Comparative study of deoxygenation behavior for graphene oxide with different oxidation degree and mildly reduced graphene oxide via solid-state microwave irradiation

Comparative study of deoxygenation behavior for graphene oxide with different oxidation degree and mildly reduced graphene oxide via solid-state microwave irradiation

Materials Chemistry and Physics 241 (2020) 122411 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.el...

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Materials Chemistry and Physics 241 (2020) 122411

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Comparative study of deoxygenation behavior for graphene oxide with different oxidation degree and mildly reduced graphene oxide via solid-state microwave irradiation Feijiao Gu a, Shuangling Jin a, **, Shan Tang a, Jiangcan Wang a, Shuo Yang a, Jiahui Wu a, Xudong Wei a, Rui Zhang a, Yan Liu a, Wenming Qiao b, Licheng Ling b, Minglin Jin a, * 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

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Hybrid microwave heating (HWH) is a effective approach to reduce graphene oxide (GO). � The lower oxidation degree of GO has higher deoxygenation efficiency via HWH. � No response of GO with low oxidation degree when it is exposed directly to microwave. � Chemically pre-reduced GO can be heated when it is solely submitted to microwave. � The partial reduction of GO leads to the enhancement of microwave absorption ability. A R T I C L E I N F O

A B S T R A C T

Keywords: Graphene oxide Oxidation degree Chemically reduced graphene oxide Hybrid microwave heating Deoxygenation efficiency

The influence of the oxidation degree of graphene oxide (GO) on its deoxygenation efficiency via hybrid mi­ crowave heating (HWH) method by using graphite powder as the external susceptor was investigated, and the response behavior was compared with the situation when GO is directly submitted to microwave irradiation. In addition, the deoxygenation behavior of mildly reduced GO (VRGO) that was obtained via chemical reduction by Vitamin C was compared with that of GO during above two microwave heating processes. No response of GO to microwave irradiation when GO samples are exposed directly to microwave, no matter how the oxidation degree is. However, the VRGO sample can be heated when it is solely submitted to microwave irradiation. The partial reduction of GO leads to enhancement of the dielectric parameters, and the residual defects and oxygencontaining functional groups not only can improve the impedance matching characteristics, but also produce defects polarization relaxation and dipole polarization relaxation, which are all beneficial to the microwave penetration and absorption. The deoxygenation efficiency of VRGO is lower in comparison with GO under the HWH condition, which should be attributed to higher amount of epoxides in GO samples that can be easily reduced in a mixed heating mode.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Jin), [email protected] (M. Jin). https://doi.org/10.1016/j.matchemphys.2019.122411 Received 5 September 2019; Received in revised form 29 October 2019; Accepted 3 November 2019 Available online 4 November 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

The exfoliation of graphite oxide followed by reduction is the most promising method for large-scale production of graphene. Two major approaches, chemical reduction and high-temperature treatment, are widely used presently for the effective healing of defective graphene oxide (GO) structure [1–9]. The chemical method is carried out by using reducing agents at relatively low temperatures (generally below 100 � C). Unfortunately this method results in mild reduction of GO, and usually involves toxic reducing agents. Annealing at a high temperature (above 1000 � C) under inert atmosphere for a prolonged period (over 12 h) has been developed for achieving the same purpose, suffering high energy cost and long time-consuming issues. Because of its selective heating, high heat transfer efficiency, short treatment time, the use of microwave heating has been introduced as a substitute to conventional heating. Based on the Maxwell–Wagner interfacial polarization theory, micro­ wave energy can be converted into heat energy through the movement of electrons in π-π conjugated structures of carbon materials [10]. Therefore, the carbon materials can be prepared or modified by the microwave heating method [11–13]. The reduction of GO by solid-state microwave heating method has been attempted by several works [14–21]. The response of graphite oxide with varying degrees of oxidation under microwave irradiation was investigated [14], it was shown that nearly no response of samples with high oxidation levels when they were directly exposed to micro­ wave in solid-state, only lower oxidized samples exhibited a weak interaction with microwave. Except adjustment of oxidation degree, pre-reduction of GO is another effective approach to enhance the mi­ crowave adsorption ability of GO to dissipate electromagnetic wave. For example, high-quality reduced graphene oxide (RGO) was produced via preliminary thermal annealing of GO followed by microwave irradiation [15]. Recently we found that the chemical pre-reduction can also improve the deoxygenation efficiency of GO under solid-state micro­ wave irradiation [16]. Besides, GO can be reduced effectively by adopting a susceptor during microwave heating, which is also referred to as the “hybrid microwave heating (HWH)” [17,18]. Susceptor is a class of highly lossy materials (e.g. SiC, activated carbon, graphite etc.) that can interact with the microwaves even at room temperature [17–19]. The susceptor can absorb microwave firstly and transfer the heat to the target material via a conventional heat transfer mode. With the increasing of temperature, the target material becomes increasingly lossy to be enable to interact with the microwaves once the coupling temperature is reached [17–19]. The susceptor can be doped in the target material or incorporated as the external assistant. For instance, the graphene nanosheets can be added to the graphite oxide, acting as a an effective microwave susceptor under microwave irradiation, to pro­ vide rapid heating for the effective exfoliation of graphite oxide [14,20]. Our previous work showed that the thermally reduced GO can also be used as a external susceptor to effectively reduce GO [21]. The interaction behavior with electromagnetic wave is crucial for the heat generation inside the materials during microwave heating process, which is dependent on the microwave absorption characteristics of materials. However, the relations between the interaction phenomena and material properties of graphene-based materials that is essential for the further improvement and optimization of microwave processing, is not fully understood. In this study, we investigated the influence of the oxidation degree of GO on its deoxygenation efficiency via HWH method by using graphite powder as the external susceptor, and the response behavior was compared with the situation when GO is solely submitted to microwave irradiation. In addition, the deoxygenation behavior of mildly reduced GO that was obtained via chemical reduction by Vitamin C (VC) was compared with that of GO during above two microwave heating processes.

2.1. Material synthesis GO with different oxidation degree were prepared via a modified Hummers’ method by varying the dose of KMnO4 (oxidant) [8,16,21]. According to the added amount (g) of KMnO4, the obtained GO samples were denoted as 3-GO, 5-GO, 9-GO and 12-GO, respectively. The aqueous dispersion of 12-GO was chemically reduced by VC with the mass ratio of VC to GO (RVC/GO) in 0.33 at 98 � C for 2 h, then washed with deionized water and freeze-dried to obtain VRGO [16]. The microwave irradiation experiments were carried out in a in­ dustrial microwave oven (2.45 GHz) under Ar atmosphere. The samples were placed in a double wall quartz cup that contains four spacings in the ring gaps. When samples were submitted to HWH, 0.5 g of graphite powders were filled in the ring gaps except the spacings permitted the microwave penetration. The samples 3-GO, 5-GO, 9-GO and VRGO with 0.5 g was exposed to microwave with graphite as susceptor at 600 W for 150 s, which was recorded as 3-GO-S600, 5-GO-S600, 9-GO-S600 and VRGO-S600, respectively. Fig. S1 (see supporting information) shows the schematic of the quartz cup filled with graphite powders. The quartz cup was placed on a turntable that was rotated during the experiment to ensure uniform heating. For comparison, the corresponding GO and VRGO samples were also irradiated directly by microwave without assistance of graphite, to obtain 3-GO-600, 5-GO-600, 9-GO-600 and VRGO-600, respectively. Besides, the 3-GO and VRGO sample was subjected to microwave directly at 2000 W for 30 s to get sample 3-GO2000 and VRGO-2000, respectively. 2.2. Characterization X-ray photoelectron spectroscopy (XPS) measurement was per­ formed on an Axis Ultra DLD X-ray photoelectron spectrometer (Kratos, Britain). Nitrogen adsorption-desorption isotherms of samples were measured on an ASAP 2020 instrument at 77 K. The samples were separately degassed at 100 � C for 4 h under vacuum before test. Brunauer-Emmett-Teller (BET) equation was used to calculate specific surface area by adopting the experimental adsorption data in the rela­ tive pressure (P/P0) range of 0.05–0.2. The pore size distribution was determined by using density functional theory (DFT) model. The inter­ layer spacing (d002) was calculated by the Bragg law: λ ¼ 2d002sinθ002. Raman spectra were tested on a Thermo Scientific DXR Raman spec­ trometer using laser excitation at 514.5 nm. Based on Raman results, the size of graphene domain (La, nm) can be estimated by La (nm) ¼ (560/ E4) (ID/IG) 1, where E is the laser energy in nanometers (λ ¼ 514.5 nm, 2.41 eV), and ID and IG is the integrated intensity (area) of the D and G bands calculated by Lorentz fitting, respectively [22]. The thermal sta­ bility of samples was characterized by a SDT Q600 thermogravimeter under nitrogen at temperature range from 30 to 800 � C with a ramp rate of 10 � C/min. The X-ray diffraction (XRD) measurements were carried out on a Rigaku-DMAX2200PC diffractometer by using Cu Kα radiation (λ ¼ 1.5418 Å) with 2θ in a range of 5-80� . High-resolution solid-state Table 1 XPS surface atomic percentages and C/O atomic ratios of samples.

2

Samples

C (at.%)

O (at.%)

C/O ratio

3-GO 3-GO-S600 3-GO-2000 5-GO 5-GO-S600 9-GO 9-GO-S600 VRGO VRGO-600 VRGO-S600

69.13 86.95 70.99 66.12 84.51 62.34 83.82 70.21 76.55 82.55

30.87 13.05 29.01 33.88 15.49 37.66 16.18 29.79 23.45 17.45

2.24 6.66 2.45 1.95 5.46 1.66 5.18 2.36 3.26 4.73

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Fig. 1. The deconvoluted C1s spectra of 3-GO, 3-GO-S600, 3-GO-2000, 5-GO, 5-GO-S600, 9-GO, 9-GO-S600, VRGO, VRGO-600 and VRGO-S600 samples. 13

C NMR spectra were examined on a Bruker Avance III (500 MHz) NMR spectrometer. A N5244A (Agilent) vector network analyzer was used to measure the complex permittivity and permeability in the frequency range of 2–16 GHz to investigate microwave absorption ability. 5 wt % of sample powder and 95 wt % of paraffin wax was mixed and pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mm for measurements.

3. Results and discussion GO samples with different oxidation degrees were prepared by varying the mass of KMnO4. As expected, the oxygen concentration in GO samples increases with the increasing amount of oxidant employed, as shown in Table 1, which is 30.87%, 33.88% and 37.66% for sample 3GO, 5-GO and 9-GO, respectively, determined by the XPS measure­ ments. When GO samples are directly exposed to microwave, no response to irradiation is observed as shown in Fig. S2 (see supporting information), which is consistent with the phenomenon reported in the literature [14,20]. In contrast, the obvious volume expansions are 3

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microwave irradiation under 2000 W for 30 s in Ar when the thermally reduced GO used as a external susceptor [21]. Qiu et al. reported that oxygen content of graphite oxide after microwave irradiation at 800 W for 2 min under N2 by mixing with 5% of graphene as the local susceptor reduced from 49.0% to 10.37% [14]. Kim et al. reported that the gra­ phene prepared by microwave solid-state irradiation at 1600 W for 50 s from graphite oxide (45.4% of oxygen) doped with 10% of graphene under an Ar atmosphere and H2/Ar atmosphere possessed oxygen con­ centration of 9.85% and 6.6%, respectively [20]. The surface area and pore structure of GO samples with different oxidation degrees before and after microwave irradiation were exam­ ined via nitrogen gas absorption/desorption technology, as displayed in Fig. S3 (see supporting information). The isotherms of all samples show an H4-type hysteresis loop, suggesting the existence of slit-like pores produced by the stacking of graphene sheets [23]. The surface area of 3-GO, 5-GO and 9-GO is 5, 77 and 125 m2/g, respectively, which is proportional to the oxidation degree of GO. The corresponding surface area increases to 272, 366, 330 m2/g for sample 3-GO-S600, 5-GO-S600 and 9-GO-S600, respectively, demonstrating the reductive exfoliation of GO samples during HWH process with graphite as the external susceptor.

Table 2 The fitted results (at.%) of C1s XPS spectra of samples. Sample

C–C/C– –C

C–OH

C–O–C

C¼O

O–C– –O

π-π*

3-GO 3-GO-S600 3-GO-2000 5-GO 5-GO-S600 9-GO 9-GO-S600 VRGO VRGO-600 VRGO-S600

44.29 53.22 46.03 34.86 52.02 28.85 51.90 43.48 50.56 53.99

10.29 7.66 6.91 12.98 8.32 15.43 8.89 11.45 9.71 8.62

24.37 16.08 23.70 29.16 16.88 25.80 14.83 16.9 11.44 8.67

13.53 5.12 12.77 14.38 7.19 19.50 8.21 11.58 10.33 9.49

7.51 5.64 7.32 8.63 6.38 10.50 7.44 12.31 11.21 6.90

– 12.28 3.28 – 9.21 – 8.73 4.28 6.75 12.32

observed for above GO samples submitted to microwave irradiation by using graphite powder as the external susceptor, demonstrating the deoxygenation occurs to these samples in this HWH process. It is shown that the oxygen content of 3-GO, 5-GO and 9-GO is reduced to 13.05%, 15.49% and 16.18%, respectively. It should be mentioned that the deoxygenation efficiency of GO is related to the microwave heating conditions and the way of using susceptor. Our previous work showed that the oxygen content of GO can be reduced from 42.55% to 6.95% via

Fig. 2. SEM images of (a1) 3-GO, (a2) 3-GO-S600, (b1) 5-GO, (b2) 5-GO-S600, (c1) 9-GO, (c2) 9-GO-S600, (d1) VRGO, (d2) VRGO-600 and (d3) VRGOS600 samples. 4

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Fig. 3. XRD patterns of different samples.

For comparison, the mildly reduced GO (VRGO) with the similar oxygen content as 3-GO, which was fabricated via reduction of 12-GO with VC, was subjected to microwave irradiation. It is interesting to note that a detectable volume expansion is observed for VRGO when it is solely exposed to microwave at 600 W for 150 s, with the oxygen con­ centration decreasing from 29.79% to 23.45%. Moreover, when VRGO is

irradiated with the aid of graphite under the same microwave condition, its oxygen concentration further reduces to 17.45%, demonstrating the higher deoxygenation efficiency via HWH method. Furthermore, even when 3-GO is irradiated under 2000 W for 30 s (3-GO-2000), it cannot interact with the microwave efficiently and almost no change of oxygen concentration is witnessed. However, the oxygen content of VRGO 5

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method to reduce GO samples. GO samples present higher deoxygen­ ation efficiency in comparison with that of VRGO sample in this process, which should be due to the fact that GO samples contain high amount of epoxides that can be readily reduced during HWH process [21]. Also, less relative content of epoxides and carboxyls can be observed for VRGO-S600 than that of VRGO-600, which has been demonstrated in our previous work that epoxides and carboxyls groups can be removed more easily in a mixed heating mode [21]. Fig. 2 displays SEM images of GO and VRGO samples before and after microwave irradiation. It can be seen that the GO samples show a smooth surface with tightly packed sheets due to the good interaction

Table 3 The interlayer spacings (d) of samples. Sample

d (nm) (2θ ¼ 26.4� )

d (nm) (2θ ¼ 11.6� )

3-GO 3-GO-600 3-GO-S600 3-GO-2000 5-GO 5-GO-600 5-GO-S600 9-GO 9-GO-600 9-GO-S600-90 9-GO-S600 VRGO VRGO-600 VRGO-S600

0.338 0.339 0.346 0.339 0.807 0.787 0.350 0.816 0.813 0.807 0.356 0.375 0.371 0.367

0.760 0.755 / 0.725 / / / / / / / / / /

Table 4 The ID/IG and La of samples.

reduces to 10.75% on this condition (sample VRGO-2000) [16], implying its stronger interaction with microwave. The concentration variation of each functional groups during different microwave processes was further investigated. As shown in Fig. 1, the C1s peak can be deconvoluted to six components located at – C/C–C 284.8, 285.5, 286.6, 287.5, 288.7 and 291.0 eV, assigned to C– – O), in aromatic, hydroxyls (C–OH), epoxides (C–O–C), carbonyls (C– – carboxyls (O–C– O) and π-π* groups, respectively [2–7,16,21], the fitted results are displayed in Table 2. It can be seen that the concentrations of oxygen functional groups for the GO samples after HWH treatment decrease evidently, demonstrating the HWH method is a effective

Sample

ID/IG

La (nm)

3-GO 3-GO-600 3-GO-S600 3-GO-2000 5-GO 5-GO-600 5-GO-S600 9-GO 9-GO-600 9-GO-S600-90 9-GO-S600 VRGO VRGO-600 VRGO-S600

1.590 1.588 1.671 1.573 1.691 1.682 1.712 1.694 1.693 1.727 1.883 1.748 1.764 1.761

10.438 10.456 9.932 10.554 9.814 9.868 9.698 9.799 9.803 9.610 8.817 9.498 9.413 9.424

Fig. 4. Raman spectra of different samples. 6

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samples increases with the increasing addition amount of KMnO4, and that of RGO samples obtained via HWH process at 600 W for 150 s de­ creases correspondingly. Moreover, no change was observed for XRD patterns of 9-GO after heating for 90 s (sample 9-GO-S600-90), indi­ cating the HWH process requires a long enough time to reduce GO. This should be attributed the fact that it needs a proper time for the graphite susceptor heat GO up to suitable temperature when GO can absorb mi­ crowave by itself, then the rest of heating occurs via the direct inter­ action of GO with microwave. Raman spectra of all the samples display a D peak at 1350 cm 1 and a G peak at 1590 cm 1 (Fig. 4), which is ascribed to the disordered carbon and graphitic sp2 domains, respectively [12,13,16,21]. It is shown that the ID/IG ratio for GO samples rises and the corresponding average size of graphene domains (La) reduces with the increase of the KMnO4 con­ sumption amount, revealing more vacancies, defects or distortions are introduced into the graphene sheets during oxidation process (Table 4). The La value further decreases after HWH treatment due to the gener­ ation of more numerous smaller sp2 domains. TGA analysis was used to examine the thermal stability of GO and VRGO samples, as shown in Fig. 5. It is generally accepted that the weight loss below 120 � C is assigned to the evaporation of adsorbed water and that above this temperature is attributed to the disintegration of unstable oxygen-containing groups. The mass loss in the temperature range of 120–800 � C for sample 3-GO, 5-GO and 9-GO is 40.35%, 55.43% and 59.51%, respectively, suggesting the stability of GO sample becomes poorer with the increasing amount of adopted oxidant. In comparison, the weight loss in this temperature range for sample VRGO is 58.91%, indicating the oxygen functional groups are mainly attached around the outside edge for sample 3-GO that possesses the similar C/O ratio with VRGO detected by the surface sensitive XPS technology. In accordance with above XRD resultsFig. S4, sample 3-GO still contains partial oxidation and incomplete exfoliation of graphite constructions [24]. In addition, the TG curves of GO samples after microwave irradi­ ation are displayed in Fig. S4 (see supporting information). It is shown that the thermal stability of irradiated GO samples is related with their oxygen content, being consistent with the above XPS results. The solid state 13C NMR was used to investigate the microstructure characteristics of 3-GO and VRGO, as shown in Fig. 6. It can be seen that four distinct peaks at the chemical shifts of 170, 135, 75 and 60 ppm are present on spectra of both samples, which are assigned to carbonyl

Fig. 5. TGA curves of 3-GO, 5-GO, 9-GO and VRGO samples under nitro­ gen atmosphere.

between layers via oxygen-containing functional groups (Fig. 2a1, b1, c1). After microwave irradiation assisted by graphite susceptor, as shown in Fig. 2a2, b2, c2, the GO samples exist as transparent and wrinkled sheets, indicating these layers are exfoliated and reduced to a certain extent. VRGO sample is composed of randomly agglomerated and crumpled sheets (Fig. 2d1), and this appearance is almost un­ changed after microwave irradiation for the resultant VRGO-600 and VRGO-S600 sample, as seen from Fig. 2 d2 and d3. The XRD patterns of different samples are shown in Fig. 3. A distinct diffraction peak at ca. 11� was observed for the GO samples, and an additional weak peak at 26.4� (d ¼ 0.338 nm) was present on the pattern of 3-GO, indicating the formation of few-layered GO due to the incom­ plete oxidation/intercalation of graphite when the oxidant is at a rela­ tively low dosage [24]. No detectable change was observed on the patterns of GO samples after direct microwave irradiation, whereas a broad peak at ca. 25� was found for the samples subjected to microwave heating with the assistance of graphite. The d-spacing of samples was calculated and listed in Table 3. It is shown that the d-spacing of GO

Fig. 6. Solid state

13

C NMR spectra of 3-GO and VRGO. 7

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Fig. 7. The real and imaginary parts of complex permittivity (a, b), complex permeability (c, d), dielectric loss tangent tanδε (e) and magnetic loss tangent tanδμ (f) of 3-GO, 5-GO, 9-GO and VRGO samples as a function of frequency.

– O), graphitic sp2-bonded carbon (C– – C), hydroxyl (C–OH) and (C– epoxide (C–O–C) groups, respectively [7,25–29]. Therefore, it can be confirmed that the presence of residual graphitic domains in 3-GO and partial recovery of sp2 carbon configurations in VRGO. To elucidate the cause for the difference of deoxygenation efficiency under microwave irradiation between GO and VRGO samples, the electromagnetic parameters of complex permittivity (εr ¼ ε0 iε00 ) and complex permeability (μr ¼ μ0 iμ00 ) for 3-GO, 5-GO, 9-GO and VRGO samples were investigated in the frequency range of 2–16 GHz, as shown in Fig. 7. The real parts of complex permittivity (ε0 ) and complex permeability (μ0 ) correspond to the storage capacity of electric and magnetic energy, and imaginary parts of complex permittivity (ε00 ) and complex permeability (μ00 ) relate to the dissipation capability of electric and magnetic energy, respectively [12,13,16,21]. It is shown that ε0 of 3-GO, 5-GO, 9-GO and VRGO is in the ranges of 2.74–2.80, 2.33–2.39, 2.31–2.39 and 2.40–2.50, respectively. The ε00 for 3-GO, 5-GO, 9-GO is close, which is in the range of 0.01–0.06, and that of VRGO is within the compass of 0.05–0.1. The larger value of ε0 for 3-GO should be related to its higher conductivity due to its incomplete oxidized structure. In addition, the dielectric-loss tangent (tanδε ¼ ε00 /ε0 ) and the magnetic loss tangent (tanδμ ¼ μ00 /μ0 ) are evaluated to estimate the microwave dissi­ pation ability of samples. It is obvious that VRGO presents the largest tanδε value of 0.024–0.045, due to its relative large ε00 value and low ε0 value, indicating its largest dielectric loss and highest capability of

converting microwave to the other forms of energy. Besides, the tanδμ value is lower than the tanδε value for all the samples, implying that the dielectric loss, instead of magnetic loss, plays a dominant role in the attenuation of microwave. To further evaluate and compare the microwave absorption capa­ bilities of different samples, the reflection loss (RL) was calculated ac­ cording to the transmission line theory with the following equations [18–22], " rffiffiffiffi pffiffiffiffiffiffiffiffi# i⋅2πfd μr εr μr Zin ¼ tanh (1) c εr � � �Zin 1� �ðdBÞ RL ¼ 20 log�� Zin þ 1�

(2)

where Zin is the input impedance, f is the frequency of the electromag­ netic wave, d is the thickness of the absorber, and c is the velocity of electromagnetic wave. Fig. 8a, b, c and d show the calculated reflection loss of 3-GO, 5-GO, 9-GO and VRGO samples with the different thick­ nesses of 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mm in the frequency range of 2–16 GHz. It can be seen that VRGO displays the largest reflection loss at each thickness, showing superior microwave absorp­ tion ability as compared with other samples. This should be attributed to good impedance matching characteristic of VRGO sample, which is determined by the compatibility of permittivity and permeability, 8

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Fig. 8. Reflection loss of (a) 3-GO, (b) 5-GO, (c) 9-GO and (d) VRGO with different thickness, (e) the reflection loss at 2.4 and 2.48 GHz and (f) the cole-cole semicircle curves of 3-GO, 5-GO and 9-GO and VRGO samples.

because overlarge difference between the permittivity and permeability is detrimental to the impedance match due to more reflection of elec­ tromagnetic wave from the surface of absorber [12,13,16,21]. In com­ parison with 3-GO, the lower permittivity of VRGO is more closer to its permeability, which is beneficial to impedance matching and ensures that the incident electromagnetic wave can permeate into the materials more easily. As shown in Fig. 8e, the reflection losses of VRGO at 2.4 and 2.48 GHz are lager than those of 3-GO, demonstrating the superior mi­ crowave adsorption capacity of VRGO under the experimental condition

(2.45 GHz). The Debye dielectric relaxation model (Cole–Cole model) was adopted to further clarify the mechanisms of the dielectric behaviors of GO and VRGO samples. According to Debye theory, the relative complex permittivity can be expressed by the following equation [30–32],

εr ¼ ε∞ þ

εs

ε∞

1 þ j2π f τ

¼ ε’ þ iε’’

(3)

where f is the frequency, εs is the static permittivity, ε∞ is the relative 9

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dielectric permittivity at the high-frequency limit, and τ is the polari­ zation relaxation time. Thus, ε0 and ε00 can be described by

ε’ ¼ ε∞ þ

ε’’ ¼

εs

ε∞

1 þ ð2πf Þ2 τ2

2πf τðεs

ε∞ Þ

1 þ ð2πf Þ2 τ2

external susceptor was investigated. The oxygen concentrations of GO is decreased from 30.87%, 33.88%, 37.66% to 13.05%, 15.49%, 16.18% for sample 3-GO, 5-GO and 9-GO, respectively, indicating the reduction happens under assistance of graphite susceptor, and the deoxygenation efficiency is closely related with their oxidation degree and the value of dielectric loss (ε0 ). No response of GO to microwave irradiation is perceived when above GO samples are directly exposed to microwave. However, the mildly reduced GO (VRGO) sample can be heated when it is solely submitted to microwave irradiation. The VRGO sample pos­ sesses relative larger dielectric parameters, and contains defects and residual oxygen-containing functional groups that not only can improve the impedance matching characteristics, but also produce defects po­ larization relaxation and dipole polarization relaxation, which are all beneficial to the microwave penetration and absorption. In addition, it is shown that the deoxygenation efficiency of VRGO is lower in compari­ son with GO samples under the HWH condition, which should be attributed to the fact that GO samples contains high amount of epoxides that can be easily reduced in a mixed heating mode. This work dem­ onstrates that the HWH method can be a effective and energy-efficient way to reduce GO samples with different oxidation degree, and for mildly reduced GO samples, the direct solid-state microwave irradiation is a easy way for their deep reduction and can be developed to a perspective approach to fabricate RGO-based nanocomposites for the particular applications such as the energy storage, catalysis and biotechnologies.

(4) (5)

According to eqs (4) and (5), the relationship between ε0 and ε00 can be speculated, � εs þ ε∞ �2 � �2 �εs ε∞ �2 ε’ þ ε’’ ¼ (6) 2 2 Thus, the plot of ε0 versus ε00 would be a single semicircle, which is denoted as the Cole–Cole semicircle, generally, each semicircle repre­ sents one Debye relaxation process. Fig. 8f shows the ε0 -ε00 curves of 3GO, 5-GO and 9-GO and VRGO samples, which contain several semi­ circles, indicating multi-relaxations are present during interactions with microwave. Microwave irradiation may cause heating by two main mechanisms, conduction (Joule heating) and dipolar polarization [30–35]. For above GO and VRGO samples, the carbon framework is partially existed, so the dielectric relaxation process that caused by the lag of delocalized π-electrons occurs to transfer the electromagnetic energy to heat energy. Meanwhile, the additional relaxation processes arising from defects and oxygen-containing groups could happen to attenuate microwave [30–35]. The defects can act as the polarization centers to produce polarization relaxation in the altering electromag­ netic field. Moreover, stretching and bending vibrations of polar oxygen-containing functional groups can occur in the electromagnetic field, and additional charge rearrangement and orbital hybridization of these groups may also happen to result in electric dipole polarization [31,32]. In addition, the Cole–Cole semicircles are curled, indicating that other loss mechanisms may exist besides dipolar relaxation, for instance, interfacial polarization between GO sheets and paraffin [30–32]. Previous XPS results show that the oxygen content of 3-GO, 5-GO and 9-GO can be reduced from 30.87%, 33.88% and 37.66% to 13.05%, 15.49% and 16.18% after HWH process, respectively, demonstrating the final oxygen concentration of RGO samples is related to their value of dielectric loss (ε0 ). This may be due to the fact that the sample with larger dielectric loss can reach the coupling temperature earlier, at which the sample can interact with the microwaves by itself. The excellent microwave absorbing performance of the VRGO sample is mainly attributed to its good electromagnetic wave attenuation and impedance matching characteristics. Compared with 5-GO and 9-GO, the partially reduced sample VRGO possesses larger ε0 and ε00 value, demonstrating its strong electromagnetic wave dissipation capability. Nevertheless, the high conductivity of 3-GO leads to a significant skin effect to lower the level of impedance matching. In comparison with 3GO, the lower permittivity of VRGO is more closer to its permeability, which ensures impedance matching feature to make the incident mi­ crowave enter the materials readily. VRGO contains obvious defects and residual oxygen-containing functional groups on the surface and inside of material, not only can improve the impedance matching character­ istics, but also produce defects polarization relaxation and dipole po­ larization relaxation which are beneficial to the microwave loss. Based on the above reasons, VRGO shows enhanced microwave absorption ability compared with above GO samples with different oxidation de­ gree, so it can interact with electromagnetic wave when it is directly exposed to microwave irradiation and give rise to a high deoxygenation efficiency.

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4. Conclusion In this study, the influence of oxidation degree of GO on the deox­ ygenation efficiency via HWH method by using graphite powder as the 10

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