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Synthesis of highly concentrated silver nanosol and its application to inkjet printing
Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 345–351
Synthesis of highly concentrated silver nanosol and its application to inkjet printing Beyong-Hwan Ryu ∗ , Youngmin Choi, Han-Sung Park, Jong-Hoon Byun, Kijeong Kong, Jeong-O. Lee, Hyunju Chang Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Taejeon 305-600, Republic of Korea Available online 18 October 2005
Abstract The synthesis of highly concentrated silver nanosol assisted by polyelectrolytes for inkjet method was studied. The role of polyelectrolytes on the synthesis of silver nanosol was studied by varying molecular weights and the molar ratios (R = [COO− ]/[Ag+ ]) of them. The concentrated silver nanosol was synthesized by reducing the cation (Ag+ ) and anionic polyelectrolytes (COO− ) complex including the step of initial nucleation and growth of silver nanoparticles to achieve dispersion stability and controlling the size of silver nanoparticles. The maximum concentration of batch-synthesized silver nanosol was possible to 40 wt%. At the same time, the particle size of silver nanoparticles was below 10–20 nm. The synthesized silver nanosol was tested for the possibility of micropatterning of electrode on ITO glass substrate. The connectivity and width of fine line depended largely on the wettability of silver nanosol on ITO glass. These relationships will be understood by wetting angle. The fine line of silver electrode as fine as 50–100 m was successfully formed on ITO glass substrate. © 2005 Elsevier B.V. All rights reserved. Keywords: Silver nanosol; Polyelectrolyte; Wetting; Ink and inkjet printing
1. Introduction Inkjet printing method is a very useful technology for microscale patterning of line or dot by ejecting tiny droplets of 10–100 m diameter onto a substrate. Recently, lots of research efforts were devoted to develop inkjet printing method as a patterning tool substituting screen printing and/or photolithography methods for making micron-sized patterns. Inkjet patterning has the following advantages over conventional photolithography methods [1–4]: (1) (2) (3) (4) (5)
it does not require a mask for direct patterning; it can be easily applied to large-size substrates; materials are used effectively; processing time is very short; the equipment is compact and requires minimal investment.
This inkjet patterning technique would also be useful for forming metal interconnects in flat panel displays, to reduce processing cost especially for plasma display (PDP) and other ∗
Corresponding author. Tel.: +82 42 860 7365. E-mail address: [email protected] (B.-H. Ryu).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.09.005
large size displays [1]. However, in order to obtain enough conductivity for bus and address electrodes of PDP interconnect, it would be necessary to develop a novel conductive ink and/or control its wetting properties on ITO glass substrate. The typical preparation methods of silver nanoparticles for conductive ink are well known, such as laser ablation or chemical reaction in solution. Although the conductive ink material has been prepared by laser ablation method already [1], it needs expensive equipment; therefore, it would be more favorable to develop direct chemical preparation method for conductive ink. The objective of this study is to develop novel conductive ink, which enables fabricating several tens of micrometer width ordered lines on ITO glass substrate by inkjet printing, as a key material for flat panel display. For an excellent conductive ink, following two conditions should be satisfied. The first conductive ink should be consisted of particles with narrow size distribution, and their dispersion stability is required for continuous ejecting at inkjet nozzles. The second, highly concentrated silver nanosol is required in order to achieve the well-connected fine microlined electrode with high conductivity after heat treatment. In this work, we have carried out the synthesis of highly concentrated silver nanoparticles assisted by polyelectrolytes. The
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complex effect of polyelectrolytes with silver ions on the particle size distribution of silver nanosol was studied. The highly concentrated silver nanosol was also prepared with varying of molecular weight of polyelectrolytes and control of initial nucleation and growth of silver nanoparticles. We can achieve the stable silver nanosol and control the size of silver nanoparticles. Furthermore, the printability of synthesized silver nanosol on ITO glass substrate was investigated. We have also tried to find out the relationship between wetting angle and line connectivity of silver nanosol on ITO glass substrate. From these results, we intended to discuss the synthesis condition of nanoparticles as well as the role of polyelectrolytes on the synthesis. The synthesized silver nanosol was patterned on ITO glass substrate by inkjet printing, to explore the possibility of using them as versatile microelectrodes. We also controlled surface properties of ITO glass substrate by treating them with cationic polyelectrolytes. Finally, we studied the relationship between formation of silver nanosol microelectrodes and wetting property of ITO glass substrate. 2. Experiment Highly concentrated silver nanosol was prepared from AgNO3 (reagent; <99%, Aldrich) as silver source materials with NaBH4 and/or hydrazine monohydrate (reagent; 97%, H2 NNH2 ·H2 O, hydrazine, Aldrich) as reducing agent. The polyelectrolytes (polyacrylic ammonium salt; Mw = 1200–30,000, Aldrich) was used as the best assisting material to prepare silver nanoparticles. The colloidal silver sol was prepared as followed. Ice-cold solution of AgNO3 (10–40 wt% Ag) that contains various molecular weights (Mw) of polyelectrolytes was reduced by adding hydrazine monohydrate and/or NaBH4 slowly. The formation of Ag(0) nanoparticles was confirmed by FE-TEM (EM912, Carl Zeiss, Germany). The particle size and zeta potential of silver nanosol were measured by the Zetasizer (ELS-800, Otsuka, Japan) after diluting them by 10,000 times. The printability of silver nanosol was explored by using custom-made inkjet printer. Bare and surfactant-modified ITO
glass substrates, and slide glass were used as substrate for inkjet printing, to compare the printability. The contact angle of water and synthesized silver nanosol, which containing anionic poyelectrolytes, on substrates was measured by contact angle analyzer (Phoenix-300, SEO, Korea). In case of modified ITO glass substrate, polyethylenimine (PEI; H( NHCH2 CH2 )n NH2 , Aldrich, cationic poyelectrolytes) was used after diluting them to 10–10,000 ppm with distilled water. To prepare the modified substrates, the ITO glass substrates were dipped in each diluted surfactant solution for 2 h, and then followed by drying at 80 ◦ C for 12 h. The microlines of silver nanosol on the proper substrate were fabricated by on-demanded type inkjet printer, and then observed by optical microscope (ICS-305A, Sometech, Korea) with 300 times magnifications. 3. Results and discussions 3.1. Synthesis of highly concentrated silver nanosol The role of polyelectrolytes on the synthesis of silver nanosol was studied by varying molecular weights and the molar ratios (R = [COO− ]/[Ag+ ]) of them. The 10 wt% of silver nanosol (nanoparticles) was synthesized by reduction the cation (Ag+ ) and anionic polyelectrolytes (COO− ) complex. To investigate the effect of complex on the synthesis of silver nanosol, the molar ratio (R = [COO− ]/[Ag+ ]) was changed in the range of 0, 0.025, 0.1, 0.25, 0.5, 1, 1.5 and 2. The particle size and shape, and particle size distribution of the silver nanosol was shown in Figs. 1 and 2, respectively. The particle size of silver nanosol assisted by the polyelectrolytes (Mw = 15,000) was typically kept below 10–20 nm as shown in Fig. 1. In preliminary experiment, we could obtain the silver nanoparticles smaller than 10 nm with the adding speed of 0.5–1.0 mL/min of 0.5 mM NaBH4 solution. The expected reducing reaction is as follows [5]: 8AgNO3 + NaBH4 + 2H2 O → 8Ag(0) ↓ + NaBO2 + 8HNO3
(1)
Fig. 1. TEM image of 10 wt% silver nanosol assisted by various concentrations of polyelectrolyte; R = [COO− ]/[Ag+ ] (R = 0–2). (a) 0, (b) 0.025, (c) 0.1, (d) 0.25, (e) 0.5, (f) 1.0, (g) 1.5 and (h) 2.0.
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Fig. 2. Particle size distribution of 10 wt% silver nanosol assisted by various concentrations of polyelectrolytes; R = [COO− ]/[Ag+ ] (R = 0.025–2). (a) 0.025, (b) 0.1, (c) 0.25, (d) 0.5, (e) 1.0, (f) 1.5 and (g) 2.0.
Since reduction potential of noble metal, such as Ag, Pt and Au, is very high, the noble metal nanoparticles are more stable than their ionic state. Therefore, the control of reducing rate of these noble metal ions could be very important to reproduce homogeneous particle size and shape [6–9]. For this reason, the polyelectrolytes will play an important role in synthesis of noble metal nanopaticles. The success of highly concentrated silver nanosol synthesis assisted by polyelectrolytes can be explained as follows: the complex effect of COO− group in polyelectrolytes with Ag+ ion will help for preparing silver nanoparticles, since the limited migration of Ag+ ion for reduction results in a control of nanoparticle nucleation and growth. In case of R = 0 (without polyelectrolytes), the particle size is 200–300 nm, which is more than 10 times larger than that of silver nanoparticles at R = 0.025,
and especially several 10 times larger than silver nanoparticles at R = 0.1–1.0. Furthermore, the polyelectrolytes, in which every segment in polyelectrolytes has anionic functional group, are more efficient compared with simple surfactant. It was considered that the polyelectrolytes provided the proper nucleation rate at initial stage and sufficient charge density on silver nanoparticles resulted in stabilizing silver colloids. Thus, the 10–20 nm sized Ag(0) nanoparticles can be produced easily by introducing reducing agent slowly. To conclude, the ratio of [COO− ] to [Ag+ ] is related with the degree of complex formation, which affects the particle size and size distribution. Another important role of polyelectrolyte is contribution to the dispersion of silver nanoparticles after the synthesis. To control the particles size and distribution, the
Fig. 3. TEM image of 10 wt% silver nanosol assisted by various molecular weights of polyelectrolyte at R = 0.5. (a) 1200, (b) 2100, (c) 5000, (d) 8000, (e) 15,000 and (f) 30,000.
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Fig. 4. Particle size distribution of 10 wt% silver nanosol assisted by various molecular weights of polyelectrolyte at R = 0.5. (a) 1200, (b) 2100, (c) 5000, (d) 8000, (e) 15,000 and (f) 30,000.
nucleation and crystal growth is very important [10,11], which is controlled by reducing power and reducing speed. Table 1 shows the Zeta potential of silver nanosol with change of R-value. It was found that the R-value of polyelectrolytes is dependent on the surface charge, which is deeply related with dispersion of silver nanosol. Below R = 0.025, the surface charge is not enough, while over R = 0.1, surface charge is greater than |−25 mV|. It means that the silver sol shows good dispersion. To consider the dispersion stability of silver sol, the surface charge should be over |−25 mV|, because the repulsion force should be greater than thermal energy barrier, kB T (kB , Boltzmann constant; T, absolute temperature) to overcome this force [12]. The silver nanosol has −39 to −45 mV in the range of R = 0.1–1.0. That means the silver nanosol in this range has good dispersion stability. In conclusion, it can be confirmed that the polyelectrolytes in the range of R = 0.1–1.0 play an important role in the synthesis of silver nanoparticles as a complex agent with Ag+ ion and COO− ion, and develop surface charges of silver nanoparticles as a dispersion agent.
To investigate the effect of Mw of polyelectrolytes on the synthesis of silver nanosol, the molar ratio (R = [COO− ]/[Ag+ ]) was kept constant at R = 0.5. The particle size and shape, and particle size distribution of the silver nanosol was shown in Figs. 3 and 4, respectively. The particle size of silver nanosol assisted by the polyelectrolytes was typically kept below 5 nm as shown in Figs. 3 and 4. Especially, Dp 50 of silver nanosol was best assisted with Mw = 15,000, while that was over 5 nm in case of Mw = 30,000. The particle sizes (Dp 50) of synthesized silver nanoparticles was summarized in Fig. 5. The Mw of polyelectrolytes and R were varied from 1200 to 30,000 and from 0.025 to 2, respectively. Most of silver nanosol could be prepared below 10 nm
Table 1 Zeta potential of silver nanosol assisted by various concentrations of polyelectrolytes Polyelectrolytes (R)
Zeta potential (mV)
Stability of silver nanosol
0 0.05 0.1 0.5 1.0
−5.5 −23.5 −39.1 −46.1 −45.2
×
(
) excellent; () good; (×) bad.
Fig. 5. Particle size (Dp 50) of sliver nanosol assisted by variation of adding molar ratio (R) and Mw of polyelectrolytes.
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Fig. 6. TEM image of various concentration of silver nanosol synthesized with Mw = 15,000: (a) 10 wt%, (b) 20 wt%, (c) 30 wt% and (d) 40 wt%.
diameter, especially in the range of R = 0.1–1.0. In this range of R, the effect of Mw of polyelectrolytes on the particle size of Ag sol can be considered. Mw of 15,000 of polyelectrolytes can provide the best Ag nanosol in this range of R. The Mw of 15,000 of polyelectrolytes was the best at R = 0.1–1.0 as shown in Fig. 5. Therefore, the size and dispersion stability of silver
nanosol have depended largely on molecular weight of polyelectrolytes, as well as their molar concentrations that determines initial nucleation and growth of silver nanoparticles. The highly concentrated silver nanosol from 10 to 40 wt% was prepared at R = 0.5 as the molar ratio of reducing agent to Ag ion was kept at 1.5. The results are shown in Figs. 6 and 7. The
Fig. 7. Particle size distribution of various concentration of silver nanosol synthesized with Mw = 15,000: (a) 10 wt%, (b) 20 wt%, (c) 30 wt% and (d) 40 wt%.
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Fig. 8. Contact angle of water (a–c) and silver nanosol (d–f) on different substrate: (a and d) slide glass, (b and e) bare ITO substrate and (c and f) ITO substrate treated with 100 ppm PEI.
particle size of all of them was below 10 nm, even though the particle size of them increased with increasing concentration. Finally, the highest concentration of batch-synthesized silver nanosol was achieved successfully to 40 wt%. 3.2. Printability of synthesized silver nanosol The 10 wt% synthesized silver nanosol with smaller than 10 nm size of silver as shown in Fig. 6a was tested to explore the possibility of micropatterning of electrode on ITO glass substrate. The line of silver electrode as fine as 50–100 m was formed on ITO glass substrate. It is found that the synthesized silver nanoparticles have good dispersion stability, due to the surface charge of −45 mV acquired from added polyelectrolytes. The rheological behavior of 10 wt% silver nanosol has a “thixotropic behavior”, which is a relatively high viscosity at low shear rate and a relatively low viscosity at high shear rate. This behavior predicts that the silver nanosol can be easily ejected with low viscosity, and then the microline is easily formed with relatively high viscosity after being ejected through the fine hole in inkjet printer. A synthesized silver nanosol was patterned on ITO glass substrates by inkjet printing. The contact angles of water and silver nanosol on three kinds of substrates were investigated as shown in Fig. 8. The contact angle shows the wettability of liquid on solid. For example, the low contact angle means high wettability. The contact angle of water and silver nanosol were as low as 27.1◦ and 21.6◦ for glass substrate, while as high as 82.7◦ and 66.1◦ for bare ITO glass substrate, respectively. The wettablity of aqueous silver nanosol has almost the same as that of water, but the wetting behavior of bare ITO glass substrate is quite different from that of slide glass substrate. The silver nanosol was printed on glass substrate by a inkjet printer,
Fig. 9. Optical microscope images of silver nanosol microlines on different substrates (×300): (a) bare ITO glass substrate and (b) ITO glass substrate coated with 100 ppm PEI.
then relatively thick line of 100 m line was formed because the wettability of silver nanosol was good. As shown in Fig. 9a, silver nanosol forms dots as large as over 100 m on bare ITO glass substrates. Due to the high wetting angles of water and silver nanosol, only dot-shaped patterns can be formed on bare ITO glass substrate. To improve wettability of bare ITO glass substrate, we have treated bare ITO glass substrate with 100 ppm of PEI. The contact angles of water and silver nanosol on 100 ppm of PEI-treated ITO glass substrate were dramatically changed from 82.7◦ and 66.1◦ to 53.4◦ and 40.3◦ , respectively. Also, uniform microline with 60 m width was formed on PEI treated ITO glass substrate as shown in Fig. 9b. This implies that the printability of silver nanosol was improved as the surface property of modified ITO glass substrate has changed to more wettable than that of bare ITO. As a further study, the bare ITO glass substrate was treated with various concentration of PEI in order to improve the wetting property of bare ITO glass substrate as shown in Fig. 9. The contact angle was decreased drastically at 10 ppm, and saturated slowly after that. Thus, it is found that the moderate concentration of PEI on bare ITO glass substrate is about 100 ppm. 4. Conclusion We have studied the synthesis of silver nanosol for micropatterning of electrode on ITO glass substrate. The size of silver nanoparticle has been depended largely on the Mw and amount of polyelectrolytes as well as adding speed and/or amount of reducing agent.
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The synthesis of silver nanoparticle assisted with polyelectroyte, which molecular weight is 15,000, was possible in the wide range of R = 0.1–1.0. The 10–20 nm sized silver(0) nanopaticles were produced in case of the slow adding of reducing agent. The maximum concentration of batch-synthesized Ag nanoparticles was up to 40 wt%. The synthesized silver nanosol was tested for printability to explore the possibility of microelectrode patterning on ITO glass substrate. The silver microelectrodes with 50–100 m line width were formed on ITO glass substrate treated with 100 ppm of PEI.
[2]
[3]
[4]
[5]
Acknowledgements This work (M1-020-KR-10-0001-02-k18-01-025-I-2) was supported from Information Display R&D Center one of the 21th Century frontier R&D Program funded by the Ministry of Commerce, Industry & Energy of Korean government. References [1] M. Furusawa, T. Hashimoto, M. Ishida, T. Shimoda, H. Hasei, T. Hirai, H. Kiguchi, H. Aruga, M. Oda, N. Saito, H. Iwashige, N. Abe, S. Fukura,
[6] [7] [8] [9] [10] [11] [12]
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K. Betsui, Technical Digest of The Society of Information Display’02, 2002, p. 753. T. Shimoda, S. Kanbe, H. Kobayashi, S. Seki, H. Kiguti, I. Yudasaka, M. Kimura, S. Miyashita, R.H. Friend, J.H. Burroughes, C.R. Towns, Technical Digest of The Society of Information Display’99 (1999), p. 376. S. Miyashita, Y. Imamura, H. Takeshita, M. Atobe, O. Yokoyama, Y. Matsueda, T. Miyazawa, M. Nishimaki, Proceedings of the Asia Display/International Display of Workshop’01, 2001, p. 1399. M. Grove, D. Hayes, R. Cox, D. Wallace, J. Caruso, M. HampdenSmith, T. Kodas, K. Kunze, A. Ludviksson, S. Pennino, D. Skamser, Proceedings of the Display Works, 1999, p. 99. C.F. Tsang, J. Kim, A. Manthiram, J. Mater. Chem. 8 (2) (1998) 425. R.C. Johnson, J. Li, J.T. Hupp, G.C. Schatz, Chem. Phys. Lett. 356 (5–6) (2002) 534. T. Ung, M. Giersig, D. Dunstan, P. Mulvaney, Langmuir 13 (6) (1997) 1773. K.-S. Chou, C.-Y. Ren, Mater. Chem. Phys. 64 (3) (2000) 241. L.M. Bronstein, O.A. Platonova, A.N. Yakunin, I.M. Yanovskaya, P.M. Valetsky, Langmuir 14 (2) (1998) 252. Y. Li, T. Ishigaki, J. Cryst. Growth 242 (3–4) (2002) 511. L. Brecevic, in: A.T. Hubbard (Ed.), Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2002, p. 1289. J.S. Reed, Introduction to the Principle of Ceramic Processing, John Willey & Sons, New York, 1988 (Chapter 10).
Synthesis of highly concentrated silver nanosol and its application to inkjet printing Beyong-Hwan Ryu ∗ , Youngmin Choi, Han-Sung Park, Jong-Hoon Byun, Kijeong Kong, Jeong-O. Lee, Hyunju Chang Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Taejeon 305-600, Republic of Korea Available online 18 October 2005
Abstract The synthesis of highly concentrated silver nanosol assisted by polyelectrolytes for inkjet method was studied. The role of polyelectrolytes on the synthesis of silver nanosol was studied by varying molecular weights and the molar ratios (R = [COO− ]/[Ag+ ]) of them. The concentrated silver nanosol was synthesized by reducing the cation (Ag+ ) and anionic polyelectrolytes (COO− ) complex including the step of initial nucleation and growth of silver nanoparticles to achieve dispersion stability and controlling the size of silver nanoparticles. The maximum concentration of batch-synthesized silver nanosol was possible to 40 wt%. At the same time, the particle size of silver nanoparticles was below 10–20 nm. The synthesized silver nanosol was tested for the possibility of micropatterning of electrode on ITO glass substrate. The connectivity and width of fine line depended largely on the wettability of silver nanosol on ITO glass. These relationships will be understood by wetting angle. The fine line of silver electrode as fine as 50–100 m was successfully formed on ITO glass substrate. © 2005 Elsevier B.V. All rights reserved. Keywords: Silver nanosol; Polyelectrolyte; Wetting; Ink and inkjet printing
1. Introduction Inkjet printing method is a very useful technology for microscale patterning of line or dot by ejecting tiny droplets of 10–100 m diameter onto a substrate. Recently, lots of research efforts were devoted to develop inkjet printing method as a patterning tool substituting screen printing and/or photolithography methods for making micron-sized patterns. Inkjet patterning has the following advantages over conventional photolithography methods [1–4]: (1) (2) (3) (4) (5)
it does not require a mask for direct patterning; it can be easily applied to large-size substrates; materials are used effectively; processing time is very short; the equipment is compact and requires minimal investment.
This inkjet patterning technique would also be useful for forming metal interconnects in flat panel displays, to reduce processing cost especially for plasma display (PDP) and other ∗
Corresponding author. Tel.: +82 42 860 7365. E-mail address: [email protected] (B.-H. Ryu).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.09.005
large size displays [1]. However, in order to obtain enough conductivity for bus and address electrodes of PDP interconnect, it would be necessary to develop a novel conductive ink and/or control its wetting properties on ITO glass substrate. The typical preparation methods of silver nanoparticles for conductive ink are well known, such as laser ablation or chemical reaction in solution. Although the conductive ink material has been prepared by laser ablation method already [1], it needs expensive equipment; therefore, it would be more favorable to develop direct chemical preparation method for conductive ink. The objective of this study is to develop novel conductive ink, which enables fabricating several tens of micrometer width ordered lines on ITO glass substrate by inkjet printing, as a key material for flat panel display. For an excellent conductive ink, following two conditions should be satisfied. The first conductive ink should be consisted of particles with narrow size distribution, and their dispersion stability is required for continuous ejecting at inkjet nozzles. The second, highly concentrated silver nanosol is required in order to achieve the well-connected fine microlined electrode with high conductivity after heat treatment. In this work, we have carried out the synthesis of highly concentrated silver nanoparticles assisted by polyelectrolytes. The
346
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complex effect of polyelectrolytes with silver ions on the particle size distribution of silver nanosol was studied. The highly concentrated silver nanosol was also prepared with varying of molecular weight of polyelectrolytes and control of initial nucleation and growth of silver nanoparticles. We can achieve the stable silver nanosol and control the size of silver nanoparticles. Furthermore, the printability of synthesized silver nanosol on ITO glass substrate was investigated. We have also tried to find out the relationship between wetting angle and line connectivity of silver nanosol on ITO glass substrate. From these results, we intended to discuss the synthesis condition of nanoparticles as well as the role of polyelectrolytes on the synthesis. The synthesized silver nanosol was patterned on ITO glass substrate by inkjet printing, to explore the possibility of using them as versatile microelectrodes. We also controlled surface properties of ITO glass substrate by treating them with cationic polyelectrolytes. Finally, we studied the relationship between formation of silver nanosol microelectrodes and wetting property of ITO glass substrate. 2. Experiment Highly concentrated silver nanosol was prepared from AgNO3 (reagent; <99%, Aldrich) as silver source materials with NaBH4 and/or hydrazine monohydrate (reagent; 97%, H2 NNH2 ·H2 O, hydrazine, Aldrich) as reducing agent. The polyelectrolytes (polyacrylic ammonium salt; Mw = 1200–30,000, Aldrich) was used as the best assisting material to prepare silver nanoparticles. The colloidal silver sol was prepared as followed. Ice-cold solution of AgNO3 (10–40 wt% Ag) that contains various molecular weights (Mw) of polyelectrolytes was reduced by adding hydrazine monohydrate and/or NaBH4 slowly. The formation of Ag(0) nanoparticles was confirmed by FE-TEM (EM912, Carl Zeiss, Germany). The particle size and zeta potential of silver nanosol were measured by the Zetasizer (ELS-800, Otsuka, Japan) after diluting them by 10,000 times. The printability of silver nanosol was explored by using custom-made inkjet printer. Bare and surfactant-modified ITO
glass substrates, and slide glass were used as substrate for inkjet printing, to compare the printability. The contact angle of water and synthesized silver nanosol, which containing anionic poyelectrolytes, on substrates was measured by contact angle analyzer (Phoenix-300, SEO, Korea). In case of modified ITO glass substrate, polyethylenimine (PEI; H( NHCH2 CH2 )n NH2 , Aldrich, cationic poyelectrolytes) was used after diluting them to 10–10,000 ppm with distilled water. To prepare the modified substrates, the ITO glass substrates were dipped in each diluted surfactant solution for 2 h, and then followed by drying at 80 ◦ C for 12 h. The microlines of silver nanosol on the proper substrate were fabricated by on-demanded type inkjet printer, and then observed by optical microscope (ICS-305A, Sometech, Korea) with 300 times magnifications. 3. Results and discussions 3.1. Synthesis of highly concentrated silver nanosol The role of polyelectrolytes on the synthesis of silver nanosol was studied by varying molecular weights and the molar ratios (R = [COO− ]/[Ag+ ]) of them. The 10 wt% of silver nanosol (nanoparticles) was synthesized by reduction the cation (Ag+ ) and anionic polyelectrolytes (COO− ) complex. To investigate the effect of complex on the synthesis of silver nanosol, the molar ratio (R = [COO− ]/[Ag+ ]) was changed in the range of 0, 0.025, 0.1, 0.25, 0.5, 1, 1.5 and 2. The particle size and shape, and particle size distribution of the silver nanosol was shown in Figs. 1 and 2, respectively. The particle size of silver nanosol assisted by the polyelectrolytes (Mw = 15,000) was typically kept below 10–20 nm as shown in Fig. 1. In preliminary experiment, we could obtain the silver nanoparticles smaller than 10 nm with the adding speed of 0.5–1.0 mL/min of 0.5 mM NaBH4 solution. The expected reducing reaction is as follows [5]: 8AgNO3 + NaBH4 + 2H2 O → 8Ag(0) ↓ + NaBO2 + 8HNO3
(1)
Fig. 1. TEM image of 10 wt% silver nanosol assisted by various concentrations of polyelectrolyte; R = [COO− ]/[Ag+ ] (R = 0–2). (a) 0, (b) 0.025, (c) 0.1, (d) 0.25, (e) 0.5, (f) 1.0, (g) 1.5 and (h) 2.0.
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Fig. 2. Particle size distribution of 10 wt% silver nanosol assisted by various concentrations of polyelectrolytes; R = [COO− ]/[Ag+ ] (R = 0.025–2). (a) 0.025, (b) 0.1, (c) 0.25, (d) 0.5, (e) 1.0, (f) 1.5 and (g) 2.0.
Since reduction potential of noble metal, such as Ag, Pt and Au, is very high, the noble metal nanoparticles are more stable than their ionic state. Therefore, the control of reducing rate of these noble metal ions could be very important to reproduce homogeneous particle size and shape [6–9]. For this reason, the polyelectrolytes will play an important role in synthesis of noble metal nanopaticles. The success of highly concentrated silver nanosol synthesis assisted by polyelectrolytes can be explained as follows: the complex effect of COO− group in polyelectrolytes with Ag+ ion will help for preparing silver nanoparticles, since the limited migration of Ag+ ion for reduction results in a control of nanoparticle nucleation and growth. In case of R = 0 (without polyelectrolytes), the particle size is 200–300 nm, which is more than 10 times larger than that of silver nanoparticles at R = 0.025,
and especially several 10 times larger than silver nanoparticles at R = 0.1–1.0. Furthermore, the polyelectrolytes, in which every segment in polyelectrolytes has anionic functional group, are more efficient compared with simple surfactant. It was considered that the polyelectrolytes provided the proper nucleation rate at initial stage and sufficient charge density on silver nanoparticles resulted in stabilizing silver colloids. Thus, the 10–20 nm sized Ag(0) nanoparticles can be produced easily by introducing reducing agent slowly. To conclude, the ratio of [COO− ] to [Ag+ ] is related with the degree of complex formation, which affects the particle size and size distribution. Another important role of polyelectrolyte is contribution to the dispersion of silver nanoparticles after the synthesis. To control the particles size and distribution, the
Fig. 3. TEM image of 10 wt% silver nanosol assisted by various molecular weights of polyelectrolyte at R = 0.5. (a) 1200, (b) 2100, (c) 5000, (d) 8000, (e) 15,000 and (f) 30,000.
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Fig. 4. Particle size distribution of 10 wt% silver nanosol assisted by various molecular weights of polyelectrolyte at R = 0.5. (a) 1200, (b) 2100, (c) 5000, (d) 8000, (e) 15,000 and (f) 30,000.
nucleation and crystal growth is very important [10,11], which is controlled by reducing power and reducing speed. Table 1 shows the Zeta potential of silver nanosol with change of R-value. It was found that the R-value of polyelectrolytes is dependent on the surface charge, which is deeply related with dispersion of silver nanosol. Below R = 0.025, the surface charge is not enough, while over R = 0.1, surface charge is greater than |−25 mV|. It means that the silver sol shows good dispersion. To consider the dispersion stability of silver sol, the surface charge should be over |−25 mV|, because the repulsion force should be greater than thermal energy barrier, kB T (kB , Boltzmann constant; T, absolute temperature) to overcome this force [12]. The silver nanosol has −39 to −45 mV in the range of R = 0.1–1.0. That means the silver nanosol in this range has good dispersion stability. In conclusion, it can be confirmed that the polyelectrolytes in the range of R = 0.1–1.0 play an important role in the synthesis of silver nanoparticles as a complex agent with Ag+ ion and COO− ion, and develop surface charges of silver nanoparticles as a dispersion agent.
To investigate the effect of Mw of polyelectrolytes on the synthesis of silver nanosol, the molar ratio (R = [COO− ]/[Ag+ ]) was kept constant at R = 0.5. The particle size and shape, and particle size distribution of the silver nanosol was shown in Figs. 3 and 4, respectively. The particle size of silver nanosol assisted by the polyelectrolytes was typically kept below 5 nm as shown in Figs. 3 and 4. Especially, Dp 50 of silver nanosol was best assisted with Mw = 15,000, while that was over 5 nm in case of Mw = 30,000. The particle sizes (Dp 50) of synthesized silver nanoparticles was summarized in Fig. 5. The Mw of polyelectrolytes and R were varied from 1200 to 30,000 and from 0.025 to 2, respectively. Most of silver nanosol could be prepared below 10 nm
Table 1 Zeta potential of silver nanosol assisted by various concentrations of polyelectrolytes Polyelectrolytes (R)
Zeta potential (mV)
Stability of silver nanosol
0 0.05 0.1 0.5 1.0
−5.5 −23.5 −39.1 −46.1 −45.2
×
(
) excellent; () good; (×) bad.
Fig. 5. Particle size (Dp 50) of sliver nanosol assisted by variation of adding molar ratio (R) and Mw of polyelectrolytes.
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Fig. 6. TEM image of various concentration of silver nanosol synthesized with Mw = 15,000: (a) 10 wt%, (b) 20 wt%, (c) 30 wt% and (d) 40 wt%.
diameter, especially in the range of R = 0.1–1.0. In this range of R, the effect of Mw of polyelectrolytes on the particle size of Ag sol can be considered. Mw of 15,000 of polyelectrolytes can provide the best Ag nanosol in this range of R. The Mw of 15,000 of polyelectrolytes was the best at R = 0.1–1.0 as shown in Fig. 5. Therefore, the size and dispersion stability of silver
nanosol have depended largely on molecular weight of polyelectrolytes, as well as their molar concentrations that determines initial nucleation and growth of silver nanoparticles. The highly concentrated silver nanosol from 10 to 40 wt% was prepared at R = 0.5 as the molar ratio of reducing agent to Ag ion was kept at 1.5. The results are shown in Figs. 6 and 7. The
Fig. 7. Particle size distribution of various concentration of silver nanosol synthesized with Mw = 15,000: (a) 10 wt%, (b) 20 wt%, (c) 30 wt% and (d) 40 wt%.
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Fig. 8. Contact angle of water (a–c) and silver nanosol (d–f) on different substrate: (a and d) slide glass, (b and e) bare ITO substrate and (c and f) ITO substrate treated with 100 ppm PEI.
particle size of all of them was below 10 nm, even though the particle size of them increased with increasing concentration. Finally, the highest concentration of batch-synthesized silver nanosol was achieved successfully to 40 wt%. 3.2. Printability of synthesized silver nanosol The 10 wt% synthesized silver nanosol with smaller than 10 nm size of silver as shown in Fig. 6a was tested to explore the possibility of micropatterning of electrode on ITO glass substrate. The line of silver electrode as fine as 50–100 m was formed on ITO glass substrate. It is found that the synthesized silver nanoparticles have good dispersion stability, due to the surface charge of −45 mV acquired from added polyelectrolytes. The rheological behavior of 10 wt% silver nanosol has a “thixotropic behavior”, which is a relatively high viscosity at low shear rate and a relatively low viscosity at high shear rate. This behavior predicts that the silver nanosol can be easily ejected with low viscosity, and then the microline is easily formed with relatively high viscosity after being ejected through the fine hole in inkjet printer. A synthesized silver nanosol was patterned on ITO glass substrates by inkjet printing. The contact angles of water and silver nanosol on three kinds of substrates were investigated as shown in Fig. 8. The contact angle shows the wettability of liquid on solid. For example, the low contact angle means high wettability. The contact angle of water and silver nanosol were as low as 27.1◦ and 21.6◦ for glass substrate, while as high as 82.7◦ and 66.1◦ for bare ITO glass substrate, respectively. The wettablity of aqueous silver nanosol has almost the same as that of water, but the wetting behavior of bare ITO glass substrate is quite different from that of slide glass substrate. The silver nanosol was printed on glass substrate by a inkjet printer,
Fig. 9. Optical microscope images of silver nanosol microlines on different substrates (×300): (a) bare ITO glass substrate and (b) ITO glass substrate coated with 100 ppm PEI.
then relatively thick line of 100 m line was formed because the wettability of silver nanosol was good. As shown in Fig. 9a, silver nanosol forms dots as large as over 100 m on bare ITO glass substrates. Due to the high wetting angles of water and silver nanosol, only dot-shaped patterns can be formed on bare ITO glass substrate. To improve wettability of bare ITO glass substrate, we have treated bare ITO glass substrate with 100 ppm of PEI. The contact angles of water and silver nanosol on 100 ppm of PEI-treated ITO glass substrate were dramatically changed from 82.7◦ and 66.1◦ to 53.4◦ and 40.3◦ , respectively. Also, uniform microline with 60 m width was formed on PEI treated ITO glass substrate as shown in Fig. 9b. This implies that the printability of silver nanosol was improved as the surface property of modified ITO glass substrate has changed to more wettable than that of bare ITO. As a further study, the bare ITO glass substrate was treated with various concentration of PEI in order to improve the wetting property of bare ITO glass substrate as shown in Fig. 9. The contact angle was decreased drastically at 10 ppm, and saturated slowly after that. Thus, it is found that the moderate concentration of PEI on bare ITO glass substrate is about 100 ppm. 4. Conclusion We have studied the synthesis of silver nanosol for micropatterning of electrode on ITO glass substrate. The size of silver nanoparticle has been depended largely on the Mw and amount of polyelectrolytes as well as adding speed and/or amount of reducing agent.
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The synthesis of silver nanoparticle assisted with polyelectroyte, which molecular weight is 15,000, was possible in the wide range of R = 0.1–1.0. The 10–20 nm sized silver(0) nanopaticles were produced in case of the slow adding of reducing agent. The maximum concentration of batch-synthesized Ag nanoparticles was up to 40 wt%. The synthesized silver nanosol was tested for printability to explore the possibility of microelectrode patterning on ITO glass substrate. The silver microelectrodes with 50–100 m line width were formed on ITO glass substrate treated with 100 ppm of PEI.
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Acknowledgements This work (M1-020-KR-10-0001-02-k18-01-025-I-2) was supported from Information Display R&D Center one of the 21th Century frontier R&D Program funded by the Ministry of Commerce, Industry & Energy of Korean government. References [1] M. Furusawa, T. Hashimoto, M. Ishida, T. Shimoda, H. Hasei, T. Hirai, H. Kiguchi, H. Aruga, M. Oda, N. Saito, H. Iwashige, N. Abe, S. Fukura,
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