Supporting Information

Supporting Information Under-Oil Switchable Superhydrophobicity to Superhydrophilicity Transition on TiO 2 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, PR China Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, PR China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China S-1

Figure S1. (a) and (b) Amplified SEM images of the TiO 2 NTAs (obtained after electrodeposition for about 2 h) viewed from the cross-section and top, respectively. It can be seen that the NTAs are composed of lots of nanoparticles with an average diameter of about 20 nm. (c) and (d) are shapes of a water droplet on the NTAs before and after UV irradiation in air, respectively. (e) and (f) are shapes of an oil droplet (hexane) on the NTAs before and after UV irradiation in air, respectively. (g) and (h) S-2

are shapes of a hexane droplet contact with the NTAs in water before and after UV irradiation, respectively. Herein, we emphasize that after heating the UV-irradiated NTAs in air at 150 for 1 h, the surface would return to the initial wetting state. From the above, it can be seen that in all conditions (before/after UV irradiation and further heating), TiO 2 NTAs surface shows superhydrophilicity, superoleophilicity in air, and superoleophobicity in water. Figure S2. (a) and (b) are SEM images of the as-prepared flat TiO 2 film with low and high magnifications, respectively. One can find that the film is composed by lots of nanoparticles with an average diameter of about 30 nm. (c) and (d) are shapes of a S-3

water droplet on the film before and after UV irradiation in air, respectively. (e) and (f) are shapes of an oil droplet (hexane) on the film before and after UV irradiation in air, respectively. From these results, it can be seen that, after UV irradiation, the film becomes superhydrophilic and superoleophilic in air. This variation has been observed in many reports, which can be ascribed to the adsorption of hydroxyl group and some coexisted molecular water. 1-3 Figure S3. SEM images of TiO 2 NTAs by electrochemical anodization of Ti foil with constant voltage of 40 V in an ethylene glycol electrolyte with addition of 0.3 wt% NH 4 F and 2 vol% deionized water for different time: (a) 0 min, (b) 20 min, (c) 40 min, (d) 60 min and (e) 120 min, respectively. The height of TiO 2 NTAs was about 0 µm, 1.8 µm, 3.4 µm 6.5 µm and 10 µm, respectively. It can be seen that the average height of TiO 2 NTAs is increased with the increase of the anodization time (f). S-4

Figure S4. The XRD patterns of TiO 2 NTAs by electrochemical anodization of Ti foil with constant voltage of 40 V for different time. It can be seen that the as-prepared TiO 2 NTAs are anatase except that prepared for 0 min. 4 S-5

Figure S5. XPS spectra of TiO 2 NTAs in C1s region: (a) before UV irradiation, (b) after UV irradiation for about 10 min (humidity = 10%), (c) after UV irradiation for about 30 min (humidity = 10%), (d) after heating the UV-irradiated surface at 150 for about 1 h, respectively. The variation of C1s on the as-prepared surface before/after UV irradiation and further heating was investigated. Herein, the C1s peak around 284.8 ev, corresponding to adventitious carbon (contaminated amorphous C) present in the film surface is not considered as it is found on the surface of all films (as a result of contamination from the atmosphere). 5 In Figure (a), the peaks at 286.4 ev and 288.6 ev, corresponding to carbonaceous species having C-O bonds 5, 6 can be observed. After UV irradiation (b and c), the two peaks disappear, indicating that the carbonaceous species having C-O bonds have been removed on the UV irradiated surface. Meanwhile, we can conclude that UV irradiation 10 min is enough to remove S-6

the carbonaceous species having C-O bonds. After further heating in air (d), one can observe that small amount of the carbonaceous species having C-O bonds re-adsorbed onto the surface. Figure S6. (a-d) XPS spectra of TiO 2 NTAs in O1s region: (a) before UV irradiation, (b) after UV irradiation for about 10 min (humidity = 10%), (c) after UV irradiation for about 30 min (humidity = 10%), and (d) after further heating at 150 for 1 h, respectively. From spectra (a), five peaks can be seen on the surface before UV irradiation. The peaks at 530.6 ev and 531.3 ev can be attributed to the Ti-O bonds; 7, 8 the peaks at 532.4 ev and 533.2 ev correspond to Ti-HO and adsorbed H 2 O, respectively; and the one at 534.2 ev can be attributed to O-C bonds resulted from the adsorption of carbonaceous contaminants. 9,10 After UV irradiation for about 10 min (b), two points S-7

need to be noticed, one is that the areas under the corresponding peaks to HO-Ti and adsorbed H 2 O is increased, meaning that the amount of HO-Ti and adsorbed H 2 O is increased; another is that the peak at 534.2 ev disappears, meaning that after UV irradiation for 10 min, the adsorbed carbonaceous contaminants have been eliminated. Further increasing the UV irradiation time to about 30 min (c), it can be found that the areas under the corresponding peaks to HO-Ti and adsorbed H 2 O are further increased. After heating the UV-irradiated surface at 150 for about 1 h, it can be seen that the spectra become similar with the original state, meaning that the surface chemical composition returns to the original state. From Figures S5 and S6, it can be concluded that on our TiO 2 NATs surface, a small amount of carbonaceous contaminants can be found, and UV irradiation 10 min is enough to remove these contaminations. Meanwhile, UV irradiation can effectively increase the amount of HO-Ti and adsorbed water. Furthermore, after heating the UV-irradiated surface at 150 for 1 h in air, a small amount of carbonaceous species can be re-adsorbed onto the surface, and the amounts of HO-Ti and adsorbed water are decreased, indicating that the heating process can help the surface chemical composition return to the initial state. S-8

Figure S7. XPS spectra of TiO 2 NTAs in Ti2p region before and after UV irradiation for about 30 min (humidity = 10%), respectively. It can be seen that after UV irradiation, the Ti2p spectrum was slightly broadened, meaning some Ti 4+ was reduced to Ti 3+ under UV irradiation, 11,12 further confirming the UV irradiation can affect the variation of TiO 2 crystal structure. S-9

Figure S8. XPS results of the flat TiO 2 film. (a) and (b) XPS survey spectrum of the surface before and after UV irradiation, respectively, proving the presence of elements Ti, O, and C. (c) and (d) High-resolution C1s XPS spectra before and after UV irradiation for about 30 min, respectively. (e) and (f) High-resolution O1s XPS spectra before and after UV irradiation for about 30 min, respectively From these figures, one can observe that before UV irradiation, similar with that on our NATs (Figures S5a, S6a), two peaks at 286.4 ev and 288.6 ev in Figure S8c and S-10

one peak at 534.2 ev in Figure S8e, corresponding to carbonaceous species having C-O bonds can be observed. After UV irradiation, these peaks disappear, meanwhile, the areas under the corresponding peaks to HO-Ti and adsorbed H 2 O in Figure S8f are increased, meaning that the amounts of HO-Ti and adsorbed H 2 O are increased, which is also similar with our NTAs (Figures S5c, S6c). These results indicate that the flat TiO 2 film and our NATs have similar surface chemical compositions in both the two conditions before and after UV irradiation. Figure S9. Statistic results of the percentage for oxygen-containing groups after UV irradiation for different time on our NATs. The data were calculated based on the XPS results (Figure S6). It can be seen that with increasing the UV irradiation time, the amount of hydroxyl group and adsorbed water is increased. S-11

Figure S10. (a) The effect of humidity on the under-oil wettability switching of TiO 2 NTAs. It can be seen that the degree of under-oil wettability switching is evidently improved with the increase of humidity under the same UV irradiation time of 10 min. Meanwhile, we found that the needed UV irradiation time can be reduced by increasing humidity to realize the transition from under-oil superhydrophobicity to superhydrophilicity (b). Without UV irradiation, even in the condition with high humidity for about 15 h, the surface still remains the under-oil superhydrophobicity (c). These results indicate that long UV irradiation time and high humidity are helpful for the transition from the superhydrophobicity to superhydrophilicity in oil. S-12

Figure S11. The fluorescence microscope images and corresponding fluorescent intensity of TiO 2 NTAs in different conditions: (a) before UV irradiation, the as-prepared surface was immersed into oil (hexane) with fluorescent molecule S-13

(fluorescent green) for a few minutes and then taken out to measure the change of fluorescence intensity. Because the TiO 2 NTAs are superoleophilic, oil can enter into the nanostructures, thus, the nanostructures can be covered by the fluorescent molecules and display a high fluorescent intensity. (b) When the oil covered surface is further immersed into water, because the surface is under-oil superhydrophobic, water cannot enter into the nanostructure and replace the oil, as a result, it can be seen that the amount of fluorescent molecules and related fluorescent intensity have no apparently variation. (c) After UV irradiation, the surface is superoleophilic, and after immersing into the same oil dyed with fluorescent molecule, oil can also wet the nanostructures. However, it should be noticed that the UV irradiation can result in the increase of the amount of the molecular water onto the TiO 2 NTAs. 1,2 These water molecules can directly impede the contact between oil and TiO 2 surface. Thus, it can be found that, compared with Figure S11a (before UV irradiation), the amount of fluorescent molecules and corresponding fluorescent intensity decreased apparently. (d) When the UV-irradiated surface with oil and fluorescent molecules was further immersed into water, water can enter into the nanostructures and replace the oil and fluorescent molecules due to the under-oil superhydrophilicity. Therefore, it can be seen that the amount of fluorescent molecules and corresponding fluorescent intensity decreased apparently (it is worthy of noting that UV irradiation can result in the increase of the adsorption of molecular water, according to the Fujishima s report, 1,2 heterogeneous wettability in microscale can be formed. Some nanoscale regions with adsorbed water have better affinity to water than oil, while other nanoscale regions with better affinity to oil than water. 1,2 After immersion into oil, some oil molecules can directly contact these regions with better affinity to oil than water, which cannot be replaced by water, therefore, some fluorescent molecules can still be observed after S-14

immersion into water). These results can help us further prove the wetting performance of our surface, that is, before UV irradiation, the nanostructure can be wetted and covered by oil, while it cannot further wetted by water. After UV irradiation, the nanostructure can still be wetted by oil, and it can also be further wetted by water, during this process, water can replace oil and enter into the nanostructure. 180 150 120 WCA( O ) 90 60 30 0 0 2 4 6 8 10 12 14 Storage time (day) Figure S12. Dependence of under-oil water contact angles on the storage time in dark environment. It can be seen that with increasing the time, the contact angle is increased and when the time reaches to about 14 days, under-oil superhydrophobicity can be achieved. Discussion about the underwater superoleophobicity on the TiO 2 NTAs In this work, before and after UV irradiation, transition from under-oil superhydrophobicity to superhydrophilicity can be observed on the TiO 2 NTAs. Different from the oil environment, when the surface is put into water, only S-15

underwater superoleophobicity can be observed (Figures S1g and S1h). The reason can be explained as follows: before UV irradiation, as shown in Figure S1c, the surface shows superhydrophilicity in air. When the superhydrophilic surface is put into water, water can certainly enter into the nanostructures and occupy the gaps among the nanotubes. In this state, when an oil droplet is used to contact the surface, it can reside the composite Cassie state and only contact the tips of the nanotubes, thus a larger oil contact angle can be observed and the surface shows underwater superoleophobicity. 13,14 According to Fujishima s reports, 1,2 the UV irradiation can result in the enhancement of the TiO 2 hydrophilicity due to the adsorption of dissociative water and molecular water. 1-3 This process can certainly enhance the hydrophilicity of the whole TiO 2 nanotubes, and it is no doubt that water can wet the whole nanotubes and occupy the gaps among the nanotubes when the surface is put in water. Meanwhile, the adsorbed molecular water can further enhance the oil-repellent ability. 15,16 Thus, as shown in Figure S1h, the oil droplet still resides in the composite Cassie state, and the surface shows the underwater superoleophobicity while has a higher oil contact angle compared with that before UV irradiation (Figure S1g). Reference (1) Sakai, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Quantitative Evaluation of the Photoinduced Hydrophilic Conversion Properties of TiO 2 Thin Film Surfaces by the Reciprocal of Contact Angle. J. Phys. Chem. B 2003, 107, 1028 1035. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Light-induced Amphiphilic Surfaces. Nature 1997, 388, 431 432. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Photogeneration of Highly Amphiphilic TiO 2 Surfaces. Adv. Mater. 1998, 10, 135 138. S-16

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