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Preparation of mesoporous TiO2 thin films by surfactant templating
Journal of Non-Crystalline Solids 285 (2001) 90±95
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Preparation of mesoporous TiO2 thin ®lms by surfactant templating Miah Muhammed Yusuf, Hiroaki Imai, Hiroshi Hirashima * Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Abstract Porous TiO2 thin ®lms have potential applications in optical-, electronic- and mechanical-components, catalysts, photoelectrodes, etc. A novel method to control pore sizes of sol±gel thin ®lms by surfactant templating was investigated. The coating sol was obtained by hydrolysis of Ti C4 H9 O4 in ethanol solution. The ®lms were prepared by spin-coating on glass substrates and drying at 90°C (xerogel ®lm) or drying after immersion in a cetyltrimethylammoniumchloride (CTAC) or benzyltrimethylammoniumchloride solution for 1±24 h (CTAC- or BTAC-modi®ed ®lm), followed by calcination at 500°C in air. The calcined ®lms were transparent and 30±120 nm in thickness. The refractive indices of the CTAC- and BTAC-modi®ed ®lms were about 1.7 and 1.9, respectively. These were 10±20% lower than those of the xerogel ®lms. A columnar structure was observed by SEM and AFM. The columns were about 30 nm in diameter. The spacing between the columns was about 10 nm, corresponding to the size of CTAC micelles. These results show that the microstructure and pore size of the sol±gel ®lms can be controlled by the surfactant-templating. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Je; 81.05.Rm; 81.16.Rf; 81.20.Fw
1. Introduction Crystalline TiO2 , especially anatase, is well known to have interesting properties and potential applications, e.g., photocatalysts [1±3], photoelectrodes [4], gas sensors [5] and electrochromic display devices [6,7]. Anatase particles show a long catalytic lifetime [8], but for many applications, porous ®lms with a large surface area are desired. In recent years, a great interest in photocatalytic activity of anatase has been growing. Many
* Corresponding author. Tel.: +81-45 563 1141; fax: +81-45 566 1551. E-mail address: [email protected] (H. Hirashima).
papers have been published on the preparation of porous titania ®lms using the sol±gel method[9], sputtering techniques [10] and direct deposition from TiF4 aqueous solutions [11]. As a process to prepare highly porous materials, supercritical drying is well known [12], but it requires high temperatures and pressures. Several other methods to prepare porous materials at ambient pressure [13], such as solvent exchange [14] and processes using polymer additives [15,16] have been proposed. The sol±gel process is one of the most appropriate technologies for the preparation of thin oxide ®lms. In this paper, a novel method to control pore sizes of sol±gel-derived thin ®lms by surfactant templating is reported. Porous TiO2 ®lms have been prepared via surface modi®cation
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 3 7 - 9
M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
with surfactant solutions. The microstructure of titania thin ®lms, including the sizes and shape of the pores and grains, can be controlled by changing the species and concentration of surfactants. 2. Experimental procedures 2.1. Film preparation TiO2 ®lms were prepared from Ti OC4 H9 4 , (99.9%, Soekawa Rikagaku, Tokyo) by hydrolysis in alcoholic solution and spin-coating. Ethanol (99.5%, Junsei Chemical, Japan) was used as solvent. The concentration of Ti OC4 H9 4 was 0.15 mol l 1 . The amount of ion-free H2 O used for hydrolysis was about ®ve times the theoretical amount. 0.17 mol HCl was added to 1 mol Ti OC4 H9 4 as a catalyst. The solution was vigorously stirred for 1 h at room temperature. The precursor solutions were transparent and were very stable in air. Silica glass plates (Matsunami Glass, 75 mm 25 mm 1 mm were used as the substrates. TiO2 coatings were prepared by spincoating at 2500 rpm for 30 s. Spinning was
91
repeated (one after one, without heating between coating) up to ®ve times. Then, the wet gel ®lms were immersed in ethanol solutions of surfactants, cetyltrimethylammonium, C16 H33 N CH3 3 Cl (CTAC) (Kanto Chemical, Tokyo) or benzyltrimethlyammoniumchloride, C6 H5 CH2 N CH3 3 Cl (BTAC) (Junsei Chemical, Japan) at room temperature under atmospheric pressure. The surfactant content in the solutions and the immersion time of the gel ®lms were varied (Table 1). After immersion, the TiO2 gel ®lms were dried at 90°C for 24 h, and annealed at temperatures up to 500°C for 1 h in air. Titania ®lms without the surfactant treatment and surfactant ®lms were also prepared by spin-coating. 2.2. Characterization of the ®lms The thickness and refractive index of the coatings were determined using an automatic ellipsometer (Shimadzu, AEP-100) using He±Ne laser light (k; 632.8 nm). X-ray diraction (XRD) measurements of the annealed ®lms were made using CUKa radiation and a rotating disk-type sample holder for thin ®lms (Rigaku, RAD-C system, RTP300).
Table 1 Preparation conditions and physical properties of samples Sample
As-dried gela
Preparation condition 1
c
Annealed gelb d
n
Porositye (%)
t (h)
n
±
±
1.880.01
608.5
2.140.04
270.9
34.9
(BTAC-immersed gels) B1 0.1 B2 0.1 B3 0.0004 B4 0.5
1.0 24 1.0 1.0
1.770.02 ± 1.700.03 ±
14515 ± 15213 ±
1.960.08 1.920.05 1.960.08 1.890.04
1378.5 865.0 1378.5 765.0
48.3 51.2 48.3 53.2
(CTAC-immersed gels) C1 0.1 C2 0.1 C3 0.1 C4 0.1 C5 0.0004 C6 0.5
0.16 1.0 5.0 24 1.0 1.0
± 1.570.05 1.500.01 1.580.01 ± ±
± 1042.6 1216.3 1435.6 ± ±
1.980.06 1.730.07 1.740.03 1.770.01 1.730.05 1.740.04
10710 583.2 995.0 1207.7 866.3 1093.5
46.9 63.7 63.1 61.2 63.7 63.1
A1
d (nm)
dd (nm)
(surfactant) mol l
a
Dried at 90°C for 24 h. Annealed at 500°C for 1 h. c Immersion time in surfactant solution. d Film thickness estimated from ellipsometry. e Estimated with Maxwell's equation, P 1 f n2 n0 is the refractive index of Anatase ( 2.55). b
1= n20
1g 100, where n is the refractive index measured by ellipsometry and
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M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
The pore and grain sizes of the ®lms were estimated from the images observed by ®eld emission scanning electron microscopy (FESEM) (Hitachi, S-4700) and atomic force microscopy (AFM) (Seiko Instruments, SPA 300A). Specimens were viewed in planar and cross-sectional modes. 3. Results 3.1. Refractive indices and thickness of the ®lms by ellipsometry The as-deposited ®lms without surfactant-immersion are colorless. The dried ®lms with and without surfactant-immersion are transparent, but light brown and colorless, respectively. The surfactant-modi®ed titania ®lms exhibit a smooth and continuous surface. Crack formation during drying is reduced, and almost crack-free ®lms are obtained with surfactant-immersion. The refractive indices of the dried and calcined ®lms immersed in the CTAC or BTAC solution for 1±24 h were 15±20% or 8±10% lower than those of the ®lms without immersion (Table 1). The refractive indices of the ®lms immersed in CTAC solution for more than 1 h hardly depend on the immersion time. The decrease in the refractive index of gel ®lms with surface treating after annealing at 500°C may be attributed to higher porosity of the surfactant-modi®ed ®lms. The thickness was 30±140 nm for the calcined ®lms. 3.2. Crystallization behavior of the ®lms determined by XRD The gel ®lms without surfactant-immersion, which were heat treated at temperatures up to 300°C, were amorphous to XRD (Fig. 1). On the other hand, diraction peaks of low h were found for the as-dried gel ®lms with surfactant-immersion (Fig. 1). Similar peaks were also found for the surfactant ®lms. These peaks disappeared after annealing at 300°C. Diraction peaks of crystalline TiO2 , anatase, were found for both of the gel ®lms with and without surfactant- immersion after annealing at 500°C for 1 h (Fig. 1). This result shows that the surfactant (CTAC) was removed by
Fig. 1. XRD patterns for CTAC-modi®ed xerogel ®lms: (a) sample A1, as-dried; (b) C2, as-dried; (c) A1, annealed at 300°C for 1 h; (d) C2, annealed at 300°C for 1 h; (e) A1, annealed at 500°C for 1 h; (f) C2, annealed at 500°C for 1 h. (, anatase; , unknown; 4, CTAC).
heating at 300°C. Diraction peaks of anatase were also found for the ®lms immersed in BTAC solution after annealing at 500°C. The XRD patterns hardly changed with the concentration of surfactant solutions and immersion time. 3.3. FESEM and AFM observations The SEM images of the gel ®lms are shown in Figs. 2(a)±(c). The ®lm thickness estimated from the SEM image coincided with the results of ellipsometry within 10%. Spherical particles about 30 nm in diameter were found in ®lms without surfactant-immersion. Larger particles, about 100 nm in diameter, were found in BTAC-modi®ed ®lms. On the other hand, a columnar structure was observed for CTAC-modi®ed ®lms. The columns are about 30 nm in diameter, and neither their diameter nor their spacing, about 10 nm, changes much with surfactant concentration or immersion time (Table 2). The AFM image shows spherical particles in the ®lms without surfactant-immersion (Fig. 3(a)). The particles in BTAC-modi®ed ®lms are rectangular, and in CTAC-modi®ed ®lms are columnar (Fig. 3(b) and (c)). The columnar grain shape can be seen in Fig. 3(c), which shows a crack near the ®lm edge.
M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
93
Fig. 2. SEM images of gel ®lms annealed at 500°C for 1 h: (a) sample A1, (b) B1, and (c) C2. (The samples were tilted about 30° to show the ®lm edge and grain shape.) Table 2 Porosity, pore size, and grain size of TiO2 gel ®lms annealed at 500°C Sample A1 B1 C2 C4 C5 C6 a
Porositya (%) 34.9 48.3 63.7 61.2 63.7 63.1
Average pore size (nm)
Average grain size (nm)
SEM
AFM
SEM
AFM
3.70.7 5.01.8 11.23.5 10.41.8 12.21.5 12.71.2
± 51.8 102.8 ± ± ±
28.03.5 10012.5 28.52.9 29.73.5 25.51.5 29.53.0
353.8 12010 302.2 ± ± ±
Estimated with Maxwell's equation, P 1 f n2 n0 is the refractive index of anatase ( 2.55).
1= n20
Grain shape Spherical Rectangular Columnar Columnar Columnar Columnar
1g 100, where n is the refractive index measured by ellipsometry and
The particle size and spacing estimated from SEM and AFM observation coincide (Table 2). 4. Discussion Mesoporous anatase ®lms, about 100 nm thickness, were obtained by immersion of wet gel ®lms
into surfactant solutions. The porosity, estimated using Maxwell's equation assuming n0 2:55, was about 50% and 60% for BTAC- and CTAC-modi®ed ®lms (Table 1). The porosity of surfactantmodi®ed ®lms are higher than that of the xerogel ®lms. These ®lms consist of rectangular or columnar grains for BTAC- or CTAC-modi®ed ®lms, respectively. The grain size and pore size, i.e., spacing
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M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
Fig. 3. AFM images of annealed gel ®lms: (a) sample A1, (b) B1, and (c) C2. (A crack near the ®lm edge is shown in (c). The ®lm was broken together with the substrate to form edges, and a few cracks were formed near the edge.)
between particles, are determined by surfactant species. These results suggest that the surfactants are `templates' for the mesopores in anatase ®lms. However, the pore size, about 5 nm for BTACmodi®ed ®lms or 10 nm for CTAC-modi®ed ®lms, is more than two times larger than the diameter of the surfactant micelles, about 2 nm for BTAC and 4 nm for CTAC. These results suggest that the surfactant micelles adsorb onto the gel surface, and they prevent shrinkage during drying. The diraction peaks attributed to the surfactant ®lm, found for the asdried ®lms, show that the surfactant remains in the ®lms after drying at 90°C. The structure of surfactant micelles, cylindrical CTAC micelles and planar BTAC micelles [17,18] may aect the micromorphology of gel ®lms. The dierence in the grain shape of BTAC- and CTAC-modi®ed ®lms may be caused by the different shapes of the micelles. The immersion time needed for substitution of pore liquid with surfactant solutions is short, one hour is enough, because the thickness of ®lms is very small. During annealing, the crystallization proceeds after removal of surfactants and the surfactant-immersion process scarcely aects the crystallization behavior of ®lms. The results of this study show that the pore size and porosity of sol±gel ®lms can be increased with
a short time immersion of wet gel ®lms in surfactant solutions before drying, and the pore size can be determined by the size and shape of surfactant micelles. The mesoporous gel shows high catalyst activity. The photocatalytic activity of CTACmodi®ed anatase ®lms for oxidation of NO, examined with a ¯ow type reactor [19], is three times higher than that of the xerogel ®lms without surfactant-immersion. It is important to control the microstructure of catalysts, and the newly developed surfactant-immersion process can be applied in the preparation of such materials. 5. Conclusions (1) A new method to prepare mesoporous thin ®lms was developed. The porous anatase ®lm was prepared by the surfactant-templating method under an atmospheric pressure. (2) The micromorphology and the pore sizes of anatase ®lms can be controlled by changing the type of the surfactant species. References [1] P. Wauthoz, M. Ruwet, T. Machej, P. Grange, Appl. Catal. 69 (1999) 149.
M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95 [2] K. Katoh, A. Tsuzuki, H. Taoda, Y. Torii, T. Kato, Y. Butsugan, J. Mater. Sci. 29 (1994) 5911. [3] T. Hisanaga, K. Harada, K. Tanaka, J. Photo-chem. Photobiol. A 53 (1990) 113. [4] B.O. Regan, M. Gratzel, Nature 353 (1991) 737. [5] L.D. Birkefeld, A.M. Azad, S.A. Akbar, J. Am. Ceram. Soc. 75 (1992) 2964. [6] K. Nagase, Y. Shimizu, N. Miura, N. Yamazoe, J. Ceram. Soc. Jpn. 101 (1993) 1032. [7] E.A. Barringer, H.K. Bower, J. Am. Ceram. Soc. 65 (1982) 199. [8] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, J. Phys. Chem. 91 (1987) 4305. [9] T. Yoko, A. Yuasa, K. Kamiya, S. Sakka, J. Electrochem. Soc. 138 (1991) 2279. [10] H. Yanagi, Y. Ohoka, T. Hishiki, K. Ajito, A. Fujishima, Appl. Surf. Sci. 113&114 (1997) 426.
95
[11] K. Shimizu, H. Imai, H. Hirashima, K. Tsukuma, Thin Solid Films 351 (1999) 220. [12] S.S. Kistler, Nature 127 (1931) 741. [13] S.S. Prakash, C.J. Brinker, A.J. Hurd, J. Non-Cryst. Solids 188 (1992) 264. [14] R. Takahashi, K. Nakanishi, N. Soga, J. Non-Cryst. Solids 189 (1995) 66. [15] S. Kure, E. Ihara, Y. Chujo, T. Saegusa, T. Yazawa, K. Eguchi, Polym. Prepr. Jpn. 39 (1990) 1681. [16] Y. Liu, A. Wang, R. Claus, J. Phys. Chem. B 101 (1997) 1385. [17] F. Reiss-Husson, V. Luzzati, J. Phys. Chem. 68 (1964) 3505. [18] V. Luzzati, H. Mustacchi, A.E. Skoulios, F. Husson, Acta Crystallogr. 14 (1961) 219. [19] N. Negishi, K. Takeuchi, T. Ibusuki, Appl. Surf. Sci. 121&122 (1997) 417.
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Preparation of mesoporous TiO2 thin ®lms by surfactant templating Miah Muhammed Yusuf, Hiroaki Imai, Hiroshi Hirashima * Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Abstract Porous TiO2 thin ®lms have potential applications in optical-, electronic- and mechanical-components, catalysts, photoelectrodes, etc. A novel method to control pore sizes of sol±gel thin ®lms by surfactant templating was investigated. The coating sol was obtained by hydrolysis of Ti C4 H9 O4 in ethanol solution. The ®lms were prepared by spin-coating on glass substrates and drying at 90°C (xerogel ®lm) or drying after immersion in a cetyltrimethylammoniumchloride (CTAC) or benzyltrimethylammoniumchloride solution for 1±24 h (CTAC- or BTAC-modi®ed ®lm), followed by calcination at 500°C in air. The calcined ®lms were transparent and 30±120 nm in thickness. The refractive indices of the CTAC- and BTAC-modi®ed ®lms were about 1.7 and 1.9, respectively. These were 10±20% lower than those of the xerogel ®lms. A columnar structure was observed by SEM and AFM. The columns were about 30 nm in diameter. The spacing between the columns was about 10 nm, corresponding to the size of CTAC micelles. These results show that the microstructure and pore size of the sol±gel ®lms can be controlled by the surfactant-templating. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Je; 81.05.Rm; 81.16.Rf; 81.20.Fw
1. Introduction Crystalline TiO2 , especially anatase, is well known to have interesting properties and potential applications, e.g., photocatalysts [1±3], photoelectrodes [4], gas sensors [5] and electrochromic display devices [6,7]. Anatase particles show a long catalytic lifetime [8], but for many applications, porous ®lms with a large surface area are desired. In recent years, a great interest in photocatalytic activity of anatase has been growing. Many
* Corresponding author. Tel.: +81-45 563 1141; fax: +81-45 566 1551. E-mail address: [email protected] (H. Hirashima).
papers have been published on the preparation of porous titania ®lms using the sol±gel method[9], sputtering techniques [10] and direct deposition from TiF4 aqueous solutions [11]. As a process to prepare highly porous materials, supercritical drying is well known [12], but it requires high temperatures and pressures. Several other methods to prepare porous materials at ambient pressure [13], such as solvent exchange [14] and processes using polymer additives [15,16] have been proposed. The sol±gel process is one of the most appropriate technologies for the preparation of thin oxide ®lms. In this paper, a novel method to control pore sizes of sol±gel-derived thin ®lms by surfactant templating is reported. Porous TiO2 ®lms have been prepared via surface modi®cation
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 3 7 - 9
M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
with surfactant solutions. The microstructure of titania thin ®lms, including the sizes and shape of the pores and grains, can be controlled by changing the species and concentration of surfactants. 2. Experimental procedures 2.1. Film preparation TiO2 ®lms were prepared from Ti OC4 H9 4 , (99.9%, Soekawa Rikagaku, Tokyo) by hydrolysis in alcoholic solution and spin-coating. Ethanol (99.5%, Junsei Chemical, Japan) was used as solvent. The concentration of Ti OC4 H9 4 was 0.15 mol l 1 . The amount of ion-free H2 O used for hydrolysis was about ®ve times the theoretical amount. 0.17 mol HCl was added to 1 mol Ti OC4 H9 4 as a catalyst. The solution was vigorously stirred for 1 h at room temperature. The precursor solutions were transparent and were very stable in air. Silica glass plates (Matsunami Glass, 75 mm 25 mm 1 mm were used as the substrates. TiO2 coatings were prepared by spincoating at 2500 rpm for 30 s. Spinning was
91
repeated (one after one, without heating between coating) up to ®ve times. Then, the wet gel ®lms were immersed in ethanol solutions of surfactants, cetyltrimethylammonium, C16 H33 N CH3 3 Cl (CTAC) (Kanto Chemical, Tokyo) or benzyltrimethlyammoniumchloride, C6 H5 CH2 N CH3 3 Cl (BTAC) (Junsei Chemical, Japan) at room temperature under atmospheric pressure. The surfactant content in the solutions and the immersion time of the gel ®lms were varied (Table 1). After immersion, the TiO2 gel ®lms were dried at 90°C for 24 h, and annealed at temperatures up to 500°C for 1 h in air. Titania ®lms without the surfactant treatment and surfactant ®lms were also prepared by spin-coating. 2.2. Characterization of the ®lms The thickness and refractive index of the coatings were determined using an automatic ellipsometer (Shimadzu, AEP-100) using He±Ne laser light (k; 632.8 nm). X-ray diraction (XRD) measurements of the annealed ®lms were made using CUKa radiation and a rotating disk-type sample holder for thin ®lms (Rigaku, RAD-C system, RTP300).
Table 1 Preparation conditions and physical properties of samples Sample
As-dried gela
Preparation condition 1
c
Annealed gelb d
n
Porositye (%)
t (h)
n
±
±
1.880.01
608.5
2.140.04
270.9
34.9
(BTAC-immersed gels) B1 0.1 B2 0.1 B3 0.0004 B4 0.5
1.0 24 1.0 1.0
1.770.02 ± 1.700.03 ±
14515 ± 15213 ±
1.960.08 1.920.05 1.960.08 1.890.04
1378.5 865.0 1378.5 765.0
48.3 51.2 48.3 53.2
(CTAC-immersed gels) C1 0.1 C2 0.1 C3 0.1 C4 0.1 C5 0.0004 C6 0.5
0.16 1.0 5.0 24 1.0 1.0
± 1.570.05 1.500.01 1.580.01 ± ±
± 1042.6 1216.3 1435.6 ± ±
1.980.06 1.730.07 1.740.03 1.770.01 1.730.05 1.740.04
10710 583.2 995.0 1207.7 866.3 1093.5
46.9 63.7 63.1 61.2 63.7 63.1
A1
d (nm)
dd (nm)
(surfactant) mol l
a
Dried at 90°C for 24 h. Annealed at 500°C for 1 h. c Immersion time in surfactant solution. d Film thickness estimated from ellipsometry. e Estimated with Maxwell's equation, P 1 f n2 n0 is the refractive index of Anatase ( 2.55). b
1= n20
1g 100, where n is the refractive index measured by ellipsometry and
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M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
The pore and grain sizes of the ®lms were estimated from the images observed by ®eld emission scanning electron microscopy (FESEM) (Hitachi, S-4700) and atomic force microscopy (AFM) (Seiko Instruments, SPA 300A). Specimens were viewed in planar and cross-sectional modes. 3. Results 3.1. Refractive indices and thickness of the ®lms by ellipsometry The as-deposited ®lms without surfactant-immersion are colorless. The dried ®lms with and without surfactant-immersion are transparent, but light brown and colorless, respectively. The surfactant-modi®ed titania ®lms exhibit a smooth and continuous surface. Crack formation during drying is reduced, and almost crack-free ®lms are obtained with surfactant-immersion. The refractive indices of the dried and calcined ®lms immersed in the CTAC or BTAC solution for 1±24 h were 15±20% or 8±10% lower than those of the ®lms without immersion (Table 1). The refractive indices of the ®lms immersed in CTAC solution for more than 1 h hardly depend on the immersion time. The decrease in the refractive index of gel ®lms with surface treating after annealing at 500°C may be attributed to higher porosity of the surfactant-modi®ed ®lms. The thickness was 30±140 nm for the calcined ®lms. 3.2. Crystallization behavior of the ®lms determined by XRD The gel ®lms without surfactant-immersion, which were heat treated at temperatures up to 300°C, were amorphous to XRD (Fig. 1). On the other hand, diraction peaks of low h were found for the as-dried gel ®lms with surfactant-immersion (Fig. 1). Similar peaks were also found for the surfactant ®lms. These peaks disappeared after annealing at 300°C. Diraction peaks of crystalline TiO2 , anatase, were found for both of the gel ®lms with and without surfactant- immersion after annealing at 500°C for 1 h (Fig. 1). This result shows that the surfactant (CTAC) was removed by
Fig. 1. XRD patterns for CTAC-modi®ed xerogel ®lms: (a) sample A1, as-dried; (b) C2, as-dried; (c) A1, annealed at 300°C for 1 h; (d) C2, annealed at 300°C for 1 h; (e) A1, annealed at 500°C for 1 h; (f) C2, annealed at 500°C for 1 h. (, anatase; , unknown; 4, CTAC).
heating at 300°C. Diraction peaks of anatase were also found for the ®lms immersed in BTAC solution after annealing at 500°C. The XRD patterns hardly changed with the concentration of surfactant solutions and immersion time. 3.3. FESEM and AFM observations The SEM images of the gel ®lms are shown in Figs. 2(a)±(c). The ®lm thickness estimated from the SEM image coincided with the results of ellipsometry within 10%. Spherical particles about 30 nm in diameter were found in ®lms without surfactant-immersion. Larger particles, about 100 nm in diameter, were found in BTAC-modi®ed ®lms. On the other hand, a columnar structure was observed for CTAC-modi®ed ®lms. The columns are about 30 nm in diameter, and neither their diameter nor their spacing, about 10 nm, changes much with surfactant concentration or immersion time (Table 2). The AFM image shows spherical particles in the ®lms without surfactant-immersion (Fig. 3(a)). The particles in BTAC-modi®ed ®lms are rectangular, and in CTAC-modi®ed ®lms are columnar (Fig. 3(b) and (c)). The columnar grain shape can be seen in Fig. 3(c), which shows a crack near the ®lm edge.
M.M. Yusuf et al. / Journal of Non-Crystalline Solids 285 (2001) 90±95
93
Fig. 2. SEM images of gel ®lms annealed at 500°C for 1 h: (a) sample A1, (b) B1, and (c) C2. (The samples were tilted about 30° to show the ®lm edge and grain shape.) Table 2 Porosity, pore size, and grain size of TiO2 gel ®lms annealed at 500°C Sample A1 B1 C2 C4 C5 C6 a
Porositya (%) 34.9 48.3 63.7 61.2 63.7 63.1
Average pore size (nm)
Average grain size (nm)
SEM
AFM
SEM
AFM
3.70.7 5.01.8 11.23.5 10.41.8 12.21.5 12.71.2
± 51.8 102.8 ± ± ±
28.03.5 10012.5 28.52.9 29.73.5 25.51.5 29.53.0
353.8 12010 302.2 ± ± ±
Estimated with Maxwell's equation, P 1 f n2 n0 is the refractive index of anatase ( 2.55).
1= n20
Grain shape Spherical Rectangular Columnar Columnar Columnar Columnar
1g 100, where n is the refractive index measured by ellipsometry and
The particle size and spacing estimated from SEM and AFM observation coincide (Table 2). 4. Discussion Mesoporous anatase ®lms, about 100 nm thickness, were obtained by immersion of wet gel ®lms
into surfactant solutions. The porosity, estimated using Maxwell's equation assuming n0 2:55, was about 50% and 60% for BTAC- and CTAC-modi®ed ®lms (Table 1). The porosity of surfactantmodi®ed ®lms are higher than that of the xerogel ®lms. These ®lms consist of rectangular or columnar grains for BTAC- or CTAC-modi®ed ®lms, respectively. The grain size and pore size, i.e., spacing
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Fig. 3. AFM images of annealed gel ®lms: (a) sample A1, (b) B1, and (c) C2. (A crack near the ®lm edge is shown in (c). The ®lm was broken together with the substrate to form edges, and a few cracks were formed near the edge.)
between particles, are determined by surfactant species. These results suggest that the surfactants are `templates' for the mesopores in anatase ®lms. However, the pore size, about 5 nm for BTACmodi®ed ®lms or 10 nm for CTAC-modi®ed ®lms, is more than two times larger than the diameter of the surfactant micelles, about 2 nm for BTAC and 4 nm for CTAC. These results suggest that the surfactant micelles adsorb onto the gel surface, and they prevent shrinkage during drying. The diraction peaks attributed to the surfactant ®lm, found for the asdried ®lms, show that the surfactant remains in the ®lms after drying at 90°C. The structure of surfactant micelles, cylindrical CTAC micelles and planar BTAC micelles [17,18] may aect the micromorphology of gel ®lms. The dierence in the grain shape of BTAC- and CTAC-modi®ed ®lms may be caused by the different shapes of the micelles. The immersion time needed for substitution of pore liquid with surfactant solutions is short, one hour is enough, because the thickness of ®lms is very small. During annealing, the crystallization proceeds after removal of surfactants and the surfactant-immersion process scarcely aects the crystallization behavior of ®lms. The results of this study show that the pore size and porosity of sol±gel ®lms can be increased with
a short time immersion of wet gel ®lms in surfactant solutions before drying, and the pore size can be determined by the size and shape of surfactant micelles. The mesoporous gel shows high catalyst activity. The photocatalytic activity of CTACmodi®ed anatase ®lms for oxidation of NO, examined with a ¯ow type reactor [19], is three times higher than that of the xerogel ®lms without surfactant-immersion. It is important to control the microstructure of catalysts, and the newly developed surfactant-immersion process can be applied in the preparation of such materials. 5. Conclusions (1) A new method to prepare mesoporous thin ®lms was developed. The porous anatase ®lm was prepared by the surfactant-templating method under an atmospheric pressure. (2) The micromorphology and the pore sizes of anatase ®lms can be controlled by changing the type of the surfactant species. References [1] P. Wauthoz, M. Ruwet, T. Machej, P. Grange, Appl. Catal. 69 (1999) 149.
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