Article Cite This: Langmuir 2017, 33, 11851-11856
pubs.acs.org/Langmuir
Guided Growth of Ag Nanowires by Galvanic Replacement on a Flexible Substrate Sanjun Yang and Qiming Liu* Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China
Langmuir 2017.33:11851-11856. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 12/17/18. For personal use only.
S Supporting Information *
ABSTRACT: An aligned Ag nanowire array was directly synthesized by galvanic replacement on curved poly(ethylene terephthalate) (PET) by using a Cu2O microcrystal as a reductant. A 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 a reductant or polystyrene as a 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.
■
nanowires were grown from the sides of the ZnO seeds.17 By controlling the vapor−liquid−solid (VLS) growth condition, aligned planar growth of InAsSb18 and GaAs19 nanowires was realized. Guided growth of 1D nanostructures was 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, the in situ growth of 1D material from the preprogrammed nanostructure template created by lithography or ion etching (the so-called top-down approach) showed powerful control over the individual elements of the 1D material.23,24 The above representative grow-in-place methods have advanced the synthesis technique of ordered 1D material but still have limitations such as low throughput, the requirement of a specific growth substrate, and a 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 conditions. Ag nanowires are one of the
INTRODUCTION One-dimensional (1D) materials including nanowires and nanotubes have been widely used in various fields.1 Practical applications require the capability to control the orientation, placement, and density of the 1D materials in order to acquire the best performance from them.2,3 Hence, to this end, great effort has 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 (postgrowth rearrangement of a 1D material to a determined place) and grow in place (manipulation of the in situ growth of 1D material).4 For the last few decades, a great many assembly methods based on the grow-and-place strategy have been developed, such as surface modification,5,6 mechanical force,7,8 the Langmuir−Blodgett (LB) technique,9,10 the blown-bubble method,11,12 evaporationinduced assembly,13,14 and external electric- or magnetic-fieldassisted assembly.15,16 However, 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 to be promising in overcoming these difficulties.4 Using ZnO seeds covered by a catalytically inactive Cr layer, aligned ZnO © 2017 American Chemical Society
Received: March 22, 2017 Revised: August 20, 2017 Published: September 27, 2017 11851
DOI: 10.1021/acs.langmuir.7b00983 Langmuir 2017, 33, 11851−11856
Article
Langmuir
Figure 1. (A) SEM image of the aligned Ag nanowires. The bottom-right corner is a magnified image of the nanowire (scale bar = 500 nm). (B) Typical XRD pattern of the Ag nanowire array. 0.5 mM AgNO3 solution for 30 min and washed with water and ethanol after the reaction ended. 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 on the microscope stage. The amount of AgNO3 solution in the culture dish was reduced to 10 mL as a result of the limited space between the objective lens and the microscope stage. Characterization. Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 1.5406 A) was used to obtain the XRD results. FE-SEM images were acquired from Zeiss-Sigma working at 10 kV. The samples were sputtered with gold for 60 s before observation. The surface chemical species of the samples were examined with an X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA) using Al Kα radiation at 1486.6 eV as the excitation source. Optical images were recorded using an Olympus BX51 microscope.
most important 1D materials and, because of their high conductivity and strong localized surface plasmon resonance (LSPR), have been widely used in transparent conductive electrodes25,26 and SERS.27,28 Because, as has been stated, orderly Ag nanowire arrays commonly exhibit novel or superior performance compared to that of disorderly ones, various methods are devised to align the Ag nanowires such as a sprayassisted method,29 capillary printing,2 the Langmuir−Blodgett (LB) technique,25 a fluid-flow method,28 three-phase evaporation,13 and electrohydrodynamic jet printing.30 All of these methods work using previously synthesized Ag nanowires and thus are attributed to the grow-and-place method. The grow-inplace synthesis method of Ag nanowires is very rare. In this study, we discovered that the Ag nanowires grown directly on a flexible substrate, poly(ethylene terephthalate) (PET), exhibit an aligned pattern if PET is curved.
■
■
RESULTS Figure 1A shows the aligned growth of Ag nanowires on PET, and the magnified image of the nanowire shown in bottomright corner indicates that the width of the nanowire is approximately 100 nm. Figure 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 microcrystals of approximately 5 μm (SEM image and XRD in Figure S1) were used to reduce 1 mM Ag+ to Ag nanowires. The XPS characterization of Cu2O microcrstals before and after reaction is shown in Figure S2A,C, respectively. From the XPS results, it can be identified that 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 reaction,31 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 Figure 1A and extend to more than 100 μm with the identical orientation. 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) theory.32−34 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 in the Ag+ concentration distribution are aroused. If the local restoring force damps the instabilities, then Ag+ will steadily diffuse to the growth front and consequently the Ag nanowire is formed. If not, then Ag+ will diffuse to the point in the vicinity of the growth front and promote branch nucleation, which leads to the growth of dendrites.33,35 A scheme of the growth of the Ag
EXPERIMENTAL SECTION
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 Microcrystals. Cu2O microcrystals were synthesized by solution-phase reduction. Under stirring in a 70 °C water bath, 25 mL of NaOH (4 g) solution was evenly added over 5 min to a conical flask that contained 25 mL of Cu2SO4 (0.49 g) solution. In this process, Cu(OH)2 gel was prepared and turned black. After that, 25 mL of a glucose (0.45 g) solution was evenly added to the above gel over 10 min. The final products were washed by centrifugation with water and absolute ethanol three times. After being dried in vacuum for 12 h, the Cu2O microcrystal was dispersed in ethanol at densities of 0.5 and 2 mg/mL. Synthesis of Ag Nanowires by Cu2O Microcrystals on PET. PET was cut into 4, 4.44, and 6.28 cm squares. A 30 μL Cu2O suspension was drop-cast on the PET. A 1 mM AgNO3 solution (40 mL) was transferred to a 50 mL beaker (diameter = 4 cm). After that, PET was carefully inserted into the beaker. In this process, the PET side with Cu2O was kept inside the curvature and thus faced the bulk of the AgNO3 solution. The reaction was kept in the 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 90°, and then the reaction proceeded for 20 min. After that, the PET was cleaned with water and ethanol. Synthesis of Ag Nanowires by Zn or on a Culture Dish. Zn microcrystal was purchased from Aladdin Chemical Reagent Co. Ltd. A 30 μL Zn suspension (0.3 mg/mL) was drop-cast on PET, and the following procedures are identical to the case of Cu2O except that 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 of a Zn suspension was dropcast on the inside of it. Then this piece of culture dish was placed in a 11852
DOI: 10.1021/acs.langmuir.7b00983 Langmuir 2017, 33, 11851−11856
Article
Langmuir
Figure 2. (A) Disorderly Ag nanowires grown on a glass slide. (B) Disorderly Ag nanowires grown on a flat PET. The top-right inset is the schematic cross-sectional view of the pet set in a beaker in a GR reaction. (C) Partially orderly Ag nanowires grown on a curved PET with a radius of curvature of 2.83 cm. (D) Orderly Ag nanowires grown on a curved PET with a radius of curvature of 2 cm.
Figure 3. (A) Optical microscopy image of Ag nanowires in a real time observation. The top right inset is a magnified image of the green square area, and the bottom right inset is the corresponding changed focus image (scale bar = 50 μm). (B) Schematic cross-sectional view of the Ag nanowire grown on a curved PET in a direction (a) perpendicular to the axis of curvature and (b) parallel to the axis of curvature.
When a Cu2O microcrystal is used to reduce Ag+, Cu2O nanoparticles break off from the microcrystal and diffuse isotropically into the solution.34,36 These Cu2O nanoparticles serve as a 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 a reductant on a glass slide incorporated the Cu element,34 which stemmed from the unoxidized Cu2O nanoparticles. Therefore, the Ag nanowires in this experiment are likely not pure either because they grow by following an identical growth mechanism. It is noteworthy that, in a real-time observation of Ag nanowire growth via the optical microscope, the nanowires dominantly grow on PET instead of free-standing in the solution, as shown in Figure 3A. In Figure 3A, we present an optical microscopy image of Ag nanowires grown on flat PET in a real time observation. It can be easily identified whether a nanowire is grown on PET. For example, in a magnified image as shown in the top right inset of Figure
nanowire based on MS theory is shown in Figure S3. From the above discussion, the reduction of Ag+ in the growth front is of a localized nature. Therefore, on the nanoscale, the growth direction of the Ag nanowire assumes a chaotic pattern, which can be easily observed in Figure 1A. On the other hand, in long-range determinations 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 curvature than in other directions. A scheme of the guided growth of Ag nanowires is shown in Figure S4A. A smaller radius of curvature leads to a more aligned Ag nanowire array. Ag nanowires grown on a flat glass slide or PET have random orientations, as shown in Figure 2a,b, respectively. With the radius of curvature decreasing (as shown in the topright inset in Figure 2b−d), the degree of orderliness of the Ag nanowire array increases as shown in Figure 2c,d. 11853
DOI: 10.1021/acs.langmuir.7b00983 Langmuir 2017, 33, 11851−11856
Article
Langmuir
Figure 4. (A) large-scale optical image of a Ag nanowire array synthesized by a higher density of Cu2O microcrystals (2 mg/mL). (B) Ag nanowire network synthesized by the second growth of a Ag nanowire array. (C) Aligned Ag nanowire array synthesized with Zn microcrystals. (D) Aligned Ag nanowire array synthesized on the inner wall of a Petri dish.
mg/mL. A large-scale transmission optical image of the correspondingly synthesized Ag nanowire array is shown in Figure 4a. The corresponding dark field optical image is shown in Figure S6. A nanowire network is useful in applications such as transparent conductive electrodes. In grow-and-place methods, the network is commonly obtained by repeatedly aligning or transferring nanowire arrays with different angles.2,25,38 Interestingly, in this experiment, a cross of the Ag nanowire array can be obtained by a second growth of the Ag nanowire array. The scheme of the growth process is shown in Figure S4B. Figure 4B shows the optical image of the cross of Ag nanowires, and the top-right inset is the corresponding SEM image. This plain network synthesis method coupled with the facilely tunable density of Ag nanowires demonstrates the merit of the grow-in-place strategy. From the above discussion, it can be seen that the mechanism dictating the aligned growth of Ag nanowires is of general interest, and thus the method can potentially be applied to other situations. As a preliminary work, Zn microcrystals (see the SEM image and XRD pattern in Figures S7 and S8A) were used to prepare the Ag nanowire array instead of Cu2O microcrystals. It has been reported that Ag dendrites can be synthesized by GR using Zn as a reductant.39 We also observed similar Ag dendrites using the same AgNO3 concentration (30 mM). According to MS theory, decreasing the reaction speed in the growth front can damp the instability of the Ag+ concentration distribution and consequently transform the dendrite morphology to a nanowire. We reduced the AgNO3 concentration from 30 to 0.5 mM, and the Ag nanowire was prepared (XRD pattern in Figure S8B). As shown in Figure 4C, the nanowires synthesized on curved PET with a radius of curvature of 2 cm are aligned. In this experiment, the Cu2O and Zn microcrystals both have the size distribution shown in the top-right inset in Figures S1a and S7, respectively. However, on the basis of observation, no noticeable influence on the alignment of Ag nanowires by the size parameter has been discerned in the case of either Cu2O or
3A, a 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 inset in Figure 3A, marked by a red arrow) and the original clear nanowires are out of focus and become blurred. Full-scale images are provided in Figure S5, which illustrates that a 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 of the nanowire attached to PET. When the PET is curved, the nanowire growing perpendicular to the axis of curvature has a higher ratio of surface exposed to the solution (>50%) as illustrated in Figure 3Ba, and the nanowire in parallel with the axis has a lower ratio (
