Inorganic Adhesives for Robust Superwetting Surfaces - ACS Nano

Jan 3, 2017 - The emulsion separation tests were carried out under gravity only. .... Biomimetic Super-Hydrophobic Engineering Materials J. Am. Chem. ...
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Inorganic Adhesives for Robust Superwetting Surfaces Mingming Liu,†,§ Jing Li,*,† Yuanyuan Hou,†,§ and Zhiguang Guo*,†,‡ †

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 S Supporting Information *

ABSTRACT: Superwetting surfaces require micro-/nanohierarchical structures but are mechanically weak. Moreover, such surfaces are easily polluted by amphiphiles. In this work, inorganic adhesives are presented as a building block for construction of superwetting surfaces and to promote robustness. Nanomaterials can be selected as fillers to endow the functions. We adopted a simple procedure to fabricate underwater superoleophobic surfaces by spraying a titanium dioxide suspension combined with aluminum phosphate binder on stainless steel meshes. The surfaces maintained their excellent performance in regard to oil repellency under water, oil/water separation, and self-cleaning properties after even 100 abrasion cycles with sandpaper. Robust superwetting surfaces favored by inorganic adhesives can be extended to other nanoparticles and substrates, which are potentially advantageous in practical applications. KEYWORDS: inorganic adhesives, underwater superoleophobic, robust surfaces, oil/water separation, self-cleaning separation.14−21 However, it remains a challenge to obtain robust underwater superoleophobic surfaces for practical applications. Inspired by seashell nacre, Xu et al. employed layer-by-layer method to prepare a “bricks-and-mortar” polyelectrolyte/clay hybrid film with underwater superoleophobic and good mechanical properties.22 Here, inorganic adhesives were used to replace organic adhesives and to construct robust underwater superoleophobic surfaces, which enjoy some distinct advantages such as strong adhesion, antioxidation, low toxicity, and outstanding resistance to oil, radiation, and high or low temperature. We fabricated an all inorganic film by mixing aluminum phosphate (AP) as an adhesive with commercial titanium dioxide (TiO2, P25) followed by spraying the suspension on stainless steel meshes (SSMs, 2300 mesh size) and heat treatment. Note that TiO2 nanoparticles are coated without low surface-energy modifiers and still possess high photocatalytic activity. The constructed surface remained superoleophobic in water after even 500 abrasion cycles with sandpaper and maintained high separation efficiency of oil-in-water emulsions and excellent self-cleaning

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rtificial superwetting surfaces have been broadly designed and constructed by learning from nature, such as lotus leaves with superhydrophobic properties and fish surfaces with underwater superoleophobic properties.1−4 Taking the advantages of superwetting surfaces with extreme water or oil repellency, the key is to preserve highly textured structures (high roughness) and steady chemical composites with low or high water affinity.5−7 Generally, these artificial surfaces are very instable because the nanocomposites possess high activity and the nanostructures are readily destroyed. In addition, such rough surfaces have considerable adsorption capacity and are easily contaminated by amphiphiles, losing the surface superwettability. To strengthen the mechanical durability of superwetting surfaces, Lu and coworkers used organic adhesives to bond the paint (perfluorosilane-coated titanium dioxide nanoparticles) to prepare robust superhydrophobic surfaces. These surfaces held their extreme water repellency after abrasion tests with sandpaper.8 In the last one year, organic adhesives have been widely employed to fabricate robust superhydrophobic surfaces.9−13 To date, many artificial superhydrophilic and underwater superoleophobic surfaces have been developed and exhibit distinguished performance in already-established areas, such as marine antifouling, antifogging, catalysis, and oil/water © 2017 American Chemical Society

Received: December 13, 2016 Accepted: January 3, 2017 Published: January 3, 2017 1113

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ature to 240 °C. An apparent endothermic peak appears at about 110 °C owing to the water evaporation. There is another noticeable endothermic peak at about 200 °C, suggesting the cross-linking of the AP binder and the removal of the crystal water. Hence, to fabricate a sturdy coating, the curing temperature held at 120 °C for 2 h and 240 °C for 1 h is desired. The curing of the TiO2−AP coating was analyzed by Xray diffraction (XRD) measurements. After curing, the obtained membrane reserves the characteristic peaks of the original SSM and P25 (anatase and rutile). The appearance of the peaks at approximately 11.7°, 14.7°, 15.5°, and 21.5° is ascribed to the transformation of the AP binder from amorphous to crosslinked crystal structures. Combining the micro-/nanohierarchical structure with the hydrophilic property of TiO2 nanoparticles and the AP binder, the TiO2−AP-treated SSMs show superhydrophilic in air and underwater superoleophobic (Figure 1e,f). A water droplet can rapidly spread on the surfaces of the TiO2−AP coatings only within 1 s (Figures S8a and S9 and movie S1).23−25 To test oil adhesion behavior, an oil droplet contacts the superhydrophilic surface in water and distorts under external pressure. After lifting, the oil droplet restores its original shape and easily detaches without any residue (Figure S8b). Meanwhile, a dichloroethane or hexane droplet (5 μL) can effortlessly roll down or up along the inclined surface (Figure S8c,d). Underwater−oil contact angles (OCAs) and sliding angles of a series of oil droplets are all above 150° and less than 10° (Figure S10), respectively. The mechanical durability of superwetting surfaces is one of important factors that limit widespread applications. However, superwetting surfaces with high roughness are mechanically weak, and their micro-/nanohierarchical structures are readily destroyed and removed. To solve this problem, we applied inorganic phosphate binder as a sophisticated and robust adhesive technique to strongly bond nanoparticles as fillers to substrates. As a result, the obtained TiO2−AP coating appeared to be undamaged after the knife-scratch test. Furthermore, we carried out sandpaper abrasion tests to systematically study the mechanically stability of the TiO2−AP coating. The TiO2−APtreated SSM (3 cm × 3.5 cm) facing down sandpaper (grit no. 320) under a 200 g weight with a diameter about 2.5 cm was moved for 10 cm along a ruler by an external drawing force and returned back (Figure 2a). After 20 abrasion cycles, underwater OCAs and the weight of the TiO2−AP-treated SSM were measured. As shown in Figure 2b, the underwater OCAs are all above 150° and the mass loss of the TiO2−AP coating is about 20% after 500 abrasion cycles, illustrating that the TiO2−AP coating does not lose underwater superoleophobic property after abrasion and displays outstanding mechanical durability. We took the photographs of the TiO2−AP-treated SSM before and after the 100th, 200th, 300th, 400th, and 500th cycle’s abrasion, respectively (Figure S12). The abrasion mainly takes place at the edge of the used weight. Once a spot is damaged, the abrasion region will be gradually aggravated. In order to examine whether the TiO2−AP-treated SSM kept underwater superoleophobicity after 500 abrasion cycles on the whole surface rather than merely on some points, a dichloroethane droplet was manipulated to travel on the whole surface in water. It is found that the oil droplet can be easily pulled without any residue at any place. Correspondingly, underwater−oil sliding angle of the TiO2−AP-treated SSM after 500 abrasion cycles is still less than 10° (Figure S11 and movie S2). Moreover, SEM measurements and element

performance against the pollution of amphiphiles after 100 abrasion cycles. Inorganic adhesives were available for other nanoparticles (such as SiO2 and Al2O3) and substrates (ceramic and glass) to form robust superwetting surfaces.

RESULTS AND DISCUSSION The spray solution was prepared by mixing AP with TiO2 nanoparticles in ethanol/water (3:1 v/v) cosolvent, forming a homogeneous TiO2−AP composite sol. Under operation with 0.2 MPa N2 gas, the TiO2−AP composite was uniformly and rapidly coated on the SSM surface using a spray gun. After heat treatment, the curing of the AP binder facilitated the formation of a robust coating with high mechanical strength. The TiO2− AP coating can be verified by X-ray photoelectron spectroscopy (XPS, Figure S3; see the Supporting Information). Figure 1a−c

Figure 1. (a, b) SEM and (c) cross-section SEM images of the TiO2−AP-treated SSM. (d) XRD patterns of the TiO2−AP-treated SSM (1), the original SSM (2), P25 (3) and AP (4). (e) A water droplet (in air) and (f) a dichloroethane droplet (in water) on the surface of the TiO2−AP-treated SSM.

and Figures S4−S6 show scanning electronic microscope (SEM) images and element distribution maps of original and TiO2−AP-treated SSMs. It is clearly seen that the original SSM with a pore size of ∼5 μm is homogeneously covered by the TiO2−AP coating, forming a micro-/nanohierarchical structure. Energy-dispersive X-ray spectroscopy (EDS) analysis at different magnifications shows nearly the same element percentage of the TiO2−AP coating, indicating that TiO2 nanoparticles and the AP binder are well dispersed throughout the prepared coating. From the cross-section SEM images and the distribution maps of characteristic elements corresponding to the coating and substrate, the average thickness of the sprayed coating is about 10 μm. We used thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) to determine the curing temperature of the AP binder combined with TiO2 nanoparticles. As shown in Figure S7, the TiO2−AP composite shows sharp weight loss and obvious heat absorption from room temper1114

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Figure 2. (a) Photographs of the TiO2−AP-treated SSM before (left) and after (right) 500 abrasion cycles with sandpaper, the circle (dash line) represents the weight place. (b) Plots of abrasion cycles with underwater OCA (solid circle) and mass loss (hollow square). m0 and mx are the mass of the TiO2−AP coating before and after abrasion cycles, respectively. Insets show photographs of dichloroethane droplet on the surface of the TiO2−AP-treated SSM after 500 abrasion cycles with sandpaper at different positions.

Figure 3. SEM images and element distribution maps of the TiO2− AP-treated SSM after 500 abrasion cycles with sandpaper at different positions (see Figure 2a).

shown in Figure S20, the TiO2−AP coating shows a dramatic rupture when the gram load reaches ∼200 g, being equivalent to a binding force of about 2 N. In reality, a force of 5 mN is equal to normal hand cleaning of surfaces. The TiO2−AP-treated SSMs were further immersed in hot water (80 °C), ice−water, salt−water (0.5 M NaCl), and acidic (1 M H2SO4) and alkaline (1 M NaOH) aqueous solutions for one month, respectively. The TiO2−AP-treated SSMs are still underwater superoleophobic and highly stable without any change (Figures S21 and S22). In addition, the TiO2−AP coating with excellent mechanical durability shows the remarkable corrosion resistance to hot water, ice−water, and salt−water. The mass losses of the TiO2−AP coating are 5.3% (original), 6.5% (hot water), 6.4% (ice water), and 8.7% (salt water) after 100 abrasion cycles, respectively. However, the stability is greatly influenced by acidic and alkaline corrosive solutions. The mass losses are increased to 37.7% for acidic solution and 32.4% for alkaline solution. It is suggested that inorganic adhesives such as phosphate binder can be simply and flexibly applied in the construction of extremely robust superwetting surfaces that can survive in various harsh environments. We also prepared the robust superwetting surfaces using SiO2 and Al2O3 nanoparticles as fillers, in which SSM, ceramic, and glass were selected as substrates. Like the TiO2−AP-treated SSM, sandpaper abrasion tests do not obviously destroy the surfaces of TiO2−AP-, SiO2−AP-, and Al2O3−AP-treated SSM, ceramic, and glass (Figures S23−S25). After 100 abrasion cycles, the TiO2−AP, SiO2−AP, and Al2O3−AP coatings still exhibit underwater superoleophobicity with OCAs of above 150° and low mass loss (less than 10%), as shown in Figure 4. SEM measurements and element distribution analysis were further adopted to study the mechanical durability of the SiO2− AP and Al2O3−AP coatings (Figure 5). Similarly, the SiO2−AP and Al2O3−AP coatings keep the integrity and continuity after 100 abrasion cycles. Only characteristic elements with homogeneous distribution corresponding to the coatings can

distribution analysis were used to investigate some obviously abrasive regions (four points) and low-wear regions in detail (Figures S13−S19). Together, most regions of the TiO2−AP coating exhibit low wear after 500 abrasion cycles, except for some scattered small abrasive regions. Figure 3 shows SEM images and element distribution maps of the TiO2−AP-treated SSM after 500 abrasion cycles at two typical positions. Positions I and II represent the low-wear region and the most serious abrasive region, respectively. For most low-wear regions, there is no obvious difference of the microscopic surface topography before and after 500 abrasion cycles. Besides, the abrasion marks can be observed (Figure S18), which is attributed to wear resistance. The detected surface elements include well-distributed P, Al, O, and Ti but not Fe, demonstrating that the TiO2−AP coating still plays a sufficiently protective role after even strong friction. On the other hand, a small serious abrasive region appears just at the edge of the used weight. At this position, the SSM substrate with a large porous structure is exposed, which can be clearly seen from the element mapping images. The detected Fe, Cr, and Ni belong to the substrate SSM. A small number of Ti are also detected, and the weight percentages of Ti, P, and Al are 2.2%, 1.3%, and 0.5% from EDS analysis, respectively. Note that the residual TiO2−AP composite at the small serious abrasive region still retains micro-/nanohierarchical structures and has the capacity for extreme oil repellency in water. The scratch test was performed to quantitatively evaluate the mechanical durability of the TiO2−AP coating. The friction force and friction coefficiency simultaneously increase as the gram load is increased. Once the diamond ball lacerates the protective coating, a sharp increase of the friction force and friction coefficiency appears and the corresponding gram load can reflect as the binding force of the protective coating. As 1115

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addition, the binders should have functional groups if continuous network structures are to be formed by a condensation−polymerization reaction. For example, the AP binder contains hydroxy groups and undergoes the condensation−polymerization reaction via intermolecular and intramolecular dehydration (Figures S26 and S27). On the other hand, the binding phase must also possess adhesive property, which is greatly dependent on the molecular attractive forces between the binding phase and the surfaces of the substrates or fillers. Rationally, there are hydrogen (or coordination) bonds between the functional groups of the AP binder and oxygen (or metal) atoms on the surfaces of the substrates or fillers (Figures S28), which probably serve to increase adhesion. The loss of superwetting properties of interface materials due to the damage of micro-/nanohierarchical structures stops the function and immensely impacts the applications. Recently, underwater superoleophobic membranes have been widely developed for oil/water separation, offering significant advantages including high flux (more than 100 L/m2h) and low external pressure (less than 1 bar, even down to gravity). Weak robustness is one of the main issues as well. For example, when solid content in feeds is higher than 0.5%, crossflow filtration is superior to dead-end filtration. However, large transverse shear puts forward high requirements on the mechanical durability. We prepared hexane-in-water (H/W), petroleum ether-inwater (P/W), and isooctane-in-water (I/W) emulsions as model feeds and investigated the oil/water separation capacity of the TiO2−AP-treated SSM and the resistivity to sandpaper abrasion. The driven force was gravity. The oil contents in the filtrates were evaluated by measuring chemical oxygen demand (COD). Figure 6a shows typical optical and microscopy images of the original emulsions and the collected filtrates. The original emulsions are milky white and turbid containing abundant oil droplets with micrometer and sub-micrometer sizes. After separation using the TiO2−AP-treated SSM, all collected filtrates are clear and transparent and no oil droplets are

Figure 4. Underwater OCA (scatter) and mass loss (column) of the TiO2−AP, SiO2−AP, and Al2O3−AP coatings after 100 abrasion cycles. The selected substrates include SSM (1), ceramic (2), and glass (3). m0 and m100 are the mass of the TiO2−AP, SiO2−AP, and Al2 O3−AP coatings before and after 100 abrasion cycles, respectively.

Figure 5. SEM images and element distribution maps of the SiO2− AP and Al2O3−AP-treated SSMs after 100 abrasion cycles.

be detected. Therefore, the inorganic adhesives can be available for various nanoparticles (such as TiO2, SiO2, and Al2O3) and substrates (SSM, ceramic, and glass) to construct robust superwetting surfaces. Cohesion and adhesion are two essential properties of binders. The former requires the formation of continuous structures throughout the binding phase. Prior to the formation of the continuous structures, the binders are in the form of a well-dispersed system which favors the ion mobility. Here, water was added in the AP binder partly for this reason. Aluminum with a relatively small cationic radius was selected to increase the bonding power of the phosphate binder. In

Figure 6. (a) Typical optical and microscopy images of oil-in-water emulsions before (1) and after (2) separation using the TiO2−APtreated SSM. (b) Change on flux of oil/water separation and COD in the collected filtrates with abrasion cycles. 1116

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ACS Nano observed. In contrast, the unmodified SSM cannot achieve the emulsion separation (Figure S29). Fluxes of the H/W emulsion and COD values in the collected filtrates were measured during the emulsion separation using the TiO2−AP-treated SSM after each 10 abrasion cycles. As shown in Figure 6b, the flux and COD value maintain more than 200 L/m2h and less than 50 mg/L, respectively. For different emulsion feeds, the TiO2−APtreated SSM still keeps high separation efficiency (more than 99.98%) after 100 abrasion cycles (Figures S31 and S32). The superwetting surfaces constructed by inorganic adhesives have promising potential for applications with steady and high performance even on severe working conditions. Besides weak robustness, the micro-/nanohierarchical structures of superwetting surfaces always suffer from fouling issues due to surface impurity adhesion, especially amphiphiles. The change of wetting properties inevitably and seriously suppresses the advantages of superwetting surfaces for longterm use. Here, inorganic adhesives (AP) are proposed as a building block to construct superwetting surfaces and promote robustness. Nanomaterials can be employed as fillers to extend the functions. P25 as one of famous TiO2 photocatalysts was used to endow the TiO2−AP-treated SSM with an outstanding antifouling performance and self-cleaning property. From SEM images in Figure 3 and Figure S5, P25 nanoparticles are uniformly distributed throughout the TiO2−AP coating, resulting in photocatalytic activity to degrade organic compounds (such as rhodamine B), as shown in Figure S33. Furthermore, we selected oleic acid as a model amphiphile with low volatility to examine the self-cleaning property of the TiO2−AP coating. Figure 7a shows the variation of water contact angles (WCAs) in air and underwater OCAs of the TiO2−AP-treated SSM after the pollution of oleic acid and then UV irradiation. The TiO2−AP-treated SSM displays superhydrophilicity in air with a nearly zero WCA and underwater superoleophobicity with an OCA of about 160°. After being polluted by oleic acid, the WCA changes to 60° in air and the underwater OCA decreases to 0°, illustrating that the obtained superhydrophilic surface suffers from fouling due to the adhesion of oleic acid. Because of the photocatalytic characteristic of TiO2 nanoparticles, the adhered amphiphiles are degraded, and the polluted surface recovers to be superhydrophilic and underwater superoleophobic after UV irradiation (movie S3), indicating the excellent self-cleaning property of the TiO2− AP coating. In addition, we studied the effect of UV irradiation time in air or in water on the underwater OCA of the TiO2− AP-treated SSM after being adhered by oleic oil (Figure S34). In air, the recovery of the polluted SSM to the original underwater superoleophobic state takes more than 40 min. When the polluted SSM is immersed in water and UV irradiated, the recovery time can be shortened to 5 min. UV irradiation excites TiO2 nanoparticles to form an electron and hole. The photogenerated electron can activate O2 to degrade the adhered amphiphiles, which allows the polluted SSM to clean and return to underwater superoleophobicity. In water, the photogenerated hole can oxidize H2O to produce highly active hydroxyl radical, which can accelerate the degradation of the adhered amphiphiles and greatly shorten the recovery time.26 We further tested the resistivity of the self-cleaning ability of the TiO2−AP coating to sandpaper abrasion. The TiO2−AP coating was polluted by oleic acid and then exposed to UV light for 1 h. Afterward, the above-mentioned abrasion test was

Figure 7. (a) A water droplet (in air) and a dichloroethane droplet (in water) on the surface of the TiO2−AP-treated SSM after being adhered by oleic oil and subsequently irradiated by UV light. (b) Change on underwater OCA of the TiO2−AP-treated SSM after being adhered by oleic oil and subsequently irradiated by UV light (dash line) with abrasion cycles (solid line).

performed 10 times. This process that combines the antifouling with an abrasion test is defined as one cycle. In each cycle, the underwater OCA changes from above 150° to 0° and then returns to above 150° (Figure 7b). Our membrane holds remarkable antifouling performance and self-cleaning property after 100 abrasion cycles. We believe that the idea of “inorganic adhesives + functional nanomaterials” provides a promising way for the construction of robust superwetting surfaces with not only the functions but also highly steady applied performance for use.

CONCLUSIONS In summary, inorganic adhesives have been used to design and build strongly robust underwater superoleophobic surfaces, which are available for various nanoparticles (such as TiO2, SiO2, and Al2O3) and substrates (SSM, ceramic, and glass). More importantly than high-efficiency oil/water separation and excellent self-cleaning property, the TiO2−AP coating shows a very steady performance after even 100 abrasion cycles with sandpaper. The flexibility of the “inorganic adhesives + functional nanomaterials” combination could be expanded to other interfacial issues and applications, such as transportation, catalysis, and sensor, using the advantages of superwetting interfaces for a long time. The surface can be readily applied in harsh and aquatic environments, especially where high robustness is required. METHODS Preparation of Robust Superwetting Surfaces. SSM (2300 mesh size), glass, ceramic, TiO2, SiO2, and Al2O3 nanoparticles were commercially available. All chemical reagents were analytical grade and 1117

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tion maps of original and TiO2−AP-treated SSMs; TGA and DSC of the TiO2−AP composite; CAs and sliding angles of the TiO2−AP-treated SSM; dynamic WCAs of the TiO2−AP-treated SSM, glass, and ceramic; photographs, SEM images, and element distribution maps of the TiO2−AP-treated SSM at different points after the 500th cycle’s abrasion; the friction force and friction coefficiency curves; weight percentage of each element from EDS analysis; the stability of the TiO2−AP-treated SSM under harsh conditions; photographs of TiO2−AP-, SiO2−AP-, and Al2O3−AP-treated SSM, ceramic, and glass before and after 100 abrasion cycles; the mechanism of the robust surfaces using the AP binder; oil/water separation using unmodified SSM and the TiO2−APtreated SSM before and after 100 abrasion cycles; photocatalytic degradation of rhodamine B using the TiO2−AP-treated SSM; self-cleaning of the TiO2−APtreated SSM (PDF) Movies S1−S3 (ZIP)

used without further purification. A certain amount of orthophosphoric acid (H3PO4, 85%) was diluted to 60% by adding deionized water. Then Al(OH)3 was added under stirring at 100 °C for 3 h. The molar ratio of H3PO4 to Al(OH)3 was 3:1. The prepared AP binder (2 g) was dissolved in 5 mL of deionized water, and nanoparticles (1 g) were dispersed in 15 mL of anhydrous ethanol. The above-mentioned solutions were mixed and then sonicated for 10 min. SSM, glass, and ceramic were ultrasonically cleaned in deionized water and anhydrous ethanol several times, which were used as substrates. Subsequently, the mixed solutions were uniformly sprayed onto the substrate surfaces with a spray gun under 0.2 MPa N2 gas. In order to realize the crosslinking and curing of the coatings, the samples were continuously heattreated at 120 °C for 2 h and 240 °C for 1 h. Sandpaper Abrasion Tests. Samples under a 200 g weight were moved for 10 cm along a ruler by an external drawing force and returned back, which was defined as one abrasion cycle. The coatings faced down sandpaper (grit no. 320). After 50 abrasion cycles, we renewed the sandpaper to maintain high roughness. Emulsion Separation. Three kinds of oil-in-water emulsions were prepared by mixing oil (hexane, petroleum ether, and isooctane) and water in a volume ratio of 1:9 under extensively shaking and stirring. The emulsion separation tests were carried out under gravity only. Water fluxes were determined by calculating the volume of water permeation per unit time using the following equation: flux = V/St, where V is the volume of water permeation, S is the film area, and t is the testing time (5 min). The emulsion height was kept at 10 cm during oil/water separation. Antifouling Tests. The TiO2−AP-treated SSMs were immersed into 40 mL of rhodamine B (5 μM) aqueous solution. After UV irradiation for given time intervals, 2 mL samples were collected and analyzed on a Cary 60 UV−vis spectrophotometer. To test the antifouling and self-cleaning properties of the TiO2−AP-treated SSMs, we chose oleic acid to pollute the membrane. The TiO2−AP-treated SSMs were immersed in 20 mL of oleic acid ethanol solution (5 wt %) for 10 min. Note that the adhered oleic acid cannot be removed by washing with ethanol or water. After UV irradiation for 1 h, the membrane was treated by 10 abrasion cycles. This process was defined as one cycle. The intensity and wavelength of UV light were 30 W and 254 nm, respectively. The distance between the TiO2−AP-treated SSMs and UV lamp was about 15 cm. Characterization. All optical photographs were taken by a digital camera (Sony, DSC-HX200). The chemical composition and crystal structure of samples were characterized by XPS (Thermo Scientific ESCALAB 250Xi) and XRD (X’SPERT PRO), respectively. The surface morphology was observed on a field emission scanning electron microscope with Au-sputtered specimens (JEOL JSM6701F). The accelerating voltage and current were 5 kV and 10 μA, respectively. The element distribution and percentage were analyzed by EDS (JSM-5600LV). The thermal property of samples was measured by TGA (Netzsch STA 449F3). The heating rate was 10 °C min−1. The binding force of the TiO2−AP coatings was studied by scratch test on a fraction testing machine (CETR UMT-2), recording the friction coefficient (μ) and friction force (g) at the different applied forces (g). The load force was varied progressively from 2 to 400 g over a distance of 10 mm in 199 s. WCAs and underwater OCAs were got on a JC20001 contact angle system (Zhongchen Digital Equipment Co., Ltd., Shanghai, China). The average CA value was obtained by measuring the sample at five different positions. Optical microscope images of oil-in-water emulsions were recorded on an OLYMPUS BX51 microscope. The oil content in the collected filtrates was calculated by measuring COD according to U.S. Environmental Protection Agency method 8000 (HACH, DRB 200).

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jing Li: 0000-0002-4183-6440 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (Nos. 51522510 and 51675513). REFERENCES (1) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus, or Escape From Contamination in Biological Surfaces. Planta 1997, 202, 1−8. (2) Guo, Z.; Zhou, F.; Hao, J.; Liu, W. Stable Biomimetic SuperHydrophobic Engineering Materials. J. Am. Chem. Soc. 2005, 127, 15670−15671. (3) Wong, T.-S.; Kang, S. H.; Tang, S. K. Y.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired Self-Repairing Slippery Surfaces with Pressure-Stable Omniphobicity. Nature 2011, 477, 443− 447. (4) Xu, L.-P.; Zhao, J.; Su, B.; Liu, X.; Peng, J.; Liu, Y.; Liu, H.; Yang, G.; Jiang, L.; Wen, Y.; et al. An Ion-Induced Low-Oil-Adhesion Organic Inorganic Hybrid Film for Stable Superoleophobicity in Seawater. Adv. Mater. 2013, 25, 606−611. (5) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Super-WaterRepellent Fractal Surfaces. Langmuir 1996, 12, 2125−2127. (6) Liu, M.; Wang, S.; Wei, Z.; Song, Y.; Jiang, L. Bioinspired Design of a Superoleophobic and Low Adhesive Water Solid Interface. Adv. Mater. 2009, 21, 665−669. (7) Wang, S.; Liu, K.; Yao, X.; Jiang, L. Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications. Chem. Rev. 2015, 115, 8230−8293. (8) Lu, Y.; Sathasivam, S.; Song, J.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P. Robust Self-Cleaning Surfaces That Function When Exposed to Either Air or Oil. Science 2015, 347, 1132−1135. (9) Chen, L.; Sun, X.; Hang, J.; Jin, L.; Shang, D.; Shi, L. Large-Scale Fabrication of Robust Superhydrophobic Coatings with High Rigidity and Good Flexibility. Adv. Mater. Interfaces 2016, 3, 1500718. (10) Chen, B.; Qiu, J.; Sakai, E.; Kanazawa, N.; Liang, R.; Feng, H. Robust and Superhydrophobic Surface Modification. ACS Appl. Mater. Interfaces 2016, 8, 17659−17667.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08348. TEM and EDS images of TiO2, SiO2, and Al2O3 nanoparticles; XPS, SEM images, and element distribu1118

DOI: 10.1021/acsnano.6b08348 ACS Nano 2017, 11, 1113−1119

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DOI: 10.1021/acsnano.6b08348 ACS Nano 2017, 11, 1113−1119