Communications
DOI: 10.1002/anie.200803202
Nanostructures
Shape-Controlled Synthesis of Gold Nanoparticles in Deep Eutectic
Solvents for Studies of Structure–Functionality Relationships in
Electrocatalysis**
Hong-Gang Liao, Yan-Xia Jiang,* Zhi-You Zhou, Sheng-Pei Chen, and Shi-Gang Sun*
Gold-based catalysts have attracted intense interest in recent
years following the discovery[1] that small supported Au
nanoparticles (NPs) can be effective catalysts for CO
oxidation at low temperatures. Gold-based catalysts have
been widely used and are regarded as a new generation of
catalysts.[2] The properties of Au NPs depend strongly on their
size and shape, which determine the surface structure of the
particles. Fundamental studies of single-crystal planes of face-
centered-cubic (fcc) metals demonstrated that the high-index
planes possess a high density of low coordination number step
atoms in comparison with low-index planes such as {111},
{100}, and even {110}, and therefore exhibit high activity for
breaking chemical bonds.[3] Tian et al.[4] reported recently that
tetrahexahedral-shaped Pt NPs display high catalytic activity
and stability, as the tetrahexahedral Pt nanoparticle is
bounded by a {730} surface and other high-index facets.
Although the shape-controlled synthesis of Au NPs has been
extensively studied,[5] the procedures often involve surfac-
tants and Au seeds in solutions. Moreover, the shapes of the
synthesized Au NPs are mainly cubes, tetrahedra, octahedra,
and rhombic dodecahedra that are enclosed by low-index
facets. Several starlike Au NPs bounded with low-index (111)
facets have also been reported recently, but their shapes are
not uniform and perfect.[6,7] It is known from crystal growth
law that the growth rate along the direction perpendicular to
the high-index planes is usually much faster than that to the
low-index planes, so the high-index planes will be eliminated
gradually during the crystal growth. Therefore, it is always
challenging to develop shape-controlled syntheses of NPs
bounded with high-index facets.
namely, high conductivity, viscosity, surface tensions, polarity,
and thermal stability and negligible vapor pressure. Unlike
the ionic liquids, DESs can nevertheless be easily prepared at
low cost and with high purity. Some of the hydrogen-bond
donors are common bulk commodity chemicals such as urea
and oxalic acid, which are suitable for large-scale processes.
DESs form extended hydrogen-bond systems in the liquid
state and are therefore highly structured “supramolecular”
solvents. This special quality can be used to form well-defined,
ordered nanoscale structures. Owing to the high thermal
stability and low vapor pressure of DESs, reactions can be
conducted at temperatures well beyond 1008Cin nonpres-
surized vessels. Low interface energies for particles can be
translated into good stabilization. DESs are promising
solvents to be used in shape-controlled synthesis of nano-
particles. However, there are as yet very few reports of the use
of DESs in the synthesis of nanomaterials.[9]
Herein we report a novel route of shape-controlled
synthesis of gold NPs without the use of any surfactants or
seeds but with a DES as solvent. Star-shaped Au NPs that are
bounded with (331) and vicinal high-index facets were
successfully synthesized for the first time. The monodisperse
star-shaped gold NPs were obtained directly by the reduction
of HAuCl4 by l-ascorbic acid at room temperature in the
DES. Au NPs of various shape and surface structure including
snowflake-like NPs and nanothorns can be obtained simply
by adjusting the content of water in the DES. Furthermore,
the electrocatalytic properties of the synthesized Au NPs
were tested by using the electroreduction of H2O2 as a probe
reaction, and it has demonstrated that the star-shaped Au NPs
exhibited a much higher catalytic activity than other shaped
NPs and polycrystalline Au.
The as-prepared samples were examined using scanning
electron microscopy (SEM, LEO-1530). It was found that
over 40% of the synthesized Au NPs exhibited regular
pentagonal symmetry and displayed a striking beauty of the
star-shaped particle, as illustrated in the Figure 1. The size of
these Au NPs was about 300 nm. High-magnified pictures at
different viewing angles (Figure S2 in the Supporting Infor-
mation) illustrate that the star-shaped Au NPs are flat and
have a slightly thicker center (about 90 nm). Besides the well-
defined pentagonal star shape, other star-shaped Au NPs of
three, four, or multiple branches are also observed in Figure 1,
which may be considered as a deformation of the pentagonal
star shape during crystal growth. Energy-dispersive X-ray
spectroscopy (EDS) indicates that the NPs are composed of
only gold (see Figure S1a in the Supporting Information). We
have also traced the synthetic process by using UV/Vis
Deep eutectic solvents (DESs)[8] were first reported by
Abbott and co-workers. They found that substituted quater-
nary ammonium salts mixed with hydrogen-bond donors such
as amides can form liquids at ambient temperatures. These
liquids have properties similar to those of ionic liquids,
[*] Dr. H. G. Liao, Prof. Y. X. Jiang, Z. Y. Zhou, S. P. Chen, Prof. S. G. Sun
State Key Laboratory of Physical Chemistry of Solid Surfaces,
Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University
Xiamen 361005 (China)
Fax: (+86)592-2180181
E-mail: yxjiang@xmu.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China (Grant Nos. 20433040, 20573085, and
20673091) and the “973” Program (No. 2009CB220102).
Supporting information for this article is available on the WWW
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 9100 –9103