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Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 111–118
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of calcination temperature on the microstructure and wetting behavior of superhydrophobic polydimethylsiloxane/silica coating Kunquan Li, Xingrong Zeng ∗ , Hongqiang Li, Xuejun Lai, Hu Xie College of Materials Science and Engineering, South China University of Technology, No. 381, Wushan Road, Tianhe District, Guangzhou 510640, China
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• The superhydrophobic coating was fabricated by spraying the PDMS and nano-SiO2 . • The coating showed various microstructure and wettability by adjusting calcination. • After calcining at 400 ◦ C, the superhydrophobic coating became highly transparent. • The calcination mechanism of the PDMS/SiO2 coating was proposed.
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Article history: Received 25 September 2013 Received in revised form 2 January 2014 Accepted 15 January 2014 Available online 24 January 2014 Keywords: PDMS/SiO2 coating Superhydrophobic Calcinations Wetting behavior Microstructure
a b s t r a c t The superhydrophobic coating was prepared through spraying the mixture of polydimethylsiloxane (PDMS) and hydrophobic nanosilica (SiO2 ) on the slide glass. The surface of the as-prepared PDMS/SiO2 coating showed hierarchical roughness with a water contact angle (WCA) of 153◦ . The resultant superhydrophobic PDMS/SiO2 coating was subsequently calcined in a muffle furnace under air atmosphere and the effects of calcination temperature on the microstructure, component, transmittance, and wetting behavior of superhydrophobic PDMS/SiO2 coating were systematically investigated. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the superhydrophobic PDMS/SiO2 coating became transparent with the visible light transmittance increasing from 40% to 80%, which was ascribed to the decomposition of PDMS and the rearrangement of the hydrophobic SiO2 particles. However, when the calcination temperature was over 500 ◦ C, the wetting behavior of the coating changed from superhydrophobicity to superhydrophilicity with a WCA of nearly 0◦ , owing to the replacement of hydrophobic Si–CH3 groups with hydrophilic groups of Si–OH. Finally, the calcination mechanism of the superhydrophobic PDMS/SiO2 composite coating is proposed, which has guiding significance for the study of other composite coatings. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Currently, solid surfaces with controlled wettability have gained much concern for their potential applications in life and industry [1]. Inspired by the natural biological species [2], such as lotus leaf and butterfly’s wing, the superhydrophobic surfaces, characterized as water contact angle (WCA) above 150◦ and sliding angle
∗ Corresponding author. Tel.: +86 20 87114248; fax: +86 20 87114248. E-mail address: [email protected] (X. Zeng). 0927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2014.01.024
(SA) below 10◦ , are thought to be ideal candidates in self-cleaning coating [3], waterproof [4], anti-corrosion material [5], water/oil separation film [6], and water transport pipeline [7]. Plenty of researches have confirmed that this special wetting behavior is mainly ascribed to the low surface energy materials and the hierarchical micro-/nanostructures on the surface [8]. According to these findings, lots of methods have been reported to fabricate superhydrophobic surfaces, including sol-gel method [9], template imitated [10], plasma etching [11], electrospinning [12], and so on. In most studies, the polydimethylsiloxanes (PDMS) was usually used as low surface energy material to replace the fluorine material in
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fabricating superhydrophobic surface due to its advantages of low cost, simple and easy-obtained [13,14]. Moreover, the properties of non-toxic, optical transparency, and high thermal stability of PDMS allow the products to be applied in various areas. Although the PDMS coating exhibits a good water-repellent property, its intrinsic WCA is only about 105◦ [15], which still cannot satisfy the requirements of superhydrophobicity. Thus, it is necessary to increase the roughness of the surface. In recent years, the introduction of inorganic particles, especially silica (SiO2 ) nanoparticle, to construct hierarchical structures on the PDMS surface is widely appreciated [16–18]. Ogihara et al. [17] prepared the superhydrophobic PDMS/SiO2 composite coatings by electrophoretic deposition. The results showed that the presence of both nanometer- and micrometer-sized roughness on the surfaces resulted from the aggregates of SiO2 particles. A highly transparent superhydrophobic surface with dual-scale roughness was also created by Karunakaran using modified SiO2 particles to construct roughness [18]. In general, the superhydrophobic surfaces made from PDMS/SiO2 composite are of great interest for both academic researches and industrial applications. The superhydrophobic PDMS/SiO2 surfaces show superhydrophobicity at room temperature, but in many cases, surfaces are required to keep their performance when exposed to heat [19]. According to the previous reports, the WCA was considered as the only standard to evaluate the thermal stability of superhydrophobic PDMS/SiO2 surface [20–22]. Xiang et al. [21] prepared superhydrophobic PDMS surfaces and found that the resultant surfaces kept superhydrophobicity after thermal treatment at 300 ◦ C while it changed to superhydrophilicity at 400 ◦ C. The organic SiO2 coating could remain constant in high WCA and low SA for calcination temperature up to 350 ◦ C, which was confirmed by Deng [19]. Lin et al., [22] also revealed that the superhydrophobic PDMS/SiO2 coating showed good thermal stability with a WCA higher than 160◦ after calcining at 400 ◦ C for 1 h. However, it is unwise to only use WCA to evaluate the thermal stability of superhydrophobic surface because the surface component and morphology structure usually have changed when the PDMS/SiO2 is calcined at high temperature, which has always been neglected by other researches [23]. In this study, the superhydrophobic PDMS/SiO2 coating was prepared through a facile way by spraying the mixture of PDMS and hydrophobic nano-SiO2 on the slide glasses. The obtained superhydrophobic PDMS/SiO2 coating was calcined in a muffle furnace under air atmosphere and the effects of calcination temperature on the microstructure, component, wetting behavior, and transmittance were investigated. The superhydrophobic PDMS/SiO2 coatings were characterized by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), WCA and thermal gravimetric (TG) analysis, energy dispersive spectroscopy (EDS), and UV−vis spectrophotometer, respectively. Finally, the calcination mechanism was also proposed.
2. Experimental 2.1. Materials The hydroxyl-terminated polydimethylsiloxane (H-PDMS) (107 Glue, molecular weight of about 1500) was provided by Jiangxi Xinghuo Organic Silicone Plant (China). Tetraethoxysilane (TEOS) and dibutyltin dilaurate (DBTDL) were acquired from Shanghai Reagent Factory (China). Hydrophobic nano-SiO2 (H15, diameter of 16–20 nm) was purchased from Wacker Company (Germany). Ethanol was obtained from Guangzhou Chemical Reagent Factory (China). The n-hexane was supplied by Sinopharm Chemical Reagent Limited Company (China). All the reagents were used as received.
2.2. Preparation of superhydrophobic PDMS/SiO2 coating The superhydrophobic PDMS/SiO2 coating was prepared according to the following procedures. Firstly, the slide glass was ultrasonically washed with ethanol–water solution and dried at 100 ◦ C for 15 min. Thereafter, 0.5 g H-PDMS, 0.01 g catalyst of DBTDL, and 0.1 g curing agent of TEOS were mixed in 10 g n-hexane by vigorous magnetic stirring for 15 min. Then, 0.2 g hydrophobic SiO2 was added into the above mixture under ultrasonic dispersion condition for about 10 min. Finally, the mixture was sprayed onto the slide glass, and the superhydrophobic PDMS/SiO2 coating was obtained after curing at 80 ◦ C for 30 min. The preparation process of PDMS/SiO2 coating was presented in Fig. 1. 2.3. Calcinations of superhydrophobic PDMS/SiO2 coating The as-prepared superhydrophobic PDMS/SiO2 coatings were calcined at a muffle furnace under air atmosphere, and the calcination temperatures were adjusted to 100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C, and 500 ◦ C, respectively. Then, the calcinations lasted for 5 h (for the coating calcined at 500 ◦ C, the calcination times varied from 1 h to 5 h). Finally, the PDMS/SiO2 coatings were transferred to the airy platform and cooled at room temperature. 2.4. Characterization Fourier transform infrared spectra were recorded on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) within a range of 4000–400 cm−1 by KBr pellet technique. Microscopic morphologies of the surfaces were observed by a field-emission scanning electron microscopy (SEM, Fei Nova Nanosem 430, the Netherlands) equipped with EDS unit at an acceleration voltage of 15 kV. A thin gold layer was sprayed on the sample surfaces to prevent charging before testing. WCA tests were carried out on a contact angle detector (DSA100, Germany) equipped with a digital camera at room temperature. For WCA studies, 5 L distilled water was used. All of the obtained WCA values were calculated from at least five different places of the samples. Thermal stability was measured by thermal gravimetric analysis (TG, TGA209F3, Germany) coupled with FTIR spectrometer at a heating rate of 10 ◦ C/min from 35 ◦ C to 800 ◦ C under air atmosphere. Transmittance of the coating calcined under different temperatures was evaluated by UV–vis spectroscopy (UV765CRT, China). The digital photographs of the transparent coating were taken by an SLR camera (Nikon D90, Japan). 3. Results and discussion 3.1. Superhydrophobic PDMS/SiO2 composite coating 3.1.1. FTIR spectra of H-PDMS and PDMS Fig. 2 illustrates the FTIR spectra of H-PDMS (a) and PDMS (b). In Fig. 2(a), the peaks at 3463 cm−1 , 2960 cm−1 , and 1108 cm−1 were ascribed to the stretching vibrations of Si–OH, –CH3 , and Si–O–Si groups in the H-PDMS [24], respectively. The absorption bands near 1245 cm−1 were attributed to the symmetric deformation of the –CH3 group in –Si(CH3 )2 of H-PDMS, and the bands located at 871 cm−1 and 806 cm−1 were assigned to the Si–C and Si–O vibration [25], respectively. Compared with the spectra of Fig. 2(a), no obvious new peaks could be observed, while the peaks near 3463 cm−1 disappeared in Fig. 2(b), indicating that the Si–OH in the H-PDMS had reacted with the Si–OC2 H5 of the TEOS. 3.1.2. SEM and WCA of PDMS/SiO2 coating The SEM photograph and WCA image of superhydrophobic PDMS/SiO2 surface are shown in Fig. 3. From Fig. 3(a), there were
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Fig. 1. Schematic diagram of fabricating PDMS/SiO2 coating.
Fig. 4. Effect of calcination temperature on the WCA of superhydrophobic PDMS/SiO2 coating. Fig. 2. FTIR spectra of H-PDMS and PDMS.
many protrusions in micro sizes irregularly distributing on the surface. The protrusions looked like some aggregates which were mainly composed of nano-SiO2 particles. These special structures were always considered as the so-called hierarchical roughness [26], which was effective in capturing the air for the increase in WCA [15]. While in Fig. 3(b), it clearly showed that the water droplet was unable to spread over the surface and kept spherical on the surface with a high WCA of 153◦ , indicating the superhydrophobicity of the PDMS/SiO2 coating. It was worthy to note that the water droplet was easy to slide when the surface inclined at a slight angle less than 5◦ . This wetting behavior endowed the surface with self-cleaning property which was known as the “lotus effect” [27].
3.2. Effect of calcinations on superhydrophobic PDMS/SiO2 coating 3.2.1. Wetting behavior The effect of calcination temperature on the WCA is shown in Fig. 4. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the surface maintained superhydrophobicity with the WCA over 150◦ , indicating the high thermal stability of the superhydrophobic PDMS/SiO2 . This stage is always thought as the process of water and solvent volatilization in the coating [20]. However, when the calcination temperature increased to 500 ◦ C, the WCA decreased to nearly 0◦ , indicating that the wetting behavior of the surface had changed from superhydrophobicity to superhydrophilicity. This was because the –CH3 groups on the surface of SiO2 particles had been replaced by –OH groups due to the
Fig. 3. SEM photograph (a) and WCA image (b) of superhydrophobic PDMS/SiO2 surface.
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Fig. 5. Effect of calcination time on the WCA of superhydrophobic PDMS/SiO2 at 500 ◦ C. Fig. 7. FTIR spectra of PDMS/SiO2 composite under different calcination temperatures.
oxidation decomposition of –CH3 groups after calcining at high temperature [28]. At this time, the water droplet quickly spread over the surface as soon as the water touched the surface, which was consistent with the Wenzel theory that the roughness on the hydrophilic surface made it more hydrophilic [29]. In fact, the oxidative decomposition of methyl groups on the surface took a certain amount of time. As shown in Fig. 5, when the coating was calcined at 500 ◦ C, it could keep superhydrophobic within 3 h. When the calcination lasted for more than 4 h, the surface of the coating became superhydrophilic with a WCA of 0◦ , indicating that the methyl groups had been completely oxidized [28]. 3.2.2. TG-FTIR The TG-FTIR technique was used to study the thermal stability of the PDMS and PDMS/SiO2 composite. As shown in Fig. 6(a), due to the volatilization of solvent/water, there was little weight loss before 200 ◦ C. The initial decomposition temperature (5% weight loss) of the PDMS/SiO2 composite was at 325 ◦ C, and the drastic weight loss occurred at nearly 380 ◦ C. The TG curves of PDMS showed a similar trend in comparison with the PDMS/SiO2 composite, and the remaining SiO2 particles at 700 ◦ C was nearly 30 wt% because of the oxidation of PDMS in air condition. While for PDMS/SiO2 composite, the residue yield at 700 ◦ C was about
70 wt%, much higher than the initial SiO2 content (25 wt%) in the composite coating, which was caused by the generation of SiO2 particles produced from the oxidation of PDMS. From Fig. 6(b), it was evident that there were no obvious peaks when the temperature was lower 300 ◦ C. However, at 400 ◦ C, many peaks appeared. The peaks near 2700–2900 cm−1 were ascribed to the stretching vibrations of –CH2 – and –CH3 from the decomposition of PDMS. The bands at about 2230 cm−1 were due to CO2 from the oxidation of alkyls. The peaks located at 1100 cm−1 and 800 cm−1 were attributed to the Si–O–Si and Si–O in the PDMS fragments produced by the decomposition of PDMS. The FTIR confirmed that the drastic weight loss was mainly caused by the decomposition of the PDMS. When the temperature increased to 600 ◦ C, no other peaks could be observed except the characteristic peaks of CO2 at 2230 cm−1 , owing to the continuous decomposition of alkyls from the hydrophobic SiO2 and PDMS. This was consistent with the result that the mass of the PDMS/SiO2 composite gradually decreased with the further increase in temperature over 500 ◦ C shown in Fig. 6(a). 3.2.3. FTIR The FTIR spectra of PDMS/SiO2 composite under different calcination temperatures are presented in Fig. 7. From the FTIR spectra
Fig. 6. TG curves of PDMS and PDMS/SiO2 composite (a) and FTIR spectra of volatile matters produced by PDMS/SiO2 composite under different temperatures (b).
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reported in Fig. 7, for the composite at 100 ◦ C, the peaks located near 2960 cm−1 , 1093 cm−1 , and 800 cm−1 were attributed to the stretching vibrations of –CH3 , Si–O–Si, and Si–O groups in the PDMS, respectively. The bands at 871 cm−1 were related to the Si–C vibration [24]. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the peaks of –CH3 near 2960 cm−1 and Si–C at 871 cm−1 became weak, owing to the gradual decomposition of the PDMS chain. Meanwhile, the FTIR spectra of PDMS/SiO2 composite calcined at 400 ◦ C further confirmed the existence of Si–CH3 groups in the superhydrophobic composite coating. When the calcination temperature increased to 500 ◦ C, both of the peaks near 2960 cm−1 and 871 cm−1 completely disappeared, which was due to the decomposition of Si–CH3 . Moreover, a new peak of Si–OH at 3421 cm−1 appeared, indicating that the Si–OH groups had replaced the Si–CH3 groups in the composite. The replacement of hydrophobic groups of Si–CH3 with hydrophilic groups of Si–OH resulted in the change of wetting behavior from superhydrophobicity to superhydrophilicity, which was consistent with the results discussed above. 3.2.4. Transmittance Fig. 8 shows the visible light transmittance of the superhydrophobic coating under different calcination temperatures and its digital photos before and after calcining. In Fig. 8(a), at a low calcination temperature of 100 ◦ C, the visible light transmittance of composite coating was only 40%, far below the visible light transmittance of original slide glass (about 90%). With the calcination temperature increased, the coating became more and more transparent. When the calcination temperature reached 400 ◦ C, the coating was rather transparent with a high visible light transmittance of about 80%. In general, the superhydrophobic coating was thought as opaque for the effect of scattering light on the roughness surface required for superhydrophobicity [30]. In this study, the coating became transparent after calcining at high temperature, which might be caused by the decomposition of PDMS and the rearrangement of nano–SiO2 particles. As the digital photos shown in Fig. 8(b), the superhydrophobic PDMS/SiO2 coating was opaque with the water droplets (dyed as blue) maintained spherical on the surface before calcining. However, it turned to transparent and still kept high superhydrophobicity after calcining at 400 ◦ C. But the superhydrophobicity of the transparent coating would be destroyed and became superhydrophilic when the calcination temperature was 500 ◦ C.
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3.2.5. SEM The SEM photographs of superhydrophobic PDMS/SiO2 coating calcined at different temperatures are presented in Fig. 9. In Fig. 9(a), at a relative low calcination temperature of 100 ◦ C, many different sizes of irregular protrusions were connected together on the surface using PDMS as binder. Fig. 9(a1 ) further confirmed that the protrusions were mainly composed of nanoparticles. From Fig. 9(b) and (b1 ), when the calcination temperature increased to 200 ◦ C, there was no significant change on the surface morphology, indicating the high thermal stability of the PDMS/SiO2 composite coating. While in Fig. 9(c), with the calcination temperature reached 300 ◦ C, the connected protrusions became scattered and discontinuous, owing to the decomposition of binder of PDMS. But the SiO2 still tightly attached to the surface of the slide glass due to the condensation between the hydroxyls from the SiO2 and slide glass under high temperature [31]. Moreover, the SEM photograph of Fig. 9(c1 ) revealed that some of the larger particles appeared on the surface. From the analysis of TG, it might be infer that these particles were SiO2 particles, which were produced by the decomposition of PDMS. The decomposition of PDMS and the rearrangement of nano-SiO2 particles made contributions to the high visible light transmittance of the superhydrophobic PDMS/SiO2 coating. When the calcination temperature was up to 400 ◦ C, in Fig. 9(d) and (d1 ), compared with Fig. 9(c) and (c1 ), the coating showed a similar surface morphology but the high temperature promoted these progresses. For the further increase in temperature to 500 ◦ C, Fig. 9(e1 ) displayed that the surface of the SiO2 particles became crude, which was caused by the adsorption between hydroxyl groups on the surface of SiO2 particles.
3.2.6. EDS The EDS spectra of the superhydrophobic PDMS/SiO2 coating after calcining at 100 ◦ C and 500 ◦ C are presented in Fig. 10. In Fig. 10(a), when the calcination temperature was 100 ◦ C, it clearly showed that the superhydrophobic PDMS/SiO2 coating was mainly composed of Si, C, and O elements, and the mass ratio of Si to C was about 1.4. However, as shown in Fig. 10(b), when the calcination temperature increased to 500 ◦ C, the C element content greatly reduced, and the mass ratio of Si to C increased to 8.2. The element change resulted from the decomposition of organic groups from PDMS and hydrophobic SiO2 particles.
Fig. 8. Effects of calcination temperature on the transmittance (a) and the digital photos of PDMS/SiO2 coating before and after calcining (b).
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Fig. 9. SEM photographs of superhydrophobic PDMS/SiO2 coating under different calcination temperatures. (a–a1 ), 100 ◦ C; (b–b1 ), 200 ◦ C; (c–c1 ), 300 ◦ C; (d–d1 ), 400 ◦ C; (e–e1 ), 500 ◦ C.
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Fig. 10. EDS spectra of superhydrophobic PDMS/SiO2 coating calcined under 100 ◦ C (a) and 500 ◦ C (b).
3.3. Calcination mechanism of the superhydrophobic PDMS/SiO2 coating Based on the results discussed above, the calcination mechanism of the superhydrophobic PDMS/SiO2 coating is proposed in Fig. 11. As shown in Fig. 11(a), the PDMS/SiO2 coating was prepared on the slide glass, and the hierarchical roughness and the low surface energy material PDMS on the surface endowed the coating with superhydrophobicity. In Fig. 11(b), after calcining at low temperature of 100–200 ◦ C, the composite showed high thermal stability and the water/solvent remaining inside the coating was released at this stage. When the calcination temperature
increased to 300–400 ◦ C, the PDMS started to decompose, and some of the PDMS fragments volatilized. Meanwhile, hydroxyl groups remaining on the surface of hydrophobic SiO2 reacted with each other at high temperature, and this reaction also occurred between the slide glass and the SiO2 nanoparticles, which was shown in Fig. 11(c). While in Fig. 11(d), with the continuous decomposition of the binder PDMS and the condensation between hydroxyls, the hydrophobic SiO2 particles tended to keep in large particle sizes. Moreover, some of the hydrophobic SiO2 particles were produced by the oxidative decomposition of PDMS. Thus, the rearrangement of the SiO2 particles took place, which made the composite coating become transparent. But the coating still kept superhydrophobicity
Fig. 11. Schematic of calcination mechanism of the superhydrophobic PDMS/SiO2 coating.
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due to the high thermal stability of Si–CH3 groups. When the temperature reached 500 ◦ C, the Si–CH3 groups started to decompose, and a mass of Si–OH generated, resulting in the superhydrophilicity of the coating. 4. Conclusion The superhydrophobic PDMS/SiO2 coating was prepared through spraying the mixture of PDMS and hydrophobic nanoSiO2 on slide glass. The superhydrophobicity of the PDMS/SiO2 coating with a WCA of 153◦ resulted from the hierarchical roughness and the low surface energy material on the surface. After calcining under different temperatures, the PDMS/SiO2 coatings showed various microstructure, transmittance, and wetting behavior. The PDMS/SiO2 coating kept superhydrophobicity when the calcination temperature was lower than 400 ◦ C, while it changed to superhydrophilicity when the calcination temperature was over 500 ◦ C. Moreover, with the increase in calcination temperature, the coating became transparent. When the calcination temperature was 400 ◦ C, the coating showed a high visible light transmittance of about 80%, resulting from the rearrangement of the SiO2 particles and the decomposition of PDMS. The replacement of Si–CH3 with Si–OH on the surface of SiO2 particles at high calcination temperature was responsible for the wetting behavior changing from superhydrophobicity to superhydrophilicity. Finally, the calcination mechanism was also proposed, which might have significances for the further study of other composite coatings. Acknowledgements The authors are thankful for the financial support of UniversityIndustry-Research Cooperation Project of Guangdong Province and Ministry of Education, China (No. 2012B091000070). References [1] Y.B. Zhang, Y. Chen, L. Shi, J. Li, Z.G. Guo, Recent progress of double-structural and functional materials with special wettability, J. Mater. Chem. 22 (2012) 799–815. [2] T.L. Sun, G.Y. Qing, B.L. Su, L. Jiang, Functional biointerface materials inspired from nature, Chem. Soc. Rev. 40 (2011) 2909–2921. [3] I.P. Parkin, R.G. Palgrave, Self-cleaning coatings, J. Mater. Chem. 15 (2005) 1689–1695. [4] H. Ogihara, J. Xie, T. Saji, Factors determining wettability of superhydrophobic paper prepared by spraying nanoparticle suspensions, Colloid. Surf. A 434 (2013) 35–41. [5] X.H. Xu, Z.Z. Zhang, J. Yang, X.T. Zhu, Study of the corrosion resistance and loading capacity of superhydrophobic meshes fabricated by spraying method, Colloid. Surf. A 377 (2011) 70–75. [6] A.K. Kota, G. Kwon, W. Choi, J.M. Mabry, A. Tuteja, Hygro-responsive membranes for effective oil–water separation, Nat. Commun. 3 (2012) 1025–1032. [7] Y.H. Xue, S.G. Chu, P.Y. Lv, H.L. Duan, Importance of hierarchical structures in wetting stability on submersed superhydrophobic surfaces, Langmuir 28 (2012) 9440–9450. [8] B. Bharat, C.J. Yong, Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction, Prog. Mater. Sci. 56 (2011) 1–108.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Effects of calcination temperature on the microstructure and wetting behavior of superhydrophobic polydimethylsiloxane/silica coating Kunquan Li, Xingrong Zeng ∗ , Hongqiang Li, Xuejun Lai, Hu Xie College of Materials Science and Engineering, South China University of Technology, No. 381, Wushan Road, Tianhe District, Guangzhou 510640, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The superhydrophobic coating was fabricated by spraying the PDMS and nano-SiO2 . • The coating showed various microstructure and wettability by adjusting calcination. • After calcining at 400 ◦ C, the superhydrophobic coating became highly transparent. • The calcination mechanism of the PDMS/SiO2 coating was proposed.
a r t i c l e
i n f o
Article history: Received 25 September 2013 Received in revised form 2 January 2014 Accepted 15 January 2014 Available online 24 January 2014 Keywords: PDMS/SiO2 coating Superhydrophobic Calcinations Wetting behavior Microstructure
a b s t r a c t The superhydrophobic coating was prepared through spraying the mixture of polydimethylsiloxane (PDMS) and hydrophobic nanosilica (SiO2 ) on the slide glass. The surface of the as-prepared PDMS/SiO2 coating showed hierarchical roughness with a water contact angle (WCA) of 153◦ . The resultant superhydrophobic PDMS/SiO2 coating was subsequently calcined in a muffle furnace under air atmosphere and the effects of calcination temperature on the microstructure, component, transmittance, and wetting behavior of superhydrophobic PDMS/SiO2 coating were systematically investigated. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the superhydrophobic PDMS/SiO2 coating became transparent with the visible light transmittance increasing from 40% to 80%, which was ascribed to the decomposition of PDMS and the rearrangement of the hydrophobic SiO2 particles. However, when the calcination temperature was over 500 ◦ C, the wetting behavior of the coating changed from superhydrophobicity to superhydrophilicity with a WCA of nearly 0◦ , owing to the replacement of hydrophobic Si–CH3 groups with hydrophilic groups of Si–OH. Finally, the calcination mechanism of the superhydrophobic PDMS/SiO2 composite coating is proposed, which has guiding significance for the study of other composite coatings. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Currently, solid surfaces with controlled wettability have gained much concern for their potential applications in life and industry [1]. Inspired by the natural biological species [2], such as lotus leaf and butterfly’s wing, the superhydrophobic surfaces, characterized as water contact angle (WCA) above 150◦ and sliding angle
∗ Corresponding author. Tel.: +86 20 87114248; fax: +86 20 87114248. E-mail address: [email protected] (X. Zeng). 0927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2014.01.024
(SA) below 10◦ , are thought to be ideal candidates in self-cleaning coating [3], waterproof [4], anti-corrosion material [5], water/oil separation film [6], and water transport pipeline [7]. Plenty of researches have confirmed that this special wetting behavior is mainly ascribed to the low surface energy materials and the hierarchical micro-/nanostructures on the surface [8]. According to these findings, lots of methods have been reported to fabricate superhydrophobic surfaces, including sol-gel method [9], template imitated [10], plasma etching [11], electrospinning [12], and so on. In most studies, the polydimethylsiloxanes (PDMS) was usually used as low surface energy material to replace the fluorine material in
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fabricating superhydrophobic surface due to its advantages of low cost, simple and easy-obtained [13,14]. Moreover, the properties of non-toxic, optical transparency, and high thermal stability of PDMS allow the products to be applied in various areas. Although the PDMS coating exhibits a good water-repellent property, its intrinsic WCA is only about 105◦ [15], which still cannot satisfy the requirements of superhydrophobicity. Thus, it is necessary to increase the roughness of the surface. In recent years, the introduction of inorganic particles, especially silica (SiO2 ) nanoparticle, to construct hierarchical structures on the PDMS surface is widely appreciated [16–18]. Ogihara et al. [17] prepared the superhydrophobic PDMS/SiO2 composite coatings by electrophoretic deposition. The results showed that the presence of both nanometer- and micrometer-sized roughness on the surfaces resulted from the aggregates of SiO2 particles. A highly transparent superhydrophobic surface with dual-scale roughness was also created by Karunakaran using modified SiO2 particles to construct roughness [18]. In general, the superhydrophobic surfaces made from PDMS/SiO2 composite are of great interest for both academic researches and industrial applications. The superhydrophobic PDMS/SiO2 surfaces show superhydrophobicity at room temperature, but in many cases, surfaces are required to keep their performance when exposed to heat [19]. According to the previous reports, the WCA was considered as the only standard to evaluate the thermal stability of superhydrophobic PDMS/SiO2 surface [20–22]. Xiang et al. [21] prepared superhydrophobic PDMS surfaces and found that the resultant surfaces kept superhydrophobicity after thermal treatment at 300 ◦ C while it changed to superhydrophilicity at 400 ◦ C. The organic SiO2 coating could remain constant in high WCA and low SA for calcination temperature up to 350 ◦ C, which was confirmed by Deng [19]. Lin et al., [22] also revealed that the superhydrophobic PDMS/SiO2 coating showed good thermal stability with a WCA higher than 160◦ after calcining at 400 ◦ C for 1 h. However, it is unwise to only use WCA to evaluate the thermal stability of superhydrophobic surface because the surface component and morphology structure usually have changed when the PDMS/SiO2 is calcined at high temperature, which has always been neglected by other researches [23]. In this study, the superhydrophobic PDMS/SiO2 coating was prepared through a facile way by spraying the mixture of PDMS and hydrophobic nano-SiO2 on the slide glasses. The obtained superhydrophobic PDMS/SiO2 coating was calcined in a muffle furnace under air atmosphere and the effects of calcination temperature on the microstructure, component, wetting behavior, and transmittance were investigated. The superhydrophobic PDMS/SiO2 coatings were characterized by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), WCA and thermal gravimetric (TG) analysis, energy dispersive spectroscopy (EDS), and UV−vis spectrophotometer, respectively. Finally, the calcination mechanism was also proposed.
2. Experimental 2.1. Materials The hydroxyl-terminated polydimethylsiloxane (H-PDMS) (107 Glue, molecular weight of about 1500) was provided by Jiangxi Xinghuo Organic Silicone Plant (China). Tetraethoxysilane (TEOS) and dibutyltin dilaurate (DBTDL) were acquired from Shanghai Reagent Factory (China). Hydrophobic nano-SiO2 (H15, diameter of 16–20 nm) was purchased from Wacker Company (Germany). Ethanol was obtained from Guangzhou Chemical Reagent Factory (China). The n-hexane was supplied by Sinopharm Chemical Reagent Limited Company (China). All the reagents were used as received.
2.2. Preparation of superhydrophobic PDMS/SiO2 coating The superhydrophobic PDMS/SiO2 coating was prepared according to the following procedures. Firstly, the slide glass was ultrasonically washed with ethanol–water solution and dried at 100 ◦ C for 15 min. Thereafter, 0.5 g H-PDMS, 0.01 g catalyst of DBTDL, and 0.1 g curing agent of TEOS were mixed in 10 g n-hexane by vigorous magnetic stirring for 15 min. Then, 0.2 g hydrophobic SiO2 was added into the above mixture under ultrasonic dispersion condition for about 10 min. Finally, the mixture was sprayed onto the slide glass, and the superhydrophobic PDMS/SiO2 coating was obtained after curing at 80 ◦ C for 30 min. The preparation process of PDMS/SiO2 coating was presented in Fig. 1. 2.3. Calcinations of superhydrophobic PDMS/SiO2 coating The as-prepared superhydrophobic PDMS/SiO2 coatings were calcined at a muffle furnace under air atmosphere, and the calcination temperatures were adjusted to 100 ◦ C, 200 ◦ C, 300 ◦ C, 400 ◦ C, and 500 ◦ C, respectively. Then, the calcinations lasted for 5 h (for the coating calcined at 500 ◦ C, the calcination times varied from 1 h to 5 h). Finally, the PDMS/SiO2 coatings were transferred to the airy platform and cooled at room temperature. 2.4. Characterization Fourier transform infrared spectra were recorded on a Bruker Tensor 27 spectrometer (Bruker Optics, Germany) within a range of 4000–400 cm−1 by KBr pellet technique. Microscopic morphologies of the surfaces were observed by a field-emission scanning electron microscopy (SEM, Fei Nova Nanosem 430, the Netherlands) equipped with EDS unit at an acceleration voltage of 15 kV. A thin gold layer was sprayed on the sample surfaces to prevent charging before testing. WCA tests were carried out on a contact angle detector (DSA100, Germany) equipped with a digital camera at room temperature. For WCA studies, 5 L distilled water was used. All of the obtained WCA values were calculated from at least five different places of the samples. Thermal stability was measured by thermal gravimetric analysis (TG, TGA209F3, Germany) coupled with FTIR spectrometer at a heating rate of 10 ◦ C/min from 35 ◦ C to 800 ◦ C under air atmosphere. Transmittance of the coating calcined under different temperatures was evaluated by UV–vis spectroscopy (UV765CRT, China). The digital photographs of the transparent coating were taken by an SLR camera (Nikon D90, Japan). 3. Results and discussion 3.1. Superhydrophobic PDMS/SiO2 composite coating 3.1.1. FTIR spectra of H-PDMS and PDMS Fig. 2 illustrates the FTIR spectra of H-PDMS (a) and PDMS (b). In Fig. 2(a), the peaks at 3463 cm−1 , 2960 cm−1 , and 1108 cm−1 were ascribed to the stretching vibrations of Si–OH, –CH3 , and Si–O–Si groups in the H-PDMS [24], respectively. The absorption bands near 1245 cm−1 were attributed to the symmetric deformation of the –CH3 group in –Si(CH3 )2 of H-PDMS, and the bands located at 871 cm−1 and 806 cm−1 were assigned to the Si–C and Si–O vibration [25], respectively. Compared with the spectra of Fig. 2(a), no obvious new peaks could be observed, while the peaks near 3463 cm−1 disappeared in Fig. 2(b), indicating that the Si–OH in the H-PDMS had reacted with the Si–OC2 H5 of the TEOS. 3.1.2. SEM and WCA of PDMS/SiO2 coating The SEM photograph and WCA image of superhydrophobic PDMS/SiO2 surface are shown in Fig. 3. From Fig. 3(a), there were
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Fig. 1. Schematic diagram of fabricating PDMS/SiO2 coating.
Fig. 4. Effect of calcination temperature on the WCA of superhydrophobic PDMS/SiO2 coating. Fig. 2. FTIR spectra of H-PDMS and PDMS.
many protrusions in micro sizes irregularly distributing on the surface. The protrusions looked like some aggregates which were mainly composed of nano-SiO2 particles. These special structures were always considered as the so-called hierarchical roughness [26], which was effective in capturing the air for the increase in WCA [15]. While in Fig. 3(b), it clearly showed that the water droplet was unable to spread over the surface and kept spherical on the surface with a high WCA of 153◦ , indicating the superhydrophobicity of the PDMS/SiO2 coating. It was worthy to note that the water droplet was easy to slide when the surface inclined at a slight angle less than 5◦ . This wetting behavior endowed the surface with self-cleaning property which was known as the “lotus effect” [27].
3.2. Effect of calcinations on superhydrophobic PDMS/SiO2 coating 3.2.1. Wetting behavior The effect of calcination temperature on the WCA is shown in Fig. 4. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the surface maintained superhydrophobicity with the WCA over 150◦ , indicating the high thermal stability of the superhydrophobic PDMS/SiO2 . This stage is always thought as the process of water and solvent volatilization in the coating [20]. However, when the calcination temperature increased to 500 ◦ C, the WCA decreased to nearly 0◦ , indicating that the wetting behavior of the surface had changed from superhydrophobicity to superhydrophilicity. This was because the –CH3 groups on the surface of SiO2 particles had been replaced by –OH groups due to the
Fig. 3. SEM photograph (a) and WCA image (b) of superhydrophobic PDMS/SiO2 surface.
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Fig. 5. Effect of calcination time on the WCA of superhydrophobic PDMS/SiO2 at 500 ◦ C. Fig. 7. FTIR spectra of PDMS/SiO2 composite under different calcination temperatures.
oxidation decomposition of –CH3 groups after calcining at high temperature [28]. At this time, the water droplet quickly spread over the surface as soon as the water touched the surface, which was consistent with the Wenzel theory that the roughness on the hydrophilic surface made it more hydrophilic [29]. In fact, the oxidative decomposition of methyl groups on the surface took a certain amount of time. As shown in Fig. 5, when the coating was calcined at 500 ◦ C, it could keep superhydrophobic within 3 h. When the calcination lasted for more than 4 h, the surface of the coating became superhydrophilic with a WCA of 0◦ , indicating that the methyl groups had been completely oxidized [28]. 3.2.2. TG-FTIR The TG-FTIR technique was used to study the thermal stability of the PDMS and PDMS/SiO2 composite. As shown in Fig. 6(a), due to the volatilization of solvent/water, there was little weight loss before 200 ◦ C. The initial decomposition temperature (5% weight loss) of the PDMS/SiO2 composite was at 325 ◦ C, and the drastic weight loss occurred at nearly 380 ◦ C. The TG curves of PDMS showed a similar trend in comparison with the PDMS/SiO2 composite, and the remaining SiO2 particles at 700 ◦ C was nearly 30 wt% because of the oxidation of PDMS in air condition. While for PDMS/SiO2 composite, the residue yield at 700 ◦ C was about
70 wt%, much higher than the initial SiO2 content (25 wt%) in the composite coating, which was caused by the generation of SiO2 particles produced from the oxidation of PDMS. From Fig. 6(b), it was evident that there were no obvious peaks when the temperature was lower 300 ◦ C. However, at 400 ◦ C, many peaks appeared. The peaks near 2700–2900 cm−1 were ascribed to the stretching vibrations of –CH2 – and –CH3 from the decomposition of PDMS. The bands at about 2230 cm−1 were due to CO2 from the oxidation of alkyls. The peaks located at 1100 cm−1 and 800 cm−1 were attributed to the Si–O–Si and Si–O in the PDMS fragments produced by the decomposition of PDMS. The FTIR confirmed that the drastic weight loss was mainly caused by the decomposition of the PDMS. When the temperature increased to 600 ◦ C, no other peaks could be observed except the characteristic peaks of CO2 at 2230 cm−1 , owing to the continuous decomposition of alkyls from the hydrophobic SiO2 and PDMS. This was consistent with the result that the mass of the PDMS/SiO2 composite gradually decreased with the further increase in temperature over 500 ◦ C shown in Fig. 6(a). 3.2.3. FTIR The FTIR spectra of PDMS/SiO2 composite under different calcination temperatures are presented in Fig. 7. From the FTIR spectra
Fig. 6. TG curves of PDMS and PDMS/SiO2 composite (a) and FTIR spectra of volatile matters produced by PDMS/SiO2 composite under different temperatures (b).
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reported in Fig. 7, for the composite at 100 ◦ C, the peaks located near 2960 cm−1 , 1093 cm−1 , and 800 cm−1 were attributed to the stretching vibrations of –CH3 , Si–O–Si, and Si–O groups in the PDMS, respectively. The bands at 871 cm−1 were related to the Si–C vibration [24]. With the increase in calcination temperature from 100 ◦ C to 400 ◦ C, the peaks of –CH3 near 2960 cm−1 and Si–C at 871 cm−1 became weak, owing to the gradual decomposition of the PDMS chain. Meanwhile, the FTIR spectra of PDMS/SiO2 composite calcined at 400 ◦ C further confirmed the existence of Si–CH3 groups in the superhydrophobic composite coating. When the calcination temperature increased to 500 ◦ C, both of the peaks near 2960 cm−1 and 871 cm−1 completely disappeared, which was due to the decomposition of Si–CH3 . Moreover, a new peak of Si–OH at 3421 cm−1 appeared, indicating that the Si–OH groups had replaced the Si–CH3 groups in the composite. The replacement of hydrophobic groups of Si–CH3 with hydrophilic groups of Si–OH resulted in the change of wetting behavior from superhydrophobicity to superhydrophilicity, which was consistent with the results discussed above. 3.2.4. Transmittance Fig. 8 shows the visible light transmittance of the superhydrophobic coating under different calcination temperatures and its digital photos before and after calcining. In Fig. 8(a), at a low calcination temperature of 100 ◦ C, the visible light transmittance of composite coating was only 40%, far below the visible light transmittance of original slide glass (about 90%). With the calcination temperature increased, the coating became more and more transparent. When the calcination temperature reached 400 ◦ C, the coating was rather transparent with a high visible light transmittance of about 80%. In general, the superhydrophobic coating was thought as opaque for the effect of scattering light on the roughness surface required for superhydrophobicity [30]. In this study, the coating became transparent after calcining at high temperature, which might be caused by the decomposition of PDMS and the rearrangement of nano–SiO2 particles. As the digital photos shown in Fig. 8(b), the superhydrophobic PDMS/SiO2 coating was opaque with the water droplets (dyed as blue) maintained spherical on the surface before calcining. However, it turned to transparent and still kept high superhydrophobicity after calcining at 400 ◦ C. But the superhydrophobicity of the transparent coating would be destroyed and became superhydrophilic when the calcination temperature was 500 ◦ C.
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3.2.5. SEM The SEM photographs of superhydrophobic PDMS/SiO2 coating calcined at different temperatures are presented in Fig. 9. In Fig. 9(a), at a relative low calcination temperature of 100 ◦ C, many different sizes of irregular protrusions were connected together on the surface using PDMS as binder. Fig. 9(a1 ) further confirmed that the protrusions were mainly composed of nanoparticles. From Fig. 9(b) and (b1 ), when the calcination temperature increased to 200 ◦ C, there was no significant change on the surface morphology, indicating the high thermal stability of the PDMS/SiO2 composite coating. While in Fig. 9(c), with the calcination temperature reached 300 ◦ C, the connected protrusions became scattered and discontinuous, owing to the decomposition of binder of PDMS. But the SiO2 still tightly attached to the surface of the slide glass due to the condensation between the hydroxyls from the SiO2 and slide glass under high temperature [31]. Moreover, the SEM photograph of Fig. 9(c1 ) revealed that some of the larger particles appeared on the surface. From the analysis of TG, it might be infer that these particles were SiO2 particles, which were produced by the decomposition of PDMS. The decomposition of PDMS and the rearrangement of nano-SiO2 particles made contributions to the high visible light transmittance of the superhydrophobic PDMS/SiO2 coating. When the calcination temperature was up to 400 ◦ C, in Fig. 9(d) and (d1 ), compared with Fig. 9(c) and (c1 ), the coating showed a similar surface morphology but the high temperature promoted these progresses. For the further increase in temperature to 500 ◦ C, Fig. 9(e1 ) displayed that the surface of the SiO2 particles became crude, which was caused by the adsorption between hydroxyl groups on the surface of SiO2 particles.
3.2.6. EDS The EDS spectra of the superhydrophobic PDMS/SiO2 coating after calcining at 100 ◦ C and 500 ◦ C are presented in Fig. 10. In Fig. 10(a), when the calcination temperature was 100 ◦ C, it clearly showed that the superhydrophobic PDMS/SiO2 coating was mainly composed of Si, C, and O elements, and the mass ratio of Si to C was about 1.4. However, as shown in Fig. 10(b), when the calcination temperature increased to 500 ◦ C, the C element content greatly reduced, and the mass ratio of Si to C increased to 8.2. The element change resulted from the decomposition of organic groups from PDMS and hydrophobic SiO2 particles.
Fig. 8. Effects of calcination temperature on the transmittance (a) and the digital photos of PDMS/SiO2 coating before and after calcining (b).
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Fig. 9. SEM photographs of superhydrophobic PDMS/SiO2 coating under different calcination temperatures. (a–a1 ), 100 ◦ C; (b–b1 ), 200 ◦ C; (c–c1 ), 300 ◦ C; (d–d1 ), 400 ◦ C; (e–e1 ), 500 ◦ C.
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Fig. 10. EDS spectra of superhydrophobic PDMS/SiO2 coating calcined under 100 ◦ C (a) and 500 ◦ C (b).
3.3. Calcination mechanism of the superhydrophobic PDMS/SiO2 coating Based on the results discussed above, the calcination mechanism of the superhydrophobic PDMS/SiO2 coating is proposed in Fig. 11. As shown in Fig. 11(a), the PDMS/SiO2 coating was prepared on the slide glass, and the hierarchical roughness and the low surface energy material PDMS on the surface endowed the coating with superhydrophobicity. In Fig. 11(b), after calcining at low temperature of 100–200 ◦ C, the composite showed high thermal stability and the water/solvent remaining inside the coating was released at this stage. When the calcination temperature
increased to 300–400 ◦ C, the PDMS started to decompose, and some of the PDMS fragments volatilized. Meanwhile, hydroxyl groups remaining on the surface of hydrophobic SiO2 reacted with each other at high temperature, and this reaction also occurred between the slide glass and the SiO2 nanoparticles, which was shown in Fig. 11(c). While in Fig. 11(d), with the continuous decomposition of the binder PDMS and the condensation between hydroxyls, the hydrophobic SiO2 particles tended to keep in large particle sizes. Moreover, some of the hydrophobic SiO2 particles were produced by the oxidative decomposition of PDMS. Thus, the rearrangement of the SiO2 particles took place, which made the composite coating become transparent. But the coating still kept superhydrophobicity
Fig. 11. Schematic of calcination mechanism of the superhydrophobic PDMS/SiO2 coating.
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due to the high thermal stability of Si–CH3 groups. When the temperature reached 500 ◦ C, the Si–CH3 groups started to decompose, and a mass of Si–OH generated, resulting in the superhydrophilicity of the coating. 4. Conclusion The superhydrophobic PDMS/SiO2 coating was prepared through spraying the mixture of PDMS and hydrophobic nanoSiO2 on slide glass. The superhydrophobicity of the PDMS/SiO2 coating with a WCA of 153◦ resulted from the hierarchical roughness and the low surface energy material on the surface. After calcining under different temperatures, the PDMS/SiO2 coatings showed various microstructure, transmittance, and wetting behavior. The PDMS/SiO2 coating kept superhydrophobicity when the calcination temperature was lower than 400 ◦ C, while it changed to superhydrophilicity when the calcination temperature was over 500 ◦ C. Moreover, with the increase in calcination temperature, the coating became transparent. When the calcination temperature was 400 ◦ C, the coating showed a high visible light transmittance of about 80%, resulting from the rearrangement of the SiO2 particles and the decomposition of PDMS. The replacement of Si–CH3 with Si–OH on the surface of SiO2 particles at high calcination temperature was responsible for the wetting behavior changing from superhydrophobicity to superhydrophilicity. Finally, the calcination mechanism was also proposed, which might have significances for the further study of other composite coatings. Acknowledgements The authors are thankful for the financial support of UniversityIndustry-Research Cooperation Project of Guangdong Province and Ministry of Education, China (No. 2012B091000070). References [1] Y.B. Zhang, Y. Chen, L. Shi, J. Li, Z.G. Guo, Recent progress of double-structural and functional materials with special wettability, J. Mater. Chem. 22 (2012) 799–815. [2] T.L. Sun, G.Y. Qing, B.L. Su, L. Jiang, Functional biointerface materials inspired from nature, Chem. Soc. Rev. 40 (2011) 2909–2921. [3] I.P. Parkin, R.G. Palgrave, Self-cleaning coatings, J. Mater. Chem. 15 (2005) 1689–1695. [4] H. Ogihara, J. Xie, T. Saji, Factors determining wettability of superhydrophobic paper prepared by spraying nanoparticle suspensions, Colloid. Surf. A 434 (2013) 35–41. [5] X.H. Xu, Z.Z. Zhang, J. Yang, X.T. Zhu, Study of the corrosion resistance and loading capacity of superhydrophobic meshes fabricated by spraying method, Colloid. Surf. A 377 (2011) 70–75. [6] A.K. Kota, G. Kwon, W. Choi, J.M. Mabry, A. Tuteja, Hygro-responsive membranes for effective oil–water separation, Nat. Commun. 3 (2012) 1025–1032. [7] Y.H. Xue, S.G. Chu, P.Y. Lv, H.L. Duan, Importance of hierarchical structures in wetting stability on submersed superhydrophobic surfaces, Langmuir 28 (2012) 9440–9450. [8] B. Bharat, C.J. Yong, Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction, Prog. Mater. Sci. 56 (2011) 1–108.
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