Zn electrodeposition in the presence of surfactants. Part I. Voltammetric and structural studies

Article abstract of DOI:10.1016/j.electacta.2006.06.025

The zinc electrodeposition onto steel substrates in the presence of surfactants with different charged head groups, namely anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (CTAB), and non-ionic octylphenolpoly(ethyleneglycolether)_n, n = 10 (Triton X-100) was studied by cyclic voltammetry. The effect of the switching potential and scanning rate on the deposition process was investigated. The structural characterisation and the chemical composition of the samples prepared potentiostaticaly, in the potential range where the voltammetric cathodic peaks appear, was performed by X-ray powder diffraction (XRD) and by energy-dispersive X-ray analysis (EDS), respectively. The experimental results show that the voltammetric behaviour, namely the deposition potential depends on the presence, nature and concentration of the tested surfactants. Zn deposition occurs at potential values more positive than the estimated equilibrium potential, peak C1, simultaneously with hydrogen formation. This fact is confirmed by XRD measurements. Zn bulk deposits prepared in the absence of surfactants and in the presence of SDS are more crystalline and with a higher grain size than the ones obtained in the presence of CTAB and Triton X-100. These facts may be justified by an increase on the overpotential deposition as the electrochemical study confirms.

Full text of DOI:10.1016/j.electacta.2006.06.025

Electrochimica Acta 52 (2006) 863–871
Zn electrodeposition in the presence of surfactants
Part I. Voltammetric and structural studies
A. Gomes , M.I. da Silva Pereira1
∗
CCMM, Departamento de Qu ´ı mica e Bioqu ´ı mica da Faculdade de Ci eˆ ncias da Universidade de Lisboa, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal
Received 23 February 2006; received in revised form 20 June 2006; accepted 21 June 2006
Available online 27 July 2006
Abstract
The zinc electrodeposition onto steel substrates in the presence of surfactants with different charged head groups, namely anionic sodium dode-
cylsulphate (SDS), cationic dodecyltrimethylammonium bromide (CTAB), and non-ionic octylphenolpoly(ethyleneglycolether) , n = 10 (Triton
n
X-100) was studied by cyclic voltammetry. The effect of the switching potential and scanning rate on the deposition process was investigated. The
structural characterisation and the chemical composition of the samples prepared potentiostaticaly, in the potential range where the voltammetric
cathodic peaks appear, was performed by X-ray powder diffraction (XRD) and by energy-dispersive X-ray analysis (EDS), respectively. The
experimental results show that the voltammetric behaviour, namely the deposition potential depends on the presence, nature and concentration of
the tested surfactants. Zn deposition occurs at potential values more positive than the estimated equilibrium potential, peak C1, simultaneously with
hydrogen formation. This fact is confirmed by XRD measurements. Zn bulk deposits prepared in the absence of surfactants and in the presence
of SDS are more crystalline and with a higher grain size than the ones obtained in the presence of CTAB and Triton X-100. These facts may be
justified by an increase on the overpotential deposition as the electrochemical study confirms.
©
2006 Elsevier Ltd. All rights reserved.
Keywords: Zinc; Surfactants; Electrodeposition; Cyclic voltammetry; Crystal structure
1
. Introduction
bright deposits [2–5]. Other additives such as polyacrylamine
6], thiourea [6] and benzylideneacetone [7] have been also used
[
It is well known that zinc electrodeposits on metals have
for zinc deposition. Common effects of the organic additives are
changes in the preferred deposit orientation, morphology and an
increase in the deposition overpotential.
The specific activity of the surfactants are generally under-
stood in terms of adsorption at the cathode surface, during
deposition, and depends on the concentration of the surfac-
tant molecules. When the concentration approaches the critical
micelle concentration (cmc) the formation of bilayers or multi-
layers at the electrode interface occurs [8].
special relevance due to their anticorrosive properties by the
formation of passive layers on contact with air, and in aqueous
solution. Moreover they are suitable metallic matrix to incorpo-
rate suspended inert particles [1]. These capacities are strongly
related to the morphological and structural characteristics of
the films, which depend on the electrodeposition conditions,
namely, the presence of organic additives at the deposition bath
and the current/potential profile applied.
The introduction of small amounts of adequate organic addi-
tives, in the electrodeposition bath, results in beneficial quality
changes on the deposits, namely the homogeneity. Surfactants
are commonly used in zinc electrodeposition to control the
metallic crystal shape and size, in order to produce smooth and
The adsorption of surfactants aggregates onto electrodes can
have large effects on the kinetics of the electron transfer and
consequently on the electrodeposition process.
The effect on the electron transfer rates includes blocking of
the active sites by the surfactants, and electrostatic interactions
between electroactive species and adsorbed surfactants [9]. Due
to those effects it is possible to modify the crystals growth mode
and tailor the morphology and structure of the electrodeposits.
Our previous work on the electrodeposition of zinc onto steel
in the presence of surfactants has shown that the charge of the
∗
Corresponding author. Tel.: +351 21 750 00 75; fax: +351 21 750 00 88.
E-mail address: abmg@fc.ul.pt (A. Gomes).
ISE member.
1
0
013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.06.025

8
64
A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
surfactant headgroups has a marked effect on the films compo-
sition, structure and morphology [10,11].
The electrodeposited films were prepared potentiostatically
in stirring solution, at room temperature under a slight N2 flux.
After, the samples were thoroughly rinsed in Millipore water,
dried in nitrogen stream and immediately transfer to a desic-
cator.
Therefore, we have undertaken a voltammetric study of the
zinc electrodeposition onto steel in acid aqueous solutions in the
presence of cationic, anionic and non-ionic surfactants. Voltam-
metric measurements were used, as an exploratory technique, to
evaluate the effect of the added surfactants on the zinc deposition
mechanism. The use of this method for the electrodeposition
studies was due to its potentialities, namely the possibility of
bothdepositionandstrippingbeingobservedinonesingleexper-
iment. Moreover the effect of blocking additives, due to its
adsorption on the cathode surface, may be estimated from the
current values measured in the presence and in the absence of
the organic molecules [12]. The effect of the switching poten-
tial and the scanning rate, on the zinc deposition process, in the
presence and absence of the surfactants, was analysed.
To complement this electrochemical study, a structural char-
acterisation of the samples obtained potentiostatically in the
potential range where the voltammetric cathodic peaks appear
was performed by X-ray powder diffraction (XRD). The sam-
ples chemical composition was obtained by energy-dispersive
X-ray analysis (EDS).
The present work illustrates how the presence, nature and
concentration of surfactant, like cationic CTAB, anionic SDS
and non-ionic Triton X-100, influences the deposition process
and consequently the structural characteristics and composition
of the electrodeposits.
Attempts to correlate electrochemical deposition conditions
and deposit structure were done. Morphological studies of the
films as well as the nucleation and growth mechanisms will be
described in a subsequent paper.
Electrodeposits X-ray diffraction analysis was carried out
using a Philips X-ray diffractometer (model PW 1710) with Cu
K␣ radiation (λ = 0.15604 nm), working at 30 mA and 40 kV.
◦
The diffractograms were obtained in the 2θ range of 20–80
using a 0.02 step and acquisition time of 2 s/step.
◦
A JEOL scanning electron microscope (model JSM-6301F)
coupled with an energy-dispersive spectroscopic (Noran/
Voyager) were used to analyse the elemental composition of the
electrodeposited samples. The energy of the primary electrons
beam was 15 keV.
3. Results and discussion
3.1. Voltammetric studies
3.1.1. Studies on Zn2+ solutions
A typical cyclic voltammogram of steel, recorded in
−
3
2+
0.06 mol dm Zn solution, between the open circuit poten-
tial (−0.4 V) and −1.4 V versus SCE, is shown in Fig. 1(a).
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 versus SCE. A voltammogram obtained in the absence
of Zn2 ions is also presented.
+
The comparison of the voltammograms obtained in the pres-
ence and absence of Zn2 ions suggests that shoulder (C1) is due
to hydrogen evolution since this reaction occurs significantly at
these potentials when the Zn2 ions are absent. Others authors
have found a similar result [15–17].
+
+
2
. Experimental
The electrochemical experiments were carried out in a three-
In order to study in more detail the shoulder C1, a scan was
performed between −0.3 and −1.12 V (Fig. 1(b)). As it can be
seen, the current increases at −0.85 V reaching a maximum near
−1.05 V, following the attainment of a plateau. On the positive
sweep no anodic peak is observed, indicating that the reduction
process is irreversible.
Similar data were obtained by Kim et al. for the zinc depo-
sition at iron electrodes, and explained in terms of Zn underpo-
tential deposition [18], although no anodic peak was observed,
as it is expected for underpotential metallic deposition [19].
In what concerns peak C2, centred at −1.26 V, it corresponds
to the zinc bulk deposition. In this potential region the hydrogen
evolution also occurs as it is detected by direct observation, and
described by the reaction [20]:
electrode glass cell with a stainless steel disc (AISI316, diameter
of 12 mm) as working electrode, a platinum mesh as counter
electrode and a commercial satured calomel electrode (SCE)
as reference electrode. All potentials are reported with respect
to this reference. Before the experiments, the stainless steel
discs were polished with silicon carbide, sonicated for 5 min
and rinsed thoroughly with Millipore Milli-Q ultrapure water
18 Mꢀ). Cyclic voltammetric studies were done at room tem-
perature with an EG and G PAR model 263 potentiostat con-
nected to a Philips PM 8271 recorder.
(
−
3
The electrolyte solution containing 0.06 mol dm ZnSO4·
−
3
7
H2O and 1.2 mol dm MgSO4·6H2O was prepared from Mil-
lipore Milli-Q water using Merck analytical grade reagents. Sep-
arately, cetyl trimethyl ammonium bromide (CTAB, Aldrich),
sodium dodecyl sulphate (SDS, Sigma) and octylphenolpoly
−
−
2
H2O + 2e → H2 + 2OH
(
ethyleneglycolether)n, n = 10 (Triton X-100, Fluka) were added
Upon the sweep reversal, two current crossovers appear indi-
cating the formation of stable growth centres, at the substrate
surface [21].
to this base solution, without further purification. The surfac-
tant concentrations were selected taking into account the critical
micelle concentration values [13,14]. All chemicals were ana-
lytical grade. For all solutions the pH value was 4. Before each
experiment, the substrate surface was cleaned and the solution
deareted with N2 for 15 min.
The anodic stripping peak (A) centred at −1.0 V, is attributed
2+
to the oxidation of metallic zinc to Zn . This potential value
is in good agreement with the results referred in the literature
[22].

A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
865
−
−1
3
Fig. 2. Linear voltammograms for stainless steel electrode in 0.06 mol dm
−
3
−1
ZnSO4 + 1.2 mol dm MgSO4, at various sweep rates: 5 mV s (1), 10 mV s
−1 −1 −1
(
2), 20 mV s (3), 50 mV s (4) and 100 mV s (5). Inset: variation of peak
current density with sweep rate for peak C1 (a) and peak C2 (b).
Fig. 1. (a) Cyclic voltammograms obtained for stainless steel electrode in
−
3
−3 −3
MgSO4 (full line) and 1.2 mol dm
0
.06 mol dm
ZnSO4 + 1.2 mol dm
MgSO4 (dotted line). (b) Cyclic voltammogram obtained for stainless steel elec-
−
3
−3
trode in 0.06 mol dm ZnSO4 + 1.2 mol dm MgSO4 in the potential range of
0.4 and −1.2 V. Scanning rate 10 mV s− . The arrows indicate directions of
1
−
scan voltages.
Beyond peak A the current approaches to zero, indicating
that the majority of the deposited zinc has been removed from
the substrate surface.
The influence of the potential scan rate (ν) on the Zn depo-
sition is shown in Fig. 2. The data reveals that an increase in
sweep rate shifts the peak potentials to more negative values
and increases the peak current. Plots of logarithm of peak cur-
rent density (ip) versus logarithm of the sweep rate, give slope
values near 0.5 for the two cathodic peaks. The relation between
the cathodic peak current and the square root of scan rate (ν1
/2
)
is presented on the inset in Fig. 2. The variation is linear but the
lines do not pass through the origin. The linearity is expected
for a reduction processes that occurs under mass transfer con-
trol. However the intercept higher than zero indicates that an
additional process other than diffusion occurs [23].
Fig. 3 shows the voltammograms obtained at different switch-
ing potential (Eλ). It can be seen that the crossover potential
(Ec) remains constant when Eλ is successively switched from
−
1.35 to −1.25 V. A similar result was obtained by Diaz-Arista
et al. for Zn electrodeposition on Pt [7]. According to Fletcher
et al. this result indicates that the Zn nucleation is charge limited
under the studied conditions [21]. Since in this circumstances
−3
Fig. 3. Cyclic voltammograms for stainless steel electrode in 0.06 mol dm
−3
ZnSO4 + 1.2 mol dm MgSO4, showing the effect of the scan negative limits:
−
1
−1.25, −1.30 and −1.35 V. Scanning rate 10 mV s
.

8
66
A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
−
3
−3
−3
SDS (a),
Fig. 4. Cyclic voltammograms obtained for stainless steel electrode in 0.06 mol dm
ZnSO4 + 1.2 mol dm
MgSO4 containing 10 mmol dm
−
3
−3
0
.1 mmol dm CTAB (b) and 0.06 mmol dm Triton X-100 (c). The dotted lines refer to the voltammograms obtained in the same solution in the absence of
−1
surfactant. Scanning rate 10 mV s
.
the crossover potential is equal to the metal/metal-ion reversible
potential, it can be assumed that the equilibrium potential for
decrease slightly more pronounced for the system containing
Triton X-100. This can be explained by an inhibition effect on
the Zn deposition, by blocking the active sites available for the
deposition, due to the surfactant adsorption on the electrode sur-
face. This inhibition depends on the type and size of the organic
molecules and, on the specific interaction between the surfactant
and the substrate.
It is expected that electrostatic interactions occur between
the CTAB, a surfactant with positive head charge group, and the
electrode surface, leading to a blockage of the active sites. On
the other hand, Triton X-100 being a large non-ionic molecule
can minimize the rate of mass transfer. The presence of hydroxyl
groups would strengthen the adsorbability on the metal surface.
Moreover hydrocarbon tails mesh with each other to form a
film, which repels the aqueous solution, establishing a diffusion
barrier to the solution flow, towards the electrode surface [25].
Besides that, Triton X-100 adsorption depends on the applied
potential as the voltammograms indicate. Similar assumptions
were proposed for the copper deposition in the presence of
polyethyleneglycol (PEG) [26–28].
2
+
Zn/Zn is −1.14 V versus SCE. This value is more negative
thanthatcalculatedfromNernstequation(−1.05 VversusSCE),
due to crystallisation overpotential related to the steel substrate.
As expected, changes on the Eλ to less negative values led to
a decrease of the anodic peak size, accompanied by a negative
shift of the peak potential, due to an easier dissolution of the
deposit.
3
.1.2. Studies on Zn2+ solutions containing surfactant
molecules
Fig. 4 contrasts the voltammograms obtained in similar con-
−
3
ditions in the absence and presence of 10 mmol dm SDS (a),
0
−3
−3
.1 mmol dm CTAB (b) and 0.06 mmol dm Triton X-100
(
c). The presence of the surfactant molecules causes significant
changes on the shape of the voltammograms and also on the
current intensity values. These results indicate that the presence
and chemical nature of the surfactant affects both the Zn depo-
sition and stripping processes. The two current crossovers in
the cathodic region are evident, for all the systems, indicating
that nucleation and growth of Zn centres occur, for the tested
conditions. Moreover, the cathodic peak C1 is similar to the one
obtained in absence of surfactant molecules, suggesting that, for
this potential range, the presence of surfactant has no effect on
the reduction process.
A more detailed inspection of the data reveals that in the
presence of SDS (Fig. 4(a)) the cathodic voltammetric profile is
analogous to the one obtained in the absence of surfactant, pre-
senting peaks C1 at −1.05 V and C2 at −1.25 V. On the anodic
sweep, a smaller oxidation peak (A) is observed at about −1.0 V
versus SCE. This result pointed out that the rate of zinc disso-
lution is inhibited by the presence of this surfactant. This is
in accordance with the results obtained by Fuchs-Godec and
Dole cˇ ek show that the SDS acts as an inhibitor for the copper
dissolution [24].
Table 1 presents the voltammetric charge densities estimated
forpeaksC1, C2andAforallthestudiedsystems, byintegrating,
over the appropriate potential range.
For all the studied systems, peak C1 presents a charge den-
−
2
sity value around 15 mC cm , similar to the one obtained in
the absence of surfactants. This value is much higher than the
−
2
calculated one (523 ␮C cm ) for the deposition of a hexagonal
close packed zinc monolayer, indicating that it results from the
zinc deposition simultaneously with hydrogen formation. The
Table 1
Voltammetric peak charge densities for the studied systems
−2
qp (mC cm
)
Surfactant
None
SDS
CTAB
Triton X-100
C1
C2
A
−14
−101
80
−15
−98
46
−16
−72
40
−16
−57
34
When CTAB and Triton X-100 are present in the solution,
smaller currents for the peaks C2 and A are recorded, being the

A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
867
incomplete oxidation of the Zn electrodeposited. Lower values
were obtained in the presence of the surfactants, which can be
related to modifications occurred in the structure/morphology
of the Zn electrodeposited films.
Fig. 6 is a collection of cyclic voltammograms obtained in
the presence of the surfactants, which illustrates the effect of the
negative potential limit on the zinc deposition.
The data show that for all the systems, the crossover potential
does not depend on the potential limit and presents the values
−
1.13 V (SDS), and −1.16 V for the systems containing CTAB
and Triton X-100. For the later systems the crossover potential
is more negative than the obtained in the absence of surfactants.
This can be explained assuming that, in the interface region, the
−
3
Fig. 5. Linear voltammograms for stainless steel electrode in 0.06 mol dm
−3
−3
ZnSO4 + 1.2 mol dm
MgSO4 (—), containing 10 mmol dm
SDS (· · ·),
−3
−3
0
.1 mmol dm CTAB (– – –) and 0.06 mmol dm Triton X-100 (– · –).
2+
Zn concentration is lower in the presence of these surfactants,
2+
as well as higher activation energy for the Zn reduction.
Voltammograms obtained at various sweep rates, for the Zn
deposition in the presence of the surfactants showed that the
cathodic peak current increases with increasing sweep rate, for
all the systems. Logarithmic plots of ip versus ν gave slopes
approaching 0.5, for the peaks C1 and C2.
charge high values are in accordance with those obtained by
Casanova et al. [15] for the zinc deposition on steel in the poten-
tial region where peak C1 appears. They have concluded that
this peak is not only due to an underpotential deposition (UPD)
process. And, the UPD peak would be masked by the current
due to hydrogen evolution on steel.
In what concerns peak C2, the maximum charge value was
found for the surfactant free solution. For the surfactant contain-
ing systems it decreases in the following order SDS, CTAB and
Triton X-100. This result could be associated with the decrease
on the amount of deposited zinc when the surfactants are present
in the solution.
In the presence of CTAB and Triton X-100, the result should
be a direct consequence of the surface substrate coverage, by
the surfactant molecules, which is responsible for the displace-
ment of the reduction potential to more negative values and the
decrease on the cathodic current as Fig. 5 shows. This cathodic
polarization effect has been reported for other organic com-
pounds in metallic electrodeposition [29].
In what concerns peak A, a charge trend similar to peak C2
is observed. A decrease of 43, 50 and 58% was estimated for the
systems containing SDS, CTAB and Triton X-100, respectively.
The ratio between the anodic and cathodic density charge,
stripping efficiency, is smaller than 1 for all the systems. For the
surfactant free system, this lower stripping efficiency can be due
tothecathodicchargeconsumedbyprotonreductionand/ortoan
Linear plots of the cathodic peak current densities (ip) as
function of ν1 , for peak C1 and peak C2 were obtained as
expected for a reduction process that occurs under mass transfer
control.
/2
For the peak C1 this dependence, is not in accordance with
the published results that show that the zinc reduction in the pres-
ence of additives and, in the underpotential region, is adsorption
controlled (ip linear with ν) [30]. Our results could be explained
assuming that peak C1, is not only due to the zinc deposition,
but simultaneously hydrogen formation occurs. Similar slopes
for the three surfactants were obtained. The values agree well
with the one obtained in the surfactant free solution.
In contrast for peak C2, the slopes depend on the nature of
the surfactant, with positive intercepts in the absence of additives
and for the SDS containing solutions. For the other two additives
the intercept approaches zero.
Although the hydrogen evolution competes with the zinc
deposition in this potential region in the absence and presence
2
+
of surfactants, a comparison between the apparent Zn diffu-
sion coefficients was done. The apparent diffusion coefficients
calculated for the Zn2 species, for the different solutions, are
+
−
3
−3
−3
−3
−1
.
Fig. 6. Cyclic voltammograms for stainless steel electrode in 0.06 mol dm ZnSO4 + 1.2 mol dm MgSO4 containing 10 mmol dm SDS (a), 0.1 mmol dm
−3
CTAB (b) and 0.06 mmol dm Triton X-100 (c), recorded with different negative limits of the voltage scan: −1.25, −1.30 and −1.35 V. Scanning rate 10 mV s

8
68
A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
Table 2
Apparent diffusion coefficients for the Zn species, in the different studied
solutions, and respective correlation coefficients for the ip vs. ν plots
Table 3
2
+
Degree of surface coverage obtained at E = −1.30 V vs. SCE for the surfactants
1
/2
studied at different concentrations
6
2
−1
R2
Surfactant
10 D/cm s
Surfactant
SDS
c (mM)
θ
None
SDS
CTAB
Triton X-100
8.53
8.68
2.28
0.56
0.99973
0.99883
0.99941
0.99808
1
10
0.05
0.09
0
1
.1
0.48
0.83
CTAB
0
0
.06
.30
0.57
0.79
Triton X-100
presented in Table 2. The values obtained in the absence and in
the presence of SDS are of the same order of the one reported by
−
6
2
−1
2+
Turnham, 6.9 × 10 cm s , for Zn species in 0.1 M KNO3
The coverage values obtained at E = −1.30 V versus SCE are
presented in Table 3. As it was expected the lowest coverage
degree was obtained for the SDS.
[
31]. However, lower values were determined in the presence of
CTAB and Triton X-100, which can be explained by viscosity
changes on the solution as well as by the possible formation of
zinc complexes [32].
Fig. 7 shows the surfactant concentration effect on the zinc
deposition. It is clear that, changes on the SDS concentration
has no effect on the voltammetric profile which contrasts with
what happens for the CTAB and Triton X-100 containing solu-
tions. The concentration effect is more obvious for the potential
region where peak C2 appears, due to changes on the adsorbed
surfactant structure at more negative potentials.
Based in these findings, linear sweep experiments were per-
−
1
formed at low sweep rates (1 mV s ), for the CTAB and Triton
X-100 containing solutions in order to fit θ values to standard
isotherms. The best fit was obtained by assuming the Langmuir
isotherm according to Eq. (2):
c
θ
1
=
c +
(2)
K
where K is the equilibrium constant of the adsorption reaction
and c is the surfactant concentration in the bulk solution [34].
The two plots c/θ versus c, presented in Fig. 8, are linear and
have slopes close to the unity as required by Eq. (2).
The inhibition effect on the zinc deposition, caused by the sur-
factant concentration increase, is due to a blockage of the elec-
trode surface. In fact the higher number of surfactant molecules
in the vicinity of the electrode hinders the access of Zn2+ species.
In addition, higher surfactant concentration affects the structure
The adsorption equilibrium constant was evaluated and val-
4
5
of the adsorbed layer and consequently the diffusion of Zn2
+
ues of 1.5 × 10 and 1.6 × 10 were obtained for CTAB and
Triton X-100, respectively, showing that Triton X-100 is more
easily adsorbed than CTAB.
species, through this surfactant compact adsorbed layer, is much
slower. Changes on the electrical properties of the surfactant
layers at the substrate surface can also occur which originate
modifications on the deposition process [33].
3.2. X-ray diffraction study
All these effects lead to a decrease on the voltammetric cur-
rent. The degree of surface coverage for the surfactants were
evaluated from Eq. (1):
A structural study of the solid phase formed in the potential
regions of peaks C1 and C2, was performed. The deposits were
grown during 1:45 and 1 h, respectively, for peaks C1 and C2.
Although bulk deposits were obtained and analysed, the infor-
mation is useful to understand the deposition process.
i − iS
θ =
(1)
i
where i and iS are the current densities without and with surfac-
tants, respectively, measured at a constant potential value [12].
Fig. 9 shows the diffractograms obtained for samples pre-
pared in the presence and absence of surfactant molecules, in
−
3
−3
Fig. 7. Cyclic voltammograms for stainless steel electrode in 0.06 mol dm ZnSO4 + 1.2 mol dm MgSO4 containing different concentrations of SDS (a), CTAB
−1
(
b) and Triton X-100 (c). Scanning rate 10 mV s
.

A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
869
Fig. 9. XRD patterns of electrodeposited samples prepared at −1.12 V during
:45 h, in absence (a) and presence of surfactant: SDS (b), CTAB (c) and Triton
1
X-100 (d). Diffraction lines due to the substrate (*), Zn4SO4(OH)6 (ꢀ), ZnO
(
᭹) and ZnSO4 (ꢁ). The bars indicate the zinc tabled diffraction lines [35].
Fig. 8. c/θ vs. c plots for (a) CTAB and (b) Triton X-100 containing solutions.
On the other hand in the surfactant free solution and in pres-
ence of SDS, the formation of zinc occurs at potentials more
positive than the equilibrium potential. This could be due to a
strong deposit–substrate interaction, which promotes the initial
zinc growth and slows down the hydrogen evolution reaction in
these conditions.
Fig. 10 shows the diffractograms obtained for samples pre-
pared in the presence and absence of surfactant molecules in the
C2 potential region, more precisely −1.27 V. As expected, for
all the samples, the zinc is the main phase present.
the potential region where peak C1 appears, more precisely at
−
1.12 V. The diffraction lines observed for the deposits obtained
in the surfactant free solution and in the presence of SDS
(
[
Fig. 9(a) and (b)) correspond to the zinc hexagonal structure
35]. Surprisingly, in the case of the electrodeposits prepared
from the solutions containing the cationic and non-ionic surfac-
tant, this diffraction pattern is not observed. The XRD measure-
ments have proven that the films consist of Zn4SO4(OH) , ZnO
6
and ZnSO4 [36]. This result has been confirms by EDS analysis.
These results are somewhat unexpected since the voltam-
metric profiles (Fig. 4), in the potential region where peak C1
appears, are similar for all the studied systems. It should be
noted, that zinc deposition is a complex process due to the simul-
taneous reactions that could occur, namely hydrogen evolution.
Takingintoaccountthatatthispotentialregion, theproductionof
hydrogen gas was observed, changes in the local pH values take
place and consequently the formation of Zn(OH)2 and/or ZnO
is possible. Moreover the possibility of zinc hydroxysulphate
A more detailed analysis of the film diffractograms shows
that the intensity of the highest diffraction line increases in the
sequence Triton X-100, CTAB, SDS and surfactant free solu-
tion. This variation can be due to an increase of the film thickness
and/orcrystallinity, which is associatedwiththe zinc ionconcen-
tration at the substrate interface and with the surface coverage
by the surfactant. It is interesting to note that these results are
correlated with the charge of the cathodic peak C2 present in
Table 1.
Changes in the crystal orientation obtained for the electrode-
posits obtained in the absence and in the presence of the organic
molecules were observed. As stated before, the changes in crys-
tallographic orientation are due to modifications on the metal’s
surface energy by the adsorption of the organic molecules [19].
Each surfactant develops specific interactions with the differ-
(
Zn4SO4(OH) ) formation cannot be discarded. Considering
6
that more negative Ec values were found for the systems con-
taining CTAB and Triton X-100, the reduction of these phases
to zinc would be more difficult, what could explain its presence
in these conditions.

8
70
A. Gomes, M.I. da Silva Pereira / Electrochimica Acta 52 (2006) 863–871
ton X-100 containing solutions. This increase goes along
with a decrease on the anionic peak charge, reflecting the
decreaseontheamountofZndeposited. Thesefindingswere
explained by the surfactant adsorption on the electrode sur-
face.
(
4) Zn bulk deposits prepared in the absence of surfactants and
in the presence of SDS are more crystalline and with a higher
grain size than the ones obtained in the presence of CTAB
andTritonX-100. Thesefactsmaybejustifiedbyanincrease
of the overpotential deposition as the electrochemical study
performed confirms.
Acknowledgements
The financial support of Funda c¸ a˜ o para a Ci eˆ ncia e a Tecnolo-
gia, Portugal, is gratefully acknowledged. A. Gomes acknowl-
edges financial support and a SFRH/BPD/11605/2002 grant
from Funda c¸ a˜ o para a Ci eˆ ncia e a Tecnologia, Portugal.
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