Substrate-Versatile Approach to Robust Antireflective and

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Substrate-Versatile Approach to Robust Antireflective and Superhydrophobic Coatings with Excellent Self-Cleaning Property in Varied Environments Tingting Ren†,‡ and Junhui He*,† †

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Robust antireflective and superhydrophobic coatings are highly desired in wide applications, such as optical devices, solar cell panels, architectural and automotive glasses, lab-on chip systems, and windows for electronic devices. Meanwhile, simple, low-cost, and substrate-versatile fabrication is also essential toward real applications of such coatings. Herein, we developed a substrate-versatile strategy to fabricate robust antireflective and superhydrophobic coatings with excellent selfcleaning property in varied environments, including air and oil and after oil contamination. A mixed ethanol suspension, which consists of 1H,1H,2H,2H-perfluorooctyltriethoxysilane modified dual-sized silica nanoparticles and acid-catalyzed silica precursor, was first synthesized. The acid-catalyzed silica precursor could help to form a highly cross-linked silica network by connecting the silica nanoparticles, thus significantly enhancing the robustness of coatings. The as-prepared coatings were able to withstand a water drop impact test, sand abrasion test, tape adhesion test, and knife and pencil scratching tests. More importantly, it was also found that the wettability and self-cleaning property of coatings after oil contamination were surprisingly different from those in air and oil. These observations are explainable by the alteration of interface; i.e., the alteration of interface has significant effects on the functional properties of coatings. Additionally, the mixed suspension could be sprayed onto various hard and soft substrates including glass, polyethylene terephthalate (PET), polycarbonate (PC), and poly(methyl methacrylate) (PMMA), opening up a feasible route toward varied practical applications in solar cell panels, optical devices, architectural and automotive glasses, droplet manipulators, and fluid control. KEYWORDS: substrate-versatile, hydrophobic silica sol, antireflection, superhydrophobicity, self-cleaning, robustness

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INTRODUCTION

Recently, researchers have paid increasing attention to superhydrophobic surfaces with antireflective property, which would expand their applications to optical devices, solar cell panels, architectural and automotive glasses, and windows for electronic devices.14,16−18 However, antireflective and superhydrophobic properties are intrinsically competitive because of light scattering that is caused by increased roughness.19

Superhydrophobic surfaces with water contact angle (WCA) larger than 150° and rolling angle (RA) less than 10° have been attracting significant attention due to their potential applications in self-cleaning,1−3 anti-icing,4−6 lab-on chip system,7 oil− water separation,8−10 and anticorrosion.11,12 They could be achieved by the rational combination of micro/nanosized surface roughness and low surface energy. Many chemical and physical methods that could generate superhydrophobic surfaces have been developed by mimicking nature through years of extensive efforts.10,13−15 © XXXX American Chemical Society

Received: July 27, 2017 Accepted: September 11, 2017

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DOI: 10.1021/acsami.7b11116 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. TEM images of as-prepared hydrophobic silica sol at low (a) and high (b) magnification.

impact, sand abrasion, and knife scratch. Additionally, the coatings could also endure the 5H pencil scratching test and score at the 5A level in the tape adhesion test, indicating their superior adhesion-to-substrate strength.

Moreover, high mechanical robustness, adhesion strength, and environmental stability of such surfaces are indispensable for practical applications. Unfortunately, when such surfaces are knife scratched, abraded, under oil, or contaminated with oil, they generally tend to lose their superhydrophobicity or high transmittance.3,20 Efforts have very recently been made to meet such challenges. For example, by using PDMS interlayer and hydrophobic silica nanoparticles suspension, Wang et al.20 reported a transparent superhydrophobic coating that could endure sandpaper abrasion, knife scratch, and strong acid/base attack. Liu et al.21 produced a healable transparent and superhydrophobic coating based on a porous silica nanotube network filled with perfluorooctyl acid; the coating remained superhydrophobic after sand abrasion test. Wong et al.22 developed an ultrarobust and transparent superhydrophobic surface which was made of sequential spraying of a novel polyurethane-acrylic colloidal suspension and a fluoro-functionalized SiO2 nanoparticle solution. The obtained surface showed outstanding abrasion resistance and excellent chemical- and photostability. Meanwhile, some other transparent superhydrophobic coatings also showed excellent mechanical robustness but required a complex or time-consuming fabrication process and specific substrates.23−25 Therefore, the fabrication of robust antireflective and superhydrophobic coatings using a substrate-versatile and simple approach is still a big challenge. Very recently, Lu et al. created a robust superhydrophobic surface and investigated its self-cleaning property when immersed in oil.3 In fact, few studies have so far addressed the superhydrophobic property as well as antireflective property in oil or when contaminated by oil. Because of the lower surface tension of oil than water, oil would easily penetrate into superhydrophobic surfaces, resulting in loss of their water repellency.20 Surfaces that still show superhydrophobicity in oil or when contaminated by oil would be promising for many applications, such as sunlight harvest, solar energy conversion, optical devices, miniature reaction systems,26 oil purification,27 and bearings and gears lubrication.10 Here, we developed an easy and versatile method to prepare substrate-versatile, robust, antireflective, and superhydrophobic coatings using a hydrophobic silica nanoparticle suspension and acid-catalyzed silica precursor at ambient atmosphere. The coatings exhibit high broadband transmittance, superhydrophobicity, and excellent self-cleaning property even in oil or after oil contamination. Most notably, the coatings feature outstanding mechanical robustness and resistance to water drop

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EXPERIMENTAL SECTION

Materials. Tetraethylorthosilicate (TEOS, 99+%) was purchased from Alfa Aesar. 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (CF3(CF2)5CH2CH2Si(OCH3)3, POTS, 97%) was obtained from Jinan Langhua Chemical Company. Aqueous ammonia (25%) and absolute ethanol (99.5%) were purchased from Beihua Fine Chemicals. Glass substrates were purchased from Shanghai Xigema Optical and Electric Company. Ultrapure water with a resistivity higher than 18.2 MΩ·cm in all experiments was obtained from a threestage Millipore Mill-Q Plus 185 purification system (Academic). Synthesis of Silica Nanoparticles with Diameter of 20 and 45 nm. 100 mL of absolute ethanol (99.5%) and 5 mL of aqueous ammonia (25%) were mixed with stirring in a round-bottomed flask. The flask was then placed in a 60 and 30 °C water bath, respectively, and 3 mL of TEOS was added when the temperature became stable. After stirring for 10 h, the suspensions of silica nanoparticles of ca. 20 and 45 nm were obtained, respectively, and the 20 nm silica sol was stirred in a fume hood to remove ammonia to form a stable sol. Preparation of Hydrophobic Silica Sol. In a typical procedure, 1−10 mL of POTS was added to 90 mL of absolute ethanol, followed by stirring at ambient temperature for 5 h. Then, certain amounts of 20 and 45 nm silica nanoparticles were added, followed by stirring for an additional 10 h to form a suspension. Finally, the obtained suspension was kept stirring in a fume hood to remove ammonia (pH = 7). Preparation of Substrate-Versatile, Robust, Antireflective, and Superhydrophobic Coatings. First, a glass substrate was washed with ultrapure water and cleaned by oxygen plasma for 5 min. Then, the suspension of 20 nm silica nanoparticles was sprayed once on both sides of the cleaned glass substrate followed by drying at room temperature. Finally, the mixture of as-prepared hydrophobic silica sol and acid-catalyzed silica sol−gel precursor was sprayed once onto one side of the as-obtained glass substrate followed by drying at room temperature. The spray distance was about 15 cm; the pressure was 0.78 MPa, and the spray speed was 3 cm/s. Characterization. For TEM observation, powder samples were placed on carbon-coated copper grids and observed on a JEOL JEM2100F transmission electron microscope at an acceleration voltage of 200 kV. Freshly fabricated coatings were examined by scanning electron microscopy (SEM) on a Hitachi S-4800 scanning electron microscope operated at 5 kV. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analyses were carried out on a Varian Excalibur 3100 spectrometer. Water contact angles on the fabricated coatings were measured at ambient temperature on a Kino SL200B/K automatic contact angle meter. Transmission spectra B


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ACS Applied Materials & Interfaces in the wavelength range of 300−2500 nm were recorded using a Varian Cary 5000 UV/vis-NIR spectrophotometer.

Table 1. Compositions of Coatings Prepared Using Different HSS/ASP Ratios

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RESULTS AND DISCUSSION Morphology and Structure of As-Prepared Hydrophobic Silica Sol and Coatings. The hydrophobic silica sol was synthesized by mixing two different SiO2 nanoparticles of ∼20 and ∼45 nm, respectively, in size in an ethanol solution of POTS. Before the preparation of hydrophobic silica sol, the 20 nm silica sol was first stirred to remove ammonia in a fume hood. With the removal of ammonia, the rate of condensation of SiO2 nanoparticles was much faster than the hydrolysis of residual TEOS. When the pH of the 20 nm silica sol became about 7, the 20 nm SiO2 nanoparticles had partly formed nanochains (as shown in Figure 1). Then, certain amounts of 20 and 45 nm SiO2 nanoparticles were added into the ethanol solution of POTS, followed by vigorously stirring for 10 h. Finally, this mixture suspension was kept stirring in a fume hood to remove ammonia, which was achieved by the 45 nm silica sol. Meanwhile, this process further facilitated the assembly of 20 nm SiO2 nanoparticles completely into nanochains and finally combined the nanochains with 45 nm SiO2 to form a compact three-dimensional network. Figure 1 shows the TEM images of the hydrophobic silica sol, demonstrating the dual-scale nature of SiO2 nanoparticles and the chain-like structure of as-made hydrophobic silica sol. FTIR spectrum of the hydrophobic silica sol is shown in Figure S1. The peak at 972 cm−1 corresponds to the C−F group from POTS, indicating that the as-prepared silica nanoparticles had been successfully modified with POTS. Furthermore, the hydrophobic silica sol was stable without any precipitation for more than 3 months. The procedure of fabrication of substrate-versatile, robust, antireflective, and superhydrophobic coatings with excellent self-cleaning property is shown in Figure 2. A glass substrate

coatings

hydrophobic silica sol/mL

TEOS/μL

HCL/μL

A B C

3

50 75 100

10 15 20

roughness and the SiO2 nanoparticles created nanosized roughness (Figure 3a,b). Meanwhile, the SiO2 nanoparticles had been modified with low surface energy substance POTS. The dual-sized surface roughness, combined with the low surface energy of the coatings, thus lead to superhydrophobicity. As the HSS/ASP ratios decreased, the coatings tended to be smooth. It could be seen that spherical nanoparticles were connected to each other by acid-catalyzed silica precursor and densely packed on the substrate. The acid-catalyzed silica could be found in the SEM image of coating C (Figure 3c, red circle). Figure 3d shows a cross-sectional SEM image of coating B, where the coating thickness was estimated to be ca. 450 nm. The RMS roughnesses of coatings A, B, and C were 110, 85.7, and 70.8 nm, respectively, on an area of 5 μm × 5 μm (Figure S3), indicating the coatings became smoother with a decrease of the HSS/ASP ratio and agreeing well with the SEM observations. Wettability and Optical Property of As-Prepared Coatings. Coatings A, B, and C on the coated glass substrates displayed antireflective and superhydrophobic properties simultaneously. As shown in Figure 4a, with a decrease of the HSS/ASP ratio, the water contact angles (WCAs) of coated substrate decrease from 166° to 162° to 124°. The rolling angles (RAs) of coatings A and B were all smaller than 5°, indicating coatings A and B showed superhydrophobicity. However, coating C did not show superhydrophobicity, which was consistent with its smooth morphology and low roughness. The low WCA and high water droplet adhesion indicated that water droplet penetrated into the nanopores of coating C as the Wenzel state describes. In contrast, because of the low surface energy and the hierarchical structure formed by dual-sized silica nanoparticles, coatings A and B demonstrated good superhydrophobicity and nearly spherical water droplets just stood on the coated substrates as the Cassie-Baxter state describes. Superhydrophobicity together with antireflection would provide coatings with favorable applications. The above coatings did exhibit antireflection. As evidenced by UV/visNIR transmission spectra (Figure 4b), the transmittance of coated glass substrates was gradually lowered with a decrease of the HSS/ASP ratio because of the decreasing porosity of asprepared coatings. However, the average transmittance of glass substrates coated with coatings A (93.5%) and B (93.0%) increased by more than 5% as compared to that of bare glass substrate (88.8%) in the wavelength range of 300−2500 nm. The maximum transmittance also reached as high as 96.7% and 96.2%, as compared to 91.4% of bare glass substrate. Due to the closely packed state and less void space of the coating C surface, the transmittance of coating C on the coated substrate (95.2%) sharply decreased but still significantly higher than bare glass substrate. Antireflective coatings could improve transmittance up to 97−99% according to the literature.28−30 However, the transmittance of coated glasses is also dependent on the transmittance of bare glasses (e.g., slide glass, K9 glass). Thus, the transmittance enhancement would be more appropriate to assess the role of antireflective coatings. In the

Figure 2. Schematic fabrication of substrate-versatile robust antireflective and superhydrophobic coatings with excellent selfcleaning property.

was sprayed first with 20 nm SiO2 nanoparticles and then with the mixture of hydrophobic silica sol and acid-catalyzed silica precursor to achieve antireflection and superhydrophobicity. Coatings were obtained by changing the proportion of hydrophobic silica sol (abbreviated as HSS) and acid-catalyzed silica precursor (abbreviated as ASP) and were named as coatings A, B, and C, respectively (Table 1). When the bare glass was first sprayed with 20 nm silica nanoparticles, a uniform layer of nanoparticles formed on the surface of glass due to quick volatilization of ethanol, as shown in Figure S2. After the mixture of hydrophobic silica sol and acid-catalyzed silica precursor was spray-coated, the coated surfaces possessed micro−nano dual-sized surface roughness, in which the numerous void spaces among nanoparticles created microsized C


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Figure 3. Top view SEM images of (a) coating A, (b) coating B, and (c) coating C. (d) Cross-sectional SEM image of coating B.

Figure 4. (a) Water contact angles (WCAs) and rolling angles (RAs) of as-prepared coatings on glass substrates. (b) Transmission spectra of coated glass substrates.

In this work, we studied the WCAs and self-cleaning properties of coated glasses as compared to bare glass under three different conditions: (1) in air, (2) in oil, and (3) contaminated by oil. The as-prepared coating was infused with oil to simulate the oil layer-contaminated condition, and the oil formed a homogeneous thin layer on the surface of coated glass. Because the surface roughness of as-prepared coating was only 85.7 nm, the thickness of the oil layer (estimated as 27 μm) would be much larger than the surface roughness of the coating. As shown in Figures 5 and S4, bare glass all showed hydrophilicity with low WCAs under three different conditions. In contrast, the coated glass demonstrated superhydrophobicity in both air and oil but lost its superhydrophobicity after being contaminated by various oils (Figures 5 and S4). Moreover, we also investigated the self-cleaning property of coated glass under three different conditions. As shown in Figure S5 and Movie S1, dyed water droplets quickly rolled off the coated

current work, the as-prepared coated glass could increase the transmittance by over 5% in the wavelength range of 300−2500 nm as compared to bare glass, which is similar to the previous study.28 Self-Cleaning Properties of Coatings. Superhydrophobic coatings are prone to lose the self-cleaning property because of immersion in oil or contamination by oil in real applications,10 which has become a recent focus in the area of self-cleaning coatings. For example, Wang et al. developed a transparent abrasion-resistant superhydrophobic coating with self-cleaning function in either an air or oil environment.20 Chen et al. fabricated robust superhydrophobic coatings by spray-coating “Paint + Adhesive”, and the coatings showed promising applications in oil−water separation and self-cleaning after oil contamination.10 However, more efforts are needed to understand the underlying mechanisms. D


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Figure 5. WCAs of bare glass (a−c) and coating B coated glass (d−f) under three different conditions. Captured images of water droplet motion on the tilted coated glass (g) and bare glass (h) after hexadecane contamination.

Figure 6. Self-cleaning test in oil. (a, b) Dyed water droplets rolled off the coated glass when immersed in oil. (c−e) Self-cleaning test at oil−solid− vapor interface. (f) The coated glass after the self-cleaning test.

oil/solid interface was replaced by the water/solid interface, and thus, water droplets would all be pinned on the surface of bare glass under three different conditions, leading to their difficulty of movement on bare glass. For the coated glass in air and oil, however, the hierarchically porous structure of coating provides space for storing air or oil, and the POTS modified SiO2 nanoparticles offer low surface energy. These two characteristics could effectively resist the penetration of water, thus presenting superhydrophobicity. Due to the hierarchically porous structure and the “air cushion” or “oil cushion”, the pinning of water droplets was significantly suppressed on the coated surface, leading to a rolling motion mode of water droplets. It is noted that the WCA of coated glass was much larger in oil than in air. Water droplets falling into oil would experience buoyancy force, which will help them shrink as a marble shape and result in a larger WCA in oil than in air. It is also noted from Movies S1 and S2 that water droplets rolled less quickly in oil than in air, because of higher resistance when water droplets rolled in oil. When the coated glass was contaminated by oil, the oil (low surface energy) had better affinity to the fluorinated surface than water,34 and water would not replace oil and contact the fluorinated surface. Thus, although falling water droplets would similarly expel the oil layer, they should eventually contact the oil layer only rather

glass in air, taking off dust (sand here). A similar phenomenon was observed when the coated glass was immersed in oil (hexadecane here), leaving the coated glass clean and transparent (Figure 6 and Movie S2). When the coated glass was contaminated by oil, however, the structure of coating was lubricated by oil, and the surface became similar to the slippery surfaces.31−33 As shown in Movie S3, water droplets slid on the hexadecane-contaminated coated glass, carrying the dust off its surface. Very interestingly, the water droplet motion on the tilted coated glass contaminated by various oils demonstrated a “rolling + sliding” motion mode (Figures 5g and S6). In contrast, water droplets just spread out slowly and stayed on the tilted bare glass (Figure 5h). The wettability and self-cleaning property of coatings after oil contamination are surprisingly different from those in air and oil, and the underlying mechanisms must be discussed. For bare glass in air and oil, water droplets (high surface energy) intended to contact the hydrophilic glass surface, and the low roughness and high surface energy of the bare glass surface resulted in the low WCAs. Upon oil contamination, the oil has poorer affinity to the bare glass surface than water.34 Falling water droplets, which had a diameter larger than the thickness of the oil layer, would expel the oil layer and contact the hydrophilic surface of the bare glass. Eventually, the original E


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Figure 7. Water contact mode on the coated glass (a−c) and bare glass (d−f) in air and oil and after oil contamination, respectively. (g−i) Selfcleaning processes on coated glass in air and oil and after oil contamination, respectively.

Figure 8. Mechanical robustness assessed by sand impact abrasion. (a) Sketch of the setup for the sand impact abrasion test. WCAs of the glass substrate with coating B before (b) and after (c) 80 g of sand impact abrasion from a 100 cm height. The 100−350 μm sized grains have a velocity of 15.9 km/h just before impingement. (d) Transmission spectra of the glass substrate with coating B before and after the sand impact abrasion test as compared to bare glass substrate. SEM images of coating B before (e) and after (f) the sand impact abrasion test.

than the fluorinated surface. The smoothness and the lubricating effect of the oil layer would allow the water droplets to spread out, leading to the observed low WCAs. When water droplets fell on the tilted coated glass, the water droplets would first contact the surface of the superhydrophobic coating under the oil layer because of gravity and momentum, resulting in an initial rolling motion. Subsequently, as the water droplets only contact the oil layer, eliminating the pinning effect, they slid on the oil-contaminated surface of the coated glass. It should be pointed out that water droplets demonstrated a marble shape in oil where the thickness of oil was much larger than the diameter (estimated as 120 μm) of water droplets, while they demonstrated a semispherical shape after oil contamination where the thickness of the oil layer was less than the diameter of water droplets. In the former case, water droplets were in a homogeneous medium, while they were in a heterogeneous medium (including air and oil) in the latter case. Figure 7 shows the water contact mode on the bare and coated glass under three different conditions, as well as the self-

cleaning process. Unlike the liquid/air/solid contact lines as the Wenzel and Cassie−Baxter states describe, the coated glass after oil contamination possesses a movable water/oil/air contact line when it interacts with a water droplet. The contact interface has significant effects on the properties of coatings. When bare glass is under three different conditions, the water/ solid interface leads to pinning of water droplets, and thus, bare glass shows hydrophilicity. When the coated glass is exposed in air and oil, the water/solid interface significantly reduced the pinning, leading to superhydrophobicity and excellent selfcleaning property of coatings. When the coated glass is contaminated by various oils, the water/oil interface eliminates the pinning of water droplets, resulting in their low WCAs and easy sliding. Therefore, the difference of wettability and selfcleaning property of the coated glass under three different conditions could all be explained by the alteration of the interface. Mechanical Robustness of Coatings. In order to reach the requirements of practical application, good mechanical F


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Figure 9. Demonstration of the knife-scratch test on the surface of coating B coated glass. Dyed water droplets easily rolled across the scratches (a− c), leaving a clean and transparent coated glass (d).

adopted to scratch the coating B coated substrate, and then, dyed water was dropped. As shown in Figure 9, water droplets rolled across the scratches on the surface, and the knifescratched coated glass substrate still remained dry and clean. Although the knife truly scratched some nanoparticles off the coating (Figure 9d), such partial damages did not significantly affect the excellent self-cleaning property. In other words, even if the coating could not withstand hard, sharp knife scratching structurally, it could endure the knife-scratch test functionally without losing its self-cleaning property. Such characteristics are extremely important for real applications. In this work, the pencil scratching test and tape adhesion test were also used to investigate the adhesion-to-substrate of asprepared coating. Figure S8 shows the surface morphology of coating B after the tape adhesion test. No damage was found on the coating even at the edge of the scratches according to the magnified SEM image. Thus, coating B scored 5A, which is the highest level in the ASTM D3359-93 tape adhesion test. Figure 10 shows SEM images of the coating B coated glass substrate before (a) and after 4H (b), 5H (c), and 6H (d) pencil scratching tests. According to the SEM images, coating B could withstand both 4H and 5H pencils scratches; merely some of surface nanoparticles were deformed. After the 6H pencil scratching test, coating B was partly scratched (the red dashed frame) but more than half of the coating remained on the surface. For comparison, SEM images of a control coating without using acid-catalyzed silica precursor as binder solution before and after the 4H pencil scratching test are shown in Figure S9. It could be seen that the coating was totally scratched and the glass substrate was completely exposed after the test, indicating its poor mechanical robustness and adhesion-to-substrate. The introduction of acid-catalyzed silica precursor, which acts as a polymer binder, leads to the formation of a highly cross-linked silica network, thus improving the adhesion property of the superhydrophobic coatings. Furthermore, the long-time durability of as-prepared coatings was also investigated. After ten months, the coating B coated glass still demonstrated antireflective property and super-

robustness of any multifunctional coatings is a prerequisite. However, due to the micro/nanoscale hierarchical structure, superhydrophobic coatings are often mechanically weak and easily lose their superhydrophobicity once invasively touched. Furthermore, the coatings are often readily abraded or removed by a piece of tape due to the weak adhesion between the superhydrophobic coating and substrate. It has been demonstrated that an acid-catalyzed silica sol, which results in linear silicate polymer chains, performs the function of polymer binder.35−37 In this work, the introduction of acid-catalyzed silica precursor led to the formation of highly cross-linked silica network and the connection between silica nanoparticles via firm Si−O−Si bridging chemical bonds, thus significantly enhancing the mechanical robustness of as-prepared coating. To evaluate the mechanical robustness of as-prepared antireflective and superhydrophobic coatings, the water drop impact test, sand abrasion test, and knife-scratch test were performed in this work. Figure S7 shows WCAs of coating B before and after the water drop impact test, which was intended to simulate the rainstorm situation.37 In the test, about 4500 water droplets (ca. 22 μL) were dropped from a height of ca. 100 cm above the coating B coated substrate. It could be seen that the WCA decreased from 162° to 160° after the test, indicating that the coating B coated substrate still retained superhydrophobicity and could endure the water drop impact test. Figure 8 shows the wettability, transmittance, and morphology of coating B coated glass after the sand abrasion test. The current test used a much larger initial height and heavier mass of sand than that used in some other work.38,39 The WCA decreased from 162° to 155°, and the transmittance also decreased by about 1% after the test; however, the coated substrate still kept its superhydrophobic and antireflective properties. Figure 8e,f shows SEM images of coating B before and after the test. The density of SiO2 nanoparticles reduced after the test, which well accounts for the slight decrease in superhydrophobicity and antireflection. However, the major coating still showed micro/nanoscale morphology, endowing the coating superhydrophobicity and antireflection. Additionally, the knife-scratch test was also performed. A knife was G


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substrates was 96.7%, 96.5%, 97.3%, and 97.5% and was all higher than those of bare substrates (91.4%, 92.5%, 92.9%, and 93.1%), respectively. Figure 11 further shows photos of dyed water droplets on the various coated substrates.

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CONCLUSION In summary, we have developed a substrate-versatile approach to the fabrication of robust antireflective and superhydrophobic coatings with excellent self-cleaning property in varied environments, including air and oil and after oil contamination, using hydrophobic dual-sized silica nanoparticles and acidcatalyzed silica precursor. The as-prepared coatings could pass the water drop impact test, sand abrasion test, and knife-scratch test and also showed good adhesion-to-substrate as proved by the pencil scratching test and tape adhesion test. The coated substrates showed superhydrophobicity not only in air but also in oil but lost superhydrophobicity after oil contamination. Water droplets rolled off the tilted coated substrate both in air and in oil, while sliding on the tilted coated substrate after oil contamination, all taking off dust. All the observed phenomena could be related to the alteration of liquid/solid interface. This novel, simple approach is applicable to various substrates toward different applications in solar cell panels, optical devices, architectural and automotive glasses, droplet manipulators, and fluid control, and the investigation of the wettability and selfcleaning property of superhydrophobic coatings in varied environments could help more deeply understand the structure−property relationship and find more applications of superhydrophobic coatings.

Figure 10. SEM images of coating B before (a) and after 4H (b), 5H (c), and 6H (d) pencil scratching tests, respectively.

hydrophobicity, as shown in Figure S10, demonstrating a good long-time durability. Applications of Coatings. Many methods used for fabricating antireflective and superhydrophobic coatings are either complex or time-consuming, such as chemical vapor deposition(CVD),30 layer-by-layer assembly (LbL assembly),40 lithography,41 and electrochemical methods.42 Among them, some require special expensive equipment and rigorous conditions. For example, CVD usually operates at high temperature and could only be applied to thermally stable substrates.43 It is not available to temperature-sensitive substrates which may degrade at high temperature, such as poly(ethylene terephthalate) (PET), polycarbonate (PC), poly(methyl methacrylate) (PMMA), and so on. The approach of the current work would, however, be adaptable to substrates of varied types or shapes. In fact, by using the versatile spraycoating technique at ambient atmosphere, we successfully fabricated robust antireflective and superhydrophobic coatings on various transparent rigid or flexible substrates including glass, PET, PC, and PMMA. Figure S11 shows the WCAs and transmission spectra of coated glass slide, PET, PC, and PMMA. The substrates can be modified into superhydrophobicity with 164 ± 1°, 158 ± 2°, 159 ± 2°, and 154 ± 1° WCAs, respectively. The maximum transmittance of these coated

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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.7b11116. ATR-FTIR spectra of as-prepared coatings; SEM images of 20 nm silica nanoparticles coated glass; 2D and 3D AFM images of as-prepared coated glasses; WCAs of bare glass and coated glass in varied environments; selfcleaning test in air; captured images of water droplet motion on tilted bare glass and coated glass when contaminated by oil; water droplet impact test of coating B; SEM images of coating B after tape abrasion test; pencil scratching test of the coating prepared without using acid-catalyzed silica precursor; WCAs and transmission spectra of coated glass, PMMA, PET, and PC substrates (PDF) Demonstration of self-cleaning property of coated glass in air (Movie S1) (AVI)

Figure 11. Applications of robust antireflective and superhydrophobic coating on a variety of different substrates. The pink and blue droplets had been dyed for easy visibility with rhodamine and copper sulfate, respectively. H


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Demonstration of self-cleaning property of coated glass in oil (Movie S2) (AVI) Demonstration of self-cleaning property of coated glass when contaminated by hexadecane (Movie S3) (AVI)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82543535. Fax: +86-10-82543535. E-mail: [email protected]. ORCID

Junhui He: 0000-0002-3309-9049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21571182, 21271177), the Science and Technology Commission of Beijing Municipality (No. Z151100003315018), the National Key Research and Development Program of China (2017YFA0207102), and a Chinese Academy of Sciences Grant (CXJJ-14-M38).

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