Synthesis of highly stable dispersions of nanosized copper particles using l-ascorbic acid

Article abstract of DOI:10.1039/c0gc00772b

Highly stable dispersions of nanosized copper particles with an average particle size less than 2 nm were synthesized using a straightforward, cost-effective, and green method. Nontoxic l-ascorbic acid was utilized as both a reducing agent and capping agent precursor in aqueous medium. The copper particles were characterized by ultraviolet-visible spectroscopy, transmission electron microscopy, and Fourier-transform infrared spectroscopy The mechanism of l-ascorbic acid on the reduction and stabilization of copper nanoparticles is also discussed. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00772b

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PAPER
Synthesis of highly stable dispersions of nanosized copper particles using
L-ascorbic acid
Jing Xiong, Ye Wang, Qunji Xue and Xuedong Wu*
Received 4th November 2010, Accepted 10th January 2011
DOI: 10.1039/c0gc00772b
Highly stable dispersions of nanosized copper particles with an average particle size less than 2 nm
were synthesized using a straightforward, cost-effective, and green method. Nontoxic L-ascorbic
acid was utilized as both a reducing agent and capping agent precursor in aqueous medium. The
copper particles were characterized by ultraviolet-visible spectroscopy, transmission electron
microscopy, and Fourier-transform infrared spectroscopy The mechanism of L-ascorbic acid on
the reduction and stabilization of copper nanoparticles is also discussed.
design of synthetic routes, chemical analyses, or chemical
1. Introduction
processes.27 Utilization of nontoxic chemicals, environmentally
Metal nanoparticles have attracted much attention in nanoscale
benign solvents, and renewable materials are key issues in the
science and engineering technology over the past decade due
nanomaterials science field when considering a green synthetic
to their unusual chemical and physical properties, such as
strategy. Following this strategy, several synthetic methods
catalytic activity, and novel electronic, optical and magnetic
relying on green chemistry have so far been reported, especially
properties.1–3 Their main application areas include catalysts,
absorbents, chemical and biological sensors, optoelectronics,
information storage, and photonic and electronic devices.4–9 Var-
ious methods, such as wet chemical reduction, reverse micelles,
and electrochemical and sonoelectrochemical techniques,10–15
for noble metal nanoparticles. Raveendran et al.28 prepared
silver nanoparticles using water as a solvent, b-D-glucose as a
reducing agent, and starch as a protecting agent. Liu et al.29
synthesized gold nanocrystals using b-D-glucose as both the
reducing and stabilizing agent. Raveendran et al.30 also prepared
have been developed to synthesize metal nanoparticles because
Au, Ag, and Au–Ag nanoparticles in water, using glucose
of the diversity and importance of these applications. Among
as the reducing agent and starch as the protecting agent.
the above methods, the wet chemical reduction method has
the advantage over the others in easy control of the reaction
Nadagouda et al.31 synthesized Ag and Pd nanoparticles using
tea/coffee extract. Moulton et al.32 fabricated Ag nanoparti-
process and production rate. The wet chemical reduction method
cles by reducing silver nitrate in solutions of tea extract or
is mostly achieved by reduction of a metal ion salt solution.
epicatechin. However, there are few reports on preparation
There are three key factors in the reducing process: (i) the
of copper nanoparticles through a green method.33,34 Copper
solvent medium, (ii) the reducing agent, and (iii) the capping
nanoparticles play a crucial role in many applications such
agent. Most of wet chemical reduction methods reported to
as lubricants, catalysts, thermal transfer nanofluids, electronic
date rely heavily on organic solvents and use environmentally
materials and optical devices.35–37 Compared to other noble
and biologically hazardous reducing agents (i.e., hydrazine,
sodium borohydride, dimethyl formamide, potassium bitartrate,
metals, copper is significantly cheaper and exhibits less of a
electromigration effect when it is used in microelectronics. In
formaldehyde, sodium hypophosphite or hydroxylamine hy-
addition, stable dispersion of nanosized copper particles can
drochloride, etc).16–22
Recently, there is an increased emphasis on the topic of green
be used as a conductive ink to manufacture low-cost electronic
components by ink-jet printing, which is a kind of additive print-
chemistry, which is defined as the design of chemical products
ing process that offers many economical and environmental ad-
and processes that reduce or eliminate the use and generation of
vantages compared to traditional mask-based photolithography
hazardous substances.23–26 There are 12 fundamental principles
methods.38–40
that any attempt must be comprehensively addressed in the
In this work, a simple, environmentally friendly, and cost-
effective method for preparing highly stable dispersions of
nanosized copper particles is reported. The reaction is carried
Ningbo Institute of Materials Technology & Engineering, Chinese
Academy of Sciences, 519 Zhuangshi Road, Zhenhai District, Ningbo,
315201, China. E-mail: xdwu@nimte.ac.cn; Fax: +86-574-86685159;
Tel: +86-574-87914083
out in an aqueous medium solution and L-ascorbic acid is used
as both reducing and capping agent to prevent the aggregation
of copper nanoparticles. No other polymer dispersant is added
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to avoid any adverse effect on the performance of products as
organic residues.
2. Experimental
Materials
CuCl2·2H2O (Sinopharm Chemical Reagent Co., Ltd) acted as
the precursor for the formation of Cu nanoparticles. L-ascorbic
acid (Sinopharm Chemical Reagent Co., Ltd) acted both as
reducing agent and capping agent. Deionized water was used in
all experiments.
Preparation of dispersions of nanosized copper particles
In a typical preparation process, CuCl2·2H2O aqueous solution
was prepared by dissolving CuCl2·2H2O (10 mmol) in 50 ml
deionized water. A flask containing CuCl2·2H2O aqueous solu-
tion was heated to 80 ◦C in an oil bath with magnetic stirring. A
50ml L-ascorbic acid aqueous solution of various concentrations
(0.4, 0.6, 0.8 and 1.0 M) was added dropwise into the flask while
stirring. The mixture was kept at 80 ◦C until a dark solution
was obtained. The resulting dispersion was centrifuged at 8000
rpm for 15 min. The supernatant was placed under ambient
conditions for 2 months.
Fig.
1 The UV-Vis absorption spectra of Cu nanoparticles
stabilized in L-ascorbic acid aqueous solution with various concentra-
tions: (a) 0.4 M, (b) 0.6 M, (c) 0.8 M and (d) 1.0 M.
energies.42–44 In our work, the resulting copper dispersion did
not show a plasmon peak at around 570 nm, but displayed a
broadened peak at a short wavelength, indicating the presence
of very small separated Cu nanoparticles (the average particle
size is less than 4 nm). Based on the above results, we can infer
that a higher L-ascorbic acid concentration leads to a more
effective capping capacity of L-ascorbic acid and then smaller
Cu nanoparticles.
The dispersion became colorless when L-ascorbic acid was
added, and gradually turned to yellow, orange, brown and
finally dark (see Fig. 2). In order to clarify the reaction process,
0.2 M CuCl2·2H2O reacted with 1.0 M L-ascorbic acid, and the
UV-Vis absorption spectra were recorded every 2 h. The time
evolution of the UV-Vis absorption spectra is shown in Fig. 2.
Initially, there is no characteristic absorption peak. After 2 h
of the reaction, the absorption peak can be observed. There
is an intensity increase and red shift of the second absorption
peaks with the reaction progressing. This phenomenon is due
to the growth of copper nanoparticles. The reaction (including
the reduction reaction process and Cu NPs growth process) was
completed after 14 h as the UV-Vis absorption curves of 14 h
and 16 h almost overlapped.
Characterization
The UV-Vis absorption spectra of the produced dispersions were
recorded on a UNICOWFZ UV-2000 spectrophotometer. The
morphology and size of the as-synthesized Cu nanoparticles
were characterized by transmission electron microscopy (TEM,
Tecnai F20, 200 kV). FT-IR spectra were performed and
recorded with a Fourier-transform infrared spectrophotometer
(Nicolet 6700) between 4000 and 400 cm-1, with a resolution of
0.09 cm-1.
3. Results and discussion
UV-Vis studies on Cu nanoparticles
Nanosized particles exhibit unique optical properties having
an exponential-decay Mie scattering profile with decreasing
photon energy. Some transition-metal nanoparticles also show
a distinct surface-plasmon band.41 UV-Vis absorbance spec-
troscopy has proved to be a very useful technique for studying
metal nanoparticles because the peak positions and shapes are
sensitive to particle size. The effect of L-ascorbic concentration
on the UV-Vis absorbance spectroscopy of the synthesized Cu
nanoparticles is shown in Fig. 1. The first absorption peaks
of different curves are all at around 330 nm corresponding to
the oxidation product of L-ascorbic acid. This can be proved
by another experiment, in which H2O2 was used as oxidant
to react with L-ascorbic acid, and the product showed single
peak at around 330 nm. The second absorption peaks are
increasingly broadening with an increasing concentration of L-
ascorbic acid. The surface plasmon peak of copper nanoparticles
has been reported to be appear at around 570 nm. However,
when the particle size is less than 4 nm, the distinctive plasmon
peak is known to be broadened and replaced by a featureless
absorbance, which increases monotonically towards higher
Size and morphology analyses
Transmission electron microscopy (TEM) was used to study
the morphology and the size distribution of the collected
particles. The typical TEM images of the as-synthesized copper
nanoparticles are shown in Fig. 3. In general, the particles are
isotropic (i.e., low aspect ratio) and spherical in shape. The mean
particle diameter observed is less than 2 nm. The histograms
of the Cu nanoparticle size distributions are also presented in
Fig. 3.
The observed patterns indicate a high degree of monodis-
persity of the Cu nanoparticles. In addition, the histograms
reveal a decrease in particle size with an increase of L-ascorbic
acid concentration. The sizes of the copper nanoparticles with
various concentrations of L-ascorbic acid (0.4 M, 0.6 M,
0.8 M, and 1.0 M) are 1.87
1.57 0.21 nm, and 1.34 0.14 nm respectively. The reason
0.35 nm, 1.68
0.35 nm,
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Fig. 2 The time evolution of the dispersion photographs and the UV-Vis absorption spectra.
be obtained. The number of Cu2+ encapsulated in L-ascorbic
acid molecules decreases with the increasing concentration
of L-ascorbic acid, leading to the formation of smaller Cu
nanoparticles (which will be further discussed in the mechanism
section).
The resulting dispersion was further examined with EDS, and
the result is shown in Fig. 4. The EDS spectrum only exhibits the
characteristic peaks of Cu, suggesting that the obtained product
is composed of pure Cu.
Fig. 4 EDS spectrum of the Cu nanoparticles stabilized in L-ascorbic
acid aqueous solution.
Fourier transform-infrared (FT-IR) characterization
FT-IR spectroscopy was used to investigate the interactions
between different species and changes in chemical compositions
of the mixtures. Fig. 5a shows the FT-IR spectrum for pure
L-ascorbic acid. The stretching vibration of the carbon-carbon
double bond and the peak of enol hydroxyl were observed at
1674 cm-1 and 1322 cm-1, respectively. These peaks disappeared
after the reaction and new peaks were observed at 3481 cm-1,
1718 cm-1, and 1681 cm-1 (Fig. 5b). These peaks correspond
to the hydroxyl, oxidated ester carbonyl groups, and conjugated
carbonyl groups, respectively. These results indicate the presence
of the polyhydroxyl structure on the surface of copper nanopar-
ticles. The polyhydroxyl structure has an excellent dispersion
effect on copper nanoparticles (which will be discussed in detail
in the mechanism section).
Fig. 3 TEM images and particle size distribution of the synthesized
copper nanoparticles with various concentrations of L-ascorbic acid.
The average particle sizes are: (a) 0.4 M, d = 1.87 0.35 nm; (b) 0.6 M,
d = 1.68 0.35 nm; (c) 0.8 M, d = 1.57 0.21 nm; (d) 1.0 M, d = 1.34
0.14 nm.
is that L-ascorbic acid molecules encapsulate Cu2+ and reduce
Cu2+ into Cu(0), then the oxidation products adsorb on the
resulting Cu nanoparticle surfaces, preventing the particles from
growing further. As a result, ultrafine Cu nanoparticles can
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L-ascorbic acid is presented in Scheme 1. L-ascorbic acid is a
highly water-soluble compound with strong polarity. It behaves
as a vinylogous carboxylic acid in which the electrons in the
double bond, hydroxyl group lone pair, and the lactone ring
carbonyl double bond form a conjugated system. As such, the
structure of L-ascorbic acid gives enough reducibility to convert
Cu2+ ions into Cu(0) nanoparticles. The redox equation of L-
ascorbic acid and copper ions can be expressed in Scheme 1.
Fig. 5 FT-IR spectra of (a) pure L-ascorbic acid and (b) L-ascorbic
acid-stabilized Cu nanoparticles.
The stability of Cu nanoparticles
The stability of nanoparticle dispersions is a key factor in
their applications. In order to prevent the agglomeration of
nanoparticles, several capping agents have been added into
reaction media. In this work, L-ascorbic acid was used as both
reducing and capping agent without any other special capping
agent. The aqueous L-ascorbic acid-stabilized Cu nanoparticle
dispersion was centrifuged at 8000 rpm for 15 min. A small
amount of precipitate was obtained from the bottom of the
centrifuge tube. The precipitate was found to be completely
soluble in aqueous solution again after the shaking of the
centrifuge tube. In addition, the supernatant obtained by
centrifuging was placed under ambient conditions and no sign
of sedimentation was observed even after 2 months of storage.
The photos of the dispersions before and after the storage are
shown in Fig. 6. This indicates that the L-ascorbic acid-stabilized
Cu nanoparticles are highly stable due to the extreme capping
effect of the L-ascorbic acid and the dispersion effect of the
polyhydroxyl structure.
Scheme 1 The reduction reaction equation for the formation of Cu
nanoparticles.
As illustrated in Scheme 1, L-ascorbic acid serves as a stable
(electron + proton) donor in interactions, and is converted into
the radical ion called “semidehydroascorbic acid” and then
dehydroascorbic acid. Dehydroascorbic acid and L-ascorbic
acid together constitute the redox system (reduction potential
~0.060 V vs. SCE) which is sufficient to reduce Cu2+ to Cu
(reduction potential is 0.340 V vs. SCE). Besides, it should be
noted that L-ascorbic acid has also played the role of stabling
agent during the reaction process. The possible mechanism of
L-ascorbic acid on the effective stability of copper nanoparticles
can be explained from two aspects. One is, the capping effect
of L-ascorbic acid in the reduction process. The lone pair
electrons in the polar groups of L-ascorbic acid can occupy
two sp orbits of the copper ion to form a complex compound.
The L-ascorbic acid is thus capped with copper ions, then
synthesizes Cu(0) nanoparticles through reduction of Cu2+
inside the nanoscopic templates. In the presence of nanoscopic
templates, small copper nanoparticles are easily formed. The
other explanation is the dispersion effect of the oxidation
product of L-ascorbic acid on the copper nanoparticles after
the completion of the reduction reaction. L-ascorbic acid is
converted into dehydroascorbic acid through oxidation. The
dehydroascorbic acid has three carbonyls in its structure. The
1,2,3-tricarbonyl is too electrophilic to survive more than a
few milliseconds in aqueous solution, and the 6-OH and the 3-
carbonyl groups form the hemiacetal rapidly. Hydration of the
Fig. 6 The photos of the dispersions (a) before and (b) after 2 months
of storage.
Possible mechanism
The above results show that well-dispersed copper nanoparticles
can be obtained through the reduction of Cu2+ using L-ascorbic
acid as both the reducing and capping agent. The structure of
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2-carbonyl is also observed.45 Finally, the polyhydroxyl structure
is obtained through irreversible hydrolysis of the ester bond
(Scheme 2).46
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Scheme 2 Irreversible hydrolysis of dehydroascorbic acid.
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4. Conclusions
In summary, we have demonstrated a facile green method to
synthesize low-cost monodispersed Cu nanoparticles (ranging
from 1.34 0.14 nm to 1.87 0.35 nm in size on average with
a narrow size distribution and a uniform shape) by employing
L-ascorbic acid as both the reducing and capping agent. The
prepared dispersions of copper nanoparticles are highly stable
and do not show any sign of sedimentation even after storage for
2 months. Since the reagents used in the reaction medium are
completely non-toxic and environmentally friendly, this green
method can be readily used for biomedical applications. More-
over, the highly stable solution of dispersed copper nanoparticles
can be used as conductive ink for applications such as printed
electronics.
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Acknowledgements
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This work was supported by the Zhejiang Provincial Natural
Science Foundation (Grant No. Y4100488) and the “Outstand-
ing Talent Recruiting Program” (2009A31004) dedicated to
Academician Q. J. Xue from Ningbo municipal government.
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