Rough Structure of Electrodeposition as a Template for an Ultra

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Rough Structure of Electrodeposition as a Template for an Ultra-Robust Self-cleaning Surface Yongquan Qing, Chuanbo Hu, Chuanning Yang, Kai An, Fawei Tang, Junyan Tan, and Changsheng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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ACS Applied Materials & Interfaces

Rough Structure of Electrodeposition as a Template for an UltraRobust Self-cleaning Surface Yongquan Qing a, Chuanbo Hu b, Chuanning Yang a, Kai An a, Fawei Tang c, Junyan Tan a and Changsheng Liu a,* a

Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China b School of Metallurgy, Northeastern University, Shenyang 110819, China c College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Beijing university of Technology, Beijing 100124, China

Abstract: Superhydrophobic surfaces with self-cleaning properties have developed, based on roughness on the micro/nanometer scales and low-energy surfaces. However, such surfaces are fragile and stop functioning when exposed to oil. Addressing these challenges, here we show an ultra-robust self-cleaning surface fabricated by a process of metal electrodeposition of a rough structure that is subsequently coated with a fluorinated metal-oxide nanoparticles. Scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction were employed to characterize the surfaces. The micro and nano scale roughness jointly with the low-surface-energy imparted by the fluorinated nanoparticles yielded surfaces with water contact angle of 164.1° and a sliding angle of 3.2°. Most interestingly, the surface exhibits fascinating mechanical stability after finger-wipe, knife-scratch, sand abrasion, and sandpaper abrasion tests. It is found that the surface with superamphiphobic properties has excellent repellency toward common corrosive liquids and low-surface-energy substances. Amazingly, the surface exhibited excellent self-cleaning ability and remained intact even after its top layer was exposed to 50 abrasion cycles with sandpaper and oil-contamination. It is believed that this simple, unique, and practical method can provide new idea to effectively solve the stability issue of superhydrophobic surfaces, and could extend to a variety of metallic materials. Keywords: Self-cleaning surface, superamphiphobic, electrodeposition, micro/nano structures, ultra-robust surface

1. Introduction Bioinspired superhydrophobic surfaces with self-cleaning function, characterized by water contact angles (CA) greater than 150° and sliding angles (SA) below 10°, have attracted enormous attention in both industrial and academic fields.1−4 Self-cleaning surfaces rely upon water droplets to form near spherical shapes, and due to repulsion these droplets easily roll on the surface, collecting dirt as they do so.5-7 The combination of roughness on the micro- and nano-scales with low-energy surfaces led to the development of artificial self-cleaning surfaces through various methods, including top-down (e.g., etching,8 lithography9) and bottom-up types (e.g., sol−gel,10 layer-by-layer deposition11). The main disadvantages of these artificial surfaces include the poor mechanical stability, easy abrasion, and contamination by oil. Till date, several investigations have attempted to solve these problems, by establishment of covalent bonding or crosslinking a coating layer between the substrate and the coating.12–15 With these attempts, the stability of surface was improved to a certain extent; however, achieving excellent surface superhydrophobicity while retaining robust stability is still difficult, thus this problem has not been fundamentally solved.

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To solve these challenges, some mechanically stable superhydrophobic surfaces have been reported by relying on an intrinsic stable media (i.e., metal meshes, polymer, or fabric).16,17 Deng et al.18 reported candle soot as a template for a robust superamphiphobic coating. A breakthrough was achieved by Lu and co-workers, who fabricated a robust self-cleaning surface from both soft and hard materials by combining the paint and adhesives.6 Zhou et al.1 developed a crosslinked elastomeric thin coating with a nanocomposite rough structure and low free-energy surface, which endowed fabrics with highly durable superhydrophobicity. Wong et al.19 prepared a large-scale ultradurable and transparent superhydrophobic surface by rapid template-free nano/micro texturing of tough polymers. Wang et al.20 reported a robust superoleophilic and superhydrophobic carbon nanotube/poly(dimethylsiloxane)-coated polyurethane sponge for use in oil-spill cleanups. These studies make use of an intermediate template to improve the stability of the superhydrophobic surfaces; however, these surfaces are easily damaged under the action of large external forces, this is mainly due to the lack of hardness in their structures. Copper is important engineering materials, owing to its good ductility, excellent thermal conductivity and relatively noble properties. It is widely used in many modern industrial areas especially decorative devices, conductors, lead frame and aerospace. Thus, it is very desirable to fabricate a superhydrophobic copper surface with superior water-repellence ability. A variety of methods have been developed to prepare superhydrophobic surfaces based on copper substrates,21– 24 but reports about mechanically stable self-cleaning copper surfaces are still scarce.25 Titanium dioxide (TiO2) nanostructures have become the focus of enormous interest in biomaterials due to their antibacterial properties, efficiency as photocatalyst, biocompatibility, and good chemical stability.26 Electrodeposition, as a classic technique for fabrication of metal matrix surface which exhibits extraordinary durablility, has the advantages of easy control, low cost, highly effective. Currently, several studies investigating the preparation of superhydrophobic surfaces via electrodeposition have been reported.25,27–29 In all these studies, a rough structure of transitionmetal layer is first constructed, which is followed by modification with the low-surface-energy material (such as stearic acid, myristic acid, fluoroalkylsilane). Su et al.25 created a superhydrophobic surface on copper substrate through electrodeposition in traditional Watts bath and the heat-treatment in the presence of (heptadecafluoro-1,1,2,2-tetradecyl) triethoxysilane. Tesler et al.30 fabricated a superhydrophobic nanoporous tungsten oxide coating on various grade steel via electrodeposition. The mechanical durability of the coating was significantly improved due to chemical bonding with the substrates. Although the as-prepared electrodeposited surface stability was very good, it was easy to lose superhydrophobicity if some external force was applied (e.g. a knife-scratch or abrasion damages). Highly inspired by this, we hypothesized that if the modified-nanoparticles could be transferred to the rough structure based on electrodeposition, a robust superhydrophobic surface could be fabricated for practical applications. To this end, herein we report a facile method for fabricating a robust superamphiphobic and self-cleaning surface by a process that involves electrodeposition technology as a tool to produce a stable rough surface, ﬂuoro-silanization of TiO2 nanoparticle and coating of these onto the rough electroplated surface creating re-entrant structures. By combining the rough structure and modified-nanoparticles, we fabricated a robust surface durable against applied external forces (i.e. finger-wipe, knife-scratch, sand abrasion and sandpaper abrasion damages), whilst retaining its self-cleaning capability. Moreover, compared to other techniques for fabricating self-cleaning surfaces, this technique has the advantages of being simple, cost-effective and unique, and could be applied to any metal surface.


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2. Experimental Section Materials: Copper plates with dimensions 100 mm × 60 mm × 1 mm were selected as the substrate. Zinc sulfate hetahydrate (ZnSO4·7H2O) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Dongguan City JiShun Import and Export Co., Ltd. supplied nano-TiO2 (rutile) with an average size of 25 nm. 1,1,2,2-Tetrahydroperfluorodecyltrimethoxysilane (FAS) was provided by Shanghai Aladdin-Reagent Co., Ltd. Pretreatment of Copper Substrate: The copper substrate was mechanically polished down to 1200–1500 grit size using abrasive silicon carbide (SiC) paper, followed by ultrasonic cleaning in acetone for 10 min and rinsing with deionized water. The substrate was processed almost immediately by electrolytic degreasing and electropolished at 1.5 A·dm−2 in a solution containing sodium carbonate (Na2CO3, 0.57 M), potassium hydroxide (KOH, 0.018 M), and sodium dodecyl sulfate (0.028 M) for 2 min. Further, it was dipped in an activation solution containing hydrochloric acid (HCl, 10 wt.%) for 30 s at room temperature to remove the oxide film. It was then quickly placed in the electrolytic bath after rinsing with deionized water. Electrodeposition of Zinc: Zinc was electrodeposited based on the method described in our previous study.31 Briefly, the Ti-IrO2 and Cu substrate were used as anode and cathode in an electrolyte solution, respectively. A distance of 2 cm separated the anode and cathode plates. The optimized bath contained ZnSO4·7H2O (0.77 M) and the pH was adjusted to 2.0 using sulfuric acid solution (H2SO4, 50 g·L−1). The electrodeposition process was performed by a direct current (DC) power with current densities of 11–23 A·dm−2 at 40 °C. After being subjected to electrolysis for 25 min, the cathode was rinsed several times with distilled water and dried, forming a rough deposited-Zn surface. Preparation of Self-cleaning FAS-TiO2/Zn Surface: We modified the surface of TiO2 particles by a hydrothermal reaction technique.32 In detail, FAS (1 g) was placed into absolute ethanol (15 mL) which was then well mixed. Then TiO2 particles (3 g) were added to this mixed solution. The mixture was magnetically stirred at 40 °C for 50 min to form a paint-like coating solution. Subsequently, the deposited-Zn sample was immersed in the coating solution at 80 °C for 20 min, and then the sample was left in air for 10 min, and placed in an oven at 120 °C for 30 min. Finally, excess residue on the sample surface was removed by ultrasonic oscillation for 10 min, leaving behind the self-cleaning FAS-TiO2/Zn surface. Characterization: The surface morphology of the as-prepared samples was examined by scanning electron microscopy (SEM, FEI Nova NanoSEM 430, 10kV), and energy dispersive Xray spectroscopy (EDS) data were acquired using the same instrument. Fourier-transform infrared spectrophotometer (ATR-FTIR, PerkinElmer, USA) and X-ray photoelectron spectroscopy (XPS, K-Alpha, USA) were used to determine the chemical composition of the surfaces. The CA and SA were measured with ~3 µL water droplets at room temperature using a JCY-2 instrument (Fangrui, China). The surface roughness (Ra) of the samples, with length of 1280 µm, was measured using an OLS3100 laser confocal scanning microscope (LSCM, Japan). X-ray diffraction (XRD, Bruker D8, Germany) was used to elucidate the crystal structure of the samples.

3. Results and discussion 3.1 Surface morphology and chemical composition It is well known that a superhydrophobic surface can be fabricated based on the combined effects of both low surface energy materials and micro/nanocomposite surface structure.33,34



Figure 1 presents the formation process of a superhydrophobic FAS-TiO2/Zn surface. TiO2 nanoparticles are hydrophilic because of the large number of hydroxyl groups existing on its surface, whose free energy can be effectively reduced by self-assembling FAS, due to its high content of –CF2– and –CF3 groups with a low surface energy of 18 and 6.7 m·Jm−2,35 respectively. First, the silicon ethoxide groups (Si–OCH2CH3) in FAS are converted to silanol groups (Si–OH) after hydrolysis, which act as reactive groups at the end of the molecule, and then react with the – OH groups of the TiO2 surface to form a self-assembled coating solution. This paint-like coating solution was directly applied onto the deposited-Zn surface using dip-coating. Then the sample was left in air to remove the solvent, followed by curing at 120 °C for 30 min. Thus, an ultrarobust self-cleaning surface was successfully fabricated. Moreover, a water drop placed on top of the FAS-TiO2/Zn surface formed a static CA of 164.1° and an SA of 3.2°, owing to the significantly reduced adhesion of water to surface. The difficulty for water droplets to deposit on the surface allows them to immediately roll off (Movie S1), indicating that the surface is expected to exhibit a water-repellent effect.

Figure 1. A schematic illustration of the superhydrophobic FAS-TiO2/Zn surface formation process via a two-step method.

Based on the model reasoned above, Figure 2 (a-d) show the tilt-view SEM images with different magnifications of deposited-Zn surfaces and FAS-TiO2/Zn surfaces. Figures 2a and b exhibit that the morphology of the deposited-Zn surface is rather rough, and composed of a large number of protrusive microstructures, with sizes in the range of 10 to 25 µm, forming microscale rough structures. There are numerous gaps, with an average size of 5–10 µm, between hill-like protrusive structures. The deposited-Zn surface (CA = ~10.2°) could not support a water droplet in contact with it in the Wenzel’s state,36 indicating that no air pockets were formed so that the surface was completely wetted. Figures 2d and e clearly show that numerous irregular nano particles (with size ranging from dozens to hundreds of nanometers) are attached to the hill-like rough structure surface, forming sub-microscale rough structures. Furthermore, nano FAS-TiO2 particles were deposited on micro-scale rough structure surface, and the covalent bonding occurred between FAS-TiO2 phase and the Zn plated layer (physical bonding or chemical


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changes), indicating the occurrence of better adhesion between layers of particles and Zn (Figure 3). According to the Cassie–Baxter model,37 the phenomena of superhydrophobicity is explained in terms of trapped air films in the holes and grooves present on sub-microscale rough surface structures. These air films can effectively prevent the surface from being wetted by water droplets, leading to larger CA and smaller SA. Further evidence of the Ra is given by LSCM images of the deposited-Zn surface and FAS-TiO2/Zn surface (Figures 2c and f). The values of Ra of depositedZn surface and FAS-TiO2/Zn surface are 31.3 and 9.1 µm, respectively. Surface rough structure is indispensable for surface to present superhydrophobicity. The air gets easily trapped in the apertures as FAS-TiO2/Zn surface has a sufficiently rough structure. The LSCM results further confirmed that the sub-microscale rough structures were formed on the composite surface.

Figure 2.Tilt-view SEM images of (a and b) deposited-Zn surface and (d and e) FAS-TiO2/Zn surface at different magnifications (the inset corresponds to the cross-section). LSCM topographical images of (c) deposited-Zn surface and (f) FAS-TiO2/Zn surface.

Figure 3.Tilt-view SEM images of FAS-TiO2/Zn surface: (a) low-magnification image; (b) a cross-sectional view of (a); and (c) magnified image of (b). Corresponding to the EDS chemical composition of line-scan (d–f) and



area-scan (g–i) signal intensity mapping showing the spatial distribution of the elements Zn, Ti, and O, respectively.

In general, the pH and concentration of precursor solution, current density, time, and temperature of electrodepositon influence the microtopography of deposited surface.28,38,39 This study, based on the previous reports,31,40 mainly considered the effect of current density on wettability to evaluate the real contribution of Ra toward superhydrophobicity. Figure S1 demonstrates that the superhydrophobicity of FAS-TiO2/Zn surface is obviously enhanced with the increase in the current density, the CA increases to 164.1° from 154.3°, and the value of Ra increases to 9.1 µm from 5.3 µm. The superhydrophobicity of FAS-TiO2/Zn surface was the best when current density was 17 A·dm−2. However, the superhydrophobicity of FAS-TiO2/Zn surface significantly decreased when the current density exceeded 17 A·dm−2, the CA was decreased to 150.1° from 164.1°, and the Ra was increased to 17.9° from 9.1° with the increase in the current density. This indicates that the surface wettability and Ra can be effectively controlled by regulating the current density. Figure 4 displays the ATR-FTIR and EDS spectra of deposited-Zn surface and FAS-TiO2/Zn surface. Compared to the spectrum of the initial deposited-Zn surface (Figure 4a), many absorption peaks are detected corresponding to the FAS-TiO2/Zn surface (Figure 4b). The peak at 1064 cm−1 is the characteristic absorption peak of Si–O–Si, peaks located at ~1317, 1243, 1207, 1143, and 1115 cm−1 are assigned to the C–F stretching vibration of the –CF3 and –CF2– groups in the fluorinated alkyl chains,41 indicating the successful attachment of the FAS-TiO2 film, thus covering the deposited-Zn surface. Figures 4c and d show the EDS spectra of electrodeposited Zn and FAS-TiO2/Zn surface, which corresponds to the concentrations of elements present on the surfaces. The results indicate that the deposited-Zn surface contains only the element Zn (Figure 4c), while the FAS-TiO2/Zn surface mainly consists of elemental C, O, F, Si, Ti, and Zn (Figure 4d), further confirming the attachment of FAS-TiO2 film, covering the Zn surface.

Figure 4. Chemical composition analysis of the surface: ATR-FTIR spectra of (a) deposited-Zn surface and (b) FAS-TiO2/Zn surface, EDS spectra of (c) deposited-Zn surface and (d) FAS-TiO2/Zn surface.


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XPS was used to further determine the chemical composition of the FAS-TiO2/Zn surface. In a wide survey, strong signals of C 1s, F 1s, Ti 2p, O 1s, and Zn 2p and a weak signal of Si 2p were detected on the surface (Figure S2). High-resolution spectra for C 1s, F 1s, Ti 2p, O 1s, and Zn 2p were deeply analyzed, as shown in Figure 5. The C 1s peak can be deconvoluted into six peaks located at 294.4, 292.0, 289.7, 287.7, 285.5, and 282.6 eV, which are ascribed to the carbon atoms of –CF3, –CF2–, –CF2–CF3, –CH2–CF2–, –CH2–, and –CH2–Si groups, respectively,42 as well as the only peak component appearing at the binding energy of ~689.4 eV in the F 1s core-level signal is associated with the C–F species (Figure 5a).43 This indicates that the FAS monolayer of low-surface-energy was immobilized on the TiO2 surface. The Ti 2p high-resolution spectrum is shown in Figure 5b, and can be deconvoluted into peaks at 465.3 eV (Ti 2p 1/2) and 459.5 eV (Ti 2p 3/2) referring to TiO2. The doublet peak energy separation is 5.8 eV, consistent with that reported in previous study.44 In the O 1s spectra (Figure 5c), new peaks that arise at 533.3, 532.3, and 530.8 eV corresponding to Si–O, ZnO(OH), and Ti–O are detected, and a strong Zn 2p signal can be deconvoluted into peaks at 1046.0 eV (Zn 2p 1/2) and 1022.8 eV (Zn 2p 3/2) indicating the presence of a Zn–O bond.45 Figure 5d displays XRD patterns of rutile TiO2, FAS-TiO2, deposited-Zn, and FAS-TiO2/Zn. The characteristic diffraction peak of TiO2 does not exhibit any change after modification with FAS. The main peaks that appeared at 2θ of 25.3°, 37.7°, 47.9°, 54.7°, and 62.7° show the expected pattern for rutile TiO2 (JCPDS 65-5714),46 and that the FAS molecules are amorphous (Figure 5d(i)). The six characteristic diffraction peaks of deposited Zn are found at 36.5°, 39.2°, 43.7°, 54.8°, 70.6°, and 82.4° corresponding to the (002), (100), (101), (102), (110), and (112) planes (JCPDS 65-5973), indicating a hexagonal crystal structure (Figure 5(iii)).47 The sharp and strong reflection peaks indicate that the FAS-TiO2/Zn composite material are well-crystallized (Figure 5d(iv)).



Figure 5. XPS spectra of the FAS-TiO2/Zn surface: (a) C 1s, (b) Ti 2p, and (c) O 1s high resolution spectra, the insets correspond to the high-resolution F 1s spectra (a) and Zn 2p spectra (c); (d) XRD patterns of (i) rutile TiO2, (ii) FAS-TiO2, (iii) deposited-Zn and (iv) FAS-TiO2/Zn surfaces.

3.2 Mechanical stability Superhydrophobic surfaces usually present a rough structure in the micro- or nanoscale, and have poor mechanical properties, limiting their use in practical applications.25,39,48 We developed a method to fabricate a self-cleaning surface by using nanoparticle-filled stable rough structure of electrodeposition, for overcoming the weak robustness of superhydrophobic surface. Figure S3 exhibits the “substrate + rough structure + modified-nanoparticle” sample preparation methods (Figure S3a). In order to investigate the mechanical stability of our coating, we performed several tests including finger-wipe (Figure S3b), knife-scratch (Figure S3c), sand abrasion (Figure S3d), and sandpaper abrasion (Figure S3e). Figure S4 and Movie 2 show the comparison of the fingerwipe and knife-scratch tests on the FAS-TiO2-treated and “rough structure + FAS-TiO2”-treated Cu substrates, respectively. After the finger-wipe and knife-scratch tests, the FAS-TiO2 directly coated on the Cu substrate was removed and exhibited a loss of superhydrophobicity. On the other hand, the FAS-TiO2/Zn surface still remained on the Cu substrate and thus still maintained excellent superhydrophobic properties. Sand grains (200–400 µm diameter) impinged upon the FAS-TiO2/Zn surface from a height of 70 to 100 cm, corresponding to an impinging energy of 4.6×10−8 to 5.3×10−7 J per grain, showed only slight changes in CA and SA of the surface (Figure S3d). Mechanical abrasion resistance test is an effective method to evaluate the stability of superhydrophobic surfaces.6,25,49 The superhydrophobic FAS-TiO2/Zn surface, weighing 100 g, was placed facedown to 400 grit SiC sandpaper. It was first moved for 10 cm along the ruler at a constant speed (Figure 6a) and then the sample face to the sandpaper was rotated by 90° and moved for 10 cm along the ruler (Figure 6b). This process is defined as one abrasion cycle (Movie S3). Surprisingly, the surface still showed good superhydrophobicity after 50 abrasion cycles, and the CAs were between 164.1° and 156.2° (Figure 6c). Further, the surface morphology after abrasion test was studied by SEM. Figure S5 shows the surface morphology, exhibiting a few of abrasion scratches, and the FAS-TiO2/Zn surface provides the concave and convex groove, which can effectively resist the mechanical abrasion. Figure 6d shows the tiny variation of surface hydrophobicity with the increase in applied pressure, the surface still exhibits superhydrophobicity with a CA of 157.8° and a SA of 8.2° after abrasion for 20 cm at 12 kPa. These results indicate the excellent mechanical abrasion resistance of the superhydrophobic surface.


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Figure 6. (a and b) Schematic illustration of sandpaper abrasion test (one cycle), (c) CA and SA as a function of mechanical abrasion cycle number for the superhydrophobic FAS-TiO2/Zn surface, the inset is water droplet traveling test after 50th abrasion cycle, and (d) CA and SA of the superhydrophobic FAS-TiO2/Zn surface with increasing applied pressure after abrasion for 20 cm.

3.3 Self-cleaning property It is important for superhydrophobic surfaces to have self-cleaning properties in practical applications. The self-cleaning tests are shown in Figures S6 (the experimental schemes), S7, and 7 (the experimental results). Figures S7a and b demonstrates that deposited-Zn and FAS-TiO2/Zn surfaces were placed in red ink dyed water, and subsequently removed. The red liquid wetted the deposited-Zn surface whilst the FAS-TiO2/Zn surface remained in its initial state with no trace of contamination. Figures S7c and d show the process of the self-cleaning effect. Dirt (graphite and dust) was placed on the sample surface and then it was cleaned by fast running water droplets. The deposited-Zn surface was wet and remained contaminated by the dirt; however, the FAS-TiO2/Zn surface very quickly became clean and stayed dry throughout. These tests indicate that the FASTiO2/Zn surface possesses excellent antifouling ability. Till date, much of the investigations in the literature conclude that superhydrophobic surfaces tend to be contaminated easily by oil, and thus lose their self-cleaning properties. This is attributed to the fact that the surface tension of oil is lower than that of water, so the oil penetrates the surface thereby removing the trapped air layer. Although many groups have reported that fabrication of superamphiphobic surfaces (that repel both non-polar liquids and water) can effectively prevent the contamination of oil on the surface,18,50,51 these surfaces lose their selfcleaning properties permanently when an external force is applied (e.g. finger-wipe, sandpaper abrasion etc.). We developed an FAS-TiO2/Zn surface with superamphiphobic properties because its outermost surface contains –CF2– and –CF3 groups. Figure 7 shows the self-cleaning effect of the superamphiphobic surface after the 50th abrasion cycle with sandpaper. Figure 7a reveals the high CA exhibited by the surface for a wide variety of liquid drops, (e.g. HCl, NaOH, and oil), demonstrating excellent repellency toward common corrosive liquids and low-surface-energy substances. Figure 7b illustrates the surface after immersion in oil. Clearly, water droplets sit on the superamphiphobic surface. This is due to the support provided by both the surface structures and oil (lower image Figure 7b); moreover, water droplets spread and wet the hydrophilic deposited-Zn surface (upper image Figure 7b). Figures7(c–f) demonstrate the immersion of superamphiphobic surface in oil for 5 min after being contaminated and subsequently cleaned. Dirt was then put onto the surface and it was removed via by passing water droplets (Movie S4). Therefore, it can be concluded that the superamphiphobic surface can protect the substrate from abrasion and oil-contamination in practical applications.



Thus, herein we described an ingenious strategy overcoming many challenges toward the realization of stable self-cleaning surface on copper substrate, which can be applied under harsh mechanical abrasion and oily environments. A combination of SEM, ATR-FTIR, XPS, and XRD characterization showed that the rough surface with sub-microscale structures was created and the low-surface-energy fluorinated components were successfully prepared on the deposited-Zn surface on copper substrate through this new method. The mechanical stability of superhydrophobic FAS-TiO2/Zn surface was found to be significantly improved due to the structure and bonding strength of the interface between deposited-Zn and the FAS-TiO2 coating. It is believed that the unique idea of “substrate + rough structure + modified-nanoparticle” can be simple, cost-effective, and widely used in variety of metal substrates (such as steel, magnesium alloy, and aluminum alloy).

Figure 7: Self-cleaning effect of the FAS-TiO2/Zn surface (superamphiphobic surface) after 50th abrasion cycle with sandpaper. (a) different types of liquid droplets with spherical shape on superamphiphobic surface: redcolored H2O droplet, black-colored HCl droplet, blue-colored NaOH droplet, and oil droplet; (b) red-colored H2O droplets sitting on the FAS-TiO2/Zn surface under oil (lower image) and on rough electrodeposited Zn surface (upper image); and (c–f) self-cleaning process of the superamphiphobic surface after being contaminated by oil.

4. Conclusion In summary, we presented an ingenious design to fabricate an ultra-robust self-cleaning surface with sub-microscale rough structures on copper substrate using nanoparticle-filled stable rough structure by electrodeposition. The resultant superhydrophobic surface, with high CA (164.1°) and small SA (3.2°), was acquired through the attachment of FAS-TiO2 film, covering the depositedZn surface that created the covalent bonding with better adhesion, and the surface wettability and Ra can be effectively controlled by regulating the current density. Furthermore, the surface maintained its superhydrophobicity after several mechanical tests, such as finger-wipe, knifescratch, sand abrasion and sandpaper abrasion, indicating that the surface possessed good mechanical stability. Also, the surface has superamphiphobic properties because its outermost surface contains –CF2– and –CF3 groups, possessing excellent repellency toward acidic, alkaline and oil. Surprisingly, the surface still exhibited excellent self-cleaning ability even after 50 abrasion cycles with sandpaper and oil-contamination. Such an ultra-robust self-cleaning surface is promising for large-scale applications using copper substrates. This simple, cost-effective, and


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unique technique can provide new ideas for designing robust superhydrophobic materials and can be further extended to a variety of metal substrates.

AUTHOR INFORMATION Corresponding Authors Prof. C. Liu, Dr. Y. Qing E-mail: [email protected], [email protected] Tel: +86 02483687365. Fax: +86 02423906313. Notes

The authors declare no competing financial interest.

Acknowledgments This work was supported by the Frontier and Key Technical Innovation of Guangdong Province (2015B010122001), and the Joint Founds of NSFC-Liaoning (U1508213).

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Graphical Abstract


Rough Structure of Electrodeposition as a Template for an Ultra

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