- Email: [email protected]
anatase complex thin film
Applied Surface Science 254 (2008) 6619–6622
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of brookite/anatase complex thin film Chiaki Ohara, Teruhisa Hongo *, Atsushi Yamazaki, Toshio Nagoya Department of Resources and Environmental Engineering, School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 October 2007 Received in revised form 9 April 2008 Accepted 11 April 2008 Available online 27 May 2008
TiO2 films were prepared on a silicon or soda-glass substrate using a sol suspension. The TiO2 film on the silicon substrate was composed of pure anatase phase and showed almost no contaminations. In contrast, the TiO2 film on the soda-glass substrate was composed of anatase and brookite phases. The diffusion of Na into the TiO2 film on the soda-glass substrate was observed by XPS, and Na was concentrated on the surface of the film. The yield of the brookite phase increased with decreasing distance from the surface of the film on the soda-glass substrate. Na promoted the formation of the brookite phase, although the preparative procedure was used for anatase synthesis. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Brookite Anatase TiO2 Thin film
1. Introduction Titanium dioxide (TiO2) has been investigated extensively for many years because of its various applications as a low-cost material in photocatalysis [1–3], in photovoltaics [4,5], in electronchromic [6] or as gas sensor [7,8]. The use of TiO2 as a catalyst during photochemical oxidation aimed at decomposing organic compounds is well known [9]. TiO2 exists in three different crystalline phases: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). In nature, rutile is the most common, while brookite is scarce. Among these phases, anatase and rutile can be obtained by several synthetic methods and their various properties have been widely studied. Recent reports on the preparation of pure brookite and the study of its characteristics are limited [10–12]. Some reports indicated that brookitetype TiO2 shows high photoinduced hydrophilicity and high photocatalytic activity compared with rutile and anatase [13,14]; thus, brookite is a good candidate material for solar-energy conversion devices. Brookite is the metastable phase of TiO2; thus, it is difficult to prepare pure brookite under laboratory conditions. Many researchers reported that it is typically obtained as an accessory second phase when rutile or anatase is synthesized [15–17]. It has been shown that the titania phase, which is the most difficult phase to prepare in thin-film form, is brookite [18,19]. Brookite/anatase complex films have been obtained by various techniques, such as
* Corresponding author. E-mail address: [email protected] (T. Hongo). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.030
pulsed laser deposition [20] and the electrochemical oxidation of titanium electrodes [21]. Djaoued et al. [22] have prepared brookite-rich films by a sol–gel method using diethanolamine and polyethylene glycol as modulators. Recently, Kuznetsova et al. [23] have prepared a spin-coated film of pure brookite with a preferred orientation. We prepared TiO2 films on silicon and soda-glass substrates using a sol suspension. This suspension was coated onto the substrates by dip coating. The TiO2 film on the silicon substrate was found to be composed of pure anatase phase by grazing incident asymmetric X-ray diffraction (GIAXRD) analysis. However, the TiO2 film on the soda-glass substrate was composed of anatase and brookite phases. In this work, we characterized these TiO2 films by GIAXRD analysis and X-ray photoelectron spectroscopy (XPS). These measurements revealed the difference between the film on the silicon substrate and that on the soda-glass substrate. We also discussed the mechanism of brookite phase generation on the soda-glass substrate. 2. Experimental 2.1. Preparation of TiO2 films TiO2 thin films were prepared by dip coating using a sol suspension on soda-glass and silicon substrates. Titanium tetraisopropoxide (TTIP) (90.6 ml) was used as the starting material. The required amount of ethanol (93.8 ml) was divided into two parts: the first part was added to the stirred TTIP. The second part was mixed with 5.2 ml of hydrochloric acid solution (38 wt.%) and then added dropwise (1.5 cm/min) into the solution with stirring
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622
6620
Fig. 1. X-ray penetration depth for TiO2 as function of incidence angle.
to promote hydrolysis. This homogenous solution was continuously stirred for 24 h at room temperature. The soda-glass and silicon substrates were first cleaned carefully in an ultrasonic bath. Using this prepared solution, the TiO2 thin films coated onto the substrates (soda glass or quartz) by dip coating. The films were then dried in a desiccator for 10 min and subsequently calcined at 500 8C in air for 2 h. After cooling to room temperature, the dip coating and calcination process were repeated. The TiO2 film thickness was about 150 nm. 2.2. Characterization The crystallographic properties of the TiO2 films were determined by GIAXRD analysis using a monochromatic Cu Ka radiation. Incidence angle was used to determine the change in the crystal phase along the film depth. The variation in the penetration depth of TiO2 as a function of incidence angle is shown in Fig. 1. Diffraction patterns were obtained in an 2u angular range from 24.08 to 32.08. The surface chemical compositions of the TiO2 films were determined by XPS. Spectra were collected using a monochromatic Mg Ka source with an energy of 1253.6 eV operated at 10 kV. For the TiO2 film on the soda-glass substrate, elemental depth profiling was carried out using Ar+ sputtering beams under the applied conditions (3 keV, 8 mA).
Fig. 2. GIAXRD patterns of TiO2 films on (a) silicon and (b) soda-glass substrates.
anatase phase (2u (Cu Ka1) = 25.2818 from JCPDS 21-1272), and the diffraction shoulder at about 25.88 is the 111 line of the brookite phase. Fig. 3 shows the intensity of the 121 diffraction line of the brookite phase relative to that of multiple lines at around 25.0–
3. Results and discussion The GIAXRD patterns of the TiO2 films are shown in Fig. 2. The GIAXRD patterns of the TiO2 film on the silicon substrate (Fig. 2(a)) were obtained at grazing incidence angles from 0.28 to 0.58. The diffraction line at about 25.58 was the 101 line of the anatase phase, and the sample was confirmed to be single-phase anatase from the upper layer to the lower layer. The diffraction patterns of the TiO2 film on the soda-glass substrate (Fig. 2(b)) were obtained at grazing angles of incidence from 0.28 to 0.68. The diffraction line at about 30.98, which was assigned to the 121 line of the brookite phase, was observed in addition to those from the anatase phase. The intensity of this line relative to the strongest 120 line of the brookite phase at 25.48 was about 90%. However, this relative intensity in Fig. 2(b) was clearly low. This result shows that the anatase and brookite phases coexist in the TiO2 film on the sodaglass substrate. Therefore, the strongest diffraction line at about 25.48 overlaps the 120 line of the brookite phase (2u (Cu Ka1) = 25.3398 from JCPDS 29-1360) and the 101 line of the
Fig. 3. Intensity of diffraction line 121 of brookite phase relative to multiple lines at around 25.0–26.08 in diffraction patterns of TiO2 film on soda-glass substrate as function of incidence angle.
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622 Table 1 Surface elemental compositions of TiO2 thin films Substrate
Ti (at.%)
O (at.%)
Si (at.%)
Na (at.%)
Silicon Soda glass
29.33 14.41
69.41 57.37
0.00 1.14
1.26 27.08
26.08 in the diffraction patterns of the TiO2 film on the soda-glass substrate as a function of incidence angle. This relative intensity decreased with increasing incidence angle. This result shows that the TiO2 film on the soda-glass substrate is not homogenous, and that the yield of the brookite phase increases with decreasing distance from the surface. The phase contents of the anatase and brookite in the surface of the film on soda-glass substrate was determined from the GIAXRD pattern obtained at grazing incidence angle of 0.28 using the method of Zhang and Banfield [24]. The proportions of the anatase and brookite phases were calculated to be 36.6 and 63.4%. The XPS spectra of the TiO2 films on soda-glass and silicon substrates showed distinct signals from Ti, O, Si and Na. The surface elemental compositions of TiO2 films are given in Table 1. In the case of the silicon substrate, a small amount of Na was observed in addition to Ti and O. This Na was considered to be produced by the dissolution of the material of the Pyrex beaker used in preparing the sol suspension. On the other hand, a large amount of Na was observed on the surface of the TiO2 film on the soda-glass substrate. Fig. 4 shows an XPS depth profile of the atomic concentrations of Ti, O, Si and Na in the TiO2 film on the soda-glass substrate. With increasing sputtering time, the amount of Na decreased significantly up to 100 s. Most of this Na was considered to be produced by the dissolution of the soda-glass substrate. A similar sodium diffusion process from the glass substrate was reported by Huber et al. [25]. They reported that the presence of contaminants in TiO2 films is substrate-dependent: in the films deposited on silicon or sapphire, the atomic fraction of impurities (most prominently Na and K) was in the range of 10 4, and, in those deposited on the glass substrate, an approximately 100-fold higher level of such species was observed, probably the result of outdiffusion during the calcination or reduction of the samples. After a sputtering time of about 800 s, a decrease in Ti concentration and an increase in Si concentration were observed, and this point was considered to be the boundary between the TiO2 film and the soda-glass substrate.
6621
From the results of XRD analysis and XPS, it is clear that the yield of the brookite phase increases with decreasing distance from the surface of the TiO2 film on the soda-glass substrate in which Na is concentrated. Na dissolved from the soda-glass substrate was considered to promote the formation of the brookite phase in calcination. This experimental result is consistent with the results indicating that brookite-phase TiO2 has been produced using highly concentrated sodium or chlorine as the reacting solution [26–29]. The relative intensity in Fig. 3 decreases slowly with incidence angle, while the amount of Na decreased rapidly with increasing sputtering time. This phenomenon is attributed to two reasons. First, Na contributes the crystal nucleation of brookite phase, but there is no correlation between Na concentration and the brookite phase content. The second is as follows. When the GIAXRD measurement is performed at grazing incidence angle of 0.68, the penetration depth is about 150 nm. The diffraction pattern is not represent the average crystal phases of the area, but is most influenced by the surface crystal phases, because X-ray decays with the distance from the surface. Although the experimental procedure is for the synthesis of anatase phase except for the substrate in this study, brookite phase is produced without using a reacting solution containing highly concentrated sodium or chlorine. This experimental result indicates that the generated TiO2 phase is sensitive to contaminations from experimental instruments as well as synthetic reagents. 4. Conclusion In this work, the diffusion of Na into a TiO2 film on a soda-glass substrate was observed by XPS, and Na was concentrated on the surface of the film. A TiO2 film on a soda-glass substrate is composed of anatase and brookite phases. The yield of the brookite phase increases with decreasing distance from the surface of the film, because Na concentrated on the surface promotes brookite phase formation. In contrast, a TiO2 film on a silicon substrate shows almost no Na and is composed of pure anatase phase. Although the preparative procedure used is for anatase synthesis, the brookite phase is obtained owing to Na contamination. Acknowledgement The authors are grateful to Shinpei Enomoto for help with XPS and valuable discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 4. XPS depth profiles for atomic concentrations of Ti, O, Si and Na in TiO2 film on soda-glass substrate.
[18]
M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735. M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 503. A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. M. Gra¨tzel, Nature 414 (2001) 338. A. Ponton, S. Barboux-Doeuff, C. Sanchez, Colloids Surf. A 162 (1999) 177. W. Go¨pel, G. Reinhardt, in: H. Baltes, W. Go¨pel, J. Hesse (Eds.), Sensors Update, Wiley, New York, 1996, p. 47. L.R. Skubal, N.K. Meshkov, M.C. Vogt, J. Photochem. Photobiol. A: Chem. 148 (2002) 103. L. Znaidi, R. Seraphimove, J.F. Bocquet, C. Colbeau-Justin, C. Pommier, Mater. Res. Bull. 36 (2001) 811. J.G. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, J. Am. Ceram. Soc. 87 (2004) 1358. J. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, J. Am. Ceram. Soc. 87 (2004) 1358. I.N. Kuznetsova, V. Blaskov, L. Znaidi, A. Kanaev, Mater. Lett. 59 (2005) 3820. T. Shibata, H. Irie, M. Ohmori, A. Nakajima, T. Watanabe, K. Hashimoto, Phys. Chem. Chem. Phys. 6 (2004) 1359. B. Ohtani, J. Handa, S. Nishimoto, T. Kagiya, Chem. Phys. Lett. 120 (3) (1985) 292. Y. Hu, H. Tsai, C. Huand, J. Eur. Ceram. Soc. 23 (2003) 291. Y. Djaoued, S. Badilescu, P. Ashrit, D. Bersani, P. Loticci, R. Bruning, J. Sol–Gel Sci. Technol. 24 (2002) 247. M. Koelsch, S. Cassaignon, J.F. Guillemoles, J.P. Jolivet, Thin Solid Films 403/404 (2002) 312. P. Lobl, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 93.
6622
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622
[19] H.K. Ha, M. Yoshimoto, H. Koinuma, B. Moon, H. Ishiwara, Appl. Phys. Lett. 68 (1996) 2965. [20] M.P. Moret, R. Zallen, D.P. Vijay, S.B. Desu, Thin Solid Films 366 (2000) 8. [21] L.D. Arsov, W. Kormann, W. Plieth, J. Electrochem. Soc. 138 (1996) 2964. [22] Y. Djaoued, R. Bruning, D. Bersani, P.P. Lottici, S. Badilescu, Mater. Lett. 58 (2004) 2618. [23] L.N. Kuznetsova, V. Blaskov, I. Stambolova, L. Znaidi, A. Kanaev, Mater. Lett. 59 (2005) 3820.
[24] H.Z. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481. [25] B. Huber, A. Brodyanski, M. Scheib, A. Orendorz, C. Ziegler, H. Gnase, Thin Solid Films 472 (2005) 114. [26] H. Kominami, M. Kohno, J. Kera, J. Mater. Chem. 10 (2000) 1151. [27] A. Pottier, C. Chaneac, E. Tronc, L. Mazerolles, J. Joliver, J. Mater. Chem. 11 (2001) 1116. [28] Y. Zheng, E. Shi, S. Cui, W. Li, X. Hu, J. Am. Ceram. Soc. 83 (2000) 1151. [29] H. Kominami, M. Konho, Y. Kera, J. Mater. Chem. 10 (2000) 1151.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of brookite/anatase complex thin film Chiaki Ohara, Teruhisa Hongo *, Atsushi Yamazaki, Toshio Nagoya Department of Resources and Environmental Engineering, School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 October 2007 Received in revised form 9 April 2008 Accepted 11 April 2008 Available online 27 May 2008
TiO2 films were prepared on a silicon or soda-glass substrate using a sol suspension. The TiO2 film on the silicon substrate was composed of pure anatase phase and showed almost no contaminations. In contrast, the TiO2 film on the soda-glass substrate was composed of anatase and brookite phases. The diffusion of Na into the TiO2 film on the soda-glass substrate was observed by XPS, and Na was concentrated on the surface of the film. The yield of the brookite phase increased with decreasing distance from the surface of the film on the soda-glass substrate. Na promoted the formation of the brookite phase, although the preparative procedure was used for anatase synthesis. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Brookite Anatase TiO2 Thin film
1. Introduction Titanium dioxide (TiO2) has been investigated extensively for many years because of its various applications as a low-cost material in photocatalysis [1–3], in photovoltaics [4,5], in electronchromic [6] or as gas sensor [7,8]. The use of TiO2 as a catalyst during photochemical oxidation aimed at decomposing organic compounds is well known [9]. TiO2 exists in three different crystalline phases: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). In nature, rutile is the most common, while brookite is scarce. Among these phases, anatase and rutile can be obtained by several synthetic methods and their various properties have been widely studied. Recent reports on the preparation of pure brookite and the study of its characteristics are limited [10–12]. Some reports indicated that brookitetype TiO2 shows high photoinduced hydrophilicity and high photocatalytic activity compared with rutile and anatase [13,14]; thus, brookite is a good candidate material for solar-energy conversion devices. Brookite is the metastable phase of TiO2; thus, it is difficult to prepare pure brookite under laboratory conditions. Many researchers reported that it is typically obtained as an accessory second phase when rutile or anatase is synthesized [15–17]. It has been shown that the titania phase, which is the most difficult phase to prepare in thin-film form, is brookite [18,19]. Brookite/anatase complex films have been obtained by various techniques, such as
* Corresponding author. E-mail address: [email protected] (T. Hongo). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.030
pulsed laser deposition [20] and the electrochemical oxidation of titanium electrodes [21]. Djaoued et al. [22] have prepared brookite-rich films by a sol–gel method using diethanolamine and polyethylene glycol as modulators. Recently, Kuznetsova et al. [23] have prepared a spin-coated film of pure brookite with a preferred orientation. We prepared TiO2 films on silicon and soda-glass substrates using a sol suspension. This suspension was coated onto the substrates by dip coating. The TiO2 film on the silicon substrate was found to be composed of pure anatase phase by grazing incident asymmetric X-ray diffraction (GIAXRD) analysis. However, the TiO2 film on the soda-glass substrate was composed of anatase and brookite phases. In this work, we characterized these TiO2 films by GIAXRD analysis and X-ray photoelectron spectroscopy (XPS). These measurements revealed the difference between the film on the silicon substrate and that on the soda-glass substrate. We also discussed the mechanism of brookite phase generation on the soda-glass substrate. 2. Experimental 2.1. Preparation of TiO2 films TiO2 thin films were prepared by dip coating using a sol suspension on soda-glass and silicon substrates. Titanium tetraisopropoxide (TTIP) (90.6 ml) was used as the starting material. The required amount of ethanol (93.8 ml) was divided into two parts: the first part was added to the stirred TTIP. The second part was mixed with 5.2 ml of hydrochloric acid solution (38 wt.%) and then added dropwise (1.5 cm/min) into the solution with stirring
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622
6620
Fig. 1. X-ray penetration depth for TiO2 as function of incidence angle.
to promote hydrolysis. This homogenous solution was continuously stirred for 24 h at room temperature. The soda-glass and silicon substrates were first cleaned carefully in an ultrasonic bath. Using this prepared solution, the TiO2 thin films coated onto the substrates (soda glass or quartz) by dip coating. The films were then dried in a desiccator for 10 min and subsequently calcined at 500 8C in air for 2 h. After cooling to room temperature, the dip coating and calcination process were repeated. The TiO2 film thickness was about 150 nm. 2.2. Characterization The crystallographic properties of the TiO2 films were determined by GIAXRD analysis using a monochromatic Cu Ka radiation. Incidence angle was used to determine the change in the crystal phase along the film depth. The variation in the penetration depth of TiO2 as a function of incidence angle is shown in Fig. 1. Diffraction patterns were obtained in an 2u angular range from 24.08 to 32.08. The surface chemical compositions of the TiO2 films were determined by XPS. Spectra were collected using a monochromatic Mg Ka source with an energy of 1253.6 eV operated at 10 kV. For the TiO2 film on the soda-glass substrate, elemental depth profiling was carried out using Ar+ sputtering beams under the applied conditions (3 keV, 8 mA).
Fig. 2. GIAXRD patterns of TiO2 films on (a) silicon and (b) soda-glass substrates.
anatase phase (2u (Cu Ka1) = 25.2818 from JCPDS 21-1272), and the diffraction shoulder at about 25.88 is the 111 line of the brookite phase. Fig. 3 shows the intensity of the 121 diffraction line of the brookite phase relative to that of multiple lines at around 25.0–
3. Results and discussion The GIAXRD patterns of the TiO2 films are shown in Fig. 2. The GIAXRD patterns of the TiO2 film on the silicon substrate (Fig. 2(a)) were obtained at grazing incidence angles from 0.28 to 0.58. The diffraction line at about 25.58 was the 101 line of the anatase phase, and the sample was confirmed to be single-phase anatase from the upper layer to the lower layer. The diffraction patterns of the TiO2 film on the soda-glass substrate (Fig. 2(b)) were obtained at grazing angles of incidence from 0.28 to 0.68. The diffraction line at about 30.98, which was assigned to the 121 line of the brookite phase, was observed in addition to those from the anatase phase. The intensity of this line relative to the strongest 120 line of the brookite phase at 25.48 was about 90%. However, this relative intensity in Fig. 2(b) was clearly low. This result shows that the anatase and brookite phases coexist in the TiO2 film on the sodaglass substrate. Therefore, the strongest diffraction line at about 25.48 overlaps the 120 line of the brookite phase (2u (Cu Ka1) = 25.3398 from JCPDS 29-1360) and the 101 line of the
Fig. 3. Intensity of diffraction line 121 of brookite phase relative to multiple lines at around 25.0–26.08 in diffraction patterns of TiO2 film on soda-glass substrate as function of incidence angle.
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622 Table 1 Surface elemental compositions of TiO2 thin films Substrate
Ti (at.%)
O (at.%)
Si (at.%)
Na (at.%)
Silicon Soda glass
29.33 14.41
69.41 57.37
0.00 1.14
1.26 27.08
26.08 in the diffraction patterns of the TiO2 film on the soda-glass substrate as a function of incidence angle. This relative intensity decreased with increasing incidence angle. This result shows that the TiO2 film on the soda-glass substrate is not homogenous, and that the yield of the brookite phase increases with decreasing distance from the surface. The phase contents of the anatase and brookite in the surface of the film on soda-glass substrate was determined from the GIAXRD pattern obtained at grazing incidence angle of 0.28 using the method of Zhang and Banfield [24]. The proportions of the anatase and brookite phases were calculated to be 36.6 and 63.4%. The XPS spectra of the TiO2 films on soda-glass and silicon substrates showed distinct signals from Ti, O, Si and Na. The surface elemental compositions of TiO2 films are given in Table 1. In the case of the silicon substrate, a small amount of Na was observed in addition to Ti and O. This Na was considered to be produced by the dissolution of the material of the Pyrex beaker used in preparing the sol suspension. On the other hand, a large amount of Na was observed on the surface of the TiO2 film on the soda-glass substrate. Fig. 4 shows an XPS depth profile of the atomic concentrations of Ti, O, Si and Na in the TiO2 film on the soda-glass substrate. With increasing sputtering time, the amount of Na decreased significantly up to 100 s. Most of this Na was considered to be produced by the dissolution of the soda-glass substrate. A similar sodium diffusion process from the glass substrate was reported by Huber et al. [25]. They reported that the presence of contaminants in TiO2 films is substrate-dependent: in the films deposited on silicon or sapphire, the atomic fraction of impurities (most prominently Na and K) was in the range of 10 4, and, in those deposited on the glass substrate, an approximately 100-fold higher level of such species was observed, probably the result of outdiffusion during the calcination or reduction of the samples. After a sputtering time of about 800 s, a decrease in Ti concentration and an increase in Si concentration were observed, and this point was considered to be the boundary between the TiO2 film and the soda-glass substrate.
6621
From the results of XRD analysis and XPS, it is clear that the yield of the brookite phase increases with decreasing distance from the surface of the TiO2 film on the soda-glass substrate in which Na is concentrated. Na dissolved from the soda-glass substrate was considered to promote the formation of the brookite phase in calcination. This experimental result is consistent with the results indicating that brookite-phase TiO2 has been produced using highly concentrated sodium or chlorine as the reacting solution [26–29]. The relative intensity in Fig. 3 decreases slowly with incidence angle, while the amount of Na decreased rapidly with increasing sputtering time. This phenomenon is attributed to two reasons. First, Na contributes the crystal nucleation of brookite phase, but there is no correlation between Na concentration and the brookite phase content. The second is as follows. When the GIAXRD measurement is performed at grazing incidence angle of 0.68, the penetration depth is about 150 nm. The diffraction pattern is not represent the average crystal phases of the area, but is most influenced by the surface crystal phases, because X-ray decays with the distance from the surface. Although the experimental procedure is for the synthesis of anatase phase except for the substrate in this study, brookite phase is produced without using a reacting solution containing highly concentrated sodium or chlorine. This experimental result indicates that the generated TiO2 phase is sensitive to contaminations from experimental instruments as well as synthetic reagents. 4. Conclusion In this work, the diffusion of Na into a TiO2 film on a soda-glass substrate was observed by XPS, and Na was concentrated on the surface of the film. A TiO2 film on a soda-glass substrate is composed of anatase and brookite phases. The yield of the brookite phase increases with decreasing distance from the surface of the film, because Na concentrated on the surface promotes brookite phase formation. In contrast, a TiO2 film on a silicon substrate shows almost no Na and is composed of pure anatase phase. Although the preparative procedure used is for anatase synthesis, the brookite phase is obtained owing to Na contamination. Acknowledgement The authors are grateful to Shinpei Enomoto for help with XPS and valuable discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 4. XPS depth profiles for atomic concentrations of Ti, O, Si and Na in TiO2 film on soda-glass substrate.
[18]
M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735. M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 503. A. Hagfeldt, M. Gra¨tzel, Chem. Rev. 95 (1995) 49. M. Gra¨tzel, Nature 414 (2001) 338. A. Ponton, S. Barboux-Doeuff, C. Sanchez, Colloids Surf. A 162 (1999) 177. W. Go¨pel, G. Reinhardt, in: H. Baltes, W. Go¨pel, J. Hesse (Eds.), Sensors Update, Wiley, New York, 1996, p. 47. L.R. Skubal, N.K. Meshkov, M.C. Vogt, J. Photochem. Photobiol. A: Chem. 148 (2002) 103. L. Znaidi, R. Seraphimove, J.F. Bocquet, C. Colbeau-Justin, C. Pommier, Mater. Res. Bull. 36 (2001) 811. J.G. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, J. Am. Ceram. Soc. 87 (2004) 1358. J. Li, C. Tang, D. Li, H. Haneda, T. Ishigaki, J. Am. Ceram. Soc. 87 (2004) 1358. I.N. Kuznetsova, V. Blaskov, L. Znaidi, A. Kanaev, Mater. Lett. 59 (2005) 3820. T. Shibata, H. Irie, M. Ohmori, A. Nakajima, T. Watanabe, K. Hashimoto, Phys. Chem. Chem. Phys. 6 (2004) 1359. B. Ohtani, J. Handa, S. Nishimoto, T. Kagiya, Chem. Phys. Lett. 120 (3) (1985) 292. Y. Hu, H. Tsai, C. Huand, J. Eur. Ceram. Soc. 23 (2003) 291. Y. Djaoued, S. Badilescu, P. Ashrit, D. Bersani, P. Loticci, R. Bruning, J. Sol–Gel Sci. Technol. 24 (2002) 247. M. Koelsch, S. Cassaignon, J.F. Guillemoles, J.P. Jolivet, Thin Solid Films 403/404 (2002) 312. P. Lobl, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 93.
6622
C. Ohara et al. / Applied Surface Science 254 (2008) 6619–6622
[19] H.K. Ha, M. Yoshimoto, H. Koinuma, B. Moon, H. Ishiwara, Appl. Phys. Lett. 68 (1996) 2965. [20] M.P. Moret, R. Zallen, D.P. Vijay, S.B. Desu, Thin Solid Films 366 (2000) 8. [21] L.D. Arsov, W. Kormann, W. Plieth, J. Electrochem. Soc. 138 (1996) 2964. [22] Y. Djaoued, R. Bruning, D. Bersani, P.P. Lottici, S. Badilescu, Mater. Lett. 58 (2004) 2618. [23] L.N. Kuznetsova, V. Blaskov, I. Stambolova, L. Znaidi, A. Kanaev, Mater. Lett. 59 (2005) 3820.
[24] H.Z. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481. [25] B. Huber, A. Brodyanski, M. Scheib, A. Orendorz, C. Ziegler, H. Gnase, Thin Solid Films 472 (2005) 114. [26] H. Kominami, M. Kohno, J. Kera, J. Mater. Chem. 10 (2000) 1151. [27] A. Pottier, C. Chaneac, E. Tronc, L. Mazerolles, J. Joliver, J. Mater. Chem. 11 (2001) 1116. [28] Y. Zheng, E. Shi, S. Cui, W. Li, X. Hu, J. Am. Ceram. Soc. 83 (2000) 1151. [29] H. Kominami, M. Konho, Y. Kera, J. Mater. Chem. 10 (2000) 1151.