Journal of Colloid and Interface Science 325 (2008) 149–156
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Facile preparation of superhydrophobic coatings by sol–gel processes Rosa Taurino a,b , Elena Fabbri a,b , Massimo Messori a,b , Francesco Pilati a,b,∗ , Doris Pospiech c , Alla Synytska c,∗ a b c
Università di Modena e Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Via Vignolese 905/A, 41100 Modena, Italy Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM), Florence, Italy Leibniz Institute of Polymer Research Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 4 March 2008 Accepted 7 May 2008 Available online 20 June 2008
Different organic/inorganic compositions and deposition methods were used to prepare superhydrophobic surfaces using metal alkoxides and the sol–gel process. Both surface roughness and composition had to be adjusted in order to obtain very high contact angles and low contact angle hysteresis as a necessary requirement for superhydrophobicity. Multilayer samples with a fluorinated organic–inorganic top layer showed water contact angles of about 157◦ with low hysteresis (2◦ ). Water drops rolled easily off their surface at a tilt angle as low as 4◦ . © 2008 Elsevier Inc. All rights reserved.
Keywords: Sol–gel process Superhydrophobic surfaces Ultrahydrophobic surfaces Self-cleaning
1. Introduction Surfaces with a water contact angle (CA) higher than 150◦ and low CA hysteresis (superhydrophobic or ultrahydrophobic surfaces) have attracted a lot of interest because of their use for design of water-repellent and self-cleaning coatings [1]. This is particularly interesting for the enhancement of surface properties of outdoor windows, glasses, automotive parts, shower boxes, textiles finishes, and many other applications. Superhydrophobic behavior was observed for the first time on the leaves of the lotus plant (Nelumbo nucifera) [2]. The leaves of this plant always remain clean because water droplets easily roll off of the ultrahydrophobic leaf surface, collecting and removing contaminations. Therefore, the effect of self-cleaning by water droplets is often called “Lotus effect.” It has been found that the main reason for the superhydrophobicity of the leaves is the special micro relief with wax crystalloids on top, representing a combination of roughness and low surface free energy. Nowadays, the requirements for obtaining superhydrophobic behavior (also called ultrahydrophobicity) have been found out in systematic investigations and the knowledge has already been transferred to synthetic materials, resulting in industrial products as mentioned above [1]. In the literature of the past few years, the conditions that should be met in order to reach self-cleaning properties have been
*
Corresponding authors. Address for correspondence: Università di Modena e Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Via Vignolese 905/A, 41100 Modena, Italy. Faxes: +39 0592056243 (F. Pilati), +49 3514658565 (A. Synytska). E-mail addresses:
[email protected] (F. Pilati),
[email protected] (A. Synytska). 0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.05.007
©
2008 Elsevier Inc. All rights reserved.
better and better understood, and an increasing number of review papers have appeared. The influence of surface free energy, surface composition, and roughness on the surface properties (mainly the water CA) was examined. It was found that a top layer with low surface free energy, i.e., high CA, is one of the requirements for achieving superhydrophobic surfaces [3]. Hare et al. [4] and many others reported that the surface free energy decreases if hydrogen in molecules is replaced by other elements, such as fluorine. Therefore, fluorocarbon polymers were typically used to prepare hydrophobic films and coatings. However, it is well known that the water CA on smooth hydrophobic surfaces does not generally exceed 125◦ [5]. The CAs of long-chain hydrocarbon and fluorocarbon self-assembled monolayers are in the range from 112◦ to 115◦ . A further increase in CA can be achieved only with a contribution deriving from surface roughness. The effect of roughness on CA has been outlined by many investigators [4,6]. It was found that roughness on different levels (micrometer level and nanometer level) is required to achieve both very high CAs and small CA hysteresis [7,8]. The proper combination of surface structure and surface free energy leads to the effect that water droplets cannot penetrate into the grooves of a rough surface filled with air (or vapor) [9] and are suspended at the top asperities. These rough surfaces show superhydrophobic behavior and exhibit little CA hysteresis; therefore, suspended drops roll off with the slightest perturbation. The earliest work on the problem of wetting on rough surfaces can be attributed to Wenzel [6] and Cassie and Baxter [9]. Later on, Shibuichi, Onda, and co-workers investigated wetting on fractal surfaces [10,11] both theoretically and experimentally. Their results showed that a fractal surface can be water-repellent if the surface is composed of hydrophobic materials. A fractal surface is a special kind of rough surface. The relationship between the CA of a flat
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surface, θe , and that of a fractal one, θ fractal , can be expressed by the equation cos θfractal = ( L /l) D −2 cos θe
(1)
where L and l are the upper and the lower limit scales of the fractal behavior on the surface, respectively, and D (2 D 3) is the fractal dimension of the surface [12]. Various methods have been employed to produce superhydrophobic rough films [13,14], including plasma-enhanced chemical vapor deposition of fluoroalkylsilanes, sublimation of aluminum acetylacetonate from boehmite, titania, or silica coatings [15,16], anodic oxidation of aluminum followed by coating with a lowsurface-free-energy top layer [17,18], photolithographically etched surfaces [8], embedding polytetrafluoroethylene oligomer particles into nickel electrodes [19], compressing submicrometer particles [1], and plasma-based etching [20] or deposition [7] techniques. Another way to enhance the CA is the creation of open porosity on a surface through wet chemical methods: isotactic polypropylene was used to form porous films with a water CA of 160◦ [21]. Most of these methods are very expensive and restricted to some specific applications. Plasma etching, for example, is limited to low-surface-energy substrates. A novel and experimentally simple approach to designing superhydrophobic repellent surfaces with core–shell particle assemblies based on submicrometer silica particles (100 nm in radius) was reported in [22–25]. The particles were modified either by thin layers of chemically anchored polystyrene or by chemisorbed (tridecafluoro-1,1,2,2-tetrahydrooctyl) dimethylchlorosilane (FSI). Irregular surfaces were obtained by uncontrolled evaporation of concentrated particle suspensions or by strong suppression of particle layers using the Langmuir–Blodgett technique. It was found that these surfaces possess fractal structures with a fractal dimension of about 2.4. Wetting measurements revealed that both advancing and receding water CAs on the fractal surface modified by FSI were in the range of 160◦ . Surprisingly, the surface showed repellent properties also with a moderately hydrophobic region, where the intrinsic CA is much less then 90◦ . Namely, the repulsion of benzyl alcohol (surface tension of 40 mN m−1 ) on this fractal surface with intrinsic CA of about 72◦ was observed. Moreover, superhydrophobicity was obtained on surfaces modified not only by FSI but also with polystyrene. Advancing and receding water CA on such a layer were measured to be 145◦ and 142◦ , respectively [24]. The main aim of the work reported here was the development of superhydrophobic surfaces by using nanostructured organic– inorganic hybrids. In a previous work, the use of tetraethyl orthosilicate (TEOS) as a silica precursor and a perfluoropolyether oligomer (PFPE) as a low surface free energy material made it possible to obtain hydrophobic coatings with very smooth surfaces and a CA of around 110◦ [26]. In this work, we report results from the preparation of superhydrophobic coatings via a sol–gel process starting from several liquid metal alkoxide precursors and α , ω trialkoxysilane-terminated PFPE. The method is simple and less expensive, compared to other conventional techniques, and it can be applied on a variety of substrates, such as silicon wafer, glass, metals, and polymer surfaces, with little or no pretreatment of the surfaces. Among the commercial metal alkoxides, TEOS, tetraethyl orthotitanate (TEOT), and tetra-n-propyl zirconate (TPOZ) were selected taking their different reactivities into account. Different reactivities are expected to result in different surface morphologies and surface free energies due to demixing of the forming phases. The influence of the preparation conditions (different combinations of spin-coating and air-brushing) on the surface properties of coatings was examined.
Fig. 1. Scheme of air-brushing apparatus: (a) glass substrate, (b) sol–gel solution, (c) air-brush applicator.
2. Experimental 2.1. Materials
α , ω-Triethoxysilane-terminated perfluoropolyether (FLKS10) having a molecular weight of 2000 g mol−1 was kindly supplied by Solvay-Solexis. Tetraethyl orthosilicate (TEOS), tetraethyl orthotitanate (TEOT), and tetra-n-propyl zirconate (TPOZ) were purchased by Aldrich. Tetrahydrofuran (THF) was purchased from Carlo Erba. All the reagents were used as received without further purification. 2.2. Preparation of organic–inorganic hybrids Several coatings with different compositions and/or numbers of layers were prepared; in the following, they are referred to as TZSx /TZF or TZx /TZF (where TZSx and TZx indicate inorganic layers, consisting of TiO2 /ZrO2 /SiO2 and TiO2 /ZrO2 , respectively; x is the number of inorganic layers deposited by air-brushing; TZF indicates the fluorinated organic–inorganic top layer). TEOT, TPOZ, TEOS, and FLKS10 were mixed in the desired ratio in a THF solution, at concentrations of 5 wt% for all-inorganic and organic–inorganic layers of the multilayer samples and 20 wt% for the monolayer samples, respectively. The solutions were allowed to react for a few seconds at room temperature before application to glass substrates, either by air-brushing or by spincoating. Microscope slides (2.5 × 7.6 cm2 ) were used as substrates: they were cleaned by washing in a standard RCA1 solution (NH4 OH:H2 O2 :H2 O = 1:1:5 by volume) at 70 ◦ C for 10 min and then rinsed several times in bidistilled water. During the airbrushing application, the glass substrate was kept at a distance of about 14 cm from the air-brush applicator and 1.5 ml of solution were deposited for each layer (see Fig. 1). Spin-coating of the solutions described above on silicon wafers or glass slides was carried out using a Laurell WS-400B-NPP-Lite with a spin rate of 3000 rpm for 30 s. After the last layer was deposited, the samples were subjected to a thermal post-treatment at 100 ◦ C for 2 h. Table 1 lists in detail the composition (the final metal oxide content was calculated assuming the completion of the sol–gel reactions of the metal alkoxide precursors) and the number of layers of the prepared samples. Multilayer samples were prepared by sequential deposition of one or more inorganic layers by air-brushing. After each layer’s application, the surface was air-dried for about 1 min with a hairdryer before a subsequent application a few minutes later. A fluorinated organic–inorganic layer was then deposited by spin-coating. For instance, for the preparation of samples TZSx /TZF, a solution of TEOT, TPOZ, and TEOS in THF (5 wt%) corresponding to a final calculated weight ratio TiO2 /ZrO2 /SiO2 = 25/36/39 was deposited
R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156
Table 1 List of the prepared samples, including number, composition, and application technique of each layer Sample ID
Number of layers
Layer
PFPE (wt%)
TiO2 (wt%)
ZrO2 (wt%)
SiO2 (wt%)
Application method
TZF TZS1sp TZS1ab TZS1 /TZF
1 1 1 2
TZS2 /TZF
3
TZS3 /TZF
4
TZS4 /TZF
5
TZS5 TZ4 /TZF
5 5
TZ5
5
1 1 1 1 2 1–2 3 1–3 4 1–4 5 1–5 1–4 5 1–5
84 0 0 0 84 0 84 0 84 0 84 0 0 84 0
4 25 25 25 4 25 4 25 4 25 4 25 40 4 40
6 36 36 36 6 36 6 36 6 36 6 36 60 6 60
6a 39 39 39 6a 39 6a 39 6a 39 6a 39 0 6a 0
Spin-coating Spin-coating Air-brushing Air-brushing Spin-coating Air-brushing Spin-coating Air-brushing Spin-coating Air-brushing Spin-coating Air-brushing Air-brushing Spin-coating Air-brushing
a
SiO2 derives from
α , ω -triethoxysilane terminated perfluoropolyether.
x times by air-brushing. Then a solution was prepared by dissolving TEOT, TPOZ, and FLKS10 in THF (corresponding to a final calculated weight ratio of PFPE/TiO2 /ZrO2/ SiO2 = 84/4/6/6). It was immediately deposited by spin-coating onto the last of the other previously applied layers. It has to be noted that the presence of SiO2 in the top layer derives from α , ω -triethoxysilane-terminated PFPE and not from TEOS. Two inorganic monolayer samples with the same composition were prepared by different techniques, TZS1ab by air-brushing and TZS1sp by spin-coating, to investigate the effect of the deposition technique on the morphological and wettability properties. The fluorinated monolayer sample, TZF, was prepared by spin-coating to study its wettability, its morphological properties, and then its effect on the final properties of multilayer samples. 2.3. Characterization Static, advancing, and receding CAs were measured by the sessile drop method using a conventional drop shape technique OCA 20 apparatus (DataPhysics Instrument GmbH, Filderstadt, Germany). To avoid any surface contamination, all specimens were rinsed in THF and accurately air-dried just before measurement. Static and dynamic CAs, measured both with water and with nhexadecane, were determined on the basis of at least 10 measurements. All CA measurements were carried out under constant conditions at 24 ± 0.5 ◦ C and relative humidity 40 ± 3%. In order to investigate the morphology of the prepared samples, environmental scanning electron microscopy (ESEM) analysis was carried out by means of a Quanta 200 (FEI, USA). The micrographs obtained at higher magnification (10,000×) were obtained with an accelerating voltage set at 20 kV, while the micrographs at lower magnification (5000×) were obtained at 5 kV. The tilt angle corresponding to the rolling of water drops on the surface of the samples was determined by tilting the ESEM sample holder, putting a drop of water on it, and reading the value at which the drop started to roll off. The surface topography was examined by using the optical imaging device MicroGlider (Fries Research & Technology GmbH, Bergisch Gladbach, Germany). The roughness characteristics were obtained from 500 × 500 μm2 (MicroGlider) scale images. The resolution of each MicroGlider image taken was 1000 × 1000 lines. Root mean square roughness R q , mean peak to valley height roughness R z , and waviness, W z , were calculated from MicroGlider large images with the Mark III software. Root mean square roughness, R q , is the standard deviation of feature height ( Z ) values within a given area. Mean peak to valley height roughness, R z , is
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Table 2 Gloss and UV–visible transmittance measurements Sample
Gloss measurement (85◦ )
Transmittance at λ = 700 nm (%)
Glass TZF TZS1 /TZF TZS2 /TZF TZS3 /TZF TZS4 /TZF TZ4 /TZF
119 107 6 3 2 2 1
100 100 20 21 24 4 12
the sum from above of the highest profile point and the depth of the lowest profile valley of the roughness profile within a single measuring distance. Additionally, the roughness factor rs [6], which is the ratio between the real (true, actual) surface area and the geometric one, and the fractal dimension as well were calculated from MicroGlider data. The definition of a fractal surface is any point set whose fractal dimension is strictly greater than its topological dimension. A fractal surface is a surface for which the lateral and vertical scaling behavior are not identical but determined by a scaling law. We applied the box-counting method (MicroGlider Software FRT Mark III) to the cross section of the investigated films to find the fractal dimension of the prepared coatings. The gloss of the samples was measured by a Novo Gloss Trio apparatus; measurements were performed by using 85◦ as standard geometry. The optical transmittance was determined by a Lambda 19 spectrophotometer (Perkin Elmer). 3. Results 3.1. Optical properties First, the optical properties of the obtained surfaces were evaluated. All the prepared samples appeared defect-free to visual inspection, and no cracks were observed on the top surface by ESEM microscopy. As the number of layers increased, the transmittance of the coatings decreased (typically below 20% compared to the glass substrate) and only the monolayer sample TZF, prepared by spin-coating, had an optical transparency close to 100%. Similar effects were observed for the gloss (see Table 2). 3.2. Surface topography The surface topography of the prepared coatings was examined using MicroGlider optical imaging microscope (Figs. 2 and 3). It was found that the surface of the fluorinated monolayer TZF was smooth and featureless (Fig. 2a). Additional incorporation of several inorganic layers into the coating led to the occurrence of aggregates and microlevel structures on the surface (Figs. 2e–2k) resulting in an increase of all roughness parameters (root mean square roughness R q , mean peak-to-valley height roughness R z , roughness factor rs and fractal dimension FD) with each additional layer. It should be noted that the increase in surface roughness was accompanied by an additional increase of the film waviness as well (Table 3). The lowest values of all roughness parameters were found for the monolayer films of fluorinated TZF and nonfluorinated TZS1sp and TZS1ab coatings (Table 3). However, the R q , R z , waviness, and roughness factor rs of the monolayer TZS1sp sample prepared by spin-coating, were much lower than those of the TZS1ab sample prepared by air-brushing, and similar to that of TZF, suggesting that the roughness was mainly generated by the application method rather than from the layer composition. The fractal dimension of the monolayer for the two-layer and three-layer samples was calculated to be 2.00, indicating that these
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Fig. 2. Representative MicroGlider topography images for monolayer samples (a) TZF, (b) monolayer TZS1sp spin-coated, (c) monolayer TZS1ab air-brushed, and multilayer samples (d) TZS1 /TZF, (e) TZS2 /TZF, (f) TZS3 /TZF, (g) TZS4 /TZF, (k) TZ4 /TZF.
surfaces are nonfractal ones (see Table 3). We found that the fractal dimension FD values for the samples with larger numbers of layers (4 and 5) are considerably higher (>2.2) than those of monolayer TZF and two-layer TZS1 /TZF coatings. It should be noted that the FD for the three-layer system TZS2 /TZF was equal to 2.00, for the four-layer TZS3 /TZF 2.25, and for the five-layer TS4 /TZF and TZS1 /TZF coatings 2.29 and 2.31, respectively (Table 3). From this result it may be concluded that the FD values for systems with 4–5 layers are almost comparable and high enough to expect superhydrophobic behavior [10,11,17]. The FD value was not significantly changed by adding the fluorinated hybrid top layer by spin-coating (compare TZS5 and TZS4 /TZF). It should be noticed that coatings with the same numbers of layers showed a little higher value of
fractal dimension when TEOS was not present in the initial solution (compare samples TZS4 /TZF and TZ4 /TZF). Scanning electron microscopy of the multilayer nonfluorinated inorganic and fluorinated coatings revealed textured surfaces with a hierarchy of structures (Fig. 3). The surface of samples TZS5 and TZ5 consisted of aggregates of smooth, round particles with a diameter of a few micrometers, as can be seen in Figs. 3a and 3b, respectively. After deposition of the last fluorinated top layer, the surface morphology changed, and a fine texture at the nanometerscale level was formed, superimposed on the micrometer roughness (see Figs. 3c and 3d). These kinds of morphologies suggest that a fast reaction within the sprayed metal alkoxide solution droplets occurred, leading to the formation of particles during or immediately after the air-brushing application. The presence of
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Table 3 Roughness parameters (R q , R z , FD, rs , W z ) and wetting properties (static (θS ), advancing (θA ), and receding (θR ) water CAs and CA hysteresis (θ = θA − θR )) obtained from MicroGlider and contact angle measurements, respectively Sample ID
Glass substrate TZS1sp TZS1ab TZF TZS5 TZ5 TZS1 /TZF TZS2 /TZF TZS3 /TZF TZS4 /TZF TZ4 /TZF
Roughness parameters, MicroGlider (500 × 500 μm2 )
Wetting properties
Rq (μm)
Rz (μm)
Wz (μm)
FD
rs
θS
θA
θR
θ
n.d. 0.19 1.09 0.05 2.24 n.d. 0.57 1.55 1.93 3.03 2.99
n.d. 0.22 1.20 1.23 43.91
n.d. 0.03 0.05 0.03 2.04
n.d. 1.05 1.03 1.05 1.43
11.19 22.39 24.72 29.16 29.05
0.54 1.74 1.93 5.32 5.50
n.d. 2.00 2.00 2.00 2.23 n.d. 2.00 2.00 2.25 2.28 2.31
21 ± 2 23 ± 2 59 ± 6 134 ± 1 100 ± 3 127 ± 7 153 ± 3 153 ± 2 153 ± 1 153 ± 1 153 ± 1
n.d. 26 ± 2 68 ± 5 143 ± 2 110 ± 7 133 ± 6 155 ± 2 156 ± 3 154 ± 3 157 ± 1 155 ± 1
n.d. 17 ± 2 55 ± 3 132 ± 1 90 ± 10 119 ± 7 151 ± 5 152 ± 6 153 ± 2 155 ± 1 154 ± 1
n.d. 9 13 11 20 14 4 4 1 2 1
1.30 1.32 1.40 1.52 1.54
(◦ )
(◦ )
(◦ )
(◦ )
Table 4 Static (θS ), advancing (θA ), and receding (θR ) n-hexadecane contact angle data for polymer/inorganic hybrid coatings Sample ID
θA (◦ )
θS (◦ )
θR (◦ )
TZF TZS5 TZS1 /TZF TZS4 /TZF TZ4 /TZF
88 ± 3 84 ± 6 104 ± 1 106 ± 3 121 ± 10
85 ± 2 79 ± 3 102 ± 1 103 ± 1 117 ± 7
75 ± 2 70 ± 11 97 ± 1 101 ± 3 113 ± 8
and 132◦ , respectively, and a CA hysteresis of 11◦ was calculated. Interestingly, the incorporation of only two inorganic layers into the system in the presence of the fluorinated top coat led immediately to superhydrophobic behavior (Table 3, wetting properties). Further increase in the number of inorganic layers (3–5) did not alter the wetting properties of the investigated multilayer systems (Table 3). Water advancing and receding CAs were measured to be in the ranges 155◦ –157◦ and 151◦ –155◦ , respectively. In contrast to those observed for water CAs, which increased after addition of the first inorganic layer and remained unalterably high after addition of each further layer, the values of nhexadecane CAs on the fluorinated surfaces were enhanced continuously with increase in the number of the layers (Table 4). Fig. 3. ESEM images of the surface (top view) of (a) TZS5 , (b) TZ5 , (c) TZS4 /TZF, and (d) TZ4 /TZF.
FLKS10 among the reactants and in particular the different application method (spin-coating) were responsible for the changes in morphology observed after the application of the last hybrid fluorinated layer. It seems that a thin layer covers the previous surface, also forming a fine texture at the nanoscale level (see insert pictures in Figs. 3c and 3d). 3.3. Surface wettability–contact angle measurements The wetting properties of the obtained surfaces in the terms of static and dynamic CAs of water and n-hexadecane were also determined (Tables 3 and 4). We started by comparing the wetting behavior of the nonfluorinated coatings TZS1sp and TZS1ab , having the same composition and prepared by spin-coating and air-brushing, respectively. While TZS1sp has CA values only slightly higher than those of the glass substrate (as expected from the coating composition), TZS1ab had CAs about 40◦ higher than those of TZS1sp , and as they have the same composition, the difference has to be attributed to the different surface topography (R q ). For the monolayer fluorinated system TZF, high values of static, advancing, and receding CAs were measured (Table 3, wetting properties). However, no superhydrophobicity effects were observed. Advancing and receding CAs were on the order of 143◦
4. Discussion The monolayer nonfluorinated system TZS1ab showed higher root mean square roughness R q and mean peak-to-valley-height roughness R z than TZS1sp (Table 3). Moreover, an increase of about 40◦ in water CAs with respect to TZS1sp was also observed (Table 3, wetting properties). This suggests that there is a significant contribution from surface roughness to CA. This result is also supported by the water CA values observed for TZS5 , with the same average chemical composition, but with higher surface roughness parameters compared to TZS1ab . A contribution from unreacted ethoxide groups of TEOS could in principle be present both in TZS1ab and in TZS5 . However, if we compare the CA values of TZS5 and TZ5 , we can deduce that this contribution is negligible, if it exists. If unreacted alkoxide groups were present, they should be contained in a greater amount in TZS5 , which contained the less reactive TEOS, than in TZ5 . As TZ5 had a much higher CA than TZS5 , it can be concluded that the contribution of residual alkoxy groups to the strong increase in CA is negligible, by far less important than the effect attributable to the surface roughness. The monolayer fluorinated TZF sample with rather low R q roughness (50 nm) and low roughness factor (rs = 1.05) possessed CA values much higher than the glass substrate (Table 3). However, the mean peak-to-valley-height roughness R z is found to be 1.23 μm for this system (Table 3). Thus, the observed wetting be-
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havior is caused by the presence of the perfluorinated polyether film and additionally by its roughness, as was suggested previously for various perfluoropolyethers (PFPE) [27]. It is evident that the incorporation of several inorganic layers (2–5) covered by the fluorinated material generated surface roughness high enough to obtain superhydrophobicity (Table 3). By comparing the CA values of fluorinated coatings with the same composition of the top layer but with different number of layers, it appears that both static and advancing CAs were very high (Table 3). To better describe the surface wettability, the CA hysteresis, that is, the difference between the advancing and receding CAs, should be considered as well [1]. On rough surfaces water droplets reside in a metastable state [28–32]. Thereby, the static or advancing CA cannot reflect the “true” wettability. It is evident that the water CA hysteresis for all multilayer fluorinated coatings was very small and in the range of 1◦ –4◦ . Note, that for coatings with 4–5 layers, CA hysteresis was found to be the lowest (1◦ –2◦ ) (Table 3, wetting properties). Interestingly, the TZS1 /TZF system showed superhydrophobic wetting behavior (θadv /θrec = 155◦ /151◦ ), even the roughness factor was very small (rs = 1.30) and the system did not possess fractal properties (FD = 2.00) (Table 3). Thus, this surface is nonfractal and could be described as random (irregular). Meanwhile, the root mean square roughness R q on the order of 570 nm and a very high peak-to-valley-height roughness R z = 11.19 μm were measured for this sample. Notably, the incorporation of two inorganic layers into the coating (TZS1 /TZF) led not only to an increase in the overall surface roughness but also to an increase in the surface waviness (540 nm) compared to that of the monolayer TZF sample (Table 3). The results obtained would suggest that the sub-micrometerlevel root mean square surface roughness R q (570 nm) and the presence of micrometer features (peak-to-valley-height roughness R z on the order of several micrometers) were enough to get superhydrophobicity. However, researchers [10,33] generally considered that surface structures make intrinsically hydrophilic surfaces more hydrophilic, and intrinsically hydrophobic surfaces more hydrophobic. Fluorinated coatings are intrinsically hydrophobic because of the presence of fluorinated material. Incorporation of a certain level of surface roughness leads to the design of more hydrophobic and even superhydrophobic surfaces. However, the above phenomena can be explained neither by the Wenzel equation [6], describing the influence of surface roughness, nor by the modified Cassie equation [9], describing the influence of air trapped in grooves of rough surfaces. No correlation was found between the geometric roughness factor rs and the values of water CAs for either 2- or 3layer fluorinated coatings. Therefore, we suggest using R q and R z roughness and waviness to understand the water wetting behavior on the investigated 2-layer coatings with a complex structure. It could be supposed that random (irregular) rough surfaces with water-repellent wetting behavior were prepared (Table 3). This conclusion is in reasonable agreement with discussions in the literature [34]. It is known that there are different kinds of rough surfaces on which it is possible to get superhydrophobicity. A rough surface can be a regular surface with special geometry or a random (irregular) rough surface. Hierarchical rough surfaces are an intermediate case. Bico et al. [35] for instance, reported on repellent properties of a regular rough surface that is a fakir carpet on the sub-micrometer scale. However, there are a number of reports in the literature about water/oil superhydrophobic wetting behavior on random irregular surfaces with sub-micrometer and micrometer levels of surface roughness/structures [1,7,36–40]. The surface root mean square roughness reported in these works was found to be in the range of 600–1120 nm, measured by AFM. Such a surface is rough enough to construct a composite material. Our roughness data, based on MicroGlider results for the systems with
Fig. 4. ESEM micrograph (cross section) of TZS4 /TZF.
2–3 layers were found to be in the sub-micrometer and micrometer scale ranges. Namely, the root mean square roughness R q was measured to be 570 and 1550 nm for the TZS1 /TZF and TZS2 /TZF films, respectively (Table 3). Furthermore, a very high peak-tovalley-height roughness R z on the micrometer scale and waviness were detected for these systems (Table 3). We believe that this additional roughness parameter R z , as well as waviness, should be also considered in the discussion of the influence of surface topography on wettability. Thus, it can be concluded that TZS1 /TZF and TZS2 /TZF coatings possess a certain level of waviness and roughness with sub-micrometer or micrometer structures. It is obvious that a further increase of the number of layers up to 4 and 5 leads to higher values of surface roughness parameters (R q , R z , FD). However, no pronounced rise in CA was observed for these systems (Table 3). The increase in roughness factor seemed to be small (Table 3). On the other hand, noticeable differences in the fractal dimension between coatings with 2–3 and 4–5 layers were observed (Table 3). Values of FD were calculated to be 2.25, 2.28, and 2.31 for the coatings with 4–5 layers (Table 3). Thus, we could conclude that for these coatings, the combination of fractality and dual-scale structures (based on ESEM images) are responsible for superhydrophobic water behavior (Table 3). Notably, very low (1◦ –2◦ ) water CA hysteresis was measured (Table 3, wetting properties) on these fractal surfaces (TZS3 /TZF, TZS4 /TZF, TZ4 /TZF). Our results are qualitatively consistent with earlier reported observations [10,11,17,41]. In the earliest works of Shibuichi et al. [10,11], a model for superhydrophobic surfaces with multiscale surface roughness was described. In the triadic Koch curve model, the value of the fractal dimension was calculated to be 2.26. It is believed that this value of FD is enough to obtain superhydrophobicity. Later on, on an experimental basis, the authors [17] showed that the value of FD = 2.19 for rough anodically oxidized aluminum surfaces covered by a fluorinated silane (1H,1H,2H,2Hperfluorooctyltrichlorosilane) was appropriate to obtain superhydrophobicity with high advancing CAs of about 160◦ and small CA hysteresis. On the other hand, prepared coatings with 4–5 layers not only are fractal but also showed dual-scale structures, as was shown in ESEM images (Fig. 4). From this observation, our results are in reasonable agreement with work of Ming et al. [40]. The authors reported rather low values for roughness factor rs for the coating prepared from raspberry-like particles (rs = 1.54), whereas root mean square roughness was in the sub-micrometer scale range (R q ≈ 185 nm), the advancing CA was found to be 165◦ , and CA hysteresis is shown to be about 2◦ . The authors explained this wetting behavior by the dual-size surface topology. Considering the nonfluorinated but multilayer coating TZS5 , it can be observed that it shows high roughness parameters including FD = 2.23. Nevertheless, the advancing CA was in the range of 110◦ and a high CA hysteresis value was observed (Table 3). Water droplets remained pinned on the surface and the tilt angle (an additional important wetting property for the evaluation of superhydrophobicity) was found to be about 90◦ . Thus, before deposition of the fluorinated top layer, the samples can present high surface roughness and fractal dimension as well (see TZS5 in Ta-
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Fig. 5. Optical image of a water drop on the TZS4 /TZF sample.
ble 3), but this is not enough to obtain superhydrophobic behavior. This wetting behavior could be explained by hydrophilic intrinsic properties of the material. Only when the last fluorinated organic–inorganic top layer was applied to these rough surfaces were the advancing and receding water CAs both very high (θA , θR > 150◦ ) and the hysteresis very low (see Table 3). In this case the minimum tilt angle to start the drop rolling was measured to be about 4◦ for the five-layer sample TZS4 /TZF. Note, a decrease of the standard deviation in CA values is observed by increasing the number of layers up to 4 and 5, suggesting that more “homogeneous” samples are obtained. Thereby, a certain hydrophobicity of the top layer and a second level (on nanometer/sub-micrometer scale) of roughness seem to be necessary to minimize contact areas, so that the droplet touches the surface only in few points. According to these results, we can conclude that a synergistic effect that leads to superhydrophobic behavior is obtained by combining fully inorganic rough surfaces with fluorinated organic–inorganic layers (see Fig. 5). Recently, it has been reported that very high CAs (only static) were obtained by grafting alkyl alkoxysilanes onto the surfaces of silica nanoparticles [42]. Those results were ascribed to the chemical modification of the surface of silica particles. No data about the surface roughness of the surfaces were reported. In view of our results, we have to suppose that a strong contribution of the surface roughness was included in the reported CA values. It also has to be emphasized that the presence of the fluorinated compound made the surfaces oil-repellent, as it appears from the static and dynamic CAs measured using n-hexadecane (see Table 4). Again, the surface roughness seems to be responsible for the large increase in CA (compare TZF with TZ4 /TZF and TZS4 /TZF in Table 4) and the CAs of the multilayer samples with the last fluorinated top layer are much higher than those on multilayer sample with only inorganic layers (compare TZS5 and TZS4 /TZF in Table 4). The CA of n-hexadecane on the fluorinated monolayer TZF was found to be in reasonable agreement (θadv /θrec = 88◦ /85◦ ) to our previously reported data for fluorinated polyesters [43]. Increase in the number of inorganic layers coated by fluorinated material leads to an increase in nhexadecane CA (compare data for TZS1 /TZF and TZS4 /TZF coatings in Table 3). Advancing n-hexadecane CAs were measured to be in the ranges of 106◦ ± 3◦ and 121◦ ± 10◦ for TZS4 /TZF and TZ4 /TZF five-layer systems, respectively. These data are found to be in good correlation with earlier reports [17]. The authors reported n-hexadecane CAs of 115◦ for fractal surfaces treated with fluorosilane. Interestingly, the sample without silica in the inorganic layers (TZ4 /TZF) showed a tendency to higher CAs than sample TZS4 /TZF (Table 4). Note that an increase in standard deviation for n-
Fig. 6. Summary of n-hexadecane static, advancing, and receding contact angles (CA) and fractal dimension (FD) values for fluorinated mono- and multilayer coatings.
hexadecane CAs is also observed for the system TZ4 /TZF compare to the TZS4 /TZF. Roughness parameters R q and R z were measured to be rather identical for both systems. However, the FD values were found to be slightly higher for the system TZ4 /TZF (FD = 2.31) compared to the TZS4 /TZF (FD = 2.28) (Table 3, Fig. 6). It can be assumed that such a small increase in fractal dimension would lead to a rise in oleophobicity for the TZ4 /TZF sample (see Table 4, Fig. 6). 5. Conclusions The present study demonstrated that the sol–gel process is an effective method for the facile preparation of superhydrophobic coatings. Due to high reactivity of the alkoxides used, catalysis to promote the hydrolysis and condensation reactions was not necessary. Superhydrophobic films were prepared by applying multilayer coatings based on TiO2 –ZrO2 –SiO2 and a silane-terminated perfluoropolyether to a glass substrate by combining air-brushing and spin-coating procedures. Only coatings that combined suitable chemical compositions and surface roughness showed both high CA and low hysteresis values. The nanometer roughness obtained with the spin-coated monolayer samples was too low to obtain superhydrophobic behavior. However, multilayer samples with at least 2–5 layers presented high values of surface roughness parameters. The prepared coatings with 2–3 layers showed random irregular structures. Incorporation in the coating of 4 and 5 inorganic layers led to fractality FD on the order of about 2.3. Water drops rolled off the surface and no pinning effects were detected. Moreover, a low CA hysteresis (about 2◦ ) was observed for these systems. We could show that superhydrophobic behavior was obtained on both random irregular and fractal surfaces. This observation may also open an interesting path for new applications. At present, random rough surfaces are much more relevant from a practical perspective, since they are cheaper to fabricate. Acknowledgments Participation and funding by the EU Network of Excellence NanoFun-Poly as well as by the CRUI/DAAD Vigoni program is gratefully acknowledged. The authors thank the technicians of CIGS–University of Modena and Reggio Emilia for the support given during some analyses of samples.
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