Synthesis of Ag nanoparticles using an electrolysis method and application to inkjet printing

Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 175–179

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Synthesis of Ag nanoparticles using an electrolysis method and application to inkjet printing Jin Min Cheon a , Jin Ha Lee a , Yongsul Song b , Jongryoul Kim a,∗ a b

Department of Metallurgy and Material Science, Hanyang University, Ansan, Kyeonggi-do 426-791, Republic of Korea Amogreentech Co., Ltd., Gimpo 415-868, Republic of Korea

a r t i c l e

i n f o

Article history: Received 31 May 2011 Received in revised form 31 July 2011 Accepted 28 August 2011 Available online 2 September 2011 Keywords: Nanoparticle Electrolysis Inkjet printing Sintering

a b s t r a c t This paper reports a new mass-production method for synthesizing Ag nano-particles using electrolysis. In this method, Ag ions were directly and continuously extracted from 99 wt% Ag metal electrodes and a reduction agent reduced them to Ag nano-particles. By changing the applied voltage and electrolyte temperature, the average size of the synthesized Ag particles was shown to vary in the range of 10–80 nm. These particles were tested for the possibility of micropatterning of the electrode on a polyimide substrate. As a result, patterns as fine as 50–100 ␮m with a relatively low resistivity of 6.5 ␮ cm were successfully obtained. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, printed electronics, particularly using ink-jet printing technology, have attracted much attention because they have significant advantages compared with conventional photolithography, such as simple process, reducing the production time and production waste [1–6]. In printed electronics it is necessary to fabricate micro-patterned metal interconnects with high conductivity. However, the development of affordable conductive ink is still a hurdle to commercializing printed electronics. Although printable conductive inks such as doped conjugated polymers or metal–organic complexes have been developed, continuous efforts to develop nano-metal ink have been made because metal ink would have high electrical conductivity and excellent electrical and thermal stabilities [7–15]. In general, metal nanoparticles, such as Au, Ag and Cu, have been chemically synthesized using salt, which requires additional washing, filtering, drying, and re-dispersion in other solvents after synthesis. Productivity improvement in nanoparticle synthesis is recognized as a critical issue for mass production of electro-magnetic devices using printing technologies. Some of recent studies was shown that the synthesis of metal ink directly using the chemical method [16]. This paper reports a new mass-production method for metal nanoparticles that utilizes electrolysis. With this method, metal ions are directly extracted from metal plates and then turned

∗ Corresponding author. Tel.: +82 31 400 4279. E-mail address: [email protected] (J. Kim). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.08.032

into metal nanoparticles by a reduction agent. This method avoids drying, cleaning and re-dispersion processes, which are generally required with chemical methods. Furthermore, Ag metal plates reduce the production cost of Ag nanoparticles without using expensive Ag compounds. The characteristics of these particles have been analyzed according to their processing conditions and a feasibility study of ink-jet printing was completed.

2. Experimental details 2.1. Synthesis of Ag nanoparticles In this experiment, two pure silver plates (5 cm × 10 cm × 2 cm, 99 wt%) were used as electrodes. The two electrodes were placed vertically face-to-face at a constant distance of 20 mm. The electrolyte consisted of citric acid (C6 H8 O7 ), dispersion agent and hydrazine monohydrate (N2 H4 ) in deionized (DI) water. Poly(Nvinylpyrrolidone) (PVP, Mw = 10,000 g/mol, Sigma–Aldrich) or Tween20 were used as the dispersion agent. In order to investigate the effects of processing conditions on the synthesis of Ag nanoparticles, the voltage, temperature and amount of hydrazine were varied. Different voltages (100, 200, 300 V) were applied in an electrolyte containing 2 mM citric acid, 1 wt% PVP and 20 mM hydrazine at 60 ◦ C. At a constant voltage of 100 V, electrolyte temperature (60, 80, 90 ◦ C) and the concentration of hydrazine (10, 20, 40 mM) at 80 ◦ C were also varied. The reaction time for all trials was fixed at 30 min.

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Fig. 1. (a) Schematic diagram of Ag nanoparticle formation by an electrolysis method. (b) Mechanism for synthesis of Ag nanoparticle.

2.2. Inkjet printing and sintering An inkjet printer with a piezo-type single nozzle with a diameter of 50 ␮m (UJ2400, Unijet) was used to draw patterns on DuPont polyimide films (PI, Kapton HN grade). The viscosity of the Ag ink (20 wt%) was controlled in the range of 22–25 mPa s. Uniform ejection of the droplets was obtained when a 35 V impulse lasting 20 ␮s at a frequency of 1 kHz was applied. After printing, patterns were dried in a vacuum oven at 60 ◦ C for 24 h and then sintered at 150–290 ◦ C for 1 h. 2.3. Characterization The microstructural changes of the metal nanoparticles as a function of reaction conditions were observed by a dynamic light scattering instrument (DLS, ELS-8000, Otsuka electronics), highresolution transmission electron microscopy (HR-TEM, 3010, JEOL) and an X-ray diffractometer (XRD, D/MAX-2500, Rigaku). The resistivity of the printing patterns was measured by a 4-point-probe (CMT-SR1000 N, Chang-min Co., Ltd.), and the pattern thickness was measured by a non-contact surface profiler (NV-P2020, Nanosystem Co., Ltd). The morphologies of the inkjet-printed conductive patterns were observed using a field emission scanning electron microscope (FE-SEM S-4700, Hitachi) 3. Results and discussion 3.1. Mechanism of electrolysis Fig. 1 shows a schematic diagram of the synthesis of silver nanoparticles by electrolysis. When voltage is applied, electrons are taken from the anode and then are replaced at the cathode. Silver in the anode goes into the electrolyte as silver cations. In the case of electro-deposition, these silver cations migrate to the cathode where reduction occurs with the formation of metallic clusters. However, in the present method, reductant (N2 H4 ) triggers the reduction of the silver cations to silver atoms in the electrolyte. These silver atoms form clusters and grow further to make insoluble particles in the solution, depending on the cluster-ion or clustercluster interaction. The growth and aggregation of these particles can be controlled by the steric hindrance effect of the dispersion agent [11]. When the electrolysis reaction is conducted in direct current (DC) for over 1 h, Ag oxides generated in the anode plate are

Fig. 2. (a) Product mass of Ag nano-particles according to the frequency of AC. (b) XRD patterns of synthesized silver nanoparticles.

observed. These oxides in the anode obstruct the flow of current and the reaction stops. In order to resolve this problem, an alternating current (AC) is applied. As shown in Fig. 2, the product mass of Ag particles is decreased by increasing the frequency of AC. However, Ag oxidation at the anode plate cannot be fully controlled at lower frequency. The optimal condition for the maximum product mass (2.15 g/h) of Ag nano-particles without formation of oxides is in a frequency range of 10–20 Hz, as shown in Fig. 2(a). The XRD pattern of the sample in AC and DC is shown in Fig. 2(b). All the peaks on the XRD pattern can be indexed to that of pure silver, respectively. 3.2. Dependence of particle size on reaction conditions In order to evaluate the effects of processing conditions on particle formation, the size changes of the Ag particles were measured, and the results are depicted in Fig. 3. As shown in Fig. 3(a) and (d)–(f), the average size of the Ag particles decreased from 55 to 31 nm as the voltage increased from 100 to 300 V. As shown in the figures, the distribution range as well as the size of Ag particles decreased with increasing voltage. When 100, 200 and 300 V were applied between electrodes, the currents at the electrode were 1.5, 3 and 4.5 A, respectively. This result shows that the particle size and distribution of Ag can be controlled by varying the applied voltage or current density. This dependence of particle size is in accord with the aggregate growth model [17].

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Fig. 3. Size distributions of Ag nanoparticles according to (a) voltage, (b) temperature and (c) the amount of reductant. Analysis of DLS: (d) 100 V, (e) 200 V and (f) 300 V and SEM micrographs of Ag nano-particles; (g) 60 ◦ C, (h) 80 ◦ C and (i) 90 ◦ C.

Fig. 3(b) shows the size changes of the synthesized Ag particles as a function of electrolyte temperature. When the electrolyte temperatures were 60, 80, and 90 ◦ C, the average sizes of the particles were 59, 37, and 23 nm, respectively. In the synthesis of nano-particles by nucleation from supersaturated solution, critical size represents the limit on how small nano-particles can be synthesized. Increasing the change of Gibbs free energy (G) is necessary to reduce the critical size and G can be increase by increasing temperature. Temperature can also influence surface energy. Surface energy of the solid nucleus can change more significantly near the roughing temperature. Fig. 3(c) indicates that particle size changes unvaried with concentrations of the reducing agent (10, 20, and 40 mM). However, contrary to the results of previous experiments, size distribution did not vary with the concentration of the reductant (33.8, 36.1, and 35.6 nm). On the other hand, Ag cations attached to the cathode were observed when the concentration of reductant was low (1 mM) in electrolyte. These results may indicate that the final size of the Ag particles does not

depend on the initial nucleus size, which is also in accord with the aggregate growth model [17]. 3.3. Ink-jet printing and sintering In order to demonstrate the applicability of metal-based conductive ink, complex patterns on flexible polyimide substrates were printed and are shown in Fig. 4. After solvent evaporation, a printed single ink droplet with a diameter of 45 ␮m produced a circular dot pattern with a diameter of 80 ␮m and a thickness of 2.4 ␮m. Line patterns were generated by reducing the dot-to-dot distance to 50 ␮m, which resulted in a continuous line with a line-width of 70 ␮m and a thickness of 1.8 ␮m. The difference in height of pattern was about 0.6 ␮m for both dot and line patterns. Heat-treatment of the printed patterns was required to remove solvent and to facilitate sintering between the metal particles. TGA was accomplished to investigate the thermal behavior of the Ag nanoparticles, as shown in Fig. 5(a). Significant weight loss was

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Fig. 4. (a) Ag conductive patterns ink-jet printed on polyimide substrates using Ag nano-particles and (b) confocal images of a single ink droplet after drying and printed lines.

Fig. 5. (a) TGA of silver nanoparticles; (b) resisitiviy variations of inkjet-printed patterns as a function of annealing temperature and (c) SEM micrographs of the conductive patterns as a fuction of annealing temperature.

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observed at 200 ◦ C in 78.9 wt% Ag nanoparticles using Tween20 as a surfactant. However, weight loss was indicated above 300 ◦ C in 89.8 wt% Ag nanoparticles using PVP as a surfactant. Fig. 5(b) shows the resistivity changes of the inkjet printed patterns as a function of sintering temperature. At around 150 ◦ C, the resistivity of the patterns was 536  cm. However, the resistivity decreased drastically at around 200 ◦ C, indicating that the conduction path between the particles was established by inter-particle neck formation; this corresponds with TGA data. Considering the melting point of the bulk silver, this result clearly shows a sintering effect for nanosized particles [18]. Resistivity of the Ag film that was annealed in air was 17.6 ␮ cm at 200 ◦ C and 6.5 ␮ cm at 250 ◦ C. Resistivity of the Ag film using PVP was relatively high at the same sintered temperature, 50.7 ␮ cm at 200 ◦ C and 10.5 ␮ cm at 250 ◦ C. Fig. 5(c) shows SEM images of the printed pattern with Ag ink sintered at different temperatures for 1 h. After annealing below 150 ◦ C, each particle seemed to be unchanged. The particles started to integrate with each other when sintered at 200 ◦ C. Significant inter-particle sintering was observed at 250 ◦ C. Most of the particles were completely connected to each other. Significant coarsening also occurred as the grain grew at the expense of the small particles. Because of this coarsening, the film became relatively porous, and this may adversely affect the conductivity. The microstructure of the Ag pattern capped by Tween20 was more densified than the Ag pattern capped by PVP, corresponding with TGA and resistivity results. 4. Conclusions We have studied the synthesis of Ag nanoparticles using an electrolysis method. Reductant (N2 H4 ) in electrolyte was shown to effectively trigger the reduction of silver cations to silver atoms. Relatively spherical Ag particles with a size range of 10–80 nm were created by controlling reaction parameters such as voltage and

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electrolyte temperature. Using this method, the maximum continuous yield ratio (2.15 g/h) of Ag nano-particles was achieved. Metal conductive ink was synthesized and printed onto a polyimide substrate. Continuous and smooth lines with a line-width of 70 ␮m were formed by inkjet printing. Upon heat treatment at 250 ◦ C for 1 h, the resistivity of the films was 6.5 ␮ cm, which was nearly three times greater than the bulk metal resistivity. These results successfully demonstrate direct writing of highly conductive lines using metal conductive ink synthesized by electrolysis. Acknowledgments This work was supported by “Development of direct nanopatterning technology” project of Korea Ministry of Knowledge Economy and the 2nd Brain Korea 21 Project in 2010. References [1] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123. [2] U. Zschieschang, H. Klauk, M. Halik, G. Schmid, C. dehm, Adv. Mater. 15 (2003) 1147. [3] H.M. Nur, J.H. Song, J. Mater. Sci. Mater. Electron. 13 (2002) 213. [4] P. Cooley, D. Wallace, B. Antohe, JALA 7 (2002) 33–39. [5] B.-J. Gans, P.C. Duineveld, U.S. Schubert, Adv. Mater. 16 (2004) 203. [6] D. Kim, S. Jeong, B. Park, J. Moon, Appl. Phys. Lett. 89 (2006) 264101. [7] M. Nakamoto, M. Yamamoto, Chem. Commun. 452 (2003). [8] K.J. Lee, B.H. Jun, T.H. Kim, J. Joung, Nanotechnology 17 (2006) 2424. [9] N.B. Bell, C.B. DiAntonio, D.B. Dimons, J. Mater. Res. J17 (2002) 2423. [10] I.K. Shim, Y. Lee, K.J. Lee, J. Joung, Mater. Chem. Phys. (2008) 316–321. [11] K.J. Lee, Y.H. Jeong, B. Lim, Nanosci. Nanotechnol. (2008) 5062. [12] Y. Lee, J.r. Choi, K.J. Lee, N.E. Stott, D. Kim, Nanotechnology 19 (2008) 415604. [13] C.H. Lo, T.T. Tsung, H.M. Lin, J. Alloy Compd. (2007) 434–435. [14] J. Simonet, J. Appl. Electrochem. 39 (2009) 1625. [15] M. Starowicz, B. Stypuła, J. Banasˇı, Electrochem. Commun. 8 (2006) 227. [16] A. Chiolerio, G. Maccioni, P. Martino, M. Cotto, P. Pandolfi, P. Rivolo, S. Ferrero, L. Scaltrito, Microelectron. Eng. 88 (2011) 2481. [17] D.L. Van Hyning, W.G. Klemperer, C.F. Zukoski, Langmuir 17 (2001) 3120. [18] P. Buffat, J-P.B. orel, Phys. Rev. 13 (1976) 2287.