- Nano Express
- Open Access
Facile Synthesis of Silver Nanowires with Different Aspect Ratios and Used as High-Performance Flexible Transparent Electrodes
© The Author(s). 2017
- Received: 28 June 2017
- Accepted: 31 July 2017
- Published: 7 August 2017
Silver nanowires (Ag NWs) are the promising materials to fabricate flexible transparent electrodes, aiming to replace indium tin oxide (ITO) in the next generation of flexible electronics. Herein, a feasible polyvinylpyrrolidone (PVP)-mediated polyol synthesis of Ag NWs with different aspect ratios is demonstrated and high-quality Ag NWs transparent electrodes (NTEs) are fabricated without high-temperature thermal sintering. When employing the mixture of PVP with different average molecular weight as the capping agent, the diameters of Ag NWs can be tailored and Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are obtained. Using these as-synthesized Ag NWs, the uniform Ag NWs films are fabricated by repeated spin coating. When the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films improve remarkably and match up to those of ITO films. Moreover, an optimal Ag NTEs with low sheet resistance of 11.4 Ω/sq and a high parallel transmittance of 91.6% at 550 nm are achieved when the aspect ratios reach almost 1000. In addition, the sheet resistance of Ag NWs films does not show great variation after 400 cycles of bending test, suggesting an excellent flexibility. The proposed approach to fabricate highly flexible and high-performance Ag NTEs would be useful to the development of flexible devices.
- Ag NWs
- Tailorable aspect ratios
- Flexible transparent electrodes
- Low-temperature welding
Flexible transparent electrodes (FTEs) play an important role in the next generation of flexible electronics [1–4]. FTEs can be applied to many optoelectronic devices as conductive components, involving touch screens [5, 6], portable solar cells [7, 8], organic light-emitting diodes (OLEDs) [9–11], fuel cell electrode [12–17], sensors [18, 19], PM filter , transparent heaters [21, 22], and wearable electronics [23, 24]. The dominant transparent electrodes (TEs) used currently is indium tin oxide (ITO) owing to the low sheet resistance (<100 Ω/sq) and high transmittance (>80%). But its intrinsic brittleness limits the applications in flexible electronics. Moreover, it requires high temperature deposition process and is challenged by the scarcity of indium [25–27]. Therefore, several new conductive films with good flexibility and optical transparency, such as metal grids [2, 28, 29], carbon nanotubes (CNTs) [30–33], graphene [34–36], Ag NWs [5, 37–41], Cu NWs [42, 43], conductive polymers [44, 45], and hybrids of these [46–48], have been fabricated, striving to replace ITO. Among these candidates, Ag NWs films have been investigated extensively in both the scientific and industrial institutions, owing to the excellent electrical conductivity and high optical transparency. In addition, Ag NWs exhibit outstanding flexibility and stretchability, which is the one of the appealing advantage to fabricate stretchable transparent conductors than fragile ITO [49–51]. Moreover, the solution-processed Ag NWs films are more cost-effective than ITO. All of these properties make Ag NWs films become promising alternatives to ITO for the applications in flexible electronics.
However, several issues need to be addressed to commercialize Ag NWs films as FTEs. Firstly, Ag NWs with different aspect ratios need to be facilely synthesized in controlled manner because the alluring properties of Ag NWs films deeply rely on the dimensions of Ag NWs and a well-designed length and diameter are of very importance for different applications [52, 53]. Generally, polyol process is the most widely used method to prepare Ag NWs. Ran et al.  synthesized thin Ag NWs with aspect ratios larger than 1000 by using the mixed PVP with the average molecular weight of 58,000 and 1,300,000 as the capping agent. However, the influence of the aspect ratios on the optoelectronic performance of Ag NTEs was not carefully investigated in their work. Although Ding et al.  prepared Ag NWs with different diameters varying from 40 to 110 nm and fabricated Ag NTEs with a transmittance of 87% and a sheet resistance of ca.70 Ω/sq, many parameters need to be simultaneously adjusted to control the diameters of Ag NWs and the optoelectronic performance of the as-obtained Ag NTEs would not be satisfactory. Li et al.  synthesized thin Ag NWs with diameters of 20 nm through altering the concentration of bromide. And they have fabricated high-quality Ag NWs films with a transmittance of 99.1% at 130.0 Ω/sq. Ko et al.  developed a multistep growth method to synthesize very long Ag NWs over several hundred micrometers and the fabricated films demonstrated superior transmittance of 90% with sheet resistance of 19 Ω/sq. The optoelectronic performance of these Ag NWs films are comparable to or even better than those of ITO films. But the minimum aspect ratio of Ag NWs, which has the ability to fabricate TEs rivaling commercial ITO in terms of sheet resistance and transmittance, is still uncertain. Therefore, it is necessary to synthesize Ag NWs with various aspect ratios and study their influence on the optoelectronic performance of Ag NWs films.
Furthermore, the electronic conductivity of Ag NWs films is relatively poor, resulting from the high nanowire junction resistance . In the polyol synthesis of Ag NWs, PVP, as the surfactant, adsorbs on the surface of Ag NWs, resulting in insulated contact between the wires in the random network [59, 60]. Consequently, different physical and chemical post-processes, involving thermal annealing [38, 39, 61, 62], mechanical press , nanosoldering with conductive polymers , plasmonic welding , laser nanowelding [66–68], and integration with other materials , have been explored to reduce the junction resistance. Among these post-treatments, thermal annealing at almost 200 °C is usually employed. It is incompatible with flexible plastic substrates which cannot withstand high temperature, and hence limits the applications of Ag NWs films in flexible optoelectronic devices.
Herein, a series of Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are controllably synthesized and used to fabricate high conductive and transparent Ag NTEs. First, Ag NWs are prepared by facile PVP-mediated polyol process where the mixture of PVP with different average molecular weight can efficiently reduce the diameters. Subsequently, the as-synthesized Ag NWs with different aspect ratios are employed to fabricate Ag NWs films without high-temperature annealing, respectively. And the corresponding optoelectronic performance are comparative investigated. The best sheet resistance and parallel transmittance can achieve 11.4 Ω/sq and 91.6% when the aspect ratios reach almost 1000. Moreover, the sheet resistance of as-fabricated Ag NWs films is nearly constant after inner-bending and outer-bending tests.
Materials and Chemicals
Silver nitrate (AgNO3, AR) and anhydrous ethanol (C2H5OH, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Copper (II) chloride dehydrate (CuCl2·2H2O, AR) and PVP (MW≈58,000, marked as PVP-58) were purchased from Shanghai Aladdin Reagents Co., Ltd. Ethylene glycol (EG, 98%) and PVP (MW≈10,000, 40,000 and 360,000, marked as PVP-10, PVP-40, and PVP-360, respectively) were purchased from Sigma-Aldrich. Deionized water (18.2 MΩ) was used in the whole experiments.
Synthesis of Ag NWs
Ag NWs with different aspect ratios are prepared by a facile one-pot PVP-mediated polyol process. Typically, 0.170 g of AgNO3 is dissolved in 10 mL of EG under magnetic stirring. Then, 0.15 M of PVP-40 and 0.111 mM of CuCl2·2H2O mixed solution in 10 mL of EG is added dropwise to the above solution. Afterwards, the mixture is transferred into Teflon-lined stainless steel autoclave with a capacity of 50 mL and heated at 160 °C for 3 h. After cooling down to room temperature naturally, pure Ag NWs are obtained by centrifugation at a speed of 2500 rpm for 5 min and washed three times with ethanol and deionized water. Finally, the products are dispersed in ethanol for further characterization and application. Moreover, the concentration and average molecular weight of PVP are very important to control the morphology and size of products. Therefore, different types of PVP molecules are simultaneously used to regulate the diameters of Ag NWs in the polyol process. Detailed experimental parameters are listed in Additional file 1: Table S1, nominated as S1–S13, respectively.
Fabrication of Ag NTEs
Polyethylene terephthalate (PET) with a thickness of 150 μm is cut to pieces with the dimension of 20 × 20 mm. Briefly, the as-prepared Ag NWs are dispersed in ethanol (6 mg/mL), and 50 μL of Ag NWs solution is spin coated at 2000 rpm for 30 s on PET substrate. Finally, the Ag NWs films are heated to 140 °C for 15 min without any additional post-process treatments. The aspect ratios of Ag NWs, rotation speed, concentration, and volume of Ag NWs solution are investigated to fabricate high-quality NTEs. Regarding to the repeated spin coating, each volume of Ag NWs solution is altered to 25 μL and the rotation speed is set to 2000 rpm. A time interval in each spin coating is needed to volatilize the ethanol. Other parameters are same as the aforementioned processes.
Characterization and Performance Test
Scanning electron microscopy (SEM) images are recorded using a cold field-emission SEM (Hitachi S-4800). The transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) images are obtained by using a JEOL JEM-2100F. The UV-vis absorption spectra of Ag NWs and the optical transmittance spectra of Ag NWs films are carried out on a Shimadzu UV-3600 spectrophotometer. The sheet resistance is measured at room temperature by using 4-point probe resistance tester (FP-001).
In addition, the influence of PVP with different molecular weight on the morphology and size of Ag NWs is also discussed. Only Ag NPs and aggregated nanorods are produced when using PVP-10 (sample S5, Additional file 1: Figure S1d). When employing separately PVP-58 (sample S6, Additional file 1: Figure S1e) and PVP-360 (sample S7, Fig. 1b), the corresponding morphology and size of products are changed from stubby Ag NWs (with average diameter of 235 nm and length of 6.7 μm) to high aspect ratio Ag NWs (with average diameter of 132.1 nm and length of 69.9 μm). According to the abovementioned results from samples S2, S5, S6, and S7, the average molecular weight of PVP not only plays a vital role in the morphology formation of Ag NWs but also has a significant influence on the diameter and length of Ag NWs products. The influence of PVP with different average molecular weight on the morphology and size of Ag NWs can be ascribed to three factors: (i) PVP as the capping agent prefers to adsorb on the side faces of MTPs . The strong chemical adsorption promotes the growth of long Ag NWs . (ii) The steric effect of PVP capping layer allows silver atoms to deposit on the side faces through the gap between adjacent PVP molecules, further resulting in the formation of thick Ag NWs . (iii) The high viscosity of PVP with high average molecular weight in EG solution would slow down the growth rate, which are benefit to form MTPs [76, 77]. As a result, the low average molecular weight of PVP, like as PVP-10, would not efficiently adsorb on the (100) crystal faces to restrict the lateral growth. Meanwhile, the small steric effect and low viscosity would not prevent the aggregation of silver nanostructures. PVP with high molecular weight, like as PVP-360, possesses strong chemical adsorption on the side faces to produce long Ag NWs. But the large steric effect of PVP-360 would lead to the increase of diameter.
σ op (λ) is the optical conductivity and σ DC is the direct current conductivity of the film . The value of σ DC/ σ op (λ) are employed as FOM. And a higher value of FOM means better optoelectronic performance. The inset in Fig. 5b exhibits the FOM values of NTEs fabricated by different volume of Ag NWs solutions. When the volume is added to 75 μL, the Ag NWs has the highest FOM value, increasing dramatically from 23.3 to 162.6. It denotes that the balance is achieved between low sheet resistance and high transmittance when implementing three times of spin coating. In addition, Fig. 5c–f shows the SEM images of Ag NWs films on PET with different densities, corresponding to the volume of Ag NWs solutions for 25, 50, 75, and 100 μl, respectively. From the images, it is obvious that the Ag NWs networks become ever denser and the distribution of Ag NWs is more uniform, as increasing the volume of Ag NWs solution. Therefore, the repeated spin-coating process is available to fabricate uniform Ag nanowire films with various transmittance and sheet resistance for different applications.
L is the length of nanowires . This equation implies that the number density of Ag NWs required for percolation network is inversely proportional to the square of length. Hence, long nanowires tend to build a sparse and effective percolation network with a low number density. It can not only increase the light transmission but also improve the conductivity through building long percolation routes with less nanowire junctions.
Z 0 is the impedance of free space (377 Ω). T and R s represent the transmittance and sheet resistance of Ag NWs films, respectively. High values of П mean low sheet resistance and high transmittance. Percolative FOM (П) and conductivity exponent (n) in this work are calculated to be 89.8 and 1.50 by using Eq. (3), respectively. The percolative FOM value is higher than other reported values of various TEs (shown in Fig. 6d). It could be attributed to two reasons: The thin PVP layer (ca. 2 nm) can effectively reduce the nanowire junction resistance. On the other hand, the long Ag NWs (ca. 71.0 μm) form long conductive routes in the percolation networks, resulting in the decrease of number of junctions. Interestingly, the value of n is a non-universal exponent which has been related to the presence of a distribution of nanowire junction resistance [82–84]. Lee et al.  used a laser nano-welding process to reduce the nanowire junction resistance, and the value of n is calculated to be 1.57. The value is close to that in our work. It further suggests that the thin PVP layer and long Ag NWs are efficient to allow low-temperature welding of Ag NWs network.
The video of Ag NWs flexible transparent electrodes. (AVI 9706 kb)
In summary, Ag NWs with different aspect ratios varying from ca. 30 to ca. 1000 are prepared via a facile PVP-mediated polyol process and are applied to the fabrication of high-performance Ag NTEs with low-temperature sintering. In the polyol process, the diameters of Ag NWs are strikingly reduced and the aspect ratios reach almost 1000 when employing mixed PVP as the capping agent. Additionally, when the aspect ratios exceed 500, the optoelectronic performance of Ag NWs films show good transmittance (81.8–87.2%) and electronic conductivity (7.4–58.4 Ω/sq), comparable to those of commercial ITO films (85%, 45 Ω/sq). Furthermore, high-performance Ag NTEs with a transmittance of 91.6% and a sheet resistance of 11.4 Ω/sq are obtained, as the aspect ratios exceed 1000. The long nanowires and thin PVP layer lead to less number of nanowire junctions and reduced junction resistance, respectively. It allows low-temperature sintering of Ag NWs network, which is advantageous for the applications in the flexible plastic substrates. Moreover, Ag NTEs show excellent flexibility against the bending test. We believe that the ability to synthesize Ag NWs with different aspect ratios and fabricate high-performance NTEs with low-temperature welding are very valuable to the development of flexible electronic devices.
This work was supported by the NSFC (51471121), Basic Research Plan Program of Shenzhen City (JCYJ20160517104459444, JCYJ20170303170426117), Natural Science Foundation of Jiangsu Province (BK20160383), and Wuhan University.
QWX completed all the experiments and wrote the manuscript. WJY, JL, QYT, LL, MXL, QL, and RP assisted with the manuscript preparation. WW conceived the study, revised the manuscript, and supervised the work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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