Transparent Superhydrophobic silica coatings on glass by sol–gel method

Applied Surface Science 257 (2010) 333–339

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Transparent Superhydrophobic silica coatings on glass by sol–gel method Satish A. Mahadik a , Mahendra S. Kavale a , S.K. Mukherjee b , A. Venkateswara Rao a,∗ a b

Air Glass Laboratory, Department of Physics, Shivaji University, Vidyanagar, Kolhapur 416 004, Maharashtra, India Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Maharashtra, India

a r t i c l e

i n f o

Article history: Received 30 December 2009 Received in revised form 22 June 2010 Accepted 22 June 2010 Available online 30 June 2010 Keywords: Superhydrophobic Transparent Wetting Sliding angle Coating

a b s t r a c t Wetting behavior of solid surfaces is a key concern in our daily life as well as in engineering and science. In the present study, we demonstrate a simple dip coating method for the preparation of Thermally stable, transparent superhydrophobic silica films on glass substrates at room temperature by sol–gel process. The coating alcosol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), trimethylmethoxysilane (TMMS), methanol (MeOH), water (H2 O) constant at 1:0.09:12.71:3.58, respectively with 13 M NH4 OH throughout the experiments and the films were prepared with different deposition time varied from 5 to 25 h. In order to improve the hydrophobicity of as deposited silica films, the films were derivatized with 10% trimethylchlorosilane (TMCS) as a silylating agent in hexane solvent for 24 h. Enhancement in wetting behavior was observed for surface derivatized silica films which showed a maximum static water contact angle (172◦ ) and minimum sliding angle (2◦ ) for 25 h of deposition time. The superhydrophobic silica films retained their superhydrophobicity up to a temperature of 550 ◦ C. The silica films were characterized by field emission scanning electron microscopy (FE-SEM), surface profilometer, Fourier transform infrared (FT-IR) spectroscopy, thermo-gravimetric and differential thermal analysis (TG–DTA), percentage of optical transmission, water contact angle measurements. The imperviousness behavior of the films was tested with various acids. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The nature of interactions between water and superhydrophobic surfaces has been a subject of enormous interest. Superhydrophobic surfaces provide an ideal model interface to study behavior of the water droplets. Surface energy and surface roughness are the dominant factors for wettability of the surface. Various methods have been employed to generate the surfaces that can mimic the structure and chemistry of natural superhydrophobic surfaces like lotus leaves [1]. Several studies have reported that by finely controlling the micro/nanostructure or chemical composition of a surface, the adhesion between the superhydrophobic surface and water can be changed. Such superhydrophobic surfaces show potential in a variety of applications from antisticking, anticontamination and self-cleaning to anti-corrosion and low friction coatings [2–10]. Beyond the existence of large water contact angles, very low adhesion, characterized by a small sliding angle is a requirement for many applications. Sol–gel technology makes it promising to produce silicate materials with properties that cannot be achieved by other methods and to improve the properties of conventional materials. The sol–gel

∗ Corresponding author. Tel.: +91 231 2609228; fax: +91 231 2609233. E-mail address: [email protected] (A.V. Rao). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.062

method was applied to the fabrication of water repellent surfaces by several groups [11–20]. The water uptake of silica films is due to the presence of Si–OH groups on the surface. Substitution of the Hs from the Si–OH groups by the hydrolytically stable Si–O–R (where, R = alkyl or aryl) groups through the oxygen bonds prevents the adsorption of water and hence results in the hydrophobic silica surfaces [21,22]. Gellermann et al. [23] modified surfaces of silica spheres by using different silane coupling agents. Orozco-Teran et al. [24] modified triethoxyfluorosilane (TEFS) films by silylation for the enhancement of hydrophobicity. In the present research work, we report an easy and efficient method to synthesize superhydrophobic surface by simple dip coating sol–gel method exhibiting a self-cleaning behavior. 2. Experimental 2.1. Materials The superhydrophobic silica films have been prepared by sol–gel process using a simple dip coating method. Chemicals used were methyltriethoxysilane (MTES) (Sigma–Aldrich Chemie, Germany), methoxytrimethylsilane (TMMS) (Fluka Chemie, Switzerland), trimethylchlorosilane (TMCS) (Sigma–Aldrich Chemie, Germany), methanol, hexane (S. D. Fine-Chem Ltd., Mumbai) and Ammonia (NH3 , Sp.Gr.0.91 Qualigens Fine Chemicals, Mumbai).

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2.2. Sample preparation

3. Results and discussions

The superhydrophobic silica coating was synthesized by following four main steps: (1) alcosol preparation, (2) dipping and withdrawing the substrates from the alcosol, (3) annealing of the as deposited silica films and (4) surface derivatization. To get uniform coating, the glass microslide substrates were cleaned by the procedure described earlier [25]. The coating alcosol was prepared at optimized molar ratio of MTES:TMMS:CH3 OH:H2 O at 1:0.09:12.71:3.58, respectively with 13 M NH4 OH at room temperature (27 ◦ C). A single step base catalyzed sol–gel process was followed to prepare the coating alcosol. In the first step, MTES and TMMS were diluted in methanol solvent at given molar ratio. Then basic water (ammonium hydroxide, 13 M) was added drop-by-drop to the solution and stirred for 15 min to get homogeneous alcosol. The alcosol cannot turns to gel within depositions time due to their higher gelation time. So efforts were made to deposit the films prior to gel formation. All the films were prepared at room temperature and deposition time was varied from 5 to 25 h. The prepared alcosol was taken in test tubes and the substrates were dipped vertically. The substrates were withdrawn from the bath containing alcosol at a speed of 5 mm/s. The withdrawn films were dried for 24 h at room temperature (27 ◦ C) to get good chemical bonding between the coating sol and the substrate. Finally, these films were kept in the oven for annealing at 100 ◦ C for 5 h and soaked in the mixture containing 10% TMCS in hexane for 24 h at room temperature. The derivatized silica films were annealed at 100 ◦ C for 1 h and taken out from the oven after cooling at ambient temperature. The surface morphology of the silica films was examined using Field Emission Scanning Electron Microscope (JEOL, JSM 7001F, Japan). The roughness of the films was determined by using a surface profilometer (Ambios XP-1 Model, India). The surface roughness was measured ten times at the same location and an average of the 5 measurements was taken for each sample. The surface chemical modification of the films was studied using infrared spectroscopy (Perkin-Elmer, Model No. 783, USA). The samples for FT-IR characterization are prepared by removing film from the substrate by using stainless steel knife. Sample powder was mixed with KBr and a homogeneous mixture formed with a mortar and pestle. This mixture was placed in a cylindrical die of 13 mm diameter and pressed for about 5 min at 200 kg/cm2 to form a pellet and then scanned in frequency range from 450 to 4000 cm−1 . The thermal stability of coating was studied by TGA-DTA (TA Instrument Q600 USA), thermal stability in terms of retention of hydrophobicity confirmed by putting sample in Furnaces (Neytech. VULCAN Benchtop Model 3-550 USA) at different temperatures. Percentage of optical transmission of the films was measured in the visible range using UV-VIS spectrophotometer (Systronic 119, USA). Wetting properties of the films was tested by measuring the static and dynamic water contact angles using computer interfaced contact angle meter equipped with a CCD camera (Ramehart Instrument Co., advanced goniometer model no. 500 F1 USA) at room temperature (∼27 ◦ C). Contact angles were measured at six different spots for each sample and the average value was adopted as the contact angle.

3.1. Static and dynamic water contact angle measurements Young [26] had described the forces acting on a liquid droplet spreading on a surface. The contact angle () of the drop is related to the interfacial energies acting between the solid–liquid ( SL ), solid–vapor ( SV ) and liquid–vapor ( LV ) interfaces given by relation: SV − SL cos  = (1) LV The expression given by Eq. (1) is strictly valid only for surfaces that are atomically smooth, chemically homogeneous, and those that do not change their characteristics due to interactions of the probing liquid with the substratum, or any other outside force. In the Wenzel regime [27], the liquid wets the surface, but the measured contact angle (*) differs from the “true” contact angle (): cos  ∗ = r cos 

(2)

where r is the ratio between the true surface area and its horizontal projection. According to the Cassie and Baxter [28] model, the surface traps air in the hollow spaces of the rough surface, so that the droplet essentially rests on a layer of air. The contact angle ( CB ) governed by the Cassie–Baxter model is expressed as follows: cos CB = rf (cos FLAT ) + f − 1

(3)

where f is the fraction of surface area that supports the liquid droplet, and  FLAT is the contact angle on a flat surface. The tilt angle refers to the critical angle between the substrate and the horizontal plane, below which the water droplet begins to move upon elevating one end of the substrate. The maximum frictional force (fmax ) required to dislodge a liquid from a surface can be calculated by, fmax = mg sin 

(4)

where m is the mass of the water droplet and g is the acceleration due to gravity, respectively. Where,  represents the minimal sliding angle of the water droplet on the surface. The effect of deposition time on static water contact angle, sliding angle and maximum frictional force to slide is given in Table 1. To evaluate the wetting behavior of the coating, the images of the contact angle () of the water droplet on the coatings synthesized before and after derivatization and at different deposition time values have been measured using contact angle meter, as shown in Fig. 1. The sliding angle was measured on derivatized silica films. The water droplets roll off from the silica film prepared at 5 h of deposition time, having a water contact angle of 161◦ and maximum frictional force required to slide the water droplet on film surface is 56 ␮N at sliding angle of 9◦ . Whereas the water droplet rolls off quickly from the silica film prepared at 25 h of deposition time having a water contact angle of 172◦ and maximum frictional force required to slide the water droplet on film surface is 18 ␮N at sliding angle of 2◦ . The as prepared silica films contain a large number of hydroxyl and alkoxy groups which are responsible for the hydrophilic char-

Table 1 Wetting properties of coating after derivatization. Deposition time (hours) 5 10 15 20 25

Roughness (nm) 10 11 18 887 1129

Contact angle () after derivatization ◦

161 164◦ 167◦ 168◦ 172◦

Sliding angle ()

Maximum friction force (fmax ) (␮N)

9 8 7 4 2

56 50 43 25 18

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Fig. 1. The effect of surface modification on the hydrophobicity.

acter of the film surface. However, using TMCS in the sol–gel processing stage the –OH groups are replaced by hydrolytically stable –O–Si–(CH3 )3 groups. The coatings surface becomes superhydrophobic because of the hydrolytic stability of Si–C bonds. At lower deposition time, the silica film surface introduces roughness required to superhydrophobicity with surface covered by silicon trimethyl groups. However, as the deposition time increases; surface roughness increases resulting large water contact angle and low sliding angle. Although, the maximum frictional force required to slide water droplet on a film surface is decreased with increas-

ing the deposition time. This strongly suggests that the contact model of a water droplet on the film prepared from T = 25 h is the Cassie–Baxter’s model. The effect of different organic liquids and acids on the contact angles of superhydrophobic surface are shown in Table 2. As regards close approach of applications and stability of a superhydrophic coating, their hydrophobicity against the organic liquids and strong acids was checked. It is clear that the coatings show superhydrophobic nature against the double distilled water, glycerin and HCL, but hydrophobic nature against the H2 SO4 and HNO3 . It shows the superhydrophilic nature with hexane. Accord-

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Table 2 Hydrophobicity of coating against different organic liquid and strong acids. Liquids

Contact angle ()

Double distilled water Glycerine Pure HCl H2 SO4 HNO3 Ethanol Hexane

172◦ 170◦ 157◦ 125◦ 90◦ 15◦ 0◦

3.4. Imperviousness against strong acids

Table 3 Table shows effect of surface roughness and transparency on superhydrophobicity of films. Deposition time in hours

Roughness (nm) Transparency (%)

Hydrophobicity ()

5 10 15 20 25

10 11.2 18.4 887.2 1129.7

161◦ 164◦ 167◦ 168◦ 172◦

90.45 90.33 90.30 90.25 90.22

superhydrophobic behavior of the silica film. The higher magnified image (5500×) of the silica film (Fig. 2(f)) shows the surface having microsized particles with inhomogeneous size distribution. According to the Cassie–Baxter model air trapped within microscale particles reduces contact surface area between liquid drop and coating surface.

ing to this result it is clear that this type of coating is effective against strong acids, and it opens a variety of potential applications such as transparent coating on a marble sculptures to avoid acidic effect. 3.2. Effect of deposition time and surface modification on wettability of the films The water repellent coatings were deposited on glass substrate for various time intervals varying from 5 to 25 h. We found that, the water contact angle increases with increase in deposition time and surface modification of the films with 10% TMCS in hexane for 24 h as shown in Table 1. The static water contact angle increases from 161◦ to 172◦ with increase in deposition time from 5 to 25 h after derivatization. This is due to the fact that, surface roughness increases with deposition time which favors to increase in the water contact angle. The roughness increases from 10 nm to 1129 nm with increase in deposition time from 5 to 25 h after derivatization. The coating shows water contact angle of 161◦ at deposition time of 5 h and shows improvement in its water contact angle with increase in deposition time. After derivatization maximum number of polar (–OH) groups are replaced by non-polar and thermally stable –(CH3 )3 groups and we get resultant low energy surface required for the superhydrophobic surface. The substrates deposited before 5 h of deposition time shows non-uniform coating due to incomplete hydrolysis and condensation reactions. However, at higher deposition time, the optical transmission of the films was decreased due to the increase in surface roughness as shown in Table 3.

It was observed that the contact angle on a coating changes due to the effect of the nitric acid (HNO3 ), sulphuric acid (H2 SO4 ) and hydrochloric acid (HCl) in water [29]. Fig. 3 shows the contact angle variations for different concentrations of nitric acid (HNO3 ), sulphuric acid (H2 SO4 ) and hydrochloric acid (HCl) in water. From Fig. 3, it is clear that the coatings show hydrophophobic nature and does not allow to wet strong acids completely on the surface of coatings even though at 100% concentration (pure acid). Such hybrid coatings show the contact angles 157◦ , 125◦ and 90◦ at 100% concentration of hydrochloric acid (HCl), sulphuric acid (H2 SO4 ) and nitric acid (HNO3 ) respectively. According to the Zisman’s rule [30], each coating surface having a critical surface tension,  C such that there is a partial wetting if  >  C , and total wetting if  <  C , where  is the surface tension of the liquid. From this, it follows that the critical surface tension,  C of the hydrophobic surface is sufficiently less so that pure water or pure acids with higher surface tension values do not wet the surface completely. But, when an organic liquid such as alcohol is mixed in water, the surface tension of water ( W ) is lowered. Considerable lowering of the surface tension is caused at low concentrations; but at intermediate and high concentrations, the surface tension of the solution,  A , changes comparatively little and mostly tends towards the value possessed by the organic liquid. The effect of concentration of acid on wettability of coating is described by Szyszkowski formula [31]:





W − A C = log +1 W k

(5)

where C is the concentration of the organic acid in water, and b and k are the constants. From Eq. (5), it is clear that the surface tension of the acid in water does not affect surface tension of the solution considerably as the concentrations of these acids increase in water. As a result, hydrophobicity of the coatings remains up to higher concentration of acid in water even as an impurity. The MTES and TMMS based surface modified coatings shows hydrophobicity even at 100% concentration (pure acid) of H2 SO4 , HNO3 and HCl because of the surface roughness and low surface energy of the coating. Thus, the coatings are more resistant to hydrochloric acid (HCl), sulphuric acid (H2 SO4 ) and nitric acid (HNO3 ) hence, they are most suitable for anti-rusting and anti-corrosive coatings on metals, alloys and avoid the effect of acid rain and acidic environments on sculptures.

3.3. Surface morphological studies

3.5. FT-IR analysis and thermal stability coating

The FE-SEM observation showed the microstructure of the MTMS/TMMS based superhydrophobic coating. Fig. 2(a)–(f) shows the SEM images of the derivatized silica films prepared at different deposition time from 5 to 25 h, respectively. Fig. 2(a) and (b) shows the non-uniform deposition over the substrate at deposition time of 5 and 10 h, respectively. At 15 and 20 h of deposition time, uniform deposition occurs on the substrate with microsized particles as shown in Fig. 2(c) and (d). Fig. 2(e) shows the film prepared at 25 h of deposition time having micro level size spherical particles (typically ranges from 1 to 10 ␮m in diameter) uniformly deposited on the substrate surface. These microsized particles provide an appropriate surface roughness which may responsible for

Fig. 4 shows FT-IR spectra of unmodified and TMCS modified silica films deposited at 25 h. The absorption band of the –OH around 3422 cm−1 and that of water adsorbed around 1600 cm−1 are decreased after modifying the surface with TMCS [32]. The FTIR spectra of unmodified and TMCS modified silica films shows a very strong absorption band at 1096 cm−1 is due to the stretching vibration of Si–O–Si bridges [33] and the peaks splits into bonds separated at 1021 and 1096 cm−1 in a hybrid coating structure. Olejniczak et al. have shown that in the hybrid gel structure, asymmetric stretching vibration of Si–O–Si splits separate into two bands due to functionalization of silanols [34]. The presence of C–H stretching vibration is supported by two peaks around 3000 and

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Fig. 2. FE-SEM images of superhydrophobic film derivatized with 10% TMCS at deposition time: (A) 5 h; (B) 10 h; (C) 15 h; (D) 20 h; (E) 25 h; (F) 25 h (5500×).

765 cm−1 . The intensity of the bands related to the C–H groups slightly increased by the TMCS modification. The thermal stability of superhydrophobic coating in air was checked by TG–DTA analysis as shown in Fig. 5. It is clear that superhydrophobic coating is thermally stable up to 300 ◦ C. The two exothermic peaks, one is broad exothermic peak at temperature 353 ◦ C corresponds to the formation of CO2 with 6% weight loss and other small exothermic peak at temperature of 544 ◦ C, causes sudden weight loss up to 36% corresponding to the oxidation of surface organic (–CH3 )3 groups present in the material with oxygen in the air [35]. The superhydrophobic coating was thermally stable up to 550 ◦ C and above this temperature they lose their hydrophobicity and become superhydrophilic at 600 ◦ C as shown in Fig. 6. This thermal stability is due to the surface silica particles covered by thermally stable and non-hydrolysable silicon trimethyl groups.

The coating starts to lose the superhydrophobicity above 550 ◦ C and becomes superhydrophilic at 600 ◦ C due to oxidation of methyl groups and other organic groups [36]. The thermal stability studies revealed that the superhydrophobic coating retain their superhydrophobicity up to 550 ◦ C was confirmed by putting the films in furnace (Neytech. VULCAN Benchtop Model 3-550, USA) at different temperatures and measured contact angle on the sample after cooling at room temperature until the superhydrophic coating turned to super hydrophilic as shown in Fig. 6. 3.6. Optical transparency and surface roughness The hydrophobicity and transparency are competitive properties from viewpoint of surface roughness. The roughness induces scattering of light and decreases intensity of transmitted light. In

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Fig. 6. Thermal stability of the derivatized silica film prepared at deposition time of 25 h. Fig. 3. Imperviousness against strong acids.

Fig. 7. Transmittance of prepared superhydrophic films at different deposition times in the visible wavelength range. Fig. 4. FT-IR spectra of the unmodified and TMCS modified silica film.

order to achieve higher transparency of the coatings in the visible range (380–760 nm), the surface roughness should be controlled below 100 nm [37] or much higher than the certain range (more than few micrometers). Fig. 7 shows the percentage of the optical transmission of the films in the visible range (380–760 nm). From Fig. 7, it is clear that transmittance of coating decreases with increasing in deposition time and surface roughness. As shown in Table 3, the surface roughness of the film increases with an increase in deposition time. With increase in the deposition time from 5 to 25 h, the surface roughness increased from 10 to 1129 nm, respectively. It is clear that hydrophobicity of the film increases with increasing surface roughness and with decrease in optical transmission. 4. Conclusions

Fig. 5. TG–DTA of the derivatized silica film prepared at deposition time of 25 h.

Simple dip coating method with single step sol–gel processing was used to prepare Thermally stable, transparent superhydrophic hybrid coating on a glass. By controlling surface roughness of resultant coatings, it is possible to achieve transparent superhydrophobic coating having excellent wetting behavior properties

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with high optical transmission, thermal stability and imperviousness against strong acids. Such non fluorinated Thermally stable transparent superhydrophobic coating potentially suitable for various industrial applications such as a lens, goggle, biker helmet, wind shield, solar panels and sculptures (made from metals, marble stones etc.) to avoid the effect of acid rain on them. Acknowledgements The authors are grateful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Mumbai, Government of India, for the financial support for this work through a major research project on “Aerogels and coatings” (No. 2007/37/18/BRNS). Mr. Satish A. Mahadik also thank to University Grant Commission (UGC), New Delhi, Government of India, for providing “UGC Research Fellowship in Sciences for Meritorious Students”. Authors also thankful to Miss. Jyoti L. Gurav for providing FE-SEM facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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