of graphene oxide-smectite composite film shielding

The Clay Science Society of Japan

NII-Electronic Library Service

The ClayScienceSociety of Japan

cray Science lS, 107-11S (2014)

-Papere

PREPARATION OF LARGE-AREA REDUCED GRAPHENE OXIDE-SMECTITE COMPOSITE FILM AND ITS ELECTROMAGNETIC

SHIELDING EFFECTIVENESS

TAKAsHi NAKAMuRA4 ',

HiRosHi NANJoa, MAsafou IsHiHARAb, TAkM/osHi YAMADAb, MAsrmKA HAsEGAwAb,

MicHiTAKA AMEyAC, YuTo KATouC, MAsAHiRo HoRiBEC, and TAKEo EBiNAa

ZReseareh

Centerfor Cbmpact Chemicai SJrstem, IVketionai Ihstitute ofAdvancedindustrial Science and 7lechnology (;4IS7), IVigatake 4-2-1, Mlyirgino-ku, Senclai, Mlyagi 983-855J, kpan

blVinnotube Research Clenteny IVlational lnstitute qfActvanced Ihdustrial Sbience and Tlechnology (14IS7]},

Centval 5, Higashi 1-1-J, foukuba, Ibaraki 305-8565, Jopan

eElectromagnetic

PPkves Division, Ndtional Adletrology of.ltu7an (?VnvLO, IVbtional institute ofAttvance industrial Science and 1lechnology "IS7), CZintral3, U}nerono 1-1-1, lsukuba, Ibaraki 305-8565, Jt\,an

(Received November 2e, 2014. Accepted January 5, 201 5)

ABSTRACT

The objective of this research was to prepare a clay film with electromagnetic shielding propenies bymaking a clay film electrically conductive. To achieve this goal, we attempted to intercalate graphene intosmectite clay and to prepare a graphene-smectite composite film with dimensions corresponding to those of

A4 paper. The composite film was prepared by rnixing graphene oxide and smectite in water, drying the solid,

and annealing at 250eC for 30 min in an air atmosphere. The film was characterized by X-ray diffi/action anal-

ysis, Fourier transform infrared spectroscopy, thermogravimetry, differential thermal analysis, and scanning

electron micrescopy, Conductivity and electromagnetic shielding were measured with a linear, 4-pin electrodemethod and the KEC method, respectively. The characterizations indicated that the reduced graphene oxidecompounded with the smectite clay at the nanometer scale. The obtained cempesite film contained 33 wt%carbon, and the sheet resistance was 1036 9'cm'2. The shielding effectiveness ofthe composite film was 67%at electrornagnetic frequencies ress than 100 MHz. At frequencies less than 1O MHz, the shielding effective-

ness ofthe film exceeded 97%. We have therefore succeeded in developing a clay film with electromagnetic

shielding properties by addition ofreduced graphene oxide to smectite clay.

Key words: graphene, reduced graphene oxide, smectite,

magnetic immunity, conductivity

composite film, electrornagnetic shielding, electro-

INTRODUCTION

Clay has been used as a raw material for pottery vesselssince ancient times and ranks with wood as one ofthe oldest

materials extensively used by human societies. [Ibday, clay is

widely used in industrial fields such as architecture, civil engi-

neering, ceramic engineering, agriculture, and chemical indus-try (Harvey and LagalM 2006; Sudo and Shimoda, 1978),

Smectite clay is characterized by a combination of prop-erties that distinguish it from all other clays, including itsability to intercept water, improve viscosity, adsorb molecules,

exchange ions, swell with the addition of water, and impart.

Correspending author: Ihlcashi Nakamura, Research Center for Com- pact Chemiear System Natienal Institute ofAdvanced Industrial Sci- ence and fechnology (AIST), Nigatake 4-2-1, Miyagino-ku, Sendai, Miyagi983-8551,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 sustained-

release preparation; and in the reforming of plastics (Sudoet al,, 1978; Weaver and Pollard, 1973).

Smectite clay consists ofnegatively charged aluminosilicate

layers cornposed of cations that are coordinated tetrahedrally

(Si, Al) and octahedrally (Al, Fe, Mg) by oxygen atoms, as isthe case with clay minerals in general. Between these Iayers,cations such as Na' and K' are found with a layer-by-layerassembly structure in smectite clay (Weaver et al., 1973), For

smectite clay, the negative charge of an aluminosilicate layer

ranges from O.2 to O,6. The properties of such smectite clays

are determined by the fact that the values of the negative

charge are intermediate between O (talc) and 1 (mica). The properties of a smectite clay such as cation exchange



TheClayScience Society of Japan

108 Z Ninkamura et al.

and molecular adsorption capacity are of interest for the de-

velopment of new materials. Researchers have been working

on fundamental (Egawa et al., 2011; Ishida et al., 2011; Tlakagi

et al., 2010) and applied aspects of smectite-based materials

(eeklovskY and Takagi, 2013; Okada et al,, 2014).

Hauser and Beau (1938; 1939) reported a unique propertyof smectite clay, They obtained a smectite film by dryinga liquid dispersion of smectite that had been spread on an

appropriate base material. Our group has carried out further

research on smectite fitms and has succeeded in developing anew smectite film by adding a small amount of polymer as a

binder (Ebina and Mizukami, 2007). The gas-barrier propertyof the smectite film fbr dry oxygen gas (O,074 cm3 m'2 day'iatm") 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 fbr applications such

as phosphorescence and light ernission. A phosphorescentsmeetite film was prepared by hybridization with CdSefZnS

nanoparticles (Tbtsuka et al., 2008a). An organic, Iight-emining diode srnectite film was developed by constructing

diode materials on a transparent smectite fi!m ('fetsuka et al.,

2008b; Venkatachalam et al., 2013).

The aim of the present study was to develop a smectite filmwith 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

eMeient 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 forobtaining the clay film with electromagnetic shielding proper-ty. We therefore fbeused on graphene-related materials, which

are layered materials equivalent to aluminosilicates. Researchon complex compounds between layered elay compounds and

carbon sheets have been attempted before the single carbonsheet is attracting considerable attention as

"graphenei',

Kyotani (1988) and Sonobe (1988a; 1988b) have been rc-

ported that the carbon-montmorillonite composite material

synthesized by heat-treating of montmorillonite intercalating

polyacrylonitrile. Alternatively carbonTclay compounds such

as graphite oxidelanionic clay (Nethravathi et al., 2008a),

graphene oxidefsmectite (Spyrou et al., 2014), exfoliated

graphitefoentonite(Seredych et al., 2008), grapheneftublar clay

(Tang et al., 2013), graphene oxidellaponite (Ybo et al., 2014),graphenefclay!polyimide (Longun et al., 20]3) have beenreported. Here we focused especially on compositing betweenreduced graphene oxides and smectite clay for the foIIowingreasons.

6raphene 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 thesesuperior properties, there are some issues to be resolved, espe-

cially handling of graphene as a single sheet ofcarbon. Whiledeveloping a graphene-based material, it is difficult to keep

graphene as "graphene"

without reaggregation. Solvation and

dispersion of graphene in appropriate liquids are particularIyimportant to the fbrmation of composite materials by mixing

of different materials. Much work has been directed at dis-

persing graphene in a liquid (Park and RuofZ 2009). Here, toobtain graphene, we fbcused on a method that uses grapheneoxide, and then the graphene oxide is reduced by annealing

(Beckett and Croft, 1952; He et al,, 1998; Matuyama, 1954;

Pe{ and Cheng, 2012). The graphene oxide is dispersed in

water in an anionic state beeause carboxylate ions are at-

tached to the carbon sheet (Li et al., 2008), We consider that

graphene oxide and smectite clay are suited to the formationof 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-oflcards stmcture (Hauseret al., 1938; Lagaly, 2006),

In this work, we atternpted to prepare a reduced grapheneoxide-smectite clay composite film to characterize the chem-

ical state of the film and to measure its electric conductivityand electrornagnetic shielding propenies. Although graphene-clay composite films have been reported befbre (Nethravathiet al., 2008b), in this study we attempted to prepare a large-area graphene-clay composite film and to measure its physicalproperties, especially its electromagnetic shielding effec-

tiveness. For the electromagnetic shielding propertyl it wastheoretically shown that the graphene-based materials possess

great ability less than 1O GHz ofthe electromagnetie frequen-

cy range (Lovat, 2012). Although the eleetrornagneti ¢ shield-

ing property of the graphene-based materia}s fbr the frequencyband 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 previouslyreported. A reason, why there is no report on the shielding

property measurement between kHz to MHz, is dienculty of

preparation of a large area sample over 150-square-mili meter.

To measure the frequeney band from kHz to MHz by using

KEC method which was developed to measure electromag-

netic interference shielding properties of film materials bythe KEC EIectronic Industry Development Center, the largearea sample is necessary, Therefbre we attempted to preparea 1arge-area graphene-smectite composite film for measuringthe eleetromagnetic shielding property ofthe ftequency rangebetween O.1 to 1000 MHz.

MiffERIALS AND METHODS

Potassium persulfate (special grade), phosphorus(V) pent-oxide (analytical grade), concentrated sulfuric acid (speeialgrade), nitric acid (special grade), potassium permanganate

(special grade), hydrogen peroxide (30 wt% in H,O, analytical

grade), and hydroehloric acid (35 wt% in H20, special grade)were purehased from Wako Pure Chemical Industries and

were used without further purification. Graphite (scaly graph-ite, SS-3) ancl smectite clay (Kunipia M) were purchased from

Japan Matex and Kunimine Industries, respeetively, and were

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Pinyi)arationqf'gFaphene-citu' ['omposite,fitm and its electromagnetic shieiding cti2ic'tii,eness 109

used without further purification.

Preparations

Prqpa"ation of graphene oxicie hv. the modijied Hummers

method. Hummers method uses sulfuric acid, nitric acid, and

potassium permanganate to oxidize graphite (Hummers andOffeman, 1958). Although it is a popular method for prepar-ing graphene oxide, obtaining fu]ly peeled graphene oxide

by means of this method is difncuLt. Many scientists have

modified the Hummers method to facilitate complctc peelingof graphite ftom graphene oxide. Here we chose a modified

Hummers method that uses potassium persulfate and phos-

phorus pentoxide as graphite pretreatment reagents to obtain

graphene oxide (Kovtyukhova et al., 1999).

Potassium persulfate (1O g) and phosphorus pentoxide {1O g)were added to concentrated sulfuric acid (30 mL) in a three-

neck flask, and the mixture was stirred until the reagents wcre

dissolved. Graphite (1O g) was added into the prepared sulfu-ric acid and stirred at 800C for 6 h and at room temperature for

12 h. Deionizcd water (2 L) was slowly added to the mixture.

with cooling by means of an ice bath. The liquid suspension

was filtered by means ofa glass fiber filter to obtain the so]id,

which was repeatedly washed with deionized water until the

pH ofthe residual liquid was neutral. The soiid was then driedin a vacuum desiccator at room temperature.

The pretreated graphite and condensed sulfuric acid (240mL) were added to a 1-L, three-neck flask. Potassium perman-

ganate (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 inixture 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 s]owly 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 centrifttge (model 7780, Kubota) at

1O,OOO x g for 20 min, and a yellow solid was obtained. The

solid was washed by adding dilute hydrochloric acid (3.5 wt9t6

in H,O) and centrifuged. This washing process was repeated

three times. Finally, deionized water (200 mL) was added, and

a graphene oxide dispersion liquid was obtained.

Piepat-ation of reduced grop)hene oxide .lftm. The grapheneoxide dispersion (O.8 wt% in H,O, 50 g) was spread onto a

polyethylene terephthalate (PET) fi]m (A4 size, 1 OO-pm thick-

ness) by using a casting knife with a c]earance gap of 2 mm.

The coating film was dried at room temperature and removed

from the PET film. The obtained film was heated at 2500C for

30 min in an electric furnace under an air atmospherc.

Pteparation of reduced graphene oxicle=smectite composite

.fitm. 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 H,O, 10 g)and the graphene oxide dispersion (O.8 wt% in H,O, 115 g)were mixed by using a planetary centrifugal mixer (ARE-31O,Thinky) for 30 min. After addition ofdeionized water (11 mL),the mixture was mixed further with a planetary centrifugal

mixer for 20 min. and a sticky liquid was obtained. The st{cky

liquid was spread onto a PET film (A4 size, leO-pm thick-ness) by using a casting knirc with a ctearance gap of 1 mm.

The coating film was dried at room temperature and rcmoved

from the PET film. The obtained film was heated at 2500C fbr

30 min in an electric furnace under an air atmosphere (Fig. 1).

Characterizationsandanalyses.\trco, difi7'action analysis. The basal spacing of the obtained

samples was characterized by X-ray diMaction (XRD) anal-ys{s (M21X, MacScicnee, Japan) w{th Cu Kor radiation at 40kV and 200 mA at 2e

= 3-700. Film samples were measured

by pasting them onto a quartz glass plate.

T7ierinogravimet(y and d4fiierential thennat anal.vsis. Ther-mogravimetry-difl'erential thermal analysis (TG-DTA) was

carried out with a Thermo Plus EVO II instrument (Rigaku)from 30 to 1OOOOC at a heating rate of 100Clmin under an air

atmosphere in a platinum pan. Alumina powder was used as a

reference material.

Fbiirier iran.Eform in.fi'ared specttwsccLtz},. Fourier transforminfi'ared (FTLIR) spectra were measured by using a Spectrum1OOO (Perkin-Elmer) equipped with an attenuated total refiec-

tance sampling accessory (Universal MR, Perkin-Elmer).Spectra of the obtained samples were recorded at room tem-

perature from 650 to 4000 cm'i with a spectral resolution of2

Cln i.

Scanning electron microscopv. The morphology of the

obtained fi]m was observed by using scanning electron micro-

scopy (SEM) with a TM 1OOO (Hitachi) operated at an acceler-

ation voltage of 1.5 kV and an emission current of20 mA.

Resi,s'tance measurement. The sheet resjstance of the obtained

films was measured with a Loresta GP MCP-T61O (MitsubishiChemical Ana]ytech) equipped with a linear, 4-pin probe elec-

trode (model PSR Mitsubishi Chemical Analytech). The sheet

Fi(;. 1. Appearance ofreduced graphene oxide-smectite cemposite film.

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110 T Nakamura et al.

resistances of the samples were estimated from the average of

threemeasurements.

Electromagnetic shieiding Elt)rectiveness, The electromagnetic

shielding effectiveness of the obtained films was measured byusing the KEC method. The KEC method measurements were

carried out with a vector network analyzer (E5071C, Agilent"fechnology)

equipped with an apparatus fbr the KEC method

(Microwave Factory). The measurement conditions of the

veetor network analyzer were set to O dBm of output power,10 Hz of intermediate frequency bandwidth, and a measure-

ment width from O.1 MHz to 1000 MHz. The obtained datawere averages of 16 measurements.

RESULTS AND DISCUSSION

T7)ermal reduction of'graphene exide.film

Figure 2 shows XRD pattems of graphene oxide filmsbefbre and after heating. There was a peak in the grapheneoxide spectrum before heating at O.838 nm (2e =

1O,540) (Fig.2a) that we attributed to interlayer spacing due to oxidationof graphite (Kim et al., 2012; Moon et al., 2010; Nethravathi

et al., 2008b). After heating at 1OOOC, which is the temperature

T.'gFs-g.l}ie-e-"

5 10 15 20 25 30 35 40

2e (o)

FiG. 2. XRD patterns for graphene oxide. (a) Asprepared, (b) heated at 1OeOC, and (c) annealed at 2SOOC. Dashed lines connecting circles

and squares are results of curve fitting for the XRD patterns corre-

sponding to d = O,402 nm and d= O.356 nm, respectively.

of water vaporization, the interlayer spacing decreased fromO.838 to O.739 nm (2e - 11.970) due to vaporization of water

and subsequent closing of the graphene oxide layers (Fig, 2b).At temperatures above 2500C, the temperature associated with

the reduction of the graphene oxide (the TG-D[[IPL paragraphbelow provides information about the reduction temperature),

the diffi/action peak of the carbon layers broadened from 2e== 15 to 300 (Fig. 2c). The broadened peak consisted of two

peaks with diffiraction peaks at 22.090 and 25.00e that we

attributed to d-spacings of O,402 and O.356 nm, respectively.

The obtained d-spacing values ofO.402 and O.356 nm indicatethat 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 wi]1 be equal to the d-spacing value of the carbon

fbr the (O02) reflection which is O.335 nm (2e = 26.550).

We used TG-DTA to elucidate the therrnal propenies of the

graphene oxide film (Fig. 3). At temperatures below 200eC,the sample lost 16% of its weight due to water vaporization,

There was an exothermic peak at 2000C associated with a

32% loss ofweight due to elimination ofthe epoxy group onthe carbon layer as carbon monoxide or carbon dioxide, Final-

ly, the combustion ofgraphene oxide at 6630C was associated

with a 529r6 loss ofweight,

Functional groups on the graphene oxide befbre and after

heating were characterized by FT-IR spectroscopy (Fig, 4).

Although the FrlLIR spectra indicated broadened peaks, a

number of locally minimal values were clearly apparent.

These peaks are summarized in [fable 1 (Chen and Yan, 201O;

Nethravathi et al., 2008b; Pretsch et al., 2000; Shim et al,,

2012). Comparison ofthe graphene oxide film at 2500C beforeand after heating revealed a change of the chemical state;

water molecules (around 3276 cmTi) and epoxy groups (1280cm'J) were elirninated by the annealing. The chemical change

from an alkenyl ether (1104 cm") to an aromatic ether (1054cm']) indicated that the oxidized atomic carbon sheets were

reduced to forrn benzene rings. Hydroxy (3672 cm'T), carboxy(1733 and 1393 cm'i), and ether groups (1257 and 119S cmM')

remained on the atomic carbon sheet. In other words, there

o

A .50gex=co.as)

.100

t150o 2oe 4oo 6oo soo

Temperature (oC)

FiG.3. TG-DIAcurveofgrapheneoxiclefilm.

1ooO

oxtu

ov

¢

m

eo¢

tso=

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Pmparation ofgmphene-ctay compositefilm and its electromcrgnetic shielding aj7Zictiveness 111

were some defects in the atomic carbon layer after therrnalreduction of the graphene oxide.

Chara¢ terization of the graphene oxide fiIm by TG-DI:A,

XRD, and FILIR Ied to the conclusion that the graphene oxidefilm was reduced by annealing at temperatures above 220eC,and then a graphene film was obtained, but the film includeddefects and some remaining chemical functional groups.These results were reflected in the preparation of reduced

graphene oxide-smectite composite films with electrical con-

ductingproperties.

enaracterization oj' neduced gmphene oxidetsmectite compo-

siteY71m

Figure 5 shows XRD pattems of an as-prepared grapheneoxide-smectite film and an annealed graphene oxide-smectite

film at 2500C. To facilitate comparison ofXRD patterns with

the raw material films, a clay (smectite) film and a grapheneoxide film are also shown in the figure.

For the clay film (Fig. 5a), we observed 1.529 nm (2e =

5.78e) ofprimary basal spacing and high-order basal spacing(d == O,512 nm, 2e = 17280; d= O.307 nm, 2e -- 29.01O). TheXRD pattern was attributed to clay layers that intercalate two

hydration layers with an inorganic exchangeable cation. Forthe graphene oxide film (described above), we observed O.838nm (2e = 1O.540) ofbasal spacing (Fig. 5b).

For the graphene oxide-smectite composite film, the basalspacing of clay before annealing (Fig. 5c) was 1,245 nm (2e=

7.09e). We attributed this spacing to aluminosilicate layers

of clay intercalating one hydration layer with inorganic ex-

changeable cations, Mixing with graphene oxide decreased thebasal spacing of the clay from 1.529 to 1.245 nm, Although

the obvious peak of graphene oxide disappeared, we observed

A.sslg'

.ugg

.g

spt

400035003000

948

1500

Wayenumber (cm-i)1000

FIG.4.FTLIR spectra of graphene oxide. (a) As-prepared and (b) annealed at 2500C.

TABLE 1,Observed wavenumbers of graphene oxide before and after heating.

Graphene oxide film Annealed graphene oxide film

Observedwavenumber

(Cm'1)Assigrirnent

Observedwavenumber (Cm'1)

Assignment

32761733162414141280122711621104

948

v(O-H) with hydrogen bonding

v(C-O)v(C-C)v(C-O)carbonyl

Epoxideringstretchingv(C-O) epexM ether

v(C-O) epoxy, ether

v(C-C-O-alC)

6(C=C-H) eut-efplane yibration

3672298729e2173316291393125711951054v(O-H) without hydrogerr bondingv(C-H)v(C-H)v(C=O)v(C=C)v(C-O)

earbeny]

v(C-O) epoxy, etherv(C-O) epoxy, etherv(arC-O-alC)

Abbreyiations. v, stretching; 6, vending yibration; al,aliphatic; ar, aromatic.

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112

as.n.fitss-g.e:es

T Ntihamura et al.

(a)saptn'rlLLts

fiN fiF- owo n' 'e o

LL 11

ts ts

(b) scoMoo'eIFts

1Ox

(c)smtrpt-･lltsfi=envcos: fitso

3xdNts Re11ts noILts

(d) s fi s- v ts

20xFthe' bso zao'

H 11 ll

ts ts ts

4 5 10 15 20 2S 30

2e{e)

FiG. 5. XRD patterns for (a) smectite film, (b) graphene oxide, (c), as-prepared graphene oxide-smectite cemposite film, anti (d) annealed graphene oxide-smectite composite film at 25OeC.

a tiny peak at 2e = 10.20 (d =

O.863 nm) on the shoulder of

the main peak that we attributed to clay basal spacing. The de-

crease of the basal spacing of the aluminosilicate layer in thecomposite film was caused by mixing with the acidic solution

of the graphene oxide dispersion liquid, Inorganic exchange-

able cations in smectite clay are known to be eliminated under

acidic conditions, In the case of the composite film preparedunder acidic conditions, the lithium iens were eliminated fromthe aluminosilicate layers with water molecules, As a result,

the basal spacing ofthe aluminosilicate layers decreased,

After annealing at 250eC (Fig. 5d), the basal spacing of

the clay layers was O.971 nm (2e = 9.100). We attributed this

spacing to the aluminosilicate layer intercalating inorganicexchangeable cations without a hydration layer. We observed

no peaks of basal spacing for the graphene oxide or reduced

graphene oxide.

We used TG-DTA te quantify the therrnal behavier of the

graphene oxide-smectite film (Fig. 6), At temperatures belowthe exothermic peak at 210eC, the weight of the sample

decreased by l2% without recognizable peaks. At 2100C, weobserved an exothermic peak and 15% loss of weight that

we attributed to elimination of the epoxy group attached to

the graphene oxide sheet. At 578eC, there was an exothermic

o

gex=. T20'U)

no

oxm

8 m"

o 2oo 4oo soo soo leeo

Ternperature("C)

FiG, 6, TG-DTA curve ofgraphene exide-srnectite composite film.

gE8=

peak with 1 1% loss ofweight that we attributed to cornbustion

ofthe reduced graphene oxide.

We used FTLIR spectroscopy to characterize the chemical

structures ofthe graphene oxide-smectite film before and after

annealing (Fig. 7). After annealing of the film, we fbund thatthe water contained in the film (3615 and 3385 cm']) and the

intercalated water in the aluminosilicate layers (1632 cm'i)

had evaporated. After annealing of the film, chemical groupssuch as hydroxyl (3670 cm'i), carboxyl (1719 cm'i), and ether

(1220 cm") associated with partly reduced graphene oxide

remained in the film.

We ¢ arried out SEM analyses to observe the morphology of

the cross-section ofthe annealed graphene oxide-smectite film

(Fig, 8). The stmcture ofthe film consisted ofa layerLby-layerassembly of layered materials such as smectite and reduced

graphene oxide, The thickness ofthe film was 1O.7 pm.

On the basis ofthe characterization results described above,

we hypothesized a model of the graphene oxide-smectite

composite film (Fig. 9). At the nanometer scale in the as-

prepared composite film, the aluminosilicates and grapheneoxide layered materials are both localized (Fig. 9, left image),

In the interlayers of the aluminosilicates, there is one hy-dration layer and inorganic exchangeable cations (lithiumions). 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 constmction of the grapheneoxide collapsed because ofmigration of the carbon sheet dueto thermal energy and the compressional force caused by the

van der Waals force of each aluminosilicate layer inducedby elimination of the interlayer waters. Conductive passagesformed in the film as a result of the migration of the carbon

sheets (the electrical conductivity ofthe film will be discussedin the following section).

Sheet nesistance and electromagnetic shielding ofectivenessofthe reducedgT"aphene exide-smectite composite.tilm

We measured the sheet resistance of the composite film toestimate electrical conductivity. We compared composite filmswith ratios of graphene oxide to smectite of 33 and 20 wt%.

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Pmparationqfgraphene-ctaycompositefitmanditselectromagneticshieldingqfi?lctiveness 113

T:gges-g8g.EgRb

(a)

3615(b)3385

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173o:

4000

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3500

29862898 l

3000 1750 ISOO

Wavenumber (cm'i)125e 1000FJG.7,

FILIR spectra of (a) as-prepared

graphene oxide-smectite composite filmand (b) graphene oxide-smectite com-

posite film annea]ed at 2SOOC.

LD3.3 x4.0k 20umFIG,8.

SEM image of

nea]ed at 2500C.grapheneOXIde-smectitecornposite fiIm an-

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250 ℃for 30 min

ny - h N

- ----

- - --

f Hi FOOH

toDeH ' eH

N

,mm-imii-mN -- t-

- - ･-

"H-t

--

-m t-

- ow -- --

-"OH oeH

-- t-

OOOH-

-"--ny-m -i--e-eiNp

-

F16.9,Schematic images ofgrapheneoxide-srnectite composite film before and after annealing at 2500C,



The ClayScience Society of Japan

114 T 7Vlakamura et aL

We calculated the sheet resistances of the composite films byaveraging three measurements. For the composite film con-

taining 33 wt% ef reduced graphene oxide, the average sheet

constant was 1036 st'cm'2, For the composite film containing20 wt% of reduced graphene oxide, the average sheet constant

was 6037 9･cm'2. Increasing the ratio of reduced grapheneoxide to smectite decreased the sheet resistance of the film,The electrical cenductivity of the smectite film was conse-

quently attributable to addition of reduced graphene oxide,

which resulted in formation ofconductive passages in the filmat the nanometer scale. In the case of only reduced grapheneoxide, the conductivity was 21O 9･cm'2.

Tb eyaluate electromagnetic shielding effectiveness of the

films, we focused on the measurement of electric field shield-

ing property because ofan ease of measurement and needs for

a researeh field of electromagnetic compatibility. Figure 10

shows the electric field shielding propenies of the eomposite

films at frequencies ranging from O.I to 1000 MHz. The elec-

tric field shielding property of the composite film containing33 wt% of reduced graphene oxide was larger than that of

the composite film containing 20 wt% of reduced grapheneoxide at all measurement frequencies. For the composite filmcontaining 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 be-cause ofhigh conductivity ofthe reduced graphene oxide film,For the smectite fi1rn 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

70

a 6ee.if

50Nfi'

40s8

3o.2g

2oas

10

oe.l

1 10Frequency

(MHz)1001000

FiG. 10. Spectra ef electric field shielding effectiyeness for reduced

graphene oxide-rsmectite composite film prepared with 33 wt%

(solid line), 20 wt% (dashed 1ine) of reduced graphene oxide and

reduced graphene oxide (chain line).

graphene oxide. We characteTized the construction ofthe com-

posite film by XRD, FTLIR, TG-DTA, and SEM and by com-

paring its construction with that of reduced graphene oxide.

The electromagnetie shielding properties ofthe cornposite filmwere increased by increasing the ratio of reduced grapheneoxide to smectite clay. For the composite film containing 33wt% of reduced graphene oxide, the electromagnetic shielding

effectiveness was over 67% for frequencies less than 100MHz. At frequencies less than 1O MHz, the effectiveness was

over 97%. In this report, we have therefore shown the capa-

bility ofthe composite film to function as an electromagnetic

shield.

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