Synthesis of novel urchin-like architecture Au by self-assembly

coupled without template

J.J. Liang a,c, L.B. Yang a, Y.H. Shen a,b,*, A.J. Xie a,b

a School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, PR Chinab State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China

c Chuzhou Vocational Technology College, Chuzhou 239000, PR China

Received 11 October 2006; received in revised form 18 April 2007; accepted 4 June 2007

Available online 7 June 2007

Abstract

The formation of novel urchin-like architecture Au by hydrothermal treated method in tungstosilicate acid (H4SiW12O40, TSA)

solution is described. The particles are characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis

(XRD) and scanning electron microscopy (SEM), respectively. The results show that it is possible to synthesize and assemble other

noble metal particles with unique superstructure with possible applications in catalysis and other fields.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Metals; A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction

1. Introduction

In recent years, one of the most important goals of material scientists is to develop ways for tailoring the structure of

materials with unusual and novel forms [1]. The shape and the texture of inorganic materials are well-known to

determine their wide electrical and optical properties [2], which are important in various applications, such as catalysis

[3], solar cells [4], light-emitting diodes [5], biological labeling [6], and so on.

TSA ions form a subset of polyoxometalates of keggin structure. It is well-known that Keggin ions undergo

stepwise multielectron redox processes without structural change [7] and can be reduced electrolytically,

photochemically and chemically (with suitable reducing agents). They are a large category of metal oxygen

cluster anions with well-defined structures and properties and have diverse applications in the fields of analytical

chemistry, biochemistry and solid-state devices, and have been used as antiviral and antitumor reagents.

Their redox chemistry is characterized by their ability to accept and release a certain number of electrons, in

distinct steps, without decomposition [8]. Recently, Papaconstantinou and co-workers have shown that

photochemically reduced polyoxometalates of the Keggin structure phosphotungstic acid (PW12O40)3� lead to

the formation of the corresponding metal nanoparticles [9]. Sastry and co-workers have used (PW12O40)3� ions

for making phase-pure core-shell nanoparticles [10]. They have also used (PW12O40)3� ions as a template for

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Materials Research Bulletin 43 (2008) 1074–1078

* Corresponding author at: School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, PR China. Tel.: +86 551 5108090;

fax: +86 551 5107342.

E-mail address: s_yuhua@163.com (Y.H. Shen).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.06.004

the in situ growth of metal nanoparticles [11,12], star-shaped calcium carbonate crystals [13] and CdS

nanoparticles [14].

In this work, special morphologies of Au containing urchin-like architecture have been prepared by utilizing TSA

ion to synthesize Au particles. To our best knowledge, a hydrothermal treatment synthesis of this novel structure Au

has not been achieved previously in TSA solution.

2. Experimental

2.1. Materials and characterization

Tungstosilicate acid [H4(SiW12O40), TSA], chloroauric acid (HAuCl4) and isopropanol (CH3CH(OH)CH3) were

all A.R. grade and obtained from Shanghai reagent Co. All the reagents in the experiment were used as received.

Double distilled water was used throughout the experiment to prepare the solutions.

FTIR spectroscopy measurements were carried out on a Nexus 870 FTIR spectrophotometer with a resolution of

4 cm�1 (America Nicolet Co.). XRD analysis of drop-coated films of the Au solution on glass substrates was carried

out on a MAP18AHF instrument (Japan MAC Science Co.). SEM measurements were performed on a Leica

Stereoscan-440 scanning electron microscope.

2.2. Preparation of Au particles

In a typical experiment, 30 ml of 1mM aqueous deaerated solution of tungstosilicate acid was taken in a test tube

along with 30 ml of 1mM aqueous deaerated solution of HAuCl4 and 2 ml of propan-2-ol. The solution of about 20 ml

was transferred into a 25 ml capacity stainless Teflon-lined autoclave. The autoclave was sealed, maintained at 160 8Cfor 6 h, and then allowed to cool to room temperature naturally. The obtained precipitates were centrifuged, washed

several times using distilled water and absolute ethanol, and dried in a vacuum at 60 8C for 6 h.

3. Results and discussion

A representative Keggin-structure (SiW12O40)4� was chosen to study the reaction between [SiW12O40]5� and

AuCl4� ions. The (SiW12O40)5� ion was obtained by hydrothermal treatment of a deaerated isopropanol/

(SiW12O40)4� aqueous solution, in the presence of, for instance, isopropanol as a sacrificial reagent [15] (Eq. (1)):

2ðSiW12O40Þ4� þ ðCH3Þ2CHOH ! 2ðSiW12O40Þ5� þ ðCH3Þ2C¼O þ 2Hþ (1)

At the same time, the above solution gradually turned from colorless to yellow (Au) (Eq. (2)), as chloroauric acid

was reduced to gold metal:

ðSiW12O40Þ5� þAuCl4� ! ðSiW12O40Þ4� þAu þ 4Cl� (2)

Furthermore TSA ions as oxidizing agent or reducing agent can be cyclically utilized via Eqs. (1) and (2).

Fig. 1 represents the FTIR spectra in the region of 700–1050 cm�1 for the pure TSA (curve 1), the precipitates

obtained in the hydrothermal treated experiments of the mixture of Au with TSA and propan-2-ol solutions (curve 2).

Curve 1 shows the W–O asymmetric stretching vibration frequency at 979 cm�1, the W–O–W bending vibration at

879 cm�1, and the Si–O asymmetric stretching vibration at 1019 cm�1 for pure tungstosilicate acid. After reaction

(curve 2), we observe W–O–bond stretching vibrations at 980 cm�1, W–O–W bending vibrations at 878 cm�1, and

Si–O asymmetric stretching vibrations at 1019 cm�1 respectively. Comparing curve 2 with curve 1, it is seen that the

position of the W–O asymmetric stretching and the W–O–W bending vibration bands occur at almost the same

wavenumbers. This indicates that the underlying keggin structure is retained after hydrothermal treatment. So we can

conclude that the TSA serves as catalysts in this reaction process.

The XRD pattern recorded from a drop-coated film of the sample on a glass substrate is shown in Fig. 2. The (1 1 1),

(2 0 0), (2 2 0) and (3 1 1) Bragg reflections of face-centered cubic (fcc) gold are observed clearly. The sharp and

strong diffraction peaks indicate the high crystallinity of the particles.

J.J. Liang et al. / Materials Research Bulletin 43 (2008) 1074–1078 1075

The Brag reflections of TSA are not seen, because the TSA is soluble and the sample for XRD is washed using

distilled water and absolute ethanol. The results, obtained from XRD, indicate that the products of Au have a high

purity.

The structure and morphology of the particles was further characterized by SEM. Fig. 3A presents many compactly

combined spherical urchin-like Au architecture collected in the TSA solvent and its corresponding image in high

magnification (Fig. 3B) displays that the sphere is built up by many interleaving slightly bending flakes with the

thickness of 80–150 nm, which entangle together to behave like a network in the sphere.

The growth on the interface of the flake crystals in different directions will terminate until colliding each other and

yield entangled network morphology. In the process of the interleaving, these newborn thin flake interfaces are

strongly undulated, which will result in the slight bend of the flake to relax the strain generated in the flakes during the

growth [16]. Finally, the urchin-like morphological Au forms as the interleaving thin flakes aggregate into

microspheres due to minimizing the interfacial free energy to the lowest state.

To ensure that the TSA present in the system act as important effect in the synthesis of Au particle, a control

experiment was performed under the condition as the same only without TSA. A lot of small irregular particles and

large flakes are obtained (see SEM, Fig. 4). Based on the above analyses, we conclude that the TSA significantly affect

the shapes of the particles.

J.J. Liang et al. / Materials Research Bulletin 43 (2008) 1074–10781076

Fig. 1. FTIR spectra recorded from the drop coated film on Si(1 1 1) wafer of pure tungstosilicate acid solution (curve 1) and the precipitates of the

hydrothermal treated process (curve 2).

Fig. 2. XRD patterns recorded from drop-coated films on glass substrates of gold nanoparticles.

4. Conclusions

In general, urchin-like architecture and a remarkably complex hierarchy of gold microstructures have been readily

fabricated in TSA solution by self-assembly coupled without template. The urchin-like architecture gold could find

applications in various areas, including catalysis [17] and surface-enhanced Raman scattering detection [18]. The

process provides a general route for the synthesis of gold superstructures with unique morphologies and complex

hierarchies. Furthermore, the synthesis may be extended to other metal systems.

Acknowledgements

This work is supported by the National Science Foundation of China (20471001, 20371001), the Specific Project

for Talents of Science and Technology of Universities of Anhui Province (2005hbz03), the Key Laboratory of

J.J. Liang et al. / Materials Research Bulletin 43 (2008) 1074–1078 1077

Fig. 3. SEM images of the Au synthesized by hydrothermal treated method. (A) Low magnification; (B) high magnification.

Fig. 4. SEM image of the Au synthesized by hydrothermal treated method without TSA.

Environment-friendly Polymer Materials of Anhui Province, and Science of Foundation of Anhui Province

(2006KJ155B).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/

j.materresbull.2007.06.004.

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