Electrochemical studies of the effects of pH and the surface structure of gold substrates on the underpotential deposition of sulfur

Article abstract of DOI:10.1021/jp004227s

The kinetics of the electrochemical deposition and desorption of sulfur monolayers on highly oriented Au(111) and polycrystalline Au electrodes in aqueous solutions containing sodium sulfide was studied by cyclic voltammetry, chronocoulometry, and chronoamperometry. Cyclic voltammetry experiments reveal that underpotential deposition (upd) and stripping of sulfur takes place at two different potentials at polycrystalline Au substrates leading to two oxidative and two reductive peaks. However, sulfur upd consists of only one oxidative and reductive peak at single crystalline Au(111) substrates. Electrosorption valancy measurements and pH dependency showed that the upd of sulfur involved two-electron and one-step mechanism. The charge corresponding to one monolayer of sulfur was determined by cyclic voltammetry and chronocoulometry, indicating a 0.33 coverage. The chronoamperometric results at polycrystalline Au electrodes indicates that deposition takes place in a Langmuir-type mechanism, whereas the stripping of sulfur follows a two-dimensional nucleation and growth mechanism which is accompanied by Langmuir adsorption. Although the appearance of current transients, observed for the deposition of sulfur on Au(111), was similar to what seen for deposition on polycrystalline Au substrates, the stripping process was totally different from deposition, involving only a two-dimensional nucleation and growth mechanism, at pH 12. From an analysis of the desorption current transient, we describe the stripping mechanism as instantaneous. Chronoamperometry experiments suggest that sulfur is first adsorbed on to a Au(111) surface randomly, then reorganizes itself to form a well-ordered structure, and finally strips off in a two-dimensional nucleation and growth mechanism.

Full text of DOI:10.1021/jp004227s

10588
J. Phys. Chem. B 2001, 105, 10588-10593
Electrochemical Studies of the Effects of pH and the Surface Structure of Gold Substrates
on the Underpotential Deposition of Sulfur
Murat Alanyalıo gˇ lu, Hilal C¸ akal, A. Emin O2 zt u1 rk, and U2 mit Demir*
Department of Chemistry, Faculty of Arts and Sciences, Atat u¨ rk UniVersity, Erzurum, Turkey 25240
ReceiVed: NoVember 8, 2000; In Final Form: August 29, 2001
The kinetics of the electrochemical deposition and desorption of sulfur monolayers on highly oriented Au(111)
and polycrystalline Au electrodes in aqueous solutions containing sodium sulfide was studied by cyclic
voltammetry, chronocoulometry, and chronoamperometry. Cyclic voltammetry experiments reveal that
underpotential deposition (upd) and stripping of sulfur takes place at two different potentials at polycrystalline
Au substrates leading to two oxidative and two reductive peaks. However, sulfur upd consists of only one
oxidative and reductive peak at single crystalline Au(111) substrates. Electrosorption valancy measurements
and pH dependency showed that the upd of sulfur involved two-electron and one-step mechanism. The charge
corresponding to one monolayer of sulfur was determined by cyclic voltammetry and chronocoulometry,
indicating a 0.33 coverage. The chronoamperometric results at polycrystalline Au electrodes indicates that
deposition takes place in a Langmuir-type mechanism, whereas the stripping of sulfur follows a two-dimensional
nucleation and growth mechanism which is accompanied by Langmuir adsorption. Although the appearance
of current transients, observed for the deposition of sulfur on Au(111), was similar to what seen for deposition
on polycrystalline Au substrates, the stripping process was totally different from deposition, involving only
a two-dimensional nucleation and growth mechanism, at pH 12. From an analysis of the desorption current
transient, we describe the stripping mechanism as instantaneous. Chronoamperometry experiments suggest
that sulfur is first adsorbed on to a Au(111) surface randomly, then reorganizes itself to form a well-ordered
structure, and finally strips off in a two-dimensional nucleation and growth mechanism.
Introduction
measurements, a total charge of approximately 148 ( 18 µC/
2
cm is passed under sulfur upd, which corresponds to a surface
There has been a great deal of interest in recent years in the
growth a thin film compound semiconductors using electro-
chemical atomic layer epitaxy (ECALE).1-5 This growth
technique takes advantage of the phenomenon of underpotential
coverage of about 0.33 (this is in good agreement with the
known coverage of the (x3 × x3)R30° - S/Au(111) adlattice).
Much of the work on the formation of sulfur adlayers at Au-
(111) has been done by elucidating the structure and voltam-
6
deposition (upd), where one element deposits onto a second
metric behavior of sulfur upd. In a few cases, two sets of peaks
have been reported in the upd region. The contrasting results
of different authors on the origin of these two peaks is still a
matter of debate and complicated, as the voltammetric behavior
element at potentials more positive than that necessary to deposit
the element on itself. The resulting deposit is generally limited
to less than a monolayer. Upd has been used to grow binary
compound thin film deposits in a layer-by-layer fashion,
1
9
is pH- and electrolyte-dependent. Arvia et al. have postulated
7
-11
12,13
14,15
including CdS,
ZnS,
and CdTe.
It is well-known that
-
a two-step mechanism involving an initial adsorption of SH
upd is strongly affected by the surface orientation of single-
crystal electrodes as well as the presence of adsorbing anions
in the electrolyte. The upd of sulfur on Au (111) is one of the
most intensively investigated by various techniques such as
followed by a two-electron transfer to form an adsorbed S based
on the results of in-situ reflectance measurements. Weaver et
1
6
2
0
al. have suggested a similar two-step mechanism by SERS.
Both sets of authors have shown no voltammetric data where
the second sets of peaks is evident.
17
scanning tunneling microscopy (STM), X-ray photoelectron
18
19
spectroscopy (XPS), modulated reflectance spectroscopy, and
surface enhanced raman spectroscopy (SERS).20 The structure
The aim of this work is to get further insight into the
voltammetric behaviors of sulfur in the upd range. Understand-
ing of this upd process requires better knowledge of the
mechanisms of basic phenomena such as the formation of two-
dimensional layers of sulfur. An exact interpretation of the
atomic surface structure has been observed by STM; however,
it requires additional and independent information on the
monolayer formation kinetics. A powerful tool for such studies
is the chronoamperometric technique since the developments
in single crystal substrate preparation have provided new insights
in to the processes involved in underpotential deposition. The
data obtained from such structurally well-defined substrates
suggest that upd is not a simple adsorption processes. One
assumption of the Bewick, Fleichman, and Thirsk model
of the initial monolayer formed by upd plays a key role in
controlling the structure of subsequently deposited layers;
therefore, a great deal of attention has focused on understanding
the kinetics of sulfur monolayer deposition. In addition, the
knowledge of the behavior of sulfur monolayers on metals is
important to understand catalysis, corrosion, and the dynamics
of self-assembled monolayers of thiols on metals.
Shannon at al have shown that S monolayers on Au(111)
have the (x3 × x3)R30° structure, which agrees with observa-
7
tions by Weaver and co-workers. On the basis of coulometric
*
To whom correspondence should be addressed. Phone: 90 442
2
311956. Fax: 90 442 233 1062. E-mail: udemir@atauni.edu.tr.
1
0.1021/jp004227s CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/09/2001

pH and the Surface Structure of Gold Substrates
BFT)21,22 for nucleation and growth is that the surface is
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10589
(
homogeneous and defect free, a high defect density on a surface
can have a significant effect on the electrochemical growth
mechanism.
Sulfur monolayers have been used to grow binary compounds,
including a number of II-IV semiconductors. The efficiency
of these materials depends on the single crystalline structure of
23
the semiconductor, in the field of optoelectronics, solar energy
conversion,2 and thin film electroluminescent display. There-
fore, the structures of initial sulfur monolayers plays a key role
in determining the structure of subsequently deposited layers,
as well as the solid-state properties of the films as a whole.
In the present investigation, chronoamperometry, coulometry,
and voltammetry techniques were employed to investigate and
compare the sulfur upd kinetics on polycrystalline as well as
on a carefully prepared single-crystal Au(111) electrodes, under
identical electrochemical conditions. An analysis of cyclic
voltammograms, obtained at various concentrations and pHs for
sulfur upd, suggests that sulfur deposition and stripping
processes follow two-electron, one-step mechanism. Current-
time transients were analyzed by using two-dimensional nucle-
ation and growth and Langmuirian model for adsorption and
desorption. Our results demonstrate that the sulfur is adsorbed
randomly on the surface of Au substrates, but it is desorbed in
a 2-D nucleation and growth and Langmurian model simulta-
neously for desorption, depending on the Au substrate structure
and pH.
4
25
Figure 1. Underpotential deposition of sulfur on polycrystalline Au
2
in 2 mM Na S at pH 12. The scan rate was 100 mV/s, and the electrode
2
area was 0.071 cm .
ments, solutions were deaerated with purified nitrogen prior to
addition of Na2S, to remove dissolved oxygen.
Results and Discussion
Sulfur upd on Polycrystalline Substrates. A typical cyclic
voltammogram for the upd of sulfur on polycrystalline Au in
sulfide solution (pH 12) is shown in Figure 1, at 100 mV/s. It
is clearly seen that the deposition takes place at two energetically
well-separated waves, shown as peak A1 at about -965 mV
and peak A2 at -860 mV. The corresponding dissolution peaks
are shown by peaks C1 (-1020 mV) and C2 (-895 mV).
Although similar voltammograms on polycrystalline Au sub-
strates were observed in the most of published data, the peaks
A2 and C2 have been assigned as upd deposition, but the origin
of peaks A1 and C1 has not been discussed in details or not
observed by some authors at all. In all of these studies, two
possible reasons for the A1/C1 wave have been proposed but
no experimental data has been reported to our knowledge.12
These proposed explanations are as follows. First, the peaks
A1/C1 may be dependent upon the identity of the anion in the
electrolyte solutions, as well as the presence of halide anions
in the electrolyte. It is known that the presence of certain
adsorbing anions in the electrolyte can affect the deposition
process as well as the structure of electrodeposited layers.2
Experimental Section
Preparation of Au(111) Substrates. Au microbead elec-
trodes used in these experiments were prepared as previously
described.1 These polycrystalline electrodes contain some large
elliptical (111) facets (visible by eye) with lateral dimensions
of approximately 1000 µm. Prior to each experiment, the
electrode was flame-annealed for 20 s, and after a short time
cooling in air, the electrode was quenched in Milli-Q water.
This procedure was repeated five times. Then, the polycrystalline
regions were coated with a chemically inert epoxy (Epoxy-
Patch), thereby allowing the voltammetric response of the
exposed single crystal facets to be measured. These surfaces,
because they have not been mechanically polished, possess an
extremely low defect density and are ideal for carrying out
kinetic studies.
1
The voltammetric responses of the (111) facets were verified
by examining the upd of Cu and Pb at the Au(111) facet
electrode. These results are consistent with earlier reports of
Cu and Pb upd with respect to the formal potentials, the relative
intensities of the voltammetric features, and the width of the
upd peaks. All surface area determinations was done by
6
-
Therefore, we intentionally added 0.05 M solutions of Cl ,
-
-
2-
3-
CH3COO , NO3 , SO4 , and PO4 anions to determine
whether any of these two waves was influenced by the
adsorption of these anions. If the A1/C1 wave arises from any
of these anions, then cyclic voltammogram will only consist of
the A2/C2 wave in the absence of corresponding anion. We
found that the A1/C1 wave was always present, When these
anions were present, the shape of peaks was affected very little,
in terms of sharpness, peak current, and peak potential.
Therefore, we can rule out the first proposal that the A1/C1
wave results from coadsorption of anions in the electrolyte
solution.
3-
chronoamperometric technique in 5.0 mM Fe(CN)6 aqueous
solutions containing 1M KCl in according to Cottrell equation
for both polycrystalline and single-crystal Au(111) electrode.
Electrochemistry. All electrochemical measurements were
carried out in a single-compartment, three-electrode Teflon cell
using a BAS 100W electrochemical analyzer. Cyclic voltam-
metric and chronoamperometric measurements were performed
with a solution-handling manifold that allows solutions to be
exchange without exposing the electrode to the laboratory
ambient and that allows all depositions to be carried out under
dry nitrogen gas. In all cases, an Ag/AgCl electrode served as
reference electrode, and a platinum wire was used as counter
electrode. The electrolyte solution was prepared from ultrapure
Second, as proposed by White and co-workers, for sulfur upd
2
7-29
-
deposition on Ag(111),
electro oxidation of HS may
proceed through a two-step mechanism, and each of these steps
leads to a voltammetric wave with a different potential,
30
involving one electron. Briss and Wright also suggested similar
two-step mechanism. To determine whether the same mecha-
nism takes place for sulfur adsorption and desorption on Au-
(
Milli-Q) water, Na2S‚9H2O (reagent grade; Aldrich), and
appropriate buffer solutions. Before each series of measure-

10590 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Alanyalıo gˇ lu et al.
with the value of 30 mV per decade change in the concentration
-
of HS anticipated for two electrons. Moreover, Figure 2b shows
a plot of Ep versus pH for the same peaks. The slopes (29 (
3
mV) are also very close to the anticipated value of 30 mV per
pH unit for a one proton per two-electron redox process.
The coulometric charge associated with the peak A1 and A2,
obtained by chronocoulometry and integration of the area of
under these two anodic deposition peaks in Figure 1, gave a
2
value of 150 ( 10 µC/cm after subtracting from the capacitive
charging contribution. This result agrees well with that obtained
for the (x3 × x3)R30° sulfur structure. Integration of the area
2
under wave A2 alone yields a charge of 110 ( 10 µC/cm ,
which is not enough to account for the 0.33 coverage, that found
for sulfur monolayer. We found that different amounts of charge
are consumed during the A1 and A2, and the charge ratio (QA1/
QA2) was dependent to the concentration, pH, and the scan
rate, but in all cases, the total charge for both peaks was around
2
1
50 ( 10 µC/cm . All of these results suggest that the
voltammetric waves involve a two-electron transfer as shown
by the following equation:
-
-
+
(
SH ) - 2e T S + H
(1)
ad
ad
where the subscript “ad” refers to an adsorbed species. Weaver
19
and et al. suggested a similar mechanism for sulfur adsorption
on gold using SERS, but they reported a single voltammetric
wave corresponding A2/C2 at pH 12. We also found that the
A1/C1 wave can be seen more clearly in the pH range 10-13.
The A1/C1 wave was scarcely observed at pH values less than
1
0 and more than 13. Because the dominant sulfur species are
-
20
HS at pH 9-12 and the A1/C1 could be observed at these
pHs, the origin of these peaks can be attributed to adsorption
of HS at different energetically distinct sites on polycrystalline
-
Au substrates.
Figure 2c shows the dependence of voltammetric response
of each wave on scan rate ν. As seen from the figure, the peak
currents for all four peaks are proportional to ν and have a linear
dependence showing that these four peaks are associated with
the surface processes. If the peak currents are plotted as a
2/3
function of ν , the plots appear nonlinear over the entire range
of scan rates we studied. However, plots for C2, A2, and A1
-
1
2/3
appear linear below 150 mVs (Figure 2d). A ν scan rate
dependency of the peak current is characteristic of a two-
3
1,32
dimensional nucleation and growth mechanism.
Sulfur Upd at Au(111) Single Crystalline Substrates. The
voltammetric response of the Au(111) single-crystal electrode
in an aqueous solution containing Na2S at pH 12 is shown in
Figure 3. It is interesting to note that we see only the A2/A2
wave for sulfur upd under the same experimental conditions as
on polycrystalline Au substrates. An anodic wave (labeled A2)
and a cathodic wave (C2), appearing at -865 and -915 mV,
correspond to the deposition and striping of sulfur layer.
Integration of the anodic and cathodic peaks (A2, C2) leading
to the formation of sulfur layer yields a charge 140 ( 10 µC/
2
cm (approximately the same charge as that for the polycrys-
1
talline surface, for two peaks) corresponding to roughly /3
Figure 2. Anodic and cathodic peak potentials for waves A1, A2,
2
coverage with respect to Au atoms per cm , provided that two
-
C1, and C2 as a function of log HS (a) and pH (b) at 100 mV/s scan
-
electrons transferred per HS . We examined the effect of the
2
rates. Scan rate (c) and /
3
power of the scan rate (d) dependency and
-
pH and the concentration of HS on the potential shift for each
of the sulfur underpotential deposition and desorption peak currents.
Ep (Figure 4a,b) and found similar results to that for polycrys-
-
(
111), we measured the dependence of Ep on the concentration
talline Au. The slopes of the Ep versus log [HS ] and pH plot
-
of HS and pH. As can be seen in Figure 2a, the potentials of
A1/C1 and A2/C2 waves are dependent on the concentration
are about 28 ( 2 and 29 ( 3mV, respectively, indicating a
two-electron redox process. Moreover, the ∆Ep (Ep,C2 - Ep,A2)
is about 30 mV at low scan rates, suggesting a reversible two-
electron transfer. The peak currents for A2 and C2 are
-
-
of HS . The slopes of the Ep versus log [HS ] plot are about
8 ( 4 mV for each peak, which is in reasonable agreement
2

pH and the Surface Structure of Gold Substrates
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10591
Figure 3. Underpotential deposition of sulfur on single crystalline
Au(111). Same electrochemical conditions as described in Figure 1.
2
Electrode area ) 0.0105 cm .
proportional to ν and have a linear dependence indicating that
these peaks are associated with the surface processes (Figure
4
c). We also tested all of the anions used on the polycrystalline
Au substrate, to see if there was coadsorption or not. We did
not observe the A1/C1 wave in any the presence of anion. The
question is what causes the A1/C1 wave to appear on the
polycrystalline Au but not on the single crystalline Au(111)
substrates? More recently, Salvarezza at al33 have observed very
small waves corresponding to A1/C1 on Au(111) prepared by
evaporation. They were able to show by STM that these very
small peaks are due to adsorption of sulfur atoms at step edges,
leading to one-dimensional structures. The upd process of sulfur
is quite sensitive to the surface structure of Au substrates.
Therefore, we believe that the origin of the differences in the
voltammetric behavior of sulfur upd on single- and polycrys-
talline Au originates from the surface structure of Au substrates.
Chronoamperometry. Chronoamperometry experiments were
performed in order to investigate the sulfur nucleation and
growth process in more detail. The potential of the working
electrode was stepped from an initial value, where no oxidation
of sulfide took place, to potentials sufficiently positive to initiate
nucleation and growth, after a short induction time. To eliminate
double layer charging currents from the transients, the back-
ground transient obtained in the absence of Na2S were taken
and subtracted electronically from the those obtained in the
presence of Na2S. Typical current density-time transients for
adsorption and desorption of sulfur at Au(111) are presented in
Figure 5. The shape of the transients for adsorption exhibit
classic Langmuir-type of adsorption.34 The current density for
this adsorption process can be modeled using a simple expo-
nential of the form
Figure 4. Anodic and cathodic peak potentials for waves A2 and C2
as a function of log HS (a) and pH (b) at 100 mV/s scan rates. Scan
rate dependency of the sulfur underpotential deposition and desorption
peak currents (c).
-
The appearance of the current density-time transients
observed for desorption of the sulfur layer is completely different
from what is seen for adsorption. Rising transients are observed
for desorption with current maximum at about 15 ms (Figure
5
). The current density increases because of the dissolution of
the sulfur monolayer. The rising current density eventually
reaches a current density maximum, jm, as the edge area of
discrete pits becomes larger. After jm has been reached, the
current decays as the edge area of these pits become smaller.
This behavior is major evidence for two-dimensional nucleation
and desorption process. Because we observed that a certain
amount of time is required to pass in order to see the transient
maximum for desorption, after the deposition process, we can
conclude that sulfur is adsorbed on to Au(111) surface randomly
first then it reorganize itself to form a well ordered structure,
then it desorbs in a two-dimensional nucleation process.
jads ) k exp(-k′t)
(2)
In this process, current density is related to the empty surface
area, and sulfur atoms are adsorbed on the Au(111) surface
wherever it is empty. Therefore, current starts from a maximum
and decays to zero exponentially at about 20 ms, which indicates
-
that diffusion of HS from bulk solution is not involved in the
A well-established and frequently used model for nucleation
2
1,22
growth of sulfur monolayers. Adsorption by this process is
considered as a random adsorption. Although it results in a layer
formation, the structure of the sulfur layer seems to be not well
ordered at the time scale of deposition.
and growth is the BFT model.
Fundamentally, two-
dimensional nucleation and growth can be described as either
instantaneous or progressive. Although the dissolution sites are
activated at the beginning of potential step experiments in the

10592 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Alanyalıo gˇ lu et al.
Figure 5. Current-density transients of the cyclic voltammogram
Figure 3) for sulfur upd on single crystalline Au(111). Potential steps
are indicated in the figure.
Figure 7. Current-density transients of the cyclic voltammogram
(Figure 1) for sulfur upd on polycrystalline Au. Potential steps are
indicated in the figure.
(
Figure 7 shows the current density transients obtained at a
polycrystalline Au electrode under the same experimental
conditions as those in Figure 5. Although the curves look similar
to what has been seen on Au(111) substrates for deposition,
the curves for dissolution show a shoulder rather than a current
maximum, and the experimental data did not fit either of the
above-described models. This behavior has previously been
observed for one of the upd peaks in Cu deposition.36 In this
case, random adsorption and 2-D nucleation and growth
processes occur simultaneously at different surface site. It is
known that surface defects such as monatomic steps, kink sites,
holes, etc. are preferred sites for adsorption. The simplest model
is to consider the current density as the sum of two independent
terms, a Langmuir-type of adsorption-desorption term and a
nucleation growth term as shown by the following equation:
jtotal ) jads + jnucl/growth
(5)
Figure 6. Reduced variable plots for the current-density transient
for the peak C2 (‚‚‚) shown in Figure 5. Values of j/jmax are plotted as
a function of t/tmax. Theoretical curves for instantaneous (s) and
progressive (- - -) nucleation are also shown for comparison.
A fit of the measured transient was attempted in order to separate
the random desorption from the instantaneous process, with this
equation using least-squares fits and working with multidimen-
sional Newton routines. Figure 8 shows excellent fit of the data
obtained for the random desorption and instantaneous desorption
process that take place simultaneously. The individual current
contributions of the random desorption and the instantaneous
process to the total current can be obtained from this numerical
fit. Figure 8 also shows that 65% of the total current is due to
the random and 35% is due to the instantaneous desorption
process. This ratio of random desorption to 2-D desorption is
dependent on the surface morphology of the polycrystalline
substrate, indicating that random desorption starts at surface
defects. These conclusions lead us to think that it is possible to
get more ordered sulfur monolayers on a single crystalline Au-
(111) substrate than on a polycrystalline Au substrate.
Effects of pH on Current Transients at Au(111). Figure 9
shows the influence of the pH on the current transients at single
crystalline Au(111) substrates for the peak C2. All of these
transients were taken under the same electrochemical conditions
except for the pH. Transients were subtracted from the transients
taken at blank solutions. The shape of the transients at pH 14
reveals that random and two-dimensional desorption take place
in parallel. When the pH drops, the charge for random
dissolution gets smaller, but the charge for dissolution with 2-D
nucleation and growth increases. At pH’s 12 and 10, the current
instantaneous model, dissolution sites become activated gradu-
ally as the experiment proceeds in the progressive model. The
expected current densities for these two cases are
2
jins ) at exp(-bt )
(3)
for the instantaneous case, and
2
3
jprog ) ct exp(-dt )
(4)
for the progressive case. A reduced variable plot of the measured
quantities (j/jmax vs t/tmax) allows us to distinguish between these
two types of nucleation behavior.3 Figure 6 shows the reduced
variables plots for the desorption transients shown in Figure 5.
Calculated transients for instantaneous and progressive nucle-
ation are shown for a direct comparison with the experimental
data. The reduced variable plot clearly indicates that the general
dissolution mechanism is instantaneous, indicating that the
maximum numbers of dissolution sites are activated or present
at the beginning of dissolution. This was shown in STM
experiments, where the sulfur layers have a large number of
pits.12
5

pH and the Surface Structure of Gold Substrates
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10593
-
pH 10-12 leads us to conclude that HS forms more ordered
sulfur monolayers than the other sulfur species.
Conclusions
We have studied the influence of pH and the substrate
structure on the kinetics of sulfur adsorption and desorption.
We found that the underpotential deposition and desorption of
sulfur takes place at two different potentials with two sets of
peaks at polycrystalline Au substrates, whereas there is only
one set of peaks for single crystalline Au substrates indicating
-
that there are two possible adsorption sites for HS at poly-
-
crystalline Au electrodes. HS concentration and pH dependency
experiments revealed that upd of sulfur involves a two-electron
one-step mechanism for each peak. Our chronoamperometric
results suggest that the deposition follows a Langmuir-type
mechanism, whereas the dissolution of sulfur takes place in a
nucleation and growth process, with both preceding at different
electrode sites depending on pH at Au(111).
Figure 8. Numerical fit of the random adsorption-nucleation model
(
eq 4) to a transient for the desorption peak of sulfur underpotential
C2). The individual contributions of adsorption and nucleation are also
(
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transients have no random desorption component and consist
of only two-dimensional desorption. The current transients starts
to have a random desorption component at pH’s less than 10,
and it increases with the decrease of pH. Finally, the transients
recorded for pH 8 have no charge corresponding to the two-
dimensional nucleation and growth process at all. As shown in
Figure 9, these current density transients exhibit the classic shape
of nucleation process; after the decay of a sharp electrode double
layer charging current, the current increases because of nucle-
ation and growth of sulfur nuclei and reaches to a maximum.
When the pH decreases maximum shifts to longer times. After
pH 10, no maximum is observed. When pH increases, the
nucleation density increases, and this shortens the time required
for the sulfur monolayer completion. These current maximum
(
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(
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(
(
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shifts are similar to those obtained when the overpotential
increases.2
2,36
The pH dependences of these transients can be
J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31.
(
(
32) Bosco, E.; Rangarajan, S. K. J. Electroanal. Chem. 1981, 129.
33) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys.
explained by taking into account that the concentration of sulfur
-
2-
species (H2S, HS , and S ) depends on the solution pH.
Chem. B 2000, 104, 302.
-
Therefore, HS is the only significant species at pH 9-12. The
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(
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36) H o¨ lzle, M. H.; Retter, U.; Kolb, D. M. J. Electranal. Chem. 1994,
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fact that we do not observe two sets of peaks at Au(111), as
seen for polycrystalline Au, and that dissolution of sulfur takes
place in a two-dimensional nucleation and growth process at
(