Potential-controllable green synthesis and deposition of metal nanoparticles with electrochemical method

Article abstract of DOI:10.1039/c0jm01293a

A controllable and environmentally friendly electrochemical method for task specific synthesis and deposition of metal nanoparticles was first demonstrated by simply adjusting the potentials for the reduction of metal precursors in ionic liquids. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0jm01293a

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COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Potential-controllable green synthesis and deposition of metal nanoparticles
with electrochemical method†
a
Ping Yu, Qin Qian, Xiang Wang, Hanjun Cheng, Takeo Ohsaka and Lanqun Mao
a
a
a
b
a
*
Received 3rd May 2010, Accepted 8th June 2010
First published as an Advance Article on the web 17th June 2010
DOI: 10.1039/c0jm01293a
This communication describes the first study on the association
between the parameters employed for the reduction of metal
precursors and the growth of metal nanoparticles, by using Pt
nanoparticles as an example. The study undertaken here is very useful
for establishing the conditions to efficiently control metal nano-
particles to solubilize into the bulk solvents (electrosynthesis) or to
deposite onto electrode surface (electrodeposition) and paves
a versatile way to task specific formation of metal nanoparticles either
in the solution or onto the electrode surface in the different electro-
chemical systems or in the same electrochemical systems in a switch-
able manner.
A controllable and environmentally friendly electrochemical method
for task specific synthesis and deposition of metal nanoparticles was
first demonstrated by simply adjusting the potentials for the reduc-
tion of metal precursors in ionic liquids.
Considerable interest has been drawn both by the synthesis of metal
nanoparticles in a bulk solution and by the deposition of metal
nanoparticles onto a conducting substrate because the distinct
physicochemical properties of metal nanoparticles have largely
enabled them to be either solubilized in a bulk solution or deposited
onto conducting substrate with promising applications in various
research and industrial areas, such as catalysis, sensing, environ-
mental processing and energy conversion and storage.1 To this end,
many methods have been demonstrated for both purposes,2 of which
the electrochemical method remains advantageous in terms of its
ease-of-operation and cost-effective properties.3 More importantly,
this method utilizes electrons as the reductants to replace chemical
reductants for metal precursors and is thus more environmentally
benign than the chemical methods.4 As a result, ever increasing
interest has been drawn by the development of effective electro-
chemical methods for the synthesis/deposition of metal nanoparticles.
In this context, research into the parameters that can efficiently
control the growth of metal nanoparticles to form task specific elec-
trosynthesis or electrodeposition process remain central, because such
research essentially offers a guide to screen and establish the condi-
tions for synthesis or deposition of metal nanoparticles in different
electrochemical systems or in the same electrochemical systems in
a switchable manner.
Investigations undertaken here were carried out with ionic liquids
(ILs) as the solvents because earlier efforts have revealed that the
excellent physical and chemical properties of ILs, such as high
polarity, low vapor pressure, good chemical stability and capability to
solubilize the organic and inorganic compounds substantially make
them very useful in the formation of metal nanoparticles.6 More
importantly, ILs are involved in the combined intrinsic high charge
plus the steric bulk, which could create an electrostatic and steric
colloid-type stabilization of metal nanoparticles, without requirement
of addition of any capping materials.7 The use of ILs as the solvents
for electrochemical synthesis/deposition of metal nanoparticles is
believed to facilitate the controllable task specific formation of
nanoparticles with less environmental pollution and simplified
experimental procedures.
The formation (i.e., synthesis/deposition) of Pt nanoparticles was
carried out in 1-n-butyl-3-methylimidazolium hexafluorophosphate
(BmimPF6) containing 20 mM H2PtCl6 with a three-electrode system
by polarizing ITO substrate at different potentials (Scheme S1†). A
silver wire was used as a quasi-reference electrode. For a convenient
comparison, the potential of the silver quasi-reference electrode was
calibrated to that of Ag/AgCl (KCl-sat.) with ferrocenecarboxylic
acid as the redox probe. When the substrate was polarized at a high
overpotential of À2.8 V (vs. Ag/AgCl), Pt nanoparticles were only
deposited onto the substrate, with no production in the bulk solution.
This was directly evident from the almost unchanged color of the IL
solution and the color change of ITO substrate from initially colorless
to black (Fig. 1A, B). The deposition of Pt nanoparticles onto ITO
substrate was further confirmed by XPS (Fig. S1 A†). Two well-
defined peaks in the XPS spectrum were recorded with binding
energy of 71.2 and 74.4 eV, which were ascribed to the Pt 4f7/2 and Pt
4f5/2 doublet in Pt0, respectively.8 This demonstration reveals that, at
a high overpotential, the as-formed Pt nanoparticles tend to deposit
onto electrode surface, rather than to solubilize into the IL solvent.
The Pt 4f XPS spectrum recorded for Pt0 was symmetric and was
unable to be resolved into different peaks by a nonlinear regression
(Fig. S1 A†), implying that the reduction of PtCl62À precursor into Pt0
was efficient, with no major reactions under the present condition.
Activated by the formation of metal nanoparticles with control-
lable size and morphology using the electrochemical method, much
effort has been devoted to the investigations on the association
between the parameters employed for electrosynthesis or electrode-
position and the properties of the as-formed nanoparticles.5
However, research on the parameters involved in the electrochemical
reduction of metal precursors to efficiently control the nanoparticles’
growth have not been reported so far.
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of
Analytical Chemistry for Living Biosystems, Institute of Chemistry, The
Chinese Academy of Sciences (CAS), Beijing, 100190, China. E-mail:
lqmao@iccas.ac.cn
bDepartment of Electronic Chemistry, Interdisciplinary Graduate School of
Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama, 226-8502, Japan
† Electronic supplementary information (ESI) available: Detailed
experiments, XRD patterns, XPS and Cyclic voltammetry results. See
DOI: 10.1039/c0jm01293a
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Fig. 2 Photographs of BmimPF6 solution (A) and ITO substrate (B)
after the substrate was polarized at À1.8 V (vs. Ag/AgCl) for 4000 s in
BmimPF6 containing 20 mM H2PtCl6. (C) A TEM image of the synthetic
Pt nanoparticles. Scale bar, 50 nm.
Fig. 1 Photographs of BmimPF6 solution (A) and ITO substrate (B)
after the substrate was polarized at À2.8 V (vs. Ag/AgCl) for 1000 s in
BmimPF6 containing 20 mM H2PtCl6. (C) A SEM image of Pt nano-
particles electrodeposited on ITO substrate. Scale bar, 10 mm.
Moreover, the diffraction lines (111, 200, 220) of metallic Pt were
clearly observed in the diffraction pattern (Fig. S1 B†), again con-
firming the formation of Pt0 onto the ITO substrate under the present
condition.7c Scanning electron microscopy of the as-deposited Pt
nanoparticles suggests the nanoparticles almost have a uniform size
(ca. 100 nm diameter on average) (Fig. 1C). All these demonstrations
reveal that when a high overpotential was applied for the reduction of
metal precursor, the as-formed nanoparticles tend to deposit onto the
substrate surface, rather than to solubilize into IL solvent, to form an
electrodeposition process for metal nanoparticles.
As far as we know, this is the first demonstration on the control-
lable synthesis of Pt nanoparticles with an electrochemical method
with IL as the solvent and capping material. In the whole process, the
use of reduction agent, additional capping materials, heating, or high
pressure was essentially avoided. This study offers a mild, renewable,
nontoxic and completely green synthetic route to formation of Pt
nanoparticles colloid. The results described above demonstrate that
the simple adjustment of the potentials for the reduction of metal
precursors could efficiently control the nanoparticles’ growth and
thus form a task specific process for electrosynthesis or electrode-
position in a same electrochemical system with a switchable manner.
As another important factor that may also determine the nano-
particles’ growth and thus control the process for synthesis or depo-
sition of metal nanoparticles, we have studied the effect of the
Interestingly, when the ITO substrate was polarized at a low
overpotential of À1.80 V (vs. Ag/AgCl), there was no color change on
the ITO surface, while the color of the IL solution was changed from
initially somewhat yellow into black (Fig. 2A and B). XPS results
show two well-defined peaks on the XPS spectrum with binding
energy of 71.1 and 74.5 eV (Fig. S2 A†), which were ascribed to the Pt
4f7/2 and Pt 4f5/2 doublet in Pt0, respectively,8 suggesting that this kind
of black solution was the dispersion of Pt nanoparticles. These results
reveal that when the electrode was polarized at a low overpotential for
2À
concentration of PtCl6 precursor on the nanoparticles’ growth.
Unlike the potentials employed for the PtCl62À reduction, the change
2À
of PtCl6 concentration (from 20 mM to 100 mM at À2.8 V and
from 5 mM to 20 mM at À1.8 V vs. Ag/AgCl) did not greatly
influence the growth of the as-formed nanoparticles, implying that the
task specific electrosynthesis or electrodeposition may not be effec-
tively controlled by such a factor. Presumably, this is due to the low
diffusion efficiency of the metal precursor in the viscous IL solvents.
The potential-controllable green synthesis and deposition of Pt
nanoparticles is considered to be based on colloid chemistry and
unique property of ILs. As shown in Scheme 1, the reduction of the
2À
the reduction of PtCl6 precursor, the as-formed Pt nanoparticles
prefer to solubilize into IL solvent to form an electrosynthesis process
for Pt nanoparticles. The Pt 4f XPS spectrum recorded for Pt0 was
symmetric and the different peaks could not be analyzed by
a nonlinear regression (Fig. S2 A†), implying that the reduction of
PtCl62À precursor into Pt0 was also efficient under the present condi-
tion. Furthermore, the diffraction lines (111, 200, 220) of metallic Pt
were clearly observed in the diffraction pattern (Fig. S2 B†),7c again
confirming the formation of Pt nanoparticles dispersion in the IL
solution. TEM image of the as-synthesized Pt nanoparticles reveals
that the nanoparticles almost have a uniform morphology with a size
ranging from 3.5 to 4 nm (Fig. 2C). The small diameter of nano-
particles may result from the low surface tension of ionic liquids.7a
2À
PtCl6 precursor diffused onto electrode surface forms so-called Pt
adatoms ([Pt0]ad). In this case, on one hand, IL creates electrostatic
and steric colloid-type stabilization toward Pt nanoparticles, resulting
in the solubilization of nanoparticles in a bulk solution to form Pt
nanoparticles colloid. On the other hand, the nanoparticles prefer to
deposit onto electrode surface due to the high Gibbs energy of solid
surface. The competition between synthesis and deposition of Pt
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systems in a switchable manner, provided the metal precursors are
electrochemically redox active.
Acknowledgements
This work is financially supported by NSF of China (Grant Nos.
20625515, 20721140650, and 20935005 for L. M. and 20805050 for
P. Y.), National Basic Research Program of China (973 Program,
2007CB935603 and 2010CB933502) and Chinese Academy of
Sciences (KJCX2-YW-H11 and KJCX2-YW-W25).
Notes and references
Scheme 1 A schematic illustration of potential-controllable task specific
electrosynthesis and electrodeposition of metal nanoparticles in IL
solvents.
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In summary, a controllable and environmentally friendly method
for task specific synthesis/deposition of metal nanoparticles has been
demonstrated by simply adjusting the potentials for the electro-
chemical reduction of metal precursors with ionic liquids as the green
solvents and with electrons as the reductant. This study essentially
paves a controllable and green route to task specific synthesis/depo-
sition of metal nanoparticles with no requirement of reducing agents,
capping materials, heating, or high pressure and with no production
of toxic substrates. The method is envisaged to be versatile for
controllable synthesis/deposition of metal nanoparticles either in the
different electrochemical systems or in the same electrochemical
5822 | J. Mater. Chem., 2010, 20, 5820–5822
This journal is ª The Royal Society of Chemistry 2010