Electrodeposition of platinum in room-temperature ionic liquids and electrocatalytic effect on electro-oxidation of methanol

Article abstract of DOI:10.1149/1.1870754

The use of room-temperature ionic liquids (RTILs) as green media for electrochemical application is desirable. The electrodeposition behavior of platinum was investigated at a glassy carbon electrode in hydrophilic 1-n-butyl-3-methylimidazolium tctrafluoroborate (BMIMBF₄) and hydrophobic 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF ₆) RTILs containing chloroplatinic acid hexahydrate by cyclic voltammetric and potentiostatic transient techniques and atomic force microscopy (AFM). For comparison, the electrodeposition of platinum in HClO₄ aqueous solution was also investigated. The cyclic voltammograms after the first scan circle in these media were different from each other, showing the obvious media effect on the electrochemical behavior of chloroplatinic acid hexahydrate. The analysis of the experimental current transients showed that the electrodeposition process of platinum in these media was characteristic of three-dimensional progressive nucleation with diffusion-controlled hemispherical growth. AFM showed that the uniform, shiny electrodeposits of platinum obtained in BMIMBF₄ and BMIMPF₆ RTILs were fairly dense and that the separate spherical nanoclusters were less than 100 nm. For the electro-oxidation of methanol in H₂SO₄ solution, the platinum films prepared in both BMIMBF₄ and BMIMPF₆ RTILs exhibited high electrocatalytic current densities.

Full text of DOI:10.1149/1.1870754

E146
Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
0013-4651/2005/152͑4͒/E146/8/$7.00 © The Electrochemical Society, Inc.
Electrodeposition of Platinum in Room-Temperature Ionic
Liquids and Electrocatalytic Effect on Electro-oxidation
of Methanol
a
a,b,z
Ping He,a Hongtao Liu,a, Zhiying Li, and Jinghong Li
*
aState Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Jilin 130022, China
bDepartment of Chemistry, Tsinghua University, Beijing 100084, China
The use of room-temperature ionic liquids ͑RTILs͒ as green media for electrochemical application is desirable. The electrodepo-
sition behavior of platinum was investigated at a glassy carbon electrode in hydrophilic 1-n-butyl-3-methylimidazolium tetrafluo-
roborate ͑BMIMBF4͒ and hydrophobic 1-n-butyl-3-methylimidazolium hexafluorophosphate ͑BMIMPF6͒ RTILs containing chlo-
roplatinic acid hexahydrate by cyclic voltammetric and potentiostatic transient techniques and atomic force microscopy ͑AFM͒.
For comparison, the electrodeposition of platinum in HClO4 aqueous solution was also investigated. The cyclic voltammograms
after the first scan circle in these media were different from each other, showing the obvious media effect on the electrochemical
behavior of chloroplatinic acid hexahydrate. The analysis of the experimental current transients showed that the electrodeposition
process of platinum in these media was characteristic of three-dimensional progressive nucleation with diffusion-controlled
hemispherical growth. AFM showed that the uniform, shiny electrodeposits of platinum obtained in BMIMBF4 and BMIMPF6
RTILs were fairly dense and that the separate spherical nanoclusters were less than 100 nm. For the electro-oxidation of methanol
in H2SO4 solution, the platinum films prepared in both BMIMBF4 and BMIMPF6 RTILs exhibited high electrocatalytic current
densities.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.1870754͔ All rights reserved.
Manuscript submitted August 18, 2004; revised manuscript received October 13, 2004. Available electronically March 9, 2005.
The study of nanoparticles of catalytically active material has
Based on these issues, it is meaningful to combine the advan-
tages of RTILs and the electrochemical method for the preparation
of materials of interest. Until now, studies of the electrodeposition of
some elements and compounds in RTILs have been reported,12,23-36
but mostly the aluminum-based RTILs were employed therein. For
the electrochemical investigation of platinum in RTILs, only two
papers have been published37,38 until now. Abbott et al.37 studied the
electrodeposition of aluminum and aluminum/platinum alloys from
AlCl3/benzyltrimethylammonium chloride RTILs. Huang and Sun38
reported the formation of nanoporous platinum by selective anodic
dissolution of a PtZn surface alloy in a Lewis acidic zinc chloride-
1-ethyl-3-methylimidazolium chloride ionic liquid, and the electro-
catalysis effect of the fabricated nanostructured platinum electrode
on methanol oxidation was examined.
It is known that 1-n-butyl-3-methylimidazolium hexafluorophos-
phate ͑BMIMPF6͒ and 1-n-butyl-3-methylimidazolium tetrafluo-
roborate ͑BMIMBF4͒ RTILs are hydrophobic and hydrophilic, re-
spectively, because the choice of anion has a dramatic effect on
water miscibility.39,40 The viscosity of BMIMPF6 is 2-3 times that of
BMIMBF4 at room temperature, and both are hundreds of times as
viscous as water. Based on the different properties between aqueous
solutions and the RTILs, it is significant to investigate the potential
difference in electrochemical behavior of materials of interest ͑for
example, platinum͒ in these media and to accumulate more funda-
mental RTIL-based data. Toward this purpose, we have tried for the
first time to electrodeposit platinum nanoparticles from BMIMBF4
and BMIMPF6 RTILs and to investigate the nucleation and growth
mechanism and the corresponding electrocatalytic effect on metha-
nol oxidation. Platinum electrodeposition in HClO4 aqueous solu-
tion has also been studied for comparison. Cyclic voltammetric and
potentiostatic transient techniques and atomic force microscopy
͑AFM͒ were used, and the electrodeposition mechanism of platinum
was characterized as three-dimensional ͑3D͒ nucleation and
diffusion-controlled hemispherical growth for a progressive model.
Based on the experimental results, it was reasonable that the cata-
lytic performance and utilization efficiency of platinum film pre-
pared in BMIMBF4 and BMIMPF6 RTILs would be much higher
than that prepared in HClO4 aqueous solution.
received increasing attention in recent years because of the impor-
tant role of nanoparticles in electrocatalysis. In this respect, the
platinum film/nanoparticle is of particular interest as it is known to
be one of the most important catalysts for many chemical reactions,
for example, production of hydrogen from methane, conversion of
automobile exhaust gas, and especially oxygen reduction and metha-
nol oxidation in a fuel cell.1 A critical problem with platinum-based
catalysts is their prohibitive cost. To date, efforts have focused on
the development of techniques to produce platinum catalysts with a
high surface area to achieve high catalytic performance and utiliza-
tion efficiency.2 The electrodeposition method is useful for the
preparation of new materials with specific characteristics ͑thin lay-
ers, dispersed materials, nanostructures͒, especially the transition el-
ements and semiconductor compounds. Compared with other meth-
ods such as chemical vapor deposition, physical vapor deposition,
and sputtering, electrodeposition has proven to be the least expen-
sive, and is highly productive and readily adoptable. Examples of
the electrochemical study of platinum film/nanoparticles in aqueous
solutions are available in the literature.3-9
10
Room-temperature ionic liquids ͑RTILs͒ have attracted inten-
sive interest in recent years as replacements for classical molecular
solvents in fundamental research and applications, including
electrochemistry,11-16 organic synthesis,17,18 and separations.19 The
advantages of RTILs include good chemical and thermal stability,
almost negligible vapor pressure, good electrical conductivity, and a
wide electrochemical window.20,21 Among the various RTILs, those
obtained by the association of aluminum halides and some organic
chlorides have received special attention.22 However, the practical
application of aluminum-based RTILs as solvents is limited because
these RTILs are moisture-sensitive and hygroscopic. Air- and water-
stable RTILs have recently been obtained by the substitution of
the chloroaluminate anion by weakly complexing anions such as
tetrafluoroborate ͑BF −4͒, hexafluorophosphate ͑PF −6͒ and bis-
͑trifluoromethanesulfone͒ imide ͓͑CF3SO2͒2N−͔. These systems
may possibly be used in various industrial purposes, for example,
electrochemical applications, including electrodeposition and batter-
ies.
Experimental
Chloroplatinic acid hexahydrate and potassium tetrachloroplati-
nate ͑Shanghai Chemical Reagent Ltd., China͒ and BMIMBF4 and
*
Electrochemical Society Student Member.
z E-mail: jhli@mail.tsinghua.edu.cn
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
E147
Figure 1. CVs of a GC electrode in ͑A͒ BMIMBF4 containing 18 mM
chloroplatinic acid hexahydrate, ͑B͒ BMIMPF6 containing 18 mM chloro-
platinic acid hexahydrate, ͑C͒ 0.1 M HClO4 aqueous solution containing 1
mM chloroplatinic acid hexahydrate, ͑D͒ BMIMBF4 containing 10 mM
potassium tetrachloroplatinate, and ͑E͒ BMIMPF6 containing 10 mM potas-
sium tetrachloroplatinate. Insets in ͑A͒ and ͑B͒ are CVs of GC electrode in
BMIMBF4 and BMIMPF6, respectively. Potential scan rate: 50 mV s−1. All
potentials referred to Ag/AgCl reference electrode.
lutions were thoroughly deoxygenated by purging the cell with ul-
trapure nitrogen. A three-electrode electrochemical configuration
was employed in BMIMBF4 and BMIMPF6 media, i.e., a glassy
carbon ͑GC͒ disk working electrode ͑⌽ 3 mm͒, a Pt counter elec-
trode, and a Pt wire immersed in BMIMPF6 as the quasi-reference
electrode, the latter being separated from the cell electrolyte by a
porosity frit. An Ag/AgCl reference electrode was adopted in HClO4
BMIMPF6 ͑both from Acros, ജ98%͒ were received. Prior to use,
BMIMBF4 and BMIMPF6 were treated according to the
literature.41,42 Activated molecular sieves were introduced to poly-
͑tetrafluoroethylene͒ vessels in which BMIMBF4 or BMIMPF6 was
stored.43
Cyclic voltammetry and chronoamperometric transient measure-
ments performed in an N2-filled dry box were carried out on a CHI
630A electrochemical workstation ͑CH Instruments, USA͒. All so-
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
aqueous solution, which was −0.35 V vs. the reference electrode
employed in BMIMBF4 and BMIMPF6 media. For comparison, all
potentials herein were referred to the Ag/AgCl reference electrode.
Prior to each experiment, the GC electrode was polished to a mirror
finish using 0.3 and 0.05 µm alumina powder and sonicated in a
water bath twice for 5 min.
Electrodeposition of platinum was carried out on a just-polished
GC substrate by shifting the cathodic limit toward more negative
potentials until platinum electrodeposition took place, i.e., −1.5,
−1.5, and −0.1 V, respectively, in BMIMBF4, BMIMPF6, and
HClO4 media. The platinum electrodeposits obtained were washed
thoroughly with ethanol and water.
X-ray photoelectron spectroscopy ͑XPS͒ was conducted using a
VG ESCALAB MK II spectrometer ͑VG Scientific, U.K.͒ employ-
ing a monochromatic Mg K␣ X-ray source ͑1253.6 eV͒. Peak posi-
tions were internally referenced to the C1s peak at 284.6 eV.
The surface topography of the platinum electrodeposits on the
GC electrode was investigated afterward under ex situ conditions
using an optical-beam deflection commercial SPM system ͑SPA400
multifunction unit/SPI3800 probe station, Seiko Instruments, Inc.,
Japan͒ operated in the ambient atmosphere. The intermittent contact
mode was employed with a resonance frequency of 23 kHz and a
scan rate of 0.5 Hz.
Figure 2. XPS spectrum of platinum electrodeposit at GC electrode obtained
in BMIMBF4 with 18 mM chloroplatinic acid hexahydrate. Electrodeposition
potential was −1.5 V vs. Ag/AgCl reference electrode.
Platinum electrodeposits on the GC electrode were electrochemi-
cally cycled between the hydrogen and oxygen evolution potentials
in a 1.0 M H2SO4 solution to activate the platinum and obtain the
real surface area. The surface area of the electrodeposited platinum
film was determined from the charge associated with the hydrogen
containing 18 mM chloroplatinic acid hexahydrate under conditions
of potentiostatic electrolysis at −0.4 V, no characteristic binding
energy was observed in the XPS spectrum. Therefore, the former
peak ͑a͒ corresponded to the reduction of platinum͑IV͒ to platinu-
m͑II͒ and the latter ͑b͒ the formation of platinum nanocrystallites.
The reason for so negative a potential for the latter was that the
electrochemical process required a certain nucleation overpotential.
Upon reverse scan, a typical crossover loop between the cathodic
and anodic branches was observed. This behavior was characteristic
of the nucleation and growth processes of platinum electrodeposi-
tion, causing an increase of the surface area of the growing 3D
clusters of the new phase.
adsorption and desorption processes at a scan rate of 100 mV s−1
44
using a conversion factor of 210 ␮C cm−2
.
The capacitive charge
was not included for the calculation. The electrocatalytic oxidation
measurements were carried out in a solution of 0.5 M methanol and
0.5 M H2SO4 solution at a scan rate of 200 mV s−1. All current
densities herein were quoted in terms of ␮A cm−2 of the apparent
geometric area of the substrate electrode except in Fig. 7 where the
current density was calculated as ␮A cm−2 of the surface area of the
deposited platinum.
For the CV obtained in BMIMBF4 containing chloroplatinic acid
hexahydrate ͑Fig. 1A͒, after the first scan circle, the potential of the
reduction peak ͑b͒ shifted anodically to about −1.1 V ͑peak d͒. This
newly appearing peak corresponded to the growth of the platinum
cluster formed during the first and the next potential scans. A fine
peak ͑c͒ at about −0.1 V was also observed, corresponding to the
oxidation of platinum͑II͒ to platinum͑IV͒. From the potential differ-
ence and the ratio of peak currents, it was reasonable that the elec-
trochemical process occurring at about −0.1 V was quasi-reversible.
With increasing scan circles, peaks a, c, and d all somewhat shifted
toward less negative potentials, indicating that during the growth
processes of platinum electrodeposition, the electroreduction of
platinum͑IV͒ and platinum͑II͒ became easier.
As shown in Fig. 1B, the voltammetric behavior of chloroplatinic
acid hexahydrate obtained in BMIMPF6 was somewhat different
from that in BMIMBF4. After the first scan circle, the potential of
the reduction peak ͑b͒ shifted anodically to about −1.1 V ͑peak e͒;
however, that of the reduction peak ͑a͒ shifted cathodically to about
−0.7 V. Compared with the oxidation peak of about −0.1 V in Fig.
1A, the peak ͑c͒ in Fig. 1B corresponding to the oxidation of plati-
num͑II͒ to platinum͑IV͒ in BMIMPF6 was observed at about 0 V.
The reason for the different electrochemical behaviors of chloropla-
tinic acid hexahydrate in BMIMBF4 and BMIMPF6 RTILs could be
due to different properties of these two RTILs, e.g., lypohydrophilic
character, size of anion ion, and viscosity.
Results and Discussion
Typical cyclic voltammograms ͑CVs͒ of a GC electrode in
BMIMBF4 and BMIMPF6 RTILs at a potential scan rate of
50 mV s−1 are shown as insets in Fig. 1A and 1B, respectively.
Considering the anodic and cathodic limits arbitrarily defined as the
potential at which the current density of 100 ␮A cm−2 was
reached,20 electrochemical windows of more than 4 V were yielded
in both BMIMBF4 and BMIMPF6 media. Both the small oxidation
peak in the anodic scan and the small reduction peak in the cathodic
scan were due to the potential limits in the scan process.45 From the
CVs of the GC electrode in the treated ͑inset curves in Fig. 1A and
1B͒ and as-received ͑not presented here͒ RTILs, it was evident that
the electrochemical window was sensitive to the residual water and
other components.46,47
Representative continuous CVs of 18 mM chloroplatinic acid
hexahydrate in BMIMBF4 and BMIMPF6 RTILs and of 1 mM chlo-
roplatinic acid hexahydrate in 0.1 M HClO4 aqueous solution, re-
corded at a just-polished GC electrode at a potential scan rate of
50 mV s−1, are shown in Fig. 1A, 1B, and 1C, respectively. As was
apparent in the CVs recorded in both BMIMBF4 and BMIMPF6
media, during the first negative potential scan a reduction peak ͑a͒
occurred at about −0.3 V and the other one ͑b͒ at a very negative
potential, about −1.5 V. The ascription of these two peaks was ex-
amined by XPS, which can give an important clue in characterizing
the electronic state of the metal particles.48 As shown in Fig. 2, after
potentiostatic electrolysis at −1.5 V for half an hour in BMIMBF4
containing 18 mM chloroplatinic acid hexahydrate, two characteris-
tic binding energies of 71.3 and 74.5 eV were obtained, which cor-
responded to Pt 4f7/2 and Pt 4f5/2, indicating the existence of plati-
num metal.49 However, as for the sample prepared in BMIMBF4
Comparatively, in the CV obtained in 0.1 M HClO4 aqueous
solution containing 1 mM chloroplatinic acid hexahydrate ͑Fig. 1C͒,
a single irreversible reduction peak ͑a͒ of platinum͑IV͒ and an oxi-
dation peak ͑b͒ occurred at about −0.15 V and 0.95 V, respectively,
and also a crossover loop was observed in the first scan circle. A
cathodic peak ͑c͒ at about 0.5 V after the first potential scan was
observed, and the peak current increased with increasing scans, in-
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
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Figure 3. AFM images of platinum electrodeposits at GC electrode ob-
tained in ͑A͒ BMIMBF4 with 18 mM chloroplatinic acid hexahydrate, ͑B͒
BMIMPF6 with 18 mM chloroplatinic acid hexahydrate, and ͑C͒ 0.1 M
HClO4 aqueous solution with 1 mM chloroplatinic acid hexahydrate. Elec-
trodeposition potentials were −1.5, −1.5, and −0.1 V vs. Ag/AgCl refer-
ence electrode, respectively, for A, B, and C.
dicating the growth of the platinum nanoclusters formed during the
first and the next potential scans. The two fine peaks of the weakly
and strongly adsorbed hydrogen monolayer were observed between
−0.2 and 0.2 V after the first scan circle when bigger nanoclusters of
platinum were formed on the GC electrode.
sium tetrachloroplatinate and in BMIMPF6 containing 10 mM po-
tassium tetrachloroplatinate, respectively. As was apparent, in the
CVs recorded in both BMIMBF4 and BMIMPF6 media, a reduction
peak ͑a͒ occurred at about −1.2 V during the negative potential
scan. This reduction peak, along with the peak d in Fig 1A and peak
e in Fig. 1B, corresponded to the reduction of platinum͑IV͒ to plati-
num͑II͒. Upon reverse scan, no oxidation peak and crossover loop
between the cathodic and anodic branches was observed. By using
potassium tetrachloroplatinate, the values of the diffusion
coefficient estimated from the potential step experiments were
2.2 ϫ 10−6 cm2 s−1 for the BMIMBF4 system and 5.0
ϫ 10−7 cm2 s−1 for the BMIMPF6 system.
The potentiostatic reduction of chloroplatinic acid hexahydrate in
BMIMBF4 and BMIMPF6 RTILs and in HClO4 aqueous solution at
−1.5, −1.5, and −0.1 V, respectively, was conducted on a GC elec-
trode, yielding gray platinum electrodeposits. Typical AFM images
of these shiny electrodeposits of platinum are shown in Fig. 3. It was
apparent that the morphology of the platinum films was uniform and
From these results, an obvious media effect due to the different
physical and chemical properties50 was observed on the electro-
chemical behavior of chloroplatinic acid hexahydrate in the two
RTILs and HClO4 aqueous solution. However, as a crystalline hy-
drate, chloroplatinic acid hexahydrate introduced a very small quan-
tity of water when it was added into the RTILs. Therefore, potas-
sium tetrachloroplatinate, an anhydrous halide, was chosen and
investigated in the two RTILs. The electrodeposition of platinum on
a GC electrode from potassium tetrachloroplatinate/H2SO4 was
studied by Jiang and Derek,7 therein a single irreversible reduction
peak was observed at about 70 mV vs. normal hydrogen electrode,
and the value of the diffusion coefficient estimated from the slope
was 2.2 ϫ 10−5 cm2 s−1. Shown in Fig. 1D and 1E in this work are
the CVs of a GC electrode in BMIMBF4 containing 10 mM potas-
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
Figure 4. Set of experimental current transients for deposition process of
platinum on GC electrode obtained in ͑A͒ BMIMBF4 with 18 mM chloro-
platinic acid hexahydrate, ͑B͒ BMIMPF6 with 18 mM chloroplatinic acid
hexahydrate, and ͑C͒ 0.1 M HClO4 aqueous solution with 1 mM chloropla-
tinic acid hexahydrate. The potential steps were initiated at −0.2, −0.2, and
0.7 V vs. Ag/AgCl reference electrode, respectively, for A, B, and C.
fairly dense, corresponding to an island ͑Volmer-Weber͒ growth
model.51 The separate platinum nanoclusters obtained in BMIMBF4
and BMIMPF6 RTILs were in evidence and the size of the platinum
nanoparticles was less than 100 nm. In contrast, the platinum nano-
particles obtained in HClO4 aqueous solution were far bigger than
100 nm. The reason for such an obvious difference in size of nano-
particles prepared in RTILs and HClO4 aqueous solution could be
the distinct viscosity, i.e., the viscosities of 154 cP ͑20°C͒ for
BMIMBF4 and 371 cP ͑20°C͒ for BMIMPF6,52 both of which are
much larger than that of water.
To further study the nucleation/growth process for the elec-
trodeposition of platinum, chronoamperometric measurements based
on the current transient recording53 were performed at a GC elec-
trode in BMIMBF4, BMIMPF6, and HClO4 media. Note that, to
avoid memory effects, the surface of the GC electrode was renewed
by additional mechanical polishing to remove the platinum nanopar-
ticles after each recording. Shown in Fig. 4 are a family of experi-
mental current transients at different step potentials for the deposi-
tion process of platinum onto a GC electrode obtained in ͑A͒
BMIMBF4, ͑B͒ BMIMPF6, and ͑C͒ 0.1 M HClO4 aqueous solution.
The potential steps were initiated at 0.2, 0.2, and 0.7 V vs. the
Ag/AgCl reference electrode, respectively. As was apparent, almost
all transients were of the same shape with characteristic and well-
defined current maxima. At higher overpotentials, the current tran-
sient maxima ͑Imax͒ were more defined but shifted toward short time
͑tmax͒. In all transients, the rapid surge and exponential decay of the
current at the onset of the potential step was mostly due to the
double-layer charging, and then the rising current arose from the
birth and growth of the new phase and/or the increasing number of
nuclei. As these grew, the coalescence of neighboring diffusion
fields with localized spherical symmetry gave rise to a current maxi-
mum. These curves presented a typical response of 3D multiple
nucleation with diffusion-controlled growth,54,55 taking into account
the eventual overlap of the diffusion zone and considering the de-
velopment of nucleation exclusion zones around already established
nuclei. It was also observed that after the current maxima all the
current transients decayed slowly and, for a longer time, almost
merged into a common curve, which was caused by diffusion con-
trol and can be described by the Cottrell equation as follows
I͑t͒ = nFD1/2C/͑␲1/2t1/2
͒
͓1͔
where I͑t͒ is the current density, n is the number of electrons in-
volved, F is the Faraday constant, D is the diffusion coefficient, C is
the concentration of species in the bulk and t is the time. Based on
the Cottrell equation, the diffusion coefficients of chloroplatinic ion
in BMIMBF4, BMIMPF6, and HClO4 media were estimated by ana-
lyzing the part after the maximum of the current transients. By av-
eraging the D values calculated from these current transients, the
values of D were ͑1.0 ± 0.3͒ ϫ 10−6, ͑3.0 ± 0.6͒ ϫ 10−7, and
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
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2
Figure 5. Nondimensional plots of ͑I/Imax͒ vs. t/tmax for current transients
of Fig. 4, illuminating comparison between experimental curves and theo-
retical lines for the limiting case of 3D instantaneous and progressive nucle-
ation and diffusion-controlled growth process. ͑A͒ Curves for platinum
deposition in BMIMBF4 with 18 mM chloroplatinic acid hexahydrate, ͑B͒
curves for platinum deposition in BMIMPF6 with 18 mM chloroplatinic
acid hexahydrate, and ͑C͒ curves for platinum deposition in 0.1 M HClO4
aqueous solution with 1 mM chloroplatinic acid hexahydrate.
͑9.7 ± 0.7͒ ϫ 10−6 cm2 s−1, respectively, for chloroplatinic acid
hexahydrate in BMIMBF4 and BMIMPF6 RTILs, and chloroplatinic
acid hexahydrate in 0.1 M HClO4 aqueous solution. The D values of
chloroplatinic acid hexahydrate in the BMIMBF4 and BMIMPF6
RTILs were smaller than that obtained in HClO4 aqueous solution,
which could be mostly ascribed to the relatively larger values of
viscosity, as described. Here, the D values of chloroplatinic acid
hexahydrate in the BMIMBF4 and BMIMPF6 RTILs were expressed
as the Stokes-Einstein product, ␩D/T, where ␩ is the viscosity and T
is the absolute temperature. The calculated ␩D/T values were 5.3
ϫ 10−9 g cm s−2 K−1 for the BMIMBF4 system and 4.0
ϫ 10−9 g cm s−2 K−1 for the BMIMPF6 system.
nism, instantaneous or progressive process. For instantaneous nucle-
ation
2
2
I2/Imax = 1.9542͑tmax/t͓͒1 − exp͑− 1.2564 t/tmax͔͒
͓2͔
and for progressive nucleation
2
2
2
I2/Imax = 1.2254͑tmax/t͓͒1 − exp͑− 2.3367 t2/tmax͔͒
͓3͔
where Imax and tmax are the current and time coordinates of the
peaks, respectively.
Figure 5 shows the representative nondimensional plots of plati-
num deposition in the BMIMBF4 and BMIMPF6 RTILs and in
HClO4 aqueous solution, along with the theoretical curves for the
limiting cases of instantaneous and progressive 3D nucleation with
diffusion-controlled growth. At the potential ranges depicted, the
analysis of the experimental results under different overpotentials
showed good agreement with the theoretical lines for progressive
nucleation in these media with diffusion-controlled growth up to
The early stages of an electrochemical reaction involving phase
formation are usually associated with 2D or 3D nucleation pro-
cesses. Several reports have been published on this subject, and a
few models have been proposed for these processes.56,57,58,59,60 The
theoretical model based on a 3D nucleation and growth process
controlled by diffusion of electroactive species55 is the most com-
monly adopted to the study of the electrodeposition of metal, espe-
cially when the deposits are good electronic conductors. According
to the characteristic shapes of the current transients presented in Fig.
4, as a preliminary step, all current transients were presented in a
t
max. By analyzing the data ͑Imax and tmax͒ from the current transient,
it was found that the nucleation rate, knNo, increased with shifting
the step potential negatively, which was consistent with 3D progres-
sive nucleation with diffusion-controlled hemispherical growth.
Note that in these nondimensional plots of platinum deposition, all
the curves exhibited a higher current at longer times after tmax; this
deviation was likely due to partial kinetic control of the growth.61,62
2
nondimensional form, normalized current vs. time plot ͓͑I/Imax͒ vs.
t/tmax͔.55 A comparison of the theoretical lines with the experimental
data was often used for the determination of the nucleation mecha-
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E152
Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
Figure 7. Representative CVs of methanol electro-oxidation in 0.5 M metha-
nol and 0.5 M H2SO4 at scan rate of 200 mV s−1. Platinum films obtained in
͑a͒ BMIMBF4 with 18 mM chloroplatinic acid hexahydrate, ͑b͒ BMIMPF6
with 18 mM chloroplatinic acid hexahydrate, and ͑c͒ 0.1 M HClO4 aqueous
solution with 1 mM chloroplatinic acid hexahydrate.
Figure 6. Representative CV for platinum film, obtained in BMIMBF4 with
18 mM chloroplatinic acid hexahydrate, in 1.0 M H2SO4 at scan rate of
100 mV s−1. Hc1 and Hc2: formation of adsorbed hydrogen, Ha1 and Ha2:
oxidation of adsorbed hydrogen, Oa: formation of adsorbed oxygen, Oc: re-
duction of oxide layer, and shaded layer: double-layer capacitive charge.
Capacitive charge was not included for calculation of surface area of elec-
trodeposited platinum films.
when adding chloroplatinic acid hexahydrate ͑H2PtCl6 · 6H2O͒ into
the RTILs in this work, hydrogen evolution was involved in the
electrodeposition process of platinum. Therefore, the value of cou-
lombs consumed for the electrodeposition of platinum in both the
BMIMBF4 and BMIMPF6 RTILs was not obtained as accurately as
that in HClO4 aqueous solution. However, based on the observed
size of platinum nanoclusters according to the AFM images, it was
reasonable that the current density per unit mass of platinum film
prepared in both the BMIMBF4 and BMIMPF6 RTILs would be
much higher than that prepared in HClO4 aqueous solution.
However, the currents in Fig. 4 did not collapse to a completely
single response, which could also result in the deviation from theo-
retical lines observed in Fig. 5 at longer times.
The surface area of the electrodeposited platinum film was de-
termined using cyclic voltammetry in aqueous H2SO4 solution. Fig-
ure 6 shows the representative CV for the platinum film, obtained in
BMIMBF4 containing 18 mM chloroplatinic acid hexahydrate, in
1.0 M H2SO4 at a scan rate of 100 mV s−1. The well-resolved peaks
between −0.17 and 0.2 V correspond to the formation and removal
of the surface adsorbed hydrogen monolayer on the nanostructured
platinum electrode. The charge associated with forming and remov-
ing such a surface monolayer was directly proportional to the active
platinum surface area. From the area under the current-potential
curve, the roughness factors of the nanostructured platinum elec-
trodes obtained in BMIMBF4, BMIMPF6, and HClO4 media were
estimated to be from 10 to more than 100, depending on the elec-
trodeposition time. In contrast, the rough factor of the just-polished
platinum electrode was about 4.5.
The nanostructured platinum electrode was tested for the electro-
oxidation of methanol. Typical CVs showing the electro-oxidation
of 0.5 M methanol in 0.5 M H2SO4 are presented in Fig. 7. The
potential scan was started from 0 toward 1.0 V and then reversed.
The CV exhibited the same feature as the electrochemical behavior
of methanol on polished polycrystalline platinum reported in many
works.63,64 Methanol oxidation commenced at about 0.4 V, giving a
peak at about 0.7 V. On the reverse-going scan, an oxidation peak
occurred at about 0.5 V, and no reduction peak was observed. This
kind of anomalous behavior was attributed to the adsorption and
interaction of various intermediate species at the electrode
surface.63,64 Based on the peak current at about 0.7 V, it was calcu-
lated that the oxidation current densities of methanol electrocatalysis
were about 280 and 260 ␮A cm−2 for the nanostructured platinum
films obtained in the BMIMBF4 and BMIMPF6 RTILs, respectively,
compared with 250 ␮A cm−2 for that prepared in HClO4 aqueous
solution. Note that the electrodeposition conditions such as the po-
larization potential and the deposition temperature have not been
well optimized according to the observed CVs of methanol electro-
oxidation, by which a much higher current density may be expected.
Also note that, because a small quantity of water was introduced
Conclusion
We have presented for the first time some results of the elec-
trodeposition of platinum in BMIMBF4 and BMIMPF6 RTILs at a
GC electrode, and the electrocatalysis effect of nanostructured plati-
num films obtained in these RTILs on the methanol electro-
oxidation was also investigated. A crossover loop was observed in
all CVs of these electrochemical systems, indicating a nucleation
process. The platinum deposition processes were characterized as a
3D nucleation and diffusion-controlled growth model for progres-
sive nucleation. AFM showed that the uniform, shiny electrodeposits
of platinum obtained in BMIMBF4 and BMIMPF6 RTILs were
fairly dense and that the separate spherical nanoclusters were less
than 100 nm, obviously different from those of far more than 100
nm prepared in HClO4 aqueous solution. As for the electro-
oxidation of methanol, the platinum films prepared in both the
BMIMBF4 and BMIMPF6 RTILs exhibited high electrocatalytic
current densities, comparable to that prepared in HClO4 aqueous
solution. Based on the observed size of platinum nanoclusters ac-
cording to the AFM images, it was reasonable that the catalytic
performance and utilization efficiency of the platinum film prepared
in BMIMBF4 and BMIMPF6 would be much higher than that pre-
pared in HClO4 aqueous solution.
Acknowledgments
We gratefully acknowledge financial support from the National
Natural Science Foundation of China ͑no. 20125513 and 20435010͒,
the Li Foundation, USA, and the Foundation for the Author of Na-
tional Excellent Doctoral Dissertation of the People’s Republic of
China. Also, Professor Alexander A. Milchev from the Institute of
Physical Chemistry, Bulgarian Academy of Sciences, and Professor
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Journal of The Electrochemical Society, 152 ͑4͒ E146-E153 ͑2005͒
E153
Werner Freyland from the Institute of Physical Chemistry, Univer-
sity of Karlsruhe, Germany, are thanked for their fruitful discussions
and help.
31. P. Y. Chen and I. W. Sun, Electrochim. Acta, 45, 3163 ͑2000͒.
32. M. H. Yang, M. C. Yang, and I. W. Sun, J. Electrochem. Soc., 150, C544 ͑2003͒.
33. J. F. Huang and I. W. Sun, J. Electrochem. Soc., 150, E299 ͑2003͒.
34. T. Tsuda, C. L. Hussey, G. R. Stafford, and J. E. Bonevich, J. Electrochem. Soc.,
150, C234 ͑2003͒.
References
1. K. Y. Chan, J. Ding, J. W. Ren, S. A. Cheng, and K. Y. Tsang, J. Mater. Chem., 14,
505 ͑2004͒.
35. S. Zein El Abedin, N. Borissenko, and F. Endres, Electrochem. Commun. 6, 510
͑2004͒.
2. X. Zhang and K. Y. Chan, Chem. Mater., 15, 451 ͑2003͒.
3. S. L. Chen and A. Kucernak, J. Phys. Chem. B, 107, 8392 ͑2003͒.
4. A. Kelaidopoulou, G. Kokkinidis, and A. Milchev, J. Electroanal. Chem., 444, 195
͑1998͒.
36. J. Dogel and W. Freyland, Phys. Chem. Chem. Phys., 5, 2484 ͑2003͒.
37. A. P. Abbott, C. A. Eardley, N. R. S. Farley, G. A. Griffith, and A. Pratt, J. Appl.
Electrochem., 31, 1345 ͑2001͒.
38. J. F. Huang and I. W. Sun, Chem. Mater., 16, 1829 ͑2004͒.
39. J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker, and
R. D. Rogers, Green Chem., 3, 156 ͑2001͒.
5. S. D. Thompson, L. R. Jordan, A. K. Shukla, and M. Forsyth, J. Electroanal.
Chem., 515, 61 ͑2001͒.
6. K. Uosaki, S. Ye, H. Naohara, Y. Oda, T. Haba, and T. Kondo, J. Phys. Chem. B,
101, 7566 ͑1997͒.
40. R. P. Swatloski, A. E. Visser, W. M. Reichert, G. A. Broker, L. M. Farina, J. D.
Holbrey, and R. D. Gogers, Green Chem., 4, 81 ͑2002͒.
41. S. Carda-Broch, A. Berthod, and D. W. Armstrong, Anal. Bioanal. Chem., 375, 191
͑2003͒.
7. L. C. Jiang and P. Derek, J. Electroanal. Chem. Interfacial Electrochem., 149, 237
͑1983͒.
8. D. Stoychev, A. Papoutsis, A. Kelaidopoulou, G. Kokkinidis, and A. Milchev,
Mater. Chem. Phys., 72, 360 ͑2001͒.
42. L. G. Lin, Y. Wang, J. W. Yan, Y. Z. Yuan, J. Xiang, and B. W. Mao, Electrochem.
Commun., 5, 995 ͑2003͒.
9. F. Gloaguen, J. M. Leger, C. Lamy, A. Marmann, U. Stimming, and R. Vogel,
Electrochim. Acta, 44, 1805 ͑1999͒.
43. B. K. Sweeny and D. G. Peters, Electrochem. Commun., 3, 712 ͑2001͒.
44. S. Trasatti and O. A. Petrii, J. Electroanal. Chem., 327, 353 ͑1992͒.
45. L. Xiao and K. E. Johnson, J. Electrochem. Soc., 150, E307 ͑2003͒.
46. Y. Katayama, S. Dan, T. Miura, and T. Kishi, J. Electrochem. Soc., 148, C102
͑2001͒.
10. J. S. Wilkes, Green Chem., 4, 73 ͑2002͒.
11. M. H. Yang and I. W. Sun, J. Appl. Electrochem., 33, 1077 ͑2003͒.
12. F. Endres, Phys. Chem. Chem. Phys., 3, 3165 ͑2001͒.
13. P. Y. Chen and I. W. Sun, Electrochim. Acta, 46, 1169 ͑2001͒.
14. I. Mukhopadhyay and W. Freyland, Langmuir, 19, 1951 ͑2003͒.
15. J. H. Shin, W. A. Henderson, and S. Passerini, Electrochem. Commun., 5, 1016
͑2003͒.
47. C. Villagran, C. E. Banks, C. Hardacre, and R. G. Compton, Anal. Chem., 76, 1998
͑2004͒.
48. W. X. Tu, K. Takai, K. Fukui, A. Miyazaki, and T. Enoki, J. Phys. Chem. B, 107,
10134 ͑2003͒.
16. Y. S. Fung and D. R. Zhu, J. Electrochem. Soc., 149, A319 ͑2002͒.
17. D. B. Zhao, M. Wu, Y. Kou, and E. Z. Min, Catal. Today, 74, 157 ͑2002͒.
18. P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 39, 3772 ͑2000͒.
19. S. V. Dzyuba and R. A. Bartsch, Angew. Chem., Int. Ed., 42, 148 ͑2003͒.
20. P. A. Z. Suarez, V. M. Selbach, J. E. L. Dullius, S. Einloft, C. M. S. Piatnicki, D.
S. Azambuja, R. F. de. Souza, and J. Dupont, Electrochim. Acta, 42, 2533 ͑1997͒.
21. J. Zhang and A. M. Bond, Anal. Chem., 75, 2694 ͑2003͒.
22. F. Endres, M. Bukowski, R. Hempelmann, and H. Natter, Angew. Chem., Int. Ed.,
42, 3428 ͑2003͒.
49. http://srdata.nist.gov/xps/
50. R. Hagiwara and Y. Ito, J. Fluorine Chem., 105, 221 ͑2000͒.
51. G. Staikov, K. Juttner, W. J. Lorenz, and E. Budevski, Electrochim. Acta, 39, 1019
͑1994͒.
52. K. R. Seddon, A. Stark, and M. J. Torres, Pure Appl. Chem., 72, 2275 ͑2000͒.
53. J. J. Lee, B. Miller, X. Shi, R. Kalish, and K. A. Wheeler, J. Electrochem. Soc.,
148, C183 ͑2001͒.
54. G. Gunawardena, G. Hills, T. Montenegro, and B. Scharifker, J. Electroanal. Chem.
Interfacial Electrochem., 138, 225 ͑1982͒.
23. G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixeira, and J. Dupont,
Chem.-Eur. J., 9, 3263 ͑2003͒.
55. B. Scharifker and G. Hills, Electrochim. Acta, 28, 879 ͑1983͒.
56. M. E. Hyde and R. G. Compton, J. Electroanal. Chem., 549, 1 ͑2003͒.
57. A. Milchev and L. Heerman, Electrochim. Acta, 48, 2903 ͑2003͒.
58. E. Budevski, G. Staikov, and W. J. Lorenz, Electrochim. Acta, 45, 2559 ͑2000͒.
59. M. Palomar-Pardave, M. Miranda-Hernandez, I. Gonzalez, and N. Batina, Surf.
Sci., 399, 80 ͑1998͒.
24. F. Endres, ChemPhysChem, 3, 144 ͑2002͒.
25. C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner, and S. R. Texeiral, Inorg.
Chem., 42, 4738 ͑2003͒.
26. W. Freyland, C. A. Zell, S. Z. El Abedin, and F. Endres, Electrochim. Acta, 48,
3053 ͑2003͒.
60. L. Heerman and A. Tarallo, J. Electroanal. Chem., 470, 70 ͑1999͒.
61. R. K. Pandey, S. N. Sahu, and S. Chandra, in Handbook of Semiconductor
Electrodeposition, Marcel Dekker, New York ͑1996͒.
27. D. L. Boxall and R. A. Osteryoung, J. Electrochem. Soc., 149, E185 ͑2002͒.
28. D. L. Boxall, J. J. O’Dea, and R. A. Osteryoung, J. Electrochem. Soc., 149, E468
͑2002͒.
62. S. Langerock and L. Heerman, J. Electrochem. Soc., 151, C155 ͑2004͒.
63. R. Parsons and T. Van der Noot, J. Electroanal. Chem. Interfacial Electrochem., 9,
257 ͑1988͒.
29. J. Zhang, A. M. Bond, W. J. Belcher, K. J. Wallace, and J. W. Steed, J. Phys. Chem.
B, 107, 5777 ͑2003͒.
30. I. Mukhopadhyay and W. Freyland, Chem. Phys. Lett., 377, 223 ͑2003͒.
64. E. Herrero, K. Franaszczuk, and A. Wieckowski, J. Phys. Chem., 98, 5074 ͑1994͒.
Downloaded on 2014-09-08 to IP 129.81.226.78 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).