Ultrathin gold nanosheets formed by photoreduction at the ionic liquid/water interface

Article abstract of DOI:10.1246/cl.2005.1234

Ultrathin, single-crystalline gold nanosheets with thickness of ca. 10 nm are successfully obtained in water by photoreduction of Au(OH)₄ ^- ions at the surface of ionic liquid (IL) μ-droplets. The nanosheets produced at the interface

Full text of DOI:10.1246/cl.2005.1234

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234
Chemistry Letters Vol.34, No.9 (2005)
Ultrathin Gold Nanosheets Formed by Photoreduction at the Ionic Liquid/Water Interface
Ã
Tetsuro Soejima and Nobuo Kimizuka
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received May 16, 2005; CL-050644)
Ultrathin, single-crystalline gold nanosheets with thickness
of ca. 10 nm are successfully obtained in water by photoreduc-
tion of Au(OH)4 ions at the surface of ionic liquid (IL) ꢀ-drop-
a
b
À
lets. The nanosheets produced at the interface are spontaneously
extracted into the ionic liquid phase, which effectively prevents
the formation of thicker nanoplates.
Nanosized golds are attracting considerable interest in nano-
technology, because of their unique optoelectronic properties
3
0 nm
1
which vary depending on the size and shapes. Extensive studies
2
have been devoted to the synthesis of nanoparticles and one-di-
3
Figure 1. (a) SEM and (b) TEM images of gold nanosheets
obtained by illuminating the macroscopic interface formed
between [C4mim]PF6 and aqueous Au(OH)4 . The inset in
mensional (1D) nanorods. The anisotropic growth of the latter is
3
achieved by employing rod-like micelles as templates, which
allowed control on their aspect ratio. In addition to these classi-
cal nanostrucures, two-dimensional (2D) nanoplates are recently
À
(
b) shows a corresponding ED pattern.
4
,5
emerging as a new family of gold nanomaterials. Thermal re-
duction of HAuCl4 in aqueous block copolymers gave gold
in vacuo. Figure 1a shows scanning electron micrographs
(SEM). Nanosheets of gold with dimensions above 700 nm and
thickness of ca. 30 nm are observed. The nanosheet structures
were also confirmed by transmission electron microscopy
(TEM, Figure 1b). They are characterized by a uniform contrast,
and electron diffraction (ED) of a nanosheet revealed hexagonal
spots (Figure 1b inset, detailed analysis shown in Figure S2).
These observations indicate that the gold nanosheets are face-
centered cubic (fcc) single crystals with an atomically flat
(111) surface.
4
nanoplates with the thickness of ca. 100 nm. Au nanoplates with
the thickness of ca. 50 nm are formed in water by using o-phe-
5
nylenediamine as reductant. However, these gold nanoplates
are fairly thick and there exist no general strategies to synthesize
ultrathin gold nanosheets with the thickness regulated at 10 nm
level.
In this study, we report the formation of ultrathin gold nano-
sheets by photoreduction of Au(III) ions at the interface of ionic
liquids (ILs) and water. Room-temperature ILs are receiving
much attention as environmentally benign solvents for organic
and inorganic synthesis, polymerization, extraction, electro-
The presence of water/ionic liquid interface is indispensable
for the formation of gold nanosheets. When aqueous HAuCl so-
lution (24.3 mM; pH, 10) is illuminated with UV light, reduction
of Au(III) ions did not occur. At the water/IL interface,
4
6
7
chemical applications, and for molecular self-assembly. We
have recently proposed the use of ILs for controlled inorganic
synthesis, and developed a one-step route to hollow metal oxide
À
Au(OH)4 ions would be electrostatically adsorbed on the cat-
À
ionic [C mim]PF phase. These Au(OH) ions are not extract-
6
4
4
8
microspheres. In this metal oxide system, organic microdroplets
ed in IL phase even after vigorous mixing, as confirmed by atom-
ic absorption analysis (Shimadzu AA-6700). On the other hand,
IL molecules excited by UV light may act as sacrificial electron
donors to reduce Au(OH)4 ions adsorbed at the interface.
Though the photoreduction occurs selectively at the macroscop-
ic water/[C mim]PF interface, it is suffered from low efficien-
cy arising from the limited contact between two layers. In addi-
tion, the thickness of gold nanosheets obtained under the static
two-layer condition (ca. 30 nm) still falls short of the intended
size-control. It is desirable that the crystal growth along the c
axis is regulated more finely.
To increase the reaction area, we have employed dynamic
mixing conditions and prepared microdroplets of ILs in water.
By vigorous mixing of [C mim]PF (1 mL) and aqueous
in ILs served as a template to direct the sol–gel reaction. Here,
microdroplets of ILs in water offer unique interfaces for control-
ling the thickness of gold nanosheets.
À
[
C4mim]PF6 (1-butyl-3-methylimidazolium hexafluoro-
9
phosphate) was synthesized according to the literature.
.
4
6
HAuCl4 4H2O was dissolved in pure water (concentration,
24.3 mM, pH adjusted to 10.0 by addition of aqueous 1 M
NaOH). Au(III) is mostly present as Au(OH)4 ions at this
À
1
0
pH. In a sample tube, aqueous HAuCl4 (1.0 mL) is layered
on 1.0 mL of [C4mim]PF6 and an interface formed between
the layers was illuminated vertically with a superhigh-pressure
mercury lamp (Ushio, USH-500D, ꢁex > 300 nm, I300{400 > 20
2
mW/cm , irradiation time, 20 min.). After photoirradiation, the
4
6
aqueous layer was removed by pipetting, and acetonitrile was
added to the mixture. The reduced products were collected by
centrifugation (at 15000 rpm) and were washed repeatedly with
acetonitrile and pure water. The samples were dispersed in water
and dropped on carbon-coated copper grids, followed by drying
4 4 6
HAuCl (24.3 mM, pH 10, 1 mL), microdroplets of [C mim]PF
with diameter of ca. 10–50 mm were formed in water (Figure S3,
The [C mim]PF phase was stained with Rhodamine B and was
4
6
observed by fluorescence microscopy.). After UV illumination
of the stirred microemulsions for 20 min, the mixture was phase
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.9 (2005)
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b
1
0 nm
Figure 3. Schematic illustration of the formation of ultrathin
gold nanosheets. Photoreduction of Au(III) ions occur at the
[
C4min]PF6/water interface and the formed nanosheets are
extracted inside the ILs microdroplet. This extraction process
prevents the growth in thickness.
c
moiré
fringes
A schematic illustration for the interfacial nanosheet synthe-
sis is shown in Figure 3. The transfer of gold nanosheets from the
water/IL interface to microdroplets in the early stage of photo-
reduction gives a reasonable account for the observed thickness
control. IL microdroplets mechanically dispersed in water would
experience dynamic equilibrium of fusion and separation by
shear forces under vigorous stirring, and this dynamic feature
may be responsible for the promoted extraction.
Figure 2. (a) A picture of aqueous dispersion, (b) SEM, and
c) TEM images of ultrathin gold nanosheets. Samples were
In conclusion, we have developed a novel and practical syn-
thetic route to ultrathin gold nanosheets by the combination of
ILs, water and photoillumination. These are the basic elements
of green sustainable chemistry and together with the enzymatic
synthesis of gold nanoparticles reported recently,12 colloid
chemistry of ILs provides promising routes to the controlled
green synthesis of inorganic nanomaterials.
(
obtained by photoilluminating mixture of [C4mim]PF6 and
À
aqueous Au(OH)4 under vigorous stirring.
separated into layers of water and [C4mim]PF6 upon standing
(
for ca. 30 s). Interestingly, the IL layer was colored blue, indi-
cating that the photoreduced gold nanosheets are extracted to
the IL phase.
The authors are grateful to Prof. M. Goto of Kyushu Univer-
sity for the use of Shimadzu AA-6700. This work was financially
supported by a Grant-in-Aid for Scientific Research (A)
(No. 16205028) from Japan Society for the Promotion of
Science and a Grant for 21st Century COE Program from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan.
The obtained material was collected as described previously
and was observed by SEM (Figure 2b). Very interestingly, ultra-
thin gold nanosheets with thickness of ca. 10 nm are abundantly
observed. Aqueous suspensions of these nanosheets displayed
broad surface plasmon absorption band ranging from 500 nm
to the near-infrared region (Figure 2a, S4). The peak observed
around at 800–900 nm is consistent with the formation of gold
nanosheets, though it is smoother than those reported for
5
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Figure 2c). Such moir e´ fringes have not been observed for pre-
(
viously reported 2D metal nanoplates probably because of their
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synthesized in the IL/water system. We assume that the thicken-
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traction of nanosheets into the IL microdroplets. The ionic liquid
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(
2002).
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9
1
(
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droplets, as similarly reported for gold and CdTe nanoparti-
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sheets indicate that the most of IL molecules are rinsed off by
repetitive washing with acetonitrile and water (Figure S1), and
this is consistent with the observed solubility in water.
(
2004).
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Published on the web (Advance View) August 6, 2005; DOI 10.1246/cl.2005.1234