A self-cleaning TiO₂ coated mesh with robust underwater superoleophobicity for oil/water separation in a complex environment

Abstract

Porous materials with underwater superoleophobicity have recently become attractive candidates for oil/water separation, but are restricted in practical applications because of their easy contamination in air and poor resistance in a complex environment. Herein, we present a facile, efficient, and environmentally friendly approach to fabricating a TiO₂ coated stainless mesh with both a photocatalytic self-cleaning property and excellent chemical resistance via a sol–gel method. The uniform, crack-free TiO₂ nanoparticle coating layer endows the mesh with underwater superoleophobicity and corrosion resistance, which enables the effective separation of oil–water mixtures not only in a gentle environment but also in a complex environment such as acidic, alkaline and salty solutions. In addition, once these meshes were contaminated by organic molecules and lost their unique surface wettability, they could readily recover their origin wettability and separation performance by ultraviolet (UV) illumination owing to the photocatalysis of TiO₂. With respect to its exceptional stability, self-cleaning property and ease fabrication, the TiO₂ coated mesh holds promising potential for practical oil/water separation applications.

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1. Introduction

The rampant increase of oily wastewater from daily life, industry processes such as petrochemical, textile and steel processing as well as the frequent oil spill accidents poses a threat to ecosystems and human health.^1,2 Traditional methods such as air flotation, skimming, gravity separation and flocculation have been used to treat oil/water mixtures, but suffer from several drawbacks such as low efficiency, high cost and large space for installation, just to name a few.^3–5 The pressure driven membrane technology, which has recently become a promising technology for treating oily wastewater with the merits of high separation efficiency and a small footprint, however, is hampered by the severe membrane fouling issue due to surfactant adsorption and pore plugging by oil droplets.^6–8 Therefore, it still remains a worldwide challenge to find a facile and effective way to separate oil/water mixtures.

In the past decade, advanced materials with superwettability have attracted broad interests in both academic research and industrial applications, providing a new opportunity for treating oily wastewater.^9–11 Typically, two different types of superwettable materials including the superhydrophobic/superoleophilic materials which only allows oil to pass through^12–15 and the underwater superoleophobic materials that are solely permeable to water,^16–19 have been successfully designed and applied to oil/water separation. It is worthy noting that the underwater superoleophobic materials are superior to the superhydrophobic/superoleophilic ones because they allow water to pass, which not only alleviates the oil fouling issue but also avoids the formation of water barrier that inevitably formed on superhydrophobic/superoleophilic materials. Nevertheless, most of the underwater superoleophobic materials would lose their low oil adhesion property when placed in air because of high surface energy, making them readily to be contaminated by oil pollutants, and therefore lose their superwettability and separation performance.^20,21 Furthermore, these oil contaminants are extremely difficult to clean once absorbed, because of their lower surface tension and higher viscosity,^22–24 thus limiting its practical applications.

As a semiconductor, TiO₂ exhibits some unique properties such as superhydrophilicity which is prerequisite for constructing underwater superoleophobic surface²⁵ and excellent photo-activity that could be utilized to decompose organic contaminants,^26,27 making it attractive for developing advanced multi-functional materials. Very recently, several studies have attempted to overcome the oil-contamination issue of underwater superoleophobic materials by using the UV induced self-cleaning function of TiO₂, such as layer-by-layer deposition²⁸ or solvothermal growth²⁹ of TiO₂ nanoparticles, coating a composite of TiO₂/carbon nanotube,³⁰ and electrochemical anodization of Ti membrane.³¹ However, the routes to fabricate these self-cleaning materials are tedious and complicated,²⁸ or require special equipment,²⁹ expensive materials³⁰ and harsh conditions (e.g. HF solution).³¹ In addition, the TiO₂ deposition generated by these methods could not fully cover the mesh surface as a protect layer to improve their chemical resistance, which makes them incapable to realize oil/water separation in harsh chemical environment such as such as acidic, alkaline and salty solutions that are always encountered in practical applications.

To this end, we developed a sol–gel coating method to generate uniform, crack-free TiO₂ nanoparticle coating on the stainless mesh, to endow the mesh with both photocatalytic self-cleaning property and excellent chemical resistance. Sol–gel method is a simple, low cost and easily scale-up way for coating thin films on a wide variety of substrates.^32–35 We demonstrated that the uniform TiO₂ coating layer significantly improved the corrosion resistance of the stainless mesh, as evidenced by the Tafel polarization curves, which enabled the effective separation of separate oil–water mixtures not only in gentle environment but also in complex environment such as acidic, alkaline and salty solutions. Meanwhile, owing to the excellent photo-activity of TiO₂, the TiO₂ coated mesh could readily recover their origin wettability and separation performance by ultraviolet (UV) illumination once they were contaminated by organic molecules, making it promising candidates for practical oil/water separation.

2. Experimental

2.1. Materials

Tween 80 was supplied by Jiangyin Huayuan Chemical Co. LTD (China). All other solvents and reagents were produced by Shanghai Sinopharm Chemical Reagent CO. LTD (China) and were used as received.

2.2. Fabrication of TiO₂ coated mesh

First, 316 L stainless mesh (pore size: 50 μm) was cut into 5 × 5 cm² species, and then ultrasonically cleaned in acetone, ethanol and water for 10 min, followed by drying at ambient temperature. Then, the TiO₂ sol was prepared by a template-based method as previously described:³⁶ 65 ml Tween 80, which serves as a pore-directing agent, was homogeneously dissolved in 170 ml isopropanol. Subsequently, 8 ml acetic acid was added into the solution to react with isopropanol for water generation. After that, 17 ml tetrabutyl titanate was added under violate stirring to obtain a transparent and stable sol. Finally, the TiO₂ coating was formed on the stainless mesh via a dip-coating procedure with a withdrawal speed of 4 mm s⁻¹. After drying naturally at room temperature for 12 h, the coated samples were heat-treated in a muffle furnace at a ramp rate of 3 °C min⁻¹ up to 500 °C, maintained at this temperature for 15 min, and cooled down naturally. This dip-coating procedure was repeated three times to increase film thickness and homogeneity.

2.3. Characterization

NMR investigations of the TiO₂ sol were carried out using Bruker AVANCE III NMR spectrometer operated at room temperature, using CDCl₃ as solvent. The surface morphology and topography of the TiO₂ coated mesh were characterized by scanning electron microscope (SEM; JSM-6360LV, Japan) and atomic force microscope (AFM; Nanoscope IIIa Multimode, USA), respectively. Surface chemical compositions were analyzed by X-ray photoelectron spectroscopy analysis (XPS; VG-miclabII, UK) and XRD patterns were recorded on a D/max-rB diffractometer (Rigaku, Japan). The water contact angles (WCAs) and underwater oil contact angles (OCAs) were determined by a JC2000D1 system (Shanghai Zhongcheng Digital Technology Apparatus Co. Ltd., China). The corrosion resistance of the TiO₂ coated mesh was tested in H₂SO₄, NaOH and NaCl solutions, and thermal resistance was confirmed in hot water (80 °C). The mechanical robustness of the as prepare samples was tested by ultrasonication in water.

2.4. Oil–water separation

The separation of oil/water mixtures in gentle environment was tested by fixing the as-prepared mesh between two glass tubes. The mesh was pre-wetted by water and then a mixture of water and oil (50% v/v) were poured onto to mesh surface. The driving force for the separation process was solely gravity. Also the separation performance of the as-prepared meshes in complex environment was tested by using several corrosive solutions, namely 1 M H₂SO₄, 1 M NaOH, 1 M NaCl solutions.

2.5. Self-cleaning experiment

The TiO₂ coated mesh was contaminated with organic molecules by immersing the mesh in an oleic acid solution (5%) for 10 min. The contaminated mesh was then UV irradiated for 2 h, and the WCAs and OCAs before and after UV illumination were measured to evaluate the self-cleaning property.

3. Results and discussion

3.1. Investigation of the sol–gel process

In common sol–gel process for TiO₂ preparation, direct addition of water always results in violent hydrolysis of highly reactive alkoxide titanium precursors and leads to uncontrolled precipitation. Therefore, acetic acid (AcOH) was used as a titania sol modifier in this study. In this approach, water molecules can be generated through the esterification reaction between isopropanol and acetic acid as follows:

iPrOH + AcOH → iPrOAc + HOH (1)

These water molecules can hydrolyze alkoxide titanium to generate Ti–OH bounds, which can further condensed to form Ti–O–Ti bridges. A direct condensation reaction between titanium bonded acetate ligands and alkoxy groups can also result in the Ti–O–Ti condensed bridge, with isopropyl acetate generated a byproduct.³⁶ Whatever is the mechanism with major contribution, there will be only one Ti–O–Ti bridge formed for every ester molecule formed.³⁷

The TiO₂ sol before coating was characterized by ¹³C-NMR-spectroscopy to investigate the hydrolysis and condensation process, and the results are shown in Fig. 1. The peaks at 79 and 24.8 ppm respectively represents the CH and CH₃ carbon from the remained isopropanol groups bonding to Ti.³⁷ The strong peaks at 178 and 22 ppm show the presence of titanium bonded acetate ligands, indicating that some of the isopropanol groups was replaced by acetate groups. The ester generation was evidenced by the peaks appear at 68 and 171 ppm representing isopropyl acetate. As proposed before, the generation of ester molecules suggested the formation of condensed Ti–O–Ti connections.

Fig. 1 ¹³C NMR spectrum for the TiO₂ sol measured at room temperature.

3.2. Surface morphology and chemistry of TiO₂ coated mesh

Fig. 2 presents the SEM images of the original stainless mesh and TiO₂ coated mesh. It can be seen from Fig. 2a that the pristine mesh possesses smooth surface with an average pore size of about 50 μm. The macroscopic morphology of the mesh barely changed after TiO₂ coating (Fig. 2b), which ensures the high permeability for gravity separation. From the magnified view (Fig. 2c and d), the single wire was covered by a detect-free layer of TiO₂ nanoparticles with grain size from several tens to several hundred nanometers. These TiO₂ nanoparticles, combined with the micro-scale mesh wire, generated a multi-scale roughness that is crucial for constructing super-wetting surface.³⁸ Furthermore, the AFM results also confirmed the increased surface roughness of the stainless mesh after TiO₂ coating (Fig. S1†). The TiO₂ coated mesh displayed a bumpy surface with average surface roughness (R_a) of 28 nm, while that of the smooth original mesh was only 4.8 nm.

Fig. 2 SEM images of (a) the original stainless mesh, the inset is a magnified view of original mesh and (b–d) TiO₂ coated mesh with different magnifications.

Fig. 3 shows the XPS wide-scan spectrum of the original stainless mesh and TiO₂ coated mesh. The two peaks at 711 eV for Fe 2p and 577 eV for Cr 2p, which are the main components of original stainless mesh, can be observed in Fig. 3a. The peak at 531 eV for O 1s is mainly attributed to the Fe–O and Cr–O bonding in the stainless mesh. Noticeably, a new peak at 458 eV for Ti 2p appears in the spectrum of the TiO₂ coated mesh (Fig. 3b), indicating the successful coating of TiO₂. From the high resolution of O 1s region, both Ti–O bonding and hydroxyl groups can be observed (Fig. S2†). These hydroxyl groups are beneficial for the enhancement of surface hydrophilicity of the as-prepared mesh.²⁵ It is noticed that the Fe 2p peak still exist in the wide-scan spectrum after TiO₂ coating, which was attributed to foreign metal diffusion during calcination process.³⁹ Also the small peak at 285 eV for C 1s appears because of the residual carbon after calcination.⁴⁰

Fig. 3 XPS spectra for the surface of (a) the original stainless mesh and (b) TiO₂ coated mesh.

3.3. Wetting behavior of TiO₂ coated mesh

The wettability of the original mesh and TiO₂ coated mesh is illustrated in Fig. 4. The original mesh displayed hydrophobicity with a WCA of 124° (Fig. 4a). After TiO₂ coating, the mesh surface became superhydrophilic with a WCA of approximately 0°, which was credited to the hydrophilicity property of TiO₂ (Fig. 4b). The underwater OCA (1,2-dichloroethane) of the TiO₂ coated mesh was measured to be 156° (Fig. 4c), which demonstrated its underwater superoleophobicity. To further investigate the dynamic wetting behavior of the as-prepared mesh, the oil drop was pressured to touch to surface and then allowed to leave, as shown in Fig. 4d. The oil drop could readily leave the surface without any deformation and residue, suggesting the ultra-low oil affinity of the TiO₂ coated mesh. In addition to 1,2-dichloroethane, various kind of oils such as cyclohexane, gasoline, petroleum and xylene were tested and the underwater OCAs of the TiO₂ coated mesh for all oils were measured to be above 150° (Fig. S3†), which verified its general underwater superoleophobicity regardless of oil type.

Fig. 4 The wettability of TiO₂ coated mesh. Photographs of water drop on (a) original stainless mesh and (b) TiO₂ coated mesh in air, (c) oil drop (1,2-dichloroethane) on TiO₂ coated mesh in water and (d) dynamic wetting behavior of oil drop in water.

The underwater superoleophobicity of the TiO₂ coated mesh is attributed to the combination of surface chemistry and hierarchical structure. When immersed in water, water can be easily trapped in the micro/nanoscale mesh surface because of the intrinsic hydrophilicity of TiO₂, which greatly decreases the contact area between oil and solid surface.³¹ This heterogeneous wetting state, can be described by the Cassie model as follows:⁴¹

cos θ′ = f cos θ + f − 1 (2)

where f is the area fraction of the solid, θ is the contact angle of the oil droplet on a smooth surface in water, and θ′ is the contact of the oil droplet on a rough surface in water. The hierarchical structure of the mesh surface implies a small contact area with oil, which significantly increases its OCA and results in the superoleophobicity underwater.

3.4. Environmental stability of TiO₂ coated mesh

The durability of super-wettable materials in harsh environment is of great significance for their practical applications. Therefore, the stability of the TiO₂ coated meshes was tested in various complex environments.

First, the as-prepared meshes were immersed in 1 M H₂SO₄, 1 M NaOH, and 1 M NaCl solutions as well as hot water for 24 h, and then the underwater OCA were measured. Fig. 5a shows that the underwater OCA of the TiO₂ coated mesh barely changed after immersion in these solutions, which was credited to the excellent chemical and thermal stability of TiO₂ film. Previous studies have shown that uniform and continuous TiO₂ coating could serve as a protective layer on metals, which significantly improved its corrosion resistance and chemical stability.⁴² The Tafel polarization curves of these meshes were measured to investigate the corrosion behavior. The results reveal that the corrosion potential of TiO₂ coated mesh shifts positively and the current densities decreases about two order of magnitudes compared to the original mesh (Fig. S4†), indicating a remarkable improvement in corrosion resistance.

Fig. 5 The stability of TiO₂ coated mesh. (a) Underwater OCA contact angle of TiO₂ coated mesh after immersion in acidic, alkaline, salty and hot environment for 24 h. (b) Variation of WCA and underwater OCA contact angle of TiO₂ coated mesh with time under ultrasonification.

Then, the mechanical durability was tested by ultrasonication in water for 120 min. As illustrated in Fig. 5b, the as-prepared meshes could retain their super wettability under violent mechanical vibration. Also the adhesion force of the TiO₂ coating was confirmed by tape stripping test. Extra strength transparent scotch tape was pressured on the as-prepared samples, maintained for 10 minutes, and then pulled off from the surface. Even after 20 cycles, the underwater OCA of the TiO₂ coated mesh were measured to be above 150°, suggesting that the TiO₂ layer is firmly bonded to the mesh substrate. These results demonstrate the excellent mechanical, chemical and thermal stability of the TiO₂ coated mesh, enabling the separation of oil/water mixtures in harsh environment as will be shown later.

3.5. Oil–water separation performance of TiO₂ coated mesh

The oil–water separation performance of the TiO₂ coated mesh was investigated using the set-up as shown in Fig. 6a. The as-prepared mesh was fixed between two glass vessels and pre-wetted by water. Once the oil–water mixture was poured onto the mesh, water quickly permeated through the superhydrophilic mesh solely driven by gravity, while oil was retained on the mesh due to the underwater superoleophobic interface. After separation, no visible oil could be observed in the permeated water, indicating its ease operation and high efficiency. To further explore the general applicability of the TiO₂ coated mesh, the separation efficiency for a variety of oil–water mixture was tested. It can be seen from Fig. 6b that the separation efficiency for all oils was above 99.5%, suggesting that the TiO₂ coated mesh is adaptable for various kinds of oil water mixtures. The recyclability of the mesh was also tested by taking gasoline/water mixture as an example. As shown in Fig. 6c, the TiO₂ coated mesh demonstrated high separation efficiency even after 40 cycles.

Fig. 6 Oil water separation studies of TiO₂ coated mesh. (a) Photographs illustrating the facile gravity-driven separation of oil–water mixture using the TiO₂ coated mesh (oil is dyed with oil red O and water is coloured with methylene blue). (b) Separation efficiency of various oil–water mixtures. (c) Separation efficiency of gasoline–water mixture after 40 cycles.

Besides high separation efficiency, the gravity-driven permeation flux of the TiO₂ coated mesh was measured to be about 1780 Lm⁻² h⁻¹, higher than that of the anodized Ti mesh (1357 Lm⁻² h⁻¹) or the hydrothermal coated TiO₂ mesh (500 Lm⁻² h⁻¹) reported in previous studies,^29,31 which was mainly attributed to the intact highly porous structure after TiO₂ sol–gel coating (Fig. 2). Moreover, the applicability of those TiO₂ meshes for oil–water separation in harsh chemical environment, which is vital for practical applications, has not been investigated. In this study, the separation performance of the TiO₂ sol–gel coated mesh was further tested in various harsh environments including acidic (1 M H₂SO₄), alkaline (1 M NaOH), and salty (10 wt% NaCl) solutions. Owing to its excellent chemical stability, the TiO₂ coated mesh can retain its high efficiency and good recyclability in these harsh environments (Fig. 7), suggesting its promising potentials for practical oil–water separation.

Fig. 7 Separation efficiency and recyclability of the TiO₂ coated mesh for the separation of cyclohexane–water mixtures (50 : 50 v/v) in different acidic, alkaline, salty and hot environment.

3.6. Self-cleaning property of TiO₂ coated mesh

As discussed earlier, underwater superoleophobic materials are often designed by coating hydrophilic materials that possess high surface energy, which makes them readily to be contaminated by oil in air. These oil foulants, once contaminated, are extremely difficult to clean and would deprive the superwettability of such materials. To this end, we aim to endow the mesh with self-cleaning property by taking advantage of the UV induced photocatalytic property of TiO₂.

To test the self-cleaning property of the TiO₂ coated mesh, the mesh was first polluted by oleic acid, which was selected as a model pollutant because of its low volatility. Fig. 8a shows that the mesh lost its original super-wettability after oleic acid contamination. The surface of contaminated mesh became relatively hydrophobic with a WCA of 82° in air. Also its underwater superoleophobic was deprived and oil could easily spread on the surface, suggesting the inability for oil–water separation. Encouragingly, the mesh could easily recover its superhydrophilicity and underwater superoleophobicity after UV irradiation, which was credited to the excellent photo-activity of TiO₂. Under UV irradiation, photo induced holes and electrons can be generated, migrate to the surface and react with oxygen to produce highly active superoxide radicals or hydroperoxyl radicals which could effectively convert organic pollutants into CO₂ and water, leading to the recovery of superhydrophilicity.²⁶ Anatase TiO₂, which is generally acknowledged as the most photo-active crystal form,²⁷ can be observed in the XRD patterns of the TiO₂ coated mesh (Fig. S5†). Moreover, this self-cleaning function can be repeated for several times without comprising the original wettability (Fig. 8b), indicating the long-term usability of the TiO₂ coated mesh.

Fig. 8 Self-cleaning ability of TiO₂ coated mesh. (a) WCAs and underwater OCAs of the as-prepared mesh, contaminated mesh and UV irradiated mesh. (b) Variation of WCAs in five cycles of oleic acid adhesion and subsequent UV irradiation.

4. Conclusions

In summary, we present a facile strategy to fabricating a TiO₂ coated stainless mesh with robust underwater superoleophobicity and self-cleaning function via sol–gel method. We demonstrate that the uniform, crack-free TiO₂ coating layer endows the mesh with durable super-wettability that enables the separation of oil/water mixtures not only in gentle environment but also in harsh conditions. More importantly, owing to the excellent photo-activity of TiO₂, the mesh presents self-cleaning property against oil contaminants. With regard to its exceptional stability, self-cleaning property and ease fabrication, the TiO₂ coated mesh shows some potentials for practical oil/water separation applications.

Acknowledgements

The authors thank for financial support by the National Science and Technology Support Project of China (2014BAB07B01 and 2015BAB09B01), Project of National Energy Administration of China (2011-1635 and 2013-117), the National Natural Science Foundation of China (21176067 and 21406060) and the Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE-14C03) for giving financial support.

References

1. M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinas and A. M. Mayes, Nature, 2008, 452, 301–310.
2. B. Dubansky, A. Whitehead, J. T. Miller, C. D. Rice and F. Galvez, Environ. Sci. Technol., 2013, 47, 5074–5082.
3. A. A. Al-Shamrani, A. James and H. Xiao, Water Res., 2002, 36, 1503–1512.
4. G. Rıos, C. Pazos and J. Coca, Colloids Surf., A, 1998, 138, 383–389.
5. T. Ichikawa, Colloids Surf., A, 2007, 302, 581–586.
6. L. Song, J. Membr. Sci., 1998, 139, 183–200.
7. D. Rana and T. Matsuura, Chem. Rev., 2010, 110, 2448–2471.
8. M. M. Pendergast and E. M. V. Hoek, Energy Environ. Sci., 2011, 4, 1946–1971.
9. Z. Xue, Y. Cao, N. Liu, L. Feng and L. Jiang, J. Mater. Chem. A, 2014, 2, 2445–2460.
10. B. Wang, W. Liang, Z. Guo and W. Liu, Chem. Soc. Rev., 2015, 44, 336–361.
11. Z. Chu, Y. Feng and S. Seeger, Angew. Chem., Int. Ed., 2015, 54, 2328–2338.
12. L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2004, 43, 2012–2014.
13. W. Zhang, Z. Shi, F. Zhang, X. Liu, J. Jin and L. Jiang, Adv. Mater., 2013, 25, 2071–2076.
14. J. Li, L. Yan, H. Li, J. Li, F. Zha and Z. Lei, RSC Adv., 2015, 5, 53802–53808.
15. L. Peng, W. Lei, P. Yu and Y. Luo, RSC Adv., 2016, 6, 10365–10371.
16. Z. Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng and L. Jiang, Adv. Mater., 2011, 23, 4270–4273.
17. W. Zhang, Y. Zhu, X. Liu, D. Wang, J. Li, L. Jiang and J. Jin, Angew. Chem., Int. Ed., 2014, 53, 856–860.
18. B. Chen, G. Ju, E. Sakai and J. Qiu, RSC Adv., 2015, 5, 87055–87060.
19. M. Liu, J. Li, L. Shi and Z. Guo, RSC Adv., 2015, 5, 33077–33082.
20. J. Yang, Z. Zhang, X. Xu, X. Zhu, X. Men and X. Zhou, J. Mater. Chem., 2012, 22, 2834–2837.
21. J. A. Howarter and J. P. Youngblood, Adv. Mater., 2007, 19, 3838–3843.
22. T. D. Blake, J. Colloid Interface Sci., 2006, 299, 1–13.
23. M. Ramiasa, J. Ralston, R. Fetzer and R. Sedev, J. Phys. Chem. C, 2011, 115, 24975–24986.
24. K. He, H. Duan, G. Y. Chen, X. Liu, W. Yang and D. Wang, ACS Nano, 2015, 9, 9188–9198.
25. L. Zhang, Y. Zhong, D. Cha and P. Wang, Sci. Rep., 2013, 3, 2326.
26. X. Lin, Y. Chen, N. Liu, Y. Cao, L. Xu, W. Zhang and L. Feng, Nanoscale, 2016, 8, 8525–8529.
27. S. J. Gao, Z. Shi, W. B. Zhang, F. Zhang and J. Jin, ACS Nano, 2014, 8, 6344–6352.
28. L. Li, Z. Liu, Q. Zhang, C. Meng, T. Zhang and J. Zhai, J. Mater. Chem. A, 2015, 3, 1279–1286.
29. K. Liu, M. Cao, A. Fujishima and L. Jiang, Chem. Rev., 2014, 114, 10044–10094.
30. S. Banerjee, D. D. Dionysiou and S. C. Pillai, Appl. Catal., B, 2015, 176–177, 396–428.
31. J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo and D. W. Bahnemann, Chem. Rev., 2014, 114, 9919–9986.
32. G. X. Shen, Y. C. Chen, L. Lin, C. J. Lin and D. Scantlebury, Electrochim. Acta, 2005, 50, 5083–5089.
33. Y. Chen and D. D. Dionysiou, Appl. Catal., B, 2008, 80, 147–155.
34. X. Cao, W. Jing, W. Xing, Y. Fan, Y. Kong and J. Dong, Chem. Commun., 2011, 47, 3457–3459.
35. Y. Cai, Y. Wang, X. Chen, M. Qiu and Y. Fan, J. Membr. Sci., 2015, 476, 432–441.
36. H. Choi, E. Stathatos and D. D. Dionysiou, Appl. Catal., B, 2006, 63, 60–67.
37. D. P. Birnie Iii and N. J. Bendzko, Mater. Chem. Phys., 1999, 59, 26–35.
38. X. J. Feng and L. Jiang, Adv. Mater., 2006, 18, 3063–3078.
39. Y. Chen and D. D. Dionysiou, Appl. Catal., B, 2006, 69, 24–33.
40. J. Yu, X. Zhao and Q. Zhao, Mater. Chem. Phys., 2001, 69, 25–29.
41. V. Hejazi, A. E. Nyong, P. K. Rohatgi and M. Nosonovsky, Adv. Mater., 2012, 24, 5963–5966.
42. G. X. Shen, Y. C. Chen and C. J. Lin, Thin Solid Films, 2005, 489, 130–136.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13847k

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