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water separation and self-cleaning
Applied Surface Science 370 (2016) 243–251
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Fabrication of polydopamine-coated superhydrophobic fabrics for oil/water separation and self-cleaning Zhanglian Xu a,∗ , Koji Miyazaki b , Teruo Hori a a b
Headquarters for Innovative Society-Academic Cooperation, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan Frontier Fiber Technology and Science Graduate School of Engineering, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2015 Received in revised form 12 February 2016 Accepted 12 February 2016 Keywords: Superhydrophobic Dopamine Fabrics Oil/water separation Self-cleaning
a b s t r a c t We report a fabric coating method inspired the superhydrophobic properties of lotus leaves and the strong adhesion of the adhesive proteins in mussels. Dopamine, which mimics the single units of the adhesive mussel proteins, was polymerized in an alkaline aqueous solution to coat the surface of fabrics. The versatile reactivity of polydopamine allows subsequent Ag deposition to form a lotus-leaf-like rough structure on the fabric surface. The composite fabric exhibited high water repellence after fluorination. Because dopamine can adhere to all kinds of materials, this method can be applied to many fabrics regardless of their properties and chemical compositions using a universal process. The modified fabrics exhibited excellent anti-wetting and self-cleaning properties with contact angles of >150◦ and sliding angles lower than 9◦ . The fabrics also efficiently separated oil from oil/water mixtures under various conditions. Our method is versatile and simple compared with other hydrophobic treatment methods, which usually only work on one type of fabric. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nature often provides biological inspiration for research. A famous example is lotus leaves, on which water droplets form spheres and roll off. The extreme water repellent behavior comes from the low surface energy of the wax on the leaf and the unique micro/nano hierarchical structure formed from sparse micropapillae covered with nanosized wax crystals. Lotus leaves have a superhydrophobic surface with a water contact angle (CA) larger than 150◦ and a low sliding angle [1–3]. Superhydrophobic surfaces have many practical uses, such as in agriculture in harsh environments [4], and for self-cleaning [5] and antifouling [6] surfaces. Although there are many studies of superhydrophobic surface fabrication, methods are usually substrate specific, and thus are limited for widespread practical use. The development of simple, versatile strategies for superhydrophobic surface modification is challenging, and there are few general methods for creating these biomimetic surface structures. The bioadhesion of marine organisms, such as mussels, has also attracted intense interest. Mussels achieve long-lasting adhesion in a wet environment, and attach to diverse substrates through
∗ Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.apsusc.2016.02.135 0169-4332/© 2016 Elsevier B.V. All rights reserved.
the proteins in its byssus. [7] The adhesive proteins contain high levels of L-3,4-dihydroxyphenylalanine (DOPA), which contributes to the crosslinking of the proteins and to form strong covalent and noncovalent interactions with surfaces. Messersmith et al. [8] reported that dopamine has similar functional groups to DOPA and that polydopamine, formed by oxygen-induced polymerization, behaves as a universal glue that adeheres to all kinds of organic and inorganic surfaces. The as-formed polydopamine contains many reactive groups that can be used for diverse secondary reactions [9–16], suitable for designing a general route to constructing a superhydrophobic lotus-leaf-like surface with superhydrophobic properties. Recently, we successfully prepared a dopamine-based superhydrophobic melamine foam for environmental treatment, and demonstrated it as a simple, convenient method for constructing superhydrophobic surfaces [17]. In the present work, fabrics with different properties and chemical compositions were modified to be superhydrophobic separately or together by using this bio-inspired treatment method. Polyethylene terephthalate (PET), PET/cotton, polypropylene (PP), and cotton, which have different properties, were chosen to demonstrate the versatility and general applicability of our approach. The fabrics was modified with a single universal procedure with no complex preparation or expensive equipment. The modified fabrics exhibited excellent superhydrophobic performance regardless of their original properties, suggesting that the method is promising for practical
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Fig. 1. SEM photographs of (a) untreated PET fabric, (b) PD/PET, (c) Ag/PD/PET, (d) Ag/PD/PET after ultrasonic treatment and (e) clear observation of F/Ag/PD/PET.
applications. The method may also pave the way for easily investigating other superhydrophobic fabrics. The general procedure for preparing superhydrophobic fabrics is shown in Scheme 1.
2. Experimental 2.1. Materials PET, PET/cotton (65/35), cotton, and PP fabrics were supplied by Kuraray Co., Ltd., Japan. 1H,1H,2H,2H-Perfluorodecanethiol and dopamine were purchased from Sigma-Aldrich Co., Ltd., USA. Tris(hydroxymethyl) aminomethane (Tris) and AgNO3 were purchased from Wako Co., Ltd., Japan.
2.2. Preparation of superhydrophobic fabrics
Scheme 1. Schematic illustration of procedure for preparing superhydrophobic fabric.
The general procedure for preparing superhydrophobic fabrics was as follows [17]. First, dopamine (2 mg/mL) was dissolved in 10 mM Tris–HCl (pH 8.5) to obtain the functional solution for fabrics. The fabric was added to the solution immediately, and then the dopamine solution was bubbled with pure O2 for 3 min and sealed. The solution was stirred for 2 h. Oxygeninduced polymerization of dopamine allows spontaneous, rapid formation of a polydopamine (PD) membrane on the surface of fabrics. The fabric was ultrasonicated in water for 5 min, washed with deionized water several times, and then placed in 1 mg/mL
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Scheme 2. (a–c) The surface traces after the water droplet left the cotton fabrics. Schematic illustration of three types of solid/liquid/air three-phase contact models: (d) “area” contact model (e) “medium area” contact model; (f) “point” contact model. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 2. FT-IR spectra of PET fabrics (black, top) and dopamine-coated PET fabrics (red, bottom). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of (a) pristine PET fabrics, (b) PD/PET fabrics, and (c) Ag/PD/PET fabrics.
2.3. Characterization
AgNO3 solution (20 mL) for 12 h. Ag nanoparticles (NPs) were deposited on the polydopamine-coated surfaces by reduction. The hydrophobic agent 1H,1H,2H,2H-perfluorodecanethiol ethanol solution (1:1000 v:v) reacted with both PD and Ag NPs to produce a superhydrophobic Ag/PD/fabric composite with a lotus-leaf-like hierarchical surface.
The chemical composition changes of the fabrics were determined by X-ray photoelectron spectroscopy (XPS; JPS-9010MCY, JEOL). The surface morphology of the samples was observed by scanning electron microscopy (SEM; JSM-6390YH, JEOL). Element mapping was carried out by field emission (FE)-SEM (Ultra plus, Carl Zeiss). Attenuated total reflectance Fourier transform infrared (ATR-FTIR; FTLA 2000, ABB Miracle) spectra of modified and unmodified PET fabrics were measured in the range
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Fig. 4. XPS spectra of (a) PET fabrics, (b) PD/PET, (c) Ag/PD/PET, (d) 1H,1H,2H,2H-perfluorodecanethiol grafted Ag/PD/PET (F/Ag/PD/PET) and (e) the clear comparison of Ag/PD/PET and F/Ag/PD/PET.
4000–1000 cm−1 . The crystalline structure of the samples was determined by powder X-ray diffraction (XRD-6100, Shimadzu) with Cu K␣ radiation. For the CA measurement, the static CAs of the fabrics were measured with a drop shape analysis system (DSA 100, Kruss) by the sessile water drop method with 3 L water drops.
Reported data were averages of five measurements at different places on the fabrics.
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Fig. 5. The element mapping analysis of F/Ag/PD/PET fabrics.
Fig. 6. Water droplets sitting on the surface of different modified PET fabrics (a) F/Ag/PD/PET fabrics, (b) F/PD/PET fabrics and (c) the picture of F/Ag/PD/PET fabrics in water.
3. Results and discussion The PET, cotton, PP, and cotton/PET fabrics were treated using the same process for each step. We characterized the superhydrophobic modification of PET fabrics. PET fabric is a difficult surface to modify owing to its lack of reactive groups. The surface morphologies of the PET fabrics before and after dopamine coating and Ag deposition were investigated by SEM. After the PET fabric was dispersed in dopamine solution for 2 h, its surface was rougher than the untreated fabric (Fig. 1a), implying that it was coated in polydopamine (Fig. 1b). The roughness of the surface was attributed to the assembly of dopamine particles. Whether the structure of polydopamine arises from dopamine polymerization or amorphous supramolecular aggregation has yet to be determined unambiguously. However, dopamine oxidation occurs via oxidation, intra-molecular cyclization, and rearrangement leading to the formation of multiple products [18,19]. These multiple products can react further with each other owing to their conjugated aromatic structures producing diverse compounds. The strong adhesion of polydopamine was confirmed by
ultrasonication (Video S1), suggesting that the coating technology has been practical and industrial applications prospect. The FTIR spectrum of the polydopamine-coated PET fabric (Fig. 2) contained a peak at 1605 cm−1 and a broad peak spanning 3200–3500 cm−1 . These peaks are consistent with the presence of reactive indole or indoline moieties and hydroxyl groups [18]. The polydopamine coating is a versatile platform for secondary reactions for further functionalization [20–28]. The strong adhesion and reactivity of polydopamine allowed the lotus leaf surface to be fabricated by depositing Ag NPs on the surface of the ultrasonicated polydopamine-coated fabrics via dipping the substrate in AgNO3 solution. No additional reducing agent was necessary, because the catechol groups in polydopamine reduced the Ag. Separate Ag NPs and aggregates with sizes from tens of nanometers to several micrometers were found on the polydopamine-coated surface after reduction (Fig. 1c). The good stability of the hierarchical structure of Ag/PD/PET fabrics arising from the chemical bonds between Ag NPs and polydopamine was also demonstrated by ultrasonication (Fig. 1d). The polydopamine assemblies and Ag NPs and clusters were the building blocks of the dual-scale
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Fig. 7. (a–d) Time sequence of the oil/water mixture separation procedure with superhydrophobic PET fabric. Time sequence of (e–g) capture oil layer (n-hexane dyed red) on water surface, and (h–j) underwater oil droplets (chloroform dyed red) with superhydrophobic PET fabrics. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
hierarchical structure, mimicking the lotus leaf micromorphology. The hierarchical structure of the fabrics did not change after hydrophobic treatment (Fig. 1e), further demonstrating that the coating was robust and stable. The crystalline structure of the fabrics was determined by XRD. Polydopamine-coated and uncoated PET fabrics have the same diffraction peaks, indicating that the polydopamine layer has no effect on the crystallinity of the PET fabric. Fig. 3c shows three distinct characteristic peaks at the 2 values of 38.2◦ , 44.4◦ , and 77.4◦ , corresponding to the (1 1 1), (2 0 0), and (3 1 1) planes of FCC phase Ag, respectively (JCPDS Card No. 4–783). No diffraction peaks for Ag oxide or halides were observed, indicating that elemental Ag was present on the surface of polydopamine-coated PET fabrics. The chemical composition of the fabric at each step of the fabrication processes was examined by XPS. The untreated PET fabrics only showed C and O signals (Fig. 4a), whereas polydopaminecoated PET fabric had C, N, and O signals from the polydopamine layer (Fig. 4b). After Ag NPs and clusters were formed on the polydopamine-coated PET fabrics, characteristic Ag peaks appeared in the spectrum (Fig. 4c). When the composite fabrics were
modified with 1H,1H,2H,2H-perfluorodecanethiol, S 2p and strong F 1s signals were observed (Fig. 4d and e). Meanwhile, a small characteristic peak at 294 eV was assigned as C 1s from F C F groups. These results demonstrate that thiol fluoroalkanes were grafted on the Ag/PD/PET fabrics. Elemental mapping (Fig. 5) for the F/Ag/PD/PET fabric showed that the N, Ag, S, and F distributions were uniform, indicating that the dopamine coating, Ag deposition, and final hydrophobic treatments were uniform on the fabric surface, leading to the formation of the superhydrophobic product. In addition, Cl was also detected (data not shown), which was attributed to residual Tris–HCl solution. The surface wetting performance of the as-prepared fabrics was evaluated by their water CA. After polydopamine coating and Ag deposition, PET fabrics were superhydrophilic owing to the abundant hydrophilic surface groups. In contrast, after modification with thiol fluoroalkanes, the surface of modified PET fibers showed a water CA of 155 ± 1.2◦ (Fig. 6a) and a low sliding angle of 7◦ . Water droplets easily rolled off the surface. This superhydrophobicity was attributed to the rough hierarchical structure and low surface free
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Fig. 8. (a–d) The self-cleaning process of superhydrophobic surfaces with low surface energy. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
energy. A stable, uniform plastron layer was formed (Fig. 6c), indicating a robust Cassie–Baxter state [29]. The polydopamine-coated PET fabric without Ag NP deposition was also modified with thiol fluoroalkanes, and the surface CA was 136 ± 1.5◦ . This result suggests that the modified hierarchical structure of polydopamine and Ag NPs played a key role in the wettability, forming a superhydrophobic surface mimicking a lotus leaf surface. Next, we discuss the mechanism that produces different wetting properties on untreated PET fabrics, F/PD/PET fabrics and F/Ag/PD/PET fabrics. A large trace of water droplet dyed with methylene blue, marked by a red circle, was observed on PET fabrics after absorbing the water with a tissue paper (Scheme 2a). No air layer between the water and the untreated PET fabrics surface was observed. Therefore, the water droplet on the untreated surface obeyed the area contact model, and had a continuous threephase contact line at the vapor–liquid–solid interface (Scheme 2d). In this state, water penetrated most of the fabric, producing a large liquid/solid contact area. In PD/PET fabrics, the surface structure was altered by the dopamine coating. Many larger PD particles or assemblies were uniformly distributed on the fabric surface. When PD/PET fabrics were directly modified with the hydrophobic agent, the trace left by the water droplet decreased substantially (Scheme 2b), indicating the formation of an air layer between the sample and the water droplet, following the medium area contact model (Scheme 2e). The large amount of air trapped inside the liquid/solid contact area prevented water from wetting the microstructure surface. The Ag NPs deposited on the PD/PET fabrics formed a lotus-leaf-like structure. Almost no water trace was observed after the fluorination of Ag/PD/PET fabrics. Fig. 6c shows a distinct plastron (air pocket) on the superhydrophobic fabric surface. When a droplet of water is placed on such surface, the water easily rolls off the tilted surface. A water droplet on a hierarchical
surface with dual scale structures is in a special lotus leaf-like Cassie state with a point contact model (Scheme 2f). In this state, the water has a small contact area with the top of the lotus-leaf-like dualscale structure. An air layer was trapped among the micro/nano particles, which prevents water from penetrating the microstructure. These results indicate that the hierarchical surface with the micro/nanostructure is crucial for the superhydrophobic samples. Superhydrophobic and underwater superoleophobic materials can be used for oil/water separation [30–35]. The as-prepared samples with special wetting properties can be used for selectively collecting oil from a floating oil layer, underwater oil droplets, or oil–water mixtures. A proof-of-concept oil/water separation experiment was performed (Fig. 7a–d). The modified PET fabric was fixed between two glass vessels, and a mixture of red-dyed chloroform and water were poured into the upper glass vessel. The chloroform droplets permeated through the modified PET fabric, and dropped into the lower container with no external force, separating the mixture. The contaminated PET fabric was cleaned with ethanol after oil/water separation and could be reused. The oil/water separation can be seen in Video S2. For specific oil–water mixtures, such as oil slicks on the surface of water or underwater, the as-prepared superhydrophilic samples cannot separate the two phases, whereas the modified superhydrophobic and superoleophilic PET fabric can. Fig. 7e–g shows the capture of red-dyed n-hexane from water. The n-hexane was immediately absorbed when it came into contact the modified fabric. For oils with a higher density than the water surface (chloroform, Fig. 7h–j), the superhydrophobic PET fabric was taken from water with tweezers, and quickly captured the oil as soon as it touched the oil surface. These results suggest that the superhydrophobic PET fabrics are promising for versatile separation of oil/water under various conditions. The capture and absorption of
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Fig. 9. SEM images and contact angles test of different surface modified fabrics (a) PP, (b) cotton and (c) PET/cotton and their respective CA images.
oils from the water surface and underwater are presented in Videos S3 and S4. To demonstrate the self-cleaning properties of the superhydrophobic F/Ag/PD/PET fabrics, methylene blue powder was used as a contaminant. The superhydrophobic fabric was placed on a transparent glass slide with a small angle of inclination. A sparse layer of methylene blue powder was dispersed on the modified fabric surface, and was cleaned off by directing water droplets with a micropipette (Fig. 8a–d and Video S5). When the droplets were moved over the powder (Fig. 8b and c), the powder was immediately trapped and carried away by the water droplets, leaving behind a cleaner surface. The water droplets rolled off the superhydrophobic fabric surface when tilted, removing the powder. Some water droplets remained spherical even after they picked up the contaminants (Video S5). Owing to the nonselective adhesion of polydopamine, our coating method is universally applicable to fabrics regardless of their properties and chemical composition. PP, cotton and cotton/PET (35/65) were modified by using the same processes for each step. The three fabrics were all coated with polydopamine, Ag NPs, and
then treated with thiol fluoroalkanes. After surface modification, the PP and cotton fabrics became superhydrophobic, with water CAs of 153 ± 1.5◦ and 152 ± 1.3◦ and sliding angles of 7◦ and 8◦ , respectively (Fig. 9a and b). Fabrics are often composed of several components, for which previous superhydrophobic coating techniques would not be suitable. However, polydopamine can coat all kinds of fabrics, including adhesion-resistant materials such as poly(tetrafluoroethylene) (PTFE), and thus provides a versatile method for fabricating superhydrophobic surfaces on mixed fabrics. Fig. 9c shows SEM images of cotton/PET (35/65) (cospun fabric) after surface modification with our method. The water CA also reach 151 ± 1.2◦ and a sliding angle of 9◦ . Different types of fibers or cospun fabric can be modified uniformly with hierarchical structures in the same vessel. Therefore, no hydrophilic areas in the final fabrics would compromise the hydrophobicity. The modified PP, cotton, and cotton/PET fabrics all showed excellent self-cleaning properties and oil/water separation similar to that of modified PET fabrics. These results show that our method for fabricating superhydrophobic surfaces is simple and universal.
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4. Conclusions Inspired by the strong adhesion of mussel adhesive protein and the hierarchical structure of lotus leaves, a versatile approach has been developed to design highly water repellent fabrics under mild conditions. This method is suitable for diverse types of fabric, regardless of their original properties and chemical compositions. Owing to the nonselective adhesion of dopamine, these fabrics can be easily modified together in each step. The modified fabrics separated oil and water mixtures, including a floating oil layer and underwater oil droplets, and showed excellent self-cleaning properties. Our results pave the way for further exploration of superhydrophobic fabrics. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.02. 135. References [1] Y. Lu, S. Sathasivam, J.L. Song, C.R. Crick, C.J. Carmalt, I.P. Parkin, Robust self-cleaning surfaces that function when exposed to either air or oil, Science 347 (2015) 1132–1135. [2] J.V.I. Timonen, M. Latikka, L. Leibler, R.H.A. Ras, O. Ikkaka, Switchable static and dynamic self-assembly of magnetic droplets on superhydrophobic surfaces, Science 341 (2013) 253–257. [3] X. Deng, L. Mammen, H.J. Butt, D. Vollmer, Candle soot as a template for a transparent robust superamphiphobic coating, Science 335 (2012) 67–70. [4] X. Sheng, J. Zhang, Superhydrophobic behaviors of polymeric surfaces with aligned nanofibers, Langmuir 25 (2009) 6916–6922. [5] R. Blossey, Self-cleaning surfaces-virtual realities, Nat. Mater. 2 (2003) 301–306. [6] M. Nosonovsky, B. Bhushan, Superhydrophobic surfaces and emerging applications: non-adhesion energy, green engineering, Curr. Opin. Colloid Interface Sci. 14 (2009) 270–280. [7] J.H. Waite, M.L. Tanzer, Polyphenolic substance of mytilus edulis: novel adhesive coating L-dopa and hydroxyproline, Science 212 (1981) 1038–1040. [8] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. [9] L. Zhang, J.J. Wu, Y.X. Wang, Y.H. Long, N. Zhao, J. Xu, Combination of bioinspiration: a general route to superhydrophobic particles, J. Am. Chem. Soc. 134 (2012) 9879–9881. [10] Q. Liu, B.X. Huang, A.S. Huang, Polydopamine-based superhydrophobic membranes for biofuel recovery, J. Mater. Chem. A 1 (2013) 11970–11974. [11] F.T. Liu, F.H. Sun, Q.M. Pan, Highly compressible and stretchable superhydrophobic coating inspired by bio-adhesion of marine mussels, J. Mater. Chem. A 2 (2014) 11365–11371. [12] Z.X. Wang, Y.C. Xu, Y.Y. Liu, L. Shao, A novel mussel-inspired strategy toward superhydrophobic surfaces for self-driven crude oil spill cleanup, J. Mater. Chem. A 3 (2015) 12171–12178. [13] Z.X. Wang, X. Jiang, X.Q. Cheng, C.H. Lau, L. Shao, Mussel-inspired hybrid coatings that transform membrane hydrophobicity into high hydrophilicity and underwater superoleophobicity for oil-in-water emulsion separation, ACS Appl. Mater. Interfaces 7 (2015) 9534–9545. [14] Z.X. Wang, C.H. Lau, N.Q. Zhang, Y.P. Baia, L. Shao, Mussel-inspired tailoring of membrane wettability for harsh water treatment, J. Mater. Chem. A 3 (2015) 2650–2657.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of polydopamine-coated superhydrophobic fabrics for oil/water separation and self-cleaning Zhanglian Xu a,∗ , Koji Miyazaki b , Teruo Hori a a b
Headquarters for Innovative Society-Academic Cooperation, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan Frontier Fiber Technology and Science Graduate School of Engineering, University of Fukui, Bunkyo 3-9-1, Fukui 910-8507, Japan
a r t i c l e
i n f o
Article history: Received 30 September 2015 Received in revised form 12 February 2016 Accepted 12 February 2016 Keywords: Superhydrophobic Dopamine Fabrics Oil/water separation Self-cleaning
a b s t r a c t We report a fabric coating method inspired the superhydrophobic properties of lotus leaves and the strong adhesion of the adhesive proteins in mussels. Dopamine, which mimics the single units of the adhesive mussel proteins, was polymerized in an alkaline aqueous solution to coat the surface of fabrics. The versatile reactivity of polydopamine allows subsequent Ag deposition to form a lotus-leaf-like rough structure on the fabric surface. The composite fabric exhibited high water repellence after fluorination. Because dopamine can adhere to all kinds of materials, this method can be applied to many fabrics regardless of their properties and chemical compositions using a universal process. The modified fabrics exhibited excellent anti-wetting and self-cleaning properties with contact angles of >150◦ and sliding angles lower than 9◦ . The fabrics also efficiently separated oil from oil/water mixtures under various conditions. Our method is versatile and simple compared with other hydrophobic treatment methods, which usually only work on one type of fabric. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nature often provides biological inspiration for research. A famous example is lotus leaves, on which water droplets form spheres and roll off. The extreme water repellent behavior comes from the low surface energy of the wax on the leaf and the unique micro/nano hierarchical structure formed from sparse micropapillae covered with nanosized wax crystals. Lotus leaves have a superhydrophobic surface with a water contact angle (CA) larger than 150◦ and a low sliding angle [1–3]. Superhydrophobic surfaces have many practical uses, such as in agriculture in harsh environments [4], and for self-cleaning [5] and antifouling [6] surfaces. Although there are many studies of superhydrophobic surface fabrication, methods are usually substrate specific, and thus are limited for widespread practical use. The development of simple, versatile strategies for superhydrophobic surface modification is challenging, and there are few general methods for creating these biomimetic surface structures. The bioadhesion of marine organisms, such as mussels, has also attracted intense interest. Mussels achieve long-lasting adhesion in a wet environment, and attach to diverse substrates through
∗ Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.apsusc.2016.02.135 0169-4332/© 2016 Elsevier B.V. All rights reserved.
the proteins in its byssus. [7] The adhesive proteins contain high levels of L-3,4-dihydroxyphenylalanine (DOPA), which contributes to the crosslinking of the proteins and to form strong covalent and noncovalent interactions with surfaces. Messersmith et al. [8] reported that dopamine has similar functional groups to DOPA and that polydopamine, formed by oxygen-induced polymerization, behaves as a universal glue that adeheres to all kinds of organic and inorganic surfaces. The as-formed polydopamine contains many reactive groups that can be used for diverse secondary reactions [9–16], suitable for designing a general route to constructing a superhydrophobic lotus-leaf-like surface with superhydrophobic properties. Recently, we successfully prepared a dopamine-based superhydrophobic melamine foam for environmental treatment, and demonstrated it as a simple, convenient method for constructing superhydrophobic surfaces [17]. In the present work, fabrics with different properties and chemical compositions were modified to be superhydrophobic separately or together by using this bio-inspired treatment method. Polyethylene terephthalate (PET), PET/cotton, polypropylene (PP), and cotton, which have different properties, were chosen to demonstrate the versatility and general applicability of our approach. The fabrics was modified with a single universal procedure with no complex preparation or expensive equipment. The modified fabrics exhibited excellent superhydrophobic performance regardless of their original properties, suggesting that the method is promising for practical
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Fig. 1. SEM photographs of (a) untreated PET fabric, (b) PD/PET, (c) Ag/PD/PET, (d) Ag/PD/PET after ultrasonic treatment and (e) clear observation of F/Ag/PD/PET.
applications. The method may also pave the way for easily investigating other superhydrophobic fabrics. The general procedure for preparing superhydrophobic fabrics is shown in Scheme 1.
2. Experimental 2.1. Materials PET, PET/cotton (65/35), cotton, and PP fabrics were supplied by Kuraray Co., Ltd., Japan. 1H,1H,2H,2H-Perfluorodecanethiol and dopamine were purchased from Sigma-Aldrich Co., Ltd., USA. Tris(hydroxymethyl) aminomethane (Tris) and AgNO3 were purchased from Wako Co., Ltd., Japan.
2.2. Preparation of superhydrophobic fabrics
Scheme 1. Schematic illustration of procedure for preparing superhydrophobic fabric.
The general procedure for preparing superhydrophobic fabrics was as follows [17]. First, dopamine (2 mg/mL) was dissolved in 10 mM Tris–HCl (pH 8.5) to obtain the functional solution for fabrics. The fabric was added to the solution immediately, and then the dopamine solution was bubbled with pure O2 for 3 min and sealed. The solution was stirred for 2 h. Oxygeninduced polymerization of dopamine allows spontaneous, rapid formation of a polydopamine (PD) membrane on the surface of fabrics. The fabric was ultrasonicated in water for 5 min, washed with deionized water several times, and then placed in 1 mg/mL
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Scheme 2. (a–c) The surface traces after the water droplet left the cotton fabrics. Schematic illustration of three types of solid/liquid/air three-phase contact models: (d) “area” contact model (e) “medium area” contact model; (f) “point” contact model. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
Fig. 2. FT-IR spectra of PET fabrics (black, top) and dopamine-coated PET fabrics (red, bottom). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. XRD patterns of (a) pristine PET fabrics, (b) PD/PET fabrics, and (c) Ag/PD/PET fabrics.
2.3. Characterization
AgNO3 solution (20 mL) for 12 h. Ag nanoparticles (NPs) were deposited on the polydopamine-coated surfaces by reduction. The hydrophobic agent 1H,1H,2H,2H-perfluorodecanethiol ethanol solution (1:1000 v:v) reacted with both PD and Ag NPs to produce a superhydrophobic Ag/PD/fabric composite with a lotus-leaf-like hierarchical surface.
The chemical composition changes of the fabrics were determined by X-ray photoelectron spectroscopy (XPS; JPS-9010MCY, JEOL). The surface morphology of the samples was observed by scanning electron microscopy (SEM; JSM-6390YH, JEOL). Element mapping was carried out by field emission (FE)-SEM (Ultra plus, Carl Zeiss). Attenuated total reflectance Fourier transform infrared (ATR-FTIR; FTLA 2000, ABB Miracle) spectra of modified and unmodified PET fabrics were measured in the range
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Fig. 4. XPS spectra of (a) PET fabrics, (b) PD/PET, (c) Ag/PD/PET, (d) 1H,1H,2H,2H-perfluorodecanethiol grafted Ag/PD/PET (F/Ag/PD/PET) and (e) the clear comparison of Ag/PD/PET and F/Ag/PD/PET.
4000–1000 cm−1 . The crystalline structure of the samples was determined by powder X-ray diffraction (XRD-6100, Shimadzu) with Cu K␣ radiation. For the CA measurement, the static CAs of the fabrics were measured with a drop shape analysis system (DSA 100, Kruss) by the sessile water drop method with 3 L water drops.
Reported data were averages of five measurements at different places on the fabrics.
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Fig. 5. The element mapping analysis of F/Ag/PD/PET fabrics.
Fig. 6. Water droplets sitting on the surface of different modified PET fabrics (a) F/Ag/PD/PET fabrics, (b) F/PD/PET fabrics and (c) the picture of F/Ag/PD/PET fabrics in water.
3. Results and discussion The PET, cotton, PP, and cotton/PET fabrics were treated using the same process for each step. We characterized the superhydrophobic modification of PET fabrics. PET fabric is a difficult surface to modify owing to its lack of reactive groups. The surface morphologies of the PET fabrics before and after dopamine coating and Ag deposition were investigated by SEM. After the PET fabric was dispersed in dopamine solution for 2 h, its surface was rougher than the untreated fabric (Fig. 1a), implying that it was coated in polydopamine (Fig. 1b). The roughness of the surface was attributed to the assembly of dopamine particles. Whether the structure of polydopamine arises from dopamine polymerization or amorphous supramolecular aggregation has yet to be determined unambiguously. However, dopamine oxidation occurs via oxidation, intra-molecular cyclization, and rearrangement leading to the formation of multiple products [18,19]. These multiple products can react further with each other owing to their conjugated aromatic structures producing diverse compounds. The strong adhesion of polydopamine was confirmed by
ultrasonication (Video S1), suggesting that the coating technology has been practical and industrial applications prospect. The FTIR spectrum of the polydopamine-coated PET fabric (Fig. 2) contained a peak at 1605 cm−1 and a broad peak spanning 3200–3500 cm−1 . These peaks are consistent with the presence of reactive indole or indoline moieties and hydroxyl groups [18]. The polydopamine coating is a versatile platform for secondary reactions for further functionalization [20–28]. The strong adhesion and reactivity of polydopamine allowed the lotus leaf surface to be fabricated by depositing Ag NPs on the surface of the ultrasonicated polydopamine-coated fabrics via dipping the substrate in AgNO3 solution. No additional reducing agent was necessary, because the catechol groups in polydopamine reduced the Ag. Separate Ag NPs and aggregates with sizes from tens of nanometers to several micrometers were found on the polydopamine-coated surface after reduction (Fig. 1c). The good stability of the hierarchical structure of Ag/PD/PET fabrics arising from the chemical bonds between Ag NPs and polydopamine was also demonstrated by ultrasonication (Fig. 1d). The polydopamine assemblies and Ag NPs and clusters were the building blocks of the dual-scale
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Fig. 7. (a–d) Time sequence of the oil/water mixture separation procedure with superhydrophobic PET fabric. Time sequence of (e–g) capture oil layer (n-hexane dyed red) on water surface, and (h–j) underwater oil droplets (chloroform dyed red) with superhydrophobic PET fabrics. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
hierarchical structure, mimicking the lotus leaf micromorphology. The hierarchical structure of the fabrics did not change after hydrophobic treatment (Fig. 1e), further demonstrating that the coating was robust and stable. The crystalline structure of the fabrics was determined by XRD. Polydopamine-coated and uncoated PET fabrics have the same diffraction peaks, indicating that the polydopamine layer has no effect on the crystallinity of the PET fabric. Fig. 3c shows three distinct characteristic peaks at the 2 values of 38.2◦ , 44.4◦ , and 77.4◦ , corresponding to the (1 1 1), (2 0 0), and (3 1 1) planes of FCC phase Ag, respectively (JCPDS Card No. 4–783). No diffraction peaks for Ag oxide or halides were observed, indicating that elemental Ag was present on the surface of polydopamine-coated PET fabrics. The chemical composition of the fabric at each step of the fabrication processes was examined by XPS. The untreated PET fabrics only showed C and O signals (Fig. 4a), whereas polydopaminecoated PET fabric had C, N, and O signals from the polydopamine layer (Fig. 4b). After Ag NPs and clusters were formed on the polydopamine-coated PET fabrics, characteristic Ag peaks appeared in the spectrum (Fig. 4c). When the composite fabrics were
modified with 1H,1H,2H,2H-perfluorodecanethiol, S 2p and strong F 1s signals were observed (Fig. 4d and e). Meanwhile, a small characteristic peak at 294 eV was assigned as C 1s from F C F groups. These results demonstrate that thiol fluoroalkanes were grafted on the Ag/PD/PET fabrics. Elemental mapping (Fig. 5) for the F/Ag/PD/PET fabric showed that the N, Ag, S, and F distributions were uniform, indicating that the dopamine coating, Ag deposition, and final hydrophobic treatments were uniform on the fabric surface, leading to the formation of the superhydrophobic product. In addition, Cl was also detected (data not shown), which was attributed to residual Tris–HCl solution. The surface wetting performance of the as-prepared fabrics was evaluated by their water CA. After polydopamine coating and Ag deposition, PET fabrics were superhydrophilic owing to the abundant hydrophilic surface groups. In contrast, after modification with thiol fluoroalkanes, the surface of modified PET fibers showed a water CA of 155 ± 1.2◦ (Fig. 6a) and a low sliding angle of 7◦ . Water droplets easily rolled off the surface. This superhydrophobicity was attributed to the rough hierarchical structure and low surface free
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Fig. 8. (a–d) The self-cleaning process of superhydrophobic surfaces with low surface energy. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
energy. A stable, uniform plastron layer was formed (Fig. 6c), indicating a robust Cassie–Baxter state [29]. The polydopamine-coated PET fabric without Ag NP deposition was also modified with thiol fluoroalkanes, and the surface CA was 136 ± 1.5◦ . This result suggests that the modified hierarchical structure of polydopamine and Ag NPs played a key role in the wettability, forming a superhydrophobic surface mimicking a lotus leaf surface. Next, we discuss the mechanism that produces different wetting properties on untreated PET fabrics, F/PD/PET fabrics and F/Ag/PD/PET fabrics. A large trace of water droplet dyed with methylene blue, marked by a red circle, was observed on PET fabrics after absorbing the water with a tissue paper (Scheme 2a). No air layer between the water and the untreated PET fabrics surface was observed. Therefore, the water droplet on the untreated surface obeyed the area contact model, and had a continuous threephase contact line at the vapor–liquid–solid interface (Scheme 2d). In this state, water penetrated most of the fabric, producing a large liquid/solid contact area. In PD/PET fabrics, the surface structure was altered by the dopamine coating. Many larger PD particles or assemblies were uniformly distributed on the fabric surface. When PD/PET fabrics were directly modified with the hydrophobic agent, the trace left by the water droplet decreased substantially (Scheme 2b), indicating the formation of an air layer between the sample and the water droplet, following the medium area contact model (Scheme 2e). The large amount of air trapped inside the liquid/solid contact area prevented water from wetting the microstructure surface. The Ag NPs deposited on the PD/PET fabrics formed a lotus-leaf-like structure. Almost no water trace was observed after the fluorination of Ag/PD/PET fabrics. Fig. 6c shows a distinct plastron (air pocket) on the superhydrophobic fabric surface. When a droplet of water is placed on such surface, the water easily rolls off the tilted surface. A water droplet on a hierarchical
surface with dual scale structures is in a special lotus leaf-like Cassie state with a point contact model (Scheme 2f). In this state, the water has a small contact area with the top of the lotus-leaf-like dualscale structure. An air layer was trapped among the micro/nano particles, which prevents water from penetrating the microstructure. These results indicate that the hierarchical surface with the micro/nanostructure is crucial for the superhydrophobic samples. Superhydrophobic and underwater superoleophobic materials can be used for oil/water separation [30–35]. The as-prepared samples with special wetting properties can be used for selectively collecting oil from a floating oil layer, underwater oil droplets, or oil–water mixtures. A proof-of-concept oil/water separation experiment was performed (Fig. 7a–d). The modified PET fabric was fixed between two glass vessels, and a mixture of red-dyed chloroform and water were poured into the upper glass vessel. The chloroform droplets permeated through the modified PET fabric, and dropped into the lower container with no external force, separating the mixture. The contaminated PET fabric was cleaned with ethanol after oil/water separation and could be reused. The oil/water separation can be seen in Video S2. For specific oil–water mixtures, such as oil slicks on the surface of water or underwater, the as-prepared superhydrophilic samples cannot separate the two phases, whereas the modified superhydrophobic and superoleophilic PET fabric can. Fig. 7e–g shows the capture of red-dyed n-hexane from water. The n-hexane was immediately absorbed when it came into contact the modified fabric. For oils with a higher density than the water surface (chloroform, Fig. 7h–j), the superhydrophobic PET fabric was taken from water with tweezers, and quickly captured the oil as soon as it touched the oil surface. These results suggest that the superhydrophobic PET fabrics are promising for versatile separation of oil/water under various conditions. The capture and absorption of
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Fig. 9. SEM images and contact angles test of different surface modified fabrics (a) PP, (b) cotton and (c) PET/cotton and their respective CA images.
oils from the water surface and underwater are presented in Videos S3 and S4. To demonstrate the self-cleaning properties of the superhydrophobic F/Ag/PD/PET fabrics, methylene blue powder was used as a contaminant. The superhydrophobic fabric was placed on a transparent glass slide with a small angle of inclination. A sparse layer of methylene blue powder was dispersed on the modified fabric surface, and was cleaned off by directing water droplets with a micropipette (Fig. 8a–d and Video S5). When the droplets were moved over the powder (Fig. 8b and c), the powder was immediately trapped and carried away by the water droplets, leaving behind a cleaner surface. The water droplets rolled off the superhydrophobic fabric surface when tilted, removing the powder. Some water droplets remained spherical even after they picked up the contaminants (Video S5). Owing to the nonselective adhesion of polydopamine, our coating method is universally applicable to fabrics regardless of their properties and chemical composition. PP, cotton and cotton/PET (35/65) were modified by using the same processes for each step. The three fabrics were all coated with polydopamine, Ag NPs, and
then treated with thiol fluoroalkanes. After surface modification, the PP and cotton fabrics became superhydrophobic, with water CAs of 153 ± 1.5◦ and 152 ± 1.3◦ and sliding angles of 7◦ and 8◦ , respectively (Fig. 9a and b). Fabrics are often composed of several components, for which previous superhydrophobic coating techniques would not be suitable. However, polydopamine can coat all kinds of fabrics, including adhesion-resistant materials such as poly(tetrafluoroethylene) (PTFE), and thus provides a versatile method for fabricating superhydrophobic surfaces on mixed fabrics. Fig. 9c shows SEM images of cotton/PET (35/65) (cospun fabric) after surface modification with our method. The water CA also reach 151 ± 1.2◦ and a sliding angle of 9◦ . Different types of fibers or cospun fabric can be modified uniformly with hierarchical structures in the same vessel. Therefore, no hydrophilic areas in the final fabrics would compromise the hydrophobicity. The modified PP, cotton, and cotton/PET fabrics all showed excellent self-cleaning properties and oil/water separation similar to that of modified PET fabrics. These results show that our method for fabricating superhydrophobic surfaces is simple and universal.
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