carboxylated graphene with enhanced dielectric performance for large energy density capacitor

Accepted Manuscript Nanocomposites of poly(vinylidene fluoride) - Controllable hydroxylated/carboxylated graphene with enhanced dielectric performance for large energy density capacitor Cheng Yang, Si-Jia Hao, Sheng-Long Dai, Xiao-Yan Zhang PII:

S0008-6223(17)30245-2

DOI:

10.1016/j.carbon.2017.03.004

Reference:

CARBON 11811

To appear in:

Carbon

Received Date: 5 August 2016 Revised Date:

1 March 2017

Accepted Date: 1 March 2017

Please cite this article as: C. Yang, S.-J. Hao, S.-L. Dai, X.-Y. Zhang, Nanocomposites of poly(vinylidene fluoride) - Controllable hydroxylated/carboxylated graphene with enhanced dielectric performance for large energy density capacitor, Carbon (2017), doi: 10.1016/j.carbon.2017.03.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Nanocomposites of Poly(vinylidene fluoride) - Controllable Hydroxylated/Carboxylated

Graphene

with

Enhanced

Dielectric Performance for Large Energy Density Capacitor Cheng Yang*, Si-Jia Hao*, Sheng-Long Dai, Xiao-Yan Zhang

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Research Center of Graphene Applications, Beijing Institute of Aeronautical Materials, Beijing 100095, China. *Corresponding authors. E-mail: [email protected] (Cheng Yang), [email protected] (Si-Jia Hao)

Abstract

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In this research, we have successfully prepared hydroxylated and carboxylated graphene (GROH and GRCOOH) with controllable functionalized level by means of

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diazonium addition was successfully prepared, and the dielectric properties of polyvinylidene fluoride (PVDF) composites fabricated via a simple solution-cast and hot-pressing method are examined. Tunable dielectric properties, low percolation threshold (0.20 wt%) and large energy density (up to 17.33 J·cm−3 of GROH-50) in the

polymer-composites

polymeric

composites

of

PVDF/GROH

and

PVDF/GRCOOH suggests strong chemical bonding are formed between the filler and

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polymer matrix. Hydroxyl groups, which act as an electron donor, is believed to improve the dielectric properties when compared to carboxyl groups with electron accepting nature, thus GROH/PVDF presents better dielectric behavior (not only high

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dielectric constant, but also low dielectric loss) than GRCOOH/PVDF. Such polymer composites consisted of functionalized graphene can be used to store charge and electrical energy and play a key role in modern electronics and electric power

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systems.

1. Introduction

High-dielectric-constant materials have drawn increasing attentions of

scientists and engineers worldwide since they are becoming the base for fabricating high performance products in many cutting-edge fields, e.g., embedded capacitors, sensors, gate dielectrics, and electric energy storage devices applied in the electronics and electrical industry.1 Owing to their flexibility, ease in processing and high dielectric strength, polymeric dielectric materials has gained special interests among those dielectric materials.2

ACCEPTED MANUSCRIPT However generally the intrinsic dielectric constant of polymer is lower than 10, still far lower than that of non-polymer composites such as dielectric ceramics, which seriously hinders its development for dielectric applications.3–5 In addition to many studies performed to improve the electromechanical actuation and to examine the mechanism of the high dielectric constant, numerous works

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have been carried out on the preparation of composites with superior dielectric properties (high dielectric constant and low dielectric loss), excellent mechanical strength, and a low percolation threshold. An effective way to is to construct polymer-based percolative composites, which can be synthesized by

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loading conducting nanofillers into the polymer matrix, giving rise to the promotion of the dielectric constants of polymer, while retaining their excellent mechanical properties. The commonly used conducting fillers are carbon

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black6, carbon nanotubes (CNTs)7,8, conducting ceramic nanopowders9,10, and metal nanoparticles11–13, all of which utilizes the phenomenon of percolation threshold to sharply raise the electrical conductivity and dielectric permittivity of the composites at considerably low filler concentrations.14 These heterogeneous inclusions, even if they are nonpolar, often cause heterogeneous

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dielectric polarization as a result of the accumulation of a virtual charge at the interface of two media with different permittivities or conductivities. Though, these polymer percolative composites, fabricated by a combination of conductive fillers within the polymer matrix, possess a remarkably high

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dielectric permittivity at low filler concentrations, but their dielectric loss is also considerably high, impeding their use as effective dielectric materials.

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Moreover, usually the interfacial bonding between the polymer matrices and conducting fillers is not good enough. Therefore a mismatch of properties between filler and polymer due to poor interfacial bonding can lead to the poor mechanical properties. CNTs is once considered as a revolutionary conductor owing to its unique

structure and outstanding integrated properties.7,8 However, the combination of a large surface area and a high aspect ratio with attractive van der Waal interaction forces presented in CNTs makes them tend toward aggregated bundles.10,15 Until recently, the discovery of graphene − an atomically thick two-dimensional (2D) sheet composed of sp2 carbon atoms arranged in honeycomb structure – has been regarded as effective nanofillers for polymer-

ACCEPTED MANUSCRIPT graphene percolative composites because of its extraordinarily high surface area, unique graphitized planar structure, low manufacturing cost and outstanding physical properties.16–21 One major challenge in the preparation and dispersion of graphene is to overcome its easily irreversible agglomeration or even restacking as results of strong van der Waals interaction and the out-of-

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plane π bond between the individual graphene nanosheet (GN). However for the functionalized graphene such as graphene oxide (GO), oxygen-containing functional groups like epoxy, hydroxyl, carboxy and carbonyl groups are distributed either on the basal plane or on the periphery of the single atom-thin

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sheet. The presence of these functional groups makes GO sheets be strongly hydrophilic, which allows GO to swell and disperse in various polar solvent readily, exhibiting much improved miscibility with polymer matrix compared

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to GNs.16,20,21 Consequently, the aggregation scenario of GNs can be reduced by the attachment of other molecules or polymers onto the sheets. Moreover, the oxygen functional groups on the GO surface provide electrical insulation and versatile sites for further chemical functionalization, improving the compatibility, and enhancing the dispersion of GO in polymeric matrices.22

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Nevertheless, it’s it is well known that on the GO the more oxygen-containing group sites are, the lower conductance it would be. Therefore the miscibility and resistance will be both increased when the functionalization of GNs is performed, which would give rise to contradicted effects on the dielectric

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behaviors of the composites.

Therefore, here we presented a simple method to synthesize the hydroxylated

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and carboxylated graphene (GROH and GRCOOH) with controllable level of functionalization by means of diazotizing route. Different from the GO which possesses uncertainty of quantity of various oxygen-containing functional group, these functionalized GNs features feature almost identical functional groups on the basal plane of nanosheets, the properties of which is are easy to be evaluated. In addition, the hydroxyl and carboxyl group is regarded as electron donating and electron accepting nature, respectively, which is expected to give diverse structure when GROH or GRCOOH is composited with polymer matrices, resulting in diverse mechanical and dielectric properties. To the best of our knowledge, dielectric composites composed of GROH or GRCOOH and polymer have not been reported yet, and the dielectric properties

ACCEPTED MANUSCRIPT of PVDF were expected to be significantly improved with the addition of GROH or GRCOOH nanofiller into the PVDF matrix. In this research, the structure and crystallization behavior of GROH and GRCOOH is was characterized, and the conductance as well as the dielectric property of GROH/PVDF and GRCOOH/PVDF composites were also investigated.

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2. Experimental Section 2.1 Synthesis of GO

Figure 1 illustrates the synthesis progress of hydroxylated and carboxylated

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graphene by means of diazotization route. Prior to the functionalization, GO was synthesized by oxidizing natural graphite flake in the manner of a modified

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Hummers method.23 Natural graphite flake (400 mesh, 99.9%, Qingdao Tiansheng Graphite Co., Ltd.) was put into a mixed solution of concentrated H2SO4, NaNO3, and KMnO4 while keeping in the temperature range of 10−15 o

C for 2 hours under continuous stirring and with the help of an ice bath,

followed by raising temperature to 35 oC and stirring for 30 minutes. The obtained mixture was diluted with deionized (DI) water, and the oxidation

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process was maintained for another 2 hours while controlling the temperature below 100 oC. Shortly after the dilution with DI water again, H2O2 was slowly added into this mixture, and the color of which became brilliant yellow along with bubbling. The mixture was then filtered as soon as possible and washed

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several times to remove the acidity with DI water. Finally, the GO was

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successfully synthesized after drying in the vacuum oven at 80 oC.

ACCEPTED MANUSCRIPT Figure 1. Schematic representation of the attachment of hydroxyl and carboxyl groups on graphene nanosheets, R represents −CH2CH2OH or –CH2COOH groups. 2.2 Synthesis of GROH and GRCOOH The hydroxylation of graphene was realized by means of diazonium addition employing

as-prepared

GO

as

starting

materials

and

2-(4-

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reaction

aminophenyl)ethanol (C8H11NO, >98%, Tokyo Chemical Industry Co., Ltd.) as diazotizing agents. GO was ultrasonically dispersed in DI water for 30 minutes in order to form a 0.1 g L−1 solution with stable suspension, and the pH of this

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solution was adjusted to around 8−9 by addition of ammonia solution. N2H4·H2O was then added and the solution was refluxed at 95 oC for 24 hours,

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followed by addition of appropriate amount of C8H11NO and isoamyl nitrite (C5H11NO2, >95%, Tokyo Chemical Industry Co., Ltd.), and then refluxed once again at 80 oC for additional 24 hours. The resultant solution underwent a dialyzing process to achieve neutrality and subsequently a freeze drying process, the successfully obtained black powder was GROH. The similar synthesis method was adopted to synthesize the GRCOOH with taking 4-

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Aminophenylacetic acid (C8H9NO2) as diazotizing agent.

Here in this research, in order to examine the effect of the functionalization, a series of GROH samples with different level of hydroxylation was prepared, where the precisely controlled amount of 2-(4-aminophenyl)ethanol was

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considered to be completely consumed in the functionalization reaction. In the light of this ideal consideration, the molar ratio between successfully

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functionalized –OH group and benzene ring on the GR sheet was expected to be 0.05, 0.15, 0.25, 0.50, 0.75 and 1.00, which was correspondingly labelled as GROH-05, GROH-15, GROH-25, GROH-50, GROH-75 and GROH-100, respectively. Similarly the sample GRCOOH-100 was prepared which means appropriate amount of 4-Aminophenylacetic acid was added to enable averagely one –COOH group to attach on each benzene ring in the GR sheet. 2.3 Preparation of GROH/PVDF and GRCOOH/PVDF The GROH/PVDF and GRCOOH/PVDF composites were prepared following a conventional method. With the help of sonication, certain amount

ACCEPTED MANUSCRIPT of GROH was fully dispersed in N,N-Dimethyl formamide (DMF, 99.5%, Sinopharm Chemical Reagent Co., Ltd.), to which polyvinylidene fluoride (PVDF, 99.5%, Sinopharm Chemical Reagent Co., Ltd.) was then added under heating and stirring to achieve solution with stable suspension. This solution was subsequently dispersed in ethanol and vacuum filtration was performed in

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order to remove DMF, and the GROH/PVDF composite was obtained by heating the resulted mixture at 40 oC in the oven for 24 hours. In order to investigate the effect of GROH filler on the dielectric performance, a series of GROH-100/PVDF composite were made with a filler concentration of 0.01%,

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0.05%, 0.10%, 0.15%, 0.20% and 0.25% were fabricated, which was labeled as 0.01 wt.%-GROH-100, 0.05 wt.%-GROH-100, 0.10 wt.%-GROH-100, 0.15 wt.%-GROH-100,

0.20

wt.%-GROH-100,

and

0.25

wt.%-GROH-100,

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respectively. Similarly, GRCOOH-100/PVDF composites with different weight content of GRCOOH-100 were prepared and marked with 0.01 wt.%GRCOOH-100, 0.05 wt.%-GRCOOH-100, 0.10 wt.%-GRCOOH-100, 0.15 wt.%-GRCOOH-100, 0.20 wt.%-GRCOOH-100, and 0.25 wt.%-GRCOOH100, respectively. In addition, a series of GROH/PVDF composites employing

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aforementioned GROH with different hydroxylation level (GROH-05, GROH15, GROH-25, GROH-50, GROH-75 and GROH-100) were fabricated, and in all the composites the weight content of GROH was precisely controlled to be 0.20%.

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2.4 Characterization

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FTIR spectra were measured with a Bruker Tensor27 FTIR spectrometer in the range of 4000−400 cm−1 with the resolution of 4 cm−1. Prior to the FTIR measurements, the GROH and GRCOOH powder sample samples was were mixed with KBr and pressed in the form of pellets. XPS patterns of the asprepared GROH and GRCOOH samples were obtained using an ESCALab250 electron spectrometer from Thermo Fisher Scientific with monochromatic 150 W Al-Kα radiation (1486.6 eV). Pass energy for the narrow scan is was 30 eV, and the base pressure was about 6.5×10−10 mbar. The binding energies were referenced to the C1s line at 284.8 eV from alkyl or adventious carbon, and the XPS results were analyzed with Avantage software version 4.15. The elemental

ACCEPTED MANUSCRIPT analysis (combustion method) of carbon, nitrogen and hydrogen was carried out, where the residual content was considered to be oxygen since GROH and GRCOOH samples are were simply composited by CHNO. The morphology of the cross sectioned composites was observed by FEI Nova NanoSEM 50 with an accelerating voltage of 15 kV. The dielectric

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properties of the as-prepared GROH/PVDF and GRCOOH/PVDF composites were examined as a function of frequency from 102 Hz to 107 Hz using an impedance analyzer (Agilent 4294A Precision Impedance Analyzer) at room temperature. Each GROH/PVDF or GRCOOH/PVDF composite was hot-

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pressed at 180 oC (pressure ~ 3.0 MPa) into cylindrical pellets of about 10.0 mm diameter and 5.0 mm thickness. Copper leads were connected to pellets of these composites on both sides using silver paste and used for dielectric and

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conductivity measurements. The electric breakdown strength was tested by a dielectric withstand voltage test (YD2013, Changzhou Yangzi Electronic Co., Ltd) with 6–8 specimens tested to increase the accuracy.

3. Results and Discussion

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3.1 Structure and Crystallization Behavior of GROH and GRCOOH

ACCEPTED MANUSCRIPT Figure 2. Comparison of FTIR spectra obtained for GROH-100 (upper curve in red) and 2-(4-aminophenyl)ethanol (lower curve in blue), and the molecule structure of 2-(4-aminophenyl)ethanol is illustrated in the inset. The hydroxylated and carboxylated aryl groups were attached on the graphene sheets by diazonium addition reaction, and the process is illustrated in

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the Figure 1. FTIR spectra of GROH-100 and 2-(4-aminophenyl)ethanol (C8H11NO) are shown in the Figure 2, it’s it is found that more absorbance bands of C8H11NO are presented than that of GROH-100. The band for C8H11NO at 3374 cm−1 is the stretching vibration band of −OH group, while the

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broad peak at 3186 cm−1 is assigned to the stretching vibration band of C−H group both in the benzene ring and in the −CH2− groups. The characteristic band observed at 1256 cm−1 is attributed to the C−N group in the C8H11NO,

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whereas the absence of such band in the GROH-100 indicates the elimination of C−N groups, thus concluding that no excessive amount of C8H11NO is left in this GROH-100/PVDF composite since the functionalization of graphene is completed. In addition, the characteristic bands of benzene ring around 1500 cm−1 and 1600 cm−1 are not evidenced in the spectrum of GROH-100, which suggests the conjugated system is constructed when the benzene rings are 24–26

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connected to the graphene surface, resulting in the vanish of those feature bands

Figure 3 shows the difference of the FTIR spectrum between 4acid

and

GRCOOH-100.

In

the

spectrum of

4-

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Aminophenylacetic

Aminophenylacetic acid, peaks around 3413 cm−1 are assigned to the O−H and N−H stretching vibration, and bands observed in the range of 2600−3100 cm−1

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is originated from the stretching vibration bands of C−H (both in the benzene ring and −CH2−) and O−H. In addition, peaks around 1690 cm-1 and 1592 cm-1 is assigned to the stretching vibration bands of C=O and C=C, and the scissoring of −CH2− is observed at 1380 cm-1, where the formation vibration of O−H is also located. The peaks in between 1000-1200 cm-1 could be associated to C−C and C-O vibration. It’s It is clearly seen that all these aforementioned peaks are also weakly distributed in the spectrum of GRCOOH-100, however, the absence of C−N stretching around 1256 cm−1 in the spectrum of GRCOOH100 confirms that the graphene is carboxylated effectively since no evidence of C−N group is presented in the GRCOOH-100. The appearance of N−H

ACCEPTED MANUSCRIPT stretching in the GRCOOH-100 is attributed to the existence of reducing agent N2H4 employed in the synthesis. Nevertheless, the absence of C=C stretching vibration band at 1516 cm−1 in the GRCOOH may be subjected to the formation of conjugated system since the benzene rings are completely connected with

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graphene sheets.

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Figure 3. Comparison of FTIR spectra obtained for GRCOOH-100 (upper curve in red) and 4-Aminophenylacetic acid (lower curve in blue), and the molecule structure of 4-Aminophenylacetic acid is illustrated in the inset.

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The XPS spectra were analyzed to elucidate the surface composition of GROH and GRCOOH. As shown in Figure 4(a), C1s and O1s transitions are the main features in the XPS survey scan of GROH-100. In addition to the observation of C KLL and O KLL Auger electron emissions in the region of high binding energy, extremely weak N1s peak at 398.4 eV suggests that nitrogen is nearly exhausted in the reaction, demonstrating the hydroxylation is completed. Figure 4b 4(b) shows the close-up view of the C1s spectrum acquired from GROH-100 with deconvolution into four Gaussian peaks corresponding to C−C (284.8 eV), C−O (286.1 eV), −C=O (287.6 eV) and −COO− (288.6 eV) bonds, respectively.22,25,27 The contributions of C−C and

ACCEPTED MANUSCRIPT C−O bonds are much larger than those of −C=O and −COO− groups, indicating the hydroxyl groups are successfully functionalized onto the graphene nanosheets. Meanwhile the evidenced trace level of −C=O and −COO− bonds suggests residue amount of carbonyl and carboxyl groups is left in GROH-100 since the GO is taken as the starting material. The peak intensities ratio between

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−COO− and C−O contribution in the Figure 4(b) is regarded as a measure of molar ratio between carboxyl and hydroxyl groups, which is 1:19, suggesting the amount of −COOH is quite low and the reduction of GO is almost fully achieved.

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In the case of GRCOOH-100, weak N1s peak in Figure 5(a) suggests the carboxylation is complete. The excellent deconvolution result of four Gaussian peak with different bonds are shown in the Figure 5(b), assigned to be the C−C

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(284.8 eV), C−O (285.3 eV), −C=O (286.3 eV) and −COO− (289.3 eV) groups, respectively. Here it should be noted is worthy to note that the ratio of measured peak intensities between the −COO− and C−O is 1.02, which is slightly larger than the ideal case (1), demonstrating GO is reduced effectively

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with extremely low level of additional −COOH groups left.

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Figure 4. (a) XPS survey scan of GR and GROH, and (b) high resolution C1s XPS spectrum between 282 and 292 eV of GROH-100. The deconvolution of the spectrum comprises four Gaussian peaks corresponding to C−C (284.8 eV), C−O (286.1 eV), −C=O (287.6 eV) and −COO− (288.7 eV) groups. To better confirm the successful hydroxylation of graphene and investigate the content of hydroxyl groups, the XPS C1s spectra of GROH-05, GROH-15, GROH-25, GROH-50, GROH-75 and GROH-100 were measured and the

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deconvolution of each spectrum was performed by employing the same method aforementioned in the figure 4(b) and 5(b), and the results are shown in Figure

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6. Each deconvolution shows excellent fitting and contains four Gaussian contributions centered at identical position, which indicates C−C (284.8 eV), C−O (285.3 eV), −C=O (286.3 eV) and −COO− (289.3 eV) groups, respectively, again demonstrating the feasibility and availability of such analysis method.

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Figure 5. (a) XPS survey scan and (b) the close-up view of the region of carbon 1s edge between 282 and 292 eV of GRCOOH-100. The data points are indicated in squares and the deconvoluted curve fit is given by the thick line. The deconvolution of the spectrum comprises four Gaussian peaks corresponding to C−C (284.8 eV), C−O (285.3 eV), −C=O (286.3 eV) and −COO− (289.3 eV) groups.

ACCEPTED MANUSCRIPT According to the discussion above, the C−O bond in the XPS spectra of GROH is mainly contributed by hydroxyl group, and the peak intensity ratio between C−O and C−C contributions in each deconvolution is considered to represent the atomic ratio of hydroxyl group and carbon, which means the average number of functionalized −OH groups on the graphene surface can be

Figure

7,

it’s

it

is

found

that

the

with

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derived easily for each GROH sample. The calculated results are plotted in slight

amount

of

2-(4-

aminophenyl)ethanol introduced, content of −OH groups of GROH-05 is increased sharply to 0.2938. However, such increment becomes moderate when

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the amount of functionalized agent is further added, and the value reaches the maximal value of 0.4227 in the case of GROH-100 sample. To better investigate the composition of these hydroxylated graphene, Figure 7 exhibits

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the variation of expected value of –OH/carbon ratio as well as the atomic ratio of oxygen/carbon obtained from elemental analysis for the GROH samples, and obvious disagreements are observed among these three datasets. The expected content of –OH is much depressed when compared with the others since the graphene without any oxygen-containing groups are considered as the starting

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material, whereas the residual oxygen-containing functional groups survived in the reduction are not taken into account in this exact situation. The results obtained from XPS are much larger than that from elemental analysis, which can be explained by the surface sensitive nature of XPS, and elemental analysis

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is believed to give more information on the bulk state. As Figure 1 shows, the hydroxyl groups are deemed to attach on the in-plane of graphene nanosheet,

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thus the –OH content on the surface is higher than that in the bulk. Another possible reason is that certain part of the C–O intensity in the XPS measurements may stem from carboxyl groups, which gives rise to the calculated value while employing the XPS results. In a word, the variation of – OH content which best describes the actual case for this series of GROH samples is the expected value that calculated in the synthesis.

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Figure 6. The deconvolution of measured XPS C1s peaks for sample GROH05 (a), GROH-15 (b), GROH-25 (c), GROH-50 (d), GROH-75 (e), and GROH100 (f) between 282 and 292 eV.

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Figure 7. The derived –OH or oxygen content obtained from expected value in the synthesis (solid square), XPS C1s deconvolution (solid triangle) and

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elemental analysis (solid circle) for the GROH samples.

ACCEPTED MANUSCRIPT 3.2 Dielectric properties of GROH-100/PVDF and GRCOOH-100/PVDF composites The dielectric constant is considered to be a measure of the amount of energy from an external electrical field stored in the material, while the dielectric loss is a measure of the amount of energy dissipated in the dielectric material due to

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an external electric field. To investigate the dielectric properties of the GROH100/PVDF composite, the devices were fabricated with the structure of silver paste/composite/silver paste and measured with an impedance analyzer, and the results were evaluated to find the optimal composition for GROH-100/PVDF

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composite.

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Figure 8. Dielectric constant (a) and dielectric loss (b) as a function of frequency for a series of GROH-100/PVDF composites with various weight percentage of GROH-100 at room temperature, and (c) shows the dependence of the dielectric constant and dielectric loss of the GROH-100/PVDF composites on the GROH-100 mass fraction, measured at room temperature and 100 Hz.

ACCEPTED MANUSCRIPT Figure 8(a) and 8(b) shows respectively the dielectric constant and dielectric loss of GROH-100/PVDF composites with increasing GROH-100 content from 0.01 wt% to 0.25 wt% at room temperature, over a frequency range of 102 to 107 Hz. For better understanding, the comparison of dielectric constant and dielectric loss of all samples obtained at frequency of 100 Hz is illustrated in

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Figure 8(c). As shown in Figure 8(a), the dielectric constant of each GROH-100/PVDF composite decreases with increasing frequency, which can be explained by the polarization relaxation occurring at the inner structure of composites, including

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interface polarization and dipole orientation polarization. The Maxwell– Wagner effect indicates that the electric charge of a composite will accumulate at the interface because of the different conductivities of each component in an

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alternating electric field.28 As the schematic diagrams shown in Figure 9, since quantities of hydroxyl groups are attached to the basal plane of GROH-100 after the hydroxylation treatment, when the GROH-100 nanosheets are mixed with PVDF matrix polymer, hydrogen bond will form between the fluorine atoms of PVDF molecular chains and the –OH groups on the GROH-100

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nanosheets.29–32 As a result, stronger interface interaction between GROH-100 nanosheets and PVDF matrix is produced. Moreover, the non-polar nature of PVDF and the polymer molecular chains hinder the contribution of electrical polarization.

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The dielectric loss of GROH-100/PVDF composite measured in the frequency range from 102 to 107 Hz at room temperature is shown in Figure

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8(b). The dielectric loss presents two different dielectric behaviors: at low frequency regions the dielectric loss decreases while suddenly increases at high frequency ranges. The low-frequency dielectric response is a typical interface polarization behavior, resulted from macroscopical heterogeneous interface of the different diphase structures. On the contrary, the high-frequency dielectric response is a Debye relaxation behavior derived from the C–F dipole orientation polarization of PVDF matrix.33 In the Figure 8(c), where the variation of the dielectric permittivity and dielectric loss of GROH-100/PVDF composite at 100 Hz is shown, one can clearly see that as the GROH-100 contents raise, the dielectric constants of the composite are first increased, then slightly declined, which is quite different

ACCEPTED MANUSCRIPT from the conventional percolative composites. This phenomenon could be understood by the percolation theory, where the dielectric properties of the conductive nanofiller/polymer composites depend on the capacitance of the micro-capacitors formed by GROH-100 nanosheets as plates and PVDF as matrix, 34–36 and the microstructures of different GROH-100/PVDF composites

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with increased GROH-100 mass fractions are schematically illustrated in the Figure 10. Since pristine graphene features flexible nature and high surface energy, the neighbour graphene nanosheets tend to aggregate rather than suspend in the PVDF matrix uniformly, thus the average thickness of PVDF

considered to be small.

37

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matrix between two micro-capacitor plates is large and the capacitance is As mentioned above, the hydroxylation to the

graphene nanosheets can effectively improve the interfacial bonding between

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the GROH-100 nanosheets and the PVDF matrix, and the attached hydroxyl groups are able to prevent these nanosheets from easy aggregating and improve the filler–matrix bonding. The microstructure of GROH-100/PVDF composites in low GROH-100 weight content is illustrated in the Figure 10(a), in which the micro-capacitors are separated from one another. When the GROH-100 is just

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introduced into PVDF matrix, some micro-capacitor structures are formed because of the addition of a small amount of GROH-100, as the content of GROH-100 increases to 0.10 wt%, both the decreased thickness (d in the Figure 10) of the dielectric layer and the raised number of micro-capacitors formed by

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GROH-100 plates and PVDF matrix will lead to the enhanced dielectric constant, which is shown in the Figure 10(b). However, as the GROH-100

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content climbs over the critical value, due to the Van der Waals force and π‒π interaction among the GROH-100 nanosheets in excessive amount, still the hydroxylated graphene nanosheets in the matrix are inclined to aggregate and contact each other to form conductive paths 38,39, as shown in the Figure 10(c). According to the micro-capacitor model, the expected number of capacitors in the composites will be significantly reduced because of the aggregations of the GROH-100 nanosheets, causing dielectric constants to be reduced. The variations of the dielectric properties with frequency ranging from 102 to 107 kHz for GRCOOH-100/PVDF composite are also studied and the results are shown in Figure 11, here the percolation threshold of GRCOOH-100/PVDF is also found to be 0.20 wt%. However the observed dielectric constant/loss of

ACCEPTED MANUSCRIPT GROH-100/PVDF is larger/lower than that of GRCOOH-100/PVDF with identical weight content, and it agrees with the fact that electron donating groups (−OH) contribute more than the electron withdrawing groups (−COOH) to the improvement of dielectric properties.40-42 Therefore, GROH shows superior properties to GRCOOH from the perspective of dielectric materials,

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and the GROH/PVDF composite with same GROH weight content but GROH

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in different –OH content will be evaluated in the next section.

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Figure 9. Schematic diagrams of the formation of hydrogen bond in GROH/PVDF composites.

ACCEPTED MANUSCRIPT Figure 10. Schematic representations of micro-structures of GROH-100/PVDF composites with different mass fraction of GROH-100, (a) below 0.10 wt%, (b) between 0.10 wt% and 0.20 wt%, and (c) above 0.20 wt%. 3.3 Resistivity, breakdown strength and energy storage of GROH/PVDF composite with different –OH content

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Before the dielectric properties of GROH/PVDF composites with different GROH are examined, the resistivity of this series of GROH samples is measured and the results are shown in the Figure 12. It can be clearly seen that with the rising amount of hydroxyl groups, the resistivity of GROH exhibits a

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relatively moderate increment from GROH-05 to GROH-50, and such increment becomes quite sharp for the sample GROH-75 and GROH-100. This

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phenomenon is mainly attributed to the formation of conjugated system induced by the attachment of hydroxyl groups, strongly blocking the transport of electrons, resulting in the conversion from conductor to insulator behavior, which could be referred to Figure 10. Therefore in contrast to GROH-75 (8807.0 Ω·m) and -100 (13760.0 Ω·m), GROH-15 (187.755 Ω·m), -25 (435.91 Ω·m) and -50 (627.12 Ω·m) gives resistivity which are in the same order of

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magnitude, which shows reasonable and suitable potential for practical use. However, the hydroxyl group makes the polarity of GROH become increased, which means more –OH is, better miscibility of GROH in organic solvents becomes. Therefore if these two opposite effects are taken into account,

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GROH-50 is believed to be the best candidate for compositing with PVDF due to its most appropriate resistivity and miscibility. Here it’s it should be noted

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that the existence of hydroxyl groups on the surface of GROH nanosheets which act as inter-particle barriers can effectively prevent the formation of conductive networks.43 Therefore, for the GROH-100/PVDF composites as Figure 8(c) shown, much more GROH-100 nanosheets than pristine graphene can be filled in the composites before forming conductive networks in the composites (about 0.20 wt%).37,39,43

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Figure 11. Dielectric constant (a) and dielectric loss (b) as a function of frequency for a series of GRCOOH-100/PVDF composites with various weight content of GRCOOH-100 at room temperature The dielectric constants and the loss of GROH/PVDF composite as a function of the GROH with different −OH content are measured over the frequency range of 102 Hz to 107 Hz at room temperature, and the results are shown in Figure 13. It should be noted that the weight content of GROH in this

ACCEPTED MANUSCRIPT series of GROH/PVDF composites is controlled to be 0.20 wt% in order to achieve the best dielectric behavior on the basis of results and discussion above. It can be seen that the dielectric constant measured at 100 Hz raises with increasing –OH content, and the maximal value of nearly 500 (which is 70 times larger than that of GR-05/PVDF) is achieved for the sample GROH-

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50/PVDF, afterwards the dielectric constant is declined for the GROH75/PVDF and GROH-100/PVDF composites. The similar behavior is observed for dielectric loss, and the maximal value about 0.7 at 100 Hz is evidenced for the case of GROH-50/PVDF. This interesting phenomenon can be easily

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understood with one fact that GROH-50 possesses moderate resistivity, as discussed in the last section. In addition, another reason for this scenario is that GROH-50 shows better miscibility in PVDF, which is clearly evidenced in the

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Figure 14. The cross sectioned image of GO/PVDF exhibits typical brittle facture and significant agglomeration of GO in the GO/PVDF composite. In contrast to that, the cross sectioned image of GROH-50/PVDF composite exhibits ductile rupture like “anchor”, which indicates the formation of a strong chemical bonding, where the formation mechanism scheme are shown in Figure

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ACCEPTED MANUSCRIPT Figure 12. The variation of resistivity as a function of –OH content for the GROH samples, and the inset gives the close-up view of the low content region. To

further

determine

the

dielectric

properties,

room-temperature

polarization-electric field (P-E) loops of PVDF, GROH-15/PVDF, GROH-

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50/PVDF and GROH-100/PVDF were carried out at 1 kHz and the results are shown in Figure 15. As shown in Table 1, with the introduction of the GROH, the breakdown strength of the composites sharply increases to a maximum of 199 kV mm−1 for GROH-50/PVDF. Such increase in breakdown strength is

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attributed to the increase in the regions around the GROH where the polymer chains are tightly bonded to the GROH. The mobility of the polymer chains is thus decreased, which reduces the probability of charge carriers transferring

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through the loose polymer chains not bonded to the GROH. Despite the careful preparation of the composites, the loading of GROH in the polymer composite inevitably introduces structural defects, such as microvoids, which leads to the concentration of the local electric field. When the –OH content of GROH is beyond GROH-50, more microvoids are introduced and lead to further field enhancement, resulting in a decrease of the breakdown strength. Similar trends

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in the dependence of the breakdown strength on the volume fraction or the weight content of dielectric fillers, i.e., the maximum value of breakdown strength at a low volume fraction, have also been observed by other

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researchers.44–46

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Figure 13. Frequency dependence of dielectric constant (a) and dielectric loss (b) for GROH-05/PVDF, GROH-15/PVDF, GROH-25/PVDF, GROH50/PVDF, GROH-75/PVDF, and GROH-100/PVDF composites at room temperature, the weight percent of GROH in each composite is 0.20%. The inset in figure (a) is the close-up view of the region of low frequency.

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Fig. 14 SEM images of cross-sectioned GO/PVDF (a) and GROH-50/PVDF (b)

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Table 1. Properties parameters of GROH/PVDF composites (weight content of GROH is 0.20%) obtained from P-E loops measurement in Figure 14 GROH15/PVDF

GROH50/PVDF

GROH100/PVDF

PVDF

Dielectric constant b)

5.44

42.90

98.91

13.91

Dielectric loss b) Breakdown strength (kV mm−1) Energy density (J cm−3)

0.06

0.16

0.501

0.161

63

88

199

125

0.096

1.47

17.33

0.96

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Performance a)

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Notes: a) sample thickness is 20 µm; b) frequency is 1 kHz Theoretically, the energy storage density of a dielectric material could be calculated by the following equation, ܷ = 1ൗ2 ߝ଴ ߝ௥ ‫ܧ‬௕ ଶ

(1)

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where ܷ , ߝ଴ , ߝ௥ , and ‫ܧ‬௕ is energy density, vacuum permittivity, relative dielectric constant (8.85×10−12 C2 N−1 m−2) and breakdown strength,

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respectively. Obviously the higher dielectric constant and breakdown strength will lead to a higher energy storage density. With increasing the −OH content of GROH, the breakdown strength and the dielectric constant firstly increases, and then decreases. The combined effect of both the dielectric constant and the breakdown strength results in a maximum value of the energy density for 0.20 wt% GROH-50/PVDF. As shown in Table 1, it’s it is clearly seen that the electric properties of GROH-50/PVDF presents the best behavior among all the composites, which gives the breakdown strength of 199 kV mm−1 and dielectric constant of 98.91. It’s It is of great importance that the energy storage density of GRPH-50/PVDF could reach 17.33 J cm−3, which promotes over 170 times

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dielectric property.

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Figure 15. Room-temperature polarization-electric hysteresis loops of PVDF, GROH-15/PVDF, GROH-50/PVDF, GROH-100/PVDF, the weight content of GROH in each GROH/PVDF composite is 0.20 wt%. The good dielectric performance of GROH/PVDF composites should be is

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probably attributed to (i) the high aspect ratio of the GROH nanosheet and the resulting increased interfacial area induced by the good dispersion of GROH in

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the PVDF matrix as well as the improved interface, which benefits the formation of more micro-capacitors; (ii) the different electric conductivity between the nanosheets and matrix entrap the free charges to accumulate in the interface; (iii) the attached hydroxyl groups on the graphene surface act as a shell to keep the sheets easily contact from each other directly, as shown in Fig. 8c. The well dispersion of GROH nanosheets in PVDF matrix and the electrical barrier layer induced by the functionalized hydroxyl groups on the sheet should can reduce the dielectric loss effectively. These results indicate that the new possibility can be created for graphene to be employed in the field of dielectric application, GROH/PVDF composites

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4. Conclusions In this research, GROH with different –OH content and tuneable dielectric property as well as GRCOOH were successfully synthesized by means of diazotizing

method,

and

dielectric

properties

of

GROH/PVDF

and

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GRCOOH/PVDF composites were examined. The results indicate that the polar functional groups in the functionalized graphene materials greatly affects the dielectric properties of functionalized graphene/PVDF composites, where

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electron donating groups (−OH) favors enhancement of dielectric property more than electron withdrawing groups (−COOH). The dielectric behavior of series of GROH-100/PVDF or GRCOOH-100/PVDF composites follows the percolation theory, and the maximal dielectric constant at percolation threshold around 0.20 wt% is improved several times compared with un-composited PVDF. The content of –OH in the GROH greatly influences the conductivity,

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which consequently affects the dielectric properties of GROH/PVDF composites seriously. In the case of GROH/PVDF with same amount of GROH (GROH-05, GROH-15…) introduced, GROH-50/PVDF presents superior

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performance than others (the maximal value observed at 100 Hz is 70 times greater than that of GROH-05) since it shows considerable resistivity and gives great dispersion in the organic substrate PVDF. These researches are very

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important for directing the synthesis and applications of graphene-based dielectric materials.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant Number 50903079) and Aviation Science Foundation of China (2015ZF21009).

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