The role of microwave absorption on formation of graphene from graphite oxide

CARBON

5 0 ( 2 0 1 2 ) 3 2 6 7 –3 2 7 3

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The role of microwave absorption on formation of graphene from graphite oxide Han Hu a, Zongbin Zhao

a,* ,

Quan Zhou a, Yury Gogotsi

a,b

, Jieshan Qiu

a,*

a

Carbon Research Laboratory, School of Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116023, China b Department of Materials Science and Engineering and A.J. Drexel Nanotechnology Institute, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

By means of manipulating the oxygen content in graphite oxides (GO) and/or graphene-

Received 30 September 2011

based materials, we demonstrate that the microwave absorption capacity of carbon

Accepted 3 December 2011

materials is highly dependent on their chemical composition and structure. The increase

Available online 8 December 2011

of oxygen in GO remarkably decreases its microwave absorption capacity due to the size decrease of the p–p conjugated structure in these materials, and vice versa. It was revealed that graphene is an excellent microwave absorbent while GO with poor microwave absorption capacity, the unoxidized graphitic region ‘‘impurities’’ in GO act as the microwave absorbents to initiate the microwave-induced deoxygenation. The addition of a small amount graphene to GO leads to avalanche-like deoxygenation reaction of GO under microwave irradiation (MWI) and graphene formation, which was used for electrode materials in supercapacitors. The interaction between microwaves and graphene or graphene-based materials may be used for the fabrication of a variety of graphene-based nanocomposites with exceptional properties and a wealth of practical applications.  2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene has attracted tremendous attention for its remarkable electronic and thermal conductivity, large specific surface area, high mobility of charge carriers, excellent chemical stability and mechanical strength due to its unique structure made of sp2 carbon atoms tightly packed into a honeycomb lattice [1]. For the sake of practical application, it is critical to find versatile methods that can produce graphene abundantly and efficiently at low cost. Until now, several strategies, such as micromechanical cleavage [2], epitaxial growth [3,4], chemical vapor deposition [5], and exfoliation of graphite oxide (GO) [6–9], have been pursued to synthesize graphene sheets. Among these methods, exfoliation of GO is the most promising method for low-cost and scalable production, and has been

researched intensively [10,11]. Thermal expansion of GO represents one of the most attractive approaches because of its straight forwardness, high efficiency, and high degree of exfoliation [12]. However, this process is energy-consuming and must be carried out at high temperature. Microwaves, an alternative energy input source, have been widely used because of their internal and volumetric heating of materials, in the field of organic synthesis [13], environmental remediation [14], preparation of catalysts [15] and activated carbon [16]. A variety of nanostructures with different compositions have also been fabricated via the energyefficient microwave irradiation (MWI) approach [17]. In this process, microwave energy is transformed into heat by using a microwave absorbent. For this very reason, it is necessary to select a proper microwave absorbent to efficiently convert

* Corresponding authors. Fax: +86 411 84986072. E-mail addresses: [email protected] (Z. Zhao), [email protected] (J. Qiu). 0008-6223/$ - see front matter  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.12.005

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microwave energy into heat. MWI has demonstrated the ability to successfully produce graphene from GO, where the main attention has been focused on the production efficiency [18–21]. However, the interaction of microwaves with GO and graphene, which is very important for the further development of this method, is not fully understood. In the present work, we investigate the response of oxidized graphite with varying degrees of oxidation under MWI. Graphene is demonstrated to behave as an excellent microwave absorbent. By dispersing tiny amounts of graphene into a GO matrix, a local heating stimulated by graphene under MWI creates an avalanche-like deoxygenating reaction of GO, thus giving rise to graphene, which shows a high specific surface area and good electrochemical performance.

2.

Experimental

2.1. Synthesis of oxidized graphite with varying degrees of oxidation The preparation of oxidized graphite with varying degrees of oxidation was achieved by adjusting the ratio of oxidant to graphite [22], the details of which are listed in Table S1. Firstly, 5 g graphite powder (180 mesh, Qingdao Black Dragon Graphite Co. Ltd.) and 2.5 g sodium nitrate (Analytic grade, Sinopharm Chemical Reagent Co. Ltd.) were mixed with 130 ml sulfuric acid (98%, Beijing Chemical Works) in an ice bath under vigorous stirring for 2 h. Weighted potassium permanganate (Analytic grade, Zhengzhou Third Chemical Reagent Factory) was then slowly added into the system while the temperature was kept from exceeding 20 C. The temperature of the mixture was subsequently raised to 35 C and maintained for 1 h after which 230 ml deionized water was slowly added, and then the mixture was heated to 98 C for 30 min. Five hundred ml deionized water and 5 ml hydrogen peroxide (30 wt.%, Sinopharm Chemical Reagent Co. Ltd.) was added into the mixture. After centrifugation and washing to remove residual salts, the wet graphite oxide was dried at 60 C for 48 h. A series of GO samples with different levels of oxidation were denoted as GO-n, where n indicates the mass ratio of KMnO4 to graphite.

2.2.

Triggered deoxygenation reaction of GO under MWI

Standard graphite oxide (GO-3.00, shorted as GO) from the Hummer method mentioned above and graphene from thermal expansion (GT) were mixed together and sonicated in an aqueous solution (For details, see supporting information). In each experiment, GT was added to the GO, with amounts of 0.5%, 2.5% and 5.0%, denoted as GT/GO-0.5, GT/GO-2.5 and GT/GO-5.0, respectively. After the mixtures were subjected to sonication (40 kHz, 100 W) in 150 ml of deionized water for 1 h, a dark brown suspension was obtained. The suspension was centrifuged to remove water, and further dried in the oven. The production of graphene was achieved in a quartz reactor (30 cm in length, 18 mm in diameter) located in a microwave oven (800 W). For comparison, both pure GO and GT/GO mixtures were exposed to the MWI under the experimental conditions mentioned above.

2.3.

Characterization and electrochemical test

Elemental analysis was conducted by Elemental Analyzer Vario EL III. The electrical conductivity of different samples was tested using a SX1934 four probe testing system. The Xray diffraction (XRD) patterns of samples with different degrees of oxidation as well as samples before and after MWI were recorded on a Rigaku D/Max2400 diffractometer. Surface functional groups were measured using a Bruker Equinox 55 Fourier transform infrared spectrometer (FTIR). Scanning electron microscopy (SEM) of GO and as-prepared graphene was performed on a JEOL S-4800 field emission SEM (FESEM), while the transmission electron imaging of graphene was obtained on a Philips Tecnai G20 transmission electron microscope (TEM). The specific surface area of graphene was obtained on a Micromeritics ASAP 2020 nitrogen adsorption equipment. The electrochemical performance of as-prepared graphene was characterized on a CHI660D electrochemical working station, using cyclic voltammetry and galvanostactic charge–discharge measurements. The working electrode was fabricated by mixing 90 wt.% active materials with 10 wt.% poly(tetrafluoroethylene) binder, and cut into disk-shaped sheets of 10 mm in diameter. The as-prepared electrode sheet was pressed between two pieces of nickel foam, which were used as current collectors. A three-electrode system was used in a 6 M KOH aqueous solution. A platinum electrode and Hg/HgO electrode served as the counter and reference electrode, respectively. Cyclic voltammetry curves at scanning rates of 5 mV s 1 and 10 mV s 1 were recorded in a potential range of 0.1 V to 0.9 V while galvanostactic charge–discharge curves between 0.1 V and 0.9 V were performed at current densities of 50 mA g 1 and 100 mA g 1.

3.

Results and discussion

GO samples with different degrees of oxidation were prepared by varying the mass ratio of KMnO4 to graphite from 0.75 to 3.00. As expected, the oxygen content in GOs increases with the increasing amount of oxidant used (listed in Fig. 1a), from 24 wt.% in mildly oxidized graphite (GO-0.75) to about 48 wt.% in highly oxidized graphite (GO-3.00). As shown in Fig. S1, the color of the as-obtained samples changes from metallic luster (pristine graphite) to dark (GO-0.75) and, finally, to brown (GO3.00), which is attributed to the blue shift of light resulting from the increasing destruction of p–p bonds in graphite by oxidation [23]. Various characterization methods were used to investigate the structure evolution and property variation of oxidized graphite during the oxidation process. The electrical conductivity of GO samples with increasing degrees of oxidation is shown in Fig. 1a. Graphite holds a conductivity value as high as 188 S cm 1, while the conductivity of GOs decreases remarkably with increasing oxidation from GO-0.75 to GO-3.00, of which GO-3.00 is almost seven orders of magnitude below graphite. XRD patterns of the samples are shown in Fig. 1b. The typical sharp (0 0 2) peak of graphite broadens and weakens, accompanied by the appearance of (0 0 1) diffraction peak around 11 due to the intercalation of functional groups into

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Conjugated region Fig. 1 – Structure evolution and property variation from graphite to highly oxidized graphite. (a) The variation of electrical conductivity with oxygen content in GOs. (b) XRD patterns of oxidized graphite with different degrees of oxidation. (c) Temperature profiles of samples as a function of time exposed to MWI. (d) Schematic illustration of the variation of the conjugated region with oxidation. Conjugated regions are shaded grey.

the graphite layers. A shoulder peak on the left of the (0 0 2) peak appears in GO-0.75 and GO-1.50 which is attributed to the stage 3 intercalation compound [24], while both the peaks of (0 0 2) and (0 0 4) are much weaker in GO-1.50 compared to GO-0.75 due to more oxygen-containing groups inserted. Finally, the (0 0 2) and (0 0 4) peaks around 26 become undetectable in GO-3.00 and are replaced by a strong GO (0 0 1) diffraction peak [12], similar to the result obtained in the case of CNTs oxidation [25]. The interaction of GOs with microwaves was examined under intense MWI (power 800 W, time 600 s). It was found that the response of GOs to MWI strongly depends on the oxidation degree of the samples employed (Fig. S2). Strong absorption was observed in the case of graphite, showing intense heating, sparking, light emission, and plasma, ascribed to the free p electrons moving out of the graphite and ionizing the surrounding atmosphere [26,27]. Compared with graphite, the lower oxidized samples of GO-0.75 and GO-1.50 exhibited a much weaker interaction with less heating and sparking, but simultaneously accompanied by a volume expansion. However, in the cases of GO-2.00 and GO-3.00 with high degrees of oxidation, no obvious variation could be observed under the same MWI process. It has been known that the temperature of a system rises as a result of the conversion of microwave energy to heat by means of the microwave absorbent during MWI. Therefore, the efficiency of the energy conversion can be used as an indicator for the evaluation and comparison of the microwave adsorption capacities of different samples [28]. The evalua-

tion processes were carried out at a low MWI power input (160 W) and for a short duration time (30 s) to avoid reactions as well as any notable volume changes of the GOs, otherwise, it would make the temperature measurement difficult and inaccurate. As shown in Fig. 1c, the temperature changes strongly depend on the oxidation degree of materials. The pure graphite and less oxidized graphite, including GO-0.75 and GO-1.50, exhibit large temperature changes of about 40 C, while the highly oxidized graphites (GO-2.00,GO-3.00) experienced negligibly smaller temperature changes, as the temperature remained nearly unchanged for GO-3.00 during MWI. The evolution of GOs structures with the oxidation can be illustrated schematically in Fig. 1d. Graphite contains abundant p electrons and a defect-free graphene area by which microwaves can be absorbed and dissipated into heat energy efficiently via Joule heating [27]. When graphite is oxidized, the addition of oxygen-containing groups leads to breaking through the p–p bonds in the graphite structure, thus disrupting the conjugated network and decreasing both the mobility and concentration of carrier. Especially in the case of the highly oxidized graphite, such as GO-3.00, most of the graphitic structures were broken down to polyaromatic islands with small p–p conjugated regions on the basal planes where the long range transport of carriers has been blocked. Consequently, the microwave absorption and heat conversion decreased to a low level, as shown by the small temperature change of 3 C of the system during MWI. We also found that the strong interaction of graphite-based materials with

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microwaves can be restored by deoxygenation treatment of the oxidized graphite at temperatures higher than 600 C (Figs. S3 and S4). The deoxygenation of GO leads to the growth of graphitic structures, which is essential for the absorption and dissipation of microwave energy, as illustrated in Fig. S4. As a result, graphene derived from the deoxygenation of GO is an excellent microwave absorbent. Based on the understanding of the interaction of microwaves with GOs and related materials, we designed a graphene-induced route to the fabrication of graphene from GO under MWI. Intriguingly, graphene can be dispersed uniformly into a stable GO aqueous colloidal solution, in which GO acts as a surfactant to enhance the solubility of graphene (Fig. S5), similar to the dispersion of carbon nanotubes in GO [29,30]. This allows for the GT-induced fabrication of graphene with the assistance of microwaves to be possible and efficient. Fig. 2a displays the temperature profiles of reaction systems under microwave irradiation with differing doses of graphene as well as pure GO. Pure GO exhibited little temperature variation (Fig. 2a) and volume expansion (Fig. 2b) during the MWI, while GT/GO-0.5 rose to 100 C from room temperature after 600 s of MWI. For the GT/GO-2.5, the reaction became violent and the temperature rose quickly to about 1000 C within 180 s, accompanied by a remarkable volume expansion (Fig. 2c). GT/GO-5.0 showed an even faster reaction, resulting in an avalanche-like reaction which ignited within 10 s. The products from the graphene-triggered MWI were characterized by microscopic observations, element analysis, Raman spectroscopy, FTIR, electrical conductivity and XRD. In Fig. 3a, the FESEM image of highly oxidized graphite shows a smooth surface with tightly packed sheets, while the exfoliated sample exists as transparent, wrinkled sheets, indicating these layers are exfoliated to a very large extent. This fact is in good agreement with the XRD analysis (Fig. S6). Also, the clear image of as-prepared graphene under high resolution suggests an enhanced conductivity ascribed to the restoration of the p–p bonds [31]. The TEM image of graphene presented in Fig. 3c exhibits a continuous, wrinkled and transparent sheet, indicating the successful synthesis of graphene. The edge-on region in Fig. 3d shows a single layer structure. As

shown in the Raman spectra, all three samples of GO, GT/ GO-5.0 and graphene show the same characteristic D-band and G-band, but with different ID/IG ratios (Fig. S7). The ID/IG ratios of GO and GT/GO-5.0 are very similar, while the ratio increases in graphene due to the increasing number of edgeatoms, thus indicating a great degree of exfoliation [32]. The elemental analysis (Fig. 3e) of GT/GO-5.0 at different MWI exposure, compared to that of GO, demonstrates the high efficiency of deoxygenation. During the entire MWI duration, the C/O mole ratio of GO was almost unchanged, while only 2 min under MWI eliminated most of oxygen in GT/GO-5.0. The variation of electrical conductivity shows similar trend with that of C/O mole ratio (Fig. S8). GO holds a conductivity of 3.6 · 10 5 S cm 1 which almost keeps invariant during the whole MWI process, while the value for GT/GO-5.0 increases 5 order of magnitude within 2 min and finally reaches 4.3 S cm 1. Fig. 3f displays the FTIR spectra of GO, GT/GO5.0, and graphene. It can be found that the band of C–O deformation vibration at 1050 cm 1 as well as the band of C=O stretching vibration at 1728 cm 1 are both greatly reduced or entirely removed [33,34], which further confirms the efficient transformation process. The avalanche-like deoxygenation of oxidized graphite triggered by graphene under MWI indicates a strong interaction of graphene with MWI. As illustrated in Fig. 4, when microwaves are irradiated onto the mixture, the highly oxidized graphite cannot respond to the microwave efficiently due to small polyaromatic islands resulting from deep oxidation [12,29]. Although there are abundant oxygen-containing groups in GO, they are immobilized on the basal planes, thus making their alignment with alternative microwaves impossible. Therefore, the contribution to the heat from the polarity mechanism can almost be excluded. The strong microwave absorption of graphene within the dominant conjugated region [35] can be induced from the effective medium theory of Maxwell–Garnett [27,28]. Acting as a molecular heater, microwaves are efficiently dissipated via p electrons moving through the graphenic structure. The transformed microwave energy results in the superheating of graphene, which contributes to a local deoxygenation reaction of GO adjacent to graphene. Once the incipient graphenic structure on the GO

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Fig. 3 – Degree of exfoliation and deoxygenation of GO. FESEM images of parent GO (a) and as-prepared graphene (b). TEM images of graphene at low (c) and high (d) magnification. (e) The variation of C content and corresponding C/O mole ratio (listed in (e)) of GO and GT/GO-5.0 with different MWI durations. (f) FTIR spectra of GO, GT/GO-5.0 and graphene.

Fig. 4 – Illustration of the graphene-triggered process: microwave absorption by GT, igniting, and self-accelerating to avalanche-like deoxygenation and the formation of graphene.

sheets is large enough for efficient Joule heating, it will act as another molecular heater, accelerating the deoxygenation and exfoliation. The self-accelerated process was reflected by an abrupt volume expansion (Fig. 2c) and a rise in temperature (Fig. 2a).

It should be noted that direct exfoliation has been found to occur with large amount of highly oxidized graphite (GO > 1.2 g) under intense MWI (800 W, 1 min) in the present work, as reported in literature [21]. What’s the reason for this? Sufficient heat inducing the deoxygenation may be

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accumulated and achieved via large amount of GO employed under MWI, although the interaction of small polyaromatic islands with microwaves is very weak. Most importantly, GO even after the harsh oxidation treatment of Hummer method still contains some sp2-bonded regions [29,30], these graphitic region ‘‘impurities’’ may actually act as the initiators of the deoxygenation reaction, as mentioned above. The absorption and conversion capacity of microwaves also reflect the structure characteristics of GO, which is in good agreement with the feature of amphiphilicity derived from the hydrophilic functional groups and hydrophobic conjugated regions in its basal plane [29]. The as-prepared graphene sheets were characterized via nitrogen absorption technology in order to investigate the surface area and pore structure. As shown in Fig. 5a, the isotherm exhibits a typical mesoporous shape. The graphene has a specific area of 415 m2/g by BET calculation between 0.05 and 0.3 P/P0. The total pore volume is 1.53 cm3/g. From the BJH method, we know that the pore size distribution of the mesopores inset is mainly centered around 2–4 nm, indicating that the graphene layers interact with each other form a porous structure. The structural characteristics and high conductivity of graphene hold the potential as materials for energy storage based on the ion adsorption mechanism [36] and have been widely demonstrated [37]. The electrochemical properties were tested in an aqueous KOH electrolyte, and relatively good supercapacitor performance was exhibited due to the large surface area and high electric conductivity [37,38]. As shown in Fig. 5b, the cyclic voltammograms show capacitive behavior with no redox reactions and their shape indicates a low equivalent series resistance [39,40]. Based on the galvanostatic charge/discharge curves in Fig. S9, a specific capacitance of 120 F/g can be obtained.

4.

Conclusion

Microwave heating was used for the preparation of graphene from graphite oxide. We have demonstrated that the microwave response of GO strongly depends on its structure, which can be tailored from a large p–p conjugated region to polyaromatic islands by controlling the degree of oxidation. The size

of the p–p conjugated region in the GOs is vital for the transformation of microwave energy to heat. Graphene addition could trigger an avalanche-like deoxygenation reaction under microwave irradiation to achieve the synthesis of graphene from GO. The material produced using this method was successfully used as an electrode in supercapacitors. The understanding of the structure-dependent interaction of carbon-based materials with microwaves may facilitate the design and synthesis of novel graphene-based composites using this energy efficient method.

Acknowledgements This work was supported by the NSFC (Grants 51072028, 20876026, 20836002, 20725619). We thank Ms. Amanda Pentecost, Drexel University, for editing the manuscript. Collaboration between Drexel University and Dalian University of Technology was supported by Cheung Kong Scholarship.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.12.005.

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