Letter pubs.acs.org/Langmuir
Flexible Transparent PES/Silver Nanowires/PET Sandwich-Structured Film for High-Efficiency Electromagnetic Interference Shielding Mingjun Hu, Jiefeng Gao, Yucheng Dong, Kai Li, Guangcun Shan, Shiliu Yang, and Robert Kwok-Yiu Li* Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, PR China S Supporting Information *
ABSTRACT: We have developed a kind of high-yield synthesis strategy for silver nanowires by a two-step injection polyol method. Silver nanowires and polyethylene oxide (PEO) (Mw = 900 000) were prepared in a homogeneouscoating ink. Wet composite films with different thicknesses were fabricated on a PET substrate by drawn-down rodcoating technology. Silver nanowires on PET substrates present a homogeneous distribution under the assistance of PEO. Then PEO was thermally removed in situ at a relatively low temperature attributed to its special thermal behavior under atmospheric conditions. As-prepared metallic nanowire films on PET substrates show excellent stability and a good combination of conductivity and light transmission. A layer of transparent poly(ethersulfones) (PESs) was further coated on silver nanowire networks by the same coating method to prevent the shedding and corrosion of silver nanowires. Sandwich-structured flexible transparent films were obtained and displayed excellent electromagnetic interference (EMI) shielding effectiveness.
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INTRODUCTION In past decades, novel materials have been a very hot research topic because of their important roles in engineering as well as in science. Those materials with a remarkable combination performance as the important components of various devices and infrastructures are attracting more and more attention.1,2 Flexible transparent conductive films, which incorporate good impact resistance, light transmission, and conductivity, have shown great promise in flexible electronics such as solar cells, organic light-emitting diodes, and flexible displays.3−9 Because of good combination properties, these films can also be used in the reception and transmission of electromagnetic waves as well as the reflection and absorption of electromagnetic interference, such as antennas and EMI shielding materials. However, their performance and application in this field, such as EMI shielding, were rarely investigated. Electromagnetic interference is an undesirable byproduct emitted by electrical circuits that carry rapidly changing current, which could cause noise signals and even the malfunction of electronic devices under normal operation and probable radiative damage to the human body.10 With the fast development of electronic and wireless technology, EMI interference is becoming a serious global problem. Currently, metals and their composites still dominate EMI shielding materials because of their high shielding effectiveness (SE). Nevertheless, excessive weight or highdemand thin-film-making procedures such as sputtering and electrodeposition introduced some inconvenience for their wide application. Subsequently, conductive polymers and polymer composites were put forward for EMI shielding © 2012 American Chemical Society
because they are lightweight, processable, and have good corrosion resistance.11−15 However, low conductivity largely limited their shielding effectiveness. To achieve the same effectiveness with metal materials, a large filler content and volume were usually required. Hence, there is a growing requirement to develop more advanced materials for convenient high-efficiency EMI shielding. As a result of the skin effect, namely, that electromagnetic radiation at high frequencies penetrates only the near surface of an electrical conductor, and multiple reflection, nanomaterials with good conductivity and a large surface area have been attractive candidates in EMI shielding.13 Obviously, silver nanowires can well meet these requirements. Moreover, silver nanowires can be homogeneously paved on flexible substrates to construct conductive networks with good light transmission so as to obtain free-standing flexible transparent conductive films for applications where good transparency was required, such as the protective covers of keyboards, displays, observation windows, and so on to shield electromagnetic interference from the outside and electromagnetic radiation of themselves. Recently, silver nanowires have been star materials in building macroscale flexible conductors because of their high conductivity, good flexibility, and mature preparation technology, and various flexible conductive materials and devices have been fabricated on the basis of silver nanowires.16−21 In this Received: February 19, 2012 Revised: April 25, 2012 Published: April 25, 2012 7101
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Figure 1. (a) As-prepared typical silver nanowires; (b, c) silver nanowire PEO composites at different magnifications. Cu Kα radiation (λ = 1.54178 Å). Thermal gravimetric analysis (TGA) was performed using a TGA Q50. EMI SE was measured at room temperature in the X-band frequency range of 8−12 GHz using an HP8510c vector network analyzer, and an electromagnetic wave was injected into the film using a waveguide setup.
article, we will focus our attention on the preparation of silver nanowire-based flexible transparent conductive films and their application in EMI shielding. A new PVA-assisted solutionprocessing method was developed for the preparation of nanowire-based films on a large scale. Excellent film stability, good conductivity, and light transmission were obtained. Then by using a layer of flexible transparent poly(ethersulfone) (PES) to coat silver nanowires, a sandwich-structured PES/ silver nanowire/PET film was fabricated and excellent EMI shielding effectiveness was achieved.
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RESULTS AND DISCUSSION Silver nanowires were obtained through a two-step-injection polyol method developed by us.22 Figure 1a shows SEM images of typical silver nanowires synthesized by this method. In our work, a drawn-down rod-coating technique was used for the fabrication of silver nanowire films, which is a well-known scalable coating technique that is widely used by laboratories for the fabrication of liquid thin films in a continuous and controlled manner. In this technique, the preparation of the coating ink is a critical aspect. A good dispersion of silver nanowires should first be required. In addition, the coating ink should have specific rheological behavior and wetting properties so as to carry sufficient silver nanowires to form a uniform, continuous coating layer. Thus, low surface tension to facilitate the spreading of coating ink on a wide range of substrates and avoid defects such as contact line recession and dewetting and high viscosity to slow secondary flows induced by surface tension and dewetting forces, allowing the film to dry without flow, were preferred.23 Herein, PEO with similar molecular structure with surfactant poly(ethylene glycol) (PEG) but with a much higher molecular weight was used to assist film formation for the following reasons: first, PEO is water-soluble at room temperature; second, PEO has a high molecular weight and can increase the viscosity of the coating ink dramatically; third, PEO can play a role similar to that of surfactants to improve the dispersion of silver nanowires; fourth, PEO is alcohol-soluble under heating but can precipitate together with silver nanowires at room temperature to generate solid composite sediments with good redispersion ability and is easy to collect, transfer, and transport, which is beneficial to commercialization (SI Figure S1); and last but especially important is that PEO has special thermal behavior and can thermally decompose at a relatively low temperature. The solvent is another important factor that affected the quality of the coating ink. Solvents with high surface tension are usually undesirable because they tend to result in the aggregation of carried solids as a result of contact line recession and dewetting during evaporation. Nontoxic ethanol with a lower boiling point and a lower surface tension than water is favorable. However, the poor solubility of high-molecularweight PEO in pure ethanol at room temperature made it inconvenient to use the alcohol solvent. The mixed solvent with 10 vol % water and 90 vol % ethanol was found to dissolve PEO well at room temperature and was used as the solvent for the silver nanowire coating ink.
EXPERIMENT SECTION
Ethylene glycol (EG), ethanol, cyclohexanone, poly(ethylene oxide) (PEO, Mw = 900 000 g/mol), and NaCl were purchased from SigmaAldrich, and AgNO3 and poly(vinylpyrrolidone) (PVP, Mw ≈ 40 000) were purchased from Shanghai Chemical Reagent Company. Poly(ethersulfone) (PES) 4100G was purchased from Sumitomo, Japan. All chemicals were used as received without further purification. Synthesis of Silver Nanowires. Silver nanowires were synthesized through a two-step-injection polyol method with some modifications.22 In a typical synthesis of silver nanowires, 1.3337 g of PVP (Mw ≈ 40 000) was dissolved in 36 mL of ethylene glycol (EG), and then the solution was heated to 160 °C. Once the temperature was stable, 80 μL of a 0.2 M sodium chloride (NaCl) EG solution was added and stirred for 1 min, and then 20 μL of a 1 M AgNO3 EG solution can be added and stirred for 5 min. Subsequently, 4 mL of a 1 M AgNO3 EG solution was added to the flask drop by drop at a rate of 0.4 mL/min. Once the reaction solution began to become turbid, all residual solution was added to the flask immediately. Then the flask was sealed until the solution glistened, indicating the formation of silver nanowires. Preparation of a Silver Nanowire PEO Composite Conductive Film. In a typical preparation process, silver nanowires were centrifuged and washed three times with ethanol and then dispersed in ethanol. A small amount of water was added to make 1:9 v/v water/ ethanol. The initial concentration of silver nanowires was obtained through measuring the weight of silver solids in 50 μL of solvent by the mass balance of TGA Q50. PEO was first dissolved in a mixed solvent of 10 vol % water and 90 vol % ethanol and then mixed with a silver nanowire solution under stirring to obtain a silver nanowire (9.6 mg/mL)/PEO (5 mg/mL) composites solution. Wet composite films with different thicknesses were prepared on PET substrates by Meyer rods and then immediately put in an oven at 50 °C to dry for 5 min before being transferred to a 160 °C oven for 10 min to remove PEO. Then the films were hot pressed. Preparation of a PES/Silver NW/PET Sandwich Transparent Film. Poly(ethersulfone) (PES) was dissolved in cyclohexanone to form a fresh 50 g/L polymer solution. A PES wet film was used to coat the silver nanowire layer by a drawn-down rod-coating technique to form a PET/silver NW/PES three-layered structure. Characterization. The morphology, dispersion, and distribution of silver nanowires were observed by scanning electrode microscopy (FESEM, JEOL JSM-820); the sheet resistances of nanowire films were detected by a standard four-probe method; the light transmission of nanowire films was measured with a PE Lambda 750 UV spectrometer. XRD patterns were recorded on a Philips X’pert diffractometer with 7102
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Figure 2. Distribution of silver nanowires at different PEO concentrations: (a) without PEO and at (b) 0.25 g/100 mL, (c) 0.5 g/100 mL, and (d) 2 g/100 mL, CAg = 9.6 mg/mL.
Figure 3. (a) Silver nanowire PEO composites film before heat treatment. (b) Silver nanowire film after heat treatment at 160 °C for 10 min. (c) Schematic diagram of the thermal removal of PEO. (d) XRD patterns of a silver nanowire composite film before and after heat treatment.
indicated that PEO begins to decompose thermally near 155 °C and can be decomposed by about 95% at 160 °C (SI Figure S3). PET can stand this temperature for a short time, but in order to avoid a possible shape change, when PEO is removed thermally, the PET film should be put on flat glass or another thermally resistant plate. In a short time, such as 10 min, the influence of the heat treatment on the color and shape of the PET film was slight (SI Figure S4), but a thin layer of PEO can be well removed. Figure 3 shows XRD patterns and SEM images of a silver nanowire film before and after heat treatment. It can be seen from SEM images that a layer of polymer film with some cracks were exhibited clearly on the PET substrate before heat treatment but disappeared afterwards. The corresponding results can also be reflected in the XRD patterns. The composite film had an additional peak between 20 and 30°, which disappeared after heat treatment. All of these results indicate that PEO can be well removed during heat treatment. Then the stability of the composite film was investigated (SI Figure S4). It was found, before heat treatment, that the composite films have been very stable, and even when scotch tape test was applied, it was hard to tear this film out of the substrate. However, because of the inherent aqueous solubility of PEO, the composite film is not resistant to water and is easily scraped off by something containing water, such as a cotton swab with water. But after heat treatment, the film became water-resistant and more stable mechanically. A small amount of residual PEO was thought to play an important role in stabilizing the silver nanowire film. Possible cross-linking of
Then we investigate the influence of the PEO concentration on the film formation of silver nanowires. Figure 2 shows the distribution of silver nanowires on PET substrates under different PEO concentrations. When no PEO was added, silver nanowires did not easily form a continuous conductive network, and obvious aggregation could be observed (Figure 2a). Evidently, a small amount of PEO can improve the distribution of silver nanowires. When the concentration is about 0.5 g/100 mL, silver nanowires presented almost the best distribution on the PET substrate on the microscale as well as on the macroscale. Different thicknesses of wet films were fabricated by different Meyer rods wound with different diameters of metallic wires (SI Figure S2). Upon increasing the thickness of the wet film, silver nanowires exhibited a denser distribution and the contact points between silver nanowires increased exponentially, which is in favor of the improvement in conductivity. However, because of the existence of isolating PEO, the conductivity of composite film is unsatisfying. There is a need to remove PEO. Two approaches can be considered to achieve this purpose: solvent removal and thermal removal. In this case, solvent removal is not very good because silver nanowires are very easy to remove together with PEO. Distinct from solution removal, thermal removal involves removing unwanted substances by pyrolysis gasification. Most polymers can be decomposed into gaseous small molecules under high temperature, but complete decomposition usually requires high temperature. PEO is, however, an exception. It can be almost completely thermally decomposed at a relatively low temperature. TGA curves 7103
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Figure 4. Conductivity and light transmission of as-prepared silver nanowire films with different thickness of silver nanowire layer.
Figure 5. (a) Photograph of a PES/PET double-layer transparent film; (b, c) photographs of a PES/silver nanowire/PET sandwich-structured film; (d) schematic diagram of a sandwich film; (e) EMI SE of the films with different thicknesses of the silver nanowire layer.
PEO during heat treatment lead to the generation of waterinsoluble substances and endowed the silver nanowire film with better solvent resistance, which will enormously benefits the further treatment of nanowire films such as electrodeposition, in situ chemical reaction, and further coating with other substances such as polymers and semiconductors. Except for the stability, the influence of the oxidation of silver nanowires during heat treatment on the conductivity may be
another problem that everybody was concerned about. In our experiments, we try to immerse the film in a hydrazine hydrate solution for several minutes to exclude possible silver oxides, but we find no obvious improvement in the conductivity. Therefore, we think that the possible oxidation of silver nanowires during short-time heat treatment has little influence on the conductivity of silver nanowire films. 7104
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absorptivity increased first and then decreased, and the transmissivity increased continuously to complete transmission. Therefore, for films with low sheet resistance (Rs), in consideration of the main contribution from the reflection on EMI SE, we try to build the relationship between EMI SE and log Rs. As was seen in Figure 6, when the sheet resistance is low,
Furthermore, the conductivity and light transmission were investigated. As-prepared conductive films were named 10c, 15c, 25c, 40c, and 60c according to the diameters of metal wires wound on Meyer rods. From Figure 4 we can see, with increasing thickness of the silver nanowire layer, that the optical transparency of the film gradually becomes poor and the sheet resistance decreases dramatically. When the sheet resistance was reduced to 14.7 Ω/square, the optical transparency was still above 80%. Although this performance may be a little poorer than for the silver nanowire film prepared by vacuum filtration, it is comparable or even better than many other methods.18,19,24−27 The uniformity of the conductivity over a large film area is not very satisfying for films with small thicknesses of the silver nanowire layer, such as 10c and 15c, but for films with a large thickness of the silver nanowire layer, such as 25c, 40c, and 60c, the reliability of the conductivity is very good and the deviation of the sheet resistance from the average value is basically within 10%. Owing to excellent film stability and good solvent resistance, PES, a kind of flexible transparent polymer with excellent hightemperature behavior and good surface smoothness, was used to coat the silver nanowire film to prevent nanowires from dropping off during scratch and collision and possible corrosion by chemicals. Sandwich-structured films were obtained, and their electromagnetic interference shielding effectiveness (EMI SE) in the X-band frequency range was investigated. Figure 5e shows the EMI SE of different transparent films with different thicknesses of the nanowire layer. As was seen, excellent EMI SE was obtained. For the 60c film at a frequency of 8 GHz, the value of EMI SE can even achieve 38 dB. Even for the 25c film with 81% light transmission, EMI SE can still be above 25 dB, which is much larger than 20 dB, which was required in commercial applications and also much better than the performance of many conductive polymer composites and carbon-based conductive films.28−30 Moreover, silver nanowire films almost without continuous conductivity over a large area were also prepared by the same rod-coating technique but without the addition of PEO to testify to the influence of the conductivity on EMI SE. The corresponding films were named 10f, 15f, 25f, 40f, and 60f. It was found that their EMI SE is much worse than for those films with obvious continuous conductivity (SI Figure S5), which indicates that EMI SE of the films was mainly contributed by the conductivity rather than the quantity of silver nanowires, which maybe bring a small effect on SE. As was known, EMI SE of a material can be expressed as SE(dB) = −10 log(Pt/P0), where Pt and P0 represent transmitted and incident electromagnetic power, respectively.4 Each 10 dB increase in SE means that the transmitted electromagnetic power attenuates to 1/10 of the former transmitted power. For the film sample, the sum of the absorptivity (A), reflectivity (R), and transmissivity (T) is 1; that is, A + R + T = 1. The total shielding effectiveness (SEtotal) consists of three main parts: SE from absorption (SEA), SE from reflection (SER), and SE from multiple reflection (SEM), thus SEtotal = SEA + SER + SEM in which SEM can be neglected when SEtotal is larger than 15 dB. According to the abovementioned equation, we can get R from the equation SER = −10 log R, T from the equation SEtotal = −10 log T, and A from the equation A = 1 − R − T. In this work, for the film of 60c, the reflectivity (R), absorptivity (A), and transmissivity (T) are 0.9074, 0.0923, 0.0003 at 10 GHz, respectively. With decreasing film conductivity, the reflectivity decreased gradually, the
Figure 6. EMI shielding effectiveness as a function of the sheet resistance of silver nanowire films at 10 GHz.
a nearly linear relationship can be found. We also tested the EMI SE of some other films with low sheet resistances, and it was found that the values of SE and log Rs can be located near this line. With increasing sheet resistance, attributed to the stronger influence of the absorption and possible multiple reflections on the EMI SE, the relationship between SE and log Rs will become more complicated.
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CONCLUSIONS We have developed a kind of novel silver nanowires PEO composite coating ink for the large-scale, large-area continuous fabrication of metallic nanowire-based films. Excellent stability, good conductivity, and light transmission can be obtained. PES/silver nanowire/PET sandwich-structured flexible transparent films were further prepared and show excellent EMI shielding effectiveness. As-prepared flexible transparent conductive films can be expected to find practical applications in many fields, such as optoelectronics, telecommunications, and EMI shielding.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images showing the dispersion and the distribution of silver nanowires on a PET substrate. TGA curves indicating the thermal behavior of PEO under atmospheric conditions. Photographs describing the film stability. EMI SE−frequency curves exhibiting the EMI shielding performance. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: + 852-3442-7830. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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