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Characterization of ramiform precipitates formed on SiO2–TiO2 gel coatings by electric field hot water treatment
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Journal of Non-Crystalline Solids 354 (2008) 1263–1266 www.elsevier.com/locate/jnoncrysol
Characterization of ramiform precipitates formed on SiO2–TiO2 gel coatings by electric field hot water treatment Atsunori Matsuda a,*, Kumpei Kobayashi b, Toshihiro Kogure c, Mototsugu Sakai a, Kiyoharu Tadanaga b, Tsutomu Minami b, Masahiro Tatsumisago b b
a Department of Materials Science, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan c Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
Available online 7 January 2008
Abstract The shape of the precipitates on sol–gel derived SiO2–TiO2 coatings at the negative electrode changed from granular to ramiform by applying an electric field to the substrates during a hot water treatment, whereas such changes in the shape of titania nanocrystals with the electric field were not observed at the positive electrode. The granular and ramiform precipitates were identified as anatase (TiO2) and hydrated titania (n(TiO2) Æ mH2O), respectively. The ramiform shape of the titania precipitates became significant with increasing the applied voltage, while the coatings gradually became dark-colored due to the reduction of Ti4+ to Ti3+. The coatings with ramiform precipitates showed an excellent wettability for water. Ó 2007 Published by Elsevier B.V. Keywords: Photocatalysis; Glass ceramics; Nanocrystals; Silicates; Titanates
1. Introduction A new approach to design the surface morphology and crystalline phase of the advanced coatings has been proposed, which is based on the chemical reaction of oxide gel materials with hot water under the controlled conditions with external stimuli. Sol–gel derived inorganic gel materials are generally porous and have a large specific surface area, so that the gel materials can easily react with water short time, resulting in the formation of textures and nanocrystals on the surface or in the bulk of residual materials. We have shown that anatase nanocrystals were formed on the sol–gel derived SiO2–TiO2 coatings with water vapor and hot water treatments [1–3]. Anatase formation proceeds through hydrolysis of Si–O–Ti bonds, dissolution of SiO2 component, migration of hydrolyzed titania species, and nucleation and growth of TiO2 [4]. Very
*
Corresponding author. E-mail address: [email protected] (A. Matsuda).
0022-3093/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2006.12.126
recently we have found that the shape of the precipitates elongates to be nanosheets of hydrated titania by applying a vector field such as vibration [5,6] and electric voltage [7] to the substrate during hot water treatment. In this study, ramiform precipitates including titania nanosheets on the SiO2–TiO2 gel coatings after the electric field hot water treatment have been characterized. The hydrophilicity and photocatalytic property of the coatings have been evaluated. 2. Experimental 75SiO2 Æ 25TiO2 (mol%) coatings were formed on indium tin oxide (ITO)-coated glass substrates. The preparation procedure was the same as reported in previous papers [5–7]. Silicon tetraethoxide and titanium tetra-nbutoxide were used as the starting materials. Ethanol and hydrochloric acid were for solvent and catalyst, respectively. The substrates with the SiO2–TiO2 coatings were dried at 90 °C for 1 h in air. A couple of substrates with the SiO2–TiO2 coatings were immersed into hot water at
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90 °C and then a DC voltage of 0–10 V was applied between the substrates which were facing at a distance of 1 cm. The changes in the texture of the coatings during the electric field hot water treatment were examined using a field-emission type scanning electron microscope (FESEM, Model S-4500, Hitachi). The precipitates on the coating were identified by using a field-emission type transmission electron microscope (FE-TEM, Model HF-2000, Hitachi). Ultraviolet–visible (UV–vis) transmission spectra of the substrates with the coatings were obtained using a UV–vis spectrophotometer (V-560, JASCO). Changes in contact angle for water of the coatings were measured using a contact angle measuring instrument (CA-C, Kyowa Surface Science). Photocatalytic activity of the coatings was evaluated from the amounts of I2 generated in the KI aqueous solution during the UV irradiation using a black blue light (BL-B 20 W, Matsushita Electric Industrial) under 0.5 mW cm 2.
3. Results FE-SEM images of the 75SiO2 Æ 25TiO2 coatings treated with hot water at 90 °C for 5 h under 10 V cm 1 are shown in Fig. 1(a) and (b) are for the coatings on ITO-coated glass substrates as positive and negative electrodes, respectively. Granular precipitates, which have been identified as anatase, of 30–50 nm in diameter are formed on the coating at the positive electrode after the electric field hot water treatment (Fig. 1(a)). This is similar to the result for the coating obtained without electric field [2]. On the other hand, ramiform precipitates are formed on the coating at the negative electrode after the treatment (Fig. 1(b)). TEM images of the ramiform precipitates formed on the coating at the negative electrode during the hot water treatment at 90 °C for 5 h under 10 V cm 1 are shown in Fig. 2. The nanosheet crystallites are observed as indicated by the arrows in Fig. 2(a). Fig. 2(b) shows an enlarged view of a portion in Fig. 2(a), where two or three nanosheets are
Fig. 1. SEM images of the surface of the 75SiO2 Æ 25TiO2 coatings treated with hot water at 90 °C for 5 h under 10 V cm 1: (a) and (b) are for the coatings on ITO-coated glass substrates as positive and negative electrodes, respectively.
Fig. 2. TEM images of the precipitates formed on the coating at the negative electrode by hot water treatment at 90 °C for 5 h under 10 V cm 1. (a) Planview of the coating, showing sheet-like precipitates as indicated by the arrows. (b) Enlarged view of the portion with the white rectangle in (a).
A. Matsuda et al. / Journal of Non-Crystalline Solids 354 (2008) 1263–1266
identified with the spacing of 0.6 nm. These nanosheet crystallites are probably hydrated titania (n(TiO2) Æ mH2O) having layered structure [8–10]. When applied voltage during hot water treatment increased from 5 to 10 V cm 1, the coatings colored and optical transmission decreased. However, the coatings remained transparent after the hot water treatment under an applied voltage lower than 5 V cm 1. In addition, the colored coating became transparent by a heat treatment in air at temperatures higher than 400 °C.
70
Contact angle / degree
60
Changes in contact angle for water of the coatings at the negative electrode with an electric filed hot water treatment are shown in Fig. 3. The treatment was carried out at 90 °C under 5 V cm 1. The contact angle of 68° monotonically decreases to be <5° and an excellent wettability for water is achieved after the electric field hot water treatment for 5 h. The decrease in contact angle for water corresponds to the formation of the ramiform structure composed of hydrated titania nanosheets. The amounts of I2 photocatalytically generated by UV irradiation on the coatings are shown in Fig. 4. Photocatalytic activities of the coatings obtained by electric hot field water treatments under 5 and 10 V cm 1 are lower than that of the coating obtained without electric field. The coating obtained 5 V cm 1 was transparent, whereas the activity of the coating was as low as that of the darkly colored one obtained at 10 V cm 1.
50
4. Discussion
40 30 20 10 0 0
1
2
3
4
5
Hot water treatment / h Fig. 3. Changes in contact angle for water of the coatings at the negative electrode with an electric field hot water treatment at 90 °C under 5 V cm 1.
0.006
0.005
I2 concentration / mg l-1cm-2
1265
0V 5V
0.004
10V 0.003
0.002
0.001
0 0
30
60
90
120
150
UV irradiation time / min. Fig. 4. The amounts of I2 photocatalytically generated by UV irradiation on the coatings. All the coatings were treated at 90 °C for 5 h.
The ramiform precipitates formed on the SiO2–TiO2 coating at the negative electrode were mainly composed of nanosheet crystallites, which were identified as hydrated titania. This phenomenon is analogous to that observed by hot water treatment under vibrations of the substrate [5,6]. In the case of the vibration hot water treatment, the formation of hydrated titania nanosheets was probably achieved by the lowering of the concentration of hydrolyzed titania species at the surface of the coating due to rapid water flow driven by the vibration. The detailed mechanisms for the formation of nanosheet crystallites under the electric filed are still not clear, but the electrostatic repulsion between the hydrolyzed, negatively charged titania species [10,11] and the negative electrode may lower the concentration of titania species at the surface of the coating and permit the preferential formation of titania nanosheets. Another important factor to be discussed is the effects of the higher pH of the hot water at the vicinity of the electrode; i.e. OH is generated from water near the negative electrode. The colored coating obtained at 10 V cm 1 became transparent after a heat treatment at 400 °C in air, so that the reduction from Ti4+ to Ti3+ in the coating caused the coloration of the coatings. The coloration of the coatings can be prevented by decreasing the applied electric field <5 V cm 1. The hydrated titania nanosheet-precipitated coating shows excellent hydrophilicity, which can be ascribed to the large roughness, unique morphology as well as large surface energy of the hydrated titania–silica layer. The control of the surface wettability is very important for the practically applications to antifogging and self-cleaning. On the other hand, a lower photocatalytic activity of the coating obtained by the electric field hot water treatment should be caused by the presence of small amounts of Ti3+ or O2 vacancy, which act as trapping sites for the exited holes and electrons.
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5. Conclusions The shape of the precipitates on the SiO2–TiO2 coating at the negative electrode changed from granular to ramiform by applying an electric field to the substrate during the hot water treatment. The granular and ramiform precipitates consisted of anatase (TiO2) and hydrated titania (n(TiO2) Æ mH2O), respectively. The ramiform shape of the precipitates became significant with increasing the applied voltage, while the coatings darkly colored due to the reduction of Ti4+ to Ti3+ in the coating. The hydrated titania nanosheets showed an excellent wettability for water because of the unique morphology and large surface energy. This work demonstrates that the crystalline phase and the morphology of nanocrystals formed on the coatings can be controlled by treating sol–gel derived multicomponent gels with hot water under an electrostatic vector field. Acknowledgments This work was partly supported by Grant-in-Aid for Scientific Research from the Japan Society for the Promo-
tion of Science (JSPS), Tatematsu Foundation, Izumi Science and Technology Foundation, and Hosokawa Powder Technology Foundation. References [1] A. Matsuda, T. Kogure, Y. Matsuno, S. Katayama, T. Tsuno, N. Tohge, T. Minami, J. Am. Ceram. Soc. 76 (1993) 2899. [2] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 83 (2000) 229. [3] Y. Kotani, A. Matsuda, T. Kogure, M. Tatsumisago, T. Minami, Chem. Mater. 13 (2001) 2144. [4] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Mater. Res. 20 (2005) 256. [5] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Sol–Gel Sci. Technol. 31 (2004) 229. [6] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, Chem. Mater. 17 (2005) 749. [7] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Ceram. Soc. Jpn. 113 (2005) 333. [8] T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izumi, S. Takenouchi, Chem. Mater. 7 (1995) 1001. [9] M. Yanagisawa, S. Uchida, S. Yin, T. Sato, Chem. Mater. 13 (2001) 174. [10] S.G.H. Heijman, H.N. Stein, Langmuir 11 (1995) 422. [11] J. Yang, S. Mei, J.M.F. Ferreira, J. Colloid Inter. Sci. 260 (2003) 82.
Journal of Non-Crystalline Solids 354 (2008) 1263–1266 www.elsevier.com/locate/jnoncrysol
Characterization of ramiform precipitates formed on SiO2–TiO2 gel coatings by electric field hot water treatment Atsunori Matsuda a,*, Kumpei Kobayashi b, Toshihiro Kogure c, Mototsugu Sakai a, Kiyoharu Tadanaga b, Tsutomu Minami b, Masahiro Tatsumisago b b
a Department of Materials Science, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan c Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
Available online 7 January 2008
Abstract The shape of the precipitates on sol–gel derived SiO2–TiO2 coatings at the negative electrode changed from granular to ramiform by applying an electric field to the substrates during a hot water treatment, whereas such changes in the shape of titania nanocrystals with the electric field were not observed at the positive electrode. The granular and ramiform precipitates were identified as anatase (TiO2) and hydrated titania (n(TiO2) Æ mH2O), respectively. The ramiform shape of the titania precipitates became significant with increasing the applied voltage, while the coatings gradually became dark-colored due to the reduction of Ti4+ to Ti3+. The coatings with ramiform precipitates showed an excellent wettability for water. Ó 2007 Published by Elsevier B.V. Keywords: Photocatalysis; Glass ceramics; Nanocrystals; Silicates; Titanates
1. Introduction A new approach to design the surface morphology and crystalline phase of the advanced coatings has been proposed, which is based on the chemical reaction of oxide gel materials with hot water under the controlled conditions with external stimuli. Sol–gel derived inorganic gel materials are generally porous and have a large specific surface area, so that the gel materials can easily react with water short time, resulting in the formation of textures and nanocrystals on the surface or in the bulk of residual materials. We have shown that anatase nanocrystals were formed on the sol–gel derived SiO2–TiO2 coatings with water vapor and hot water treatments [1–3]. Anatase formation proceeds through hydrolysis of Si–O–Ti bonds, dissolution of SiO2 component, migration of hydrolyzed titania species, and nucleation and growth of TiO2 [4]. Very
*
Corresponding author. E-mail address: [email protected] (A. Matsuda).
0022-3093/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2006.12.126
recently we have found that the shape of the precipitates elongates to be nanosheets of hydrated titania by applying a vector field such as vibration [5,6] and electric voltage [7] to the substrate during hot water treatment. In this study, ramiform precipitates including titania nanosheets on the SiO2–TiO2 gel coatings after the electric field hot water treatment have been characterized. The hydrophilicity and photocatalytic property of the coatings have been evaluated. 2. Experimental 75SiO2 Æ 25TiO2 (mol%) coatings were formed on indium tin oxide (ITO)-coated glass substrates. The preparation procedure was the same as reported in previous papers [5–7]. Silicon tetraethoxide and titanium tetra-nbutoxide were used as the starting materials. Ethanol and hydrochloric acid were for solvent and catalyst, respectively. The substrates with the SiO2–TiO2 coatings were dried at 90 °C for 1 h in air. A couple of substrates with the SiO2–TiO2 coatings were immersed into hot water at
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90 °C and then a DC voltage of 0–10 V was applied between the substrates which were facing at a distance of 1 cm. The changes in the texture of the coatings during the electric field hot water treatment were examined using a field-emission type scanning electron microscope (FESEM, Model S-4500, Hitachi). The precipitates on the coating were identified by using a field-emission type transmission electron microscope (FE-TEM, Model HF-2000, Hitachi). Ultraviolet–visible (UV–vis) transmission spectra of the substrates with the coatings were obtained using a UV–vis spectrophotometer (V-560, JASCO). Changes in contact angle for water of the coatings were measured using a contact angle measuring instrument (CA-C, Kyowa Surface Science). Photocatalytic activity of the coatings was evaluated from the amounts of I2 generated in the KI aqueous solution during the UV irradiation using a black blue light (BL-B 20 W, Matsushita Electric Industrial) under 0.5 mW cm 2.
3. Results FE-SEM images of the 75SiO2 Æ 25TiO2 coatings treated with hot water at 90 °C for 5 h under 10 V cm 1 are shown in Fig. 1(a) and (b) are for the coatings on ITO-coated glass substrates as positive and negative electrodes, respectively. Granular precipitates, which have been identified as anatase, of 30–50 nm in diameter are formed on the coating at the positive electrode after the electric field hot water treatment (Fig. 1(a)). This is similar to the result for the coating obtained without electric field [2]. On the other hand, ramiform precipitates are formed on the coating at the negative electrode after the treatment (Fig. 1(b)). TEM images of the ramiform precipitates formed on the coating at the negative electrode during the hot water treatment at 90 °C for 5 h under 10 V cm 1 are shown in Fig. 2. The nanosheet crystallites are observed as indicated by the arrows in Fig. 2(a). Fig. 2(b) shows an enlarged view of a portion in Fig. 2(a), where two or three nanosheets are
Fig. 1. SEM images of the surface of the 75SiO2 Æ 25TiO2 coatings treated with hot water at 90 °C for 5 h under 10 V cm 1: (a) and (b) are for the coatings on ITO-coated glass substrates as positive and negative electrodes, respectively.
Fig. 2. TEM images of the precipitates formed on the coating at the negative electrode by hot water treatment at 90 °C for 5 h under 10 V cm 1. (a) Planview of the coating, showing sheet-like precipitates as indicated by the arrows. (b) Enlarged view of the portion with the white rectangle in (a).
A. Matsuda et al. / Journal of Non-Crystalline Solids 354 (2008) 1263–1266
identified with the spacing of 0.6 nm. These nanosheet crystallites are probably hydrated titania (n(TiO2) Æ mH2O) having layered structure [8–10]. When applied voltage during hot water treatment increased from 5 to 10 V cm 1, the coatings colored and optical transmission decreased. However, the coatings remained transparent after the hot water treatment under an applied voltage lower than 5 V cm 1. In addition, the colored coating became transparent by a heat treatment in air at temperatures higher than 400 °C.
70
Contact angle / degree
60
Changes in contact angle for water of the coatings at the negative electrode with an electric filed hot water treatment are shown in Fig. 3. The treatment was carried out at 90 °C under 5 V cm 1. The contact angle of 68° monotonically decreases to be <5° and an excellent wettability for water is achieved after the electric field hot water treatment for 5 h. The decrease in contact angle for water corresponds to the formation of the ramiform structure composed of hydrated titania nanosheets. The amounts of I2 photocatalytically generated by UV irradiation on the coatings are shown in Fig. 4. Photocatalytic activities of the coatings obtained by electric hot field water treatments under 5 and 10 V cm 1 are lower than that of the coating obtained without electric field. The coating obtained 5 V cm 1 was transparent, whereas the activity of the coating was as low as that of the darkly colored one obtained at 10 V cm 1.
50
4. Discussion
40 30 20 10 0 0
1
2
3
4
5
Hot water treatment / h Fig. 3. Changes in contact angle for water of the coatings at the negative electrode with an electric field hot water treatment at 90 °C under 5 V cm 1.
0.006
0.005
I2 concentration / mg l-1cm-2
1265
0V 5V
0.004
10V 0.003
0.002
0.001
0 0
30
60
90
120
150
UV irradiation time / min. Fig. 4. The amounts of I2 photocatalytically generated by UV irradiation on the coatings. All the coatings were treated at 90 °C for 5 h.
The ramiform precipitates formed on the SiO2–TiO2 coating at the negative electrode were mainly composed of nanosheet crystallites, which were identified as hydrated titania. This phenomenon is analogous to that observed by hot water treatment under vibrations of the substrate [5,6]. In the case of the vibration hot water treatment, the formation of hydrated titania nanosheets was probably achieved by the lowering of the concentration of hydrolyzed titania species at the surface of the coating due to rapid water flow driven by the vibration. The detailed mechanisms for the formation of nanosheet crystallites under the electric filed are still not clear, but the electrostatic repulsion between the hydrolyzed, negatively charged titania species [10,11] and the negative electrode may lower the concentration of titania species at the surface of the coating and permit the preferential formation of titania nanosheets. Another important factor to be discussed is the effects of the higher pH of the hot water at the vicinity of the electrode; i.e. OH is generated from water near the negative electrode. The colored coating obtained at 10 V cm 1 became transparent after a heat treatment at 400 °C in air, so that the reduction from Ti4+ to Ti3+ in the coating caused the coloration of the coatings. The coloration of the coatings can be prevented by decreasing the applied electric field <5 V cm 1. The hydrated titania nanosheet-precipitated coating shows excellent hydrophilicity, which can be ascribed to the large roughness, unique morphology as well as large surface energy of the hydrated titania–silica layer. The control of the surface wettability is very important for the practically applications to antifogging and self-cleaning. On the other hand, a lower photocatalytic activity of the coating obtained by the electric field hot water treatment should be caused by the presence of small amounts of Ti3+ or O2 vacancy, which act as trapping sites for the exited holes and electrons.
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5. Conclusions The shape of the precipitates on the SiO2–TiO2 coating at the negative electrode changed from granular to ramiform by applying an electric field to the substrate during the hot water treatment. The granular and ramiform precipitates consisted of anatase (TiO2) and hydrated titania (n(TiO2) Æ mH2O), respectively. The ramiform shape of the precipitates became significant with increasing the applied voltage, while the coatings darkly colored due to the reduction of Ti4+ to Ti3+ in the coating. The hydrated titania nanosheets showed an excellent wettability for water because of the unique morphology and large surface energy. This work demonstrates that the crystalline phase and the morphology of nanocrystals formed on the coatings can be controlled by treating sol–gel derived multicomponent gels with hot water under an electrostatic vector field. Acknowledgments This work was partly supported by Grant-in-Aid for Scientific Research from the Japan Society for the Promo-
tion of Science (JSPS), Tatematsu Foundation, Izumi Science and Technology Foundation, and Hosokawa Powder Technology Foundation. References [1] A. Matsuda, T. Kogure, Y. Matsuno, S. Katayama, T. Tsuno, N. Tohge, T. Minami, J. Am. Ceram. Soc. 76 (1993) 2899. [2] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 83 (2000) 229. [3] Y. Kotani, A. Matsuda, T. Kogure, M. Tatsumisago, T. Minami, Chem. Mater. 13 (2001) 2144. [4] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Mater. Res. 20 (2005) 256. [5] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Sol–Gel Sci. Technol. 31 (2004) 229. [6] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, Chem. Mater. 17 (2005) 749. [7] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Ceram. Soc. Jpn. 113 (2005) 333. [8] T. Sasaki, M. Watanabe, Y. Michiue, Y. Komatsu, F. Izumi, S. Takenouchi, Chem. Mater. 7 (1995) 1001. [9] M. Yanagisawa, S. Uchida, S. Yin, T. Sato, Chem. Mater. 13 (2001) 174. [10] S.G.H. Heijman, H.N. Stein, Langmuir 11 (1995) 422. [11] J. Yang, S. Mei, J.M.F. Ferreira, J. Colloid Inter. Sci. 260 (2003) 82.