Accepted Manuscript Short communication Ultrasonication Assisted Production of Silver Nanowires with Low Aspect Ratio and Their Optical Properties Miso Park, Youngku Sohn, Jinhwan Lee, Weon Gyu Shin PII: DOI: Reference:

S1350-4177(14)00165-5 http://dx.doi.org/10.1016/j.ultsonch.2014.05.007 ULTSON 2603

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

8 April 2014 13 May 2014 13 May 2014

Please cite this article as: M. Park, Y. Sohn, J. Lee, W.G. Shin, Ultrasonication Assisted Production of Silver Nanowires with Low Aspect Ratio and Their Optical Properties, Ultrasonics Sonochemistry (2014), doi: http:// dx.doi.org/10.1016/j.ultsonch.2014.05.007

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Ultrasonication Assisted Production of Silver Nanowires with Low Aspect Ratio and Their Optical Properties Miso Park,1 Youngku Sohn,2 Jinhwan Lee,3 and Weon Gyu Shin1,* 1

Department of Mechanical Engineering, Chungnam National University, Daejeon 305764, South Korea 2

Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, South Korea 3

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea

Keywords : Ultrasonication, Fragmentation, Silver nanowires, UV- absorption, SEM Address correspondence to Weon Gyu Shin. Department of Mechanical Engineering, Chungnam National University, Daejeon 305-764, South Korea. E-mail:[email protected]

Abstract

We investigated a facial method to produce silver nanowires with low aspect ratio by fragmentizing as produced long silver nanowires. The length of silver nanowires can be shortened in a controllable manner by increasing ultrasonication time or ultrasonication power. However, excessively large ultrasonication power caused a problem of agglomeration of nanowires. From UV absorption spectra, it was found that the UV absorption characteristic of silver nanowires is not affected by ultrasonication assisted fragmentation indicating that one dimensional structure of silver nanowires is maintained during the fragmentation process.

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1. Introduction The properties of nanometer-sized metallic nanoparticles are affected by their morphology. Especially, when metal nanomaterials have one-dimensional structure (nanowire or nanorod) they have unique chemical, physical and mechanical properties. For the reason, particles which have onedimensional structure are utilized in various fields. For example, Chen et al. (2009) synthesized Ag/SiO2 one-dimensional metallic-dielectric photonic crystals to enhance light absorption characteristic better than the corresponding metal [1]. Kolmakov et al. (2004) fabricated a sensitive and selective chemical or biological sensor based on metal-oxide nanowires [2]. Li et al. (2009) investigated chemical properties of one-dimensional nano-structured silver titanates which are (Ag,H)2Ti4O9-nanowire and (Ag,H)2Ti4O9 -nanorod. One-dimensional silver titanates show strong and fast adsorption for methylene blue and Alizarin red dyes and effective photocatalytic activities [3]. Sepulveda-Mora et al. (2012) fabricated thin films of uniform silver nanowires on glass substrate using a dip-coating technique. It was shown that thin films made of longer nanowires have higher performance of a transparent conductive electrode. [4]. Chen et al. (2013) proposed a novel architecture of graphene paper composed of 1D silver nanowires-doped graphene. The graphene paper has excellent electrical conductivity and desirable flexibility [5]. Because silver has the highest thermal and electrical conductivity among all metals, silver has a lot of applications in electronic components such as printed circuit board and capacitor. Furthermore, due to high ductility and malleability of silver, thin films and wires can be easily fabricated. When the metallic particles such as platinum and indium are in the form of a wire, their electrical properties can be enhanced [6], [7]. Similarly, it would be advantageous to make use of silver nanowires with one dimensional nanostructure rather than silver spheres. Various synthesis methods have been developed to produce silver nanowires or nanorods including chemical synthesis [8], [9], hydrothermal method [10], DNA template [11], porous materials template [12], and polyol process [13]. For examples, Sun et al. (2002) produced bicrystalline silver nanowires with dimensions of 30-40 nm in diameter and up to 20-50 micrometer in length through solution-phase approach[14]. Yvonne et al. (2002) produced silver selenide nanowires through the template directed synthesis by using a porous alumina membrane as a template [15]. Cui et al. (2007) synthesized the silver nanowires by using DNA as a template through a kind of electrochemical method. The method has the advantage of high growth rates [16]. By changing the electrochemical parameters such as potential or electrolysis time, the size and shape of silver nanowires could be controlled. Wang et al.(2005) successfully produced uniform silver nanowires with the diameter of 100 nm and up to 500 micrometer in length by reducing the silver chloride with glucose in the absence of a surfactant or a polymer at 180°C for 18 hours. Wang et al.’s method is simple and environmentally friendly [17]. Caswell et al. (2003) reported that crystalline silver nanowires can be made in water with no surfactant or polymer [18]. Coskun et al. 2

(2011) synthesized silver nanowires through polyol process by controlling various parameters such as reaction temperature, injection rate, molar ratio of poly (vinylpyrrolidone) to silver nitrate, the amount of sodium chloride and stirring rate of solution [19]. When PVP:AgNO3 molar ratio is increased, the diameter of the silver nanowires is decreased. However, at the same time micrometer-sized silver particles can be formed. The surface of silver nanoparticles can be covered with the PVP molecules when excess PVP molecules exist. For the reason, the anisotropic growth (1D) of the nanowires can be limited. The above mentioned methods are usually suitable for the synthesis of nanowires with high aspect ratio, however it is very difficult to make the short nanowires with low aspect ratio. In the present study, we suggested a novel method to make short nanowires by fragmentizing as produced silver nanowires with large aspect ratio using an ultrasonic processor. We investigated the effect of input power and irradiation time in ultrasonication process on the size reduction of silver nanowires through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) image analysis. We also characterized optical properties of silver nanowires according to their size and morphology using UV-visible absorption spectroscopy.

2. Experimental method Ag nanowires (Ag NWs) used in experiments were synthesized in a modified polyol reduction process. 45 ml of glycerol and 1.83 g of PVP were injected to the clean flask, which was then immersed into oil bath to dissolve PVP at 100 °C. While dissolving, 1.7 mM NaCl solution in 5 mL of glycerol was prepared and added drop wise into PVP solution after the temperature of the flask dropped to 50°C. After injecting 0.4 g of AgNO3 into the flask, the solution was gradually heated up to 160°C. The color of the solution turned from pale white into bright orange, red, and finally into gray-green. To control the length of Ag NWs, PVP:AgNO3 molar ratio was modulated. Methanol was added to as-obtained Ag NW solution with 1:9 volume fraction to remove the PVP residue, and the mixture was centrifuged. After several centrifuge processes, the washed Ag NWs were collected and dispersed into a methanol solution of 0.02 g/mL. The nanowires were synthesized to have a very small distribution of diameter but a wide distribution of length. In our experiments, 5 ml of asproduced Ag NWs was diluted with 200 ml of deionized water and then the solution was well dispersed using an ultrasonic bath. An ultrasonic processor delivers ultrasonic energy to materials. When powerful ultrasonic wave is applied in liquid, bubbles are generated by repeated compression and expansion force. Bubbles cause shock wave by rupturing of bubbles when bubbles grow up to a certain size. The generated shock waves accelerate the fragmentation of particles. Silver nanowires were fragmentized using an ultrasonic processor (Model ULH-700S, Jeio Tech, Korea) which can generate the ultrasound with frequency of 20 kHz and maximum power of 700 W. The ultrasonic processor was operated under the 3

continuous mode. In order to investigate how Ag NWs are fragmentized according to ultrasonication conditions, input power was set to the 25%, 50% or 75% of the maximum power (700W) and irradiation time to 0.5 hr, 1.0 hr, 1.5 hr, or 2 hr. The diluted solution was placed on a support jack in order to adjust the height between solution and ultrasonic horn. The titanium horn with a tip diameter of 10 mm and length of 135 mm was immersed 10 mm into the diluted Ag NW solution. Every 30 minutes, some solutions about 8 ml was taken out of the bottle to make samples according to the irradiation time. In order to make specimens for scanning electron microscope (Sirion, USA) image analysis, several drops of each sample was deposited on a silicon wafer and dried in an oven at 100°C during 2 hours in order to remove the moisture on samples. SEM images were obtained according to the operating condition of an ultrasonic processor. We also obtained TEM image in order to compare the morphology of silver nanowires before and after fragmentation. The diluted Ag solution in ethanol were dropped on a TEM grid and dried at room temperature. TEM images and diffraction patterns of as-produced silver nanowires and silver nanowires which were fragmentized for 30 minutes were obtained respectively.

3. Experimental results Figure 1 shows the SEM image and the electron diffraction pattern of as-produced silver nanowires using polyol process. From the image analysis, Ag NWs were found to have the average diameter of 44 nm and average length of 2613 nm. The electron diffraction reflects higly single crystalline nature of the Ag NWs with cubic crystal structure. Figure 2 shows the TEM images of the edges of Ag NWs before and after applying ultrasonication for 30 minutes. Although the wire morpholgy was not critically changed the fragmentized edge appeared to be somwhat rougher plausibly due to breakage by ultrasoniation fragmentation process. Figure 3 (a)-(c) show the effect of ultrasonication time on the length distribution of Ag NWs for a given ultrasonication power. For each condition, more than 200 particles were used for the SEM image analysis. As the ultrasonication time is increased up to 120 minutes for the ultrasonication power of 75%, the fraction of silver nanowires with the length smaller than 500 nm reaches to 75 %. On the other hand, for the ultrasonication power of 25%, the fraction of Ag NWs with the length below 500 nm was not increased significantly. Figure 3(d) shows the average length of fragmentized silver nanowires according to ultrasonication time at each ultrasonic power 25%, 50% and 75%. Figure 4 (a)-(d) show the SEM images of Ag NWs with a different ultrasonication time given when the ultrasonication power was fixed at 50% of the maximum power. Since all four SEM images were taken at the same magnification level, one can see that Ag NWs become shorter as the ultrasonication power is increased.

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We confirm that the higher power is applied for the fragmentation of nanowires, the more size reduction occurs. The reduction of nanowire length can be also obtained by increasing ultrasonication time. When the size reduction rate is analyzed in Table 1, we can conclude that ultrasonication power is more dominant parameter to determine the reduction rate. However, there exists agglomeration problem when using high ultrasonication power as shown in Figure 5. The higher ultrasonication power can help to make nanowires shorter but to make nanowires be more agglomerated. Prior study shows that particles tend to be more agglomerated as irradiation time is increased [20]. SEM images in Figure 5 show the formation of agglomerated nanowires when 75% power was used. Based on these results, one should note that it is important to choose a suitable condition to avoid the agglomeration of fragmented nanowires. Figure 6 shows normalized UV-visible absorption spectra for the synthesized Ag NWs dispersed in water with sonication time (0, 30, 60, 90 and 120 minutes). No clear detectable change in color was observed with ultrasonication time, as seen in the photo images. Two peaks were commonly found at 350 nm (weak) and 390 nm (strong). The smaller peak at 350 nm was attributed to the longitudinal mode or the out-of-plane quadrupole mode [21, 22] with one-dimensional (1D) structure. The strong peak at 380 nm was ascribed to transverse mode of surface plasmon resonance of Ag NWs [21, 22]. Although the major peaks were not significantly changed the broad tails in the longer wavelength was clearly increased as the sonication time was increased. The results could indicate that the size of Ag NWs became shorter with no change in the wire morphology.

4. Conclusions In this study, we suggested a new facial way to produce silver nanowires with a low aspect ratio by using an ultrasonic processor. We also investigated the fragmentation properties of Ag nanowires according to the ultrasonication power and operation time. Both longer ultrasonication operation time and higher power can contribute to make Ag nanowires shorter significantly. Ultrasonication operation time has a more dominant effect on the decrease in the nanowire length compared to ultrasonication operation time. However, using high power can cause the agglomeration of fragmentized silver nanowires. Based on UV absorption spectra of fragmentized silver nanowires, it was found that the UV absorption characteristic of silver nanowires is not affected by ultrasonication assisted fragmentation. TEM images of fragmentized silver nanowires show no significant change in the structure and morphology especially around the fragmentized edge. We conclude that one dimensional structure of silver nanowires is maintained during the fragmentation process. It is expected that our method will be useful since the process can be directly applied to as-produced long nanowires.

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5. References [1] S. Chen, Y. Wang, D. Yao, Z. Song, Absorption enhancement in 1D Ag/SiO2 metallic-dielectric photonic crystals. Opt. Appl. 39. 3 (2009): 473-479. [2] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by one-dimensional metal-oxide nanostructures. Annu. Rev. Mater. Res. 34 (2004) 151-180. [3] Q. Li, T. Kako, J. Ye, Strong adsorption and effective photocatalytic activities of one-dimensional nano-structured silver titanates. Appl. Catal. A: 375.1 (2010): 85-91. [4] S.B. Sepulveda-Mora, S.G. Cloutier, Figures of merit for high-performance transparent electrodes using dip-coated silver nanowire networks. J. Nanomater. 2012 (2012): 9. [5] J. Chen, H. Bi, S. Sun, Y. Tang, W. Zhao, T. Lin, D. Wan, F. Huang, X. Zhou, X. Xie, M. Jiang,Highly Conductive and Flexible Paper of 1D Silver-Nanowire-Doped Graphene. ACS Appl. Mater. Interfaces 5.4 (2013): 1408-1413. [6] M.A. El-Sayed, , Some interesting properties of metals confined in time and nanometer space of different shapes. Acc. Chem. Res. 34.4 (2001): 257-264. [7] C. Li, D. Zhang, X. Liu, S. Han, T. Tang, J. Han, In2O3 nanowires as chemical sensors. Appl. Phys. Lett. 82.10 (2003): 1613-1615. [8] N.R. Jana, L. Gearheart, C.J. Murphy, Wet chemical synthesis of silver nanorods and nanowires of controllable aspect ratio Electronic supplementary information (ESI) available: UV–VIS spectra of silver nanorods. Chem. Commun. 7 (2001): 617-618. [9] S.H. Kim, B.S. Choi, K. Kang, Y.S. Choi, S.I. Yang, Low temperature synthesis and growth mechanism of Ag nanowires. J. Alloy. Compd. 433.1 (2007): 261-264. [10] J. Xu, J. Hu, C. Peng, H. Liu, Y. Hu, A simple approach to the synthesis of silver nanowires by hydrothermal process in the presence of gemini surfactant. J. Colloid Interface Sci. 298.2 (2006): 689-693. [11] K. Keren, M. Krueger, R. Gilad, G. Ben-Yoseph, U. Sivan, E. Braun, Sequence-specific molecular lithography on single DNA molecules. Science 297.5578 (2002): 72-75. [12] L.B. Kong, M. Lu, M.K. Li, H.L. Li, X.Y. Guo, Branched silver nanowires obtained in porous anodic aluminum oxide template. J. Mater. Sci. Lett. 22.9 (2003): 701-702. [13] Y. Sun, Y. Xia, Large-scale synthesis of uniform silver nanowires through a soft, self-seeding, polyol process. Nature 353.1991 (1991): 737. [14] Y. Sun, Y. Yin, B.T. Mayers, T. Herricks, Y. Xia, Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly (vinyl pyrrolidone). Chem. Mater. 14.11 (2002): 4736-4745. [15] Y.J. Glanville, D.G. Narehood, P.E. Sokol, A. Amma, T. Mallouk, Preparation and synthesis of Ag2Se nanowires produced by template directed synthesis. J. Mater. Chem. 12.8 (2002): 24332434. [16] S. Cui, Y. Liu, Z. Yang, X. Wei, Construction of silver nanowires on DNA template by an 6

electrochemical technique. Mater. Design 28.2 (2007): 722-725. [17] Z. Wang, J. Liu, X. Chen, J. Wan, Y. Qian, A Simple Hydrothermal Route to Large‐Scale Synthesis of Uniform Silver Nanowires. Chem.-Eur. J. 11.1 (2005): 160-163. [18] K.K. Caswell, C.M. Bender, C.J. Murphy., Seedless, surfactantless wet chemical synthesis of silver nanowires. Nano Lett. 3.5 (2003): 667-669. [19] S. Coskun, B. Aksoy, H.E. Unalan, Polyol synthesis of silver nanowires: an extensive parametric study. Cryst. Growth. Des. 11.11 (2011): 4963-4969. [20] V.S. Nguyen,D. Rouxel, R. Hadji, B. Vincent, Effect of ultrasonication and dispersion stability on the cluster size of alumina nanoscale particles in aqueous solutions. Ultrason. Sonochem. 18.1 (2011): 382-388. [21] P. Ramasamy, D.M. Seo, S.H. Kim, J. Kim, Effects of TiO2 shells on optical and thermal properties of silver nanowires. J. Mater. Chem. 22.23 (2012): 11651-11657. [22] Y. Sun, Silver nanowires–unique templates for functional nanostructures. Nanoscale 2.9 (2010): 1626-1642.

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List of Figures Figure 1 SEM image (left) and the corresponding electron diffraction pattern (right) of as-produced silver nanowires using polyol process Figure 2 TEM images showing the edge of untreated silver nanowire (left) and silver nanowire after 30 minutes of ultrasonciation (right) Figure 3 The effect of ultrasonication time on the fragmentation of silver nanowires: (a) 25% of the maximum power (b) 50% of the maximum power (c) 75% of the maximum power and (d) the average length of nanowires according to ultrasonication time at each power 25%, 50% and 75%. Figure 4 SEM images of Ag nanowires according to ultrasonication operation time at 50% of the maximum power: (a) 30 minutes (b) 60 minutes (c) 90 minutes and (d) 120 minutes Figure 5

Figure 6

SEM images showing agglomeration of nanowires when high power (75%) is applied: (a) for 60 minutes (b) and (c) for 90 minutes (d)for 120 minutes. Normalized UV-visible absorption spectra of Ag nanowires dispersed in water with ultrasonication time (0, 30, 60, 90 and 120 min). Photo images for the corresponding samples

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List of Table Table 1 Average length of the nanowires according to ultrasonication time and ultrasonication power

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Figure1_right

Figure2_left

Figure2_right

Figure3_a

Figure3_b

Figure3_c

Figure3_d

Figure4_a

Figure4_b

Figure4_c

Figure4_d

Figure5_a

Figure5_b

Figure5_c

Figure5_d

Figure6

Table 1 Average length of the nanowires according to time and ultrasonication power Time

Ultrasonication power

Average length

Standard deviation

(minutes)

(%)

(nm)

(nm)

30

25

1696.9

862.9

30

50

1250.5

600.5

30

75

944.3

494.9

60

25

1517.6

787.5

60

50

934.5

457.8

60

75

777.1

346.3

90

25

1147.6

692.6

90

50

791.7

383.9

90

75

612.3

264.0

120

25

1019.0

533.6

120

50

697.3

279.2

120

75

500.1

232.3

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Highlights ► An ultrasonic processor was used to produce silver NWs with low aspect ratio. ► UV absorption characteristic of fragmentized silver NWs was not changed. ► One dimensional structure of a nanowire was not influenced by the fragmentation process.

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Ultrasonication assisted production of silver nanowires with low aspect ratio and their optical properties.

We investigated a facial method to produce silver nanowires with low aspect ratio by fragmentizing as produced long silver nanowires. The length of si...
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