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Formation mechanism of titania nanosheet cryatallites on silica–titania gel films by vibration hot-water treatment
Materials Science and Engineering B 161 (2009) 170–174
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Formation mechanism of titania nanosheet cryatallites on silica–titania gel films by vibration hot-water treatment Niki Prastomo a,∗ , Yusuke Daiko a,b , Toshihiro Kogure c , Hiroyuki Muto a , Mototsugu Sakai a , Atsunori Matsuda a,∗ a b c
Department of Materials Science, Toyohashi University of Technology, Japan Department of Materials Science and Chemistry, University of Hyogo, Japan Department of Earth and Planetary Sciences, The University of Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 13 October 2008 Accepted 21 November 2008 Keywords: Sol–gel coating Hot-water treatment Titania External field Nanocrystal Formation mechanism
a b s t r a c t The control of the nanostructures of the titania-based coatings further offers an exciting opportunity to enhance their unique physical and chemical properties. By immersing silica–titania gel film coatings in hot-water concurrently with applying a vibration, nanosheet crystallites of hydrated titania were precipitated at the surface of the gel films. The main objective of this study is to design the functionality of titania-based coatings through the control of their surface morphology and crystallinity at low processing temperatures. Two types of vibration treatments, i.e. perpendicular and parallel directions against the gel films, were used. On 87.5SiO2 ·12.5TiO2 composition, parallel vibration direction enhances the nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration. The attachment of the precipitates from dissolved titania species should be an important driving force for the formation of the sheet-like structure. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Sol–gel method is a promising wet chemical technique for fabricating silica–titania nanocoatings. This route offers a control of the process on the molecular level and impurities can be avoided. Compared with conventional vacuum coating techniques, this technique is more cost-effective because coatings could be obtained practically easy since it can be formed on various substrates with large surface area and non-flat surface [1–3]. Generally, crystallization and densification of sol–gel derived titania coatings require heat-treatments at relatively high temperatures. Recently, the lowering process temperature and controlling of surface morphology attract considerable interests for designing advanced titania-based nanocoatings [4–8]. We have shown that anatase nanocrystals are formed on sol–gel derived SiO2 –TiO2 coatings at low temperatures using hot-water treatment at 90 ◦ C (HWT) [9,10]. The process temperature can be further lowered at 38 ◦ C when the SiO2 –TiO2 coatings are treated with the water for long period of time [11]. Substrates with coatings are immersed into hot, pure water and kept at rest for given time. The resultant coatings
∗ Corresponding authors. E-mail addresses: [email protected] (N. Prastomo), [email protected] (A. Matsuda). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.028
show unique characteristic, such as high photocatalytic activity and excellent wettability for water [12]. We also have reported that by applying external-field during HWT, the shape of the titania precipitates interestingly changed. Roundish precipitates changed to ramiform upon applying electric field to the substrate during HWT [13]. By applying vibration to the substrate during HWT, the precipitates tend to be elongated [14]. These ramiform and elongated precipitates are found to contain hydrated titania nanosheet with lepidocrocite structure [1]. The formation mechanism of this titania nanocrystals in the gel films with HWT is essentially important to control and design the required physical and chemical properties with purpose to enhance the functionalities of the advanced titania-based nanocoatings. We have proposed the formation mechanism of titania nanocrystals growth on SiO2 –TiO2 coatings with hot water as follows: (i) hydrolysis of Si–O–Ti bonds and dissolution of SiO2 component, (ii) migration of hydrolyzed titania species from the interior to the surface of the coating, and (iii) nucleation and growth of titania nanocrystals at the residual coating [1]. In the present work, we have newly found that before the nucleation and the growth of the titania crystalline phase, the titania species are dissolved into the hot-water first, and the dissolved titania species re-precipitate on the high titania concentration area on the surface of the coatings. Furthermore, the formation mechanism of elongated sheet-like titania structure obtained from vibration
N. Prastomo et al. / Materials Science and Engineering B 161 (2009) 170–174
171
Fig. 1. Schematic diagram of HWT experimental method.
HWT is described. The effects of vibration direction are also discussed on the basis of changes in the surface morphology. 2. Experimental The chemical composition 87.5SiO2 ·12.5TiO2 mol% was applied in this study. Based on the preliminary field emission scanning electron microscopy (FESEM) study, this composition provides well-dispersed titania precipitates on the silica–titania coating surface resulting an easy and straightforward observation. Ethanol was used as a solvent for silicon ethoxide and titanium n-buthoxide. Diluted hydrochloric acid (3.6 wt%) was then introduced as a catalyst during hydrolysis process. SiO2 –TiO2 coating films were formed on silica glass substrate and indium tin oxide (ITO)-coated glass substrates using the colloidal suspension by dipping and withdrawing at a constant speed of 4 cm/min and on ITO-coated quartz microbalance (QCM) by a constant speed of 1.5 cm/min to produce smooth and crack-free coatings. The sol–gel derived coatings were subsequently aged in a drying oven at 90 ◦ C for 1 h. To confirm dissolution and reprecipitation of titania species before nucleation and growth on the surface of the coatings, an experimental setup shown in Fig. 1 was used. “Face-to-face” method was done by using two kinds of coating. A 100SiO2 film was placed in front of 87.5SiO2 ·12.5TiO2 films with 2 mm distance. Both films then immersed into hot water for 5 h without applying any external-fields. For comparison, two 100SiO2 films were placed face-to-face also under the same condition.
Fig. 2. Schematic diagram of vibration HWT experimental method.
For vibration HWT by using ITO substrates, Fig. 2 shows the schematic diagram of the experimental method. The coatings were immersed into a hot-water environment for various times, 6 Hz with 2 mm amplitude vibration was afterward introduced to the coating by 2 methods, the vibration directions of which were perpendicular and parallel with the coatings. All the HWTs were carried out without stirring at normal pH environment (6.5–7). Field emission scanning electron microscopy (FESEM, S-4800 Hitachi), water contact angle measurements (DropMaster 300, Kyowa Interface Science), energy dispersed X-ray spectrometry (EDX, Emax Energy EX-250, Horiba) and quartz crystal microbalance (QCM, UEQ-400 Easy, USI) have been used to determine surface changes of the coating. 3. Results and discussion The formation of anatase nanoparticles and hydrated titania nanosheets is dominated by nucleation and growth of hydrolyzed titania species at the surface of the coatings. Before the nucleation and the growth of the anatase and/or hydrated titania phases, the titania species should dissolve into the hot-water and reprecipitate on the highly titania concentrated area on the surface of the coatings. By placing pure silica (100SiO2 )-coated substrate in front of silica–titania-coated substrate during HWT, validity of the dissolution and reprecipitation mechanism could be demonstrated.
Fig. 3. FESEM images of SiO2 coatings before and after face-to-face HWT for 5 h; (a) before treatment, (b) after treatment facing a 100SiO2 coating and (c) after treatment facing a 87.5SiO2 ·12.5TiO2 coating.
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Fig. 3 shows the FESEM images of the surface of 100SiO2 coatings before face-to-face treatments, after face-to-face treatment with other 100SiO2 and with 87.5SiO2 ·12.5TiO2 at 90 ◦ C for 5 h. Small amounts of precipitates were observed at places on the 100SiO2 -coated substrate treated face-to-face with the 87.5SiO2 ·12.5TiO2 film. EDX characterization results also verify the presence of titanium on the surface of the SiO2 coating, which confirms the mechanism of dissolution, migration and reprecipitation of the hydrolyzed titania species from SiO2 –TiO2 coatings. Vibration plays an important role to the formation mechanism of the sheet-like structure. Fig. 4 shows the difference on weight loss that measured by using ITO-coated QCM substrate between samples that were treated without vibration, with perpendicular vibration and parallel vibration. Total weight losses are 17, 71, and 73 m/cm2 for the samples without vibration, with perpendicular, and parallel vibrations, respectively, after HWT for 100 min. Comparing with the nonvibration sample, weight loss rates from both samples treated with dissimilar vibration show significant difference. These high rates and faster changes were provided by water flow from vibration mechanism that supplied larger driving force for dissolving titania species on to the hot-water in shorter time. To gain more understanding of formation mechanism, detail experiments using short treatment period were conducted. Fig. 5
Fig. 4. Normalized weight loss of the ITO-coated QCM samples without vibration (), and with perpendicular (♦) and parallel () vibrations.
shows the FESEM images of non-vibration and vibration parallel direction treated of 87.5SiO2 ·12.5TiO2 coatings for a range of times. Precipitation occurred earlier on the non-vibration sample. Anatase precipitates start to be observed on the 2 min treated sample, whereas the parallel vibration sample needs 4 min treatment to precipitate the hydrated titania. From the weight loss and FESEM observation results, it was shown that by applying vibration during
Fig. 5. FESEM images of 87.5SiO2 ·12.5TiO2 coating on ITO substrate, treated by hot-water with parallel direction vibration and without vibration at 90 ◦ C for various times.
N. Prastomo et al. / Materials Science and Engineering B 161 (2009) 170–174
173
Fig. 6. FESEM image characterization of 87.5SiO2 ·12.5TiO2 , (a) before treatment and (b) after treated by hot-water parallel vibration at 90 ◦ C for 3 h.
HWT, higher dissolution and lower precipitation rates of the titania species were detected. Cross-sectional FESEM images were obtained to provide better perspective regarding the formation mechanism of sheet-like structure. It was observed that the hot-water treated coatings contain amorphous film and hydrated titania nanocrystals sheet-like structure layer. Fig. 6 provides clear images to distinguish coatings before and after parallel vibration HWT. The layers thickness changed with increasing of treatment time. Fig. 7 summarizes the measurements. Parallel vibration gives faster thickness reduction of the amorphous layer. This phenomenon shows that rapid water flow driven by this direction offers larger dissolution rate of the titania species. Parallel vibration direction enhances the nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration. The water flow moves the dissolved titania species into larger area and lowers the surface concentration of hydrolyzed titania species. Higher probability for the titania species to attach and combine each other should be achieved by parallel direction vibration under the present conditions. Based on the data, proposed mechanism for formation of sheetlike structure from vibration HWT on sol–gel derived silica–titania coatings is shown in Fig. 8.
Fig. 7. Amorphous film thickness changes for () perpendicular and () parallel vibrations and nanosheet structure thickness changes for (♦) perpendicular and () parallel vibrations by using 87.5SiO2 ·12.5TiO2 composition at 90 ◦ C.
The schematic shows that in early stage of treatment, washing and hydrolysis of the surface coatings happened. Afterward, precipitation starts on a microcrack area and then initiation of precipitates occurs. EDX results on the untreated sample show no significant difference of Ti element concentration between microcrack and the smooth area. This suggests that higher surface charge or defects in microcrack part may provide suitable driving force to the nucleation of the precipitate. Subsequently, the nucleation section probably become highly concentrated titania area and further precipitation growth takes place in this region. In between 10 and 20 min of treatment, precipitates start to elongate. Later cross-shape sheet-like structure starts on 25–35 min of treatment. The sheet-like structure keeps growing until at around 55 min treatment, and starts to combine each other. Amorphous film thickness decreases by increasing of treatment time. SiO2 –TiO2 films probably dissolved to form H3 SiO4 − and H3 TiO3 − ions in hot water, whereas the solubility of TiO2 component is very small. Based on pourbaix phase diagram of TiO2 in water at 25 ◦ C without any electric charge applied [15], at normal pH environment (6.5–7) titania species are stable in oxide form. Therefore, the formation of anatase (TiO2 ) and hydrated titania (TiO2 ·nH2 O) by HWT should be obtainable.
Fig. 8. Proposed formation mechanism of titania nanosheet cryatallites on silica–titania gel films by vibration HWT.
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4. Conclusions Dissolution into water, migration and reprecipitations of the titania species happened during the growth of titania nanocrystals on SiO2 –TiO2 coatings with hot-water treatment. The precipitation starts on a microcrack area. These provide the missing link of the previous proposed mechanism. On the vibration HWT, higher dissolution and lower precipitation rates of the titania species were detected. The proposed mechanism is as follows: (i) formation of roundish precipitates (ii) elongated precipitates formed (iii) elongated precipitates connected each other (iv) sheetlike structure occurred from combination of connected elongated precipitates. Parallel vibration direction enhances the hydrated titania nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration treatment. Based on the vibration direction study, parallel direction provides an optimum condition for hydrolyzed titania to precipitate on the silica–titania coating producing sheet-like structure. Acknowledgements This work was partially funded by The Japan Society for the Promotion of Science (JSPS) (Grant-in-Aid for Scientific Research (B) No. 20360268). N.P. acknowledges ASEAN University Net-
work/Southeast Asia Engineering Education Development Network (AUN/SEED-Net) and Japan International Cooperation Agency (JICA) for supporting his PhD study at Toyohashi University of Technology. References [1] A. Matsuda, M. Sakai, T. Kogure, K. Tadanaga, M. Tatsumisago, J. Ceram. Soc. Jpn. 114 (2006) 26–35. [2] R.R.A. Syms, A.S. Holmes, J. Non-Cryst. Solids 170 (1994) 223–233. [3] A.M. Seco, M.C. Gonc¸alves, R.M. Almeida, Mater. Sci. Eng. B 76 (2000) 193– 199. [4] H. Imai, H. Hirashima, K. Awazu, Thin Solid Films 351 (1999) 91–94. [5] H. Imai, H. Morimoto, A. Tominaga, H. Hirashima, J. Sol–Gel Sci. Technol. 10 (1997) 45–54. [6] Y. Sheng, L. Liang, Y. Xu, D. Wu, Y. Sun, Opt. Mater. 30 (2008) 1310–1315. [7] Y.J. Yun, J.S. Chung, S. Kim, S.H. Hahn, E.J. Kim, Mater. Lett. 58 (2004) 3703–3706. [8] L. Ge, M. Xu, M. Sun, H. Fang, Mater. Res. Bull. 41 (2006) 1596–1603. [9] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 83 (2000) 229–231. [10] Y. Kotani, A. Matsuda, M. Tatsumisago, T. Minani, T. Kogure, J. Sol–Gel Sci. Technol. 19 (2001) 585–588. [11] A. Matsuda, T. Matoda, K. Kogure, K. Tadanaga, T. Minami, M. Tatsumisago, J. Am. Ceram. Soc. 88 (2005) 1421–1426. [12] A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago, T. Minami, J. Sol–Gel Sci. Techmnol. 22 (2001) 41–46. [13] A. Matsuda, K. Kobayashi, T. Kogure, M. Sakai, K. Tadanaga, T. Minami, M. Tatsumisago, J. Ceram. Soc. Jpn. 113 (2005) 519–524. [14] A. Matsuda, T. Matoda, T. Kogure, K. Tadanaga, T. Minami, M. Tastumisago, J. Sol–Gel Sci. Technol. 31 (2004) 229–233. [15] A. Fujishima, Electrochemistry Handbook, Electrochemical Society of Japan, fifth ed., Maruzen, Tokyo, 2000, pp.71–77.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Formation mechanism of titania nanosheet cryatallites on silica–titania gel films by vibration hot-water treatment Niki Prastomo a,∗ , Yusuke Daiko a,b , Toshihiro Kogure c , Hiroyuki Muto a , Mototsugu Sakai a , Atsunori Matsuda a,∗ a b c
Department of Materials Science, Toyohashi University of Technology, Japan Department of Materials Science and Chemistry, University of Hyogo, Japan Department of Earth and Planetary Sciences, The University of Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 13 October 2008 Accepted 21 November 2008 Keywords: Sol–gel coating Hot-water treatment Titania External field Nanocrystal Formation mechanism
a b s t r a c t The control of the nanostructures of the titania-based coatings further offers an exciting opportunity to enhance their unique physical and chemical properties. By immersing silica–titania gel film coatings in hot-water concurrently with applying a vibration, nanosheet crystallites of hydrated titania were precipitated at the surface of the gel films. The main objective of this study is to design the functionality of titania-based coatings through the control of their surface morphology and crystallinity at low processing temperatures. Two types of vibration treatments, i.e. perpendicular and parallel directions against the gel films, were used. On 87.5SiO2 ·12.5TiO2 composition, parallel vibration direction enhances the nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration. The attachment of the precipitates from dissolved titania species should be an important driving force for the formation of the sheet-like structure. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Sol–gel method is a promising wet chemical technique for fabricating silica–titania nanocoatings. This route offers a control of the process on the molecular level and impurities can be avoided. Compared with conventional vacuum coating techniques, this technique is more cost-effective because coatings could be obtained practically easy since it can be formed on various substrates with large surface area and non-flat surface [1–3]. Generally, crystallization and densification of sol–gel derived titania coatings require heat-treatments at relatively high temperatures. Recently, the lowering process temperature and controlling of surface morphology attract considerable interests for designing advanced titania-based nanocoatings [4–8]. We have shown that anatase nanocrystals are formed on sol–gel derived SiO2 –TiO2 coatings at low temperatures using hot-water treatment at 90 ◦ C (HWT) [9,10]. The process temperature can be further lowered at 38 ◦ C when the SiO2 –TiO2 coatings are treated with the water for long period of time [11]. Substrates with coatings are immersed into hot, pure water and kept at rest for given time. The resultant coatings
∗ Corresponding authors. E-mail addresses: [email protected] (N. Prastomo), [email protected] (A. Matsuda). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.11.028
show unique characteristic, such as high photocatalytic activity and excellent wettability for water [12]. We also have reported that by applying external-field during HWT, the shape of the titania precipitates interestingly changed. Roundish precipitates changed to ramiform upon applying electric field to the substrate during HWT [13]. By applying vibration to the substrate during HWT, the precipitates tend to be elongated [14]. These ramiform and elongated precipitates are found to contain hydrated titania nanosheet with lepidocrocite structure [1]. The formation mechanism of this titania nanocrystals in the gel films with HWT is essentially important to control and design the required physical and chemical properties with purpose to enhance the functionalities of the advanced titania-based nanocoatings. We have proposed the formation mechanism of titania nanocrystals growth on SiO2 –TiO2 coatings with hot water as follows: (i) hydrolysis of Si–O–Ti bonds and dissolution of SiO2 component, (ii) migration of hydrolyzed titania species from the interior to the surface of the coating, and (iii) nucleation and growth of titania nanocrystals at the residual coating [1]. In the present work, we have newly found that before the nucleation and the growth of the titania crystalline phase, the titania species are dissolved into the hot-water first, and the dissolved titania species re-precipitate on the high titania concentration area on the surface of the coatings. Furthermore, the formation mechanism of elongated sheet-like titania structure obtained from vibration
N. Prastomo et al. / Materials Science and Engineering B 161 (2009) 170–174
171
Fig. 1. Schematic diagram of HWT experimental method.
HWT is described. The effects of vibration direction are also discussed on the basis of changes in the surface morphology. 2. Experimental The chemical composition 87.5SiO2 ·12.5TiO2 mol% was applied in this study. Based on the preliminary field emission scanning electron microscopy (FESEM) study, this composition provides well-dispersed titania precipitates on the silica–titania coating surface resulting an easy and straightforward observation. Ethanol was used as a solvent for silicon ethoxide and titanium n-buthoxide. Diluted hydrochloric acid (3.6 wt%) was then introduced as a catalyst during hydrolysis process. SiO2 –TiO2 coating films were formed on silica glass substrate and indium tin oxide (ITO)-coated glass substrates using the colloidal suspension by dipping and withdrawing at a constant speed of 4 cm/min and on ITO-coated quartz microbalance (QCM) by a constant speed of 1.5 cm/min to produce smooth and crack-free coatings. The sol–gel derived coatings were subsequently aged in a drying oven at 90 ◦ C for 1 h. To confirm dissolution and reprecipitation of titania species before nucleation and growth on the surface of the coatings, an experimental setup shown in Fig. 1 was used. “Face-to-face” method was done by using two kinds of coating. A 100SiO2 film was placed in front of 87.5SiO2 ·12.5TiO2 films with 2 mm distance. Both films then immersed into hot water for 5 h without applying any external-fields. For comparison, two 100SiO2 films were placed face-to-face also under the same condition.
Fig. 2. Schematic diagram of vibration HWT experimental method.
For vibration HWT by using ITO substrates, Fig. 2 shows the schematic diagram of the experimental method. The coatings were immersed into a hot-water environment for various times, 6 Hz with 2 mm amplitude vibration was afterward introduced to the coating by 2 methods, the vibration directions of which were perpendicular and parallel with the coatings. All the HWTs were carried out without stirring at normal pH environment (6.5–7). Field emission scanning electron microscopy (FESEM, S-4800 Hitachi), water contact angle measurements (DropMaster 300, Kyowa Interface Science), energy dispersed X-ray spectrometry (EDX, Emax Energy EX-250, Horiba) and quartz crystal microbalance (QCM, UEQ-400 Easy, USI) have been used to determine surface changes of the coating. 3. Results and discussion The formation of anatase nanoparticles and hydrated titania nanosheets is dominated by nucleation and growth of hydrolyzed titania species at the surface of the coatings. Before the nucleation and the growth of the anatase and/or hydrated titania phases, the titania species should dissolve into the hot-water and reprecipitate on the highly titania concentrated area on the surface of the coatings. By placing pure silica (100SiO2 )-coated substrate in front of silica–titania-coated substrate during HWT, validity of the dissolution and reprecipitation mechanism could be demonstrated.
Fig. 3. FESEM images of SiO2 coatings before and after face-to-face HWT for 5 h; (a) before treatment, (b) after treatment facing a 100SiO2 coating and (c) after treatment facing a 87.5SiO2 ·12.5TiO2 coating.
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Fig. 3 shows the FESEM images of the surface of 100SiO2 coatings before face-to-face treatments, after face-to-face treatment with other 100SiO2 and with 87.5SiO2 ·12.5TiO2 at 90 ◦ C for 5 h. Small amounts of precipitates were observed at places on the 100SiO2 -coated substrate treated face-to-face with the 87.5SiO2 ·12.5TiO2 film. EDX characterization results also verify the presence of titanium on the surface of the SiO2 coating, which confirms the mechanism of dissolution, migration and reprecipitation of the hydrolyzed titania species from SiO2 –TiO2 coatings. Vibration plays an important role to the formation mechanism of the sheet-like structure. Fig. 4 shows the difference on weight loss that measured by using ITO-coated QCM substrate between samples that were treated without vibration, with perpendicular vibration and parallel vibration. Total weight losses are 17, 71, and 73 m/cm2 for the samples without vibration, with perpendicular, and parallel vibrations, respectively, after HWT for 100 min. Comparing with the nonvibration sample, weight loss rates from both samples treated with dissimilar vibration show significant difference. These high rates and faster changes were provided by water flow from vibration mechanism that supplied larger driving force for dissolving titania species on to the hot-water in shorter time. To gain more understanding of formation mechanism, detail experiments using short treatment period were conducted. Fig. 5
Fig. 4. Normalized weight loss of the ITO-coated QCM samples without vibration (), and with perpendicular (♦) and parallel () vibrations.
shows the FESEM images of non-vibration and vibration parallel direction treated of 87.5SiO2 ·12.5TiO2 coatings for a range of times. Precipitation occurred earlier on the non-vibration sample. Anatase precipitates start to be observed on the 2 min treated sample, whereas the parallel vibration sample needs 4 min treatment to precipitate the hydrated titania. From the weight loss and FESEM observation results, it was shown that by applying vibration during
Fig. 5. FESEM images of 87.5SiO2 ·12.5TiO2 coating on ITO substrate, treated by hot-water with parallel direction vibration and without vibration at 90 ◦ C for various times.
N. Prastomo et al. / Materials Science and Engineering B 161 (2009) 170–174
173
Fig. 6. FESEM image characterization of 87.5SiO2 ·12.5TiO2 , (a) before treatment and (b) after treated by hot-water parallel vibration at 90 ◦ C for 3 h.
HWT, higher dissolution and lower precipitation rates of the titania species were detected. Cross-sectional FESEM images were obtained to provide better perspective regarding the formation mechanism of sheet-like structure. It was observed that the hot-water treated coatings contain amorphous film and hydrated titania nanocrystals sheet-like structure layer. Fig. 6 provides clear images to distinguish coatings before and after parallel vibration HWT. The layers thickness changed with increasing of treatment time. Fig. 7 summarizes the measurements. Parallel vibration gives faster thickness reduction of the amorphous layer. This phenomenon shows that rapid water flow driven by this direction offers larger dissolution rate of the titania species. Parallel vibration direction enhances the nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration. The water flow moves the dissolved titania species into larger area and lowers the surface concentration of hydrolyzed titania species. Higher probability for the titania species to attach and combine each other should be achieved by parallel direction vibration under the present conditions. Based on the data, proposed mechanism for formation of sheetlike structure from vibration HWT on sol–gel derived silica–titania coatings is shown in Fig. 8.
Fig. 7. Amorphous film thickness changes for () perpendicular and () parallel vibrations and nanosheet structure thickness changes for (♦) perpendicular and () parallel vibrations by using 87.5SiO2 ·12.5TiO2 composition at 90 ◦ C.
The schematic shows that in early stage of treatment, washing and hydrolysis of the surface coatings happened. Afterward, precipitation starts on a microcrack area and then initiation of precipitates occurs. EDX results on the untreated sample show no significant difference of Ti element concentration between microcrack and the smooth area. This suggests that higher surface charge or defects in microcrack part may provide suitable driving force to the nucleation of the precipitate. Subsequently, the nucleation section probably become highly concentrated titania area and further precipitation growth takes place in this region. In between 10 and 20 min of treatment, precipitates start to elongate. Later cross-shape sheet-like structure starts on 25–35 min of treatment. The sheet-like structure keeps growing until at around 55 min treatment, and starts to combine each other. Amorphous film thickness decreases by increasing of treatment time. SiO2 –TiO2 films probably dissolved to form H3 SiO4 − and H3 TiO3 − ions in hot water, whereas the solubility of TiO2 component is very small. Based on pourbaix phase diagram of TiO2 in water at 25 ◦ C without any electric charge applied [15], at normal pH environment (6.5–7) titania species are stable in oxide form. Therefore, the formation of anatase (TiO2 ) and hydrated titania (TiO2 ·nH2 O) by HWT should be obtainable.
Fig. 8. Proposed formation mechanism of titania nanosheet cryatallites on silica–titania gel films by vibration HWT.
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4. Conclusions Dissolution into water, migration and reprecipitations of the titania species happened during the growth of titania nanocrystals on SiO2 –TiO2 coatings with hot-water treatment. The precipitation starts on a microcrack area. These provide the missing link of the previous proposed mechanism. On the vibration HWT, higher dissolution and lower precipitation rates of the titania species were detected. The proposed mechanism is as follows: (i) formation of roundish precipitates (ii) elongated precipitates formed (iii) elongated precipitates connected each other (iv) sheetlike structure occurred from combination of connected elongated precipitates. Parallel vibration direction enhances the hydrated titania nanosheet structure formation to 180 nm thickness after 5 h, whereas the perpendicular vibration provides 150 nm thickness, which is about 18% lower than that of nanosheet structure formed by the parallel vibration treatment. Based on the vibration direction study, parallel direction provides an optimum condition for hydrolyzed titania to precipitate on the silica–titania coating producing sheet-like structure. Acknowledgements This work was partially funded by The Japan Society for the Promotion of Science (JSPS) (Grant-in-Aid for Scientific Research (B) No. 20360268). N.P. acknowledges ASEAN University Net-
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