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Synthesis of spongy titanium oxide
Materials Science and Engineering B 172 (2010) 163–166
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of spongy titanium oxide Qingchun Zhao a,b,∗ , Tian Cao a,b a b
Anhui Key Laboratory of Advanced Building Materials, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China Department of Material Science and Engineering, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 30 April 2010 Accepted 3 May 2010 Keywords: Spongy Titanium oxide Polyacrylic acid gels
a b s t r a c t Spongy titanium oxide was prepared via using polyacrylic acid gels as template. Effect of Ti3+ concentration on product morphology was investigated. Scanning electron microscopy (SEM) images of the obtained product clearly show that TiO2 possesses a three-dimensional network structure and these networks are found to form through the aggregation of TiO2 nanorods and nanodots. X-ray diffraction (XRD) analysis was used to examine the crystal structure of the products. The obtained product is pure anatase TiO2 . The composition of the products was analyzed by X-ray photoelectron spectroscopy (XPS). A possible mechanism to explain the growth of the networks materials was investigated. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The discovery that nanoporous materials can be produced using a surfactant-templated approach has opened up a new era in the synthesis of ordered nanoscale materials [1–4]. Many investigations have explored the preparation of nanoporous materials with novel chemical composition [5], the fundamental nature of the reaction processes [6–8], and potential applications, such as catalysis [9,10], separation technology [11], and drug delivery [12] which is expected to open up further application possibilities. TiO2 has been investigated for applications in many areas, such as optical materials [13], superhydrophobic and superhydrophilic materials [14,15], photocatalytic devices [16,17], dye-sensitized solar cells [18], and lithium-ion batteries [19,20]. In particular, many researchers have focused their studies on the control of the nanostructure of TiO2 for potential applications, because the high surface area resulting from nanostructure control improves the properties of the TiO2 . In this structure, we could achieve both a large interface between the semiconducting materials and straight carrier paths to the electrodes. The structural dimension of less than 20 nm is desirable, because the diffusion length of excitons in a semiconducting polymer is smaller than 20 nm [21]. TiO2 is one of the best-studied inorganic materials for hybrid metal-oxide/polymer photovoltaic devices because of its electron accepting and conducting ability [22]. It is well known that the polyacrylic acid gels are threedimensional network structures. In this paper, three-dimensional
titanium oxide networks materials were prepared by using the interpenetrating polymer network method. This will offer a facile method to prepare nanoporous networks materials in the material field. Gels of polyacrylic acid with Ti3+ and Cl− ions in the pores were prepared by free radical polymerization in water using ␥ rays as initiator. 2. Experimental 2.1. Materials and physical techniques To prepare spongy titanium oxide, TiCl3 (2 g, 1 g, 0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water, respectively. After the solutions were bubbled with N2 for 20 min to eliminate oxygen, they were irradiated in the field of a 2.22 × 1015 Bq 60 Co ␥-rays source with 4000 Gy. After the polyacrylic acid gels were obtained and were immersed in 1 mol L−1 ammonia for 3 days, colloid of titanium acid was produced in pores of the polyacrylic acid gels and the interpenetrating network polymers were produced. After the interpenetrating network polymers were taken out and were washed with water, the interpenetrating network polymers were heated to 600 ◦ C and held at this temperature for 24 h. The polyacrylic acid gels are degradated, volatilized and oxidized by high temperature and oxygen, and TiO2 nanomaterials were obtained. 2.2. Instrumentation
∗ Corresponding author at: Department of Material Science and Engineering, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China. E-mail address: [email protected] (Q. Zhao). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.05.003
The synthesized products were characterized by scanning electron microscopy (SEM) (JEOL JSM-6300), X-ray powder diffraction (XRD), using a Dmax ␥A X-ray diffractometer with Cu-K␣ radiation
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Q. Zhao, T. Cao / Materials Science and Engineering B 172 (2010) 163–166
( = 1.54178 Å) and X-ray photoelectron spectroscopy (XPS) was acquired in Vgescalab MkII instrument in which Mg K-Alpha was employed. 3. Results and discussions 3.1. SEM
Fig. 1. SEM images of TiO2 three-dimensional networks that are obtained by calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (1 g), and acrylic acid 20 g, which were dissolved in 40 g of water.
The product morphology was determined by scanning electron microscopy (SEM). Fig. 1 is SEM image of TiO2 three-dimensional networks, SEM observation shows that there are a large quantity of porous in the obtained samples and the porous array is irregular. Fig. 1 is a typical SEM image of the products, clearly showing that TiO2 possesses three-dimensional networks structure. It indicates that well-defined TiO2 networks structures can be obtained under the present experimental conditions. The growth of the networks structures is rather unique. TiO2 nanostructures prepared in the present study are networks, which are formed by aggregation of TiO2 nanorods and nanodots, respectively. This result is different from our previous work [23]. Polyacrylic acid molecular chains may act as a structure-directing agent of the TiO2 nanocrystal. Fig. 2a shows a SEM image of the product obtained by irradiating the solution containing TiCl3 (2 g), and acrylic acid 20 g, which were dissolved in 40 g of water, Fig. 2b shows a SEM image of the product
Fig. 2. (a) Shows a SEM image of the product obtained by calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (2 g), and acrylic acid 20 g, which were dissolved in 40 g of water. (b) Shows a SEM image of the product obtained calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water.
Q. Zhao, T. Cao / Materials Science and Engineering B 172 (2010) 163–166
165
Fig. 3. XRD patterns of the obtained TiO2 nanonetworks.
obtained by irradiating the solution containing TiCl3 (0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water, respectively. 3.2. Mechanisms The concentration of Ti3+ is found to play a significant role in the formation spongy titanium oxide. A key question here is why the concentration of Ti3+ effect formation in spongy titanium oxide. The most likely mechanism to explain the growth of the networks materials may be the control of the network of the polyacrylic acid gels. It is well known that polyacrylic acid gels are three-dimensional network structure. When polyacrylic acid gels with Ti3+ and Cl−1 ions in the pores were immersed into the 1 mol L−1 ammonia solutions, colloid of titanium acid was produced in pores of the polyacrylic acid gels and formed interpenetrating polymer network. After the interpenetrating network polymer were heated to 600 ◦ C and held at this temperature for 24 h, the polyacrylic acid gels were degradated, volatilized and oxidized by high temperature and oxygen, and networks TiO2 materials were obtained and the pores may formed by the polyacrylic acid molecular chains. When the concentration of Ti3+ is higher 2 g (20 g acrylic acid, 40 g of water), Ti3± may baffle cross-link of polyacrylic acid molecular chains, the network of the polyacrylic acid gels cannot be formed. The spongy titanium oxide cannot be obtained. When the concentration of Ti3+ is lower 0.5 g (20 g acrylic acid, 40 g of water), colloid of titanium acid in pores of the polyacrylic acid gels cannot form interpenetrating polymer network, monodisperse titanium oxide particles can be obtained. 3.3. X-ray powder diffraction X-ray diffraction (XRD) analysis was used to examine the crystal structure of the products. The XRD pattern shown in Fig. 3 indicates that the crystallographic phase of the obtained TiO2 threedimensional networks structure prepared in this work belongs to the anatase-type (SG: I41/amd; JCPDS no. 21-1272). 3.4. X-ray photoelectron spectroscopy The average composition of the particles was confirmed using X-ray photoelectron spectroscopy (XPS). Previous XPS studies have demonstrated that various Ti oxides can be distinguished based on their Ti 2p XPS spectrum [24]. For example, the Ti 2p3/2 peak in TiO2 is at approximately 459.0 eV, in Ti2 O3 is at approximately 457.6 eV, and in TiO is at approximately 455.3 eV. Metallic Ti exhibits a 2p3/2
Fig. 4. XPS spectra of as-grown networks: (a) Ti (2p) binding energy spectrum; (b) O (1s) binding energy spectrum.
peak at 453.8 eV. As reported in Fig. 4a, XPS investigation on the titanium chemical state shows that the binding energy of Ti 2p3/2 is equal to 458.9 eV and the binding energy of Ti 2p1/2 is equal to 464.5 eV, which are identical to that reported in the literature for the same anatase phase [25]. The chemical state of Ti IV is thus confirmed. The typical O 1s spectrum (530.4 eV) (Fig. 4b) indicates that the networks are composed of TiO2 and there are Ti–O bonds. 4. Conclusions In summary, we have presented a simple process for preparing unusual TiO2 networks via interpenetrating network. A possible mechanism for the formation has been proposed. However, the exact mechanism for the formation of the TiO2 networks is still not fully understood and further studies are needed to explain the observed phenomena. The experimental results (SEM) clearly show that the TiO2 nanostructures prepared in the present study is networks, which are aggregated by TiO2 nanorods and nanodots. Our study may provide a new method for direct growth of 3D nanostructured materials. References [1] C.T. Kresge, M.E. Leonowitz, W.J. Roth, J.S. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [2] K.M. Mcgrath, D.M. Dabbs, N. Yao, K.J. Elder, I.A. Aksay, S.M. Gruner, Langmuir 16 (2000) 398–406. [3] D. Roux, C. Coulon, M.E. Cates, J. Phys. Chem. 96 (1992) 4174–4187. [4] R. Gomati, N. Bouguera, A. Gharbi, Phys. B: Condens. Matter 322 (2002) 262–269. [5] S.S. Kim, W. Zhang, T.J. Pinnavaia, Science 282 (1998) 1302–1305.
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Q. Zhao, T. Cao / Materials Science and Engineering B 172 (2010) 163–166
[6] A. Firouzi, F. Atef, A.G. Oertly, G.D. Stucky, B.F. Chmelka, J. Am. Chem. Soc. 119 (1997) 3596–3610. [7] D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Graces, M.M. Olken, N. Cooms, Adv. Mater. 7 (1995) 842–846. [8] J.H. Chang, L.O. Wang, Y. Shin, B. Jeong, G. Birnbaum, Adv. Mater. 14 (2002) 378–382. [9] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56–77. [10] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403–1419. [11] X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276 (1997) 923–926. [12] J.H. Chang, H.S. Chang, B.J. Kim, Y. Shin, G.J. Exarhos, K.J. Kim, Adv. Mater. 17 (2005) 634–637. [13] Y. Dai, J. Zeng, Y. Sun, Y. Xia, Nano Lett. 9 (6) (2009) 2455–2459. [14] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432. [15] X. Feng, J. Zhai, L. Jiang, Angew. Chem. Int. Ed. 44 (2005) 5115–5118.
[16] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [17] K. Fujishima, Honda, Nature 238 (1972) 37–38. [18] M. Oregan, Grätzel, Nature 353 (1991) 737–740. [19] Kavan, M. Grätzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc., 118 (1996) 6716–6723. [20] S. Zhou, D.L. Li, M. Hibino, I. Honma (Eds.), Angew. Chem. Int. E 44 (2005) 797–802. [21] C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.H. Horhold, Adv. Mater. 12 (2000) 1689–1692. [22] Adachi, Y. Murata, J. Takao, J. Jiu, M Sakamoto, F. Wang, J. Am. Chem. Soc. 126 (45) (2004) 14943–14949. [23] Q.C. Zhao, W.M. Chen, Q.R. Zhu, Mater. Lett. 57 (2003) 3606–3608. [24] H.A. Bullen, S.J. Garrett, Nano. Lett. 2 (7) (2002) 739–745, NANO. [25] G.S. Herman, Y. Gao, Thin Solid Films 397 (2001) 157–161.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of spongy titanium oxide Qingchun Zhao a,b,∗ , Tian Cao a,b a b
Anhui Key Laboratory of Advanced Building Materials, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China Department of Material Science and Engineering, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 30 April 2010 Accepted 3 May 2010 Keywords: Spongy Titanium oxide Polyacrylic acid gels
a b s t r a c t Spongy titanium oxide was prepared via using polyacrylic acid gels as template. Effect of Ti3+ concentration on product morphology was investigated. Scanning electron microscopy (SEM) images of the obtained product clearly show that TiO2 possesses a three-dimensional network structure and these networks are found to form through the aggregation of TiO2 nanorods and nanodots. X-ray diffraction (XRD) analysis was used to examine the crystal structure of the products. The obtained product is pure anatase TiO2 . The composition of the products was analyzed by X-ray photoelectron spectroscopy (XPS). A possible mechanism to explain the growth of the networks materials was investigated. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The discovery that nanoporous materials can be produced using a surfactant-templated approach has opened up a new era in the synthesis of ordered nanoscale materials [1–4]. Many investigations have explored the preparation of nanoporous materials with novel chemical composition [5], the fundamental nature of the reaction processes [6–8], and potential applications, such as catalysis [9,10], separation technology [11], and drug delivery [12] which is expected to open up further application possibilities. TiO2 has been investigated for applications in many areas, such as optical materials [13], superhydrophobic and superhydrophilic materials [14,15], photocatalytic devices [16,17], dye-sensitized solar cells [18], and lithium-ion batteries [19,20]. In particular, many researchers have focused their studies on the control of the nanostructure of TiO2 for potential applications, because the high surface area resulting from nanostructure control improves the properties of the TiO2 . In this structure, we could achieve both a large interface between the semiconducting materials and straight carrier paths to the electrodes. The structural dimension of less than 20 nm is desirable, because the diffusion length of excitons in a semiconducting polymer is smaller than 20 nm [21]. TiO2 is one of the best-studied inorganic materials for hybrid metal-oxide/polymer photovoltaic devices because of its electron accepting and conducting ability [22]. It is well known that the polyacrylic acid gels are threedimensional network structures. In this paper, three-dimensional
titanium oxide networks materials were prepared by using the interpenetrating polymer network method. This will offer a facile method to prepare nanoporous networks materials in the material field. Gels of polyacrylic acid with Ti3+ and Cl− ions in the pores were prepared by free radical polymerization in water using ␥ rays as initiator. 2. Experimental 2.1. Materials and physical techniques To prepare spongy titanium oxide, TiCl3 (2 g, 1 g, 0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water, respectively. After the solutions were bubbled with N2 for 20 min to eliminate oxygen, they were irradiated in the field of a 2.22 × 1015 Bq 60 Co ␥-rays source with 4000 Gy. After the polyacrylic acid gels were obtained and were immersed in 1 mol L−1 ammonia for 3 days, colloid of titanium acid was produced in pores of the polyacrylic acid gels and the interpenetrating network polymers were produced. After the interpenetrating network polymers were taken out and were washed with water, the interpenetrating network polymers were heated to 600 ◦ C and held at this temperature for 24 h. The polyacrylic acid gels are degradated, volatilized and oxidized by high temperature and oxygen, and TiO2 nanomaterials were obtained. 2.2. Instrumentation
∗ Corresponding author at: Department of Material Science and Engineering, Anhui Institute of Architecture & Industry, Hefei 230022 Anhui, PR China. E-mail address: [email protected] (Q. Zhao). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.05.003
The synthesized products were characterized by scanning electron microscopy (SEM) (JEOL JSM-6300), X-ray powder diffraction (XRD), using a Dmax ␥A X-ray diffractometer with Cu-K␣ radiation
164
Q. Zhao, T. Cao / Materials Science and Engineering B 172 (2010) 163–166
( = 1.54178 Å) and X-ray photoelectron spectroscopy (XPS) was acquired in Vgescalab MkII instrument in which Mg K-Alpha was employed. 3. Results and discussions 3.1. SEM
Fig. 1. SEM images of TiO2 three-dimensional networks that are obtained by calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (1 g), and acrylic acid 20 g, which were dissolved in 40 g of water.
The product morphology was determined by scanning electron microscopy (SEM). Fig. 1 is SEM image of TiO2 three-dimensional networks, SEM observation shows that there are a large quantity of porous in the obtained samples and the porous array is irregular. Fig. 1 is a typical SEM image of the products, clearly showing that TiO2 possesses three-dimensional networks structure. It indicates that well-defined TiO2 networks structures can be obtained under the present experimental conditions. The growth of the networks structures is rather unique. TiO2 nanostructures prepared in the present study are networks, which are formed by aggregation of TiO2 nanorods and nanodots, respectively. This result is different from our previous work [23]. Polyacrylic acid molecular chains may act as a structure-directing agent of the TiO2 nanocrystal. Fig. 2a shows a SEM image of the product obtained by irradiating the solution containing TiCl3 (2 g), and acrylic acid 20 g, which were dissolved in 40 g of water, Fig. 2b shows a SEM image of the product
Fig. 2. (a) Shows a SEM image of the product obtained by calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (2 g), and acrylic acid 20 g, which were dissolved in 40 g of water. (b) Shows a SEM image of the product obtained calcination the polyacrylic acid gels at 600 ◦ C that prepared using TiCl3 (0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water.
Q. Zhao, T. Cao / Materials Science and Engineering B 172 (2010) 163–166
165
Fig. 3. XRD patterns of the obtained TiO2 nanonetworks.
obtained by irradiating the solution containing TiCl3 (0.5 g), and acrylic acid 20 g, which were dissolved in 40 g of water, respectively. 3.2. Mechanisms The concentration of Ti3+ is found to play a significant role in the formation spongy titanium oxide. A key question here is why the concentration of Ti3+ effect formation in spongy titanium oxide. The most likely mechanism to explain the growth of the networks materials may be the control of the network of the polyacrylic acid gels. It is well known that polyacrylic acid gels are three-dimensional network structure. When polyacrylic acid gels with Ti3+ and Cl−1 ions in the pores were immersed into the 1 mol L−1 ammonia solutions, colloid of titanium acid was produced in pores of the polyacrylic acid gels and formed interpenetrating polymer network. After the interpenetrating network polymer were heated to 600 ◦ C and held at this temperature for 24 h, the polyacrylic acid gels were degradated, volatilized and oxidized by high temperature and oxygen, and networks TiO2 materials were obtained and the pores may formed by the polyacrylic acid molecular chains. When the concentration of Ti3+ is higher 2 g (20 g acrylic acid, 40 g of water), Ti3± may baffle cross-link of polyacrylic acid molecular chains, the network of the polyacrylic acid gels cannot be formed. The spongy titanium oxide cannot be obtained. When the concentration of Ti3+ is lower 0.5 g (20 g acrylic acid, 40 g of water), colloid of titanium acid in pores of the polyacrylic acid gels cannot form interpenetrating polymer network, monodisperse titanium oxide particles can be obtained. 3.3. X-ray powder diffraction X-ray diffraction (XRD) analysis was used to examine the crystal structure of the products. The XRD pattern shown in Fig. 3 indicates that the crystallographic phase of the obtained TiO2 threedimensional networks structure prepared in this work belongs to the anatase-type (SG: I41/amd; JCPDS no. 21-1272). 3.4. X-ray photoelectron spectroscopy The average composition of the particles was confirmed using X-ray photoelectron spectroscopy (XPS). Previous XPS studies have demonstrated that various Ti oxides can be distinguished based on their Ti 2p XPS spectrum [24]. For example, the Ti 2p3/2 peak in TiO2 is at approximately 459.0 eV, in Ti2 O3 is at approximately 457.6 eV, and in TiO is at approximately 455.3 eV. Metallic Ti exhibits a 2p3/2
Fig. 4. XPS spectra of as-grown networks: (a) Ti (2p) binding energy spectrum; (b) O (1s) binding energy spectrum.
peak at 453.8 eV. As reported in Fig. 4a, XPS investigation on the titanium chemical state shows that the binding energy of Ti 2p3/2 is equal to 458.9 eV and the binding energy of Ti 2p1/2 is equal to 464.5 eV, which are identical to that reported in the literature for the same anatase phase [25]. The chemical state of Ti IV is thus confirmed. The typical O 1s spectrum (530.4 eV) (Fig. 4b) indicates that the networks are composed of TiO2 and there are Ti–O bonds. 4. Conclusions In summary, we have presented a simple process for preparing unusual TiO2 networks via interpenetrating network. A possible mechanism for the formation has been proposed. However, the exact mechanism for the formation of the TiO2 networks is still not fully understood and further studies are needed to explain the observed phenomena. The experimental results (SEM) clearly show that the TiO2 nanostructures prepared in the present study is networks, which are aggregated by TiO2 nanorods and nanodots. Our study may provide a new method for direct growth of 3D nanostructured materials. References [1] C.T. Kresge, M.E. Leonowitz, W.J. Roth, J.S. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [2] K.M. Mcgrath, D.M. Dabbs, N. Yao, K.J. Elder, I.A. Aksay, S.M. Gruner, Langmuir 16 (2000) 398–406. [3] D. Roux, C. Coulon, M.E. Cates, J. Phys. Chem. 96 (1992) 4174–4187. [4] R. Gomati, N. Bouguera, A. Gharbi, Phys. B: Condens. Matter 322 (2002) 262–269. [5] S.S. Kim, W. Zhang, T.J. Pinnavaia, Science 282 (1998) 1302–1305.
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[6] A. Firouzi, F. Atef, A.G. Oertly, G.D. Stucky, B.F. Chmelka, J. Am. Chem. Soc. 119 (1997) 3596–3610. [7] D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Graces, M.M. Olken, N. Cooms, Adv. Mater. 7 (1995) 842–846. [8] J.H. Chang, L.O. Wang, Y. Shin, B. Jeong, G. Birnbaum, Adv. Mater. 14 (2002) 378–382. [9] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56–77. [10] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403–1419. [11] X. Feng, G.E. Fryxell, L.Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276 (1997) 923–926. [12] J.H. Chang, H.S. Chang, B.J. Kim, Y. Shin, G.J. Exarhos, K.J. Kim, Adv. Mater. 17 (2005) 634–637. [13] Y. Dai, J. Zeng, Y. Sun, Y. Xia, Nano Lett. 9 (6) (2009) 2455–2459. [14] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432. [15] X. Feng, J. Zhai, L. Jiang, Angew. Chem. Int. Ed. 44 (2005) 5115–5118.
[16] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [17] K. Fujishima, Honda, Nature 238 (1972) 37–38. [18] M. Oregan, Grätzel, Nature 353 (1991) 737–740. [19] Kavan, M. Grätzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc., 118 (1996) 6716–6723. [20] S. Zhou, D.L. Li, M. Hibino, I. Honma (Eds.), Angew. Chem. Int. E 44 (2005) 797–802. [21] C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, S.A. Carter, H.H. Horhold, Adv. Mater. 12 (2000) 1689–1692. [22] Adachi, Y. Murata, J. Takao, J. Jiu, M Sakamoto, F. Wang, J. Am. Chem. Soc. 126 (45) (2004) 14943–14949. [23] Q.C. Zhao, W.M. Chen, Q.R. Zhu, Mater. Lett. 57 (2003) 3606–3608. [24] H.A. Bullen, S.J. Garrett, Nano. Lett. 2 (7) (2002) 739–745, NANO. [25] G.S. Herman, Y. Gao, Thin Solid Films 397 (2001) 157–161.