Structural and electrochemical characterisation of Pt and Pd nanoparticles electrodeposited at the liquid/liquid interface

Article abstract of DOI:10.1016/j.electacta.2004.02.050

We present the first reported characterisation by x-ray diffraction and high resolution transmission electron microscopy of metals electrodeposited at the bare and templated liquid/liquid interfaces. Additional structural information is also obtained using ion transfer voltammetry as an in situ characterisation tool. In particular, the metallic deposits are shown to consist of aggregates of discrete nanoparticles, predominantly between 3 and 5nm in diameter. Deposition of platinum at the liquid/liquid interface is reported for the first time, which enables a preliminary comparison to be made between the growth mechanism of this metal and the growth of palladium, previously reported at this interface.

Full text of DOI:10.1016/j.electacta.2004.02.050

Electrochimica Acta 49 (2004) 3937–3945
Structural and electrochemical characterisation of Pt and Pd
nanoparticles electrodeposited at the liquid/liquid interface
a
a,∗,1
, Edward P.L. Robertsb
Mark Platt , Robert A.W. Dryfe
a
Department of Chemistry, University of Manchester Institute of Science & Technology, P.O. Box 88, Manchester M60 1QD, UK
b
Department of Chemical Engineering, University of Manchester Institute of Science & Technology, P.O. Box 88, Manchester M60 1QD, UK
Received 19 October 2003; received in revised form 8 February 2004; accepted 12 February 2004
Abstract
We present the first reported characterisation by x-ray diffraction and high resolution transmission electron microscopy of metals electrode-
posited at the bare and templated liquid/liquid interfaces. Additional structural information is also obtained using ion transfer voltammetry as
an in situ characterisation tool. In particular, the metallic deposits are shown to consist of aggregates of discrete nanoparticles, predominantly
between 3 and 5 nm in diameter. Deposition of platinum at the liquid/liquid interface is reported for the first time, which enables a preliminary
comparison to be made between the growth mechanism of this metal and the growth of palladium, previously reported at this interface.
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Liquid/liquid interface; Electrodeposition; Nanoparticle; Template; Nucleation
1
. Introduction
second liquid phase [5,6]. The metal nucleates and grows
in the absence of a solid support, while the electrochem-
ical control permits the extent and rate of the reduction
to be controlled through the electrolysis time and applied
potential difference, respectively.
Electrodeposition continues to attract considerable inter-
est, both from a fundamental and an applied standpoint [1].
Notably, the fundamental aspects of this field have been
transformed by the application of in situ scanning probe mi-
croscopies to the deposition process [2]. Electrodeposition
represents an alternative to chemical reduction methods as
a solution phase route to the preparation of small metallic
particles, although the resultant nanoparticles are generally
immobilised on the substrate electrode [3]. In contrast, the
preparation of well-defined metallic nanoparticles by chem-
ical reduction generally requires the presence of capping
ligands to prevent particle aggregation [4]. An interest-
ing approach that combines aspects of the chemical and
electrochemical reduction processes is the electrodeposi-
tion of metals using the interface between two immiscible
electrolyte solutions (ITIES). In this approach, a metallic
precursor initially present in one liquid phase is reduced to
the zero-valent metal by a reducing agent dissolved in the
This method, originally applied to the deposition of cop-
per at the water/organic interface [7], has been extended to
the deposition of gold and palladium [5,8,9]. Previous re-
ports from this laboratory have demonstrated that the ITIES
deposition approach can be combined with the “template”
method, devised for solid electrodes [10], to localise the
deposition process. In this manner, the templated ITIES
has been used to deposit palladium, within the pores of
␥-alumina membranes, using both an electrochemically
driven and a spontaneous approach [6,11,12]. Micron and
sub-micron scale pipettes have also been employed to con-
strain the deposition of silver at the ITIES [13]. An alterna-
tive approach to deposition at immiscible liquid interfaces
is the micro-droplet strategy developed by Compton and
co-workers where a redox active organic phase is dispersed
over an electrode surface, which is subsequently immersed
in an aqueous phase. Silver deposition has been reported at
the boundary between the liquids [14].
∗
Corresponding author. Tel.: +44 161 200 4522;
fax: +44 161 200 4559.
Questions which arise from this work involve the struc-
tural characterisation of the metallic deposits formed at the
E-mail address: robert.dryfe@umist.ac.uk (R.A.W. Dryfe).
1
ISE member.
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.02.050

3
938
M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
ITIES, with or without a template, and how the resultant
structures relate to the conditions under which the metals
were deposited. Rao and co-workers have recently deposited
films of nanocrystalline gold, silver and copper at the water/
toluene interface: the particles were stable and remained ad-
sorbed at the interface [15]. The metals deposited were trans-
ferred to a solid substrate to allow subsequent characterisa-
tion by transmission electron microscopy (TEM) and atomic
force microscopy. Films of semiconductors have been de-
posited using spontaneous reactions at the liquid/liquid
interface [16]. Detailed structural characterisation has also
been performed of the metallic “nano-rods” produced by
templated electro-deposition at solid electrodes, using ␥-
alumina membranes as templates [17]. An advantage of the
use of the templated electrodeposition method, with respect
to deposition at the liquid/liquid interface, is that transfer to
a solid substrate is not required for subsequent microscopic
characterisation. Previous work from this group suggested
that the palladium deposits formed at the templated ITIES
may have had some nano-scale porosity, although the scale
of these features was at the resolution limit of the TEM
apparatus previously employed [11]. In this manuscript,
the first characterisation by X-ray diffraction (XRD) and
high-resolution TEM of metals electrodeposited at the liq-
uid/liquid interface is presented. Additional structural char-
acterisation using ion transfer at the ITIES is also described.
Furthermore, the first reported results on the electrodeposi-
tion of platinum at the ITIES are presented, which permit a
preliminary comparison to be made between the mechanism
of palladium and platinum reduction at the ITIES.
reference electrodes. The counter electrodes were con-
structed in-house by spot welding an approximately 1 cm
2
area of platinum gauze (Advent, Eynsham, UK) to a length
of platinum wire (Advent). Commercial 13 mm diameter
␥-alumina membranes, with nominal mean pore diameters
of 100 nm and 20 nm, denoted A01 and A002 respectively
(Whatman International Ltd, Maidstone, UK), were sealed
to a 10 mm inner diameter glass tube with a silicone sealant
(RS components, Corby, UK). Membranes were washed
in water, and a solution of the supporting electrolyte be-
fore use, to ensure removal of any contaminants. Relevant
properties of the membranes employed are summarised
in a previous publication [18]. It should be noted that the
diameter of the pores is not uniform along their length:
the 100 nm pore diameter refers to the “active” diameter,
which determines the membrane’s use in filtration. Only
the final 0.5 ␮m length of the pore is of this diameter, the
remaining “support” layer is two to three times larger than
the diameter of the active pore [19,20].
The cell was set up as shown in Fig. 1. The aqueous
phase comprised of either ammonium tetrachloropalladate
or ammonium tetrachloroplatinate as appropriate (supplied
by Fluka, Gillingham, UK), at concentrations between 1 ×
−
3
−3
−3
10 and 5 × 10 mol dm . In the ion transfer experi-
+
ments tetraethylammonium, TEA , (Fluka) was added as
the chloride salt to the aqueous phase at a concentration of
−
3
−3
−2
−3
,
1 × 10 mol dm . Lithium chloride (5 × 10 mol dm
Lancaster Synthesis, Morecambe, UK) and lithium sul-
−
2
−3
fate (5 × 10 mol dm , Aldrich, Gillingham, UK) were
present in all aqueous solutions as supporting electrolytes.
Water, with a resistivity of 18 Mꢀ cm, was obtained from
a Milli-Q purification system (Millipore, Watford, UK).
All aqueous solutions were left for two days to age, to en-
sure that an equilibrium was reached for ligand exchange
[21]. The liquid/liquid interface was formed with water and
1,2-dichloroethane (DCE, HPLC grade, obtained from Lan-
caster Synthesis). The organic electrolyte was either bis(tri-
phenylphosphoranylidene) ammonium tetrakis(pentafluoro)-
phenylborate (BTPPA TPBF ), or bis(triphenylphosphor-
2
. Experimental
Voltammetric experiments were performed using a
PC-controlled EG&G Model 273 potentiostat (Princeton
Applied Research Corp., Princeton, NJ, USA) operating
in four-electrode mode. The reference electrodes were sil-
ver/silver chloride wires produced in-house by oxidation
of a 1 mm diameter silver wire in a solution of lithium
chloride. All potentials are reported with respect to these
20
anylidene) ammonium tetraphenylborate (BTPPA TPB).
The salts were prepared according to a reported metathesis
Fig. 1. Schematic of cell design and set up.

M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
3939
◦
◦
procedure [22] from equimolar amounts of bis(triphenyl-
phosphoranylidene) ammonium chloride (Lancaster, 97%)
and either lithium tetrakis(pentafluoro)phenyl borate (Boul-
der Scientific, Boulder, CO, USA, 98% pure) or lithium
tetraphenylborate (Aldrich), respectively. The concentration
of the organic electrolyte in the DCE phase of all cells was
tilt was typically between 10 and 30 . Particle size analy-
sis was carried out using the Soft Imaging Systems software
of the Quanta 200 SEM. XRD was performed by placing
the metal-loaded membranes, or filtration membranes plus
residue, on the analysis stage of a Bruker AXS D8 Discover
diffractometer (Bruker, Karlsruhe, Germany).
−2
−3
2
× 10 mol dm . For the palladium deposition experi-
ments, butylferrocene (BuFc, Lancaster, 98%) was added as
−
2
−3
,
an electron donor at a concentration of 1 × 10 mol dm
3. Results
whereas bis(pentamethylcyclopentadienyl)iron (DmFc,
Aldrich, 97%) was added to the organic phase at a concen-
3.1. Electrochemical characterisation of Pd Particles
−
2
−3
tration of 1 × 10 mol dm to perform this function for
platinum deposition. The reference electrode for the organic
phase was formed by placing the silver/silver chloride wire
in an aqueous solution of bis(triphenylphosphoranylidene)
ammonium chloride (BTPPACl) and lithium chloride (1 ×
Liquid/liquid electrochemistry was employed as an ini-
tial tool to analyse the products of the deposition process,
and thereby address the previous suggestion that there may
be some nanometre scale porosity within the deposited
palladium particles [11]. This approach offers a unique
way to investigate materials of a porous nature, using ion
transfer voltammetry to characterise the porosity, Θ, of the
−3
−2
−3
1
0
and 1 × 10 mol dm , respectively).
Three experimental cells can be designated:
Cell 1
Cell 2
Cell 3
Electrodeposition was performed, for Cells 1 and 2, using the
bare and alumina-templated ITIES. In the templated cases,
following electrodeposition, the membranes were separated
from the cell and affixed to 12 mm diameter imaging studs
with carbon tabs (Agar scientific, Essex, UK), for analy-
sis using scanning electron microscopy (SEM) and energy
dispersive X-ray (EDX) with a Quanta 200 Environmental
SEM (FEI Corp., Hillsboro, OR, USA). For the bare ITIES,
the contents of the cell were filtered, with a 100 nm pore di-
ameter polyester membrane (Poretics Corp., Livermore, CA,
USA), and analysis was undertaken on the metallic residue
retained on the membrane surface. TEM was performed on
the particles deposited at the templated ITIES using a Tec-
nai F20 field emission gun apparatus (FEI Corp.), operating
at 200 keV. The metal-loaded membranes were broken with
the resultant fragments being set in resin. The fragments
were set such that the cross section of the alumina was at
the sectioned region (perpendicular to the membrane face),
however the fragments were not completely vertical: their
material [18]. Cell 1 was used to deposit palladium at the
alumina-templated ITIES, forming a membrane with pores
that appeared blocked when viewed under the SEM, as
shown in Fig. 2. The cell was then emptied, washed and
+
further voltammetric experiments performed with TEA as
+
the transferring ion (Cell 3). TEA was transferred voltam-
metrically from the aqueous phase to the organic phase
via the Pd-loaded membrane: this approach has previously
been reported as a characterisation tool for porous struc-
tures prior to deposition [18]. Ion transfer cyclic voltammo-
grams (CV) obtained for the Pd-loaded membrane of Fig. 2,
before and after metal deposition, are shown in Fig. 3. It
is clear that even when all pores appear blocked under the
SEM, a distorted yet significant voltammetric feature at-
+
tributable to TEA transfer is observed, providing evidence
that the metallic deposits are indeed porous. The CV in
Fig. 3 also indicates that there is a significant increase in
capacitive current associated with metal deposition. Com-
parison of the capacitive currents obtained with a membrane

3
940
M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
Fig. 4. Cyclic voltammograms obtained from Cell 3 when z is equal to
zero: (a) blank alumina membrane A01, (b) palladium loaded membrane,
deposited using identical conditions to those of Fig. 2. The voltage scan
−
1
rate in both cases was 0.1 V s
.
The fraction of the membrane surface that corresponds
to pores, Θ, of an A01 membrane prior to deposition has a
value of 0.27 ± 0.02 [18]. The currents observed under the
conditions of Fig. 3 as a function of voltage scan rates also
correspond to a Θ value of ca. 0.27. This lack of change
in porosity confirms that the structure of the metal deposit
Fig. 2. Palladium deposited into an A01 membrane, using Cell 1 where
−3
−3
x was 5.0 × 10 mol dm . The deposition was performed by applying
a potential difference of 0.85 V for 60 min. The dark grey colouration is
due to Pd, a few remaining unblocked pores are visible, directly above
the scale bar.
+
does not appreciably affect the flux of TEA across the
water/DCE interface. The constancy of the ionic flux can
be viewed in terms of the radial diffusion fields to each
inter-metallic pore overlapping to produce a linear diffusion
field relative to the size of the pore [23].
before and after deposition demonstrates that the increase
in membrane capacitance is due to the presence of palla-
dium. Fig. 4 shows CVs, for Cell 3 in the absence of TEA
+
(
i.e. where z = 0) before and after the deposition of palla-
3
.2. Structural characterisation of Pd particles by TEM
dium within the alumina membrane: this increase in capaci-
tive current suggests that the metal particles deposited at the
ITIES charge up when a potential difference is applied to the
ITIES.
and XRD
An A002 membrane was electrochemically loaded with
palladium under the conditions of Fig. 2: the metal deposits
within the pores were then analysed using high resolution
TEM, Fig. 5. The microscopy indicates that, rather then be-
ing blocked with one large porous particle, an aggregate of
smaller particles each ca ∼ 2–5 nm in diameter, is formed.
The porous nature arises from the packing of these spherical
particles meaning voids are always present between neigh-
bouring particles. XRD can provide complementary data on
the particle sizes, the Scherrer equation (Eq. (1.1)) relates
the peak width at half-height (ꢁθ) in the diffraction pattern
and the particle size [24]:
0
.9λ
t = 2
(1.1)
ꢁθ(cos θ)
where t is the particle thickness/diameter, λ is the X-ray
wavelength, and θ is the peak angle. Fig. 6(a) shows a typi-
cal diffraction pattern obtained for palladium deposited into
an A01 membrane using the conditions of Fig. 2. The Scher-
rer calculation yields a mean t value of 4 (± 0.5) nm, which
confirms the suggestion that the particles grow as small
nanoparticles with a relatively narrow size distribution. For
Fig. 3. Cyclic voltammograms obtained for the membrane described
in Fig. 2(a) before and (b) after the deposition described above. The
voltammograms show transfer of TEA across the water/DCE interface,
+
obtained when the washed and dried membrane was re-assembled into
−1
Cell 3. A voltage scan rate of 0.10 V s was employed, with z equal to
−
3
−3
.
1
.0 × 10 mol dm

M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
3941
Fig. 5. High resolution TEM image of an A002 membrane loaded with palladium. The deposition conditions were as for Fig. 2. The inset shows a
histogram of the particle size distribution.
comparison, XRD was performed on the metallic filtration
residue obtained following deposition under the same condi-
tions at the bare ITIES, Fig. 6(b). Despite the poorer signal
to noise ratio, Eq. (1.1) can be used to yield a mean t value
of 4 (± 0.8) nm for the palladium particles grown at the bare
ITIES. Hence XRD suggests that the presence of the tem-
plate does not change the nanoparticle size appreciably and
merely acts as a barrier which defines the maximum aggre-
gate size, on coalition of the particles. The features of the
diffraction patterns shown in Fig. 6 coincide well with those
of the authentic palladium samples [25].
characteristic of nucleation on the initial voltammetric scan
[11], the figure shows the nucleation loop occurred on the
second cycle. The occurrence of the nucleation loop sug-
gests that platinum can also be deposited at the ITIES, al-
though no nucleation loop was observed on the first scan.
The broad voltammetric feature observed on the initial cycle,
only seen in the presence of both tetrachloroplatinate and
the electron donor, does not show any hysteresis or loop. To
investigate the origin of this current, the cell was set up as
shown in Cell 2, and one CV was run, with a positive cur-
rent identical to that of scan “a” of Fig. 7 being observed.
The cell was then left to stand for 5 h: no material was
observed immediately after the scan was performed, how-
ever over the five hour period a thin grey film developed
at the water/DCE interface. The film of material was col-
lected by filtration (see Section 2) and analysed using EDX
(Fig. 8; Table 1), which demonstrated that the film consisted
of platinum particles. The deposit was strongly adsorbed at
3
.3. Deposition and characterisation of Pt particles
Electrodeposition of platinum has also been performed
using the templated ITIES approach: the CV response ob-
tained from Cell 2 is shown in Fig. 7. By contrast with pal-
ladium electrodeposition at the ITIES, which gives a loop

3
942
M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
320
270
220
170
120
70
34
39
44
49
54
59
(a)
2θ
2
40
20
00
80
60
40
20
00
2
2
1
1
1
1
1
30
35
40
45
50
(b)
2θ
Fig. 6. Diffraction pattern obtained when (a) a Pd-loaded A01 membrane was analysed using XRD and (b) Pd deposited at the bare ITIES was subjected
to XRD. Both samples were prepared using the conditions described in Fig. 2.
the ITIES and stable for days, even after agitation of the
solutions.
By way of contrast, when the solutions of Cell 2 were left
in contact over several days without an initial voltammet-
ric cycle being performed, no solid material was formed:
the only change observed in the cell was due to the aerial
oxidation of DmFc. This lack of reaction in the absence of
an initial voltammetric sweep was seen for both the bare
ITIES and the alumina-modified ITIES, showing that as
with palladium deposition, the alumina was not respon-
sible for the deposition of the metal and thus plays little
or no part in the nucleation process. Platinum particles
were analysed using TEM following electrodeposition at
the alumina-modified ITIES within an A002 membrane.
Once more the metal was observed to grow as aggre-
gates of smaller particles within the alumina pores rather
than as single particles within each pore, with a mean
particle diameter of ca. 3 nm, as shown in Fig. 9. XRD
supported this finding: the resultant diffraction pattern is
shown in Fig. 10, application of Eq. (1.1) gives a t value
of 2.5 (± 0.5) nm. Once more, comparison with reported
patterns indicated that a platinum phase was deposited
[25].
Table 1
EDX results for the filtration membrane and residue grown at the ITIES
using Cell 2
Element
Weight percentage
Atom percentage
C
O
Pt
55.73
8.96
35.32
86.23
10.40
3.36
Fig. 7. CVs obtained using cell 2, y = 1 × 10− mol dm : (a) first scan
3
−3
and (b) second scan. The scans were performed consecutively, with a
−1
scan rate in both cases of 0.1 V s
.

M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
3943
Fig. 8. EDX of material formed at the ITIES using Cell 3 after a single voltammetric scan.
−
3
−3
Fig. 9. High resolution TEM of platinum deposited within the pores of an A002 alumina membrane using Cell 3 where y was 5 × 10 mol dm
.
Deposition was allowed to proceed for 60 min at a potential difference of 0.45 V.

3
944
M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
3
3
2
2
1
1
50
considerable debate about the reduction mechanism of the
chloro-complexes of palladium and platinum, with some
experimental and computational work suggesting that the
MCl2 species plays a central role in the reduction process
00
50
00
50
00
[29,31], with dimer complexes being proposed for platinum
formation, while other work on platinum complexes has sug-
gested that all the ligating ions must be lost for reduction to
proceed [30]. Recent in situ STM studies have shown that
tetrachloropalladate and tetrachloroplatinate anions adsorb
on gold single crystal electrode surfaces [2,32–34]. In fact,
use of the reported formation constants for the chloro com-
plexes of platinum and palladium under the conditions of
Cells 1 and 2 [29,35], gives similar figures for the speciation
of the two metals among their chloro complexes.
30
35
40
45
50
2θ
Fig. 10. Diffraction pattern obtained when a Pt-loaded A002 membrane
was analysed using XRD. The sample was prepared as described in Fig. 9.
The insert shows the particle size distribution obtained.
We can thus identify two possible factors which may con-
tribute to the slower kinetics of platinum reduction com-
pared to palladium. Firstly, although the speciation of the
chloro complexes of the two metals is similar, previous re-
ports [30,36] have suggested that the rate of loss of chloride
from the platinum complexes is very slow (on the order of
days), whereas the corresponding processes for palladium
are believed to occur in under a second. Hence, if chloride
loss is a necessary precursor to metal reduction, it is con-
ceivable that this effect is significant. However, we note that
recent studies on single crystal electrode surfaces have im-
plied that reduction proceeds via the adsorbed tetrachloro
species [2,32–34], which suggests the kinetics of halide loss
may not affect the overall process.
A second factor which may underlie the observed higher
overpotentials for platinum deposition is the difference in
surface energy of the metals. The classical model of metal
nucleation and growth includes a surface term and a bulk
term for each metal [1]. Eqs. (1.2) and (1.3) indicates that
the latter term should favour platinum deposition, thus the
difference observed between the two metals is conceivably
attributable to the differences in the surface energies of the
growing phases. Surface energy effects will be particularly
significant at the ITIES, where the nascent metal clusters
are not stabilised by an interaction with a solid substrate.
Notably, key differences in the morphology of platinum and
palladium deposits from the corresponding tetrachloro pre-
cursors have been identified on gold electrode surfaces, with
the former occurring as three-dimensional clusters, whereas
the latter grow in a pseudo-morphic two-dimensional man-
ner [2,33,34]. The dramatic morphological changes previ-
ously observed on gold surfaces were attributed to the higher
surface energy of platinum [34], a phenomenon which may
explain the much lower reactivity observed for this metal
at the ITIES. This observation may explain the slower rate
of Pt formation at the ITIES, although it is at variance with
the observed particle sizes since the Pt particles are slightly
smaller than those of Pd. Finally, a striking feature of the
deposit morphology is the ease of formation of such small
(5 nm or less) particles, with a relatively narrow size distri-
bution, in the absence of capping ligands. It is possible that
the growing particles are stabilised by their interaction with
4
. Discussion
The deposition of platinum and palladium at the bare and
templated ITIES allows a comparison between the reduction
mechanisms of the two metals to be made: this is of particu-
lar relevance given that nanoparticulate deposits are formed,
thus electrodeposition at the ITIES conceivably represents a
promising way to understand the growth mechanism of col-
loidal metals [26]. The lower reactivity of platinum towards
ferrocene derivatives, compared to palladium which reacts
spontaneously with DmFc [12], was also reflected in the re-
activity towards the organic phase supporting electrolytes.
Deposition of tetrachloropalladate required the use of the
fluorinated tetraphenylborate anion, TPBF20, because use of
TPB led to a spontaneous reduction of tetrachloropalladate.
Aqueous solutions of tetrochloroplatinate were, however,
stable when left in contact with organic solutions of TPB
for several weeks. Similarly no reduction (spontaneous or
electrochemically driven) between tetrochloroplatinate and
BuFc, which is less readily oxidised than DmFc [27], was
observed. The absence of a nucleation loop for platinum on
the initial voltammetric scan may also be related to the rel-
ative rate of platinum deposition, compared to palladium.
This trend of lower reactivity for the platinum precursor is
0
at variance with the standard E values (versus NHE) for
the two systems [28]:
PdCl4 (aq)2 + 2e → Pd(s) + 4Cl(
−
−
−
,
E = 0.62 V
0
aq)
(
1.2)
1.3)
PtCl4 (aq)2 + 2e → Pt(s) + 4Cl(
−
−
−
,
E = 0.80 V
0
aq)
(
The two standard reduction potentials show that platinum re-
duction is favoured over palladium reduction thermodynam-
ically, which suggests that the contrasting trend observed
for the two metals is due to the kinetics of the formation
of the new phases. The E values are, however, functions
of the speciation of the two metals [29,30], an effect man-
ifested in the need for an ageing process. There has been
0

M. Platt et al. / Electrochimica Acta 49 (2004) 3937–3945
3945
the organic/water interface [37], which may lead to a restric-
tion in their size, although “magic number” effects may also
be present [38].
[7] M. Guainazzi, G. Silvestri, G. Serravalle, J. Chem. Soc. Chem.
Commun. (1975) 200.
[8] Y. Cheng, D.J. Schiffrin, J. Chem. Soc. Farad. Trans. 92 (1996) 3865.
9] C. Johans, P. Liljeroth, K. Kontturi, Phys. Chem. Chem. Phys. 4
[
(
2002) 1067.
10] R.M. Penner, C.R. Martin, J. Electrochem. Soc. 133 (1986) 2206.
[11] M. Platt, R.A.W. Dryfe, E.P.L. Roberts, Electrochim. Acta 48 (2003)
037.
12] R.A.W. Dryfe, A.O. Simm, B. Kralj, J. Am. Chem. Soc. 125 (2003)
3014.
13] J.D. Guo, T. Tokimoto, R. Othman, P.R. Unwin, Electrochem. Com-
mun. 5 (2003) 1005.
[14] C.E. Banks, T.J. Davies, R.G. Evans, G. Hignett, A.J. Wain, N.S.
Lawrence, J.D. Wadhawan, F. Marken, R.G. Compton, Phys. Chem.
Chem. Phys. 5 (2003) 4053.
[
5
. Conclusions
3
[
[
The heterogeneous reduction of tetrachloropalladate and
1
tetrachloroplatinate at the water/DCE interface has been
studied: this is the first report, to our knowledge, of plat-
inum deposition at the ITIES. Characterisation of the de-
posits formed by both metals indicates that particles with
mean diameters in the range of 3–5 nm are formed, with plat-
inum particles being slightly smaller than the palladium par-
ticles. Despite the thermodynamic preference for platinum
reduction, the kinetics of the reduction process indicate that
palladium is more readily formed: it is suggested that dif-
ferences in the surface energies of the nascent metals may
be responsible for this phenomenon. The alumina template
is shown merely to act as a restriction on the agglomeration
of the particles. Further quantitative studies of platinum and
palladium deposition at the ITIES using combined electro-
chemical and spectrophotometric approaches are currently
underway [12,39].
[15] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, V. Varun Agrawal, P.
Saravanan, J. Phys. Chem. B 107 (2003) 7391.
[16] A.B. Mandale, S.D. Sathaye, K.R. Patil, Mat. Lett. 55 (2002) 30.
[17] R.E. Benfield, D. Grandjean, J.C. Dore, Z. Wu, T. Sawitowski, M.
Kröll, G. Schmid, Eur. Phys. J. D 16 (2001) 399.
[
[
18] M. Platt, R.A.W. Dryfe, E.P.L. Roberts, Langmuir 19 (2003) 8019.
19] A. Hernández, J.I. Calvo, R. Prádanos, L. Palacio, M.L. Rodr ´ı guez,
J.A. de Saja, J. Membr. Sci. 137 (1997) 89.
[20] Communication from manufacturer: see http://www.whatman.co.uk.
[21] A. Henglein, M. Giersig, J. Phys. Chem. B 104 (2000) 6767.
[
[
[
[
22] D.J. Ferm ´ı n, H.D. Duong, Z.F. Ding, P.-F. Brevet, H.H. Girault,
Phys. Chem. Chem. Phys. 1 (1999) 1461.
23] C. Amatore, J.-M. Savéant, D. Tessier, J. Electroanal. Chem. 147
(1983) 39.
24] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New
York, 1954.
25] See International Centre for Diffraction Data (ICDD): http://www.
icdd.com.
Acknowledgements
[
26] M.A. Watzky, R.G. Finke, J. Am. Chem. Soc. 119 (1997) 10382.
We thank the EPSRC for financial support, Dr G. Goodlet
´
[27] N. Eugster, D.J. Fermın, H.H. Girault, J. Phys. Chem. B 106 (2002)
(Johnson Matthey Technology Centre, Sonning Common,
3428.
[
[
[
[
28] J.F. Llopis, F. Colom in: A.J. Bard (Ed.), Encyclopaedia of Electro-
chemistry of the Elements, vol. VI, Marcel-Dekker, NY, 1976.
29] Y. Gimeno, A. Hernández Creus, P. Carro, S. González, R.C. Sal-
vazerra, A.J. Arvia, J. Phys. Chem. B 106 (2002) 4232.
30] A.J. Gregory, W. Leavson, R.E. Noftle, R. Le Penven, D. Pletcher,
J. Electroanal. Chem. 399 (1995) 105.
UK) and Mr. P. Hill (Chemical Engineering Dept., UMIST)
for performing the TEM and SEM/EDX, respectively. MP
acknowledges support from UMIST’s Margaret Elizabeth
Lee bequest.
31] L.C. Ciacchi, W. Pompe, A. De Vita, J. Am. Chem. Soc. 123 (2001)
7
371.
References
[
[
32] H. Naohara, S. Ye, K. Uosaki, Colloids Surf. A 154 (1999) 201.
33] L.A. Kibler, M. Kleinert, D.M. Kolb, Surf. Sci. 461 (2000) 155.
[1] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase For-
mation and Growth, VCH, Weinheim, 1996.
[34] H.-F. Waibel, M. Kleinert, L.A. Kibler, D.M. Kolb, Electrochim.
Acta 47 (2002) 1461.
[
2] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Surf. Sci. 443
[35] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced
Inorganic Chemistry, sixth ed., Wiley, NY, 199, p. 1071.
[36] J.M. Cosden, R.H. Byrne, Geochim. et Cosmochim. Acta 67 (2003)
1331.
[37] Y. Lin, H. Skaff, T. Emrick, A.D. Dinsmore, T.P. Russell, Science
299 (2003) 226.
[38] A. Howie, Faraday Disc. 92 (1991) 1.
[39] Z. Ding, D.J. Ferm ´ı n, P.F. Brevet, H.H. Girault, J. Electroanal. Chem.
458 (1998) 139.
(1999) 19.
[
[
3] K.H. Ng, H. Lui, R.M. Penner, Langmuir 16 (2000) 4016.
4] M. Brust, M. Walker, D.J. Schiffrin, R. Whyman, J. Chem. Soc.
Chem. Commun. (1994) 801.
[5] C. Johans, R. Lahtinen, K. Kontturi, D.J. Schiffrin, J. Electroanal.
Chem. 488 (2000) 99.
[6] M. Platt, R.A.W. Dryfe, E.P.L. Roberts, J. Chem. Soc. Chem. Com-
mun. (2002) 2324.