Facile Inkjet Printing Using Silver Precursor with Controllable Surface Tension for Fabricating Ultra Pliable Paper Electrode Seoungwoong Park, Siyong Park, Taeheon Kim, Dain Kwak, Sangki Park, Hochung Ryu, and Jong-Jin Park* School of Polymer Science and Engineering, Chonnam National University, Gwangju 61186, Korea (E-mail:
[email protected])
M
Home inkjet printer
Silver precursor
Y
We were able to create surfactants and electrodes, and to prove the probability of correlations between conductivities by using contact angle measurements. Using this approach, where the silver precursor and reducing agent formed an in situ chemical reduction, the printed solution state pre-electrode was able to form a long-range percolation network.
Printed electrode Reducing agent
REPRINTED FROM
Vol.46 No.3
2017 p.299–302 CMLTAG March 5, 2017
The Chemical Society of Japan
Received: October 20, 2016 | Accepted: November 28, 2016 | Web Released: December 10, 2016
CL-160964
Facile Inkjet Printing Using Silver Precursor with Controllable Surface Tension for Fabricating Ultra Pliable Paper Electrode Seoungwoong Park,# Siyong Park,# Taeheon Kim, Dain Kwak, Sangki Park, Hochung Ryu, and Jong-Jin Park* School of Polymer Science and Engineering, Chonnam National University, Gwangju 61186, Korea (E-mail:
[email protected]) We used surfactants to increase the spreading property when drop-jetting using the drop-on-demand printing method in order to create a flexible paper electrode that is low-cost and does not require high temperatures. The method results from a silver percolation network by adjusting the surface tension value of water. In addition, we were able to create surfactants and electrodes, and prove the probability of the correlation between conductivities by using contact angle measurement. Using this approach, where the silver precursor and the reducing agent formed by in situ chemical reduction, the printed solution state before using electrodes was able to form a long-range percolation network. The resistance of the electrode formed at this time was 2.2 ³ cm, demonstrating a resistance of about 25.8 ³ cm after 500 durability measurements were performed through a process of folding the electrode. Keywords: Pliable paper electrode | Inkjet printing | In situ chemical reduction
Fabrication of patterned electrodes with inkjet printing technology is less expensive and easier than fabrication by the current electrode and pattern method.15 Inkjet printing technology does not involve complicated processing stages of chemical vapor deposition (CVD) and etching,69 as is required in the widely applied electrode pattern method in the existing industry. While research on various methods related to the field of is in progress, process issues with these methods remain. Addressing these process issues has attracted many researchers’ attention, with the aim of developing an alternative to the inkjet printing method for forming electrodes. In previous studies, nanoparticledispersed conductive ink was mainly used with the thermal reduction method. However, this method requires a printed electrode because of the high-temperature heat treatment; additional before/after processing for substrates and electrodes is thus required.1014 Furthermore, this method requires the manufacturing of conductive ink from carbon-based materials,1,5,15 followed by compositing and dispersing metal nanoparticles such as silver9,13,14 and copper16,17 have issues regarding the high price and complicated synthetic procedures due to a significantly high volume of dispersing substances. Compared to intrinsically conductive polymers and carbon-based materials, metal-based inks offer superior conductive coatings as they need to address a wide variety of commercial applications according to three orders of high conductive magnitude,5,15 such as PEDOT:PSS18 and PANI.19 While a small amount as possible of conductive materials is used in the above-mentioned methods, various other methods are also receiving attention: the method of nanodripping20 (involving near-field electrospinning droplets with a volume of submicron scale similar to the nanodripping mode of an electro-hydrodynamic jet (e-jet)),10,21 as well as the printing method in which the discharge of the printing solution Chem. Lett. 2017, 46, 299–302 | doi:10.1246/cl.160964
is reduced in an effort to make electrodes, whereby a line form with a high aspect ratio or electrode formation of the micro dot is formed. However, commercialization is difficult due to the lack of accessibility and affordability. In order to solve the above problems pertaining to advanced printing technology, a user-friendly electrode was manufactured in this study by using inkjet printing (HP ENVY 5530) with a simple design based on the bubble jet method and dropon-demand (DOD) technology typically used domestically and commercially.1,5,22 Of the various methods of electrode formation, inkjet printing is the simplest and least expensive method with excellent accessibility; the accessibility and cost of the technology are reasonable and thus provide economic efficiency. Moreover, our method is environmentally friendly, where we treated (2:8) silver trifluoroacetate (STA) dissolved with water. It contains a soluble metal salt,23 in pure water for 15 min, using a bath-type sonicator (NIONS), and manufactured a printing solution without complicated mixing. The solution was naturally ionized, having transparent characteristics and chemical stability. A silver precursor existed in an ion state in the water. Because the surface tension of water is very high, we added about 0.01 wt % surfactants (BYK 9170) prior to inserting the sonicator in order to effectively control the surface tension to optimize the printing condition and prevent the back flow of the ink while printing. To reduce this simply made silver precursor (SPS), we mixed hydrazine mono-dehydrate at a high reducing power and mixed ethanol at a low reducing power at the ratio of 1:9. Unlike the original silver precursor, this reducing agent has toxicity.24,25 Each solution contains SPS and a reducing agent; the solutions were injected into the ink cartridge and installed in the print head. This was injected via a 1 mL syringe. We then drew the desired circuits using a personal computer and printed the circuits to make electrodes. Fabrication of our printed electrodes procedure consists of the computer commanding the input of red and then the silver precursor and reducing agent to be ejected from the cartridge containing magenta and yellow while printing the desired shapes, where the silver electrode is formed in situ by the chemical reduction. The electrodes are quickly and easily formed without the need of skilled operators because the method is the same as printing pictures or documents; also, any desired shape and size is possible. Other solutions can be ejected from a new cartridge according to the printing command of the computer. In addition, colors and electrodes can be used that express black at the same time by using different black ink cartridges, as shown in Figure 1. Figure 2 shows the resistance change according to the amount of silver precursor used. The two connected plots shown in Figure 2a demonstrate the resistance when printed on a regular printing paper of A4 size and when printed on a glossy paper by making a reducing agent by placing hydrazine in
© 2017 The Chemical Society of Japan | 299
(a)
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Figure 1. Schematic of inkjet printing for fabricating foldable paper based on a silver electrode.
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Figure 2. (a) Compared to dissolved silver salt in the two cases of solvent printable pre-conductive ink while printing with a sheet resistance reducing agent measured to optimize the blending of the silver precursor. (b) Optimized water based on a silver precursor and a sheet resistance reducing agent measured before/after injected surfactant. ethanol and using tetrahydrofuran (THF) as the silver precursor solvent. The x-axis shows the amount of overprinting that occurred. The glossy paper showed resistance from the second overprinting, while the A4 paper showed resistance from the third overprinting. The two cases did not demonstrate conductivity during a one-off print due to ink absorption by the paper in the polyethyleneimine (PEI) layer (the hydrophilic polymer layer). However, in both cases, a percolation network of silver nanoparticles was formed with the increase of the amount of ink due to overprinting. We examined the possibility of using solvents such as THF, ethanol, and water to optimize the continuous silver precursor phase. However, when we use THF solution, the head of the plastic cartridge is damaged due to the dissolving in the organic solvents. We attempted to use ethanol to avoid melting of the print head, but mild reduction occurs due to the OH moiety in the ethanol, as shown in 300 | Chem. Lett. 2017, 46, 299–302 | doi:10.1246/cl.160964
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Figure 3. (a) Schematic of an in situ chemical reduction with inkjet printing on glossy paper substrate. (b) Measurement of the silver precursor contact angle in a pure state, (c) injected surfactant, (d) top view of the FE-SEM image patterned without surfactants, and (e) injected surfactant. Figure 2a (inset), whereby the OH moiety reduces the amount of silver salt ionized in the solvent and decreases the stability. To enhance the stability of the solution, we used water as the solvent for the silver precursor. This was measured in terms of stability after one month had elapsed in an ambient condition. The solution was still in a transparent state caused by the present ion state in the water phase, as shown in Figure 2b (inset). Figure 2b shows that the resistance of the formed electrode decreased further and became even more noticeable with only one occurrence of overprinting. The printed electrode thickness 90 ¯m and overprinted electrodes had a similar thickness. is ¼ Surfactants were added because the amount of solution used for printing when using the DOD method seemed smaller. Since water has a higher surface tension than THF or ethanol, we observed less resistance of the formed electrode. The resistance of the electrode formed by the five occurrences of overprinting was 2.2 ³ cm. Figure 3a shows our reduction mechanism. The silver precursor in Figure 3a shows our reduction mechanism. The silver precursor is dropped on the paper substrate. Spreading26,27 then occurs because the reducing agent is randomly dropped on the silver trace. The printed initial electrode causes in situ chemical reduction when forming the silver electrode, as shown in Figure 3a. Figures 3b and 3c show the contact angle of water due to the existence on the silver trace. The printed initial electrode causes in situ chemical reduction when forming the silver electrode, as shown in Figure 3a. Figures 3b and 3c show the contact angle of water due to the existence of surfactants when using a contact measuring instrument or when examining the change of electrodes printed before and after the surfactant is added. The contact angle measurement according to the status of surfactant is shown in eq 1:1,26,28 £ sv ¼ £ sl þ £ lv cos ªY
ð1Þ
© 2017 The Chemical Society of Japan
Chem. Lett. 2017, 46, 299–302 | doi:10.1246/cl.160964
Resistance/Ω cm
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The above formula is Young’s contact angle eq 1, which is controlled according to the change of cos ªY when surfactants are added to the silver precursor and reducing agent. The contact angle of the silver precursor with the surfactant injected was 45.55°, as shown in Figure 3c, showing a reduced contact angle of about 14° compared to that of 59.55°, which is the initial state without mixing the surfactants, as shown in Figure 3b. This contact angle and viscosity are optimized for a jetting silver precursor, as shown in Figure S1 (viscosity < 5 mPa s). We were therefore able to control the spreading condition of the printed ink drop by controlling the interfacial energy using surfactants in the short-range percolation network formed by numerous fine dots of the electrode’s initial state. Such a mechanism not only signifies smaller surface tension caused by the surfactant in the ink, but also shows a larger space with the increase of wetting when the solution comes in contact with the paper. The number of silver nanoparticles with surfactants is greater than the number of nanoparticles without surfactants, as shown in Figure S2a, and the total area of silver nanoparticles is larger than the area of silver nanoparticles without the solvent. The long-range percolation network is accomplished with a proportional area of surfactant concentration. When ink drops on a piece of paper, the PEI layer is invaded by the liquid-state preelectrode due to the difference between the surface tension in the polyimide and that in the liquid-state pre-electrode. When a surfactant is used, the surface tension will deteriorate, ensuring easy invasion of the PEI by the liquid-state pre-electrode. The sizes of silver nanoparticles that occupy the same area increase due to spreading and more areas are created to form a long-range percolation network.14,29,30 Figures 3d and 3e show the field emission scanning electronic microscope (FE-SEM) images of the electrode printed on an A4 sheet of paper using the DOD method. When compared with Figure 3d, a larger particle was formed and the probability of forming a long-ranged percolation network increased, as shown in Figure 3e. We also connected electrodes by the various absorbed silver percolation network layers. Of the sheets of paper we used, the glossy paper coated with the polymer having a hydrophilic property plays a role in fixing the printed silver electrodes. When compared to roughness between the glossy paper and the general paper, because the photo glossy paper finish a coating with a hydrophilic polymer layer such as (PEI) or poly(vinyl alcohol) as a role of planarization, the glossy paper shows the property of very smooth roughness with Ra = 13 nm and look of true photographic prints. Without any thermal annealing process, we can make a smooth electrode with roughness Ra = 43 nm (Figure S3). The aforementioned processes have disadvantages of being complicated and not user-friendly. However, we can solve such problems efficiently by using commercialized substrates optimized for high-resolution image printing. Figure 4 shows the measurement of durability using a press force meter without pre- or post-treatment for improving the durability of electrodes formed through in situ chemical reduction on a glossy paper. In order to ensure flexible electrodes, we measured the electrodes; durability could be fully measured compared to electrodes in a folded state. When a printed electrode is folded, the radius of curvature R is 0 mm.31 The resistance of the line-formed electrodes consequently changes, and the resistance of the measured surface-formed
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Figure 4. Measurement of durability test via press force tester at 92.5 kPa, pristine state, and folded state (180°): (a) lineresistance of printed electrode showing electrode durability and (b) variation of electrode of R/R0 during 500 tests. electrodes was 2.2 ³ cm. Formed electrodes durability that act vertically for contacting and the fall of electrodes can be caused by folded and the bent area of electrodes. The results of 100 durability measurements showed that each electrode demonstrated an increase in resistance by 2.2 ³ cm. After 500 measurements, the line-formed and surface-formed electrodes showed resistance changes of 25.8 ³ cm. Also, after 500 folded tests, R/R0 is less than 10% (Figure 4b and Figure S4). This result suggests that when using the measuring instrument that operated at a pressure of less than 92.5 kPa, the hydrophilic polymer layer of electrodes provided a stable support for the silver layer. In summary, we used a printing method in which nozzle clogging is minimized by using a metal precursor with excellent chemical stability, usability, affordability, and convenience of use. We also formed electrodes without the need to use thermal energy and heat treatment by using a metal precursor with excellent chemical stability, usability, affordability, and convenience of use. We also formed electrodes without the need to use thermal energy and heat treatment by using a process at room temperature. We controlled the interfacial energy of the silver precursor and reducing agent in the printing process, thereby utilizing a resistance value in the range of 2.2 ³ cm. Post-treatment was not necessary for additionally printed substrates. As a result of 500 measurements conducted by bending electrodes 100% with layer-by-layer stacking by reduced silver nanoparticles in a water-soluble polymer layer, the electrodes’ line resistance was able to maintain a percolation network of less than 25.8 ³ cm in the folded area and a durability for the external force of 92.5 kPa. Furthermore, the resistance of the electrode printed on the glossy paper is similar to that of the ITO electrode of the current brittle form, which can be used as a heating element or an electrode for touch screens.
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This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF2015R1D1A1A01057245) and the Leades INdustry-university Cooperation Project supported by the Ministry of Education, Science and Technology (MEST). Additional support was provided by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under the Industrial Technology Innovation Program. No. 10067151, smart device connected and textile-based smart function mounted outdoor garment for smart life with ergonomic design.
13 14 15 16 17
Supporting Information is available on http://dx.doi.org/ 10.1246/cl.160964.
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