Article pubs.acs.org/Langmuir
Preparation and Characterization of Transparent Hydrophilic Photocatalytic TiO2/SiO2 Thin Films on Polycarbonate Razan Fateh, Ralf Dillert, and Detlef Bahnemann* Institut für Technische Chemie, Gottfried Wilhelm Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany ABSTRACT: Transparent hydrophilic photocatalytic TiO2 coatings have been widely applied to endow the surfaces self-cleaning properties. A mixed metal oxide (TiO2/SiO2) can enhance the photocatalytic performance improving the ability of surface adsorption and increasing the amount of hydroxyl surface groups. The present work introduces a systematic study concerning the effect of the SiO2 addition to TiO2 films on the wettability, the photocatalytic activity, the adhesion strength, and the mechanical stability of the films. Transparent hydrophilic photocatalytic TiO2/SiO2 thin films were used to coat the polycarbonate (PC) substrate which was precoated by an intermediate SiO2 layer. The TiO2/SiO2 thin film was prepared employing a bulk TiO2 powder (Sachtleben Hombikat UV 100) and different molar ratios of tetraethoxysilane in acidic ethanol. A dipcoating process was used to deposit the films onto the polycarbonate substrate. The films were characterized by UV/vis spectrophotometry, FTIR spectroscopy, ellipsometry, BET, AFM, XRD, and water contact angle measurements. The mechanical stability and the UV resistance were examined. The photocatalytic activity of the coated surface was calculated from the kinetic analysis of methylene blue photodegradation measurements and compared with the photocatalytic activity of Pilkington Activ sheet glass. The coated surfaces displayed considerable photocatalytic activity and superhydrophilicity after exposure to UV light. The addition of SiO2 results in an improvement of the photocatalytic activity of the TiO2 film reaching the highest value at molar ratio TiO2/SiO2 equal to 1:0.9. The prepared films exhibit a good stability against UV(A) irradiation.
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INTRODUCTION In recent years polycarbonate (PC) has become a very attractive material for a range of industrial applications.1 The global polycarbonate market in 2009 amounted to 2.9 million tons. The global demand for polycarbonate is predicted to grow at a compound annual growth rate of 6% up to 2015.2 The applications of polycarbonate range from plastic vessels and machine parts to optical grades for compact discs and optical fibers. It is expected for the near future that rigid plastic optical components will replace parts made from glass whenever improved properties or lower costs can be achieved. In particular, transparent plastic materials with optical-grade surfaces will be used on a large scale for covering car instruments as well as for refractive and diffractive elements for optical sensors. Already, for more than 50 years transparent polymers such as polymethyl methacrylate and bisphenol A polycarbonate (PC trade names Makrolon, Lexan, Diflon) have been used as precision optical parts.3 Since polycarbonate is much softer than polymethyl methacrylate, it is commonly utilized in the eyeglass industry. As a result of its high impact strength and its considerable thermal stability, bisphenol A-polycarbonate is important for industrial applications. However, the disadvantages of this material are related to its low mechanical hardness and its sensitivity to UV light.4 In order to prevent the gradual destruction of the PC interfaces for outdoor applications © 2013 American Chemical Society
employing UV light, coatings exhibiting a high resistance to photocorrosion are required.5 These poor physical properties pose a barrier for the adoption of PC in a wide range of industrial applications. Therefore, an urgent need exists to change or improve the properties of the surface of these polymers without altering their bulk properties.1 Recent advances in material science offer a wide array of new coatings with properties that accommodate these important criteria. Several routes are possible to apply these coatings such as physical or chemical vapor deposition. However, while these techniques exhibit many advantages they are expensive and cumbersome. In addition, the range and shape of materials that can be coated by these techniques is limited.6 An alternative coating method is the sol−gel route allowing novel materials, such as organic−inorganic hybrids, to be deposited on various substrates from a solution at or near room temperature. Consequently, this technique is suitable for coating polymers, which usually have limited thermal stability.6 It enables the coating of large surfaces and also the attainment of thin layers with good thermal and mechanical stabilities as well as high optical quality.7 Furthermore, the sol−gel method is relatively simple and inexpensive. Received: June 14, 2012 Published: January 30, 2013 3730
dx.doi.org/10.1021/la400191x | Langmuir 2013, 29, 3730−3739
Langmuir
Article
newly prepared photocatalyst films have been compared with Pilkington Activ sheet glass by the determination of their photonic efficiencies for the photocatalytic decolorization of a methylene blue solution and by measuring the light-induced changes of the surface wettability. The efficiency of the photocatalytic reactions was found to be about three times higher in the case of the mesoporous TiO2 films compared with films prepared from UV-100 or Pilkington Activ. Matsuda et al.21 have prepared transparent TiO2/SiO2 films with a 1:5 molar ratio on various types of substrates, including PC, by dipcoating and post-treatment at temperatures 97.9% at 500 nm, and a water contact angle of (57 ± 5)°. According to the XRD pattern (data not shown) the SiO2 film consists of amorphous particles without any crystalline phase. Figure 1 shows the FTIR transmission spectrum of the SiO2 intermediate layer. A peak around 1082 cm−1 attributed to Si−O−Si, and a broad peak around 3430 cm−1 attributed to the stretching mode of water and hydroxyl groups are observed. The AFM images (Figure 2) illustrate that coating the PC with a SiO2 intermediate layer decreases the roughness of the polycarbonate surface from 9.1 to 1.2 nm. After the deposition of the SiO2 intermediate layer, mixtures of a commercial TiO2 powder with varying amounts of tetraethoxysilane in acidic ethanol were prepared and applied 3732
dx.doi.org/10.1021/la400191x | Langmuir 2013, 29, 3730−3739
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Article
decrease of the transmission in the range from 420 to 380 nm corresponding to the absorption intrinsic to the polycarbonate substrate. To test the stability and the adhesion of the thin films before and after UV(A) irradiation, the adhesion quality was measured according to ISO 2409.24 The thin film samples were irradiated with UV(A) light for three months, and then a network of small squares was scratched into the surface in order to facilitate the removal of the functional and the intermediate layers. An adhesive tape was pressed on the coated surface and subsequently removed applying a certain constant force under an angle of 60°. Crumbling of less than 5% of the prepared films was observed (Figure 4) revealing the prepared films to be quite stable and adhesive even after prolonged UV(A) irradiation. Figure 4 in connection with Table 1 allows a ranking of the quality of the prepared TiO2/SiO2 thin films on PC as 1 (with 0 being the most stable and 5 the least stable films according to DIN EN ISO 2409). No effect of the SiO2 content of the inorganic layer on the mechanical stability was observed. The three chemical elements Ti, Si, and O are uniformly distributed in the covering thin films as revealed by EDX mapping (Figure 5). The EDX measurement at a TiO2/ SiO2(1:0.9) thin film showed the molar ratio of Ti, Si, and O being 15.5, 14.5, and 70.0, respectively, corresponding to a calculated Ti−Si ratio of 1: 0.935. The FTIR spectra of TiO2, SiO2, and TiO2/SiO2 films in the wavenumber range 400−4000 cm−1 are presented in Figure 6. The spectra show broad peaks between 500 cm−1 and 1000 cm−1 attributed to the stretching vibration of Ti−O, a peak at 964 cm−1 corresponding to the vibration of Si−O−Ti, a peak around 1082 cm−1 attributed to Si−O−Si, and broad peaks around 2840−3430 cm−1 attributed to the stretching mode of water and hydroxyl groups.26
Figure 1. FTIR transmission spectra of a SiO2 intermediate layer (dash line) compared with the that of pure silica (solid line).
to coat precoated PC sheets with functional TiO2/SiO2 thin films. The cumulative thicknesses of the prepared films as determined by ellipsometry were found to range between 140 and 220 nm (Table 2). The thickness of the prepared films decreases with an increase in the molar ratio of SiO2 in the TiO2/SiO2 films. The optical properties of the prepared films were determined by recording the absorption spectra in the 400−800 nm spectral range. Figure 3 exemplarily shows the transmission spectra of a bare TiO2 and a TiO2/SiO2(1:0.9) thin film on PC as well as a photographic image of a PC sheet coated with a TiO2/ SiO2(1:0.9) thin film. This figure and the transmission values of uncoated and coated polycarbonate plates at λ = 500 nm summarized in Table 2 clearly demonstrate that all coatings were highly transparent in the visible range of the spectrum with a transmission >94% at 500 nm. All spectra show a drastic
Figure 2. Two-dimensional and three-dimensional AFM image of the polycarbonate substrate and the intermediate layer SiO2 film deposited onto the PC surface. 3733
dx.doi.org/10.1021/la400191x | Langmuir 2013, 29, 3730−3739
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Article
Table 2. Characteristic Data of the Thin Films on PC ξ/%
CA/deg photocatalytic thin film none (PC) none (SiO2) TiO2 TiO2/SiO2 (1:0.2) TiO2/SiO2 (1:0.5) TiO2/SiO2 (1:0.7) TiO2/SiO2 (1:0.8) TiO2/SiO2 (1:0.9) TiO2/SiO2 (1:1) Pilkington Activ
thickness/ nm
transmission at 500 nm/%
BET surface area/m2 g−1
33 ± 9 186 ± 19 222 ± 5
98 98 95 95
n.d. 270 n.d.
85 57 54 55
216 ± 11
94
n.d.
205 ± 10
95
170 ± 30
before irradiation ± ± ± ±
after irradiation for after storage in the 700 h dark for 326 h
after felt abraison test
before felt abraison test
after felt abraison test
n.d. n.d.
