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Polydimethylsiloxane-Based Superhydrophobic Surfaces on Steel Substrate: Fabrication, Reversibly Extreme Wettability and Oil-Water Separation Xiaojing Su, Hongqiang Li, Xuejun Lai, Lin Zhang, Tao Liang, Yuchun Feng, and Xingrong Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13901 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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ACS Applied Materials & Interfaces

Polydimethylsiloxane-Based Superhydrophobic Surfaces on Steel Substrate: Fabrication, Reversibly Extreme Wettability and Oil-Water Separation Xiaojing Su, Hongqiang Li*, Xuejun Lai, Lin Zhang, Tao Liang, Yuchun Feng, Xingrong Zeng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China KEYWORDS: superhydrophobic surfaces, polydimethylsiloxane, UV irradiation, reversibly extreme wettability, oil-water separation

ABSTRACT: Functional surfaces for reversibly switchable wettability and oil-water separation have attracted much interest with pushing forward an immense influence on fundamental research and industrial application in recent years. This article proposed a facile method to fabricate superhydrophobic surfaces on steel substrates via electroless replacement deposition of copper sulfate (CuSO4) and UV curing of vinyl terminated polydimethylsiloxane (PDMS). PDMS-based superhydrophobic surfaces exhibited water contact angle (WCA) nearly to 160o and water sliding angle (WSA) lower than 5o, preserving outstanding chemical stability which maintained superhydrophobicity immersing in different aqueous solutions with pH values from 1 to 13 for 12 h. Interestingly, the superhydrophobic surface could dramatically switched to

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superhydrophilic state under UV irradiation, and then gradually recovered to highly hydrophobic state with WCA at 140o after dark storage. The underlying mechanism was also investigated by scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Additionally, the PDMS-based steel mesh possessed high separation efficiency and excellent reusability in oil-water separation. Our studies provide a simple, fast and economical fabrication method for wettability-transformable superhydrophobic surfaces and have the potential applications in microfluidics, biomedical field and oil spill clean-up.

INTRODUCTION Superhydrophobic surfaces with excellent water repellency are usually defined as water contact angle (WCA) above 150o and water sliding angle (WSA) below 10o,1 receiving enormous attention for their wide applications such as self-cleaning,2,3 anti-corrosion,4,5 anti-icing,6,7 antipollution,8 oil-water separation,9,10 microfluidics,11,12 etc. Inspired by well-known “lotus effect” in nature, we know hierarchical roughness and low surface-energy materials commonly endow surfaces with superhydrophobicity.13,14 To date, the representative techniques including templating,15 sol-gel process,16 self-assembly,17 vapor deposition18 and lithography,19 have been developed to mimick organism surfaces, expecting to obtain the special wettability. Most of these techniques show very good results in terms of superhydrophobicity, but the tedious and time-consuming fabrication process, special and intricate instruments, and rigorous reaction conditions to create hierarchical roughness limit their potential for industry applications. Comparatively, electroless replacement deposition on metallic substrates is an easy and highly efficient way, promising to be practically applicable for large-scale production.20 However, to


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the best of our knowledge, the superhydrophobic surfaces by the method is difficult to achieve favorable nature properties such as durability, chemical stability, flexibility, etc. Consequently, it is passionately expected to fabricate robust superhydrophobic surfaces with a simple, low-cost and environmentally-friendly method. Meanwhile, in the past ten

years, enlightened by Namib Desert beetle with

hydrophilic/hydrophobic patterning on the back,21 many methods such as plasma treating,22,23 UV exposure,24,25 temperature stimuli,26 and electric fields induction,27 have been applied to transform surface wettability between superhydrophobicity and superhydrophilicity, exerting a significant role in water harvesting, microfluidic devices, cell adhesion and protein adsorption.2832

Tokudome et al.33 reported a novel superhydrophobic film made up of vertically oriented

titanate nanotubes (TNTs) with high aspect ratio after fluoroalkylsilane (FAS) modification, which could be switched to superhydrophilic by UV illumination with a xenon lamp. Unfortunately, such photo-catalytic decomposition of FAS molecules permanently changed the wettability of surfaces. Although Lv et al.34 demonstrated a pH-responsive surface layers with reversible switching between superhydrophobicity and superhydrophilicity based on the combination of lauryl methacrylate and 10-undecylenic or 2-[(methacryloyloxy)ethyl]dimethyl(3-sulfopropy), the usage of porogens will largely influence the construction of rough structure and deteriorate the mechanical properties of the surface. Currently, with the increasing oil spill accidents and environmental pollution, the demand of functional membrane for oil-water separation is becoming more urgent.35 Various approaches (e.g. electrospinning, sol-gel process, and one-step spray or dip coating)36-39 and materials (e.g. textiles, metals and sponges)9, 35, 40 have been used for the preparation of artificial membranes. For example, Fang et al.41 prepared a superhydrophobic membrane by electrospinning N-


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perfluorooctyl-substituted polyurethanes for oil-water separation, and the membrane had selfhealing ability dependent on the migration of low-surface-energy fluorine-containing polymer. Wu et al.42 reported superhydrophobic PU@Fe3O4@SiO2@FP sponges by CVD of TEOS to bind Fe3O4 nanoparticles and then dip-coating in a fluoropolymer (FP) aqueous solution, which could be magnetically driven to separate water/oil mixture. However, it is inevitable that the expensive and toxic fluorine-containing reagents will bring raised serious risks to human health and natural environment. Herein, we present a facile and economical approach to fabricate polydimethylsioxane-based superhydrophobic surfaces on steel substrates with hierarchical micro-nano structures. It was based on the utilization of electroless replacement deposition of CuSO4 to construct roughness on steel substrate and UV cross-linking reaction of vinyl-terminated PDMS to contribute flexible and hydrophobic surfaces with trimethylolpropane triacrylate (TMPTA) and 2-hydroxy-2methylproplophenone (Darocur 1173) as cross-linking agent and photo-initiator, respectively. The surface morphology and water contact angle (WCA) of the PDMS-based surfaces on steel plates were manipulated by controlling the deposition time of steel substrate in CuSO4 solution as well as the concentration of CuSO4 solution and V-PDMS solution, and the chemical resistance of the surfaces was studied at different pH values. The effect of UV radiation time on the wettability of the PDMS-based superhydrophobic surfaces on steel plates and its hydrophobic recovery behavior were investigated by Fourier transform infrared spectroscopy (FT-IR) and Xray photoelectron spectroscopy (XPS). In addition, the PDMS-based surface on steel mesh was applied for oil-water separation, and the separation efficiency and reusability were studied. The fabrication method is simple, fast, low-cost and available for large-scale production. The fabricated superhydrophobic surfaces on steel substrate exhibited excellent chemical stability and


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special UV-stimulated wettability transformation as well as high separation efficiency and reusability in oil-water separation. Our findings greatly develop the superhydrophobic materials and surfaces with reversibly extreme wettability. EXPERIMENTAL SECTION Materials. Vinyl terminated polydimethylsiloxane (V-PDMS, average Mw=6000) was supplied by Meryer (Shanghai) Chemical Technology Co., Ltd. Trimethylolpropane triacrylate (TMPTA, 85%), 2-hydroxy-2-methylproplophenone (Darocur 1173, 97%), hexane (A.R.), ethanol (A.R.) and sodium chloride (NaCl, A.R.) were provided by Aladdin reagent Co., Ltd (China). Chloroform (A.R.), hydrochloric acid (HCl, 37%, A.R.), copper (II) sulfate pentahydrate (CuSO4·5H2O, A.R.), acetone (A.R.) and sodium hydroxide (NaOH, A.R.) were purchased from Guangzhou chemical reagent factory (China). Methylene blue was obtained from Tianjin Tianxin fine chemical development center (China). Steel plate and steel mesh (80 meshes) were supplied by Guangzhou WINNER Stainless Co., Ltd (China) and Anping Dinghan Wire Mesh Product Co., Ltd (China), respectively. Sandpapers (400 and 2000 meshes) were bought from local stores. All chemicals were used as received without further purification, and deionized water was used for all the experiments and tests. Fabrication of PDMS-Based Superhydrophobic Surfaces on Steel Plates. The fabrication process of PDMS-based superhydrophobic surfaces on steel plates is presented in Figure 1. Steel plates (7.0 cm × 3.2 cm) were mechanically polished by sandpaper with 400 meshes until the surfaces became mirror flashed, and then ultrasonically washed with acetone, ethanol and deionized water for 5 min, respectively. After that, the plates were dried at 80 oC for 1 h and then deposited in CuSO4 solution containing equivalent molar concentration of NaCl for certain time under ambient condition, followed by water washing and air drying. Next, the plates were


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immersed in V-PDMS solutions including appropriate amount of V-PDMS, TMPTA, Darocur 1173 and hexane (see Table 1) for 60 sec. Subsequently, the taken-out plates were exposed at UV light (INTELLI-RAY 400, Uvitron International, Inc., USA) for 120 sec with a distance of 15 cm between the sample and the center of UV light lamp, and then dried at 80 oC for 4 h. UV-Stimulated Extreme Wettability Transformation of PDMS-Based Superhydrophobic Surfaces on Steel Plates and the Recovery Behavior. A 2 KW mercury lamp generating 365 nm wavelength was used as UV source. The PDMS-based superhydrophobic surfaces on steel plates were irradiated with the intensity of 140 mW/cm2 at a distance of 5 cm between the sample and UV source for different time, and the WCAs were measured to confirm the changing wettability. Specially, to intuitively exhibit the extreme wettability transformation, the photomask was used to fabricate two different regions with desired patterns, and water was dropped onto the masked and irradiated regions to form the selective wetting of surface. Furthermore, to further evaluate the recovery behavior of wettability, the irradiated steel plates were stored in dark environment to ensure constant light condition for 7 days to observe the change of WCAs. Oil-Water Separation of PDMS-Based Surface on Steel Mesh. The preparation process of PDMS-based surface on steel mesh was similar to that of the superhydrophobic surfaces on steel plates except polishing with sandpaper. The PDMS-based surface on steel mesh was folded to be a miniature flat groove for oil-water separation. Hexane and chloroform were selected as representatives in light oil-water separation and heavy oil-water separation, respectively. A mixture of 50 mL of deionized water colored with methylene blue and 50 mL of oil was separated through the steel mesh, and then the steel mesh was washed in ethane and dried. The separation process was repeated for 20 cycles to evaluate the reusability of steel mesh. The oil-


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water separation efficiency of the steel mesh was calculated according to the following equation (1):

Separation efficiency =

m1 × 100% m0

(1)

where m0 and m1 represented the initial oil mass and the collected oil mass in beaker, respectively. Characterizations. The surface morphology of the superhydrophobic surfaces on steel plate or mesh was observed with a EV018 scanning electron microscope (SEM, Carl Zeiss Jena, Germany) at an acceleration voltage of 10.0 kV under high vacuum condition, and the samples were coated with thin gold layer before SEM observation. The surface microstructure and roughness were analyzed by atomic force microscopy (AFM, Bruker Multimode 8, USA) in tapping mode with a scanning rate of 0.977 Hz in 5 µm × 5 µm scale. Chemical analysis of the surfaces was performed on X-ray photoelectron spectroscopy (XPS, Kratos Axis Ulra DLD, UK) with Mg Kα monochromatic X-ray source (1200 eV) and three electron take-off angles (30o, 60o and 90o). Fourier transform infrared spectroscopy (FT-IR) was recorded on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) from 4000 to 400 cm-1 with a resolution of 4 cm-1. X-ray diffraction (XRD) spectra were tested on a D8 Advance diffractometer (Bruker, Germany) using Cu Kα radiation source (λ1 = 1.54060 Å, λ2 = 1.54439 Å) and a LynxEye_XE detector. Static water contact angle (WCA) and dynamic water sliding angle (WSA) were measured with a contact angle meter (DSA100, Germany) equipped with a video capture using 5 µL of water as probe liquid at room temperature. The WCAs and WSAs of all samples were obtained from at least three different locations to calculate the mean value. To test chemical stability, the PDMS-


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based superhydrophobic surfaces on steel plates were immersed in aqueous solutions at different pH values (1, 3, 5, 7, 9, 11 and 13) for 12 h, and then washed and dried for WCA measurement. RESULTS AND DISCUSSION Fabrication of PDMS-Based Superhydrophobic Surfaces on Steel Plates. Roughness and low surface-energy material are two critical factors for the formation of the superhydrophobic surfaces. To construct the roughness on steel plates, the electroless replacement deposition was carried out to generate plentiful copper particles on the steel plates in CuSO4 solution containing NaCl for providing a corrosive environment. The surface chemical composition after deposition of Cu particles is analyzed by XRD, as shown in Figure S1. It is notable that three diffraction peaks appear in the region of 43o, 50o and 74o, indexed as (111), (200) and (220) planes of facecentered cubic crystal structure of copper particles. The morphology of Cu particles deposited on the steel surface with different deposition time is exhibited in Figure S2. It can be observed that Cu particles with irregular shape are uniformly dispersed on the steel substrate to construct a rough surface. However, limited by the polarity of metal materials, the obtained rough steel plates were difficult to reach superhydrophobic state even hydrophobic state. Therefore, hydrophobic V-PDMS was selected to dress a crosslinking “coat” on the rough copper-deposited steel plates under UV light with TMPTA and Darocur 1173 as crosslinking agent and photoinitiator, respectively. Figure 2 presents the SEM images of PDMS-based surfaces on steel plates with different deposition time in 0.05 M CuSO4 solution. It is clear that the surface of the steel plate without deposition appeared some micro-grade grooves formed by polishing, and the WCA was 93.5o (Figure 2a), indicating that the superhydrophobic surfaces was unable to achieve without enough roughness. Obviously, when the steel plate was deposited in CuSO4 solution for 10 s, copper


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particles were irregularly generated on steel plate and the surface roughness increased, the WCA reached 126.5o (Figure 2b). With the deposition time prolonging to 20 s and 40 s, the PDMSbased surfaces became rougher, and the WCAs further increased to 154o and 153.5o, respectively (Figure 2c and Figure 2d). Importantly, in Figure 2d, the micro-nano architecture can be observed clearly, which is beneficial for the construction of superhydrophobic surfaces. Figure 2e. shows the effect of deposition time on the WCAs of PDMS-based steel plates in different molar concentrations of CuSO4 solutions. It is notable that the WCAs of the steel plates are highly dependent on the deposition time and molar concentration of CuSO4 solution. With high molar concentration of CuSO4 solution at 0.08 M, the superhydrophobic state was quickly obtained in 10 s of deposition time and the surface reached 157.5o, nearly to 160o, at 20 s of deposition time. Contrarily, with the low molar concentration at 0.03 M, at least 30 s was needed for the surface to achieve superhydrophobic state. However, with the deposition time further increasing, the WCAs of the steel plates changed little, indicating that prolonging deposition time merely increased the accumulation degree of copper particles on steel surface. To further find out the effect of the roughness on the surface wettability, AFM is used to probe the steel plates with different deposition time in 5 µm data scale, the three-dimensional images and two-dimensional images are shown in Figure 3 and Figure S3. It is calculated the surface area are about 26.2 µm2, 28.0 µm2, 38.3 µm2 and 53.7 µm2, corresponding to the surfaces with deposition times of 0 s, 10 s, 20 s and 40 s, respectively. Accordingly, the roughness factor (r), defined as the ratio of actual surface area to projected surface area,43 are 1.05, 1.12, 1.53 and 2.15, respectively. Wenzel model43 can be expressed as the following equation (2),

cos(θ ) = r cos(θ0 )

(2)


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where θ and θ0 are the contact angle of rough and smooth surfaces, respectively. It is well known that the advancing CA increases with r increasing if θ0 > 90o. However, as r increases to a critical level, the liquid will be unable to penetrate into the surface cavities, resulting in the formation of air pockets between liquid and solid surface. Thus, the solid-liquid-air interface dramatically decreases the CA hysteresis and leads to a small rolling angle.44,45 Based on this, Cassie-Baxter model 46 is proposed as follows,

cos(θ ) = rf cos(θ0 ) + f −1

(3)

where f is the fraction of solid surface area when contacting with liquid. In our study, r gradually increases with the deposition time of steel plates increasing in CuSO4 solution. When r reaches 1.53 at 20 s, the surface possesses WCA of 154o and WSA above 30o, belonging to Wenzel state. However, when r increases to 2.15 at 40 s, plenty of air is trapped between water and Cu particles coated with PDMS, attributing to a high WCA of 153.5o and a small WSA lower than 10o in Cassie- Baxter state. Meanwhile, according to Cassie- Baxter equation, f can be calculated to be 0.11 (θ0 = 92o as tested on smooth surface), indicating that only 11% of steel plates is in contact with water and 89% with air. Consequently, the micro-nano architecture is extremely important to amplify roughness factor to obtain a superhydrophobic surface with high WCA and low WSA. Additionally, the root mean square roughness (RMS), another vital parameter obtained from AFM analysis, increases from 82.3 nm to 535 nm, resulting to the increase of the WCAs from 93.5o to 153.5o (see Figure S3e). Except for deposition time and molar concentration of CuSO4 solution, the mass ratio of VPDMS solution also largely influences the surface morphology of steel plates, and the SEM images of the steel plates after being immersed in different mass ratios of V-PDMS solutions are shown in Figure 4. It is interesting to note that when the mass ratio is 0.01, the surface possesses


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obvious roughness and appears clear flower-shaped patterns, and the WCA reaches 155o. However, the PDMS-based crosslinking film is too thin and the surface of the steel plates has the possibility to be not completely covered. With the mass ratio of V-PDMS solution increasing to 0.02, a large number of irregular bugles are observed on the surface with a lower WCA of 145o, as shown in Figure 4b. With the mass ratio of V-PDMS solution further increasing to 0.03 and 0.05, due to being covered by more amount of PDMS, the surfaces gradually flatten and the flower-shaped patterns disappear, thus the contact angles accordingly decrease to 119.5o and 108o, respectively (Figure 4c and 4d). Figure 4e shows the effect of mass ratio of V-PDMS solutions on the WCAs of PDMS-based steel plates at different deposition time in 0.05 M CuSO4 solution. With deposition time increasing, the WCAs of steel plates immersed in V-PDMS solution with mass ratio at 0.01 quickly increases to above 150o at 10 s, and the steel plates immersed in V-PDMS solution with mass ratio at 0.02 and 0.03 attain superhydrophobic state at 20 s and 40 s, respectively. However, when the mass ratio further increases to 0.05, the WCA of steel plates is less than 130o and the superhydrophobic state is unable to obtain even prolonging the deposition time, indicating that the increasing deposition time has no obvious improvement for the roughness of steel plates, and the higher mass ratio of V-PDMS solution diminishes the constructed roughness by electroless replacement deposition. The steel plates deposited for 40 s in 0.05 M CuSO4 solution and immersed in V-PDMS solution with mass ratio at 0.02 exhibit micro-nano architecture and superhydrophobicity, and are chosen for the following test. Figure 5 shows the optical images for a water droplet rolling off the PDMS-based superhydrophobic surface on steel plate. In Figure 5a, 5b, 5c, and 5d, the water droplet rapidly slides across and off from the steel surface within 0.38 s, which is due to


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the highly rough structure with micro-nano hierarchical topographies of Cassie’ state. Under this circumstance, the water droplet on the PDMS-based steel plates will trap many air pockets underneath rough surfaces and form a non-wet-contact mode, then easily slide with a minimal angle, which can be applied in self-cleaning field.47,48 The resistance of superhydrophobic surfaces to acid/base media is very important in practical applications especially being used at outdoors and heavily contaminated environment. To evaluate the effect of acid/base media on the superhydrophobicity of the PDMS-based steel plates, the samples were separately immersed in aqueous solutions with different pH values from 1 to 13 for 12 h, the measured WCAs are shown in Figure 5e. The WCAs are almost unchanged at different pH values, and the steel surface keeps superhydrophobicity with WCA above 150o even in the strong acid solution with pH value at 1 and base solution with pH value at 13 for 12 h, indicating that the superhydrophobic surface has excellent chemical stability. Obviously, the chemical inertness of Si-O-Si bonds is one of the important factors to endow the property for the superhydrophobic surfaces. Additionally, the cross-linking network structure formed with VPDMS and TMPTA under UV curing is another factor, which has been confirmed by Feng 49 and Gao.50 UV-Stimulated Extreme Wettability Transformation and Recovery Behavior of PDMSBased Superhydrophobic Surfaces on Steel Plates. To investigate in detail the wettability transformation of the PDMS-based surface with micro-nano architecture, the steel plates were irradiated under UV light from 0 min to 180 min, and the WCAs at different irradiation time are measured and shown in Figure 6a. In the first 60 min, the WCAs of the superhydrophobic surface are almost unchanged. However, with irradiation time further increasing, it can be amazedly observed that the WCAs gradually decrease to 0o at 180 min and the surface achieves


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superhydrophilic state. The superhydrophobic steel plate without UV irradiation and that after UV irradiation for 180 min, respectively, were put into water, and the photos are presented in Figure 6b. The former floats all the time and the latter sinks immediately. Moreover, the superhydrophobic surfaces on steel plates were shaded selectively and irradiated under UV light for 180 min. When water is continuously dripped onto the surfaces, the water droplets spontaneously condense on the superhydrophilic regions to form a smooth water layer, but roll down from the superhydrophobic regions, as shown in Figure 6c. It also demonstrates the extreme wettability transformation from superhydrophobicity to superhydrophilicity under UV irradiation. Interestingly, the obtained superhydrophilic surfaces with UV irradiation for 180 min is found to recover highly hydrophobicity with WCA of 140o after being stored in dark environment for 7 days (see Figure S4), and there is almost no obvious change with storage time further increasing. The morphologies of the surfaces before UV irradiation, after UV irradiation and being stored in dark environment for 7 days are illustrated in Figure 7. As shown in Figure 7, the surface morphologies are similar and still appear micro-nano architecture, indicating the physical morphology is not damaged by UV irradiation and the extreme wettability transformation has no relationship with the microstructure of the surfaces. To study the underlying mechanism of the UV-stimulated wettability transformation and the recovery behavior, FT-IR was used to compare the characteristic absorption peaks of the surfaces (see Figure S5). There are three main bands of groups changing a lot: the first absorption bands at 2985 and 1259 cm-1 are associated to stretching vibration peak and bending deformation peak of methyl groups, the second absorption band at 3480 cm-1 is attributed to the existence of hydroxyl groups, the third bands at 3181 and 1624 cm-1 belong to carboxyl hydroxyl groups and carboxyl carbonyl groups, respectively.51


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Compared with the spectrum of the surface before UV irradiation, the peak intensity of methyl groups after UV irradiation for 180 min is weakened obviously, but the new bands of hydroxyl and carboxyl groups are observed, demonstrating that the CH3 groups of the surfaces are gradually decomposed and change into OH and COOH groups under UV irradiation. After being stored in dark environment for 7 days, the intensity of methyl and carboxyl peaks is basically unchanged whereas the peak at 3480 cm-1 slightly shifts to the high wavenumber region. XPS were carried out to further investigate the underlying transformation mechanism, and the C 1s peaks of the PDMS-based superhydrophobic surface on steel plate are presented in Figure 8. The XPS C 1s peak can be divided into three peaks at 284.5, 286.1 and 288.7 eV, corresponding to C-H, C-O and O-C=O bonds, respectively. After UV irradiation for 180 min, the peak at 284.5 eV is weakened strongly and the peak at 288.6 eV is strengthened slightly. Furthermore, the XPS Si 2p peak (see Figure S6) shifts from 102.0 eV to 102.8 eV related to SiOx (1≤x≤2). It indicates that CH3 groups on the superhydrophobic PDMS-based surface gradually decompose and photochemically change into OH and COOH groups which partially form the silica-like structure in the surface region.52 When the surface after UV irradiation is stored in dark environment for 7 days, the C 1s peak is substantially same with that of the surface after UV irradiation for 180 min. It demonstrates that there is no change for the chemical structure during dark storage, which is in accord with the FT-IR results. Hillborg et al.53,54 found that the WCA of PDMS could decrease from 103±2o to 21±4o under UV exposure for 60 min and then recover the WCA to about 60o after 50 days, and investigated the hydrophobic recovery of UV/ozone treated crosslinked PDMS. They firstly illustrated the formation of a homogeneous oxidized surface at a sub-50-nm level on the crosslinked PDMS sample after UV exposure, then proposed that a liquidlike layer consisting of


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free siloxanes formed by chain scission reactions during short-time UV treatment gradually constituted on top of the oxidized surface after storage. Furthermore, the silica-like hydrophilic layer formed during long-time UV/ozone treatment collapsed to form plenty of tiny domains less than 100 nm size, and then PDMS dispersed around these regimes to recover hydrophobicity. On the basis of the above analyzed results and the reported work about hydrophobic recovery of PDMS after UV/ozone treatment, the mechanism of the extreme wettability transformation and recovery behavior is proposed as follows: with UV exposure time gradually increasing to 180 min, some organ-silica molecules with low molecular weight are firstly generated due to chain scission reactions, and then the silica-like layer with plenty of hydroxyl groups is further formed on the PDMS-based surface and exhibited the characteristics of superhydrophilicity with the help of the micro-nano architecture by electroless replacement deposition. When being stored in dark environment, the organ-silica molecules gradually migrate across the incomplete and loose silica-like layer to the surface due to the higher molecular activity and lower surface energy. Additionally, strain is created between more and less transformed regions when the surface undergoes the transition to a new inorganic silica-like layer, and the strain release leads to the collapse of silica-like layer and pushes some organ-silica chains to the surface. Therefore, with the increase of the hydrophobic organ-silica molecules or chains, the wettability of the PDMSbased surface gradually recovered to highly hydrophobic state with WCA of 140o. Certainly, the micro-nano architecture is also the crucial factor to promote the extreme wettability recovery of the surface. Unfortunately, the full hydrophobic recovery is not achieved to above 150o of WCA, which is possible that some tiny regions are not covered by the organ-silica molecules or chains. The schematic diagram for the mechanism of the wettability transformation of the PDMS-based surface is shown in Figure 9.


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PDMS-Based Surface on Steel Mesh and Its Oil-Water Separation Behavior. Oil-water separation of the superhydrophobic surfaces is one of the most important practical applications with the deteriorating marine environment. Similar to the fabrication process of the PDMS-based superhydrophobic surfaces on steel plates, the PDMS-based surfaces on steel meshes are fabricated, and the oil-water separation behavior is investigated. Figure 10a shows the photographs of the water droplets on neat steel mesh and PDMS-based steel mesh, it is clear that the former is hydrophilic and the latter exhibits superhydrophobicity. From the microstructure of the steel meshes shown in Figure 10c and 10d, the surface of the neat steel mesh is smooth, and that of PDMS-based steel mesh appears hierarchical rough morphology. Additionally, it can be observed that the intersection of PDMS-based steel mesh appears thicker and rough structure. It might be caused by the accumulation of V-PDMS solution with low mass ratio when the mesh was being cured under UV light. The abrasion resistance of the mesh is also studied according to the schematic diagram in Figure 10b, and the morphology is shown in Figure 10e. Herein, we used sandpaper (2000 meshes) as abrasion material to testify the mechanical durability of steel mesh, and the mesh was pulled at a speed of 2 cm/s under a 200 g stainless steel weight through a distance of 15 cm for 50 cycles. With abrasion cycles increasing, the WCAs appear a tiny decreasing trend, and the PDMS-based steel mesh still keeps highly hydrophobic state after 50 cycles (see Figure S7). From the SEM images of the most serious abrasion part of the PDMSbased mesh after 50 cycles as shown in Figure 10e and Figure S8, it is notable that the external microstructure of the frictional area is only slightly destroyed and the considerable amount of Cu particles coated with PDMS are observed. The ability against abrasion is mainly contributed to the strong adhesive force to steel substrates and mechanical stability of the crosslinked PDMS


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layer as well as the three-dimensional braided structure protecting most regions of the steel mesh.55 Hexane and chloroform are selected as light oil and heavy oil, respectively, for the oil-water separation of the PDMS-based surface on steel mesh. As shown in Figure 11a and 11b, the oil and water can be easily separated just by the gravity-driven function. The process of oil-water separation with the PDMS-based surface on steel mesh is also exhibited in Video S1 and S2. The separation efficiency of the mesh reaches 96.8% for hexane and 95.8% for chloroform. Moreover, after 20 cycles of oil-water separation, the hierarchical morphology of PDMS-based steel mesh in SEM images is only weakened slightly (see Figure S9), and the steel mesh still possesses high separation efficiency at 97.0% and 92.2% (see Figure S10), respectively, showing excellent recyclability and reusability. CONCLUSIONS To summarize, we demonstrated a facile approach to fabricate the superhydrophobic surfaces on steel substrate by electroless replacement deposition of CuSO4 and UV curing of V-PDMS. The WCAs of the PDMS-based surfaces on steel plates were found to be largely influenced by the molar concentration of CuSO4 solution, deposition time and mass ratio of V-PDMS solution. The superhydrophobic steel plates appeared clear micro-nano architecture deposited for 40 s in 0.05 M CuSO4 solution and immersed in V-PDMS solution with mass ratio at 0.02. The PDMS-based superhydrophobic surfaces on steel plates possessed outstanding chemical stability due to chemical inertness of Si-O-Si bonds and cross-linking network structure. Importantly, with the UV irradiation time increasing to 180 min, the PDMS-based superhydrophobic surfaces could extremely transform to superhydrophilic state, and then recover to highly hydrophobicity with the WCA at 140o. The special patterned surfaces with switchable wettability have the potential


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applications in various fields such as microfluidic devices and biomedicine. Additionally, the PDMS-based surface on steel mesh exhibited high oil-water separation efficiency and excellent reusability. The fabrication procedure is simple, fast, low-cost, available to produce in large scale, and can be further applied to superhydrophilic surface and other functional surfaces on various metal substrates.

FIGURES

Figure 1. Schematic illustration for fabricating PDMS-based superhydrophobic surfaces on steel plates.


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Figure 2. SEM images (inset on the top right was the higher magnification image and the bottom right was the WCA optical image) of PDMS-based steel plates with different deposition time: (a) 0 s, (b) 10 s, (c) 20 s and (d) 40 s in 0.05 M CuSO4 solution. (e) Effect of deposition time on the WCAs of PDMS-based steel plates in different molar concentrations of CuSO4 solutions.


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Figure 3. AFM three dimensional (3D) images of PDMS-based steel plates with different deposition time: (a) 0 s, (b) 10 s, (c) 20 s and (d) 40 s in 0.05 M CuSO4 solution.


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Figure 4. SEM images (inset on the bottom right was the WCA optical image) of PDMS-based steel plates immersed in V-PDMS solutions with different mass ratios at (a) 0.01, (b) 0.02, (c) 0.03 and (d) 0.05, respectively. The steel plates were firstly deposited in 0.05 M CuSO4 solution for 15 s. (e) Effect of mass ratio of V-PDMS solutions on the WCAs of PDMS-based steel plates at different deposition time in 0.05 M CuSO4 solution.


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Figure 5. Optical images for a water droplet rolling off the PDMS-based superhydrophobic surface on steel plate. (e) WCAs of PDMS-based superhydrophobic surfaces on steel plates after being immersed in aqueous solutions with different pH values for 12 h.


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Figure 6. (a) WCAs of PDMS-based superhydrophobic surfaces on steel plates with different irradiation time from 0 min to 180 min. (b) Photographs of the floating superhydrophobic surface on steel plate without UV irradiation in water and the sank surface on steel plate after UV irradiation for 180 min. (c) Images of the patterned superhydrophobic and superhydrophilic surfaces by shading selective regions when being irradiated under UV light.


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Figure 7. SEM images (inset on the bottom right was the WCA optical image) of the PDMSbased surfaces on steel plates (a) before UV irradiation, (b) after UV irradiation for 180 min and (c) after storage in dark for 7 days, respectively. (a1), (b1) and (c1) were the corresponding higher magnification images.


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Figure 8. XPS C 1s spectra of PDMS-based superhydrophobic surfaces (a) before UV irradiation, after (b) UV irradiation and (c) being stored in dark environment for 7 days, respectively.


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Figure 9. Schematic illustration for the mechanism of the wettability transformation.


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Figure 10. (a) Photographs of water on neat steel mesh (left) and PDMS-based surface on steel mesh (right). (b) Schematic diagram of sandpaper abrasion test for PDMS-based surface on steel mesh. SEM images (inset on the top right was the higher magnification image) of (c) neat mesh, (d) PDMS-based surface on steel mesh, and (e) PDMS-based surface on steel mesh after 50 cycles of sandpaper abrasion.

Figure 11. Oil-water separation process of PDMS-based surface on steel mesh for (a) hexanewater and (b) chloroform-water.


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TABLES Table 1. Formulations for PDMS-Based Superhydrophobic Surfaces on Steel Plates Mass(V-PDMS) V-PDMS (g) Hexane (g) TMPTA (g) Darocur 1173 (g) /Mass(Hexane) 0.01

0.125

12.5

0.0125

0.01

0.02

0.25

12.5

0.0250

0.01

0.03

0.375

12.5

0.0375

0.01

0.05

0.625

12.5

0.0625

0.01

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Additional XRD spectrum (Figure S1) and SEM images (Figure S2) of steel plate after deposition showing the type and micro-morphology of the obtained Cu particles, AFM two dimensional images of PDMS-based steel plates (Figure S3), recovery process of PDMS-based surface after UV irradiation from superhydrophilicity to high hydrophocity (Figure S4), FT-IR spectra (Figure S5) and XPS spectra (Figure S6) illustrating reversibly extreme wettability, WCAs (Figure S7) and SEM images (Figure S8) of PDMS-based steel mesh after abrasion, SEM images (Figure S9) of PDMS-based steel mesh for oil-water separation, separation efficiency after different cycles (Figure S10), and separation process for hexane-water (Video S1) and chloroform-water (Video S2).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

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2016, 466, 36-43.


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