Layer-by-Layer Strippable Ag Multilayer Films Fabricated by Modular

Dec 23, 2013 - People,s Republic of China ... is called modular assembly in this study. On the ... The key to fabricate multilayer films by modular as...
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Layer-by-Layer Strippable Ag Multilayer Films Fabricated by Modular Assembly Yan Li,† Xiaoyan Chen,§ Qianqian Li,† Kai Song,† Shihui Wang,† Xiaoyan Chen,† Kai Zhang,† Yu Fu,†,* Yong-Hua Jiao,‡,* Ting Sun,†,* Fu-Chun Liu,⊥ and En-Hou Han⊥ †

College of Sciences and ‡College of Life and Health Sciences, Northeastern University, Shenyang 110819, People’s Republic of China § College of Engineering, Peking University, Beijing 100871, People’s Republic of China ⊥ State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China ABSTRACT: We have developed a new method to fabricate multilayer films, which uses prepared thin films as modular blocks and transfer as operation mode to build up multilayer structures. In order to distinguish it from the in situ fabrication manner, this method is called modular assembly in this study. On the basis of such concept, we have fabricated a multilayer film using the silver mirror film as the modular block and poly(lactic acid) as the transfer tool. Due to the special double-layer structure of the silver mirror film, the resulting multilayer film had a well-defined stratified architecture with alternate porous/compact layers. As a consequence of the distinct structure, the interaction between the adjacent layers was so weak that the multilayer film could be layer-by-layer stripped. In addition, the top layer in the film could provide an effective protection on the morphology and surface property of the underlying layers. This suggests that if the surface of the film was deteriorated, the top layer could be peeled off and the freshly exposed surface would still maintain the original function. The successful preparation of the layer-by-layer strippable silver multilayer demonstrates that modular assembly is a feasible and effective method to build up multilayer films capable of creating novel and attractive micro/nanostructures, having great potential in the fabrication of nanodevices and coatings.

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INTRODUCTION In the past decades, micro/nanostructured multilayers have attracted increasing attention due to the crucial role they play in the applications of sensors, catalysis, microelectronics, electroluminescence, photovoltaic devices, and so on.1−3 At present, the widely used methods to prepare multilayers include layerby-layer self-assembly,4−6 physical vapor deposition (PVD),7 chemical vapor deposition (CVD),8 spin or dip coating,9,10 and so on. Most of these methods build up the multilayer based on the in situ strategy, in which the latter layer is fabricated directly on the surface of the former layer. Although the in situ strategy to fabricate the multilayer is facile and convenient, it is restricted by a couple of limitations. First, the former layer has to be robust enough to survive the fabrication process of the latter layer. To obtain the multilayer structure, there are many possible influences that need be considered, such as the solubility of the former layer in the preparation solvent of the latter layer, the thermal stability of the former layer in the preparation temperature of the latter layer, and so on. With spin coating as an example, the former layer should be insoluble for the preparation solution of the latter layer. Otherwise, the former layer would be resolved, and the layer structure would not be achieved.9,10 Second, the influence of the former layer on the structure of the latter layer should not be neglected. It is © 2013 American Chemical Society

known that the substrate is a key factor in the fabrication of nanostructured thin films, and hence the physical and chemical properties and morphology of the former layer would influence the structure of the latter layer inevitably.11,12 Sometimes the influence is negative and unavoidable. In addition, some thin films with special structures have particular requirements on the fabrication substrate, which cannot be met by an arbitrary surface. This makes such films quite difficult to be integrated into a multilayer through an in situ fabrication method. Another strategy to build up multilayer films is to employ the prepared thin films as modular blocks to fabricate multilayer structures. Namely, the component layers of the target film were first prepared on other substrates separately and then integrated into one multilayer structure. Because this strategy does not need the layer to be fabricated directly on the existing film, the negative influence and restriction of the preparation process on the former layers can be avoided, and moreover, the micro/nanostructures of the modular block films that rely on the substrate can be preserved. Therefore, this strategy circumvents the limitations of the in situ fabrication to some Received: November 26, 2013 Revised: December 20, 2013 Published: December 23, 2013 548

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Scheme 1. Schematic Process of Fabrication and Layer-by-Layer Stripping of the Modular Assembly Multilayer Film

extent. According to the feature of this strategy, such a film fabrication method was called modular assembly. The key to fabricate multilayer films by modular assembly is to find a method to transfer the prepared film from the asgrown substrate onto the target film. Our group has developed a transfer method by using soluble polymer of poly(lactic acid) (PLA) as the delivery tool, illustrated in Scheme 1 (steps 1− 4).13 The method could transfer metal thin films, which were prepared by electrochemical and solution chemical deposition, to an arbitrary surface without visible change in the morphology and surface properties, making it qualified for modular assembly of multilayer films. In this study, we have used modular assembly to fabricate a multilayer silver film with distinct structure of alternate porous and compact layers. The module block was the thin film prepared by silver mirror reaction on an activated glass. The modular assembly is the process of superimposing the silver mirror films by the PLA-assisted transfer method as mentioned above (shown Scheme 1, step 5). Because the used silver mirror film has a double-layer structure, including a porous upper layer and a compact underlying layer, the resulting multilayer film has a distinct structure with alternate porous and compact layers. This structure is supposedly difficult to obtain by in situ fabrication because a compact and flat metal layer should be quite difficult to form on a porous and rough substrate. The special stratified structure could bring out attractive features. For example, the silver multilayer film could be stripped in a layer-by-layer manner, as shown in steps 6 and 7 in Scheme 1. In addition, the top layer in the film has the capability of protecting the morphology and surface properties of the under layers from environmental corrosion. These imply that, if the surface of the multilayer is deteriorated, the surface functions can be recovered by peeling off the top layer and exposing the underlying surface. This could be an attractive feature for

functional coatings. For example, the self-cleaning coating based on a superhydrophobic surface has suffered short service life because of the passivation of the surface chemistry and the abrasion of the micro/nanostructures. The fabrication of the layer-by-layer strippable coatings provides an alternative solution to this issue. It allows the self-cleaning properties conveniently renewed when the top layer is out of service, multiplying the service life by the layer numbers. Therefore, the modular assembly provides a possible approach to make recoverable coatings and devices, having potential in the fields of self-cleaning, antifouling, and antibacterial coatings.

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EXPERIMENTAL SECTION

Materials. AgNO3 was obtained from Hensel Tianjin Chemical Industry Co., Ltd. Poly(lactic acid) (Mw = 200 000 g mol−1) was purchased from Shandong Medical Apparatus Research Institute. nDodecanethiol was purchased from Alfa Aesar. All of the materials were used as received without further purification. The SEM images were taken on a Hitachi S-3400N scanning electron microscope. The contact angle measurements were conducted with a Dataphysics OCA20 contact angle system. Fabrication of Silver Mirror Film. A glass substrate (1 cm × 2 cm) was first immersed into piranha solution (3:7 mixture of 30% H2O2 and 98% H2SO4) and then heated until no bubbles were released. Caution! Piranha solution reacts violently with organic materials and should be handled caref ully. After being rinsed with ample water and ethanol and dried, the substrate was activated by exposure to a solution of SnCl2 (0.01%) for 15 min, followed by rinsing with water. At the same time, silver ammonia solution (10 mL of 0.1 M AgNO3 solution and 5 mL of 1 M ammonia) was prepared in a 100 mL Teflon bottle. The activated glass substrate was then immersed in the silver ammonia solution and cooled to 10 °C in ice water. Afterward, 0.1 mL of fresh HCHO solution (2%) was added dropwise into the above solution containing the substrates under slight oscillation. After the glass substrate surface was covered by a layer of bright silver, another 0.9 mL of HCHO solution was dropped rapidly. After the resulting mixture was kept at 30 °C in a water bath for 3 h, the silver mirror film 549

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Figure 1. SEM images of the silver mirror films (fabricated on the activated glass): surface (A), cross-sectional view (B), after peeling off the upper layer (C), and the reverse side (D). was obtained. Prior to transfer, the silver film was heated in vacuum at 150 °C for 3 h to enhance the stability. Transfer of Silver Mirror Film. The transfer process is illustrated in Scheme 1 (steps 1−4). First, a layer of PLA thin film was deposited onto the silver mirror film by drop-casting 35 mg/mL PLA solution in dichloromethane, followed by drying under ambient conditions. After being immersed in water for 3 h, the PLA film along with the silver film was then carefully peeled off from the as-grown substrate using tweezers, resulting in a Ag/PLA hybrid freestanding film. Next, the target surface was covered with a certain amount of water, onto which the hybrid film was spread out (note: avoid producing bubbles). After the water was absorbed away with filter paper, the hybrid film was attached on the target surface. Last, the PLA film was removed by immersing in or washing with dichloromethane to yield the target surface covered by the silver mirror film. Fabrication of Modular Assembly Multilayer Film. The modular assembly process is illustrated in Scheme 1 (step 5). The first layer of the multilayer film was the silver mirror film transferred from the as-grown substrate as described above. The second layer was built up by transfer of the silver mirror film using the first layer as the substrate. First, a few drops of the mixed solution (Vwater/Vethanol = 1:1) were dropped on the surface of the first layer. Then, the Ag/PLA hybrid freestanding film was spread out onto the liquid, followed by drying with filter paper. Last, the film was immersed in dichloromethane to remove the PLA layer. Repeating the above steps would produce the modular assembly multilayer film. Layer-by-Layer Stripping. The layer-by-layer stripping process is illustrated in Scheme 1 (steps 6 and 7). First, adhesive tape was attached on the surface of the multilayer film gently. Then the adhesive tape was lifted up, which would make the top layer of the film separate from the underlying layers because it was glued on the adhesive tape. Repeating the above steps would strip the film in a layer-by-layer manner.

SnCl2. As-prepared silver mirror film has a double-layer structure. The upper layer, composed of silver particles, is thick and porous, oppositely, the underlying layer is continuous and compact. Figure 1A shows the SEM image of the surface of the silver mirror film. What can be observed on the surface is an agglomeration of nanoparticles. Figure 1B shows the crosssectional view of the silver mirror film. Besides the upper silver particles, there is a thin but compact silver layer above the substrate. The upper porous layer could be separated from the underlying layer by being peeled off with adhesive tape. The surface after peeling is shown in Figure 1C. Although there is still some nanoparticle residue, the continuous and compact surface of the underlying layer could be clearly observed. Furthermore, the whole silver mirror film was peeled off by PLA (which will be discussed in the next section), and the reverse side of the film was observed by SEM, as shown in Figure 1D. It also confirms the compact and smooth morphology of the underlying layer. The formation of the double-layer structure could be attributed to the SnCl2 activation of the glass surface, which induced the reduced silver deposition on the surface densely. After the surface was covered by the compact silver layer, the granular silver was piled on it, producing the porous layer. If the mirror reaction was performed on the glass directly without activation, there would be only the porous layer on the surface. Transfer of Silver Mirror Film. Our previous research indicated that the silver mirror film could be transferred from the as-grown substrate to an arbitrary surface by using the polymer of poly(lactic acid) as the delivery tool. The transfer process is illustrated in Scheme 1. Briefly, the silver mirror film was first covered by a drop-casted poly(lactic acid) layer, and then the resulting hybrid film was peeled off from the as-grown substrate with the tweezers. The peeled hybrid film is freestanding in air and can be operated by tweezers without any visible damage. Afterward, the resulting freestanding film

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RESULTS AND DISCUSSION Fabrication of Silver Mirror Film. The used modular block is the silver film prepared by silver mirror reaction (i.e., chemical deposition method) on a piece of glass activated by 550

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Figure 2. SEM images of the reverse sides of the porous silver films (fabricated on the glass directly without activation) peeled off by the adhesive tape (A) and the PLA (B).

Figure 3. SEM images of the silver mirror multilayer films with three layers (A), four layers (B), the cross-sectional view (C), and a photo of lasagna (D).

was adhered onto the target surface with the aid of water. Finally, the polymer layer was washed with solvent. The reasons that the silver mirror film could be transferred by PLA are discussed as follows. First, the silver mirror film was obtained through the deposition of reduced silver ions on the glass surface; the interaction between the silver film and the substrate was nonspecific and unstable. Second, when the PLA solution was dipped onto the surface of the double-layer silver mirror film, the solution could penetrate into the porous structure of the upper layer. Therefore, when the PLA layer was formed, it interlaced with the upper layer, which enhanced the interaction between the polymer layer and the silver mirror film. To prove the penetration of the PLA into the porous layer, the porous silver film was prepared on the glass substrate without SnCl2 activation. As-prepared silver films were peeled off by adhesive tape and PLA separately, and the reverse sides of the films were observed by SEM, as shown in Figure 2. In the image of the one peeled with the tape, only the porous silver layer was observed (Figure 2A), whereas for the other one with

PLA, there was an obvious organic substance between the silver particles, which confirmed that the polymer could penetrate into the porous layer, forming an interlaced hybrid film (Figure 2B). Fabrication of Modular Assembly Multilayer Film. The modular assembly multilayer film was fabricated by superimposing the silver mirror films through the PLA-assisted transfer as shown in the step 5 in Scheme 1. The first layer was prepared by transferring the silver mirror film on a glass surface. Using the first layer as the target substrate, the second layer was built up in the same way. Repeating the transfer process could produce the multilayer structure. The surface images of the three- and four-layer films are shown in Figure 3A,B. The morphologies of the films were the same as that before transfer, implying that the transfer and superimposition did not destroy the film structure. In order to check the architecture of the multilayer film, the cross section of a three-layer film was observed by SEM, as shown in Figure 3C. A well-defined stratified structure with alternate porous/compact layers, just 551

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Figure 4. SEM images of the film after peeling (A) and the reverse side of the stripped layer (B). The insets are their respective photos.

Figure 5. SEM images of the silver multilayer films after exposed in an organic synthesis lab for 140 day before (A) and after (B) peeling off the top layer. The insets are their respective photos.

interaction between them, making it hard for the film to separate from the substrate. Protection Effect of the Top Layer. Because the underlying layers are fully covered by the top layer, the top layer could protect the inside of the multilayer from the corrosion of the atmosphere. To test it, a double-film multilayer was exposed in an organic synthesis lab, where the air and organic vapor could corrode silver remarkably.14−16 With the exposure time, the color of the silver film became darker and darker, and meanwhile, the surface morphology made a dramatic change. As Figure 5A shows, after 140 days of exposure, most silver particles of the top porous layer faded away and parts of the compact layer were even exposed. Although the top layer was seriously damaged, the underlying layers were well preserved. As shown in Figure 5B, after the corroded top layer was stripped off, the underlying surface still maintained the original morphology. Besides the morphology, the top layer could help keep the surface properties of the underlying layers, as well. To display this effect, we prepared a superhydrophobic mirror silver film by modifying the film surface with n-dodecanethiol and building up the corresponding double-film mutilayer by modular assembly.17−20 Due to the hydrophobic modification and the micro/nanostructure, the multilayer film had a high contact angle of 146.7 ± 1.3°. Because of the instability of the n-dodecanethiol monolayer, the hydrophobic property of the surface would degrade with the exposure time in atmosphere. After 14 days of exposure, the contact angle of the film reduced to 114.3 ± 1.8°. However, after the top layer was stripped off, the contact angle of the freshly exposed surface was still 142.7 ± 1.2°.

like the delicious food of lasagna (Figure 3D), was found. All of these characterizations indicate the successful fabrication of the silver multilayer film by modular assembly. Layer-by-Layer Stripping. Due to the distinct structure, the modular assembly silver film exhibited some special and attractive features, such as layer-by-layer stripping (Scheme 1). The top layer of the multilayer film could be stripped off by adhesive tape. The stripping process is facile, speedy, and gentle and does not cause any visible damages on the underlying structure. As shown in Figure 4A, the surface morphology of the multilayer film after stripping was the same as that before stripping. At the same time, the reverse side of the stripped layer (shown in Figure 4B) also exhibited the same morphology as the compact layer of the silver mirror film without any residues of the porous layer. In order to prove that only the top layer was stripped off, the SEM image of the cross section of the film after stripping was taken. The cross-sectional image of the four-layer film after striping was the same as the one of the three-layer film, which suggests that the stripping process just separates the top layer from the film without destroying the underlying layers. The stripping step could be layer-by-layer repeated on the multilayer film until the bottom layer on the substrate was not able to be peeled off entirely by the adhesive tape. This could be ascribed to the alternate porous/compact layer structure. Because the interaction between two adjacent layers is nonspecific, and moreover the touching area is small, the multilayer film is prone to be separated at the interface of two adjacent layers by the adhesive tape. As for the bottom layer, the touching area on the substrate is large, which enhances the 552

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(11) Knor, M.; Nowakowski, R.; Nowicka, E.; Dus, R. SurfaceMediated Thin Terbium Hydride Film Formation. Langmuir 2010, 26, 3302−3307. (12) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. Polyelectrolyte Multilayer as Matrix for Electrochemical Deposition of Gold Clusters: Toward Super-hydrophobic Surface. J. Am. Chem. Soc. 2004, 126, 3064−3065. (13) Zhao, S.; Hu, C. Y.; Chen, X. Y.; Zhou, J.; Jiao, Y. H.; Zhang, K.; Fu, Y. Transfer of Inorganic Thin Films by Soluble Polymer Layer for Arbitrary Surface Coating. Soft Matter 2012, 8, 937−941. (14) Ando, E.; Miyazaki, M. Moisture Degradation Mechanism of Silver-Based Low-Emissivity Coatings. Thin Solid Films 1999, 351, 308−312. (15) Sahm, H.; Charton, C.; Thielsch, R. Oxidation Behaviour of Thin Silver Films Deposited on Plastic Web Characterized by Spectroscopic Ellipsometry (SE). Thin Solid Films 2004, 455, 819− 823. (16) Yang, X.; Zhao, Q.; Han, B.; Zhao, X. Oxidation Mechanism of Silver Thin Films under Room Temperature and Atmospheric Environment. J. Chin. Ceram. Soc. 2008, 36, 954−959. (17) Shen, L. Y.; Ji, J.; Shen, J. C. Silver Mirror Reaction as an Approach To Construct Superhydrophobic Surfaces with High Reflectivity. Langmuir 2008, 24, 9962−9965. (18) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Combining Layer-byLayer Assembly with Electrodeposition of Silver Aggregates for Fabricating Superhydrophobic Surfaces. Langmuir 2005, 21, 4713− 4716. (19) Xiao, M.; Cheng, M. J.; Zhang, Y. J.; Shi, F. Combining the Marangoni Effect and the pH-Responsive Superhydrophobicity− Superhydrophilicity Transition to Biomimic the Locomotion Process of the Beetles of Genus Stenus. Small 2013, 9, 2509−2514. (20) Xiao, M.; Guo, X. P.; Cheng, M. J.; Ju, G. N.; Zhang, Y. J.; Shi, F. pH-Responsive On−Off Motion of a Superhydrophobic Boat: Towards the Design of a Minirobot. Small 2013, DOI: 10.1002/ smll.201302132.

CONCLUSIONS In summary, we have developed a new concept of modular assembly and used it to fabricate a silver multilayer film successfully. The prepared film has a well-defined stratified structure with alternate porous and compact layers, which is supposed to be difficult to achieve by the in situ fabrication technology. One of the attractive features of this film is that the multilayer could be stripped layer-by-layer with adhesive tape because of the weak interaction and small touching area between the adjacent layers. In addition, the underlying layers in the film are under the protection of the top layer. Once the top layer was corroded, it could be peeled off and the freshly exposed layer would still maintain the original morphology and surface properties. This endows the multilayer the ability to renew and gives it potential in the field of surface coating. This study has provided a new strategy to fabricate multilayer films, which is promising in the preparation of micro/nanodevices and coatings and has great potential in the creation of novel micro/nanostructures.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (20974017, 21174024), Supporting Projects for Talents in the Universities of Liaoning Province (LR2012008), Fundamental Research Funds for the Central Universities (N120505005, N120405007), and SKLCP2012KF04.

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