Switchable Wettability Surface with Chemical Stability and Antifouling

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A switchable wettability surface with chemical stability and antifouling property for controllable oil-water separation Hanpeng Gao, Yan Liu, Guo Yong Wang, Shuyi Li, Zhiwu Han, and Luquan Ren Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00094 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Langmuir

Type: Article

A switchable wettability surface with chemical stability and antifouling property for controllable oil-water separation

Hanpeng Gao, † Yan Liu,*,† Guoyong Wang,‡ Shuyi Li, † Zhiwu Han, † and Luquan Ren†

†Key

Laboratory of Bionic Engineering (Ministry of Education), Jilin University,

Changchun 130022, P. R. China ‡Key

Laboratory of Automobile Materials (Ministry of Education) and College of

Materials Science and Engineering, Jilin University, Changchun, 130022, P. R. China

KEYWORDS: biomimetic; switchable wettability; chemical stability; controllable oil-water separation; antifouling

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ACS Paragon Plus Environment

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ABSTRACT Membrane materials with special wettability for separating oil-water mixtures have gradually become one of the research hotspots. However, oily wastewater usually contains very strong corrosiveness, which puts forward high requirements for the chemical stability of the separation membrane. In addition, oil droplets may block the pores, resulting in the decrease of separation efficiency or even separation failure. Herein, a biomimetic TiO2-titanium meshes (BTTM) with switchable wettability was successfully fabricated by one-step dip coating of PVDF and modified TiO2 suspension on the titanium meshes. The simple and efficient preparation method will facilitate the promotion of this smart material. Due to the controlled wettability, the BTTM can separate water or oil from oil-water mixture as required. Immersed in strong corrosive solution or liquid nitrogen, the wettability did not change much, showing good stability. Furthermore, the BTTM also has self-healing ability, self-recovery anti-oil-fouling property and self-cleaning behavior, which can resist oil pollution and improve recyclability. This study provides a simple and efficient strategy to fabricate a stable smart surface for on-demand controllable treatment of corrosive oily wastewater.

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INTRODUCTION The organisms in nature always try various evolutionary schemes and choose the optimal solution to promote their survival and development. Some organisms with special wettability can achieve such things as water walking, water collecting, self-cleaning and anti-oil-fouling, which makes them more conducive to survival.1-6 By developing various technologies, human beings are also conducive to their own survival and solving various challenges. Unfortunately, many problems are caused by human development. The problem of oily wastewater discharge and oil spills seriously threatens people's health.7-10 Inspired by organisms with special wettability in nature, membrane materials have been widely used to treat oily wastewater.11-14 Many methods have successfully prepared membrane materials with special wettability for treating oily wastewater.15-20 In addition, multifunctional membrane materials are gradually becoming a research trend.21-23 In general, research on oil-water separation of super-wetting materials can be divided into two types: separation of water or oil from oily wastewater.24-26 Separation of oil or water from oily wastewater requires superhydrophobic (or underwater superoleophilic) and superhydrophilic (or underwater superoleophobic) surfaces, respectively.27,

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Therefore, the single

wettability membrane material limits the development of oil-water separation technology to some extent.

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Recently, biomimetic surfaces with switchable wettability have attracted wide attention due to their potential application value.33, 34

Many external stimuli can

change the wettability of smart surfaces.29-34 Among these stimuli, light has attracted increasing attention because they have many advantages, such as non-contact type, not contaminating the surface and remote-control mechanism. Membrane material with switchable wettability has many advantages for oil-water separation. However, industrially discharged oily wastewater is usually highly corrosive. Despite some encouraging progress, obtaining a smart stable membrane materials for on-demand oil-water separation remains a challenge. At the same time, the problem of oil adhesion and contamination limits the repetition and long-term use of membrane materials. Herein, we fabricated a biomimetic TiO2-titanium meshes (BTTM) with controllable wettability to realize on-demand oil-water separation. This surface was prepared by one-step dip coating of PVDF and modified TiO2 suspension on the titanium meshes. The wettability of the BTTM can switch reversibly between underwater superoleophobicity to underwater superoleophilicity. It can be attributed to changes in the surface hydroxyl content under different conditions. Therefore, the BTTM can separate water or oil from oil-water mixture as required. Immersed in acid, alkali and salt solution for 12 hours or liquid nitrogen for 60 minutes, the surface wettability did not change much, showing good chemical stability. Therefore, the BTTM maintains high separation efficiency (> 92 %) even for highly corrosive oily wastewater. The BTTM also has self-healing ability, 4


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self-recovery anti-oil-fouling property and self-cleaning behavior, which can resist oil pollution and improve recyclability. Due to the simple and efficient preparation method, the as-prepared smart material can realize large-area preparation, and this method can be easily extended to other substrates. EXPERIMENTAL METHODS Materials. Titanium meshes (99.88% purity) were purchased from Hebei Oufulang metal mesh manufacturing Co. Ltd. TiO2 (25 nm) were purchased from Evonik Degussa, Germany (Figure S1). Perfluorooctanoic acid (C7F15COOH, 90%) and Polyvinylidene Fluoride (PVDF) were purchased from Aladdin and Taiyuan Yingze Source battery sales department. N-Methyl Pyrrolidone (NMP) were purchased from Guang Fu fine chemical research institute. Diesel was products of SINOPEC. Hexadecane was purchased from FU CHEN chemical reagents factory. Other chemicals used in this work were all purchased from Beijing Chemical Works. Preparation of UV-responsive BTTM. The experimental process was shown in Figure 1a. TiO2 nanoparticles tend to form agglomerates due to high surface energy. Perfluorooctanoic acid (PFOA) had a very low surface energy and was easily modified on surfaces with hydroxyl groups. Therefore, PFOA was used to modify nanoparticles to reduce surface energy and agglomeration, which was also conducive to enhancing the dispersion of Nano-TiO2 in solvents. Nano-TiO2 particles (2g) was modified with perfluorooctanoic acid (0.1 M) for 120 minutes (under ultrasonic agitation). After 5


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modified by perfluorooctanoic acid, TiO2 particles were washed with alcohol and then centrifuged to obtain modified TiO2. The modified TiO2 particles were added to PVDF solutions (PVDF was dissolved in NMP, 10% wt) and vigorously shaken for 5 min using ultrasonic cell crusher, then the mixtures was shaken with ultrasonic for 3 hours to obtain a uniform suspension. Finally, titanium meshes were immersed in the mixtures and then were dried at 120 °C for 30 min. As-prepared biomimetic TiO2-titanium meshes (BTTM) showed superhydrophobicity and underwater superoleophilicity. Switchable Wettability. The original surface has underwater superoleophilicity (superhydrophobicity). After 70 minutes of UV irradiation, the BTTM exhibits underwater superoleophobicity (superhydrophilicity). The UV high pressure mercury lamp was 250 W, and the spectral energy distribution was centered on 365nm. The wettability of the BTTM can be restored by heating. After heating at 120 °C for 90 minutes, the BTTM can recover underwater superoleophilicity (superhydrophobicity). By this simple method, the wettability of the surface can be controlled. Oil-water Mixtures Separation. The separation equipment consisted of iron stands, quick clamping hoop, beakers, two glass tubes and tested membrane. The oil (25 mL) and the aqueous solution (acids, bases or salts, 25 mL) were poured together into the beaker. The as-prepared membrane was placed in a separation apparatus. The beaker was used to collect the solution after separation. In addition, different dyes (Sudan III, Rhodamine B and methylene blue) were used to dye

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aqueous solutions and oils. The oil-water separation efficiency was calculated using the following equation: 𝑀

𝑅 = 𝑀0 × 100 %

(1)

where M0 and M were original oil weight and collected oil weight, respectively. In order to ensure the reliability of the data, all experiments were carried out ten times. Characterization. The morphology was observed by scanning electron microscopy (SEM, EVO18, ZEISS). Surface roughness and 3D morphology were detected using confocal laser scanning microscopy (CLSM, OLS3000). The XRD patterns was used to characterize the crystal structure of the BTTM. FTIR spectroscopy (FTIR-4100, JASCO), X-ray photoelectron spectroscopy (XPS, SPECSXR50) and energy dispersive spectrometer (EDS) were used to analyze the chemical composition of the BTTM. The modified TiO2 was separated by using high speed centrifuge (ZONKIA, HC3514, China). The TiO2 was dispersed in PVDF solutions by using ultrasonic cell crusher (Shanghai Hanuo instruments Co. Ltd, China). Oxygen was used as a gas source for plasma treatment (PDC-002). Contact angles and sliding angles were measured by the contact angle meter (DSA 255, Germany) and the results were obtained by averaging five values obtained on the same surface at different positions. The volume of underwater oil droplets (carbon tetrachloride) or water droplets was 3μL. The UV high pressure mercury

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lamp (250 W) was purchased from Shanghai Yuanyuan company. All experimental digital photos were taken with Canon EOS80D. RESULTS AND DISCUSSION Surface Morphology and Chemical Composition. The characterization of the as-prepared surface morphology was carried out by SEM and CLSM. Figure 1b displayed the SEM image of the original titanium meshes, it could be seen that wire diameter was ∼ 80 μm and the pore diameter was ∼ 100 μm. The water contact angles (WCAs) of original surface was 88.5 ± 2.5°. After processing, many nanoscale structures could be observed (Figure 1c, Figure 1d and Figure 1e). The as-prepared surface showed superhydrophobicity, and the WCAs was 152.3 ± 3°. Confocal laser scanning microscope (CLSM) indicated that the original titanium meshes were smooth, with a surface roughness Ra was 67.95 μm (Figure 1f) and 2.64 μm (Figure 1g). After processing, the surface roughness Ra was 79.48 μm (Figure 1h) and 3.47 μm (Figure 1i). Micro-nano hierarchical structure was considered to be more advantageous for obtaining superhydrophobic surfaces with good performance.35 We further analyzed the BTTM using the Cassie–Baxter equation:36 cos 𝜃𝑟 = 𝑓1cos 𝜃0 - 𝑓2

(2)

where 1 and 2 were the fractions of the solid and gas in contact with the liquid; θr and θ0 were the CAs on BTTM and original titanium mesh. Given that 1 + 2 =1, 1 is calculated to be 0.1117. The calculations show that most of the droplets are in

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contact with the air layer (area fraction is 88.83%), and only a few of them are in contact with the BTTM (area fraction is 11.17%). Surface chemical composition was detected using EDS, FTIR and XRD. The chemical composition of the original titanium mesh (Figure 2a) and the BTTM (Figure 2b) were analyzed using EDS. The elements of Ti and O appeared on the original titanium mesh. It may be attributed to the oxidation of the surface of the titanium mesh. The elements of Ti and O also appeared on the BTTM. Compared to the original titanium mesh, the distribution of oxygen on the surface was significantly increased, demonstrating that TiO2 particles had been dispersed on the BTTM. In addition, the elements of C and F appeared on the BTTM. It could be attributed PVDF and PFOA. EDS results of the original titanium mesh (Figure 2c) and the BTTM (Figure 2d) also demonstrated the change in surface element content. Although surface element distribution and content were measured, these were not comprehensive for surface composition analysis, so FT-IR and XRD were used to further analyze the surface composition. The adsorption peaks at 612 cm1 and 762 cm1 were ascribed to CF2 group (it could be attributed to PVDF and Perfluorooctanoic acid, Figure 2e). The adsorption peak at 1070 cm1 was ascribed to CF2 and CH2groups.37 The adsorption peak at 1207 cm1 and 1402 cm1 were attributed to the CH2 of PVDF. The adsorption peaks at 1631 cm-1 were ascribed to COO, indicating that PFO successfully modified the TiO2 particles. The X-ray diffraction (XRD) was used to further determine the crystal 9


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structure of the TiO2 on the BTTM (Figure 2f). The pure PVDF (Figure 2f, black line) exhibited three main characteristic diffraction peaks at 18.4°, 19.9°, and 26.5°, which were assigned to the (020), (110) and (021) reflections of the a-phase crystal, respectively.38 From the XRD patterns of the BTTM (Figure 2f, red line), it can be seen that the diffraction peaks of the as-prepared surface matched well with anatase (JCPDS 21:1272) and rutile TiO2 (JCPDS 21:1276).39 The mixed crystals (anatase and rutile) have better photocatalytic properties, which can be attributed to increased surface charge transfer efficiency.40 Switchable Wettability and Mechanism. The original BTTM showed underwater superoleophilicity (~ 0°) and superhydrophobicity (152.2 ± 3°). With the extension of UV irradiation time, the underwater oil contact angles (UOCAs) gradually increased and WCAs gradually decreased (Figure 3a). After 70 minutes of UV irradiation, the UOCAs was 151.6 ± 3° and the WCAs was 0° (Figure S2) . Interestingly, the wettability of the BTTM could be restored after heating at 120 °C for 90 minutes (Figure 3b). The change in surface extreme wettability could be seen more intuitively in Figure 3c. It was worth noting that water droplets in the air would have opposite wettability transitions. (Figure S3). In addition, the switchable wettability has good repeatability (Figure 3d). In order to further analyze the above interesting phenomena, the following calculation formula was introduced:41

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cos 𝜃OW =

𝛾OAcos 𝜃O - 𝛾WAcos 𝜃W 𝛾OW

(3)

where 𝜃𝑂𝑊, 𝜃𝑂, 𝜃𝑊 were the CAs of oil in water, oil in air and water in air. And 𝛾𝑂𝐴, 𝛾𝑊𝐴 and 𝛾𝑂𝑊 were surface tensions of the oil-air, water-air and oil-water interfaces. The UOCAs was ~ 0° (Figure S4). According to the equation (3), when the BTTM is hydrophobic, 𝛾OAcos 𝜃O - 𝛾WAcos 𝜃W is always positive, indicating an

underwater

oleophilicity.

When

the

BTTM

is

hydrophilic,

𝛾OAcos 𝜃O - 𝛾WAcos 𝜃W is always negative, indicating an underwater oleophobicity (the surface tension of oil is usually much smaller than water). Therefore, it is reasonable to have extreme wettability in combination with the rough structure of the surface. The results obtained by the formula were consistent with our experimental dates. In order to further study the mechanism of switchable wettability, the following experiments were carried out. The surface chemistry before and after UV irradiation was further analyzed by XPS, revealing the reason for the surface switching wettability (for wide survey of XPS, you can see Figure S5). The O1s peak were shown in Figure 3e, Figure 3f and Figure 3g, respectively. On original BTTM, the distinct fitted values at 531.3 eV and 529.9 eV, were corresponded to Ti-OH and Ti-O (Figure 3e). After UV irradiation, the area fraction of the Ti–OH peak was significantly increased (Figure 3f). However, after UV irradiation and then heat treatment, the area fraction of Ti–OH peak was almost consistent with

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the original surface (Figure 3g). The changed in the surface hydroxyl content were considered to be the cause of the wettability transition. The mechanism of switchable wettability was further analyzed (Figure 3h). Several studies have shown that the H2O and O2 in the environment will compete for dissociative adsorption on oxygen vacancies of TiO2.42, 43 After UV irradiation, the oxygen vacancies will increase the probability of absorbing the surrounding H2O, making Ti-OH content increase. The increase of surface hydroxyl groups will enhance the hydrophilicity. In addition, the surface has a very rough structure, which magnifies the hydrophilicity of the surface. Therefore, after UV irradiation, the

wettability

of

surface

changes

from

superhydrophobicity

to

superhydrophilicity. However, after heating, the oxygen vacancies will increase the probability of absorbing the surrounding O2. This will cause "Ti-O" to be replaced by "Ti-OH", resulting in surface restoration of superhydrophobicity. According to the discussion of the equation (3), the UOCAs will change inversely. Thus, the BTTM surface enables a reversible transition of the UOCAs and WCAs. Chemical Stability. Oily wastewater is usually highly corrosive, which requires high chemical stability of membrane materials. However, due to the fact that rough structure and low surface energy substances were easily destroyed by corrosive solutions, some studies often used pH = 1 or pH = 13 solutions to verify the chemical stability of the surface. It is still a challenge to prepare membrane materials that can resist high concentration corrosive solutions by simple methods. 12


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The BTTM was wetted with alcohol so that the surface could be completely immersed in the solution. After immersing in the acid (HCl, 2 mol/L), alkali (NaOH, 2 mol/L) or salt solution (NaCl, 2 mol/L) for 12 hours, the BTTM was washed with plenty of water and then dried with N2 (Figure 4a). It can be found that there is almost no change in surface wettability (Table S1). In addition, after UV irradiation, the BTTM were also immersed in different solutions for 12 hours (Figure 4b). The results also showed that immersion in corrosive solutions had little effect on surface wettability (Table S2). A similar experiment was performed on the pure PVDF (Figure S6, Figure S7 and Table S3). The above results indicated that the BTTM have good chemical stability, which was attributed to the good stability of PVDF, titanium mesh and Nano-TiO2.44, 45 The stability of surface wettability in storage was also studied. In order to eliminate environmental interference (sunlight or UV), the entire storage was carried out in a dark environment. After six weeks storage, the WCAs and WSAs of the BTTM did not change significantly (Figure 4c). In addition, the BTTM was immersed in liquid nitrogen to test its low temperature resistance. The results showed that storage at low temperature had little effect on surface wettability (Figure 4d). In addition, there was no significant change in surface topography (Figure S8). More importantly, the BTTM has self-healing ability after oxygen plasma etching. After plasma damage for 5 s, the surface became superhydrophilic (~ 0° ). 13


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It was worth noting that heating at 120°C for one hour could restore surface wettability. The above process was repeated ten times, demonstrating the stable self-healing ability of BTTM (Figure 4e). Surface self-healing mechanism was also analyzed (Figure 4f). After plasma etching, many oxygen-containing groups appeared on the surface (such as −OH and C=O), which caused the surface to transform into superhydrophilic. It is speculated that during the heating process, the surface will spontaneously transfer the internal low surface energy PVDF to the surface, while the hydrophilic group on the surface will enter the interior of the coating. The above may be similar to the self-healing properties of polysiloxanes reported in many studies.46, 47 This self-healing behavior is a spontaneous process under heating in order to reduce surface energy. Good stability and self-healing ability facilitates the promotion and application of the BTTM. Controllable On-demand Oil-water Separation. BTTM was placed in the mixtures of water-oil. For the mixtures of heavy oil (carbon tetrachloride) and water, the BTTM would float on the water. After UV irradiation, the BTTM would be immersed in water but would float on the oil-water interface (Figure 5a). It was important to emphasize that this process was reversible by heating. In addition, for the mixtures of light oil (petroleum ether) and water, the BTTM would be immersed in water and float on the oil-water interface (Figure 5b). This phenomenon did not change through UV irradiation and heating. These phenomena were further analyzed. The BTTM has superhydrophobicity and 14


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underwater superoleophilicity, which will float on the water to form a solid-gas-water three-phase interface. After 70 minutes of UV irradiation, the BTTM has superhydrophilicity and underwater superoleophobicity. Therefore, the as-prepared surface will be immersed in water but float on the water-oil interface. The solid-water-oil three-phase interface forms between the BTTM and the oil (Figure 5c). Surface wettability can be restored by heating, so this phenomenon is reversible. However, for the mixtures of light oil and water, due to superoleophilicity, the BTTM will be immersed in the oil. The superoleophilicity of the surface does not change after UV light or heating. When the surface contacted with water, the solid-oil-water interface was formed, which caused the surface to float on the oil-water interface (Figure 5d). The above interesting experimental phenomena provide a basis for controlled oil-water separation. Due to the switchable wettability, BTTM has a controllable permeability to oil-water mixtures. When the BTTM was placed in the oil-water separation device, heavy oil (carbon tetrachloride) could pass through the surface while water remained in the glass tube (Figure 6a). After UV irradiation, the BTTM was wetted by water and placed in the oil-water separation equipment. When light oil (petroleum ether) and water were poured into the equipment, water could pass through and the oil remained in the glass tube (Figure 6b). The whole process was driven by gravity, which had the advantages of convenience and low-energy consumption. The mechanism of controllable oil-water separation was further 15


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explained. According to the Laplace equation, the critical-resistant pressure was calculated according to the following formula:48 ∆𝑝 =

2𝛾 𝑅

=―

𝑙𝛾𝑐𝑜𝑠 𝜃𝑎 𝐴

(4)

where γ was the interfacial energy, R was the the void radius, l was the circumference of the mesh pore, A was the cross-sectional area of pore and θa was the contact angle. For the original BTTM, the WCA was 152.3 ± 3°, and the UOCA was ~ 0°. According to the equation (4), it can be known that △p > 0 for water, and △p < 0 for oil. It means that the surface needs to withstand some external pressure before it is completely wetted by water (Figure 6c). However, oil can be directly passed through the surface without external pressure (Figure 6d). Therefore, when the mixtures of heavy oil and water was poured into the separation device, heavy oil could pass through the surface while water remained in the glass tube. After UV irradiation, the BTTM has superhydrophilicity (~ 0°) and underwater superoleophobicity (151.6 ± 3°). According to the equation (4), it can be known that △p < 0 for water, and △p > 0 for underwater oil. It means that the water can be directly passed through the surface without external pressure (Figure 6e). However, underwater oil requires some external pressure to pass through the wetted surface (Figure 6f). Therefore, when the mixtures of light oil and water was poured into the separation device, water could pass through the surface while light oil remained in the glass tube.

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BTTM maintained high separation efficiency (> 93%) for different types of oil. Even after ten consecutive separation experiments, there was no significant decrease in separation efficiency (Figure 6g). This indicated that the BTTM has good reusability. The difference of separation efficiency may be attributed to the different adhesion and volatility of different oils. In addition, an interesting phenomenon was the increased separation efficiency of chloroform and carbon tetrachloride in the second separation. This may be because some of the oil was stored on BTTM during the first separation, resulting in a decline in the quality of the separated oil. And in the second continuous separation, this loss was avoided because the surface had been wetted by the oil. In contrast, when separating light oil and water, oil was difficult to store on the BTTM because it was wetted with water. Therefore, there was no significant difference in the separation efficiency of ten cycles. After ten cycles of separation test, the BTTM was repeatedly rinsed with alcohol and placed at 120 °C for 90 minutes. It is worth noting that BTTM remained switchable wettability, which enabled the surface to change different oil-water separation methods on demand after repeated use (Figure 6h). Corrosive oily wastewater is more common in real life. Therefore, the strong acid solution (2mol/L HCl), strong alkali solution (2mol/L NaOH) and salt solution (2mol/L NaCl) were mixed with different oils. The separation efficiencies of different oil-water mixtures were greater than 92%, and some even exceed 97% (Figure 6i).

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The results showed that BTTM maintained high separation efficiency even with highly corrosive oil-water mixtures. Anti-oil-fouling Property of BTTM. For conventional oil-water separation materials, oil adhesion reduces the reusability of the membrane and also blocks the pores to reduce separation efficiency. Therefore, it is very important to carry out the antifouling test. First, the volatilization of toluene on the original titanium mesh and BTTM was tested using the following method. The tested samples were cut to the same size and placed on a high precision balance. Subsequently, the same weight of toluene (40 mg) was dropped on the surface, and the change in the data of the balance was recorded with a video camera. The results showed that toluene on the original titanium meshes took about 880 s to completely evaporate. Compared to the original titanium mesh, the evaporation rate of toluene on the BTTM was significantly accelerated. Toluene on the BTTM took about 642 s to completely evaporate (Figure 7a). This may be due to the increase of the spreading area of oil droplets on BTTM, and the larger spreading area is more conducive to the volatilization of oil, so the BTTM has better self-recovery anti-oil-fouling property. In addition, UV irradiation could restore surface wettability when the surface was contaminated with hexadecane. The absorbed hexadecane changed the wettability of the BTTM (after UV irradiation 70 minutes ) from superhydrophilicity (WCAs: 0°) to hydrophilicity (WCAs: 46.7 ± 2°). After seven hours of UV irradiation, the WCAs became 0°, indicating that the original 18


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wettability was restored (Figure 7b). The reason for the decomposition of photocatalytic organic compounds was analyzed (Figure 7c). TiO2 nanoparticle is a well-known photocatalyst that converts pollutants into CO2 and H2O under UV irradiation.49 Reduction (O2-) and oxidation (OH·) reactions can both occur on the surface of the photoexcited TiO2 (for more detail you can see Table S4). Therefore, the as-prepared surface has the ability to decompose oil contamination. In summary, for volatile oils, the BTTM can accelerate the evaporation of oils, and for oils that are not easily volatilized, the oil contamination on the BTTM can be decomposed under UV irradiation (Figure 7d). CONCLUSIONS In conclusion, a smart biomimetic TiO2-titanium meshes (BTTM) is reported, which can reversibly switch the wettability between underwater superoleophilicity and underwater superoleophobicity. The switchable wettability is ascribed to the changes of surface hydroxyl content during UV irradiation or heating. Furthermore, based on the special switchable wettability, the application of the BTTM as a controllable oil-water separation membrane was also demonstrated. Immersed in acid, alkali and salt solution for 12 hours or liquid nitrogen for 60 minutes, the surface wettability did not change much, showing good chemical stability. Therefore, the BTTM maintains high separation efficiency (> 92 %) even for highly corrosive oily wastewater. Furthermore, the BTTM also has self-healing ability, self-recovery anti-oil-fouling property and self-cleaning behavior, which 19


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can resist oil pollution and improve recyclability. This study provides a simple and efficient strategy to fabricate a stable smart surface for controllable treatment of corrosive oily wastewater. It will also provide an important reference for the promotion and development of stable smart switchable wettability surfaces.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: FE-SEM images of the Nano-TiO2 particles; wettability of the BTTM (after UV irradiation); transformation between superhydrophobicity and superhydrophilicity; the oil droplet contact angle of the BTTM; XPS survey spectra of original BTTM, after UV irradiation and followed by heating, respectively; the WCAs and WSAs of the BTTM; the UOCAs and UOSAs of the BTTM; PVDF immersed in corrosive solution; the process of chemical reaction between PVDF and alkali; WCAs and WSAs of the coated glass; surface morphology after soaking in alkali solution and liquid nitrogen treatment; the wettability change of surface after immersion in corrosive solution for seven days; WCAs and WSAs of hot and cold droplets on the BTTM; sandpaper abrasion test; water stream flushing experiment and the mechanism of photocatalytic degradation of organic matter by TiO2. Author Contributions 20


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The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS The authors thank the National Key Research and Development Program of China (2016YFE0132900), the National Natural Science Foundation of China (Nos. 51775231, 51761135110, 51475200), the Science and Technology Development Project of Jilin Province (No. 20160204005SF), the 111 project (B16020) of China and JLU Science and Technology Innovative Research Team (No. 2017TD-04).

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For Table of Contents only

The as-prepared surface has switchable wettability, good chemical stability and antifouling ability. It can be used for strong corrosive controlled on-demand oil-water separation.

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Figure 1. (a) Schematic illustration of the preparation process. (b) SEM images of the original titanium meshes. Insets was the photographs of water droplets on the original titanium meshes. (c) SEM images of the as-prepared biomimetic TiO2-titanium meshes (BTTM). Insets was the photographs of water droplets on the BTTM, indicating that the as-prepared surface was superhydrophobic. (d) and (e) Amplified images corresponding to (c) at different magnified scales. Three-dimensional (3D) confocal laser scanning microscopy (CLSM) images of original titanium meshes (f-g) and the BTTM (h-i). The results showed that the surface roughness of the BTTM was significantly increased.


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Figure 2. Chemical composition analysis of original titanium mesh and BTTM. (a) EDS element mapping of the original titanium mesh. The elements of Ti (pink) and O (green) appear on the surface. (b) EDS element mapping of the BTTM. The elements of Ti (pink), C , F (Indigo) and O (green) appear on the surface. The scare bar is 250 μm. (c), (d) EDS results of the original titanium mesh and the BTTM. (e) FT-IR spectra of the BTTM. (f) XRD spectrum of the BTTM with pure PVDF as reference, indicating that the nano-TiO2 has been successfully prepared on the surface. 85x121mm (300 x 300 DPI)


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Figure 3. (a) WCAs and UOCAs of the BTTM as a function of different UV irradiation time (min). (b) WCAs and UOCAs of the BTTM as a function of different heating time (min). (c) Photograph of the underwater oil droplet (3 μL) on the BTTM. (d) Reversible transformation between underwater superoleophilicity and underwater superoleophobicity realized by altering the UV irradiation and heating. XPS spectra of the O 1s peak on the BTTM with different conditions: (e) original surface, (f) after UV irradiation for 70 minutes and (g) after UV irradiation and then heating to 120 °C for 90 minutes. (h) Schematic illustration of the surface wettability transition. Dynamic transition of the hydrophobic oxygen groups and hydrophilic hydroxyl groups occurred on the BTTM via heating or UV irradiation. Thus, the smart surface has reversibly switchable wettability.


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Figure 4. (a) WCAs and UOCAs were measured on BTTM after 12 hours immersion in different solutions. (b) WCAs and UOCAs were measured on BTTM (after UV irradiation) after 12 hours immersion in different solutions. (c) The WCAs and WSAs of the BTTM as a function of the different dark storage time (week). (d) The WCAs and WSAs of the BTTM as a function of liquid nitrogen immersion time. (e) Variation of WCAs with plasma/self-healing cycles. (f) Schematic illustrations of the O2 plasma etching /self-healing process of the BTTM. 177x86mm (300 x 300 DPI)


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Figure 5. (a) For a mixture of heavy oil and water (blue), the BTTM will float on the water. After UV irradiation, the BTTM will be immersed in water but will float on the oil-water interface. It is worth noting that this process is reversible by heating. (b) For light oil and water (blue) mixtures, the BTTM will be immersed in water and float on the oil-water interface. UV irradiation and heating had no effect on this phenomenon. (c) Schematic illustrations of the BTTM with a mixture of heavy oil and water. (d) Schematic illustrations of the BTTM with a mixture of light oil and water. The interesting phenomena provide the basis for controlled on-demand oil-water separation.


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Figure 6. Controllable on-demand oil-water separation. (a) A mixture of heavy oil and water was poured into the upper glass tube. The heavy oil passed through the BTTM, whereas the water remained in the glass tube. (b) The BTTM was irradiated with UV and then wetted with water. A mixture of light oil and water was poured into the upper glass tube. The water passed through the BTTM, whereas light oil remained in the glass tube. (c)-(f) Schematic diagram of oil and water wetting models: when Δp < 0, water or oil can pass through the BTTM; when Δp > 0, water or oil cannot pass through the BTTM. The different models further analyze the mechanism of controlled oil-water separation. (g) Recycled experiments for separating oil-water mixture by using the above mentioned device. Separated oils included petroleum ether, toluene, diesel, chloroform and carbon tetrachloride. (h) After ten separation experiments, the as-prepared surface remained switchable wettability. (i) For highly corrosive oil-water mixtures, the as-prepared surface still maintains high separation efficiency. The results indicate that the as-prepared surface has good reusability and can efficiently separate corrosive oil-water mixtures. 177x120mm (300 x 300 DPI)


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Figure 7. (a) Self-recovery anti-oil-fouling property of the BTTM. The toluene on the BTTM will evaporate more quickly than the original titanium mesh. (b) WCAs evolution of hexadecane prewetted BTTM (after UV irradiation) under UV light irradiation. Cycles of variation of WCAs on the BTTM as it was prewetted by hexadecane and irradiated by UV irradiation for 7 h. (c) Schematic illustrations of the self-cleaning behavior of the BTTM and the photocatalytic mechanism of TiO2 nanoparticles. (d) Schematic illustration of the selfrecovery anti-oil-fouling property and self-cleaning behavior for the BTTM.


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The as-prepared surface has switchable wettability, good chemical stability and antifouling ability. It can be used for strong corrosive controlled on-demand oil-water separation.


Switchable Wettability Surface with Chemical Stability and Antifouling

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