Research Article www.acsami.org
One-Pot Synthesis and Purification of Ultralong Silver Nanowires for Flexible Transparent Conductive Electrodes Ye Zhang, Jiangna Guo, Dan Xu, Yi Sun, and Feng Yan* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *
ABSTRACT: Metal nanowires (NWs) have become the most promising candidates for the next generation of flexible transparent conductive electrodes (FTCEs), with high transmittance and low sheet resistance. In this work, ultralong silver NWs (Ag NWs), ∼220 μm (even larger than 400 μm) in length and ∼55 nm in diameter (aspect ratio: ∼4000), were synthesized via a one-pot polyol process using high molecular weight poly(vinylpyrrolidone) (Mw = 1 300 000) and an appropriate concentration of FeCl3 (12.5 μM) through hydrothermal reaction. The prepared Ag NWs were purified by a filter cloth (pore size: about 30 × 50 μm2) to remove the Ag nanoparticles and short-length Ag NWs. The FTCE based on the ultralong Ag NWs without any posttreatments exhibits low sheet resistance of 155.0 Ω sq−1 and transmittance of 97.70% at 550 nm. The outstanding performance of FTECs demonstrated that the ultralong Ag NWs are ideal materials for applications in flexible transparent optical devices. KEYWORDS: silver nanowires, ultralong, hydrothermal reaction, filter cloth, flexible transparent conductive electrodes
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INTRODUCTION Flexible transparent conducting electrodes (FTCEs) have shown a spectacular rapid next-generation growth in lowemissivity windows, liquid crystal display, light-emitting diodes (LEDs), large touch screen areas and thin film solar cells.1−10 To date, indium tin oxide (ITO) is still the most commonly used material in transparent conductive electrodes due to its high transmittance and relative low sheet resistance.11,12 However, the high-cost, hard-fabrication process and poor mechanical property of ITO films limit their wider applications.13 With the rapid development of FTCEs, ITO is now being gradually substituted with flexible transparent materials, such as conducting polymers,14,15 carbon nanotube networks,16,17 graphene electrodes,18,19 and metal (silver or copper) nanowires (NWs).20−25 Among all candidates investigated, metal NWs, especially silver NWs (Ag NWs), have attracted considerable attention because of an environment-friendly solution-based fabrication process and their excellent conductivity and easy large-scale preparation in FTCEs.26−32 The attractive characteristic of Ag NW-based FTCEs is strongly dependent on the size and distribution of individual Ag NWs because the Ag NW-based FTCEs are network structures by morphology rather than a continuous array.33−35 It has been generally acknowledged that ultralong Ag NWs with a high aspect ratio possess a promising strategy for improving the transmittance and reducing the sheet resistance of FTCEs. Compared with short NWs, ultralong Ag NWs provide more “open area” for light to pass through.36 In © 2017 American Chemical Society
addition, ultralong Ag NWs can also reduce the internanowire junctions where the major contact resistance occurs.37−39 However, most Ag NWs reported are relatively short in length (100 μm).49,50 Lee et al. synthesized Ag NWs via multisteps, and the NWs showed a broad distribution from 10 to 200 μm of length.49 Bari et al. synthesized Ag NWs through the hydrothermal method.50 However, high reaction temperatures (>200 °C) and long reaction times are usually applied for the preparation of long Ag NWs.51 In addition, both the Received: May 20, 2017 Accepted: July 11, 2017 Published: July 11, 2017 25465
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Filter cloth used for the filtration of Ag NWs, pore size of 50 × 30 μm2, scanning electron microscopy (SEM) images of Ag NWs before (b) and after (c) filtration (the inset shows the pentatwinned structure (red circles)), (d) SEM image of ultralong Ag NWs with diameter of ∼55 nm, the distribution of (e) length, and (f) diameters in a count of 200 Ag NWs. ethanol (EtOH, 99%), and acetone (98%) were acquired from Sinopharm Chemical Reagent Co., Ltd.; anhydrous ferric chloride (FeCl3, 99%), ferric nitrate (Fe(NO3)3, 99%), thermal water kettle (volume: 50 mL), and poly(ethylene glycol)terephthalate (PET) were purchased from Shanghai Chemical Reagent Co., Ltd.; deionized (DI) water was used throughout the experiments. Synthesis of Ag NWs. Ultralong Ag NWs were synthesized by the following method: first, 0.3 g PVP and AgNO3 (0.20 g, 1.18 mmol) were dissolved in 50 mL of EG. Then, FeCl3 (0−25.0 μM) solution was added and stirred at room temperature. The obtained mixture was then transferred into a 50 mL Teflon thermal water kettle and reacted at 130 °C for 8 h till the reaction was completed. Purification of Ag NWs. After the completion of reaction, acetone was used to wash the product. This washing process was repeated twice to remove the extra solvent and chemical agents (PVP and other reactants). The Ag NWs were then redispersed in ethanol for filtration. Here, a piece of filter cloth (see Figure 1a) with pore size of 30 × 50 μm2 was used to remove the short-length Ag NWs and NPs. The purified ultralong Ag NWs left on the filter cloth were collected for the next fabrication process of FTCEs. Fabrication of FTCEs. PET membranes (50 × 50 mm2) were used as the substrate for the ultralong Ag NW-based FTCEs. The PET
short-length Ag NWs and nanoparticles (NPs) remaining in the products affected the performance of FTCEs. Herein, we report the synthesis of ultralong Ag NWs (∼220 μm in length, ∼55 nm in diameter, and an aspect ratio of ∼4000) using PVP and FeCl3 as the capping agents through the hydrothermal reaction. Compared to that in the results reported earlier, our method could be carried out at a relatively lower reaction temperature (130 °C). For the first time, filter clothes were used for the purification of synthesized Ag NWs to remove the unwanted short-length Ag NWs and Ag NPs (see Figure 1a). The purified ultralong Ag NWs with uniform length were further applied to fabricate FTCEs through spin-coating, which show a low sheet resistance of 155 Ω sq−1 and high transmittance of 97.70% at 550 nm.
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EXPERIMENTAL SECTION
Materials. Silver nitrate (AgNO3, 98%), sodium chloride (NaCl, 98%), and potassium chloride (KCl, 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd.; PVP (K85−K95, Mw = 1 300 000) was provided by J&K Scientific; EG (analytical reagent grade), 25466
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
Research Article
ACS Applied Materials & Interfaces
400 μm (Figure S2b) and diameters of ∼55 nm (Figure 1d) can be seen. The histograms below (Figure 1e,f) show the distribution of ultralong Ag NWs in length and diameter after filtration in a count of 200 Ag NWs. Most of the Ag NWs are about 250 μm in length and 55 nm in diameter, further indicating the high efficiency of the filtration. XRD measurement was performed to analyze the structure and crystallinity of the ultralong Ag NWs. As shown in Figure S3, five diffraction peaks assigned to {111}, {200}, {220}, {311}, and {222} planes of the face-centered cubic (fcc) Ag can be observed. The lattice constant was calculated to be 4.081 Å, which is consistent with the standard value (4.086 Å, Joint Committee on powder diffraction standards file No. 04-0783). High-resolution TEM (HR-TEM) was further used to understand the ultralong Ag NWs. Figure 2a shows the HRTEM image of the Ag seed. The Ag seed showed a pentagonal structure and was capped by five {111} facets at one end. Figure 2b shows the structure models of fivefold twinned Ag NWs, and the relative five subunits are labeled from T1 to T5. The electron beam running perpendicular (Figure 2c,d) and parallel (Figure 2e,f) to one side surface of Ag NWs was applied to investigate the selected area electron diffraction (SAED) pattern of ultralong Ag NWs. The lattice spacing of Ag NWs was determined to be 0.238 nm (Figure 2c,e), which is in agreement with the d value of the {111} plane of Ag.53 The Fourier patterns (white boxes) embedded in Figure 2c,e give rise to the diffraction pattern corresponding to the single crystal with the fcc structure along the ⟨110⟩ zone axis direction.54 Figure 2d shows the SAED pattern of the electron beam that runs perpendicular to the Ag NWs; the SAED pattern is overlapped by two fcc patterns composed of zone [001] generated from T1 and zone [112] generated from T3 and T4. Figure 2f was acquired from the [110] zone axis direction generated from T5 and the ⟨111⟩ direction generated from T2 and T3 when the electron beam ran parallel to the Ag NWs. The remaining relative weak diffraction spots in the parallel lines can be attributed to the double-diffraction effects through the subcrystals and the twin boundary-related diffraction. The pentatwinned crystals cause relatively weak double diffraction spots in between the stronger primary spots.55 Figure 3 shows the proposed growth mechanism of Ag NWs via a polyol process. In the first stage of the reaction, decahedral silver seeds with pentagonal structures are formed in the solution. Because of the imperfect space-filling among the five single-crystal subunits that form a Ag decahedral seed, inherent internal strain is distributed over their twin boundaries, which provides the highest energy sites for the deposition of Ag atoms and subsequent growth. As the reaction proceeds, PVP and Cl− anions adsorb and bind preferentially on the newly formed {100} facets, which can inhibit the lateral growth rate and the Ag atoms continually deposit along the ⟨111⟩ direction. Then, the obtained Ag NPs elongate into ultralong Ag NWs with pentagonal cross sections.44 It has been widely recognized that halide ions play an important role in controlling the diameter of Ag NWs.28,56 Halide ions can passivate the {100} surface of Ag, which can induce the anisotropic growth and prevent the lateral growth of Ag NWs. Ag nanocrystals with small diameters will be generated and thus long Ag NWs with a high aspect ratio can be obtained.53 Here, the concentration effect of FeCl3 on the diameter of Ag NWs was investigated and the results are shown in Figure 4 and summarized in Table 1. Figure 4 shows SEM images of the Ag NWs prepared using various
membranes were thoroughly cleaned for 10 min with DI water, acetone, and ethanol and dried at room temperature. Then, 0.5 g of ultralong Ag NWs dispersed in 50 mL of ethanol was dropped on the PET membrane surface and spin-coated at 400 rpm. The as-prepared FTCEs were dried at room temperature before the characterization. Characterization of the Ag NWs and FTCEs. The shape and distribution of the Ag NWs were investigated using field emission SEM (FE-SEM; Hitachi S-4700) and a Tecnai G220 transmission electron microscope (TEM). X-ray diffraction (XRD) measurements were performed to analyze the structure and crystallinity of the Ag NWs at a scan rate of 20 min−1, with a Cu Kα (k = 1.54056 Å) radiation (Rigaku Model D/MAX-2500V/PC). The solid UV/vis absorption spectrum was obtained using a UV-3150 UV−vis-NIR spectrophotometer from Shimadzu Scientific Instruments. The sheet resistance of the FTCEs were measured using the four-probe method (Loresta GP T610; Mitsubishi Chemical Analytech Co., Ltd.).
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RESULTS AND DISCUSSION In this work, Ag NWs were prepared through a typical polyol method: a mixture containing PVP, AgNO3, and FeCl3 in EG was reacted in a hydrothermal kettle at 130 °C for 8 h. The obtained products were further purified by filtration. As can be seen from Scheme 1, the cooled reaction mixture was first Scheme 1. Purification and Filtration Process of the Prepared Ultraong Ag NWsa
a Acetone is first used to remove the remaining chemical reactants. The short-length Ag NWs and Ag NPs could pass through the filter cloth.
poured into acetone to remove the unreacted reactants. After the removal of supernatant acetone, the aggregated Ag NWs were redispersed in ethanol for filtration. Figure 1a shows the SEM image of the filter cloth used in this work. Pore sizes with 50 μm length and 30 μm width were observed. Therefore, it is expected that most of short-length Ag NWs and Ag NPs could pass through the filter cloth. The ultralong Ag NWs retained on the surface of the filter cloth can be easily removed by ethanol, and the yield of Ag NWs was calculated to be about 92.1% (Figure S1). Figure 1b shows the SEM image of the as-synthesized Ag NWs before filtration. Both Ag NWs and Ag NPs can be clearly seen. The Ag NPs almost disappeared after the filtration (Figure 1c). The inset in Figure 1c shows that the Ag NWs with the twin boundary of single Ag NWs along the longitudinal direction can be found (red circles), indicating the pentatwinned structure of Ag NWs.52 Figure S2a shows the SEM image of the filtrate; short-length Ag NWs and Ag NPs can be observed, demonstrating high filtration efficiency. Furthermore, Ag NWs with lengths of over 25467
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
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ACS Applied Materials & Interfaces
Figure 2. (a) HRTEM image of a Ag seed with pentagonal structures and capped by five {111} facets at one end. (b) Model of the fivefold twinned Ag NWs consists of five subunits labeled T1−T5, and the electron beam runs perpendicular and parallel to one side surface of Ag NWs. (c, e) HRTEM images of Ag NWs (insets show fast Fourier transform spectra of the white boxes). (d, f) Typical SAED patterns; the electron beam runs perpendicular and parallel to one side surface of Ag NWs, respectively.
Figure 3. Proposed growth mechanism for Ag NWs with pentagonal structures. Five-twinned seeds generated in the first stage, and PVP and Cl− tend to adsorb on the {100} facets. Then, Ag NPs continue to deposit and recrystallize on {111} facets to form the Ag NWs. 25468
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
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Figure 4. (a) Proposed FeCl3 contents’ effect on the synthesis of Ag NWs, (b) without FeCl3, (c) 9.0 μM FeCl3, (d) 12.5 μM FeCl3, and (e) 25.0 μM FeCl3. The diameter of Ag NWs decreased with the increasing concentration of FeCl3 (from (b) to (d)), and the main products are Ag NPs (e).
It has been widely recognized that PVP with high molecular weight can serve as a stabilizer and capping agent for an Ag {100} surface through a stronger binding strength resulting from the polyvalency effect.62 In addition, PVP can also increase the yield of Ag NWs; however, it does not influence the diameter of Ag nanomaterials.63 Here, high molecular weight PVP (Mw = 1 300 000) was used as the capping ligand.52 The molar ratio of PVP/AgNO3 also highly affected the aspect ratio of Ag NWs. Figure 6 shows the SEM images of Ag NWs prepared under various PVP/AgNO3 ratios. As it can be seen, the higher the PVP/AgNO3 ratio, the shorter the length of Ag NWs. The results are consistent with the previous results reported.53 A higher concentration of PVP means that PVP may not only adsorb onto and passivate {100} facets but also hinder the deposition of Ag atoms onto the {111} facets, resulting in short-length Ag NWs. It is anticipated that the ultralong Ag NWs with diameter of ∼55 nm synthesized in this work are flexible and could not be broken into several segments while being bent. Simultaneously, the flexible Ag NWs have more opportunities to connect with each other directly during the fabrication process of FTCEs, which makes it possible to form FTCEs with higher transmittance by using fewer Ag NWs. Furthermore, the smaller diameter of Ag NWs can effectively avoid the scattering effect of light that prevents the penetration of light and the haze of FTCEs can also be effectively reduced.51 Ethanol solution containing uniform ultralong Ag NWs was prepared for the fabrication of FTCEs through spin-coating. Figure 7a shows the transmittance of FTCEs with various filtration numbers (FNs), using a pristine PET membrane as the reference. Several waves between 500 and 800 nm derived from the pristine PET membrane were observed (inset in Figure 7a). In addition, the transmittance of FTCEs decreased with the increasing of FNs; for example, transmittance decreased from 97.70 (FN = 1) to 85.76% (FN = 8). Figure 7b shows the transmittance versus sheet resistance for FTCEs
Table 1. Effect of FeCl3 on the Ag NWs Ag NWs FeCl3 (μM) 0 9.0 12.5 25.0
length (μm)
diameter (nm)
215 ± 20 180 ± 25 120 ± 15 215 ± 20 220 ± 20 55 ± 5 Ag NPs (size: ∼3 μm)
aspect ratio ∼1200 ∼1800 ∼4000
concentrations of FeCl3. Without the FeCl3, Ag NWs with a diameter of about 180 ± 25 nm were obtained (Figure 4b), which agrees well with the results reported by Wiley et al.11 It can be seen that the diameter of Ag NWs decreased with the increasing concentration of FeCl3. For example, the diameter decreased from ∼180 to ∼55 nm with the concentration of FeCl3 increased from 0 to 12.5 μM. Further increase of the concentration to 25.0 μM mainly produced Ag NPs (∼3 μm in diameter, see Figure 4e). The results may be due to oxidative etching of the oxygen dissolved in the solvent, especially under the high concentration of Cl−.57,58 It has been reported that Fe3+ may also affect the formation of Ag NWs.59−61 Here, the effect of Fe3+ on the diameter of Ag NWs was investigated too. Figure 5a shows the SEM image of Ag NWs prepared with 12.5 μM Fe(NO3)3 (without Cl−). A diameter of about 175 ± 25 nm was observed, which is about 5 nm smaller than that of the Ag NWs observed in Figure 4b (synthesized without FeCl3). The result suggested that Fe3+ could slightly decrease the diameter of Ag NWs to a certain degree due to the oxidative etching of Fe3+.60 Figure 5b,c shows SEM images of Ag NWs, with 37.5 μM NaCl and KCl, respectively. Compared with Figure 5a, the diameter of Ag NWs was dramatically decreased to about 57 ± 5 nm. Figure 5d exhibits Ag NWs synthesized with both 12.5 μM Fe(NO3)3 and 37.5 μM NaCl. The synthesized Ag NWs exhibit the same diameter as that with 12.5 μM FeCl3 (depicted in Figure 4d). Therefore, it can be concluded that Cl− has the dominant influence on the Ag NWs’ diameter than Fe3+. 25469
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
Research Article
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Figure 5. SEM images of Ag NWs prepared with (a) 12.5 μM Fe(NO3)3, (b) 37.5 μM NaCl, (c) 37.5 μM KCl, and (d) 12.5 μM Fe(NO3)3 + 37.5 μM NaCl.
Figure 6. SEM images of Ag NWs prepared under various molar ratios of PVP/AgNO3. (a) PVP/AgNO3 = 1.5:1, (b) PVP/AgNO3 = 2:1, (c) PVP/ AgNO3 = 2.5:1, and (d) PVP/AgNO3 = 3:1. A higher molar ratio of PVP/AgNO3 leads to shorter Ag NWs and a reduced aspect ratio.
(or TCEs) in the reported results.11,33,37,51 It can be seen that our FTCE is only slightly poorer than that of Ag NWs-based TCEs reported by Li et al.11 These results further confirm the advantageous effects of the ultralong Ag NWs with a high aspect ratio on the performance of Ag NWs-based FTCEs.
We also used an LED lamp to detect the conductivity of the FTCEs. Figure S4 shows four FTCEs with different transmittances. The results revealed that the LED lamp becomes brighter with the decreasing of transmittance because of the decreased sheet resistance. To prove the flexibility of the Ag NWs, the fabricated FTCEs were bent to various angles. The 25470
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
Research Article
ACS Applied Materials & Interfaces
Figure 7. (a) Transmittance of FTCEs fabricated in this work (vs that of pristine PET as the reference). Inset shows the transmittance of the pristine PET membrane in air. (b) Transmittance vs sheet resistance for FTCEs of our work and reported results.
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sheet resistance versus 1000 bending−releasing cycle numbers was carried out to examine the stability of sheet resistance of the FTCEs. Figure 8 shows the normalized sheet resistance of
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07146. Additional characterization, including SEM images, and XRD characterization of the Ag NWs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jiangna Guo: 0000-0002-4052-252X Feng Yan: 0000-0001-9269-7025 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation for Distinguished Young Scholars (No. 21425417), the National Natural Science Foundation of China (No. 21274101), and the National Nature Science Youth Foundation of China (No. 21603156) and the project was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Figure 8. Normalized sheet resistance of FTCEs (A−C represent FTCEs bended with different angles. A: 45, B: 90, and C: 135°) after applying multiple numbers of bend−release cycles. The sheet resistance of the electrodes remains at their original value during the whole experiment. All FTCEs tested were made from the same batch of ultralong Ag NWs, with the same fabrication process.
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FTCEs after the multiple bending cycles. We can see that the sheet resistance is nearly maintained at its original level after 1000 bending cycles. The results may be due to the high flexibility of ultralong Ag NWs with smaller diameters.
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CONCLUSIONS In conclusion, we have synthesized ultralong Ag NWs with a high aspect ratio (>4000). Key factors to the success of this synthesis are using high molecular weight PVP and proper concentration of FeCl3 to cover the {100} plane and the oxidative etching of Ag to inhibit the lateral growth of Ag. In addition, filtration is an effective way to remove the Ag NWs and short-length Ag NWs. The uniform ultralong Ag NWfabricated FTCEs show the best performance, with a sheet resistance of 155.0 Ω sq−1 and transmittance of 97.70% at 550 nm without any post-treatments. In addition, the fabricated FTCEs are flexible and can maintain their original sheet resistance after 1000 bending−releasing cycles. The ultralong Ag NWs with high aspect ratios offer the potential for many other applications in flexible transparent optical devices. 25471
DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
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DOI: 10.1021/acsami.7b07146 ACS Appl. Mater. Interfaces 2017, 9, 25465−25473
