Zengmin Tang‡
,
Aasim Shahzad‡,
Woo-Sik Kim* and
Taekyung Yu*
Department of Chemical Engineering, College of Engineering, Kyung Hee University, Youngin, 446-701, Korea. E-mail: wskim@khu.ac.kr; tkyu@khu.ac.kr
First published on 29th September 2015
This work describes a simple and aqueous-phase route for the synthesis of Cu nanowires (Cu NWs) having a long length of 140−180 μm, a high aspect ratio of more than 350, and long-term stability. High-quality Cu NWs were synthesized by reduction of CuCl2 with ascorbic acid in the presence of branched polyethyleneimine (BPEI) in an aqueous solution at 90 °C. The synthesized uniform Cu NWs showed long-term stability without the formation of Cu oxides on the surface of the NWs after being stored at room temperature for 40 days. Interestingly, we found that Cl− in the reacting solution played a key role in the formation of long Cu NWs. We also investigated the influence of various experimental conditions including the weight ratio of BPEI/CuCl2, the pH of the reacting solution, and the reaction temperature on the length, morphology, and stability of Cu NWs.
Recently, various aqueous-phase synthetic methods have been developed for fabrication of Cu NWs, but these methods are not appropriate for large-scale production of high quality long Cu NWs with precise morphological control. For example, methods for synthesizing Cu NWs by adding a reducing agent into an aqueous solution containing a Cu salt and a stabilizer often suffer from short NWs length (below 20 μm), long reaction time (16 h), and complicated procedures.3,11,12 The template method, which typically uses soft and/or hard templates for electrochemical deposition and reduction of copper compounds in the channels of the template, can provide uniform Cu NWs with controllable dimensions.13,14 However, these processes need multiple tedious steps including formation and removal of the templates to obtain Cu NWs. In addition, during the template removal process, the surface of the Cu NWs can be damaged. Moreover, some of the templates are expensive and are composed of toxic materials.15,16 There have been some reports of the synthesis of Cu NWs using amine-based capping agents. Xia and co-workers synthesized Cu NWs using hexadecylamine (HDA).17 However, their synthesis took a very long time (30 h) to complete. Wiley and co-workers also reported the synthesis of Cu NWs using ethylenediamine (EDA) as a capping agent and polyvinylpyrrolidone (PVP) as a stabilizer, however, this synthesis required multiple-steps and the resulting Cu NWs were short (10−20 μm).18 Therefore, developing a simple and economic route for synthesizing high-quality long Cu NWs still remains a significant challenge.
In this work, we describe a simple and aqueous-phase route to the synthesis of Cu NWs with a long length of 140−180 μm, a high aspect ratio of more than 350, and long-term stability. Our simple synthetic protocol involves the reduction of CuCl2 with ascorbic acid in the presence of branched polyethyleneimine (BPEI) in an aqueous-phase at 90 °C. The synthesized uniform Cu NWs showed long-term stability without formation of Cu oxides on the surface of the NWs after being stored at room temperature for 40 days. Interestingly, we found that Cl− in the reacting solution played a key role in the formation of long Cu NWs. We also investigated the influence of various experimental conditions including the weight ratio of BPEI/CuCl2, the pH of the reacting solution, and the reaction temperature on the length, morphology, and stability of Cu NWs.
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Fig. 2 (a) XPS spectra of BPEI-stabilized Cu NWs as shown in Fig. 1. (b) FT-IR spectra of Cu NWs as shown in Fig. 1. |
We observed a growth behavior of the Cu NWs by taking samples at various reaction stages and analyzing them by TEM and SEM. Fig. 3 shows TEM and SEM images of the products sampled at 15 min, 20 min, 30 min, and 3 h respectively. At an early stage of t = 15 min, only small Cu nanoparticles with sizes of around 20 nm were synthesized (Fig. 3(a)). At t = 20 min, a TEM image shows the presence of short Cu nanorods with length of 5 μm and diameter of around 400 nm (Fig. 3(b)). As the reaction proceeded to t = 30 min and t = 3 h, the short Cu nanorods started to grow to long nanowires while keeping their diameter of around 400 nm (Fig. 3(c) and (d)). These results suggest that the Cu NWs were grown from small Cu nanoparticles formed at the initial stages of the reaction by addition of monomer attachment without any change in the diameter.
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Fig. 3 TEM images (a and b) and SEM images (c and d), showing different stages of the growth for Cu NWs. The reaction time was, (a) 15 min, (b) 20 min, (c) 30 min, (d) 3 h. |
In reports on the synthesis of metal nanocubes, octahedra, and nanoplates, it has been reported that the selective chemisorption of capping agents such as citric acid and bromide ions reduce the growth rate along the specific direction, thus leading to the formation of nanoparticles with controllable shapes.22 In the present synthesis, we believe chloride ions (Cl−) from CuCl2 can act as a major capping agents for the formation of 1-D structures. When the synthesis was conducted in the presence of Cu(NO3)2 as a precursor instead of CuCl2 while keeping the other experimental conditions unchanged, Cu nanoparticles with cubic and pyramidal shapes were observed as shown in Fig. 4(a). On the other hand, long Cu NWs were observed when the synthesis was conducted in the presence of Cu(NO3)2 and KCl, clearly showing the importance of Cl− in the formation of Cu NWs (Fig. 4(b)).
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Fig. 4 (a) SEM image of a sample prepared under the same conditions as those in Fig. 1 except that synthesis was conducted in the presence of Cu(NO3)2 as a precursor instead of CuCl2. (b) SEM image of a sample prepared under the same conditions as those in Fig. 1 except that synthesis was conducted in the presence of Cu(NO3)2 and an equivalent amount of KCl instead of CuCl2. |
In addition to the role of Cl−, we also investigated the effect of the weight ratio of BPEI/CuCl2 on the formation and stabilization of Cu NWs. When the synthesis was conducted in the absence of BPEI (BPEI/CuCl2 weight ratio was 0), a SEM image showed the formation of irregular and aggregated particles (Fig. 5(a)). XRD patterns show that the product was a mixture of Cu and CuCl (F43m, a = 5.405 Å, JCPDS file number 77-2383) instead of pure Cu metal (Fig. 5(b)). This result indicates that BPEI acts as a co-reducing agent for the complete reduction of Cu2+ to Cu metal. At a low weight ratio of 0.1, a small number of short Cu NWs (about 20−80 μm in length)and nanoparticles were produced (Fig. 5(c)). At low weight ratios, there was not enough BPEI to effectively cap the Cu seeds, resulting in the formation of a small portion of Cu NWs. Upon increasing the weight ratio to 0.3, long Cu NW shaving long-term stability were successfully synthesized, as discussed earlier (Fig. 1). At high a BPEI/CuCl2 ratio of 1.2, large particles can be seen with a small number of short Cu NWs (Fig. 5(d)). It would be seem that the formation of a stable BPEI–Cu complex occurred via thermal dynamic growth of Cu nanoparticles, thus limiting the formation and growth of long nanowires.3,23
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Fig. 5 (a) SEM image and (b) XRD patterns of a sample prepared under the same conditions as those in Fig. 1, except that synthesis was conducted in the absence of BPEI. (c and d) SEM images of samples prepared under the same conditions as those in Fig. 1, except that synthesis was conducted at various weight ratios of BPEI/CuCl2: (c) 0.1 and (d) 1.2, respectively. |
In an aqueous-phase synthesis, the formation and stabilization of metal nanoparticles are strongly dependent on the pH of the resulting solution.21,24,25 In previous research on the synthesis of Ag nanoparticles using BPEI in an aqueous-phase, we reported that the protonated amine groups of BPEI have a weaker ability to stabilize Ag nanoparticles under strong acidic conditions.21 When the synthesis was conducted under strong acidic conditions (pH = 1.8), thick and short Cu NWs were observed, showing the weak stabilization ability of BPEI (Fig. 6(a)). In contrast, as the pH was increased to 5.3, SEM images showed the formation of large particles (Fig. 6(b)). We believe that increased pH led to the fast reduction rate of Cu2+ by ascorbic acid, which was not favorable for the formation of anisotropic structures.24–26
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Fig. 6 SEM images of Cu NWs prepared under the same conditions as those in Fig. 1, except that synthesis was conducted at different pH values: (a) 1.8 and (b) 5.3, respectively. |
The influence of reaction temperature on the formation and growth of Cu NWs was also studied. At low reaction temperatures, the amine groups of BPEI were not sufficiently activated to stabilize the Cu NWs, thus leading to the formation of small and non-uniform Cu NWs (Fig. 7(a)). On the other hand, at a high reaction temperature (100 °C), we observed a large portion of nanoparticles with long Cu NWs in SEM measurements (Fig. 7(b)). These results indicated that temperature was also an important factor for the formation of long Cu NWs.
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Fig. 7 SEM images of Cu NWs prepared under the same conditions as those in Fig. 1, except that synthesis was conducted at various reaction temperatures: (a) 60 and (b) 100 °C, respectively. |
Footnotes |
† Electronic supplementary information (ESI) available: Additional photograph, TEM image, and XRD result of Cu NWs. See DOI: 10.1039/c5ra15751j |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |