Continuous synthesis of a co-doped TiO₂ photocatalyst and its enhanced visible light catalytic activity using a photocatalysis microreactor

Abstract

Three kinds of co-doped TiO₂ nanomaterials were synthesized by using a continuous precipitation method with a valve assisted micromixer in the presence of additives and titanate sources. The research results revealed that co-doped elements were incorporated into the lattice of TiO₂ by substituting Ti and O atoms in the lattice of TiO₂. Compared with conventional methods, the substitution process was realized within a few seconds. The resulting nanoparticles were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nitrophysisorption measurements, X-ray photoelectron spectroscopy (XPS) and diffused reflectance UV-visible spectra (DRS). The synthesized samples were evaluated in the degradation of methyl orange dye (MO) under UV and visible light irradiation at room temperature. Fe, S co-doped TiO₂ (TiO₂–Fe–S) showed nearly the same catalytic efficiency compared with pure TiO₂ (5–10 nm) under UV irradiation. The visible photocatalytic activity was higher than pure TiO₂ (5–10 nm) under visible light irradiation.

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1. Introduction

Recently heterogeneous semiconductor photocatalysis has become a popular technique in controlling aqueous contaminates and air pollutants. TiO₂ has been an attractive material in water purification in the past decade due to its relatively low cost, strong oxidation ability, photostability and nontoxicity among various oxide semiconductors.¹ More defects were observed in anatase compared with rutile,^2,3 resulting in a higher photocatalytic activity. Meanwhile, higher absorptivity and adsorption rate were detected when nanoparticles with smaller particle size was used in the degradation of organic pollutants. However, decreased photocatalytic activity was observed with further smaller particle size. Recombination probability of electron–hole pairs increased when the specific surface area increased. Besides, light absorption spectral blueshifted, resulting in lower degree of photosensitization.^4,5

However, low photocatalytic activity under visible light was the major limitation of TiO₂.⁶ Therefore, a great deal of efforts have been made to improve the utilization of solar energy, such as, dye sensitized,^7,8 transition metals doping^9–11 and noble metal deposition.^12–15 More recently, TiO₂ doped with multiple elements simultaneously has attracted considerable interests, since higher photocatalytic activity under visible light could be obtained. It has been reported that TiO₂ doped with non-metal elements showed a relatively higher photocatalytic activity under visible light.^16–18 Through mixing p orbital of nonmetal with O2p orbital of TiO₂, these non-metal elements have been proved to be beneficial dopants in the TiO₂ lattice to reduce the band gap energy. An optical absorption edge change from UV to visible light was observed with the doping of various transitional metal ions into TiO₂.^19,20 However, remarkable changes in TiO₂ band gap have not been observed. Also, substitution of Ti⁴⁺ in the TiO₂ lattice by metal ion, such as Fe, Ni^21,22 has been reported since it could increase visible light absorbance of TiO₂. Furthermore, co-doped TiO₂ nanoparticles with non-metal,^23–28 metal–nonmetals,^29–34 metal ions,^35,36 and metal–semiconductor^37,38 have attracted more attention since lower band gap energy and a shift in band gap from UV into visible light have been observed.

Numerous methods have been developed to prepare and modify TiO₂ nanoparticles, such as sol–gel method,³⁹ chemical vapor depositions,⁴⁰ solvothermal processes,⁴¹ sputtering,^42,43 reverse micelle method,⁴⁴ liquid phase deposition,⁴⁵ electrochemical method⁴⁶ and hydrothermal treatment.^47,48 However, it is still a big challenge to control size and shape of nanoparticles, especially for nanoparticles co-doped with multiple elements. Another problem in degradation of organic water pollutants was that the degradation process was a complicated process involved in gas–liquid–solid phases.

Far-reaching progress in the development of microfluidic systems in the chemical synthesis have been obtained over the past two decades.^49–51 Recently, microflow system (MFS) has been employed as a new method for preparation of nanomaterials with benefits of better control of shape and size.⁵² Due to extremely fast mixing and more efficient heat transfer, co-doped catalysts have been prepared by a continuous precipitation method with a valve assisted micromixer.^53,54 Meanwhile, investigation concerning three-phase reaction in MFS has been launched. The effect of reactor inner diameter on space-time yield of hydrocarbon and microalgae oil conversion was studied to confirm the superiority of the microreactor for three-phase reaction.⁵⁵

In this study, N–S–TiO₂, B–S–TiO₂ and Fe–S–TiO₂ were prepared at different reaction concentrations in the MFS to increase visible light absorbance. Meanwhile, these prepared nanoparticles were employed to perform photocatalytic degradation of methyl orange (MO) dye under UV and visible light in MFS to study its potential utility in purification of organic water pollutant.

2. Experimental

2.1 Sample preparation

N–S–TiO₂ catalysts were prepared in a micromixer by co-precipitation of titanium precursor solution and an alkaline solution (ammonia water, 25–28%, Sinopharm Chemical Reagent Co., Ltd). Titanous sulfate (96%, Sinopharm Chemical Reagent Co., Ltd) solution and a precipitating agent were pumped (HPLC pump, TBP 5010T, Shanghai Tauto Biotech Co., Ltd) into the valve assisted micromixer (Fig. 1). After centrifugation, white precipitates were washed with water and ethanol. Subsequently the precipitates were dried at 40 °C overnight under vacuum. Finally, N–S–TiO₂ catalysts were successfully prepared by calcining white precipitates at certain temperatures for 1 h in a muffle furnace with a heating rate of 3 °C min⁻¹.

Fig. 1 Schematic drawing of the continuous precipitation method with the valve assisted micromixer.

B–S–TiO₂ catalysts were prepared in the micromixer by co-precipitation of tetrabutyl titanate (98%, Sinopharm Chemical Reagent Co., Ltd) solution and a solution consisting of different concentrations of boric acid (99.8%, Sinopharm Chemical Reagent Co., Ltd) and sulfuric acid (95–98%, Sinopharm Chemical Reagent Co., Ltd). Furthermore, Fe–S–TiO₂ catalysts were prepared in the micromixer by co-precipitation of tetrabutyl titanate solution and a solution including different concentrations of iron sulfate (21–23%, Sinopharm Chemical Reagent Co., Ltd).

2.2 Photocatalyst characterization

The prepared samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen physisorption measurements, X-ray photoelectron spectroscopy (XPS) and diffused reflectance UV-visible spectra (DRS). The appropriate calcination temperature was determined according to the DSC results recorded in a differential thermal analysis meter (TG/DTA 7300, Exstar). XRD patterns were acquired on a Bruker D8 Advance power X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). TEM images and SAED patterns were obtained with a JEOL JEM-200CX microscope. Nitrogen physisorption measurements were realized in a Micromeritics Asap-2020 surface area & pore size analyzer. XPS conducted using a PHI-500 VersaProbe system was employed to characterize the chemical states in the compounds. The DRS of the samples was recorded on a Perkin-Elmer Lambda-950 UV-vis spectrophotometer with an integrating sphere attachment. The extent of MO degradation was monitored using UV-vis spectrophotometer (Agilent Cary 60).

2.3 Photocatalytic experiments

The photodegradation experiments were performed in a photoreactor containing an UV lamp (30 W). Better mixing between MO solution and air was obtained through the slit plate micromixer. Subsequently, the gas–liquid mixture was pumped into three-phase microreactor. The degradation experiments were performed at room temperature and pH of MO solutions was adjusted to 3.⁵⁶ At given irradiation time intervals, the solutions were taken out and analyzed by an UV-vis spectrometer after centrifugation.

3. Results and discussion

3.1 Optimization of flow system conditions

Table 1 showed the effects of calcination temperature and calcination time on the physical properties of TiO₂–N–S powders. TiO₂–N–S powders calcined at 500 °C for 1 h displayed a larger specific surface area, and its value reached 182 m² g⁻¹ (Table 1, entry 5). Interestingly, TiO₂–N–S synthesized by sol–gel method (Table 1, entry 6) showed a lower specific area than that of TiO₂–N–S prepared in the MFS. Smaller nanoparticles were obtained within few seconds in MFS, while, larger nanoparticles were prepared within several hours in the conventional method. Better mixing may be the primary cause. With increased calcination temperature and extended calcination time, the specific surface areas steadily decreased due to the growth of TiO₂–N–S crystallite. It was easy to note that all powders showed a monotonic decrease in the surface areas.

Table 1 The effects of calcination conditions on the physical properties of the TiO₂–N–S powders

Entry	Temperature (°C)	Calcination time (h)	SBET^a (m^2 g^−1)	Crystalline size^b (nm)	Crystalline size^f (nm)
a The BET surface area was determined by multipoint BET method.b Average crystalline size of TiO2–N–S was determined by XRD using Scherrer equation.c The samples calcined at such temperatures were amorphous particles.d The samples were prepared by a continuous precipitation method with a valve assisted micromixer.e The samples were synthesized by sol–gel method.f Average crystalline size of TiO2–N–S was determined by TEM.
1	400	2	—	—^c	—^c
2	500	2	178.3	8.589	8.789
3	600	2	104.9	—	10.825
4	500^d	1	182.4	8.189	—
5	500	0.5	—	—	—
6	500^e	1	103.8	10.479	—
7	500	3	165.7	9.539	—

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The changes of phase structure of the as-prepared TiO₂–N–S powder before and after heat-treatment were investigated according to the XRD results. Fig. 2 showed the effects of calcination temperature and calcination time on the phase structure of TiO₂–N–S powders. The crystallinity and phase composition of the TiO₂–N–S powders were obviously influenced by the calcination temperature and calcination time. With increased calcination temperatures and extended calcinations time, the intensity of anatase phase increased and the width of the (101) plane diffraction peak of anatase (2θ = 25.4°) become narrower, while, an incomplete transformation from amorphous phase to anatase phase appeared at 400 °C for 2 h. When calcined at 600 °C, two peaks were observed which may demonstrate that brookite phase was formed. The grain size of TiO₂–N–S nanoparticles was estimated according to the Scherrer equation. The size of TiO₂–N–S nanoparticles varied from 8.189 nm to 9.539 nm accompanying increased calcination temperatures and extended calcinations time (Table 1, entry 2, 4, 7). At the same time, co-doped nanoparticles calcined at different temperatures were also evaluated by TEM. When the nanoparticles were calcined at 400 °C, amorphous particles were observed in the TEM images (Fig. 3-a). When the calcination temperatures increased to 500 °C, the size was 8.789 nm (Fig. 3-b). However, increased temperature (600 °C) resulted in large particles (10.825 nm) (Fig. 3-c), which was in agreement with the results revealed by XRD. Therefore, the best nanoparticles were obtained when calcined at 500 °C for 1 h. Just as the trend displayed by conventional methods, specific surface area was parabolic correlated to reactant concentration in MFS.

Fig. 2 The XRD patterns of co-doped nanoparticles calcined at different temperatures and time.

Fig. 3 The TEM images of co-doped nanoparticles calcined at different temperatures (a) 400 °C (b) 500 °C (c) 600 °C.

3.2 DSC and XRD analysis

Fig. 4 showed the DTA-TG curves of the TiO₂–N–S powders prepared in a micromixer at room temperature and dried under vacuum at 40 °C overnight. An endothermic peak at about 100 °C was due to desorption of the physically adsorbed water and residual alcohol.⁵⁷ The relatively small endothermic peak at 305 °C probably come from the thermal decomposition of unhydrolyzed Ti(SO₄)₂ remained in the co-doped TiO₂ powders.⁵⁸ The DTA curve also showed two exothermic peaks at about 351 °C and 440 °C, respectively. The two peaks are probably caused by the phase transformation of amorphous to anatase.^57,59 The exothermic peak at 351 °C can be assigned to the phase transformation from amorphous to anatase,⁶⁰ while the other exothermic peak at a higher temperature is probably ascribed to the fact that the phase transformation of the TiO₂–N–S from amorphous to anatase is suppressed by the presence of N and S elements.⁶¹ It can be concluded from the DTA results that the co-doped TiO₂ sample was amorphous, and it can transform from amorphous to anatase at the appropriate temperature (351 °C, 440 °C).

Fig. 4 The DTA-TG curves of the TiO₂–N–S powders prepared by a continuous precipitation method.

3.3 HRTEM and SAED analysis

The crystallite sizes of co-doped TiO₂ samples were also evaluated by HRTEM. Fig. 5 showed HRTEM images of samples in which TiO₂–N–S, TiO₂–B–S and TiO₂–Fe–S had a crystallite sizes around 7.939, 9.321 and 8.471 nm, respectively. Fig. 5(a–c) showed the HRTEM images and SAED patterns of the TiO₂–N–S, TiO₂–B–S and TiO₂–Fe–S nanoparticles, respectively. The doping of other elements does not leave any change in the morphology of the catalyst, which was in agreement with Hamadanian.⁶² Besides, a crystallite size of pure TiO₂ was 10.479 nm, while, a crystallite size of Fe, S-codoped TiO₂ was 8.189 nm. It can be concluded that the addition of dopants to titania hinders the growth of TiO₂ nanoparticles.⁶² Besides, HRTEM images in Fig. 5 showed that spherical shape was the most form, while, the rest had uneven shapes. The phase structure of co-doped nanoparticles was also confirmed to be anatase phase by SAED analysis. Interestingly, the same concentric Scherrer's rings observed in the SAED images showed strong evidence for the nanocrystallinity. A set of rings instead of spots in the SAED patterns were assigned to random orientation of crystallites, which demonstrated that diffraction from different planes of nanoparticles. Meanwhile, interplanar spacings of these three nanoparticles were calculated, which were 3.42 Å, 3.46 Å and 3.51 Å, respectively. The differences between standard (101) crystal plane spacing (3.52 Å) and experiments might be attributed to structural modification caused by the co-doped elements. The location of plane corresponding to such as (101), (004), (200) and (211) was in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS no. 21-1272) reference diagrams for the corresponding anatase phase. As shown in Table 1, TiO₂–N–S had a crystallite size of 10.479 nm in conventional method, while a crystallite size of 7.939 nm was obtained within few seconds in MFS. This is probably because of that fast mixing in MFS leads to fast nucleation.

Fig. 5 The HRTEM image and SAED pattern of co-doped TiO₂ (a) TiO₂–N–S (b) TiO₂–B–S (c) TiO₂–Fe–S.

3.4 DRS analysis

The DRS of the samples was shown in Fig. 6. It was obvious that the absorbance of co-doped TiO₂ in the visible region increased. DRS results showed a red shift in the absorption in the case of Fe–S–TiO₂ that indicated that the dopant decreased band gap of the catalyst. The enhancement of absorbance in the UV-vis region increases the number of photogenerated electrons and holes to participate in the photocatalytic reaction, which can enhance the photocatalytic activity of TiO₂.⁶³

Fig. 6 The DRS of co-doped nanoparticles prepared by a continuous precipitation method.

3.5 XPS analysis

XPS analysis was performed in order to investigate the chemical composition and chemical states of TiO₂ samples (Fig. 7). For pure TiO₂ sample, it only contained C, O and Ti elements with sharp photoelectron peaks appearing at binding energies of 458(Ti2p), 529(O1s) and 284 eV(C1s). The C1s peak was attributed to the adventitious carbon from the XPS instrument itself. On the contrary, the TiO₂–N–S samples not only contained C, O and Ti elements, but also a small amount of N and S atoms.

Fig. 7 The XPS survey spectra of different co-doped TiO₂.

Curve b in Fig. 7 showed the high-resolution XPS spectra of the N1s region, taken on the surface of TiO₂ powders. The N1s region consisted of two peaks, one was attributed to the Ti–N,⁶⁴ which was probably formed by a nucleophilic substitution reaction between NH₃ and Ti (SO₄)₂ during the hydrolysis.⁶⁰ The other was probably assigned to some NH₃ adsorbed on the surface of TiO₂.^65,66 Two S2p peaks located at 168 and 169 eV were observed. The major peak was attributed to S₆⁺ and the peak at 169 eV was assigned to the S₄⁺.^67–70 However, it was reported that when SO₂ molecules were adsorbed on the surface of TiO₂, the S2p peak was in the region of 166–170 eV.⁷¹ In this study, the samples were calcined in air at 500 °C for 1 h. Under the circumstance, SO₂ could not be steady absorbed on the surface of TiO₂. Thus, it was more reasonable that the S₄⁺ and S₆⁺ were incorporated into the lattice of TiO₂ and substituted for titanium atoms.

Curve a in Fig. 7 showed the XPS spectra of TiO₂–B–S. A peak at 192 eV was contributed to B1s, indicating that B atoms was incorporated into the lattice of TiO₂ and substituted for oxygen atoms.⁷² Two S2p peaks located at 168 and 169 eV were observed as well as the TiO₂–N–S, indicating that the S₄⁺ and S₆⁺ were incorporated into the lattice of TiO₂ and substituted for titanium atoms.

Curve c in Fig. 7 showed the XPS spectra of TiO₂–Fe–S. Fe2p and S2p core level XPS were measured. As shown in Fig. 6, two peaks at 710.9 and 723.9 eV were assigned to Fe2p3/2 and Fe2p1/1 photoelectrons,^73,74 respectively, demonstrating that Fe was incorporated into the lattice of TiO₂ through substituting titanium atoms existing as Fe₃⁺. Two S2p peaks located at 168 and 169 eV were observed as well as the TiO₂–N–S, indicating that the S₄⁺ and S₆⁺ were incorporated into the lattice of TiO₂ and substituted for titanium atoms.

3.6 Photocatalytic activity

Degradation of MO in the presence of co-doped TiO₂ photocatalyst under UV and visible light irradiation was evaluated by UV-visible spectroscopy. The results were depicted in Fig. 8. Less MO was detected with increased ammonium hydroxide concentration, while the degradation level decreased when the ammonium hydroxide concentration exceeded 20 ml/100 ml. Less MO was detected with more titanous sulfate initially. However, when titanous sulfate concentration increased above 15 g/100 ml, a decrease in degradation efficiency was observed. This phenomenon indicated that the degradation process would proceed better with smaller nanoparticles. Highest degradation level was obtained when the concentration of sulfuric acid and boric acid was 1 g/100 ml. Moreover, a parabolic trend in degradation by TiO₂–Fe–S was observed and the optimal degradation level was 96.92%. Mechanistically, a decrease in degradation under UV irradiation was observed with a red shift in the absorption. As shown in DRS experiments, a red shift was observed in the three co-doped TiO₂. It could be seen from Fig. 9 that an increase in MO degradation by coped TiO₂ was detected compared with pure TiO₂ under visible light.

Fig. 8 Degradation of MO in the presence of co-doped TiO₂ under UV irradiation.

Fig. 9 Degradation of MO in the presence of photocatalysts in 3 min under visible light irradiation.

3.7 Possible mechanism

Mechanistically, during the photocatalytic process, the absorption of photons by the photocatalysts leads to the excitation of electrons from the valence band to the conduction band and generates electron–hole pairs. Thus, an increase in MO degradation is observed due to the fact that S element coped in TiO₂ lattice increases the surface acidity.⁷⁵ Moreover, the best photocatalyst was TiO₂–Fe–S, whose degradation value of 90% was obtained. With increasing Fe content, absorption in visible region and generation of electron–hole pairs increases. The high photocatalytic activity may be due to the following factors:⁷⁶ (1) increased light absorption capability; (2) action of both h⁺/e⁻ traps to reduce the recombination rate of h⁺/e⁻ pairs during the process. Compared with TiO₂–Fe–S, non-elements coped TiO₂ shows a lower photocatalytic activity. This is probably because coped non-elements decreased the energy gap of TiO₂, resulting in a decrease in the activity of oxidizability.⁷⁷ Moreover, the lower stability of non-elements coped TiO₂ may also contribute to the lower photocatalytic activity.

4. Conclusions

In conclusion, a novel process for nanoparticles in MFS was investigated. Light absorption capacity increased in the visible region and an increase in MO degradation under visible light irradiation was observed with some elements co-doped TiO₂ in comparison with pure TiO₂. Compared with non-element coped TiO₂, TiO₂–Fe–S shows a higher photocatalytic activity both in UV and visible light irradiation.

Acknowledgements

The research has been supported by the Joint Funds of the National Natural Science Foundation of China (Grant U1463201); National Key Basic Research Program of China (973 Program) 2012CB725204 and 2011CB710803; National High Technology Research and Development Program of China (863 Program) 2013AA031901; the National Natural Science Foundation of China (Grant no. 81302632 and 21402240); the youth in Jiangsu Province Natural Science Fund (Grant no. BK20130913); a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

1. A. F. Shojaie and M. H. Loghmani, Chem. Eng. J., 2010, 157, 263.
2. P. H. Maruska and A. K. Ghosh, Sol. Energy, 1978, 20, 20.
3. K. Tanaka, M. F. V. Capule and T. Hisanaga, Chem. Phys. Lett., 1991, 187, 73.
4. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891.
5. A. Wold, Chem. Mater., 1993, 5, 280.
6. H. Yang, X. C. Zhang and Q. F. Tao, Inorg. Mater., 2009, 45, 1139.
7. D. F. Watson, A. Marton, A. M. Stux and G. J. Meyer, J. Phys. Chem. B, 2003, 107, 10971.
8. J. N. Clifford, E. Palomares, K. Nazeeruddin, R. Thampi, M. Grtzel and J. R. Durrant, J. Am. Chem. Soc., 2004, 125, 5670.
9. M. Anpo and M. Takeuchi, J. Catal., 2003, 216, 505.
10. W. Zhao, C. Chen, X. Li and J. Zhao, J. Phys. Chem. B, 2002, 106, 5022.
11. G. G. Lenzi, C. V. B. Favero, L. M. S. Colpini, H. Bernable, M. L. Baesso, S. Specchia and O. A. A. Santos, Desalination, 2011, 270, 241.
12. V. Subramanian, E. E. Wolf and P. Kamat, J. Am. Chem. Soc., 2004, 126, 4943.
13. M. Jakob, H. Levanon and P. V. Kamat, Nano Lett., 2003, 3, 353.
14. C. Sahoo, A. K. Gupta and A. Pal, Desalination, 2005, 181, 91.
15. P. Wongwisate, S. Chavadej, E. Gulari, T. Sreethawong and P. Ransunvigit, Desalination, 2011, 272, 154.
16. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269.
17. D. Chen, D. Yang, Q. Wang and Z. Jiang, Ind. Eng. Chem. Res., 2006, 45, 4110.
18. V. Iliev, D. Tomova and S. Rakovsky, Desalination, 2010, 260, 101.
19. M. I. Litter and J. A. Navio, J. Photochem. Photobiol., A, 1996, 98, 171.
20. J. C. S. Wu and C. H. Chen, J. Photochem. Photobiol., A, 2004, 163, 509.
21. X. H. Wang, J. G. Li, H. Kamiyama, Y. Moriyoshi and T. Ishigaki, J. Phys. Chem. B, 2006, 110, 6804.
22. W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 1994, 98, 13669.
23. J. Yu, M. Zhou, B. Cheng and X. Zhao, J. Mol. Catal. A: Chem., 2006, 246, 176.
24. D. Chen, Z. Jiang, J. Geng, Q. Wang and D. Yang, Ind. Eng. Chem. Res., 2007, 46, 2741.
25. X. Li, R. Xiong and G. Wei, Catal. Lett., 2008, 125, 104.
26. T. Ohno, T. Tsubota, Y. Nakamura and K. Sayama, Appl. Catal., A, 2005, 288, 74.
27. L. Lin, R. Y. Zheng, J. L. Xie, Y. X. Zhu and Y. C. Xie, Appl. Catal., B, 2007, 76, 196.
28. F. Wei, L. Ni and P. Cui, J. Hazard. Mater., 2008, 156, 135.
29. C. Liu, X. Tang, C. Mo and Z. Qiang, J. Solid State Chem., 2008, 181, 913.
30. P. Wu, J. Tang and D. Zhi, Mater. Chem. Phys., 2007, 103, 264.
31. H. Xia, H. Zhuang, D. Xiao and T. Zhang, J. Wuhan Univ. Technol., Mater. Sci. Ed., 2008, 23, 467.
32. D. B. Hamal and K. J. Klabunde, J. Colloid Interface Sci., 2007, 311, 514.
33. K. Obata, H. Irie and K. Hashimoto, Chem. Phys., 2007, 339, 124.
34. X. Yang, C. Cao, K. Hohn, L. Erickson, R. Maghrang, D. Hamal and K. Klabunde, J. Catal., 2007, 252, 296.
35. D. R. Zhang, Y. H. Kim and Y. S. Kang, J. Curr. Appl. Phys., 2006, 6, 801.
36. R. Khan, S. W. Kim, T. Kim and C. Nam, Mater. Chem. Phys., 2008, 112, 167.
37. X. Yang, L. Xu, X. Yu and Y. Guo, Catal. Commun., 2008, 9, 913.
38. B. Zhao, B. Shi, X. Zhang, X. Cao and Y. Zhang, Desalination, 2011, 268, 55.
39. K. Abbas, Colloids Surf., A, 2009, 346, 130.
40. W. L. Gladfelter, Surf. Sci., 2011, 605, 1146.
41. S. Perera and E. G. Gillan, Solid State Sci., 2008, 10, 864.
42. Y. Hoshi, D. Ishihara, T. Sakai, O. Kamiya and H. Lei, Vacuum, 2010, 84, 1377.
43. X. Yu and Z. Shen, Vacuum, 2011, 85, 1026.
44. X. Sui, Y. Chu, S. Xing and C. Liu, Mater. Lett., 2004, 58, 1255.
45. M. Mallak, M. Bockmeyer and P. Labmann, Thin Solid Films, 2007, 515, 8072.
46. N. Lu, H. Zhao, J. Li, X. Quan and S. Chen, Sep. Purif. Technol., 2008, 62, 668–673.
47. T. Putta, M. C. Lu and J. Anotai, J. Environ. Manage., 2011, 92, 2272.
48. S. S. Mali, P. S. Shinde, C. A. Betty, P. N. Bhosale, W. J. Lee and P. S. Patil, Appl. Surf. Sci., 2011, 257, 9737.
49. K. S. Elvira, X. C. Solvas, R. C. R. Wootton and A. J. deMello, Nat. Chem., 2013, 5, 905.
50. B. Xu, Y. Zhang, S. Wei, H. Ding and H. Sun, ChemCatChem, 2013, 5, 2091.
51. N. Wang, X. Zhang, Y. Wang, W. Yu and H. L. W. Chan, Lab Chip, 2014, 14, 1074.
52. G. Simson, E. Prasetyo, S. Reiner and O. Hinrichsen, Appl. Catal., A, 2013, 450, 1.
53. Scientific Bases for the Preparation of Heterogeneous Catalysts, ed. E. M. Gaigneaux, M. Devillers, D. E. DeVos, S. Hermans, P. A. Jacobs, J. A. Martens and P. Ruiz, Elsevier, 2006, p. 175.
54. S. Kaluza and M. Muhler, J. Mater. Chem., 2009, 19, 3914.
55. L. Zhou and A. Lawal, Energy Fuels, 2015, 29, 262.
56. L. Rizzo, J. Koch, V. Belgiorno and M. A. Anderson, Desalination, 2007, 211, 1.
57. J. Yu, X. Zhao and Q. Zhao, Thin Solid Films, 2000, 379, 7.
58. J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang and L. Z. Zhang, Chem. Mater., 2002, 14, 3808.
59. K. Terabe, K. Kato, H. Miyazaki, S. Yamaguchi, A. Imai and Y. Iguchi, J. Mater. Sci., 1994, 29, 1617.
60. J. Yu, J. C. Yu, W. Ho, M. K. P. Leung, B. Cheng, G. Zhang and X. Zhao, Appl. Catal., A, 2003, 255, 309.
61. J. Yu, M. Zhou, B. Cheng and X. Zhao, J. Mol. Catal. A: Chem., 2006, 246, 176.
62. M. Hamadanian, A. Reisi-Vanani, M. Behpour and A. S. Esmaeily, Desalination, 2011, 281, 319.
63. D. Li, H. Haneda, S. Hishita and N. Ohashi, Mater. Sci. Eng. B, 2005, 117, 67.
64. S. Sakthivel, M. Janczarek and H. Kisch, J. Phys. Chem. B, 2004, 108, 19384.
65. O. Diwald, T. L. Thompson, E. Zubkov, E. G. Goralski, S. D. Walck Jr and J. T. Yates, J. Phys. Chem. B, 2004, 108, 6004.
66. R. Nakamura, T. Tanaka and Y. Nakato, J. Phys. Chem. B, 2004, 108, 10617.
67. T. Ohno, T. Mitsui and M. Matsumura, Chem. Lett., 2003, 32, 364.
68. M. Zhou and J. Yu, J. Hazard. Mater., 2008, 152, 1229.
69. S. Liu and X. Chen, J. Hazard. Mater., 2008, 152, 48.
70. T. Ohno, T. Tsubota, K. Nishijima and Z. Miyamoto, Chem. Lett., 2004, 33, 750.
71. D. I. Sayago, P. Serrano, O. Bohme, A. Goldoni, G. Paolucci, E. Roman and J. A. Martin-Gago, Phys. Rev. B, 2001, 64, 205402.
72. B. Wang, X. Lu, J. Xuan and M. K. H. Leung, Mater. Lett., 2014, 115, 57.
73. T. K. Ghorai, S. K. Biswas and P. Pramanik, Appl. Surf. Sci., 2008, 254, 7498.
74. M. Descostes, F. Mercier, N. Thromat, C. Beaucaire and M. Gautier-Soyer, Appl. Surf. Sci., 2000, 165, 288.
75. T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui and M. Matsumura, Appl. Catal., A, 2004, 265, 115.
76. M. Hamadanian, A. Reisi-Vanani, M. Behpour and A. S. Esmaeily, Desalination, 2011, 281, 319.
77. R. Nakamura, T. Tanaka and Y. Nakato, J. Phys. Chem. B, 2004, 108, 10617.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05956a

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