Effective reduction of graphene oxide via a hybrid microwave heating method by using mildly reduced graphene oxide as a susceptor

Effective reduction of graphene oxide via a hybrid microwave heating method by using mildly reduced graphene oxide as a susceptor

Applied Surface Science 473 (2019) 222–229 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 473 (2019) 222–229

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Effective reduction of graphene oxide via a hybrid microwave heating method by using mildly reduced graphene oxide as a susceptor

T



Shan Tanga, Shuangling Jina, , Rui Zhanga, Yan Liua, Jiangcan Wanga, Zhen Hua, Wangzhao Lua, ⁎ Shuo Yanga, Wenming Qiaob, Licheng Lingb, Minglin Jina, a b

School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene oxide reduction Solid-state microwave irradiation Hybrid microwave heating Susceptor Reduced graphene oxide

Due to the poor microwave absorption ability of graphene oxide (GO), direct reduction of GO by solid-state microwave irradiation is inefficient. In this study, GO was reduced effectively by a hybrid microwave heating (HWH) method by using mildly reduced graphene oxide (MG) membrane as a external susceptor, in which MG was obtained by annealing of GO at 300 °C for 1 h under nitrogen by a conventional heating method. When the mass ratio of MG to GO is 0.045, the C/O atomic ratio of GO increases from 1.8 to 17.84 by microwave irradiation under argon at 2000 W for 30 s. However, when GO was irradiated solely under the same conditions, the ratio of C/O was unchanged (1.88). For comparison, the MG was further irradiated by microwave at 2000 W for 30 s, its C/O ratio is enhanced from 4.17 to 8.46, demonstrating the higher reduction efficiency due to a mixed heating mode in the HWH process. The conventional heating of GO occurs during the initial stage and a two-way heating mode during the later stage, in which MG heats the GO from the surface and the microwave heating from the GO itself. Therefore, more amount of carboxyls and epoxides can be removed in the HWH process to achieve higher C/O ratio compared with that of the product obtained by direct microwave irradiation of MG.

1. Introduction Graphene, as a new type of single atomic layer of two-dimensional carbon material, possesses unique properties such as ultra high specific surface area, high strength, high electrical and thermal conductivity, good flexibility and chemical stability, making it has great application potential in electronics, photonics, mechanics, and so on [1–6]. At present, many methods have been developed for preparing graphene, among which the exfoliation and reduction of graphene oxide (GO) is recognized as the easiest way to prepare graphene in large-scale. Many established procedures such as chemical reduction or high-temperature thermal reduction have been adopted to remove the surface oxygencontaining groups of GO to recover the intrinsic properties of graphene. However, these reduction methods often involve complicated processes, high energy cost, long time-consuming, toxic reducing agents and other issues [7–15]. Microwave heating as an alternative method has received significant attention based on the internal heating and energy efficient volumetric processing [16–24]. GO is mainly reduced by a solvent medium microwave reaction, but the quality of reduced graphene oxide (RGO) obtained by this method is not high. On the one hand, the ⁎

reaction temperature is limited since most of irradiated microwave is absorbed by solvent [25,26]. On the other hand, due to the presence of oxygen-containing functional groups in GO, its microwave absorption capacity is poor, because of which the response of pure GO to microwave irradiation is strongly depends on the oxidation degree of the samples employed [27]. Chhowalla et al obtained high-quality RGO by slightly reducing GO via thermal annealing prior to exposure to microwaves [28]. The GO after mild reduction can absorb microwaves which leads to rapid heating of the GO, causing desorption of oxygen functional groups as well as reordering of the graphene basal plane. However, this process also need a long-time pre-reduction of GO prior to microwave heating, which reduces the production efficiency greatly. One of proposal to overcome this issue is to enhance the dielectric properties of GO by using the susceptor assisted microwave heating method which is also referred to as the “hybrid microwave heating (HWH)”. The HWH method combines microwave heating with an additional heating from an effective microwave absorber (susceptor). Upon microwave irradiation, the temperature of the susceptor rises sharply first and the susceptor transfers heat to the target material through a conventional heating conduction mode. Then, the dielectric properties of target material is changed at the elevated temperature,

Corresponding authors. E-mail addresses: [email protected] (S. Jin), [email protected] (M. Jin).

https://doi.org/10.1016/j.apsusc.2018.12.096 Received 4 October 2018; Received in revised form 5 December 2018; Accepted 11 December 2018 Available online 12 December 2018 0169-4332/ © 2018 Published by Elsevier B.V.

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2. Experimental

stability of the films was characterized by a SDT Q600 thermogravimeter, and all the measurements were carried out under nitrogen gas over a temperature range from 30 to 800 °C with a ramp rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) measurements was measured on Axis Ultra DLD X-ray photoelectron spectrometer manufactured by Kratos, Britain. Nitrogen adsorption-desorption isotherms were measured at 77 K with an ASAP 2020 instrument. The samples were separately degassed at 100 °C in a vacuum environment for a period of at least 4 h prior to measurements. Experimental adsorption data in the relative pressure (P/P0) range of 0.05–0.2 was used to calculate surface area values using the Brunauer-Emmett-Teller (BET) equation. The pore size distribution was determined by applying density functional theory (DFT) model assuming slit-pore geometry. The electric conductivity of samples was performed on a resistivity tester produced by Wuhan Jiayitong Technology Co., Ltd. using a four-point probe head with a pin-distance of about 6 mm. The powdered samples were pressed into blocks under a pressure of 20 MPa, and then cut into rectangular pieces for testing.

2.1. Material synthesis

3. Results and discussion

2.1.1. Preparation of GO Graphite oxide was prepared by a modified Hummers method [32]. In brief, 2.0 g of flake graphite powder (∼2000 mesh) and 2.0 g of NaNO3 were mixed with 96 mL of concentrated H2SO4 under mechanical stirring in an ice bath. Then 12.0 g of KMnO4 was added slowly to the suspension with stirring for 1.45 h. Then the above mixture was heated to 35 °C and stirred for 2 h. Afterwards, 80 mL of deionized water was slowly added to the solution with vigorous agitation. After 30 min, an additional 200 mL of deionized water was added accompanying dropwise addition of 30% H2O2 (10 mL), which produced a dark yellowish brown solution of oxidized graphite. After cooling to room temperature, the mixture was centrifuged and washed with 5% HCl and then deionized water to neutral, and then suspended in deionized water, followed by sonication in an ultrasonic cleaner (100 W, 40KHz) for 1 h. The above solution was further centrifuged by highspeed centrifugation (3000 rpm, 25 min) to remove unreacted graphite impurities. Then the GO powder was obtained by drying the solution at 60 °C for 48 h.

The experimental process and the digital photographs of corresponding products are illustrated in Fig. 1. The thermal reduction of dark yellow GO at 300 °C obtains MG with metallic luster, which is further irradiated by microwave to get metallic gray RMG. When GO is irradiated solely by microwave, the color of product RGO-0 does not change. While GO is heated under microwave irradiation by using MG as susceptor, the color of product is black, indicating the restoration of π–π conjugation of graphene sheets upon reduction [12,34]. The microwave absorption ability change of GO before and after thermal reduction at 300 °C is demonstrated by measuring its permittivity and permeability on a vector network analyzer. The microwave absorption property of an absorber is highly associated with its complex permittivity (εr = ε′−ε″i) and complex permeability (µr = µ′−µ″i)), where the real parts of complex permittivity (ε′) and complex permeability (μ') represent the storage capability of electric and magnetic energy, and imaginary parts (ε″ and µ″) stand for the loss capability of electric and magnetic energy [23,24]. Fig. 2 shows the real and imaginary parts of complex permittivity and complex permeability of GO and MG in the frequency range of 2–16 GHz. The ε′ and ε″ of GO is ca.

which leads to an effective absorption of microwaves [29,30]. It is reported that dispersion of a small amount graphene to GO can enhance localized heating and deoxygenation reaction of GO under microwave irradiation [27,31]. However, this technique alters the chemical composition of the original material and hence, restricts its application. Alternatively, the microwave absorbing material can be incorporated as the external susceptors, so that the necessary heat can be provided without altering the chemical composition of the target material. In this study, GO was reduced effectively by the HWH method, in which a layer of mildly reduced graphene oxide (MG) membrane was adopted as a external susceptor. After microwave irradiation, the obtained RGOs could be easily separated from the MG membrane. The effect of mass ratios of MG to GO (RMG/GO) on the reduction efficiency was investigated, and the evolution of oxygen-containing groups on the surface of GO in the HWH process was compared with that treated with thermal pre-reduction plus microwave irradiation procedure.

2.1.2. Preparation of RGO MG was obtained by annealing GO at 300 °C for 1 h under nitrogen in a horizontal heating furnace. The reduction of GO was carried out in a industrial microwave oven (2.45 GHz) in argon. GO powder was covered with MG membrane in different mass ratios of MG to GO (RMG/ GO) by microwave irradiation at 2000 W for 30 s. The RGO samples obtained from RMG/GO of 0, 0.045, 0.09 and 0.2 was denoted as RGO-0, RGO-0.045, RGO-0.09, RGO-0.2, respectively. When the RMG/GO is 0.2, the corresponding product of MG after microwave irradiation is recorded as RMG-0.2. For comparison, MG was irradiated solely by microwave under 2000 W for 30 s to obtain RMG. 2.2. Characterization A N5244A (Agilent) vector network analyzer was applied to determine the complex permittivity and permeability in the frequency range of 2–16 GHz for the investigation of microwave absorption ability. 5 wt% sample powders with 95 wt% paraffin wax using as the binder was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for microwave measurement. Raman spectra were examined with a Thermo Scientific DXR Raman spectrometer using laser excitation at 514.5 nm. According to Raman spectra, the size of graphitic domain (La, nm) can be estimated by La (nm) = (560/E4)(ID/IG)−1, where E is the laser energy in nanometers (λ = 514.5 nm, 2.41 eV), and ID and IG is the integrated intensities (areas) of the D and G bands, respectively [33]. The thermal

Fig. 1. Schematic diagram of experimental section. 223

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Fig. 2. The real and imaginary parts of complex permittivity (a) and complex permeability (b) of GO and MG as a function of frequency.

2.25 and 0.01, respectively, implying its poor dielectric loss. Both ε′ and ε′′ are simultaneously improved after GO is thermal annealed at 300 °C for 1 h, which is 2.69–2.87 and 0.07–0.18, respectively, attributed to the partial recovery of π-π* structures and the consequent increase of electrical conductivity [23,24]. In addition, it is discovered that the μ' and µ″ of GO and MG are close to 1 and 0, respectively, indicating their negligible magnetic loss for incident electromagnetic wave. Therefore, the GO and MG samples work mainly in dielectric loss mechanism under microwave irradiation, and the absorption ability of MG is

definitely improved in comparison with GO. Raman measurement was used to study the structural evolution of GO before and after reduction, as shown in Fig. 3. The average size of the sp2 domains (La) estimated from Raman results and electric conductivities of samples are listed Table 1. Raman spectra of all samples show a D band at 1348 cm−1 and a G band at 1594 cm−1, attributed to disordered carbon and sp2 clusters, respectively [35]. It can also be seen that the La of samples after reduction becomes smaller than that of GO due to creation of new graphitic domains during reduction process 224

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Fig. 4. TGA curves of different samples under nitrogen.

Fig. 3. The Raman spectra of different samples.

Table 2 Weight losses of samples under nitrogen atmosphere.

Table 1 The La estimated from Raman results and electric conductivities of samples. Sample

ID/IG

La (nm)

Conductivity (S/cm)

Sample

Weight loss below 120 °C (%)

Weight loss of 120–800 °C (%)

GO MG RMG RGO-0 RGO-0.045 RGO-0.09 RGO-0.2

1.857 1.934 2.170 1.831 2.209 2.211 2.280

8.939 8.583 7.650 9.066 7.515 7.508 7.281

1.73 × 10−8 0.06 1.2 1.71 × 10−8 5.49 8.12 2.98

GO MG RMG RGO-0 RGO-0.045 RGO-0.09 RGO-0.2

23.15 4.41 2.09 20.72 2.26 2.12 4.32

72.48 38.74 7.60 73.65 13.20 9.92 6.91

samples all show a predominant graphitic C 1 s peak at ca. 285 eV and an O 1 s peak at ca. 532 eV, respectively (Fig. 5a). The C/O atomic ratios of samples calculated from the XPS spectra (by using ratio of area of the C1s peak to area of the O1s peak) are listed in Table 3. The deconvolution of the C1s peak (Fig. 5b) shows the presence of six peaks at C1 (284.8 eV), C2 (285.5 eV), C3 (286.6 eV), C4 (287.5 eV), C5 (288.7 eV) and C6 (291.0 eV), corresponding to C=C/C-C in aromatic, hydroxyls (C-OH), epoxides (C-O-C), carbonyls (C=O), carboxyls (OC=O) and π-π* groups, respectively [37–40]. The O1s spectrum can be divided into O1 (530.7 eV), O2 (531.1–531.8 eV), O3 (532.6 eV), O4 (533.5 eV), O5 (535.2 eV), which is assigned to quinone, C=O, C-O, -OH and adsorbed H2O, respectively (Fig. 5c) [40,41]. The summary of XPS fitting results, which quantitate the relative amount of each species, are listed in Table 3 and Table 4. As shown in Table 3, compared with GO, the C/O ratio of MG rise to 4.17, owing to the partial removal of hydroxyls, epoxides, carbonyls and carboxyls after thermal annealing at 300 °C. The C/O ratio of RMG further increases to 8.46, which is attributed to the enhanced microwave absorption capacity of MG due to its partial recovery of π-π* conjugated structure [21–24]. It is shown that the C/O ratio of RGO-0 (∼1.88) is almost unchanged compared with that of GO (∼1.8), and the relative proportion of the oxygen-containing groups on the surface of RGO-0 is similar with that of GO, which is due to the poor microwave absorbing properties of GO, resulting no obvious reduction of GO under direct microwave irradiation. When GO is under microwave irradiation by using MG as a susceptor, the C/O ratios of obtained RGO samples are dramatically enhanced compared with that of GO, achieving 17.84 for RGO-0.045. And it is found that the C/O ratio of RGO samples is decreased with the increasing of RMG/GO, which should be ascribed to the less microwave penetration onto the surface of GO due to increased thickness of covered MG [42–44]. In addition, the C/O ratios of RGO samples are higher than that of RMG, and RGO samples possess less relative amount of carboxyls and epoxides groups than that of RMG. It

[7–15]. The electric conductivity of GO is about 1.73 × 10−8 S/cm as determined by a 4-probe method, which is dramatically increased by 8 magnitude after reduction (e.g. 8.12 S/cm for sample RGO-0.09) except RGO-0 sample. It should be noted that the conductivity of RGOs is in the range of 10−2–103 S/cm [11–14]. The limited electric conductivity enhancement of samples obtained in this study may be attributed to the wrinkles, folds and interspaces existing in the sample blocks pressed for testing that greatly increase resistances [13,36]. Furthermore, since the conductivity measurement is related with the sample preparation, instrument settings, electrical contact quality and so on, direct comparison of different studies is inaccurate. Thermogravimetric analysis (TGA) was used to characterize the thermal stability of different samples. Fig. 4 shows TGA curves of samples under nitrogen atmosphere with a heating rate of 10 °C/min. Two major weight losses can be observed on the TGA curves. The weight loss below 120 °C is mainly attributed to the removal of adsorbed water, and that in the temperature range of 120–800 °C is due to the pyrolysis of unstable oxygen-containing groups to generate CO, CO2, and H2O vapors [9–11]. The corresponding mass loss results are listed in Table 2. The TGA curve of GO and RGO-0 shows obvious mass loss below 120 °C attributed to the evaporation of absorbed water, due to their higher hydrophilicity. It can also be seen that the mass loss of RMG between 120 and 800 °C is obviously less than that of MG, suggesting the further reduction of MG under microwave irradiation. In addition, the mass loss in the range of 120–800 °C decreases with the increase of RMG/GO for RGO samples, reaching 6.91% for RGO-0.2, which is lower than that of RMG (7.60%), demonstrating the effective reduction of GO by the HWH method. In accordance with the above Raman results, the volumetric reduction degree of RGO samples increases with the increasing of RMG/GO. XPS was performed to further characterize the surface chemical change of GO before and after reduction. The XPS survey spectra of 225

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Fig. 5. The XPS survey (a), deconvoluted C1s (b) and O1s XPS spectra (c) of different samples.

heating process, whereas they are relatively stable under microwave irradiation, demonstrating the decomposition of the oxygen-containing groups simultaneous in a mixed heating mode when using MG as susceptor. The conventional heating of GO happens during the initial stage and a two-way heating mode during the later stage, in which MG heats the GO from the surface and the microwave heat from the GO itself. In addition, the MG susceptor can reduce the heat loss from the GO surface

is reported that the thermal stability of oxygen functional groups during conventional heating process is in the order of hydroxyls < epoxides < carboxyls < carbonyls [45,46]. However, the sequence of stability under microwave irradiation for the oxygen-containing groups on carbon surface by theoretical simulations is carbonyls < hydroxyls < carboxyls < epoxides [47]. So the carboxyls and epoxides can be decomposed at low temperatures during conventional

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Fig. 5. (continued) Table 3 Fitted results (at.%) of C1s XPS spectra of samples. Binding Energy (eV)

C1 (284.8)

C2 (285.5)

C3 (286.6)

C4 (287.5)

C5 (288.7)

C6 (291.0)

Assignment GO MG RMG RMG-0.2 RGO-0 RGO-0.045 RGO-0.09 RGO-0.2

C-C/C=C 18.67 44.07 53.96 58.23 19.10 59.61 59.62 61.03

C-OH 11.01 7.92 7.39 7.06 11.33 8.96 7.95 5.24

C-O-C 15.30 13.60 12.59 10.62 15.10 8.34 8.19 7.38

C=O 8.93 2.29 2.50 2.49 8.98 2.96 2.20 2.21

O-C=O 10.39 8.29 6.30 5.14 10.31 5.20 4.69 4.72

π-π* – 4.48 6.68 7.11 – 9.52 8.92 8.84

C/O 1.80 4.17 8.46 9.70 1.88 17.84 11.08 10.90

also higher than that of RMG (8.46), demonstrating the deeper reduction of MG happens during HWH process. Moreover, the fitting results of O1s spectra can help to track the evolution of oxygen groups during different heating processes more accurately. As shown in Table 4, small amount of quinone-type oxygens are introduced after the thermal annealing or HWH of GO. This may be

to avoid thermal runaway which is beneficial to maintain the temperature stability during the hybrid heating. Therefore, more amounts of carboxyls and epoxides can be removed in the HWH process to achieve higher reduction efficiency compared with the situation for direct microwave heating of MG. Moreover, in comparison with MG, the C/O atomic ratio of RMG-0.2 is further enhanced to 9.7, which is Table 4 Fitted results (at.%) of O1s XPS spectra of samples. Binding Energy (eV)

O1 (530.7)

O2 (531.1–531.8)

O3 (532.6)

O4 (533.5)

O5 (535.2)

Assignment GO MG RMG RMG-0.2 RGO-0 RGO-0.045 RGO-0.09 RGO-0.2

quinone – 1.81 1.41 0.98 – 0.68 1.22 1.27

C=O 6.49 3.13 1.41 1.00 6.28 0.72 1.07 1.16

C-O 20.14 6.42 2.50 2.55 19.20 1.42 2.19 1.78

-OH 8.77 5.99 3.29 3.37 8.51 1.87 2.91 2.81

H2O 0.29 2.00 1.96 1.44 0.70 0.60 0.88 1.37

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4. Conclusion In summary, the GO was reduced effectively by a HWH method by using MG membrane as a external susceptor. With the increase of RMG/ GO, the volumetric reduction degree of GO increases gradually, whereas reduction degree on the surface decreases, which should be ascribed to the less microwave penetration onto the surface of GO due to increased thickness of covered MG. More relative amount of carboxyls and epoxides can be removed in the HWH process to achieve higher C/O ratio compared with that of the product obtained by direct microwave irradiation of MG, demonstrating the higher reduction efficiency due to a mixed heating mode in the HWH process. The conventional heating of GO occurs during the initial stage and a two-way heating mode during the later stage, in which MG heats the GO from the surface and the microwave heating from the GO itself. The conductivity of RGOs obtained by this method is 108 times higher than that of GO. Therefore, the HWH method provides a new perspective on low-energy and highefficient reduction of GO. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. U1710252, No. 21203120) and the Collaborative Innovation Fund (No. XTCX2016-6, No. XTCX2017-3). References [1] S. Wang, P.K. Ang, Z. Wang, A.L.L. Tang, J.T.L. Thong, K.P. Loh, High mobility, printable, and solution-processed graphene electronics, Nano Lett. 10 (1) (2010) 92–98. [2] K.S. Kim, H.-K. Hong, H. Jung, I.-K. Oh, Z. Lee, H. Kim, G.Y. Yeom, K.N. Kim, Surface treatment process applicable to next generation graphene-based electronics, Carbon 104 (2016) 119–124. [3] X.Q. Wu, S.L. Yu, H.R. Yang, W.L. Li, X.M. Liu, L.M. Tong, Effective transfer of micron-size graphene to microfibers for photonic applications, Carbon 104 (2016) 1114–1119. [4] Q.L. Bao, K.P. Loh, Graphene photonics, plasmonics, and broadband optoelectronic devices, ACS Nano 6 (5) (2012) 3677–3694. [5] G. Anagnostopoulos, P.-N. Pappas, Z.L. Li, I.A. Kinloch, R.J. Young, K.S. Novoselov, C.Y. Lu, N. Pugno, J. Parthenios, C. Galiotis, K. Papagelis, Mechanical stability of flexible graphene-based displays, ACS Appl. Mater. Interf. 8 (2016) 22605–22614. [6] D.G. Papageorgiou, I.A. Kinloch, R.J. Young, Mechanical properties of graphene and graphene-based nanocomposites, Prog. Mater. Sci. 90 (2017) 75–127. [7] S. Pei, H.M. Cheng, The reduction of graphene oxide, Carbon 50 (9) (2012) 3210–3228. [8] S. Park, J. An, J.R. Potts, A. Velamakanni, S.T. Murali, R.S. Ruoff, Hydrazine-reduction of graphite- and graphene oxide, Carbon 49 (9) (2011) 3019–3023. [9] S.L. Jin, Q. Gao, X.Y. Zeng, R. Zhang, K.J. Liu, X. Shao, M.L. Jin, Effects of reduction methods on the structure and thermal conductivity of free-standing reduced graphene oxide films, Diam. Relat. Mater. 58 (2015) 54–61. [10] K. Kanishka, H. De, H.-H. Silva, M. Yoshimura Huang, Progress of reduction of graphene oxide by ascorbic acid, Appl. Surf. Sci. 447 (2018) 338–346. [11] H.A. Becerril, J. Mao, Z.F. Liu, R.M. Stoltenberg, Z.N. Bao, Y.S. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (3) (2008) 463–470. [12] G.Y. He, H.Q. Chen, J.W. Zhu, F.L. Bei, X.Q. Sun, X. Wang, Synthesis and characterization of graphene paper with controllable properties via chemical reduction, J. Mater. Chem. 21 (2011) 14631–14638. [13] S.F. Pei, J.P. Zhao, J.H. Du, W.C. Ren, H.M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (15) (2010) 4466–4474. [14] C.M. Chen, J.Q. Huang, Q. Zhang, W.Z. Gong, Q.H. Yang, M.Z. Wang, Y.G. Yang, Annealing a graphene oxide film to produce a free standing high conductive graphene film, Carbon 50 (2) (2012) 659–667. [15] R. Larciprete, S. Fabris, T. Sun, P. Lacovig, A. Baraldi, S. Lizzit, Dual path mechanism in the thermal reduction of graphene oxide, J. Am. Chem. Soc. 133 (2011) 17315–17321. [16] M. Oghbaei, O. Mirzaee, Microwave versus conventional sintering: a review of fundamentals, advantages and applications, J. Alloys Compd. 41 (21) (2010) 175–189. [17] M.L. Jin, X.T. Wang, Z.Y. Wang, H.H. Jiang, K.J. Liu, Effects of atmosphere on the microstructure and magnetic properties of strontium ferrites with microwave-assisted sintering, J. Supercond. Nov. Magn. 28 (10) (2015) 3059–3063. [18] M.L. Jin, Z.Y. Wang, H.H. Jiang, Y.J. Sun, X.R. Wang, H.C. Qian, Q.Z. Chen, K.J. Liu, X-Ray pole figure analysis and magnetic properties of microwave sintered Sr-M-type hexagonal ferrites, J. Supercond. Nov. Magn. 26 (8) (2013) 2779–2783. [19] C.H.A. Wong, O. Jankovsky, Z. Sofer, M. Pumera, Vacuum-assisted microwave

Fig. 6. Nitrogen adsorption-desorption isotherms (a), and the corresponding DFT pore size distributions (b) of GO and RGO-0.045.

because some unadulterated free radicals generate on the edge and basal plane of graphene during the elimination of functionalities, and the oxygen atoms in the circumstance are tend to adsorb on these radicals to produce the carbyne-type coordinate intermediates. Then the rearrangement reactions occur between the delocalized π electrons from the basal plane of graphene and these coordinate intermediates to form thermodynamically stable quinones [40,48,49]. After microwave heating of MG or HWH of GO, most of the thermally unstable oxygen components in graphene have been thoroughly removed. As a result, the O-H, C-O, and C=O related functional groups, which are respectively attributed to more thermally stable isolated phenols, ethers, and carbonyls, finally become the major components in the final samples. Furthermore, the influence of reduction by HWH method on stack structure of GO sheets was analyzed by nitrogen adsorption/desorption. As shown in Fig. 6a, the isotherms of GO and RGO-0.045 exhibit a pronounced hysteresis and a high nitrogen uptake volume at low relative pressure, indicating the existence of micro- and mesopores [50]. An H4 and H3-type hysteresis loop is observed on the curve of GO and RGO-0.045, respectively, which confirms the slit-like pores formed by aggregate of plate-like graphene platelets. In addition, the nitrogen uptake on the RGO-0.045 curve is observed at high relative pressure of 0.95 due to the macropores. Meanwhile, the pore size distributions calculated by DFT model (Fig. 6b) indicate mesopores and macropores are generated during reduction due to the release of small gas molecules such as CO, CO2, and H2O, resulting the increase of BET surface area of GO from 151 to 313 m2/g.

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