One-step solid state preparation of reduced graphene oxide

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One-step solid state preparation of reduced graphene oxide Jianfeng Shen, Tie Li, Yu Long, Min Shi, Na Li, Mingxin Ye

*

Center of Special Materials and Technology, Fudan University, 200433 Shanghai, China

A R T I C L E I N F O

A B S T R A C T

Article history:

We have developed an easy and scalable chemical reduction method assisted by microwave

Received 18 October 2011

irradiation for the synthesis of reduced graphene oxide (RGO) nanosheets in solid state.

Accepted 5 January 2012

The as-synthesized RGO is characterized by Fourier transform infrared spectroscopy,

Available online 11 January 2012

Raman spectroscopy, thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. It is revealed that the bulk of the oxygen-containing functional groups are removed from graphene oxide with this one-step reduction method and monolayer RGO sheets are got from its N,N-dimethyl formamide solution. It is found that the ammonium bicarbonate plays a key role in the preparation of RGO. Considering the analysis results, a mechanism for the formation of RGO is proposed. Besides being eco-friendly, when compared to previous chemical techniques, this process has several advantages like low cost, simplicity and short processing times, which may find practical applications in the preparation of graphene-based composites.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene, a unique single layer of two-dimensional carbon lattice, has stimulated great interest due to its extraordinary electronic, thermal and mechanical properties [1–6]. It has shown intensive promising applications in electronic devices [7], batteries [8], and composites [9,10]. Researchers have developed many methods to prepare this promising novel nanomaterial, such as mechanical exfoliation [11], chemical vapor deposition (CVD) [12], transfer printing [13], epitaxial growth [14], organic synthesis [15], and oxidation–dispersion–reduction [16]. Actually, among these methods, chemical reduction of graphene oxide sheets (GOS) can produce graphene in large quantities by using graphite as a raw material. Since graphite is cheap and readily available, this chemical approach is likely the least expensive and most effective method for the large-scale production of graphene [17]. Chemical exfoliation methods based on the Hummers’ method, oxidation of graphite into graphite oxide (GO), followed by sonication and reduction, have recently drawn much attention due to the advantages of potentially

low-cost and solution-processed fabrication [18]. In many cases, graphene is not directly employed to construct nanoand macro-structures because it tends to aggregate with each other and is hard to disperse in aqueous solution. Subsequently, graphene oxide sheets (GOS) that are easily dispersible in an aqueous environment are employed as starting units for graphene-based nano-structured materials, including graphene based membranes and three-dimensional (3D) macrostructures. GOS are kind of insulators since they consist of intact graphitic regions interspersed with sp3hybridized carbons related to hydroxyl and epoxy functional groups. The introduction of such sp3 components destroys the p-system of the sp2 framework, which leads to the disruption of conduction pathways. Toward this end, GOS (oxidized form of graphene) can be converted to reduced GO (RGO), which is less expensive and exhibits surface properties amenable to graphene [16]. Generally, the chemical reduction of GOS was carried out using strong reductant. Typical reduction reagents are hydrazine [19], hydroquinone [20], and NaBH4 [21]. However, these reagents are very hazardous or highly poisonous, and therefore their use should be avoided in the

* Corresponding author: Fax: +86 021 55664094. E-mail address: [email protected] (M. Ye). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.01.019

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large-scale production of RGO. Recently, several ‘‘green’’ agents, such as vitamin C, ethylene glycol, aluminum powder, hydrohalic acid and alkali, have been developed as the reductant in the synthesis of RGO [22–26]. But these reactions are very slow. In addition, to obtain stable RGO colloidal dispersions, either an organic stabilizer (polymer or surfactant) or an exact control of pH value of the dispersions is usually needed to prevent irreversible agglomeration of RGO sheets in solution [27–29]. The presence of foreign chemical species is generally undesirable for most applications. Moreover, the as-synthesized RGO cannot be re-dispersed in solution after the stabilizers are removed. Therefore, it is still a great challenge to develop a green and easy approach to the large-scale synthesis of stable RGO dispersions. Microwave irradiation (MWI) has been demonstrated for the synthesis of many nanomaterials with controlled shape and size without the need for high pressure or high temperature [30]. It has several advantages, such as higher reaction rates, selectivities, and realizing reactions in a very short time. However, to the best of our knowledge, only a few studies related to the preparation of graphene by using microwave synthesis method have been reported [30–32]. In addition, most of these methods use pretreated graphite as the precursor and the mechanism of these methods are not clearly addressed. With the aim of expanding the processability and application of RGO, in this study, we investigate the largescale synthesis of RGO with a solid-state reaction. In this study, GO and ammonium bicarbonate were used as the precursors. Under MWI, they will exfoliate violently due to volatile gaseous species (such as NH3 and CO2) released and well reduced RGO can be got quickly. Comparing with previous methods, this chemical reduction method assisted by MWI in solid state shows such advantages as easy and scalable, which may find practical applications in the preparation of graphene-based composites.

vacuum. The whole process is described in Fig. 1. As a control experiment, RGO-2 was prepared with the same method without using ammonium bicarbonate.

2.3.

Experimental section

2.1.

Chemicals and materials

Pristine graphite was purchased from Qingdao BCSM Co., Ltd. The purity of the pristine graphite is 98.5%. Ammonium bicarbonate was supplied by Shanghai Chemical Reagent. All other reagents were at least of analytical reagent grade and used without further purification.

2.2.

Preparation of RGO

GO was obtained by the modified Hummers method [33–36]. GO (100 mg) and ammonium bicarbonate (1 g) was ground to a fine powder. This powder was put into a microwave oven (WP800P23-K, 2450 MHz, 800 W) for 60 s. Under MWI, the precursors reacted rapidly, accompanied by lightening (caution: excessive microwave intensity, subjecting the reaction mixture to longer times or large volumes of reacting mixtures may cause violent explosions). On completion of the reaction, the mixture was cooled to room temperature and then washed with water for several times until, the pH of the filtrate was neutral. The remaining black solid was dried under

Characterization

Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Thermo Nicolet IS10 spectrometer. Solid samples were imbedded in KBr disks. The spectrum was generated, collected 16 times, and corrected for the background noise. Raman spectra were recorded on a Dilor LABRAM-1B multi-channel confocal microspectrometer with 514 nm laser excitation. Thermo-gravimetric analysis (TGA) was conducted in nitrogen atmosphere with a heating rate of 10 C/ min using a Netzsch TG 209F1. Before the tests, all the samples were carefully grinded to powders to ensure sufficient diffusion of heat. X-ray diffraction (XRD) powder patterns were taken on D/max-cB diffractometer using Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) was recorded on XR 5 VG (UK). The XPSPEAK software was used to deconvolute the narrow-scan XPS spectra of the C1s of the samples, using adventitious carbon to calibrate the C1s binding energy (284.5 eV). Scanning electron microscopy (SEM) was performed with a Philips XL30 FEG FE-SEM instrument at an accelerating voltage of 25 kV. Sample was sputter-coated with gold to improve the contrast. Transmission electron microscopy (TEM) was performed with a JEOL JEM-2100F. To evaluate the dispersion state of the sheets in the suspensions, atomic force microscopy (AFM) was carried out using a Multimode Nano4 in the tapping mode. The AFM samples were prepared by spin coating the dispersion solutions of GO and RGO. GO was dispersed in deionized water, while RGO was dispersed in N,N-dimethyl formamide (DMF). Water bath sonication was performed with a JL-60 DTH sonicator (50 W).

3. 2.

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Results and discussion

While earlier fundamental researches have been focused on the micromechanical cleavage of highly crystalline graphite for high-quality graphene sheets, recent efforts are geared toward producing the graphene sheets in a controlled and scalable manner. Chemical reduction of GOS has been one of the most effective methods for large-scale production of RGO. However, until now, most reduction of GOS or modification of RGO composite involves complicated processing steps and harsh reaction condition with lengthy experiment time. In this study, we used MWI method to minimize the reaction time. The main advantage of MWI over other conventional heating is that the reaction is uniform and rapid. Following MWI for 60 s in the presence of ammonium bicarbonate, the sample changed to black, indicating the reduction of GO. This reaction is supposed to lead to deoxygenation of GO and to significant restoration of the sp2 carbon sites [37]. Fig. 2 shows the FTIR spectra of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d). In the spectrum of pristine graphite (Fig. 2a), the peak at 1350 cm 1 is from skeletal vibration of unoxidized graphitic domains. The peaks at 2300 cm 1 is because of the CO2 absorption in air. It is often used as a

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Fig. 1 – Procedures used in this study for preparation of RGO.

Fig. 2 – FTIR spectra of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d).

reference to considering the intensity of other absorption peaks [38]. The spectrum of GO (Fig. 2b) shows a broad absorption band at 3400 cm 1, which is related to the O–H groups. Besides, the oxygen-containing functional groups of GO are revealed by the bands at 1075, 1235, 1400, and 1735 cm 1. These bands correspond to C–O stretching vibrations, C–OH stretching peak, carboxyl C–O, and C@O groups [36]. The peak at 1620 cm 1 can be assigned to the vibrations of the adsorbed water molecules and also the contributions from the skeletal vibrations of un-oxidized graphitic domains. In contrast, after the GO was chemically reduced (Fig. 2c), the absorption band of O–H at 3400 cm 1 almost vanished, demonstrating that the bulk of the oxygen containing functional groups were removed from GOS with reduction [39]. Moreover, when comparing with RGO, the intensity of those oxygen-containing peaks in RGO-2 are still very high (Fig. 2d), suggesting that the reduction of GO is rarely complete without adding ammonium bicarbonate. Raman spectroscopy is a powerful non-destructive technique widely used to distinguish ordered and disordered crystal structures of carbon. In the spectrum of raw graphite (Fig. 3a), the G band at 1580 cm 1 (arising from the first order scattering of the E2g phonon) is related to of sp2 C atoms. The D band at 1350 cm 1 (arising from a breathing mode of

j-point photons of A1g symmetry) corresponds to defects in the curved graphene sheet. Comparing with raw graphite, in the Raman spectrum of GO (Fig. 3b), the G band is broadened and upshifted to 1600 cm 1 due to the presence of isolated double bonds that resonate at higher frequencies than that of graphite. At the same time, the intensity of the D band of GO at 1350 cm 1 increases substantially, indicating the reduction in size of the in-plane sp2 domains, which is due to the extensive oxidation and ultrasonic exfoliation. Comparing with GO, RGO does not show noticeable changes in the D/G ratio (Fig. 3c). This observation suggests that, while most of the oxygenated groups have been removed, the vacant lattice sites produced with carbon atom removal in the form of CO2 during graphite oxidation remain unchanged during the reduction process [40]. As to RGO-2 (Fig. 3d), the D/G ratio is very close to GO. Shape and position of the 2D band are the key parameters indicating information of the layer number of graphene sheets [41,42]. The 2D position of single-layer graphene sheets is usually observed at 2679 cm 1, while the 2D band of multilayer shifts to higher wavenumbers by 19 cm 1. From Fig. 3c, the observed 2D bands of RGO with a nearly symmetrical shape were centered at 2680 cm 1, suggesting formation of single or bilayer RGO sheets. While the shape of 2D bands of GO and RGO-2 are similar with each other, suggesting that RGO-2 has not been greatly reduced, which is consistent with the results of FTIR, TGA and XRD analysis. Therefore, Raman spectroscopy confirms that RGO is greatly exfoliated and the D band arises mostly from the edges and steps present in the RGO sheets. TGA curves of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d) are shown in Fig. 4. Comparing with the raw graphite (Fig. 4a), the greyish GO powder is thermally unstable and its thermal decomposition starts at 140 C, and the major mass loss occurs at 200 C under nitrogen atmosphere. This results from pyrolysis of the unstable oxygen functionalities, generating gases including CO and CO2. Compared to GO, RGO exhibits clearly increased thermal stability without significant mass loss between 200 and 400 C (Fig. 4c). Additionally, the residual content at 800 C for RGO is 92%, much higher than 68% of GO at 800 C. As to RGO-2 (Fig. 4d), its main decomposition tendency is similar with that of GO, suggesting that there

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Fig. 3 – Raman spectra of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d).

Fig. 4 – TGA curves of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d). are still much remaining oxygen-containing groups on RGO-2. The distance between two layers is an important parameter to evaluate the structural information of the graphenecontaining materials. Fig. 5 shows XRD patterns of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d). Pristine graphite (Fig. 5a) showed the very strong 0 0 2 peak at 26.44. As to GO (Fig. 5b), while a small change in the position of the principal reflection is observed with oxidation, the most

Fig. 5 – XRD patterns of pristine graphite (a), GO (b), RGO (c) and RGO-2 (d).

striking difference is the intensity and broadness of the peak observed at 2h = 10.45, corresponding to an average interlayer spacing of 0.78 nm, which is significantly larger than that of graphite due to the intercalating oxygen functional groups. The RGO sheets show a broad (0 0 2) diffraction peak at about 2h = 24 (Fig. 5c), corresponding to a d-spacing of 0.35 nm, which is slightly larger than that (0.34 nm) of graphite (JCPDS No. 75-1621).This suggests that GOS were well reduced, and the restacking of as-reduced graphene sheets was effectively prevented [43]. Besides, both the d (0 0 2) value and broadening of this reflection are typical for randomly ordered (turbostatic) graphitic platelets, suggesting that the re-aggregation of RGO can be greatly limited because of volatile gaseous species (such as NH3 and CO2) released from the intercalate. As to RGO-2 (Fig. 5d), its main peaks are similar with that of GO, suggesting that the reduction of GO without ammonium bicarbonate is incomplete. To intuitively evaluate the reduction level and determine the composition of the as-prepared samples, XPS was utilized. C/O ratio is often used as a convenient measure of the oxidation degree of GO samples [44]. In our case, the C/O ratio of RGO is about 10 (Fig. 6a), while for GO it is about 2 (Fig. 6b). In the C1s spectrum of GO (Fig. 6c), the binding energy of 284.5 eV is attributed to the C–C, C=C, and C–H bonds. The deconvoluted peaks center at the binding energies of 286.0, 287.7 and 289.2 eV are assigned to the C–OH, C=O, and O=C– OH functional groups [45]. While in the spectrum of RGO (Fig. 6d), in comparison with GO, it obviously exhibits a decreased intensity for peaks corresponding to functional groups. Most of the C–O and C=O peaks are greatly weakened, indicating effective deoxygenation of GO, which is in accordance with the FTIR and XRD analysis. However, the presence of the XPS C1s binding energy profile at 286.2 eV (C–O) for the as-prepared RGO ensemble in Fig. 6d proves that there still exist small amounts of oxygenated carbon species in the carbon lattice structure. The investigation of the structure of GO had been performed by SEM and TEM. From SEM image (Fig. 7), the layered structure of the solid GO sample with stacked GOS can be clearly seen. SEM image appears to have rough surface which might be due to the oxidation of sheets. Comparing with GO, RGO also shows layered structure, but with lower height. To

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Fig. 6 – XPS spectra of GO (a, c) and RGO (b, d).

Fig. 7 – SEM and TEM images of GO (a, c) and RGO (b, d). further characterize the exact structures of nanocarbons in the dispersions, we conducted TEM analysis. TEM samples were prepared by pipetting a few milliliters of dispersions onto holey carbon mesh grids. Large sheets are observed to be situated on the top of the grid, where they resemble silk veil waves, illustrating the flake-like shapes of graphene. TEM micrograph of GO clearly indicates that the GOS are partially folded due to large area occupied. While the TEM image of RGO shows that RGO sheets are also homogeneous and quite smooth, with little aggregation. AFM characterization has been one of the most direct methods of quantifying the degree of exfoliation to graphene

level after the dispersion of the powder in a solvent. AFM images are obtained to measure the thickness of the samples, and images are shown in Fig. 8. The GO was dispersed in water while the as-prepared RGO was dispersed in DMF. A droplet of the suspension was placed on a freshly cleaved mica surface. GO had a greyish appearance and was not as shiny as the starting material raw graphite. AFM image confirm that evaporated dispersion of GO is comprised of isolated GOS (Fig. 8a) and they have lateral dimensions of hundreds of nanometers. The cross-sectional view of the typical AFM image of the exfoliated GO indicated that the average thickness of GO sheets is 1.2 nm, which is larger than 0.8 nm as the

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Fig. 8 – AFM images of GO (a) and RGO (b) with concentrations of 0.05 mg mL 1.1 lm · 1.1 lm.

typical thickness of the observed single-layer GOS, but it is still smaller than the typical thickness of a bilayer GOS [46]. As described previously, the interlayer spacing along the caxis will change during oxidation. During this process, hydroxyl, carbonyl, epoxy groups are bonded to the edges of basal planes of the graphite structure. Simultaneously, carbon hydroxylation occurred and the sp2 bonds changed to sp3 bonds. From the fact that the thickness is uniformly distributed, it is believed that these typical GO sheets are fully exfoliated. As to RGO sheets, its AFM image (Fig. 8b) reveals that the sheets display height variations at two length scales: some flat areas covered with 0.1–0.2 nm ‘‘bumps’’ and some high points showing average height of about 1.2 nm. We relate the bumpy texture of the flat regions to the presence of dead space because of the extensive edge functionalization employed during our approach. Thus, it can be concluded that complete exfoliation of RGO nanoplatelets down to monolayers are indeed achieved under these conditions, since the intersheet distance for RGO nanoplatelets is 0.34 nm. The mechanism of the reaction between GO and ammonium bicarbonate is still unclear, we tentatively speculate that the whole process contains three steps: (1) when the ammonium bicarbonate is mixed with GO, the melted ammonium bicarbonate may intercalate into the nanosheets under MWI; (2) when the temperature of the system suddenly gets higher, exfoliation of GO occurs because of the decomposition of the intercalated ammonium bicarbonate. Upon heating, they will exfoliate violently due to volatile gaseous species (such as NH3 and CO2) released from the intercalate; (3) the produced NH3 may react with oxygen-containing groups on GOS. However, our analysis still leaves open the question of how the produced NH3 react with oxygen-containing groups on GOS. The answer to this intriguing query constitutes a fertile research area that awaits further investigations by both experimenters and theorists.

4.

1

. Image dimensions are 1.8 lm · 1.8 lm and

Conclusions

In this study, we present a MWI route to isolated RGO sheets that is amenable to bulk production. This work suggests that ordinary GO, when treated directly by appropriate chemical means, can readily generate RGO without the need of any solvents. These results should facilitate the processing of graphene-based materials for different kinds of applications. With further surface modifications, RGO sheets that are soluble in different solvents should be accessible, thereby further expediting the application of graphene.

Acknowledgments We gratefully acknowledge the financial sponsor by the fundamental research funds for the Central Universities (No. 10FX 011) and National Basic Research Program of China (2011CB 605704).

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