A ZnO-based nanocomposite coating with ultra water repellent properties

A ZnO-based nanocomposite coating with ultra water repellent properties

Applied Surface Science 258 (2012) 5723–5728 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevi...

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Applied Surface Science 258 (2012) 5723–5728

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

A ZnO-based nanocomposite coating with ultra water repellent properties Gelareh Momen ∗ , Masoud Farzaneh NSERC/Hydro-Quebec/UQAC Industrial Chair on Atmospheric Icing of Power, Network Equipment (CIGELE) and Canada Research Chair on Atmospheric Icing, Engineering of Power Networks (INGIVRE) www.cigele.ca, Université du Québec à Chicoutimi, Chicoutimi, QC, Canada

a r t i c l e

i n f o

Article history: Received 4 October 2011 Received in revised form 23 January 2012 Accepted 14 February 2012 Available online 22 February 2012 Keywords: Superhydrophobic coating ZnO UV Stability Nanoparticle

a b s t r a c t In this paper a simple and low-cost approach for the elaboration of a superhydrophobic nanocomposite coating is reported. This method can be used for preparing self-cleaning superhydrophobic coatings on large areas for different kinds of substrates. The synergistic effect of the micro-nano-binary scale roughness was produced by a silicone rubber/ZnO/SiO2 composite. A high static contact angle of about 162◦ and low contact angle hysteresis of about 7.5◦ was obtained for the prepared superhydrophobic surface on which water droplets were observed to easily roll off and bounce. This type of coating showed exceptional stability against UV exposure, humidity and heating. Also, good stability was observed after immersion in different aqueous solution. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The wettability of solid surfaces is an interesting property governed by a combination of surface chemistry and roughness on multiple scales. A surface is superhydrophobic when the static contact angle is greater than 150◦ . An ultra-water-repellent (UWR) surface has a water contact angle of more than 150◦ on which water drops roll over and over [1]. Such surfaces have a number of practical applications. The most common areas where superhydrophobic surfaces attract attention are bio-fouling, drug reduction, corrosion resistance, eyeglasses, self-cleaning windshields for automobiles, anti-sticking of snow or ice, and many others [2].These surfaces are commonly prepared in a two-steps process: surface roughness creation and coating with a low surface energy material. The most popular methods to create superhydrophobic surfaces are: wet chemical reaction [3], electrochemical deposition [4,5], selfassembly [6], layer-by-layer methods [7], plasma treatment [8], chemical vapor deposition [9], sol–gel method [10], polymerization reaction [11], templates [12], electrospraying [13] and sandblasting [14]. However, the above mentioned methods are still subject to certain limitations, such as time consumption and high facility costs. Therefore, it would be very beneficial and practical if superhydrophobic surfaces were obtained in just one-step using inexpensive materials and application methods. The spray coating

∗ Corresponding author. E-mail address: [email protected] (G. Momen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.074

method using the nanocomposite can fulfill this demand because of its rapidity and simplicity of application to large surface areas. A nanocomposite based on ZnO nanoparticles can be a good candidate due to its excellent semiconducting, optical, electrical and piezoelectrical properties and its potential applications in a variety of areas. Several studies have been reported on the improvement of thermal conductivity and relative permittivity of polymer matrix by the incorporation of ZnO nanoparticles [15]. A superhydrophobic nanocomposite coating was prepared by simple spraying. An appropriate combination of ZnO and SiO2 nanoparticles to silicone rubber leads to ultra-high water-repellent properties of the resulting coating. A big challenge in this field is to develop an easy, practical and industrially feasible method to prepare superhydrophobic coatings with good stability against adverse environmental influences. Concerning the stability of superhydrophobic coatings for outdoor applications, not much research has been done [16–19]. The present study was expanded to include several environmental factors such as immersion in acid solution, basic solution, tap water and deionized water, as well as exposition to UV light, humidity and heating treatment.

2. Experimental Commercial RTV silicone rubber containing 40–70 wt% alumina hydrate as well as zinc oxide (ZnO) nanoparticles with an average size of 100 nm (surface area of 15–25 m2 g−1 ) and SiO2 nanoparticles with an average size of 10 nm were used in the present study.

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Fig. 1. SEM images of the prepared coating with:(a) RTV SR; (b) RTV SR/ZnO; (c) RTV SR/ZnO/SiO2; (d) the high magnification of (a); (e) the high magnification of (b); (f) the high magnification of (c).

Glass substrates were ultrasonically cleaned in acetone and water. Three samples a, b and c were prepared as follows: Sample a It was prepared by adding 8 ml RTV silicone rubber (SR) to 100 ml hexane. Then, the suspension was sonicated and agitated magnetically for 40 min and 15 min respectively. Sample b It was prepared by dispersing 1 g of ZnO in 100 ml Hexane. Then, it was stirred magnetically for 20 min at 600 rpm followed by sonication for 30 min. A quantity of 8 ml of RTV silicone rubber was then added to this solution and sonicated for 40 min and agitated for 15 min (600 rpm). Sample c It was prepared by separately dispersing 0.2 g of SiO2 dioxide and 1 g of ZnO each in 50 ml of hexane. Then, the two solutions were agitated on a magnetic stirrer at 600 rpm for 20 min followed by sonication for 30 min. Afterward the ZnO and SiO2 prepared solutions were mixed together by a means of ultrasonic bath for 30 min

after which 8 ml of RTV silicone rubber was added. The resulting solution was put in ultrasonic bath for 40 min and then stirred for 15 min at 600 rpm. The three solutions (a, b and c) were coated on clean dry glass slides with nitrogen gas by means of a spray gun. Heat treatment of the coatings was done at 85 ◦ C in air overnight to remove residual solvents. The wettability of theses coatings was investigated using distilled water (water drop volume ∼4 ␮L) and a DSA -100 contact angle (CA) measuring instrument from Krüss. Contact angle hysteresis (CAH) was measured using a common experimental procedure [20]. The morphological characterization was carried out using an atomic force microscope (AFM) (Digital Nanoscope IIIa by Digital Instruments) to observe the surface roughness of the prepared coatings. FTIR spectra were obtained by using the Fourier transformation infrared spectrometer (Perkin-Elmer, Spectrum One). The reflected beam was collected for 24 scans at a resolution of 4 cm−1 .

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UV tests were performed by using standard tests ASTM G154 for the fluorescent UV exposure of non-metallic materials. A UVA-340 fluorescent lamp was used to simulate the short and middle UV wavelength region corresponding to daylight exposure. The test cycle was an 8-hour UV exposure with the uninsulated black panel temperature controlled at 60 ◦ C followed by a 4-hour condensation time with the uninsulated black panel temperature controlled at 50 ◦ C. A radiometer was used to monitor and control the amount of radiant energy received on the sample. 3. Results and discussion 3.1. Surface morphology FESEM investigations of the glass surfaces coated with SR, SR/ZnO and SR/ZnO/SiO2 composites showed the evolution of various morphological features as shown in Fig. 1(a–f). The images shown in Fig. 1d–f were obtained at higher resolution. Fig. 1a exhibits a surface with a uniform coating of SR with the alumina hydrate microparticles (1–2 ␮m) already present in its composition. A similar microstructure has been observed after ZnO nanoparticles incorporation as shown in Fig. 1b. However, the image obtained at higher resolution showed the presence of ZnO nanoparticles (Fig. 1e). Indeed, little morphological change was observed by the presence of ZnO nanoparticles and rock-like nanostructure appeared at nanometer scale while the SR coating (Fig. 1d) appeared to be smooth at this scale. By adding SiO2 nanoparticles, highly porous “coral-like” structure consisting of micro and nanostructures was formed (Fig. 1c) and both appearance and surface morphology of coating modified. Fig. 1f suggests that the coral-like morphology observed in Fig. 1c consists of nanostructures implying the presence of a twolength-scale hierarchical structure on the surface. It is believed that such a structure exists in the lotus leaf. The variation in the morphological features of surface has been demonstrated in Fig. 2a–c, using AFM analysis. The roughness values correspond to the root mean square values (Rq) of the surface height measurements. Fig. 2a shows a uniform silicone rubber coating with dispersed microparticles of alumina hydrate (1–2 ␮m) already present in its composition. As shown in Fig. 2b, the roughness of sample b, coated with SR/ZnO, is significantly higher than that of sample a, coated with SR. The quantitative analysis based on the AFM images indicates that the root-mean-square roughness (Rq) is 20 nm for the RTV SR coating while it increases to 103 nm for the SR/ZnO nanocomposite coating. The incorporation of SiO2 nanoparticles (of size smaller than 10 nm) to the SR/ZnO composite substantially contributed to increase the surface roughness of the coating, up to a value of 304 nm, as shown in Fig. 2c. 3.2. 3.2.Wettability Fig. 3 shows water droplets (4 ␮L) deposited on the samples a, b and c. Contact area between the water droplet and the solid surface was observed to be minimal for sample c. The water droplet shape was almost spherical for sample c which was not the case for samples a and b. The combination of low surface energy silicone rubber and appropriate surface roughness can produce a superhydrophobic surface, as shown with sample c. Contact angle hysteresis, which is the difference between the advancing and receding contact angle, was measured by moving water droplets on the samples [20]. Fig. 4 shows how the water droplets move on the samples. It was observed that water droplets

Fig. 2. 3D AFM topography images of the (a) SR coating, (b) SR/ZnO coating and (c) SR/ZnO/SiO2 coating. Table 1 Static contact angles and contact angle hysteresis of the prepared coatings. Sample

Description

Static contact angle (◦ )

Contact angle hysteresis (◦ )

(a) (b) (c)

RTV Silicone rubber RTV SR/ZnO RTV SR/ZnO/SiO2

117.3 ± 4.6 132.5 ± 2 162.7 ± 4.5

24.9 ± 3.5 39 ± 4.5 7.5 ± 5.6

tend to stick to the surfaces of samples a and b which implies relatively high contact angle hysteresis while the water droplet easily rolls off sample c. In Table 1, CA and CAH values are listed for the three samples. Theses contact angle values correspond the average of at least four measurements at various positions on each sample. The static contact angle on the flat surface coated with RTV silicone rubber doesn’t exceed 117◦ . Incorporation of ZnO nanoparticles to silicone rubber increased the static contact angle to 132.5 ± 2◦ and also increased slightly contact angle hysteresis to39 ± 4.5◦ . It should be noticed that increasing the ZnO content up to 2 g caused a slight increase of the contact angle (140.1 ± 1.3◦ ) and reduction of the contact angle hysteresis (36 ± 2.1◦ ). However, the appropriate level of micro/nanoscale roughness to create a high water repellent coating was not accomplished by the incorporation of ZnO nanoparticles. After a small amount (0.2 g) of SiO2 nanoparticle of small size (10 nm) was added, an important increase of contact angle, up to

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Fig. 3. Images of a water droplet sitting on a surface coated with (a) RTV SR, (b) SR/ZnO and (c) SR/ZnO/SiO2.

Fig. 4. Images of water droplets moving surfaces coated with (a) RTV SR, (b) SR/ZnO and (c) SR/ZnO/SiO2.

110

about 162◦ , was obtained and the surface became superhydrophobic. Furthermore, contact angle hysteresis was reduced to 7◦ with water droplets rolling off the surface. Two distinct models, developed by Wenzel and Cassie-Baxter, are commonly used to describe superhydrophobic surfaces which correspond to the homogeneous and heterogeneous wetting as shown in Fig. 5a and b, respectively. In Wenzel’s model (homogeneous interface), a wetting liquid will be completely absorbed by the rough surface cavities. However in the Cassie-Baxter model (composite interface), air pockets are trapped in the rough surface cavities, leading to a composite solid–liquid–air interface, as opposed to the homogeneous solid–liquid interface [21]. The former has a high contact angle hysteresis, while the latter has a low contact angle hysteresis which makes the presence of a composite interface evident. As a matter of fact, the incorporation of ZnO nanoparticles into silicon rubber produced nanoscale roughness that lead to an increase in static contact angle. However, the surface coated with silicone rubber/ZnO remained hydrophobic with a relatively high contact angle hysteresis which is characteristic of the Wenzel wetting regime. Indeed, due to the hierarchically structured surface, more air was trapped between the adjacent SR/ZnO/SiO2 composite in comparison to the SR/ZnO composite, as shown in Fig. 4. The prepared SR/ZnO/SiO2 composite was wetted following the CassieBaxter model.

Static Contact Angle (°)

Fig. 5. Schematic representation of wetting models, with (a) homogeneous interface (Wenzel) and (b) heterogeneous interface (Cassie-Baxter).

90

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pH=4 pH=10 pH=7 (D.I Water) pH=6 (Tap water)

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0 0

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Fig. 6. Contact angle values as a function of immersion time in pH solutions.

3.3. Stability test 3.3.1. Immersion in aqueous solutions To assess the stability of the prepared superhydrophobic coating, it was immersed in solutions of different pH levels 10, 7, 6 and 4). These values were selected considering that washing liquor has a pH of 9–11 [22], deionized water and tap water have pH values of 7 and 6, respectively, and typical acid rain has a pH value of 4 [23]. The results showed a slight decrease in contact angle for the samples immersed in pH buffer solutions after 10 days. However, this decrease was more pronounced for tap water solutions (down to 145 ± 1.4◦ ) than other solutions (see Fig. 6). Otherwise, all the samples immersed in pH solutions maintained their superhydrophobic property, except for the sample immersed in tap water due to ionic contamination. The variation of contact angle hysteresis along with different pH levels are illustrated in Fig. 6. It was observed that contact angle hysteresis was more affected by the immersion. Following

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Si-CH3 Before treatment After treatment

100

50

C-H

Static Contact Angle(°)

% Absorbance

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500 1000 1500 2000 2500 3000 3500 4000 4500

1

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4

Contact Angle Hysteresis (°)

35

200

0

-1

Wavenumber (cm )

Fig. 8. UV stability test results for superhydrophobic coating.

Fig. 7. FTIR spectra of the prepared superhydrophobic coating before and after pH solution immersion. The upper spectrum corresponds to coating before the immersion.

3.3.2. UV and humidity One of the most important characteristics of the coating is its wetting behaviour in response to UV exposure. In order to assess the stability of the hydrophobic properties of the surface under UV irradiation, accelerated aging tests were conducted according to the ASTM G154 standard. Indeed, every test performed by QUV-accelerated device includes two parts of UV rayon and condensation. The value of relative humidity (RH%) during condensation was 63 ± 2%. Superhydrophobicity of the surfaces is retained even after 4 test cycles, as shown in Fig. 8. Moreover, the contact angle hysteresis is <7◦ throughout the test period which implies that the roll-off properties of the sample are stable. Also, there was no change in the color of the sample.

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Contact Angle Hysteresis (°)

Static Contact Angle(°)

the immersion, contact angle hysteresis values had considerably increased while the static contact angle values were still very high (>150◦ ). Transition from the superhydrophobic Cassie to the superhydrophobic Wenzel state was therefore induced (see Section 3.2). The samples had lost their roll-off property nearly after 2 days. To find out how this test can influence the composition and hydrophobic performance of the nanocomposite coatings, FTIR spectroscopy has been used before and after the pH treatment (pH 4) in the range of 4000–450 cm−1 , as shown in Fig. 7. From the obtained spectra, the stronger absorption band at 1270–1255 cm−1 can be assigned to the Si CH3 bonds. Other typical absorption bands can be attributed to the CHx symmetric and asymmetric stretching at 2900–2960 cm−1 . The decrease in the intensity of the band at 1260 cm−1 was obtained for the sample after the pH treatment. Indeed, the decrease in Si CH3 bonds relatively to the total C H bonds results in lower hydrophobic properties for the sample treated in the pH solutions. Since the Si CH3 groups are responsible for lowering the wetting properties, there is a good correlation between the results of the FTIR analysis and the water droplet contact angle measurements. AFM analysis also showed that the roughness of (the value of Rq) the coating has been decreased form 304 nm to around 150 nm after pH treatment. The transition of the Cassie wetting model to the Wenzel model was observed during the pH test which may be attributed to a slight elimination of the coating. Consequently, a portion of the nanostructured area started to get damaged, resulting in increasing contact angle hysteresis. Such results indicate that the coating has medium-good stability against a wide range of pH solutions.

35 160

0

Fig. 9. Thermal stability of the prepared superhydrophobic surface vs. time.

These results reveal that there was no significant effect of UV and humidity on the as-prepared coatings. 3.3.3. Thermal stability test Thermal stability of a coating is also an important property which was investigated by heating the samples at temperature up to 150 ◦ C for different periods of time. The superhydrophobic coating exhibited excellent stability following the heat treatment as shown in Fig. 9. A high static contact angle of 158◦ and low contact angle hysteresis of 6◦ was obtained for the sample placed in the oven after almost a month. So, the prepared nanocomposite coating very much retained its superhydrophobic and roll-off properties against the heating treatment. 4. Summary and conclusions A stable nanocomposite coating with ultra water repellent properties was prepared by simple spraying in a one-step process. This coating has potential for widespread industrial applications because of its simplicity and practicability. It was found that the hierarchical scale roughness is essential to achieving the superhydrophobicity level demonstrated by this SR/ZnO/SiO2 composite. Scanning electron microscopy image analysis showed “coral-like” structures appeared on the SR/ZnO/SiO2 composite. The

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typical morphology of its surface showed important roughness of around 300 nm. The stability of the coating was tested in different conditions, immersion in various aqueous solutions, exposure to UV irradiation, as well as to heating and humidity. The coating demonstrated an excellent behaviour against these factors. In particular, it retained it’s superhydrophobicity after being immersed 10 days in solutions of different pH levels, though its roll-off properties were lost after 2 days of immersion. The slight deterioration of contact angle and contact angle hysteresis values observed after the 10-day immersion could be attributed to a weakening of the Si CH3 bond strength which is the responsible for the coating’s hydrophobic properties. Acknowledgments This work was carried out within the framework of the NSERC/Hydro-Quebec/UQAC Industrial Chair on Atmospheric Icing of Power Network Equipment (CIGELE) and the Canada Research Chair on Engineering of Power Network Atmospheric Icing (INGIVRE) at Université du Québec à Chicoutimi. The authors would like to thank the CIGELE partners (Hydro-Québec, Hydro One, Réseau Transport d’Électricité (RTE) and Électricité de France (EDF), Alcan Cable, K-Line Insulators, Tyco Electronics, Dual-ADE, and FUQAC) whose financial support made this research possible. The authors also wish to thank Fatima MADIDI for her assistance in sample preparation and characterization. References [1] T. Ishizaki, N. Saito, Yasushi Inoue, M. Bekke, O. Takai, Fabrication and characterization of ultra-water-repellent alumina–silica composite films, J. Phys. D: Appl. Phys. 40 (2007) 192–197. [2] G. Momen, M. Farzaneh, A simple process to fabricate a superhydrophobic Silicone rubber coating, Micro Nano Lett. 6 (6) (2011) 405–407. [3] S. Wang, L. Feng, L. Jiang, Adv. Mater. 18 (2006) 767. [4] R. Jafari, R. Menini, M. Farzaneh, Superhydrophobic and icephobic surfaces prepared by RF-sputtered polytetrafluoroethylene coatings, Appl. Surf. Sci. 257 (2010) 1540–1543. [5] G. Momen, M. Farzaneh, R. Jafari, Wettability behaviour of RTV silicone rubber coated on nanostructured aluminium surface, Appl. Surf. Sci. 257 (15) (2011) 6489–6493.

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