Synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by water-in-oil microemulsion

Article abstract of DOI:10.1149/1.1534600

The synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by reduction with hydrazine of H₂PtCl₆ and K₂PdCl₄ in a water-in-oil microemulsion of water/poly(ethylene glycol)-dodecyl ether (BRIJ 30)/n-heptane is reported. X-ray photoelectron spectroscopy experiments were carried out to determine the composition of the nanoparticles obtained. Due to the presence of surfactant molecules coating on the nanoparticles, a decontamination procedure was developed. The decontamination procedure applied allows a cleaning of the surface of the nanoparticles without modification or damage for the crystalline surface structure as a result of synthesis conditions. The influence of the classical electrochemical activation or cleaning of the surface on the electrochemical behavior of the nanoparticles is also compared. The results obtained show a deep modification of the electrocatalytic behavior of the nanoparticles after the electrochemical activation treatment. They demonstrate that this activation process should not be applied in studies where the influence of the size, surface structure, and surface composition of the nanoparticles on their electrocatalytic and catalytic properties are studied.

Full text of DOI:10.1149/1.1534600

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Journal of The Electrochemical Society, 150 ͑2͒ E104-E109 ͑2003͒
0
013-4651/2003/150͑2͒/E104/6/$7.00 © The Electrochemical Society, Inc.
Synthesis and Electrochemical Decontamination of
Platinum-Palladium Nanoparticles Prepared by Water-in-Oil
Microemulsion
,z
J. Solla-Gull o´ n, V. Montiel, A. Aldaz,* and J. Clavilier
Departamento de Qu ´ı mica-F ´ı sica, Universidad de Alicante, Alicante 03080, Spain
The synthesis and electrochemical decontamination of platinum-palladium nanoparticles prepared by reduction with hydrazine of
H PtCl and K PdCl in a water-in-oil microemulsion of water/poly͑ethylene glycol͒-dodecyl ether ͑BRIJ 30͒/n-heptane is re-
2
6
2
4
ported. X-ray photoelectron spectroscopy experiments were carried out to determine the composition of the nanoparticles obtained.
Due to the presence of surfactant molecules coating on the nanoparticles, a decontamination procedure was developed. The
decontamination procedure applied allows a cleaning of the surface of the nanoparticles without modification or damage for the
crystalline surface structure as a result of synthesis conditions. The influence of the classical electrochemical activation or cleaning
of the surface on the electrochemical behavior of the nanoparticles is also compared. The results obtained show a deep modifi-
cation of the electrocatalytic behavior of the nanoparticles after the electrochemical activation treatment. They demonstrate that
this activation process should not be applied in studies where the influence of the size, surface structure, and surface composition
of the nanoparticles on their electrocatalytic and catalytic properties are studied.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1534600͔ All rights reserved.
Manuscript submitted April 1, 2002; revised manuscript received July 25, 2002. Available electronically January 3, 2003.
The properties of nanoparticles have been the subject of innu-
merable studies in electrochemistry and heterogeneous catalysis, not
only because of their importance in the design of practical gas-
diffusion electrodes and catalysts, but also for understanding of their
fundamental interest. Between these the influence of the size and
surface structure of nanoparticles on their electrocatalytic and cata-
lytic properties has been the aim of several works. Thus, the size
effect and the electrochemical behavior of these nanoparticles has
Here we report the synthesis and decontamination under electro-
chemical control of platinum-palladium nanoparticles prepared in a
w/o microemulsion.
Experimental
Preparation of nanoparticles.—In this work the concentration of
surfactant is referred to the overall volume of the microemulsion,
while the concentration of metal precursors and hydrazine is re-
ferred to the volume of the aqueous solution present in the micro-
emulsion. The amount of surfactant, in volume, amounts to 16.54%
of the total volume of the microemulsion.
1
2
been reviewed by Kinoshita and Mukerjee, and the size effect and
electrocatalytic properties of Pt have also been studied by different
3,4
5
6-8
9
groups: Friedrich et al., Schmidt et al., Rice et al., Kinoshita,
and Takasu et al.
1
0,11
Platinum and palladium nanoparticles were obtained by reduc-
tion of either H PtCl or K PdCl with hydrazine using a w/o mi-
The problem of bimetallic nanoparticles is somewhat more com-
plex because of the characterization of their surface composition
throughout the process leading to their formation until their use
2
6
2
4
croemulsion of water/poly͑ethylene glycol͒-dodecyl ether ͑BRIJ
0͒/n-heptane. The synthesis was achieved by mixing equal volumes
3
of the two microemulsions that have the same water-to-surfactant
molar ratio (␻0 ϭ 3.8) and surfactant concentration. One micro-
emulsion contains an aqueous solution of the metallic precursors ͑Pt
or Pd͒ and the other an aqueous solution of hydrazine. The concen-
trations of H PtCl , K PdCl , and hydrazine were 0.1, 0.1, and 2.5
under electrochemical conditions. Several techniques have been
used for the preparation of multimetallic nanoparticles.12-15 We have
used in this work the water-in-oil microemulsions.
Water-in-oil ͑w/o͒ microemulsion is an optically transparent iso-
2
6
2
4
tropic and thermodynamically stable dispersion of two nonmiscible
M, respectively. For the preparation of alloyed nanoparticles, an
liquids, which are stabilized by a surfactant.1
6-18
In these systems the
aqueous solution of H PtCl ϩ K PdCl with the convenient atomic
2
6
2
4
aqueous phase is dispersed as nanosized droplets surrounded by a
monolayer of surfactant molecules in a continuous apolar organic
phase. The method used here consists of mixing two different mi-
croemulsions carrying the specific reactants ͑metallic salt and reduc-
ing agent͒ dissolved in the aqueous phase.
proportion of the two elements was employed. In order to have
micelles of the same size, i.e., nanoparticles of approximately the
same size, the water/surfactant molar ratio was maintained constant.
The reduction reactions are
Since the platinum-group metal nanoparticles were synthesized
in a water-in-oil microemulsion by Boutonnet et al., the micro-
H PtCl ϩ N H OH → Pt ϩ 6HCl ϩ N ϩ H O
͓1͔
2
6
2
5
2
2
19
2
K PdCl ϩ N H OH → 2Pd ϩ 4KCl ϩ 4HCl ϩ N ϩ H O
emulsion technique has been used to prepare many nanomaterials,
2
4
2
5
2
2
1
9,20
20-22
23
͓2͔
such as metals,
hydroxides,
alloys,
metal borides, metal oxides and
2
4-29
30
organic polymers, etc.
After complete reduction that takes place in a few minutes, ac-
Very few works related to the electrochemical characterization of
nanoparticles prepared by w/o microemulsion have been published.
When this technique is used, the nanoparticles obtained are coated
with surfactant molecules that block the surface active sites, modi-
fying their electrocatalytic properties. In a previous work, we devel-
oped a procedure to clean Pt nanoparticles prepared with this meth-
odology. This procedure allows cleaning of the surface of the
particles without damaging the crystalline surface structure resulting
from generation conditions.31
etone was added to the solution to cause phase separation. The pre-
cipitate formed by the metallic nanoparticles was washed several
times with acetone and finally put in a suspension of ultrapure water.
Preparation of the electrodes and electrochemical measure-
ments.—For the electrochemical adsorption characterization of the
nanoparticles the procedure used is similar to that employed by
3
,4
Friedrich et al. in their work on particle structure and reactivity on
CO oxidation. A polycrystalline gold disk electrode ͑л 6 mm, thick-
ness 0.25 mm͒ on which the nanoparticles are deposited by gravity
was used as a substrate whose double-layer contribution is perfectly
known in the potential range where the particle adsorption proper-
ties are studied.
*
Electrochemical Society Active Member.
E-mail: aldaz@ua.es
z
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Journal of The Electrochemical Society, 150 ͑2͒ E104-E109 ͑2003͒
Metal nanoparticle/electrodes were prepared by depositing a
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drop of the nanoparticle suspension, typically 8-12 ␮L, on the sur-
face of the polycrystalline gold disk. An argon atmosphere was used
to evaporate the drop of water. This evaporation procedure gives a
good contact between the gold support and the nanoparticles. The
amount of nanoparticles deposited on the substrate can be varied by
changing the particle concentration or by successive additions of
drops of nanoparticles suspension. The counter electrode was a gold
wire and as reference electrode a reversible hydrogen electrode
͑
RHE͒ connected to the cell through a Luggin capillary was used.
All potentials are quoted vs. this reference electrode.
All electrochemical measurements were performed in a 0.5 M
H SO solution at room temperature. Electrolyte solutions were
2
4
made from Milli-Q water. The electrode potential was controlled
using a PGSTAT30 Autolab system. The experiments were carried
out in a three-electrode electrochemical cell. Adsorption of CO on
the electrode was carried out by bubbling CO gas for 2 min at 0.03
V. To remove CO argon was bubbled for 10 min.
Before each experiment the gold substrate was mechanically pol-
ished with alumina and rinsed with ultrapure water to remove nano-
particles from the previous experiments. After that the Au electrode
was cleaned by the flame treatment and cooled in air. The so-
prepared gold surface was characterized by cyclic voltammetry be-
tween 0.05 and 1.7 V at 50 mV/s. The upper potential limit corre-
3
2
sponds to that of the formation of a monolayer of AuO. In this
way, the real surface area of the gold substrate can be obtained from
the charge of the oxide reduction peak during the negative-going
Ϫ2
scan ͑400 ␮C cm ͒.
The effective composition of the nanoparticles was determined
from X-ray photoelectron spectroscopy ͑XPS͒ spectra using a VG-
Microtech Multilab electron spectrometer.
Results and Discussion
Composition analysis.—Figure 1 shows the XPS profiles of the
Pd 3d5/2 and Pt 4f7/2 regions of Pt/Pd ͑50/50͒ bimetallic nanopar-
ticles. From the intensities of XPS peaks, the elemental ratio of
Pt/Pd can be obtained. The results are shown in Table I.
Figure 1. XPS profiles of Pt/Pd ͑50/50͒ nanoparticles.
Electrochemical results.—In Ref. 31 was demonstrated that the
decontamination of nanoparticles is a sine qua non condition to
study the relation between surface structure and electrocatalytic
properties because the presence of contamination on the surface pro-
duces a deep modification of the nanoparticle electrocatalytic activ-
ity. In the case of an alloy and in order to study the relation between
surface composition and reactivity, this decontamination procedure
should be able to clean the surface without any modification of the
surface composition of the nanoparticles.
It was established that platinum nanoparticles with a mean size
of 3 nm prepared by the microemulsion method can be decontami-
nated by using a cleaning procedure consisting of a two-step pro-
cess: a chemical stage, washing the nanoparticles several times with
acetone and finally with ultrapure water, followed by an electro-
chemical stage, cycling the electrode between 0.05 and 1.0 V until a
stable voltammogram was obtained.31
In the course of the present work it has been shown that the
procedure described that works quite well with platinum nanopar-
ticles could not be applied to the Pt/Pd nanoparticles because of the
dissolution of Pd during the positive sweep, even up to 1.0 V. Thus,
for cleaning these Pt/Pd nanoparticles a modified decontamination
procedure was applied. The first step was the same washing step
described previously while the second one was a modification of the
electrochemical treatment. Thus, after deposition of the nanopar-
ticles on the gold support, the electrode was immersed into the sul-
furic acid solution and a constant potential of 0.03 V was applied for
into the solution during 2 min and holding the electrode at a poten-
tial of 0.03 V. After that the CO was electrochemically stripped from
the surface.
In Fig. 2 are reported the voltammograms of nanoparticles for
pure platinum and palladium as well as for three compositions of
their alloys: Pt/Pd ͑80/20͒, Pt/Pd ͑50/50͒, and Pt/Pd ͑20/80͒ before
the adsorption of CO. The curve obtained for pure platinum nano-
particles shows the characteristic voltammetric profile of a clean
surface of platinum and is similar to that obtained with the previous
method of decontamination using several cycles up 1 V for 35 min
Ϫ1 31
at 0.5 V s
.
The latter seems to be less efficient for decontami-
nation than the former. Palladium nanoparticles were cycled be-
tween 0.1 and 0.5 V. The potential of 0.1 V was chosen to avoid
absorption of hydrogen, whose oxidation interferes with the volta-
mmetric profile of the adsorbed hydrogen on palladium. The volta-
mmetric profile obtained for palladium nanoparticles shows a par-
tially blocked surface due to the incomplete removal of the
Table I. Quantitative XPS data of PtÕPd nanoparticles.
Theoretical atomic
composition
Pt/Pd
Calculated atomic
composition
Pt/Pd
Observed band
3
min. After that, the potential was stepped to 0.05 V and several
80/20
60/40
50/50
40/60
20/80
Pt 4f7/2 , Pd 3d5/2
Pt 4f7/2 , Pd 3d5/2
Pt 4f7/2 , Pd 3d5/2
Pt 4f7/2 , Pd 3d5/2
Pt 4f7/2 , Pd 3d5/2
75.3/24.7
56.6/43.4
45/55
38/62
21.4/78.6
voltammograms were recorded between 0.05 and 0.4 V to observe
the surface condition. This procedure was repeated at least three
times. After that and for improving the surface cleanliness of the
electrode, CO was adsorbed on the nanoparticles by bubbling CO
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Journal of The Electrochemical Society, 150 ͑2͒ E104-E109 ͑2003͒
Figure 2. Voltammograms of ͑a͒ platinum nanoparticles, ͑b͒ palladium nanoparticles, ͑c͒ Pt/Pd ͑80/20͒, ͑d͒ Pt/Pd ͑50/50͒, and ͑e͒ Pt/Pd ͑20/80͒ nanoparticles
Ϫ1
after immersion at 0.03 V during t ϭ 3 min. Test solution 0.5 M H SO , sweep rate 50 mV s
.
2
4
Figure 3. Voltammograms of CO oxidation of ͑a͒ platinum, ͑b͒ palladium, ͑c͒ Pt/Pd ͑80/20͒, ͑d͒ Pt/Pd ͑50/50͒, and ͑e͒ Pt/Pd ͑20/80͒ nanoparticles. Test solution
Ϫ1
0
.5 M H SO , sweep rate 20 mV s .
2 4
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Journal of The Electrochemical Society, 150 ͑2͒ E104-E109 ͑2003͒
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Figure 4. Effect of electrochemical activation on ͑a͒ Pt/Pd ͑50/50͒ and ͑b͒
Pt/Pd ͑20/80͒ nanoparticles. Test solution 0.5 M H SO , sweep rate 50 mV
Figure 5. Voltammograms of CO oxidation of ͑a͒ Pt/Pd ͑50/50͒ and ͑b͒
2
4
Ϫ1
s
.
Pt/Pd ͑20/80͒ alloy nanoparticles ͑—͒ before and ͑- - -͒ after electrochemical
Ϫ1
activation. Test solution 0.5 M H SO , sweep rate 20 mV s
.
2
4
surfactant from the surface of the palladium nanoparticles. That
means that the decontamination procedure seems to be much less
effective with palladium than with platinum. Results obtained with
of palladium. This behavior fits with two potential ranges of CO
oxidation on pure platinum and pure palladium, respectively.
In order to check the influence of the classical electrochemical
activation on the surface structure and surface composition, the elec-
trode was cycled at more positive potential ͑1.5 V͒ and the CO
oxidation peak potential was used as a surface-composition-
sensitive reaction test. Figure 4 shows the voltammograms of Pt/Pd
͑50/50͒ and Pt/Pd ͑20/80͒ alloy nanoparticles during the electro-
chemical activation.
Pt/Pd nanoparticles show
decontamination.
a
reasonable level of surface
Figure 3 shows the voltammetric profiles obtained after CO ad-
sorption and oxidation for nanoparticles of platinum, palladium, and
three Pt/Pd alloyed nanoparticles. Because of the slowness of the
CO oxidation and in order to avoid the risk of Pd dissolution, a
lower seep rate was used ͑20 mV/s͒ for CO oxides, the oxidation at
this lower sweep rate starts at less positive potentials. Before CO
oxidation, one cycle was triggered between 0.05 and 0.35 V in order
to observe the surface blocking due to adsorbed CO. After that the
electrodes were cycled to more positive potential to oxidize CO. It
may be observed that the voltammetric profile obtained is similar
but with a slight increase in the hydrogen adsorption/desorption
charge to that obtained before CO adsorption. This clearly shows
that this additional CO adsorption/desorption treatment does not
modify the initial surface structure nor the surface composition and
only increases the decontamination of the surface. It is interesting to
note the slightly different effect of CO adsorption-oxidation with
pure palladium nanoparticles and with Pt/Pd nanoparticles with
higher palladium composition. As it can be observed in Fig. 2, pal-
ladium nanoparticles appear to be more contaminated than the other
nanoparticles, Pt and Pt/Pd. The CO adsorption/oxidation treatment
completely removes the surfactant from the surface, as seen in Fig.
The results show a very clear loss of charge during the electro-
chemical activation and a modification of the voltammetric profile to
that of platinum nanoparticles. Thus, the voltammetric profile in the
hydrogen adsorption-desorption region changes and reaches the pro-
file of pure platinum. Samples with higher palladium content show
the most significant loss of charge, and the modification of the vol-
tammetric profile occurs more rapidly with cycling. This behavior
indicates that palladium dissolves from the surface of the nanopar-
ticles during the electrochemical activation. Simultaneously, a shift
of the position of the oxide reduction peak of the Pt/Pd surface to a
more positive potential range corresponding to pure platinum is
clearly observed. It is important to note that these experiments were
carried out at 50 mV/s and with a slight bubbling of argon. These
experimental conditions allow avoiding the redeposition of Pd on
3
3
the Pt surface. This redeposition is clearly observed when the elec-
trode is cycled at higher sweep rate ͑ϳ200 mV/s͒ and without
bubbling.
3
b. The same tendency but taking place to a lesser extent can be
observed with Pt/Pd ͑20/80͒ nanoparticles, Fig. 3d. As seen in Fig.
, CO oxidation is positively shifted with the increase of the amount
In Fig. 5, the voltammetric CO oxidation curves for Pt/Pd ͑50/
50͒ and Pt/Pd ͑20/80͒ nanoparticles before and after electrochemical
activation are reported. The voltammograms obtained show an im-
3
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Journal of The Electrochemical Society, 150 ͑2͒ E104-E109 ͑2003͒
stable structure which would reduce the surface area. The second
one would be related to the dissolution of the platinum surface at-
3
4
oms with the lower coordination. More work is in progress in
order to confirm these two hypotheses.
Conclusions
We have reported the synthesis and decontamination of
platinum-palladium nanoparticles prepared by reduction of H PtCl6
2
and K PdCl by hydrazine in w/o microemulsions of water/
2
4
poly͑ethylene glycol͒-dodecyl ether ͑BRIJ 30͒/n-heptane. A tech-
nique was proposed for elimination of the surfactant from the sur-
face of the nanoparticles without modification of the initial surface
structure. XPS results show that the effective composition of bime-
tallic nanoparticles was in agreement with that predicted from the
composition of the metallic precursor solutions. The classical elec-
trochemical activation that works with pure platinum ͑nanoparticles
or bulk͒ was shown to be irrelevant for surface decontamination or
activation of the nanoparticles of platinum-palladium alloys because
of the deep modifications it produces both on surface composition
and surface structure.
Acknowledgments
J.S-G. is grateful to the Oficina de Ciencia y Tecnolog ´ı a de la
Generalitat Valenciana for his research grant. J.C. is also grateful to
the Oficina de Ciencia y Tecnolog ´ı a de la Generalitat Valenciana for
his Professor Visitante grant. The authors acknowledge financial
support from Programa Nacional de Promoci o´ n General del Cono-
cimiento del Ministerio de Ciencia y Tecnolog ´ı a ͑BQU2000-0460͒
and Programa d’Investigaci o´ Cient ´ı fica i Desenvolupament Tecno-
l o` gic de la Generalitat Valenciana ͑GV01-278͒.
Universidad de Alicante assisted in meeting the publication costs of this
article.
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Figure 6. Effect of electrochemical activation on ͑a͒ palladium and ͑b͒ plati-
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