Ultrafast microwave reduction process for high-quality graphene foam with outstanding electromagnetic interference shielding and good adsorption capacity

Ultrafast microwave reduction process for high-quality graphene foam with outstanding electromagnetic interference shielding and good adsorption capacity

FlatChem 17 (2019) 100117 Contents lists available at ScienceDirect FlatChem journal homepage: www.elsevier.com/locate/flatc Ultrafast microwave red...

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FlatChem 17 (2019) 100117

Contents lists available at ScienceDirect

FlatChem journal homepage: www.elsevier.com/locate/flatc

Ultrafast microwave reduction process for high-quality graphene foam with outstanding electromagnetic interference shielding and good adsorption capacity☆ ⁎

Taolin Zhanga,b,1, Xiongying Qiua,1, Zhichang Xiaoc, Yingjie Maa, Debin Konga, , Linjie Zhia,b,

T



a

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100039, China c College of Science, Agricultural University of Hebei, Baoding 071001, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Microwave reduction Catalytic pre-reduction High-quality graphene foam Electromagnetic interference shielding Oil absorption capacity

Mass production of high-quality graphene and its assemblies is a key step for practical application of graphene in many fields. Herein, a facile and ultrafast microwave reduction process with the aid of catalytic pre-reduction at room temperature is reported towards the preparation of high-quality graphene foam. The catalytic pre-reduction aided microwave method enables an efficient reduction process within a few seconds, whilst the original architecture of three-dimensional graphene foam remains, along with a superhigh C/O ratio (31.3), low lattice defects, and significant electrical conductivity (50,000 S/m) as well as excellent dispersibility and processability. Due to the intrinsic structure advantages of MW-H-GF, when applied to electromagnetic shielding, it could deliver a shielding efficiency of around 50 dB in the low frequency region while 70 dB in the high frequency region, indicating excellent electromagnetic shielding performance. Furthermore, MW-H-GF also demonstrated remarkable oil adsorption capacity which can reach an adsorption capacity of 480 times chloroform of the selfweight.

1. Introduction Graphene [1–3], as a unique two-dimensional monoatomic thick carbon plate, has been expected to revolutionize a wide range of technological areas due to its various attractive physical and chemical properties. Mass production of high-quality graphene and its assemblies is a key step for practical application of graphene in many fields. Several typical methods, such as liquid phase exfoliation [4–6], chemical vapor deposition [7,8], reduction of graphite oxide (GO) [9,10], and electrochemical exfoliation [11–14] have been widely applied in the preparation and application of graphene. Among these, exfoliation and reduction of GO has attracted intense attentions in the communities of chemistry and materials due to its capability for affordable and highthroughput production of reduced graphene oxide (rGO) or chemically converted graphene. Particularly, owing to its abundant functional groups, rGO could be also utilized for the design and preparation of structurally rich assemblies, which exhibit intensive practical potential in the applications of adsorption [15–19], electromagnetic shielding

[20–29] as well as energy storage and conversion. Additionally, efficient reduction of GO into high-quality graphene is a key factor for the enhancement of performance. Typical GO reduction methods generally undergo chemical reduction [9] or high temperature thermal expansion [10], which involve either potentially harmful reductants or high energy consumption. Very recently, microwave radiation [11] has been proved to be a rapid and facile method to realize the successful exfoliation and reduction of GO. For example, Ruoff et al. [30] for the first time reported a strategy of directly treating GO powder in commercial microwave ovens to prepare conductive (274 S/m) microwave flaky graphite oxide (MEGO) with a medium deoxidization degree (2.75C/O ratio) and specific surface area (463 m2/g). However, the low conductivity of graphene oxide itself leads to an unsatisfactory performance in microwave reduction, especially for the reduction efficiency. Mixing rGO with graphite [31] has been reported to be an effective strategy to improve the instinct conductivity. Despite these progresses achieved in recent years [32–35], it is still highly desired to develop new methods for fast mass production



Conflicts of interest: The authors declare no conflicts of interest. Corresponding authors. E-mail addresses: [email protected] (D. Kong), [email protected] (L. Zhi). 1 Contribute equally to this work. ⁎

https://doi.org/10.1016/j.flatc.2019.100117 Received 3 March 2019; Received in revised form 29 May 2019; Accepted 3 June 2019 Available online 07 June 2019 2452-2627/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The typical preparation process diagram of MW-H-GF.

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of high quality graphene, particularly and its assemblies involving lessor non-toxic chemicals. Herein, high-quality graphene foam (MW-H-GF) with large-scale production is successfully developed through an efficient microwave reduction process with the aid of catalytic pre-reduction at room temperature, which can unexpectedly promote microwave exfoliation and reduction of GF significantly. Even within a few seconds, the reduction process towards three-dimensional GF can still be enabled. Remarkably, advantageous structure characters, namely, superhigh C/O ratio (31.3), low lattice defects, and significant electrical conductivity (50,000 S/m) as well as excellent dispersibility and processability are retained after reduction process. Consequently, an excellent electromagnetic shielding efficiency (EMI SE) of around 50 dB in the low frequency region and 70 dB in the high frequency region is observed when applied to electromagnetic shielding. Furthermore, an oil adsorption capacity which can reach an adsorption capacity of 480 times chloroform of the self-weight is also demonstrated.

2.3. Adsorption performance test In order to test the adsorption capacity of the samples, oil, soybean oil, olive oil, chloroform, acetone, twelve alkane and other organic reagents were utilized. Typically, the foam with a mass of m0 was immersed in the solvent. After sufficient adsorption (around 1 min), the weight of the foam was recorded as m1 after drying the surface. Subsequently, the adsorption capacity of Q was defined as the mass of adsorbed material divided by the weight of the foam:

Q= (m1 − m 0)/m 0 2.4. Electromagnetic shielding performance test According to the size of the coaxial line (Agilent 85051–60007, 50 Ohm Airline, inner diameter of 3 mm, outer diameter of 7 mm), the as-fabricated film was cut by laser engraving machine, subsequently put in the sample preparation mold of coaxial testing. The melted paraffin wax was poured into the mold, cooled and solidified at room temperature, so that the film and paraffin wax were fully bonded, and an EMI shielding coaxial testing sample was gained. The EMI shielding effectiveness of the film was tested using PNA-X Network Analyzer (Agilent Technologies, N5247A).

2. Experimental section 2.1. Preparation of MW-H-GF Graphene oxide (GO) was prepared by the oxidation of natural graphite flakes using the modified Hummers method [36]. With the addition of 1.25 mg PdCl2, 50 mL GO solution (2.5 mg mL−1) suffered from the freeze-drying process to produce GO foam, which is denoted as GF. Then, the hydrogen-involved catalytic reduction of GF (H-GF) was conducted at room temperature in the reaction bottle with the existence of hydrogen. Finally, H-GF was irradiated by microwave (1000 W) in H2 atmosphere to yield MW-H-GF for 10 s.

3. Results The typical preparation process diagram is illustrated in Fig. 1. To obtain homogeneous dispersion of GO and catalysts, PdCl2 was used as the precursor because PdCl2 is easily reduced to Pd metal by hydrogen at room temperature [35]. Typically, the synthesis of MW-H-GF experiences a two-step reduction process. With the addition of precursor PdCl2, a brown cylinder of GF was obtained by the direct freeze-drying of the homogeneous mixture. GF is initially reduced to H-GF with enhanced conductivity at hydrogen atmosphere in the presence of Pd catalyst, and the experimental details can be seen in our previous work [35]. The enhanced conductivity can effectively help the material to adsorb microwaves [34]. Finally, H-GF was irradiated by microwave (1000 W) in H2 atmosphere to yield MW-H-GF. In microwave process, H-GF with improved conductivity can make the temperature rise to thousands of degrees in a very short time, thus realizing the quick reduction and repair of graphene to yield MW-H-GF. Compared with the

2.2. Instrument The morphologies of the samples were observed with scanning electron microscopy (SEM, Hitachi SU8220) and Field-emission transmission electron microscopy (FE-TEM, Tecnai G2 T20-TWIN). The thickness of the graphene sheet of MW-H-GF was examined through atomic force microscopy (AFM, Bruker Fastscan). The chemical information of the samples was characterized by Fourier transform infrared spectrometer (FT-IR, Perkin, Spectrum One), Raman spectra (Renishaw, Renishaw invia plus) and X-ray photoelectron spectroscopy (XPS, HI1600). The thermal stability of the samples was measured by means of thermogravimetric analysis (TGA, Rigaku Thermal plus TG 2

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Fig. 2. Morphology and microstructure of MW-H-GF. (a–c) SEM images of MW-H-GF, (d and e) TEM images of MW-H-GF, the inset is the SAED image (e), (f) AFM image of MW-H-GF.

electron microscopy (TEM) further illustrated the detailed microstructure of MW-H-GF. As shown in Fig. 2d and e, the graphene sheet in MW-H-GF is dominated by monolayer form with wrinkles, which is also consistent with the foam-like morphology from SEM images. The lattice fringes of graphene are observed in the HR-TEM image (Fig. S5). It shows that the lattice structure of graphene is effectively repaired by microwave reduction, and the crystallization area is significantly increased compared with that of graphene oxide. To further zoom in the edge of each graphene sheet, no stripe is observed, indicating the existence of single layer graphene in MW-H-GF. The selected area electron diffraction (SAED) image of MW-H-GF displays a six-fold symmetry and the presence of only a single set of spots (inset of Fig. 2e). This observation implies that the graphene sheets in MW-H-GF restore a crystal structure after the thorough reduction during microwave treatment. In stark contrast, the SAED image of the precursor GO generally exhibits one ring (Fig. S6), which reflects the degree of structural distortion/ degradation of graphene sheets. In order to obtain the direct information about the thickness of graphene sheet, the dispersed droplets of graphene were coated on silicon wafers and characterized by AFM. From the characterization results of Fig. 2f, we can see that the thickness of graphene sheet is around 2 nm, which is thicker than the theoretical value of monolayer graphene. This phenomenon can be probably attributed to the large amount winkles in our graphene sheet as well as the increased distance due to the oxide layer on the surface of the substrate and the van der Waals force between the graphene sheets. XPS and FTIR measurements were performed to investigate the reduction degree and chemical information of GF, H-GF and MW-H-GF. For comparison, the raw graphite and the sample suffering from heat treatment of 800 °C (denoted as 800-GF) were also investigated. The XPS survey spectra of GF, H-GF and MW-H-GF (Fig. 3a) exhibited that the C/O atomic ratio was increased from 2.0 for GF to 5.0 and 31.3 for H-GF and MW-H-GF, respectively, which verify that MW-H-GF is much more reduced than H-GF by microwave reduction reaction. It is worthy note that our MW-H-GF shows a comparable atomic ratio even with raw graphite (29.0), which proves the impressive reduction degree through our microwave reduction strategy. Interestingly, after microwave reduction process, the ultralow content of palladium in the obtained samples can be attributed to the rapid sublimation of palladium chloride at instantaneous high temperature which may reach thousands

traditional microwave process that needs 15 min, surprisingly, this reaction could be triggered immediately and completed within 10 s with the aid of the catalytic pre-reduction strategy. Interestingly, the direct reduction of GF could effectively avoid the stacking of reduced graphene due to Van der Waals force, and thus enhance the dispersibility greatly. The color of GF changed from brown to light black after hydrogen pre-reduction, indicating that H-GF has been reduced to a certain extent in hydrogen. After microwave irradiation reduction (as schemed in Fig. S1), the macrostructure of MW-H-GF remained intact and the color is further turned to black owing to the thorough reduction of H-GF. The morphology information of the MW-H-GF is first observed by means of scanning electron microscopy (SEM). As shown in Fig. 2a, after intense microwave reduction process, the MW-H-GF demonstrated a three-dimensional (3D) network structure. The foamed and loose architecture could greatly weaken Van der Waals' force, and thus effectively inhibit the restacking of graphene sheets during reduction. Additionally, graphene sheets intend to exhibit irregular morphology with many wrinkles, which may be attributed to the utilization of freezedrying technology to fix the original morphology of GF (Fig. S2). It is also noted that the existence of wrinkles of graphene sheets in MW-HGF could separate the sheets from each other to ensure the yield of single or few layers graphene. Graphene sheets with lateral dimensions as high as tens of micrometers can be observed (Fig. 2b) while the edge of the graphene sheets shows a thickness of less than 2 nm (Fig. 2c), suggesting single or few layers graphene in MW-H-GF. Besides, the macro- and mesoporous feature of original GF has been demonstrated by nitrogen adsorption-desorption measurements, as shown in Fig. S3. The adsorption data indicate the specific surface area of GF is around 370 m2 g−1, which is in consistent with the previous report and reflects the good quality of precursor GO. Moreover, GF is obviously highly macroporous, due to the random stacking of graphene oxide with large sheet size. Interestingly, our MW-H-GF shows surprisingly good dispersity and efficient processability. The dispersion of graphene can be easily obtained with a high concentration of 2.5 mg ml−1 by ultrasonication (only 5 min at the power of 60 W in NMP), which could be stable for several weeks without agglomeration or precipitation. The SEM images (Fig. S4) show that the size of graphene sheet after ultrasonication is comparable to that before dispersion, indicating that the dispersion process does not damage the graphene sheets. Transmission 3

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Fig. 3. Chemical structure information and reduction degree. (a) XPS spectra of GF, H-GF and MW-H-GF; (b) C1s XPS spectrum of GF, H-GF and MW-H-GF; (c) Raman spectra of GF, H-GF and MW-H-GF; (d) TGA pattern of GF, H-GF and MW-H-GF; (e–g) Raman mapping images of MW-H-GF.

atmosphere were used respectively. As shown in Fig. S9, the samples obtained in hydrogen atmosphere possess the least defects, while the samples obtained in air atmosphere show the most defects. When microwave reduction is carried out in hydrogen atmosphere, oxygencontaining functional groups on graphene oxide are more likely to combine with hydrogen, rather than remove carbon atoms from graphene lattice to form defects at the same time. Therefore, microwave reduction in hydrogen atmosphere can further reduce the defect density of graphene and improve the quality of graphene products. The intensity ratio of 2D peak to G peak (I2D:IG) is about 1:1, which indicates the few layer structure [9]. Moreover, the Raman mapping images of D, G and 2D peaks of MW-H-GF further demonstrated the uniform distribution as well as the high intensity of G and 2D peaks, suggesting the entire crystal structure with high quality. On the contrary, removal of oxygen-containing functional groups in inert and air atmosphere usually takes away carbon atoms in graphene lattice structure, resulting in the massive generation of defects. To further confirm the efficient reduction of GF, the TGA analysis is conducted in N2 atmosphere using a heating rate of 10 °C min−1, as shown in Fig. 3d. The GF showed a large weight loss upon heated to 800 °C owing to the removal of oxygen functional groups. Obviously, as reflected in TGA curves, the weight loss of GF and H-GF reached 60% and 20% at 800 °C, respectively, while almost no weight loss (~1% weight loss) can be observed for MW-H-GF after the microwave reduction process. This results clearly demonstrate the complete removal of oxygen-containing functional groups and the efficient recovery of graphene sheet (Fig. 3e–g). In a word, all these results unambiguously demonstrate hydrogen atmosphere plays a key role in the recovery of conjugate network. Benefiting from the interconnected three dimensional structure as well as the high hydrophobicity due to the recovery of conjugate carbon network and ultralow oxygen content, MW-H-GF is expected to be competitive in applications of high-efficiency adsorption, which is critical for the separation/extraction of oils or other organic pollutants [40]. MW-H-GF exhibits an impressive performance for adsorption of model oil molecules. As illustrated in Fig. 4a–c, after the addition of a piece of MW-H-GF, the oil will be quickly adsorbed in 10 s,

of degree. As the typical C1s spectrum of GF (Fig. 3b) generally exhibits additional peaks that correspond to the carbon bonded to hydroxyl and epoxy groups (C-O), carbonyl carbon (C]O), and carboxyl carbon (O]CeOH), the C1s XPS spectrum of MW-H-GF (Fig. 3b) only shows a single peak (284.7 eV) corresponding to the nonoxygenated carbons in aromatic rings (CeC/C]C), indicating most of oxygen-containing functional groups have been removed through the microwave and catalytic microwave process [37]. For the FT-IR spectra of GF, H-GF and MW-H-GF (Fig. S7), the strong absorption peak at 3440, 1732, 1630 and 1055 cm−1 of GF, corresponding to OeH, C]O, CeOH and CeOeC vibrations respectively, are greatly reduced in H-GF and MWH-GF [38]. When focused on MW-H-GF, we find that all peaks were weaker than H-GF, which indicated a better removal of oxygen-containing functional groups and is consistent with the XPS results. We further employed Raman spectroscopy to gain more insight into the structural differences among these samples [39]. As shown in Fig. 3c, GF generally exhibits two peaks at 1585 cm−1 and 1325 cm−1, respectively. The peak at about 1585 cm−1 (G band), corresponding to an E2g mode of graphite, is related to the vibration of the sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, while the peak at about 1325 cm−1 (D band) is related to the defects and disorder in the hexagonal graphitic layers. After H2 pre-reduction, the high D peak of H-GF indicates the excessive removal of carbon atoms in the lattice during the reduction process, which leads to higher defect density and obvious broadening of G peak. In sharp contrast, the D peak of MW-HGF obtained by microwave reduction almost disappeared, showing a very low defect density. The obvious 2D peak accompanied with the sharp G peak implies that the lattice structure of graphene was relatively complete. This indicated that the microwave process in hydrogen reduction atmosphere could repair the structure of graphene primely. Raman results from different regions exhibit almost the same shape with the similar peak intensity, confirming the homogeneity of the sample (Fig. S8). In order to investigate the effect of atmosphere on the quality of graphene during microwave irradiation, hydrogen, argon and air 4

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Fig. 4. Oil adsorption demonstration of the sample MW-H-GF. (a) digital photos of 250 μL oil in water; (b) after adding a piece of MW-H-GF, oils were subsequently adsorbed; (c) images of reach complete adsorption; (d) adsorption capacity of MW-H-GF, H-GF, 800-GF for various organic liquids.

EMI SE values of MW-H-GF film, H-GF film, and GF foil are slightly larger than 70, 30, 25 dB, respectively. Therefore, regardless of the band range, MW-H-GF film is the best in all performance. Consequently, MW-H-GF film maintains prominent EMI SE (more than 52 dB) over an ultrawide frequency range (1–18 GHz), and the highest EMI SE value of 82.922 dB recorded at 18 GHz, enough to block and adsorb 99.99999949% of incident electromagnetic wave with only 0.00000051% transmission. In conclusion, we successfully developed an efficient microwave strategy with the aid of catalytic pre-reduction at room temperature towards the preparation of high-quality graphene foam (MW-H-GF). Intriguingly, the proposed catalytic pre-reduction aided microwave method can not only shorten the reaction time from more than ten minutes without catalyst to a few seconds, but also maintain the original structure of three-dimensional graphene foam with a superhigh C/ O ratio (31.3), low lattice defects, and significant electrical conductivity (50,000 S/m) as well as excellent dispersibility and processability. Benefiting from these unique structures and properties, MW-H-GF exhibits promising advantage in electromagnetic shielding and oil adsorption areas. Specially, a shielding efficiency of around 50 dB in the low frequency region and 70 dB in the high frequency region is achieved, indicating excellent electromagnetic shielding performance. Meanwhile, MW-H-GF shows remarkable oil adsorption capacity which can reach an adsorption capacity of 480 times chloroform of the selfweight.

demonstrating an ultrafast adsorption kinetics. In order to verify the good oil adsorption ability of MW-H-GF, several different oils and organic solvents, including chloroform, acetone, motor oil, dodecane etc. have been adopted to test its adsorption capacity with comparison of HGF and 800-GF. Notably, as shown in Fig. 4d, MW-H-GF exhibits a superhigh adsorption capacity towards chloroform with 480 times of the self-weight, which is much higher than that of H-GF and 800-GF (around 200 times). For common oil pollution such as oil, soybean oil and olive oil, MW-H-GF can also achieve an adsorption capacity of 300 times of the self-weight. Such superior adsorption performance with a high adsorption capacity as well as the fast adsorption kinetics could mainly attributed to the following aspects: firstly, super-lipophilicity and hydrophobicity owing to the recovery of conjugated carbon skeleton and the almost complete removal of oxygen; secondly, preservation of macro-morphology with three dimensional network featuring the adsorption rate and conducive to the storage of oil pollution. Meanwhile, conjugate carbon skeleton with fewer defects also endow MW-H-GF with excellent conductivity for electromagnetic shielding applications. We have tested the electrical conductivity of our MW-H-GF and measured the film thickness by electronic thickness gauge. The self-supported film obtained by physical extrusion under 50 MPa pressure (Fig. 5a) holds a square resistance of around 0.8 Ω □−1, and a thickness of ca. 25 μm, which can be observed in the SEM cross section images (Fig. 5b), resulting in an electrical conductivity of around 50,000 S m−1. As presented in Fig. 5c and d, MW-H-GF film exhibits excellent EMI SE from 0 to 18 GHz, including L-band (1–2 GHz), S-band (2–4 GHz), C-band (4–8 GHz), X-band (8–12 GHz), Ku-band (12–18 GHz) according to IEEE 521–2002 standard [41]. In Lband, in terms of the EMI SE, MW-H-GF film shows a 45–50 dB which is significantly higher than that of graphene reduced by hydrogen (H-GF film) and annealed at high temperature (800-GF), while graphene oxide shows microwave transparency without any shielding effect. The highest EMI SE value of 50.349 dB recorded at 2 GHz, enough to block and adsorb 99.99908% of incident electromagnetic wave with only 0.00092% transmission. The average shielding efficiency is about 70 dB, showing excellent electromagnetic wave shielding performance at high-frequency test, while the other samples have no significant change compared with the low-frequency test results. In X-band, the

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51425302, 51302045, 51702062), the Beijing Municipal Science and Technology Commission (Z121100006812003), Opening Project of State Key Laboratory of Advanced Technology for Float Glass, Talents Introduction Plan of Hebei Agricultural University (YJ201819) and the Chinese Academy of Sciences.

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Fig. 5. Electromagnetic shielding performance. (a) Optical image of MW-H-GF obtained through physical extrusion under 50 MPa pressure; (b) The SEM cross section image of MW-H-GF film; (c) Electromagnetic shielding performance of GF, H-GF and MW-H-GF in the low frequency test of 0–3000 MHz; (d) Electromagnetic shielding performance of GF, H-GF and MW-H-GF in the high frequency test of 3–18 GHz.

Appendix A. Supplementary data

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