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Highly ordered TiO2 nanotube array as recyclable catalyst for the sonophotocatalytic degradation of methylene blue
Catalysis Communications 10 (2009) 1188–1191
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Highly ordered TiO2 nanotube array as recyclable catalyst for the sonophotocatalytic degradation of methylene blue Shuai Yuan a,*, Le Yu a, Liyi Shi a,b,*, Jun Wu a, Jianhui Fang a,b, Yin Zhao a a b
Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 10 January 2009 Accepted 15 January 2009 Available online 20 January 2009 Keywords: TiO2 nanotube array Ultrasound Photocatalytic Synergy Recyclability
a b s t r a c t TiO2 nanotube array was prepared by the anodic oxidation method and was characterized by XRD and TEM. The inner pore diameter, wall thickness and length of TiO2 nanotube are about 60 nm, 19 nm and 1.93 lm, respectively. The porosity and specific surface area calculated are 55% and 27.4 m2 g1, respectively. The sonophotocatalytic activity of TiO2 nanotube array was evaluated by the degradation of methylene blue. The results show that the frequency has great effect on the synergy between sonolytic and photocatalytic degradation. At 27 kHz, the synergistic effect is as high as 22.1%. The catalyst also shows good recyclability. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is widely investigated as a promising photocatalyst to purify waste waters. A lot of achievements about the photocatalytic reaction have been obtained from the researches based on TiO2 nanoparticles [1,2]. However, the main drawback of TiO2 nanoparticles during practical applications is the inevitable difficulty for recycling the photocatalysts, which is a more important problem than the photocatalytic activity. Porous TiO2 with large secondary structure or TiO2 immobilized on other substrates can be separated easily from reactants [3–5]. Since Grimes and coworkers firstly reported the formation of highly ordered TiO2 nanotube array via anodic oxidation of Ti sheet in a hydrofluoric electrolyte [6], the preparation of TiO2 nanotube array attracts many attentions because the radii, lengths, wall thicknesses of the ordered pores perpendicular to the substrate are easy to be modified by choosing suitable electrochemical conditions [7]. The applications of TiO2 nanotube array as photoelectrode in the dye sensitized solar cell (DSC) have received wide investigation [8,9]. Recently, TiO2 nanotube array is also used as photocatalyst for the controllable porous structure. For example, Fujishima and coworkers investigated the photocatalytic degradation of phenol combined with extra electric field [10]. In practice, the transparency of waste water is always very low and the light cannot penetrate into the water deeply. In contrast,
* Corresponding authors. Tel./fax: +86 21 66134852. E-mail addresses: [email protected] (S. Yuan), [email protected] (L. Shi). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.01.016
ultrasound (sound with a frequency greater than 20 kHz) can penetrate deeply into the water without the influence of transparency. The organic compounds in aqueous solutions can be decomposed by sonolytic degradation in the presence of oxides [11,12]. And some reports have shown that the combination of ultrasound and photocatalysis leads to high efficiency of decomposing organic pollutants [13–15]. However, most of the works are based on the TiO2 particles. The synergistic effect of ultrasound and photocatalysis based on TiO2 nanotube array still needs more investigation in detail. In this paper, the effects of ultrasound frequencies on the synergistic effect between sonolytic degradation and photocatalytic degradation were investigated. And the recycle performance was also investigated. 2. Experimental section 2.1. Chemicals Titanium foil (99.5%, 0.3 mm thick) was purchased from Shanghai Lushi metal materials limited company. Potassium fluoride (KF), sodium sulfate (Na2SO4), sulfuric acid (H2SO4), and absolute alcohol (C2H5OH) were all AR grade and used without any pretreatment. 2.2. Preparation of TiO2 nanotube array film Titanium foil was treated by ultrasonic cleaning in C2H5OH and deionized water for 30 min, respectively. Highly ordered TiO2
S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191
nanotube array was prepared by anodization method in a twoelectrode electrochemical cell. The titanium foil (3 cm 7 cm) was used as a working electrode. The interval between working electrode and counter electrode (stainless steel foil, 3 cm 7 cm) was 3 cm. The voltage was applied by a stabilized direct-current power supply (GPS-2302C, Insteck). The TiO2 nanotube array film was formed by anodizing the titanium foil in 136 mL solution containing 0.12 mol L1 KF and 1.0 mol L1 Na2SO4 at 20 V for 6 h. H2SO4 was used to adjust the pH value of electrolyte in the range of 4.5–5.0. After the anodic oxidation, the TiO2 nanotube array film was washed by using deionized water to remove the electrolyte. At last, the film was calcined at 673 K for 2 h. 2.3. Characterization Glancing angle X-ray diffraction of TiO2 nanotube array film was measured by DLMAX-2550 diffractometer with glancing angle 2° (Cu Ka radiation, k = 1.5406 Å). The nanotube structures were observed on scanning electron microscope JSM-6700F. 2.4. Catalytic activity The reactor was a cylindrical quartz vessel with a quartz cooling jacket. The apparatus CY-5D (China Ningbo Xinzhi) with five emission frequencies (18, 20, 24, 27, and 30 kHz) was used as the ultrasound source. The catalytic activity of catalysts was evaluated by the degradation of methylene blue (MB). The prepared TiO2 nanotube array film was immersed in 100 mL of aqueous MB solution (6.0 mg L1). After stirring under dark conditions for 30 min, the solution was irradiated by ultrasonic. In the case of investigating the synergy of ultrasonic and UV light, the MB solution was irradiated by ultrasound and UV light (16 W UV lamp, kmax = 365 nm) together. The progress of the reactions was monitored by a spectrophotometer (Spectrumlab 22pc, Shanghai Lengguang). 3. Results and discussion 3.1. The morphology and crystalline phase of TiO2 nanotube array film Fig. 1 shows the SEM images of TiO2 nanotube array prepared by the anodic oxidation method. After calcination at 673 K for
1189
2 h in airflow, the ordered tube array structure is still reserved. The inset SEM image of TiO2 nanotube after recycle uses shows that the ordered structure can sustain the impact of ultrasound. The inner pore diameter is about 60 nm, the wall thickness is about 19 nm and the tube length is about 1.93 lm. The film porosity P of a hexagonal nanotube array can be determined from
pffiffiffi 2 P ¼ 1 2pðdp w þ w2 Þ=ð 3l Þ
ð1Þ
where P, dp, w and l are porosity, inner pore diameter, wall thickness and center-to-center distance between nanotubes, respectively [8]. The specific surface area of TiO2 nanotube array As can be given by As = 2/(wq) that is deduced from the equations described by Jennings and coworkers, where q is the density of anatase [9]. For the sample presented in Fig. 1, the porosity is estimated to be 55% and the specific area is about 27.4 m2 g1. After the anodic oxidation, an amorphous TiO2 nanotube array film was obtained, and there is no characteristic peaks belonging to crystallized TiO2 in the glancing angle XRD pattern. After being annealed at 673 K in airflow for 2 h, the new peaks at 2h = 25.4°, 37.0°, etc. (shown in Fig. 2) assigned to anatase phase TiO2 (PDF 89-4921) appeared. Combined with the SEM analysis results, the phase transition from amorphous to anatase did not destroy the order of nanotube array, indicating that the pore walls of TiO2 nanotube array are consisted of anatase nanocrystals. The anatase nanocrystal size calculated by Scherrer formula is about 17 nm. 3.2. Catalytic activity of TiO2 nanotube array film As shown in Fig. 3A, the degradation of methylene blue under ultrasound irradiation with different frequency was investigated. Under every frequency, the Ln(C0/C) value possesses a good linearity against the irradiation time, suggesting the ultrasonic degradation processes follow the first-order kinetics. The reaction rate constants are listed in Table 1. When the frequency increases from 27 to 30 kHz, the reaction rate constant decreases sharply from 7.2 104 to 1.4 104 min1. It has been reported that the sonication of water produces OH radicals in the acoustic cavitation process, and the frequency of ultrasound has great effects on the acoustic cavitation [16,17]. The resonance size of an acoustically cavitation bubble (Rr) is in reverse ration to the ultrasonic frequency [16]. The bubble size at 30 kHz is only 0.6 times the size at 18 kHz, which means the energy resulted from bubble collapse
Fig. 1. The top-viewed (left) and cross-sectional (right) SEM images of TiO2 nanotube array after calicination at 673 K for 2 h. The SEM image of TiO2 nanotube array after recycle uses (inset).
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S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191 Table 1 Reaction rate constants (k) under different reaction conditions (104 min1). Reaction conditions
Ultrasound Ultrasound + TiO2 Ultrasound + UV + TiO2
k under different ultrasound frequency 18 kHz
20 kHz
24 kHz
27 kHz
30 kHz
0.9 3.3 4.3
3.1 3.6 2.4
4.1 20.1 26.5
7.2 23.3 35.6
1.4 1.9 5.6
The power of ultrasound was fixed at 24 W.
Fig. 2. The glancing angle XRD pattern of TiO2 nanotube array after calcination at 673 K for 2 h.
may change in a wide range. In this investigation, the optimal frequency for the ultrasonic degradation of MB is 27 kHz in the range from 18 to 30 kHz. Fig. 3B shows the first-order kinetic plots of MB degradation under the irradiation of varied frequency ultrasound in the presence of TiO2 nanotube array film. Compared with the reaction rate constants in the case of sole ultrasound, the reaction rate constants in
the presence of TiO2 nanotube array film increased. This phenomenon is similar with the previous reports on the TiO2 nanoparticles [18–20]. The most suitable frequency is still 27 kHz, and the reaction rate constant is 23.3 104 min1. Previous researches show that the presence of metal oxides can increase the performance of ultrasonic degradation due to the generation of holes on the surface of metal oxides [11,12,18–20]. When the system was irradiated by ultrasound and UV light simultaneously, the reaction rate constants present in Fig. 3C and Table 1 also show extremely high degradation rate at 27 kHz. The histogram in Fig. 3D shows the degradation for 3 h. The synergistic effect between ultrasound and UV light is evaluated by the following equation [21]:
DUSþUVþTiO2 Synergy ¼ 1 DUSþTiO2 þ DUVþTiO2
ð2Þ
Here DUSþTiO2 , DUVþTiO2 and DUSþUVþTiO2 represent the degradation of methylene blue in the case of ultrasonic degradation, photocatalytic degradation and sonophotocatalytic degradation, respectively. At 24 and 27 kHz, the synergistic effects calculated by Eq. (1) are 12.4% and 22.1%, respectively. At 18 and 20 kHz, the synergistic ef-
Fig. 3. First-order kinetic plots of methylene blue (MB) degradation under varied frequency ultrasound without TiO2 nanotube array film (A), in the presence of TiO2 nanotube array film (B), in the presence of TiO2 nanotube array film and UV light irradiation (C). And the degradation of MB solution under different condition after 3 h (D).
S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191
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of ultrasound without peeling off and works well as recyclable catalyst. The frequencies of ultrasound have great effects on the synergy of sonolytic and photocatalytic degradation of methylene blue. TiO2 nanotube array shows high sonophotocatalytic activity at 27 kHz and the synergy is as high as 22.1%. So, TiO2 nanotube array can be used as recyclable catalyst for the sonodegradation and sonophotodegradation of waste waters.
Acknowledgements
Fig. 4. Duplicated experiments of methylene blue (MB) degradation under varied frequency ultrasound in the presence of TiO2 nanotube array film.
fects are not observed. At 30 kHz, the degradation rate is so much lower than that at 27 kHz, although the synergistic effect calculated (35.0%) is higher than that at 27 kHz. Previous reports suggested that the synergy of UV light and ultrasound irradiation are caused by the ultrasonic dispersion of catalyst particles, the enhancement of mass-transfer between the bulk liquid and the surface of catalyst, the formation of OH radicals from H2O2 produced by photocatalysts [13]. In this study, the catalyst is TiO2 nanotube array fixed on Ti substrate, and ultrasound has no effect on the dispersion of catalyst. Ultrasound was found to improve the mass-transfer coefficient by creating high-speed microscopic turbulence at solid–liquid interface in a porous structure [22,23]. In addition, higher frequencies favor OH radical production. However, the resonant bubbles will not grow large enough to produce enough energy during collapse to form sufficient numbers of OH radicals at high frequencies. So, there is an optimal frequency for the formation of OH radical [16]. In this experiment, the reaction rate constants of degradation in the presence or absence of TiO2 and UV light are significantly higher at 27 kHz than at other frequencies. It can be concluded that OH production increases with the increasing frequency from 18 to 27 kHz and decreases when the frequency increases to 30 kHz. So 27 kHz is more favorable for the mass-transfer and formation of OH radicals. Fig. 4 shows the degradation of methylene blue in duplicated experiments. The results reveal that ultrasound does not have obvious effects on the repeatability of degradation experiments at different frequencies and the combination between TiO2 nanotube array and Ti substrate is strong enough to sustain the impact of ultrasound. Actually, the ICP–AES analysis result confirmed that there was no TiO2 nanotube array film peeling off from the substrate in the methylene blue solution. 4. Conclusion The TiO2 nanotube array prepared by anodic oxidation combines with the Ti substrate strongly enough to sustain the impact
The Project Sponsored by Academic Leader Program of Shanghai Science and Technology Committee (07XD14014); Technical Innovation Team Project of Shanghai Science and Technology Committee (06DZ05902); Key Subject of Shanghai Municipal Education Commission (J50102); Shanghai-unilever Research and Development Fund (06SU07001); The Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; Innovation Program of Shanghai Municipal Education Commission (08YZ09); Science Foundation for The Excellent Youth Scholars of University (Shanghai); Innovative Foundation of Shanghai University.
References [1] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. [3] S. Yuan, Q. Sheng, J. Zhang, H. Yamashita, D. He, Micropor. Mesopor. Mater. 111 (2008) 501. [4] S. Yuan, L. Shi, K. Mori, H. Yamashita, Micropor. Mesopor. Mater. 117 (2009) 356. [5] Q. Zhang, W. Fan, L. Gao, Appl. Catal. B: Environ. 76 (2007) 168. [6] D. Gong, C.A. Grimes, O.K. Varghese, W. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. [7] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Solar Energy Mater. Solar Cells 90 (2006) 2011. [8] K. Zhu, N.R.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [9] J.R. Jennings, A. Ghicov, L.M. Peter, P. Schmuki, A.B. Walker, J. Am. Chem. Soc. 130 (2008) 13364. [10] Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D.A. Tryk, T. Murakami, A. Fujishima, J. Phys. Chem. C 112 (2008) 253. [11] P. Theron, P. Pichat, C. Guillard, C. Petrier, T. Chopin, Phys. Chem. Chem. Phys. 1 (1999) 4663. [12] M. Abbasi, N.R. Asl, J. Hazard. Mater. 153 (2008) 942. [13] A. Maezawa, H. Nakadoi, K. Suzuki, T. Furusawa, Y. Suzuki, S. Uchida, Ultrason. Sonochem. 14 (2007) 615. [14] C. Berberidou, I. Poulios, N.P. Xekoukoulotakis, D. Mantzavinos, Appl. Catal. B: Environ. 74 (2007) 63. [15] A. Paleologou, H. Marakas, N.P. Xekoukoulotakis, A. Moya, Y. Vergara, N. Kalogerakis, P. Gikas, D. Mantzavinos, Catal. Today 129 (2007) 136. [16] Y.G. Adewuyi, Environ. Sci. Technol. 39 (2005) 3409. [17] Y.G. Adewuyi, Environ. Sci. Technol. 39 (2005) 8557. [18] M. Kubo, K. Matsuoka, A. Takahashi, N. Shibasaki-Kitakawa, T. Yonemoto, Ultrason. Sonochem. 12 (2005) 263. [19] A. Nakajima, H. Sasaki, Y. Kameshima, K. Okada, H. Harada, Ultrason. Sonochem. 14 (2007) 197. [20] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Ultrason. Sonochem. 14 (2007) 184. [21] E. Selli, Phys. Chem. Chem. Phys. 4 (2002) 6123. [22] B.S. Schueller, R.T. Yang, Ind. Eng. Chem. Res. 40 (2001) 4912. [23] X. Wang, J.C. Yu, H.Y. Yip, L. Wu, P.K. Wong, S.Y. Lai, Chem. Eur. J. 11 (2005) 2997.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Highly ordered TiO2 nanotube array as recyclable catalyst for the sonophotocatalytic degradation of methylene blue Shuai Yuan a,*, Le Yu a, Liyi Shi a,b,*, Jun Wu a, Jianhui Fang a,b, Yin Zhao a a b
Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 10 January 2009 Accepted 15 January 2009 Available online 20 January 2009 Keywords: TiO2 nanotube array Ultrasound Photocatalytic Synergy Recyclability
a b s t r a c t TiO2 nanotube array was prepared by the anodic oxidation method and was characterized by XRD and TEM. The inner pore diameter, wall thickness and length of TiO2 nanotube are about 60 nm, 19 nm and 1.93 lm, respectively. The porosity and specific surface area calculated are 55% and 27.4 m2 g1, respectively. The sonophotocatalytic activity of TiO2 nanotube array was evaluated by the degradation of methylene blue. The results show that the frequency has great effect on the synergy between sonolytic and photocatalytic degradation. At 27 kHz, the synergistic effect is as high as 22.1%. The catalyst also shows good recyclability. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is widely investigated as a promising photocatalyst to purify waste waters. A lot of achievements about the photocatalytic reaction have been obtained from the researches based on TiO2 nanoparticles [1,2]. However, the main drawback of TiO2 nanoparticles during practical applications is the inevitable difficulty for recycling the photocatalysts, which is a more important problem than the photocatalytic activity. Porous TiO2 with large secondary structure or TiO2 immobilized on other substrates can be separated easily from reactants [3–5]. Since Grimes and coworkers firstly reported the formation of highly ordered TiO2 nanotube array via anodic oxidation of Ti sheet in a hydrofluoric electrolyte [6], the preparation of TiO2 nanotube array attracts many attentions because the radii, lengths, wall thicknesses of the ordered pores perpendicular to the substrate are easy to be modified by choosing suitable electrochemical conditions [7]. The applications of TiO2 nanotube array as photoelectrode in the dye sensitized solar cell (DSC) have received wide investigation [8,9]. Recently, TiO2 nanotube array is also used as photocatalyst for the controllable porous structure. For example, Fujishima and coworkers investigated the photocatalytic degradation of phenol combined with extra electric field [10]. In practice, the transparency of waste water is always very low and the light cannot penetrate into the water deeply. In contrast,
* Corresponding authors. Tel./fax: +86 21 66134852. E-mail addresses: [email protected] (S. Yuan), [email protected] (L. Shi). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.01.016
ultrasound (sound with a frequency greater than 20 kHz) can penetrate deeply into the water without the influence of transparency. The organic compounds in aqueous solutions can be decomposed by sonolytic degradation in the presence of oxides [11,12]. And some reports have shown that the combination of ultrasound and photocatalysis leads to high efficiency of decomposing organic pollutants [13–15]. However, most of the works are based on the TiO2 particles. The synergistic effect of ultrasound and photocatalysis based on TiO2 nanotube array still needs more investigation in detail. In this paper, the effects of ultrasound frequencies on the synergistic effect between sonolytic degradation and photocatalytic degradation were investigated. And the recycle performance was also investigated. 2. Experimental section 2.1. Chemicals Titanium foil (99.5%, 0.3 mm thick) was purchased from Shanghai Lushi metal materials limited company. Potassium fluoride (KF), sodium sulfate (Na2SO4), sulfuric acid (H2SO4), and absolute alcohol (C2H5OH) were all AR grade and used without any pretreatment. 2.2. Preparation of TiO2 nanotube array film Titanium foil was treated by ultrasonic cleaning in C2H5OH and deionized water for 30 min, respectively. Highly ordered TiO2
S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191
nanotube array was prepared by anodization method in a twoelectrode electrochemical cell. The titanium foil (3 cm 7 cm) was used as a working electrode. The interval between working electrode and counter electrode (stainless steel foil, 3 cm 7 cm) was 3 cm. The voltage was applied by a stabilized direct-current power supply (GPS-2302C, Insteck). The TiO2 nanotube array film was formed by anodizing the titanium foil in 136 mL solution containing 0.12 mol L1 KF and 1.0 mol L1 Na2SO4 at 20 V for 6 h. H2SO4 was used to adjust the pH value of electrolyte in the range of 4.5–5.0. After the anodic oxidation, the TiO2 nanotube array film was washed by using deionized water to remove the electrolyte. At last, the film was calcined at 673 K for 2 h. 2.3. Characterization Glancing angle X-ray diffraction of TiO2 nanotube array film was measured by DLMAX-2550 diffractometer with glancing angle 2° (Cu Ka radiation, k = 1.5406 Å). The nanotube structures were observed on scanning electron microscope JSM-6700F. 2.4. Catalytic activity The reactor was a cylindrical quartz vessel with a quartz cooling jacket. The apparatus CY-5D (China Ningbo Xinzhi) with five emission frequencies (18, 20, 24, 27, and 30 kHz) was used as the ultrasound source. The catalytic activity of catalysts was evaluated by the degradation of methylene blue (MB). The prepared TiO2 nanotube array film was immersed in 100 mL of aqueous MB solution (6.0 mg L1). After stirring under dark conditions for 30 min, the solution was irradiated by ultrasonic. In the case of investigating the synergy of ultrasonic and UV light, the MB solution was irradiated by ultrasound and UV light (16 W UV lamp, kmax = 365 nm) together. The progress of the reactions was monitored by a spectrophotometer (Spectrumlab 22pc, Shanghai Lengguang). 3. Results and discussion 3.1. The morphology and crystalline phase of TiO2 nanotube array film Fig. 1 shows the SEM images of TiO2 nanotube array prepared by the anodic oxidation method. After calcination at 673 K for
1189
2 h in airflow, the ordered tube array structure is still reserved. The inset SEM image of TiO2 nanotube after recycle uses shows that the ordered structure can sustain the impact of ultrasound. The inner pore diameter is about 60 nm, the wall thickness is about 19 nm and the tube length is about 1.93 lm. The film porosity P of a hexagonal nanotube array can be determined from
pffiffiffi 2 P ¼ 1 2pðdp w þ w2 Þ=ð 3l Þ
ð1Þ
where P, dp, w and l are porosity, inner pore diameter, wall thickness and center-to-center distance between nanotubes, respectively [8]. The specific surface area of TiO2 nanotube array As can be given by As = 2/(wq) that is deduced from the equations described by Jennings and coworkers, where q is the density of anatase [9]. For the sample presented in Fig. 1, the porosity is estimated to be 55% and the specific area is about 27.4 m2 g1. After the anodic oxidation, an amorphous TiO2 nanotube array film was obtained, and there is no characteristic peaks belonging to crystallized TiO2 in the glancing angle XRD pattern. After being annealed at 673 K in airflow for 2 h, the new peaks at 2h = 25.4°, 37.0°, etc. (shown in Fig. 2) assigned to anatase phase TiO2 (PDF 89-4921) appeared. Combined with the SEM analysis results, the phase transition from amorphous to anatase did not destroy the order of nanotube array, indicating that the pore walls of TiO2 nanotube array are consisted of anatase nanocrystals. The anatase nanocrystal size calculated by Scherrer formula is about 17 nm. 3.2. Catalytic activity of TiO2 nanotube array film As shown in Fig. 3A, the degradation of methylene blue under ultrasound irradiation with different frequency was investigated. Under every frequency, the Ln(C0/C) value possesses a good linearity against the irradiation time, suggesting the ultrasonic degradation processes follow the first-order kinetics. The reaction rate constants are listed in Table 1. When the frequency increases from 27 to 30 kHz, the reaction rate constant decreases sharply from 7.2 104 to 1.4 104 min1. It has been reported that the sonication of water produces OH radicals in the acoustic cavitation process, and the frequency of ultrasound has great effects on the acoustic cavitation [16,17]. The resonance size of an acoustically cavitation bubble (Rr) is in reverse ration to the ultrasonic frequency [16]. The bubble size at 30 kHz is only 0.6 times the size at 18 kHz, which means the energy resulted from bubble collapse
Fig. 1. The top-viewed (left) and cross-sectional (right) SEM images of TiO2 nanotube array after calicination at 673 K for 2 h. The SEM image of TiO2 nanotube array after recycle uses (inset).
1190
S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191 Table 1 Reaction rate constants (k) under different reaction conditions (104 min1). Reaction conditions
Ultrasound Ultrasound + TiO2 Ultrasound + UV + TiO2
k under different ultrasound frequency 18 kHz
20 kHz
24 kHz
27 kHz
30 kHz
0.9 3.3 4.3
3.1 3.6 2.4
4.1 20.1 26.5
7.2 23.3 35.6
1.4 1.9 5.6
The power of ultrasound was fixed at 24 W.
Fig. 2. The glancing angle XRD pattern of TiO2 nanotube array after calcination at 673 K for 2 h.
may change in a wide range. In this investigation, the optimal frequency for the ultrasonic degradation of MB is 27 kHz in the range from 18 to 30 kHz. Fig. 3B shows the first-order kinetic plots of MB degradation under the irradiation of varied frequency ultrasound in the presence of TiO2 nanotube array film. Compared with the reaction rate constants in the case of sole ultrasound, the reaction rate constants in
the presence of TiO2 nanotube array film increased. This phenomenon is similar with the previous reports on the TiO2 nanoparticles [18–20]. The most suitable frequency is still 27 kHz, and the reaction rate constant is 23.3 104 min1. Previous researches show that the presence of metal oxides can increase the performance of ultrasonic degradation due to the generation of holes on the surface of metal oxides [11,12,18–20]. When the system was irradiated by ultrasound and UV light simultaneously, the reaction rate constants present in Fig. 3C and Table 1 also show extremely high degradation rate at 27 kHz. The histogram in Fig. 3D shows the degradation for 3 h. The synergistic effect between ultrasound and UV light is evaluated by the following equation [21]:
DUSþUVþTiO2 Synergy ¼ 1 DUSþTiO2 þ DUVþTiO2
ð2Þ
Here DUSþTiO2 , DUVþTiO2 and DUSþUVþTiO2 represent the degradation of methylene blue in the case of ultrasonic degradation, photocatalytic degradation and sonophotocatalytic degradation, respectively. At 24 and 27 kHz, the synergistic effects calculated by Eq. (1) are 12.4% and 22.1%, respectively. At 18 and 20 kHz, the synergistic ef-
Fig. 3. First-order kinetic plots of methylene blue (MB) degradation under varied frequency ultrasound without TiO2 nanotube array film (A), in the presence of TiO2 nanotube array film (B), in the presence of TiO2 nanotube array film and UV light irradiation (C). And the degradation of MB solution under different condition after 3 h (D).
S. Yuan et al. / Catalysis Communications 10 (2009) 1188–1191
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of ultrasound without peeling off and works well as recyclable catalyst. The frequencies of ultrasound have great effects on the synergy of sonolytic and photocatalytic degradation of methylene blue. TiO2 nanotube array shows high sonophotocatalytic activity at 27 kHz and the synergy is as high as 22.1%. So, TiO2 nanotube array can be used as recyclable catalyst for the sonodegradation and sonophotodegradation of waste waters.
Acknowledgements
Fig. 4. Duplicated experiments of methylene blue (MB) degradation under varied frequency ultrasound in the presence of TiO2 nanotube array film.
fects are not observed. At 30 kHz, the degradation rate is so much lower than that at 27 kHz, although the synergistic effect calculated (35.0%) is higher than that at 27 kHz. Previous reports suggested that the synergy of UV light and ultrasound irradiation are caused by the ultrasonic dispersion of catalyst particles, the enhancement of mass-transfer between the bulk liquid and the surface of catalyst, the formation of OH radicals from H2O2 produced by photocatalysts [13]. In this study, the catalyst is TiO2 nanotube array fixed on Ti substrate, and ultrasound has no effect on the dispersion of catalyst. Ultrasound was found to improve the mass-transfer coefficient by creating high-speed microscopic turbulence at solid–liquid interface in a porous structure [22,23]. In addition, higher frequencies favor OH radical production. However, the resonant bubbles will not grow large enough to produce enough energy during collapse to form sufficient numbers of OH radicals at high frequencies. So, there is an optimal frequency for the formation of OH radical [16]. In this experiment, the reaction rate constants of degradation in the presence or absence of TiO2 and UV light are significantly higher at 27 kHz than at other frequencies. It can be concluded that OH production increases with the increasing frequency from 18 to 27 kHz and decreases when the frequency increases to 30 kHz. So 27 kHz is more favorable for the mass-transfer and formation of OH radicals. Fig. 4 shows the degradation of methylene blue in duplicated experiments. The results reveal that ultrasound does not have obvious effects on the repeatability of degradation experiments at different frequencies and the combination between TiO2 nanotube array and Ti substrate is strong enough to sustain the impact of ultrasound. Actually, the ICP–AES analysis result confirmed that there was no TiO2 nanotube array film peeling off from the substrate in the methylene blue solution. 4. Conclusion The TiO2 nanotube array prepared by anodic oxidation combines with the Ti substrate strongly enough to sustain the impact
The Project Sponsored by Academic Leader Program of Shanghai Science and Technology Committee (07XD14014); Technical Innovation Team Project of Shanghai Science and Technology Committee (06DZ05902); Key Subject of Shanghai Municipal Education Commission (J50102); Shanghai-unilever Research and Development Fund (06SU07001); The Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; Innovation Program of Shanghai Municipal Education Commission (08YZ09); Science Foundation for The Excellent Youth Scholars of University (Shanghai); Innovative Foundation of Shanghai University.
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