Facile fabrication hybrids of TiO₂@ZnO tubes with enhanced photocatalytic properties

Abstract

Hollow nano-tubes of TiO₂ and TiO₂@ZnO hybrids were produced by a facile and mild approach combining an electrospinning technique and soaking method, followed by calcination. During the synthesis process, the electrospinning procedure provides the templates for the fabrication of hollow nano-tubes by producing the PS fibres, which are porous and enable the immobilization of amorphous TiO₂ through the soaking method. After that, the composite fibers of TiO₂/PS and TiO₂@ZnO/PS were calcined into pure TiO₂ and TiO₂@ZnO tubes after the complete combustion of organic materials. The samples were then characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). Ultimately, the photocatalytic performance of the hybrids was evaluated from the degradation of methylene blue (MB), where they exhibited excellent photocatalytic activity. The proposed mechanism for the enhanced photocatalytic efficiency of the samples is explained at the end of the paper. In principle, the approach to the fabrication of TiO₂ tubes is demonstrated to be an effective and robust strategy to fabricate long and uniform metal-oxide micro-nano tubes on a large scale.

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Introduction

With the development of industrialization, environmental problems have increased, which has had a negative influence on the health of human beings. Especially in terms of water pollution, a variety of contaminants have been discharged into the environment, which severely threatens the stability of the ecosystem.¹ Therefore, a series of strategies have been resorted to in order to reduce pollutants, such as physisorption and biological methods.^2–4 However, such methods might leave secondary waste problems. Photocatalytic oxidation, as an efficient avenue for removing the contaminants of sewage water, has attracted a great deal of attention.^5–7 Relative to the remediation of water pollution, the procedure of photocatalysis is based on an advanced oxidative process, which could generate free radical species, such as ˙OH and O₂⁻ groups, and these radicals could oxidize contaminants into small molecules such as H₂O and CO₂ in the bulk solutions or at the interfacial surface of the photocatalysts.^8–11

In recent years, a number of research groups have made great efforts to study the photocatalytic processes and the fabrication of photocatalysts with unique nano-structures,⁹ because the corresponding photocatalytic activity depends on the surface area, crystalline size, particle shape, crystalline structure, surface hydroxyl groups, and other factors.^12–18 Titanium dioxide (TiO₂) has been regarded as an excellent candidate for the photocatalytic degradation of water pollutants though its wide band gap, because the photoexcitation of electron–hole pairs at the surface of TiO₂ could sensitize and catalyze the UV/visible light-induced redox procedure.^19,20 Although enormous progress has been made in enhancing the photocatalytic activity of TiO₂, the recombination of photo-generated electron–holes is still the main problem, which greatly obstructs its application.²¹ It is well known that this recombination is responsible for the low quantum yields and numerous studies have confirmed that photoelectron trapping is an effective approach to decrease the recombination rate of charge carriers on the interior of TiO₂. Generally, the electron-traps capture the photo-induced charge carriers, which promotes the interfacial charge-transfer processes.^22–24 In order to realize this goal, some researchers suggested that modifications to the surface of TiO₂ via dispersed fine particles, such as noble metals and some oxide semiconductor materials, could improve the charge transfer and reduce the recombination of charge carriers.^25–27 As a photocatalyst, ZnO also exhibits similar or even better activity compared with TiO₂ in principle.²⁸ Furthermore, ZnO has additional properties such as its relatively low cost, high reactivity, and non-toxicity which favors its applications in photocatalysis.²⁹ However, the band gaps of both ZnO and TiO₂ are relatively high, and both of them require a high-energy light source, such as UV light, to generate photo-generated electrons and holes during their application.³⁰ The semiconductor–semiconductor (S–S) hetero-junction has been confirmed as an effective strategy to enhance the photocatalytic activity for the charge-transfer and spatial separation of photo-generated charge carriers.³¹

Herein, we firstly report a straightforward fabrication of hybrid ZnO/TiO₂ hollow nanotubes via an electrospinning technique combined with a soaking process followed by calcination in a muffle furnace with a program control device. The electrospinning technique provides an efficient and versatile approach to fabricate micro-fibers with a porous structure, which were used as templates for the nano-tubes.^32,33 Because of the porosity of the PS fibers, the solution of tetrabutyl titanate could easily enter the internal fibers and immobilize amorphous TiO₂ effectively. However, the challenges associated with high-efficiency nano-hybrids as photocatalysts still mainly remain in the construction of desirable interfacial structures, because inappropriate structures of photocatalysts will limit the charge-transfer process of the photo-generated electron–holes and then worsen the photocatalytic properties. Compared to other methods such as electrode reaction,³⁴ chemical vapor deposition³⁵ and laser evaporation,³⁶ the present work provides a facial process for the construction of hollow nano-tubes, which could serve as a channel for the organic dye fluid during the process of photocatalytic degradation and greatly improve the degradation efficiency via enlarging the interfacial adsorption of methylene blue (MB). Simultaneously, the strategy of fabrication of hollow nano-tubes may have huge potential for the formation of outstanding materials after self-assembly modification in the future.

Experimental section

Materials and methods

PS and tetrabutyl titanate (Ti(OBu)₄ 97%) were purchased from Sigma Aldrich and the PS with M_w = 350 000 g mol⁻¹; N,N-dimethylformamide (DMF), absolute methanol and ethyl alcohol were bought from Beijing chemical works. Zinc acetate and polyvinylpyrrolidone (PVP) used in the experiment were purchased from Aladdin. All of chemicals used in the experiments were analytical grade and used without further purification.

The following is the synthesis process:

Firstly, the PS fibers were fabricated using electrospinning equipment with a high voltage power supply, a spinning nozzle and a collector connected with the ground. The precursor of the spinning solution was made up with 2.3 g PS and 7.7 g DMF. After the complete dissolution of PS in DMF, it was transferred into the syringe. The feeding rate of the solution was controlled at 1.5 mL h⁻¹. The voltage applied to the spinning needle (number 10) was 14.20 kV, and the distance from the tip to the collector was 20 cm with the air humidity at 20%.

Then, the membrane was put into a solution of tetrabutyl titanate and ethyl alcohol for 2 h, where the volume ratio of tetrabutyl titanate and ethyl alcohol was 1 : 10. After the soaking process, the composite membrane was shifted into a drying oven for one hour.

Ultimately, the TiO₂/PS fibers were immersed in a mixture of zinc acetate solution. The concentrations of Zn²⁺ were 0.01 M and 0.1 M, respectively. The concentration of PVP, which was used as a stabilizer agent for Zn²⁺, was 1.2% in terms of mass. The solvent of the solution was absolute methanol, which has a good solubility for zinc acetate.

The last step is calcination. The TiO₂/PS and TiO₂@ZnO/PS membranes were calcined at 550 °C for 0.5 h at the rate of 2 °C min⁻¹ to burn the organic compounds of the templates of PS completely; then, the hybrids of hollow nano-tubes were fabricated.

Characterization

The samples were measured by a D/MAX 2250 V diffractometer (Rigaku, Japan, XRD) to confirm the crystal phase of the hybrids. The morphologies of PS and the hollow tubes were characterized via Scanning Electron Microscopy (SEM JEOL JSM-7500F) and transmission electron microscopy (TEM JEM-2010) under a working voltage of 200 kV equipped with an energy dispersive X-ray (EDX) spectrometer. UV-vis spectra were recorded on a UV-vis spectrophotometer (UV-2550, Shimadzu) with a scanning range from 300 to 800 nm. Photoluminescence (PL) spectra were recorded by a fluorescence spectrometer (Jobin Yvon Fluoro Max-4) with the excitation wavelength of 320 nm. The degradation process was monitored by the UV-vis spectrophotometer measuring the absorption of MB at 664 nm.

Photocatalytic tests for the degradation of MB

The tests were carried out under UV illumination by a wide-band lamp bulb (125 W Philips TL/05) with a predominant wavelength of 365 nm, and the photo-reactor was an ordinary glass of 250 mL equipped with circulating water at room temperature. The photocatalytic activities of the samples were tested under visible light, which was generated by an internal 150 W high-pressure xenon lamp with a UV cut-off glass filter transmitting λ > 400 nm. During the degradation process, the methylene blue (MB) was used as a degradation agent to evaluate the photocatalytic activity of samples. Firstly, the four tests were put in darkness for 2 h to ensure the establishment of adsorption–desorption equilibrium between the MB dye and the as-prepared tubes. An aliquot (3 mL) of the solution was taken out at half an hour intervals during the experiment and tested using a UV-vis spectrophotometer (Hitachi U-3010); after that, the analyzed aliquot would be poured back into the glass reactor immediately to ensure a roughly equivalent volume of solution. Finally, the photocatalytic activities were determined by the degradation efficiency from the absorbance data.

Results and discussion

Fig. 1 shows typical microstructures of the PS fibres and pure TiO₂ tubes. The PS fibers with high porosity were produced by the electrospinning technique and used as templates for fabrication of hollow tubes, which could provide channels for TBOT diffusion (Fig. 1B). The scanning electron microscopy image of the calcined TiO₂ nanotubes reveals that the diameters of the tubes is in the range of 1–1.3 μm, and the thickness of the shell of the tube was estimated to be 20–50 nm, observed from the SEM and TEM images. Furthermore, the morphology of the hybrids and pure TiO₂ tubes were still maintained in good condition after calcination. As shown in Fig. 2A and B, the surface of the hybrid of TiO₂@ZnO was still maintained as smooth, which was attributed to the shrinkage of fibres during the process of calcination. Compared with Fig. 1 and 2, there were no differences in the morphology of the fibrous surface after the addition of ZnO, which indicated that there was no obvious aggregation of for the ZnO nanoparticles and the Zn²⁺ ions were distributed on the surface of fibres evenly. After soaking in the solution of Zn²⁺ ions, the TiO₂ particles would react with the zinc acetate to form the lattice type of ZnTiO₃ by calcination to remove the organic matter at 550 °C, proved from the crystal phase in the Fig. 3.³⁷ Importantly, the outer wall of the hollow tube could be tuned by varying the concentration of tetrabutyl titanate (TBOT) and electrospinning parameters, because the diameter of the tubes was ascribed to the diameter of electrospinning fibres. Generally, the electrospinning technique is a versatile approach for generating fibers with specific structures via adjusting the concentration of polymer, the conductivity and the surface tension of solutions.

Fig. 1 SEM images of samples: (A) SEM image of PS fibers; (B) morphology of a cross section of PS fiber; (C) low magnification SEM image of TiO₂ tubes; (D) high magnification SEM image of a TiO₂ tube.

Fig. 2 SEM and TEM images of samples: (A) SEM image of pure TiO₂@ZnO tubes; (B) SEM image of cross section of TiO₂@ZnO; (C) TEM image of TiO₂@ZnO, and (D) high magnification TiO₂@ZnO TEM image.

To further investigate the interior crystal structure, XRD was used to measure the phases of the TiO₂@ZnO hybrids and pure TiO₂ tubes. The EDX spectra were also used to confirm the composition of the samples after calcination. As we can see from Fig. S1,† the organic matter had been burned completely. Fig. 3 shows the XRD spectra of the pure tube of TiO₂ and the hybrid of TiO₂@ZnO calcined at 550 °C for 0.5 h with the heating rate of 2 °C min⁻¹. The green curve in Fig. 3 represents the XRD pattern of the pure TiO₂ tubes, and the XRD pattern match the JCPDS file no. # 21-1272, which corresponds to the anatase in the phase. However, there is a certain amount of rutile phase in the pure TiO₂ tubes. In comparison with the diffraction profiles of TiO₂@ZnO hybrids, the peaks of ZnTiO₃ (JCPDS no. # 26-1500) have been enhanced gradually with the increase of ZnO. In the case of the hollow nano-tubes, the XRD patterns of all three samples show the rutile peak at 2θ = 27.4°; however, the peak intensity was substantially decreased due to the addition of the metal ions of Zn²⁺, which could impede the transition for TiO₂ from anatase to rutile.³⁸

Fig. 3 The XRD patterns of the as-synthesized products.

As for the photocatalysts, the photocatalytic efficiency of the TiO₂@ZnO hollow tubes is closely related to the light absorption amount and wavelength range. UV-vis DRS is an effective measurement for detecting the interfacial properties of photocatalysts, which could provide reasoning for the photo-oxidation performance of the materials. As observed from Fig. 4, the absorption spectra of samples are depicted. The UV-vis adsorption spectrum indicated that the TiO₂@ZnO (0.1 M) had a broad absorption in the range of 350–600 nm, especially in the absorption of visible light compared with pure TiO₂ tubes, which demonstrated that the hybrids need a lower energy than pure TiO₂ tubes to be excited. The absorption edges of the samples appear at 409, 420 and 442 nm for TiO₂, TiO₂@ZnO (0.01 M) and TiO₂@ZnO (0.1 M), respectively. With the addition of ZnO, the absorption intensity for UV and visible light has been enhanced. Furthermore, there is a significant red-shift with a lower band energy of TiO₂@ZnO hybrids compared with the TiO₂ tubes. Therefore, the results indicate that the hollow tubes of TiO₂@ZnO could absorb light in the visible range, and the light could be utilized more efficiently for photocatalytic purposes. Simultaneously, as suggested by the XRD patterns, the ZnTiO₃ crystallites have been formed in the hybrids; thus, this might induce defects in the crystal lattice structure of TiO₂ or ZnO, leading to an increase of the absorption edge.

Fig. 4 UV-vis DRS of TiO₂ tubes, TiO₂@ZnO (0.01 M) and TiO₂@ZnO (0.1 M).

PL spectra are related to the transfer behaviour of photo-generated electrons and holes, and therefore, the PL spectra can reflect the separation and recombination of charge carriers. As we can see from Fig. 5, the PL spectra of the TiO₂ tubes have a strong emission peak at 387 nm with the excitation wavelength at 320 nm, however, the PL intensity of the hybrid decreased greatly with the addition of ZnO.³⁹ The most important reason for this phenomenon is the defects in the crystal structure of TiO₂, which would act as traps for capturing the photo-excited electrons, and thus inhibit the recombination of e⁻/h⁺ pairs. The PL spectrum results are consistent with the photocatalytic activity of the TiO₂ tubes and the TiO₂@ZnO hybrids under UV and visible light.

Fig. 5 Photoluminescence (PL) spectra of (a) TiO₂ and (b) TiO₂@ZnO (0.1 M) λ_EX = 320 nm.

Owing to the absorption peaks of MB in the visible range, its degradation rate can be easily monitored with a UV-vis spectrophotometer. Moreover, MB is poorly biodegradable and a main contaminant in wastewater, being a large portion of textile dye and industrial dye stuffs. Therefore, in order to evaluate the photocatalytic properties of the TiO₂@ZnO hollow tubes, MB was used as a test contaminant during the degradation process under UV illumination. For comparison, four photocatalytic degradation tests were carried out for the decomposition of MB dye in solution, with hybrids and pure TiO₂ tubes as photocatalysts, in darkness without a photocatalyst, and a blank test, respectively. As can be observed from the photocatalytic degradation curves, the hybrids of TiO₂@ZnO decolorized MB faster than the pure TiO₂ hollow tubes and after three hours UV irradiation, the degradation efficiency values for MB were 95.27% and 83.25%, respectively. In order to investigate the stability of the hybrid TiO₂@ZnO tubes, the sample was repeatedly used four times after separation via centrifugation and drying. The photocatalytic degradation efficiency was evaluated by E_c/E₀ (E_c is the degradation efficiency of the reuse and E₀ is the first time degradation efficiency). Regretfully, the photocatalytic activity of TiO₂@ZnO reduced obviously after the second repeated use under UV irradiation. To find the above reason, we examined the morphology of the sample after a second repeated use. As shown from Fig. S2,† the structure of the hybrid TiO₂@ZnO had been destroyed, which would have some influence on the photocatalytic performance. Additionally, the loss of a small amount of the sample in the process of recycling would also lower the photocatalytic efficiency relative to before. The detailed plots of degradation efficiency for the MB dyes are shown in Fig. 6. Interestingly, the intensity of the maximum absorption peaks of MB have a distinct blue-shift as the photo-degradation continues, especially for that of TiO₂@ZnO, as revealed in Fig. 6. This is because of the formation of the demethylated dyes during the process of degradation, which would lead to a multicomponent system and a blue-shift.⁴⁰ Furthermore, the hybrid as a catalyst exhibited higher efficiency than that of pure TiO₂ for the degradation of MB under visible light in the same conditions. The degradation efficiency of TiO₂@ZnO for MB reached 36.52%, whereas the TiO₂ tubes’ degradation of MB was just 24.54%. Therefore, the hybrids exhibited better photocatalytic activity than that of the pure TiO₂ tubes. So, the fabrication of TiO₂@ZnO provided a new method to enhance the photocatalytic activity.

Fig. 6 Photocatalytic degradation of MB in the presence of TiO₂ tubes and TiO₂@ZnO hybrids. (A–D) show degradation curves of samples and degradation efficiency under UV light, (E) and (F) show the degradation efficiency of TiO₂ tubes and TiO₂@ZnO (0.1 M) respectively.

Compared with the degradation efficiency of the two samples, the TiO₂@ZnO nanotubes exhibited a higher photocatalytic efficiency under UV light, which was consistent with the results of UV-vis DRS.

As for the hybrids of TiO₂@ZnO, the enhanced photocatalytic activity for the decomposition of MB dye could be ascribed to the certain amount of ZnO.⁴¹ On the surface of the TiO₂ hollow tubes, there are some zinc oxide particles distributed uniformly, which would form a semiconductor–semiconductor (S–S) heterostructure with TiO₂.³¹ Fig. 7 shows a schematic of the photocatalytic mechanism. In principle, ZnO could be regarded as a modification for the surface of the TiO₂ tubes, which belongs to the type II semiconductor hetero-junction.⁴² When the hybrids of TiO₂@ZnO are excited under UV illumination with higher energy photons than the band gap, there would be a great deal of electrons promoted from the valence band (VB) to the conduction band (CB) of ZnO and TiO₂, respectively.⁴³ Then, the electrons would be transferred from the CB of ZnO to the CB of TiO₂. Additionally, the photo-generated holes existing in the VB of TiO₂ are transferred to the VB of ZnO. After two transfers of electrons and holes, the photo-generated electron/hole could be effectively separated, and therefore, the recombination rate of the electron–hole pair should be decreased to some degree.⁴⁴ Simultaneously, the electrons and holes located in the CB of TiO₂ and the VB of ZnO, respectively, would react with O₂ and H₂O to produce radical species, such as OH˙, HO₂⁻ and O₂⁻. These radical species would oxidize the organic dyes absorbed on the surface of the photocatalyst directly.⁴⁵ Although the hetero-junction effect improves the photocatalytic activity, the special structure of the hybrids could also promote the photocatalytic process for the hollow structures, providing a channel for the dye distribution and enlarging the interfacial surface area with bulk solutions. Thus, the redox process has been enhanced vastly.

Fig. 7 Schematic of electron–hole separations and energy band matching of TiO₂@ZnO heterostructures under UV illumination.

Conclusions

In summary, a facile fabrication procedure was demonstrated to prepare hollow nano-tubes of TiO₂@ZnO hybrids effectively. During the preparation process, the electrospinning technique provides an effective approach to generate the micro-nano fibres, which serve as templates for the fabrication of micro-tubes. Furthermore, the hybrids of TiO₂@ZnO exhibited excellent photocatalytic activity in the degradation of MB dyes compared with pure TiO₂ tubes. The most significant aspect is the methodology, which furnishes a new avenue for the preparation of hollow nano-tubes or core–shell materials in a relatively mild and environmentally benign approach. Therefore, it possesses great potential for applications in various fields in the future.

Acknowledgements

M. H. Wang thanks the financial support of Doctoral funding (No. 201218).

References

1. C. J. Vörösmarty, P. B. McIntyre, M. O. Gessner, D. Dudgeon, A. Prusevich, P. Green and P. M. Davies, Nature, 2010, 467, 555.
2. M. Rafatullah, O. Sulaiman, R. Hashim and A. Ahmad, J. Hazard. Mater., 2010, 177, 70.
3. A. Ahmad, S. Hamidah Mohd-Setapar, C. Sing Chuong, A. Khatoon, W. A. Wani, R. Kumar and M. Rafatullah, RSC Adv., 2015, 5, 30801.
4. K. C. Kemp, H. Seema, M. Saleh, N. H. Le, K. Mahesh, V. Chandra and K. S. Kim, Nanoscale, 2013, 5, 3149.
5. S. Vadahanambi, S. H. Lee, W. J. Kim and I. K. Oh, Environ. Sci. Technol., 2013, 47, 10510.
6. V. Corcé, L. M. Chamoreau, E. Derat, J. P. Goddard, C. Ollivier and L. Fensterbank, Angew. Chem., 2015, 127, 11576.
7. A. Di Paola, M. Bellardita, B. Megna, F. Parrino and L. Palmisano, Catal. Today, 2015, 252, 195.
8. L. Ismail, A. Rifai, C. Ferronato, L. Fine, F. Jaber and J. M. Chovelon, Appl. Catal., B, 2016, 185, 88.
9. Q. Sun, K. Lv, Z. Zhang, M. Li and B. Li, Appl. Catal., B, 2015, 164, 420.
10. T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2, 527.
11. L. Zhou, W. Wang, H. Xu, S. Sun and M. Shang, Chem.–Eur. J., 2009, 15, 1776.
12. S. Girish Kumar and K. S. R. Koteswara Rao, RSC Adv., 2015, 5, 3306.
13. Y. Liu, J. Goebl and Y. Yin, Chem. Soc. Rev., 2013, 42, 2610.
14. A. Kar and A. Patra, J. Mater. Chem. C, 2014, 2, 6706.
15. A. Primo, A. Corma and H. García, Phys. Chem. Chem. Phys., 2011, 13, 886.
16. M. Wang, J. Ioccozia, L. Sun, C. Lin and Z. Lin, Energy Environ. Sci., 2014, 7, 2182.
17. P. Kar, S. Sardar, S. Ghosh, M. R. Parida, B. Liu, O. F. Mohammed, P. Lemmens and S. K. Pal, J. Mater. Chem. C, 2015, 3, 8200.
18. W. Ong, L. Tan, S. Chai, S. Yong and A. Mohamed, Nanoscale, 2014, 6, 1946.
19. Y. Zhang, Z. Jiang, J. Huang, L. Y. Lim, W. Li, J. Deng, D. Gong, Y. Tang, Y. Lai and Z. Chen, RSC Adv., 2015, 5, 79479.
20. D. María Hernández-Alonso, F. Fresno, S. Suárez and J. M. Coronado, Energy Environ. Sci., 2009, 2, 1231.
21. W. Zhou, H. Liu, J. Wang, D. Liu, G. Du and J. Cui, ACS Appl. Mater. Interfaces, 2010, 2, 2385.
22. Y. Hou, X. Li, Q. Zhao, X. Quan and G. Chen, J. Mater. Chem., 2011, 21, 18067.
23. J. Xiao, Y. Xie, H. Cao, F. Nawaz, S. Zhang and Y. Wang, J. Photochem. Photobiol., A, 2016, 315, 59.
24. S. G. Kumar and L. G. Devi, J. Phys. Chem. A, 2011, 115, 13211.
25. W. Zhao, W. Ma, C. Chen, J. Zhao and Z. Shuai, J. Am. Chem. Soc., 2004, 126, 4782.
26. F. Zhang, Z. Cheng, L. Kang, L. Cui, W. Liu, G. Hou, H. Yang and X. Xu, RSC Adv., 2014, 4, 63520.
27. A. Bumajdad and M. Madkour, Phys. Chem. Chem. Phys., 2014, 16, 7146.
28. F. Lu, W. Cai and Y. Zhang, Adv. Funct. Mater., 2008, 18, 1047.
29. S. Malato, P. Fernández-Ibáñez, M. I. Maldonado, J. Blanco and W. Gernjak, Catal. Today, 2009, 147, 1.
30. S. Girish Kumar and K. S. R. Koteswara Rao, RSC Adv., 2015, 5, 3306.
31. C. Cheng, A. Amini, C. Zhu, Z. Xu, H. Song and N. Wang, Sci. Rep., 2014, 4, 4181.
32. J. Lin, B. Ding and J. Yu, ACS Appl. Mater. Interfaces, 2010, 2, 521.
33. W. Liu, C. Huang and X. Jin, Nanoscale Res. Lett., 2014, 9, 1.
34. Y. C. Nah, A. Ghicov, D. Kim, S. Berger and P. Schmuki, J. Am. Chem. Soc., 2008, 130, 16154.
35. J. W. Do, D. Estrada, X. Xie, N. N. Chang, J. Mallek, G. S. Girolami and J. W. Lyding, Nano Lett., 2013, 13, 5844.
36. A. Bjelajac, R. Petrovic, G. Socol, I. N. Mihailescu, M. Enculescu, V. Grumezescu and D. Janackovic, Ceram. Int., 2016, 42, 9011.
37. M. H. Hossein and M. Mikhak, Curr. Nanosci., 2011, 7, 603.
38. K. Wilke and H. D. Breuer, J. Photochem. Photobiol., A, 1999, 121, 49.
39. L. Jing, Y. Qu, B. Wang, S. Li, B. Jiang, L. Yang, W. Fu, H. Fu and J. Su, Sol. Energy Mater. Sol. Cells, 2006, 90, 1773.
40. R. Ullah and J. Dutta, J. Hazard. Mater., 2008, 156, 194.
41. Z. Zhang, Y. Yuan, Y. Fang, L. Liang, H. Ding and L. Jin, Talanta, 2007, 73, 523.
42. K. Shen, K. Wu and D. Wang, Mater. Res. Bull., 2014, 51, 141.
43. R. Liu, H. Ye, X. Xiong and H. Liu, Mater. Chem. Phys., 2010, 121, 432.
44. D. L. Liao, C. A. Badour and B. Q. Liao, J. Photochem. Photobiol., A, 2008, 194, 11.
45. A. Adel Ismail and D. W. Bahnemann, J. Mater. Chem., 2011, 21, 11686.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11065g

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