Journal of Colloid and Interface Science 325 (2008) 149–156

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Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

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 Università di Modena e Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Via Vignolese 905/A, 41100 Modena, Italyb Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM), Florence, Italyc 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 2008Accepted 7 May 2008Available online 20 June 2008

Keywords:Sol–gel processSuperhydrophobic surfacesUltrahydrophobic surfacesSelf-cleaning

Different organic/inorganic compositions and deposition methods were used to prepare superhydrophobicsurfaces using metal alkoxides and the sol–gel process. Both surface roughness and composition had tobe adjusted in order to obtain very high contact angles and low contact angle hysteresis as a necessaryrequirement for superhydrophobicity. Multilayer samples with a fluorinated organic–inorganic top layershowed water contact angles of about 157◦ with low hysteresis (2◦). Water drops rolled easily off theirsurface at a tilt angle as low as 4◦.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

Surfaces with a water contact angle (CA) higher than 150◦ andlow CA hysteresis (superhydrophobic or ultrahydrophobic surfaces)have attracted a lot of interest because of their use for design ofwater-repellent and self-cleaning coatings [1]. This is particularlyinteresting for the enhancement of surface properties of outdoorwindows, glasses, automotive parts, shower boxes, textiles finishes,and many other applications.

Superhydrophobic behavior was observed for the first time onthe leaves of the lotus plant (Nelumbo nucifera) [2]. The leavesof this plant always remain clean because water droplets easilyroll off of the ultrahydrophobic leaf surface, collecting and remov-ing contaminations. Therefore, the effect of self-cleaning by waterdroplets is often called “Lotus effect.” It has been found that themain reason for the superhydrophobicity of the leaves is the spe-cial micro relief with wax crystalloids on top, representing a com-bination of roughness and low surface free energy. Nowadays, therequirements for obtaining superhydrophobic behavior (also calledultrahydrophobicity) have been found out in systematic investiga-tions and the knowledge has already been transferred to syntheticmaterials, resulting in industrial products as mentioned above [1].

In the literature of the past few years, the conditions thatshould be met in order to reach self-cleaning properties have been

* Corresponding authors. Address for correspondence: Università di Modenae Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, ViaVignolese 905/A, 41100 Modena, Italy. Faxes: +39 0592056243 (F. Pilati), +493514658565 (A. Synytska).

E-mail addresses: francesco.pilati@unimore.it (F. Pilati), synytska@ipfdd.de(A. Synytska).

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.05.007

better and better understood, and an increasing number of reviewpapers have appeared. The influence of surface free energy, surfacecomposition, and roughness on the surface properties (mainly thewater CA) was examined. It was found that a top layer with lowsurface free energy, i.e., high CA, is one of the requirements forachieving superhydrophobic surfaces [3]. Hare et al. [4] and manyothers reported that the surface free energy decreases if hydro-gen in molecules is replaced by other elements, such as fluorine.Therefore, fluorocarbon polymers were typically used to preparehydrophobic films and coatings. However, it is well known thatthe water CA on smooth hydrophobic surfaces does not generallyexceed 125◦ [5]. The CAs of long-chain hydrocarbon and fluorocar-bon self-assembled monolayers are in the range from 112◦ to 115◦ .A further increase in CA can be achieved only with a contributionderiving from surface roughness. The effect of roughness on CA hasbeen outlined by many investigators [4,6]. It was found that rough-ness on different levels (micrometer level and nanometer level) isrequired to achieve both very high CAs and small CA hysteresis[7,8]. The proper combination of surface structure and surface freeenergy leads to the effect that water droplets cannot penetrate intothe grooves of a rough surface filled with air (or vapor) [9] andare suspended at the top asperities. These rough surfaces show su-perhydrophobic 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 surfacescan be attributed to Wenzel [6] and Cassie and Baxter [9]. Lateron, Shibuichi, Onda, and co-workers investigated wetting on fractalsurfaces [10,11] both theoretically and experimentally. Their resultsshowed that a fractal surface can be water-repellent if the surfaceis composed of hydrophobic materials. A fractal surface is a specialkind of rough surface. The relationship between the CA of a flat

150 R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156

surface, θe, and that of a fractal one, θ fractal, can be expressed bythe equation

cos θfractal = (L/l)D−2 cos θe (1)

where L and l are the upper and the lower limit scales of thefractal behavior on the surface, respectively, and D (2 � D � 3) isthe fractal dimension of the surface [12].

Various methods have been employed to produce superhy-drophobic rough films [13,14], including plasma-enhanced chemi-cal vapor deposition of fluoroalkylsilanes, sublimation of aluminumacetylacetonate from boehmite, titania, or silica coatings [15,16],anodic oxidation of aluminum followed by coating with a low-surface-free-energy top layer [17,18], photolithographically etchedsurfaces [8], embedding polytetrafluoroethylene oligomer parti-cles into nickel electrodes [19], compressing submicrometer par-ticles [1], and plasma-based etching [20] or deposition [7] tech-niques. Another way to enhance the CA is the creation of openporosity on a surface through wet chemical methods: isotacticpolypropylene was used to form porous films with a water CA of160◦ [21]. Most of these methods are very expensive and restrictedto some specific applications. Plasma etching, for example, is lim-ited to low-surface-energy substrates.

A novel and experimentally simple approach to designing su-perhydrophobic repellent surfaces with core–shell particle assem-blies based on submicrometer silica particles (100 nm in radius)was reported in [22–25]. The particles were modified either bythin layers of chemically anchored polystyrene or by chemisorbed(tridecafluoro-1,1,2,2-tetrahydrooctyl) dimethylchlorosilane (FSI).Irregular surfaces were obtained by uncontrolled evaporation ofconcentrated particle suspensions or by strong suppression of par-ticle layers using the Langmuir–Blodgett technique. It was foundthat these surfaces possess fractal structures with a fractal di-mension of about 2.4. Wetting measurements revealed that bothadvancing and receding water CAs on the fractal surface modi-fied by FSI were in the range of 160◦ . Surprisingly, the surfaceshowed repellent properties also with a moderately hydrophobicregion, where the intrinsic CA is much less then 90◦ . Namely, therepulsion of benzyl alcohol (surface tension of 40 mN m−1) onthis fractal surface with intrinsic CA of about 72◦ was observed.Moreover, superhydrophobicity was obtained on surfaces modifiednot only by FSI but also with polystyrene. Advancing and reced-ing 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 developmentof superhydrophobic surfaces by using nanostructured organic–inorganic hybrids. In a previous work, the use of tetraethyl or-thosilicate (TEOS) as a silica precursor and a perfluoropolyetheroligomer (PFPE) as a low surface free energy material made it pos-sible to obtain hydrophobic coatings with very smooth surfacesand a CA of around 110◦ [26]. In this work, we report results fromthe preparation of superhydrophobic coatings via a sol–gel processstarting from several liquid metal alkoxide precursors and α,ω-trialkoxysilane-terminated PFPE. The method is simple and lessexpensive, compared to other conventional techniques, and it canbe applied on a variety of substrates, such as silicon wafer, glass,metals, and polymer surfaces, with little or no pretreatment of thesurfaces. Among the commercial metal alkoxides, TEOS, tetraethylorthotitanate (TEOT), and tetra-n-propyl zirconate (TPOZ) were se-lected taking their different reactivities into account. Different re-activities are expected to result in different surface morphologiesand surface free energies due to demixing of the forming phases.The influence of the preparation conditions (different combinationsof spin-coating and air-brushing) on the surface properties of coat-ings 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 suppliedby 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 thereagents were used as received without further purification.

2.2. Preparation of organic–inorganic hybrids

Several coatings with different compositions and/or numbersof layers were prepared; in the following, they are referred to asTZSx/TZF or TZx/TZF (where TZSx and TZx indicate inorganic lay-ers, consisting of TiO2/ZrO2/SiO2 and TiO2/ZrO2, respectively; x isthe number of inorganic layers deposited by air-brushing; TZF in-dicates the fluorinated organic–inorganic top layer).

TEOT, TPOZ, TEOS, and FLKS10 were mixed in the desired ra-tio in a THF solution, at concentrations of 5 wt% for all-inorganicand organic–inorganic layers of the multilayer samples and 20 wt%for the monolayer samples, respectively. The solutions were al-lowed to react for a few seconds at room temperature beforeapplication to glass substrates, either by air-brushing or by spin-coating. Microscope slides (2.5 × 7.6 cm2) were used as sub-strates: they were cleaned by washing in a standard RCA1 solu-tion (NH4OH:H2O2:H2O = 1:1:5 by volume) at 70 ◦C for 10 minand then rinsed several times in bidistilled water. During the air-brushing application, the glass substrate was kept at a distance ofabout 14 cm from the air-brush applicator and 1.5 ml of solutionwere deposited for each layer (see Fig. 1).

Spin-coating of the solutions described above on silicon wafersor glass slides was carried out using a Laurell WS-400B-NPP-Litewith a spin rate of 3000 rpm for 30 s.

After the last layer was deposited, the samples were subjectedto a thermal post-treatment at 100 ◦C for 2 h. Table 1 lists in detailthe composition (the final metal oxide content was calculated as-suming the completion of the sol–gel reactions of the metal alkox-ide precursors) and the number of layers of the prepared samples.

Multilayer samples were prepared by sequential deposition ofone or more inorganic layers by air-brushing. After each layer’sapplication, the surface was air-dried for about 1 min with a hair-dryer before a subsequent application a few minutes later. A fluori-nated organic–inorganic layer was then deposited by spin-coating.For instance, for the preparation of samples TZSx/TZF, a solutionof TEOT, TPOZ, and TEOS in THF (5 wt%) corresponding to a finalcalculated weight ratio TiO2/ZrO2/SiO2 = 25/36/39 was deposited

R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156 151

Table 1List of the prepared samples, including number, composition, and application tech-nique of each layer

Sample ID Numberof layers

Layer PFPE(wt%)

TiO2

(wt%)ZrO2

(wt%)SiO2

(wt%)Applicationmethod

TZF 1 1 84 4 6 6a Spin-coatingTZS1sp 1 1 0 25 36 39 Spin-coatingTZS1ab 1 1 0 25 36 39 Air-brushingTZS1/TZF 2 1 0 25 36 39 Air-brushing

2 84 4 6 6a Spin-coatingTZS2/TZF 3 1–2 0 25 36 39 Air-brushing

3 84 4 6 6a Spin-coatingTZS3/TZF 4 1–3 0 25 36 39 Air-brushing

4 84 4 6 6a Spin-coatingTZS4/TZF 5 1–4 0 25 36 39 Air-brushing

5 84 4 6 6a Spin-coatingTZS5 5 1–5 0 25 36 39 Air-brushingTZ4/TZF 5 1–4 0 40 60 0 Air-brushing

5 84 4 6 6a Spin-coatingTZ5 5 1–5 0 40 60 0 Air-brushing

a SiO2 derives from α,ω-triethoxysilane terminated perfluoropolyether.

x times by air-brushing. Then a solution was prepared by dis-solving TEOT, TPOZ, and FLKS10 in THF (corresponding to a finalcalculated weight ratio of PFPE/TiO2/ZrO2/SiO2 = 84/4/6/6). It wasimmediately deposited by spin-coating onto the last of the otherpreviously applied layers. It has to be noted that the presence ofSiO2 in the top layer derives from α,ω-triethoxysilane-terminatedPFPE and not from TEOS.

Two inorganic monolayer samples with the same compositionwere prepared by different techniques, TZS1ab by air-brushing andTZS1sp by spin-coating, to investigate the effect of the depositiontechnique on the morphological and wettability properties. The flu-orinated monolayer sample, TZF, was prepared by spin-coating tostudy its wettability, its morphological properties, and then its ef-fect on the final properties of multilayer samples.

2.3. Characterization

Static, advancing, and receding CAs were measured by thesessile drop method using a conventional drop shape techniqueOCA 20 apparatus (DataPhysics Instrument GmbH, Filderstadt, Ger-many). To avoid any surface contamination, all specimens wererinsed in THF and accurately air-dried just before measurement.Static and dynamic CAs, measured both with water and with n-hexadecane, were determined on the basis of at least 10 mea-surements. All CA measurements were carried out under constantconditions at 24 ± 0.5 ◦C and relative humidity 40 ± 3%.

In order to investigate the morphology of the prepared sam-ples, environmental scanning electron microscopy (ESEM) analysiswas carried out by means of a Quanta 200 (FEI, USA). The micro-graphs obtained at higher magnification (10,000×) were obtainedwith an accelerating voltage set at 20 kV, while the micrographsat lower magnification (5000×) were obtained at 5 kV. The tiltangle corresponding to the rolling of water drops on the surfaceof the samples was determined by tilting the ESEM sample holder,putting a drop of water on it, and reading the value at which thedrop started to roll off.

The surface topography was examined by using the opticalimaging device MicroGlider (Fries Research & Technology GmbH,Bergisch Gladbach, Germany). The roughness characteristics wereobtained from 500 × 500 μm2 (MicroGlider) scale images. The res-olution of each MicroGlider image taken was 1000 × 1000 lines.

Root mean square roughness Rq, mean peak to valley heightroughness Rz , and waviness, W z , were calculated from MicroGliderlarge images with the Mark III software. Root mean square rough-ness, Rq, is the standard deviation of feature height (Z ) valueswithin a given area. Mean peak to valley height roughness, Rz , is

Table 2Gloss and UV–visible transmittance measurements

Sample Gloss measurement (85◦) Transmittance at λ = 700 nm(%)

Glass 119 100TZF 107 100TZS1/TZF 6 20TZS2/TZF 3 21TZS3/TZF 2 24TZS4/TZF 2 4TZ4/TZF 1 12

the sum from above of the highest profile point and the depthof the lowest profile valley of the roughness profile within a sin-gle measuring distance. Additionally, the roughness factor rs [6],which is the ratio between the real (true, actual) surface area andthe geometric one, and the fractal dimension as well were cal-culated from MicroGlider data. The definition of a fractal surfaceis any point set whose fractal dimension is strictly greater thanits topological dimension. A fractal surface is a surface for whichthe lateral and vertical scaling behavior are not identical but de-termined by a scaling law. We applied the box-counting method(MicroGlider Software FRT Mark III) to the cross section of theinvestigated films to find the fractal dimension of the preparedcoatings.

The gloss of the samples was measured by a Novo Gloss Trioapparatus; measurements were performed by using 85◦ as stan-dard geometry. The optical transmittance was determined by aLambda 19 spectrophotometer (Perkin Elmer).

3. Results

3.1. Optical properties

First, the optical properties of the obtained surfaces were eval-uated. All the prepared samples appeared defect-free to visual in-spection, and no cracks were observed on the top surface by ESEMmicroscopy. As the number of layers increased, the transmittanceof the coatings decreased (typically below 20% compared to theglass substrate) and only the monolayer sample TZF, prepared byspin-coating, had an optical transparency close to 100%. Similar ef-fects were observed for the gloss (see Table 2).

3.2. Surface topography

The surface topography of the prepared coatings was exam-ined using MicroGlider optical imaging microscope (Figs. 2 and 3).It was found that the surface of the fluorinated monolayer TZFwas smooth and featureless (Fig. 2a). Additional incorporation ofseveral inorganic layers into the coating led to the occurrence ofaggregates and microlevel structures on the surface (Figs. 2e–2k)resulting in an increase of all roughness parameters (root meansquare roughness Rq, mean peak-to-valley height roughness Rz ,roughness factor rs and fractal dimension FD) with each additionallayer. It should be noted that the increase in surface roughness wasaccompanied by an additional increase of the film waviness as well(Table 3).

The lowest values of all roughness parameters were found forthe monolayer films of fluorinated TZF and nonfluorinated TZS1spand TZS1ab coatings (Table 3). However, the Rq, Rz , waviness, androughness factor rs of the monolayer TZS1sp sample prepared byspin-coating, were much lower than those of the TZS1ab sam-ple prepared by air-brushing, and similar to that of TZF, suggest-ing that the roughness was mainly generated by the applicationmethod rather than from the layer composition.

The fractal dimension of the monolayer for the two-layer andthree-layer samples was calculated to be 2.00, indicating that these

152 R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156

Fig. 2. Representative MicroGlider topography images for monolayer samples (a) TZF, (b) monolayer TZS1sp spin-coated, (c) monolayer TZS1ab air-brushed, and multilayersamples (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 fractaldimension FD values for the samples with larger numbers of layers(4 and 5) are considerably higher (>2.2) than those of mono-layer TZF and two-layer TZS1/TZF coatings. It should be noted thatthe FD for the three-layer system TZS2/TZF was equal to 2.00, forthe four-layer TZS3/TZF 2.25, and for the five-layer TS4/TZF andTZS1/TZF coatings 2.29 and 2.31, respectively (Table 3). From thisresult it may be concluded that the FD values for systems with 4–5layers are almost comparable and high enough to expect superhy-drophobic behavior [10,11,17]. The FD value was not significantlychanged by adding the fluorinated hybrid top layer by spin-coating(compare TZS5 and TZS4/TZF). It should be noticed that coatingswith the same numbers of layers showed a little higher value of

fractal dimension when TEOS was not present in the initial solu-tion (compare samples TZS4/TZF and TZ4/TZF).

Scanning electron microscopy of the multilayer nonfluorinatedinorganic and fluorinated coatings revealed textured surfaces witha hierarchy of structures (Fig. 3). The surface of samples TZS5 andTZ5 consisted of aggregates of smooth, round particles with a di-ameter of a few micrometers, as can be seen in Figs. 3a and 3b,respectively. After deposition of the last fluorinated top layer, thesurface morphology changed, and a fine texture at the nanometer-scale level was formed, superimposed on the micrometer rough-ness (see Figs. 3c and 3d). These kinds of morphologies suggestthat a fast reaction within the sprayed metal alkoxide solutiondroplets occurred, leading to the formation of particles during orimmediately after the air-brushing application. The presence of

R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156 153

Table 3Roughness parameters (Rq, Rz , FD, rs , W z ) and wetting properties (static (θS), advancing (θA), and receding (θR) water CAs and CA hysteresis (�θ = θA − θR)) obtained fromMicroGlider and contact angle measurements, respectively

Sample ID Roughness parameters, MicroGlider (500 × 500 μm2) Wetting properties

Rq

(μm)Rz

(μm)W z

(μm)FD rs θS

(◦)θA

(◦)θR

(◦)�θ

(◦)

Glass substrate n.d. n.d. n.d. n.d. n.d. 21 ± 2 n.d. n.d. n.d.TZS1sp 0.19 0.22 0.03 2.00 1.05 23 ± 2 26 ± 2 17 ± 2 9TZS1ab 1.09 1.20 0.05 2.00 1.03 59 ± 6 68 ± 5 55 ± 3 13TZF 0.05 1.23 0.03 2.00 1.05 134 ± 1 143 ± 2 132 ± 1 11TZS5 2.24 43.91 2.04 2.23 1.43 100 ± 3 110 ± 7 90 ± 10 20TZ5 n.d. n.d. 127 ± 7 133 ± 6 119 ± 7 14TZS1/TZF 0.57 11.19 0.54 2.00 1.30 153 ± 3 155 ± 2 151 ± 5 4TZS2/TZF 1.55 22.39 1.74 2.00 1.32 153 ± 2 156 ± 3 152 ± 6 4TZS3/TZF 1.93 24.72 1.93 2.25 1.40 153 ± 1 154 ± 3 153 ± 2 1TZS4/TZF 3.03 29.16 5.32 2.28 1.52 153 ± 1 157 ± 1 155 ± 1 2TZ4/TZF 2.99 29.05 5.50 2.31 1.54 153 ± 1 155 ± 1 154 ± 1 1

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 appli-cation method (spin-coating) were responsible for the changes inmorphology observed after the application of the last hybrid fluo-rinated layer. It seems that a thin layer covers the previous surface,also forming a fine texture at the nanoscale level (see insert pic-tures in Figs. 3c and 3d).

3.3. Surface wettability–contact angle measurements

The wetting properties of the obtained surfaces in the termsof static and dynamic CAs of water and n-hexadecane were alsodetermined (Tables 3 and 4). We started by comparing the wet-ting behavior of the nonfluorinated coatings TZS1sp and TZS1ab,having the same composition and prepared by spin-coating andair-brushing, respectively. While TZS1sp has CA values only slightlyhigher than those of the glass substrate (as expected from thecoating composition), TZS1ab had CAs about 40◦ higher than thoseof TZS1sp, and as they have the same composition, the differencehas to be attributed to the different surface topography (Rq).

For the monolayer fluorinated system TZF, high values of static,advancing, and receding CAs were measured (Table 3, wettingproperties). However, no superhydrophobicity effects were ob-served. Advancing and receding CAs were on the order of 143◦

Table 4Static (θS), advancing (θA), and receding (θR) n-hexadecane contact angle data forpolymer/inorganic hybrid coatings

Sample ID θA (◦) θS (◦) θR (◦)

TZF 88 ± 3 85 ± 2 75 ± 2TZS5 84 ± 6 79 ± 3 70 ± 11TZS1/TZF 104 ± 1 102 ± 1 97 ± 1TZS4/TZF 106 ± 3 103 ± 1 101 ± 3TZ4/TZF 121 ± 10 117 ± 7 113 ± 8

and 132◦ , respectively, and a CA hysteresis of 11◦ was calculated.Interestingly, the incorporation of only two inorganic layers intothe system in the presence of the fluorinated top coat led imme-diately to superhydrophobic behavior (Table 3, wetting properties).Further increase in the number of inorganic layers (3–5) did notalter the wetting properties of the investigated multilayer systems(Table 3). Water advancing and receding CAs were measured to bein the ranges 155◦–157◦ and 151◦–155◦ , respectively.

In contrast to those observed for water CAs, which increasedafter addition of the first inorganic layer and remained unalter-ably high after addition of each further layer, the values of n-hexadecane CAs on the fluorinated surfaces were enhanced con-tinuously with increase in the number of the layers (Table 4).

4. Discussion

The monolayer nonfluorinated system TZS1ab showed higherroot mean square roughness Rq and mean peak-to-valley-heightroughness Rz than TZS1sp (Table 3). Moreover, an increase of about40◦ in water CAs with respect to TZS1sp was also observed (Table 3,wetting properties). This suggests that there is a significant contri-bution from surface roughness to CA. This result is also supportedby the water CA values observed for TZS5, with the same averagechemical composition, but with higher surface roughness parame-ters compared to TZS1ab. A contribution from unreacted ethoxidegroups of TEOS could in principle be present both in TZS1ab and inTZS5. However, if we compare the CA values of TZS5 and TZ5, wecan deduce that this contribution is negligible, if it exists. If unre-acted alkoxide groups were present, they should be contained ina greater amount in TZS5, which contained the less reactive TEOS,than in TZ5. As TZ5 had a much higher CA than TZS5, it can beconcluded that the contribution of residual alkoxy groups to thestrong increase in CA is negligible, by far less important than theeffect attributable to the surface roughness.

The monolayer fluorinated TZF sample with rather low Rqroughness (50 nm) and low roughness factor (rs = 1.05) possessedCA values much higher than the glass substrate (Table 3). How-ever, the mean peak-to-valley-height roughness Rz is found to be1.23 μm for this system (Table 3). Thus, the observed wetting be-

154 R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156

havior is caused by the presence of the perfluorinated polyetherfilm and additionally by its roughness, as was suggested previouslyfor various perfluoropolyethers (PFPE) [27].

It is evident that the incorporation of several inorganic lay-ers (2–5) covered by the fluorinated material generated surfaceroughness high enough to obtain superhydrophobicity (Table 3).By comparing the CA values of fluorinated coatings with the samecomposition of the top layer but with different number of layers,it appears that both static and advancing CAs were very high (Ta-ble 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 dropletsreside in a metastable state [28–32]. Thereby, the static or advanc-ing CA cannot reflect the “true” wettability. It is evident that thewater CA hysteresis for all multilayer fluorinated coatings was verysmall and in the range of 1◦–4◦ . Note, that for coatings with 4–5layers, CA hysteresis was found to be the lowest (1◦–2◦) (Table 3,wetting properties).

Interestingly, the TZS1/TZF system showed superhydrophobicwetting behavior (θadv/θrec = 155◦/151◦), even the roughness fac-tor was very small (rs = 1.30) and the system did not possessfractal properties (FD = 2.00) (Table 3). Thus, this surface is non-fractal and could be described as random (irregular). Meanwhile,the root mean square roughness Rq on the order of 570 nm anda very high peak-to-valley-height roughness Rz = 11.19 μm weremeasured for this sample. Notably, the incorporation of two inor-ganic layers into the coating (TZS1/TZF) led not only to an increasein the overall surface roughness but also to an increase in the sur-face waviness (540 nm) compared to that of the monolayer TZFsample (Table 3).

The results obtained would suggest that the sub-micrometer-level root mean square surface roughness Rq (570 nm) and thepresence of micrometer features (peak-to-valley-height roughnessRz on the order of several micrometers) were enough to get super-hydrophobicity. However, researchers [10,33] generally consideredthat surface structures make intrinsically hydrophilic surfaces morehydrophilic, and intrinsically hydrophobic surfaces more hydropho-bic. Fluorinated coatings are intrinsically hydrophobic because ofthe presence of fluorinated material. Incorporation of a certainlevel of surface roughness leads to the design of more hydrophobicand even superhydrophobic surfaces. However, the above phenom-ena can be explained neither by the Wenzel equation [6], describ-ing the influence of surface roughness, nor by the modified Cassieequation [9], describing the influence of air trapped in grooves ofrough surfaces. No correlation was found between the geometricroughness factor rs and the values of water CAs for either 2- or 3-layer fluorinated coatings. Therefore, we suggest using Rq and Rz

roughness and waviness to understand the water wetting behavioron the investigated 2-layer coatings with a complex structure.

It could be supposed that random (irregular) rough surfaceswith water-repellent wetting behavior were prepared (Table 3).This conclusion is in reasonable agreement with discussions inthe literature [34]. It is known that there are different kinds ofrough surfaces on which it is possible to get superhydrophobicity.A rough surface can be a regular surface with special geometry ora random (irregular) rough surface. Hierarchical rough surfaces arean intermediate case. Bico et al. [35] for instance, reported on re-pellent properties of a regular rough surface that is a fakir carpeton the sub-micrometer scale. However, there are a number of re-ports in the literature about water/oil superhydrophobic wettingbehavior on random irregular surfaces with sub-micrometer andmicrometer levels of surface roughness/structures [1,7,36–40]. Thesurface root mean square roughness reported in these works wasfound to be in the range of 600–1120 nm, measured by AFM. Sucha surface is rough enough to construct a composite material. Ourroughness 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 microme-ter scale ranges. Namely, the root mean square roughness Rq wasmeasured to be 570 and 1550 nm for the TZS1/TZF and TZS2/TZFfilms, respectively (Table 3). Furthermore, a very high peak-to-valley-height roughness Rz on the micrometer scale and wavinesswere detected for these systems (Table 3). We believe that this ad-ditional roughness parameter Rz , as well as waviness, should bealso considered in the discussion of the influence of surface topog-raphy on wettability. Thus, it can be concluded that TZS1/TZF andTZS2/TZF coatings possess a certain level of waviness and rough-ness with sub-micrometer or micrometer structures.

It is obvious that a further increase of the number of layers upto 4 and 5 leads to higher values of surface roughness parameters(Rq, Rz , FD). However, no pronounced rise in CA was observed forthese systems (Table 3). The increase in roughness factor seemedto be small (Table 3). On the other hand, noticeable differences inthe fractal dimension between coatings with 2–3 and 4–5 layerswere 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, wecould conclude that for these coatings, the combination of fractal-ity and dual-scale structures (based on ESEM images) are respon-sible for superhydrophobic water behavior (Table 3). Notably, verylow (1◦–2◦) water CA hysteresis was measured (Table 3, wettingproperties) on these fractal surfaces (TZS3/TZF, TZS4/TZF, TZ4/TZF).

Our results are qualitatively consistent with earlier reported ob-servations [10,11,17,41]. In the earliest works of Shibuichi et al.[10,11], a model for superhydrophobic surfaces with multiscale sur-face roughness was described. In the triadic Koch curve model,the value of the fractal dimension was calculated to be 2.26. Itis believed that this value of FD is enough to obtain superhy-drophobicity. Later on, on an experimental basis, the authors [17]showed that the value of FD = 2.19 for rough anodically oxidizedaluminum surfaces covered by a fluorinated silane (1H,1H,2H,2H-perfluorooctyltrichlorosilane) was appropriate to obtain superhy-drophobicity with high advancing CAs of about 160◦ and small CAhysteresis.

On the other hand, prepared coatings with 4–5 layers not onlyare fractal but also showed dual-scale structures, as was shownin ESEM images (Fig. 4). From this observation, our results are inreasonable agreement with work of Ming et al. [40]. The authorsreported rather low values for roughness factor rs for the coat-ing prepared from raspberry-like particles (rs = 1.54), whereas rootmean square roughness was in the sub-micrometer scale range(Rq ≈ 185 nm), the advancing CA was found to be 165◦ , and CAhysteresis is shown to be about 2◦ . The authors explained this wet-ting behavior by the dual-size surface topology.

Considering the nonfluorinated but multilayer coating TZS5, itcan be observed that it shows high roughness parameters includ-ing FD = 2.23. Nevertheless, the advancing CA was in the rangeof 110◦ and a high CA hysteresis value was observed (Table 3).Water droplets remained pinned on the surface and the tilt an-gle (an additional important wetting property for the evaluation ofsuperhydrophobicity) was found to be about 90◦ . Thus, before de-position of the fluorinated top layer, the samples can present highsurface roughness and fractal dimension as well (see TZS5 in Ta-

R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156 155

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 intrinsicproperties of the material.

Only when the last fluorinated organic–inorganic top layer wasapplied to these rough surfaces were the advancing and recedingwater CAs both very high (θA, θR > 150◦) and the hysteresis verylow (see Table 3). In this case the minimum tilt angle to start thedrop rolling was measured to be about 4◦ for the five-layer sampleTZS4/TZF. Note, a decrease of the standard deviation in CA valuesis observed by increasing the number of layers up to 4 and 5, sug-gesting that more “homogeneous” samples are obtained.

Thereby, a certain hydrophobicity of the top layer and a secondlevel (on nanometer/sub-micrometer scale) of roughness seem tobe necessary to minimize contact areas, so that the droplet touchesthe surface only in few points.

According to these results, we can conclude that a synergis-tic effect that leads to superhydrophobic behavior is obtainedby combining fully inorganic rough surfaces with fluorinatedorganic–inorganic layers (see Fig. 5). Recently, it has been reportedthat very high CAs (only static) were obtained by grafting alkylalkoxysilanes onto the surfaces of silica nanoparticles [42]. Thoseresults were ascribed to the chemical modification of the surfaceof silica particles. No data about the surface roughness of the sur-faces were reported. In view of our results, we have to supposethat a strong contribution of the surface roughness was includedin the reported CA values.

It also has to be emphasized that the presence of the fluo-rinated compound made the surfaces oil-repellent, as it appearsfrom the static and dynamic CAs measured using n-hexadecane(see Table 4). Again, the surface roughness seems to be respon-sible for the large increase in CA (compare TZF with TZ4/TZFand TZS4/TZF in Table 4) and the CAs of the multilayer sam-ples with the last fluorinated top layer are much higher thanthose on multilayer sample with only inorganic layers (compareTZS5 and TZS4/TZF in Table 4). The CA of n-hexadecane on thefluorinated monolayer TZF was found to be in reasonable agree-ment (θadv/θrec = 88◦/85◦) to our previously reported data forfluorinated polyesters [43]. Increase in the number of inorganiclayers coated by fluorinated material leads to an increase in n-hexadecane CA (compare data for TZS1/TZF and TZS4/TZF coat-ings in Table 3). Advancing n-hexadecane CAs were measured tobe in the ranges of 106◦ ± 3◦ and 121◦ ± 10◦ for TZS4/TZF andTZ4/TZF five-layer systems, respectively. These data are found to bein good correlation with earlier reports [17]. The authors reportedn-hexadecane CAs of 115◦ for fractal surfaces treated with fluorosi-lane.

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 compareto the TZS4/TZF. Roughness parameters Rq and Rz were measuredto be rather identical for both systems. However, the FD valueswere 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 canbe assumed that such a small increase in fractal dimension wouldlead 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 aneffective method for the facile preparation of superhydrophobiccoatings. Due to high reactivity of the alkoxides used, catalysis topromote the hydrolysis and condensation reactions was not neces-sary.

Superhydrophobic films were prepared by applying multilayercoatings based on TiO2–ZrO2–SiO2 and a silane-terminated per-fluoropolyether to a glass substrate by combining air-brushingand spin-coating procedures. Only coatings that combined suitablechemical compositions and surface roughness showed both highCA and low hysteresis values. The nanometer roughness obtainedwith the spin-coated monolayer samples was too low to obtainsuperhydrophobic behavior. However, multilayer samples with atleast 2–5 layers presented high values of surface roughness pa-rameters. The prepared coatings with 2–3 layers showed randomirregular structures. Incorporation in the coating of 4 and 5 inor-ganic layers led to fractality FD on the order of about 2.3. Waterdrops rolled off the surface and no pinning effects were detected.Moreover, a low CA hysteresis (about 2◦) was observed for thesesystems.

We could show that superhydrophobic behavior was obtainedon both random irregular and fractal surfaces. This observationmay also open an interesting path for new applications. At present,random rough surfaces are much more relevant from a practicalperspective, since they are cheaper to fabricate.

Acknowledgments

Participation and funding by the EU Network of ExcellenceNanoFun-Poly as well as by the CRUI/DAAD Vigoni program isgratefully acknowledged. The authors thank the technicians ofCIGS–University of Modena and Reggio Emilia for the supportgiven during some analyses of samples.

156 R. Taurino et al. / Journal of Colloid and Interface Science 325 (2008) 149–156

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