Reduced Graphene Oxide Coated Carbon

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Flexible Polyvinyl Alcohol/Reduced Graphene Oxide Coated Carbon Composites for Electromagnetic Interference Shielding Dengguo Lai, Xiaoxiao Chen, Xuejiao Liu, and Yin Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01499 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 22, 2018

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ACS Applied Nano Materials

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Flexible Polyvinyl Alcohol/Reduced Graphene Oxide Coated Carbon

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Composites for Electromagnetic Interference Shielding

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Dengguo Lai, † Xiaoxiao Chen, † Xuejiao Liu, †, ‡ Yin Wang †, *

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†

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Academy of Sciences, Xiamen 361021, China

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‡

Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese

University of Chinese Academy of Sciences, Beijing 100049, China

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ABSTRACT

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Flexible polyvinyl alcohol/reduced graphene oxide coated activated carbon (PVA/RGO@AC)

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composite films with extremely low graphene amounts were prepared by using AC as segregators

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and substrates. Decoration of AC with graphene to create an individual RGO sheet coated AC

12

structure leads to a dramatic increase in the conductivity of AC and effectively prevents the

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restacking and agglomeration of graphene. The percolation threshold of the PVA/RGO@AC

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composites is as low as 0.17 wt% for RGO@AC, and in particular, only 0.017 wt% RGO is

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needed. A high conductivity of 10.90 S/m and impressive electromagnetic interference shielding

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effectiveness (EMI SE) of 25.6 dB with an absorption-dominated mechanism are achieved for

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PVA/RGO@AC composites with a low RGO loading of 1.0 wt%. The specific EMI SE of the

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composite reaches 17.5 dB/mm, outperforming most of the reported pioneering graphene-based

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polymer composites with such low RGO amount. The excellent electrical property and outstanding

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EMI shielding performance are attributed to the internal well-constructed three-dimensional

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RGO-AC-RGO interconnected conductive network. Intriguingly, the fabricated composites exhibit

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a stable EMI SE even after 1000 bend-release cycles. These results demonstrate that our approach

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is a novel and promising method for producing highly conductive, high shielding performance and

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cost-effective materials with very low graphene loading.

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KEYWORDS: electromagnetic interference shielding, reduced graphene oxide, activated carbon,

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segregated structure, hybrid composites

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ACS Paragon Plus Environment

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1. INTRODUCTION

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Currently, the extensive development of electrical equipment and personal electronic devices

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generates ubiquitous electromagnetic radiation pollution that significantly affects the normal

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functioning of sensitive electronics and the health of human beings.

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made to develop highly efficient electromagnetic interference (EMI) materials for eliminating

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unwanted electromagnetic radiation.

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materials, electrically conductive polymer composites containing carbon-based nanofillers have

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attracted intense attention for EMI shielding applications due to their advantages of excellent

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processability, low weight, and corrosion resistance. 2, 6

5

1-4

Great efforts have been

As promising substitutes for the traditional metal-based

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Graphene possesses a variety of fascinating properties such as a large aspect ratio, low weight,

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and outstanding electrical, thermal, and mechanical properties and has therefore been widely

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investigated for fabrication of high-performance graphene-based EMI shielding materials.

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first graphene-based EMI shielding composite displayed an EMI shielding effectiveness (SE) of

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~21 dB for 15 wt% graphene loading in an epoxy matrix.

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devoted to developing graphene or reduced graphene oxide (RGO) based polymer composites

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(GPC) for EMI shielding. 8-12 Nevertheless, it has been observed that an EMI SE of approximately

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20 dB that is satisfactory for commercial applications is always found for a high loading of

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graphene (~5-30 wt%) in the polymer matrices, inevitably resulting in high production costs that

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restrict the practical applications of these composites. 13 Although many great improvements have

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been achieved, the facile preparation of GPC with high EMI shielding performance at low

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graphene loadings (< 5 wt%) is still challenging.

2

7

The

Since then, many studies have been

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The EMI SE and electrical conductivity of composites depend critically on the intrinsic

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electrical conductivity, aspect ratio, conductive filler content, and the dispersion of the fillers in the

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matrix and their interfacial compatibility. 14 To be suitable for commercial application, the volume

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electrical conductivity of the composite must typically be least 1 S/m. 5 Such a high conductivity

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can be realized through superior conductive interconnected networks that are achieved by

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graphene nanosheets (GNS) linking through face-to-face or edge-to-edge contacts. However,

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irreversible restacking of GNS occurs during the reduction of insulating graphene oxide (GO) to 2


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highly conductive graphene, and graphene has a strong tendency to aggregate in the polymer

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matrix due to its high surface area and lack of functional sites on the surface.

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and agglomeration significantly lower the effectiveness of graphene as a reinforcement for

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high-performance composites and strongly degrade the efficiency of the constructed conductive

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networks.

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loading. To fully tap the potential of graphene as a superior functional filler, the restacking and

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aggregation of GNS must be prevented, so that the properties of composites can be tailored to

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obtain the desired effects at low graphene loadings. 15, 20

18-19

15-17

The restacking

As a result, satisfactory conductivity and EMI SE always require a high graphene

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Some researchers have preferred to construct a segregated structure to improve the dispersion

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of graphene and enhance the conductive network formation in the polymer with a reduced fraction

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of graphene.

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the polymer granules, resembling a cell-like configuration. 22 Graphene was first used to construct

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segregated conductive networks in an ultrahigh molecular weight polyethylene matrix with a

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thickness of 2.5 mm, with an EMI SE of 28.3-32.4 dB obtained at a low graphene amount of 1.5

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wt%.

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45.1 dB was achieved with a graphene loading of 7 wt% due to its multi-faceted segregated

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architecture.

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nanocomposites, successfully obtaining an EMI SE of 63.2 dB at 4.7 wt% graphene with a

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thickness of 2.9 mm. 23 In these studies, they coated polymer powders with GO sheets and restored

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GO on the granules, so that graphene tends to be distributed only at the interfaces of the polymer

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granules, and the restacking of GNS was prevented to some extent. However, the poor interaction

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between the GO nanosheets and polymer granules leads to the weak adhesion of GO on the

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polymer surface and the shedding of graphene. The agglomeration of GNS at the polymer powder

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interfaces restricts molecular diffusion between the granules, resulting in the poor mechanical

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performance and unsatisfactory conductivity of these materials. 16, 24 Also, these polymer granules

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only act as segregators and do not contribute to the conductivity of the composites.

16

21

In this type of structure, graphene tends to be distributed only at the interfaces of

In another study, an RGO/polystyrene composite was fabricated and a high EMI SE of

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Recently, Sharif et al. develop segregated poly(methyl methacrylate)/RGO

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Thus, the conductivity of the composites can be improved much further by substituting

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conductive particles for polymer granules to form a segregated graphene architecture followed by

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dispersing the obtained complex filler into the polymer. Using this type of ternary composite not

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only dramatically reduces the loading of graphene by optimizing its inherent property but also 3



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enhances the formation of a continuous conductive network without impeding the molecular

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diffusion of the polymer.

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surface area, high intrinsic conductivity and low weight. The aromatic structure and large surface

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area of AC provide the opportunity for intensive π-π interactions between AC and GO sheets and

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the adsorption of GO on the AC surface for effectively preventing GNS restacking. 25 Additionally,

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good conductivity and multiple functional sites of AC facilitate the formation of a conductive

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network and good dispersion of the fillers in the polymer.

21-23

Activated carbon (AC) presents the characteristics of large specific

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This work reports an original study about the fabrication of flexible and low-cost

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graphene-based composite films with extremely low graphene amount for EMI shielding

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application. The restacking and agglomeration of graphene were effectively inhibited by

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decorating segregated single-layer graphene nanosheets on AC surface via a facile one-step

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method in which almost entirely individual GO nanosheets were coated on AC and then reduced in

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situ. The segregated RGO-coated AC (RGO@AC) complex with an excellent electrical

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conductivity and flexible PVA/RGO@AC EMI shielding composite films with functionalities of

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high conductivity and remarkable EMI shielding performance with very low graphene loading

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were realized using an environmentally friendly all-aqueous casting method. The resultant flexible

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composite films with only 1.0 wt% graphene exhibit a high conductivity of 10.90 S/m and an

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average EMI SE of 25.6 dB, greatly lowering the production cost of the composites and showing a

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competitive high EMI SE compared to pure graphene-based EMI shield at an RGO loading of 10

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wt%. In addition, the optimization of RGO and AC ratio, a comprehensive study of the

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fundamental shielding mechanism, the effects of filler content and sample thickness, and the

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reliability of composites under mechanical deformation were conducted to well present the

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feasibility and superiority of this kind of processing method and composites.

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2. EXPERIMENTAL SECTION

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2.1 Preparation of RGO@AC and RGO nanofillers

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Graphene oxide (GO) was synthesized according to a modified Hummers method, and the 26-28

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synthesis process is described in detail in the Supporting Information.

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procedure for the RGO@AC nanofillers was as follows: GO suspension was diluted in deionized

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(DI) water by stirring and ultrasonication for 30 min at room temperature to obtain a homogeneous 4


The preparation

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dispersion with individual GO nanosheets. Then, the calculated quantity of wood-based activated

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carbon (AC, supplied by Jiangxi New Taisheng Carbon Technology Co., Ltd., size below 10 µm,

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shown in Figure S1 in the Supporting Information was added to the GO suspension and vigorously

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stirred for an additional 2 h at room temperature to form a GO-coated AC (GO@AC) suspension.

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An in situ reduction was carried out to convert GO to RGO with L-Ascorbic acid at 90 oC for 4 h.

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The RGO@AC was finally obtained by filtering and was dried in a vacuum oven at 80 oC

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overnight. To further restore the RGO and increase the conductivity of the nanofillers, thermal

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annealing was performed in a tube furnace by heating from room temperature to 800 oC at a rate of

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7 oC/min and then holding at 800 oC for 20 min under high-purity nitrogen flow. RGO was

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similarly prepared via the same method as that used for RGO@AC but without adding AC.

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Various RGO@AC with different RGO weight fractions of 2.5%, 5%, 10%, and 20% were

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fabricated and designated as RGO@AC-X, where X is the content of RGO in the RGO@AC

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complex.

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2.2 Fabrication of PVA composite films

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Polyvinyl alcohol (PVA, polymerization degree 1799, purchased from Aladdin Industrial

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Corporation) was dissolved in DI water at 90 oC for 2 h to give a 10 wt% solution. Then, a

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calculated mass of the RGO@AC complex was added to the PVA solution and stirred for another 2

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h to form a homogeneous solution. Finally, the uniformly mixed solution of the PVA/RGO@AC

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was poured onto a polystyrene petri dish and kept at 60 oC in a vacuum oven for film formation

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until its weight equilibrated. This film was then peeled off from the substrate and cut for testing. A

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series of PVA/RGO@AC composites with different RGO@AC loadings were prepared in a similar

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manner. For comparison, PVA/AC and PVA/RGO composites were also prepared by a similar

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route. The thickness of the composites was controlled by the content of the solutions.

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2.3 Material characterizations

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The X-ray diffraction (XRD) patterns of the fillers and PVA composites were recorded in the

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range of 2θ = 5o-80o using an X-ray diffractometer (X’ Pert Pro, Panalytical) with Cu Kα radiation

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at 30 kV and 15 mA. Raman spectra were obtained using a LabRAM Aramis spectrometer (Horiba

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Jobin Yvon S.A.S., France) with a laser at a wavelength of 532 nm. Scanning electron microscopy

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(SEM) observations were carried out using a field emission SEM (Hitachi S-4800) at an

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accelerating voltage of 5 kV. The volume electrical conductivity above 10-6 S/m of the fillers and 5



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composite films was measured using a classical four-point probe instrument (ST2263, Suzhou,

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China), and the conductivity below 10-6 S/m was measured using a resistivity meter (ZC-90C,

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Shanghai, China). EMI shielding measurements were performed at room temperature over the

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frequency range of 8.2-12.4 GHz (X-band) with a vector network analyzer (Agilent Technologies

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N5244A PNA, America) using the waveguide method, and the samples with different thicknesses

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were cut into small pieces with dimensions of 22.9 × 10.2 mm2 to fit well into the waveguide

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holders. To determine the shielding components for the composites, the scattering parameters, S11

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and S21 were obtained. The power coefficients of reflectivity (R), transmissivity (T), and

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absorptivity (A) were calculated from the measured scattering parameters, and their relationship

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was described as R + A + T = 1. The values of SE absorption (SEA) and SE reflection (SER) were

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thus determined as follows:

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R = |S11|2, T = |S21|2

(1)

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A = 1 – R – T

(2)

SER = –10 log (1–R), SEA = –10 log (T/(1–R))

160 161 162 163 164 165 166 167 168

(3)

SETotal is the sum of the SE contributions due to absorption, reflection, and multiple reflection. SETotal = SEA + SER + SEM

(4)

When SETotal > 10 dB, SEM can be neglected 29-31, and it was generally assumed that SETotal ≈ SEA + SER

(5)

The effective absorbance (Aeff), with respect to the power of the incident electromagnetic wave inside the shielding material is described as 32-33 Aeff = A / (A+T)

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3. RESULTS AND DISCUSSION

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3.1 Characteristics of conductive fillers and PVA composites

(6)

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Figure 1 comparatively illustrates the synthetic routes of the conventional RGO and the

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restacking-inhibited RGO-coated AC complex (RGO@AC). Both synthetic routes start from the

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GO suspension in which GO nanosheets are almost entirely exfoliated, and this homogeneous

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suspension can be stably stored. For conventional RGO, the direct reduction of GO easily leads to

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the inevitable and severe restacking of graphene (Figure 1b) due to the intermolecular van der 6


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Waals forces between layers of graphene nanosheets.

By contrast, a simple water solution

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mixing is first used to prepare GO-coated AC (GO@AC) in which the exfoliated GO sheets

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closely adhere to the AC surface. Then, the GO-covered AC structure is retained during the

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subsequent in situ reduction, and thus, the RGO-coated AC complex is readily obtained. The

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restacking of GNS during reduction is effectively inhibited by introducing AC particles as

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substrates to separate the GNS, and RGO with a segregated structure is constructed (Figure 1c).

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The AC surfaces are almost entirely covered with GNS, and no distinct graphene stacking is

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observed. Therefore, the novel and facile one-step process of coating GO on AC and in situ

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reduction effectively prevents GNS from restacking. The possible mechanism underlying this

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phenomenon is attributed to the strong π-π interaction between the GO nanosheets and the

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aromatic group structures of AC, as well as the intensive adsorption induced by the abundant pores

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and large surface area of AC. The closely covered GNS on AC are observed as a wave-structured

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gauze connecting the AC particles to form small conductive clusters. AC pores (Figure 1a and

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Figure S1) are still clearly visible on the surface of RGO@AC even through the coated GNS

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(Figure 1c), demonstrating that the decorated RGO is found in the form of ultrathin individual

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sheets or in a few layer morphology. The final conductive complexes are fabricated by dispersing

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RGO@AC in the PVA polymer, and the obtained composite films can be bent and twisted many

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times as shown in Figure 1d, confirming their good flexibility.

7



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Figure 1. Schematic fabrication illustration of the nanofillers and PVA composites. SEM images

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of (a) AC, (b) RGO and (c) RGO@AC-10 complex at different magnifications. (d) Optical graphs

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of PVA/RGO@AC composite film with 10 wt% filler loading, presenting a high level of bending

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and torsion deformations.

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The surfaces and cross-sections of PVA/AC, PVA/RGO and PVA/RGO@AC-10 composite

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films at 10 wt% total filler content are illustrated in Figure 2. The image of the PVA/AC film

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surface indicates the homogeneous dispersion of AC in the PVA polymer (Figure 2a) due to the

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multiple functional groups on AC. However, the large surface area and lack of functional sites lead

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to the agglomeration of RGO in PVA, as observed in Figure 2b. Astoundingly, the filler of the

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RGO@AC-10 complex is regularly dispersed in the PVA matrix as shown in Figure 2c,

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demonstrating that the introduction of AC enhances the interfacial adhesion between the

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RGO@AC-10 and the polymer and overcomes the problem of RGO agglomeration. It is necessary

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to note that the RGO@AC-10 complex is distributed in the form of continuous small clusters,

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constructed through linking the AC particles by the covered GNS as depicted in Figure 1c. The

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cross-sections of the PVA/AC film shown in Figures 2d and 2g further confirm the good dispersion

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of AC throughout the entire polymer. In Figures 2e and 2h, the cross-sections of the PVA/RGO 8


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composite clearly present the stacked layers of RGO, making it difficult to efficiently disperse

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RGO in the PVA polymer. The polymer matrix only connects the outer layer and edges of the

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stacked RGO, while the tightly stacked inner layers hamper the entry of the polymer. As a result, a

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large amount of RGO is necessary to construct a conductive network in the polymer. As for

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PVA/RGO@AC-10, the filler is well-dispersed throughout the polymer according to the images of

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the surface and the cross-section presented in Figures 2c, 2f and 2i. Surprisingly, an examination at

217

further magnification finds that the coated thin graphene on AC interconnects through face-to-face

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or edge-to-edge links over large distances serving as bridges to form electrical conductive

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pathways (Figures 2j and 2k), while conductive AC is present as substrates or piers to link the

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coated GNS over small distances. These images support the formation of a three-dimensional (3D)

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RGO-AC-RGO well-constructed conductive network structure in the composites that is the origin

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of the high electrical conductivity and EMI shielding performance.

9



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Figure 2. Typical SEM images of (a-c) surfaces and (d-i) cross-sections of PVA/AC, PVA/RGO

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and PVA/RGO@AC-10 composites at different magnifications for the filler loading of 10 wt%.

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The area circled in red in (k) indicates the connected RGO forming the electrical conductive

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network.

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X-ray diffraction (XRD) is an important tool for determining whether GNS are indeed present 10


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as individual graphene sheets in the complex and composite. 20 Figure 3a shows the XRD patterns

230

of RGO, AC and RGO@AC with 10 wt% RGO loading. The typical diffraction peak of RGO is

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observed at approximately 2θ = 26.1o, indicating the restacking of GNS to form a stacked layer

232

structure or a graphite structure (graphite powders, 2θ = 26.5o). However, the XRD pattern of

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RGO@AC-10 only shows a broad AC diffraction peak that is similar to that of AC, whereas the

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sharp diffraction peak of RGO disappears. The XRD results clearly demonstrate that AC coated by

235

individual GNS is achieved and the stacking of graphene during reduction is effectively prevented

236

by incorporating AC. Similar XRD results were obtained for nanofillers with other RGO loadings

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from 2.5 wt% to 20 wt%. In Figure 3b, the peak of pure PVA appears at 2θ = 19.8o. The XRD

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pattern of PVA/RGO not only shows a similar PVA diffraction peak but also presents the typical

239

peak of RGO that is the same as that of pure RGO, indicating the stacking structure of RGO in the

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PVA polymer. This result is in good agreement with the results shown in Figures 2e and 2h.

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However, after RGO@AC is dispersed into the PVA matrix, the XRD pattern of the

242

PVA/RGO@AC-10 composite only shows a PVA diffraction peak that is the same as that of

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PVA/AC, whereas the characteristic diffraction peak of RGO disappears. This finding further

244

proves that RGO is well-dispersed as individual GNS on AC and in the final PVA matrix.

245 246

Figure 3. XRD patterns of (a) graphite, RGO, AC and RGO@AC-10 powders and (b) composites

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of PVA, PVA/RGO, PVA/AC and PVA/RGO@AC-10 with the total filler loading of 10 wt% and

248

thickness of 0.8 mm.

249

3.2 Electrical conductivity of fillers and composite films 11



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It is well-known that formation of a well-established conductive network is of the utmost

251

importance for achieving improved EMI shielding that critically depends on the intrinsic electrical

252

conductivity and dispersion of conductive fillers. The electrical conductivity values for the

253

RGO@AC complexes with different RGO weight fractions are shown in Figure S2 (Supporting

254

Information). The conductivity of RGO@AC improves substantially with the increase in the RGO

255

content. For instance, the conductivity increases significantly from 37 to 1051 S/m, corresponding

256

to an enhancement of nearly 28 times, when the RGO content increases from 0 to 20 wt%. This

257

high conductivity of the RGO@AC complex and significant improvement are attributed to the

258

high intrinsic conductivity of coated graphene and the formation of conductive clusters through the

259

linking of AC by wave-structured GNS. Additionally, the removal of oxygen-containing groups on

260

the RGO and the graphitic sp2 network of graphene restored by annealing is another important

261

reason for the greatly enhanced conductivity, as confirmed by Raman analysis of RGO and

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RGO@AC-10 before and after annealing presented in Figure S3 and the related discussion in the

263

Supporting Information.

264

S/m due to the annealing, showing a significant improvement by nearly 4 times and thus further

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revealing the effect of annealing. The conductivity values of RGO@AC-20 and RGO@AC-10 are

266

1051 S/m and 758 S/m, respectively, corresponding to 32.1% and 23.1% of the conductivity of

267

pure annealed RGO. It is clear that RGO@AC-10 presents a more remarkable enhancement in the

268

conductivity and better performance than the other RGO@AC complexes in manifesting the high

269

intrinsic conductivity of graphene when compared based on the unit loading of RGO, as displayed

270

in Table S1 (Supporting Information). Furthermore, the inset in Figure S2 also shows that the

271

increase in the conductivity tends to be gentle when the RGO loading exceeds 10 wt%. Thus,

272

taking the RGO loading in the final PVA composite, the electrical conductivity and the production

273

cost into consideration, RGO@AC-10 is chosen for the fabrication of the composite films. It

274

should be noted that this highly conductive RGO@AC is an universal filler which can be easily

275

and well dispersed in other kinds of polymers, not only PVA, to gain some specific properties of

276

flame retardance and tolerance against high humidity and temperature.

35

Moreover, the conductivity of pure RGO increases from 605 to 3278

277

Figure 4 shows the variation of the electrical conductivity of the PVA/RGO and

278

PVA/RGO@AC-10 composites with filler weight fractions. An increase by nearly 10 orders of

279

magnitude is observed for the conductivity as the RGO and RGO@AC-10 filler content increases 12


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from 0 to 10 wt%, indicating a typical percolation behavior. The power-law equation: σ = σ0 (m-mc)

281

t

282

content, where σ is the electrical conductivity of the composites, σ0 is a scaling factor, m is the

283

mass fraction, mc is the percolation threshold, and t is the critical exponent.

284

result obtained using the power-law equation is presented in the inset of Figure 4. The fitted mc is

285

0.28 wt% for RGO, while a lower value of 0.17 wt% is found for RGO@AC-10; these values are

286

comparable to most of the reported percolation threshold values of GPC.

287

threshold value is believed to be due to the large aspect ratio of graphene and the well-formed

288

conducting RGO region in the matrix. The even lower percolation threshold of PVA/RGO@AC-10

289

is ascribed to its better dispersion compared to the conventional RGO and the efficient formation

290

of the RGO-AC-RGO highly conductive network in the polymer matrix. It should be noted that the

291

threshold value for the fabricated composites is only 0.017 wt% RGO when calculated on the

292

RGO basis, which is among the lowest values reported in the literature for GPC with segregated

293

structure. 22, 36, 41 Thus, it is elucidated that RGO with a similar segregated architecture is found in

294

the PVA/RGO@AC composite. In addition to the low percolation threshold, we observe that the

295

fitted value of t is 2.31 and 3.17 for RGO and RGO@AC-10, respectively, which are higher than

296

the universal critical exponents (1.1-2.0) derived from the classical conduction model.

297

large aspect ratio of RGO and its special geometry (wave-structured graphene linking AC and 3D

298

RGO-AC-RGO conducting network) are thought to be responsible for the high critical exponent

299

and are in good agreement with other reports in the literature. 8, 22, 39, 43 A similar calculation for the

300

PVA/AC composites is shown in Figure S4 (Supporting Information), and a much higher

301

percolation threshold of 2.5 wt% and a lower t value of 1.5 are obtained because of the low aspect

302

ratio and limited conductivity of AC.

was then used to evaluate the relationship between the electrical conductivity and the total filler

14, 36-37

23, 38-40

The best fit

Such a low

22, 42

The

303

In addition, the PVA/RGO@AC-10 composites present a superior electrical conductivity (2-6

304

times higher) than that of PVA/RGO at the same total filler content, demonstrating that

305

PVA/RGO@AC-10 composites construct a better conductive network. An electrical conductivity

306

of 1.47 S/m is obtained when filled with 5 wt% RGO@AC-10, which is comparable to that of the

307

composite with 10 wt% RGO loading (1.77 S/m). Notably, the RGO amount in the PVA composite

308

of the former is only 0.5 wt%, which is only one-twentieth of the latter. Therefore, the properties

309

of the newly synthesized RGO@AC-10 complex are comparable to or even better than those of the 13



310

pure conventional RGO for forming a continuous conductive network, and the RGO amount is

311

decreased considerably, exhibiting the advantage of low production cost. A further increase in the

312

RGO content to 1.0 wt% results in the conductivity of 10.83 S/m, far surpassing the target

313

conductivity (1 S/m) for highly efficient EMI shielding applications. The conductivity obtained in

314

this work is among the highest previously reported for GPC, especially for such low graphene

315

loadings, as shown in Table S2 (Supporting Information). It should be highlighted that the

316

fabricated PVA/RGO@AC-10 composites have an outstanding advantage of a very low RGO

317

amount (< 1.0 wt%) even though the total filler content is similar to those of the others. Therefore,

318

it is concluded that the conductive interconnected structure of RGO-AC-RGO in the polymer

319

matrix is composed of the interconnecting GNS and well-dispersed RGO@AC, while the

320

irreversible restacking and agglomeration of RGO significantly lower the effectiveness of

321

graphene as a superior conductive filler and are likely to be responsible for the inferior

322

conductivity of the obtained PVA/RGO composites.

323 324

Figure 4. Electrical conductivity of PVA/RGO and PVA/RGO@AC-10 composite films as a

325

function of the filler content. The dimensions of the testing sample are 22.9 (length) × 10.2 (width)

326

× 0.8 (thickness) mm3.

327

To further elucidate the formation mechanism of the conductive network in PVA/RGO@AC

328

composite, Figure 5 compares the electrical conductivity of PVA composites filled with RGO,

329

RGO@AC-10 complex, AC, and a mixture of AC and RGO. At 1.0 wt% RGO loading, the

330

electrical conductivity of PVA/RGO@AC-10 (10.83 S/m) is at least three orders of magnitude

331

higher than that of PVA/RGO (0.0039 S/m). Intriguingly, by adding 9 wt% AC into the PVA/RGO 14


Page 14 of 29

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332

composite, the conductivity of PVA/RGO/AC with 1.0 wt% RGO and 9 wt% AC increases

333

noticeably by a factor of 180 to 0.71 S/m. The addition of AC increases the effective RGO

334

concentration in the polymer using the volume exclusion method and thus enhances the formation

335

of a conductive network, more importantly, this remarkable improvement in conductivity is

336

ascribed mainly to the interconnecting of RGO by the added conductive AC. 44-45 It is important to

337

note that the conductivity of PVA/RGO@AC-10 is at least one order of magnitude higher than that

338

of PVA/RGO/AC at the same RGO amount of 1.0 wt% and total filler content of 10 wt%, further

339

proving the superiority of the RGO-coated AC structure in exploiting the intrinsic outstanding

340

conductivity of graphene and realizing the well-formed RGO-AC-RGO conductive network in

341

PVA/RGO@AC via the coated ultrathin GNS serving as bridges to facilitate charge transfer over

342

large distances while conductive AC serves as a pier to link GNS through charge transport over

343

small distances.

344 345

Figure 5. Comparison of the electrical conductivity of PVA composites filled with RGO,

346

RGO@AC-10, AC and mixture of AC and RGO at different total filler contents and RGO

347

amounts.

348

3.3 EMI shielding performance of PVA composites

349

The EMI SE values of the PVA/RGO@AC-10 composites with various filler contents were

350

determined in the X band (8.2-12.4 GHz) with the results shown in Figure 6. The dimensions of

351

the testing sample are 22.9 (length) × 10.2 (width) × 0.8 (thickness) mm3. All of the composites

352

exhibit a weak frequency dependence across the measured frequency range, allowing the average

353

EMI SE to be used for evaluating the EMI shielding effect. It is observed that shielding by the 15



354

PVA/RGO@AC-10 composite increases with increasing filler content, which is attributed to the

355

increase in the electrical conductivity. The average EMI SE of pure PVA is only 0.15 dB. Similarly,

356

the average EMI SE values of the PVA/RGO@AC-10 composites with 5 wt% and 10 wt% loading

357

are 8.4 dB and 14.0 dB, respectively, indicating that 85.5% and 96.0% of the electromagnetic

358

radiation is blocked by the shielding material. The RGO amount of these PVA/RGO@AC-10

359

composites is one-tenth of the corresponding total filler content, that is, only 0.5 wt% and 1.0 wt%

360

RGO is present in the composites with 5 wt% and 10 wt% total filler content, implying a high

361

performance of RGO by constructing the RGO-AC-RGO conducting structure. The outstanding

362

EMI shielding performance of these thin films (0.8 mm) at such low RGO loading (

Reduced Graphene Oxide Coated Carbon

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