Fabrication of Highly Transparent Superhydrophobic Coatings from

Apr 26, 2012 - The largest water contact angle (WCA) of coating reached as high as. 156° with a sliding ... concentration of APTS and the withdrawing...
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Fabrication of Highly Transparent Superhydrophobic Coatings from Hollow Silica Nanoparticles Ligang Xu†,‡ and Junhui He*,† †

Functional Nanomaterials Laboratory 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 ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: We herein report a simple and effective method to fabricate excellent transparent superhydrophobic coatings. 3-Aminopropytriethoxysilane (APTS)-modified hollow silica nanoparticle sols were dip-coated on slide glasses, followed by thermal annealing and chemical vapor deposition with 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (POTS). The largest water contact angle (WCA) of coating reached as high as 156° with a sliding angle (SA) of ≤2° and a maximum transmittance of 83.7%. The highest transmittance of coated slide glass reached as high as 92% with a WCA of 146° and an SA of ≤6°. A coating simultaneously showing both good transparency (90.2%) and superhydrophobicity (WCA: 150°, SA: 4°) was achieved through regulating the concentration of APTS and the withdrawing speed of dip-coating. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were used to observe the morphology and structure of nanoparticles and coating surfaces. Optical properties were characterized by a UV−visible spectrophotometer. Surface wettability was studied by a contact angle/interface system. The effects of APTS concentration and the withdrawing speed of dip-coating were also discussed on the basis of experimental observations.

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transparency decreases.32−36 Hollow silica nanoparticles (HSNs) with lower refractive index than solid silica nanoparticles were previously used to fabricate highly transparent coatings on poly(methyl methacrylate) (PMMA) and glass substrates via relatively time-consuming layer-by-layer assembly.37,38 Very recently, Deng et al.39 prepared porous silica capsules using polystyrene spheres as a template, and fabricated transparent, thermally and mechanically robust superhydrophobic surfaces with these porous silica capsules. Such a strategy was also previously employed by He’s group to increase the roughness of coatings and enhance the hydrophilicity to superhydrophilicity.40These approaches need, however, presynthesized polystyrene spheres, which makes them less attractive for large-scale applications. Thus it is currently still a technical challenge toward a large scale fabrication of coatings with both superhydrophobicity and transparency by a cost-effective method. As a result, the practical application of superhydrophobic coatings has so far been limited on optically transparent materials (such as solar cell glass panels). Herein, we report a simple, versatile strategy for creating a transparent superhydrophobic surface via sol−gel dip-coating of 3-aminopropytriethoxysilane (APTS)-modified HSNs on glass

INTRODUCTION Superhydrophobic surfaces, showing a water contact angle (WCA) higher than 150° and a sliding angle (SA) lower than 10°,1−3 have numerous applications in self-cleaning,1 anticorrosion,4 improving blood compatibility,5 and so forth.6−9 In nature, many plants and animals utilize superhydrophobic surfaces for special purposes, such as lotus leaves and water striders.1,10,11 Generally, the wetting behavior of a surface is dependent on both its surface chemistry (i.e., surface energy) and surface topography (i.e., surface roughness). A dual-scale structure has been revealed to be an essential feature in generating superhydrophobic coatings and especially important for obtaining surfaces of low water SAs.12−18 Fascinated by discoveries pertaining to natural surfaces, varied techniques were developed to fabricate artificial superhydrophobic surfaces, such as colloidal self-assembly,19,20 wet chemical etching,21 inorganic or organic template method,22,23 electrospinning,24,25 and phase separation.26 Nanoparticles are frequently used to control the surface roughness27−30 because they are readily available commercially and can also be synthesized with uniform size and tunable surface chemistry by the Stöber method31 followed by postmodification. In addition, fabrication of superhydrophobic coatings by spin-coating or dip-coating is straightforward, effective, and inexpensive. The roughness (and thus the hydrophobicity) and transparency are well-known to be competitive properties. When the roughness increases, the hydrophobicity increases, whereas the © XXXX American Chemical Society

Received: February 20, 2012 Revised: April 22, 2012

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Figure 1. SEM (a) and TEM (b) images and size distribution histogram (c) of HSNs prepared using 0.40 g PAA and 1.80 mL TEOS. Characterization. Transmission spectra in the wavelength range of 200−800 nm were recorded using a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co.). The as-prepared coatings were examined by scanning electron microscopy (SEM) on a Hitachi S-4300 scanning electron microscope operated at 10 kV. The specimens were coated with a layer of gold by ion sputtering before SEM observations. For transmission electron microscopy (TEM) observations, the sols were diluted by ethanol without violent stirring or vibration to avoid possible damage to the colloidal aggregation, and added onto carbon-coated copper grids. After drying at 60 °C overnight, they were observed on a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 150 kV. WCAs of coating surfaces were measured at ambient temperature on a JC2000C contact angle/interface system (Shanghai Zhongchen Digital Technique Apparatus Co.), the angle precision of which is ±0.5°. Water droplets of 4 μL were dropped carefully onto the coating surfaces. The roughness and morphology of the surfaces were characterized by atomic force microscopy (AFM) on an MM8-SYS scanning probe microscope (Bruker AXR).

substrates, followed by thermal annealing and chemical vapor deposition with 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (POTS). The largest WCA of the coating reached as high as 156° with an SA of ≤2° and a maximum transmittance of 83.7%. The highest transmittance of the coated slide glass reached as high as 92% with a WCA of 146° and an SA of ≤6°. A coating simultaneously showing both good transparency (90.2%) and superhydrophobicity (WCA: 150°, SA: 4°) was achieved through regulating the concentration of APTS and the withdrawing speed of dip-coating.

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EXPERIMENTAL SECTION

Materials. Tetraethyl orthosilicate (TEOS, 98+%), APTS (98%), and POTS (97%) were obtained from Alfa Aesar. Aqueous ammonia (25%), acetic acid (98%), and absolute ethanol (99.5%) were purchased from Beihua Fine Chemicals. Poly(acrylic acid) (PAA, 30 wt % in water, Mw = 5000) was purchased from Shandong Heli water treatment company. Ultrapure water with a resistivity higher than 18.2 MΩ·cm was used in all experiments, and was obtained from a threestage Millipore Mill-Q Plus 185 purification system (Academic) Preparation of HSN Sols. In a typical procedure, 0.40 g of PAA dissolved in 4.5 mL aqueous ammonia was mixed with 90 mL of absolute ethanol, followed by injection of five TEOS aliquots totaling 1.80 mL at a time interval of 1 h under vigorous magnetic stirring at room temperature. After 10 h, 45 nm HSNs formed. Ammonia (NH3) was removed by stirring the sol in a ventilating cabinet to improve its stability. The solid concentration of the target HSN sol was estimated to be 0.8 wt %. APTS was used to aggregate HSNs in the sol.35 Different amounts of APTS were added dropwise, and then a certain amount of acetic acid was used to adjust the pH value to 4. The mixed sol was stirred at 60 °C for 30−60 min. Then the sol was cooled in ice water immediately and redispersed by ultrasonication for 20 min. Dip-Coating and Hydrophobic Modification. In a typical procedure, glass substrate was sonicated in deionized water for at least 15 min, and then treated by oxygen plasma (84 W, 5 min) under an oxygen flow of 800 mL min−1. The cleaned substrate was immersed into the sol for 5 min before withdrawal for the first dip-coating and for 5 s before withdrawal for four subsequent dip-coatings, i.e., each substrate was subject to a total of five dip-coatings. The withdrawing speed was kept at 3−12 cm/min. After each dip-coating, the substrate was dried for 2 min at room temperature (25 °C). Then the coating was heated at 120 °C for 30 min to remove residual solvent and solidify the coating. The slide glass with coating was then placed in a Teflon container, and sealed by a stainless steel autoclave on the bottom of which was dispensed a few droplets (10−20 μL) of POTS. There was no direct contact between the substrate and the POTS droplets. The autoclave was put in an oven at 120 °C for 2 h to enable the vapor of POTS to react with the hydroxyl groups on the coating surface. Finally, the autoclave was opened and placed in an oven at 150 °C for an additional 1.5 h to volatilize unreacted POTS molecules on the coating.

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RESULTS AND DISCUSSION Synthesis of HSNs. There are a number of synthetic methods41,42 that have been used to synthesize HSNs. In this

Figure 2. TEM images of sols prepared with varied APTS concentrations without ultrasonication: (a) 0.3 wt %, (b) 0.5 wt %, (c) 0.7 wt %, and (d) 0.9 wt %. The black arrows point to the polymer matrix. B

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1 shows HSNs synthesized with 0.40 g PAA/1.8 mL TEOS. The mean diameter and shell thickness were measured to be ca. 45 ± 8.62 nm and ca. 12 ± 2.70 nm. HSN Aggregation by APTS and Redispersion. APTS was added in the hollow silica sols to aggregate the HSNs, and the possible chemical reactions are shown in Figure S1 (Supporting Information). When APTS meets water contained in the HSNs sol, the ethoxy groups of APTS hydrolyze immediately, generating hydroxyl groups in each molecule. Then, dehydration or dealcohol polycondensation may occur between the hydrolyzed APTS molecules and the hydroxyl groups on HSNs colloid particles or between the hydrolyzed APTS molecules themselves. As a result, the HSNs can be covered and aggregated by the formed poly-APTS. It was found that the APTS concentration was an important parameter that could affect the hydrolyzation and polycondensation process. Different amounts of APTS were added in the HSNs sols, and the effect of APTS was investigated by TEM on the aggregation of HSNs. The morphologies obtained by using varied APTS concentrations are shown in Figure 2 and Scheme 1. Figure 2a shows the product obtained when a small amount of APTS (0.30 wt %) was added into the hollow silica sol. HSNs showed a clear trend to aggregate with each other. When the APTS concentration increased to 0.5 wt %, the HSNs were more aggregated (Figure 2b). Some HSNs were connected by polymer net. When the APTS concentration increased to 0.7 wt %, most of the HSNs were not only aggregated but also

Scheme 1. Schematic Illustration of Sols Prepared with Varied APTS Concentrations of (a) 0.3 wt %, (b) 0.5 wt %, (c) 0.7 wt % and (d) 0.9 wt %, Respectivelya

a

Blue: HSN; green: APTS network.

article we used a facile one-step solution-based synthetic route to produce HSNs.40 The nanoparticles size and shell thickness could be regulated by changing the ratio of PAA/TEOS. Figure

Figure 3. SEM images of coatings prepared using sols of vaired APTS concentrations of (a) 0.30 wt %, (d) 0.70 wt %, and (g) 0.90 wt %. Panels b, e, and h are the corresponding manified images of a, d, and g, respectively. Panels c, f, and i are the corresponding AFM images of a, d, and g, respectively. C

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the HSNs and the formed poly-APTS could also bind to the substrate by van der Waals forces. Effect of APTS Concentration on Coating Morphology and Properties. When the withdrawing speed of dip-coating was kept at 3 cm/min, the sols containing varied concentrations of APTS were used to fabricate different coatings. The surface morphologies of the coatings were examined by SEM and AFM, and the results are shown in Figure 3. Figure 3a−c shows the surface of the coating prepared from the sol containing 0.30 wt % APTS. It can be seen that the surface was very smooth, and the root-mean square (rms) was only 26 nm. A comparison of Figure 3a,d,g shows that the surface changes from smooth to striated gradually, and the surface roughness increases significantly with increasing the APTS concentration. The rms roughness was estimated on an area of 10 × 10 μm2 (Figure 3c,f,i). It increases from 26 to 58 nm to 68 nm when the APTS concentration increases from 0.30 wt % to 0.70 wt % to 0.90 wt %. AFM images show that the number of bumps increases with increase of the APTS concentration, and the height of bumps was strongly affected by the size of aggregates. Thus, the sol prepared using a higher APTS concentration could be used to fabricate a coarser surface to meet the morphological requirement of superhydrophobic coatings. The water-repellent properties of coatings after POTS modification were researched by measuring the static WCAs of 4.0 μL water droplets on these coatings. For the APTS concentrations of 0.30, 0.50, 0.70, and 0.90 wt %, the WCAs were measured to be 141 ± 2, 146 ± 2, 153 ± 2, and 156 ± 2°, respectively. The digital images of water droplets on slide glasses with varied coatings are shown in Figure 4a−d. When the APTS concentration was equal to or higher than 0.70 wt %, the WCAs were larger than 153°. The SAs were very low (≤2°). Water droplets of 4 μL rolled off the coated slide glasses easily and quickly, which are shown in Figure 4e−h. The sliding speed of the water droplet was calculated to be 56 mm/s from panel f to g. The sliding speed of the water droplet is probably related to the adhesion forces of the surface, and it may increase with a decrease of the adhesion forces. Water droplets of 15 μL also rolled off the surfaces easily and quickly. Therefore, these POTS-treated coatings exhibit superhydrophobicity and low SA, which bestow a self-cleaning function like that of lotus leaves. It has been reported that void fraction and surface roughness would significantly affect the wettability of a surface with a water droplet. Wenzel’s43 and Cassie−Baxter’s44 models are two typical empirical models to explain the relationship between superhydrophobicity and surface structure. When coatings were prepared from the sols with a 0.7 wt % or even higher APTS concentration, the coating surfaces were composed of nanoparticles and many bumps. When a water droplet was placed on these coatings, these nanoparticles can discontinue the contact area between water and the surface, as in the case of Cassie’s model. A high WCA was thus observed, and the adhesion force between the substrate and water droplet was very small and could be neglected. The roughness (and thus the hydrophobicity) and transparency are competitive properties. When the roughness increases, the hydrophobicity increases, whereas the transparency decreases. Therefore, precise control over the roughness is required to satisfy both properties. Since the visible light wavelength range is 400−750 nm, the surface roughness must be controlled to be less than 100 nm in order to minimize light scattering and achieve good transparency.45 The digital images

Figure 4. Digital images of water droplets on surfaces prepared from sols of varied APTS concentrations of (a) 0.30 wt %, (b) 0.50 wt %, (c) 0.70 wt %, and (d) 0.90 wt %, respectively, and (e-h) images of the rolling process of water droplet on the surface coated from the sol containing 0.70 wt % APTS and tilted by an angle of 2°. The withdrawing speed was kept at 3 cm/min. The average sliding speed of water droplet was estimated to be 56 mm/s from panel f to g.

connected by polymer net (Figure 2c). When the APTS concentration increased to 0.9 wt %, almost all of the HSNs were connected by polymer net and a denser polymer net formed (Figure 2d). The effect of the APTS concentration on the morphology of the sol is also schematically illustrated in Scheme 1. Clearly, the HSNs were aggregated and connected by polymer net as the APTS concentration increased. All of the sols were redispersed by ultrasonication for 20 min. The redispersed sols were used for fabrication of coatings on glass substrates by a simple dip-coating method. The hydroxy groups of APTS and HSNs are easy to react with the hydroxy groups of the glass substrate under acidic conditions, and amino groups of the formed poly-APTS and APTS-modified HSNs could bind to the substrate by hydrogen bonding. Meanwhile D

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Figure 5. Digital images of glass substrates with superhydrophobic coatings prepared from sols containing (a) 0.70 wt % APTS and (b) 0.90 wt % APTS, respectively. (c) Transmission spectra of coatings on glass substrates: (0) control slide glass and (1−4) slide glasses dip-coated from the sols containing 0.30 wt %, 0.50 wt %, 0.70 wt %, 0.90 wt % APTS, respectively. The withdrawing speed was kept at 3 cm/min.

spectrum of coated glass showed a maximum transmittance of 91.8%, slightly higher than that (91.3%) of an uncoated slide glass. When the APTS concentration increased, the transmittance of coated glass decreased gradually but the surface roughness increased gradually. As a result, light scattering increased, leading to lowered transparency. The coating prepared by the sol containing 0.70 wt % APTS showed both superhydrophobicity and good transparency, the WCA was as high as 153°, and the highest transmittance was 87.8%. The mechanical stability is important for application of the transparent superhydrophobic coatings. Thus a pencil hardness test was carried out to evaluate the mechanical stability of the coatings. The results showed that the coating well stood for a 1 H pencil test. When a 2H pencil was used, however, nearly 70% of the coating was scratched away (Figure S2). Thus, the coating had a pencil hardness between 1H and 2H. Effect of the Withdrawing Speed of Dip-Coating on Coating Morphology and Properties. When the APTS concentration was 0.70 wt % and the withdrawing speed of dipcoating was kept 3 cm/min, the WCA of coating was as high as 153° (Figure 4c), and the highest transmittance was 87.8%. In order to increase the transmittance of the coating, the sol containing 0.70 wt % APTS was used to fabricate coatings of high transparency and superhydrophobicity by varying the withdrawing speed. The transmittance and WCA of prepared coatings are shown in Figures 6 and 7. It can be seen that the transmittance of coating increased, while the WCA decreased with increase of the withdrawing speed. When the withdrawing speed was kept at 9 cm/min, the coating showed both good transparency (90.2%) and superhydrophobicity (WCA: 150°, SA: 4°). However, when the withdrawing speed increased to 12 cm/min, the WCA decreased to 146° and the SA increased to 6°, but the maximum transmittance reached as high as 92.0%, showing enhanced light transmission (Table 1). The structures of coatings fabricated by varied withdrawing speeds of dip-coating were examined by SEM and AFM, and the results are shown in Figure 8. When the withdrawing speed was kept at 5 cm/min, the surface appeared striated, and the surface was very rough. However, when the withdraw speed increased to 9 cm/min or 12 cm/min, the surface appeared smoother. From Figure 8c,f,i, it could also be seen clearly that the surface changes from striated to smooth gradually, and the rms roughness decreases from 52 to 42 nm to 35 nm when the withdrawing speed increases from 5 cm/min to 9 cm/min to 12 cm/min. These experimental results show that the nanoparticle density might decrease on the substrate with increase of the withdrawing speed, which makes the surface gradually change

Figure 6. Transmission spectra of coatings on glass substrates dipcoated from the sol containing 0.70 wt % APTS using varied withdrawing speeds of (a) 3 cm/min, (b) 5 cm/min, (c) 9 cm/min, and (d) 12 cm/min, respectively.

Figure 7. Digital images of water droplets on surfaces dip-coated from the sol containing 0.70 wt % APTS using varied withdrawing speeds of (a) 5 cm/min, (b) 9 cm/min, and (c) 12 cm/min, respectively.

Table 1. Maximum Transmittances and Wetting Properties of Coatings Prepared from the Sol Containing 0.70 wt % Using Varied Withdrawing Speeds rms roughness contact angle sliding angle maximum transmittance

3 cm/min

5 cm/min

58 nm 153°