Hierarchically structured superhydrophobic coatings fabricated by successive

Langmuir–Blodgett deposition of micro-/nano-sized particles and surface silanization

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Nanotechnology 18 (2007) 465604 (7pp) doi:10.1088/0957-4484/18/46/465604

Hierarchically structuredsuperhydrophobic coatings fabricated bysuccessive Langmuir–Blodgett depositionof micro-/nano-sized particles and surfacesilanizationPing-Szu Tsai, Yu-Min Yang1 and Yuh-Lang Lee

Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan

E-mail: ymyang@mail.ncku.edu.tw

Received 15 June 2007, in final form 19 September 2007Published 12 October 2007Online at stacks.iop.org/Nano/18/465604

AbstractThe present study demonstrates the creation of a stable, superhydrophobicsurface by coupling of successive Langmuir–Blodgett (LB) depositions ofmicro- and nano-sized (1.5 μm/50 nm, 1.0 μm/50 nm, and 0.5 μm/50 nm)silica particles on a glass substrate with the formation of a self-assembledmonolayer of dodecyltrichlorosilane on the surface of the particulate film.Particulate films, in which one layer of 50 nm particles was deposited overone to five sublayers of larger micro-sized particles, with hierarchical surfaceroughness and superhydrophobicity, were successfully fabricated.Furthermore, the present ‘two-scale’ (micro- and nano-sized particles)approach is superior to the previous ‘one-scale’ (micro-sized particles)approach in that both higher advancing contact angle and lower contact anglehysteresis can be realized. Experimental results revealed that thesuperhydrophobicity exhibited by as-fabricated particulate films withdifferent sublayer particle diameters increases in the order of0.5 μm > 1.0 μm > 1.5 μm. However, no clear trend between sublayernumber and surface superhydrophobicity could be discerned. An explanationof superhydrophobicity based on the surface roughness introduced bytwo-scale particles is also proposed.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

A fully superhydrophobic surface should exhibit both highcontact angle (higher than about 150 ◦C) and very low slidingangle, which can also be expressed as the contact anglehysteresis (the difference between advancing and recedingcontact angles). Plant leaf surfaces that exhibit unusual wettingcharacteristics of superhydrophobicity have been documentedand investigated [1, 2]. In these leaves (including the lotus leaf)even dew and fog, but especially rain, lead to the complete

1 Author to whom any correspondence should be addressed.

removal of particulate contamination. This self-cleaning orLotus effect [1, 3] is caused by both the hierarchical roughnessof the leaf surface from micrometer-sized papillae havingnanometer-sized branch like protrusions and the intrinsicmaterial hydrophobicity of a surface layer of epicuticular waxcovering these papillae [4, 5].

Studies of plant leaf surfaces, therefore, provide auseful approach to constructing superhydrophobic surfaces.Three strategies have been proposed to fabricate the artificialsuperhydrophobic surfaces that mimic the surface structureof plant leaves. The first one is to create a roughsurface first and then modify it with low surface energy

0957-4484/07/465604+07$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 465604 P-S Tsai et al

Figure 1. Schematic diagram of roughness effects on contact angleand contact angle hysteresis for hydrophobic surfaces. θW, θC&B arecontact angles calculated from Wenzel’s and Cassie and Baxter’sequations. θA, θR, and θH are advancing contact angle, recedingcontact angle, and contact angle hysteresis, respectively. (AfterJohnson and Dettre [39].)

molecules [5–18]. The second one is to roughen thesurface of hydrophobic materials [19–25] and the third oneis to generate well-ordered microstructured surfaces witha small ratio of liquid–solid contact area [26–34]. Ithas also been proven that nanostructures are essential infabricating superhydrophobic surfaces with high contact angle,and multiscale structures can effectively reduce the slidingangle of water droplets [5, 13–18, 22, 24, 25, 30–33].A recent review of the design and creation of surfaceswith special wettability such as superhydrophilicity andsuperhydrophobicity is available [35].

It is well-known that increasing the roughness of a hy-drophobic surface can dramatically increase the hydrophobic-ity [36, 38]. Two distinct models, which provided expres-sions for the apparent contact angle, have been proposed toexplain the phenomenon. The Wenzel [36, 37] model describesa roughness regime in which the liquid fills up the grooves onthe rough surface (the wetted contact). The Cassie and Bax-ter [38] model, on the other hand, describes a roughness regimein which the liquid does not fill the grooves and consequentlyforms a composite surface on the rough substrate (the compos-ite contact).

Johnson and Dettre simulated the variation of the watercontact angle for hydrophobic surfaces with various roughness(r ), which can be defined as the ratio between the actualsurface area and the projected surface area values, byassuming idealized sinusoidal surfaces [39]. Their resultsare summarized schematically in figure 1 for the hydrophobicsurfaces. During the roughness regime where Wenzel’smode is dominant, they showed that the contact angle andits hysteresis on hydrophobic rough surfaces increase as theroughness increases. They also demonstrated that when theroughness exceeds a certain level, the contact angle continues

Figure 2. Schematic diagram of the hierarchical roughness of LBdeposited two-scale SiO2 particulate film and the hydrophobicity ofthe surface from a SAM of alkylsilane.

to increase while the hysteresis starts decreasing. This decreasein hysteresis occurs as a consequence of the switching of thedominant hydrophobicity mode from Wenzel’s to Cassie andBaxter’s due to the increase of the air fraction at the interfacebetween solid and water. Figure 1 explains the outstandingsuperhydrophobicity exhibited by the hierarchical roughness ofthe surface [5, 13–18, 22, 24, 25, 30–33] mentioned previously.As surface roughness is further increased beyond a critical levelby the ‘multiscale structure’ preparation methods, the highcontact angle and low contact angle hysteresis are approachedas required by a superhydrophobic surface.

In a previous work of the authors [34], a method wasproposed for fabricating hydrophobic surfaces by couplingLangmuir–Blodgett (LB) deposition of silica particles ona glass substrate with the formation of a self-assembledmonolayer (SAM) of alkylsilane. The roughness, r , ofthe hexagonally close-packed (hcp) particulate monolayer hasbeen calculated to be 1.9. These experimental results revealedthat a static contact angle of about 130◦ resulted from theparticulate films regardless of the particle size and particlelayer number. This is consistent with the predictions ofboth the Wenzel model and the Cassie and Baxter model inthat roughness of the hydrophobic surface can increase itshydrophobicity and a switching of the dominant mode fromWenzel’s to Cassie and Baxter’s. In general, advancing contactangle about 150◦, receding contact angle about 110◦, andcontact angle hysteresis about 40◦, were exhibited by theparticulate films fabricated. Although considerable success inthe preparation of the hydrophobic ‘one-scale’ (micro-sizedparticles) particulate films has been achieved, somewhat lowcontact angle and relatively high contact angle hysteresis,however, were found for the as-prepared surfaces. In thiswork, ‘two-scale’ (micro- and nano-sized particles) rather thanone-scale particulate films were fabricated by LB depositionof silica multi-layered particle (0.5, 1.0, and 1.5 μm indiameter) coatings on a glass substrate followed by subsequentdeposition of another layer of smaller silica nanoparticles(50 nm in diameter). As shown schematically in figure 2,a sintered and hydrophobically finished particulate film withhierarchical roughness was finally fabricated by sinteringand surface silanization. The silane molecules are linkedto the silica particle surface through Si–O–Si linkage withhydrophobic tails. An increase in roughness of the particulatefilm surface and a consequently enhanced superhydrophobicityare expected. The present work aims to fabricate fullysuperhydrophobic surfaces.

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2. Experimental section

The Langmuir–Blodgett technique is commonly used toprepare molecular films on solid substrates. Recently, it hasalso been successfully applied for the deposition of micro-and nanoparticulate films. Details of the experiments werementioned elsewhere [34, 40]. Only brief description is givenas follows.

2.1. Materials

A suspension of silica particles (20 wt%) with 50 nm diameterwas purchased from Taiwan Nissan Chemical Ind. Co.Silica powders of 0.5, 1.0, and 1.5 μm in diameter werepurchased from Lancaster Synthesis, Inc. These powderswere free from organic contamination and can be easilydispersed in an aqueous medium. Cationic surfactanthexadecyltrimethylammonium bromide (HTMAB, 99% pure,Sigma) was used as the physical modification agent for silicaparticle surfaces. Chloroform (99.9% pure), methanol (99.8%pure), hydrochloric acid (36.5%–38.0%), and 2-propanol(99.8% pure) were supplied by Baker, Mallinckrodt, Backer,and Fluka, respectively. Dodecyltrichlorosilane (C12H25Cl3Si)was purchased from Fluka and used as the SAM componentwithout further purification. All experiments were conductedwith pure water that was passed through a Milli-Q pluspurification system (Millipore, USA) with a resistivity of18.2 M� cm.

2.2. Surface modification of silica particles

Bare silica particles were firstly dispersed in 2-propanol bysonication for 24 h, followed by sonication in the presenceof the surfactant for 1 h. All solutions were left to stand toattain equilibrium for another 24 h. The solvent was thenremoved by a vacuum system. Finally, these surface-modifiedsilica particles were redispersed in chloroform by sonication.The surfactant concentration and particle concentration were1000 ppm and 20 mg ml−1, respectively, in all experiments.It is noteworthy that surfactants were added to the suspensionto control the hydrophilic–hydrophobic balance of the particlesurfaces. In the case of particle suspension, the suspension wasfirstly placed in a vacuum oven for 1 day to remove the solventthen followed by the same procedure for surface modificationas the bare silica particles. The dispersion of the silica particlesin such a solution is stable, as revealed by the variation ofparticle size with time by using a computerized particle sizeanalyser (model Zetasizer 3000 HS, Malvern, UK).

2.3. Particulate monolayer preparation andLangmuir–Blodgett film deposition

The silica suspensions were agitated in an ultrasonic bath priorto use. The monolayer experiments and the film depositionwere conducted in a Langmuir trough (KSV mini-trough, KSVInstruments Ltd, Finland) with a working area of 32 cm ×7.5 cm on a vibration isolation table. The film pressure atthe air/water interface was measured by the Wilhelmy plateattached to a microbalance. Water subphase temperature wasalways controlled at 25 ◦C by a thermostat. In a typicalexperiment, an appropriate amount of the silica suspension

in chloroform was spread on a pure water subphase by usinga microsyringe (1705N, Hamilton Co., USA) and a waitingperiod of 20 min was allowed for solvent evaporation. Themonolayer at the air/water interface was then continuouslycompressed at a barrier speed of 5 mm min−1 to yield a π-Aisotherm.

For film deposition, a 24 mm × 50 mm × 0.1 mm glassmicroscope slide was thoroughly cleaned with rinsing liquidsas indicated by a measured contact angle of less than 5◦ withwater. The particulate film was deposited in the up-strokedirection at a speed of 1 mm min−1, at a selected surfacepressure of 10 mN m−1, which was kept constant during thedeposition by automatic adjusting of the barriers. Multilayerswere deposited successively on a microscope glass slide in thesame way as described for the monolayers. A 10 min waitingperiod at the end of each up-stroke deposition, however, wasallowed to dry the film and the dipping rate of the slide wascontrolled at 75 mm min−1.

2.4. Sintering of particulate film

The LB deposited particulate films were heated in a furnaceunder air atmosphere for 30 min at 450 ◦C to obtain sintered,stable, and organics-free silica films.

2.5. Silanization of particulate films

The clean glass and as-fabricated particulate films were fin-ished by a self-assembled monolayer of dodecyltrichlorosi-lane. Typically, the clean glass and particle deposited glasssubstrates were dipped into a 0.25 wt% solution of dodecyl-trichlorosilane in chloroform at room temperature for 30 min.They were then removed from the silane solution, washed withchloroform twice, and dried with nitrogen gas.

2.6. Characterization of particulate film

Surface morphology and cross-sections of the particledeposited films were examined with a scanning electronmicroscope (SEM, Hitachi S4100, Japan). The static contactangle and dynamic contact angles were measured with waterby using a contact angle meter (GBX, PX610, France) anda dynamic contact angle analyser (Thermo Cahn, WinDCA300, USA), respectively. The dynamic method, commonlyreferred to as the Wilhelmy technique, is the basis of thedynamic contact angle analysis of the fabricated particulatefilms. This is a dynamic approach in which the wetting forceat the solid/liquid/gas interface is automatically recorded viaan electrobalance as a function of immersion depth. While thesolid sample is held in a fixed position by the electrobalance,the wetting liquid contained in a beaker scans along thesolid at a constant speed via a computer-controlled stage.The meniscus formed at the interface is characterized by thedynamic contact angle at the interface. Calculations fromthe Wilhelmy technique are derived directly from the Youngequation. A simple equation may relate the cosine of thecontact angle to the magnitude of the wetting force recordedby the balance, the surface tension of the probe liquid, andthe wetted perimeter of the solid sample. Contact anglesare measured in two directions—in one direction as the stagemoves up, advancing the liquid across the solid surface, and

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Nanotechnology 18 (2007) 465604 P-S Tsai et al

Figure 3. SEM micrographs in top view of a particulate film withthree layers of 1.5 μm SiO2 particles plus one layer of 50 nm SiO2

particles on a glass substrate by LB deposition. Magnification:(a) 10 k, (b) 40 k.

in the opposite direction as the stage moves down, recedingthe liquid across the previously wetted surface. The differencebetween these two extreme contact angles is known as contactangle hysteresis, and is a universal property of most surfaces.By running multicycle immersions on the same sample, thedynamic contact angle can be used to readily distinguishbetween various kinetic and true thermodynamic hysteresiseffects.

3. Experimental results and discussion

In each π-A isotherm of the particulate monolayer, threeregimes, that is gaseous phase, condensed phase, andcollapse phase, could be clearly distinguished. Moreover,there is insignificant hysteresis in the compression andexpansion isotherms, indicating good retention and re-expansion of surface modified particles on the air/waterinterface. Furthermore, a transfer ratio close to 1 was alwaysfound for each upstroke deposition, implying nearly perfecttransfer of silica particulate monolayers.

The SEM micrographs in top view and in cross-sectionalview of a silica particulate film with three layers of 1.5 μmparticles plus one layer of 50 nm particles on a glass substrateare shown in figures 3 and 4, respectively. Actually, thefabrication of this particulate film was realized by successiveLB deposition of particles. Hexagaonally close-packed (hcp)

Figure 4. SEM micrographs in cross-sectional view of a particulatefilm with three layers of 1.5 μm SiO2 particles plus one layer of50 nm SiO2 particles on a glass substrate by LB deposition.Magnification: (a) 10 k, (b) 50 k.

ordering of the larger silica particles was observed. However,defects in them can also be seen. This is found to beeven more serious in the cases of 1.0 and 0.5 μm particles,as shown in figures 5(a) and (b). Smaller silica particles(50 nm), on the other hand, were found to cover mainlythe upper surfaces of the larger particles lain underneath. Itis noteworthy that aggregates of smaller particles were notinfrequently encountered, as also can be seen in the SEMmicrographs. This may be due to nonuniformity in size andshape of the smaller particles. As compared to the one-scale counterparts fabricated previously [34], however, thetwo-scale particulate film obviously gained much more surfaceroughness. Moreover, the surface of LB deposited particulatefilm was always made hydrophobic through the self-assemblyof dodecyltrichlorosilane molecules with hydrophobic tailgroups.

An image of a water droplet with volume of 12 μl on anas-fabricated surface with one layer of 0.5 μm particles plusone layer of 50 nm particles is shown in figure 6 for example.A static contact angle of 148◦ was observed. It is worthnoting that the static contact angle on the silanized ‘two-scale’particulate surface is much higher than that on the silanized‘one-scale’ particulate surfaces, as reported previously by theauthors [34]. About a 20◦ increase in static contact angle mayresult from two-scale instead of one-scale particle deposition.An interesting finding is that the water droplet slides easily

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Figure 5. SEM micrographs in top view of particulate films with(a) three layers of 1.0 μm SiO2 particles plus one layer of 50 nmSiO2 particles, (b) five layers of 0.5 μm SiO2 particles plus one layerof 50 nm SiO2 particles on a glass substrate by LB deposition.Magnification: 40 k.

Figure 6. Water droplet with a volume of 12 μl on a silanizedparticulate film with one layer of 0.5 μm particles plus one layer of50 nm particles on a glass substrate.

on some of the silanized two-scale particulate surfaces and is,therefore, difficult to photograph.

Figure 7 shows, for example, the three repeatedimmersion–emersion cycles during dynamic contact anglemeasurements of the silanized particulate films with (1) onelayer of 0.5 μm particles plus one layer of 50 nm particlesand (2) two layers of 0.5 μm particles plus one layer of50 nm particles. While very high and noticeably differentadvancing and receding contact angles were measured for the

(a)

(b)

Figure 7. Dynamic contact angle measurements of silanizedparticulate film with (a) one layer of 0.5 μm SiO2 particles plus onelayer of 50 nm SiO2 particles, (b) two layers of 0.5 μm SiO2

particles plus one layer of 50 nm SiO2 particles on a glass substrateobtained by LB deposition.

former (figure 7(a)), very high and nearly equal advancingand receding contact angles were measured for the latter(figure 7(b)). The silanized particulate film with two layers of0.5 μm particles plus one layer of 50 nm particles thus qualifiesas a superhydrophobic surface.

The experimental results of average advancing contactangle, receding contact angle, and hysteresis for the elevensilanized two-scale particulate films are summarized infigure 8. Overall advancing contact angles from 162◦ to 176◦,receding contact angles from 128◦ to 169◦, and hysteresisfrom 6◦ to 37◦ were measured (see details in table 1).As shown in figure 8, sublayer particle diameter had aninsignificant effect on advancing contact angle but had asignificant effect on receding contact angle and, consequently,on hysteresis. For two-scale particulate films prepared fromsublayer particles of 0.5, 1.0, and 1.5 μm in diameter, itis apparent that the smaller the sublayer particle diameterthe higher the receding contact angle and the lower thehysteresis. It should be mentioned that the different sublayerparticle size enables ready adjustment of the size ratiobetween the small and large particles and, subsequently,enables us to tune the surface roughness. The diameterratio of the outmost layer particle to the sublayer particle,thus increases from 1/30 to 1/10 as the sublayer particle

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Table 1. Dynamic contact angle data. (Two-scale SiO2 particulate films with a SAM of dodecyltrichlorosilane on the outer surface.)

(a) 1.5 μm(1–3 layers) + 50 nm (1 layer)

1 layer 2 layers 3 layers

Contact angle (deg) Adv. Rec. Hys. Adv. Rec. Hys. Adv. Rec. Hys.164 128 36 170 135 35 163 132 31163 130 33 166 133 33 166 134 32163 130 33 167 131 36 166 134 32

Average (deg) 163 129 34 168 133 35 165 133 32

Standard deviation (deg) 1 1 2 2 2 2 2 1 1

(b) 1.0 μm(1–3 layers) + 50 nm (1 layer)

1 layer 2 layers 3 layers

Contact angle (deg) Adv. Rec. Hys. Adv. Rec. Hys. Adv. Rec. Hys.169 132 37 170 147 23 172 159 13162 133 29 164 153 11 170 157 13163 135 28 165 152 13 167 162 5

Average (deg) 165 133 31 166 151 16 170 159 10

Standard deviation (deg) 4 2 5 3 3 6 3 3 5

(c) 0.5 μm(1–5 layers) + 50 nm (1 layer)

1 layer 2 layers 3 layers 4 layers 5 layers

Contact angle (deg) Adv. Rec. Hys. Adv. Rec. Hys. Adv. Rec. Hys. Adv. Rec. Hys. Adv. Rec. Hys.168 146 22 171 165 6 172 156 16 170 162 8 176 165 11167 143 24 171 165 6 170 152 18 175 166 9 174 158 16167 140 27 171 164 7 169 149 20 175 165 10 175 157 18

Average (deg) 167 143 24 171 165 6 170 152 18 173 164 9 175 160 15

Standard deviation (deg) 1 3 3 0 0 0 2 4 2 3 2 1 1 4 4

Figure 8. Average advancing and receding contact angles andhysteresis of two-scale particulate films with different sublayerparticle diameters and sublayer numbers.

diameter decreases from 1.5 to 0.5 μm. Better hydrophobicityis, therefore, exhibited by two-scale films with 0.5 μmsublayer particles. This is consistent with the predictions asshown in figure 1 for surfaces with higher roughness. Sincethe superhydrophobic phenomenon basically only concernsoutmost surface, no clear trend between sublayer numberand surface superhydrophobicity could be discerned. Thedifferent hydrophobicity values exhibited by particulate filmswith different sublayer numbers, however, may be the resultof different surface roughnesses, which developed by possibledefects in the monolayers of sublayer particles during thesuccessive LB depositions.

A comparison between the hydrophobicity exhibited bythe particulate films fabricated by one-scale and two-scaleapproaches is shown in figure 9. Significant enhancement ofhydrophobicity for higher advancing contact angle and lowercontact angle hysteresis has been achieved through the two-scale approach for 1.5 μm/50 nm (figure 9(a)), 1.0 μm/50 nm(figure 9(b)), and 0.5 μm/50 nm (figure 9(c)) two-scaleparticle combinations. The present ‘two-scale’ approach is,obviously, superior to the previous ‘one-scale’ approach.

4. Conclusions

Silica particles of 50 nm, 0.5 μm, 1.0 μm, and 1.5 μmin diameter were surface-modified and used for fabricatingsuperhydrophobic surfaces by coupling the LB deposition ofthese particles on glass substrates with the formation of a self-assembled monolayer of dodecyltrichlorosilane on the outersurface of the particulate films. The effects of sublayer particlesize and sublayer number on the wetting behavior of thetwo-scale (0.5 μm/50 nm, 1.0 μm/50 nm, 1.5 μm/50 nm)particulate films were systematically studied. Someconclusions can be drawn from the study as follows. Firstly,superhydrophobic two-scale particulate films with hierarchicalroughness were shown to be successfully fabricated by theproposed method. The present ‘two-scale’ approach is superiorto the previous ‘one-scale’ approach in that both higheradvancing contact angle and lower contact angle hysteresis canbe realized. Secondly, the superhydrophobicity exhibited byas-fabricated particulate films with different sublayer particlediameters increases in the order of 0.5 μm > 1.0 μm >

1.5 μm. However, no clear trend between sublayer number

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Nanotechnology 18 (2007) 465604 P-S Tsai et al

(a)

(b)

(c)

Figure 9. Enhancement of hydrophobicity for higher advancingcontact angle and lower contact angle hysteresis through the‘two-scale’ approach. (a) 1.5 μm/50 nm, (b) 1.0 μm/50 nm,(c) 0.5 μm/50 nm.

and surface superhydrophobicity could be discerned. Finally,an explanation of superhydrophobicity based on the surfaceroughness introduced by two-scale particles was proposed.

Acknowledgments

This work was supported by the National Science Council ofTaiwan through Grants NSC 94-2214-E-006-017 and NSC 95-2221-E-006-301. Zhung-Ching Du is acknowledged for hisassistance with the Langmuir trough.

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