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Electrochemistry Communications 10 (2008) 655–658

Facile electrochemical route to directly fabricate hierarchical sphericalcupreous microstructures: Toward superhydrophobic surface

Liang Wang, Shaojun Guo, Shaojun Dong *

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun, Jilin, 130022, China

Received 11 January 2008; received in revised form 25 January 2008; accepted 28 January 2008Available online 5 February 2008

Abstract

A templateless, surfactantless, electrochemical route is proposed to directly fabricate hierarchical spherical cupreous microstructures(HSCMs) on an indium tin oxide (ITO) substrate. The as-prepared HSCMs have been characterized by scanning electron microscopy(SEM), energy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The HSCMsprepared by simple potentiostatic technique exhibit hierarchical spherical microstructures with higher roughness. After further chemi-sorption of a self-assembled monolayer of n-dodecanethiol, the as-prepared compact surface becomes superhydrophobic with a contactangle as high as 152�.� 2008 Elsevier B.V. All rights reserved.

Keywords: Electrodeposition; Hierarchical spherical cupreous microstructures; Superhydrophobic surface

1. Introduction

Currently, superhydrophobic surfaces with a water con-tact angle (CA) higher than 150� have generated a frenzy ofexcitement due to their elegant properties, such as antis-ticking, anti-contamination, and self-cleaning [1–10]. Arti-ficial superhydrophobic surfaces are fabricated bycombining rough surface morphology and low-surface-energy coatings. The superhydrophobic surfaces are usu-ally covered with hierarchical micro/nano-structures [1,2].Benefiting from the advancing progress in hierarchicalmicro- and nanofabrication techniques, diverse methodsfor fabrication rough surfaces have been achieved, includ-ing colloidal chemistry method [11–14] and electrochemicalapproach [15–19], etc.

Hierarchical micro/nanostructures assemblies usingmicro/nanoparticles, nanorods, nanobelts as buildingblocks with well-defined shape and inner structure can be

1388-2481/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2008.01.034

* Corresponding author. Tel.: + 86 431 85262101; fax: +86 43185689711.

E-mail address: dongsj@ciac.jl.cn (S. Dong).

obtained through colloidal chemistry strategy [20,21]. It isof importance to directly assemble on substrate for super-hydrophobic application, which is difficult for colloidalchemistry strategy. Electrochemical deposition methodsare widely used to form rough structures [22,23]. Althoughelectrodeposition is a general route to fabricate hierarchicalmicro/nanostructures, most of which are synthesized in thepresence of organic additives or surfactants [17], and othersare based on template [16]. However, the use of organicadditives or surfactants may introduce heterogeneousimpurities, and the use of templates may complicate thesynthetic procedure and limit the synthesis of micro/nano-structured materials in large quantities. In order to realizepractical applications, the fabrication methods of superhy-drophobic coatings should be as cheap and facile aspossible.

Herein a very simple and promising electrochemicalroute is proposed to directly fabricate hierarchical sphericalcupreous microstructures (HSCMs) on a ‘‘clean” surface ofindium tin oxide (ITO) substrate. The as-prepared HSCMsown unique local morphology with many potential applica-tions. As an initial application of this surface coated with

Fig. 1. Typical SEM images of the HSCMs located at ITO substrate atlow (A) and higher (B) magnification, respectively, prepared by potentio-static technique for 30 min. The deposition potential is �0.25 V and theconcentration of Cu(NO3)2 is 50 mM.

656 L. Wang et al. / Electrochemistry Communications 10 (2008) 655–658

the HSCMs aggregates, the wetting property has beeninvestigated after further simple surface modification withn-dodecanethiol.

2. Experimental section

Cu(NO3)2 and n-dodecanethiol were purchased fromBeijing Chemical Factory (Beijing, China). ITO was pur-chased from Shenzhen Hivac Vacuum Photo-electronicsCo. Ltd (Shenzhen, China).

For a typical electrochemical fabrication of HSCMs,50 mM of Cu(NO3)2 aqueous solution was used as thesource of Cu and also as electrolyte. ITO was used as work-ing electrode. Before used, ITO was cleaned by sonicatingsequentially for 20 min each in 10% NaOH, acetone anddistilled water. The clean platinum wire and Ag/AgCl(sat. KCl) electrodes were used as counter and referenceelectrode, respectively. Potentiostatic technique wasemployed to electrochemical deposition HSCMs with thepotential at �0.25 V for 30 min.

For a typical preparation of the HSCMs to superhydro-phobic surface, the ITO electrodes coated with compactHSCMs aggregates were immersed into the ethanol solu-tion of 1 mM n-dodecanethiol overnight, then taken outand washed repeatedly with ethanol, and finally dried inair.

Electrochemical experiments were performed with aCHI 832 electrochemical analyzer (CH Instruments, Chen-hua Co. Shanghai, China). The morphology and theenergy-dispersive X-ray (EDX) analysis of the HSCMswere characterized with a XL30 ESEM FEG scanning elec-tron microscopy (SEM). The X-ray diffraction (XRD)analysis of the resulting product was carried out on a D/MAX 2500 V/PC X-ray diffractmeter. Analysis of the X-ray photoelectron spectra (XPS) was performed on anESCLAB MKII. Contact angles were measured on a DropShape Analysis System G10/DSA10 contact angle system.

3. Results and discussion

Fig. 1 is the typical SEM images of the as-preparedHSCMs sample fabricated directly on ITO substrate at dif-ferent magnifications. Low-magnification image (Fig. 1A)indicates that the ITO surface was uniformly, compactlyand stably coated by rough microstructures. Higher-mag-nification image (Fig. 1B) demonstrates that these micro-structures exhibit spherical structures with accidentedsurface and the diameter is about 7.2 lm.

The oxidation state of copper in the HSCMs was deter-mined by XPS as shown in Fig. 2A. The XPS spectrum ofthe as-prepared HSCMs shows the Cu2p doublet with thebinding energies of 932.3 and 952.2 eV, respectively. Theseare typical values for Cu0. The unique HSCMs were furthercharacterized to determine their crystal direction. Fig. 2Bshows the XRD pattern obtained. The peaks located at43.0�, 50.3�, 74.1� and 90.1� are assigned to {111},{20 0}, {22 0} and {31 1} faces of the HSCMs, respectively.

The intensity ratio (2.5) of the {111} to the {200} diffrac-tion line indicates that the deposited cupreous microstruc-ture has the tendency to grow with the surfaces dominatedby the lowest energy {111} facets. No impurities could bedetected in this pattern, which implies HSCMs could beobtained without impurities under the current electrochem-ical route. The EDX analysis in a selected domain of the as-prepared sample furthermore testified the existing of theCu, in which the peak of the corresponding elemental Cuwas distinct and no impurities could be detected; elementmass analysis shows the content of Cu is 100% (Fig. 2C).

In order to investigation the growth process of theHSCMs on the ITO surface, the morphologies of as-pre-pared samples obtained from different conditions werecharacterized by SEM. Fig. 3 shows the typical SEMimages of as-prepared samples located at ITO substrateprepared by potentiostatic technique at �0.25 V for differ-ent conditions: 50 mM Cu(NO3)2, 5 min (Fig. 3A and D);50 mM Cu(NO3)2, 60 min (Fig. 3B and E); 5 mMCu(NO3)2, 30 min (Fig. 3C and F). As observed by SEM,in the initial stage (Fig. 3A and D), the as-prepared prod-uct consisting of microspheres (with the diameter about 6.6lm) and irregular particles were generated and locatedsparsely on the ITO surface. With the increase of electrode-position time, the ITO surface was compactly coated by

930 945 960 975

Cu2p

952.2 eV

932.3 eV

Inte

nsi

ty /

a.u

Binding energy / eV

A

45 60 75 90

B

Cu(311)Cu(220)

Cu(200)

Cu(111)

Inte

nsi

ty /

a.u

2θ / degree

Element Mass (%) Cu 100

C

Fig. 2. XPS (A) and XRD (B) pattern of the HSCMs, respectively. Panels (C) shows the EDX analysis in a selected domain of the sample.

Fig. 3. Typical SEM images of as-prepared samples located at ITO substrate prepared by potentiostatic technique at �0.25 V for different condition: (A)50 mM Cu(NO3)2, 5 min; (B) 50 mM Cu(NO3)2, 60 min; (C) 5 mM Cu(NO3)2, 30 min. Panels (D), (E) and (F) are the magnified views of panels (A), (B)and (C), respectively.

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nonuniform microstructures (Fig. 3B and E). Thus, theelectrodeposition time is one key factor to fabricateHSCMs surface. In addition, the low concentration ofCu(NO3)2 (5 mM) was also investigated by potentiostatictechnique at �0.25 V for 30 min. However, no HSCMswere observed and only some regular cubic particles andnear-spherical particles (with the diameter about 2.9 lm)were located sparsely on the ITO surface (Fig. 3C andF). So the concentration of Cu(NO3)2 plays another keyrole in the electrochemical fabrication of HSCMs. Theeffect of deposition potential on the structure and size of

the HSCMs has also been investigated. SEM testified theHSCMs can be obtained only at the potential of �0.25 Vfor 30 min electrodeposition; when other deposition poten-tial is used, only compact particle aggregates can beobtained.

As an initial application of the HSCMs, it would beinteresting to explore if the as-prepared hierarchical roughmicrostructures could be used for fabricating superhydro-phobic surface. Then the investigation of the wetting prop-erty of the ITO surface coated compactly and uniformlywith the hierarchical microstructures (corresponding to

Fig. 4. Shape of a water droplet on the HSCMs surface (corresponding toFig. 1A) modified with n-dodecanethiol (drop weight 5 mg).

658 L. Wang et al. / Electrochemistry Communications 10 (2008) 655–658

Fig. 1) was done by contact angle measurement after it wasmodified with n-dodecanethiol. As shown in Fig. 4, thecontact angle of the surface, using water droplet as an indi-cator (drop weight 5 mg), is 152�, suggesting that theas-proposed method can lead to the formation of a super-hydrophobic surfaces. This result shows the rough micro-structures of the as-prepared thin film surface wereresponsible for the superhydrophobic properties and aircan be trapped in the hierarchical rough microstructureson the films surface. In other words, the surface morphol-ogy plays a very important role in attaining the superhy-drophobic surface.

4. Conclusions

In conclusion, a cost-effective, templateless and surfac-tantless electrochemical route to directly fabricate HSCMson ITO substrate is proposed. After further simple surfacemodification, this surface coated with the HSCMs aggre-gates has showed remarkable superhydrophobicity with acontact angle as high as 152�. The advantages of theapproach to fabricate superhydrophobic surface are obvi-ous. First, this approach is very simple and fast. The‘‘one-step fabrication” conforms to this trend to makethe fabrication process easy and widely feasible. Second,it is grateful to design superhydrophobic surface for prac-tical application since it is possible to save precious metal,e.g. Au and Ag. Furthermore, by taking the advantage ofthe fact that the electrochemical deposition is independentof the shape and size of the substrate, applications aimed at

different mimicking nature, such as the legs of water strid-ers, can be obtained [1].

Acknowledgements

This research was supported by National Natural Sci-ence Foundation of China (Nos. 20575064 and20675076). We are grateful to Mr. Longjian Xue for hiskind help with the contact angle experiments.

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