Electrochemical behavior of thin polycrystalline rhodium layers studied by cyclic voltammetry and quartz crystal microbalance

Article abstract of DOI:10.1016/j.electacta.2006.12.066

Electrochemical behavior of thin polycrystalline Rh layers has been studied in 0.5 M H₂SO₄ solution by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The properties of surface oxide formed on freshly electrodeposited Rh are different than on electrochemically aged Rh. The analysis of frequency changes in both hydrogen and oxygen regions is presented. It is suggested that hydrogen desorption occurs simultaneously with the adsorption of HSO₄^- ions, whose maximum surface coverage reaches ca. 8%. The EQCM results indicate that RhO is the main species formed during Rh surface oxidation. Metal dissolution proceeds during electrode cycling to sufficiently high potentials. The amount of dissolved metal increases with an increase in potential and a decrease in scan rate.

Full text of DOI:10.1016/j.electacta.2006.12.066

Electrochimica Acta 52 (2007) 4560–4565
Electrochemical behavior of thin polycrystalline rhodium layers
studied by cyclic voltammetry and quartz crystal microbalance
a,∗,1
M. Łukaszewskia, H. Siweka,b, A. Czerwinski
´
a Warsaw University, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
b Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland
Received 2 October 2006; received in revised form 24 November 2006; accepted 23 December 2006
Available online 21 January 2007
Abstract
Electrochemical behavior of thin polycrystalline Rh layers has been studied in 0.5 M H2SO4 solution by cyclic voltammetry (CV) and the
electrochemical quartz crystal microbalance (EQCM). The properties of surface oxide formed on freshly electrodeposited Rh are different than on
electrochemically aged Rh. The analysis of frequency changes i−n both hydrogen and oxygen regions is presented. It is suggested that hydrogen
desorption occurs simultaneously with the adsorption of HSO4 ions, whose maximum surface coverage reaches ca. 8%. The EQCM results
indicate that RhO is the main species formed during Rh surface oxidation. Metal dissolution proceeds during electrode cycling to sufficiently high
potentials. The amount of dissolved metal increases with an increase in potential and a decrease in scan rate.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Rhodium; Anion adsorption; Surface oxide; Electrochemical dissolution; Cyclic voltammetry; Electrochemical quartz crystal microbalance
1. Introduction
potentials RhOH species are formed, which are converted with
the potential increase into Rh(OH)3, whose thickness does not
exceed three monolayers. At high potentials a layer of RhO(OH)
isgeneratedonthetopofRh(OH)3 species. However, thispicture
is not shared by all authors and other schemes of Rh oxidation
are also proposed in the literature [1,4,9–12,16], involving the
existence of such species as Rh2O, RhO, Rh2O3 and RhO(OH)3.
It is known that during polarization at sufficiently high poten-
tials the electrochemical dissolution of noble metal electrodes
takes place [1,18,32,33]. Rand and Woods [32] using cyclic
voltammetry together with analytical techniques found the rela-
tionship between the amount of dissolved noble metals and the
upper potential limit of electrode polarization in acidic solutions
(1 M H2SO4). Juodkazis et al. [18] used the EQCM method for
investigations of Rh dissolution in 0.5 M H2SO4. They reported
that during potential cycling through the oxygen region the
electrode mass decreased with each cycle indicating metal dis-
solution.
The electrochemical properties of Rh have been widely
investigated [1–33], mostly using cyclic voltammetry and
chronoamperometry, also combined with an additional non-
electrochemical technique, such as XPS, radiotracer method
or IR spectroscopy. Recently, several reports [16–19,25] have
appeared on the use of the electrochemical quartz crystal micro-
balance (EQCM) in the studies on electrochemical behavior of
Rh.
Similarly to other platinum group metals, Rh is known to
adsorb hydrogen and oxygen on its surface [1–4]. Under elec-
trochemicalconditionsthereactionsofhydrogenelectrosorption
and surface oxidation occur at potentials between those required
for hydrogen and oxygen evolution processes.
The electrochemical processes of Rh surface oxide formation
and reduction have been studied by many authors [1,2,4–19].
According to Peuckert [5] and Jerkiewicz [2,6–8] surface oxi-
dation of Rh electrode proceeds in three steps. Initially, at low
In this work we present the results of the experiments per-
formed on thin polycrystalline Rh layers electrodeposited on Au
substrate. We discuss the frequency changes of the quartz crys-
tal resonator during electrode polarization through the hydrogen
and oxygen regions. In particular, our goal was to derive from
frequency–charge analysis quantitative information concerning
∗
Corresponding author. Tel.: +48 22 8220211; fax: +48 22 8225996.
E-mail address: aczerw@chem.uw.edu.pl (A. Czerwin´ski).
ISE member.
1
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.12.066

M. Łukaszewski et al. / Electrochimica Acta 52 (2007) 4560–4565
4561
anion adsorption and surface oxide formation, including the
identification of Rh-oxygen species present on the oxidized
surface. We also show the EQCM data on Rh electrochemi-
cal dissolution under the conditions of a cyclic voltammetric
experiment. We have examined the influence of electrode poten-
tial, scan rate and number of potential cycles on the amount of
dissolved metal.
2. Experimental
All experiments were performed at room temperature in
0.5 M H2SO4 solutions deoxygenated using an Ar stream. A
Hg|Hg2SO4|0.5 M H2SO4 was used as the reference electrode.
A Pt gauze was used as the auxiliary electrode. All potentials
are recalculated with respect to the SHE.
A 10 MHz AT-cut Au-plated crystals were used in the elec-
trochemical quartz crystal microbalance (EQCM) experiments.
The calibration constant determined by Ag and Pd deposition
was 1.2 ng Hz−1, very close to the theoretical value based on
Sauerbrey equation [34]. Thin (0.2–0.3 ␮m, i.e. ca. 1000–1500
atomic layers) Rh layers were deposited potentiostatically on
quartz crystal surfaces from a bath containing RhCl3 and HCl.
The real surface area of Rh electrodes was calculated from
adsorbed hydrogen oxidation charge [1] using a conversion fac-
tor 221 ␮C cm−2 for the removal of a monolayer of adsorbed
hydrogen. The roughness factor of Rh deposits was in the range
50–80. It was confirmed using scanning electron microscope
that the deposit covered tightly Au substrate, therefore no signals
from Au could be detected in CV and EQCM responses.
Fig. 2. Cyclic voltammogram (a) and frequency–potential relationship (b) for
Rh electrode. Potential cycling range from 0.02 to 1.27 V; scan rate 0.05 V s−1
Number of cycle indicated.
.
surface oxide reduction is expected. This peak decreases during
potential cycling together with the appearance of a second reduc-
tion signal (b) at higher potential similar to that typical of surface
oxide reduction on Rh. After ca. 20 cycles the CV curve becomes
characteristic of the literature curve for Rh. This behavior sug-
gests that the surface oxide formed on a freshly deposited Rh
electrode has different electrochemical properties than the oxide
formed at electrochemically treated one. In particular, a lower
potential of its reduction indicates higher stability of that oxide,
and therefore higher irreversibility of the process. The results
presented below concern Rh electrodes subjected to continuous
potential cycling until a stable CV curve was obtained.
Fig. 2a presents cyclic voltammogram and frequency–
potential relationship for Rh electrode. At low potentials on the
anodic part of a CV curve one can distinguish the region of
hydrogen desorption (A), while at high potentials surface oxida-
tion (C) occurs. Between the hydrogen and oxygen regions there
is a narrow potential range (B), called “double layer region”,
which seems to be free from faradaic currents. However, accord-
3. Results and discussion
3.1. General behavior of Rh-EQCM electrode
Fig. 1 presents cyclic voltammograms recorded for a freshly
prepared Rh electrode. It is clearly visible that first cycles
obtained after Rh deposition differ markedly from the CV curve
typical for polycrystalline Rh [1,20]. One should pay attention
to a sharp reduction signal (a) placed at the potential range where
´
ing to Wasberg and Horanyi [23] in that range both OH and H
species at low coverage are present simultaneously on the sur-
face of Rh electrode. The overlapping of hydrogen and oxygen
regions is particularly well pronounced in the cathodic scan after
prior polarization into high potentials. The currents due to sur-
face oxide reduction (D) are superimposed with currents due
to hydrogen adsorption (E) and the double layer region is not
visible.
In the course of the frequency–potential curve (Fig. 2b) a
monotonic decrease in frequency is observed during an anodic
voltammetric scan. According to Sauerbrey equation [34] fre-
quency drop corresponds to mass gain. The frequency decline in
the hydrogen and double layer regions is explained in the liter-
Fig. 1. Cyclic voltammograms for freshly deposited Rh. Potential cycling range
from 0.01 to 1.26 V; scan rate 0.1 V s−1
.

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M. Łukaszewski et al. / Electrochimica Acta 52 (2007) 4560–4565
CV curve: (A) hydrogen desorption, (B) double layer charg-
ing, (C) surface oxidation, (D) surface oxide reduction and (E)
hydrogen adsorption. Due to overlapping of the hydrogen and
oxygen regions in the cathodic scan the analysis of data based on
the apparent molar mass changes presented below concerns only
the anodic scan (regions A–C), where the processes of hydro-
gen desorption and surface oxide formation are much better
separated.
In the hydrogen desorption region (A) the apparent molar
mass is 7.8 g mol−1 of electrons, which far exceeds the value
−1 g mol−1 predicted for mass decrease due to removal of
hydrogen atoms in a one-electron reaction. This discrepancy can
be explained assuming that: (a) hydrogen desorption occurs with
a simultaneous adsorption of other species of higher molar mass
and (b) some non-mass effects influence the resonator frequency.
According to the first explanation hydrogen replacement with
adsorbed HSO4− anions or water molecules might be taken into
account. The following schemes can be proposed for region A:
Fig. 3. Frequency–charge correlation obtained from cyclic voltammetric and
EQCM responses for Rh electrode. Potential cycling range from −0.03 to 1.37 V;
scan rate 0.05 V s−1
.
ature [16,19,25] by the replacement of adsorbed hydrogen with
water molecules and anions from the solution. In the oxygen
region frequency drops due to surface oxide formation. In the
cathodic scan the frequency increases but in the oxygen region
this rise is retarded which mirrors the electrochemical irre-
versibility of the processes of surface oxide formation/reduction
[1]. A quantitative discussion on frequency changes is presented
inSection 3.2. Anotherimportantfeatureofthefrequencycurves
shown in Fig. 2 is that they are not closed within a single
cycle and the frequency measured at the anodic vertex poten-
tial increases during continuous cycling. This behavior indicates
that a resultant loss of mass occurs, i.e. electrochemical disso-
lution of Rh takes place [18]. It should be stressed that marked
changes in the frequency–potential curve observed within sev-
eral cycles are not accompanied by any noticeable changes
in the course of the cyclic voltammogram. It means that cur-
rents due to Rh dissolution strongly overlap with currents due
to surface oxide formation. However, with the use of such a
sensitive device like the EQCM it is possible to detect these
small effects. Rh dissolution is discussed in detail in Section
3.3.
−
Rh-H [1 g] + xHSO4 → Rh (HSO4−)x[97x g] + H+ + e−
(2)
Rh-H [1 g] + yH2O → Rh (H2O)y[18y g] + H+ + e−
Fromthe apparentmolarmass inthe hydrogenregion wehave
obtained x = 0.09 or y = 0.49, which means that the adsorption
of one anion or one water molecule accompanies the removal of
hydrogen from ca. 11 or 2 surface sites, respectively.
The fact of sulfate ion adsorption occurring simultaneously
with hydrogen desorption is generally accepted in the literature
and has been proved by the radiotracer method [24,25,30,31].
(3)
−
However, the number of H atoms displaced by a HSO4 ion
found in our study is twice higher than the value (ca. 5) obtained
under similar conditions by Salgado et al. [19]. On the other
hand, the total mass increase in the hydrogen and double layer
regions (A and B) related to the number of Rh surface atoms (cal-
culated from the charge of hydrogen desorption) is 8.2 g mol−1
,
which corresponds to ca. 8% surface coverage with adsorbed
HSO4− anions, in a good agreement with the radiotracer results
for Rh [25,30,31] and the EQCM results [35] for Pt.
3.2. Analysis of frequency–charge correlations
The alternative possibility is that two hydrogen atoms are dis-
placed by one water molecule. Although this explanation might
seem realistic, it should be excluded, since it would mean that
water is adsorbed only on bare metal and does not interact with
the layer of adsorbed hydrogen present earlier on the electrode
surface. In fact, such interactions are expected due to probably
enhanced hydrophilicity of the surface covered with hydrogen
atoms, similarly to the case of Pt electrode [37–41], where this
phenomenon is believed to be responsible for frequency lower-
ing in the potential region of the existence of weakly adsorbed
hydrogen.
One may suggest that also in the case of Rh electrode the
non-mass effects like changes in the metal–solution interactions
resulting in the variation of solution density and viscosity in the
vicinity of the electrode play an important role in the observed
frequency changes in the hydrogen and double layer regions. To
estimate these contributions an equation postulated by Tsionsky
In the literature [16,19,35,36] the resonator response is often
correlated with the faradaic charge passed during electrode pro-
cesses. From the ratio between frequency shift and consumed
charge the value of the apparent molar mass per number of
electrons can be obtained (Ma/z), according to the equation:
Ma
z
ꢀfCF
ꢀQ
= −
(1)
where ꢀf is the total frequency shift, C is the calibration constant
of the quartz crystal resonator, F is the Faraday constant and ꢀQ
is the charge consumed in the reaction.
Fig. 3 presents the frequency–charge relationship for Rh
polarization in the potential range from −0.03 to 1.37 V. On
the plot one can distinguish several regions connected with dif-
ferent processes, corresponding to the respective regions on a

M. Łukaszewski et al. / Electrochimica Acta 52 (2007) 4560–4565
4563
et al. [42] can be used:
oxidation. However, the electrodes used in their works were
smooth Rh wires and it is possible that thin Rh layers of con-
siderable roughness have different electrochemical properties
towards oxygen adsorption. The differences in electrode rough-
ness can also be responsible for the differences in the course
of frequency–charge relationships presented by various authors
[16–19]. On the other hand, Woods [1,4] reported that a mono-
layer of adsorbed oxygen on Rh is formed at potentials from the
range between 1.2 and 1.5 V, i.e. similar to that applied in this
work. Moreover, our results are in line with the recent findings
for Pt [36], Pd [44] and Au [45], where using the EQCM it was
stated that O rather than OH species are electrosorbed in the
early stages of surface oxide formation on those metals.
ꢀ ꢁ
ꢂ
ꢃ
2f02
μqρq
η
ꢀf = −
ρ
1 −
+ (ρf − ρ) Lf
(4)
√
ηf
where f0 is the fundamental frequency of the resonator (here
10 MHz), μq is the shear modulus of the quartz crystal
(2.95 × 1011 g cm−1 s−2), ρq is the density of the quartz crystal
(2.65 g cm−3), η, ρ are the viscosity and density of the bulk of
solution, ηf, ρf are the viscosity and density of a thin film of
liquid near the electrode surface and Lf is the thickness of the
film of liquid whose viscosity and density differ from the values
in the bulk of solution. It was shown in Ref. [42] that at potential
higher by 0.5 V than potential of zero charge (for Rh the value of
0.0 V is reported [43]) the concentration of anions at the outer
Helmholtz plane can exceed by one order of magnitude their
concentration in the bulk of solution, leading to corresponding
changes in local density. Simultaneously, the changes of solu-
tion composition together with high electric field strength at the
interface result in changes in local viscosity by 20–60%. Tak-
ing into account the above facts and assuming Lf as 1 nm (i.e.
a typical thickness of double layer) we obtain a maximum neg-
ative frequency shift in regions A and B by ca. 10–15 Hz. This
value is only ca. 5% of the total measured frequency drop in this
potential range, so it can be neglected.
Thus, it can be concluded that a dominant contribution to
frequency decrease in the hydrogen and double layer regions
originates from mass increase due to adsorption of HSO4− ions.
This process is facilitated by positive charge on the electrode
surface at potentials higher than zero charge potential.
From the analysis of mass versus charge correlation in the
oxygen region Oliveira et al. [16] were able to identify vari-
ous Rh-oxygen species present on the electrode surface, namely
Rh(OH)3 and RhO(OH)3. On the other hand, Salgado et al. [19]
on the basis of a similar data treatment concluded that Rh oxides
cannot be directly identified by the EQCM method. Our results
show that in the anodic scan through the oxygen region (C) the
apparent molar mass is 6.8 g mol−1 of electrons, which is close
to the value of 8 g mol−1 predicted for a two-electron reaction
of surface oxide formation according to the scheme:
3.3. Electrochemical dissolution of Rh
Owing to its high sensitivity to mass changes the EQCM
is a very suitable method for studying the dissolution process.
In order to explore this phenomenon Rh electrode was polar-
ized to various upper potential limits. Fig. 4 presents a series of
cyclic voltammograms and frequency–potential curves recorded
inthisexperiment. TheCVcurvesdemonstratethattheprocesses
of surface oxide formation/reduction become electrochemically
more irreversible with the increase in the upper potential limit.
In particular, the surface oxide reduction peak is shifted into
lower potentials, strongly overlapping with hydrogen adsorp-
tion currents. From the analysis of the EQCM response it is
evident that for a sufficiently low potential (Fig. 4a) no changes
in frequency during a single cycle are observed indicating that
metal dissolution does not occur. On the other hand, for higher
anodic potentials the effect of frequency increase corresponding
to electrode mass decrease is visible, being the more pronounced
the higher the upper potential limit (Fig. 4b and c).
Fig. 5 shows the potential dependence of the amount of dis-
solved metal (expressed as a fraction of monolayer of Rh surface
atoms) calculated using Sauerbrey equation from the difference
in frequency between two consecutive scans (0.05 V s−1) mea-
sured at anodic vertex potential. The potential of the onset of
Rh dissolution is ca. 0.6 V and then a monotonic, almost linear
mass loss is observed with the potential increase. At a potential
slightly lower than the oxygen evolution potential the amount of
dissolved metal corresponds to ca. 4.5% of monolayer of surface
atoms.
Fig. 6 presents the influence of scan rate on the amount of
dissolved metal for electrode cycling to a fixed upper potential
limit (1.37 V). It is clearly seen that with the decrease in scan
rate the effect of Rh dissolution increases. It results from the fact
that during polarization at lower scan rate the electrode remains
at sufficiently high potentials for a longer time. Moreover, at
low scan rates the effect of partial redeposition of previously
dissolved metal is diminished, since Rh ions have more time to
diffuse away from the vicinity of the electrode.
Fig. 7 shows total amount of dissolved metal after a given
number of cycles in the potential range from 0.02 to 1.27 V
(0.05 V s−1). Itcanbeseenthatelectrodemassdecreaseslinearly
with the progress of cycling procedure and after ca. 50 cycles a
full monolayer of Rh atoms is removed from the surface.
Rh + H2O → RhO [16 g] + 2H+ + 2e−
(5)
Moreover, the mass increase in the oxygen region related to
the number of surface Rh atoms was 14.8 g mol−1, again close
to 16 g mol−1 expected for a monolayer of RhO. The small dif-
ferences could be explained by the following effects: (a) partial
desorption of anions during surface oxide formation, (b) changes
in the metal–solution interactions occurring when oxygen atoms
are adsorbed on Rh surface and (c) metal dissolution after polar-
ization to sufficiently high potentials. It should be added that
the anodic charge consumed in the processes in the oxygen
region during polarization to 1.37 V was 460 ␮C cm−2, i.e. only
slightly higher than a value corresponding to the formation of a
monolayer of RhO (442 ␮C cm−2).
Our results are not consistent with the data reported by Peuck-
ert [5] and Jerkiewicz [2,6–8], who postulated the formation
of Rh(OH) and then Rh(OH)3 as initial stages of Rh surface

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M. Łukaszewski et al. / Electrochimica Acta 52 (2007) 4560–4565
Fig. 5. Dependence of the loss in Rh electrode mass (expressed as a fraction of
monolayer of surface atoms) on upper potential limit under the conditions of a
cyclic voltammetric experiment; scan rate 0.05 V s−1
.
Fig. 6. The influence of scan rate on the amount of dissolved Rh (expressed as a
fraction of monolayer of surface atoms) during cyclic voltammetric polarization
to 1.37 V.
Fig. 4. Cyclicvoltammograms(solidline)andfrequency–potentialrelationships
(dotted line) for Rh polarization to various upper potential limits: (a) 0.67 V, (b)
0.97 V and (c) 1.42 V; scan rate 0.05 V s−1
.
It should be added that the effect of Rh dissolution was well
reproducible. In that context it is surprising that such effect was
not reported by some other authors [16,19], who claimed that
Rh EQCM electrode was stable during potential cycling in the
oxygen region. In those works Rh was deposited on the quartz
crystal plated with Pt, while in our studies resonators covered
with Au were used, similarly to the work by Juodkazis et al.
[18], who also observed the dissolution process. Therefore, it
may be suggested that the kind of substrate can influence cer-
tain electrochemical properties of thin Rh deposits. However,
according to Woods [1] only for very thin layers (several mono-
layers) the cyclic voltammetric behavior of a bimetallic system
can be influenced by both the plated and substrate metals. When
multilayer deposits are formed, completely covering the under-
Fig. 7. Dependence of the loss in Rh electrode mass (expressed as a fraction of
monolayer of surface atoms) on number of potential cycles. Potential cycling
range from 0.02 to 1.27 V; scan rate 0.05 V s−1
.

M. Łukaszewski et al. / Electrochimica Acta 52 (2007) 4560–4565
4565
lying metal, the properties of the resulting surface are those for
an electrode composed of the pure plated metal. It should be
stressed that our results are consistent with the data reported for
Rh wires by Rand and Woods [32], who using analytical tech-
niques confirmed metal dissolution during cycling voltammetry
in the oxygen region.
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ˇ
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4. Conclusions
Surface oxide formation on freshly electrodeposited Rh is
more irreversible than on electrochemically aged Rh. From the
analysis of the EQCM response it can be concluded that hydro-
gen desorption occurs simultaneously with the adsorption of
HSO4− ions, whose maximum surface coverage reaches ca. 8%.
RhO is suggested as the main species formed during Rh surface
oxidation. The EQCM experiments indicate Rh dissolution dur-
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