Nafion-assisted deposition of microemulsion-synthesized platinum nanoparticles on BDD: Activation by electrogenerated OH radicals

Article abstract of DOI:10.1016/j.electacta.2004.10.008

Platinum nanoparticles have been synthesized by the water-in-oil (w/o) microemulsion technique and deposited onto boron-doped diamond (BDD) electrodes. Transmission electron microscopy (TEM) has shown that Pt particles size is limited to 2-5 nm range with narrow distribution. Anodic treatment at high overpotentials activates Pt deposit that is mechanically stabilized by a Nafion layer. Such activation results in enhancement of activity towards methanol electrooxidation, due to additional cleaning of the particles by oxidation of the residual surfactant by electrogenerated hydroxyl radicals.

Full text of DOI:10.1016/j.electacta.2004.10.008

Electrochimica Acta 50 (2005) 2249–2254
®
Nafion -assisted deposition of microemulsion-synthesized platinum
nanoparticles on BDD
•
Activation by electrogenerated OH radicals
∗
,1
G. Sin e´ , Ch. Comninellis
SB-ISIC-GGEC, Ecole Polytechnique F e´ d e´ rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
Received 15 June 2004; received in revised form 15 September 2004; accepted 2 October 2004
Available online 11 November 2004
Abstract
Platinum nanoparticles have been synthesized by the water-in-oil (w/o) microemulsion technique and deposited onto boron-doped diamond
(
BDD)electrodes. Transmissionelectronmicroscopy(TEM)hasshownthatPtparticlessizeislimitedto2–5 nmrangewithnarrowdistribution.
®
Anodic treatment at high overpotentials activates Pt deposit that is mechanically stabilized by a Nafion layer. Such activation results in
enhancement of activity towards methanol electrooxidation, due to additional cleaning of the particles by oxidation of the residual surfactant
by electrogenerated hydroxyl radicals.
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Microemulsion; Pt nanoparticles; Nafion® film; Hydroxyl radicals electrogenerated on diamond electrode; Methanol electrooxidation
1
. Introduction
which Pt adsorbs methanol, and the other metal dissociates
water molecules at lower overpotentials than Pt, and the elec-
tronic effect, in which the Pt CO bond is weakened by the
modification of electronic properties of Pt induced by the
presence of the alloying metal [6]. On the other hand, the
necessity of alloying metals instead of just mixing them is
now an open question [7]. Special requirements in cataly-
sis science consist in obtaining monodispersed nanoparticles
with well-defined compositions. The microemulsion method
is well known, since the first work of Boutonnet et al. [8],
to be an efficient and simple way to prepare metallic par-
ticles in the nanometer scale with narrow size distribution.
A water-in-oil (w/o) microemulsion is a thermodynamically
stable, optically transparent and isotropic dispersion of aque-
ous nanodroplets coated by a layer of surfactant in a continu-
ousoilyphase[9]. Thismethodhasbeensuccessfullyusedfor
the synthesis of nanomaterials like pure metals [8,10], alloys
Since a few decades, there is a growing interest in the study
of supported catalyst nanoparticles (Pt, Ru, IrO2, RuO2) due
to the different and, sometimes, novel properties of the later
compared to bulk materials. Special attention was focused
on Pt nanoparticles due to their applicability for the Direct
Alcohol Fuel Cell (DAFC) technology [1,2]. However, the
complete oxidation of methanol to carbon dioxide passes
through several adsorbed intermediates [3] and CO poison-
ing that blocks methanol adsorption onto reactive sites dra-
matically decreases the activity of the catalyst [4]. Alloy-
ing Pt with other transition metals (Ru, Sn, Mo, W or Ni)
enhances CO tolerance of pure Pt, but the reasons are still
under debate. Two approaches are currently privileged by re-
searchers: the so-called bifunctional mechanism [5], during
[
11–13] or metal oxides [14]. However, the surfactant layer
∗
Corresponding author. Tel.: +41 21 693 36 74; fax: +41 21 693 31 90.
E-mail addresses: guillaume.sine@epfl.ch (G. Sin e´ ),
thatlimitstheparticlesgrowthtothedropletsowndimensions
becomes a problem for their electrochemical and electrocat-
alytic characterization. Thus cleaning of such synthesized
christos.comninellis@epfl.ch (Ch. Comninellis).
1
ISE member.
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.10.008

2
250
G. Sin e´ , Ch. Comninellis / Electrochimica Acta 50 (2005) 2249–2254
particles appears as a major challenge in order to allow their
electrochemical study. Electrochemical aging or activation
heptane, following Eq. (2):
H2PtCl + NH2NH2 → Pt + 6HCl + N2
(2)
(i.e. potential sweep between typically 0 and 1.3–1.5 V ver-
6
sus SHE) has been used to clean Pt surfaces but it implies sur-
face rearrangements or recombination [15], and aggregation
of particles [16]. Recently, a cleaning procedure that includes
CO adsorption and stripping has been successfully used for
the decontamination of Pt-based nanoparticles prepared in
microemulsion systems [17,18].
The microemulsions were prepared using ultra-pure water
®
®
(Millipore system). BRIJ 30 and n-heptane were pur-
chased from Fluka and Riedel-de-Ha e¨ n, respectively, and
were used as received. The reactants (Fluka, analytical grade)
were dissolved into the aqueous phases of two different mi-
croemulsions that had the same composition and water-to-
surfactant (ω0 = 3.8) molar ratio. The quantity of surfactant
represented 16.54% of the volume of the microemulsion, and
We have recently started some investigations on BDD-
supported Pt and Pt-based nanoparticles synthesized via the
microemulsion technique. The main advantages of BDD as
support consist in the inert character of its surface, a very
low background current and a large electrochemical window
available [19], and the BDD electrode has been widely stud-
ied for its use on electrochemical oxidation of organic pol-
lutants [20–22]. The use of BDD as substrate can be a good
approach to solve the problems of corrosion (glassy carbon)
or oxide formation (Au and Ti) encountered with usual sub-
strates, problems that can result in the modification of the
catalytic activity of the deposited particles (metal–support
interactions). Diamond substrates have thus been used as
substrate for the deposition of various metallic particles in-
cluding mercury and silver [23], lead dioxide [24], ruthenium
oxides [25,26] or gold [27]. In most of the papers dealing with
BDD-supported platinum particles, electrodeposition was the
employed technique, but yields to particles with a large size
distribution and that cannot be considered strictly as nanopar-
ticles (i.e. d > 100 nm) [28,29].
the concentrations of H2PtCl and hydrazine in the aqueous
6
phases were 0.1 and 2.5 M, respectively. The synthesis was
realised mixing equal volumes of the two microemulsions.
After complete reduction, acetone was added to the solution
to cause phase separation and precipitation of the particles.
The precipitate was rinsed several times with acetone and
ultra-pure water, and then centrifuged for a few minutes to
eliminate most of the surfactant. Finally, particles were put
in a small volume of ultra-pure water as a suspension.
2.2. Electrode preparation
BDD films were synthesized by the hot filament chem-
ical vapor deposition technique (HF-CVD) on single crys-
tal p-type Si ꢀ1 0 0ꢁ wafers (1–3 mꢀ cm, Siltronix). The
doping level of boron expressed as B/C ratio was about
3
1
500 ppm. The obtained diamond film thickness was about
␮m with a resistivity in the range 10–30 mꢀ cm. This as-
In the present article, we report some preliminary results
obtained for pure platinum nanoparticles, which can be acti-
vated by electrogenerated hydroxyl radicals on BDD. It was
indeed showed in a recent paper from our laboratory [30]
that free and highly reactive hydroxyl radicals are produced
on BDD electrodes by water discharge at high overpotentials
2
grown BDD contains some graphitic (sp ) phase and is hy-
drogen terminated. Activation of BDD by anodic polarization
− ◦
2
(10 mA cm in 1 M H2SO4 at 25 C during 30 min) elimi-
2
nates most of the sp and adsorbed hydrogen from the sur-
face, and is necessary to obtain reproducible electrochemical
measurements [33].
(>2.3 V versus SHE), as described in Eq. (1):
Platinum nanoparticles were deposited onto the BDD sub-
strate putting a droplet (5 ␮l) of the suspension onto diamond.
The excess water was dried under nitrogen atmosphere. The
•
+
−
H2O → OH + H + e
(1)
®
These electrogenerated hydroxyl radicals at the BDD surface
participate in the oxidation of the residual surfactant layer
present on the Pt nanoparticles, resulting in a clean surface.
Furthermore, in order to avoid detachment of Pt nanoparticles
during this treatment, we have also considered the use of a
BDD–Pt–Nafion electrode was realized adding 5 ␮l of a
®
commercial solution of Nafion (5 wt.% in aliphatic alco-
hols, Aldrich), diluted ten more times in pure ethanol, to the
BDD–Pt electrode. The excess alcohol was also removed un-
der nitrogen atmosphere. The BDD–Pt or BDD–Pt–Nafion®
electrode was then transferred to the electrochemical cell.
®
Nafion film to mechanically stabilize these ones, following
a similar way than Swain et al. who entrapped their particles
in an additional diamond layer [31,32].
2.3. Measurements
Electrochemical measurements were performed in
2
. Experimental
a conventional three-electrode cell using a computer-
controlled EG&G potentiostat model M 273. BDD–Pt or
®
2
2
.1. Nanoparticles synthesis
BDD–Pt–Nafion (exposed area of BDD: 0.4 cm ) was used
as working electrode, Hg/Hg2SO4·K2SO4 (saturated) as ref-
erence and Pt wire as counter electrode. All potentials in
this work are referred with respect to the Standard Hydrogen
Electrode (SHE). All solutions were made with ultra-pure
Platinum nanoparticles were obtained by reduction
of H2PtCl6 with hydrazine in a w/o microemulsion of
water/tetraethyleneglycol-monododecylether (BRIJ 30)/n-
®

G. Sin e´ , Ch. Comninellis / Electrochimica Acta 50 (2005) 2249–2254
2251
water and analytical grade reagents, and were saturated with
nitrogen gas prior to each experiment.
this picture that Pt particles are present on the copper grid
as small spherical isolated units. The size domain of these
units is limited to the range 2–5 nm that corresponds well
with the commonly accepted definition of a nanoparticle in
electrocatalysis.
Transmission Electron Microscopy (TEM) analyses were
conducted using a Philips CM 300 microscope. A small
droplet of the aqueous particles suspension was deposited
onto an amorphous carbon-coated copper grid, and the ex-
cess water was removed before analysis. Atomic absorption
spectroscopy (AAS) was also used to determine the concen-
tration of Pt particles in the suspension.
The cleaning of BDD–Pt and BDD–Pt–Nafion® elec-
trodes under potential conditions for which hydroxyl radicals
are produced on BDD has been investigated. For this purpose,
a potential of 2.5 V versus SHE was applied in 1 M HClO4
to the BDD–Pt electrode during a determined time. The po-
tential was then cycled until a stable voltammogram was ob-
tained and the charge in the H adsorption–desorption region
was calculated. Fig. 3 shows the evolution of the normalized
charge versus time of activation treatment for a BDD–Pt (thin
line with triangle markers) and a BDD–Pt–Nafion® (thick
line with square markers) electrode. The decreases with time
of treatment indicate detachment of particles from the sub-
strate caused by oxygen evolution on BDD. Nevertheless,
this phenomenon is less pronounced when the polymer layer
3
. Results and discussion
The principle of nanoparticle synthesis by the microemul-
sion method is schematised in Fig. 1. The synthesis involves
the following steps: (1) mixing of two w/o microemulsions
of identical formulation, each one containing one of the reac-
tant (H2PtCl and N2H4) dissolved in its aqueous phase, (2)
6
collision between aqueous droplets due to Brownian motion,
formation of metastable fused dimers and exchange of reac-
®
is added and the beneficial effect of Nafion on the sta-
2+
tants, (3) nucleation of Pt clusters after reduction of Pt ions
with hydrazine (see Eq. (2)), and (4) growth of the particles
into the inverse micelles.
bility of the Pt deposit on BDD is clearly visible. A zoom
onto the 0–5 s domain is given in inset of Fig. 3. The ini-
®
tial increase of charge for the BDD–Pt–Nafion electrode
A TEM micrograph of Pt particles synthesized in w/o mi-
croemulsion system is shown in Fig. 2. It can be seen from
can be related to cleaning of the surface, i.e. oxidation of
the residues of surfactant by electrogenerated hydroxyl rad-
icals at the BDD electrode, whereas non-coated Pt is imme-
diately removed in important quantities. It seems possible in
®
the case of the BDD–Pt–Nafion electrode to define an op-
timum activation time of approximately 3 s. The CV of the
®
BDD–Pt–Nafion electrode recorded in HClO4 1 M after 0 s
(thin line) and 3 s (thick line) of activation at 2.5 V are shown
in Fig. 4. After activation, the Pt signal exhibits better-defined
H adsorption–desorption peaks of higher intensities, indicat-
ing that the process effectively cleans the surface.
Fig. 1. Schematic representation of nanoparticles synthesis by the w/o mi-
croemulsion technique: (1) mixing of the two microemulsions; (2) collision
between droplets, exchange of reactants and reduction of H2PtCl6 following
Eq. (2); (3) nucleation of Pt and (4) growth of the Pt particles into the aque-
ous droplets. A: H2PtCl6 aq., B: N2H4 aq.; inverse micelles containing the
reactants are dispersed into an n-heptane phase and stabilized by a surfactant
Fig. 2. TEM micrograph of platinum nanoparticles synthesized by the mi-
croemulsion method.
®
(BRIJ 30 of molecular formula C12H25(OCH2CH2)4OH) layer.

2
252
G. Sin e´ , Ch. Comninellis / Electrochimica Acta 50 (2005) 2249–2254
Fig. 3. Evolution of normalized charge in the H adsorption–desorption
region of Pt on a BDD–Pt (thin line with triangle markers) and a
BDD–Pt–Nafion (thick line with square markers) electrode vs. time of ac-
−1
◦
tivation at 2.5 V vs. SHE. Scan rate of 50 mV s in 1 M HClO4 at 25 C.
Inset: zoom on the 0–5 s domain.
The catalytic activity of the BDD–Pt electrode was tested
both for methanol and ethanol electrooxidations that are the
main reactions of interest for the DAFC technology. CV
of a BDD–Pt (thin lines) and of a BDD–Pt–Nafion (thick
lines) electrode in HClO4 1 M + CH3OH 0.1 M and HClO4
Fig. 5. Cyclic voltammograms of
a BDD–Pt (thin lines) and a
®
BDD–Pt–Nafion (thick lines) electrode in the presence of methanol (a)
®
−1
and ethanol (b). Scan rate of 50 mV s in 1 M HClO4 + 0.1 M CH3OH at
◦
◦
25 C (a), and in 1 M HClO + 0.1 M C H OH at 25 C (b).
4
2
5
1
5
M + C2H OH 0.1 M solutions, recorded at a scan rate of
5
0 mV s , are shown in Fig. 5a and b, respectively. The
−1
in reactions (3.1) and (3.2):
voltammograms of Fig. 5 exhibit the characteristic features of
methanol and ethanol oxidations on pure platinum. The CV of
methanol electrooxidation can for instance be described as a
sequence of multiple and complex physicochemical phenom-
ena [34]. At very low overpotentials (0.05 V versus SHE),
methanol easily adsorbs on Pt, adsorption that is followed by
alcohol dehydrogenation (during a fast process) as described
3Pt(H2O) + CH3OH → Pt3(CH3OH)ads + 3H2O (3.1)
Pt3(CH3OH)ads → Pt(CO)ads + 2Pt + 4H + 4e (3.2)
+
−
As a consequence, carbon monoxide (CO) blocks the Pt
active sites, explaining the flat feature of CV of Fig. 5a be-
tween 0.05 and 0.45 V versus SHE, approximately. CO to
be removed needs to be oxidized by hydroxyl radicals gener-
ated from water discharge on Pt. However, water dissociation
does not occur on Pt at potentials below 0.4 V versus SHE.
When this potential is attained, the following sequence can
take place (reactions (4.1) and (4.2)):
•
+
−
Pt + H2O → Pt( OH) + H + e
(4.1)
ads
•
→ 2Pt + CO2 + H + e−
+
Pt(CO)ads + Pt( OH)
ads
(4.2)
This explains the current increase after 0.45 V: Pt can
readsorb and reoxydate methanol. The current decrease after
0.9 V versus SHE can be related to the Pt surface state: cy-
cling the potential in acidic media up to 1 V versus SHE and
more, Pt oxides formation begins. Consequently, there is no
Pt free to dissociate water and the current decreases. During
the reverse scan of potential, Pt oxides are reduced, so water
Fig. 4. Cyclic voltammograms of a BDD–Pt–Nafion® electrode after 0 s
(
thin line) and 3 s (thick line) of activation at 2.5 V vs. SHE. Scan rate of
− ◦
1
5
0 mV s in 1 M HClO4 at 25 C.

G. Sin e´ , Ch. Comninellis / Electrochimica Acta 50 (2005) 2249–2254
2253
dissociation and methanol complete oxidation can take place
again. Finally, the current decreases again due to CO poison-
ing of the surface when the potential is not sufficiently high
to allow water dissociation. The second oxidation peak is of
higher intensity than the first one and located at less posi-
tive potential because this second oxidation takes place on a
“
fresh” Pt surface that is more active.
A similar mechanism should be theoretically expected in
the case of ethanol electrooxidation, however, ethanol com-
plete electrooxidation is a much more complicated case as it
needs the breaking of the C C bond. On platinum, this reac-
tion first yields to several adsorbed intermediates, namely ac-
etaldehyde and acetic acid that have been identified by means
of Fourier transform infra-red spectroscopy (FTIR) [35,36].
The breaking of the C C bond of these adsorbates is one
of the key problems in ethanol-feed DAFC. A good elec-
trocatalyst for the complete oxidation of ethanol is indeed
defined as an activator of the C C bond breaking [37]. Thus,
we can reasonably postulate that difference in activity of Pt
particles towards the oxidation of methanol and ethanol is
due to the inability of pure Pt to promote the breaking of the
C C bond of ethanol. This incomplete oxidation would lead
to acetaldehyde and/or acetic acid that only need two elec-
trons per molecule of adsorbate formed. This explains the
fact that in Fig. 5, activity of Pt is three times more impor-
tant towards methanol electrooxidation, and that the oxida-
tion current peaks are located at approximately 0.9 (forward
scan) and 0.75 V versus SHE (reverse scan) for methanol ox-
idation, and at 0.95 (forward) and 0.8 V versus SHE (reverse)
for ethanol oxidation.
Fig. 6. Evolution of normalized first oxidation peak currents of methanol on
a BDD–Pt (thin lines with triangle markers) and a BDD–Pt–Nafion® (thick
lines with square markers) electrode vs. time of activation at 2.5 V vs. SHE.
Scan rate of 20 mV s in 1 M HClO4 + 0.1 M CH3OH at 25 C. Inset: zoom
on the 0–5 s domain.
−
1
◦
(4 s at 2.5 V versus SHE) are shown in Fig. 7. It can be seen
that the onset of first oxidation peak is shifted to less positive
potentials by the activation procedure whereas the second
oxidation peak position is not changed. It can also be no-
ticed that after activation, the first oxidation peak intensity is
much important that the second one. Such behaviour can be
•
explained by the fact that electrogenerated OH radicals on
BDD may spillover onto the Pt surface, participating in the
methanol oxidation process itself and explaining both the in-
crease in oxidation current and the shift in peak position. The
oxidation peak recorded during the reverse scan is of lower
intensity than the first one, certainly due to the fact that all
the electrogenerated free hydroxyl radicals were consumed
during the forward scan. The current measured during the
reverse scan is only due to the Pt deposit and assistance of
It can also be seen in Fig. 5 that the intensities of CV for the
coated electrode corresponds to 70% of the signal of the non-
coated one, due to partial blockage of the catalytic surface
®
by the polymer. Nevertheless, Nafion does not affect peaks
shapes and positions. The same observations were made from
CV measured in pure supporting electrolyte (data not shown).
Fig. 6 shows the evolution of normalized oxidation cur-
rent peaks for methanol electrooxidation with and without the
•
the oxidation process by OH radicals may not occur. Never-
theless, the catalyst surface is more active than it was before
activation. Similar behaviour is also observed in the case of
ethanol electrooxidation (data not shown).
®
Nafion adlayer. Forclarity, onlytheevolutionofthefirstoxi-
dation peak (during the forward potential scan) is shown here,
but the same tendency is also observed, with lower levels of
®
increase, for the second one. Without Nafion , the curve of
methanol oxidation (thin line with triangle markers in Fig. 6)
begins with a strong increase followed by a continuous de-
crease. When the polymer is added (thick line with square
markers in Fig. 6), the initial increase of activity is of higher
importance and the decrease with time is limited. Initial en-
hancement of catalytic activity can be due to assistance of
oxidation by hydroxyl radicals when the polymer is absent,
and by combination of this process with additional cleaning
®
of the particles when Nafion is present. The final decrease in
activity can be related to the loss of catalytic material. Zoom-
ing on the 0–5 s region allows the definition of an optimum
activation time of 4 s.
Fig. 7. Cyclic voltammograms of a BDD–Pt–Nafion® electrode after 0 s
®
CVofaBDD–Pt–Nafion recordedin1 MHClO4 + 0.1 M
(
thin line) and 4 s (thick line) of activation at 2.5 V vs. SHE. Scan rate of
−
1
−1 ◦
4 3
20 mV s in 1 M HClO + 0.1 M CH OH at 25 C.
CH3OH at 20 mV s before (thin line) and after activation

2
254
G. Sin e´ , Ch. Comninellis / Electrochimica Acta 50 (2005) 2249–2254
4
. Conclusion
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The microemulsion method is a simple, efficient way to
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have revealed that the size of such synthesized Pt particles is
included in the 2–5 nm range that corresponds well with the
definition of a nano-object in catalysis. Activation of the Pt
deposit by hydroxyl radicals produced by water discharge be-
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comes feasible when a Nafion layer is added to the BDD–Pt
electrode. The polymer layer strongly stabilizes the particles
under these conditions, and optimum activation times, for
which activity reaches by a maximum, have been found to
be close to 3 s in all cases. Enhancement of activity thanks
to activation by hydroxyl radicals can be attributed to the
combination of two effects: cleaning of particles from the
remaining surfactant and assistance of the electrooxidation
reaction itself, and is characterized by an increase of oxida-
tion peak currents and a shift of onset of oxidation to lower
potentials. With the non-coated electrode, beneficial effect of
activation is counterbalanced by important loss of catalytic
material.
Further studies will help in investigating the relationship
between surface properties of the substrate and behaviour of
the Pt deposits, in terms of stability and catalytic activity. We
have, for instance, recently reported the possibility to modify
BDD characteristics by electrochemical treatment in organic
media, leading to polishing and enrichment in C O bonds
of the surface [38]. Current experiments are conducted on
platinum-based alloys microemulsion-synthesized nanopar-
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since they have been designated as the best candidate cata-
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Authors thank the CSEM Centre Suisse d’Electronique et
de Microtechnique SA, Neuch aˆ tel, Switzerland, for prepar-
ing the boron-doped diamond electrodes, and Professor
Ph. A. Buffat from the Interdisciplinary Centre of Elec-
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