Ultralight graphene micro-popcorns for multifunctional composite applications

Carbon 139 (2018) 545e555

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Ultralight graphene micro-popcorns for multifunctional composite applications Chen Chen a, Jiabin Xi a, Yin Han b, Li Peng a, Weiwei Gao a, Zhen Xu a, Chao Gao a, * a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, PR China b Beijing Institute of Space Long March Vehicle, Beijing, 100076, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 April 2018 Received in revised form 25 June 2018 Accepted 9 July 2018

Graphene has been widely applied in polymer nanocomposites due to its charming physical and chemical properties. However, the performance of graphene/polymer composite is hampered by the strong stacking tendency of graphene sheets. Here we introduce graphene micropopcorns (GMPs) with a hollow structure and a low stacking degree as an efficient additive for multifunctional composites. GMPs are massively fabricated via a facile thermal treatment of spray-dried graphene oxide (GO) powder. GMPs feature a low tap density (6e8 mg cm
3), a high specific surface area (947 m2 g
1) and a good solvent absorption capability (62 g g
1). This micro form of graphene can act as electrical conductive fillers with a low percolation threshold of 0.18 vol% and microcapsules of phase change materials with a 358% enhancement in thermal conductivity. Besides, the microwave absorption (MA) performance of GMPs/ paraffin composites outperforms most graphene-based materials ever reported but with a record-low filler content (2.5 wt%). The large-scale producible GMPs enable an efficient method to utilize graphene to grant conventional materials with better performances and new functions. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction Graphene has attracted substantial interest in the field of polymer nanocomposites [1,2]. The introduction of graphene can bring remarkable enhancements in mechanical, electrical and thermal properties of composites, together with many other functions, such as gas barrier, electromagnetic shielding and antimicrobial merits [3e5]. For the unique two-dimensional topology and atomic thickness, the effective filler content of graphene and its derivates is much lower than conventional fillers such as glass fiber, carbon fiber and carbon blacks, showing advantages in light weight and easy processing [6]. However, the performances of graphene/ polymer composites are usually limited by the severe selfagglomeration of graphene, which arises from the vast exposed surface with strong interlayer p-p attraction between graphene sheets [7,8]. In order to reduce the stacking degree of graphene in polymer matrix, several strategies are widely applied. One strategy is surface modification of graphene via either noncovalent or covalent

* Corresponding author. E-mail address: [email protected] (C. Gao). https://doi.org/10.1016/j.carbon.2018.07.020 0008-6223/© 2018 Elsevier Ltd. All rights reserved.

functionalization [3,9,10]. Bao and coworkers incorporated polyaniline (PANI) nanowires onto GO through a hydrothermal route to act as spacers to prevent the agglomeration of graphene layers [11]. Xu et al. prepared graphene-nylon 6 nanocomposite with outstanding mechanical performances by in-situ polymerization [12]. The highly grafted nylon-6 arms hindered the stacking of graphene. However, the functionalization process usually involves choice reagents with complex reactions, during which the stacking of graphene can hardly be avoided. Another strategy is constructing a three-dimensional (3D) framework of graphene before compositing with polymers [9]. The highly porous graphene scaffold with extremely low density can efficiently accommodate matrix to form homogeneous composites with outstanding mechanical, thermal and electrical properties [13]. For example, Wang et al. infiltrated 3D reduced graphene oxide (rGO) with epoxy resin to prepare 3D rGO/epoxy composite [14]. The unusual isotropic rGO aerogel/ epoxy composite with a filler content of 1.4 wt% showed a high electrical conductivity of 20 S m
1 and a remarkable 64% enhancement in fracture toughness. The efficient graphene scaffolds are generally produced via chemical vapor deposition, template sacrificing and lyophilization, whereas these time and energy consuming methods are undesirable for scalable manufacturing

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and processing [15,16]. Apart from methods mentioned above, introducing folded structure in graphene is also conducive to alleviating graphene aggregation [17,18]. Luo et al. obtained crumpled graphene particles by capillary compression in rapidly evaporating aerosol droplets [19]. The graphene particles presented a significantly lower aggregation tendency in either solution or solid state compared with flat graphene sheets. Wang et al. synthesized porous polyacrylonitrile/graphene spheres by means of spray drying [20]. The graphene sheets wrapped the polyacrylonitrile nanoparticles loosely and formed a continuous conductive network, resulting in excellent rate capability and cycling stability for sulfur-lithium battery. However, in comparison with the continuous graphene network, folded graphene with a relatively compact structure manifests higher density, and fails to fulfill the expected enhancement in electrical conductivity. Apart from these conventional strategies, construction of hollow spheres has greater potential for their light weight, high porosity and low effective filling ratio [21,22]. Considerable research efforts have been devoted so far to obtain hollow graphene spheres via multifarious approaches. Song and coworkers adopted a water-inoil (W/O) emulsion technique to fabricate hollow GO spheres, which was applied as anode materials in lithium-ion batteries [23]. Template-sacrifice method is also widely utilized in the preparation of graphene hollow spheres [24,25]. Huang and coworkers synthesized graphene capsules by sacrificing the polystyrene bead templates, and the obtained material showed good oil absorbing ability [26]. For all that, there still remain drawbacks in the above approaches. On the one hand, the complicated procedures are inefficient and time-consuming. On the other hand, both emulsion assembly and template sacrificing are small-batch production methods due to the demands in solvent and templates. Scalable self-assembly of graphene hollow spheres is rarely reported. Herein we present a facile strategy to produce GMPs in a large scale. Aqueous dispersion of GO was first spray-dried into flowershaped graphene oxide (fGO) powder. With the annealing temperature increasing, we found that fGO gradually transferred to hollow shell structure with openings, which is similar to the cooking process of popcorns. The fabricated GMPs feature an ultralow tap density (6e8 mg cm
3), a high surface area (947 m2 g
1) and a high solvent absorption capability (62 g g
1). The incorporation of GMPs into paraffin exhibits a low percolation threshold of 0.18 vol% (0.51 wt%) and a 358% enhancement in thermal conductivity. The hollow shells of GMPs can enclose paraffin inside with enhanced thermal stability and latent heat storage, showing potential in phase change materials. Besides, the 2.5 wt% GMPs/ paraffin composite shows peak absorption up to
45.4 dB with a wide efficient absorption bandwidth (EAB, reflection loss 
10 dB) of 6 GHz for the MA application. Such an MA performance accompanied with an ultralow filler content surpasses all functionalized graphene and most graphene-based composite fillers ever reported. Therefore, GMPs can be applied as high-performance oil absorbents, electrical conductive fillers, microcapsules of phasechange materials and microwave absorbents, etc. Besides, GMPs exhibit advantages in low density, low filler content, mass productivity and facile processibility compared with other graphene based materials. 2. Experimental section 2.1. Preparation of fGO GO was synthesized via the modified Hummers method, as described elsewhere [27]. The obtained GO aqueous dispersion (3 mg/g) was nebulized through a nozzle under the pressure of 0.2 MPa. The sprayed micron-sized droplets were carried by pre-

heated air (150  C) through a cyclone separator. During this process water evaporated in seconds, making GO sheets fold and shrink to microflowers [28]. 2.2. Synthesis of GMPs To obtain GMPs, fGO was directly subjected to thermal treatment with a heating rate of 5  C/min in a tube furnace under nitrogen atmosphere. The treatment temperature varied from 150  C to 1300  C. The annealing duration was 1 h. GMPs refer to fGO particles treated under 1300  C if there is no special statement. The annealing above 1600  C was conducted on a graphitization furnace under argon atmosphere. The heating rate was 500  C/h and the annealing duration was 30 min. 2.3. Synthesis of GMPs/paraffin composites GMPs were added into melted paraffin (melting point: 62e65  C) under 70  C with filling ratio varied from 0 to 5 wt% (weight percentage). The mixtures were stirred for 10 min and underwent a vacuum defoamation for 10 min. The obtained compounds were molded into cylinder and cylinder toroidal for electrical conductivity test and MA measurement, respectively. 2.4. Characterization The morphologies of samples were obtained from on a Hitachi S4800 field emission scanning electron microscope (SEM) system. The accelerating voltage was 3 kV. High resolution transmission electron microscope (HRTEM) images were taken on a JEM-2010 HRTEM with an accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI 5000C ESCA system operated at 14.0 kV. All binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV. Thermal gravimetric analysis (TGA) was performed using a thermogravimetric analyzer (TA-Q500) from room temperature to 850  C at 10  C min
1 heating rate under nitrogen atmosphere. Before TGA tests, all samples were dried under 100  C for 30 min. X-Ray diffraction (XRD) data were collected with an X'Pert Pro (PANalytical) diffractometer using monochromatic Cu Ka1 radiation (l ¼ 1.5406 Å) at 40 kV. The scan rate was 0.1 /s. Raman spectra were recorded on a Labram HRUV spectrometer operating at 514 nm. Brunauer-Emmett-Teller (BET) nitrogen cryoadsorption was tested on an AUTOSORB-IQ-MP (Quantachrome Inc., USA) and all samples were outgassed under 120  C for 1 h. Transmission infrared spectra were recorded on a TENROS Ⅱ (BRUKER CO.). Particle diameter distribution analysis was operated on an LS-230 (Beckman Coulter, Inc.) and the dispersion medium was water. Contact angle test was conducted on an OCA20 (DATAPHYSI). 2.5. Solvent and oil absorption analysis GMPs were sealed in a copper strainer to form a small bag, which was then immersed in solvents or oil for 15 min. Before weighing, the bag was kept in ambient for 2e5 min to remove the liquid on the bag surface. The absorption capability was calculated from the equation: (m1-m0)/m2, where m1 is the mass of the bag filled with GMPs after absorption, m0 is the mass of the empty copper bag after absorption, and m2 is the mass of encapsulated GMPs. 2.6. Electrical conductivity measurement The electrical conductivity was measured on an electrochemical workstation (CHI660e, CH Instruments, Inc.). All samples were

C. Chen et al. / Carbon 139 (2018) 545e555

tested by connecting the electrodes on both ends via copper tapes. Based on the percolation theory, the electric conductivity (d) of the composite and volume fraction of the conductive filler (4) follows the power law: d f (4 - 4c)t, where 4c is the percolation volume fraction and t is the critical exponent [29]. The fitting of log d e 4 curves gives 4c, and t can be obtained by further fitting log d vs log (4 - 4c). The transformation between 4 and weight fraction (w) follows: 4 ¼ wrm/(wrm þ rf), where rm and rf are the density of matrix (0.78 g cm
3) and filler (2.2 g cm
3) respectively. 2.7. Heat storage test 5 g GMPs were mixed with 65 g melted paraffin under 70  C. After kept for 30 min, the mixture was filtrated under 80  C to remove excess paraffin. The final GMPs content was calculated as 8.9 wt%. The obtained composite was compressed in a cylinder mold, forming a round disk. Differential scanning calorimetry (DSC) was operated on a TA-Q100 MDSC under nitrogen atmosphere. The temperature range was -10e80  C and the heating/cooling rate was 10  C/min. Infrared images and temperature profile were recorded on an infrared camera (FLIR T630sc). Thermal conductivity (l) was calculated using equation (1):

l ¼ r$Cp $D

(1)

D is the thermal diffusivity, which is measured by laser flash method using Netzsch LFA 467 NanoFlash instrument. The sample was heated by a light pulse, giving rise to a temperature rise. D can be calculated from the temperature rise time and sample thickness [30]. r is the density of the material and Cp represents the specific heat capacity obtained by DSC. 2.8. Microwave absorption measurement The complex permittivity was tested via a coaxial line method on a vector network analyzer (ZNB 40, Rohde & Schwarz). GMPs were first mixed with paraffin in different filler contents, and shaped as cylindrical toroidal specimens with an outer diameter of 7.0 mm, inner diameter of 3.04 mm, and thickness of 4.0 mm. The complex permeability was considered to be that of free space since no ferromagnetic materials were involved. The reflection loss (RL) of microwave can be calculated from:

Zin ¼

rffiffiffiffiffi    mr 2p pffiffiffiffiffiffiffiffiffi fd mr εr tanh j εr c

  Z
1  RL ðdBÞ ¼ 20 log in Zin þ 1

(2)

(3)

where Zin refers to the normalized input impedance of a metalbacked microwave absorbing layer, mr and εr are referred to complex permeability and complex permittivity respectively, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of light in free space. 3. Results and discussion 3.1. The structural evolution of GMP GMPs were obtained via a two-step assembly of GO sheets, as illustrated in Fig. 1a. During the spray-drying process, water molecules evaporate in seconds and the induced capillary force crumples GO sheets into flower-like micropowders [28,31]. In micro-sized droplets GO sheets fold centripetally to form highly crumpled particles with graphene ridges and boundaries sticking

547

out, resembling a bloom (Fig. 1c). The subsequent 1300  C thermal annealing leads to intense volume expansion in fGO and the formation of GMP. The two-step technique produced GMPs in large scale, as shown in massive powder (10 g) in Fig. 1b. The obtained GMPs feature a hollow shell structure comprised of crumpled graphene sheets, and some holes can be observed on its surface (Fig. 1d and e). The diameter of GMPs is up to 20 mm, indicating a maximum 100-fold volume expansion compared with fGO (2e5 mm in diameter) (Fig. S1). Graphene folds spread out and the corrugated surface is flattened with some outstretched graphene sheets on it. From the opening on the GMPs particles, the wafery graphene walls can be observed. As can be found from HRTEM observation, the transparent wrinkle graphene sheets in the center of GMP demonstrates a hollow structure (Fig. 1f). Besides, there are many small winkles of nanometer width on GMP surface (Fig. 1g). There are 3e5 indistinct stripe patterns in HRTEM observation of GMP shell, indicating a few-layer stacking of graphene sheets with poor crystallization (Fig. 1h). The structural evolution of GMPs is closely related with the thermal reduction of fGO, as proved in the TGA analysis and SEM observations of fGO under different annealing temperatures. There is a sharp weight loss from 130  C to 300  C owing to the elimination of labile oxygen-containing groups of GO, and thermal decomposition continues slowly to remove the remaining groups under higher temperature (Fig. 2a). We found that the morphology of the obtained graphene turned from microflower to hollow sphere with the treatment temperature increasing from 130  C to 600  C (Fig. 2bed), indicating the formation of GMPs should be resulted from the generated gas of the decomposed groups on GO. Below 130  C, little gas is released and fGO keeps its original shape (Fig. 2b). When thermal annealing goes to 250  C, fGO particles only partly turned into GMPs, implying the gas release is not sufficient enough to fully expand fGO (Fig. 2c). Under 600  C, most groups have already been eliminated and the weight loss tends to be negligible, thus hollow graphene bubbles are generated (Fig. 2d). Further increasing the annealing temperature to 1200  C exhibits inapparent influences on the morphology of expanded fGO, as the SEM images shown in Fig. S2. However, if the treatment temperature increases above 1600  C, remarkable crumpling effect can be observed on the graphene shell (Fig. S3). Because most oxygencontaining groups have already been removed below 1600  C, the shrinkage of graphene sheets can be ascribed to the healing of defects on rGO. In consequence, we choose 1300  C as the proper treatment temperature. We consider the structural evolution from fGO to GMPs in a “popcorn-like” manner. When fGO particles are treated under a direct heating process, the decomposition rate of the oxygen groups exceeds the releasing rate of the generated gas (H2O, and CO and CO2), providing a pressure to push graphene layers outwards. Finally, expanded GO spheres burst like blown balloons under high internal pressure but cannot shrink back again owing to the rigidity of the graphene shell. Resembling popcorns, pores generate on GMP shells. To verify this mechanism, fGO was kept under 120  C for 48 h to mildly eliminate the oxygen containing groups on GO. The mildly reduced fGO (RfGO) remains the flower-shaped structure under the following 1300  C annealing, confirming the popcorn-like expansion mechanism (Fig. S4). Different from the thermal expansion of graphite oxide and GO paper in normal direction [32,33], the explosive pyrolysis of the groups in fGO causes a tremendous expansion in omnidirections. Besides, the folded graphene sheets in fGO stack loosely, which lowers the energy barrier of the expansion. Considerable research efforts have been devoted so far to obtain hollow carbon materials via multifarious approaches, including template-sacrifice method, emulsion assembly, hydrothermal reaction and polymerization [34]. The spray-drying

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Fig. 1. (a) Schematic illustration of the formation of GMPs. (b) Photograph of 10 g GMPs powder. (c) SEM image of fGO, and the inset is an individual fGO particle (scale bar: 500 nm). (d,e) SEM images of GMPs, and the inset is the photograph of popcorns. (f) TEM image of a GMP particle. (g) HRTEM image of the nano-sized folds on GMP. (h) HRTEM image of the boundary of the GMP shell. (A colour version of this figure can be viewed online.)

and subsequent thermal annealing applied here are both highly scalable fabrication techniques, thus the facile two-step preparation strategy avoids complex reaction, solvent removal and the usage of templates, showing advantages in cost-effectiveness and mass productivity. 3.2. Characterization of GMPs BET absorption-desorption analyses prove the specific surface area and pore volume of fGO increase with the treatment temperature increasing from 200 to 1300  C (Fig. 3a). The specific surface area of GMPs (947 m2 g
1) is over 12 times that of fGO (76.5 m2 g
1), and the pore volume of GMPs (4.71 cm3 g
1) is also much higher than fGO (0.4 cm3 g
1). The emergence of hysteresis loops in the absorption/desorption curves of fGO with increasing temperature demonstrate the formation of mesopores (shown in Fig. S5a, b). The specific surface area (S) of graphene has an inverse relationship with the stacking layers (n) as: S ¼ 2600/n m2 g
1 [26,35]. The average thickness of GMPs shells is calculated as around 3 layers, which agrees with the TEM observation in Fig. 1g. The obtained GMPs powder is highly fluffy and displays a tap density of 6e8 mg cm
3, close to graphene aerogel and much lower than graphite (2.2 g cm
3) and crumpled graphene (0.5e1 g cm
3) (Fig. S6) [36]. XRD analyses provide more information about the microstructure transformation of fGO. As can be seen in Fig. 3b, the characteristic (001) peak in fGO gradually disappears and the

stacking peak of graphene centering around 26  C becomes prominent with the annealing temperature increasing. However, the (002) peak of GMPs is nearly negligible compared with 1300  C annealed RGO film, proving a loose stacking of crumpled graphene layers in GMP shells (shown in Fig. 3c) [37,38]. The half peak width of GMPs reaches 6.5 , whereas thermally treated RGO film shows a half peak width of 1. Such loose stacking of graphene results from the interlayer expansion during annealing and the formation of small crumples. When the annealing temperature rises to over 1600  C, the (002) peak becomes intense because of the stacking of rGO sheets with fewer defects (Fig. S7a). Such restacking behavior is responsible for the shrinkage of GMPs under high temperature in Fig S3. Raman spectroscopy analyses further verify the thermal reduction of GO. The intensity ratio of D band (1350 cm
1, representing the defects in graphene) and G band (1580 cm
1, corresponding to the graphitic sp2 structure) is widely accepted as the metric of the disorder degree in graphene [39]. As calculated from Fig. 3d and e, ID/IG increases from 1.44 to 2 after annealing fGO under 900  C, attributing to the increment in the number of smaller sp2 domains [40,41]. When the crystalline size La is smaller than 2 nm, ID/IG increases with La increasing because graphene sheet is dominated by the structurally disordered areas. With further increasing the treatment temperature to 1300  C, ID/IG sharply decreases to 1.46 owing to the elimination of residual functional groups and the healing of defects. When the annealing is conducted

C. Chen et al. / Carbon 139 (2018) 545e555

549

Fig. 2. (a) TGA plot of fGO is divided into three parts according to the microstructures of the products. (b) SEM image of fGO after a thermal treatment under 100  C, corresponding to the region I in (a). (c) SEM image of fGO after a thermal treatment under 250  C, corresponding to the region II in (a). (d) SEM image of fGO after a thermal treatment under 600  C, corresponding to the region III in (a). (A colour version of this figure can be viewed online.)

Fig. 3. (a) Specific surface area and pore volume of fGO annealed under different temperatures. (b) XRD patterns of fGO annealed under various temperatures. (c) XRD patterns of fGO, GMPs and RGO film. (d,e) Raman spectra of fGO annealed under different temperatures. (f) XPS curves of fGO, fGO annealed under 200  C and GMPs. (A colour version of this figure can be viewed online.)

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above 2000  C, the ID/IG tumbles below 0.1 (Fig. S7b). XPS patterns reveal that the epoxy groups transfers into hydroxy groups under 200  C annealing, as indicated in Fig. 3f, and increasing annealing temperature further eliminates residual oxygen-containing groups on fGO. The carbon and oxygen contents in GMPs are 94.6% and 3.7% respectively, verifying the reduction of fGO. Similar result can be obtained from Fourier transform infrared spectrometry (Fig. S7c). GMPs with a hollow shell structure exhibit noticeable low density and high pore volume, thus GMPs can be applied as absorbents of solvents and oil. A square bulk of GMPs (120 mg) obtained after compression can spontaneously absorb 3 ml petroleum ether within 5 min, as illustrated in Fig. 4a. The absorption capability toward the commonly used solvents and oil is up to 62 g g
1, higher than many powdery oil absorbents, for example, expanded graphite (47 g g
1), oil absorption resin (10e21 g g
1) and activated carbon (10e20 g g
1) (Fig. 4b and detailed data see Table S1). The absorbed petroleum ether can be completely burned in air, and the regenerated GMPs can be recycled for oil absorption (Fig. S8a,b). The weight gain of GMPs toward petroleum ether decreases by 13.4% in the first five absorption-burning cycles and reaches steady values in the following circles. During ten absorption-burning cycles there is no weight loss in GMPs and the morphology of the hollow shell is well preserved, verifying superior thermal stability and recycling of GMPs (Fig. 4c and d). Moreover, the oil absorption ability may be further enhanced by crosslinking GMPs into a threedimensional network. 3.3. Electrical conductivity and heat storage properties of GMPs/ paraffin composite The highly porous graphene microstructure is beneficial for the formation of conductive network, which is very useful for preparing conductive composites. GMPs can disperse homogeneously in

the paraffin matrix without any precipitates owing to the hydrophobicity and anti-aggregation nature of the hollow spheres (Fig. S9). As exhibited in Fig. 5a, the conductivity dramatically increases with the incorporation of GMPs in paraffin. With addition of only 0.7 wt% GMPs, the electrical conductivity of the composite meets the demand of anti-static materials (above 10
7 S m
1). The electric conductivity reaches 12.47 S m
1 with a mere 5 wt% GMPs. The calculated percolation threshold 4c is 0.18 vol% (0.51 wt%), which is lower than that of graphite [42], graphene nanoplates [43], thermally reduced GO [44], etc (Table S2). Such phenomenon is easy to understand, the hollow spheres with conductive graphene shells can form a network under much lower weight ratio (Fig. 5c and d). On the contrary, the inner parts of the graphitic materials with high stacking degree have no contribution to the formation of conductive network, thus higher percolation threshold is obtained. The percolation threshold of GMPs/paraffin is even lower than that of graphene aerogel/epoxy composite [14]. Considering the low density, scalable productivity and high cost-efficiency, GMPs can act as a superior conductive filling material for the fabrication of conductive or anti-static composites. Furthermore, the hollow GMPs with open structure can act as ideal microcapsules for phase change materials. After excess paraffin is removed, paraffin/GMP composite with 8.9 wt% GMPs inside can be molded into a thermally stable bulk material. It is proved that GMPs keep the hollow structure after paraffin is dissolved by petroleum ether, demonstrating great shape preservation of GMPs in the composite (Fig. S10). As indicated in Fig. 6a, under 80  C pure paraffin melts while the paraffin/GMP composite keeps good integrity, attributing to the entrapment of paraffin in the hollow graphene shells. The key for improving the energy storage ability of phase change materials is enhancing the thermal conductivity and latent heat capability [45]. The thermal conductivity of paraffin/GMPs is measured as 0.757 W m
1 k
1, which is 358% higher than that of paraffin (0.165 W m
1 k
1). The highly porous

Fig. 4. (a) Absorption process of petroleum ether (stained with Sudan Black B) on water by GMPs. (b) Absorption capability of GMPs toward solvents and oil. (c) The absorption capability and mass of GMPs after repetitively absorbing and burning of petroleum ether for ten cycles. (d) SEM observation of GMPs after absorbing and burning of petroleum ether for ten cycles. (A colour version of this figure can be viewed online.)

C. Chen et al. / Carbon 139 (2018) 545e555

551

Fig. 5. (a) DC conductivity (d) vs volume fraction (4) of GMPs/paraffin composites. (b) Log-log plots of d vs (4-4c) for GMPs/paraffin composites. (c,d) Schematic models of a polymer matrix filled with solid spheres (c) and hollow spheres (d). (A colour version of this figure can be viewed online.)

graphene structure favors the heat diffusion via the network, leading to higher thermal conductivity. The latent heat and phase change temperature were obtained from DSC analysis. The melting and solidifying heats of paraffin/GMPs are slightly higher than pure paraffin, as verified in Fig. 6b. We can calculate the latent heat of paraffin in the composite by subtracting the weight of GMPs (summarized in Table 1). The melting and solidifying heat of paraffin increase by 9.8% and 11.4% after encapsuled by 8.9 wt% GMPs. In addition, the melting point of paraffin/GMPs is 0.9  C higher than paraffin. Based on polymer physics and previous reports, the enhanced latent heat and melting temperature could be ascribed to the interaction between graphene and paraffin molecules, as well as the confine effect of GMPs [15]. By contrast, many previous reports claimed that graphene had little or even negative effects on the latent heat of paraffin [46,47]. Therefore, the enhancement in thermal conductivity, latent heat and melting point of paraffin renders GMPs applicable for the micro-packages of phase change materials with high heat storage performance and excellent dimensional stability. In order to evaluate the heat storage performance, a round disk of paraffin/GMPs was placed on a steel backboard, which was then heated to 90  C. Infrared imaging was applied to track the distribution and variation of temperature during heating and cooling. There is remarkable temperature hysteresis between the composite and steel, as revealed in Fig. 6c. The lags of heating and cooling arise from two aspects. One is the contrast in the specific heat capacity (Cp) of the paraffin/GMPs composite and steel. The paraffin/GMPs composite with higher Cp (2.12 J g
1 k
1) compared with steel (0.46 J g
1 k
1) means it requires more heat to raise the temperature. The other aspect is the storage and release of heat during phase change. In the cooling region, there is a distinct peak corresponding to the phase change of paraffin, coinciding with DSC

curves. During heating, the temperature rises so rapidly that the peak is hard to distinguish. Infrared images of the paraffin/GMPs during cooling process demonstrate that the stable composite can efficiently store and release latent heat (Fig. 6deg). The significant temperature contrast of paraffin/GMPs and steel is up to 8.6  C during phase change, while the contrast is no higher than 3  C without phase change (Fig. 6hek). Collectively, the efficient enwrapping of paraffin, good thermal stability, enhanced thermal conductivity and high latent heat altogether make GMPs excellent microcapsules of phase change materials. The paraffin/GMPs composite shows promise in energy storage applications including thermal management device, smart textile and waste heat recovery [48,49]. 3.4. Microwave absorption (MA) performance of GMPs The combination of hollow sphere structure and conductive graphene shell is applicable for MA materials. In order to evaluate the MA performance, GMPs/paraffin composites with different filler contents were shaped into cylindrical rings and measured via the coaxial line method. Since no magnetic matter is introduced here, the complex permeability of all samples is 1 and the microwave absorption performance is determined by the complex permittivity. As proved in Fig. 7a and b, the real parts (ε0 ) and imaginary parts (ε”) of all GMPs/paraffin composites decrease with increasing frequency owing to the dielectric relaxation [50]. There are remarkable increases in the values of both ε0 and ε” with the filler content increasing, attributing to the enhanced electrical conductivity and polarization relaxation of composites. According to the classic electromagnetic theory, the real permittivity represents the storage capability of the incident electromagnetic wave, while the imaginary part influences the dissipation ability [51].

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Fig. 6. (a) Digital image of paraffin/GMPs composite and paraffin heated to 80  C. (b) DSC curves of paraffin and paraffin/GMPs. (c) Temporal temperature curves of the center of paraffin/GMPs and steel backboard during heating and cooling. (deg) Infrared images of paraffin/GMPs during cooling. (hek) Temperature profile of the blue dash line in (deg). (A colour version of this figure can be viewed online.)

Table 1 Thermal properties of neat paraffin and paraffin/GMPs composite with a filler content of 8.9 wt%. Sample

Paraffin Paraffin/GMPs

Melting

Solidifying

Tm

DHm

DHm-ca

Tf

DHf

DHf-cb

61 61.9

204.4 204.7

e 224.7

56.5 55

200.9 203.9

e 223.8

a DHm-c is the calculated latent melting heat of the encapsuled paraffin by subtracting the weight of GMPs. b DHf-c is the calculated latent freezing heat of the encapsuled paraffin.

With an ultralow filler content of 2.5 wt%, GMPs/paraffin presents a high ε” ranging between 3 and 6, which is even higher than other materials with contents higher than 20 wt%, indicating high dissipation capability of microwave in GMPs [52,53]. However, overhigh permittivity (ε0 and ε”) results in great reflection and scattering of incident microwave, which comes from the mismatch between air and sample interface. In order to obtain a high-performance MA material, the filler content should be optimized to settle the permittivity in rational region. Based on the transmission line theory, we can calculate the reflection loss (RL) performances of GMPs/paraffin composites, as shown in Fig. 7cee. With the sample thickness increasing, the situation of RL peak shifts to lower frequency due to the quarter wavelength attenuation phenomena. With addition of 2.2 wt% GMPs, the 2 mm thick composite demonstrates an EAB of 4.36 GHz and maximum RL of
17.71 dB (Fig. 7c). When the filler content further rises to 2.5 wt%, the 2.14 mm thick composite displays an RL

of
31.1 dB and an EAB of 6 GHz (12e18 GHz), which completely covers Ku band (12e18 GHz, primarily used for satellite communications) (Fig. 7d). The 3 mm composite shows a wide EAB of 3.74 GHz (8.18e11.92 GHz), covering most X band (8.2e12.4 GHz). Hence, efficient absorption in specified frequency range can be obtained by simply adjusting the sample thickness. The RL of GMPs/ paraffin is up to
45.4 dB with the thickness of 2.5 mm. The MA performance begins to degrade when the filling ratio exceeds 2.5 wt%. The EAB and minimum RL of 3 wt% GMPs/paraffin decreases to 3.33 GHz and
12.9 dB respectively, owing to the broadened impedance gap and enhanced reflection of microwave (Fig. 7e). The detailed MA performances of GMPs/paraffin composites with various filler contents are summarized in Fig. 7f. The optimized filling ratio is 2.5 wt%, which is far lower than commercialized MA materials, such as metallic compounds and ferrites (50e70 wt%) [54]. The low filler content not only favors the processing and cost reduction, but also improves the integrity and homogeneity of the composite. We compared our results with the performances of graphenebased materials in previous reports, as displayed in Fig. 8a. It is noteworthy that many efforts have been focused on improving the MA performance via compositing with other materials. However, these complex syntheses only brought about moderate results with high filler contents. GMPs/paraffin composite exhibits a top-three EAB value with the lowest filler content among all functionalized graphene and graphene composites (summarized in Table S3). Here we introduce the specific EAB, which is obtained by dividing EAB with filler content, as a metric of cost-efficiency for MA filling materials. The calculated specific EAB of GMPs is 240 GHz,

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Fig. 7. (a,b) Real part and imaginary part of permittivity of GMPs/paraffin under different filler content. (c) Reflection loss curves of 2.2 wt% GMPs/paraffin with different thickness. (d) Reflection loss curves of 2.5 wt% GMPs/paraffin with different thickness. (e) Reflection loss curves of 3 wt% GMPs/paraffin with different thickness. (f) MA performances of GMPs/ paraffin under different filler content. (A colour version of this figure can be viewed online.)

Fig. 8. (a) The highest EABs and corresponding filler contents of GMPs, functionalized graphene and graphene composites in literature. (b) Comparison of the specific EABs of GMPs and other materials with hollow shell structure. (c) Dielectric tangent loss of GMPs/paraffin under different filler content. (d) Schematic illustration of the MA mechanism of GMPs. (A colour version of this figure can be viewed online.)

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outperforming all materials ever reported (Fig. S11 and Table S3). On the other hand, although hollow shell structure has already been obtained in glass hollow spheres, carbon spheres, Fe3O4/PANI and graphene composites, their specific EABs are only one-tenth of GMPs (Fig. 8b). High specific EAB represents high MA performance with low cost, low bulk density and facile processing. Taking its low density into consideration, GMPs have great potential in lightweight microwave attenuation applications, especially in fighter, radar, spacecraft and telecommunication. Dielectric loss tangent (tan de ¼ ε”/ε0 ) is an important parameter for evaluating the dielectric loss capability of microwave absorbents [55]. Materials with a high tan de can efficiently absorb and convert microwave into other kinds of energy, but excessive tan de may result in impedance mismatch with free space. As can be seen from Fig. 8c, GMPs/paraffin composites with higher filler contents exhibit enhanced dielectric loss tangent, implying increased MA performance. Whereas the tan de of 3 wt% GMPs/paraffin is up to 1, inferior performance is obtained. The outstanding microwave absorption performance of GMPs arises from the combination of crumpled RGO sheets and hollow shell microstructure. The thermal treatment eliminates the oxygen-containing groups on GO and concurrently results in the dimensional expansion of fGO. On the one hand, the enhanced polarization in GMPs promotes the dielectric loss of microwave. The plot of ε” versus ε’ (also called Cole-Cole plots) should be one semi-circle if there is only one Debye relaxation process. However, there are at least four semi-circles in the Cole-Cole plot of GMPs, demonstrating the existence of multiple relaxation processes (Fig. S12) [56,57]. Such relaxation processes come from the interfacial polarization and the dipole polarization of residual polar groups and defects on GMPs. On the other hand, GMPs can act as “traps” to efficiently attenuate microwaves. Microwaves permeate into GMPs through the holes on the surface and undergo interference loss of multiple reflections inside (Fig. 8d) [58e60]. In addition, highly porous GMP constructs the conductive network by low filler ratios, as described in Fig. 5. Based on the free electron theory, ε” is proportional to the conductivity (ε” z d/(2pε0f)). Therefore, high electrical conductivity leads to high ε” and efficient conductive loss [61e63]. The network is beneficial for the energy transformation of microwave, leading to the enhanced MA performance. 4. Conclusions In this paper, we offer a facile and scalable method to prepare GMPs with a hollow shell structure. A spray-drying process accompanied with a direct thermal treatment produces GMPs in short time and large scale. Resembling the thermal expansion mechanism of popcorn, GMPs are produced under the internal pressure induced by the decomposed oxygen-containing groups. GMPs possess high surface area, low density, excellent shape preservation and can be applied as outstanding oil absorbent and electrical conductive filler. Besides, GMPs act as microcapsules of paraffin with excellent thermal stability, remarkably enhanced thermal conductivity and latent heat. With only addition of 2.5 wt% GMPs in paraffin, the composite shows a broad EAB of 6 GHz, outperforming most graphene-based materials. The multifunctional performance of GMPs can be further promoted via chemical doping and composition with other materials. This work not only offers a new type of graphene assembly for diverse applications, but also uncovers the boundless potential in the applications of graphene via innovative morphology control. Acknowledgements This work is supported by MOST National Key Research and

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