Under-Oil Switchable Superhydrophobicity to Superhydrophilicity

Recently, smart interfacial materials that can reversibly transit between the superhydrophobicity and superhydrophilicity have aroused much attention...
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Under-Oil Switchable Superhydrophobicity to Superhydrophilicity Transition on TiO2 Nanotube Arrays Hongjun Kang,† Yuyan Liu,† Hua Lai,† Xiaoyan Yu,† Zhongjun Cheng,*,‡ and Lei Jiang§ †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China ‡ Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, P. R. China § Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China S Supporting Information *

ABSTRACT: Recently, smart interfacial materials that can reversibly transit between the superhydrophobicity and superhydrophilicity have aroused much attention. However, all present performances happen in air, and to realize such a smart transition in complex environments, such as oil, is still a challenge. Herein, TiO2 nanotube arrays with switchable transition between the superhydrophobicity and superhydrophilicity in oil are reported. The switching can be observed by alternation of UV irradiation and heating process, and the smart controllability can be ascribed to the cooperative effect between the surface nanostructures and the chemical composition variation. By using the controllable wetting performances, some applications such as under-oil droplet-based microreaction and water-removal from oil were demonstrated on our surface. This paper reports a surface with smart water wettability in oil, which could start some fresh ideas for wetting control on interfacial materials. KEYWORDS: TiO2 nanotube arrays, under-oil, wettability switching, superhydrophobicity, superhydrophilicity

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research about surface wetting control in oil is still extremely rare. In fact, the control of surface wetting behavior in oil is also very important. For example, under-oil superhydrophobic interface may be helpful for the machine equipment that needs to be lubricated with oil due to the corrosion caused by the water in the long-time run, and under-oil superhydrophilic materials are promising to be used in the purification of watercontaining oil and other confined chemical reactions.32 Research about surface wetting control in oil can not only provide some functional materials for the development of smart devices but also help us further understand the inner mechanism that affects the wetting performance. Recently, Zhang et al. have systematically studied the impact of surface morphologies (micro, nano and micro/nanostructures) and hydrophilicity/hydrophobicity on the wettability of silicon surfaces for different liquids under various media (air, water,

ecently, smart interfacial materials that can switch between the superhydrophobicity and superhydrophilicity have been extensively researched for their important applications in microfluidic devices,1−4 biological engineering,5−8 oil/water separation,9−12 and so on. Until now, by combining the responsive materials and rough structures, a variety of stimuli-responsive materials with such smart wettability have been prepared on inorganic (TiO2,13,14 ZnO,15,16 etc.) and organic materials (polypyrrole,17 polyaniline,18 etc.) that respond to photo,19,20 pH,21 temperature,22 electric,23,24 etc. However, all these works are concentrated on wettability switch in air. To realize the same variation in other complex environments, such as oil, is still a challenge. In the past few years, control of surface wettability has become a research focus, since it can directly affect many other properties of materials and related applications, such as catalysis,25 electrochemistry,26 and friction.27 By controlling surface chemical composition and microstructure alone, or together, lots of surfaces with tunable water wettability in air and oil wettability in water have been reported.28−31 However, © XXXX American Chemical Society

Received: August 15, 2017 Accepted: January 10, 2018

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Figure 1. (a and b) SEM images of the as-prepared TiO2 NTAs at low and high magnifications, respectively. (c) Cross-sectional view of TiO2 NTAs. (d) XRD pattern of TiO2 NTAs. Results indicate that the NTAs are ascribed to the anatase TiO2.

and oil).33 Results show that by creating micro/nanostructures on both hydrophobic/hydrophilic substrates, only under-oil superhydrophobicity can be observed, since oil can enter into both hydrophilic/hydrophobic micro/nanostructures and thus enhance the water-repellent ability. From this research, it seems difficult to realize superhydrophilicity in oil, not to say the reversible transition between superhydrophobicity and superhydrophilicity in oil. Herein, TiO2 nanotube arrays (NTAs) that can reversibly switch between the under-oil superhydrophobicity and superhydrophilicity are reported. The TiO2 NTAs surface was prepared by a simple anodizing and heating process, which shows under-oil superhydrophobicity. Upon UV irradiation, it turns into under-oil superhydrophilicity and further heating would make it return to original under-oil superhydrophobicity. Reversible transition between the two extreme states can be achieved by alternation of UV irradiation and heating treatment. Furthermore, the wetting switch for different water droplets including acid, basic, and salt under various oils was also investigated, and the smart control can still be observed, indicating that the TiO2 NTAs surface has a good stability and the control is universal regardless of oil type. It is worth noting that, in air, after both UV irradiation and heating, the obtained TiO2 NTAs surface shows the same superhydrophilicity. Different results between air and oil environments demonstrate that in addition to the surface microstructure and chemical composition, the environmental medium also has a significant effect on the wetting performance. To the best of our knowledge, although many efforts have been devoted to investigate the wettability and related applications on TiO2based surfaces, all of them are restricted in water/oil wetting in air or oil wetting in water,13,14,30 and research about water wetting control in oil on TiO2 is still extremely rare.

RESULTS AND DISCUSSION Figure 1a,b shows the scanning electron microscope (SEM) images of the obtained TiO2 NTAs at low and high magnifications, respectively, showing highly ordered TiO2 NTAs with the inner diameter and wall thickness of about 82 and 35 nm (Figure S1a,b in Supporting Information), respectively. Figure 1c is a cross-sectional view of TiO2 NTAs, suggesting that TiO2 NTAs grow vertically onto Ti substrate with the height of about 10 μm. As confirmed by Xray diffraction (XRD) in Figure 1d, the obtained NTAs are ascribed to the anatase TiO2.34 The surface wettability of the obtained TiO2 NTAs was evaluated by a contact angle meter. As shown in Figure S1c,d, in air, a water droplet can spread on the surface before and after UV irradiation, showing the same superhydrophilicity with a water contact angle (WCA) of almost 0°. When the surface is placed into oil, an interesting phenomenon can be observed. The as-prepared surface presents the superhydrophobicity with a WCA of about 159° in oil (Figure 2a, hexane was used as an example). Upon UV irradiation for about 30 min (humidity = 10%), it turns into the under-oil superhydrophilicity with a WCA of about 6.5°(Figure 2b). Further heating the surface at 150 °C for about 1 h, it returns to the original under-oil superhydrophobicity. As a comparison, a flat anatase TiO2 film was prepared according to the report, and the under-oil WCAs on the film were also examined (Figure S2a,b in Supporting Information).35 It can be seen that the WCAs can only be controlled in a narrow range, between about 125° and 36°, under the action of UV irradiation and heating treatment (Figure 2c,d), confirming that similar as that in air condition,1 a proper surface rough structure is also significant in oil condition for superhydrophobicity/superhydrophilicity transition. The above results indicate that in oil, reversible transition between B

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Figure 2. (a and b) Shapes of a water droplet on TiO2 NTAs in oil under the alternation of UV irradiation and heating, indicating that reversible transition between the under-oil superhydrophobicity and superhydrophilicity can be realized. (c and d) Shapes of a water droplet on a flat TiO2 film under the alternation of UV irradiation and heating; one can observe that reversible switching can only be achieved in a narrow range between the under-oil hydrophobicity and hydrophilicity. (e) The cycles of the reversible wettability switch on the TiO2 NTAs, demonstrating a good controllability. In each cycle, the times needed for the transition from the superhydrophobicity to the superhydrophilicity and the reverse process are about 30 min (with UV light intensity of about 10 mW cm−2 in air) and 1 h (heated at 150 °C in air), respectively.

Figure 3. (a) Dependence of WCA on UV irradiation and heating time on the surface. (b) The influence of TiO2 NTAs height on the WCA before and after UV irradiation, respectively. (c) The wettability switch of TiO2 NTAs under various oils. (d) The wettability switch of TiO2 NTAs for different water droplets containing 1 M HCl, 1 M NaOH, and 10 wt % NaCl in hexane. C

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Figure 4. Schematic illustration of surface wetting switching: (a) before UV irradiation, on the flat TiO2 film (composed by TiO2 NPs, inset in figure a), a small amount of carbonaceous contaminants can increase the surface’s affinity to oil and lead to the trapping of oil in the gaps between TiO2 NPs, which results in the under-oil hydrophobicity. (b) Corresponding to the NTAs, the under-oil hydrophobicity of the tube wall (which is similar to the flat TiO2 film) would provide an upward Laplace force to suspend the water droplet, and the NTAs show the under-oil superhydrophobicity (c). (d) Schematic diagram of the anatase TiO2 (004) facet crystal structure variation under UV irradiation and heating. UV irradiation can lead to the formation of oxygen vacancies and the adsorption of hydroxyl group and molecular water. As for the flat TiO2 film, the increased amounts of hydroxyl group and molecular water (inset in figure e) would help the water droplet further wet the surface due to the formation of hydrogen bonds between them. As a result, it becomes under-oil hydrophilicity (e). Corresponding to the NTAs, the under-oil hydrophilicity would offer a downward capillary force to help the water enter into the NTAs (f) and result in the underoil superhydrophilicity (g). Further heating can help the NTAs return to the original state, and thus, reversible transition can be realized.

the inner mechanism that affects the surface WCA in oil was analyzed carefully. Before UV irradiation, the flat TiO2 surface shows both hydrophilicity and oleophilicity, which has the macroscopic WCA and oil contact angle (OCA) of about 51° and 18° in air, respectively (Figures S2c,e in Supporting Information). Under the action of capillary effect of the NTAs,37 it would be easy to understand the superhydrophilicity and superoleophilicity of the TiO2 NTAs surface in air (Figure S1c,e in Supporting Information). When such a superoleophilic surface is put in oil, oil can easily enter into the nanostructures and wet the NTAs due to the superoleophilicity. In this condition, when a water droplet is put onto the NTAs, a solid/ oil/water interface can be formed, and according to the Yang− Laplace equation, the oil/water interface in a cylindrical pore can form the Laplace pressure (ΔP), which can be described as38

two extreme states can be realized by alternation of UV irradiation and heating treatment of our TiO2 NTAs. Moreover, this process can be repeated several times (Figure 2e), demonstrating a good controllability on our surface. To have a better understanding of the smart switch, the effect of UV irradiation (humidity = 10%) and heating time on water wettability in oil of the TiO2 NTAs was investigated in detail (Figure 3a). It is observed that the under-oil WCA gradually decreases from the superhydrophobicity as required by controlling the time of UV irradiation or heating treatment. In addition, it is found that the geometric structure parameter is also significant for the smart controllability. As shown in Figure 3b, with the increase of TiO2 NTAs height (Figures S3 and S4 in Supporting Information), the wetting control can be varied from the limited hydrophobicity/hydrophilicity to the amplified superhydrophobicity/superhydrophilicity when the TiO2 NTAs height gets to about 10 μm, further confirming that the surface microstructure also has the enhanced effect on surface wettability in oil environment.36 In addition to the hexane, the surface wettability was also examined in some other oils such as benzene, petroleum ether, cyclohexane, and gasoline, and the smart controllability can also be realized (Figure 3c), indicating that the controllability is universal and regardless of oil type. Furthermore, for different water droplets (including strong acid (pH = 1), basic (pH = 14), and high salty solutions (10 wt % NaCl)), the under-oil wetting switch between the two extreme states can also be achieved (Figure 3d, hexane was used as the oil), demonstrating a good stability of TiO2 NTAs surface. As shown in Figure S1c,d, the TiO2 NTAs surface always shows the same superhydrophilicity in air, while it displays smart wetting switch between the superhydrophobicity and superhydrophilicity in oil. To clarify the interesting phenomena,

ΔP = −

2γ cos θwo r

(1)

Herein, θwo is the intrinsic WCA on tube wall in oil, γ is the water−oil interface tension, and r is the radius of the NTAs. From the eq 1, it can be found that when θwo > 90°, the ΔP > 0, and water would be prevented from wetting the NTAs. In the contrary, when θwo < 90°, ΔP < 0, water would wet the NTAs spontaneously. From the above, it can be found that θwo is very important for understanding the NTAs wetting performances in oil. However, to directly examine the WCA on tube wall is difficult, given the small size of the nanotubes. Herein, considering that the tube walls of NTAs and the flat TiO2 film are composed of the same anatase TiO2 nanoparticles (TiO2 NPs, Figures S1a,b and S2b in Supporting Information) with similar chemical compositions (Figures S5, S6, and S8 in Supporting D

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condition.49 On our TiO2 NTAs surface, a small amount of adsorbed carbonaceous contaminants can be observed on both high-resolution C 1s and O 1s spectra (Figures S5a and S6a in Supporting Information). As expected, the UV irradiation can effectively decompose these adsorbed contaminants, and when the UV irradiation time is about 10 min, these adsorbed contaminants would disappear (Figures S5b and S6b in Supporting Information). At this time, the under-oil WCA of TiO2 NTAs surface is about 42° (Figure 3a, the least UV irradiation time for realization of the under-oil superhydrophilicity is about 30 min), which is much larger than the value for the superhydrophilicity. These results indicate that on our surface, by singly eliminating adsorbed carbonaceous contaminants, the surface cannot achieve the under-oil superhydrophilicity, and we believe that the increased amount of the adsorbed hydroxyl group (dissociated water) and coexisted molecular water on our NTAs may play a more important role for achieving the under-oil superhydrophilicity (Figure S9 in Supporting Information). Our results are apparently different from Tom Mathews’s reports. In their research,50,51 the destruction of adsorbed carbonaceous contaminants was considered to be the key factor for the superhydrophilicity in air. This discrepancy indicates that in different environments (air and oil), the wetting transition mechanism may not be exactly the same. To further understand the reconstruction of TiO2 surface during the hydroxylation process and its effect on the wetting switching, based on the XRD result (Figure 1d), the dominated exposed (004) facet for our NTAs was used as an example and modeled in Figure 4d. As is known, when the TiO2 surface is irradiated by UV light, the electrons can be excited from the valence band to the conduction band, leaving behind the holes in the valence band. These formed electrons and holes can diffuse randomly to the surface, and the holes can cleave the Ti−O bonds, resulting in the formation of oxygen vacancies, which can further adsorb the hydroxyl group and some coexisting molecular water.43,44,47,48,52,53 In oil, due to the increased amount of hydroxyl group and co-existed molecular water (inset in Figure 4e), on flat TiO2 film, oil cannot wet the TiO2 nanoparticles completely.48,54 In this case, when a water droplet contacts the surface, the hydroxyl group and molecular water would help the water droplet further wet the surface due to the formation of hydrogen bonding between them. Thus, the flat TiO2 surface becomes under-oil hydrophilic with the WCA of about 36° (Figure 4e). In air, as confirmed by Fujishima et al.,43,44 such a hydroxylation process can result in the superhydrophilicity and superoleophilicity on flat TiO2 film (Figure S2d,f in Supporting Information),47 not to say our TiO2 NTAs with three-dimensional (3D) capillary effect (Figure S1d,f in Supporting Information). When such a superoleophilic TiO2 NTA surface is put into oil, oil can certainly wet the surface and enter into the NTAs (Figure 4f). However, different from the condition before UV irradiation, when a water droplet contacts the UV-irradiated NTAs, because the tube wall becomes under-oil hydrophilic after UV irradiation (which is similar to that on flat TiO2 film as stressed above), the θwo < 90°, and ΔP < 0 (eq 1), meaning that water can spontaneously enter into the NTAs under the capillary effect (Figure 4g). Therefore, as displayed in Figure 2b, after UV irradiation, the TiO2 NTAs show under-oil superhydrophilicity. In order to further confirm our speculation that the adsorb dissociated water and some co-existed molecular water play the key role for the under-oil super-

Information, as proved by XPS results, under both two conditions before and after UV irradiation, the flat TiO2 film and our NTAs have similar chemical compositions), it is rational to believe that the intrinsic wettability of the tube wall of our NTAs is similar to that on the flat TiO2 film. Before UV irradiation, little hydroxyl group, adsorbed molecular water can be found on the flat TiO2 film (Figure S8e in Supporting Information), and the film is hydrophilic/oleophilic (Figures S2c and S2e in Supporting Information). When the film is put into oil, oil can wet most of TiO2 NPs (inset in Figure 4a, as mentioned above, the film is in fact composed of TiO2 NPs). Meanwhile, a small amount of carbonaceous contaminants can also increase the oil affinity to the film (Figure S8c in Supporting Information), resulting in the trapping of oil into the gaps between the TiO2 NPs and ultimately increasing the film’s water-repellent ability. Therefore, as illustrated in Figure 4a, the flat TiO2 film shows under-oil hydrophobicity (the WCA in oil is about 125°, Figure 2c). Similar phenomena have also been observed on many other nanostructured films including silicon nanowirs,33 porous PVDF membrane,39 and nanostructured ZnO-coated nonwoven fabrics.40 On our TiO2 NTAs, as emphasized above, the tube wall has similar wettability with the flat TiO2 film and should show similar under-oil hydrophobicity before UV irradiation. According to the eq 1, θwo is higher than 90°, ΔP > 0, meaning that an upward Laplace force can support the water on the NTAs (Figure 4b). Under this condition, the water droplet would reside in the composite Cassie state (Figure 4c), and the high WCA can be explained by the following modified equation:41 cos θ′wo = f cos θwof + f − 1

(2)

where θ′wo and θwof are the WCAs of the nanostructured TiO2 NTAs and flat TiO2 substrate in oil, respectively, and f represents the area fraction of the TiO2 NTAs in contact with water, which can be calculated according to the following equation:42 f=

2πw(w + D) 1/2

3

(D + 2w + x)2

(3)

where w, D, and x are average tube wall thickness, tube inner diameter, and spacing between tubes, respectively. These parameters were obtained from the SEM images, and according to this equation, f is calculated to be 0.254, meaning that about 74.6% of the contact area is the water−oil contact interface. Therefore, as shown in Figure 2a, the TiO2 NTAs display the under-oil superhydrophobicity (the mechanism is similar to that for the underwater superoleophobicity as shown in Figures S1g,h, more discussion in Supporting Information). It is well-known that UV irradiation can lead to the superhydrophilicity in air on both flat and rough TiO2 surfaces regardless of single crystalline or polycrystalline structures.43,44 Two popular theories are often used to explain this phenomenon. One is the destruction of the adsorbed carbonaceous contaminants under the effect of UV irradiation;45,46 and the other is the adsorption of hydroxyl group (dissociated water) and some co-existed molecular water on UV-irradiation resulted oxygen vacancies sites.47,48 To clearly understand the under-oil superhydrophilicity on our TiO2 NTAs, the surface chemistry variation before and after UV irradiation was carefully analyzed by the XPS (Figures S5−S7 in Supporting Information). As is well-known, TiO2 surface is easily contaminated by airborne stains under ambient E

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ACS Nano hydrophilicity, the effect of the environmental humidity on the wetting switch was also investigated (Figure S10 in Supporting Information). As the humidity is increased, more water molecules can be easily absorbed, and the needed time for realizing the under-oil superhydrophilicity is apparently decreased. However, in relatively dry condition such as N2 atmosphere, to achieve this transition is more difficult. These results indicate that the adsorbed water plays a significant role for the wetting switch to the superhydrophilicity in oil, which can be further confirmed by the results of fluoresce detection (Figure S11 in Supporting Information). Noticeably, UVirradiated TiO2 NTAs surface is an energetically metastable state in thermodynamics,53,54 and the absorbed dissociated water and molecular water will be replaced by the atmosphere oxygen gradually in dark, that is, the surface defects at oxygen vacancies sites would be healed, and heating can accelerate such a process (Figure S12 in Supporting Information).49 Thus, after heating, the original under-oil superhydrophobicity would be recovered, and as shown in Figure 2e, the repeated wetting switch can be realized by alternation of UV irradiation and heating process. From the above, one can find that UV irradiation can lead to the transition from the slight hydrophilicity to the high hydrophilicity on flat TiO2 surface, and on corresponding NTAs, superhydrophilicity in air and oil, respectively, can be observed. Based on the above experimental results and discussion, it can be deduced that under the same enhanced effect of surface rough structure, weak hydrophilic surface chemical composition would endow the rough surface with the superhydrophilicity in air, while in oil, to realize the same superhydrophilicity, a highly hydrophilic surface chemical composition is necessary. This finding would provide us with a design principle of superhydrophilic surfaces for applications in different environments. The smart wettability of our surface can endow it with many applications, such as droplet-based microreactor and water capture in oil based on the surface with under-oil superhydrophobicity and superhydrophilicity, respectively. The droplet-based microreactor has shown significant advancement in biochemical reactions,55 which has been successfully used in air condition, nevertheless, in complex oil condition, the report is still extremely rare. Figure 5a shows such an example. Water droplets containing FeCl3 and KSCN (dyed with methyl blue) were placed on the superhydrophobic TiO2 NTAs surfaces in hexane, respectively. Then, using a pair of under-oil superhydrophobic TiO2 surfaces, such as the “tweezers”, the KSCN droplet can be picked up and put onto the FeCl3 droplet. After contact and coalescence, KSCN would react with FeCl3 and form the fuchsia [Fe(SCN)6]3−, thus completing a dropletbased microreaction in oil (more details in Movie S1). The surface with under-oil superhydrophilicity can be used to directly capture water from oil. As shown in Figure 5b, when the UV-irradiated TiO2 NTAs surface touches the water in hexane, the water quickly spreads out on the surface and ultimately is removed from the oil along with the surface (more details in Movie S2). Herein, what needs to be stressed is that previous reports on oil/water separation by absorption were mainly focused on the selective removal of oil using “oilabsorbing” materials.56,57 The opposite case, that is, removal of water from bulk oil, has been seldom reported. Our surface advances a concept based on under-oil superhydrophilicity and provides a good example, which possesses great potential for applications in purification of water-containing oils.

Figure 5. Application of the smart surface: (a) Process of under-oil droplet-based microreaction by means of the as-prepared under-oil superhydrophobic TiO2 NTAs surface. (b) Removal of water from oil by UV-irradiated under-oil superhydrophilic TiO2 NTAs surface, and hexane was used as the oil.

CONCLUSIONS In summary, TiO2 NTAs arrays with switchable wettability between the under-oil superhydrophobicity and superhydrophilicity are reported. The transition can be repeated by alternately treating the surface with UV-irradiation and heating. The smart control can be attributed to the cooperative effect between the surface nanostructures and chemical composition variation. Finally, based on the special wettability switching on the TiO2 NTAs surface, some applications including dropletbased microreaction in oil and water-removal from oil are demonstrated. This work reports a surface with tunable wettability in oil from the superhydrophobicity to superhydrophilicity, which can not only provide some fresh ideas for development of functional materials for application in complex environments but also help us further understand the inner mechanism affecting the surface wettability. Furthermore, such a smart control can be easily extended to other TiO2-based materials, such as 3D porous materials and hybrid materials. EXPERIMENTAL SECTION Fabrication of Highly Ordered TiO2 Nanotube Arrays (NTAs). Titanium sheets of 0.25 mm in thickness and a purity of 99.7% were purchased from Qing Yuan Metal Materials Co., Ltd., China. The highly ordered TiO2 NTAs were fabricated by electrochemical anodization of Ti foil in an ethylene glycol electrolyte with addition of 0.3 wt % NH4F and 2 vol % deionized water. Prior to anodization, Ti foil (2.5 cm × 2.5 cm) was ultrasonically cleaned in acetone, alcohol, and distilled water sequentially and then was chemically polished in a mixed solution of HF, HNO3, and H2O (volume ratio = 1:4:5). The anodization process contains two steps. The first anodization step was conducted at 40 V for 2 h in a two-electrode cell with Ti foil as the working electrode and platinum foil (2 cm × 2 cm) as the counter electrode at room temperature. After this, the produced TiO2 NTAs were removed ultrasonically in deionized water, and then it was anodized again at 40 V for 2 h in above electrolyte. The obtained TiO2 NTAs were immediately rinsed with abundant deionized water and dried in air. After that, the as-prepared TiO2 F

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ACS Nano NTAs surface was first annealed at 350 °C for 1 h in air with a heating rate of 5 °C min−1, and then the surface was cooled down to 150 °C with a cooling rate of 2 °C min−1. When the temperature was reached, the surface was further kept at 150 °C for about 1 h and then cooled to room temperature. Finally, the TiO2 NTAs would be obtained. The flat TiO2 surface was prepared by the spraypyrolysis method,35 in which very small uniform droplets of TiO2 precursor emulsion (titanium(IV) oxyacetylacetonate/ethanol) are sprayed onto a heated glass substrate at a constant rate through a glass atomizer. The asprepared flat TiO2 surface was annealed at 500 °C for 1 h in air atmosphere with a heating rate of 2 °C min−1 and cooled down to room temperature with a cooling rate of 2 °C min−1. Characterization. SEM images of TiO2 NTAs morphology were obtained on a field emission SEM apparatus (HITACHI, SU8010). The crystal phase of samples is identified by using an X-ray diffractometer (XRD, X’ Pert-Pro MRD, Philips) with Cu Ka radiation (λ = 0.1542 nm). Under-oil surface wettability of TiO2 NTAs is evaluated at room temperature by measuring the contact angle of water droplet (ca. 4 μL) using a contact angle meter (JC 2000D5, Shanghai Zhongchen Digital Technology Apparatus Co., Ltd.). The WCA and OCA measurements were performed at five different points on the same surface, and an average value was adopted. For wettability investigation in air, liquid droplets were directly put on the membrane. For under-oilwater wetting performance measurements, the substrates were first fixed in a quartzose container that was transparent and full of oil. Then a water droplet was used to contact the membrane. UV light irradiation was carried out in air with a 500 W mercury xenon lamp under controlled light intensity with 10 mW cm−2 (CHF-XM-500W, Beijing Chang Tuo technology Co., Ltd.,), employing a filter to obtain light with a wavelength centered at 365 nm. For the recovery process of surface wettability, TiO2 NTAs were heated in air, at 150 °C for different times in an oven. The X-ray photoelectron spectroscopy is performed with a K-Alpha electron spectrometer (XPS, Thermo Fisher Scientific Company) using Al Kα radiation. Fluorescence microscope (FM, Nikon 80i, Japan) was used to prove under-oil wettability change of TiO2 NTAs before and after UV irradiation after immersion into n-hexane dyed with fluorescent green. The operating process is as follows: The as-prepared TiO2 NTAs were placed in hexane dyed with fluorescent green and then taken out quickly. Next, TiO2 NTAs wetted by hexane dyed fluorescent green were immersed into water and, a few minutes later, taken out once again to observe the change of fluorescence intensity by using fluorescence microscope (more details see Supporting Information).

Droplet-based microreaction in oil (AVI) Surface with under-oil superhydrophilicity directly captures water from oil (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yuyan Liu: 0000-0003-3030-8551 Zhongjun Cheng: 0000-0001-5550-2989 Lei Jiang: 0000-0003-4579-728X Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC grant nos. 21674030 and 51573035). REFERENCES (1) Wang, S. T.; Liu, K. S.; Yao, X.; Jiang, L. Bioinspired Surfaces with Superwettability: New Insight on Theory, Design and Applications. Chem. Rev. 2015, 115, 8230−8293. (2) Xin, B. W.; Hao, J. C. Reversibly Switchable Wettability. Chem. Soc. Rev. 2010, 39, 769−782. (3) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. What Do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36, 1350−1368. (4) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67−70. (5) Geyer, F. L.; Ueda, E.; Liebel, U.; Grau, N.; Levkin, P. A. Superhydrophobic−Superhydrophilic Micropatterning: Towards Genome-on-a-Chip Cell Microarrays. Angew. Chem., Int. Ed. 2011, 50, 8424−8427. (6) Huang, Q. L.; Lin, L. X.; Yang, Y.; Hu, R.; Vogler, E. A.; Lin, C. J. Role of Trapped Air in the Formation of Cell-and-Protein Micropatterns on Superhydrophobic/Superhydrophilic Microtemplated Surfaces. Biomaterials 2012, 33, 8213−8220. (7) Lai, Y. K.; Lin, L. X.; Pan, F.; Huang, J. Y.; Song, R.; Huang, Y. X.; Lin, C. J.; Fuchs, H.; Chi, L. F. Bioinspired Patterning with Extreme Wettability Contrast on TiO2 Nanotube Array Surface: A Versatile Platform for Biomedical Applications. Small 2013, 9, 2945−2953. (8) Falde, E. J.; Yohe, S. T.; Colson, Y. L.; Grinstaff, M. W. Superhydrophobic Materials for Biomedical Applications. Biomaterials 2016, 104, 87−103. (9) Wang, B.; Liang, W. X.; Guo, Z. G.; Liu, W. M. Biomimetic Super-Lyophobic and Super-Lyophilic Materials Applied for Oil/ Water Separation: A New Strategy Beyond Nature. Chem. Soc. Rev. 2015, 44, 336−361. (10) Xu, Z. G.; Zhao, Y.; Wang, H. X.; Wang, X. G.; Lin, T. A Superamphiphobic Coating with an Ammonia-Triggered Transition to Superhydrophilic and Superoleophobic for Oil-Water Separation. Angew. Chem., Int. Ed. 2015, 54, 4527−4530. (11) Ma, Q. L.; Cheng, H. F.; Fane, A. G.; Wang, R.; Zhang, H. Recent Development of Advanced Materials with Special Wettability for Selective Oil/Water Separation. Small 2016, 12, 2186−2202. (12) Yong, J. L.; Chen, F.; Yan, Q.; Huo, J. L.; Hou, X. Superoleophobic Surfaces. Chem. Soc. Rev. 2017, 46, 4168−4217. (13) Liu, K. S.; Cao, M. Y.; Fujishima, A.; Jiang, L. Bio-Inspired Titanium Dioxide Materials with Special Wettability and Their Applications. Chem. Rev. 2014, 114, 10044−10094. (14) Lai, Y. K.; Huang, J. Y.; Cui, Z. Q.; Ge, M. Z.; Zhang, K. Q.; Chen, Z.; Chi, L. F. Recent Advances in TiO2-Based Nanostructured

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05813. The magnified SEM images of TiO2 NTAs and flat TiO2 surface; the water and oil wetting performance of TiO2 NTAs and flat TiO2 surface before and after UV irradiation in air; the oil contact angle (OCA) of the as-prepared TiO2 NTAs surface before and after UV irradiation under water; SEM images of TiO2 NTAs by electrochemical anodization for different time; the XRD patterns of TiO2 NTAs by electrochemical anodization for different time; XPS spectra of TiO2 NTAs in C 1s, O 1s and Ti 2p region before and after UV irradiation, respectively; statistic results of the percentage for oxygencontaining groups after UV irradiation for different times; the effect of humidity on the under-oil wettability switching of TiO2 NTAs; the fluorescence microscope images and corresponding fluorescent intensity of TiO2 NTAs in different conditions; dependence of under-oil− water contact angles on the storage time in dark environment; discussion about the underwater superoleophobicity on the TiO2 NTAs (PDF) G

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