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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 3076−3082
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Extremely Simple and Rapid Fabrication of Flexible Transparent Electrodes Using Ultralong Copper Nanowires Thanh-Hung Duong and Hyun-Chul Kim* High Safety Vehicle Core Technology Research Center, Department of Mechanical and Automotive Engineering, Inje University, Gimhae-si 50834, South Korea ABSTRACT: Recently, flexible transparent electrodes have been fabricated successfully by using silver and copper nanowires. Silver is expensive and scarce, while copper is inexpensive but it requires a complex post-treatment process to obtain copper nanowire electrodes with an equivalent performance. In this study, by using polyvinylpyrrolidone-based ink and ultralong copper nanowires, a simple and low-cost approach has been presented for the fabrication of excellent copper nanowire-based flexible transparent electrodes. When ultrahigh aspect ratio copper nanowires and 0.5 wt % polyvinylpyrrolidone-based ink are applied to coat onto a substrate, the post-treatment process can be simplified to dipping once in acetic acid for 60 s. When compared to previously reported studies, our copper nanowire electrodes proved to be one of the best quality transparent electrodes. Moreover, these electrodes were applied successfully in the fabrication of self-capacitance button touch sensors. on a flexible substrate by using a Meyer rod. However, to make the film conductive, the film should be placed in a plasma cleaner under inert gas and then in a tube furnace. To overcome this expensive fabrication method, our previous research showed that polyvinylpyrrolidone (PVP)-based ink can be used as an alternative film former for Cu NWs coatings and it can easily be removed by putting it in an oven at 102 °C for 30 min, followed by dipping it in acetic acid for 15 min.11 Although this method is simple and affordable, it still requires a lot of time and energy. In theory, longer and thinner Cu NWs are preferred to fabricate excellent performance transparent electrodes.12,13 However, because the ultralong Cu NWs have a very poor dispersibility in ethanol or water, it is difficult to handle them. Li et al. decreased the aspect ratio of Cu NWs in synthesis from 1700 to 300 to achieved high-quality nanowires network.14 Dou et al. applied ultrasonication on Cu NWs/isopropyl alcohol to form a homogeneous suspension.15 On the other hand, in previous reported studies, thermal spray coating and vacuum filtration are the most common technique to spread ultralong Cu NWs on the substrate. Nevertheless, the thermal spray coating requires costly set up and wastes many nanowires, while vacuum filtration technique needs expensive membrane filter and only can be applied on a small area. Therefore, using ultralong Cu NWs in transparent electrodes fabrication is still a challenge for many researchers.
1. INTRODUCTION In the last few decades, the touch sensor has become an indispensable component in every electronic device such as smartphones, laptops, navigation systems, and televisions. Compared to the button-type sensor, the outstanding properties of touch sensors are coincident multiple sensing, high response rate, good durability, high sensitivity, lightweight, and compact.1−4 As the trend for developing flexible hand-held devices increases, the demand for flexible transparent touch sensors (FTTSs) is inevitable. Fabricating FTTSs requires highperformance flexible transparent electrodes (FTEs) and a patterning system that can create patterns on the surface of electrodes without damaging them. Although indium tin oxide (ITO) has dominated the material market of transparent electrodes for a long time, metal nanowires have been considered as a new trend for the fabrication of FTEs due to the brittleness of ITO. Recently, by using silver nanowires (Ag NWs), researchers have succeeded in the fabrication of FTEs that are on par with ITO transparent electrodes in terms of performance.5−7 However, like the ITO, silver is also expensive and scarce. Hence, copper nanowires (Cu NWs) have risen as a potential alternative material to Ag NWs and ITO. Cu NWs-based electrodes also have been applied in the fabrication of solar cell, display device or various type of sensors.8 Copper is well-known for its abundance and excellent conductivity. In 2010, Rathmell et al.9 for the first time reported that Cu NWs can be synthesized at low cost and be printed onto a substrate to produce FTEs that perform comparable to Ag NW electrodes. Later, in 2011, they improved the synthesis process and demonstrated a novel method to fabricate FTEs.10 A nitrocellulose-based ink has been developed and applied to distribute Cu NWs uniformly © 2018 American Chemical Society
Received: Revised: Accepted: Published: 3076
November 14, 2017 January 14, 2018 February 1, 2018 February 1, 2018 DOI: 10.1021/acs.iecr.7b04709 Ind. Eng. Chem. Res. 2018, 57, 3076−3082
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Industrial & Engineering Chemistry Research
Figure 1. SEM images of as-synthesized Cu NWs at scale 50 μm.
2.3. Preparation of Flexible Transparent Electrode. First, 0.5 wt % PVP-based ink was prepared by dissolving 0.5 g of PVP-K90 in 99.5 g of IPA. The stored Cu NWs were transferred to a 1.5 mL tube and washed once more with the 0.5 wt % PVP-based ink solution by centrifuging at 2000 rpm for 5 min. Lastly, depending on the desired concentration, the required amount of PVP-based ink was pipetted into the tube containing the copper nanowires to make the final coating solution. Meyer rods were utilized to coat the copper nanowire solution uniformly onto a laboratory glass slide or PET substrate. After coating, the film was dipped in acetic acid for a minute and self-dried in air. 2.4. Characterization. The synthesized Cu NWs were analyzed using a scanning electron microscope (SEM, Hitachi S-4800). X-ray diffraction (XRD) of Cu NWs samples was measured in the range of 2θ = 20−80° by step scanning on the X’Pert PRO MPD X-ray diffractometer of PANalytical (Netherlands). The optical specular transmittance of the electrode was measured using a T60 UV/vis Spectrophotometer of PG Instruments Limited.
In this paper, we introduce a simple and inexpensive method to fabricate FTTSs by using ultralong Cu NWs. Cu NW flexible transparent electrodes can be produced in a few minutes by using PVP-based ink, a Meyer rod, and acetic acid. The ultralong Cu NWs was synthesized via hexadecylamine (HDA)mediated method. Moreover, a simple application of Cu NWs as a touch sensor was demonstrated.
2. MATERIALS AND METHODS 2.1. Materials. Acetic acid (1002-4400) and isopropyl alcohol (IPA, 5035-4400) were purchased from Daejung Chemical & Metal (South Korea). 1-Hexadecylamine, tech. 90%, and 1-octadecylamine (HDA, B22459) were purchased from Alfa Aesar. Copper(II) chloride dihydrate (CuCl2·2H2O) was purchased from Junsei (Japan). D-(+)-Glucose (C6H12O6, G8270-100G), n-hexane (anhydrous, 95%, 110-54-3), and polyvinylpyrrolidone (PVP, Mw = 360 000) were procured from Sigma-Aldrich (USA). Polyethylene terephthalate (PET) with 50 μm thickness was obtained from KAIST (Deajeon, South Korea). Meyer rods of sizes #12, 16, 20, and 24 (30.48, 40.64, 50.80, and 60.96 μm of wet film thickness) were supplied by RD Specialties. 2.2. Synthesis of Copper Nanowires. The flasks, beakers, measuring cylinders, and stir bars used for the process were cleaned thoroughly and dried at room temperature. In a typical synthesis of Cu NWs, 0.24 g of CuCl2·2H2O, 0.1 g of glucose, and 1.5 g of HDA were dissolved in 80 mL of deionized (DI) water in a 100 mL DURAN bottle. After being capped, the bottle was placed in a water bath that was preheated to 60 °C; then, the solution was magnetically stirred at 800 rpm for 2 h. Next, the resulting blue emulsion was transferred to a 100 mL Teflon-lined stainless steel autoclave and kept in an oven at 140 °C for 12 h. After cooling down to room temperature, the resulting solution was centrifuged and washed with DI water three times. To separate Cu NWs and Cu nanoparticles (NPs),10 mL nhexane and 15 mL DI water were added to the solution and vortexed for 30 s. Then, the mixture was left to stand for 30 min. Later, the solution separated into two phases: Cu NWs floated in the top n-hexane phase, while Cu NPs settled down in the bottom water phase. The n-hexane phase was transferred to a 50 mL tube using a pipet. After that, the obtained solution was centrifuged and washed with isopropyl alcohol (IPA) three times. Finally, Cu NWs were dispersed in IPA for later use.
3. RESULTS AND DISCUSSION 3.1. Cu NW Synthesis. In terms of Cu NW synthesis, the fabrication technique mainly relies on the solution-phase synthesis due to its merits, which are facile functionalization, scalability, and bioapplication.16 Among the solution-phase synthesis approaches, the hexadecylamine (HDA)-mediated method has been considered as one of the most efficient
Figure 2. XRD of as-synthesized Cu NWs. 3077
DOI: 10.1021/acs.iecr.7b04709 Ind. Eng. Chem. Res. 2018, 57, 3076−3082
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Figure 3. Principle of our fabrication techniques: (a) normal Cu NWs coated by using 2.5 wt % PVP/IPA and then treated by oven and acetic acid; (b) ultralong Cu NWs coated by 2.5 wt % PVP/IPA and treated by oven and acetic acid; (c) ultralong Cu NWs coated by 0.5 wt % PVP/IPA and treated by acetic acid.
Figure 6. Comparison of electrodes performance (sheet resistance and specular transmittance at wavelength of 550 nm) between our study and previously reported studies.
Figure 4. Effect of acid dipping conditions on the sheet resistance of Cu NW electrodes.
Figure 7. Plot of specular transmittance vs wavelength for slide glass, ITO, and three Cu NWs-based films.
synthesis methods for ultralong Cu NWs.13,17 Synthesis of Cu NWs using HDA as a capping agent was presented the first time in 2010 by Mohl et al.18 By using glucose as a reducing agent and a Teflon-lined stainless steel autoclave, Cu NWs that have a uniform diameter of 64 ± 8 nm and length of a few micrometers can be produced easily. Later, to improve the Cu NW aspect ratio and productivity, researchers have modified this method through three different approaches. First, expensive equipment such as an oil bath, a microwave, and an ultrasonic probe was used.19−22 Second, hexadecyltrimethylammonium bromide (CTAB) was used in the synthesis process.23 Finally,
Figure 5. Influence of dipping time on (a) the sheet resistance and (b) the difference between specular transmittances before and after acid treatment of Cu NW electrodes at a wavelength of 550 nm − ΔT%.
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are costly, time-consuming, or toxic. Therefore, developing an HDA-mediated synthesis method that is cost-effective, green, and simple is still challenging for many researchers. A standard synthesis of copper nanowires was carried out based on the hydrothermal method of Mohl et al.,18 with several modifications. First, 0.24 g of CuCl2·2H2O, 0.1 g of glucose, 1.5 g of HDA, and 80 mL of DI water were mixed in a 100 mL DURAN bottle. As stated on the CAMEO Chemicals Web site, the solubility of HDA is less than 1 mg mL−1 at 22 °C. Because of this insolubility of HDA, the mixture was normally stirred for 5 h to overnight at room temperature.18−21,24,25 Therefore, to reduce the mixing time while making sure that all the chemicals are well mixed, the capped bottle was put in a water bath that was preheated to 60 °C. The solution was then magnetically stirred at 800 rpm for 2 h. After 2 h, the mixture changed its color from colorless to light blue, which meant that all chemicals were well dispersed. Next, the
Figure 8. Oxidation over time of Cu NW electrodes coated by Meyer rod #12 and #16.
the last method involves increasing the mixing time to 16 h and reaction time to 24 h or more.24,25 However, these techniques
Figure 9. (a) Simple self-capacitance flexible button touch sensor; (b) chart of signal over time. 3079
DOI: 10.1021/acs.iecr.7b04709 Ind. Eng. Chem. Res. 2018, 57, 3076−3082
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impossible to distribute Cu NWs uniformly on the substrate. After several preliminary experiments, we found that if 0.5 wt % PVP-based ink was used to coat Cu NWs onto the substrate, the coated film can be made conductive by simply dipping it in acetic acid without worrying about any deformation of the nanowire network. In addition, this technique does not require any preheating treatment, which means that the fabrication cost is significantly reduced. Moreover, reducing the concentration of PVP-based ink reduces the ink cost to 90% of a 2.5 wt % PVP-based ink and 38% of a nitrocellulose-based ink.10,11 To understand practically this fabrication technique, several groups of experiments were carried out. First, to determine the effect of different acid treatments on the performance of Cu NWs electrode, various dipping times and the number of dipping was investigated. Six transparent electrodes were fabricated under various conditions. After Meyer rod coating, two films were plunged into acetic acid once for 60 s and 5 min, respectively, while the other four films were dipped for 10 s one, three, five, and seven times, respectively. Next, these electrodes were dried in normal air and measured their sheet resistance. Figure 4 compares the sheet resistance of these Cu NW electrodes. As expected, when the dipping time increased from zero to 60 s, the conductivity of the electrode increased. In contrast, as the number of dips increases, the sheet resistance reduces remarkably, but if the dip time is more than 3 min, the sheet resistance will rise significantly. Among these electrodes, the electrode that was dipped once for 60 s exhibited the highest conductivity. Therefore, we decided to focus on the dipping-once technique over the multiple-dips method. Figure 5 shows the influence of dipping time from 2 s to 5 min on the performance of the Cu NW electrodes coated with different concentrations of Cu NW ink. In all cases, the electrodes dipped once from 60 to 300 s showed the lowest sheet resistances. The difference in the conductivity of the FTEs between the 60-s case and the 300-s case is only slight, and range from 0.3 to 4 Ω/sq. Meanwhile, the difference in the specular transmittance of the electrodes before and after acid dipping at a wavelength of 550 nm, ΔT%, increased as the dipping time increased, as illustrated in Figure 5b. ΔT% of the 2 s case and the 60 s case were the lowest among the electrodes, and the difference between them was below 0.2%. Based on these results, to save fabrication time while maintaining the excellent performance of Cu NW electrodes, dipping once for 60 s was chosen for further study. Moreover, Figure 5 also shows that when the concentration of Cu NW ink is increased, the film sheet resistance decreases, the film specular transmittance decreases, and the difference between specular transmittances before and after treatment increases. Next, to determine the performance of the as-fabricated Cu NWs electrodes, Cu NWs obtained from synthesis were used to fabricate transparent electrodes by using a Meyer rod #16 (wet film thickness 40.64 μm). The relation between sheet resistance and specular transmittance of those electrodes is demonstrated in Figure 6. This performance was compared to previously reported studies that used an ITO-based electrode,27 Ag NWbased electrodes,28 and Cu NW-based electrodes from HDAmediated synthesis.20,22,23 As shown in Figure 6, our transparent electrode’s performance is on par with or even better than the previously reported Cu NW-based electrodes. For example, at a specular transmittance of 90% at a wavelength of 550 nm, our electrode exhibited a sheet resistance of 25 Ω/sq, while those of the electrodes reported by Won et al.20 was 25.4
blue emulsion was transferred to an autoclave and placed in an oven at 140 °C for 12 h. After obtaining a reddish solution from the autoclave, n-hexane was added to separate Cu NWs and Cu NPs based on the multiphase separation technique of Qian et al.26 The morphology and dimensions of the as-synthesized Cu NWs were characterized by scanning electron microscopy. Figure 1 exhibits the SEM images which showed uniform, straight, and ultralong Cu NWs. By using ImageJ software, the average diameters were calculated from 100 nanowires randomly selected from a dozen SEM images. The average diameter of as-synthesized Cu NWs was 31.2 ± 6.8 nm, while their average length was over 100 μm. Hence, it means that our modification method can produce Cu NWs with ultrahigh aspect ratios, length/diameter (L/D) > 3200, one of the highest aspect ratios of Cu NWs obtained through the HDA-mediated method. Figure 2 depicts the XRD pattern of as-synthesized copper nanowires. Three peaks at 2θ = 43.42°, 50.57°, and 74.32°, corresponding to diffraction from the {111}, {200}, and {220} planes of face-centered cubic Cu, respectively, were observed in the XRD patterns. Meanwhile, no other peaks were detected, proving that our method produced only pure Cu NWs or Cu NPs, with no sign of CuO, Cu2O, or another unwanted product. Combined with SEM images, it can be said that our final product contains mostly Cu NWs. 3.2. Fabrication of Cu NW Electrodes. Generally, bar coating or Meyer rod coating is one of the most popular techniques for large-scale fabrication of transparent electrodes. In the case of copper nanowires, it requires a special ink formation, such as nitrocellulose-based ink or PVP-based ink, to be a binder so that Cu NWs can be distributed uniformly on a substrate. Our latest study11 showed that PVP-based ink is an excellent low-cost binder for Cu NW coatings. The principle of that technique is shown in Figure 3a. First, Cu NWs were mixed with a 2.5 wt % PVP/IPA solution, and then the Meyer bar was used to coat this mixture on a PET or glass substrate. Right after that, to settle down these nanowires, the coated film was placed in the oven for 30−60 min at 105 °C. Next, it was dipped in acetic acid for 15 min at room temperature to remove unwanted chemicals, expose the Cu NWs layer at the bottome and become conductive. However, when we applied this fabrication technique to the Cu NWs obtained from our HDAmediated synthesis, the Cu NWs network floated away right after extract from acetic acid solution. This may be due to the difference in the aspect ratios of the Cu NWs. The copper nanowires used in our previous study has an aspect ratio of around 650,11 while the aspect ratios of Cu NWs from HDAmediated synthesis in the present study were higher than 3200. As illustrated in Figure 3b, because the PVP/IPA solution has high viscosity and the ultralong Cu NWs was lighter than the average Cu NWs, it is more difficult for the ultralong Cu NWs to settle down at the bottom by using an oven. Hence, after dipping in the acetic acid, a large amount of nanowires was floated away. To solve this problem, we decided to reduce the thickness of coated Cu NWs/PVP layer. Figure 3c shows the principle of this idea. If the thickness of Cu NWs/PVP layer is around once or twice the diameter of Cu NWs, the floating away issue can be avoided. By reducing the concentration of PVP-K90 in PVPbased ink, the thickness of Cu NWs/PVP layer can be controlled. However, if the concentration of PVP is too low, the Cu NWs will disperse poorly in the coating ink, which led to 3080
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Industrial & Engineering Chemistry Research Ω/sq and by Zhang et al.23 was 100 Ω/sq. At the sheet resistance over about 40 Ω/sq, our electrodes show performance equivalent to that of Ag NW-based and ITO-based electrodes. Furthermore, the specular transmittance of Cu NWs-based electrodes with different sheet resistances over the wavelength from 325 to 1100 nm is shown in Figure 7. Therefore, to the best of the author’s knowledge, it can be said that our method produced excellent electrodes, which have the best quality reported Cu NW-based electrodes. In order to evaluate the oxidation rate, two Cu NW transparent electrodes with different Cu NW network thickness were fabricated by using a Meyer rod #12 and #16 from RD Specialties (wet film thickness: 30.48 and 40.64 μm). As Figure 8 indicates, after the post-treatment step, the sheet resistances of these two electrodes were measured daily. With time, the conductivity of the two films decreased, and after 5 days, the sheet resistance increased nearly 3 times the original. Compared to other previous studies, the increase rate of sheet resistance of the electrodes reported in the present study is rapider.11,23 The reason for this may come from the difference of the contact area of junctions between two cases. Because our Cu NWs possess a thin shape, the area of each wire−wire junction is much smaller than the contact area of Cu NWs network in previous studies. Hence, even at the same oxidation rate, the junction that have smaller area will be oxidized completely faster than the large one. Consequently, after a period, the number of junctions that allow electrical current pass through well will decrease dramatically and then the sheet resistance will increase significantly. Therefore, the Cu NW electrodes fabricated by our method should be kept under an inert gas atmosphere right after fabrication for further use. 3.3. Application in Flexible Transparent Touch Sensor Fabrication. To demonstrate the possibility of applying our Cu NW electrodes in FTE fabrication, a simple self-capacitance button touch sensor was made. A flexible transparent electrode (14.6 Ω/sq of sheet resistance and 85% of specular transmittance at a wavelength of 550 nm) was connected to a flash microcontroller (ATSAMD20J18 of Atmel), as illustrated in Figure 9a. Through the Atmel Studio software, the electrode became a button-type touch sensor. Figure 9b shows that the acquired signal by Atmel Studio software changed at high response rate every time a finger approaches closely or touches it. To test the stability of the transparent electrode, it was covered by a scotch tape and was checked the touch-test every day for a week. At the end of the week, the Cu NWs electrode still functioned well. This example shows that ultralong Cu NWs obtained through HDA-mediated synthesis can be applied to fabricate flexible transparent touch sensors at a low cost.
Moreover, HDA-mediated synthesis has been optimized to reduce the total synthesis time down to14 h, while achieve the aspect ratio (L/D) of over 3200. Furthermore, a single selfcapacitance button touch sensor was made to demonstrate the potential of our approach in the touch screen industry.
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AUTHOR INFORMATION
Corresponding Author
*Hyun-Chul Kim. E-mail:
[email protected], Tel: +82-55320-3988, Fax: +82-55-324-1723. ORCID
Hyun-Chul Kim: 0000-0003-0637-7872 Author Contributions
The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Thanh-Hung Duong and Hyun-Chul Kim contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program & Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education and MSIP(Ministry of Science, ICT & Future Planning) (2017R1D1A1B03029074, 2017R1C1B5014970, N170626010)
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REFERENCES
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4. CONCLUSIONS Briefly, this paper introduced an exceptionally simple and rapid fabrication approach for flexible transparent electrodes. By using ultralong copper nanowires and reducing the concentration of PVP in ink formation, the fabrication of flexible transparent electrodes can be simplified significantly. It only consists of two steps: (i) the as-synthesized Cu NWs are dispersed well in a 0.5 wt % PVP-based ink and then coated onto glass or PET by a Meyer rod, and (ii) the coated film is dipped into acetic acid for 1 min at room temperature and left for 3 min to self-dry. The electrodes fabricated by this technique exhibited excellent performance (sheet resistance of 25 Ω/sq and specular transmittance of 90% at a wavelength of 550 nm) and are comparable with the best-reported electrodes. 3081
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