One-pot synthesis, optical property and self-assembly of monodisperse silver nanospheres

Article abstract of DOI:10.1016/j.jssc.2011.05.041

High-quality spherical silver (Ag) nanocrystals have been synthesized by using a one-pot approach, in which pre-synthesis of organometallic precursors is not required. This reaction involves the thermolysis of a mixed solution of silver acetate and n-dodecanethiol in a non-coordinating organic solvent. The size of the as-obtained Ag nanospheres can be controlled by adjusting the reaction time, reaction temperature and the amount of silver acetate added. The growth and nucleation process of the resultant Ag nanospheres have been studied by employing UV-vis absorption spectra and transmission electron microscopy (TEM) images. Furthermore, these Ag nanospheres have good self-assembly behaviors, and they are easily self-assembled into two- or three-dimensional superlattice structures due to the bundling and interdigitation of thiolate molecules adsorbed on Ag nanospheres. This one-pot synthetic procedure is simple and highly reproducible, which may be extended to prepare other noble-metal nanocrystals.

Full text of DOI:10.1016/j.jssc.2011.05.041

Journal of Solid State Chemistry 184 (2011) 1956–1962
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
One-pot synthesis, optical property and self-assembly of monodisperse
silver nanospheres
a,b,n
b
c
c,nn
c
b
Aiwei Tang
, Shengchun Qu , Yanbing Hou , Feng Teng
, Yongsheng Wang , Zhanguo Wang
a
Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing 100044, China
b
c
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing JiaoTong University, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history:
High-quality spherical silver (Ag) nanocrystals have been synthesized by using a one-pot approach, in
which pre-synthesis of organometallic precursors is not required. This reaction involves the thermo-
lysis of a mixed solution of silver acetate and n-dodecanethiol in a non-coordinating organic solvent.
The size of the as-obtained Ag nanospheres can be controlled by adjusting the reaction time, reaction
temperature and the amount of silver acetate added. The growth and nucleation process of the
resultant Ag nanospheres have been studied by employing UV–vis absorption spectra and transmission
electron microscopy (TEM) images. Furthermore, these Ag nanospheres have good self-assembly
behaviors, and they are easily self-assembled into two- or three-dimensional superlattice structures
due to the bundling and interdigitation of thiolate molecules adsorbed on Ag nanospheres. This one-pot
synthetic procedure is simple and highly reproducible, which may be extended to prepare other noble-
metal nanocrystals.
Received 15 February 2011
Received in revised form
2
8 April 2011
Accepted 29 May 2011
Available online 7 June 2011
Keywords:
Ag nanospheres
Self-assembly
Optical property
Superlattice structures
&
2011 Elsevier Inc. All rights reserved.
1
. Introduction
associated with their morphologies and size [19,20]. In those
aforementioned non-aqueous notable approaches, alkanethiol
often played an important role in producing monodisperse Ag
nanocrystals. Interestingly, the thiolate molecules adsorbed on
the nanocrystals’ surface not only acted as the protective layer
of the nanocrystals from aggregation but also directed the Ag
nanocrystals to assemble into two- or three-dimensional (2D or
3D) superlattice structures [17,21].
In the past few decades, colloidal nanocrystals have attracted
much attention due to their potential applications in optoelec-
tronic and biological fields [1–4]. Among these well-known
colloidal nanocrystals, noble-metal nanocrystals may be one of
the most important category. To date, these noble-metal nano-
crystals have been widely used in the fields of catalysis [5],
memory devices [6,7], and surface-enhanced Raman scattering
Although so much success has been made in the synthesis
of Ag nanocrystals, yet it is still challenging to develop a simple
but low-cost technique for preparing uniform and large-scale Ag
nanocrystals with tunable size less than 10 nm. The size of the
metal nanocrystals that can be self-assembled into superlattice
structures is usually smaller than 10 nm, and it is in this size
range that some exciting and unusual physical properties are
enhanced [22]. In this paper, we reported a facile one-pot reaction
for synthesis of Ag nanospheres with excellent control of their
size at high temperature (4200 1C). This process was designed by
thermolysis of a mixture of silver acetate and n-dodecanethiol in
a high boiling-point solvent [23,24]. This synthetic process does
not need any extra steps for preparing metal precursors. Impor-
tantly, this method represents a highly reproducible approach for
producing large-scale and high-quality Ag nanocrystals. The size
of the as-obtained Ag nanospheres could be conveniently tailored
via adjusting the reaction time, temperature and the precursor
ratios. Due to the interaction between the neighboring nanospheres,
the Ag nanospheres have a high tendency to be self-assembled into
highly ordered superlattice structures.
(
SERS) [8,9], and also used as building blocks for nanocrystal
assemblies [10]. Up to now, a large number of physical and
chemical synthetic methods have been explored to effectively
control the size and morphology of noble-metal nanocrystals,
such as polyol process [11,12], aerosol technique [13], thermolysis
of a single-source organometallic precursor [14,15], surfactant-
promoted reductive route [16,17], seed-growth method [18] and so
on. These strategies make it possible to produce high-quality metal
nanocrystals with uniform size and different shapes ranging
spheres to wires, rods, disks, cubes and prisms [11–18]. As an
important member of metal nanocrystals, silver (Ag) nanocrystals
have been extensively studied and are of particular interest
because of their unusual properties and applications presumably
n
Corresponding author at: Department of Chemistry, School of Science, Beijing
JiaoTong University, Beijing 100044, China.
nn
Corresponding author.
E-mail addresses: awtang@bjtu.edu.cn (A. Tang), fteng@bjtu.edu.cn (F. Teng).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.05.041

A. Tang et al. / Journal of Solid State Chemistry 184 (2011) 1956–1962
1957
2
. Materials and methods
photoelectron spectroscopic (XPS) measurements were performed
on a VG ESCALAB 220i-XL spectrometer with a 300 W AlK
a
2
.1. Materials
radiation source. All binding energies for different elements were
calibrated with respect to C 1s line at 284.8 eV from the contami-
nant carbon. The thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) were measured using a NETZSCH TG209 F3
instrument. Crystal structure of the samples was measured using
Silver acetate (Ag(Ac), Sinopharm Chemical Reagent Co., Ltd.,
China, Z99%), 1-Octadecene (ODE, J&K Chemical Ltd., 90%),
n-dodecanethiol (DDT, Sinopharm Chemical Reagent Co., Ltd.,
China, 98%) were used as purchased without further purification.
Other solvents such as ethanol and chloroform were commer-
cially available products in analytical grade, which were pur-
chased from Beijing Chemical Reagent, China.
Bruker D8 Discover X-ray Diffractometer with a CuK
a radiation
˚
source (
l
¼1.54056 A). Fourier transform infrared spectra (FTIR)
were collected using a Varian Excalibur 3100 spectrometer. The
UV–vis absorption spectra of the Ag nanospheres in chloroform
were obtained on a Shimadzu-UV 3101 spectrophotometer. All the
measurements were carried out under ambient conditions at room
temperature.
2.2. Synthesis of Ag nanospheres
In a typical synthesis of sample A, 15 mmol of Ag(Ac) was
dissolved in 10 ml of DDT and 20 ml of ODE. The mixture was
degassed by nitrogen gas for about 20 min. Then the mixture was
heated up to 200 1C and kept at the temperature for several hours.
Aliquots were extracted from the mixture at different reaction
intervals for monitoring the shape and size evolutions of the
resultant samples. Right after the reaction, the aliquots solution
was collected and then centrifuged at 6000 rpm for 10 min. The
supernatant was discarded and the precipitation was fully dispersed
in chloroform, and then precipitated by adding some ethanol,
following by centrifugation at 6000 rpm for 10 min. The precipita-
tion and washing procedures were repeated three times to remove
surfactant residuals, un-reacted reactants and the by-products.
Finally, the resultant Ag nanospheres obtained at different reaction
time were re-dispersed into chloroform or dried in vacuum for next
characterization. Different-sized Ag nanospheres (samples B and C)
were also synthesized using the same conditions as the process for
sample A. For the sake of convenience, the detailed experimental
conditions are summarized in Table 1.
3. Results and discussion
In this work, Ag nanospheres were obtained by heating silver
acetate and n-dodecanethiol in a non-coordinating solvent (ODE)
at high temperature, in which DDT was used as a surface-capping
ligand, and ODE acted as a solvent to allow the reaction to be
performed. In order to study the effects of reaction conditions on
the size of Ag nanospheres, different Ag nanospheres were
prepared at different reaction temperatures and different pre-
cursor ratios. Generally speaking, metal precursors can react with
DDT initially to form an intermediate complex [25]. Herein, the
reaction of Ag(Ac) and DDT leads to the formation of intermediate
complex, as demonstrated by the color change from turbid
suspension to a yellow and transparent mixture at early stage.
Afterwards, the color of the system changed from yellow to dark-
brown while the reaction was going on, and finally black nano-
crystal ink was obtained. The schematic illustration of the
synthetic procedure for Ag nanospheres is shown in Fig. 1. The
intermediate products obtained at the initial stage can be identi-
m 25 n
fied as the formation of Ag-thiolate [Ag (SC12H ) ] precursor
2.3. Sample characterization
with a layered structure, which is confirmed by the sharp
diffraction peaks of small-angle XRD patterns of the sample taken
from at the beginning of the reaction (see Figure. S1 in Supporting
information). The sharp diffraction peaks at lower angles corre-
spond to successive orders of diffractions from layered structure
of Ag-thiolate observed previously by Wu and co-workers [14].
During the next refluxing process, the thermal decomposition of
the resultant Ag-thiolate precursor took place, in which the Ag
ions could be reduced by the thiolate group with the formation of
disulfide [14]. The following reaction seems to be responsible for
the synthesis of Ag nanocrystals:
The morphology of the samples was studied using a Hitachi-
7
650 transmission electron microscope operating at an accelerating
voltage of 100 kV. High-resolution transmission electron microscopy
HRTEM) images and selected-area electron diffraction (SAED)
patterns were recorded using a JEM-2010 transmission electron
microscope working at an accelerating voltage of 200 kV. The X-ray
(
Table 1
Experimental conditions for preparing different-sized Ag nanospheres at different
reaction temperatures and amounts of Ag(Ac).
Ag(CH
3
COO)þ2C12
H25SH-AgþC12
H
25SSC12
H25þCH
3
COOH.
Samples
Amount of Ag(Ac) (mmol)
Reaction temperature (1C)
As a result, the monodisperse Ag nanospheres were obtained
after washing and precipitation process (as shown in Fig. 1).
In order to obtain detailed information of the thermal behavior
of Ag-thiolate precursor, the TGA and DTA analyses of the Ag
Sample A
Sample B
Sample C
15
15
3
200
240
240
Fig. 1. A schematic illustration of the process for preparing monodisperse Ag nanospheres.

1
958
A. Tang et al. / Journal of Solid State Chemistry 184 (2011) 1956–1962
Fig. 2. Typical TGA and DTA curves of sample A.
nanospheres are given in Fig. 2. A rapid weight loss is observed
mainly in the temperature range from 250 to 350 1C. Since the
boiling point of n-dodecanethiol is in the range 266–283 1C,
the above weight loss observed in TGA curve may be caused by
the evaporation and volatilization of n-dodecanethiol or other
organic molecules [26]. On the other hand, an endothermal peak
is observed at about 90 1C, which may be attributed to the
desorption of dodecanethiol molecules from the surface of Ag
nanospheres [27], and another endothermal peak at about 150 1C,
probably due to the reorganization of the chains of dodecanethiol
molecules. The broad exothermal range may result from the
dissociation of the ligands from the core of the nanospheres and
the evaporation of rudimental ODE.
The nucleation and growth process of the as-obtained Ag
nanospheres was characterized by using TEM images. The TEM
results of sample A extracted at different reaction time are
presented in Fig. 3. The aforementioned results indicate that the
m 25 n
intermediate polymeric precursor [Ag (SC12H ) ] was formed at
early stage through the reaction between Ag(Ac) and DDT. As
shown in Fig. 3(a), irregularly shaped crystals appeared in the
samples extracted at 20 min. In fact, some very small particles are
observed, and they are encapsulated in the organic phase. It is
believed that the accumulation of monomer species took place as
partially demonstrated by the transition of the mixture color from
yellow to dark-brown, which eventually led to nucleation of
nanocrystals upon supersaturation. After nucleation, the mono-
mer species were adsorbed onto the surface of Ag nanospheres,
promoting the progressive growth of the nanocrystals, which
Fig. 3. TEM images of sample A extracted at different reaction time: (a) 20 min;
(b) 60 min; (c, d) 100 min; (e)180 min and (f) the corresponding SAED pattern.
2
has been studied in the case of Cu S nanocrystals [24,28].
Fig. 3(b) shows the TEM images of sample A collected at 60 min,
and the Ag nanospheres are connected to a network structure
with a larger area due to the cohesive interaction of layered
thiolate molecules adsorbed on the surface of Ag nanospheres. As
m 25 n
the reaction time was prolonged to 100 min, the [Ag (SC12H ) ]
complex was partially consumed, uniform Ag nanospheres with a
mean diameter of 4.070.4 nm are obtained, as shown in Fig. 3(c).
Fig. 3(d) gives the HRTEM image of Ag nanospheres obtained at
100 min, careful inspection suggests that the lattice fringes of this
sample are clearly resolved, confirming their crystallinity. Interest-
ingly, the Ag nanospheres can be self-assembled into hexagonal
close-packed arrays with one particle surrounded symmetrically by
the other six particles. Further prolonging the reaction time to
Fig. 4. Temporal evolution of UV–vis absorption spectra of sample A.
1
80 min allowed the average diameter of the Ag nanospheres to
UV–vis spectroscopy was employed to monitor the optical
property of the Ag nanospheres during their nucleation and growth
process. Fig. 4 shows the temporal evolution of the absorption
spectra for sample A. For the sample A obtained at 20 min, an
absorption band at 348 nm is observed, which may be related with
increase from 4.070.4 nm (100 min) to 5.470.4 nm (180 min), but
the shape of the samples still remained in spherical, as depicted in
Fig. 3(e). The corresponding SAED pattern of sample A obtained at
180 min has been performed, which is given in Fig. 3(f). All the
diffraction rings can be assigned to (1 1 1), (2 0 0), (2 2 0) and (3 1 1)
reflections, confirming the formation of face-centered cubic silver
m 25 n
the formation of dominant presence of [Ag (SC12H ) ] complex. As
the Ag-thiolate precursor is consumed, the absorption band at
348 nm disappears for sample A collected after 60 min, indicating
the formation of monomer species. After nucleation, the growth of
Ag nanospheres makes the absorption of Ag nanospheres become
(JCPDS Card no.87-0597) [9]. It is noted that some well-resolved
diffraction spots are observed in the SAED pattern, demonstrating
the formation of metallic silver.

A. Tang et al. / Journal of Solid State Chemistry 184 (2011) 1956–1962
1959
apparent. After reaction for 100 min, an obvious absorption band at
27 nm appears, which is typical of the absorption of metallic Ag
4
nanospheres. This absorption band is attributed to the excitation of
the surface plasma resonance (SPR) of Ag nanospheres [12].
A narrow and symmetric absorption peak implies that the size of
the Ag nanospheres is very uniform [9].
In order to get a molecular level understanding of the capping
ligands on the nanocrystal surface, FTIR spectroscopies of samples A
and C were measured, which are shown in Fig. 5. It is found that
ꢀ
1
both samples have two sharp bands at 2916 and 2848 cm due to
asymmetric methyl stretching vibration and the asymmetric and
symmetric methylene stretching vibration from the DDT molecule
ꢀ
1
[
28]. The band at 1467 cm can be assigned to methylene scissor-
ꢀ
1
ing of DDT [29]. The absence of the band at 2577 cm arising from
the S–H vibrations of DDT suggests the deprotonation of DDT, and
this indicates that DDT may serve as the capping ligand for the
growth of Ag nanospheres [25,28].
XPS analysis was employed to detect the oxidation state and
composition of the as-obtained Ag nanospheres. The XPS survey
spectrum, Ag 3d signals and S 2p signals of sample A are shown in
Fig. 6(a)–(c), respectively. The XPS survey spectrum shown in
Fig. 6(a) confirms the formation of Ag nanospheres and reveals
the presence of thiolate ligands on the surface of the as-obtained
Ag nanospheres. The peak position and peak to peak separation
(about 6 eV) are the judgement standard for indentifying the
oxidation state of Ag [29]. As shown in Fig. 6(b), the core levels of
Ag 3d5/2 and Ag 3d3/2 located at 373.8 and 367.8 eV correspond to
an Ag(0) oxidation state, which is in agreement with those of
previous reported values [29,30]. In Fig. 6(c), the S 2p core level
signal can be resolved into two peaks at 162.6 and 161.3 eV,
which are assigned to the binding energy of S 2p1/2 and S 2p3/2
,
respectively. This result matches well with the values for chemically
bound thiolate sulfur in previous reports [31,32]. The absence of the
peak above 163 eV excludes the possibility of the presence of free
alkanethiol, disulfides and oxidized sulfur [29,32].
XRD patterns were used to determine the atomic structure of
the final product, which may further confirm the formation of Ag
nanospheres. Representative XRD patterns of Ag nanospheres
(
sample A and sample B) are given in Fig. 7. For both of the
samples, the four diffraction peaks correspond to (1 1 1), (2 0 0),
2 2 0) and (3 1 1) reflections, indicating that the formation of Ag
(
particles with face-centered cubic structure (JCPDS Card no.87-0597,
˚
lattice parameter a¼4.0862 A) [14]. This result matches well with
2 2
the SAED results. In addition, no other species, such as Ag S or Ag O,
is observed in the as-obtained samples. The above XPS analysis and
the XRD results thus clearly prove the formation of Ag nanospheres
with the surface passivation of DDT.
According to previous reports, the nanocrystal size and shape
can be controlled using the following parameters as the variables:
reaction temperature, reaction time and ratio of ligand to metal
Fig. 6. Representative XPS spectra of Ag nanospheres (sample A): (a) survey
spectrum; (b) Ag 3d core level and (c) S 2p core level with the experimental result
(solid line) and the fitting result (dotted line).
Fig. 5. FTIR spectra of sample A and sample C.
Fig. 7. XRD patterns of sample A and sample B.

1
960
A. Tang et al. / Journal of Solid State Chemistry 184 (2011) 1956–1962
salt [24,28,31]. Fig. 8 presents the TEM images and the corre-
sponding size distribution histograms of the as-obtained Ag
nanospheres by changing the reaction temperature and the ratio
of Ag(Ac) to DDT. As the reaction temperature was raised from
200 to 240 1C while all other reaction conditions were kept the
same, the mean size of the as-obtained Ag nanospheres was
increased from 4.070.2 nm (sample A) to 4.870.3 nm (sample
B). The particles display a narrow size distribution and an overall
spherical shape. This indicates that the reduction of the
Ag-thiolate precursors and the growth of Ag nanospheres can be
accelerated by increasing the reaction temperature, which is
characteristic for a decomposition reaction. At a higher tempera-
ture, the stability of the intermediate complexes is decreased,
which provides more monomer species to enhance the rates of
both nanocrystal nucleation and growth [33,34]. On the other
hand, increasing the temperature may weaken the binding of the
surfactants to the surface of the nanocrystals, which may quicken
the growth of the nanocrystals [24]. In addition, when the
reaction temperature was kept at 240 1C, the amount of Ag(Ac)
was decreased to 20% of that used for sample B, the mean size of
sample C was increased to 8.470.5 nm with a narrow size
distribution (o10%), but the spherical shape was still unchanged.
This suggests that the amount of the Ag(Ac) present in the
solution is important for controlling the size of Ag nanospheres.
Therefore, the size of the as-obtained Ag nanospheres can be
easily controlled by varying the reaction conditions.
The self-assembly behaviors of noble-metal nanocrystals were
reported before, in which the noble-metal nanocrystals could be
self-assembled into highly ordered superlattice structures due to
the narrow size distribution or the cohesive interaction through
interdigitation or interpenetration of the capping ligand [35–41].
With respect to the Ag nanospheres in this work, almost perfect
3D superlattices were also observed by slow evaporation of
carrier solvent from a dense solution, which was deposited onto
carbon-coated Cu grids. Fig. 9 displays the high- and low-resolu-
tion TEM images of Ag nanocrystal superlattices (sample B). As
shown in Fig. 9(b), irregularly shaped 3D superlattices with clearly
defined edges are observed, and the domain size of the superlattice
structures can reach up to several micrometers. Fig. 9(a) and (c)
represent the magnified images of the superlattice edge in Fig. 9(b).
This nanocrystal superlattice is remarkable in the perfect arrange-
ment of monodispersed Ag nanospheres [38]. As stated in previous
reports, the presence of excess alkylthiol played an important
role in the Ag or Au nanocrystal superlattice formation due to
the cohesive interaction through interdigitation or interpenetration
of alkyl chains of thiol molecules [21,22]. Herein, the adjacent
Ag nanospheres are separated by a distance of less than 3 nm
estimated from TEM results in Fig. 9(c). The length of alkanethiol
Fig. 8. TEM images and the corresponding size distribution histogram of Ag
nanospheres with different reaction conditions at 100 min: (a, b) sample A; (c, d)
sample B; and (e, f) sample C.
Fig. 9. TEM images of typical superlattice structures of sample B obtained at 100 min: (b) a low-magnification image, and (a, c) magnified images of superlattice edges
from (b).

A. Tang et al. / Journal of Solid State Chemistry 184 (2011) 1956–1962
1961
Ag-thiolate precursor forms at the early stage of the reaction and
its decomposition leads to further nucleation and growth of Ag
nanospheres. During this process, the absorption spectra were
employed to monitor the growth of Ag nanospheres. The size of
the Ag nanospheres can be tuned by simply changing the experi-
mental diameters such as reaction temperature, reaction time and
the amount of Ag(Ac) added. The resultant Ag nanospheres can be
self-assembled into highly ordered superlattice structures, which is
independent on the particle size. The bundling and interdigitation of
thiolate molecules adsorbed on the surface of Ag nanospheres may
play an important role in the formation of superlattice structures of
Ag nanospheres. This synthetic procedure is simple and highly
reproducible, which may extend to prepare other inorganic func-
tional nanocryatal. Furthermore, the resulting nanocrystals prepared
using this method can be produced in a large scale, and it is
important for industrial applications.
Acknowledgments
This work was partly supported by NSFC projects (60954001,
60825407, 60736034, 61076009 and 50990064), ‘‘973’’ projects
(
2010CB933800), Program for NCET (08-0717) and the ‘‘111’’
Project (No.B08002). S.Q. thanks the research fund from Key
Laboratory of Photochemical Conversion and Optoelectronic
Materials, TIPC, CAS. A.T. is also grateful to China Postdoctoral
Science Foundation (201003148 and 20090460499).
Appendix A. Supporting information
Fig. 10. TEM images of superlattice structures of (a, b) sample A and (c) sample C
obtained at 100 min.
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.05.041.
molecule can be evaluated from the following empirical equation: l
(
nm)¼0.25þ0.127n, where n is the number of –CH
2
– unit (for
References
DDT, n¼12), thus the length of DDT molecule is calculated to be
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on the surface of two adjacent nanospheres interdigitate with each
other, which contributes to the formation of superlattice structures
of the Ag nanospheres [21]. For samples A and C, the Ag nano-
spheres can also be self-assembled into highly ordered multi-
layered superlattice structures by adjusting the nanocrystal
concentration and the speed of the solvent evaporation, and the
TEM images for typical multilayered superlattice structures of
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the other hand, the synergistic effects of the steric repulsions, the
van der Waals attraction and directional dipolar interactions are
not excluded in the formation of highly ordered superlattice
structures.
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