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Functional Inorganic Materials and Devices
Copper Nanowire Dispersion through an Electrostatic Dispersion Mechanism for High-Performance Flexible Transparent Conducting Films and Optoelectronic Devices Zhongmin Yin, Shanyong Chen, Youwei Guan, Qinqin Ran, Qingsong Zhang, Xingwu Yan, Rong Jin, Hong Yu, Lu Li, and Junsheng Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19277 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019
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ACS Applied Materials & Interfaces
Copper Nanowire Dispersion through an Electrostatic Dispersion Mechanism for High-Performance Flexible Transparent Conducting Films and Optoelectronic Devices Zhongmin Yin,†,‡ Shanyong Chen,*,‡ Youwei Guan,‡ Qinqin Ran,‡ Qingsong Zhang,‡ Xingwu Yan, ‡ Rong Jin,‡ Hong Yu,‡ Lu Li,*,†,‡ Junsheng Yu† State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China †
‡ Research
Institute for New Materials Technology, Chongqing University of Arts and Sciences, Yongchuan 402160, P. R. China Keywords: copper nanowires, electrostatic dispersion mechanism, post-treatment temperatures, transparent conducting films, flexible optoelectronic devices ABSTRACT: Highly dispersed copper nanowire (CuNW) is an essential prerequisite for its practical application in various electronic devices. At present, the dispersion of CuNW is almost realized through the steric hindrance effect of polymers. However, the high post-treatment temperature of polymers makes this dispersion mechanism impractical for many actual applications. Here, after investigating the relationships between the electrostatic dispersion force and influence factors, an electrostatic dispersion mechanism is refined by us. Under the guidance of this mechanism, high dispersion of CuNW and a record low post-treatment temperature (80 ℃ ) are realized simultaneously. The high dispersity endows CuNW with good stability (–45.66 mV) in water-based ink, high uniformity (65.7 ± 2.5 Ω sq-1) in the prepared transparent conducting film (TCF) (23 cm × 23 cm) and industrial film-preparation process which are the issues that hinder the widespread application of CuNW-based TCF at present. The low post-treatment temperature makes CuNW possible for applying on any substrate. In addition, the charge modifier, 2-mercaptoethanol, enables CuNW to resist oxidation well. Finally, flexible optoelectronic devices employing the CuNW film as the electrode are fabricated and show efficiencies comparable to those of optoelectronic devices on ITO/glass.
INTRODUCTION Transparent conducting film (TCF) is essential for many electronic devices, such as touch sensors, wearable devices, optoelectronic devices and electronic skins1-8. Currently, the main TCF is indium tin oxide (ITO) film. Although ITO film possesses excellent conductivity and transmittance, it still suffers from some crucial drawbacks, such as high cost, low mechanical flexibility, scarcity of indium, and difficulty to grow on large-scale substrate9. These inherent drawbacks of ITO motivate the development of the next-generation conductive materials, such as metal nanowire, metal mesh, graphene, carbon nanotube and conductive polymers10-17. Among these options, copper nanowire (CuNW) gains much attention because of its outstanding opto-electrical performance, high flexibility and low cost18-20. Recently, the reported conductivity and transmittance of CuNW-based TCF have met the requirements of practical application21-23. However, the widespread application of CuNW-based TCF is still hindered by two critical issues: oxidation and dispersion24,25. The first problem is a research hotspot now and many methods have been developed to prevent the oxidation of CuNW26. At present, the oxidation-
resistance of CuNW has been greatly improved. In contrast with the first problem, few studies about the second problem are reported. Poor dispersity makes CuNW impractical for broad applications: first, it can’t be stored stably because poor dispersity makes CuNW aggregate and settle easily in the ink; second, the inhomogeneous dispersion of CuNW greatly reduces the uniformity of final TCF; third, because of the poor dispersity, current CuNW-based TCF is mainly prepared by nonindustrial methods (vacuum filtration, spin coating and drop casting) which are laboratory use only, and industrial processing methods (such as roll-to-roll printing) are not suitable for present CuNW ink19,27. To improve the dispersity of CuNW, researchers have carried out excellent work. Until now, methods that researchers adopted to improve the dispersity of CuNW were similar: polymeric dispersants, such as polyvinylpyrrolidone and nitrocellulose, were absorbed onto the surface of CuNW and the steric hindrance effect of polymers could prevent CuNWs from getting close to each other28,29. This method was very effective and after the addition of polymers, highly dispersed CuNW was
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obtained by researchers. However, this method has a fatal shortcoming. As we know, polymers must be eliminated after the film is prepared because the existence of polymers will dramatically lower the performance of final TCF. However, due to the high decomposition temperature (usually higher than 200 ℃) of polymers, it is difficult to eliminate them under the protection temperature (< 150 ℃) of PET substrate30. Therefore, this method is not suitable for general flexible application. Meanwhile, even for glass substrate, the high posttreatment temperature is unfavorable for industrial production yet. Thus, a novel strategy is eager to obtain highly dispersed CuNW and low post-treatment temperature simultaneously. Here, for the first time, we utilized electrostatic force to realize the high dispersion of CuNW. To achieve this goal, the relationships between the electrostatic dispersion force and influence factors were systematically investigated firstly. Then, under the guidance of these theoretical results, highly dispersed CuNW in water was obtained by using 2-mercaptoethanol (0.2%) to modify its surface. Because 2-mercaptoethanol could become vapor easily at low temperature and then be eliminated, the post-treatment temperature for our TCF was as low as 80 ℃ . Meanwhile, 2-mercaptoethanol endowed the CuNW with good oxidation resistance. Finally, large-scale flexible TCFs (23 cm × 23 cm) were prepared by Meyer rod coating and flexible optoelectronic devices which employed the prepared CuNW film as bottom electrode were fabricated. RESULTS AND DISCUSSION The design for the electrostatic dispersion of CuNW Different from silver nanowire, charge on the surface of CuNW is little. As a consequence, aggregation and settlement of CuNW happen easily in the ink. Currently, CuNW is usually stored in ethanol or isopropanol because it has a certain degree of dispersity in organic solvent31. However, its dispersity in organic solvent is still far from the requirement of practical application. In fact, organic solvent is not a suitable dispersion medium for CuNW because of the following reasons. In organic solvent, matters which possess charge often have bad solubility and ionization is also difficult. Therefore, in organic solvents, electrostatic dispersion almost can’t work and
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the dispersion of CuNW has to rely on the steric hindrance effect of polymers that will bring about high post-treatment temperature. Compared with organic solvents, water can efficiently solve problems above. Hence, water was chosen as the dispersion medium of CuNW in our work. In previous reports, water was rarely utilized as the dispersion medium of CuNW because once CuNW touched water, serious aggregation happened immediately (Figure S1). In water, the zeta potential of fresh prepared CuNW was just –3.17 mV. To solve this problem, electrostatic dispersion mechanism was introduced by us (Figure 1): if surface modifier that could generate charge was added, charge in water and charge on the surface of CuNW would generate and then affect the dispersion of CuNW simultaneously; for the surface charge, the similar charge would generate electrostatic repulsion force which could prevent CuNWs from getting close to each other, thus dispersed CuNW. Following this mechanism, the surface modifier can be polymer or organic small molecule. It means through this mechanism, the dispersion of CuNW can be realized by organic small molecule which can be easily eliminated at low temperature. Obviously, this mechanism is suitable for practical application. The relationships between the dispersion force and influence factors
electrostatic
Two kinds of charge determine the electrostatic dispersion force of CuNW: the surface charge and the charge in water (Figure 1). Obviously, the surface charge is the driving force of dispersion because it brings about electrostatic repulsive force. In contrast, according to DLVO theory and our previous work, the charge in water is the resistance force of dispersion because it will compress the surface electric layer and lower the dispersion force5,32. The final dispersion force is determined by the competition of these two forces. To obtain high dispersion force, the surface charge should be enhanced and the charge in water should decrease. From these discussions, a principle for the electrostatic dispersion of CuNW is concluded by us: except for surface modifier, other matters that can generate charge should not be added because these matters will make the charge in water increase.
Figure 1. The electrostatic dispersion mechanism for CuNW in water.
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ACS Applied Materials & Interfaces
Following this principle, with no other charge sources, both the surface charge and charge in water are almost from the surface modifier. Hence, adjusting these two kinds of charge is actually altering the nature and amount of surface modifier, including three factors: the bond strength between surface modifier and CuNW, charge per single modifier molecule, and the amount of surface modifier. To understand how these factors worked, the relationships between the dispersion force and these three factors were studied. Studying these relationships needs to determine the amount of surface charge and charge in water. However, it’s difficult to measure the amount of charge directly. Hence, in this work, ink conductivity was utilized to represent the amount of charge in the ink because the amount of charge was directly proportional to the ink conductivity. Specifically, the ink conductivity before centrifugation (Call), the ink conductivity after CuNW was removed by centrifugation (Cwater), the difference between the two conductivities (Csurface) represented the sum of surface charge and charge in water, the charge in water, and the charge on surface of CuNW respectively. Csurface/Call represented the ratio of surface charge in all of the ink charge. For the bond strength between CuNW and surface modifier, if the bond strength is weak, the surface modifier can easily fall off from the surface of CuNW.
This will result in the decrease of surface charge and the increase of charge in water. According to discussion above, under this condition, the dispersion force will decrease. Therefore, we speculate that strong bond is essential for obtaining high dispersion force. To verify our speculation, surface modifiers (sodium sulfamate, sodium n-cyclohexyl sulfamate, sodium formaldehyde bisulfite and sodium 3-mercapto-1-propanesulfonate) with different bonding groups (–NH2, –NH, –OH, –SH respectively) but same charge group (–SO3–) and concentration (0.2%) were utilized (Table 1 and Figure 2). For these four surface modifiers, ratios of surface charge were 10.29%, 16.31%, 12.96% and 23.66% respectively (Table 1). Higher ratio meant stronger bond strength. Hence, the order of bond strength was as follows: SH–Cu>NH–Cu>OH–Cu>NH2–Cu. To investigate the influence of bond strength on the dispersity of CuNW, we tested the dispersion degree of CuNW in these four inks through observing their dispersion by naked eyes, measuring their zeta potentials and the uniformities of films prepared from these inks (Table 1) (Figure 2a, b, c and d). Results of experiments indicated that with the enhancement of bond strength, zeta potential gradually increased (Table 1) and the dispersion degree of CuNW was also gradually enhanced (Figure 2a, b, c and d). This was in agreement with our speculation. Hence, strong bond was favorable for obtaining high dispersion force and –SH was a suitable bonding group for CuNW.
Table 1. The surface modifiers used in this work and their functional groups, the ink conductivities before and after centrifugation, the ratios of surface charge, and the zeta potential of CuNW inks.
Surface modifier
Bonding group
Charge group
Ink conductivity before centrifugation (Call)
Ink conductivity after centrifugation (Cwater)a)
μs/cm
μs/cm
μs/cm
The difference between Call and Cwater (Csurface)
Ratios of surface charge
Zeta potential mV
Sodium sulfamate
–NH2
–SO3–
1153.7
1035
118.7
10.29%
–2.67
Sodium n-cyclohexyl sulfamate
–NH
–SO3–
534.8
447.6
87.2
16.31%
–4.23
Sodium formaldehyde bisulfite
–OH
–SO3–
935
813.8
121.2
12.96%
–3.52
Sodium 3-mercapto-1propanesulfonate
–SH
–SO3–
611.1
466.5
144.6
23.66%
–17.45
Sodium thiosulfate
–S
2×O–
1099.3
864.2
235.1
21.39%
–6.82
2-Mercaptoethanol
–SH
–OH
16.87
13.08
3.79
22.46%
–45.66
n-Octyl mercaptan
–SH
–CH3
9.50
7.39
2.11
22.21%
–2.26
a) The
conductivity measured in the inks whose CuNW had been removed by centrifugation.
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Figure 2. The CuNW inks with different surface modifiers (a: sodium sulfamate, b: sodium n-cyclohexyl sulfamate, c: sodium formaldehyde bisulfite, d: sodium 3-mercapto-1-propanesulfonate, e: sodium thiosulfate, f: 2-mercaptoethanol, g: n-octyl mercaptan) (0.2%) (inset: the molecular structures of these surface modifiers). The distributions of CuNW in the films prepared from these inks through roller coating.
For the electrostatic dispersion of CuNW, common viewpoint believes that with the increase of surface modifier, the electrostatic repulsion force will increase and the dispersity of CuNW will be enhanced. However, when we tried to disperse CuNW, we discovered that things were not what we might thought. When a modifier molecule absorbed on the surface of CuNW produced one negative charge, one positive charge generated in water inevitably. Meanwhile, from studies above, we discovered that most of surface modifier existed in water and only part (< 24%) of modifier was absorbed onto the surface of CuNW (Table 1). Hence, charge generated from surface modifier mainly distributed in water and only a small part of charge existed on the surface of CuNW. For this situation, with the increase of surface modifier, the surface charge and charge in water would increase simultaneously, and the increase amplification of charge in water would be several times higher than that of surface charge. When the amount of total charge was little, the charge density in water (charge amount/ink volume) was low and the resistance force for dispersion was weak. In contrast, because the volume of CuNW was small, the density of surface charge was high. Therefore, during this period, the driving force generated from the surface charge dominated the dispersion of CuNW. As a result, with the increase of Csurface, the dispersion force would increase. However, when the amount of total charge was large and the influence of charge in water was obvious, as the amount of total charge increased, the
increase amplification of resistance force would be larger than that of driving force. During this period, with the increase of Csurface, the dispersion force would decrease. To verify speculations above, surface modifiers (sodium thiosulfate, sodium 3-mercapto-1-propanesulfonate, 2mercaptoethanol and n-octyl mercaptan respectively) with different charge groups (2×O–, 2e; –SO3–, 1e; –OH, 0e
