Potentiostatic and AFM morphological studies of Zn electrodeposition in the presence of surfactants

Article abstract of DOI:10.1149/1.2752984

Zinc electrodeposition onto a steel substrate in the presence of surfactants with different charged head groups, namely, anionic sodium dodecylsulfate (SDS), cationic dodecyltrimethylammonium bromide (CTAB), and nonionic octylphenolpoly (ethyleneglycolether)n, n=10 (Triton X-100), was studied by both chronoamperometric and atomic force microscopy (AFM) techniques. AFM analysis shows that Zn electrodeposition, in the absence of surfactants, begins in the underpotential deposited (UPD) region with the formation of different-sized circular aggregates with a random distribution, which indicates that progressive nucleation takes place. In the presence of SDS, CTAB, and Triton X-100 no change of the electrodeposit morphology is observed as a function of the surfactant presence. The Zn UPD deposition is confirmed by chronoamperometric measurements performed in this potential region. In what concerns the bulk deposition region, where the hydrogen evolution that goes together with the zinc deposition, the analysis of the current-time transients indicates that the Zn electrodeposition occurs by instantaneous nucleation and three-dimensional growth controlled by diffusion in the surfactant-free solution and in the presence of SDS. When the CTAB and Triton X-100 were added to the bath, change from instantaneous to progressive nucleation arises as a consequence of the simultaneous adsorption of the surfactant that inhibits the nucleation sites. The kinetic parameters values obtained from the Heerman and Tarallo model are in accordance with the dimensionless analysis of the transients. AFM confirms the effect of surfactants on the zinc bulk deposition.

Full text of DOI:10.1149/1.2752984

D452
Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
0013-4651/2007/154͑9͒/D452/10/$20.00 © The Electrochemical Society
Potentiostatic and AFM Morphological Studies of Zn
Electrodeposition in the Presence of Surfactants
A. Gomes,a,z A. S. Viana,b and M. I. da Silva Pereiraa,
*
aC.C.M.M., Departamento de Química e Bioquímica da Faculdade de Ciências, Universidade de Lisboa,
Campo Grande Ed. C8, 1749-016 Lisboa, Portugal
bLaboratório de SPM, Faculdade de Ciências, Universidade de Lisboa, Campo Grande Ed. ICAT,
1749-016 Lisboa, Portugal
Zinc electrodeposition onto
a steel substrate in the presence of surfactants with different charged head groups,
namely, anionic sodium dodecylsulfate ͑SDS͒, cationic dodecyltrimethylammonium bromide ͑CTAB͒, and nonionic
octylphenolpoly͑ethyleneglycolether͒n, n = 10 ͑Triton X-100͒, was studied by both chronoamperometric and atomic force micros-
copy ͑AFM͒ techniques. AFM analysis shows that Zn electrodeposition, in the absence of surfactants, begins in the underpotential
deposited ͑UPD͒ region with the formation of different-sized circular aggregates with a random distribution, which indicates that
progressive nucleation takes place. In the presence of SDS, CTAB, and Triton X-100 no change of the electrodeposit morphology
is observed as a function of the surfactant presence. The Zn UPD deposition is confirmed by chronoamperometric measurements
performed in this potential region. In what concerns the bulk deposition region, where the hydrogen evolution that goes together
with the zinc deposition, the analysis of the current–time transients indicates that the Zn electrodeposition occurs by instantaneous
nucleation and three-dimensional growth controlled by diffusion in the surfactant-free solution and in the presence of SDS. When
the CTAB and Triton X-100 were added to the bath, change from instantaneous to progressive nucleation arises as a consequence
of the simultaneous adsorption of the surfactant that inhibits the nucleation sites. The kinetic parameters values obtained from the
Heerman and Tarallo model are in accordance with the dimensionless analysis of the transients. AFM confirms the effect of
surfactants on the zinc bulk deposition.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2752984͔ All rights reserved.
Manuscript submitted January 30, 2007; revised manuscript received May 8, 2007. Available electronically July 19, 2007.
Surfactants are commonly used in the zinc electrodeposition to
more positive than the estimated equilibrium and thermodynamic
potentials, and the deposition at overpotential region depends on the
presence and concentration of the tested surfactants.26
The aim of this paper is to further explore the systematic com-
prehension of the deposition mechanism and kinetics and, in particu-
lar, the role of the surfactants. To reach this goal a study of the initial
stages of the zinc electrodeposition was undertaken using both chro-
noamperometric and AFM techniques.
control the metallic crystal shape and size in order to produce
smooth and bright deposits.1-4 A variety of studies on zinc elec-
trodeposition onto different substrates, in the presence of a range of
surfactants, can be found in the literature.4-6
In order to analyze the role played by the surfactants, study of
the early stages of the deposition process is very important, because
they determine the morphology and physicochemical properties of
the electrodeposits.
Different mechanisms are considered to explain the interaction of
the surfactants with electrode surface: electrostatic adsorption,
chemisorption bonding, electronic polarization of ␲ electrons, and
hydrophobic/hydrophilic interactions.7 When the interaction be-
tween ionic surfactants and the electrode surface is electrostatic it is
expected that the potential controls the electrode coverage.8 The
adsorption of the surfactants at the electrode surface has an effect on
the growth site density and the adion concentration on the surface,
as well as the adion surface diffusion activation energy and the
diffusion coefficient.9
It is well accepted that the association of different techniques,
namely, chronoamperometry and atomic force microscopy ͑AFM͒,
is very useful for the understanding of the mechanism and kinetics
of the early stages of metal deposition.5,6,10-15 The transient’s analy-
sis provides reliable information on the nucleation and growth
mechanisms and kinetics, which can be complemented by AFM im-
aging. The interpretation of the current-time transients for metal
deposition is developed by using different established theoretical
models.16-22 Hyde and Compton reviewed the subject in 2003.23
More recently the contribution of secondary redox processes occur-
ring simultaneously with metallic nucleation and growth and its ef-
fects on the shape of the transients has been discussed.24,25
In a previous paper we reported a voltammetric and structural
study on the zinc electrodeposition onto a steel substrate in the pres-
ence of different surfactants: anionic sodium dodecylsulfate ͑SDS͒,
cationic dodecyltrimethylammonium bromide ͑CTAB͒, and non-
ionic octylphenolpoly͑ethyleneglycolether͒n, n = 10 ͑Triton X-100͒.
The results show that the zinc deposition occurs at potential values
Experimental
The electrochemical experiments were carried out in a three-
electrode glass cell with a stainless steel disk ͑AISI 316, diameter of
12 mm͒ as working electrode, a platinum mesh as counter electrode,
and a commercial saturated calomel electrode ͑SCE͒ as reference.
All potentials are reported with respect to this reference. Before
each experiment, the stainless steel disks were washed with deter-
gent, etched in HNO3, and finally polished to a mirror finish using
silica powder and ultrasonically cleaned with Millipore Milli-Q ul-
trapure water ͑18 M⍀͒ during 10 min. The electrochemical cell was
connected to a Voltalab 32 Radiometer apparatus connected to an
IMT 102 interface, controlled by a personal computer through the
VoltaMaster 2 software.
The electrolyte solution containing 0.06 mol dm−3 ZnSO4·7H2O
and 1.2 mol dm−3 MgSO4·6H2O was prepared from Millipore
Milli-Q water using Merck analytical grade reagents. Separately,
1.0 mmol dm−3 cetyl trimethyl ammonium bromide ͑CTAB, Ald-
rich͒, 10 mmol dm−3 sodium dodecyl sulfate ͑SDS, Sigma͒, and
0.06 mmol dm−3 octylphenolpoly͑ethyleneglycolether͒n, n = 10
͑Triton X-100, Fluka͒ were added to this solution. All chemicals
were analytical grade. The surfactants were used without further
purification. For all solutions the pH value was 4. Before each ex-
periment, the solution was deaerated with N2 for 15 min.
AFM images were recorded in tapping mode in a Nanoscope IIIa
multimode microscope ͑Digital Instruments, Veeco͒. Measurements
were performed in air using etched silicon cantilevers ͑RTESP,
Veeco͒ with a resonance frequency of ca. 300 kHz.
A JEOL scanning electron microscope ͑model JSM-6301F͒ was
used to characterize the surface morphology of the zinc electrode-
posits. The energy of the primary electron beam was 15 keV.
*
Electrochemical Society Active Member.
z E-mail: aboavida@fc.ul.pt
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
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Figure 1. Cyclic voltammograms for a steel electrode in 0.06 mol dm−3 ZnSO4 + 1.2 mol dm−3 MgSO4 ͑full line͒ and 1.2 mol dm−3 MgSO4 ͑dotted line͒. Scan
rate 10 mV s−1. XRD patterns of electrodeposited samples prepared at −1.12 and −1.27 V for tdep = 90 and 60 min, respectively, in surfactant-free solution.
Results and Discussion
indicate that the steel presents a smooth enough surface ͑root-mean-
square roughness on a 10 ϫ 10 ␮m2 scan area was 7.9 nm͒, appro-
priated for the analysis of the very initial formation of the zinc
electrodeposits by AFM.27
The samples observed by AFM were prepared potentiostatically
in the UPD ͑C1͒ and overpotential deposited ͑OPD͒ ͑C2͒ regions.
In a previous paper we presented a voltammetric study of the Zn
deposition on steel in the presence and absence of various
surfactants.26 This study allowed the definition of the deposition
potential region for the different studied systems.
Figure 1 shows a cyclic voltammogram for zinc deposition on a
steel substrate from the surfactant-free solution and the correspond-
ing X-ray diffractograms of the deposit, prepared at −1.12 and
−1.27 V during 90 and 60 min, respectively. The voltammogram
main features are the sharp cathodic peak C2 and the corresponding
anodic stripping peak A. A shoulder C1, without anodic counterpart
is observed between −0.8 and −1.2 V vs SCE. By means of X-ray
diffraction analysis it was possible to identify the solid phases
formed on peaks C1 and C2. For all the studied systems, the samples
prepared potentiostatically, in the region of peak C2, were consti-
tuted by zinc.
In what concerns the potential interval, where peak C1 appears,
the studies were not totally conclusive. Taking into account that we
are dealing with an underpotential deposition ͑UPD͒ region it is
important to get a better understanding of the processes occurring on
that region. For this purpose, potentiostatic studies and AFM analy-
sis were performed.
Underpotential deposition.— Figure 2 presents AFM images ob-
tained, at 2 ϫ 2 ␮m2 scanned area, from the deposits prepared for
6 s, at the peak C1 potential region, more precisely, at −1.00, −1.05,
−1.10, and −1.12 V vs SCE, in the absence of surfactants.
As it can be seen, the coverage of the steel surface, as well as the
shape and size of the clusters, strongly depends on the applied po-
tential. At the lowest value, −1.00 V, the presence of isolated, small
zinc circular clusters, with different size, dispersed at the substrate
surface is observed. Cross-sectional profiles of the surface, along a
line, show that the cluster width is always greater than its height,
indicating a tendency toward two-dimensional ͑2D͒ growth.
With increasing potential, more clusters appear and they merge
into larger ones. At the highest potential value ͑−1.12 V͒ a more
complex morphology is found. The analysis of this sample by scan-
ning electron microscopy ͑SEM; Fig. 3͒ in parallel with the obser-
vation of the corresponding three-dimensional ͑3D͒ AFM images
͑data not shown͒, enables a flat deposit zone and different-sized
vertical structures to be observed. Small, circular clusters and crys-
tallites with an almost hexagonal shape form the flat zone. The last
ones probably correspond to zinc crystallites with a ͑00.2͒ crystal-
lographic orientation and are aligned parallel to the substrate.
This is an expected result considering the previous structural
characterization.26 The 3D images suggest that the “vertical” struc-
Ex situ AFM morphological studies.— AFM was used in order
to look closely into the surface morphology of the zinc deposits on
the steel substrate, in the presence and absence of various surfactant
molecules, in the early stages of the deposition process.
AFM examination of the pretreated steel surface was performed
before the deposition process with the purpose of determining the
substrate surface roughness and morphology. The obtained images
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
Figure 2. ͑Color online͒ 2D AFM images
͑2 ϫ 2 ␮m2͒ of Zn electrodeposits on
steel substrate prepared at different poten-
tials, in peak C1 region, in surfactant-free
solution.
tures exhibit polyhedral shapes, namely, pyramidal. Because we are
dealing with ex situ analysis, it is impossible to know exactly which
growth mechanism was involved in the formation of the hexagonal
and pyramidal agglomerates.
These results obtained near the equilibrium potential indicates
that the zinc deposition could follow the Stranski-Krastanov growth,
which predicts the nucleation and growth of the 3D clusters on the
top of the 2D phase. This result is similar to the observations made
by Kibler et al. for rhodium electrodeposits from in situ scanning
tunneling microscopy investigations.28
Based on the previous discussion it can be concluded that the
nucleation and growth of Zn deposits occurs at potential values less
negative than both the equilibrium and thermodynamic potential,
−1.14 V and −1.05 V vs SCE, respectively, as it has been proposed
previously.26 In the potential region, where peak C1 appears, 2D
nucleation occurs for the less negative values and a 2D–3D nucle-
ation transition is observed for more negative potentials. The 2D
growth in the early stages of Zn deposition can be understood in
terms of a relation between the Zn deposit close-packed arrange-
ment and the planes of the steel substrate. This is in accordance with
the literature which reports that in the initial stages of electrodepo-
sition, the Burgers orientation relationship always holds between
zinc crystals and the ferrite substrate.29,30
A similar study has been performed in the presence of surfac-
tants. AFM images obtained for the deposits prepared at −1.120 V
͑peak C1͒, for 6 s, in the presence of SDS, CTAB, and Triton
X-100, show that the deposit morphology is very similar to the one
obtained in the absence of surfactant molecules ͑see Fig. 4͒. A flat
deposit and a random distribution of different sized vertical aggre-
gates were observed, which indicates that progressive nucleation
takes place. These results demonstrate that the presence of the sur-
factant molecules does not have a great effect on the zinc deposition
in this potential region.
Overpotential deposition.— The AFM images obtained from de-
posits prepared at −1.27 V ͑peak C2͒ during 3 s are shown in Fig. 5.
All samples present a similar and peculiar morphology, character-
ized by randomly arranged long, curved ridges. No isolated nuclei
can be detected. The analysis of Fig. 5 shows that the vertical
growth is promoted against the lateral growth. For the deposit pre-
pared in the absence of surfactant molecules, the morphology can be
assigned to the formation and adsorption of hydrogen bubbles, on
the steel substrate and growing deposit, which inhibit and/or condi-
tion the zinc nucleation due to physical interaction between hydro-
Figure 3. SEM image of Zn electrodeposit on steel substrate prepared in
surfactant-free solution at −1.12 V during 6 s.
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
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Table I. Root-mean-square roughness „Rq… for the electrodepos-
its prepared at −1.27 V, obtained from a 10 Ã 10 ␮m2 area.
Surfactant-
Sample
free
SDS
137
CTAB
123
Triton X-100
105
Rq ͑nm͒
128
age of the active sites, the deposit morphology is similar to the ones
obtained in the presence of SDS and in the surfactant-free solution.
When Triton X-100 is added to the bath, a less compact, ridged
morphology is observed. This effect is more obvious for high con-
centrations of the nonionic surfactant. This should be related with an
easier adsorption of the Triton X-100, at the steel surface and/or at
the growing deposit, in this potential region.26
This result is confirmed by the data shown in Table I where the
electrodeposit root-mean-square roughness ͑Rq͒, obtained for a
10 ϫ 10 ␮m2 area, is compared.
The AFM morphological study performed at the C2 peak region
indicates that the presence of the surfactant molecules affects the
deposition process in the OPD region.
Figure 4. ͑Color online͒ 2D AFM amplitude image ͑1 ϫ 1 ␮m2͒ for a flat
zone of the Zn electrodeposit prepared in surfactant-free solution at −1.12 V,
during 6 s.
Potentiostatic studies.— Potentiostatic studies were used to elu-
cidate the nucleation mechanism of zinc onto a steel substrate in the
UDP and OPD regions, in the presence and the absence of surfactant
molecules.
gen bubbles and the steel substrate as well as the zinc growing
deposit.14,31 According to Grujicic and Pesic, in the case of the
nickel electrodeposition the hydrogen evolution is responsible for
the aggregation of small clusters into larger ones due to physical
interaction between hydrogen bubbles and the nickel clusters.15
For the surfactant-containing systems, adsorption of the surfac-
tant molecules occurs, in addition to the previous effect, as sug-
gested by the earlier voltammetric studies.26 A detailed analysis of
these images shows exclusion areas on the substrate surface and that
the coverage by the deposit crystallites depends on the type of sur-
factant in the solution. Although electrostatic interactions between
the CTAB and the electrode surface are expected, leading to a block-
Underpotential deposition.— In order to obtain kinetic informa-
tion on the UPD region, potential-step measurements were done in
the interval where the voltammetric peak C1 appears. Figure 6a
shows a set of current-time transients obtained from surfactant-free
solutions, when the potential is stepped from the open-circuit poten-
tial to −1.00, −1.05, −1.10, or −1.12 V vs SCE. In the inset, current
transients obtained in the absence of Zn2+ ions under similar experi-
mental conditions are presented. In this case no current peaks are
observed, as expected. Furthermore, the current densities increase
Figure 5. ͑Color online͒ 3D AFM images
of electrodeposits prepared onto a steel
substrate, in the absence and in the pres-
ence of the studied surfactant molecules,
at −1.27 V.
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
Figure 6. ͑Color online͒ ͑a͒ Potentiostatic current transients for the electrodeposition of zinc onto a steel substrate, obtained from 0.06 mol dm−3 ZnSO4 and
1.2 mol dm−3 MgSO4 at different potentials in the UPD region. The inset presents potentiostatic current transients obtained at the same potentials in
1.2 mol dm−3 MgSO4 solution. ͑b͒ Theoretical and experimental dimensionless plots ͑i/im͒ vs t/tm for Zn deposition onto steel substrate.
with applied potential. This result suggests the occurrence of a redox
process, namely, hydrogen evolution, as the previous voltammetric
study has shown.26
Fig. 1 of Ref. 34, shows that the agreement is good for the falling
part but at short times a deviation is apparent. The reason for this
may well be that initially other processes may take place on the
substrate surface, namely, hydrogen adsorption and/or evolution.
Figure 7 shows a set of current-time transients obtained from
Zn2+ solutions containing the surfactant molecules ͑a͒ SDS, ͑b͒
CTAB, and ͑c͒ Triton X-100, at different potentials, within the zinc
UPD region. For all the systems the transient shapes are similar to
the ones obtained from surfactant-free solutions. As it can be seen,
the maximum current density is lower and tm longer for the systems
containing CTAB or Triton X-100 than for the SDS system. A com-
parison between the experimental current transient in a dimension-
less form, with the Bewick, Fleischmann, and Thirsk theoretical
model, was performed. As expected, the transients do not show a
good fit to the shape predicted.
Table II shows the charge density values under the peak for all
the systems at the different applied potentials. Examination of these
results showed that the charge density increases with the applied
potential. For potentials higher than −1.00 V the values are bigger
than those estimated for the deposition of one hexagonal close-
packed zinc monolayer: 523 ␮C cm−2. This result compares well
with the AFM analysis, particularly the results obtained at −1.12 V
͑Fig. 4͒, for which a multilayer growth is observed.
For the Zn2+-containing solution, each transient presents a well-
defined current maximum followed by a current decay that stabilizes
on a steady-state current plateau. With increasing deposition poten-
tial the maximum current and the steady-state current plateau in-
crease.
The behavior of the current maximum, particularly the shift to
shorter times and higher current densities with increasing potential,
suggests the existence of nucleation and growth processes in the
UPD region.32 This conclusion is supported by the AFM morpho-
logical study, which shows that in this potential region 2D zinc
clusters are formed.
A direct comparison of the experimental current transient in a
dimensionless form, i/im vs t/tm, with the theoretical curves given
by the well-known model developed by Bewick, Fleischmann, and
Thirsk for potentiostatic growth of a two-dimensional circular
island,33 has been performed.
This comparison is presented in Fig. 6b which shows that the
theoretical curves for 2D nucleation do not fit the experimental re-
sults. One possible reason for this behavior could be related to other
parallel processes occurring simultaneously that are beyond the the-
oretical model used.
Abyaneh and Fleischmann proposed a description of the current-
time transient due to 2D nucleation and growth together with hydro-
gen evolution, on the top and at the edge of the growing centers.34,35
An attempt to qualitatively compare our experimental transients
with Abyaneh and Fleischmann’s predicted transient, presented in
Overpotential deposition.— A family of current-time transients
obtained from surfactant-free solutions, at different potentials within
the zinc bulk deposition region, is depicted in Fig. 8a. The transients
present the typical response of a nucleation and growth process, i.e.,
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
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Figure 7. Potentiostatic current transients obtained from 0.06 mol dm−3 ZnSO4 and 1.2 mol dm−3 MgSO4 solution containing ͑a͒ SDS, ͑b͒ CTAB, and ͑c͒
Triton X-100 at different potentials in the UPD region.
a rising current, due to the nucleation and growth of the nuclei
followed by a falling portion due to linear diffusion to a planar
electrode.16
Current transients were recorded, also, in the base electrolyte,
i.e., in the absence of Zn2+ ions, under similar experimental condi-
tions ͑inset͒. No current peaks are observed, as expected. Further-
more, the current densities increase with applied potential and are of
the same order of those measured, in the presence of Zn2+ ions, at
times longer than that corresponding to the current maximum. It
appears that the falling currents, recorded in the base electrolyte, are
related with other faradaic processes, namely, proton reduction and
hydrogen evolution. This result suggests that the proton reduction
reaction and hydrogen bubble formation take place simultaneously
Table II. Transient charge densities for the studied systems.
−q/mC cm−2
Surfactant-
−E/V vs SCE
free
SDS
CTAB
Triton X-100
1.00
1.05
1.10
1.12
0.669
1.185
1.987
2.242
0.562
1.541
2.025
2.280
0.261
0.958
1.209
1.707
0.359
0.878
1.579
1.885
Figure 8. ͑Color online͒ ͑a͒ Potentiostatic current transients for the electrodeposition of zinc onto a steel substrate, obtained from 0.06 mol dm−3 ZnSO4 and
1.2 mol dm−3 MgSO4 at different potentials in the bulk deposition region. The inset presents potentiostatic current transients obtained at the same potentials in
2
1.2 mol dm−3 MgSO4 solution. ͑b͒ Theoretical and experimental dimensionless plots ͑i/im͒ vs t/tm for Zn deposition onto steel substrate.
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
Figure 9. Potentiostatic current transients obtained from 0.06 mol dm−3
ZnSO4 and 1.2 mol dm−3 MgSO4 solution containing ͑a͒ SDS, ͑b͒ CTAB,
and ͑c͒ Triton X-100 at different potentials in the bulk deposition region.
Figure 10. ͑Color online͒ Theoretical and experimental dimensionless plots
͑i/im͒ vs t/tm for Zn deposition onto a steel substrate in the presence of
surfactants ͑a͒ SDS, ͑b͒ CTAB, and ͑c͒ and Triton X-100 ͑data from Fig. 9͒.
2
with the zinc nucleation. However the well-defined maximum on the
current-time transients clearly indicates that zinc nucleation is the
preferential process.
This assumption is in accordance with the recent publications of
Palomar-Pardavé et al.24 and Milchev and Zapryanova25 which have
discussed the influence of secondary redox processes taking place
prior to and simultaneous with the nuclei formation and their con-
tribution to the total current.
It would be expected that the growth of the zinc films onto the
steel substrate, in the investigated conditions ͑metallic ion concen-
tration and potential range͒, occur by nucleation and 3D growth
controlled by diffusion.16 The transients presented in Fig. 8a appear,
at first sight, to resemble the predicted theoretical curves. In fact, the
current at the maximum increases in magnitude as the potential is
stepped to more negative values and inversely, as tm decreases. A
closer inspection shows that at longer times, after the maximum, the
current for the different growth potentials does not decay to a single
limiting current according to the Cottrell equation, in disagreement
with the model proposed by Scharifker and Hills ͑SH model͒.14 The
reason for this may well be that the SH model is not adequate to
describe the experimental transient after the maximum.
Assuming that the hydrogen evolution contribution is minor
initially,6,36 the fitting of the experimental transients to the dimen-
sionless theoretical curves derived by Scharifker and Hills for 3D
growth, instantaneous and progressive nucleations were attempted.16
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
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Figure 11. ͑Color online͒ Comparison between the experimental transient obtained during deposition of zinc at −1.27 V and the theoretical ones ͑solid line͒
obtained from the nonlinear fitting ͑Eq. 2͒ for the systems studied.
Dimensionless plots of ͑i/im͒2 vs t/tm for the experimental transients
Trejo et al. obtained similar results for the electrodeposition of zinc
onto Pt in the presence of several ethoxylated additives of different
shown in Fig. 8a are presented in Fig. 8b, with the corresponding
theoretical curves. The rising parts of the transients follow the the-
oretical curve for instantaneous nucleation, up to the current maxi-
mum, in good agreement with the results obtained by others.4,5 In
what concerns the falling parts, the data lie between the two theo-
retical curves, approaching the instantaneous one for the most nega-
tive growth potential. For the less negative potentials, the nucleation
process varies with time from an instantaneous to a progressive one.
Figure 9 shows current-time transient families obtained from
Zn2+ solutions containing the surfactant molecules: ͑a͒ SDS, ͑b͒
CTAB, and ͑c͒ Triton X-100.
The transients are similar to the ones recorded in the absence of
surfactant molecules, consistent with nucleation and 3D controlled
by diffusion.16 In the same way, at long times, the current for the
different growth potentials do not decay to a single current. The
maximum current is lower than in surfactant-free solution and de-
creases in the following order: SDS, CTAB, and Triton X-100. The
inverse variation is observed for the time of the maximum current
density. These results are expected, taking into account the previous
voltammetric studies,26 and are interpreted by the decrease of the
nucleation and growth rates due to the presence of the surfactant
molecules at the electrode surface, mostly CTAB and Triton X-100.
molecular weights.4
Figure 10 compares data from the experimental transients for the
different systems with the dimensionless plots for instantaneous and
progressive nucleation, where the differences in the nucleation
mechanism are clearly seen.
The data fits well the instantaneous nucleation in the case of the
SDS-containing system, for the whole transient, whereas in the pres-
ence of CTAB the transients follow the plot for progressive nucle-
ation. With regard to Triton X-100, the data lie between the two
calculated curves for the entire transient. This change in the nucle-
ation mechanism may occur due to CTAB and Triton X-100 adsorp-
tion which prevents the Zn2+ species from approaching the steel
electrode surface and consequently, a competition for sites takes
place. These results point out the dominant role played by the pres-
ence of the surfactant molecules in the zinc nucleation mechanism.
Similar effects have been reported for Zn electrodeposition in the
presence of other surfactants.4-6
Although the SH model usually used in the form of dimension-
less curves provides a satisfactory diagnostic for the “progressive/
instantaneous” character of the process, in practice it is difficult to
extract the nucleation rate constant ͑A͒ and the number density of
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D460
Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
active sites ͑No͒, because the two parameters appear as a product in
the equation that relates the current density and time, and the esti-
mation of the electrocrystallization parameters is based only on a
single point, the maximum current.23
More recently, Heerman and Tarallo ͑HT͒ proposed a new model
that assumes that the expansion of the diffusion layer should be a
function not only of time, but also of the nucleation rate constant.20
It is commonly accepted that the experimental transients can be
described and the kinetics parameters extracted using this theoretical
model for diffusion-controlled crystal nucleation and 3D
growth.5,6,20,21,37
The analytical expression for the current transient derived by the
HT model is given by
⌽
−͑1/2͒
i͑t͒ = zFDc͑␲Dt͒
͓1 − exp͑−␲kN Dt⌰͔͒
͓1͔
ͩ ͪ
0
⌰
where
1/2
8␲cM
ͩ ͪ
␳
k =
1 − exp͑−At͒
⌰ = 1 −
At
and
1/2
⌽ = ⌽͓͑At͒
͔
related to Dawson’s integral, is given by
1/2
͑At͒
exp͑−At͒
2
⌽ = 1 −
exp͑␭ ͒d␭
͵
1/2
͑At͒
0
where A is the nucleation rate constant, N0 is the number density of
active sites on the surface of the substrate, D is the diffusion coef-
ficient, c is the concentration of the metal ions in solution, and M
and ␳ are the molecular weight and density of the metal, respec-
tively.
Figure 12. Variation of kinetic parameters A and No with the applied poten-
tial. The vertical bars represent the standard deviation.
Commercial software based on the Levenberg-Marquardt algo-
rithm was used to obtain the parameters, A, No, and D according to
Eq. 1 for the current density due to 3D growth diffusion control
͑i3D͒. The agreement between the experimental transients and the
nonlinear fit is very satisfying. In order to increase the correlation
values, the contribution of the double-layer charging current ͑idl͒ has
been considered for the total current density
indicate that for the surfactant-free system an instantaneous nucle-
ation mechanism ͑large A and small No͒ occurs, while for CTAB
and Triton X-100 a progressive nucleation ͑small A and large No͒
takes place.39 This is in accordance with the dimensionless plot
analysis.
With regard to the nucleation rate, the values obtained for the
systems containing CTAB and Triton X-100, in comparison to the
others, indicate a blocking effect of the surfactants and/or a decrease
of the ion rate transferred across the electric double layer. A similar
interpretation of the influence of different additives on the Zn nucle-
ation and growth has been given by other authors.4,5 These results
are in accordance with the AFM analysis ͑Fig. 5͒, where the effect
of the surfactants, namely, CTAB and Triton X-100, in the bulk zinc
deposition at −1.27 V is clearly observed.
itotal = idl + i3D
͓2͔
Based on the literature, the descending current observed at the initial
stages of the transients corresponds to the double-layer charging
effect. Hölzle et al. showed that such a charging effect could be
correlated quantitatively to the adsorption-desorption process of ions
on the electrode surface.38 The estimation of the double-layer charg-
ing effect was based on a Langmuir-type adsorption-desorption
equilibrium and is given by
The values of the Zn2+ ion diffusion coefficient, D, estimated
from the nonlinear fitting ͑Table III͒ show the expected variation
caused by the presence of the surfactants, and they are in good
agreement with earlier reports.31 They are lower, however, than
those previously calculated from voltammetric data.26 These differ-
idl = kads exp͑−kdest͒
͓3͔
where kads = kdesQads
.
Figure 11 shows a comparison of the experimental current den-
sity transients obtained for all the systems at −1.27 V vs SCE with
the theoretically generated curve from the nonlinear fitting of the
experimental data to Eq. 2. These results clearly show that the ap-
plied model successfully describes the experimental current tran-
sients.
Table III. Diffusion coefficients for the Zn2+ species estimated
from the nonlinear fitting.
The variation of the kinetic parameters A and No, obtained from
the fitting, with the applied potential is presented in Fig. 12. In
general, both A and No values increase for more negative deposition
potentials. It is also observed that the highest A value was obtained
in the absence of surfactants. For the surfactant-containing systems
the following order is verified: SDS, CTAB, and Triton X-100. An
inverse behavior is encountered for the No parameter. These results
D ϫ 106/cm−2 s−1
Surfactant-
−E/V vs SCE
free
SDS
CTAB
Triton X-100
1.25
1.27
1.29
3.36
4.02
4.50
1.87
2.29
3.95
1.97
2.54
2.49
2.00
2.33
2.36
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Journal of The Electrochemical Society, 154 ͑9͒ D452-D461 ͑2007͒
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Zinc electrodeposition onto a steel substrate using both electro-
chemical and AFM techniques has been studied.
AFM analysis shows that Zn electrodeposition, in the absence of
surfactants, begins in the UPD region with the formation of nano-
metric circular clusters homogeneously distributed on the steel sur-
face. This result indicates a strong interaction between the steel sub-
strate and the growing deposit. In the presence of SDS, CTAB, and
Triton X-100 no meaningful changes of the electrodeposit morphol-
ogy are observed in this potential region.
In all cases free areas on the steel surface coexist with small
aggregates and the impingement of growing aggregates, indicating
that progressive nucleation takes place. The Zn UPD is confirmed by
the chronoamperometric measurements performed in this potential
region.
With regard to the bulk deposition region, where the hydrogen
evolution goes together with the zinc deposition, the analysis of the
current-time transients indicates that the Zn electrodeposition occurs
by instantaneous nucleation and 3D growth controlled by diffusion
in the surfactant-free solution and in the presence of SDS. When the
CTAB and Triton X-100 were added to the bath, change from in-
stantaneous to progressive nucleation arises as a consequence of the
simultaneous adsorption of the surfactant that inhibits the nucleation
sites. The kinetic parameter values obtained from the HT model are
in accordance with the dimensionless analysis of the transients.
AFM confirms the effect of surfactants on the zinc bulk deposition.
In summary, this work shows that the application of different
techniques to the study of zinc deposition has been rewarded by a
considerable improvement in our understanding of the early stages
of deposition, in particular, allowing us to confirm Zn UPD onto a
steel substrate.
Acknowledgments
Financial support from Fundação para a Ciência e a Tecnologia,
Portugal, is gratefully acknowledged. A.G. acknowledges financial
support and a SFRH/BPD/11605/2002 grant from Fundação para a
Ciência e a Tecnologia, Portugal.
Universidade de Lisboa assisted in meeting the publication costs of this
article.
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