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Photocatalytic degradation of methylene blue in a sparged tube reactor with TiO2 fibers prepared by a properly two-step method
Available online at www.sciencedirect.com
Catalysis Communications 9 (2008) 1178–1183 www.elsevier.com/locate/catcom
Photocatalytic degradation of methylene blue in a sparged tube reactor with TiO2 fibers prepared by a properly two-step method Shiying Zhang a,b, Zhenhua Chen b,*, Yunlong Li a, Qun Wang b, Long Wan b a
Department of Chemistry and Environmental Engineering, Changsha University, Hunan 410003, PR China b College of Material Science and Engineering, Hunan University, Hunan 410082, PR China Received 12 January 2007; received in revised form 31 October 2007; accepted 1 November 2007 Available online 19 November 2007
Abstract The photocatalytic oxidation of methylene blue has been performed in the presence of suspended TiO2 fibers prepared by a properly two-step method in an air-sparged tube reactor. The key factors affecting the methylene blue oxidation efficiency were investigated, including the initial concentration of methylene blue, the pH value and the electric power of UV lamp. For photodegradation of methylene blue, it was observed that TiO2 fibers have a higher decomposition efficiency than the self-made TiO2 powders, as well as the commercial P25. The optimal conditions were a concentration of 9 103 mol/l at pH 6 with 40 W of illumination for the fastest rate of methylene blue photodegradation. Diffuse reflectance UV–Vis analysis showed a shift in the threshold absorption value and a corresponding increase in bandgap energy for TiO2 fibers, which is considered to be responsible for the decrease in TiO2 fibers photoactivity after 15 trials. Ó 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 fibers; Two-step method; Methylene blue; Photochemical technology
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources. Various kinds of contaminants enter water, most of them in industrial wastewater. Developments in the field of chemical water purification have led to an improvement in oxidative degradation processes applying catalytic and photochemical methods [1,2]. Recently, environmental purification using TiO2 as a photocatalyst has attracted a great deal of attention because of its high activity, chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the shortcomings of conventional powder catalysts include low efficiency of light use, difficulty of stirring during reaction and separation *
Corresponding author. Tel.: +86 13568410438; fax: +86 731 4261219 (Z. Chen). E-mail address: [email protected] (Z. Chen). 1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.009
after reaction, and low-concentration contamination near TiO2 due to its low surface [5]. These disadvantages of TiO2 result in low efficiency of photocatalytic activity in practical applications. In order to achieve rapid and efficient decomposition of organic pollutants and easy manipulation in a total photocatalytic process, it may be effective to prepare photocatalysts with high surface areas to concentrate the pollutants around the photocatalysts. Therefore, much recent work has focused on the preparation of composites such as TiO2/SiO2, TiO2/zeolite, TiO2/SnO2 and TiO2/polymer [6–9]. Additional, pure TiO2 fibers with high surface area [10–12] have also been prepared. It has been reported that the TiO2 fibers showed high surface area and the efficient degradation of some organic compounds in the photocatalytic process [13]. However, the photoactivity of TiO2 fibers strongly depends on the preparing technology, post treatment and photocatalytic reaction parameters (TiO2 concentration, light intensity, pH value, etc.) [14–17]. By far, performance and structure of TiO2 fibers prepared by a two-step method is few reported.
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Therefore, there is novel and necessary to elucidate the influence of the preparation process, post treatment and reaction conditions on TiO2 fibers photoactivity. This work examined the photocatalytic activity of prepared TiO2 fibers by a properly two-step method for degradation of methylene blue in a sparged tube reactor. We also investigated the effects of parameters such as the initial methylene blue concentration, the electric power of UV lamp and the pH value on the degradation of methylene blue from aqueous solutions in an air-sparged reactor with suspended TiO2 fibers.
Secondly (hydrothermal method), TiO2 powder was put into different content of KOH solution by refluxing process for 72 h at 150 °C, and then was washed by ethanol and hydrochloric acid for three times to produce TiO2 fibers. Finally, the fibers obtained were heat–treated at 550 °C in air for 2 h. In addition, pure TiO2 powders were prepared by the same miniemulsion procedure for tetrabutyl orthotitanate. Suitable concentrations of KOH in refluxing process, heat-treatment temperature and TiO2 powders surface area are key steps for preparing TiO2 fibers with high photoactivity.
1.1. Preparation of TiO2 fibers
1.2. Characterization of the structure
In this paper, TiO2 fibers were prepared by a properly two-step method. A high pressure kettle (WHF20125 L, Weihai Reactor Ltd., China) and analytical reagent grade tetrabutyltitanate, triethylamine, cyclohexanol, hexadecyl trimethyl ammonium bromide (HTAB), acetone, potassium hydroxide and distilled water were used. As shown in Fig. 1, the TiO2 fibers were prepared by a properly two-step method. Firstly (miniemulsion method), TiO2 powders were prepared from A solution reacted with B solution, and then adjusted pH in reaction solution via triethylamine and subsequently washed by acetone for three times. After centrifugation, white precipitation was dried to form TiO2 powder in oven at 450 °C. A solution was synthesized from 10 g tetrabutyltitanate dropped in 50 ml cyclohexanol during stirring. Meanwhile, a certain amount of hexadecyl trimethyl ammonium bromide were dissolved into 20 ml distilled water, and then dropped in 50 ml cyclohexanol to produce B solution.
The structure of the TiO2 fibers was investigated by Raman spectroscopy (super labran, France). The chemical states of titanium and oxygen on the surface and near surface of TiO2 fibers were studied by XPS measurements, using a VG Scientific ESCALAB MarkIIspectrometer (England). The morphology, rate of length to diameter and crystalline structure of the fibers were investigated by TEM (JEOLJEM–2010, Japan) and HRTEM (FEI Tecnai12 operating at 120 kV, or FEI Technai20F operating at 200 kV, Japan).
Tetrabutyltitanate
Cyclohexanol
1.3. Evaluation of the photocatalytic activity Methylene blue was chosen as a model organic compound to evaluate the photoactivity of the prepared TiO2 fibers. Fig. 2 shows a scheme of the experimental equipment (280 ml volume). The main component of the system was the reactor, with internal stirring by air sparging with a
HTAB
A solution
Distilled water
Cyclohexanol
B solution
Stirring
TiO2 miniemulsion
First step (miniemulsion)
Precipitation Wash TiO2 powders Refluxing Wash
TiO2 fibers Fig. 1. The TiO2 fibers prepared by two-step method.
Second step (hydrothermal)
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S. Zhang et al. / Catalysis Communications 9 (2008) 1178–1183 UV− lamp
8000
Water outflow
634.27
Entering materials
Glass wall bleb catalyst Reactant solution Air sparging
Air sparging
Scatter intensity (a.u)
exhaust
6000
392.45
4000
514.10
2000
Cooling water inpour
0 200
Fig. 2. Experimental equipment for photocatalytic reaction.
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2. Results and discussion 2.1. Characterization of the samples Raman spectroscopy is available for clarifying structure of fibers. According to factor group analysis, anatase and rutile have six Raman active modes (A1g + 2B1g + 3Eg) and six Raman active modes (A1g + B1g + B2g + 3Eg), respectively. Fig. 3 shows three typical raman spectra measurements for the fibers prepared with two-step synthesis method. The peaks located at 392.45, 514.10 and 634.27 cm1 was assigned as B1g, A1g (or B1g), and Eg modes in anatase phase. It was obvious that TiO2 in fibers was consisted of anatase. Additional, it is well known that P25 is consisted of anatase (30%) and rutile (70%) [18]. It can be seen from Fig. 4 that TiO2 fibers contains Ti, O and C elements. The photoelectron peak for Ti2p appeared
Fig. 3. Raman spectra of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
O1s Ti2p Ti2s
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OKLL
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flow capacity of 56 ml/s. A 40 W ultraviolet lamp (Institute of Electric Light Source, Beijing) was positioned inside a Pyrex cell at irradiance of 135 mW/cm2. The wavelength range and peak wavelength of the UV lamp were 320– 400 nm and 365 nm, respectively. A 0.5 g sample of the photocatalyst (TiO2 fibers, self-made TiO2 powder or commercial P25) was suspended in 250 ml of air-sparged aqueous solution containing methylene blue in the above Pyrex reaction vessel. The temperature of photocatalytic reaction was maintained at 25 °C by water circulation. To determine the change in methylene blue concentration in solution during the process, a few milliliters of the solution was taken from the reaction mixture, subsequently centrifuged, filtered through a Millipore filter (pore size 0.22 lm) to separate the TiO2 fibers, and loaded in a UV–Vis spectrometer (JascoV-500, Japan). The methylene blue concentration was calculated from the absorbance at 660 nm using a calibration curve. The pH value was controlled by the addition of a weak acid or weak alkali. The bandgap energy of the TiO2 fibers photocatalyst, both fresh and after photocatalytic reaction, was determined using a UV–Vis spectrometer (Specord M40, Carl–Zeiss) equipped with an integrating sphere accessory for diffuse reflectance.
200
0
Binding Energy (eV) Fig. 4. XPS spectra of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
clearly at a binding energy, Eb, of 458 eV, O1s at Eb = 531 eV and C1s at Eb = 284 eV. The element C in the TiO2 fibers was attributed to the residual carbon. Meanwhile, there is not Na element existed in fibers. It can be concluded that the Na+ ions in fibers was all exchanged with H+ ions by means of washing with ethanol and hydrochloric acid. The binding energies of Ti3p, Ti3s and Ti2p of TiO2 fibers were same as those of TiO2 powders [19], indicating the integrity of the TiO2 structure, which was not modified by KOH solutions reflux. The microstructure of TiO2 fibers is shown in Fig. 5. The image shows clearly that large quantity (>95%) fibers materials with about 5 nm in diameter and 100 to several hundreds nanometers in length have been synthesized (Fig. 5a). It is obvious that rate of length to diameter is about 200. Meanwhile, It is observed that the TiO2 fibers have meshy holes from the intercross space which is responsible for a surface area of approximately 257.90 m2/g as determined by Bet surface area (HZG–4 diffractometer, Zeiss, Germany). This is a proper reason why surface area of TiO2 fibers are higher than that of TiO2 powders (about 87.53 m2/g). In addition, as indicated in corresponding HREM image (Fig. 5b), prepared by a properly two-step
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Fig. 5. TEM image (a) and HREM image (b) of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
method and subsequently heat treatment at 550 °C for 2 h, the TiO2 fibers were well crystalline, solid materials with high surface area, which are not the same as nanotube reported. 2.2. Photocatalytic activity of TiO2 fibers Under identical experimental conditions, it was found that only 1.8% of methylene blue in solution absorbed on TiO2 powders in the dark after 1 h, while the amount of methylene blue removed by the TiO2 fibers was 6.5%. This suggests that adsorption of methylene blue is mainly relate on the surface of the TiO2 fibers. The results of methylene blue removal by the photocatalysts are presented in Fig. 6. The remnant rate of the methylene blue decomposed by TiO2 powders is 35% after 200 min under UV radiation. This is due to the low adsorbability of TiO2 powders. However, TiO2 fibers achieved almost 100% methylene blue removal. This seems to suggest that the adsorbing effect of the high surface area is remarkable. To demonstrate further the photocatalytic activity of the TiO2 fibers, photodecomposition of methylene blue was studied using P25. As shown in Fig. 6, the reaction rate
for P25 was increased and the remnant rate for methylene blue decolorized was 18%, but the enhancement was not as great as that obtained for TiO2 fibers. This may be attributed to the high surface area of the TiO2 fibers, which worked well as an effective adsorbant to concentrate methylene blue around the activated sites of TiO2under ultraviolet radiation. Additional, the TiO2 fibers (pure anatase) photoactivity is more than P25 (anatase + rutile) possible due to fast recombination rate of generated electrons and holes of rutile [20]. 2.3. Effect of initial concentration of methylene blue The experiments on photocatalytic oxidation of methylene blue were conducted for solutions with various methylene blue concentrations: 3, 6, 9, 12 and 15 103 mol/l. Fig. 7 shows the changes in methylene blue concentration during the photocatalytic reaction. It is apparent from the results that the reaction time lengthened with increasing methylene blue concentration. In addition, the photodegradation velocity for 9 mM methylene blue was highest among the different initial concentrations. For example, after 4 h of illumination, for a concentration of 3 mM, the degree of methylene blue decomposition was 100%; for a concentration of 6 mM, after the same length of time,
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Time (min) Fig. 6. Photocatalytic degradation properties of TiO2 fibers, TiO2 powders and P25 for 5 mM methylene blue under UV radiation. The amount of the all suspended was 2 g/l ( TiO2 fibers: d; TiO2 powders: j; P25: ).
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Time (h) Fig. 7. Degree of methylene blue decomposition for different concentrations. The amount of the all suspended was 2 g/l (TiO2 fibers: d 15 mM, } 12 mM, N 9 mM, j 6 mM, * 3 mM;TiO2 powders: 15 mM).
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methylene blue loss was 100%. With further increases in concentration to 9, 12 and 15 mM, the degree of methylene blue decomposition was 90.5%, 66.8%, and 33.1%, respectively. These results indicate that when the concentration of methylene blue was 9 103 mol/l, TiO2 fibers were the most effective for methylene blue decomposition. This can be explained as follows. For a certain TiO2 fibers, the amount of active centers on the photocatalysts is finite, so photodegradation rate increases with initial concentration of methylene blue increase before reaching the concentration of about 9 103 mol/l. The photodegradation rate decreases when the concentration of methylene blue is more than 9 103 mol/l, possibly because the molecules of methylene blue is excessive in comparison with the amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. Thus, the photocatalytic process was influenced by the initial concentration of methylene blue. For photocatalytic degradation of 15 mM methylene blue, the degradation rate on TiO2 powders was faster than that on TiO2 fibers in the beginning, while after 3.5 h the degradation rate on TiO2 powders was slower than that on TiO2 fibers. This was possibly because the methylene blue concentration near TiO2 powders was high during the early photocatalytic reaction, so the synergistic effect of the surface area was not apparent. In order to verify the activity of used TiO2 fibers and determine their lifetime, decomposition of 5 mM methylene blue was followed. Results are presented in Fig. 8. For TiO2 fibers photocatalyst after 4 h of illumination, in the third trial, decomposition of methylene blue was 100%, while in the fifth trial decomposition was 93.8%. This value remained at approximately 84.3% during the 15th trial after10 h. This suggests that the lifetime of the fibers was greatly lengthened and the used catalyst retained high photocatalytic activity. This was mainly due to the fact that the microporous in fibers were not blocked for TiO2 decomposition of methylene blue on them, when they provided a high concentration of the organic compound for TiO2 distributed on them by the absorption process.
2.4. Effect of solution pH The pH is an important factor in influencing the photocatalytic process. It is clearly observed that pH 6 is an advantage for the photocatalytic reaction of methylene blue (Fig. 9). The degradation degree of methylene blue for different pH is increasing with reaction time length. Strong acid or alkali is not available for decomposing methylene blue, It is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [21]. The low and high pH values are not available for methylene blue absorption and hydroxy produce on TiO2 fibers, respectively. So there is optimum pH in the photocatalytic process of methylene blue because high concentration hydroxy and plentiful methylene blue which are absorbed on TiO2 fibers are available for the photocatalytic reaction. 2.5. Effect of electric power of the UV lamp Light intensity is a major factor in photocatalytic reactions, because electron–hole pairs are produced by light energy [22]. The relation between electric power of the UV lamp and its light intensity fits direct ratio, when positions of UV lamp and reactor are invariable in Fig. 2, so the electric power of the UV lamp also influences photodegradation of methylene blue during photocatalytic process. Fig. 10 shows an almost linear relation between the electric power and the apparent constant k for methylene blue degradation, which was 0.0102 min1 for the 40 W UV lamp. The concentration of methylene blue decreased with radiation time increase. With increasing electric power of the UV lamp, the decomposition rate of methylene blue increased. For example, the decomposition rate at 40 W was faster than at 25, 15 and 10 W. This was because higher electric power provides higher energy for more TiO2 in the fibers to produce electron–hole pairs. 2.6. Catalyst characterization
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Time (h) Fig. 8. Degree of methylene blue decomposition for initial methylene blue concentration of 5 mM. The amount of the all suspended was 2 g/l (TiO2 fibers: 3rd trial, j 5th trial, N 8th trial, 12th trial, 15th trial).
Concentration/ mmol·L
Degree of decomposition/ %
An important feature of materials used in photocatalytic processes is the bandgap energy. The lower the value of this
5 4 3 2 1 0 0
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Time (min) Fig. 9. Effect of pH on the photodegradation of methylene blue. The concentration of the TiO2 fibers was 2 g/l (pH 2, 4, N 6, d 9).
S. Zhang et al. / Catalysis Communications 9 (2008) 1178–1183
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Time (min) Fig. 10. Effect of electric power of the UV lamp on the photodegradation of methylene blue. The concentration of TiO2 fibers was 2 g/l (Watt: j 10 W, 15 W, d 25 W, N 40 W).
Absorbing intensity (a.u)
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Acknowledgement
References
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idation of methylene blue. In addition, the lifetime of the fibers was considerably lengthened. The initial concentration of methylene blue influenced the photoactivity of TiO2 fibers. With an increase in the initial methylene blue concentrations from 3 to 15 mM, the photocatalytic degradation time lengthened. However, for a methylene blue concentration of 9 103 mol/l, TiO2 fibers showed the highest decomposition velocity. In addition, the pH values and electric power of UV lamp also influenced the photoactivity of TiO2 fibers. Values of pH 6 and 40 W were suitable for methylene blue degradation. The photoactivity of used fibers in methylene blue photodegradation decreased due to bandgap energy increase of TiO2.
The authors would like to express their thanks to the Hunan Provincial Natural Science Foundation of China (No: 06JJ50150, 06JJ4068).
0.25 0.20
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Fig. 11. Diffuse reflectance UV–Vis spectra of photocatalyst sample: (a) fresh TiO2 fibers; used TiO2 fibers, (b) 8th trial used, (c) 12th trial used, (d) 15th trial used).
energy, the broader the range of light that can be absorbed to activate the photocatalyst. Fig. 11 shows derivatives determined from the diffuse reflectance UV–Vis spectra of TiO2 fibers samples. Shift in the threshold absorption value between the fresh and used photocatalyst was observed and the corresponding bandgap energy had changed from 3.18 eV for fresh TiO2 fibers to 3.26 eV for 15-used photocatalyst. It indicated that with used degree increasing, bandgap energy of TiO2 fibers increased and their photoactivity decreased. 3. Conclusions Due to the reciprocity of high surface area, the TiO2 fibers prepared showed high photoactivity for the photoox-
[1] V. Vamathevan, R. Amal, D. Beydoun, G. Low, J. Photochem. Photobiol. A: Chem. 148 (2002) 237. [2] P.J. Senogles, J.A. Scott, G. Shaw, H. Stratton, Water. Res. 35 (2001) 246. [3] J. Yu, X. Zhao, Mater. Res. Bull. 36 (2001) 99. [4] C. Trapalis, A.D. Modestov, O. Lev, J. Mater. Sci. 28 (1993) 1276. [5] K.B. Kyoko, S. Kazuhiro, K. Hitoshi, O. Kiyomi, A. Hironori, Appl. Catal. A: General 165 (1997) 394. [6] Y. Xu, W. Zheng, W. Liu, J. Photochem. Photobiol. A: Chem. 122 (1999) 57. [7] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Appl. Catal. B: Environ. 46 (2003) 458. [8] H. Tada, A. Hattori, Y. Tokihisa, J. Phys. Chem. 104 (2000) 4586. [9] Y. Liu, A. Wang, R. Claus, J. Phys. Chem. 101 (1997) 1385. [10] K. Kamiya, K. Tanimoto, Mater. Sci. Lett. 5 (1986) 403. [11] Y.F. Chen, C.Y. Lee, Mater. Chem. Phys. 81 (2003) 40. [12] Y.Q. Wang, G.Q. Hu, Chem. Phys. Lett. 365 (2002) 429. [13] S. Madhugiri, B. Sun, Panagiotis, G. Smirniotis, Micropor. Mesopor. Mater. 69 (2004) 82. [14] A. Fujishima, K. Honda, Nature 238 (1972) 37. [15] C.M. Jian, F.O. David, H.R. Wim, B. Harry, Water. Res. 33 (1999) 1177. [16] B.G. Miray, O. Lhan, Water. Sci. Technol. 33 (1996) 189. [17] M.R. Hoffman, S.T. Martin, W. Choi, Chem. Rev. 95 (1995) 70. [18] M.V. Shankar, K.K. Cheralathan, A. Banumathi, J. Mol. Catal. A: Chem. 223 (2004) 195. [19] Y.J. Li, X.D. Li, J.W. Li, J. Yin, Mater. Lett. 59 (2005) 2660. [20] A. Sclafani, L. Palmisano, M. Schiavello, J. Phys. Chem. 94 (1990) 829. [21] M.S. Kim, K.M. Hong, J.G. Chung, Water. Res. 37 (2003) 3524. [22] T. Zhang, T. Oyama, A. Aoshima, J. Photohem. Photobiol. A: Chem. 140 (2001) 163.
Catalysis Communications 9 (2008) 1178–1183 www.elsevier.com/locate/catcom
Photocatalytic degradation of methylene blue in a sparged tube reactor with TiO2 fibers prepared by a properly two-step method Shiying Zhang a,b, Zhenhua Chen b,*, Yunlong Li a, Qun Wang b, Long Wan b a
Department of Chemistry and Environmental Engineering, Changsha University, Hunan 410003, PR China b College of Material Science and Engineering, Hunan University, Hunan 410082, PR China Received 12 January 2007; received in revised form 31 October 2007; accepted 1 November 2007 Available online 19 November 2007
Abstract The photocatalytic oxidation of methylene blue has been performed in the presence of suspended TiO2 fibers prepared by a properly two-step method in an air-sparged tube reactor. The key factors affecting the methylene blue oxidation efficiency were investigated, including the initial concentration of methylene blue, the pH value and the electric power of UV lamp. For photodegradation of methylene blue, it was observed that TiO2 fibers have a higher decomposition efficiency than the self-made TiO2 powders, as well as the commercial P25. The optimal conditions were a concentration of 9 103 mol/l at pH 6 with 40 W of illumination for the fastest rate of methylene blue photodegradation. Diffuse reflectance UV–Vis analysis showed a shift in the threshold absorption value and a corresponding increase in bandgap energy for TiO2 fibers, which is considered to be responsible for the decrease in TiO2 fibers photoactivity after 15 trials. Ó 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 fibers; Two-step method; Methylene blue; Photochemical technology
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources. Various kinds of contaminants enter water, most of them in industrial wastewater. Developments in the field of chemical water purification have led to an improvement in oxidative degradation processes applying catalytic and photochemical methods [1,2]. Recently, environmental purification using TiO2 as a photocatalyst has attracted a great deal of attention because of its high activity, chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the shortcomings of conventional powder catalysts include low efficiency of light use, difficulty of stirring during reaction and separation *
Corresponding author. Tel.: +86 13568410438; fax: +86 731 4261219 (Z. Chen). E-mail address: [email protected] (Z. Chen). 1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.11.009
after reaction, and low-concentration contamination near TiO2 due to its low surface [5]. These disadvantages of TiO2 result in low efficiency of photocatalytic activity in practical applications. In order to achieve rapid and efficient decomposition of organic pollutants and easy manipulation in a total photocatalytic process, it may be effective to prepare photocatalysts with high surface areas to concentrate the pollutants around the photocatalysts. Therefore, much recent work has focused on the preparation of composites such as TiO2/SiO2, TiO2/zeolite, TiO2/SnO2 and TiO2/polymer [6–9]. Additional, pure TiO2 fibers with high surface area [10–12] have also been prepared. It has been reported that the TiO2 fibers showed high surface area and the efficient degradation of some organic compounds in the photocatalytic process [13]. However, the photoactivity of TiO2 fibers strongly depends on the preparing technology, post treatment and photocatalytic reaction parameters (TiO2 concentration, light intensity, pH value, etc.) [14–17]. By far, performance and structure of TiO2 fibers prepared by a two-step method is few reported.
S. Zhang et al. / Catalysis Communications 9 (2008) 1178–1183
1179
Therefore, there is novel and necessary to elucidate the influence of the preparation process, post treatment and reaction conditions on TiO2 fibers photoactivity. This work examined the photocatalytic activity of prepared TiO2 fibers by a properly two-step method for degradation of methylene blue in a sparged tube reactor. We also investigated the effects of parameters such as the initial methylene blue concentration, the electric power of UV lamp and the pH value on the degradation of methylene blue from aqueous solutions in an air-sparged reactor with suspended TiO2 fibers.
Secondly (hydrothermal method), TiO2 powder was put into different content of KOH solution by refluxing process for 72 h at 150 °C, and then was washed by ethanol and hydrochloric acid for three times to produce TiO2 fibers. Finally, the fibers obtained were heat–treated at 550 °C in air for 2 h. In addition, pure TiO2 powders were prepared by the same miniemulsion procedure for tetrabutyl orthotitanate. Suitable concentrations of KOH in refluxing process, heat-treatment temperature and TiO2 powders surface area are key steps for preparing TiO2 fibers with high photoactivity.
1.1. Preparation of TiO2 fibers
1.2. Characterization of the structure
In this paper, TiO2 fibers were prepared by a properly two-step method. A high pressure kettle (WHF20125 L, Weihai Reactor Ltd., China) and analytical reagent grade tetrabutyltitanate, triethylamine, cyclohexanol, hexadecyl trimethyl ammonium bromide (HTAB), acetone, potassium hydroxide and distilled water were used. As shown in Fig. 1, the TiO2 fibers were prepared by a properly two-step method. Firstly (miniemulsion method), TiO2 powders were prepared from A solution reacted with B solution, and then adjusted pH in reaction solution via triethylamine and subsequently washed by acetone for three times. After centrifugation, white precipitation was dried to form TiO2 powder in oven at 450 °C. A solution was synthesized from 10 g tetrabutyltitanate dropped in 50 ml cyclohexanol during stirring. Meanwhile, a certain amount of hexadecyl trimethyl ammonium bromide were dissolved into 20 ml distilled water, and then dropped in 50 ml cyclohexanol to produce B solution.
The structure of the TiO2 fibers was investigated by Raman spectroscopy (super labran, France). The chemical states of titanium and oxygen on the surface and near surface of TiO2 fibers were studied by XPS measurements, using a VG Scientific ESCALAB MarkIIspectrometer (England). The morphology, rate of length to diameter and crystalline structure of the fibers were investigated by TEM (JEOLJEM–2010, Japan) and HRTEM (FEI Tecnai12 operating at 120 kV, or FEI Technai20F operating at 200 kV, Japan).
Tetrabutyltitanate
Cyclohexanol
1.3. Evaluation of the photocatalytic activity Methylene blue was chosen as a model organic compound to evaluate the photoactivity of the prepared TiO2 fibers. Fig. 2 shows a scheme of the experimental equipment (280 ml volume). The main component of the system was the reactor, with internal stirring by air sparging with a
HTAB
A solution
Distilled water
Cyclohexanol
B solution
Stirring
TiO2 miniemulsion
First step (miniemulsion)
Precipitation Wash TiO2 powders Refluxing Wash
TiO2 fibers Fig. 1. The TiO2 fibers prepared by two-step method.
Second step (hydrothermal)
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S. Zhang et al. / Catalysis Communications 9 (2008) 1178–1183 UV− lamp
8000
Water outflow
634.27
Entering materials
Glass wall bleb catalyst Reactant solution Air sparging
Air sparging
Scatter intensity (a.u)
exhaust
6000
392.45
4000
514.10
2000
Cooling water inpour
0 200
Fig. 2. Experimental equipment for photocatalytic reaction.
300
400
600
500
700
800
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2. Results and discussion 2.1. Characterization of the samples Raman spectroscopy is available for clarifying structure of fibers. According to factor group analysis, anatase and rutile have six Raman active modes (A1g + 2B1g + 3Eg) and six Raman active modes (A1g + B1g + B2g + 3Eg), respectively. Fig. 3 shows three typical raman spectra measurements for the fibers prepared with two-step synthesis method. The peaks located at 392.45, 514.10 and 634.27 cm1 was assigned as B1g, A1g (or B1g), and Eg modes in anatase phase. It was obvious that TiO2 in fibers was consisted of anatase. Additional, it is well known that P25 is consisted of anatase (30%) and rutile (70%) [18]. It can be seen from Fig. 4 that TiO2 fibers contains Ti, O and C elements. The photoelectron peak for Ti2p appeared
Fig. 3. Raman spectra of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
O1s Ti2p Ti2s
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Ti3s
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flow capacity of 56 ml/s. A 40 W ultraviolet lamp (Institute of Electric Light Source, Beijing) was positioned inside a Pyrex cell at irradiance of 135 mW/cm2. The wavelength range and peak wavelength of the UV lamp were 320– 400 nm and 365 nm, respectively. A 0.5 g sample of the photocatalyst (TiO2 fibers, self-made TiO2 powder or commercial P25) was suspended in 250 ml of air-sparged aqueous solution containing methylene blue in the above Pyrex reaction vessel. The temperature of photocatalytic reaction was maintained at 25 °C by water circulation. To determine the change in methylene blue concentration in solution during the process, a few milliliters of the solution was taken from the reaction mixture, subsequently centrifuged, filtered through a Millipore filter (pore size 0.22 lm) to separate the TiO2 fibers, and loaded in a UV–Vis spectrometer (JascoV-500, Japan). The methylene blue concentration was calculated from the absorbance at 660 nm using a calibration curve. The pH value was controlled by the addition of a weak acid or weak alkali. The bandgap energy of the TiO2 fibers photocatalyst, both fresh and after photocatalytic reaction, was determined using a UV–Vis spectrometer (Specord M40, Carl–Zeiss) equipped with an integrating sphere accessory for diffuse reflectance.
200
0
Binding Energy (eV) Fig. 4. XPS spectra of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
clearly at a binding energy, Eb, of 458 eV, O1s at Eb = 531 eV and C1s at Eb = 284 eV. The element C in the TiO2 fibers was attributed to the residual carbon. Meanwhile, there is not Na element existed in fibers. It can be concluded that the Na+ ions in fibers was all exchanged with H+ ions by means of washing with ethanol and hydrochloric acid. The binding energies of Ti3p, Ti3s and Ti2p of TiO2 fibers were same as those of TiO2 powders [19], indicating the integrity of the TiO2 structure, which was not modified by KOH solutions reflux. The microstructure of TiO2 fibers is shown in Fig. 5. The image shows clearly that large quantity (>95%) fibers materials with about 5 nm in diameter and 100 to several hundreds nanometers in length have been synthesized (Fig. 5a). It is obvious that rate of length to diameter is about 200. Meanwhile, It is observed that the TiO2 fibers have meshy holes from the intercross space which is responsible for a surface area of approximately 257.90 m2/g as determined by Bet surface area (HZG–4 diffractometer, Zeiss, Germany). This is a proper reason why surface area of TiO2 fibers are higher than that of TiO2 powders (about 87.53 m2/g). In addition, as indicated in corresponding HREM image (Fig. 5b), prepared by a properly two-step
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Fig. 5. TEM image (a) and HREM image (b) of TiO2 fibers prepared by the two-step method and heat treatment at 550 °C.
method and subsequently heat treatment at 550 °C for 2 h, the TiO2 fibers were well crystalline, solid materials with high surface area, which are not the same as nanotube reported. 2.2. Photocatalytic activity of TiO2 fibers Under identical experimental conditions, it was found that only 1.8% of methylene blue in solution absorbed on TiO2 powders in the dark after 1 h, while the amount of methylene blue removed by the TiO2 fibers was 6.5%. This suggests that adsorption of methylene blue is mainly relate on the surface of the TiO2 fibers. The results of methylene blue removal by the photocatalysts are presented in Fig. 6. The remnant rate of the methylene blue decomposed by TiO2 powders is 35% after 200 min under UV radiation. This is due to the low adsorbability of TiO2 powders. However, TiO2 fibers achieved almost 100% methylene blue removal. This seems to suggest that the adsorbing effect of the high surface area is remarkable. To demonstrate further the photocatalytic activity of the TiO2 fibers, photodecomposition of methylene blue was studied using P25. As shown in Fig. 6, the reaction rate
for P25 was increased and the remnant rate for methylene blue decolorized was 18%, but the enhancement was not as great as that obtained for TiO2 fibers. This may be attributed to the high surface area of the TiO2 fibers, which worked well as an effective adsorbant to concentrate methylene blue around the activated sites of TiO2under ultraviolet radiation. Additional, the TiO2 fibers (pure anatase) photoactivity is more than P25 (anatase + rutile) possible due to fast recombination rate of generated electrons and holes of rutile [20]. 2.3. Effect of initial concentration of methylene blue The experiments on photocatalytic oxidation of methylene blue were conducted for solutions with various methylene blue concentrations: 3, 6, 9, 12 and 15 103 mol/l. Fig. 7 shows the changes in methylene blue concentration during the photocatalytic reaction. It is apparent from the results that the reaction time lengthened with increasing methylene blue concentration. In addition, the photodegradation velocity for 9 mM methylene blue was highest among the different initial concentrations. For example, after 4 h of illumination, for a concentration of 3 mM, the degree of methylene blue decomposition was 100%; for a concentration of 6 mM, after the same length of time,
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Remnant rate/ %
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Time (min) Fig. 6. Photocatalytic degradation properties of TiO2 fibers, TiO2 powders and P25 for 5 mM methylene blue under UV radiation. The amount of the all suspended was 2 g/l ( TiO2 fibers: d; TiO2 powders: j; P25: ).
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Time (h) Fig. 7. Degree of methylene blue decomposition for different concentrations. The amount of the all suspended was 2 g/l (TiO2 fibers: d 15 mM, } 12 mM, N 9 mM, j 6 mM, * 3 mM;TiO2 powders: 15 mM).
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methylene blue loss was 100%. With further increases in concentration to 9, 12 and 15 mM, the degree of methylene blue decomposition was 90.5%, 66.8%, and 33.1%, respectively. These results indicate that when the concentration of methylene blue was 9 103 mol/l, TiO2 fibers were the most effective for methylene blue decomposition. This can be explained as follows. For a certain TiO2 fibers, the amount of active centers on the photocatalysts is finite, so photodegradation rate increases with initial concentration of methylene blue increase before reaching the concentration of about 9 103 mol/l. The photodegradation rate decreases when the concentration of methylene blue is more than 9 103 mol/l, possibly because the molecules of methylene blue is excessive in comparison with the amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. Thus, the photocatalytic process was influenced by the initial concentration of methylene blue. For photocatalytic degradation of 15 mM methylene blue, the degradation rate on TiO2 powders was faster than that on TiO2 fibers in the beginning, while after 3.5 h the degradation rate on TiO2 powders was slower than that on TiO2 fibers. This was possibly because the methylene blue concentration near TiO2 powders was high during the early photocatalytic reaction, so the synergistic effect of the surface area was not apparent. In order to verify the activity of used TiO2 fibers and determine their lifetime, decomposition of 5 mM methylene blue was followed. Results are presented in Fig. 8. For TiO2 fibers photocatalyst after 4 h of illumination, in the third trial, decomposition of methylene blue was 100%, while in the fifth trial decomposition was 93.8%. This value remained at approximately 84.3% during the 15th trial after10 h. This suggests that the lifetime of the fibers was greatly lengthened and the used catalyst retained high photocatalytic activity. This was mainly due to the fact that the microporous in fibers were not blocked for TiO2 decomposition of methylene blue on them, when they provided a high concentration of the organic compound for TiO2 distributed on them by the absorption process.
2.4. Effect of solution pH The pH is an important factor in influencing the photocatalytic process. It is clearly observed that pH 6 is an advantage for the photocatalytic reaction of methylene blue (Fig. 9). The degradation degree of methylene blue for different pH is increasing with reaction time length. Strong acid or alkali is not available for decomposing methylene blue, It is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [21]. The low and high pH values are not available for methylene blue absorption and hydroxy produce on TiO2 fibers, respectively. So there is optimum pH in the photocatalytic process of methylene blue because high concentration hydroxy and plentiful methylene blue which are absorbed on TiO2 fibers are available for the photocatalytic reaction. 2.5. Effect of electric power of the UV lamp Light intensity is a major factor in photocatalytic reactions, because electron–hole pairs are produced by light energy [22]. The relation between electric power of the UV lamp and its light intensity fits direct ratio, when positions of UV lamp and reactor are invariable in Fig. 2, so the electric power of the UV lamp also influences photodegradation of methylene blue during photocatalytic process. Fig. 10 shows an almost linear relation between the electric power and the apparent constant k for methylene blue degradation, which was 0.0102 min1 for the 40 W UV lamp. The concentration of methylene blue decreased with radiation time increase. With increasing electric power of the UV lamp, the decomposition rate of methylene blue increased. For example, the decomposition rate at 40 W was faster than at 25, 15 and 10 W. This was because higher electric power provides higher energy for more TiO2 in the fibers to produce electron–hole pairs. 2.6. Catalyst characterization
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Time (h) Fig. 8. Degree of methylene blue decomposition for initial methylene blue concentration of 5 mM. The amount of the all suspended was 2 g/l (TiO2 fibers: 3rd trial, j 5th trial, N 8th trial, 12th trial, 15th trial).
Concentration/ mmol·L
Degree of decomposition/ %
An important feature of materials used in photocatalytic processes is the bandgap energy. The lower the value of this
5 4 3 2 1 0 0
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Time (min) Fig. 9. Effect of pH on the photodegradation of methylene blue. The concentration of the TiO2 fibers was 2 g/l (pH 2, 4, N 6, d 9).
S. Zhang et al. / Catalysis Communications 9 (2008) 1178–1183
K×102min-1
Concentration/mol·L-1
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Time (min) Fig. 10. Effect of electric power of the UV lamp on the photodegradation of methylene blue. The concentration of TiO2 fibers was 2 g/l (Watt: j 10 W, 15 W, d 25 W, N 40 W).
Absorbing intensity (a.u)
a b c d
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Acknowledgement
References
0.10
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idation of methylene blue. In addition, the lifetime of the fibers was considerably lengthened. The initial concentration of methylene blue influenced the photoactivity of TiO2 fibers. With an increase in the initial methylene blue concentrations from 3 to 15 mM, the photocatalytic degradation time lengthened. However, for a methylene blue concentration of 9 103 mol/l, TiO2 fibers showed the highest decomposition velocity. In addition, the pH values and electric power of UV lamp also influenced the photoactivity of TiO2 fibers. Values of pH 6 and 40 W were suitable for methylene blue degradation. The photoactivity of used fibers in methylene blue photodegradation decreased due to bandgap energy increase of TiO2.
The authors would like to express their thanks to the Hunan Provincial Natural Science Foundation of China (No: 06JJ50150, 06JJ4068).
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Fig. 11. Diffuse reflectance UV–Vis spectra of photocatalyst sample: (a) fresh TiO2 fibers; used TiO2 fibers, (b) 8th trial used, (c) 12th trial used, (d) 15th trial used).
energy, the broader the range of light that can be absorbed to activate the photocatalyst. Fig. 11 shows derivatives determined from the diffuse reflectance UV–Vis spectra of TiO2 fibers samples. Shift in the threshold absorption value between the fresh and used photocatalyst was observed and the corresponding bandgap energy had changed from 3.18 eV for fresh TiO2 fibers to 3.26 eV for 15-used photocatalyst. It indicated that with used degree increasing, bandgap energy of TiO2 fibers increased and their photoactivity decreased. 3. Conclusions Due to the reciprocity of high surface area, the TiO2 fibers prepared showed high photoactivity for the photoox-
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