Photo-induced hydrophilicity of polycrystalline SrTiO3 thin films - J-Stage

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Journal of the Ceramic Society of Japan 118 [1] 43-47 2010

Photo-induced hydrophilicity of polycrystalline SrTiO 3 thin films Ken-ichi KATSUMATA,† Tetsuya SHICHI* and Akira FUJISHIMA*,** Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503 Technology Research and Development Department, Central Japan Railway Company, 1545-33, Ohyama, Komaki City, Aichi 485-0801 ** Kanagawa Academy of Science and Technology, KSP Building West 614, 3-2-1, Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012 *

In the current work, we investigate the photo-induced hydrophilicity of SrTiO3 thin films prepared by the sol–gel process. The SrTiO3 thin films were prepared with different grain sizes but the same crystalline phase, almost the same crystallinity and band gap. With increasing heated temperature, the grain sizes on the film surfaces increased and became more heterogeneous microstructure. Fraction of methylene blue (MB) decomposed on the films was almost the same under UV-A or UV-B light irradiation. Although the films did not show highly photo-induced hydrophilic conversion under UV-A light irradiation, they exhibited the photo-induced hydrophilicity under UV-B light irradiation. The films with smaller grains were highly hydrophilic and higher hydrophilicizing rate than those with larger grains. These results indicate that the SrTiO3 films exhibited the photo-induced hydrophilicity, and it was affected by the surface microstructure. ©2010 The Ceramic Society of Japan. All rights reserved.

Key-words : Photocatalyst, Photo-induced hydrophilicity, SrTiO3, Sol–gel process, Surface microstructure [Received August 13, 2009; Accepted November 19, 2009]

1. Introduction Titanium dioxide (TiO2) is a well-known photocatalyst material.1) Upon ultraviolet light (UV) irradiation of TiO2, electron hole pairs are generated, which reduce and oxidize adsorbates on the surface, respectively, thereby producing radical species, such as OH radicals and O2–. These radicals can decompose most organic compounds and bacteria.2)–4) Photo-induced hydrophilicity was discovered in 1995,5)–8) soon after the time that conventional applications of TiO2 as a photocatalyst began to be developed. Upon UV light irradiation, a TiO2 surface becomes highly hydrophilic state. In addition to the TiO2 photocatalyst, perovskite-type (ABO3) oxides have been studied to determine their photocatalytic oxidation properties.9)–12) However, few have been determined to be capable of photoinduced hydorophilicity. Recently, we reported that the perovskite-type oxide NaNbO3 films exhibit photo-induced hydrophilic conversion under UV-A or UV-B light irradiation, but with little photocatalytic oxidation activity.13) The photo-induced hydrophilic conversion rate under UV-B light irradiation was faster than that under UV-A. From this report, it is considered that the photo-induced hydrophilicity is dependent on the UV wavelength range. On the other hand, Miyauchi et al.14) have reported that SrTiO3 films did not become hydrophilic under UV light irradiation, even though it rapidly decomposed methylene blue adsorbed on its surface. Interestingly, the photocatalytic properties have been found to be different between TiO2 and NaNbO3. However, they have not investigated the photocatalytic activity under UV-B light irradiation. In this study, we focused on the well-known perovskite-type †

Corresponding author: K. Katsumata; E-mail: katsumata.k.ab@m. titech.ac.jp

©2010 The Ceramic Society of Japan

oxide SrTiO3, and have investigated the photocataytic oxidation activity and the photo-induced hydrophilicity of SrTiO3 films under UV-A or UV-B light irradiation.

2.

Experimental procedure

A coating solution was prepared by the sol–gel method. First, strontium chloride hexahydrate (SrCl2∙6H2O, 1.06 g) was dissolved in ethanol (C2H5OH, 35 mL), and acetylacetone (C5H8O2, 0.82 mL) was added into it. After the solution was stirred for 30 min, titanium tetraisopropoxide [Ti(OCH(CH3)2)4, 1.19 mL] was added to the solution and was vigorously stirred for an additional 30 min. The molar ratio of the mixtures of SrCl2∙6H2O: C5H8O2:Ti(OCH(CH3)2)4 was 1:2:1. This mixed solution was hydrolyzed by adding a drop of hydrochloric acid solution (5.00 mL, pH 1) with stirring at room temperature. Finally, reagent grade polyethylene glycol 200 [H(OCH2CH2)nOH, 5.00 mL] was added into the solution to facilitate the coating of the glass substrates. Stirring for 15 min yielded the coating solution. Films were prepared by spin-coating this solution onto silica glass followed by heating at 500, 700 and 900°C for 1 h. The surface microstructure of the films was examined by scanning electron microscopy (SEM, S–4800; Hitachi HighTechnologies Co.). The average surface roughnesses (Ra) of the films were evaluated in 5 μ m × 5 μ m regions by atomic force microscopy (AFM, JSPM–5200; JEOL Ltd.) in the tapping mode using a conventional Si cantilever. The crystalline phase was evaluated by a high power X-ray diffraction system for thin film (XRD, D8 DISCOVER Hybrid Super Speed, Bruker AXS K. K.) analysis, using monochromated Cu Kα radiation. The measuring method was a detector scan (ω = 0.2°), and the power was 50 kV–100 mA. The band gaps of the films were calculated from the spectra using a UV-visible scanning spectrophotometer (U– 3310, Hitachi High-Technologies Co.) following a published procedure.15) 43

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Katsumata et al.: Photo-induced hydrophilicity of polycrystalline SrTiO3 thin films

The photo-induced hydrophilicity of the prepared films was evaluated by the water contact angle (θ ) using a commercial automatic contact angle system (OCA 15plus, Dataphysics Instruments GmbH, Germany). The water contact angle of the films was measured under UV light intensity (1.0 mW/cm2) utilizing UV-A or UV-B lamp, with peaks in the emission spectra at 360 or 306 nm, respectively. The water droplet volume used for the measurements was 1.0 μ L. The values of the water contact angle were averages of three separate measurements. The photocatalytic oxidation of the prepared films was evaluated by the decomposition of methylene blue (MB, C16H18ClN3S). The prepared films were immersed in 0.02 mM MB aqueous solution over night, in order to equilibrate adsorption on the film surfaces. After washing with ultrapure water, a cylinder (φ 40 × 30 mm) was contacted with the film surface sealing by silicone grease and a 0.01 mM MB aqueous solution was poured into the cylinder. Irradiating with UV-A or UV-B light, the absorption spectra of MB were monitored by a UV-visible spectrophotometer after 24 h.

3.

Results and discussion

Figure 1 shows the SEM photographs of the prepared film

Fig. 1. SEM images of the film surfaces heated at 500, 700 and 900°C. The electron acceleration voltage was 1.0 kV.

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surfaces. For a film heated at 500°C, a homogeneous microstructure with an average grain size of 10–20 nm was obtained. More heterogeneous microstructures were, however, obtained for films heated at 700 and 900°C, and the average grain sizes were 40– 100 nm and 100–300 nm, respectively. The average roughness factors (Ra) of the films heated at 500, 700, and 900°C were 1.11, 5.68, and 7.42 nm, respectively. These results are compatible with the expectation that grain growth is caused by higher heating temperatures. Figure 2(a) shows the XRD patterns of the films. The crystalline phase detected in the films was a SrTiO3 cubic perovskite phase with Pm3m space group (ICDD card: 00-035-0734), having no preferred orientation. Figure 2(b) shows a detailed measurement of the full width at half maximum (FWHM) values of the SrTiO3 (110) peak, for the films heated at 500, 700, and 900°C. The resulting FWHM values were 0.52°, 0.60°, and 0.62°, respectively. The FWHM value of the film heated at 500°C was smaller than those of 700°C and 900°C. This result indicates that the film heated at 500°C is higher crystallinity than those of 700°C and 900°C, being incompatible with the commonly known tendency as higher crystallinity with higher heating temperatures. The grain sizes of the films heated at 700°C and 900°C were larger than that at 500°C as shown in Fig. 1. A lot of investigations in the field of fracture mechanics in polycrystalline ceramic materials revealed that the effect of residual stress among grains caused by thermal expansion increases with increasing grain size and becomes the highest when the stress level is limited in the elastic region.16) Since grain growth clearly occurs in the films heated at 700°C and 900°C, higher residual stress will be generated during cooling in the film heated at 700°C and 900°C than

Fig. 2. X-ray diffraction patterns of the films: (a) wide scan 30–80°, (b) detailed scan corresponded to the (110) reflection of SrTiO3.

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that at 500°C due to the larger temperature difference. It is, thus, thought that the lower crystallinity of the films heated at 700°C and 900°C is caused by residual stress. Figure 3 shows the UV-Vis optical absorption spectra of the films heated at 500, 700, and 900°C. An absorption peak attributed to the SrTiO3 phase was observed at around 250 nm in all the samples though the peak intensity was lower in the film heated at 900°C than the other two films. This result indicates that the film thickness of the films heated at 500°C and 700°C was almost the same but they were thicker than that at 900°C. The optical band gap could be estimated from the absorption spectra using the following equation:

α hν = A(hν – Eg)n/2

(1)

where A, α, hν and Eg are constant, absorption coefficient, the photon energy and the optical band gap, respectively. In this equation, n is determined by the transition types and in the cases of direct and indirect transition, the n values are equal to 1 and 4, respectively. The n and Eg values were estimated by the following steps: first, the plot of ln(α hν ) vs. ln(hν –Eg) was drawn using an approximate value of Eg, and then the value of n was estimated from the slope of the straight line near the band edge; second, the plot of (α hν)2/n vs. hν was drawn and then a tangential line was plotted near the band edge, the x-intercept of the tangential line corresponded to the optical band gap. The estimated band gaps of the films heated at 500, 700, and 900°C were 3.15, 3.18, and 3.09 eV, respectively. These values were in good agreement with the reported values 3.0–3.2 eV.17),18) These

Fig. 3. UV-Vis optical absorption spectra of the SrTiO3 films coated on silica glass substrates.

Fig. 4. Fraction of methylene blue (MB) decomposed on the SrTiO3 films under UV-A or UV-B light irradiation after 24 h. The light intensity was 1.0 mW/cm2.

results indicate that all of these films are consisted of a single phase of crystalline SrTiO3. Figure 4 shows the fraction of MB decomposed by the photocatalytic oxidation on the films under UV-A or UV-B light irradiation. The MB decomposition ratios of the films heated at 500, 700, and 900°C by UV-A irradiation were 22, 18, and 17%, respectively. In the case of UV-B light irradiation, they were 26, 24, and 32%, respectively. Since their band gaps are almost the same, photocatalytic oxidation activity of the SrTiO3 films is mainly affected by the crystallinity, surface microstructure, and film thickness. With higher crystallinity, finer surface microstructure, and thicker film thickness, the activity becomes higher. In this case, these correlative relationships were not shown under UV-A or UV-B light irradiation. Figure 5 shows the change of the water contact angle (θ ) of the films under 1.0 mW/cm2 UV-A light irradiation (1.0 mW/cm2). The initial contact angles of the 500, 700, and 900°C were 45, 56, and 52°, respectively, and somewhat different among the three films. The reason for the difference may be related to the dark storage time in the vacuum desiccators, slowly converting to a hydrophobic state. When UV-A light was irradiated on the films, the contact angles began to decrease. The contact angles decreased to 20 –27° after irradiation for 300 min but they did not become θ < 10° corresponding to highly hydrophilic state. This result was corresponded with the previous report.14) In the case of UV-B light irradiation, however, the films heated at 500°C and 700°C became highly hydrophilic state after 300 min and higher hydrophilicizing rate than that at 900°C as shown in Fig. 6. Thus, the films showed photo-induced hydrophilic conversion by UV-B light irradiation. In the case of TiO2, the photo-induced hydrophilicity is conjectured to be due to either by decomposition of organic compounds on the film surface by photocatalytic oxidation19),20) or by hydrophilicity induced by photo-induced modification of the surface structure, causing an increase in the number of hydroxyl groups.21) Although the photocatalytic oxidation activities of the prepared SrTiO3 films were almost the same under UV-A or UVB light irradiation (Fig. 4), their photo-induced hydrophilicities differed greatly (Figs. 5 and 6). The effect of the photocatalytic oxidation activity for the photo-induced hydrophilicity of the films was little in the present films. The adsorbed photon numbers (N) at 360 nm (UV-A) or 306 nm (UV-B) were calculated by following equation: N = Pλ /hc

(2)

Fig. 5. Variation of water contact angle on the SrTiO3 films under UVA light irradiation. Values of the water contact angle were averages of three measurements.

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Katsumata et al.: Photo-induced hydrophilicity of polycrystalline SrTiO3 thin films

Fig. 6. Variation of water contact angle on the SrTiO3 films under UVB light irradiation. Values of the water contact angle were averages of three measurements.

where P, λ, h, and c are light intensity, wavelength, Planck constant, and light speed, respectively. Calculated values at 360 nm or 306 nm were 1.81 × 1015 or 1.54 × 1015. Absorbance of the films at 360 nm or 306 nm was almost the same, and the values were 0.093 or 0.251 (Fig. 3). We estimated the absorbed photon numbers from the following equation: (Photon number) × (Absorbance)

(3)

The values of the absorbed photon numbers at 360 nm or 306 nm were 1.68 × 1014 or 3.86 × 1014. This result indicates that the amounts of radiation absorbed from UV-B light was more than that of UV-A. This is one of the factors for the different photoinduced hydrophilicity by UV-A and UV-B lights. Wenzel has discussed the relationship between the contact angle and surface roughness and proposed the following equation22): cosθ 1 = r cosθ 0

(4)

where θ 1, θ 0, and r are the observed contact angle on a rough surface, the contact angle on a smooth surface, and the roughness factor, respectively. Since r is more than 1, the observed contact angle on a rough surface is smaller than that on a smooth surface in the case of θ 0 < 90°. Although the surface roughness for the 900°C film was largest among the three films, the hydrophilic conversion was the slowest. Thus, the differences in the hydrophilic conversion of the films cannot be explained on the basis of differences in the surface roughness. It is appropriate to consider that these results are due to a difference in the intrinsic hydrophilic conversion behavior between these films. Shibata et al.23),24) investigated the stress effect on the photoinduced hydrophilicity of TiO2 thin films by using substrate materials with different thermal expansion coefficients. They prepared TiO2 films by sputtering, and found that external tension enhances and compression inhibits photo-induced hydrophilicity. Thermal expansion coefficients of silica glass25) and SrTiO326) are 0.4–0.55 × 10–6 and 8.75 × 10–6, respectively. Thus, SrTiO3 films are loaded external tension from the silica glass substrate after firing. And then, higher residual stress will be generated during cooling in the film heated at 700°C and 900°C than that at 500°C due to the larger temperature difference. Therefore, the films heated at 700°C and 900°C should exhibit higher hydrophilic conversion rate than that at 500°C if this external tension is important. The result shown in Fig. 5 and 6 implies that the dominant stress origin is not the external one from silica glass substrate. 46

Moreover, the photo-induced hydrophilicity of the polycrystalline anatase thin films is affected by the surface microstructure under UV irradiation.27) In this report, homogeneous structure with fine grains (25– 40 nm) exhibited a higher photo-induced hydrophilic conversion rate than heterogeneous structure with coarse grains (150 –200 nm). The results showing higher hydrophilicity and hydrophilicizing rate in the films heated at 500°C and 700°C than that at 900°C are therefore, supportive. The photo-induced hydrophilic conversion of the prepared SrTiO3 films was found to be affected by the surface microstructure. Further investigation is, however, necessary to elucidate the cause and mechanisms of photo-induced hydrophilicity in various semiconductor metal-oxide films.

4. Conclusions In the present study, we have prepared SrTiO3 thin films with different grain sizes by sol–gel process. The microstructures of the resulting SrTiO3 films varied largely depending on the heating temperatures; homogeneous fine grained microstructure (10– 20 nm) in the film heated at 500°C while it became more heterogeneous including coarser grains in the films heated at 700°C (40 –100 nm) and 900°C (100 –300 nm). The photocatalytic oxidation activities of the resulting SrTiO3 films were almost the same under UV-A and UV-B light irradiation. The films did not show highly hydrophilic state under UV-A light irradiation while they exhibited the photo-induced hydrophilicity under UV-B light irradiation. The films heated at 500°C and 700°C were highly hydrophilic and higher hydrophilicizing rate than that at 900°C. These different behaviors are attributed to the amounts of UV absorption and the surface microstructure. Acknowledgements The authors are grateful to Professor Kiyoshi Okada of Tokyo Institute of Technology for critical reading and editing of the manuscript. The authors thank Mr. Tachiyanagi and Ms. Hattori for their help with XRD and water contact angle measurements.

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