Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
C333
0013-4651/2004/151共5兲/C333/9/$7.00 © The Electrochemical Society, Inc.
Effects of Ethoxylated ␣-Naphtholsulfonic Acid
on Tin Electroplating at Iron Electrodes
Joo-Yul Lee,a Jae-Woo Kim,a Byoung-Yong Chang,a Hyun Tae Kim,b
and Su-Moon Parka,*,z
a
Department of Chemistry and Center for Integrated Molecular Systems, Pohang University of Science
and Technology, Pohang 790-784, Korea
b
POSCO Research Laboratory, POSCO, Pohang 790-785, Korea
Effects of ethoxylated ␣-naphtholsulfonic acid 共ENSA兲 on the initial stages of tin plating have been studied on iron electrodes in
an acidic stannous sulfate solution containing phenolsulfonic acid as a supporting electrolyte using potentiodynamic polarization,
electrochemical quartz crystal microbalance 共EQCM兲, scanning probe microscopy 共SPM兲, and electrochemical impedance spectroscopy techniques. The smallest exchange current density and a larger transfer coefficient are observed at a typical ENSA
concentration used in industrial plating baths, i.e., 0.013 M. The SPM imaging and EQCM measurements show that ENSA
molecules form a compact structure by interacting with neighboring molecules at the iron surface, which controls the mass
transport for Sn共II兲 reduction. The EQCM studies indicate that the ENSA molecules remain stably adsorbed on the electrode
surface at considerably high overpotentials. The ENSA molecules present in both the tin layers and the solution are found to slow
the hydrogen evolution reaction at, as well as the corrosion process of, the tin-plated electrode, acting as an anticorrosion agent in
commercial tin plating baths.
© 2004 The Electrochemical Society. 关DOI: 10.1149/1.1690289兴 All rights reserved.
Manuscript submitted July 24, 2003; revised manuscript received November 27, 2003. Available electronically April 12, 2004
Stannous sulfate solutions have been widely used for tin electroplating in electronics and related industries.1-4 Tin is electrodeposited with little activational polarization from acidic stannous sulfate
solutions in the absence of additives, and the deposits obtained under such conditions are porous, coarse, and poorly adherent, with
formation of needles, whiskers, and dendrites that cause short circuits between the anode and cathode.1 It is well known that the
addition of certain organic molecules to the electrolyte results in an
increase in polarization potentials, leading to low current
densities.5-8 The increase in polarization potentials is due to the
adsorption of additive molecules on the electrode surface, blocking
the high-energy sites for the electrocrystallization. In the case of
surfactant additives, the molecules form micelles at the critical micelle concentration 共CMC兲, affecting the metal deposition mechanism and properties.1,5,9-13
Various organic compounds have been used in tin electroplating
baths for the purpose of enhancing throwing powers, grain refinement, surface brightness, and antioxidative properties of stannous
solution.5,8,14,15 Tin electrodeposition has been studied at steel, copper, dropping mercury, aluminum, platinum, gold, nickel phosphide,
carbon, and n-gallium arsenide 共GaAs兲 electrodes to examine the
deposition kinetics using cyclic voltammetry 共CV兲, Tafel plots, dc
polarography, ac impedance measurements, scanning electron microscopy, and atomic force microscopy 共AFM兲.5,8,14-22
Extensive effort has been directed to find more effective organic
molecules for tin electroplating and to examine their effects on the
structural properties of the deposits.4,14,15,17 Although the initial
deposition process is critical in determining the overall quality of
deposited layers, little attention has been paid to establishing the
way organic molecules act on the kinetics of the initial tin electrodeposition process at the iron electrode, which is one of the most
important industrial materials.
The purpose of the present work is to investigate the effects of an
organic compound, ethoxylated ␣-naphthol sulfonic acid, also
known as a commercial name ENSA, on the polarization behavior,
kinetic parameters of the electrode reaction, interfacial properties,
and surface morphology when tin is electrochemically deposited at
pure iron electrodes in acidic stannous sulfate solutions.
* Electrochemical Society Active Member.
z
E-mail: [email protected]
Experimental
The plating solution was prepared by dissolving appropriate
amounts of stannous sulfate 共Aldrich, 95⫹%兲, phenolsulfonic acid
共PSA, Aldrich, 65 wt % in water兲, and ENSA-6 关3-共␣-naphthol兲
sulfonic acid, ethoxylated with a chain of six ethoxyl groups, Daiichi Kogyo Seiyaku, 49.7 wt % in water: see scheme I兴. Potassium
ferrocyanide 关 K4 Fe(CN) 6 , Aldrich, 99%兴 and potassium chloride
共KCl, Acros, 99⫹%兲 were used as received. The industrial tinplating solution has 0.14 M SnSO4 as a source of tin, 0.033 M PSA
as a supporting electrolyte, and 0.013 M ENSA as an additive. The
PSA solution thus prepared had pH values of 1.41 and 1.43, respectively, in the absence and presence of 0.14 M Sn2⫹. In this study,
solutions were prepared by dissolving SnSO4 and PSA to the previous composition and a different amount of ENSA to make its final
concentration of 0, 1.30 ⫻ 10⫺4 , 1.30 ⫻ 10⫺3 , 1.30 ⫻ 10⫺2 , or
0.13 M before each experiment. Solutions were deaerated for 20 min
with purified nitrogen, and all the experiments were conducted under the nitrogen atmosphere. Tin deposits were prepared onto the
stationary iron electrodes by potentiodynamic or galvanostatic polarization in the stock solution with variable concentrations of
ENSA, followed by rinsing with purified water and drying with the
nitrogen gas.
An electrochemical cell having iron rotating disk working 共FeRDE, Mateck, diameter, 5.0 mm兲, platinum foil counter, and Ag/
AgCl 共in saturated KCl兲 reference electrodes was used. The iron
electrode was polished successively with 1.0, 0.3, and 0.05 ␮m alumina slurries 共Fischer兲 and then cleaned ultrasonically with doubly
distilled, deionized water before drying by purging with N2 . A 9
MHz AT-cut, gold-plated electrochemical quartz crystal microbalance 共Au-EQCM兲 electrode was used as a resonator and a working
electrode, which was assembled on a Seiko EG&G model QA-CL4
electrode holder. The Au-EQCM electrode was cleaned before each
experiment with an H2 O2 /H2 SO4 共3:7 v/v兲 solution a few times to
remove any impurities. The sensitivity factor of the EQCM electrodes was calibrated to be 3.70 ng/Hz/cm2 by depositing silver from
2.8 mM AgNO3 in an aqueous PSA 共0.033 M兲 solution. For
EQCM experiments on an iron or tin electrode, a layer of iron 共FeEQCM兲 or tin 共Sn-EQCM兲 was electrodeposited on the Au-EQCM
electrode first by sweeping the potential from ⫺0.40 to ⫺1.40 V at
10 mV/s in either an iron- or tin-plating solution. The iron-plating
solution contained 0.10 M FeSO4 , 0.40 M H3 BO3 , and
0.50 M Na2 SO4 , while the tin-plating solution had a composition
of 0.14 M SnSO4 , 1.30 ⫻ 10⫺2 M ENSA, and 0.033 M PSA.
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Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
Figure 1. LSVs at a rotating Fe-RDE at 2 mV/s in solutions containing 0.14
M Sn2⫹, 0.033 M PSA and 共a兲 0, 共b兲 1.30 ⫻ 10⫺4 , 共c兲 1.30 ⫻ 10⫺3 , 共d兲
1.30 ⫻ 10⫺2 , and 共e兲 1.30 ⫻ 10⫺1 M ENSA. Inset: enlarged version for the
circled region. Rotation rate 1200 rpm, T ⫽ 293 K.
potential and were treated with Visual SPM software from PicoSPM.
Results and Discussion
Scheme 1. An ENSA molecule. The large dark balls with two large lightcolored balls attached 共hydrogen兲 represent carbon atoms and same-size balls
with two small balls attached 共lone pair electrons兲 are oxygen. The large
light-colored ball attached to the ␣-position of the naphthalene ring is the
sulfur atom.
A Pine Instruments model AFMSRX rotator was used for the
rotating disk electrode 共RDE兲 experiments. Tafel plots were obtained for tin deposition at the Fe-RDE electrode from the data
acquired between ⫺0.40 and ⫺0.60 V at 2 mV/s with a rotation
speed of 1200 rpm. Galvanostatic polarization experiments were run
in stock solutions containing different amounts of ENSA at 20
mA/cm2 at 1200 rpm.
The impedance measurements were made using a Solatron SI
1255 HF frequency response analyzer along with an EG&G model
273A potentiostat-galvanostat. The instruments were controlled with
an EG&G M398 software program between 100 kHz and 100 mHz
with an ac wave of 5 mV peak-to-peak overlaid on a dc bias potential, and the impedance data were obtained at a rate of 10 points per
decade change in frequency.
AFM measurements were carried out using an Autoprobe CP
Research 共Thermomicroscopes, Inc.兲 scanning probe microscopy
共SPM兲 in a noncontact mode. The spring constant of the silicon
cantilevers was ca. 3.2 nN/m and a resonance frequency of ca. 90
kHz was used for scanning the selected area with a distance between
tip and the sample of 10-40 nm. An accurately calibrated 100 ␮m
scanner was used, and the image processing and data analysis were
done using an IP 2.0 software program.
Scanning tunneling microscopy 共STM兲 images were obtained on
an Fe共110兲 single-crystal 共Mateck, 99.98%, 9 mm diam兲 surface. In
situ electrochemical STM 共EC-STM兲 experiments were performed
using a PicoSPM 共Molecular Imaging, Inc.兲 in the EC-STM cell
consisting of a compartment housing the Ag/AgCl 共saturated with
KCl兲 reference and Pt gauze auxiliary electrodes, which was connected through a solution channel to the working compartment having a single-crystal iron electrode. Tungsten tips, etched at 5 V dc in
fresh 3 M KOH, were coated with an Apiezon wax to limit the tip
area exposed to the solution during the in situ imaging. All the
images were obtained in a constant current mode at an open-circuit
Potentiodynamic measurements.—Figure 1 shows a series of potentiodynamic curves recorded for tin deposition at an iron disk
electrode rotating at 1200 rpm in a solution containing various concentrations of ENSA while the potential was swept at 2 mV/s from
an open-circuit potential 共OCP: ⫺0.40 to ⫺0.45 V depending on
关ENSA兴; vide infra兲 to ⫺1.20 V; shown in the inset is an enlarged
version of the circled region. There are a few points to be noted in
these linear sweep voltammograms 共LSVs兲 at the RDE. The current
increases slowly without any break when no ENSA is present in
solution. When ENSA is added, however, two steps of electron
transfer are clearly noted. The first increase in currents starting from
about ⫺0.4 V reaches a peak and levels off until a new current
increase begins again a little beyond about ⫺0.8 V. This trend,
which is seen even at the ENSA concentration of 1.30 ⫻ 10⫺4 M,
becomes quite clear when 关ENSA兴 is 0.013 M. Another peculiar
phenomenon is that small peak currents are observed in the RDE
voltammograms, where only current plateaus would have been observed under normal experimental conditions. Once the peak potential is past, the currents are seen to be convection-diffusion limited
and thus, maintain a constant value until the next process comes in
at about ⫺0.8 V. This is an indication that the mechanism of Sn2⫹
reduction changes when ENSA is added. The change in the mechanism is clearer in the results obtained from galvanostatic experiments, which are described later. The small peak currents observed
at the outset of the current increase 共⬃⫺0.54 V兲 have been ascribed
to the nuclei formation, which was shown to be dependent on the
electrode materials and organic additives.6,22 The currents at around
⫺0.54 V, as well as those in the bulk tin deposition region, show an
ENSA concentration dependency.
The higher the 关ENSA兴 is, the more distinctive the current peaks
and the limiting currents are. Also, the peak potential appears to
move around ⫺0.54 V as well. Limiting currents for tin deposition
were reported to be smaller when the surfactant concentration approached the CMC,10 which was interpreted to control the mass
transfer of Sn2⫹. It is interesting to note that the rate of nuclei
formation at the surface, which appears to be related to the peak
current at ⫺0.54 V, seems to be the lowest at 1.30 ⫻ 10⫺2 M
ENSA. This suggests that ENSA molecules control the mass transfer
of Sn2⫹ for nuclei formation by keeping Sn2⫹ off the electrode
surface by perhaps forming sheaths that hold Sn2⫹. This is con-
Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
Figure 2. OCPs measured at a rotating iron disk electrode as a function of
the ENSA concentration in a 0.033 M PSA solution 共-䊊-兲 with and 共-䊏-兲
without 0.14 M Sn2⫹ present. Rotation rate 1200 rpm, T ⫽ 293 K.
firmed by STM images as shown later. The aggregates of ENSA
molecules adsorbed on the electrode surface appear to act as sheaths
and control Sn2⫹ transport by surrounding Sn2⫹, presenting effective barriers to the Sn2⫹ reduction. It is interesting to note also that
a rather higher peak current is observed at a high ENSA concentration of 0.13 M. The facilitation of mass transport of Sn2⫹ in this
case may perhaps result from the conformational transformation of
the Sn2⫹-ENSA aggregates on the electrode surface, as discussed
later using the STM images, in which protruded pillar-shaped
Sn2⫹-ENSA complexes are formed in an ordered manner. This configuration may direct the Sn2⫹ toward the surface and facilitate its
mass transport, resulting in a higher peak current density. The rather
ill-defined current peaks at lower ENSA concentrations appear to be
due to the insufficient control for the mass transport of Sn2⫹.
Figure 2 displays OCPs measured at the iron electrode when the
concentration of ENSA is varied in a 0.033 M PSA solution with
and without 0.14 M Sn2⫹. A change of 35 mV in OCPs is observed
for a 1000-fold change in the ENSA concentration. When only the
关 Sn2⫹兴 was varied in a control experiment without ENSA and PSA
present, a change of 31 mV/dec in OCPs was observed 共not shown兲,
which is an excellent confirmation of the Nernst equation when
n ⫽ 2. Thus, we conclude from Fig. 2 that an increase in the
关ENSA兴 by three orders of magnitudes causes the effective concentration of the potential-determining species, i.e., Sn2⫹, to decrease to
only about 1/10 of the original concentration at the electrode surface, resulting in a change of the OCP by ⫺35 mV. Apparently a
certain form of complexation of ENSA with Sn2⫹ leads to the decrease in the concentration of free Sn2⫹ at the electrode surface,
because the change in the OCP would have been about 100 mV for
three orders of change in Sn2⫹ without ENSA present.
Effects of ENSA on Tafel plots in earlier stages of tin deposition
shown in Fig. 1 are displayed in Fig. 3. Assuming that n ⫽ 2, the
exchange currents, Tafel slopes, and transfer coefficients 共␣兲 were
obtained for a 0.14 M Sn2⫹ solution containing different amounts
of ENSA and the results are summarized in Table I. From the results, definite trends are noted. The ␣-value undergoes a rather
abrupt change between the ENSA concentration of 1.30 ⫻ 10⫺3
and 1.30 ⫻ 10⫺2 M. The same is true for the Tafel slopes. At
the same time, the exchange current goes through a minimum at
1.30 ⫻ 10⫺2 M ENSA. Because the variation of Tafel slopes indicates the change in the reaction mechanism for the active species,10
we conclude that the electron-transfer mechanism changes above a
critical ENSA concentration, which is 1.3 ⫻ 10⫺2 M. The Tafel
slope obtained at an ENSA concentration above 1.3 ⫻ 10⫺2 M is
C335
Figure 3. Tafel plots for reduction of Sn2⫹ in stock solutions with 共a兲 0, 共b兲
1.30 ⫻ 10⫺4 , 共c兲 1.30 ⫻ 10⫺3 , 共d兲 1.30 ⫻ 10⫺2 , and 共e兲 1.30 ⫻ 10⫺1 M
ENSA present. Potential sweep rate 2 mV/s; rotation rate 1200 rpm; and
T ⫽ 293 K. The E 0 value taken was ⫺0.39 V.
similar to the one that would have been obtained had one electron
transfer been involved in the activation step; thus, Sn共I兲 might have
been involved as an intermediate species during reduction of Sn2⫹
in a complexed state. The stepwise electron transfer via Sn共I兲 would
slow down the electron-transfer rate above 1.30 ⫻ 10⫺2 M ENSA.
Galvanostatic polarization experiments.—The trends displayed
in potentiodynamic experiments are also observed in galvanostatic
experiments. Figure 4 shows galvanostatic polarization curves recorded at 20 mA/cm2 at the rotating Fe-RDE in acidic plating solutions containing variable concentrations of ENSA. Note that these
are not the chronopotentiograms recorded in an unstirred solution,
and thus, the first transition would indicate depletion of oxidants in
a diffusion layer on the surface of the RDE. The diffusion layer
thickness is estimated to be 1.0 ⫻ 10⫺3 cm at 1200 rpm, and the
potential transition would be observed when the supply of electroactive species cannot catch up with the rate of reduction within this
layer.23 It is seen from the figure that the potential reaches a steadystate value of about ⫺0.57 V in less than 1 s in all cases, and then
the potential undergoes a transition when ENSA is present. For the
solution without ENSA 共Fig. 4a兲, no transition is observed, indicating that Sn2⫹ is not depleted within the compact diffusion layer and
proceeds directly to bulk deposition. However, the potential increases continuously without reaching a transition at 关ENSA兴 of
1.3 ⫻ 10⫺4 M 共Fig. 4b兲. Then the time to the potential transition to
about ⫺0.75 V becomes increasingly longer after the inflection at
about ⫺0.57 V as the 关ENSA兴 increases 共Fig. 4c-e兲. The instantaneous nucleation of tin appears to take place by directly reducing
free Sn2⫹ on the surface when no additives are present. However,
the ENSA molecules on the electrode surface seem to control nucleation, as shown later. The instantaneous nucleation is now inhibited
Table I. Kinetic parameters for the tin electrodeposition in the
solution containing different amounts of ENSA.
关ENSA 共M兲兴
0
1.30 ⫻
1.30 ⫻
1.30 ⫻
1.30 ⫻
10⫺4
10⫺3
10⫺2
10⫺1
I O 共A/cm2兲
2.63 ⫻
2.24 ⫻
1.58 ⫻
8.31 ⫻
2.11 ⫻
10⫺3
10⫺3
10⫺3
10⫺4
10⫺3
Tafel slope
共mV/dec兲
␣
113.4
122.4
117.4
77.4
72.4
0.27
0.24
0.25
0.38
0.41
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Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
Table II. Analysis of AFM images shown in Fig. 4.
Electrolysis
time 共s兲
0.5
2.0
Figure 4. Galvanostatic polarization curves for tin deposition at a constant
current density of 20 mA/cm2 in a stock solution with: 共a兲 0, 共b兲 1.30
⫻ 10⫺4 , 共c兲 1.30 ⫻ 10⫺3 , 共d兲 1.30 ⫻ 10⫺2 , and 共e兲 1.30 ⫻ 10⫺1 M ENSA
present. Rotation rate 1200 rpm, T ⫽ 293 K.
by ENSA molecules at the electrode surface and the progressive
nucleation takes over while the potential is maintained at a plateau
potential of about ⫺0.75 V. The nucleation mechanism has been
reported to affect the quality of the deposits; electrodeposits obtained via progressive nucleation have better quality than those obtained by instantaneous nucleation.24-26 Bulk reduction of Sn2⫹ then
takes place at higher polarizations. Thus, larger nuclei are formed
when no or a low concentration of ENSA is present, which leads to
the poor quality of the tin bulk deposition. A controlled number of
smaller-sized nuclei is formed on the surface, which controls the rate
and quality of bulk deposition at higher polarization. This observation is consistent with that observed during the potentiodynamic
experiments, in which there is a small peak at around ⫺0.55 V,
followed by a constant potential to about ⫺0.78 V.
To investigate the changes caused by the potential transition, we
obtained AFM images of the electrode surface before and after the
current step in the tin-plating solution containing 0.013 M ENSA.
Figure 5 shows a chronopotentiogram and AFM images taken during the electrolysis at the time indicated. Both the images A1 and B1
were taken ex situ in the air from the surface after the galvanostatic
deposition for 0.5 and 2 s, respectively, at a stationary electrode. The
AFM images are clearly different in that only a few grains of about
240 nm are found in image A1, while the whole surface is covered
with large grains in image B1. This indicates that the deposition
proceeded only to the formation of nuclei, which cannot be easily
Average
surface
roughness
共nm兲
Average
peak
height
共nm兲
Average
valley
depth
共nm兲
Average
grain
size
共nm兲
25.8
60.6
31.6
32.9
⫺20.6
⫺30.7
209
244
seen by the AFM of the scale used before the potential transition
takes place at 0.5 s, whereas grains of larger sizes grew on the
surface by the time the transition in potential took place at 2 s 共see
Table II兲. From the results shown in Fig. 1, 4, and 5, we conclude
that small tin nuclei are formed 共images A1兲 and distributed evenly
at around ⫺0.54 V at the iron electrode before they grow to large
grains 共image B1兲 after the potential transition. Thus, ENSA molecules affect the nucleation process around the peak current region
by adsorption at the iron electrode, leading to the formation of tin
grains across the whole surface after the potential transition.
We also took AFM images after tin was deposited by sweeping
the potential to ⫺0.52 V as shown in Fig. 1 with and without ENSA
present 共images not shown兲, and the root mean square roughness
(R rms) was the smallest when 关ENSA兴 was 0.013 M at 7.5 nm. The
R rms values ranged from about 18 nm when 关ENSA兴 was 0.13 M to
as large as several tens of nanometers in the absence of ENSA. This
indicates that the presence of ENSA inhibits the instantaneous
nucleation, whose effect is maximal when the 关ENSA兴 is close to the
CMC.
Surface chemistry and physics of ENSA molecules at the
electrolyte/electrode interfaces.—Because the ENSA molecules are
shown to play important roles in controlling the mass transport of
Sn2⫹ at its critical concentration, it is important to probe what happens to these molecules at what might be called the ‘‘critical’’ concentration. We ran a set of experiments similar to the ones in which
CMCs were determined in case a given number of molecules form a
micelle.27 Figure 6 shows typical CVs of a 20.0 mM K4 Fe(CN) 6
solution containing 0.50 M KCl and various concentrations of
ENSA. A K4 Fe(CN) 6 solution, whose pH was 6.49, has been used
as an electrochemical probe for the determination of the CMC of
various kinds of cationic surfactants.27
Figure 7 shows the changes in peak currents (i p) and half-wave
potentials (E 1/2) upon increasing the concentration of ENSA at the
constant probe concentration. The sharp decrease in i p with an increase in 关ENSA兴 is observed up to 1.30 ⫻ 10⫺2 M, where the
Figure 5. AFM images taken during galvanostatic electrolysis in the stock solution containing 0.013 M ENSA at a stationary Fe electrode 共roughness 1.7 nm兲.
AFM image taken after 共A1兲 0.5 and 共B1兲
2 s of electrolysis at 20 mA/cm2.
Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
C337
Figure 7. Relationship between the i p as well as E 1/2 , and the concentration
of ENSA for oxidation of the K4 Fe(CN) 6 probe. Experimental conditions as
in Fig. 1.
some aggregates above the critical concentration, which somehow
inhibit the access of the electroactive Fe(CN) 2⫺
6 . The structure of an
ENSA molecule that takes the minimum in water is shown in
scheme Ib. These structures were obtained by simulating for the
Figure 6. CVs of 20 mM K4 Fe(CN) 6 in a 0.50 M KCl solution containing:
共a兲 0.0014 M 共
兲 and 0.0070 M 共• • • •兲; 共b兲 0.028 M 共
兲 and 0.10
M 共• • • •兲; and 共c兲 0.15 M 共
兲 and 0.21 M 共• • • •兲 ENSA. Scan rate
100 mV/s.
slope becomes smaller. Although the increase in i p was observed
with an increase in surfactant concentration beyond the CMC for
most surfactants, only a slight decrease in i p has been reported in the
case of cetylpyridinium chloride, which forms a rigid micelle in
aqueous media.28 Whether the i p and E 1/2 increases or decreases
upon increase in surfactant concentration are shown to be determined by their molecular dynamics and the strength of counterion
association. Therefore, the concentration where the two lines cross,
i.e., 1.30 ⫻ 10⫺2 M, should be a critical concentration for ENSA, at
which the rate of decrease in the Sn2⫹ slows significantly due perhaps to the formation of spherical micelles.29 Considering its molecular structure 共scheme I兲, however, it does not seem to be the
micelles that have been formed because the ENSA molecule has
hydrophilic groups at both ends 共scheme Ia兲. Even if ENSA molecules had formed micelles, it would be difficult to imagine that the
negatively charged Fe(CN) 2⫺
6 would be encased within the micelles
of negative charges. We thus believe that the ENSA molecules form
Figure 8. In situ STM images and depth profiles for the Fe共110兲 singlecrystal surface in solutions containing: 共a兲 nothing, 共b兲 0.013 M ENSA, 共c兲
0.013 M ENSA ⫹ 0.14 M Sn2⫹, 共d兲 0.13 M ENSA, and 共e兲 0.13 M ENSA
⫹ 0.14 M Sn2⫹. Image sizes: 共a兲 200 ⫻ 200 nm; 共b兲 30 ⫻ 30 nm 共ENSA
molecular width 1.3 nm, length 3.2 nm兲; 共c兲 200 ⫻ 200 nm 共lump width
33.1 nm, height 4.9 nm兲; 共d兲 200 ⫻ 200 nm 共enlarged view 30 ⫻ 30 nm);
and 共e兲 200 ⫻ 200 nm.
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Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
Figure 9. 共a兲 The frequency change recorded at the Fe-EQCM electrode
when 100 ␮L ENSA 共0.06 g ENSA/100 ␮L兲 was injected into 10 mL of a
0.033 M PSA solution at ⫺1.50 V. 共b兲 The frequency change at the SnEQCM electrode when 100 ␮L ENSA was injected into 0.033 M PSA solution 共10 mL兲 at ⫺1.10 V.
minimum energies using the ChemDraw program. This program
outputs an optimum structure whose energy has been minimized
when the chemical formula of a compound is inputted.30
In efforts to see how these molecules actually form aggregates
and are adsorbed on the iron surface, we obtained in situ STM
images in the solution environment. Because the iron undergoes
dissolution in an acidic medium, the in situ STM experiments were
performed in pure water deaerated thoroughly by purging with an
argon gas. Figure 8 shows the STM images of: 共a兲 an Fe共110兲
single-crystal surface at an OCP before exposure to ENSA molecules, 共b兲 the Fe共110兲 with 1.30 ⫻ 10⫺2 M ENSA alone, 共c兲 with
both 0.14 M Sn2⫹ and 1.30 ⫻ 10⫺2 M ENSA, 共d兲 with 0.13 M
ENSA alone, and 共e兲 with both 0.14 M Sn2⫹ and 1.30 ⫻ 10⫺1 M
ENSA, respectively. The width and length of the ENSA molecule
are calculated to be 0.72 and 2.9 nm, respectively, by simulation
with Chemdraw software, but most images obtained are significantly
larger than that of a single ENSA molecule. It appears that the
ENSA molecules form large, cloud-shaped aggregates segregated
from each other 共Fig. 8b and d兲. Addition of Sn2⫹ appears to accelerate the formation of the even larger aggregates of pillar shapes to
perhaps encase the tin ions 共Fig. 8c and e兲, forming complexes with
Sn2⫹ to assemble fluffy lumps. The ENSA molecules may wrap
around Sn2⫹ ions by complexing it with oxygen atoms of the ethylene oxide groups, forming pillar-shaped aggregates. These pillarshaped aggregates appear to control the mass transport of electroac-
Figure 10. Curves 共a兲 and 共b兲 are CVs recorded during a potential sweep
from 0.05 to ⫺0.55 V at 20 mV/s in a stock solution containing no and 0.013
M ENSA, respectively, while curves 共c兲 and 共d兲 show the accompanying
frequency changes for CV curves 共a兲 and 共b兲, respectively.
tive Sn2⫹ during the nucleation as well as the bulk deposition. This
supports our contention that the nucleation is limited by the mass
transport due to the compact barrier of ENSA molecules present on
the iron electrode during tin deposition in the solution containing
ENSA.
Figure 8d and e shows conformational changes when the concentrations of Sn2⫹ and ENSA are about the same at 0.14 and 0.13 M,
respectively. With Sn2⫹ added 共Fig. 8e兲, aggregated ENSA molecules covering the surface are shown to self-assemble to form ordered rod-shaped structures. These images suggest that the mode of
interaction of ENSA molecules with metal ions can be modulated by
the relative ratio of 关 Sn2⫹兴 / 关 ENSA兴 in the solution.
As shown by these images, there is no question that the presence
of Sn2⫹ helps form more organized aggregates than its absence.
While the data shown in Fig. 6 and 7 support the formation of some
aggregates, they are not likely to be micelles considering that both
-SO3 H and -(OCH2 CH2 ) n-OH groups on the molecule are hydrophilic. At present, we do not have an understanding of how the
aggregates are formed on the surface and how Sn2⫹ interacts with
ENSA molecules during the formation of the aggregates.
Quartz crystal analysis/EQCM analysis.—To see if the ENSA
molecules show an affinity to iron and tin surfaces, quartz crystal
analysis 共QCA兲 experiments have been carried out at iron- and
tin-coated QCA electrodes. Figure 9 shows QCA results for the adsorption of ENSA molecules on 共a兲 the Fe-EQCM electrode at
⫺1.50 V and 共b兲 the Sn-EQCM electrode at ⫺1.10 V, respectively.
The Fe-EQCM electrode was prepolarized at ⫺1.50 V for 5 min
in the 0.033 M PSA solution, in which no adsorption of PSA was
Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
C339
Figure 11. Impedance diagrams recorded at the electrodeposited tin layers at: 共a兲 ⫺1.00, 共b兲 ⫺0.90, 共c兲 ⫺0.70, and 共d兲 ⫺0.50 V. The impedance measurements
were made for tin layers in solutions containing: 共1,䊏 and 2,䊉兲 only PSA and 共3,⽧ and 4,䉱兲 PSA and ENSA. Tin layers were deposited from the stock solution
in 共1, 3兲 the absence and 共2, 4兲 presence of 0.013 M ENSA at an applied current of 30 mA/cm2 for 30 s. The electrode area was 0.20 cm2. Equivalent circuits
EC1 and EC2 are for interpretation of impedance responses at ⫺1.00 and ⫺0.90 V, and ⫺0.70 and ⫺0.50 V, respectively.
observed in a control QCA experiment, to obtain a reproducible
surface state by removing an iron oxide layer. When a concentrated
ENSA solution was injected into the 0.033 M PSA solution so
that its final concentration would become 0.013 M, a frequency
change of 55 Hz was observed over 100 s, which corresponds
to 5.4 ⫻ 1013 ENSA molecules/cm2
共Fig. 9a兲 关 ⫽(55 Hz
⫻ 3.7 ng/Hz/cm2 ⫻ 0.20 cm2 ⫻ Avogadro’s number兲/共molar mass
of ENSA兲兴. This is consistent with our STM result that ENSA molecules are indeed adsorbed on the iron surface even without Sn2⫹.
Because it is difficult to determine which form of the two shown in
scheme I the ENSA molecules would take at the iron surface, it is
difficult to estimate the surface coverage by ENSA molecules.
Figure 9b shows a frequency response measured at a tin-coated
QCA electrode at ⫺1.10 V in a 0.033 M PSA solution containing
0.13 M ENSA. The observed frequency change was 184 Hz, corresponding to 1.8 ⫻ 1014 ENSA molecules/cm2 . This indicates that
the ENSA molecules show stronger adsorption and remain on the tin
surface even at a considerably high overpotential, where hydrogen
evolution occurs concurrently with tin electrodeposition. The ENSA
molecules adsorbed would make the kinetics of not only proton
reduction but also tin electrodeposition slow. Thus, the ENSA molecules appear to affect the bulk deposition by adsorbing at the tin
deposits as well as by complexing with Sn2⫹ in the solution as
described previously.
Because of the active dissolution of the Fe-EQCM surface, it was
difficult to study the effects of ENSA molecules on tin deposition at
the iron electrode in a less negative potential range; we thus used a
Au-EQCM electrode to study the role of ENSA molecules in deposition and subsequent stripping of the tin deposits. Figure 10 shows
how tin deposition and subsequent dissolution proceeded when the
potential was cycled at the gold electrode at 20 mV/s between 0.05
and ⫺0.55 V. More tin residues remain on the surface in the presence of ENSA after the anodic scan has been completed than without it. The frequency changes after the anodic sweep were 860 and
200 Hz for the solution with and without ENSA, corresponding to
15.9 and 3.7 ␮g/cm2, respectively. Summarizing the results, less tin
is deposited during the cathodic sweep due to slow electrokinetics
for electrodeposition but more remains on the surface after the anodic scan is completed, when ENSA is present in the solution,
thanks to higher stability of the film formed. Thus, the ENSA molecules help tin interlayers hold more tightly, playing an important
role in determining physical characteristics. This is also supported
by the effects of 关ENSA兴 on the nuclei formation as already described previously, in which the nuclei of the smallest sizes as well
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Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
in the absence of ENSA. Also, the tin layers present larger resistances to hydrogen evolution when ENSA is in the electrolyte solution during the measurements. This suggests that ENSA, whether it
was in the plating solution during electrodeposition of tin or is in the
electrolyte solution during the measurements, helps increase the resistances to the HER.
The charge-transfer resistance (R ct) value goes through a maximum at ⫺0.70 V and decreases at ⫺0.50 V due perhaps to iron
oxidation coupled with proton reduction. The iron surface, which
would undergo active dissolution at this potential, appears to be
protected by the ENSA molecules contained in either the solution or
the tin layer. This supports our previous contention that more stable
and compact tin layers are obtained from the ENSA-containing solutions during the initial deposition process.
As already pointed out, the presence of ENSA in both the plating
solution and the electrolyte solution raises the R ct values compared
to those without it. The steeper decrease in the R ct values beyond
⫺0.70 V suggests that the nature of the reaction changes from tin
dissolution to hydrogen evolution. The double-layer capacitance values plotted as a function of potential show a similar trend to that
shown by the R ct values. The presence of ENSA in both the plating
solution and the electrolyte solution decreases the C dl values, indicating that the surface areas become smaller when ENSA is present
in either the plating solution or the electrolyte solution. The decrease
in double-layer capacitances results from more compact structures
or smaller pores of tin layers when ENSA is present in the plating
solution, while its adsorption obviates the formation of the double
layer, making the capacitances smaller.
Conclusions
Figure 12. Potential dependencies of 共a兲 the double-layer capacitances (C dl)
and 共b兲 the charge-transfer resistances (R ct) for the electrodeposited tin layers. The data were extracted by simulation of the impedance plots shown in
Fig. 11 using equivalent circuits EC1 and EC2.
as the smallest roughness were formed at a proper concentration of
ENSA. When the nuclei are evenly small with smaller internuclei
porosities, the adhesion between the interlayers would be tight and
the quality would be improved.
AC impedance measurements.—In order to see how the metal
electrode/electrolyte interface is affected by the presence the ENSA
molecules, electrochemical impedance spectroscopy 共EIS兲 measurements were made. Figure 11 shows the results of impedance measurements as well as two equivalent circuits, EC1 and EC2, for
analysis of impedance data, depending on the potential applied
during the measurements. Tin layers were prepared from the
plating solution in the presence and absence of 1.30
⫻ 10⫺2 M ENSA at 30 mA/cm2 for 30 s. The impedance spectra
were recorded successively at a few bias potentials between ⫺1.00
and ⫺0.50 V in a 0.033 M PSA solution with and without 0.013 M
ENSA. Figure 12 summarizes the potential dependency of chargetransfer resistance (R ct) and double-layer capacitance (C dl) values
extracted from Fig. 11 at various potentials.
Because no electroactive species are present in the solution during the impedance measurements, the semicircles shown in Fig.
11a-c should represent the hydrogen evolution reaction 共HER兲 at
⫺0.90 V on the tin surface. The depressed semicircles shown here
indicate that the galvanostatically deposited tin layers have many
pores.31 The tin layers deposited in the presence of ENSA molecules
display larger resistances for hydrogen evolution than those obtained
From the observations described thus far, we conclude that the
ENSA molecules control the mass transport during both the nucleation and bulk deposition of tin on the iron surface. Following is the
summary of the conclusions:
1. ENSA molecules affect both the nucleation and bulk deposition processes. The nucleation step is influenced by the adsorption of
aggregated ENSA molecules at the surface. Also, the presence of
ENSA in the interlayers of the tin deposit increases the overpotential
for bulk deposition of the tin.
2. A critical concentration of ENSA may be defined for its optimum effectiveness as an additive, where the lowest exchange current density and a large transfer coefficient were observed and the
Tafel slope becomes smaller. At this concentration and above, ENSA
molecules appear to interact intimately with neighboring ones, leading to the formation of compact aggregates at the surface, requiring
a large overpotential for reduction of Sn2⫹ due perhaps to its complexation with ENSA. The compact structure is evidenced by
smaller capacitance values of the tin layer-electrolyte interface as
well as the STM images. While the aggregates formed without Sn2⫹
had 3-5 nm sizes and are separated from each other, the aggregates
form a larger and thicker blanket when Sn2⫹ is added to the solution. This appears to control the access of Sn2⫹ to the electrode
surface.
3. During the initial tin layer formation, the deposition and stripping processes of the tin layer appear to proceed in a progressive
way with ENSA present; the QCA results indicate that the ENSA
molecules appear to enhance physical adherence among depositing
layers.
4. The presence of ENSA molecules in the tin-plating solution, as
well as in the electrolyte solution, makes the electron-transfer process more resistive with respect to hydrogen evolution and iron as
well as tin dissolution 共or corrosion兲 processes. The ENSA molecules in commercial tin-plating baths appear to act as an anticorrosion agent.
In conclusion, the ENSA molecules enhance the quality of tin
electroplating by primarily affecting the initial stage, i.e., its nucleation as well as its bulk deposition. The ENSA molecules form
aggregates on the electrode surface and control the mass transport of
Journal of The Electrochemical Society, 151 共5兲 C333-C341 共2004兲
electroactive material, i.e., Sn2⫹, which in turn affects the initial
stage of the tin plating and the bulk deposition. The nature of the
interaction between the Sn2⫹ and ENSA molecules is not fully understood and further studies are in progress in our laboratory to
elucidate how they interact and why the interaction is maximal at a
certain ENSA concentration.
Acknowledgment
Grateful acknowledgment is made for supporting this research to
KOSEF for a grant through the Center for Integrated Molecular
Systems, to POSCO for a research contract, and to the Korea Research Foundation for the BK21 program through which the graduate stipends were paid to JYL and JWK.
Pohang University of Science and Technology assisted in meeting the
publication costs of this article.
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