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One-step facile route to fabricate functionalized nano-silica and silicone sealant based transparent superhydrophobic coating
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One-step facile route to fabricate functionalized nano-silica and silicone sealant based transparent superhydrophobic coating ⁎
Zhenzhen Lu , Lijie Xu, Yang He, Jianting Zhou College of Civil Engineering, ChongQing JiaoTong University, Chongqing 400074, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent coatings Superhydrophobicity Silicon dioxide Silicone Composite coatings Dip coating Chemical stability Thermal stability
A highly transparent and stable superhydrophobic surface was prepared on substrates of different types including tile, concrete, wood, and ferroalloy via a facile and low-cost method using hydrophobic silica nanoparticles as building blocks for construction of micro/nano-structures and neutral silicone sealant as binder. The surface wettability, morphology, topography, and transmittance was characterized by means of contact angle measurement, scanning electron microscope, atomic force microscope, and ultraviolet-visible spectrophotometer respectively. All the as-prepared SiO2/Silicone composite coatings exhibited well water-repellency with a static water contact angle above 150° and a sliding angle below 10°. Besides, a highest average transmittance of 88.7% of the superhydrophobic coatings was obtained. The results of the performance testing showed that the superhydrophobic coatings possess good mechanical resistance to water drops, as well as excellent chemical/thermal stability and UV durability. Moreover, the method to fabricate superhydrophbic coatings on various substrates is facile and cost-effective, which provides a possibility of large-scale industrial applications.
1. Introduction
Combination of organic polymer and textured surface is thought to be one of the simple and high-efficiency approaches to fabricate superhydrophobic surface due to the full utilization of their advantages. For instance, Y. Lu et al. [15] prepared a robust superhydrophobic surface through “paint+adhesive” combination, in which TiO2 nanoparticles and polymeric glue were served as paint and adhesive respectively. However, they did not mention the transparency of the superhydrophobic surface. X. C. Tian et al. [16] discovered that thermal treatment on silicone sealant could construct a robust superhydrophobic micro/nano-structure, but the transparency was neither mentioned. M. Im et al. [17] fabricated a flexible superhydrophobic substrate with polydimethylsiloxane (PDMS), yet the practicability is reduced due to the complication of lithography method. S. J. Yang et al. [18] reported a self-cleaning superhydrophobic coating by air brushing based on PDMS and TiO2/SiO2 nanoparticles, without mentioning the transparency yet. Z. K. He et al. [19] fabricated a superhydrophobic coating with mixture of PDMS and high percent silica nanoparticles, but its application is restricted by the low transparency. Y. Lin et al. [20] used a femtosecond laser to obtain a surface with a modest roughness, and then hydrophobized it by a layer of fluoroalkylsilane molecules to get a superhydrophobic surface with high transparency. Nevertheless, the practical application of the as-prepared surface is limited by the complex multi-laser process. Recently, Hooda et al. [21,22] fabricated
The research on superhydrophobic surfaces has been accumulating fast recently because of their great potential applications in selfcleaning material, anticorrosion, water harvesting, and wood protection, etc. [1–4] Also, they would be promising for usages on optical device, solar panel, architectural glass, and automotive windshield [5–8] if parameter requirements for transparency and antireflection could be simultaneously satisfied. A superhydrophobic surface with a water droplet should show a contact angle above 150° and sliding angle below 10°. Inspired by biological surfaces those are endowed with superhydrophobic characteristic, such as lotus leaf, strider legs and cicada wings [9,10], superhydrophobicity is revealed and believed to be derived from the hierarchical micro/nano-structures and the layering of low surface energy materials. As a consequence, many delicate attempts based on the studies had been made to create superhydrophobic surfaces artificially, such as chemical deposition [11], colloidal assembly [12], layer by layer deposition [13], atmospheric arc discharge processing [14] and so on. However, in many cases, the preparation methods require complicated growth conditions such as high temperature, harsh chemical treatment, and complex techniques under ultra clean reaction environment, which extremely limits their widespread applications.
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Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Lu).
https://doi.org/10.1016/j.tsf.2019.137560 Received 17 January 2019; Received in revised form 20 August 2019; Accepted 9 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhenzhen Lu, et al., Thin Solid Films, https://doi.org/10.1016/j.tsf.2019.137560
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Scheme. 1. Schematic procedure of superhydrophobic coating preparation.
Specialty Chemicals Co., Ltd. (Shanghai, China). Neutral silicone sealant (contains α, ω-dihydroxydimethylpolysiloxane, fumed silica, CaCO3, cross-linkers, Plasticizer, etc.) was purchased from Heshan Honghua Industrial Co., Ltd. (Guangdong, China). Sodium hydroxide (96%), hydrochloric acid (36–38%), and sodium chloride (99.8%) and ethanol (99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was laboratory-prepared. Glass slides, ferroalloy, wood, tile and concrete were used as the substrates. All of the materials were used as received without further purification.
polystyrene/ZnO coatings and modified nano-silica embedded polystyrene based coatings with superhydrophobicity and high transparency by a simple sol-gel method. Although efforts have been made to overcome various obstacles in the practical application of superhydrophobic materials, creating durable and transparent superhydrophobic coatings in a simple and low-cost way is still a meaningful research topic. It is well known that SiO2 nanoparticles are water-soluble since their surfaces are covered with hydroxyl groups. In addition, due to the high surface energy and high specific surface area of SiO2 nanoparticles, they tend to aggregate in the mixing process with the polymer matrix, leading to phase separation. Therefore, combining functionalized SiO2 nanoparticles with surfactants is an effective way to improve the dispersion and compatibility. Neutral silicone sealant, as a kind of commercial adhesive with high bonding strength to substrates, is extensively applied in industry and people's daily life, which is mainly composed of silicone and other components including fumed silica, plasticizer, and cross-linkers, etc. [23]. In contrast to the organosilicon polymer such as PDMS commonly used, and other binding matrix such as fluorinated and epoxy polymer used in superhydrophobic coatings [17–20,24–26], neutral silicone sealant is absolutely costless, environment friendly, and more convenient to operate without curing agents, as well as qualified with weathering resistance and good mechanical properties. Therefore, neutral silicone sealant herein was chosen as the desirable polymeric bonding materials. Up to now, there are limited reports on the combination of silicone sealant and SiO2 nanoparticles in the study of superhydrophobicity. In this work, we adopted a facile and cost-effective method to fabricate highly transparent superhydrophobic surface based on commercial silicone sealant and functionalized silica nanoparticles. The superhydrophobic surface was prepared on the glass slide by dip-coating method with the mixture obtained via simple mechanical blending (stirring and ultrasonication) of SiO2 nanoparticles and silicone sealant binder. Moreover, the mixture could be easily sprayed on any bottom materials including ferroalloy, wood, concrete, glass, and tile using spray-coating method to produce large-area superhydrophobic surface. In addition, a series of durability tests were designed and carried out to verify its good practical performance.
2.2. Preparation of the superhydrophobic coating In a typical experiment, samples were prepared via the following procedures: Firstly, silica nanoparticles (1.01 g) were dispersed in ethanol (100 g) at different content levels (0 wt%, 0.5 wt%, 1 wt%, 2 wt %, 5 wt%) and treated by ultrasonication for 20 min. And then, 1.5 wt% Silicone sealant (1.52 g) was added into the above suspension and stirred for a few hours to form homogeneous mixture. Then, dip the glass slide into the mixture at a speed of 150 mm/min then stay for 1 min, and then take it out at a speed of 100 mm/min. At last, the SiO2/ sealant superhydrophobic composite coating was obtained after drying at room temperature for 12 h.
2.3. Characterization The water static contact angle (CA) and sliding angle (SA) measurements were carried out on a contact angle goniometer (DSA100, Krüss, Hamburg, Germany) at ambient temperature. 5 μL deionized water droplet was transferred by a microsyringe for the CA test and 10 μL deionized water droplet was delivered with the needle of the goniometer for the SA test, respectively. The presented results of the CA and the SA were mean values of five data points collected at different positions. The surface physical morphology was characterized with a scanning electron microscopy (SEM, SU8010, Hitachi, Tokyo, Japan) operated under an acceleration voltage of 5.0 kV. A thin layer of Au was sputter-coated onto samples to increase the conductivity of surface. The surface topography of the coatings was examined with an atomic force microscopy (AFM, Dimension Icon, Bruker, Karlsruhe, Germany) in tapping mode with a silicon nitride (Si3N4) tip, under ambient conditions. The surface roughness of the coatings was calculated according to the AFM images with the NanoScope-Analysis software. The optical transmission of the coatings was investigated with a UV–vis spectrophotometer (Inesa N4, Inesa Analytical Instrument Company, Shanghai, China) at visible wavelength ranging from 380 nm to 820 nm.
2. Experimental section 2.1. Materials Hydrophobic dimethyldichlorosilane-treated fumed SiO2 nanoparticles (Aerosil® R972) (JT-SQ: particles size range 30–70 nm, verified by scanning electron microscopy) were obtained from Evonik 2
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Fig. 1. SEM images of the as-prepared coatings with 1.5 wt% sealant and various SiO2 contents: (a1,a1) 0 wt%, (b1,b2) 0.5 wt%,1 wt%, (d1,d2) 2 wt%, (e1,e2) 5 wt%.
3. Results and discussion
in Scheme. 1. The functionalized silica nanoparticles were dispersed into the ethanol and treated by ultrasonic machine for 20 min to obtain a homogeneous solution. Subsequently, the silicone sealant was
The fabrication procedure of the superhydrophobic surface is shown
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Fig. 2. AFM images of the as-prepared coatings: (a) Height image of the sealant coating (sealant: 1.5 wt%); (b) Height image of the SiO2/sealant coating (SiO2:2 wt%, sealant: 1.5 wt%).
bring about decreasing of transparency of the coatings. Based on these points, fixed content of 1.5 wt% silicone sealant was chosen for subsequent experiments. The surface microstructures of the as-prepared coatings with various SiO2 contents and 1.5 wt% silicone sealant are presented in Fig. 1. As shown in Fig. 1a, the surface exhibited a continuous morphology. At low nano-SiO2 content (0.5 wt%), nanoparticles crossed in the polymeric silicone sealant backbones with a small quantity of micro/nano-scale air voids as shown in Fig. 1b2. As the nano-SiO2 content increased to 1 wt%, more nanoparticles and more air voids appeared in the mixture coating surface as shown in Fig. 1c1, 1c2. Further increasing the nano-SiO2 would lead to similar variation trend as shown in Fig. 1d1, 1d2. When the nano-SiO2 content was increased to 5 wt%, the silicone sealant backbones were completely covered by a granular layer of nano-SiO2 aggregates with decreased amount of air voids as shown in Fig. 1e1, 1e2. To understand the relationship between the morphology and the nano-SiO2 content, evaporation of the solvent and nano-SiO2 deposition are inferred to be involved in the formation processes of different surface structures. The prepared mixture contains solvent, varying contents of nano-SiO2 and fixed mass fraction of silicone sealant. In the mixed suspension, nanoSiO2 aggregates would produce some weak bonding area between the coating and the substrate, and these regions would capture the humidity of air then air voids would be the formed when the solvent is evaporating. Therefore, at lower content (0.5 wt% and 1 wt%), increasing nano-SiO2 content led to more air voids in the coating surface. When the content of nano-SiO2 was increased to much higher levels (2 wt% and 5 wt%), stable amount of air voids was observed because more nano-SiO2 deposited on the surface and homogeneously overspread on the substrate bonded by the silicone sealant. Additionally, this superhydrophobic surface could be analyzed by the Cassie–Baxter model. A mass of air was trapped in the hierarchical micro/nanostructures (as shown in SEM images) formed by the combined action of SiO2 and silicone sealant, which would greatly decrease the contact area between the surface and the water droplet and then lead to the superhydrophobicity of the coating. The surface morphology of the prepared coatings was further investigated by AFM shown in Fig. 2. The results of surface roughness parameters including root-mean-square roughness (Rq), arithmetic average roughness (Ra) and maximum roughness (Rmax) are listed in Table 1. A dual-scale structure could be observed with uniformly
Table 1 Roughness parameters of sealant coating (sealant: 1.5 wt%) and SiO2/sealant (SiO2:2 wt%, sealant: 1.5 wt%). Coating
Rq/ nm
Ra/ nm
Rmax/ nm
Sealant SiO2/sealant
2.48 81.4
1.94 63.7
30.5 601.0
Fig. 3. Variation of static contact angle with changing nano-SiO2 contents in the coatings.
injected into the suspension and stirred for a few hours. After that, the mixture of silica nanoparticles and silicone sealant was deposited onto the glass slides by dip-coating method, then dried at room temperature for 12 h. During the drying process of the surface, the silicone sealant was served as a bonding material between the silica nanoparticles and the substrate which could enhance the stability and adhesion strength of the coating. In SiO2/silicone sealant composites, the content of silicone sealant is an important factor for the preparation of excellent coatings. From macroscopic view, content below 1.5 wt% will lead to discontinuous and nonuniform surface of the coatings, and content over 1.5 wt% will
Scheme. 2. Mechanism of variation of the CA derived from different nano-SiO2 content
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Fig. 4. (a) The optical image of spherical water droplets (dyed with methyl blue) rested on the surface of the superhydrophobic SiO2/sealant coating. (b) A column of water bounced off the surface other than spread. (c)-(f) The self-cleaning process of the as-prepared surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Photographs of water droplets (dyed with methyl blue) dripped on the coatings with various SiO2 contents exhibiting different wettability and visible transmittance from top view. (b) Transmittances of bare glass and coated glasses tested by UV–vis transmission spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dispersed nano-sized protrusions on the SiO2/sealant coated substrate, producing a fairly rough surface with larger roughness value than that of the sealant coated substrate. The AFM results verify that the
incorporation of silica nanoparticles in coating formula was conducive to achieve highly hydrophobic features, which is in accordance with the SEM study. 5
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Fig. 6. The mixture (formula: SiO2:2 wt%, sealant: 1.5 wt%) was sprayed onto different types of substrates: (a) Ferroalloy, (b) Wood, (c) Tile, (d) Concrete. Table 2 Variation of the average transmittance in the visible wavelength range from 380 to 760 nm of the coatings with different silicone sealant contents (SiO2/ sealant = 4:3, wt/wt) before and after impact. Sealant content/wt%
0 1 2 3 4
Average transmittance/% Before impact
After impact
87.8 88.2 86.5 82.9 72.5
86.6 87.8 85.0 83.5 72.4
from 0.5 wt% to 2 wt%, the CA was improved significantly from 155.2° to 169.8°, indicating that the superhydrophobic coatings were actually achieved. As illustrated in Scheme. 2, when the contents of nano-SiO2 changing from little amount to moderate amount, the number of nano air pockets formed by nano-SiO2 is gradually growing homogeneously on the surface of the coating, which could serve as superhydrophobic touch points. However, the CA values of the coatings had a slight and continuous reduction along with the rising of the content of SiO2. When the SiO2 content was increased to 5 wt%, the coated glass has a CA of 169.0°. Thus excessive SiO2 content can no longer enhance the hydrophobicity of the coatings, and the explanation could be that the aggregates of nano-SiO2 changed the surface morphology and reduced the surface roughness. In addition, it was observed that the super-hydrophobic coating prepared all displayed no stickiness to water droplets, and even when the sample tilted about 10°, water droplets could quickly roll off the surface, indicating that the super-hydrophobic coating was indeed in the Cassie-Baxter state. It is observed from Fig. 4a that water droplets kept near spherical on the surface prepared, indicating an excellent superhydrophobicity. As shown in Fig. 4b, when a thin water flow dropped onto the as-prepared surface, it bounced out other than spread, owing to the trapped air pockets in the microroughness structures on the surface. Dust is an important existence in our living environment, which could gradually contaminate the surface, and then resulting in loss of aesthetics and
Fig. 7. (a) Schematic of the water injector in front and side views. (b) The variation of the CA of the coatings with different silicone sealant contents (the other four lines except “green line” corresponding to SiO2/sealant = 4:3, wt/ wt, as what mentioned before, SiO2:2 wt%, silicone sealant:1.5 wt%) as a function of impact time.
Fig. 3 presents the influence of SiO2 contents on the superhydrophobicity of the as-prepared coatings. It can be seen that the CA of the sealant-coated glass was 112.5° as shown in Fig. 3, which was already hydrophobic probably due to water-repellent property of PDMS mainly contained in the silicone sealant. Increasing the content of SiO2 6
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Fig. 9. The variation of the CA and SA after heating treatment at different temperatures. (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
water droplet exhibited more spherical shape on the glass slide coated with SiO2 nanoparticles/silicone sealant while the shape of the droplet on the slide coated with silicone sealant is not so shape and round. Meanwhile, letters are more clearly through sealant-coated slide and sealant/0.5 wt%, 1 wt% and 2 wt% of SiO2 slides than those through sealant/5 wt% of SiO2. Testing results of transmittance spectra for the uncoated and coated glasses in visible light range (380–760 nm) [28], which is regarded as a conventional characterization of transparency, are presented in Fig. 5b. When the content of SiO2 was less than 5 wt%, similar high transmittance values were obtained from the prepared samples, among which the sealant-coated ones had the highest average transmittance of 88.7% in the range of visible wavelengths (380–760 nm), very close to that (91.4%) of the bare glass. It should be mentioned that with the increase of SiO2 content, the transmittance decreased gradually. The lowest average transmittance of 66.7% was obtained when SiO2 content reached 5 wt%. As shown in Fig. 1e, high SiO2 content led to superfluously aggregated SiO2 nanoparticles, which generated a higher surface roughness then resulted in heavy light scattering. Taking both the transparency and superhydrophobicity of the coating into consideration, the formula with 2 wt% SiO2 could be optimal for further study with CA of 169.8° and good transmittance of 82.9%. Spraying [29] is a commonly-used method with advantages such as convenience, efficiency and cost-effectiveness, and is especially appropriate for construction of superhydrophobic coatings on large area. As shown in Fig. 6a-d, the as-prepared mixture was sprayed onto a half part of different types of substrates including ferroalloy, wood, tile and concrete. In contrast to the scattering or permeating of water on/into the bare part of the substrates, water droplets (dyed with pink paint) could readily stay spherically on the treated surfaces with CA above 160° and SA below 5°, which proved the substrates were endowed with superhydrophobicity. The practical applications of the superhydropphobic coatings have been extremely impeded by the poor durability to long-term water impact. Here, a water resistance test was conducted on the apparatus reported before [30]. As shown in Fig. 7a, the water was controlled to drip one drop per second through the designed infusion set, which was composed of bottles with a certain volume and needles. The as-prepared samples were placed 8 cm below four uniformly-spaced syringe fixed on a ruler, and scotch tapes were used to connect the coated glass slides with the other two parts of bare glass, tilted at 30°. As exhibited in Fig. 7b, the coatings with 0 wt%, 1 wt% and 2 wt% of silicone sealant sharply lost their superhydrophobicity after 1 h (~3600 droplets), 3 h (~10,800 droplets) and 3 h (~10,800 droplets) of the water impact, respectively, and CA values were decreased to 40°, same to that of the
Fig. 8. The superhydrophobic glass is immersed in water by an external force and appears like being coated with a silver film from the top (a) and the side (b). (c) The effect of pH values and (d) immersion time in 3.5 wt% NaCl aqueous solution on the CA and SA of the superhydrophobic coating (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
functions. Therefore, a dust removal experiment should be conducted to examine the self-cleaning performance of the as-prepared surface. As demonstrated in Fig. 4c-f, some carbon powder was employed as dust and placed on the surface of the as-prepared sample. Rolling water droplets supplied by an injector easily carried away the dust, and then left a clean surface with no trace of water. Moreover, no damages on the surface were observed after the experimental process, which suggests a strong adhesion strength between the coating and the substrate. An ideal balance between surface transparency and wettability is normally not easy to achieve since the roughness usually competes with the transparency. As previous investigations reported [19,27], ideal transparency can be obtained by controlling the size of surface roughness patterns down to 100 nm, which could minimize the adverse effect of the visible light scattering without degrading hydrophobic performance. Transparency is a crucial factor for superhydrophobic film applying on glass, lens, solar panels, etc. Fig. 5 shows the transparency of the as-prepared samples in visible range by means of optical photographs and UV–vis spectrophotometer. As shown in Fig. 5a, the dyed 7
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Fig. 10. (a) Schematic illustration of the UV test system used for the as-prepared coatings and (b) change of the CA and SA of the coated surface as a function of UV irradiation time. Enlarged drawing in (a) shows spherical water droplets deposited on the surface after 52 h UV irradiation. The inset graph in (b) illustrates in a smaller scale the variation of the contact angle with UV irradiation time. (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
which suggests the superhydrophobicity of the coating lost owing to a transition from Cassie state to Wenzel state. After 15 days of immersion, SA became 33°. The above results indicate that the as-prepared coatings display a good NaCl resistance and have a potential for application in marine environments. The thermal stability of the coating was studied, which provided a basis for its practical application in high temperature environment. A high-temperature experiment was arranged by heating the as-prepared samples in an oven for an hour. The temperature gradient was set at 60 °C, 120 °C, 180 °C, 240 °C, 300 °C, 360 °C, 420 °C and 480 °C. Each sample was measured at room temperature after cooling for a few minutes, and the results are shown in Fig. 9. When the treatment temperature is lower than 360 °C, the CA value of the coating is higher than 150 °C and the SA value is lower than 10 °C, indicating that the coating has good thermal stability, which could be attributed to fine heat-resistance performance of SiO2 nanoparticles and the cross-linked Si-O-Si networks in the silicone sealant [33]. A distinct transition to the Wenzel state was then observed as the heating temperature increased to 420 °C, reaching to the decomposition temperature of the Si-CH3 group in the composite coating, and leading to the damage of the surface micro-morphology, subsequently, the loss of superhydrophobicity [34]. When the treatment temperature is further increased to 480 °C, the coating changed from superhydrophobic to superhydrophilic, which proves the complete decomposition of the Si-CH3 group, and the formation of the hydrophilic silica. It can be concluded that the treating temperature below 360 °C had almost no influence on the performance of the coating. In order to evaluate the durability of the as-prepared surface under UV light, an UV-resistant test was designed as displayed in Fig. 10a. The as-prepared sample was placed into a test chamber, and then was irradiated under a medium pressure Hg lamp (250 W) for 52 h at 50 °C. As can be seen from Fig. 10b, the UV light had little influence on the CA and SA of the coatings, and the coating could retain its superhydrophobicity after exposure for 52 h, as proved by the inset of Fig. 10a. The results suggest a prominent UV-stability of the coating, the reason is the higher bond energy of 460 kJ/mol possessed by the SieO bond cannot be broken by the UV light (314–419 kJ/mol) [35]. A little enhancement of the CA was also observed compared to the initial 156° after UV irradiation, as shown in the inset of Fig. 10b, which could be ascribed to the change of the surface micro-morphology under the irradiation impact.
bare glass, indicating the coatings were seriously destroyed. When the water resistance time increased to 5 h (~18,000 droplets), the CA of the coating with 3 wt% silicone sealant was decreased to 40° as well. And we found that a strong hydrophobicity still can be achieved when the silicone sealant content increased to 4 wt%, with only a slight decrease in CA, which is 142°. The results present a remarkable improvement in the mechanical resistance against the water droplets by an indispensable addition of the silicone sealant. Table 2 shows the variation of the transparency in visible wavelength range from 380 nm to 760 nm of the coatings with various silicone sealant contents before and after impact. The average transmittance of the coatings with the silicone sealant content of 4 wt% remained almost unchanged, indicating that the higher silicone sealant content could lead to the better mechanical strength of the coatings. To assess the chemical stability of the superhydrophobic coating, the as-prepared samples were immersed in various acid-base aqueous solutions (adjusted by HCl and NaOH) for an hour. As shown in Fig. 8c, it was observed that the coating lost its superhydrophobicity in a strong alkali environment at pH = 13. This phenomenon should be clarified by that the rapid non oxidation-reduction reaction between SiO2 and NaOH, leading to the loss of hydrophobic SiO2. In addition, superhydropbicity could be retained when the coatings were immersed in the solutions at pH value lower than 13, proving that the coating could maintain its stability in a wide range of pH values. The reason for good chemical stability is also intuitively shown by Fig. 8a and b, surface of the submerged superhydrophobic sample looks like to be covered with a thin silver film, mainly attributed to the total internal reflection of tiny bubbles caused by the trapped air pockets in the micro/nanogrooves [31], and the surface could be effectively protected by this airinduced barrier of preventing the penetration of the liquid. The CassieBaxter model [32] herein is adopt to illustrate the superhydrophobicity of the submerged SiO2/sealant rough porous system as a whole. The apparent contact angle θc is described as:
cos θc = ϕs cos θe − (1 − ϕs )
(1)
where θe is the eigen contact angle for the smooth solid surface, and ϕs is the area fraction of the projected wetting area between liquid and solid. Given that values of θc and θc are 156° and 40°, respectively, 0.0490 and 0.9510 are calculated for the ϕs and (1-ϕs), respectively. The results suggest that this mico-nano structure captured 95.10% air pockets under the water, leading to an air barrier between water and coating. Fig. 8d shows the influence of 3.5 wt% NaCl solution on the CA and SA of the superhydrophobic coating. It can be seen that after 6 days of immersion in 3.5 wt% NaCl solution, CA decreased slightly from 156° to 154.3°, and SA increased from 5° to 8°.When the immersion time increased to 9 days, the SA increased to 12° and CA decreased to 153°,
4. Conclusions A facile and low-cost one-step process was developed for large-scale fabrication of transparent superhydrophobic coatings on different types 8
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of substrates such as glass, concrete, wood, and ferroalloy. The method preparing the superhydrophobic material is as simple as blending hydrophobic silica with neutral silicone sealant. The surface morphology and the variation trend of the water contact angle could be easily controlled by adjusting mass ratio of SiO2 nanoparticles to silicone sealant in the as-prepared mixture. A series of tests were carried out to evaluate the practical performance of the constructed coatings, results show that the coatings have not only good mechanical resistance to water drops, but also excellent chemical, thermal and UV stability, indicating they may have broad application prospects.
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Gupta, A facile approach to develop modified nano-silica embedded polystyrene based transparent superhydrophobic coating, Mater. Lett. 233 (2018) 340, https://doi.org/10.1016/j.matlet.2018.09.043. [23] F.D. Buyl, Silicone sealants and structural adhesives, Int. J. Adhes. Adhes. 21 (2001) 411, https://doi.org/10.1016/S0143-7496(01)00018-5. [24] J. Mertens, J. Hubert, N. Vandencasteele, M. Raes, H. Terryn, F. Reniers, Chemical and physical effect of SiO2 and TiO2 nanoparticles on highly hydrophobic fluorocarbon hybrid coatings synthesized by atmospheric plasma, Surf. Coat. Technol. 315 (2017) 274, https://doi.org/10.1016/j.surfcoat.2017.02.040. [25] P. Wang, M.J. Chen, H.L. Han, X.L. Fan, Q. Liu, J.F. Wang, Transparent and abrasion-resistant superhydrophobic coating with robust self-cleaning function in either air or oil, J. Mater. Chem. A 4 (2016) 7869, https://doi.org/10.1039/c6ta01082b. [26] C.J. Cai, N. Sang, S.C. Teng, Z.G. Shen, J. Guo, X.H. Zhao, Z.H. Guo, Superhydrophobic surface fabricated by spraying hydrophobic R974 nanoparticles and the drag reduction in water, Surf. Coat. Technol. 307 (2016) 366, https://doi. org/10.1016/j.surfcoat.2016.09.009. [27] D. Wang, Z.B. Zhang, Y.M. Li, C.H. Xu, Highly transparent and durable superhydrophobic hybrid nanoporous coatings fabricated from polysiloxane, ACS Appl. Mater. Interfaces 6 (2014) 10014, , https://doi.org/10.1021/am405884x. [28] S.A. Mahadik, M.S. Kavale, S.K. Mukherjee, A.V. Rao, Transparent Superhydrophobic silica coatings on glass by sol-gel method, Appl. Surf. Sci. 257 (2010) 333, https://doi.org/10.1016/j.apsusc.2010.06.062. [29] J. Li, Z.Y. Huang, F.P. Wang, X.Z. Yan, Y. Wei, One-step preparation of transparent superhydrophobic coatings using atmospheric arc discharge, Appl. Phys. Lett. 1074 (2015) 051603, , https://doi.org/10.1063/1.4927745. [30] D. Ebert, B. Bhushan, Transparent, superhydrophobic, and wear-resistant coatings on glass and polymer substrates using SiO2, ZnO, and ITO nanoparticles, Langmuir 28 (2012) 11391, , https://doi.org/10.1021/la301479c. [31] M.N. Qu, S.S. Liu, J.M. He, J. Feng, Y.L. Yao, L.G. Hou, X.R. Ma, X.R. Liu, Fabrication of recyclable superhydrophobic materials with self-cleaning and mechanically durable properties on various substrates by quartz sand and polyvinylchloride, RSC Adv. 6 (2016) 79238, , https://doi.org/10.1039/c6ra12767c. [32] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546, https://doi.org/10.1039/tf9444000546. [33] Y.B. Wang, F. Lin, Y.P. Dong, Z. Liu, W. Li, Y.D. Huang, A multifunctional polymeric nanofilm with robust chemical performances for special wettability, Nanoscale 8 (2016) 5153, https://doi.org/10.1039/c5nr07601c. [34] Z.L. Jiang, S.Y. Fang, C.S. Wanga, H.P. Wang, C.C. 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Acknowledgments This research was financially supported by National Science Fund for Distinguished Young Scholars of China (51425801), China Postdoctoral Science Foundation (2015M580775), Chong Qing Postdoctoral Science Special Foundation (Xm2015004), Science and Technology Project of Gui Zhou Provincial Transportation Department (2016-123-040), Science Technology Project of Gui Zhou Province ([2017]2035). References [1] L. Torun, N. Celik, M. Hancer, F. Es, C. Emir, R. Turan, M.S. Onses, Water impact resistant and antireflective superhydrophobic surfaces fabricated by spray coating of nanoparticles: Interface engineering via end-grafted polymers, Macromolecules 51 (2018) 10011, , https://doi.org/10.1021/acs.macromol.8b01808. [2] D.F. Zhi, Y. Lu, S. Sathasivam, I.P. Parkin, X. Zhang, Large-scale fabrication of translucent and repairable superhydrophobic spray coatings with remarkable mechanical, chemical durability and UV resistance, J. Mater. Chem. A 5 (2017) 10622, , https://doi.org/10.1039/c7ta02488f. [3] H.C. Qian, D. Xu, C.W. Du, D.W. Zhang, X.G. Li, L.Y. Huang, L.P. Deng, Y.C. Tu, J.M.C. Mol, H.A. Terryn, Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (2017) 2355, https://doi.org/10.1039/c6ta10903a. [4] S.M. Shah, U. Zulfiqar, S.Z. Hussain, I. Ahmad, H. Rehman, I. Hussain, T. Subhani, A durable superhydrophobic coating for the protection of wood materials, Mater. Lett. 203 (2017) 17, https://doi.org/10.1016/j.matlet.2017.05.126. [5] L. Zhang, C.H. Xue, M. Cao, M.M. Zhang, M. Li, J.Z. Ma, Highly transparent fluorine-free superhydrophobic silica nanotube coatings, Chem. Eng. J. 320 (2017) 244, https://doi.org/10.1016/j.cej.2017.03.048. [6] S. Sutha, S. Suresh, B. Raj, K.R. Ravi, Transparent alumina based superhydrophobic self–cleaning coatings forsolar cell cover glass applications, Sol. Energy Mater. Sol. Cells 165 (2017) 128, https://doi.org/10.1016/j.solmat.2017.02.027. [7] M. Fathi, M. Abderrezek, M. Friedrich, Reducing dust effects on photovoltaic panels by hydrophobic coating, Clean Techn. Environ. Policy 19 (2017) 577, https://doi. org/10.1007/s10098-016-1233-9. [8] T.T. Ren, J.H. He, Substrate-versatile approach to robust antireflective and superhydrophobic coatings with excellent self-cleaning property in varied environments, ACS Appl. Mater. Interfaces 9 (2017) 34367, , https://doi.org/10.1021/acsami. 7b11116. [9] C. Neinhuis, W. Barthlott, Characterization and distribution of water-repellent, selfcleaning plant surfaces, Ann. Bot. 79 (1997) 667, https://doi.org/10.1006/anbo. 1997.0400. [10] X.F. Gao, L. Jiang, Water-repellent legs of water striders, Nature 432 (2004) 36, https://doi.org/10.1038/432036a. [11] L. Huang, J.L. Song, Y. Lu, F. Chen, X. Liu, Z.J. Jin, D.Y. Zhao, C.J. Carmalt, I.P. Parkin, Superoleophobic surfaces on stainless steel substrates obtained by chemical bath deposition, Micro & Nano Lett. 12 (2017) 76, https://doi.org/10. 1049/mnl.2016.0576. [12] P. Kothary, X. Dou, Y. Fang, Z.X. Gu, S.Y. Leo, P. Jiang, Superhydrophobic hierarchical arrays fabricated by a scalable colloidal lithography approach, J. Colloid Interface Sci. 487 (2017) 484, https://doi.org/10.1016/j.jcis.2016.10.081. [13] H.C. Guo, E.Y. Ye, Z.B. Li, M.Y. Han, X.J. Loh, Recent progress of atomic layer deposition on polymeric materials, Mater. Sci. Eng. C 70 (2017) 1182, https://doi. org/10.1016/j.msec.2016.01.093.
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One-step facile route to fabricate functionalized nano-silica and silicone sealant based transparent superhydrophobic coating ⁎
Zhenzhen Lu , Lijie Xu, Yang He, Jianting Zhou College of Civil Engineering, ChongQing JiaoTong University, Chongqing 400074, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent coatings Superhydrophobicity Silicon dioxide Silicone Composite coatings Dip coating Chemical stability Thermal stability
A highly transparent and stable superhydrophobic surface was prepared on substrates of different types including tile, concrete, wood, and ferroalloy via a facile and low-cost method using hydrophobic silica nanoparticles as building blocks for construction of micro/nano-structures and neutral silicone sealant as binder. The surface wettability, morphology, topography, and transmittance was characterized by means of contact angle measurement, scanning electron microscope, atomic force microscope, and ultraviolet-visible spectrophotometer respectively. All the as-prepared SiO2/Silicone composite coatings exhibited well water-repellency with a static water contact angle above 150° and a sliding angle below 10°. Besides, a highest average transmittance of 88.7% of the superhydrophobic coatings was obtained. The results of the performance testing showed that the superhydrophobic coatings possess good mechanical resistance to water drops, as well as excellent chemical/thermal stability and UV durability. Moreover, the method to fabricate superhydrophbic coatings on various substrates is facile and cost-effective, which provides a possibility of large-scale industrial applications.
1. Introduction
Combination of organic polymer and textured surface is thought to be one of the simple and high-efficiency approaches to fabricate superhydrophobic surface due to the full utilization of their advantages. For instance, Y. Lu et al. [15] prepared a robust superhydrophobic surface through “paint+adhesive” combination, in which TiO2 nanoparticles and polymeric glue were served as paint and adhesive respectively. However, they did not mention the transparency of the superhydrophobic surface. X. C. Tian et al. [16] discovered that thermal treatment on silicone sealant could construct a robust superhydrophobic micro/nano-structure, but the transparency was neither mentioned. M. Im et al. [17] fabricated a flexible superhydrophobic substrate with polydimethylsiloxane (PDMS), yet the practicability is reduced due to the complication of lithography method. S. J. Yang et al. [18] reported a self-cleaning superhydrophobic coating by air brushing based on PDMS and TiO2/SiO2 nanoparticles, without mentioning the transparency yet. Z. K. He et al. [19] fabricated a superhydrophobic coating with mixture of PDMS and high percent silica nanoparticles, but its application is restricted by the low transparency. Y. Lin et al. [20] used a femtosecond laser to obtain a surface with a modest roughness, and then hydrophobized it by a layer of fluoroalkylsilane molecules to get a superhydrophobic surface with high transparency. Nevertheless, the practical application of the as-prepared surface is limited by the complex multi-laser process. Recently, Hooda et al. [21,22] fabricated
The research on superhydrophobic surfaces has been accumulating fast recently because of their great potential applications in selfcleaning material, anticorrosion, water harvesting, and wood protection, etc. [1–4] Also, they would be promising for usages on optical device, solar panel, architectural glass, and automotive windshield [5–8] if parameter requirements for transparency and antireflection could be simultaneously satisfied. A superhydrophobic surface with a water droplet should show a contact angle above 150° and sliding angle below 10°. Inspired by biological surfaces those are endowed with superhydrophobic characteristic, such as lotus leaf, strider legs and cicada wings [9,10], superhydrophobicity is revealed and believed to be derived from the hierarchical micro/nano-structures and the layering of low surface energy materials. As a consequence, many delicate attempts based on the studies had been made to create superhydrophobic surfaces artificially, such as chemical deposition [11], colloidal assembly [12], layer by layer deposition [13], atmospheric arc discharge processing [14] and so on. However, in many cases, the preparation methods require complicated growth conditions such as high temperature, harsh chemical treatment, and complex techniques under ultra clean reaction environment, which extremely limits their widespread applications.
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Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Lu).
https://doi.org/10.1016/j.tsf.2019.137560 Received 17 January 2019; Received in revised form 20 August 2019; Accepted 9 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Zhenzhen Lu, et al., Thin Solid Films, https://doi.org/10.1016/j.tsf.2019.137560
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Scheme. 1. Schematic procedure of superhydrophobic coating preparation.
Specialty Chemicals Co., Ltd. (Shanghai, China). Neutral silicone sealant (contains α, ω-dihydroxydimethylpolysiloxane, fumed silica, CaCO3, cross-linkers, Plasticizer, etc.) was purchased from Heshan Honghua Industrial Co., Ltd. (Guangdong, China). Sodium hydroxide (96%), hydrochloric acid (36–38%), and sodium chloride (99.8%) and ethanol (99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was laboratory-prepared. Glass slides, ferroalloy, wood, tile and concrete were used as the substrates. All of the materials were used as received without further purification.
polystyrene/ZnO coatings and modified nano-silica embedded polystyrene based coatings with superhydrophobicity and high transparency by a simple sol-gel method. Although efforts have been made to overcome various obstacles in the practical application of superhydrophobic materials, creating durable and transparent superhydrophobic coatings in a simple and low-cost way is still a meaningful research topic. It is well known that SiO2 nanoparticles are water-soluble since their surfaces are covered with hydroxyl groups. In addition, due to the high surface energy and high specific surface area of SiO2 nanoparticles, they tend to aggregate in the mixing process with the polymer matrix, leading to phase separation. Therefore, combining functionalized SiO2 nanoparticles with surfactants is an effective way to improve the dispersion and compatibility. Neutral silicone sealant, as a kind of commercial adhesive with high bonding strength to substrates, is extensively applied in industry and people's daily life, which is mainly composed of silicone and other components including fumed silica, plasticizer, and cross-linkers, etc. [23]. In contrast to the organosilicon polymer such as PDMS commonly used, and other binding matrix such as fluorinated and epoxy polymer used in superhydrophobic coatings [17–20,24–26], neutral silicone sealant is absolutely costless, environment friendly, and more convenient to operate without curing agents, as well as qualified with weathering resistance and good mechanical properties. Therefore, neutral silicone sealant herein was chosen as the desirable polymeric bonding materials. Up to now, there are limited reports on the combination of silicone sealant and SiO2 nanoparticles in the study of superhydrophobicity. In this work, we adopted a facile and cost-effective method to fabricate highly transparent superhydrophobic surface based on commercial silicone sealant and functionalized silica nanoparticles. The superhydrophobic surface was prepared on the glass slide by dip-coating method with the mixture obtained via simple mechanical blending (stirring and ultrasonication) of SiO2 nanoparticles and silicone sealant binder. Moreover, the mixture could be easily sprayed on any bottom materials including ferroalloy, wood, concrete, glass, and tile using spray-coating method to produce large-area superhydrophobic surface. In addition, a series of durability tests were designed and carried out to verify its good practical performance.
2.2. Preparation of the superhydrophobic coating In a typical experiment, samples were prepared via the following procedures: Firstly, silica nanoparticles (1.01 g) were dispersed in ethanol (100 g) at different content levels (0 wt%, 0.5 wt%, 1 wt%, 2 wt %, 5 wt%) and treated by ultrasonication for 20 min. And then, 1.5 wt% Silicone sealant (1.52 g) was added into the above suspension and stirred for a few hours to form homogeneous mixture. Then, dip the glass slide into the mixture at a speed of 150 mm/min then stay for 1 min, and then take it out at a speed of 100 mm/min. At last, the SiO2/ sealant superhydrophobic composite coating was obtained after drying at room temperature for 12 h.
2.3. Characterization The water static contact angle (CA) and sliding angle (SA) measurements were carried out on a contact angle goniometer (DSA100, Krüss, Hamburg, Germany) at ambient temperature. 5 μL deionized water droplet was transferred by a microsyringe for the CA test and 10 μL deionized water droplet was delivered with the needle of the goniometer for the SA test, respectively. The presented results of the CA and the SA were mean values of five data points collected at different positions. The surface physical morphology was characterized with a scanning electron microscopy (SEM, SU8010, Hitachi, Tokyo, Japan) operated under an acceleration voltage of 5.0 kV. A thin layer of Au was sputter-coated onto samples to increase the conductivity of surface. The surface topography of the coatings was examined with an atomic force microscopy (AFM, Dimension Icon, Bruker, Karlsruhe, Germany) in tapping mode with a silicon nitride (Si3N4) tip, under ambient conditions. The surface roughness of the coatings was calculated according to the AFM images with the NanoScope-Analysis software. The optical transmission of the coatings was investigated with a UV–vis spectrophotometer (Inesa N4, Inesa Analytical Instrument Company, Shanghai, China) at visible wavelength ranging from 380 nm to 820 nm.
2. Experimental section 2.1. Materials Hydrophobic dimethyldichlorosilane-treated fumed SiO2 nanoparticles (Aerosil® R972) (JT-SQ: particles size range 30–70 nm, verified by scanning electron microscopy) were obtained from Evonik 2
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Fig. 1. SEM images of the as-prepared coatings with 1.5 wt% sealant and various SiO2 contents: (a1,a1) 0 wt%, (b1,b2) 0.5 wt%,1 wt%, (d1,d2) 2 wt%, (e1,e2) 5 wt%.
3. Results and discussion
in Scheme. 1. The functionalized silica nanoparticles were dispersed into the ethanol and treated by ultrasonic machine for 20 min to obtain a homogeneous solution. Subsequently, the silicone sealant was
The fabrication procedure of the superhydrophobic surface is shown
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Fig. 2. AFM images of the as-prepared coatings: (a) Height image of the sealant coating (sealant: 1.5 wt%); (b) Height image of the SiO2/sealant coating (SiO2:2 wt%, sealant: 1.5 wt%).
bring about decreasing of transparency of the coatings. Based on these points, fixed content of 1.5 wt% silicone sealant was chosen for subsequent experiments. The surface microstructures of the as-prepared coatings with various SiO2 contents and 1.5 wt% silicone sealant are presented in Fig. 1. As shown in Fig. 1a, the surface exhibited a continuous morphology. At low nano-SiO2 content (0.5 wt%), nanoparticles crossed in the polymeric silicone sealant backbones with a small quantity of micro/nano-scale air voids as shown in Fig. 1b2. As the nano-SiO2 content increased to 1 wt%, more nanoparticles and more air voids appeared in the mixture coating surface as shown in Fig. 1c1, 1c2. Further increasing the nano-SiO2 would lead to similar variation trend as shown in Fig. 1d1, 1d2. When the nano-SiO2 content was increased to 5 wt%, the silicone sealant backbones were completely covered by a granular layer of nano-SiO2 aggregates with decreased amount of air voids as shown in Fig. 1e1, 1e2. To understand the relationship between the morphology and the nano-SiO2 content, evaporation of the solvent and nano-SiO2 deposition are inferred to be involved in the formation processes of different surface structures. The prepared mixture contains solvent, varying contents of nano-SiO2 and fixed mass fraction of silicone sealant. In the mixed suspension, nanoSiO2 aggregates would produce some weak bonding area between the coating and the substrate, and these regions would capture the humidity of air then air voids would be the formed when the solvent is evaporating. Therefore, at lower content (0.5 wt% and 1 wt%), increasing nano-SiO2 content led to more air voids in the coating surface. When the content of nano-SiO2 was increased to much higher levels (2 wt% and 5 wt%), stable amount of air voids was observed because more nano-SiO2 deposited on the surface and homogeneously overspread on the substrate bonded by the silicone sealant. Additionally, this superhydrophobic surface could be analyzed by the Cassie–Baxter model. A mass of air was trapped in the hierarchical micro/nanostructures (as shown in SEM images) formed by the combined action of SiO2 and silicone sealant, which would greatly decrease the contact area between the surface and the water droplet and then lead to the superhydrophobicity of the coating. The surface morphology of the prepared coatings was further investigated by AFM shown in Fig. 2. The results of surface roughness parameters including root-mean-square roughness (Rq), arithmetic average roughness (Ra) and maximum roughness (Rmax) are listed in Table 1. A dual-scale structure could be observed with uniformly
Table 1 Roughness parameters of sealant coating (sealant: 1.5 wt%) and SiO2/sealant (SiO2:2 wt%, sealant: 1.5 wt%). Coating
Rq/ nm
Ra/ nm
Rmax/ nm
Sealant SiO2/sealant
2.48 81.4
1.94 63.7
30.5 601.0
Fig. 3. Variation of static contact angle with changing nano-SiO2 contents in the coatings.
injected into the suspension and stirred for a few hours. After that, the mixture of silica nanoparticles and silicone sealant was deposited onto the glass slides by dip-coating method, then dried at room temperature for 12 h. During the drying process of the surface, the silicone sealant was served as a bonding material between the silica nanoparticles and the substrate which could enhance the stability and adhesion strength of the coating. In SiO2/silicone sealant composites, the content of silicone sealant is an important factor for the preparation of excellent coatings. From macroscopic view, content below 1.5 wt% will lead to discontinuous and nonuniform surface of the coatings, and content over 1.5 wt% will
Scheme. 2. Mechanism of variation of the CA derived from different nano-SiO2 content
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Fig. 4. (a) The optical image of spherical water droplets (dyed with methyl blue) rested on the surface of the superhydrophobic SiO2/sealant coating. (b) A column of water bounced off the surface other than spread. (c)-(f) The self-cleaning process of the as-prepared surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Photographs of water droplets (dyed with methyl blue) dripped on the coatings with various SiO2 contents exhibiting different wettability and visible transmittance from top view. (b) Transmittances of bare glass and coated glasses tested by UV–vis transmission spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dispersed nano-sized protrusions on the SiO2/sealant coated substrate, producing a fairly rough surface with larger roughness value than that of the sealant coated substrate. The AFM results verify that the
incorporation of silica nanoparticles in coating formula was conducive to achieve highly hydrophobic features, which is in accordance with the SEM study. 5
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Fig. 6. The mixture (formula: SiO2:2 wt%, sealant: 1.5 wt%) was sprayed onto different types of substrates: (a) Ferroalloy, (b) Wood, (c) Tile, (d) Concrete. Table 2 Variation of the average transmittance in the visible wavelength range from 380 to 760 nm of the coatings with different silicone sealant contents (SiO2/ sealant = 4:3, wt/wt) before and after impact. Sealant content/wt%
0 1 2 3 4
Average transmittance/% Before impact
After impact
87.8 88.2 86.5 82.9 72.5
86.6 87.8 85.0 83.5 72.4
from 0.5 wt% to 2 wt%, the CA was improved significantly from 155.2° to 169.8°, indicating that the superhydrophobic coatings were actually achieved. As illustrated in Scheme. 2, when the contents of nano-SiO2 changing from little amount to moderate amount, the number of nano air pockets formed by nano-SiO2 is gradually growing homogeneously on the surface of the coating, which could serve as superhydrophobic touch points. However, the CA values of the coatings had a slight and continuous reduction along with the rising of the content of SiO2. When the SiO2 content was increased to 5 wt%, the coated glass has a CA of 169.0°. Thus excessive SiO2 content can no longer enhance the hydrophobicity of the coatings, and the explanation could be that the aggregates of nano-SiO2 changed the surface morphology and reduced the surface roughness. In addition, it was observed that the super-hydrophobic coating prepared all displayed no stickiness to water droplets, and even when the sample tilted about 10°, water droplets could quickly roll off the surface, indicating that the super-hydrophobic coating was indeed in the Cassie-Baxter state. It is observed from Fig. 4a that water droplets kept near spherical on the surface prepared, indicating an excellent superhydrophobicity. As shown in Fig. 4b, when a thin water flow dropped onto the as-prepared surface, it bounced out other than spread, owing to the trapped air pockets in the microroughness structures on the surface. Dust is an important existence in our living environment, which could gradually contaminate the surface, and then resulting in loss of aesthetics and
Fig. 7. (a) Schematic of the water injector in front and side views. (b) The variation of the CA of the coatings with different silicone sealant contents (the other four lines except “green line” corresponding to SiO2/sealant = 4:3, wt/ wt, as what mentioned before, SiO2:2 wt%, silicone sealant:1.5 wt%) as a function of impact time.
Fig. 3 presents the influence of SiO2 contents on the superhydrophobicity of the as-prepared coatings. It can be seen that the CA of the sealant-coated glass was 112.5° as shown in Fig. 3, which was already hydrophobic probably due to water-repellent property of PDMS mainly contained in the silicone sealant. Increasing the content of SiO2 6
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Fig. 9. The variation of the CA and SA after heating treatment at different temperatures. (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
water droplet exhibited more spherical shape on the glass slide coated with SiO2 nanoparticles/silicone sealant while the shape of the droplet on the slide coated with silicone sealant is not so shape and round. Meanwhile, letters are more clearly through sealant-coated slide and sealant/0.5 wt%, 1 wt% and 2 wt% of SiO2 slides than those through sealant/5 wt% of SiO2. Testing results of transmittance spectra for the uncoated and coated glasses in visible light range (380–760 nm) [28], which is regarded as a conventional characterization of transparency, are presented in Fig. 5b. When the content of SiO2 was less than 5 wt%, similar high transmittance values were obtained from the prepared samples, among which the sealant-coated ones had the highest average transmittance of 88.7% in the range of visible wavelengths (380–760 nm), very close to that (91.4%) of the bare glass. It should be mentioned that with the increase of SiO2 content, the transmittance decreased gradually. The lowest average transmittance of 66.7% was obtained when SiO2 content reached 5 wt%. As shown in Fig. 1e, high SiO2 content led to superfluously aggregated SiO2 nanoparticles, which generated a higher surface roughness then resulted in heavy light scattering. Taking both the transparency and superhydrophobicity of the coating into consideration, the formula with 2 wt% SiO2 could be optimal for further study with CA of 169.8° and good transmittance of 82.9%. Spraying [29] is a commonly-used method with advantages such as convenience, efficiency and cost-effectiveness, and is especially appropriate for construction of superhydrophobic coatings on large area. As shown in Fig. 6a-d, the as-prepared mixture was sprayed onto a half part of different types of substrates including ferroalloy, wood, tile and concrete. In contrast to the scattering or permeating of water on/into the bare part of the substrates, water droplets (dyed with pink paint) could readily stay spherically on the treated surfaces with CA above 160° and SA below 5°, which proved the substrates were endowed with superhydrophobicity. The practical applications of the superhydropphobic coatings have been extremely impeded by the poor durability to long-term water impact. Here, a water resistance test was conducted on the apparatus reported before [30]. As shown in Fig. 7a, the water was controlled to drip one drop per second through the designed infusion set, which was composed of bottles with a certain volume and needles. The as-prepared samples were placed 8 cm below four uniformly-spaced syringe fixed on a ruler, and scotch tapes were used to connect the coated glass slides with the other two parts of bare glass, tilted at 30°. As exhibited in Fig. 7b, the coatings with 0 wt%, 1 wt% and 2 wt% of silicone sealant sharply lost their superhydrophobicity after 1 h (~3600 droplets), 3 h (~10,800 droplets) and 3 h (~10,800 droplets) of the water impact, respectively, and CA values were decreased to 40°, same to that of the
Fig. 8. The superhydrophobic glass is immersed in water by an external force and appears like being coated with a silver film from the top (a) and the side (b). (c) The effect of pH values and (d) immersion time in 3.5 wt% NaCl aqueous solution on the CA and SA of the superhydrophobic coating (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
functions. Therefore, a dust removal experiment should be conducted to examine the self-cleaning performance of the as-prepared surface. As demonstrated in Fig. 4c-f, some carbon powder was employed as dust and placed on the surface of the as-prepared sample. Rolling water droplets supplied by an injector easily carried away the dust, and then left a clean surface with no trace of water. Moreover, no damages on the surface were observed after the experimental process, which suggests a strong adhesion strength between the coating and the substrate. An ideal balance between surface transparency and wettability is normally not easy to achieve since the roughness usually competes with the transparency. As previous investigations reported [19,27], ideal transparency can be obtained by controlling the size of surface roughness patterns down to 100 nm, which could minimize the adverse effect of the visible light scattering without degrading hydrophobic performance. Transparency is a crucial factor for superhydrophobic film applying on glass, lens, solar panels, etc. Fig. 5 shows the transparency of the as-prepared samples in visible range by means of optical photographs and UV–vis spectrophotometer. As shown in Fig. 5a, the dyed 7
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Fig. 10. (a) Schematic illustration of the UV test system used for the as-prepared coatings and (b) change of the CA and SA of the coated surface as a function of UV irradiation time. Enlarged drawing in (a) shows spherical water droplets deposited on the surface after 52 h UV irradiation. The inset graph in (b) illustrates in a smaller scale the variation of the contact angle with UV irradiation time. (SiO2/sealant = 4:3, wt/wt, sealant: 4 wt%.)
which suggests the superhydrophobicity of the coating lost owing to a transition from Cassie state to Wenzel state. After 15 days of immersion, SA became 33°. The above results indicate that the as-prepared coatings display a good NaCl resistance and have a potential for application in marine environments. The thermal stability of the coating was studied, which provided a basis for its practical application in high temperature environment. A high-temperature experiment was arranged by heating the as-prepared samples in an oven for an hour. The temperature gradient was set at 60 °C, 120 °C, 180 °C, 240 °C, 300 °C, 360 °C, 420 °C and 480 °C. Each sample was measured at room temperature after cooling for a few minutes, and the results are shown in Fig. 9. When the treatment temperature is lower than 360 °C, the CA value of the coating is higher than 150 °C and the SA value is lower than 10 °C, indicating that the coating has good thermal stability, which could be attributed to fine heat-resistance performance of SiO2 nanoparticles and the cross-linked Si-O-Si networks in the silicone sealant [33]. A distinct transition to the Wenzel state was then observed as the heating temperature increased to 420 °C, reaching to the decomposition temperature of the Si-CH3 group in the composite coating, and leading to the damage of the surface micro-morphology, subsequently, the loss of superhydrophobicity [34]. When the treatment temperature is further increased to 480 °C, the coating changed from superhydrophobic to superhydrophilic, which proves the complete decomposition of the Si-CH3 group, and the formation of the hydrophilic silica. It can be concluded that the treating temperature below 360 °C had almost no influence on the performance of the coating. In order to evaluate the durability of the as-prepared surface under UV light, an UV-resistant test was designed as displayed in Fig. 10a. The as-prepared sample was placed into a test chamber, and then was irradiated under a medium pressure Hg lamp (250 W) for 52 h at 50 °C. As can be seen from Fig. 10b, the UV light had little influence on the CA and SA of the coatings, and the coating could retain its superhydrophobicity after exposure for 52 h, as proved by the inset of Fig. 10a. The results suggest a prominent UV-stability of the coating, the reason is the higher bond energy of 460 kJ/mol possessed by the SieO bond cannot be broken by the UV light (314–419 kJ/mol) [35]. A little enhancement of the CA was also observed compared to the initial 156° after UV irradiation, as shown in the inset of Fig. 10b, which could be ascribed to the change of the surface micro-morphology under the irradiation impact.
bare glass, indicating the coatings were seriously destroyed. When the water resistance time increased to 5 h (~18,000 droplets), the CA of the coating with 3 wt% silicone sealant was decreased to 40° as well. And we found that a strong hydrophobicity still can be achieved when the silicone sealant content increased to 4 wt%, with only a slight decrease in CA, which is 142°. The results present a remarkable improvement in the mechanical resistance against the water droplets by an indispensable addition of the silicone sealant. Table 2 shows the variation of the transparency in visible wavelength range from 380 nm to 760 nm of the coatings with various silicone sealant contents before and after impact. The average transmittance of the coatings with the silicone sealant content of 4 wt% remained almost unchanged, indicating that the higher silicone sealant content could lead to the better mechanical strength of the coatings. To assess the chemical stability of the superhydrophobic coating, the as-prepared samples were immersed in various acid-base aqueous solutions (adjusted by HCl and NaOH) for an hour. As shown in Fig. 8c, it was observed that the coating lost its superhydrophobicity in a strong alkali environment at pH = 13. This phenomenon should be clarified by that the rapid non oxidation-reduction reaction between SiO2 and NaOH, leading to the loss of hydrophobic SiO2. In addition, superhydropbicity could be retained when the coatings were immersed in the solutions at pH value lower than 13, proving that the coating could maintain its stability in a wide range of pH values. The reason for good chemical stability is also intuitively shown by Fig. 8a and b, surface of the submerged superhydrophobic sample looks like to be covered with a thin silver film, mainly attributed to the total internal reflection of tiny bubbles caused by the trapped air pockets in the micro/nanogrooves [31], and the surface could be effectively protected by this airinduced barrier of preventing the penetration of the liquid. The CassieBaxter model [32] herein is adopt to illustrate the superhydrophobicity of the submerged SiO2/sealant rough porous system as a whole. The apparent contact angle θc is described as:
cos θc = ϕs cos θe − (1 − ϕs )
(1)
where θe is the eigen contact angle for the smooth solid surface, and ϕs is the area fraction of the projected wetting area between liquid and solid. Given that values of θc and θc are 156° and 40°, respectively, 0.0490 and 0.9510 are calculated for the ϕs and (1-ϕs), respectively. The results suggest that this mico-nano structure captured 95.10% air pockets under the water, leading to an air barrier between water and coating. Fig. 8d shows the influence of 3.5 wt% NaCl solution on the CA and SA of the superhydrophobic coating. It can be seen that after 6 days of immersion in 3.5 wt% NaCl solution, CA decreased slightly from 156° to 154.3°, and SA increased from 5° to 8°.When the immersion time increased to 9 days, the SA increased to 12° and CA decreased to 153°,
4. Conclusions A facile and low-cost one-step process was developed for large-scale fabrication of transparent superhydrophobic coatings on different types 8
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of substrates such as glass, concrete, wood, and ferroalloy. The method preparing the superhydrophobic material is as simple as blending hydrophobic silica with neutral silicone sealant. The surface morphology and the variation trend of the water contact angle could be easily controlled by adjusting mass ratio of SiO2 nanoparticles to silicone sealant in the as-prepared mixture. A series of tests were carried out to evaluate the practical performance of the constructed coatings, results show that the coatings have not only good mechanical resistance to water drops, but also excellent chemical, thermal and UV stability, indicating they may have broad application prospects.
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