Protective agent-free preparation of gold nanoplates and nanorods in aqueous HAuCl4 solutions using gas - Liquid interface discharge

Article abstract of DOI:10.1246/cl.2007.1088

Gas - liquid interface discharge at atmospheric pressure above HAuCl ₄ solutions containing no protective agents generates triangular and hexagonal nanoplates and nanorods of gold. Experimental results under several different conditions indicat

Full text of DOI:10.1246/cl.2007.1088

1088
Chemistry Letters Vol.36, No.9 (2007)
Protective Agent-free Preparation of Gold Nanoplates and Nanorods
in Aqueous HAuCl4 Solutions Using Gas–Liquid Interface Discharge
Kenji Furuya,ꢀ Yuko Hirowatari, Toshio Ishioka, and Akira Harata
Department of Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580
(Received May 15, 2007; CL-070530; E-mail: furuya@mm.kyushu-u.ac.jp)
Gas–liquid interface discharge at atmospheric pressure
above HAuCl4 solutions containing no protective agents gener-
ates triangular and hexagonal nanoplates and nanorods of gold.
Experimental results under several different conditions indicate
that the plates and rods rapidly grow on the surface of the
solution. In contrast, the spherical nanoparticles slowly grow
in the solution.
(Millipore Systems, resistivity 18 Mꢀꢂcm). The other was
0.33 mM HAuCl4 and 1.33 mM sodium hydrogen carbonate
(Kishida Chemical, NaHCO3, 99.5%). The anode of Cu was
fixed at 1–2 mm above the gas–solution interface, while the Cu
cathode, which was grounded, was immersed in a 25-mL solu-
tion in a 50-mL beaker. A DC voltage was applied to the anode
using a DC high voltage power supply (MAX-ELECTRONICS,
HVꢀ-10K50P/DPM/100), where the power supply confined the
electric current to 50 mA. The applied voltage depended on the
distance between the end of the anode and the surface of the
solution and on the distance between the two electrodes, which
was typically 1 to 1.5 kV. A steady discharge was generated
between the anode and the surface of the solution. The solution
was stirred using a magnetic stirrer during the discharge. Within
15 s, the solution changed from faint yellow to dark red, a
well-known color for gold nanoparticle solutions.
Recently, the preparation and characterization of nanostruc-
tured materials have received significant attention. Especially,
gold nanoparticles have unique optical, electrical, catalytic
properties, and biocompatible features that should be applicable
to biosensors and gene therapy.1 Various methods have been
reported on the preparation of the gold nanoparticles,2 where
a polymer or surfactant added to the reactant solution plays a
crucial role in preparing triangular and hexagonal nanoplates
and nanorods.
Glow discharge electrolysis (GDE) is not very common
but has been a part of the electrochemistry field since 1887.3,4
In GDE, one electrode is set in the gas phase above the electro-
lyte solution, and the other is immersed in the solution. The glow
discharge is generated between the surface of the solution and
the end of the electrode placed in the gas phase in a pressure
range of 3:3 ꢁ ð103{104Þ Pa. Charged particles produced by
the discharge in the gas phase are accelerated toward the surface
of the solution and then collide with solvent molecules to pro-
duce various radicals, ions, and excited species, which cannot
be generated by ordinary electrolysis in the solution. The forma-
tion of hydrogen peroxide (H2O2) in various aqueous solutions
and of hydrazine (N2H4) in liquid ammonia has been examined
by GDE in detail.3 However, this method has very limited appli-
cations in chemical synthesis to date. Other applications of the
discharge in systems containing liquid have recently been tried
on the formation of carbon nanotubes using an arc discharge
in solution5 and on the decomposition of phenol (C6H5OH)
using a pulsed gas–liquid interface discharge.6,7
In this letter, we show that the gold nanoplates and nanorods
can be prepared in simple aqueous HAuCl4 solutions containing
no protective agents such as a polymer and surfactant, using
a gas–liquid interface discharge at atmospheric pressure. The
experimental method used here is very similar to that of GDE.
However, nucleation and crystallization instead of electrolysis
has occurred in the present experiment. In addition, the dis-
charge form differs from that of an ordinary glow discharge.
Hence, the expression ‘‘GDE’’ is not appropriate in this report.
Therefore, we describe the present experimental method as a
Discharge-Induced Chemical REaction in Solution (DICRES).
Two kinds of solutions were prepared. One was an aqueous
solution of 0.33 mM hydrogen tetrachloroaurate(III) tetrahydrate
The transmission electron microscope (TEM; JEOL JEM-
2100XS) images and electron diffraction patterns of the particles
prepared by the DICRES method were observed at an electron
energy of 200 kV.
Figures 1a and 1b show the TEM images of the nanoparti-
cles prepared by the discharge of the HAuCl4/NaHCO3 solution
in the air during 14-s and 10-min discharges, respectively. Trian-
gular and hexagonal plates and rods are clearly observed.
Observing the selected-area electron diffraction (SED) patterns
of these nanoparticles, we confirmed that these are gold crystals.
Figure 2 shows the TEM image of a triangle nanoplate and
its SED pattern. The hexagonal symmetry of the SED pattern in-
dicates that the triangle nanoplate is a single crystal and that the
incident electron beam is perpendicular to the {111} facet of the
plate. The obtained colloidal gold solution was dispersed a few
days, and it took about one month for the complete aggregation
of the nanoparticles. Very recently, Irie et al.8 have reported the
preparation of gold nanoparticles by pulsed discharge where
both electrodes were immersed into aqueous HAuCl4/KCl/gel-
atin solutions. However, only spherical particles with about 10-
nm diameter have been observed in the TEM image. To our best
knowledge, this is the first report on the protective agent-free
(a)
(b)
100 nm
100 nm
Figure 1. TEM images of gold nanoparticles prepared by the
discharge of the HAuCl4/NaHCO3 solution in the air during
(a) 14 s and (b) 10 min.
.
(Wako Chemicals, HAuCl4 4H2O, 99%) using Millipore water
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.9 (2007)
1089
100 nm
200 nm
Figure 2. TEM image (left bottom) and SED pattern of a trian-
gle gold nanoplate.
Figure 3. TEM image of gold nanoparticles collected near the
gas–liquid surface.
above the solution are accelerated in the gas phase and then
collide with the water molecules which are the solvent mole-
cules. Chemically active species such as the H and OH radicals
and their positive ions are produced by the collisions. (2) The
preparation of gold nanoplates and nanorods. The size of the
plates and rods in Figure 1a is almost the same as those in
Figure 1b, although the discharge time was quite different. In
contrast, the fraction of small spherical particles with less than
50-nm diameter in Figure 1a is significantly greater than that
in Figure 1b. This finding indicates that the growth rate of the
plates and rods is much faster than that of the spherical particles.
Yang et al.9 have demonstrated that the gold nanoparticles
can be prepared by synchrotron X-ray irradiation to aqueous
HAuCl4/NaHCO3 solutions, but only spherical particles with
5–10-nm diameters have been observed in the TEM image. An
analogy of the reaction mechanism between GDE and radiation
chemistry has been pointed out about the decomposition of
solvent molecules by the collisions with energetic particles.3
The formation of the gold nanoplates and nanorods presented
here demonstrates that DICRES can lead to a unique nucleation
or crystallization of gold nanoparticles, which is difficult by
X-ray irradiation.
In order to investigate the formation mechanism of the gold
nanoplates and nanorods in detail, we further conducted experi-
ments under several different conditions. Firstly, similar experi-
ments were conducted using a U-tube without stirring the solu-
tion, where two electrodes were placed at two open sides of the
U-tube one by one. Consequently, the solution changed to red
only in the anode side, especially near the gas–solution interface
where the discharge was generated. At the other side of the
U-tube where the cathode was immersed, the solution did not
change color and boiled near the electrode. Secondly, the elec-
trode material was exchanged from copper to gold. The solution
changed to dark red after the discharge, similarly to that in the
case of the copper electrodes. Thirdly, a colloidal gold solution,
which contained only spherical gold nanoparticles with 10–20-
nm diameter and was prepared in advance by a citrate reduction
method, was used for the DICRES experiment. As a result, the
aggregation of the nanoparticles was observed in the TEM im-
age, but no nanoplates and nanorods were observed. Finally,
the DICRES experiment was conducted under an atmosphere
of Ar or N2. The formation of gold nanoplates and nanorods
was confirmed in their atmospheres, similarly to the experiment
conducted in the air. Especially, the TEM image of the nanopar-
ticles collected near the gas–liquid surface after a one-minute
discharge using the HAuCl4 solution without stirring the solu-
tion in the Ar atmosphere contained rich nanoplates, as shown
in Figure 3.
ꢃ
Au3þ ions, which are present in the form of AuCl4 ions in
the solution, are reduced to Au0 by the H radicals. This reduction
mechanism is the same as that induced by the X-ray irradiation
and by the pulsed discharge in the solution. (3) The gold nano-
plates and nanorods rapidly grow only on the surface of the solu-
tion. This growth mechanism is supported by the fact that the
nanoplates and nanorods were not produced by the X-ray radia-
tion and by the pulsed discharge in the solution. The report that
the gold crystal is easy to grow on the surface of the solution in a
chemical reduction method10 also supports this growth mecha-
nism in the DICRES method. Once the plates and rods grow
on the surface of the solution up to a certain size, they sink into
the solution because of their weight, and then their growth stops.
(4) The spherical particles slowly grow in the solution. This is
the same growth mechanism of the spherical particles as that
by the X-ray irradiation and by the pulsed discharge in the
solution.
In conclusion, we have successfully demonstrated that the
DICRES method can prepare gold nanoplates and nanorods in
aqueous HAuCl4 solutions containing no protective agents. This
method is promising as a simple, rapid preparation method of
nanocrystals.
We gratefully acknowledge Prof. Masaharu Tsuji, Dr.
Takeshi Tsuji, Mr. Mitsutaka Umezu, and Mr. Takeshi Tanaka
of Institute for Materials Chemistry and Engineering, Kyushu
University for helpful suggestions and TEM measurements.
This work was partly supported by the National Institute for
Fusion Science (No. NIFS06KOAP014).
References and Notes
1
2
M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293, and references therein.
Z. Zhong, K. B. Male, J. H. T. Luong, Anal. Lett. 2003, 36, 3097, and references
therein.
3
A. Hickling, in Modern Aspects of Electrochemistry, ed. by J. O’M. Bockris,
B. E. Conway, Butterworths, London, 1971, Vol. 6, pp. 329–373, and references
therein.
4
5
M. A. Mottaleb, J. S. Yang, H.-J. Kim, Appl. Spectrosc. Rev. 2002, 37, 247.
D. Bera, S. C. Kuiry, M. McCutchen, S. Seal, H. Heinrich, G. C. Slane, J. Appl.
Phys. 2004, 96, 5152.
6
7
Z. He, J. Liu, W. Cai, J. Electrostatics 2005, 63, 371.
T. Kuroki, K. Yoshida, H. Watanabe, M. Okubo, T. Yamamoto, Jpn. J. Appl.
Phys. 2006, 45, 4296.
8
9
K. Irie, M. Oda, T. Ishizaki, N. Saito, O. Takai, The 24th Symposium on Plasma
Processing (SPP-24), Osaka, Japan, January, 2007, Proceedings, No. A1-02.
Y.-C. Yang, C.-H. Wang, Y.-K. Hwu, J.-H. Je, Mater. Chem. Phys. 2006,
100, 72.
Judging from these findings, the formation mechanism of
the gold nanoparticles in the DICRES method is summarized
as follows; (1) The positive ions generated through the discharge
10 H. Yao, S. Sato, K. Kimura, in Nanoparticle assemblies and superstructures, ed.
by N. A. Kotov, Taylor and Francis, London, 2006, pp. 601–614.
Published on the web (Advance View) July 28, 2007; doi:10.1246/cl.2007.1088