Article pubs.acs.org/crystal
Large-Scale Synthesis and Characterization of Very Long Silver Nanowires via Successive Multistep Growth Jin Hwan Lee,† Phillip Lee,† Dongjin Lee,§ Seung Seob Lee,† and Seung Hwan Ko*,†,‡ †
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea ‡ KAIST Institute for the NanoCentury, KAIST, Daejeon 305-701, Korea § School of Mechanical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 143-701, Korea S Supporting Information *
ABSTRACT: In this research, we developed a novel successive multistep growth method to synthesize very long silver nanowires (AgNWs) over several hundred micrometers (maximum length of 400−500 μm) and performed a systematic parameter study to optimize the dimension of nanowires synthesized at a large scale. It was demonstrated that AgNWs continued to grow through successive multistep growth as long as Ag ion rich conditions were maintained continuously. We successfully attained an extremely high aspect ratio of 1000−3000 with length of over 300 μm and diameter of less than 150 nm. This value demonstrated an order of magnitude length enhancement from previous AgNW synthesis research. Furthermore, we demonstrated that the very long AgNW mesh can be used for a transparent conductor as an alternative to metal oxide conductors. The production of very long metal nanowires at a large scale has significant impact on their potential application in flexible transparent conductors.
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INTRODUCTION Metallic nanostructures have attracted much attention for a decade for promising applications such as interconnects in nanoelectronics,1 active components in nanophotonics,2 and transparent conductors.3−6 In particular, one-dimensional metal nanowires play promising roles in providing devices or existing materials with novel functionality like high mechanical stiffness, optical transparency, and electrical conductivity. In these regards, synthesis and characterization of metallic nanowires have been of great importance along with precise control over the morphology and dimension. Silver nanowires (AgNWs) have been widely studied because they play important roles in practical devices such as capacitors, batteries, transparent and flexible electrodes, and water filters.7−9 Many ways to synthesize AgNWs have been developed such as template-guided synthesis,10 ultraviolet photoreduction,11 solid−liquid phase arc-discharge,12 and electrochemical method.13 Among them, the polyol process is considered an ideal method due to low cost, high yield, and process simplicity. Fievet14 et al. initially performed polyol process to synthesize sub-micrometer-sized metallic nanoparticles. Crystalline Ag nanostructures with uniform size and shape were successfully synthesized in the form of spherical particles, cubes, and rods15 by Xia et al. using poly(vinylpyrrolidone) (PVP) as a protecting agent. Furthermore, they demonstrated high yield synthesis of Ag nanowires by introducing salt ions that enabled rapid injection of AgNO3.16 It has been demonstrated that crystalline AgNWs could be © 2012 American Chemical Society
directed to grow into nanocrystals enclosed by (100) facets, where PVP bound directly forming into 1D shape. High aspect ratio with long length and small diameter plays a most important role to allow AgNWs with novel functionality. However, to the best of our knowledge, the length of AgNWs has been limited to few tens of micrometers at best, and there has been no successful report of controlling the length of nanowires to the several hundred micrometers at large scale. In this paper, we developed a novel successive multistep growth (SMG) method to synthesize very long AgNWs over several hundred micrometers with dramatically increased aspect ratio. Furthermore, we performed a systematic parameter study to optimize the dimension of nanowires synthesized at a large scale. As a result of parametric study, the optimal condition was obtained for very long AgNWs in terms of various synthesis conditions. Very long AgNWs with a maximum length of 400− 500 μm were synthesized at scaled-up process, which is significant progress from previous works17−20 where maximum 40−50 μm wires were produced. Many researchers synthesized 4−20 μm long AgNWs for their applications, even though they followed the same process in previous study,3,5,21,22 which demonstrates the need for special control of synthesis conditions. Indeed, we showed that precise control of experimental conditions is required for the synthesis of uniformly shaped nanostructures with excellent ideal geometry. Received: August 5, 2012 Revised: September 16, 2012 Published: September 20, 2012 5598
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Figure 1. The effect of AgNO3 sonication before injection: (A) optical image of 0.1 M AgNO3 solutions after sonication for 0, 2, 7, 10, and 30 min and 30 min with heating. SEM images of synthesized silver nanowires from AgNO3 sonicated for (B) 0 min, (C) 10 min, and (D) 30 min with heating. The scale bars in panels B, C, and D are 10, 20, and 100 μm, respectively. The inset in panel C represents TEM image (scale bar of 50 nm) of AgNO3 sonicated for 10 min. was slightly ivory to red ivory within 30 min, beginning dark gray within 90 min, and finally changed to bright opaque gray. Successive Multistep Growth (SMG) for Very Long AgNWs Synthesis. For successive multistep growth (SMG), the first grown AgNWs were used as seed NWs to grow very long NWs by repeating the modified growth process. Simple addition of chemicals and simple repetition of the regular AgNW growth process failed longer AgNW growth but generated many byproduct nanoparticles. Cautious chemical reaction parameter control could lead to the very long AgNW growth. The synthesized AgNW solution was stored in a vial in a 95 °C oven. A clean flask was filled with 50 mL of EG, suspended in an oil bath, and heated under continuous magnetic stirring. The injection of Cu-additive solution and PVP solution was performed followed by the injection of 10 mL of previously synthesized nanowires. Then 15 mL of AgNO3 solution was added at the injection rate of 0.5 mL/min using a syringe pump. As the second growth step progressed, the color of the reaction solution changed from bright ivory to opaque gray. After nanowire formation, the reaction was quenched by cooling the reaction flask in a water bath at a room temperature. Acetone was added to the synthesized nanowires with 1:9 volume fraction to remove excess chemicals like EG and PVP, and the mixture was centrifuged. The precipitate was redispersed in ethanol followed by centrifugation several times. The cleaned nanowires were stored in ethanol until characterization. These steps can be repeated to synthesize very long AgNWs. For SEM and TEM studies, a drop of the ethanol-suspended wires was placed on a piece of silicon wafer and a carbon-coated grid, respectively, and dried under ambient condition. 1-Hexadecanethiol Functionalized Very Long AgNWs. Very long AgNWs were functionalized with 1-hexadecanethiol for surfaceenhanced Raman spectroscopy as follows. 1-Hexadecanethiol (10 mM) was prepared in chloroform solution. Cleaned AgNWs were mixed with 30 mL of 1-hexadecanethiol solution to change the surface from PVP to 1-hexadecanethiol. After 12 h, the mixed solution was cleaned with ethanol three times to remove excess chemicals, and then a drop of solution was spread on a piece of bare silicon wafer.
Furthermore, it was demonstrated that AgNWs continued to grow through SMG as long as Ag+-rich conditions were maintained continuously. We successfully attained an extremely high aspect ratio of 1000−3000 with length of over 300 μm and diameter of less than 150 nm. This value demonstrated an order of magnitude length enhancement from previous AgNW works.17−20 The production of very long metal nanowires at a large scale has significant impact on their potential application in flexible transparent conductors as an alternative to ITO (indium tin oxide) transparent conductors in optoelectronics.
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MATERIALS AND METHODS
The synthesis of the very long AgNWs is composed of two stages: first seed NW growth and second successive multistep growth for additional length growth along the seed NWs. Seed NW Synthesis for Very Long AgNWs. In a typical seed NW synthesis, 50 mL of ethylene glycol (EG, J. T. Baker, 9300-03) in a glass flask was preheated in an oil bath at 151.5 °C for 1 h under continuous magnetic stirring. The reagent solutions of CuCl2 (SigmaAldrich, 487847), PVP (Sigma-Aldrich, 856568), and AgNO3 (SigmaAldrich, 10220) were prepared in EG during the preheating step. Then, 400 μL of 4 mM CuCl2 solution was added to the flask. After 15 min, 15 mL of 0.147 M PVP solution (concentration was calculated based on the repeating unit) was injected into the flask. Finally 15 mL of 0.094 M AgNO3 solution was added to the flask with the injection time of 15−30 min. The color of AgNO3 solution in EG changed to transparent red in the shaking step. The effect of seed solution on the silver nanowire produced was carefully studied in this work by adjusting AgNO3 solution sonication time while conditions were kept constant unless they are noted. In previous works,17−19 a small amount of AgNO3 (0.05 g or less) was used to synthesize AgNWs for a onepot process. Five to ten times scale-up production of AgNW was performed in this work as follows. In contrast to the previous research where the continuous color changes show the reaction progress,16 we did not observed same color changes in this current work. At a controlled speed of AgNO3 injection, a broad spectrum of grown nanowire status carries different colors leading to a different trend from the previous work. The color
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RESULTS AND DISCUSSION The critical chemical reaction parameters for successful very long AgNW growth include (1) the sonication time, (2) 5599
dx.doi.org/10.1021/cg301119d | Cryst. Growth Des. 2012, 12, 5598−5605
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Figure 2. SEM images (A, C, and E) and length distribution (B, D, and F) of synthesized AgNWs by different experimental processes: (A, B) short AgNWs by a conventional AgNW synthesis process following ref 25 without optimization., (C, D) long AgNWs by an optimized single step growth conditions developed in this work, and (E, F) very long AgNWs by the successive multistep growth (SMG) optimized process developed in this work. All scale bars in SEM images are 100 μm and a scale bar in inset of panel E is 4 μm. The length of silver nanowires in panel A is under 30 μm.
injection speed effects on the initial silver nitrate precursor, and (3) heating conditions. Extensive parametric study of their influence on the nanowire growth was carried out to find the optimum conditions. It was experimentally reported that the initial status of silver nitrate in EG influenced the shape of grown nanostructures,23 and according to the previous research, the presence of Cl during polyol synthesis of Ag nanowire induces chemical reaction as follows: (i) Ag+ are reduced into Ag0 atoms with EG and Ag+ ion and Cl− form AgCl by utilization of its Cl−, (ii) chemicals such as Na, Fe, and Cu remove atomic oxygen from the surface of silver nanostructures and are reduced again by EG, (iii) initial decreased Ag+ is slowly released from AgCl after formation of Ag seeds through ionization.16,24,25 Herein, we studied the sonication effect of AgNO3 in EG on the shape and quality of AgNWs synthesized as shown in Figure 1. AgNO3
was dissolved in EG by mild hand shaking (2 min) and subsequent ultrasonication for various duration of time. As shown in Figure 1A, the color of the AgNO3 solution changed due to the formation of tiny particles as sonication time increased. The color of AgNO3 in EG changed from colorless to transparent red as sonication progressed as shown in Figure 1A. Particularly, prolonged sonication with heating (60 °C, 15 min) makes the seed solution look black (Figure 1A(vi)) which signifies the formation of big particles. Figure 1B−D shows the synthesized AgNWs from AgNO3 seed solutions that were sonicated for 0, 10, and 30 min, respectively. We found that 5− 7 min sonication of the AgNO3 precursor solution could minimize the byproduct particles and thus grew very long AgNWs. Too much longer or shorter than the optimum sonication time would lead to dramatic generation of many short AgNWs with a large number of byproduct nanoparticles 5600
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growth of the particles and NWs that were shorter than 10 μm was possible, to the best of our knowledge, multistep growth of AgNW 1D structure with over 50 μm length has not been reported yet. Preinjection of additional PVP in SMG helped to prevent the additional growth of {100} facets and continuous SMG process made transition of length distribution clear as shown in Figure 2. According to the progress, the observed very long AgNWs are not many because each step uses AgNW solution from prior steps as seed solution. Moreover, during successive multistep growth, both successive long AgNWs growth from prior steps and the fresh short AgNW growth from seed nanoparticles occur simultaneously. These combined growths caused broader AgNW length distribution. However, the total frequency of long AgNWs ranging from 100 to above 200 μm is almost 50% as shown in Figure S6, Supporting Information (colored AgNWs). Based on previous research, the mechanism of AgNW growth is clear with several factors such as metal precursor concentration, reduction rate, and the presence of a capping agent. Therefore, we speculated that SMG successfully provided optimal ion-rich environment and to prevent the growth in {100}. It is illustrated in Figure 3A that AgNWs grown in an optimized single-step polyol process acted as the seeds on
and thus lower the NW synthesis yield as shown in Figure 1B,D. When sonication time was much shorter than the optimum sonication time or the AgNO3 was dissolved only by mild hand shaking, many byproduct nanoparticles along with the many short NWs were observed (Figure 1B). This is because there would be undissolved AgNO3 clumps, where free Ag+ could easily deposit by Ostward ripening resulting in many particles. When the sonication time was longer than the optimum duration, the AgNO3 color change to dark (Figure 1A(vi)). This might be influenced by the initial seed particle formation and growth to large and irregular bad seed particles due to the long sonication and heating. However, wellsonicated AgNO3 seed solutions dramatically reduced byproduct nanoparticles (Figure 1C). The 5−7 min sonication could change the color to transparent red (Figure 1A(iv,v)) and produce AgNWs with minimized or no byproduct particles (Figure 1C). Well-sonicated AgNO3 in EG had uniform tiny particles that could act as seed for nanowire growth, where about 2 nm tiny uniform particles were observed (Figure 1C, inset, TEM picture). The tiny twinned particles in wellsonicated AgNO3 play an important role as the nucleation site for high-quality very long AgNWs, while others will be etched uniformly. The synthesis of AgNWs with few or no byproduct particles is important considering practical application. Particularly, industrial application requires an economical and simple synthesis process. Once byproduct particles are generated, it is hard to separate them perfectly by filtration or centrifugation. The minimization of byproduct particles means that most silver ions are used to grow only AgNWs without the nonideal byproducts. To enhance this effect, injection speed was controlled in this work, although rapid injection is still widely used to synthesize AgNWs. Previously, slow injection and dropby-drop injection were regarded as a way to increase wire-toparticle ratio.25 Furthermore, this condition should be adjusted depending on the scale or reaction conditions. The parametric study we performed is detailed in Table S1 (Supporting Information). As a result, we successfully accomplished a large scale synthesis of almost byproduct particle-free AgNWs with the injection rate of under 1 mL/min. The formation of byproduct particles might be minimized by successive and slow Ag+ supply that was enabled by intermediate AgCl formation. In consequence, we could synthesize long AgNWs with average length of 50 μm by means of minimized particle formation and optimized external conditions. Even though AgNWs with average length of over 50 μm were synthesized by an optimized single step, NW growth was limited by precursor chemical quantity in the one-pot process. Therefore, to grow very long AgNWs, we introduce a successive multistep growth (SMG) method, where presynthesized AgNWs played a role of nucleation sites for longer NW growth in Ag ion-rich environment. The SEM images and length distribution of AgNWs by SMG method are shown in Figure 2. For comparison, we synthesized short AgNWs by a conventional method26 using rapid injection of AgNO3 without optimization of external conditions. The average length of AgNWs is 4−10 μm as shown in Figure 2A,B. As aforementioned, however, long AgNWs with average length of 50 μm were synthesized in our optimized process as shown in Figure 2C,D. Through SMG, the AgNW length was significantly increased as shown in Figure 2E,F, where over 400 μm long AgNWs were found with average length of 120 μm (Figure S4, Supporting Information). Although selective
Figure 3. Schemes of very long AgNW synthesis process: (A) illustration of polyol process and a successive multistep growth process and (B) transition of AgNW length distribution through optimization and the SMG method. In the SMG process, synthesized silver nanowires can serve as seed wire for longer NWa and additional ions selectively deposit on AgNWs.
which Ag+ ions are reduced to produce longer NWs. The evolution of length distribution through SMG method is estimated as shown in Figure 3B. By repetition of the SMG process, it is possible to make AgNWs much longer. Although there is a difficulty of identifying exact the shift of Gaussian distribution in length due to a wide distribution of NW length, we observed that fairly uniform 4−10 μm long wires grew to 20−50 μm (average length) after 3 times of SMG method and to 100−150 μm (average length) after 7 times of SMG steps in our experiment. 5601
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Figure 4. (A) High-resolution transmission electron microscopy image of very long AgNW grown under optimized conditions. Insets are TEM image and electron diffraction pattern of AgNWs. The scale bars are 2 and 100 nm, respectively. (B) X-ray diffraction pattern of synthesized AgNWs.
Figure 5. Thermal stability analysis of very long AgNWs: (A) thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), and SEM images of AgNWs treated at (B) 300, (C) 350, and (D) 400 °C for 1 h. The scale bars in B, C, and D are 5, 0.5, and 30 μm, respectively. The scale bar in inset is 2 μm.
The very long AgNWs grown by the SMG method possessed highly crystalline structure as shown in the high-resolution TEM image with a selected area electron diffraction image (SAED) in Figure 4A. The diameter of AgNW from the SMG method was found to be 150 nm, which is the exactly same as that produced in a normal polyol process. From that result, it was concluded that the AgNWs maintained the initial diameter through the SMG process. Therefore, it could be deduced that
SMG method grows AgNWs preferentially lengthwise while the diameter-wise growth was suppressed. The HRTEM image demonstrates the crystalline structure with lattice distance of 1.45 Å. The SAED (Figure 4A, bottom inset) pattern suggests [022̅] growth direction. Furthermore, X-ray diffraction (XRD) was performed on AgNWs on silicon substrate, resulting in the XRD pattern as shown in Figure 4B. Four distinct peaks of (111), (200), (220), and (311) explain that the synthesized 5602
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at wavenumbers of 708, 891, 1063, 1098, 1129, 1298, and 1435 cm−1. These peaks are in good agreement with those of 1hexadecanethiol in previous studies.22,28 The observed Raman bands can be assigned to C−Strans, C−Ctrans, and C−Htrans bands. The strongest peak around 500 cm−1 was from a silicon substrate. Consequently, the synthesized AgNW was used as a substrate to inspect the SERS signal of 1-hexadecanethiol molecules, and it was demonstrated that the synthesized single wire performed well as SERS platform. The major application of AgNWs is in optoelectronics as a transparent conductor. The optical and electrical properties of very long AgNWs were characterized. To compare transmittance of AgNW solution, two types of AgNW were synthesized from the same amount of AgNO3 precursors; the regular short AgNWs (50 μm) by controlled optimized method developed in this paper. UV−vis spectroscopy was used to compare the transmittance of AgNW solutions as shown in Figure 7. It is clearly observed that the
AgNWs have pure face-centered-cubic (FCC) structure, which is in good agreement with the previous studies.17,27 Thermal stability of synthesized AgNWs was characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) along with SEM observation as shown in Figure 5. The mass loss and heat flow are depicted in Figure 5A. AgNWs (50 mg) were heated at ca. 10 K/min in an air environment. The mass loss and endothermic peak before 200 °C corresponds to the loss of ethanol used as a dispersant. The following mass loss and exothermic peak before 400 °C in the process of heating the sample is presumably due to the melting and decomposition of capping PVP. At about 950 °C, the distinct endothermic peak is due to melting of silver, and this value is comparable to the physical properties of bulk silver. The direct observation of the melting behavior of the AgNWs is shown in the SEM images (Figure 5B−D). The SEM images of AgNWs baked at 300, 350, and 400 °C for 1 h are in Figure 5, panels B, C, and D, respectively. It is clearly observed that most of the PVP capping layer was removed and accumulated into chunks (blue colored parts in Figure 5B) on the surface of AgNWs or scattered on the substrate (blue colored parts in Figure 5C). The AgNWs shows distinctive melting behavior to form discrete Ag molten droplets over 400 °C heating as shown in Figure 5D. The significantly lower melting temperature of AgNW (
