Smectite clays as the quasi-templates for platinum electrodeposition

Article abstract of DOI:10.1016/j.electacta.2011.11.105

Smectite clays suspended in a platinum deposition solution are here shown to increase the specific surface area of electrodeposited Pt on gold and porous carbon paper supports, and to enhance the specific activity of Pt towards methanol oxidation under steady-state conditions. Organomodified natural bentonite shows a more pronounced effect with respect to natural montmorillonite and synthetic laponite. The effect of clay, as evidenced by microscopy and X-ray diffraction, is to decrease the average size of the Pt crystallites and simultaneously the degree of their mutual coalescence. This effect is associated with suppression of Pt secondary nucleation observed in deposition current transients, and the electrocatalytic activity is suggested to be affected by the intergrain boundaries.

Full text of DOI:10.1016/j.electacta.2011.11.105

Electrochimica Acta 61 (2012) 94–106
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Smectite clays as the quasi-templates for platinum electrodeposition
a
a
a
b
c
d
a
E.K. Lavrentyeva , S.Y. Vassiliev , E.E. Levin , A.A. Tsirlin , S.N. Polyakov , M. Leoni , K.S. Napolskii ,
a,1
a,∗,1
O.A. Petrii , G.A. Tsirlina
a
Moscow State University, Chemical Faculty, Leninskie Gory, 1-str. 3, Moscow 119991, Russia
Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, D-01187 Dresden, Germany
Technological Institute for Superhard and Novel Carbon Materials, Centralnaya Street, 7а, Troitsk, Moscow region, 142190, Russia
University of Trento, Via Mesiano 77, I-38123 Trento, Italy
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2011
Received in revised form
Smectite clays suspended in a platinum deposition solution are here shown to increase the specific sur-
face area of electrodeposited Pt on gold and porous carbon paper supports, and to enhance the specific
activity of Pt towards methanol oxidation under steady-state conditions. Organomodified natural ben-
tonite shows a more pronounced effect with respect to natural montmorillonite and synthetic laponite.
The effect of clay, as evidenced by microscopy and X-ray diffraction, is to decrease the average size of the
Pt crystallites and simultaneously the degree of their mutual coalescence. This effect is associated with
suppression of Pt secondary nucleation observed in deposition current transients, and the electrocatalytic
activity is suggested to be affected by the intergrain boundaries.
2
0 November 2011
Accepted 24 November 2011
Available online 6 December 2011
Keywords:
Clay
Template
©
2011 Elsevier Ltd. All rights reserved.
Platinum
Electrodeposition
Methanol oxidation
1
. Introduction
least those fabricated from the most usual chloride baths) demon-
2
−1
strate the specific surface area of several m g or higher.
Grain boundaries are certainly non-equilibrium structural fea-
tures of electrodeposits. The same features can be observed also
in carbon-supported catalysts when the metal loading is high. On
the other hand, high loading is recognized today as the preferable
(at least for fuel cell catalysts). For electrodeposited samples, the
grain boundaries are created in the course of secondary nucleation
of Pt on freshly formed surface of primary nuclei (initially deposited
crystals).
Our recent studies of Pt [5] and PtRu [6] have already confirmed
that, under some circumstances, the catalytic activity of dispersed
metals towards the oxidation of methanol and CO correlates with
the amount of grain boundaries. It is therefore clear that an optimi-
sation of the grain boundary should lead to the control of catalytic
activity. However, it is difficult to vary the quantity of grain bound-
aries without affecting crystal size and lattice defects that, in turn,
influence the catalytic activity as well. Those three interrelated fea-
tures are usually controlled by a single parameter, the deposition
potential.
Electrodeposited platinum catalysts can be fabricated in more
controllable manner as compared to chemically reduced materi-
als. For applications (e.g. in the context of fuel cells research), the
prospects of Pt electrodeposits on smooth supports are however
rather limited. Nevertheless, dispersed Pt electrodeposits attract
systematic attention [1–3], because they can be used as a model
system to study complex structural effects in electrocatalysis.
Structural features of these materials are in fact representative for
real catalysts that cannot be properly modelled by the macroscopic
size single crystals or well-arranged ensembles of separate single
crystalline nanoparticles.
Platinum deposits prepared from conventional electrolytes are
actually the assemblies of nanocrystals having an average effec-
tive diameter of ca. 10 nm and a size distribution highly dependent
on the deposition mode. These crystals hold together due to the
formation of intergrain boundaries [4,5]. Some portion of the inter-
nal surface of these dispersed porous materials is screened by the
boundaries, but about a half of the total true surface takes part in
the interfacial electrochemical processes. Basically, the deposits (at
In order to understand the structural complexity of electrode-
posited Pt, it is highly desirable to vary the content of grain
boundaries over a broad range keeping the same deposition poten-
tial. This is what we want to accomplish in this study with the use
of a quasi-templating procedure. The quasi-templating approach
was worked out earlier for the Pd-poly(ethylene glycol) [7,8] and
∗ Corresponding author. Tel.: +7 495 9391321; fax: +7 495 9328846.
E-mail address: tsir@elch.chem.msu.ru (G.A. Tsirlina).
ISE member.
1
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.105

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
95
When clay platelets are captured by the growing metal deposit,
there is a chance to increase the electrocatalytic activity due
to bifunctionality. The latter is known for clay-supported met-
als (intercalated clay catalysts) applied in heterogeneous catalysis
[
18,19]: bifunctional Pt-intercalated clay catalysts are commonly
used for the hydrogenation of aldehydes [20] and alkynes [21],
and sometimes demonstrate a synergetic increase in the activity
[
[
20,22]. Catalytic sites of clays are Lewis and Brønsted acid centers
23] located at the edges and surfaces of clay platelets, or formed by
hydrated exchangeable cations. Despite all clays are insulating, a lot
of examples of clay-modified electrodes (CME) can be found, espe-
cially in electroanalysis and in molecular recognition applications
[
24–26]. CME conductivity results from the permeability of clay for
small molecular/ionic species and by fast proton transport along the
water-hydroxide net. However, some aspects of the CME electro-
chemistry remain unclear, especially those related to the electron
transfer mechanism: for instance an electron-hopping mechanism
is widely accepted for zeolite-containing electrodes [27,28].
Smectite clays are here used as a suspension added to the plat-
inum deposition bath. The clay particles in these suspensions are
expected to be 30–50 nm long and a few nm thick [29–31], i.e. of a
size comparable with that of Pt nanocrystals in the electrodeposits
Fig. 1. The crystal structure of the smectite clay. T and O denote the layers of silicate
tetrahedra and aluminium (magnesium) octahedra, respectively; circles show the
exchangeable cations accommodated in the interlayer space.
(5–20 nm [1]). To establish the effect of clays on the properties
of the Pt electrodeposit, we compared the influence of three dif-
ferent smectites, namely Laponite (Lap), Montmorillonite (Mont)
and Bentonite (Bent), on the structure of the Pt deposits. Lap is a
single-phase synthetic clay that closely resembles the natural clay
hectorite, Mont is a purified natural montmorillonite clay, and Bent
is a natural montmorilonite modified with an organic component
to improve the stability of clay suspensions in water [32].
the Pt-dodecatungstate [9] systems. Quasi-templating is based on
the effect of strongly adsorbing substances on the mutual location
and ordering of growing nanocrystals. In quasi-templated elec-
trodeposits, the mean crystallite size and/or the degree of surface
screening by grain boundaries tend to decrease as compared to
non-templated deposits formed at the same potential. Compared
to non-templated deposits, a higher specific surface area can be
achieved for the same size distribution.
2. Experimental
Our hypothesis (qualitative at this stage) on the quasi-
templating effect consists in the suppression of the growth of Pt
crystallites by large templating molecules. We assume that the
templating agent should not be necessarily dissolved; the same
role can be played by colloid particles immobilized on the freshly
formed deposit surface. Electrocrystallization-accompanied elec-
trophoretic deposition of colloid particles is a wide research area
Electrodeposition of Pt and electrochemical measurements
◦
were carried out at 25 C with an EG&G potentiostat PARC 273 in
a three-electrode cell. The counter electrode was a Pt foil, and the
reference electrode was a saturated calomel electrode (SCE) con-
nected to the cell via a Luggin capillary. Hereinafter, the potentials
E are given versus RHE (ESCE − ERHE = 0.247 V in aqueous solution of
0
.5 M H SO ).
2 4
(
see e.g. [10,11]), but silicate and alumosilicate colloid modifiers
2
Samples of polycrystalline gold foil (0.2 mm thick, 2 cm ) and
are only scarcely studied [12].
2
carbon paper (∼0.19 mm thick, 2 cm ; Toray Industries, Inc.) were
used as supports. Prior to electrodeposition, the gold foils were
etched in hot aqua regia for 5 s to obtain a reproducible surface
state. The carbon paper supports were used without any pretreat-
ment. Platinum was electroplated from aqueous solutions 0.01 M
Na PtCl + 0.02 M HCl and from clay suspensions of 0.2, 0.5 and
Some clays, zeolites and various oxohydrated silicates have been
previously used as the additives to plating solutions [13,14] or in
the form of gel containing the discharging metal ion [15]. Various
types of metal dispersions were obtained due to the effects of the
colloidal medium on the diffusion of the reagent and the specific
bonding of metal ions to the particles in suspension.
Our choice of smectite clays as a templating agent is moti-
vated by their stability in acids and their general chemical inertness
that can avoid the risk of Pt poisoning and catalyst degradation.
Smectites are layered aluminosilicates with a general formula
Six(Al/Mg)yOp(OH)q. They have a 2:1 layered structure with a sin-
2
6
1
wt.% in this solution. The suspensions were mixed with a mag-
netic stirrer in closed flasks for 2, 3 and 6 h, respectively. An Ocean
Optics spectrophotometer was used to control the concentration
of hexachloroplatinate in the deposition solutions. Unless other-
wise noted, all samples specified as Pt/clay refer to those deposited
from the 0.2 wt.% suspensions. The electrodeposition of Pt was per-
formed at constant potentials Ed = 100 mV and 250 mV. The total
gle layer of AlO (MgO ) octahedra (Fig. 1, O-layers) sandwiched in
6
6
2
between two layers of silicate tetrahedra (Fig. 1, T-layers). A vari-
ety of smectites results from Al(Mg) substitution in the O-layers
and/or Si in the T-layers by lower-charged cations. The substitution
results in excess of electrons in the layers [17] that is compensated
by loosely bound exchangeable cations located in the interlayer
space (Fig. 1). Clay particles suspended in aqueous medium are usu-
ally aggregates of several platelets arranged parallel to each other,
with a nanometer range periodicity.
charge Q spent for electrodeposition was always 2C for gold sup-
d
ports; for all the deposits on carbon paper supports, the values are
tabulated below. The weight of Pt in the samples was close to 1 mg
for Au support) and to 3 mg (for carbon paper support), therefore
(
the accuracy of Pt weighting hardly exceeded 20%. An averaged
mass of 5–6 samples for each type of deposit was employed to esti-
mate the specific surface areas of Pt (S(C)) and the current efficiency.
True surface area SH of electrodeposited Pt and Pt/clay samples
was determined coulometrically from the hydrogen desorption
−
1
region of cyclic voltammograms (CV) measured at 0.1 V s within
2
Images were obtained using the VESTA software [16].
the potential range of 0.06–1.24 V in a 0.5 M H SO4 solution
2

9
6
E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
deaerated with Ar for 1 h. The potential was cycled until a stationary
voltammogram was obtained (usually in the fiftieth cycle). The sur-
face area of all freshly deposited samples was determined prior to
the structural analysis. For prolonged ageing experiments, the sam-
1
00.0
(A)
8
0.0
−1
ples were cycled at 0.02 V s in the potential range of 0.06–1.4 V.
Steady-state polarization curves were measured in 0.1 M
CH OH + 0.5 M H SO under potentiostatic mode; the current was
6
0.0
3
2
4
4
0.0
considered stationary if its further change with time was less than
Bent
0.7% per minute.
To prepare the plating bath, MilliQ water, Na PtCl6 (Merc, GR
20.0
0.0
2
for analysis), H SO (Merc, GR for analysis), HCl (Merc, GR for anal-
2
4
®
+
ysis), CH OH (distilled), Montmorillonite (Cloisite Na , Southern
3
1
0.0
20.0
30.0
40.0
50.0
60.0
70.0
Clay Products, Inc.), Bentonite (Fluka Chemika-BioChemika Corp.)
and Laponite (Laponite RD, Conservation Resources INT’L LLC.) were
used.
STM imaging was carried out ex situ with an “Umka” instru-
ment (“NanoIndustry”, Moscow), using −0.3 V bias and tunneling
currents up to 0.5 nA (no effects of bias sign were observed).
Pt–Ir STM probes were sharpened by mechanical cutting of Pt–Ir
wire (10 wt.% Ir). To construct size distributions, six images of
2
θ / degree
(
B)
100.0
80.0
60.0
1
20 × 120–180 nm × 180 nm size obtained from two identical sam-
ples of each type of the deposit were used, with about 50–120
isolated particles visualized in each image. Submicron-resolution
scanning electron microscopy was performed using both a LEO
Supra 50 VP (Carl Zeiss, Germany) and a JEOL JSM6380 (Jeol, Japan)
instrument equipped with INCA 250 EDS (Oxford Istruments, UK)
analysis module. TEM imaging was carried out with a digital LEO
40.0
Lap
20.0
Mont
0.0
1
0.0
20.0
30.0
40.0
50.0
60.0
70.0
9
12 AB (Carl Zeiss, Germany) microscope.
X-ray powder diffraction (XRD) data for Pt electrodeposits were
2
θ / degree
collected in step-scan mode at room temperature using a ꢀ–2ꢀ
Bragg–Brentano RIGAKU D/max RC diffractometer operating with
CuK␣ radiation and equipped with a graphite analyser and a scintil-
Fig. 2. XRD patterns of Bent (A) and Mont, Lap (B). Solid vertical lines show the
reflections of Mont [33] (#29-1498). Dashed vertical lines, solid circles, and open
circles indicate impurities in the Bent sample: opal [33] (#38-448), calcite [33] (#72-
1937), and quartz [33] (#46-1045), respectively.
◦
lation detector. XRD patterns were recorded in the 2.6–140 range
◦
of 2ꢀ with a step of 0.02 . For phase identification, the ICDD PDF-2
database [33] was used. The instrumental line profile broaden-
shapes and intensities of the reflections are somewhat different for
Mont, Lap and Bent due to the difference in their chemical composi-
tion and microstructure. For Bent, additional reflections are found,
indicating the presence of opal (#38-448), quartz (#46-1045) and
calcite (#72-1937) as impurities.3 All these impurities are typical
for bentonite as a natural multi-phase material [36]. The ambiguity
in chemical composition and the irregular shape of the reflections
prevented us from a quantitative phase analysis. A rough estimate
using the strongest reflections yields 30–60 wt.% of the smectite-
type phase in the Bent sample. The Mont sample contains a small
amount of opal, while Lap is free from crystalline impurities.
Chemical composition of the clays (Table 1) is more complex
for Mont and Bent as compared to Lap. According to TG-DTA, Bent
and Mont samples contain, respectively, ca. 20 and ca. 2 wt.% of
organic substances, while in Lap only traces of organic contami-
nations are found. Unfortunately, the composition of the organic
stabilizer in Bent is not provided by the manufacturer. Accord-
ing to our Raman spectroscopy and chromatography investigation,
this stabilizer consists mostly of aromatic compounds. Mass spec-
troscopy in the course of TG-DTA measurements confirms thermal
decomposition of these compounds with the formation of hydro-
carbons and carbon dioxide. Correspondingly, in what follows, one
should keep in mind the possible influence of organic substances
on the deposition of Pt templated with Bent.
ing effects were measured on the NIST SRM660a (LaB ) standard.
6
Microstructural analysis (crystallites size distribution in terms of
coherently scattering domains, dislocation density and stacking
faults concentration calculations), was done within the framework
of the Whole Powder Pattern Modelling (WPPM) approach [34]
using the PM2K software package [35].
The Philips PW2400 X-ray fluorescence spectrometer (INEOS
RAS, Moscow) was used for the chemical analysis of the clays when
no data from the manufacturer were available. A Netzsch STA 449
PC instrument was used for the TG-DTA analysis of Mont and Bent:
thermal decomposition products were monitored online using a
Netzsch QMS 403C mass spectrometer. In this analysis, a 112 mg
◦
◦
−1
sample of Bent was heated up to 900 C (10 C min ) in oxygen
−1
flow (40 ml min ). Additionally, we investigated the organic sta-
bilizer contained in the Bent sample by liquid chromatography
(
Shimadzu LC-2010A) and Raman spectroscopy (Raman RXN1 Sys-
tems, Kaiser Optical Systems, Inc.). To extract the stabilizer, Bent
was mixed in a proportion 1:100 with carbon tetrachloride (for liq-
uid chromatography) or water (for Raman spectroscopy), and kept
at room temperature for three days under slow stirring. Before the
measurements, the clay particles were separated by centrifugation.
3
. Results and discussion
To estimate the size of clay fragments in suspension, we put
a drop of the aqueous suspension (0.1 wt.%) on a highly oriented
pyrolitic graphite (HOPG) substrate. We were able to observe some
aggregates (Fig. 3) even in the ex situ STM configuration: the
3.1. Clays characterization
The complete characterization of clay samples is complicated
by their possible multi-phase nature and the flexibility of chemical
composition. The XRD patterns of all three clays (Fig. 2) show the
features typical of layered systems (diffraction bands), although the
3
The numbers provided in parentheses are those of the corresponding card in the
ICDD PDF-2 database [33].

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
97
Table 1
Chemical composition of clays (wt.%).
Clay
Na2O
MgO
Al2O3
SiO2
P2O5
S
K2O
CaO
TiO2
Fe2O3
LiO2
LIa
Mont
Bent
Lap
5.61
6.35
2.8
1.05
1.11
27.5
17.36
9.07
–
56.74
43.75
59.5
0.03
0.24
–
0.32
0.20
–
0.07
0.81
–
1.73
1.62
–
0.12
0.27
–
4.43
1.05
–
–
–
0.8
13.14
35.43
8.2
b
a
◦
Losses on ignition (for Mont and Bent were determined at 950 C).
www.laponite.com
b
natural moisture in the tunneling gap may indeed facilitate
the imaging of insulating fragments (in this case, as the STM
device operates like a Scanning ElectroChemical Microscope, SECM
attached electrostatically. An example for the Pt/Bent system at
100 mV (Fig. 4B) demonstrates a less pronounced inhibition than at
Ed = 250 mV. Here, we also have to consider the effect of the organic
stabilizer equally adsorbing at both 100 and 250 mV. The adsorp-
tion of organic molecules will result in additional screening of the
growing surface. However, when comparing the Bent and Mont
systems, we found no correlation between the deposition kinetics
and the amount of organic component, although Bent contains ten
times more organic component than Mont (Fig. 4A).
[
37,38]). The thickness of Mont particles is ca. 15 nm, and their
diameter is ca. 70–100 nm (Fig. 3a). Lap particles are of ca. 70 nm
size and ca. 5 nm thick (Fig. 3b), even if their mean size reported by
the manufacturer (http://www.laponite.com) is significantly lower
(
ca. 25 and 1 nm, respectively). Most likely, we cannot resolve
some boundaries between individual particles in the aggregates
immobilized on HOPG, so the above values for Mont and Lap can
be overestimated. Characteristic size of Bent particles (typically
rectangular) is 40–60 nm, and the thickness is ca. 10 nm (Fig. 3c).
This is in agreement with more detailed studies reported e.g. in
Refs. [31,39]. Opal, a component of Bent, is a partly amorphous
phase of hydrated silica formed by cristobalite-like network of
SiO4 tetrahedra. This network is three-dimensional, but differ-
ent shapes (fibrous, plate-like) of opal microcrystals have been
reported (see [40] and references therein). Therefore, an assign-
ment of the imaged particles from Bent suspension to a certain
phase remains somewhat ambiguous.
Another phenomenon, which can accompany electrodeposition,
is the electrophoretic movement of the negatively charged clay
particles from the cathode where Pt deposition occurs. A rough
estimate based on Smoluchowski equation [43] and typical zeta
potential of smectite clays (ca. 10 mV in suspensions at pH ∼ 2 and
the same ionic strength as for the deposition solution under inves-
−
1
tigation [44,45]) yields 10 nm s rate of electroporetic movement.
As soon as this value is comparable with the rate of the deposit
growth, the mechanical capture of the clay particles by the growing
deposit is surely possible.
Current efficiency determined gravimetrically is 70-100% (see
discussion in Ref. [1]), but the accuracy of these values is fairly
low because of the low deposit thickness (200–250 nm). Within
this accuracy, no systematic effect of clay on current efficiency was
found.
3.2. Electrodeposition
Experimental transients for Pt and Pt/clay deposition at
Ed = 250 mV on Au are shown in Fig. 4A. Despite the pronounced
scatter of current values, some tendencies in transients’ behavior
are well reproducible in all the experiments. For deposition from
Bent- and Mont-containing suspensions, the current remains lower
than for clay-free solutions and the suspensions of Lap. Bent and
Mont affect the shape of the transients as well: the decrease in
current at the initial period (0–100 s) appears to be less sharp;
when approaching the plateau region (>300 s), the current starts
to decrease sharper. The lowest currents were observed for Mont
suspensions.
3.3. True surface area
The voltammograms of Pt and Pt/clay samples deposited at
250 mV (for equal geometric areas and deposition charge) are given
in Fig. 5A. Independently of deposition potential (Fig. 5b, Table 2),
the presence of clay in the deposition solutions leads to an increase
in SH as determined from hydrogen desorption with identical dou-
ble layer correction. Owing to the inaccuracy of the weighting
procedure, the accuracy of the recalculated specific surface area S
(C) is much lower than that of SH. However, the increase in S (C) due
to the Bent and Mont addition (Table 2) exceeds the possible error
We should stress that no pronounced effect of the deposition
potential on the current value is observed in the absence of Bent,
2
−1
while in Bent-containing medium the current at E = 250 mV is
of few m g . In contrast to the Pt/Bent sample, Pt/Mont and Pt/Lap
samples show a pronounced difference in the shape of the hydro-
gen desorption region of voltammograms (as compared to typical
Pt deposits): the relative height of the second peak is reduced, and
both peaks are shifted to lower potentials (Fig. 5A).
d
significantly lower during the whole deposition period than the
current at E = 100 mV (compare Fig. 4A and B). Clay effect on the
d
transient shape is less pronounced at E = 100 mV compared to
d
Ed = 250 mV; in the former case, no current decrease during the
initial period (0–160 s) is observed (Fig. 4B).
Among Pt/clay samples, Pt/Bent demonstrates the highest spe-
cific surface area, while Pt/Lap is similar to usual Pt deposits (Fig. 5A,
Table 2). In the former, one cannot exclude an additional effect
of the organic stabilizer, as some organic surfactants are known
to support platinum nanostructuring [46–49]. Additional experi-
ments were performed in order to check for the possible templating
effect of the organic stabilizers dissolved in the deposition solu-
tion. First, the suspension of Bent in the same portion of Milli-Q
water was prepared three times, with a subsequent separation of
the precipitate by centrifugation. Each time a new portion of Bent
was suspended to achieve a higher concentration of the stabilizer in
water. Then two deposition solutions were prepared: (a) by adding
Spectrophotometry demonstrates that clays suspended in the
deposition solution do not lead to any decrease in the concentration
of the hexachloroplatinate anion. This means that the “clay effect”
on the nucleation-growth rate is hardly induced by complexing of
the hexachloroplatinate anion with the functional groups at the
surface of clay particles. Electrostatic reasons look more plausible.
Although the potential of zero free charge (pzfc) was not evaluated
precisely for solutions under investigation, this value can be esti-
mated as ca. 220 mV by taking into account the pH-dependence
of pzfc [41,42]. Therefore, at E = 250 mV the free charge of plat-
d
inum surface is already positive, and electrostatic adsorption of clay
should be expected, thus inducing the screening of newly formed
growing surface. In contrast, electrodeposition at 100 mV obviously
takes place at the negatively charged support, and no clays can be
Na PtCl6 into water separated from washed Bent, and (b) by sus-
2
pending washed Bent in the usual platinum plating solution. These
experiments confirmed that organic stabilizers can increase the

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E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
Table 2
True surfaces (SH) and specific surfaces S(C), determined coulometrically from the hydrogen desorption region of CV; S(STM) and degree of coalescence calculated from STM
data for Pt and Pt/clay samples deposited on Au. Total charge spent for electrodeposition (Qd) was 2 C for all samples.
SH (cm2)
S(C)a (m2
g
−1
)
S(STM) (m2
g
−1
)
Degree of coalescence,
-S/S(STM)
Sample
1
Pt (Ed = 250 mV)
92 ± 8
122 ± 2
151 ± 8
98 ± 0
13.0 ± 4.8
17.5 ± 5.3
21.6 ± 7.3
14.0 ± 4.0
18.0 ± 5.6
11.6 ± 4.0
15.7 ± 4.8
22 ± 2
24 ± 0
24 ± 0
22 ± 0
–
0.41 ± 0.27
0.27 ± 0.22
0.10 ± 0.30
0.36 ± 0.18
–
Pt/Mont (Ed = 250 mV)
Pt/Bent (Ed = 250 mV)
Pt/Lap (Ed = 250 mV)
Pt (Ed = 250 mV) control experiment (see text)
Pt (Ed = 100 mV)
125 ± 3
105 ± 13
157 ± 17
23.5 ± 0.5
0.51 ± 0.12
Pt/Bent (Ed = 100 mV)
24 ± 0
0.35 ± 0.13
a
S(C) is normalized per the average mass of 5–6 samples.
specific surface area of Pt. However, their effect is less pronounced
than the effect of non-washed Bent in suspension, despite of much
higher concentration of the organic substances in the deposition
solution (a). The specific surface area obtained in these control
solution (Fig. 6C). Si-containing inclusions remained visible even
after a thorough washing of the samples under ultrasonic stirring,
thus indicating that the clay was strongly attached to the Pt deposit.
Since the resolution of the SEM images is still too low to visualize
separate Pt crystals, we applied ex situ STM. Despite of a small (and
unavoidable) drift, resulting in the apparent egg-shape, particle size
can be reliably determined from cross-sections. Mutual location of
the particles can be determined from this data as well. Figs. 7 and 8
2
−1
experiments does not exceed 18 m g , whereas for Pt/Bent it is
2
−1
clearly above 20 m g (Table 2). In the control experiment (b), no
templating effect was observed. This is not surprising, because the
suspension of washed Bent is unstable (it precipitates in ca. 10 min).
Since not only organically modified Bent, but also Mont (with
much lower content of organic substances) increases the specific
surface area of Pt deposits (Table 2), the templating effect can not
be attributed only to the presence of the organic additives in the
Bent suspension. We conclude that, at least for Bent, templating
is induced by both clay and stabilizer in parallel. The presence of
the latter can be risky for Pt catalytic activity, as organic molecules
could strongly adsorb on the active sites of the catalysts and act as
a poison. Fortunately, our voltammetric experiments with Pt foil
in 0.5 M H SO containing suspended clays demonstrate a slightly
present Pt and Pt/clay images and corresponding size distributions
4
of the Pt particles. Samples deposited at E = 250 mV and 100 mV
d
are compared in Figs. 7 and 8. Like usual Pt deposits [1], Pt/clay
materials consist of nm-size crystals. No STM features indicating
the presence of clay particles are found.
As a first approximation, we fitted all distributions using lognor-
mal functions, and determined the mean particle size (D) (Table 3).
These D values were used to estimate the weight of Pt corre-
sponding to the formation of one monolayer of Pt particles, and
corresponding deposition charge. The latter was compared to the
time dependence of the experimental deposition charge obtained
by integrating the curves in Fig. 4. We found that, for all samples,
the hypothetic first monolayer of Pt particles can be formed during
the first 30–50 s of deposition (this time range is shown by dashed
vertical lines in Fig. 4). The primary nucleation maximum should
surely precede the completion of the monolayer of primary crystals.
Moreover, secondary nucleation always starts before a complete
monolayer of primary Pt crystals is formed. Therefore, broad max-
ima at the deposition current transients cannot be assigned to
primary nucleation, and traditional nucleation models should not
be applied in this case. The most pronounced and reproducible clay
effect on the deposition kinetics (current decrease after the maxi-
mum) should be discussed in the context of secondary nucleation
exclusively. A comparison of transients normalized per current
maxima demonstrates that current growth is much stronger in the
absence of clays (Fig. 4C). For pure Pt samples, when ca. 4–5 effec-
tive monolayers of Pt particles are deposited, the current changes
only slightly (this region can be already solidly assigned to repeated
secondary nucleation events). Contrary, for Pt/Bent sample the
current decrease is systematically observed as the number of
effective monolayers of Pt particles increases. Deposition poten-
tial demonstrates no influence on the current in the initial
region for both types of samples. This inhibitory effect of clays
2
4
distorted hydrogen adsorption region, but satisfactory purity of
the surface is easily achieved by using the conventional cycling
pretreatment (usually in the fiftieth cycle). This means that the
presence of the organic stabilizer in Bent is not detrimental for the
catalytic activity.
Actually, the increase in the specific surface area is rather typical
for template-assisted deposits [7,9]. To propose a more controllable
quasi-templating procedure, it is important to understand whether
such an increase results from smaller size of Pt crystals, from lower
degree of coalescence, or from both factors simultaneously. To clar-
ify the problem, the results of SEM, STM and XRD characterization
of deposits are discussed below.
3.4. Morphology
SEM images of Pt/Bent samples deposited on Au (Fig. 6) clearly
show that the adhesion of Pt/Bent deposited at 250 mV (Fig. 6B)
is weaker as compared to samples deposited at 100 mV (Fig. 6E)
as well as to usual Pt deposited at the same potential (Fig. 6A).
All these images differ qualitatively from the images of Au sup-
port reported earlier [4,5]. The appearance of cracks gives indirect
evidence of a lower coalescence of the Pt particles in the Pt/Bent
samples deposited at 250 mV, and/or of the relaxation of high
residual stresses previously present in the deposit. Despite the
morphology of our thin deposits is still strongly affected by the
roughness of the etched gold support, it is possible to recognize a
more distinct globular microstructure for Pt/Bent (Fig. 6E) than for
is independent of E , so we assume that it can be related
d
only to mechanical capture of the clay particles by the growing
deposit.
Size distributions (Figs. 7 and 8) demonstrate two effects of
quasi-templating with clay. First, for E = 250 mV the distribution
d
Pt (Fig. 6D) at E = 100 mV.
d
Microanalysis (EDX) failed to discover any appreciable quan-
tities of Si or other clay-related chemical elements inside the
platinum deposits. The presence of clay inclusions (huge Bent agre-
gates) was confirmed only for higher Bent content in the plating
4
We hope that the resolution of STM makes it possible to visualize separate single
crystalline Pt particles, this possibility was demonstrated in Ref. [50].

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
99
-
-
2.0
1.6
Pt(250 mV)
Pt/Bent(250 mV)
Pt/Lap(250 mV)
Pt/Mont(250 mV)
-
-
-
2.0
1.5
1.0
-
-
-
1.2
0.8
0.4
0
50
100
(
A)
0.0
0
1000
2000
3000
4000
t / s
-
-
2.0
1.6
Pt(100 mV)
Pt/Bent(100 mV)
-
-
-
1.2
0.8
0.4
-
-
-
2.0
1.6
1.2
(
B)
0
50
100
0.0
0
500
1000
1500
t / s
1
1
0
.5
Pt(100 mV)
1
.5
.2
.9
.6
Pt/Bent(100 mV)
Pt(250 mV)
.2
.9
Pt/Bent(250 mV)
1
0
0
0
50
100
(C)
0
300
600
900
1200
t / s
Fig. 4. Comparison of deposition current transients measured on Au electrode in
.01 M Na2PtCl6 + 0.02 M HCl solution (Pt) and in clay suspensions in this solution
Pt/Mont, Pt/Bent and Pt/Lap). Deposition potential (mV vs. RHE) is indicated in
0
(
parentheses: 250 mV (A) and 100 mV (B). Plot (C) demonstrates deposition tran-
sients for Pt and Pt/Bent normalized per the current value in transient maximum.
Vertical dashed line corresponds to the hypothetic formation of one monolayer of
crystals of ca. 10 nm size, as estimated from STM and XRD data for Pt deposits. Insets:
magnified intervals of t = 0–100 s.
Fig. 3. STM images of highly oriented pyrolitic graphite with the clays immobilized
from aqueous suspensions of Mont (a), Lap (b) and Bent (c). The concentration of
clay in suspensions is 0.1 wt.%.
becomes more narrow (particles of diameter exceeding 17 nm
disappear (Fig. 7b–d) as compared to the distribution for pure
Pt deposited in the absence of clay (Fig. 7a). The effect is more
pronounced for Pt/Bent and Pt/Mont and is minimal for Pt/Lap. Sec-
ond, for the Pt/Bent sample deposited at E = 100 mV (Fig. 8b) the
d

1
00
E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
Pt(250 mV)
of the particles by means of STM. This unavoidable underestimation
is confirmed independently by XRD data.
2
1
0
0
0
Pt/Mont(250 mV)
Pt/Bent(250 mV)
Pt/Lap(250 mV)
3.5. XRD characterization
The XRD patterns of the Pt deposits clearly show just the peaks
of Au and Pt (Fig. 9). No traces of alumosilicate or silicate phases
were found, despite the patterns were registered starting from
◦
2
ꢀ ∼ 3 . According to Ref. [51], the reflections of clay appear for elec-
trophoretically deposited clay films when the thickness is above
25 nm. The content of clay mixed with Pt to be detectable by XRD,
estimated using the PowderCell software [52] and the RIR tech-
nique [53], is quite high (ca. 10 wt.%). Therefore, our XRD data do
not exclude the presence of clay in the composite deposits.
∼
-
-
10
20
(
A)
In order to quantify the lattice defects in the Pt deposits, we
applied the WPPM technique which performs a microstructure
analysis from powder diffraction data on the basis of physically-
grounded models of the material [34]. We assumed that diffraction
line profile broadening results from three factors, namely, the
small crystallite size and the presence of dislocations and stack-
ing faults. For comparison with STM data, we assumed Pt domains
being spherical and showing a lognormal distribution of sizes. The
microstructural parameters extracted from the data were: mean
crystallite size D (XRD) and variance of the lognormal distribution
0
.0
0.3
0.6
0.9
1.2
E vs. RHE / V
2
1
4
2
0
Pt(100 mV)
Pt/Bent(100 mV)
[
54], dislocation density ꢁdisl, deformation fault ˛ and twin fault ˇ
probabilities.
The lattice parameters a(Pt) are slightly different for the sam-
ples under investigation (Table 3). However, the accuracy of these
values can be lower than specified in Table 3 in terms of the formal
statistical error from the least-squares refinement. We assume the
real precision to be 0.01 A˚ , thus the deviation of the cell parameters
from the bulk value (abulk(Pt) = 3.9231 A˚ (PDF2# 4-802)) is marginal
-
-
12
24
(
B)
and can be hardly considered as a signature of lattice compression
that was previously found in Pt electrodeposits fabricated at higher
potentials [5].
0
.0
0.3
0.6
0.9
1.2
E vs. RHE / V
The results of the WPPM analysis demonstrate that Bent has
the most pronounced effect of on the microstructure of Pt deposits,
as it causes a decrease in crystallite size and an increase in disloca-
tions density, as compared to the samples deposited from Bent-free
solutions. We should note that for Pt/Bent and Pt (250 mV) sam-
ples D (XRD) is significantly lower than D (STM). For other clays,
the specific microstructural features are less pronounced, and D
(XRD) approaches D (STM). Variance is always lower for templated
Fig. 5. CVs of Pt and Pt/clay samples. Deposition potential (mV vs. RHE) is indicated
in parentheses: 250 mV (A) and 100 mV (B).
maximum of the distribution shifts towards smaller particle sizes
as compared to pure Pt (Fig. 8a).
Using the experimental size distributions, we can also estimate
the degree of coalescence of the Pt particles. The expected specific
surface area S(STM) is calculated under an assumption of zero coa-
lescence, i.e. assuming that the whole surface of all the particles is
accessible. Thus, S(STM) is the sum of the surface area of all par-
ticles normalized per their total weight. This value is compared to
the experimental S(C): when S(C)/S(STM) approaches 1, the degree
of coalescence tends to zero. Using this approach, we found that
Bent and Mont decrease the degree of coalescence in Pt deposits,
while Lap does not affect this value significantly (Table 2).
In contrast to Pt samples, a significant effect of the deposition
potential on the degree of coalescence is observed for Pt/Bent sam-
samples, and this decrease is more pronounced at E = 250 mV (like
d
other E effects mentioned above).
d
The calculated values of ꢁdisl are similar to those frequently
observed in severely deformed metals [55] or oxide materials [56].
The clays generally increase ꢁdisl (with Pt/Mont sample being a
reproducible exception), and simultaneously decrease the proba-
bility of deformation faults.5 Thus, the templating with clays not
only increases or decreases the defectiveness of the system, but
also induces a redistribution of various types of defects. XRD data
demonstrate that Lap also affects the microstructure of deposited
Pt, although its effect on the surface area and deposition kinetics
could not be discerned.
ples: Pt particles deposited at E = 100 mV are more coalesced than
d
those deposited at E = 250 mV (Table 2). The decrease in the coa-
d
lescence should be attributed to a smaller overlap of the secondary
nucleation centers. This finding agrees with our assumption that
Unfortunately, the present analysis of the XRD data does not give
any detailed information concerning intergrain boundaries that
represent highly disordered regions of the sample and marginally
contribute to XRD patterns. However, the observed dislocation den-
sity can be due to grain boundaries that, basically are macroscopic
defects for the system.
electrostatic adsorption is more pronounced at E = 250 mV.
d
Coalescence degrees of 0.3/0.7 are typical for Pt electrodeposits
[
5], while the value for Pt/Bent (E = 250 mV) is anomalously low
d
(
Table 2) and corresponds formally to the lack of any coalescence.
This value does not certainly look realistic, as the deposit is hardly
stable without the grain boundaries. However, S(STM) values could
be underestimated if the size of some particles is overestimated
5
We should bear in mind that PM2K utilizes the formalism of Warren to evaluate
(
due to non-zero curvature radius of the STM probe). It may be dif-
the stacking faults concentration, and therefore results are meaningful only for low
concentrations (below 5–10%).
ficult to identify the boundaries between coalesced crystals in some

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
101
Table 3
Cell parameters and averaged crystallite sizes for XRD patterns of Pt and Pt/clay samples deposited on Au, D (XRD) (lognormal mean) and variance (distribution half-width),
dislocation density ꢁdisl, deformation fault ˛ and twin fault ˇ probabilities. D (STM) values are listed for comparison.
Sample
a (Pt), A˚
D (XRD), nm
Variance (XRD), nm
ꢁdisl (×10−16, m−2
)
˛ (%)
ˇ (%)
D (STM), nm
Pt
3.9197 ± 0.0001
3.9168 ± 0.0002
3.9186 ± 0.0001
3.9213 ± 0.0001
3.9149 ± 0.0001
3.9151 ± 0.0001
8.6 ± 0.6
13.1 ± 0.4
7.1 ± 0.4
11.7 ± 0.8
11.8 ± 0.7
7.0 ± 0.3
4.3 ± 0.3
3.9 ± 0.1
2.8 ± 0.1
3.1 ± 0.2
3.6 ± 0.2
3.4 ± 0.1
4.00 ± 0.01
3.30 ± 0.01
4.78 ± 0.01
4.41 ± 0.05
4.42 ± 0.01
7.35 ± 0.01
1.8
0
0.8
0
0
0
0.3
0
0.7
0
0.6
0.6
11.7
11.7
11.3
12.1
10.8
10.2
(
Ed = 250 mV)
Pt/Mont
Ed = 250 mV)
Pt/Bent
Ed = 250 mV)
Pt/Lap
Ed = 250 mV)
Pt
(
(
(
(Ed = 100 mV)
Pt/Bent
(Ed = 100 mV)
Fig. 6. SEM images of Pt (A and D) and Pt/Bent (B, C and E) samples. Deposition potential (mV vs. RHE) is indicated in each photo. (C) SEM and local analysis data for Pt/0.5 wt.%
Bent sample (i.e., higher Bent content as compared to usual 0.2 wt.%).
3.6. Possible nature of clay effect on Pt dispersity
(iii) it prevents the neighboring nanoparticles of Pt from coales-
cence in the course of deposition, especially at E = 250 mV;
d
Based on the experimental data, we conclude that the clay in the
this hypothesis is confirmed by our analysis of the S(C)/S(STM)
deposition solution can template platinum in at least three ways:
ratios.
(
i) it screens some active centers of the Au support, and therefore
it decreases the number of contacts between the deposit and
the support, resulting in lower adhesion;
Point (iii) can be easily understood in terms of the screening of
some nucleation centers even without any appreciable effect on
the secondary nucleation rate constant. Otherwise, one is unable
to explain the decrease in the mean crystal size evidenced above.
(
ii) it decreases the growth rate of secondary Pt crystallites;

1
02
E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
Fig. 7. Typical STM images of Pt (a), Pt/Bent (b), Pt/Mont (c) and Pt/Lap (d) samples deposited at Ed = 250 mV and the size distributions for corresponding samples (constructed
using the data from at least six images). The distributions are approximated by Gauss (dashed lines) and LogNormal (solid lines) functions. Vertical dashed lines are guides
for the comparison of size distributions.
These results allow us to conclude that the electrostatic adsorp-
tion and the mechanical capture of the clay particles by the growing
deposit are the key reasons for the templating effect of clays.
The former phenomenon is affected by the electrode free charge
(deposition potential) and by the platelet charge. The latter depends
on concentration, size and charge of clay particles, because it is
regulated by an electrophoretic withdrawal of clay particles from
the cathode. In the case of the organoclay Bent, the additional

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
103
Fig. 8. Typical STM images of Pt (a) and Pt/Bent (b) samples deposited at Ed = 100 mV and size distributions for these samples (constructed using the data from at least
six images). The distributions are approximated by Gauss (dashed lines) and LogNormal (solid lines) functions. Vertical dashed lines are guides for the comparison of size
distributions.
adsorption of organic stabilizer could also affect the templating
process.
pores. The latter blocking is occasional and results in lower repro-
ducibility. The deposition current on CP is 10 times higher than on
Au with the same geometric area, i.e. a lot of Pt is deposited on
the internal surface. Besides, the region of the current growth is
longer for CP, and the growth is sharper. We cannot exclude that
the increase in the internal surface accessible for the electrochem-
ical process takes place in the course of the deposition due to the
gradual increase of hydrophilicity of CP.
The voltammograms of CP Pt and CP Pt/Bent samples are pre-
sented in Fig. 10B. The deposition from Bent suspensions results in
the formation of deposits with higher specific surface area as com-
pared to CP Pt samples. The effect is similar to the deposits on Au
(compare Tables 2 and 4).
3.7. Carbon supported Pt/clay catalysts
Carbon-supported Pt electrocatalysts are extensively used in
practice [57–59]. To test the applicability of the clay as a templating
additive for the fabrication of this type of catalysts, we deposited
Pt/Bent samples on porous carbon paper (CP). These samples are
named below CP Pt/Bent, whereas pure Pt deposits prepared for
comparison are named CP Pt.
Since the majority of typical carbon-supported catalysts contain
2
3
0–40 wt.% of Pt, we studied a model CP Pt/Bent material with ca.
0 wt.% metal loading. The samples were deposited at E = 250 mV.
d
140.0
Au(111)
The porous carbon paper, as well as most carbon supports, is
expected to provide a lower concentration of nucleation centers
than the Au supports [60]. The true surface of the porous CP support
exceeds considerably its geometric area, but the material is more
or less hydrophobic, and electrolyte penetration into the pores is
incomplete. Deposition transients obtained for this complex elec-
trode material demonstrate worse reproducibility as compared to
the deposition on Au, but the basic qualitative tendencies of the
clay templating effect can be formulated and compared with the
trends described above.
The presence of Bent in the deposition solution reduces the rate
of electrodeposition on CP (Fig. 10A), similar to the effect found for
Au (Fig. 4A). In the majority of transients, the onset of the “plateau”
region is observed earlier for the deposition from the Bent suspen-
sion than from the clay-free solution. At this stage, we assume that
the components of Bent block not only nucleation centers at the
external and internal surface of the CP paper, but also some narrow
Au(200)
100.0
Pt(111)
6
2
0.0
0.0
Pt(200)
Pt/Bent(100 mV)
Pt(100 mV)
Pt/Bent(250 mV)
Pt(250 mV)
36.0
40.0
44.0
48.0
52.0
2
θ / degree
Fig. 9. XRD patterns for Pt and Pt/Bent deposits. Deposition potential (mV vs. RHE)
is indicated in parentheses.

1
04
E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
Table 4
Total charge spent for electrodeposition (Qd); true surfaces (SH) and specific surfaces S(C), determined coulometrically from the hydrogen desorption region of CV; degree of
a
coalescence calculated from STM and TEM data and microstructural parameters calculated from XRD patterns of Pt and Pt/clay samples deposited on CP at Ed = 250 mV.
−16
,
Sample
Qd (C)
SH
(
S (C)a
Degree of
coalescence,
Degree of
coalescence,
1-S/S(STM)
a (Pt), A
˚
D (XRD),
nm
Variance
(XRD), nm
ꢁdisl (×10
˛ (%)
ˇ (%)
cm2)
m
2
g
−1
−2
m )
1-S/S(TEM)
CP Pt
33
251
356
7.2 ± 0.4
0.87 ± 0.01
0.77 ± 0.02
0.68 ± 0.04
0.36 ± 0.08
3.9191 ± 0.0001
3.9207 ± 0.0001
15.1 ± 0.04
10.7 ± 0.03
3.4 ± 0.1
4.6 ± 0.1
3.77 ± 0.01
3.68 ± 0.01
1.2
0
0.8
CP Pt/Bent 26
12.7 ± 0.9
0
.8
a
See notations in the title of Table 3.
As CP allows an easy detaching of fragments, it was possible
to use TEM in addition to STM for a microscopic investigation of
the deposits. The TEM data mainly show the internal surface, while
STM visualizes the external surface. Therefore, the mean particle
size D (STM) differs significantly from D (TEM). Treating the data
for all fragments of nonuniform deposits on CP, we conclude that
the template-assisted deposition yields a lower degree of Pt coales-
cence (Table 4) than for deposits on Au (compare Tables 2 and 4).
However, our Pt deposits on Au are much thinner than on CP.
D (XRD) values confirm that the addition of Bent in the deposi-
tion solution decreases the size of the Pt crystallites (Table 4), like
in the case of deposition on Au. The pure Pt deposits on CP show
lower values of dislocation density and faulting probability as com-
pared to the sample deposited on Au at the same potential (compare
Tables 2 and 4 for E = 250 mV). This less pronounced imperfection
d
can be related to the higher deposit thickness and, correspondingly,
to a weaker effect of the lattice mismatch between the support and
deposit. However, the general effect of Bent on the microstructure
(redistribution of various imperfections) remains the same as found
for Au.
At this stage, we can suggest that for the samplesdepositedon CP
the mechanical capture of the clay particles by the growing deposit
is the only reasonable explanation for the templating effect of clay.
However, the porosity of carbon paper can limit clay penetration
and complicate the capture of clay platelets by the growing deposit.
In contrast to samples deposited on Au, the electrostatic adsorption
of clays on the carbon surface in the course of primary Pt nucleation
is hardly possible. Electrostatic attraction is expected to appear only
in the course of the secondary nucleation and growth, when a small
amount of Pt is already accumulated at the interface.
-
-
-
-
5
4
3
2
CP_Pt
CP_Pt/Bent
3.8. Pt/clay prospects for methanol electrocatalysis: activity and
ageing behavior
To test the electrocatalytic prospects of the fabricated Pt/clay
materials, the steady-state currents of methanol oxidation in
0.1 M CH OH + 0.5 M H SO solution were measured in a narrow
(
A)
3
2
4
540–640 mV potential region (Fig. 11). These tests assumed the
comparative estimate of Pt and Pt/clay specific activity. Surely, for
practical goals one should compare the methanol oxidation rates
at lower potentials, but this test is difficult to arrange at this stage
because of the too small surface areas of the electrodes. Despite
this problem, we consider the applied tests as rather reasonable
because the main kinetic features (i.e. nature of the limiting step
and tendency to self-poisoning) remain the same up to ca. 0.7 V
0
2000
4000
t / s
6000
8000
12
CP_Pt
CP_Pt/Bent
Au_Pt
[
59], and the formal kinetics (e.g. Tafel parameters for the steady-
Au_Pt/Bent
6
state polarization curves) determined in the interval under study
are typical just for low potentials interval.
The higher specific activity in respect to methanol oxidation was
found for template-assisted deposits than for pure Pt deposits on
gold, independently of the deposition potential and the nature of
0
6
the support. The samples deposited at E = 100 mV demonstrate
d
-
higher specific activities as compared to the samples obtained at
Ed = 250 mV (Fig. 11A). This agrees with earlier known tendencies
[
61] for Pt deposits on metallic supports. For deposits on CP, the
-
12
(B)
effect looks rather small (but still positive). However, if one com-
pares the activities per Pt weight, the difference is more evident
(
see S(C) values in Table 4).
Taking into account the XRD data, we can speculate that the
0.0
0.3
0.6
0.9
1.2
E vs. RHE / V
clay-induced defects play a positive role in methanol electrocatal-
ysis. At this stage, the only correlation we have found between the
activity and the microstructure is the one with the increased dislo-
cation density (Table 3). As dislocations measured by XRD are both
those inside the grains (with possible outcrop on the surface) and
those forming the grain boundaries, we cannot exclude that grain
Fig. 10. Deposition transients (A) measured on CP electrode in 0.01 M
Na2PtCl6 + 0.02 M HCl solution (CP Pt) and in clay suspensions in this solution
CP Pt/Bent); CVs (B) of thus obtained samples (CVs for Pt and Pt/Bent deposited
on Au are shown for comparison). Deposition currents are normalized per mass of
the deposits.
(

E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
105
boundaries play a major role here [61]. The sizes of the Pt particles,
240
200
even being slightly different for various samples, can be hardly con-
sidered as a factor affecting the activity, because they lie beyond the
range where pronounced size effects are expected.
In contrast to usual Pt deposits, for clay-templated Pt the bifunc-
tionality can be considered as a reason of the increased activity.
According to Ref. [62], Pt catalysts supported on zeolite exhibit
higher electrocatalytic activity towards methanol oxidation in the
range of potentials from ∼270 to ∼640 mV in comparison with
Pt supported on carbon. Faster oxidation in the presence of zeo-
lites was explained by the interaction of CO adsorbed on platinum
with hydroxyls at the zeolites surface. This assumption is in agree-
ment with the previously known fact [63,64] that the exchangeable
cations, silanols and the Lewis acid centers of zeolites can serve as
CO bonding sites. Since the interfacial chemistry of smectite clays
and zeolites is rather similar, one can speculate that at not-too-low
potentials the higher electrocatalytic activity of Pt/clay samples
in comparison to Pt samples results from the interaction of CO
with the OH-groups of clay. The enhanced electrocatalytic activ-
ity can be also a result of a weaker self-inhibition by products of
strong methanol chemisorption in the presence of clay (“third-body
effect”). However, a true evidence of this bifunctionality requires
further efforts to characterize the location and content of clay in
templated deposits.
Pt/Bent
1
1
60
20
Pt
Pt
Pt/Bent
8
4
0
0
0
40
80
120
160
200
240
Number of cycles
Fig. 12. Decrease in the true surface areas SH of Pt and Pt/Bent samples deposited
on Au (open symbols) and on CP (filled symbols) in the course of cycling (N, number
of cycles). Ed = 250 mV. Total charge spent for electrodeposition of Pt and Pt/Bent
samples on CP was 23 and 17 correspondingly.
mated for the potential range from 540 to 590 mV. This difference
is hardly sufficient to assume changes in the catalysis mechanisms
and can be attributed to the reduction of oxygen traces, being higher
for thinner electrodes.
Formally determined Tafel slopes of the steady-state methanol
oxidation curves for platinized platinum of high roughness are
known to be ca. 65–70 mV in the potential range under study
(
(
540–640 mV) [65]. For thinner deposits of platinum on Au
Ed = 100–400 mV), these slopes were found to be 65–85 mV [61].
To check the stability of the Pt/Bent material under polarization
in electrolyte solution, we arranged ageing tests like described in
[2,6] (under a potential cycling). The decrease in the Pt true sur-
face area with the number of cycles occurs with the same rate
for all samples under study (Fig. 12). Qualitatively, it demonstrates
the absence of mechanical destruction risk that could be expected
because of the lower adhesion of clay-templated samples. The
decay of the normalized true surface SH(N) with the cycle num-
ber was fitted, like in Ref. [2], according to the phenomenological
model of particles coalescence [66]:
The Tafel slopes for Pt/clay on Au (Fig. 11A) are 80–95 mV, as esti-
-
-
-
-
-
-
4.0
4.5
5.0
5.5
6.0
6.5
(
A)
dSH(N)
dt
n
−
= const · (SH(N))
Pt/Bent(100 mV)
Pt/Mont(250 mV)
Pt(100 mV)
Pt/Bent(250 mV)
Pt(250 mV)
where n is a constant, independent of the thickness of the electrode-
posited layer and related to the mechanism of Pt reconstruction.
The value of n for the samples deposited on Au was found to be
3
.2 ± 0.4 for the Pt sample and 1.8 ± 0.2 for the Pt/Bent sample.
According to [67], this can imply different mechanisms of surface
reconstruction (smoothing) for Pt and Pt/Bent: the slow surface
diffusion of Pt atoms and the so-called ‘atom capture’, respectively.
One can speculate that the higher density of dislocations and the
lower content of intergrain boundaries support faster surface diffu-
sion, but further microstructural studies are necessary in future to
support this hypothesis. For samples deposited on CP, the accuracy
of n determination does not allow a reliable comparison.
0
.54
0.57
0.60
0.63
0.66
E vs. RHE / V
-
-
-
-
-
-
4.0
4.5
5.0
5.5
6.0
6.5
(
B)
4. Conclusions
Pt/Bent(250 mV)
Pt(250 mV)
Quasi-templating of Pt with clays results in higher true sur-
faces and higher specific electrocatalytic activity as compared to
the pure Pt samples electrodeposited under similar mode. Taking
into account the satisfactory ageing rate, these results make the
new material a promising candidate for electrocatalytic applica-
tions. Clay covers a part of active nucleation centers and, therefore,
0
.54
0.57
0.60
0.63
(i) prevent the coalescence of Pt particles in the course of sec-
E vs. RHE / V
ondary nucleation; (ii) decrease the growth rate of the nuclei,
supporting the formation of smaller crystals. Primary nucleation
centers are also partly screened, resulting in a more poor adhe-
sion. Organomodified smectite clay is also a promising templating
Fig. 11. Fragments of steady-state polarization curves of Pt and Pt/clay samples
deposited on Au (A) and on CP (B). Current densities per true surface area are
presented. Deposition potential (mV vs. RHE) is indicated in parentheses.

1
06
E.K. Lavrentyeva et al. / Electrochimica Acta 61 (2012) 94–106
agent as soon as its organic components could provide an additional
templating effect.
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d
d
the clay itself, even if trapped inside the Pt deposit, does not affect
the adsorption properties or even enhances electrocatalysis. We
have not observed the pronounced spillover effects known for other
Pt/oxide systems, as the ratio of charges in hydrogen and oxygen
regions was close to usual, but a possibility of spillover-like behav-
ior should be taken into account in future studies of the increased
catalytic activity.
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estimation of the most important practical characteristic of catalyst
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[
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(
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to use solid alumina template [66] and demonstrates the variety of
advantages of inorganic templates for electrocatalytic applications.
[
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Acknowledgements
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This study is supported by the Russian Foundation for Basic
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RFBR No. 07-03-91583 – ASP 01-360. The authors are grateful to
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