sources 2012;218:168–73. [49] Yang Z, Yao Z, Li G, Fang G, Nie H, Liu Z, et al. Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction. ACS Nano 2012;6:205–11. [50] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80. 1339-1339. [51] Min S, Lu G. Dye-sensitized reduced graphene oxide photocatalysts for highly efficient visible-light-driven water reduction. J Phys Chem C 2011;115:13938–45. [52] Mukherji A, Seger B, Lu GQM, Wang L. Nitrogen doped Sr2Ta2O7 coupled with graphene sheets as photocatalysts for increased photocatalytic hydrogen production. ACS Nano 2011;5:3483–92. [53] Liao S-H, Liu P-L, Hsiao M-C, Teng C-C, Wang C-A, Ger M-D, et al. One-step reduction and functionalization of graphene oxide with phosphorus-based compound to produce flameretardant epoxy nanocomposite. Ind Eng Chem Res 2012;51:4573–81. [54] Shan C, Yang H, Song J, Han D, Ivaska A, Niu L. Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene. Anal Chem 2009;81:2378–82. [55] Zhou Y, Bao QL, Tang LAL, Zhong YL, Loh KP. Hydrothermal dehydration for the ‘‘green’’ reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties. Chem Mater 2009;21:2950–6. [56] Whitby RLD, Korobeinyk A, Glevatska KV. Morphological changes and covalent reactivity assessment of single-layer graphene oxides under carboxylic group-targeted chemistry. Carbon 2011;49:718–36. [57] Whitby RLD, Gun’ko VM, Korobeinyk A, Busquets R, Cundy AB. K. La´szlo, et al., Driving forces of conformational changes in single-layer graphene oxide. ACS Nano 2012;6:3967–73. [58] Svedberg F, Alaverdyan Y, Johansson P, Kall M. Raman spectroscopic studies of terthiophenes for molecular electronics. J Phys Chem B 2006;110:25671–7. [59] Chen W, Yan L, Bangal PR. Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves. Carbon 2010;48:1146–52. [60] Byon HR, Suntivich J, Horn YS. Graphene-based non-noblemetal catalysts for oxygen reduction reaction in acid. Chem Mater 2011;23:3421–8. [61] Deab MSE, Sotomura T, Ohsaka T. Oxygen reduction at electrochemically deposited crystallographically oriented Au(1 0 0)-like gold nanoparticles. Electrochem Commun 2005;7:29–34. [62] Dhavale VM, Kurungot S. Tuning the performance of low-Pt polymer electrolyte membrane fuel cell electrodes derived from Fe2O3@Pt/C core–shell catalyst prepared by an in situ anchoring strategy. J Phys Chem C 2012;116:7318–26. [63] Ahmed MS, Kim D, Jeon S. Covalently grafted platinum nanoparticles to multi walled carbon nanotubes for enhanced electrocatalytic oxygen reduction. Electrochim Acta 2013;92:168–75.