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CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

Ceramics International 41 (2015) 983989

A facile and eco-friendly synthesis of graphenesilver hybrid materialsfor transparent conductive lms

Munkhshur Myekhlaia, Sinil Leea, Taejin Leea, Hanshik Chungb,1, Hyomin Jeongb,n,1

aDepartment of Energy and Mechanical Engineering, Gyeongsang National University, Cheondaegukchi-Gil 38, Tongyeong, Gyeongnam 650-160,

Republic of KoreabDepartment of Energy and Mechanical Engineering, Institute of Marine 10 Industry, Gyeongsang National University, Cheondaegukchi-Gil 38,

Tongyeong, Gyeongnam 650-160, Republic of Korea

Received 5 August 2014; received in revised form 2 September 2014; accepted 2 September 2014Available online 18 September 2014

Abstract

Recently, graphenesilver (GNAg) hybrid material has attracted a great attention to substitute indium tin oxide (ITO) and/or uorine tin oxide(FTO) in the transparent conductive electrodes (TCE) due to their multifunctional extraordinary properties. However, the synthesis process of thishybrid material requires some toxic chemicals and complicated processes. Herein, we report a simple, an efcient and an environment-friendlyapproach to synthesize the graphenesilver nanoparticles (GNAgNPs) hybrid material for transparent, conductive and exible lms, which canfurther be used in various modern optoelectronic devices. The morphological and structural characteristics of hybrids are studied via severaltechniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Ramanspectroscopy. The results of these analysis conrmed that silver AgNPs were successfully decorated on the surface of GN. Furthermore, the sheetresistance of the exible lms with this hybrid structure was signicantly lower (10-fold) than that of the GN-only deposited lms at the widerange of transmittance.&2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Graphene-silver nanoparticles (GN-AgNPs) hybrid material; Silver nanoparticles (AgNPs); Transparent conductive lm (TCF)

1. Introduction

Transparent conductive lm (TCF) is one of the keycomponents in the modern technologies including touchpanels, organic light emission diode (OLED), solar cells andso on [13]. High technology-electronic devices requireoptically transparent and electrically conductive lms that

possess several unique properties, namely lightweight, exibleand economically efcient[4,5]. Until now, indium tin oxide(ITO) and uorine tin oxide (FTO) have been commonly usedas TCE materials owing to their excellent conductivity andhigh transmittance[68]. However, there are some drawbackssuch as brittleness structure, high cost and limited availability

in the use of ITO and FTO. Due to these issues, researches onnding alternatives that can replace the traditional materialshave been progressively growing [9,10]. Various structuressuch as carbon nanotubes (CNTs)[11,12], graphene (GN) andits derivatives[1315], metallic nanostructures[16,17]as wellas their hybrid materials [1820] have been intensivelyinvestigated as the alternatives to ITO and FTO. Among them,

composite material based on GN and AgNPs has beendemonstrated as one of the most promising candidates becauseof its unusual properties[21,22].

GN[23], which is a hexagonally arrayed single atom thicktwo dimensional layers of sp2-bonded carbon, has attractedsignicant attention from both the scientic and industrialcommunities due to its special physical and chemical proper-ties including unique mechanical stiffness, excellent conduc-tivity and high optical transparency [2427]. On the otherhand, AgNPs have been used in various research elds such as

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nCorresponding author.E-mail address: hmjeong3184@gmail.com(H. Jeong).1Tel.: 82 55 640 3184.

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heat transfer uids[28], antibacterial applications[29], TCEs[17]and solar cells [30]because of their several outstandingproperties including high conductivity, good transparency andspecial plasmonic effect. In comparison to single structures ofGN or AgNPs, their hybrid materials exhibit considerablybetter properties due to the following several reasons: (i)

aggregation is decreased by intercalating nanoparticles betweenGN sheets; (ii) GN-based hybrid materials show novelsynergistic properties [3134]. Therefore, hybrid materialscomposed of GN and AgNPs exhibit excellent combinationalproperties of electrical conductivity, optical transparency andgood stability [3539]. The sample studies of GN/AgNPshybrid as TCEs are as follows.

Recently, Chen et al. [36]prepared hybrid materials basedlms by co-percolating silver nanowires (AgNWs) networkbetween GN sheets. As a result, this hybrid structure providedrobust, scalable, low-cost as well as high performance TCEs.Tien et al.[37]fabricated TCEs based on graphene nanosheets(GNs) decorated with AgNPs. They found that the electricalconductivity of this electrode was signicantly improvedbecause of AgNPs decreased aggregation of GNs. Moreover,a number of studies have investigated the GN/AgNPs hybridmaterials as promising candidates to the next generation TCEs.[21,22,31,32,3537]. However, the synthesis method of hybridmaterials based on GN and AgNPs contains various issuessuch as high cost, long time consumption, the use of toxicchemicals and complicated process. For this reason, it isimportant to develop a cost effective; a simple and eco-friendly method to prepare the GN/AgNPs hybrid based lms.

In the present study, we rst synthesized GN/AgNPs hybridmaterials in an aqueous solution using a pulsed power wire

(PWE) evaporation one-step method. Then, the exible lmswere prepared with the GN/AgNPs via spin coating technique.The thicknesses of the lms were simply adjusted by the dropsof GN/AgNPs solution. By using this method, 10-fold decre-ment in the sheet resistance of GN-only lms was observedafter AgNPs grafted on GN. Furthermore, we anticipate thatsuch exible TCE based on GN/AgNPs can be used in variousmodern devices.

2. Experimental details

2.1. Materials

GN nanopowder (AO-2) with 8 nm ake (2030 mono-layers), average particle (lateral) size 550 nm and purity-

99.9% was purchased from Graphene supermarket, USA. Ag

wire with a diameter of 0.2 mm was received from NanoTechnology inc. Korea. Polyethylene terephthalate (PET) lmwas obtained from Hanseung Techno Co., Ltd, Korea.Distilled water (DI water) was used as a base uid.

2.2. Surface modication of GN

A schematic diagram of experimental process is described inFig. 1. The surface modication of GN was performed using aplanetary ball milling (PBM) (Fig. 1a and b). PBM (HPM-700)

was made by Haji Engineering, Korea. Based on our previousstudy [40], samples were ground at a grinding speed of500 rpm under wet condition for 1 h. Details of grindingprocess was described in the previously published literatures[40,41].

Fig. 2. Flexible PET lm.

Fig. 1. Experimental general schema of preparation of GNAg composite: (a) pristine GN, (b) ground GN, (c) GN suspension in H 2O, (d) GNAgNPs suspension,

(e) GNAgNPs hybrid and (f) GNAgNPs hybrid after thermal treatment.

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2.3. Preparation of GNAgNPs composite

First, the ground GN was dispersed in DI water underultrasonication for 1 h (Fig. 1c). Then, the AgNPs weresynthesized directly in the previously prepared GN-aqueoussolution using PWE method (Portable Nano Colloid Maker,

NTI-miniP, Nanotechnology Inc., Korea) (Fig. 1d). Thedetailed description of PWE method can be found in ourprevious study[28]. The weight ratio of GN and AgNPs waschosen based on the reference literature [37]. In addition, amagnetic stirrer (Hot plate stirrer, SMSH-20A, Scilab Korea.,Ltd) was utilized to prevent agglomeration. Furthermore, thenanouid containing GNAgNPs composite was evaporatedunder mild temperature (Fig. 1e). Finally, the thermal treat-ment was applied at 250 1C for 30 min to strengthening theinteraction between GN and AgNPs (Fig. 1f) in a furnace(Dongwon Scientic Co.)[42].

2.4. Preparation of the exible TCF

Pristine GN and GNAgNPs hybrid were suspended inethanol solution for 2 h and exible PET lm (Fig. 2) wascleaned by ethanol solution for 20 min under ultrasonication,respectively. The cleaned PET lm was dried at 50 1C in thefurnace. Therefore, GNAgNPs hybrid was decorated from itsaqueous solution (1 mg/ml) on exible PET lm using spincoating (Spin coating JSPA type), operated with a rotation

speed of 2500 rpm. Thickness of GN/PET and GNAgNPs/PET TCFs was controlled by drop numbers.

2.5. Characterization

Morhphological analysis was studied by eld emission

scanning electron microscopy (FESEM) (JSM-6710 F, JEOL)and transmission electron microscopy (TEM) (JEM-2100 F,JEOL; Tokyo, Japan). The structural characteristics wereperformed with X-ray diffractometer (XRD), (Bruker AXS,D8 advance powder diffractometer) using Cu K radiation(1.5406 ) from 101 to 901 and Raman Spectrometer(Labram HR800) with a 514 nm Argon ion laser ranging from1200 to 1700 cm1 was used to reveal the defect of GN afterintroduction of AgNPs. The transmittance and sheet resistancestudy were measured using a UV spectrophotometer (X-ma3000 Series Spectrophotometer, Human Co., Ltd., Korea) andsheet resistance measurement system (CMT-SR1000N).

3. Results and discussion

The SEM micrographs of pristine GN, AgNPs and GNAgNPs composite are shown in Fig. 3ad. As is shown inFig. 3a, the pristine GN was observed as aggregated andwrinkled structure which is known to be due to its strong interaction and vander Waals force (Fig. 3a). On the otherhand, AgNPs can be seen as slightly aggregated powders with

Fig. 3. SEM micrographs of (a) pristine GN, (b) AgNPs, (c) GN

AgNPs composite and, (d) magni

ed SEM image of GN

AgNPs composite.

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a particle size ranging from a few nanometers to a severalmicrometers (Fig. 3b). It can be seen fromFig. 3c and d thatAgNPs were successfully decorated onto the GN surfacehaving good dispersion in spite of decreasing wrinkled and

aggregated GN.

The pristine GN was found as aggregated and stackedstructure (Fig. 4a) which is very good agreement with theprevious SEM images. TEM analysis is also conrming thatAgNPs were successfully synthesized from Ag-wire usingPWE method (Fig. 4b). As shown inFig. 4c and d, the AgNPswere uniformly embedded into GN surface and akes.

It was reported in the literatures[37,42]that the AgNPs cansignicantly be grafted into GN surface having homogenously

dispersion. Therefore, GN akes distance can be increased and

Fig. 4. TEM micrographs of (a) pristine GN, (b) AgNPs, (c) GNAgNPs composite and, (d) a high resolution TEM of GNAgNPs composite.

Fig. 5. The XRD pattern of (a) pristine GN, (b) AgNPs and (c) GNAgNPshybrid.

Raman shift (cm-1

)

1200 1300 1400 1500 1600 1700

Intensity(a.u

)

0

500

1000

1500

2000

2500

(a) Pristine GN

(b) GN-AgNPs composite

Fig. 6. Raman spectra of (a) pristine GN and (b) GNAgNPs composite.

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thus agglomeration of GN is considerably decreased as a resultof penetration of AgNPs into GN akes.

The XRD patterns of the pristine GN, AgNPs and GN

AgNPs composite were depicted in Fig. 5. The crystalline

plane peaks, (0 0 2), (1 0 1), (0 0 4), (1 1 0) and (1 1 2) of GNwere observed at 226.5, 44.6, 54.6, 77.6 and 83.81in theXRD pattern for pristine GN, respectively (Fig. 5a)[40]. Onthe other hand, the peaks at 238.3, 44.4, 63.5, 77.7 and81.61were determined in the XRD pattern of AgNPs (Fig. 5b),which were assigned to (1 1 0), (2 0 0), (2 2 0), (3 1 1) and(2 2 0) crystalline plane peaks of Ag, respectively [43,44].Furthermore, all crystalline plane peaks of GN and AgNPswere revealed in the XRD pattern of GNAgNPs composite(Fig. 5c). This indicates that AgNPs were signicantly associatedwith the GN surface. Moreover, average crystalline size ofAgNPs in hybrid material was calculated about 43.89 nm inaccording to Scherrers equation[43].

Raman spectroscopy is powerful tool to study structuralcharacteristics such as their quality analysis and structuraldefects of carbon based materials. It is well established thatcomposite materials based on GNAg nanostructures exhibitunique property of Surface Enhanced Raman Scattering(SERS) [32,40,43,45]. Therefore, to conrm XRD measure-ment as well as to study SERS property, Raman spectroscopymeasurement was performed. As shown inFig. 6, the D and Gmain peaks of GN were observed around at 1350 and1580 cm1 in the Raman spectra of both the pristine GN

(Fig. 6a) and GNAgNPs composite (Fig. 6b). This result may

Fig. 7. Inuence of drop number on thickness and transmittance of (a) GN/PET, (b) (GNAg)/PET, and (c) PET lms.

Wavelength, nm

400 500 600 700 800

Transmittance,

%

40

50

60

70

80

90

100

T550=61.28 %

T550=70,99 %

T550=80,89 %

T550=88.43 %

T550=94.64 %

Wavelength, nm

400 500 600 700 800

Tra

nsmittance,

%

40

50

60

70

80

90

100

T550=58.54 %

T550=69.52 %

T550=81.41 %

T550=91.11 %

T550=93.52 %

Fig. 8. Transmittance of (a) GN/PET TCF and (b) (GNAg)/PET TCFs.

Transmittance at 550 nm, %

50 60 70 80 90 100

0

50

100

150

200

250

GN/PET films

(GN-Ag)/PET films

Transmittance at 550 nm, %

60 70 80 900

5

10

15

20

25R

S1/ R

S2

Average

(a)(b)

Sheetresistance(R

s),

k

/sq

Fig. 9. Sheet resistance of (a) GNPET lm and GNAg-PET lm at varioustypes of transmittance at 550 nm; (b) improvement factor (RS1/RS2) andaverage improvement factor.

Table 1Sheet resistance (RS), /sq of the GN/PET and (GNAg)/PET TCFs at varioustypes of transmittance at 550 nm

Transmittance at 550 nm (%) Sheet resistance (k/sq) RS1/RS2

GN/PET (RS1) (GNAg)/PET (RS2)

60.0 34.94 2.91 12.0070.0 53.15 3.41 15.5980.0 97.65 10.34 9.4491.0 161.58 24.32 6.6494.0 206.53 27.79 7.43Average (RS1/RS2) 10.22

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be able to conrm that the GN based composite materialsuccessfully synthesized.

Notably,D peak is a breathing point ofk-point phonons ofA1gsymmetry which is revealed disorder of carbon atoms, andGband indicates sp2-bonded carbon atoms. Intensity ratio (ID/

IG) of D peak and G peak illustrates structural defects and

purity [32,40,45]. The ID/IGratio of pristine GN was deter-mined 0.320, which was considerably associated with pre-viously published literature [40]. Moreover, ID/IG ratio wasincreased till around 0.595 for GNAgNPs composite materialdue to some structural changes including local defects anddisorders. In addition to, the intensity ratio for GNAgNPscomposite increased around 400% compared with GN accord-ing to the SERS activity[32,43,45].

In order to apply our samples in a possible practicalapplication (TCF), we further studied the transmittance andsheet resistance of GNAgNPs composite. TCF is known asone of the mostly attracting research interests in the eld ofoptoelectronics. The most important criteria of TCF materialspossess of a low sheet resistance at a high transmittance. Thethickness of TCFs was adjusted by drop numbers. Theinuence of drop number on thickness and transmittance isrevealed inFig. 7. As shown inFig. 7, the transmittance of thelms is decreased with increasing the lm thickness, whichwas controlled by drop numbers. Fig. 8 depicts that thetransmittance of various lms at wavelength ranging from300 to 800 nm as a function of lm thickness.

The comparison of sheet resistance of GN/PET and (GNAg)/PET lms was studied at the wide range of transmittancefrom around 60.0 to 95.0% (Fig. 9a). The lms of hybridmaterial were shown considerably lower sheet resistance than

the lms of pristine GN at each transmittance. In particular to,improvement factor in hybrid material was calculated fromsheet resistance (RS) ratio of GN/PET and (GNAg)/PET TCFsand average improvement factor was determined around 10(Fig. 9b). This demonstrated that (GNAg)/PET lm can beexhibit high electrical conductivity. On the other hand, the lmof hybrid material can be shown more efcient property thatpristine GN.

In addition to, the combined data of sheet resistance andimprovement factors were revealed in Table 1.

4. Conclusion

In this paper, we synthesized GNAgNPs hybrid materialusing a facile and an environment friendly method.

According to morphological and structural study, AgNPshaving pure crystalline structure and around 43.89 nm crystal-line size signicantly decorated on the GN surface withhomogeneously dispersion as well as aggregated structure ofGN was meaningful decreased in GNAgNPs composite.Additionally, the SERS activity was in hybrid materialenhanced around 400%.

Finally, we prepared exible TCFs based on pristine GNand GNAgNPs composite and therefore investigated itstransparent and conductive characteristics. As the results of a

transmittance and a sheet resistance measurements, sheet

resistance for (GNAg)/PET TCFs was considerably (10fold) decreased in comparison to GN/PET TCF.

Based on the study, we expected that as-prepared GNAgNPshybrid material can be further used in the optoelectronics eld.

Acknowledgements

This research was supported by Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science andTechnology (NRF-2014R1A1A4A03005148).

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