Synthesis of Ultralong Copper Nanowires for High-Performance

Mar 19, 2018 - The prepared sensors exhibit high sensitivity and a fast response time. ... Large-Scale Synthesis of High-Quality Ultralong Copper Nano...
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Synthesis of Ultralong Copper Nanowires for High-Performance Flexible Transparent Conductive Electrodes: The Effects of Polyhydric Alcohols Ye Zhang, Jiangna Guo, Dan Xu, Yi Sun, and Feng Yan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00344 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Synthesis of Ultralong Copper Nanowires for High-Performance Flexible Transparent Conductive Electrodes: The Effects of Polyhydric Alcohols Ye Zhang, Jiangna Guo, Dan Xu, Yi Sun and Feng Yan* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China. E-mail: [email protected]

Abstract: Copper nanowires (Cu NWs) have become a promising material for flexible transparent conductive electrodes (FTCEs) owing to the outstanding transparency and conductivity properties. In this work, ultralong Cu NWs with an average length over 250 μm and diameter around 50 nm (aspect ratio ~5000) were synthesized in water/polyhydric alcohol co-solvent. The effects of polyhydric alcohols (including ethanol, ethylene glycol and glycerol) on the aspect ratio of Cu NWs were investigated. The diameter of Cu NWs decreased with the increased number of hydroxyl groups of polyhydric alcohols. In addition, the capping ligands (oleylamine and oleic acid) and glucose also exhibit important effects on the

dispersity

and

morphology

of

Cu

NWs.

The

ultralong

Cu

NW

based

polydimethylsiloxane (PDMS) FTCEs exhibit the high performance with a low sheet resistance of 92.1 Ω sq−1 at transmittance of 91.524%. Inspired by the stretchable ability of PDMS, wearable sensors were fabricated to detect the movement of finger joint through the 1 ACS Paragon Plus Environment

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chronoamperometry method. The prepared sensors exhibit high sensitivity and fast-response time. The excellent performance of FTCEs and wearable sensors suggest that the ultralong Cu NWs have a bright future in the application of next generation of flexible optoelectronic devices. Keywords: Copper nanowires, polyhydric alcohols, ultralong, flexible transparent conductive electrodes, sensors

Introduction The flexible transparent conductive electrodes (FTCEs) have become an essential building block for the fabrication of next generation flexible optical devices, such as low-emissivity windows, touch screens, organic light-emitting diodes (OLEDs), light-emitting diodes (LEDs) and other electronic devices.1-4 Currently, the most widely used technologies still rely on the indium tin oxide (ITO) based thin films.5-8 However, the scarcity of indium, high cost of manufacturing process and low flexibility of ITO have hindered its application in flexible devices.9-11 In decades, a variety of conductive materials, such as conducting polymers, carbon nanotubes, graphene, and metal nanowires have been investigated as candidates for the preparation of FTCEs.12-15 Among all these materials, metal nanowires, especially copper nanowires (Cu NWs), have attracted growing attention due to their superior high conductivity, relatively low cost and easy synthesis, light transmission and large abundance on Earth.16-18 Compared with other metal materials, Cu NWs are cheaper and abundant while keeps the high conductivity and low sheet resistance.1, 19 According to the previous reports, Cu NWs with high aspect ratio can effectively reduce the sheet resistance and provide more areas for 2 ACS Paragon Plus Environment

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light passing through the FTCEs.20-22 Besides, both the theoretical simulation and experimental studies have already demonstrated that the transparency/conductivity performance of metal nanowire mesh films are largely determined by the aspect ratio of metal nanowires.23-24 Therefore, fabrication of FTCEs based on ultralong Cu NWs with high aspect ratio is highly desirable. Recently, many efforts have been conducted for the synthesis of long Cu NWs with high aspect ratio. Ye et al. synthesized Cu NWs with about 35 nm in diameter and 80 µm in length (aspect ratio ~2500) via reducing Cu2+ with ethylenediamine (EDA) in extreme alkaline environment and the transparent electrode based on glass shows 100 sq−1 at 95% transmittance.16 Cui and co-workers used tris(trimethylsilyl)silane as a mild reducing reagent. The obtained Cu NWs show 17 µm in length and 17.5 nm in diameter (aspect ratio ~1000). The fabricated electrodes exhibits 34.8 sq−1 in sheet resistance at 90% in transmittance.21 Zhang et al. synthesized Cu NWs by using copper acetyacetonate with the assistant of hexadecylamine (HDA) and cetyltriamoninum bromide (CTAB). The obtained Cu NWs with nearly 40 µm in length and 78 nm in diameter display the aspect ratio more than 500.25 The electrodes fabricated in their work show the performance of 90 Ω sq−1 in sheet resistance and 90% in transmittance. Based on the results reported, it can be concluded that the ultralong Cu NWs with high aspect ratio is a key factor to obtain FTCEs with excellent performance. However, the tedious purification process (such as removal of the residue of copper nanoparticles

and

narrowing

the

length

distribution),

and

the

relatively

poor

environmental stability, restricted the preparation of Cu NWs for practical applications. Therefore, it is desirable to develop an effective method for the synthesis of Cu NWs with 3 ACS Paragon Plus Environment

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high aspect ratio and FTCEs with good antioxidant ability. Here, for the first time, ultralong Cu NWs (over 250 μm in length and around 50 nm in diameter) with high aspect ratio (~5000) were synthesized in water/polyhydric alcohol co-solvent. Compared with previous works reported, the water/polyhydric alcohol co-solvent system is more cheap and environmental friendly. The effects of polyhydric alcohols (including ethanol, ethylene glycol and glycerol) on the aspect ratio of Cu NWs were studied in detail. The molar ratio of capping ligands (oleylamine and oleic acid) and the concentration effects of glucose on the dispersity and morphology of Cu NWs were investigated. The obtained ultralong Cu NWs were well dispersed in n-hexane and used for the fabrication of FTCEs based on PDMS. For the first time, a method combining filtration, hydrogen reduction and spin coating was applied for the fabrication of FTCEs. Compared with the methods reported,16,21 our strategy could effectively remove the oxide layers of Cu NWs and prevent the Cu NWs from being oxidized. The FTCEs show the high performance of sheet resistance of 92.1 Ω sq−1 and transmittance of 91.524% at 550 nm. Inspired by the stretchable ability of PDMS, wearable sensor was further fabricated and applied to detect the movement of thumb joint through the chronoamperometry method. The as-prepared sensor exhibits high sensitivity and fast-response time, demonstrating the potential applications of the ultralong Cu NWs.

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Experimental Section Materials Copper cholride (CuCl2) was purchased from ACROS Chemical Reagent Co., Ltd. Oleylamine (OLA) was provided by J&K; Oleic Acid (OA) was supplied by TCI. Glucose was acquired from Sinopharm Chemical Reagent Co., Ltd. Ethanol, ethylene glycol, glycerol and n-Hexane were purchased from Enox. PDMS, nitrocellulose membrane (NC membrane) and thermal water kettle (volume: 50 mL) were purchased from Shanghai Chemical Reagents Co., Ltd. Deionized water was used throughout the experiments.

Synthesis of Copper Nanowires Ultralong Cu NWs were synthesized as follows: 0.8 g glucose and 0.6 g CuCl 2 were dissolved in 40 mL deionized water. Then 10 mL alcohol (ethanol, ethylene glycol or glycerol), OLA (8 mL) and OA (80 μL) were added. After stirring for 60 seconds, the mixture was transferred into a Teflon thermal water kettle. The kettle was placed in an oven at 120 oC for 4 h till the reaction was completed. After the reactor was cooled to room temperature, the solution was poured into n-hexane. The Cu NWs sank down, and the supernatant was removed with a pipe. Such a cycle was repeated 2 or 3 times until the supernatant become clear. Then the obtained ultralong Cu NWs were dispersed in n-hexane.

Flexible Transparent Conductive Electrodes Fabrication The Cu NWs were dispersed in n-hexane (0.01 mg/mL) and filtered through a NC membrane. After the filtration, the NC membrane was carefully transferred on the bottom of crystal dish 5 ACS Paragon Plus Environment

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and placed under hydrogen atmosphere for annealing 30 min to remove the oxidized layers. Then PDMS solution was spin-coated on the surface of NC membrane and cured at 165 oC for 30 min. The prepared FTCEs were obtained after peeling off from the NC membrane.

Fabrication of Stretchable Wearable Sensor The stretchable wearable sensor was fabricated by cutting the prepared FTCEs into strips.26 The sensors were fixed to the index finger with adhesive tape to detect the movement of the joint through the chronoamperometry method.

Characterization of Cu NWs and FTCEs The shape and the distribution of the Cu NWs were measured by the field emission scanning electron microscopy (FE-SEM, Hitachi SU8010) and Tecnai G20 transmission electron microscope (TEM). The structure and the crystallinity of Cu NWs were defined by the X-ray diffraction (XRD) measurement at a scan rate of 20 min-1 with a Cu Ka (k = 1.54056 Å) radiation (Rigaku Model D/MAX-2500V/PC). The UV/VIS absorption spectrum was carried out through a UV-3150 UV-Vis-Nir spectrophotometer (Shimadzu). The optical transmission of the FTCEs was determined by the different volume of Cu NWs solution. The sheet resistance of the FTCEs was measured using a four-probe method (Loresta GP T610, Mitsubishi Chemical Analytech Co. Ltd.). The chronoamperometry method was conducted by the Autolab Pgstat 302N (Metrohm).

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Results and Discussion

Figure 1. a) The proposed growth mechanism of ultralong Cu NWs with pentagonal structures; b) and c) TEM images of the radial and parallel growth of Cu NWs; d) TEM image of the final Cu NWs.

Figure 1a shows the synthesis process applied for the ultralong Cu NWs. At the first stage, copper ions (Cu2+) and OLA molecules form OLA/Cu2+ complexes, then Cu2+ are reduced to five-twinned seeds. With the reaction going on, the small seeds recrystallized into relative large copper nanoparticles due to the Ostwald ripening process.27 The obtained copper nanoparticles with decahedron structure show higher energy sites on the surface, leading to the continuous twin growth on the surface. The five-twinned seeds prefer to grow along the parallel axis to maintain a stable state which can form new {110} plane and prevent the formation of large nanoparticles.28-29 Here, OLA is used as a capping ligand which plays the same role as that of PVP in the formation of Ag NWs.30 OLA can adsorb onto and passivate the {100} plane instead of {111} facets leading to the one-dimensional nanowire growth.5, 31 OA was added as the co-capping ligand which could prevent the Cu NWs from aggregating.

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Figure 1b and c show the products of the reaction stopped at 2 h. The thinner Cu NWs and small nanoparticles (~5 nm in diameter) can be observed, indicating the radial and parallel growth of Cu NWs. Figure 1d shows the TEM image of the final Cu NWs with uniform diameter.

Figure 2. SEM images of synthesized Cu NWs a) large-scale and b) in detail. Cu NWs show about 250 µm in length and ~50 nm in diameter.

Figure 2a shows the SEM images of the synthesized Cu NWs in water/glycerol co-solvent. Figure S1 shows a combined SEM image, in which a single Cu NW with a length over 250 μm could be clearly observed. Figure 2b indicates that the diameter of Cu NWs is around 50 nm. Figure S2a shows the X-ray diffraction (XRD) pattern of Cu NWs with the typical face-centred cubic (FCC) structure. Three typical diffraction peaks at 43.3°, 50.5°, 74.1°can be assigned to {111}, {200} and {220} planes of the FCC copper (JCPDS file No. 03-1018), respectively. No characteristic peaks of CuO or Cu2O can be detected, indicating the high purity and stability of the synthesized Cu NWs. Figure S2b shows the UV-visible spectrum of the obtained Cu NWs and an absorption peak at 558 nm was observed.32 8 ACS Paragon Plus Environment

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Figure 3. HR-TEM images and SAED patterns of the synthesized Cu NWs. The electron beam runs perpendicular (a and b), and parallel (c and d) to one side of the five-fold twinned nanowire. The embedded images in (a) and (c) show the fast Fourier transformation (FFT) patterns of the selected area of Cu NWs.

High resolution transmission electron microscope (HR-TEM) was further applied to investigate the crystal structure of ultralong Cu NWs. Figure 3 shows the HR-TEM images and the selected area electron diffraction (SAED) patterns of Cu NWs with the electron beam runs perpendicular (Figure 3a, b) and parallel (Figures 3c, d) to one side of the Cu NWs surface. The Cu NWs contain five single-crystalline subunits can be labelled as T1-T5 (Figures 3b,d).27 Figure 3a shows that the [111] twin plane of Cu NWs is oriented parallel to the longitudinal axis. Figure 3b shows the typical SAED pattern composed of two zones [001] and [112] which are generated from T1, T3 and T4, respectively. Different with Figure 3a and b, Figures 3cand d show the [111] direction generated from T2 and T3 and the SAED 9 ACS Paragon Plus Environment

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pattern of the electron beam runs parallel to the side facet. The diffraction suggests the pattern corresponds to the overlap of [110] and [111] zone axis direction generated from T5. The remaining reflection spots in Figures 3b and d can be explained by the double-diffraction effects through the subcrystals and the twin boundary-related diffraction.33-34 All these results revealed that those Cu NWs have a multiple-twinned structure (bounded by five side surfaces planes {100} and capped by five planes {111} at both ends) which is consistent with the previous results.27

Figure 4. SEM images of Cu NWs synthesized in water/alcohol (4:1, volume ratio) co-solvent: (a) pure water (~120 nm in diameter); (b) water/ethanol (~95 nm in diameter); (c) water/ethylene glycol (~70 nm in diameter); and (d) water/glycerol (~50 nm in diameter). The insert SEM images show the copper nanoparticles obtained after 30 min reaction.

The effect of alcohols on the diameter of Cu NWs was studied. Figure 4 shows the SEM images of Cu NWs synthesized in various water/alcohol co-solvent. It can be seen that the 10 ACS Paragon Plus Environment

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diameters of Cu NWs decreased gradually from ~120 to ~50 nm as from in water to in glycerol. The decreasd diameter of Cu NWs can be attributed to the following proposed factors. At the firststage of the reaction, OLA molecules and Cu2+ can form OLA/Cu2+ complexes which show poor solubility in pure water due to the long hydrophobic alkyl chains of OLA. Addition of alcohols could improve the solubility of the OLA/Cu2+ complexes in co-solvent than in pure water.35 As shown in the insert images that the increased concentration of copper complexes can suppress the Ostwald ripening process and lead to smaller copper nanoparticles, which can induce the seeds grow along the longitudinal plane to form thinner Cu NWs.36 In addition, compared with pure water and enthanol, both ethylene glycol and glycerol can act as the reducing agents at high temperature to accelerate the reduction rate of Cu2+ at the nucleation stage.37-38 Such a higher reduction rate favors the formation of smaller copper nanoparticles and resulting in the growth of thinner Cu NWs. Furthermore, it is assumed that addition of ethylene glycol and glycerol increased the viscosity of the solution (viscosity of ethylene glycol and glycerol at 20 oC and 101.325 kPa is 25.66 and 1412 mPa.s, respectively).39-40 The high viscosity could prohibit the aggregation of nanoparticles and lead to the formation of smaller copper nanoparticles.41,42 The volume ratio effects of water/alcohol were further studied. Figure S3 shows the SEM images of Cu NWs prepared with various volume ratio of water/glycerol co-solvent. In the case of V(water):V(glycerol) = 5:1 (Figure S3a), the Cu NWs mantain the length around 250 μm, but most of the Cu NWs exhibited thick diameter as shown in the insert SEM image. As the volume ratio V(water):V(glycerol) reached to 4:1, the Cu NWs show uniform diameter around 50 nm (Figure S3b and the insert SEM image) and keep the length unchanged. However, further 11 ACS Paragon Plus Environment

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increasing the amount of glycerol yielded shorter length Cu NWs and copper nanoparticles (Figure S3c-e). Since the higher proporation of water is necessary for the formation of copper micelle,36 an optimized volume ratio of water/glycerol is critical for obtaining ultralong Cu NWs with thin diameter.

Figure 5. SEM images of Cu NWs prepared with different molar ratio of OLA and OA. a) OLA served as the capping ligand solely. Molar ratio of OLA:OA (b) 100:1, (c) 75:1 and (d) 50:1, respectively.

The molar ratio of capping ligands also plays an important role in obtaining ultralong Cu NWs with good dispersion in solution.43-44 Figure 5 exhibits the SEM images of Cu NWs prepared from different molar ratio of capping ligands. In the case of only OLA used as the capping ligand, the main product was bundled Cu NWs (Figure 5a). These bundled Cu NWs show poor dispersion in water or organic solvents, and cannot be used to fabricate FTCEs.43 To synthesize Cu NWs with good dispersity, OA was added as a co-capping ligand. Figures 12 ACS Paragon Plus Environment

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5b and c show the images of prepared Cu NWs based on two capping ligands (molar ratio of OLA:OA, 100:1 and 75:1, respectively). It can be seen that with the increased concentration of OA, the bundled number of the Cu NWs is reduced effectively. When the molar ratio of OLA and OA reached to 50: 1, well dispersed Cu NWs were obtained (Figure 5d). Further increase the molar ratio of OLA and OA (molar ratio of OLA:OA, 30:1) does not change the dispersity of Cu NWs and the remained aggregated Cu NWs can be attributed to the strong interdigitation between the Cu NWs (Figure S4a),45 Figure S4 b shows the SEM image of copper nanostructures with OA served as the capping ligand solely, we can see the main products are copper nanoparticles and short nanowires which demonstrate OLA plays an important role in obtaining Cu NWs.

Figure 6. SEM images of the Cu NWs prepared with different concentration of glucose. a) 140 mM; b) 120 mM; c) 100 mM and d) 80 mM. The insets in a, b and c show the detail of tadpole-like Cu NWs. 13 ACS Paragon Plus Environment

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Moreover, the concentration of the reducing agent, glucose, is another key factor affecting the morphology of Cu NWs.20 Figure 6 shows the SEM images of Cu NWs synthesized with different concentration of glucose. We can see tadpole-like Cu NWs when the concentration of glucose was 140-100 mM (Figure 6a-c). Such tadpole-like Cu NWs were originated from the tapered nanocrystals.46-47 Cu2+ was reduced by glucose to form tapered Cu nanoparticles. Then the formed nanoparticles grew into lager tadpole-like copper particles with pentagonal structures, similar to the results reported by Xia46 and Wiley’s47. The tapered Cu NWs have seed-wire interface consists of a single crystal, also similar to the growth of Pt NWs from spherical. With the reaction proceeded, the Cu NWs showed at the thinner end of the tadpole-like copper nanoparticles, and thus produced tadpole-like Cu NWs. With the concentration of glucose decreased, the number of tadpole-like Cu NWs reduced significantly (see the embedded images in Figure 6a-c). When the concentration of glucose decreased to 80 mM, such tadpole-like structure disappeared and turned into uniform Cu NWs(Figure 6d). At the lower concentration of glucose (60 mM), the formation of copper nanoparticles could be observed probably due to the insufficient reducing agent (Figure S5). Table S1 summarized the experimental conditions and their corresponding results.

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Scheme 1. The fabrication process of FTCEs based on PDMS. The FTCEs were fabricated through the filtration of Cu NWs solution. The Cu NWs deposited on the surface of NC membrane were fixed on the bottom of crystal dish and placed under hydrogen atmosphere for annealing for 30 min. PDMS solution was spin-coated on the surface of NC membrane and cured at 165 oC for 30 min. The prepared FTCEs were obtained after peeling off from the NC membrane.

Figure 7. Ultralong Cu NWs fabricated FTCEs and their optical and electrical performance. a) SEM images of the cross section of FTCE. b) Wavelength-dependent transmittance of as-prepared FTCEs and c) Transmittance versus sheet resistance for FTCEs of our FTCEs. 15 ACS Paragon Plus Environment

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The synthesized Cu NWs were further used to fabricate FTCEs based on PDMS (Scheme 1). Figure S6a shows the optical images of FTCEs with different transmittance. In this work, the transmittance could be adjusted by filtering amount of Cu NWs solution on the NC membrane. The higher amount of Cu NWs deposited, the lower transmittance of FTCEs. The FTCEs also exhibit good stability compared with the traditional filtration method. Figure S6b shows the tape test of the FTCEs fabricated by two different methods. We can see that there are lots of Cu NWs are adhered on the tape for filtration method fabricated FTCE while there are few Cu NWs can be found on the tape for the FTCE fabricated by this method. Figure 7a shows the cross section of as-prepared FTCEs. It can be seen that the most of Cu NWs were imbedded in PDMS which can prevent the Cu NWs detached from the PDMS substrate and further prevent the oxidation of Cu NWs. Here, all the FTCEs show high transmittance between 350 and 900 nm (Figure 7b). Figure 7c shows the sheet resistance of the FTCEs versus different transmittance at 550 nm. For a FTCE with 82.041% transmittance, the sheet resistance was only 3.4 Ω sq−1. With the transmittance of FTCEs increased, the sheet resistance increased gradually. When the transmittance finally increased to 91.524%, we obtain the FTCE with a lower sheet resistance of 92.1 Ω sq−1. The performance of our FTCEs stands out when compared with other FTCEs based on stretchable materials which showed 62.4 Ω sq−1 at 80%,1 4.1 Ω sq−1 at 70%,36 190.0 Ω sq−1 at 90.0%,48 and 56.2 Ω sq−1 at 84.5%.49 The ultralong Cu NWs is the main factor to explain the FTCEs with excellent performance. Ultralong Cu NWs can ensure the conductivity of FTCEs with fewer nanowires leading to more “free area” on the surface of PDMS which dose good for the light passing through and the fewer nanowires means less contact sites (the main sources of resistance) and 16 ACS Paragon Plus Environment

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can reduce the sheet resistance effectively. These advantages do significantly affect the performance of FTCEs.

Figure 8. a) Photo images of the FTCE based devices. A LED lamp was used to detect the conductivity of FTCEs under different state (pure PDMS, FTCEs under normal, twisting and stretching states, respectively. b) The R-V curves of the FTCEs under normal, twisting and stretching states, respectively. c) The R-V curve of the FTCE under the stretching state (elongation degree: 25%) after various cycles. d) The resistance versus 1000 cycles of FTCEs under twisting and stretching states.

Figure 8a shows the photo images of the FTCEs fabricated in this work. A LED lamp was used to detect the conductivity of FTCEs. It can be seen that the LED lamps still shine while the FTCEs under twisting or stretching state. To better understand the practicability of FTCEs, the FTCEs were chosen as a part of the LED lamp by applying the voltage from 0 to 3V. Figure 8b shows the R-V curve of the FTCEs under three different states. We can see that 17 ACS Paragon Plus Environment

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there is an obvious increasement in resistance under the stretching process. Figure 8c records the change of resistance during the stretching process by applying numbers of cycles. We can see that with the cycles increased, the resistance almost kept unchanged after 10 cycles. The FTCEs were test under twisting and stretching states (1000 cycles) to monitor the changes of resistance. Figure 8d records the normalized resistance of FTCEs with increased cycle numbers. It is worth noting that the resistance shows the same tendency with Figure 8b and c. The resistance almost maintained at its original levelfor twisting while there is a significant increasement and kept unchanged in resistance after stretching. The increased resistance under stretching state can be explained by the following reasons:50-51 during the stretching-releasing process, the PDMS is under compressive stress along the stretching direction which leading the Cu NWs to detach and buckle out of the film.52-53 Consider the ideal situation, all the Cu NWs should slide back to their original positions after releasing. However, there is friction force between the film and Cu NWs leading to the Cu NWs buckled at the certain position which cannot slide back to their original sites.53-54 Furthermore, the Cu NWs may break and fracture due to the extra stress. We can see the broken and fractured Cu NWs after the stretching-releasing process as depicted in Figure S7a. The broken and fractured Cu NWs will decrease the number of electrical pathways and increase the contact sites with each other, resulting in the increased resistance of FTCEs. Figure S7b shows the stability test of FTCEs after the stretching-releasing, we can see that the conductivity of the film only degrades little after exposuring in ambient air for 30 days. Figure S8 also displays the FTCE based on PDMS with two different tensile elongations.

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The FTCEs fabricated in this work show excellent stretch ability with a breaking elongation about 230 % (Figure S8c). All these results exhibit the great potential of our FTCEs.

Figure 9. Wearable and stretchable FTCE sensor for detecting the movement of finger joint. a) display the whole movement of finger joint and (b) the current signal changes during three cycles and (c) after 50 cycles.

Inspired by the good performance of our FTCEs, a wearable sensor was fabricated to detect the movement of finger joint through the chronoamperometry method. Figure 9a show the movement of finger joint. The change of current intensity during the process of finger bonding were showed in Figures 9b,c. As shown in Figure 9a that when bending the finger naturally, a slight curvature on the finger joint can be seen. The movement of the finger results in the slight tension strain of the wearable sensor, leading to the decrease in current. After releasing the finger to a straight state, the strain decreased and accompanied by the

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increasement in current. From the measurement, we can obtain the skin strain involved in the movement of finger joint easily. From Figures 9b,c, it can be seen that the current showed a decreasing trend and stabilized after 50 cycles. This kind of wearable sensor can also be used to detect other finger joints similarly. We thought the wearable sensor has promising potential for the application in next generation flexible and stretchable optical devices to realize better human control.

Conclusions In summary, ultralong Cu NWs were prepared in water/polyhydric alcohol co-solvent. Through choosing proper polyhydric alcohol (glycerol) and the controlled volume ratio of water/glycerol, Cu NWs with about 250 μm in length and 50 nm in diameter (aspect ratio:~5000) were obtained. The suitable molar ratio of capping ligands (OLA and OA) and concentration of glucose also ensure the dispersion and morphology of Cu NWs. The produced ultralong Cu NWs were used to fabricate PDMS based flexible FTCEs. The best performance of as-prepared FTCE exhibits a low sheet resistance of 92.1 Ω sq−1 and high transmittance of 91.52% at 550 nm which shows a certain degree of competitiveness compared with other metal nanowires based transparent electrodes. The prepared FTCEs were further used to fabricate the wearable stretchable sensors and exhibit high sensitivity and fast response to the movement of finger joint. All these results confirmed the promising potential utilities of such ultralong Cu NWs for many other flexible optical devices.

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Associated content The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.xxxxxxx.

Additional characterization, including SEM images,

and the FTCEs characterization.

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

Notes The authors declare no competingfinancial interest.Any additional relevant notes should be placed here.

Acknowledgements This work was supported by the National Nature Science Foundation for Distinguished Young Scholars (No. 21425417), the National Natural Science Foundation of China (No. 21274101), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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