New strategy to prepare platinum salts by electrochemical methods and subsequent synthesis of platinum nanoparticles

Article abstract of DOI:10.1016/j.materresbull.2011.11.056

As shown in the literature, most of Pt nanoparticles (NPs) were synthesized from precursors of commercial Pt salts. However, the impurity of the commercial Pt salts is a concerned issue. In this work, we report a new pathway based on electrochemical methods to prepare Pt-containing complexes with high purity in aqueous solutions from bulk Pt substrates. Experimental results indicate that Pt complexes with higher concentration can be obtained in 0.1 N HCl by using square-wave oxidation-reduction cycles (ORCs) under a frequency of 8 Hz with a step potential of 6.3 mV. Moreover, concentrations of other heavy metals of Hg and Cr in 65 ppm Pt complexes-containing solution are just 0.65 and 0.78 ppb, respectively. These Pt complexes were further reduced to Pt NPs by using NaBH₄ and poly(vinylpyrrolidone) (PVP) as reducing agent and stabilizer, respectively. The concentration and the particle size of synthesized Pt (1 1 1) NPs are ca. 60 ppm and smaller than 5 nm, respectively.

Full text of DOI:10.1016/j.materresbull.2011.11.056

Materials Research Bulletin 47 (2012) 167–171
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
New strategy to prepare platinum salts by electrochemical methods and
subsequent synthesis of platinum nanoparticles
a,d
b
a,d,
c
e
Fu-Der Mai , Chung-Chin Yu , Yu-Chuan Liu *, Ting-Chu Hsu , Yi-Hao Wu
a
Department of Biochemistry, School of Medicine, College of Medicine, Taipei Medical University, No. 250, Wu-Hsing St., Taipei 11031, Taiwan
Department of Environmental Engineering, Vanung University, No. 1, Van Nung Rd., Chung-Li City, Taiwan
General Education Center, Vanung University, No. 1, Van Nung Rd., Chung-Li City, Taiwan
b
c
d
e
Biomedical Mass Imaging Research Center, Taipei Medical University, No. 250, Wu-Hsing St., Taipei 11031, Taiwan
Department of Pharmacognosy Science, Taipei Medical University, No. 250, Wu-Hsing St., Taipei 11031, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history:
As shown in the literature, most of Pt nanoparticles (NPs) were synthesized from precursors of
commercial Pt salts. However, the impurity of the commercial Pt salts is a concerned issue. In this work,
we report a new pathway based on electrochemical methods to prepare Pt-containing complexes with
high purity in aqueous solutions from bulk Pt substrates. Experimental results indicate that Pt complexes
with higher concentration can be obtained in 0.1 N HCl by using square-wave oxidation–reduction cycles
Received 26 May 2011
Received in revised form 14 November 2011
Accepted 22 November 2011
Available online 2 December 2011
(
ORCs) under a frequency of 8 Hz with a step potential of 6.3 mV. Moreover, concentrations of other
heavy metals of Hg and Cr in 65 ppm Pt complexes-containing solution are just 0.65 and 0.78 ppb,
respectively. These Pt complexes were further reduced to Pt NPs by using NaBH and poly(vinylpyrro-
Keywords:
A. Metals
4
B. Chemical synthesis
C. Photoelectron spectroscopy
lidone) (PVP) as reducing agent and stabilizer, respectively. The concentration and the particle size of
synthesized Pt (1 1 1) NPs are ca. 60 ppm and smaller than 5 nm, respectively.
ß 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Recently, electrocatalysts of Pt NPs were popularly used in
proton-exchange membrane fuel cells [21,22] and oxidation of
Recently, applications of gold and platinum nanoparticles (NPs)
in the fields of protein detection [1,2] and catalyst modification
3,4] are rapidly growing because of their unique electronic
small organic molecules [3,23]. Sanles-Sobrido et al. [24]
proposed a one-step method for the preparation of single-crystal
dendritic Pt NPs with no need of organic solvents, templates, or
seeded growth. It is shown that Pt NPs with two different shapes
(spherical or dendritic) can be efficiently supported onto the
sidewalls of carbon nanotubes. Notably, the supported dendritic
Pt NPs yield unprecedented catalytic activity, evidenced through
the lowest activation energy within an electron-transfer reaction
as compared with those reported in the literature for Pt NPs. Hsu
et al. [25] developed a novel method to debundle carbon
nanotubes (CNTs) and load Pt NPs on them without damaging
their graphene structures. In this article, the aniline acts as a very
efficient dispersing agent to debundle CNTs from 200 to 50 nm at a
very low concentration of 0.5% in an isopropanol (IPA)/water
solution. The result indicates that aniline is an efficient dispersant
and stabilizer for the preparation of Pt NPs deposited on CNTs.
Additionally, the whole process, which could be easily scaled up
for industrial production, is simple, efficient, and inexpensive.
Zhang et al. [26] reported a novel experimental protocol for the
preparation of stable and exfoliated nano-Pt catalysts supported
on layered silicate clay surfaces. Uniformly dispersed highly
crystalline Pt NPs with diameters between 2 and 5 nm were
chemically adsorbed on to the layered silicate surfaces using a
chemical vapor deposition (CVD) method employing organoclay
[
structure and extremely large surface areas. As shown in the
literature, the developed methods for fabrications of NPs of noble
metals include chemical [5], electrochemical [6], sonochemical [7]
and sonoelectrochemical [8] reductions, laser ablation [9],
annealing from high-temperature solutions [10], metal evapora-
+
tion [11], Ar ion sputtering [12], and UV light irradiation [13].
Meanwhile, some stabilizers, like sodium dodecyl sulfate [14],
sugar ball [15] and poly(vinylpyrrolidone) (PVP) [16] were used,
and some stabilization technologies of thiol-ligand coatings [17]
and polymer capping agents [18] were developed to prevent the
prepared NPs from aggregating. Also, it is necessary to develop
effective methods for size- and shape-controlled synthesis of metal
NPs due to these properties can significantly influence their
corresponding characterization [19,20].
*
Corresponding author at: Department of Biochemistry, School of Medicine,
College of Medicine, Taipei Medical University, No. 250, Wu-Hsing St., Taipei 11031,
Taiwan. Tel.: +886 2 27361661x3155; fax: +886 2 27356689.
E-mail addresses: liuyc@tmu.edu.tw, liuyc@mail.vnu.edu.tw (Y.-C. Liu).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.11.056

1
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F.-D. Mai et al. / Materials Research Bulletin 47 (2012) 167–171
as the initial support. The as-prepared catalysts were found to be
extremely stable against both intensive mechanical agitation and
high temperature treatment.
compensate for surface charging effects, all HRXPS spectra are
referred to the C 1s neutral carbon peak at 284.8 eV.
As shown in the literature, most of Pt NPs were synthesized
from precursors of commercial platinum salts. The impurity of the
commercial platinum salts is a concerned issue. It would reduce
the purity of the synthesized Pt NPs. Therefore, the synthesized Pt
NPs from commercial platinum salts are unfavorable for their
subsequent applications. In this work, we propose a new pathway
based on electrochemical methods to prepare Pt-containing
complexes with high purity in aqueous solutions from pure Pt
substrates. These complexes can be further reduced to platinum
NPs.
3. Results and discussion
3.1. Preparation of platinum-containing complexes
As shown in the literature [27,23], most of noble metal NPs
were conveniently synthesized from precursors of commercial
metal salts. However, the impurity of the commercial platinum
salts is a considerable issue for their practical applications. The aim
of this work is to find a suitable electrochemical method to prepare
Pt-containing complexes with high purity in aqueous solutions
from pure bulk Pt substrates. Certainly, these prepared complexes
should be successfully reduced to Pt NPs. In the triangular-wave
ORCs treatment, the chloride electrolyte was selected since this
facilitates the metal dissolution–deposition process that is known
to produce surface-enhanced Raman scattering (SERS)-active
surfaces [28]. Meanwhile, the Au-containing complexes were also
left in the solution after roughening the Au substrate, as reported in
our previous study [29]. These prepared precursors of Au-
containing complexes can be further reduced to Au NPs by
sonoelectrochemical reactions [12]. For preparing Ag-containing
complexes based on triangular-wave ORCs treatment electrolytes
2
. Experimental
2.1. Chemical reagents
4
Chemical reagents of HCl, NaBH and PVP (p.a. grade) purchased
from Acros Organics were used as received without further
purification. All of the solutions were prepared using deionized
1
8.2 MV cm water provided from a MilliQ system.
2.2. Preparation of platinum nanoparticles
All of the electrochemical experiments were performed in a
3
of HNO were suggested because the concentration of the prepared
three-compartment cell at room temperature (22 8C) and were
Ag-containing complexes would be reduced due to the formation
of settlement of AgCl in electrolytes containing chloride. These
prepared precursors of Ag-containing complexes can be also
reduced to Ag NPs by heating under aid of chitosan [30]. Tian et al.
[31] reported a square-wave ORCs treatment to Pt substrates in
controlled by a potentiostat (model PGSTAT30, Eco Chemie). First,
2
a sheet of platinum with bare surface area of 4 cm , a sheet of
2
cm  4 cm platinum, and a KCl-saturated silver–silver chloride
(Ag/AgCl) rod were employed as the working, counter and
reference electrodes, respectively. Before the square-wave oxida-
tion–reduction cycles (ORCs), the Pt electrode was mechanically
polished (model Minimet 1000, Buehler) successively with 1 and
2 4
electrolytes of H SO to prepare weakly SERS-active Pt substrates.
However, there are no Pt-containing complexes available in the
solution after roughening the Pt substrate. These experimental
experiences encourage us to find some suitable electrochemical
ORCs method to prepare Pt-containing complexes in aqueous
solutions. First, we employed different triangular-wave ORCs
treatments to Pt substrates in general electrolytes of chloride, or
0
.05
m
m of alumina slurries to a mirror finish. Then the electrode
deoxygenated aqueous solution of 40 mL
was cycled in
a
containing 0.1 N HCl from À0.1 to +2.4 V vs Ag/AgCl for 300
scans. A frequency of 8 Hz and a step potential of 6.3 mV was used.
The solution was slightly stirred during the ORCs treatment. After
this dissolving procedure, Pt-containing complexes were left in
this aqueous solution at pH 0.95. Immediately, 1 mg stabilizer of
PVP was added in this solution under slight stirring for 5 min. Then
3 2 4
HNO or H SO to prepare Pt-containing complexes in aqueous
solutions. However, these treatments did not succeed. Encourag-
ingly, Pt-containing complexes with considerable concentrations
in aqueous solutions can be obtained by using suitable square-
wave ORCs treatment to Pt substrates in specific electrolytes, as
discussed below.
1
4
mg reducing agent of NaBH was subsequently added to prepare
Pt NPs in solution. The prepared colloidal solutions were further
centrifuged at 15,000 rpm for 3 min and then ultrasonically
redispersed in aqueous solutions for three cycles to remove
impurities.
Fig. 1 shows the typical I–E curve in roughening the Pt substrate
in 0.1 N HCl by using the square-wave ORCs treatment under a
frequency of 8 Hz. In experiment, the upper and lower potentials
are 2.4 and À0.1 V vs Ag/AgCl, respectively. The step potential is
6.3 mV. This I–E curve is quite different from that shown in the
SERS study based on a roughened Pt electrode reported by Tian
et al. [31]. In our system, considerable amount of Pt-containing
complexes can be obtained in solution after the square-wave ORCs
treatment to a Pt substrate in an acidic solution of 0.1 N HCl.
Fig. 2 shows the UV–vis spectra of Pt-containing complexes
after the square-wave ORCs treatments to Pt substrates in
solutions containing different electrolytes. As shown in spectrum
a of Fig. 2 (in a solution containing electrolytes of acidic HCl), the
absorbance maximum appears at ca. 261 nm, which can be
assigned to Pt-containing complexes due to the ligand-to-metal
charge–transfer transition [32]. However, there are no absorbance
maxima, as shown in spectra b and c of Fig. 2 (in solutions
2.3. Characteristics of platinum nanoparticles
The concentrations of Pt NPs and impurities of Hg and Cr were
measured from inductively coupled plasma-mass spectrometer
ICP-MS) analyses. A single drop of the sample-containing solution
(
was placed on a 300 mesh Cu/carbon film transmission electron
microscopy (TEM) sample grid and was allowed to be dried in a
vacuum oven. Then the sample was examined by using a Philips
Tecnai G2 F20 electron microscope with an acceleration voltage of
2
00 kV. Ultraviolet–visible absorption spectroscopic measure-
ments were carried out on a Perkin-Elmer Lambda 35 spectropho-
tometer in 1 cm quartz cuvettes. Before recording the high
resolution X-ray photoelectron spectroscopy (HRXPS) spectra
sample powders were obtained via centrifugation collections at
2 4
containing electrolytes of neutral KCl and acidic H SO , respec-
1
5,000 rpm for 3 min and being rinsed with deionized water for
three cycles. For HRXPS measurements, a ULVAC PHI Quantera
SXM spectrometer with monochromatized Al K radiation, 15 kV
and 25 W, and an energy resolution of 0.1 eV was used. To
tively). Therefore, electrolytes of acidic HCl are favorable for
preparing Pt-containing complexes in solutions with higher
concentration based on our proposed square-wave ORCs treat-
ment.
a

F.-D. Mai et al. / Materials Research Bulletin 47 (2012) 167–171
169
0
0
0
0
0
0
.12
.11
.10
.09
.08
.07
261 nm
a
b
c
-
0.40 0.00
0.40
0.80
1.20
1.60
2.00
2.40
E / V vs Ag/AgCl
200
300
400
500
600
Fig. 1. Square-wave oxidation–reduction cycles (ORCs) of the 300th scan under a
frequency of 8 Hz with a step potential of 6.3 mV for electrochemically roughening
platinum substrate in 0.1 N HCl.
Wavelength / nm
Fig. 3. UV–vis spectra of Pt-containing complexes in solutions after roughening the
Pt substrates by square wave ORCs under a frequency of 8 Hz for 300 scans with
different step potentials: (a) 0.63 mV; (b) 6.3 mV; (c) 63 mV.
As shown in our previous report [33], the SERS effects observed
on the silver substrates are dependent on the scan rates used in the
triangular-wave ORCs preparations. Thus the effects of the step
potentials used in these square-wave ORCs treatments to Pt
substrates on the corresponding Pt-containing complexes obtained
in solutions were also examined in this work. Fig. 3 shows the UV–
vis spectra of Pt-containing complexes in 0.1 N HCl solutions by
using the square-wave ORCs treatments with different step
potentials. Basically, these three spectra are similar. The absor-
bance maxima appear at ca. 261 nm, which can be assigned to Pt-
containing complexes [32]. The only differences between them are
the absorbance values, which are proportional to the concentra-
tions of Pt-containing complexes. The highest absorbance can be
obtained with a low step potential of 0.63 mV. The absorbance
values were reduced to 70% and 8% of magnitudes when the step
potentials were increased from 0.63 to 6.3 and 63 mV, respectively.
Although a higher concentration of Pt-containing complexes can
be prepared with a lower step potential of 0.63 mV, it takes much
time to perform the experiment. Therefore, a suitable step
potential of 6.3 mV was adopted in this work to prepare
considerable concentration of Pt-containing complexes in solu-
tions. The concentrations of prepared Pt-containing complexes
were also investigated by increasing the frequency from 8 to 80 Hz
with a step potential of 6.3 mV. The obtained concentration of
complexes was correspondingly reduced to 60% of its original
value. Conclusively, Pt-containing complexes with high purity can
261 nm
a
b
c
200
300
400
500
600
Wavelength / nm
200
300
400
500
600
Fig. 2. UV–vis spectra of Pt-containing complexes in solutions after roughening the
Pt substrates by square wave ORCs under a frequency of 8 Hz with a step potential
of 6.3 mV for 300 scans in different solutions: (a) 0.1 N HCl; (b) 0.1 N KCl; (c) 0.1 N
Wavelength / nm
H
2
SO
4
.
Fig. 4. UV–vis spectrum of chemically prepared Pt(0) nanoparticles.

1
70
F.-D. Mai et al. / Materials Research Bulletin 47 (2012) 167–171
Fig. 5. TEM micrograph of synthesized Pt(0) nanoparticles, showing size and
Fig. 6. High-resolution TEM micrograph of synthesized Pt(0) nanoparticles,
dispersion; scale of nano-bar being 50 nm.
showing the (1 1 1) lattice fringes with an interplanar spacing of 2.22 A˚ ; scale of
nano-bar being 5 nm.
be efficiently prepared by using square-wave ORCs methods in
indicate that the concentrations of the prepared Pt-containing
complexes and the elemental Pt NPs in solutions are ca. 65 and
0
.1 N HCl under a frequency of 8 Hz and a step potential of 6.3 mV.
60 ppm, respectively. Meanwhile, concentrations of other heavy
3
.2. Preparation of platinum nanoparticles
metals of Hg and Cr are just 0.65 and 0.78 ppb, respectively.
However, these concentrations are 13 and 29 ppb for Hg and Cr,
respectively, in commercial Pt salts based on the same Pt
complexes concentration prepared in this work. In the electro-
chemical ORCs method, the impurity mainly comes from the
dissolution of the Pt substrate. Therefore, the content of impurity
can be controllably reduced by using pure Pt substrate. The
concentrations of impurities discussed above are average values
based on three measurements.
The dispersion and the particle size of prepared Pt NPs in
solutions are examined by using HRTEM images, as shown in Fig. 5.
The particle sizes of the prepared Pt NPs are smaller than 5 nm.
They demonstrate no aggregation and fairly even dispersion. Fig. 6
shows the moir e´ patterns of the Pt NPs. It exhibits a one-
Subsequently, as-prepared Pt-containing complexes were
further reduced to Pt NPs by reducing agent of NaBH
protected by stabilizer of PVP. Fig. 4 demonstrates the UV–vis
spectrum of the synthesized elemental Pt NPs in solutions. Along
4
and
4
with the adding of NaBH , the color of the solution becomes
yellow-brown and finally becomes black after 5 min. The absor-
bance band at 261 nm disappears. It indicates that all of the Pt-
containing complexes are reduced. Instead, the synthesized Pt NPs
have absorption band in all range of the UV–vis spectrum.
Moreover, the absorption increases gradually with the decrease
of the wavelength. These phenomena are consistent with the
reports shown in the literature [32,34]. Further ICP-MS analyses
Fig. 7. EDX spectrum of high-resolution TEM of synthesized Pt(0) nanoparticles, showing the nanoparticles being platinum.

F.-D. Mai et al. / Materials Research Bulletin 47 (2012) 167–171
171
4. Conclusions
In this work, we have successfully developed a new electro-
chemical pathway to prepare Pt-containing complexes with high
purity from pure bulk Pt substrates. Pt-containing complexes can
be efficiently prepared by using square-wave ORCs methods in
0.1 N HCl under a frequency of 8 Hz and a step potential of 6.3 mV.
This method can overcome the general issue of impurity in
synthesizing Pt NPs from precursors of commercial Pt salts. The
concentrations of the prepared Pt-containing complexes and the
synthesized elemental Pt NPs in solutions are ca. 65 and 60 ppm,
respectively. Meanwhile, concentrations of other heavy metals of
Hg and Cr in 65 ppm Pt complexes-containing solution are just
a
b
c
0.65 and 0.78 ppb, respectively. Also, the particle sizes of the
synthesized Pt(1 1 1) NPs are smaller than 5 nm.
Acknowledgment
The authors thank the National Science Council of the Republic
of China (NSC-97-2622-E-238-008-CC3) for its financial support.
66.0
68.0
70.0
72.0
74.0
76.0
78.0
80.0
Binding energy / eV
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