Nano Hierarchical

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Fabrication of ZIF-8@SiO2 Micro/Nano Hierarchical Superhydrophobic Surface on AZ31 Magnesium Alloy with Impressive Corrosion Resistance and Abrasion Resistance Cuiqing Wu,† Qi Liu,† Rongrong Chen,*,†,‡ Jingyuan Liu,† Hongsen Zhang,† Rumin Li,† Kazunobu Takahashi,‡ Peili Liu,‡ and Jun Wang*,†,‡ Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, and ‡Institute of Advanced Marine Material, Harbin Engineering University, Harbin 150001, People’s Republic of China

ACS Appl. Mater. Interfaces 2017.9:11106-11115. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/21/19. For personal use only.

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ABSTRACT: Superhydrophobic coatings are highly promising for protecting material surfaces and for wide applications. In this study, superhydrophobic composites, comprising a rhombic-dodecahedral zeolitic imidazolate framework (ZIF-8@SiO2), have been manufactured onto AZ31 magnesium alloy via chemical etching and dip-coating methods to enhance stability and corrosion resistance. Herein, we report on a simple strategy to modify hydrophobic hexadecyltrimethoxysilan (HDTMS) on ZIF-8@SiO2 to significantly improve the property of repelling water. We show that various liquids can be stable on its surface and maintain a contact angle higher than 150°. The morphologies and chemical composition were characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FI-IR). In addition, the anticorrosion and antiattrition properties of the film were assessed by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization and HT, respectively. Such a coating shows promising potential as a material for large-scale fabrication. KEYWORDS: AZ31 magnesium alloy, ZIF-8@SiO2, superhydrophobic, antiattrition, corrosion resistance Alexander et al.26 prepared aluminum oxide nanoparticles modified with hydrocarbon chains. The modified nanoparticles can be coated onto various substrates. Yoon et al.27 reported about a transparent coating that was successfully formed onto a substrate via electrospraying. Our group28 have formed transition metal ion functionalized hydrocarbon chains via a hydrothermal method. Unfortunately, few works have studied the abrasive resistance of the superhydrophobic coating. The weak antifriction effect of the superhydrophobic coating is attributed to low adhesion strength between coating and substrate. This problem impeded the practical application of the superhydrophobic surface. The waterproof properties of the superhydrophobic surface, however, do not meet the needs of society. With the development of science and technology, the multifunctionality of the superhydrophobic surface has become an important requirement. Nowadays, a wide variety of

1. INTRODUCTION Liquid repellent surfaces, also known as lotus effect and superhydrophobic surfaces, have been widely reported.1−3 It is generally regarded that a superhydrophobic surface has a water contact angle (CA) higher than 150°and a sliding angle (SA) less than 10°.4,5 At the same time, the superhydrophobic surfaces have attracted a lot of attention because of their practical application in anticorrosion,6−8 anti-icing,9−11 drag reduction,12 self-cleaning,13−16 etc. However, most superhydrophobic surfaces tend to lose their property of nonwettability due to contamination, which seriously affects their applicability. There are two methods for fabrication of superhydrophobic surfaces: one is to modify in situ the surface of various substrates, including glass,17,18 copper,19 aluminum alloy,20 magnesium alloy,21,22 etc. The second is to modify a rough substrate with superhydrophobic powder or particles,23−25 which is easily realized. Fabrication of a superhydrophobic surface depends on the surface chemical compositions and morphology. Recently, a number of studies have reported on superhydrophobic coatings. © 2017 American Chemical Society

Received: December 31, 2016 Accepted: March 6, 2017 Published: March 6, 2017 11106

DOI: 10.1021/acsami.6b16848 ACS Appl. Mater. Interfaces 2017, 9, 11106−11115

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ACS Applied Materials & Interfaces

mL of methanol, and 162.2 mg of 2-methylimidazole was dissolved in 50 mL of methanol under continuous stirring for 10 min, respectively. Then the latter was poured into the former solution and stirred for 5 min. The solution mixture was kept for 24 h without stirring. Subsequently, the turbid liquid was centrifuged and washed with methanol three times. After that, the final product ZIF-8 was dried in an oven at 60 °C overnight. ZIF-8 was coated with SiO2 based on a sol−gel process that included hydrolysis and condensation of tetraethylorthosilicate (TEOS) in ethanol, forming a core−shell structure of ZIF-8@SiO2. 80 mg of ZIF-8 particles and 100 mg CTAB were dispersed in ethanol (80 mL) and ammonium hydroxide (0.2 mL) used to adjust pH 11.The mixture was ultrasonicated for 2 min, then 0.125 mL of TEOS was added to the above solution under vigorous stirring for 18 h. Finally, the product was centrifuged and rinsed with a molar ratio (1:1) of deionized water and ethanol three times, and dried in an oven at 70 °C. The core−shell structure of ZIF-8@SiO2 was formed. 2.3. Modification of ZIF-8@SiO2 Particles. The ZIF-8@SiO2 particles were modified by HDTMS to improve their hydrophobicity. The silane solution included a 5 v% silane coupling agent that dissolved in a mixed solution which contained 75/25 (v/v) ethanol/ water and acetic acid to adjust pH 4.5. The mixed solution was stirred for 24 h and 2 mg of ZIF-8@SiO2 particles were added to the above solution under stirring for 12 h. Finally, the as-prepared particles were centrifuged and washed with deionized water and ethanol, and dried at 70 °C for 12 h. 2.4. Pretreatment of the AZ31Mg Alloy. First, magnesium alloy substrate was abraded with 600#, 1000#, and 2000# silicon carbide paper to remove the oxide/hydroxide layer. Second, The Mg alloy was ultrasonically degreased in absolute ethanol for 10 min to remove impurities. Finally, it was washed with deionized water and dried with a hairdryer. 2.5. Fabrication of Superhydrophobic Surface. A mass of 4 mg of ZIF-8@SiO2 particles coated with HDTMS were dispersed in 10 mL of n-hexane by ultrasonication for 10 min. The treated AZ31 alloy was dipped into the solution at 80 °C for 2 h in an oven. The superhydrophobic coating was successfully formed on AZ31 magnesium alloy. 2.6. Characterization. The phase composition and structures of the samples were investigated by X-ray diffraction analysis (XRD, Rigaku TTR-III, Cu Kα, λ = 0.15406 nm). Images from a scanning electron microscope (SEM) characterized the surface morphologies, carried out using a JEOL JSM-6480A microscope equipped with an energy-dispersive X-ray spectrum (EDS). Transmission electron microscope (TEM) was used to observe the morphology of the specimen. Fourier transform infrared spectroscopy (FTIR) was carried out for ZIF-8, ZIF-8@SiO2 and ZIF-8@SiO2 coated with HTMS. Static contact angles were recorded on a FTA200 drop shape analysis system at room temperature. The property of antiattrition was measured by HT Scratch Testing Machine (HT-1000) and the corrosion resistances of properties of sample and bare magnesium alloy substrate were measured by an electrochemical workstation (Zennium, IM6, Germany) using 3.5 wt % NaCl aqueous solution in a three-electrode system, where the sample with exposed area of 1 cm2 as the working electrode (WE), saturated calomel electrode (SCE) as reference electrode (RE), a platinum mesh as counter electrode (CE). Potentiodynamic polarization tests were performed at a scan rate of 1 mV/s. The potential range was studied from −400 mV of the open circuit potential (OCP) value in the cathodic regime, to 1600 mV in the anodic regime.

inorganic particles are used to prepare a functional surface, such as nano silica,29 carbon black,30 and MOFs.31 Among them, metal−organic frameworks (MOFs), as novel organic− inorganic particles have received more attention in potential applications but seldom have appeared in superhydrophobic reports due to the water-sensitivity of most MOFs. Given the polyhedral structure of MOFs, the modification of MOFs foreshadows a significant role. MOFs, also called porous crystalline coordination polymers32 include Zeolitic Imidazolate Frameworks (ZIFs),33 and consist of metal ions and various organic ligands yields, which exhibit an orderly arrangement of pores and great high surface area.34 MOFs and nanoparticles also play an important role for combining the functionalities of each component. In particular, the combination of ZIFs with other components, which can enhance chemical and biological properties, have recently received much attention.35 Zeolite imidazole frameworks (ZIFs), which belong to porous crystalline coordination polymers (MOFs), exhibit good chemical stability and flexibility, and have wide application in catalysis and separation.36,37 In all of the ZIFs, zeolitic imidazolate framework-8 (ZIF-8) has been widely applied in many applications due to its rhombic dodecahedral structure and porosity. Fortunately, we selected SiO2 in situ formed on ZIF-8 with a special core/shell structure because of its cheap and facile fabrication.38 In addition, it does not change the primary morphology, and the shell of SiO2 can be easily modified. Studies about superhydrophobic silica and compounds have been reported, but few studies have reported on the successful synthesis of ZIF-8@SiO2, with regard to abrasive resistance. In this work, we describe our recent progress in the synthesis of the superhydrophobic surface, which is nanometer ZIF-8 covered with SiO2 (ZIF-8@SiO2). This was synthesized for tackling the poor durability and weak abrasion performance of the superhydrophobic surface. Hydrophobic modification was achieved in alkoxysilane, which involved 5 vol % hexadecyltrimethoxysilane (HDTMS) dissolved in 75/25 (v/v) ethanol/ water solvent and the use of acetic acid to adjust pH 3.5. Finally, the etched Mg alloy, which presented micrometer morphology, was dipped into the hydrophobic ZIF-8@SiO2 nhexane solution. The as-prepared superhydrophobic surface with hierarchical structure on magnesium alloy not only performed well as a water repellant but also exhibited advantageous properties, including excellent corrosion resistance, mechanical, and chemical stability, which satisfies the needs of society and offered promising application potential.

2. EXPERIMENTAL SECTION 2.1. Materials. Magnesium alloy AZ31 (composition: 2.98 wt % Al, 0.88 wt % Zn, 0.38 wt % Mn, 0.0135 wt % Si, 0.0027 wt % Fe, 0.002 wt % Ni, 0.001 wt % Cu, Mg) was cut into 30 mm × 30 mm × 2 mm sheets and used as the substrate. Zinc nitrate tetrahydrate(Zn(NO3)2.6H2O, 98%), 2-methylimidazole (H-MeIM, 99%), tetraethoxysilane (TEOS, 95%), and hexadecyltrimethoxysilane (HDTMS, 98%)were purchased from Aladdin, cetyltrimethylammonium bromide (CTAB), methanol, hydrochloric acid, acetic acid, deionized water, ammonium hydroxide (25%), and absolute ethanol were used as analytical grade and purchased from Tianjin Benchmark Chemical Regent, China. 2.2. Synthesis of ZIF-8@SiO2 Core−Shell Particles. At room temperature, ZIF-8 nanocrystals were synthesized via a coprecipitation procedure, which is depicted in a previous report.39 The specific process is as follows: 732 mg of Zn(NO3).6H2O was dissolved in 50

3. RESULTS AND DISCUSSION 3.1. Characterization of ZIF-8@SiO2 Particles. Figure 1 describes the XRD of ZIF-8 and ZIF-8@SiO2 particles and modified ZIF-8@SiO2 particles. As shown in Figure 1a, the image exhibits a number of sharp peaks between 5°and 90°, indicating their high crystallinity and all of the sharp diffraction peaks could be assigned to (011), (002), (112), (022), (013), and (222) peaks. On the basis of these results, it is shown that a 11107


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of ZIF-8@SiO2, which is in agreement with that previously reported.43 3.2. Morphology of Superhydrophobic Surface and Wettability. Surface measurements and water contact angle (CA) of the bare AZ31 surface, etched AZ31 surface, ZIF-8 and ZIF-8@SiO2 particles at different magnifications are presented in Figure 3. As shown in Figure 3a, there is no obvious morphology on the bare AZ31 alloy, and the CA is about 35° (inset in Figure 3a). However, after the etching process, the surface is covered with lots of irregularly shaped micro holes (Figure 3b). Because Mg atoms on the substrate may be dissolved in the acid solution, the surface becomes more rougher, but the surface energy is still high, so the CA is about 0°(inset in Figure 3b). On the basis of the ZIF-8 particles (Figure 3c), the SEM image shows a hexagonal shape with an average size of 500 nm, a higher resolution image is shown in Figure 3d. With SiO2 coated onto ZIF-8, it is clearly seen that the image still retains its rhombic dodecahedral structure (Figure 3e). However, compared with the uncoated particles, there is a significant appearance that the edges and corners of the dodecahedron structure are not obvious (Figure 3f). It further certified that SiO2 is successfully coated on ZIF-8. These results are consistent with XRD diagrams. Figure 3g shows that when the particles of ZIF-8@SiO2 are modified with HDTMS, the morphology remains unchanged and the CA was approximately 130° (inset in Figure 3g). As shown in Figure 3h, when hydrophobic particles of ZIF-8@SiO2 are deposited on the etched Mg alloy, roughness and low surface energy result in superhydrophobicity with a CA value of 153° (insert in Figure 3h). Another important observation is that the superhydrophobic surface can also repel daily liquids (Figure 3i). To further confirm the preparation of ZIF-8@SiO2 crystals, we carefully characterized as-obtained ZIF-8 particles and those modified with SiO2 by TEM. Figure 4a shows well the rhombic dodecahedral morphology of ZIF-8 seeds with size about 500 nm, which corresponds to the size of SEM image. In contrast, when incorporated with SiO2, the edge of ZIF-8 with rough structure is clearly observed (Figure 4b). It indicates that nanoscaled structure SiO2 is successfully formed. These results are in good agreement with XRD and FT-IR. The superhydrophobicity of the sample can be attributed to the hierarchical structure, when a water droplet or other daily used liquids are put on the surface, a three-phase of solid/ water/air develops, which is consistent with the Cassie model.44 The aerated rough structure filled with air can protect the substrate from wetting by liquid droplet. Thus, the liquid drops are in contact with the air phase without contact with the solid surface. The high CA can be interpreted according to the Cassie−Baxter equation45

Figure 1. XRD patterns of the (a) ZIF-8 particles, (b) ZIF-8@SiO2 particles, (c) modified ZIF-8@SiO2 particles.

rhombic dodecahedral structure ZIF-8 is formed. From Figure 1b, silica is an amorphous solid, therefore, we can detect a new broad diffraction peak at around 23°and the residual peaks can be indexed to diffraction peaks of ZIF-8. Notably, after the ZIF8@SiO2 decoration process (Figure 1c), the diffraction peak tends to migrate to a low angle (around 22°), which suggests that HDTMS has formed a new crystalline structure. From the literature, it may be due to the self-polymerization of silane coupling agent.40 To further confirm ZIF-8 and SiO2 particles were prepared, we used FTIR analysis. Figure 2 shows the FTIR spectra of the

Figure 2. (a) FTIR of the ZIF-8 particles, (b) ZIF-8@SiO2 particles, (c) modified ZIF-8@SiO2 particles.

cos θ = f cos θ1 + f − 1

ZIF-8 and ZIF-8@SiO2 and modified particles. For the ZIF-8 particles, there are four characteristic peaks at 2926, 1600, 1145, and 995 cm−1, assigned to vibrations of C−H, CN and C−N in the imidazole ring, respectively.41 These peaks are also found from ZIF-8@SiO2 particles. Meanwhile, two new peaks at about 1090 and 471 cm−1 appear that are attributed to stretching vibrations of Si−O−Si and bending vibration of the Si−O. For the modified particles, we observe two peaks at 2918 and 2850 cm−1,42 which belong to the −CH2− asymmetric and symmetric vibrations of the HDTMS. In addition, the peaks at 1115 and 1030 cm−1 are assigned to the asymmetric and symmetric vibrations of Si−O−Si in HDTMS. These results indicate that silica covers ZIF-8 and HDTMS coats the surface

(1)

Where θ and θ1 are the contact angles of a liquid droplet on the treated surface and untreated surface, respectively. f is the fraction of liquid contact with solid. From the contact angle measurements, θ = 153°, θ1= 35° and f = 0.06 can be calculated from eq 1. The low value of f indicates that about 6% of the contact area of the liquid droplet contact with the superhydrophobic surface and about 94% contact with air. These results indicate that a micro/nano rough structure contributes to the superhydrophobility of the film. 3.3. Stability of Superhydrophobic Surface. In view of the application of the superhydrophobic surface in the harsh 11108


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Figure 3. SEM images of samples: (a) nare Mg alloy, (b) etched Mg alloy, (c, d) ZIF-8 particles with different magnification; (e, f) ZIF-8@SiO2 particles with different magnification; (g) ZIF-8@SiO2 particles modified with HDTMS; (h) coating; (i) daily used liquids on the superhydrophobic surface, and the inset in a, b, g, and h show the CA of water droplet on the samples.

Figure 4. TEM images of (a) ZIF-8 and (b) ZIF-8@SiO2 particles.

the air layer that trapped in the microstructure reduces the contact area between the droplet and the solid surface. At the same time, the contact angle (CA) is greater than 150°, indicating a good chemical stability. In addition, an immersion test was carried out to estimate the long-term stability of the superhydrophobic coating. Figure 5b shows the relationship between the CA of superhydrophobic film and the immersion time in ethanol solution. The CA was almost constant after

conditions, the chemical and long-term stability as well as mechanical durability were measured. 3.3.1. Chemical and Long-Term Stability. The chemical stability of the superhydrophobic surface was investigated by liquids at different pH values (pH 2−12). From Figure 5a, there is no obvious difference between water droplets and different pH values droplets and the contact angles varying from 150 to 153°. According to these results, the existence of 11109


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Figure 5. (a) Relationship between the CA and the different pH values of liquids on the as-prepared specimen, and (b) variation of CA after immersion into ethanol solution with different times.

Figure 6. Friction coefficient and abrasion loss of samples: (a) Friction coefficient, (b) Abrasion loss.

The images shown in Figure 7 together with SEM tracks caused by friction and wear tests in superhydrophobic layers further show that the superhydrophobic coating has the function of reducing wear. The SEM track (Figure 7a) depicts the superhydrophobic film after testing, which shows clearly that from EDS analysis, the film is not removed and there are many cracks and granules in the SEM image. In addition, compared to Figure 7b, Zn, Si, C, O, and N have not disappeared and Mg appeared after testing (Figure 7c). The quality is much less, indicating that the layer has not been completely worn. So the film exhibits a good response for outdoor environments situation, where there is exposure for a long time. 3.4. Ice-Repellent Properties of the Superhydrophobic Coating. Considering industrial applications, the anti-icing property has attracted great interest recently. In this work, we speculate that the film possesses anti-icing performance.46 Two plates of bare Mg alloy (on the left) and superhydrophobic Mg alloy (on the right) were horizontally placed on a glass dish, in an ambient of temperature of −15 °C. Then, to assist in observing the freezing process, two drops of water doped with methyl orange, were carefully set on the bare alloy and superhydrophobic coating, respectively. The color of the water would changes from deep orange to light orange.47 It was observed that the water on the two surfaces does not change for about 10s (Figure 8a). After 300 s, the left sample freezes at the edge, whereas there is no change with the right sample (Figure 8b). At 500 s, the surface of bare Mg alloy appears misty; meanwhile, the water droplet completely freezes on the bare substrate. At the same time, the as-prepared sample has not begun to change (Figure 8c). As shown in Figure 8d, the water droplet on the right sample has not completely frozen after

immersion in ethanol solution for 3 days. It can be seen from Figure 5b, accompanied by the extension of the immersion time, the contact angle slightly changed. When the superhydrophobic magnesium alloy was immersed in ethanol solution, the ethanol solution will wet the surface due to the surface tension of the ethanol below the solid surface tension. However, the ethanol molecules stayed on the surface will evaporate rapidly and not destroy the surface structure that increases the physical barrier to the water droplet to some extent so the initial contact angle did not change. The above results suggest that the coated sample can resistant to ethanol, which is important for the practical application of the superhydrophobic film. 3.3.2. Mechanical Stability. The friction and wear test was conducted to explain the mechanical stability of the superhydrophobic surface. Figure 6a shows the friction coefficients of bare alloy, etched alloy and superhydrophobic alloy. It is clearly observed that the friction coefficients of bare alloy and etched alloy have fluctuated considerably, while the superhydrophobic alloy has only fluctuated slightly and the friction coefficients are lower than the bare and etched samples. Figure 6b shows the relationship between friction coefficient and abrasion loss. The bare sample and etched samples have friction coefficients of 0.488 and 0.52 and abrasion losses of 1 × 10−3 mm3/Nm and 1.4 × 10−3 mm3/Nm, respectively. However, for the asprepared sample, the friction coefficients is 0.288 and abrasion loss is 5 × 10−4 mm3/Nm, lower than those of bare and etched samples. These results demonstrates that the inherent wear resistance of silicon dioxide and the micropores interact strongly with the nano polyhedron, resulting in the superhydrophobic layer protect the AZ31 alloy from wearing, and therefore increasing wearability.. 11110


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Figure 7. (a) SEM image for superhydropobic Mg alloy after friction and wear test for 20 min, (b, c) EDS spectra (b) before and (c) after friction and wear test.

Figure 8. Comparison of surface icing property between the untreated substrate (left) and superhydrophobic surface substrate (right). The frozen time of the two samples are shown in (a) 10 s, (b) 300 s, (c) 500 s, (d) 900 s, (e) 1200 s, and (f) the surface after deicing, (g) 10 s, (h) 1200 s.

To further prove the potential of ice-repellent of the superhydrophobic surface, we put the two samples at the edge of the glass dish. Two drops of liquids were carefully dropped on the untreated AZ31 Mg alloy and superhydrophobic AZ31 Mg alloy, respectively. At 10 s, the liquid

about 900 s. From Figure 8e, the coated and uncoated alloys were both frozen at 1200 s. In addition, it was easy to remove the ice from the superhydrophobic surface, and after deicing, the surface is still clean. But for the untreated Mg alloy, the ice is difficult to remove from the surface (Figure 8f). 11111


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Figure 9. Potentiodynamic polarization curves for the bare and modified surface after immersion into 3.5 wt % NaCl aqueous solution for different times.

samples were approximately 2.69 × 10−7 and 4.49 × 10−7 A/ cm2, respectively. In addition, a significant change in the icorr after immersion in the 3.5 wt % NaCl aqueous solution for 24, 72h, and 144 h (Figure 9b), the icorr of the coated samples increased by about one order and two orders magnitude compared to that sample that immersion in NaCl solution for 3h.Yet, it still lower than that that of bare Mg alloy. These results indicate that coated surface has a high resistance to corrosion for a long time. From Figure 9, the value of the icorr of the superhydrophobic surface is much lower than that of bare magnesium alloy after immersion in the 3.5 wt % NaCl aqueous solution for different times. From literature reported,48,49 lower corrosion current densities signify lower corrosion rate, that is, good anticorrosion property. So we can draw a conclusion that the composite surface formed on the substrate improves the corrosion resistance of AZ31, which is ascribed to the effective inhibition of infiltration of corrosive-acting ions. For explaining the microstructure of the superhydrophobic substrates after immersion in 3.5 wt % NaCl aqueous solution, SEM images were measured. Figure 10 displays the SEM images of the superhydrophobic specimens after immersion in NaCl solution for (a) 3, (b) 6, (c) 9, (d) 24, (e) 72, and (f) 144 h, as well as (g, h) the bare and the etched samples after immersion for 3 h, respectively. Compared with the morphology before test (Figure 3a, b, h), there is little change for coated alloys after immersion for 3 h (Figure 10a). After 6 h, a few pits appear on the surface but the surface remains uniform (Figure 10b). When the immersion time was increased to 9 h, a few pits appear on the coating (Figure 10c). Accompanied by an increase in immersion time, the pits gradually become larger with the emergence of corrosion products (Figure 10d, e). It can be clearly seen that after immersed for 144 h (Figure 10f), pitting corrosion phenomenon appeares on the coated surface, which is likely to result in the decrease of impedance value. To some extent, these pores can provide an area of accumulation of corrosion products, which protect the substrate from invasion of a corrosive solution, but the protection time will not last long. As shown in Figure 10g, h, for both the bare Mg alloy and etched Mg alloy surfaces, a large number of corroded products and spiral-shaped pits appear after immersion for 3 h, respectively. The SEM images further prove that a superhydrophobic surface can effectively improve the corrosion resistance of the Mg alloy substrate. To further characterize corrosion resistance, we used EIS to analyze the anticorrosion of the sample. Figure 11 depicts the

stays on the bare alloy, nevertheless, the spherical liquid rolls on the glass dish (Figure 8g). About 1200 s later, we observed the water has frozen on both alloy samples surfaces (Figure 8h). These results indicate that the superhydrophobic surface can suppress ice formation and is more resistant to freezing than the bare sample. These results are consistent with the Cassie model, as mentioned above. 3.5. Corrosion Resistance of the Superhydrophobic Surface. Magnesium alloy is easy to corrode, especially in a corrosive medium. Here, the anticorrosion property was conducted by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in a three electrode system in 3.5 wt % NaCl aqueous solution. Figure 9 shows that potentiodynamic polarization curves of the as-prepared superhydrophobic sample after immersion in 3.5 wt % NaCl aqueous solution for 3, 6, 9, 24, 72, and 144 h and of the bare and etched samples after immersion for 3 h. The results of the test are summed up in Table 1. Table 1. Electrochemical Parameters of Bare Mg Alloy, Etched Mg Alloy, and Superhydrophobic Surface Mg Alloy in 3.5 wt % NaCl Aqueous Solution sample

time (h)

Ecorr (V/(SCE))

icorr (μA cm−2)

untreated AZ31 etched AZ31 coated AZ31

3 3 3 6 9 24 72 144

−1.174 −1.172 −1.196 −1.154 −1.189 −1.159 −1.156 −1.156

15.7 15.2 0.217 0.269 0.449 1.78 3.13 10.8

On the basis of Figure 9 and Table 1, it is shown that the corrosion current density (icorr) decreases after the magnesium alloy being coated. Due to the corrosion potential (Ecorr) mainly explains the thermodynamic performance of the materials. Hence the anticorrosion performance cannot be assessed by Ecorr. Another evaluation standard is icorr, which for the untreated alloy and etched alloy are 1.57 × 10−5 A/cm2 and 1.52 × 10−5 A/cm2 after immersion in 3.5 wt % NaCl aqueous solution for 3h, respectively, whereas that of the coated substrate is 2.17 × 10−7 A/cm2 (Figure 9a). The icorr of the superhydrophobic film formed on AZ31 Mg alloy decreases 2 orders of magnitude compared to the untreated and etched substrates. After immersion for 6 and 9 h, the icorr of the coated 11112


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Figure 10. Microstructure of the superhydrophobic samples, untreated Mg alloy, and etched Mg alloy with immersion in 3.5 wt % NaCl solution for different times.

11). It contains a capacitive loop at high frequency and an inductive loop at medium frequency. The emergence of the inductive loop is ascribed to intermediate products, derived from the dissolved of Mg, deposited on the specimen surface. In comparison, the as-prepared superhydrophobic film has a single capacitive semicircle at high frequency, with diameter of loop much larger than the bare and etched Mg alloys, indicating that the treated Mg alloy possesses good anticorrosion property. According to previous works,50 the larger of the diameter of the capacitive loop in the Nyquist plots reply the greater corrosion resistance of the working electrode. Figure 12 shows that Nyquist plot of immersion in 3.5 wt % NaCl solution with different times at room temperature. From Figure 12a, with the increase of immersion time, the diameter of capacitive loop decreases, which demonstrates that the corrosion resistance of the as-prepared superhydrophobic film is degraded. However, it is still more than that of bare Mg substrate. The above-mentioned results are consistent with the results of the potentiodynamic polarization curves and the description of the images (Figure 10). Figure 12b is an enlarged Nyquist plot that immersion in 3.5 wt % NaCl solution for 24, 72, 144, 480, and 600 h. It can be seen that the radius of curvature for the treated Mg alloy clearly decreases, indicating that the air layer on the solid surface gradually disappears and

Figure 11. Nyquist plots for the bare and modified surfaces in 3.5 wt % NaCl aqueous solution for 3 h.

Nyquist plots of the superhydrophobic surface, untreated specimen and etched specimen after immersion in 3.5 wt % NaCl aqueous solution for 3 h. The bare and etched specimens shown two capacitive semicircles in high and medium frequency ranges in the enlarged Nyquist plot (inset in Figure

Figure 12. EIS measurement for as-prepared superhydrophobic surface after immersion in 3.5 wt % NaCl solution for different times: (a) Nyquist plots, and (b) magnified Nyquist plots. 11113


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Figure 13. EIS diagrams of the bare, etched, superhydrophobic surfaces after immersion in 3.5 wt % NaCl aqueous solution for 3 h. (a) Bode plots of |Z| versus frequency, (b) Bode plots of phase angle versus frequency.

Notes

the electrolyte penetrates into the product layer. It is worth considering that immersion in 3.5 wt % NaCl solution for 3 h, the |Z| value for superhydrophobic specimen approximates 1 × 105 Ω cm2, 3 orders of magnitude higher than those of untreated and etched specimens after 3 h of immersion, which are about 102 Ω cm2 (Figure 13a), demonstrating that coated AZ31 alloy is not corroded and the corrosion resistance of the AZ31 substrate is enhanced. For the untreated and etched Mg alloys, two time constants exist around low- and high-frequency regions, respectively. Compared to the above-mentioned samples, the new time constant of the superhydrophobic sample appeares at a low frequency region of about 1 Hz (Figure 13b), which is attributed to the charge transfer process at the interface between Mg alloy and the superhydrophobic surface. Another time constant appears at high frequency region about 1 × 104 Hz, which is ascribed to the resistance of the superhydrophobic surface. These results demonstrate that the superhydrophobic coating protects the substrate from corroding.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC 51301050), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang Province (B2015021), International Science & Technology Cooperation Program of China (2015DFA50050), and the Magor Project of Science and Technology of Heilongjiang Province (GA14A101).

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4. CONCLUSIONS The core/shell structure of ZIF-8@SiO2 superhydrophobic film was successfully formed on etched AZ31 Mg alloy substrate by a simple dip-immersion method. The as-prepared rhombic dodecahedral zeolitic imidazolate framework surface with micro/nano hierarchical structure had a contact angle of 153°. Ascribed to the micro/nanostructure coating induced by chemical etching and dip-immersion, the surface showed excellent chemical stability and long-term durability with long-term exposure to ethanol as well as mechanical durability by friction and wear test. The surface also developed a significantly improved anticorrosion property compared to untreated Mg alloy. We believe that the method is adaptable to large-scale fabrication of a superhydrophobic coating on magnesium alloy. In addition, we anticipate a promising potential to transfer this technology to other metal substrates for important applications.

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REFERENCES

(1) Liu, K.; Cao, M.; Fujishima, A.; Jiang, L. Bio-inspired Titanium Dioxide Materials with Special Wettability and Their Applications. Chem. Rev. 2014, 114, 10044−10094. (2) Tian, Y.; Su, B.; Jiang, L. Interfacial Material System Exhibiting Superwettability. Adv. Mater. 2014, 26, 6872−6897. (3) Chen, Z.; Dong, L.; Yang, D.; Lu, H. Superhydrophobic Graphene-based Materials: Surface Construction and Functional Applications. Adv. Mater. 2013, 25, 5352−5359. (4) Geng, W.; Hu, A.; Li, M. Superhydrophiliicity to Superhydrophobicity Transition of a Surface with Ni Micro/Nano Cones Array. Appl. Surf. Sci. 2012, 263, 821−824. (5) Tian, A.; Sun, W.; Hu, Z.; Quan, B.; Xia, X.; Li, Y.; Han, D.; Li, J.; Gu, C. Morphology Modulating the Wettability of a Diamond Film. Langmuir 2014, 30, 12647−12653. (6) Su, B.; Tian, Y.; Jiang, L. Bio-Inspired Interfaces with SuperWettability: From Materials to Chemistry. J. Am. Chem. Soc. 2016, 138, 1727−1748. (7) Gao, R.; Liu, Q.; Wang, J.; Zhang, X. F.; Yang, W. L.; Liu, J. Y.; Liu, L. H. Fabrication of Fibrous Szaibelyite with Hierarchical Structure Superhydrophobic Coating on AZ31 Magnesium Alloy for Corrosion Protection. Chem. Eng. J. 2014, 241, 352−359. (8) Yang, N.; Li, Q.; Chen, F. N.; Cai, P.; Tan, C.; Xi, Z. X. A Solving- reprecipitation Theory for Self-recovering Functionality of Stannate Coating with a High Environmental Stability. Electrochim. Acta 2015, 174, 1192−1201. (9) Lv, J. Y.; Song, Y. L.; Jiang, L.; Wang, J. J. Bio-inspired Strategies for Anti-icing. ACS Nano 2014, 8, 3152−3169. (10) Tang, Y.; Zhang, Q.; Zhan, X.; Chen, F. Superhydrophobic and Anti-icing Properties at Overcooled Temperature of a Fluorinated Hybrid Surface Prepared via a Sol−gel Process. Soft Matter 2015, 11, 4540−4550. (11) Wang, N.; Xiong, D. S.; Deng, Y. L.; Shi, Y.; Wang, K. Mechanically Robust Superhydrophobic Steel Surface with Anti-icing, UV-durability and Corrosion Resistance Properties. ACS Appl. Mater. Interfaces 2015, 7, 6260−6272.

AUTHOR INFORMATION

Corresponding Authors

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

Jun Wang: 0000-0002-5192-0574 11114


Research Article

ACS Applied Materials & Interfaces (12) Cheng, M. J.; Zhang, S. S.; Dong, H. G.; Han, S. H.; Wei, H.; Shi, F. Improving the Durability of a Drag-reducing Nanocoating by Enhancing Its Mechanical Stability. ACS Appl. Mater. Interfaces 2015, 7, 4275−4282. (13) Wang, Y.; Liu, X. W.; Zhang, H. F.; Zhou, Z. P. Superhydrophobic Surfaces Created by a One-step Solution-immersion Process and Their Drag-reduction Effect on Water. RSC Adv. 2015, 5, 18909−18914. (14) Hong, D.; Bae, K. E.; Hong, S. P.; Park, J. H.; Choi, I. S.; Cho, W. K. Mussel-inspired, Perfluorinated Polydopamine for Self-cleaning Coating on Various Substrates. Chem. Commun. 2014, 50, 11649− 11652. (15) Yoon, H.; Kim, H. Y.; Latthe, S.; Kim, M. W.; Al-Deyab, S.; Yoon, S. A Mechanically Bendable Superhydrophobic Steel Surface with Self-cleaning and Corrosion Resistant Properties. J. Mater. Chem. A 2015, 3, 11403−11410. (16) Lu, Y.; Sathasivam, S.; Song, J. L.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P. Robust Self-cleaning Surfaces that Function When Exposed to Either Air or Oil. Science 2015, 347, 1132−1135. (17) Darmanin, T.; Guittard, F. Superhydrophobic and Superoleophobic Properties in Nature. Mater. Today 2015, 18, 273−285. (18) Ji, H.; Chen, G.; Yang, J.; Hu, J.; Song, H.; Zhao, Y. A Simple Approach to Fabricatate Stable Superhydrophobic Glass Surfaces. Appl. Surf. Sci. 2013, 266, 105−109. (19) Zhu, X.; Zhang, Z.; Xu, X.; Men, X.; Yang, J.; Zhou, X.; Xue, Q. Rapid Control of Switchable Oil Wettability and Adhesion on the Copper Substrate. Langmuir 2011, 27, 14508−14513. (20) Liu, W.; Luo, Y.; Sun, L.; Wu, R.; Jiang, H.; Liu, Y. Fabrication of the Superhydrophobic Surface on Aluminum Alloy by Anodizing and Polymeric Coating. Appl. Surf. Sci. 2013, 264, 872−878. (21) She, Z.; Li, Q.; Wang, Z.; Tan, C.; Zhou, J.; Li, L. Highly Anticorrosion Self-cleaning Superhydrophobic Ni-Co Surface Fabricated on AZ91D Magnesium Alloy. Surf. Coat. Technol. 2014, 251, 7−14. (22) Wang, Z.; Li, Q.; She, Z.; Chen, F.; Li, L. Low-cost and Largescale Fabrication Method for an Environmentally-friendly Superhydrophobic Coating on Magnesium Alloy. J. Mater. Chem. 2012, 22, 4097. (23) Yuan, Z.; Chen, H.; Tang, J.; Chen, X.; Zhao, D.; Wang, Z. Facile Method to Fabricate Stable Superhydrophobic Polystyrene Surface by Adding Ethanol. Surf. Coat. Technol. 2007, 201, 7138−7142. (24) Sugikawa, K.; Furukawa, Y.; Sada, K. SERS-Active MetalOrganic Frameworks Embedding Gold Nanorods. Chem. Mater. 2011, 23, 3132−3134. (25) Yeom, C.; Kim, Y. Purification of Oily Seawater, Wastewater Using Superhydrophobic Nano-Silica Coated Mesh and Sponge. J. Ind. Eng. Chem. 2016, 40, 47−53. (26) Alexander, S.; Eastoe, J.; Lord, A. M.; Guittard, F.; Barron, A. R. Branched Hydrocarbon Low Surface Energy Materials for Superhydrophobic Nanoparticle Derived Surfaces. ACS Appl. Mater. Interfaces 2016, 8, 660−666. (27) Yoon, H.; Kim, H.; Latthe, S. S.; Kim, M.; Al-Deyab, S.; Yoon, S. S. A Highly Transparent Self-cleaning Superhydrophobic Surface by Organosilane-coated Alumina Particles Deposited via Electrospraying. J. Mater. Chem. A 2015, 3, 11403−11410. (28) Li, J. H.; Liu, Q.; Wang, Y. L.; Chen, R. R.; Takahashi, K.; Li, R. M.; Liu, L. H.; Wang, J. Formation of a Corrosion Resistant and Antiicing Superhydrophobic Surface on Magnesium Alloy via a Single Step Method. J. Electrochem. Soc. 2016, 163, C213−C220. (29) Tu, M.; Wannapaiboon, S.; Khaletskaya, K.; Fischer, R. A. Engineering Zeolitic-Imidazolate Framework (ZIF) Thin Film Devices for Selective Detection of Volatile Orangic Compunds. Adv. Funct. Mater. 2015, 25, 4470−4479. (30) Eddaoudi, M.; Sava, D. F.; Eubank, J. F.; Adil, k.; Guillerm, V. Zeolite-like Metal-Organic Frameworks (ZMOFs): Design, Systhesis, and Propertied. Chem. Soc. Rev. 2015, 44, 228−249. (31) Sugikawa, K.; Furukawa, Y.; Sada, K. SERS-Active MetalOrangic Frameworks Embedding Gold Nonorods. Chem. Mater. 2011, 23, 3132−3134.

(32) Wei, Y.; Han, S.; Walker, D.; Fuller, P.; Grzybowski, B. Nanoparticle Core/Shell Architectures within MOF Crystals Synthesized by Reaction Diffusion. Angew. Chem., Int. Ed. 2012, 51, 7435− 7538. (33) Lee, H.; Cho, W.; Oh, M. Advanced Fabrication of MetalOrganic Frameworks: Template-directed Formation of Polystyrene@ ZIF-8 Core-shell and Hollow ZIF-8 Microspheres. Chem. Commun. 2012, 48, 221−224. (34) Kitagawa, S. Engineering Zeolitic-Imidazolate Framework (ZIF) Thin Film Devices for Selective Detection of Volatile Organic Compounds. Angew. Chem., Int. Ed. 2015, 54, 10686−10687. (35) Tu, M.; Wannapaiboon, S.; Fischer, R. A. Liquid Phase Stepwise Growth of Surface Mounted Metal-Organic Frameworks for Exploratory Research and Development of Applications. Inorg. Chem. Front. 2014, 1, 442−463. (36) Chen, B.; Yang, Z.; Zhu, Y.; Xia, Y. Zeolitic Imidazolate Framework Materials: Recent Progress in Synthesis and Applications. J. Mater. Chem. A 2014, 2, 16811−16831. (37) Hughes, J. T.; Bennett, T. D.; Cheetham, A. K.; Navrotsky, A. Thermochemistry of Zeolitic Imidazolate Frameworks of Varying Porosity. J. Am. Chem. Soc. 2013, 135, 598−601. (38) Cliffe, M. J.; Mottillo, C.; Stein, R. S.; Bu£ar, D.-K.; Friscic, T. Accelerated Aging: A Low Energy, Solvent-free Alternative to Solvothermal and Mechanochemical Synthesis of Metalorganic Materials. Chem. Sci. 2012, 3, 2495−2500. (39) Bell, M. S.; Shahraz, A.; Fichthorn, K. A.; Borhan, A. Effects of Hierarchical Surface Roughness on Droplet Contact Angle. Langmuir 2015, 31, 6752−6762. (40) Liu, L.; Chen, Y.; Huang, W. P. Preparation and Tribological Properties of Organically Modified Graphite Oxide in Liquid Paraffin at Ultra-low Concentrations. RSC Adv. 2015, 5, 90525. (41) Cravillon, J.; Munzer, S.; Lohmeier, S.; Feldhoff, A.; Huber, K.; Wiebcke, M. Rapid Room-temperature Synthesis and Characterization of Nanocrystals of a Prototypical Zeolitic Imidazolate Framework. Chem. Mater. 2009, 21, 1410−1412. (42) Tang, Y.; Yang, J.; Yin, L.; Chen, B.; Tang, H.; Liu, C.; Li, C. Fabrication of Superhydrophobic Polyurethane/MoS2 Nanocomposite Coatings with Wear Resistance. Colloids Surf., A 2014, 459, 261−266. (43) Jesionowski, T.; Zurawska, J.; Krysztafkiewicz, A.; Pokora, M.; Waszak, D.; Tylus, W. Physicochemical and Morphological Properties of Hydrated Silica Precipitated Following Alkoxysilane Surface Modification. Appl. Surf. Sci. 2003, 205, 212−224. (44) Zhao, L.; Liu, Q.; Gao, R.; Wang, J.; Yang, W.; Liu, L. One-step Method for the Fabrication of Superhydrophobic Surface on Magnesium Alloy and Its Corrosion Protection, Antifouling Performance. Corros. Sci. 2014, 80, 177−183. (45) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (46) Quang, D. V.; Lee, J. E.; Kim, J. K.; Kim, Y. N.; Shao, G. N.; Kim, H. T. A Gentle Method to Graft Thiol-functional Groups onto Silica Gel for Adsorption of Silver Ions and Immobilization of Silver Nanoparticles. Powder Technol. 2013, 235, 221−227. (47) Zhu, H.; Guo, Z.; Liu, W. Biomimetic Water-collecting Materials Inspired by Nature. Chem. Commun. 2016, 52, 3863−3879. (48) Si, Y.; Guo, Z. Bio-Inspired Writable Multifunctional Recycled Paper with Outer and Inner Uniform Superhydrophobicity. RSC Adv. 2016, 6, 30776−630784. (49) Yan, H. J.; Bai, J. W.; Wang, J.; Zhang, X. Y.; Wang, B.; Liu, Q.; Liu, l. H. Graphene Homogeneously Anchored with Ni(OH)2 Nanoparticles as Advanced Supercapacitor Electrodes. CrystEngComm 2013, 15, 10007−10015. (50) She, Z.; Li, Q.; Wang, Z.; Li, L.; Chen, F.; Zhou, J. Researching the Fabrication of Anti-corrosion Superhydrophobic Surface on Magnesium Alloy and Its Mechanical Stability and Durability. Chem. Eng. J. 2013, 228, 415−424.

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