- Email: [email protected]
Preparation of TiO2 nanotubes coated on polyurethane and study of their photocatalytic activity
Applied Surface Science 258 (2012) 9593–9598
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of TiO2 nanotubes coated on polyurethane and study of their photocatalytic activity Pei Liu a , Haijin Liu a , Guoguang Liu a,b,∗ , Kun Yao b , Wenying Lv b a School of Chemistry and Environmental Science, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huaihe River Water Environment and Pollution Control, Ministry of Education, Xinxiang 453007, PR China b Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 29 May 2012 Accepted 30 May 2012 Available online 6 June 2012 Keywords: TiO2 nanotube Silane coupling agents Polyurethane membrane Photocatalysis
a b s t r a c t TiO2 nanotubes have been synthesized by a hydrothermal method and their surface has been effectively modified with silane coupling agents. TiO2 nanotubes–polyurethane photocatalytic composites were then successfully synthesized through a series of activation reactions using polyurethane (PU) membrane as a solid carrier. All of these products have been characterized and identified by means of scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The results have shown that the TiO2 nanotubes treated by silicon alkylation could efficiently combine with the solid carrier (PU) and that the surface multilayer structure was very stable and compact. In addition, the new composite showed very good photocatalytic activity and was recyclable, which was demonstrated by the photocatalytic degradation of Rhodamine B. © 2012 Published by Elsevier B.V.
1. Introduction
silica gel [15], stainless steel [16] and polymer materials [17], etc. The ideal support for photocatalysis must satisfy several criteria, such as strong adherence between catalyst and support; nondegradation of the catalyst reactivity by the attachment [18]; the capacity to resist oxidation of the support during the photocatalysis process. Polyurethane (PU), a widely used macromolecular material, has good flexibility, elasticity, surface resistance, and age resistance, and is easily shaped [19]. In the present work, we have introduced certain functional groups into TiO2 nanotubes by using silane coupling agents, improved the surface properties of PU, and synthesized a new composite of TiO2 nanotubes–PU. The photocatalytic activity of the product has been evaluated by measuring the rate of photocatalytic degradation of Rhodamine B under laboratory simulated conditions. The objective of this work has been to develop a new solid photocatalyst and to evaluate its function.
Nano-semiconductor materials have attracted increasing attention since their photocatalytic activity was identified in the 1970s [1–5]. Nano-TiO2 , as one of the well-known nano-materials, holds great promise in environmental pollution control due to its high photocatalytic activity, stability, and almost harmless nature. Novel TiO2 nanotubes, first synthesized by Kasuga et al. [6,7] in 1998 by a hydrophilic-thermal method, have a higher specific surface area and stronger adsorption ability, which significantly increases the photocatalytic activity and photoelectrical transformation efficiency of nano-TiO2 . The preparation, renaturalization, and development of nanoTiO2 have gradually become a focus of research in recent years [8–12]. However, at present, nano-TiO2 is still difficult to be freely applied in practice on account of its lack of reusability, this result in costs and environmental pollution. Finding appropriate solid carriers to fix nano-TiO2 powder might be an effective way to resolve these problems. To date, many materials have been studied as supports for nano-TiO2 , including glass [13], activated carbon [14],
2.1. Materials
∗ Corresponding author at: School of Chemistry and Environmental Science, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huaihe River Water Environment and Pollution Control, Ministry of Education, Xinxiang 453007, PR China. Tel.: +86 373 3325971; fax: +86 373 3326335. E-mail address: [email protected] (G. Liu).
TiO2 powder was purchased from Yili Fine Chemical Co., Ltd. (Beijing, China). ␥-Aminopropyl triethoxysilane (KH-550) was purchased from LiPai Chemicals Co., Ltd. (Nanjing, China). Polyurethane (PU) membrane was obtained from JiMing Chemical Industry Co., Ltd. (Luoyang, China). The other reagents used in the experiments, such as toluene-2,4-diisocyanate (PDI), triethylamine, toluene, methanol, and ethanol, were all of analytical grade.
0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.05.154
2. Experimental
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Fig. 1. Mechanism of silanization of TiO2 nanotubes.
2.2. Preparation of TiO2 nanotubes First, TiO2 powder (2 g) was added to a polytetrafluoroethene bottle containing 10 M NaOH solution (100 mL), and the mixture was kept in a thermostat at 180 ◦ C for 24 h. Thereafter, the deposited material was washed with deionized water until pH. 7. Second, the material was subjected to acidic conditions with 0.1 M HCl for 12 h, then washed with deionized water until neutrality, collected by filtration on a water membrane, and finally dehydrated in a baking oven at 80 ◦ C for 10 h[20]. 2.3. Silanization of TiO2 nanotubes According to the reported method [21], silanization of the TiO2 nanotubes was carried out as follows: TiO2 nanotubes (0.2 g) were placed in a three-necked flask containing toluene (200 mL) and were ultrasonicated for 30 min, a 1% solution of KH-550 in toluene (30 mL) was then added, and the mixture was stirred for 6 h. After the reaction, methanol (120 mL) was added to the mixture to scavenge the residual silane coupling agent, and the treated nanotubes were collected by filtration, washed three times with methanol, deionized water, and acetone, respectively, and finally dried at 80 ◦ C under vacuum for 12 h. The mechanism of the silanization is illustrated in Fig. 1. 2.4. Pretreatment of PU membrane and surface activation The surface of the PU was activated according to Deng’s method [22]. A PU membrane of area of 1 cm × 1.5 cm was ultrasonicated for 15 min in toluene and ethanol, respectively, which helped to scavenge any organic chemicals on its surface, and then the treated membrane was dehydrated at 40 ◦ C under vacuum for 15 h. The pre-treated PU membrane were placed in a mixed solution of toluene (20 mL), PDI (2 mL), and anhydrous triethylamine (0.5 mL) in a 150 mL flask, and stirred continuously by means of a magnetic stirrer. The solution was then heated to 60 ◦ C under nitrogen as a protective gas and maintained at this temperature for 1 h in order to introduce isocyanate groups on the surface of the PU. After this procedure, the surface-activated PU was washed repeatedly with toluene and then immersed in toluene for about 20 h for dehydration.
Fig. 2. Mechanism of TiO2 nanotubes coated on PU.
total reflectance Fourier-transform infrared spectroscopy (ATRFTIR; Nicolet 5700, Thermo ESI, USA) was also employed to probe the surface structure of the activated PU and TiO2 nanotubes–PU. The surface morphology of the final TiO2 nanotubes–PU composite was observed by scanning electron microscopy (SEM; JSM-5610LV, JEOL, Japan).
2.7. Photocatalytic degradation of Rhodamine B The photocatalytic activity of TiO2 nanotubes–PU was evaluated by measuring the photocatalytic degradation rate of Rhodamine B. The photoreactor basically consisted of a quartz cooling well, the reactor, and a lamp source (Fig. 3). The reactor was put next to the cooling well and the distance was about 10 cm. A test solution of 1.25 × 10−5 M Rhodamine B (100 mL) was placed in the photoreactor and stirred by a magnetic stirrer throughout the experiments. Pristine PU membrane or TiO2 nanotubes–PU membrane was fixed in the middle of the reactor. Oxygen flowed into the Rhodamine B solution at a flow rate of 0.2 L/min. A 300 W mercury vapor lamp (Shanghai Yaming Lighting Co., Ltd., China) with a main wavelength of 365 nm was placed in the cooling well, and the average irradiation intensity for the test solutions was 8 mW cm−2 . The temperature of the photoreactor was regulated at 25 ± 2 ◦ C by a circulating water system. After initiating the test, we took samples at intervals of 10 min and measured their absorbance at 554 nm by a 722 UV-vis spectrophotometer.
2.5. Preparation of TiO2 nanotube-PU The silanized TiO2 nanotubes (0.2 g) were added to a flask containing toluene (100 mL), and dispersed by ultrasonication for 10 min. The pre-treated PU was then added, and the mixture was stirred and heated at 60–65 ◦ C under a protective atmosphere of nitrogen. At the same time, potassium persulfate solution (5 mL) was added dropwise to the mixture under exclusion of light. On completion of the reaction, the solid material was collected, washed repeatedly with ethanol, and dried under vacuum. The mechanism of the coating of TiO2 nanotubes onto PU is illustrated in Fig. 2. 2.6. Characterization Fourier-transform infrared spectroscopy (FTIR; Bio-RAD, USA) was used to characterize the silanized TiO2 nanotubes. Attenuated
Fig. 3. Schematic representation of photoreactor.
P. Liu et al. / Applied Surface Science 258 (2012) 9593–9598
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Fig. 4. FTIR spectra of TiO2 nanotubes (a) and silanized TiO2 nanotubes (b).
2.8. Regeneration of TiO2 nanotubes–PU membrane The TiO2 nanotubes–PU membrane was put into ethanol and sonicated for 8–10 min after photocatalytic reaction, and then sonicated by deionized water for 10 min for next use. 3. Results and discussion 3.1. FTIR of TiO2 nanotubes Fig. 4 shows the FTIR spectra of the TiO2 nanotubes before and after silanization. For the pristine TiO2 nanotubes (Fig. 4a), the band
at 3421 cm−1 may be attributed to O H stretching vibrations, while that at 1635 cm−1 may be attributed to the presence of hydroxyl groups ( OH) on the surface of TiO2 nanotubes, which are believed to originate from ambient atmospheric moisture. In addition, the band at 472 cm−1 is assigned to Ti O Ti stretching vibrations. From Fig. 4b, the band at 910 cm−1 suggests that the hydrolyzed silane coupling agent reacted with hydroxyl groups on the surface of the TiO2 nanotubes through an intermolecular condensation, resulting in Ti O Si bonds [23]. Similarly, the band at 1095 cm−1 may be assigned to Si O Si resulting from condensation of molecules of the hydrolyzed silane coupling agent. The mutual bond crossover helped to construct a multilayer network protecting
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Fig. 5. ATR-FTIR spectrum of pristine PU.
the TiO2 nanotubes from oxidation. It was very difficult to confirm the presence of an N H bond solely based on the presence of the bands at 3426 cm−1 and 1629 cm−1 , because the bands of free hydroxyl groups on the surface of the nano-TiO2 were very similar to those of the N H bond of amidocyanogen. However, it seems
likely that the band at 1514 cm−1 is due to a monoamine group on account of the theory that mutual interaction between the hydroxyl from the silane and the hydroxyl from the TiO2 is responsible for the hypsochromic shift of the monoamine group band [24]. The band at 3029 cm−1 can be assigned as a methylene stretching vibration.
Fig. 6. ATR-FTIR spectrum of activated PU.
P. Liu et al. / Applied Surface Science 258 (2012) 9593–9598
Fig. 7. ATR-FTIR spectrum of TiO2 nanotube–PU.
Fig. 8. SEM image of pristine PU (a) and TiO2 nanotube–PU (b).
All of these characteristic peaks showed that the target functional group had been coated on the surface of the nano-TiO2 through a silanization reaction. 3.2. ATR-FTIR analysis The ATR-FTIR spectra of pristine PU and activated PU are shown in Figs. 5 and 6, respectively. No spectral features due to a reactive functional group were found for pristine PU. However, after subjecting the PU surface to a chemical reaction with excess PDI, which resulted in the introduction of free isocyanate groups, two strong characteristic peaks were observed at 2268 and 1412 cm−1 , implying effective activation [25]. The ATR-FTIR spectrum of TiO2 nanotubes coated onto PU is shown in Fig. 7. It was found that the characteristic bands seen at 2268 and 1412 cm−1 in Fig. 6 had disappeared, and that a C O stretching vibration at 1682 cm−1 and a C N H bending vibration at 1558 cm−1 [26] appeared instead, which showed that the activated PU membrane had been grafted through the monoamine group of the TiO2 nanotubes with the formation of a diamide. The
Fig. 9. Degradation of Rhodamine B with pristine PU and TiO2 –PU.
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Fig. 10. Degradation ratio of Rhodamine B without photocatalyst cleaning (a) and with photocatalyst cleaning (b).
peaks at 911 and 1102 cm−1 could be attributed to Ti O Si and Si O Si, respectively. The above results indicated successful grafting between PU and the TiO2 nanotubes. 3.3. Morphologies of pristine PU and TiO2 nanotubes–PU
composite was very stable and could be used for many times. Immobilization of TiO2 nanotubes on a PU film greatly facilitated reuse of TiO2 catalyst. At the same time, it also provided a feasible and low-cost means of applying TiO2 in effluent treatment. Acknowledgments
SEM images of pristine PU and TiO2 nanotubes–PU are shown in Fig. 8. The surface of the pristine PU is seen to be very smooth with few impurities (Fig. 8a). In comparison, the surface of the PU coated with TiO2 nanotubes shows a very compact multilayer network structure due to mutual grafting between TiO2 nanotubes, the silane coupling agent, and PU, although the nanotubes seemed not to be very uniformly dispersed on the surface of the PU. 3.4. Photocatalytic degradation of Rhodamine B Fig. 9 indicates direct photolysis and photocatalytic degradation of Rhodamine B. There was an obvious decrease in the amount of Rhodamine B, despite pristine PU having no photocatalytic activity. In 1 h, about 38% of the added Rhodamine B was degraded under UV light irradiation. In the presence of PU coated with TiO2 nanotubes, the photodegradation rate of Rhodamine B increased markedly and about 70% of the added Rhodamine B was degraded in 1 h. It indicated that TiO2 –PU membrane had a good photocatalytic activity. Fitting a line with ln(C/C0 ) with respect to time, a very good linear relationship was observed, the first-order kinetic constants of pristine PU and TiO2 –PU were 0.0032 and 0.0088 K min−1 , respectively. 3.5. Reuse of the photocatalyst The extent of the degradation of Rhodamine B at 60 min was about 70% (Fig. 10a) for the first use of TiO2 nanotubes–PU. However, after reuse for 20 times, the photocatalytic activity of the TiO2 nanotubes–PU had decreased greatly compared with the first time. On the contrary, if this composite was cleaned with ethanol after each use, no significant decrease of photocatalytic activity was observed after reuse for 20 times (Fig. 10b), which suggested that the active sites on the surface of this composite might be occupied by the products and reactants during photoreaction. Hence, it appeared that the TiO2 nanotubes showed little tendency to be cleaved from the surface of TiO2 nanotubes–PU during the photocatalytic reaction and that the graft between PU and the TiO2 nanotubes was very compact and solid, facilitating reuse. 4. Conclusion In this work, we successfully immobilized TiO2 nanotubes onto PU membrane and the photocatalytic degradation of Rhodamine B showed that the new composite catalyst had good photocatalytic activity. The repeated experiment indicated that the synthesized
This work is supported by the National Natural Science Foundation of China (No. 20677012), the Scientific Research Project of Guangdong Province (Nos. 2009B090300342, 2009B030802045), and the Government Key Project of Water Pollution Controlling of China (No. 2009ZX07211-005-03). References [1] M. Fujihira, Y. Satoh, T. Osa, Bulletin of the Chemical Society of Japan 55 (1982) 666. [2] H. Gerischer, A. Heller, Journal of Physical Chemistry 95 (1991) 5261. [3] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahneman, Chemical Reviews 95 (1995) 69. [4] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Applied Catalysis B 17 (1998) 25. [5] C.A.K. Gouvêa, F. Wypych, S.G. Moraes, N. Durán, N. Nagata, P. Peralta-Zamora, Chemosphere 40 (2000) 433. [6] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. [7] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Advanced Materials 11 (1999) 1307. [8] W.J. Lee, W.H. Smyrl, Current Applied Physics 8 (2008) 818. [9] M. Uzunova-Bujnovaa, R. Kralchevskaa, M. Milanovaa, R. Todorovskab, D. Hristova, D. Todorovsky, Catalysis Today 151 (2010) 14. [10] Q. Zhao, M. Li, J.Y. Chu, T.S. Jiang, H. Yin, Applied Surface Science 255 (2009) 3773. [11] C.A. Linkous, G. Carter, D.B. Locuson, A.J. Ouellette, D.K. Slattery, L.A. Smitha, Environmental Science and Technology 34 (2000) 4754. [12] I.K. Konstantinou, T.M. Sakellarides, V.A. Sakkas, T.A. Albanis, Environmental Science and Technology 35 (2001) 398. [13] D.S. Kim, Y.S. Park, Chemical Engineering Journal 116 (2006) 133. [14] A.H. El-Sheikh, Y.S. Al-Degs, A.P. Newman, D.E. Lynch, Separation and Purification Technology 54 (2007) 117. [15] Y. Chen, K. Wang, L. Lou, Journal of Photochemistry and Photobiology A 163 (2004) 271. [16] Z. Ding, X. Hu, G.Q. Lu, P.L. Yue, P.F. Greenfield, Langmuir 16 (2000) 6216. [17] A.H. Fostier, M.S.S. Pereira, S. Rath, J.R. Guimaraes, Chemosphere 72 (2008) 319. [18] A.Y. Shan, T.M. Ghazi, S.A. Rashid, Applied Catalysis A 389 (2010) 1. [19] S.X. Li, Y.J. Liu, Polyurethane Resin and its Application, Chemical Industry Press, Beijing, 2002 (In Chinese). [20] D. Wang, F. Zhou, Y. Liu, W. Liu, Materials Letters 62 (2008). [21] C.M. Peng, Carbon 44 (2006) 3232. [22] F.J. Deng, W.F. Li, S.H. Xu, Y.W. Chen, New Chemical Materials 36 (2008) 68. [23] Z.J. Li, B. Hou, Y. Xu, D. Wu, Y.H. Sun, W. Hu, F. Deng, Journal of Solid State Chemistry 178 (2005) 1395. [24] E. Ukaji, T. Furusawa, M. Sato, N. Suzuki, Applied Surface Science 254 (2007) 563. [25] L.Z. Meng, S.L. Gong, B.Y. He, Organic Spectral Analysis, Wuhan University Press, Wuhan, 1996 (In Chinese). [26] Y.C. Ning, Identification of Organic Compound Structure and Study of Organic Spectrum, Science Press, Beijing, 2000 (In Chinese).
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of TiO2 nanotubes coated on polyurethane and study of their photocatalytic activity Pei Liu a , Haijin Liu a , Guoguang Liu a,b,∗ , Kun Yao b , Wenying Lv b a School of Chemistry and Environmental Science, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huaihe River Water Environment and Pollution Control, Ministry of Education, Xinxiang 453007, PR China b Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2011 Received in revised form 29 May 2012 Accepted 30 May 2012 Available online 6 June 2012 Keywords: TiO2 nanotube Silane coupling agents Polyurethane membrane Photocatalysis
a b s t r a c t TiO2 nanotubes have been synthesized by a hydrothermal method and their surface has been effectively modified with silane coupling agents. TiO2 nanotubes–polyurethane photocatalytic composites were then successfully synthesized through a series of activation reactions using polyurethane (PU) membrane as a solid carrier. All of these products have been characterized and identified by means of scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). The results have shown that the TiO2 nanotubes treated by silicon alkylation could efficiently combine with the solid carrier (PU) and that the surface multilayer structure was very stable and compact. In addition, the new composite showed very good photocatalytic activity and was recyclable, which was demonstrated by the photocatalytic degradation of Rhodamine B. © 2012 Published by Elsevier B.V.
1. Introduction
silica gel [15], stainless steel [16] and polymer materials [17], etc. The ideal support for photocatalysis must satisfy several criteria, such as strong adherence between catalyst and support; nondegradation of the catalyst reactivity by the attachment [18]; the capacity to resist oxidation of the support during the photocatalysis process. Polyurethane (PU), a widely used macromolecular material, has good flexibility, elasticity, surface resistance, and age resistance, and is easily shaped [19]. In the present work, we have introduced certain functional groups into TiO2 nanotubes by using silane coupling agents, improved the surface properties of PU, and synthesized a new composite of TiO2 nanotubes–PU. The photocatalytic activity of the product has been evaluated by measuring the rate of photocatalytic degradation of Rhodamine B under laboratory simulated conditions. The objective of this work has been to develop a new solid photocatalyst and to evaluate its function.
Nano-semiconductor materials have attracted increasing attention since their photocatalytic activity was identified in the 1970s [1–5]. Nano-TiO2 , as one of the well-known nano-materials, holds great promise in environmental pollution control due to its high photocatalytic activity, stability, and almost harmless nature. Novel TiO2 nanotubes, first synthesized by Kasuga et al. [6,7] in 1998 by a hydrophilic-thermal method, have a higher specific surface area and stronger adsorption ability, which significantly increases the photocatalytic activity and photoelectrical transformation efficiency of nano-TiO2 . The preparation, renaturalization, and development of nanoTiO2 have gradually become a focus of research in recent years [8–12]. However, at present, nano-TiO2 is still difficult to be freely applied in practice on account of its lack of reusability, this result in costs and environmental pollution. Finding appropriate solid carriers to fix nano-TiO2 powder might be an effective way to resolve these problems. To date, many materials have been studied as supports for nano-TiO2 , including glass [13], activated carbon [14],
2.1. Materials
∗ Corresponding author at: School of Chemistry and Environmental Science, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huaihe River Water Environment and Pollution Control, Ministry of Education, Xinxiang 453007, PR China. Tel.: +86 373 3325971; fax: +86 373 3326335. E-mail address: [email protected] (G. Liu).
TiO2 powder was purchased from Yili Fine Chemical Co., Ltd. (Beijing, China). ␥-Aminopropyl triethoxysilane (KH-550) was purchased from LiPai Chemicals Co., Ltd. (Nanjing, China). Polyurethane (PU) membrane was obtained from JiMing Chemical Industry Co., Ltd. (Luoyang, China). The other reagents used in the experiments, such as toluene-2,4-diisocyanate (PDI), triethylamine, toluene, methanol, and ethanol, were all of analytical grade.
0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apsusc.2012.05.154
2. Experimental
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Fig. 1. Mechanism of silanization of TiO2 nanotubes.
2.2. Preparation of TiO2 nanotubes First, TiO2 powder (2 g) was added to a polytetrafluoroethene bottle containing 10 M NaOH solution (100 mL), and the mixture was kept in a thermostat at 180 ◦ C for 24 h. Thereafter, the deposited material was washed with deionized water until pH. 7. Second, the material was subjected to acidic conditions with 0.1 M HCl for 12 h, then washed with deionized water until neutrality, collected by filtration on a water membrane, and finally dehydrated in a baking oven at 80 ◦ C for 10 h[20]. 2.3. Silanization of TiO2 nanotubes According to the reported method [21], silanization of the TiO2 nanotubes was carried out as follows: TiO2 nanotubes (0.2 g) were placed in a three-necked flask containing toluene (200 mL) and were ultrasonicated for 30 min, a 1% solution of KH-550 in toluene (30 mL) was then added, and the mixture was stirred for 6 h. After the reaction, methanol (120 mL) was added to the mixture to scavenge the residual silane coupling agent, and the treated nanotubes were collected by filtration, washed three times with methanol, deionized water, and acetone, respectively, and finally dried at 80 ◦ C under vacuum for 12 h. The mechanism of the silanization is illustrated in Fig. 1. 2.4. Pretreatment of PU membrane and surface activation The surface of the PU was activated according to Deng’s method [22]. A PU membrane of area of 1 cm × 1.5 cm was ultrasonicated for 15 min in toluene and ethanol, respectively, which helped to scavenge any organic chemicals on its surface, and then the treated membrane was dehydrated at 40 ◦ C under vacuum for 15 h. The pre-treated PU membrane were placed in a mixed solution of toluene (20 mL), PDI (2 mL), and anhydrous triethylamine (0.5 mL) in a 150 mL flask, and stirred continuously by means of a magnetic stirrer. The solution was then heated to 60 ◦ C under nitrogen as a protective gas and maintained at this temperature for 1 h in order to introduce isocyanate groups on the surface of the PU. After this procedure, the surface-activated PU was washed repeatedly with toluene and then immersed in toluene for about 20 h for dehydration.
Fig. 2. Mechanism of TiO2 nanotubes coated on PU.
total reflectance Fourier-transform infrared spectroscopy (ATRFTIR; Nicolet 5700, Thermo ESI, USA) was also employed to probe the surface structure of the activated PU and TiO2 nanotubes–PU. The surface morphology of the final TiO2 nanotubes–PU composite was observed by scanning electron microscopy (SEM; JSM-5610LV, JEOL, Japan).
2.7. Photocatalytic degradation of Rhodamine B The photocatalytic activity of TiO2 nanotubes–PU was evaluated by measuring the photocatalytic degradation rate of Rhodamine B. The photoreactor basically consisted of a quartz cooling well, the reactor, and a lamp source (Fig. 3). The reactor was put next to the cooling well and the distance was about 10 cm. A test solution of 1.25 × 10−5 M Rhodamine B (100 mL) was placed in the photoreactor and stirred by a magnetic stirrer throughout the experiments. Pristine PU membrane or TiO2 nanotubes–PU membrane was fixed in the middle of the reactor. Oxygen flowed into the Rhodamine B solution at a flow rate of 0.2 L/min. A 300 W mercury vapor lamp (Shanghai Yaming Lighting Co., Ltd., China) with a main wavelength of 365 nm was placed in the cooling well, and the average irradiation intensity for the test solutions was 8 mW cm−2 . The temperature of the photoreactor was regulated at 25 ± 2 ◦ C by a circulating water system. After initiating the test, we took samples at intervals of 10 min and measured their absorbance at 554 nm by a 722 UV-vis spectrophotometer.
2.5. Preparation of TiO2 nanotube-PU The silanized TiO2 nanotubes (0.2 g) were added to a flask containing toluene (100 mL), and dispersed by ultrasonication for 10 min. The pre-treated PU was then added, and the mixture was stirred and heated at 60–65 ◦ C under a protective atmosphere of nitrogen. At the same time, potassium persulfate solution (5 mL) was added dropwise to the mixture under exclusion of light. On completion of the reaction, the solid material was collected, washed repeatedly with ethanol, and dried under vacuum. The mechanism of the coating of TiO2 nanotubes onto PU is illustrated in Fig. 2. 2.6. Characterization Fourier-transform infrared spectroscopy (FTIR; Bio-RAD, USA) was used to characterize the silanized TiO2 nanotubes. Attenuated
Fig. 3. Schematic representation of photoreactor.
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Fig. 4. FTIR spectra of TiO2 nanotubes (a) and silanized TiO2 nanotubes (b).
2.8. Regeneration of TiO2 nanotubes–PU membrane The TiO2 nanotubes–PU membrane was put into ethanol and sonicated for 8–10 min after photocatalytic reaction, and then sonicated by deionized water for 10 min for next use. 3. Results and discussion 3.1. FTIR of TiO2 nanotubes Fig. 4 shows the FTIR spectra of the TiO2 nanotubes before and after silanization. For the pristine TiO2 nanotubes (Fig. 4a), the band
at 3421 cm−1 may be attributed to O H stretching vibrations, while that at 1635 cm−1 may be attributed to the presence of hydroxyl groups ( OH) on the surface of TiO2 nanotubes, which are believed to originate from ambient atmospheric moisture. In addition, the band at 472 cm−1 is assigned to Ti O Ti stretching vibrations. From Fig. 4b, the band at 910 cm−1 suggests that the hydrolyzed silane coupling agent reacted with hydroxyl groups on the surface of the TiO2 nanotubes through an intermolecular condensation, resulting in Ti O Si bonds [23]. Similarly, the band at 1095 cm−1 may be assigned to Si O Si resulting from condensation of molecules of the hydrolyzed silane coupling agent. The mutual bond crossover helped to construct a multilayer network protecting
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Fig. 5. ATR-FTIR spectrum of pristine PU.
the TiO2 nanotubes from oxidation. It was very difficult to confirm the presence of an N H bond solely based on the presence of the bands at 3426 cm−1 and 1629 cm−1 , because the bands of free hydroxyl groups on the surface of the nano-TiO2 were very similar to those of the N H bond of amidocyanogen. However, it seems
likely that the band at 1514 cm−1 is due to a monoamine group on account of the theory that mutual interaction between the hydroxyl from the silane and the hydroxyl from the TiO2 is responsible for the hypsochromic shift of the monoamine group band [24]. The band at 3029 cm−1 can be assigned as a methylene stretching vibration.
Fig. 6. ATR-FTIR spectrum of activated PU.
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Fig. 7. ATR-FTIR spectrum of TiO2 nanotube–PU.
Fig. 8. SEM image of pristine PU (a) and TiO2 nanotube–PU (b).
All of these characteristic peaks showed that the target functional group had been coated on the surface of the nano-TiO2 through a silanization reaction. 3.2. ATR-FTIR analysis The ATR-FTIR spectra of pristine PU and activated PU are shown in Figs. 5 and 6, respectively. No spectral features due to a reactive functional group were found for pristine PU. However, after subjecting the PU surface to a chemical reaction with excess PDI, which resulted in the introduction of free isocyanate groups, two strong characteristic peaks were observed at 2268 and 1412 cm−1 , implying effective activation [25]. The ATR-FTIR spectrum of TiO2 nanotubes coated onto PU is shown in Fig. 7. It was found that the characteristic bands seen at 2268 and 1412 cm−1 in Fig. 6 had disappeared, and that a C O stretching vibration at 1682 cm−1 and a C N H bending vibration at 1558 cm−1 [26] appeared instead, which showed that the activated PU membrane had been grafted through the monoamine group of the TiO2 nanotubes with the formation of a diamide. The
Fig. 9. Degradation of Rhodamine B with pristine PU and TiO2 –PU.
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Fig. 10. Degradation ratio of Rhodamine B without photocatalyst cleaning (a) and with photocatalyst cleaning (b).
peaks at 911 and 1102 cm−1 could be attributed to Ti O Si and Si O Si, respectively. The above results indicated successful grafting between PU and the TiO2 nanotubes. 3.3. Morphologies of pristine PU and TiO2 nanotubes–PU
composite was very stable and could be used for many times. Immobilization of TiO2 nanotubes on a PU film greatly facilitated reuse of TiO2 catalyst. At the same time, it also provided a feasible and low-cost means of applying TiO2 in effluent treatment. Acknowledgments
SEM images of pristine PU and TiO2 nanotubes–PU are shown in Fig. 8. The surface of the pristine PU is seen to be very smooth with few impurities (Fig. 8a). In comparison, the surface of the PU coated with TiO2 nanotubes shows a very compact multilayer network structure due to mutual grafting between TiO2 nanotubes, the silane coupling agent, and PU, although the nanotubes seemed not to be very uniformly dispersed on the surface of the PU. 3.4. Photocatalytic degradation of Rhodamine B Fig. 9 indicates direct photolysis and photocatalytic degradation of Rhodamine B. There was an obvious decrease in the amount of Rhodamine B, despite pristine PU having no photocatalytic activity. In 1 h, about 38% of the added Rhodamine B was degraded under UV light irradiation. In the presence of PU coated with TiO2 nanotubes, the photodegradation rate of Rhodamine B increased markedly and about 70% of the added Rhodamine B was degraded in 1 h. It indicated that TiO2 –PU membrane had a good photocatalytic activity. Fitting a line with ln(C/C0 ) with respect to time, a very good linear relationship was observed, the first-order kinetic constants of pristine PU and TiO2 –PU were 0.0032 and 0.0088 K min−1 , respectively. 3.5. Reuse of the photocatalyst The extent of the degradation of Rhodamine B at 60 min was about 70% (Fig. 10a) for the first use of TiO2 nanotubes–PU. However, after reuse for 20 times, the photocatalytic activity of the TiO2 nanotubes–PU had decreased greatly compared with the first time. On the contrary, if this composite was cleaned with ethanol after each use, no significant decrease of photocatalytic activity was observed after reuse for 20 times (Fig. 10b), which suggested that the active sites on the surface of this composite might be occupied by the products and reactants during photoreaction. Hence, it appeared that the TiO2 nanotubes showed little tendency to be cleaved from the surface of TiO2 nanotubes–PU during the photocatalytic reaction and that the graft between PU and the TiO2 nanotubes was very compact and solid, facilitating reuse. 4. Conclusion In this work, we successfully immobilized TiO2 nanotubes onto PU membrane and the photocatalytic degradation of Rhodamine B showed that the new composite catalyst had good photocatalytic activity. The repeated experiment indicated that the synthesized
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