Optimization of silver nanowire synthesis for flexible transparent conductive electrodes

Cite this paper: Vietnam J. Chem., 2021, 59(1), 98-105 Article DOI: 10.1002/vjch.202000131 98 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Optimization of silver nanowire synthesis for flexible transparent conductive electrodes Nguyen Thanh Nhan1, Pham Duy Linh1, Doan Tien Dat1, Min Ju Cho2, Dong Hoon Choi2*, Hoang Mai Ha1* 1Institute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 2Department of Chemistry, Research Institute for Natural Science, Korea University Submitted July 31, 2020; Accepted October 13, 2020 Abstract In this work, a range of factors was investigated for the synthesis of silver nanowires. Nickel chloride showed to be a promising controlling agent for the scale-up synthesis of silver nanowires. Other factors such as reaction temperature, concentrations of surfactant, precursor, controlling agent, and the effect of degassing were studied and optimized. The synthesized silver nanowire sample was applied for flexible electrode fabrication. The fabricated electrode showed to have high transparency (90 %), low sheet resistance (26 Ω/sq), low surface roughness (Rq = 6 nm), and high bending stability (withstand 100 bending cycles at bending radius of 4 mm), which satisfied the requirement for applications in flexible optoelectronic device. Keywords. Silver nanowires, flexible, transparent, conductive electrodes. 1. INTRODUCTION Flexible transparent conductive electrode (FTCE) is an indispensable part of many electronic and optoelectronic devices. Among various materials for the fabrication of FTCE, silver nanowires (AgNWs) emerges as the most potential candidate due to its high conductivity, high flexibility and transparency.[1] Researches have focused on the application of AgNW-based electrode for organic light-emitting diodes,[2] thin-film solar cells,[3,4] touch panels,[5] sensors,[6] transparent heaters,[7] etc. Until now, AgNW synthesis procedures are mostly based on the polyol method. The core difference between procedures is the use of halide salt as controlling agent. Recently, several groups reported the use of Br- or Cl- as co-additives to produce AgNW with different diameters.[8–14] Those procedures also concentrate on the effect of some other factors such as reaction time,[9,10] controlling agent,[11,15] surfactant chain length,[11,12] precursor and surfactant concentration on the morphology of AgNWs.[13,14] However, this work still attracts much effort to find the best condition to synthesize AgNWs for specific applications. Herein, we try to optimize the procedure for the synthesis of AgNWs by investigating various factors like controlling agent, precursor, surfactant, solvent, temperature, and degassing process. The optimized procedure utilized chloride salt as controlling agent to achieve AgNWs with low content of by-product, and hence simplifying the purification step and achieving AgNWs in good yield. Moreover, the synthesis required no degassing step and the purification used only ethanol, which made our method greener and more applicable in industry. The optimized AgNW sample was applied for the fabrication of FTCEs through spin-coating technique, showing good electrode morphology, low sheet resistance and high transparency. 2. MATERIALS AND METHODS 2.1. Materials Silver nitrate (AgNO3, 99.9 %), ethylene glycol (EG) and diethylene glycol (DEG) (analytical grade) were provided by Fisher Scientific. Sodium bromide (NaBr, 99 %), sodium chloride (NaCl, 99 %), anhydrous ferric chloride (FeCl3, 99.99 %), nickel chloride (NiCl2, 98 %), and poly(vinylpyrrolidone) with MW ~ 55000 and 360000 g/mol (PVP 360 and PVP 55, respectively) were obtained from Sigma- Aldrich. Ethanol, acetone, and isopropanol were of high purity and used without further purification. PET substrate 300 mm×300 mm, thickness of 0.25 Vietnam Journal of Chemistry Hoang Mai Ha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 99 mm was purchased from Sigma-Aldrich. 2.2. Characterization The morphology and distribution of the AgNWs on PET substrate were investigated using scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The UV-Vis absorption/transmittance spectra of electrodes were obtained using UV-Vis spectrophotometer (SP-3000 nano). The surface roughness was measured by atomic force microscopy (AFM, XE-100, PSIA). The sheet resistance of FTCEs was measured using a four- point probe (Keithley, 2634B). 2.3. Synthesis of silver nanowire 2.3.1. Using sodium bromide as controlling agent AgNWs were synthesized with some modification from the procedure reported by Xia et al.[16] Firstly, the solutions of 100 mM AgNO3 and 50 mM NaBr were prepared in EG. Then 250 mg PVP 360 was dissolved in 40 mL of EG and heated to 160 °C for 1 h. Next, 500 µl NaBr was pipetted to the PVP solution. After 5 min, 5 mL AgNO3 solution was added dropwise into the PVP solution at the speed of 0.15 mL/min. The magnetic stirrer was turned off after 10 min of stirring, and the reaction was allowed to react for a further 30 min at 160 °C. 2.3.2. Using sodium chloride as controlling agent The method for synthesizing AgNWs using NaCl as controlling agent was based on the procedure reported by Yan et al. with some modifications.[17] First, 1800 µM sodium chloride solution was prepared (solution A). Then, 0.2 g PVP 360 was dissolved in 25 mL EG (solution B). After the complete dissolution of PVP in EG, 0.25 g AgNO3 was added to solution B, followed by the addition of 3 mL of solution A. This solution was stirred for 15 min before heating to 140 °C. Once it reached 140 °C, the magnetic stirrer was turned off and the reaction was allowed to react for 5 h. 2.3.3. Using ferric chloride as controlling agent The procedure for synthesizing AgNWs using FeCl3 was similar to the above procedure but the controlling agent was FeCl3 instead of NaCl. 2.3.4. Using nickel chloride as controlling agent The procedure reported by Zhang et al. was modified for the synthesis of AgNWs using NiCl2 as controlling agent.[13] Specifically, 416.5 mg PVP was dissolved in 30 mL EG. Then, 254.5 mg AgNO3 was dissolved in 20 mL EG. After all compounds were completely dissolved, 0.65 mL of 7.5 mM NiCl2 solution was pipetted to the PVP solution, followed by the addition of AgNO3 solution. This final solution was stirred well for 10 min before heating to 140 °C. The reaction system was held at this temperature for 6 h at low stirring speed. All the raw solutions were purified. Specifically, 10 mL of each raw solution was transferred into a centrifugation tube and mixed with 20 mL ethanol. The solution was shaken gently before being centrifuged at speed of 7000 rpm for 5 min. The supernatant was discarded. The precipitation was washed twice with ethanol by centrifugation at 4000 rpm for 5 min before being dispersed in 5 mL isopropanol for further usage. 2.4. Fabrication of transparent electrodes PET substrates were cut into small pieces with the size of 2.5×2.5 cm. They were thoroughly cleaned with DI water for 10 min twice and isopropanol for 10 min before being dried with a nitrogen gun at room temperature. Then, the purified silver nanowire solution was dropped on the PET substrate and spin-coated at 2000 rpm. The as-prepared FTCEs were annealed at proper temperature for 10 min before characterization. 3. RESULTS AND DISCUSSION 3.1. Synthesis of silver nanowires 3.1.1. Effect of controlling agent on the formation of silver nanowires The effect of controlling agents on the morphology of AgNWs was shown in figure 1. Comparing between chloride and bromide salt, bromide ions promote the formation of thinner silver nanowires (figure 1a, 1b). However, in the case of bromide ions, a significant amount of nanoparticles was formed as a by-product. The formation of these silver nanoparticles and silver bromide nanoparticles was explained by the difference in the interaction affinity between silver nanocrystal and halide ions.[18] Generally, halide ions can bind with silver ion to generate silver halide nanoparticles which reduce the free silver ion concentration and slow down the reduction rate.[8] Because silver bromide has a lower solubility than silver chloride, the synthesis of AgNWs using sodium bromide as Vietnam Journal of Chemistry Optimization of silver nanowire synthesis for © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 100 controlling agent led to the production of a larger amount of silver halide nanoparticles, which lowered the yield of AgNW synthesis. The effect of metal ions in halide salts on the morphology of AgNWs was evaluated. There was a slight difference in the effect of sodium, iron, and nickel ion. The average diameters of nanowires using sodium chloride, ferric chloride, and nickel chloride were 52, 40, and 35 nm, respectively. When using NaCl, the surface of AgNW was very rough with many tiny particles grew on it (figure 1b-1 and 1b-2), suggesting the growth of silver nanoparticles on the surface of AgNW crystal. Noticeably, in the case of FeCl3, the morphology of AgNWs was unusual. As obviously seen from figures 1c-1 and 1c-2, the silver nanowires were slightly wavy with many bending points; and the diameter was uneven throughout a nanowire. The size distribution of silver nanowires in this sample was unequal as well. The reason for this unusual morphology is suggested to be the uneven oxidative etching of Fe3+ on AgNWs.[17] In both the case of NaCl and FeCl3, the morphology of AgNWs led to a higher proportion of electron scattering, thus increasing the resistivity of silver nanowire electrode. However, when using NiCl2 (figures 1d-1 and 1d-2), the surface of AgNWs was very smooth, and AgNWs had a narrow size distribution with the mean value of 35 nm in diameter and 15 µm in length. Moreover, the by- product was removed effectively after a simple purification process. This observation proved that nickel chloride is the suitable controlling agent for the synthesis of AgNWs. 10 µm 200 nm c-1) c-2) 5 µm 200 nm d-1) 5 µm b-1) 5 µm a-1) a-2) 5 µm 200 nm b-2) 200 nm d-2) Figure 1: The morphology of silver nanowires synthesized using NaBr (a-1, a-2), NaCl (b-1, b-2), FeCl3 (c-1, c-2), NiCl2 (d-1, d-2) as controlling agents 3.1.2. Effect of temperature The effect of temperature on the synthesis of AgNW was studied. At 120 °C, there were no AgNWs formed, implying that this temperature was not sufficient for the formation of AgNWs (figure 2a). When the temperature was above 130 °C, AgNWs with different diameters were obtained, and the diameter rose in association with the increase in temperature. Noticeably, at 130 °C, AgNWs with unequal diameters were obtained while at 140 °C, the diameters of nanowires were more equal (figures 2b and 2c). It is explained that at the first stage of the synthesis, the nucleation rate was significantly higher at 140 °C than that at 130 °C. The fall in the concentration of precursor at 140 °C hampered the formation of nuclei at the later state, thereby forming nanowires with more uniform morphology. However, at 150 °C, the quality of AgNWs was lower with higher average diameter; and some tiny c) a) d) b) 200 nm 200 nm 200 nm200 nm Figure 2: SEM images of AgNWs synthesized using NiCl2 as controlling agent at different temperatures: 120 °C (a), 130 °C (b), 140 °C (c), 150 °C (d) Vietnam Journal of Chemistry Hoang Mai Ha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 101 nanoparticles formed on the surface of nanowires (figure 2d). It was reasoned that at such high temperature, silver atom deposited on silver nanowire less selectively, thereby increasing the AgNW diameter. In the meantime, this fast deposition process also favored the epitaxial growth of nanoparticles on silver nanowires. 3.1.3. Effect of precursor concentration The morphology of AgNWs synthesized at different AgNO3 concentrations is shown in figure 3. At low concentration of Ag+, the formation of nanoparticles was more favorable while the nanowires formed with a short length and large diameter (figure 3a). It is reasoned that after forming seeds, the concentration of Ag+ was too low for the growth of silver nanowires. At high concentration (0.04 M), the high deposition rate of Ag atom favored the formation of not only AgNWs but also nanoparticles (figure 3d). At moderate concentrations (0.02 M and 0.03 M), the AgNWs formed with less byproduct (figures 3b and 3c). Comparing these two concentrations, the AgNWs synthesized at the concentration of 0.03 M formed with higher homogeneity. So, this concentration was used for further optimization. d) a) b) c) 500 nm 500 nm 500 nm500 nm Figure 3: SEM images of AgNWs synthesized with different concentrations of silver nitrate: 0.01 M (a), 0.02 M (b), 0.03 M (c), 0.04 M (d) 3.1.4. Effect of Ag+/Cl- As mentioned earlier, controlling agents play a crucial role in the formation of nanoparticles and nanowires. To investigate the effect of Cl- ion on the morphology of AgNWs in more detail, silver nanowires with four different Ag+/Cl- molar ratios were synthesized (figure 4a-d). At low Ag+/Cl- molar ratio, a large amount of seed was formed, and Ag+ precursor depleted quickly, leading to short-length silver nanowire. Moreover, Cl- at high concentration also passivated all surfaces of crystal, thereby producing more silver nanoparticle (Figure 4a). Decreasing Cl- concentration enabled to reduce the amount of by-product (figure 4b and 4c). Nevertheless, at very high Ag+/Cl- molar ratio, a smaller number of seed produced, leading to big-size silver nanowires (figure 4d). The optimized ratio of Ag+/Cl- was 150/1. c) a) b) d) 500 nm 500 nm 500 nm 500 nm Figure 4: SEM images of AgNWs synthesized at different Ag+/Cl- molar ratios: 50/1 (a), 100/1 (b), 150/1 (c), 200/1 (d) 3.1.5. Effect of PVP/AgNO3 molar ratio PVP surfactant plays a crucial role in the anisotropic growth of AgNWs by strong binding to the lateral surfaces of crystals (these surfaces have higher energy than the end surfaces), thereby inhibiting the lateral growth.[19] The effect of PVP/AgNO3 molar ratio was investigated and illustrated in figure 5 (these AgNW samples was not purified to observe the byproducts). Note that the calculation of [PVP] is monomer based. When this ratio was small, a significant amount of nanoparticles was produced (figure 5a) since there was not enough PVP to passivate the lateral crystal surfaces. Increasing the PVP/AgNO3 molar ratio led to a decrease in by- product (figures 5b and 5c). However, when the ratio of PVP/AgNO3 was higher, a large amount of silver nanoparticle and short-length silver nanowires were obtained because PVP layer covering the silver nanocrystal was too thick and it passivated not only the lateral but also the end surfaces (figure 5d). Hence, the optimum molar ratio of PVP/AgNO3 was 2.5/1. 3.1.6. Effect of PVP chain length To investigate the effect of PVP chain length on the morphology of AgNWs, two types of PVP with Vietnam Journal of Chemistry Optimization of silver nanowire synthesis for © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 102 different average molecular weights were used and the SEM images of produced AgNWs were shown in figure 6. In the case of PVP 55, the obtained product contained a large amount of byproducts; and the average diameter of the nanowire was large (50 nm) (figure 6a). However, when using PVP 360, byproducts were produced but much less; and the average diameter was also smaller (figure 6b). It is explained by the steric effect.[12] Short-chain PVP imposed incomplete covering of nanowire surface, passivate the lateral surfaces ineffectively, thereby allowing silver atom to deposit onto the lateral surfaces and producing a great deal of nanoparticle and large-diameter silver nanowire. d) a) b) 300 nm 300 nm 300 nm c) 300 nm Figure 5: SEM images of AgNWs synthesized at different PVP/AgNO3 molar ratio: 0.8/1 (a), 1.5/1 (b), 2.5/1 (c), 3.5/1 (d) a) b) 5 µm 5 µm c) d) 5 µm 5 µm Figure 6: SEM images of AgNWs synthesized using different chemicals: different PVP: PVP 55 (a) and PVP 360 (b), AgNWs synthesized in DEG (c), AgNWs synthesized in innert environment by nitrogen bubbling into the system (d). The scale bar in the insets: 500 nm 3.1.7. Effect of solvent EG plays a vital role in the success of silver nanowire synthesis because it is not only a solvent for the reaction to take place but also a reducing agent. For a deeper understanding of the role of EG, AgNW was synthesized in DEG instead of EG and the result was shown in figure 6c. It was interesting that there were no AgNW produced when using DEG as a solvent. It is suggested that the reduction potential of DEG is slightly different from that of EG, which led to the tendency of forming nanoparticles rather than nanowires. 3.1.8. Effect of removing oxygen Generally, oxygen has a profound impact on the synthesis of silver nanowire. It oxidizes ethylene glycol, thereby lowering the reducing power of EG.[13] Thus, the impact of oxygen-removing was studied to guarantee the best condition for the synthesis of AgNWs. So far, most of the procedures for the synthesis of AgNWs reported the degassing process by bubbling nitrogen into the solution. Nevertheless, after investigation, it was found that bubbling nitrogen directly into solution led to the generation of a great amount of big-size silver nanoparticles which are very difficult to be removed by usual purification methods like centrifugation or sedimentation (Figure 6d). This result indicates that bubbling nitrogen into the solution created a negative impact on the AgNWs, and the step of removing oxygen was not necessary for this procedure. 3.2. Electrode fabrication After optimization process, we synthesized AgNWs with the following condition: NiCl2 as controlling agent, PVP 360 as surfactant, AgNO3 concentration of 0.04 M, Ag+/Cl- ratio of 150/1, PVP/AgNO3 molar ratio of 2.5/1, and the reaction was conducted at 140 °C in ethylene glycol. The AgNW sample was used for the fabrication of electrodes on flexible substrates. To find the best condition for electrode fabrication, AgNW solutions with different concentrations were prepared. The electrodes fabricated with these solutions were characterized. The transmittance spectra and FoM values (FoM = T10/R, where T is transmittance measured at 550 nm and R is sheet resistance [20]) of those electrodes were shown in figures 7a and 7b for better comparison. The optimized concentration for the fabrication of the electrode was 3 mg/mL. The electrode obtained from this solution was further checked. SEM images of the AgNW electrode (figure 7c) showed even distribution of silver Vietnam Journal of Chemistry Hoang Mai Ha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 103 nanowire on the PET substrate. Through AFM image (figure 7d), the surface roughness of the electrode was 6.12 nm, qualified for the application of the elec