Effect of AgNPs/Reduced Graphene Oxide ...

Effect of AgNPs/Reduced Graphene Oxide Nanocomposites on the Electrical Performance of Electrically Conductive Adhesives Jinfeng Zeng, Xiaopeng Chen, Xin Ren, Yanqing Ma * School of Chemistry and Chemical Engineering, Shihezi University /Key Laboratory for Green Processing of Chemical Engineering of XinJiang Bingtuan /Engineering Research Center of Materials — Oriented Chemical Engineering of Xinjiang Bingtuan

Shihezi 832003, P. R. China Y. Q. Ma (E-mail:[email protected])

Abstract—Rapid development in electronic products in recent years leads to an increasing demand for electrical conductive adhesives (ECAs) with large electrical conductivity and environmentally friendly. Therefore, how to increase its electrical property meanwhile its other properties can not be affected and even can be enhanced has become the focus of attention. In this study, we chose a green and mild reducing agent glucose, which was synthesised AgNPs/reduced graphene oxide (rGO) nanocomposites based on one-pot method in aqueous solution. The ECAs were prepared by mixing AgNPs/rGO, silver flakes and epoxy. Then we measured the bulk resistivity of ECAs after curing it at 150 oC for two hours. The results showed that when the total amount of the conductive filler maintained at 70 wt%, the mass fraction of AgNPs/rGO reached 0.2 wt%, and the bulk resistivity (8.76×10-5 Ω·cm) was lower than that of filled pure silver flake (1.11×10-4 Ω·cm ). Keywords—AgNPs/rGO; one-pot; Electrically conductive adhesives

electrical

properties;

I. INTRODUCTION In recent years, electrical conductive adhesives (ECAs) have attracted more attention in electronics industry due to many more advantages than Tin-lead solder, such as more environmentally friendly, lower processing temperature [1], and finer pitch printing. The electrical properties of ECA are mainly determined by conductive fillers [2], and many ohter kinds of materials, such as silver, copper, carbon black, carbon nanotubes, graphene and Ag-plated graphene [3-4]. Among these materials, silver is widely used due to its chemical stability and high electrical conductivity compared to other filler materials. Boxin Zhao [5] reported that the resistivity of ECA is observed at 1.80 × 10-4 Ω·cm with content of 80 wt%. Z. X. Zhang [6] studied the conductivity of composites reached 2.5×105 S/cm by incorporating 1.5 wt% muti-walled carbon nanotubes assembled with functionalized nano-Ag particles, and 79.5 wt% micron-sized silver powders as conductive fillers. Ching-Ping Wong [3] developed the ECAs with content of 70 wt% Ag-coated Cu flakes pre-modified by silane coupling agent exhibited bulk resistivity is 8.4×10-3 Ω·cm. Cheng Yang [7] reported the resistivity of ECA at 80 wt% of total silver content is 3.6 × 10-5 Ω·cm. These methods are intended to use nanocomposites to improve the

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conductivity of ECAs at low content of conductive fillers. However, its lower conductivity and higher content of conductive fillers were its main limitations. Graphene is a quasi-two-dimensional (2D) structure material, in which monolayers of carbon atoms are packed into honeycomb crystal planes [8]. Unique electrical, mechanical and thermal properties of graphene also have been discovered [9]. However, the graphene sheets were also aggregated due to its vander waals force and strong π-π interaction. To explore the potential of graphene, many efforts have been made to functionalize graphene with metal nanoparticals [10, 11]. A large number of metal nanoparticals were assembled on the surface of graphene oxide sheets based on large specific surface area. And nano-sized silver particles can prevent graphene sheets from aggregating, so the hybrids nanocomposites are particularly attractive in terms of their distinctive optical and electrochemical properties, high catalytic activity, and strong surface-enhanced effect. To the best of our knowledge, there is seldom report systemically investigates the effects of AgNPs/rGO as fillers on the electrical properties of the ECAs. In this paper, AgNPs with the size from 15 to 20 nm and narrow size distribution, which were successfully decorated on rGO sheets by a one-pot method in aqueous solution. Then ECAs were prepared by mixing AgNPs/rGO, silver flake and epoxy. The optimum content of conductive filler and electrical properties of ECAs were studied after cured at 150 oC. Ⅱ. EXPERIMENTAL DETAILS A. Chemicals and materials Glucose was supplied by Tianjin Shengyu Chemical Co., Ltd.. AgNO3 was supplied by Xi’An Chemical Corporation. 1(2-Cyanoethyl)-2- ethyl- 4-methylimidazde (2E4MZ-CN) and 4-Methylcyclohexane-1, 2-dicarboxylic Anhydride (MHHPA) were supplied by TCI (Shanghai) Development Co., Ltd.. Epoxy resin (862), used as epoxy binder was purchased from Guangzhou Topyon trade Co., Ltd.. The silver flakes were purchased from Strem Chemicals, Inc.. Graphite flakes (natural, -325mesh) were purchased from Alfa Aesar. HCl (38 %) and H2O2 (30 %) were supplied by Tianjin Fuyu Fine

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Scheme 1. A schematic of AgNPs decoration over the surface of rGO

Figure 1. FE-SEM image of the AgNPs/rGO nanocomposites

Chemical Co., Ltd.. NaNO3 was purchased from Aladdin industrial Corporation, H2SO4 (98 %) was purchased from Chengdu area of the industrial development zone xindu mulan, without all of the chemicals further purified. The morphology and dispersion of the AgNPs were characterized by field emission scanning electron microscope (FE-SEM) on Hitachi limited and high resolution transmission electron microscopy (HR-TEM) on a FEI Tecnai G20. X-ray diffraction (XRD) (D8 ADVANCE) was used to investigate the crystalline structure of the AgNPs and layer spacing of graphene. The thickness of curing film was measured by thickness gauge (Shanghai Chuanlu Measuring Tool Limited Liability Co.). And resistance was measured by the RTS-8 4point probe Resistivity Measurement system (Guangzhou). B. Fabrication Graphene oxide (GO) was prepared from natural graphite flake according to modified Hummers method [12]. The AgNPs/rGO was synthesized by using a green and mild reducing agent via one-step method according to a previous literature [13]. The ECAs were prepared based on the following procedures: the matrix resin was composed of epoxy, MHHPA and 2E4MZ. Then different content of AgNPs/rGO was mixed with matrix resin. The prepared ECA was printed onto a glass substrate and cured at 150 oC for 2 h. Ⅲ.

RESULTS AND DISCUSSION

A. Morphology and Structure Characterization Glucose acts as a reducing agent. By adding AgNO3 into the aqueous solution, silver ions start to form on graphene nanosheets (Scheme 1). SEM and TEM images show the surface morphology of AgNPs/rGO and the distribution and size of the AgNPs. The results show that the size of AgNPs can be controlled by one-pot method in aqueous solution. The SEM image of the as-prepared AgNPs is presented in Figure 1, from which the morphology and diameter of AgNPs can be obtained. Figure 1 shows the AgNPs were spherical and ranged in size from 15 to 20 nm. TEM images (Figure 2) shown the black spheres are AgNPs and rGO like crumpled silk (wrinkles indicated by arrows). Almost transparent graphene nanosheets were decorated with a large amount of

2015 16th International Conference on Electronic Packaging Technology

Figure 2. HR-TEM image of the AgNPs/rGO nanocomposites

AgNPs. It is obviously seen that almost no AgNPs distributed outside of rGO sheets, which reveals the strong interaction between the rGO and AgNPs [14]. We also found that AgNPs deposited both sides of rGO sheets, AgNPs could efficiently keep rGO sheets from stacking . At the same time, The GO sheets bear epoxy functional groups, carbonyl, carboxyl groups and hydroxyl on their basal planes which have been used as anchors for adsorption of AgNPs. AgNPs/rGO nanocomposites could fill into gaps and voids from stacking of silver flakes and thus formed new channels. Large specific surface area of rGO sheets anchored a large number of AgNPs, which improved the conductivity of ECA filled with AgNPs/rGO. XRD patterns of graphite, GO and AgNPs/rGO nanocomposites are shown in Figure 3. The diffraction peak at 2θ = 26.684 °from a line, which is corresponding to (002), (101), (004), (110) reflection of graphite [15, 16]. According to Bragg equation, the d-spacing is 0.33 nm. GO (b line) showed the 001 peak around 11.093 °, and the d-spacing is 0.80 nm. The d-spacing of GO is obviously larger than that of the graphite. It indicates that oxygen-containing groups attached on rGO sheets. There are no clear characteristic peaks of graphite and GO from c line, the reason could be the diffraction peaks of GO become weak or disappear after reduced via glucose. However, the peaks observed at about 2θ=38.0 °, 44.2 °, 64.5 °and 77.4 °are attributed to the (111), (200), (220) and (311) crystalline planes of face-centered

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Figure 4. XPS wide-scans of AgNPs/rGO nanocomposites (a) and GO (b) Figure 3. XRD patterns of graphite (a), GO (b), AgNPs/rGO (c)

Silver (corresponding to PDF card) [17]. The results demonstrated that the AgNPs were successfully decorated to rGO sheet. XPS was applyed to further confirm the reduction of GO. From the wide-scan spectra of AgNPs/rGO in Figure 4, it can be observed that characteristic peaks of Ag 3d emerged which indicated that the AgNPs were successfully decorated on the rGO sheets, which was consistent with TEM analysis. The CC content (21.10 %) in AgNPs/rGO increased by 14.40 % , compared with GO (6.66 %), so electric properties of rGO were recovered. Ag 3d binding energies at 368.3 and 374.3 eV can be assigned to metallic Ag 3d5/2 and Ag 3d3/2. Doublet components about spin-orbit splitting were measured to be 6 eV indicating the complete formation of metallic silver supported on rGO sheets. There was no Ag2O on the surface of rGO sheets. The observation is consistent with the result of XRD patterns. B. Electrical performance characterization On the average, five specimens were made for each sample, and every specimen was measured at least 5 points (region in a length (L) of 1.5 cm, a width (W) of 0.3 cm for each specimen). The resistance (ρ) of electrical conductivity adhesive could be calculated following (1) below:

ρ 

RTW L

Figure 5. XPS spectra of Ag 3d

relationship between AgNPs/rGO content and electrical resistivity with 70 wt% of the total content. The electrical resistivity increased after the first decreased with the increase of AgNPs/rGO mass fraction. The electrical resistivity fell to 8.76×10-5 Ω·cm when the mass fraction of AgNPs/rGO reached to 0.2 wt%. Then the electrical resistivity slowly increased after the mass fraction exceeded 0.2 wt%. It was attributed to uneven dispersion and agglomerated of AgNPs/rGO nanocomposites filler in the ECAs at higher loading [17].

(1)

ρ is resistivity (Ω·cm), R is resistance (Ω), T is the thickness of ECA layer (cm). As all we know, electrically conductive fillers are combined together by the bonding of matrix resin to form electrically conductive channels and achieve electrically conductive connects. But fillers can’t contact with each other completely after only doped micron silver flakes in the conductive adhesive because micron silver flakes formed many gaps and voids. It decreased the number of electrically conductive channels. AgNPs/rGO nanocomposites can fill into these gaps and voids and thus formed new channels and exhibit excellent electrical conductivity. Figure 6 shows the


Figure 6. Relationship between resistivity and the mass fraction of AgNPs/rGO nanocomposites

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Ⅳ. CONCLUSIONS In brief, AgNPs/rGO nanocomposites were successfully prepared by a environmentally friendly chemical reduction method. SEM and TEM analysis of AgNPs/rGO showed AgNPs were successfully attached to the surface of rGO, and the size of the AgNPs is from 15 to 20 nm with narrow distribution via glucose as the reductant. XPS was employed to further confirm the reduction of GO. Then ECAs were prepared by mixing AgNPs/rGO, silver flake and epoxy. The resistivity of ECA at 70 wt% of total conductive filler and the content of AgNPs/rGO nanocomposites maintains at 0.2 wt% is 8.76×10-5 Ω·cm. This confirmed that glucose as a kind of environment-friendly reductant, which could decrease the electrical resistivity of ECAs and the ECAs can be an alternative to lead-containing solders. ACKNOWLEDGMENT The authors would like to thank the funding support of Shi hezi University projects (SRP2015118 and 201510759021). REFERENCES [1]

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