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Ice-templated Bimodal-porous Silver Nanowire/ PDMS Nanocomposites for Stretchable Conductor Jae Young Oh, Dongju Lee, and Soon Hyung Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06536 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Applied Materials & Interfaces

Ice-templated Bimodal-porous Silver Nanowire/PDMS Nanocomposites for Stretchable Conductor Jae Young Oh†, Dongju Lee*,‡ and Soon Hyung Hong,*, † † Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Advanced Materials Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju, Chungbuk 28644, Republic of Korea KEYWORDS Silver nanowire, 3-D nanostructure, Nanocomposite, Porous, Stretchability, Conductivity

ABSTRACT

A three dimensional (3-D) bimodal-porous silver nanowire (AgNW) nanostructures with superior electrical properties are fabricated by freeze drying of AgNW aqueous dispersion with macro-sized ice spheres for bimodal-porous structure. The ice sphere dispersed AgNW solution yields a 3-D AgNW network at the surface of ice sphere and formation of macro-pores by

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removal of ice sphere during freeze drying process. The resulting nanostructures exhibit excellent electrical properties due to their low electrical percolation threshold by the formation of macro-pores, which results in an efficient and dense 3-D AgNW network with a small amount of AgNWs. The highly conductive and stretchable AgNW/PDMS nanocomposites are made by impregnating

the

3D

porous

conductive

network

with

highly

stretchable

PDMS

(polydimethylsiloxane) matrix. The AgNW/PDMS nanocomposites exhibit high conductivity of 42 S/cm with addition of relatively small amount of 2 wt.%. The high conductivity is retained when stretched up to 120% elongation even after 100 stretching-releasing cycles. Due to high electrical conductivity and superior stretchability of AgNW/PDMS nanocomposites, it is expected to be used in stretchable electronic devices.

1. INTRODUCTION Stretchable conductors maintain their electrical characteristics when stretched or deformed. This characteristic could potentially invalidate the widely accepted assumption that electronic devices must be rigid. Electronic devices are becoming increasingly portable and research on wearable devices1-2 and skin patch devices3-4 employing stretchable conductors is underway. Stretchable conductors would allow electronic devices to be used in environments that are not conducive to conventional electronic devices, such as in flexible structures or devices that are subject to strong external forces. Stretchable conductors can be manufactured by incorporating conductive fillers, such as carbon and/or metal nanomaterials, into a stretchable matrix. Of the various metal nanomaterials that have been studied, silver nanowires (AgNWs) have shown particular promise as conductive fillers in stretchable conductors due to their excellent electrical properties,


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moderate oxidation stability, and high aspect ratio. Despite these advantages, fabricating stretchable and conductive nanocomposites from AgNWs present several problems. For example, AgNWs5 are not highly dispersible in most stretchable matrices. Therefore, at present, most stretchable conductors employing AgNWs are manufactured by transferring6-8 or coating9-11 AgNWs onto stretchable substrates. However, stretchable conductors made in this way are only partially conductive, which limits their application space.12 In addition, repeated stretching and flexing of these materials tends to detach the AgNWs from the substrate, resulting in significant stability loss. Several studies have sought to improve the applicability of AgNWs as conductive fillers in stretchable conductors by incorporating three-dimensional (3-D) porous AgNW nanostructures into a stretchable matrix.13-17 Methods for fabricating 3-D AgNW nanostructures can be classified into two types: dip-coating processes with polymer templates,13-16 and freezedrying processes without polymer templates.17 Of the former methods, Jin Ge et al. fabricated 3D AgNW nanostructures by coating AgNWs onto the surface of a 3-D polyurethane sponge (PUS).13 This method resulted in 3-D AgNW networks with low AgNW content, but the network was limited to the PUS surface and the properties for stretchable conductors were negatively affected by the presence of the polymer template. In contrast, H. L. GaO et al. fabricated 3-D AgNW nanostructures using a freeze-drying process without a polymer template.17 They produced anisotropic micro-porous 3-D AgNW nanostructures by forming AgNW networks at the interface of a frozen solvent. The solvent was then removed by freeze-drying. While the resulting nanomaterial is unaffected by a polymer template, the structure of 3-D AgNW nanostructures is hard to control because there are no templates involved. This report details the fabrication of 3-D bimodal-porous AgNW nanostructures by introducing ice sphere to control pore size distribution. Polymer templates are not conducive to freeze-drying


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methods and require additional removal processes. Conversely, ice spheres are removed together with the frozen solvent during freeze-drying and form additional macro-pores of different size from micro-pores formed during solvent removal. This allows structural control of the 3-D AgNW nanostructures and the AgNW content can be lowered without degrading the density of the AgNW network. This improves the electrical properties and stretchability of the AgNW based nanocomposites.

2. EXPERIMENTAL SECTION 2.1. Fabrication of ice spheres Ice spheres were prepared by dropping deionized (DI) water into liquid nitrogen at a rate of 0.3 ml/min using a syringe pump. 2.2. Fabrication of bimodal-porous AgNW nanostructures The AgNWs by N&B company had a length of 5~15 µm and a thickness of 40~60 nm (Figure S1). The initial isopropanol (IPA) solvent in the AgNW solution was replaced with DI water to allow rapid freezing. To change the solvent, the AgNW solution was centrifuged at 8,000 rpm for 5 min to allow sedimentation of the AgNWs. After removing the IPA, the AgNWs were redispersed in DI water using a vortexer. The ice spheres were added to the redispersed AgNW solution, which was then rapidly frozen in liquid nitrogen. Bimodal-porous AgNW nanostructures were then prepared by freeze-drying the mixture at -80ºC and 0.05 mTorr. 2.3. Optical welding of bimodal-porous AgNW nanostructures


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Optical welding was performed with an LFA 467 xenon flash lamp. The pulse voltage was controlled to 150 V, 210 V or 270 V with a pulse width of 100, 200, 800 or 1500 µs. 2.4. Fabrication of bimodal-porous AgNW/PDMS nanocomposites A PDMS is elastic matrix with high stretchability but non-conductive polymer. A PDMS mixture made by mixing PDMS precursor and curing in a weight ratio of 10 to 1 was allowed to infiltrate the bimodal-porous AgNW nanostructures in a vacuum oven for 2 hours. The AgNW/PDMS nanocomposites were then cured at 90ºC for 2 h. 2.5. Characterization Microstructural analyses of AgNW nanostructures were conducted with a scanning electron microscope (SEM) (S-4800; Hitachi). The electrical conductivity of the AgNW/PDMS nanocomposites was calculated from the sheet resistance using a four-point probe system (CMTSR1000N; Advanced Instrument Technology) and the thickness of the sample. The stretchability of the AgNW/PDMS nanocomposites was measured using a universal testing machine (Instron 8848; Instron).

3. RESULTS AND DISCUSSION The illustration in Scheme 1 shows the fabrication of 3-D bimodal-porous AgNW nanostructures with a stretchable PDMS matrix. Macro-sized ice spheres were fabricated by using a syringe pump to form water droplets followed by rapid freezing of the droplets in liquid nitrogen. The ice spheres were added to an AgNW solution, which was then rapidly frozen in liquid nitrogen to


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create an AgNW network at the ice interface. Micro-sized ice crystals in the frozen solvent and the macro-sized ice spheres were removed by freeze-drying without destroying the 3-D AgNW nanostructures, resulting in micro- and macro-pores, respectively. Optical welding done using a xenon flash lamp was introduced to improve the electrical properties of the AgNW nanostructures by reducing the interfacial resistance between AgNWs. The welding conditions were optimized by controlling the power of the xenon flash lamp. Stretchable and conductive AgNW/PDMS nanocomposites were fabricated by impregnating the 3-D bimodal-porous AgNW nanostructures with PDMS and thermally curing.

Scheme 1. Schematic images of 3-D bimodal-porous AgNW/PDMS nanocomposites Figures 1a showed the 3-D bimodal-porous AgNW nanostructures fabricated by macro-sized ice spheres (Figure S2). The cross section of AgNW/PDMS nanocomposites (Figure 1b) showed that macro-pores were well formed in the AgNW nanostructure by the rapid freezing using liquid nitrogen with the floating interference of ice templates through homogeneously dispersed AgNW. Microstructural analyses confirmed that the AgNW networks were well-formed with both micro-pores (Figure 1c) and macro-pores (Figure 1d) in a 3-D AgNW nanostructure. These pores enabled the formation of 3-D AgNW nanostructures with only a small amount of AgNW. Figures 1e and 1f show quantitative histograms of pore diameter. Micro-pores ranged in diameter


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from 1 to 16 µm, with an average diameter of 9.97 µm, as shown by the quantitative histogram in Figure 1e. Macro-pores ranged from of 0.6 to 2.4 mm, with an average diameter of 1.35 mm, as shown in Figure 1f.

Figure 1. (a) Top view image of bimodal-porous AgNW nanostructures. (b) Cross section image of bimodal-porous AgNW/PDMS nanocomposites. (c) Microstructure of micro-pores in bimodal-porous AgNW nanostructure. (d) Microstructure of macro-pores in bimodal-porous AgNW nanostructure. (e) Histogram of micro-pore diameter. (f) Histogram macro-pore diameter. Optical welding significantly improved the interconnectivity of the 3-D bimodal-porous AgNW nanostructure network. The interfacial resistance of the as-formed AgNW network is largely due to impurities such as polyvinylpyrrolidone (PVP), which is a capping agent using in the synthesis of AgNWs,18 and the contact distance between AgNWs. The bonding of AgNWs through optical welding improves electrical performance by reducing this interfacial resistance, as shown in Figure 2a. AgNW nanostructures can be welded at lower energies than AgNW films. This was confirmed by observing microstructure changes and measuring the conductivity of


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AgNW films (Figures S3, S4) and AgNW nanostructures (Figure S5, S6, S7) as a function of the pulse voltage and pulse width of the xenon flash lamp. This is because the AgNW nanostructures contained a lower density of AgNWs, resulting in lower heat transfer efficiencies and higher heat concentrations in the AgNW nanostructures. After optical welding process in the optimized welding condition, bonding of AgNW networks in 3-D bimodal-porous AgNW nanostructures were also observed as shown in the microstructure (Figure 2b). The magnified image in Figure 2c shows this bonding clearly. Comparisons of electrical conductivity were made between optically welded, bimodal-porous AgNW nanostructures, non-welded, micro-porous AgNW nanostructures without ice spheres and non-welded, bimodal-porous AgNW nanostructures (Figure 2d). The optically welded, bimodal-porous AgNW nanostructures exhibited the highest conductivity (42.36 S/cm). The non-welded, micro-porous AgNW nanostructures exhibited the lowest conductivity of (18.58 S/cm). These data show that reduction of the contact resistance between AgNWs and structural control of 3-D bimodal-porous AgNW nanostructures enhance the electrical conductivity of 3-D AgNW/PDMS nanocomposites.


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Figure 2. (a) Schematic images of optically welded AgNWs (b) Microstructures of optically welded AgNWs (c) Magnification of optically welded AgNWs (d) Electrical conductivity change of 2wt.% AgNW/PDMS nanocomposites by ice sphere and optical welding. To better understand the effects of ice templating on structural control, the microstructural and electrical properties of 3-D AgNW nanostructures, prepared with or without ice templating, were compared. Note that AgNW content was carefully controlled such that all systems contained the same amount of AgNWs. Figure 3a and 3b are schematic images and microstructures of micro-porous AgNW nanostructures and bimodal-porous AgNW nanostructures prepared with the same contents of AgNWs, respectively. The 3-D bimodal-porous AgNW nanostructures were prepared by adding up to a 50% (v/v) ice spheres, so that up to half of the AgNW nanostructure volume was composed of macro-pores. In regions where the AgNW network was formed, the AgNW density in the bimodal-porous AgNW nanostructures was twice that in the micro-porous AgNW nanostructures. As the density of the AgNW network increased, the number of AgNWs per unit volume also increased, in turn increasing the electrical path and decreasing the contact distance between AgNWs. These structural changes should enhance the interconnectivity of the AgNW


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network. A simple model was employed to explain these effects theoretically. To allow comparison of the theoretical and experimental results, AgNW/PDMS nanocomposites were fabricated with various amounts of AgNWs. The electrical conductivities of micro-porous AgNW/PDMS nanocomposites and bimodal-porous AgNW/PDMS nanocomposites were fitted by applying a scaling law,19 shown in Equation (1), that is commonly used with conductive filler/polymer matrix nanocomposites, σ = ߪ଴ ∙ ሺΦ − Φ௖ ሻ௧

(1)

where σ is the electrical conductivity of the conductive filler/polymer matrix nanocomposite, σ0 is the electrical conductivity of the conductive fillers, Φ is the fraction of filler in the nanocomposites, Φc is the electrical percolation threshold of the nanocomposite, and t is the critical exponent. Fitting our data to Equation 1 resulted in a σ0 of 1.11 × 104 S/cm, Φc of 0.97, and t of 1.34 for the micro-porous AgNW/PDMS nanocomposite, as shown in Figure 3c. The bimodal-porous AgNW/PDMS nanocomposite yielded a σ0 of 1.37 × 104 S/cm, Φc of 0.48, and t of 1.37, as shown in Figure 3d. σ0 differs only slightly between the micro-porous and bimodal-porous AgNW nanostructures because σ0 represents the expected conductivity at 100 wt.% AgNW. However, the maximum fraction of AgNW in the bimodal-porous AgNW nanostructures was 50%. The electrical percolation threshold of 0.48 wt.% in the bimodal-porous AgNW/PDMS nanocomposite is about half that of the micro-porous AgNW/PDMS nanocomposite (0.97 wt.%). This is because, compared to a micro-porous system with the same AgNW content, the bimodalporous AgNW/PDMS nanocomposite has twice the AgNW density. Therefore, the bimodalporous system can form an electrical network with the same density as that of a micro-porous


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AgNW nanostructure but with half of the amount of AgNW. By reducing the electrical percolation threshold of the bimodal-porous AgNW/PDMS nanocomposite, electrical conductivity increased from 0.81 to 12.12 S/cm at 1 wt.% AgNW, and from 23.78 to 42.36 S/cm at 2 wt.% AgNW (Figure 3e). These results show that the addition of macro-pores can enhance the electrical properties of AgNW nanostructures through a concurrent lowering of the percolation threshold and an increase in AgNW density along the electrical path. Furthermore, controlling the volume ratio of ice spheres in the precursor AgNW solution allows tailoring of macro-pore volumes and electrical percolation thresholds.

Figure 3. Schematic images and microstructure of (a) micro-porous AgNW nanostructures and (b) bimodal-porous AgNW nanostructures with same AgNW contents. (c) Fitting electrical conductivity of micro-porous AgNW/PDMS nanocomposites using scaling law in log scale. (d) Fitting electrical conductivity of bimodal-porous AgNW/PDMS nanocomposites using scaling law in log scale. (e) Electrical conductivity of micro-porous AgNW/PDMS nanocomposites and bimodal-porous AgNW/PDMS nanocomposites with fitted curves.


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The electrical properties of a stretchable, bimodal-porous 2 wt.% AgNW/PDMS nanocomposite were measured as a function of strain. Figure 4a shows that a high stretchability of 138% was attained with a maximum conductivity of 42.36 S/cm. It has been found that optical welding improves conductivity without significant changes in stretchabililty due to the formation of an electrically improved junction which is only partially melted at the AgNW contact site (Figure S8a). Furthermore, it was analyzed that the electrical network maintenance characteristics were improved by the decrease of the distance between AgNW network through bimodal-porous structure and junction contact improvement by optical welding as shown in Figure S8b. Due to these structural properties, electrical conductivity was highly maintained at 25 S/cm, even at 120% elongation. The relative change in resistance, given by strain (R)/initial resistance (R0), was measured during the first and second stretches. The data in Figure 4b show that R/R0 was maintained at 1.08 at up to 40% strain, and increased to 1.64 at 120% strain. When strain was removed, R/R0 decreased from 1.64 to 1.44. This shows that some of the AgNW networks were restored when the stain was removed. However, these data also show that some electrical degradation occurred after the first stretch. Comparing the microstructures of AgNW/PDMS nanocomposites before stretching (Figure S9a,9b) and after the first stretch (Figure S9c, S9d) reveals an increase in the contact distance between AgNWs. The changes in R/R0 were less severe after the second stretch. Changes in the contact characteristics between AgNWs after the second stretching (Figure S9e, S9f), as they relate to microstructural differences, are not as clear as after the first stretch. This implies that the weaker components of the AgNW network were largely destroyed during the first stretch, with relatively less network degradation occurring during the second stretch.


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These results suggest that the electrical characteristics of our AgNW/PDMS nanocomposites will become more stable with increasing stretch cycles. This hypothesis was confirmed by the data in Figures 4c and 4d. Electrical conductivity was practically constant after 10 stretch cycles, maintaining above 20 S/cm even after 100 cycles of stretching to 120%. These results are significantly enhanced than those found in most reports of AgNW nanostructure based stretchable conductors due to enhancement of electrical network by macro-pore formation and reduction of interfacial resistance between AgNWs by optical welding process (Figure 4e).13, 1517, 20-24

The 3D bimodal-porous AgNW nanostructure is advantageous in reducing the content of

conductive filler in a stretchable conductor by having high conductivity at low density (12.12 S/cm at 10 mg/cm3, 42.36 S/cm at 20 mg/cm3) compared to reported micro-porous copper nanowire nanostructure (1.13 S/cm at 9.4 mg/cm3)22,25 or micro-porous AgNW nanostructure (25 S/cm at 40 mg/cm3)17. Therefore, our results show that fabricating 3-D bimodal-porous nanostructures by freeze-drying with an ice sphere reduces manufacturing costs by allowing for lower proportions of conductive filler, while simultaneously improving the performance of stretchable electronic devices. The bimodal-porous AgNW/PDMS nanocomposites were applied to the components of stretchable circuit for light bulb as shown in Figure S10. Due to these stable electrical signal transmission characteristics, it is expected to be applicable to various stretchable electronic devices.


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Figure 4. (a) Electrical conductivity of 2wt.% bimodal-porous AgNW/PDMS nanocomposites as strain. (b) R (changed resistance as strain)/R0 (initial resistance) of 2wt.% bimodal-porous AgNW/PDMS nanocomposites as strain. (c) Electrical conductivity of bimodal-porous 2 wt.% AgNW/PDMS nanocomposites as a function of strain with increasing the number of stretches. (d) Electrical conductivity changes of bimodal-porous 2wt.% AgNW/PDMS nanocomposites as a function of stretching cycles. (e) Electrical conductivity and stretchability of 3-D AgNW nanostructure based nanocomposites.

4. CONCLUSIONS 3-D bimodal-porous AgNW nanostructures containing macro- and micro-pores are prepared by solution-based methods; freeze drying of AgNW aqueous dispersion with removable macrosized ice spheres. The ice sphere yields a 3-D AgNW network at the surface and then formation of macro-pores during freeze drying. Our 3-D bimodal-porous AgNW nanostructures exhibited high AgNW densities along the electrical path and highly interconnected AgNW networks with a small amount of AgNWs, consequently lower electrical percolation thresholds. In addition, to


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further improving the electrical properties of the 3-D AgNW nanostructures by reducing contact resistance, optical welding are performed. 3-D bimodal-porous AgNW/PDMS nanocomposites are fabricated by PDMS infiltration into the 3-D bimodal-porous AgNW nanostructure for stretchable conductors. The resulting AgNW/PDMS nanocomposites exhibited high conductivities of up to 42 S/cm with a stretchability of up to 138%, with conductivity being maintained after 100 stretching cycles at relatively small amount of 2wt.%. These highly conductive and stretchable AgNW/PDMS nanocomposites are suitable for applications in stretchable electronic devices.

ASSOCIATED CONTENT Supporting Information. Microstructure and electrical properties of optically treated AgNW films and 3-D AgNW nanostructures; Microstructure of AgNW network changes in PDMS matrix with strain The following files are available free of charge.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected]


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ACKNOWLEDGMENT This research was supported by Nanomaterial Technology Development Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609).

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PDMS

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