Synthesis of oxidation-resistant electrochemical

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Nov 22, 2018 - The polyPDA-coated Cu NWs are electrochemically active even after a long-term storage in .... Copper chloride (CuCl2, from Alfa Aesar), glucose (from TCI), para- ... beam SEM) and X-ray powder diffractometer (Bruker D8 focus Bragg .... (For interpretation of the references to color in this figure legend, the ...
Materials and Design 162 (2019) 154–161

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

Synthesis of oxidation-resistant electrochemical-active copper nanowires using phenylenediamine isomers Tan Zhang a, Farhad Daneshvar a, Shaoyang Wang b, Hung-Jue Sue a,⁎ a b

Polymer Technology Center, Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Cu nanowire/polyphenylenediamine core/shell structures were first time synthesized under a mild condition by forming a Cu (I) intermediate. • As-synthesized Cu nanowires show excellent oxidation-resistant in boiling water (3 days), and long-term stability in water at room temperature (10 months). • As-synthesized Cu nanowires are electrochemically active even after 10 months of storage in water.

a r t i c l e

i n f o

Article history: Received 19 August 2018 Received in revised form 18 November 2018 Accepted 19 November 2018 Available online 22 November 2018 Keywords: Copper nanowires Phenylenediamine Anti-oxidation Cu (I) complex Electrochemical properties

a b s t r a c t Phenylenediamine (PDA) was chosen as a coordinating, reducing, and capping agent to effectively direct growth and protect against oxidation of Cu nanowires (Cu NWs) in an aqueous solution. PDA was found to reduce Cu (II) to Cu (I) at room temperature, and stabilize the resulting Cu (I) by forming a coordination complex. The presence of a stable, intermediate Cu (I) complex is an essential step in the synthesis of Cu NWs under mild conditions. Incorporation of different PDA isomers leads to different growth paths of forming Cu NWs. Both pPDA and mPDA-synthesized Cu NWs are covered with a thin layer of polyphenylenediamine and show excellent oxidation-resistant in boiling water (3 days), and long-term stability in water at room temperature (10 months). The polyPDA-coated Cu NWs are electrochemically active even after a long-term storage in water. The usefulness of the oxidation-resistant electrochemical-active Cu NWs prepared here for a variety of nanotechnology applications is discussed. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction One-dimensional nanocrystals of transition metals, such as platinum (Pt), silver (Ag) and copper (Cu) nanowires (NWs), are important conducting materials for fabrication of electronic nano-devices. Compared with bulk metal, metal NWs exhibit unique electronic and optical ⁎ Corresponding author. E-mail address: [email protected] (H.-J. Sue).

properties due to their high aspect ratios and nanoscale diameters [1–3]. Among these metallic NWs, Cu NWs are of particular interest not only because of their relatively low cost but also their excellent electrical and electrochemical properties. For example, individual Cu NW has been shown to possess an ampacity nearly 40 times higher than that of bulk Cu [4]. Cu NWs also show novel electrochemical properties for use as battery electrode and catalyst [5,6]. Cu NWs are therefore being considered as a preferred candidate material of choice for “multifunctional building block material” in nano-device fabrication.

https://doi.org/10.1016/j.matdes.2018.11.043 0264-1275/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

T. Zhang et al. / Materials and Design 162 (2019) 154–161

Recent rapid advances in battery, sensor and catalytic systems demand high performance electrochemical materials that are stable in an aqueous environment [5,7,8]. A high performance electrochemical device requires a long service life and a good electrochemical response. The former requires oxidation-resistance and the latter needs electrochemical activity. However, both properties are usually mutually exclusive from each other. Cu NWs exhibit poor oxidation resistance in the presence of water or humid air, thus compromising its intended functionalities [1,9]. Several methods have been proposed to prevent the oxidation of Cu NWs, such as deposition of a layer of protective metallic or metal oxide nanoparticles [10,11], or wrapping them with graphene oxide [12]. On one hand, due to the rapid oxidation of Cu NWs, typically within hours [9], these post-synthesis methods cannot effectively protect freshly prepared Cu NWs, and are not suitable for large-scale production. On the other hand, forming a protective layer on Cu NWs usually blocks the charge transfer from solution to Cu NW surfaces which deteriorates the electrochemical properties of Cu NWs [13]. How to prepare the Cu NWs which are not only anti-oxidative but also electrochemically active still remains challenge. Cu NWs synthesized by the hydrothermal method are usually covered with alkyl amines. During Cu NW synthesis, these alkyl amines can coordinate with Cu ions, thereby not only promoting chemical reduction of Cu ions but also serve as a facet-selective capping agent to direct one-dimensional growth of Cu nanocrystals [14]. These alkyl amines remain on the Cu NW surface after synthesis. It has been reported that long-chain alkyl amines, such as hexadecylamine (HDA), form a densely packed monolayer on Cu NW surfaces [15]. These adsorbed hydrophobic alkyl chains can repel oxygen and water molecules from attacking Cu, which slows down the oxidation of Cu NWs to some extent [15,16]. However, these long-chain alkyl amines could also inhibit the charge transfer from the solution to the Cu NW surfaces [13]. Short-chain alkyl diamines, such as ethylenediamine (EDA), have also been used to synthesize and protect Cu NWs [9,17–19]. The Cu NWs synthesized from EDA were found to be oxidation-resistant in polar organic solvents, but oxidized easily in nonpolar organic solvents and water [9]. One solution to prepare oxidation-resistant electrochemically active Cu NWs is to decorate a thin layer of conducting polymer on Cu NWs in-situ [20]. However, it is difficult to identify a conductive molecule that can facilitate one-dimensional growth of Cu nanocrystals, and also coat Cu NWs to inhibit oxidation. Until now, the only successful attempt in literature uses pyrrole with the assistance of a strong reducing agent hydrazine, but the stability of the resulting polypyrrole-coated Cu NWs was not reported [20]. Herein, we report using phenylenediamine (PDA) isomers to prepare coated Cu NW structures. Similar to alkyl amines, the amine group(s) on PDA is able to coordinate with Cu (II) to serve as a coordinating and capping agent during the Cu NW synthesis. In addition, PDA alone also can be used as a reducing agent [21]. A redox reaction has been reported to occur between PDA and Ag (I), where Ag (I) is reduced to Ag atom, and PDA itself is oxidized and polymerized to form poly(phenylenediamine) (polyPDA) [21]. PolyPDA is a conductive polymer, which has been used for applications in pseudo capacitor, dye or metal ion absorption, catalysis and biosensors [21–23]. Similar to polyaniline, polyPDA is also an efficient anticorrosion material to protect metals [23,24]. PolyPDA can be an effective coating to protect Cu NWs without compromising their electrochemical properties. All three isomers of PDA (para-, meta- and ortho-) have different oxidation potentials (oPDA N pPDA N mPDA) [25], Therefore, it is of interest to explore the role of PDA isomers in Cu NW synthesis for multifunctional applications. 2. Experimental section 2.1. Materials Copper chloride (CuCl2, from Alfa Aesar), glucose (from TCI), paraphenylenediamine (pPDA, from Sigma), meta-phenylenediamine

155

(mPDA, from Acros), ortho-phenylenediamine (oPDA, from Merck), hexadecylamine (HDA, from Merck) and potassium hydroxide (KOH, from Sigma) were used as received. Indium tin oxide coated glass slides with a resistance of 5–15 Ω were provided by Delta Technologies. 2.2. Cu nanowire synthesis In the precursor solution, 0.75 mmol CuCl2 and 3.0 mmol PDA with or without 0.75 mmol glucose were dissolved in 60 mL deionized water. The precursors were first stirred for 3 h at room temperature, and then kept at 110 °C for 24 h or 60 h without stirring. The products were washed with deionized water and ethanol to remove unreacted reagents and free polymeric byproducts. Detailed synthetic procedures for Cu NWs using HDA have been reported elsewhere [4]. 2.3. Characterization and methods The morphologies and the properties of the as-synthesized samples were characterized using ultraviolet-visible near-infrared spectrophotometer (Shimadzu UV-3600), X-ray photoelectron spectroscopy (XPS, Omicron's DAR 40), transmission electron microscopy (JEOL JEM-2010 TEM), scanning electron microscopy (Tescan FERA-3 focused ion beam SEM) and X-ray powder diffractometer (Bruker D8 focus Bragg Brentano). The electrochemical properties were tested using Solartron 1287 potentiostat by placing a Cu NWs coated indium tin oxide slide as working electrode in 0.1 M KOH solution with a scan rate of 20 mV/s. A platinum wire was used as counter electrode and the reference electrode is Ag/AgCl. The simulation was conducted in Avogadro where one Cu ion and four PDA molecules were placed randomly, and the most stable structures were calculated using a universal force field (UFF). 3. Results and discussion The aqueous precursor solutions containing CuCl2 and PDA were stirred at room temperature for 3 h to obtain uniform suspensions. The UV–Vis spectra for the precursor solutions having different stirring time durations were recorded and plotted in Fig. 1. The absorption bands for PDA isomers are located below 300 nm [26] and that for CuCl2 is above 800 nm [27]. Due to a lone electron pair on the nitrogen atom, PDA can coordinate with electrophilic Cu (II) as illustrated in Fig. 2. The presence of a broad absorption band centered at 580 nm suggests the formation of Cu (II)-amine occurs within 2 min of mixing [28], which is consistent with the observations for the Cu (II) coordinated with alkyl amines [27,29]. Cu (II)-PDA coordination complex is known to be unstable [30] because the electron donation from the coordinated PDA leads to a ligand-to-metal charge transfer (LMCT) where PDA is oxidized and Cu (II) is reduced. The redox reaction between Cu (II) and PDA occurs spontaneously in an aqueous environment. As seen in Fig. 1, the absorption band for Cu (II)-PDA complexes at 580 nm disappeared rapidly with increased stirring time, or was not observed in the precursor solutions in the case of mPDA. In addition, an absorption band between 400 and 450 nm was observed within 2 min of stirring. This band is attributed to the π-π* transition from the phenazine unit conjugated to the bridging nitrogen [25], a typical structural characteristic for the polymers or oligomers from the oxidation of PDA, suggesting that PDA was immediately oxidized by Cu (II) and polymerized [22,25], evidenced by the polymer formation in the precursor solution (Fig. 3b). The short lifetime for Cu (II)-PDA complexes makes their structures difficult to analyze experimentally. Fig. 2 shows a 1:3 coordination ratio for Cu (II)-PDA complex based on a simulation using a universal force field. The proposed structure from simulation shows that Cu (II) serves as a core, and is surrounded by three PDA molecules through the coordination bonding with their amine groups. The simulated 1:3 ratio is in line with the others' work where Cu (II) can coordinate with up to three diamines [30].

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Fig. 1. UV–Vis spectra of the precursor solutions containing (a) pPDA, (b) mPDA and (c) oPDA.

With increased stirring time, the absorption band for the oxidized PDA is shifted toward higher wavelengths, and the shift range varies with the types of PDA isomers. The redshift for PDA absorption band suggests a higher degree of oxidation and oligomerization [23]. Based on the oxidation potential [25], the ease of oxidation should follow the order of oPDA N pPDA N mPDA. However, from Fig. 2, the redshift range for the wavelength is found to be pPDA N mPDA N oPDA, which does not match their oxidation potential order. It is possible that some other factor also affect the redox reaction between Cu (II) and PDA other than thermodynamics solely. One possibility is the differences in their coordination structures. Due to the steric effect of aromatic ring, mPDA and oPDA can only coordinate with one Cu (II) ion. For pPDA, in addition to form the coordination structures as shown in Fig. 2, the para positioned amine group is also active, and can coordinate with the second Cu (II) ion [30,31]. This bridging structure can form stable colloids between oxidized pPDA and Cu ions [31], which may further enhance the extent of redox reaction in between. The coordination structure likely also affects the ease of the redox reaction between Cu (II) and PDA. Although PDA has been oxidized in the precursor solution, Cu (0) was not observed after 3 h of stirring at room temperature (Fig. S1), which is consistent with others' work [31–33]. The suspended solids in the precursor solutions were collected and analyzed using XPS. As seen in Fig. 3a, the suspended solids in the precursor containing mPDA or oPDA show two groups of peaks at (932.0 w/952.0 eV) and (934.7 w/954.3 eV). The former is attributed to Cu (I) and the latter originates from Cu (II) [34,35]. By comparing the area under the each peak, only half of Cu (II) was reduced to Cu (I) by mPDA and oPDA at room temperature, respectively. For the precursor containing pPDA, N85% of Cu (II) was reduced to Cu (I), which is agreed with the UV–Vis spectra

in which the redox reaction in pPDA precursor is much more intense compared with the other two isomers. Cu (I) is very unstable in an aqueous environment with respect to disproportionation into Cu (0) and Cu (II). The presence of a large fraction of Cu (I) in the precursor solutions indicates that these Cu (I) are stabilized by the polymer or oligomer forms of PDA. The nitrogen atoms on C_N bond are found to coordinate with Cu (I) in all repeating units of polyPDA (Fig. S2), which agrees with the experimental observations [22,31,38,39]. Due to the complexity in the oxidative polymerization of PDA [25], the detailed structure for Cu (I)-polyPDA complexes is difficult to characterize. After 110 °C for 60 h, only the precursor solution containing pPDA produced Cu NWs (see the inset in Fig. 4b). The as-synthesized Cu NWs are coated with a thin layer of polyPDA. The successful synthesis of Cu NWs indicates that Cu (I)-polyPDA complexes are still redoxactive in pPDA precursors. It has been shown that the charge transfer between transition metal ions and amines can be enhanced at elevated temperatures [40]. At 110 °C, the Cu (II) previously reduced to Cu (I) by PDA in the precursor solution is further reduced to Cu (0) through the coordinated nitrogen atoms. For the solutions containing mPDA or oPDA, no Cu (0) diffraction was detected using XRD (Fig. S3). This finding is consistent with the observed relatively weak reducing strength for mPDA or oPDA at room temperature (Figs. 1 and 3a), at least in the precursor solution used in this study. In order to verify whether the reducing strength is the reason for the differences in producing Cu NWs among the PDA isomers, an additional reducing agent, glucose, was added to the precursor solutions. As seen in Fig. 4, with glucose, Cu NWs were obtained from the precursor solutions containing pPDA or mPDA within 24 h of reaction at 110 °C. Introducing glucose enhances the reducing strength in the precursor

T. Zhang et al. / Materials and Design 162 (2019) 154–161

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Fig. 2. The coordination structures of the complexes of Cu (II) and PDA isomers. The spheres in brown, blue, dark gray and light gray color represents Cu (II), nitrogen, carbon and hydrogen atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

solutions. The reaction time needed to produce Cu NWs decreases from 60 h to 24 h for pPDA, and Cu NWs are also obtained using mPDA/glucose. Note that glucose is a mild reducing agent which does not reduce Cu (II) to Cu (0) at 110 °C [27]. A stabilized Cu (I) in the precursors allows glucose to produce Cu NWs, because compared with Cu (II), Cu (I) is easier to be reduced thermodynamically, with or without ligand [41]. This is a great benefit to avoid using a harsh reduction condition to directly reduce Cu (II) to Cu (0), such as high temperature (N 165 °C), strong base and strong reducing agent (hydrazine etc.) as reported in literature [12,20,40]. The reducing strength of glucose, however, is compromised in the presence of oPDA due to a condensation reaction [42]. The synthesis results from different reducing conditions

are summarized in Table 1. Reduction of Cu (II) to Cu (I) using PDA at room temperature is the key step to synthesize Cu NWs in a mild condition. The diameters for the as-synthesized Cu NWs with glucose are found to be 313 ± 43 and 475 ± 60 nm for pPDA and mPDA, respectively. These Cu NWs synthesized with glucose are also found to be coated with a thin layer of poly PDA. The appearance of the as-synthesized Cu NWs can be found in Fig. S4. Although bulk polyPDA is black in color, the polyPDA-coated Cu NWs are still reddish brown, because the polyPDA coatings are too thin so the optical properties of the Cu NWs are not affected. The XRD pattern, as seen in Fig. 5a, confirms the presence of Cu with strong diffractions for Cu (111) and (200) facet. It is

Fig. 3. (a) XPS spectra and (b) FTIR spectra of the suspension in the precursor solution containing pPDA, mPDA and oPDA after 3 h of stirring at room temperature. For FTIR spectra, the assigned wavenumber range for each chemical structure is highlighted [22,23,25,32,36,37].

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Fig. 4. Microscopic images of the as-synthesized Cu NWs: (a) SEM of Cu NWs synthesized using pPDA (Scale bar = 20 μm); (b) TEM of Cu NWs synthesized using pPDA and the inset for that synthesized without glucose (Scale bar = 200 nm); (c) TEM of Cu NWs synthesized using mPDA (Scale bar = 200 nm); (d) a polymer layer on the Cu NWs (Scale bar = 50 nm).

known that Cu NWs exhibit stronger diffractions for (111) facets while Cu nanoparticles have more (200) facets exposed [43]. The stronger (111) diffraction observed in the as-synthesized Cu NWs suggests a high yield of Cu NWs, which is similar to the high-quality Cu NWs synthesized using HDA [4]. SEM confirms the high yield of Cu NWs with a small amount of Cu nanoparticles (Fig. 4a). The one-dimensional growth of Cu nanocrystals is likely due to the capping effect of PDA. Ethylenediamine (EDA) has been recently found to act as a facet-selective promoter instead of a capping agent. EDA selectively adsorbs onto the (111) facets at the end of a growing Cu NW to prevent oxides deposition [17,18]. If PDA followed the same mechanism as EDA, then polyPDA should form on (111) facets, which blocks the anisotropic growth of Cu NWs. The formation of a coreshell structure in-situ suggests a different mechanism for PDA. Cu (I) ions coordinated with polymer can adsorb onto Cu surfaces in an aqueous environment [44]. It is possible that once Cu seeds were formed in the precursor solution at an elevated temperature or with the assistance of glucose, Cu (I)-polyPDA complexes can deposit onto Cu seeds and reduce to Cu (0), as demonstrated in Fig. 6. As a result of Cu (I) reduction, the coordinated PDA unit is oxidized and polymerizes likely on the high surface energy (100) facets of a Cu seed [4,45]. Similar to a capping agent, the adsorbed polyPDA can prevent the deposition of Cu (I) on (100) facets, resulting in an anisotropic growth through (111) facets. This process can proceed repeatedly to grow into Cu NW/

polyPDA core-shell hybrids. The length and yield of Cu NWs from this work (Table 1) is smaller than these using alkyl amines (up to 150 μm in length and N40% in yield) [16]. It is likely because compared with the relatively short alkyl amines, the steric hindrance from polyPDA makes the coordinated Cu (I) more difficult to diffuse onto a growing seed consequently limits the growth of Cu nanocrystals. Further studies are needed to investigate the detailed mechanism for the formation of Cu NW/polyPDA core-shell structures. The Cu NWs synthesized with glucose were selected for antioxidation tests, and their properties were compared with that synthesized using HDA. As seen in Fig. 5b, no significant oxidation was observed for the Cu NWs after exposure to air at room temperature (21 °C, humidity 40–60%) for 14 days, except a small hump was observed for Cu NWs synthesized using HDA, which is the diffraction peak of Cu2O (111) facet. Water and high temperature are known to be two important factors in causing oxidation of metals [19,46]. Thus, the expected performance of neat Cu NWs is likely to be compromised in a hot and humid environment. Most of the Cu NWs synthesized using alkyl amines alone can only be stable for a short period of time in such a harsh condition. As demonstrated in Fig. 5c, HDAsynthesized Cu NWs kept in boiling water (100 °C) are completely oxidized to CuO within 3 days. Under the same condition, only a small Cu2O peak was observed for the PDA-synthesized Cu NWs because polyPDA coating can significantly inhibit oxidizing species transferring

Table 1 Properties of the Cu NWs synthesized using different PDA isomers and reaction conditions. PDA isomer

pPDA mPDA oPDA

21 °C

NC NC NC

110 °C (60 h)

110 °C with glucose (24 h)

Diameter (nm)

Length (μm)

Yield (%)a

Diameter (nm)

Length (μm)

Yield (%)a

287 ± 89

11 ± 9 NC NC

b20

313 ± 43 475 ± 60

35 ± 15 18 ± 13 NC

38 31

NC: No Cu formation. a Yield was calculated based on the total mass of Cu produced in the reaction.

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Fig. 5. XRD patterns of Cu NWs synthesized using pPDA, mPDA, oPDA and HDA. (a) as-synthesized with glucose, (b) after exposure to air for 14 days, (c) after treated in water at 100 °C for 3 days, and (d) after long-term stored in water at room temperature.

from aqueous solution to Cu NW surfaces [24]. Compared with the thickness of long-chain alkyl amine monolayer (~2 nm), the 20 nm thick polyPDA coating also forms a better barrier against oxidants from attacking Cu NWs. Additionally, the possibility that alkyl amines desorb from Cu NWs in an aqueous environment becomes greater at an elevated temperature [47], leaving unoccupied sites on Cu NW surfaces for oxygen and water molecules to attack. The in-situ coated polyPDA layers remain intact with Cu NWs after 3 days in boiling water (Fig. S5), providing an excellent environmental stability for Cu NWs. Long-term stability was tested by storing the Cu NWs in water at room temperature for 3 or 10 months. Fig. 5d shows that almost no

peak for oxides was observed for polyPDA-coated Cu NWs even after 10 months of storage in water. As far as we know, the anti-oxidation property for the Cu NWs synthesized in this work is the best among the literatures. For comparison, Cu NWs synthesized from EDA were mostly oxidized after stored in water for only 1 month [9]. The polyPDA shells on Cu NW surfaces provide an efficient protection to prevent them from oxidation in a hygrothermal environment. The electrochemical properties of the Cu NWs which have been stored in water for 10 months (Fig. 5d) were further characterized using cyclic voltammetry. PolyPDA itself does not show redox activities in the applied scan range [37]. As shown in Fig. 7, the polyPDA coated Cu

Fig. 6. Possible mechanism for the formation of Cu NW/polyPDA core-shell structures. The spheres in brown, yellow, blue, dark gray and light gray color represents Cu, Cu (I), nitrogen, carbon and hydrogen atoms, respectively. For simplicity, a Cu (I)-pPDA (1–1) complex is used to represent Cu (I)-polyPDA complex. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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of metallic nanowires as a multi-functional building block for various electrochemical applications, especially for these requiring a long standby or service life. CRediT authorship contribution statement Tan Zhang: Methodology, Data curation, Writing - original draft. Farhad Daneshvar: Data curation. Shaoyang Wang: Data curation. Hung-Jue Sue: Supervision, Writing - review & editing. Acknowledgment

Fig. 7. Cyclic voltammetry curves for polyPDA-coated Cu NWs in 0.1 M KOH solution.

NWs exhibit characteristic anodic current peaks at −0.32 and −0.07 V, corresponding to Cu (0)/Cu (I) and Cu (I)/Cu (II), respectively. The cathodic peaks for Cu (II)/Cu (I) and Cu (I)/Cu (0) is at −0.50 and −0.81 V, respectively [48]. The electrochemical response of the Cu NWs remained repeatable until the intensities for the cathodic peaks started to decease after the 15th cycles. It is noted that electrochemical activity and oxidation resistance are usually mutually exclusive from each other. PolyPDA itself can accommodate ions and soluble species to swell, allowing their applications in pseudo capacitors and watersoluble pollutes removal [22,37]. It has been shown that Cu electrodes coated with a 75 nm thick polyPDA layer remain electrochemically active [24]. The polyPDA layers on the as-synthesized Cu NWs in this study are around 20 nm thick which is thin enough to maintain the electrochemical activity of Cu NWs, and also able to slow down the oxidizing species diffusion through the polyPDA layer to prevent fast oxidation of Cu NW [24]. The excellent oxidative stability makes these PDA-synthesized Cu NWs remain electrochemically active after stored in water for a long period of time (10 months). The Cu NWs synthesized in this work are a promising candidate for the applications in a high temperature, highly humid or aqueous environment, such as batteries, catalysts and sensors. 4. Conclusion We have demonstrated the synthesis of Cu NWs using aromatic diamines for the first time. PDA has a multifunctional role in the Cu NW synthesis, which is coordinating, reducing and capping. PDA reduces Cu (II) to Cu (I) at room temperature, and stabilizes the resulting Cu (I) in an aqueous environment. The formation of stable Cu (I) complex is the key step to synthesize Cu NWs with a mild reducing agent at lower temperatures. Despite their identical chemical compositions, PDA isomers show different chemical reduction strength toward Cu (II). This indicates that not only the oxidation potential for PDA but also the coordination structures play important roles in the chemical reduction of Cu (II). The as-synthesized Cu NWs are covered with a thin layer of conducting polyPDA polymer, which exhibit excellent anti-oxidation properties even in the presence of water, suggesting the suitability of Cu NWs for applications in aqueous or humid environment. The as-synthesized Cu NWs are electrochemically active even after 10 months of storage in water, which can be used for biosensors, catalysis and batteries. The present work shows that a conductive polymer coating not only provides much better anti-oxidation properties than alkyl ligands but also retain the electrochemical properties to the as-synthesized Cu NWs. It opens a new route for the core-shell

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