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Superwettability-Induced Confined Reaction towards High-Performance Flexible Electrodes Weiwei Xiong, Xiqi Zhang, Hongliang Liu, Yahong Zhou, Yi Ding, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02515 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 28, 2016

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ACS Applied Materials & Interfaces

Superwettability-Induced Confined Reaction towards High-Performance Flexible Electrodes Weiwei Xiong, † Xiqi Zhang,*†Hongliang Liu,† Yahong Zhou,† Yi Ding,‡ Lei Jiang*‡ †

Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. ‡

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic

Solid, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P.R. China KEYWORDS: superwettability, confinement, flexible electrode, conductivity, interfacial adhesion

ABSTRACT: To find a general strategy to realize confinement of conductive layer for highperformance flexible electrode, with improved interfacial adhesion and high conductivity, is of important scientific significance. In this work, superwettability-induced confined reaction is used to fabricate high-performance flexible Ag/polymer electrodes, showing significantly improved silver conversion efficiency and interfacial adhesion. The as-prepared flexible electrodes by superhydrophilic polymeric surface under oil are highly conductive with an order of magnitude higher than the Ag/polymer electrodes obtained from original polymeric surface. The high conductivity achieved via superhydrophilic confinement is ascribed to that superhydrophilic polymeric surface can enhance reaction rate of silver deposition, and reduce the size of silver

1 Environment ACS Paragon Plus


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nanoparticles to achieve the densest packing. This new approach will provide a simple method to fabricate flexible and highly conductive Ag/polymer electrodes with excellent adhesion between conductive layer and substrate, and can be extended to other metal/polymeric electrodes or alloy/polymeric electrodes.

1. Introduction The development of flexible electrodes has attracted extensive attention owing to the distinct advantages, including flexibility, light weight, diversity of conductive layer and substrate, and shown great application prospects in modern optoelectronics devices, compact energy devices, and portable medical products.1-4 Flexible electrode fabrication by vacuum coating method like physical vapor deposition (PVD) can achieve uniform and tight conductive layer on the substrate. However, the PVD process suffers from high costs and large energy supplement.5 Solution-based electrode coating method like electroless deposition is one of the most frequently adopted industrial processes for flexible electrodes owing to low cost and simplicity of processing.6 The crucial factors to achieve high performance of flexible electrodes by solution-based electrode coating method are to improve interfacial adhesion between conductive layer and substrate, and the uniformity and compactness of conductive layer to obtain high conductivity.7 In this regard, some works have been carried out, such as modification of substrate morphology,8 optimization of reactants,9 and operating conditions10 like temperature, pH, external field, and so on. However, these conventional improvements often induce complexity of operation and energy waste.11-12 More importantly, previous fabrication of flexible electrodes by solution-based electrode coating method usually involves bulk reaction, and can not achieve confinement of conductive layer, which inevitably leads to the waste of raw materials, and can not meet the


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growing demand for new materials and devices.13-14 Recently, Hu et al. reported "Regioselective Deposition"

method

to

pattern

silver

electrodes

onto

flexible

substrates

through

hydrophobic/hydrophilic reaction, and fabricated flexible, large-area and high-performance polymer transistors.15 Chen et al. reported site-selective electroless metallization on porous organosilica films by vacuum plasma to control the wettability of polymeric surface, and fabricated well-defined Cu metallization patterns.16 However, to realize confinement of conductive layer for high-performance flexible electrode with improved interfacial adhesion and high conductivity is still limited, but has important scientific significance. Fabrication of new materials and devices with special surface and interface by controlling structure of material and integration of heterogeneous material has become a hot research topic in the field of materials science.17-20 In recent years, construction and application of novel functional materials based on superwettability have attracted significant attention in environmental protection, energy and green industry areas, and achieved unprecedented applications.21 Superwettable interfaces including superhydrophobic interface (contact angle > 150°) and superhydrophilic interface (contact angle < 10°) have been prepared and employed for applications in self-cleaning,22-23 corrosion resistance,24 drag reduction,25-26 cancer cells capture,27-28 anti-icing,29-30 anti-bacterial,31 and so on. Recently, several innovations about superhydrophobicity-based interfacial chemistry involved for reaction, crystallization, and nanofabrication have been developed in our group, such as miniature droplet reactor built on superhydrophobic pedestals,32 superoleophobic surfaces to pattern polymeric semiconductors,33 superhydrophobicity-mediated electrochemical reaction,34 and elaborate positioning of nanowire arrays by superhydrophobic pillar-structured substrates.35 To the best of our knowledge, underoil chemical reactions and fabrication based on superhydrophilic surface have not yet been



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reported, which would provide a confined environment with the capability to perform evaporation-free reaction. Here, we report superwettability-induced confined reaction towards flexible electrodes for the first time, based on under-oil electroless silver deposition upon various polymeric substrates (Figure 1a). Three types of polymers have been employed as the substrates (Scheme S1), which were polyethylene terephthalate (PET), polyimide (PI), and polyethylene (PE). The polymeric surface is pretreated by air plasma to afford superhydrophilic surface, then silver-ammonia solution is dripping on the polymeric surface and will spread out immediately and cover the entire surface. Subsequently, the polymeric film is immersed into hexane and followed by addition of glucose solution, which is spreading out quickly under oil, and thereby the confinement of electroless silver deposition is carried out upon the polymeric surface. The obtained Ag/PET film directly indicates excellent reflectance and flexibility (Figure 1b,c). We also demonstrate these Ag/polymer electrodes facilely fabricated under oil have excellent silver conversion efficiency, prominent interfacial adhesion between the conductive layer and the substrate, and extremely high electrical conductivity.


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Figure 1. a) Schematic illustration of under-oil electroless silver deposition upon superhydrophilic polymeric surface to fabricate Ag/polymer electrodes. The polymeric surface is pretreated by air plasma to afford superhydrophilic surface, then silver-ammonia solution is dripping on the polymeric surface and will spread out immediately and cover the entire surface. Subsequently, the polymeric film is immersed into hexane and followed by addition of glucose solution, which is spreading out quickly under oil, and the confinement of electroless silver deposition is carried out upon the polymeric surface. The prepared Ag/polymer electrodes show excellent silver conversion efficiency, prominent interfacial adhesion, and extremely high conductivity. b,c) Photographs of Ag/PET films obtained through this superwettability-induced confined reaction, indicating excellent reflectance and flexibility.

2. Experimental Section



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Materials: The PET film used in this study was purchased from Shenzhen Hongmei Film Co., Ltd. The PI film (Kapton) was purchased from Sui On Insulating Materials Co., Ltd. The PE film was purchased from Boxing Shuangjie Plastics Co., Ltd. Positive photoresist (SPR220-7.0) was purchased from Suzhou Ruicai Semiconductor Material Co., Ltd. Silver nitrate (AR, 99.8%), sodium hydroxide ( ≥ 96%, AR), ammonia solution (25-28%, AR), and glucose (AR) were purchased from Aladdin Industrial Inc. Ethanol (AR) and acetone (AR) was purchased from Beijing Chemical Works. Water used in this study was Milli-Q-deionized. All the reagents were used as received without any further purification unless specially mentioned. Characterizations: The contact angles were measured with an OCA20 machine (DataPhysics, Germany) at ambient temperature. Five independent measurements were used to calculate the average advancing and receding contact angles. XRD was carried out on a D8 Advanced X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm, Bruker) at a scanning speed of 2 degrees min−1. Adhesion between the silver layer and the substrate was assessed qualitatively using a tape peel test. Adhesive tape (Scotch 600, 3M) was carefully attached to the Ag/polymer film and subsequently removed, which was repeated for five times. The mass fraction of the remaining silver was used to evaluate the adhesion. Electrical conductivity was measured by a four-point probe technique. Morphology of the silver layer was observed using scanning electron microscopy (SEM, JF7500). Atomic force microscopy (AFM) was performed with Dimension Fastscan Bio (Bruker, Germany). The electrical properties were obtained by a Hall effect measurement system (Hall 8800, SWIN, Taiwan). The thickness of silver layer was measured by a Profilometer Dektak XT (Brucker). Electroless Silver Plating on Polymeric Surface: The polymeric films were cut into strips of about 1.5 cm ×1.5 cm in size. The surface of the film was cleaned by ultrasonic for 5 min in


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ethanol and dried in vacuum at room temperature before use. The electroless silver plating was performed in the silver-ammonia solution and glucose to directly fabricate the silver thin films. Specifically, the silver-ammonia solution was first prepared by mixing 10 mL silver nitrate solution (2%), 1 mL sodium hydroxide solution (10%), and 6 mL aqueous ammonia solution (2%). Then, 0.1 mL silver-ammonia solution was dripping on the polymeric surface. Subsequently, two strategies were carried out to prepare Ag/polymer electrodes at room temperature. One was reacting under oil by immersion of polymeric film into hexane and addition of 0.1 mL glucose solution (5%), while the other one was directly adding 0.1 mL glucose solution (5%) and reacting in air. Preparation of silver electrode arrays: Photolithography technology and superhydrophilic confinement were utilized to prepare silver electrode arrays. Firstly, a photoresist layer was spincoated onto the PET surface, which was subsequently exposed under UV light to develop a high resolution pattern. Secondly, the surface was treated with air plasma (30 W, 60 s) to achieve superhydrophilic surface and followed by electroless silver plating. Finally, the photoresist was rinsed off with acetone to obtain the silver electrode arrays. 3. Results and Discussion Air plasma treatment was firstly conducted to modify the surface wettability of the polymers by altering the treating time. The result showed that the hydrophilicity of PET surface enhanced by increasing plasma treating time, and superhydrophilic PET surface could be achieved by air plasma treatment for 60 s with a contact angle (CA) of 9.5° ± 3.1° in air (Figure 2a). The superhydrophilicity of PET surface still retained under oil with a even lower CA value of 4.5° ± 0.4°. With respect to PI and PE, superhydrophilic surface could also be obtained by air plasma treatment for 60 s with CA values of 9.1° ± 0.9° and 9.0° ± 2.1°, respectively (Figures S1-2). The



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under-oil superhydrophilicity could also be achieved as 5.1° ± 0.8° and 2.7° ± 0.3° for PI and PE, respectively. After superhydrophilic polymeric surface was achieved by plasma treatment, we compared the silver yield vs reaction time upon superhydrophilic polymeric surface under oil and original polymeric surface in air, respectively. The in-air reaction of electroless silver deposition on polymeric surface is conducted as control experiment for comparison by dripping silverammonia solution on the polymeric surface and followed by addition of glucose solution in air. Figure 2b presents the silver mirror reaction upon PET surface with different wettability is completed in 20 min either in air or under oil. Although the reaction completed time is almost the same, silver deposition upon superhydrophilic PET surface exhibits higher silver yield than that of original surface. Meanwhile, electroless silver deposition upon PI and PE has also been carried out in the same condition (Figures S3-4). Results showed both reactions completed in 30 min, with higher silver yield upon the superhydrophilic surface than that of original one, which is in accord with PET. To verify the successful preparation of silver-plated polymers, we preformed XRD patterns of these original polymers and Ag/polymer films (Figure 2c). The result revealed obvious changes of XRD patterns after silver deposition, as four distinct diffraction peaks (2θ) at 38.1, 44.2, 64.5, and 77.5° were attributed to the (111), (200), (220), and (311) crystal planes, which were reflections of face-centered cubic (fcc) silver (JCPDS, silver file No: 04-0783). No characteristic peaks of impurities like Ag2O were observed in the XRD patterns, suggesting high purity of the obtained silver layer.


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Figure 2. a) The hydrophilicity of PET surface enhances by increasing plasma treating time, and superhydrophilic PET surface can be achieved by air plasma treatment (30 W) for 60 s. Inset: inair and under-oil water droplet on PET surface after plasma treatment. b) The reaction is completed in 20 min upon both PET surface, and the silver yield upon superhydrophilic PET is higher than that of original PET. c) XRD spectra of the polymers and Ag/polymers indicate four peaks corresponding to (111), (200), (220), and (311) are reflections of face-centered cubic silver (JCPDS, silver file No: 04-0783). d) Reaction rate of electroless silver deposition upon original and superhydrophilic polymeric surface in air and under oil. Under-oil superhydrophilic polymeric surfaces exhibit highest reaction rate of electroless silver deposition, among them, PET has the highest reaction rate.

To quantitatively understand the reaction rate of silver mirror reaction upon different original and superhydrophilic polymeric surfaces, the comparison of reaction rate under different



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conditions is showed in Figure 2d. The result demonstrates superhydrophilic polymeric surface has much higher reaction rate. Specifically, in the case of PET, under-oil reaction upon superhydrophilic surface gains a highest reaction rate of 3.9 ± 0.4 mmol L-1min-1, which is higher than that of in-air reaction upon original PET by one order of magnitude. PE surface exhibits similar amplified reaction rate for the under-oil reaction upon superhydrophilic surface as compared to the in-air reaction upon original surface by one order of magnitude. In contrast, PI surface only acquires two-fold amplification of reaction rate upon superhydrophilic surface, which could be attributed to the different wettability of the polymers. According to the results of contact angle experiments (Figure 2a, Figures S1 and S2), the original PI surface with water contact angle (WCA) of about 68o is more hydrophilic than original PET (WCA = 77o) and PE (WCA = 104o). So the reaction rate of silver deposition on original PI is obviously higher that of PET and PE. However, after plasma treatment, all the three polymer surfaces turn to be superhydrophilic, and the reaction rate of silver deposition on three polymers has no much differences. Therefore, the reaction rate on original and superhydrophilic PI surface shows smaller difference than that of PET and PE. During traditional process of electroless silver deposition, various strategies have been employed, such as roughening of substrate, optimization of bath formulation and operating conditions, to improve interfacial adhesion between the conductive layer and the substrate, and the uniformity of conductive layer to achieve high electrical conductivity. To our best knowledge, the relationship of silver conversion efficiency, interfacial adhesion and electrical conductivity with the superwettability of the polymeric substrate is yet to be established. Therefore, we determined silver conversion efficiency of the silver mirror reaction upon different wettable polymeric surface in air and under oil. Figure 3a demonstrates higher silver yield is obtained


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upon superhydrophilic PET surface under oil, which is 2.5 times as high as the silver conversion efficiency obtained upon original PET surface by in-air reaction. The silver conversion efficiency upon PI and PE surface with different wettability were also tested, suggesting similar increasement of silver yield upon superhydrophilic surface under oil than that on original surface in air, with 1.6-fold and 1.8-fold increasement for PI and PE, respectively. Comparison of silver yield on three superhydrophilic polymeric surfaces after electroless silver deposition under oil is also showed in Figure 3a, indicating under-oil reaction upon superhydrophilic PE surface has the highest silver yield of ~ 90%. To systematically study the relationship of silver conversion efficiency and surface wettability of the polymeric substrate, the silver yields upon polymeric surface in air and under oil with different air plasma treating time are investigated (Figures S5-7). The result demonstrates the silver yield on different polymeric surface increases by increasing plasma treating time, and the maximum silver yield is achieved on the superhydrophilic surface.

Figure 3. a) Silver yield of electroless silver deposition upon original and superhydrophilic polymeric surface in air and under oil. Under-oil superhydrophilic polymeric surfaces have highest silver conversion efficiency. b) Normalized adhesion of electroless silver deposition upon original and superhydrophilic polymeric surface in air and under oil. Under-oil superhydrophilic polymeric surfaces have highest interfacial adhesion.



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We further investigated the relationship of interfacial adhesion between the conductive layer and the substrate with the superwettability of the polymeric substrate. In a quantitative analysis, the interfacial adhesion between silver layer and superhydrophilic PET surface exhibits 7.5 times as high as that of original PET surface (Figure 3b). Another two polymers of PI and PE were also adopted to study the relationship of interfacial adhesion with the wettability of the polymeric substrate. The results are in accord with that of PET, indicating 1.4 and 9.6 times of interfacial adhesion based on superhydrophilic surface as high as that of original surface of PI and PE, respectively. The different increasement of interfacial adhesion for PI and PE is suggested to be caused by the different wettability of the polymers. According to the results of contact angle experiments, the original PI surface is more hydrophilic than original PE. So the interfacial adhesion of silver layer on original PI are obviously higher that of PE. However, after plasma treatment, the polymer surfaces turn to be superhydrophilic, and the interfacial adhesion of silver layer on the polymers have no much differences. Therefore, the interfacial adhesion of silver layer on original and superhydrophilic PI surface shows smaller difference than that of PE. Systematic investigation of the change of interfacial adhesion between silver layer and polymeric surface at different air plasma treating time with reaction in air and under oil are showed in Figures S8-10, which demonstrates the interfacial adhesion increases as the improvement of hydrophilicity of polymeric surface, and reaches a maximum value at the superhydrophilic surface after plasma treatment for 60 s. Moreover, Ag/polymer electrodes obtained under oil has higher interfacial adhesion than those of in-air reaction. Beyond those benefits of excellent silver conversion efficiency and prominent interfacial adhesion achieved by superhydrophilic polymeric surface, improvement of electrical conductivity of the silver-plated electrode based on superhydrophilic polymeric surface is also


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demonstrated. Figure 4a presents comparison of electrical conductivity of Ag/polymer electrodes obtained on original and superhydrophilic polymeric surface. We demonstrates Ag/PET electrode based on the superhydrophilic PET surface gains much higher electrical conductivity, which is 28 times as high as that based on original PET surface. In the cases of Ag/PI and Ag/PE electrodes, similar improvements of higher conductivity are obtained upon superhydrophilic surface through under-oil reaction, reaching 19 and 34 times as high as that through in-air reaction upon original surface for PE and PET, respectively. In order to better evaluate this high conductivity of Ag/polymer electrodes via superhydrophilic confinement, comparison of electrical conductivity of Ag/polymer electrodes based on superhydrophilic polymeric surfaces with Ag/polymer electrodes fabricated by printed method in the previous literature (Ref.)36 is also showed in Figure 4a, suggesting that all of the electric conductivities of Ag/polymer electrodes based on superhydrophilic polymeric surfaces are higher than that of reported printed method. for one order of magnitude. Among them, Ag/PET electrode obtained under oil has the highest electrical conductivity of (1.33 ± 0.22) × 105 S cm-1. The relative conductivities of Ag/polymeric electrodes with different tortuous angle are shown in Figure S11, with increasing tortuous angle, all the three Ag/polymeric electrodes remains high conductivity with almost no changes, indicating excellent flexibility of the electrodes. Moreover, the carrier concentration and mobility performance of Ag/polymer electrodes based on superhydrophilic polymeric surfaces have been examined by Hall measurement, showing 3.995×1022 cm-3 and 22.30 cm2 V−1s−1 for PET, 3.030×1022 cm-3 and 14.44 cm2 V−1s−1 for PI, 3.855×1022 cm-3 and 21.18 cm2 V−1s−1 for PE, respectively. These results are consistent with the electrical conductivity results in this order: Ag/PET > Ag/PE > Ag/PI. In addition, the relative conductivities of Ag/polymeric electrodes with thickness of silver layer are determined and shown in Figure S12. Different



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thicknesses of silver layer were prepared via under-oil electroless silver deposition upon superhydrophilic polymeric surface at different reaction time: 2 min, 4 min, 6 min, and 30 min, respectively. The result indicates the conductivity of Ag/polymeric electrode increases as the thickness of silver layer increases. To deeply understand the mechanism of high conductivity achieved on the superhydrophilic polymeric surface, we further examined SEM images of silver layer upon polymeric surface with different wettability. Figure 4b presents the formation of incompact silver nanoparticles with size of 300-500 nm after electroless silver deposition on the original PET surface in air. However, the silver nanoparticles size reduces to 100-200 nm after electroless silver deposition on the superhydrophilic PET surface under oil, leading to closer accumulation of silver nanoparticles on the substrate without any exposed substrate (Figure 4c). Similar trends also exist in the SEM images of Ag/PE and Ag/PI electrodes (Figures S13-14), demonstrating the superiority of superhydrophilic polymeric surface for the densest packing of silver nanoparticles, further resulting in high electrical conductivity on the superhydrophilic surface. Moreover, the reaction rate of silver mirror reaction upon polymeric surface with different wettability is also in accord with the changing tendency of the SEM images, as the superhydrophilic polymeric surface can dramatically enhance the reaction rate of silver deposition, and reduces the size of silver nanoparticles to achieve the densest packing for the highest conductivity.


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Figure 4. a) Comparison of electrical conductivity among Ag/polymer electrodes based on original and superhydrophilic polymeric surface. The “Ref.” represents Ag/polymer electrodes fabricated by printed method in the previous literature.36 The result suggests the electrical conductivity of under-oil silver deposition on superhydrophilic polymeric surface is higher than that on original surface and “Ref.” by one order of magnitude. b) SEM image of silver layer on original PET surface. c) SEM image of silver layer on superhydrophilic PET surface, which indicates smaller and denser packing of silver nanoparticles than that on original PET surface. d) Ag/PE electrodes obtained under oil have higher electrical conductivity than those obtained in air. The PE surface is pretreated by air plasma for 0 s, 5 s, and 60 s to obtain different wettability. e,f) SEM images of silver layer on PE surfaces (pretreated by air plasma for 5 s) obtained in air and under oil, respectively, indicating the morphology of silver nanoparticles obtained under oil has closer accumulation.



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Bar view illustrations of electrical conductivity of Ag/polymer electrodes obtained in air and under oil are showed in Figure 4d and Figures S15-16. The surface of original polymer is pretreated by air plasma for 0 s, 5 s, and 60 s to obtain different wettability. The result demonstrates Ag/polymer electrodes obtained under oil has higher electrical conductivity than those obtained in air, which can be evidenced by SEM images, as the morphology of silver nanoparticles obtained under oil has closer accumulation (Figure 4e, 4f, Figures S13-14, S17). Table 1. XPS data for original and plasma treated polymer films Original

Plasma treated

C (%)

N (%)

O (%)

C (%)

N (%)

O (%)

PET

73.66

3.81

22.53

68.62

2.81

28.57

PI

80.79

6.15

13.06

71.22

9.07

19.71

PE

94.71

1.46

3.83

86.7

2.59

10.71

To extend this work to other polymers, the relationship between surface molecular composition and performance, and relationship between surface morphology and performance were determined by XPS, SEM, and AFM, respectively, to find out whether the striking performance comparison here us just a change in surface composition or one of both composition and topography. For the three kinds of polymer surfaces, i.e., PET, PI and PE, XPS data showed that weight ratios of oxygen were obviously increased after air plasma treatments (See Table 1). Typical C1s spectra for the original and superhydrophilic polymers are shown in Figures S18-20. The C1s spectrum for the original PET film is deconvoluted into three components, which appear at 284.8 eV (C-H), 286.2 eV (C-O), and 288.8 eV (C=O). The C1s spectrum for the superhydrophilic PET film also possesses three deconvoluted components at the same peak location, indicating decreased content of C-H group and increased content of C=O for the


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superhydrophilic PET (Figure S18). The C1s spectrum for the original PI film is deconvoluted into three components, which appear at 284.7 eV (C-H), 285.3 eV (C-O and C-N), and 288.3 eV (C=O), and the C1s spectrum for the superhydrophilic PI also suggests decreased content of C-H group and increased content of C=O (Figure S19). In comparison, The C1s spectrum for the original PE film is deconvoluted into two components, which appear at 284.7 eV (C-H) and 285.1 eV (C-C), while the superhydrophilic PE has a new C=O peak at 288.9 eV (Figure S20). This XPS result suggested air plasma treatments enhanced the surface energy by generating – CHO and –COOH groups on the polymeric surfaces. The XPS data further support the statement that the superhydrophilic polymeric surface produces a dense distribution of Ag nanoparticles with smaller sizes. After plasma treatment, the surface energy of the superhydrophilic polymeric surface increases by generating –CHO and –COOH groups on the surface, it appears that the functional end groups play an important role in the higher absorption of silver ammonium ion and better adherence of the silver layer on the polymeric surface, and the generating rate of silver seeds is faster than the growing rate of silver seeds, which results in the decrease of silver particle size and denser distribution of Ag nanoparticles.37-38 To make clear the issue of surface morphology, we compared the surface morphology of PET, PI and PE before and after air plasma treatments by using SEM. As shown in Figures S21-S23, all the three polymer surfaces had no obvious differences at the microscale before and after air plasma treatments, however, there exist slight differences at the nanoscale. In order to determine the morphological feature of polymer surface more clearly, AFM were carried out to obtain highly magnified images. Figure 5 shows AFM images of the surface morphology of PET films before and after plasma treatment, indicating increased roughness of polymeric surface after plasma treatment. The AFM results of PI and PE are consistent with that of PET, demonstrating increased surface roughness after



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plasma treatment (Figure S24 and S25). So, we conclude that the effect of air plasma treatments to achieve striking performance of the flexible electrodes is a collective effect in surface composition and surface morphology.

Figure 5. AFM images of the surface morphology of PET films (a) before plasma treatment with Ra = 0.94 nm and (b) after plasma treatment with Ra = 2.11 nm, indicating increased roughness of polymeric surface after plasma treatment.

Here, the relationship of silver conversion efficiency, interfacial adhesion and electrical conductivity with the superwettability of the polymeric substrate is clarified, and flexible Ag/polymer electrodes with excellent silver conversion efficiency, prominent interfacial adhesion, and extremely high electrical conductivity can be facilely fabricated via confinement of electroless silver deposition upon superhydrophilic polymeric surface under oil. In addition, silver electrode arrays were patterned efficiently on PET surface through photolithography and superhydrophilic confinement, which is showed in Figure 6 with 10 µm channel length, indicating this superhydrophilic confinement can be applied to the preparation of flexible electrode arrays. We believe superwettability-induced confined reaction towards high-


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performance flexible electrodes in this work can find great potential applications in the area of addressing back-plane in display, smart cards, and various flexible electronic devices.15, 39

Figure 6. a) Patterning of Ag/PET electrodes fabricated via superhydrophilic confinement with channel length of 10 µm on the PET substrate, the scale bar is 1 mm. b-d) Enlarged images of silver electrodes in Figure 6a from left to right, respectively. All of the scale bars are 500 µm.

4. Conclusion In conclusion, based on confinement of electroless silver deposition upon superhydrophilic polymeric surface under oil, we have fabricated flexible Ag/polymer electrodes with excellent silver conversion efficiency and extremely high interfacial adhesion. The as-prepared flexible electrodes based on superhydrophilic surface under oil are highly conductive with an order of magnitude higher than the Ag/polymer electrodes obtained from original polymeric surface. The mechanism of high conductivity achieved via superhydrophilic confinement is that superhydrophilic polymeric surface can enhance the reaction rate of silver deposition, and reduce the size of silver nanoparticles to achieve the densest packing. We believe this new approach will provide a simple method to fabricate flexible and highly conductive Ag/polymer electrodes with excellent adhesion between the conductive layer and the substrate, and can be extended to other metal/polymeric electrodes or alloy/polymeric electrodes, which will greatly promote the



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preparation and application of flexible electrodes to meet the growing demand for new materials and devices.

ASSOCIATED CONTENT Supporting Information Available: Scheme S1 and Figures S1-S25 showing molecular structures, contact angle, silver yield, interfacial adhesion, conductivity, XPS spectra, SEM and AFM images. The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: X. Zhang ([email protected]); L. Jiang ([email protected]) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research is supported by the National Research Fund for Fundamental Key Projects (2013CB933000,

2014CB932203,

2013CB834700,

2012CB933800,

2012CB933200,

2012CB934100), the National Natural Science Foundation (21421061, 91127025, 21431009, 21404109), and the key Research Program of the Chinese Academy of Sciences (KJZD-EWM01, KJZD-EW-M03), the 111 project (B14009), China Postdoctoral Science Foundation (2015M570157) and Youth Innovation Promotion Association, CAS (2016026). 20 Environment ACS Paragon Plus

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Superwettability-Induced Confined Reaction ... - ACS Publications

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