Article pubs.acs.org/Langmuir
Transparent Superhydrophobic/Translucent Superamphiphobic Coatings Based on Silica−Fluoropolymer Hybrid Nanoparticles Seung Goo Lee,†,§ Dong Seok Ham,†,§ Dong Yun Lee,‡ Hyojin Bong,† and Kilwon Cho*,† †
Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea Samsung Advanced Institute of Technology, Yongin 446-712, Korea
‡
S Supporting Information *
ABSTRACT: This paper describes a simple approach to prepare a transparent superhydrophobic coating and a translucent superamphiphobic coating via spraying silica− fluoropolymer hybrid nanoparticles (SFNs) without any preor post-treatment of substrates; these nanoparticles create both microscale and nanoscale roughness, and fluoropolymer acts as a low surface energy binder. We also demonstrate the effects of varying the concentration of the SFN sol on the water and hexadecane repellency and on the transparency of the coated glass substrates. An increase in the concentration of the sol facilitates the transition between the superhydrophobic/transparent and superamphiphobic/translucent states. This transition results from an increase in the discontinuities in the three-phase (solid−liquid−gas) contact line and in the light scattering properties due to micropapillae tuned by varying the concentration of the sol. This versatile and controllable approach can be applied to a variety of substrates over large areas and may provide a wide range of applications for self-cleaning coatings of optoelectronics, liquid-repellent coatings, and microfluidic systems.
1. INTRODUCTION Superhydrophobic surfaces with high contact angles and low contact angle hystereses for water have recently attracted significant attention because of their numerous applications in self-cleaning,1−3 anticorrosion,4 anti-icing,5,6 and so forth. It is now well-known that superhydrophobic surfaces result from the combination of a low surface energy and dual- or multiscale surface roughness. Various research groups have tried to develop fabrication techniques for superhydrophobic surfaces based on a variety of top-down and bottom-up approaches.7−15 Recently, there has been intense interest in transparent superhydrophobic coatings for optically transparent applications such as solar cells, lenses, and windows.16−23 However, most techniques for the fabrication of transparent superhydrophobic surfaces require multistep or lithographic procedures to prepare the required small surface features (≤100 nm) and/or an additional surface post-treatment procedure to reduce the surface energy. Very recently, onestep coating methods of hydrophobic silane-treated nanoparticles that yield transparent superhydrophobic properties have been reported,21−23 and the effects of the surface structures on the hydrophobicity and transparency have been described; however, no studies have yet demonstrated how control over the surface structures of hydrophobic nanoparticle coatings affect both amphiphobicity and transparency. In contrast to superhydrophobic surfaces, studies on superamphiphobic (i.e., super-repellent to both water and oil) surfaces have been rather limited because organic liquids with low surface tensions, such as alcohol or oil, tend to spread © 2013 American Chemical Society
readily over solid surfaces. To fabricate superamphiphobic surfaces, re-entrant surface microstructures with a negative slope and a low surface energy are essential.24 Although there have been a few reports of the fabrication of superamphiphobic surfaces,24−37 most of the reported approaches to the preparation of superamphiphobic surfaces require lithographic, etching, or multistep procedures or are limited to certain substrates, which could limit their practical applications. Recently, superamphiphobic nanocomposite coatings have been demonstrated by the spray atomization of fluoropolymer blends with nanoparticles (or carbon nanotubes) using cosolvents, which are intrinsically cheap, low-expertise routes to mass production, and are applicable to almost any substrate type.34−37 In this blend system, a difference in solubility between the fluoropolymer and the nanoparticle leads to the formation of microscale aggregates with re-entrant structures that not only increases amphiphobicity but also decreases transparency of coatings. Up until now, however, the simple coating method to prepare both transparent superhydrophobic surfaces and translucent (or opaque) superamphiphobic surfaces has not been reported due to their different structural requirements and preparations. It requires control of the assembly of hydrophobic nanoparticles on the substrate to optimize surface structures for each property. Received: August 25, 2013 Revised: November 13, 2013 Published: November 14, 2013 15051
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Figure 1. (a) Schematic illustration of the fabrication process comprising silane treatment and polymerization of the silica nanoparticles. TEM (b) and SEM (c) images of fabricated silica−fluoropolymer hybrid nanoparticles (SFNs). filtered, and dried under vacuum (see the Supporting Information for the characterization of the prepared SFNs). 2.3. Spray-Coating of SFNs. For spray-coating, the SFNs were dispersed in AK-225 (20 g) at various weight percentages and sonicated for 3 h prior use. The clean substrate (2.5 cm × 2.5 cm) was coated by using an airbrush (Iwata Eclipse HP-BH) with a nozzle of 0.2 mm diameter that uses compressed nitrogen gas (0.15 MPa, see the Supporting Information for the effect of applied gas pressures on coated surfaces). The distance between the airbrush and the substrate was approximately 15 cm. 2.4. Characterization. The morphologies of the samples were examined with a scanning electron microscope (SEM, Hitachi S-4800) at 8 kV and an atomic force microscope (AFM, Digital Instruments Multimode Nanoscope III) in tapping mode. Each diluted SFN sol was coated onto a carbon grid for transmission electron microscopy (TEM) analysis (Hitachi-7600, operated at 100 kV). Contact angles and roll-off angles were measured by using a contact angle meter (SEO 300A, SEO Co.). The droplet size of the liquid was controlled to be 5 μL. The advancing/receding contact angles were measured by slowly (volume flow rate ∼0.2 μL/s) adding/removing the liquid to/from the sessile liquid on the surface, using a syringe. The roll-off angles were measured by a tilting stage method, as the tilt angle of the stage at which a liquid droplet rolled off the surface. The optical transparencies of the coated substrates were measured with UV−vis spectrophotometry (Cary-5000, Varian). IR spectroscopic analysis of the MPStreated SNs was performed by using a Fourier transform IR (Nicolet 6700, Thermo Scientific).
In this paper, we present a simple and broadly applicable spray method that uses hydrophobic nanoparticles for the preparation of transparent superhydrophobic surfaces and translucent superamphiphobic surfaces. We first report the preparation of hydrophobic silica−fluoropolymer hybrid nanoparticles (SFNs) which consist of inner silica nanoparticles surrounded by fluoropolymers. The fluoropolymer shell acts as a compatibilizer to render the particle dispersible in a fluorosolvent and prevents particle aggregation. Our method does not require additional surface treatments to increase the adhesive force between the substrate and the coated material or to produce surface hydrophobization because the fluoropolymer acts as a low surface energy binder. We also demonstrate a simple approach that confers significant transparency or superoleophobicity to superhydrophobic coatings by simply tuning the surface morphology, which can be controlled by varying the concentration of the SFNs in the spray solution.
2. EXPERIMENTAL SECTION 2.1. Materials. Fumed silica nanoparticles (SNs, 11 nm diameter), 2-(perfluorooctyl)ethyl methacrylate, absolute ethanol, tetrahydrofuran (THF), chloroform, and the initiator 2,2′-azobis(isobutyronitrile) (AIBN) were obtained from Sigma-Aldrich. Ammonia hydroxide (NH4OH) catalyst was provided by Samcheon Inc. (Korea). The coupling agent 3-methacryloxypropyltrimethoxysilane (MPS) was purchased from Gelest, Inc. The fluorinated solvent, Asahiklin AK225, was provided by Asahi Glass Co. (Japan). 2.2. Preparation of Silica−Fluoropolymer Hybrid Nanoparticles (SFNs). MPS-treated SNs were prepared with the modified Stöber method.38 SNs (1.0 g) were dispersed in absolute ethanol (100 mL), followed by injection of NH4OH (10 g). Then MPS (2.88 g, 11.60 mmol) was added into the SN solution (pH 10) and allowed to react at room temperature for 24 h. The MPS-treated SNs were centrifuged and washed three times with ethanol, and the particles were dried overnight under vacuum (see the Supporting Information for the characterization of the MPS-treated SNs). The MPS-treated SNs (0.1 g) were dispersed in THF (50 mL). Then, AIBN (0.01 g, 0.06 mmol) and 2-(perfluorooctyl)ethyl methacrylate (0.16 g, 0.30 mmol) were added. The polymerization was performed under a nitrogen atmosphere with continuous stirring at 60 °C for 10 h. The overall size of the silica−fluoropolymer hybrid nanoparticles (SFNs) and the thickness of the fluoropolymer shell can easily be increased by increasing the proportion of the fluoroacrylate monomer. To obtain small SFNs, small amounts of fluoroacrylate monomer are desirable. The SFNs solution was added dropwise to chloroform (500 mL),
3. RESULTS AND DISCUSSION The multifunctional superhydrophobic coatings are fabricated via a facile spray process using silica−fluoropolymer hybrid nanoparticles (SFNs). As shown in Figure 1a, the SFNs are obtained in two steps through the methacryloxypropyltrimethoxysilane (MPS) treatment of silica nanoparticles (SNs, 11 nm in diameter) and the radical polymerization of the MPStreated SNs with fluoroacrylate monomers (see the Experimental Section for additional details). The morphology of the as-prepared SFNs was characterized with TEM and SEM. The TEM image in Figure 1b shows silica−polymer hybrid nanostructures. There is a strong contrast difference within each nanoparticle between the dark inner center and the relatively light shell, which confirms that the fluoroacrylate monomers copolymerize with the MPS on the surfaces of the SNs. The mean particle size of the SFNs is 60 nm, and the sprayed and aggregated SFNs produce multiscale roughness and high porosity (Figure 1c). Small SFNs favor high 15052
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transparency with respect to visible light and enable finer control of the surface roughness of the coated sample. The micro- and nanoscale surface roughness of the spraycoated samples are enhanced by increasing the concentration of the SFNs in solution. The surface topography of the coated SFNs was investigated with SEM and AFM (Figure 2 and
key geometric characteristic of superamphiphobicity.24 These results indicate that the surface morphology of spray-coated SFNs can be controlled by varying the concentration of SFNs in the solution. The water- and oil-repellent properties of the coatings were investigated by measuring the advancing and receding contact angles (CAs) of water and hexadecane droplets. As shown in Figure 4a, the advancing water CA increases and the CA hysteresis of water decreases to reach a saturated value as the surface becomes rougher (i.e., as the concentration of SFNs in the solution increases). The film coated with a 0.05 wt % solution of SFNs is hydrophilic with a small CA of 35° due to the exposure of the uncoated hydrophilic substrate (see the arrows in Figure 2a). When the SFN sol concentration is equal to or higher than 0.1 wt %, the coated surfaces are superhydrophobic with water CAs exceeding 150° and sufficiently low water CA hystereses. In the superhydrophobic state, water rolls away very quickly when the sample is tilted slightly (below 10°, Figure 4c). These properties are attributed to the large volume of air trapped beneath the liquid, which prevents penetration of water droplets into the nanopores (the Cassie state).39 According to the Cassie equation, cos θ* = −1 + ϕs(1 + cos θ), the apparent CA (θ*) of samples with a given Young’s contact angle (θ) increases as the surface roughness increases because the fraction (ϕs) of solid−liquid contact decreases as a result of the increase in surface roughness (see the illustrations for water in regimes I−III in Figure 4e). The advancing CAs of hexadecane droplets, which have a low surface tension of 27.5 mN/m, also increase as the surface structure becomes rougher (Figure 4b). In parallel, the contact angle hysteresis as a function of liquid adhesion to the surface showed two distinct trends.39−44 For the less rough surfaces in regimes I and II, the CA hysteresis increases as the surface roughness increases because the solid−liquid contact area increases as the surface becomes rougher (fully wetted Wenzel state).40 Hexadecane droplets are expected to easily penetrate the nanopores and the valleys between the sparse micropapillae to create a continuous stable three-phase (solid−liquid−gas) contact line (TCL) with high adhesion (Figure 4e, I and II). Therefore, in this situation the hexadecane droplet is firmly pinned to the surface. On the other hand, when the concentration of SFNs is in the range 0.5−0.6 wt % (regime III), the coated surfaces exhibit superoleophobicity due to the high density and large size of micropapillae (Figure 4e, III). These re-entrant structures enable the Cassie state to be stabilized, even for low surface tension liquids. Therefore, in the Cassie state both water and oil roll readily off the surfaces (superamphiphobic surfaces) due to the discontinuous TCL and the large volume of air trapped beneath the liquid (Figure 4d). These results indicate that the liquid repellency of this material can be systematically controlled by varying the concentration of the sprayed sol. To explain our observation on superoleophobicity, we present a theoretical analysis based on the dimensionless parameter, robustness factor A*,24−26 which is the ratio of the breakthrough pressure (Pbreakthrough) required to force the transition from the Cassie state to the Wenzel state and the reference pressure (Pref) across the interface from the effects of gravity and the Laplace pressure within the droplet; a given droplet is predicted to be in the Wenzel state if A* ≤ 1 or in the Cassie state if A* > 1. Also, large values of A* indicate the formation of a robust composite interface with high break-
Figure 2. SEM images of spray-deposited SFNs on Si wafers with various sol concentrations: (a) 0.05, (b) 0.1, (c) 0.3, and (d) 0.6 wt %.
Figure S4). When the concentration of the sol is low, 0.05 wt %, the substrate is not fully covered with SFNs (see the arrows in Figure 2a). When the concentration is increased to 0.1 wt %, the SFNs fully cover the substrate, and the coated surface contains nanostructures but no microstructures (Figure 2b). This relatively smooth surface is attributed to the remaining solvent of sprayed liquid droplet. When the sprayed liquid droplets reach the substrate, the SFNs within the liquid droplets are prone to stack flatways due to their fluidity and impingement (Figure 3a). As the concentration is increased
Figure 3. Schematic illustration of the possible mechanism of the spray-deposited SFNs on substrates with low (a) and high (b) sol concentrations.
beyond 0.1 wt %, both the nanostructures and microstructures are roughened by numerous irregular bumps ranging in size from several to tens of micrometers, and there are a large number of nanopores on the surface (Figure 2 and Figure S4). The formation of the hierarchical structure can be explained in terms of fast solvent evaporation.36 The solvent evaporates more readily during flight as the concentration of SFNs in the solution increases, so SFNs within the flying droplets are more likely to aggregate in the air. These particles then tend to stack and adhere together irregularly on the substrate, resulting in numerous micropapillae and a relatively rough surface (Figures 2d and 3b). The papillae exhibit re-entrant curvature, which is a 15053
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Figure 4. (a, b) Contact angles of water (a) and hexadecane (b) on surfaces prepared from sols with various SFN concentrations. (c, d) Series of photographs showing the rolling behavior of a liquid droplet (5 μL) on the spray-deposited substrates with 0.1 wt % solution of SFNs (c) and a 0.6 wt % solution of SFNs (d). (e) Schematic illustrations of the possible solid−liquid contact modes in regimes I−III in (a) and (b).
through pressures. For an idealized microsphere geometry composed of numerous SFNs (Figure 5a), A* is given as24−26 A* = ×
Pbreakthrough
=
2πlcap
Pref R(2 3 D* − π ) 1 − cos θ D* − 1 + 2 sin θ
(1)
where lcap = (γlv/ρg)1/2 is the capillary length of liquid (hexadecane, lcap = 1.91 mm; here γlv is the surface tension of liquid, ρ is the liquid density, and g is the acceleration of gravity), θ is the Young’s contact angle (hexadecane, θ = 70°), and D* = [(R + D)/R]2 is the spacing ratio (here R is the radius of the microsphere and D is half the intermicrosphere spacing). From eq 1, A* can be calculated with respect to D and R numerically. As shown in Figure 5b, A* increases with the decrease of D and the increase of R. Interestingly, the critical D at which A* = 1 increases as R increases. These results correspond with the wetting transition of the oil from the Wenzel state to the Cassie state observed previously for the SFN-coated surfaces with high sol concentrations. In addition to their superamphiphobic properties, we investigated the transparency of the SFN-coated glass substrates with UV−vis transmittance spectra, with a view to assessing their potential applications in window treatments or optical devices. Figure 6 shows the variations in the transmittance
Figure 5. Theoretical calculation of wetting transition. (a) Schematic illustrations of the idealized microsphere geometry with side (upper illustration) and top (lower illustration) views. (b) Computed values of robustness factor (A*) as a function of half the intermicrosphere spacing (D) with different radius (R) of the microsphere.
profiles of the samples with increases in the concentration of the sprayed sol. When the concentration of the SFNs is 0.05 or 0.1 wt %, the transmittance of the coated glass is comparable to uncoated glass; the glass is transparent with a clear transmitted image (relative transmittance ≈98% at 550 nm). When the concentration is greater than 0.1 wt %, both the surface roughness and the film thickness are higher (Figures S4 and S5 15054
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Figure 6. Transmittance spectra of coatings on glass substrates with SFN sols of various concentrations. The insets show optical images of the coated glass substrates overlaying printed paper.
in the Supporting Information). As a result, light scattering increases, leading to reduced transparency (see the inset in Figure 6). According to Mie theory, when the surface roughness is comparable to or larger than the wavelength of light, the scattering cross section (σs) can be determined by using the equation45,46
Figure 7. (a) Optical photograph of water droplets (dyed with methylene blue) on the glass substrate coated with SFNs (0.1 wt %). (b) Optical photograph of droplets of water (dyed with methylene blue) and hexadecane (dyed with Oil Red O) on the surfaces of steel substrates without (top) and with (bottom) SFNs (0.6 wt %).
∞
σs = (πd 2/2) ∑ (2n + 1)(an + bn) n=1
(2)
additional poly(dimethylsiloxane) (PDMS) coating and a curing process (see Figure S8 and Table S1 for more details).
where an and bn are the Mie coefficients of order n and are related to the electromagnetic properties of the material and d is the diameter of the spherical particles (i.e., aggregated SFNs in this study). In eq 2, σs is proportional to the square of d. These results show that the concentration of the sol has to be optimized with respect to the specific application because surface roughness and transparency are competing properties. To demonstrate the versatility of our method for preparing multifunctional superhydrophobic surfaces, we coated SFNs onto various substrates including glass, metal, and polymer substrates. On the glass substrate, the SFN film spray-coated from a 0.1 wt % sol exhibits both good transparency and superhydrophobicity (Figure 7a). Figure 7b shows optical photographs of water and hexadecane droplets on uncoated steel and steel coated with a 0.6 wt % solution of SFNs. On the uncoated steel surface, both the water and hexadecane droplets spread with small contact angles. In contrast, on coated steel the water and hexadecane droplets have high contact angles of 163° and 151°, respectively. This superamphiphobicity can be achieved on not only hydrophilic but also hydrophobic substrates, such as a polystyrene Petri dish, without any preor post-treatment (Figure S6). Lastly, we tested the durability of the fabricated multifunctional superhydrophobic surfaces under ambient conditions. The properties of the transparent superhydrophobic surface coated with the sol containing 0.1 wt % SFNs and the superamphiphobic surface coated with the sol containing 0.6 wt % SFNs are durable without notable changes over 6 months (Figure S7). In addition, the mechanically robust superhydrophobic SFN-coated surfaces can be obtained by an
4. CONCLUSIONS In this paper, we have reported a simple method for the fabrication of transparent superhydrophobic or superamphiphobic surfaces via the spray-coating of SFNs without any preor post-treatment of the substrate. This method requires no further surface modification and can be applied to a variety of substrates over large areas. We tested the effects of varying the concentration of the SFN sol on the water and hexadecane repellency and on the transparency of the coated glass substrates. An increase in the concentration of the sol facilitates the transition between the superhydrophobic/transparent and superamphiphobic/translucent states. This transition results from an increase in the discontinuities in the TCL and in the light scattering properties due to micropapillae tuned by varying the concentration of the sol. The present approach is anticipated to have practical applications in the preparation of highly transparent superhydrophobic or superamphiphobic coatings.
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ASSOCIATED CONTENT
S Supporting Information *
FT-IR data of MPS-treated SNs, XPS analysis of SFN-coated surfaces, SEM images of the SFN-coated surfaces with different sol concentrations and gas pressures, AFM images and coating thickness of the SFN-coated surfaces with different sol concentrations, optical photograph of droplets of water and hexadecane on the surface without and with SFNs (0.6 wt %), dynamic contact angles of water and hexadecane on the SFN15055
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coated samples (0.1 and 0.6 wt %) over a time period of 6 months, and shear resistance data of PDMS/SFN coatings. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.C.). Author Contributions §
S.G.L. and D.S.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant (Code No. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT & Future Planning, Korea.
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dx.doi.org/10.1021/la404005b | Langmuir 2013, 29, 15051−15057
