Facile Fabrication of Superhydrophobic and Underwater

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Cite This: ACS Appl. Nano Mater. 2018, 1, 4894−4899

Facile Fabrication of Superhydrophobic and Underwater Superoleophobic Coatings Lu Tie,†,§ Jing Li,*,† Mingming Liu,†,§ Zhiguang Guo,*,†,‡ Yongmin Liang,†,∥ and Weimin Liu*,†

ACS Appl. Nano Mater. 2018.1:4894-4899. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/02/18. For personal use only.

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State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ‡ Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: Water-repellent and water-loving components are usually used to prepare superhydrophobic and underwater superoleophobic surfaces, respectively. In this work, a facile approach for the fabrication of coatings with the opposite superwettabilities is presented. Hydrophilic nanoparticles (TiO2, SiO2, and Al2O3) and fluorocarbon surfactants were mixed in water. The obtained aqueous suspensions were simply dipped, brushed, or sprayed onto various substrates, such as fabric, sponge, cotton, nickel foam, stainless steel mesh, copper sheet, glass, and ceramic. The prepared coatings show superhydrophobic and underwater superoleophobic properties with contact angles above 150° and sliding angles below 10°, and they exhibit high resistance to liquid impact, sandpaper abrasion, and acid/base treatment. Taking advantage of its unusual superwettability, the coated fabric can be used for on-demand oil−water separations without a continuous external stimulus. The proposed coatings with opposite superwettabilities will demonstrate the complementary advantages in a variety of interfacial applications. KEYWORDS: superhydrophobic, underwater superoleophobic, hydrophilic nanoparticles, fluorocarbon surfactants, on-demand oil−water separation

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INTRODUCTION Bioinspired superwetting surfaces are of great importance to fundamental research and demonstrate distinguished performance in a broad range of interfacial applications, such as antifouling, anti-icing, anticorrosion, catalysis, microfluidic devices, drag reduction, water harvesting, and oil−water separation.1−10 Nanomaterials and nanotechnologies combined with surface modification technologies have been widely developed to fabricate superhydrophobic and underwater superoleophobic surfaces with high roughness, which reduces the contact area between the liquid droplets and the solid.11−15 Inspired by the self-cleaning effect of lotus leaves, superhydrophobic surfaces are prepared by using low-surface-energy materials, such as polytetrafluoroethylene (PTFE), copper, carbon nanotubes, and low-surface-energy modifiers.16−20 Because the unique underwater−oil-repellent properties of clam shells and fish scales were discovered, a variety of hydrophilic materials with high surface energies were employed to construct underwater superoleophobic surfaces, such as hydrogels, zeolites, graphene oxide, metal−organic frameworks, and inorganic oxides.21−26 Superhydrophobicity © 2018 American Chemical Society

in air and superoleophobicity in water are opposite extreme wetting properties that show different advantages in interfacial applications. Thus, there is a high demand for preparing materials with opposing superwetting properties on the same surface. Recently, intelligent surfaces with switchable superwettability, between superhydrophobicity and underwater superoleophobicity, were developed using external stimuli, such as temperature, light, pH, and electricity.27−31 However, achieving superhydrophobic and underwater superoleophobic states simultaneously on one surface without external stimuli is rare. In this work, we present a facile approach for fabricating both superhydrophobic and underwater superoleophobic coatings. Foraperle 323 (F323), Zonyl 321 (Z321), and Zonyl 9977 (Z9977), commercial water-soluble fluorocarbon surfactants from DuPont, were used as low-surface-energy materials. Hydrophilic nanoparticles, such as TiO2, SiO2, and Received: June 26, 2018 Accepted: August 13, 2018 Published: August 13, 2018 4894

DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899

Article

ACS Applied Nano Materials Al2O3, were employed to construct rough surfaces. After the fluorocarbon surfactants and hydrophilic nanoparticles were mixed in water, the water-repellent and water-loving components were integrated on the same surface simply by dipping, brushing, and spraying the aqueous suspensions on various substrates, including fabric, sponge, cotton, nickel foam, stainless steel mesh (SSM), copper sheet, glass, and ceramic. The obtained coatings maintained their superhydrophobic and underwater superoleophobic properties throughout liquid impact, sandpaper abrasion, and acid/base treatment. Moreover, the coated fabric was used for ondemand oil−water separation without continuous external stimuli.

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EXPERIMENTAL SECTION

Materials. Fabric, sponge, cotton, nickel foam, SSM, copper sheet, glass, ceramic, diesel, and TiO2, SiO2, and Al2O3 nanoparticles were commercially available. F323, Z321, and Z9977 were purchased from Shanghai Jianbang Industry (China). A PTFE aqueous suspension (60 wt %) was provided by Aladdin. All other chemical reagents were of analytical grade and were used without further purification. Deionized water was used throughout the work. Preparation of Superhydrophobic and Underwater Superoleophobic Coatings. An aqueous suspension with a weight ratio of F323:P25:H2O = 1:1:100 was prepared. Afterward, the cleaned substrates were immersed in the aqueous suspension and then dried in an oven at 60 °C. The F323−P25-coated substrates were heattreated at 130 °C for 3 h. The aqueous suspension was also sprayed on various substrates using a spray gun with 0.2 MPa of N2 gas and then heat-treated at 130 °C for 3 h. SiO2 and Al2O3 were also used as hydrophilic nanoparticles to replace P25. Other commercial fluorocarbon surfactants, such as Z321 and Z9977, were employed to prepare superhydrophobic and underwater superoleophobic coatings. Furthermore, the weight ratio of F323:P25:H2O was adjusted to 1:1:2 to generate a viscous suspension. Then, the aqueous suspension was brushed on the surfaces of the substrates, and then they were heat-treated at 130 °C for 3 h. Characterization. Photographs were obtained using a digital camera. X-ray diffraction (XRD, X’SPERT PRO) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) measurements were performed to investigate the chemical compositions of the samples. The surface morphologies and compositions were studied by scanning electron microscopy (SEM, JEOL JSM-6701F and JSM-5600LV). The samples were sputtercoated with Au. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed to determine the element distribution and percentages. The morphologies of the used nanoparticles were observed using transmission electron microscopy (TEM, Tecnai G2 TF20, FEI). Contact angles and sliding angles were recorded on a contact angle system (JC20001, Zhongchen Digital Equipment Co., Ltd., Shanghai, China).

Figure 1. Schematic diagram of the preparation of superhydrophobic and underwater superoleophobic coatings.

Figure 2. Photographs of the F323−TiO2-coated samples by the dipping (a), brushing (b), and spraying (c) methods. In (a), the substrates are fabric (1), nickel foam (2), sponge (3), and cotton (4). In (b), the substrates are SSM (1), copper sheet (2), glass (3), and ceramic (4). In (c), the substrates are SSM (1), nickel foam (2), fabric (3), and ceramic (4). The rows below are photographs of a water droplet in air (left) and an oil droplet (1,2-dichloroethane, right) in water on the sample surfaces.

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RESULTS AND DISCUSSION Common substrates, whether soft or hard, porous or flat, hydrophilic or hydrophobic, can be used to build superhydrophobic and underwater superoleophobic coatings (Figure S1). In a typical synthesis (Figure 1), an aqueous suspension with a weight ratio of F323:P25:H2O = 1:1:100, where P25 is commercial TiO2 nanoparticles with an average diameter of approximately 20 nm (Figure S2), is prepared. Then, porous substrates are immersed in the aqueous suspension. Figure 2a shows photographs of the F323−P25coated fabric, nickel foam, sponge, and cotton. The spherical water droplets (dyed blue) easily roll off the dip-coated porous substrates with slight vibrations. An advantage of the F-rich F323 is that the formed stable solid/air composite surfaces can

fully support the water droplets. When the coated substrates are prewetted by ethanol and then immersed in water, the ethanol layer can be replaced with water to form stable solid/ water composite surfaces, resulting in their unique oil-repellent property. In water, oil droplets (such as droplets of 1,2dichloroethane) are efficiently repelled by the water layer on the surfaces of the dip-coated samples and take on obviously spherical shapes. The weight ratio of F323:P25:H2O was adjusted to 1:1:2 to generate a viscous suspension, which was brushed on the hard substrates to fabricate the superwettable coatings. As shown in Figure 2b, water (in air) and oil (in water) droplets on the surfaces of the brush-coated SSM, copper sheet, glass, and 4895

DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899

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ACS Applied Nano Materials

makes the spray-coated SSM superhydrophobic and underwater superoleophobic. To further assess the wetting properties of the prepared coatings, the static contact angles and sliding angles were measured. Figure 3 shows the water contact angles in air and the oil contact angles in water as well as the corresponding sliding angles of the dip-coated, brush-coated, and spray-coated samples. As expected, all of the water contact angles in air and the oil contact angles in water of the prepared samples were above 150°, and all of the sliding angles were below 10° (Figure S3 and Movie S1). The aqueous F323−P25 suspensions can be used to fabricate both superhydrophobic and underwater superoleophobic coatings by simple dipping, brushing, and spraying methods. The surface structures and chemical components play an important role in the formation of the superwettable surfaces. Figure 4a,b shows the SEM images of the dip-coated fabric. Compared to the original fabric, which has smooth surfaces, the nanoparticles are well distributed and tightly bound on the fabric wires, resulting in micro-/nanohierarchical structures of the F323−P25 coating. The samples obtained by the dipping, brushing, and spraying methods exhibited similar surface structures (Figures S7−S12). Furthermore, XRD, XPS, and EDS measurements were conducted to investigate the chemical components on the surfaces. As shown in Figure 4c, the F323−P25-coated SSM retains the characteristic XRD peaks at 25.2° and 27.4° of P25 (anatase and rutile). In addition, the elements detected on the superwettable coatings

Figure 3. Water contact angles in air and oil contact angles in water and corresponding sliding angles of the dip-coated, brush-coated, and spray-coated samples.

ceramic maintain almost spherical shapes. In addition, spraying can be used to form uniform coatings. Figure 2c shows photographs of the spray-coated SSM, nickel foam, fabric, and ceramic. The sprayed coatings can be used to regulate the surface properties of the substrates. For example, the original SSM is hydrophobic and oleophobic underwater. In contrast, the F323−P25 coating does not change the appearance but

Figure 4. (a, b) SEM images of the dip-coated fabric. (c) XRD patterns of the original and spray-coated SSMs. (d) C 1s XPS spectrum of the dipcoated fabric. 4896

DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899

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ACS Applied Nano Materials

Figure 5. (a−d) Photographs of a water droplet in air (left) and an oil droplet (1,2-dichloroethane, right) in water on the sample surfaces: (a, b) the F323−SiO2-coated and F323−Al2O3-coated fabrics by the dipping method and (c, d) the Z321−TiO2-coated and Z9977−TiO2coated ceramics by the spraying method. (e) Water contact angles in air and oil contact angles in water and corresponding sliding angles of the F323−SiO2, F323−Al2O3, Z321−TiO2, and Z9977−TiO2 coatings.

Figure 6. Variation of the water contact angles in air and the oil contact angles in water of the Z321−TiO2-coated ceramic as functions of the number of cycles of the ethanol (a) and water (b) impact tests. Insets show the processes of the ethanol and water impact tests.

mainly include O, C, Ti, and F (Figure S13). From the EDS analysis, their weight ratios of the spray-coated fabric were 42.9%, 30.6%, 22.2%, and 4.3%, respectively. For the C 1s XPS spectrum, the peaks at 284.8, 286.0, 289.0, 291.7, and 294.2 eV correspond to C−C, C−O, CO, −CF2, and −CF3 (Figure 4d), respectively. The low-surface-energy −CF2 and −CF3 groups lead to excellent water repellency. On the other hand, the C−O and CO groups make the prepared coatings hydrophilic. In addition to TiO2 and F323, other hydrophilic nanoparticles (SiO2 and Al2O3) and fluorocarbon surfactants (Z321 and Z9977) were also used in the construction of unusual superwettable coatings (Figure 5 and Figures S14−S22). The fabrics were immersed in aqueous suspensions containing SiO2 (or Al2O3) nanoparticles and F323. In addition, the ceramic surfaces were sprayed with the aqueous suspensions containing P25 and Z321 (or Z9977). The obtained coatings have water contact angles in air and oil contact angles in water of more than 150° and corresponding sliding angles of less than 10°. We believe that the synergy between the high-surface-energy (hydrophilic nanoparticles) and low-surface-energy (fluorocarbon surfactants) components facilitates the fabrication of superhydrophobic and underwater superoleophobic coatings. In contrast, hydrophobic PTFE was used to replace the hydrophilic nanoparticles (Figure S23). The prepared F323− PTFE, Z321−PTFE, and Z9977−PTFE coatings displayed superhydrophobic and underwater superoleophilic properties (Figure S24), further demonstrating the significance of the hydrophilic nanoparticles in the fabrication of the super-

hydrophobic and underwater superoleophobic coatings. The formation of superhydrophobic surfaces does not require the low-surface-energy components to be fully covered. In water, the hydrophilic nanoparticles on the formed superhydrophobic surfaces can trap water molecules to repel oil, showing underwater superoleophobic properties. Fluorocarbon surfactants were used to prepare robust superamphiphobic surfaces.32 For example, Z321 consists of a fluoropolymer core and a polyurethane shell that can adhere to the surfaces of most substrates. Here, ethanol and water impact tests were performed to evaluate the mechanical stability of the Z321−P25-coated ceramic in which the flows of ethanol and water using a washing bottle rapidly and steadily impacted the ceramic surface. The contact angles were measured for 100 mL of ethanol and 100 mL of water. As shown in Figure 6, ethanol easily wets and effectively impacts the ceramic surface. In contrast, water droplets randomly impact the ceramic surface and promptly bounce without any residual small water droplets (Movies S2 and S3). After 10 cycles of impact tests, the Z321−P25 coatings still retain their micro-/nanohierarchical structures and thus maintain their unique superwettability (Figure S25). A sandpaper abrasion test was further used to test the mechanical stability of the F323−P25-coated fabric. The dip-coated fabric (3 cm × 3 cm) was placed against sandpaper (grit no. 320) under a 50 g weight and was moved 10 cm by an external force and returned 4897

DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899

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ACS Applied Nano Materials

Figure 7. (a, b) Separation of hexane (dyed red)−water (a) and 1,2-dichloroethane−water (dyed blue, b) mixtures using the dip-coated fabric. (c, d) Cycling performance of the dip-coated fabric in alternating separations of hexane−water (blue) and 1,2-dichloroethane−water (red) mixtures. In each cycle, the flux (c) and separation efficiency (d) were measured.

superhydrophobic and underwater superoleophobic properties after 10 cycles of oil−water separation (Figure S31). Therefore, the proposed coatings with both superhydrophobic and underwater superoleophobic properties demonstrate great potential for high-efficiency on-demand separations of oil and water.

back to its starting position (Figure S26). After 100 abrasion cycles, the composite coating maintains its superhydrophobic and underwater superoleophobic properties. There were no obvious differences in the microscopic surface topographies of most regions. The weight ratios of C, O, Ti, and F were 48.4%, 39.7%, 9.2%, and 2.7%, respectively (Figure S27). In addition, the F323−P25-coated fabric still exhibits superhydrophobic and underwater superoleophobic properties for acid/base solutions even after immersion in acid/base solutions for 48 h (Figure S28). The good mechanical and chemical stability makes the prepared coatings highly applicable. Because of the superhydrophobic and underwater superoleophobic properties, various oil−water mixtures were used to evaluate the separation abilities of the prepared fabric. When a hexane−water mixture was poured onto fabric with the F323− P25 coating that had been prewetted with water, water quickly passed through the fabric, whereas the hexane (dyed red) was blocked and could not be detected in the collected water (Figure 7a and Movie S4). In contrast, a 1,2-dichloroethane− water mixture can be directly separated by the F323−P25coated fabric without prewetting (Figure 7b and Movie S5). Flux was calculated by measuring the time required to separate mixtures of 30 mL of water and 30 mL of oil. The separation efficiency was determined according to mseparation/minitial, where mseparation was the weight of the collected water or oil after oil− water separation and minitial was the weight of water or oil before the oil−water separation. As shown in Figure S30, the fluxes and separation efficiencies are more than 3000 L/(m2 h) and 99.0%, respectively. To further test the recyclability of the on-demand oil−water separation methodology, the coated fabrics were used to separate alternating mixture of hexane and water and 1,2-dichloroethane and water. After 10 cycles, the water and oil fluxes remained approximately 4000 and 10000 L/(m2 h), respectively, and the separation efficiencies reach ∼99.6%. Moreover, the used fabric continued to display

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CONCLUSIONS In summary, a facile approach for preparing materials with two opposite superwettabilities, such as superhydrophobicity and underwater superoleophobicity, on one same surface is proposed. Aqueous suspensions containing low-surface-energy fluorocarbon surfactants and high-surface-energy nanoparticles (such as TiO2, SiO2, and Al2O3) are simply dipped, brushed, or sprayed on various common substrates, forming unusual superwettable coatings. The obtained coatings are resistant to liquid impact, sandpaper abrasion, and acid/base treatment and can be used for on-demand oil−water separations without a continuous external stimulus. This study will lead to extensive development of interfacial materials with opposite superwettabilities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01074. Contact angles and SEM images of the original substrates; TEM images of the used nanoparticles; sliding angles, SEM images, element distribution maps, and XPS spectra of the prepared coatings by the dipping, brushing, and spraying methods; SEM images and contact angles of the prepared PTFE-based coatings; SEM images of the coatings after the liquid impact test; the coatings subjected to sandpaper abrasion and acid/ 4898

DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899

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base treatments; on-demand oil−water separations using the coated fabric; contact angles of the coated fabric after 10 cycles of oil−water separations (PDF) Movies S1−S5 (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.L.). *E-mail [email protected] (Z.G.). *E-mail [email protected] (W.L.). ORCID

Lu Tie: 0000-0001-5478-5622 Jing Li: 0000-0002-4183-6440 Zhiguang Guo: 0000-0002-4438-3344 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (Nos. 51735013, 51522510, and 51675513).

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DOI: 10.1021/acsanm.8b01074 ACS Appl. Nano Mater. 2018, 1, 4894−4899