Influence of base electrolytes on the electrodeposition of iron onto a silicon surface

Article abstract of DOI:10.1021/jp061614e

Comparative electrodeposition of iron onto the surface of silicon was investigated at large overpotential in the presence of some base electrolytes. The nanostructure of the iron electrodeposits was analyzed with SEM and FE-SEM measurements. The results highlight the influences of ion specificity on the rate of hydrogen evolution and of selective ion adsorption on the morphology of the iron electrodeposits.

Full text of DOI:10.1021/jp061614e

J. Phys. Chem. B 2006, 110, 14779-14786
14779
Influence of Base Electrolytes on the Electrodeposition of Iron onto a Silicon Surface
J. L. Trompette* and H. Vergnes
Laboratoire de G e´ nie Chimique UMR 5503, UniVersit e´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse
Cedex, France
ReceiVed: March 15, 2006; In Final Form: June 15, 2006
Comparative electrodeposition of iron onto the surface of silicon was investigated at large overpotential in
the presence of some base electrolytes. The nanostructure of the iron electrodeposits was analyzed with SEM
and FE-SEM measurements. The results highlight the influences of ion specificity on the rate of hydrogen
evolution and of selective ion adsorption on the morphology of the iron electrodeposits.
1
. Introduction
structure of substrates and the morphology of metal electro-
deposits to be obtained. It was inferred from these studies that
the nucleation of the new metallic phase and its growing form
play a dominant role in the size distribution and structure of
the metallic deposit.
Both the size and the shape of metal or semiconductor
colloidal particles have received much attention in recent years
because of their strong influence on the physical and chemical
properties of materials. The properties and applications of metal
nanoparticles with various morphologies can be used in a variety
of areas including catalysis, electronics, and optics.
preparation of many monodisperse inorganic colloids of various
shapes (spheres, rods, cubes, triangles, disks) by precipitation
Although the electrolyte phase was assumed to affect the
characteristics of the substrate-electrolyte interface and prob-
ably the properties of the metallic deposits, very few studies
have been focused on investigating the influence of the nature
of the background electrolyte in the electrocrystallization
process. Very often, the metal is deposited by using an
electrolyte having the same anion as the metallic salt. However,
it has been known for some time that some components can
have a pronounced effect on the resulting crystal shape because
of their specific adsorption onto a particular crystallographic
1
-3
The
and/or reduction reactions from homogeneous solutions has been
reported in the literature.4
-7
The use of a soft template or
capping agent (polymers, surfactants) is often required in these
reaction pathways. The control of the shape of the nanoparticles
was found to be related to that of the templates, and this allows
the generation of various morphologies in different experimental
1
7
8
,9
plane. Moreover, the nature of an ion can have a significant
influence on the behavior of a colloidal system, particularly at
concentrations exceeding 0.10 M, where the range of electro-
conditions.
In parallel, the presence of metal nanoparticles directly
deposited onto a substrate is also required in some technological
applications (catalysts for the synthesis of carbon nanotubes,
electrochemical sensors). In this sense, the electrochemical
synthesis of metal nanoparticles through the electrocrystallization
process has gained renewed attention and is considered to be a
18
static interactions is greatly reduced. These ion-specific effects,
related to the Hofmeister series, are relevant to many systems
and processes, implying colloidal interactions where short-range
hydration contributions were shown to play a crucial role.1
Both ions and substrate surfaces change the solvent properties
around them, and this can have an influence on solvent-surface,
surface-ion, and solvent-ion interactions during the electro-
crystallization process.
8-21
promising way because of its simplicity, convenience, and use
of inexpensive equipment.10,11 Metal electrocrystallization takes
place at an electrode-electrolyte interface under the influence
of an electric field and is directly related to nucleation and
crystal growth processes.1
2,13
In this study, the influence of the nature of some base
electrolytes on the electrodeposition of iron onto the hydrophobic
surface of silicon without any capping agent or template was
investigated. The experiments were performed with the same
imposed overpotential (-670 mV). The base electrolyte con-
centration was taken as 0.6 M to avoid any migration current
in the electrochemical system and to ensure the eventual
manifestation of ionic specificity. Iron electrodeposits were
analyzed through SEM and FE-SEM measurements.
On the basis of energetic consid-
erations, general trends can be expected. When the interfacial
bonding between the foreign substrate and the deposited metal
is weaker than the bonding in the metallic deposit itself, the
surface concentration of adsorbed metal, Meads, is low, and the
growth of isolated 3D islands is more favored.14 Recent
fundamental studies have been directed toward identifying and
understanding the modulation of the key parameters allowing
control of the dimensions and the narrowing of the size
distribution of many metal nanoparticles without the use of any
additives or template.1
0,15,16
2. Experimental Section
Moreover, various surface analytical
techniques, such as SEM and TEM (scanning and transmission
electron microscopies, respectively), EXAFS (extended X-ray
absorption fine structure) analysis, and AFM (atomic force
microscopy), have allowed important information on the
The iron sulfate salt FeSO4‚7H2O and the base electrolytes
(
NH4)2SO4 (ammonium sulfate), NH4SCN (ammonium thiocy-
anate), NH4ClO4 (ammonium perchlorate), NH4Br (ammonium
bromide), and NH4CH3CO2 (ammonium acetate) were obtained
from Sigma-Aldrich (France) and were used as received. The
electrolytic baths (50 mL) were prepared by dissolving the
required amount of iron salt and base electrolyte in distilled
*
To whom correspondence should be addressed. E-mail: JeanLuc.
Trompette@ensiacet.fr. Phone: (33) 05 62 88 58 59. Fax: (33) 05 61 55
1 39.
6
1
0.1021/jp061614e CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/07/2006

14780 J. Phys. Chem. B, Vol. 110, No. 30, 2006
Trompette and Vergnes
and deionized water. It should be noted that the electrolytic
solution composed of ammonium thiocyanate and iron sulfate
salts exhibited the characteristic red color corresponding to the
formation of the soluble iron(III) thiocyanate complex Fe-
2
+
(
SCN) originating from the rapid oxidation of iron(II) by
22
-3
dissolved oxygen in water (0.26 × 10 M maximum solubility
at 25 °C). The concentration of the iron(III) thiocyanate complex
was determined spectroscopically (466.7-nm maximum absorp-
-
3
tion wavelength) and was found to correspond to 1.5 × 10
M, a negligible amount with respect to the 0.4 M iron(II)
concentration used.
Figure 1. Water droplet deposited on the silicon surface after
hydrofluoric acid treatment.
Prior to each experiment, electrolytic baths were degassed
by bubbling of nitrogen gas during 10 min. The pH of the
various electrolyte solutions was between 3 and 5.2.
The electrochemical setup was a standard three-electrode cell
disruption of the adjacent water network, so that the layer of
water molecules is of much lower density. Consequently, the
hydrophilic or hydrophobic character of the surface can exert
an influence on the surrounding ions because surfaces of
different characters do not present the same affinity with water
molecules. Ions have been divided into two broad classes
2
with a silicon plate as the working electrode (1 × 2 cm ),
platinum foil as the counter electrode, and a saturated calomel
reference electrode (SCE). The working electrode, n-type Si-
(
Hofmeister series) according to their ability to structure the
(100) single crystals with a resistivity of 0.003 Ω‚cm (Neyco
2
6
water molecules in their vicinity. Ions with a large surface
electric field that interact more strongly with water molecules
than water molecules interact with each other are called structure
makers or cosmotropes (Mg , Li , Na , SO4 , CH3CO2 , F ),
whereas those presenting the opposite effect, with a small
surface electric field, are called structure breakers or chaotropes
Co.), was first sonicated in trichloroethylene for 10 min to
remove dirt and rinsed with acetone. It was then immersed for
1
min in dilute (1/40) hydrofluoric acid (HF) to dissolve the
2+
+
+
2-
-
-
(eventual) hydrophilic superficial silica layer, thoroughly rinsed
with distilled water, and finally dried in a stream of nitrogen
gas. The electrode was immersed (1-cm depth) in electrolytic
bath at 298 K just before experiments. After electrodeposition,
the electrode was rinsed with water and then dried by nitrogen
gas.
The electrochemical experiments (cyclic voltammetry and
chronoamperometry) were executed at ambient (room) illumina-
tion with a potentioscan (Radiometer Analytical S.A. Copen-
hagen, Tacussel DEA 332) coupled with a digital converter
+
+
+
-
-
-
(
NH4 , Cs , Rb , SCN , ClO4 , Br ). These hydration effects
were shown to be more pronounced with anions because anions
can bind more strongly with water molecules through hydrogen-
2
6
bond-like interactions.
The wetting behavior of the studied silicon substrate was
determined by measuring the contact angle of a water droplet
deposited on the surface. The average value was found to be
around 87° after the hydrofluoric acid treatment (see Figure 1),
thus reflecting the hydrophobicity of the electrode surface. As
chaotropic ions disrupt the water structure around them, it might
be suggested that such ions would tend to be embedded more
favorably than cosmotropic ions in this interfacial region with
a low-density water layer at the contact of the hydrophobic
silicon surface. First, comparative iron electrodeposition experi-
ments were carried out by using FeSO4 salt at 0.4 M and (NH4)2-
SO4 or NH4SCN base electrolyte at 0.6 M to satisfy similar
(Radiometer Analytical, IMT 102) and controlled by a PC
running the electrochemical software (Radiometer Analytical,
VoltaMaster 2).
Iron electrodeposits were examined by scanning electron
microscopy (SEM) with a LEO 435 VP electron microscope
and by field-emission scanning electron microscopy (FE-SEM)
with a JEOL JSM 6700F apparatus, using various magnifications
to observe the morphology of the deposits. An energy-dispersive
X-ray spectroscopy (EDS) apparatus (INCA Energy 250, Oxford
Instruments), attached to the SEM instrument, was used to obtain
information on the composition of the electrodeposits.
Contact angles of water droplets deposited on the electrode
surface were measured with a Digidrop apparatus (GBX, France)
to determine the wetting behavior.
11
conditions encountered in previous studies and to enable easier
observation of electrodeposit morphology. The cosmotropic
2
-
-
SO4 and chaotropic SCN anions were chosen to investigate
the influence of ion specificity because they are in the extreme
26
of the Hofmeister series. The ammonium counterion was taken
as the same common chaotropic cation.
To determine characteristic electrochemical potentials such
as the deposition potential, the onset of cathodic reduction, and
the nucleation overpotential, comparative cyclic voltammograms
were obtained at the same scanning rate (20 mV/s) for both
systems. The results of current-voltage (CV) experiments are
shown in Figure 2. The onset of iron deposition is found to
occur at essentially the same potential during the direct cathodic
scan, namely, -1082 mV/SCE for (NH4)2SO4 (A) and -1072
mV/SCE for NH4SCN (B), and no cathodic peak is observed
because of the high concentration of iron salt. Upon sweep
reversal, the cathodic current gradually decreases until it crosses
0 and becomes an anodic current. The beginning of iron
oxidation is found to occur at -790 mV/SCE for (NH4)2SO4
(A), whereas it corresponds to -870 mV/SCE for NH4SCN (B),
3
. Results and Discussion
3
.1. Comparative Electrodeposition of Iron. The different
phases occurring during the electrocrystallization of metals have
been described elsewhere.1
2,23
The specificity of the electro-
crystallization process comes from the presence of the charged
electrode, which generates the well-known electrical double
layer. The distribution of ions around the electrode is governed
by the balance between electrostatic interactions and thermal
agitation. Counterions having a charge of opposite sign to that
of the surface are attracted, whereas co-ions with a charge of
the same sign as the surface are depleted. However, depending
on the nature of the substrate surface, interfacial water molecules
can exhibit spatial orientations significantly different from that
of bulk water.2 When a hydrophilic (polar) surface is immersed
in water, a high-density layer of water molecules around it is
promoted through an extensive network of hydrogen bonds. In
contrast, when a hydrophobic surface is used, there is a local
24
5
27
thus indicating a greater nucleation overpotential in the
presence of (NH4)2SO4. Further sweeping in the positive
direction results in a different shape of the anodic peak. In the
case of (NH4)2SO4, this peak is sharper and higher. It corre-

Electrodeposition of Iron onto a Silicon Surface
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14781
Figure 4. Water droplet deposited on the silicon surface after iron
electrodeposition.
4
Figure 2. Cyclic voltammogram with (A) 0.4 M FeSO and 0.6 M
4 2 4 4
(NH ) SO and (B) 0.6 M NH SCN.
Figure 3. Current versus time transients for iron electrodeposition onto
silicon at -1500 mV/SCE in the presence of (NH SO (thin curve)
and NH SCN (thick curve) base electrolyte.
Figure 5. FE-SEM images of iron electrodeposits obtained with (A)
4
)
2
4
0
4 4 2 4 4
.4 M FeSO and 0.6 M (NH ) SO and (B) 0.6 M NH SCN, top view.
4
observed on the electrode surface in the case of NH4SCN. These
aspects will be studied in more detail in section 3.3. After iron
electrodeposition, the measurement of a low contact angle for
a water droplet on the electrode surface (see Figure 4) reveals
the dramatic modification of the wetting behavior due to the
presence of the hydrophilic metallic phase, regardless of the
electrolytic bath used.
sponds to a greater current magnitude (1420 mC) than in the
case of NH4SCN (950 mC). Moreover, the current past the peak
is nil with (NH4)2SO4 (A), whereas this is not the case with
NH4SCN (B). These results suggest hindered and incomplete
oxidative dissolution of deposited metallic iron at the electrode
surface in the presence of NH4SCN.
All of the following experiments were performed at -1500
mV/SCE, corresponding to an overpotential η of about 670 mV.
Current versus time transients for iron electrodeposition onto
the silicon surface for both electrolytic baths are shown in Figure
3.2. Comparative Morphology of Iron Deposits. The top
view from FE-SEM measurements (see Figure 5) shows that
many iron cubes are obtained on the surface in the presence of
(NH4)2SO4 (A), whereas round-shaped deposits are present when
NH4SCN is used (B). The sizes of the larger cubes and round-
shaped deposits are around 200 and 500 nm, respectively, with
(NH4)2SO4 and NH4SCN. In the case of NH4SCN (B), the larger
3
. The curve with (NH4)2SO4 starts at a lower value of the
current density and begins to level off later than that with NH4-
SCN. Moreover, it should be noted that some bubbles were

14782 J. Phys. Chem. B, Vol. 110, No. 30, 2006
Trompette and Vergnes
Figure 6. FE-SEM image of iron electrodeposits obtained with 0.4 M
FeSO and 0.6 M (NH SO , magnification.
4
4
)
2
4
particles are located in regions of lower population density (two
large particles are observed to “coalesce”), and the size
distribution is wider. This observation has already been pointed
1
5
out, and it was ascribed to interparticle diffusional coupling
10
during the growth phase. Closer inspection shows highly
faceted cubic iron deposits (see Figure 6). The side view (Figure
7A) and profile view (Figure 7B) reveal that the electrodeposits
with NH4SCN correspond to hemispheres (domes) protruding
toward the aqueous phase, the highest corresponding to about
400 nm. As the applied overpotential is quite large, iron particles
are assumed to nucleate instantaneously and to grow through a
diffusion-controlled reaction. Under these conditions, the metal
deposits are expected to adopt the cubic shape corresponding
to the natural crystal habit of iron.28 According to Wulff’s rule,
the morphology of a crystal depends on the growth rates of the
different crystallographic faces, the slow-growing faces having
the greater influence.17 The presence of some ions or additives
can have a profound effect on the growth of a crystal and thus
modify the crystal habit.1 Some additives can suppress growth
entirely, whereas others can be adsorbed selectively onto
different crystal faces and retard their growth rates because
adsorption reduces the interfacial tension. The adsorption of
Figure 7. FE-SEM images of iron electrodeposits obtained with 0.4
M FeSO and 0.6 M NH SCN: (A) side view and (B) profile view.
4 4
7,29
adsorption of thiocyanate ions during growth of the iron metallic
phase is found to inhibit the formation of the crystal habit,
without privileging any face, but rather to generate equal growth
rates in all directions with respect to the substrate surface, thus
allowing hemispherical electrodeposits to be obtained.
thiocyanate ions on large bare and passive iron surfaces has
been reported in the literature.3
0,31
The amount adsorbed was
found to increase significantly with thiocyanate concentration
and electrode potential. It was inferred from these studies that
SCN adsorption should occur with the S-C-N axis normal
to the surface, and the standard Gibbs free energy of adsorption
was found to correspond to -1.26 and -0.7 kcal/mol for bare
and passive iron, respectively. Therefore, energy-dispersive
X-ray spectroscopy (EDS) was used to detect the eventual
presence of thiocyanate in the composition of electrodeposits.
The EDS patterns of electrodeposits on silicon substrate showed
only the presence of iron (Fe) when the base electrolyte was
To obtain more information, experiments were performed
with the same electrolyte concentration, say, 0.6 M, but with
-
2-
-
2-
varying proportion of SO4 and SCN anions such that [SO4 ]/
-
[SCN ] ranged from 0.33 to 600 in the electrolytic bath. Only
-
-3
at the lowest thiocyanate proportion ([SCN ] ) 10 M and
2
-
[SO4 ] ) 0.6 M) were cubic deposits obtained, but yet many
of these deposits exhibited a rounded shape (see Figure 9A).
As the amount of SCN increased, the formation of iron domes
-
on the surface was promoted, and cubic deposits were no longer
observed (see Figure 9B). Electrodeposition of iron was
conducted in the presence of a quite chaotropic electrolyte such
as NH4ClO4. As can be seen in Figure 10, cubic iron deposits
were obtained (the same results were found in the presence of
ammonium bromide NH4Br), thus indicating that the modulation
of the particle shape is not correlated with the ion-dependent
water structure at the interface. These results thus emphasize
(
(
NH4)2SO4 (Figure 8A), whereas the presence of iron, sulfur
S), and carbon (C) was systematically detected for various
measurements when the NH4SCN electrolyte was used (Figure
B). These results thus confirm the presence of thiocyanate ions
8
in iron electrodeposits (even after water rinsing of the elec-
trodeposits). This might justify the previous observed differences
-
-
in the comparative CV curves (Figure 2A,B) because of SCN
the significant impact of SCN adsorption on the morphology
poisoning as soon as iron becomes electrodeposited. As such,
of the electrodeposits. This might offer the opportunity to test

Electrodeposition of Iron onto a Silicon Surface
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14783
4 4 2 4 4
Figure 8. EDS pattern of iron electrodeposits obtained with (A) 0.4 M FeSO and 0.6 M (NH ) SO and (B) 0.6 M NH SCN.
the controlled effect of electrodeposit shape on catalytic
performance, for instance.
the same concentration (say, 0.6 M); the electrodeposition time
was increased (30 s) at the same applied overpotential. The
results are shown in Figure 11. Two distinct behaviors can be
observed depending on the nature of the anions: in the presence
of chaotropic anions of the ammonium salt, NH4SCN, NH4-
ClO4 and NH4Br, the current increases (in absolute value) very
steeply from the beginning and becomes nearly constant after
a few seconds, whereas with cosmotropic anions, (NH4)2SO4
and NH4CH3CO2, the increase is much less pronounced.
Moreover, it should be noted that a continuous flow of tiny
3.3. Effect of Ion Specificity during Iron Electrodeposition.
As mentioned previously, the variation of the current as a
function of time was found to exhibit different behaviors during
iron electrodeposition in the presence of (NH4)2SO4 and NH4-
SCN electrolytes (see Figure 3), and some bubbles were noticed
in the latter case. To investigate more deeply the primary instants
of electrodeposition, a low iron salt concentration was taken as
-4
5
× 10 M in the presence of varying base electrolytes having

14784 J. Phys. Chem. B, Vol. 110, No. 30, 2006
Trompette and Vergnes
Figure 10. SEM image of iron electrodeposits obtained with 0.4 M
FeSO and 0.6 M NH ClO .
4 4 4
Figure 11. Current as a function of time for iron electrodeposition
onto silicon at -1500 mV/SCE in the presence of (NH SO , NH
CH CO , NH SCN, NH ClO , and NH Br base electrolytes.
4
)
2
4
4
-
3
2
4
4
4
4
Figure 9. SEM images of iron electrodeposits obtained with (A) 0.4
-
3
M FeSO
4
+ 0.6 M (NH
4
)
SO
2
SO
4
4
+ 10 M NH
4
SCN and (B) 0.4 M
SCN.
FeSO + 0.55 M (NH
4
4
)
2
+ 0.05 M NH
4
ascending hydrogen (H2) bubbles was observed quasispon-
taneously along the electrode surface in the presence of
chaotropic electrolytes. As time proceeds, the association of
growing hydrogen bubbles and their screening of the electrode
surface resulted in the rapid appearance of a plateau region with
chaotropic salts (see Figure 11), whereas bubbling was signifi-
cantly delayed and fewer hydrogen bubbles were produced in
the presence of (NH4)2SO4 and NH4CH3CO2.
For sufficiently high overpotential, hydrogen evolution takes
+
-
place through the reaction of proton reduction (2H + 2e f
H2) on the freshly deposited iron particles at the cathode surface.
As an indication, hydrogen evolution was checked to be absent
when the experiment was performed without any iron elec-
trodeposits on the silicon surface or without any iron salt in
solution when the potential was applied. To obtain comple-
mentary information, the same experiments were carried out
using varying proportions of (NH4)2SO4 and NH4ClO4 salts in
solution, but with the same 0.6 M total concentration. The curves
are shown in Figure 12. As the amount of perchlorate anions
decreased, bubbling was delayed, and the current intensity
Figure 12. Current as a function of time for iron electrodeposition
onto silicon at -1500 mV/SCE in the presence of varying proportions
4 2 4 4 4
of (NH ) SO and NH ClO base electrolytes. The amount indicated
corresponds to the perchlorate ion concentration.
increased less strongly. When the perchlorate concentration was
lower than 0.1 M, the behavior became similar to that of
cosmotropic base electrolytes. These results thus reflect the

Electrodeposition of Iron onto a Silicon Surface
J. Phys. Chem. B, Vol. 110, No. 30, 2006 14785
should break the bonds with the molecules of solvent before
being attached to the nucleus, so that an energy barrier for
desolvation must be overcome. Accordingly, it can be expected
that dehydration of iron adatoms should be more favored when
the network of water molecules is disrupted at the electrode-
solution interface, as in the case of a high concentration of
chaotropic electrolytes. This might contribute to improving the
rate of iron electrodeposition, although it is difficult to find
evidence for this experimentally.
4
. Conclusions
In this study, the nature of base electrolytes was varied so as
to investigate the implication of ion-specific effects on renewed
electrochemical approaches for the synthesis of nanoparticles.
Two incidences have been highlighted in the process of iron
electrodeposition onto hydrophobic silicon surface:
The morphology of electrodeposits can be modulated through
the selective adsorption of some ions: the presence of thio-
cyanate anions was shown to change the shape from cubes to
hemispheres, thus allowing further comparative effects of
catalyst shape.
The ion-specific character of electrolytes affects the rate of
hydrogen evolution near the electrode surface: in the presence
of chaotropic species, faster production of hydrogen bubbles
occurs, and dehydration of adatoms is presumably enhanced.
Figure 13. Current as a function of time for comparative hydrogen
evolution reaction at -1500 mV/SCE in the presence of (NH
4 2 4
) SO
(
thin curve) and NH ClO (thick curve) base electrolytes at 0.6 M.
4
4
influence of the nature of the base electrolyte on iron elec-
trodeposition occurring concomitantly with hydrogen evolution.
To investigate the separate contribution of hydrogen evolu-
tion, the influence of the nature of the base electrolyte on the
hydrogen evolution reaction alone was investigated. For that
purpose, hydrogen evolution was compared at the same applied
potential (-1500 mV/SCE) in the presence of (NH4)2SO4 or
NH4ClO4 electrolyte (0.6 M concentration) by taking similar
silicon plates with sufficient and identical amounts of iron
already electrodeposited on the same surface area. As an
indication, the pH was 5.2 for both electrolytic solutions (i.e.,
the amounts of free protons are the same). The results reported
in Figure 13 (and also from direct observation of bubbling)
clearly show that the intensity of the hydrogen evolution reaction
was greater in the presence of NH4ClO4 electrolyte. This
indicates that the production of hydrogen gas is enhanced when
the water network at the interface is significantly disrupted as
in the presence of chaotropic electrolytes at 0.6 M. The origin
of this behavior is still unclear and merits deeper studies because
the release and the motion of hydrogen bubbles near the
electrode surface were found to generate a beneficial convective
transport of ions and to improve the narrowing of the size
distribution of various electrodeposits.10
Acknowledgment. The authors are very grateful to M. L.
de Solan-Bethmale (Laboratoire de G e´ nie Chimique) and S. Le
Blond du Plouy (Service de Microscopie Electronique, UPS
Toulouse) for SEM and FE-SEM measurements, respectively,
and to B. Fenouillet (Laboratoire de G e´ nie Chimique) for
technical assistance.
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(
(
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