Research Article www.acsami.org
One-Pot Sintering Strategy for Efficient Fabrication of HighPerformance and Multifunctional Graphene Foams Yang Li,†,‡ Hao-Bin Zhang,§ Lihua Zhang,† Bin Shen,*,† Wentao Zhai,† Zhong-Zhen Yu,§ and Wenge Zheng*,† †
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang Province 315201, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § State Key Laboratory of Organic−Inorganic Composites, College of Materials Sciences and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: Macroscopic three-dimensional (3D) graphene foams (GFs) were fabricated efficiently by immediately sintering low-temperature exfoliated graphene powder under inert atmosphere at the temperature over 500 °C. The one-pot sintering process not only integrated two-dimensional (2D) graphene sheets into 3D GF, but also accelerated the structural integrity of graphene by inducing its deoxygenation and repairing the defects. More importantly, the whole process could be finished within hours, usually less than 12 h, and the resultant GFs with interconnected graphene framework as well as meso- and macroporous structure exhibited exceptional attenuating performance for high-frequency electromagnetic interference and adsorption capacities for organic pollutants. In comparison with conventional hydro/solvothermal, sol−gel chemistry, sol-freezing, and templating methods, our sintering strategy possesses more advantages in maneuverability, efficiency, and repeatability, benefiting for the mass production of high-performance and multifunctional GFs. KEYWORDS: graphene foam, sintering, electromagnetic interference, organics, adsorption ods.6,12,23−28 The former three have some similarities as all of them start from graphite oxide (GO) and consist mainly of four steps: (a) ultrasonic exfoliation of GO, (b) self-assembly of graphene,7,15 or gelation/directional freezing of graphene oxidecontaining sols,18−22 (c) freeze/supercritical CO2 drying of wet GFs or graphene oxide foams, and (d) subsequent chemical or thermal reduction of dried foams for the purpose of improving the electrical conductivity of GFs.13,18,20−22,29,30 These methods are quite complex and time-consuming, especially for the selfassembly process, the freeze or supercritical drying treatment, and the post reduction procedure. Generally, the larger are the samples, the longer is the preparation period. The resultant GFs generally show an interconnected hierarchical microstructure
1. INTRODUCTION Graphene, a two-dimensional (2D) nanomaterial with sp2bonded carbon atoms packed into a hexagonal lattice, has numerous wonderful properties,1 such as the record carrier mobility at room temperature (∼10 000 cm2 V−1 s−1),2 thermal conductivity (5300 W m−1 K−1),3 Young’s modulus (∼1 TPa),4 and large theoretical specific surface area (2630 m2/g).5 Therefore, it is frequently regarded as an ideal building block for constructing macroscopic three-dimensional (3D) graphene foams (GFs),6 by which the advantages of nanoscale graphene sheets are expected to be transferred to the resulting macroscopic GFs that are fairly promising in the fields of energy storage,7,8 water purification,9−11 electromagnetic interference (EMI) shielding,12 and microwave absorbing.13 Recently, several strategies have been developed for fabricating GFs, including hydro/solvothermal,7−11,13−17 sol− gel chemistry,18,19 sol-freezing,20−22 and templating meth© 2017 American Chemical Society
Received: February 17, 2017 Accepted: March 28, 2017 Published: March 28, 2017 13323
DOI: 10.1021/acsami.7b02408 ACS Appl. Mater. Interfaces 2017, 9, 13323−13330
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Sketch of the preparation route of macroscopic 3D GF. (b) Digital pictures to show the morphological changes in compressed G-200 powder (inside and outside a graphite crucible) after sintering at different temperatures from 500 to 2200 °C. (c) Weight loss and volume shrinkage of compressed G-200 powder as a function of HTTs. (d) Compressive stress−strain curves of GF and the compressed G-200 powder with a similar density. (e) N2 adsorption isotherms of GFs at 77 K. The inset is the corresponding BJH pore size distribution. (f−h) Representative SEM images of GF in different magnifications: ×300 (f), ×1000 (g), and ×15 000 (h).
with both mesopores (2−50 nm) and macropores (>50 nm).7,8,16 As to the templating methods, they can be carried out either by growing graphene on Ni foam via chemical vapor deposition (CVD),6,12,24−26 or coating graphene oxide on the scaffold of polyurethane (PU) sponge,27,31−33 or by incorporating polystyrene (PS) or silicon microspheres into graphene monoliths.23,28 After sacrificing the templates by acid etching or thermal pyrolysis, macroporous 3D GFs are obtained, and their structures strongly rely on the templates with the pore size ranging from several tens of nanometers to hundreds of micrometers. Although these approaches are effective, it is hard to remove the metallic, polymeric, and ceramic templates without any damage to the fragile graphene frameworks.6,27
Sometimes, the growth of CVD derived graphene, as well as the removal of the template, is also time-consuming and challenging. More recently, a leavening method was also reported for making a porous graphene film but not bulk GF.34 Herein, we developed a novel strategy for efficient fabrication of high-performance 3D GFs by immediately sintering lowtemperature exfoliated graphene (LTEG) powder at high temperature over 500 °C. On the basis of our previous technology,35 the LTEG powder could be simply obtained by exfoliating GO at a temperature as low as 130 °C within minutes under ambient atmosphere, and the followed one-pot sintering process, which integrated 2D LTEG into 3D GFs and simultaneously accelerated the structural integrity of graphene 13324
DOI: 10.1021/acsami.7b02408 ACS Appl. Mater. Interfaces 2017, 9, 13323−13330
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a−c) XPS analysis and Raman spectra of GO and graphene samples thermally treated at different HTTs: full XPS spectra (a), highresolution C 1s XPS spectra (b), and Raman spectra (c). frequency range of 8.2−12.4 GHz, and the absorption and reflection coefficients of A and R were evaluated as in our previous work.37 The samples were cut into rectangular plates with a dimension of 22.5 × 10.0 mm to fit the waveguide chamber, and calibration was needed before the measurement. The electrical conductivity was measured by a standard four-probe method on a Napson Cresbox Measurement System. The adsorption capacity was determined by weight measurements before and after the saturated adsorption of organics in GFs. Typically, a dry bulk GF was immersed into organics for ∼5 s and then quickly weighed after the removal of excessive liquids, to reduce the evaporation of some adsorbed organics. For all of the data, three samples were tested to obtain average data and standard deviation.
by inducing its deoxygenation and repairing the defects, could be finished within hours, usually less than 12 h. The resultant GFs with meso- and macroporous microstructure exhibited exceptional EMI shielding properties and adsorption efficiency for organic solvents. In comparison with the aforementioned approaches, our sintering strategy possesses more advantages in maneuverability, efficiency, and repeatability, benefiting for the mass production of high-performance and multifunctional GFs.
2. EXPERIMENTAL SECTION 2.1. Material Preparation. The whole preparation route of GF is schematically illustrated in Figure 1a. GO was prepared from natural flake graphite (Qingdao Huatai Lubricant Sealing S&T) via a modified Hummers’ method,36 and then thermally exfoliated into fluffy black powder (noted as G-200) at a low temperature of 200 °C in air for 5 min. Afterward, the G-200 (0.6 g) was filled into a cylindrical graphite crucible (ϕinner4 × ϕouter5 × 5 cm) under a pressure of ∼4.7 KPa and heat-treated at 500, 1000, 1500, and 2200 °C in flowing high-purity nitrogen at a rate of 10 °C/min. For each temperature, the sample was annealed for 1 h. It was found that the successful formation of GF is largely dependent on the heat treatment temperature (HTT) (Figure 1b), due to its significant influence on the weight loss and volume shrinkage of G-200 during the sintering process (Figure 1c). For the sample treated at 500 °C (noted as G-500), it remained still in compressed powder, and only slight weight loss (∼6.4%) and volume shrinkage (∼2.5%) were observed. Further increasing the HTT may cause larger weight loss and volume shrinkage, resulting in three different GFs. The GFs were noted in terms of the HTTs as GF-1000 (ϕ3.3 × 3.6 cm, 11.3 mg cm−3), GF-1500 (ϕ3.0 × 3.5 cm, 19.4 mg cm−3), and GF-2200 (ϕ2.6 × 2.7 cm, 33.9 mg cm−3). Moreover, their dimension could be facilely tuned by the mold. For example, a GF as large as ϕ8.2 × 7.3 cm (the inset in Figure 1a) was also fabricated with a larger crucible (ϕinner10 × ϕouter11 × 10 cm). 2.2. Characterization. The density of GFs was calculated from the mass and volume of the samples. Compressive stress−strain curves were recorded on a universal material experiment machine (Instron 5567) at a rate of 0.5 mm min−1. For each curve, three cylindrical samples were tested, and the mean one was selected. Nitrogen adsorption−desorption measurement was carried out with a Micromeritics ASAP2020HD88 analyzer. Microstructure was observed by a Hitachi S-4800 field-emission scanning electron microscopy (SEM) at an accelerating voltage of 8 kV. Raman spectra were excited with a laser of 532 nm in a Super LabRam II system. X-ray photoelectron spectroscopy (XPS) analysis was performed using Al (mono) Kα radiation under 1.2 × 10−9 Torron. The EMI shielding performance was measured with a R&S ZVA67 vector network analyzer in the
3. RESULTS AND DISCUSSION 3.1. Fabrication of GFs. To investigate the influence of sintering treatment on the mechanical properties of GFs, compressed powder of G-200 with a density similar to that of GF (GF-2200) was prepared, and their compressive properties were measured as shown in Figure 1d. As expected, GF shows a yield strength of 15 KPa and an elastic modulus of 0.28 MPa, about 2 times higher than those of the G-200, demonstrating the advantage of the sintering process in fabricating mechanically strong GFs with the microstructure observed with SEM in Figure 1f−h. Clearly, the typical GF shows an interconnected graphene framework with macroporous structure (Figure 1f) that is further composed of randomly coalesced or overlapped worm-like graphene agglomerates (Figure 1g) with rippled and wrinkled texture (Figure 1h). Moreover, the N2 adsorption isotherms of the GFs present a type IV isotherm with a distinct hysteresis loop of H2 over the P/P0 range of 0.4−1.0 (Figure 1e), indicating the existence of mesopores in the GFs.16,28 Barret−Joyner−Halenda (BJH) analysis further suggests a narrow pore size distribution of 2.0−4.4 nm (the inset in Figure 1e). These results demonstrate the hierarchical microstructure of our GFs, which finally renders the foams with high Brunauer−Emmett−Teller (BET) specific area of 108− 508 m2 g−1, showing a decreasing tendency with the HTT increasing. These properties make the meso- and macroporous GFs comparable to those fabricated by hydrothermal,7,8,16 sol− gel chemistry,18,19 and sol-freezing method,20−22 as given in Table S1. XPS analysis was conducted to monitor the deoxygenation of graphene samples. As seen in Figure 2a, pristine GO shows two strong peaks at ∼284 eV (C 1s) and ∼532 eV (O 1s) and a low 13325
DOI: 10.1021/acsami.7b02408 ACS Appl. Mater. Interfaces 2017, 9, 13323−13330
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a−c) EMI SE, R, and A of epoxy/GF composites with thicknesses of 0.5, 1.0, and 2.0 mm in X band: GF-1000 (a), GF-1500 (b), and GF2200 (c). (d) Electrical conductivity of epoxy/GF composites (GF-1000, GF-1500, and GF-2200) and the epoxy/graphene counterparts (G-1000, G-1500, and G-2200). (e) Comparison of the average EMI SE of epoxy/GF composites in thickness of 0.5−2.0 mm and the epoxy/graphene counterparts in thickness of 2.0 mm. (f) Comparison of the EMI SE of GF-1500 and GF-2200 with other materials in X band as summarized in Table S3.
structure in GF-1500 and GF-2200 due to the random overlapping of graphene sheets during the sintering process. After piecing all of the information together, we proposed a possible mechanism for the molding of 3D GFs. During the sintering process, G-200 would be extensively reduced, leading to the weight loss due to the removal of oxygen-containing groups. It, in turn, notably enhanced the physical interactions (e.g., π−π stacking interactions) among the compressed and adjacent graphene sheets.7,9,15 Moreover, interlaminar consolidation could occur at a high temperature of 1500−2200 °C.45,46 The synergetic effect of them could construct effective bindings among graphene sheets, driving worm-like graphene agglomerates randomly overlapped with each other and consequently forming the GFs with stable graphene framework. The whole process was accompanied by the release of gases like CO and CO2, due to the pyrolysis of residual oxygencontaining groups of G-200 and the evaporation of carbon atom.35,38,39 The gases further acted as one sort of “foaming agent”, contributing to the formation of GFs. It should be noted that there are three key factors in the sintering process: (a) the low-temperature exfoliated graphene powder (to reserve some oxygen-containing groups), (b) the sufficient preloaded pressure on the powder (to force graphene sheets contact tightly together), and (c) the high HTT larger than 500 °C (to trigger the interactions and bindings among graphene sheets). Consequently, the interconnected graphene framework, together with its excellent electrical property, make our GFs very promising for handling high-frequency electromagnetic pollution. 3.2. EMI Shielding Performance of GFs. To investigate the EMI shielding performance conveniently, wave-transmitting epoxy resin was infused into GFs to protect their brittle graphene skeleton. The resulting epoxy/GF composite shows a typical cross-section where an interconnected network is tightly
C/O atom ratio of 2.3, implying a high degree of oxidation. After thermal treatment, the C 1s/O 1s peak becomes much stronger/weaker and the C/O atom ratio progressively increases to 68.9 for GF-2200, suggesting the improved degree of the reduction of graphene at higher HTT, in accordance with others.38,39 The corresponding high-resolution C 1s spectra in Figure 2b further confirm the above conclusion by the increase in C−C/CC (284.6 eV) component, but the decrease in C− O (286.4 eV) and CO (288.9 eV) components (Table S2). Particularly, almost 100% C−C/CC component is left for GF-1500 and GF-2200, which is very similar to the case in graphite (Figure S1a), denoting their extremely high degree of reduction. In Figure 2c, Raman spectroscopy was employed to investigate the structural regularity of graphene, where all samples exhibit two characteristic peaks around 1350 cm−1 (Dband) and 1580 cm−1 (G-band). Besides, the ID/IG ratio of GO (0.67) is lower than 1.33−1.67 of the samples treated at 200− 1000 °C, but higher than 0.13−0.46 of those annealed at 1500−2200 °C. The reason for the former may be attributed to the appearance of more numerous but smaller size graphitic domains during the initial reduction process of GO,22,40−42 while that for the latter is assigned to the extensive repairing of the residual defects. Furthermore, the G-band shifts from 1592 cm−1 for G-200 to 1579 cm−1 for GF-2200 (approximately that of graphite in Figure S1b), and the corresponding half width at half-maximum (HWHM) line width (Figure S1c) declines gradually from ∼66 to ∼12 cm−1, revealing the increased sp2 ordered domains at improved HTT. The conclusion is also supported by the steep G′-band (∼2700 cm−1) in the secondorder Raman spectra of GF-1500 and GF-2200, whereas the symmetric G′-band is unable to be fitted into two Lorentz peaks like nature graphite (the inset of Figure S1b) with AB Bernal stacking,43,44 revealing the presence of turbostratic 13326
DOI: 10.1021/acsami.7b02408 ACS Appl. Mater. Interfaces 2017, 9, 13323−13330
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Adsorption of ∼12.02 g of benzene (dyed with Sudan IV) in ∼0.38 g of GF. As expected, about ∼100% of the benzene flowing on diluted water was adsorbed in ∼10 s. (b) Photographs to show the hydrophobicity of GF by forming a contact angle of 130° ± 2° with a water droplet and the superwetting behavior for organic solvents by completely adsorbing an acetone droplet. (c) Adsorption capacities of GF-1500 in terms of weight gain for various organic solvents. The inset shows a cubic GF-1500 before and after saturate adsorption for ethanol. The well preserved framework ensures the accurate measurement of the adsorption capacities of GF-1500. (d) Adsorption capacities of GF-1000, GF-1500, and GF-2200 for benzene, chloroform, ethanol, and acetone. (e) Comparison of the adsorption capacities of different adsorbents and GF-1500 for common organic pollutants as listed in Table S4.
embedded into epoxy resin (Figure S2a−c), verifying the wellpreserved graphene framework of the GF. With this method, three GF composites were fabricated and termed as GF-1000, GF-1500, and GF-2200, respectively, and their EMI shielding effectiveness (SE) was measured by a waveguide method over the frequency range of 8.2−12.4 GHz (X band). As shown in Figure 3a−c, the SE is largely dependent on the thickness of samples, presenting an increasing tendency from 6−16 dB for GF-1000 to 21−34 dB for GF-1500 and 26−39 dB for GF2200 as the thickness increases from 0.5 to 2.0 mm. This principle is fairly valuable for facilely adjusting the SE of EMI shields to meet the commercial standard of 20 dB, and that is why large thickness is generally required for the materials with insufficient SE. Moreover, all of the samples exhibit higher reflectivity R (>0.5) than absorptivity A (
