Hybrid of silver nanowire and pristine-graphene by liquid-phase exfoliation for synergetic effects on electrical conductive composites

Abstract

One-dimensional (1D) silver nanowire (AgNW) and 2D defect-free pristine-graphene have been integrated together to form a 3D hybrid network. The AgNWs were introduced into pristine graphene dispersion and then the sedimentation containing the two ocurred, leading to the formation of an AgNW/graphene hybrid. The graphene sheets embedded within the AgNW network fill the open spaces and enhance the interfacial contact between the AgNWs, leading to a uniform electrical conductive network. The AgNW network can prevent the re-stacking of graphene sheets, and the graphene sheets act as a protective layer to prevent the AgNWs from surface oxidation. The prepared hybrid was used as reinforcing nanoscale fillers to prepare epoxy-based conductive composites. The results show that the hybrid has synergetic effects on the improvement of the electrical conductivity and shear strength of the composites. The composites filled with the AgNW/graphene hybrid have lower volume resistivity and larger shear strength than the composites which contain graphene or AgNWs alone. It is found that the synergetic effect of the hybrid results from the uniform dispersion of the hybrid filler in the epoxy matrix and the formation of more effective electron-transport channels in the composites. This synthetic approach to the 1D/2D nanohybrids opens up opportunities for various potential applications ranging from devices to transparent electrodes and conductive composites.

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Introduction

For a long time, many material scientists and engineers have devoted their efforts to develop advanced electrically interconnecting materials to substitute traditional tin/lead (Sn/Pb) solders in microelectronics.^1,2 The so-called electrically conductive composites (ECCs), which are composed of the conductive fillers and dispersant matrix, become a key material to the printed electronics.^3,4 Improving the electrical conductivity of the ECCs has become a major challenge for general application.^5,6 Generally, adopting the multi-model filler systems is rather effective in improving the electrical conductivity by the reduction of contact resistance between the fillers, e.g. the introduction of silver nanoparticles (AgNPs) into micron-sized Ag flakes.^7,8 However, the decreased average size of the nanoscale fillers will increase the viscosity of the fillers. Moreover, the increased isotropicity of the NPs leads to the rise of the percolation threshold.²

Recently, Ag nanowires (AgNWs) have been demonstrated as excellent printed conductors, which are able to form a reticular conductive network at a lower percolation threshold than AgNPs.⁹ The ECCs filled with 75% of the silver fillers (3 : 2 weight ratios of micro-flakes and AgNWs) reached 5.8 × 10⁻⁶ Ω cm.¹⁰ However, the AgNW networks still suffer from some disadvantages, such as low breakdown voltages, typically high NW–NW junction resistance, high contact resistance between the network and active materials and material instability in harsh environments.^11–15 A novel architecture consisting of one dimensional (1D) metallic NW/two dimensional (2D) graphene has emerged as a promising material for numerous applications, such as electrode and conducting film.^16–22 Hur's group has demonstrated that combining the two will significantly decrease the percolation threshold, thus leading to the high electrical conductivity at low content.²³ The nanowires (silver and cooper) play the role of framework to enhance the electrical conductivity.^24–26 Graphene provides the large flat geometry to fabricate nanostructure with extraodinary properties.^27,28 Until now, graphene oxide (GO), reduced GO and chemical vapor deposition (CVD)-grown graphene were used as precursors. However, chemical oxidation dramatically deteriorates the electronic properties of graphene although partial restoration is accomplished by reduction.^29,30 Moreover, the CVD method involves a very complicated and time-consuming transfer process, which limits its application in polymer composites.

As the literature and our previous study demonstrated, the liquid phase exfoliation of graphite in organic solvent by sonication without oxidation is considered a promising method to obtain high quality graphene sheets.^31–34 Herein, pristine graphene was prepared by liquid phase exfoliation of natural graphite in a low boiling-point solvent. The hybrid consisting of 1D AgNWs and 2D defect-free pristine-graphene was fabricated by simple filtration of the simultaneous sediments of AgNWs and graphene. The structure and morphology of the hybrid were investigated. The incorporation of the AgNW/graphene hybrid into epoxy-based polymer composites for microelectronic interconnects application is achieved. The synergetic effects of the hybrid on electrical and mechanical properties of the composites were studied. The structure of hybrid dispersion in the composites and the reinforcing mechanism were also discussed.

Experimental

Materials

Natural graphite powder (size <20 μm, purity >99.8%) was purchased from Qingdao Xinghua Graphite Product Co., Ltd. 1-Cyanoethyl-2-ethyl-4-methylimidazole (2E4MZ-CN, 98%) was purchased from Tokyo Chemical Industry Co. Ltd. Silver nitrate (AgNO3, 99.5%) was purchased from Aladdin Chemistry Co. Ltd. Glycerol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Polyvinylpyrrolidone (PVP) was purchased from Shanghai Yuanju Biotechnology Co., Ltd. Sodium chloride (NaCl) was purchased from Tianjin Fuchen Chemical Reagents Factory. The epoxy resin (E-51, epoxide equivalent:196) was supplied by Guangzhou Lushan Chemical Co. Ltd. Methylhexahydrophthalic anhydride used as a curing agent was obtained from Lunliqi Chemical Company. Silver flakes used in this study were purchased from Kunming Noble metal electronic materials Co. Ltd.

Pristine graphene dispersion

Pristine graphene dispersion was prepared by liquid-phase exfoliation method on the basis of our previous study.³⁴ Graphite dispersions were prepared by dispersing natural graphite in acetonitrile at an initial concentration of 3.5 mg ml⁻¹. 2E4MZ-CN was subsequently added into the initial dispersion at a certain concentration. These dispersions were sonicated for 2 h, followed by standing for 24 h to allow the formation of any unstable aggregates in the bottom. The dispersion was then centrifuged at a speed of 4000 rpm for 30 min. The supernatant obtained was the pristine graphene dispersion.

Synthesis of the AgNWs

The AgNWs were prepared according to the reported work by Wong's group.³⁵ Typically, 5 g PVP were added to 190 ml glycerol in a 500 ml round three-necked flask assisted with tender stirring and heating at 50 °C until PVP was all dissolved. Then, the temperature was dropped down to room temperature. 1.58 g silver nitrate powder was added to the solution. Then, a 10 ml glycerol solution containing 59 mg NaCl and 0.5 ml H₂O was added to the flask. Then, the solution temperature was raised from room temperature to 210 °C with gentle stirring (50 rpm). When the temperature reached 210 °C, the heating was stopped and the temperature was dropped down to room temperature. Water was added into the solution in 1 : 1 ratio, and then the mixture was centrifuged at 8500 rpm for 30 min. The as-obtained AgNWs were washed by water to remove the PVP residue. The AgNWs were collected and re-dispersed into an aqueous solution with 7 mg ml⁻¹.

Fabrication of AgNW/graphene hybrid

The AgNW/graphene hybrid was prepared by introducing the AgNW aqueous solution into the graphene dispersion. The mixture was sonicated for 30 min to allow the assembling process. After sedimentation for 24 h, the AgNW/graphene hybrid was precipitated at the bottom of the flask and then filtered, washed by acetonitrile to remove 2E4MZ-CN, and then dried under vacuum at 50 °C overnight. The AgNW/graphene hybrid was obtained for further use. The different proportion of the AgNWs to graphene was controlled by adjusting the initial amount of AgNWs dispersion and graphene dispersion.

Preparation of highly conductive epoxy-based composites

The AgNW/graphene hybrid was first re-dispersed in acetonitrile by ultrasonication for 20 min. The epoxy resin was subsequently added into the hybrid dispersion. The mixture was sonicated again for 20 min to allow the hybrid to mix well with epoxy resin. Then, the solvent was removed by vacuum distillation. The curing agent, catalyst and microscale silver flakes were added into the mixture successively. The composite was homogeneously mixed by ultrasonication at 20 °C for 40 min. The relative ratio of epoxy, curing agent and catalyst was 1 : 0.85 : 0.0185. The concentration of microscale silver flakes was kept constant at 75 wt% of the total amount of the composite. The composites were then cured at 150 °C for 3 h.

Characterization

X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation. Differential scanning calorimetry (DSC) was performed on a TA Q20 at a heating rate of 15 °C min⁻¹ from 25 °C to 250 °C with two circles. UV-Vis absorption spectroscopy was carried out on a UNICAN UV-500 spectrophotometer using a quartz cell with a 1 cm optical path. X-ray photoelectron spectrometer (XPS) analysis was carried out on a Kratos Axis Ultra DLD with Al Ka radiation. The XPS peaks were fitted by XPS peak 41 software. The morphology of the AgNWs and hybrid was investigated by field emission scanning electron microscopy (FESEM, Nano SEM 430). Atomic force microscopy (AFM) was conducted on a NanoScope (R) III instrument operated in a tapping mode. The samples for SEM and AFM measurement were prepared by depositing a drop of much diluted hybrid dispersion on mica substrate and dying at ambient environment. The electrical conductivity was measured with a standard four-point method at room temperature using a low resistance system composed of Keithley 2182A (Nanovoltmeter) and Keithley 6220 (DC Current Source). The shear strength was measured on a Zwick/Roell Z010 tensile instrument.

Results and discussion

Synthesis of AgNW/graphene hybrid

Homogeneous pristine graphene dispersion is achieved by bath sonication of natural graphite in acetonitrile assisted with the stabilization of 2E4MZ-CN. Fig. 1a shows the TEM image of the prepared pristine graphene. Additional Raman spectra is shown in ESI, Fig. S1.† The AgNWs were prepared according to the reported work by Wong and his co-workers.³⁵ Glycerol with two primary hydroxyl groups and one secondary hydroxyl group was used as the solvent due to its stronger reducing ability compared to ethylene glycol. Water was added as an electrolyte to modulate the reaction process and facilitate the growth of the AgNWs along the (110) direction.^36,37 The hybrid of AgNWs and graphene was fabricated by simply shaking and filtering graphene and the AgNWs mixture as illustrated in Scheme 1. The homogenous state of the graphene dispersion is broken down after introducing AgNWs. The shaking procedure allows the assembling of the AgNW/graphene hybrid. After shaking and standing by for sedimentation, the top regions of the vial become a uniform yellow. This is the color of the stabilizer (2E4MZ-CN). The simultaneous sedimentation of graphene sheets with the AgNWs at the bottom of the vials is observed. The occurrence of the simultaneous sedimentation process indicates that there is interaction between the graphene and AgNWs. Fig. 1b and c show the TEM and AFM images of the produced AgNWs. The AgNWs have a uniform width of 50 ± 10 nm. The selected electron diffraction pattern (Fig. 1b inset) suggests that the AgNWs are highly crystalline. A SEM overview image (Fig. 1d) shows that the AgNWs assemble into a big network. They have an average length of 8 ± 5 μm. Fig. 2 shows the SEM images of the AgNW/graphene hybrid with the initial proportion of AgNWs to graphene in the range from 6 : 1 to 2 : 1 (Fig. 2a–c). It can be seen that the graphene sheets are embedded into the open spaces within the AgNW network, leading to a 3D hybrid network. As the relative concentration of graphene in the hybrid increases, the graphene tightly entangled with the AgNW network can fill the non-contacted blank region, and thus there are fewer holes in the AgNW network, and a layered 3D network structure is observed (Fig. 2b). When the concentration of graphene is nearly equal to that of AgNWs, it can be seen that there are excess graphene sheets covered on the surface of the AgNW network (Fig. 2c). The addition of graphene increases the contact areas between the AgNWs. The graphene sheets bridge the neighboring AgNWs, which leads to the formation of effective electron-transport channels in the network. The TEM images (Fig. 2d and e) also confirm the formation of the AgNW/graphene hybrid network. The TEM results are consistent with the above SEM observation.

Fig. 1 (a) TEM image of the prepared pristine graphene. (b) TEM images of the AgNWs and the electron diffraction pattern of a single AgNW (inset). (c) AFM image of AgNWs which shows the height of the AgNWs. (d) An overview of SEM images of AgNWs on a mica matrix.

Scheme 1 Schematic illustration of the preparation of the AgNW/graphene hybrid.

Fig. 2 (a)–(c): SEM images of the AgNW/graphene hybrid with different proportions of AgNWs to graphene ranging from 6 : 1 to 2 : 1. (a) 6 : 1, (b) 4 : 1, (c) 2 : 1. (d) and (e): TEM images of the AgNW/graphene hybrid network with the proportion of AgNWs to graphene at 2 : 1.

The interactions between AgNWs and graphene in the hybrid network were investigated by XRD, UV-Vis and XPS. Fig. 3a shows the XRD patterns of the different samples. The pristine graphene has a peak at 26°, the as same as natural graphite, indicating that there is re-stacking of exfoliated graphene sheets after filtration.^38,39 However, in the AgNW/graphene hybrid, the peak density becomes weaker and weaker, suggesting the successful prevention of the graphene re-stacking due to the isolation of the AgNW network. The peak nearly disappears when the relative ratio of AgNWs to graphene reaches 4 : 1. The significant decrease in density suggests that the graphene sheets are well exfoliated and embedded in the hybrid network, and the AgNW network can prevent the self-stacking of the graphene sheets after filtration. The peaks at 38.2°, 44.4°, 64.6° and 77.6° for AgNWs and the AgNW/graphene hybrid correspond to the planes (111), (200), (220) and (311) of face-centered cubic Ag, implying the formation of AgNWs in the as-synthesized composite.^40,41 However, in the pure AgNWs sample, a small peak at 32.8°, corresponding to Ag₂O, is observed. The formation of Ag₂O is due to the oxidation of AgNWs in the ambient environment.¹⁸ The small peak disappears when the AgNWs are hybridized with graphene, indicating that the graphene sheets can act as an encapsulation layer that protects the AgNWs from surface oxidation, thus improving the chemical stability of the hybrid. Fig. 3b shows the UV-Vis spectra of AgNWs and the AgNW/graphene hybrid. The peak at 379.4 nm in the pure AgNW sample corresponds to surface plasmon resonance (SPR) bands of Ag. However, the peak in the hybrid has a shift from 379.4 nm to 385.3 nm. This behavior indicates that there is an interaction between AgNWs and graphene sheets. Since the AgNWs were prepared by the polyols method, there were hydroxyl groups (–OH) absorbed on the surface of AgNWs. Thus there is possibly a hydrogen bond between the AgNWs and graphene.

Fig. 3 (a) XRD patterns of graphene, AgNWs and the AgNW/graphene hybrid. (b) UV-Vis spectra of AgNWs and the AgNW/graphene hybrid. Inset: the magnified image of the circle-marked region.

XPS was used to further confirm the interactions between AgNWs and graphene which leads to the formation of the 3D hybrid network. Fig. 4 shows the C 1s spectra of pristine graphene (Fig. 4a) and the AgNW/graphene hybrid (Fig. 4b). As expected, the most dominant peak in the pristine graphene is observed at 284.5 eV, corresponding to graphitic carbon (C–C). The C 1s spectrum suggests low levels of oxidation of pristine graphene, although it shows additional small peaks corresponding to C–O (286 eV). In the AgNW/graphene hybrid, high-resolution scans of the C 1s region show the dominant C–C peak, accompanied by three additional small features at 1.1, 2.2 and 3.9 eV higher binding energies than the graphite peak, which correspond to C–N, C–O and C=O, respectively. The change of the C 1s spectra for the hybrid can be attributed to the synthesized AgNWs. The O1s spectra is shown in Fig. S2.† Both of the AgNWs and hybrid have the same O1s spectra, indicating that the O and N result from the residual Glycerol and PVP attached with AgNWs. This observation demonstrates that the AgNWs can interact with graphene to form the hybrid. Fig. 4c shows XPS patterns of the synthesized AgNWs and AgNW/graphene hybrid. The shift of the binding energy of Ag 3d signals from 367.9 eV to 368.4 eV also indicates that there are interactions between the synthesized AgNWs and graphene, which lead to the formation of the 3D hybrid network.

Fig. 4 High resolution XPS C 1s spectra of pristine graphene (a) and the AgNW/graphene hybrid (b). (c) XPS Ag 3d patterns of AgNWs and the AgNW/graphene hybrid.

Properties of epoxy-based conductive composites filled with AgNW/graphene hybrid

The AgNW/graphene hybrid is used as a nanoscale conductive filler to prepare epoxy-based conductive composites with better performance. The epoxy-based composites filled with AgNW/graphene hybrid are easily prepared by a solution mixing process because of the re-dispersion capacity of the hybrid in low boiling-point solvent. The synergetic effects of the AgNW/graphene hybrid on the thermal, mechanical and electrical properties of the composites are discussed, as shown in Fig. 5. DSC was used to investigate the effect of the hybrid on the thermal properties of epoxy resin. As shown in Fig. 5a, the original exothermic peak from curing of epoxy shifts to a higher temperature as the content of hybrid increases. This is due to the thermal conductivity of the hybrid filler which leads to the higher curing temperature. When the concentration further increases, the dominant exothermic peak shifts to a lower temperature. This is the outcome that the aggregation hinder the formation of the thermal conductive paths within the network. DSC was also adopted to investigate the glass transition process of the composites (Fig. 5b). The Tg decreases when the AgNWs are added to the epoxy network. This is due to the fact that the addition of AgNWs leads to the increase of the flexibility of the network. The addition of the AgNW/graphene hybrid further decreases the Tg of the epoxy system to a great extent. Both the slippage of graphene layers and the flexibility of AgNWs lead to a large decrease in Tg. However, as the concentration of the hybrid further increases, the hinder effects of aggregates on the mobility of epoxy chains lead to the increase of Tg.

Fig. 5 (a) Heat curves of the curing of epoxy resin with different contents of the AgNW/graphene hybrid. (b) Glass transition temperature of the cured epoxy resin filled with different contents of the AgNW/graphene hybrid. (c) Shear strength of the epoxy-based composites filled with the hybrid with different proportions of AgNWs to graphene. (d) Shear strength of the composites filled with different contents of the hybrid when the ratio of AgNWs and graphene is kept at 4 : 1. (e) Volume resistivity of the epoxy-based composites filled with the hybrid with different proportions of AgNWs to graphene when the content of the hybrid is kept at 1.3 wt%. (f) Volume resistivity of the composites filled with different contents of the hybrid when the ratio of AgNWs and graphene is kept at 2 : 1.

While the total amount of the hybrid remains constant, the effects of different proportions of AgNWs to graphene in the hybrid on the shear strength of the composites are investigated as shown in Fig. 5c. The shear strength of the control sample with only AgNWs is 8.6 MPa. When the relative ratio between AgNWs and graphene is fixed at 2 : 1, the shear strength of the composites filled with single component are 10.3 and 9.9 MPa, respectively. However, the shear strength of the composites filled with AgNW/graphene hybrid is 12 MPa, an increase of approximately 39.5% compared with the control sample. When the AgNWs integrate with graphene sheets at a relative ratio of 4 : 1 into the hybrid, the shear strength of the composite reaches 13 MPa, better than that of the composites filled with a single component (11 and 9.4 MPa, respectively). The composites filled with the AgNW/graphene hybrid have a shear strength of 12.7 MPa at a ratio of 6 : 1 (AgNWs to graphene). This is nearly equal to that filled with pure AgNWs. The results indicate that the AgNW/graphene hybrid has a synergistic effect on enhancing the shear strength of the composites. This is due to the enhanced strength of the hybrid network when graphene is embedded in AgNW network. The proportion between the AgNWs and graphene has a significant impact on the shear strength. When there is too much graphene sheets in the hybrid, the slippage between the graphene layers and graphene aggregates leads to the reduction of the shear strength. If there are not enough graphene sheets so as to result in a close 3D network with strong strength, the blank region in the hybrid will weaken the interfacial interaction of the network. The effect of the total amount of the AgNW/graphene hybrid on the shear strength is studied when the relative ratio of AgNWs to graphene keeps constant at 4 : 1. As shown in Fig. 5d, as the content of the hybrid increases, the shear strength increases. When the content of the hybrid is 0.9 wt%, the composites have the largest shear strength. The further increase of the content of the hybrid leads to the formation of aggregates which decrease the shear strength.

Fig. 5e shows the volume resistivity of the composites filled with the AgNW/graphene hybrid with different ratios between the AgNWs and graphene when the total amount remains constant. Epoxy-based conductive composites were prepared by using the obtained AgNW/graphene hybrid as a nanoscale filler and microscale Ag flakes as a primary filler. The concentration of Ag flakes keeps constant at 75 wt%. As a control, the composites without adding graphene and AgNWs have a volume resistivity of 3.08 × 10⁻⁶ Ω cm. When the proportion of AgNWs to graphene is fixed at 2 : 1, the volume resistivity of the composites with the addition of AgNWs and graphene reaches 1.1 × 10⁻³ Ω cm and 1.7 × 10⁻³ Ω cm, an increase of 63.7% and 43.9%, respectively. However, the composites filled with the hybrid have a lower volume resistivity (3.05 × 10⁻⁴ Ω cm) than the two kind of composites which contain graphene or AgNWs, an increase of 80.1%. The composite filled with the hybrid has a volume resistivity of 7.45 × 10⁻⁴ Ω cm at the ratio of 4 : 1 (AgNWs to graphene), also lower than the two (9.2 × 10⁻⁴ Ω cm and 2.03 × 10⁻³ Ω cm). When the content of AgNWs in the hybrid further increases, the composites filled with the hybrid have the same volume resistivity as those filled with pure AgNWs. The synergistic effect on improving the electrical conductivity of the composites results from the formation of effective conductive channels in the 3D hybrid network. The graphene sheets embed in the open spaces of the AgNW network and enhance the interfacial contacts between the neighboring AgNWs, thus leading to the formation of more effective conductive paths in the 3D hybrid network. Moreover, the graphene can protect the AgNWs from oxidation which will deteriorate the electrical conductivity. The different ratio between the AgNWs and graphene results in different degrees of improvement on the electrical conductivity. When the proportion of AgNWs to graphene is fixed at 2 : 1, the largest improvement of the electrical conductivity is obtained. When the graphene is not sufficient to fill the open holes within the network, there is the presence of high contact resistivity and oxidation. However, the large amount of graphene sheets in the network will give rise to the aggregation of graphene sheets, which deteriorates the formation of conductive channels. The effect of the total amount of the AgNW/graphene hybrid on the electrical conductivity is studied when the relative ratio AgNWs to graphene keeps constant at 2 : 1 as shown in Fig. 5f. As the concentration of the hybrid increases, the volume resistivity decreases. The further increase of the content of hybrid leads to the increase of volume resistivity. When the content of the hybrid is 1.3 wt%, the composites have the lowest volume resistivity of 6.05 × 10⁻⁴ Ω cm. The volume resistivity has a decrease of 80%. In other words, the electrical conductivity of composites has an increase of 400.8% at the low content of the AgNW/graphene hybrid.

Structure of epoxy-based conductive composites

The excellent performance of composites is achieved not only by the inherent properties of the nanoscale filler but more importantly by the good dispersion and interface chemistry.⁴² SEM images of the fractured surfaces were used to characterize the dispersion morphology of the AgNW/graphene hybrid in the epoxy matrix (Fig. 6). The fracture surface of the controlled sample is smooth (Fig. 6a). In the composite filled with AgNWs, the aggregates of AgNWs are observed as shown in Fig. 6b and c. However, compared to the two samples, the fracture surfaces become relatively rough after the addition of the AgNW/graphene hybrid as shown in Fig. 6d. High magnified images (Fig. 6e and f) show that the AgNW/graphene hybrid is uniformly embedded into and tightly bound to the epoxy matrix. The 3D layered hybrid structure is also observed in the composites. There are no large clusters and agglomerates of graphene or AgNWs across the whole fracture surface of the composites.

Fig. 6 SEM images of the cross section of epoxy-based composites before adding micro-scale Ag flakes. (a) Control sample, (b) and (c) filled with AgNWs, (d)–(f) filled with the AgNW/graphene hybrid.

XRD investigations were also performed to further analyze the AgNW/graphene hybrid structure in the composites (Fig. 7). The composites containing 0.9 wt% AgNW/graphene hybrid with the proportion of AgNWs to graphene in the range of 6 : 1 to 2 : 1 does not show the peak at 26°, corresponding to nonexfoliated graphite (Fig. 7a). Moreover, all the samples with hybrid loading amounts of 0.2 to 1.7 wt% do not show the presence of the peak (Fig. 7b). These results indicate that there are no aggregates in the composites as the content increases to 1.3 wt%. The four peaks corresponding to Ag remain the same, suggesting that there is no change of structure of AgNWs. These results reveal that the synergic effect of AgNWs and graphene results in homogenous dispersion of the hybrid filler in the epoxy matrix and a strong combination with the matrix, thus providing a mechanical improvement. SEM observation of the composites filled with AgNW/graphene hybrid and micro-scale Ag flakes is conducted to reveal the reinforcing mechanism of the electrical conductivity of the composites (Fig. 8). In the control sample and sample filled with AgNWs, many un-contacted regions between the Ag flakes are observed. However, in the samples filled with AgNW/graphene hybrid, there is a 3D hybrid network embedded between the Ag flakes. The hybrid network provides the contact between Ag flakes, leading to the formation of effective electrical conductive channels in the conductive fillers. Because of the anisotropy of 1D NWs, the electrical conductivity of the composites is largely improved at a relatively low amount.

Fig. 7 XRD patterns of the epoxy composites filled with the AgNW/graphene hybrid. (a) Different proportions of AgNWs to graphene. (b) Different content of the hybrid (2 : 1).

Fig. 8 SEM images of the cross section of epoxy-based conductive composites after being filled with micro-scale Ag flakes. (a) Control sample, (b) filled with AgNWs, (c) filled with AgNW/graphene hybrid. The circle marked region represents the AgNW/graphene hybrid.

Conclusions

The AgNW/graphene hybrid consisting of 1D AgNWs and 2D defect-free pristine-graphene is fabricated. Pristine graphene was prepared by liquid phase exfoliation of natural graphite in low boiling-point solvent. When the AgNWs are introduced into the homogenous dispersion of pristine graphene, the graphene dispersion is broken down, and the simultaneous sediments of the hybrid take place. This is a simple approach to the AgNW/graphene hybrid without any surface modification on the pristine-graphene or the AgNWs. The intrinsic structures of the pristine-graphene and the AgNWs are well preserved in the hybrid system. The graphene sheets are tightly embedded in the synthesized AgNWs network, thereby filling the empty spaces within the AgNW network and leading to 3D hybrid network. The addition of graphene sheets serves as a bridge to enhance the interfacial contact between the neighboring AgNWs, leading to the formation of effective electron-transport channels in the network. The AgNWs network can also prevent the re-stacking of graphene sheets and protect the AgNWs from surface oxidation. The AgNW/graphene hybrid displays synergistic effects on enhancing the electrical conductivity and mechanical properties of the epoxy-based conductive composites for electrical packaging application due to the uniform dispersion of the hybrid filler within the matrix and effective electrical conductive channels formed in the composites.

Acknowledgements

The authors would like to thank the Science and Technology innovation key project of universities of Guangdong province (CXZD1106) and the Fundamental Research Funds for the Central Universities, South China University of Technology (2012ZZ0006) for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03849e

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