Copper Nanowires
Facile Synthesis of Oxidation-Resistant Copper Nanowires toward Solution-Processable, Flexible, Foldable, and Free-Standing Electrodes Zhenxing Yin, Chaedong Lee, Sanghun Cho, Jeeyoung Yoo, Yuanzhe Piao,* and Youn Sang Kim* Solution-processable, flexible, foldable and free-standing electrodes have been great interests in the development of next generation electronic devices with high electrical conductivity, and flexibility, foldability and free-standing ability.[1] For advanced applications, such as smart clothes, wearable devices, and patchable medical devices, various materials for flexible, foldable or free-standing electrodes have been proposed.[2,3] In general, the conventional flexible electrodes have been based on conductive polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI). In spite of a good process-ability, they suffer from a relative high sheet resistance (∼10 Ω/䊐).[4] Meanwhile, conductive carbon materials, such as carbon nanotubes and graphene, are feasible to use for foldable electrodes with their low sheet resistance (∼1 Ω/䊐) and good mechanical propery. However, the carbon based conductive material loses the flexibility and foldability without flexible substrate, also the wide range of their applications is still limited to a high cost and complicate process.[5,6] In conductive ink using traditional metals, nanoparticles (NPs) and nanowires (NWs) shaped silver (Ag) or copper (Cu) have been developed for conductive ink materials toward flexible and foldable electrodes. They exhibit a very low sheet resistance (∼0.01 Ω/䊐) and superior mechanical properties compared to other conductive materials.[7–9] Specially, Ag NPs and NWs exhibit outstanding electrical conductivity and flexibility.[8,10,11] However, Ag also has the problems of scarcity and high cost. Therefore, Cu has recently received more attention, because the cost of Cu is cheaper than that
Z. Yin, C. Lee, S. Cho, Dr. J. Yoo, Prof. Y. Piao, Prof. Y. S. Kim Program in Nano Science and Technology Graduate School of Convergence Science and Technology Seoul National University Seoul 151–742, Republic of Korea E-mail: [email protected]; [email protected] Prof. Y. Piao, Prof. Y. S. Kim Advanced Institutes of Convergence Technology 864–1 Iui-dong, Yeongtong-gu, Suwon-si Gyeonggi-do 443–270, Republic of Korea DOI: 10.1002/smll.201401276 small 2014, DOI: 10.1002/smll.201401276
of Ag and electrical conductivity of Cu is similar with that of Ag.[12] But, unfortunately, the oxidation of Cu ink materials, harsh reduction process and high sintering temperature are still a critical problem of wide applications for conductive inks. Most of the research for flexible and foldable electrodes using Cu have been flourished toward on sphere-shaped Cu NPs.[9] However, these flexible films are strongly depended on conductive network of their structures. To fully connected, conventional sintering temperature for proper electrical conductivity reaches over 300 °C,[13] which is a limitation to apply for flexible substrates like plastic and fabric. In Comparison with sphere-shaped Cu NPs, the Cu NWs not only show the proper electrical conductivity and flexibility under a relatively low sintering temperature, but also have the freestanding ability, due to their structural advantages of wire shape.[14,15] Up to now, the several methods to synthesize Cu NWs have been introduced. The hydrazine-reduced Cu NWs were proposed by Wiley. B. J. group.[15] However, this method has the critical limitations for oxidation and aggregation. To prevent oxidation, the Cu NWs must be preserved in toxic hydrazine solution. Also, the sintering process should be assisted by the pure H2 reduction gases for removing the oxidized surface layer. Recently, another synthesis of ultra-long Cu NWs using the hexadecylamine reduction of Cu(acac)2 was proposed.[16] To synthesize the long Cu NWs, high cost Pt catalyst and long reaction time of 10 h were required. Also, these Cu NWs should be sintered at 95% N2 and 5% H2 reduction gases. Furthermore, the dispersion-ability in solvent, which is a critical for solution-processable, flexible, foldable and free-standing electrodes, was not studied. Consequently, in previously reported methods, the various limitations, such as rapid oxidation, poor dispersion-ability, high price catalyst and long reaction time, had still blocked the Cu NWs to widely apply to conductive inks for flexible, foldable and free-standing electrodes. Herein, we introduce the novel synthesis of oxidationresistant and single-crystalline Cu NWs toward solutionprocessable, flexible, foldable and free-standing electrodes by a salt-assisted polyol reduction method through control the structure-induced factors of capping ratio and nucleation rate. In addition, our method for Cu NWs successfully gives solutions for the previously mentioned problems, such as the
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rapid oxidation, the poor dispersion-ability, expensive catalyst and a long reaction time. The whole reaction time for the synthesis of Cu NWs was less than 2 hours without any catalysts. Also, the Cu NWs were well dispersed in isopropyl alcohol (IPA), preserved over 7 days without any oxidation, and easily re-dispersed in solvent after a short time shaking even passed a long time. The flexible, foldable and free-standing electrodes, which were fabricated by these Cu NWs ink with a low temperature sintering process as maximum as 220 °C, exhibited a high electrical performance (0.11 Ω/䊐), excellent flexibility and foldability even after 1000 time bending and all 90° folding. Moreover, these Cu NWs based electrode should have a free-standing ability; still maintain intrinsic performance of conductivity, flexibility and foldability, due to their structure merits. In synthetic process of Cu NWs, the reaction conditions of the molar ratio of capping agents and nucleation rate are the significant factors as a guiding role in structures of Cu. Particularly, the capping agents of bromide preferentially adsorbed on the {100} plane of metal NPs to change the morphology.[17] As the structure-induced factors, they effectively guided the ions to form the decahedron shaped seeds, and further be growth to the wire shaped Cu. Our method depended on the reduction potential of ethylene glycol (EG) with surfactants like oleylamine and potassium bromide (KBr) which could control the morphology of metal NWs.[16,17] The molar ratio of the capping agents is one of dominant factors in reaction conditions. Through modification of
various synthetic conditions, different morphologies of reaction products were clearly observed. As we known, the surface energy of copper is {110} > {100} > {111}.[16] To be a stable state, the lowest crystal plane is natural to be the most widespread exposed, and different proportions of crystal planes on the surface decide various shapes of Cu nanomaterial. Consequently, the shape is directly depended on surface energy gap. Bromide could effectively change the surface energy gap to synthesize the nanowire shape, due to their unique adsorption ability on {100} plane of metal NPs.[18] The reaction products were obtained from reaction media included a fixed amount of 30 ml EG, 2 mmol copper (I) chloride (CuCl) and 8 mmol capping agents which are consisted of different mole ratio of oleylamine and KBr. Figure 1a and S1 show the effects of molar ratio variation of two type capping agents in totally fixed 8 mmol. In case of the 7 mmol oleylamine and 1 mmol KBr (Figure S1a), only little part of seed was capped by bromide was grown in the [110] direction, due to the small amount of bromide as a capping agent.[17] In addition, the growth of [110] direction was blocked and stopped in the midway of reaction, so finally formed the tadpole shaped Cu NPs. In optimized condition of the molar ratio of 6 mmol oleylamine and 2 mmol KBr (Figure 1a), the bromide as the ionic capping agent was preferentially adsorbed in the {100} plane of Cu nanocrystals.[18] Therefore, the plane with lowest energy was exposed on the surface, and the Cu NWs elongated to [110] direction.[19] Figure S1b shows the molar ratio of 4 mmol oleylamine and 4 mmol KBr. After the synthesis,
Figure 1. (a) SEM image of Cu NWs. (b) SAED pattern of the Cu NWs along the [110] zone axis. (c) HR-TEM images of the top right white boxed areas, and corresponding FFT pattern of the bottom right. (d) XRD pattern of Cu NWs.
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small 2014, DOI: 10.1002/smll.201401276
Oxidation-Resistant Copper Nanowires for Electrodes
above 50% contents of the Cu NPs as impurities were observed, because the exceeded bromide not only capped in the {100} plane but also capped in the {110} and {111} planes. The sole effect of individual capping agent, oleylamine and KBr is shown in Figure S2, respectively. Besides the mole ratio of capping agents, a nucleation rate is another significant factor in the synthesis of the Cu NWs. We controlled the nucleation rate by changing of heating rate of 18, 9 and 4.5 °C/min from 110 to 198 °C. The products were obtained in the fixed condition of 30 ml EG, 2 mmol CuCl, 2 mmol KBr and 6 mmol oleylamine, which was the optimum condition induced from the above mention experiments. When the heating rate was 18 °C/min (Figure S3a), the Cu precursor was reduced rapidly by EG before being controlled the morphology of the seed. As a result, the flake-like or irregular-shaped Cu structures were obtained. Otherwise, at the heating rate of 9 °C/min (Figure 1a), the precursor was firstly reduced to the quasi-stable decahedral shaped Cu.[20,21] With the effect of the capping agent, decahedral shaped Cu seed was grown along the [110] direction to form the wire shape; increased their aspect ratio and reached to the relatively stable state at the 198 °C. In the case of the heating rate of 4.5 °C/min (Figure S3b), the approximately 60% of Cu NPs and 40% of Cu wires were obtained, because the Cu+ ions formed Cu NPs in the stable state of sphere shape by the over much time for the nucleation. On the analyses of above experimental conditions and SEM images, we confirmed the optimum synthetic conditions for Cu NWs as follow; 2 mmol CuCl, 30 ml EG, 6 mmol oleylamine, 2 mmol KBr, and the heating rate of 9 °C/ min from 110 to 198 °C. As well known, the Cu NWs were grown under unstable specific state of decahedral shaped Cu seed.[18,20] Therefore, the synthesis of Cu NWs required more harsh synthetic conditions than that of other shapes like spheres. The crystal structure of Cu NWs was investigated by high resolution transmission electron microscope (HR-TEM). The average 92 nm in width and 30 µm in length were measured from the dozens of Cu NWs. A selected area of electron diffraction (SAED) pattern from the Cu NWs is shown in Figure 1b, which indicates that electron beam was oriented along the [110] zone axis, and the single-crystalline Cu NWs were grown along the [110] direction.[19] Through the analyses of SEM, TEM, SAED and experimental conditions, we could assume the reaction steps and mechanism as follow. CuCl was reacted with the oleylamine to form the blue-color Cu-amine complex in the EG solution, and then, these Cu-amine complex were reduced by EG with a high reduction ability at high temperature (approximately 180 °C).[22] Generally, the order of the surface energy of face-centered-cubic metal is {110} > {100} > {111}.[23] When the lowest surface energy of {111} plane was exposed to the crystal surface, the decahedral shaped Cu seeds were obtained.[18,21] Simultaneously, the bromide molecules selectively capped in the {100} plane with the growth of Cu seed, and the surface energy of the {100} plane tended to be relatively stable.[24] When the growth rate of seed and the molar ratio of capping molecules reached to an optimized point, the Cu seed began to expose as the {100} plane to crystal surface small 2014, DOI: 10.1002/smll.201401276
and grew along [110] direction, because the {100} plane was more stabilized by the capping agents.[18,20] The HR-TEM image of the Cu NWs in Figure 1c clearly exhibits the two different fringe lattices spacing of 0.21 and 0.18 nm of Cu crystals, which corresponds to the {111} and {200} and planes.[25] Through the fast Fourier transform (FFT), the pattern also matches with {111} and {200} planes. Figure 1d shows the X-ray diffraction (XRD) pattern of the face-centered-cubic (fcc) Cu (JCPDS # 03–1028). The three diffraction peaks at 2θ = 43.5, 50.7, and 74.4° are respectively correspond to the {111}, {200}, and {220} planes.[16,19,26] Although the surface of synthesized Cu NWs was more activated with the increase of the reaction temperature, the Cu NWs were not oxidized by the strong reduction condition of the EG solution at above 180 °C.[27] To be a stable state, the plenty of capping molecules were effectively bound to the activated surface of Cu NWs at high temperature of 198 °C. In this reason, the total surface energy of Cu NWs was reduced. After fully stable at 198 °C, the surface energy of Cu NWs was suddenly lower, because of rapid decreasing of temperature in quenching process of experimental. Then, Cu NWs still maintain the effectively capped molecules. Therefore, this type of Cu NWs possessed an excellent oxidation resistance in wide range of temperature. In general, other Cu NWs were synthesized in amine, hydrazine or aqueous solution, these solutions could not provide a proper reduction potential and capping conditions, further not to reduce the surface energy to enough stable state.[15,16,25,28] Consequently, they were easy to be oxidized and aggregate with each other. For these problems, they were difficult to be used for conductive ink solution. However, our method for Cu NWs were well dispersed in IPA for relatively a long time of 1 h and redispersed easily after a short time shaking even preserved in 7 days (Figure S4). This phenomenon also indirectly proved that these synthesized Cu NWs had the lower surface energy and successfully stabilized by capping molecules. The oxidation resistance depending on time of Cu NWs preserved in isopropyl alcohol (IPA) was measured by XRD, ultraviolet-visible spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS) and conductivity meter (4-point probe). On the basis of XRD, UV-vis and XPS analyses, there were no significant changes on the characteristics of the Cu NWs, and they also had high conductivity after 7 days without any anti-oxidants, such as toxic hydrazine and ascorbic acid. Figure 2a,b show the XRD and UV-vis analysis of the Cu NWs after preserving for 2 and 7 days, respectively. As a result, the (111), (200) and (220) of crystal structure and 585 nm of light absorbance had no changes after 7 days. In general, the range of optical absorption for Cu NPs is 550–590 nm, which was reported previously.[29,30] In this study, the absorption peak at 585 nm was attributed to the plasma resonance of Cu NWs.[30] The XPS analysis is shown in Figure S5, and the peaks were corrected with the 284.6 eV of C 1s as the reference, which is the internal standard method.[31] On the basis of the XPS analysis, the characteristic peaks of Cu 2p1/2 and Cu 2p3/2 were matched with the binding energy of 952.2 eV and 931.8 eV, means that the composition of nanowires is mainly Cu (0). The satellite like peak with the binding energy of 944.0 eV indicates the existence of Cu2+.[32] In addition,
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Figure 2. The analysis of Cu NWs preserved in IPA after 2 days and 7 days, (a) XRD pattern (b) UV-vis absorption spectrum (c) XPS pattern. (d) The sheet resistance for Cu NWs based electrode (5 mg/cm2) preserved within 7 days and sintering at 220 °C.
O 1s was observed in the binding energy of 530.4 eV, means that a very thin natural Cu2O layer was formed at the surface of the Cu NWs.[33] The natural oxide layer at the surface prevents the Cu NWs from further oxidation, and is not detected by XRD pattern and UV-vis adsorption spectrum, because it is very thin.[34] The XPS spectrum of Cu 2p1/2 and Cu 2p3/2 preserved in 2 days and 7 days are shown in Figure 2c. The characteristic peaks of the spectrum are still located the binding energy of 952.2 eV and 931.8 eV, even preserved for 7 days. It was directly proven that the most of Cu NWs still kept the original state preserved in IPA during 7 days without any anti-oxidants. For further demonstration of the Cu NWs with high oxidation resistance, the electrode (5 mg/cm2) was fabricated by preserved Cu NWs sintering at 220 °C. Despite the Cu NWs were preserved for 7 days in IPA without any anti-oxidants, the electrode still exhibited high conductivity (0.11–0.44 Ω/䊐) (Figure 2d). Certainly, any types of Cu NWs were not absolutely free from the oxidation. Therefore, the thin oxide layer became gradually thicker and the sheet resistance of electrode increased with the passage of preserved time. However, this type of Cu NWs relatively sustained the electrical conductivity for a long period of time, although it was also slightly degraded. The strongly capped molecules by amine and bromide and a thin oxide layer (below 1nm) could reduce the surface energy and delay the further oxidation.[35] In this reason, this type of Cu NWs had an enough potential to be used for conductive ink materials with their outstanding oxidation resistance.
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In general, the sintering temperature of conventional Cu NPs for proper conductivity is more than 300 °C, because of their high contact resistance.[13] Compared with the spherical Cu NPs, the Cu NWs have outstanding structural advantages. Normally, Cu nanomaterial-based electrodes have a plenty of point-to-point contacts as a current pathway through the partial sintering process.[13,15,16,36] Since the spherical Cu NPs have large surface area compared to that of the Cu NWs with same volume, show higher sheet resistance because of their excessive contact points, thus need to be sintered at more increased temperature for exhibiting a proper conductivity.[13,15] Therefore, the nanowire structures, which need relative low sintering temperature, are more applicable to the fabrication of electrodes better than using spherical nanoparticles. In this reason, Cu NW based electrode exhibited superior electrical conductivity even at the remarkable low sintering temperature (180 °C) with excellent flexibility, foldablity and free-standing ability (Figure 3a). As a special structure property of nanowires, the free-standing film depended on Cu NWs randomly connecting and supporting for each other, and it still maintained intrinsic high electrical conductivity, flexibility and foldability. In comparison with bulk metal, these Cu NW based electrodes had no feeling of metallic contacts, and as light as a thin paper or textile. Therefore, the flexible, foldable and free-standing electrodes had a great potential to be used for smart clothes and wearable devices.
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small 2014, DOI: 10.1002/smll.201401276
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Figure 3. (a) Photographs of flexible, foldable and free-standing electrode (5 mg/cm2), lighting of a LED with an external power supply. (b) The sheet resistance for various surface densities of Cu NWs sintered at different temperatures and SEM image (180 °C sintering) on the top right. (c) The relative resistance of 180 °C sintered Cu NW electrode (5 mg/cm2) after 1, 500, 1000 bending test. (d) The relative resistance of 180 °C sintered Cu NW electrode (5 mg/cm2) under 1–5 folding 90°-cross sections and the schematic images on the top left. The brown section is Cu NWs and the white section is a filter paper.
Obviously, the sheet resistance of Cu NW electrodes depended on the amount of Cu NWs and sintering temperatures (180, 200 and 220 °C) (Figure 3b). To test mechanical properties, flexible Cu NW electrode (5 mg/cm2) was stressed by different bending radius and bending time using a self-designed bending machine (Figure 3c). Although there was a little increase of the sheet resistance by decreasing the bending radius and increasing the bending time, the electrical conductivity of these electrodes was maintained. For general spherical Cu NPs, too much bending induced intergranular fracture at the points of contact, because of a plenty of contact points.[36] However, Cu NWs were grown up to several micrometers along the one direction, made current pathways via simple stacking structures, which can resist the bending effectively.[37] Most of the bending occurred at the linear parts of NWs, not the sintered contact points, the electrode exhibited more stable performance under the excessive bending with inherent malleability and flexibility of copper materials. These advantages of Cu NWs could also be applied for folding test. Figure 3d shows the folding test of the Cu NW electrode with all 90° folding sections. After the folding test, the resistivity of the electrode was slightly increased when folded in the in-direction mode. In contrast, the resistivity was increased by 6 times after five 90°-out-direction folding mode, because folding induced creases which develop the deformation of cracks and remain permanently.[6] Although small 2014, DOI: 10.1002/smll.201401276
the electrode was slightly degraded with the folding test, compared with other foldable electrodes, it still exhibited high conductivity and a tiny degradation.[38,39] Here, we just introduced the fabrication of solutionprocessable, flexible, foldable and free-standing electrodes using Cu NWs inks. As the synthesis of Cu NWs have several unique advantages, such as low sintering temperature, high oxidation resistance, and good dispersion-ability, we believe that they can easily apply to other substrates like a plastic or fabric. Also, as the aspect ratio of Cu NWs can be easily controlled in the introduced synthetic method, it would be good for the control of metal ink properties. In summary, we synthesized an oxidation-resistant Cu NWs for solution-processable, flexible, foldable and free-standing electrodes using the salt-assisted polyol reduction method. The synthesized Cu NWs showed excellent oxidation resistance and well dispersion-ability without any anti-oxidants, because the Cu NWs are stabilized by strongly capped molecules of amine and bromide. Also, on the condition of a low sintering temperature (180–220 °C) in the vacuum oven without any reduction gases, the Cu NWs exhibited a low sheet resistance (0.11 Ω/䊐) with a plenty of contact areas. We believe that the suggested Cu NWs-based electrodes in this study are promising in wide applications due to their superior electrical conductivity, anti-oxidation property, low sintering temperature processibility, flexibility, foldability and free-standing ability.
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Experimental Section Preparation of Cu NWs: In a typical synthesis, CuCl (2 mmol), KBr (2 mmol), and oleylamine (6 mmol) were dissolved by EG (30 mL) in 100 mL of round-bottom flask, while forming Cu-amine complex. Then, it was magnetically stirred for 10 min, and the temperature was raised up to 110 °C for degassing and enhancing the purity, kept for 0.5 h. Finally, the reaction temperature was raised to 198 °C (9 °C/min of heating rate), reflux and kept it for 1 h. When the synthesis of Cu NWs was finished, the products must be rapid quenched by cold water and be washed by hexane, then centrifuged for 3 min in 10 000 rpm, repeated 3 times. Finally, the NWs were re-dispersed and preserved in IPA or n-hexane. Whole synthetic process was carried out in air without any protective atmosphere. Fabrication of Electrodes: The flexible, foldable and freestanding Cu NW electrode was prepared by vacuum filtration method. Typically, the well-dispersed Cu NWs filtered onto a filter paper (pore size 8 µm) to form the Cu NW film. After filtration, the Cu NW film was sintered under vacuum at 180, 200 and 220 °C for 1 h. The high conductive flexible and foldable electrodes were obtained with paper substrate. Through the separation of filter paper, the free-standing electrode could be easily obtained. Characterization of Cu NWs: The Cu NWs and electrodes were measured by x-ray diffraction measurements (Bruker D8 DISCOVER), ultraviolet-visible spectroscopy (PerkinElmer Lambda 35), X-ray photoelectron spectroscopy (VG Multilab ESCA 2000 system), scanning electron microscope (Hitachi S-4800 FE-SEM), and high resolution transmission electron microscope (JEOL JEM2100F instrument operated at 200 kV). The sheet resistance of the films was measured by a conductivity meter (4-point probe) (AiTCo., Ltd CMT-SR2000N).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by Basic Research Program (2011– 0018113) and Center for Advanced Soft Electronics as Global Frontier Research Program (2013M3A6A5073177) funded by the Ministry of Science, ICT and Future Planning of Korea. Also, Y.P. is grateful for the financial support by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CISS-2012M3A6A6054193) of Korea.
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Received: May 9, 2014 Revised: June 29, 2014 Published online: small 2014, DOI: 10.1002/smll.201401276
Facile Synthesis of Oxidation-Resistant Copper Nanowires toward Solution-Processable, Flexible, Foldable, and Free-Standing Electrodes Zhenxing Yin, Chaedong Lee, Sanghun Cho, Jeeyoung Yoo, Yuanzhe Piao,* and Youn Sang Kim* Solution-processable, flexible, foldable and free-standing electrodes have been great interests in the development of next generation electronic devices with high electrical conductivity, and flexibility, foldability and free-standing ability.[1] For advanced applications, such as smart clothes, wearable devices, and patchable medical devices, various materials for flexible, foldable or free-standing electrodes have been proposed.[2,3] In general, the conventional flexible electrodes have been based on conductive polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI). In spite of a good process-ability, they suffer from a relative high sheet resistance (∼10 Ω/䊐).[4] Meanwhile, conductive carbon materials, such as carbon nanotubes and graphene, are feasible to use for foldable electrodes with their low sheet resistance (∼1 Ω/䊐) and good mechanical propery. However, the carbon based conductive material loses the flexibility and foldability without flexible substrate, also the wide range of their applications is still limited to a high cost and complicate process.[5,6] In conductive ink using traditional metals, nanoparticles (NPs) and nanowires (NWs) shaped silver (Ag) or copper (Cu) have been developed for conductive ink materials toward flexible and foldable electrodes. They exhibit a very low sheet resistance (∼0.01 Ω/䊐) and superior mechanical properties compared to other conductive materials.[7–9] Specially, Ag NPs and NWs exhibit outstanding electrical conductivity and flexibility.[8,10,11] However, Ag also has the problems of scarcity and high cost. Therefore, Cu has recently received more attention, because the cost of Cu is cheaper than that
Z. Yin, C. Lee, S. Cho, Dr. J. Yoo, Prof. Y. Piao, Prof. Y. S. Kim Program in Nano Science and Technology Graduate School of Convergence Science and Technology Seoul National University Seoul 151–742, Republic of Korea E-mail: [email protected]; [email protected] Prof. Y. Piao, Prof. Y. S. Kim Advanced Institutes of Convergence Technology 864–1 Iui-dong, Yeongtong-gu, Suwon-si Gyeonggi-do 443–270, Republic of Korea DOI: 10.1002/smll.201401276 small 2014, DOI: 10.1002/smll.201401276
of Ag and electrical conductivity of Cu is similar with that of Ag.[12] But, unfortunately, the oxidation of Cu ink materials, harsh reduction process and high sintering temperature are still a critical problem of wide applications for conductive inks. Most of the research for flexible and foldable electrodes using Cu have been flourished toward on sphere-shaped Cu NPs.[9] However, these flexible films are strongly depended on conductive network of their structures. To fully connected, conventional sintering temperature for proper electrical conductivity reaches over 300 °C,[13] which is a limitation to apply for flexible substrates like plastic and fabric. In Comparison with sphere-shaped Cu NPs, the Cu NWs not only show the proper electrical conductivity and flexibility under a relatively low sintering temperature, but also have the freestanding ability, due to their structural advantages of wire shape.[14,15] Up to now, the several methods to synthesize Cu NWs have been introduced. The hydrazine-reduced Cu NWs were proposed by Wiley. B. J. group.[15] However, this method has the critical limitations for oxidation and aggregation. To prevent oxidation, the Cu NWs must be preserved in toxic hydrazine solution. Also, the sintering process should be assisted by the pure H2 reduction gases for removing the oxidized surface layer. Recently, another synthesis of ultra-long Cu NWs using the hexadecylamine reduction of Cu(acac)2 was proposed.[16] To synthesize the long Cu NWs, high cost Pt catalyst and long reaction time of 10 h were required. Also, these Cu NWs should be sintered at 95% N2 and 5% H2 reduction gases. Furthermore, the dispersion-ability in solvent, which is a critical for solution-processable, flexible, foldable and free-standing electrodes, was not studied. Consequently, in previously reported methods, the various limitations, such as rapid oxidation, poor dispersion-ability, high price catalyst and long reaction time, had still blocked the Cu NWs to widely apply to conductive inks for flexible, foldable and free-standing electrodes. Herein, we introduce the novel synthesis of oxidationresistant and single-crystalline Cu NWs toward solutionprocessable, flexible, foldable and free-standing electrodes by a salt-assisted polyol reduction method through control the structure-induced factors of capping ratio and nucleation rate. In addition, our method for Cu NWs successfully gives solutions for the previously mentioned problems, such as the
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rapid oxidation, the poor dispersion-ability, expensive catalyst and a long reaction time. The whole reaction time for the synthesis of Cu NWs was less than 2 hours without any catalysts. Also, the Cu NWs were well dispersed in isopropyl alcohol (IPA), preserved over 7 days without any oxidation, and easily re-dispersed in solvent after a short time shaking even passed a long time. The flexible, foldable and free-standing electrodes, which were fabricated by these Cu NWs ink with a low temperature sintering process as maximum as 220 °C, exhibited a high electrical performance (0.11 Ω/䊐), excellent flexibility and foldability even after 1000 time bending and all 90° folding. Moreover, these Cu NWs based electrode should have a free-standing ability; still maintain intrinsic performance of conductivity, flexibility and foldability, due to their structure merits. In synthetic process of Cu NWs, the reaction conditions of the molar ratio of capping agents and nucleation rate are the significant factors as a guiding role in structures of Cu. Particularly, the capping agents of bromide preferentially adsorbed on the {100} plane of metal NPs to change the morphology.[17] As the structure-induced factors, they effectively guided the ions to form the decahedron shaped seeds, and further be growth to the wire shaped Cu. Our method depended on the reduction potential of ethylene glycol (EG) with surfactants like oleylamine and potassium bromide (KBr) which could control the morphology of metal NWs.[16,17] The molar ratio of the capping agents is one of dominant factors in reaction conditions. Through modification of
various synthetic conditions, different morphologies of reaction products were clearly observed. As we known, the surface energy of copper is {110} > {100} > {111}.[16] To be a stable state, the lowest crystal plane is natural to be the most widespread exposed, and different proportions of crystal planes on the surface decide various shapes of Cu nanomaterial. Consequently, the shape is directly depended on surface energy gap. Bromide could effectively change the surface energy gap to synthesize the nanowire shape, due to their unique adsorption ability on {100} plane of metal NPs.[18] The reaction products were obtained from reaction media included a fixed amount of 30 ml EG, 2 mmol copper (I) chloride (CuCl) and 8 mmol capping agents which are consisted of different mole ratio of oleylamine and KBr. Figure 1a and S1 show the effects of molar ratio variation of two type capping agents in totally fixed 8 mmol. In case of the 7 mmol oleylamine and 1 mmol KBr (Figure S1a), only little part of seed was capped by bromide was grown in the [110] direction, due to the small amount of bromide as a capping agent.[17] In addition, the growth of [110] direction was blocked and stopped in the midway of reaction, so finally formed the tadpole shaped Cu NPs. In optimized condition of the molar ratio of 6 mmol oleylamine and 2 mmol KBr (Figure 1a), the bromide as the ionic capping agent was preferentially adsorbed in the {100} plane of Cu nanocrystals.[18] Therefore, the plane with lowest energy was exposed on the surface, and the Cu NWs elongated to [110] direction.[19] Figure S1b shows the molar ratio of 4 mmol oleylamine and 4 mmol KBr. After the synthesis,
Figure 1. (a) SEM image of Cu NWs. (b) SAED pattern of the Cu NWs along the [110] zone axis. (c) HR-TEM images of the top right white boxed areas, and corresponding FFT pattern of the bottom right. (d) XRD pattern of Cu NWs.
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above 50% contents of the Cu NPs as impurities were observed, because the exceeded bromide not only capped in the {100} plane but also capped in the {110} and {111} planes. The sole effect of individual capping agent, oleylamine and KBr is shown in Figure S2, respectively. Besides the mole ratio of capping agents, a nucleation rate is another significant factor in the synthesis of the Cu NWs. We controlled the nucleation rate by changing of heating rate of 18, 9 and 4.5 °C/min from 110 to 198 °C. The products were obtained in the fixed condition of 30 ml EG, 2 mmol CuCl, 2 mmol KBr and 6 mmol oleylamine, which was the optimum condition induced from the above mention experiments. When the heating rate was 18 °C/min (Figure S3a), the Cu precursor was reduced rapidly by EG before being controlled the morphology of the seed. As a result, the flake-like or irregular-shaped Cu structures were obtained. Otherwise, at the heating rate of 9 °C/min (Figure 1a), the precursor was firstly reduced to the quasi-stable decahedral shaped Cu.[20,21] With the effect of the capping agent, decahedral shaped Cu seed was grown along the [110] direction to form the wire shape; increased their aspect ratio and reached to the relatively stable state at the 198 °C. In the case of the heating rate of 4.5 °C/min (Figure S3b), the approximately 60% of Cu NPs and 40% of Cu wires were obtained, because the Cu+ ions formed Cu NPs in the stable state of sphere shape by the over much time for the nucleation. On the analyses of above experimental conditions and SEM images, we confirmed the optimum synthetic conditions for Cu NWs as follow; 2 mmol CuCl, 30 ml EG, 6 mmol oleylamine, 2 mmol KBr, and the heating rate of 9 °C/ min from 110 to 198 °C. As well known, the Cu NWs were grown under unstable specific state of decahedral shaped Cu seed.[18,20] Therefore, the synthesis of Cu NWs required more harsh synthetic conditions than that of other shapes like spheres. The crystal structure of Cu NWs was investigated by high resolution transmission electron microscope (HR-TEM). The average 92 nm in width and 30 µm in length were measured from the dozens of Cu NWs. A selected area of electron diffraction (SAED) pattern from the Cu NWs is shown in Figure 1b, which indicates that electron beam was oriented along the [110] zone axis, and the single-crystalline Cu NWs were grown along the [110] direction.[19] Through the analyses of SEM, TEM, SAED and experimental conditions, we could assume the reaction steps and mechanism as follow. CuCl was reacted with the oleylamine to form the blue-color Cu-amine complex in the EG solution, and then, these Cu-amine complex were reduced by EG with a high reduction ability at high temperature (approximately 180 °C).[22] Generally, the order of the surface energy of face-centered-cubic metal is {110} > {100} > {111}.[23] When the lowest surface energy of {111} plane was exposed to the crystal surface, the decahedral shaped Cu seeds were obtained.[18,21] Simultaneously, the bromide molecules selectively capped in the {100} plane with the growth of Cu seed, and the surface energy of the {100} plane tended to be relatively stable.[24] When the growth rate of seed and the molar ratio of capping molecules reached to an optimized point, the Cu seed began to expose as the {100} plane to crystal surface small 2014, DOI: 10.1002/smll.201401276
and grew along [110] direction, because the {100} plane was more stabilized by the capping agents.[18,20] The HR-TEM image of the Cu NWs in Figure 1c clearly exhibits the two different fringe lattices spacing of 0.21 and 0.18 nm of Cu crystals, which corresponds to the {111} and {200} and planes.[25] Through the fast Fourier transform (FFT), the pattern also matches with {111} and {200} planes. Figure 1d shows the X-ray diffraction (XRD) pattern of the face-centered-cubic (fcc) Cu (JCPDS # 03–1028). The three diffraction peaks at 2θ = 43.5, 50.7, and 74.4° are respectively correspond to the {111}, {200}, and {220} planes.[16,19,26] Although the surface of synthesized Cu NWs was more activated with the increase of the reaction temperature, the Cu NWs were not oxidized by the strong reduction condition of the EG solution at above 180 °C.[27] To be a stable state, the plenty of capping molecules were effectively bound to the activated surface of Cu NWs at high temperature of 198 °C. In this reason, the total surface energy of Cu NWs was reduced. After fully stable at 198 °C, the surface energy of Cu NWs was suddenly lower, because of rapid decreasing of temperature in quenching process of experimental. Then, Cu NWs still maintain the effectively capped molecules. Therefore, this type of Cu NWs possessed an excellent oxidation resistance in wide range of temperature. In general, other Cu NWs were synthesized in amine, hydrazine or aqueous solution, these solutions could not provide a proper reduction potential and capping conditions, further not to reduce the surface energy to enough stable state.[15,16,25,28] Consequently, they were easy to be oxidized and aggregate with each other. For these problems, they were difficult to be used for conductive ink solution. However, our method for Cu NWs were well dispersed in IPA for relatively a long time of 1 h and redispersed easily after a short time shaking even preserved in 7 days (Figure S4). This phenomenon also indirectly proved that these synthesized Cu NWs had the lower surface energy and successfully stabilized by capping molecules. The oxidation resistance depending on time of Cu NWs preserved in isopropyl alcohol (IPA) was measured by XRD, ultraviolet-visible spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS) and conductivity meter (4-point probe). On the basis of XRD, UV-vis and XPS analyses, there were no significant changes on the characteristics of the Cu NWs, and they also had high conductivity after 7 days without any anti-oxidants, such as toxic hydrazine and ascorbic acid. Figure 2a,b show the XRD and UV-vis analysis of the Cu NWs after preserving for 2 and 7 days, respectively. As a result, the (111), (200) and (220) of crystal structure and 585 nm of light absorbance had no changes after 7 days. In general, the range of optical absorption for Cu NPs is 550–590 nm, which was reported previously.[29,30] In this study, the absorption peak at 585 nm was attributed to the plasma resonance of Cu NWs.[30] The XPS analysis is shown in Figure S5, and the peaks were corrected with the 284.6 eV of C 1s as the reference, which is the internal standard method.[31] On the basis of the XPS analysis, the characteristic peaks of Cu 2p1/2 and Cu 2p3/2 were matched with the binding energy of 952.2 eV and 931.8 eV, means that the composition of nanowires is mainly Cu (0). The satellite like peak with the binding energy of 944.0 eV indicates the existence of Cu2+.[32] In addition,
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Figure 2. The analysis of Cu NWs preserved in IPA after 2 days and 7 days, (a) XRD pattern (b) UV-vis absorption spectrum (c) XPS pattern. (d) The sheet resistance for Cu NWs based electrode (5 mg/cm2) preserved within 7 days and sintering at 220 °C.
O 1s was observed in the binding energy of 530.4 eV, means that a very thin natural Cu2O layer was formed at the surface of the Cu NWs.[33] The natural oxide layer at the surface prevents the Cu NWs from further oxidation, and is not detected by XRD pattern and UV-vis adsorption spectrum, because it is very thin.[34] The XPS spectrum of Cu 2p1/2 and Cu 2p3/2 preserved in 2 days and 7 days are shown in Figure 2c. The characteristic peaks of the spectrum are still located the binding energy of 952.2 eV and 931.8 eV, even preserved for 7 days. It was directly proven that the most of Cu NWs still kept the original state preserved in IPA during 7 days without any anti-oxidants. For further demonstration of the Cu NWs with high oxidation resistance, the electrode (5 mg/cm2) was fabricated by preserved Cu NWs sintering at 220 °C. Despite the Cu NWs were preserved for 7 days in IPA without any anti-oxidants, the electrode still exhibited high conductivity (0.11–0.44 Ω/䊐) (Figure 2d). Certainly, any types of Cu NWs were not absolutely free from the oxidation. Therefore, the thin oxide layer became gradually thicker and the sheet resistance of electrode increased with the passage of preserved time. However, this type of Cu NWs relatively sustained the electrical conductivity for a long period of time, although it was also slightly degraded. The strongly capped molecules by amine and bromide and a thin oxide layer (below 1nm) could reduce the surface energy and delay the further oxidation.[35] In this reason, this type of Cu NWs had an enough potential to be used for conductive ink materials with their outstanding oxidation resistance.
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In general, the sintering temperature of conventional Cu NPs for proper conductivity is more than 300 °C, because of their high contact resistance.[13] Compared with the spherical Cu NPs, the Cu NWs have outstanding structural advantages. Normally, Cu nanomaterial-based electrodes have a plenty of point-to-point contacts as a current pathway through the partial sintering process.[13,15,16,36] Since the spherical Cu NPs have large surface area compared to that of the Cu NWs with same volume, show higher sheet resistance because of their excessive contact points, thus need to be sintered at more increased temperature for exhibiting a proper conductivity.[13,15] Therefore, the nanowire structures, which need relative low sintering temperature, are more applicable to the fabrication of electrodes better than using spherical nanoparticles. In this reason, Cu NW based electrode exhibited superior electrical conductivity even at the remarkable low sintering temperature (180 °C) with excellent flexibility, foldablity and free-standing ability (Figure 3a). As a special structure property of nanowires, the free-standing film depended on Cu NWs randomly connecting and supporting for each other, and it still maintained intrinsic high electrical conductivity, flexibility and foldability. In comparison with bulk metal, these Cu NW based electrodes had no feeling of metallic contacts, and as light as a thin paper or textile. Therefore, the flexible, foldable and free-standing electrodes had a great potential to be used for smart clothes and wearable devices.
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Figure 3. (a) Photographs of flexible, foldable and free-standing electrode (5 mg/cm2), lighting of a LED with an external power supply. (b) The sheet resistance for various surface densities of Cu NWs sintered at different temperatures and SEM image (180 °C sintering) on the top right. (c) The relative resistance of 180 °C sintered Cu NW electrode (5 mg/cm2) after 1, 500, 1000 bending test. (d) The relative resistance of 180 °C sintered Cu NW electrode (5 mg/cm2) under 1–5 folding 90°-cross sections and the schematic images on the top left. The brown section is Cu NWs and the white section is a filter paper.
Obviously, the sheet resistance of Cu NW electrodes depended on the amount of Cu NWs and sintering temperatures (180, 200 and 220 °C) (Figure 3b). To test mechanical properties, flexible Cu NW electrode (5 mg/cm2) was stressed by different bending radius and bending time using a self-designed bending machine (Figure 3c). Although there was a little increase of the sheet resistance by decreasing the bending radius and increasing the bending time, the electrical conductivity of these electrodes was maintained. For general spherical Cu NPs, too much bending induced intergranular fracture at the points of contact, because of a plenty of contact points.[36] However, Cu NWs were grown up to several micrometers along the one direction, made current pathways via simple stacking structures, which can resist the bending effectively.[37] Most of the bending occurred at the linear parts of NWs, not the sintered contact points, the electrode exhibited more stable performance under the excessive bending with inherent malleability and flexibility of copper materials. These advantages of Cu NWs could also be applied for folding test. Figure 3d shows the folding test of the Cu NW electrode with all 90° folding sections. After the folding test, the resistivity of the electrode was slightly increased when folded in the in-direction mode. In contrast, the resistivity was increased by 6 times after five 90°-out-direction folding mode, because folding induced creases which develop the deformation of cracks and remain permanently.[6] Although small 2014, DOI: 10.1002/smll.201401276
the electrode was slightly degraded with the folding test, compared with other foldable electrodes, it still exhibited high conductivity and a tiny degradation.[38,39] Here, we just introduced the fabrication of solutionprocessable, flexible, foldable and free-standing electrodes using Cu NWs inks. As the synthesis of Cu NWs have several unique advantages, such as low sintering temperature, high oxidation resistance, and good dispersion-ability, we believe that they can easily apply to other substrates like a plastic or fabric. Also, as the aspect ratio of Cu NWs can be easily controlled in the introduced synthetic method, it would be good for the control of metal ink properties. In summary, we synthesized an oxidation-resistant Cu NWs for solution-processable, flexible, foldable and free-standing electrodes using the salt-assisted polyol reduction method. The synthesized Cu NWs showed excellent oxidation resistance and well dispersion-ability without any anti-oxidants, because the Cu NWs are stabilized by strongly capped molecules of amine and bromide. Also, on the condition of a low sintering temperature (180–220 °C) in the vacuum oven without any reduction gases, the Cu NWs exhibited a low sheet resistance (0.11 Ω/䊐) with a plenty of contact areas. We believe that the suggested Cu NWs-based electrodes in this study are promising in wide applications due to their superior electrical conductivity, anti-oxidation property, low sintering temperature processibility, flexibility, foldability and free-standing ability.
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Experimental Section Preparation of Cu NWs: In a typical synthesis, CuCl (2 mmol), KBr (2 mmol), and oleylamine (6 mmol) were dissolved by EG (30 mL) in 100 mL of round-bottom flask, while forming Cu-amine complex. Then, it was magnetically stirred for 10 min, and the temperature was raised up to 110 °C for degassing and enhancing the purity, kept for 0.5 h. Finally, the reaction temperature was raised to 198 °C (9 °C/min of heating rate), reflux and kept it for 1 h. When the synthesis of Cu NWs was finished, the products must be rapid quenched by cold water and be washed by hexane, then centrifuged for 3 min in 10 000 rpm, repeated 3 times. Finally, the NWs were re-dispersed and preserved in IPA or n-hexane. Whole synthetic process was carried out in air without any protective atmosphere. Fabrication of Electrodes: The flexible, foldable and freestanding Cu NW electrode was prepared by vacuum filtration method. Typically, the well-dispersed Cu NWs filtered onto a filter paper (pore size 8 µm) to form the Cu NW film. After filtration, the Cu NW film was sintered under vacuum at 180, 200 and 220 °C for 1 h. The high conductive flexible and foldable electrodes were obtained with paper substrate. Through the separation of filter paper, the free-standing electrode could be easily obtained. Characterization of Cu NWs: The Cu NWs and electrodes were measured by x-ray diffraction measurements (Bruker D8 DISCOVER), ultraviolet-visible spectroscopy (PerkinElmer Lambda 35), X-ray photoelectron spectroscopy (VG Multilab ESCA 2000 system), scanning electron microscope (Hitachi S-4800 FE-SEM), and high resolution transmission electron microscope (JEOL JEM2100F instrument operated at 200 kV). The sheet resistance of the films was measured by a conductivity meter (4-point probe) (AiTCo., Ltd CMT-SR2000N).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by Basic Research Program (2011– 0018113) and Center for Advanced Soft Electronics as Global Frontier Research Program (2013M3A6A5073177) funded by the Ministry of Science, ICT and Future Planning of Korea. Also, Y.P. is grateful for the financial support by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CISS-2012M3A6A6054193) of Korea.
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Received: May 9, 2014 Revised: June 29, 2014 Published online: small 2014, DOI: 10.1002/smll.201401276
