- Email: [email protected]
Broadband and strong electromagnetic wave absorption of epoxy composites filled with ultralow content of non-covalently modified reduced graphene oxides
Carbon 154 (2019) 115e124
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Broadband and strong electromagnetic wave absorption of epoxy composites filled with ultralow content of non-covalently modified reduced graphene oxides Xu Zhang a, Xiaoqun Wang a, *, Fanbo Meng b, Jiajun Chen a, Shanyi Du a a b
School of Materials Science and Engineering, Beihang University, Beijing, 100083, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2019 Received in revised form 5 July 2019 Accepted 21 July 2019 Available online 29 July 2019
High-efficiency microwave absorption epoxy nanocomposites filled with ultralow content of noncovalently modified reduced graphene oxides (rGO) are prepared via an in-situ polymerization method. Here, the porous and folded rGO nanosheets are successfully functionalized by strong p-p interaction between conjugated imidazole and graphene sheets. The combination of electron microscope and synchronous radiation X-ray small angle scattering (SAXS) techniques unambiguously illustrates that the non-covalently modified graphene nanosheets in the epoxy composites are well-dispersed and extensively winkle. Hence, compared with pristine rGO/epoxy composites, the composites containing the highly dispersed rGO demonstrate more ideal impedance matching and stronger microwave dissipation based on the experimental and simulated results. The composites remarkably achieve excellent microwave absorptions (minimum reflection loss of 65 dB, with a matching thickness of 1.8 mm), an extremely broad absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possess high hydrophobicity, endowing them attractive functions of selfcleaning. This work provides a promising, facile and scalable approach for designing and fabricating graphene-based nanocomposites with hydrophobicity and excellent microwave absorption capacities. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Graphene nanocomposites Microwave absorption X-ray small angle scattering
1. Introduction To address the rising electromagnetic (EM) radiation pollution from the rapid progress of electronic industry and wireless communication technology, high-performance microwave absorbing (MA) materials are desperately demanded [1,2]. Conventionally, MA materials are mainly classified as structure composites (honeycomb structure, pyramidal structure, multilayer structure and frequency selective surface, etc.) and coatings [3e5]. MA structure composites are mechanically strong and display wide absorption bandwidths, but suffer from the drawbacks of large thickness and selective absorption of microwave in some certain direction or polarization [6]. In contrast, MA coatings truly eliminate the microwave signal in nearly all directions and polarization modes, and are widely applied to portable electronic devices and military vehicles (aircrafts, ships and tanks) [7e9].
* Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.carbon.2019.07.076 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
MA coatings are fabricated by insulating polymer and lossy fillers. Traditional lossy fillers consist of magnetic loss materials (ferrites, magnetic metal powders, etc.) and dielectric loss materials (TiO2, CuS, SiC, etc.) [10,11]. Despite good microwave attenuation performance in some cases, the practical applications of those traditional microwave absorbers are severely hindered by their large mass filling ratio, high density and poor environmental adaptability [10,12]. Graphene, as a typical graphitic carbon material, has anisotropic two-dimensional (2-D) shapes, and consists of 2-D conjugated sp2 hybrid carbons [13e15]. The unique chemical structure and morphology provide graphene great potential in the field of MA due to its high electrical conductivity, good chemical durability, large specific surface, high thermal conductivity and ultralight weight [16e18]. However, MA properties of pristine graphene materials are limited by the impedance mismatch and single attenuation mechanism, which are mainly caused by its improper electrical conductivity and poor dispersion in the matrix [19,20]. Compared with pristine graphene nanosheets, reduced graphene oxide (rGO) possesses residual functional groups and defects (missing carbon
116
X. Zhang et al. / Carbon 154 (2019) 115e124
atoms and sheet corrugation), and thus presents decreased electrical conductivity and enhanced microwave attenuation ability [17,21]. As a typical example, Kuang et al. fabricated rGO/wax composites and systematically investigated the effect of reduction degrees and content of rGO on MA properties of the composites [22]. The results display that the minimum reflective loss (RL) value is 37 dB with 3.5 mm thickness at 5.92 GHz, and the filling ratio of rGO is 30 wt%. Nevertheless, the MA performance can still hardly reach to the requirement of practical applications owing to the narrow absorption bandwidth, large thickness and filling ratio. To improve the MA capacities of the rGO, firstly, most researchers combine it with magnetic or other dielectric loss materials to tune the impedance matching behavior and enhance microwave attention [23,24]. Although the compositing strategy can improve the MA performance of rGO, there are many disadvantages for the way, such as the increasing of density, weakening of chemical durability, and the unhandy compositing technology [25,26]. Besides, recently, significant progress has been made toward 3D interconnected rGO foams by microstructural design strategies. Evidently, the method can avoid the agglomeration of rGO and provide heterogeneous interfaces and voids, which are conducive to creating low effective permittivity and matched impedance. Unfortunately, despite the fact of broadband and strong EM wave absorption and great reduction of the density, rGO foam structures inevitably increases the thickness of MA materials [27e31]. Hence, it is extremely significant to prepare sole rGO-filled materials with highly efficient MA performance. As abovementioned, despite the lower electrical conductivity and more defects than pure graphene, the rGO still has a pronounced tendency to agglomerate in the matrix owing to the poor interfacial compatibility. Therefore, to improve MA capacities of rGO-filled materials, it is of key importance to form homogeneous composites. Surface modification of rGO plays a crucial role in maintaining its uniform and stable dispersion in the matrix [32]. Generally, in most application areas, Covalent interactions on the surfaces of rGO are favored because of the presence of hydroxyl, epoxy, and carboxylic groups [33]. Unfortunately, covalent functionalization inevitably destroys the structure of rGO and adversely affects its electronic properties, which drastically decreases its microwave dissipation properties [34]. In contrast, non-covalent modification plays an important role in enhancing the solubility and preventing the aggregation without weakening the MA abilities of the rGO [35,36]. Consequently, in this work, we creatively prepare epoxy MA composites filled with ultralow content of non-covalently modified reduced graphene oxides. Firstly, we synthesize the porous rGO by Hummers and green chemical reduction methods. Afterwards, imidazole, as the non-covalent modifier of rGO and curing accelerator of epoxy, was introduced into the in-suit polymerization process. The effect of imidazole contents on the MA performance is investigated. Results exhibit that our composites show strong microwave absorptions (RLmin ¼ 65 dB, with a matching thickness of 1.8 mm), an extremely wide absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possess strong hydrophobicity with a water contact angle of 112.5 , which is larger than that (z87 ) of the pure epoxy resin. In addition, we all know that the excellent MA performance of the rGO-filled composites results from the highly dispersed rGO with wrinkled and defective structures. Therefore, to statistically confirm the presence of well exfoliated and wrinkled sheets in the epoxy matrices, synchronous radiation X-ray small angle scattering (SAXS), as an effective and non-destructive technique is employed to give an average conformation of rGO in the nanocomposites [37,38].
2. Experimental section 2.1. Preparation of rGO/acetone suspension The rGO suspension was fabricated by a Hummers' method and chemical reduction method. Briefly, 6 g expanded graphite (EG) powders (325 mesh) were oxidized using a modified Hummers' method [39]. As-synthesized graphite oxide was washed 3 times by diluted 1 wt% HCl aqueous solution (400 mL) and deionized water to completely remove residual salts and acids. The resultant water suspension was passed through a weak basic ion-exchange resin with deionized water as mobile phase to remove the remaining impurities. The purified deionized water dispersion of rGO was exfoliated by ultrasonication for 1 h and subsequently reduced by adding ascorbic acid (VC) with a concentration of 10 mg/g for 4 days at room temperature. The rGO dispersion was centrifuged with ethanol and acetone washed by several times. Then, the rGO/ acetone suspension was prepared by dispersing the 100 mg washed rGO and a certain amount of imidazole (0 mg/g, 4 mg/g or 8 mg/g) into acetone with the assistance of ultrasonication. 2.2. Synthesis of rGO/epoxy composites Different concentrations of bisphenol A liquid epoxy resin (E51) were added into the rGO/acetone suspension and sonicated for 30 min at a power of 600 W. Next, the acetone is evaporated off by drying the suspension at room temperature for about 12 h in air. Then, the polyetheramine (D400), as an additional low viscosity curing agent, was added into the rGO/epoxy slurry and grinded for approximately 10 min. The mixture was transformed to a cylindrical stainless steel mould and pressed into toroidal-shaped specimen with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Finally, the sample was cured at 75 C for 1 h, 110 C for 1 h, followed by 1 h of post-cure at 150 C. 2.3. Characterization The morphology and microstructure of the samples were investigated by a scanning electron microscope (SEM, JEOLJSM7500) and a transmission electron microscopy (TEM, JEM2100) equipped with energy-dispersive X-ray spectroscopy (EDS). For the graphene/Epoxy composites, the samples were cut using an ultra-microtome (Thickness z 100 nm) for the TEM measurement, and the fracture surfaces were prepared for the SEM measurement. XRD patterns were collected by a D/MAX2200pc XRD machine with Cu Ka as the X-ray sources (wavelength ¼ 1.5418 Å). Raman spectra (Renishaw inVia Raman Spectrometer) were recorded by He-Ne laser excitation at 514 nm. The BrunauereEmmetteTeller (BET) surface area analysis was performed on a QUADRASORB EVO specific surface and pore size analyser. X-ray photon spectroscopy was performed with a Thermo Scientific ESCALAB 250XI XPS system. Water contact angle measurements were carried out on a contact angle goniometer (JC2000D). A small-angle X-ray scattering (SAXS) experiment was performed at the 1W2A beamline of the Beijing Synchrotron Radiation Facility (BSRF, China). The wavelength was 1.54 Å, and the sample-to-detector distance was 1.58 m. The electromagnetic parameters of the toroidal-shaped samples were measured by a vector network analyzer (Agilent N5244A) in the frequency range of 2e18 GHz with coaxial wire method. 3. Results and discussion Fig. 1 schematically presents the preparation process of noncovalently functionalized rGO/epoxy composites. The morphology and microstructure of rGO are characterized. As displayed in
X. Zhang et al. / Carbon 154 (2019) 115e124
117
Fig. 1. Schematic illustration of the synthesis of epoxy nanocomposites filled with non-covalently functionalized rGO. (A colour version of this figure can be viewed online.)
Fig. 2aed, the scanning electron microscopy (SEM) images exhibit that GO possesses extremely small thickness and wrinkled topology, and rGO shows interconnected and crumpled porous microstructures with graphene sheets partially overlapping in 3D space. Fig. 2e depicts the nitrogen adsorption/desorption isotherm curves of the rGO powders, which shows an IV-type isotherm with a long and narrow hysteresis loop at relative pressure (P/P0) from 0.4 to 1.0. Pore size distribution (Fig. 2f) confirms that there are several mesopores mainly distributing in 3e21 nm. The structure evolution of the oxidation and reduction process from EG to rGO is characterized by XRD and Raman (Fig. 3a and b). It can be seen that EG exhibits a sharp (002) peak at 2q ¼ 26.4 . After oxidation, GO displays a characteristic peak at a much smaller 2q of ~9.8 , which corresponds to a remarkedly enlarged interlayer distance due to the formation of oxygen functional groups between graphite layers. rGO appears a very weak (002) peak as a result of a high degree of reduction and exfoliation. As shown in the Raman spectra, the intensity ratio ID/IG of EG is 0.14, revealing the perfect graphitic lattice. However, GO shows a high ID/IG of 1.33 due to the appearance of massive lattice-defects. For the rGO, the intensity ratio ID/IG is up to 1.92, which indicates the increased disorders and defect sites mainly caused by porous microstructures. Furthermore, X-ray photoelectron spectroscopy (XPS) analyses of C atom (Fig. S1, Supporting Information) reveal the reduction behavior and degree of rGO samples. The C1s peak is split into four components at 284.4 eV, 286.7 eV, 287.3 eV and 288.6 eV, corresponding to CeC/C]C in aromatic, CeO (epoxy and hydroxyl), C]O
(carbonyl) and OeC]O (carboxyl) groups, respectively. In the case of graphene oxide (GO), the strongest peak of CeO bond is observed. After being reduced, the intensity the oxygen-containing groups, especially the CeO, obviously decrease while the CeC/C]C bond in aromatic increase for the rGO. Owing to the high volatility and good dissolution with epoxy, acetone is widely used as solvent to fabricate epoxy nanocomposites [40,41]. Here, three groups of rGO/acetone dispersion containing various contents of imidazole are prepared. As the imidazole amount increases, corresponding suspension is marked as rGO (without imidazole), rGO-im-1 and rGO-im-2 (detailed component ratio is shown in Table S1 in the Supporting Information). To investigate the effect of imidazole contents on the stability of the suspension, we disperse rGO-im-2, rGO-im-1 and rGO into acetone media with the concentration of 1 mg/ml, respectively. As shown in Fig. 3c, leaving these vials for about 10 min, abundant aggregation and sedimentation appear in the suspension of rGO. After 2 h of storage, the dispersion of rGO-im-1 shows slight aggregation and sedimentation, and no obvious precipitation is observed in the case of the rGO-im-2 suspension. The results indicate that enough concentration of imidazole can effectively decorate the rGO by non-covalent bond, and maintain their uniform dispersion in the poor solvent of acetone. XPS spectra of N 1s (Fig. 4a and b) and EDX elemental mapping of N element (Fig. 4cef) are characterized to investigate nitrogen distributions in rGO-im-1 and rGO-im-2. As presented in the XPS curves, both rGO-im-2 and rGO-im-1 display clear peaks at
Fig. 2. SEM images of a,b) GO and c,d) rGO. e) N2 absorption/desorption curves and f) corresponding calculated pore diameter distribution of rGO. (A colour version of this figure can be viewed online.)
118
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 3. a) XRD and b) Raman curves. c) Effects of imidazole on the acetone dispersions stability of rGO, rGO-im-1 and rGO-im-2. (A colour version of this figure can be viewed online.)
Fig. 4. XPS spectra of N 1s of a) rGO-im-2 and b) rGO-im-1, SEM images and corresponding EDS maps of c,d) rGO-im-2 and e,f) rGO-im-1. (A colour version of this figure can be viewed online.)
401.28 eV and 399.8 eV, corresponding to N1 (graphitic quaternary N) and N2 (pyrrolic N), respectively. The overall N content of rGOim-2 is 4.12 atom %, which is higher than 2.1 atom % of rGO-im-1. In addition, the elemental mapping images evidence the homogeneous distribution of N element on the surface of rGO-im-2 and rGO-im-1. Due to the outstanding physicochemical performance and ability of being cured by imidazole, epoxy is chosen as a representative polymer to fabricate the microwave absorption nanocomposites [42,43]. To investigate the microwave absorption performance of the prepared absorbers, various contents of the three kinds of acetone dispersions are mixed with epoxy to form nanocomposites followed by solvent removal and in-situ polymerization. Fig. 5 shows the SEM images of fracture surfaces of the nanocomposite samples with 2% weight of expanded graphite (EG), rGO, rGO-im-1 and rGO-im-2, respectively. It can be clearly seen that the EG-filled epoxy composite exhibits typical brittle fractured surface with thick graphite sheets (Fig. 5a). The rGO/epoxy sample contains relatively rough and river-like structures, but it is noted that sporadic agglomerates of rGO sheets exist mainly caused by
strong inter-sheet van der Waals forces (Fig. 5b). Comparatively, in the case of composites of rGO-im-1 and rGO-im-2, Fig. 5c and d present that both of the two samples show well-dispersed and obvious wavy edge structures (The inserts are high-resolution SEM images.). The TEM images further display the porous and winkled microstructures of the rGO-im-2/epoxy nanocomposites (Fig. S2, Supporting Information). Despite the good visualization of nanostructures, electron microscope technique cannot statistically obtain the actual structure in nanocomposites containing high aspect ratio fillers [44,45]. Therefore, synchronous radiation X-ray small angle scattering (SAXS), as an effective and non-destructive technique for investigating detail microstructural features of materials, is used to further study the average conformation of the rGO-filled nanocomposites. The original 2-D scattering images and averaged 1-D scattering curves are presented in Fig. S3 (Supporting Information). The correction of Porod deviation is carried up before subsequent analysis. Firstly, as shown in the Guinier curves (Fig. S3, Supporting Information), the plots of I(q) vs q of all samples are nonlinear, indicating that the rGO-filled nanocomposites are polydisperse
X. Zhang et al. / Carbon 154 (2019) 115e124
119
Fig. 5. SEM images of the a) graphite/epoxy composites, b) rGO/epoxy nanocomposites, c) rGO-im-1/epoxy nanocomposites and d) rGO-im-2/epoxy nanocomposites. The filling weight content of each sample is 2 wt%. (A colour version of this figure can be viewed online.)
system. Thus, we then analyze scatters size distribution by the logarithm Gaussian distribution method [46,47]. The distributions of the gyration radius are exhibited in Table S2 (Supporting Information) and the corresponding insert images in Fig. 6bee. When the filling content is 2 wt%, in the case of the rGO/epoxy composite, the gyration radius of the scatters is mainly distributed in the range of 6e9 nm, and the most probable value is 7.3 nm. In contrast, for the rGO-im-1/epoxy and rGO-im-2/epoxy composites, the most probable values of gyration radius are 0.8 nm and 0.2 nm, respectively, and the corresponding distributions of the gyration radius are narrower compared with that of the rGO/epoxy composite. When the weight content decreases to 1 wt%, the most probable value of gyration radius of the rGO-im-2/epoxy reaches as low as 0.1 nm. Hence, the results confirm the fact that the non-covalent functionalization effectively reduces the aggregation of rGO in epoxy. Moreover, the statistical self-similarity and scale invariance are quantitatively described by the fractal dimension. The doublelogarithmic plot of I vs q power-law scattering of the form qa resulting in self-similar structures appears as a linear region of gradient a. A a value of between 3 and 4 is indicative of fractal surfaces with self-similar roughness, and it manifests the existence of a smooth surface when a ¼ 4. The fractal dimension D is related to a as D ¼ 6 a [48,49]. Fig. 6a sketches the corresponding fractal characteristics of the rGO filled epoxy composites at different length scales. As illustrated in Fig. 6bee, with a filling content of 2 wt%, the fractal dimensions D of rGO, rGO-im-1 and rGO-im-2 epoxy nanocomposites are 2.48, 2.51 and 2.71, respectively, in the q region from about 0.1 nm1 to 1 nm1. And in the higher q region, the corresponding fractal dimensions D are 2.08, 2.51 and 2.31, respectively. Distinctly, for the rGO-im-2 epoxy nanocomposites, the D values dramatically increase when the filler concentration decreases from 2 wt% to 1 wt% (Fig. 6d and e). Consequently, the analysis further supports the existence of well-exfoliated, rough and extensively winkle graphene nanosheets in the non-covalently modified rGO/epoxy composites, which is extremely consistent with the SEM results. More than these, compared with the SEM
technique, the SAXS surely distinguishes the more detailed structural conformations between different non-covalently modified rGO/epoxy composites, which are the key to achieving highperformance microwave absorption. Microwave absorption properties are evaluated with the measured data of complex permittivity and permeability based on the transmission line theory [50,51].
Z 1 RL ¼ 20 log in Zin þ 1
(1)
where Zin is the normalized input impedance, which can be expressed as
rffiffiffiffiffi Zin ¼
2p pffiffiffiffiffiffiffiffiffi tanh j mr εr fd εr c
mr
(2)
where εr, mr, j, c, f and d are relative complex permittivity, relative complex permeability, imaginary unit, light velocity in vacuum, frequency and thickness of the absorber, respectively. Fig. S4 (Supporting Information) and 7 show the RL contour plots and curves of epoxy nanocomposites containing the fillers with a loading of 1 wt%, 2 wt% and 5 wt%, respectively. We mainly focus on the MA properties in the thickness range of 1e3 mm according to the actual application. The minimum RL value of rGO/ epoxy is 19.3 dB at 11.88 GHz with a thickness of 2 mm and a filler loading of 2 wt% (Fig. 7a2). In the case of rGO-im-1/epoxy, when the weight content is 1 wt%, the minimum RL value reaches as low as 34.2 dB at 11.88 GHz and 47.2 dB at 11.8 GHz with a thickness of 2.7 mm and 3 mm, respectively (Fig. 7b1). Its effective absorption bandwidth (abbreviated as EAB, meaning the frequency range that corresponding RL values are lower than 10 dB) remarkably achieves 7.48 GHz at 2.7 mm and 6.04 GHz at 3 mm. When the weight content increases to 2 wt%, the rGO-im-1/epoxy composite achieves a strong absorption with minimum RL value of 49 dB at 17.8 GHz with a thickness of 1.7 mm (Fig. 7b2), and the maximum EAB value is 5.76 GHz at 2 mm. The rGO-im-2/epoxy exhibits an
120
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 6. a) Schematic diagrams on winkle graphene/polymer nanocomposites at different length scales, bee) fitted SAXS profiles and predicted distributions of particle sizes (insert) of the epoxy nanocomposites filled with rGO, rGO-im-1 and rGO-im-2, respectively. (A colour version of this figure can be viewed online.)
extremely strong absorption with minimum RL value of 65 dB at 15.32 GHz with a thickness of 1.9 mm and a filler loading of 2 wt%, and the corresponding EAB value is 4.8 GHz (Fig. 7c2). However, due to the impairment of impedance matching characteristics, when the filling content increases to 5 wt%, absorption properties of rGO-im-1/epoxy and rGO-im-2/epoxy composites obviously decrease to some extent. After a comprehensive analysis of the MA performance of these composites, it can be concluded that, non-covalently functionalization of rGO by imidazole can enhance the MA properties of composites effectively. Furthermore, in the case of the two kinds of composites, the MA capacities are of slight distinction but both excellent with the increasing contents of imidazole. Therefore, the non-covalent modification method is indeed an efficient approach to fabricate highly dispersed rGO/polymer nanocomposites with outstanding microwave absorption. Compared with other typical graphene-based composites reported in the recent literature
(Table S3, Supporting Information), it is doubtless that our rGO-im2/epoxy and rGO-im-1/epoxy composites are more promising MA materials owing to their strong absorption, thin thickness, wide EAB and ultralow loading contents. To further evaluate the comprehensive MA performance, specific reflection loss and specific EAB values are defined by dividing RL and EAB by the layer thickness and filler loading (assuming that the mass of the sample is 100 mg), respectively [52,53]. As shown in Fig. 10, the rGO-im-2/ epoxy and rGO-im-1/epoxy composites possess much higher specific reflection loss and specific EAB values than other reported graphene-based materials. Besides, the water-repellent properties of the composites are investigated and quantified by the contact angle (CA) q of a water droplet on the surface. As illustrated in Fig. 8, the pure epoxy resin possesses poor hydrophobicity with the water CA of 87.1. However, the q values of rGO-im-1/epoxy and rGO-im-2/epoxy composites increase to 110.4 and 112.5 , respectively. The high water CA is
X. Zhang et al. / Carbon 154 (2019) 115e124
121
Fig. 7. Frequency dependence of RL curves of the epoxy composites containing different contents of a1-a3) rGO, b1-b3) rGO-im-1 and c1-c3) rGO-im-2, respectively. (A colour version of this figure can be viewed online.)
Fig. 8. The contact angle of water on the prepared surface of pure epoxy resin and the non-covalently modified rGO/epoxy composites. (A colour version of this figure can be viewed online.)
mainly ascribed to the material roughness and low surface polarity caused by uniformly dispersed and wrinkled rGO. The hydrophobicity of the imidazole-modified rGO/epoxy composites can substantially improve their waterproof, anti-icing, and corrosion resistance and thus ensure long-term excellent MA capacities when being exposed to environment [54]. Microwave absorption properties of dielectric MA materials are correlated with the complex permittivity, where the real parts (ε0 ) and imaginary parts (ε00 ) of complex permittivity represent the
storage ability and the dissipation capability of electric energy, respectively [55,56]. The frequency-dependence relative complex permittivity and dielectric dissipation factor (tan d ¼ ε00 /ε0 , tangent loss) for the composites with different weight contents of fillers are shown in Fig. S5 (Supporting Information). Overall, the ε0 of all the composites drastically decreases, and the ε00 exhibits obvious resonant peaks with the increasing frequency. The dielectric relaxation behavior associates with polarization relaxation and conductive loss that described by Debye relaxation theory [57].
122
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 9. Frequency dependence of a) attenuation constant a and b) relative input impedance jZin/Z0j of the samples. (A colour version of this figure can be viewed online.)
frequency when the filling contents are increased from 1 wt% to 2 wt%. The difference is probably attributed to the aggregation of graphene sheets. The dielectric relaxation loss in the noncovalently functionalized rGO/epoxy composites probably originates from the increased defect dipoles caused by chemical reduction and massive heterointerfaces between graphene and epoxy, and dipolar polarization resulting from the different electronegativity between graphene and imidazole or the residual oxygen functional groups in an unsaturated coordination (Fig. 11a). Both high dissipation and matched characteristic impedance are important to produce considerable microwave absorption. The calculated electromagnetic wave attenuation constant a [58,59].
a¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðm00 ε00 m0 ε0 Þ þ ðm00 ε00 m0 ε0 Þ2 þ ðm0 ε00 þ m00 ε0 Þ2 c (8)
Fig. 10. Comparison of specific jRLminj and specific effective bandwidth (EAB) of various carbon composites in previous references and this work. (A colour version of this figure can be viewed online.)
εs ε∞ 1 þ u2 t2
(3)
εs ε∞ s ut þ uε0 1 þ u2 t2
(4)
0
ε ¼ ε∞ þ 00
ε ¼ 00
εp ¼ 00
εc ¼
εs ε∞ ut 1 þ u2 t2
(5)
s uε0
(6)
ε0
ε ε 2 εs þ ε∞ 2 2 s ∞ þ ðε00 Þ ¼ 2 2
(7)
where ε0, εs and ε∞ are the dielectric constant in vacuum, the static permittivity and relative dielectric permittivity at the highfrequency limit, respectively. In Equations (3)e(7), u refers to the angular frequency; s stands for the electrical conductivity; t is the 00 00 relaxation time; εc refers to the conductive loss part and εp is the polarization loss part. The ε0 and ε00 values of rGO-im-1/epoxy and rGO-im-2/epoxy composites aggrandize gradually with the increasing filler loading nearly in the whole frequency range of 2e18 GHz, which is consistent with the effective medium theory [22]. However, the ε0 values of rGO/epoxy appear abrupt increasing in the corresponding
is a key parameter to determine the dissipation properties of MA materials. The rGO-im-2/epoxy and rGO-im-1/epoxy samples present high dielectric loss factors of nearly 0.4 with a filler loading of 2 wt% (Fig. S5, Supporting Information), but the former shows larger a and especially more ideal impedance matching of a jZin/Z0j value of 1.0 (Fig. 9). However, the optimal jZin/Z0j value of rGO/ epoxy is 0.8 with the thickness of 1.8 mm and a filler loading of 2 wt % mainly caused by high electrical conductivity, indicating the existence of stacked graphene sheets. Furthermore, we set up two 2-D finite element models of epoxy composites filled with high-dispersed and heavily-stacked graphitic carbon sheets, respectively, to simulate the effect of graphene dispersity on MA characteristics of the composites schematically and intuitively. The detailed geometric size, material parameters and boundary conditions are listed in Fig. S6 (Supporting Information). The results show that the maximum absorption coefficient of high-dispersed and heavily-stacked graphitic carbon/epoxy composites are 0.58 and 0.08 at 15 GHz (Fig. S6c, Supporting Information), respectively, and the corresponding electric displacement of former is stronger and more even-distributed (Fig. 11b and c). 4. Conclusion In summary, we prepared epoxy MA composites filled with ultralow content of non-covalently modified reduced graphene oxides (rGO) by an in-situ polymerization method. Overall, the surfaces of rGO nanosheets were successfully functionalized by strong p-p interaction between conjugated imidazole and graphene sheets. The combination of SEM and SAXS unambiguously demonstrated that the graphene sheets in the non-covalently
X. Zhang et al. / Carbon 154 (2019) 115e124
123
Fig. 11. a) Defect-related and dipolar polarization relaxation of non-modified rGO sheets, b, c) schematic representation of the MA mechanism and simulated electric displacement distribution for the highly dispersed and heavily stacked rGO/epoxy composites, respectively. (A colour version of this figure can be viewed online.)
modified rGO/epoxy composites were well-dispersed and extensively winkle. Compared with rGO/epoxy composites, the highly dispersed non-covalently functionalized rGO/epoxy showed more ideal impedance matching and stronger microwave dissipation based on the experimental and simulated results. The composites achieved excellent microwave absorptions (RLmin ¼ 65 dB, with a matching thickness of 1.8 mm), an extremely wide absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possessed high hydrophobicity with a water contact angle of 112.5 , which is larger than that (z87 ) of the pure epoxy resin. Acknowledgements This work was supported by 1W2A beamline of the Beijing Synchrotron Radiation Facility (BSRF, China). We gratefully acknowledge Mr. Xiongjie Yang, Cong Ge and Miss Xiaoqi Zhang for their assistance in preparing the nanocomposites. We are also indebted to Dr. Guang Mo and Dr. ZhiHong Li for their help in the measurements of SAXS. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.076. References [1] F. Ye, Q. Song, Z.C. Zhang, W. Li, S.Y. Zhang, X.W. Yin, et al., Direct growth of edge-rich graphene with tunable dielectric properties in porous Si3N4 ceramic for broadband high-performance microwave absorption, Adv. Funct. Mater. 28 (2018), 1707205. [2] H.H. Chen, Z.Y. Huang, Y. Huang, Y. Zhang, Z. Ge, B. Qin, et al., Synergistically assembled MWCNT/graphene foam with highly efficient microwave absorption in both C and X bands, Carbon 124 (2017) 506e514. [3] H. Luo, F. Chen, X. Wang, W.Y. Dai, Y. Xiong, J.J. Yang, et al., A novel two-layer honeycomb sandwich structure absorber with high-performance microwave absorption, Composites Part A 119 (2019) 1e7. [4] J. Choi, H.T. Jung, A new triple-layered composite for high-performance broadband microwave absorption, Compos. Struct. 122 (2015) 166e171. [5] A. Shah, A. Ding, Y.H. Wang, L. Zhang, D.X. Wang, J. Muhammad, et al., Enhanced microwave absorption by arrayed carbon fibers and gradient dispersion of Fe nanoparticles in epoxy resin composites, Carbon 96 (2016) 987e997. e, C. Bailly, I. Huynen, Thin and flexible multilayer polymer composite [6] Y. Danle structures for effective control of microwave electromagnetic absorption, Compos. Sci. Technol. 100 (2014) 182e188.
[7] W.J. Lee, J.W. Lee, C.G. Kim, Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band, Compos. Sci. Technol. 68 (2008) 2485e2489. [8] Z.B. Huang, W.C. Zhou, W.B. Kang, X.F. Tang, F. Luo, D.M. Zhu, et al., Dielectric and microwave-absorption properties of the partially carbonized PAN cloth/ epoxyesilicone composites, Composites Part B 43 (2012) 2980e2984. [9] Y.C. Qing, D.D. Min, Y.Y. Zhou, F. Luo, W.C. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxyesilicone composites as thinethickness and wide-bandwidth microwave absorber, Carbon 86 (2015) 98e107. [10] S.S. Wang, Y. Zhao, M.M. Gao, H.L. Xue, Y.C. Xu, C.H. Feng, et al., Green synthesis of porous cocoon-like rGO for enhanced microwave absorbing performances, ACS Appl. Mater. Interfaces 10 (2018) 42865e42874. [11] X. Qiu, L.X. Wang, H.L. Zhu, Y.K. Guan, Q.T. Zhang, Lightweight and efficient microwave absorbing materials based on walnut shell-derived nanoporous carbon, Nanoscale 9 (2017) 7408e7418. [12] B. Wen, M.S. Cao, M.M. Lu, W.Q. Cao, H.L. Shi, J. Liu, et al., Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures, Adv. Mater. 26 (2014) 3484e3489. [13] A.K. Geima, K.S. Novselov, The rise of graphene, Nat. Mater. 6 (2007) 183e191. [14] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nat. Mater. 10 (2011) 569e581. [15] S. Nardecchia, D. Carriazo, M.L. Ferrer, M.C. Gutierrez, F.D. Monte, Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications, Chem. Soc. Rev. 42 (2013) 794e830. [16] M.S. Cao, C. Han, X.X. Wang, M. Zhang, Y.L. Zhang, J.C. Shu, et al., Graphene nanohybrids: excellent electromagnetic properties for the absorbing and shielding of electromagnetic waves, J. Mater. Chem. C 6 (2018) 4586e4602. [17] B. Wen, X.X. Wang, W.Q. Cao, H.L. Shi, M.M. Lu, G. Wang, et al., Reduced graphene oxides: the thinnest and most lightweight materials with highly efficient microwave attenuation performances of the carbon world, Nanoscale 6 (2014) 5754e5761. [18] S.H. Lee, D.H. Lee, W.J. Lee, S.O. Kim, Tailored assembly of carbon nanotubes and graphene, Adv. Funct. Mater. 21 (2011) 1338e1354. [19] C. Chen, J.B. Xi, E.Z. Zhou, L. Peng, Z.C. Chen, C. Gao, Nano-Micro Lett. 10 (26) (2018) 1e11. [20] Y. Zhang, Y. Huang, T.F. Zhang, H.C. Chang, P.S. Xiao, H.H. Chen, et al., Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (2015) 2049e2053. [21] C. Wang, X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, et al., The electromagnetic property of chemically reduced graphene oxide and its application as microwave absorbing material, Appl. Phys. Lett. 98 (2011), 072906. [22] B.Y. Kuang, W.L. Song, M.Q. Ning, J.B. Li, Z.J. Zhao, D.Y. Guo, et al., Chemical reduction dependent dielectric properties and dielectric loss mechanism of reduced graphene oxide, Carbon 127 (2018) 209e217. [23] Z.X. Hou, X.W. Yin, H.L. Xu, H.J. Wei, M.H. Li, L.F. Cheng, et al., Reduced graphene oxide/silicon nitride composite for cooperative electromagnetic absorption in wide temperature spectrum with excellent thermal stability, ACS Appl. Mater. Interfaces 11 (2019) 5364e5372. [24] S. Zhao, H.B. Zhang, J.Q. Luo, Q.W. Wang, B. Xu, S. Hong, et al., Highly electrically conductive three-dimensional Ti3C2Tx MXene/reduced graphene oxide hybrid aerogels with excellent electromagnetic interference shielding performances, ACS Nano 12 (2018) 11193e11202.
124
X. Zhang et al. / Carbon 154 (2019) 115e124
[25] L. Quan, F.X. Qin, D. Estevez, H. Wang, H.X. Peng, Magnetic graphene for microwave absorbing application: towards the lightest graphene-based absorber, Carbon 125 (2017) 630e639. [26] H.H. Chen, W.L. Ma, Z.Y. Huang, Y. Zhang, Y. Huang, Y.S. Chen, Graphene-based materials toward microwave and terahertz absorbing stealth technologies, Adv. Opt. Mater. 7 (2019), 1801318. [27] Y. Zhang, Y. Huang, H.H. Chen, Z.Y. Huang, Y. Yang, P.S. Xiao, et al., Composition and structure control of ultralight graphene foam for high-performance microwave absorption, Carbon 105 (2016) 438e447. [28] H.H. Chen, Z.Y. Huang, Y. Huang, Y. Zhang, Z. Ge, W.L. Ma, et al., Consecutively strong absorption from gigahertz to terahertz bands of a monolithic threedimensional Fe3O4/graphene material, ACS Appl. Mater. Interfaces 11 (2019) 1274e1282. [29] Z.Y. Huang, H.H. Chen, Y. Huang, Z. Ge, Y. Zhou, Y. Yang, et al., Ultra-broadband wide-angle terahertz absorption properties of 3D graphene foam, Adv. Funct. Mater. 28 (2018), 1704363. [30] X.L. Li, X.W. Yin, C.Q. Song, M.K. Han, H.L. Xu, et al., Wenyan duan, selfassembly coreeshell graphene-bridged hollow MXenes spheres 3D foam with ultrahigh specific EM absorption performance, Adv. Funct. Mater. 28 (2018), 1803938. [31] Z.Y. Huang, H.H. Chen, S.T. Xu, L.Y. Chen, Y. Huang, Z. Ge, et al., Graphenebased composites combining both excellent Terahertz shielding and stealth performance, Adv. Opt. Mater. 6 (2018), 1801165. [32] V.D. Punetha, S. Rana, H.J. Yoo, A. Chaurasia, J.T. M Jr., M.S. Ramasamy, et al., Functionalization of carbon nanomaterials for advanced polymer nanocomposites: a comparison study between CNT and graphene, Prog. Polym. Sci. 67 (2017) 1e47. [33] T. Kuilla, S. Bhadra, D.H. Yao, N.H. Kim, S. Bose, J.H. Lee, Recent advances in graphene-based polymer composites, Prog. Polym. Sci. 35 (2010) 1350e1375. [34] B. Long, M. Manning, M. Burke, B.N. Szafranek, G. Visimberga, D. Thompson, et al., Non-covalent functionalization of graphene using self-assembly of alkaneamines, Adv. Funct. Mater. 22 (2012) 717e725. [35] I. Zaman, H.C. Kuan, Q. Meng, A. Michelmore, N. Kawashima, T. Pitt, L. Zhang, S. Gouda, L. Lee, J. Ma, Adv. Funct. Mater. 21 (2011) 1338. [36] M. Beidaghi, C.L. Wang, Micro-supercapacitors based on interdigital electrodes of reduced graphene oxide and carbon nanotube composites with ultrahigh power handling performance, Adv. Funct. Mater. 22 (2012) 4501e4510. [37] L. Xu, H. Nakajima, E. Manias, R. Krishnamoorti, Tailored nanocomposites of polypropylene with layered silicates, Macromolecules 42 (2009) 3795e3803. [38] H.J. Salavagione, Q.D. Susana, E.J. Patricia, G. Martínez, F. Ania, A. Flores, et al., Development of advanced elastomeric conductive nanocomposites by selective chemical affinity of modified graphene, Macromolecules 49 (2016) 4948e4956. [39] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [40] Y.M. Wang, S.F. Chen, X.C. Chen, Y.F. Lu, M.H. Miao, D.H. Zhang, Controllability of epoxy equivalent weight and performance of hyperbranched epoxy resins, Composites Part B 160 (2019) 615e625. [41] S. Zainuddin, A. Fahim, T. Arifin, M.V. Hosur, M.M. Rahman, J.D. Tyson, et al., Optimization of mechanical and thermo-mechanical properties of epoxy and E-glass/epoxy composites using NH2-MWCNTs, acetone solvent and combined dispersion methods, Compos. Struct. 110 (2014) 39e50. [42] Y. Chen, H.B. Zhang, Y.B. Yang, M. Wang, A.Y. Cao, Z.Z. Yu, High-performance epoxy nanocomposites reinforced with three-dimensional carbon nanotube sponge for electromagnetic interference shielding, Adv. Funct. Mater. 26 (2016) 447e455.
[43] M.S.S. Dorraji, M.H. Rasoulifard, A.R. Amani-Ghadim, M.H. Khodabandeloo, M. Felekari, M.R. Khoshrou, et al., Microwave absorption properties of polypyrrole-SrFe12O19-TiO2-epoxy resin nanocomposites: optimization using response surface methodology, Appl. Surf. Sci. 383 (2016) 9e18. [44] M.P. Weir, D.W. Johnson, S.C. Boothroyd, R.C. Savage, R.L. Thompson, S.R. Parnell, et al., Extrinsic wrinkling and single exfoliated sheets of graphene oxide in polymer composites, Chem. Mater. 28 (2016) 1698e1704. [45] T. Li, A.J. Senesi, B. Lee, Small angle X-ray scattering for nanoparticle research, Chem. Rev. 116 (2016) 11128e11180. [46] Z.H. Li, A program for SAXS data processing and analysis, Chin. Phys. C 37 (2013), 108002. [47] J. Polte, T.T. Ahner, F. Delissen, S. Sokolov, F. Emmerling, A.F. Thünemann, et al., Mechanism of gold nanoparticle formation in the classical citrate synthesis method derived from coupled in situ XANES and SAXS evaluation, J. Am. Chem. Soc. 132 (2010) 1296e1301. [48] X. Xia, J.H. Yin, B. Su, D. Hui, R.T. Yu, X.X. Liu, Quantitative determining interface information of nano composite by synchrotron radiation small-angle X-ray scattering, Composites Part B 120 (2017) 92e96. [49] R. Wei, X.Q. Wang, X. Zhang, S.J. Wang, S.Y. Zhu, S.Y. Du, A novel method for manufacturing high-performance layered silicate/epoxy nanocomposites using an epoxyediamine adduct to enhance compatibility and interfacial reactivity, Macromol. Mater. Eng. 303 (2018), 1800065. [50] Z.C. Wu, W. Hu, T. huang, P. Lan, K. Tian, F.F. Xie, et al., Hierarchically porous carbon with controlled structure for efficient microwave absorption, J. Mater. Chem. C 6 (2018) 8839e8845. [51] S.S. Dai, Y. Cheng, B. Quan, X.H. Liang, W. Liu, Z.H. Yang, et al., Porous-carbonbased Mo2C nanocomposites as excellent microwave absorber: a new exploration, Nanoscale 10 (2018) 6945e6953. [52] Y. Li, X.F. Liu, X.Y. Nie, W.W. Yang, Y.D. Wang, R.H. Yu, et al., Multifunctional organiceinorganic hybrid aerogel for self-cleaning, heat-insulating, and highly efficient aicrowave absorbing material, Adv. Funct. Mater. 29 (2018), 1807624. [53] Y.C. Yin, X.F. Liu, X.J. Wei, Y. Li, X.Y. Nie, R.H. Yu, et al., Magnetically aligned CoC/MWCNTs composite derived from MWCNT-interconnected zeolitic imidazolate frameworks for a lightweight and highly efficient electromagnetic wave absorber, ACS Appl. Mater. Interfaces 9 (2017) 30850e30861. [54] Y. Cheng, T.X. Zhu, S.H. Li, J.Y. Huang, J.J. Mao, H. Yang, et al., A novel strategy for fabricating robust superhydrophobic fabrics by environmentally-friendly enzyme etching, Chem. Eng. J. 355 (2019) 290e298. [55] D.W. Xu, S. Yang, P. Chen, Q. Yu, X.H. Xiong, J. Wang, Synthesis of magnetic graphene aerogels for microwave absorption by a in-situ pyrolysis, Carbon 146 (2019) 301e312. [56] B. Quan, X.H. Liang, G.B. Ji, J.N. Ma, P.Y. Ouyang, H. Gong, et al., Strong electromagnetic wave response derived from the construction of dielectric/ magnetic media heterostructure and multiple interfaces, ACS Appl. Mater. Interfaces 9 (2017) 9964e9974. [57] D.W. Liu, Y.C. Du, P. Xu, N. Liu, Y.H. Wang, H.H. Zhao, et al., Waxberry-like hierarchical Ni@C microspheres with high-performance microwave absorption, J. Mater. Chem. C 7 (2019) 5037e5046. [58] L.W. Yan, C.Q. Hong, B.Q. Sun, G.D. Zhao, Y.H. Cheng, S. Dong, et al., In situ growth of coresheath heterostructural SiC nanowire arrays on carbon fibers and enhanced electromagnetic wave absorption performance, ACS Appl. Mater. Interfaces 9 (2017) 6320e6331. [59] D.W. Xu, J.L. Liu, P. Chen, Q. Yu, J. Wang, S. Yang, et al., In situ growth and pyrolysis synthesis of super-hydrophobic graphene aerogels embedded with ultrafine b-Co nanocrystals for microwave absorption, J. Mater. Chem. C 7 (2019) 3869e3880.
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Broadband and strong electromagnetic wave absorption of epoxy composites filled with ultralow content of non-covalently modified reduced graphene oxides Xu Zhang a, Xiaoqun Wang a, *, Fanbo Meng b, Jiajun Chen a, Shanyi Du a a b
School of Materials Science and Engineering, Beihang University, Beijing, 100083, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2019 Received in revised form 5 July 2019 Accepted 21 July 2019 Available online 29 July 2019
High-efficiency microwave absorption epoxy nanocomposites filled with ultralow content of noncovalently modified reduced graphene oxides (rGO) are prepared via an in-situ polymerization method. Here, the porous and folded rGO nanosheets are successfully functionalized by strong p-p interaction between conjugated imidazole and graphene sheets. The combination of electron microscope and synchronous radiation X-ray small angle scattering (SAXS) techniques unambiguously illustrates that the non-covalently modified graphene nanosheets in the epoxy composites are well-dispersed and extensively winkle. Hence, compared with pristine rGO/epoxy composites, the composites containing the highly dispersed rGO demonstrate more ideal impedance matching and stronger microwave dissipation based on the experimental and simulated results. The composites remarkably achieve excellent microwave absorptions (minimum reflection loss of 65 dB, with a matching thickness of 1.8 mm), an extremely broad absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possess high hydrophobicity, endowing them attractive functions of selfcleaning. This work provides a promising, facile and scalable approach for designing and fabricating graphene-based nanocomposites with hydrophobicity and excellent microwave absorption capacities. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Graphene nanocomposites Microwave absorption X-ray small angle scattering
1. Introduction To address the rising electromagnetic (EM) radiation pollution from the rapid progress of electronic industry and wireless communication technology, high-performance microwave absorbing (MA) materials are desperately demanded [1,2]. Conventionally, MA materials are mainly classified as structure composites (honeycomb structure, pyramidal structure, multilayer structure and frequency selective surface, etc.) and coatings [3e5]. MA structure composites are mechanically strong and display wide absorption bandwidths, but suffer from the drawbacks of large thickness and selective absorption of microwave in some certain direction or polarization [6]. In contrast, MA coatings truly eliminate the microwave signal in nearly all directions and polarization modes, and are widely applied to portable electronic devices and military vehicles (aircrafts, ships and tanks) [7e9].
* Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.carbon.2019.07.076 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
MA coatings are fabricated by insulating polymer and lossy fillers. Traditional lossy fillers consist of magnetic loss materials (ferrites, magnetic metal powders, etc.) and dielectric loss materials (TiO2, CuS, SiC, etc.) [10,11]. Despite good microwave attenuation performance in some cases, the practical applications of those traditional microwave absorbers are severely hindered by their large mass filling ratio, high density and poor environmental adaptability [10,12]. Graphene, as a typical graphitic carbon material, has anisotropic two-dimensional (2-D) shapes, and consists of 2-D conjugated sp2 hybrid carbons [13e15]. The unique chemical structure and morphology provide graphene great potential in the field of MA due to its high electrical conductivity, good chemical durability, large specific surface, high thermal conductivity and ultralight weight [16e18]. However, MA properties of pristine graphene materials are limited by the impedance mismatch and single attenuation mechanism, which are mainly caused by its improper electrical conductivity and poor dispersion in the matrix [19,20]. Compared with pristine graphene nanosheets, reduced graphene oxide (rGO) possesses residual functional groups and defects (missing carbon
116
X. Zhang et al. / Carbon 154 (2019) 115e124
atoms and sheet corrugation), and thus presents decreased electrical conductivity and enhanced microwave attenuation ability [17,21]. As a typical example, Kuang et al. fabricated rGO/wax composites and systematically investigated the effect of reduction degrees and content of rGO on MA properties of the composites [22]. The results display that the minimum reflective loss (RL) value is 37 dB with 3.5 mm thickness at 5.92 GHz, and the filling ratio of rGO is 30 wt%. Nevertheless, the MA performance can still hardly reach to the requirement of practical applications owing to the narrow absorption bandwidth, large thickness and filling ratio. To improve the MA capacities of the rGO, firstly, most researchers combine it with magnetic or other dielectric loss materials to tune the impedance matching behavior and enhance microwave attention [23,24]. Although the compositing strategy can improve the MA performance of rGO, there are many disadvantages for the way, such as the increasing of density, weakening of chemical durability, and the unhandy compositing technology [25,26]. Besides, recently, significant progress has been made toward 3D interconnected rGO foams by microstructural design strategies. Evidently, the method can avoid the agglomeration of rGO and provide heterogeneous interfaces and voids, which are conducive to creating low effective permittivity and matched impedance. Unfortunately, despite the fact of broadband and strong EM wave absorption and great reduction of the density, rGO foam structures inevitably increases the thickness of MA materials [27e31]. Hence, it is extremely significant to prepare sole rGO-filled materials with highly efficient MA performance. As abovementioned, despite the lower electrical conductivity and more defects than pure graphene, the rGO still has a pronounced tendency to agglomerate in the matrix owing to the poor interfacial compatibility. Therefore, to improve MA capacities of rGO-filled materials, it is of key importance to form homogeneous composites. Surface modification of rGO plays a crucial role in maintaining its uniform and stable dispersion in the matrix [32]. Generally, in most application areas, Covalent interactions on the surfaces of rGO are favored because of the presence of hydroxyl, epoxy, and carboxylic groups [33]. Unfortunately, covalent functionalization inevitably destroys the structure of rGO and adversely affects its electronic properties, which drastically decreases its microwave dissipation properties [34]. In contrast, non-covalent modification plays an important role in enhancing the solubility and preventing the aggregation without weakening the MA abilities of the rGO [35,36]. Consequently, in this work, we creatively prepare epoxy MA composites filled with ultralow content of non-covalently modified reduced graphene oxides. Firstly, we synthesize the porous rGO by Hummers and green chemical reduction methods. Afterwards, imidazole, as the non-covalent modifier of rGO and curing accelerator of epoxy, was introduced into the in-suit polymerization process. The effect of imidazole contents on the MA performance is investigated. Results exhibit that our composites show strong microwave absorptions (RLmin ¼ 65 dB, with a matching thickness of 1.8 mm), an extremely wide absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possess strong hydrophobicity with a water contact angle of 112.5 , which is larger than that (z87 ) of the pure epoxy resin. In addition, we all know that the excellent MA performance of the rGO-filled composites results from the highly dispersed rGO with wrinkled and defective structures. Therefore, to statistically confirm the presence of well exfoliated and wrinkled sheets in the epoxy matrices, synchronous radiation X-ray small angle scattering (SAXS), as an effective and non-destructive technique is employed to give an average conformation of rGO in the nanocomposites [37,38].
2. Experimental section 2.1. Preparation of rGO/acetone suspension The rGO suspension was fabricated by a Hummers' method and chemical reduction method. Briefly, 6 g expanded graphite (EG) powders (325 mesh) were oxidized using a modified Hummers' method [39]. As-synthesized graphite oxide was washed 3 times by diluted 1 wt% HCl aqueous solution (400 mL) and deionized water to completely remove residual salts and acids. The resultant water suspension was passed through a weak basic ion-exchange resin with deionized water as mobile phase to remove the remaining impurities. The purified deionized water dispersion of rGO was exfoliated by ultrasonication for 1 h and subsequently reduced by adding ascorbic acid (VC) with a concentration of 10 mg/g for 4 days at room temperature. The rGO dispersion was centrifuged with ethanol and acetone washed by several times. Then, the rGO/ acetone suspension was prepared by dispersing the 100 mg washed rGO and a certain amount of imidazole (0 mg/g, 4 mg/g or 8 mg/g) into acetone with the assistance of ultrasonication. 2.2. Synthesis of rGO/epoxy composites Different concentrations of bisphenol A liquid epoxy resin (E51) were added into the rGO/acetone suspension and sonicated for 30 min at a power of 600 W. Next, the acetone is evaporated off by drying the suspension at room temperature for about 12 h in air. Then, the polyetheramine (D400), as an additional low viscosity curing agent, was added into the rGO/epoxy slurry and grinded for approximately 10 min. The mixture was transformed to a cylindrical stainless steel mould and pressed into toroidal-shaped specimen with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Finally, the sample was cured at 75 C for 1 h, 110 C for 1 h, followed by 1 h of post-cure at 150 C. 2.3. Characterization The morphology and microstructure of the samples were investigated by a scanning electron microscope (SEM, JEOLJSM7500) and a transmission electron microscopy (TEM, JEM2100) equipped with energy-dispersive X-ray spectroscopy (EDS). For the graphene/Epoxy composites, the samples were cut using an ultra-microtome (Thickness z 100 nm) for the TEM measurement, and the fracture surfaces were prepared for the SEM measurement. XRD patterns were collected by a D/MAX2200pc XRD machine with Cu Ka as the X-ray sources (wavelength ¼ 1.5418 Å). Raman spectra (Renishaw inVia Raman Spectrometer) were recorded by He-Ne laser excitation at 514 nm. The BrunauereEmmetteTeller (BET) surface area analysis was performed on a QUADRASORB EVO specific surface and pore size analyser. X-ray photon spectroscopy was performed with a Thermo Scientific ESCALAB 250XI XPS system. Water contact angle measurements were carried out on a contact angle goniometer (JC2000D). A small-angle X-ray scattering (SAXS) experiment was performed at the 1W2A beamline of the Beijing Synchrotron Radiation Facility (BSRF, China). The wavelength was 1.54 Å, and the sample-to-detector distance was 1.58 m. The electromagnetic parameters of the toroidal-shaped samples were measured by a vector network analyzer (Agilent N5244A) in the frequency range of 2e18 GHz with coaxial wire method. 3. Results and discussion Fig. 1 schematically presents the preparation process of noncovalently functionalized rGO/epoxy composites. The morphology and microstructure of rGO are characterized. As displayed in
X. Zhang et al. / Carbon 154 (2019) 115e124
117
Fig. 1. Schematic illustration of the synthesis of epoxy nanocomposites filled with non-covalently functionalized rGO. (A colour version of this figure can be viewed online.)
Fig. 2aed, the scanning electron microscopy (SEM) images exhibit that GO possesses extremely small thickness and wrinkled topology, and rGO shows interconnected and crumpled porous microstructures with graphene sheets partially overlapping in 3D space. Fig. 2e depicts the nitrogen adsorption/desorption isotherm curves of the rGO powders, which shows an IV-type isotherm with a long and narrow hysteresis loop at relative pressure (P/P0) from 0.4 to 1.0. Pore size distribution (Fig. 2f) confirms that there are several mesopores mainly distributing in 3e21 nm. The structure evolution of the oxidation and reduction process from EG to rGO is characterized by XRD and Raman (Fig. 3a and b). It can be seen that EG exhibits a sharp (002) peak at 2q ¼ 26.4 . After oxidation, GO displays a characteristic peak at a much smaller 2q of ~9.8 , which corresponds to a remarkedly enlarged interlayer distance due to the formation of oxygen functional groups between graphite layers. rGO appears a very weak (002) peak as a result of a high degree of reduction and exfoliation. As shown in the Raman spectra, the intensity ratio ID/IG of EG is 0.14, revealing the perfect graphitic lattice. However, GO shows a high ID/IG of 1.33 due to the appearance of massive lattice-defects. For the rGO, the intensity ratio ID/IG is up to 1.92, which indicates the increased disorders and defect sites mainly caused by porous microstructures. Furthermore, X-ray photoelectron spectroscopy (XPS) analyses of C atom (Fig. S1, Supporting Information) reveal the reduction behavior and degree of rGO samples. The C1s peak is split into four components at 284.4 eV, 286.7 eV, 287.3 eV and 288.6 eV, corresponding to CeC/C]C in aromatic, CeO (epoxy and hydroxyl), C]O
(carbonyl) and OeC]O (carboxyl) groups, respectively. In the case of graphene oxide (GO), the strongest peak of CeO bond is observed. After being reduced, the intensity the oxygen-containing groups, especially the CeO, obviously decrease while the CeC/C]C bond in aromatic increase for the rGO. Owing to the high volatility and good dissolution with epoxy, acetone is widely used as solvent to fabricate epoxy nanocomposites [40,41]. Here, three groups of rGO/acetone dispersion containing various contents of imidazole are prepared. As the imidazole amount increases, corresponding suspension is marked as rGO (without imidazole), rGO-im-1 and rGO-im-2 (detailed component ratio is shown in Table S1 in the Supporting Information). To investigate the effect of imidazole contents on the stability of the suspension, we disperse rGO-im-2, rGO-im-1 and rGO into acetone media with the concentration of 1 mg/ml, respectively. As shown in Fig. 3c, leaving these vials for about 10 min, abundant aggregation and sedimentation appear in the suspension of rGO. After 2 h of storage, the dispersion of rGO-im-1 shows slight aggregation and sedimentation, and no obvious precipitation is observed in the case of the rGO-im-2 suspension. The results indicate that enough concentration of imidazole can effectively decorate the rGO by non-covalent bond, and maintain their uniform dispersion in the poor solvent of acetone. XPS spectra of N 1s (Fig. 4a and b) and EDX elemental mapping of N element (Fig. 4cef) are characterized to investigate nitrogen distributions in rGO-im-1 and rGO-im-2. As presented in the XPS curves, both rGO-im-2 and rGO-im-1 display clear peaks at
Fig. 2. SEM images of a,b) GO and c,d) rGO. e) N2 absorption/desorption curves and f) corresponding calculated pore diameter distribution of rGO. (A colour version of this figure can be viewed online.)
118
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 3. a) XRD and b) Raman curves. c) Effects of imidazole on the acetone dispersions stability of rGO, rGO-im-1 and rGO-im-2. (A colour version of this figure can be viewed online.)
Fig. 4. XPS spectra of N 1s of a) rGO-im-2 and b) rGO-im-1, SEM images and corresponding EDS maps of c,d) rGO-im-2 and e,f) rGO-im-1. (A colour version of this figure can be viewed online.)
401.28 eV and 399.8 eV, corresponding to N1 (graphitic quaternary N) and N2 (pyrrolic N), respectively. The overall N content of rGOim-2 is 4.12 atom %, which is higher than 2.1 atom % of rGO-im-1. In addition, the elemental mapping images evidence the homogeneous distribution of N element on the surface of rGO-im-2 and rGO-im-1. Due to the outstanding physicochemical performance and ability of being cured by imidazole, epoxy is chosen as a representative polymer to fabricate the microwave absorption nanocomposites [42,43]. To investigate the microwave absorption performance of the prepared absorbers, various contents of the three kinds of acetone dispersions are mixed with epoxy to form nanocomposites followed by solvent removal and in-situ polymerization. Fig. 5 shows the SEM images of fracture surfaces of the nanocomposite samples with 2% weight of expanded graphite (EG), rGO, rGO-im-1 and rGO-im-2, respectively. It can be clearly seen that the EG-filled epoxy composite exhibits typical brittle fractured surface with thick graphite sheets (Fig. 5a). The rGO/epoxy sample contains relatively rough and river-like structures, but it is noted that sporadic agglomerates of rGO sheets exist mainly caused by
strong inter-sheet van der Waals forces (Fig. 5b). Comparatively, in the case of composites of rGO-im-1 and rGO-im-2, Fig. 5c and d present that both of the two samples show well-dispersed and obvious wavy edge structures (The inserts are high-resolution SEM images.). The TEM images further display the porous and winkled microstructures of the rGO-im-2/epoxy nanocomposites (Fig. S2, Supporting Information). Despite the good visualization of nanostructures, electron microscope technique cannot statistically obtain the actual structure in nanocomposites containing high aspect ratio fillers [44,45]. Therefore, synchronous radiation X-ray small angle scattering (SAXS), as an effective and non-destructive technique for investigating detail microstructural features of materials, is used to further study the average conformation of the rGO-filled nanocomposites. The original 2-D scattering images and averaged 1-D scattering curves are presented in Fig. S3 (Supporting Information). The correction of Porod deviation is carried up before subsequent analysis. Firstly, as shown in the Guinier curves (Fig. S3, Supporting Information), the plots of I(q) vs q of all samples are nonlinear, indicating that the rGO-filled nanocomposites are polydisperse
X. Zhang et al. / Carbon 154 (2019) 115e124
119
Fig. 5. SEM images of the a) graphite/epoxy composites, b) rGO/epoxy nanocomposites, c) rGO-im-1/epoxy nanocomposites and d) rGO-im-2/epoxy nanocomposites. The filling weight content of each sample is 2 wt%. (A colour version of this figure can be viewed online.)
system. Thus, we then analyze scatters size distribution by the logarithm Gaussian distribution method [46,47]. The distributions of the gyration radius are exhibited in Table S2 (Supporting Information) and the corresponding insert images in Fig. 6bee. When the filling content is 2 wt%, in the case of the rGO/epoxy composite, the gyration radius of the scatters is mainly distributed in the range of 6e9 nm, and the most probable value is 7.3 nm. In contrast, for the rGO-im-1/epoxy and rGO-im-2/epoxy composites, the most probable values of gyration radius are 0.8 nm and 0.2 nm, respectively, and the corresponding distributions of the gyration radius are narrower compared with that of the rGO/epoxy composite. When the weight content decreases to 1 wt%, the most probable value of gyration radius of the rGO-im-2/epoxy reaches as low as 0.1 nm. Hence, the results confirm the fact that the non-covalent functionalization effectively reduces the aggregation of rGO in epoxy. Moreover, the statistical self-similarity and scale invariance are quantitatively described by the fractal dimension. The doublelogarithmic plot of I vs q power-law scattering of the form qa resulting in self-similar structures appears as a linear region of gradient a. A a value of between 3 and 4 is indicative of fractal surfaces with self-similar roughness, and it manifests the existence of a smooth surface when a ¼ 4. The fractal dimension D is related to a as D ¼ 6 a [48,49]. Fig. 6a sketches the corresponding fractal characteristics of the rGO filled epoxy composites at different length scales. As illustrated in Fig. 6bee, with a filling content of 2 wt%, the fractal dimensions D of rGO, rGO-im-1 and rGO-im-2 epoxy nanocomposites are 2.48, 2.51 and 2.71, respectively, in the q region from about 0.1 nm1 to 1 nm1. And in the higher q region, the corresponding fractal dimensions D are 2.08, 2.51 and 2.31, respectively. Distinctly, for the rGO-im-2 epoxy nanocomposites, the D values dramatically increase when the filler concentration decreases from 2 wt% to 1 wt% (Fig. 6d and e). Consequently, the analysis further supports the existence of well-exfoliated, rough and extensively winkle graphene nanosheets in the non-covalently modified rGO/epoxy composites, which is extremely consistent with the SEM results. More than these, compared with the SEM
technique, the SAXS surely distinguishes the more detailed structural conformations between different non-covalently modified rGO/epoxy composites, which are the key to achieving highperformance microwave absorption. Microwave absorption properties are evaluated with the measured data of complex permittivity and permeability based on the transmission line theory [50,51].
Z 1 RL ¼ 20 log in Zin þ 1
(1)
where Zin is the normalized input impedance, which can be expressed as
rffiffiffiffiffi Zin ¼
2p pffiffiffiffiffiffiffiffiffi tanh j mr εr fd εr c
mr
(2)
where εr, mr, j, c, f and d are relative complex permittivity, relative complex permeability, imaginary unit, light velocity in vacuum, frequency and thickness of the absorber, respectively. Fig. S4 (Supporting Information) and 7 show the RL contour plots and curves of epoxy nanocomposites containing the fillers with a loading of 1 wt%, 2 wt% and 5 wt%, respectively. We mainly focus on the MA properties in the thickness range of 1e3 mm according to the actual application. The minimum RL value of rGO/ epoxy is 19.3 dB at 11.88 GHz with a thickness of 2 mm and a filler loading of 2 wt% (Fig. 7a2). In the case of rGO-im-1/epoxy, when the weight content is 1 wt%, the minimum RL value reaches as low as 34.2 dB at 11.88 GHz and 47.2 dB at 11.8 GHz with a thickness of 2.7 mm and 3 mm, respectively (Fig. 7b1). Its effective absorption bandwidth (abbreviated as EAB, meaning the frequency range that corresponding RL values are lower than 10 dB) remarkably achieves 7.48 GHz at 2.7 mm and 6.04 GHz at 3 mm. When the weight content increases to 2 wt%, the rGO-im-1/epoxy composite achieves a strong absorption with minimum RL value of 49 dB at 17.8 GHz with a thickness of 1.7 mm (Fig. 7b2), and the maximum EAB value is 5.76 GHz at 2 mm. The rGO-im-2/epoxy exhibits an
120
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 6. a) Schematic diagrams on winkle graphene/polymer nanocomposites at different length scales, bee) fitted SAXS profiles and predicted distributions of particle sizes (insert) of the epoxy nanocomposites filled with rGO, rGO-im-1 and rGO-im-2, respectively. (A colour version of this figure can be viewed online.)
extremely strong absorption with minimum RL value of 65 dB at 15.32 GHz with a thickness of 1.9 mm and a filler loading of 2 wt%, and the corresponding EAB value is 4.8 GHz (Fig. 7c2). However, due to the impairment of impedance matching characteristics, when the filling content increases to 5 wt%, absorption properties of rGO-im-1/epoxy and rGO-im-2/epoxy composites obviously decrease to some extent. After a comprehensive analysis of the MA performance of these composites, it can be concluded that, non-covalently functionalization of rGO by imidazole can enhance the MA properties of composites effectively. Furthermore, in the case of the two kinds of composites, the MA capacities are of slight distinction but both excellent with the increasing contents of imidazole. Therefore, the non-covalent modification method is indeed an efficient approach to fabricate highly dispersed rGO/polymer nanocomposites with outstanding microwave absorption. Compared with other typical graphene-based composites reported in the recent literature
(Table S3, Supporting Information), it is doubtless that our rGO-im2/epoxy and rGO-im-1/epoxy composites are more promising MA materials owing to their strong absorption, thin thickness, wide EAB and ultralow loading contents. To further evaluate the comprehensive MA performance, specific reflection loss and specific EAB values are defined by dividing RL and EAB by the layer thickness and filler loading (assuming that the mass of the sample is 100 mg), respectively [52,53]. As shown in Fig. 10, the rGO-im-2/ epoxy and rGO-im-1/epoxy composites possess much higher specific reflection loss and specific EAB values than other reported graphene-based materials. Besides, the water-repellent properties of the composites are investigated and quantified by the contact angle (CA) q of a water droplet on the surface. As illustrated in Fig. 8, the pure epoxy resin possesses poor hydrophobicity with the water CA of 87.1. However, the q values of rGO-im-1/epoxy and rGO-im-2/epoxy composites increase to 110.4 and 112.5 , respectively. The high water CA is
X. Zhang et al. / Carbon 154 (2019) 115e124
121
Fig. 7. Frequency dependence of RL curves of the epoxy composites containing different contents of a1-a3) rGO, b1-b3) rGO-im-1 and c1-c3) rGO-im-2, respectively. (A colour version of this figure can be viewed online.)
Fig. 8. The contact angle of water on the prepared surface of pure epoxy resin and the non-covalently modified rGO/epoxy composites. (A colour version of this figure can be viewed online.)
mainly ascribed to the material roughness and low surface polarity caused by uniformly dispersed and wrinkled rGO. The hydrophobicity of the imidazole-modified rGO/epoxy composites can substantially improve their waterproof, anti-icing, and corrosion resistance and thus ensure long-term excellent MA capacities when being exposed to environment [54]. Microwave absorption properties of dielectric MA materials are correlated with the complex permittivity, where the real parts (ε0 ) and imaginary parts (ε00 ) of complex permittivity represent the
storage ability and the dissipation capability of electric energy, respectively [55,56]. The frequency-dependence relative complex permittivity and dielectric dissipation factor (tan d ¼ ε00 /ε0 , tangent loss) for the composites with different weight contents of fillers are shown in Fig. S5 (Supporting Information). Overall, the ε0 of all the composites drastically decreases, and the ε00 exhibits obvious resonant peaks with the increasing frequency. The dielectric relaxation behavior associates with polarization relaxation and conductive loss that described by Debye relaxation theory [57].
122
X. Zhang et al. / Carbon 154 (2019) 115e124
Fig. 9. Frequency dependence of a) attenuation constant a and b) relative input impedance jZin/Z0j of the samples. (A colour version of this figure can be viewed online.)
frequency when the filling contents are increased from 1 wt% to 2 wt%. The difference is probably attributed to the aggregation of graphene sheets. The dielectric relaxation loss in the noncovalently functionalized rGO/epoxy composites probably originates from the increased defect dipoles caused by chemical reduction and massive heterointerfaces between graphene and epoxy, and dipolar polarization resulting from the different electronegativity between graphene and imidazole or the residual oxygen functional groups in an unsaturated coordination (Fig. 11a). Both high dissipation and matched characteristic impedance are important to produce considerable microwave absorption. The calculated electromagnetic wave attenuation constant a [58,59].
a¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðm00 ε00 m0 ε0 Þ þ ðm00 ε00 m0 ε0 Þ2 þ ðm0 ε00 þ m00 ε0 Þ2 c (8)
Fig. 10. Comparison of specific jRLminj and specific effective bandwidth (EAB) of various carbon composites in previous references and this work. (A colour version of this figure can be viewed online.)
εs ε∞ 1 þ u2 t2
(3)
εs ε∞ s ut þ uε0 1 þ u2 t2
(4)
0
ε ¼ ε∞ þ 00
ε ¼ 00
εp ¼ 00
εc ¼
εs ε∞ ut 1 þ u2 t2
(5)
s uε0
(6)
ε0
ε ε 2 εs þ ε∞ 2 2 s ∞ þ ðε00 Þ ¼ 2 2
(7)
where ε0, εs and ε∞ are the dielectric constant in vacuum, the static permittivity and relative dielectric permittivity at the highfrequency limit, respectively. In Equations (3)e(7), u refers to the angular frequency; s stands for the electrical conductivity; t is the 00 00 relaxation time; εc refers to the conductive loss part and εp is the polarization loss part. The ε0 and ε00 values of rGO-im-1/epoxy and rGO-im-2/epoxy composites aggrandize gradually with the increasing filler loading nearly in the whole frequency range of 2e18 GHz, which is consistent with the effective medium theory [22]. However, the ε0 values of rGO/epoxy appear abrupt increasing in the corresponding
is a key parameter to determine the dissipation properties of MA materials. The rGO-im-2/epoxy and rGO-im-1/epoxy samples present high dielectric loss factors of nearly 0.4 with a filler loading of 2 wt% (Fig. S5, Supporting Information), but the former shows larger a and especially more ideal impedance matching of a jZin/Z0j value of 1.0 (Fig. 9). However, the optimal jZin/Z0j value of rGO/ epoxy is 0.8 with the thickness of 1.8 mm and a filler loading of 2 wt % mainly caused by high electrical conductivity, indicating the existence of stacked graphene sheets. Furthermore, we set up two 2-D finite element models of epoxy composites filled with high-dispersed and heavily-stacked graphitic carbon sheets, respectively, to simulate the effect of graphene dispersity on MA characteristics of the composites schematically and intuitively. The detailed geometric size, material parameters and boundary conditions are listed in Fig. S6 (Supporting Information). The results show that the maximum absorption coefficient of high-dispersed and heavily-stacked graphitic carbon/epoxy composites are 0.58 and 0.08 at 15 GHz (Fig. S6c, Supporting Information), respectively, and the corresponding electric displacement of former is stronger and more even-distributed (Fig. 11b and c). 4. Conclusion In summary, we prepared epoxy MA composites filled with ultralow content of non-covalently modified reduced graphene oxides (rGO) by an in-situ polymerization method. Overall, the surfaces of rGO nanosheets were successfully functionalized by strong p-p interaction between conjugated imidazole and graphene sheets. The combination of SEM and SAXS unambiguously demonstrated that the graphene sheets in the non-covalently
X. Zhang et al. / Carbon 154 (2019) 115e124
123
Fig. 11. a) Defect-related and dipolar polarization relaxation of non-modified rGO sheets, b, c) schematic representation of the MA mechanism and simulated electric displacement distribution for the highly dispersed and heavily stacked rGO/epoxy composites, respectively. (A colour version of this figure can be viewed online.)
modified rGO/epoxy composites were well-dispersed and extensively winkle. Compared with rGO/epoxy composites, the highly dispersed non-covalently functionalized rGO/epoxy showed more ideal impedance matching and stronger microwave dissipation based on the experimental and simulated results. The composites achieved excellent microwave absorptions (RLmin ¼ 65 dB, with a matching thickness of 1.8 mm), an extremely wide absorbing bandwidth of 7.48 GHz (RL < 10 dB) and ultralow filler loading (1e2 wt%). Moreover, the composites possessed high hydrophobicity with a water contact angle of 112.5 , which is larger than that (z87 ) of the pure epoxy resin. Acknowledgements This work was supported by 1W2A beamline of the Beijing Synchrotron Radiation Facility (BSRF, China). We gratefully acknowledge Mr. Xiongjie Yang, Cong Ge and Miss Xiaoqi Zhang for their assistance in preparing the nanocomposites. We are also indebted to Dr. Guang Mo and Dr. ZhiHong Li for their help in the measurements of SAXS. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.076. References [1] F. Ye, Q. Song, Z.C. Zhang, W. Li, S.Y. Zhang, X.W. Yin, et al., Direct growth of edge-rich graphene with tunable dielectric properties in porous Si3N4 ceramic for broadband high-performance microwave absorption, Adv. Funct. Mater. 28 (2018), 1707205. [2] H.H. Chen, Z.Y. Huang, Y. Huang, Y. Zhang, Z. Ge, B. Qin, et al., Synergistically assembled MWCNT/graphene foam with highly efficient microwave absorption in both C and X bands, Carbon 124 (2017) 506e514. [3] H. Luo, F. Chen, X. Wang, W.Y. Dai, Y. Xiong, J.J. Yang, et al., A novel two-layer honeycomb sandwich structure absorber with high-performance microwave absorption, Composites Part A 119 (2019) 1e7. [4] J. Choi, H.T. Jung, A new triple-layered composite for high-performance broadband microwave absorption, Compos. Struct. 122 (2015) 166e171. [5] A. Shah, A. Ding, Y.H. Wang, L. Zhang, D.X. Wang, J. Muhammad, et al., Enhanced microwave absorption by arrayed carbon fibers and gradient dispersion of Fe nanoparticles in epoxy resin composites, Carbon 96 (2016) 987e997. e, C. Bailly, I. Huynen, Thin and flexible multilayer polymer composite [6] Y. Danle structures for effective control of microwave electromagnetic absorption, Compos. Sci. Technol. 100 (2014) 182e188.
[7] W.J. Lee, J.W. Lee, C.G. Kim, Characteristics of an electromagnetic wave absorbing composite structure with a conducting polymer electromagnetic bandgap (EBG) in the X-band, Compos. Sci. Technol. 68 (2008) 2485e2489. [8] Z.B. Huang, W.C. Zhou, W.B. Kang, X.F. Tang, F. Luo, D.M. Zhu, et al., Dielectric and microwave-absorption properties of the partially carbonized PAN cloth/ epoxyesilicone composites, Composites Part B 43 (2012) 2980e2984. [9] Y.C. Qing, D.D. Min, Y.Y. Zhou, F. Luo, W.C. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxyesilicone composites as thinethickness and wide-bandwidth microwave absorber, Carbon 86 (2015) 98e107. [10] S.S. Wang, Y. Zhao, M.M. Gao, H.L. Xue, Y.C. Xu, C.H. Feng, et al., Green synthesis of porous cocoon-like rGO for enhanced microwave absorbing performances, ACS Appl. Mater. Interfaces 10 (2018) 42865e42874. [11] X. Qiu, L.X. Wang, H.L. Zhu, Y.K. Guan, Q.T. Zhang, Lightweight and efficient microwave absorbing materials based on walnut shell-derived nanoporous carbon, Nanoscale 9 (2017) 7408e7418. [12] B. Wen, M.S. Cao, M.M. Lu, W.Q. Cao, H.L. Shi, J. Liu, et al., Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures, Adv. Mater. 26 (2014) 3484e3489. [13] A.K. Geima, K.S. Novselov, The rise of graphene, Nat. Mater. 6 (2007) 183e191. [14] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nat. Mater. 10 (2011) 569e581. [15] S. Nardecchia, D. Carriazo, M.L. Ferrer, M.C. Gutierrez, F.D. Monte, Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications, Chem. Soc. Rev. 42 (2013) 794e830. [16] M.S. Cao, C. Han, X.X. Wang, M. Zhang, Y.L. Zhang, J.C. Shu, et al., Graphene nanohybrids: excellent electromagnetic properties for the absorbing and shielding of electromagnetic waves, J. Mater. Chem. C 6 (2018) 4586e4602. [17] B. Wen, X.X. Wang, W.Q. Cao, H.L. Shi, M.M. Lu, G. Wang, et al., Reduced graphene oxides: the thinnest and most lightweight materials with highly efficient microwave attenuation performances of the carbon world, Nanoscale 6 (2014) 5754e5761. [18] S.H. Lee, D.H. Lee, W.J. Lee, S.O. Kim, Tailored assembly of carbon nanotubes and graphene, Adv. Funct. Mater. 21 (2011) 1338e1354. [19] C. Chen, J.B. Xi, E.Z. Zhou, L. Peng, Z.C. Chen, C. Gao, Nano-Micro Lett. 10 (26) (2018) 1e11. [20] Y. Zhang, Y. Huang, T.F. Zhang, H.C. Chang, P.S. Xiao, H.H. Chen, et al., Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (2015) 2049e2053. [21] C. Wang, X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, et al., The electromagnetic property of chemically reduced graphene oxide and its application as microwave absorbing material, Appl. Phys. Lett. 98 (2011), 072906. [22] B.Y. Kuang, W.L. Song, M.Q. Ning, J.B. Li, Z.J. Zhao, D.Y. Guo, et al., Chemical reduction dependent dielectric properties and dielectric loss mechanism of reduced graphene oxide, Carbon 127 (2018) 209e217. [23] Z.X. Hou, X.W. Yin, H.L. Xu, H.J. Wei, M.H. Li, L.F. Cheng, et al., Reduced graphene oxide/silicon nitride composite for cooperative electromagnetic absorption in wide temperature spectrum with excellent thermal stability, ACS Appl. Mater. Interfaces 11 (2019) 5364e5372. [24] S. Zhao, H.B. Zhang, J.Q. Luo, Q.W. Wang, B. Xu, S. Hong, et al., Highly electrically conductive three-dimensional Ti3C2Tx MXene/reduced graphene oxide hybrid aerogels with excellent electromagnetic interference shielding performances, ACS Nano 12 (2018) 11193e11202.
124
X. Zhang et al. / Carbon 154 (2019) 115e124
[25] L. Quan, F.X. Qin, D. Estevez, H. Wang, H.X. Peng, Magnetic graphene for microwave absorbing application: towards the lightest graphene-based absorber, Carbon 125 (2017) 630e639. [26] H.H. Chen, W.L. Ma, Z.Y. Huang, Y. Zhang, Y. Huang, Y.S. Chen, Graphene-based materials toward microwave and terahertz absorbing stealth technologies, Adv. Opt. Mater. 7 (2019), 1801318. [27] Y. Zhang, Y. Huang, H.H. Chen, Z.Y. Huang, Y. Yang, P.S. Xiao, et al., Composition and structure control of ultralight graphene foam for high-performance microwave absorption, Carbon 105 (2016) 438e447. [28] H.H. Chen, Z.Y. Huang, Y. Huang, Y. Zhang, Z. Ge, W.L. Ma, et al., Consecutively strong absorption from gigahertz to terahertz bands of a monolithic threedimensional Fe3O4/graphene material, ACS Appl. Mater. Interfaces 11 (2019) 1274e1282. [29] Z.Y. Huang, H.H. Chen, Y. Huang, Z. Ge, Y. Zhou, Y. Yang, et al., Ultra-broadband wide-angle terahertz absorption properties of 3D graphene foam, Adv. Funct. Mater. 28 (2018), 1704363. [30] X.L. Li, X.W. Yin, C.Q. Song, M.K. Han, H.L. Xu, et al., Wenyan duan, selfassembly coreeshell graphene-bridged hollow MXenes spheres 3D foam with ultrahigh specific EM absorption performance, Adv. Funct. Mater. 28 (2018), 1803938. [31] Z.Y. Huang, H.H. Chen, S.T. Xu, L.Y. Chen, Y. Huang, Z. Ge, et al., Graphenebased composites combining both excellent Terahertz shielding and stealth performance, Adv. Opt. Mater. 6 (2018), 1801165. [32] V.D. Punetha, S. Rana, H.J. Yoo, A. Chaurasia, J.T. M Jr., M.S. Ramasamy, et al., Functionalization of carbon nanomaterials for advanced polymer nanocomposites: a comparison study between CNT and graphene, Prog. Polym. Sci. 67 (2017) 1e47. [33] T. Kuilla, S. Bhadra, D.H. Yao, N.H. Kim, S. Bose, J.H. Lee, Recent advances in graphene-based polymer composites, Prog. Polym. Sci. 35 (2010) 1350e1375. [34] B. Long, M. Manning, M. Burke, B.N. Szafranek, G. Visimberga, D. Thompson, et al., Non-covalent functionalization of graphene using self-assembly of alkaneamines, Adv. Funct. Mater. 22 (2012) 717e725. [35] I. Zaman, H.C. Kuan, Q. Meng, A. Michelmore, N. Kawashima, T. Pitt, L. Zhang, S. Gouda, L. Lee, J. Ma, Adv. Funct. Mater. 21 (2011) 1338. [36] M. Beidaghi, C.L. Wang, Micro-supercapacitors based on interdigital electrodes of reduced graphene oxide and carbon nanotube composites with ultrahigh power handling performance, Adv. Funct. Mater. 22 (2012) 4501e4510. [37] L. Xu, H. Nakajima, E. Manias, R. Krishnamoorti, Tailored nanocomposites of polypropylene with layered silicates, Macromolecules 42 (2009) 3795e3803. [38] H.J. Salavagione, Q.D. Susana, E.J. Patricia, G. Martínez, F. Ania, A. Flores, et al., Development of advanced elastomeric conductive nanocomposites by selective chemical affinity of modified graphene, Macromolecules 49 (2016) 4948e4956. [39] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [40] Y.M. Wang, S.F. Chen, X.C. Chen, Y.F. Lu, M.H. Miao, D.H. Zhang, Controllability of epoxy equivalent weight and performance of hyperbranched epoxy resins, Composites Part B 160 (2019) 615e625. [41] S. Zainuddin, A. Fahim, T. Arifin, M.V. Hosur, M.M. Rahman, J.D. Tyson, et al., Optimization of mechanical and thermo-mechanical properties of epoxy and E-glass/epoxy composites using NH2-MWCNTs, acetone solvent and combined dispersion methods, Compos. Struct. 110 (2014) 39e50. [42] Y. Chen, H.B. Zhang, Y.B. Yang, M. Wang, A.Y. Cao, Z.Z. Yu, High-performance epoxy nanocomposites reinforced with three-dimensional carbon nanotube sponge for electromagnetic interference shielding, Adv. Funct. Mater. 26 (2016) 447e455.
[43] M.S.S. Dorraji, M.H. Rasoulifard, A.R. Amani-Ghadim, M.H. Khodabandeloo, M. Felekari, M.R. Khoshrou, et al., Microwave absorption properties of polypyrrole-SrFe12O19-TiO2-epoxy resin nanocomposites: optimization using response surface methodology, Appl. Surf. Sci. 383 (2016) 9e18. [44] M.P. Weir, D.W. Johnson, S.C. Boothroyd, R.C. Savage, R.L. Thompson, S.R. Parnell, et al., Extrinsic wrinkling and single exfoliated sheets of graphene oxide in polymer composites, Chem. Mater. 28 (2016) 1698e1704. [45] T. Li, A.J. Senesi, B. Lee, Small angle X-ray scattering for nanoparticle research, Chem. Rev. 116 (2016) 11128e11180. [46] Z.H. Li, A program for SAXS data processing and analysis, Chin. Phys. C 37 (2013), 108002. [47] J. Polte, T.T. Ahner, F. Delissen, S. Sokolov, F. Emmerling, A.F. Thünemann, et al., Mechanism of gold nanoparticle formation in the classical citrate synthesis method derived from coupled in situ XANES and SAXS evaluation, J. Am. Chem. Soc. 132 (2010) 1296e1301. [48] X. Xia, J.H. Yin, B. Su, D. Hui, R.T. Yu, X.X. Liu, Quantitative determining interface information of nano composite by synchrotron radiation small-angle X-ray scattering, Composites Part B 120 (2017) 92e96. [49] R. Wei, X.Q. Wang, X. Zhang, S.J. Wang, S.Y. Zhu, S.Y. Du, A novel method for manufacturing high-performance layered silicate/epoxy nanocomposites using an epoxyediamine adduct to enhance compatibility and interfacial reactivity, Macromol. Mater. Eng. 303 (2018), 1800065. [50] Z.C. Wu, W. Hu, T. huang, P. Lan, K. Tian, F.F. Xie, et al., Hierarchically porous carbon with controlled structure for efficient microwave absorption, J. Mater. Chem. C 6 (2018) 8839e8845. [51] S.S. Dai, Y. Cheng, B. Quan, X.H. Liang, W. Liu, Z.H. Yang, et al., Porous-carbonbased Mo2C nanocomposites as excellent microwave absorber: a new exploration, Nanoscale 10 (2018) 6945e6953. [52] Y. Li, X.F. Liu, X.Y. Nie, W.W. Yang, Y.D. Wang, R.H. Yu, et al., Multifunctional organiceinorganic hybrid aerogel for self-cleaning, heat-insulating, and highly efficient aicrowave absorbing material, Adv. Funct. Mater. 29 (2018), 1807624. [53] Y.C. Yin, X.F. Liu, X.J. Wei, Y. Li, X.Y. Nie, R.H. Yu, et al., Magnetically aligned CoC/MWCNTs composite derived from MWCNT-interconnected zeolitic imidazolate frameworks for a lightweight and highly efficient electromagnetic wave absorber, ACS Appl. Mater. Interfaces 9 (2017) 30850e30861. [54] Y. Cheng, T.X. Zhu, S.H. Li, J.Y. Huang, J.J. Mao, H. Yang, et al., A novel strategy for fabricating robust superhydrophobic fabrics by environmentally-friendly enzyme etching, Chem. Eng. J. 355 (2019) 290e298. [55] D.W. Xu, S. Yang, P. Chen, Q. Yu, X.H. Xiong, J. Wang, Synthesis of magnetic graphene aerogels for microwave absorption by a in-situ pyrolysis, Carbon 146 (2019) 301e312. [56] B. Quan, X.H. Liang, G.B. Ji, J.N. Ma, P.Y. Ouyang, H. Gong, et al., Strong electromagnetic wave response derived from the construction of dielectric/ magnetic media heterostructure and multiple interfaces, ACS Appl. Mater. Interfaces 9 (2017) 9964e9974. [57] D.W. Liu, Y.C. Du, P. Xu, N. Liu, Y.H. Wang, H.H. Zhao, et al., Waxberry-like hierarchical Ni@C microspheres with high-performance microwave absorption, J. Mater. Chem. C 7 (2019) 5037e5046. [58] L.W. Yan, C.Q. Hong, B.Q. Sun, G.D. Zhao, Y.H. Cheng, S. Dong, et al., In situ growth of coresheath heterostructural SiC nanowire arrays on carbon fibers and enhanced electromagnetic wave absorption performance, ACS Appl. Mater. Interfaces 9 (2017) 6320e6331. [59] D.W. Xu, J.L. Liu, P. Chen, Q. Yu, J. Wang, S. Yang, et al., In situ growth and pyrolysis synthesis of super-hydrophobic graphene aerogels embedded with ultrafine b-Co nanocrystals for microwave absorption, J. Mater. Chem. C 7 (2019) 3869e3880.