A general strategy for metallic nanocrystals synthesis in organic medium

Article abstract of DOI:10.1039/c0cc00307g

Herein we report a general synthesis strategy for metallic nanocrystals in a two-phase liquid-liquid system, which involves a quite fast nucleation stage overlapped with the growth stage within 10 min.

Full text of DOI:10.1039/c0cc00307g

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A general strategy for metallic nanocrystals synthesis in
organic mediumw
Peng Huang,a Jing Lin,b Zhiming Li,a Hengyao Hu,a Kan Wang,a
Guo Gao,a Rong Hea and Daxiang Cui*a
Received 10th March 2010, Accepted 4th May 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/c0cc00307g
Herein we report a general synthesis strategy for metallic
nanocrystals in a two-phase liquid–liquid system, which involves
a quite fast nucleation stage overlapped with the growth stage
within 10 min.
protocol for transferring metal ions from water to an organic
medium, which involves mixing a two-phase system containing
metal salts with an aqueous solution of sodium oleate (SO),
and then extracting the metal ions into an organic layer
(toluene, hexane or other non-polar solvents). It was success-
fully applied towards the synthesis of a variety of metallic
nanocrystals. Compared with other general approaches, this
protocol allowed nanocrystals to be synthesized in an organic
medium using aqueous soluble metal salts as starting materials,
which are relatively inexpensive and easily obtained. It was
easily extended to synthesize a large scale of nanomaterials for
industrial production and application.
The development of efficient methods for the controlled
synthesis of nanocrystals with monodispersity, stability, and
predictable morphology is one of the key goals in the advance-
ment of nanoscience and nanotechnology.1 Over the past
20 years, great efforts have been devoted to the development
of various synthesis methods for preparing monodisperse
nanocrystals. Recently, a two-phase approach has become a
promising technique to challenge this key scientific problem.2
Compared with other methods,3 it has the following advantages:
(1) Reaction of molecular precursors were spatially separated
in the organic and aqueous phases; (2) The nucleation stage of
nanocrystals overlapped with the growth stage only occurring
at the liquid–liquid interface; (3) Product prepared in organic
media can be easily self-assembled into close-packed hexa-
gonal monolayers on solvent evaporation for various applica-
tions. To date, the two-phase approach has been exploited
for the synthesis of a series of metallic, alloy, semiconductor
and semiconductor–metal hybrid nanoparticles.2,4 How-
ever, in the case of synthesizing metallic nanocrystals, the
two-phase approach is not quite successful.5 Further develop-
ments of the two-phase synthesis for metallic nanocrystals are
desirable.
Our strategy is based on a general phase transfer and
separation mechanism occurring at the interfaces of liquid–
liquid systems without the separation of nucleation and growth
processes, as shown in ESI Scheme 1.w The whole reaction
goes on at room temperature and only needs about ten
minutes. The concrete synthesis process consists of two steps:
the first step is that sodium oleate is added into a vigorously
stirring two-phase system with metal cations in the aqueous
layer, two immiscible phases in the presence of the surfactant
reverse micelles are formed, which needs about 3 min, as soon
as the color of the aqueous phase becomes weak, the metal
cations are quantitatively removed from this phase. The
second step is the reduction of metal cations within reverse
micelles by adding hydrazine hydrate (N2H4ÁH2O). The source
of the electron, N2H5+, is also captured by free surfactant
molecules that are floating at the interface. The reduction of
metal cations starts at the aqueous cores of reverse micelles, as
a result of thermal Brownian motion. This step takes about
5 min to ensure that the reduction reaction is completely
finished.
Metal nanoparticles such as silver and copper have attracted
much attention because of their unique physical and chemical
properties, and their tremendous applications in the area of
catalysis, optical, magnetic, thermal, electronic devices, sensoric
devices and SERS (surface enhanced Raman scattering).6 Such
properties and applications strongly depend on their size, size
distribution and morphologies. However, the design and
synthesis of nanostructured materials with controlled properties
remain a significant challenge. Herein we report a general
Sodium oleate plays many important roles throughout this
process. It is firstly used as a phase-transfer agent, the Ag+,
Cu2+, Ni2+, Co2+, Fe3+, or Fe2+ ions in aqueous solution
are firstly extracted into toluene. Then, the extracted metal
ions are reduced into a critical composition of atomic species
by rapidly adding hydrazine hydrate. Because metal atoms
have a high surface activity, the extractant SO also acts as a
capping agent to modify the surface of newly generated
particles and to coordinate their carboxyl end groups, the
hydrophobic carbon tails of the sodium oleate were pointed
outwards from the surface of the synthesized particles. In
addition, SO acts as a surfactant to form fluctuating structures
that were postulated to behave as reactors or templates
during synthesis. Meanwhile, charged surfactants often form
complexes with the metal salt precursors, which may affect
a Department of Bio-Nano Science and Engineering, Key Laboratory
for Thin Film and Microfabrication Technology of Ministry of
Education, National Key Laboratory of Micro/Nano Fabrication
Technology, Research Institute of Micro/Nano Science and
Technology, Shanghai Jiao Tong University, Shanghai 200240,
China. E-mail: dxcui@sjtu.edu.cn
b College of Chemistry, Chemical Engineering and Biotechnology,
Donghua University, Shanghai 201620, China
w Electronic supplementary information (ESI) available: Experimental
details, characterization and part results and discussions. See DOI:
10.1039/c0cc00307g
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reduction kinetics. SO’s function in this process was generally
proposed on the basis of the following reactions.
necessary. Further studies on the quality of obtained metallic
nanocrystals were implemented. The temperature of the ripening
reaction was determined at B150 1C, only contraposing the
organic phase (including the formed product) with using an
autoclave, the time of extraction is adjustable for accurately
controlling the size of the as-prepared nanocrystals.
Mn+
+ nC17H33COOÀ
(toluene)
(aq.)
- M(C17H33COO)n(toluene)
(1)
(2)
+
+ 5OHÀ
(aq.)
TEM images of Ag nanocrystals after heat treatment for 2 h
are shown in Fig. 2a and b, which indicates the Ag nano-
particles are spherical with a narrow size distribution (B6.5 nm).
Compared with Fig. 1e, the tiny nanoparticles disappeared
and are consumed by larger particles during the growth
process. The inset is the corresponding SAED pattern in an
area including many Ag nanocrystals, revealing that the
nanocrystals are polycrystalline. The observed diffraction
rings can be indexed as (111), (200), (220) and (311) reflections,
respectively. Fig. 2c shows the UV-vis absorption spectra of
Ag nanocrystals before (1) and after heat treatment (2) in the
300–700 nm wavelength range, using toluene as the solvent.
Curve (1) has a special absorption peak at 412 nm. Curve (2)
exhibits no obvious shift of absorption peak position and
significant enhancement of absorption at 412 nm. The absorp-
tion intensity and half-peak width are both correlated to the
particle size of the nanomaterials. According to this principle,
we can conclude that the size of the latter is bigger than the
former, which is consistent with TEM image results. Fig. 2d is
the XRD pattern of Ag nanocrystals after heat treatment. The
characteristic (111), (200), (220) and (311) peaks could be
indexed as the face-centered cubic silver, which are in agree-
ment with the standard card (JCPDS: 04–0783).
nM(C17H33COO)n(toluene) + N2H5
(aq.)
- nM-(C17H33COO)n(toluene) + N2 + 5H2O
A variety of metallic nanocrystals were obtained in a liquid–
liquid system at room temperature. Typical reaction nodes
were recorded by a camera shown in Fig. 1. From a to d, they
respectively corresponded to Ag, Cu, Ni and Fe. The whole
reaction was intentionally recorded into three nodes. Before
adding the surfactant, the systems display two layers clearly.
The lower color layer is the water phase containing metal ion
and the upper colorless layer is the toluene phase. After adding
the surfactant, the whole system turns into a consistent color
without distinct layers. After the reducing agent was added,
the system comes back to two layers. The lower layer turns
clear and the upper layer contains a color, different from the
former color of the metal ions. Fig. 1e–f show transmission
electron microscope (TEM) images of typical samples of Ag,
Cu, Ni, and Fe nanocrystals and indicates the large quantity
and small size (o10 nm). The Ag nanocrystals are usually in
round shapes with smooth surfaces, and self-assemble into
ordered two-dimensional (2D) arrays on the surface of the
TEM grid (Fig. 1e).
As shown in Fig. 1e–f, the metallic nanocrystals are approxi-
mately spherical with an average diameter of 6.2 Æ 1.6, 3.2 Æ 0.6,
4.4 Æ 0.8 and 2.5 Æ 0.5 nm, estimated by measuring 200
randomly selected particles in enlarged TEM images, corres-
ponding to Ag, Cu, Ni and Fe, respectively. Thorough high
resolution (HR) TEM characterizations revealed the highly
crystalline nature of Ag and Cu nanocrystals (see supple-
mentary data Fig. 1 and 2w). The histograms of Ag and Cu nano-
crystals show the particle size distribution fitted by a Gaussian
curve (see supplementary data Fig. 3 and 4w), but they are quite
broad, which are identical with the references.5 Therefore, the
quality of obtained metallic nanocrystals should be substantially
improved.
TEM images of Cu nanocrystals after heat treatment for 10 h
are shown in Fig. 3a and b, which indicate the Cu nano-
particles are spherical with a narrow size distribution (B10 nm).
Compared with Fig. 1f, the tiny nanoparticles disappeared
and are consumed by larger particles during the growth
process. The inset is the corresponding SAED pattern in an
area including many Cu nanocrystals, revealing that the
Based on Ostwald ripening (also referred to as coarsening)
theory,7 smaller particles are essentially consumed by larger
particles during the growth process. Post-treatment at a high
temperature to control size and promote crystallization is
Fig. 2 (a) Low magnification TEM image of Ag nanocrystals after
heat treatment and corresponding electron diffraction pattern (inset).
(b) High magnification TEM image, (c) UV-vis spectra of Ag nano-
crystals before (1) and after heat treatment (2), and (d) XRD pattern
of Ag nanocrystals after heat treatment.
Fig. 1 Photographs showing typical reaction nodes in toluene–water
system (a.Ag; b.Cu; c.Ni; d.Fe) and TEM images of metallic nano-
crystals (e.Ag; f.Cu; g.Ni; h.Fe)
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nanocrystals. Our results lay the foundation for industrial
production and application of the metal nanoparticles.
This work is supported by China National 973 Project
(No. 2010CB933901), National 863 Hi-tech Project (2007AA0-
22004), National Natural Scientific Fund (No. 20771075 and No.
20803040), New Century Excellent Talent of Ministry of Educa-
tion of China (NCET-08-0350), Shanghai Science and Technology
Fund (No. 10XD1406100 and No. 072112006-6).
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Fig. 3 (a) Low magnification TEM image of Cu nanocrystals after
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(b) High magnification TEM image, (c) UV-vis spectra of before (1)
and after heat treatment (2), and (d) XRD pattern of Cu nanocrystals
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absorption spectra of Cu nanocrystals before (1) and after
heat treatment (2) in the 200–700 nm wavelength range. Curve
(1) has a special absorption peak at 280 nm. Curve (2) exhibits
no obvious shift of absorption peak position and significant
enhancement of absorption at 280 nm. Fig. 3d is the XRD
pattern of Cu nanocrystals after heat treatment, the charac-
teristic (111), (200), (220) and (311) peaks are typical of Cu2O
crystallinity.
In addition, we also used the above-mentioned method to
synthesize large scale nanoparticles such as Ag, Cu, Ni, Fe and
Co nanoparticles with the amounts of 5 g with a yield of >95%
(see supplementary data Fig. 5 and 6w). Based on the synthesis
of metallic nanocrystals, we also successfully prepared alloy,
semiconducting, and magnetic nanocrystals in supplementary
data Fig. 7.w
In summary, we established a simple, general, and con-
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nanocrystals with a narrow size distribution such as Ag, Cu,
Ni, Fe and Co nanoparticles. A possible mechanism for
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4802 | Chem. Commun., 2010, 46, 4800–4802