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Superhydrophilic anatase TiO2 film with the micro- and nanometer-scale hierarchical surface structure
Materials Letters 62 (2008) 3503–3505
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Superhydrophilic anatase TiO2 film with the micro- and nanometer-scale hierarchical surface structure Shu Song, Liqiang Jing ⁎, Shudan Li, Honggang Fu ⁎, Yunbo Luan The Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China
A R T I C L E
I N F O
Article history: Received 23 November 2007 Received in revised form Accepted 5 March 2008 Available online 8 March 2008 Keywords: TiO2 Thin films Surfaces Superhydrophilicity
A B S T R A C T Nanosized anatase TiO2 film on the ITO glass has been fabricated via spin coat process, with sodium dodecylbenzenesulfonate modified TiO2 nanoparticles, which is synthesized by a sol–hydrothermal method, and also characterized mainly by means of field emission scanning electron microscopy (FESEM). The results show that the as-prepared anatase TiO2 film exhibits superhydrophilic characteristic although it is not exposed to ultraviolet irradiation. The high roughness resulting from hierarchical surface structure is responsible for its superhydrophilicity. This work would provide a new route to fabricate newly nanostructured semiconductor films. © 2008 Published by Elsevier B.V.
1. Introduction In recent two decades, in addition to environmental purification and solar energy conversion [1–3], the coating of nanosized TiO2, featuring a contact angle (CA) of water lower than 5°, has developed as a new and very attractive application [4], due to its antifogging and self-cleaning properties [5]. It has been well recognized that the wettability of solid surfaces is mainly governed by both the chemical composition and geometrical microstructure [6–8]. The obviously enhanced hydrophilic TiO2 films can be obtained by utilizing polyethylene glycol (PEG) [9,10]. The cetyltrimethyl ammonium bromide as a cationic surfactant can increase the roughness of titania sol–gel films [11]. However, the surfactants have seldom been employed, especially for combining with organic macromolecule substances, to modify surface microstructure during the preparation processes of TiO2 films, up to date. Moreover, the superhydrophilic TiO2 surfaces can be achieved easily by ultraviolet (UV) illumination, even displaying amphiphilic for both water and oil droplets [12,13]. Nevertheless, there is the limitation that the TiO2 film tends to lose its superhydrophilicity, when UV illumination is switched off. Actually, it is desired to prepare the superhydrophilic TiO2 films independent of UV illumination from the practical application of view. However, to the best of our knowledge, few works on superhydrophilic TiO2 films with no UV radiation are reported until now. Very recently, Wee Yong Gan et al
⁎ Corresponding authors. Tel.:+86 451 86608616; fax: +86 451 86673647. E-mail addresses: [email protected] (L. Jing), [email protected] (H. Fu). 0167-577X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matlet.2008.03.005
reported TiO2 thin film with non-UV activated superwettability fabricated via multilayer assembly of TiO2 nanoparticles and PEG, and the multilayer structure is responsible for its wettability [14]. Sodium dodecylbenzenesulfonate (DBS), a typical anion surfactant, is commonly used to modify TiO2 nanoparticles so that the optical performance, dispersed situation and photocatalytic activity are changed accordingly [15–17]. The DBS-modified TiO2 nanoparticles, synthesized by a hydrothermal process in our group, can exhibit high photocatalytic activity, superior to the un-modified ones [15]. In this work, a superhydrophilic nanosized TiO2 film was synthesized by adopting DBS-modified TiO2 nanoparticles instead of un-modified ones, followed by a spin coat process. It can be suggested that the formation of the rough surface microstructure, which contributes mainly to the superhydrophilicity, is close related to the surface modification of TiO2 with DBS groups. 2. Experimental The nanosized anatase TiO2 films on the ITO glasses were fabricated by spin coating a certain amount of the mixture, which consists of 1 g of freshly prepared TiO2 nanoparticles modified with DBS groups (or without DBS groups) via a sol–hydrothermal method [15], 1 mL of PEG 400 and 5 μL of acetylacetone, with the rotation speed of 1000 rpm for 15 s and 3000 rpm for 30 s in succession, followed by drying at 80 °C and thermal treatment at 400 °C for 2 h. To enhance the adhesion between the resulting TiO2 nanoparticles and the ITO substrate, prior to the spin coating, a TiO2 gel layer was pasted onto the well-cleaned ITO glasses by dip coating the prepared semitransparent TiO2 sol, composed of ethanol, tetrabutyl titanate,
3504
S. Song et al. / Materials Letters 62 (2008) 3503–3505 smaller than that in the TF, and the TiO2 crystallinity becomes weak [19], which demonstrates that the DBS addition has an inhibiting effect on the crystallization and growth of anatase TiO2. In Fig. 2, the peaks at 147, 400, 520 and 644 cm− 1 could be ascribed to the Eg, B1g, B2g and Eg Raman vibration modes, respectively, which are the characteristics of the anatase TiO2 [20]. It should be noted that the slight blue shift of the B1g, B2g and Eg modes in the DTF takes place compared with the TF, which is because of the increase in the surface oxygen deficiency content [21], mainly resulting from the smaller particle size. It is consistent with the XRD results. Although the added DBS groups cannot be present in the DTF after thermally treated at 400 °C for 2 h, they are prone to link with the TiO2 crystal nucleus and crystallites by quasi-sulphonate bonds prior to the thermal treatment, hence suppressing the subsequent growth of the crystalline TiO2 [15]. It can be seen from Fig. 3 that both the TF and the DTF are composed of spherical nanoparticles with narrow size distribution of about 6 nm and 5 nm, respectively. And, the TF has a smooth surface with a compact configuration. By contrast, it can be confirmed that the surface of the DTF is considerable rough. Interestingly, the DTF, with
Fig. 1. XRD patterns of the as-prepared films.
acetylacetone, water and polyethylene glycol in a certain proportion, subsequently placed vertically in air for 1 h and dried at 80 °C for 2 h. The as-prepared films resulting from the TiO2 nanoparticles modified with and without DBS were referred as DTF and TF, respectively. The samples were characterized by X-ray powder diffraction (XRD) with a Rigaku D/MAX-rA powder diffractormeter(Japan), using Cu Kα radiation (λ = 0.15418 nm). An accelerating voltage of 30 kV and emission current of 20 mA were employed. The Raman spectra of the samples were recorded with JOBIN YVON HR800 Raman spectrophotometer (France), and the used excitation wavelength was 458 nm. The surface morphology of the TiO2 films were observed by field emission scanning electron microscopy (FESEM) with a Philip XL-30ESEM-FEG microscope (Holland), operated at 20 kV. The contact angle (CA) measurements were performed with a JY-82 contact angle meter (China). And water droplets were dribbled onto three different positions for each film sample and the average value was recorded as the contact angle. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared TiO2 films. The XRD peak at 2θ = 25.28° confirms that there is only anatase phase in the two films [18]. Apart from the XRD peak at about 25.28°, the other two XRD peaks at about 30° and 35.2° result from the ITO glass. Compared with the TF, the characteristic XRD peaks of the DTF is slightly lower and weaker, indicating that the average crystallite size in the DTF is
Fig. 2. Raman spectra of the as-prepared films.
Fig. 3. The top view of the FE-SEM images of the TF (a), the DTF (b) and the selected enlarged part view of the DTF (c).
S. Song et al. / Materials Letters 62 (2008) 3503–3505 the micro- and nanometer-scale hierarchical surface structure, has many micropapillae (about 0.6 μm), resulting from the aggregation of many TiO2 nanoparticles. Its possible formation mechanism is preliminarily deduced on the basis of DBS groups with amphiphilic property as follows. TiO2 crystallites are first capped by an appropriate amount of DBS groups through quasi-sulphonate bonds during the hydrothermal processes, further changing surface attribute from hydrophilic to hydrophobic. Thus, a number of the DBS-capped TiO2 nanoparticles can easily connect with the PEG molecule chain via hydrophobic part (C–C chains) of the DBS molecule. Then, during the solvent volatilizing processes, the DBS-capped TiO2 nanoparticles would conglomerate due to the shrinkage of the PEG molecule chains [22], which would possibly lead to the formation of the micropapillae after the thermal treatment at 400 °C. On the contrary, in the absence of DBS, the hierarchical surface structure cannot be formed as shown in Fig. 3a. The wettability of the TF and DTF, both prepared freshly, is examined by the water CA measurement. The results show that the CA value of the TF is around 35°, indicating that the TF is hydrophilic. Interestingly, although the DTF is not exposed to ultraviolet irradiation prior to the CA measurement, it exhibits superhydrophilicity since its CA value closes to 0°. It is well known that the increase in the surface roughness is favorable to improve the wettability [23]. For the DTF, its surface is very rough because of the micro- and nanometer-scale hierarchical structure, which is characterized by lots of micropapillae dispersed uniformly on the film surface. Every micropapilla results from the aggregation of a great number of nanoparticles so that there are many threedimensional tiny voids between nanoparticles. Thus, when a water droplet is dribbled onto the DTF, the droplet would spread instantly due to the capillary effects of the three-dimensional tiny voids as well as the hydrophilic effect of TiO2 in nature [24]. Moreover, the increase in the oxygen deficiency content would also be beneficial for wettability [25]. Therefore, it can be suggested that the distinguishing surface structure with high roughness and many three-dimensional tiny voids, as well as the oxygen vacancy content increase, are responsible for the superhydrophilicity.
4. Conclusion In summary, a non-UV activated superhydrophilic TiO2 film has been prepared via spin coat processes with DBS-modified TiO2. It can be suggested that the superhydrophilic performance is mainly attributed to the high roughness resulting from hierarchical surface structure. The formation of characteristic surface structure is close related to the DBS addition. This will be useful to fabricate other newly nanostructured semiconductor film materials, and promote the practical application in the wetting areas of TiO2 films. Acknowledgments This work is supported from the National Nature Science Foundation of China (No. 20431030, 20501007), the programme for New Century Excellent Talents in universities (NCET-07-0259), the Key Project of Science & Technology Research of Ministry of Education
3505
of China (No. 207027) and the Science Foundation of Excellent Youth of Heilongjiang Province of China (JC200701), for which we are very grateful. References [1] S.N. Frank, A.J. Bard, J. Am. Chem. Soc. 99 (1977) 303–304. [2] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahneman, Chem. Rev. 95 (1995) 69–96. [3] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758. [4] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432. [5] J. Premkumar, Chem. Mater. 16 (2004) 3980–3981. [6] H.Y. Lee, Y.H. Park, K.H. Ko, Langmuir 16 (2000) 7289–7293. [7] K.S. Liu, H.G. Fu, K.S. Shi, B.F. Xin, L.Q. Jing, W. Zhou, Nanotechnology 17 (2006) 3641–3648. [8] H. Tokuhisa, P.T. Hammond, Langmuir 20 (2004) 1436–1441. [9] J.C. Yu, J. Yu, H.Y. Tang, L. Zhang, J. Mater. Chem. 12 (2002) 81–85. [10] D.S. Kommireddy, A.A. Patel, T.G. Shutava, D.K. Mills, Y.M. Lvov, J. Nanosci. Nanotechnol. 5 (2005) 1081–1089. [11] J. Medina-Valtierra, C. Frausto-Reyes, S. Calixto, P. Bosch, V.H. Lara, Mater. Charact. 58 (2007) 233–242. [12] R. Wang, K. Hashimoto, A. Fujishima, Adv. Mater. 10 (1998) 135–138. [13] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 3023–3026. [14] W.Y. Gan, S.W. Lam, K. Chiang, R. Amal, H.J. Zhao, M.P. Brungs, J. Mater. Chem. 17 (2007) 952–954. [15] B.Q. Wang, L.Q. Jing, Y.C. Qu, S.D. Li, B.J. Jiang, L.B. Yang, B.F. Xin, H.G. Fu, Appl. Surf. Sci. 252 (2006) 2817–2825. [16] G. Ramakrishna, H.N. Ghosh, Langmuir 19 (2003) 505–508. [17] C.Y. Xu, P.X. Zhang, L. Yan, J. Raman Spectrosc. 32 (2001) 862–865. [18] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808–3816. [19] Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B 26 (2000) 207–215. [20] C.C. Hyun, M.J. Young, B.K. Seung, Vibr. Spectrosc. 37 (2005) 33–38. [21] J.K. Zhou, Y.X. Zhang, X.S. Zhao, A.K. Ray, Ind. Eng. Chem. Res. 45 (2006) 3503–3511. [22] C.C. Weng, K.H. Wei, Chem. Mater. 15 (2003) 2936–2941. [23] X.J. Feng, L. Jiang, Adv. Mater. 18 (2006) 3063–3078. [24] A. Nakajima, S. Koizumi, T. Watanabe, K. Hashimoto, Langmuir 16 (2000) 7048–7050. [25] M. Miyauchi, H. Tokudome, J. Mater. Chem. 17 (2007) 2095–2100.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Superhydrophilic anatase TiO2 film with the micro- and nanometer-scale hierarchical surface structure Shu Song, Liqiang Jing ⁎, Shudan Li, Honggang Fu ⁎, Yunbo Luan The Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China
A R T I C L E
I N F O
Article history: Received 23 November 2007 Received in revised form Accepted 5 March 2008 Available online 8 March 2008 Keywords: TiO2 Thin films Surfaces Superhydrophilicity
A B S T R A C T Nanosized anatase TiO2 film on the ITO glass has been fabricated via spin coat process, with sodium dodecylbenzenesulfonate modified TiO2 nanoparticles, which is synthesized by a sol–hydrothermal method, and also characterized mainly by means of field emission scanning electron microscopy (FESEM). The results show that the as-prepared anatase TiO2 film exhibits superhydrophilic characteristic although it is not exposed to ultraviolet irradiation. The high roughness resulting from hierarchical surface structure is responsible for its superhydrophilicity. This work would provide a new route to fabricate newly nanostructured semiconductor films. © 2008 Published by Elsevier B.V.
1. Introduction In recent two decades, in addition to environmental purification and solar energy conversion [1–3], the coating of nanosized TiO2, featuring a contact angle (CA) of water lower than 5°, has developed as a new and very attractive application [4], due to its antifogging and self-cleaning properties [5]. It has been well recognized that the wettability of solid surfaces is mainly governed by both the chemical composition and geometrical microstructure [6–8]. The obviously enhanced hydrophilic TiO2 films can be obtained by utilizing polyethylene glycol (PEG) [9,10]. The cetyltrimethyl ammonium bromide as a cationic surfactant can increase the roughness of titania sol–gel films [11]. However, the surfactants have seldom been employed, especially for combining with organic macromolecule substances, to modify surface microstructure during the preparation processes of TiO2 films, up to date. Moreover, the superhydrophilic TiO2 surfaces can be achieved easily by ultraviolet (UV) illumination, even displaying amphiphilic for both water and oil droplets [12,13]. Nevertheless, there is the limitation that the TiO2 film tends to lose its superhydrophilicity, when UV illumination is switched off. Actually, it is desired to prepare the superhydrophilic TiO2 films independent of UV illumination from the practical application of view. However, to the best of our knowledge, few works on superhydrophilic TiO2 films with no UV radiation are reported until now. Very recently, Wee Yong Gan et al
⁎ Corresponding authors. Tel.:+86 451 86608616; fax: +86 451 86673647. E-mail addresses: [email protected] (L. Jing), [email protected] (H. Fu). 0167-577X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matlet.2008.03.005
reported TiO2 thin film with non-UV activated superwettability fabricated via multilayer assembly of TiO2 nanoparticles and PEG, and the multilayer structure is responsible for its wettability [14]. Sodium dodecylbenzenesulfonate (DBS), a typical anion surfactant, is commonly used to modify TiO2 nanoparticles so that the optical performance, dispersed situation and photocatalytic activity are changed accordingly [15–17]. The DBS-modified TiO2 nanoparticles, synthesized by a hydrothermal process in our group, can exhibit high photocatalytic activity, superior to the un-modified ones [15]. In this work, a superhydrophilic nanosized TiO2 film was synthesized by adopting DBS-modified TiO2 nanoparticles instead of un-modified ones, followed by a spin coat process. It can be suggested that the formation of the rough surface microstructure, which contributes mainly to the superhydrophilicity, is close related to the surface modification of TiO2 with DBS groups. 2. Experimental The nanosized anatase TiO2 films on the ITO glasses were fabricated by spin coating a certain amount of the mixture, which consists of 1 g of freshly prepared TiO2 nanoparticles modified with DBS groups (or without DBS groups) via a sol–hydrothermal method [15], 1 mL of PEG 400 and 5 μL of acetylacetone, with the rotation speed of 1000 rpm for 15 s and 3000 rpm for 30 s in succession, followed by drying at 80 °C and thermal treatment at 400 °C for 2 h. To enhance the adhesion between the resulting TiO2 nanoparticles and the ITO substrate, prior to the spin coating, a TiO2 gel layer was pasted onto the well-cleaned ITO glasses by dip coating the prepared semitransparent TiO2 sol, composed of ethanol, tetrabutyl titanate,
3504
S. Song et al. / Materials Letters 62 (2008) 3503–3505 smaller than that in the TF, and the TiO2 crystallinity becomes weak [19], which demonstrates that the DBS addition has an inhibiting effect on the crystallization and growth of anatase TiO2. In Fig. 2, the peaks at 147, 400, 520 and 644 cm− 1 could be ascribed to the Eg, B1g, B2g and Eg Raman vibration modes, respectively, which are the characteristics of the anatase TiO2 [20]. It should be noted that the slight blue shift of the B1g, B2g and Eg modes in the DTF takes place compared with the TF, which is because of the increase in the surface oxygen deficiency content [21], mainly resulting from the smaller particle size. It is consistent with the XRD results. Although the added DBS groups cannot be present in the DTF after thermally treated at 400 °C for 2 h, they are prone to link with the TiO2 crystal nucleus and crystallites by quasi-sulphonate bonds prior to the thermal treatment, hence suppressing the subsequent growth of the crystalline TiO2 [15]. It can be seen from Fig. 3 that both the TF and the DTF are composed of spherical nanoparticles with narrow size distribution of about 6 nm and 5 nm, respectively. And, the TF has a smooth surface with a compact configuration. By contrast, it can be confirmed that the surface of the DTF is considerable rough. Interestingly, the DTF, with
Fig. 1. XRD patterns of the as-prepared films.
acetylacetone, water and polyethylene glycol in a certain proportion, subsequently placed vertically in air for 1 h and dried at 80 °C for 2 h. The as-prepared films resulting from the TiO2 nanoparticles modified with and without DBS were referred as DTF and TF, respectively. The samples were characterized by X-ray powder diffraction (XRD) with a Rigaku D/MAX-rA powder diffractormeter(Japan), using Cu Kα radiation (λ = 0.15418 nm). An accelerating voltage of 30 kV and emission current of 20 mA were employed. The Raman spectra of the samples were recorded with JOBIN YVON HR800 Raman spectrophotometer (France), and the used excitation wavelength was 458 nm. The surface morphology of the TiO2 films were observed by field emission scanning electron microscopy (FESEM) with a Philip XL-30ESEM-FEG microscope (Holland), operated at 20 kV. The contact angle (CA) measurements were performed with a JY-82 contact angle meter (China). And water droplets were dribbled onto three different positions for each film sample and the average value was recorded as the contact angle. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared TiO2 films. The XRD peak at 2θ = 25.28° confirms that there is only anatase phase in the two films [18]. Apart from the XRD peak at about 25.28°, the other two XRD peaks at about 30° and 35.2° result from the ITO glass. Compared with the TF, the characteristic XRD peaks of the DTF is slightly lower and weaker, indicating that the average crystallite size in the DTF is
Fig. 2. Raman spectra of the as-prepared films.
Fig. 3. The top view of the FE-SEM images of the TF (a), the DTF (b) and the selected enlarged part view of the DTF (c).
S. Song et al. / Materials Letters 62 (2008) 3503–3505 the micro- and nanometer-scale hierarchical surface structure, has many micropapillae (about 0.6 μm), resulting from the aggregation of many TiO2 nanoparticles. Its possible formation mechanism is preliminarily deduced on the basis of DBS groups with amphiphilic property as follows. TiO2 crystallites are first capped by an appropriate amount of DBS groups through quasi-sulphonate bonds during the hydrothermal processes, further changing surface attribute from hydrophilic to hydrophobic. Thus, a number of the DBS-capped TiO2 nanoparticles can easily connect with the PEG molecule chain via hydrophobic part (C–C chains) of the DBS molecule. Then, during the solvent volatilizing processes, the DBS-capped TiO2 nanoparticles would conglomerate due to the shrinkage of the PEG molecule chains [22], which would possibly lead to the formation of the micropapillae after the thermal treatment at 400 °C. On the contrary, in the absence of DBS, the hierarchical surface structure cannot be formed as shown in Fig. 3a. The wettability of the TF and DTF, both prepared freshly, is examined by the water CA measurement. The results show that the CA value of the TF is around 35°, indicating that the TF is hydrophilic. Interestingly, although the DTF is not exposed to ultraviolet irradiation prior to the CA measurement, it exhibits superhydrophilicity since its CA value closes to 0°. It is well known that the increase in the surface roughness is favorable to improve the wettability [23]. For the DTF, its surface is very rough because of the micro- and nanometer-scale hierarchical structure, which is characterized by lots of micropapillae dispersed uniformly on the film surface. Every micropapilla results from the aggregation of a great number of nanoparticles so that there are many threedimensional tiny voids between nanoparticles. Thus, when a water droplet is dribbled onto the DTF, the droplet would spread instantly due to the capillary effects of the three-dimensional tiny voids as well as the hydrophilic effect of TiO2 in nature [24]. Moreover, the increase in the oxygen deficiency content would also be beneficial for wettability [25]. Therefore, it can be suggested that the distinguishing surface structure with high roughness and many three-dimensional tiny voids, as well as the oxygen vacancy content increase, are responsible for the superhydrophilicity.
4. Conclusion In summary, a non-UV activated superhydrophilic TiO2 film has been prepared via spin coat processes with DBS-modified TiO2. It can be suggested that the superhydrophilic performance is mainly attributed to the high roughness resulting from hierarchical surface structure. The formation of characteristic surface structure is close related to the DBS addition. This will be useful to fabricate other newly nanostructured semiconductor film materials, and promote the practical application in the wetting areas of TiO2 films. Acknowledgments This work is supported from the National Nature Science Foundation of China (No. 20431030, 20501007), the programme for New Century Excellent Talents in universities (NCET-07-0259), the Key Project of Science & Technology Research of Ministry of Education
3505
of China (No. 207027) and the Science Foundation of Excellent Youth of Heilongjiang Province of China (JC200701), for which we are very grateful. References [1] S.N. Frank, A.J. Bard, J. Am. Chem. Soc. 99 (1977) 303–304. [2] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahneman, Chem. Rev. 95 (1995) 69–96. [3] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735–758. [4] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432. [5] J. Premkumar, Chem. Mater. 16 (2004) 3980–3981. [6] H.Y. Lee, Y.H. Park, K.H. Ko, Langmuir 16 (2000) 7289–7293. [7] K.S. Liu, H.G. Fu, K.S. Shi, B.F. Xin, L.Q. Jing, W. Zhou, Nanotechnology 17 (2006) 3641–3648. [8] H. Tokuhisa, P.T. Hammond, Langmuir 20 (2004) 1436–1441. [9] J.C. Yu, J. Yu, H.Y. Tang, L. Zhang, J. Mater. Chem. 12 (2002) 81–85. [10] D.S. Kommireddy, A.A. Patel, T.G. Shutava, D.K. Mills, Y.M. Lvov, J. Nanosci. Nanotechnol. 5 (2005) 1081–1089. [11] J. Medina-Valtierra, C. Frausto-Reyes, S. Calixto, P. Bosch, V.H. Lara, Mater. Charact. 58 (2007) 233–242. [12] R. Wang, K. Hashimoto, A. Fujishima, Adv. Mater. 10 (1998) 135–138. [13] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 105 (2001) 3023–3026. [14] W.Y. Gan, S.W. Lam, K. Chiang, R. Amal, H.J. Zhao, M.P. Brungs, J. Mater. Chem. 17 (2007) 952–954. [15] B.Q. Wang, L.Q. Jing, Y.C. Qu, S.D. Li, B.J. Jiang, L.B. Yang, B.F. Xin, H.G. Fu, Appl. Surf. Sci. 252 (2006) 2817–2825. [16] G. Ramakrishna, H.N. Ghosh, Langmuir 19 (2003) 505–508. [17] C.Y. Xu, P.X. Zhang, L. Yan, J. Raman Spectrosc. 32 (2001) 862–865. [18] J.C. Yu, J.G. Yu, W.K. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2002) 3808–3816. [19] Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B 26 (2000) 207–215. [20] C.C. Hyun, M.J. Young, B.K. Seung, Vibr. Spectrosc. 37 (2005) 33–38. [21] J.K. Zhou, Y.X. Zhang, X.S. Zhao, A.K. Ray, Ind. Eng. Chem. Res. 45 (2006) 3503–3511. [22] C.C. Weng, K.H. Wei, Chem. Mater. 15 (2003) 2936–2941. [23] X.J. Feng, L. Jiang, Adv. Mater. 18 (2006) 3063–3078. [24] A. Nakajima, S. Koizumi, T. Watanabe, K. Hashimoto, Langmuir 16 (2000) 7048–7050. [25] M. Miyauchi, H. Tokudome, J. Mater. Chem. 17 (2007) 2095–2100.