Ultrathin Copper Nanowire Synthesis with Tunable Morphology Using

Jul 25, 2018 - Parts a–d of Figure 1 show the TEM and SEM images of the samples obtained at ... Five single-crystal structures are bound by the {111...
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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Ultrathin Copper Nanowire Synthesis with Tunable Morphology Using Organic Amines for Transparent Conductors Haiyan Xiang,† Tingting Guo,‡ Minjie Xu,† Haozi Lu,† Song Liu,*,† and Gang Yu*,† †

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Institute of Chemical Biology and Nanomedicine (ICBN), State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China ‡ Key Laboratory of Yunnan Provincial Higher Education Institutions for Organic Optoelectronic Materials and Devices, Kunming University, Kunming 650000, P. R. China S Supporting Information *

ABSTRACT: High-quality monodispersed copper nanowires with an ultrathin diameter of 13.5 nm, lengths up to 30 μm (aspect ratio >104) were successfully synthesized by a facile and controllable hydrothermal reduction procedure. The synthesis utilized glucose in the presence of hexadecylamine (HDA) and octadecylamine (ODA) as the capping agents. The copper decahedra nanoparticles with a low-surface-energy {111} plane formed pentatwinned one-dimensional nanowires, which was exactly verified by selected-area electron diffraction. Furthermore, the diameter and relative film conductivity of copper nanowires are sensitive to the HDA/ODA molar ratio. The conductor film made of the high-quality and ultrathin copper nanowires shows high transmittance and low resistance (83.83%, 61 Ω/□), exhibiting great potential in the applications of nanofabrication, transparent and flexible conductors, organic light-emitting diodes, and more. KEYWORDS: copper nanowires, HDA, ODA, pentatwinned nanostructure, transparent conductor

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the capability of synthesizing small-diameter copper nanowires in a controllable manner is appealing in both nanotechnologies and advanced materials research.22 With the aim of obtaining uniform, high-yield, aspect ratio copper nanowires, the majority of the works were focused on aqueous reduction approaches, which largely rely on proper surfactants.14 Recently, several groups synthesized copper nanowires with a diameter of around 20 nm. For example, Cui et al. successfully synthesized copper nanowires with an average diameter of 17.5 nm using oleylamine as the surfactant and tris(trimethylsilyl)silane as the mild reductant.24 Zhang and co-workers reported the production of 24-nm-diameter copper nanowires with controllable morphology using two different surfactants of hexadecylamine (HDA) and cetyltriammonium bromide.25 Herein, we report a facile and lowprocessing-cost hydrothermal reduction approach to synthesize ultrathin and homogeneous copper nanowires (average diameter of 13.5 nm and length of 30 μm), adopting two different mixing capping agents of octadecylamine (ODA) and HDA. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and selected-area electron diffraction (SAED) were implemented to explore the formation mechanism and characterize the morphologies and structures of the copper

ne-dimensional (1D) metal nanostructures have drawn a great deal of interest because of their distinctive and superior optical,1 electrical,2 physical,3 catalytic,4,5 and chemical properties6 compared to their bulk counterparts. They have great potential in fabricating nanoscale electronic,7 optoelectronic,8 and magnetic9 devices and provide an ideal test bed for investigating many basic physical phenomena.10 Copper is probably the most frequently used metallic nanowire owing to its earthly abundance, superior thermal and electrical conductivity,11 and potential applications as an interconnector12 and replacement for traditional transparent electrodes made of indium−tin oxide (ITO)13 in nanodevices. Although ITO possesses low resistivity and excellent optical transmittance, it suffers from the high cost of indium, complicated manufacturing processes, and brittleness. It is reported that the properties of copper nanowires are mostly determined by their dimensions, aspect ratio, structure, and crystallinity.14 In particular, copper nanowires with high aspect ratios have shown huge potential in nanofabrication,15 nanocatalysis,16 nanobiotechnology,17 and electronics.2,18 For example, theoretical simulation and experimental results19 demonstrate that good transparency/conductivity performance can be achieved for nanowire mesh films with wire aspect ratios of 400 or higher. Copper nanowires with high aspect ratios and thin diameters also became an ideal material for studying fundamental mechanical and electrical properties.20 Utilizing ultrathin high-aspect-ratio copper nanowires helps to minimize light scattering and maintain high transparency.21 Therefore, © XXXX American Chemical Society

Received: May 1, 2018 Accepted: July 25, 2018 Published: July 25, 2018 A

DOI: 10.1021/acsanm.8b00722 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 1. TEM images and the inset SEM images (400 nm) of copper nanowires prepared at different reaction times: (a) 1 h; (b) 2 h; (c) 3 h; (d) 4 h. (e and f) SAED images shown in the boxed areas in parts a and c, respectively. (g) Copper nanowire diameter distribution with different reaction times.

nanowires. We found out that the nanowire diameter and film conductivity were highly dependent on the molar ratio of the mixed organic amines. The resultant ultrathin copper nanowire conducting film exhibited high transmittance and low resistance. The approach described here provides a solution to produce ultrathin copper nanowires in high quantities, which is promising in transparent and flexible conductors and electronics applications. The morphology and dimensions of the as-prepared copper nanowires were examined by TEM and scanning electron microscopy (SEM). Images were taken after the hydrothermal reaction proceeded for 1, 2, 3, and 4 h. Parts a−d of Figure 1 show the TEM and SEM images of the samples obtained at different reaction times, which clearly exhibited the evolution of copper nanostructures from nanoparticles to nanowires over time. The initial product after 1 h mainly consisted of nanoparticles (94.2%) with sizes of 62−80 nm (refer to Figures S2 and S3 and Table S2). When the reaction time increased, more uniform-shaped copper nanowires were obtained and the nanowires became thinner. The diameter decreased from 25 to 13.5 nm. It has been reported that the metallic nanowire formation is mainly attributed to multiply pentatwinned nanoparticle generation.26 At the initial reaction stage, Cu atoms acquired low thermodynamic energy and favored relatively large nanoparticles, having more active sites to deposit. When the reaction time proceeded, more Cu atoms acquired high thermodynamic energy and deposited on the small nanoparticles to form a majority of ultrathin copper nanowires with an average diameter of around 13.5 nm. The mechanism of pentatwinned structure nuclei growth is proposed.23 Stating that Cu atoms favor isotropic growth in the early-seed-formation stage. The most compelling mechanism is the intrinsic equilibrium that accounts for the primary atoms favoring the relatively poor surface energy of facets.27 The SAED pattern shown in Figure 1e demonstrates one face of the decahedra structure nanoparticles bounded by the {111}

plane. With increasing reaction time, the SEM images show both single-crystal and decahedra seeds with multiple twinning (inset in Figure 1b). Beyond 3 h, the copper nanoparticles gradually decreased. As mentioned earlier, metallic nanowire growth is mostly based on decahedra particles, while the singlecrystal disappearance should be effected by Ostwald ripening at an oxygen-deficient environment. Because high yields of singlecrystal nanoparticles were always obtained at high temperature or oxidative etching, without oxygen, decahedra pentatwinned seeds should be more active than single-crystal nanoparticles. To better understand the detailed structure of copper nanowires, we performed SAED, as shown in Figure 1f, and analyzed the diffraction spots. The sample is not a simple single-crystal structure because the diffraction spots image contains two sets of zone axes, [112] and [001]. The typical twinned structural pattern suggests that two sets of facecentered-cubic (fcc) patterns exist along the [112] and [001] zone axes, respectively, when the electron beam is perpendicular to one of the side facets.28 Therefore, on the basis of the decahedra multiple seeds, nanowires grow along some longitudinal direction and form a 5-fold-twinned structure. Five single-crystal structures are bound by the {111} plane from the pentatwinned decahedra via some stacking fault.27 Each adjacent perfect {111} lattice plane shares an angle of 70.5° and a 7.5° gap (the criterion angle of the five {111} planes at the planar cyclic pentatwinned structure is 72°). Various models of the gap in the multiply twinned seeds have been proposed elsewhere,23 and various nanoparticles in Figure 1a show a mismatch of five different irregular single structures. The unfilling space provides high activity sites for Cu atoms anisotropically along the direction ⟨110⟩ and from the pentatwinned nanowires. To elucidate the nanowire structure, we further obtained the TEM and corresponding high-resolution TEM (HRTEM) images of two different copper nanowires. Figure 2b illustrates the HRTEM image; the lattice spacings of 0.13, 0.18, and 0.21 B

DOI: 10.1021/acsanm.8b00722 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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nanowires were identified on three diffraction peaks at 2θ = 43.5, 50.7, and 74.4°, with lattice constant = 0.3615 nm, respectively corresponding to the {111}, {200}, and {220} crystal planes of fcc metallic copper, in all reaction times (JCPDS 03-1018). After the reaction time proceeded for 1 h, another obvious peak at 2θ = 21° was observed that did not belong to the fcc copper pattern. With increasing reaction time to 4 h, the nonmetallic peak disappeared. The extra peaks may lead to some intermediate products, for example, organic amine or other byproducts. The peak of the {220} plane gradually emerged, and the diffractions of the {111} and {200} planes intensified with increasing reaction time. Such observations are well consistent with the results from SAED and HRTEM. In our synthesis, HDA and ODA could not dissolve in distillation simultaneously with the initial reactant in the hypothermal reaction. After stirring overnight, the solution turned clear blue. Positive CuII mixed with negative organic amines form a CuII-ODA and CuII-HDA mixture. Previous reports had pointed out that the formation of a metallic 1D nanostructure largely depended on the corresponding capping agent.21 Poly(vinylpyrrolidone) bromide was determined to be the primarily reactant for nanowire formation of silver, gold, platinum, and palladium,28 while the aliphatic amine HDA played a crucial role as the capping reagent for the mature of copper 1D structure.29 Besides HDA, another aliphatic amine of ODA was introduced in our synthesis. A long pair of electrons on the N-atom amine of ODA also has been reported as the reductant to obtain copper nanowire in hydrothermal conditions and served as a ligand-assisted template.30 A systematic investigation of the influence of the amount of HDA and ODA on the morphology of the final product was obtained from analysis of the SEM images (Figures 3 and S5 and Table S3). Figure S5 shows the nanowire morphology of the products obtained at different molar ratios of HDA/ODA (i.e., 1.2:0, 1:0.2, 0.8:0.4, 0.6:0.6, 0.4:0.8, 0.2:1.0, and 0:1.2), while other conditions were fixed. By analysis of the distributions of nanowires and nanoparticles (Figure 3g), we detect that the nanowire yield is only 14% in the absence of ODA. With increasing molar ratio of ODA, the percentage of nanoparticles gradually decreased (45.5%, 29.4%, 10.2%, 6.1%, 1%, and 0%) and the compositions of the nanowires were enhanced. The addition of ODA could be beneficial to 1D copper nanowire formation. The inset image in Figure 3h illustrates the relationship between the average diameter of the copper nanowires and the HDA/ODA molar ratio. The diameter of the nanowires gradually became thinner with increasing ODA molar ratio until the molar ratio of HDA/ ODA reached 04:0.8. A further increase of ODA caused the sizes of the nanowires to begin to grow large. Therefore, the final ultrathin nanowire formation was achieved by the proper addition of ODA, and diameter variation was mainly attributed to the steric hindrance effect. Two additional alkyl chains belonging to ODA compared to HDA played a significant role in increasing the high density oil phase and reducing the interspacing that favored nanowire formation at the amphiphilic system. However, disordered organic amines of HDA and ODA at the same molar ratio compared to purified HDA or ODA could lead to a narrow area for ultrathin copper nanowire formation. The above hypothesis is demonstrated in Figure S6. Parts d−f of Figure 3 show the TEM images of the product at molar ratios of HDA/ODA (1.2:0, 0.4:0.8, and 0:1.2) after

Figure 2. TEM (a and c) and HRTEM (b and d) images taken from the region marked in parts a and c. The inset in part b shows that one of the side surfaces of the copper nanowires is parallel to the electron beam. The inset in part d shows that the electron beam is perpendicular to one of the side surfaces of the copper nanowires. (e) EDS analysis showing the composition of pure copper nanowires. (f) XRD patterns of copper nanowires at different reaction times.

nm, which could be indexed to the {220}, {200}, and {111} planes, respectively, are observed. The orientation of the copper nanowires is perpendicular to the lattice plane of {220}. The inset image depicts that the incident electron beam is parallel to one of the side surfaces of the copper nanowires. Another HRTEM image shown in Figure 2d presents two sets of fringes with spacings of 0.13 and 0.21 nm, corresponding to the existence of the {220} plane perpendicular to the growth direction of the nanowires and {111} facets parallel to the long axis of the nanowires, when the incident electron beam is perpendicular to one of the bottom side surfaces. The results agreed well with the reports that pentatwinned copper nanowires have two series of five {111} planes at the head and five {200} planes at the side surface.23 The primarily decahedra seeds were bound by low-energy {111} planes. With increasing reaction time, the metallic nanowires of the fcc cubic structure were formed. The composition of the nanowire were tested by energydispersive X-ray spectroscopy (EDS; Figure 2e). Typical spectra of carbon (0.254 keV), molybdenum (2.317 keV), and copper (0.811, 0.832, 0.93, 8.905, and 8.048 keV) are presented; redundant elements, e.g., carbon and molybdenum, are ascribed to the substrate of a carbon-coated molybdenum microgrid; therefore, purified copper nanowires were obtained. The XRD patterns of the resultant samples at 1, 2, 3, and 4 h were also measured in order to understand the mature stages of the pentatwinned copper nanowires (Figure 2f). Copper C

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Figure 3. SEM images without acid processing and TEM images after immersion in acetic acid with the different molar ratios of HDA/ODA: (a and d) 1.2:0; (b and e) 0.4:0.8; (c and f) 0:1.2. (g) Distribution of copper nanowires and nanoparticles. (h) Sheet resistance of different molar ratios of HDA/ODA. The inset image is the copper nanowire diameter with different molar ratios of HDA/ODA.

(9070, 8470, 6970, 156, 14, 1007, and 83 Ω/□) at different HDA/ODA molar ratios (1.2:0, 1:0.2, 0.8:0.4, 0.6:0.6, 0.4:0.8, 0.2:1.0, and 0:1.2), which presented some artful relationships with the diameter of the copper nanowires (Figure 3h). From the experiments, excellent conductivity of the copper nanowire films was always obtained at a proper organic amine molar ratio, i.e., HDA:ODA = 0.4:0.8. On the basis of the above results, we proposed the mechanism concerning the formation of the pentatwinned structure of 1D copper nanowires under the influence of different molar ratios of HDA/ODA (Scheme 1). At the beginning of the reaction, CuII ions bonded with organic amines after overnight stirring and mixing, and then the solution turned a clear sky blue. With an increase in the reaction temperature and a reduction by glucose, more CuII ions became Cu atoms and formed multiple nanoparticles such as decahedra, single-crystal, or pentatwinned structures. With increasing reaction time, the organic amines HDA and ODA as capping reagents strongly adsorbed on the {200} planes of the growing nanowires, leading to deposition of the Cu atoms on the low-surface-energy {111} planes of pentatwinned nanoparticles and forming 1D nanowires along the orientation of ⟨110⟩. At the proper content of alkylamine, the ultrathin pentatwinned nanowires were enclosed by 5 {200} planes as side surfaces and 10 {111} planes as head surfaces. Long-chain aliphatic alkylamines played multiple roles in copper nanowire formation, serving as both a soft reducing agent and an adsorption agent. Figure S8 shows the UV−visible absorption spectrum taken from the copper nanowires dispersed in distilled water of different concentrations. The spectrum stays at the same position, and the adsorption maxima at 585 nm were in good

immersion in acetic acid. Distinct junctions between nanowires and nanoparticles and between nanowires and nanowires are observed. Furthermore, we tested the copper nanowire film (the fabricated process is given in Figure 4a) sheet resistance

Figure 4. (a) Copper nanowire/poly(ethylene terephthalate)fabricated process. (b) UV−visible transmittance, corresponding sheet resistance, and optical images for copper conductor films with different loading amounts. D

DOI: 10.1021/acsanm.8b00722 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 1. Illustration of the Copper Nanowire Evolution Process

diameter and high aspect ratio, the obtained nanowire films exhibit high transmittance and low resistance (83.83%, 61 Ω/ □). Their excellent performance and low cost make copper nanowires suitable for use as promising building blocks in flexible and stretchable electronics, optoelectronic devices, organic light-emitting devices, and more.

agreement with the reported values for copper nanowires.31 This was attributed to the plasma excitation of copper nanowires. Figure S8b plots the extinction transmittance of copper nanowires with different concentrations at the position of 585 nm. A highly concentrated solution has more copper nanowires with deep-red color and shows low transmittance. Besides, we also acquired the UV−visible spectrum of a copper nanowire solution at different growth stages to further explore the nanowire mechanism (Figure S9). An obvious blue shift of the spectra with the reaction time was observed (604, 588, 585, and 585 nm). As mentioned above, when the diameter of nanowires decreases, the quantum size will influence the energy gap and result in a short spectral shift. This well explains why the primary inducement of the spectral position changes at different growth stages. Figures 4b and S10 show copper nanowire transparent conduction (the fabrication process is depicted in Figure 4a) and its transmittance over the visible spectrum and corresponding sheet resistance. Low-concetration copper nanowire films show high transmittance (95%) and high sheet resistance (4300 Ω/□). Increased loading resulted in lowered transmittance and sheet resistance, for example, (86.09%, 131 Ω/□), (83.83%, 61 Ω/□), (58.69%, 30 Ω/ □), and (3.04%, 5 Ω/□). The copper nanowire conductors are comparable to commercial conductors made of ITO (88%, 10 Ω/□), which shows the promising possibility of substituting ITO in flexible and transparent applications. In summary, high-quality and uniform ultrathin copper nanowires with an average diameter of about 13.5 nm were successfully prepared by a simple hydrothermal reduction reaction in high yield. This is the first demonstration of copper nanowires with such small diameters, to the best of our knowledge. Comprehensive investigations were applied to understand the effect of the reaction time and capping agents. The copper nanowires followed a pentatwinned growth mechanism. The organic amines served a crucial role in the synthetic process, which not only contributed to the capping effect but also dramatically affected the nanowire diameter and conductivity of the transparent film. Owing to their ultrathin

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00722. Experimental details, tables of different HDA/ODA molar ratios, SEM and TEM images, picture of the storage time, UV−visible spectra, and sheet resistance of a copper nanowire transparent conductor film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.L.). *E-mail: [email protected] (G.Y.). ORCID

Song Liu: 0000-0003-3112-8907 Gang Yu: 0000-0002-3184-9045 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21476066, 51271074, and 21705036) and Fundamental Research Funds for the Central Universities from Hunan University.

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DOI: 10.1021/acsanm.8b00722 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX