Fluffy and Ordered Graphene Multilayer Films with ... - ACS Publications

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Fluffy and Ordered Graphene Multilayer Films with Improved Electromagnetic Interference Shielding over X‑Band Zicheng Wang, Renbo Wei,* and Xiaobo Liu* Research Branch of Advanced Functional Materials, School of Microelectronics and Solid-State Electronics, High Temperature Resistant Polymer and Composites Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, China S Supporting Information *

ABSTRACT: Highly ordered nitrogen-doped graphene multilayer films with large interlayer void are successfully fabricated by thermal annealing of the compact stacking graphene oxide/copper phthalocyanine (GO/CuPc) multilayer films. Scanning electron microscopic (SEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopic (XPS), and electrical conductivity measurements indicate that the breakaway of oxygen functional groups on/in the GO sheets at high temperature and the in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets synergistically facilitate the restoration of GO in graphitization, the effective nitrogen doping by replacing carbon atoms in graphene frameworks, the retention of layer-by-layer stacking structure of graphene sheets in plane, and the formation of interlayer voids, leading to the enhancement in the electrical conductivity (3.64 × 103 S/m). In addition, due to the formation of a Fabry−Pérot resonance cavity in the unique layer-by-layer stacking structure with larger interlayer voids, constructive interference of internal reflections aligned between parallel reflecting planes endows the fluffy graphene multilayer films with excellent electromagnetic interference (EMI) shielding effectiveness (exceeds 25 dB in all Xbands). The optimal shielding effectiveness is up to 55.2 dB with a smaller thickness of 0.47 mm, which makes it possible to become a practical EMI shielding material with a distinct competitive advantage. KEYWORDS: graphene, interlayer void, electrical conductivity, Fabry−Pérot resonance, electromagnetic interference

1. INTRODUCTION With the wide use of electronic and communication facilities in commercial and military areas, electromagnetic shielding materials have become a significant concern for protecting devices and/or human from undesirable electromagnetic interference (EMI). On the basis of the fundamental mechanism of EMI shielding, the EMI shielding effectiveness (EMI SE) is dependent on the synergistic effect of EMI absorption, reflection, and/or multiple reflection.1−3 For the reflection, it can be enhanced through the improvement in the electrical conductivity, thereby increasing the reflection loss and attenuating the energy of electromagnetic radiations. To date, varieties of conductive materials, especially metal, conductive polymers, and carbon-based nanomaterials, have been prepared as high-performance electromagnetic shielding materials. However, the use of metals has some limitations such as their high density, high cost, and low corrosion resistant in harsh environment even though they have high conductivity and exhibited excellent EMI shielding effectiveness. The conductive polymers are of low cost and lightweight, while the conductivity, chemical, and thermal stability are not high enough for the high-temperature field application.4,5 Recently, carbon-based nanomaterials as alternative EMI shielding materials to traditional metals and conductive polymers have attracted increasing attention due to their high corrosion © 2017 American Chemical Society

resistance, low weight, and low cost apart from excellent electrical conductivity. Among those carbon-based nanomaterials, graphene gradually becomes more promising as a novel lightweight EMI shielding material based on the stable two-dimensional carbon nanostructure, high specific surface area, and excellent electrical conductivity.6,7 Lots of works focus on the fabrication of the graphene composites by dispersing graphene nanosheets into polymeric matrixes to form effective conductive network, thereby attenuating electromagnetic irradiation.8−10 However, the graphene-based composites are unsatisfactory in shielding due to the low electrical conductivity, high brittleness, and large effective thickness of several millimeters. Recently, researchers have been attracted in fabricating macroscopic layered graphene films through different methods to make full use of the large aspect ratio of graphene sheets and their alignment into layered structure, which plays a positive role in improving the electrical conductivity of the systems.11−14 In comparison with general graphene-based composites, the orientation configuration endows layered graphene films with superior electrical conductivity of ∼200−1500 S/cm and excellent EMI shielding Received: March 21, 2017 Accepted: June 22, 2017 Published: June 22, 2017 22408

DOI: 10.1021/acsami.7b04008 ACS Appl. Mater. Interfaces 2017, 9, 22408−22419

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ACS Applied Materials & Interfaces

Figure 1. SEM images (a and b) of cross-section morphology of layer-by-layer GO/CuPc multilayer films; SEM images (c and d) of cross-section morphology of layer-by-layer GO/CuPc multilayer films annealed at 800 °C for 6 h (NG-6h).

performance of ∼20−60 dB. Meanwhile, the effective thickness decreases to the order of micrometers successfully. On the other hand, more novel explorations have been attempted to optimize the preparation process of highperformance graphene-based EMI shielding materials. As reported in the literature,15,16 the foaming of layered graphene films into porous graphene foam and/or fabricating of multiplelayered graphene papers (GP) via tuning the thickness of the wax spacing or the stacking number of GP pieces could substantially improve the internal multiple reflection due to the existence of the free space (microcellular structures or interlayer voids), thereby enhancing EMI shielding effectiveness. This opens a new way for the exploitation of graphene as superlight, adjustable, and broadband high-performance EMI materials. In this study, we report a facile and effective approach to fabricate layer-by-layer nitrogen-doped graphene multilayer films with large interlayer voids by thermal annealing of the compact stacking GO/CuPc multilayer films at high temperature. The breakaway of oxygen functional groups on/in the GO sheets at high temperature and the in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets can synergistically facilitate the restoration of GO in graphitization, the effective nitrogen doping by using CuPc with aromatic heterocyclic macromolecule as nitrogen source for replacing carbon atoms in graphene frameworks, and the retention of orientation stacking structure of graphene sheets in plane, thereby enhancing the electrical conductivity of graphene. In addition, the retention of layer-by-layer structure of graphene sheets and generation of interlayer voids contribute to the formation of a Fabry−Pérot resonance cavity, which motivates the further performance enhancement in graphene-based EMI shielding effectiveness.

2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide (GO) and copper phthalocyanine (CuPc) were synthesized in our laboratory.17 N-Methyl-2-pyrrolidone (NMP) and methylbenzene were supplied by Tianjin BODI chemicals, Tianjin, China. All of the materials were used without any further purification. 2.2. Graphene Oxide/Copper Phthalocyanine (GO/CuPc) Multilayer Films. Multilayer films of graphene oxide/copper phthalocyanine were prepared as reported.17 The homogeneous liquid crystal dispersions of GO/CuPc (100 mg of GO and 50 mg of CuPc) in NMP were cast on a clean preheated glass of quartz plate (30 mm × 30 mm), and the solvent was evaporated using a sequential mode of temperature programmed at 80, 100, 120, 140, 160, 180, and 200 °C for 2 h, respectively. The obtained layer-by-layer GO/CuPc multilayer films (labeled as GO/CuPc) loaded on the quartz plate were annealed at a higher temperature of 800 °C for 3, 6, and 9 h under the nitrogen atmosphere, which can be marked as NG-3h, NG-6h, and NG-9h, respectively. For comparison, GO, CuPc, and disordered GO/CuPc composites were also prepared as follows: homogeneous liquid crystal dispersions of GO/NMP (100 mg of GO) were cast on a quartz plate with the same size, and the solvent was evaporated using a sequential mode of temperature programmed at 80, 100, 120, 140, and 160 °C for 2 h and 180 °C for 4 h, respectively, to give the final assembled GO multilayer films (labeled as GO). The sample then was annealed at the same temperature of 800 °C for 6 h under the nitrogen atmosphere. The final obtained products were named as graphene. The pure CuPc was also annealed at the same temperature of 800 °C for 3, 6, and 9 h under the nitrogen atmosphere, and marked as CuPc 800 °C 3h, CuPc 800 °C 6h, and CuPc 800 °C 9h, respectively. As to the disordered GO/CuPc composite, a homogeneous dispersion of GO/CuPc (100 mg of GO and 50 mg of CuPc) was prepared under previous protocol, then the dispersion was rapidly dropped into methylbenzene, and disordered GO/CuPc composite (disordered GO/CuPc) was obtained by casting the precipitate on the quartz plate and evaporating the solvent as previous. It was also annealed at 800 °C for 6 h under the nitrogen atmosphere, which can be denoted as disordered NG-6h. 2.3. Characterization. Scanning electron microscopic (SEM) images were taken on a JSM 6490LV (JEOL, Japan) field emission 22409


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Figure 2. SEM images of cross-section morphology of ordered GO/CuPc multilayer films annealed at 800 °C for 0 h (a), 3 h (b), 6 h (c), and 9 h (d). microscope. X-ray diffraction (XRD) was recorded on a RINT 2400 vertical goniometer (Rigaku, Japan) with Cu Kα radiation. Raman spectra were recorded from 0 to 3250 cm−1 on a Renishaw Invia Raman microprobe using a 532 nm argon ion laser. X-ray photoelectron spectroscopic (XPS) measurements were carried out on an ESCA 2000 (VG Microtech, UK) using a monochromic Al Kα (hν = 1486.6 eV) X-ray source. The electrical conductivity was measured using the standard four-point contact method (Swin Hall 8800 system). The S parameters of the samples were measured with a vector network analyzer VNA Agilent E8363B in the 8−12 GHz region (X-band). During the measurement, the films were sandwiched between the waveguide sample holders. Electromagnetic interference shielding effectiveness (EMI SE) was calculated using the listed equations:

CuPc molecules were employed at high temperature. As shown in Figure 1c and d, the compact stacking nanostructures are successfully exfoliated through thermal expansion and lots of sphere-like nanoparticles with a diameter of ∼200 nm homogeneously and tightly dispersed on the two-side surfaces of graphene sheets, while the layer-by-layer stacking structure of graphene sheets is reserved. It can be mainly attributed to the fact that the extensive thermal reduction of oxygen functional groups in the GO sheets and in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets will yield CO, CO2, NOx, and water vapor during the thermal annealing process, which generates enough inner pressure to overcome the interaction between graphene sheets, thereby resulting in the formation of interlayer voids.15,18,19 Furthermore, the layer-to-layer distance exhibits a tendency to increase with the increasing annealing time at 800 °C as shown in Figure 2, which once again confirms the above assumption of expansion exfoliation. For the system, the compact layer-by-layer stacking GO/CuPc multilayer films prepared by liquid crystalline assembly are considered as a critical factor for the formation of highly fluffy ordered graphene multilayer films to improve the final EMI shielding effectiveness. As a control experiment, a disordered GO/CuPc composite has also been prepared through dropping into methylbenzene rapidly. However, after thermal treatment under the same conditions, the ordered layer-by-layer configuration is not observed as shown in Figure S1. In addition, pure GO multilayer films are assembled into honeycomb foam (Figure S2) under the same annealing conditions, which are fluffier than those of NG-6h. The difference in morphologies as revealed by SEM images can be ascribed to the absence of CuPc molecules, which serve as potential sites for cross-linking graphene sheets through π−π interaction. More importantly, CuPc molecules, as nitrogen

R = |S11|2 = |S22|2 , T = |S21|2 = |S12|2 , A = 1 − R − T SER = − 10 log(1 − R ), SEA = − 10 log(T /(1 − R ))

SET = SER + SEA = − 10 log T = − 20 log|S21| where R is the reflection coefficient, T is the transmission coefficient, and A is the absorption coefficient.

3. RESULTS AND DISCUSSION 3.1. SEM Images of GO and GO/CuPc Composites. The three-dimensional (3D) layer-by-layer multilayer films of GO/ CuPc composites with a thickness of ∼0.1 mm were successfully fabricated by self-assembling in the orientation liquid crystalline state as a precursor and immobilizing the ordered structure upon simple casting and drying as reported by our previous work.17 A compact layer-by-layer configuration can be confirmed by scanning electron microscopy (SEM) as shown in Figure 1a and b. To prepare highly fluffy ordered layer-by-layer nitrogen-doped graphene multilayer films, indispensable thermal annealing for achieving the effective reduction of GO nanosheets and pyrolysis of nitrogen-enriched 22410


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Figure 3. XRD patterns of GO multilayer film, CuPc, layer-by-layer GO/CuPc multilayer film, and disordered GO/CuPc composite (a) and the corresponding patterns (b) annealed at 800 °C for 6 h; XRD patterns of layer-by-layer GO/CuPc multilayer films (c) and CuPc (d) annealed at 800 °C with different times.

corresponding to the (001) planes of graphene.20,21 The result determines that the thermal annealing process can effectively promote the restoration and exfoliation of graphene sheets at high temperature. In comparison with the samples of graphene and disordered NG-6h, NG-6h exhibits a similar evaluation but a sharper diffraction peak (001) at the same angle. According to the Scherrer formula, the narrower full width at half maxima (fwhm) is the higher stacking height (Lc) of graphene layers in the c-direction. In general, the thermal expansion at high temperature will facilitate the exfoliation of graphene sheets in the form of inter vapor pressure, thereby leading to a drastic attenuation or disappearance in intensity of the characteristic diffraction peak (001) as displayed by the results of graphene and disordered NG-6h. However, for the layer-by-layer GO/ CuPc multilayer films, the thermal expansion exfoliation of graphene sheets can also occur at high temperature, while the pushing of inter vapor pressure will result in the formation of a microcosmic layer-by-layer stacking structure of adjacent graphene sheets in c-direction (as demonstrated in Figure S3) due to the layer-by-layer stacking structure in the previous process. As a result, microcosmic layer-by-layer stacking in cdirection can be detected as displayed in Figure 3b. More importantly, the formation of microcosmic layer-by-layer stacking structure will contribute to retention of macroscopic orientation stacking structure with large interlayer voids as shown in Figure 1c and d. Meanwhile, in comparison with the

source, play a vital role in the nitrogen doping process for enhancing the electrical conductivity of the graphene sheets. As a result, we can conclude that the formation of compact layered structure and the introduction of CuPc molecules will contribute to the unique structural characteristics of the final product and realize the effective thermal expansion exfoliation and nitrogen doping. 3.2. X-ray Diffraction Patterns of GO and GO/CuPc Composites. To further determine the changing microstructures of GO and GO/CuPc composites annealed at high sintering temperature, the identification of the samples was carried out by an X-ray diffraction (XRD) with Cu Kα radiation. As shown in Figure 3a, the original GO multilayer film shows a sharp diffraction peak indexed to (002) centered at 10.46°, corresponding to a d-spacing of 0.84 nm. After incorporation with CuPc by liquid crystalline assembly, a well-defined diffraction peak of the obtained GO/CuPc multilayer film shifts down to a smaller angle (8.56°), while the stacking peaks (002) of GO in disordered GO/CuPc composites prepared by dropping into methylbenzene rapidly disappear, indicating that the liquid crystalline assembly with CuPc facilitates effectively exfoliation of GO sheets and the formation of the layer-by-layer stacking structure of GO/CuPc. After thermal annealing at high temperature, the characteristic diffraction peak (002) of GO completely disappears, while a new broad peak around 26.25° arises as shown in Figure 3b, 22411


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Figure 4. Raman spectra of GO and graphene (a); CuPc and CuPc at 800 °C 6 h (b); GO/CuPc multilayer films and NG-6h (c); and GO/CuPc annealed at 800 °C for 3 h (NG-3h), 6 h (NG-6h), and 9 h (NG-9h) (d).

amorphous graphene and CuPc annealed at high temperature, the diffraction curve of GO/CuPc multilayer films annealed at 800 °C with different times shows a prominent transition from broad hill to sharp peak around 26°, revealing a gradually decreasing tendency in fwhm with the increase of annealing time. The increase of the crystalline nature (Lc) can be attributed to the fact that more graphene sheets are pushed and stacked together with the increase in the pyrolysis degree of GO and CuPc molecules, which can be indirectly confirmed by the gradually increasing interlayer voids as shown in Figure 2. 3.3. Raman Spectra of GO and GO/CuPc Composites. To obtain further information during the annealing process, Raman spectroscopy was employed to investigate the structural changes that occurred in graphene frameworks. As shown in Figure 4, significant structural changes appear before and after the annealing process from GO, CuPc, to GO/CuPc multilayer films. The initial GO displays a remarkable D peak at 1339 cm−1 and a weak G peak at 1583 cm−1 as shown in Figure 4a. The G band related to the E2g mode in the basal-plane bondstretching motion of graphite (sp2 carbon atoms) can be used to explain the degree of graphitization, whereas the D band is identified as a disorder-induced feature due to the structural defects and/or partially disordered structures in the graphene framework.22 The intensity ratio of the D peak to the G peak (ID/IG) provides a direct indication for the amount of structural defects and a quantitative measure of edge plane exposure.23

Representative spectra of GO and graphene are shown in Figure 4a. In comparison, the sample of graphene displays a higher ID/IG (1.03) as compared to that of GO (about 0.92). According to the modified Tuinstra−Koenig model,24 the inplane nanocrystalline size (La) can be estimated by an empirical relation La [nm] = (2.4 × 10−10)λ4(ID/IG)−1. The changes suggest a decrease in the average size of the sp2 domains in graphene sheets, which can be ascribed to the loss of carbon atoms by the decomposition of oxygen-functional groups in GO sheets and the generation of more defects upon the effectively thermal reduction of GO.25 Yet the new graphitic domains are more numerous in number.26 For the GO/CuPc multilayer films, the location of G band is stiffened by about 5 cm−1 relative to that of GO due to the intercalation of CuPc between the graphene sheets as shown in Figure 4c. However, after pyrolysis, the G band of NG-6h treated under the same annealing conditions is down-shifted to 1581 cm−1 as compared to that of pristine GO/CuPc (1588 cm−1), which is also lower than that of graphene (1583 cm−1) and CuPc 800 °C 6 h (1596 cm−1). The downshift of the G band in NG-6h can be assigned as the result of nitrogen doping during the pyrolysis process, which is consistent with the previous reports of nitrogen-doped graphene and CNT.27,28 Meanwhile, it is worthy to note that NG-6h displays a higher ID/IG ratio of 1.05, obviously larger than that of graphene (1.03) and CuPc 800 °C 6 h (0.95) annealed at the same 22412


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Figure 5. XPS survey spectra of GO and graphene (a); CuPc and CuPc at 800 °C 6 h (b); GO/CuPc and NG-6h (c); and GO/CuPc annealed at 800 °C for 3 h (NG-3h), 6 h (NG-6), and 9 h (NG-9h) (d).

conditions. The further increased ID/IG ratio observed for NG6h can be also assigned as the result of the structural defects and edge plane exposure caused by the incorporation of nitrogen heteroatoms into the graphene sheets, leading to a lower graphitic crystalline size (La) in plane. A similar phenomena can be observed for the other samples annealed with different times as shown in Figure 4d, revealing that the nitrogen doping of graphene sheets in plane gradually increases with the prolongation of annealing time. 3.4. XPS Spectra of GO and GO/CuPc Composites. To further probe the elemental composition and nitrogen bonding configuration in the system, XPS measurements were carried out. As shown in Figure 5, the XPS spectrum of the pristine GO exhibits only two peaks centered at 287 and 534.5 eV, corresponding to C 1s and O 1s, respectively. For CuPc, the appearance of a new peak of N 1s located at 398.0 eV in the spectrum (Figure 5b) indicates the important status of CuPc molecules used as nitrogen source in this system. After thermal annealing, the carbon content (atom %) of GO and CuPc increases while the oxygen and nitrogen content decrease as from the XPS results of Figure 5a,b and Table S1. As calculated, the area ratio of C 1s peak and O 1s peak (C/O ratio) of GO and CuPc increases from 2.04 to 16.51, and from 6.91 to 7.76, respectively. As a result, it can be ascribed to the efficiency in degradation of GO and CuPc at high temperature, which are consistent with the above results of XRD and Raman. As incorporating CuPc molecules into GO sheets, GO/CuPc

annealed at the same temperature with different time displays a similar tendency to increase in C/O ratio as shown in Figure 5c,d and Table S1, indicating the improvement in the degree of graphitization with the increasing annealing time. In addition, the nitrogen content gradually decreases from 3.28% to 2.55% with the increase of annealing time, but there is relatively little change in the oxygen content. To gain more details about the changes in bonding configuration of GO, CuPc, and GO/CuPc composites during the pyrolysis process, the XPS spectra of C 1s and N 1s were particularly analyzed in high-resolution as shown in Figures 6, 7, and S4. As shown in Figure 6a, the C 1s spectrum of GO could be quantitatively differentiated into five different carbon species (OC−O, CO, C−O−C (epoxy/ether), C−OH, and CC/C−C) (Figure S5). After pyrolysis, the peaks of different C−O bonding configuration considerably decrease. Only the peak located at 284.53 eV, corresponding to sp2 carbon atoms, is still standing as shown in Figure 6b. This once again verifies that most of the oxygen groups have been removed, and the graphitic carbon network is partially restored during the pyrolysis process. Furthermore, the C 1s spectrum of CuPc can be also fitted into five peaks at 289.63, 287.14, 286.32, 285.48, and 284.52 eV as shown in Figure 6c, which are assigned to π → π* satellite C, aromatic ether C−O−C, phthalocyanine N−CN, nitrile CN, and phenyl CC of the CuPc molecule (Figure S6), respectively.29,30 After pyrolysis under the same conditions, it can be clearly observed 22413


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Figure 6. High-resolution C 1s spectra of GO (a) and CuPc (c) and the corresponding patterns (b and d) annealed at 800 °C for 6 h.

spectra of CuPc 800 °C 6 h as shown in Figure 6d. For NG-6h (Figure 7d), the binding energies of N atoms are shifted to lower values (398.76, 399.99, and 402.75 eV except for 401.70 eV), demonstrating the increase of electron density in nitrogen dopant within NG-6h, which can be due to the incorporation of graphene phase into CuPc. In addition, it is worthy to note that the proportion of graphite-like nitrogen displays considerably increases from 12.39 to 25.84 at. %, while the content of pyrrole-like nitrogen remarkably decreases as calculated (Table S2), which once again confirms that the pyrrole-like nitrogen is unstable at high temperature and can be easily converted to more stable graphite-like nitrogen.35 The increase in content of graphite-like nitrogen for NG-6h indicates that the effective nitrogen doping in graphene frameworks by using CuPc as nitrogen source has been successfully realized, which is consistent with the results of Raman spectra (Figure 4c). It can be also found that the proportion of graphite-like nitrogen will increase with the prolonging annealing time, resulting in more graphite-like nitrogen incorporated into the graphene frameworks (Figure S4 and Table S2). As illustrated in Figure 8, graphite-like nitrogen is also called “quaternary nitrogen”, which is incorporated into the graphene layer by substituting a carbon atom in the graphene plane and merges two π-electrons to the π-conjugated system, thereby contributing to improve the electrical conductivity of the graphene plane.36 3.5. Electrical Conductivity of GO and GO/CuPc Composites. As an intrinsic ability of materials for absorbing

that the peaks of aromatic ether C−O−C, phthalocyanine N− CN, and nitrile CN disappear, whereas two small new peaks arise at 285.18 and 287.52 eV obtained by peak fitting as shown in Figure 6d, suggesting the bonding formation of nitrogen atoms to be sp2-C atoms (CN) and sp3-C atoms (C−N), respectively. As a result, the effective degradation of CuPc molecules provides enough nitrogen atoms for reconstruction with carbon atoms at high annealing temperature, thereby achieving nitrogen doping in graphene frameworks. To further understand the nitrogen doping process in this system, the bonding configurations of nitrogen atoms in CuPc and GO/CuPc were also characterized by high-resolution N 1s spectrum. For CuPc, before sintering treatment, as shown in Figure 7a, three peaks arise at 399.32, 398.68, and 399.94 eV, which correspond to the nitrogen functionalities of nitrile (C N), C−N, and CN in CuPc molecules.31,32 In contrast to CuPc molecules, incorporation of GO into CuPc led to a series of distinct red-shifts and a significant intensity enhancement of the peak at 399.32 eV (Figure 7c), which can be attributed to the electron-withdrawing effect of GO and CuPc.32 As can be seen from Figure 7b, after sintering treatment, the N 1s spectrum of CuPc can be deconvoluted into four different signals with binding energies of 398.72, 400.39, 401.60, and 404.07 eV, revealing the presence of pyridine-like, pyrrole-like, graphite-like nitrogen, and pyridine oxides species, respectively,33,34 which are consistent with the results of the C 1s 22414


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Figure 7. High-resolution N 1s spectra of GO (a) and GO/CuPc multilayer films (c) and the corresponding patterns (b and d) annealed at 800 °C for 6 h.

Table 1. Electrical Conductivity of Graphene, GO/CuPc Multilayer Films, and Disordered GO/CuPc Composite Annealed at 800 °C samples

conductivity (S/m)

graphene NG-3h NG-6h NG-9h disordered NG-6h

2.67 1.68 3.64 × 103 8.45 × 103 3.50 × 103

microcellular structure during the annealing process would impair the conductive network.15 As a result, graphene foam shows a low electrical conductivity of 2.67 S/m, which can be attributed to overexpansion during the annealing process. As to GO/CuPc multilayer films, due to the introduction of CuPc molecules, the degree of volume expansion of NG-6h is distinctly lower than that of graphene foam as shown in Figure 1. Meanwhile, the in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets facilitates the retention of layerby-layer stacking structure of graphene sheets and the effective nitrogen doping by replacing carbon atoms in graphene frameworks. Under the synergistic effect of enriching, orientation, and effective nitrogen doping, the electrical conductivity of NG-6h gets a conspicuous enhancement (to 3.64 × 103 S/m). As a control, the disordered GO/CuPc films are also treated under the same conditions. The final electrical conductivity (3.50 × 103 S/m) of the disordered NG-6h is a

Figure 8. Schematic structure of nitrogen doped graphene.

electromagnetic radiation, electrical conductivity is a critical parameter for EMI shielding materials, which was determined by the composition of the nanocomposites, the intrinsic properties of the components, and the distribution or alignment in morphology. In this Article, the influence of in situ nitrogen-doping and the formation of layer-by-layer multilayer structure on the electrical conductivity of graphene films were investigated, and the results were listed in Table 1. For GO film, the thermal reduction at high temperature could effectively facilitate the improvement in the electrical conductivity of graphene, whereas the formation of fluffy 22415


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Figure 9. Total EMI shielding effectiveness (SE total) (a), SE absorption and SE reflection (b) of graphene, NG-6h, and disordered NG-6h; and the SE total (c), and SE absorption and SE reflection (d) of GO/CuPc multilayer films annealed at 800 °C with different times (NG-3h, NG-6h, and NG-9h).

shielding effectiveness ≥20 dB is about 3.5 GHz from 8.5 to 12 GHz, reaching the target required for practical application. To further clarify the shielding mechanism, the SE reflection (SER) and SE absorption (SEA) were also calculated from the scattering parameters S11 and S21 and displayed in Figure 9b. It can be obviously observed that the value (>10 dB) of SE absorption is higher than that (∼5 dB) of SE reflection. Therefore, we can conclude that both SE reflection and SE absorption contribute to the total EMI shielding effectiveness, but the SE absorption plays a dominant role in EMI shielding of graphene foam, which is consistent with the results of previous literature.15 The excellent SE absorption can be attributed to the formation of microcellular structure in graphene foam. In comparison with the compact stacking layered graphene film,12,13,15 graphene foam with microcellular configuration possesses a large internal interface area. Electromagnetic wave entering the foam would be reflected at the corresponding interface due to the impedance mismatch between air and graphene sheets. Because of the presence of the closed microcellular structure, the random electromagnetic reflections would repeatedly occur at the internal interfaces of the cell wall, resulting in the transfer of electromagnetic energy to be dissipated as heat in the form of microcurrent, thereby enhancing the final SE absorption. Moreover, two distinct peaks arise in the frequency range of 8.5−10 GHz, and the optimal EMI shielding effectiveness is up to 36.1 dB as shown in Figure

little lower than that of NG-6h, which once again confirms the influence of orientation configuration in the electrical conductivity. It can be mainly attributed to the fact that compact contact between aligned graphene sheets along the plane direction can minimize the barrier for electron transfer, thereby improving the electrical conductivity of NG-6h effectively. Moreover, on the basis of the same orientation configuration in layer-by-layer GO/CuPc multilayer films, NG annealed with different times shows a tendency to increase in electrical conductivity from 1.68 S/m (NG-3h) to 8.45 × 103 S/m (NG-9h). Therefore, it can be concluded that the electrical conductivity is determined by the degree of reduction of GO in graphitization, orientation, volume expansion, and nitrogen doping. In this study, the positive enhancement of reduction of GO in graphitization, orientation, and nitrogen doping plays a more dominant role than the negative impairment of volume expansion in electrical conductivity. 3.6. EMI Shielding Effectiveness of GO and GO/CuPc Composites. To investigate the EMI shielding performance, the samples were measured with VNA (Agilent E8363B) using the waveguide method in the frequency range of 8−12 GHz (X-band). The EMI shielding effectiveness as a function of frequency was calculated and displayed in Figure 9. As shown in Figure 9a, the graphene foam with a thickness of 0.73 mm exhibits excellent EMI shielding effectiveness over the measured X-band. The qualified frequency bandwidth of EMI 22416


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Table 2. EMI Shielding Performance with Shielding Thickness in Recently Reported Graphene-Based Shielding Materials in XBanda name

thickness

conductivity (S/m)

EMI SE (dB)

EMI SEmax(dB)

refs

PUG-10 foam epoxy/TAGA (radial 0.8 wt %) GP|wax|GP GA-CT GN-BaTiO3 PLA/GNP G-foam NG-6h rLGO film GF-2000

60 mm 4 mm 2.1 mm 2.0 mm 1.5 mm 1.5 mm 0.6 mm 0.47 mm ∼15 μm ∼8.4 μm

0.06 9.8 × 102

>50 32 >20 26−27 42 >11.5 ∼36 >25 ∼20 ∼19

57.7 32 47.7 27 42 15.5 36.5 55.2 20.2 19.1

37 38 16 39 40 41 15 this work 42 12

1.9 7.4 310 3.64 2.43 1.0

× 103

× 103 × 104 × 105

a

PUG, polyurethane/graphene; TAGA, thermally annealed anisotropic graphene aerogels; GP, graphene papers; GA-CT, graphene aerogels-carbon texture; GN, graphene nanosheets; PLA/GNP, poly lactide/graphene nanoplatelet; rLGO, large-area reduced graphene oxide; GF, graphene film.

SE reflection as shown in Figure 9b, which once again confirms the significant influence of orientation configuration with interlayer voids in EMI shielding effectiveness. In addition, a relatively smooth resonance peak also appears in the shielding curve of disordered NG-6h, which can be attributed to the loose and disordered configuration of graphene sheets with small free space as displayed in graphene foam as shown in Figure S1. Moreover, on the basis of the same orientation configuration in layer-by-layer GO/CuPc multilayer films, NG shows a tendency to increase in total EMI shielding effectiveness, and the corresponding resonance peak gradually shifts toward higher frequency with increasing annealing times as shown in Figure 9c. For the SE absorption and SE reflection, similar trends can be also observed in Figure 9d, but the increase of SE reflection with prolonging the annealing times is slight and pallid in comparison with the conspicuous enhancement in SE absorption. The conspicuous evolution in EMI shielding effectiveness can be ascribed to the increase of electrical conductivity and interlayer spacing as shown in Figure 2 and Table 1, which is determined by the degree of reduction in graphitization, nitrogen doping, and volume expansion. Thus, it provides an effective strategy to tune the EMI shielding effectiveness of NG by altering the annealing time.

9a. However, such unique peaks are observed in previous literature,15 which may be attributed to the difference in the size of the hollow microcellular structure in the graphene foam. In this study, the diameter of the microcellular structure decreases to a value of 2.5 μm as shown in Figure S2, being 1 order of magnitude smaller than that (25 μm) of the G-foam in ref 15. The smaller microcellular structure could result in a larger interface area, which may further impact the random multiple reflections of electromagnetic wave among them, leading to the greater fluctuation in the shielding curve as shown in Figure 9a. As for GO/CuPc multilayer films, NG-6h treated under the same conditions exhibits a significant improvement in total EMI shielding performance in comparison with the graphene foam as shown in Figure 9a. The EMI shielding effectiveness of NG-6h with a thickness of 0.47 mm exceeds the value of 25 dB over the measured band. Meanwhile, the shielding curve also exhibits a stronger resonance peak, and the optimal EMI shielding effectiveness is up to 55.2 dB. In comparison with other graphene-based shielding materials as displayed in Table 2, the excellent shielding performance and smaller thickness make it possible for NG-6h to become an effective EMI shielding material with a distinct competitive advantage.15,37−42 Moreover, in comparison with SE reflection and SE absorption of graphene foam, the improvement of NG-6h in SE absorption is distinct larger than that in SE reflection as shown in Figure 9b. In other words, the improvement of EMI shielding effectiveness in NG-6h can be mainly ascribed to the enhancement in SE absorption. Such a dramatic enhancement in SE absorption can be attributed to the excellent electrical conductivity and the formation of the layer-by-layer stacking structure with large interlayer voids. The unique configuration would contribute to the formation of a Fabry−Pérot resonance cavity.43,44 Electromagnetic wave entering the parallel cavity would be also reflected at the corresponding interface, but the subsequent internal reflections would be aligned between parallel reflecting planes, producing constructive interference, which is different from the mostly random reflections as reported in other graphene-based shielding materials. As a result, the corresponding internal reflection in NG-6h was substantially attenuated, leading to a dramatic enhancement in SE absorption in comparison with that of graphene foam as shown in Figure 9b. As a control, the disordered NG-6h is fabricated and measured under the same conditions. The final EMI shielding effectiveness (SE total) is lower than that of ordered NG-6h. With further analysis, it can be found that the only difference between them is the SE absorption rather than

4. CONCLUSIONS In summary, a novel kind of layer-by-layer nitrogen-doped graphene multilayer films with large interlayer voids was successfully fabricated by thermal annealing the compact stacking GO/CuPc multilayer films. The results of SEM, XRD, Raman, and XPS spectra indicated that thermal annealing treatment effectively removed the oxygen-functional groups on/in GO sheets at high temperature, leading to restoration of the π-conjugated network of graphene. Meanwhile, the in situ pyrolysis of CuPc molecules in the interlayer of graphene sheets facilitated the retention of orientation stacking structure of graphene sheets and achieved the effective nitrogen doping by replacing carbon atoms in graphene frameworks. Moreover, the breakaway of oxygen functional groups in the GO sheets and pyrolysis of CuPc molecules in the form of CO, CO2, NOx, and water vapor at high temperature could generate enough inner pressure to overcome the interaction between graphene sheets, thereby resulting in the formation of interlayer voids. As compared to the graphene foam with larger volume expansion, the restoration of GO in graphitization, the retention of orientation stacking structure and nitrogen doping in plane, and enrichment in volume synergistically endowed the fluffy layered 22417


Research Article

ACS Applied Materials & Interfaces graphene films with excellent electrical conductivity of 3.64 × 103 S/m. In addition, due to the formation of a Fabry−Pérot resonance cavity in the unique orientation stacking structure with larger interlayer voids, the internal reflections would be aligned between parallel reflecting planes and produce constructive interference, leading to a dramatic enhancement in EMI shielding effectiveness (exceeds 25 dB in all X-bands) in comparison to that of graphene foam and the disordered films. The optimal shielding effectiveness of 55.2 dB and smaller thickness of 0.47 mm made NG-6h a practical EMI shielding material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04008. SEM images of disordered GO/CuPc composite and GO multilayer films before and after thermal annealing; schematic illustration of the procedure to prepare the GO/CuPc multilayer films; high-resolution N 1s spectra of GO/CuPc multilayer films; structure of GO and CuPc; and atomic percentage and nitrogen atomic percentage of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Renbo Wei: 0000-0002-5975-7054 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (project nos. 51603029 and 51403029) is gratefully acknowledged.

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REFERENCES

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Fluffy and Ordered Graphene Multilayer Films with ... - ACS Publications

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