5422
Langmuir 1999, 15, 5422-5425
Notes Photoreactivity of Sol-Gel TiO2 Films Formed on Soda-Lime Glass Substrates: Effect of SiO2 Underlayer Containing Fluorine Akihiko Hattori,† Koji Shimoda,† Hiroaki Tada,*,‡ and Seishiro Ito§ Nippon Sheet Glass Techno-Research Co., Ltd., 1, Kaidoshita, Konoike, Itami, Hyogo 664-8520, Japan, Environmental Science Research Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka, 577-8502, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka, 577-8502, Japan Received September 2, 1998. In Final Form: April 30, 1999
Introduction A great deal of attention has been focused in recent years on the development of heterogeneous photocatalysts for application to environmental problems.1 Among various semiconductor photocatalysts, TiO2 is believed to be the most promising photocatalyst owing to its strong oxidizing power, nontoxicity, and long-term stability. If a TiO2 film with a high photocatalytic activity can be coated on soda-lime (SL) glass, it would be quite useful for selfcleaning or indoor air cleaning windows.2 However, it is well-known that the diffusion of Na+ ions into the TiO2 film from the SL-glass substrate during its production significantly deteriorates the photoreactivity.3 On the other hand, to improve the photoreactivity of TiO2 and/or to extend its light absorption into the visible region, doping of different transition metal cations has been attempted. As opposed to the expectation, the activity is reduced except for a few cases,4,5 because the dopant usually acts as a recombination center of photogenerated charge carriers. In contrast, there have been few studies on anion doping.6 We have recently reported that the photoreactivity of a sol-gel (SG)-TiO2 film coated on quartz is enhanced with addition of a small amount of NH4F into the starting SG solution.7 This is the first report on the preparation of a highly photoreactive SG-TiO2 film on the SL-glass substrate with a SiO2 film containing fluorine (F:SiO2). Particular emphasis is placed on the action mode of the F:SiO2 underlayer. * To whom correspondence should be addressed. Tel: +81-6-67212332. Fax: +81-6-6721-3384. E-mail:
[email protected]. † Nippon Sheet Glass Techno-Research Co., Ltd. ‡ Environmental Science Research Institute, Kinki University. § Department of Applied Chemistry, Kinki University. (1) In Photocatalytic Purification and Treatment of Water and Air; Ollis, F. D., Al-Ekabi, H., Eds.; Elsevier Science: Amsterdam, 1993. (2) Paz, Y.; Lu, Z.; Heller, A. J. Mater. Res. 1995, 10, 2842. (3) Tada, H.; Tanaka, M. Langmuir 1997, 13, 360. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (5) Anpo, M. Catal. Catal. 1998, 40, 140. (6) Wang, C. M.; Mir, Q.-C.; Maleknia, S.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 3710. (7) Hattori, A.; Yamamoto, M.; Tada, H.; Ito, S. Chem. Lett. 1998, 707.
Experimental Section [Ti(OiPr)4] (85.6 g, 0.30 mol) was stabilized by adding 60.0 g of acetylacetone (acacH, 0.60 mol). The stabilization results from chelate formation ([Ti(OiPr)3(acac)]),8 which is manifested by a color change of the solution from pale yellow to orange. The solution was diluted with ethanol (100/30 v/v) and used for the coating. After the films had been coated on substrates by a dipping method (pulling up speed ) 26 mm min-1), they were calcined at 773 K for 30 min. Quartz plates (Sumikin Quartz Products Inc.), SL glass, F:SiO2-film-coated SL glass (Nippon Sheet Glass Co.), and SG-SiO2-film-coated SL glass were used as the substrates. The SG-SiO2 film was prepared according to the method by Sakka et al.9 The SG-TiO2 films formed on F:SiO2coated SL glass, quartz, SG-SiO2-coated SL glass, and SL-glass substrates were respectively designated as samples A-D (see Table 1). X-ray diffraction (XRD) measurements of the samples were carried out using a Rigaku Rotaflex RTP 300 RC. The crystallinity was evaluated by the relative intensity of the diffraction from the anatase (101) plane at 2θ ) 25.3° (I101) and the average size of crystallites (d101/nm) that was calculated from Scherrer’s equation of d101 ) Kλ/b cos θ (λ ) 0.154 nm, θ ) 12.65°, K ) 0.9, and b is the half-width of the diffraction peak). The apparent rate constant of the photoinduced oxidation of 1,3,5,7tetramethylcyclotetrasiloxane (TMCTS) chemisorbed on the surface of the SG-TiO2 film (k1) was determined by analyzing the variation of H2O contact angle (θ) with photoirradiation (λ > 300 nm).10 The light intensity integrated from 320 to 400 nm (I320-400) was 2.1 mW cm-2. A monotonic decrease in θ, i.e., increase in hydrophilicity, with illumination was previously ascribed to the TiO2-photoinduced oxidation of the outermost Si-CH3 groups. The value of k1 was obtained from the slope in the plot of ln(f0/ft) vs time; f0 and ft denote the coverages of the Si-CH3 groups at t ) 0 and t ) t, respectively. The f0 and ft values were calculated from the equation of ft ) (1 - cos θt)/(1 - cos θ0), where θ0 and θt are the H2O contact angles at t ) 0 and t ) t, respectively. The activity of the SG-TiO2 film in the photoinduced oxidation of CH3CHO was also evaluated. A 500ppm standard CH3CHO gas (CH3CHO/N2) was introduced into a reaction chamber (3.3 L) and diluted with air so that its initial concentration was controlled in the 1200 ( 100 ppm range. After the adsorption equilibrium of CH3CHO had been achieved under the dark, front-face irradiation (λ > 300 nm, I320-400 ) 19.7 mW cm-2) of the sample (50 mm × 50 mm) was carried out at room temperature. The concentration of CH3CHO was determined by gas chromatography (Shimadzu GC-14B). Further, TiO2 photoinduced decomposition of triolein (TOL, C17H33COOH(CH2OCOC17H33)2, >80%, Tokyo Kasei) was examined. After a TOL liquid layer (2.5 mg) had been applied uniformly on the surface of the TiO2 film (size ) 70 mm × 70 mm), UV light was irradiated from the direction of the film using a 20 W black light (I320-400 ) 3 mW cm-2). The variation in the weight of the residual TOL after t h of illumination (w(t)) was followed by an electronic balance. The value of [(w(0) - w(168 h))/w(0)] × 100% was defined as ∆w and used as an indicator of the activity of the TiO2 film. The depth profiles of the compositions of the SG-TiO2 films were measured by Rutherford backscattering (RBS) using a model RBS AN-2500 (Nissin-High Voltage Co.). A beam of accelerated 4He+ ions (accelerated energy ) 2.3 MeV, beam current ) 10 nA) was irradiated on the sample area of 1 mm φ with an incident angle ) 0° and the scattered ions were detected with a scattering angle of 110° using a passivated implanted planar silicon detector (8) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, F. J. Non-Cryst. Solids 1988, 100, 65. (9) Yamamoto, Y.; Kamiya, K.; Sakka, S. Yogyo Kyokai Shi 1982, 90, 328. (10) Tada, H. Langmuir 1996, 12, 966.
10.1021/la981148n CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999
Notes
Langmuir, Vol. 15, No. 16, 1999 5423 Table 1. Photoreactivity and Properties of the TiO2 Films Formed on Various Substrates film thicknes
sample
substrate
k1/min-1 a
A B C D
F:SiO2/SL glass quartz SGSiO2/SL glass SL glass
2.8 × 10-2 1.7 × 10-2 4.8 × 10-3 3.3 × 10-3
k2/min-1 b
∆wc
cryst
formd
d101e
I101
TiO2/nm
SiO2/nm
4.2 × 10-3 2.2 × 10-3
89.3 36.4 ∼0
14.6 13.7 12.3
1.24 1 0.53 0
75 ( 5 75 ( 5 75 ( 5 75 ( 5
45 ( 5
5.4 × 10-4
anatase anatase anatase amorphous
100 ( 10
a The light intensity integrated from 320 to 400 nm (I -2 (front-face illumination). b The I 320-400) was 2.1 mW cm 320-400 value was 19.7 mW cm-2 (front-face illumination). c The I320-400 value was 3 mW cm-2 (front-face illumination). d The size of the crystallites was determined from XRD data using Scherrer’s equation. e The relative intensity of the diffraction peak from 101 plane (reference ) sample B).
Figure 2. Plots of ln(C0/C) vs time, where C0 and C are the concentrations of CH3CHO at t ) 0 and t ) t, respectively (A, sample A; B, sample B; D, sample D).
Figure 1 shows scanning electron micrographs (SEMs) of the cross-sections of samples A and D. Samples A and D have respectively a double layer structure (TiO2(75 ( 5 nm)/F:SiO2(45 ( 5 nm)/SL glass) and a single layer structure (TiO2(75 ( 5 nm)/SL glass). XRD measurements clarified that the crystallinity of the TiO2 film is very
sensitive to the kind of the substrates. The TiO2 film of sample A consists of many anatase crystallites with an average diameter of 14.6 nm, while sample D is amorphous. Although grain boundaries are unclear in sample D, well-grown grains are observed in sample A. In a previous paper, it was demonstrated that the photoinduced oxidation of the TMCTS monolayer chemisorbed on the TiO2 film follows first-order kinetics with an apparent rate constant of k1 (see Experimental Section). The k1 was in good agreement with the rate constant determined spectroscopically from the decrease in the absorbance of the Si-CH3 groups in the TMCTS monolayer with irradiation.10 In this system, the decrease in θ with irradiation is governed not by the photoinduced hydrophilicity of TiO2 itself12 but by the oxidation of the SiCH3 groups. Figure 2 shows plots of ln(C0/C) vs time, where C0 and C are the concentrations of CH3CHO at t ) 0 and t ) t, respectively (A, sample A; B, sample B; D, sample D). Upon illumination in the absence of the photocatalyst, no decomposition of CH3CHO took place (data not shown). Clearly, the TiO2 photoinduced oxidation of CH3CHO also obeys good first-order kinetics. The apparent rate constant of k2 was obtained from the slope of the straight line, which is strongly dependent on the kind of substrate. The surface area of the SG-TiO2 film (,1 m2 g-1) is much smaller than those of powder samples usually used as photocatalysts (>10 m2 g-1). Then the influence of the photoadsorption of CH3CHO on the TiO2 film will be neglected, even if it occurs to some degree. The TiO2 photoinduced decomposition of the TOL liquid layer does not involve the adsorption process as in the case of the TMCTS monolayer oxidation. Both TiO2 and illumination were required also for the decomposition of TOL. Table 1 summarizes the photoreactivity of the TiO2 films coated on the various substrates together with some
(11) Kimmel, S.; Peter, A.; Fletcher, W. W. Glastech. Ber. 1986, 59, 252.
(12) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Langmuir 1998, 14, 5918.
Figure 1. Scanning electron micrographs of cross sections of samples A and D. (Canberra Industries, Inc., PD300-16-100). X-ray photoelectron spectroscopic (XPS) measurements were performed using a PHI 5600Ci spectrometer with an monochromated Al KR X-ray source operated at 14 kV and 150 W. The take off angle was 45°, and typical operating pressures were 1 × 10-8 Pa. Multiplex (narrowscan) spectra were obtained for C1s, O1s, Ti2p3/2, Ti2p1/2, and F1s photopeaks. The line shapes used for curve fitting were Gaussian, and the integrated background were employed. All the binding energies were referenced with respect to the carbon peak of C-H at 285.0 eV. The alkali-barrier property of the F:SiO2 film was examined by a leaching method.11 After the F:SiO2/SL-glass and SL-glass samples had respectively been contacted with water at 96 °C, a spectrochemical buffer solution was added to the extract. The contents of Na+ and F- ions leached out into the water were analyzed by flame spectrometry and ion chromatography, respectively.
Results and Discussion
5424 Langmuir, Vol. 15, No. 16, 1999
Notes
Figure 4. Amounts of Na+ ions leached out directly (a) and through the F:SiO2 film (b) from SL glass into water at 96 °C. (c) shows the amount of F- ions leached out into water from the F:SiO2 film under the same conditions.
Figure 3. RBS depth profiles of the compositions of the SGTiO2 films formed on various substrates: A, SG-TiO2/F:SiO2/ SL glass; C, SG-TiO2/SG-SiO2/SL glass; D, SG-TiO2/SL glass; R, F:SiO2/SL glass.
physical properties. Both k1 and k2 of sample D are extremely small. Figure 3 shows the RBS depth profile of the film compositions in each sample. In sample D, as much as 4.2 atom % of Na+ ions diffuses into the TiO2 film from the SL-glass substrate. The inhibition of the TiO2 crystallization due to the Na+ ions (I101 ∼ 0, Table 1) would reduce the diffusion length of the photogenerated charge carriers (Lp).3 It has been established that heterogeneous photocatalytic reactions generally obey the LangmuirHinshelwood mechanism.13 Under the surface reaction controlled conditions, the rate is proportional to the steadystate surface concentration of the charge carriers that decreases with decreasing Lp. Consequently, the low photoreactivity of sample D can be attributed to Na+ ions diffusing into the TiO2 film from the SL-glass substrate. The photoreactivity in each oxidation (k1 and k2) increases in the same order of sample D < C < B. This order is in agreement with those of the crystallinity (I101(D) ∼ (13) Matthews, R. W. In Photochemical Conversion and Storage of Solar Energy; Pelizzetti, E., Schiavello, M., Eds.; Kluwer Academic: Dordrecht, The Netherlands, 1988.
0 < I101(C) ) 0.53 < I101(B) ) 1) and the average size of the crystallites (d101(C) ) 12.3 nm < d101(B) ) 13.7 nm) of the TiO2 films. The higher photoreactivity of sample C than of sample D results from the suppression of the diffusion of Na+ ions from the SL-glass substrate by the SG-SiO2 underlayer (see the RBS depth profile of sample C in Figure 3). Further the difference in the photoreactivity of samples B and C suggests incompleteness of the Na+ion barrier property of the SG-SiO2 underlayer. The effect of various oxide underlayers was previously studied, however, the photoreactivity of the SG-TiO2 film coated on the SL glass having underlayers has never exceeded that on the quartz substrate.2,14 Noticeably, sample A has a higher photoreactivity than that of sample B; the k1 and k2 of sample A are 1.7 and 1.9 times as great as the corresponding values of sample B, respectively. Also, the ∆w for sample A is larger than of sample B by a factor of 2.5. These cannot be explained only in terms of the excellent Na+-ion barrier property of the F:SiO2 underlayer.15 The RBS depth profile of the SL glass with a F:SiO2 film (sample R) indicates the presence of 6.6 atom % fluorine in the film. The F:SiO2 film was uniquely deposited on the SL-glass substrate at room temperature from a H2SiF6 solution saturated with silica gel by adding H3BO3 as an initiator.15 The small amount of fluorine is spontaneously incorporated into the film in the course of the deposition process. In the RBS depth profile of sample A, fluorine diffuses into the TiO2 film from the F:SiO2 underlayer at the level of 3.0 atom %, while the diffusion of Na+ ions into the TiO2 film from the SL-glass substrate is almost completely blocked by it. Infrared spectroscopy detected the signals due to the νSi-F (∼930 cm-1),16 νSiO-H (∼3650 cm-1) and νSi-OH (∼980 cm-1)16 vibrations, whose intensities decrease upon heating at 773 K. Then the SG-TiO2 film is thought to be furnished with fluorine as HF during the heat treatment (eq 1). The 773 K
≡Si-F + ≡Si-OH 98 ≡Si-O-Si≡ + HF (1) excellent Na+-ion-blocking property of the F:SiO2 underlayer is more clearly demonstrated in Figure 4. This figure shows the amounts of Na+ ions leached out into hot water directly (a) and through the F:SiO2 film (b) from SL glass. (14) Negishi, N.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1995, 841. (15) Nagayama, H.; Honda, H.; Kawahara, H. J. Electrochem. Soc. 1988, 135, 2013. (16) Krol, D. M.; Rabinovich, E. M. J. Non-Cryst. Solids 1986, 82, 143.
Notes
Langmuir, Vol. 15, No. 16, 1999 5425
TiO2-xFx.19 In the present SG system, a similar event may take place (eq 3). From the XPS surface analyses of
Table 2. Results of XPS Analyses of the TiO2 Films sample binding energy/eV A B
atomic ratio
Ti2p3/2
O1s
F1s
O/Ti
(O + F)/Ti
458.3 458.5
529.4 529.7
655.5
1.87 1.76
1.89
The amount of Na+ ions leached out is remarkably decreased by the coating of the F:SiO2 film. Also, it is evident that the film operates as a diffusion source of Fions for the distinct phase in contact with it upon heating (c). It has recently been reported that a small amount addition of NH4F or CF3COOH into the starting SG solution significantly increases the photoreactivity of the SG-TiO2 film.7,17 The effect was mainly ascribable to the improvement of the anatase crystallinity caused by Fions. This is also true for sample A; both the average size of crystallites (d101 ) 14.6 nm) and the relative crystallinity (I101 ) 1.24) increase as compared to those of sample B (d101 ) 13.7 nm, I101 ) 1) as shown in Table 1. Arai et al. observed a similar phenomenon in the formation of a SnO2 film, whose crystallinity was improved with adsorption of NH4F on the substrate prior to the chemical vapor deposition.18 When the gel-TiO2 film is annealed, carbonaceous residue involved in it will act as a reductant to generate oxygen vacancies, which are partly responsible for the recombination of the photogenerated charge carriers (eq 2). Table 2 summarizes the results of XPS surface 773 K
gel-TiO2 98 TiO2-x (x ∼ 0.24 for sample B at the surface)
(2)
analyses. The O/Ti atomic ratios below 2 show the existence of the oxygen vacancy at the surface. Also, the x for sample A (x ∼ 0.11) is significantly smaller than that of sample B (x ∼ 0.24). It is evident that HF promotes the oxidation of the gel-TiO2 film to reduce the surface oxygen vacancy; however, the detail mechanism is unclear. An additional possibility is that F- ions decrease the rate of the recombination as the result of the filling of oxygen vacancies.7,17 Subbarao et al. reported an increase in the photocurrent of a TiO2 single-crystal electrode with its gas-phase HF treatment followed by the formation of (17) Hattori, A.; Shimoda, K.; Tada, H. Proceedings of XVIII International Congress on Glass; 1998; AB117. (18) Arai, M.; Hamada, S.; Nishiyama, Y. J. Colloid Interface Sci. 1996, 178, 315.
773 K
gel-TiO2 + HF 98 TiO2-x-yFy (x ∼ 0.11, y ∼ 0.02 for sample A at the surface) (3) sample A (TiO2-x-yFy), x and y were estimated to be 0.11 and 0.02, respectively (see Table 2). The fact that the ionic radius of F- ion (1.33 nm) is close to that of O2- ion (1.40 nm) explains the F--ion filling of oxygen vacancies without decreasing the anatase crystallinity. On the basis of the results above, three effects of the F:SiO2 underlayer can be listed. The first is to inhibit the diffusion of Na+ ions into the TiO2 film from the SL-glass substrate (effect I). The second is to stimulate the anatase crystallization by supplying F- ions (effect II). The third is to decrease the oxygen vacancy via the action of HF for the gel-TiO2 film upon heating (effect III). All these effects are considered as increasing Lp to lead to an increase in the charge separation efficiency that is prerequisite for all the photochatalytic reactions to occur efficiently. Consequently, the SG-TiO2/F:SiO2/SL-glass sample would show high photoreactivities for various reactions irrespective of the detail mechanisms. Conclusions A highly photoreactive SG-TiO2 film was successfully formed on the SL-glass substrate by employing a F:SiO2 underlayer (SG-TiO2/F:SiO2/SL glass). The RBS depth profile of the compositions indicated that 3 atom % of fluorine diffuses into the TiO2 film from the F:SiO2 underlayer, while the diffusion of Na+ ions from the SLglass substrate is completely blocked by it. Also, the XRD analyses confirmed that the underlayer improves the anatase crystallinity of the SG-TiO2 film. The photoreactivity superior to that of the TiO2 film coated on the quartz substrate was chiefly attributable to the enhancement of the anatase crystallinity caused by F- ions. Although the detail action mode of F- ions deserves further scrutiny, this study will provide fundamental information on the anion doping effect on the photoreactivity of the TiO2 film and its application to the self-cleaning and the indoor air cleaning windows. Acknowledgment. The authors express sincere gratitude to Dr. M. Iwasaki of Kinki University for valuable comments. LA981148N (19) Subbarao, S. N.; Yun, Y. H.; Kershaw, R.; Dwighta, K.; Wold, A. Inorg. Chem. 1979, 18, 488.
