Lanthanum ion doped nano TiO₂ encapsulated in zeozyme and impregnated in a polystyrene film as a photocatalyst for the degradation of diuron in an aquatic ecosystem

Abstract

The occurrence of chlorinated herbicide diuron in water bodies is considered serious pollution and a major health hazard to flora, fauna and mankind. In the present investigation, we studied the photocatalytic degradation of diuron in an aquatic ecosystem using lanthanum ion doped nano TiO₂ (Lnp) encapsulated in NaY zeolite pores (1 : 10) and impregnated in polystyrene film (ZLT). The hydrophobic nature of the polystyrene support resulted in an efficient and highly recoverable heterogeneous system. Catalyst characterization was carried out by FT-IR, XRD, DRS-UV, fluorescence, BET, SEM-EDAX and XPS. BET results revealed the successful loading of lanthanum ion doped TiO₂ (Lnp) inside the NaY zeolite pores via a decrease in surface area for the zeolite encapsulated Lnp (ZLnp) as compared to NaY zeolite alone. DRS UV supported the impregnation of ZLnp in the polystyrene films; the bathochromic shift (Δλ) was 4 nm and the hypochromic shift decreased in intensity 10 fold. The photocatalytic reaction was carried out at a concentration of 20 mg L⁻¹ of diuron, with 0.01 M H₂O₂ and a catalytic amount of 500 mg L⁻¹ ZLT under unstirred conditions. Degradation of diuron by ZLT reached 40% after 2 hours. Noteworthy features are the good results under optimized conditions and that the same film models were used successfully in the presence of zebra fish (Danio rerio). The present investigation also demonstrated successful re-use of the photocatalytic film six times without any appreciable loss in catalytic activity. From the abovementioned results, it was proven that ZLT is an efficient and ecofriendly catalyst.

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Introduction

Among xenobiotic pollutants, diuron, or 3-(3,4-dichlorophenyl)-1,1-dimethylurea, is of major concern due to its adverse effects on human health and the ecosystem, as reported and listed in the Contaminant Candidate List 3 from the US EPA (US Environmental Protection Agency, 2009), which identifies potential drinking water contaminants for future regulation.¹ As a halogenophenylurea, diuron represents an important class of contact herbicides that have been used worldwide for more than 40 years. Due to the low solubility and chemical stability of halogenophenylureas, they penetrate slowly through the soil and contaminate underground sources of drinking water. Their photo reactivity has already been studied several times.² Halogenophenylureas are also commonly found in crops that are grown and stored for human or animal consumption, as well as in processed foods.³ However, environmental exposure to diuron specifically has not been fully investigated. Thus far, the main input sources of diuron into the aquatic environment have been identified as runoff and drainage from fields sprinkled with the herbicide, and from sewer systems. In particular, the production and emission is from infected agricultural plots, as well as their occurrence in surface waters has hardly been investigated systematically. Furthermore, it is difficult to treat this contaminated water by conventional techniques. Nevertheless, it has been reported that diuron can be degraded via photocatalytic reactions on oxide semiconductors (Sayeh, et al., 2007).⁴ There are several available oxidation technologies suitable for wastewater treatment such as wet air oxidation,⁵ supercritical water oxidation,⁶ incineration⁷ and advanced oxidation processes (AOPs).^8–14 Since Frank and Bard¹⁵ first examined the possibilities for the decomposition of cyanide in water with the aid of TiO₂, interest in this material for environmental applications has grown. Photocatalysis of TiO₂ can be explained as a “catalytic reaction involving the production of a catalyst by absorption of light”.¹⁶ The TiO₂ semiconductor photocatalyst remains outstanding among other catalysts due to its excellent oxidative properties, Honda–Fujishima effect and physical stability.^17–19 However, application of TiO₂ is limited by fast recombination of electron–hole pairs and their wide band gaps, which correspond to UV light.²⁰ Direct photolysis of diuron was studied under various conditions by Jirkovský et al.²¹ In the present investigation, La³⁺ has been doped onto TiO₂ and encapsulated in NaY zeolite, on the hypothesis that the presence of a dopant can retard the recombination between e⁻–h⁺ pairs generated by irradiated TiO₂ nanoparticles encapsulated into the zeolite, consequently improving the catalytic activity.²² Metal doping of TiO₂ has generally been shown to reduce recombination and sensitization of electron–hole pairs under visible light.^23–26 The co-incorporation of both La and Ti into the framework of the zeolite occupied most of the internal and external surface area with active metals, thus enhancing the catalytic activity. Zeolites are good adsorbents and eco-friendly materials possessing high surface area and high thermal stability. The behavior of the catalyst and its practical applications will be influenced by the properties of the zeolites such as particle size, surface area, pore diameter, mechanical strength, microbial resistance, thermal stability, chemical durability, hydrophobic/hydrophilic character, ease of regeneration, loading capacity, and cost.²⁷ Herein, we report the excellent photocatalytic degradation of diuron by lanthanum ion doped TiO₂ encapsulated in NaY zeolite and impregnated in polystyrene film via a realistic and eco friendly route that also offers reproducibility and reusability. Furthermore, the research was extended to assess the fish (Danio rerio) acute toxicity of the catalyst.

Experimental

Materials

Titanium tetraisopropoxide (97%) – Aldrich; ammonia solution (25%) – Merck Specialities, Mumbai; 2-propanol 99.5% – Merck Specialities, Mumbai; lanthanum(III) nitrate (98%) – Alfa Aesar; methanol – Hi Media; dichloromethane – Merck Specialities, Mumbai; tetrahydrofuron – Merck Specialities, Mumbai; polystyrene – Sigma Aldrich, USA.

Physical measurements

IR spectra were recorded on a Jasco FT-IR-6600. Powder XRD diffraction data were collected on a Scintag XDS 2000 X-ray diffractometer using CuKR radiation. Absorption spectra were recorded using a DRS UV Shimadzu UV-VIS-NIR SPECTROPHOTOMETER UV-3600 PLUS. Fluorescence spectra were measured in ethanol using Perkin Elmer LS45 Fluorescence Spectrophotometer. Nitrogen adsorption isotherms were measured at −196 °C using an ASAP 2020 POROSIMETER. The Scanning Electron Micrographs (SEM) of the samples were recorded using an F E I Quanta FEG 200 High Resolution Scanning Electron Microscope. ICP-OES analyses were performed on a Perkin-Elmer Optima 2000 DV model. X-ray photoelectron spectra of the catalysts were recorded using an ESCA-3 Mark II spectrometer (VG Scientific LT., England) using Al Kα (1486.6 eV) radiation as the source. Spectra were referenced with binding energy of C 1s (284 eV).

The quantification of residues of diuron was done using a Shimadzu Prominence High Performance Liquid Chromatograph equipped with two pumps (model LC-20AT), oven (CTO-20A), Ultra Violet detector (SPD-20A), and a C18 reverse phase column (25 cm length × 4.6 mm i.d. × 5 μm particle size, Phenomenex). The eluent was a mixture of acetonitrile and water (80 : 20 v/v), with 1.0 ml min⁻¹ flow rate and oven temperature of 40 °C. Detection was at 235 nm, with an injection volume of 20 μL. The peak of diuron was eluted at 4.6 minutes, whereas the degradant (3,4-dichloroaniline) appeared at 4.1 minutes.

Methods

Preparation of nano particle TiO₂ (np)

TiO₂ powder was prepared by a sol–gel process,²⁸ in which sol was prepared by mixing titanium tetraisopropoxide (TTIP), 2-propanol, and ammonia at room temperature. 10 cm³ (0.036 mol) of titanium tetraisopropoxide was dissolved in 80 cm³ (0.82 mol) of 2-propanol in a round bottom flask and homogenized by stirring for 10 minutes. To the abovementioned mixture, 10 ml of ammonia solution was added gradually at a rate of 1 cm³ min⁻¹. The solution changed from being clear to appearing as a white precipitate. After 1 hour of stirring, 2-propanol was evaporated by buchi rotavapour. At the terminal stage, the sample was dried by convection at 105 °C for 18 h. The sample was then calcined at 600 °C for 2 h.

Preparation of lanthanum ion doped TiO₂ nano particle (Lnp)

1 g (np) was taken into a round bottomed flask containing 20 ml of 2-propanol, and was kept over the magnetic stirrer for 10 minutes. To the abovementioned mixture, 200 mg of lanthanum(III) nitrate was added and stirred for five hours. 2-Propanol was evaporated by buchi rotavapour. At the terminal stage, the sample was dried by convection at 105 °C for 18 h. The sample was then calcined at 600 °C for 2 h.

Preparation of lanthanum ion doped TiO₂ nano particles encapsulated in NaY zeolite (ZLnp)

500 mg of zeolite was taken into a 100 ml round bottom flask containing 30 ml of methanol and the mixture was sonicated for 10 minutes. To the abovementioned mixture, 50 mg of lanthanum doped nano particles (Lnp) was added and sonicated for 5 hours. Methanol was then evaporated using a buchi rotavapour. The residual methanol was removed by drying in the oven at 105 °C for 3 h.

Preparation of ZLnp impregnated polystyrene film (ZLT)

Thin film polystyrene was impregnated with ZLnp by adding 200 mg of ZLnp to a 5% polystyrene in a tetrahydrofuron solution. The abovementioned mixture was sonicated for 20 minutes. An aliquot of above mixture was then spread on a uniform smooth surface and the solvent was evaporated in the oven at 50 °C for 2 h. The same procedure was employed without the addition of ZLnp for the preparation of bare polystyrene film.

Design of degradation experimental procedure

The selective photocatalytic degradation activities of the catalysts were investigated as follows. A total of 4 aquarium tanks sized (60 × 30 × 45 cm, lbh) were filled with 10 L of water for use in the studies. The aquarium tanks were aerated and nine zebra fish (Danio rerio) were taken into each aquarium. The first aquarium contained 20 ppm of diuron solution, 0.1 g of ZLT and 20 mM of H₂O₂, added dropwise. The second aquarium contained 20 ppm of diuron solution and 20 mM of H₂O₂, added dropwise. The third aquarium was used as a control without ZLT, diuron, or H₂O₂. The fourth aquarium contained 20 ppm of diuron solution, 0.1 g of ZLT and 20 mM of H₂O₂, added dropwise, but in the absence of fish. All fishes were fed with Pinar Yem at a concentration of 1% of their body mass per day. All four aquariums were kept under direct sunlight without stirring. The course of the diuron degradation was monitored at regular intervals and the pattern of degradation was analyzed using HPLC method.

Results and discussion

IR spectra

Infrared spectra of the ZLnp impregnated polystyrene film, bare polystyrene film, Lnp encapsulated in NaY zeolite and bare NaY zeolite are shown in Fig. 1(a)–(d). A new absorption band was detected in zeolite catalysts loaded with Lnp at a wavelength of 996 cm⁻¹ that was due to Ti–O–Si bonding. This result was in agreement with a previous report by Wang et al.²⁹ The peak was ascribed to the replacement of tetrahedral Si with Ti during the ion-exchange method.²⁹ For bare NaY zeolite, the peak at 996 cm⁻¹ is attributed to Si–OH bonding³⁰ (Fig. 1(d)). From Fig. 1(b), it is clear that there was no such peak in the bare polystyrene film. Thus, this peak was an indication of successful impregnation of ZLnp in to the polystyrene film.

Fig. 1 Infrared spectra of (a) ZLnp impregnated polystyrene film (ZLT); (b) bare polystyrene film; (c) NaY zeolite with loading of Lnp (ZLnp); (d) NaY zeolite.

XRD studies

The X-ray powder diffraction (XRD) patterns of the encapsulated Lnp in NaY zeolite and the bare NaY zeolite are presented in Fig. 2(a) and (b). XRD patterns of the zeolite host before and after encapsulation of Lnp indicated no remarkable differences, although a slight change in intensity of the typical lines 220, 311 and 331 in the encapsulated Lnp was noticed. This confirmed that NaY zeolite encapsulation of Lnp had little influence on crystallinity, and that the zeolite host can accommodate Lnp. The diffraction pattern for the metal could not be detected in the Lnp loaded zeolite. This could possibly be due to low loading levels of metal in the zeolite. In the X-ray studies of the samples of np, Lnp and ZLT (Fig. S1 in the ESI†), it was observed that a mixed structure of anatase (A) and rutile (R) crystalline phases coexisted in La-doped TiO₂, with the weight percent of the anatase phase being 86–89% according to the intensity of the highest peak. No rutile phase appeared in undoped TiO₂. The high-angle XRD results for the as-synthesized samples revealed the coexistence of TiO₂–anatase (JCPDS, no. 21-1272). The downward facing arrows in Fig. 3 represent the existence and position of TiO₂ anatase in ZLT. The average grain size as calculated from the broadening of the (101) peak of anatase using Scherrer's equation was 6–19 nm.

Fig. 2 Powder XRD spectra of (a) ZLnp; (b) NaY zeolite.

Fig. 3 Fluorescence spectra of ZLT and bare polystyrene film.

DRS-UV and fluorescence

The UV-Vis diffuse reflectance spectra for NaY zeolite, Lnp, ZLnp and np are shown in Fig. S2 in the ESI.† A blue shift towards a shorter wavelength of 370 nm was observed for ZLnp compared to the np peak, which had a wavelength of about 390 nm. The result was in good agreement with the findings of Easwaramoorthi and Natarajan.³¹ The origin of such a blue shift was the quantum size effect for semiconductors, as the particle size decreased after loading of TiO₂ into the pores of the zeolite. The rapid decrease in reflectance at certain wavelengths indicated the presence of an optical band gap after the ion-exchange of the Ti species into the zeolite.³² The reduction of energy gap was facilitated by lanthanum doping of semiconductors, which caused the red shift in the reflection spectra and led to excitation of the electrons from the valance band to the conduction band (Fig. S2 in the ESI†).

The absorption spectra for bare polystyrene film and ZLnp impregnated polystyrene thin film (ZLT) are shown in Fig. S3 in the ESI.† A hypochromic shift occurred, with tenfold decrease in intensity and additional bathochromic shift of Δλ = 4 nm, which revealed quenching of the polystyrene signal by the impregnated ZLT.

The fluorescence emission occurred due to the recombination of electron hole pairs in the semiconductor. Emission spectra of np and Lnp (Fig. S4 in the ESI†) revealed that the emission intensity of the Lnp was decreased compared with np. This was due to the suppression of recombination of photo-generated electron hole pairs by the lanthanum loaded onto TiO₂.³³

The fluorescence spectra of the bare polystyrene film and ZLT were shown in Fig. 3. Comparing the emission spectra revealed that there was a significant drop in the intensity at 611 nm, which justified the conclusion of fluorescence quenching due to the impregnation of ZLnp. This was in accordance with the results obtained from UV spectra.

Surface textural studies – BET (Brunauer–Emmett–Teller)

The surface textural properties of the zeolite with encapsulated Lnp are shown in Table S1 in the ESI.† Clearly, inclusion of Lnp in the zeolite cavities considerably reduced the pore volume and the surface area of the zeolite host. This change confirmed the existence of Lnp in the zeolite cavities rather than on the external surface.³⁴ The decreasing values for surface area pore volume and adsorption capacity depended on the amount of incorporated Lnp as well as their molecular size and geo-metrical conformation inside the zeolite host. Fig. 4 showed the N₂ adsorption/desorption isotherms and pore size distribution for ZLnp, which were typical type I according to the IUPAC classification and are characteristic of the microporous nature of the materials. This supported the observation that the Lnp are present within the zeolite cages and not on the external surface since the zeolite crystallinity was retained.

Fig. 4 A linear plot of BET surface area.

SEM analysis

The SEM images of the ZLT that indicated encapsulation of ZLnp in the bare polystyrene film are shown in Fig. 5. These images confirmed that the Lnp were preserved inside the cages of the zeolite pores without leaching of Lnp, and were further protected by the polystyrene film. This absence of morphology change was also observed by IR spectroscopy. The EDAX spectrum of Lnp showed that different elements were present on the surface of the catalyst: C, Si, O, Ti, and La. This result was another indication that the metal loading was successful. Carbon was present in the polystyrene film, while silicon and oxygen were the main elements of the zeolite frame work. Titanium and lanthanum was successfully encapsulated inside the supercages of the zeolite (Fig. S5 in the ESI†). This conclusion was consistent with results of XRD and UV-Vis reflectance spectra.

Fig. 5 (a) FE-SEM analysis image at 1 μm of ZLT; (b) FE-SEM analysis image at 10 μm of ZLT.

XPS analysis

The catalyst was analysed using X-ray photoelectron spectroscopy (XPS), a surface sensitive technique³⁵ for determination of elemental composition and oxidation states. Fig. 6 and 7 present the selected area scan for ZLT (C 1s, Ti, La, O 1s, Al and Si). The C 1s peak that was observed in the region of 284.6 eV was due to C–C neutral bonding. Its greater intensity signified that the ZLnp was loaded into the polystyrene film (Fig. 6(a)). The Ti core level spectrum in Fig. 6(b) contained two peaks at 458.4 and 464.5 eV, which corresponded to Ti 2p_3/2 and Ti 2p_1/2 binding energy regions. These values matched well with the previously reported data for Ti⁴⁺ ions in TiO₂.³⁶ Fig. 6(c) presents the binding energy of La 3d_5/2 and La 3d_3/2, which were 834.8 eV and 851.2 eV, respectively.³⁷

Fig. 6 (a) C 1s peak image; (b) Ti peak image; (c) La peak image.

Fig. 7 (a) O 1s peak image; (b) Al 2p peak image; (c) Si 2p peak image.

In Fig. 7(a), one intense peak was located at 531.2 eV that corresponded to O 1s of the NaY zeolite.^38,39 Fig. 7(b) showed the binding energy of Al 2p at 74.2 eV. An intense peak was observed in Fig. 7(c) at 102.5 eV, which was due to Si 2p binding energy.

Catalytic activity

The photocatalytic activity of the newly fabricated ZLT was tested in aquatic system for the degradation of diuron in the presence of zebra fish. There was a constant decrease in the diuron concentration over every interval of sample collection.

The HPLC analysis shown in Fig. 8 revealed the photocatalytic degradation pattern. The influence of the catalyst was studied by comparison of the experiments with and without ZLT in the tank under identical conditions. A sluggish degradation profile was observed without ZLT and the final diuron concentration remained high at more than 90%, even after two days. Catalyst reproducibility was estimated by conducting the experiment in the presence of catalyst for six replicates. Concordance was achieved for the reproducibility experiments. In addition to its reusability, the catalyst was also easily recoverable under all conditions; even after more than three cycles it was reused without any degeneration (Fig. S6 in the ESI†). The catalyst selectivity for diuron was tested for all the catalysts (Fig. S7(a)–(d) and Table S3 in the ESI†). ZLT showed the highest selectivity with regards to the DT₅₀ value, reaction completion time and low catalyst amount. The reusability of ZLT was high due to excellent recovery when compared with other catalysts. ZLT satisfied the key elements required for an effective heterogeneous catalyst. Assessment of acute toxicity of catalyst on fishes was conducted with high amount of catalyst loading in the tank in presence of zebra fishes for a week and no visible abnormalities were observed, such as loss of equilibrium, swimming behavior, respiratory function, and pigmentations as per OECD Guideline no. 203.⁴⁰ In addition, leaching of the impregnated zeolite was tested by soaking ZLT in 0.1 M hydrochloric acid for 2 hours at 40 °C. The collected samples were analyzed by ICP-OES, which showed less than 0.01 ppm of silica and 0.0003 ppm of titanium in solution. Therefore, the toxicity of the metal was decreased due to its excellent encapsulation during the fabrication process. The degradation of diuron experimentally gave a successful result, with DT₅₀ value of 2.01 hours (Fig. S8 and Table S3 in the ESI†). The influence of hydrogen peroxide was not significant in terms of decrease of DT₅₀ value, but in presence of catalyst a dropwise addition of the hydrogen peroxide yielded very rapid degradation of diuron. The final concentration of diuron under these conditions was found to be below 5% within five hours. The major degradant was identified as 3,4-dichloroaniline. Furthermore, any abnormalities and mortality to the fish was assessed during and after the completion of the experiment. All the fish were alive and active, even a month after completion of the experiment.

Fig. 8 HPLC analysis of diuron.

Conclusion

The present investigation concluded that the lanthanum ion doped TiO₂ encapsulated in NaY zeolite and impregnated in polystyrene film had greater impact on the hazardous pollutant diuron, which has been found in running water bodies and the aquatic food chain. The same was confirmed by analytical characterization, which exhibited fairly clear evidence in the spectral (FTIR, Drs-UV, fluorescence, XPS) and physico-chemical studies (XRD, BET, SEM) for the well-defined inclusion and distribution of lanthanum ion doped TiO₂ in the nanopores of the zeolite matrix. It was also found that the ZLT acted as an efficient heterogeneous catalyst for photo degradation of diuron under direct sunlight for unstirred aquatic systems. The observed DT₅₀ value was 2.01 hours in presence of the catalyst. We also made an attempt to demonstrate the acute toxicity of the catalyst on fishes as per the OECD Guideline no. 203. Based on the results of our study, it could be concluded that ZLT was non toxic to aquatic species. Excellent reproducibility of results was obtained when tested for catalyst reusability.

Acknowledgements

The authors are thankful to the management of IIBAT and Director Dr A. Ramesh for providing necessary facilities during the present investigation. The authors are also thankful to IIT-Chennai, SRM institute of Nanotechnology Potheri, and B. S. Abdur Rahman University, Chennai. We also extend our sincere thanks to Y. Gowthami, Research Scholar, IIBAT for her valuable inputs in the preparation of the manuscript.

References

1. US Environmental Protection Agency, 2009. Candidate Contaminant List 3, accessed on 25 September 2009, http://www.epa.gov/safewater/ccl/ccl3.html.
2. Metabolic Pathways of Agrochemicals. Part 1. Herbicides and Plant Growth Regulators, ed. T. R. Roberts, The Royal Society of Chemistry, Bookcraft (Bath) Ltd., Cambridge, 1998, p. 705.
3. L. Turner, Diuron Analysis of Risks to Endangered and Threatened Salmon and Steelhead, Environmental Field Branch Office of Pesticide Programs, 2003.
4. R. Sayeh, B. Coulomb, J. L. Boudenne and F. Belkadi, Eng. Appl. Sci., 2007, 2, 664–670.
5. S. Zalouk, S. Barbati, M. Sergent and M. Ambrosio, Chemosphere, 2009, 74, 193–199.
6. H. Erkonak, O. O. Sǒgut and M. Akgun, J. Supercrit. Fluids, 2008, 46, 142–148.
7. T. Lennart, C. Harald, B. Elisabet and S. John, Pestic. Outlook, 2002, 13, 108–111.
8. S. Chiron, A. Fernandez-Alba, A. Rodriguez and E. Garcia-Calvo, Water Res., 2000, 34, 366–377.
9. S. Malato, J. Caceres, A. R. Fernandez-Alba, L. Piedra, M. D. Hernando, A. Aguera and A. Vial, Environ. Sci. Technol., 2003, 37, 2516–2524.
10. E. Neyens and J. Bayeans, J. Hazard. Mater., 2003, 98, 33–50.
11. M. A. Tarr, Chemical degradation methods for wastes and pollutants, in Environmental and Industrial Applications, Marcel Dekker Inc., New York, 2003, pp. 165–200.
12. J. J. Pignatello, E. Oliveros and A. MacKay, Environ. Sci. Technol., 2006, 36, 1–84.
13. E. Brillas, I. Sires and M. A. Oturan, Chem. Rev., 2009, 109, 6570–6631.
14. D. Melgoza, A. Hernandez-Ramirez and J. M. Peralta-Hernandez, Photochem. Photobiol. Sci., 2009, 8, 596–599.
15. S. N. Frank and A. J. Bard, J. Am. Chem. Soc., 1977, 99, 303–304.
16. J. W. Verhoeven, Pure Appl. Chem., 1996, 68, 2223–2286.
17. N. Serpone and E. Pelizzetti, Photocatalysis: Fundamentals and Applications, JohnWiley & Sons, New York, NY, USA, 1989.
18. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
19. A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol., C, 2000, 1, 1–21.
20. Z. Zou, J. Ye, K. Sayama and H. Arakawa, Nature, 2002, 414, 625–627.
21. J. Jirkovský, V. Faure and P. Boule, Pestic. Sci., 1997, 50, 42.
22. G. Zhang, W. Choi, S. H. Kim and S. B. Hong, J. Hazard. Mater., 2011, 188, 198–205.
23. Y. Wang, Y. Hao and H. Cheng, et al., J. Mater. Res., 1999, 34, 2773–2779.
24. P. Yang, C. Lu, N. P. Hua and Y. K. Du, Mater. Lett., 2002, 57, 794–801.
25. J. Moon, H. Takagi, Y. Fujishiro and M. Awano, J. Mater. Sci., 2001, 36, 949–955.
26. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
27. A. Pourahmad and S. Sohrabnezhad, Int. J. Nano Dimens., 2010, 1, 143–152.
28. K. Siwinska-Stefanska, D. Paukszta, A. Piazecki and T. Jesionowski, Physicochem. Probl. Miner. Process., 2014, 50, 265–276.
29. C. C. Wang, C. K. Lee, M. D. Lyu and L. C. Juang, Dyes Pigm., 2008, 76, 817–824.
30. S. Anandan and M. Yoon, J. Photochem. Photobiol., C, 2003, 4, 5–18.
31. S. Easwaramoorthi and P. Natarajan, Microporous Mesoporous Mater., 2005, 86, 185–190.
32. G. P. Joshi, N. S. Saxena, T. P. Sharma and S. C. K. Mishra, Indian J. Pure Appl. Phys., 2006, 44, 786–790.
33. D. T. Dimitrov, M. M. Milanova and R. P. Kralshevska, Bulg. Chem. Commun., 2011, 43, 489–501.
34. S. P. Varkey, C. Ratnasamy and P. Ratnasamy, J. Mol. Catal. A: Chem., 1998, 135, 295–306.
35. M. Anpo, Catal. Surv. Jpn., 1997, 1, 169.
36. (a) J. Liu, Q. Zhang, J. Yang, H. Ma, M. O. Tade, S. Wang and J. Liu, Chem. Commun., 2014, 50, 13971–13974; (b) J. Liu, L. Han, H. Ma, H. Tian, J. Yang, Q. Zhang, B. J. Seligmann, S. Wang and J. Liu, Sci. Bull., 2016, 61(19), 1543–1550.
37. M. Salavathi-Niasari, G. Hosseinzadeha and F. Davar, J. Alloys Compd., 2011, 509, 134–140.
38. H. Shimada, N. Matsubayashi, M. Imamura, T. Sato and A. Nishijima, Catal. Lett., 1996, 39, 125–128.
39. Y. Okamoto, M. Ogawa, A. Maezawa and T. Imanaka, J. Catal., 1988, 112, 427–436.
40. OECD Guidelines for Testing of Chemicals (No. 203, Adopted: 17th July 1992) Fish, Acute Toxicity Test, http://www.oecd-ilibrary.org/environment/test-no-203-fish-acute-toxicity-test_9789264069961-en.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21258a

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