107-115 Nakamura.indd

Clay Science 18, 107−115 (2014) –Paper–

PREPARATION OF LARGE-AREA REDUCED GRAPHENE OXIDE–SMECTITE COMPOSITE FILM AND ITS ELECTROMAGNETIC SHIELDING EFFECTIVENESS Takashi Nakamuraa, *, Hiroshi Nanjoa, Masatou Ishiharab, Takatoshi Yamadab, Masataka Hasegawab, Michitaka Ameyac, Yuto Katouc, Masahiro Horibec, and Takeo Ebinaa a

Research Center for Compact Chemical System, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi 983-8551, Japan b Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan c Electromagnetic Waves Division, National Metrology of Japan (NMIJ), National Institute of Advance Industrial Science and Technology (AIST), Central 3, Umezono 1-1-1, Tsukuba, Ibaraki 305-8565, Japan (Received November 20, 2014. Accepted January 5, 2015)

ABSTRACT The objective of this research was to prepare a clay film with electromagnetic shielding properties by making a clay film electrically conductive. To achieve this goal, we attempted to intercalate graphene into smectite clay and to prepare a graphene–smectite composite film with dimensions corresponding to those of A4 paper. The composite film was prepared by mixing graphene oxide and smectite in water, drying the solid, and annealing at 250°C for 30 min in an air atmosphere. The film was characterized by X-ray diffraction analysis, Fourier transform infrared spectroscopy, thermogravimetry, differential thermal analysis, and scanning electron microscopy. Conductivity and electromagnetic shielding were measured with a linear, 4-pin electrode method and the KEC method, respectively. The characterizations indicated that the reduced graphene oxide compounded with the smectite clay at the nanometer scale. The obtained composite film contained 33 wt% carbon, and the sheet resistance was 1036 Ω·cm−2. The shielding effectiveness of the composite film was 67% at electromagnetic frequencies less than 100 MHz. At frequencies less than 10 MHz, the shielding effectiveness of the film exceeded 97%. We have therefore succeeded in developing a clay film with electromagnetic shielding properties by addition of reduced graphene oxide to smectite clay. Key words: graphene, reduced graphene oxide, smectite, composite film, electromagnetic shielding, electromagnetic immunity, conductivity

INTRODUCTION Clay has been used as a raw material for pottery vessels since ancient times and ranks with wood as one of the oldest materials extensively used by human societies. Today, clay is widely used in industrial fields such as architecture, civil engineering, ceramic engineering, agriculture, and chemical industry (Harvey and Lagaly, 2006; Sudo and Shimoda, 1978). Smectite clay is characterized by a combination of properties that distinguish it from all other clays, including its ability to intercept water, improve viscosity, adsorb molecules, exchange ions, swell with the addition of water, and impart *

Corresponding author: Takashi Nakamura, Research Center for Compact Chemical System National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi 983-8551, Japan. e-mail: [email protected]

thixotropic behavior to fluids. Smectite clay has therefore been applied as a thickening agent for paints, cosmetics, and adhesives; as a caking additive for molds; as a sustainedrelease preparation; and in the reforming of plastics (Sudo et al., 1978; Weaver and Pollard, 1973). Smectite clay consists of negatively charged aluminosilicate layers composed of cations that are coordinated tetrahedrally (Si, Al) and octahedrally (Al, Fe, Mg) by oxygen atoms, as is the case with clay minerals in general. Between these layers, cations such as Na+ and K+ are found with a layer-by-layer assembly structure in smectite clay (Weaver et al., 1973). For smectite clay, the negative charge of an aluminosilicate layer ranges from 0.2 to 0.6. The properties of such smectite clays are determined by the fact that the values of the negative charge are intermediate between 0 (talc) and 1 (mica). The properties of a smectite clay such as cation exchange

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and molecular adsorption capacity are of interest for the development of new materials. Researchers have been working on fundamental (Egawa et al., 2011; Ishida et al., 2011; Takagi et al., 2010) and applied aspects of smectite-based materials (Čeklovský and Takagi, 2013; Okada et al., 2014). Hauser and Beau (1938; 1939) reported a unique property of smectite clay. They obtained a smectite film by drying a liquid dispersion of smectite that had been spread on an appropriate base material. Our group has carried out further research on smectite films and has succeeded in developing a new smectite film by adding a small amount of polymer as a binder (Ebina and Mizukami, 2007). The gas-barrier property of the smectite film for dry oxygen gas (0.074 cm3 m−2 day−1 atm−1) is higher than that of polymer-based, gas-barrier films. In parallel with this research, our group has also worked on functionalization of smectite films for applications such as phosphorescence and light emission. A phosphorescent smectite film was prepared by hybridization with CdSe/ZnS nanoparticles (Tetsuka et al., 2008a). An organic, lightemitting diode smectite film was developed by constructing diode materials on a transparent smectite film (Tetsuka et al., 2008b; Venkatachalam et al., 2013). The aim of the present study was to develop a smectite film with electromagnetic shielding properties. Films with these properties are most effectively prepared by using materials with high conductivity, highly complex permittivity, and highly complex permeability (Metaxas, 1996; Nakamura et al., 1994). Above all things, a way to use conductivity is easy and efficient to give electromagnetic shielding property on matters (Tumidajski, 2005). Metals(Geetha et al., 2009), conductive polymers (Wang and Jing, 2005), and carbon-based materials (Chung, 2001) have been mainly studied. In this study, our goal was to produce a smectite film with high conductivity for obtaining the clay film with electromagnetic shielding property. We therefore focused on graphene-related materials, which are layered materials equivalent to aluminosilicates. Research on complex compounds between layered clay compounds and carbon sheets have been attempted before the single carbon sheet is attracting considerable attention as “graphene”. Kyotani (1988) and Sonobe (1988a; 1988b) have been reported that the carbon–montmorillonite composite material synthesized by heat-treating of montmorillonite intercalating polyacrylonitrile. Alternatively carbon–clay compounds such as graphite oxide/anionic clay (Nethravathi et al., 2008a), graphene oxide/smectite (Spyrou et al., 2014), exfoliated graphite/bentonite(Seredych et al., 2008), graphene/tublar clay (Tang et al., 2013), graphene oxide/laponite (Yoo et al., 2014), graphene/clay/polyimide (Longun et al., 2013) have been reported. Here we focused especially on compositing between reduced graphene oxides and smectite clay for the following reasons. Graphene consisting of a single sheet of lattice-shaped carbon has attracted considerable attention from researchers because of its ideal mechanical and electrical properties and the fact that it is transparent. Although graphene has these superior properties, there are some issues to be resolved, especially handling of graphene as a single sheet of carbon. While developing a graphene-based material, it is difficult to keep graphene as “graphene” without reaggregation. Solvation and

dispersion of graphene in appropriate liquids are particularly important to the formation of composite materials by mixing of different materials. Much work has been directed at dispersing graphene in a liquid (Park and Ruoff, 2009). Here, to obtain graphene, we focused on a method that uses graphene oxide, and then the graphene oxide is reduced by annealing (Beckett and Croft, 1952; He et al., 1998; Matuyama, 1954; Pei and Cheng, 2012). The graphene oxide is dispersed in water in an anionic state because carboxylate ions are attached to the carbon sheet (Li et al., 2008). We consider that graphene oxide and smectite clay are suited to the formation of the composite because both layered materials bear anionic charges on their layers. In a mixed dispersion, these negatively charged layers are mutually repulsive and mix homogenously with each other at the scale of nanometers. During drying of the liquid dispersion after it has been spread on a suitable substance, the layered materials (i.e., the graphene oxide and the aluminosilicate derived from the smectite) approach each layer in the film through a house-of-cards structure (Hauser et al., 1938; Lagaly, 2006). In this work, we attempted to prepare a reduced graphene oxide–smectite clay composite film to characterize the chemical state of the film and to measure its electric conductivity and electromagnetic shielding properties. Although graphene– clay composite films have been reported before (Nethravathi et al., 2008b), in this study we attempted to prepare a largearea graphene–clay composite film and to measure its physical properties, especially its electromagnetic shielding effectiveness. For the electromagnetic shielding property, it was theoretically shown that the graphene-based materials possess great ability less than 10 GHz of the electromagnetic frequency range (Lovat, 2012). Although the electromagnetic shielding property of the graphene-based materials for the frequency band of GHz have been mainly reported (Hong et al., 2012; Song et al., 2014a; Song et al., 2013; Song et al., 2014b), the shielding property from kHz to MHz has not been previously reported. A reason, why there is no report on the shielding property measurement between kHz to MHz, is difficulty of preparation of a large area sample over 150-square-mili meter. To measure the frequency band from kHz to MHz by using KEC method which was developed to measure electromagnetic interference shielding properties of film materials by the KEC Electronic Industry Development Center, the large area sample is necessary. Therefore we attempted to prepare a large-area graphene–smectite composite film for measuring the electromagnetic shielding property of the frequency range between 0.1 to 1000 MHz. MATERIALS AND METHODS Potassium persulfate (special grade), phosphorus(V) pentoxide (analytical grade), concentrated sulfuric acid (special grade), nitric acid (special grade), potassium permanganate (special grade), hydrogen peroxide (30 wt% in H2O, analytical grade), and hydrochloric acid (35 wt% in H2O, special grade) were purchased from Wako Pure Chemical Industries and were used without further purification. Graphite (scaly graphite, SS-3) and smectite clay (Kunipia M) were purchased from Japan Matex and Kunimine Industries, respectively, and were

Preparation of graphene–clay composite film and its electromagnetic shielding effectiveness

used without further purification. Preparations Preparation of graphene oxide by the modified Hummers method. Hummers method uses sulfuric acid, nitric acid, and potassium permanganate to oxidize graphite (Hummers and Offeman, 1958). Although it is a popular method for preparing graphene oxide, obtaining fully peeled graphene oxide by means of this method is difficult. Many scientists have modified the Hummers method to facilitate complete peeling of graphite from graphene oxide. Here we chose a modified Hummers method that uses potassium persulfate and phosphorus pentoxide as graphite pretreatment reagents to obtain graphene oxide (Kovtyukhova et al., 1999). Potassium persulfate (10 g) and phosphorus pentoxide (10 g) were added to concentrated sulfuric acid (30 mL) in a threeneck flask, and the mixture was stirred until the reagents were dissolved. Graphite (10 g) was added into the prepared sulfuric acid and stirred at 80°C for 6 h and at room temperature for 12 h. Deionized water (2 L) was slowly added to the mixture, with cooling by means of an ice bath. The liquid suspension was filtered by means of a glass fiber filter to obtain the solid, which was repeatedly washed with deionized water until the pH of the residual liquid was neutral. The solid was then dried in a vacuum desiccator at room temperature. The pretreated graphite and condensed sulfuric acid (240 mL) were added to a 1-L, three-neck flask. Potassium permanganate (30 g) was added piecemeal into the mixture, which was cooled in an ice bath and stirred for 1 h. Deionized water (500 mL) was slowly added to the mixture while it was cooled in an ice bath. The mixture was transferred to a 5-L beaker, and deionized water (1400 mL) was added while the mixture was stirred. Hydrogen peroxide (50 mL) was slowly added to the mixture with stirring. The color of the mixture then changed from brown to yellow. The mixture was centrifuged in a universal refrigerated centrifuge (model 7780, Kubota) at 10,000 × g for 20 min, and a yellow solid was obtained. The solid was washed by adding dilute hydrochloric acid (3.5 wt% in H2O) and centrifuged. This washing process was repeated three times. Finally, deionized water (200 mL) was added, and a graphene oxide dispersion liquid was obtained.

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liquid was spread onto a PET film (A4 size, 100-μm thickness) by using a casting knife with a clearance gap of 1 mm. The coating film was dried at room temperature and removed from the PET film. The obtained film was heated at 250°C for 30 min in an electric furnace under an air atmosphere (Fig. 1). Characterizations and analyses X-ray diffraction analysis. The basal spacing of the obtained samples was characterized by X-ray diffraction (XRD) analysis (M21X, MacScience, Japan) with Cu Kα radiation at 40 kV and 200 mA at 2θ = 3−70°. Film samples were measured by pasting them onto a quartz glass plate. Thermogravimetry and differential thermal analysis. Thermogravimetry–differential thermal analysis (TG-DTA) was carried out with a Thermo Plus EVO II instrument (Rigaku) from 30 to 1000°C at a heating rate of 10°C/min under an air atmosphere in a platinum pan. Alumina powder was used as a reference material. Fourier transform infrared spectroscopy. Fourier transform infrared (FT-IR) spectra were measured by using a Spectrum 1000 (Perkin-Elmer) equipped with an attenuated total reflectance sampling accessory (Universal ATR, Perkin-Elmer). Spectra of the obtained samples were recorded at room temperature from 650 to 4000 cm−1 with a spectral resolution of 2 cm−1. Scanning electron microscopy. The morphology of the obtained film was observed by using scanning electron micro scopy (SEM) with a TM1000 (Hitachi) operated at an acceleration voltage of 1.5 kV and an emission current of 20 mA. Resistance measurement. The sheet resistance of the obtained films was measured with a Loresta GP MCP-T610 (Mitsubishi Chemical Analytech) equipped with a linear, 4-pin probe electrode (model PSP, Mitsubishi Chemical Analytech). The sheet

Preparation of reduced graphene oxide film. The graphene oxide dispersion (0.8 wt% in H2O, 50 g) was spread onto a polyethylene terephthalate (PET) film (A4 size, 100-μm thickness) by using a casting knife with a clearance gap of 2 mm. The coating film was dried at room temperature and removed from the PET film. The obtained film was heated at 250°C for 30 min in an electric furnace under an air atmosphere. Preparation of reduced graphene oxide–smectite composite film. Here we describe the typical preparation of a reduced graphene oxide–smectite film containing 33 wt% of reduced graphene oxide. Semisolid smectite (20 wt% in H2O, 10 g) and the graphene oxide dispersion (0.8 wt% in H2O, 115 g) were mixed by using a planetary centrifugal mixer (ARE-310, Thinky) for 30 min. After addition of deionized water (11 mL), the mixture was mixed further with a planetary centrifugal mixer for 20 min, and a sticky liquid was obtained. The sticky

Fig. 1.

Appearance of reduced graphene oxide–smectite composite film.

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resistances of the samples were estimated from the average of three measurements. Electromagnetic shielding effectiveness. The electromagnetic shielding effectiveness of the obtained films was measured by using the KEC method. The KEC method measurements were carried out with a vector network analyzer (E5071C, Agilent Technology) equipped with an apparatus for the KEC method (Microwave Factory). The measurement conditions of the vector network analyzer were set to 0 dBm of output power, 10 Hz of intermediate frequency bandwidth, and a measurement width from 0.1 MHz to 1000 MHz. The obtained data were averages of 16 measurements. RESULTS AND DISCUSSION Thermal reduction of graphene oxide film Figure 2 shows XRD patterns of graphene oxide films before and after heating. There was a peak in the graphene oxide spectrum before heating at 0.838 nm (2θ = 10.54°) (Fig. 2a) that we attributed to interlayer spacing due to oxidation of graphite (Kim et al., 2012; Moon et al., 2010; Nethravathi et al., 2008b). After heating at 100°C, which is the temperature

Fig. 2. XRD patterns for graphene oxide. (a) As-prepared, (b) heated at 100°C, and (c) annealed at 250°C. Dashed lines connecting circles and squares are results of curve fitting for the XRD patterns corresponding to d = 0.402 nm and d = 0.356 nm, respectively.

of water vaporization, the interlayer spacing decreased from 0.838 to 0.739 nm (2θ = 11.97°) due to vaporization of water and subsequent closing of the graphene oxide layers (Fig. 2b). At temperatures above 250°C, the temperature associated with the reduction of the graphene oxide (the TG-DTA paragraph below provides information about the reduction temperature), the diffraction peak of the carbon layers broadened from 2θ = 15 to 30° (Fig. 2c). The broadened peak consisted of two peaks with diffraction peaks at 22.09° and 25.00° that we attributed to d-spacings of 0.402 and 0.356 nm, respectively. The obtained d-spacing values of 0.402 and 0.356 nm indicate that the heating incompletely reduced the graphene oxide because some hydroxyl and carboxyl groups remained on the carbon sheets (the FT-IR measurement paragraph below provides data). If the graphene oxide was reduced to complete graphene, it will be equal to the d-spacing value of the carbon for the (002) reflection which is 0.335 nm (2θ = 26.55°). We used TG-DTA to elucidate the thermal properties of the graphene oxide film (Fig. 3). At temperatures below 200°C, the sample lost 16% of its weight due to water vaporization. There was an exothermic peak at 200°C associated with a 32% loss of weight due to elimination of the epoxy group on the carbon layer as carbon monoxide or carbon dioxide. Finally, the combustion of graphene oxide at 663°C was associated with a 52% loss of weight. Functional groups on the graphene oxide before and after heating were characterized by FT-IR spectroscopy (Fig. 4). Although the FT-IR spectra indicated broadened peaks, a number of locally minimal values were clearly apparent. These peaks are summarized in Table 1 (Chen and Yan, 2010; Nethravathi et al., 2008b; Pretsch et al., 2000; Shim et al., 2012). Comparison of the graphene oxide film at 250°C before and after heating revealed a change of the chemical state; water molecules (around 3276 cm−1) and epoxy groups (1280 cm−1) were eliminated by the annealing. The chemical change from an alkenyl ether (1104 cm−1) to an aromatic ether (1054 cm−1) indicated that the oxidized atomic carbon sheets were reduced to form benzene rings. Hydroxy (3672 cm−1), carboxy (1733 and 1393 cm−1), and ether groups (1257 and 1195 cm−1) remained on the atomic carbon sheet. In other words, there

Fig. 3.

TG-DTA curve of graphene oxide film.


were some defects in the atomic carbon layer after thermal reduction of the graphene oxide. Characterization of the graphene oxide film by TG-DTA, XRD, and FT-IR led to the conclusion that the graphene oxide film was reduced by annealing at temperatures above 220°C, and then a graphene film was obtained, but the film included defects and some remaining chemical functional groups. These results were reflected in the preparation of reduced graphene oxide–smectite composite films with electrical conducting properties. Characterization of reduced graphene oxide–smectite compo site film Figure 5 shows XRD patterns of an as-prepared graphene oxide–smectite film and an annealed graphene oxide–smectite film at 250°C. To facilitate comparison of XRD patterns with

Fig. 4.

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the raw material films, a clay (smectite) film and a graphene oxide film are also shown in the figure. For the clay film (Fig. 5a), we observed 1.529 nm (2θ = 5.78°) of primary basal spacing and high-order basal spacing (d = 0.512 nm, 2θ = 17.28°; d = 0.307 nm, 2θ = 29.01°). The XRD pattern was attributed to clay layers that intercalate two hydration layers with an inorganic exchangeable cation. For the graphene oxide film (described above), we observed 0.838 nm (2θ = 10.54°) of basal spacing (Fig. 5b). For the graphene oxide–smectite composite film, the basal spacing of clay before annealing (Fig. 5c) was 1.245 nm (2θ = 7.09°). We attributed this spacing to aluminosilicate layers of clay intercalating one hydration layer with inorganic exchangeable cations. Mixing with graphene oxide decreased the basal spacing of the clay from 1.529 to 1.245 nm. Although the obvious peak of graphene oxide disappeared, we observed

FT-IR spectra of graphene oxide. (a) As-prepared and (b) annealed at 250°C.

Table 1. Observed wavenumbers of graphene oxide before and after heating. Graphene oxide film

Annealed graphene oxide film

Observed wavenumber (cm−1)

Assignment

Observed wavenumber (cm−1)

Assignment

3276 1733 1624 1414 1280 1227 1162 1104 948

ν(O-H) with hydrogen bonding ν(C=O) ν(C=C) ν(C-O) carbonyl Epoxide ring stretching ν(C-O) epoxy, ether ν(C-O) epoxy, ether ν(C=C-O-alC) δ(C=C-H) out-of-plane vibration

3672 2987 2902 1733 1629 1393 1257 1195 1054

ν(O-H) without hydrogen bonding ν(C-H) ν(C-H) ν(C=O) ν(C=C) ν(C-O) carbonyl ν(C-O) epoxy, ether ν(C-O) epoxy, ether ν(arC-O-alC)

Abbreviations. ν, stretching; δ, vending vibration; al, aliphatic; ar, aromatic.

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Fig. 6.

Fig. 5. XRD patterns for (a) smectite film, (b) graphene oxide, (c), as-prepared graphene oxide–smectite composite film, and (d) annealed graphene oxide–smectite composite film at 250°C.

a tiny peak at 2θ = 10.2° (d = 0.863 nm) on the shoulder of the main peak that we attributed to clay basal spacing. The decrease of the basal spacing of the aluminosilicate layer in the composite film was caused by mixing with the acidic solution of the graphene oxide dispersion liquid. Inorganic exchangeable cations in smectite clay are known to be eliminated under acidic conditions. In the case of the composite film prepared under acidic conditions, the lithium ions were eliminated from the aluminosilicate layers with water molecules. As a result, the basal spacing of the aluminosilicate layers decreased. After annealing at 250°C (Fig. 5d), the basal spacing of the clay layers was 0.971 nm (2θ = 9.10°). We attributed this spacing to the aluminosilicate layer intercalating inorganic exchangeable cations without a hydration layer. We observed no peaks of basal spacing for the graphene oxide or reduced graphene oxide. We used TG-DTA to quantify the thermal behavior of the graphene oxide–smectite film (Fig. 6). At temperatures below the exothermic peak at 210°C, the weight of the sample decreased by 12% without recognizable peaks. At 210°C, we observed an exothermic peak and 15% loss of weight that we attributed to elimination of the epoxy group attached to the graphene oxide sheet. At 578°C, there was an exothermic

TG-DTA curve of graphene oxide–smectite composite film.

peak with 11% loss of weight that we attributed to combustion of the reduced graphene oxide. We used FT-IR spectroscopy to characterize the chemical structures of the graphene oxide–smectite film before and after annealing (Fig. 7). After annealing of the film, we found that the water contained in the film (3615 and 3385 cm−1) and the intercalated water in the aluminosilicate layers (1632 cm−1) had evaporated. After annealing of the film, chemical groups such as hydroxyl (3670 cm−1), carboxyl (1719 cm−1), and ether (1220 cm−1) associated with partly reduced graphene oxide remained in the film. We carried out SEM analyses to observe the morphology of the cross-section of the annealed graphene oxide–smectite film (Fig. 8). The structure of the film consisted of a layer-by-layer assembly of layered materials such as smectite and reduced graphene oxide. The thickness of the film was 10.7 μm. On the basis of the characterization results described above, we hypothesized a model of the graphene oxide–smectite composite film (Fig. 9). At the nanometer scale in the asprepared composite film, the aluminosilicates and graphene oxide layered materials are both localized (Fig. 9, left image). In the interlayers of the aluminosilicates, there is one hydration layer and inorganic exchangeable cations (lithium ions). Annealing of the composite film (Fig. 9, right image) eliminated the interlayer waters in the aluminosilicates and epoxy groups on the carbon layers of the graphene oxide. During the annealing, the layer construction of the graphene oxide collapsed because of migration of the carbon sheet due to thermal energy and the compressional force caused by the van der Waals force of each aluminosilicate layer induced by elimination of the interlayer waters. Conductive passages formed in the film as a result of the migration of the carbon sheets (the electrical conductivity of the film will be discussed in the following section). Sheet resistance and electromagnetic shielding effectiveness of the reduced graphene oxide–smectite composite film We measured the sheet resistance of the composite film to estimate electrical conductivity. We compared composite films with ratios of graphene oxide to smectite of 33 and 20 wt%.


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Fig. 7. FT-IR spectra of (a) as-prepared graphene oxide–smectite composite film and (b) graphene oxide–smectite composite film annealed at 250°C.

Fig. 8. SEM image of graphene oxide–smectite composite film annealed at 250°C.

Fig. 9.

Schematic images of graphene oxide–smectite composite film before and after annealing at 250°C.

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We calculated the sheet resistances of the composite films by averaging three measurements. For the composite film containing 33 wt% of reduced graphene oxide, the average sheet constant was 1036 Ω·cm−2. For the composite film containing 20 wt% of reduced graphene oxide, the average sheet constant was 6037 Ω·cm−2. Increasing the ratio of reduced graphene oxide to smectite decreased the sheet resistance of the film. The electrical conductivity of the smectite film was consequently attributable to addition of reduced graphene oxide, which resulted in formation of conductive passages in the film at the nanometer scale. In the case of only reduced graphene oxide, the conductivity was 210 Ω·cm−2. To evaluate electromagnetic shielding effectiveness of the films, we focused on the measurement of electric field shielding property because of an ease of measurement and needs for a research field of electromagnetic compatibility. Figure 10 shows the electric field shielding properties of the composite films at frequencies ranging from 0.1 to 1000 MHz. The electric field shielding property of the composite film containing 33 wt% of reduced graphene oxide was larger than that of the composite film containing 20 wt% of reduced graphene oxide at all measurement frequencies. For the composite film containing 33 wt% of reduced graphene oxide, the electric field shielding property exceeded 30 dB (97% of shielding effectiveness) and 10 dB (67% of shielding effectiveness) below frequencies of 12 and 275 MHz, respectively. For the reduced graphene oxide film without the smectite clay, the shielding property was better than of the composite films because of high conductivity of the reduced graphene oxide film. For the smectite film without reduced graphene oxide, the shielding effectiveness was less than 5 dB at all measurement frequencies. CONCLUSIONS We prepared a clay film with effective electromagnetic shielding properties by mixing smectite clay with reduced

Fig. 10. Spectra of electric field shielding effectiveness for reduced graphene oxide–smectite composite film prepared with 33 wt% (solid line), 20 wt% (dashed line) of reduced graphene oxide and reduced graphene oxide (chain line).

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