La-doped TiO 2 hollow fibers and their photocatalytic activity under UV and visible light

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1 Indian Journal of Chemistry Vol. 51A, August 2012, pp La-doped TiO 2 hollow fibers and their photocatalytic activity under UV and visible light Shuhua Yao, Xueying Jia, Lulu Jiao, Chuang Zhu & Zhongliang Shi* School of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang , China shzhl2000@163.com Received 20 March 2012; revised and accepted 19 July 2012 A series of La-doped TiO 2 photocatalytic material with a hollow fiber structure has been successfully prepared using La(NO 3 ) 3 6H 2 O and Ti(OC 4 H 9 ) 4 as precursors and cotton fiber as the template. Scanning electron microscopy, X-ray diffraction, N2 adsorption-desorption measurements, UV-vis diffuse absorption spectra and X-ray photoelectron spectroscopy have been used to characterize the morphology, crystal structure, surface structure and optical absorption properties of the samples. The photocatalytic activity of the samples with varying amounts of La-doped has been studied by photodegradation of methyl orange in water under UV and visible light. Results show that La doping restrains the growth of grain size and extends the photoabsorption edge of TiO 2 hollow fiber into the visible light region. The fiber structured material shows better photocatalytic properties for the degradation of methyl orange than pure TiO 2 under UV and visible light. The amount of La-doping significantly affected the catalytic property. Under the experiment conditions, the photocatalytic activity of 0.5 mol% La-doped TiO 2 fiber is optimal amongst the studied samples. The prepared La-doped TiO 2 fiber is a low energy consuming, high activity and green environmental-friendly catalytic material. Keywords: Photocatalysts, Titania, Lanthanum doping, Fiber templates, Photocatalytic activity, Methyl orange Within the ecosystem, colored wastewater released in textile effluents is a major source of pollution, eutrophication and disturbance in aquatic life. Moreover, a variety of organic chemicals are produced during the dyeing process, and some have been shown to be carcinogenic 1. Methyl orange (MO) a representative dye compound, includes the benzene ring, which shows a strong inhibitive function for biologic degradation, and hence is very difficult to be degraded into smaller inorganic molecules by using common methods. With the growing awareness about decreasing available water resources, many methods, including physical, chemical, and biological methods, are being used for wastewater treatment and recycling. Among these, heterogeneous photocatalysis appears to be an emerging destructive technology leading to the total mineralization of most organic pollutants 2-4. TiO 2 is a widely used photocatalyst due to its noticeable photocatalytic properties such as strong oxidizing power, nontoxicity, and long term photostability 5. Its applications for air clean-up, water purification, antibacteria, deodorization disinfection sheet, soil-proofing household furnishing, antialgal oil-proofing plate, antifogging coating, deodorant fiber etc. are widely studied 6. However, practical applications of photocatalysis are limited because of two obvious problems arising from using fine TiO 2 powders, i.e., (i) low photoefficiency, and, (ii) separation of fine particles of used TiO 2 after the treatment process and the recycling of the photocatalyst 7. An efficient process that effectively improves the photocatalytic activity of fine TiO 2 powder is doping. Doping can extend the light absorption towards the visible light range and prevent the recombination of electron-hole pairs 8-10, and hence improve the photocatalytic performance. Rare earth metals having incompletely occupied 4f and empty 5d orbitals often serve as catalyst or promote catalysis. Some results showed that the photocatalytic activity of TiO 2 can be improved by doping with some rare earth metals Besides, it has been proved that lanthanum as one of the rare earth metals, has the ability to enhance the photocatalytic activity of TiO 2 14,15. Lanthanide doping increases the surface area, pore volume, adsorption capacity for organic compounds as well as suppresses electron-hole recombination rates during the process of photocatalytic reaction 16,17. In field applications of the photocatalyst, the second problem can be eliminated by use of supported TiO 2. Therefore, much recent work has been devoted

2 1050 INDIAN J CHEM, SEC A, AUGUST 2012 to immobilize TiO 2 photocatalyst on porous supporting matrices, such as silica 18, alumina 19, zeolites 20, activated carbon (AC) 21,22 and activated carbon fiber (ACF) 23,24. However these methods largely decrease the surface area of TiO 2, which is disadvantage (smaller the surface area, lower is the photocatalytic activity) for use in decomposing waste. The TiO 2 fibers show good application in wastewater disposal due to their high surface area and easy recovery besides promising photoelectrochemical applications, as well as applications as solar cell and gas sensors. The common methods of synthesizing TiO 2 fibers include sol-gel method 25, hydrothermal method 26, and two-step synthesis method reported by Zhang et al. 27 In this work, a series of La-doped TiO 2 hollow fibers have been prepared by template method using La(NO 3 ) 3 6H 2 O and Ti(OC 4 H 9 ) 4 as precursors and cotton fiber as template. The effect of concentration of La-doping on the photocatalytic activity of TiO 2 hollow fibers is investigated. The photocatalytic activity has been evaluated by photodegradation of MO over the prepared samples under UV and visible light irradiation. Additionally, the cyclic performance of TiO 2 hollow fibers for the degradation of MO has also checked. Materials and Methods Preparation and characterization of photocatalyst TiO 2 hollow fiber was prepared by a sol-gel method at low temperature using Ti(OC 4 H 9 ) 4 (Analytic Grade, Shanghai Xingta Co. Ltd, China) as precursor and cotton fiber as the template. Ti(OC 4 H 9 ) 4 (0.02 mol) was added to anhydrous ethanol (50 ml) under vigorous stirring, followed by addition of triethylamine (0.01 mol) as a stabilizer. The solution was stirred at 200 rpm for 2 3 min under an inert atmosphere by flowing argon. A second solution was prepared by addition of hydrochloric acid (3.0 ml) and water (0.72 ml) to anhydrous ethanol (50 ml) and mixed well by a magnetic stirrer at 200 rpm. The two solutions were then mixed together and stirred vigorously for 30 min under flowing argon. The TiO 2 sol formed was transparent, quite stable and highly sensitive to triethylamine and water. The cotton fiber (1.0 g) was then immersed in the TiO 2 containing liquid sol for 1 h. The TiO 2 sol/cotton fiber turned into TiO 2 gel/cotton fiber after aging for 7 days at 25 ºC. The TiO 2 hollow fiber catalyst was obtained by desiccation of the TiO 2 gel/cotton fiber system at 100 ºC for 6 h and then calcination at 500 ºC in air atmosphere for 2 h. The La-doped TiO 2 hollow fibers were synthesized by a similar method. The appropriate amount of La(NO 3 ) 3 6H 2 O was dissolved in anhydrous ethanol, which was added dropwise into the distilled water prior to the hydrolysis of Ti(OC 4 H 9 ) 4 under vigorous stirring. Scanning electron microscopic (SEM) microimages of TiO 2 hollow fiber and 0.5 mol% La-doped TiO 2 fiber were obtained on a Philips XL-30 (25 kv, LaB6 filament) scanning electron microscope. Wideangle X-ray powder diffraction (XRD) patterns of all samples were obtained at room temperature with a Rigaku D/Max-RB X-ray diffractometer using Cu-Kα radiation which operated at 45 kv and 40 ma. The size of the crystallite was calculated from X-ray line broadening from the Scherrer equation, D = 0.89 λ/βcosθ, where D is the average crystal size in nm, λ is the Cu-Kα wavelength ( nm), β is the full-width at half-maximum, and, θ is the diffraction angle. Specific surface area and pore volume of the catalysts were calculated using the BET method from nitrogen adsorption-desorption isotherms measured at 77 K with a Micromeritics 2000 instrument (ASAP 2000, Micromeritics, USA). The diffuse absorption spectra were recorded on a UV-2550 spectrophotometer. Reflectance spectra were referenced to BaSO 4. The band gap energies (Eg) of the prepared photocatalysts were calculated by the formula Eg = /λg from the wavelength values corresponding to the intersection point of the vertical and horizontal parts of the spectra (λg) 28. A Thermo Electron Corporation Theta Probe equipped with Mg-Kα excitation was employed to record the X-ray photoelectron spectra of the 0.5 mol% La-doped TiO 2 fiber. Detailed scans were recorded for Ti 2p, O 1s and La 3d. Photocatalytic efficiency The photocatalytic activity of the prepared catalysts under UV light was estimated by measuring the degradation rate of MO (50 mg L -1 ) in aqueous solution. Experiments were carried out using a magnetically stirred quartz reactor and an ultraviolet mercury lamp (150 W, 365 nm) as the light source at ambient temperature of ca. 20 C. The ph of the suspension was maintained constant (8 ± 0.5) by adjusting either with dilute 0.01 mol L -1 HNO 3 or 0.1 mol L -1 NaOH throughout the tests. Sixty minute adsorption time in dark condition was allowed before the start of photoreactions. Then, samples of the suspension were withdrawn after a definite time

3 YAO et al.: PHOTOCATALYTIC ACTIVITY OF La-DOPED TiO 2 HOLLOW FIBERS 1051 (a) (b) Fig. 1 SEM images of (a) pure TiO 2 fiber, and, (b) 0.5 mol% La-doped TiO 2 fiber samples. interval and filtered through 0.22 µm filter paper. The filtrates were analyzed for residual MO using a UV-vis spectrophotometer (UV762, Shanghai Analysis Co.) at nm. All the experiments were carried out in triplicate. The results presented herein are the mean values with a total error of less than 5 %. For comparison, the pure TiO 2 fiber prepared by the same method and Degussa P25 were used as reference systems. The amount of TiO 2 fiber and P25 was chosen as 1.0 g L -1, which was adequate under the present conditions without disturbing the UV light entering the reactor. The La-doped TiO 2 fiber sample was used repeatedly, and each cycle lasted 4 h. Before the beginning of the next cycle, the remaining solution was replaced with 50 mg L -1 fresh MO solution. The experiments with visible light irradiation were performed at ambient temperature of ca. 20 C by using a 150 W metal halide lamp as the light source. To limit the irradiation wavelength, the light beam was passed through a 410 nm cut filter (L41) to ensure cutoff wavelengths shorter than 410 nm. Then, samples of the suspension were withdrawn after a definite time interval and filtered through 0.22 µm filter paper. Results and Discussion SEM and TEM analyses Figure 1 shows the SEM images of the pure and 0.5 mol% La-doped TiO 2 fibers. It shows that all products possess a fibrous shape inherited from the cotton fiber. The cotton-like products are about 10 µm in size, and have similar width and are at least several hundred nanometers long. Figure 1 clearly shows that the prepared TiO 2 fibers have a hollow structure. The TEM image of the 0.5 mol% La-doped TiO 2 fiber shown in Fig. 2 also indicates that the TiO 2 fiber is effectively obtained. Fig. 2 TEM photograph of 0.5 mol% La-doped TiO 2 fiber. XRD analyses To obtain information on the crystal structure of the La-doped photocatalysts, their X-ray diffraction patterns were recorded. The XRD patterns of prepared samples are shown in Fig. 3. The diffraction peaks at 25.28, 37.80, 48.05, 53.37, 62.63, and are consistent with the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 5), (2 2 0) and (3 0 1) lattice planes of TiO 2, which are attributed to the anatase phase. The La-doped photocatalysts have anatase TiO 2 structures with low intensity due to the smaller crystallite size as compared to the pure TiO 2 fiber. The XRD results suggest that the La-doping has little influence on the nature of crystal formation. No characteristic peak of La oxide was found in the XRD patterns, implying that either the La ions were incorporated in the crystallinity of TiO 2, or, the La oxide was very small and highly dispersed. In addition, the mean crystallite size was calculated as a function of peak width, specified as the full width at half maximum peak intensity based on the Scherrer equation. The physical properties determined from

4 1052 INDIAN J CHEM, SEC A, AUGUST 2012 XRD data of the samples were listed in Table 1. The crystallite size decreases because of the doping, which implies that La doping restrained the increase in grain size and refined crystallite size. The change of crystal parameters being more in the doped samples as compared to in the undoped implies that the crystal matrix of the doped samples can be expanded. In fact, it seems impossible for La 3+ to be incorporated in the titania matrix because of the mismatch of the ionic radii of Ti 4+ and La 3+. It is expected that the surrounding La ions will inhibit the phase transition of anatase-rutile due to the formation of a Ti-O-La bond. On the other hand, the La 2 O 3 lattice locks the Ti-O species at the interface with titania domains, preventing the nucleation that is necessary for the transformation of anatase to rutile, resulting in the decrease of the crystallite size of TiO Nitrogen adsorption-desorption As shown in Table 2, the La-doped samples had higher surface area than pure TiO 2 fiber, with the 0.5 mol% La-doped sample displaying the highest specific surface area of m 2 g -1 and the highest pore volume of cm 3 g -1. When the La/Ti percentage increased beyond 0.5 mol%, a decrease in the surface area was observed. The N 2 adsorptiondesorption isotherms and Barret-Joyner-Halenda (BJH) pore size distribution plots (calculated from the adsorption branch) of pure TiO 2 fiber and 0.5 mol% La-doped TiO 2 fiber are shown in Fig. 4. The adsorption-desorption isotherms of all samples have a Sample Table 1 XRD analyses of the prepared samples Crystallite size (nm) Lattice parameters a (nm) c (nm) V (nm 3 ) TiO 2 fiber mol% La-TiO 2 fiber mol% La-TiO 2 fiber mol% La-TiO 2 fiber Fig. 3 XRD patterns of TiO 2 fiber samples. [1, pure TiO 2 fiber; 2, 0.4 mol% La-doped TiO 2 fiber; 3, 0.5 mol% La-doped TiO 2 fiber; 4, 0.6 mol% La-doped TiO 2 fiber]. Table 2 Physicochemical properties of the prepared samples Sample S BET (m 2 /g ) Total pore vol. (cm 3 /g) Band gap energy (ev) TiO 2 fiber mol% La-TiO 2 fiber mol% La-TiO 2 fiber mol% La-TiO 2 fiber Fig. 4 (a) N 2 adsorption-desorption isotherrms of TiO 2 fiber samples. [1, pure TiO 2 fiber; 2, 0.5 mol% La-doped TiO 2 fiber]. (b) BJH pore size distributions of TiO 2 fiber samples. [1, pure TiO 2 fiber; 2, 0.5 mol% La-doped TiO 2 fiber].

5 YAO et al.: PHOTOCATALYTIC ACTIVITY OF La-DOPED TiO 2 HOLLOW FIBERS 1053 hysteresis loop between 0.4 and 0.8 relative pressure, indicating the formation of mesopores. The narrow pore size distribution range indicates that the present materials have uniform pore channels and most pores have a diameter of about 8.0 nm. UV-vis absorption spectra UV-vis absorption spectra were recorded to characterize the light absorption ability of the prepared photocatalysts (Supplementary Data Fig. S1). The UV-vis absorption spectra show the influence of La ion on the UV-vis absorption. Modification of TiO 2 fiber with La ion significantly affected the absorption properties of photocatalysts. It was noticeable that light absorption in the visible region ( nm) was higher than that of pure TiO 2 fiber, and the light absorption increased with increasing La ion content. The band gap energies (Eg) of the prepared photocatalysts calculated by the formula Eg = /λg are listed in Table 2. For pure TiO 2 fiber prepared without any dopant, the Eg was 3.19 ev. In the case of La-doped TiO 2 fibers, Eg decreased from 3.05 to 2.79 ev, depending on the dopant amount. The lowest Eg (2.79 ev) was observed for sample 0.5 mol% La-doped TiO 2 fiber (see Table 2). The narrowing of band gaps of the prepared La-doped TiO 2 fibers may be due to the interaction of TiO 2 with La in lower oxidation state, suggesting that they are likely to be responsive to visible light. La ion modified photocatalyst caused absorption spectra to shift to the visible region implying that La-doping was favourable to visible light absorption. For pure TiO 2, the absorption in the ultraviolet range (λ = 378 nm) was associated with the excitation of the O 2p electron to the Ti 3d level. The red shift of the absorption spectrum can be ascribed to the charge transfer between the TiO 2 valence or conduction band and the La ion 4f level 30. As a result, the La-doped samples have trapping levels which decrease the TiO 2 band gap. The La ions in the doped photocatalysts increase the visible light absorption ability of the photocatalysts. Moreover, due to the broad absorption band of the rare earths, their effect on being incorporated into the TiO 2 is similar to adding a photosensitizer to the reaction solution. Therefore, the La ions surrounding the TiO 2 fiber grains could absorb a larger range of light radiation, which brings about the higher light absorption in the nm region of the La-doped samples 31. This extended absorbance indicates the possible enhancement in the photocatalytic activity of mesoporous TiO 2 illuminated by visible light. X-ray photoelectron spectroscopy In the XPS spectrum of Ti 2p of 0.5 mol% La-doped TiO 2 fiber, the Ti 2p peaks appear as a single, well defined, spin-split doublet with the typical interval of 6 ev between its two peaks which corresponds to Ti 4+ in a tetragonal structure (i.e., Ti 2p1/2 and Ti 2p3/2) (Supplementary Data Fig. S2). The binding energies of the peaks within the doublet were found to be ev for Ti 2p1/2 and ev for Ti 2p3/2. The XPS spectrum of O of TiO 2 shows an intense signal at about ev due to O 2- ions of TiO 2. This peak is attributed to the Ti-O in TiO 2 and OH groups on the surface of the catalysts 32,33. The XPS spectrum of La 3d shows that La 3+ ions incorporated into TiO 2 lattice interact with oxidic sites of TiO 2 (Ti-O-La) and appear to provide a broadened spectrum of La 3d. Photocatalytic activity Photocatalytic activity of La-doped TiO 2 fibers was estimated by measuring the degradation of MO in the presence of UV and visible light irradiation without looking into the degradation intermediates in detail. Pure TiO 2 fiber synthesized by the same method and Degussa P25 were used as the reference systems. Effect of La-doping MO degradation under UV irradiation in the presence of La-doped TiO 2 fibers with La-doped content in the range of mol% was investigated. The photocatalytic activity of TiO 2 fibers enhanced after La-doping with the optimum content of La-doping being 0.5 mol% (Fig. 5). It is proposed that the presence of the La ions on the TiO 2 fibers influenced the photocatalytic activity by creating oxygen vacancies. Crystal expansion in crystal matrix of La-doped TiO 2 fibers is observed obviously as shown in in Table 1. The expansion in the crystal matrix creates oxygen vacancies, which generates low energy states at the bottom of the conduction band and serves as electron trap sites in nanocrystalline TiO 2. The low energy states introduced by rare earth ion in the top valence band also serve as hole trap sites 34. The separation of the charge carriers is attributed to such trapping. Subsequently, the charge carriers transfer to the surface of photocatalyst initiating redox reactions, and

6 1054 INDIAN J CHEM, SEC A, AUGUST 2012 Fig. 5 Effect of La-doped content on the degradation rate of MO. [Cond.: Initial conc. of MO: 50 mg/l; catalyst: 1.0 g/l; calcination temp. 500 C. Curve 1, TiO 2 fiber; 2, 0.1 mol% La-TiO 2 fiber; 3, 0.2 mol% La-TiO 2 fiber; 4, 0.3 mol% La-TiO 2 fiber; 5, 0.4 mol% La-TiO 2 fiber; 6, 0.5 mol% La-TiO 2 fiber; 7, 0.6 mol% La-TiO 2 fiber]. thereby promoting photocatalytic activity of doped TiO 2 fibers. The UV-vis absorption spectra also show that La-doping improved the ultraviolet absorption and the red-shift of the absorption profile (see Fig. S1), which further improves the photocatalytic activity of TiO 2 fibers. However, the photoactivity of the catalyst decreases when the content of La-doping reached 0.6 mol%. This may be attributed to the surrounding lanthanum ions, which act as recombination centers for the photogenerated electrons and holes at high doping. Increase in available lanthanum ions with the higher La-doping concentration will lead to lower photocatalytic activity. Effect of UV and visible light irradiation The degradation percentages of MO with varying irradiation time under UV irradiation in the presence of La-doped TiO 2 fiber is illustrated in Fig. 6(a). It can be clearly seen that the degradation of MO with TiO 2 fibers is relatively higher as compared to that for Degussa P25. Maximum degradation of MO was shown by La-doped TiO 2 fiber. In the present study, photocatalytic reaction mechanism for the degradation of MO over La-doped photocatalyst is proposed as follows: Under light illumination, La-doped TiO 2 fibers are exposed and the electrons are excited from the valence band state to conduction band state. Chen et al. 35 reported that the introduction of new impurity states due to the Fig. 6 Degradation curves of MO under (a) UV irradiation, and, (b) visible light with varying irradiation time. [Cond.: Initial conc. of MO: 50 mg/l; catalyst: 1.0 g/l; calcination temp.: 500 C. Curve 1, P-25 Degussa; 2, TiO 2 fiber; 3, 0.5 moi% La-TiO 2 fiber]. incorporation of La 3+ decreased the recombination of photogenerated electrons and holes. Hence, in the present study, it is proposed that the excited electrons in the conduction band transfered to the La 3+ states and the holes present in the valence band available for the photocatalytic oxidation of MO. The rare earth metal (here, La) which usually act as a reservoir for photogenerated electrons, promotes an efficient charge separation in La-doped TiO 2 photocatalysts. In addition, La-doping increases the ultraviolet absorption and the red-shift of the absorption profile (see Fig. S1), which improved the photocatalytic activity of TiO 2 fibers. Degradation of MO under visible light was studied and is shown in Fig. 6(b). It is observed that La-doped

7 YAO et al.: PHOTOCATALYTIC ACTIVITY OF La-DOPED TiO 2 HOLLOW FIBERS 1055 samples exhibit much higher photocatalytic activity than pure TiO 2 fiber and Degussa P25 under visible light. This may be due to the positive effect of La 3+ on the photocatalytic activity since the La-doping not only increased the surface area of TiO 2 fiber but also extended its photoabsorption to visible light region. The surface area is one of the key factors to control the photocatalytic activity of a photocatalyst. The larger the surface area, higher is the photocatalytic activity. From the UV-vis absorption spectra of samples (Fig. S1), it may be seen that the intensity and region of visible light photoabsorption on La-doped TiO 2 fiber was stronger and larger, respectively than those of pure TiO 2 fiber. The strong absorption is beneficial to the photocatalytic activity because the available photons are proportional to photoabsorption. In addition, the surrounding lanthanum ions influence the activity significantly by acting as recombination centers for photogenerated electrons and holes at high doping, thus decreasing the photocatalytic activity. The highest photocatalytic activity of the 0.5 mol% La-doped TiO 2 fiber under visible light irradiation may be a combination of the above factors. Recyclability of the La-doped TiO 2 fiber catalyst Four cycles of photocatalytic degradation of MO were carried out with the La-doped TiO 2 fiber photocatalyst. The photocatalytic reactivity of the present photocatalyst was only slightly reduced in stirred aqueous solution, and the La-doped TiO 2 fiber. After being used four times, ca. 91 % of photocatalytic activity of the fresh sample remained (Supplementary Data Fig. S3). The degradation percentage of MO reached % when irradiation time was 4 h. This indicates that the final removal of MO from solutions is caused by the photocatalytic degradation rather than the adsorption process that leads to saturated adsorption of MO on the photocatalyst. These results indicated that cyclic usage of the La-doped TiO 2 fiber was possible and its stability in treating polluted water was satisfactory. Therefore, it shows potential for application in continuous photocatalytic degradation processes. Conclusions A composite photocatalyst of La-doped TiO 2 fiber with a hollow structure was successfully synthesized by template method using La(NO 3 ) 3 6H 2 O and Ti(OC 4 H 9 ) 4 as precursor and cotton fiber as the template, followed by calcination at 500 ºC in air atmosphere for 2 h. The photocatalyst thus prepared was applied for degradation of methyl orange. The results showed that La-doping enhanced the photocatalytic activity of TiO 2 fibers under UV and visible light irradiation. The 0.5 mol% La-doped sample exhibited the highest photocatalytic activity for MO degradation. The La-doping caused absorption spectra of TiO 2 fibers to shift to the visible region. Factors such as surface area, La-doping content and photoabsorption affect the photocatalytic activity of La-doped TiO 2 fibers. Acknowledgement The authors gratefully acknowledge financial support for this work from National Natural Science Foundation of China, China (No ). Supplementary Data Supplementary data associated with this article, viz., Figs S1 S3, are available in the electronic form at 51A(08) _Suppl Data.pdf. References 1 Lachheb H, Puzenat E, Houas A, Ksibi M, Elaloui E, Guillard C & Herrmann J M, Appl Catal B, 39 (2002) Fukahori S, Ichiura H, Kitaoka T & Tanaka H, Appl Catal B, 46 (2003) Alhakimi G, Studnicki L H & Al-Ghazali M, J Photochem Photobiol A, 154 (2003) Shiraishi F, Yamaguchi S & Ohbuchi Y, Chem Eng Sci, 58 (2003) Tryba B, Morawski A W & Inagaki M, Appl Catal B, 46 (2003) Shimizu K, Imai H, Hirashima H & Tsukuma K, Thin Solid Films, 351 (1999) Rachel A, Subrahmanyam M & Boule P, Appl Catal B, 37 (2002) De Vos D E, Dams M, Sels B F & Jacobs P A, Chem Rev, 102 (2002) Asahi R, Morikawa T, Ohwaki T, Aoki K & Taga Y, Science, 293 (2001) Sakthivel S & Kisch H, Angew Chem Ent Ed, 42 (2003) Xu A W, Gao Y & Liu H Q, J Catal, 207 (2002) Xie Y & Yuan C, Appl Catal B, 46 (2003) Shi Z L, Liu F M & Yao S H, J Rare Earths, 28 (2010) Ranjit K T, Willner I, Bossmann S H & Braun A M, Environ Sci Technol, 35 (2001) Ando T, Wakamatsu T, Masuda K, Yoshida N, Suzuki K, Masutani S, Katayama I, Uchida H, Hirose H & Kamimot A, Appl Surf Sci, 255 (2009) Kim H R, Lee T G & Shul Y G, J Photochem Photobiol A, 185 (2007) Li F B, Li X Z & Hou M F, Appl Catal B, 48 (2004) Alemany L J, Banares M A, Pardo E, Martin F, Galan- Fereres M & Blasco J M, Appl Catal B, 13 (1997) 289.

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