- Email: [email protected]
High transmittance superhydrophilic thin film with superior mechanical properties
TSF-35161; No of Pages 5 Thin Solid Films xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
High transmittance superhydrophilic thin film with superior mechanical properties Ya-Chen Chang a,b, Hung-Sen Wei a,b,⁎, Chien-Cheng Kuo b,c, Wei-Bo Liao a,b, Sing-Rong Huang a,b, Cheng-Chung Lee a,b,⁎ a b c
Department of Optics and Photonics, National Central University, Taoyuan 320, Taiwan Thin Film Technology Center, National Central University, Taoyuan 320, Taiwan Graduate Institute of Energy Engineering, National Central University, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 11 April 2016 Accepted 19 April 2016 Available online xxxx Keywords: Mechanical properties Superhydrophilic Anti-reflection coating Thin films Sol–gel processes
a b s t r a c t A two-layer structure of superhydrophilic thin film with high transmittance and superior mechanical properties was fabricated and developed. Superhydrophilic thin film has been deposited on a pre-coated anti-reflection coating substrate. The average transmittance in the visible range (400–700 nm) was increased from 91.3 ± 0.1% to 94.2 ± 0.1%, because an anti-reflection coating was inserted between the superhydrophilic thin film and substrate. In particular, the mechanical properties of the high transmittance superhydrophilic thin film were also deeply investigated in this paper. The hardness of the thin film was greater than 3H. The superhydrophilic property was maintained after the thin film was pressed by a 500 g weight steel wool and moved back and forth for more than 100 times. According to ISO 2409 standard, the adhesion of the thin film is given a quality of rank 1. The water contact angle of the thin film was less than 10° after ultraviolet light irradiation and the superhydrophilicity was maintained for 76 h when stored in a dark place. The high transmittance superhydrophilic thin film with superior mechanical properties can be utilized in optical, environmental, superhydrophilic and photocatalytic applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In 1972, the research on the semiconductor photocatalyst of TiO2 was presented by Honda-Fujishima and named as the HondaFujishima effect [1]. It was found that when the TiO2 thin film was irradiated by ultraviolet, UV, light, the water contact angle, WCA, decreased gradually and finally, it became almost 0° [2–4]. The phenomenon was called superhydrophilicity. The superhydrophilic property of the surface allows water to spread across the surface rather than remaining as a droplet [5]. The superhydrophilic phenomenon brings about self-cleaning and anti-fogging properties [6]. In addition, transparent TiO 2 thin films have a high potential for practical applications such as on windows and windshields of automobile, mirrors [7]. Many methods have been used to fabricate superhydrophilic thin films, such as chemical vapor deposition [8,9], plasma-enhanced chemical vapor deposition [10,11], sputtering [12,13] and so on. However, they are expensive and bulky. Moreover, the shapes and species of substrates that can be coated using these methods are limited [14]. Sol–gel process is widely known as practical ⁎ Corresponding authors at: Department of Optics and Photonics, National Central University, Taoyuan 320, Taiwan. E-mail addresses: [email protected] (H.-S. Wei), [email protected] (C.-C. Lee).
and effective for fabricating superhydrophilic thin films on various substrates [15–17]. Sol–gel technique provides unique benefits such as fabrication on large-area substrates and low-temperature synthesis, and it is simple to implement [18,19]. The solar flux of UV incidents at the earth's surface is less than 5%. To establish the fully developed superhydrophilic state, the superhydrophilic thin film had to be irradiated for several days. Besides, the superhydrophilic thin film is not always irradiated by UV light such as in a rainy or cloudy day. Therefore, it is desirable that the WCA stays low for a long time in dark. In general, a superhydrophilic thin film which consists of only TiO2, the WCA increases quickly in a dark place. It was found that adding SiO2 to TiO2, WCA was lower and the hydrophilicity was maintained longer in a dark place. However, the transmittance and mechanical properties were not investigated in [20]. For the optical applications, the transmittance is an important factor to be considered. Optical reflection is a fundamental phenomenon occurring when light propagates across a boundary between different refractive index media. Anti-reflection, AR, coatings play a pivotal role in a wide variety of optical technologies by reducing reflective losses at interfaces [21]. Optical elements based on glass and common plastics have refraction indices (n) in the range of 1.45–1.7 [22]. As the results, there are 4% to 6.5% reflectance from each air–substrate interface. The reflection of substrate can lead to a blur image and the reduction of
http://dx.doi.org/10.1016/j.tsf.2016.04.028 0040-6090/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
2
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
transmittance. The reduction of surface reflection can be achieved by applying an AR coating on the substrates [23–25]. It is therefore necessary to reduce the intensity of reflected light to improve the overall quality of systems. Macleod software was used to simulate the thickness of each layer of the AR coating. Not only transmittance but also the mechanical properties of superhydrophilic thin films such as adhesion, hardness and abrasion resistance must be considered for practical applications. However, the research of TiO2 + SiO2 composite thin film about mechanical properties is seldom discussed. In previous studies, Fateh et al. [26] reported that the thin film made by different molar ratio of TiO2 and SiO2 on PC. Although the adhesion of the film was measured, the other mechanical properties were not discussed in their study. Besides, several works have shown that the addition of SiO2 into TiO2 films and focus on the characterizations of function group, surface analysis, crystalline and the ratio of TiO2 + SiO2 [3,5,27]. Damchan et al. [28] reported the WCA change of TiO2 ± SiO2 composite film in different mol% of SiO2 and investigated the photocatalytic activity and hydrophilicity fraction. Yu et al. [29] studied the anatase peak change of X-ray diffraction, XRD, of a TiO2 ± SiO2 composite thin film in different mol% of SiO2. Tricoli et al. [30] reported that the WCA and antifogging property of SiO2, TiO2 and TiO2 ± SiO2 with different thickness. Eshaghi et al. [31] prepared nanocomposite TiO2 ± SiO2 thin film by sol–gel process and reported the surface analysis of the film by XRD and X-ray photoelectron spectroscopy, XPS. Pakdel et al. [32] used TiO2 and TiO2 ± SiO2 nanocomposites to coat on wool fabrics and studied the self-cleaning property and hydrophilicity of the TiO2 ± SiO2 nanocomposite films based on different molar ratio percentages of TiO2 ± SiO2. Liu et al. [13] also reported XRD patterns and XPS characterization of TiO2 ± SiO2 films. Unfortunately, these studies had never focused on mechanical properties. In this work, the paper deeply studied and discussed mechanical properties including hardness, adhesion and abrasion resistance and the transmittance of superhydrophilic thin film which was increased when an AR coating was inserted.
Fig. 1. SEM image of high transmittance superhydrophilic thin film.
2.4. Preparation of TiO2 + SiO2 superhydrophilic thin film Superhydrophilic thin film was fabricated by TiO2 + SiO2 solution which was prepared by mixing a TiO2 dispersion solution and a SiO2 sol. To obtain a good TiO2 dispersion solution, the pH value of EtOH was adjusted to pH = 1 and the surfactant (SDS) was added to the EtOH. Additionally, SiO2 sol was typically prepared by the sol–gel process. The molar ratio of SiO2 sol was TEOS:EtOH:deionized water = 1:7.88:5. Subsequently, a 0.02 ml volume of HCl (3.6%) was added into above SiO2 sol to catalyze the hydrolysis followed by stirring the solution 1 h at room temperature. Finally, TiO2 + SiO2 solution was obtained by TiO2 dispersion solution mixing with SiO2 sol. The optimized molar ratio of TiO2:SiO2:EtOH was 1:1.78:371. 2.5. High transmittance superhydrophilic thin film preparation
2. Experiment 2.1. Chemicals and materials Titanium dioxide nanoparticles (Sachtleben Hombikat UV100) that were smaller than 10 nm were obtained from Sachtleben Chemie. Tetraethoxysilane (TEOS) was purchased from Seedchem. Hydrochloric acid (HCl) and anhydrous ethanol (EtOH) were purchased from Showa Chemical Co., Ltd. and Champion Yuan Co., Ltd., respectively. Sodium dodecyl sulfate (SDS) was obtained from Acros Organics. The water used herein was deionized. The above chemicals were all used without further purification. 2.2. High transmittance superhydrophilic thin film structure The SEM image of structure of high transmittance superhydrophilic thin film was formed on a Si wafer/AR coating/superhydrophilic thin film, as displayed in Fig. 1. The superhydrophilic thin film and AR coating was fabricated by sol–gel process and electron gun evaporation, respectively.
Substrate (B270 glass) was cleaned in acetone and isopropanol for 10 min by ultrasonic bath and then dried by using a N2 gun. The TiO2 + SiO2 solution was deposited onto a pre-coated AR coating glass by dip-coating with 2 mm/s withdrawal rate. And then the thin films thus formed by dried in an oven at 150 °C for 1 h. 2.6. Instrumentation and measurements WCA was measured by a water contact angle measurement apparatus (Pentad Scientific Corporation, FTA-125) and the volume of deionized water droplets was 2 μL. The transmission spectra were measured using a UV–vis spectrophotometer (Hitachi, U-4100). The adhesion and hardness of the film were estimated quantitatively by using ISO 2409 standard and the pencil test by using ASTM D 3363 standard, respectively. The film was illuminated by UV light with a UV intensity of 4 mW/cm2 under germicidal lamps that efficiently transmit ultraviolet rays at 253.7 nm (Sankyo Denki, G10T8). 3. Results and discussion
2.3. Preparation of AR coating
3.1. Optical properties of the high transmittance superhydrophilic thin film
The structure of AR coating is substrate/(HL)2/air. Ta2O5 (n = 2.21) and SiO2 (n = 1.46) were selected as the high (H) refractive index and low (L) refractive index materials at the central wavelength λ0 = 550 nm, respectively. AR coating was produced by electron gun evaporation with ion-beam-assisted deposition. From substrate to air, the thicknesses of each layer were 13.24 nm, 33.71 nm, 119.92 nm and 87.65 nm. And the optical thickness was monitored by an optical monitor.
For the practical application of the superhydrophilic thin film, the transmittance is an important factor to be considered. The transmittance was reduced when a TiO2 + SiO2 superhydrophilic thin film was coated on a substrate. The refraction index of TiO 2 + SiO2 superhydrophilic thin film is 1.75 as calculated by using Macleod software from the transmittance spectra. In this work, AR coating was the key point to eliminate the reflection from the substrate and to increase the transmittance of the substrate. Macleod software
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
was utilized to simulate the thickness of each layer of AR coating. AR coating was composed of 4 layers (HLHL) and was only coated on one side of the substrate. The experimental and simulated transmittance spectra of the substrate (B270 glass)/AR coating were shown in Fig. 2(a). It showed that the experimental and simulated transmittance spectra of the substrate (B270 glass)/AR coating were almost the same. And then, the superhydrophilic thin film was deposited on a bare glass and AR-coated glass, respectively. The average transmittance spectra of the glass/TiO2 + SiO 2 assembly (5 samples) and glass/AR coating/TiO2 + SiO2 assembly (5 samples) are shown in Fig. 2(b). It is clearly that the transmittance of the glass/AR coating/TiO 2 + SiO 2 assembly is obviously higher than the glass/TiO2 + SiO2 assembly. The result indicated that the average transmittance in visible wavelength (400–700 nm) was increased from 91.3 ± 0.1% to 94. ± 0.1%, when AR coating was pre-coated on substrate.
3
Fig. 3. XRD pattern of high transmittance superhydrophilic thin film.
3.2. WCA of high transmittance superhydrophilic thin film The superhydrophilicity of high transmittance superhydrophilic thin film was resulted from a hybrid TiO2 ± SiO2 layer after being irradiated by UV light. The XRD pattern of the thin film was shown as Fig. 3. It exhibited that diffraction peaks at 25° and 48° indicating the TiO2 of TiO2 ± SiO2 layer in the anatase phase. Fig. 4 shows that the change of WCA of the high transmittance superhydrophilic thin film during UV light irradiation immediately after the preparation and during storage in a dark place (opaque acrylic box). The results indicated that the WCA of the thin film was only marginally wettable after preparation. While, upon UV light irradiation for 4 h, the WCA of the thin film decreased to values less than 10°. In addition, the thin film showed superhydrophilicity during storage in a dark place. After 76 h in a dark place, the thin film still exhibited significantly low WCA. The phenomenon of this superhydrophilicity is a characteristic feature of TiO2 + SiO2 thin film. For 96 h storage in dark, the WCA of the thin film increased to approximately 40°. As the thin film was irradiated by UV light again, the WCA of the thin film returned to its initial value and then showed superhydrophilicity again. The above result indicated that the superhydrophilicity of our coatings can be maintained at a low WCA for more than three days and is repeatable as irradiated by a UV light. 3.3. Mechanical properties of high transmittance superhydrophilic thin film For actual application, the mechanical properties such as adhesion, hardness and abrasion resistance of high transmittance superhydrophilic thin films must also be considered. The hardness of the thin film was obtained by performing pencil hardness test according to ASTM D 3363 standard [33]. An optical microscope was utilized to observe the degree of damages after performing pencil hardness test as shown in Fig. 5. Compared with optical microscope image of the thin film before and after treatment by 3H pencil, the thin film revealed no damage. It indicated that the hardness of the thin film was greater than 3H. In addition, the
Fig. 4. Change of WCA of highly transmittance superhydrophilic thin film (a) during UV light irradiation after the preparation, (b) during storage in the dark, and (c) during subsequent UV light irradiation.
adhesion of the thin film was also estimated quantitatively by using ISO 2409 method [34]. According to the standard, the quality of adhesion is ranked by different numbers ranging from 0 (excellent with 0%) to 5 (very poor with N65%). The thin film was crisscrossed with a razor blade to form small squares to facilitate the removal. 3M scotch tape was pressed and pasted on the thin film surface and then removed applying a constant force at an angle of 60°. Fig. 6 shows the optical images of the thin film after adhesion test. It was observed that the crumbling of the thin film was less than 5%. It indicated that the thin film was quite stable and adhered well. According to ISO 2409, the quality of the thin film can be ranked as 1. Furthermore, the abrasion resistance of the
Fig. 2. (a) Transmittance spectra of simulated and experimental AR coating and (b) transmittance spectra of glass/TiO2 + SiO2 assembly and glass/AR coating/TiO2 + SiO2 assembly.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
4
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
Fig. 5. Optical image of high transmittance superhydrophilic thin film before and after a pencil hardness test.
thin film was also examined by a simple method. Using steel wool which was pressed by a 500 g weight parallel to substrate back and forth to friction the thin film. And the WCA was used to determine the abrasion resistance of the thin film. Fig. 7 shows that the change of WCA of the thin film in different friction times. And optical images of water droplets are inserted in Fig. 7. The WCA of the thin film was still smaller than 10° after 100 times friction. The results indicated that the superhydrophilic phenomenon can be maintained after the thin film was treated by the above-mentioned abrasion resistance test more than 100 times. In this work, the sol–gel process was utilized to improve the mechanical properties of the thin film. The hydrolysis reaction is faster than
condensation reaction and the dense structure is obtained in acid catalyzed [35]. Here, the TiO2 + SiO2 solution was in acid catalyzed reaction (pH = 1) to make the TiO2 + SiO2 thin film more densified. It means that the hydroxyl group in the hybrid film was increased. And the hydroxyl groups can result in the Van der Waals force and the hydrogen bonding between the thin films and substrate. In addition, a TiO2 + SiO2 solution was also prepared without any catalyst in this work. But, the experimental results showed that the thin film has poor mechanical properties, when a solution was prepared without any catalyst. Therefore, the result of the thin film which was fabricated in an acid catalyzed reaction showed not only high transmittance and lasting superhydrophilicity but also superior mechanical properties. Our results suggest that the high transmittance superhydrophilic thin film with superior mechanical properties can be applied for optical, environmental, superhydrophilic and photocatalytic applications. 4. Conclusions
Fig. 6. Optical image of high transmittance superhydrophilic thin film after applying a cross cut.
High transmittance superhydrophilic thin film with superior mechanical properties was developed and fabricated. When AR coating was applied, the average transmittance of the thin film in the visible wavelength (400–700 nm) was increased from 91.3 ± 0.1% to 94.2 ± 0.1%. The thin films have favorable superhydrophilicity (WCA b 10°) and can be maintained 76 h when stored in a dark place. And the 3H hardness of the thin film was obtained by ASTM D 3363. The thin film can tolerate more than 100 times steel wool abrasion test which was pressed by 500 g weight. According to the ISO 2409 standard, the adhesion of the thin film is in a quality of rank 1. Above results show that the high transmittance superhydrophilic thin film has superior mechanical properties and optical property. Acknowledgments The authors would like to thank the Central Taiwan Science Park and Ministry of Science and Technology, for financially supporting this research under Contract No. 103RB02 and MOST 104-2221-E-008-002. References
Fig. 7. Abrasion resistance of high transmittance superhydrophilic thin film in different friction times.
[1] A. Fujishima, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] K. Hashimoto, H. Irie, A. Fujishima, TiO2 photocatalysis: a historical overview and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269. [3] S. Permpoon, G. Berthomé, B. Baroux, J.C. Joud, M. Langlet, Natural superhydrophilicity of sol–gel derived SiO2–TiO2 composite films, J. Mater. Sci. 41 (2006) 7650–7662. [4] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431–432. [5] K. Guan, Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films, Surf. Coat. Technol. 191 (2005) 155–160. [6] J. Son, S. Kundu, L.K. Verma, M. Sakhuja, A.J. Danner, C.S. Bhatia, H. Yang, A practical superhydrophilic self cleaning and antireflective surface for outdoor photovoltaic applications, Sol. Energy Mater. Sol. Cells 98 (2012) 46–51.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx [7] T. Watanabe, S. Fukayama, M. Miyauchi, A. Fujishima, K. Hashimoto, Photocatalytic activity and photo-induced wettability conversion of TiO2 thin film prepared by sol–gel process on a soda-lime glass, J. Sol-Gel Sci. Technol. 19 (2000) 71–76. [8] Z. Ding, X. Hu, G.Q. Lu, P.-L. Yue, P.F. Greenfield, Novel silica gel supported TiO2 photocatalyst synthesized by CVD method, Langmuir 16 (2000) 6216–6222. [9] P. Evans, M.E. Pemble, D.W. Sheel, Precursor-directed control of crystalline type in atmospheric pressure CVD growth of TiO2 on stainless steel, Chem. Mater. 18 (2006) 5750–5755. [10] P. Hajkova, P. Spatenka, J. Krumeich, P. Exnar, A. Kolouch, J. Matousek, The influence of surface treatment on photocatalytic activity of PE CVD TiO2 thin films, Plasma Process. Polym. 6 (2009) S735–S740. [11] A. Borras, A. Barranco, A.N.R. González-Elipe, Reversible superhydrophobic to superhydrophilic conversion of Ag@ TiO2 composite nanofiber surfaces, Langmuir 24 (2008) 8021–8026. [12] N. Kanai, T. Nuida, K. Ueta, K. Hashimoto, T. Watanabe, H. Ohsaki, Photocatalytic efficiency of TiO2/SnO2 thin film stacks prepared by DC magnetron sputtering, Vacuum 74 (2004) 723–727. [13] Y. Liu, L. Qian, C. Guo, X. Jia, J. Wang, W. Tang, Natural superhydrophilic TiO2/SiO2 composite thin films deposited by radio frequency magnetron sputtering, J. Alloys Compd. 479 (2009) 532–535. [14] J.D. Mackenzie, E.P. Bescher, Chemical routes in the synthesis of nanomaterials using the sol–gel process, Acc. Chem. Res. 40 (2007) 810–818. [15] M. Qu, B. Zhang, S. Song, L. Chen, J. Zhang, X. Cao, Fabrication of superhydrophobic surfaces on engineering materials by a solution–immersion process, Adv. Funct. Mater. 17 (2007) 593–596. [16] Q.F. Xu, J.N. Wang, K.D. Sanderson, Organic–inorganic composite nanocoatings with superhydrophobicity, good transparency, and thermal stability, ACS Nano 4 (2010) 2201–2209. [17] H. Zhou, H. Wang, H. Niu, A. Gestos, X. Wang, T. Lin, Fluoroalkyl silane modified silicone rubber/nanoparticle composite: a super durable, robust superhydrophobic fabric coating, Adv. Mater. 24 (2012) 2409–2412. [18] M. Langlet, A. Kim, M. Audier, C. Guillard, J. Herrmann, Transparent photocatalytic films deposited on polymer substrates from sol–gel processed titania sols, Thin Solid Films 429 (2003) 13–21. [19] H.S. Wei, C.C. Kuo, C.C. Jaing, Y.C. Chang, C.C. Lee, Highly transparent superhydrophobic thin film with low refractive index prepared by one-step coating of modified silica nanoparticles, J. Sol-Gel Sci. Technol. 71 (2014) 168–175.
5
[20] M. Machida, K. Norimoto, T.e. Watanabe, K. Hashimoto, A. Fujishima, The effect of SiO2 addition in super-hydrophilic property of TiO2 photocatalyst, J. Mater. Sci. 34 (1999) 2569–2574. [21] J.A. Hiller, J.D. Mendelsohn, M.F. Rubner, Reversibly erasable nanoporous antireflection coatings from polyelectrolyte multilayers, Nat. Mater. 1 (2002) 59–63. [22] H. Macleod, T.-F.O. Filters, American Elsevier Publishing Company, Inc., New York, New York, pp. ix-x (1969) 37–42. [23] Z. Liu, X. Zhang, T. Murakami, A. Fujishima, Sol–gel SiO2/TiO2 bilayer films with selfcleaning and antireflection properties, Sol. Energy Mater. Sol. Cells 92 (2008) 1434–1438. [24] M.S. Park, Y. Lee, J.K. Kim, One-step preparation of antireflection film by spin-coating of polymer/solvent/nonsolvent ternary system, Chem. Mater. 17 (2005) 3944–3950. [25] B.G. Prevo, Y. Hwang, O.D. Velev, Convective assembly of antireflective silica coatings with controlled thickness and refractive index, Chem. Mater. 17 (2005) 3642–3651. [26] R. Fateh, R. Dillert, D. Bahnemann, Preparation and characterization of transparent hydrophilic photocatalytic TiO2/SiO2 thin films on polycarbonate, Langmuir 29 (2013) 3730–3739. [27] S.S. Latthe, S. Liu, C. Terashima, K. Nakata, A. Fujishima, Transparent, adherent, and photocatalytic SiO2–TiO2 coatings on polycarbonate for self-cleaning applications, Coatings 4 (2014) 497–507. [28] J. Damchan, L. Sikong, K. Kooptarnond, S. Niyomwas, Contact angle of glass substrate coated with TiO2/SiO2 thin film, CMU J. Nat. Sci. 7 (2008) 19–23. [29] J. Yu, X. Zhao, J.C. Yu, G. Zhong, J. Han, Q. Zhao, The grain size and surface hydroxyl content of super-hydrophilic TiO2/SiO2 composite nanometer thin films, J. Mater. Sci. Lett. 20 (2001) 1745–1748. [30] A. Tricoli, M. Righettoni, S.E. Pratsinis, Anti-fogging nanofibrous SiO2 and nanostructured SiO2–TiO2 films made by rapid flame deposition and in situ annealing, Langmuir 25 (2009) 12578–12584. [31] A. Eshaghi, A. Dashti, A. Eshaghi, R. Mozaffarinia, Photo-induced superhydrophilicity of nanocomposite TiO2–SiO2 thin film, Mater. Sci. Pol. 29 (2011) 22–28. [32] E. Pakdel, W.A. Daoud, X. Wang, Self-cleaning and superhydrophilic wool by TiO2/SiO2 nanocomposite, Appl. Surf. Sci. 275 (2013) 397–402. [33] ASTM D 3363, Standard Test Method for Film Hardness by Pencil Test, ASTM international, 2000. [34] DIN EN ISO 2409, Cross-cut Test, 2007. [35] J.D. Wright, N.A. Sommerdijk, Sol–Gel Materials: Chemistry and Applications, CRC press, 2000.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
High transmittance superhydrophilic thin film with superior mechanical properties Ya-Chen Chang a,b, Hung-Sen Wei a,b,⁎, Chien-Cheng Kuo b,c, Wei-Bo Liao a,b, Sing-Rong Huang a,b, Cheng-Chung Lee a,b,⁎ a b c
Department of Optics and Photonics, National Central University, Taoyuan 320, Taiwan Thin Film Technology Center, National Central University, Taoyuan 320, Taiwan Graduate Institute of Energy Engineering, National Central University, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 16 November 2015 Received in revised form 11 April 2016 Accepted 19 April 2016 Available online xxxx Keywords: Mechanical properties Superhydrophilic Anti-reflection coating Thin films Sol–gel processes
a b s t r a c t A two-layer structure of superhydrophilic thin film with high transmittance and superior mechanical properties was fabricated and developed. Superhydrophilic thin film has been deposited on a pre-coated anti-reflection coating substrate. The average transmittance in the visible range (400–700 nm) was increased from 91.3 ± 0.1% to 94.2 ± 0.1%, because an anti-reflection coating was inserted between the superhydrophilic thin film and substrate. In particular, the mechanical properties of the high transmittance superhydrophilic thin film were also deeply investigated in this paper. The hardness of the thin film was greater than 3H. The superhydrophilic property was maintained after the thin film was pressed by a 500 g weight steel wool and moved back and forth for more than 100 times. According to ISO 2409 standard, the adhesion of the thin film is given a quality of rank 1. The water contact angle of the thin film was less than 10° after ultraviolet light irradiation and the superhydrophilicity was maintained for 76 h when stored in a dark place. The high transmittance superhydrophilic thin film with superior mechanical properties can be utilized in optical, environmental, superhydrophilic and photocatalytic applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In 1972, the research on the semiconductor photocatalyst of TiO2 was presented by Honda-Fujishima and named as the HondaFujishima effect [1]. It was found that when the TiO2 thin film was irradiated by ultraviolet, UV, light, the water contact angle, WCA, decreased gradually and finally, it became almost 0° [2–4]. The phenomenon was called superhydrophilicity. The superhydrophilic property of the surface allows water to spread across the surface rather than remaining as a droplet [5]. The superhydrophilic phenomenon brings about self-cleaning and anti-fogging properties [6]. In addition, transparent TiO 2 thin films have a high potential for practical applications such as on windows and windshields of automobile, mirrors [7]. Many methods have been used to fabricate superhydrophilic thin films, such as chemical vapor deposition [8,9], plasma-enhanced chemical vapor deposition [10,11], sputtering [12,13] and so on. However, they are expensive and bulky. Moreover, the shapes and species of substrates that can be coated using these methods are limited [14]. Sol–gel process is widely known as practical ⁎ Corresponding authors at: Department of Optics and Photonics, National Central University, Taoyuan 320, Taiwan. E-mail addresses: [email protected] (H.-S. Wei), [email protected] (C.-C. Lee).
and effective for fabricating superhydrophilic thin films on various substrates [15–17]. Sol–gel technique provides unique benefits such as fabrication on large-area substrates and low-temperature synthesis, and it is simple to implement [18,19]. The solar flux of UV incidents at the earth's surface is less than 5%. To establish the fully developed superhydrophilic state, the superhydrophilic thin film had to be irradiated for several days. Besides, the superhydrophilic thin film is not always irradiated by UV light such as in a rainy or cloudy day. Therefore, it is desirable that the WCA stays low for a long time in dark. In general, a superhydrophilic thin film which consists of only TiO2, the WCA increases quickly in a dark place. It was found that adding SiO2 to TiO2, WCA was lower and the hydrophilicity was maintained longer in a dark place. However, the transmittance and mechanical properties were not investigated in [20]. For the optical applications, the transmittance is an important factor to be considered. Optical reflection is a fundamental phenomenon occurring when light propagates across a boundary between different refractive index media. Anti-reflection, AR, coatings play a pivotal role in a wide variety of optical technologies by reducing reflective losses at interfaces [21]. Optical elements based on glass and common plastics have refraction indices (n) in the range of 1.45–1.7 [22]. As the results, there are 4% to 6.5% reflectance from each air–substrate interface. The reflection of substrate can lead to a blur image and the reduction of
http://dx.doi.org/10.1016/j.tsf.2016.04.028 0040-6090/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
2
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
transmittance. The reduction of surface reflection can be achieved by applying an AR coating on the substrates [23–25]. It is therefore necessary to reduce the intensity of reflected light to improve the overall quality of systems. Macleod software was used to simulate the thickness of each layer of the AR coating. Not only transmittance but also the mechanical properties of superhydrophilic thin films such as adhesion, hardness and abrasion resistance must be considered for practical applications. However, the research of TiO2 + SiO2 composite thin film about mechanical properties is seldom discussed. In previous studies, Fateh et al. [26] reported that the thin film made by different molar ratio of TiO2 and SiO2 on PC. Although the adhesion of the film was measured, the other mechanical properties were not discussed in their study. Besides, several works have shown that the addition of SiO2 into TiO2 films and focus on the characterizations of function group, surface analysis, crystalline and the ratio of TiO2 + SiO2 [3,5,27]. Damchan et al. [28] reported the WCA change of TiO2 ± SiO2 composite film in different mol% of SiO2 and investigated the photocatalytic activity and hydrophilicity fraction. Yu et al. [29] studied the anatase peak change of X-ray diffraction, XRD, of a TiO2 ± SiO2 composite thin film in different mol% of SiO2. Tricoli et al. [30] reported that the WCA and antifogging property of SiO2, TiO2 and TiO2 ± SiO2 with different thickness. Eshaghi et al. [31] prepared nanocomposite TiO2 ± SiO2 thin film by sol–gel process and reported the surface analysis of the film by XRD and X-ray photoelectron spectroscopy, XPS. Pakdel et al. [32] used TiO2 and TiO2 ± SiO2 nanocomposites to coat on wool fabrics and studied the self-cleaning property and hydrophilicity of the TiO2 ± SiO2 nanocomposite films based on different molar ratio percentages of TiO2 ± SiO2. Liu et al. [13] also reported XRD patterns and XPS characterization of TiO2 ± SiO2 films. Unfortunately, these studies had never focused on mechanical properties. In this work, the paper deeply studied and discussed mechanical properties including hardness, adhesion and abrasion resistance and the transmittance of superhydrophilic thin film which was increased when an AR coating was inserted.
Fig. 1. SEM image of high transmittance superhydrophilic thin film.
2.4. Preparation of TiO2 + SiO2 superhydrophilic thin film Superhydrophilic thin film was fabricated by TiO2 + SiO2 solution which was prepared by mixing a TiO2 dispersion solution and a SiO2 sol. To obtain a good TiO2 dispersion solution, the pH value of EtOH was adjusted to pH = 1 and the surfactant (SDS) was added to the EtOH. Additionally, SiO2 sol was typically prepared by the sol–gel process. The molar ratio of SiO2 sol was TEOS:EtOH:deionized water = 1:7.88:5. Subsequently, a 0.02 ml volume of HCl (3.6%) was added into above SiO2 sol to catalyze the hydrolysis followed by stirring the solution 1 h at room temperature. Finally, TiO2 + SiO2 solution was obtained by TiO2 dispersion solution mixing with SiO2 sol. The optimized molar ratio of TiO2:SiO2:EtOH was 1:1.78:371. 2.5. High transmittance superhydrophilic thin film preparation
2. Experiment 2.1. Chemicals and materials Titanium dioxide nanoparticles (Sachtleben Hombikat UV100) that were smaller than 10 nm were obtained from Sachtleben Chemie. Tetraethoxysilane (TEOS) was purchased from Seedchem. Hydrochloric acid (HCl) and anhydrous ethanol (EtOH) were purchased from Showa Chemical Co., Ltd. and Champion Yuan Co., Ltd., respectively. Sodium dodecyl sulfate (SDS) was obtained from Acros Organics. The water used herein was deionized. The above chemicals were all used without further purification. 2.2. High transmittance superhydrophilic thin film structure The SEM image of structure of high transmittance superhydrophilic thin film was formed on a Si wafer/AR coating/superhydrophilic thin film, as displayed in Fig. 1. The superhydrophilic thin film and AR coating was fabricated by sol–gel process and electron gun evaporation, respectively.
Substrate (B270 glass) was cleaned in acetone and isopropanol for 10 min by ultrasonic bath and then dried by using a N2 gun. The TiO2 + SiO2 solution was deposited onto a pre-coated AR coating glass by dip-coating with 2 mm/s withdrawal rate. And then the thin films thus formed by dried in an oven at 150 °C for 1 h. 2.6. Instrumentation and measurements WCA was measured by a water contact angle measurement apparatus (Pentad Scientific Corporation, FTA-125) and the volume of deionized water droplets was 2 μL. The transmission spectra were measured using a UV–vis spectrophotometer (Hitachi, U-4100). The adhesion and hardness of the film were estimated quantitatively by using ISO 2409 standard and the pencil test by using ASTM D 3363 standard, respectively. The film was illuminated by UV light with a UV intensity of 4 mW/cm2 under germicidal lamps that efficiently transmit ultraviolet rays at 253.7 nm (Sankyo Denki, G10T8). 3. Results and discussion
2.3. Preparation of AR coating
3.1. Optical properties of the high transmittance superhydrophilic thin film
The structure of AR coating is substrate/(HL)2/air. Ta2O5 (n = 2.21) and SiO2 (n = 1.46) were selected as the high (H) refractive index and low (L) refractive index materials at the central wavelength λ0 = 550 nm, respectively. AR coating was produced by electron gun evaporation with ion-beam-assisted deposition. From substrate to air, the thicknesses of each layer were 13.24 nm, 33.71 nm, 119.92 nm and 87.65 nm. And the optical thickness was monitored by an optical monitor.
For the practical application of the superhydrophilic thin film, the transmittance is an important factor to be considered. The transmittance was reduced when a TiO2 + SiO2 superhydrophilic thin film was coated on a substrate. The refraction index of TiO 2 + SiO2 superhydrophilic thin film is 1.75 as calculated by using Macleod software from the transmittance spectra. In this work, AR coating was the key point to eliminate the reflection from the substrate and to increase the transmittance of the substrate. Macleod software
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
was utilized to simulate the thickness of each layer of AR coating. AR coating was composed of 4 layers (HLHL) and was only coated on one side of the substrate. The experimental and simulated transmittance spectra of the substrate (B270 glass)/AR coating were shown in Fig. 2(a). It showed that the experimental and simulated transmittance spectra of the substrate (B270 glass)/AR coating were almost the same. And then, the superhydrophilic thin film was deposited on a bare glass and AR-coated glass, respectively. The average transmittance spectra of the glass/TiO2 + SiO 2 assembly (5 samples) and glass/AR coating/TiO2 + SiO2 assembly (5 samples) are shown in Fig. 2(b). It is clearly that the transmittance of the glass/AR coating/TiO 2 + SiO 2 assembly is obviously higher than the glass/TiO2 + SiO2 assembly. The result indicated that the average transmittance in visible wavelength (400–700 nm) was increased from 91.3 ± 0.1% to 94. ± 0.1%, when AR coating was pre-coated on substrate.
3
Fig. 3. XRD pattern of high transmittance superhydrophilic thin film.
3.2. WCA of high transmittance superhydrophilic thin film The superhydrophilicity of high transmittance superhydrophilic thin film was resulted from a hybrid TiO2 ± SiO2 layer after being irradiated by UV light. The XRD pattern of the thin film was shown as Fig. 3. It exhibited that diffraction peaks at 25° and 48° indicating the TiO2 of TiO2 ± SiO2 layer in the anatase phase. Fig. 4 shows that the change of WCA of the high transmittance superhydrophilic thin film during UV light irradiation immediately after the preparation and during storage in a dark place (opaque acrylic box). The results indicated that the WCA of the thin film was only marginally wettable after preparation. While, upon UV light irradiation for 4 h, the WCA of the thin film decreased to values less than 10°. In addition, the thin film showed superhydrophilicity during storage in a dark place. After 76 h in a dark place, the thin film still exhibited significantly low WCA. The phenomenon of this superhydrophilicity is a characteristic feature of TiO2 + SiO2 thin film. For 96 h storage in dark, the WCA of the thin film increased to approximately 40°. As the thin film was irradiated by UV light again, the WCA of the thin film returned to its initial value and then showed superhydrophilicity again. The above result indicated that the superhydrophilicity of our coatings can be maintained at a low WCA for more than three days and is repeatable as irradiated by a UV light. 3.3. Mechanical properties of high transmittance superhydrophilic thin film For actual application, the mechanical properties such as adhesion, hardness and abrasion resistance of high transmittance superhydrophilic thin films must also be considered. The hardness of the thin film was obtained by performing pencil hardness test according to ASTM D 3363 standard [33]. An optical microscope was utilized to observe the degree of damages after performing pencil hardness test as shown in Fig. 5. Compared with optical microscope image of the thin film before and after treatment by 3H pencil, the thin film revealed no damage. It indicated that the hardness of the thin film was greater than 3H. In addition, the
Fig. 4. Change of WCA of highly transmittance superhydrophilic thin film (a) during UV light irradiation after the preparation, (b) during storage in the dark, and (c) during subsequent UV light irradiation.
adhesion of the thin film was also estimated quantitatively by using ISO 2409 method [34]. According to the standard, the quality of adhesion is ranked by different numbers ranging from 0 (excellent with 0%) to 5 (very poor with N65%). The thin film was crisscrossed with a razor blade to form small squares to facilitate the removal. 3M scotch tape was pressed and pasted on the thin film surface and then removed applying a constant force at an angle of 60°. Fig. 6 shows the optical images of the thin film after adhesion test. It was observed that the crumbling of the thin film was less than 5%. It indicated that the thin film was quite stable and adhered well. According to ISO 2409, the quality of the thin film can be ranked as 1. Furthermore, the abrasion resistance of the
Fig. 2. (a) Transmittance spectra of simulated and experimental AR coating and (b) transmittance spectra of glass/TiO2 + SiO2 assembly and glass/AR coating/TiO2 + SiO2 assembly.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
4
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx
Fig. 5. Optical image of high transmittance superhydrophilic thin film before and after a pencil hardness test.
thin film was also examined by a simple method. Using steel wool which was pressed by a 500 g weight parallel to substrate back and forth to friction the thin film. And the WCA was used to determine the abrasion resistance of the thin film. Fig. 7 shows that the change of WCA of the thin film in different friction times. And optical images of water droplets are inserted in Fig. 7. The WCA of the thin film was still smaller than 10° after 100 times friction. The results indicated that the superhydrophilic phenomenon can be maintained after the thin film was treated by the above-mentioned abrasion resistance test more than 100 times. In this work, the sol–gel process was utilized to improve the mechanical properties of the thin film. The hydrolysis reaction is faster than
condensation reaction and the dense structure is obtained in acid catalyzed [35]. Here, the TiO2 + SiO2 solution was in acid catalyzed reaction (pH = 1) to make the TiO2 + SiO2 thin film more densified. It means that the hydroxyl group in the hybrid film was increased. And the hydroxyl groups can result in the Van der Waals force and the hydrogen bonding between the thin films and substrate. In addition, a TiO2 + SiO2 solution was also prepared without any catalyst in this work. But, the experimental results showed that the thin film has poor mechanical properties, when a solution was prepared without any catalyst. Therefore, the result of the thin film which was fabricated in an acid catalyzed reaction showed not only high transmittance and lasting superhydrophilicity but also superior mechanical properties. Our results suggest that the high transmittance superhydrophilic thin film with superior mechanical properties can be applied for optical, environmental, superhydrophilic and photocatalytic applications. 4. Conclusions
Fig. 6. Optical image of high transmittance superhydrophilic thin film after applying a cross cut.
High transmittance superhydrophilic thin film with superior mechanical properties was developed and fabricated. When AR coating was applied, the average transmittance of the thin film in the visible wavelength (400–700 nm) was increased from 91.3 ± 0.1% to 94.2 ± 0.1%. The thin films have favorable superhydrophilicity (WCA b 10°) and can be maintained 76 h when stored in a dark place. And the 3H hardness of the thin film was obtained by ASTM D 3363. The thin film can tolerate more than 100 times steel wool abrasion test which was pressed by 500 g weight. According to the ISO 2409 standard, the adhesion of the thin film is in a quality of rank 1. Above results show that the high transmittance superhydrophilic thin film has superior mechanical properties and optical property. Acknowledgments The authors would like to thank the Central Taiwan Science Park and Ministry of Science and Technology, for financially supporting this research under Contract No. 103RB02 and MOST 104-2221-E-008-002. References
Fig. 7. Abrasion resistance of high transmittance superhydrophilic thin film in different friction times.
[1] A. Fujishima, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] K. Hashimoto, H. Irie, A. Fujishima, TiO2 photocatalysis: a historical overview and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269. [3] S. Permpoon, G. Berthomé, B. Baroux, J.C. Joud, M. Langlet, Natural superhydrophilicity of sol–gel derived SiO2–TiO2 composite films, J. Mater. Sci. 41 (2006) 7650–7662. [4] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature 388 (1997) 431–432. [5] K. Guan, Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films, Surf. Coat. Technol. 191 (2005) 155–160. [6] J. Son, S. Kundu, L.K. Verma, M. Sakhuja, A.J. Danner, C.S. Bhatia, H. Yang, A practical superhydrophilic self cleaning and antireflective surface for outdoor photovoltaic applications, Sol. Energy Mater. Sol. Cells 98 (2012) 46–51.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028
Y.-C. Chang et al. / Thin Solid Films xxx (2016) xxx–xxx [7] T. Watanabe, S. Fukayama, M. Miyauchi, A. Fujishima, K. Hashimoto, Photocatalytic activity and photo-induced wettability conversion of TiO2 thin film prepared by sol–gel process on a soda-lime glass, J. Sol-Gel Sci. Technol. 19 (2000) 71–76. [8] Z. Ding, X. Hu, G.Q. Lu, P.-L. Yue, P.F. Greenfield, Novel silica gel supported TiO2 photocatalyst synthesized by CVD method, Langmuir 16 (2000) 6216–6222. [9] P. Evans, M.E. Pemble, D.W. Sheel, Precursor-directed control of crystalline type in atmospheric pressure CVD growth of TiO2 on stainless steel, Chem. Mater. 18 (2006) 5750–5755. [10] P. Hajkova, P. Spatenka, J. Krumeich, P. Exnar, A. Kolouch, J. Matousek, The influence of surface treatment on photocatalytic activity of PE CVD TiO2 thin films, Plasma Process. Polym. 6 (2009) S735–S740. [11] A. Borras, A. Barranco, A.N.R. González-Elipe, Reversible superhydrophobic to superhydrophilic conversion of Ag@ TiO2 composite nanofiber surfaces, Langmuir 24 (2008) 8021–8026. [12] N. Kanai, T. Nuida, K. Ueta, K. Hashimoto, T. Watanabe, H. Ohsaki, Photocatalytic efficiency of TiO2/SnO2 thin film stacks prepared by DC magnetron sputtering, Vacuum 74 (2004) 723–727. [13] Y. Liu, L. Qian, C. Guo, X. Jia, J. Wang, W. Tang, Natural superhydrophilic TiO2/SiO2 composite thin films deposited by radio frequency magnetron sputtering, J. Alloys Compd. 479 (2009) 532–535. [14] J.D. Mackenzie, E.P. Bescher, Chemical routes in the synthesis of nanomaterials using the sol–gel process, Acc. Chem. Res. 40 (2007) 810–818. [15] M. Qu, B. Zhang, S. Song, L. Chen, J. Zhang, X. Cao, Fabrication of superhydrophobic surfaces on engineering materials by a solution–immersion process, Adv. Funct. Mater. 17 (2007) 593–596. [16] Q.F. Xu, J.N. Wang, K.D. Sanderson, Organic–inorganic composite nanocoatings with superhydrophobicity, good transparency, and thermal stability, ACS Nano 4 (2010) 2201–2209. [17] H. Zhou, H. Wang, H. Niu, A. Gestos, X. Wang, T. Lin, Fluoroalkyl silane modified silicone rubber/nanoparticle composite: a super durable, robust superhydrophobic fabric coating, Adv. Mater. 24 (2012) 2409–2412. [18] M. Langlet, A. Kim, M. Audier, C. Guillard, J. Herrmann, Transparent photocatalytic films deposited on polymer substrates from sol–gel processed titania sols, Thin Solid Films 429 (2003) 13–21. [19] H.S. Wei, C.C. Kuo, C.C. Jaing, Y.C. Chang, C.C. Lee, Highly transparent superhydrophobic thin film with low refractive index prepared by one-step coating of modified silica nanoparticles, J. Sol-Gel Sci. Technol. 71 (2014) 168–175.
5
[20] M. Machida, K. Norimoto, T.e. Watanabe, K. Hashimoto, A. Fujishima, The effect of SiO2 addition in super-hydrophilic property of TiO2 photocatalyst, J. Mater. Sci. 34 (1999) 2569–2574. [21] J.A. Hiller, J.D. Mendelsohn, M.F. Rubner, Reversibly erasable nanoporous antireflection coatings from polyelectrolyte multilayers, Nat. Mater. 1 (2002) 59–63. [22] H. Macleod, T.-F.O. Filters, American Elsevier Publishing Company, Inc., New York, New York, pp. ix-x (1969) 37–42. [23] Z. Liu, X. Zhang, T. Murakami, A. Fujishima, Sol–gel SiO2/TiO2 bilayer films with selfcleaning and antireflection properties, Sol. Energy Mater. Sol. Cells 92 (2008) 1434–1438. [24] M.S. Park, Y. Lee, J.K. Kim, One-step preparation of antireflection film by spin-coating of polymer/solvent/nonsolvent ternary system, Chem. Mater. 17 (2005) 3944–3950. [25] B.G. Prevo, Y. Hwang, O.D. Velev, Convective assembly of antireflective silica coatings with controlled thickness and refractive index, Chem. Mater. 17 (2005) 3642–3651. [26] R. Fateh, R. Dillert, D. Bahnemann, Preparation and characterization of transparent hydrophilic photocatalytic TiO2/SiO2 thin films on polycarbonate, Langmuir 29 (2013) 3730–3739. [27] S.S. Latthe, S. Liu, C. Terashima, K. Nakata, A. Fujishima, Transparent, adherent, and photocatalytic SiO2–TiO2 coatings on polycarbonate for self-cleaning applications, Coatings 4 (2014) 497–507. [28] J. Damchan, L. Sikong, K. Kooptarnond, S. Niyomwas, Contact angle of glass substrate coated with TiO2/SiO2 thin film, CMU J. Nat. Sci. 7 (2008) 19–23. [29] J. Yu, X. Zhao, J.C. Yu, G. Zhong, J. Han, Q. Zhao, The grain size and surface hydroxyl content of super-hydrophilic TiO2/SiO2 composite nanometer thin films, J. Mater. Sci. Lett. 20 (2001) 1745–1748. [30] A. Tricoli, M. Righettoni, S.E. Pratsinis, Anti-fogging nanofibrous SiO2 and nanostructured SiO2–TiO2 films made by rapid flame deposition and in situ annealing, Langmuir 25 (2009) 12578–12584. [31] A. Eshaghi, A. Dashti, A. Eshaghi, R. Mozaffarinia, Photo-induced superhydrophilicity of nanocomposite TiO2–SiO2 thin film, Mater. Sci. Pol. 29 (2011) 22–28. [32] E. Pakdel, W.A. Daoud, X. Wang, Self-cleaning and superhydrophilic wool by TiO2/SiO2 nanocomposite, Appl. Surf. Sci. 275 (2013) 397–402. [33] ASTM D 3363, Standard Test Method for Film Hardness by Pencil Test, ASTM international, 2000. [34] DIN EN ISO 2409, Cross-cut Test, 2007. [35] J.D. Wright, N.A. Sommerdijk, Sol–Gel Materials: Chemistry and Applications, CRC press, 2000.
Please cite this article as: Y.-C. Chang, et al., High transmittance superhydrophilic thin film with superior mechanical properties, Thin Solid Films (2016), http://dx.doi.org/10.1016/j.tsf.2016.04.028