Preparation and characterisation of platinum- And gold-coated copper, iron, cobalt and nickel deposits on glassy carbon substrates

Article abstract of DOI:10.1016/j.electacta.2008.04.015

Pt- and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent partial replacement by Pt or Au upon immersion into a chloroplatinic or chlorolauric solution. This process resulted in a thin Pt or Au shell over the bimetallic particle as electrochemical evidence suggests, while indicative sputter-etch Auger Electron Spectroscopy points to the coexistence of the two metals in the particle core. SEM/EDS characterisation of the deposits revealed the existence of Pt(M) and Au(M) extensively agglomerated nanoparticles (M: Cu, Fe, Co, Ni). The surface electrochemistry of the deposits (hydrogen adsorption/desorption on Pt and oxide formation/stripping on Au) proved the complete coverage of the bimetallic particles by Pt or Au and allowed an estimate of their electroactive surface area. The study of hydrogen evolution at these deposits points to a modification of the electronic properties of the Pt and Au surface layers by the core metal (due to strain effects and/or ligand-electronic interactions) and further confirmed that these layers form a very thin outer shell.

Full text of DOI:10.1016/j.electacta.2008.04.015

Electrochimica Acta 53 (2008) 6559–6567
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Preparation and characterisation of platinum- and gold-coated copper, iron,
cobalt and nickel deposits on glassy carbon substrates
S. Papadimitrioua, A. Tegoua, E. Pavlidoub, S. Armyanovc, E. Valovac, G. Kokkinidisa, S. Sotiropoulosa,∗
a Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 55124, Greece
b Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 55124, Greece
c Rostislaw Kaischew Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
Pt- and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by
electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent par-
tial replacement by Pt or Au upon immersion into a chloroplatinic or chlorolauric solution. This process
resulted in a thin Pt or Au shell over the bimetallic particle as electrochemical evidence suggests, while
indicative sputter-etch Auger Electron Spectroscopy points to the coexistence of the two metals in the par-
ticle core. SEM/EDS characterisation of the deposits revealed the existence of Pt(M) and Au(M) extensively
agglomerated nanoparticles (M: Cu, Fe, Co, Ni). The surface electrochemistry of the deposits (hydrogen
adsorption/desorption on Pt and oxide formation/stripping on Au) proved the complete coverage of the
bimetallic particles by Pt or Au and allowed an estimate of their electroactive surface area. The study of
hydrogen evolution at these deposits points to a modification of the electronic properties of the Pt and Au
surface layers by the core metal (due to strain effects and/or ligand–electronic interactions) and further
confirmed that these layers form a very thin outer shell.
Received 5 March 2008
Received in revised form 7 April 2008
Accepted 7 April 2008
Available online 14 April 2008
Keywords:
Platinum catalysts
Gold catalysts
Electroless deposition
Bimetallic catalysts
Hydrogen evolution
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
underlayers/surface impurities on the electronic properties of pre-
cious metal overlayers, based on density function theory (DFT).
Noble metal-based catalysts are used in many reactions of
practical interest such as hydrogenations [1,2], combustion engine
exhaust treatment [3], fuel cell reactions [4,5], etc. to mention a
few. Reducing the cost of these catalysts is paramount if the asso-
ciated processes were to find widespread commercial/industrial
application. To that direction, an increase of precious catalyst activ-
ity with a consequent decrease in their loading has been a major
target of catalysis research. Binary or even ternary catalytic sys-
tems often show improved catalytic activity, attributed either to
synergism, modification of electronic properties (of a strain or/and
ligand–electronic interaction origin) or “third body”-geometric
effects (associated with a perturbation of surface geometry on an
atomic scale) [6].
Pt-or Au-based binary electrocatalysts tested in reactions rele-
vant to fuel cell technology (oxygen reduction, see for example[7,8];
methanol oxidation, see for example [9,10], borohydride oxidation
[11]) constitute a large part of recent electrochemical literature.
At the same time, mainly due to Nørskov and co-workers [12–15],
there are now detailed estimates of the effect of transition metal
These effects have been successfully correlated with modifications
of experimentally determined catalytic activity towards CO adsorp-
tion [13] and electrochemical oxygen reduction [16,17] for a number
of bimetallic systems.
Bimetallic samples of clearly defined metal layers, deposited
by thermal or magnetron sputtering, have almost exclusively been
used in fundamental studies (see, for example [18,19]. On the other
hand, practical bimetallic catalysts are usually of a surface alloy
or mixture type with both metals exposed to the reaction medium,
thus excluding the possibility of studying the effect of one metal on
the electronic properties of the other one alone (although in some
cases, depending on preparation or treatment conditions, prefer-
ential surface segregation of one metal may still occur [19,20]). In
particular, fuel cell electrocatalysts are almost exclusively prepared
by chemical methods which involve impregnation of carbon sup-
ports with the metal ions and their subsequent reduction either in
a hydrogen atmosphere at elevated temperatures or by a reducing
agent in solution phase [4,5,21].
A recent alternative method for the preparation of precious
metal coatings (e.g. Pt, Pd, Au) is based on their spontaneous elec-
troless deposition by surface replacement of less precious metal
layers (e.g. Ru, Ag, Cu, Pb, Ti), following immersion of the samples
into a solution containing the precious metal ions [22–37]. When Cu
∗
Corresponding author. Tel.: +30 2310 997742; fax: +30 2310 443922.
E-mail addresses: eczss@chem.auth.gr, eczss@otenet.gr (S. Sotiropoulos).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.04.015

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S. Papadimitriou et al. / Electrochimica Acta 53 (2008) 6559–6567
or Pb was the metal to undergo surface exchange, this has been elec-
trodeposited on other electrode substrates (Au [24,28–30,32,34], Pd
[8,31–33], Pt [32,34], Ru, Rh, Ir [34] or glassy carbon [35,36]). In the
series of papers by Adzic and co-workers a Cu UPD monolayer has
been electrodeposited on noble metal substrates and subsequently
fully replaced by a precious metal layer [8,24,28,31–34]. Using a
UPD monolayer of the non-precious metal restricts the applica-
tion of the method to noble metal electrode substrates and does
not allow the formation of a precious metal—non-precious metal
bimetallic catalyst since the UPD metal is fully exchanged. Also, in
the general case that the stoichiometry of the replacement reac-
tion is not 1:1 (e.g. when Cu is oxidised to Cu(II) by Pt(IV) which
is deposited as Pt, or by Au(III) which is converted to Au), then
the electrode surface is not fully covered by the precious metal
after the exchange process and the uncovered parts of the elec-
trode substrate may also show a synergism, once again excluding
the possibility of studying the effect on electronic properties alone.
(Note however that this problem may be alleviated in the case
of Pt if Pt(II) complexes are used.) On the other hand, the elec-
trodeposition of non-precious metal bulk deposits is free from the
above limitations. This variation was first introduced by Kokkini-
dis and co-workers who exchanged bulk Cu and Pb deposits on
Au by Pt [28,29] and further developed by Sotiropoulos and co-
workers who exchanged similar bulk deposits on glassy carbon
substrates by Pt and Au [35,36]. It allows the use of substrates more
relevant to fuel cell technology (such as carbon materials) and the
preparation of bimetallic catalysts that have a precious metal shell
and a non-precious metal core. The above-mentioned electrode-
position/electroless exchange process has not yet been tested on
other than Cu or Pb electrodeposits and its expansion to other early
transition non-precious metals would be of both fundamental and
practical interest. In the former case it will allow the study of the
effect of the second metal present in the particle core on the elec-
tronic properties of the pure Pt or Au skin (similar studies have used
up to now either successively sputtered layers or annealed alloys).
In the latter case the inexpensive metal M could be deposited on
high surface area carbons (of fuel cell type) by a standard adsorp-
tion/reduction technique and its surface layers effectively replaced
by Pt or Au following immersion in an appropriate solution, poten-
tially lowering the amount of precious metal used.
radius of Cu as 0.1278 nm and assuming the FCC crystal struc-
ture for the Cu deposit [37]). Electrodeposition of Fe was carried
out from 0.01 M FeSO4·7H2O + 0.1 M K2SO4 + 0.01 M HClO4 deaer-
ated solutions at −1.15 V vs. Ag/AgCl (3 M NaCl). The total charge
density passed was in the 410–552 mC cm−2 range, correspond-
ing to the equivalent of 1500–2000 flat Fe monolayers (taking
the atomic radius of Fe as 0.124 nm and assuming the BCC crys-
tal structure for the Fe deposit [37]). Co was electrodeposited
from 5 mM CoCl2·6H2O + 0.1 M MgSO4·7H2O + 0.1 M H3BO3 deaer-
ated solutions at −1.00 V vs. Ag/AgCl (3 M NaCl) at a total charge
density in the 588–878 mC cm−2 range, corresponding to the equiv-
alent of 1000–1500 flat Co monolayers (taking the atomic radius of
Co as 0.1253 nm and assuming the HCP crystal structure for the
Co deposit [37]). Finally, Ni was plated from 0.01 M Ni sulfamate
+0.227 mM NiCl2 + 0.025 M H3BO3 deaerated solutions at −1.1 V vs.
Ag/AgCl (3 M NaCl). The total charge density passed was in the
378–567 mC cm−2 range producing the equivalent of 1000–1500
flat Ni monolayers (the atomic radius of Ni is taken as 0.1246 nm and
its crystal structure assumed as FCC [37]). The number of metal lay-
ers deposited was chosen to be sufficient for complete GC substrate
coverage and eventual clear Pt and Au surface electrochemistry
(unaffected by the substrate).
Following their preparation, the Cu/GC, Fe/GC, Co/GC and Ni/GC
electrodes were subsequently immersed in a 0.1 M HCl + 10−3 M
K2PtCl6 solution for 30 min where spontaneous Cu, Fe, Co and Ni
(denoted as M below) replacement by Pt occurred:
2M/GC + PtCl62− → Pt(M)/GC + 2M2+ + 6Cl−
(1)
In another set of experiments, the electroplated samples were
immersed in a 0.1 M HCl + 10−3 M AuCl3 solution for 30 min so that
Cu, Fe, Co and Ni replacement by Au took place:
3M/GC + 2AuCl4− → 2Au(M)/GC + 3M2+ + 8Cl−
(2)
Note that all of the above reactions are thermodynamically
favoured since the standard potentials of the Cu++/Cu, Fe++/Fe,
Co++/Co and Ni++/Ni, which are +0.340, −0.440, −0.277 and
−0.257 V vs. SHE respectively, are lower than the standard poten-
tial of the Pt(IV)Cl62−/Pt and Au(III)Cl4−/Au couple (+0.744 V vs. SHE
and +1.002 V vs. SHE respectively) [38]. The fact that they also occur
in practice after 30 min is confirmed by the results of EDS elemental
analysis (see below) that point to partial replacement of Cu, Fe, Co,
Ni by Pt or Au.
The aim of this work has been to prepare and characterise bi-
component Pt(Cu), Pt(Fe), Pt(Co), Pt(Ni) and Au(Cu), Au(Fe), Au(Co),
Au(Ni) deposits on glassy carbon (GC) electrode supports. Specific
objectives have been: i. the microscopic (SEM) and spectroscopic
(EDS) characterisation of Pt- and Au-coated deposits prepared by
surface replacement of pre-deposited thick Cu, Fe, Co and Ni lay-
ers on GC, ii. The electrochemical characterisation of the bimetallic
particles with respect to hydrogen adsorption/desorption (Pt) and
oxide formation/stripping (Au) andiii. the investigation of any effect
of the Cu, Fe, Co, Ni-containing core onto the electronic properties
of the Pt or Au shell, using as a probe their activity towards hydrogen
evolution.
2.2. Electrochemical characterisation of coatings
An Autolab 100 (EcoChemie) system has been employed for
all voltammetric and constant potential deposition experiments.
These were performed in small three-compartment cells equipped
with a luggin capillary at the end of the reference electrode (Ag/AgCl
(3 M NaCl) in 3 M NaCl, BAS Inc.) chamber. The counter electrode
was a Pt coil and the working electrode a glassy carbon disc sealed
in a glass tube. Three cells were used: one for the core metal (M: Cu,
Fe, Co, Ni) deposition, one for preliminary Pt(M)/GC or Au(M)/GC
activation-cleaning in acid and one for studying Pt or Au surface
electrochemistry and hydrogen evolution in acid.
2. Experimental
Following M deposition and immersion into the chloroplatinic
or chlorolauric acid exchange solution, the as prepared Pt(M)/GC
or Au(M)/GC electrodes were scanned repeatedly (typically, more
than 60 times) at 1 V s−1 in the “cleaning” 0.1 M HClO4 solu-
tion between hydrogen and oxygen evolution (between −0.35 and
+1.25 V for Pt and between −0.45 and +1.60 V for Au), ensuring that
any un-exchanged surface M had been anodically dissolved during
exposure to positive potentials. The electrode was then transferred
to a clean deaerated 0.1 M HClO4 solution and scanned again at
1 V s−1 for another 30 times until a steady-state picture, typical of
2.1. Electrodeposition of Cu/GC, Fe/GC, Co/GC, Ni/GC and
electroless deposition of Pt(Cu)/GC, Pt(Fe)/GC, Pt(Co)/GC,
Pt(Ni)/GC and Au(Cu)/GC, Au(Fe)/GC, Au(Co)/GC, Au(Ni)/GC
Electrodeposition of Cu on GC disc electrodes (5 mm diameter)
was carried out from 0.1 M HClO4 + 0.01 M CuSO4 deaerated solu-
tions at −0.40 V vs. Ag/AgCl (3 M NaCl). The total charge density
passed was in the 340–597 mC cm−2 range, corresponding to the
equivalent of 1000–1500 flat Cu monolayers (taking the atomic

S. Papadimitriou et al. / Electrochimica Acta 53 (2008) 6559–6567
6561
Pt or Au surface electrochemistry, was obtained. Finally, the elec-
trode was swept at 5 mV s−1 (between +0.1 and −0.35) V vs. Ag/AgCl
(3 M NaCl) for Pt(M) and between +0.1 and −0.45 V vs. Ag/AgCl (3 M
NaCl) for Au(M)) to obtain near steady-state potential vs. current
curves for the hydrogen evolution reaction. Stirring was found not
to affect the results (at various stirring rates used) and this was
also the case with electrode rotation speed in an indicative Pt RDE
experiment.
2.3. Microscopic and spectroscopic characterisation of coatings
Scanning Electron Microscopy (SEM) was carried out using a
JEOL JSM-5510 microscope and elemental analysis of the coatings
was performed by the accompanying EDS system.
Auger electron spectroscopy (AES) was performed using a PHI
model 650 scanning Auger electron microscope with a cylindri-
cal mirror analyzer with multichannel Auger electron detection.
Sputter-etching by Ar+ ions was used. PHI PC-Access 7.0 software
was applied to collect the spectra yielding quantitative profiles. The
subsequent data treatment was performed with PHI Multipak 5.0 A
software.
2.4. Electrode materials and chemicals
Glassy carbon from Alfa Aesar (1 mm thick) was cut into 5 mm
diameter discs and sealed into glass tubes with epoxyresin glue.
Electrodes to be used for SEM/EDS experiments were prepared by
connecting the carbon disc (Alfa Aesar) to a glass tube via a bridge
made of a shrinkable thermoplastic tube. The latter was filled with
mercury to ensure electrical contact between the disc and a com-
mercial wire inserted from the open end of the glass tube.
HClO4 from Riedel (puriss p.a., ACS reagent, ≥70%) was
employed in the preparation of electrodeposition solutions.
CuSO4·6H2O from Sigma–Aldrich (ACS reagent) was used in the
preparation of Cu deposition solutions, FeSO4·7H2O from Aldrich
(p.a. 98%), K2SO4 from Merck (p.a. > 99%) for the Fe deposition
solutions, CoCl2·6H2O and MgSO4·7H2O from Merck (p.a. 99%) and
H3BO3 (puriss 98%) from Aldrich for the Co deposition solutions and
Ni sulfamate (p.a. 99%) from Fluka, NiCl2 (puriss > 97%) from Merck
and H3BO3 (puriss 98%) from Aldrich were used in the preparation
of Ni deposition solutions.
Pt and Au exchange solutions were prepared with H2PtCl6 hex-
ahydrate from Sigma–Aldrich (ACS reagent, ≥37.50% as Pt) and
H(AuCl4)·3H2O from Merck respectively.
3. Results and discussion
3.1. Microscopic and spectroscopic characterisation of Pt(M) and
Au(M) deposits; M: Cu, Fe, Co, Ni
Figs. 1(A), 2(A), 3(A) and 4(A) show SEM micrographs of the thick
Cu, Fe, Co and Ni electrodeposits formed by constant potential elec-
trodeposition of 340, 522, 588 and 378 mC cm−2 onto GC electrodes
(as described in Section 2). Fig. 1(A) depicts a nodular Cu deposit
consisting of sub-micron particles aggregated to larger formations.
Fig. 2(A) shows a Fe deposit consisting of a dense layer of smaller
nanoparticles and sparse large aggregates on top of them. (The high
oxygen content of the latter, as confirmed by EDS, points that they
are most likely iron oxides or hydroxides precipitated during depo-
sition by local solution alkalisation during concurrent hydrogen
evolution and/or formed during sample exposure to air before SEM
experiments). As shown in Figs. 3(A) and 4(A) the Co and Ni deposits
formed in this work were very smooth when compared to those of
Cu and Fe.
Fig. 1. SEM micrographs of (A) electrodeposited Cu on GC (at −0.40 V vs. Ag/AgCl;
total charge density 340 mC cm−2, equivalent of 1000 Cu monolayers), (B) Pt(Cu) pre-
pared after immersion of the deposit of (A) in a 0.1 M HCl + 10−3 M K2PtCl6 solution)
followed by electrochemical conditioning and (C) Au(Cu) prepared after immersion
of the deposit of (A) in a 0.1 M HCl + 10−3 M AuCl3 solution followed by electrochem-
ical conditioning.
Figs. 1(B) 2(B), 3(B) and 4(B) show SEM micrographs of similar
samples (Pt(M); M: Cu, Fe, Co, Ni) after the surface replacement of
M by Pt and subsequent conditioning by exposure to positive poten-
tials so that M from any defective Pt(M) particles (not fully coated
with Pt) is anodically dissolved and causes the particle to collapse.
The Pt(Cu) deposit of Fig. 1(B) retains its morphology but becomes

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cles (indicative such locations are denoted as a in Fig. 1(B)) and areas
of a thin deposit (denoted as b). At both types of locations Pt and
Cu were detected by EDS. On the contrary, the Pt(Co) (Fig. 3(B)),
Pt(Ni) (Fig. 4(B)) and especially the Pt(Fe) (Fig. 2(B)) deposits are
nano-particulate and retain a high coverage of the substrate. This
Fig. 2. SEM micrographs of (A) electrodeposited Fe on GC (at −1.15 V vs. Ag/AgCl;
total charge density 522 mC cm−2, deposition charge equivalent to that of 1500 Fe
monolayers), (B) Pt(Fe) prepared after immersion of the deposit of (A) in a 0.1 M
HCl + 10−3 M K2PtCl6 solution) followed by electrochemical conditioning and (C)
Au(Fe) prepared after immersion of the deposit of (A) in a 0.1 M HCl + 10−3 M AuCl3
solution followed by electrochemical conditioning.
notably sparser than its Cu precursor of Fig. 1(A). This indicates that
the surface exchange of Cu by Pt is a slow process and uncovered
Cu areas dissolve away, either by the competitive corrosion pro-
cess in the presence of oxygen (during the exchange step) or due to
anodic dissolution during the electrochemical treatment. Also, the
deposit is not uniform and consists of areas with aggregated parti-
Fig. 3. SEM micrographs of (A) electrodeposited Co on GC (at −1.00 V vs. Ag/AgCl;
total charge density 588 mC cm−2, deposition charge equivalent to that of 1000 Co
monolayers), (B) Pt(Co) prepared after immersion of the deposit of (A) in a 0.1 M
HCl + 10−3 M K2PtCl6 solution) followed by electrochemical conditioning and (C)
Au(Co) prepared after immersion of the deposit of (A) in a 0.1 M HCl + 10−3 M AuCl3
solution followed by electrochemical conditioning.

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Table 1
Relative atomic percentage composition (%) of Pt(M) and Au(M) deposits, M: Cu,
Fe, Co, Ni as obtained from EDS analysis of samples prepared from electrodeposits
equivalent to 1500 M flat monolayers
Cu
Fe
Co
Ni
Pt
Au
70–30
74–26
77–23
91–9
78–22
98–2
92–8
97–3
The first number shows the precious metal (Pt or Au) percentage and the second
that of the core metal M.
higher standard potential (see Section 2.1 above). The deposition
of Pt (cathodic process) with simultaneous M dissolution (anodic
process) has a direct analogy with metal corrosion (in that case M
dissolution is supported by oxygen or proton reduction). As such,
its rate (analogous to the corrosion current) is expected to depend
exponentially on the difference between the standard potentials
of the cathodic and anodic processes (EPtCl /Pt − EMn+/M) (see
−2
6
Ref. [40] pp. 501–502). This difference is +0.404 V for Pt(Cu) and
+1.184 V, +1.021 V, +1.001 V for Pt(Fe), Pt(Co) and Pt(Ni) respectively
(see Section 2.2 above and [38]).
The nano-particulate Au-exchanged deposits shown in
Figs. 1(C), 2(C), 3(C) and 4(C) (for Au(Cu), Au(Fe), Au(Co) and
Au(Ni) respectively) differ from their Pt-exchanged counterparts
in that they are generally formed by smaller particles and show
a higher surface coverage. This is indicative of a faster metal
exchange by Au. An explanation could be again the much higher
standard potential of the AuCl4−/Au couple (+1.002 V vs. SHE) than
that of the PtCl62−/Pt (+0.744 V vs. SHE) [38] which leads to high
EAuCl
− EMn+
values (+0.598, +1.442, +1.279, +1.259 V for
−3
/M
/Au
Au(C4u), Au(Fe), Au(Co) and Au(Ni) respectively).
Finally, we should stress that EDS analysis of both large sample
areas and individual particles confirmed the existence of Pt or Au in
all deposit locations. Table 1 presents the relative atomic percent-
agecomposition ofPt andM orAuandMasobtained byEDS analysis
of Pt(M) or Au(M) samples resulting from initial electrodeposits of
the equivalent of 1500 flat monolayers of M. EDS results are not
surface sensitive and the type of the components of the bimetal-
lic layers (of sufficiently high atomic weight) ensures that the EDS
spectrum provides quantitative information for the entire deposit.
Hence, some composition trends can be identified. Au-exchanged
deposits contain less core metal than Pt-exchanged ones and the
content of Fe, Co and Ni is much less than that of their Cu counter-
parts. Both these observations are in line with the above-mentioned
higher rates of M dissolution and precious metal deposition as the
difference between their standard potential increases.
The space distribution of elements on various particle locations
of indicative Pt(Cu) coatings was further examined by AES mea-
surements. On the surface survey of several locations of a Pt(Cu)/GC
sample (prepared from 1000 originally electrodeposited Cu layers)
before sputtering, Pt was always detected but Cu either gave an
extremely weak signal (at locations corresponding to large parti-
cles, see locations a in Fig. 1(B)) or could not be detected at all (at
locations where the deposit was thinner, see locations b in Fig. 1(B)
again). Since as a first approximation one could take the Cu Auger
electrons escape depth to be equal or smaller than the electron
inelastic mean free path (IMFP) through Pt at the respective energy,
the above finding indicates that the Pt shell is of different thickness
at different deposit locations and features. Furthermore, since for
Auger electrons of 900 eV (Cu1 Auger electrons are of 910 eV) the
IMFP value through Pt is 1.22 nm [39], Cu should lie closer to the
surface than this distance at locations a and deeper than that at
locations b. (Note that, taking into account that the atomic radius
of Pt is 0.139 nm [37], the maximum thickness of 1.22 nm estimated
above, corresponds to ca. 4.4 Pt monolayers.) Thus, and despite AES
Fig. 4. SEM micrographs of (A) electrodeposited Ni on GC (at −1.10 V vs. Ag/AgCl;
total charge density 378 mC cm−2, deposition charge equivalent to that of 1000 Ni
monolayers), (B) Pt(Ni) prepared after immersion of the deposit of (A) in a 0.1 M
HCl + 10−3 M K2PtCl6 solution) followed by electrochemical conditioning and (C)
Au(Ni) prepared after immersion of the deposit of (A) in a 0.1 M HCl + 10−3 M AuCl3
solution followed by electrochemical conditioning.
indicates that Fe, Co and Ni exchange with Pt is faster than that of
Cu and protective Pt layers are more readily formed on the former
metals. The slowness of Cu exchange when compared to that of Fe,
Co, Ni may originate from reactant (chloroplatinate) diffusion lim-
itations into the porous Cu electrodeposit or, more likely, due to its

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Table 2
Pt or Au electroactive area (Ae) per substrate geometric area (Ags), r = Ae/Ags, for
Pt(M)/GC or Au(M)/GC electrodes prepared from initially electrodeposited M (elec-
trodeposition charge equivalent to 1000 flat monolayers for M: Cu, Co, Ni and 2200
for M: Fe)
Cu
Fe
1.8 0.7
Co
Ni
Pt
Au
5.1a
4.4 0.7
1.0b
3.6 1.0
1.8 0.4
5.2 2.0
3.7 0.6
1.5 0.5
a
Average of 5 measurements.
Difference between average and maximum or minimum value (whichever the
b
largest).
than 100 nm of particle outer layers. The simultaneous decrease of
Pt and Cu and increase of C indicates that the particles are of dif-
ferent size and shape and they are not covering the substrate in
a uniform manner (these are also supported by the SEM picture
of Fig. 1(B)). The ratio of atomic percentage composition of Pt–Cu
(taken from the profile) varies from ca. 10.2 close to the particle
surface to ca. 7.5 after 7.5 min of sputter-etching, i.e. 30 nm into the
particle. This ratio is much higher than that of 3.1 (75–24%) obtained
from EDS analysis of similar Pt(Cu) coatings (prepared from the
originally electrodeposited equivalent of 1000 Cu monolayers) and
referring to the coating as a whole. All this proves that there is coat-
ing enrichment in Pt as one moves to the outer shell but that Cu and
Pt are mixed in the particle core. The latter is indicative of a slow,
uneven replacement of Cu by Pt and the creation of interconnected
Cu pockets surrounded by Pt regions within the particles, until a
protective continuous outer Pt shell is formed.
Fig. 5. Auger sputter-etch depth profiles of elements at a Pt(Cu) coating on a GC
substrate, following 30 s of initial sputter-etch cleaning.
not having the surface sensitivity needed in this case (LEIS should
be used ideally), the fact that a very weak or no Cu signal is recorded
before sputtering, supplements the electrochemical evidence pre-
sented below (Section 3.2.1) for a Cu-free surface. Furthermore,
the small IMFP value and, especially, the fact that (even weak) Cu
signals are observed at various locations, does not exclude the pos-
sibility of the Pt shell being thin enough (a few atomic layers, at
least at locations) for a substrate effect on shell electronic prop-
erties to be operative. This effect can be understood to be either
of a geometric perturbation or ligand–electronic interaction ori-
gin at locations that the shell is 1–2 monolayers thick, and almost
exclusively of a geometric perturbation origin for thicker overlay-
ers. (Note, that modification of the catalytic properties of surface
layers of a metal by the presence of underlayers of a different metal,
has been reported to occur even up to a thickness of 5 surface mono-
layers, see discussion in Section 3.2.2 below and references therein.)
The change in Pt electronic properties is further supported by the
alteration of Pt catalytic activity shown in Section 3.2.2 below.
Fig. 5 illustrates the thickness distribution of Pt and Cu, obtained
by sputter-etching from the surface down to the substrate. Profiling
has been started after 30 s of sputtering which removes ca. 2 nm of
material. Carbon dominates since the coating is apparently not con-
tinuous and the spot of AES analysis has a ∼250 nm diameter. Given
that each sputter-etch cycle lasts 30 s the profile corresponds to less
3.2. Electrochemical characterisation of Pt(M)/GC and Au(M)/GC
electrodes
3.2.1. Surface electrochemistry in acid solutions
Fig. 6 shows the voltammetric picture (at 1 V s−1) of Pt(Cu),
Pt(Fe), Pt(Co) and Pt(Ni) samples in deaerated 0.1 M HClO4 solu-
tions. This picture coincides with that of polycrystalline Pt, showing
all the characteristic features of Pt surface electrochemistry (hydro-
gen adsorption/desorption peaks and oxide formation/stripping
wave/peak) and pointing to complete surface platinisation of the
initial Cu, Fe, Co and Ni electrodeposits. Note that, in the case of
Pt(Cu), the lack of any Cu sites on the surface has been further sup-
ported by electrochemical evidence in Ref. [36], whereby hydrazine
oxidation is not hindered at Pt(Cu) electrodes when compared to
Pt ones (as it would happen if Cu was present on the surface). Also,
formic acid oxidation was not significantly enhanced at similarly
prepared Pt(Pb) electrodes as one would expect if Pb resided in
the surface [35]. An estimate of Pt electroactive surface area, Ae,
could be obtained from the charge associated to the formation or
stripping of a H monolayer (210 ␮C cm−2 [41]) and has been quan-
tified by the ratio r = Ae/Ags where Ags is the substrate geometric
area. Depending on the type of M, Pt(M)/GC electrodes with r val-
ues in the 5–2 range were obtained as listed in Table 2. Generally,
Pt(Cu) electrodes (SEM pictures showed them to have a particulate
and uneven morphology) had higher values than Pt(Fe) electrodes
(smoother and more even according to SEM), with Pt(Co) and Pt(Ni)
electrodes having intermediate values. The rather low r values
could be due to extensive particle agglomeration and/or incomplete
substrate coverage.
Fig. 7 shows the voltammetric picture (at 1 V s−1) of Au(Cu),
Au(Fe), Au(Co) and Au(Ni) samples in deaerated 0.1 M HClO4 solu-
tions. This picture is again identical with that of polycrystalline Au,
showing the typical oxide formation wave and associated stripping
peak, and indicating again complete surface coverage of the elec-
trodeposits by Au. The estimate of Au electroactive surface area,
Ae, can be done in this case using the charge corresponding to the
Fig. 6. Voltammograms at 1 V s−1 potential scan rate, in a deaerated 0.1 M HClO4
solution, at Pt(Cu)/GC, Pt(Fe)/GC, Pt(Co)/GC and Pt(Ni)/GC electrodes, as indicated
on the graphs.

S. Papadimitriou et al. / Electrochimica Acta 53 (2008) 6559–6567
6565
Table 3
Tafel slopes (mV per decade) for hydrogen evolution reaction from Pt(M) and Au(M)
electrodes from 0.1 M deaerated HClO4 solutions (M: Cu, Fe, Co, Ni)
Bulk metal
33
Cu
39
Fe
38
137
Co
44
128
Ni
Pt
44
Au
62 (low overpotentials, ꢀ)
98 (high overpotentials, ꢀ)
70 (low ꢀ)
85 (high ꢀ)
62 (low ꢀ)
90 (high ꢀ)
to the Volmer–Heyrovski mechanism (Ref. [40], pp. 38–44) with the
Heyrovski reaction (Reaction (6) below) being the rds (40 mV dec−1
predicted slope at low overpotentials):
H
+ + e− + Pt(M) ↔ Pt(M) − Hads
(5)
(6)
Pt(M) − Hads + H+ + e−→ Pt(M) − H2
It is well-known (see, for example Ref. [43]) that H2 evolution on
Pt is either controlled by the hydrogen recombination step or, for
very activated electrodes, by diffusion of molecular hydrogen. On
occasions that the latter mechanism was observed this was asso-
ciated with the observation of three Tafel slope regions of ca. 29,
0 and 120 mV dec−1 [44]. In the case of our electrodes no effect of
stirring or electrode rotation has been observed indicating that, for
the particular activation state of these electrodes, hydrogen evo-
lution is not affected by mass transport effects. The most striking
finding shown in Fig. 8 is the significant suppression of hydrogen
evolution currents as one passes from bulk Pt to platinized metal
Pt(M).
Both the possible change of mechanism and definite current
suppression of hydrogen evolution at Pt(M) electrodes (when com-
pared to bulk Pt) points to a change of the Pt(M)–Hads bond strength
when compared to that of Pt–Hads, since it is well established that
bulk Pt shows the optimum bond strength for hydrogen evolution in
the respective volcano plot (reaction rate vs. M–Hads bond strength)
[45]. This behaviour should in turn be due to a modification of the
electronic properties of the Pt shell by M (having a strain and/or
ligand–electronic interaction origin).
It has long been established that Pt d-orbitals play a key role
in H adsorption [46–48]. At the same time, it has been argued
by Nørskov and co-workers [12–15] that transition metal affinity
towards a variety of adsorbates (O, CO, H, etc.) is best described
not by the degree of d-orbital filling or the density of states at
the Fermi level but by the energy of the “d-band center”, εd.
The lower this center lies the lower is the affinity of the metal
for the adsorbate. The same group of researchers has estimated
the shift in εd of a metal overlayer M1 by the presence of a
bulk metal M2 underlayer for a variety of M1(M2) bimetallic sys-
tems, based on density function theory (DFT) calculations [14,15].
When considering simple strain (tensile or compressive) effects
caused by the bulk substrate M2 to the overlayer atoms M1, these
calculations show that Cu, Fe, Ni and Co substrates lower the
εd of a Pt overlayer by 1.15, 1.48, 1.53 and 1.89 eV respectively,
i.e.|ꢁεd|Cu < |ꢁεd|Fe < |ꢁεd|Ni < |ꢁεd|Co. Taking also into account the
difference in electronegativity between M1 and M2, which however
has a much weaker effect on εd [15], this trend can be modified into
|ꢁεd|Cu < |ꢁεd|Fe ≈ |ꢁεd|Ni < |ꢁεd|Co. Hence, according to this the-
ory, all of our Pt(M) deposits should have a lower εd value than
bulk Pt and consequently form a weaker Hads bond and show lower
hydrogen evolution currents, as is indeed the case of Fig. 8. As a
result, the observed trend in hydrogen evolution current densi-
ties (ie)Pt > (ie)Pt(Cu) ≈ (ie) Pt(Fe) ≈ (ie)Pt(Ni) > (ie)Pt(Co) can be partially
explained by the trend in |ꢁ␧d|. Note that it has recently been
shown both theoretically and experimentally that Pt–Ni bimetallic
catalysts show a lower affinity for hydrogen adsorption than pure
Pt [48].
Fig. 7. Voltammograms at 1 V s−1 potential scan rate, in a deaerated 0.1 M HClO4
solution, at Au(Cu)/GC, Au(Fe)/GC, Au(Co)/GC and Au(Ni)/GC electrodes, as indicated
on the graphs.
stripping of the Au surface oxide (420 ␮C cm−2 [42]). Depending
on the type of M, Au(M)/GC electrodes with r values in the 4.5–1.5
range were obtained as listed in Table 2. Again, Au(Cu) electrodes
have higher r values than Au(Fe) electrodes while those of Au(Co)
and Au(Ni) electrodes lie in-between.
3.2.2. Hydrogen evolution from acid solutions
Fig. 8 presents log ie vs. E plots, constructed from slow (at
5 mV s−1) potential sweep voltammograms in deaerated 0.1 M
HClO4 solutions at Pt (M)/GC electrodes and, for comparison, at a
bulk Pt disc electrode. The current density ie was calculated on the
basis of electroactive Pt area, Ae, as determined from the hydro-
gen adsorption charge (see Section 3.2.1 above). The slope of the
plots (Tafel slopes) are given in Table 3 and varies from 33 mV per
decade for bulk Pt to 44 mV per decade for Pt(Ni) and Pt(Co). This
slight variation of the Tafel slope may signify the change of hydro-
gen evolution mechanism from the Volmer–Tafel mechanism (Ref.
[40], pp. 38–44) with the Tafel reaction (Reaction (4) below) being
the rds (30 mV dec−1 predicted slope):
H
+ + e− + Pt ↔ Pt − Hads
(3)
(4)
2Pt − Hads→ 2Pt + H2
Fig. 8. Tafel plots for hydrogen evolution at Pt and Pt(M)/GC (M: Cu, Fe, Co, Ni) elec-
trodes in deaerated 0.1 M HClO4 solutions, constructed from slow potential sweep
voltammograms (at 5 mV s−1). Current density, ie, is per electroactive Pt area.

6566
S. Papadimitriou et al. / Electrochimica Acta 53 (2008) 6559–6567
the basis of electroactive Au area, Ae as determined from the gold
oxide stripping peak charge (see Section 3.2.1 above). The slope
of the plots (Tafel slopes) are given in Table 3. For bulk Au the
Tafel plot shows a continuously changing slope and two values
can be estimated, 62 mV at low overpotentials and 98 mV at high
overpotentials. This is similar to those reported by Bockris and co-
workers for 0.1 N HCl (76 and 97 mV respectively [63]) although it
should be noted that a variety of slopes, ranging from 30 to 120 mV,
have appeared in the literature, depending on the preparation and
pretreatment of Au electrodes [64]. In general however, all these
findings point to the Volmer–Heyrovski mechanism [65] with the
latter (Reaction (8) below) being the rds:
H
+ + e− + Au ↔ Au − Hads
(7)
(8)
Au − Hads + H+ + e− → Au + H2
Fig. 9. Tafel plots for hydrogen evolution at Au and Au(M)/GC (M: Cu, Fe, Co, Ni) elec-
trodes in deaerated 0.1 M HClO4 solutions, constructed from slow potential sweep
voltammograms (at 5 mV s−1). Current density, ie, is per electroactive Au area.
From Table 3 it follows that a definite statement for a change
in mechanism when passing from bulk Au to Au(M) cannot
be made since the observed Tafel slopes are within the wide
30–120 range of slopes reported for Au [64]. Also, since Au
is known to have a very small affinity for hydrogen adsorp-
tion, it is unlikely that the d-band center energy (εd) lowering
(predicted too for Au(Cu), Au(Fe), Au(Ni) and Au(Co) in [14]) deter-
mines the observed behaviour but there must be other factors
dominating hydrogen evolution activity. This is also indicated
by the fact that there is no coincidence of the estimated εd
trends with the observed catalytic activity of Au(M) electrodes:
(ie)Au > (ie)Au(Ni) > (ie) Au(Cu) > (ie)Au(Co) > (ie)Au(Fe)). Recent experi-
mental and theoretical studies have shown that Au nanoparticles,
or conversely very thin Au films, exhibit extraordinary catalytic
properties (for example for oxygen adsorption [66–70]) especially
when they are negatively charged [69,70]. This behaviour has been
attributed to quantum-size effects [66] or/and changes in step den-
sity [71]. Note also that we have recently reported a moderate
increase of Au(Cu) catalytic activity towards oxygen reduction [36],
in contrast to an opposite anticipation based simply on the esti-
mates by Nørskov and co-workers [14]. In any case, Fig. 8 shows
that the presence of M in Au(M) electrodes modifies their catalytic
properties towards hydrogen evolution.
A recent series of papers has also appeared that applies DFT cal-
culations specifically on the adsorption strength of H on a number
of surface [49] or near-surface alloys [50,51]. In these works too a
decrease in H adsorption tendency on Pt(M) systems (M: Cu, Fe,
Co, Ni) has been predicted. In another series of papers, DFT has
been used to investigate the effect of noble metal underlayers on
the catalytic activity of Pd towards hydrogen evolution [52–54]. In
[52,54] the effect of the underlying metal is found to be operative
for up to 5 Pd surface monolayers, supporting the view that such
effects are likely to be present in our deposits too, since the Auger
sputter-etch experiments discussed in Section 3.1 point to a Pt shell
thickness smaller than ca. 4.4 monolayers at locations. Of course,
in the case of our deposits, the layers below the Pt shell consist
of a Pt–M bimetallic rather than pure M (as the indicative sputter-
etching AES experiments of Section 3.1 suggest). However, the effect
of M underlayers on the electronic properties of the Pt shell is still
expected to be operative to some extent and follow the above trends
as it has already been estimated theoretically (and proven exper-
imentally for oxygen reduction) in the case of Pt “skin” on Pt3M
(M: Fe, Co, Ni) alloys [15,17,55–57]. At this point we would like to
mention that, although the application of DFT to the interpretation
of a number of catalytic systems is gaining pace and its predictions
are in qualitative agreement with experiment in most cases, there
are still reservations regarding its quantitative predictions for the
hydrogen evolution reaction, as indicated by the recent scientific
discussion that has appeared in the literature [58–60].
Finally, as far as the electrochemical behaviour of our Pt(M)
electrodes is concerned, we would like to comment on the
apparent discrepancy observed between hydrogen adsorption
and hydrogen evolution trends. As follows from the Pt(M) sur-
face electrochemistry presented in Fig. 6, the second metal has
almost no effect on the voltammetric picture associated with the
adsorption/desorption of the underpotentially deposited (UPD) H
monolayer. On the contrary, there is a large effect of M on hydro-
gen evolution at Pt electrodes (Fig. 8). This may be due to different
types of adsorbed H that are critical for the rate of the two pro-
cesses which in turn are likely to be affected to a different extent by
the change in Pt electronic properties. It has already been reported
by Conway and co-workers that H UPD is different from weakly
adsorbed over-potentially deposited, OPD, H which is kinetically
important for hydrogen evolution [61,62].
4. Conclusions
i. A two-step process, involving electroless coating of metal (M)
electrodeposits by Pt or Au upon the immersion of the former
into chloro-complex solutions of the latter, has been expanded
from M: Cu and Pb [28,29,35,36] to the early transition metals
M: Fe, Co, Ni. Adherent platinum- or gold-coated copper, iron,
cobalt and nickel deposits have been prepared on glassy carbon
substrates.
ii. The prepared Pt(M)/GC and Au(M)/GC electrodes all exhibited
typical Pt and Au surface electrochemistry in deaerated acid
solutions, after initial treatment by repetitive potential cycling
between hydrogen and oxygen evolution. This indicates that
Pt(M) and Au(M) particles not fully covered by Pt or Au, col-
lapse during the electrochemical treatment (due to M anodic
dissolution) and the remaining deposit is fully protected by a Pt
or Au shell.
iii. Pt(M)/GC electrodes exhibited a decrease in hydrogen evolution
rates, attributed to a strong change of Pt electronic proper-
ties (d-band lowering) caused by the M component on Pt, as
predicted by the theoretical calculations of Nørskov and co-
workers [14,15,49] and Mavrikakis and co-workers [50,51]. This
decreases the strength of the Pt H bond below that corre-
sponding to bulk Pt (top of the corresponding volcano plot)
thus retarding hydrogen evolution. The behaviour of Au(M) elec-
Fig. 9 presents log ie vs. E plots, constructed from slow (at
5 mV s−1) potential sweep voltammograms in deaerated 0.1 M
HClO4 solutions at Au(M)/GC electrodes and, for comparison, at a
bulk Au disc electrode. The current density ie was calculated on

S. Papadimitriou et al. / Electrochimica Acta 53 (2008) 6559–6567
6567
trodes with respect to hydrogen evolution is more complicated
but a negative effect of M on the catalytic properties of Au is also
observed.
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two-step technique and having a Pt or Au shell over a Pt–M
or Au–M core (where M is an early transition metal) should
be tested in other reactions of technological interest in view of
possible favorable modifications of Pt and Au catalytic activity.
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ꢂꢂET Greece for ꢃENEꢁ-03Eꢁ378 and Greece-Ukraine-146-␥
Projects; IKY, Greece for a PhD Fellowship to S.P. The authors express
their gratitude to Prof. Dr. Annik Hubin from the Department of
Metallurgy, Electrochemistry and Materials Science of Vrije Uni-
versiteit Brussel, Belgium for the possibility to conduct AES studies
and to Mr. Eng. Oscar Steenhaut for his assistance in performing
them.
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