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Applied Surface Science 257 (2011) 4801–4806

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

abrication of superhydrophobic surfaces on zinc substrates

enguo Xu ∗, Tao Ning, Xiaochan Yang, Shixiang Lu ∗

ey Laboratory of Cluster Science of Ministry of Education, School of Science, Beijing Institute of Technology, Department of Chemistry, 5 South Street of Zhongguanchun, Beijing00081, PR China

r t i c l e i n f o

rticle history:eceived 6 October 2010eceived in revised form0 December 2010

a b s t r a c t

Stable superhydrophobic surfaces were fabricated on the zinc substrates through simple silver replace-ment deposition process with the modification of octadecyl mercaptan. The effects of reaction conditionson the surface morphology and wettability of the prepared surfaces were carefully studied. The results

ccepted 12 December 2010vailable online 8 January 2011

eywords:uperhydrophobicurface morphology

show that the fabrication of a best superhydrophobic surface depends largely on the moderate reactantconcentration. When the concentration of AgNO3 solution was 2 mmol/L, the zinc substrate was cov-ered by a dendritic outline structure. Aggregated silver nanoparticles were formed on the substrate inaccordance with some certain laws, exhibiting great surface roughness. The typical hierarchical micro-nanostructures, flower-like structures and porous structures also could be found from the SEM images.The maximal water contact angle (CA) value of about 161 ± 2◦, and the minimal sliding angle (SA) of

nder

ater contact angle about 2◦ were obtained u

. Introduction

Surface wetting behavior is of great importance in determin-ng the application of numerous materials. Many physicochemicalrocesses, such as adsorption, lubrication, adhesion, dispersion,riction and so forth, are closely related to the wettability of mate-ials [1]. Superhydrophobic surfaces, characterized by high staticontact angles (CA) (above 150◦), just as Nelumbo nucifera (lotus)nd Colocosia escuenta, have a rough or microtextured surface withlow interfacial free energy [2,3]. Due to their potential applica-

ions in self-cleaning coatings, paints, microfluidic devices, textilendustries, biocompatible materials and energy conservation ando on, many efforts have been dedicated during recent years to thertificial synthesis [1,4–8]. Generally, a superhydrophobic surfacean be prepared by the combination of reducing surface energynd enhancing surface roughness. In the last decade, various ofechniques have been developed to fabricate artificial superhy-rophobic surfaces, including plasma etching [9], laser etching [10],hemical vapor deposition [11], electrospinning [12], anodic oxi-ation [13], electrochemical reaction and deposition [14], sol–gelethod [15], layer-by-layer deposition [16], etc. However, many

rtificial methods have certain limitations, like severe conditions,

omplex process control, special equipment and poor durability.herefore, inventing some simple and effective methods to fab-icate superhydrophobic surfaces with durable performance is annevitable tendency.

∗ Corresponding authors. Tel.: +86 10 68912667; fax: +86 10 68912631.E-mail addresses: xuwg60@bit.edu.cn (W. Xu), shixianglu@bit.edu.cn (S. Lu).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.12.059

the same reaction condition.© 2011 Elsevier B.V. All rights reserved.

So far, various superhydrophobic surfaces based on polymers,glasses, metals, semiconductors, carbon nanotubes and waxes havebeen fully investigated [17,18]. Among these materials, metallicsuperhydrophobic surfaces have attracted a great deal of inter-est owing to their technological applicability and easy control ofmorphology. Zhao et al. fabricated the branchlike structure of Agaggregating on the matrix of a layer-by-layer (LbL) polyelectrolytemultilayer by an electrodeposition technique [19]. Praig et al. pre-pared a surface using the electrochemical approach of depositingAu nanostructures on ITO with the modification of tetradecanethiol[20]. Qian et al. prepared superhydrophobic surfaces through adislocation-selective chemical etching method on aluminum, cop-per, and zinc substrates [21]. A solution-immersion process forthe fabrication of superhydrophobic steel, copper, and titaniumsurfaces was proposed by Zhang et al. [22]. Similarly, our groupalso obtained superhydrophobic surfaces on copper and galvanizediron substrates by immersing metal substrates into a methanolsolution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltrichlorosilane(CF3(CF2)5(CH2)2SiCl3, FOTMS). Superhydrophobic surfaces werealso prepared on zinc and steel substrates by Liu et al. usingthe solution immersion technique [23–25]. As previewed, all ofthese metallic super-hydrophobic surfaces have showed remark-able superhydrophobicity with a contact angle higher than 150◦.In order to make the practical application in industrial field, itis valuable to explore more techniques for the fabrication of

metallic superhydrophobic surfaces. As is well known, replace-ment deposition is a basic and simple method for the synthesisof nanostructured materials in various systems. For instance, thesilver-engaged replacement deposition can, in principle, extendto any metal whose redox potential is lower than that of the

4802 W. Xu et al. / Applied Surface Science 257 (2011) 4801–4806

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ig. 1. SEM images of the prepared surfaces with different concentrations of A.05 mmol/L.

g+/Ag pair. Up to now, there have been some reports abouthe preparation of metal nanostructures via this simple replace-

ent deposition process. Xia group reported pioneering workn the synthesis of gold hollow nanostructures with a range ofifferent shapes by replacement reaction [26]. Fang et al. pre-ared a dendriticlike Au nanostructure using similar methody means of adjusting the monomer concentration and reac-ion time [27]. Moreover, this simple and effective process alsoan be used for the fabrication of metallic superhydrophobicurfaces. For example, Song et al. fabricated large-area superhy-rophobic metal surfaces via this electroless replacement reactionetween copper substrate and HAuCl4 solution [28]. Even so, byar, little attention has been concentrated on the similar pro-ess.

Herein, we report on this replacement deposition processor the fabrication of superhydrophobic silver surfaces on theinc substrates. In order to reduce the toxicity and envi-onmental pollution, we applied octadecyl mercaptan as theodified material instead of common fluoride. Only three essen-

ial ingredients are required in the process: silver nitrate, zincnd octadecyl mercaptan. The whole experimental process onlyeeds dozens of minutes. Especially, through changing the reac-

ion condition, we carefully studied the effects on morphologynd wettability of prepared surfaces. As expected, the optimaltructure, ideal CA value and stability of fabricated surfacesith both the surface roughness and low surface energy were

btained.

solution under various magnifications: (a)–(c) 20 mmol/L; (d)–(f) 2 mmol/L; (g)

2. Experimental

2.1. Materials and sample preparation

Zinc specimens (99.9%; Tianjin Kermel Chemical Reagent Co.,China) with a size of 2.0 cm × 7.0 cm × 0.02 cm were first treated byhydrochloric acid (20%, AR, Beijing Fine Chemical. Co. LTD, China)for 1 min to remove surface contamination and were rinsed withdistilled water. After drying, the specimens were cleaned ultrasoni-cally with ethanol (99.5%, AR, Beijing Fine Chemical. Co. Ltd, China)for 10 min; subsequently, they were washed with distilled waterrepeatedly for 5 min. After that, the clean specimens were dried atambient temperature in air.

A series of AgNO3 (aq) with a certain Ag+ concentration (from0.05 mmol/L to 20 mmol/L) were prepared using analytically pureAgNO3 reagent (99.7%, AR, Beijing Fine Chemical. Co. Ltd, China).The prepared zinc specimens were immersed in AgNO3 solutionkeeping erect position for a few minutes. Then, they were rinsedwith distilled water and dried at ambient temperature in air.After drying, the specimens were immersed in the ethanol solu-tion (2 mmol/L) of octadecyl mercaptan (Sigma-Aldrich, USA) for30 min. Subsequently, they were dried at room temperature in air.

2.2. Surface characterization

The surface morphology was observed using a SEM (X650,Hitachi, Japan), and the corresponding element distributions

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the formation of the final nanostructure. Thus, clearly, the low sur-face roughness and the closely packed structure are observed fromSEM images. Similarly, when the concentration of AgNO3 (aq) is0.05 mmol/L, the water CA only reaches a maximum value of about

W. Xu et al. / Applied Surfac

f the surface were determined by EDX (EDAX9100, Hitachi,apan).

Contact-angle measurements were measured using deion-zed water by a remote computer-controlled goniometer systemOCA20, Dataphysics, Germany) under static conditions (3 dropsach time, averaging). The accuracy is ±2◦. All measurements wereade under an ambient atmosphere at room temperature.The surface structure was analyzed using a XRD (Xı Pert PRO

PD, PANalytical, Holland, operating at 5.0 kV and 30 mA with au anticathode, � = 1.5418 A).

. Results and discussion

.1. Morphological analyses

The SEM images of products (Fig. 1) can intuitively illustratehe relation between the surface morphology and hydrophobicity.

e carefully monitored the effect of reactant concentration on theorphology of prepared surfaces under different magnifications

effect of the reaction time on the morphology is little). From them,t can be found that aggregated particles are formed on the zinc sub-trate, exhibiting various kinds of special structures. Accordingly,he hydrophobicity of prepared surfaces would be different. Whenhe concentration of AgNO3 (aq) is 20 mmol/L and the reaction times 3 min or 5 min, one can see that the fine nanogrooves form aerfect dendritic structure on the resulting surface (Fig. 1a and b).his dendritic structure is constructed by countless tiny particlesith a higher density. The size of these particles is about 100 nm

Fig. 1c). Fig. 1a and b demonstrate a typical SEM image of silverendrites, exhibiting a single morphology with uniform feather-

ike branches. The individual dendrite length is about 4–6 �m ands composed of symmetrical branches and leaves. However, thedeal surface roughness and porous structures for the superhy-rophobicity cannot be found. All the aggregated particles on theinc substrate form a closely packed structure. Under reducing theoncentration of AgNO3 solution to 2 mmol/L, it is clearly shownhat the zinc substrate is covered by a dendritic outline structurehich is not flawless (Fig. 1d and e). Compared with Fig. 1a and b,

ewer particles are formed and aggregated. Yet, from the images,t is found that aggregated particles are formed on the substrate inccordance with some certain laws, exhibiting great surface rough-ess. In particular, interesting hierarchical micro-nanostructuresnd flower-like structures also can be found in some ways. Seenrom the magnified SEM image (Fig. 1f), it is very valuable thathe dispersed nanoparticles with an average diameter of around00 nm are clearly observed. The porous composite structures alsoan be seen in this magnified image, which maybe contribute to theetter superhydrophobicity of the resulting surface together withhe great surface roughness and hierarchical micro-nanostructures.

hen the concentration of AgNO3 solution is 0.2 mmol/L, the pre-ared surface merely shows the symptom of the formation of theendritic structure, and only a few aggregated nanoparticles can beeen from the image without any rules (Fig. 1g).

.2. Hydrophobicity

The surface wettability of the obtained surfaces on the zinc sub-trate has been studied by CA measurements. As shown in Fig. 2,he water CAs of the surfaces vary quite significantly dependingpon the concentration of AgNO3 solution under the condition of

he constant reaction time (reaction time is 3 min). When the con-entration of AgNO3 (aq) is 20 mmol/L, the water CA only reaches aaximum value of about 147 ± 2◦ (Fig. 3a) as well as a SA of about

◦ (Fig. 3b), implying that this prepared surface cannot be used assuperhydrophobic surface absolutely. The result is caused by the

concentration/mmol/L

Fig. 2. The contact angle as a function of the concentration of AgNO3 solution withthe constant reaction time.

quick formation and fast drop of a black fluey layer on the zinc sub-strate due to the great reactant concentration. The reaction is soviolent that the rough structure on the zinc substrate obtained bythe rinsing with hydrochloric acid cannot play an important rolein the formation of the final prepared surfaces. Because the roughstructure takes part in the replacement reaction rapidly, the deep-seated smooth face of zinc substrate would have main effect for

Fig. 3. The shape of a water droplet on the surface of different samples. (a) Con-centration of AgNO3 solution is 20 mmol/L; (b) sliding water droplet on the surfacetilted at 4◦; (c) concentration of AgNO3 solution is 0.05 mmol/L; (d) sliding waterdroplet on the surface tilted at 4◦; (e) concentration of AgNO3 solution is 2 mmol/L;(f) sliding water droplet on the surface tilted at 2◦ .

4804 W. Xu et al. / Applied Surface Science 257 (2011) 4801–4806

th different rinsing times: (a) 0.5 min; (b) 1 min; (c) 5 min; (d) 10 min.

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Fig. 4. The shape of a water droplet on the resulting surfaces wi

48 ± 2◦ (Fig. 3c) as well as a SA of about 4◦ (Fig. 3d). Because theomparatively slow replacement deposition process, as a result ofmall concentration of AgNO3 (aq), results in the formation of thextremely few aggregated nanoparticles. However, significantly,hen the concentration of AgNO3 (aq) is moderate 2 mmol/L, theA reaches a maximum value of about 161 ± 2◦ (Fig. 3e) as well asSA of about 2◦ (Fig. 3f). The high water CA and low SA indicate

hat such obtained surface can be considered as a genuine super-ydrophobic surface undoubtedly. The low SA value, which reflectshe difference between the advancing and reducing angles, pro-ides further evidence for the superhydrophobicity of this surface.he wetting behavior of the resulting surface can be expressed byhe Cassie and Baxter equation [29], cos �r = f1 cos � − f2. Here f1 ishe fraction of the solid surface in contact with water, and f2 is theraction of air in contact with water; �r and � are the CAs on theough and smooth surface, respectively. Since f1 + f2 = 1, � = 64 ± 2◦,2 was calculated to be about 0.95. The high value of f2 suggestshat the resulting surface (the concentration of AgNO3 solution ismmol/L) is responsible for the better superhydrophobicity.

As above observation, we may reasonably arrive at the conclu-ion that the fabrication of the superhydrophobic surface is stronglyssociated with the reaction condition owing to the continuity ofhe preparation. In view of this, we studied the effects of other fac-ors, including the rinsing time with hydrochloric acid and reactionime, on the surface wettability by CA measurements. After rins-ng the zinc plate with hydrochloric acid, the oxidation film on theinc substrate can be removed completely. When the rinsing times 0.5 min and the concentration of AgNO3 (aq) is 2 mmol/L, the

ater CA reaches a maximum value of about 154 ± 2◦ (Fig. 4a).hen the rinsing time is 1 min, the water CA is about 161 ± 2◦

Fig. 4b). However, upon increasing the rinsing time to 5 min, theA reaches a maximum value of about 151 ± 2◦ (Fig. 4c). Further,hen the rinsing time is 10 min, the water CA just reaches a max-

mum value of about 142 ± 2◦ (Fig. 4d). From above, it is clear thathe rinsing time obviously influences the surface wettability of the

btained surfaces. The hydrochloric acid can not only remove thexidation film on the zinc substrate, but can increase the rough-ess of the substrate due to the effect of chemical etch, which isropitious to the formation of the rough structure on the final pre-ared surfaces for superhydrophobicity. With the increase of the

Fig. 5. SEM images of the zinc substrates after the rinsing w

rinsing time/min

Fig. 6. The contact angle as a function of the rinsing time.

rinsing time, the roughness of the substrate can be increased effec-tively. But, in contrast, the rough structure on the zinc substratewould be destroyed gradually owing to the chemical corrosion ifthe rinsing time is more than 1 min. Therefore, the CA values of theobtained surfaces present a high-to low trend. 1 min can be deter-mined as the optimum rinsing time in our experiment. The SEMimages of the zinc substrates with different rinsing times are givenin Fig. 5. The CA results of the prepared surfaces with different rins-ing times are given in Fig. 6. Besides, interestingly, the reaction timehas little influence on the hydrophobicity of the zinc surfaces. Fig. 7illustrates the CA values for different reaction times from 1 min to15 min (the concentration of AgNO3 solution is 2 mmol/L). It is obvi-ous that the change of the CA values along with the reaction timeis indistinctive, and not an apparent law of the change could be

confirmed. But we can make sure that the optimal reaction time is3 min from the figure.

The CA values of the zinc substrates after silver replacementdeposition process without modification of octadecyl mercaptan

ith hydrochloric acid: (a) 0 min; (b) 1 min; (c) 10 min.

W. Xu et al. / Applied Surface Science 257 (2011) 4801–4806 4805

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reaction time/min

Fig. 7. The contact angle as a function of the reaction time.

ave been studied. When the concentration of AgNO3 (aq) ismmol/L, reaction time is 3 min, the water CA can reaches a maxi-um value of about 87 ± 2◦ (Fig. 8a). As the water CA of the smooth

inc substrate is about 64 ± 2◦, and it just reaches a maximum valuef about 71 ± 2◦ after the immersion in octadecyl mercaptan with-ut silver replacement deposition process (Fig. 8b), it is obvious thatoth the structure (after the silver replacement deposition process)nd composition (after the modification of octadecyl mercaptan)re the functions on superhydrophobic surface.

.3. Sample structure

X-ray powder diffraction (XRD) patterns and energy dispersive-ray (EDX) spectroscopy were used to characterize the surfaceomposition of the prepared surfaces before the modification. Weave obtained several XRD patterns by changing the concentrationf AgNO3 (aq). When the concentration of AgNO3 (aq) is 2 mmol/Lnd the reaction time is 3 min, XRD demonstrates that the silvertructures are of high crystallinity (Fig. 9b). Since the substrates zinc plate, several strong peaks from 35◦ to 45◦ can be seenbviously from the XRD pattern, which can be regarded as the over-apping peaks attributed to zinc substrate and silver together. Theeaks at about 33◦ and 47◦ are ascribed to zinc oxide on the surfacef the zinc substrate. In addition, remarkably, the diffraction peakst about 59◦, 64.5◦ can be indexed to the (1 0 5), (1 1 0) planes ofg according to the JCPDS card, which can be used to testify the

ormation of crystal silver nanoparticles on the resulting zinc sur-ace reasonably. Fig. 9 also shows the different XRD patterns of therepared surfaces under other reaction conditions (different con-

entration of AgNO3 solution, the same reaction time). We can findhe same diffraction peaks at about 59◦, 64.5◦ which are ascribedo the (1 0 5), (1 1 0) planes of Ag. To further confirm the composi-ion of the obtained surface (the concentration of AgNO3 solutions 2 mmol/L), we conducted EDX experiment and detected Ag ele-

ig. 8. The shape of a water droplet on different surfaces: (a) after silver replace-ent deposition process without modification; (b) after immersion in octadecylercaptan without replacement deposition process.

Fig. 10. EDX spectra of the distributed elements for the prepared surface on zincsubstrate.

ment, which entirely confirmed the existence of nano Ag particleson zinc substrate (Fig. 10). The EDX spectrum of the distributed ele-ments reveals that the surface mainly consists of Ag, Zn, C and Oelements with a ratio of 4.27:33.56:47.82:14.35. Above EDX resultsupports the idea that the nano Ag particles have been successfullyintroduced to the metal surface through the simple replacementdeposition process. Additionally, combined with the SEM results,the typical micro-nanostructure along with the low surface energyleads to the surface superhydrophobicity.

3.4. Stability

The environmental stability and durability of the superhy-drophobic surface on zinc substrate have been investigated. Afterseveral months of storage in air, the value of the CA did not change,which indicated that the superhydrophobicity of the hierarchicalstructures is stable in air.

4. Conclusions

In summary, with the modification of octadecyl mercaptan, thestable super-hydrophobic surface on zinc substrate with the typicalmicro-nanostructure, consisting of aggregated silver nanoparti-cles, has been successfully fabricated using a simple and effective

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eplacement deposition process. It is based on the use of octadecylercaptan which can be seemed as the substitute for the com-on fluoride to a certain extent. And most important of all, in our

esearch, the fabrication of the superhydrophobic surface on zincubstrate is strongly associated with the reactant concentration.he great surface roughness and porous micro-nanostructure areust formed on zinc substrate only if the reactant concentrations moderate. The optimal resulting surface exhibits a special den-ritic outline structure, which is composed of countless tiny crystalarticles. The maximal CA value of about 161 ± 2◦ also could bebtained at the same reaction condition as well as the minimalA value of about 2◦, which represents the best superhydrophobicurface in our experiment. This method can also be applied to fab-icate other superhydrophobic metal surfaces. And it holds greatrospect for the application in practice due to its simple processnd environmental conservation.

cknowledgments

We gratefully thank the National Natural Science Foundation ofhina (No. 20773014 and 20933001) and the 111 Project of ChinaNo. B07012) for their support of this work.

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