Effect of Cl- ions and benzylideneacetone/ethanol on the underpotential deposition of zinc in acidic media: Cyclic voltammetry and EQCM studies

Article abstract of DOI:10.1016/j.electacta.2004.11.029

The influence of chloride ions and a benzylideneacetone (BDA)/ethanol (EtOH) mixture on the underpotential deposition (UPD) of Zn²⁺ ions on Pt electrodes in acidic media was investigated by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). In the potential region of the UPD of H and Zn, the surface coverage of Zn adatoms on Pt was evaluated based on the correspondence between charge and mass for several different solutions. In the absence of Cl^- ions and BDA/EtOH, the maximum surface coverage of the Zn deposited by UPD was 0.29. In addition, in the presence of Cl^- ions, the UPD of Zn²⁺ ions occurred simultaneously with the adsorption of Cl^-, and the presence of Cl^- did not modify the quantity of Zn deposited by UPD. In the presence of Cl^- ions and BDA/EtOH, the maximum surface coverage of the Zn deposited by UPD was 0.16. The partial inhibition of the UPD of Zn ²⁺ ions is associated with the adsorption of BDA/EtOH or products of the decomposition of BDA/EtOH during the UPD process.

Full text of DOI:10.1016/j.electacta.2004.11.029

Electrochimica Acta 50 (2005) 2815–2822
−
Effect of Cl ions and benzylideneacetone/ethanol on the
underpotential deposition of zinc in acidic media: cyclic
voltammetry and EQCM studies
a
a
a
a
b
a,∗
P.F. M e´ ndez , J.R. L o´ pez , Y. Meas , R. Ortega , L. Salgado , G. Trejo
a
Centro de Investigaci o´ n y Desarrollo Tecnol o´ gico en Electroqu ´ı mica (CIDETEQ), Parque Tecnol o´ gico Sanfandila,
Pedro Escobedo, A. P. 064. Quer e´ taro C.P. 76700, Mexico
Universidad Aut o´ noma Metropolitana, Area de Electroqu ´ı mica. Apdo. Postal 55-534, M e´ xico D.F. 09340, Mexico
b
Received 6 August 2004; received in revised form 4 November 2004; accepted 6 November 2004
Available online 15 December 2004
Abstract
The influence of chloride ions and a benzylideneacetone (BDA)/ethanol (EtOH) mixture on the underpotential deposition (UPD) of Zn2+
ions on Pt electrodes in acidic media was investigated by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). In
the potential region of the UPD of H and Zn, the surface coverage of Zn adatoms on Pt was evaluated based on the correspondence between
−
charge and mass for several different solutions. In the absence of Cl ions and BDA/EtOH, the maximum surface coverage of the Zn deposited
−
2+
−
by UPD was 0.29. In addition, in the presence of Cl ions, the UPD of Zn ions occurred simultaneously with the adsorption of Cl , and
−
−
the presence of Cl did not modify the quantity of Zn deposited by UPD. In the presence of Cl ions and BDA/EtOH, the maximum surface
coverage of the Zn deposited by UPD was 0.16. The partial inhibition of the UPD of Zn ions is associated with the adsorption of BDA/EtOH
or products of the decomposition of BDA/EtOH during the UPD process.
2
+
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Additives; Benzylideneacetone; EQCM; Zinc underpotential deposition
1
. Introduction
the order of parts per million and their presence promotes the
formation of soft and shiny coatings. In recent years, mixtures
of benzylideneacetone (BDA) and ethanol (EtOH) have been
increasingly used as an additive in the electrodeposition of
zinc [10–13] and Zn–Co alloy coatings in acidic electrolytic
baths [14]. The superior quality of the coatings obtained in
the presence of BDA/EtOH has generated growing interest in
the mechanism by which this additive affects zinc electrode-
position. However, despite the work carried out in this area,
the adsorption characteristics of BDA/EtOH and its mech-
anism of action during electrodeposition remain largely un-
known. One potentially useful method for obtaining informa-
tion on the function of BDA/EtOH during electrodeposition
is to study the formation of the initial states of the deposit, that
is, the underpotential deposition (UPD) of Zn2+ ions under
the influence of the organic additive.
Coatings of zinc and its alloys are of great practical impor-
tanceduetotheircapacitytoprotectferroussubstratesagainst
corrosion [1–4]. Various factors affect the mechanism of zinc
electrodeposition, and hence influence the composition and
morphology of the coatings obtained. These factors include
the concentration of zinc ions [5], complexing agents [6], an-
ions [7,8], and additives [9], all of which play fundamental
roles in zinc electrodeposition. The use of additives in elec-
trolytic baths is very important due to their influence on the
growth and structure of the deposits obtained. Typically, ad-
ditives are added to the electrolytic bath at concentrations on
∗
Corresponding author. Tel.: +52 442 211 6028; fax: +52 442 211 6001.
E-mail address: gtrejo@cideteq.mx (G. Trejo).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.11.029

2
816
P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
The UPD of Zn2+ ions has been studied by Aramata and
sides with a platinum film (Maxtek) was used as the working
electrode (Pt-EQCM). The geometric area of the platinum
coworkers [15,16] on polycrystalline and monocrystalline Pt,
Pd and Au substrates. They investigated the effect of pH on
the anodic displacement of the peak corresponding to the
dissolution of Zn deposited by UPD onto Pt, and suggested
that the UPD of Zn2+ ions on Pt involves the transfer of two
electrons. In addition, it has been reported that the UPD of
2
electrode (Pt-EQCM) was 1.37 cm . The real area of the Pt-
EQCM electrode was estimated from the desorption charge
of H UPD and the charge corresponding to the value predicted
forthedesorptionofamonolayerofH frompolycrystalline
ads
−
2
platinum (0.210 mC cm ) [23]. The value obtained for the
real area was 8.05 cm and the roughness factor, fr, was found
2
2+
Zn ions is sensitive to the crystallographic orientation of
the surface, with the (1 1 0) orientation being the most favored
to be 5.9.
The EQCM signal was recorded as ꢀf (=f − f0) as a func-
tion of the electrode potential. The relation between the
change in surface mass and the resonance frequency for an
EQCM electrode is given by the Sauerbrey equation [24]:
[
17].
The influence of anions on UPD of Zn2+ ions has been
studied by various groups. Taguchi and Aramata [18] studied
the UPD of Zn2 in phosphate media by cyclic voltammetry.
They proposed a model involving three stages: (i) desorption
of anions from the substrate, (ii) UPD of Zn2+ ions, and (iii)
adsorption of anions onto the Zn deposited by UPD. This
model is supported by results obtained using voltammetry
+
ꢀ
f = −C ꢀm
(1)
f
where ꢀf (Hz) is the frequency change, ꢀm (ng cm 2) the
−
−1
2
mass change per unit area, and C (Hz ng cm ) is the sensi-
[
[
18], the radiotracer technique [19,20], FT-IR, and EQCM
21]. In addition, Taguchi and Aramata [18] demonstrated
f
tivity factor of the quartz crystal used for the measurements.
−
−
2−
−
The value of C was determined in independent experiments
that the adsorption of Cl , HSO4 (SO4 ) and H2PO4
onto Pt is induced by the coadsorption of adatoms of Zn. The
results show that the specific adsorption of HSO4 (SO4
and H2PO4 ions onto the Zn adatoms is greater than the
adsorption of Cl ions. In a study using voltammetry and
rotating ring-disc voltammetry, Mascaro et al. [22] showed
that the coadsorption of HSO4 , ClO4 and F onto the
substrate (Pt) or adatoms influenced the nature of the Zn
adlayer.
f
on the Faradaic deposition of Ag onto the Pt-EQCM elec-
−
1
2
−
2−
trode (C = 0.051 Hz ng cm ). Recently, Vatankhah et al.
)
f
−
[
25] reported that the value of C for Pt-EQCN electrodes
f
−
depended on the electrochemical technique used, as well as
on the thickness of the deposited Ag film. They suggested
that an adequate experimental procedure for determining Cf
could be based on chronopotentiometric or chronoampero-
metric measurements and Ag deposits on the order of 7–16
monolayer equivalents. In the present work, chronoamper-
−
−
−
Previous studies of the UPD of Zn2+ ions have shed light
on the fundamental aspects of this process, including effects
related to the nature of the substrate, crystallographic ori-
entation of the surface, pH of the solution, and the anions.
However, only limited information is available regarding the
influence of organic compounds in the electrolytic medium
ometry was used to obtain the value of C . The Ag deposits
f
−2
were obtained using a solution comprised of 10 M AgNO3
in 0.2 M H2SO4. The potential was stepped from an initial
value, E = 0.7 V, to the final one, E = 0.51 V versus NHE.
i
f
The time of duration of the step went between 10 and 150 s.
In the series of experiments, the changes in the frequency re-
sponse were recorded simultaneously with the electrochem-
ical response over the same time intervals. The value of Cf
was then calculated from the slope of the plot of the change
on UPD of Zn2 ions. The study of UPD Zn ions in elec-
trolytic media similar to the baths used for massive depo-
sition should contribute valuable information that will aid
in elucidating the action of these species in the Zn deposi-
tion process. The aim of the present work was to study the
UPD of Zn2 ions onto Pt electrodes in acid media under
the influence of chloride ions and BDA/EtOH. This study
was carried out using cyclic voltammetry in conjunction with
EQCM.
+
2+
in charge (ꢀq) versus the frequency change (ꢀf).
+
A mercury–mercurous sulfate electrode (Hg/Hg2SO4/
K2SO4) and a spectroscopic-grade graphite rod (PARC) were
used as the reference and counter electrodes, respectively. All
potentialsweremeasuredwithrespecttomercury–mercurous
sulfateelectrodeatroomtemperature. Inthetext, thepotential
values are referred to the normal hydrogen electrode (NHE).
In order to minimize iR-drop effects, a Luggin capillary was
employed to connect the reference-electrode compartment to
the working-electrode one.
2
. Experimental
The electrogravimetric study was performed in a conven-
tional three electrode cell with a water jacket. An EQCM
Maxtek, Mod. 710) and a potentiostat/galvanostat (PAR,
Mod. 273A) controlled by independent computers running
the software PM710 and M270, respectively, were used to si-
multaneously record electrochemical measurements and the
resonance frequency of the quartz crystal. An AT-cut quartz
crystal of nominal frequency f0 = 5 MHz, covered on both
The solutions were prepared prior to each experi-
ment using ultrapure water (18 Mꢁ cm), sulfuric acid
(Backer), ZnSO4·7H2O (Backer), KCl (Backer). A stock
BDA/EtOH solution containing 5 g of benzylideneacetone
(
(BDA) (C H CH CHCOCH3) (Aldrich) and 100 ml of
6
5
ethanol (EtOH) (Aldrich) was prepared (6 wt.% BDA in
EtOH). All reageants were of analytic grade. Prior to each
experiment, the solution in the cell was purged for 30 min

P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
2817
with ultrapure nitrogen (Praxair) and the experiments were
carried out under a nitrogen atmosphere. In all experiments,
the potential scans were performed over the potential range
0.06–1.5 V and were initiated in the negative direction begin-
ning from the resting potential. The plots shown correspond
to the first scan cycle.
3
. Results and discussion
3
.1. Electrochemical characteristics of the Pt-EQCM
electrode in 0.5 M H2SO4
Before commencing the study of the UPD of Zn2+ ions and
−
the influence of Cl and BDA/EtOH, the Pt-EQCM electrode
was subjected to several potential cycles in the base solution
S0 of composition 0.5 M H2SO4. Fig. 1a shows a typical
cyclic voltammogram (CV) of Pt in this medium [23,26,27].
Three characteristic potential regions are observed: region A
Fig. 2. Cyclic voltammograms of Pt-EQCM for three solutions: (a) S0
−3
2+
−4
M
(
=0.5 M H2SO4), (b) S1 (=S0 + 10 M Zn ) and (c) S2 (=S1 + 10
−1
KCl). v = 50 mV s
.
(
0.06–0.4 V), in which electroadsorption–electrodesorption
for the desorption of hydrogen, obtained by integrating the i
versus E curve in region A (anodic scan) was 0.175 mC cm
after correcting for the double layer charge.
−2
of hydrogen (H UPD) occurs; region B (0.4–0.8 V) associ-
ated with the region of the double layer; region C (0.8–1.5 V)
associated with the formation and reduction of platinum ox-
ide (PtO) at the surface [27,28]. In region A, the process of H
UPD is characterized by the cathodic peaks H and H asso-
ciated with two types of adsorbed hydrogen (Hads): strongly
boundHandweaklyboundH, respectively[29–32]. PeaksH
and H are the anodic equivalents of H and H , respectively.
Peak H corresponds to the desorption of H from sites
different from those described above. Recently, Jerkiewicz
et al. [33,34] calculated the standard entropy of the UPD of
H onto Pt (poly), ꢀS (Hads), as a function of the surface
coverage of Hads, θHads . The plot of ꢀS (Hads) versus θHads
revealed two waves, which the authors associated with two
adsorption–desorption peaks in the CV profiles. In addition,
they found that the Pt–Hads binding energy (EPt–H ) was in
the range of 245–265 kJ mol . The electric charge density
Fig. 1b shows the variation in mass as a function of poten-
tial recorded simultaneously with the cyclic voltammogram
in Fig. 1a. The offset ꢀm = 0 was fixed at E = 1.5 V. Dur-
ing the potential scan in the positive direction starting from
0.06 V, the mass (ꢀm) is observed to continuously increase
in the potential range 0.06–1.5 V, with different values of the
slope (dꢀm/dE) in each potential region. In region A, the in-
c
1
c
2
a
1
a
2
c
1
c
2
a
3
ads
−
2
crease in mass (17.10 ± 0.57 ng cm ) has been attributed to
the incorporation (adsorption) of water molecules during the
desorptionofHads (substitutionofHads forthewatermolecule
[35]);inregionB, theincreaseinꢀmhasbeenassociatedwith
the incorporation of anions in the region of the double layer;
and in region C, the increase in ꢀm has been attributed to
the formation of PtO at the surface [27,28,36–38]. In a recent
study using Auger electron spectroscopy, cyclic voltamme-
try and EQCN, Jerkiewicz et al. [27] generated a detailed
description of the mass response of a cyclic voltammogram
as part of the study of the formation of the surface oxide, PtO.
◦
Ads
◦
Ads
ads
−
1
3
.2. UPD of Zn on Pt in the absence and presence of
−
Cl ions
Fig. 2 shows the CVs of Pt in the base solution S0 (=0.5 M
−
3
2+
H2SO4) (curve a), in solution S1 (=S0 + 10 M Zn ) (curve
−
4
b), and in solution S2 (=S1 + 10 M KCl) (curve c), recorded
−
1
at 50 mV s
.
Comparison of the CVs obtained for solutions S0 (curve
a, Fig. 2) and S1 (curve b, Fig. 2) shows the effect of the
2+
2+
Zn ions on H UPD. In the presence of Zn ions (curve
b), the anodic and cathodic current densities increase when
E < 0.3 V (H UPD, zone A). The formation of three anodic
peaks is observed (peak Ia, 0.2 V; peak IIa, 0.25 V; peak IIIa,
0
.28 V) as well as the corresponding cathodic peaks (peak
Fig. 1. (a) Cyclic voltammogram, and (b) ꢀm vs. E plot for a Pt-EQCM
−1
electrode in solution S0 (=0.5 M H2SO4). v = 50 mV s
.
Ic, 0.16 V; peak IIc, 0.2 V; peak IIIc, 0.25 V). Peaks IIIc and

2
818
P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
Fig. 3 shows the profiles of mass versus potential (ꢀm ver-
sus E) corresponding to the cyclic voltammograms in Fig. 2.
Curves a–c in Fig. 3 have been grouped with an offset of
ꢀm = 0 at the upper potential limit, E = 1.5 V. Curve a shows
the change in mass as a function of potential for Pt in solu-
tion S0. The behavior of this system was discussed above in
relation to Fig. 1b; the data are included in Fig. 3 for compar-
ison purposes. Comparison of the curves obtained for solu-
tions S0 and S1 (curves a and b, respectively, Fig. 3) reveals
that the mass changes in regions B and C do not depend on
the presence of Zn2+ ions in the solution. However, in re-
2+
gion A and in the presence of Zn ions (curve b, Fig. 3),
c
−2
an increase (gain) in mass (ꢀm = 25.10 ± 0.57 ng cm
)
A
is observed during the potential scan in the negative direc-
tion. In addition, on switching the direction of the poten-
tial scan to the positive direction starting from 0.06 V, a de-
crease (loss) in mass of the same magnitude is observed
Fig. 3. ꢀm vs. E curves corresponding to the cyclic voltammograms
−
3
2+
in Fig. 2: (a) S0 (=0.5 M H2SO4), (b) S1 (=S0 + 10 M Zn ), (c) S2
−
4
−1
a
A
−2
(=S1 + 10 M KCl). v = 50 mV s
.
(ꢀm = −25.10 ± 0.57 ng cm ), suggesting that in this
potential range the process of Zn deposition and dissolution
is reversible.
IIIa are associated with the increase in the current density of
The voltammetry results indicate that the dissolution of
Zn deposited by UPD occurs simultaneously with hydro-
gen desorption. Thus, the experimentally determined mass
change during the potential scan in the positive direction
c
a
peaks H and H , respectively, which correspond to the ad-
1
1
sorption and desorption of strongly bound hydrogen [29–32]
Fig. 1). In addition, peaks Ic and Ia correspond to the de-
(
c
2
a
2
a
−2
crease in the current density of peaks H and H , respec-
(ꢀm = 25.10 ± 0.57 = ng cm ) will depend on two fac-
A
tively, which are associated with the adsorption and desorp-
tion of weakly bound hydrogen [29–32] (Fig. 1). The anodic
tors: mass loss due to the dissolution of Zn deposited by
Zn
UPD (ꢀm
) and mass gain due to the adsorption of wa-
diss,upd
a
charge density (Q ) in this potential range (0.06–0.4 V)
H O
F,A
2
−2
ter molecules (ꢀm
= 17.10 ± 0.57 ng cm ) during the
−
2
ads
was 0.205 ± 0.002 mC cm . This value is 17% greater than
process of hydrogen desorption from the Pt surface (Fig. 1b).
Considering that the masses of H2O and H2O + Hads can-
not be distinguished within the error of this experiment, the
total mass of dissolution of Zn deposited by UPD was deter-
mined using the following equation:
2+
that obtained in the absence of Zn ions.
The data in Fig. 2 additionally indicate that the presence
of Zn2 ions has no significant effect on the processes in the
region of the double layer electric charge (zone B) or on the
formation and reduction of surface oxide (zone C). Similar
results, showinganassociationbetweenthechangesobserved
in zone A of voltammograms with UPD of Zn2+ ions and the
dissolution of Zn deposited by UPD, have been reported by
Aramata et al. [15].
+
a
Zn
H2O
ꢀ
m = ꢀm
+ ꢀmads
(2)
A
diss,upd
a
where ꢀm is the net decrease in mass recorded by EQCM
A
−
2
(
=−25.10 ± 0.57 ng cm ) during the positive scan in re-
H2O
ads
−
gion A, ꢀm
is the mass of adsorbed water molecules
Introduction of Cl ions into the solution (solution S2)
−
2
Zn
(
17.10 ± 0.57 ng cm ), and ꢀm
is the total mass
causes important changes in the voltammograms (curve c,
diss,upd
−
of dissolution of Zn deposited by UPD. The absolute
Fig. 2). The voltammogram obtained in the presence of Cl
Zn
diss,upd
−2
value of ꢀm
obtained was 42.20 ± 0.57 ng cm . The
ions has the following features. In region A, only two ca-
thodic peaks (Ic and IIc) and two anodic peaks (Ia and IIa)
electric charge density associated with the total dissolved
Zn
diss,upd
Zn
−2
−
mass (ꢀm
), Q
= 0.124 ± 0.002 mC cm , is
are observed. This behavior suggests that the Cl ions inhibit
diss,upd
both: the UPD of Zn2+ ions and H UPD associated with peak
less than that obtained by voltammetry for the same pro-
cess (Qa = 0.205 ± 0.002 mC cm ). The excess elec-
−2
IIIc and promotes the process giving rise to peaks Ic and IIc
in curve b. In addition, the current densities of these peaks are
F,A
tric charge is associated with the charge due to hydrogen
desorption from the Pt surface (QH = Q − Q
a
F,A
Zn
=
−
greater than those observed in the absence of Cl ions (curve
upd
diss,upd
0
.081 mC cm 2).
−
b). Moreover, the potential range corresponding to the charge
of the double layer (zone B) increases; this effect is charac-
From the EQCM results, and assuming complete dis-
charge of the Zn2+ ions [15,16,19] the surface coverage of
the zinc, θZn-upd, can be evaluated using the following equa-
tion, which is valid in the potential range of 0.06–0.4 V:
−
teristic of the adsorption of Cl [28] and is associated with
the progressive blocking of the initial states (up to 0.85 V)
of the formation of the PtO film [26–28,39]. This picture is
corroborated by the decrease (23%) observed in the intensity
of the peak corresponding to surface oxide reduction (peak
Oc).
ꢀ
ꢁ
Zn
Q
diss,upd
θZn-upd =
= 0.29
(3)
2
QH,s

P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
2819
Zn
diss,upd
where Q
is the anodic charge density associated with
the dissolution of Zn adatoms, QH,s the charge density asso-
ciated with the adsorption of a H monolayer onto polycrys-
−
2
talline Pt (0.210 mC cm ) and a transfer of two electrons
from the Zn [15,16,19].
−
Curve c in Fig. 3 shows the effect of adding Cl ions
(
solution S2) on the UPD of Zn2+ ions. The mass changes
produced in potential regions B and C of the ꢀm versus
E curve (curve c, Fig. 3) are similar to those reported by
Zolfaghari at al. [28] for Pt in 0.05 M H2SO4 in the presence
−
−
of Cl ions. Specifically, when Cl ions are present in the
solution a hysteresis is observed in the double layer region
(region B) between the potential scans in the positive and
negative directions. This behavior is attributed to transitory
−
desorption of Cl due to reduction of coadsorbed O species
generated during the anodic scan. At more positive potentials
Fig. 4. Cyclic voltammograms of Pt-EQCM in solution S2 (=0.5 M
−
−3
2+
−4
H2SO4 + 10
M
Zn + 10 M
KCl) with different concentrations
(region C), the adsorbed Cl ions inhibit the surface oxide
−
7
−5
of BDA/EtOH: (a) 0.0, (b) 6.8 × 10 M BDA/3.4 × 10 M EtOH,
formation during the positive potential scan; as a result, the
mass changes due to the formation and reduction of surface
−
6
−5
−6
M
(
c) 1.36 × 10
M
BDA/6.86 × 10
M
EtOH, (d) 2.05 × 10
−4
−6
−4
BDA/1 × 10 M EtOH, (e) 2.7 × 10 M BDA/1.37 × 10 M EtOH.
−
oxide are smaller in the presence of Cl ions than in their
−1
v = 50 mV s
.
absence.
Fig. 4 shows the cyclic voltammograms of Pt in solution
S2 containing different concentrations of BDA/EtOH. The
presence of BDA/EtOH in the solution significantly modi-
fies the voltammograms. In region A, as the concentration
of BDA/EtOH in the solution is increased, a progressive de-
crease is observed in the intensities of the cathodic and anodic
peaks (I and II) corresponding to the UPD of Zn2+ ions and
the dissolution of Zn deposited by UPD, respectively, indicat-
ing that BDA/EtOH partially inhibits the UPD of Zn ions.
Additionally, the cathodic current at the lower potential limit
(0.06 V) increases with increasing BDA/EtOH concentration.
TheinhibitionoftheUPDofZn2+ ionsbytheadsorptionof
BDA/EtOH can be utilized to estimate the degree of blocking
of the Pt surface by the additive, using the following equation:
In region A, during the potential scan in the negative di-
rection, the ꢀm versus E curve shows a mass increase of
−
2
2
6.70 ± 0.57 ng cm in the potential range of 0.40–0.06 V.
−
This implies that the presence of Cl ions causes a mass
increase 6% greater than that observed in the absence of
Cl ions. However, on switching the potential scan to
−
the positive direction starting from 0.06 V, a mass loss of
−
2
2
5.10 ± 0.57 ng cm isobservedinthesamepotentialrange.
2
+
−
This value is similar to that obtained in the absence of Cl
ions (curve b, Fig. 3), which suggests that the quantity of
−
Zn deposited does not depend on the presence of Cl ions
in the solution. The mass of adsorbed species that are not
desorbed during the potential scan in the positive direction
−
2
−
(1.6 ng cm ) may be associated with the adsorption of Cl
ions onto the electrode surface that then remain on the elec-
trode surface [19,20,28], causing a hysteresis between the
deposition and dissolution of Zn (curve c, Fig. 3). Thus, the
increase in charge observed in the voltammogram recorded
a,BDA
F,A
a
Q
− Q
F,A
F,A
Θ
blo
=
(4)
a
Q
where Qa and QF,A are the anodic electric charge densi-
a,BDA
F,A
in the presence of ions chloride (curve c, Fig. 2) corresponds
to an increase in the quantity of Hads and not to UPD of Zn2+
ties in region A in the absence and presence of BDA/EtOH,
respectively. Both charges were measured from the voltam-
mograms shown in Fig. 4. The difference between Q
ions.
a
F,A
a,BDA
and QF,A is proportional to the number of sites blocked
−
3
.3. Effect of BDA/EtOH and Cl ions on the UPD of Zn
by BDA/EtOH. The value of Θblo increases with increas-
ing BDA/EtOH concentration, up to 0.37 at the highest
BDA/EtOH concentration studied (see Table 1). This behav-
ior can be explained by considering that, as for most addi-
on Pt
The influence of BDA/EtOH on the UPD of Zn2+ ions onto
Pt was analyzed using solutions comprised of solution S2
1
tives, BDA is adsorbed on the cathode. Using IR and H NMR
−
7
with different concentrations of BDA/EtOH (6.8 × 10
≤
spectroscopy, Mockute et al. [10,12] showed that during the
electrolysis of Zn in baths containing BDA/EtOH, the addi-
tive decomposes into diverse compounds, including benzy-
lacetone (BAC), benzaldehyde (BZ), and 1-phenylbutanol-3
(FL). They proposed that BAC is formed at the cathode by
reduction of BDA, and that BAC is subsequently reduced
to FL, with this process likely being accelerated by the pro-
−
6
[
BDA] ≤ 2.7 × 10 M and its corresponding concentrations
−
5
−4
of EtOH (3.4 × 10 ≤ [EtOH] ≤ 1.3 × 10 M)). The po-
tential scans were performed in the negative direction starting
from the resting potential. Prior to each experiment, the Pt-
EQCM electrode was regenerated in solution S0 to ensure
that it was clean.

2
820
P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
Table 1
Influence of the concentration of BDA/EtOH on the anodic charge of Pt in solution S2 (=0.5 M H2SO4 + 10 M Zn2+ + 10 M KCl) in different regions of
−
3
−4
the voltammogram
5
−1
a,BDA
F,A
(± 0.002 mC cm−2
a,BDA
F,(0.4–1.5 V)
(± 0.002 mC cm−2
a,exc
[
BDA]/[EtOH] (×10 mol l
)
Q
)
Θblo
Q
)
%Q
F,(0.4–1.5 V)
0
0
0
0
.068/3.43
.137/6.86
.200/10.0
.270/13.7
a
0.164
0.146
0.135
0.129
0.20
0.28
0.34
0.37
0.445
0.510
0.521
0.547
26.19
44.54
47.46
54.88
Q
= 0.205 ± 0.002 mC cm−2, Qa
= 0.354 ± 0.002 mC cm−2
.
F,A
F,(0.4–1.5 V)
tonation of the carbonyl group of BAC. On the other hand,
BZ is formed in the cathodic layer, where the pH is higher
than in the solution. In addition, the reduction of EtOH oc-
curs at E < 0.3 V; Schmidt et al. [40] proposed that this pro-
cess proceeds via a strong interaction of ethanol with the
electrode/electrolyte interface, leading to the formation of
adsorbed intermediates that react with adsorbed hydrogen
atoms. Thus, the additive BDA/EtOH or the reaction prod-
ucts partially block the activated sites on the Pt surface for
the adsorption of H and the UPD of Zn2+ ions.
During the potential scan in the positive direction, in
region B (0.4–1.0 V) the presence of BDA/EtOH causes
an increase in the current density with respect to that ob-
served in the absence of BDA/EtOH (curve a, Fig. 4). Two
current density bands are observed, one between 0.4 and
have identified ethanal, acetic acid and CO2 as products of
this reaction.
Fig. 5 shows a series of ꢀm versus E plots obtained si-
multaneously with the voltammograms in Fig. 4 at various
concentrations of BDA/EtOH. For comparison purposes the
offset ꢀm = 0 was fixed at the upper potential limit (1.5 V). In
region A, ꢀm increases (mass gain) during the potential scan
a,BDA
inthenegativedirectionanddecreases(massloss)(ꢀmA
)
during the scan in the positive direction, indicating that the
2+
−
process UPD of Zn ions occurs in the presence of Cl ions
and BDA/EtOH.
From the plot of ꢀm versus E recorded in the absence of
BDA (Fig. 1b), the quantity of water adsorbed in the poten-
−2
tial range of 0.06–0.4 V can be estimated as 17.1 ng cm . In
addition, considering that only a fraction of hydrogen adsorp-
tion sites (H sites) are blocked by the presence of BDA/EtOH
in the solution, and that the remaining sites can adsorb water
0
.7 V (HA), whose intensity is independent of the con-
centration of BDA/EtOH, and another between 0.85 and
.0 V (HB), whose intensity increases slightly with in-
H2O
1
molecules (Θads = 1 − Θblo), it is possible to determine the
creasing BDA/EtOH concentration. At more positive po-
tentials (region C, 1.0–1.5 V), a progressive increase is
observed in the current density as the concentration of
BDA/EtOH is increased. In the potential range 0.4–1.5 V,
quantity of adsorbed water as a function of the adsorbed frac-
H2O,BDA
ads
tion of BDA/EtOH: ꢀm
= 17.1 × (1 − Θblo). The
H O,BDA
2
values of ꢀm
obtained at different BDA/EtOH con-
ads
centrations are listed in Table 2. Besides, Table 2 shows the
mass change due to dissolution of Zn deposited by UPD
the anodic charge density obtained in the presence of
a,BDA
F,(0.4–1.5 V)
BDA/EtOH (Q
) was between 26 and 54% greater
a,exc
(%Q
) than that obtained in the absence of the ad-
F,(0.4–1.5 V)
a
ditive (Q
) (Table 1).
F,(0.4–1.5 V)
On switching the potential scan to the negative direction
starting from 1.5 V, a cathodic peak, Oc, is observed at 0.8 V,
associatedwiththereductionofthesurfaceoxide(Fig. 4). The
intensity of this cathodic peak decreases only slightly (<10%)
when BDA/EtOH is added to the solution. This suggests that
the oxidation products formed in regions B and C during
the positive potential scan are produced simultaneously in
−
the presence of adsorbed Cl ions and with the formation of
surface oxide. Thus, the present findings suggest that there is
−
no interference between the BDA/EtOH and the Cl ions.
The behavior observed in regions B and C of the voltam-
mograms in Fig. 4 is characteristic of the oxidation reactions
of ethanol on platinum electrodes in acidic media [40–45].
Previous studies by Schmidt et al. [40] indicate that the ox-
idation of EtOH (0.01 M) on Pt/HClO4 (1 M) commences
at 0.42 V during the positive potential scan. Analyses of the
products of the oxidation of ethanol on Pt by Fourier trans-
form infrared spectroscopy (FT-IRS) [40–42] and on-line dif-
ferential electrochemical mass spectroscopy (DEMS) [43,44]
−
3
Fig. 5. ꢀm vs. E curves for Pt in solution S2 (=0.5 M H2SO4 + 10
M
2
+
−4
Zn + 10 M KCl) with different concentrations of BDA/EtOH cor-
responding to the cyclic voltammograms in Fig. 4: (a) absence of
BDA/EtOH, (b) 1.36 × 10 M BDA/6.86 × 10 M EtOH, and (c).
−6
−5
−6
−4
−1
2
.7 × 10 M/1.37 × 10 M EtOH. v = 50 mV s
.

P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
2821
Table 2
Influence of the concentration of BDA/EtOH on the mass of Zn deposited by UPD and surface coverage
a,BDA
A
(± 0.57 ng cm
H2O,BDA
Ads
(± 0.57 ng cm
Zn,BDA
diss,upd
± 0.57 ng cm−2
Zn,BDA
dep,upd
(± 0.002 mC cm−2
Zn,BDA
diss,upd
(± 0.57 mC cm−2
BDA
[
BDA]/[EtOH]
ꢀm
ꢀm
ꢀm
ꢀm
Q
θ
Zn-upd
5
−1
−2
−2
)
(×10 mol l
)
)
(
)
)
)
0
0
0
0
.068/3.43
.137/6.86
.200/10.0
−20.27
−17.21
−14.44
−12.91
Zn
13.68
12.14
11.29
10.80
−33.95
−29.35
−25.73
−23.71
35.02
30.63
28.05
26.63
0.100
0.086
0.075
0.069
0.23
0.20
0.17
0.16
.270/13.70
Zn
dep,upd
= 43.8 ± 0.57 ng cm−2, ꢀm
= −42.2 ± 0.57 ng cm−2
ꢀ
m
.
diss,upd
Zn,BDA
), evaluated from ꢀm aA ,BDA using the following
correspondingtothechargedensityobservedinregionsBand
C. This suggests that in these potential regions a Faradaic pro-
cess takes place during the positive potential scan that does
not involve a mass change. In agreement with Iwasita et al.
(
ꢀm
diss,upd
equation:
a,BDA
A
Zn,BDA
diss,upd
H2O,BDA
ꢀ
m
= ꢀm
+ ꢀmads
(5)
[46], oxidativeethanoladsorptiononPtelectrodestakesplace
in two separate potential regions: 64% of the adsorbates are
oxidized below a maximum at 0.70–0.80 V, and the oxidation
of the remaining intermediates leads to a broad current peak
superimposed on the peak associated with platinum oxide
formation. CO2 and ethanal have been identified as oxida-
tion products [40]. In agreement with Snook et al. [47], the
processes that involve gas-phase species make an insignif-
icant contribution to the mass-potential response. However,
these processes make a significant contribution in the voltam-
mogram.
a,BDA
where ꢀmA
is the net decrease in mass recorded by
EQCM during the positive scan in region A at a particular
BDA/EtOH concentration (Table 2), ꢀmads
H2O,BDA
is the mass
Zn,BDA
of adsorbed water molecules, and ꢀmdiss,upd is the total mass
of dissolution of Zn deposited by UPD.
Increasing the concentration of BDA/EtOH causes
decrease in the absolute value of ꢀmdiss,upd ,
Zn,BDA
a
that is, in the quantity of Zn deposited by UPD.
At the highest BDA/EtOH concentration studied
−
6
−4
(
2.7 × 10 MBDA/1.37 × 10 MEtOH), the absolute
Table 2 shows the surface coverage of zinc adatoms
Zn,BDA
diss,upd
−2
BDA
Zn-upd
value of ꢀm
was 23.71 ± 0.57 ng cm , which is
(θ
) as a function of the concentration of BDA/EtOH,
4
4% less than the mass change observed in the absence of
evaluated based on the anodic charge density associated with
the mass of the Zn deposited by UPD that was dissolved dur-
−
2
BA/EtOH (42.20 ± 0.57 ng cm ). In addition, it is impor-
Zn,BDA
tant to observe that at a particular BDA/EtOH concentration,
ing the positive scan (Qdiss,upd ) using Eq. (3). The surface
Zn,BDA
the absolute value of ꢀmdiss,upd is less than that due to the
UPD of Zn2+ ions (ꢀm
increases with increasing BDA/EtOH concentration. This
suggests that the UPD of Zn2+ ions occurs at the same time
as the adsorption of one or more other species from the
medium or reaction products that remain on the electrode
surface during the dissolution of Zn deposited by UPD
e.g. Cl ions or products of reactions of the BDA/EtOH
mixture). In addition, the values of the charge density
coverage of zinc decreases with increasing concentration of
Zn,BDA
BDA/EtOH in the solution.
), and that this difference
dep,upd
2+
The inhibition of the UPD of Zn ions by BDA/EtOH is
an interesting result from a technological viewpoint because
in general the UPD of metals plays a fundamental role in the
anomalous codeposition of alloys [48]. Thus, the inhibition
of the UPD of Zn2+ ions could potentially be exploited to
modify the mechanism of the anomalous codeposition of Zn
alloys (e.g., Zn–Co [14]).
−
(
Zn,BDA
diss,upd
(Q
) (Table 2) obtained using the total dissolved mass
at the various BDA concentrations (ꢀm Zd ins ,s B, uDp Ad ) are less than
the corresponding values obtained by voltammetry (Fig. 4)
4. Conclusions
a,BDA
(Q
; Table 1). This excess charge is associated with the
F,A
−
charge due to the desorption of hydrogen (Hads) from the Pt
surface.
In region C (0.8–1.5 V), the mass change due to the for-
mation and reduction of surface oxides is approximately
The influence of Cl ions and BDA/EtOH on UPD of
Zn2+ ions was investigated by EQCM and voltammetry. The
2+
results show that the process UPD of Zn ions occurs simul-
taneously with H UPD. The maximum surface coverage of
the Zn onto Pt (0.29) was obtained in the absence of Cl and
BDA/EtOH. In addition, the EQCM study revealed that the
presence of Cl ions does not interfere with the UPD of Zn
∼
−2
−
the same (=30.00 ± 0.57 ng cm ) in the absence (curve a,
Fig. 5) and presence (curves b and c. Fig. 5) of BDA/EtOH,
indicating that the quantity of surface oxide (PtO) formed
does not depend on the concentration of BDA/EtOH in solu-
tion. This result is similar to that observed by voltammetry
−
2+
−
ions. However, Cl ions remain adsorbed on the Pt surface,
inhibiting the formation of surface oxides (PtO).
(
peak Oc, Fig. 4).
Comparison between the voltammograms (Fig. 4) and the
m versus E plots (Fig. 5) shows that there is no mass change
In the presence of BDA/EtOH, oxidation of EtOH was
observed during the positive potential scan in regions corre-
sponding to the electric charge of the double layer and to the
ꢀ

2
822
P.F. M e´ ndez et al. / Electrochimica Acta 50 (2005) 2815–2822
formation of PtO. The experiments additionally revealed that
theadditiveBDA/EtOHdoesnotinterferewiththeadsorption
of Cl or the formation of surface oxides.
[14] G. Trejo, R. Ortega, Y. Meas, E. Chainet, P. Ozil, J. Appl. Elec-
trochem. 33 (2003) 373.
[15] D.A. Quaaiyyum, A. Aramata, S. Moniwa, S. Sathochi, M. Enyo, J.
Electroanal. Chem. 373 (1994) 61.
−
The EQCM study in the UPD region of the voltammo-
grams showed that the presence of BDA/EtOH reduced the
quantity of Zn deposited by UPD by up to 44%, an effect that
is associated with the adsorption of BDA/EtOH or reaction
products onto the Pt surface.
[
16] A. Aramata, Md.A. Quaiyyum, W.A. Balais, T. Atoguchi, M. Enyo,
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[21] A. Aramata, S. Teuri, S. Taguchi, T. Kawaguchi, K. Shimazu, Elec-
trochim. Acta 41 (1996) 761.
TheinhibitionoftheUPDofZn2+ ionsbyBDA/EtOHmay
have important technological implications because the UPD
of metals plays an important role in the anomalous code-
[
[
[
22] L.H. Mascaro, M.C. Santos, S.A.S. Machado, L.A. Avaca, J. Braz.
position of alloys. Thus, addition of BDA/EtOH to inhibit
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Chem. Soc. 13 (2002) 529.
23] R. Woods, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 9,
Marcel Dekker, New York, 1977, p. 1.
24] G. Sauerbrey, Z. Phys. 155 (1959) 206.
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making it possible to obtain Zn alloy coatings of more ho-
mogeneous composition.
[25] G. Vatankhah, J. Lessard, G. Jerkiewicz, A. Zolfaghari, B.E. Conway,
Electrochim. Acta 48 (2003) 1613.
[
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Acknowledgement
[28] A. Zolfaghari, B.E. Conway, G. Jerkiewicz, Electrochim. Acta 47
The authors are grateful for financial assistance provided
by CONACyT (Consejo Nacional de Ciencia y Tecnolog ´ı a),
M e´ xico, Proyects: 46350-Q end 45994. P.F. M e´ ndez and J.R.
L o´ pez also acknowledge CONACYT for scholarship support.
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