Development of Durable, Fluorine-free, and Transparent Superhydrophobic Surfaces for Oil/Water Separation.

Although artificial superhydrophobic materials have extensive and significant applications in antifouling, self-cleaning, anti-icing, fluid transport, oil/water separation, and so forth, the poor robustness of these surfaces has always been a bottleneck for their development in practical industrial applications. Here, we report a facile, economical, efficient, and versatile strategy to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surfaces by combining adhesive and hydrophobic paint, which is applicable for both hard and soft substrates. The coated substrates exhibit excellent superhydrophobic property and ultralow adhesion with water (contact angle ≈ 160° and sliding angle <2°). Additionally, the coated surface maintained its superhydrophobicity even after 325 sandpaper abrasion cycles, showing remarkable mechanical robustness. Furthermore, the coated surfaces were applied to separate oil/water mixtures because of their unique characteristics of being simultaneously superhydrophobic and superoleophilic. In addition, it is believed that this fabrication method is significant, promising, and feasible for mass production of superhydrophobic surfaces for industrial applications.

Introduction

Artificial superhydrophobic materials have been widely designed and fabricated by getting motivation from the nature such as lotus leaves,[1] butterfly wings,[2] legs of the water strider,[3] and so on,[4] wherein the extreme water repellency is resulted from low-surface-energy chemical components and dual-scale hierarchical surface structures.[5,6] These materials have engaged increasing consideration because of their exciting applications in self-cleaning,[7] antifouling,[8] anti-icing,[9] drag reduction in fluid transport,[10] and oil/water separation.[11−13] In addition, if transparent superhydrophobic surfaces could be produced, the range of their possible applications could be further extended to glass-based substrates such as windshields for automobile, windows, goggles, and solar panels.[14−17] In the past decades, various kinds of methods including dip-coating,[18] vapor-phase deposition,[19] electrospinning,[20] self-assembly,[21] and so on[22] have been applied to construct superhydrophobic surfaces such as fluoroalkylsilane (FAS)-grafted ceramic membrane,[23] polytetrafluoroethylene-coated glass,[24] stearic acid-modified steel,[25] SiO2-coated sponge,[26,27] TiO2-modified fabric,[28] and so forth.[29] Unfortunately, many fabricated superhydrophobic surfaces can be easily damaged by the application of an external force because of the poor mechanical stability of the rough surface structure and weak adhesion between low-surface-energy coating and the substrates, which greatly limits their application and development.[30−32] It is challenging to realize both robustness and superhydrophobicity simultaneously by a facile and cost-effective approach.

Number of conventional strategies are employed to enhance the robustness of superhydrophobic surfaces, such as creating covalent bond between superhydrophobic coating and substrate,[31] microscale protuberance to protect nanoscale structures,[33] adding elastic and soft materials to the coatings,[34] fabrication of bulk superhydrophobic materials,[35] fabrication of self-healing superhydrophobic surfaces,[36] and so on.[37−40] For instance, Deng et al.[31] prepared a mechanically robust superhydrophobic surface by the chemical vapor deposition method. As-prepared coating was composed of porous silica capsules where the particles were chemically bonded with each other and to the substrate surface through silica bridges that formed by the hydrolysis and condensation of tetraethoxysilane. The obtained surface maintained its superhydrophobicity after performing abrasion tests with double-sided tape and sand paper. Besides, Zhang et al.[41] fabricated a TiO2/SiO2/polypropylene composite bulk material with superhydrophobic property based on the powder pressing approach, which could retain desirable water repellency even after being mechanically damaged. Additionally, Lin et al.[42] reported a self-healing superhydrophobic coating which contains hydrophobic nanoparticles, fluorinated alkyl silane (FAS), and fluorocarbon surfactant. The coating exhibited self-healing ability against both chemical and physical damages after being heat treated at 135 °C. However, the above-mentioned fabrication methods require complex processes or expensive equipment, which greatly narrows their wide application in industry.

Recently, Lu et al.[43] developed an ethanol-based suspension of dual-scale TiO2 nanoparticles modified with perfluorooctyltriethoxysilane to form a paint that could be coated onto various substrates to create a superhydrophobic and superoleophobic surface. They proposed the use of commercial adhesives to bond the paint and substrates for the first time. After enhancing the robustness by using adhesives, the painted surface could maintain its superhydrophobicity even after being scratched by a knife and also 40 abrasion cycles with sandpaper. Similarly, Chen et al.[44] prepared a superhydrophobic surface by spraying “adhesive + hydrophobic CaCO3 nanoparticles (modified with 1H,1H,2H,2H-perfluorooctyltriethoxysilane)”. The as-prepared surface exhibits good self-cleaning property and shows durability against sandpaper abrasion and knife scratch. However, above-mentioned superhydrophobic coatings are not transparent, and the fluorinated reagents used in the coating are quite expensive and not environmentally friendly, which limit their practical applications related to glass or solar energy. Consequently, to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surface is highly desirable.

Inspired by the pioneer research, we present a versatile strategy to prepare mechanically robust, environmentally friendly, and transparent superhydrophobic coating applicable for both hard and soft substrates. In this work, we propose a ferroconcrete structure that is composed of adhesive particles which act as the skeleton and functional particles for the filler to further enhance the robustness of hydrophobic surfaces. Moreover, inexpensive and fluorine-free SiO2 nanoparticles modified with dimethyldichlorosilane (DDS) was selected as a hydrophobic functional paint. To construct the ferroconcrete structure, the commercial adhesive particles and an ethanol suspension of SiO2 nanoparticles were sprayed consecutively and repetitively onto the substrates. This superhydrophobic surface fabricating strategy is applicable for both soft and hard substrates such as glass slides, stainless steel meshes, and sponges. As-prepared superhydrophobic surfaces display significant likely applications in oil/water separation and self-cleaning. This versatile method provides a facile, low-cost, and highly efficient solution for the fabrication of environmentally friendly superhydrophobic surfaces and possesses promising potential in large-scale industrial applications.

Experimental Section

Materials

All required materials were of commercial standard and were utilized without any purification, which could lower the cost of fabrication. The hydrophobic fumed silica nanoparticles having an average size of 16 nm (modified with DDS, AEROSIL R972) were purchased from Degussa, Germany. Spray adhesive 75 was obtained from 3M Co., Ltd. Ethanol and acetone were purchased from Beijing Chemical Reagent Co., Ltd., China. Sandpaper (grit no. 800) was purchased from Beijing Dongxin Grind Instrument Co., Ltd., China. Carbon tetrachloride and decane were purchased from J&K, China. The glass slides (CAT. no. 7101) were purchased from Sail Brand. The stainless steel mesh and PU sponge were bought from the local supermarket. All silicon wafers, glass slides, stainless meshes, and sponges were ultrasonically cleaned before utilizing.

Preparation of the Superhydrophobic Coating

In brief, a paint-like suspension was formulated by adding 5 g of DDS-modified SiO2 into 95 g of absolute ethanol and magnetically stirred for 1 h. Then, all of the superhydrophobic surfaces were assembled by the spray-coating method. The adhesive was first sprayed on the surface of all substrates for 3 s with a distance of 400 mm. Then, the hydrophobic SiO2 nanoparticle suspension was sprayed on the surface of adhesive-modified substrates for 6 s with a distance of 400 mm. This mechanism was characterized as one cycle of preparation. The ferroconcrete-structured superhydrophobic coating on the substrates was fabricated by 1–10 cycles. All as-prepared substrates were dried in air for 5 min before characterization.

Characterization

The morphologies of all substrates were examined by a HITACHI SU8220 field emission scanning electron microscope. The surface composition and chemical states were identified by energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS, PHI Quantera II), and Fourier transform infrared spectroscopy (FTIR, IRTracer-100, SHIMADZU). All of the samples were sputtered with Pt before scanning electron microscopy (SEM) tests. The transmittance of pristine and as-prepared glass was measured by a UV–Vis–NIR spectrophotometer (UV-3600 plus, SHIMADZU). The water sliding angle and water contact angle (WCA) were evaluated by OCA 25 (Data-Physics, Germany) at ambient temperature.

Robustness Test

The sandpaper of grit 800 was elected as an abrasive surface to test the durability of the as-prepared superhydrophobic glass slides. The as-prepared glass slide surface was placed face down onto the sandpaper. The robustness test is achieved by performing a reciprocating motion of the glass slides at a velocity of 10 mm/s with a round trip distance of 20 cm. The above round trip movement was characterized as one cycle of abrasion. The wettability of the sample was evaluated by measuring the WCA after every 15 cycles.

Oil/Water Separation

The coated and uncoated stainless steel meshes were placed between the two glass tubes, and this assembly was placed on a beaker. The oil/water mixture was drained gradually into the above glass tube. In order to calculate the separation efficiency, following relation has been utilizedwhere m0 is the initial mass of water and m1 is the water mass after separation. For measurement of separation efficiency, the oil/water mixture that consists of 10 g of water and 10 g of oil was prepared and used in each separation experiment.

Results and Discussion

Characterization of Modified Surfaces: Morphology, Composition, and Wettability

The mechanically durable superhydrophobic surface was prepared by following the procedure, as shown in Figure . The hydrophobic SiO2 nanoparticles were used to construct the hierarchical rough structure with low-surface-energy property on the substrate surface. Meanwhile, the spray adhesive was used to enhance the strength of coating and the bonding force between SiO2 nanoparticles and substrate. The surface morphology and element analysis of adhesive and modified SiO2 are shown in Figure S1. It has been observed that the sprayed adhesive exhibits smooth surface with the main element of carbon, whereas the hydrophobic SiO2 nanoparticles with an average diameter size of 16 nm contain carbon, oxygen, and silicon. Furthermore, the FTIR spectra of spray adhesive and modified SiO2 are shown in Figure S2a; it can be seen that the main composition of spray adhesive is hydrocarbon. In the spectra of hydrophobic SiO2 nanoparticles, the two intense peaks of 3471 and 1105 cm–1 can be attributed to the silanol group and Si–O–Si structure.[27,45] C–H stretching and C–H bending were detected at 2962 and 1373 cm–1, respectively, which further indicated the presence of CH3 due to the silanization of SiO2 nanoparticles by DDS. Inspired by the reinforced concrete structure that widely used in construction engineering, here, we present a superhydrophobic coating with a ferroconcrete structure where the adhesive particles are used as a skeleton and the SiO2 nanoparticles are used as the filler. In order to fabricate this structure, the adhesive particles and SiO2 nanoparticles were sprayed consecutively and repetitively onto the substrate. This composite coating is well mixed and randomly distributed the adhesive and SiO2 nanoparticles.

Figure 1

Schematic illustration of the fabrication procedure of the superhydrophobic surface.

The silicon wafer coated with six cycles of spray adhesive/SiO2 composite coating was used to investigate the properties of coating. The cross-sectional SEM images showed that the average thickness of as-prepared coating is about 180 μm (as shown in Figure a,b) According to the surface topography, it is obvious that the coating is composed of region A (smooth) and region B (rough). Moreover, the higher resolution SEM images and EDS analysis of region A and region B are shown in Figure c,d, respectively. It can be confirmed that region A belongs to spray adhesive and region B belongs to SiO2 nanoparticles. From the distribution maps of characteristic elements corresponding to spray adhesive and SiO2 (Figure e), it can be clearly seen that the SiO2 nanoparticles are immobilized over the substrate by spray adhesive. Besides, the spray adhesive and SiO2 nanoparticles were superimposed and well mixed.

Figure 2

SEM images and element distribution maps of super 75 and SiO2 nanoparticle-coated silicon wafer. (a,b) Cross-sectional SEM images of the coated silicon wafer. (c) SEM images and EDX analysis of region A; (d) SEM images and EDX analysis of region B. (e) Element distribution maps of the cross section of the coated silicon wafer.

The surface morphologies of coated glass slide were also analyzed by field emission SEM and are shown in Figure . It can be seen that the surface of pristine glass slide was smooth and neat (Figure a), whereas the as-prepared glass slide coated with SiO2 nanoparticles exhibits a dual-scale rough structure (Figure b,c). The nanoscale surface roughness of coating was further confirmed by the atomic force microscopy (AFM) measurement (Figure d). Meanwhile, as shown in Figure e, before spray-coating treatment, the signals of O, Na, Mg, Al, Si, and Ca, which were the dominant elements of glass, were detected.[44] However, after being coated with spray adhesive and SiO2 nanoparticles, obvious increase of C signals from 3.90 to 29.36% was detected by EDX analysis, which can be contributed to the existence of hydrophobic SiO2 coating and spray adhesive (Figure f). This conclusion can be further confirmed by the XPS spectrum of as-prepared superhydrophobic glass slides, as shown in Figure S2b.

Figure 3

SEM images of (a) pristine and (b,c) coated glass slide. (d) AFM image of coated glass slide. EDX measurements of (e) pristine and (f) coated glass slide.

The wettability of pristine and coated substrates was also measured. The pristine glass slide exhibited hydrophilic property with a WCA of ∼38° (Figure a). However, the water droplets stayed in spherical shape on the as-prepared glass slide with a WCA of ∼160°, indicating that the coated glass slide performs excellent superhydrophobic property (Figure b). Additionally, the as-prepared slide exhibited not only superhydrophobicity but also ultralow adhesion to a water droplet. The rolling process of a water droplet (5 μL) on the as-prepared slide is shown in Figure a (Movie S1, Supporting Information). The extreme tilt angle of the slide when the water droplet began to roll was only 1.5°, which means the (sliding angle) SA is 1.5°. There was no obvious change in superhydrophobicity by the increase of spraying cycles (Figure S3). As shown in Figure b, a water droplet (4 μL) was forced to sufficiently contact with the coated slide; when the droplet was lift up, it remained in spherical shape and there was not any residual water on the coated slide (Movie S2, Supporting Information), confirming the ultralow adhesion of this superhydrophobic surface. This superhydrophobic state can be analyzed by the Cassie–Baxter state where the air is trapped in the valley of the rough structures and greatly curtails the contact area between solid and water surface.[4,46,47] The wettability transformation of glass slide from hydrophilic to superhydrophobic could be linked to the two factors: the dual-scale rough structures obtained from SiO2 nanoparticles and the low surface energy of DDS. On the contrary, it was found that the as-prepared slide exhibits excellent superoleophilic characteristic. Figure c shows that when an oil droplet was added dropwise on the coated surface, it would quickly spread out and fully wet the surface within 0.24 s (Movie S3, Supporting Information). Besides, the transmittance curves and optical images of coated glass slides with different spraying cycles are shown in Figure S4; it can be seen that the coated glass slides were highly transparent and their transparency decreased with the increase in spraying cycles.

Figure 4

WCA measurements of (a) pristine and (b) coated glass slides.

Figure 5

(a) Water droplet rolling on the as-prepared glass slide surface with a tilt angle of ∼1.5°. (b) Water droplet making contact and losing contact with the as-prepared glass slide surface. (c) Dripping an oil droplet on the coated glass slide surface.

Mechanical Robustness

Abrasion test was examined to be a universal method to assess the durability of the as-prepared superhydrophobic surface against the physical force.[48−50] In this work, a glass slide was engaged for abrasion test by using a sand paper, as shown in Figure . The as-prepared glass sample was situated face down to the sandpaper (aluminum oxide sandpaper of 800 mesh) under a weight of 100 g and moved for 20 cm along the ruler (Figure a,b). The one reciprocating slide is prescribed as one cycle of abrasion, and the WCAs were measured after every 15 cycles. The WCAs of the as-prepared glass slide with different spraying cycles and after abrasion are shown in Figure c. It can be clearly seen that the robustness of the as-prepared glass slide increased greatly with the increase in number of spraying cycles. Moreover, the coated glass slide with six spraying cycles remained its superhydrophobic property (WCA > 150° and SA < 10°) even after 325 abrasion cycles, displaying great mechanical abrasion resistance of the coated glass slide. This ferroconcrete structure superhydrophobic coating exhibited outstanding performance in enhancing the mechanical robustness. The coated glass slide with six spraying cycles could withstand over 325 abrasion cycles, which is nearly 10 times longer than that reported in previous refs (44) and (51). Additionally, the characterization of the glass slides coated with six spraying cycles including their appearance, wettability, and surface morphology is shown in Figure . It can be seen that more and more black substance appears on the surface with the increase of abrasion cycle, which can be attributed to the residual of abrasion grains from sand paper. After 600 times of abrasion cycle, although the WCA of the as-prepared glass slide is slightly decreased, its surface was still completely protected by spray adhesive/SiO2 nanoparticle composite coating, showing excellent mechanical robustness.

Figure 6

(a,b) Sandpaper abrasion test as-prepared glass slide (one cycle of test). (b) Plot the WCA of coated glass slide after scratch cycles.

Figure 7

(a–c) Characterization of a glass slide coated with six spraying cycles (appearance, wettability, and morphology before abrasion, after 200 abrasion cycles, and 400 abrasion cycles, respectively). The water (dyed blue) was used to test the wettability.

This universal superhydrophobic and superoleophilic coating fabrication method could be utilized to prepare oil/water separation materials. By using the spray “adhesive + hydrophobic paint” method, we coated the hydrophobic silica nanoparticles on porous substrate surfaces (e.g., stainless steel mesh and sponge). Then, the coated porous substrates were employed to selectively filtrate or adsorb oil from the oil/water mixture.

After spray coating treatment, the mesh wires were wrapped by SiO2 nanoparticles and showed hierarchical rough structure (Figure S5). Moreover, the coated mesh exhibited excellent water-repellent property and had a low sliding angle of ∼2° (Figure S6). The oil/water separation experiments are conducted, as shown in Figure a,b. When the oil/water mixture was poured into the upper tube, it can be seen that only the oil (CCl4) could permeate the as-prepared mesh and flow inside the beaker placed below. All the water was hold above the surface because of the superhydrophobic property of the coated mesh (see Figure a and Movie S4). Hence, the coated mesh could realize the separation of oil and water by a simple filtering method. On the contrary, when the oil/water mixture was drained onto the pristine mesh, both oil and water could pass though the mesh, resulting in the failure of separation (Figure b and Movie S5). In addition, a superhydrophobic and superoleophilic sponge was prepared through the same method. As shown in Figure c, the coated sponge exhibited great superhydrophobic property. When the as-prepared sponge was brought into contact with the oil/water mixture, the oil (decane) was fully absorbed by the coated sponge within few seconds and left pure neat water in the beaker. The as-prepared superhydrophobic/superoleophilic mesh/sponge could separate diverse oil/water mixtures with high efficiency of above 98% (Figure a).

Figure 8

Oil/water mixture separation test of (a) pristine and (b) as-prepared mesh. (c) Images of water droplets on the pristine and coated sponge surface. (d) Coated sponge absorbs oil (red) from an oil/water mixture.

Figure 9

(a) Separation efficiency of the as-prepared mesh for various oil/water mixtures. (b) Relationship between the pore diameters and intrusion pressure of coated meshes (the inset is the experimental setup).

To further thoroughly understand the separation mechanism of as-prepared superhydrophobic and superoleophilic porous materials, the liquid-wetting model is shown in Figure S7. The pores were assumed to have cylindrical geometry; the breakthrough pressure (ΔPC) can be determined by the Young–Laplace equation[52]where γL is the surface tension of the liquid and θ is the intrinsic contact angle of the liquid on the flat surface. The rp represents the pore radius. As inferred from eq , the superhydrophobic surface can withstand a certain external pressure because of ΔPC > 0 (Figure S6a). In the meanwhile, the oil can spontaneously pass through this superoleophilic surface because of ΔPC < 0 (positive capillary effect), as shown in Figure S6b. Furthermore, the oil can be trapped into the rough structure and form oil film on the surface during the separation process, which will prevent the water droplet contacting with the solid surface, thus resulting in under-oil superhydrophobicity. Therefore, the water cannot permeate through the surface that immersed in oil (Figure S6c). If the superhydrophobic surface is drenched in water, the air will be trapped into the rough structure and form an air layer between surface microstructure and surrounding water.[53] In this scenario, once the oil was dropped on this surface, it will enter into this thin air layer and quickly spread out under water pressure and capillary effect (Figure S6d). Therefore, the superhydrophobic and superoleophilic porous materials could allow the oil to pass through freely, whereas repel water completely. Intrusion pressure is a crucial property for the oil/water separation material, which is defined as the maximal pressure that the material surface can support. In order to ensure the separation of oil and water, the external liquid pressure should be kept less than intrusion pressure. The intrusion pressure of as-prepared meshes with different pore sizes is shown in Figure b; it can be seen that the coated mesh could support a high external pressure up to 3430 Pa, and their intrusion pressure decreased with the increase of pore size.

Conclusions

In conclusion, we have presented a facile and versatile strategy to prepare environmentally friendly, mechanically robust, and superhydrophobic surfaces with excellent transparency by the spraying method, which is applicable for both hard and soft substrates. The coated substrates exhibited excellent superhydrophobic property and ultralow adhesion with water. Moreover, they retained superhydrophobicity even after 325 sandpaper abrasion cycles, showing outstanding mechanical robustness. Furthermore, the coated surfaces can also be employed to separate oil/water mixtures because of their characteristics of being simultaneously superhydrophobic and superoleophilic. In short, it is believed that this superhydrophobic surface fabrication method is significant, promising, and feasible for plenty of industrial applications.