Sonoelectrochemical (20 kHz) production of platinum nanoparticles from aqueous solutions

Article abstract of DOI:10.1016/j.electacta.2009.07.001

The present work describes the production of platinum nanoparticles from aqueous chloroplatinic solutions in the presence of low-frequency high-power ultrasound (20 kHz) on titanium alloy electrodes. The production of this new type of Pt nanoparticles was

Full text of DOI:10.1016/j.electacta.2009.07.001

Electrochimica Acta 54 (2009) 7201–7206
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Sonoelectrochemical (20 kHz) production of platinum nanoparticles from
aqueous solutions
Valentina Zin , Bruno G. Pollet , Manuele Dabalàa
a
b,∗
a
DPCI – University of Padua, via Marzolo, 9, 35131 Padova, Italy
Fuel Cells Group, School of Chemical Engineering, The University of Birmingham, Edgbaston Road, Edgbaston, Birmingham B15 2TT, UK
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The present work describes the production of platinum nanoparticles from aqueous chloroplatinic solu-
tions in the presence of low-frequency high-power ultrasound (20 kHz) on titanium alloy electrodes. The
production of this new type of Pt nanoparticles was performed galvanostatically at (298 ± 1) K using a
newly designed experimental set-up and ‘sonoelectrode’ producing ultrasonic pulses triggered and fol-
lowed immediately by short applied current pulses. From galvanostatic studies, it was shown that Pt mean
grain size ranging from 11 to 15 nm was produced. Morphological and structural studies of the produced
nanoparticles were performed by TEM, SEM, XRD and SAED and showed that Pt nanoaggregates were
predominantly formed, with no redissolution of the nanoaggregates. Globular clusters had a mean size
ranging between 100 and 200 nm which in turn aggregated and built complex structures.
© 2009 Elsevier Ltd. All rights reserved.
Received 25 February 2009
Received in revised form 22 May 2009
Accepted 1 July 2009
Available online 9 July 2009
Keywords:
Platinum
Nanoparticles
Sonoelectrode
Power ultrasound
Sonochemistry
1. Introduction
examples investigating mass transfer phenomena under sonica-
tion. Power ultrasound is known to decrease the diffusion layer
The use of ultrasound on electrochemical systems or Sonoelec-
trochemistry was first observed by Moriguchi as early as 1934 [1]
and continues to be an active and exciting research area [2]. Exten-
sive work has been carried out in which high power ultrasound
was applied to various electrochemical processes leading to several
industrial applications and many publications over a wide range
of subject areas such as electrodeposition, electroplating, electro-
chemical dissolution and corrosion testing.
It was shown that the effects of high intensity ultrasonic irra-
diation on electrochemical processes lead to both chemical and
physical effects, for example, mass-transport enhancement, sur-
face cleaning and radical formation. Many workers have also
investigated the distribution of ultrasonic waves or energy in var-
ious electrochemical reactors operating in the lower ultrasonic
frequency range (20–100 kHz) and at high ultrasonic powers. Sev-
eral methods for such determination have been proposed, e.g.
aluminum foil erosion, sonoluminescence, calorimetric methods,
chemical dosimetry [2] and laser-sheet visualization [3].
thickness (ı) thereby giving substantial increase in limiting cur-
rent (Ilim) attributed due to effects of cavitation and/or micro- and
macro-streaming. It is known in the field that both cavitational and
acoustic streaming effects contribute significantly to the increase in
observed experimental currents [4–7]. The experimental decrease
in the diffusion layer thickness is also known to be due to asymmet-
rical collapse of cavitation bubbles at the electrode surface leading
to the formation of high velocity jets of liquid being directed toward
its surface. This jetting, together with acoustic streaming, is thought
to lead to random punctuation and disruption of the mass transfer
boundary layer at the electrode surface at close electrode-to-horn
separations. Birkin et al. [8,9] also showed that the nature of the
solvent is paramount in assigning limiting currents. More recently,
Pollet et al. [10] showed, with aid of mathematical models based
on mass-balance equations, that a Levich-like equation relating
the limiting current density, the square root of ultrasonic intensity
and the inverse square root of the electrode–horn distance, may
be generated for ultrasonic frequencies of 20 and 40 kHz allowing
the generation of an ‘equivalent’ flow velocity under sonication, an
important and useful parameter in chemical engineering.
Many of the observed effects in Sonoelectrochemistry may be
explained by the enhancement of mass-transport in diffusion-
controlled processes. The extensive work of Coury and co-worker
Recently, an upsurge of interests has been observed in the syn-
thesis of metallic nanoparticles [11–13]. In the last decade, the
production of metallic platinum nanoparticles has been inves-
tigated as they offer high surface-to-volume ratios and have
considerable potential for usage in various areas such as fuel
cells, reforming, catalysis and medicine [14,15]. There are a range
[
4] and Compton et al. [5,6] were probably the first “modern”
∗ Corresponding author. Tel.: +44 781 495 2112; fax: +44 121 414 5377.
E-mail address: b.g.pollet@bham.ac.uk (B.G. Pollet).
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.001

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V. Zin et al. / Electrochimica Acta 54 (2009) 7201–7206
of methods of producing metallic platinum nanosized mate-
rials including thermal decomposition, physical and thermal
evaporation, laser ablation, laser-assisted catalytic growth (LCG),
vapor–liquid–solid growth (VLS), ultrahigh-vacuum (UHV), ion
implantation, biochemical, electrochemical, sonochemical, radiol-
ysis, chemical reduction/oxidation and sol–gel [15]. However, most
of these techniques tend to be expensive and time-consuming. An
alternative method, which is both simple and cost-effective, is the
use of Sonoelectrochemistry. For example, based on the original
work of Reisse et al. [16,17] for the production of copper, Dabalà
and Pollet [18] produced for the first time nanosized Fe–Co metallic
particles sonoelectrochemically, in which the ultrasonic horn was
used as the working electrode. This ‘sonoelectrode’ was subjected
to ultrasonic pulses which were each followed by short applied
current pulses. They showed that, during cavitation, a jet of liq-
uid penetrates inside the cavitation bubble perpendicular to the
operating in galvanostatic mode and a 20 kHz ultrasonic gener-
ator (Sintec Generator EG36) and (ii) a platinum mesh acting as
the anode linked to the potentiostat. Electrochemical experiments
were performed using a cylindrical vessel (200 ml) [Fig. 1(a)]. The
electrochemical cell was placed in a Faraday cage. Temperature
was regulated by a glass cooling coil (C) placed inside the elec-
trochemical cell and linked to a thermostatted bath operating at
preset temperatures. The temperature of the electro-analyte was
measured with a Fluke 51 digital thermometer fitted to a K-type
thermocouple. A heat-shrinkable sleeve surrounded the side walls
of the extreme part of the sonoelectrode, leaving only a flat active
surface for the electrodeposition equal to 1.227 cm2 (ultrasonic
horn tip area determined coulometrically) [Fig. 1(a)]. A constant
galvanostatic current was applied to the sonoelectrode and the
maximum ultrasonic power ultrasound employed was 76 W. A trig-
ger acted like a switch with the role of closing alternatively the
circuits in which the potentiostat and the piezoelectric power sup-
ply operated. The pulse drivers allowed applied galvanic current
and ultrasonic pulsing.
‘sonoelectrode’ surface and the resulting impact was responsible for
dislodging any nanopowder material which had been electrochem-
ically deposited on the surface.
This paper reports for the first time, a study on the synthesis of
platinum nanoparticles by using the new method combining metal-
lic electrodeposition with low-frequency high-power ultrasound
All electrodes were electrochemically cleaned by cycling in sul-
−
3
phuric acid (1.0 mol dm ) for 10 min prior to the experiments. They
were then washed with high quality MilliQ water and all Ti elec-
trodes were polished to a mirror finish first with grinding paper
(Buelher-Met, P600) and then sequentially with 25 ␮m down to
(
20 kHz). In this study the platinum nanopowders produced were
characterized both morphologically and chemically.
0
.3 ␮m alumina oxide paste and were cleaned with immersion in
nitric acid for 5 min in order to remove any traces of contaminants.
For the production of nanopowders, the time management
sequence employed was as follows [Fig. 1(b)]:
2
. Experimental
The apparatus employed for the production of platinum nano-
sized materials was similar to that employed by Reisse et al. and
Pollet and Dabalà [17,18] [Fig. 1(a)]. The system consisted of a
two-electrode set-up, namely (i) a titanium alloy (Ti–6Al–4V) horn
acting both as the cathode and the ultrasonic emitter (described
therein as the sonoelectrode) linked to a AMEL 7060 potentiostat
−2
1
. A short current pulse of |i| = 50 mA cm was applied to the sono-
electrode, and here the titanium horn acted as an electrode only
tON); the time of this phase typically varied between 0.3 and
.5 s.
(
0
2
. Immediately after the electrochemical pulse was turned off, an
ultrasonic pulse was sent to the sonoelectrode and here it acted
only as a vibrating ultrasonic horn (tUS); this second phase lasted
no more than 0.5 s.
3
. A rest time, tp, followed the two previous phases (this was useful
to restore the initial electrolyte conditions close to the sonoelec-
trode).
A characteristic time management parameter of the process, ꢀ,
was employed according to Eq. (1) [17,18]:
tON
ꢀ
= t
(1)
ON
+ tOFF
where tOFF = tUS + tp
By controlling the varying process parameter, ꢀ, and the applied
current, it was possible to produce sonoelectrochemically high
purity and high surface/volume ratio suspended nanoparticles
which were filtered with 0.05 ␮m Millipore filters under vacuum.
The filters were then washed with pure ethanol, dried for 48 h in
a silica-gel drier and stored under vacuum. Each filter was weighted
after dehydration and the efficiency of the process was calculated
as the ratio of the produced mass of powder to the faradic yield
according to Eq. (2) [19]:
ꢀ
ꢀ
· I · t
i(xi · PAi)
m =
· ꢀ
(2)
f
F
(x · n )
i
i
ei
where ꢀ is tON/(t + tOFF), I is the applied current in A, t is the total
ON
−
1
time in s, F is the Faraday constant (96,500 C mol ), x is the molar
fraction, PA is the atomic weight of Pt in g mol (195.09 g mol
i
−
1
−1
)
Fig. 1. (a) Schematic of the sonoelectrochemical nanoparticle production setup and
i
(b) time management for Pt sonoelectrosyntheses.
and nei is the number of electron transferred (=2).

V. Zin et al. / Electrochimica Acta 54 (2009) 7201–7206
7203
Table 1
Cathode efficiency (ꢁ) of Pt nanoparticles showing variation of time management.
tON (s) is the applied current pulse time (s); tUS (s) is the applied ultrasonic pulse
time (s); tp is the rest time (s); ꢀ (chi) is the varying process parameter; mf is the
faradic yield (mg); mr is the actual yield (mg); ꢁ is the cathode efficiency (%).
Test #
tON/s
tUS/s
tr/s
ꢀ
ꢁ/%
1
2
3
4
5
0.2
0.2
0.3
0.3
0.5
0.5
0.3
0.5
0.3
0.3
0.2
0.2
0.2
0.2
0.2
0.222
0.2857
0.3
0.375
0.5
67.5
80.5
59.3
83.4
73.8
For each run, the cathode efficiency, ꢁ in % was determined using
Eq. (3) [20]:
mr
ꢁ
= m · 100%
(3)
Fig. 2. Cathode efficiency (ꢁ) vs. time management parameter (ꢀ) of Pt nanoparticles
production. Inset figure: cathode efficiency (ꢁ) vs. tON.
f
where m is the faradaic yield in g and mr is the actual metallic mass
produced during the sonoelectrochemical tests in g.
f
to the formation of nanoparticles in the bulk electrolyte. On the
other hand, for larger values of tON, higher quantity of Pt nuclei
was formed on the sonoelectrode surface causing water reduction
at the nuclei–electrolyte interface due to lower overvoltages. In
other words, ‘secondary reactions’ became predominant and metal
reduction hindered. Also, growth of Pt nuclei is promoted by longer
electrochemical pulses leading to an increase in Pt nanoparticles
mean size causing difficulties in ablating nuclei from the sonoelec-
trode surface, causing a decrease in cathode efficiency (see inset
Fig. 2). This observation could explain the low value of process effi-
ciency of ꢀ = 0.3. In fact, the test is characterized of long duration of
tUS coupled to a longer duration of tON when compared to Test #1.
The two syntheses are both affected by the same decrease of ꢁ due
to a tUS that lasts for 0.5 s, but Test #3 has also a higher tON which
causes a further drop in cathode efficiency.
Fig. 3 shows the dependency of ꢀ with tON and tUS. The figure
also shows that as ꢀ increases with tON the phenomenon is more
noticeable when tUS is short, while ꢀ decreases, i.e. when tUS ꢀ tON,
in particular when tON is tending to zero. From our observations,
the most efficient conditions to produce Pt nanoparticles with high
yield are between the two extreme tested values of studied param-
eters, i.e. with tON = 0.2–0.3 s and tUS = 0.3 s.
The produced powders were analyzed by X-EDS (Philips EDAX
PV9800) in views of identifying any contaminant. Morphological
studies of the Pt nanopowders were performed on both a scanning
electron microscope (Stereoscan 440 SEM, Cambridge, equipped
with a Philips EDAX PV9800) and a transmission electron micro-
scope (JEOL 2000FX operating at 160 kV). X-ray diffractometer
(
Siemens D500 XRD) with Cu K␣ radiation (ꢂ = 1.5418 Å, 40 kV and
0 mA) was used for the identification of the phases and the mea-
surement of grain size in the powders. 2ꢃ/ꢃ diffractograms were
3
◦
◦
obtained in the 35–85 range with a step of 0.03 with a measuring
time of 15 s per step.
Finally, for all experiments, chemical reagents were of AnalaR
grade or equivalent. All Pt nanopowders were synthesized from
−
3
a chloride bath. Aqueous chloroplatinic solutions of 0.1 mol dm
−3
K PtCl and 0.5 mol dm
NaCl (background electrolyte) were
2
4
freshly prepared using high quality MilliQ water (R = 12 Mꢄ). The
pH(1 ± 0.1)ofthechloroplatinicsolutionswasmeasuredbeforeand
after each experiments and showed no variations from the value set
at the start. Ultrasonic powers were determined calorimetrically
using the method of Margulis et al. [21,22] and ultrasonic powers
are quoted as W or otherwise stated.
Clearly, sonication has the sole role to induce cavitation phe-
nomenon in the electrolytic solution [6,23] and to ablate the
metallic nuclei from the cathodic surface. Nuclei formed on the
titanium surface during the tON time are ejected into the electrolyte
during tUS. Cavitation event takes place and gas bubbles collapse in
3
. Results and discussion
3.1. Cathode efficiency
Syntheses of Pt nanoparticles were performed at various time
managements (ꢀ) between 0.22 and 0.5 in views of optimising the
production of Pt nanoparticles with the highest cathode efficiency,
ꢁ
. The total time of the sonoelectrochemical run was set to 60 min.
In these experiments no titanium particles arising from cavitational
erosion from the ultrasonic horn surface were detected on the filters
(see later). The experimental conditions for the sonoelectrochemi-
cal investigations, i.e. the process time management parameter (ꢀ)
and the cathode efficiency (ꢁ) are summarized in Table 1 and Fig. 2.
From the data, it is clearly evident that the cathode efficiency is
well over 50% in all applied conditions, particularly exceeding 80%
when time management parameter ꢀ is either 0.2875 or 0.375. This
is due to the fact that, for low values of ꢀ, tON is notably lower than
tUS, i.e. the electrodeposition time is shorter and ultrasonic irradi-
ation lasts longer leading to a few Pt nuclei being formed at the
sonoelectrode with (a) a large amount of energy being dissipated
to the electrolytic solution, (b) an increase in local temperature
and (c) redissolution of nanoparticles. Furthermore, from the data,
the lowest cathode efficiencies of 59.3% and 67.5% were obtained
with tUS = 0.5 s which is in good agreement with previous findings
Fig. 3. Cathode efficiency (ꢁ) as a function of tON and tUS.
[
18], according to which longer ultrasonic pulses are detrimental

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V. Zin et al. / Electrochimica Acta 54 (2009) 7201–7206
the liquid resulting in high energy release [23–26]. Locally, shock
waves propagate through the liquid and generate high-intensity
shearing forces causing a displacement of the electrolyte, known
as acoustic streaming providing agitation of the solution near the
sonoeletrode, improving the mass-transport and allowing the sys-
tem to work under charge transfer control [24].
3.2. Characterization of Pt nanoparticles
Compositional analyses of the filters containing Pt nanopar-
ticles were performed with the aim to rule out any possible
contamination from the sonoelectrode material or any other bath’s
components. Analyses were carried out by X-EDS and XRD and
showed that all produced nanoparticles were exclusively consti-
tuted with platinum and no titanium from the sonoelectrode and
any other chemicals were detected in tested samples.
However, two other elements (Al and Cu) were detected by
EDAX analyses, as shown in Fig. 4(a). Al and Cu peaks are caused
by the sample-carrier which is made of Al–Cu alloy, on which Pt
nanoparticles were mounted on. XRD measurements [Fig. 4(b)]
were performed to investigate the microstructure of Pt nanopar-
ticles. The patterns did not reveal foreign elements other than the
characteristic peaks for Pt; this observation indicates that the Pt
nanoparticles produced in this study are of high purity. The grain
size was calculated from the peak broadening by using the Rietveld
method [27,28] and the patterns for the five tests as shown in
Fig. 4(b); due to small size effect, the XRD peaks are low and broad.
Spectra revealed the presence of four peaks in each scan. These
are typical of a single phase Pt with face centred cubic (fcc) struc-
ture, according to the JCPDS database (card number 01-1194),
◦
◦
◦
where the peaks with 2ꢃ values of 40.08 , 46.58 , 67.92 and
8
◦
1.59 correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes
Fig. 5. (a) Selected area electron diffraction pattern of run #1 nanoparticles and (b)
TEM images for Test #4.
of the fcc Pt phase, respectively [29]. In addition, Pt nanoparticles
showed relatively broad peaks, and this feature of the spectra was
useful in using the Rietveld method to describe sample microstruc-
ture. Furthermore, no signals from other contaminants or Ti-alloy
(constituting the sonoelectrode) were detected. Once again, this
observation shows that, in our conditions, pure Pt nanoparticles
can be produced sonoelectrochemically.
Grain size and lattice parameter were calculated for each test
and the data are shown in Table 2. The table shows that Pt nanopar-
ticles exhibited a mean grain size ranging from 13 to 15 nm, which is
concordant with TEM analyses discussed earlier and measured lat-
tice parameter was in good agreement with the theoretical value
obtained from literature of 3.91 Å [30,31].
It can also be emphasized that time management did not affect
the microstructural feature of the produced Pt nanopowders, in
other words, the samples did not reveal any noticeable variations
Table 2
Structural parameters of Pt nanoparticles obtained from XRD analysis.
Test #
ꢀ
tON/s
tUS/s
aa/Å
Grain size/nm
1
2
3
4
5
0.5
0.375
0.3
0.222
0.2857
0.5
0.3
0.3
0.2
0.2
0.3
0.3
0.5
0.5
0.3
3.89
3.88
3.88
3.87
3.90
13.6
14.3
14.5
13.1
13.3
Fig. 4. (a) X-ray energy dispersive spectrometry of Pt nanoparticles and (b) XRD
patterns of Pt nanoparticles.
a
Lattice parameter (a).

V. Zin et al. / Electrochimica Acta 54 (2009) 7201–7206
7205
Table 3
Morphological characterization was performed by both SEM and
TEM. Fig. 5(b) shows a representative TEM micrograph, referring
to Pt nanopowders from Test 4. The figure shows that produced
Pt nanoparticles are spherical and with minimum diameter of ca.
Comparison of interplanar spacings (d h k l) with standard data.
Data
JCPDS 01-1194
d/Å
d/Å
Intensity
h k l
13 nm with an average size ranging from 10 to 20 nm. For exam-
2
1
1
1
1
.26
.93
.44
.21
.06
2.25
1.95
1.38
1.18
1.13
100
30
16
16
3
111
200
220
311
222
ple, it was found that be Pt nanoparticles had mean average size of
ca. 14.06 nm with a standard deviation of 2.78 which is in excel-
lent agreement with values obtained from data analysis carried
out by XRD characterization [Fig. 6(a)]. Using the Rietveld method,
the mean crystallite size was calculated and it was observed that
the nanoparticle dimensions were similar suggesting that platinum
nanopowders were mono-crystalline rather than poly-crystalline.
From the picture it can be seen that nanoparticles have aggre-
gated into secondary particles which are clearly visible in SEM
images shown in Fig. 6(b). Globular clusters observed by SEM,
and defined as secondary particles, have a mean size ranging
between 100 and 200 nm and in turn aggregated and built complex
structures; these sphere-chains are the results of the attractive sur-
face tension between the ultra-fine particles, i.e. the high surface
energy that leads to agglomeration (in order to minimize system
energy).
The difference between TEM and SEM images is due to (a) the
low resolution of the SEM compared to TEM and (b) the agglomer-
ation of nanoparticles; in fact during filtration, nanoparticles were
compacted on the filter and formed large clusters by weak chemical
bonding. These large agglomerates could not be separated before
sample preparation leading to an apparent larger particle size on
SEM pictures; in other words, one visible particle of 200 nm usually
contained several primary nanoparticles of about 15 nm diameter
size.
in grain size values and lattice parameters. This means that metal
particles can be synthesized with nanometric dimension providing
that tON and tUS remain below 0.5 s, i.e. at a maximum ꢀ of 0.5. This
finding suggests that under drastic conditions induced by cavita-
tion leading to the “hot spot” mechanism [26,32] do not affect the
crystalline structure of Pt nanoparticles.
Fig. 5(a) shows the corresponding selected-area electron diffrac-
tion (SAED) pattern, referring to TEM image in Fig. 5(b). It can be
indexed to the reflection of a fcc-structure which was further con-
firmed by XRD. The random orientation of the small particles causes
the broadening of the diffraction rings. Electron diffraction images
were elaborated and the values of interplanar spacing dh k l were cal-
culated from the diameter of the diffraction rings and compared to
the theoretical ones obtained from JCPDS 01-1194 (Table 3) [33–35].
ꢁ
2
2
2
For fcc structure, dh k l = a/ h + k + l , the lattice parameter
(
a) can be calculated from measured values for the spacing of the
detected planes; for example, for Test 1, a, value measured in TEM-
SAED analysis was found to be 3.854 Å which is in good agreement
with XRD result and the theoretical data of 2.89 Å.
Fig. 6. (a) XRD pattern of Pt nanopowders and (b) SEM micrograph of Pt nanoparticles.

7
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V. Zin et al. / Electrochimica Acta 54 (2009) 7201–7206
Table 4
structure with a lattice parameter of ca. 3.91 Å. Furthermore, it was
observed that sonication enables the production of Pt nanoparticles
with high purity, controlled structure and homogenous nanometric
crystalline size.
Particle sizes and surface areas of Pt nanoparticles showing variation of ꢀ (chi) [the
varying process parameter] and ꢁ [the cathode efficiency (%)].
Test #
ꢀ
ꢁ/%
Particle size/nm
Surface area/m2
g
−1
1
2
3
4
5
0.222
0.2857
0.3
0.375
0.5
67.5
80.5
59.3
83.4
73.8
10.1
11
10.6
9.5
27.76
25.49
26.45
29.51
26.97
Acknowledgements
The authors would like to thank the European Community Sixth
Framework Program through a STREP grant to the SELECT-NANO,
Contract No. 516922.03/25/2005 for their kind financial support.
10.4
To support these observations, the volume-averaged particle
size was calculated from the full width half-maximum of the (1 1 1)
peak for various time management parameters (ꢀ) using Fig. 4(b)
and the Scherrer equation [36,37]:
References
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kꢂ
d (Å ) =
(4)
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ˇ cos ꢃ
[
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coefficient (0.9, assuming spherical spheres), ꢂ the wavelength of
radiation used (1.54184 Å), ˇ is the full width half-maximum of the
peak in radian and ꢃ is the angle at the position of peak maximum
in radian.
Table 4 gives the average particle size for several management
parameters (ꢀ) using the Scherrer equation. For a time management
parameter of 0.375 corresponding to a cathode efficiency of 83.4%,
the lowest average particle size was found to be 11 nm, which is in
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[
From, the calculated average particle sizes, it is possible to
2
−1
determine the surface area (SA in m g ) of platinum, assuming
homogeneously distributed and spherical particles, as follows:
[
14] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C.A. Lucas, G. Wang, P.N.
Ross, N.M. Markovic, Nat. Mater. 6 (2007) 241.
[
15] R. Ferrando, J. Jellinek, R.L. Johnston, Chem. Rev. 108 (2008) 845.
SA = 6
× 1000
(5)
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4
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