A method for the catalytic reduction of graphene oxide at temperatures below 150 °C

A method for the catalytic reduction of graphene oxide at temperatures below 150 °C

CARBON 4 9 ( 2 0 1 1 ) 3 0 2 4 –3 0 3 0 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon A method for the catal...

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CARBON

4 9 ( 2 0 1 1 ) 3 0 2 4 –3 0 3 0

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

A method for the catalytic reduction of graphene oxide at temperatures below 150 C Junfeng Li a, Hong Lin

a,* ,

Zhilong Yang a, Jianbao Li

a,b

a

State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Key Laboratory for Hainan Advantage Resources and Applied Technology in Chemical Engineering Material of Ministry of Education, Hainan Provincial Key Laboratory of Research on Utilization of Si–Zr–Ti, Materials and Chemical Engineering Institute, Hainan University, Haikou 570228, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Anhydrous AlCl3 was used to increase the reducing ability of sodium borohydride (NaBH4)

Received 23 November 2010

for removing oxygen functional groups on graphene oxide (GO) at a reaction temperature

Accepted 13 March 2011

below 150 C, which provided an extendable, mild, and controllable route for large-scale

Available online 17 March 2011

production of graphene. The influences of reducing temperature and reducing time on the electrical conductivity of reduced GO were examined. Structural evolution during the reduction of GO was studied by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and elemental analysis.  2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene, as an atomic-layer-thick 2D carbon material, has attracted a great deal of attention due to its excellent electrical, thermal, optical and mechanical properties, and has extensive potential application to electronics [1–5], catalysis [6], sensors [7–11], supercapacitors [12], and batteries [13,14]. In the past few years, many methods, such as micromechanical exfoliation, chemical vapor disposition, chemical reduction of graphene oxide (GO), have been used to prepare single or few-layer graphene. Chemical reduction of GO is an efficient approach to the large-scale production of graphene. Several reducing agents, such as hydrazine [15,16], hydrazine derivate [17], vitamin C [18], bacteria [19], sodium hydride [20], NaBH4 [21,22], reducing sugar [23], and HI [24,25], were used to reduce GO. Among the above reducing agents, NaBH4 is an effective and stable reducing agent in general organic reducing reactions, and can selectively remove different functional groups

with the aid of different catalysts. NaBH4 in combination with anhydrous AlCl3 may conveniently remove the hydroxyl in diaryl–arylalkylcarbinols and the carbonyl bonded to diaryl and aryl alkyl ketones [26], whereas the NaBH4–ZnCl2 reagent system can reduce the aromatic acid into the corresponding alcohols in refluxing THF [27]. Herein, an easy, extendable, and controllable method for reducing GO in air atmosphere at relatively low temperatures (6150 C) using anhydrous AlCl3 as catalyst and NaBH4 as reducing agent is presented. The reducing agent system, NaBH4 with anhydrous AlCl3, is not highly poisonous and explosive like hydrazine and its hydrazine derivate, not violent like sodium hydride, more extendable compared with vitamin C and reducing sugar, and not strongly corrosive like HI. The effects of anhydrous AlCl3, reducing temperatures, and reducing time on the electrical conductivity and oxygen functional groups have been investigated in detail. The variation of GO structure in the reducing process has also been detailedly demonstrated.

* Corresponding author: Fax: +86 10 62772672. E-mail address: [email protected] (H. Lin). 0008-6223/$ - see front matter  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.03.022

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2.

Experimental

2.1.

Reduction of GO

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GO was prepared using a modified Hummers and Offeman’s method [28] from natural graphite (2 g, 300 mesh and 500 mesh). Typically, 120 ml GO suspension (2 mg/ml) was dispersed in 200 ml N, N-dimethylformamide (DMF) by ultrasonication, and 3 g NaBH4 and 0.2 g anhydrous AlCl3 were dissolved in 120 and 80 ml DMF, respectively. The above three kinds of solutions were mixed together and heated at 30–150 C for 0.25–3 h. Twenty-five milliliter suspension (1 g/ml) of reduced graphene oxide (RGO) was filtered by mixed cellulose ester filter membrane, and the filtration film was dried at 50 C for 10 h.

2.2.

Characterization

The electrical conductivity of square RGO paper (2 · 2 cm in size) dried under vacuum at 100 C for 6 h was measured by a digital four-point probe system (SX1934, Suzhou, China). The thickness of the RGO paper was measured by a step profiler (XP1, MTS, USA). To get reliable electrical conductivity data, eight different sites of each sample were measured by the four-point probe system, and three different sites of each sample were measured by the step profiler. The Fourier-transform infrared (FT-IR) spectroscopy were obtained on a Fourier transform infrared spectrometer (NICOLET560, Nicolet, USA) with a resolution of 4 cm 1. The X-ray photoelectron spectroscopy (XPS) were performed on X-ray photoelectron spectrometer (PHI-5300, PE-PHI, USA), whose resolution is 0.8 eV.

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The binding energy is calibrated with C 1s = 284.8 eV. The layer-to-layer distance of GO sheets (d-spacing) was determined by X-ray diffraction (XRD) on an X-ray diffractometer (D/max-2550, Japan), operated at 200 kV, 45 mA, with experimental condition as follows: a step width 0.02 and a scanning range of 5–60. Raman spectroscopy was performed on a laser micro-Raman spectroscope (Hr800, Horiba Jobin Yvon, France) with a laser beam wavelength of 633 nm. Transmission electron microscopy (TEM) images were obtained using a transmission electron microscope (JEM-2010F, JEOL Ltd., Japan) operated at 200 kV. Scanning electron microscopy (SEM) images were obtained using a scanning electron microscope (SS-550, Shimadzu, Japan). Atomic force microscopic (AFM) images of the RGO reduced at 140 C were taken on an Atomic force microscopy (SPI3800, Seiko Instruments, Japan) with the scanning rate at 0.8–1 Hz. The samples for AFM were prepared by dropping aqueous suspension (0.05 mg/mL) of the RGO reduced at 140 C on freshly cleaved mica surface and dried under vacuum at 80 C. The elemental analysis was measured on an elemental analyser (CE440, Exeter analytical, Inc.,USA).

3.

Results and discussion

Fig. 1a shows the electrical conductivity of RGO reduced with and without anhydrous AlCl3 at 140 C for 3 h. The FT-IR spectra of the two samples show that the peaks at 1390 and 1406 cm 1 in Fig. 1b, which belong to the vibrations of C–OH and O–H [29,30], are much smaller in the RGO reduced with anhydrous AlCl3 than that without it. The electrical conductivity of RGO reduced with anhydrous AlCl3, therefore, is

Fig. 1 – Electrical conductivity of RGO reduced with and without anhydrous AlCl3 as catalyst (a), and the influences of reducing temperatures (b) and reducing time (c) on the electrical conductivity of RGO.

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Fig. 2 – FT-IR spectra of RGO reduced at different temperatures (a) and time (c), and (b) is an enlarging part of (a) between 1250 and 1800 cm 1.

improved more than four times, which is attributed to that more hydroxyls have been removed in the presence of anhydrous AlCl3, as shown in Fig. 1b. The influences of reducing temperatures and reducing time on the electrical conductivity of RGO reduced with anhydrous AlCl3 are shown in Fig. 1c and d. It shows that the highest electrical conductivity is obtained at 140 C in Fig. 1c, whereas the electrical conductivity of RGO in Fig. 1d increases with increasing the reducing time from 0.25 to 3 h. The differences of the electrical conductivity of RGO reduced at 140 C in Fig. 1c (580 S m 1) and Fig. 1d (620 S m 1) are due to that two different kinds of natural graphite were used as starting materials. Fig. 2 depicts FT-IR spectra of GO and RGO reduced at different temperatures and time. The peaks at 1060, 1186, 1226, and 1290 cm 1 are assigned to the stretching vibration of C–O (alkoxy), phenolic OH [31], C–O (epoxy) [30], and C–OH bending [32], respectively. The peaks at 1720, 1640, 1620, 1566, and 1393 cm 1 are attributed to the stretching vibration of [email protected] [30], aromatic C–C, Ph–CO [29], deformed C–C [33], C–OH [29], respectively. The deformed C–C stretching vibration at 1566 cm 1 is due to the presence of the neighbouring epoxy groups [33]. The peaks (1060–1290 cm 1) corresponding to oxygen functional groups dramatically decrease with increasing the reducing temperatures from 80 to 140 C in Fig. 2a and with increasing the reducing time from 0.25 to 3 h in Fig. 2c. The [email protected] peak at 1720 cm 1 in Fig. 2b disappears when the reducing temperature is above 55 C, while the Ph–

Fig. 3 – C/O atom ratio of GO and RGO reduced at different temperatures measured by elemental analysis. CO and C–OH peaks in Fig. 2b have no obvious changes at different reducing temperatures. In addition, the [email protected] peak in Fig. 2c completely disappears until the reducing time reaches 3 h. The deformed C–C peak at 1566 cm 1 in Fig. 2c decreases with increasing the reducing time and the aromatic C–C at 1640 cm 1 simultaneously increases, indicating that the GO is gradually reduced into graphene. All above FT-IR results suggest that most of the oxygen functional groups can be removed, especially a carbonyl group being easily reduced, but a

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Fig. 4 – Relative contents of bonds in GO and RGO obtained by fitting C 1s peaks in the XPS spectra of GO and RGO.

Fig. 5 – XRD patterns of GO and RGO reduced at (a) different reducing temperatures (30–150 C) and (b) different reducing time (0.25–3 h).

hydroxyl group is relatively difficult to be removed. The elemental analysis result in Fig. 3, showing that the C/O atom ratios from GO to the RGO reduced at 140 C change from 1.09 to 5.58, is also consistent with the above FT-IR results. Fig. 4 shows the relative contents of different bonds (C–C, C–OH, C–O–C, [email protected], C(O)–O) in GO and RGO obtained by fitting C 1s peaks in the XPS spectra of GO and RGO (Supplementary Fig. S1). Two principles should be met due to different fitting results being able to be obtained for the same XPS spectra [34]: (1) the changing tendency of the relative content of C–C bond in GO and RGO should be consistent with the changing tendency of the electrical conductivity in GO and RGO; (2) according to the FT-IR results, the relative content of [email protected] bond should be as low as possible among different fitting results for the same XPS spectrum. The changing tendencies of the C–C bond contents in Fig. 4a and b are able to be accordant with the changing tendencies of the RGO electrical conductivity in Fig. 1c and d in the case of keeping the [email protected] content as low as possible for each XPS spectrum, which implies that the C–C content is exactly a main factor that affects the electrical conductivity of RGO. The C–O–C and [email protected] contents of the RGO reduced at 100 C in Fig. 4a markedly decrease on contrary to that in GO, whereas the C–OH and C(O)–O contents correspondingly

Fig. 6 – Raman D/G intensity ratios of GO and RGO reduced at different temperatures. The inset shows the Raman spectra of GO, graphite, and RGO reduced at 140 C. Error bars are based on Raman spectra taken at 3–9 different spots on each sample. increase, all of which support that the C–O–C and [email protected] bonds probably change into the C–OH and C(O)–O bonds. Most of the C–O–C, [email protected], and C(O)–O bonds are reduced at 140 C, and the

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Fig. 7 – Different images of RGO sheets reduced at 140 C, (a) TEM image, (b) SEM image on mica, (c) a tapping mode AFM image on mica. C–OH bond (20%) is the major residual oxygen functional group, which is the main defect that restricts the electrical conductivity of RGO. The decrease of the electrical conductivity of RGO at 150 C in Fig. 1c is due to that the reducing ability of NaBH4 with Anhydrous AlCl3 for removing oxygen functional groups decreases at the overhigh reducing temperature, which is supported by the small increase of the hydroxyl, carbonyl and epoxy contents from 140 to 150 C in Fig. 4a. The increase of the electrical conductivity of RGO in Fig. 1d with increasing the reducing time from 2 to 3 h is primarily attributed to a large decrease (10%) of the hydroxyl content (as shown in Fig. 4b). The XRD patterns of graphite, GO and RGO reduced at different reducing temperatures and time are displayed in Fig. 5. The d-spacing of graphite sharp feature peak (0 0 2) at 26.38 ˚ . The GO feature peak at 9.26, whose corresponding is 3.38 A ˚ , disappears when the reducing temperad-spacing is 9.55 A ture increases above 80 C, which indicates that most of the oxygen functional groups, having marked effects on the d-spacing, have been reduced above 80 C. In addition, the d-spacing of RGO reduced from 30 to 140 C gradually gets ˚ , and reaches 3.65 A ˚ at 140 C, which implies close to 3.38 A that the structure of RGO is gradually getting close to that of graphene with increasing the reducing temperatures. ˚) Afterwards, the d-spacing of RGO reduced at 150 C (3.73 A is bigger than that at 140 C, which implies that there are more oxygen functional groups in the RGO at 150 C. The feature peaks (0 0 2) of the RGO reduced for 0.25–3 h almost have no shift (Fig. 4b), which indicates that the content of carbonyl and epoxy in GO or RGO is the main factor that affects the dspacing values of GO and RGO, combined with the changes of the relative contents of carbonyl ([email protected]), epoxy (C–O–C), and hydroxyl (C–OH) bonds in GO and RGO (as shown in Fig. 4). The average intensity ratios of the prominent Raman bands, namely D and G bands at wave numbers of 1339 and 1586 cm 1, for GO and RGO reduced from 30 to 150 C are

shown in Fig. 6. The D/G intensity ratios increase from GO to the RGO reduced at 100 C, decrease for the RGO reduced from 100 to 140 C, and increase again for the RGO reduced from 140 to 150 C. The first increase stage of the D/G intensity ratios is attributed to an increase in the number of small crystalline graphene domains, the next decrease is due to an increase of the average size of the crystalline graphene domains with increasing the reducing temperatures [35], and the last increase of the D/G intensity ratios at 150 C is resulted from the decrease of reducing ability of NaBH4 with Anhydrous AlCl3 because of overhigh reducing temperature. This result is also accordant with the above FT-IR result, XPS result and XRD result. The inset in Fig. 6 shows the Raman spectra of GO, RGO reduced at 140 C, and graphite, which indicate that GO is well reduced into graphene. Fig. 7 displays TEM image, SEM image, and AFM image of RGO reduced at 140 C. TEM and SEM images show that single or few-layer graphene with lots of wrinkles was obtained. The thickness of graphene sheets at thinnest part is measured to be around 0.6 nm by the height profile of AFM image, which is accordant with the thickness of the graphene reduced by pure hydrazine reported in the previous literature [36].

4.

Conclusions

An effective and mild catalytic reducing method for GO using anhydrous AlCl3 as catalyst and NaBH4 as reducing agent at low reducing temperatures has been demonstrated. The reducing temperatures and reducing time intensively affect the electrical conductivity of RGO. In addition, the vast majority of carbonyl and epoxy groups have been changed into hydroxyl groups, and most of the carboxyl groups and part of the hydroxyl groups are able to be removed in the presence of anhydrous AlCl3. Moreover, the other catalysts may also be used to further improve the reducing ability of NaBH4 and selectively remove remaining oxygen functional groups,

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primarily hydroxyl groups, to obtain pure graphene, although the electrical conductivity of RGO is not so high at present. We believe that the extendable reducing method presented here may pave the way for a low temperature large-scale preparation of graphene in future.

Acknowledgements This work was supported by the national ‘‘863’’ (No. 2007AA03Z524) program and the research fund for the doctoral program of higher education of China (No. 20090002110010).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.03.022.

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