Constructing Fluorine-Free and Cost-Effective Superhydrophobic

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Constructing Fluorine-Free and Cost-Effective Superhydrophobic Surface with Normal-Alcohol-Modified Hydrophobic SiO2 Nanoparticles Hui Ye, Liqun Zhu, Weiping Li, Huicong Liu, and Haining Chen* Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Haidian District, Beijing 100191, People’s Republic of China S Supporting Information *

ABSTRACT: Superhydrophobic coatings have drawn much attention in recent years for their wide potential applications. However, a simple, cost-effective, and environmentally friendly approach is still lacked. Herein, a promising approach using nonhazardous chemicals was proposed, in which multiple hydrophobic functionalized silica nanoparticles (SiO2 NPs) were first prepared as core component, through the efficient reaction between amino group containing SiO2 NPs and the isocyanate containing hydrophobic surface modifiers synthesized by normal alcohols, followed by simply spraying onto various substrates for superhydrophobic functionalization. Furthermore, to further improve the mechanical durability, an organic−inorganic composite superhydrophobic coating was fabricated by incorporating cross-linking agent (polyisocyanate) into the mixture of hydrophobicfunctionalized SiO2 NPs and hydroxyl acrylic resin. The hybrid coating with cross-linked network structures is very stable with excellent mechanical durability, self-cleaning property and corrosion resistance. KEYWORDS: superhydrophobic, fluorine-free, SiO2 nanoparticles, organic−inorganic composite coating, mechanical durability

1. INTRODUCTION The design and development of superhydrophobic surfaces with a static water contact angle greater than 150° and sliding angle below 10° have garnered tremendous interest in both academic and commercial communities.1 They show much promise in the applications ranging from anticorrosion, selfcleaning, antifogging, anti-icing, drag reduction, medicine, oil− water separation, water harvesting, and other fields.2−11 Recent studies inspired by the functional natural biological surfaces, including lotus leaf, wings of butterfly, legs of water strider, gecko’s feet, and nepenthes leaf, revealed that surface chemistry and surface roughness are two main contributors to govern surface wettability.12−16 To date, many artificial superhydrophobic surfaces have been developed by various methods, including physical, chemical, or both.15,17−20 Despite great advances in scientific community, plenty of limitations still remain in the widespread practical use of superhydrophobic surfaces because of the strict conditions and complicated treatment in most available methods. Therefore, developing simple, low-cost, and suitable methods to fabricate superhydrophobic surfaces in large scale is © 2016 American Chemical Society

important for industrial application. Among various methods, the method to prepare superhydrophobic coating by the spray deposition of hydrophobic nanoparticles is thought to be the most promising one.21−23 What is more, the superhydrophobicity of those surfaces can be easily repaired after degradation by respraying the hydrophobic nanoparticle-based coating. Fluorocarbons, the most typical low surface energy materials, are often employed to prepare such superhydrophobic materials. For example, fluor-alkylsilanes modified nanoparticles with hydrophobicity have been widely employed to fabricate superhydrophobic coating.2,24−29 However, long-chain polyfluorinated compounds are expensive and may lead to bioaccumulation and toxicity.30−32 Fortunately, numerous functional and natural interface materials with superhydrophobicity have been disclosed, whose chemical compositions are fluorine-free. Inspired by this fact, many researchers have been exploring methods for constructing superhydrophobic surface Received: October 9, 2016 Accepted: December 13, 2016 Published: December 13, 2016 858

DOI: 10.1021/acsami.6b12820 ACS Appl. Mater. Interfaces 2017, 9, 858−867

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration for the Fabrication of Superhydrophobic Surfaces

Figure 1. Preparation of hydrophobic silica NPs. (a) Schematic illustration depicting the procedure of one-step approach to fabricating NH2-SiO2 NPs. (b) Schematic illustration showing the synthesis route of surface modifier. (c) Schematic illustration for the fabrication of hydrophobic SiO2 NPs.

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the −OH (from alcohol) or the −NH2 (from amines) or the −COOH (from carboxylic acid)/NCO ratio was maintained at 1:2. The appropriate IPDI and BuA were first introduced into a four-neck round-bottomed flask. The solution was bubbled with N2 to remove the vapor in air. The flask was then heated to 70 °C. After that, a mixture of normal alcohol (amine or carboxylic acid) and BuA was added dropwise into the flask with continuous magnetic stirring. The reaction was continued for another 2 h after feeding. Finally, a clear solution containing surface modifier was obtained. 2.4. Preparation of Hydrophobic Silica Nanoparticles. Hydrophobic silica nanoparticles were prepared by incorporating the surface modifier to modify NH2-SiO2 NPs. The modification route is shown in Figure 1c. First, NH2-SiO2 NPs were dispersed in BuA by ultrasonic treatment. Under stirring, the BuA solution containing surface modifier was added in dropwise. Then, the mixture was stirred at 75 °C for 24 h in the presence of dibutyltindilaurate. After cooling down to room temperature, the resultant was centrifuged (5 min, 10 000 rpm) and washed by BuA for three times. By drying at 60 °C overnight, hydrophobic nanoparticles were obtained. 2.5. Preparation of Different Coatings. Nanocoatings. After purification, hydrophobic silica nanoparticles were dispersed in ethanol with ultrasonication to make a paintlike suspension (1.5 wt %). Then the nanoparticle suspension was sprayed onto substrates, including glass slides, steel, filter paper, fabric, cotton wool, and wood. All the substrates were treated by spraying except cotton wool. The cotton wool was treated by dip-coating. The coating was dried in air before testing. Organic−inorganic Composite Coatings. To prepare the organic−inorganic composite coatings, a paint consisting of part A and part B was needed. For preparation of part A, the hydrophobic SiO2 NPs were first dispersed in BuA by ultrasonic treatment for 15 min, then the hydroxyl acrylic resin (the synthetic procedure is shown in Scheme S1) was added with another 15 min ultrasonic treatment. Hexamethylene diisocyanate tripolymer (N3375, Bayer), employed as a cross-linking agent, was part B. After mixing part A and part B at room temperature, the organic−inorganic composite coatings could be prepared by spraying the paint on glass slides, followed by curing at 60 °C for 8 h. 2.6. Characterizations. The morphologies of superhydrophobic surfaces on glass slides were observed by using a field emission scanning electron microscopy (FESEM, JSM-7500F, Japan) instrument. Morphologies of APTES functionalized silica nanoparticles and hydrophobic silica nanoparticles were performed by transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). Fourier transform infrared spectra (FT-IR) were recorded on a NEXUS-470 FTIR analyzer (Nicolet, USA) from 4000 to 500 cm−1 at a resolution of 8 cm−1 by KBr pellet method. Thermogravimetric analysis (TGA) was performed on the STA 3 Jupiter (Netzsch, Germany) from 25 to 800 °C at a heating rate of 10 °C/min with argon protection. Film surface morphologies were investigated by atomic force microscopes (AFM, Veeco DI) in tapping mode at ambient temperature by keeping the scan rate of 256 Hz and scan size of 5 μm × 5 μm. Measurements of contact angle and sliding angle: The static contact angle and sliding angle of water were measured using an optical contact angle meter system KRÜ SS DSA20 (KRÜ SS, Germany) by sessile drop method at room temperature. The sliding angles were measured by tilting the stage until the droplet rolled off from the surface. The volume of droplets used for the static contact angle and sliding angle were 5 uL and 10 uL, respectively. Five measurements were performed on each surface. Mechanical durability tests: Knife-scratch and sandpaper abrasion were chosen as mechanical durability tests, according to the previous work.24 The superhydrophobic surface was coated on glass slides. For the knife-scratch test, the coating was scratched with a knife along meshy paths. For the sandpaper abrasion test, the sample weighing 100 g was placed face-down to the sandpaper (800 meshes, 0.5 kPa). The surface was moved for 10 cm along the ruler, and then rotated by 90°.

with fluorine-free chemicals, which were low-cost and environmentally friendly. As a result, some strategies with fluorine-free superhydrophobic coating have been developed by the hydrophobic modified nanoparticles, and most of these surface chemical modification techniques were achieved via the reaction between the surface hydroxyl of nanoparticles and the silanol groups or carboxyl from low-surface-energy materials. However, commercially available materials for developing those surperhydrophobic surfaces are usually limited.33−39 Therefore, though some strategies have been exploited to fabricate fluorine-free superhydrophobic coating, it is still necessary to develop more and versatile approaches to further promote the application of fluorine-free superhydrophobic surfaces. Herein, we have developed a new class of fluorine-free superhydrophobic surfaces with hydrophobic SiO2 nanoparticles. First, a novel and efficient route was exploited to synthesize surface-hydrophobic-functionalization SiO2 NPs by the reaction between the amino groups containing SiO2 NPs and the isocyanate containing hydrophobic surface modifiers synthesized by normal alcohols, which is cost-effective and environmentally friendly. An ethanol-based suspension containing as-prepared hydrophobic functionalized SiO2 NPs can be directly sprayed onto different substrates to prepare superhydrophobic surfaces. What is more, a superhydrophobic organic−inorganic composite coating with excellent mechanical durability, self-cleaning property and corrosion resistance could be fabricated by introducing polyisocyanate into the mixture of hydrophobic functionalized SiO2 NPs and hydroxyl acrylic resin. The schematic illustration of this approach is shown in Scheme 1. This low-cost, eco-friendly, and scalable superhydrophobic surface is supposed to have great potential in practical application.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraethoxysilane (TEOS) and Benzoyl peroxide (BPO) were obtained from Xilong Chemical Co., Ltd. (3-aminopropyl) triethoxysilane (APTES, 99%), and isophorone diisocyanate (IPDI, 99%) were obtained from Aladdin industrial corporation. nhexyl alcohol, n-octyl alcohol, n-decyl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, myristic acid, and myristylamine were purchased from Sinopharm chemical reagent Co., Ltd. (Shanghai, China). Butyl acetate (BuA), xylene (Xyl), ammonia (25 wt %), and alcohol were supplied from Beijing chemical works. Methyl methacrylate (MMA), styrene (St), butyl acrylate (BA), hydroxyethyl methyl acrylate (HEMA), and acrylic acid (AA) were provided by DongfangYakeli chemicals limited corporation (Beijing, China), which were employed to prepared hydroxyl acrylic resin. The hexamethylene diisocyanate tripolymer (N3375, Bayer) was used as a curing agent. All chemicals were used without further purification. 2.2. Synthesis of APTES-Functionalized Silica Nanoparticles. APTES functionalized silica nanoparticles (NH2-SiO2 NPs) were prepared by co-condensation of TEOS with APTES, and the reaction mechanism is depicted in Figure 1a. Typically, SiO2 NPs were prepared according to the Stöber procedure. Ethanol (50 mL) and ammonia hydroxide (2 mL) were mixed to form a homogeneous solution at room temperature. Then tetraethoxysilane (4.2 mL) was added dropwise. After continuously stirring for a few minutes, a certain amount of APTES was added into the reactor. The reaction lasted for 24 h under magnetic stirring at room temperature. After that, the temperature was increased to 75 °C and stirred for 2 h. Finally, the product was centrifuged and washed with ethanol three times and then dried at 50 °C overnight. Eventually the obtained NH2-SiO2 was ground to powder before use. 2.3. Surface Modifier Preparation. The synthetic route of surface modifier was exhibited in Figure 1b. In all the addition of IPDI, 860


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Figure 2. Characterization of hydrophobic silica NPs. (a) FTIR spectra of SiO2, NH2-SiO2, C12-SiO2. (b, c) FTIR spectra of the surface modifier −C12IPDI. TEM images of (d) NH2-SiO2 and (e) C12-SiO2.

NPs (NH2-SiO2 NPs) were first prepared by co-condensation of TEOS with APTES. It is noteworthy that in the process of TEOS hydrolysis, both feeding time and the content of APTES would influence the modification process. Therefore, an appropriate co-condensation procedure and the content of APTES during the preparation of NH2-SiO2 NPs are significantly important. FTIR spectra of silica nanoparticles (SiO2 NPs) before and after APTES modification (NH2-SiO2) were presented in Figure 2a. The strongest peak at 1069 cm−1 corresponds to the asymmetric stretching vibration of Si−O−Si bonds, while the appearance of the peak at 456 cm−1 indicates the presence of Si−OH bonds. Compared with the unmodified silica nanoparticles, N−H bending vibration around 696 cm−1 and the symmetric NH2 bending vibration at 1562 cm−1 are observed in the FTIR spectrum of APTES-functionalized silica nanoparticles, demonstrating the existence of amino groups. The stretching vibration of OH at 3434 cm−1 is further widened in the spectrum of APTES-functionalized silica nanoparticles, which is attributed to the overlap with the NH2 stretching vibration signals at around 3373 and 3300 cm−1. In addition, after modification, the symmetric stretching vibration of −CH at 2924 cm−1 gets stronger than that of the unmodified silica, which supports the occurrence of grafting. All of these observations prove that the silica has been successfully modified with APTES. Morphology and nanoparticle size of the NH2-SiO2 NPs are shown in Figure 2d. The NH2-SiO2 NPs present a mean size of 27 nm (Figure S1a), aggregating into small clusters. SEM image (Figure S2) reveals an obviously rough surface on NH2-SiO2 NPs. In contrast with the smooth surface of silica nanoparticles acquired by the Stöber method in alcohol-based media, this rough surface of NH2-SiO2 NPs results from the occurrence of the uncompleted silica condensation reactions, which is attributed to the hydrolysis and condensation process of TEOS resulting from the addition of APTES and the presence of a small amount of water.

We defined this process as one cycle of the sandpaper abrasion test. To investigate the abrasion resistance of the coating, the static water contact angles were measured after each abrasion cycle. Self-cleaning test: For the self-cleaning test, hydrophobic graphite particles (50 μm) and hydrophilic silicon carbide (SiC) particles (10 μm) were chosen as the artificial contaminants separately. The superhydrophobic coating surface was contaminated by a simple device. The sample was placed in a plastic chamber, and then the artificial contaminant would blow into it by nitrogen. The samples adequately got stained within seconds. Water was then dropped onto the surface to test the self-cleaning property. Electrochemical corrosion test: Electrochemical measurements were performed to compare an acid barrier property of the as-prepared hybrid superhydrophobic coating with bare tinplate during their immersion in pH 1 aqueous solution. The electrochemical test was conducted in an electrochemical workstation, model CHI660E (Shanghai Chenhua instruments Ltd., China), where 3.5 wt % NaCl solutions were used as electrolyte. The electrochemical workstation was a three-electrode system. A saturated calomel electrode (SCE) and a platinum plate were used as reference and counter electrodes, respectively. The specimen acted as working electrode with an exposed area of 1 cm2, which was immersed in the electrolyte at room temperature. The tafel measurements were performed on potentiostatic mode at the open circuit potential (OCP) with a scan rate of 5 mV/s.

3. RESULTS AND DISCUSSION 3.1. Hydrophobic Silica NPs. 3.11. APTES-Functionalized Silica NPs. IPDI was chosen as the bridge of SiO2 NPs and various alcohols were applied for hydrophobic modification. The hydrophobic SiO2 NPs were synthesized using an efficient route by employing the reaction between the surface reactive groups on SiO2 NPs and the hydrophobic surface modifiers prepared by IPDI and normal alcohols. The reaction between isocyanate and amine groups is much more active than that between isocyanate and hydroxyl groups. Therefore, in order to enhance the grafting efficiency and grafting density of the surface modifiers on SiO2 NPs, APTES-functionalized silica 861


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Figure 3. Hydrophobic silica (Cn-SiO2) NPs. (a) SEM images of C12-SiO2 nanocoating coated on glass slide. (b) Surface roughness parameters and contact angle measurements for Cn-SiO2 nanocoatings. (c) Water droplets placed on the C12-SiO2 nanocoatings surfaces of various substrates.

-NH-. The appearance of the CO (from −CO−NH−) peak at 1703 cm−1, the stronger C−H stretching vibration peak at around 2922 cm−1 and the disappearance of the − NCO characteristic vibration peak at around 2265 cm−1, indicate that the surface modifier has been successfully grafted onto the silica surface. Apparently, this methodology can be applied to a variety of NPs for surface-hydrophobic-functionalization NPs. A majority of reactive groups or polymeric molecules can also be grafted onto the surface of the nanoparticles (According to the synthesis route of the modified NPs, the nanoparticles containing vinyl can be obtained, if the normal alcohol is replaced by 3-buten-1-ol). Therefore, the potential applications of this type of functional nanoparticles will be the subject of further investigations. As presented in Figure 2e, the C12-SiO2 NPs aggregated to small clusters structure with a larger mean size of 33 nm (Figure S1b) than that of NH2-SiO2 NPs, which indicates the encapsulation of NH2-SiO2 NPs with surface modifier. To further evaluate the hydrophobicity of the surface functionalized SiO2 NPs (C12-SiO2), “water marbles” are prepared according to literature procedure.40 (Figure S3) It is well-known that silica nanoparticles are naturally wetted by water due to the numerous hydroxyl groups (−OH) on the surface. However, when partially functionalized with a hydrophobic surface modifier, the nanoparticles may simultaneously have both hydrophobic and hydrophilic characteristics. When water drops interact with these particles, “water marbles” are formed.41 Due to the micro- and nanoscale hierarchical structures from the small clusters and low surface energy −CH2 and −CH3 groups, Cn-SiO2 NPs exhibit an excellent superhydrophobic property. As such, C6-SiO2, C8-SiO2, C10-SiO2, C14-SiO2, C16SiO2, and C18-SiO2 NPs were prepared to evaluate the versatility of the above-mentioned method. C14(NH2)-SiO2 NPs, whose surface modifier was synthesized by myristylamine with IPDI, and C14(COOH)-SiO2 NPs, whose surface modifier

3.12. Synthesis of Surface Modifier. Surface modifier was synthesized by the reaction between isocyanate groups from IPDI and hydroxyl from normal alcohols (CnH2n+1OH). To make sure that there is a -NCO group remaining on each of the surface modifier molecule, CnH2n+1OH solution was needed to be added dropwise to ensure that isophorone diisocyanate was excessive during the reaction. The surface modifier (C12IPDI) synthesized by IPDI and C12H25OH was used as an example to illustrate the general synthesis route for producing the hydrophobic surface modifier, and the FTIR spectra are displayed in Figure 2b, c. The peaks at 3336 cm−1 is assigned to the stretching vibration of -NH-, while the peak at 2266 cm−1 corresponds to the absorption band of the asymmetrical stretching vibration of -NCO. The CO bond stretching at 1718 cm−1 is clearly observed for the surface modifier. The peak at 1536 cm−1 is due to the deformation vibration of N−H from-CO-NH-, whereas, the presence of strong peaks at 1240 cm−1 is assigned to the characteristic vibration of C−N bond. The peaks at 1380 and 1110 cm−1 are assigned to the vibrations of −CH2. Furthermore, the presence of characteristic vibration peaks for -NCO and -NH−CO- proves that C−OH has fully reacted with IPDI. The surface modifiers (CnIPDI) with different carbon chain length have been further synthesized by reactions of IPDI with various alcohols (CnH2n+1OH) having different carbon chain length (n = 6, 8, 10, 14, 16, 18), demonstrating the versatility of our present strategy. (Myristic acid and myristylamine have been also employed to prepare the surface modifiers to verify the general applicability of this method). 3.13. Hydrophobic Silica (Cn-SiO2) NPs. Hydrophobic silica (Cn-SiO2) NPs were obtained by the reaction of the surface modifier (CnIPDI) with hydroxyl and amino groups on the surface of NH2-SiO2 NPs in the presence of dibutyltindilaurate. As shown in the FTIR spectrum in Figure 2a, the peak at 3434 cm−1 is assigned to stretching vibration of −OH, while the peaks at 3381 cm−1 is attributed to the stretching vibration of 862


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Figure 4. Organic−inorganic composite superhydrophobic coating. (a) Possible bonding structure of the composite coating (b) SEM image of C12SiO2 composite coating. Inset is its water contact angle. (c) AFM images of the C12-SiO2 composite coating.

cotton wool, and fabric coated superhydrophobic surface are long-lasting and durable, which is attributed to the reduced direct friction between the coating and the surface by their inherent flexibility.42,43 The successful construction of superhydrophobic surface on various substrates highlights the wide adaptability of Cn-SiO2 nanocoatings. 3.2. Superhydrophobic Organic−Inorganic Composite Coating. The nanocoating with micro- and nanoscale roughness structures is mechanically weak and readily abraded, especially for hard substrates. The organic−inorganic nanocomposites may be a possible solution for paving the way for superhydrophobic coating to practical applications. They present improved performance by grafting synthetic polymers on nanoparticles or by incorporating modified nanoparticles into a polymer matrix.44 In order to improve the mechanic properties of superhydrophobic coating, we developed an organic−inorganic composite superhydrophobic coating by mixing acrylic resin (adhesive resin) and hydrophobic SiO2 NPs with incorporating N3375 as cross-linking agent, which can obtain strong bonding between the functionalized-silica nanoparticles and the acrylic resin matrix. In the FTIR spectrum of the superhydrophobic organic− inorganic composite coating (Figure S8), the peaks at 3381 and 1555 cm−1 represent the −NH bending vibration, whereas the peak at 1702 cm−1 is attributed to the stretching vibration of CO structure in the amino ester −O−C(O)−NH− unit. There is no characteristic vibration peak of −NCO at around 2265 cm−1, indicating that the entire cross-linking agent N3375 has been incorporated into the composite coatings. Thus, a cross-linked network structure was formed, and the possible bonding structure of the hybrid coatings was illustrated in

was synthesized by myristic acid with IPDI, were also prepared (Figures S4 and S5). The superhydrophobic nanocoatings coated on glass slide were obtained by these Cn-SiO2 NPs, with a typical rough surface morphology (Figure 3a). AFM images are recorded to reveal the roughness of the nanocoating (Figure S6), and the corresponding surface root-mean-square roughness (Rq ) was calculated. The quantitative roughness parameters and the surface wettability of different nanocoating are exhibited in Figure 3b. It is well-known that the hydrophobicity increases with increasing hydrocarbon tail length. However, the results show that the water contact angles here do not increase linearly with the tail length of the surface modifier. This phenomenon is probably attributed to the different grafting density of Cn-SiO2 NPs (Calculated by the TGA curves, Figure S7 and eq S1) and a large difference in surface roughness. The grafting density decreases with increasing the tail length of the surface modifier due to the steric effect of long carbon chain. Meanwhile, though the spraying deposition is a simple method that is applicable for a variety of substrates and large-scale preparation, it is hard to fine-tune the surface roughness. To investigate the adaptability of Cn-SiO2 nanocoatings, we used different substrates, such as glass, steel, filter paper, fabric, wood, and cotton wool, for the nanocoating. A 1.5 wt % ethanol-based suspension of C12-SiO2 NPs was prepared as a representative. The nanocoatings on various substrates show the super-repellency to water droplets at room temperature with sphere-like droplets (water dyed by methylene blue) forming on the surface (Figure 3c). Representatively, the coated surface of glass slide shows a static water contact angle of 157° with a sliding angle of about 7°, while for filter paper, 863


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Figure 5. Characterizations of organic−inorganic composite superhydrophobic coating. (a) Average contact angle and sliding angle of composite coatings with the number of abrasion cycles. (b) SEM image of the composite coatings after 30 cycles of abrasion. The inset is the water contact angle of the coating. (c) Self-cleaning action of the composite coatings performed on artificially contaminated surfaces with SiC and graphite particles. (d) Potentiodynamic polarization curves of different immersion time in pH 1 hydrochloric acid aqueous solution.

away after many more abrasion cycles. As well-known, it is widely recognized that superhydrophobic coating could be applied for anti-icing. The extraordinary water repellency and mechanical stability of the composite superhydrophobic surfaces may enable them with high anti-icing property because water droplets that drop onto their surfaces could retract or bounce away from the surface, which could avoid ice accumulation and attain anti-icing functionality.46 It is noticed that the water contact angle and sliding angle have a large fluctuating with the number of abrasion cycles increasing. SEM image in Figure 5b shows the coating surface becomes smoother after 30 abrasion cycles with the presence of some microscale roughness structures. Obviously, the drastically decreased water contact angle indicates that the top layer was removed, leading to destruction of the micro- and nanoscale roughness structure. Furthermore, the heterogeneity of the internal structure in the coating may reduce the number of cavities on the new surface after several abrasion cycles. Interestingly, as the number of abrasion cycles increases, sometimes the contact angle of the coating would increase and its sliding angle would decrease rapidly. This phenomenon can be explained that the new surface was still composed of hydrophobic SiO2 NPs and a new micro- and nanoscale roughness structure was established by sandpaper wearing. Moreover, it is found that the organic−inorganic composite coating would not show superhydrophobicity with a low RMS roughness, which is consistent with previous reports.47 3.22. Self-Cleaning Test. Self-cleaning is another desirable property for functional coating due to their wide potential applications, such as windows, solar panels and paints. We also evaluate the self-cleaning property by depositing such semitransparent and composite superhydrophobic coatings on glass

Figure 4a. The as-prepared composite coating displays a water contact angle than 165° and a sliding angle lower than 10°. SEM image of the coating exhibits a dual-scale multiscale structure (Figure 4b). Compared with the nanocoatings, the composite coating possesses a more dense structure. The AFM height image was recorded to evaluate the specific roughness structure, which shows a root-mean-square roughness of 110 nm (Figure 4c). As is well-known, when water droplets drop onto a solid surface without wetting it, they will rebound because the water droplets’ kinetic energy was transformed into vibrational energy.45 Therefore, the bouncing ability of a droplet can also indicate the superhydrophobicty property (Movie S1). 3.21. Mechanical Property. Weak mechanical durability is the main issue for limiting the widespread practical applications of superhydrophobic surfaces. Knife-scratch and sandpaper abrasion tests were employed to evaluate the mechanical durability of the organic−inorganic composite coatings. After the cross-cut scratching in knife-scratch test, the coating still retained superhydrophobicity (Figure S9a, b, and Movie S2), while after 30 cycles of abrasion in the sandpaper abrasion test (Figure S9c, d), the coatings still show a water contact angle of 153° and a sliding angle of 9° (Figure 5a), which indicate that the superhydrophobic behavior is resistant to mechanical forces (scratching and abrasion) (Movie S3). This high resistance to mechanical forces should be attributed to the ‘volumetric superhydrophobic’ coating created by this method, in which the surface and inner parts of this coating is superhydrophobic owning to the hydrophobized SiO2 nanoparticles and lots of cavities. It is obvious that such a coating will retain their superhydrophobicity even if the top layer gets abraded away. Nevertheless, the composite coatings would be eventually worn 864


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4. CONCLUSION We have introduced a facile and efficient approach to fabricate a low-cost and nonfluorine superhydrophobic surface, based on the hydrophobic silica NPs with low surface energy and high surface roughness at the micro and nanoscales. A majority of commercially alternatives and nonhazardous chemicals such as normal alcohols (CnH2n+1OH, n = 6, 8, 10, 12, 14, 16, 18) could be employed for surface modification to prepare hydrophobic silica NPs. On the basis of a spraying technique, superhydrophobic nanocoatings could be easily fabricated on different substrates, such as glass, steel, wood, filter paper and fabric, using hydrophobic modified silica NPs. By incorporating cross-linking agent (N3375) for a composite coating, the mechanical durability of the superhydrophobic coating was well improved with forming a stable cross-linked network structure. As a result, this composite coating not only exhibited a good self-cleaning property but also retained its superhydrophobic property after 30 cycles of abrasion using sandpaper with 100 g weight. Besides, this coating also presented a good corrosion resistance with its water roll-off property retaining after 240 h of immersion in acid solution (pH 1). Therefore, this composite superhydrophobic coating is anticipated to have widely practical applications because of its satisfactory low-cost, environmentally friendly, weather-durable, and scalable fabrications.

slides. During self-cleaning test, hydrophobic graphite particles (50 μm) and hydrophilic silicon carbide particles (10 μm) were placed on the samples as artificial contaminant in a simple designed device (Figure S10). As observed, the composite superhydrophobic coating allows water droplets to take dirt away from its rolling and bouncing path, which demonstrates an excellent self-cleaning property regardless of hydrophilic or hydrophobic contaminants (Figure 5c). 3.23. Anticorrosion Test. Because of the increasing attention on the superior anticorrosion capacity of superhydrophobic coatings, the as-prepared composite superhydrophobic coatings on tinplate electrodes were further examined by the electrochemical corrosion tests after the acid barrier tests with a comparison with a bare tinplate plate. In the acid barrier tests, the electrodes coated with the composite coatings were immersed into hydrochloric acid (pH 1) for different time. After immersion for 240 h, the coating surface was still dry and exhibited water roll-off property, indicating the superhydrophobic performance is still remained. However, the partially wetting phenomenon was observed on the surface of coating after immersion for 288h indicating that the air trapped in the coating surface micro- and nanoscale roughness structure was taken up by the diffusion of water molecules. Interestingly, the composite coating can recover its water-repellent property after being dried or placed at room temperature for a period of time, which confirms the explanation above. Potentiodynamic polarization curves (Figure 5d) were recorded to study the corrosion resistance properties and some representative parameters were calculated (Table S1). The corrosion inhibition efficiency (η) was calculated, which gradually decreased with immersion time due to the increase in the coating capacitance caused by water uptake. It is noticed that the electrode with intact superhydrophobic coating exhibits a significantly high natural corrosion potential Ecorr (0.182 V/ SCE) at the beginning of the immersion. This should be attributed to an air film that was formed as a barrier at the interface of the electrode surface and the surrounding corrosive environment. The corrosion potential of the bare tinplate (−0.587 V/SCE) was lower than all the superhydrophobic sample, which indicates that the sample with superhydrophobic coating has higher anticorrosion tendency. The bare tinplate exhibits active dissolution in the anodic region with a much higher anodic current density (1 × 10−4.461 A cm−2) than that for the sample coated with superhydrophobic coating. It is clear that the superhydrophobic coatings can effectively prevent acidic media to penetrate into the superhydrophobic surface for 240 h (presented superior corrosion resistance to acid for 240 h), which suggests that the superhydrophobic coatings are chemically stable in the presence of water. As wellknown, the deterioration of superhydrophobicity is commonly governed by the desorption of hydrophobic molecules under the persistent attack of water or other corrosive media.48 The high chemical stability of our present superhydrophobic coating suggests the strong chemical bond formed between hydrophobic molecules and rough surface. Furthermore, there is no significant change in the morphologies after anticorrosion test (Figure S11), implying a good mechanical stability. Therefore, the high chemical and mechanical stabilities afford the composite superhydrophobic coatings with excellent anticorrosion ability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12820. Supplementary SEM images, AFM images, FT-IR spectrum, TGA curves, schematics, and electrochemical corrosion measurements (PDF) Movie S1, water dropping tests (AVI) Movie S2, knife-scratch test (AVI) Movie S3, sandpaper abrasion test (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 1082317113. Fax: +86 1082317113. ORCID

Haining Chen: 0000-0002-7543-3674 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding of this research by National Science Foundation of China (Grant No. 21603010) is gratefully acknowledged. REFERENCES

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