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Atmospheric-pressure plasma-enhanced chemical vapor deposition of UV-shielding TiO2 coatings on transparent plastics
Materials Letters 228 (2018) 479–481
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Atmospheric-pressure plasma-enhanced chemical vapor deposition of UV-shielding TiO2 coatings on transparent plastics Hiroki Nagasawa, Jing Xu, Masakoto Kanezashi, Toshinori Tsuru ⇑ Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2018 Received in revised form 12 June 2018 Accepted 14 June 2018 Available online 15 June 2018 Keywords: Atmospheric pressure plasma Plasma-enhanced chemical vapor deposition TiO2 thin film UV-shielding coating
a b s t r a c t UV-shielding TiO2 coatings for the protection of the UV-degradable transparent plastic polymethylmethacrylate (PMMA) were prepared by atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD). The structure and morphology of AP-PECVD-derived TiO2 films and TiO2-coated PMMA were characterized by XRD, FTIR, SEM, and water contact angle measurements, and their optical properties were investigated by UV–vis spectroscopy. The results showed that AP-PECVD-derived TiO2 had an amorphous structure, and a TiO2 layer with a compact columnar structure without any visible cracks was successfully formed on PMMA without damaging the structure of the polymer. The film deposited on PMMA exhibited excellent UV-shielding performance with 99% absorption of UV light in the wavelength range of 200–280 nm and with visible light transmittance above 90%. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Transparent plastics, such as polycarbonate (PC) and polymethylmethacrylate (PMMA), are being used to replace glass in both industrial and domestic applications as they have several advantages over glass [1]. They provide excellent optical properties and have a lower density and higher shock resistance than glass; however, they undergo degradation upon exposure to UV light. Hence, the protection of these transparent plastics against UVinduced photodegradation for practical applications is of high importance. UV-shielding coatings, which absorb UV light and transmit visible light, are most frequently used for this purpose. UV-shielding coatings are composed of organic or inorganic materials or a mixture thereof [2,3]. Titanium oxide (TiO2) is a suitable material for this because of its wide energy band gap (anatase, 3.2 eV and rutile, 3.0 eV) which exhibits a sharp cutoff in the UV light region [4]. It has been reported that TiO2 thin films with several hundred nanometers in thickness could effectively shield UV light and protect UV-degradable materials [5,6]. Several methods are available for the preparation of TiO2 coatings, such as sol-gel [7], sputtering [8], chemical vapor deposition [9], layer-by-layer assembly [5], and atomic layer deposition [6]. However, these methods often require either multiple steps, high-temperature or vacuum processing to obtain films.
⇑ Corresponding author. E-mail address: [email protected] (T. Tsuru). https://doi.org/10.1016/j.matlet.2018.06.053 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) is recognized as an attractive and versatile coating technique because of its outstanding advantages, namely a single-step and solvent-free dry coating process that can be operated at room temperature and at ambient pressure. AP-PECVD has been utilized for the preparation of various types of inorganic thin films for use in microelectronics [10], sensing [11], and separation [12]. AP-PECVD has also been adopted for the deposition of TiO2 thin films for photocatalysis [13] and dye-sensitized solar cells [14]. However, AP-PECVD of TiO2 films for UV-shielding applications to protect UV-degradable transparent plastics have not yet been reported. Herein, AP-PECVD of TiO2 thin films were demonstrated for the preparation of UV-shielding coatings onto transparent plastics. The shielding ability in the UV light region and the transparency in the visible light region, which are important for UV-protective coatings of transparent plastics for optical use, were investigated. 2. Experimental The deposition of TiO2 thin films was conducted using a nonthermal atmospheric-pressure plasma jet (Plasma Concept Tokyo) equipped with a precursor injection port at the afterglow region of the discharge, as shown in Fig. 1. Argon gas was employed as a plasma working gas with a flow rate of 5 L min 1, and the plasma was driven by a sinusoidal voltage with a maximum value of 6.0 kV at a frequency of 50 kHz. Titanium (IV) isopropoxide (TTIP, SigmaAldrich) was used as the precursor and was fed from a bubbler
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H. Nagasawa et al. / Materials Letters 228 (2018) 479–481
Fig. 1. Schematic illustration of the AP-PECVD system.
maintained at 25 °C (vapor pressure 6.0 Pa) using argon as the carrier gas at a flow rate of 500 mL min 1. The concentration of TTIP in the afterglow region was 5.4 ppm. The distance from the nozzle exit to the substrate surface was 5 mm. Silicon wafers with (1 0 0) orientation and quartz glass were used as substrates for the characterization of the deposited layer. PMMA (Tokyo Kasei Kogyo) was dissolved in toluene at 10 wt% and spin-coated over the silicon wafer and quartz glass substrates at 3000 rpm for 60 s, in order to perform the AP-PECVD of TiO2 onto transparent plastics. The structure and morphology of the films were investigated by X-ray diffraction (XRD, D2 PHASER, Bruker), Fourier transform infrared (FTIR) spectrometry (FT/IR-4100, JASCO) and field emission scanning electron microscopy (FE-SEM, S5000, Hitachi). Contact angles were measured to evaluate the surface properties of the films using a contact angle meter (DM-300, Kyowa. Interface Science). The optical properties were characterized using an ultraviolet and visible (UV–vis) spectrophotometer (UV-3600 plus, Shimadzu).
3. Results and discussion 3.1. Characterization of AP-PECVD-derived TiO2 thin films TiO2 films were deposited on bare silicon wafer substrates in order to analyze the structural properties of the AP-PECVDderived films. The AP-PECVD time for XRD analysis was 30 min, which was sufficient to obtain a thickness suitable for accurate measurement. The XRD patterns of an as-deposited film as well
as films after heat treatment at temperatures of 200, 400, and 600 °C are shown in Fig. 2(a). No obvious diffraction peaks were observed for the as-deposited film and the film annealed at 200 °C, indicating the amorphous structure of the samples. The diffraction peaks corresponding to the anatase phase [15] were observed for the films annealed at 400 and 600 °C, indicating a transformation of amorphous to the anatase phase. Films were deposited on quartz glass substrates with an APPECVD time of 3 min to investigate the optical transmittance. The UV–vis transmittance spectra of an AP-PECVD-derived TiO2 film are shown in Fig. 2(b). The transmittance of the TiO2 film on quartz glass at a wavelength of 600 nm decreased by only 4.1% compared to that of a bare quartz glass substrate, showing high transmittance in the visible light region. The transmittance of TiO2 films on quartz glass in the UV light region between 200 and 280 nm was less than 1%, and the transmittance gradually increased with increasing wavelength. This result indicated that the AP-PECVD-derived TiO2 film was capable of absorbing shortwavelength UV light. The absorbance edge of the film was approximately 330 nm, which was slightly shorter than that for single crystal anatase (388 nm). This agreed with the previously reported larger band gap of amorphous TiO2 thin films [8]. 3.2. TiO2 deposition onto transparent plastics and their UV-shielding performance TiO2 was deposited onto PMMA films under the same deposition conditions as for the quartz glass substrates. Fig. 3(a) shows FTIR spectra of a PMMA film before and after AP-PECVD of TiO2. A comparison of the spectra before and after AP-PECVD showed essentially no change in the absorption of C@O at 1720 cm 1, OACH3 at 1440 cm 1, and the ester group at 1270–1140 cm 1, which are the characteristic bands of PMMA, suggesting that the PMMA substrate was not affected by plasma exposure. For the film after AP-PECVD, the absorption band around 438 cm 1, which is characteristic of TiO2, was observed. As shown in the inset of Fig. 3(a), the water contact angle decreased from 73° to 29° after AP-PECVD, suggesting an increase in the hydrophilicity due to the formation of a thin TiO2 layer on top of the PMMA film. The morphology of the TiO2 deposited on the PMMA film was investigated by SEM, and the surface and cross-sectional images of TiO2-deposited PMMA are shown in Fig. 3(b) and (c), respectively. The TiO2 layer had a thickness of 0.5–1 lm, and had a compact columnar structure with grain size between 0.1 and 0.4 lm and with no visible cracks. These results indicated that a TiO2 layer was formed on PMMA via AP-PECVD without damaging the structure of the polymer substrate.
Fig. 2. (a) XRD spectra of as-deposited TiO2 films prepared on bare silicon wafer and films annealed at 200, 400, and 600 °C in air. (b) UV–vis spectra of an as-deposited TiO2 film deposited on quartz glass and a bare quartz glass substrate. The spectrum of ambient air was subtracted as a background.
H. Nagasawa et al. / Materials Letters 228 (2018) 479–481
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Fig. 3. (a) FTIR spectra of a PMMA film without coating and TiO2-coated PMMA. The spectrum of silicon wafer was subtracted as a background. The inset images are water contact angles (CA) for the corresponding films. (b) Surface and (c) cross-sectional SEM images of TiO2-coated PMMA. The samples were directly used for the measurements without any further annealing.
4. Conclusions Herein, we demonstrated the preparation of a UV-shielding TiO2 coating on transparent plastics via the AP-PECVD technique. XRD analysis of as-deposited and heat-treated TiO2 films prepared on bare silicon substrates revealed that the as-deposited film had an amorphous structure, which changed to the anatase phase after annealing at 400 °C. An AP-PECVD-derived TiO2 layer absorbed UV light with an absorbance edge of approximately 330 nm. A TiO2 layer with a compact columnar structure was successfully formed on PMMA without damaging the structure of the polymer. TiO2-coated PMMA exhibited excellent UV-shielding performance with 99% absorption of UV light in the wavelength range of 200–280 nm, and with visible light transmittance above 95%. Therefore, TiO2 thin films prepared via AP-PECVD are an effective UV-shielding layer for the protection of UV-degradable transparent plastics. Fig. 4. UV–vis spectra of a PMMA film without coating and TiO2-coated PMMA. The spectrum of bare quartz glass was subtracted as a background. The samples were directly used for the measurements without any further annealing.
The optical properties of TiO2-coated PMMA were evaluated by UV–vis spectroscopy. Fig. 4 shows UV–vis transmittance spectra of original PMMA and TiO2-coated PMMA. The TiO2 coating slightly reduced the transmittance in the visible light region, with a transmittance of 95.9% at a wavelength of 600 nm. In contrast, the coating significantly reduced the transmittance in the UV light region. UV light between 200 and 280 nm was almost completely (up to 99%) adsorbed and the transmittance at 350 nm was reduced by half. The high absorption in the short-wavelength region was associated with the bandgap absorption of the TiO2 layer, while absorption in the long-wavelength region may be due to scattering effects of the granular surface of the AP-PECVD-derived TiO2 layer. It is also interesting to note that the interference fringes in the visible light region were not observed in the AP-PECVD-derived TiO2 probably due to the light interference was significantly reduced by the rough surface. These results clearly demonstrated that a UVshielding TiO2 coating with excellent transmittance in the visible light region can be formed on transparent plastics such as PMMA via single-step AP-PECVD.
References [1] C. Seubert, K. Nietering, M. Nichols, R. Wykoff, S. Bollin, Coatings 2 (2012) 221– 234. [2] M. Zayat, P. Garcia-Parejo, D. Levy, Chem. Soc. Rev. 36 (2007) 1270–1281. [3] H. Hayashi, O. Yoshida, H. Suzuki, J. Ceram. Soc. Jpn. 117 (2009) 978–982. [4] H. Yang, S. Zhu, N. Pan, J. Appl. Polym. Sci. 92 (2004) 3201–3210. [5] D. Cheng, G. Cai, J. Wu, J. Ran, X. Wang, Colloid. Polym. Sci. 295 (2017) 2163– 2172. [6] X. Xiao, X. Liu, F. Chen, D. Fang, C. Zhang, L. Xia, W. Xu, ACS Appl. Mater. Interface 7 (2015) 21326–21333. [7] R.C. Suciu, E. Indrea, T.D. Silipas, S. Drever, M.C. Rosu, V. Popescu, G. Popescu, H. I. Nascu, J. Phys. Conf. Ser. 182 (2009) 012080. [8] K. Eufinger, D. Poelman, H. Poelman, R. De Gryse, G.B. Marin, Appl. Surf. Sci. 254 (2007) 148–152. [9] K.L. Siefering, G.K. Griffin, J. Electrochem. Soc. 137 (1990) 1206–1208. [10] S. Dong, M. Watanabe, R.H. Dauskards, Adv. Funct. Mater. 24 (2014) 3075– 3081. [11] P. Heier, N.D. Boscher, T. Bohn, K. Heinze, P. Choquet, J. Mater. Chem. A 2 (2014) 1560–1570. [12] H. Nagasawa, Y. Yamamoto, N. Tsuda, M. Kanezashi, T. Yoshioka, T. Tsuru, J. Membr. Sci. 524 (2017) 644–651. [13] L.B. Di, X.S. Li, C. Sjo, Y. Xu, D.Z. Zhao, A.M. Zhu, J. Phys. D: Appl. Phys. 42 (2009) 032001. [14] H.K. Seo, C.M. Elliot, H.S. Shin, ACS Appl. Mater. Interface 2 (2010) 3397–3400. [15] D. Wang, Q. Yang, Y. Guo, X. Liu, J. Shi, J. Zhang, Mater. Lett. 65 (2011) 2526– 2529.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Atmospheric-pressure plasma-enhanced chemical vapor deposition of UV-shielding TiO2 coatings on transparent plastics Hiroki Nagasawa, Jing Xu, Masakoto Kanezashi, Toshinori Tsuru ⇑ Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan
a r t i c l e
i n f o
Article history: Received 17 May 2018 Received in revised form 12 June 2018 Accepted 14 June 2018 Available online 15 June 2018 Keywords: Atmospheric pressure plasma Plasma-enhanced chemical vapor deposition TiO2 thin film UV-shielding coating
a b s t r a c t UV-shielding TiO2 coatings for the protection of the UV-degradable transparent plastic polymethylmethacrylate (PMMA) were prepared by atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD). The structure and morphology of AP-PECVD-derived TiO2 films and TiO2-coated PMMA were characterized by XRD, FTIR, SEM, and water contact angle measurements, and their optical properties were investigated by UV–vis spectroscopy. The results showed that AP-PECVD-derived TiO2 had an amorphous structure, and a TiO2 layer with a compact columnar structure without any visible cracks was successfully formed on PMMA without damaging the structure of the polymer. The film deposited on PMMA exhibited excellent UV-shielding performance with 99% absorption of UV light in the wavelength range of 200–280 nm and with visible light transmittance above 90%. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Transparent plastics, such as polycarbonate (PC) and polymethylmethacrylate (PMMA), are being used to replace glass in both industrial and domestic applications as they have several advantages over glass [1]. They provide excellent optical properties and have a lower density and higher shock resistance than glass; however, they undergo degradation upon exposure to UV light. Hence, the protection of these transparent plastics against UVinduced photodegradation for practical applications is of high importance. UV-shielding coatings, which absorb UV light and transmit visible light, are most frequently used for this purpose. UV-shielding coatings are composed of organic or inorganic materials or a mixture thereof [2,3]. Titanium oxide (TiO2) is a suitable material for this because of its wide energy band gap (anatase, 3.2 eV and rutile, 3.0 eV) which exhibits a sharp cutoff in the UV light region [4]. It has been reported that TiO2 thin films with several hundred nanometers in thickness could effectively shield UV light and protect UV-degradable materials [5,6]. Several methods are available for the preparation of TiO2 coatings, such as sol-gel [7], sputtering [8], chemical vapor deposition [9], layer-by-layer assembly [5], and atomic layer deposition [6]. However, these methods often require either multiple steps, high-temperature or vacuum processing to obtain films.
⇑ Corresponding author. E-mail address: [email protected] (T. Tsuru). https://doi.org/10.1016/j.matlet.2018.06.053 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) is recognized as an attractive and versatile coating technique because of its outstanding advantages, namely a single-step and solvent-free dry coating process that can be operated at room temperature and at ambient pressure. AP-PECVD has been utilized for the preparation of various types of inorganic thin films for use in microelectronics [10], sensing [11], and separation [12]. AP-PECVD has also been adopted for the deposition of TiO2 thin films for photocatalysis [13] and dye-sensitized solar cells [14]. However, AP-PECVD of TiO2 films for UV-shielding applications to protect UV-degradable transparent plastics have not yet been reported. Herein, AP-PECVD of TiO2 thin films were demonstrated for the preparation of UV-shielding coatings onto transparent plastics. The shielding ability in the UV light region and the transparency in the visible light region, which are important for UV-protective coatings of transparent plastics for optical use, were investigated. 2. Experimental The deposition of TiO2 thin films was conducted using a nonthermal atmospheric-pressure plasma jet (Plasma Concept Tokyo) equipped with a precursor injection port at the afterglow region of the discharge, as shown in Fig. 1. Argon gas was employed as a plasma working gas with a flow rate of 5 L min 1, and the plasma was driven by a sinusoidal voltage with a maximum value of 6.0 kV at a frequency of 50 kHz. Titanium (IV) isopropoxide (TTIP, SigmaAldrich) was used as the precursor and was fed from a bubbler
480
H. Nagasawa et al. / Materials Letters 228 (2018) 479–481
Fig. 1. Schematic illustration of the AP-PECVD system.
maintained at 25 °C (vapor pressure 6.0 Pa) using argon as the carrier gas at a flow rate of 500 mL min 1. The concentration of TTIP in the afterglow region was 5.4 ppm. The distance from the nozzle exit to the substrate surface was 5 mm. Silicon wafers with (1 0 0) orientation and quartz glass were used as substrates for the characterization of the deposited layer. PMMA (Tokyo Kasei Kogyo) was dissolved in toluene at 10 wt% and spin-coated over the silicon wafer and quartz glass substrates at 3000 rpm for 60 s, in order to perform the AP-PECVD of TiO2 onto transparent plastics. The structure and morphology of the films were investigated by X-ray diffraction (XRD, D2 PHASER, Bruker), Fourier transform infrared (FTIR) spectrometry (FT/IR-4100, JASCO) and field emission scanning electron microscopy (FE-SEM, S5000, Hitachi). Contact angles were measured to evaluate the surface properties of the films using a contact angle meter (DM-300, Kyowa. Interface Science). The optical properties were characterized using an ultraviolet and visible (UV–vis) spectrophotometer (UV-3600 plus, Shimadzu).
3. Results and discussion 3.1. Characterization of AP-PECVD-derived TiO2 thin films TiO2 films were deposited on bare silicon wafer substrates in order to analyze the structural properties of the AP-PECVDderived films. The AP-PECVD time for XRD analysis was 30 min, which was sufficient to obtain a thickness suitable for accurate measurement. The XRD patterns of an as-deposited film as well
as films after heat treatment at temperatures of 200, 400, and 600 °C are shown in Fig. 2(a). No obvious diffraction peaks were observed for the as-deposited film and the film annealed at 200 °C, indicating the amorphous structure of the samples. The diffraction peaks corresponding to the anatase phase [15] were observed for the films annealed at 400 and 600 °C, indicating a transformation of amorphous to the anatase phase. Films were deposited on quartz glass substrates with an APPECVD time of 3 min to investigate the optical transmittance. The UV–vis transmittance spectra of an AP-PECVD-derived TiO2 film are shown in Fig. 2(b). The transmittance of the TiO2 film on quartz glass at a wavelength of 600 nm decreased by only 4.1% compared to that of a bare quartz glass substrate, showing high transmittance in the visible light region. The transmittance of TiO2 films on quartz glass in the UV light region between 200 and 280 nm was less than 1%, and the transmittance gradually increased with increasing wavelength. This result indicated that the AP-PECVD-derived TiO2 film was capable of absorbing shortwavelength UV light. The absorbance edge of the film was approximately 330 nm, which was slightly shorter than that for single crystal anatase (388 nm). This agreed with the previously reported larger band gap of amorphous TiO2 thin films [8]. 3.2. TiO2 deposition onto transparent plastics and their UV-shielding performance TiO2 was deposited onto PMMA films under the same deposition conditions as for the quartz glass substrates. Fig. 3(a) shows FTIR spectra of a PMMA film before and after AP-PECVD of TiO2. A comparison of the spectra before and after AP-PECVD showed essentially no change in the absorption of C@O at 1720 cm 1, OACH3 at 1440 cm 1, and the ester group at 1270–1140 cm 1, which are the characteristic bands of PMMA, suggesting that the PMMA substrate was not affected by plasma exposure. For the film after AP-PECVD, the absorption band around 438 cm 1, which is characteristic of TiO2, was observed. As shown in the inset of Fig. 3(a), the water contact angle decreased from 73° to 29° after AP-PECVD, suggesting an increase in the hydrophilicity due to the formation of a thin TiO2 layer on top of the PMMA film. The morphology of the TiO2 deposited on the PMMA film was investigated by SEM, and the surface and cross-sectional images of TiO2-deposited PMMA are shown in Fig. 3(b) and (c), respectively. The TiO2 layer had a thickness of 0.5–1 lm, and had a compact columnar structure with grain size between 0.1 and 0.4 lm and with no visible cracks. These results indicated that a TiO2 layer was formed on PMMA via AP-PECVD without damaging the structure of the polymer substrate.
Fig. 2. (a) XRD spectra of as-deposited TiO2 films prepared on bare silicon wafer and films annealed at 200, 400, and 600 °C in air. (b) UV–vis spectra of an as-deposited TiO2 film deposited on quartz glass and a bare quartz glass substrate. The spectrum of ambient air was subtracted as a background.
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Fig. 3. (a) FTIR spectra of a PMMA film without coating and TiO2-coated PMMA. The spectrum of silicon wafer was subtracted as a background. The inset images are water contact angles (CA) for the corresponding films. (b) Surface and (c) cross-sectional SEM images of TiO2-coated PMMA. The samples were directly used for the measurements without any further annealing.
4. Conclusions Herein, we demonstrated the preparation of a UV-shielding TiO2 coating on transparent plastics via the AP-PECVD technique. XRD analysis of as-deposited and heat-treated TiO2 films prepared on bare silicon substrates revealed that the as-deposited film had an amorphous structure, which changed to the anatase phase after annealing at 400 °C. An AP-PECVD-derived TiO2 layer absorbed UV light with an absorbance edge of approximately 330 nm. A TiO2 layer with a compact columnar structure was successfully formed on PMMA without damaging the structure of the polymer. TiO2-coated PMMA exhibited excellent UV-shielding performance with 99% absorption of UV light in the wavelength range of 200–280 nm, and with visible light transmittance above 95%. Therefore, TiO2 thin films prepared via AP-PECVD are an effective UV-shielding layer for the protection of UV-degradable transparent plastics. Fig. 4. UV–vis spectra of a PMMA film without coating and TiO2-coated PMMA. The spectrum of bare quartz glass was subtracted as a background. The samples were directly used for the measurements without any further annealing.
The optical properties of TiO2-coated PMMA were evaluated by UV–vis spectroscopy. Fig. 4 shows UV–vis transmittance spectra of original PMMA and TiO2-coated PMMA. The TiO2 coating slightly reduced the transmittance in the visible light region, with a transmittance of 95.9% at a wavelength of 600 nm. In contrast, the coating significantly reduced the transmittance in the UV light region. UV light between 200 and 280 nm was almost completely (up to 99%) adsorbed and the transmittance at 350 nm was reduced by half. The high absorption in the short-wavelength region was associated with the bandgap absorption of the TiO2 layer, while absorption in the long-wavelength region may be due to scattering effects of the granular surface of the AP-PECVD-derived TiO2 layer. It is also interesting to note that the interference fringes in the visible light region were not observed in the AP-PECVD-derived TiO2 probably due to the light interference was significantly reduced by the rough surface. These results clearly demonstrated that a UVshielding TiO2 coating with excellent transmittance in the visible light region can be formed on transparent plastics such as PMMA via single-step AP-PECVD.
References [1] C. Seubert, K. Nietering, M. Nichols, R. Wykoff, S. Bollin, Coatings 2 (2012) 221– 234. [2] M. Zayat, P. Garcia-Parejo, D. Levy, Chem. Soc. Rev. 36 (2007) 1270–1281. [3] H. Hayashi, O. Yoshida, H. Suzuki, J. Ceram. Soc. Jpn. 117 (2009) 978–982. [4] H. Yang, S. Zhu, N. Pan, J. Appl. Polym. Sci. 92 (2004) 3201–3210. [5] D. Cheng, G. Cai, J. Wu, J. Ran, X. Wang, Colloid. Polym. Sci. 295 (2017) 2163– 2172. [6] X. Xiao, X. Liu, F. Chen, D. Fang, C. Zhang, L. Xia, W. Xu, ACS Appl. Mater. Interface 7 (2015) 21326–21333. [7] R.C. Suciu, E. Indrea, T.D. Silipas, S. Drever, M.C. Rosu, V. Popescu, G. Popescu, H. I. Nascu, J. Phys. Conf. Ser. 182 (2009) 012080. [8] K. Eufinger, D. Poelman, H. Poelman, R. De Gryse, G.B. Marin, Appl. Surf. Sci. 254 (2007) 148–152. [9] K.L. Siefering, G.K. Griffin, J. Electrochem. Soc. 137 (1990) 1206–1208. [10] S. Dong, M. Watanabe, R.H. Dauskards, Adv. Funct. Mater. 24 (2014) 3075– 3081. [11] P. Heier, N.D. Boscher, T. Bohn, K. Heinze, P. Choquet, J. Mater. Chem. A 2 (2014) 1560–1570. [12] H. Nagasawa, Y. Yamamoto, N. Tsuda, M. Kanezashi, T. Yoshioka, T. Tsuru, J. Membr. Sci. 524 (2017) 644–651. [13] L.B. Di, X.S. Li, C. Sjo, Y. Xu, D.Z. Zhao, A.M. Zhu, J. Phys. D: Appl. Phys. 42 (2009) 032001. [14] H.K. Seo, C.M. Elliot, H.S. Shin, ACS Appl. Mater. Interface 2 (2010) 3397–3400. [15] D. Wang, Q. Yang, Y. Guo, X. Liu, J. Shi, J. Zhang, Mater. Lett. 65 (2011) 2526– 2529.