Synthesis of gold nanoparticles from gold(I)-alkanethiolate complexes with supramolecular structures through electron beam irradiation in TEM

Article abstract of DOI:10.1021/ja042423x

Monodisperse gold nanoparticles were prepared via electron beam irradiation of Au(I)-SR (R = -C_nH_2n+1) polymers with highly ordered supramolecular structures in transmission electron microscopy. The Au(I)-SR polymers were synthesized

Full text of DOI:10.1021/ja042423x

Published on Web 06/25/2005
Synthesis of Gold Nanoparticles from Gold(I)-Alkanethiolate Complexes with
Supramolecular Structures through Electron Beam Irradiation in TEM
Jong-Uk Kim, Sang-Ho Cha, Kyusoon Shin, Jae Young Jho, and Jong-Chan Lee*
School of Chemical and Biological Engineering and Hyperstructured Organic Materials Research Center,
Seoul National UniVersity, Shilim-9-Dong, Gwanak-Gu, Seoul 151-744, Korea
Received December 16, 2004; E-mail: jongchan@snu.ac.kr
It is well-known that the physicochemical properties of gold
nanoparticles are highly related to their sizes and size distributions.1
Therefore, the ability to synthesize nanoparticles in a size-controlled
manner has been one of the main goals of materials science over
the past decade due to their possible applications in electronic and
1
optical devices. One of the most successful synthetic methods
allowing for precise control of the size was achieved by Schaaff et
2
3
al. through a modification of the well-known Brust method. They
reported that gold nanoparticles with an extremely narrow size
distribution could be prepared through the reductive decomposition
of polymeric Au(I)-SRs, which are made by the reaction of the
gold salt and alkanethiol prior to the reduction step, although to
the best of our knowledge the structure and chemical properties of
the Au(I)-SRs have not been clearly elucidated.
Recently, we found that a gold polymer, Au(I)-SC18 (C18
)
-
C H37), prepared from the well-known chemical reaction of
18
LiAuCl
+ 3 RSH f Au(I)-SR + RS-SR + 3 HCl + LiCl,2
b,4
4
shows mesomorphic behavior and luminescent properties upon UV
irradiation. Furthermore, when the Au(I)-SC18 polymer was
irradiated by an electron beam in a transmission electron microscope
Figure 1. (a) The XRD pattern of Au(I)-SC18 at room temperature. (b)
The DSC curve of Au(I)-SC18 on a heating scan at 5 °C/min. (c) Emission
spectrum of powder Au(I)-SC18 sample with λex ) 350 nm. (Inset)
Photograph of the luminescence of Au(I)-SC18 placed on a slide glass when
photoirradiated with a UV lamp (>350 nm).
(TEM), highly monodisperse gold nanoparticles were obtained.
The Au(I)-SC18 polymer was prepared simply by mixing a gold
salt (LiAuCl
37SH, 1.0 mmol) in THF (5 mL). When the mixture was
stirred for 24 h, the yellowish color of LiAuCl totally disappeared,
4
, 0.1 mmol) with an excess amount of octadecanethiol
18
(C H
4
Au(I)-SC18 in the isotropic state was cooled at 5 °C/min, a grainy
texture of bi-refringent domains was observed from about 157 °C
using polarized optical microscopy. It is well-known that highly
viscose polymer melts can prevent the formation of the normal
and the Au(I)-SC18 polymer was obtained as a white, solid
precipitate. The chemical structure of Au(I)-SC18 was confirmed
from the solid-state 13C NMR and FT-IR experiments (see Sup-
porting Information). Additionally the elemental analysis and
thermal gravimetric analysis results showed that the stoichiometry
corresponds to one octadecane-thiolate group per gold atom within
experimental error.5
The X-ray diffraction (XRD) curve of the powdery Au(I)-SC18
at room temperature shows a series of ordered reflections that can
be assigned to the (0k0) (k ) 1-7) planes in a small angle region
8
texture for their mesomorphic phases. In addition to this result,
both the mechanically soft nature of the sample and the broad wide-
angle X-ray reflections (see Supporting Information) from room
temperature to the isotropic temperature also indicate that Au(I)-
SR18 has mesomorphic phases. The two wide angle reflections
below the isotropic temperature possibly represent the Au- - -Au
distances corresponding to aurophilic interactions, which in turn
cause the luminescence of Au(I)-SC18 upon UV irradiation (Figure
1c). Bachman et al. also reported the luminescence property of Au-
(I)-phenylthiolate (Au(I)-SPh) prepared from PhSNa and Et-
(Figure 1a), indicating that the sample has a highly ordered layer
structure. In the wide angle region, two reflections were observed
at d-spacings of 3.45 and 3.95 Å, respectively, while their intensities
were very small. The layer d-spacing of Au(I)-SC18 (49.0 Å) is
about twice the calculated length of a fully extended octadecanethi-
olate group (25.3 Å). Therefore, Au(I)-SC18 would be expected
to have a bilayer structure. Similar layer structures have been found
6
NCAuCl. In the case of Au(I)-SC , as the temperature increases,
18
the intensity of the luminescence decreases, only to disappear
completely when the sample is heated above the isotropic temper-
ature. Therefore, the luminescent property of Au(I)-SC18 is
observed only in the LC state. Detailed studies on the luminescence
property of Au(I)-SC18 are currently under way.
6
for other polymeric metal complexes, such as RNCAu(I)-SPh and
7
Ag(I)-SRs. In the DSC curve of the Au(I)-SC18 polymer, a sharp
endotherm was observed at 157 °C (∆H ) 116 J/g) (Figure 1b).
Above the endotherm, Au(I)-SC18 was determined to be isotropic
from the XRD experiments. On heating the powdery Au(I)-SC18
from room temperature, the ordered small-angle and wide-angle
reflections were maintained until 157 °C, at which point both the
small angle peaks and the wide-angle peaks disappeared. When
We performed TEM experiments (JEOL JEM-3000F, 300 keV)
for Au(I)-SC18 to observe the ordered structure of the sample,
because normally the ordered phase of a block copolymer can be
observed by TEM. For the sample preparation for TEM, the
powdery Au(I)-SC18 was dispersed in THF, and a droplet of the
solution was dropped onto a carbon-coated copper grid. When Au-
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10.1021/ja042423x CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
2
of the gold nanocrystals. When Au(I)-SC , which was found to
have a much less ordered structure (only a weak (010) reflection
was observed from the XRD pattern), was irradiated with the
electron beam, a mixture of gold nanocrystals with various sizes
and irregular shapes were obtained (Figure 2d). Currently, we are
working on the reduction of Au(I)-SRs using an electron beam
accelerator of high capacity with different energy values (0.3 to
tens of MeV) to perform the controlled-synthesis of a large quantity
of gold nanoparticles with a monodisperse size distribution using
this solvent-free process.
In conclusion, the one-pot fabrication of thiol-stabilized mono-
disperse gold nanoparticles was achieved through the electron beam
irradiation of Au(I)-SRs with highly ordered supramolecular
structures. The Au(I)-SRs were synthesized simply by mixing
LiAuCl and alkanethiols in THF. Au(I)-SRs were found to be
4
mesomorphic and showed luminescent behavior upon UV irradia-
tion. The novel luminescent properties and LC behavior of the Au-
(I)-SRs with various alkyl groups and various functional groups
are also being studied. The results will be reported shortly.
Acknowledgment. This work was supported by the Korea
Science and Engineering Foundation (KOSEF) through the HOM-
RC and the Research Institute of Engineering Science at Seoul
National University. Experiments at Pohang Accelerator Laboratory
were supported in part by MOST and POSTECH. The electron
beam accelerator of high capacity was used in KAERI supported
by KOSEF.
Figure 2. (a) High-resolution TEM images of gold nanoparticles formed
from Au(I)-SC18 by electron beam irradiation in TEM of 300 keV (JEM-
3
000F). The characteristic lattice fringes (lattice spacing ) 2.5 Å) of the
fcc crystal structure of gold are clearly shown. TEM images of gold
nanoparticles formed (b) from Au(I)-SC18 by electron beam irradiation in
TEM of 80 keV (JEM-1010), (c) from Au(I)-SC6 in TEM of 300 keV,
and (d) from Au(I)-SC2 in TEM of 300 keV.
Supporting Information Available: Detailed synthetic procedures,
EDX and NMR spectra, POM and TEM images, XRD spectra (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(
I)-SC18 was irradiated by the electron beam in the TEM, to our
interest, spherical nanoparticles with an average diameter of 1.9
nm ((0.3 nm) were observed (Figure 2a). The spherical shapes of
the gold nanoparticles are not clearly seen in these TEM images
probably due to the three-dimensionally assembled structure of the
gold nanoparticles, as reported by others.9
We found that the size of the gold nanoparticles can be controlled
by adjusting the accelerating voltage of the electron beam in the
TEM. For example, the average sizes of the gold nanoparticles
formed in the TEM of 80 keV (JEOL JEM-1010) and 200 keV
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6
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(
0
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Au(I) atoms, which in turn probably influences the uniform growth
(
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