- Email: [email protected]
Preparation of superhydrophilic mesoporous SiO2 thin films
Applied Surface Science 258 (2012) 4334–4338
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of superhydrophilic mesoporous SiO2 thin films Peiyi Chen, Yun Hu ∗ , Chaohai Wei The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 2 June 2011 Received in revised form 23 December 2011 Accepted 26 December 2011 Available online 31 December 2011 Keywords: Mesoporous SiO2 thin films Transparent Superhydrophilicity Antifogging
a b s t r a c t Using a simple sol–gel/spin-coating method, mesoporous SiO2 thin films were prepared on glass slides. All of the prepared thin films were colorless and transparent as original glass substrates. XRD and TEM measurements revealed that the prepared SiO2 thin films coated 3–12 times possess hexagonal mesoporous structure. The mesoporous SiO2 thin films performed the superhydrophilicity and antifogging property in the absence of UV light irradiation. The results suggested that the appropriate film thickness and mesoporous structures can improve the surface superhydrophilic behavior of SiO2 thin films. © 2012 Published by Elsevier B.V.
1. Introduction Surfaces with extreme wetting characteristics such as superhydrophilicity and superhydrophobicity play an important role in various applications. These phenomena have already been applied to some self-cleaning products such as windows glasses, tiles, mirrors and plastics. It is known that TiO2 films show a superhydrophilicity under ultraviolet (UV) light irradiation [1–3]. However, current applications are mainly limited to out-door use because the superhydrophilicity of TiO2 films rapidly vanishes if the UV light irradiation is removed [4,5]. Thus, many efforts have been taken to overcome this problem. Although some researchers reported visible light induced hydrophilicity on nitrogen doped TiO2 films, visible light irradiation did not cause the surfaces of these thin films to become superhydrophilic with water contact angles below 5◦ [6]. On the other hand, since SiO2 contains a mass of hydroxyl groups, many articles reported that SiO2 addition on TiO2 films could maintain the photo-induced superhydrophilicity for a certain time if the UV light irradiation was removed [7,8]. Daeyeon et al. reported that multifunctional thin films were fabricated from layer-by-layer deposition of TiO2 and SiO2 nanoparticles with superhydrophilic wetting characteristics, but the contact angle of water on a planar SiO2 coated surface was approximate 20◦ [9]. Early theory established by Quéré and co-workers suggests that it is possible to enhance the wetting property of a surface with
water by introducing roughness at the right scale[10,11]. Several research groups reported that micro/nanostructured surfaces exhibited superhydrophilic properties due to the capillary effect [12–15]. Based on above theories, both roughness and microstructure of the surfaces can affect the superhydrophilicity of thin films [13,16]. Recent attempts of a multilayer assembly of SiO2 nanoparticles and polymer showed superhydrophilicity in the absence of UV light irradiation [17,18]. Dong H et al. reported that the superhydrophilic polymer-SiO2 nanocomposite coatings were prepared by building up a hierarchical micro/nanostructure within the coating [18]. However, these superhydrophilic materials containing polymer were aged gradually over time. The methods related to multi-step coating procedures or specialized equipment, limiting the practical applications. Due to the importance of hydroxyl groups and surface microstructures or nanostructures on the superhydrophilic property, we designed mesoporous SiO2 thin films spin-coated on glass substrates with a simple sol–gel method. The mesoporous structure as well as the optical, superhydrophilic and antifogging properties of the thin films were investigated.
2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +86 20 39380573; fax: +86 20 39380573. E-mail address: [email protected] (Y. Hu). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.apsusc.2011.12.109
Tetraethyl orthosilicate (TEOS, AR) was obtained from Tianjin Fu Chen chemical reagent factory. Anhydrous ethanol (EtOH, AR) was purchased from Tianjin Bai Shi chemical limited
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
4335
corporation. C12 H25 (OCH2 CH2 )4 OH (BrijR 30, AR) was obtained from Sigma–Aldrich. 2.2. Preparation of SiO2 thin films SiO2 sol was prepared via a sol–gel process. TEOS, EtOH and BrijR 30 were used as precursor, solvent and surfactant, respectively. A clear solution with the molar ratio of 8.0 TEOS:50 EtOH:0.9 BrijR 30:0.8 HCl was obtained after stirring for 10 min at 30 ◦ C. The sol was dripped onto a glass substrate at a spinning rate of 4000 rpm for 1 min. For the thin films, the spin-coating step was repeated until the desired coating times were obtained. The coating times were 1, 3, 6, 9, and 12. Therefore the obtained mesoporous SiO2 thin films were labeled as MS-1, MS-3, MS-6, MS-9, and MS-12, respectively. All the thin films were dried at room temperature for 2 h, and then calcined at 450 ◦ C for 5 h with a heating rate of 1 ◦ C/min. 2.3. Characterizations The mesostructures of the thin films were identified by X-ray diffraction (XRD, Bruker D8) measurement and transmission electron microscopy (TEM, Jeol, JEM-2100). The thickness of the films was measured by scanning electrons microscope (SEM, Nano430). Transmission measurements were carried out on a UV–vis spectrophotometer (Shimadzu, UV-2550). Water contact angles were obtained in atmospheric air at room temperature using a commercial contact angle meter (Dataphysics OCA 15plus) with ±1◦ accuracy. A droplet of water was injected on the surface of glass slide with a 1 l micro-injector. 3. Results and discussion 3.1. Mesoporous structure Fig. 1 shows the XRD patterns of the prepared SiO2 thin films on glass substrates. The XRD pattern of the film coated 1 time exhibited no peak, suggesting that the thickness of MS-1 was too thin to form mesoporous structure. The mesoporous SiO2 thin films coated 3–12 times reflected a single (1 0 0) diffraction peak at 2 = 2–3◦ , indicating the presence of ordered hexagonal mesoporous structure [19] The XRD pattern of MS-3 showed a weak and broad peak because of the unstable structure. With the increase in coating time, the XRD peak of the films became narrower and sharper and shifted to small degree, indicating that the highly ordered mesoporous structure existed within the films [20]. The lattice parameter (a0 ) of MS-3, MS-6, MS-9 and MS-12 estimated by the formula of √ a0 = (2d1 0 0 / 3) were about 4.3 nm, 4.4 nm, 4.5 nm and 4.5 nm, respectively.
Fig. 1. XRD patterns of mesoporous SiO2 thin films: (A) glass; (B) MS-1; (C) MS-3; (D) MS-6; (E) MS-9; (F) MS-12.
The TEM image of MS-6 (Fig. 2a) shows that the film had highly ordered mesoporous structure. Besides, it provided direct evidence for the presence of ordered hexagonal arrays aligned in the one-dimensional channel [21]. The average pore diameter of mesoporous structure in the SiO2 thin film estimated from the TEM image was about 4 nm, which was consistent with the XRD results. From the XRD results, the average pore diameter of MS6 was about 3.4 nm by using the formula (d = a0 − 1) [22]. These results confirmed the formation of mesoporous SiO2 thin film. The thicknesses of MS-6 (Fig. 2b), MS-9 and MS-12 determined by SEM measurements were about 340 nm, 480 nm, and 590 nm, respectively, indicating that the thickness of the thin films increased with the increase in the coating time.
3.2. Optical properties The photo images of the prepared mesoporous SiO2 thin films are shown in Fig. 3a. Although the transparency of the thin films became a little lower with increase in the number of coated time,
Fig. 2. (a) TEM image of MS-6 and (b) SEM image of MS-6.
4336
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
Fig. 3. (a) Photograph of mesoporous SiO2 thin films and (b) transmittance of mesoporous SiO2 thin films: (A) glass; (B) MS-1; (C) MS-3; (D) MS-6; (E) MS-9; (F) MS-12.
the films on the glass were colorless and transparent as original glass slide. Fig. 3b shows the transmittance spectra of a glass substrate and the mesoporous SiO2 thin films supported on the glass. The transmittance of glass substrate in the wavelength range of 350–800 nm was about 90%. The transmittance of the glass coated with mesoporous SiO2 thin films on one side became a little lower than that of the uncoated glass. However, all the films remained high level of 83–89%, indicating that the transmission level of the glass slide was unaffected by the coated mesoporous SiO2 thin films. 3.3. Superhydrophilic and antifogging properties Fig. 4a shows the changes in the contact angle of water droplets on the mesoporous SiO2 thin films without UV light irradiation. The contact angle of water became below 5◦ in 4 s on the film of MS-1, while it became below 5◦ in less than 1 s on the films coated 6–12 times, indicating that the superhydrophilicity increased with the increase in the coating times. The images (Fig. 4b) obtained from the video data showed that a drop of water wets the surface of MS6 thin film in less than 1 s to completely spread on the surface. The mechanism of such behavior can be understood from the simple relation derived by Wenzel and co-workers. It is well established that the apparent contact angle of a liquid on a surface depends on the roughness of the surface according to the following relation [23] cos a = r cos
(1)
where a is the apparent water contact angle on a rough surface and is the intrinsic contact angle as measured on a smooth surface. r
is the surface roughness defined as the ratio of the actual surface area over the project surface area. r becomes infinite for porous materials meaning that the surface will be completely wetted (i.e., a ≈ 0) with any liquid that has a contact angle (as measured on a smooth surface) of less than 90◦ . According to the previous works, the water contact angle of a nonporous SiO2 surface was 20–25◦ [9,24]. These observations indicated that the mesoporous SiO2 thin films could perform the superhydrophilic property even in the absence of UV light irradiation. The superhydrophilicity of the SiO2 thin films with meosoporous structure established ideal conditions for extreme wetting behavior. The images in Fig. 4c illustrate the antifogging behavior of the prepared thin films. Two glass slides, one (right) with a coating of the superhydrophilic mesoporous SiO2 thin films and the other (left) without such a coating, were cooled in a refrigerator at about −18 ◦ C for 24 h and then moved into humid laboratory air. The uncoated slide fogged immediately, whereas the slide coated with a superhydrophilic thin film remained transparent. These results indicated that the presence of mesopores in these films improved the superhydrophilicity of the films, leading to the instantaneous spreading of water droplets on the surface of the films. To check the persistence of superhydrophilicity of the thin films, they were exposed to air for longer times and the average contact angle measurement was performed during the time. Fig. 5 shows the measured average contact angles of water droplets on the surface of the films as a function of elapsed time. As can be seen, these mesoporous SiO2 thin films could keep superhydrophilic after exposed to air for 3 months. However, the superhydrophilic property of the films exposed to dirty air for 15 days was lost, due to the adsorption of organic pollutants on the surface of the films in air.
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
4337
Fig. 4. Results indicating the superhydrophilicity of mesoporous SiO2 thin films. (a) Changes in the contact angle of a water droplet on the mesoporous SiO2 thin films without UV light irradiation. (b) Images of a water droplet instantaneously wetting MS-6. (c) Fogging behavior of a glass slide coated with MS-6 (right) and an uncoated glass slide (left).
4. Conclusions The mesoporous SiO2 thin films were spin-coated on glass substrates by a simple sol–gel method. All of the prepared thin films were transparent. With the increase in the film thickness, thin films exhibited higher ordered mesoporous structure and more efficient superhydrophilicity. These films possess potential in the applications for anti-fogging and self-cleaning products.
Acknowledgements
Fig. 5. Contact angle of MS-6 of fresh and after exposed to air for 15 days and 3 months.
This work was supported by National Natural Science Foundation of China (Nos. 20807015, 21037001), Ph.D. Programs Foundation of Ministry of Education of China (No. 200805611055), the Fundamental Research Funds for the Central Universities, SCUT (2012ZZ0049), and the “Hundred-Scholar” Talent-Recruiting Program of South China University of Technology.
4338
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
References [1] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135. [3] I.P. Parkin, R.G. Palgrave, J. Mater. Chem. 15 (2005) 1689. [4] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188. [5] K. Guan, B. Lu, Y. Yin, Surf. Coat. Technol. 173 (2003) 219. [6] A. Borras, C. Lopez, V. Rico, F. Gracia, A.R. Gonzalez-Elipe, E. Richter, G. Battiston, R. Gerbasi, N. McSporran, G. Sauthier, J. Phys. Chem. C 111 (2007) 1801. [7] M. Maeda, S. Yamasaki, Thin Solid Films 483 (2005) 102. [8] M. Houmard, D. Riassetto, F. Roussel, A. Bourgeois, G. Berthome, J.C. Joud, M. Langlet, Appl. Surf. Sci. 254 (2007) 1405. [9] D. Lee, M.F. Rubner, R.E. Cohen, Nano Lett. 6 (2006) 2305. [10] J. Bico, C. Marzolin, D. Quere, Europhys. Lett. 47 (1999) 743. [11] J. Bico, U. Thiele, D. Quere, Colloid Surf. A 206 (2002) 41. [12] S. Ganjoo, R. Azimirad, O. Akhavan, A.Z. Moshfegh, J. Phys. D: Appl. Phys. 42 (2009) 0253022.
[13] N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry, P. Roach, Chem. Commun. 2005 (2005) 3135. [14] X.T. Zhang, O. Sato, M. Taguchi, Y. Einaga, T. Murakami, A. Fujishima, Chem. Mater. 17 (2005) 696. [15] V. Zorba, X.B. Chen, S.S. Mao, Appl. Phys. Lett. 96 (2010) 0937029. [16] L. Zhai, F.C. Cebeci, R.E. Cohen, M.F. Rubner, Nano Lett. 4 (2004) 1349. [17] F.C. Cebeci, Z.Z. Wu, L. Zhai, R.E. Cohen, M.F. Rubner, Langmuir 22 (2006) 2856. [18] H.C. Dong, P.L. Ye, M.J. Zhong, J. Pietrasik, R. Drumright, K. Matyjaszewski, Langmuir 26 (2010) 15567. [19] H. Yamashita, S. Nishio, I. Katayama, N. Nishiyama, H. Fujii, Catal. Today 111 (2006) 254. [20] R. Mokaya, J. Phys. Chem. B 103 (1999), 10204. [21] N. Nishiyama, S. Tanaka, Y. Egashira, Y. Oku, K. Ueyama, Chem. Mater. 14 (2002) 4229. [22] M. Grunm, I. Laure, K.K. Unger, Adv. Mater. 9 (1997) 254. [23] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. [24] Z.Y. Liu, X.T. Zhang, T. Murakami, A. Fujishima, Sol. Energ. Mat. Sol. C 92 (2008) 1434.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of superhydrophilic mesoporous SiO2 thin films Peiyi Chen, Yun Hu ∗ , Chaohai Wei The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, College of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 2 June 2011 Received in revised form 23 December 2011 Accepted 26 December 2011 Available online 31 December 2011 Keywords: Mesoporous SiO2 thin films Transparent Superhydrophilicity Antifogging
a b s t r a c t Using a simple sol–gel/spin-coating method, mesoporous SiO2 thin films were prepared on glass slides. All of the prepared thin films were colorless and transparent as original glass substrates. XRD and TEM measurements revealed that the prepared SiO2 thin films coated 3–12 times possess hexagonal mesoporous structure. The mesoporous SiO2 thin films performed the superhydrophilicity and antifogging property in the absence of UV light irradiation. The results suggested that the appropriate film thickness and mesoporous structures can improve the surface superhydrophilic behavior of SiO2 thin films. © 2012 Published by Elsevier B.V.
1. Introduction Surfaces with extreme wetting characteristics such as superhydrophilicity and superhydrophobicity play an important role in various applications. These phenomena have already been applied to some self-cleaning products such as windows glasses, tiles, mirrors and plastics. It is known that TiO2 films show a superhydrophilicity under ultraviolet (UV) light irradiation [1–3]. However, current applications are mainly limited to out-door use because the superhydrophilicity of TiO2 films rapidly vanishes if the UV light irradiation is removed [4,5]. Thus, many efforts have been taken to overcome this problem. Although some researchers reported visible light induced hydrophilicity on nitrogen doped TiO2 films, visible light irradiation did not cause the surfaces of these thin films to become superhydrophilic with water contact angles below 5◦ [6]. On the other hand, since SiO2 contains a mass of hydroxyl groups, many articles reported that SiO2 addition on TiO2 films could maintain the photo-induced superhydrophilicity for a certain time if the UV light irradiation was removed [7,8]. Daeyeon et al. reported that multifunctional thin films were fabricated from layer-by-layer deposition of TiO2 and SiO2 nanoparticles with superhydrophilic wetting characteristics, but the contact angle of water on a planar SiO2 coated surface was approximate 20◦ [9]. Early theory established by Quéré and co-workers suggests that it is possible to enhance the wetting property of a surface with
water by introducing roughness at the right scale[10,11]. Several research groups reported that micro/nanostructured surfaces exhibited superhydrophilic properties due to the capillary effect [12–15]. Based on above theories, both roughness and microstructure of the surfaces can affect the superhydrophilicity of thin films [13,16]. Recent attempts of a multilayer assembly of SiO2 nanoparticles and polymer showed superhydrophilicity in the absence of UV light irradiation [17,18]. Dong H et al. reported that the superhydrophilic polymer-SiO2 nanocomposite coatings were prepared by building up a hierarchical micro/nanostructure within the coating [18]. However, these superhydrophilic materials containing polymer were aged gradually over time. The methods related to multi-step coating procedures or specialized equipment, limiting the practical applications. Due to the importance of hydroxyl groups and surface microstructures or nanostructures on the superhydrophilic property, we designed mesoporous SiO2 thin films spin-coated on glass substrates with a simple sol–gel method. The mesoporous structure as well as the optical, superhydrophilic and antifogging properties of the thin films were investigated.
2. Experimental 2.1. Materials
∗ Corresponding author. Tel.: +86 20 39380573; fax: +86 20 39380573. E-mail address: [email protected] (Y. Hu). 0169-4332/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.apsusc.2011.12.109
Tetraethyl orthosilicate (TEOS, AR) was obtained from Tianjin Fu Chen chemical reagent factory. Anhydrous ethanol (EtOH, AR) was purchased from Tianjin Bai Shi chemical limited
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
4335
corporation. C12 H25 (OCH2 CH2 )4 OH (BrijR 30, AR) was obtained from Sigma–Aldrich. 2.2. Preparation of SiO2 thin films SiO2 sol was prepared via a sol–gel process. TEOS, EtOH and BrijR 30 were used as precursor, solvent and surfactant, respectively. A clear solution with the molar ratio of 8.0 TEOS:50 EtOH:0.9 BrijR 30:0.8 HCl was obtained after stirring for 10 min at 30 ◦ C. The sol was dripped onto a glass substrate at a spinning rate of 4000 rpm for 1 min. For the thin films, the spin-coating step was repeated until the desired coating times were obtained. The coating times were 1, 3, 6, 9, and 12. Therefore the obtained mesoporous SiO2 thin films were labeled as MS-1, MS-3, MS-6, MS-9, and MS-12, respectively. All the thin films were dried at room temperature for 2 h, and then calcined at 450 ◦ C for 5 h with a heating rate of 1 ◦ C/min. 2.3. Characterizations The mesostructures of the thin films were identified by X-ray diffraction (XRD, Bruker D8) measurement and transmission electron microscopy (TEM, Jeol, JEM-2100). The thickness of the films was measured by scanning electrons microscope (SEM, Nano430). Transmission measurements were carried out on a UV–vis spectrophotometer (Shimadzu, UV-2550). Water contact angles were obtained in atmospheric air at room temperature using a commercial contact angle meter (Dataphysics OCA 15plus) with ±1◦ accuracy. A droplet of water was injected on the surface of glass slide with a 1 l micro-injector. 3. Results and discussion 3.1. Mesoporous structure Fig. 1 shows the XRD patterns of the prepared SiO2 thin films on glass substrates. The XRD pattern of the film coated 1 time exhibited no peak, suggesting that the thickness of MS-1 was too thin to form mesoporous structure. The mesoporous SiO2 thin films coated 3–12 times reflected a single (1 0 0) diffraction peak at 2 = 2–3◦ , indicating the presence of ordered hexagonal mesoporous structure [19] The XRD pattern of MS-3 showed a weak and broad peak because of the unstable structure. With the increase in coating time, the XRD peak of the films became narrower and sharper and shifted to small degree, indicating that the highly ordered mesoporous structure existed within the films [20]. The lattice parameter (a0 ) of MS-3, MS-6, MS-9 and MS-12 estimated by the formula of √ a0 = (2d1 0 0 / 3) were about 4.3 nm, 4.4 nm, 4.5 nm and 4.5 nm, respectively.
Fig. 1. XRD patterns of mesoporous SiO2 thin films: (A) glass; (B) MS-1; (C) MS-3; (D) MS-6; (E) MS-9; (F) MS-12.
The TEM image of MS-6 (Fig. 2a) shows that the film had highly ordered mesoporous structure. Besides, it provided direct evidence for the presence of ordered hexagonal arrays aligned in the one-dimensional channel [21]. The average pore diameter of mesoporous structure in the SiO2 thin film estimated from the TEM image was about 4 nm, which was consistent with the XRD results. From the XRD results, the average pore diameter of MS6 was about 3.4 nm by using the formula (d = a0 − 1) [22]. These results confirmed the formation of mesoporous SiO2 thin film. The thicknesses of MS-6 (Fig. 2b), MS-9 and MS-12 determined by SEM measurements were about 340 nm, 480 nm, and 590 nm, respectively, indicating that the thickness of the thin films increased with the increase in the coating time.
3.2. Optical properties The photo images of the prepared mesoporous SiO2 thin films are shown in Fig. 3a. Although the transparency of the thin films became a little lower with increase in the number of coated time,
Fig. 2. (a) TEM image of MS-6 and (b) SEM image of MS-6.
4336
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
Fig. 3. (a) Photograph of mesoporous SiO2 thin films and (b) transmittance of mesoporous SiO2 thin films: (A) glass; (B) MS-1; (C) MS-3; (D) MS-6; (E) MS-9; (F) MS-12.
the films on the glass were colorless and transparent as original glass slide. Fig. 3b shows the transmittance spectra of a glass substrate and the mesoporous SiO2 thin films supported on the glass. The transmittance of glass substrate in the wavelength range of 350–800 nm was about 90%. The transmittance of the glass coated with mesoporous SiO2 thin films on one side became a little lower than that of the uncoated glass. However, all the films remained high level of 83–89%, indicating that the transmission level of the glass slide was unaffected by the coated mesoporous SiO2 thin films. 3.3. Superhydrophilic and antifogging properties Fig. 4a shows the changes in the contact angle of water droplets on the mesoporous SiO2 thin films without UV light irradiation. The contact angle of water became below 5◦ in 4 s on the film of MS-1, while it became below 5◦ in less than 1 s on the films coated 6–12 times, indicating that the superhydrophilicity increased with the increase in the coating times. The images (Fig. 4b) obtained from the video data showed that a drop of water wets the surface of MS6 thin film in less than 1 s to completely spread on the surface. The mechanism of such behavior can be understood from the simple relation derived by Wenzel and co-workers. It is well established that the apparent contact angle of a liquid on a surface depends on the roughness of the surface according to the following relation [23] cos a = r cos
(1)
where a is the apparent water contact angle on a rough surface and is the intrinsic contact angle as measured on a smooth surface. r
is the surface roughness defined as the ratio of the actual surface area over the project surface area. r becomes infinite for porous materials meaning that the surface will be completely wetted (i.e., a ≈ 0) with any liquid that has a contact angle (as measured on a smooth surface) of less than 90◦ . According to the previous works, the water contact angle of a nonporous SiO2 surface was 20–25◦ [9,24]. These observations indicated that the mesoporous SiO2 thin films could perform the superhydrophilic property even in the absence of UV light irradiation. The superhydrophilicity of the SiO2 thin films with meosoporous structure established ideal conditions for extreme wetting behavior. The images in Fig. 4c illustrate the antifogging behavior of the prepared thin films. Two glass slides, one (right) with a coating of the superhydrophilic mesoporous SiO2 thin films and the other (left) without such a coating, were cooled in a refrigerator at about −18 ◦ C for 24 h and then moved into humid laboratory air. The uncoated slide fogged immediately, whereas the slide coated with a superhydrophilic thin film remained transparent. These results indicated that the presence of mesopores in these films improved the superhydrophilicity of the films, leading to the instantaneous spreading of water droplets on the surface of the films. To check the persistence of superhydrophilicity of the thin films, they were exposed to air for longer times and the average contact angle measurement was performed during the time. Fig. 5 shows the measured average contact angles of water droplets on the surface of the films as a function of elapsed time. As can be seen, these mesoporous SiO2 thin films could keep superhydrophilic after exposed to air for 3 months. However, the superhydrophilic property of the films exposed to dirty air for 15 days was lost, due to the adsorption of organic pollutants on the surface of the films in air.
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
4337
Fig. 4. Results indicating the superhydrophilicity of mesoporous SiO2 thin films. (a) Changes in the contact angle of a water droplet on the mesoporous SiO2 thin films without UV light irradiation. (b) Images of a water droplet instantaneously wetting MS-6. (c) Fogging behavior of a glass slide coated with MS-6 (right) and an uncoated glass slide (left).
4. Conclusions The mesoporous SiO2 thin films were spin-coated on glass substrates by a simple sol–gel method. All of the prepared thin films were transparent. With the increase in the film thickness, thin films exhibited higher ordered mesoporous structure and more efficient superhydrophilicity. These films possess potential in the applications for anti-fogging and self-cleaning products.
Acknowledgements
Fig. 5. Contact angle of MS-6 of fresh and after exposed to air for 15 days and 3 months.
This work was supported by National Natural Science Foundation of China (Nos. 20807015, 21037001), Ph.D. Programs Foundation of Ministry of Education of China (No. 200805611055), the Fundamental Research Funds for the Central Universities, SCUT (2012ZZ0049), and the “Hundred-Scholar” Talent-Recruiting Program of South China University of Technology.
4338
P. Chen et al. / Applied Surface Science 258 (2012) 4334–4338
References [1] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135. [3] I.P. Parkin, R.G. Palgrave, J. Mater. Chem. 15 (2005) 1689. [4] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188. [5] K. Guan, B. Lu, Y. Yin, Surf. Coat. Technol. 173 (2003) 219. [6] A. Borras, C. Lopez, V. Rico, F. Gracia, A.R. Gonzalez-Elipe, E. Richter, G. Battiston, R. Gerbasi, N. McSporran, G. Sauthier, J. Phys. Chem. C 111 (2007) 1801. [7] M. Maeda, S. Yamasaki, Thin Solid Films 483 (2005) 102. [8] M. Houmard, D. Riassetto, F. Roussel, A. Bourgeois, G. Berthome, J.C. Joud, M. Langlet, Appl. Surf. Sci. 254 (2007) 1405. [9] D. Lee, M.F. Rubner, R.E. Cohen, Nano Lett. 6 (2006) 2305. [10] J. Bico, C. Marzolin, D. Quere, Europhys. Lett. 47 (1999) 743. [11] J. Bico, U. Thiele, D. Quere, Colloid Surf. A 206 (2002) 41. [12] S. Ganjoo, R. Azimirad, O. Akhavan, A.Z. Moshfegh, J. Phys. D: Appl. Phys. 42 (2009) 0253022.
[13] N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry, P. Roach, Chem. Commun. 2005 (2005) 3135. [14] X.T. Zhang, O. Sato, M. Taguchi, Y. Einaga, T. Murakami, A. Fujishima, Chem. Mater. 17 (2005) 696. [15] V. Zorba, X.B. Chen, S.S. Mao, Appl. Phys. Lett. 96 (2010) 0937029. [16] L. Zhai, F.C. Cebeci, R.E. Cohen, M.F. Rubner, Nano Lett. 4 (2004) 1349. [17] F.C. Cebeci, Z.Z. Wu, L. Zhai, R.E. Cohen, M.F. Rubner, Langmuir 22 (2006) 2856. [18] H.C. Dong, P.L. Ye, M.J. Zhong, J. Pietrasik, R. Drumright, K. Matyjaszewski, Langmuir 26 (2010) 15567. [19] H. Yamashita, S. Nishio, I. Katayama, N. Nishiyama, H. Fujii, Catal. Today 111 (2006) 254. [20] R. Mokaya, J. Phys. Chem. B 103 (1999), 10204. [21] N. Nishiyama, S. Tanaka, Y. Egashira, Y. Oku, K. Ueyama, Chem. Mater. 14 (2002) 4229. [22] M. Grunm, I. Laure, K.K. Unger, Adv. Mater. 9 (1997) 254. [23] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. [24] Z.Y. Liu, X.T. Zhang, T. Murakami, A. Fujishima, Sol. Energ. Mat. Sol. C 92 (2008) 1434.