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Room temperature synthesis of nearly monodisperse rodlike rutile TiO2 nanocrystals
Materials Letters 63 (2009) 127–129
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 ev i e r. c o m / l o c a t e / m a t l e t
Room temperature synthesis of nearly monodisperse rodlike rutile TiO2 nanocrystals Sen Zhang a,b, Chun-Yan Liu a,⁎, Yun Liu a, Zhi-Ying Zhang a, Li-Juan Mao a,b a b
Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China Graduate School of the Chinese Academy of Sciences, Beijing 100806, China
a r t i c l e
i n f o
Article history: Received 17 December 2007 Accepted 24 September 2008 Available online 27 September 2008 Keywords: Titania Rutile Room temperature Monodisperse nanocrystal Photocatalyst
a b s t r a c t A new method was developed to synthesize uniform rodlike rutile TiO2 nanocrystals by the hydrolysis of tetrabutyl titanate [Ti(OC4H9)4] in hydrochloric acid–alcohol aqueous solutions at room temperature. The hydrolytic sol–gel reaction generated 44 nm (diameter) × 200 nm (length) sized rutile TiO2 nanocrystals. Transmission electron microscopic images showed that the particles have a uniform shape and narrow size distribution. X-ray diffraction and electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the rodlike TiO2 nanoparticles prepared at room temperature were crystalline rutile structure grown along the [001] direction. The morphology and photocatalytic activity of the TiO2 nanocrystals formed at different urea concentrations were showed. The rutile TiO2 nanocrystals formed in the absence of urea exhibited higher photocatalytic activity than the commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, titanium dioxide (TiO2) has been one of the most intensively researched substances due to its applications in photocatalysts [1,2], photovoltaic devices [3–5], gas sensors [6,7], photochromic devices [8] and lithium batteries [9]. TiO2 exists mainly in three crystal phases, namely brookite, anatase and rutile, with rutile being the thermodynamically stable form. In contrast to the anatase phase, rutile TiO2 has attracted less attention in producing photocatalysts [10] and dye-sensitized solar cells (DSCs) [11]. However, rutile TiO2 has some advantages over anatase such as higher refractive index, higher dielectric constant, higher electric resistance, higher chemical stability and cheaper production cost [12]. It has, therefore, found application in pigments, capacitors, filter and power circuits, polarizers and temperature compensating condensers [13–15]. Furthermore, rutile TiO2 has been proved to be comparable to anatase in application to DSCs [16]. The chemical routes to prepare rutile TiO2 can be mainly classified into two groups. One is to obtain rutile TiO2 by high-temperature calcination of first synthesized anatase or amorphous TiO2 [17–20]. In this way, a two-step process is needed and high-temperature calcination unavoidably leads to agglomeration and growth of nanocrystallites. It is difficult to obtain uniform nanocrystals with regular shapes by this route. The other route is the direct hydrolysis of
⁎ Corresponding author. Tel.: +86 10 82543573; fax: +86 10 62554670. E-mail address: [email protected] (C.-Y. Liu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.09.032
titanium salts such as titanium tetrachloride under hydrothermal or moderate conditions [21–24]. However, these methods produce normally polydisperse spherical or rodlike particles. Obviously, lowtemperature synthesis of uniform rutile TiO2 nanocrystals is still a large challenge. Herein, we reported a simple one-step method to synthesize nearly monodisperse pure rutile TiO2 nanocrystals at room temperature and demonstrated the dependence of product morphology on urea concentration. In most cases, rutile TiO2 showed poor photocatalytic activities[25,26]. Interestingly, the single-phase rutile TiO2 nanocrystals formed in the absence of urea in our study exhibited higher photocatalytic activity than the famous commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B. 2. Experimental section In a typical synthesis, 1 mL of tetrabutyl titanate [Ti(OC4H9)4] was dissolved in 10 mL absolute ethanol. After stirred for 30 min, the resulting solution was dropwise added into 100 mL of 0.5 M hydrochloric acid aqueous solution containing 0.25 M urea in an ice bath under vigorous stirring to form a misty mixture. After stirred in the ice bath for 4 h, the mixture was allowed to stand at room temperature (25 °C) for 12 days. The resulting white precipitate was collected by means of centrifugation, washed with deionized water, and dried in air at room temperature. The as-prepared products were characterized by X-ray powder diffraction (XRD; Rigaku DMAX-2000 X-ray diffractometer, with Cu Kα radiation (λ = 1.54056 Å)), transmission electron microscopy (TEM;
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S. Zhang et al. / Materials Letters 63 (2009) 127–129
Fig. 1. (a, b) TEM images of the as-prepared rodlike TiO2 nanocrystals; (c) electron diffraction pattern of a single TiO2 nanocrystal; (d) HRTEM image of one of the rodlike TiO2 nanocrystals, in which the inset is a low-magnification TEM image of the rodlike nanocrystal; (e) XRD pattern of the as-prepared rodlike TiO2 nanocrystals.
JEOL JEM-200CX, with an accelerating voltage of 160KV), and highresolution transmission electron microscopy (HRTEM; JEOL JEM-2010, with an accelerating voltage of 200 kV). The photocatalytic activities of the products were evaluated by the photodegradation of Rhodamine B in water. A cylindrical Pyrex flask was used as a photoreactor vessel. The aqueous system containing RhB (2.0 × 10− 5 M, 40 mL) and the as-prepared rutile TiO2 nanocrystals (20 mg) was magnetically stirred in the dark for 30 min to reach the adsorption equilibrium of RhB with the catalyst, and then exposed to
UV light from an high-pressure mercury lamp (λ b 330 nm). Commercially available TiO2 (Degussa P25) was adopted as a reference to compare the photocatalytic activity. During the photocatalytic reaction, the concentration of Rhodamine B was monitored by a Shimadu UV-1601 PC UV–VIS spectrophotometer. 3. Results and discussion The morphology and structure of the products were characterized with TEM and XRD. Fig. 1a indicates that the product is entirely comprised of nearly monodisperse
Fig. 2. TEM images of the rutile TiO2 nanocrystals formed with different urea concentrations: (a) 0, (b) 0.12, and (c) 0.5 M.
S. Zhang et al. / Materials Letters 63 (2009) 127–129 rodlike nanoparticles. The mean length and diameter of the particles are respectively 200 nm and 44 nm, which is the statistical result of more than 150 nanoparticles in Fig. 1a. The morphology of the rodlike TiO2 nanoparticles can be seen more clearly from the higher-magnification TEM image (Fig. 1b). Fig. 1c shows an electron diffraction (ED) pattern of an individual rodlike nanoparticle, which indicates that the rodlike TiO2 nanoparticles synthesized at room temperature are crystalline. X-ray diffraction pattern (Fig. 1e) of the product indicates the nearly monodisperse rodlike nanoparticles are tetragonal rutile TiO2 (JCPDS No. 21-1276). A HRTEM image of an individual rodlike nanocrystal is presented in Fig. 1d. The area observed in the image corresponds to the region indicated by the white frame in the inset of Fig. 1d. Lattice fringes, which are perpendicular to the wall of the rodlike nanocrystal, can be clearly seen due to phase contrast. The distance between the adjacent lattice fringes is 2.958 Å, corresponding to (001) planes of rutile TiO2, which indicates that the preferential growth direction of the nanocrystal is [001]. The hydrolytic sol–gel reactions using titanium alkoxides as precursors have been extensively studied and generally can be considered as a two-step process, namely a hydrolysis reaction and a condensation reaction [27]. Oliver and co-workers calculated the specific surface energies of rutile TiO2 based on atomistic simulation and claimed the surface energies of {001}, {100}, {221}, {011} and {110} were 2.40, 2.08, 2.02, 1.85 and 1.78 J m− 2, respectively [28]. Since the {001} and {110} surfaces are suggested to have the highest and the lowest surface energies respectively, the [001] direction is the favored growth direction. Consequently, the formed nanocrystals should have a high aspect ratio along [001]-axis. The rutile TiO2 nanocrystals prepared in our case show a mean aspect ratio of 4:1 along [001]-axis, which agrees very well with the theoretical calculation. For comparison, the same experiments were performed at the urea concentrations: 0, 0.12, and 0.5 M. Fig. 1a and Fig. 2 display the effect of urea concentration on product morphology. The rodlike nanoparticles formed in the absence of urea have lots of lateral branches (Fig. 2a). Obviously, the growth of lateral branches is inhibited effectively and the monodispersity of TiO2 nanocrystals is improved with the urea concentrations of 0.12 (Fig. 2b) and 0.25 M (Fig. 1a), but the higher urea concentration of 0.5 M results in the aggregation and high polydispersity of the particles (Fig. 2c). Thus, urea concentration is a key factor to control product morphology in the present experiments. However, the crystalline form of the rodlike nanoparticles is not related to urea concentration. As shown in Fig. 1e and Fig. 3, the products obtained at the different urea concentrations have the same composition and crystalline phase, namely tetragonal rutile TiO2 (JCPDS No. 21-1276). The photocatalytic activity of the products obtained with different urea concentrations was evaluated by the photocatalytic degradation of RhB. Fig. 4 clearly shows that all products possess obvious photocatalytic activity. Generally, rutile TiO2 shows poor photocatalytic activities. However, in our case, the single-phase rutile TiO2 nanocrystals formed in the absence of urea exhibit greater photocatalytic activity than the commercially available photocatalyst P25.
4. Conclusion In summary, nearly monodisperse rodlike rutile TiO2 nanocrystals have been synthesized at room temperature by a simple method. The dependence of product morphology on urea concentration is demonstrated. The product prepared in the absence of urea shows higher photocatalytic activity than the commercial photocatalyst P25. The adopted synthetic process is facile and proceeds in aqueous solutions
Fig. 3. XRD patterns of the rutile TiO2 nanocrystals formed with different urea concentrations: (a) 0, (b) 0.12, and (c) 0.5 M.
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Fig. 4. The photocatalytic degradation of RhB in the presence of commercial P25 and the rutile TiO2 nanocrystals (formed with different urea concentrations: 0, 0.12, 0.25 and 0.5 M).
at room temperature, which is very beneficial for the further study of rutile TiO2 nanocrystals such as surface modification and doping. Acknowledgments The work was supported by the National Nature Science Foundation of China (20573126), Chinese Academy of Sciences and National Basic Research Program of China (973 Program). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Subramanian V, Wolf E, Kamat PV. J Phys Chem, B 2001;105:11439–46. Tang J, Wu Y, McFarland EW, Stucky GD. Chem Commun 2004;14:1670–1. O'Regan B, Grätzel M. Nature 1991;353:737–8. Stergiopoulos T, Arabatzis IM, Katsaros G, Falaras P. Nano Lett 2002;2:1259–61. Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2006;6:215–8. Zhu Y, Shi J, Zhang Z, Zhang C, Zhang X. Anal Chem 2002;74:120–4. sharma RK, Bhatnagar MC, Sharma GL. Sens Actuators B: Chem 1998;46:194–201. Iuchi K, Ohko Y, Tatsuma T, Fujishima A. Chem Mater 2004;16:1165–7. Wagemaker M, Krol RVD, Kentgens APM, Well AAV, Mulder FM. J Am Chem Soc 2001;123:11454–61. Sun J, Gao L, Zhang Q. J Am Ceram Soc 2003;86:1677–82. Park NG, van de Lagemaat J, Frank AJ. J Phys Chem, B 2000;104:8989–94. Kim KJ, Benksten KD, van de Lagemaat J, Frank AJ. Chem Mater 2002;14:1042–7. Gonzalez RJ, Zallen R, Berger H. Phys Rev B 1997;55:7014–7. Gopal M, Chan MWJ. J Mater Sci 1997;32:6001–6. Sato T, Baba K, Hirozawa T, Kawakami S. IEEE J Quantum Electron 1993;29:175–81. Byun HY, Vittal R, Kim DY, Kim KJ. Langmuir 2004;20:6853–7. Zhang H, Banfield JF. J Phys Chem B 2000;104:3481–7. Pottier A, Chaneac C, Tronc E, Mazerolles L, Jolivet JP. J Mater Chem 2001;11:1116–21. Wu M, Lin G, Chen D, Wang G, He D, Feng S, et al. Chem Mater 2002;14:1974–80. Navrotsky A. Geochem Trans 2003;4:34–7. Cheng H, Ma J, Zhao Z, Qi L. Chem Mater 1995;7:663–71. Li Y, Fan Y, Chen Y. J Mater Chem 2002;12:1387–90. Yang K, Zhu JM, Zhu JJ, Huang S, Zhu X, Ma G. Mater Lett 2003;57:4639–42. Wang Y, Zhang L, Deng K, Chen X, Zou Z. J Phys Chem C 2007;111:2709–14. Xiaoxu L, Yujie X, Zhengquan L, Yi X. Inorg Chem 2006;45:3493–5. Andersson M, Osterlund L, Ljungstrom S, Palmqvist A. J Phys Chem, B 2002;106:10674–9. Livage J, Henry M, Sanchez C. Prog Solid State Chem 1988;18:259–341. Oliver PM, Watson GW, Kelsey ET, Parker SC. J Mater Chem 1997;3:563–8.
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 ev i e r. c o m / l o c a t e / m a t l e t
Room temperature synthesis of nearly monodisperse rodlike rutile TiO2 nanocrystals Sen Zhang a,b, Chun-Yan Liu a,⁎, Yun Liu a, Zhi-Ying Zhang a, Li-Juan Mao a,b a b
Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China Graduate School of the Chinese Academy of Sciences, Beijing 100806, China
a r t i c l e
i n f o
Article history: Received 17 December 2007 Accepted 24 September 2008 Available online 27 September 2008 Keywords: Titania Rutile Room temperature Monodisperse nanocrystal Photocatalyst
a b s t r a c t A new method was developed to synthesize uniform rodlike rutile TiO2 nanocrystals by the hydrolysis of tetrabutyl titanate [Ti(OC4H9)4] in hydrochloric acid–alcohol aqueous solutions at room temperature. The hydrolytic sol–gel reaction generated 44 nm (diameter) × 200 nm (length) sized rutile TiO2 nanocrystals. Transmission electron microscopic images showed that the particles have a uniform shape and narrow size distribution. X-ray diffraction and electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the rodlike TiO2 nanoparticles prepared at room temperature were crystalline rutile structure grown along the [001] direction. The morphology and photocatalytic activity of the TiO2 nanocrystals formed at different urea concentrations were showed. The rutile TiO2 nanocrystals formed in the absence of urea exhibited higher photocatalytic activity than the commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In the past decades, titanium dioxide (TiO2) has been one of the most intensively researched substances due to its applications in photocatalysts [1,2], photovoltaic devices [3–5], gas sensors [6,7], photochromic devices [8] and lithium batteries [9]. TiO2 exists mainly in three crystal phases, namely brookite, anatase and rutile, with rutile being the thermodynamically stable form. In contrast to the anatase phase, rutile TiO2 has attracted less attention in producing photocatalysts [10] and dye-sensitized solar cells (DSCs) [11]. However, rutile TiO2 has some advantages over anatase such as higher refractive index, higher dielectric constant, higher electric resistance, higher chemical stability and cheaper production cost [12]. It has, therefore, found application in pigments, capacitors, filter and power circuits, polarizers and temperature compensating condensers [13–15]. Furthermore, rutile TiO2 has been proved to be comparable to anatase in application to DSCs [16]. The chemical routes to prepare rutile TiO2 can be mainly classified into two groups. One is to obtain rutile TiO2 by high-temperature calcination of first synthesized anatase or amorphous TiO2 [17–20]. In this way, a two-step process is needed and high-temperature calcination unavoidably leads to agglomeration and growth of nanocrystallites. It is difficult to obtain uniform nanocrystals with regular shapes by this route. The other route is the direct hydrolysis of
⁎ Corresponding author. Tel.: +86 10 82543573; fax: +86 10 62554670. E-mail address: [email protected] (C.-Y. Liu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.09.032
titanium salts such as titanium tetrachloride under hydrothermal or moderate conditions [21–24]. However, these methods produce normally polydisperse spherical or rodlike particles. Obviously, lowtemperature synthesis of uniform rutile TiO2 nanocrystals is still a large challenge. Herein, we reported a simple one-step method to synthesize nearly monodisperse pure rutile TiO2 nanocrystals at room temperature and demonstrated the dependence of product morphology on urea concentration. In most cases, rutile TiO2 showed poor photocatalytic activities[25,26]. Interestingly, the single-phase rutile TiO2 nanocrystals formed in the absence of urea in our study exhibited higher photocatalytic activity than the famous commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B. 2. Experimental section In a typical synthesis, 1 mL of tetrabutyl titanate [Ti(OC4H9)4] was dissolved in 10 mL absolute ethanol. After stirred for 30 min, the resulting solution was dropwise added into 100 mL of 0.5 M hydrochloric acid aqueous solution containing 0.25 M urea in an ice bath under vigorous stirring to form a misty mixture. After stirred in the ice bath for 4 h, the mixture was allowed to stand at room temperature (25 °C) for 12 days. The resulting white precipitate was collected by means of centrifugation, washed with deionized water, and dried in air at room temperature. The as-prepared products were characterized by X-ray powder diffraction (XRD; Rigaku DMAX-2000 X-ray diffractometer, with Cu Kα radiation (λ = 1.54056 Å)), transmission electron microscopy (TEM;
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S. Zhang et al. / Materials Letters 63 (2009) 127–129
Fig. 1. (a, b) TEM images of the as-prepared rodlike TiO2 nanocrystals; (c) electron diffraction pattern of a single TiO2 nanocrystal; (d) HRTEM image of one of the rodlike TiO2 nanocrystals, in which the inset is a low-magnification TEM image of the rodlike nanocrystal; (e) XRD pattern of the as-prepared rodlike TiO2 nanocrystals.
JEOL JEM-200CX, with an accelerating voltage of 160KV), and highresolution transmission electron microscopy (HRTEM; JEOL JEM-2010, with an accelerating voltage of 200 kV). The photocatalytic activities of the products were evaluated by the photodegradation of Rhodamine B in water. A cylindrical Pyrex flask was used as a photoreactor vessel. The aqueous system containing RhB (2.0 × 10− 5 M, 40 mL) and the as-prepared rutile TiO2 nanocrystals (20 mg) was magnetically stirred in the dark for 30 min to reach the adsorption equilibrium of RhB with the catalyst, and then exposed to
UV light from an high-pressure mercury lamp (λ b 330 nm). Commercially available TiO2 (Degussa P25) was adopted as a reference to compare the photocatalytic activity. During the photocatalytic reaction, the concentration of Rhodamine B was monitored by a Shimadu UV-1601 PC UV–VIS spectrophotometer. 3. Results and discussion The morphology and structure of the products were characterized with TEM and XRD. Fig. 1a indicates that the product is entirely comprised of nearly monodisperse
Fig. 2. TEM images of the rutile TiO2 nanocrystals formed with different urea concentrations: (a) 0, (b) 0.12, and (c) 0.5 M.
S. Zhang et al. / Materials Letters 63 (2009) 127–129 rodlike nanoparticles. The mean length and diameter of the particles are respectively 200 nm and 44 nm, which is the statistical result of more than 150 nanoparticles in Fig. 1a. The morphology of the rodlike TiO2 nanoparticles can be seen more clearly from the higher-magnification TEM image (Fig. 1b). Fig. 1c shows an electron diffraction (ED) pattern of an individual rodlike nanoparticle, which indicates that the rodlike TiO2 nanoparticles synthesized at room temperature are crystalline. X-ray diffraction pattern (Fig. 1e) of the product indicates the nearly monodisperse rodlike nanoparticles are tetragonal rutile TiO2 (JCPDS No. 21-1276). A HRTEM image of an individual rodlike nanocrystal is presented in Fig. 1d. The area observed in the image corresponds to the region indicated by the white frame in the inset of Fig. 1d. Lattice fringes, which are perpendicular to the wall of the rodlike nanocrystal, can be clearly seen due to phase contrast. The distance between the adjacent lattice fringes is 2.958 Å, corresponding to (001) planes of rutile TiO2, which indicates that the preferential growth direction of the nanocrystal is [001]. The hydrolytic sol–gel reactions using titanium alkoxides as precursors have been extensively studied and generally can be considered as a two-step process, namely a hydrolysis reaction and a condensation reaction [27]. Oliver and co-workers calculated the specific surface energies of rutile TiO2 based on atomistic simulation and claimed the surface energies of {001}, {100}, {221}, {011} and {110} were 2.40, 2.08, 2.02, 1.85 and 1.78 J m− 2, respectively [28]. Since the {001} and {110} surfaces are suggested to have the highest and the lowest surface energies respectively, the [001] direction is the favored growth direction. Consequently, the formed nanocrystals should have a high aspect ratio along [001]-axis. The rutile TiO2 nanocrystals prepared in our case show a mean aspect ratio of 4:1 along [001]-axis, which agrees very well with the theoretical calculation. For comparison, the same experiments were performed at the urea concentrations: 0, 0.12, and 0.5 M. Fig. 1a and Fig. 2 display the effect of urea concentration on product morphology. The rodlike nanoparticles formed in the absence of urea have lots of lateral branches (Fig. 2a). Obviously, the growth of lateral branches is inhibited effectively and the monodispersity of TiO2 nanocrystals is improved with the urea concentrations of 0.12 (Fig. 2b) and 0.25 M (Fig. 1a), but the higher urea concentration of 0.5 M results in the aggregation and high polydispersity of the particles (Fig. 2c). Thus, urea concentration is a key factor to control product morphology in the present experiments. However, the crystalline form of the rodlike nanoparticles is not related to urea concentration. As shown in Fig. 1e and Fig. 3, the products obtained at the different urea concentrations have the same composition and crystalline phase, namely tetragonal rutile TiO2 (JCPDS No. 21-1276). The photocatalytic activity of the products obtained with different urea concentrations was evaluated by the photocatalytic degradation of RhB. Fig. 4 clearly shows that all products possess obvious photocatalytic activity. Generally, rutile TiO2 shows poor photocatalytic activities. However, in our case, the single-phase rutile TiO2 nanocrystals formed in the absence of urea exhibit greater photocatalytic activity than the commercially available photocatalyst P25.
4. Conclusion In summary, nearly monodisperse rodlike rutile TiO2 nanocrystals have been synthesized at room temperature by a simple method. The dependence of product morphology on urea concentration is demonstrated. The product prepared in the absence of urea shows higher photocatalytic activity than the commercial photocatalyst P25. The adopted synthetic process is facile and proceeds in aqueous solutions
Fig. 3. XRD patterns of the rutile TiO2 nanocrystals formed with different urea concentrations: (a) 0, (b) 0.12, and (c) 0.5 M.
129
Fig. 4. The photocatalytic degradation of RhB in the presence of commercial P25 and the rutile TiO2 nanocrystals (formed with different urea concentrations: 0, 0.12, 0.25 and 0.5 M).
at room temperature, which is very beneficial for the further study of rutile TiO2 nanocrystals such as surface modification and doping. Acknowledgments The work was supported by the National Nature Science Foundation of China (20573126), Chinese Academy of Sciences and National Basic Research Program of China (973 Program). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Subramanian V, Wolf E, Kamat PV. J Phys Chem, B 2001;105:11439–46. Tang J, Wu Y, McFarland EW, Stucky GD. Chem Commun 2004;14:1670–1. O'Regan B, Grätzel M. Nature 1991;353:737–8. Stergiopoulos T, Arabatzis IM, Katsaros G, Falaras P. Nano Lett 2002;2:1259–61. Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2006;6:215–8. Zhu Y, Shi J, Zhang Z, Zhang C, Zhang X. Anal Chem 2002;74:120–4. sharma RK, Bhatnagar MC, Sharma GL. Sens Actuators B: Chem 1998;46:194–201. Iuchi K, Ohko Y, Tatsuma T, Fujishima A. Chem Mater 2004;16:1165–7. Wagemaker M, Krol RVD, Kentgens APM, Well AAV, Mulder FM. J Am Chem Soc 2001;123:11454–61. Sun J, Gao L, Zhang Q. J Am Ceram Soc 2003;86:1677–82. Park NG, van de Lagemaat J, Frank AJ. J Phys Chem, B 2000;104:8989–94. Kim KJ, Benksten KD, van de Lagemaat J, Frank AJ. Chem Mater 2002;14:1042–7. Gonzalez RJ, Zallen R, Berger H. Phys Rev B 1997;55:7014–7. Gopal M, Chan MWJ. J Mater Sci 1997;32:6001–6. Sato T, Baba K, Hirozawa T, Kawakami S. IEEE J Quantum Electron 1993;29:175–81. Byun HY, Vittal R, Kim DY, Kim KJ. Langmuir 2004;20:6853–7. Zhang H, Banfield JF. J Phys Chem B 2000;104:3481–7. Pottier A, Chaneac C, Tronc E, Mazerolles L, Jolivet JP. J Mater Chem 2001;11:1116–21. Wu M, Lin G, Chen D, Wang G, He D, Feng S, et al. Chem Mater 2002;14:1974–80. Navrotsky A. Geochem Trans 2003;4:34–7. Cheng H, Ma J, Zhao Z, Qi L. Chem Mater 1995;7:663–71. Li Y, Fan Y, Chen Y. J Mater Chem 2002;12:1387–90. Yang K, Zhu JM, Zhu JJ, Huang S, Zhu X, Ma G. Mater Lett 2003;57:4639–42. Wang Y, Zhang L, Deng K, Chen X, Zou Z. J Phys Chem C 2007;111:2709–14. Xiaoxu L, Yujie X, Zhengquan L, Yi X. Inorg Chem 2006;45:3493–5. Andersson M, Osterlund L, Ljungstrom S, Palmqvist A. J Phys Chem, B 2002;106:10674–9. Livage J, Henry M, Sanchez C. Prog Solid State Chem 1988;18:259–341. Oliver PM, Watson GW, Kelsey ET, Parker SC. J Mater Chem 1997;3:563–8.