Li et al.

Nanoscale Research Letters 2012, 7:612

http://www.nanoscalereslett.com/content/7/1/612

NANO EXPRESS Open Access

Large-scale synthesis and self-organization of

silver nanoparticles with Tween 80 as a reductant
and stabilizer
Hui-Jun Li1,2, An-Qi Zhang3, Yang Hu1,2, Li Sui4, Dong-Jin Qian1,2 and Meng Chen1,2*

Abstract
Tween 80 (polysorbate 80) has been used as a reducing agent and protecting agent to prepare stable
water-soluble silver nanoparticles on a large scale through a one-pot process, which is simple and environmentally
friendly. Silver ions can accelerate the oxidation of Tween 80 and then get reduced in the reaction process. The
well-ordered arrays such as ribbon-like silver nanostructures could be obtained by adjusting the reaction conditions.
High-resolution transmission electron microscopy confirms that ribbon-like silver nanostructures (approximately 50
nm in length and approximately 2 μm in width) are composed of a large number of silver nanocrystals with a size
range of 2 to 3 nm. In addition, negative absorbance around 320 nm in the UV-visible spectra of silver
nanoparticles has been observed, probably owing to the instability of nanosized silver colloids.
Keywords: Silver, Nanoparticles, Tween 80, Discrete dipole approximation (DDA)

Background preparation process, silver nanoparticles can be modified

In recent years, nanoparticles (NPs) of noble metals have and stabilized by surfactants or polymers [23-25]. Surfac-
attracted considerable particular attention due to their tants are a kind of organic compounds possessing both
unique physical and chemical properties different from hydrophilic polar groups and hydrophobic nonpolar
those of bulk substances [1-5]. Among those noble metal groups. Under certain conditions, the surfactant molecules
nanoparticles, silver (Ag) NPs have been mostly studied can form into various structures of ordered agglomera-
owing to their high electrical and thermal conductivity tions such as micelles, reversed micelles, microemulsions,
as well as their extraordinary optical properties, which vesicles or liquid crystals [26,27] whose microenvironment
lead to extensive application in the fields of catalysis, can serve as microreactors and templates for the control-
electronic and optical materials [6-10]. However, the lable formation of nanomaterials [28-30]. On the other
applications mentioned above depend on to a great ex- hand, the surface of nanocrystals is composed of different
tent the properties of Ag NPs especially on the particle lattices, and different lattices may have different growing
shape, size, and size distribution [11-13]. rates on its vertical direction. Some kinds of surfactants
There have been some shape-controlled synthetic meth- can stabilize certain lattice faces by hindering their growth,
ods of silver NPs including template-assisted methods, and the proportion of the faster ones will gradually de-
seed-mediated methods, polyol routes, and so on [14-20]. crease until all will disappear, making most of the surface
Different silver nanostructures such as spheres, hollow of the nanocrystals be composed of the more slowly grow-
spheres, cubes, wires, rods, tubes, and plates would dis ing ones [31,32]. Surfactant molecules are also capable of
play different optical phenomena [21,22]. During the improving the stability of system through static electricity
repulsive forces, steric hindrance, and Van der Waals force
* Correspondence: chenmeng@fudan.edu.cn by absorbing onto the surface of nanomaterials [33,34].
1
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, In most of the previous works, surfactants only acted
Department of Chemistry, Fudan University, Shanghai 200433, People’s
Republic of China
as stabilizers or protecting agents. Actually, many
2
Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s nonionic surfactants, such as poly-(10)-oxyethylene
Republic of China oleyl ether (Brij 97, Sigma-Aldrich Co., MO, USA)
Full list of author information is available at the end of the article

© 2012 Li et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Li et al. Nanoscale Research Letters 2012, 7:612 Page 2 of 13
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[35], polyoxyethylene-(20)-sorbitan monooleate (Tween appropriate amount of AgNO3 in 0.2 mL of water before
80, Sigma-Aldrich Co.) [35], polyoxyethylene tert- mixing. The solid ones are designated as the dry sys-
octylphenyl ether (Triton X-100, Sigma-Aldrich Co.) tems, and the aqueous solution ones the wet systems.
[36], can function as reductants to synthesize noble No stirring but shaking was necessary to homogenize
metal nanoparticles. Premkumar et al. [37] had used the solution.
Tween 80 as a reductant to react with a gold salt at The mixture was then kept at different temperatures
room temperature and synthesized well-dispersed gold with different reaction times. When the reaction started,
nanoparticles. Luo et al. [38] reported that silver nano- the color of the solution turned from yellow to orange,
particles were prepared under mild conditions by and finally to dark red-brown, which indicated the for-
exploiting poly(ethylene glycol) as a reducing agent at mation of silver nanoparticles.
temperature (>17°C). Debnath et al. [39] showed a After the reaction mixture has cooled to room
solid-state high-speed vibration milling method for the temperature, a large amount of absolute ethanol was
synthesis of Ag NPs, in which poly(vinylpyrrolidone) poured into the mixture, which was centrifuged
(PVP) functioned as a reductant. Therefore, using at 10,000 rpm for 20 min. The resulting pellet was dis-
those surfactants or polymers with reductive properties persed in ethanol by gentle bath sonication, and the
for the synthesis of metal nanoparticles is not only suspension was centrifuged again. The dispersion-
theoretically possible but also practical. centrifugation process was repeated three times to wash
In this study, we have synthesized well-dispersed off the surfactants and the remaining residues. In the
water-soluble silver nanoparticles using polysorbate 80 final step of centrifugation, the final pellet was dried and
(Tween 80) as both the reducing agent and the protect- then dissolved in deionized water.
ing agent, and performed a systematic study on the for-
mation process of silver nanoparticles, thus, produced. A Instruments
trace of water in the system is very important to the The optical properties of silver particles were monitored
homogeneity and dispersity of obtained Ag NPs. By by UV-vis spectroscopy. The UV-vis absorption spectra
changing the temperature, new arrays such as network- were taken at room temperature on a UV-3150 spectro-
and ribbon-shaped self-organization have been observed. photometer (Shimadzu, Kyoto, Japan) with a variable
In addition, we consider that the formation mechanism wavelength between 200 and 800 nm using a glass cu-
of Tween 80-stabilized Ag NPs be similar to the oxida- vette with a 1-cm optical path. A UV/visible spectrum
tion of nonionic surfactants reported by Currie et al. diode-array spectrophotometer (Model HP 8453; Agilent
[40]. The initial step in the oxidation of Tween 80 is the Technologies, Palo Alto, CA) was also employed for
formation of a free radical in α-position to the ether oxy- some specific samples. All of the UV-vis absorption
gen. Silver ions can accelerate the oxidation process and spectra in this paper, except those mentioned otherwise,
silver nanoparticles slowly form afterwards [40]. were recorded on a UV-3150 spectrophotometer.
The one-pot synthesis is clean and green, and the prep- The selected area electron diffraction (SAED) pattern,
aration is simple with an easy operation. Tween 80 can act elemental analysis, and transmission electron micro-
as both a reducing agent and a stabilizer without adding scope (TEM) images were acquired on a JEOL 2100F
any additional reducing agents into the reaction. More- microscope (JEOL Co., Ltd., Tokyo, Japan) operating at
over, the method can provide a high concentration of an accelerating voltage of 200 kV. All TEM samples
nanosized silver colloids; thus, it may be developed into were made using aqueous colloids of the metal nanopar-
industrial mass production of noble metal nanoparticles. ticles directly without size selection, which were depos-
ited onto a 230-mesh copper grid covered with Formvar.
Methods The Fourier transform infrared (FT-IR) spectra were
Materials collected on an IRPRESTIGE-21 FT-IR spectrophotom-
Polysorbate 80 (Tween 80) and AgNO3 were purchased eter in the wavenumber range of 400 to 4,000 cm−1 at a
from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, resolution of 4 cm−1. The samples were prepared in the
China) and used without further purification. Deionized form of pellets together with KBr. The X-ray powder
water was used throughout the work. diffraction (XRD) pattern was recorded using a Rigaku
D/max γB-ray diffractometer (Rigaku Corporation,
Synthesis of silver nanoparticles Tokyo, Japan) in transition mode and Cu K radiation
In a typical experiment, silver particles were prepared by (γ = 1.54056 Å). Samples for measurement were prepared
simply mixing AgNO3 in a state of solid or an aqueous by dropping silver colloids on quartz plates and allowing
solution with Tween 80. In all systems, the fixed amount them to dry at 40°C. The X-Ray photoelectron spectros-
of Tween 80 was 2 mL. In the aqueous solution, the sil- copy (XPS) was performed on a ESCALAB MKII X-ray
ver salt solutions were prepared by dissolving an photoelectron spectrometer (VG Instruments, CA, USA)
Li et al. Nanoscale Research Letters 2012, 7:612 Page 3 of 13
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using non-monochromatized Mg-Kα X-rays as the excita- In the present study, the binding energies of Ag 3d for
tion source. The binding energies for the samples were the Tween 80-stabilized Ag nanoparticles are between
calibrated by setting the measured binding energy of C 1s those for metal silver (368.2 eV for Ag 3d5/2 and 374.2
to 284.60 eV. Particle size distribution (PSD) analysis was eV for Ag 3d3/2) and for silver (I) oxide (367.5 eV for
carried out by manually digitizing the TEM image with Ag 3d5/2, 373.5 eV for Ag 3d3/2) [44]. In our opinion,
Image Tool from which the average size and standard de- this implies the strong coordination of Ag atoms with
viation of metal nanoparticles were generated. the oxygen atoms of the carbonyl (C=O) groups in the
Tween 80 or oxidized Tween 80 chains. Actually Tween
80 molecules cannot provide enough carbonyl groups to
Results and discussions interact with Ag nanoparticles, while the oxidized Tween
Characterization and study of Tween 80-stabilized silver 80 should contain a sufficient number of carbonyl
nanoparticles groups, which further confirms the silver ions-mediated
The XRD pattern of as-prepared nanoparticles (shown oxidative degradation of Tween 80.
in Figure 1) confirms the formation of Ag nanoparticles The peak in the C 1s spectrum (shown in Figure 2c)
with the cubic close packed (fcc) type, as in the bulk me- can be fitted to several symmetrical peaks with binding
tallic Ag. Five well-resolved broad peaks, which can be energies of 284.6, 286.1, and 288.7 eV, which are
indexed as (111), (200), (220), (311), and (222) diffrac- assigned to the C 1s of the adventitious reference hydro-
tion peaks, are shown (JCPDS No. 4–783). The peak at carbon, that of carbon –C–O–, and that of carboxyl car-
2θ = 69.89° belongs to the supporting silicon substrate. bon–O–C=O, respectively [45,46]. The peak of O 1s
XPS studies have been carried out to study the chem- (Figure 2d) has also been deconvoluted into two bands
ical composition of Tween 80-stablized silver nano with binding energies in agreement with those reported
particles. Figure 2 shows the XPS spectra of obtained sil- in the literature [41]. One of the O 1s curves located at
ver nanoparticles. The survey spectrum (shown in 532.7 eV should possibly be attributed to a combination
Figure 2a) reveals the high content of C and O, owing to of the backbone –CH2–O–CH2–[47], plus the oxygen in
the surfactant molecules attached to the surface of the the carboxyl group (−C=O–) which interacted with sur-
nanoparticles and absorbed gaseous molecules such as face of the silver nanoparticles [41].
oxygen and carbon dioxide [25]. Figure 3 shows the FT-IR spectra of Ag NPs obtained
Higher resolution spectra of the Ag 3d region in from the system added with 70 mg of AgNO3 and the
Figure 2b showed that the binding energies of Ag standard spectrum of neat Tween 80. A comparison of
3d5/2 and Ag 3d3/2 of the silver nanoparticles are panels a and b of Figure 3 confirms that Tween 80 is
367.8 and 373.8 eV, respectively, with a spin-orbit sep- absorbed on the surface of Ag nanoparticles, as both of
aration of 6.0 eV. These results are essentially the the fingerprint absorptions are nearly identical in both
same as those reported for the PVP-stabilized Ag spectra. As shown in the spectrum of the sample, the
nanoparticles, which indicate a strong interaction be- strong O–H and –CH2 stretching vibrations are repre-
tween the carboxyl oxygen atoms in the PVP chain sented at 3,432, 2,925 (asymmetrical stretch), and 2,863
and silver nanoparticles [41-43]. cm−1 (symmetrical stretch), respectively [48-50]. The ab-
sorption at 1,458 and 1,351 cm−1 can be attributed to
the symmetrical and asymmetrical bending vibrations of
–CH3. The bands at 1,637 to approximately 1,654 cm−1
should belong to the stretching vibrations of C=C or the
O–H bending mode [48-50]. The sharp and symmetric
characteristic absorption peak at 1,735 cm−1 is due to
the stretching vibration of C=O in the ester carbonyl
group [48]. The absorption peak at 1,105 cm−1 is the
stretching variation of C–O–C, and the 1,249 cm−1 is
from the ester group.
In agreement with the XPS observation, the inter-
action of C=O group with the surface of Ag nanoparti-
cles can be further proved by the FT-IR analysis. The
relative intensity of the peaks (after background correc-
tion and normalization relative to the lowest value
around 1,105 cm−1) assigned to C=O in the sample
Figure 1 XRD patterns of the samples prepared from the
decreases significantly, possibly implying that the oxygen
mixtures. The mixtures include 50 and 100 mg AgNO3, respectively.
in the carbonyl group along the long Tween 80 chains
Li et al. Nanoscale Research Letters 2012, 7:612 Page 4 of 13
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Figure 2 XPS spectra of the Tween 80-stablized silver nanoparticles. (a) The survey spectrum and the close-up spectra of (b) Ag 3d, (c) C 1s,
and (d) O 1s. Red and green lines represent deconvolution of the spectra into Gaussian bands.

can provide coordinative saturation of dangling bonds

on the surface of the silver nanoparticles, favoring the
stability of the Ag nanoparticles in water. Similar result
has already been reported for Tween 80 interacting with
other kinds of nanoparticles in which the decrease of the
relative band intensity and even the disappearance of the
C=O absorption peak around 1,735 cm−1 have been
observed [49,50].
Based on the above analysis, as well as additional rele-
vant literature [35,37,40], we consider that the possible
reduction mechanism is related to the Ag+-medicated
oxidation of Tween 80. Tween 80, a kind of non-ionic
surfactant derived from polyethoxylated sorbitan and
oleic acid, is often used as emulsifier in food and
pharmacological applications. Its hydrophilic groups are
polyether known as polyoxyethylene (−CH2–CH2–O–)
Figure 3 FT-IR spectra. Ag NPs obtained from the system added chains, which are capable of reducing metal ions to
with 70 mg AgNO3 and pure Tween 80.
elemental metals [40,51]. As shown in Figure 4, the
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[52,53]. There are no shoulder peaks near the 350-nm

Ag+ 0 absorption peaks, indicating that no bulk silver is gener-
CH2CH2O CH2CHO + Ag
ated [38]. Similar profiles of the absorption spectra for
O the silver nanoparticles, obtained in the wet system with
Ag+
20, 50, and 70 mg of AgNO3, imply that the experimen-
CH2C O OR O2 tal preparation could be readily controllable and repro-
0
Ag + ducible. It is worth noting that the locations of the
O
absorption peaks, shown in the inset of Figure 5b, were
CH2C H essentially unchanged.
Figure 4 Schematic illustration of the proposed mechanism for It can also be found that the increase of the amount of
the formation of Tween 80-stabilized silver nanoparticles. AgNO3 in the dry systems could induce a slight widen-
ing and a red shift of the characteristic peaks of silver
(as shown in Figure 5a and the inset), indicating the de-
initial step in the oxidation of Tween 80 is the gener- velopment of larger crystalline silver nanoparticles, and
ation of a free radical in α-position to the ether oxygen, that the size of particles was not uniform. This can be
which is formed by dehydrogenation. Then, the radicals explained by the poor dispersing of silver nitrate in
thus formed can be further oxidized into esters or Tween 80. In the dry system, the solid silver salt could
degraded to form aldehydes. Noble metal ions such as not be dispersed homogeneously in Tween 80. The
Ag+ can accelerate the oxidation process and then get unreacted salt might be gathered around the new gener-
reduced. ated silver particles and reduced on their surface, which
resulted in a larger size distribution of the as-formed sil-
Concentration effects on the formation of Tween ver nanoparticles.
80-stabilized silver nanoparticles Figure 6 shows the TEM images of silver nanoparticles
In the reaction system, there are only three kinds of obtained at 100°C for 3 days in the wet system. The
reactants, AgNO3, Tween 80, and traces of water with- nanoparticles in these photos appear well dispersed over
out additional reductants. Thus, it is convenient to a large area of the substrate. The size of silver NPs is
evaluate the effect of the concentrations of the reactants mainly between 20 and 40 nm. Comparing the silver
on the products. nanoparticles obtained in the dry system (shown in Add-
Figure 5 shows the UV-vis spectra of the silver disper- itional file 1: Figure S1) with those in the wet system, it
sions obtained with different concentrations of AgNO3 can be found that the wet system produced smaller
in the dry or wet system, which exhibit similar profiles nanoparticles with narrower size distribution than the
with single narrow and sharp absorption peaks. The dry one. The silver nanoparticles obtained in the dry
420-nm absorption peaks correspond to the surface system were more irregular, and some of them were
plasma resonance absorption of silver nanoparticles joined together. While in the wet system, the silver

Figure 5 UV-vis spectra of silver aqueous solutions obtained dry and wet systems. The (a) dry and (b) wet systems at 100°C. 1, 2, 3, 4, 5
respectively correspond to the UV-vis spectra of systems added with 10, 20, 50, 70, and 100 mg of AgNO3. The inset is the diagram of
wavelength of absorption peaks versus its corresponding AgNO3 amount.
Li et al. Nanoscale Research Letters 2012, 7:612 Page 6 of 13
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Figure 6 TEM images of silver particles. These particles were synthesized in the wet systems added with (a) 10, (b) 20, (c) 50, and (d) 100 mg
of AgNO3 at 100°C for 3 days. The insets in (b) and (c) are the high-resolution transmission electron microscopic image and SAED patterns taken
from a single particle.

nanoparticles were more stable with a narrower disper- well-resolved fringe spacing (0.23 nm), which agrees well
sity. These differences can easily be explained by the rea- with the spacing of the (111) lattice places of silver
son that silver salt in the wet system can react with nanoparticles, and indicates that the nanoparticles are
Tween 80 more effectively than the dry system. In single nanocrystals. From the SAED image, we can see
addition, the AgNO3 concentrations show no significant some diffraction rings respectively corresponding to the
impact on the morphology of the formed Ag particles. different lattice of a centered cubic silver nanoparticle
Figure 7 is the corresponding PSD of the nanoparticles that is (111), (200), (220), (311), and (222).
shown in Figure 6. It is obvious that the increase of
the amount of AgNO3 could induce the decrease of Self-organization of Tween 80-stabilized silver
particle size. A larger amount of silver salts tends to nanoparticles
produce more nuclei in the first stage, and the growth A correlation between the dissolution rate of the surfac-
rate of the silver particles is more rapid [54]. The sys- tant and the temperature has already been proved to
tem with a smaller amount of AgNO3 might endure exist [55]. In order to study the effects of temperature
Ostwald ripening, leading to an increase of particle on the system, we have carried out the reactions at 90°C
size. Moreover, when mixing 100 mg AgNO3 with 2 and 110°C without changing any other conditions.
mL Tween 80, that is to say, the synthetic concentra- Figure 8 shows the UV-vis extinction spectra of aque-
tion of silver ions was up to 0.29 M, the obtained sys- ous suspensions of silver nanoparticles produced with
tem could still remain stable and yield comparatively different concentrations at different reaction tempera-
well-dispersed nanoparticles. tures. The absorption peaks of the suspension obtained
The insets are the high-resolution transmission elec- at 90°C in the wet system are wide, and the absorbance
tron microscopy image (HRTEM, shown in Figure 6b) of around 320 nm is below zero (shown in Figure 8a). We
a nanoparticle and SAED patterns (shown in Figure 6c) have repeated the process including the synthesis and
taken from silver nanoparticles. The HRTEM gives a characterization under the same conditions and always
Li et al. Nanoscale Research Letters 2012, 7:612 Page 7 of 13
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Figure 7 PSDs of silver particles. Ag particles were synthesized in the wet systems added with (a) 10, (b) 20, (c) 50, and (d) 100 mg of AgNO3
at 100°C for 3 days.

obtained the negative data for the suspension. Careful absorbance in the UV-vis absorption spectrum of the Ag
inspection reveals that the negative absorbance can nanoparticles. Li and Xia have reported a negative ab-
probably be attributed to the instability of the nanoparti- sorption of light from the cubic gold nanosystem with a
cles, which was proved by the perceptible slow precipita- gain material [56], which is different from our study.
tion during the testing process. To further address the The Ag nanoparticles, produced in the wet system
issue of the negative absorbance in this system, the UV- with 50 and 100 mg of AgNO3 at 90°C, show two ab-
vis spectra (shown in Figure 8c,d) of the same samples sorption peaks around 410 and 560 nm, respectively,
were also recorded on a diode-array spectrophotometer with a long tail at the long-wavelength side of the band
(model HP 8453, Agilent Technologies, CA, USA) with (shown in Figure 8a,c). The absorption features are in
scan rate of 100 ms. The profiles of the as-obtained UV- good agreement with the results for the aggregation of
vis spectra (Figure 8c,d) of the silver colloids are almost polymer-coated silver nanoparticles [24] or the salt-
identical to those (Figure 8a,b) acquired on a UV-3150 induced silver aggregates [57,58]. In addition, the red
spectrophotometer. However, all the intensities of the shift of the band in the long-wavelength side indicates
absorption obtained with fast scan rate of 100 ms are that the size of the silver aggregates increase with the
positive, which further implies that negative absorbance concentration of AgNO3 [57,58].
around 320 nm acquired on a UV-3150 spectrophotom- On the other hand, the absorption spectra of the
eter indicates the instability of the as-produced Ag nano- Ag nanoparticles, obtained with 50 and 100 mg of
particles. The correlation may be used as an indicator of AgNO3 at 110°C in the wet system, exhibit a single
the instability of silver colloids by varying the scan rate, sharp, symmetrical peak around 420 nm. While the
although there has been no report about negative Ag nanoparticles produced with 20 mg of AgNO3
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Figure 8 UV-vis spectra of aqueous solutions of silver nanoparticles synthesized at different temperatures. (a) and (c) 90°C, (b) and (d)
110°C. The spectra of (a) and (b) were acquired on a UV-3150 spectrophotometer, while (c) and (d) on a diode-array spectrophotometer.

show an absorption profile with very low absorbance silver particles in wet systems at different temperatures.
and similar to those observed for silver nanoparticles Some interesting arrays of particles could be clearly seen
produced at 90°C, presumably owing to the aggrega- from the images. When the temperature is 90°C, a few
tion of Ag nanoparticles. The silver nanoparticles gen- one-dimensional spindle-shaped aggregates consisting of
erated with 20 mg of AgNO3 were more instable than roughly close-packed silver nanoparticles of about 10
those obtained with high concentration of silver salt, nm could be found in Figure 9b,c. These aggregates are
which has also been observed in the silver nanoparticle relatively rigid, around 15 to 300 nm in length and 20 to
production at relative lower temperature. The observed 30 nm in width. Similar aggregates have been reported
behavior can be explained by the slow initial nucle- for the surfactant-stabilized nanoparticles, such as
ation rate. In detailed speaking, the fewer silver salts BaSO4 [59] and hematite [60]. In the present study, the
existed in the starting stage, the slower the nucleation surfactant concentration is very high; some of the surfac-
process occurred, and the fewer nuclei were generated tant aggregates or micelles may be deformed and can
in the nucleation stage. The slow nucleation rate and stay as different shapes at relatively low temperature,
the fewer nuclei result in the larger-size distribution of thus making it possible to generate different nanoparti-
Ag nanoparticles, followed by the instability of silver cle aggregates [61].
colloids. Then, in Figure 9d, a mixture of different shapes of
To better understand the effect of temperature on the particles is observed, making sense the particular profile
morphology of silver particles, the TEM characterization of its absorption peak. Moreover, when the silver salts
has been done. Figure 9 shows the TEM images of the with high concentration were used in the system at
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Figure 9 TEM images of silver nanoparticles obtained in wet systems. (a) 20, (b) 50, and (c) 100 mg of AgNO3 at 90°C; (d) 20, (e) 50, and (f)
100 mg of AgNO3 at 110°C for 3 days.

110°C, the obtained silver nanoparticles have a ten- be entirely evaporated, which finally caused the forma-
dency to self-organize into 2D well-ordered arrays. The tion of nanonetworks [62]. On the other hand, strong
formation of these nanoparticle arrays may be attribu- adsorption of dendritic-structured Tween 80 on silver
ted to the synergetic effects of interparticle, particle- nanoparticles facilitates the cohesive interaction be-
substrate, and solvent-substrate (wetting) interactions. tween the particles such as interdigitation and inter-
When the solvent began to evaporate during the sam- penetration [63], favoring the self-assembly of Ag
ple preparation for TEM characterization, the solvent nanoparticles. Similar arrays, including some hexagonal
film forming on the surface of the copper grid became close-packed arrays, of noble metal nanoparticles have
unstable, and small droplets generated. The particles been reported before [64,65].
contained in the droplets would be left behind fast at To investigate the effect of reaction time on the aggre-
the margin of the merge point after ethanol and would gate shape of silver nanoparticles and to better
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understand the evolution process of nanoparticle aggre- reaction time is longer, the silver nanoparticles would like
gates, we varied the reaction time from 1 to 4 days at to assemble into rigid spindles (shown in Additional file
90°C. The experimental results reveal that shorter time 2: Figure S2). Researchers found that the presence of a la-
could produce silver ribbons (shown in Figure 10). It is mellar liquid crystalline phase might play an important
noteworthy that many ribbons are twisted or folded, im- role in the formation of the silver nanoparticle ribbons
plying a good flexibility. There are also a few large parti- because the nanometer-sized water layers confined the
cles attached on the ribbons. Careful investigation with packing of the particles in the direction perpendicular to
HRTEM reveals that the ribbon mainly consists of a large the water layers, leading to the ribbon packing of the
number of tiny silver nanoparticles with a size range of 3 nanoparticles [66]. This conclusion is proved to be rea-
to 4 nm (Figure 10f ). The inset in Figure 10d also con- sonable by the TEM images of silver nanoparticles
firms that the nanoparticles are crystals. When the synthesized in a dry system (shown in Figure 10b).

Figure 10 TEM images of silver nanoparticles. Obtained in (a) wet systems; (b) dry system with 50 mg of AgNO3 at 90°C for 1 day. (c) to (f)
are the images with higher magnification. The insets are the ED patterns of the ribbons.
Li et al. Nanoscale Research Letters 2012, 7:612 Page 11 of 13
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A plausible formation process for the silver ribbons is axis equals to 2:1. Then, mix it with Ag-Tween 80 alloy
briefly presented as following: It is more likely that the sphere particles (Ag/Tween 80 = 8:2). Initial parameters
concentrated Tween 80 molecules aggregated into one- are set as effective radius of ellipsoid = 25 nm, effective
dimensional micelles in the presence of silver ions. radius of sphere = 10 nm, and refractive index of ambi-
When the temperature reached about 90°C, a large num- ent medium = 1.33 (H2O).
ber of tiny silver nanoparticles generated quickly around For homogeneous Ag-Tween 80 alloy particles, the di-
the micelles of Tween 80. Subsequently, the tiny silver electric constants can be calculated as follows [69-71]:
nanocrystals started to form aggregates, most of them


ordering in ribbon shape. The silver nanocrystals could ∈Alloy xAg ; ω ¼ xAg ∈Ag ðωÞ þ 1  xAg ∈Tween80 ðωÞ;
also act as the cross-linker of Tween 80 molecules,
where xAg means the Ag fraction of Ag-Tween 80 alloy;
which kept the ribbon-shaped aggregates in water after
ω means the frequency of incident light.
purification. As the reaction proceeded, some tiny parti-
The relationship between dielectric constant and re-
cles grew larger because of coalescing or Ostwald ripen-
fractive index can be described as follows:
ing. More and more discrete silver nanoparticles with
the size of 20 nm or so appeared in the products ∈ðωÞ ¼ nðωÞ2 ;
obtained in the three- or four-day reaction. Only a small
fraction of nanoparticles, which had not grown into lar- where n means complex refractive index.
ger ones, tended to form into quite rigid spindles with By simply calculating the dielectric constants of Ag-
bits of stabilizers around them (see Additional file 2: Tween 80 alloy, we can get its refractive index table as
Figure S2). the input file. Figure 11 shows the simulation results for
the silver nanoparticles obtained from the starting solu-
Theoretical studies of the dependence of the UV-vis tion with AgNO3 concentration of 20, 50, and 100 mg.
spectra upon aggregate shape and size of Tween 80- The extinction curve shows two peaks, appearing
stabilized silver nanoparticles around 400 and 500 to 700 nm, respectively. This simu-
Based on TEM data about the size of silver nanoparticles lation result corresponds well with the real UV-vis spec-
and their aggregate shape, we use computer simulations tra of the silver nanoparticles with one-dimensional
to see whether the real UV-vis spectra corresponded aggregates (shown in Figure 8a,c).
well with the theoretical calculations. The UV-vis spectra Figure 11 also shows the simulation results of extinc-
show the extinction efficiency of the obtained product. tion efficiency of the mixed system when the fraction of
The relation of extinction, scattering, and absorption ef- ellipsoid was decreased while the fraction of sphere was
ficiency factors are as follows: increased. By increasing the fraction of the sphere parti-
cles, the extinction intensity of the peak at ca. 410 nm
Qext ¼ Qsca þ Qabs : grows a lot. It illustrates the shape of the UV-vis spectra
when the amount of AgNO3 equals to 100 mg.
The simulation methods of calculating the absorption The extinction spectra in Figure 11 show more distinct
and scattering efficiency usually belong to two categor- profiles, compared with the real absorption spectra of
ies: the exact and approximated solutions [4]. In this
paper, we use DDSCAT7.2 [67], an open-source Fortran
90 software applying the discrete dipole approximation
(DDA) [68], to calculate scattering and absorption of
electromagnetic waves by targets with arbitrary geom-
etries and complex refractive index.
In our experiment, we have synthesized Ag nanoparti-
cles coated with Tween 80. According to the TEM
images of the products, some silver nanoparticles aggre-
gated to form one-dimensional spindle-like structures.
The structure was a mixture of large amount of silver
nanoparticles and organic coat, so that we may consider
it as silver-Tween 80 alloy. Besides, the solution we
obtained also contained large amount of dispersive Ag
nanoparticles and aggregated molecules of Tween 80.
Thus, we established a model this way: calculate the ex-
Figure 11 Simulated absorption curves of silver nanoparticles
tinction efficiency of a Ag-Tween 80 alloy ellipsoid (Ag/
obtained with different AgNO3 concentrations.
Tween 80 = 7:3), whose ratio of the major axis to minor
Li et al. Nanoscale Research Letters 2012, 7:612 Page 12 of 13
http://www.nanoscalereslett.com/content/7/1/612

the silver nanoparticles (shown in Figure 8a,c). We Republic of China. 2Advanced Materials Laboratory, Fudan University,
found in the simulation that if more Ag-Tween 80 alloy Shanghai 200433, People’s Republic of China. 3Department of Materials
Science, Fudan University, Shanghai 200433, People’s Republic of China.
sphere particles in the simulation model, the extinction 4
School of Medical Instrument and Food Engineering, University of Shanghai
peaks tend to be less distinct. That is because alloy parti- for Science and Technology, Shanghai 200093, People’s Republic of China.
cles have a larger complex refractive index than the am-
Received: 3 September 2012 Accepted: 25 October 2012
bient environment, water, but smaller than the complex Published: 6 November 2012
refractive index of pure silver; thus, the extinction peak
appears at the middle area. In addition, we use an ideal
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