Novel Method for the Fabrication of Flexible Film with OrientedArrays of Graphene in Poly(vinylidene fluoride-co-hexafluoropropylene) with Low Dielectric LossWangshu Tong,† Yihe Zhang,*,† Li Yu,† Xinglong Luan,† Qi An,† Qian Zhang,† Fengzhu Lv,†

Paul K. Chu,‡ Bo Shen,† and Zhilei Zhang†

†National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing100083, China‡Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China

*S Supporting Information

ABSTRACT: Carbon−polymer nanocomposites with good dielectric propertieshave potential applications in the electronic and electrical industry because of theirgood mechanical properties and low cost. The morphology, structure, dielectricproperties, and mechanical strength of reduced-graphene oxide nanosheet/poly-(vinylidene fluoride-co-hexafluoropropylene) nanocomposites (rGO/PVDF-HFP)were investigated. The rGO nanosheets were well dispersed and strongly orientedin the matrix, thanks to the unique spin-assistant preparation process. A dielectricconstant of 54 (100 Hz) which was four times higher than that of pure PVDF-HFPwas obtained when the concentration of rGO was 0.7 vol % and the dielectric loss wasas low as 0.27. The good dielectric performance of the nanocomposites was attributedto the homogeneous dispersion and good alignment of rGO. The shear forceprovided by spin-coating, the thickness decreasing process, and thickness controlwere assumed to be key factors in the alignment of rGO nanosheets in thenanocomposite films. At the same time, the aligned rGO sheets increased the percolation threshold of the composite which shedlight on the mechanism for obtaining low loss materials.

1. INTRODUCTION

Since the early 2000s, there have been many attempts toprepare effective charge storage films with characteristicproperties of flexibility, tunability, and easy processing ofpolymers.1,2 These composites are considered promisingcandidates for capacitors and charge-storage applications.3−9

Poly(vinylidene fluoride) (PVDF) copolymers exhibit certainadvantages compared with the homopolymer such as enhancedpiezoelectricity and improved mechanical behavior.10−12 PVDFcopolymers are considered ideal candidates for severalapplications including electroacoustic and electromechanicalconvertors, actuators, ferroelectric memory devices, mecha-tronics, and artificial muscles.13 The improvement of thedielectric properties of the polymers is the pressing issue inthese applications. The most promising way to enhance thedielectric constant is to add conductive fillers to the polymersto make composite materials.Graphene, a two-dimensional sheet of sp2-bonded carbon

atoms packed in a honeycomb crystal lattice, has attractedconsiderable attention because of its extraordinary flexibility,high aspect ratio, large specific surface area (2600 m2/g),excellent mechanical strength, and good electrical and thermalconductivities.14 Graphene oxide (GO) has a large number offunction groups, such as hydroxyl, carboxyl, and epoxidegroups, on the surfaces, rendering good dispersibility in polar

solvents and matrix. After the subsequent reduction of GO in apolymer matrix, the GO changes to reduced-graphene oxide(rGO) which can be used as conductive fillers.15,16 Thanks tothese advantages, GO has been widely used to prepare“graphene”−polymer composites. Various chemical methodshave been used to modify GO for “graphene”−polymercomposites with high dielectric constant and low dielectricloss, such as graphene−TiO2 nanorod hybrid nanostructures,17

hyperbranched aromatic polyamide-functionalized graphenesheets, graphene oxide-encapsulated carbon nanotube hy-brids,18 and poly(vinyl alcohol)-functionalized graphene.19

The internal microstructure of composites, such as moleculararrangement of polymers and the spatial distribution of thefillers, plays a crucial role in defining the macroscopic physicalproperties of the composites.20−22 A few researchers havestudied the properties of composites with aligned fillers.Composites of high-density polyethylene-mica of plate-likeparticles with a certain orientation were obtained by hot stressand influenced the elastic and viscous properties of thecomposites.23 The distribution of the fillers in a matrix playsa key role in determining its dielectric properties, which are

Received: December 2, 2013Revised: April 27, 2014Published: May 1, 2014

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influenced mainly by geometric parameters such as particle size,shape, and orientation.1 For example, Dang and colleaguesstudied the orientation effect on the macroscopic dielectricproperties of multiwall carbon nanotube/PVDF nanocompo-sites by characterizing the inside microstructure shift duringuniaxial mechanical stretching.24

The orientation of planar fillers in composites is difficult andremains a challenge. In this paper, we report a novel andeffective way to prepare well-aligned rGO/poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) composites byrepeated spin-coating followed by thermal reduction of GO inPVDF-HFP. It is a simple way to prepare rGO/PVDF-HFPcomposite film with advantageous dielectric properties withoutthe need for modification. A dielectric constant of 54 (100 Hz)and low dielectric loss of 0.27 were obtained. The gooddielectric properties of rGO/PVDF-HFP composites areassumed to derive from three factors: (1) the homogeneousdispersion of rGO in PVDF-HFP and the strong interfacialinteractions between the two components, (2) good alignmentof rGO fillers provided by an effective spin-assistant method,and (3) the good conductivity of rGO, which was reduced byheating during the fabrication of rGO/PVDF-HFP at 220 °Cand increased the accumulation of interfacial charges incomposites. The majority of research has focused on rGO’sorientation in the composite and the corresponding effects ondielectric properties. The orientation of rGO in PVDF-HFPwas ascertained by scanning electron microscopy (SEM) of thefracture surface, low-angle X-ray diffraction (LAXRD), and thecomparison of dielectric properties of rGO/PVDF-HFPcomposites by repeated spin-coating (SC) and traditionaldrop-coating (TDC). These findings provide a simple,environmentally benign, and commercially viable process forproducing well-aligned rGO reinforced polymers with advanta-geous dielectric and mechanical properties and shed light onthe mechanism for obtaining low loss materials.

2. EXPERIMENTAL SECTION

Materials. Natural flake graphite (300 mesh) was providedby the Shuangxing Graphite Processing Plant, China, andPVDF-HFP (density 1.78 g cm−3, 5−20% molar ofhexafluoropropene) was purchased from Sigma-Aldrich,America. Potassium persulfate (KPS, ≥99.5%), phosphoruspentoxide (PPO, ≥98%), potassium permanganate (KMnO4,≥99.5%), sulfuric acid (H2SO4, 95−98%), perhydrol (H2O2,30%), and N,N-dimethylformamide (DMF, ≥99.5%) wereobtained from local commercial sources and used as received.

Preparation of GO. GO was prepared according to theHummers method.25 For complete oxidization, an additionalgraphite oxidation procedure was needed, and the preoxidizedgraphite was prepared by KPS and PPO in concentratedH2SO4.

26 This preoxidized graphite was then subjected tooxidation by the Hummers method. The GO was demonstratedthrough X-ray photoelectron spectroscopy (XRD) and atomicforce microscopy (AFM), as shown in Figure S1 (SupportingInformation), and Raman spectra of GO are shown in FigureS2 (Supporting Information). A measured amount of GO wasloaded in a 100 mL round-bottom flask, and DMF (20 mL) wasadded, yielding an inhomogeneous yellow-brown dispersion.The dispersion was sonicated with an SB-5200 DTDNultrasonic bath cleaner (160 W, Ningbo Scientz BiotechnologyCo., Ltd., China) for 3 h to form a homogeneous suspension.After mixing with PVDF-HFP particles (3 g) and stirring at 80°C for 2 h, the GO/PVDF-HFP solution was obtained.

Preparation of Oriented rGO/PVDF-HFP Nanocompo-sites. The rGO/PVDF-HFP composite films were fabricatedby repeated spin-coating followed by a high-temperaturemodeling (220 °C) process. Briefly, the GO/PVDF-HFPmixture was spin-coated on a glass substrate, followed by anaging process at 80 °C for 1 h, as shown in Scheme 1. Then thespin-coating and aging process were repeated five times. Theas-obtained film was then kept in the oven at 220 °C for 1 h,during which process the GO was presumably reduced to rGO.Finally, the rGO/PVDF-HFP composite film was peeled fromthe glass substrate to reveal a free-standing composite film. Thedetailed fabrication process of the composite film is shown inthe Supporting Information.

Characterization of GO and rGO/PVDF-HFP. Themorphology and structure of the GO were examined by AFMobtained by a scanning probe microscope in the tapping mode,TEM (JEOL JEM2100F), FTIR (PerkinElmer Spectrum 100),XRD (Rigaku D/max RA X-ray diffractometer), as well asRaman spectroscopy recorded on a microspectrometer (HoribaJobin Yvon, LavRAM Aramis). The structure of composites wasexamined by SEM (Hitachi S-4300) and XRD. The wide-angleand low-angle XRD (WAXRD/LAXRD) spectra were acquiredon a Rigaku D/max-rA X-ray diffractometer. The samples werescanned at 1° min−1/8° min−1, respectively, with Cu Kα

radiation (0.154 nm) emitted from a filament (40 kV and100 mA). The dynamic mechanical thermal analysis (DMA)was conducted on a TA Instrument DMA 2980 in the filmtension mode at a fixed frequency of 1 Hz. The sampledimensions were 20 × 8 mm2, and they were tested in thetemperature range between −50 and 90 °C under N2 at a

Scheme 1. Schematic Representation of the Fabrication of Layered rGO/PVDF-HFP Composite Films

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heating rate of 2 °C/min. The dielectric properties weredetermined by an impedance analyzer (Agilent 4294 A) atfrequencies ranging from 102 to 107 Hz. The film strips werecut accurately from the samples. Prior to the measurement,silver electrodes were fabricated on the sides of these strips withconductive silver paint (Agar no. 0443).

3. RESULTS AND DISCUSSIONMorphology and Structure of GO and rGO/PVDF-HFP.

The TEM of GO and the morphology and thickness of therGO/PVDF-HFP composite film investigated by SEM areshown in Figure 1. The large soft layer of GO can be observed

in Figure 1a, and the diameter is larger than severalmicrometers. Figure 1b shows a uniform and flexible rGO/PVDF-HFP composite film in black. The functional groups onthe surface of GO nanosheets, including hydroxyl, epoxy, andcarbonyls, improve compatibility with the PVDF-HFP matrixwhich has a decisive effect on the good dispersion of GO in thepolymer matrix.27 During the high-temperature (220 °C)modeling process, GO was reduced in situ (as proved in thefollowing paragraph), diminishing aggregation because of thehigh viscosity of the polymer matrix. Thus, uniform, flexible,and black rGO/PVDF-HFP composite films were prepared bya repeated spin-coating process. At the same time, the thicknesswas also controlled by a spin-coating process and measuredabout 40 μm, as shown in Figure 1c.To demonstrate that GO can be partially reduced during the

high-temperature (220 °C) process, the GO samples weresubjected to the same thermal treatment as the film fabricationprocess. The FTIR of GO before and after thermal treatment ispresented in Figure 2. Several significant changes occur afterthermal reduction because of the removal and reactions ofvarious functional groups. The spectrum of GO illustrates thepresence of C−O−C corresponding to cyclic ether moieties(vC−O−C at 1225 cm−1), C−OH (vC−OH at 1052 and 3400cm−1), and CO in carboxylic acid and carbonyl moieties(vCO at 1650 and 1720 cm−1).28,29 These results indicatethat GO is effectively oxidized. After thermal reduction, a largemajority of the hydroxyl groups (∼3400 and 1052 cm−1) areremoved. Most carbonyl moieties are removed, and a significantpeak at 1720 cm−1, attributed to CO stretching, decreasesafter the high-temperature process. Two closely overlappingpeaks are observed at ∼1650 and ∼1620 cm−1, attributed toCOOH and aromatic CC stretching, respectively.15 Thisresult is consistent with previous reports: reduction removes

the unstable COOH groups from the material, while theincreased sp2 hybridization pushes the CC peak from 1620to 1530 cm−1.29,30 FTIR spectra of GO and rGO show thatthermal reduction with relatively low temperatures couldeffectively remove the oxygen moieties from GO and partiallyrestore the sp2-hybridized structure.

Structure of Orientation rGO Nanosheets in thePVDF-HFP Matrix. The SEM image of the rGO/PVDF-HFPfilms shown in Figure 3 demonstrates that the rGO sheets are

well dispersed and highly aligned in the PVDF-HFP matrix.Large rGO sheets are arranged in parallel along the direction ofthe film surface. In the enlarged micrographs, the rGO sheetswith lamella dimensions in the range of 1−6 μm in the polymermatrix can be observed, where the good orientation of largerGO sheets and good compatibility between the polymermatrix and rGO are shown.The orientation of the rGO sheets and their parallel

distribution in the composite films are proved by LAXRD

Figure 1. (a) TEM of GO. (b) Photograph of flexible rGO/PVDF-HFP film (2.1 vol %). (c) SEM micrographs of the fractured surface ofrGO/PVDF-HFP film (2.1 vol %).

Figure 2. FTIR spectra of GO and rGO.

Figure 3. Fracture surface topography of rGO/PVDF-HFP film (2.1vol %).

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measurement, as shown in Figure 4.27,31 Composite films withrGO of 0.7 vol %, 1.8 vol %, and 2.1 vol % all display a peak

centered around 1.2 degrees, whereas PVDF-HFP displays nopeak. The appearance of the peak was believed to be caused bythe orientation of rGO sheets along the direction of the filmsurface. The spacing between pieces of rGO is around 7 nmaccording to the Bragg equation. The layers are separated byPVDF-HFP, leading to wider spacing between each layer.When the layers are planarly aligned, a crystal lattice is created,leading to low-angle XRD patterns. The small shift around 1.2could be caused by changes in contents, aggregation status, andorientations of rGO which could affect the intensity, position,and width of the peak of LAXRD.The aligned structure of the composite can be explained by

the fabrication process. The shear force provided by spin-coating, the thickness decreasing process, and thickness controlare believed to be the three factors responsible for thealignment of rGO nanosheets in the nanocomposite films. Thefirst one is the shear force provided by the spin-coating process.The shear force induces GO sheets to lie flat and orient alongthe direction of the film surface because of GO’s high aspectratio.32−34 Second, the rGO/PVDF-HFP film with a thicknessof about 40−60 μm is composed of six layers, each with athickness of about 6−10 μm. Thus, every single layer is verythin, and the thickness decreasing process caused by the spin-coating and the evaporation of solvent leads to the dimensionalconfinement of GO and facilitates its alignment. Finally, tosome extent, rGO nanosheets with diameters of severalmicrometers have limited space which hinders their remainingrandomly in a single film which helps the orientation of thelarge rGO sheets.In addition, the crystallinity of PVDF-HFP was slightly

inhibited by the presence of fully exfoliated rGO, as evidencedby the WAXRD measurements shown in Figure 5. The purePVDF-HFP sample shows three peaks at 18.4, 19.9, and 26.5corresponding to 020, 110, and 021 diffractions, respectively,indicating that the α-phase of PVDF-HFP has been formed.After dispersion of rGO in the PVDF-HFP matrix, the WAXRDpatterns of the rGO/PVDF-HFP composites show only thePVDF-HFP diffraction peaks, and the diffraction peak of rGOdisappears, clearly demonstrating the formation of fullyexfoliated rGO sheets in the polymer matrix.27 Theincorporation of rGO in the composite inhibits the formation

of α-phase PVDF-HFP during crystallization of the polymer, asindicated by the decrease in the relative intensity of peaks withincreasing contents of rGO.

Enhanced Dielectric Properties of the rGO/PVDF-HFPComposite. The rGO with homogeneous dispersion andorientation in the PVDF-HFP matrix effectively increased thedielectric constant of the polymer. Figure 6a shows thedielectric constant of the rGO/PVDF-HFP composite films as afunction of rGO volume fraction and frequency at roomtemperature. When the volume fraction of rGO is lower than0.8 vol %, the dielectric constant of composite films increasessteadily compared with that of pure PVDF-HFP. When thevolume fraction of rGO is higher than 0.8 vol %, the dielectricconstant of composite films dramatically increases and isapparently dependent on frequency. The dielectric constantrises from 54 (0.70 vol %) to 236 (0.84 vol %) at 100 Hz. Forcomposites filled with conductive fillers, the percolation theorystates that the variations of dielectric constant with frequencyfollow a power law as the filler content approaches thepercolation threshold.35 Thus, the dramatic increase in thedielectric constant can be explained by the percolation theory,as follows

ε ε= − <−f f f f( ) forsm c c (1)

where ε and εm are the dielectric constants of the compositesand polymer matrix; f and fc are the concentration andpercolation threshold concentration of the filler in the matrix,respectively; and s is the scaling constant related to the materialproperties. Equation 1 indicates that a large ε enhancement willbe obtained as f Graphene/fc = 1. The best fit of the dielectricconstant with the log−log plots of the power law gives fc = 0.8vol % and s = 0.54 according to eq 1, as shown in the inset ofFigure 6b. The percolation threshold, fc = 0.8 vol %, is evidentlydue to the extremely high aspect ratio of the graphene sheetsand their homogeneous dispersion in the composites. The largeenhancement in the dielectric constant at low frequencies ismainly attributed to the Maxwelle−Wagnare−Sillars interfacialpolarization due to the different conductivity of rGO andPVDF-HFP.36

As shown in Figure 6c, for composites with f > fc, the lossesare high at low frequencies, primarily owing to DC or quasi DCconduction caused by the percolation effect. As frequenciesincrease from 102 to 104 Hz, tan δ decreases due to the failureof the induced charges to follow the reversing field and thus

Figure 4. LAXRD patterns of rGO/PVDF-HFP composites with 0 vol%, 0.7 vol %, 1.8 vol %, and 2.1 vol % rGO.

Figure 5. WAXRD patterns of rGO/PVDF-HFP composites with 0vol %, 0.35 vol %, 0.7 vol %, and 2.1 vol % rGO.

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leading to reduced electronic oscillations. However, forcomposites with f < fc, the dielectric loss is lower than 0.4 at102−107 Hz, which is appealing to a lot of applications.36,37

rGO/PVDF-HFP has the edge in terms of dielectric loss overgraphene/PVDF and rGO/PVDF, which were previouslyreported to have losses of 1.4 (ε = 63) and 0.8 (ε = 90),respectively.15,27 The oriented arrays of rGO lead to a fewconductive paths between neighboring layers, resulting in a lowloss. For comparison, the dielectric constant and loss of rGO/PVDF-HFP composites fabricated by both the repeated spin-coating (SC) and traditional drop-coating (TDC) methods(the detailed fabrication processes of the composite film areshown in the Supporting Information), following heating, areshown in Tables 1 and 2. It can be seen that the SC samplespresent smaller loss than that of TDC rGO/PVDF-HFP. Theloss of the TDC sample of 0.7 vol % rGO is 126, and the loss ofthe SC sample of 0.7 vol % rGO is only 0.27. These resultsindicate that the conductive network has formed, and thepercolation threshold concentration of rGO/PVDF-HFP byTDC is less than 0.7 vol % ( fc < 0.7 vol %). The aligned rGOsheets increase the percolation threshold of the composite,

which increases to 0.8 vol %. In addition, the conductivity ofthe rGO nanocomposite is shown in Figure 6d. The electricalconductivity of the rGO-PVDF-HFP composite ( f > 0.8 vol %)is 3 orders of magnitude higher than that of pure PVDF-HFPbecause of the existence of the rGO sheets which have partiallyrestored the sp2-hybridized structure and recovered conductiv-ity ability.The increment of dielectric permittivity can be explained by

the Maxwell−Wagner−Sillars (MWS) effect. This effect isassociated with the entrapment of free charges between theinsulator/conductor interfaces and can be characterized by thefrequency dependence of the dielectric constant in the low-frequency range.36 For the rGO/PVDF-HFP nanocomposites,when f < fc, the dielectric constant is almost independent offrequency, indicating that there is insufficient accumulation ofinterfacial charges in the nanocomposites. When the f of GO isclose to fc, there are many rGOs separated by very thin PVDF-HFP layers, and the dielectric permittivity in the low-frequencyrange increases rapidly. A large number of charges are blockedat the interfaces between the rGO and PVDF-HFP matrixbecause of the MWS effect, which accounts for a remarkable

Figure 6. (a) Dependence of effective dielectric constant on frequency at room temperature. (b) Effective dielectric constant of the rGO/PVDF-HFP composites as a function of the rGO volume fraction, measured at 1000 Hz and 25 °C. The inset shows a log−log plot of the dielectric constantas a function of fc − f with the exponent s = 0.54 and a critical volume content fc = 0.8 vol % according to eq 1. (c) Dependence of dielectric loss onfrequency at room temperature. (d) Dependence of AC conductivity on frequency at room temperature.

Table 1. Dielectric Constant of rGO/PVDF-HFP Composites by Repeated Spin-Coating (SC) and Traditional Drop-Coating(TDC)

100 Hz (SC, TDC) 1 kHz (SC, TDC) 10 kHz (SC, TDC) 100 kHz (SC, TDC) 1 MHz (SC, TDC)

0.35 vol % 20 24 18 22 17 20 15 17 12 130.7 vol % 54 210 41 191 32 174 25 115 17 332.1 vol % 6368 - 825 322 159 521 99 503 57 445

Table 2. Loss of rGO/PVDF-HFP Composites by Repeated Spin-Coating (SC) and Traditional Drop-Coating (TDC)

100 Hz (SC, TDC) 1 kHz (SC, TDC) 10 kHz (SC, TDC) 100 kHz (SC, TDC) 1 MHz (SC, TDC)

0.35 vol % 0.09 0.1 0.05 0.07 0.06 0.08 0.11 0.13 0.25 0.250.7 vol % 0.27 126 0.18 14 0.18 1.7 0.2 0.7 0.3 12.1 vol % 367 - 233 313 109 30 19 3.3 4 0.6

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increase in dielectric permittivity with the increase of rGOcontent at low frequency.36

Dynamic Mechanical Thermal Analysis. In addition, thedynamic mechanical properties of the rGO/PVDF-HFPcomposite were measured in the temperature range −50−90°C at 1 Hz. Figure 7a presents the results of DMA tests which

show the temperature dependence of the storage modulus ofrGO/PVDF-HFP nanocomposites. The storage modulus ofneat PVDF-HFP was also measured and displayed forcomparison. The 2.1 vol % GO/PVDF-HFP has a greaterstorage modulus over the entire temperature range examined.The storage modulus increases from 1.7 × 108 (neat PVDF-HFP) to 5.6 × 108 (2.1 vol % rGO/PVDF-HFP) at roomtemperature (20 °C). The enhancement of storage modulus of2.1 vol % rGO/PVDF-HFP results from strong interfacialinteractions between the polymer and rGO platelets and thereduced mobility of polymer chains confined between rGO orbonded to rGO surfaces.38 These measurements also identifythe variation of glass transition temperature (Tg) with rGOloading, as shown in Figure 7b. The Tg of neat PVDF-HFP thatappears at about −3.3 °C is broadened, and the peak of rGO/PVDF-HFP shifts toward higher temperatures (−2.6 to 0.7°C), which are associated with the content of rGO. Thebroadening and increase of Tg observed in the nanocompositesare generally ascribed to the restricted segmental motions in theinterlayer spacing of the rGO.39 The reduced mobility ofpolymer chains also leads to a decrease of the value at the peakof tan δ with the increasing load of rGO.

4. CONCLUSIONIn conclusion, rGO/PVDF-HFP nanocomposite films withhigh dielectric constants and low dielectric loss have beenprepared by repeated spin-coating followed by a high-temperature modeling process. The process produced rGOsthat were fully exfoliated, sufficiently reduced, and aligned wellin the PVDF-HFP matrix. When the concentration of rGO was0.7 vol %, a dielectric constant of 54 and a loss of 0.27 wereobtained at 100 Hz. At the same time, the aligned rGOincreased the fc of composites by means of a repeated spin-coating process. The shear force provided by spin-coating, thethickness decreasing process, and thickness control are believedto be the key factors in the alignment of rGO nanosheets.These findings provide a simple and commercially viableprocess for producing well-aligned rGO-reinforced polymerswith advantageous dielectric properties and are also meaningfulin relation to the effects of aligned fillers on dielectricproperties.

■ ASSOCIATED CONTENT

*S Supporting InformationFabrication of rGO/PVDF-HFP composite films with repeatedspin-coating (SC) method; fabrication of rGO/PVDF-HFPcomposite films with traditional drop-coating (TDC) method;WAXRD patterns of graphite and graphite oxide; AFM of GO;and Raman spectra of GO. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: zyh@cugb.edu.cn. Phone: 86-10-82323433.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This study was jointly supported by the Key Project of theChinese Ministry of Education (no. 107023), the FundamentalResearch Fund for the Central Universities (nos. 2011PY0181,2011PY0180, 2010ZY46, 2010ZD08), the Special Fund forConstruction of Beijing Education Committee, the CityUniversity of Hong Kong Research Grant (no. 9360110), theNSFC (no. 21303169), the Fundamental Research Funds forthe Central Universities (no. 2652013115), and Beijing NovaProgram (no. Z141103001814064).

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Figure 7. DMA: variation in (a) storage modulus and (b) tan δ as afunction of temperature for composite films measured at 1 Hz.

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp411828e | J. Phys. Chem. C 2014, 118, 10567−1057310573

1 1

Supporting Information

Novel Method for the Fabrication of Flexible Film with Oriented Arrays of Graphene in

Poly(vinylidene fluoride-co-hexafluoropropylene) with Low Dielectric Loss

Wangshu Tonga, Yihe Zhang

*a, Li Yu

a, Xinglong Luan

a, Qi An

a, Qian Zhang

a, Fengzhu Lv

a,

Paul K. Chub, Bo Shen

a and Zhilei Zhang

a

aNational Laboratory of Mineral Materials, School of Materials Science and Technology,

China University of Geosciences, Beijing 100083, China

bDepartment of Physics & Materials Science, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong 999077, China

*E-mail: zyh@cugb.edu.cn. Phone: 86-10-82323433.

2 2

Table of Contents

1. Fabrication of rGO/PVDF-HFP composite films with repeated spin-coating (SC) method

2. Fabrication of rGO/PVDF-HFP composite films with traditional drop-coating (TDC)

method

3. Morphology and structure of GO (Figure S1)

4. Raman spectra of GO (Figure S2)

3 3

1. Fabrication of rGO/PVDF-HFP composite films with repeated spin-coating (SC)

method

rGO/PVDF-HFP composite films were prepared by a spin-coating process. Typical

procedures were as follows and are shown in Scheme 1: 1) the as-prepared GO/PVDF-HFP

solution was spin-coated on a glass substrate with spin speed of 1400 rpm; 2) the glass

substrate was kept in an oven at 80oC for 1h to evaporate the solvent slowly and GO plates

were fixed in PVDF-HFP; 3) GO/PVDF-HFP solution was spin-coated on the same glass

substrate used in 1) and then a second layer of GO/PVDF-HFP on the first one was obtained;

4) steps 1 and 2 were repeated five times and a GO/PVDF-HFP film with six layers was

obtained; and 5) the GO/PVDF-HFP film with six layers was kept in an oven at 80 o

C for 3h

and then at 220oC for 1 h. Finally, rGO/PVDF-HFP composite films were obtained.

2. Fabrication of rGO/PVDF-HFP composite films with traditional drop-coating (TDC)

method

The as-prepared GO/PVDF-HFP solution was drop cast on a glass plate and kept in an oven at

80 oC for 3 h to evaporate the solvent slowly to obtain the GO/PVDF-HFP composite films.

Then the film was kept in an oven at 220oC for 1 h and rGO/PVDF-HFP composite films (one

layer) were obtained by TDC.

3. Morphology and structure of GO

Figuer S1. (a) WAXRD patterns of graphite and GO. (b) and (c) AFM of GO.

GO was synthesized from flake graphite by the Hummers method, and its structures were

characterized by XRD (Figure S1a), AFM (Figure S1b and c), and raman spectra (Figure S2).

The XRD pattern of GO only shows a peak at 2θ = 10.3o, corresponding to the inter-planar

spacing of 8.6Å. The XRD patterns of GO reveal the transformation of the interlayer spacing

from 3.4 Å to 8.6 Å, indicating the incorporation of various functional groups in the sheet of

4 4

graphite. The expanded smooth XRD peak of the GO compared with the sharp peak of

graphite indicates the poor crystallinity of the oxidized graphite.1 AFM images show that the

GO is a flake structure with about 0.5-1μm of the dimensions of GO diameter, and about 1nm

in thickness (Figure S1c).

4. Raman spectra of GO

Figure S2. Raman spectra of GO.

The typical features of the raman spectra are the D band (about 1344 cm-1

) assigning to the

disorder-induced D band from the multiple photo scattering of defects or amorphous carbon

and the G band (about 1600 cm-1

) assigning to the stretching of conjugated double bonds

corresponding to sp2 hybridization.

2-3 The intensity ratio of D and G bands (ID/IG) is often

taken to characterize the number of defects and impurities in GO.4 The ID/IG=0.83 suggests

that the GO was successfully oxidized with function groups defecting the ordered structure of

sp2 hybridization.

[1] Chen, D.; Zhu, H.; Liu, T. X. In Situ Thermal Preparation of Polyimide Nanocomposite

Films Containing Functionalized Graphene Sheets. ACS Appl. Mater. Inter. 2010, 2, 3702-

3708.

[2] Yao, S. H.,; Yuan, J. K.; Zhou, T.; Dang, Z. M.; Bai, J. B. Stretch-Modulated Carbon

Nanotube Alignment in Ferroelectric Polymer Composites: Characterization of the

Orientation State and Its Influence on the Dielectric Properties. J. Phys. Chem. C. 2011, 115,

20011-20017.

[3] Yu, L.; Zhang, Y. H.; Tong, W. S.; Shang, J. W.; Shen, B. Lv, F. Z.; Chu, P. K. Green

Dielectric Materials Composed of Natural Graphite Minerals and Biodegradable Polymer.

RSC Advances. 2012, 2, 8793-8796.

[4] Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the

Properties of Graphene. Nat. Nanotechnol. 2013, 8, 235-246.