Silver Nanowire Gel-Like Composite with

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Applications of Polymer, Composite, and Coating Materials

Binder-free Graphene/Silver Nanowire Gel-like Composite with Tunable Properties and Multifunctional Applications Gui-Wen Huang, Na Li, Yu Liu, Cheng-Bing Qu, Qing-Ping Feng, and Hong-Mei Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22053 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Materials & Interfaces

Binder-free Graphene/Silver Nanowire Gel-like Composite with Tunable Properties and Multifunctional Applications Gui-Wen Huang,† Na Li,† Yu Liu,† Cheng-Bing Qu,† Qing-Ping Feng*,† and Hong-Mei Xiao*,† †Technical

Institute of Physics and Chemistry, Chinese Academy of Sciences, No. 29

Zhongguancun East Road, Beijing 100190, P. R. China. KEYWORDS: graphene, silver nanowire, gel, strain sensor, thermal conduction

ABSTRACT: To realize macroscopically utilization of the excellent properties of graphene, various forms of graphene assemblies have been investigated. Among them, the gel form assemblies show great advantages for their shapeable and self-healable properties, and facile and simple manufacturing processes. For the conventional gel formed graphene assemblies, a relatively large content of binders including hydrophilic polymers, celluloses, or and amorphous inorganic materials are necessary in achieving the gelation. However, these binders are electrically nonconductive and electrochemically inactive, which would weaken the favorable functionalities of the composite and the potential advantages of graphene cannot be fully utilized. Herein, a binder-free silver nanowire (Ag-NW)/reduced graphene oxide (rGO) gel-like composite is designed and successfully fabricated by employing the ultra-long Ag-NWs to enhance the hierarchical synergistic effects. The fabrication technique is highly efficient and

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repeatable, and the obtained composite is flexible, stretchable and self-healable. Furthermore, the overall properties of the composite can be easily adjusted in a wide range by controlling the mass ratio between Ag-NW and rGO, which makes it multipurpose and suitable in different applications. Several demonstrations have been carried out and some special performances including linear strain sensing range and rapid transformation from wet to dry state are found in this unique composite. This binder-free structure could also be expanded to other material systems, which may offer a valuable inspiration for the development of functional devices based on nanocomposite.

Introduction Graphene is well known for its two-dimensional (2D) plane structure and excellent mechanical, thermal and electrical properties.1-3 To realize the full potential of graphene in practical applications, assemblies that could bring the graphene from microscopic nanosheets to macroscopic material is of great significant.4-5 Lots of graphene-based assemblies have been designed and fabricated with various macroscopic structures, and the frequently-used fabricating assembly processes including evaporating,6-8 filtration,9-12 freeze drying13-17 and spray,18-21 etc. However, although reasonable properties have been achieved by these methods, disadvantages such as complicated and time-consuming manufacturing process, lacking of re-shape, recycling and self-healing abilities in resultant material still limit their further developments. By contrast, gel-like assemblies show multiple advantages in both fabricating process and resultant materials. For instance, the gel-like assemblies can be easily transformed into required shapes such as film, noodle or bulk structures with simple processes, and could be re-shaped as needed during applications, which cannot be achieved by the solid assemblies. Except that, the self-healing


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property enables the ability of rapid recovery from damage, which is an ideal peculiarity for functional materials.22-24 To date, several graphene-based gel-like assemblies have been fabricated and performances including super stretching, self-healing and free shaping were demonstrated.25 However, for the vast majority of the present gel-like graphene assemblies, graphene has to be blended with binders to achieve gelation such as hydrophilic polymers including polyvinyl alcohol (PVA),22 polyacrylic acid (PAA),25 or other amphiphilic copolymers;26 celluloses;27 or amorphous inorganic materials.28 These additives usually take a relatively large proportion in the composites but are electrically nonconductive and electrochemically inactive. Thus, although they offer the gel-like assemblies with proper mechanical properties, the favorable functionalities of the composite will be weakened and the potential advantages of the material cannot be fully utilized. Therefore, gel-like graphene composites with no binder or nonconductive additive are highly desired. Taking advantages of the physical interactions between materials is the efficient strategy to achieve this goal. Hierarchical synergistic effects, which exit in composites consisting of multiscale or multidimensional components, could bring synergistic mechanical, electrical or thermal properties to the composites, and have attracted increasing attention especially in onedimensional (1D) and 2D hybrid systems.29-30 Carbon nanotubes (CNTs), silver nanowires (AgNWs), and copper nanowires are the most investigated 1D conducting materials and lots of applications have been demonstrated. Among them, Ag-NWs are of particular interest due to silver possesses the highest electrical conductivity in metal and owing good resistance to corrosion.31-32 These 1D nanoconductors have been combined with the 2D graphene and positive hierarchical synergistic effects exhibited in the composite assemblies. For example, by assembling graphene with CNTs, neat carbon aerogels with retractable large elongation and


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composite film with enhanced buckling and bundling resistance were respectively obtained attributing to the synergistic reinforcement effect.13,33 Also, synergistic effect was found to be effective in reducing the electrical percolation between graphene and CNTs.34 In addition, graphene was introduced to the Ag-NW network and both the mechanical and electrical properties were enhanced, especially at the nanowire joints.35 Besides, synergistic disperse effect was reported between graphene and Ag-NWs, which facilitates the stability of the mixture system, avoids the aggregations and enables the fully utilizing of the active materials.36 However, although the synergistic effects could bring multiple advantages, they were reported mostly in dispersing liquid form or solid form assemblies, since the interactions are not strong enough to form gelation and replace the function of binders. Therefore, strengthening the physical interactions between nanomaterials is the key to achieve binder-free gel form composites. Herein, a new type of graphene/Ag-NW binder-free gel-like assembly is designed and fabricated. Different from the previously reported graphene/Ag-NW composites, synthesized ultra-long Ag-NWs (with average length over 200 μm and diameter of ~100 nm) were used in the composite material. Taking advantage of its super large aspect ratio, synergistic effects between the graphene and Ag-NWs were strongly enhanced. As a result, the gelation could take place in the aqueous system with only neat reduced graphene oxide (rGO) and Ag-NWs without binders or additives. Benefiting from this, a gel-like assembly free of any inactive material was successfully obtained. The whole process is simple and highly repeatable, and the resultant composite is shapeable and self-healable. Furthermore, by controlling the ratio of rGO to AgNWs, the functional properties of the gel could be easily adjusted, making it multipurpose in


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various applications. In addition, several interesting and special properties were found from this unique composite and were investigated in detail.

Figure 1. a) and b) schematically show the fabricating process of rGO/Ag-NW composite with normal and ultra-long Ag-NWs, respectively. c) Scanning probe microscope image of the rGO. d) SEM image of the ultra-long Ag-NWs. e) SEM image of the gel-like rGO/AgNW composite. f) TEM image of the gel-like rGO/Ag-NW composite.


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Results and discussion Figure 1 schematically shows the fabricating process of the composite gel. As comparison, normal Ag-NWs with length of ~20 μm was also performed the same process and shown in path a. The preparation sequence is quite simple: Generally, the rGO powder was firstly dispersed in water by sonicating to form the rGO dispersion. Similarly, Ag-NWs were also dispersed in water to obtain the Ag-NW dispersion. Then, the two dispersions were directly mixed together with a certain mass ratio. After vigorously shaking, uniform mixtures were obtained as displayed in the digital pictures. Finally, the mixtures were centrifuged to remove the excessive water. The only difference between path a and path b is the length of the Ag-NWs, which, however, is the key factor that determines the final physical form of the composite. As described in path a, when the rGO was mixed with the normal Ag-NWs, the sheets and the wires were randomly dispersed in the mixture, and the hierarchical synergistic effects are relatively week since the normal Ag-NWs and the rGO possess similar magnitude in geometric size. Thus, after centrifugation, the rGO and Ag-NWs were just simply stacked together, and only amorphous paste can be obtained with no mechanical integrity. By contrast, when rGO was mixed with the ultra-long Ag-NWs, the nanowires could be interlaced among the nanosheets by reason of their super large aspect ratio, resulting in the great enhancement on the hierarchical synergistic effects. As a result, with the help of the centrifugal force, the rGO and ultra-long AgNWs were entangled together, and a gel-like composite with good mechanical integrity was gained. Figure 1c presents the scanning probe microscope image of the used rGO, in which the typical 2D structure can be clearly observed. Figure 1d shows the scan electron microscopy (SEM) image of the ultra-long Ag-NWs, wherein the winding long wires can be seen. The ultralong Ag-NWs were synthesized by using a multi-step growth method and the detail can be found


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in Experimental Section. The distributions of the length and diameter of the normal and ultralong Ag-NWs were shown in Figure S1 in Supporting Information. It can be seen that the diameter of the ultra-long Ag-NW is only 13 nm larger than that of short Ag-NW, which is negligible compared with their length difference. Thus the length is the main factor that influences the gelation performance. Figure 1e and 1f display the SEM and TEM images of the resultant composite, respectively, which confirm the Ag-NWs were interlacedly distributed in the composite and wrapped by the rGO sheets. This hierarchical structure offers strong interactions that enables the formation of the gel-like composite.

Figure 2. a-d) show the SEM images of Ag-NWs with average length of about 20 μm, 60 μ m, 120 μm, 200 μm, respectively, and the appearance forms of their composites at fixed mass ratio to rGO of 1:1. e) Digital photographs of the 200 μm Ag-NW composites with a


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series of mass ratio to rGO. f) Sheet resistance of the A1G as a function of the water content in the composite. To further explore the formation mechanism of the gel-like form composite, and find the boundaries in material size, mass ratio to achieve required mechanical and electrical performances, a series of quantitative experiments have been carried out. Firstly, Ag-NWs with different lengths were used to investigate the critical size that could lead to gelation in composite. Figure 2 a-d show the SEM images of Ag-NWs with average lengths of about 20 μm, 60 μm, 120 μm and 200 μm, respectively. These Ag-NWs were used to fabricate composite with rGO at a fixed mass ratio of 1:1 and the appearance forms of the obtained composite were shown in the corresponding digital photographs below Figure 2 a-d. It can be seen that the gellike form composite could be achieved when the length of Ag-NW is over 120 μm. While with shorter Ag-NWs, only paste-like composite can be obtained. Since the size of rGO is several micrometers (Figure 1c), the increase of Ag-NW length greatly improved the twisting and binding effects with rGO, which could lead to the gelation in the mixture system. This verified that the gel-like form was induced by the strong synergistic effects between Ag-NW and rGO. Length over 120 μm has been demonstrated to be a critical requirement to achieve gelation. To ensure better performance, Ag-NWs with average length of ~200 μm was used for further investigations. Secondly, mass ratio of Ag-NW to rGO is another key parameter that determines the final form of composites. To investigate the influence of content ratio on performance, composites with series of Ag-NW to rGO mass ratios ranging from 0.5:1 to 8:1 were fabricated and the samples were named correspondingly as A0.5G to A8G, respectively. The weight content of Ag-NW was set to be higher than rGO because the silver possesses larger density than carbon. The appearance forms of the samples were shown in Figure 2e. As can be seen, gel-like


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composites could be obtained in mass ratio range of about 1:1 to 4:1. While with smaller or higher mass ratio, paste-like composites with enrichments of rGO or Ag-NW were gotten. This is because the strong synergistic effects between the components only exist when they are in a proper range of mass ratio, or the synergistic effects will be weakened. Besides, as a gel-like composite, water content is important in influencing the formation of gel-like state of the composite. We found that the gel-like behavior can be achieved when the water content in the composite is below 96%. Meanwhile, the water content obviously affects the composite performances especially the electrical properties. A1G composite with a series of water contents were manufactured by adjusting the centrifugal speed, and their electrical properties were measured as shown in Figure 2f. As described in the results, the sheet resistance dropped dramatically with decreasing the water content, because the reduction of water shortened the distances between the nanomaterials. After the water content was down to 92%, the change in electrical resistance became gentle, indicating the distances between the nanomaterials reached the critical tunneling distance. Thus, the 92% water content was chosen to be the proper value as lower water content could weaken the deformability of the composite. Furthermore, the residual amount of PVP on Ag-NWs during the solvothermal synthetic process may also influence the overall performance. To quantify the residual amount of PVP, thermogravimetry (TG) test was conducted and the result was shown in Figure S2 in Supporting Information. It can be seen that after repeated rinses, the residual amount of PVP on 200 μm Ag-NW was only 0.89 wt%. Therefore, the influence of PVP on the gel-like composite performance could be ignored due to such a small content. X-ray diffraction (XRD) test was performed to confirm the crystalline structure of the materials, as shown in Figure S3 of the Supporting Information. For Ag-NWs, strong peaks was


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observed, indicating the pure phase structure of silver. While for the rGO, a broad gentle peak appears in the range of 20° to 30°, showing the typical XRD pattern of the rGO.37 The pattern of the Ag-NW/rGO composite with mass ratio of 1:1 shows a simple combination of the peaks of pure Ag-NW and rGO, demonstrating no new phase was formed in the composite. Fourier transform infrared spectroscopy (FT-IR) was used to indicate the functional groups in the composite, which is shown as Figure S4 in the Supporting Information. In the FT-IR spectrum, the strong absorption at 1640 cm-1 was caused by the water existing in the composite. The band at ~1415 cm-1 corresponds to the O–H stretching vibration of carboxyl, the bands at ~1100 cm-1 and 1027 cm-1, indicating the exist of C–O group.38 By comparing the spectrum the composite and the raw materials, it can be seen that no obvious shift or change in peaks can be found, demonstrating the Ag-NW and the rGO was physically combined in the system. X-ray photoelectron spectroscopy (XPS) analysis was employed to further confirm the chemical bonding in the system. Figure S5 in Supporting Information shows the survey spectrum, highresolution Ag 3d and C 1s of the composite, respectively. The survey spectrum in Figure S5a confirms the existence of C, O, N and Ag in the composite. From the high-resolution Ag 3d spectra in Figure S5b it can be seen that there are two characteristic peaks at 368.2 eV (Ag 3d 5/2) and 374.2 eV (Ag 3d3/2), which match the values of pristine Ag atom,

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and two

corresponding satellite peaks were found at 368.8 eV and 374.6 eV, which could be attributed to the mild oxidation on the surface of Ag-NWs.40 Figure S5c presents the high-resolution C 1s spectra. In the spectra, the spectral deconvolution curves indicated three profile peaks which correspond to C–C (284.7 eV), C–N (285.7 eV) and C=O (288.2 eV), respectively. From which the C–N and C=O came from the PVP absorbed on the Ag-NWs. To investigate the surface charge of the material and composite, Zeta potential tests were conducted. As shown in Figure


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S6, the Zeta potential of Ag-NWs, rGO and their 1:1 mixture are -4.5 mV, -37.28 mV and -20.1 mV, respectively. This relatively large Zeta potential value could be helpful for the stability and homogeneity of the Ag-NW/rGO dispersion for the synthesis of gel like composite.41 By comparing the XPS, XRD and FTIR data, no evidence could be found to reveal the existence of chemical reaction between Ag-NWs and rGO. Thus the gelation mechanism between Ag-NWs and rGO could be only attributed to the physically synergistic effects.

Figure 3. a-c) SEM images and digital photographs of the composites with serial Ag-NW to rGO mass ratios. d) Sheet resistances of the composites. e) Storage modulus (G’) and loss modulus (G”) of the composites as functions of frequency. f) Complex viscosities of the composites as functions of shear frequency.


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Because the gel-like composite was composed of two neat functional materials, the overall properties of the composite could be easily controlled by adjusting the content ratio of the components. Figure 3 a-c show the SEM and digital images of the A1G, A2G and A4G, respectively. As presented in the SEM images, for the A1G, nanowires were sparsely distributed among the rGO, showing a vein-like structure. With the increasing of Ag-NW ratio, the nanowire network was gradually becoming dense and rGO sheets alternately warped among the nanowires. The digital photographs present the macroscopic appearance of the materials, and all composites are in gel-like form with good mechanical integrity. The electrical properties of the gel form composites were measured and displayed in Figure 3d. It can be seen that with the increasing of Ag-NW ratio from A1G to A4G, the sheet resistance dropped dramatically from kilo-ohm to dozens ohm, demonstrating the wide adjustable range in electrical property of the composite. Furthermore, to investigate the interactions between the components in the composite system, rheology tests were carried out. Figure 3e shows the storage modulus (G’) and loss modulus (G”) of the composites as functions of frequency, and the loss tangent data was shown in Figure S7 of Supporting Information. As can be seen, for the three samples in the full range of testing frequency, the G’ are higher than G”, indicating the good mechanical integral behavior of the composites. Figure 3f presents the complex viscosity of the composites as functions of frequency. The difference in the viscosities of the gels with different compositions is slight and all the samples show shear-thinning performance, namely the viscosity decreased as the increasing of testing frequency. This phenomenon might be related to the tendency of alignment of Ag-NW and rGO along the shear stress direction at high shear frequency.


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Figure 4. a) Schematic illustration of the fabrication process of the flexible conductor. b) Photograph of the conductor working as a part of circuit to light a LED. c) Demonstration of the stretchable and self-healing abilities of the conductor. d) Schematic diagram describing the self-healing process of the gel-like composite. Benefiting from the gel-like state and adjustable performance of the composites, they could be fitted to various multifunctional applications. For example, the A4G with high electrical conductivity can be used as a flexible conductor. As described in Figure 4a, the A4G was shaped into a “noodle” and placed on a cured elastomeric Ecoflex layer in a mold. Two thin copper wires were embedded in the gel serving as electrodes. Afterwards, liquid Ecoflex was poured into the mold to submerge the gel composite and cured at room temperature. Then, the flexible conductor with embedded structure was obtained. Figure 4b shows the digital


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photograph of the flexible conductor working as a part of circuit to light a LED. Due to the gel state of the conductor, it could be stretched to a relatively high strain with good stability in electrical property. Figure 4c shows the conductor under 100% tensile strain and the LED light can still be lit, which demonstrates the stretchable ability of the conductor. Moreover, this gellike conductor possesses a noteworthy self-healing capability. As displayed in Figure 4c, the conductor was cut off at the middle and the complete circuit was broken. Interestingly, the gel conductor can be quickly self-healed just by joining the broken surfaces together and the circuit would be connected again. After adding additional Ecoflex at the junction area and going through a secondary curing process, the substrate could be healed, too. Finally, the mechanical and electrical properties of the conductor were both recovered as shown in the picture. The selfhealing capability of this gel-like composite could be attributed to the interaction between AgNWs and rGO nanosheets in wet state. As illustrated in Figure 4d, when the gel noodle was cut off, free nanowires and nanosheets generated at the fresh surfaces. In the gel-like composite, because of the large surface area and hydrophilicity of the rGO, water was physically absorbed on their surfaces and interspaces. This keeps the composite in wet state and offers it with certain of fluidity. Therefore, when the two fractured surfaces were joined together, the nanosheets at the surfaces attract with each other due to the hydrophilic effect and the two parts of the noodle could be fused together gradually. During this procedure, the inter network of the Ag-NWs could also be rebuilt by self-entangling and synergistic effects with rGO nanosheets due to the ultralarge aspect ratio, and finally the composite was completely self-healed. To evaluate the selfhealing time of the gel-like composite, the resistance change during cut and healing process was monitored and shown as Figure S8 in the Supporting Information. As can be seen, when the composite was cut off, the resistance increased to infinity at once. While when the disconnected


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composite were contacted together again, the resistance immediately dropped down and then recovered to the original value within 3.6 s, which can be identified as the healing time of the composite. This quick recovering time demonstrates the good self-healing performance of the composite, which is similar in value with the reported CNT/hydrogel self-healing sensor.22 The amazing self-healing ability makes this unique composite an attractive candidate for various potential self-healable electronic devices, such as self-healable electronic skin, wearable electronics, portable electronics, etc.


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Figure 5. a) and b) respectively show the horizontal and vertical strain spatial distributions on the strain sensor during stretching and releasing recorded and analyzed by a digital image correlation system. c) Plot of relative resistance change as a function of applied strain for the Ag-NW/rGO composite based strain sensor. The equations presented are parabolic fitted and linear fitted results of the piecewise data. In the equations, y is the relative resistance changes and ε is the tensile strain. d) Schematic illustration of the


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compress effect on the composite in the direction perpendicular to stretching and the change in tunneling distance between Ag-NWs and rGO. In addition, the conductive and highly stretchable composite is suitable for fabricating strain sensors. While for strain sensors, gauge factor instead of the absolute resistance value is the key parameter that mainly focused. The gauge factor is defined as (ΔR/R0)/ε, where ΔR stands for the difference between the resistance under stretch and that at initial state, and ε stands for the applied strain. A proper large and regular gauge factor is favorable for strain sensors since which could clearly reflect the change in strain. In view of this, the A4G with high electrical conductivity and good electrical stability is not suitable here because it will lead to a relatively small gauge factor. Hence, the A1G with higher resistance but better sensibility was used to meet the application requirement of strain sensor. The strain sensor was fabricated through the similar process as the flexible conductor mentioned above. Surprisingly, a special sensing performance was found here compared with the other gel-based strain sensors: a relationship of piecewise function was observed between the relative resistance change and the applied strain. Normally, for strain sensors, the relationship between relative resistance changes and applied strains can be fitted to a parabolic equation y = Aε2 + Bε + C, where y is the relative resistance changes and ε is the applied strain.22,42 Since no resistance change happened in the initial state, the intercept C is zero. As shown in Figure 5c, when the strain is low, the sensing data was following the typical tendency and could be fitted to the parabolic equation. However, when the strain is higher than about 180%, the sensing data changed into a nearly linear track, which can be well fitted to a linear equation as presented in the figure. This unique phenomenon could be explained by the multidimensional deformation of the gel composite. The horizontal and vertical strain spatial distributions on the sensor during strain applying were recorded and analyzed by a digital image


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correlation (DIC) system and shown in Figure 5a and 4b, respectively. In all the images, the color gradation from blue to red reflects the numerical value of strain from small to large. So the color gradation of strain distribution during stretching in horizontal and vertical direction is opposite. In the vertical direction, because the tensile strain is positive in value, the red color stands for the maximum tensile strain, and the blue color stands for the minimum tensile strain. While in the horizontal direction, because the contraction strain is negative in value, then the red color stands for the minimum contraction strain, and the blue color stands for the largest contraction strain. As can be seen in Figure 5a, the contraction strain at the gel channel under low tensile strain is smaller than that of the substrate, showing a hysteretic phenomenon. When the stain goes larger, the hysteretic phenomenon gradually disappeared. While for the vertical strain, no obvious hysteretic in strain was observed. More details can be found in Movie S1 and Movie S2 of Supporting Information. For nanocomposite-based sensors, there are mainly two factors that determine the resistance change under strain: one is the macroscopic geometrical changes of the composite and the other is the microscopic tunneling resistance change between nanomaterials.43 The macroscopic resistance of the composite can be expressed as18,44 𝑅 = 𝜌𝐿 𝑆

(1)

Where R is the total resistance, ρ is the appearance resistivity, L is the length of the composite in testing direction and S is the sectional area perpendicular to the testing direction. While the tunneling resistance can be determined by 43,45-46 (2) Where Rt is the tunneling resistance, V is the electrical potential difference, A is the crosssectional area of the tunnel, J is tunneling current density, h is Planck’s constant, d is the distance


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between nanomaterials, e is the single electron charge, m is the mass of one electron and 𝜆 is the height of the energy barrier. At the early stage of stretching, since the horizontal contraction value is not very large, coupled with the hysteretic effect of the gel in horizontal strain, the tunneling resistances between the nanowires and the nanosheets are not influenced obviously. Thus, at this stage, the geometrical change plays the major role in effecting the total resistance change, leading to the resistance increment following a parabolic tendency. When the tensile strain goes higher, the contraction strain increased, and the hysteretic effect disappeared, so the gel will be obviously compressed by the substrate in horizontal direction. Because there was no binder in the gel system, under compression stress, the distance between the nanomaterials will be shortened. The schematic diagrams in Figure 5d describe this effect, and the distance between the Ag-NW and rGO and that between the adjacent Ag-NWs were denoted as d1 and d2, respectively. Under large strain, both the d1 and d2 were shortened. With the decrease of d1 and d2, the tunneling resistance in the composite will be reduced according to Equation 2, which displays a converse effect compared to the geometrical change. Therefore, the simultaneous existence of these two competing effects resulted in the approximately linear behavior of the strain sensor. As linearity is a favorable property for sensors,47 this special piecewise performance of the strain sensor offers an extra choice for strain sensing applications. The gauge factors of the sensor as a function of strain were calculated and shown as Figure S9 in Supporting Information. It can be seen that the gauge factor increased obviously with the increasing of applied strain before 180%, and became relatively stable after 180% strain. The gauge factor values are 0.7 at 100% strain, 1.22 at 200% strain and 1.3 at 300% strain, respectively. It is higher than the reported strain sensors with high stretchability:


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graphene/hydrogel based sensor (0.45 at 300% strain), Ag-NW/hydrogel based sensor (1.15 at 300% strain), CNT/hydrogel based sensor (0.6 at 300% strain) and piezoresistive electronic sensor (0.06 at 200% strain).22,48 This shows the advantage in sensitivity of the Ag-NW/rGO gellike sensor. Furthermore, the reliability of the sensor was evaluated by repeatedly applying 300% stretching strain to the sensor and the largest resistance was recorded and shown as Figure S10 in Supporting Information. As can be seen, the resistance at 300% strain generally remains stable with slightly decrease after 100 stretching cycles. The decrease of resistance might be relative to the strain induced alignment and reconstruction of Ag-NW and rGO in the composite.

Figure 6. a) Schematic illustration of the Ag-NW/rGO composite working as interface material. b) Sheet resistance and thermal conductivity of the composites in dry state at


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room temperature. c) and d) show the proceeding of heat conduction between two copper plates over time without and with the Ag-NW/rGO composite at their interface, respectively. Besides, there is another particular property of this binder-free gel-like composite. As mentioned above, because there is no binder in the composite, the water is physically absorbed on the surfaces and interspaces of rGO and Ag-NWs. Consequently, the water in the composite could be rapidly removed by mechanical force. For example, the gel-like composite can be easily transformed into dry state just by applying external pressure in a limited space. What’s more, the dried composite will spontaneously fill the space at the interface as described in Figure 6a. In the dried state, since the Ag-NWs and the rGO were closely compressed together, the electrical property of the composite was greatly enhanced compared with that in gel state, which reaches 1 × 10-4 ohm sq-1 for A4G at room temperature as shown in Figure 6b. In addition, the thermal conductivity of the composites in dry state were measured and a high thermal conductivity of 4.2 W m-1 K-1 can be achieved. Photographs of the test samples in dry state are presented in Figure S11 in Supporting Information, where the compact structure of the composite could be seen. As the A4G possesses the best electrical and thermal properties at room temperature, its corresponding properties at cryogenic temperature (77 K) were measured. The sheet resistance of A4G at 77 K is 9.8×10-5 ohm sq-1, which is slightly lower compared to that at room temperature. The thermal conductivity at 77 K is 4.32 W m-1 K-1, which is a little higher than the room temperature value. This differences can be mainly attributed to the property change of the silver at cryogenic temperature, which follows an electron conducting mechanism. These advantages make it an ideal plastic electrical and thermal interface material. Figure 6c and 6d present a demonstration of the A4G composite applied as thermal material through a set of contrast test. In


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the test, one piece of copper plate was heated in oven and placed upon another copper plate at room temperature. Figure S12 in Supporting Information shows the digital photographs of the visible light photographs for the contrast experiment. The change in temperature distribution between the two copper plates along with the time was recorded by a thermal imaging system. Figure 6c shows the proceeding of heat conduction between the bare copper plates and Figure 6d shows that with A4G at their interface. As can be seen, for the bare copper plates, the heat conduction through the interface was slow and obvious temperature difference still existed between the two plates after 30 s. By contrast, by adding A4G at the interface, the heat conduction was dramatically accelerated. After only 10 s, most of the heat had been conducted through the interface, and the temperature of the two plates almost reached to the same level after 30 s. The detailed temperature change data and process can be found in Figure S13, Movie S3 and Movie S4 in Supporting Information. This experiment proved the excellent thermal conducting property of the composite, making it a potential thermal interface material with good operability and adaptability. Conclusion In summary, a new type of binder-free Ag-NW/rGO gel-like composite is reported here and several special properties have been demonstrated. The using of ultra-long Ag-NWs is the key factor that leads to the gelation without any inactive binder in the composite. The resultant composite owns outstanding reshaping and self-healing capabilities. Moreover, by controlling the content ratio of Ag-NW and rGO, the performance of the composite can be easily adjusted in a wide range, which offers it abilities to meet various application requirements. Demonstrations including stretchable and healable conductor, strain sensor and thermal interface material are presented. Unique properties of the binder-free composite such as linear strain sensing range and


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rapid transforming to dry state are discussed in detail. Furthermore, it worth noting that the manufacturing process is highly efficient and repeatable, which can be conveniently expanded to mass production. This attempt of binder-free gel-like composite may also contribute a valuable reference for the design and realization of other functional materials.

MATERIALS AND METHODS Synthesis of normal Ag-NWs. 50 ml ethylene glycol (EG, Sinopharm Chemical Reagent Co.,Ltd, China) containing 5 × 10−6 mol of FeCl3•6H2O (Beijing Chemical works, China) and 7 × 10−3 mol of PVP (Mw≈58,000, Alfa Aesar, USA) was dropwise mixed with another 50 ml EG containing 5 × 10−3 mol of AgNO3 (Beijing Chemical works, China) under vigorous stirring to form the reacting mixture. Then the mixture was transferred into an autoclave and heated at 160 °C for 3 h letting growth of the Ag-NWs. The Ag-NWs were then separated from the solution by adding a large amount of acetone. The synthesized Ag-NWs have an average length of 22.1 µ m and an average diameter of 89.2 nm. Synthesis of ultra-long Ag-NWs. The ultra-long AgNWs were synthesized through a multistep growth based on the as-prepared normal AgNWs. 100 ml reacted mixture of the normal AgNWs was concentrated to 20 ml and added to 50 ml EG containing 3.75 × 10−6 mol of FeCl3•6H2O and 5.25 × 10−3 mol of PVP. The solution was heated to 160 °C and 30 ml EG containing 5 × 10−3 mol AgNO3 was injected at a rate of 1 ml min-1 letting further growth of the AgNWs. This procedure was repeated for three times and AgNWs with average length of 202 μm and average diameter of 102.4 nm were finally obtained after acetone rinsing. Fabrication of the gel-like composite. The rGO used was purchased from Chengdu Organic Chemicals Co. Ltd., China, and was dispersed in purified water by tip sonicating for two


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hours to form the rGO dispersion with concentration of 10 mg ml-1. On the other hand, the ultralong Ag-NWs were also dispersed in water with concentration of 10 mg ml-1 under gentle sonication for avoiding the break of the long nanowires. Then, the two dispersions were directly added into a centrifuge tube in a certain mass ratio. With vigorously shacking, a composite paste was formed in the tube. After centrifuging at 5000 r/min for 5 min, the excessed water could be removed and the gel-like composite was obtained. Composites with series of Ag-NW to rGO mass ratios ranging from 0.5:1 to 8:1 were fabricated and the samples were named correspondingly as A0.5G to A8G, respectively, where A stands for Ag-NW, G stands for rGO. Fabrication of the stretchable conductor. A room temperature curing silicon resin (Ecoflex A30, USA) was used as the substrate of the stretchable conductor. Firstly, a bottom layer of Ecoflex was cured in a Teflon mold. Then, the A4G composite was shaped into a noodle by injector and placed on the bottom Ecoflex layer. Two thin copper wires were embedded in the noodle serving as electrodes. Afterwards, liquid Ecoflex was poured into the mold to submerge the composite and cured at room temperature for 4 hours. Finally, the stretchable conductor was obtained by peeling off from the mold. Characterizations. The morphology of the samples were observed by a scanning electron microscope (Hitachi S-4800, Japan), transmission electron microscope (TEM, JEM2100, Japan) and a scanning probe microscope (SPM-8000FM, Japan). Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using a Bruker Tensor 27 spectrometer. The X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Focus diffractometer with Cu Kα radiation. Resistances of the samples were measured with Keithley SourceMeter 2400 (USA) using the standard four-probe method. The rheological behaviors of the composites were investigated by a hybrid rheometer (HR2, TA, USA) using parallel-plate at 25 °C in oscillation


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mode. Zeta Potential measurements were performed using a Zetasizer Nano ZS. The X-ray photoelectron spectroscopy (XPS) was tested on ESCALAB 250Xi. The TG test was performed on NETASCH STA 409 PC (Germany). The strain spatial distributions on the stain sensor during stretching and releasing was recorded by a digital image correlation system (MTI, Belgium). The thermal conductivity was measured by a Physical Property Measurement System (PPMS, Quantum Design, USA). The heat conducting process was recorded by a thermal imaging system (FOTRIC, USA) and analyzed by a software of AnalyzIR. ASSOCIATED CONTENT Supporting Information. Movie S1. The horizontal strain distribution on the strain sensor during stretching. Movie S2. The vertical strain distribution on the strain sensor during stretching. Movie S3. The heat conduction process between bare copper plates. Movie S4. The heat conduction process between copper plates with A4G at the interface. Figure S1: The length and diameter distributions of the normal and super-long Ag-NWs. Figure S2: TG test to quantify the residue amount of PVP on Ag-NWs. Figure S3. XRD patterns of the Ag-NWs, rGO and their composite with mass ratio of 1:1. Figure S4. FT-IR spectrum of the Ag-NW, rGO and their composite with mass ratio of 1:1. Figure S5. XPS characterization of the raw Ag-NWs, rGO and their composite. Figure S6. Zeta potential tests for the raw Ag-NWs, rGO and their composite. Figure S7: Loss tangent values of the composites as functions of frequency. Figure S8. Evaluation of the self-healing time of the gel-like composite. Figure S9. Gauge factor as a function of strain of the strain sensor based on A1G. Figure S10. Relative resistance change at 300% strain as a function of stretch number. Figure S11: Digital photographs of the thermal conductivity testing samples of the composites. Figure S12: Digital photographs of the heat conduction contrast experiment. Figure S13: The temperature changing data for the contrast heat conducting experiment.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (nos. 51503213 and 11572321). REFERENCES (1) Ameri, S. K.; Ho, R.; Jang, H.; Tao, L.; Wang, Y.; Wang, L.; Schnyer, D. M.; Akinwande, D.; Lu, N. Graphene Electronic Tattoo Sensors. ACS Nano 2017, 11, 7634-7641. (2) Mu, J.; Hou, C.; Wang, G.; Wang, X.; Zhang, Q.; Li, Y.; Wang, H.; Zhu, M. An Elastic Transparent Conductor Based on Hierarchically Wrinkled Reduced Graphene Oxide for Artificial Muscles and Sensors. Adv. Mater. 2016, 28, 9491-9497. (3) Li, C.; Jiang, D.; Liang, H.; Huo, B.; Liu, C.; Yang, W.; Liu, J. Superelastic and ArbitraryShaped Graphene Aerogels with Sacrificial Skeleton of Melamine Foam for Varied Applications. Adv. Funct. Mater. 2018, 28,1704674. (4) Wan, S.; Fang, S.; Jiang, L.; Cheng, Q.; Baughman, R. H. Strong, Conductive, Foldable Graphene Sheets by Sequential Ionic and pi Bridging. Adv. Mater. 2018, 30, 1802733. (5) Jin, H.; Bu, Y.; Li, J.; Liu, J.; Fen, X.; Dai, L.; Wang, J.; Lu, J.; Wang, S. Strong Graphene 3D Assemblies with High Elastic Recovery and Hardness. Adv. Mater. 2018, 30, 1707424. (6) Chen, C. M.; Yang, Q. H.; Yang, Y. G.; Lv, W.; Wen, Y. F.; Hou, P. X.; Wang, M. Z.; Cheng, H. M. Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 2009, 21, 3007-3011. (7) Sui, L.; Wang, Y.; Ji, W.; Kang, H.; Dong, L.; Yu, L. N-Doped Ordered Mesoporous Carbon/Graphene Composites with Supercapacitor Performances Fabricated by Evaporation Induced Self-Assembly. Int. J. Hydrogen Energ. 2017, 42, 29820-29829. (8) Kim, S. K.; Chang, H.; Kim, C. M.; Yoo, H.; Kim, H.; Jang, H. D. Fabrication of Ternary Silicon-Carbon Nanotubes-Graphene Composites by Co-Assembly in Evaporating Droplets for Enhanced Electrochemical Energy Storage. J. Alloy. Compound. 2018, 751, 43-48. (9) Tao, L.-Q.; Zhang, K.-N.; Tian, H.; Liu, Y.; Wang, D.-Y.; Chen, Y.-Q.; Yang, Y.; Ren, T.-L. Graphene-Paper Pressure Sensor for Detecting Human Motions. ACS Nano 2017, 11, 8790-8795. (10) Wan, S. J.; Li, Y. C.; Peng, J. S.; Hu, H.; Cheng, Q. F.; Jiang, L. Synergistic Toughening of Graphene Oxide-Molybdenum Disulfide-Thermoplastic Polyurethane Ternary Artificial Nacre. ACS Nano 2015, 9, 708-714.


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Silver Nanowire Gel-Like Composite with

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