Full text of DOI:10.1007/BF03183543

NOTES
diffraction (XRD) technique was employed to roughly
estimate the size of the nanoparticles. In this work, we
selected 1-nonanethiol as the stabilizer to prepare ex-
pected gold nanoparticles in size of 5 nm through adjust-
ing suitable Au/S ratio. The size selective precipitation
technique was used to produce the nearly monodisperse
gold nanoparticles. UV-Vis, FT-IR, X-ray photoelectron
spectroscopy (XPS) and transmission electron microscopy
(TEM) were employed to characterize the gold nanoparti-
cles and gold colloidal-based ordered structure.
Two-dimensional self-organi-
zation of 1-nonanethiol-
capped gold nanoparticles
1
1
2
JIANG Peng , XIE Sishen , YAO Jiannian ,
1
1
1
HE Shengtai , ZHANG Haoxu , SHI Dongxia ,
1
1
PANG Shijin & GAO Hongjun
1
. Beijing Laboratory of Vacuum Physics, Institute of Physics & Center
for Condensed Matter Physics, Chinese Academy of Sciences, Beijing
00080, China;
1
Experimental
1
2
. Center for Molecular Sciences, Institute of Chemistry, Chinese Acade-
my of Sciences, Beijing 100101, China
4 2
(ν) Reagents. HAuCl g3H O (Acros, 99.8%),
tetra-n-octylammonium bromide (Acros, 98%), NaBH
Acros, 99%), 1-nonanethiol (C 19SH) (Aldrich, 99%)
4
Correspondence should be addressed to Jiang Peng or Gao Hongjun
e-mail: pjiang@ aphy.iphy.ac.cn or hjgao@aphy.iphy.ac.cn)
(
9
H
(
were used as received. AR grade solvents and deionized
and subsequently distilled water (with the resistance more
Abstract A two-dimensional (2D) ordered hexagonal
close-packed structure, formed by 1-nonanethiol-capped
gold nanoparticles, is reported. The structure was con-
structed only by dipping the gold nanoparticle colloidal solu-
tion on flat substrate. The gold nanoparticles were synthe-
than 18 M:gcm) were used.
(
ξ) Preparation of sample. Colloidal gold nano-
particles were synthesized using the method suggested by
Brust as follows: First, 10 mL tetraoctylammonium bro-
mide toluene solution (0.1 mol/L) was added to a vigor-
ꢀ
1
sized as follows: First, AuCl
4
was transferred from aqueous
solution to toluene by the phase-transfer reagent of tetraoc-
tylammonium bromide. Then it was reduced with aqueous
sodium borohydride in the presence of a given amount of
4 2
ously stirred 15 mL HAuCl g3H O aqueous solution (0.03
mol/L). Immediately, the upper layer became orange/red
organic phase and the lower buff aqueous phase. The sys-
tem was shaken repeatedly in order to remove the color
from the aqueous phase. After the complete separation of
the two phases, the organic phase was collected and 0.12
mmol 1-nonanethiol was injected into the organic phase,
followed by the addition of 12.5 mL sodium borohydride
aqueous solution (0.4 mol/L). After the reaction mixture
was successively stirred under ambient condition for 24 h,
the deep red organic phase was separated from the aque-
ous phase and concentrated to ̚5 mL by evaporating in
a rotary evaporator. Sequentially, 350 mL methanol was
poured into the dispersion, and the mixture was kept in a
refrigerator for 4 h to effectively precipitate the gold
nanoparticles. The black precipitate was collected by fil-
tering the mixture with 2.5 Pmꢀpolypropylene filter paper
and washed several times with ethanol. The size selective
precipitation technique employing chloroform/methanol
as the solvent was used to narrow the product. Finally, the
nearly monodisperse 5 nm sized gold nanoparticles were
thus prepared for various characterizations.
1
-nonanethiol molecules which was used to control the nu-
cleation and growth of the gold nanoparticles for the desired
size. The experimental techniques, such as UV-Vis, FT-IR,
and X-ray photoelectron spectroscopy (XPS), were employed
to characterize the obtained product. Transmission electron
microscopy (TEM) measurement demonstrated the size of
the gold nanoparticle and the formation of two-dimensional
ordered hexagonal close-packed gold nanoparticle structure.
Keywords: gold nanoparticle, characterization, self-organization.
According to some people’s design in advance, using
nanoparticles as building blocks to prepare under control
two-dimensional and three-dimensional ordered structures
has been among the most hot points in material chemistry
today. Its realization will raise a new revolution in the
fields such as optics, electronics, catalysis, chemical sen-
[
1ü
sors, and nanometer-scaled information storage devices
.
5]
In recent years, with the development of the techniques
for preparing and characterizing metal and semiconductor
nanoparticles, scientists have found some special ways
to control the size and monodispersity of the nanoparticles.
Among them, the wet chemistry approach proves quite
effective. Brust et al. first performed the research work.
Based on the ability of stable adsorption of thiol molecule
monolayer on gold surface, they synthesized gold
nanoparticles with the size varying from 1 to 3 nm in the
presence of dodecanethiol by the chemical reduction
method. After that, Leff et al. found that the gold particle
size in a diameter range from 1.5 to 20 nm can be con-
trolled by simply varying the initial gold-to-thiol ratio. At
the same time, in their experiments, the X-ray powder
(
ο ) Characterizations. The UV-Vis absorption
spectrum of the colloidal gold nanoparticles dispersed in
cyclohexane was measured using a Shimadzu UV-1601
PC double beam spectrophotometer in the range of 200ü
800 nm with 2 nm resolution. FT-IR spectra were re-
corded on a Bio-Rad FT-IR spectrophotometer using a
150 mg KBr disk dispersed with the colloidal gold cyclo-
hexane solution or a drop of 1-nonanethiol liquid. The
background correction was performed using a reference
blank KBr pellet. The XPS spectrum of gold nanoparticles
996
Chinese Science Bulletin Vol. 46 No. 12 June 2001

was obtained on a VG ESCALAB 5 multi-technique elec-
tron spectrum meter that focused monochromatic Mg
molecules and the thiol-capped gold nanoparticles are
shown in fig. 2. The comparison of the FT-IR spectra re-
veals extremely similar feature. The only difference
between them is that the intensities of peaks at various
frequencies increase to some extent while widths of them
become narrow in the case of gold nanoparticles. A possi-
ble qualitative explanation is originated from the special
orientation and state of the alkanethiol molecules ad-
sorbed on the gold nanoparticle surfaces. The FT-IR ex-
perimental evidence strongly suggests that 1-nonanethiol
has been an essential component of the composite nanopar-
ticles. The XPS spectrum can present the information on
the existence of Au and S elements in the nanoparticles.
Fig. 3 shows the scanning range of binding energy of the
particle sample from 80 to 95 eV, in which a double-peak
occurs at 83.7 and 87.3 eV, corresponding to Au4f7/2 and
Au4f5/2, respectively, a typical character of the presence of
K
2
DꢁX-rays onto the sample, the main C(1s) peak was set to
84.8 eV as the final calibration of the energy scale. The
sample for TEM experiment was prepared by depositing a
drop of the toluene solution of the colloidal gold nanopar-
ticles onto a carbon-coated copper grid and letting it dry
completely under ambient condition. TEM images were
obtained from a Jeol 2000EX TEM system operated at
2
00 kV.
Results and discussion
ν) Synthesis and properties of thiol-capped gold
2
(
nanoparticles. The synthesis of the gold nanoparticles
was completed in toluene solution in the presence of a
given amount of 1-nonanethiol molecules. The overall
process from aqueous phase to organic phase can be
summarized by equations as follows:
0
Au state. It is worth noting that we do not find the obvi-
ꢀ
ꢂ
AuCl (aq) ꢂ N(C H ) ꢂ (C H Me) o
ous peak of Au oxidation state at 84.9 eV for Au(ĉ) oc-
tanethiolate complex, which presumably suggests that the
0
gold atoms must be present largely as Au . In addition, the
4
8
17
4
6
5
(
1)
ꢂ
ꢀ
N(C H ) AuCl (C H Me)
6
8
17
4
4
5
ꢀ
ꢀ1
sulfur region (not shown here) exhibits weaker broad
double peaks at 163.1 and 162.1 eV, showing the exis-
tence of S element on the surfaces of the gold nanoparti-
cles.
mAuCl (C H Me) ꢂ nC H SH(C H Me) ꢂ 3me
o
4
6
5
9
19
6
5
ꢀ
1
4
mCl (aq) ꢂ (Au )(C H SH) (C H Me)
(2)
m
9
19
n
6
5
The size of the gold nanoparticles can be adjusted by
an initial Au/S (m/n) ratio. In this experiment, we selected
the ratio of 4/1(Au/S). Many other experiments have
shown that thiol molecules can form stable, self-assem-
bled monolayer on gold surface due to the strong interac-
tion between gold and sulfur. In the process of formation
and growth of gold nuclei, if thiol molecules exist in the
system, the growth of gold nuclei will be restricted
quickly due to the interaction between gold and sulfur. As
a result, small thiol-capped gold nanoparticles are pro-
duced. In general, the product thus-obtained is with dark
waxy texture, which is soluble very easily in nonpolar
solvents such as benzene, cyclohexane, hexane but in-
soluble in polar solvents, including water and alcohol,
suggesting that the gold particles have been indeed sur-
rounded by the hydrophobic organic molecules.
Fig. 1. UV-Vis spectrum of 1-nonanethiol gold colloidal in cyclohex-
ane.
(
ξ) UV-Vis, FT-IR and XPS spectral studies. UV-
Vis characterization of the gold nanoparticles was con-
ducted using a Shimadzu UV-1601 PC double beam spec-
trophotometer. In the experiment, 1 mg sample was first
dissolved in cyclohexane, then adjusted to a suitable con-
centration for the measurement; meanwhile, cyclohexane
was selected as a correction. Fig. 1 gives the typical
UV-Vis spectrum of the system. It can be seen that a clear
peak maximum, corresponding to the plasmon excitation,
occurs at around 518 nm. In addition, a continuous rising
background toward shorter wavelength can also be ob-
served, which might result from the Mie scattering of the
nanoparticle suspension. The FT-IR spectrum can provide
us the knowledge about the presence of thiol molecules on
the nanoparticles. FT-IR spectra of pure 1-nonanethiol
Fig. 2. FT-IR spectra of free 1-nonanethiol (1) and thiol-capped gold
nanoparticles (2).
(ο) TEM characterization. A TEM image of the
gold nanoparticles is shown in fig. 4, in which near
spherical gold nanoparticles with the average size of 5.0f
Chinese Science Bulletin Vol. 46 No. 12 June 2001
997

NOTES
0
.5 nm are clearly observed. From a further observation of
3
Conclusions
fig. 4, it is found that the nanoparticles do not fuse into
larger particles, but in the way of the lowest energy,
forming the hexagonal close-packed structure, in which
some defects have also been found. The average distance
between nanoparticles is about 1 nm, corresponding to the
length of one 1-nonanethiol molecule. This implies that
the thiol molecules attached to the adjacent nanoparticles
likely interpenetrate with each other due to the intermo-
lecule interaction. Moreover, in TEM measurement, we
found that the ordered array could be destroyed by higher
operating voltage (˚200 kV) and some nanoparticles
even fused with each other. The smaller the nanoparticle,
the faster the rate of fusion. The experimental result
showed that higher electron irradiation would evaporate
the attached organic ligands, leading to the development
of bigger particles.
In conclusion, we have used the method of liq-
uid-liquid extraction to successfully synthesize the gold
nanoparticles with the size of 5.0f0.5 nm in the presence
of a given amount of 1-nonanethiol molecules. After that,
the step-by-step precipitation technique has been em-
ployed to narrow the distribution of obtained nanoparti-
cles. At the same time, the techniques including UV-Vis,
FT-IR, XPS have also been utilized to characterize the
product. The results provide the direct evidence of the
formation of the thiol-capped gold nanoparticles. Fur-
thermore, after dissolving the particles into toluene and
depositing the colloidal solution on carbon-coated copper
grid, we found the formation of ordered hexagonal close-
packed structure by TEM. We conclude that the gold
nanoparticles can self-organize into ordered two-dimen-
sional close-packed structure on the flat substrate after
solvent evaporates under suitable concentration. The for-
mation of the ordered structure is closely related to the
thiol molecules attached on gold surfaces.
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Fig. 4. A TEM image of gold nanoparticles.
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Chinese Science Bulletin Vol. 46 No. 12 June 2001