Surfactant-free synthesis of ultralong silver nanowires for durable transparent conducting electrodes

Sian-Hong Tseng a, Lian-Ming Lyu a, Kai-Yuan Hsiao a, Wan-Hua Ho a and Ming-Yen Lu *ab
aDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. E-mail:
bHigh Entropy Materials Center, National Tsing Hua University, Hsinchu 300, Taiwan

Received 13th March 2020 , Accepted 5th May 2020

First published on 5th May 2020

The present study employed the surfactant-free growth of ultralong (∼50 μm) silver nanowires (AgNWs) with a high aspect ratio (more than 1000) by galvanic replacement. AgNW conducting films were fabricated as electrodes using drop-casting. The AgNW film had 90% transmittance to 550 nm light with a sheet resistance of 232 Ω sq−1. Further, the flexibility test of the transparent flexible AgNW/MoS2 device array indicated that the variation of the current was within 5% when a strain of 0.5% was applied to the device; additionally, the device showed a 13% decrease in current after experiencing 50[thin space (1/6-em)]000 bending cycles. This study indicated that the ultralong AgNWs synthesized using galvanic replacement have potential for applications as transparent and flexible electrodes.

Electrodes are essential parts of electronic devices. To effectively transport the carriers, materials with excellent conductivity are required. With technological development, electrodes are expected to possess excellent transmittance and flexibility for various applications. Therefore, developing transparent conducting electrodes has become one of the major goals. Indium tin oxide (ITO)1,2 and conductive polymers3–5 are often used as transparent conducting electrodes (TCEs) in organic light-emitting diodes,6,7 solar cells,8,9 and touch panels.10,11 ITO is highly conductive but expensive and brittle.12–15 Besides, conductive polymers are relatively inexpensive and flexible;16 however, their major disadvantages are poor carrier transport capacity and stability.17

Because of the limitations of ITO and conductive polymers, the development of metal nanowires as TCEs has drawn more attention. Among all metals, silver (Ag) has the highest conductivity and ductility.18,19 Therefore, AgNW-based electrodes possess not only high conductivity but also excellent transmittance and flexibility.20 Previously, Y. H. Duan et al. employed a AgNW–photopolymer (NOA63) film to enhance the mechanical robustness of AgNW films.21 J. Li et al. employed a low energy plasma process to improve the conductance and transmittance of AgNW films.22 Decreasing the diameter and increasing the aspect ratio of AgNWs have been used to improve the performance of TCEs, because the thin AgNWs scatter less light and the high aspect ratio diminishes the number of high-resistance nanowire–nanowire contacts in the electrode.23 The polyol method is commonly used for the synthesis of AgNWs.24–27 However, if polyvinylpyrrolidone (PVP) is not thoroughly removed after the synthesis, the electrical performance is compromised.28 Hence, synthesizing large amounts of ultralong AgNWs without surfactants is crucial for development.

The present study employed the galvanic replacement reaction to successfully synthesize surfactant-free ultralong AgNWs, and as long as the total electrode potential is positive, the replacement takes place spontaneously.29,30 Compared with the polyol method, the proposed method in this study is facile and suitable for mass production. Transmission electron microscopy (TEM) analysis indicated that the obtained AgNWs have highly-dense stacking faults, which can effectively enhance the endurance of AgNWs in electrodes.31 The growth of AgNWs over time is displayed in Fig. S1 (ESI).

When vanadium (V) foil is immersed in a Ag ion solution, a galvanic replacement reaction takes place and then AgNWs are produced (brown products in the solution). The amount of AgNWs did not increase after 4 hours, which implied that the Ag ions in the solution were fully consumed. To understand the growth mechanism of the AgNWs, Ag ion concentration and reaction temperature were adjusted in the synthesis. The scanning electron microscopy (SEM) images in Fig. 1 show the experimental results with different parameters. When the concentration of AgNO3 is 1 mM, a wire-like morphology with a length of ∼50 μm and aspect ratios of more than 1000 could be obtained at different synthesis temperatures (i.e., 25 °C, 50 °C, and 75 °C), as presented in Fig. 1a, d and g, respectively. As shown in Fig. S2 (ESI), when the concentration of Ag ions is too low (e.g., 0.1 mM), irregular shaped nanoparticles are observed. This may be due to insufficient supply of Ag ions. In particular, Ag ions formed nuclei on the surface of the V foil; subsequently, Ag ions continued to react and grew into AgNWs heterogeneously. As the initial concentration of Ag ions increased (e.g., 3 mM or 5 mM), the morphology changed from nanowires to dendrites, as shown in Fig. 1f, h and i. This evolution can be explained by Mullins–Sekerka instability theory.32 During the growth of AgNWs, the Ag ions at the growth front are constantly consumed. Accordingly, the concentration of Ag ions at the front is lower than that on the nanowire side if the Ag ion supply is insufficient, resulting in concentration instability locally at the growth front. Subsequently, new growth fronts are formed at the side of the NW to eliminate the concentration discrepancy, which forms the nanodendrites. As the reaction temperature is increased to 50 °C and 75 °C, the morphology of the product shows wire-like structures, as shown in Fig. 1b, c and e. Although the Ag ions (3 mM and 5 mM) at the growth front continued to be consumed during the reaction, higher reaction temperatures provide higher kinetic energy for Ag ion diffusion, which supplies sufficient ions for growth at the growth front of the NW. Hence, the final product has NWs even at high Ag ion concentrations.

image file: d0cc01915a-f1.tif
Fig. 1 SEM images of the Ag nanostructures synthesized at different Ag ion concentrations and reaction temperatures.

In addition to the effects of Ag ion concentration and reaction temperature on the morphology of the Ag structures, the present study also changed the Ag ion sources (AgClO4 and Ag2SO4) and metal foil (Cu and Zn) to probe their effects on the morphology. The experiments (Fig. S3 and S4, ESI) yielded products with different morphologies, including nanowires, nanoparticles, and nanoflakes. As a result, by controlling the experimental parameters, Ag nanostructures with different morphologies could be synthesized easily and effectively. In particular, the optimized growth conditions for ultralong and uniform AgNWs were reaction with 1 mM AgNO3 solution and V foil at room temperature for a period of time.

Subsequently, TEM was employed to analyze the structure of the AgNWs. Fig. 2a shows a TEM image of a single AgNW with a diameter of ∼50 nm, which is consistent with the results of SEM observations (Fig. 1g). In addition, many fringes can be observed in the NW in the image. Fig. 2b shows the diffraction pattern (DP) of the AgNWs along the [110] zone axis. The diffraction spots correspond to the (002), (1[1 with combining macron]1), and ([1 with combining macron]11) crystal planes of the face-centered cubic-structured Ag. Furthermore, AgNWs were determined to grow along the [1[1 with combining macron]1] direction and the streaks in the DP were likely attributed to the presence of [1[1 with combining macron]1] planar defects in the NW. Fig. 2c shows a high-resolution (HR) TEM image of the AgNWs. The lattice spacing of 0.24 nm corresponds to the (111) crystal plane of Ag, and high-density planar defects (indicated as white arrows in the image) can be observed. Energy-dispersive X-ray spectroscopy elemental analysis (EDS, Fig. 2d) indicates that except for Cu, C, O, and Si signals from the Cu grid and contamination, only the Ag signal is detected and no V signal is detected in the spectrum, implying that pure AgNWs were successfully synthesized by galvanic replacement. The scanning transmission electron microscopy (STEM) image and the corresponding EDS linescan in Fig. 2e reveal the uniform elemental distribution of the AgNWs. The HRSTEM image (Fig. 2f) shows the atomic arrangements of the AgNWs, and an ABC stacking order along the [1[1 with combining macron]1] direction was observed. In addition, the planar defect in the NWs was determined to be an extrinsic stacking fault.

image file: d0cc01915a-f2.tif
Fig. 2 (a) TEM image, (b) diffraction pattern, (c) high-resolution TEM image, (d) EDS image, (e) results of the line scan analysis on EDS, and (f) STEM image of AgNWs.

The conductivity of AgNWs is crucial for application, and the electrical measurement results of the AgNWs (Fig. S5a, ESI) display a linear relationship, that is, the current increases as voltage increases. When 0.1 V of voltage was applied, the current value reached approximately 1 mA, indicating that AgNWs are highly conductive. The linear correlation of the resistances as a function of the geometric parameters [length and sectional area (L/A)] of four AgNW devices is shown in Fig. S5b (ESI), and the relationship can be described as

R = ρ × L/A(1)
where ρ is the resistivity of the AgNW. The calculated resistivity of the AgNWs is approximately 2.07 × 10−7 Ω m. Although the resistivity is higher than that of Ag bulk (1.59 × 10−8 Ω m), the obtained value is similar to that from the literature.33–35 AgNWs were then used to fabricate TCEs by drop-casting the AgNW solution on a PET substrate. It should be noted that surfactants or other organic molecules are usually needed to confine the wire-like morphology for AgNW synthesis in the literature,36–38 and further treatments are required to remove the organic molecule residuals and hence improve the contacts between NWs for fabricating TCEs; however, in the present study, since no surfactant is used during the synthesis, further treatment is not necessary. The distribution of AgNWs on the PET substrate is displayed in the inset of Fig. S5c (ESI). The density of AgNWs on the PET substrate affects the transmittance and conductivity of the TCEs, and Fig. S5d (ESI) presents the transmittance results of the samples with different deposition densities. When the deposition density is 248 mg m−2, TCEs have a transmittance of 90% to 550 nm light and a sheet resistance of 232 Ω sq−1. Fig. S5d (ESI) indicates that transmittance and sheet resistance change with the deposition density; when the deposition density is 992 mg m−2, the transmittance at 550 nm reduces to approximately 50%, and the sheet resistance considerably decreases to 29 Ω sq−1. These results suggest that a higher sheet resistance may be due to loose contacts and non-uniform AgNW TCEs obtained by drop-casting.

On the other hand, MoS2 films were used to test the potential of AgNWs as TCEs for flexible devices. The pyrolysis synthesis of MoS2 films and the fabrication of a transparent flexible device array are described in detail in the ESI. Fig. S6a and b (ESI) show a photograph and an atomic force microscopy (AFM) analysis image of the MoS2 film, respectively, the uniform contrast of the photograph indicates an even thickness of the MoS2 films, and the AFM results reveal the thickness of the films to be about 5 nm, which confirms that the films are multilayered MoS2. Fig. S6c and d (ESI) illustrate the Raman spectrum and the X-ray photoelectron spectrum, which confirmed that the films are composed of MoS2. An illustration and a photograph of the transparent flexible device array are shown in Fig. 3a and b, respectively. The highlighted area in Fig. 3b is the device array region, and the pattern under the device array can be clearly seen, indicating its excellent transparency. Fig. 3c shows a dark-field optical microscopy (OM) image of the device array, and the square areas with a higher contrast are the AgNW electrodes with a density of 248 mg m−2. Fig. 3d shows the transmittance spectra of the AgNW films, 1.7 and 5 nm MoS2 films, and the transparent flexible AgNWs/1.7 and 5 nm MoS2 device array; the transmittance at 550 nm for the AgNW films and the 1.7 nm MoS2 films is about 90%, and for the 5 nm MoS2 films, the transmittance is 75%; however, for the device arrays, the transmittance values approximately decrease by 15%, which indicates that the low transmittance of the device array is mainly influenced by the multi-layered MoS2 films. Furthermore, the MoS2 films show intense absorption at approximately 400 nm, which is attributed to the phonon–electron coupling and C excitons.39–42

image file: d0cc01915a-f3.tif
Fig. 3 (a) Illustration and (b) photograph of the transparent device arrays, (c) dark-field OM image of the transparent device arrays, the electrodes are AgNW films, and (d) transmittance of the AgNW film, 1.7 and 5 nm MoS2 film, and the transparent flexible AgNWs/1.7 and 5 nm MoS2 device arrays.

The electrical transport result of the transparent flexible AgNWs/MoS2 device array is plotted in Fig. 4a. When a +5 V bias is applied to the devices, the current is about 25 nA, and the linear curve suggests that MoS2 forms an ohmic contact with the AgNWs. Moreover, 36 devices in the device array were measured to evaluate the uniformity of electrical performance. Fig. 4b shows the distribution of current of the abovementioned devices when +1 V is applied. Importantly, differences in electrical performance between the devices can be ignored, and all the currents are between 1 and 6 nA with an average current of 3.68 nA. Additionally, the stability of the flexible devices during bending is a crucial factor to show their potential for practical applications. The present study conducted electrical measurements to evaluate the device array under different degrees of bending, and the results in Fig. 4c reveal that the discrepancies between the electrical performances of the devices are all within 5% regardless of the applied tensile strains or compressive strains. The flexibility of MoS2 films and AgNWs enables the device to be durable under strain and helps in maintaining the stability of the device. Devices with different bending cycles were also measured to assess their durability. Fig. 4d displays the changes of the sensitivity of the AgNW electrodes and flexible devices as a function of the bending cycles, suggesting that the differences between the currents are within 3% after 103 bending cycles under 0.33% strain, and then they exhibit an obvious 7% and 13% decrease, respectively, after 5 × 104 bending cycles. Notably, the decrease in the device performance may be facilitated by the deterioration of the contacts between the AgNWs and the MoS2 films. Nevertheless, the flexible device made of the 1.7 nm MoS2 film exhibited notable degradation in electrical performance after frequent bending (Fig. S7, ESI). According to the preceding results, AgNWs used as electrodes for a flexible device array can not only withstand bending strain but also maintain electrical performance after frequent bending tests.

image file: d0cc01915a-f4.tif
Fig. 4 (a) IV curve of the MoS2 transparent device arrays. (b) Current distributions of MoS2 transparent device arrays when 1 V is applied. (c) The variations in electrical properties of the devices under different degrees of bending. (d) The changes of electrical properties of the AgNW electrodes and MoS2 device arrays as a function of the number of bending cycles. I0 and I represent the current of the device before and after a bending test, respectively.

In summary, the present study used a galvanic replacement reaction to effectively synthesize large amounts of ultralong AgNWs with a high aspect ratio. By manipulating the synthesis parameters (i.e., Ag ion concentration, reaction temperature, Ag ion source, and reactive metal types), Ag nanostructures with different morphologies are obtained. The as-synthesized AgNWs contain numerous stacking faults, which is beneficial to stabilizing the structure of the AgNWs during current injection for electrodes. When the deposition density of AgNWs on the PET substrate is 248 mg m−2, the film possesses a transmittance of 90% at 550 nm and a sheet resistance of 232 Ω sq−1. Subsequently, AgNW films as the electrodes were combined with MoS2 film to form a transparent flexible device array. The reliability results of the device under bending revealed that the variations in the current values are less than 5% when the strain on the device reaches 0.5%. In addition, the differences in the currents of the device are within 3% after 103 bending cycles, and the devices are still functional even after 5 × 104 bending cycles. These findings in the present study reveal that the surfactant-free AgNWs are stable and reliable in applications for transparent conducting electrodes.

We acknowledge the financial support from the Ministry of Science and Technology (MOST) in Taiwan under the Young Scholar Fellowship Program (Columbus Program, MOST 109-2636-E-007-017).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cc01915a

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