Facile fabrication of superhydrophobic meshes with different water adhesion and their influence on oil/water separation

Abstract

The control of water adhesion is important for superhydrophobic surfaces in numerous applications. Compared with the abundant research on oil/water separation through the use of superhydrophobic materials, research relating to the influence of the water adhesion of superhydrophobic materials on their oil/water separation performance is extremely rare. Herein, superhydrophobic ZnO coated meshes with different water adhesion were successfully prepared by spraying ZnO nanoparticles (NPs) on a stainless steel mesh. By simply changing the percentage of hydrophobic ZnO NPs in the hydrophobic/hydrophilic ZnO NP mixture, superhydrophobic ZnO meshes with different water adhesion have been fabricated successfully. In addition, the separation mechanism for oil/water mixtures is elaborated by interpreting the different states of a water droplet on the surface before and during separation. Furthermore, the influence of the water adhesive property of superhydrophobic meshes on their oil/water separation performance is studied. To the best of our knowledge, this issue has scarcely been considered. It is found that the influence of the adhesive property of the superhydrophobic meshes on their oil/water separation performance can be neglected. The superhydrophobic ZnO coated meshes whether with low or high water adhesion showed nearly the same separation efficiency, which is up to 98.5% for a kerosene and water mixture. The water intrusion pressure and oil flux study exhibit that the low adhesive meshes show a little higher water intrusion pressure and oil flux than the high adhesive meshes. This report assesses the influence of the water adhesion of superhydrophobic materials on their oil/water separation performance, which not only helps us to further understand the mechanism of oil/water separation, but also to design and prepare superhydrophobic surfaces for the effective separation of water from oil.

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1. Introduction

Recently, superhydrophobic surfaces with water contact angles (CA) larger than 150° have attracted considerable interest in both fundamental research and potential practical applications in the fields of self-cleaning,^1,2 anticorrosion,³ antifogging,⁴ antisticking,⁵ anti-icing,⁶ fluidic drag reduction,⁷ water collection,⁸ oil/water separation,⁹ and no loss micro droplet transportation,¹⁰ etc.^11–13 According to the different water adhesion on the surfaces, superhydrophobic surfaces are normally classified into two classes: low adhesion to water and high adhesion to water. Superhydrophobic surfaces with low adhesion are generally inspired by lotus leaves.¹⁴ Water droplets spontaneously roll away removing loose dust particles on low adhesive superhydrophobic surfaces. In addition, high adhesive superhydrophobic surfaces are usually inspired by the gecko's feet and rose petals.^15,16 On these surfaces, water droplets are firmly pinned without any movement, even when the surfaces are tilted vertically or turned upside down. Generally, water adhesion on superhydrophobic surfaces is mainly governed by the surface geometrical structure and surface composition. Therefore, by dynamically tuning these two factors, the water adhesion on superhydrophobic surfaces could be effectively controlled.^17–28

Oil/water separation has become an increasingly important and urgent issue in modern chemical industrial processes and environmental protection due to the increase in industrial oily wastewater as well as frequent oil spill accidents.²⁹ Therefore, there is an urgent need of energy-efficient and environmentally friendly methods for oily sewage treatment. Inspired by nature, making use of the advantages of novel materials with special wettability is a feasible and effective approach for the separation of oil and water mixtures.^30–35 Recently, Jiang et al., for the first time, used superhydrophobic/superoleophilic mesh films in the field of oil/water separation, where the water phase was repelled while the oil phase penetrated though the mesh easily, which exhibited high oil/water separation efficiency and selectivity.³⁶ Inspired by this work, various materials with both superhydrophobic and superoleophilic properties have been utilized for the separation water from water/oil mixtures,^37–42 such as mesh-based materials,^43–45 fabrics,^46,47 filter paper,⁴⁸ carbon-based materials,^49,50 sponge-based materials,^51,52 and foam-based materials.^53,54 Water adhesion is important for superhydrophobic surfaces, because it is the adhesive property that ultimately determines the dynamic action of the water on the surface. However, thus far, the influence of the water adhesion of superhydrophobic surfaces on oil/water separation performance has been largely neglected. To the best of our knowledge, we are unaware of any previously reported studies about the influence of the water adhesion of superhydrophobic/superoleophilic surfaces on oil/water separation performance. In fact, such research is of great importance not only to help us to further understand the mechanism of oil/water separation, but also to design and prepare superhydrophobic surfaces for the separation of water from oil effectively.

Herein, we fabricate superhydrophobic mesh surfaces with tunable water adhesion successfully through a one-step spray-coating process, and study the influence of the water adhesive property of the superhydrophobic mesh surfaces on their oil/water separation performance. In detail, a mixture of hydrophobic ZnO NPs (hydrophilic ZnO NPs modified with stearic acid) and hydrophilic ZnO NPs suspension was sprayed onto stainless steel mesh. By simply changing the percentage of hydrophobic ZnO NPs in the hydrophobic/hydrophilic ZnO NP mixture, tunable adhesive superhydrophobic ZnO meshes have been fabricated successfully. Since water remains exclusively on the ZnO coated mesh surface and oil permeates through the mesh quickly, the as-prepared superhydrophobic/superoleophilic mesh with tunable water adhesion could be used for the separation of oil from water successfully. Besides, the separation mechanism for the oil/water mixture is elaborated by interpreting the different states of a water droplet on the mesh surface before and during separation. Moreover, the influence of the water adhesion of superhydrophobic surfaces on oil/water separation performances such as separation efficiency, water intrusion pressure and oil flux is studied. It is found that the influence of the adhesive property of the superhydrophobic mesh on the oil/water separation performance can be neglected. The superhydrophobic ZnO coated meshes with low or high water adhesion showed nearly the same separation efficiency, which was up to 98.5% for a kerosene and water mixture and higher than 96.0% for other oil and water mixtures. The water intrusion pressure and oil flux study exhibits that the low adhesive meshes have a few advantages over the meshes with high water adhesion.

2. Experimental

2.1 Materials

Stainless steel mesh (300 mesh size) was purchased from a local hardware store, and was ultrasonicated in acetone and ethanol before use, sequentially. Acetone and ethanol were purchased from Guangzhou Jin Ju Chemical Co., Ltd, China. ZnO nanoparticles (NPs) (average diameter of about 70 nm) were purchased from NanoTek. Stearic acid was obtained from Tianjin Chemical Reagent Co., Ltd. All chemicals used for the experiments were of standard commercial grade, and were used as received without any further purification.

2.2 Fabrication of the superhydrophobic ZnO coated meshes

Hydrophobic ZnO NPs were prepared by modifying hydrophilic ZnO NPs with stearic acid (Fig. S1, ESI†). The wettability of the hydrophobic and hydrophilic ZnO NPs is shown in Fig. S2, ESI.† Superhydrophobic ZnO coated meshes with tunable water adhesion were prepared according to the previous method with some modifications.¹⁹ In a typical process, 0.3 g ZnO NPs were first dispersed in 20 mL of ethanol solution under stirring for 30 min to obtain a homogeneous suspension. The mixed suspension was then sprayed on stainless steel mesh substrates (3 × 3 cm²) using a spray gun of 0.2 MPa air pressure, as shown in Fig. 1. By simply changing the percentage of hydrophobic ZnO NPs in the ZnO NP mixture, superhydrophobic ZnO coated meshes with low water adhesion (pure hydrophobic ZnO NPs) or high water adhesion (40 wt% hydrophobic ZnO NPs) were fabricated successfully. Finally, the ZnO coated meshes were dried at ambient temperature for 30 min for the ethanol to evaporate entirely. Superhydrophobic ZnO meshes with different water adhesion were prepared by tuning surface composition without changing surface morphology, as shown in Fig. 1.

Fig. 1 Schematic for the fabrication of superhydrophobic ZnO coated meshes with low and high water adhesion through a facile spray-coating method.

2.3 Separation of oil/water mixtures

The as-prepared ZnO coated meshes with different water adhesion were fixed between two Teflon flanges. Two flanges were attached to two glass tubes and placed with a tilt angle of about 15° to make it easier for oil to be in good contact with the mesh surface. A series of oils and organic solvents, including kerosene, toluene, hexane, petroleum ether, chloroform and tetrachloroethane, were used in this experiment. They were dyed with oil red O and mixed with water that was colored with methylene blue. The oil/water mixtures (50%, v/v) were poured onto the as-prepared superhydrophobic ZnO coated meshes. The separation force was only gravity during the separation process. The separation efficiency was calculated according to η = (m₁/m₀) × 100%, where, m₀ and m₁ represent the mass of water before and after the separation process, respectively.

2.4 Characterization

The morphology of the as-prepared ZnO mesh surfaces was observed by field emission scanning electron microscopy (FE-SEM, Zeiss). The surface chemical composition of the meshes was determined by X-ray photoelectron spectroscopy (XPS) analysis on a PHI-5702 electron spectrometer using Mg Kα radiation as the excitation source and the binding energies were referenced to C 1s at 284.80 eV. Water and oil contact angles (CAs), water sliding angle and slipping angle were measured using a SL200KB apparatus at ambient temperature. The water droplet volume was 5 μL, and an average of five measurements was done to determine the surface wettability in five different positions.

3. Results and discussion

3.1 Morphology characterization

Superhydrophobic ZnO meshes with different water adhesion were prepared by the subsequent introduction of roughness and low energy surface through the spray-coating deposition of a mixture containing both hydrophobic and hydrophilic ZnO NPs. The surface morphologies of the original and ZnO coated meshes were investigated by FE-SEM. As shown in Fig. 2a, the original mesh average pore diameter is about 50 μm (300 mesh size), and the high magnification of the image in the inset of Fig. 2a reveals that the surface of the original mesh wire is smooth. After coating the mixtures of the hydrophobic and hydrophilic ZnO NPs, the original mesh becomes completely covered by densely and randomly distributed ZnO NPs at the microscale, as shown in Fig. 2b. The high magnification FE-SEM image in Fig. 2c shows that the ZnO NPs are relatively aggregated with a diameter ranging from below 50 nm to 150 nm, which results in hierarchical micro- and nano-scale roughness on the ZnO coated meshes. This hierarchical binary roughness of the ZnO coated mesh surface is essential for superhydrophobicity. In addition, the surface structure of the ZnO coated mesh before and after oil/water separation was nearly the same, as shown in Fig. S3, ESI.†

Fig. 2 Typical FE-SEM images of stainless steel meshes. (a) The original mesh with the magnified image inserted. (b and c) images of the ZnO coated meshes with low and high magnification, respectively.

3.2 Composition characterization

The heterogeneous chemical composition of the as-prepared ZnO NP mesh surfaces was analyzed by XPS. To obtain more information on the changes in surface composition, we collected high-resolution XPS data of the O 1s peak for the ZnO surfaces prepared with the percentage of hydrophobic ZnO NPs = 100 and 40, separately. Fig. 3a shows the survey XPS spectra of the ZnO coated meshes with low and high water adhesion. It can be seen that the C 1s, O 1s and Zn 2p peaks are detected in both of the meshes. Fig. 3b exhibits the multi-element spectra of the O 1s peak for the ZnO coated mesh surfaces with low and high water adhesion, which were fitted to Zn–O–C (532.5 eV), Zn–OH (531.2 eV) and Zn–O–Zn (530.0 eV), respectively. When the percentage of hydrophobic ZnO NPs in mixture decreased from 100 to 40%, the relative amount of oxygen in the hydroxyl group (Zn–OH) increased from 33.6% to 43.8%. As a result, the water adhesion of the superhydrophobic ZnO mesh surfaces could be tuned from low adhesion to high adhesion by controlling the mass percentage of hydrophobic ZnO NPs in the mixtures.

Fig. 3 Survey (a) and O 1s (b) XPS spectra of the ZnO coated meshes with low and high water adhesion.

3.3 Mesh wettability performance

The wettability of the as-prepared ZnO coated meshes was tested by CA measurements. The ZnO coated mesh prepared with the deposition of pure hydrophobic ZnO NPs showed a water CA of 158 ± 1° (Fig. 4a) and water sliding angle of 5 ± 1° (Fig. 4b), respectively. When the mass percentage of hydrophobic ZnO NPs was 40%, the ZnO coated mesh exhibited a water CA of 154 ± 1° and high adhesion to water (Fig. 4d and e). However, the superhydrophobic ZnO coated meshes with low and high water adhesion both showed the same wettability to oil with the oil CA of nearly to 0° (Fig. 4c and f). Consequently, superhydrophobic/superoleophilic ZnO coated meshes with low and high water adhesion were fabricated successfully by altering the mass percentage of hydrophobic NPs in the mixture of hydrophobic and hydrophilic ZnO NPs, which was deposited on stainless steel meshes.

Fig. 4 Wettability of the as-prepared superhydrophobic ZnO coated mesh surfaces with low (upper row) and high water adhesion (lower row). Water CA (a), water SA (b) and oil CA (c) on the low adhesive ZnO coated meshes. Water CA (d), water SA (e) and oil CA (f) on the high adhesive ZnO meshes.

3.4 Separation of oil and water

The influence of the water adhesion of the superhydrophobic ZnO meshes on oil/water separation performance was studied. The oil/water separation process is shown in Fig. 5a and b.

Fig. 5 Oil/water separation studies of the as-prepared ZnO coated mesh (water was dyed with methylene blue and oil was dyed with oil red O). (a) During separation. (b) After separation. (c) Separation efficiency of the ZnO coated meshes with low and high water adhesion for various oil/water mixtures.

Mixtures of water (dyed with methylene blue) and kerosene (dyed with oil red O) were poured onto the ZnO coated mesh fixed between two Teflon flanges, both of which were fitted with glass. Since the density of the majority of oils is less than water, the device was obliquely fixed to make it easier for the oil to be in good contact with the coated mesh. No external force was employed during the separation process, which only relies on its own gravity. Due to the use of ZnO coated meshes with superhydrophobicity and superoleophilicity simultaneously, the oil (kerosene) penetrated through the surface quickly and flowed into the lower beaker, while the water was rejected on the mesh surface. Furthermore, neither oil nor water was observed in the collected water or oil, which indicates the high purity and effectiveness of the separation for oil/water mixtures. Similarly, mixtures of toluene, hexane, petroleum ether, chloroform, and tetrachloroethane and water were also separated effectively. Furthermore, mixtures of light oil (kerosene) or heavy oil (chloroform) and water (10 mL oil and 10 mL water) were separated within a few seconds using the meshes with high water adhesion (Movies S1 and S2, ESI†). The separation efficiency was calculated according to η = (m/m₀) × 100%, where, m₀ and m₁ are the mass of water before and after the separation process respectively. The separation efficiencies of the as-prepared ZnO coated meshes for the kerosene and water mixture was up to 98.5% and higher than 96.6% for other oil and water mixtures. More importantly, the superhydrophobic ZnO meshes with low and high water adhesion have nearly the same high separation efficiencies within experimental error for a series of oil and water mixtures, as shown in Fig. 5c. Therefore, from discussion above, we can conclude that the water adhesion of the superhydrophobic/superoleophilic meshes has nearly no effect on the separation efficiency for oil/water mixtures.

A schematic of the separation mechanism for an oil/water mixture is shown in Fig. 6. Whether a droplet could be pinned on a superhydrophobic surface is ascribed to the distinct contact modes (Wenzel state and Cassie state).¹⁹ In the Cassie state, the water droplet is suspended by vapor pockets trapped on the surface (in composite contact mode), thus the water droplet easily rolls off the mesh surface due to the low water adhesion (Fig. 6a). In contrast, in the Wenzel state, the water droplet fully penetrates into the valleys of a textured surface (in wet contact mode), thus the water droplet is pinned on the surface without any movement (Fig. 6b). However, during the separation process, no matter which water adhesion the superhydrophobic mesh shows, the mesh surfaces are infiltrated with oil due to their superoleophilic property. Meanwhile, when a water droplet is dropped onto the mesh surface whether in air or in oil, it prefers to remain in the Cassie state within the oil beneath it (Fig. 6c and d). The oil filled in the roughness of the mesh prevents the water droplet from contacting the rough mesh surfaces directly and the surfaces become relatively flat to allow the water droplet to slide easily.⁴³ In air, both ZnO coated mesh surfaces with different water adhesion wetted by oil showed the same water CA of 74° ± 2° (Fig. 6e). In addition, the water droplet can easily slip away from the oil wetted mesh surfaces with the water slipping angle is as low as 10° (Fig. 6f). Under oil, both ZnO coated mesh surfaces showed superhydrophobicity with a water CA larger than 150° (Fig. 6g) and low adhesion properties with a water sliding angle lower than 10° (Fig. 6h), no matter the water adhesion (low or high water adhesion) the ZnO coated mesh surfaces possessed in air. Fig. 6i–l demonstrates the approach, contact, deformation and departure processes of a 6 μL water droplet suspended on a syringe with respect to the ZnO coated mesh surfaces under oil. It can be seen that the water droplet suspended on the syringe was difficult to be pulled down to both ZnO coated mesh surfaces under oil in all cases, even though the water droplet was deformed severely. This result also confirms that the adhesion force between water and both of the ZnO coated mesh surfaces under oil was extremely low. Therefore, water droplets were repelled and stayed on the ZnO coated mesh surface, while oil droplets permeated through the mesh quickly. As a result, during the process of oil and water separation, the water adhesion of the ZnO coated mesh surfaces has almost no effect on oil/water separation. This is also a reasonable explanation why the surfaces with different water adhesion have the same separation efficiency for oil and water mixtures.

Fig. 6 Schematic of the mechanism of oil/water mixture separation by the ZnO coated meshes with low and high water adhesion.

To further study the influence of the ZnO coated meshes with different water adhesion on oil/water separation performance, water intrusion pressure and oil flux were also measured. Water intrusion pressure is determined by the maximum height (h_max) of water column that the ZnO coated meshes can bear. The water intrusion pressure (P) was calculated using the following eqn (1):

P = ρgh_max (1)

where, ρ is the density of water, g is the acceleration of gravity, and h_max is the maximum water column height that the ZnO coated mesh can support. The maximum bearable height achieved for the low adhesive ZnO coated mesh was about 25.0 cm (Fig. 7a), while the value for the high adhesive ZnO coated mesh was about 20.0 cm (Fig. 7b). Thus, the water intrusion pressure for the low and high water adhesive ZnO coated meshes were 2.45 kPa and 1.96 kPa, respectively. Water cannot flow through the mesh under this pressure. In addition, oil flux is another parameter that can be used to evaluate the oil/water separation ability of the mesh. Oil flux (F) was measured under a fixed column of water and from the resulting data from five repeated experiments. The oil flux value was calculated using eqn (2):where, V is the volume of oil that permeates through the membrane, and here, we fixed V to 0.1 L. S is the area of the mesh, and t is the required time for the permeation of 0.1 L of oil (kerosene). Here, the oil flux for the low and high water adhesive ZnO coated meshes were about 11.5 m⁻² s⁻¹ and 10.6 L m⁻² s⁻¹, respectively. The water intrusion pressure and oil flux study exhibit that the water adhesive property of the superhydrophobic ZnO coated meshes has a little effect on their oil/water separation performance, in which the low adhesive meshes showed a little higher water intrusion pressure and oil flux than the high adhesive meshes. From the discussion above, we can conclude that the influence of the adhesive property of the superhydrophobic meshes on their oil/water separation performance (such as separation efficiency, water intrusion pressure and oil flux) can be neglected. Therefore, the superhydrophobic ZnO coated meshes whether with low or high water adhesion could be used to separate a large amount of oil/water mixture.

Fig. 7 Water intrusion pressure of the ZnO coated meshes with low (a) and (b) high water adhesion (water is dyed with methylene blue).

To further understand the mechanism of the oil/water separation of the ZnO coated meshes, the process was modelled in Fig. 8 (all the pores on the meshes were assumed to be a regular square array). The intrusion pressure (ΔP) can be calculated by the following equation (eqn (3)):^55–58

where γ_L1L2 represents the interfacial tension of water and oil, θ represents the water CA on the ZnO coated meshes, R represents the radius of the water meniscus, C represents the circumference of the mesh pore and A represents the cross-sectional area of the pore. When the liquid, such as water, is static on the mesh surfaces, the meniscus of the liquid is stable and the intrinsic contact angle θ, is definite no matter where the three phase contact line position due to the surface curvature. It can be seen from Fig. 8a and eqn (3) that the intrusion pressure is ΔP > 0 (negative capillary effect), when θ > 90°. Obviously, water cannot spontaneously permeate the ZnO coated meshes whether with low or high water adhesion, unless an external pressure is applied. Conversely, the ZnO coated mesh cannot withstand any pressure, when θ < 90° because the static pressure ΔP < 0 (capillary effect) in the special oil/water/solid three phase system, and the liquid like oil will permeate the film spontaneously. As shown in Fig. 8b, because the ZnO coated meshes, whether with low or high water adhesion, are superoleophilic, θ is nearly 0°, thus ΔP < 0 and the ZnO coated cannot support any oil pressure. During the separation process of oil/water mixtures, oil or organic solvent is trapped in nano-structures, which increase the repellent force between water and the coated meshes. Therefore, oil and water mixtures can be separated efficiently by the ZnO coated meshes regardless of if the surfaces showing high or low water adhesion.

Fig. 8 Schematic of the wetting model of the ZnO coated meshes. (a) Water cannot permeate the meshes, because the static pressure ΔP > 0 and (b) the meshes are permeable to oil, because the static pressure ΔP < 0. O is the center of the spherical cap of the meniscus, O₁ and O₂ are the cross section center of the stainless steel wires, M and N are the three-phase contact line positions, and r and R are the radius of the stainless steel wires and curvature radius of the spherical cap of the meniscus, respectively.

4. Conclusions

In summary, superhydrophobic ZnO meshes with different water adhesion were prepared by a facile spray-coating process through tuning the surface composition without changing surface morphology. Specifically, by simply changing the mass percentage of hydrophobic ZnO NPs in the hydrophobic/hydrophilic ZnO mixture, superhydrophobic/superoleophilic meshes with low or high water adhesion have been fabricated successfully. In addition, the influence of the water adhesive property of the superhydrophobic meshes on their oil/water separation performance was studied. The results show that the water adhesive property of the superhydrophobic meshes has nearly no effect on the separation efficiency, however, a little effect on the water intrusion pressure and oil flux. In general, the low adhesive ZnO coated mesh slightly outperformed the high adhesive mesh. The reason why the adhesive property of the superhydrophobic ZnO coated meshes has almost no effect on their oil/water separation is explicated by their oil/water separation mechanism. We propose that this study would open a new avenue to design and prepare superhydrophobic materials with different water adhesion for the separation of oil/water mixtures.

Acknowledgements

The work is supported by the Natural Science Foundation of China (Grants 21301141, 21261021) and the Nature Science Foundation of Gansu Province, China (Grant 145RJYA241).

Notes and references

1. K. Liu, X. Yao and L. Jiang, Chem. Soc. Rev., 2010, 39, 3240.
2. S. S. Latthe, P. Sudhagar, A. Devadoss, A. M. Kumar, S. Liu, C. Terashima, K. Nakata and A. Fujishima, J. Mater. Chem. A, 2015, 3, 14263.
3. J. Li, R. Wu, Z. Jing, L. Yan, F. Zha and Z. Lei, Langmuir, 2015, 31, 10702.
4. J. A. Howarter and J. P. Youngblood, Adv. Mater., 2007, 19, 3838.
5. J. Li, L. Yan, Q. Ouyang, F. Zha, Z. Jing, X. Li and Z. Lei, Chem. Eng. J., 2014, 246, 238.
6. L. B. Boinovich, A. M. Emelyanenko, K. A. Emelyanenko and K. I. Maslakov, Phys. Chem. Chem. Phys., 2016, 18, 3131.
7. N. J. Shirtcliffe, G. McHale, M. I. Newton and Y. Zhang, ACS Appl. Mater. Interfaces, 2009, 1, 1316.
8. J. Ju, K. Xiao, X. Yao, H. Bai and L. Jiang, Adv. Mater., 2013, 25, 5937.
9. R. Raninga, K. Page and I. P. Parkin, Chem. Commun., 2014, 50, 12656.
10. J. Li, X. Liu, Y. Ye, H. Zhou and J. Chen, Colloids Surf., A, 2011, 384, 109.
11. H. Bellanger, T. Darmanin, E. T. Givenchy and F. Guittard, Chem. Rev., 2014, 114, 2694.
12. P. Ragesh, V. A. Ganesh, S. V. Nair and A. S. Nair, J. Mater. Chem. A, 2014, 2, 14773.
13. H. Cho, J. Lee, S. Lee and W. Hwang, Phys. Chem. Chem. Phys., 2015, 17, 6786.
14. W. Barthlott and C. Neinhuis, Planta, 1997, 202, 1.
15. K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing and R. J. Full, Nature, 2000, 405, 681.
16. L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia and L. Jiang, Langmuir, 2008, 24, 4114.
17. Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin and L. Jiang, Adv. Mater., 2009, 21, 3799.
18. J. Li, X. Liu, Y. Ye, H. Zhou and J. Chen, J. Phys. Chem. C, 2011, 115, 4726.
19. J. Li, Z. Jing, F. Zha, Y. Yang, Q. Wang and Z. Lei, ACS Appl. Mater. Interfaces, 2014, 6, 8868.
20. Z. Cheng, R. Hou, Y. Du, H. Lai, K. Fu, N. Zhang and K. Sun, ACS Appl. Mater. Interfaces, 2013, 5, 8753.
21. J. Li, Z. Jing, Y. Yang, L. Yan, F. Zha and Z. Lei, Mater. Lett., 2013, 108, 267.
22. J. Huang, Y. Lai, L. Wang, S. Li, M. Ge, K. Zhang, H. Fuchs and L. Chi, J. Mater. Chem. A, 2014, 2, 18531.
23. J. Li, Z. Jing, Y. Yang, F. Zha, L. Yan and Z. Lei, Appl. Surf. Sci., 2014, 289, 1.
24. J. Yong, Q. Yang, F. Chen, D. Zhang, U. Farooq, G. Du and X. Hou, J. Mater. Chem. A, 2014, 2, 5499.
25. H. Li, W. Li, L. Yan and J. Li, J. Adhes. Sci. Technol., 2016, 30, 1087.
26. J. Li, Y. Yang, F. Zha and Z. Lei, Mater. Lett., 2012, 75, 71.
27. H. Zhu, Z. Guo and W. Liu, Chem. Commun., 2014, 50, 3900.
28. J. Li, Z. Jing, Y. Yang, Q. Wang and Z. Lei, Surf. Coat. Technol., 2014, 258, 973.
29. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinas and A. M. Mayes, Nature, 2008, 452, 301.
30. B. Wang, W. Liang, Z. Guo and W. Liu, Chem. Soc. Rev., 2015, 44, 336.
31. Z. Xue, Y. Cao, N. Liu, L. Feng and L. Jiang, J. Mater. Chem. A, 2014, 2, 2445.
32. J. Li, D. Li, Y. Yang, J. Li, F. Zha and Z. Lei, Green Chem., 2016, 18, 541.
33. Z. Chu, Y. Feng and S. Seeger, Angew. Chem., Int. Ed., 2015, 54, 2328.
34. Q. Ma, H. Cheng, A. G. Fane, R. Wang and H. Zhang, Small, 2016, 12, 2186.
35. J. Li, L. Yan, H. Li, W. Li, F. Zha and Z. Lei, J. Mater. Chem. A, 2015, 3, 14696.
36. L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2004, 43, 2012.
37. M. Tenjimbayashi, K. Sasaki, T. Matsubayashi, J. Abe, K. Manabe, S. Nishioka and S. Shiratori, Nanoscale, 2016, 8, 10922.
38. J. Li, R. Kang, X. Tang, H. She, Y. Yang and F. Zha, Nanoscale, 2016, 8, 7638.
39. J. Li, L. Yan, X. Tang, H. Tang, D. Hu and F. Zha, Adv. Mater. Interfaces, 2016, 3, 1500770.
40. J. Lin, F. Tian, Y. Shang, F. Wang, B. Ding and J. Yu, Nanoscale, 2013, 5, 2745.
41. J. Gu, P. Xiao, J. Chen, F. Liu, Y. Huang, G. Li, J. Zhang and T. Chao, J. Mater. Chem. A, 2014, 2, 15268.
42. X. Tang, Y. Si, J. Ge, B. Ding, L. Liu, G. Zheng, W. Luo and J. Yu, Nanoscale, 2013, 5, 11657.
43. J. Li, L. Yan, H. Li, J. P. Li, F. Zha and Z. Lei, RSC Adv., 2015, 5, 53802.
44. C. Gao, Z. Sun, K. Li, Y. Chen, Y. Cao, S. Zhang and L. Feng, Energy Environ. Sci., 2013, 6, 1147.
45. Y. Lu, S. Sathasivam, J. Song, F. Chen, W. Xu, C. J. Carmalt and I. P. Parkin, J. Mater. Chem. A, 2014, 2, 11628.
46. J. Huang, S. Li, M. Ge, L. Wang, T. Xing, G. Chen, X. Liu, S. S. Al-Deyab, K. Zhang, T. Chen and Y. Lai, J. Mater. Chem. A, 2015, 3, 2825.
47. J. Li, L. Yan, Y. Zhao, F. Zha, Q. Wang and Z. Lei, Phys. Chem. Chem. Phys., 2015, 17, 6451.
48. K. Rohrbach, Y. Li, H. Zhu, Z. Liu, J. Dai, J. Andreasen and L. Hu, Chem. Commun., 2014, 50, 13296.
49. B. Lee, S. Lee, M. Lee, D. H. Jeong, Y. Baek, J. Yoon and Y. H. Kim, Nanoscale, 2015, 7, 6782.
50. H. Sun, A. Li, Z. Zhu, W. Liang, X. Zhao, P. La and W. Deng, ChemSusChem, 2013, 6, 1057.
51. D. Wu, Z. Yu, W. Wu, L. Fang and H. Zhu, RSC Adv., 2014, 4, 53514.
52. J. Li, D. Li, W. Hu, J. P. Li, Y. Yang and Y. Wu, New J. Chem., 2015, 39, 9958.
53. Q. An, Y. Zhang, K. Lv, X. Luan, Q. Zhang and F. Shi, Nanoscale, 2015, 7, 4553.
54. J. Li, D. Li, W. Li, H. She, H. Feng and D. Hu, Mater. Lett., 2016, 171, 228.
55. M. A. Gondal, M. S. Sadullah, M. A. Dastageer, G. H. McKinley, D. Panchanathan and K. K. Varanasi, ACS Appl. Mater. Interfaces, 2014, 6, 13422.
56. D. Tian, X. Zhang, Y. Tian, Y. Wu, X. Wang, J. Zhai and L. Jiang, J. Mater. Chem., 2012, 22, 19652.
57. Z. Guo, X. Zheng, D. Tian, Y. Song, J. Zhai, X. Zhang, W. Li, X. Wang, S. Dou and L. Jiang, Nanoscale, 2014, 6, 12822.
58. D. Tian, X. Zhang, X. Wang, J. Zhai and L. Jiang, Phys. Chem. Chem. Phys., 2011, 13, 14606–14610.

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Footnote

† Electronic supplementary information (ESI) available: Additional figures and movies. See DOI: 10.1039/c6ra17153b

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