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Synthesis of immobilized TiO2 nanowires by anodic oxidation and their gas phase photocatalytic properties
Electrochemistry Communications 11 (2009) 1692–1695
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Synthesis of immobilized TiO2 nanowires by anodic oxidation and their gas phase photocatalytic properties Zhongbiao Wu, Sen Guo, Haiqiang Wang *, Yue Liu Key Laboratory of Polluted Environmental Remediation and Ecological Health of Ministry of Education, College of Environmental and Resource Sciences, Zhejiang University, Zheda Road, No. 38, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 6 June 2009 Received in revised form 23 June 2009 Accepted 24 June 2009 Available online 27 June 2009 Keywords: Anodic oxidation Titanium dioxide Nanowires Photocatalysis
a b s t r a c t In this paper, anodic oxidation method was successfully employed to the direct growth of immobilized TiO2 nanowires on titanium foil in ethylene glycol electrolyte solution contained HF and water. The morphologies of the TiO2 nanowires could be tuned by changing the content of HF and water. The structures, morphologies and optical properties of TiO2 nanowires were characterized by SEM, XRD, UV–vis and PL. It was found that the nanowires originally grew from the splitting of TiO2 nanotubes. The gas phase photocatalytic activities were investigated by photodegradation of gaseous toluene under UV irradiation, and irregular TiO2 nanowires showed the best photocatalytic ability. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is an important semiconductor for various applications [1– 4]. By far, many efforts have been made to modify the structure of TiO2 and develop immobilized method with the aim of practical application [5,6]. Recent years, researchers show great interest in TiO2 nanowires for their unique properties and higher photocatalytic activities which may be attributed to the larger surface area and efficient charge separation [7,8]. Generally, TiO2 nanowires could be fabricated by vapor techniques or template methods. However, vapor techniques usually require special equipment and high temperatures. And template methods often encounter difficulties of prefabrication and post removal of the templates resulting in impurities [9]. Furthermore, most of the nanowires prepared by these methods are in the form of powder, which need further immobilization for practical application. The immobilization of nanowires may lower their photocatalytic activities due to the loss of surface area [6]. Anodic oxidization is a promising method for fabricating immobilized TiO2-based photocatalysts, since it can lead to direct the growth of immobilized self-organized TiO2 on different shaped Ti substrates. This method is originally used to produce high ordered nanoporous and nanotubular TiO2 materials. The systematic preparation of TiO2 with other morphologies, like nanowires and nanograss, by this method was rarely reported [10–12]. Generally, the formation of TiO2 by anodic oxidation depends on the reactions
* Corresponding author. Tel./fax: +86 571 87953088. E-mail address: [email protected] (H. Wang). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.06.031
of electrochemical etching and chemical dissolution. The surface morphologies and structures of the anodic TiO2 films are significantly influenced by the interaction between these two reactions. This promises the synthesis of tunable TiO2 morphologies under different conditions. For example, by changing the balance of the electrochemical etching and chemical dissolution reactions, extremely long TiO2 nanotube arrays could be obtained in nonaqueous organic polar electrolytes [13], while the maximum thickness of immobilized TiO2 was only 500 nm in aqueous HF [14]. This new strategy for the preparation of long TiO2 nanotube arrays is based on the decrease of hydrogen ions that derived from the electrolyte solution. Because the decrease of hydrogen ions can inhibit the chemical dissolution reaction of formed TiO2: 2
TiO2 þ 6F þ 4Hþ ! TiF6 þ 2H2 O
ð1Þ
This principle enlightens us that specific morphology would be obtained by adjusting the hydrogen ions content. This paper intends to demonstrate the preparation of immobilized TiO2 nanowires in ethylene glycol electrolyte solution contained certain amount of HF and water by anodic oxidation method. Furthermore, the photocatalytic degradation of gas phase toluene for these TiO2 nanowires has been investigated for the potential application.
2. Experimental Three types of immobilized TiO2 nanostructured films, nanowires (NW), irregular nanowires (IRNW) and nanotubes (NT) were prepared by potentiostatic anodization in a two-electrode electro-
Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
chemical cell. For preparation, 0.1 mm thickness titanium foil (6 cm 6 cm, 99.99% purity, Deli Co., Ltd, China) was used as a working electrode connected to a DC power supply (Hongbao Co., Ltd, China). NW and IRNW were prepared in ethylene glycol electrolyte solution contained 0.2 wt% HF and 0.3 wt% water at 100 V for 3 h and 2 h, respectively. NT was prepared in ethylene glycol electrolyte solution contained 0.08 wt% HF and 0.12 wt% water at 100 V for 3 h. After anodization, the samples were washed by ethanol carefully, and then annealed at 400 °C for 2 h to obtain anatase TiO2. For comparison, we prepared another type of TiO2 film (denoted as P25) by coating 0.15 g commercial photocatalyst Degussa P25 TiO2 powder on the surface of Ti foil. The microstructures were observed by scanning electron microscopes (Sirion200, USA). The crystal phases were analyzed by X-ray diffraction (XRD) with Cu Ka radiation (Model D/max RA, Japan). The UV–vis diffuse reflection spectra (UV–vis DRS) were obtained with a UV–vis spectrophotometer (TU-1901, China) that equipped with an integrating sphere assembly. BaSO4 was used as a reflectance sample. The photoluminescence (PL) spectra were measured at room temperature on a fluorospectrophotometer (Fluorolog-3Tau, France) with excitation source of Xe lamp. Photocaytalytic degradation of toluene was carried out in a 1.8 L photocatalytic reactor sealed with quartz plate. The UV irradiation was provided by a 150 W Xe lamp, and an optical glass filter was inserted to cut off the infrared (k > 800 nm). The sample was placed on the bottom of the reactor for test. The analysis of toluene concentration was conducted with a GCFID (FULI 9790, China). The initial concentration of toluene was controlled at about 200 mg m3. 3. Results and discussion Fig. 1a–d shows microstructures of NW, IRNW, NT and P25. The thickness of the films is 11.1, 3.6, 4.1 and 5.4 lm, respectively. It can be observed from Fig. 1a that the tops of the wires are bundled together for NW. The wires are irregular shaped for IRNW as shown in Fig. 1b. Fig. 1a and b shows that TiO2 nanotubes can be
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still detected under the nanowires. NT was yielded under the same conditions as that of NW, except the content of HF and water decreased from 0.2 wt% and 0.3 wt% to 0.08 wt% and 0.12 wt%. However, it can be seen that there are no nanowires on the surface of NT. These results demonstrate the key role of hydrogen ions for the anodic formation of TiO2 as shown in Equation (1). In addition, only irregular nanowires (Fig. 1b) can be observed for 2 h anodization, while more uniform nanowires (Fig. 1a) are produced for 3 h anodization under the same conditions. It revealed that anodic time is an important factor for the preparation process. The results above are helpful for understanding the formation mechanism of TiO2 nanowires. It seems that the nanowires were grown originally from the vertical splitting of nanotubes. The increase of hydrogen ions initially broke the balance of the electrochemical etching and chemical dissolution reactions during the anodic oxidation process. It led to chemical dissolution on the wall of TiO2 tubes, and formed IRNW. After that, the edge of the irregular nanowires might get smoother with the increase of anodic time, which facilitated the formation of NW. This mechanism is similar to that proposed by Lim and Choi [10]. Above all, nanowires and irregular nanowires could be produced by optimizing the electrolyte solution content and anodic time. Fig. 2 shows the XRD patterns of different types of TiO2 nanowires: NW and IRNW, with thicknesses of 11.1 and 3.6 lm, respectively. For comparison, the XRD patterns of NT and P25 are also listed in Fig. 2. It can be seen that the crystallizations of NW, IRNW and NT are mainly anatase phase, as evidenced by the strong diffraction peaks at 2h = 25.5, 37.1, 48.3, 54.1, and 55.3, which can be indexed attributed to the crystal faces of anatase TiO2 (JCPDS Files No. 21-1272). The high intensity of anatase (0 0 4) crystal faces of NW and IRNW promises the possibility of the growth of single crystalline TiO2 [15]. The diffraction peaks of P25 film reveal the existent phase of rutile TiO2. The average crystalline sizes can be calculated from the broadening of the anatase (1 0 1) peaks by using the Scherrer equation. The results are 28.3, 28.9, 20.0 and 21.0 nm for NW, IRNW, NT and P25, respectively. The XRD spectra
Fig. 1. SEM images of top view and cross view of NW (a), IRNW (b), NT (c), and P25 (d) the length of each sample was marked in the figure.
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Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
Fig. 2. X-ray diffraction patterns of TiO2 films of NW, IRNW, NT and P25.
indicate that the anodic time has less influence on the crystalline sizes than on the lengths and surface morphologies of the samples. The optical properties of NW and IRNW is characterized by UV– vis DRS and PL spectrum. The optical properties of NT and P25 are also tested for comparison. Fig. 3a shows the UV–vis adsorption spectra of NW, IRNW, NT and P25, respectively. It can be seen that the strong adsorption wavelengths of NW and IRNW are in the range of 240 to 360 nm, which are wider than that of P25 film and similar to that of NT. The band gap absorption edges of NW, IRNW and NT were around 386 nm, which are close to the band gap absorption edge of anatase TiO2 with the band gap energy of 3.23 eV [16]. Moreover, stronger visible light absorptions of NW, IRNW and NT can be observed from Fig. 3a. This might be caused by the incomplete removal of electrolyte, which brought carbon impurities to the surface of the samples during the annealing process. Fig. 3b shows the photoluminescence of NW, IRNW, NT and P25. Notable PL peak shifting of NW and NT towards that of P25 is observed. The PL peak shifting might ascribe to the different phase types of the samples [17], which were further revealed by the XRD analysis above. The PL intensity of IRNW is too low to determine its peak position. Generally, the PL intensity can be employed to understand the fate of electron/hole pairs in semiconductor particles [18], and low PL intensity indicates that the separations of free carriers are promoted. The promotion usually results in the improvement of photocatalytic ability. The photcatalytic activities of NW and IRNW for air purification were investigated by photodegradation of gaseous toluene under UV irradiation. NT and P25 films were also tested under identical conditions for comparison. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics may be expressed by ln (C0/C) = kt, where k is the apparent reaction rate constant, C0 and C are the initial concentration and the reaction concentration of toluene [18]. The photocatalytic activities of the samples can be quantitatively evaluated by comparing the apparent reaction rate constants k. The results are shown in Fig. 4. The k values are 0.0741, 0.1256, 0.0816, and 0.0776 min1 for NW, IRNW, NT, and P25, respectively. It is widely accepted that the absorption of the incident photons would be increased with the increase of film thickness [19]. However, it is noticeable that the thickest film of NW (11.1 lm thick, Fig. 1a) has similar photocatalytic activity to NT (4.1 lm thick, Fig. 1c) and P25 (5.4 lm thick, Fig. 1d). This could be attributed to the fact that the congregation of the bundled nanowires and the elimination of nanotube mouths reduced the surface area. The best photocatalytic activity is demonstrated by
Fig. 3. UV–vis DRS spectra (a) and photoluminescence spectra (b) of TiO2 films of NW, IRNW, NT, and P25.
Fig. 4. The apparent reaction rate constants of NW, IRNW, NT, and P25 for photodegradation of gaseous toluene.
IRNW (3.6 lm thick, Fig. 1b) with mixed morphologies of irregular nanowires on top and nanotubes on bottom. The unique morphology endowed IRNW larger surface area, hollow interior wall, and hierarchical porous structures, which results in a faster diffusion of various gas phase species during the photocatalytic reaction.
Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
The unique morphology of IRNW also gains an advantage in recombination of the electron and hole pairs, which were proved by its lower PL intensity (See Fig. 3). All these factors contributed to the enhancement of photocatalytic activity.
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151 Talent Project of Zhejiang Province and the New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549). References
4. Conclusions In this paper, it was demonstrated that immobilized TiO2 nanowires and irregular TiO2 nanowires could be prepared by anodic oxidation of titanium foil in ethylene glycol electrolyte solution with suitable content of HF and water. The balance of electrochemical etching reaction and chemical dissolution reaction during the anodic oxidation process was adjusted by hydrogen ions, which derived from varying the content of HF and water. This balance led to chemical dissolution of the wall of TiO2 nanotubes, resulting in the formation of nanowires. It was also found that the anodic time was also an important factor of the morphologies of TiO2 nanowires. The photodegradation of gaseous toluene experimental tests showed the irregular nanowires were of the highest photocatalytic activity, which might be the results of their larger surface area, hollow interior wall, and hierarchical porous structures. Acknowledgments The project was financially supported by the National Natural Science Foundation of China (NSFC-50808156), the New Century
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33. M.H. Zhou, X.J. Ma, Electrochem. Commun. 11 (2009) 921. X. Quan, S.G. Yang, X.L. Ruan, H.M. Zhao, Environ. Sci. Technol. 39 (2005) 3770. J. Jakubowicz, Electrochem. Commun. 10 (2008) 735. M. Sleiman, P. Conchon, C. Ferronato, J.M. Chovelon, Appl. Catal. B 86 (2009) 159. Z.Y. Liu, X.T. Zhang, S. Nishimoto, T. Murakami, A. Fujishima, Environ. Sci. Technol. 42 (2008) 8547. J. Jitputti, Y. Suzuki, S. Yoshikawa, Catal. Commun. 9 (2008) 1265. B. Liu, J.E. Boercker, E.S. Aydil, Nanotechnology 19 (2008) 505604. Y.N. Zhao, J. Jin, X.Q. Yang, Mater. Lett. 61 (2007) 384. J.H. Lim, J. Choi, Small 3 (2007) 1504. Y.Y. Song, R. Lynch, D. Kim, P. Roy, P. Schmuki, Electrochem. Solid State 12 (2009) C17. D. Kim, A. Ghicov, P. Schmuki, Electrochem. Commun. 10 (2008) 1835. M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, T.A. Desai, C.A. Grimes, J. Phys. Chem. C 111 (2007) 14992. D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. J.L. Zhao, X.H. Wang, T.Y. Sun, L.T. Li, Nanotechnology 16 (2005) 2450. Z.H. Zhang, Y. Yuan, G.Y. Shi, Y.J. Fang, L.H. Liang, H.C. Ding, L.T. Jin, Environ. Sci. Technol. 41 (2007) 6259. K. Fujihara, S. Izumi, T. Ohno, M. Matsumura, J. Photoch. Photobio. A 132 (2000) 99. F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607. H.C. Liang, X.Z. Li, J. Hazard. Mater. 162 (2009) 1415.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Synthesis of immobilized TiO2 nanowires by anodic oxidation and their gas phase photocatalytic properties Zhongbiao Wu, Sen Guo, Haiqiang Wang *, Yue Liu Key Laboratory of Polluted Environmental Remediation and Ecological Health of Ministry of Education, College of Environmental and Resource Sciences, Zhejiang University, Zheda Road, No. 38, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 6 June 2009 Received in revised form 23 June 2009 Accepted 24 June 2009 Available online 27 June 2009 Keywords: Anodic oxidation Titanium dioxide Nanowires Photocatalysis
a b s t r a c t In this paper, anodic oxidation method was successfully employed to the direct growth of immobilized TiO2 nanowires on titanium foil in ethylene glycol electrolyte solution contained HF and water. The morphologies of the TiO2 nanowires could be tuned by changing the content of HF and water. The structures, morphologies and optical properties of TiO2 nanowires were characterized by SEM, XRD, UV–vis and PL. It was found that the nanowires originally grew from the splitting of TiO2 nanotubes. The gas phase photocatalytic activities were investigated by photodegradation of gaseous toluene under UV irradiation, and irregular TiO2 nanowires showed the best photocatalytic ability. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is an important semiconductor for various applications [1– 4]. By far, many efforts have been made to modify the structure of TiO2 and develop immobilized method with the aim of practical application [5,6]. Recent years, researchers show great interest in TiO2 nanowires for their unique properties and higher photocatalytic activities which may be attributed to the larger surface area and efficient charge separation [7,8]. Generally, TiO2 nanowires could be fabricated by vapor techniques or template methods. However, vapor techniques usually require special equipment and high temperatures. And template methods often encounter difficulties of prefabrication and post removal of the templates resulting in impurities [9]. Furthermore, most of the nanowires prepared by these methods are in the form of powder, which need further immobilization for practical application. The immobilization of nanowires may lower their photocatalytic activities due to the loss of surface area [6]. Anodic oxidization is a promising method for fabricating immobilized TiO2-based photocatalysts, since it can lead to direct the growth of immobilized self-organized TiO2 on different shaped Ti substrates. This method is originally used to produce high ordered nanoporous and nanotubular TiO2 materials. The systematic preparation of TiO2 with other morphologies, like nanowires and nanograss, by this method was rarely reported [10–12]. Generally, the formation of TiO2 by anodic oxidation depends on the reactions
* Corresponding author. Tel./fax: +86 571 87953088. E-mail address: [email protected] (H. Wang). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.06.031
of electrochemical etching and chemical dissolution. The surface morphologies and structures of the anodic TiO2 films are significantly influenced by the interaction between these two reactions. This promises the synthesis of tunable TiO2 morphologies under different conditions. For example, by changing the balance of the electrochemical etching and chemical dissolution reactions, extremely long TiO2 nanotube arrays could be obtained in nonaqueous organic polar electrolytes [13], while the maximum thickness of immobilized TiO2 was only 500 nm in aqueous HF [14]. This new strategy for the preparation of long TiO2 nanotube arrays is based on the decrease of hydrogen ions that derived from the electrolyte solution. Because the decrease of hydrogen ions can inhibit the chemical dissolution reaction of formed TiO2: 2
TiO2 þ 6F þ 4Hþ ! TiF6 þ 2H2 O
ð1Þ
This principle enlightens us that specific morphology would be obtained by adjusting the hydrogen ions content. This paper intends to demonstrate the preparation of immobilized TiO2 nanowires in ethylene glycol electrolyte solution contained certain amount of HF and water by anodic oxidation method. Furthermore, the photocatalytic degradation of gas phase toluene for these TiO2 nanowires has been investigated for the potential application.
2. Experimental Three types of immobilized TiO2 nanostructured films, nanowires (NW), irregular nanowires (IRNW) and nanotubes (NT) were prepared by potentiostatic anodization in a two-electrode electro-
Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
chemical cell. For preparation, 0.1 mm thickness titanium foil (6 cm 6 cm, 99.99% purity, Deli Co., Ltd, China) was used as a working electrode connected to a DC power supply (Hongbao Co., Ltd, China). NW and IRNW were prepared in ethylene glycol electrolyte solution contained 0.2 wt% HF and 0.3 wt% water at 100 V for 3 h and 2 h, respectively. NT was prepared in ethylene glycol electrolyte solution contained 0.08 wt% HF and 0.12 wt% water at 100 V for 3 h. After anodization, the samples were washed by ethanol carefully, and then annealed at 400 °C for 2 h to obtain anatase TiO2. For comparison, we prepared another type of TiO2 film (denoted as P25) by coating 0.15 g commercial photocatalyst Degussa P25 TiO2 powder on the surface of Ti foil. The microstructures were observed by scanning electron microscopes (Sirion200, USA). The crystal phases were analyzed by X-ray diffraction (XRD) with Cu Ka radiation (Model D/max RA, Japan). The UV–vis diffuse reflection spectra (UV–vis DRS) were obtained with a UV–vis spectrophotometer (TU-1901, China) that equipped with an integrating sphere assembly. BaSO4 was used as a reflectance sample. The photoluminescence (PL) spectra were measured at room temperature on a fluorospectrophotometer (Fluorolog-3Tau, France) with excitation source of Xe lamp. Photocaytalytic degradation of toluene was carried out in a 1.8 L photocatalytic reactor sealed with quartz plate. The UV irradiation was provided by a 150 W Xe lamp, and an optical glass filter was inserted to cut off the infrared (k > 800 nm). The sample was placed on the bottom of the reactor for test. The analysis of toluene concentration was conducted with a GCFID (FULI 9790, China). The initial concentration of toluene was controlled at about 200 mg m3. 3. Results and discussion Fig. 1a–d shows microstructures of NW, IRNW, NT and P25. The thickness of the films is 11.1, 3.6, 4.1 and 5.4 lm, respectively. It can be observed from Fig. 1a that the tops of the wires are bundled together for NW. The wires are irregular shaped for IRNW as shown in Fig. 1b. Fig. 1a and b shows that TiO2 nanotubes can be
1693
still detected under the nanowires. NT was yielded under the same conditions as that of NW, except the content of HF and water decreased from 0.2 wt% and 0.3 wt% to 0.08 wt% and 0.12 wt%. However, it can be seen that there are no nanowires on the surface of NT. These results demonstrate the key role of hydrogen ions for the anodic formation of TiO2 as shown in Equation (1). In addition, only irregular nanowires (Fig. 1b) can be observed for 2 h anodization, while more uniform nanowires (Fig. 1a) are produced for 3 h anodization under the same conditions. It revealed that anodic time is an important factor for the preparation process. The results above are helpful for understanding the formation mechanism of TiO2 nanowires. It seems that the nanowires were grown originally from the vertical splitting of nanotubes. The increase of hydrogen ions initially broke the balance of the electrochemical etching and chemical dissolution reactions during the anodic oxidation process. It led to chemical dissolution on the wall of TiO2 tubes, and formed IRNW. After that, the edge of the irregular nanowires might get smoother with the increase of anodic time, which facilitated the formation of NW. This mechanism is similar to that proposed by Lim and Choi [10]. Above all, nanowires and irregular nanowires could be produced by optimizing the electrolyte solution content and anodic time. Fig. 2 shows the XRD patterns of different types of TiO2 nanowires: NW and IRNW, with thicknesses of 11.1 and 3.6 lm, respectively. For comparison, the XRD patterns of NT and P25 are also listed in Fig. 2. It can be seen that the crystallizations of NW, IRNW and NT are mainly anatase phase, as evidenced by the strong diffraction peaks at 2h = 25.5, 37.1, 48.3, 54.1, and 55.3, which can be indexed attributed to the crystal faces of anatase TiO2 (JCPDS Files No. 21-1272). The high intensity of anatase (0 0 4) crystal faces of NW and IRNW promises the possibility of the growth of single crystalline TiO2 [15]. The diffraction peaks of P25 film reveal the existent phase of rutile TiO2. The average crystalline sizes can be calculated from the broadening of the anatase (1 0 1) peaks by using the Scherrer equation. The results are 28.3, 28.9, 20.0 and 21.0 nm for NW, IRNW, NT and P25, respectively. The XRD spectra
Fig. 1. SEM images of top view and cross view of NW (a), IRNW (b), NT (c), and P25 (d) the length of each sample was marked in the figure.
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Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
Fig. 2. X-ray diffraction patterns of TiO2 films of NW, IRNW, NT and P25.
indicate that the anodic time has less influence on the crystalline sizes than on the lengths and surface morphologies of the samples. The optical properties of NW and IRNW is characterized by UV– vis DRS and PL spectrum. The optical properties of NT and P25 are also tested for comparison. Fig. 3a shows the UV–vis adsorption spectra of NW, IRNW, NT and P25, respectively. It can be seen that the strong adsorption wavelengths of NW and IRNW are in the range of 240 to 360 nm, which are wider than that of P25 film and similar to that of NT. The band gap absorption edges of NW, IRNW and NT were around 386 nm, which are close to the band gap absorption edge of anatase TiO2 with the band gap energy of 3.23 eV [16]. Moreover, stronger visible light absorptions of NW, IRNW and NT can be observed from Fig. 3a. This might be caused by the incomplete removal of electrolyte, which brought carbon impurities to the surface of the samples during the annealing process. Fig. 3b shows the photoluminescence of NW, IRNW, NT and P25. Notable PL peak shifting of NW and NT towards that of P25 is observed. The PL peak shifting might ascribe to the different phase types of the samples [17], which were further revealed by the XRD analysis above. The PL intensity of IRNW is too low to determine its peak position. Generally, the PL intensity can be employed to understand the fate of electron/hole pairs in semiconductor particles [18], and low PL intensity indicates that the separations of free carriers are promoted. The promotion usually results in the improvement of photocatalytic ability. The photcatalytic activities of NW and IRNW for air purification were investigated by photodegradation of gaseous toluene under UV irradiation. NT and P25 films were also tested under identical conditions for comparison. The photocatalytic oxidation of toluene is a pseudo-first-order reaction and its kinetics may be expressed by ln (C0/C) = kt, where k is the apparent reaction rate constant, C0 and C are the initial concentration and the reaction concentration of toluene [18]. The photocatalytic activities of the samples can be quantitatively evaluated by comparing the apparent reaction rate constants k. The results are shown in Fig. 4. The k values are 0.0741, 0.1256, 0.0816, and 0.0776 min1 for NW, IRNW, NT, and P25, respectively. It is widely accepted that the absorption of the incident photons would be increased with the increase of film thickness [19]. However, it is noticeable that the thickest film of NW (11.1 lm thick, Fig. 1a) has similar photocatalytic activity to NT (4.1 lm thick, Fig. 1c) and P25 (5.4 lm thick, Fig. 1d). This could be attributed to the fact that the congregation of the bundled nanowires and the elimination of nanotube mouths reduced the surface area. The best photocatalytic activity is demonstrated by
Fig. 3. UV–vis DRS spectra (a) and photoluminescence spectra (b) of TiO2 films of NW, IRNW, NT, and P25.
Fig. 4. The apparent reaction rate constants of NW, IRNW, NT, and P25 for photodegradation of gaseous toluene.
IRNW (3.6 lm thick, Fig. 1b) with mixed morphologies of irregular nanowires on top and nanotubes on bottom. The unique morphology endowed IRNW larger surface area, hollow interior wall, and hierarchical porous structures, which results in a faster diffusion of various gas phase species during the photocatalytic reaction.
Z. Wu et al. / Electrochemistry Communications 11 (2009) 1692–1695
The unique morphology of IRNW also gains an advantage in recombination of the electron and hole pairs, which were proved by its lower PL intensity (See Fig. 3). All these factors contributed to the enhancement of photocatalytic activity.
1695
151 Talent Project of Zhejiang Province and the New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549). References
4. Conclusions In this paper, it was demonstrated that immobilized TiO2 nanowires and irregular TiO2 nanowires could be prepared by anodic oxidation of titanium foil in ethylene glycol electrolyte solution with suitable content of HF and water. The balance of electrochemical etching reaction and chemical dissolution reaction during the anodic oxidation process was adjusted by hydrogen ions, which derived from varying the content of HF and water. This balance led to chemical dissolution of the wall of TiO2 nanotubes, resulting in the formation of nanowires. It was also found that the anodic time was also an important factor of the morphologies of TiO2 nanowires. The photodegradation of gaseous toluene experimental tests showed the irregular nanowires were of the highest photocatalytic activity, which might be the results of their larger surface area, hollow interior wall, and hierarchical porous structures. Acknowledgments The project was financially supported by the National Natural Science Foundation of China (NSFC-50808156), the New Century
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33. M.H. Zhou, X.J. Ma, Electrochem. Commun. 11 (2009) 921. X. Quan, S.G. Yang, X.L. Ruan, H.M. Zhao, Environ. Sci. Technol. 39 (2005) 3770. J. Jakubowicz, Electrochem. Commun. 10 (2008) 735. M. Sleiman, P. Conchon, C. Ferronato, J.M. Chovelon, Appl. Catal. B 86 (2009) 159. Z.Y. Liu, X.T. Zhang, S. Nishimoto, T. Murakami, A. Fujishima, Environ. Sci. Technol. 42 (2008) 8547. J. Jitputti, Y. Suzuki, S. Yoshikawa, Catal. Commun. 9 (2008) 1265. B. Liu, J.E. Boercker, E.S. Aydil, Nanotechnology 19 (2008) 505604. Y.N. Zhao, J. Jin, X.Q. Yang, Mater. Lett. 61 (2007) 384. J.H. Lim, J. Choi, Small 3 (2007) 1504. Y.Y. Song, R. Lynch, D. Kim, P. Roy, P. Schmuki, Electrochem. Solid State 12 (2009) C17. D. Kim, A. Ghicov, P. Schmuki, Electrochem. Commun. 10 (2008) 1835. M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, T.A. Desai, C.A. Grimes, J. Phys. Chem. C 111 (2007) 14992. D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. J.L. Zhao, X.H. Wang, T.Y. Sun, L.T. Li, Nanotechnology 16 (2005) 2450. Z.H. Zhang, Y. Yuan, G.Y. Shi, Y.J. Fang, L.H. Liang, H.C. Ding, L.T. Jin, Environ. Sci. Technol. 41 (2007) 6259. K. Fujihara, S. Izumi, T. Ohno, M. Matsumura, J. Photoch. Photobio. A 132 (2000) 99. F. Dong, W.R. Zhao, Z.B. Wu, Nanotechnology 19 (2008) 365607. H.C. Liang, X.Z. Li, J. Hazard. Mater. 162 (2009) 1415.