Multi-Dimensional Ternary Hybrids with Synergistically Enhanced

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Applications of Polymer, Composite, and Coating Materials

Multi-Dimensional Ternary Hybrids with Synergistically Enhanced Electrical Performance for Conductive Nanocomposites and Prosthetic Electronic Skin Yougen Hu, Xuebin Liu, Lan Tian, Tao Zhao, Hui Wang, Xianwen Liang, Fengrui Zhou, Pengli Zhu, Guanglin Li, Rong Sun, and Chingping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14932 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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ACS Applied Materials & Interfaces

Multi-Dimensional

Ternary

Hybrids

with

Synergistically Enhanced Electrical Performance for Conductive

Nanocomposites

and

Prosthetic

Electronic Skin Yougen Hu,† Xuebin Liu,†, ‡ Lan Tian,† Tao Zhao,† Hui Wang,† Xianwen Liang,† Fengrui Zhou,† Pengli Zhu,*, † Guanglin Li,† Rong Sun,† and Ching-Ping Wong†,⊥

†Shenzhen

Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, China

‡Shenzhen

College of Advanced Technology, University of Chinese Academy of

Sciences, Shenzhen 518055, China

⊥School

of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,

Georgia 30332, USA

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KEYWORDS: conductive polymer nanocomposites, prosthetic electronic skin, flexible strain sensor, multi-dimensional hybrids, synergistic effect

ABSTRACT: Graphene and silver nanowires (AgNWs) are idea fillers for conductive polymer composites but they tend to aggregate in polymer matrix due to the lack of surface functional groups and large specific surface area, which is hard for the polymer composites filled with them to reach their full potential. Here, ternary hybrids with multi-dimensional architectures including 3D polystyrene (PS) microspheres, 2D reduced graphene oxide (RGO) nanosheets and 1D AgNWs are obtained using a simple but effective electrostatic attraction strategy. The electrical conductivity (136.25 S m-1) of the ternary hybrid conductive nanocomposites filled with RGO and AgNWs is significantly higher than that of the nanocomposites containing only RGO (3.255 S m-1) at the same total filler loading due to the synergistic effect of RGO and AgNWs. The conductive nanocomposites simultaneously present a low percolation threshold of 0.159 vol% and a maximum electrical conductivity of 1230 S m-1 at 3.226 vol% filler loading. Moreover, a flexible electronic skin based on the multi-dimensional ternary hybrids is presented, and it exhibits large stretchability, high gauge factor and excellent cyclic working durability, which is successfully demonstrated in monitoring prosthetic finger motions. 2 ACS Paragon Plus Environment

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1.

INTRODUCTION

Electrically conductive polymer nanocomposites are widely used in a variety of application areas such as antistatic protection, electromagnetic shielding, electrodes, sensors, electronic devices, and so on, due to their fascinating advantages of low cost, light weight and ease processibility, which have attracted great attention and been extensively studied in the past few decades.1-5 The general strategy to construct the electrical conductivity is adding conductive fillers into an electrically insulating polymer matrix. The conductive fillers mainly include metallic fillers (Au, Ag, Cu, Ni, Al, etc.) and carbon fillers (carbon black, carbon nanotube, carbon fiber, graphite, graphene, etc) with different morphologies and dimensionalities, such as zero-dimension (nanoparticle), one-dimension (nanowire, nanofiber, nanotube, nanorob, etc) and two-dimension (nanosheet, nanoplate, etc).6-9 As conductive fillers, carbon-based materials have outstanding merits of light-weight and easy surface functionalization, and metallic materials show significant advantages of high electrical conductivity. However, there are still some defects of relatively low electrical conductivity for carbon-based fillers, high cost and high density for metallic fillers, 3 ACS Paragon Plus Environment


resulting into poor mechanical property of the nanocomposites at high filler loading.10-13 So, one of the critical issues in developing high performance conductive polymer composites is reducing the filler content and increasing the conductivity, as well as enhancing mechanical and thermal properties. In addition, employing conductive nanomaterial network is one of mainstream strategies to construct flexible strain sensors or stretchable conductors which widely used in emerging electronic skins (e-skins), due to they can maintain conductance under mechanical deformations such as stretching, bending and folding when integrated with elastomer matrix.14-16 If the e-skins possess high electrical conductivity, they could generate effective current with a low voltage, indicating their low energy consumption and portable energy harvesting devices possibility. Therefore, it is vital to reduce the percolation threshold and improve the electrical conductivity of whether the common conductive polymer nanocomposites or the emerging flexible e-skins. Graphene is regarded as the thinnest material in the universe with extraordinary electronic, electrochemical, thermal, and mechanical properties, and it is considered valuable in various applications, such as electronics, catalysis, sensors, solar cells, biomedicines, and composite materials.17-22 Unfortunately, pristine graphene as a bulk material has a remarkable tendency to agglomerate because of its π-π stacking between layers. Many efforts have been made to achieve good dispersion of graphene in polymer, such as melt-blending, solution mixing and in situ polymerization.23-26 Graphene oxide (GO) and reduced graphene oxide (RGO), as derivatives of graphene, maintain many merits of pristine graphene and overcomes the difficult dispersion problem due to its oxygen containing functional groups on the surface and edge. In order to build a continuous conductive network using as little graphene as possible, 3D architecture was usually designed and made for the graphene-based polymer composites. For instance, Vickery et al. 4 ACS Paragon Plus Environment

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demonstrated an ice-segregation-induced self-assembly technology to produce graphene-polymer nanocomposites with higher-order 3D architectures.27 Ju et al. presented a simple approach for the fabrication of monodisperse electroconductive hybrid spheres with electrical conductivity of 1.33-4.21 S m-1 using graphene sheet-wrapping via ionic interaction-based self-assembly.28 Wu et al. fabricated a highly conductive polymer nanocomposites with 3D compactly interconnected graphene networks using polystyrene (PS) and ethylene vinyl acetate (EVA) as polymer matrixes by self-assembly process.29 Yang et al. developed a unique “particle-constructing” method for fabricating highly ordered 3D graphene-based polymer composite materials, throughout which the graphene nanosheets formed intact, uniform and well defined network structure.30 Although the above graphene-polymer composites posses relatively low percolation threshold due to the existence of unique 3D graphene conductive network, the electrically conductivity of the nanocomposites still relatively low due to defects and incomplete reduction of RGO compared with high temperature thermal reduction of GO

31, 32

and chemical vapor deposition (CVD)

grown graphene,33 and saturated filler loading in the 3D framework. Silver nanowires (AgNWs) are the most promising candidates for conductive materials due to their high electrical conductivity, transparency, mechanical flexible and excellent optical properties. Many previous researches have proved that the nanocomposites can obtain a high electrical conductivity after introducing AgNWs whether into pure polymer matrix34-36 or graphene-based materials,37-40 but usually high concentration AgNWs is required. In this work, we report a simple approach to fabricate ternary hybrid conductive networks for conductive polymer nanocomposites by using 3D cationic polystyrene (CPS), 2D RGO and 1D AgNWs. Due to the unique multi-dimensional architecture and synergistic effect between RGO and AgNWs, the CPS/RGO/AgNWs nanocomposites exhibit low percolation threshold, high electrically 5 ACS Paragon Plus Environment


conductivity and robust mechanical-thermal properties. Furthermore, prosthetic electronic skin based on the CPS/RGO/AgNWs hybrids shows large stretchability, high gauge factor and excellent long-term working stability, exhibiting great potential application in emerging flexible electronics such as prosthetic rehabilitation, internet of things (IoT), flexible circuits, human motion monitoring and soft robotics. 2.

EXPERIMENTAL SECTION

2.1 Materials Styrene (St, CP), 2,2’-azobls(2-methylpropionltrile) (AIBN, 99%) and hydroiodic acid (HI, ≥47.0%) were purchased from Aladdin. Methacroylcholine chloride (DMC, 80% in water) was obtained from TCI. Silver nitrate (AgNO3, ≥99.8%), Graphite powder (8000, mesh, 99.5%), anhydrous cupric chloride (CuCl2, 99%), ethyl alcohol (EtOH, AR), polyvinylpyrrolidone (PVP K-30), ethylene glycol (EG, AR), potassium permanganate (KMnO4, ≥99.5%), sodium nitrate (NaNO3), hydrochloric acid (HCl, 36.0~38.0%), sulfuric acid (H2SO4, ≥95.0%) and hydrogen peroxide (H2O2, ≥30.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Liquid PDMS (Sylgard 184) was provided by Dow Corning. All of the above chemicals were used as received. 2.2 Synthesis of CPS Microspheres, GO and AgNWs CPS microspheres with positive charge were synthesized by a modified dispersed polymerization method.41 GO was prepared with modified Hummer’s method from natural graphite.42 AgNWs were obtained via a modified CuCl2-mediated polyol process as described in our previous report.43 The details are shown in the Supporting Information. 2.3 Fabrication of CPS/RGO/AgNWs nanocomposites 6 ACS Paragon Plus Environment

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First, a given volume of diluted aqueous dispersion of GO (0.5 mg mL-1) was added dropwise into the 200 mL CPS microsphere suspension (1.0 wt%) under magnetic stirring of 400 rpm at room temperature for 1 h, and they were self-assembled to form CPS/GO particles due to electrostatic attraction and quickly precipitated at the bottom of the breaker after stop stirring. Next, the dried CPS/GO powders were immersed into HI solution at 90 ℃ for 24 h with magnetic stir to reduce GO. The CPS/RGO hybrid particles were collected via vacuum filtration with cacuum filtration with vacuum pressure of about -0.1 MPa and adequately washed with deionized water and ethanol to remove the residues. Subsequently, the appropriate amount of AgNWs ethanol suspension was homogeneously mixed with the CPS/RGO ethanol dispersion by a vortex mixer to obtain the CPS/RGO/AgNWs hybrids ethanol suspension. After vacuum filtration of the CPS/RGO/AgNWs suspension, the filter cake was vacuum dried at 60 ℃ for 12 h. Finally the dried CPS/RGO/AgNWs hybrids were compressed into disk-like specimens at room temperature for 5 min under 25 MPa pressure by an electri tablet press machine (DY-20) followed by heating treatment at 130 ℃ for 2 h to obtain the CPS/RGO/AgNWs nanocomposites. 2.4 Fabrication of CPS/RGO/AgNWs/PDMS Flexible Strain Sensor First, a partially cured PDMS film was prepared by mixing PDMS base and curing agent with a mass ratio of 10:1 and spin-coated on a glass substrate at 800 rpm for 12 s followed by curing at 60 ℃ for 30 min. Then, 4 mL CPS/RGO/AgNWs hybrids ethanol suspension (6.5 mg mL-1) was dipped into a mold (20 mm × 5 mm × 0.3 mm) on the partially cured PDMS film. After ethanol complete evaporation at room temperature in a vacuum oven under -100 kPa vacuum pressure for 2 h and mold removal, liquid PDMS mixture was poured into the CPS/RGO/AgNWs hybrids and placed in a vacuum oven to accelerate infiltration of liquid PDMS into voids of the hybrids.



Ultimately, the PDMS was fully solidified at 80 ℃ for 2 h and carefully peeled off from the glass substrate to obtain the CPS/RGO/AgNWs/PDMS sensor. 2.5 Characterization The morphology and microstructure were investigated by FE-SEM (FEI Nova Nano SEM 450), TEM (FEI Tecnai G2 F20 S-TWIN), AFM (NanoFocus, Bruker). Raman spectra were performed on iHR550 spectrometer (HORIBA) using 633 nm incident laser wavelength. FTIR spectrum was measured by a spectrometer (Bruker VERTEX 70). Thermogravimetric analysis was carried out with a TA instrument (SDT Q600) at a heating rate of 10 ℃ min-1 in nitrogen atmosphere. Differential scanning calorimetry (TA Q2000) was employed to measure glass transition temperature (Tg) of the samples, which was conducted by heating at 5 ℃ min-1. Size distribution and Zeta potential measurements were performed using a Malvern instrument (Zetasizer Nano ZS). The coefficient of thermal expansion (CTE) was conducted using a thermo-mechanical analyzer (TMA 402F1, NETZSCH Instruments) by recording the change in thick dimension of the specimens from room temperature to 100 ℃ at a rate of 5 ℃ min-1. The samples in CTE study were cut into square shape with size of about 5 mm × 5 mm × 0.5~1.0 mm (length × width × thickness). Two thin layers of commercial conductive silver paste were coated on two sides of the compressed disk-like samples, and copper wires were bonded with two ends of the sensor samples by silver paste for electrical performance test by a digital multimeter (Keithley 34401A). The electrical conductivity can be calculated with the equation on basis of the resistance as follows: 1

d

(1)

σ = ρ = R×S

where σ is the electrical conductivity, ρ is the resistivity, R is the resistance, d is the thickness of sample and S is the cross-sectional area of the sample. The electro-mechanical properties measure 8 ACS Paragon Plus Environment

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of the strain sensor was carried out on a universal testing machine connected with the digital multimeter (Agilent 34401A), and the results were recorded by a computer in real time. A prosthetic hand (i-LIMB hand, Tough Bionics) was used to demonstrate the sensing ability of the sensor as a prosthetic e-skin. 3.

RESULTS AND DISSCUSSION

3.1 Preparation and Characterization of CPS/GO, CPS/RGO and CPS/RGO/AgNWs Hybrids CPS/GO hybrids were prepared via electrostatic attraction strategy between positive charged CPS microspheres and negative charged GO nanosheets. After chemical reduced of CPS/GO, CPS/RGO hybrids were formed, and AgNWs were subsequently integrated into the CPS/RGO conductive network by latex mixing to obtain CPS/RGO/AgNWs hybrids. The fabrication process is illustrated in Figure 1a and the details see Experimental Section. Figure 1b shows the SEM image of the synthesized CPS spheres, which exhibit perfect spherical shape and nicely uniform size of ~1.0 μm. Figure S1 in the Supporting Information is the size distribution curve of the CPS aqueous suspension, which shows a very narrow distribution range with an average size of ~1.2 μm and polydispersity index of 0.15, further confirming an excellent size uniformity of the PS microspheres. The FTIR spectrum of the CPS powder is presented in Figure S2 in the Supporting Information. It is clearly observed that peaks located at 3060, 3025, 2921, 2849 cm-1, and 1601, 1493, 1453 cm-1, which are the typical characteristic bands of the styrene unit. In addition, the absorbance peaks at 1718 and 964 cm-1 are also observed, which assigned to carbonyl and N+(CH3)3 in the DMC monomer, respectively. These results indicated that the DMC functional groups had been successfully grafted onto the surface of PS microspheres. The AFM image of GO sheets was collected by spinning coating dilute GO solution on a mica plate and 9 ACS Paragon Plus Environment


shown in Figure 1c. AFM image and its corresponding height profile show that GO has few-layer structure with size of several micrometers and their thickness of ~1.0 nm, suggesting that graphite has been completely exfoliated and consistent with previous reports. Generally, there are abundant hydroxyl and carboxyl groups randomly distributed on the basal plane of GO nanosheets, resulting in negative charge of GO aqueous solution. It is can be proved by the FTIR spectrum of GO as shown in Figure S3, which exhibits characteristic peaks at around 3414 and 1400 cm-1 for hydroxyl groups and peak at around 1726 cm-1 for carboxyl groups. As a control, the characteristic peak of RGO at 3420 cm-1 was decreased and peaks at 1400 and 1726 cm-1 were nearly disappeared, indicating hydroxyl and carboxyl groups were mostly removed after chemical reduction of GO. The matched pair surface charge between GO nanosheets and CPS microspheres is important for the self-assembly process by the electrostatic interaction. The zeta potential of GO and CPS suspension were measured and the results are shown in Figure S4 in the Supporting Information. It can be seen that GO nanosheets present negative charge (-12.1 to 50.9 mV) while CPS microspheres show positive charge (11.8 to 62.6 mV) over the pH range (111). Moreover, the potentials of CPS and GO aqueous suspensions almost simultaneously reach their maximum value at the same pH of 6, indicating the electrostatic assembly can spontaneously occur under weakly acidic medium. The synthesized AgNWs have length in the range of 10-60 μm and average diameter of 90 ± 20 nm, as shown in Figure 1d. And there are barely particles are observed, indicating pure AgNWs with large aspect ratio were obtained by the polyol process. The crystalline property of the AgNWs was also collected by XRD as shown in Figure S5a in the Supporting Information. The XRD pattern shows that four major peaks corresponded to the (111), (200), (220) and (311) planes of silver crystal (JCPDS card No. 04-0783), which indicates that the synthesized AgNWs 10 ACS Paragon Plus Environment

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have a pure crystalline structure and can be assigned as face-centered cubic (fcc) silver. XPS further confirms the purity and chemical state of the synthesized AgNWs. Figure S5b-d in the Supporting Information show the XPS wide scan spectrum, the Ag 3d and O 1s deconvoluted spectra of the AgNWs, respectively. The Ag 3d spectrum with two peaks at 373.98 and 367.99 eV corresponding to Ag 3d3/2 and Ag 3d5/2, respectively, demonstrates the presence of metallic silver. Moreover, O 1s spectrum with peak at 531.33 eV suggests that residual PVP was adsorbed onto the surfaces of the AgNWs, revealing strong chemical interaction between PVP and AgNWs. Just because of the existence of PVP, it restricts the radial growth of metallic silver and ultimately formed the AgNWs. Figure S6a-f in the Supporting Information shows the SEM images of the assembled CPS/GO with different ratios. Creamy GO is uniformly spread over the CPS microspheres surface forming considerably rougher surfaces compared with pure CPS spheres. And the CPS microspheres adsorb more GO sheets with an increase of GO content, ultimately fully covered by GO sheets at the high concentration of 4.555 vol%. From the corresponding photographs of the CPS/GO suspensions as shown in Figure S6g in the Supporting Information, it can be observed that all the CPS/GO samples generated coagulations and precipitations after electrostatic assembly between negative GO and positive CPS. Additionally, there are clear supernatants of the suspensions under low GO content no more than 1.179 vol%, and the supernatants gradually become brown with increasing of GO content, implying incomplete assembly of CPS microspheres and GO nanosheets at higher GO content. CPS/RGO hybrids were obtained by chemical reducing of CPS/GO in HI solution. Figure 1f exhibits the SEM image of the typical CPS/RGO hybrids with 0.946 vol% of RGO, i.e. mass ratio of CPS:RGO=50:1. Less creamy overlays are observed in the CPS/RGO spheres as 11 ACS Paragon Plus Environment


compared with the corresponding CPS/GO hybrids (Figure 1e). Figure 1g is the SEM image of the hollow RGO prepared by removing CPS polymer microspheres with DMF solvent (details see the Supporting Information), which shows the circles with diameter of about 1 μm are just the holes remained by the CPS microspheres, demonstrating that the CPS microspheres were fully covered by the RGO and completely removed by DMF. The hollow structure did not collapse after the removal of the CPS core because of the mechanical strength of the RGO walls in spite of their thin thickness. In addition, the CPS microspheres in the CPS/RGO hybrids prevent RGO from re-stacking during chemical reduction through inter-layer π-π interactions. The TEM images of CPS/RGO (Figure 1i) are further confirmed that there are less conglutinations among the CPS microspheres surfaces than that of CPS/GO (Figure 1h). These results can be explained as the flexible GO nanosheets with abundant -COOH and -OH groups were successfully modified to rigid RGO sheets due to the loss of oxygenated functional groups of the GO as proved by the FTIR spectra in Figure S3 in the Supporting Information, ultimately resulting into the tighter and wizened cover layers on CPS surfaces. In order to further verify the effect of the CPS microspheres in CPS/RGO/AgNWs hybrids, GO nanosheets were chemical reduced to RGO at the same condition as CPS/GO reduction, GO/AgNWs (GO:AgNWs=1:20, w/w) and RGO/AgNWs (RGO:AgNWs=1:20, w/w) hybrids were also fabricated by mixing GO and RGO with AgNWs without CPS microspheres, respectively. The results are showed in Figure S7 in the Supporting Information. It can be seen from Figure S7a that GO nanosheets are smoothly paved on the Si substrate with a little stacking. However, RGO obtained from chemical reduction of GO by HI solution at 90 ℃ for 24 h without assistance of CPS microspheres exhibits obvious agglomeration and forming a disordered solid bulk due to existence of strong Van der Waals’ force among GO nanosheets (Figure S7b). As shown in Figure S7c, AgNWs were uniformly dispersed to form a conductive network and GO nanosheets were patched well on AgNWs. 12 ACS Paragon Plus Environment

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However, for the RGO/AgNWs hybrids as shown in Figure S7d, it can be seen that the aggregated RGO bulks were randomly and non-homogenously trapped into AgNWs conductive network. Therefore, it is important to introduce CPS microspheres into the RGO/AgNWs hybrids to build the three-dimensional architectures. Figure 2a-e present the SEM images of CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) hybrids with various AgNWs contents before mechanical compress treatment. It clearly shows that the AgNWs are well individually and randomly dispersed in the CPS/RGO microspheres compared with the sample without AgNWs (Figure 1f), forming uniform distribution and numerous interconnected networks with random contacts between neighboring AgNWs. With typically increasing of the contents of AgNWs from 0.471 vol% to 3.644 vol% in the hybrids, the density of AgNW networks gradually increased and formed to continuous conductive networks, which is beneficial to enhance the electrical conductivity of the ultimate nanocomposites. It should be emphasized out that there are no aggregations of AgNWs are observed even for the composites with high loading of 3.644 vol% of AgNWs. The excellent distribution of AgNWs in the CPS/RGO hybrids can be mainly attributed to the good dispersion both of AgNWs and CPS/RGO in ethanol solvent, and the AgNWs are successfully separated by the CPS/RGO microspheres, avoiding electrostatic affinities among AgNWs and preventing them from agglomeration. Moreover, from photographs of Figure S8 in the Supporting Information, it can be seen that the coagulations were quickly generated after mixing of the well dispersed CPS/RGO and AgNWs ethanol dispersion, suggesting existence of the possible electrostatic interactions between positively charged CPS/RGO and negatively charged AgNWs which confirmed by zeta measurement results as shown in Figure S9 in the Supporting Information. The opposite charges facilitate formation of the agglomeration. The charges of CPS/RGO and 13 ACS Paragon Plus Environment


AgNWs are generated from excess CPS charge and slightly negative charge of PVP capped on the surface of AgNWs, respectively. The presence of GO and RGO coverage on CPS microspheres is confirmed the evidence of by Raman spectra as shown in Figure 2f. For the CPS microspheres, there are some sharp signals at about 622, 798, 1003, 1032, 1200, and 1584 cm-1, the typical strong signal at 1003 cm-1 can be assigned to ν1 ring-breathing mode of polystyrene.44 For pure GO, there are two classical peaks of D peak and G peak at about 1330 and 1584 cm-1, respectively. Whereas two distinct peaks at about 1330/1595 cm-1, 1328/1588 cm-1 and 1330/1585 cm-1 accompanied with a very weak peak at 1003 cm-1 are displayed obviously for CPS/GO, CPS/RGO and CPS/RGO/AgNWs hybrid particles, respectively, indicating the existence of GO or RGO in the hybrids. Interestingly, the characteristic peaks of CPS almost disappear due to the Raman signal belonging to CPS could be absorbed by the full and dense coverage of GO or RGO on CPS microspheres surfaces. Moreover, an increased ID/IG intensity ratio of the CPS/RGO hybrid spheres (1.524) compared to that of GO (1.244) and CPS/GO (1.165) indicates the decrease of the sp2 in-plane domain induced by the introduction of defects and disorder of the sp2 domains i.e. the successful reduction of GO. XPS analyses were further conducted to analyze the elemental compositions of the resulting samples. The peaks at 286.9 and 532.7 eV shown in Figure 2g can be assigned to the binding energies of C 1s and O 1s of GO, respectively. For CPS microspheres, one sharp peak at 284.8 eV and two relatively weak peaks at 399.7 and 532.8 eV can be assigned to the binding energies of C 1s, O 1s and N 1s, respectively. The O 1s and N 1s peaks in CPS can be attributed to the carbonyl and N+(CH3)3 groups in the DMC, which again proved that the DMC functional monomers were successfully grafted to the PS spheres. After self-assembly of GO and CPS, the resulting products of CPS/GO shows three peaks at ~284.8, 399.9 and 532.8 eV, which are the 14 ACS Paragon Plus Environment

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combination of GO and CPS, suggesting the existence of only physical interaction between GO and CPS during their self-assembly process. After chemical reduction of CPS/GO with HI, the CPS/RGO shows three similar peaks with CPS/GO, but the peak of O 1s is greatly reduced, indicating that most of the oxygen-containing groups in GO were removed by the reduction process. Moreover, the atomic ratio (C/O) of CPS/RGO in the C 1s peak is increased significantly compared to CPS/GO from 6.38 to 31.35, also revealing the decrease of the oxygencontaining groups. For the final sample of CPS/RGO/AgNWs, in addition to the peaks of C 1s, O 1s and N 1s, there are two obvious peaks of Ag 3d and Ag 3p. In order to further evaluate the reduction degree of GO on CPS microspheres after chemical reduction, the RGO samples were extracted by through removing CPS microspheres in DMF solvent prior to XPS test to eliminate the interference of CPS. Figure 2h and i show the C 1s XPS spectra of GO and RGO respectively. Three fitted peaks at 288.8 eV (O=C-O), 287.4 eV (C=O) and 286.8 eV (C-O-C) are clearly identified on GO for different groups. After chemical reduction, these peaks are dramatically decreased even disappeared, accompanying with two new peaks at 285.2 eV and 289.4 eV, which are responsible for sp2 C-C bond and π-π* shake up satellite, respectively. The above results reveal that the most of oxygen-containing groups of GO have been successfully removed and remarkable restoration of the graphitic structure of RGO through the HI reduction reaction, which lead to excellent electric conductivity of the CPS/RGO compared to the electrically insulating GO. 3.2 Fabrication of CPS/RGO/AgNWs Nanocomposites CPS/RGO/AgNWs

nanocomposites

were

fabricated

by

mechanical

compressing

the

CPS/RGO/AgNWs hybrids at room temperature followed by heating treatment as shown in Figure 1. Figure 3 shows the across-sectional SEM images of the representatively molded pure 15 ACS Paragon Plus Environment


CPS microspheres, CPS/RGO (0.946 vol%) and CPS/RGO/AgNWs (0.94 vol% RGO and 0.94 vol% AgNWs) nanocomposite specimens. In order to clearly observe the cross-sectional morphology, the compressed samples were cut and polished by GATAN Ilion II 697 system, and the SEM analysis was operated at a high accelerating voltage of 10 kV without advance sputter coating with gold of these samples. As shown in Figure 3a, c and e, for the molded samples only treated by mechanical pressing at room temperature but did not undergo post heating treatment, it can be observed that the CPS, CPS/RGO microspheres and AgNWs were compactly contacted with each other compared to their corresponding powders, respectively, due to the plastic deformation of the CPS polymer under mechanical compression. However, there still are obvious micro-gaps at the interfaces among the microspheres as shown in yellow dotted circles, indicating that it is difficult to form thoroughly and tightly contact when the powders were only treated by mold pressing at room temperature. After the molded samples were further treated by heating at 130 ℃ for 2 h, the packing voids in the specimens are hardly observed as shown in Figure 3b, d and e on account of the micro-flow of the molecular chains of the thermoplastic CPS microspheres under heat treating above Tg for sufficient time. In addition, it can be seen that there are scarcely any interfaces in the pressed CPS samples (Figure 3a and b), however, numerous distinct boundaries exist between neighboring microspheres in the CPS/RGO samples (Figure 3c and d) and they form a continuous interconnected network. That’s the thin RGO layers wrapped on the surfaces of CPS microspheres. The interconnected RGO network provides high electrical conductivity for the CPS/RGO samples. Figure 3e and f show the SEM images of the CPS/RGO/AgNWs nanocomposites. The AgNWs are observed uniformly distributed along the compressed interfaces of neighboring CPS/RGO microspheres, forming abundant conductive networks for the CPS/RGO/AgNWs nanocomposites. It should be emphasized that after postheating treatment of the compressed samples, their disk-like shape can be perfectly maintained, 16 ACS Paragon Plus Environment

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as shown in their corresponding insert. The pressed pure CPS sample change from white to transparency, further reflecting the voids are eliminated during heating treatment. The samples of CPS/RGO and CPS/RGO/AgNWs do not appear any change in color due to black of RGO. 3.3 Thermal-Mechanical Properties of CPS/RGO/AgNWs Nanocomposites The presence of RGO and AgNWs in the hybrids was quantitatively measured by TGA of representative CPS/RGO (50:1, w/w) and CPS/RGO/AgNWs (50:1:9.55, w/w) nanocomposites. As shown in Figure 4a, pure CPS microspheres can be fully decomposed below 450 ℃. However, for the CPS/RGO particles, the residual content is about 1.64 wt%, which is slightly smaller than the theoretical value of 1.96 wt%, the difference can be attributed to the removal of oxygen functional groups remaining in RGO. In order to further accurately calculate the content of RGO in CPS/RGO hybrids, a certain amount of CPS/RGO powders were immersed in toluene solvent to completely dissolve CPS polymer and remove it by several centrifugal and washing cycles. After dried of the centrifugal precipitate in a vacuum oven, the content of RGO in CPS/RGO hybrids is calculated to be ~1.75 wt%, which is basic consistent with the result measured by TGA. In curve of the CPS/RGO/AgNWs, the residual weight percentage is 16.3 wt%, which is also less than the theoretical value of about 17.4 wt%. It can be explained by the degradation both of the oxygen functional groups in RGO and PVP encapsulating the AgNWs. Moreover, the thermal stability of the nanocomposites can be inferred by the onset decomposition temperature (defined as the temperature at 5% weight loss) from the TG curves. The decomposition temperature for neat CPS is ~367 ℃, and ~389 ℃ for CPS/RGO hybrids, indicating the RGO on the surfaces of the CPS microspheres can obviously improve the thermal stability of the pristine polymer. However, after added AgNWs, the onset decomposition temperature for the CPS/RGO/AgNWs nanocomposites is essentially the same as the CPS/RGO, suggesting that the impact of AgNWs on the thermal stability of the CPS/RGO hybrids is inappreciable. 17 ACS Paragon Plus Environment


In order to further confirm the effect of RGO and AgNWs on the thermal properties, Tg of the nanocomposites was tested by DSC. As presented in Figure 4b, the endothermic peaks in DSC curves gradually shift to higher temperature region from 104 ℃ to 109.4 ℃ with increasing the RGO concentration from zero to 1.873 vol%. However, for the CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) nanocomposites, Tg values remain approximately constant with various AgNWs loadings as shown in Figure 4c. The above results clearly reveal that RGO can surely enhance the thermal stability including the onset decomposed temperature and Tg of the nanocomposites, whereas the effects of AgNWs on the thermal performance of the nanocomposites is nearly negligible. The improved thermal stability can be mainly attributed to the unique core-shell structure of CPS/RGO particles and the corresponding 3D continuous networks of RGO in the nanocomposites, there are strong interfacial bonding between CPS microspheres chains and RGO shells, and the rigid RGO nanosheets restrict the random flow of the amorphous CPS polymer segments at the temperature beyond its Tg. However, AgNWs in the nanocomposites hardly obstruct the motion of the polymer chains due to their high aspect ratio. The coefficient of thermal expansion (CTE) is an issue for polymer nanocomposites in industrial production and practical applications. Reducing CTE of polymers is an important goal to improve the dimensional stability due to the CTE of polymers is usually large and they are easy to expand when suffered heating. The effect of RGO and AgNWs on the CTE of CPS/RGO/AgNWs composites was analyzed by TMA as shown in Figure 4d and the results are presented in Figure 4e and 4f, respectively. It can be seen that the CTE values of the molded CPS/RGO/AgNWs (CPS:AgNWs=50:9.55, w/w) pieces decreased monotonically with increased RGO concentration, which decrease from ~92 ppm K-1 for the CPS/AgNWs composites without RGO to ~57 ppm K-1 with 4.474 vol% RGO. Such a significant ~38 % reduction in the CTE is 18 ACS Paragon Plus Environment

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attributed to the intrinsic negative CTE of graphene45,46 and the polymer chain segments bounded and restricted by the 3D RGO networks as described in DSC analysis. However, CTE values of molded CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) samples are almost constant with increasing the AgNWs contents from 0% to 7.032 vol%, indicating the effect of AgNWs on the CTE of CPS/RGO/AgNWs nanocomposites is insignificant due to the spherical CPS matrix is fully wrapped by RGO and unique 1D structure of AgNWs with high aspect ratio. 3.4 Electrical Properties of CPS/RGO/AgNWs Nanocomposites The post-heating treatment temperature and time predicted affects the electrical properties of the polymer nanocomposites and 130 ℃ for 2 h was selected in this work as discussed in Figure S10 in the Supporting Information. Figure 5a shows the electrical conductivity of CPS/RGO/AgNWs (RGO:AgNWs=1:1, v/v) ternary hybrid nanocomposites and the control samples of CPS/RGO and CPS/AgNWs nanocomposites as a function of total filler volume fraction. The electrical conductivity of these three types of nanocomposites increases with the increasing filler loading and shows a typical insulator-conductor transition usually described as percolation behavior. The percolation threshold of the nanocomposites is evaluated using the classical percolation theory by a scaling law:47 𝜎 = σ0(φ ― φc)t

(2)

where σ0 is the proportionality constant related to the intrinsic conductivity of the conductive filler, φ is the volume fraction of the filler, φc is the critical volume fraction of the filler at percolation threshold, and t is the critical exponent relating to the system dimensionality of composites. The fitted plots inserted in bottom right corner display the percolation behaviors of the composites. From the experimental data and fitted plots, the φc and t of CPS/RGO, CPS/AgNWs and CPS/RGO/AgNWs nanocomposites is calculated to be 0.0795 vol% and 2.03, 0.662 vol% and 2.25, 0.159 vol% and 2.62, respectively. All the t values of the three samples are 19 ACS Paragon Plus Environment


higher than 2, indicating these nanocomposites have 3D conductive networks. A higher t value of CPS/RGO/AgNWs compared with that of CPS/RGO and CPS/AgNWs composites, implying a broader tunneling distance distribution in the ternary hybrid composites than the composites containing single RGO or AgNWs fillers. The low percolation threshold value (0.0795 vol%) of CPS/RGO nanocomposites can be mainly attributed to the homogeneous dispersion and 3D network of RGO in CPS matrix. Although the CPS/RGO nanocomposites possess the smallest percolation threshold among these three samples, the electrical conductivity is obviously much lower than that of CPS/AgNWs and CPS/RGO/AgNWs nanocomposites, especially in high filler loading range. It should be pointed out that higher RGO filling volume in CPS/RGO composites is difficult to obtain due to the saturation adsorption between positive charged CPS and negative charged GO. For the CPS/AgNWs composites, there is a relatively high percolation threshold of 0.662 vol% AgNWs and reach maximum value of 950 S m-1 at 3.226 vol% AgNWs. Surprisingly, the ternary hybrid nanocomposites of CPS/RGO/AgNWs containing both RGO and AgNWs conductive fillers with 1:1 (v/v) have a low percolation threshold value of 0.159 vol% which slightly higher than that of CPS/RGO nanocomposites (0.0795 vol%) but much lower than that of CPS/AgNWs nanocomposites (0.662 vol%), and also lower than many previous reports of 0.16 vol% for the PMMA/RGO composites,48 0.23 vol% for the rubber-graphene composites,49 0.56 vol% for the few-layered-graphene/MgO composites,50 and so on. Moreover, CPS/RGO/AgNWs nanocomposites possess a higher electrical conductivity value than that of both CPS/RGO and CPS/AgNWs nanocomposites at the same total filler loading. For example, the electrical conductivity of CPS/RGO/AgNWs nanocomposites is 43.2 and 136.25 S m-1 at total filler loading of 0.946 vol% and 1.566 vol%, which is 38 (1.12 S m-1) and 42 (3.26 S m-1) times higher than that of CPS/RGO at the same filler loading, respectively. The maximum electrical conductivity of CPS/RGO/AgNWs reaches 1230 S m-1 at 3.226 vol% total filler loading, which is 20 ACS Paragon Plus Environment

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higher than that of CPS/AgNWs at the same filler content. The low percolation threshold and high electrical conductivity of CPS/RGO/AgNWs nanocomposites can be explained by the synergistic effect of 1D AgNWs on 2D RGO in a 3D microsphere matrix. The unique 3D continuous network structure of RGO distributed in the polymer matrix offers a low percolation threshold for the resulting nanocomposites, and the AgNWs with high aspect ratio provide high electrical conductivity for the hybrid nanocomposites. It is important to point out that AgNWs content in the CPS/RGO/AgNWs (RGO:AgNWs=1:1, v/v) is only half of that of CPS/AgNWs with the same total filler loading. In addition, the cost of AgNWs is generally dramatically higher than that of RGO derived from GO. Therefore, the ternary hybrid nanocomposites of CPS/RGO/AgNWs not only have low percolation threshold, but also have less AgNWs consumption to obtain high electrical conductivity, which can effectively reduce the cost of the resulting conductive polymer composites. These above results confirm the synergetic effect of RGO and AgNWs in the ternary hybrid nanocomposites of CPS/RGO/AgNWs, and provide a simple strategy to fabricate conductive polymer composites with low percolation threshold and high electrical conductivity at a low expense. To better understand the synergistic effect between RGO and AgNWs, the idealized electrically conductive model of the nanocomposites was presented, as illustrated in Figure 5b. In order to simplify the model of the conductive paths, we presume the CPS microspheres were arranged in a straight line. If the composites contain only RGO, the electrically conductive paths are along the mutual contact CPS/RGO microsphere borders. There are a large number of contact interfaces in a long-range whole conductive path due to the small size of CPS microspheres and RGO nanosheets, resulting in a large contact resistance and low electrical conductivity of the CPS/RGO nanocomposites. In the CPS/AgNWs nanocomposites, AgNWs with high aspect ratio 21 ACS Paragon Plus Environment


are randomly segregated by the CPS microspheres. A few AgNWs is failed to obtain a whole conductive network, implying a high percolation threshold for the CPS/AgNWs nanocomposites. However, for the CPS/RGO/AgNWs ternary hybrid nanocomposites, RGO can easily form interconnected conductive network themselves, and AgNWs act as “conductive bridges” to connect the non-adjacent RGO nanosheets together, leading to less contact interfaces and shorter conductive paths. In addition, the intrinsic electrical conductivity of AgNWs is higher than that of RGO chemical reduced from GO. Therefore, the electrical conductivity of the CPS/RGO/AgNWs nanocomposites exhibits a huge improvement due to the synergistic effect between RGO and AgNWs as compared with nanocomposites containing sole fillers. To vividly demonstrate the electrically conductive of the CPS/RGO/AgNWs ternary hybrid nanocomposites, a disk-like sample was used as an electrode to light up a blue LED lamp (2.5 V, 20 mA) using a 3 V battery, as shown in Figure 6. It can be observed that no matter in x-y direction (in-plane, Figure 6a) or z direction (out-of-plane, Figure 6b), both of the LED lamps can be illumined, indicating excellent isotropic electrical connectivity of the CPS/RGO/AgNWs nanocomposites. Figure 6c shows the magnified photo of the disk-like sample clamped by a holder in z direction, and Figure 6d exhibits the schematic diagram of the compressed disk-like CPS/RGO/AgNWs nanocomposites in x-y-z direction. 3.5 CPS/RGO/AgNWs/PDMS Flexible Strain Sensor and Electronic Skin Due to its abundant conductive pathways, the representative CPS/RGO/AgNWs (50:1:17, w/w) hybrids were used to fabricate a e-skin with strain-sensing capability by integrating the hybrids with elastic PDMS matrix, as shown in Figure 7a. Figure 7b-d shows the cross-sectional SEM images of the CPS/RGO/AgNWs/PDMS-based e-skin strain sensor. It can be seen that the sensor has a classical sandwich structure including two outer PDMS layers and a middle sensitive layer 22 ACS Paragon Plus Environment

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and the thickness of the sensitive layer is about 50 μm (Figure 7b). The sandwich structure could improve the sensing quality due to the smaller hysteresis of the outer PDMS layer than that of middle layer of CPS/RGO/AgNWs/PDMS composites, which has been proved by previous reports.11, 51 From Figure 7c of a magnified SEM image of the middle sensitive layer, we can see that PDMS surrounds CPS/RGO microspheres and AgNWs altogether and solidified as an elastic matrix of the sensor rather than a loose CPS/RGO/AgNWs powder, owing to liquid PDMS infiltrates into the voids of the stacked hybrids powder. Figure 7d presents strong interface bonding between the middle layer and the outer PDMS layers due to the residual groups in the partially cured PDMS layer can react with fresh liquid PDMS in interface area after heating, which effectively avoids the delamination in the interface area during mechanical deformations of the sensor. As revealed in Figure 7e-h, CPS/RGO/AgNWs/PDMS sensor exhibits outstanding mechanical flexibility of stretchable (Figure 7e), twistable (Figure 7f), curlable (Figure 7g) and foldable (Figure 7h), which are the reqiurements for the e-skin applied in wearable electronics. Figure S11a in the Supporting Information exhibits the strain-stress curve to further precisely evaluate the mechanical flexibility of the CPS/RGO/AgNWs/PDMS sensor. It can be seen that the stress gradually increases with the increase in strain until the sample rupture at a large breaking strain of ~141%. From the magnified strain-stress curve of Figure S11b, the elastic modulus is calculated to be 2.35 MPa by the slope of the linear fitted regression line of the strainstress curve. The large breaking strain and low elastic modulus again quantitatively prove the excellent mechanical flexibility of the CPS/RGO/AgNWs/PDMS sensor. In order to vividly demonstrate the electro-mechanical performance of the sensor, it was connected into a simple circuit consisting of a LED. The LED exhibits bright luminescence at initial state (Figure 7i), indicating a low initial resistance. When the sensor was stretched by 50% 23 ACS Paragon Plus Environment


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strain, the LED becomes dim (Figure 7j) and then turns off at 100% strain (Figure 7k), implying great increase in resistance of the sensor with increasing of stretching strain. The resistive response of the sensor was further determined quantitatively. From Figure 7l, it can be seen that the resistance change ratio of the sensor increase with increasing strain. And the slope of the resistance versus strain curve represents the gauge factor (GF) of the sensor, which is defined as follows: 𝐺𝐹 =

(R ― R0)/R0

(3)

ε

where R0 is the initial resistance and R is the resistance at a specified strain level, and ε is the applied strain. The response curve can be divided into four linear regions and the GF value increases with increasing of the strain: 4.1 (ε=0-25%), 20.8 (ε=25-40%), 93 (ε=40-75%) and 354 (ε=75-100%), which are obviously higher than that of 5 (ε=0-60%) for sandwich structured AgNWs-PDMS nanocomposite strain sensors,52 0.54 (ε=0-400%) and 64 (ε=400-960%) for CNT-fiber-based sensor,53 9.6 (ε=0-250%) and 37.5 (ε=250-500%) for carbonized silk fabrics sensor,54 and 1.35 (ε=0-100%) for MWNTs/PSs/PDMS strain sensor.55 The high GF values of the CPS/RGO/AgNWs/PDMS sensor can be mainly attributed to the slippage and disconnection of conductive networks under stretching stain. In addition, the CPS/RGO/AgNWs/PDMS sensor exhibits good working stability and reproducibility during the repeated 150 stretch/release cycles test (~58.5 sec/cycle) under 20% strain, as shown in Figure 7m. From its inserts of the first 5th cycles and the last 5th cycles, it can be seen that there is only a slight drifting of relative resistance change. It is believed that the CPS/RGO/AgNWs/PDMS strain sensor provides a promising prospect in flexible electronics, and can operate effectively with high reliability for practical applications.


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To demonstrate the sensing ability of the CPS/RGO/AgNWs/PDMS strain sensor in actual applications, a strip sensor was fixed on a prosthetic forefinger as e-skin to monitor its bending motions as shown in Figure 8a. The initial angle of the forefinger joint is about 40 º, and it was bended to different angels manipulated by the prosthesis control system. It can be seen that all the changes in resistance of the e-skin at various prosthetic forefinger bending angles periodically increase to maximum values and nearly fully recover to their initial values after finger release. And the maximal relative resistance change values of the e-skin increase from about 5% (Figure 8b), 8% (Figure 8c), 12% (Figure 8d), 15% (Figure 8e) to 18% (Figure 8f) for bending angle of 10°, 20°, 30°, 40° and 50°, respectively, which clearly indicates excellent discernment ability of the e-skin in accurately monitoring various prosthetics finger bending motions with slightly differences. It should be pointed out that the electrical resistance change of the sensor was generated by stretching rather than compressing during prosthetic finger bending. The sensor also shows good cyclic working durability in all the above different bending angles. In addition, the eskin exhibits rapid synchronous response ability during bending process for different bending angles with only a slight hysteresis, as shown in Video S1 (20°) and Video S2 (30°) in the Supporting Information. 4.

CONCLUSIONS

In summary, we have demonstrated ternary hybrids of CPS/RGO/AgNWs with multidimensional architectures based on 1D AgNWs, 2D RGO and 3D CPS microspheres, which could be fabricated by a simple and effective electrostatic attraction strategy. The CPS/RGO/AgNWs (RGO:AgNWs=1:1, v/v) nanocomposites exhibited a low percolation threshold of 0.159 vol% which was slightly higher than that of CPS/RGO nanocomposites (0.0795 vol%) but obviously lower than that of CPS/AgNWs nanocomposites (0.662 vol%). The CPS/RGO/AgNWs nanocomposites also displayed high electrical conductivity of 1230 S m-1 at 25 ACS Paragon Plus Environment


3.226 vol% total filler loading, which is much better than that of CPS/RGO (3.255 S m-1 at 1.556 vol% RGO) or CPS/AgNWs (950 S m-1 at 3.226 vol% AgNWs) nanocomposites with sole fillers. The excellent electrical properties of the CPS/RGO/AgNWs nanocomposites origin from the synergistic effect between 1D AgNWs and 2D RGO. CPS/RGO/AgNWs nanocomposites with 0.94 vol% RGO and 0.94 vol% AgNWs also presented enhanced thermal-mechanical properties with high thermal stability (Tg≈108 ℃) and low CTE (~71 ppm K-1). Moreover, the CPS/RGO/AgNWs hybrids were successfully used to fabricate flexible strain sensor of CPS/RGO/AgNWs/PDMS, which exhibits large stretchability (＞100%), high gauge factor of 4.1 (ε=0-25%), 20.8 (ε=25-40%), 93 (ε=40-75%) and 354 (ε=75-100%), and excellent long-term working stability more than 150 cycles under 20% strain. Its ability as a e-skin to monitor the motions of prosthetic finger is further demonstrated. This strategy is facile, environment friendly, large-scale and low-cost, providing a new synthesis protocol for conductive polymer composites with low percolation threshold and simultaneously high electrical conductivity, and also for flexible e-skins with promising potential applications in prosthetic rehabilitation, soft robot and motions monitoring. ■ ASSOCIATED CONTENT *Supporting Information Synthesis of CPS Microspheres, GO, AgNWs and hollow RGO; size distribution curve of the CPS microspheres aqueous suspension; FTIR spectrum of the CPS, GO and RGO powder; Zeta potentials of CPS microspheres and GO aqueous suspension under different pH values; XRD and XPS spectrum of AgNWs; SEM images of CPS/GO hybrid particles with various GO volume fractions; SEM images of GO, RGO, GO/AgNWs and RGO/AgNWs hybrids; Photograph of 26 ACS Paragon Plus Environment

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CPS/RGO, AgNWs and CPS/RGO/AgNWs dispersed in ethanol; Zeta potentials of AgNWs, CPS/RGO

and CPS/RGO/AgNWs ethanol suspensions; Optimization of the post-heating

treatment temperature and time of the compressed nanocomposites; Strain-stress curve of the CPS/RGO/AgNWs/PDMS sensor; Prosthetic finger bending motions monitoring in real time by the as-fabricated flexible electronic skin. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Address correspondence to: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors acknowledge the financial support from the National Natural Science Foundation of China

(Nos.

61701488

and

21571186),

Shenzhen

Basic

Research

Plan

(No.

JCYJ20170818162548196), National and Local Joint Engineering Laboratory of Advanced Electronic Packaging Materials (Shenzhen Development and Reform Committee 2017-934), Leading Scientific Research Project of Chinese Academy of Sciences (No. QYZDY-SSWJSC010), Guangdong Provincial Key Laboratory (No. 2014B030301014), and SIAT Innovation Program for Excellent Young Researchers (No. 2016005). REFERENCES G. Lian, C.-P. Wong, C.-C.Chem. Tuan,Mater. L. Li, S. 2016, Jiao,28, Q.6096 6096. Wang, K.-S. Moon, D. Cui, 27 ACS Paragon Plus Environment


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Figure Captions Figure 1. (a) Schematic illustration of fabrication process of the CPS/RGO/AgNWs conductive nanocomposites; (b) SEM image of CPS microspheres; (c) AFM image of GO nanosheets; (d) SEM image of AgNWs; (e-g) SEM images of CPS/GO, CPS/RGO and the corresponding hollow RGO after removing the CPS cores, respectively. (h) and (i) TEM images of CPS/GO and CPS/RGO, respectively. Figure 2. (a-e) SEM images of the CPS/RGO/AgNWs hybrids with various AgNWs content: 0.471, 0.936, 1.857, 2.759 and 3.644 vol%, respectively. (f) and (g) Raman spectra and XPS survey scan spectra of CPS, GO, CPS/GO, CPS/RGO and CPS/RGO/AgNWs samples, respectively. (h) and (i) Deconvoluted XPS C 1s spectra of GO and RGO, respectively. Figure 3. Cross-sectional SEM images of mould pure CPS microspheres, CPS/RGO (0.946 vol%) and CPS/RGO/AgNWs (0.94 vol% RGO and 0.94 vol% AgNWs) hybrids. (a), (c) and (e) compressed at room temperature; (b), (d) and (f) compressed at room temperature and followed by post heating treatment, respectively. Inserts are their corresponding photographs of the disklike samples. Figure 4. Mechanical-thermal properties of the samples. (a) TGA curves of CPS, CPS/RGO, CPS/RGO/AgNWs, RGO and AgNWs; (b) DSC curves of CPS/RGO nanocomposites with various RGO contents; (c) DSC curves of CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) nanocomposites with various AgNWs concentrations; (d) Sketch map of the CTE test; (e) CTE values of CPS/RGO/AgNWs (CPS:AgNWs=50:9.55, w/w) nanocomposites with various RGO



contents; (f) CTE values of CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) nanocomposites with various AgNWs contents. Figure 5. (a) The electrical properties of CPS/RGO, CPS/AgNWs and CPS/RGO/AgNWs nanocomposites. (b) Illustration of electrically conductive paths of CPS/RGO, CPS/AgNWs and CPS/RGO/AgNWs nanocomposites. Figure 6. Demonstration of electrical conductive for the CPS/RGO/AgNWs disk in different directions. (a) in-plane direction; (b) out-of-plane direction; (c) magnified photo of the fixture at out-of-plane direction and (d) Schematic diagram of the disk-like sample in x-y-z directions. Figure 7. (a) Fabrication process of the CPS/RGO/AgNWs/PDMS flexible strain sensor; (b-d) Cross-sectional SEM images of the sensor; (e-h) Photographs of the sensor under different mechanical deformations of stretching, twisting, curling and folding, respectively; (i-k) Photographs of LED demonstration under different stretching strains; (l) Relative change of resistance versus strain for the sensor. The inset shows the magnified curve of 0-40% strain range; (m) Relative resistance changes of the sensor with 150 stretching/releasing cycles under 20% strain. The inset shows the first 5th and the last 5th cycles of the resistance change curves. Figure 8. Monitoring of prosthetic finger bending motions in real time. (a) Schematic of the measure system; (b-f) The changes in resistance of the e-skin at prosthetic forefinger for various bending angles of 10°, 20°, 30°, 40° and 50°, respectively.


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Figure 1. (a) Schematic illustration of fabrication process of the CPS/RGO/AgNWs conductive nanocomposites; (b) SEM image of CPS microspheres; (c) AFM image of GO nanosheets; (d) SEM image of AgNWs; (e-g) SEM images of CPS/GO, CPS/RGO and the corresponding hollow RGO after removing the CPS cores, respectively. (h) and (i) TEM images of CPS/GO and CPS/RGO, respectively.



Figure 2. (a-e) SEM images of the CPS/RGO/AgNWs hybrids with various AgNWs content: 0.471, 0.936, 1.857, 2.759 and 3.644 vol%, respectively. (f) and (g) Raman spectra and XPS 38 ACS Paragon Plus Environment

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survey scan spectra of CPS, GO, CPS/GO, CPS/RGO and CPS/RGO/AgNWs samples, respectively. (h) and (i) Deconvoluted XPS C 1s spectra of GO and RGO, respectively.

Figure 3. Cross-sectional SEM images of mould pure CPS microspheres, CPS/RGO (0.946 vol%) and CPS/RGO/AgNWs (0.94 vol% RGO and 0.94 vol% AgNWs) hybrids. (a), (c) and (e) compressed at room temperature; (b), (d) and (f) compressed at room temperature and followed by post heating treatment, respectively. Inserts are their corresponding photographs of the disklike samples. 39 ACS Paragon Plus Environment


Figure 4. Mechanical-thermal properties of the samples. (a) TGA curves of CPS, CPS/RGO, CPS/RGO/AgNWs, RGO and AgNWs; (b) DSC curves of CPS/RGO nanocomposites with various RGO contents; (c) DSC curves of CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) nanocomposites with various AgNWs concentrations; (d) Sketch map of the CTE test; (e) CTE 40 ACS Paragon Plus Environment

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values of CPS/RGO/AgNWs (CPS:AgNWs=50:9.55, w/w) nanocomposites with various RGO contents; (f) CTE values of CPS/RGO/AgNWs (CPS:RGO=50:1, w/w) nanocomposites with various AgNWs contents.

Figure 5. (a) The electrical properties of CPS/RGO, CPS/AgNWs and CPS/RGO/AgNWs nanocomposites. (b) Illustration of electrically conductive paths of CPS/RGO, CPS/AgNWs and CPS/RGO/AgNWs nanocomposites.




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Figure 6. Demonstration of electrical conductive for the CPS/RGO/AgNWs disk in different directions. (a) in-plane direction; (b) out-of-plane direction; (c) magnified photo of the fixture at out-of-plane direction and (d) Schematic diagram of the disk-like sample in x-y-z directions.



Figure 7. (a) Fabrication process of the CPS/RGO/AgNWs/PDMS flexible strain sensor; (b-d) Cross-sectional SEM images of the sensor; (e-h) Photographs of the sensor under different mechanical deformations of stretching, twisting, curling and folding, respectively; (i-k) Photographs of LED demonstration under different stretching strains; (l) Relative change of resistance versus strain for the sensor. The inset shows the magnified curve of 0-40% strain range; (m) Relative resistance changes of the sensor with 150 stretching/releasing cycles under 20% strain. The inset shows the first 5th and the last 5th cycles of the resistance change curves.


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Figure 8. Monitoring of prosthetic finger bending motions in real time. (a) Schematic of the measure system; (b-f) The changes in resistance of the e-skin at prosthetic forefinger for various bending angles of 10°, 20°, 30°, 40° and 50°, respectively.



Multi-dimensional ternary hybrids including 3D CPS microspheres, 2D RGO nanosheets and 1D AgNWs are presented by a simple electrostatic attraction strategy. The hybrid conductive nanocomposites simultaneously show low percolation threshold and high electrical conductivity due to synergistic effect between RGO and AgNWs. A flexible electronic skin based on the CPS/RGO/AgNWs hybrids exhibits large stretchability, high gauge factor and excellent working durability, which demonstrates high discerning ability and rapid response ability in monitoring prosthetic finger bending motions. 461x187mm (300 x 300 DPI)

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Multi-Dimensional Ternary Hybrids with Synergistically Enhanced

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