Fabrication of octahedral gold nanostructures using an alcoholic ionic liquid

Article abstract of DOI:10.1246/cl.2008.106

Octahedral gold nanocrystals have been fabricated in an alcohol ionic liquid by a one-step method in the absence of any extra capping agent. The method is based on a reaction of HAuCl₄·H₂O in 1-(3-hydroxylpropyl)-3-methylimidazolium tetrafluoroborate (C ₃OHmimBF₄ by microwave heating. Upon adjusting HAuCl ₄·4H₂O concentration and reaction temperature, nanoplates and micrometer-sized particles have also been obtained. Copyright

Full text of DOI:10.1246/cl.2008.106

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Chemistry Letters Vol.37, No.1 (2008)
Fabrication of Octahedral Gold Nanostructures Using an Alcoholic Ionic Liquid
ꢀ
Lanzheng Ren, Lingjie Meng, and Qinghua Lu
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University,
00 Dongchuan Road, Shanghai 200240, P. R. China
8
(Received October 29, 2007; CL-071195; E-mail: qhlu@sjtu.edu.cn)
Octahedral gold nanocrystals have been fabricated in an
alcohol ionic liquid by a one-step method in the absence of
any extra capping agent. The method is based on a reaction of
.
HAuCl4 4H2O in 1-(3-hydroxylpropyl)-3-methylimidazolium
tetrafluoroborate (C3OHmimBF4) by microwave heating. Upon
.
adjusting HAuCl4 4H2O concentration and reaction tempera-
ture, nanoplates and micrometer-sized particles have also been
obtained.
Figure 1. SEM images of the octahedral gold nancrystals ob-
.
tained via 50 mg of HAuCl4 4H2O in 1 mL of C3OHmimBF4
at 300 C for 1 min. (a) Low magnification, and (b) high magni-
fication.
ꢁ
Metal nanomaterials, owing to their unique chemical and
physical properties such as catalytic, electronic, optical, and oth-
er properties, have attracted intensive study in extending their
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structures and applications in the past decade. Among these
metal nanomaterials, gold nanoparticles are of particular interest
built-in magnetic stirring, and the temperature of the reaction
mixture was accurately controlled by automatically adjusting
microwave power output with the aid of a thermocouple sensor.
The mixture was allowed to cool to room temperature, and
the products were separated by centrifugation and washed with
anhydrous ethanol and distilled water several times.
The typical scanning electron microscopy (SEM, JSM-
7401F, operating voltage of 5 kV) images of octahedral gold
nanocrystals are shown in Figure 1. It can be seen from
Figure 1a that the as-prepared products consist of octahedral
nanocrystals with an edge length of ca. 150–500 nm (ca. 61%),
coexisting with particles of other shapes (ca. 39%). Figure 1b
demonstrates the high-magnification image of the octahedral
particles, from which one can clearly resolve the exact shape.
It clearly demonstrates that each octahedron has eight equilateral
triangle faces.
To further characterize the octahedral gold nanocrystals,
transmission electron microscopy (TEM, JEOL-2010, operating
voltage of 200 kV) and energy-dispersive X-ray spectrography
(EDS, Oxford INCA) have also been used to illustrate the prod-
ucts. Figures 2a and 2b demonstrate the typical TEM image of a
single octahedral gold nanocrystal as well as its corresponding
geometrical model. The size of the octahedral nanocrystal is in
agreement with the results of the SEM characterization. The
EDS (Figure 2c) obtained from the octahedral gold nanocrystal
only shows the peaks corresponding to Au, thus demonstrating
that the octahedral gold nanocrystal consists only of metallic
Au and that there are no residues on its surface (the peaks of
Cu come from Cu grid).
The high crystalline nature of the gold nanocrystals was
also confirmed by the corresponding X-ray diffraction (XRD,
D/max-2200/PC, Rigaku) spectrum in Figure 3. The five sharp
peaks can be assigned to the {111}, {200}, {220}, {311}, and
{222} diffraction peaks of pure crystalline Au with face-cen-
tered cubic (fcc) structure. Note that the intensity ratio between
the {111} and other peaks is much higher than the conventional
bulk intensity ratio (JCPDS, file no. 04-0784). This observation
indicates that the surfaces of the particles are primarily dominat-
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because of their many potential applications. As we know, the
physical and chemical properties of nanostructures are closely
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related to their size and shape. Many papers describing the
controlled growth of gold nanostructures using a wide range of
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approaches have been published, mostly focusing on rods,
wires,4 and plates. However, there remain some shapes such
as octahedron that are difficult to obtain by existing methods.
It is interesting to note that only very recently, octahedral metal
nanocrystals have been reported. As surface capping agents or
templates, polymers are usually needed in the previous studies.
Ionic liquids (ILs) have received increasing attention in re-
cent years. They are liquid organic salts at ambient temperature
and are regarded as ‘‘green solvent’’ for their nonflammability,
high specific solvent abilities, etc. Recently, ILs have been used
as excellent media for the preparation and stabilization of tran-
sition-metal nanoparticles. It has been suggested that both
the electrostatic and coordination effects of ILs contribute to
nanoparticles stabilization.
Here, we describe the novel and facile fabrication of gold
nanostructures, octahedral gold nanocrystals in particular, by
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microwave irradiation of HAuCl4 4H2O in 1-(3-hydroxypro-
pyl)-3-methylimidazolium tetrafluoroborate (C3OHmimBF4).
Alcohol is one of the best reducing agents for preparation of met-
al nanomaterials.5 Thus, C3OHmimBF4 can serve as solvent,
reducing agent, and stabilizer in this procedure, and polymer sur-
facing capping agent is not required. In addition, ILs are also ex-
cellent microwave-absorbing agents owing to their high ionic
conductivity and polarizability. Thus may result a high heating
rate and a significantly shortened reaction time by introducing
,6
5
microwave heating. C3OHmimBF4 was synthesized according
to the literature.12 In a typical experiment for preparing octahe-
.
dral gold nanocrystals, HAuCl4 4H2O (50 mg) was dissolved in-
to 1 mL of C3OHmimBF4 in a 10-mL tube. After homogeniza-
tion of the mixture, the tube was put into the microwave reactor
(Apex, Shanghai EU Microwave Chemistry Technology Co.,
Ltd.) for a desired time. The system was equipped with a
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.1 (2008)
b)
107
ethylene glycol via the microwave heating of HAuCl4 without
5
(
the presence of polymer surfactant (PVP). Thus, it can be specu-
lated that ions of C3OHmimBF4 play important roles like the
‘‘capping agents’’ directing the crystal growth and consequently
determine its final shape. It has been well demonstrated that
the octahedra and plates mainly bounded with {111} facets
and that the surface free energy of cubic crystal is in the order
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of ꢀ
> ꢀ
> ꢀ
f111g
.
When the reduction performs at
f110g
f100g
ꢂ
high temperature, the AuCl4 is reduced by C3OHmimBF4 to
generate Au atoms at a sufficiently high rate, and the final prod-
0
ucts will take the thermodynamically favored shapes which have
the lowest surface free energy. Thus, the octahedra and plates
which mainly bounded with {111} facets are formed at high
temperature. While the reduction becomes substantially slow
at low temperature, the nucleation and growth will be turned into
kinetic control and the final product can take a range of shapes
deviated from the thermodynamic ones. Therefore, microme-
ter-sized particles with various shapes were obtained at low tem-
perature. On the basis of above results, it demonstrates that reac-
Figure 2. TEM (a) image of the octahedral gold nanocrystal
and (b) its cogeometrical structure; (c) EDS spectrum of the gold
nanocrystal.
.
tion temperature, HAuCl4 4H2O concentration and special prop-
erties of C3OHmimBF4 play important roles in the formation of
gold nanostructures. It is still unclear about the role of each re-
action parameter in detail, and the crystal evolution mechanism
needs further research.
In conclusion, octahedral gold nanostructures were fabricat-
ed in an alcoholic ionic liquid by microwave heating without any
.
extra surfactant or polymer. By adjusting HAuCl4 4H2O con-
centration and reaction temperature, nanoplates and micro-
mter-sized particles were obtained. This environmentally friend-
ly fabrication procedure is a rapid and one-pot reaction route.
The simple and facile procedure may be extended to the fabrica-
tion of other metal nanocrystals of different size.
Figure 3. X-ray diffraction pattern of the octahedral gold nano-
crystals.
This work was supported by the NSFC (No. 60577049) and
Shanghai Municipal Science and Technology Commission
(
Nos. 034319224 and 0652nm017).
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A common point in many different reports highlights the im-
portant role of polymers or surfactants as templates or capping
agents for achieving anisotropic growth. To our knowledge, only
spherical gold particles are obtained in the traditional solvent
Published on the web (Advance View) December 15, 2007; doi:10.1246/cl.2008.106