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Guided growth of Ag nanowires by galvanic replacement on flexible substrate Sanjun Yang, and Qiming Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00983 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017
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Guided growth of Ag nanowires by galvanic replacement on
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flexible substrate
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Sanjun Yang, Qiming Liu*, Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China *Corresponding author:
[email protected] (Qiming Liu)
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Abstract
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Aligned Ag nanowire array was directly synthesized by galvanic replacement on curved Polyethylene terephthalate (PET) using Cu2O microcrystal as reductant. More orderly aligned nanowire array was obtained when the curvature radius was reduced. A second growth with different orientation produced Ag nanowire networks. The guided growth was also achieved when using Zn as reductant or Polystyrene as substrate. This plain method with facile control over the orientation and density of the Ag nanowire array enriches the “grow-in-place” methodology and can potentially be applied to various fields.
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INTRODUCTION
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One-dimensional (1D) materials including nanowire or nanotube have been widely used in various fields1. Practical applications require the capability to control the orientation, placement and density of the 1D materials in order to acquire the best performance of them2, 3. Hence, to this end, great efforts have been made. Recently the “bottom-up approach”, which involves creating nanostructures from atoms or molecules, stands as a promising candidate. Two common strategies of this approach are “grow-and-place” (post-growth rearrangement of 1D material to determined place) and “grow-in-place” (manipulation of in situ growth of 1D material)4. For the last decades a great many assembly methods based on “grow-and-place” strategy have been developed, such as surface modification5, 6, Mechanical Force7, 8, the Langmuir−BlodgeE (LB) technique9, 10, blown-bubble method11, 12, evaporation-induced assembly13, 14 and external electric or magnetic field assisted assembly15, 16. But, despite the remarkable success achieved by these “grow-and-place” methods, generally they face difficulties in controlling the density or placement of the 1D material. On the other hand, the “grow-in-place” strategy, by directly growing 1D nanostructures on the substrate, proves promising in overcoming these difficulties4. Using ZnO seeds covered by catalytically inactive Cr layer, aligned ZnO nanowires were grown from the sides of the ZnO seeds17. By controlling the vapor−liquid−solid (VLS) growth condition, aligned planar growth of InAsSb18 and GaAs19 nanowires were realized. Guided growth of 1D nanostructures were also demonstrated through epitaxial or graphoepitaxial growth, in which the lattice of crystal20, 21 (more specifically, sapphire) or nanosteps20-22 and nanogrooves20, 21 were used to direct the growth direction of 1D materials. Furthermore, in situ growth of 1D material from the preprogrammed nanostructure template created by lithography or ion etching (so called “top-down approach”) showed powerful control over the individual elements of the 1D material23, 24. The above representative “grow-in-place” methods have advanced the synthesis
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technique of ordered 1D material, but still have limitations like low throughput, requirement of specific growth substrate or restriction to specific 1D material. Therefore, searching new “grow-in-place” methods is still appealing. We present in this study a novel “grow-in-place” method of synthesizing aligned Ag nanowires by galvanic replacement (GR) under ambient condition. Ag nanowire is one of the most important 1D material and due to its high conductivity and strong localized surface plasmon resonance (LSPR), has been widely used in transparent conductive electrodes25, 26 and SERS27, 28. Since, as has been stated, orderly Ag nanowire array commonly exhibits novel or superior performance compared to that of disorderly one, various methods are devised to align the Ag nanowires such as spray-assisted method29, capillary printing2, the Langmuir−BlodgeE (LB) technique25, fluid-flow method28, three-phase evaporation13, Electrohydrodynamic Jet Printing30. All these methods work using previously synthesized Ag nanowires and thus are attributed to “grow-and-place” method. The “grow-in-place” synthesis method of Ag nanowire is very rare. In this study, we discovered that the Ag nanowires grown directly on flexible substrate, Polyethylene terephthalate (PET), exhibit aligned pattern if PET is curved.
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EXPERIMENTAL SECTION
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Materials. All chemicals and solvents were of analytical grade, purchased from Aladdin Chemical Reagent Co. Ltd., and used as received without further purification. Synthesis of Cu2O microcrystal Cu2O microcrystal was synthesized by solution phase reduction. Under stirring and 700C waterbath, 25 ml NaOH (4 g) solution was evenly added in 5 min to a conical flask, which contained 25 ml Cu2SO4 (0.49 g) solution. In this process, Cu(OH)2 gel was prepared and turned black. After that, 25 ml glucose (0.45 g) solution was evenly added to the above gel in 10 min. The final products were washed by centrifugation with water and absolute ethanol for 3 times. After dried in vacuum for 12 hours, the Cu2O microcrystal was dispersed in ethanol with density of 0.5 mg/ml and 2 mg/ml. Synthesis of Ag nanowires by Cu2O microcrystals on PET PET was cut into 4 cm, 4.44 cm and 6.28 cm squares. 30 μL Cu2O suspension was drop-casted on the PET. 40 ml 1 mM AgNO3 solution was transferred to 50 ml beaker (diameter = 4 cm). After that, the PET was carefully inserted into the beaker. In this process, the PET side with the Cu2O was kept as inside of the curvature and thus faced the bulk of the AgNO3 solution. The reaction was kept in dark and lasted for 30 min. After the reaction ended, the PET was cleaned with water and ethanol. In the synthesis of Ag networks, after the reaction lasted for 20 min, the PET was carefully rotated 900, and then the reaction proceeded for 20 min. After that, the PET was cleaned with water and ethanol. Synthesis of Ag nanowires by Zn and on culture dish Zn microcrystal was purchased from Aladdin Chemical Reagent Co. Ltd.. 30 μL Zn suspension (0.3 mg/ml) was drop-casted on PET and the following procedures were identical to the case of Cu2O except the AgNO3 concentration was 0.5 mM. A piece of the wall of a culture dish (made of Polystyrene (PS); diameter= 35 cm) was cut and 10 ml Zn suspension was drop-casted on the inside of it. Then this piece of culture dish was put in 0.5 mM AgNO3 solution for 30 min and washed with water and ethanol after the reaction ended.
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Real-time observation Real-time observation of the growth of nanowires was conducted by putting a piece of PET with Cu2O (0.5 mg/ml) on it in a smaller culture dish, which was set at the microscope stage. The amount of AgNO3 solution in the culture dish was reduced to 10 ml due to the limited space between objective lens and microscope stage. Characterization Bruker D8 advance X-ray diffractometer by Cu Kα radiation (λ = 1.5406 A) was used to obtain XRD results. FE-SEM images were acquired from ZEISS-∑IGMA working at 10 kV. The samples were sputtered with gold for 60 sec before observation. The surface chemical species of the samples were examined on X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA) using Al Kα radiation of 1486.6eV as the excitation source. Optical images were recorded using Olympus BX51 microscope.
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RESULTS
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Fig. 1A shows the aligned growth of Ag nanowires on PET and the magnified image of the nanowire shown on bottom-right corner indicates that the width of the nanowire is approximately 100 nm. Fig. 1B is a typical XRD pattern of the Ag nanowire and the peaks in it can be indexed to pure Ag (JCPDS no. 04-0783) and Cu2O (JCPDS no. 05-0667). In this experiment, octahedral Cu2O microcrystal approximately 5 μm (see the SEM image and XRD in Fig. S1) was used to reduce 1 mM Ag+ into Ag nanowire. The XPS characterization of Cu2O microcrstals before and after reaction is shown in Fig. S2A and C respectively. From the XPS results, it can be identified that the Cu2O before reaction was pure Cu2O ( 2P3/2, 932.2 eV) and was partially transformed to Cu2O (933.4 eV) and Cu(OH)2 (934.8 eV) after reaction31, which proves that the Ag nanowires grow by galvanic replacement, namely, Cu+ + Ag+ = Cu2+ + Ag. The Ag nanowires sprout from the Cu2O microcrystals as marked in Fig. 1A and extend to more than 100 μm with identical orientation.
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Fig.1 (A) SEM image of the aligned Ag nanowires. Bottom-right is a magnified image of the nanowire (scale bar=500 nm). (B) Typical XRD pattern of the Ag nanowire array. In the synthesis of Ag nanostructures by GR, the morphology of Ag nanostructures can be transformed from dendrite to nanowire by controlling the reaction rate in the growth front of the Ag nanowire based on Mullins-Sekerka (MS) theory32-34. This theory predicts that in the process of the Ag growth front extending to the solution where more Ag+ is available, Ag+ diffuses to the growth front and instabilities of Ag+ concentration distribution is aroused. If the local restoring force damps the instabilities, the Ag+ will steadily diffuse to the growth front and consequently
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Ag nanowire is formed; if not, the Ag+ will diffuse to the point in the vicinity of the growth front and promote the branch nucleation, which leads to the growth of dendrite33, 35. A scheme of the growth of Ag nanowire based on MS theory is shown in Fig. S3. From the above discussion, the reduction of Ag+ in the growth front is of localized nature, therefore in nanoscale, the growth direction of the Ag nanowire assumes a chaotic pattern, which can be easily observed in Fig. 1A. On the other hand, in long-range one nanowire commonly grows following a single direction and this direction can be controlled by curving the flexible growth substrate, i.e., PET. The Ag nanowires are more inclined to grow in parallel with the axis of the curvature than other directions. Scheme of the guided growth of Ag nanowires is shown in Fig. S4A. Smaller curvature radius leads to more aligned Ag nanowire array. Ag nanowires grown on flat glass slide or PET have random orientations, as shown in Fig. 2a and Fig. 2b, respectively. With the curvature radius decreasing (as shown in the top-right insert in Fig. 2b-d), the degree of orderliness of Ag nanowire array increases as shown in Fig. 2c and Fig. 2d.
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Fig. 2 (A) disorderly Ag nanowires grown on glass slide. (B) disorderly Ag nanowires grown on flat PET. Top-right insert is the schematic cross-section view of the pet set in a beaker in GR reaction. (C) partly orderly Ag nanowires grown on curved PET with curvature radius of 2.83 cm. (D) orderly Ag nanowires grown on curved PET with curvature radius of 2 cm. When Cu2O microcrystal is used to reduce Ag+, Cu2O nanoparticles break off from the microcrystal and diffuse isotropically into the solution34, 36. These Cu2O nanoparticles serve as reductant and growth substrate in the reduction of Ag+, which leads to the growth of Ag nanowires in random directions if PET is flat. In our previous study, we discovered the Ag nanowires grown by Cu2O as reductant on glass slide incorporated Cu element34, which stemmed from the unoxidized Cu2O nanoparticles. Therefore the Ag nanowires in this experiment are highly probably not pure either, because they grow following an identical growth mechanism. It is noteworthy that, in a real-time observation of the Ag nanowire growth by optical microscope, the nanowires dominantly grow on PET instead of free-standing in the solution, as shown in Fig. 3A. In Fig. 3A, we present an optical microscopy image of Ag nanowires grown on flat PET in real time observation. It can be easily identified whether a nanowire is grown on PET. For example, in a
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magnified image as shown in the top right insert of Fig. 3A, the majority of the nanowires are clear; when the microscope stage is slightly lowered, a small portion of the nanowires, i.e. the free-standing grown nanowires, become clearer (as shown in the bottom right insert in Fig. 3A; marked by red arrow) while the original clear nanowires are out of focus and turn blurred. Full scale images are provided in Fig. S5, which illustrates that the majority of the nanowires are grown on PET instead of free-standing. Therefore, thermodynamically, it can be inferred that the surface energy of the Ag nanowire exposed to the solution is higher than that attached to PET. When the PET is curved, the nanowire growing perpendicular to the curvature axis has a higher ratio of surface exposed to the solution (>50%) as illustrated in Fig. 3Ba, and the nanowire in parallel with the axis has a lower ratio (
