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SnO2 composite thin films
Materials Research Bulletin 37 (2002) 2255±2262
Preparation and super-hydrophilic properties of TiO2/SnO2 composite thin ®lms Qingju Liu*, Xinghui Wu, Baoling Wang, Qiang Liu Department of Material Science and Engineering, Yunnan University, Kunming, Yunnan 650091, PR China (Refereed) Received 5 April 2002; accepted 9 September 2002
Abstract SnO2±TiO2 composite thin ®lms were fabricated on soda-lime glass with sol±gel technology. By measuring the contact angle of the ®lm surface and the degradation of methyl orange, we studied the in¯uence of SnO2 doping concentration, heat-treatment temperature and ®lm thickness on the super-hydrophilicity and photocatalytic activity of the composite ®lms. The results indicate that the doping of SnO2 into TiO2 can improve their hydrophilicity and photocatalytic activity, and the composite ®lm with 1±5 mol% SnO2 and heat-treated at 4508C is of super-hydrophilicity. The optimal SnO2 concentration for the photocatalytic activity is 10 mol% and larger ®lm thickness is helpful to reduce the contact angle of the composite ®lms. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Composites; A. Thin ®lms; D. Catalytic properties; D. Surface properties
1. Introduction In recent years, nano-TiO2 thin ®lms with super-hydrophilic and photocatalytic characteristics have attracted a great deal of attention. They have many advanced functions and features, including anti-fouling, deodorizing, sterilizing and antifogging. When exposed to UV light, TiO2 ®lms can break down organic compounds and enable water to spread evenly on their super-hydrophilic surface to easily realize *
Corresponding author. Tel.: 86-871-5036510; fax: 86-871-5153832. E-mail address: [email protected] (Q. Liu). 0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 7 2 - 8
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surface self-cleaning. There have been numerous research papers reporting improvement in the photocatalytic activity and super-hydrophilicity of TiO2 thin ®lms. Fujishima et al. [1] reviewed the current progress in the area of TiO2 photocatalysis and observed a photo-induced super-hydrophilic phenomenon in TiO2 and discussed its potential applications. NavõÂo et al. [2] studied the photocatalytic ef®ciency of iron-doped titania containing Fe prepared from TiCl4 and Fe(III) acetylacetonate by sol±gel method. Watanabe et al. [3] evaluated photo-induced hydrophilic conversion on different crystal faces of single crystal rutile and polycrystalline anatase titanium dioxide to clarify the dependence of the crystal structure on the photo-induced hydrophilic conversion. Yu and Zhao [4] reported that the hydrophilicity can be enhanced by processing the TiO2 surface with acid, the contact angle of porous TiO2 coating ®lm can be reduced by increasing the adsorbed content of hydroxyl, capillary force and roughness of the resultant coating ®lms [5]. However, the super-hydrophilicity of the composite TiO2/SnO2 ®lm has not been reported yet. In this work, we are going to report a novel composite TiO2/SnO2 thin ®lm, which was prepared on soda-lime glass using sol±gel technology. The TiO2/SnO2 powder was heat-treated at 4508C and characterized with an X-ray diffractometer (XRD). The composite ®lms were characterized with a spectroscopic ellipsometer and an UV±VIS spectrophotometer. The effects of SnO2 doping in the precursor solution, heat treatment temperature and ®lm thickness on the super-hydrophilicity and photocatalytic activity of the composite ®lms were evaluated by measuring the contact angle for water of TiO2/SnO2 ®lms and the photocatalytic decolorization of aqueous methyl orange. 2. Experiments 2.1. Preparation of composite TiO2/SnO2 thin ®lms The preparation of precursor solutions for TiO2/SnO2 composite ®lms is described as follows. First, TiO2 and SnO2 sols were prepared separately. For the preparation of the TiO2 sol, tetranbutyl titauate was dissolved in ethanol ®rst, stirred vigorously for 30 min at room temperature, followed by the dropping reaction with the addition of HNO3 to the solution until pH 3:5. Distillated water was ®nally added to the solution over stirring through a burette. The chemical composition of the resultant alkoxid solution was (C4H9O)4Ti:C2H5OH:H2 O 1:82:3 in mole ratio. The TiO2 sol was obtained by keeping stirring the alkoxid solution for hydrolysis reaction for 4 h. The SnO2 sol was prepared in a similar way. Stannic chloride was dissolved in ethanol ®rst, followed by stirring and re¯uxing vigorously for 2 h at 808C. Over stirring distillated water was added to the solution with a burette. The chemical composition of the resultant solution was SnCl4:C2H5OH:H2 O 1:66:3 in mole ratio. Keeping stirring for hydrolysis reaction for 24 h, we got the SnO2 sol. Then mixture sols of TiO2 and SnO2 were made with different mole ratios of SnO2 sol. The mole ratios of SnO2 to TiO2 were 0, 1, 3, 5, 10, 20, 30 mol%. Finally, the TiO2/SnO2 ®lms were
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formed on the substrates with the prepared sol solutions by dipping-withdrawing at room temperature. The withdrawal speed was 3 mm/s. The thickness of the TiO2 and the TiO2/SnO2 ®lms is dependent on and approximately linearly proportional to the number of the cycle from dipping to heat treatment. For the single layer ®lm, the thickness of the pure TiO2 is about 180 nm, and that of the mixed is about 160 nm. The substrates coated with TiO2/SnO2 composite gel ®lms and the corresponding composite xerogel were dried with infrared light and then heat-treated together at different temperature for 1 h in air using an electric oven. 2.2. Characterization Constituent and crystallinity of the TiO2/SnO2 composite ®lms were analyzed with XRD (type D/maxIII, Japan) using Cu Ka radiation. The accelerating voltage was 35 kV and the applied current was 25 mA. The refractivity and thickness of the ®lm were measured with a spectroscopic ellipsometer (type TP-77, China) at 632.8 nm. The measurement of the ®lms' light transmittance was performed with an UV±visible spectrometer (type UV±VIS8500, China) in the wavelength range of 200±1100 nm. The hydrophilic properties of various TiO2/SnO2 ®lms were evaluated by measuring the contact angle for water of TiO2/SnO2 thin ®lms with a contact angle device (type JC2000A, China). Prior to measuring, the ®lms had been illuminated with natural light for 30 min after being placed in dark for 24 h. The photocatalytic activities of various TiO2/SnO2 ®lms were examined by using methyl orange degradation decolorization described below. The pure TiO2 ®lm and the TiO2/SnO2 composite ®lms were placed in aqueous methyl orange that has an initial density of 46 mg/l in quartz cells. An UV lamp (30 W) was used as the light source. The averaged intensity of UV irradiance was 78 mW/ cm2 measured with an UV irradiance meter. Its wavelength range is 320±400 nm, and peak wavelength is 360 nm. The density of methyl orange was determined with the spectrometer. 3. Results and discussion 3.1. Constituent and crystallinity of TiO2/SnO2 composite powder The XRD spectra for powders and ®lms of pure TiO2 and composite TiO2/SnO2 with SnO2:TiO2 20 mol% and heat-treated at 4508C revealed the present of TiO2 and SnO2 in the composite TiO2/SnO2. It has been con®rmed that the crystal form of TiO2 in both the pure TiO2 and the composite TiO2/SnO2 are all of anatase. The diffraction patterns also show that the diffraction peak width of the composite TiO2/ SnO2 is broader than that of the pure TiO2, indicating that the average diameter of the former is smaller than that of the latter. This can be attributed to the existence of SnO2 that reduces the growth rate of TiO2 particles. The reason for above phenomenon is that the introduction of tin ions changes the surface charge of the TiO2 sol particles in
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Fig. 1. UV±VIS spectra of pure TiO2 and TiO2/SnO2 ®lms deposited on soda lime glass (three coating cycle times, SnO2:TiO2 0:05, heat-treated at 4508C).
the composite sol, and distances them from each other. In this way, the TiO2 particles are most likely formed in smaller sizes. Fig. 1 shows the UV±VIS spectra of the pure TiO2 and the composite TiO2/SnO2 ®lms (three layers and heat-treated at 4508C) deposited on soda lime glass in the wavelength range 300±800 nm. An absorption edge of the composite TiO2/SnO2 ®lm with SnO2:TiO2 0:05 was observed at a shorter wavelength than that of the pure TiO2 coating ®lm. The shift is agreeable with the above XRD result and further con®rms that the particle size of the pure TiO2 is larger than that of the TiO2/SnO2 ®lms. 3.2. Effect of SnO2 content on hydrophilicity The contact angle for water of TiO2/SnO2 thin ®lms (three layers, heat-treated at 4508C) is listed in Table 1. We should note here that all the samples on hydrophilicity discussed below have been put in dark for 24 h and irradiated under natural light for 30 min prior to measurement. It is found that the contact angle for water is 9.58 for the pure TiO2 ®lm, and 08 for the TiO2/SnO2 ®lms with SnO2 doping levels of 1±5 mol%. The ®lm with higher SnO2 doping, for example, SnO2:TiO2 10±30 mol%, the contact angle increases from 8.5 to 13.58. The results indicate that low level doping of SnO2 can improve the hydrophilicity of the TiO2 ®lms, most probably due Table 1 Contact angle of TiO2/SnO2 ®lms with different doping levels SnO2 content (mol%) Contact angle (8)
0 9.5
1 0
3 0
5 0
10 8.5
20 9
30 13.5
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Table 2 Effect of heat-treated temperature on contact angle Heat-treated temperature (8C) (SnO2:TiO2 1 mol%) (SnO2:TiO2 3 mol%) (SnO2:TiO2 5 mol%)
150
250
350
450
550
650
69 65 68.5
36.5 36.5 48
14.5 10.5 8
0 0 0
13.5 19 5.5
10.5 17 31
to the increase of hydroxyl in the composite ®lms [6]. It is usually expected that there is an optimal range in doping density for super-hydrophilicity. In our case it is SnO2:TiO2 1±5 mol%. 3.3. Effect of heat-treatment temperature on hydrophilicity The contact angle for water of the TiO2/SnO2 thin ®lms (three layers, heat-treated for 60 min) at different heat-treated temperature is listed in Table 2. It is found that the contact angle of the composite ®lms with SnO2:TiO2 1, 3 and 5 mol% decreases as the heat-treated temperature increase from 150 to 4508C and increase as temperature above 4508C. Therefore the optimal heat-treated temperature for super-hydrophilicity is around 4508C. The change in the contact angle with the heat-treatment temperature can be explained in terms of two factors: the crystalline phase and the density of the composite ®lms. First, the hydrophilicity is dependent on the ®lm's crystalline phase. The structure of the TiO2 ®lms heat-treated below 2508C is almost amorphism and there is no super-hydrophilic feature. From 250 to 4508C, the content of the anatase TiO2 increases with the heat-treated temperature, and TiO2 heat-treated at 4508C completely become anatase, result in super-hydrophilc ®lm. However, When the temperature further increases above 4508C, the anatase TiO2 starts to transform gradually to rutile TiO2, leading to the rise in the contact angle. This is because hydrophilicity is related to the density of surface hydroxyl in TiO2 ®lms. The surface hydroxyl can easily combine with water molecule to form hydrogen bond, resulting in good wettability. So the more surface hydroxyl, the better the hydrophilicity. On the same UV illumination, although changes in hydrophilicity have been observed on both the anatase and rutile TiO2 surface [1], the former with more surface hydroxyl is more active in achieving hydrophilic features than the latter with less surface hydroxyl. The hydrophilicity is also dependent on the density of the composite ®lm. Fig. 2 illustrates the effect of heat-treatment temperature on the refractive index of TiO2/ SnO2 ®lms. As can be seen, the contact angle for water increases with the refractive index. It is well known that the index is related to the mass density of the materials. In the present case, the ®lms become denser as the temperature increases due to the decline in porosity [7]. So the refractive index of the ®lms increases with the temperature. Therefore, we can conclude from Fig. 2 that the contact angle of the composite ®lm increases with the ®lm's density.
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Fig. 2. Effect of heat-treatment temperature on refractive index of SnO2±TiO2 ®lms.
3.4. Effect of thickness on hydrophilicity The change in contact angle for water of TiO2/SnO2 thin ®lms (SnO2:TiO2 5 mol% and heat-treated at 4508C) with the number of coating cycle is listed in Table 3. It can be seen that the contact angle decreases with the increasing in the coating cycle. It is 08 for the ®lms coated more than three layers, while it is 128 for the single layer ®lms. The thickness of the single layer ®lm is about 160 nm by calculating the measurement data using the spectroscopic ellipsometer. In this case, UV ray can easily penetrate through the single layer so that the availability of the ray energy is low. With the number of coating cycle increasing, the availability of the irradiation light energy is heightened and more hydroxyl is produced resulting in the contact angle decreases to 08. 3.5. Effect of SnO2 concentration on photocatalytic activity Fig. 3 shows the results of photocatalytic decolorization of methyl orange by TiO2/ SnO2 ®lms with different doping levels of SnO2 (four layers and heat-treated at 4508C). After illumination for 120 min with UV light, the decolorization rate of methyl orange increases by the ®lms with low levels of SnO2 and then decreases with high levels of SnO2. The maximum rate of decolorization is 58.33% by the ®lm with 10 mol% SnO2, while that by the pure TiO2 ®lm is 44.35%. The photocatalytic activities of all the TiO2/ SnO2 composite ®lms are higher than that of the pure TiO2 ®lm. This phenomenon is probably attributed to two reasons stated below. Firstly, it is due to the increase of surface hydroxyl of the composite ®lms [6]. Secondly, it is because of the decrease in Table 3 Effect of ®lm thickness on contact angle of TiO2/SnO2 ®lm Number of coating cycle Contact angle (8)
1 12
3 0
5 0
7 0
9 0
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Fig. 3. Decolorization of methyl orange degraded by TiO2/SnO2 ®lm doped with different levels of SnO2 (four layers and heat-treated at 4508C).
recombination rate of photo-induced carriers in the TiO2/SnO2 composite ®lms. The conduction band energy level for SnO2 and for TiO2 is different, for example, it is 0 eV for SnO2 (versus NHE, pH 7) and 0.5 eV for TiO2 (versus NHE, pH 7) [8]. Due to this potential difference, the photo-induced electrons transfer from the surface of TiO2 to that of SnO2 can take place easily when SnO2 is doped into TiO2. On the other hand, however, if there is too much doped SnO2, the relative density of TiO2 decreases which cause the activity to decrease. In our case it is around 10 mol% of SnO2 that makes the photocatalytic activity turn from increase to decrease. 3.6. UV±VIS spectra The UV±VIS spectra of TiO2/SnO2 composite ®lms from 320 to 760 nm are shown in Fig. 4 for different doping levels of SnO2. The ®lms were double layered on soda
Fig. 4. UV±VIS spectra of TiO2/SnO2 composite ®lms with mole percentage (mol%) of SnO2: (1) 0, (2) 1, (3) 3, (4) 5, (5) 10, and (6) 20%.
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lime glass and heat-treated at 4508C. It is observed that the visible light transmittances of the pure TiO2 and TiO2/SnO2 composite ®lms are almost the same, indicating the doping of SnO2 does not change the transmission of the ®lms. The average transmission of the ®lms is about 80%. 4. Conclusions The TiO2/SnO2 composite coating ®lms can be fabricated from precursor solutions by doping with different concentrations of SnO2 on soda lime glass with the sol±gel technology. Their super-hydrophilicity, photocatalytic activity and light transmission of the composite ®lms have been studied. It has been shown that the hydrophilicity and photocatalytic activity of the TiO2/SnO2 composite ®lms are superior to the pure TiO2 ®lm. A contact angle of 08 has been achieved in the TiO2/SnO2 composite ®lms with 1±5 mol% SnO2 and heat-treated at 4508C. It has been also found that a thicker composite ®lm is of a smaller contact angle, the best photocatalytic activity is observed in the ®lms doped with 10 mol% SnO2. The doping of SnO2 in the composite ®lms does not decrease the visible light transmittance and the average transmission of the pure TiO2 and the TiO2/SnO2 composite ®lms is about 80%. Acknowledgments This work was ®nancially supported by National Natural Science Foundation of China (Grant No. 50162002) and the Main Natural Science Foundation of Yunnan Province, China (Grant No. 2000E0002Z). References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1±21. [2] J.A. NavõÂo, J.J. Testa, P. Djedjeian, J.R. PadroÂn, D. RodrõÂguez, M.I. Litter, Appl. Catal. A 178 (1999) 191±203. [3] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260±263. [4] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97±107. [5] J.G. Yu, X.J. Zhao, Q.N. Zhao, G. Wang, Mater. Chem. Phys. 68 (2001) 253±259. [6] P.Y. Zhang, G. Yu, Z.P. Jiang, Chinese J. Progress Environ. Sci. 5 (3) (1997) 3±7. [7] Q.J. Liu, Y.B. Zhang, X.H. Wu, Q. Liu, Key Eng. Mater. 224/226 (2002) 215±218. [8] L.Y. Shi, H.C. Gu, C.Z. Li, Chinese J. Cat. 20 (3) (1999) 338±342.
Preparation and super-hydrophilic properties of TiO2/SnO2 composite thin ®lms Qingju Liu*, Xinghui Wu, Baoling Wang, Qiang Liu Department of Material Science and Engineering, Yunnan University, Kunming, Yunnan 650091, PR China (Refereed) Received 5 April 2002; accepted 9 September 2002
Abstract SnO2±TiO2 composite thin ®lms were fabricated on soda-lime glass with sol±gel technology. By measuring the contact angle of the ®lm surface and the degradation of methyl orange, we studied the in¯uence of SnO2 doping concentration, heat-treatment temperature and ®lm thickness on the super-hydrophilicity and photocatalytic activity of the composite ®lms. The results indicate that the doping of SnO2 into TiO2 can improve their hydrophilicity and photocatalytic activity, and the composite ®lm with 1±5 mol% SnO2 and heat-treated at 4508C is of super-hydrophilicity. The optimal SnO2 concentration for the photocatalytic activity is 10 mol% and larger ®lm thickness is helpful to reduce the contact angle of the composite ®lms. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Composites; A. Thin ®lms; D. Catalytic properties; D. Surface properties
1. Introduction In recent years, nano-TiO2 thin ®lms with super-hydrophilic and photocatalytic characteristics have attracted a great deal of attention. They have many advanced functions and features, including anti-fouling, deodorizing, sterilizing and antifogging. When exposed to UV light, TiO2 ®lms can break down organic compounds and enable water to spread evenly on their super-hydrophilic surface to easily realize *
Corresponding author. Tel.: 86-871-5036510; fax: 86-871-5153832. E-mail address: [email protected] (Q. Liu). 0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 7 2 - 8
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surface self-cleaning. There have been numerous research papers reporting improvement in the photocatalytic activity and super-hydrophilicity of TiO2 thin ®lms. Fujishima et al. [1] reviewed the current progress in the area of TiO2 photocatalysis and observed a photo-induced super-hydrophilic phenomenon in TiO2 and discussed its potential applications. NavõÂo et al. [2] studied the photocatalytic ef®ciency of iron-doped titania containing Fe prepared from TiCl4 and Fe(III) acetylacetonate by sol±gel method. Watanabe et al. [3] evaluated photo-induced hydrophilic conversion on different crystal faces of single crystal rutile and polycrystalline anatase titanium dioxide to clarify the dependence of the crystal structure on the photo-induced hydrophilic conversion. Yu and Zhao [4] reported that the hydrophilicity can be enhanced by processing the TiO2 surface with acid, the contact angle of porous TiO2 coating ®lm can be reduced by increasing the adsorbed content of hydroxyl, capillary force and roughness of the resultant coating ®lms [5]. However, the super-hydrophilicity of the composite TiO2/SnO2 ®lm has not been reported yet. In this work, we are going to report a novel composite TiO2/SnO2 thin ®lm, which was prepared on soda-lime glass using sol±gel technology. The TiO2/SnO2 powder was heat-treated at 4508C and characterized with an X-ray diffractometer (XRD). The composite ®lms were characterized with a spectroscopic ellipsometer and an UV±VIS spectrophotometer. The effects of SnO2 doping in the precursor solution, heat treatment temperature and ®lm thickness on the super-hydrophilicity and photocatalytic activity of the composite ®lms were evaluated by measuring the contact angle for water of TiO2/SnO2 ®lms and the photocatalytic decolorization of aqueous methyl orange. 2. Experiments 2.1. Preparation of composite TiO2/SnO2 thin ®lms The preparation of precursor solutions for TiO2/SnO2 composite ®lms is described as follows. First, TiO2 and SnO2 sols were prepared separately. For the preparation of the TiO2 sol, tetranbutyl titauate was dissolved in ethanol ®rst, stirred vigorously for 30 min at room temperature, followed by the dropping reaction with the addition of HNO3 to the solution until pH 3:5. Distillated water was ®nally added to the solution over stirring through a burette. The chemical composition of the resultant alkoxid solution was (C4H9O)4Ti:C2H5OH:H2 O 1:82:3 in mole ratio. The TiO2 sol was obtained by keeping stirring the alkoxid solution for hydrolysis reaction for 4 h. The SnO2 sol was prepared in a similar way. Stannic chloride was dissolved in ethanol ®rst, followed by stirring and re¯uxing vigorously for 2 h at 808C. Over stirring distillated water was added to the solution with a burette. The chemical composition of the resultant solution was SnCl4:C2H5OH:H2 O 1:66:3 in mole ratio. Keeping stirring for hydrolysis reaction for 24 h, we got the SnO2 sol. Then mixture sols of TiO2 and SnO2 were made with different mole ratios of SnO2 sol. The mole ratios of SnO2 to TiO2 were 0, 1, 3, 5, 10, 20, 30 mol%. Finally, the TiO2/SnO2 ®lms were
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formed on the substrates with the prepared sol solutions by dipping-withdrawing at room temperature. The withdrawal speed was 3 mm/s. The thickness of the TiO2 and the TiO2/SnO2 ®lms is dependent on and approximately linearly proportional to the number of the cycle from dipping to heat treatment. For the single layer ®lm, the thickness of the pure TiO2 is about 180 nm, and that of the mixed is about 160 nm. The substrates coated with TiO2/SnO2 composite gel ®lms and the corresponding composite xerogel were dried with infrared light and then heat-treated together at different temperature for 1 h in air using an electric oven. 2.2. Characterization Constituent and crystallinity of the TiO2/SnO2 composite ®lms were analyzed with XRD (type D/maxIII, Japan) using Cu Ka radiation. The accelerating voltage was 35 kV and the applied current was 25 mA. The refractivity and thickness of the ®lm were measured with a spectroscopic ellipsometer (type TP-77, China) at 632.8 nm. The measurement of the ®lms' light transmittance was performed with an UV±visible spectrometer (type UV±VIS8500, China) in the wavelength range of 200±1100 nm. The hydrophilic properties of various TiO2/SnO2 ®lms were evaluated by measuring the contact angle for water of TiO2/SnO2 thin ®lms with a contact angle device (type JC2000A, China). Prior to measuring, the ®lms had been illuminated with natural light for 30 min after being placed in dark for 24 h. The photocatalytic activities of various TiO2/SnO2 ®lms were examined by using methyl orange degradation decolorization described below. The pure TiO2 ®lm and the TiO2/SnO2 composite ®lms were placed in aqueous methyl orange that has an initial density of 46 mg/l in quartz cells. An UV lamp (30 W) was used as the light source. The averaged intensity of UV irradiance was 78 mW/ cm2 measured with an UV irradiance meter. Its wavelength range is 320±400 nm, and peak wavelength is 360 nm. The density of methyl orange was determined with the spectrometer. 3. Results and discussion 3.1. Constituent and crystallinity of TiO2/SnO2 composite powder The XRD spectra for powders and ®lms of pure TiO2 and composite TiO2/SnO2 with SnO2:TiO2 20 mol% and heat-treated at 4508C revealed the present of TiO2 and SnO2 in the composite TiO2/SnO2. It has been con®rmed that the crystal form of TiO2 in both the pure TiO2 and the composite TiO2/SnO2 are all of anatase. The diffraction patterns also show that the diffraction peak width of the composite TiO2/ SnO2 is broader than that of the pure TiO2, indicating that the average diameter of the former is smaller than that of the latter. This can be attributed to the existence of SnO2 that reduces the growth rate of TiO2 particles. The reason for above phenomenon is that the introduction of tin ions changes the surface charge of the TiO2 sol particles in
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Fig. 1. UV±VIS spectra of pure TiO2 and TiO2/SnO2 ®lms deposited on soda lime glass (three coating cycle times, SnO2:TiO2 0:05, heat-treated at 4508C).
the composite sol, and distances them from each other. In this way, the TiO2 particles are most likely formed in smaller sizes. Fig. 1 shows the UV±VIS spectra of the pure TiO2 and the composite TiO2/SnO2 ®lms (three layers and heat-treated at 4508C) deposited on soda lime glass in the wavelength range 300±800 nm. An absorption edge of the composite TiO2/SnO2 ®lm with SnO2:TiO2 0:05 was observed at a shorter wavelength than that of the pure TiO2 coating ®lm. The shift is agreeable with the above XRD result and further con®rms that the particle size of the pure TiO2 is larger than that of the TiO2/SnO2 ®lms. 3.2. Effect of SnO2 content on hydrophilicity The contact angle for water of TiO2/SnO2 thin ®lms (three layers, heat-treated at 4508C) is listed in Table 1. We should note here that all the samples on hydrophilicity discussed below have been put in dark for 24 h and irradiated under natural light for 30 min prior to measurement. It is found that the contact angle for water is 9.58 for the pure TiO2 ®lm, and 08 for the TiO2/SnO2 ®lms with SnO2 doping levels of 1±5 mol%. The ®lm with higher SnO2 doping, for example, SnO2:TiO2 10±30 mol%, the contact angle increases from 8.5 to 13.58. The results indicate that low level doping of SnO2 can improve the hydrophilicity of the TiO2 ®lms, most probably due Table 1 Contact angle of TiO2/SnO2 ®lms with different doping levels SnO2 content (mol%) Contact angle (8)
0 9.5
1 0
3 0
5 0
10 8.5
20 9
30 13.5
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Table 2 Effect of heat-treated temperature on contact angle Heat-treated temperature (8C) (SnO2:TiO2 1 mol%) (SnO2:TiO2 3 mol%) (SnO2:TiO2 5 mol%)
150
250
350
450
550
650
69 65 68.5
36.5 36.5 48
14.5 10.5 8
0 0 0
13.5 19 5.5
10.5 17 31
to the increase of hydroxyl in the composite ®lms [6]. It is usually expected that there is an optimal range in doping density for super-hydrophilicity. In our case it is SnO2:TiO2 1±5 mol%. 3.3. Effect of heat-treatment temperature on hydrophilicity The contact angle for water of the TiO2/SnO2 thin ®lms (three layers, heat-treated for 60 min) at different heat-treated temperature is listed in Table 2. It is found that the contact angle of the composite ®lms with SnO2:TiO2 1, 3 and 5 mol% decreases as the heat-treated temperature increase from 150 to 4508C and increase as temperature above 4508C. Therefore the optimal heat-treated temperature for super-hydrophilicity is around 4508C. The change in the contact angle with the heat-treatment temperature can be explained in terms of two factors: the crystalline phase and the density of the composite ®lms. First, the hydrophilicity is dependent on the ®lm's crystalline phase. The structure of the TiO2 ®lms heat-treated below 2508C is almost amorphism and there is no super-hydrophilic feature. From 250 to 4508C, the content of the anatase TiO2 increases with the heat-treated temperature, and TiO2 heat-treated at 4508C completely become anatase, result in super-hydrophilc ®lm. However, When the temperature further increases above 4508C, the anatase TiO2 starts to transform gradually to rutile TiO2, leading to the rise in the contact angle. This is because hydrophilicity is related to the density of surface hydroxyl in TiO2 ®lms. The surface hydroxyl can easily combine with water molecule to form hydrogen bond, resulting in good wettability. So the more surface hydroxyl, the better the hydrophilicity. On the same UV illumination, although changes in hydrophilicity have been observed on both the anatase and rutile TiO2 surface [1], the former with more surface hydroxyl is more active in achieving hydrophilic features than the latter with less surface hydroxyl. The hydrophilicity is also dependent on the density of the composite ®lm. Fig. 2 illustrates the effect of heat-treatment temperature on the refractive index of TiO2/ SnO2 ®lms. As can be seen, the contact angle for water increases with the refractive index. It is well known that the index is related to the mass density of the materials. In the present case, the ®lms become denser as the temperature increases due to the decline in porosity [7]. So the refractive index of the ®lms increases with the temperature. Therefore, we can conclude from Fig. 2 that the contact angle of the composite ®lm increases with the ®lm's density.
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Fig. 2. Effect of heat-treatment temperature on refractive index of SnO2±TiO2 ®lms.
3.4. Effect of thickness on hydrophilicity The change in contact angle for water of TiO2/SnO2 thin ®lms (SnO2:TiO2 5 mol% and heat-treated at 4508C) with the number of coating cycle is listed in Table 3. It can be seen that the contact angle decreases with the increasing in the coating cycle. It is 08 for the ®lms coated more than three layers, while it is 128 for the single layer ®lms. The thickness of the single layer ®lm is about 160 nm by calculating the measurement data using the spectroscopic ellipsometer. In this case, UV ray can easily penetrate through the single layer so that the availability of the ray energy is low. With the number of coating cycle increasing, the availability of the irradiation light energy is heightened and more hydroxyl is produced resulting in the contact angle decreases to 08. 3.5. Effect of SnO2 concentration on photocatalytic activity Fig. 3 shows the results of photocatalytic decolorization of methyl orange by TiO2/ SnO2 ®lms with different doping levels of SnO2 (four layers and heat-treated at 4508C). After illumination for 120 min with UV light, the decolorization rate of methyl orange increases by the ®lms with low levels of SnO2 and then decreases with high levels of SnO2. The maximum rate of decolorization is 58.33% by the ®lm with 10 mol% SnO2, while that by the pure TiO2 ®lm is 44.35%. The photocatalytic activities of all the TiO2/ SnO2 composite ®lms are higher than that of the pure TiO2 ®lm. This phenomenon is probably attributed to two reasons stated below. Firstly, it is due to the increase of surface hydroxyl of the composite ®lms [6]. Secondly, it is because of the decrease in Table 3 Effect of ®lm thickness on contact angle of TiO2/SnO2 ®lm Number of coating cycle Contact angle (8)
1 12
3 0
5 0
7 0
9 0
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Fig. 3. Decolorization of methyl orange degraded by TiO2/SnO2 ®lm doped with different levels of SnO2 (four layers and heat-treated at 4508C).
recombination rate of photo-induced carriers in the TiO2/SnO2 composite ®lms. The conduction band energy level for SnO2 and for TiO2 is different, for example, it is 0 eV for SnO2 (versus NHE, pH 7) and 0.5 eV for TiO2 (versus NHE, pH 7) [8]. Due to this potential difference, the photo-induced electrons transfer from the surface of TiO2 to that of SnO2 can take place easily when SnO2 is doped into TiO2. On the other hand, however, if there is too much doped SnO2, the relative density of TiO2 decreases which cause the activity to decrease. In our case it is around 10 mol% of SnO2 that makes the photocatalytic activity turn from increase to decrease. 3.6. UV±VIS spectra The UV±VIS spectra of TiO2/SnO2 composite ®lms from 320 to 760 nm are shown in Fig. 4 for different doping levels of SnO2. The ®lms were double layered on soda
Fig. 4. UV±VIS spectra of TiO2/SnO2 composite ®lms with mole percentage (mol%) of SnO2: (1) 0, (2) 1, (3) 3, (4) 5, (5) 10, and (6) 20%.
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lime glass and heat-treated at 4508C. It is observed that the visible light transmittances of the pure TiO2 and TiO2/SnO2 composite ®lms are almost the same, indicating the doping of SnO2 does not change the transmission of the ®lms. The average transmission of the ®lms is about 80%. 4. Conclusions The TiO2/SnO2 composite coating ®lms can be fabricated from precursor solutions by doping with different concentrations of SnO2 on soda lime glass with the sol±gel technology. Their super-hydrophilicity, photocatalytic activity and light transmission of the composite ®lms have been studied. It has been shown that the hydrophilicity and photocatalytic activity of the TiO2/SnO2 composite ®lms are superior to the pure TiO2 ®lm. A contact angle of 08 has been achieved in the TiO2/SnO2 composite ®lms with 1±5 mol% SnO2 and heat-treated at 4508C. It has been also found that a thicker composite ®lm is of a smaller contact angle, the best photocatalytic activity is observed in the ®lms doped with 10 mol% SnO2. The doping of SnO2 in the composite ®lms does not decrease the visible light transmittance and the average transmission of the pure TiO2 and the TiO2/SnO2 composite ®lms is about 80%. Acknowledgments This work was ®nancially supported by National Natural Science Foundation of China (Grant No. 50162002) and the Main Natural Science Foundation of Yunnan Province, China (Grant No. 2000E0002Z). References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1±21. [2] J.A. NavõÂo, J.J. Testa, P. Djedjeian, J.R. PadroÂn, D. RodrõÂguez, M.I. Litter, Appl. Catal. A 178 (1999) 191±203. [3] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260±263. [4] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97±107. [5] J.G. Yu, X.J. Zhao, Q.N. Zhao, G. Wang, Mater. Chem. Phys. 68 (2001) 253±259. [6] P.Y. Zhang, G. Yu, Z.P. Jiang, Chinese J. Progress Environ. Sci. 5 (3) (1997) 3±7. [7] Q.J. Liu, Y.B. Zhang, X.H. Wu, Q. Liu, Key Eng. Mater. 224/226 (2002) 215±218. [8] L.Y. Shi, H.C. Gu, C.Z. Li, Chinese J. Cat. 20 (3) (1999) 338±342.