Facile electrochemical approach to fabricate hierarchical flowerlike gold microstructures:...

Available online at www.sciencedirect.com

www.elsevier.com/locate/elecom

Electrochemistry Communications 10 (2008) 95–99

Facile electrochemical approach to fabricate hierarchical flowerlikegold microstructures: Electrodeposited superhydrophobic surface

Liang Wang, Shaojun Guo, Xiaoge Hu, Shaojun Dong *

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

Changchun, Jilin 130022, China

Received 8 October 2007; received in revised form 31 October 2007; accepted 1 November 2007Available online 6 November 2007

Abstract

A templateless, surfactantless, electrochemical approach is proposed to directly fabricate hierarchical flowerlike gold microstructures(HFGMs) on an indium tin oxide (ITO) substrate. The as-prepared HFGMs have been characterized by scanning electron microscopy(SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and cyclic voltammetry. The HFGMs prepared by simplesquare wave voltammetry technique exhibit flowerlike microstructures and are built with many staggered nanosheets as building blocks.The diameter and density of the HFGMs can be easily controlled via simply controlling the repetitive run times of square wave voltam-metry. Moreover, after further chemisorption of a self-assembled monolayer of n-dodecanethiol, the as-prepared compact surfacebecomes superhydrophobic with a contact angle as high as 154�.� 2007 Elsevier B.V. All rights reserved.

Keywords: Electrodeposition; Square wave voltammetry; Hierarchical flowerlike gold microstructures; Superhydrophobic surface

1. Introduction

Hierarchical micro/nanostructures assemblies usingnanoparticles, nanorods, nanobelts as building blocksand complex nanocrystals with well-defined shape andinner structure are of great significance for the realizationof nanodevices and have been obtained through differentways, such as colloidal chemistry method and electrochem-ical approach, etc. [1–11]. Some semiconductor materialswith flowerlike nanoarchitectures (FNs) have been pre-pared by colloidal chemistry strategy [12–16]. However,fabrication of metallic FNs is still a challenge. Especially,FNs growing directly on a substrate, which is difficult forcolloidal chemistry strategy, is of importance for some spe-cial applications, such as surface-enhanced Raman scatter-ing (SERS) active substrate, superhydrophobicity, and thenanodevices realization [17]. So, an additional procedure is

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.11.002

* Corresponding author. Tel.: +86 431 85262101; fax: +86 431 85689711.E-mail address: dongsj@ciac.jl.cn (S. Dong).

required for colloidal chemistry strategy to immobilizenanocatalysts to a solid substrate to form two-dimensional(2D) or 3D nanostructures [18].

Electrochemical approach is a good alternative strategyfor one-step fabrication of FNs grown on a substratedirectly. Several electrochemical methods are employed toprepare complex hierarchical gold micro/nanostructures[19]; most of which are synthesized in the presence oforganic additives or surfactants [12], and others are basedon template [20]. However, the use of organic additivesor surfactants may introduce heterogeneous impurities,and the use of templates may complicate the synthetic pro-cedure and limit the synthesis of micro/nanostructuredmaterials in large quantities [21,22].

Herein a very simple and promising electrochemicalapproach is proposed to directly fabricate hierarchical flow-erlike gold microstructures (HFGMs) on the surface ofindium tin oxide (ITO) substrate. Recent achievement inpreparation of tetrahexahedral platinum nanocrystals andplatinum nanothorn by square wave voltammetry electrode-position technique can throw light on the preparation of

96 L. Wang et al. / Electrochemistry Communications 10 (2008) 95–99

noble metals FNs [23,24]. Similar tactics can now beharnessed to prepare HFGMs. When compared with otherelectrochemical method for fabricating gold microstruc-tures, the advantages of the proposed approach are obvious.For example, when compared with electrochemical growthof gold microstructures prepared at a constant current den-sity on a thin polypyrrole film modified ITO [19] or on thesurface of a vanadium doped TiO2/Ti electrode [10], thisproposed approach is on a ‘‘clean’’ ITO surface, indicatingmore simple and facilely controlled; when compared withpotentiostatic mode for preparation gold microstructures[25], this proposed approach can obtain very compactHFGMs clusters within 1 h (prepared by 144 repetitivelyrun times), indicating more rapid and low-cost. The as-pre-pared HFGMs own unique local morphology with potentialapplication, such as matrices for SERS, superhydrophobic-ity, and nanodevices, etc. As an initial application of thissurface coated with the HFGMs aggregate, the wettingproperty has been investigated after further simple surfacemodification with n-dodecanethiol.

Fig. 1. Typical SEM images of the HFGMs located at ITO substrate atlow (A) and higher (B) magnification, respectively, prepared by squarewave voltammetry for 72 times of repetitive run. EL = �0.2 V, EU

= 0.8 V; the concentration of HAuCl4 is 24.3 mM.

2. Experimental section

HAuCl4 and n-dodecanethiol were purchased from Bei-jing Chemical Factory (Beijing, China). ITO was pur-chased from Shenzhen Hivac Vacuum Photo-electronicsCo., Ltd. (Shenzhen, China).

For a typical electrochemical fabrication of HFGMs,24.3 mM of HAuCl4 aqueous solution was used as the sourceof Au and also as electrolyte. ITO was used as working elec-trode. Before used, ITO was cleaned by sonicating sequen-tially for 10 min each in acetone, 10% NaOH in ethanoland distilled water. The clean platinum wire and Ag/AgCl(sat. KCl) electrodes were used as counter and reference elec-trode, respectively. Square wave voltammetry technique wasemployed to electrochemical deposition HFGMs with thepotential between 0.8 and �0.2 V at 10 Hz for 72 run times.

For a typical preparation of the HFGMs to superhydro-phobic surface, the ITO electrodes coated with compactHFGMs (prepared by 144 run times) were immersed intothe ethanol solution of 1 mM n-dodecanethiol overnight,then taken out and washed repeatedly with ethanol, andfinally dried in air.

30 60 90

222

311220

200

111

Inte

nsi

ty /

a.u

2θ / degree

84 88

0

10000

20000 Au4f7/2

Inte

nsi

ty /

a.u

Bonding Energy / ev

Au4f5/2

A

B

Fig. 2. XPS (A) and XRD (B) pattern of the HFGMs sample located atITO substrate (corresponding to Fig. 1).

L. Wang et al. / Electrochemistry Communications 10 (2008) 95–99 97

Electrochemical experiments were performed with aCHI 832 electrochemical analyzer (CH Instruments, Chen-hua Co., Shanghai, China). The morphology of theHFGMs was characterized with a XL30 ESEM FEG scan-ning electron microscopy (SEM). The X-ray diffraction(XRD) analysis of the resulting product was carried outon a D/MAX 2500 V/PC X-ray diffractometer. Analysisof the X-ray photoelectron spectra (XPS) was performedon an ESCLAB MKII. Contact angles were measured ona drop shape analysis system G10/DSA10 contact anglesystem.

3. Results and discussion

Fig. 1 is the typical SEM images of the as-preparedHFGMs sample fabricated directly on ITO substrate at dif-ferent magnifications. Low-magnification image (Fig. 1A)indicates that the as-prepared product consists of a largequantity of well dispersed microstructures with the diame-ter about 8.3 lm. Higher-magnification images (Fig. 1B)

Fig. 3. Typical SEM images of the HFGMs located at ITO substrate preparEL = �0.2 V, EU = 0.8 V; the concentration of HAuCl4 is 24.3 mM. (A) n = 12(B), respectively.

demonstrate that these microstructures exhibit flowerlikestructures and are built with many staggered nanoflakesas building blocks.

The oxidation state of gold in the HFGMs was deter-mined by XPS as shown in Fig. 2A. The XPS spectrumof the as-prepared HFGMs shows the Au 4f7/2 and 4f5/2

doublet with the binding energies of 84.3 and 87.9 eV,respectively. These are typical values for Au0 [26]. Theunique HFGMs were further characterized to determinetheir crystal direction. Fig. 2B shows the XRD patternobtained. The peaks located at 38.2�, 44.3�, 64.7�, 77.8�,and 81.8� are assigned to {111}, {200}, {220}, {311}and {222} faces of HFGMs, respectively. The intensityratio (3.5) of the {111} to the {200} diffraction line ishigher than that 1.9 of the standard diffraction of goldpowders, indicating that the deposited gold structure hasthe tendency to grow with the surfaces dominated by thelowest energy {111} facets [27].

In order to investigation the growth process of theHFGMs on the ITO surface, the morphologies of

ed by square wave voltammetry for different times (n) of repetitive run;; (B) n = 144. Panels (C) and (D) are the magnified views of panels (A) and

Fig. 4. Shape of a water droplet on the surface of HFGMs (correspondingto Fig. 3B) modified with n-dodecanethiol (drop weight 5 mg).

98 L. Wang et al. / Electrochemistry Communications 10 (2008) 95–99

HFGMs obtained from differently repetitive run timeswere characterized by SEM. Fig. 3 shows the SEM imagesof HFGMs obtained from differently repetitive run times,12 times and 144 times for Fig. 3A and B, respectively. Asobserved by SEM, in the initial stage (Fig. 3A and C), theHFGMs consisting of staggered nanoflakes were gener-ated and located sparsely on the ITO surface with thediameter about 4.5 lm. With the increase of repetitiverun times (prepared by 144 repetitively run times), theITO surface was uniformly, compactly and stably coatedby the microstructures within 1 h (Fig. 3B and D). Thesize of HFGMs further increased and the average sizewas about 10 lm. Thus, the size and density of theHFGMs can be easily controlled via simply changingthe repetitive run times.

Besides the repetitive run times, the potential range ofsquare wave voltammetry might be a factor for controllingof the morphology of HFGMs aggregates. To study themorphology of the HFGMs aggregates under differentpotential ranges, the changes both the lower (EL) andupper (EU) potential limits of the square wave voltammetryare studied while keeping the square wave voltammetryrepetitive run times constant as 72. It was found that themorphology and diameter only slightly changed with thedecrease of EL or with the increase of EU while lower EL

produced higher density HFGMs due to the lower poten-tial is more suitable for reduction (data not shown). Inaddition, the low concentration of HAuCl4 (1 mM) wasalso investigated by square wave voltammetry techniquefor 72 times of repetitive run with the potential range from0.8 to �0.2 V. However, no HFGMs were observed andonly some irregular particles and particle aggregates wereobtained (data not shown). So the concentration ofHAuCl4 plays the key role in the electrochemical fabrica-tion of HFGMs.

As an initial application of the HFGMs, it would beinteresting to explore if the as-prepared hierarchical roughstructures could be used for fabricating superhydrophobicsurfaces. Then the investigation of the wetting propertyof the ITO surface coated compactly and uniformly withthe hierarchical microstructures (corresponding toFig. 3B) was done by contact angle measurement after itwas modified with n-dodecanethiol. As shown in Fig. 4,the contact angle of the surface, using water droplet asan indicator (drop weight 5 mg), is 154�, suggesting thatthe as-proposed method can lead to the formation of asuperhydrophobic surfaces. For the smooth gold surfacemodified with n-dodecanethiol, the contact angle is onlyabout 95� suggesting that the surface morphology is neces-sary for superhydrophobicity [28]. To thoroughly under-stand the superhydrophobicity of the surface of theHFGMs modified with n-dodecanethiol, the contact anglewas described in terms of the Cassie equation: coshr = f1-

cosh � f2, where hr (154�) and h (95�) are the contact angleson the self-assembled monolayer of n-dodecanethiol on arough surface with HFGMs aggregates and on a smoothgold surface, respectively; f1 and f2 are the fractional inter-

facial areas of the HFGMs aggregates and of the air in theinterspaces among the HFGMs aggregates, respectively(i.e. f1 + f2 = 1). It is easy to deduce from this equation thatincreasing the fraction of air (f2) will increase the contactangle of the rough surface (hr). According to the Cassieequation, the f2 value of the rough surface with HFGMsclusters is estimated to be 0.89. Therefore it can be realizedthat the fraction of air in the surface is important to thesuperhydrophobicity. In other words, the surface morphol-ogy plays a very important role in attaining the superhy-drophobic surface [29].

4. Conclusions

Overall, the present results demonstrate a very simple,templateless, surfactantless, electrochemical approach todirectly fabricate of HFGMs on ITO substrate. After fur-ther simple surface modification, this surface coated withthe HFGMs aggregate has showed remarkable superhy-drophobicity with a contact angle as high as 154�. Themethod can be readily extended to fabricate other inter-esting nanostructures of noble metals, transition metalsor metal alloys for generating a myriad of interestingmaterials, used in, for example, matrices for SERS, sur-face catalysis and nanodevices. It is believed that thiswork offers new electrochemical strategy and promisingapplications for the fabrication of micro/nanostructuredsurfaces.

Acknowledgements

This research was supported by National Natural Sci-ence Foundation of China (Nos. 20575064 and20675076). We are grateful to Mrs. Lili Li for her kind helpwith the contact angle experiments.

L. Wang et al. / Electrochemistry Communications 10 (2008) 95–99 99

References

[1] L. Lu, R. Capek, A. Kornowski, N. Gaponik, A. Eychmuller, Angew.Chem. Int. Ed. 44 (2005) 5997.

[2] L. Lu, I. Randjelovic, R. Capek, N. Gaponik, J. Yang, H. Zhang, A.Eychmuller, Chem. Mater. 17 (2005) 5731.

[3] T.D. Ewers, A.K. Sra, B.C. Norris, R.E. Cable, C.H. Cheng, D.F.Shantz, R.E. Schaak, Chem. Mater. 17 (2005) 514.

[4] H.G. Yang, H.C. Zeng, Angew. Chem., Int. Ed. 43 (2004) 5930.[5] X.J. Yang, Y. Makita, Z.H. Liu, K. Sakane, K. Ooi, Chem. Mater. 16

(2004) 5581.[6] X.Y. Chen, X. Wang, Z.H. Wang, X.G. Yang, Y.T. Qian, Cryst.

Growth Des. 5 (2005) 347.[7] H.T. Shi, L.M. Qi, J.M. Ma, H.M. Cheng, J. Am. Chem. Soc. 125

(2003) 3450.[8] J. Zhang, L.D. Sun, J.L. Yin, H.L. Su, C.S. Liao, C.H. Yan, Chem.

Mater. 14 (2003) 4172.[9] Y. Yamauchi, K. Kuroda, Electrochem. Commun. 8 (2006) 1677.

[10] J. Haber, P. Nowak, P. Zurek, Langmuir 19 (2003) 196.[11] Y. Li, G. Lu, X. Wu, G. Shi, J. Phys. Chem. B 110 (2006) 24585.[12] J. Yuan, W. Li, S. Gomez, S.L. Suib, J. Am. Chem. Soc. 127 (2005)

14184.[13] M. Mo, J.C. Yu, L.Z. Zhang, S.K. Li, Adv. Mater. 17 (2005) 756.

[14] Z. Zhang, X. Shao, H. Yu, Y. Wang, M. Han, Chem. Mater. 17(2005) 332.

[15] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124.[16] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 16744.[17] G.T. Duan, W.P. Cai, Y.Y. Luo, Z.G. Li, Y. Li, Appl. Phys. Lett. 89

(2006) 211905.[18] J. Schulz, A. Roucoux, H. Patin, Chem. Rev. 102 (2002) 3757.[19] Y. Li, G. Shi, J. Phys. Chem. B 109 (2005) 23787.[20] G.T. Duan, W.P. Cai, Y.Y. Luo, Y. Li, Y. Lei, Appl. Phys. Lett. 89

(2006) 181918.[21] Y. Sun, Y. Xia, Adv. Mater. 14 (2002) 833.[22] S. Huang, H. Ma, X. Zhang, F. Yong, X. Feng, W. Pan, X. Wang, Y.

Wang, S. Chen, J. Phys. Chem. B 109 (2005) 19823.[23] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Science 316

(2007) 732.[24] N. Tian, Z.Y. Zhou, S.G. Sun, L. Cui, B. Ren, Z.Q. Tian, Chem.

Commun. (2006) 4090.[25] S.J. Guo, L. Wang, E.K. Wang, Chem. Commun. (2007) 3163.[26] W.L. Chen, S.J. Dong, E.K. Wang, Langmuir 19 (2003) 9434.[27] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176.[28] X. Zhang, F. Shi, X. Yu, H. Liu, Y. Fu, Z. Wang, L. Jiang, X. Li, J.

Am. Chem. Soc. 126 (2004) 3064.[29] N. Zhao, F. Shi, Z. Wang, X. Zhang, Langmuir 21 (2005) 4713.