Thin Solid Films 483 (2005) 50 – 59
www.elsevier.com/locate/tsf
Study of the electrodeposition of rhenium thin films by electrochemical
quartz microbalance and X-ray photoelectron spectroscopy
R. Schreblera,T, P. Curya, C. Suáreza, E. Muñoza, F. Veraa, R. Córdovaa, H. Gómeza,
J.R. Ramos-Barradob, D. Leinenb, E.A. Dalchielec
b
a
Instituto de Quı́mica, Facultad de Ciencias, Universidad Católica de Valparaı́so, Casilla 4059, Valparaı́so, Chile
Laboratorio de Materiales y Superficie, Unidad asociada al CSIC, Departamento de Fı́sica Aplicada and Departamento de Ingenierı́a Quı́mica,
Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, España
c
Instituto de Fı́sica, Facultad de Ingenierı́a, Herrera y Reissig 565, C.C. 30, 11000 Montevideo, Uruguay
Received 10 December 2003; accepted in revised form 9 December 2004
Available online 10 March 2005
Abstract
Rhenium thin films were prepared by electrodeposition from an aqueous solution containing 0.1 M Na2SO4+H2SO4, pH 2 in presence of
y mM HReO4. As substrates polycrystalline gold ( y=0.75 mM HReO4) and monocrystalline n-Si(100) ( y=40 mM HReO4) were used. The
electrochemical growth of rhenium was studied by cyclic voltammetry and electrochemical quartz microbalance on gold electrodes. The
results found in the potential region before the hydrogen evolution reaction (her) showed that ReO3, ReO2 and Re2O3 with different hydration
grades can be formed. In the potential region where the her is occurring, either on gold or n-Si(100) the electrodeposition of metallic rhenium
takes place. On both substrates, rhenium films were formed by electrolysis at constant potential and X-ray photoelectron spectroscopy
technique was used to characterise these deposits. It was concluded that the electrodeposited films were of metallic rhenium and only the
uppermost atomic layer contained rhenium oxide species.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Rhenium; Electrodeposition; X-ray photoelectron spectroscopy; Electrochemical quartz microbalance
1. Introduction
In the last time, the electrodeposition of rhenium species
from acid solutions of perrhenate on different electrodic
substrates is being studied with relative interest because
both oxides or metallic rhenium present an important
catalytic activity in bimetallic catalyst systems [1,2]. Also,
electrocatalytic activities for reduction reactions such as
hydrogen evolution [3,4], carbon dioxide [5,6], perchlorate
[7] and nitrate [8] ions have been observed. The effect of
rhenium species in the electrooxidation of methanol [9,10]
and formic acid [11] on platinum has also been reported.
Besides the rhenium applications mentioned above, there is
T Corresponding author. Fax: +56 32 273422.
E-mail address: [email protected] (R. Schrebler).
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2004.12.061
a great interest on rhenium–semiconductor interfaces to be
used in the silicon microelectronics: in field emitters,
barriers, high resistant layers, stabilisation of porous silicon
and in thermoelectric applications [12–14]. Electrochemical
metal deposition on semiconductors is a single and
promising technique for the production of high quality
ohmic contacts, Schottky barriers, and gas sensors. Furthermore, in the case of rhenium, in spite of its high melting
point (3190 8C), rhenium deposits can be obtained by
electrochemical methods at room temperature.
In relation to the electrode processes of rhenium species,
in a previous communication [11], it was found that when
the electroreduction of ReO4 ion takes place onto a
platinum electrode in sulphuric acid media, the species
being deposited was ReO2. These results have been
confirmed by Szabo et al. [15]. They suggested that in 0.5
M H2SO4 media in the potential range prior to the hydrogen
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
evolution process on Pt, the species initially adsorbed was
ReO2, which evolves towards a bulk phase of ReO2d xH2O
and ReO3. In more concentrated H2SO4 solutions (10 M),
the ReO4 ion is exclusively reduced to Re2O5. Other
authors, on platinum electrodes, and based in ellipsometric
studies, concluded that a composite layer has been formed,
with a volumetric fraction of about 30% of metallic rhenium
and hydrogen occlusion [16].
Our experience on the electroreduction of ReO4 ion in
slightly acid solutions (pH 2) on Au, C, Ti and Cu
electrodes [17], has indicated that two processes (rhenium
electrodeposition and her) are simultaneously observed at
potentials more negative than 0.750 V vs. saturated
calomel electrode (SCE). In this region, metallic rhenium
is thermodynamically stable. When electrochemical quartz
microbalance experiments were carried out, the process
occurring on the electrode surface at more positive
potentials was attributed to the adsorption of ReO4 ion.
However, in this case the studies were focused into the
potential region where metallic rhenium electrodeposition
takes place. The potential region prior to this process was
not studied in detail.
On the other hand, when nucleation and growth
mechanism studies of this metal on polycrystalline gold
electrode were carried out [18], the energy-dispersive X-ray
spectrum analysis showed the presence of rhenium.
Unfortunately, X-ray diffraction peaks corresponding to
metallic rhenium were not observed, and it was concluded
that the electrodeposited films were in an amorphous or
nanocrystalline phase.
It is important to mention that in our previous works
[17,18], on rhenium electrodeposition two aspects have not
been considered. One of them corresponds to the process
taking place prior to metallic rhenium formation, while the
other is related to the characterisation of the metallic
rhenium obtained in the potential region where this process
takes place simultaneously with the her.
For these reasons, the aim of this work was first to
confirm by X-ray photoelectron spectroscopy (XPS) technique that rhenium electrodeposits obtained on polycrystalline gold or monocrystalline n-Si(100) surfaces in slightly
acid solution (pH 2) correspond to a metallic or oxide phase,
when these films were formed in the potential region where
the her is also occurring. Additionally, in this same media,
on polycrystalline gold and using an electrochemical quartz
crystal microbalance (EQCM), the potential region previous
to the metallic rhenium deposition was also studied with the
purpose to establish that in this potential region, rhenium
oxides are participating as intermediates in the reduction of
perrhenate anion.
2. Experimental details
All electrochemical experiments were carried out at 25
8C in a conventional three electrodes cell system. In the
51
cyclic voltammetry and chronoamperometric measurements
with simultaneous nanogravimetric experiments, the working electrode was a polycrystalline gold thin film deposited
onto quartz resonator (QC-10-AuB, ELCHEMA) with a
geometric area of 0.23 cm2. The AT-cut quartz crystal
resonating at a fundamental frequency of 10 MHz was
employed in this study. The particular crystal had a mass
sensitivity of 5.8 ng cm2 Hz1. This parameter was
obtained employing two reactions whose faradaic efficiencies are practically 100%. These reactions correspond to
copper and silver electrodeposition. In the Cu case, the
frequency changes (Df) and the currents of the electrodeposition process were obtained directly from the equipment by the step potential method. From the currents were
calculated the cathodic charges corresponding to each step
potential. The mass were calculated from these charges
(Dm CAL), which were in agreement with those obtained
directly by the EQCM (Dm EQCM). Both mass changes were
associated with the frequency changes, establishing that
follow the Sauerbrey law [19],
Dm ¼ kDF
where k corresponds to the slope of the mass changes as
function of the frequency changes and in this work was
assigned as the EQCM sensitivity. Table 1 shows the
frequency changes and the mass changes obtained both
directly from the EQCM and those calculated from the I–t
transients.
On the other hand, the area of the electrode was
determined by the voltammetric charge either of the
formation or reduction of the gold oxide in sulphuric
acid. Before each experiment the gold electrode was
cleaned with a freshly prepared H2SO4–HNO3 (1:1)
mixture for 2 min. Then this crystal face was extensively
rinsed with deionized water (18.3 MV cm1) and finally
dried with argon. For the rhenium thin film electrodeposition, two types of substrates have been used: (a)
an electrodeposited polycrystalline gold thin film onto an
alpaca substrate (2 cm2) (from now on: balpaca/AuQ) and
(b) (100) monocrystalline n-type silicon (1.0–5.5 V cm Pdoped, Int. Wafer Service, CA, USA); n-type silicon has
Table 1
Frequency changes and mass changes obtained both directly from the
EQCM and from the I–t transients by the step potential method in 0.01 M
CuSO4+0.5 M H2SO4
E (V)
Df (Hz)
Dm EQCM (ng)
Dm calc (ng)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
245
368
437
653
718
825
951
1120
1306
342.6
501.7
598.7
886.9
973.6
1116
1284
1510
1778
358
547
615
935
994
1144
1327
1536
1808
52
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
been chosen because the n-type character gives a nonblocking electrode for cathodic reactions. The silicon wafer
was cut into 11 cm2 squares that were first degreased in
boiling isopropanol, then rinsed with a 0.5:1:4 HCl/H2O2/
H2O mixture heated up to 80 8C, in order to remove any
trace of heavy metals. Afterwards, the oxide film of the
polished face was removed by etching with a 2 M NH4F
acid (pH 4.5) solution and thoroughly rinsed with ultra
pure water. The ohmic contact was made through the
application of a Ga–In eutectic in the non-polished face of
the samples that were mounted in a copper support. The
edges of the silicon squares and the copper were isolated
with an adhesive Teflon tape leaving a defined exposed
area facing the solution. Before the electrochemical
experiments the electrode surface was again etched for
15 min with the 2 M NH4F solution (after this procedure
an atomically smooth and hydrogen terminated surface has
been obtained).
A SCE, connected to the cell by a Luggin capillary, was
used as a reference electrode. All the potential values in our
study are referred to this electrode. A platinum coil,
separated from the working electrode compartment by a
fine glass frit, was used as a counter electrode.
The electrolyte was an aqueous solution of 0.1 M
Na2SO4+H2SO4 (pH=2), containing either 0.75 mM HReO4
for gold electrode or 40 mM NH4ReO4 for silicon. Argon
was flushed through the cell and the electrolyte prior to the
experiments and an argon flow was maintained over the
solution during the measurements.
For the electrochemical growth of the rhenium thin films,
electrolysis with constant potential programs were applied.
In the case of electrochemical growth onto alpaca/Au
substrates, a constant potential of 0.750 V vs. SCE has
been imposed for ca. 10 h. For the electrodeposition of
rhenium onto n-Si(100), a potential step from 0.200 V for
5 min to 1.200 V for 4 h was applied.
The voltammetry and chronoamperometric experiments
were performed with a PINE (Pine Instrument Company,
Model RDE4) potentiostat system and the mass measurements were carried out in an electrochemical quartz microbalance ELCHEMA (system EQCN-501).
XPS studies were carried out with a PHI 5700 equipment. Survey and multiregion spectra were recorded at 458
take-off-angle with a concentric hemispherical energy
electron analyzer operating in the constant pass energy
mode at 187.85 eV and 29.35 eV, respectively, using a 720
Am diameter analysis area. The spectrometer energy scale
was calibrated with Cu 2p3/2, Ag 3d5/2 and Au 4f7/2
photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. A standard X-ray source 15 kV, 300 W, Mg Ka
(1253.6 eV) was used. The pressure in the chamber was
about 107 Pa. XPS depth profiling was carried out by 4
keV Ar+ sputtering. The sputter rate is assumed to be
approximately 1 nm/min as determined for Ta2O5 under
identical sputter conditions. Binding energies of unsputtered surfaces were referenced to the C 1s peak at 284.8
eV [20]. Spectra were handled and fitted by PHI-Access
V.6 and Multipak software, both from Physical Electronics
[21]. The atomic concentrations were determined from C
1s, O 1s, Zn 2p, In 3p5/2 and Re 4f XPS peak areas using
Shirley background subtraction [22] and sensitivity factors
provided by the spectrometer manufacturer (PHI) [21]. The
techniques (X-ray and XPS) used for characterisations are
ex situ and cannot give the real state of the rhenium
electrodeposited.
3. Results and discussion
3.1. Electrochemical study
Fig. 1 shows the j–E and Dm–E profiles corresponding
to the first cathodic scan of a gold electrode in 0.1 M
Na2SO4+H2SO4 (pH=2) in the absence (curve 1) and in the
presence (curve 2) of 0.75 mM HReO4. The voltammetry
was performed in the potential range from 0.800 V to
1.000 V and the potential sweep was initiated at 0.800 V
towards the negative direction at 0.050 V s1. In the
absence of Re(VII) species, the j–E profile shows only the
hydrogen evolution reaction, which begins at ca. 0.22 V.
With the addition of Re(VII) species, the voltammetric
response reveals the presence of a cathodic current peak at
0.750 V. This signal has been associated, on a gold
Fig. 1. Cathodic potentiodynamic profile j–E (a) and Dm–E (b) for rhenium
electrodeposition on a gold electrode obtained at 0.05 V s1. Curve (1) 0.1 M
Na2SO4+H2SO4, pH=2. Curve (2) 0.75 mM HReO4+0.1 M Na2SO4+H2SO4,
pH=2.
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
electrode, with both rhenium electrodeposition and the
hydrogen evolution reactions [17,18].
Simultaneously, the Dm–E profile (Fig. 1b, curve 1)
shows that in the potential region between 0.800 V and
0.400 V, mass decreases. Afterwards the Dm remains
constant until 0.2 V. At more negative potentials a slight
mass increase is observed. This mass increase begins
approximately at the same potential where the her is
initiated. However, in the presence of perrhenate, a
significant change in the Dm–E profile can be observed.
Initially, a mass increase until 0.7 V is observed, followed
by a decrease, after which the mass remains approximately
constant until 0.69 V. These facts could be indicating
that in this potential region, the ReO4 species remains
adsorbed onto the electrode surface. Another possibility
could be that the reduction of this species with oxide
formation is taking place in this region. Both hypotheses
could explain the shift of the hydrogen evolution reaction
from 0.30 V to 0.500 V on gold, which is in agreement
with Jusys et al. [23].
At more negative potentials than 0.69 V, an abrupt
mass increase is observed. However, this mass change
(Dm=206 ng) is not in agreement with the mass change
calculated from the voltammetric charge of the cathodic
peak (5.89 mC or 1622 ng), where the faradaic efficiency
for rhenium electrodeposition corresponds to 12.7%. In a
previous work [18], it was demonstrated that the mass
change obtained from the cathodic Dm–E profile is in
agreement with the mass calculated from the Re to ReO4
stripping process. These results allowed us to conclude that
the mass measurements are not affected by the presence of
adsorbed hydrogen. This could be a consequence of the high
Re/H mass ratio (186:1), and also that the adsorbed
hydrogen quickly evolves to molecular hydrogen that leaves
the electrode interface. Indeed, in Fig. 1 (curve 1), a great
faradaic current and a slight mass increase associated to her
on gold electrode were observed.
As previously indicated, in rhenium electrodeposition
there are two aspects which have not been considered. One
of them has to do with the processes taking place prior to
metallic rhenium formation. These processes probably
occur in the potential range indicated in the rectangle of
Fig. 1a. The other aspect is related to the characterisation
of metallic rhenium obtained in the potential region where
the cathodic peak is developed. Both aspects will now be
discussed.
Fig. 2 shows the j–E and Dm–E profiles corresponding
to the cathodic first scan in the potential region from 0.8 V
to 0.4 V at a higher sensibility. These profiles were
obtained under the same conditions indicated in Fig. 1. It
can be seen that in curve 1 (Fig. 2a), there is a small
cathodic peak located at 0.42 V, probably associated with
the reduction of species containing oxygen and formed at
the initial potential. Afterwards, the current remains
approximately constant until 0.37 V, then the her on
gold takes place. When rhenium is present in the electro-
53
Fig. 2. Potentiodynamic profile j–E (a) and Dm–E (b) for rhenium
electrodeposition on a gold electrode corresponding to an expansion of the
rectangle shown in Fig. 1. Curve (1) 0.1 M Na2SO4+H2SO4, pH=2. Curve
(2) 0.75 mM HReO4+0.1 M Na2SO4+H2SO4, pH=2.
lyte solution, the initial cathodic current decreases and a
set of current plateaux are observed. This could be
associated with rhenium oxide formation, as has been
proposed by another author [7]. On the other hand, the
Dm–E profiles obtained simultaneously with the voltammetric scans show more drastic changes (Fig. 2b). On
gold, after a small increase, the mass decreases until it
reaches a constant value. These mass changes had been
principally attributed to the adsorption and desorption
processes of sulphate and/or bisulphate anions on the
electrode surface [24]. When the perrhenate ion is present
in the electrolyte solution, the mass changes are more
attenuated, which could indicate that the perrhenate ion
remains adsorbed onto the electrode surface, as was
initially proposed [18]. However, the Dm–Q plot (Fig. 3)
shows that this is only true in the first potential region,
from 0.8 V to 0.4 V. In this region, an initial increase of
both Dm and Q until 0.58 V can be observed, which can
be attributed to the adsorption of perrhenate and sulphate/
bisulphate anions. For 0.58zEz0.4 V the mass decreases
until it reaches a minimum level because sulphate anion
desorption begins, but the ReO4 ion remains adsorbed
onto the electrode substrate. From Eb0.4 V both mass and
charge begin to increase and four linear regions can be
observed. The slope values obtained in each of these
regions are summarised in Table 2. This indicates that the
reactions occurring on the electrode surface correspond to
54
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
B
ReO4- .H20
A
ads
ReO3 n H2O
ReO2 n H2O
ads
C
ReO 3 (n-x) H2O
ads
+ ReO
4 (aq)
D
- 6 H 2O
ads
Re2O3 n H2O
ads
ads
ReO
4
Scheme 1. Reaction formalism for the reduction of adsorbed
and the
reduction of species formed onto electrode surface. A, B, C and D
correspond to different region described in Fig. 3.
Fig. 3. Dm–Q relation obtained from Fig. 2.
the reduction of adsorbed ReO4 ion or to the reduction of
species formed from this anion. A reaction formalism that
takes into account these slopes is shown in Scheme 1. In
this scheme the rhenium oxide species that can be formed
according to the E–pH diagram shown in Fig. 4 (obtained
from reference [25]) are postulated.
n corresponds to the number of water molecules of each
rhenium oxide species formed on the electrode surface. These
values were calculated from the slopes of the Dm–Q
relationships deduced from the most probable reaction taking
place in each potential region. This deduction was made
considering the Faraday law, as proposed in a previous work
[18], and is summarised in Table 2. Thus, the species that
would form on the surface correspond to ReO3d 1.96H2O
(region A), ReO2d 4.96H2O (region B) and Re2O3d 2.97H2O
(region D). For region C, the reaction that occurs would
correspond to the reduction of ReO3 to ReO2. In this case, it
was calculated that the previously formed ReO3 lost 0.4 water
molecules. For this reason, the reaction should correspond to
the reduction of ReO3d 1.56H2O to ReO2d 4.96H2O. More-
over, in the last region (D) it was considered that the
Re2O3d 2.97H2O species is formed from adsorbed ReO2 and
ReO4 ion. This last anion comes from the bulk solution and
removes six water molecules that were previously adsorbed
onto the electrode surface [23].
There is no doubt that the n values determined by
voltammetric or step potential techniques with simultaneous
mass measurements can be less precise than other techniques, because both Dm and the electrical charge involved are
also relatively low. Indeed, the charges involved in this
potential region are less than a monolayer [11–15]. However,
in most of the postulated oxide the hydration grade seems
reasonable for these species. In the particular case of ReO2,
our results indicated that the adsorbed oxide on the electrode
surface corresponds to ReO2d 5H2O. For this oxide the
hydration grade is higher than those reported by Mazzochin
et al. [26], who proposed values close to one or two water
molecules. This situation can be explained by considering
that our measurements detect the initial formation of this
oxide, which can contain a hydration grade greater than that
observed by these authors, who used ex situ techniques (Karl
Fischer titrimetry).
It is important to notice that after the formation of these
oxides, a slight increase in the pH in the interfacial region can
be attained. For this reason, under the experimental conditions used in this work, it was not possible to form the Re3+
ion, as has been postulated for more acidic media by other
Table 2
n values determined from the slopes obtained from Fig. 3, and their corresponding equations deduced from the reactions represented in Scheme 1
Region
A
BDm
1
2
ng mC cm
BQ
13.5
Slope relationship
n
BDm
106 ¼
MM ReO3 þ nMM H2 O MM ReO4 d H2 O
BQ
F
BDm
106 ¼
MM ReO2 þ nMM H2 O MM ReO4 d H2 O
BQ
3F
1.96
B
133
C
257
BDm
106
¼
½MM ReO2 þ ð3 þ xÞMM H2 O MM ReO3 BQ
2F
1.56
117
BDm
10
¼
½MM Re2 O3 þ ðn 11ÞMM H2 O MM ReO2 BQ
5F
2.97
4.96
6
D
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
55
is equal to Dm obtained in the absence of perrhenate.
Likewise, the total electric charge Q T obtained in the
presence of Re(VII) can be expressed as:
QT ¼ QS þ QRe
ð2Þ
where Q S and Q Re can be determined from the subtraction
of the integration of the j–t transient obtained in the absence
and presence of rhenium species, respectively. Fig. 6 shows
the corresponding Dm Re–Q Re plots obtained at both
potentials. At 0.2 V and 0.3 V one and two linear regions,
respectively, are clearly observed, which can be explained
according to the following general equation:
x tReO
H2 Ob ads þ 2ð 4x yÞHþ þ ð 7x 2yÞe
4
Y Rex Oy nH2 O ads þ ð 5x y nÞH2 O:
Fig. 4. E–pH diagram of rhenium species obtained from equations of
reference [25].
authors [15,24]. Consequently, Re2O3 must be the species
that is formed, prior to the hydrogen evolution reaction.
In order to verify these results, the potential step
method was employed. Fig. 5 shows the j–t and Dm–t
transients obtained at two different potential steps starting
from 0.8 V. It can be seen that in the presence of Re(VII)
species either at 0.2 V or 0.3 V, the currents are always
higher than in the absence of this species. At the same
time, the mass changes show that in the absence of
Re(VII) for both potentials, the mass decreases until it
reaches a constant value. This has been attributed to
sulphate species desorption. In the presence of Re(VII),
after an initial mass fall, a minimum is reached after which
the mass begins to increase.
In the voltammetric measurements the reduction of the
adsorbed perrhenate ions takes place in a potential region
where the sulphate ions have been practically desorbed.
However, in the potential step method, it is necessary to
consider that at the final potential (0.2 or 0.3 V) and at
t=0, both species (SO42/HSO4 and ReO4 anions) are
adsorbed. For tN0, the desorption of sulphate species and
the reduction of adsorbed perrhenate ions begin simultaneously. For this reason, the Dm measurements in the
presence of Re(VII) correspond to the total mass change,
Dm T, which can be expressed as:
DmT ¼ DmS þ DmRe
ð3Þ
Table 3 summarises the possible reactions that justify the
slope values obtained for both potentials. It can be
appreciated that at 0.2 V two rhenium species can be
formed according to the general reaction (3). In both cases
the slope value is justified and these species would
correspond to ReO3d 2.46H2O or ReO2d 4.84H2O. The first
slope found at 0.3 V can only be explained by the
formation of ReO3d 2.05H2O according to Eq. (3). On the
other hand, for the second slope two possible reactions
leading to the formation of ReO2d nH2O can take place. One
of them corresponds to the reaction mentioned above where
an n value of 3.25 is obtained. The other reaction
corresponds to the reduction of the previously formed
ð1Þ
where Dm S and Dm Re correspond to the mass changes due
to sulphate species desorption and to the reduction of
perrhenate adsorbed, respectively. An approximation to
determine Dm Re from Dm TDm S is to consider that Dm S
Fig. 5. j–t and Dm–t transients obtained on a gold electrode. (a, aV) E=0.2 V
and (b, bV) E=0.3 V. Curve (1) 0.1 M Na2SO4+H2SO4, pH=2. Curve (2)
0.75 mM HReO4+0.1 M Na2SO4+H2SO4, pH=2.
56
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
Fig. 6. Dm Re–Q Re relation obtained from Fig. 5.
ReO3d 2.05H2O to ReO2d 3.55H2O. These results, with
some differences, are in agreement with the voltammetric
measurements. However, the formation of Re2O3 was not
found using the potential step method, but it could be
attained at slightly more negative potentials.
The results found by voltammetric or potential step
methods with simultaneous mass measurements suggest that
prior to the hydrogen evolution reaction, rhenium species
might be present on the electrode surface. This indicates that
the reduction processes of perrhenate ion can take place
independent of the presence of hydrogen adatoms, at least
until the formation of ReO2 or Re2O3 species. Then, at
potential values more negative than 0.4 V, where the her
begins to take place, two hypothetical situations could
explain the metallic rhenium deposition. One possibility is
that the reduction of perrhenate ion to metallic rhenium
occurs in a parallel way to the her. In this case, the
successive intermediate oxides that are formed on the
electrode surface are electrochemically reduced or undergo
a chemical reaction of disproportionation until the metallic
state is attained. The second hypothesis is that hydrogen
adatoms might participate in the reduction of rhenium
oxides, as has been proposed by other authors [15].
Unfortunately, under the actual experimental conditions, it
is not possible to distinguish between the two situations.
Nevertheless, the evidence shows that in this potential
region metallic rhenium is formed as was reported in a
previous work [18] and as will be demonstrated by XPS
measurements.
3.2. XPS study
XPS studies were done in order to determine the
oxidation state of the rhenium species electrodeposited onto
the alpaca/Au and silicon substrates. The chemical composition of the film surface and, in combination with 4 keV Ar+
bombardment, of the subsurface of the electrodeposited film
has been studied by XPS. XPS survey spectra revealed
besides photoelectron peaks of Re also peaks of In, Zn, O
and C. Carbon is mainly due to adventitious surface
contamination albeit about 10% of the C 1s signal
corresponds to carbonate species which have been found
only at the film surface. Zinc and indium impurities in the
rhenium salt precursor can explain their presence in the
Table 3
n values determined from the slopes obtained from Fig. 6, and their corresponding equations deduced from the general reaction (3)
E (V)
BDmRe
(ng mC1 cm2)
BQRe
Equation
0.2
129
BDm
106 ¼
MM ReO3 þ nMM H2 O MM ReO4 d H2 O
BQ
F
BDm
106 ¼
MM ReO2 þ nMM H2 O MM ReO4 d H2 O
BQ
3F
X
n
3
2.46
2
4.84
30.0
BDm
10 ¼
MM ReO3 þ nMM H2 O MM ReO4 d H2 O
BQ
F
3
2.05
58.0
BDm
10 ¼
MM ReO2 þ nMM H2 O MM ReO4 d H2 O
BQ
3F
2
3.25
BDm
10
¼
½MM ReO2 þ nMM H2 O MM ReO3 d 2H2 O BQ
2F
2T
3.55
6
0.3
6
6
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
Fig. 7. XPS Re 4f spectra of a rhenium thin film electrodeposited onto an
alpaca/Au substrate from a 0.75 mM NH4ReO4+0.1 M Na2SO4+H2SO4,
(pH=2) bath, and applying a potential pulse 0.2 V, 5 min, 0.75 V vs.
SCE, 4 h; before (original) (a), after 1 min (b), and 6 min (c) of 4 keV
Ar+ sputtering. The fitting curves are also depicted. Re 4f doublets
according to the different contributions (assignment to Re 4f7/2): Re metal
(band A), ReO2 (band B), ReO3 (band C); on metal (subindex M), on
oxide (subindex O).
films. XPS high resolution multiregion spectra of the most
intense photoelectron lines showed that In and Zn are
present according to its binding energies as In2O3 and ZnO
(In 3d5/2 measured at 445.2 eV, Zn 2p3/2 at 1022.5 eV, C 1s
at 284.8 eV) [20].
Fig. 7 shows XPS results of a rhenium thin film
electrodeposited onto an alpaca/Au substrate, by means of
the potential pulse program as indicated in the experimental
section.
Rhenium was found at the film surface as ReO2 and
ReO3 as can be concluded from Fig. 7a which shows the Re
4f spectrum of the as-deposited film surface (original) and
the curve fitting due to ReO2 and ReO3. These species are
present on both conducting substrates, i.e. metallic rhenium
57
as deduced below, and on In2O3 and ZnO particles. For each
of the four contributions to the measured Re 4f signal, a 4f
doublet was considered for the curve fitting with Re 4f5/2 to
Re 4f7/2 separation of 2.43 eV [20] and area ratio of 0.75 as
fixed parameters. The fit reveals independently for each Re
4f doublet of ReO2 and ReO3 species a shift of about 1.3 eV
to higher binding energies when adhered to the oxide
particles (In2O3 and ZnO). This fact serves as argument of
confidence for the fit taking into account that eight fitting
bands have been involved although linked by the two
parameters as mentioned above. The displacement of 1.3 eV
to higher binding energies is due to differential charging
[27], in that case, for the Re species on the oxide particles
compared to Re species on conducting substrate, i.e. Re 4f7/
2 bands BO and CO compared to Re 4f7/2 bands BM and CM
in Fig. 7. The binding energies of Re 4f7/2 BM and CM bands
of 42.2 eV and 45.8 eV are in the range of previously
published values for ReO2 and ReO3 [20]. The fit indicates
that about 30% of rhenium is found as ReO3 both on the
metallic substrate as well as on the oxide particles.
However, when sputtering the film surface with 4 keV
Ar+ for only 1 min, the Re 4f spectrum shows the Re 4f7/2
peak at 40.4 eV (band AM) characteristic of metallic
rhenium [20] (see Fig. 7b). Again, two doublets are used
for fitting the spectrum corresponding now to metallic Re on
conducting substrate and on oxide particles. In this case, a
shift of 1.2 eV due to differential charging is obtained by the
fit, a value slightly lower than that obtained for the
unsputtered surface. This is not surprising as the conduction
behavior between oxide particles and conductive substrate
may change through ion bombardment. The fit curve (see
Fig. 7b) shows only a weak deviation from the measured
spectrum of around 46 eV which may be the result of a
small amount of oxidized Re species at the film surface not
been reached by the ion beam during sputtering due to
shadowing effects. One has to take into account that
electrodeposited films do not have a totally flat surface
and that the sample was not rotated during sputtering.
Finally, after 6 min of 4 keV Ar+ only metallic rhenium is
left, making coincidence over the whole energy range the fit
curve with the measured Re 4f spectrum (Fig. 7c).
In Table 4 atomic ratios relative to the amount of Re are
given for the film surface before and after 1 and 6 min of 4
keV Ar+ sputtering. It can be seen that carbon contamination
has been considerably reduced during sputtering, i.e. by a
factor of 10 by 1 min of 4 keV Ar+. Besides, no contribution
in the C 1s signal due to carbonates (~289 eV) could no
Table 4
Atomic ratios estimated from In 3p5/2, Zn 2p, C 1s and Re 4f XPS peak
areas
Atomic ratio
In/Re
Zn/Re
C/Re
Original
1 min 4 keV Ar+
6 min 4 keV Ar+
1.05
0.72
0.26
0.15
0.15
0.04
4.6
0.5
0.2
58
R. Schrebler et al. / Thin Solid Films 483 (2005) 50–59
longer be discerned. Consequently, carbon is due to surface
contamination of the rhenium film.
The fact that after 1 min of 4 keV Ar+ practically only
metallic rhenium is detected at the film surface could be a
result of ion bombardment induced reduction, which always
occurs (to less or more extent) in oxides subjected to ion
bombardment [28–30]. In this case, it is not the right
conclusion as we can see by comparing thermodynamic data
as, for instance, the standard enthalpy of formation DH f with
the observed behaviour of the different oxides present at the
film surface when subjected to 4 keV Ar+ bombardment. In
that comparison DH f is used as a parameter indicating the
chemical stability of the oxide subjected to ion bombardment. We can state that: on the one hand, while rhenium is
found as metal, In2O3 and ZnO species have not suffered
any reduction after 1 min of 4 keV Ar+ sputtering, i.e. they
stay as oxides; on the other hand, DH f values of ReO2,
ReO3, In2O3 and ZnO with respect to the corresponding
metal (ReO2: 445 kJ/mol; ReO3: 582 kJ/mol; In2O3:
923 kJ/mol; ZnO: 356 kJ/mol) [31] are quite similar.
Thus, Ar ion bombardment induced reduction should be of
similar importance for the different oxides. Since not
observed for In2O3 and ZnO, we can disregard ion
bombardment induced reduction as the main reason for
the findings of metallic rhenium after 1 min of 4 keV Ar+
and therefore conclude that the rhenium oxide species have
been simply sputtered away leaving exposed to the surface
the metallic rhenium film on the surface.
Another interesting point is the evolution with depth
(equivalent to sputter time) of the amount of indium and zinc
oxides in the film (see Table 3). It diminishes to about a
quarter by 6 min of 4 keV Ar+ sputtering showing that those
oxide particles are restricted to a certain surface depth of the
rhenium film. This decrease can also be seen in relation to the
amount of Re as oxide or metal found on those oxide species
compared to the amount found on the metallic film as has
been deduced by the curve fittings represented in Fig. 7. The
latter reveals that for the unsputtered surface 25% of rhenium
oxides (ReO2 and ReO3) is found on the In2O3 and ZnO
particles, while 23% of metallic Re after 1 min of 4 keV Ar+
and 13% of metallic Re is found after 6 min of 4 keVAr+. It is
possible that for 6 min of 4 keV Ar+ some ion bombardment
induced mixing [32] also occurs, which is why we do not
observe the same decrease in Re on In2O3 and ZnO particles
than in the amount of indium and zinc oxides.
The XPS results carried out in rhenium thin films
electrodeposited onto n-Si(100) surface are similar to those
showed above for the rhenium grown onto the alpaca/Au
surface.
4. Conclusions
The electroreduction of ReO4 ions involves the participation of different rhenium oxides prior to the metallic
rhenium electrodeposition. Electrochemical and XPS meas-
urements allow us to suggest that these oxides correspond
principally to ReO2 and ReO3, with different hydration
grades. These oxides were always found on the surface of
the metallic rhenium either electrodeposited on Au or Si
substrate.
On the other hand, the fact that only 1 min of 4 keV Ar+
sputtering has been sufficient in order to eliminate practically
all rhenium oxide let us to conclude that the electrodeposited
film is of metallic rhenium and only the uppermost atomic
layer of the film contains rhenium oxide species.
Acknowledgements
The authors are thankful to FONDECYT-Chile for
financial support of this work (complementary line project
no. 8000022) and are also grateful to DGI-Universidad
Católica de Valparaı́so, Chile. E.A. Dalchiele thanks
CSIC—Universidad de la República and PEDECIBA-Fı́sica
(Uruguay) for financial support. C. Suárez and E. Muñoz
specially thank CONICYT for their Doctoral Scholarships.
J.R. R-B and D.L. are grateful to the European Union and
CICYT (Spain) (grant MAT2000-1505) and the Junta de
Andalucia through the research group FQM-192.
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