The effect of the Cu2+/Cu+ step on copper electrocrystallization in acid noncomplexing electrolytes

Article abstract of DOI:10.1149/1.2755873

The effect of different electrolytes (ClO4-, NO3-, and SO4 2- at 1 M) at pH 3 on the kinetics, growth type, and morphology of copper electrodeposition on glassy carbon was studied. The concentration was 0.1 M Cu2+ in all of the solutions employed. In cyclic voltammetry, a larger reduction peak was obtained in the nitrate electrolyte, in comparison with the other two media. This higher current has been associated with the catalytic reduction of nitrate ions on recently deposited metallic copper. This effect was confirmed in the potential-step analysis. Despite the fact that Cu+ is not thermodynamically stable in these three solutions, the reduction step Cu2+ Cu+ has been considered as an electron-transfer reaction. The anion type has a direct effect on this electron-transfer reaction and consequently, on the electrocrystallization process; sulfates provoke the most important current decrease associated with the Cu2+ Cu+ step and also strongly influence the nuclei formation, while the nitrate ions accelerate the Cu2+ Cu+ step. The scanning electron microscopy images obtained for copper deposits after different deposition times corroborate the type of copper nucleation predicted by the electrochemical techniques.

Full text of DOI:10.1149/1.2755873

Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
D473
0013-4651/2007/154͑9͒/D473/9/$20.00 © The Electrochemical Society
The Effect of the Cu2+/Cu+ Step on Copper
Electrocrystallization in Acid Noncomplexing Electrolytes
*
*
Jorge Vazquez-Arenas, Gerardo Vázquez,
**,z
Angel M. Meléndez, and Ignacio González
Departamento de Química, Universidad Autónoma Metropolitana, 09340 México Distrito Federal, Mexico
The effect of different electrolytes ͑ClO−4, NO3−, and SO42− at 1 M͒ at pH 3 on the kinetics, growth type, and morphology of copper
electrodeposition on glassy carbon was studied. The concentration was 0.1 M Cu2+ in all of the solutions employed. In cyclic
voltammetry, a larger reduction peak was obtained in the nitrate electrolyte, in comparison with the other two media. This higher
current has been associated with the catalytic reduction of nitrate ions on recently deposited metallic copper. This effect was
confirmed in the potential-step analysis. Despite the fact that Cu+ is not thermodynamically stable in these three solutions, the
reduction step Cu2+/Cu+ has been considered as an electron-transfer reaction. The anion type has a direct effect on this electron-
transfer reaction and consequently, on the electrocrystallization process; sulfates provoke the most important current decrease
associated with the Cu2+/Cu+ step and also strongly influence the nuclei formation, while the nitrate ions accelerate the Cu2+/Cu+
step. The scanning electron microscopy images obtained for copper deposits after different deposition times corroborate the type
of copper nucleation predicted by the electrochemical techniques.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2755873͔ All rights reserved.
Manuscript submitted February 9, 2007; revised manuscript received May 17, 2007. Available electronically July 26, 2007.
Copper is a base metal, which nowadays is essential in the elec-
tronic industry. Its low cost and excellent conductive properties have
assured its use in almost any electronic device. For this reason,
copper electrochemistry concentrates on refinements of the tech-
niques allowing better control of the deposit ͑i.e., monolayers͒.
Because most of the industrial processes for copper are carried
out in acid media, predominately sulfates, the initial stages of copper
electrodeposition in acid media have been the subject of many
studies.1-9
However, to our knowledge the influence of the supporting elec-
trolyte ͑anion͒ on the morphology and nucleation of copper under
similar conditions ͑Cu2+ concentration, pH͒ has been scarcely
investigated.3,4 In addition, much of the work in the literature as-
sumes that the cathodic deposition of copper in acidic media occurs
in two consecutive charge-transfer steps, through a soluble interme-
diate ͑i.e., Cu+͒,10-12 despite the fact that uncomplexed Cu+ is ther-
modynamically unstable.13 These studies have been performed using
ac techniques such as electrochemical impedance spectroscopy, and
few authors2,11,12 have employed direct current techniques. Soares
and Doblhofer et al.14,15 reported that during the Cu2+ ion reduction
in sulfate medium, the addition of chloride ions in concentrations on
the order of 10−3 M, which is known to stabilize Cu+, causes an
increase of the reduction current, which may be associated to a
by adding NaOH until a pH of 3 was reached. A similar procedure
was carried out for the other two solutions, starting with Cu͑NO3͒2
or Cu͑ClO4͒2 and adding HNO3 or HClO4, respectively, for a final
solution concentration of 0.1 M Cu2+ in 1 M of the respective an-
ion. All solutions were prepared with AR grade reagents and high-
purity water ͑18.2 M⍀ cm−1͒ from a Millipore Milli-Q water puri-
fication system.
A conventional three-electrode cell was used for the electro-
chemical experiments. A GC rod ͑Alfa Aesar, type 1͒ covered with a
resin and embedded in Teflon was used to generate a rotating disk
working electrode, with a 0.385 cm2 exposed area. A graphite rod
͑Alfa Aesar, 99.999%͒ was used as counter electrode, and a sulfate
saturated electrode ͑SSE, 640 mV vs standard hydrogen electrode,
SHE͒ as reference; in the case of ClO−4, a saturated calomel elec-
trode ͑SCE, 241 mV vs SHE͒ was used. All reported potentials are
referred to SHE. The reference electrode was placed in a Luggin
capillary in order to avoid the contamination of the SSE or SCE.
Prior to each experiment, the working electrode surface was me-
chanically polished with a 5 ␮m abrasive paper, carefully polished
to a mirror finish using Buehler alumina powder ͑final granulometry
Յ0.05 ␮m͒, rinsed with ultrapure water, and placed in an ultrasonic
cleaner for 5 min to ensure a reproducible surface in every experi-
ment. All solutions were previously bubbled with nitrogen gas
͑Praxair, grade 4.8͒ and a nitrogen atmosphere was maintained dur-
ing the entire experiment.
second mechanism ͓CuCl mediation͔, parallel to the chloride-free
͑s͒
path ͑direct Cu2+/Cu0͒.
However, a quantitative analysis for the Cu2+/Cu+ reduction step
has not been reported for the case of the copper nucleation growth,
in the presence of perchlorate and nitrate ions, in acid solution.
In this work, the influence of the ClO−4, NO3−, and SO42− anions
͑all of which do not form complexes with either the Cu2+ or the Cu+
ions͒, on the Cu2+/Cu+ step is analyzed and their effect on the ki-
netics, nucleation growth, and morphology of copper electrocrystal-
lization determined using glassy carbon ͑GC͒ as the working elec-
trode. GC was employed because it is frequently used to perform
studies of copper nucleation,16 and in this manner the influence of
the substrate is eliminated.
The electrochemical experiments were performed using an Auto-
lab model PGSTAT 30 potentiostat/galvanostat, coupled to a per-
sonal computer and the GPES software.
The deposits were morphologically and chemically characterized
using a scanning electron microscope ͑SEM, Philips XL 30; primary
electron-beam energy 20 keV͒, coupled with an energy-dispersive
X-ray fluorescence spectrometer ͑EDX DX460͒. To avoid the pre-
cipitation of copper species, the samples of the deposits were rinsed
with ethanol ͑J. T. Baker͒.
Results and Discussion
Voltammetric study.— Cyclic voltammetry ͑CV͒ studies were
performed in 0.1 M Cu2+, and 1 M of each different electrolytic
media, at pH 3, on a GC substrate. The scan rate ͑␯͒ was
100 mV s−1. The potential scan was initiated from the open-circuit
potential ͑OCP͒ in the negative direction; the potential scan was
reversed at different potentials ͑E−␭͒, and then the cycle was com-
pleted ͑Fig. 1͒. In the voltammetric study of each medium, a ca-
thodic peak appears as a result of the Cu2+ reduction process. The
copper deposit comprehends the transfer of two consecutive
electrons,9,10 with the Cu+ as a soluble intermediate;9-12 however,
Experimental
The sulfate solution was prepared by adding 0.1 M CuSO4 and
the appropriate quantity of H2SO4 in order to achieve 1 M SO24− and
0.1 M Cu2+; subsequently, the pH of the acid solution was adjusted
*
**
Electrochemical Society Student Member.
Electrochemical Society Active Member.
z E-mail: igm@xanum.uam.mx
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D474
Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
Figure 1. Typical CVs obtained at 0.1 M Cu2+ and 1 M ClO4− ͑pH 3͒ on a
GC substrate ͑0.385 cm2͒. Different inversion potentials ͑E−␭͒, which are
Figure 3. Variation of the accumulated charge, evaluated from the oxidation
͑Qa͒ and reduction ͑Qc͒ voltammetric branches, as a function of the inver-
sion potentials ͑E−␭͒ obtained in 0.1 M Cu2+ and 1 M anion at pH 3: ͑a͒
ClO−4, ͑b͒ SO42−, and ͑c͒ NO−3.
labeled in the figure, were applied to the substrate. = 0.1 V s−1
.
v
with the CV is not possible to detect this rapid process ͑Fig. 1 and
2͒. When the direction of the potential scan is reversed ͑now in the
positive direction͒, a wide peak is observed in the case of the ClO4−
and SO24− solutions ͑Fig. 2͒, which is probably related to the oxida-
tion process of Cu0 to two species, one of which may be a solid. In
the case of NO−3, the situation is more evident, because two oxida-
tion peaks are shown ͑Fig. 2͒. Nevertheless, the identification of
these species is beyond the scope of this work.
to an additional faradaic process in the reduction peak, which is
possibly ͑i͒ the chemical oxidation of the freshly deposited copper
nuclei by the nitrate ions, occurring similarly in cobalt deposition in
a Watts-type bath,17 or in copper deposition in ammonia baths,18 or
͑ii͒ the reduction of the nitrate ions on the electrode surface.
In order to distinguish between the two possibilities for the as-
The reduction process to produce metallic copper starts from
Cu2+, because the formation of complexes in these media does not
occur.4 Therefore, the influence of the anion on the reduction mecha-
nism can be more clearly observed in the voltammograms in Fig. 2.
The peak current reached in the reduction process ͑Ic͒ is different
sociated reaction,
a potential program was imposed to the
GC/Cu͑II͒-nitrate solution interface, as shown in Fig. 4. The reduc-
tion potential was initially applied during 10 s and the cell circuit
was subsequently opened in order to relax the diffusion layer during
a time ͑tw͒, allowing the nitrate ions to diffuse to the interface; then,
a potential step was imposed during 10 s to oxidize the remaining
metallic copper on the surface. This experiment was performed with
different tw. In each experiment, the charge ͑Qc͒ associated with the
direct potential step and the charge ͑Qa͒ attributed to the applied
oxidation potential were calculated. The recoveries at all tw were
similar and always less than 100% ͑Fig. 5͒, independent of the time
͑tw͒ that the nitrate ions were in contact with the freshly deposited
copper nuclei. This eliminates the conjecture regarding the chemical
oxidation of the copper nuclei and confirms the catalytic nitrate
reduction on the copper nuclei.
for each medium; in the case of the SO24− solution, the Ic is the
lowest, followed by that of the ClO−4 solution. The nitrate solution
has the highest Ic.
Recovery of the deposited copper.— In order to analyze the influ-
ence of the anion on the deposition of copper, the recovery ͑i.e.,
ratio of charge involved in the oxidation/reduction voltammetric
peaks͒ of metallic copper was evaluated in the three different media.
Figure 3a shows the recovery of copper in the ClO−4 solution, where
the highest value was obtained ͑ϳ99%͒ practically independent of
E−␭. A similar situation occurs in SO24−, where the recovery is very
high; at less negative E−␭, the recovery declines, probably due to the
lack of stability of the nuclei, in comparison with that obtained at
more negative potentials.
Additionally, a voltammetric study was performed in the nitrate
solution ͑without Cu2+͒ on GC ͑not shown͒, where no reduction
In the NO−3 solution, at all E−␭ the recovery is low ͑i.e., Qa/Qc
Ͻ 1͒ in comparison with the two other media. This may be related
Figure 4. Typical transients obtained by the double-potential step on the GC
electrode in 0.1 MCu2+ and 1 M NO3− at pH 3, recorded at 0.01 V ͑forward
step during 10 s͒ and 0.8 V vs SHE ͑reverse step during 10 s͒; relaxation
time of the GC/solution interface, tw = 0 s.
Figure 2. Typical CVs in the different media, for 0.1 M Cu2+ and 1 M anion
͑pH 3͒, on a GC substrate ͑0.385 cm2͒. = 0.1 V s−1. The negative scan
v
potential was reversed at same potential E−␭ = −0.16 V, in each case.
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Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
D475
͑Ic͒ ͑Eq. 1͒, it was possible to calculate a rough approximation for Ic
involving a reduction process from a soluble species to an insoluble
species
Ic = 367z3/2aD
C0
͓1͔
1/2v1/2
where z is the number of the electrons transferred, a is the area
͑cm2͒, D is the diffusion coefficient ͑cm2 s−1͒, is the scan rate
v
͑V s−1͒, and C0 is the concentration of the electroactive species in
the bulk ͑mol L−1͒. Using the D values obtained in the rotating disk
study and Eq. 1, a ratio of Ic ͑ClO−4͒/Ic ͑SO24−͒ of 2.25 was obtained
for the cathodic peak currents in ClO−4 and SO42−. However, the
experimental voltammograms show a ratio of 1.55 for the reduction
peak current; thus, the modification of the reduction mechanism is
not caused by diffusional phenomena. Probably, this modification is
due to the adsorption of SO24− ions on the substrate,7,20 because it
decreases the Cu2+ activity and increases the concentration overpo-
tential of Cu2+; meanwhile, in the ClO−4 solution, the adsorption of
perchlorate ions is negligible.7
In order to elucidate the influence of the anion in the reduction
mechanism of copper, potential-step studies ͑current transients͒ were
performed. It is important to mention that a voltammetric study
always involves the competition between the formation rate of spe-
cies on the interface and the polarization rate of the electrode, the
reason the different oxidation processes cannot be identified. Mean-
while, in the potential-step studies the energetic conditions are im-
posed at the electrode and the evolution of the chemical species
formed on the surface is monitored with the time.
Figure 5. Charge ratio associated with the cathodic ͑Qc͒ and anodic ͑Qa͒ I-t
curves, as a function of the relaxation time ͑tw͒. The charges were evaluated
from the current transients obtained with a potential-step program. The pro-
gram was applied to the GC electrode in 0.1 M Cu2+ and 1 M NO3− ͑pH 3͒ at
0.01 V ͑forward step during 10 s͒ and 0.8 V ͑reverse step during 10 s͒, with
different tw.
peak was found. This means that the reduction of nitrates occurs
catalytically, immediately following the deposition of the copper
nuclei.
Notwithstanding the previous study, it was not possible to ex-
plain the difference between the Ic of the copper reduction processes
in SO24− and ClO−4 media. Thus, a rotating disk analysis was per-
formed in order to compare the diffusion coefficients ͑D͒ which
were calculated from the Levich equation and the stationary current
of the experimental transients. The experiments were performed at
constant potential and different rotation speeds: 100, 500, 1000,
1500, and 3000 rpm ͑not shown͒. This technique allows control of
the amount of copper deposited on the substrate; therefore, with this
technique, the determination of the diffusion coefficients is more
reliable, unlike in linear voltammetry. Potentials of −108 and
−80 mV were applied for the ClO−4 and SO42− solutions, respectively.
With this method, values of 4.5 ϫ 10−6 and 2 ϫ 10−6 cm2 s−1 were
obtained in ClO−4 and SO42−, respectively. Using the equation pro-
posed by Berzins and Delahay19 for the voltammetric reduction peak
Potentiostatic study.— The nucleation kinetics and growth of the
first metallic nuclei formed on the substrate are critical because they
determine the properties of the electrodeposited copper. The poten-
tial step is a technique that reveals the formation of the first nuclei
on the substrate, which determines the growth in subsequent stages.
Additionally, it permits the determination of the limiting step in the
deposition reaction ͑i.e., adsorption, diffusion, or charge transfer͒.
Figure 6a-c shows a set of current transients obtained at different
potential steps, for the reduction of Cu2+ ions, from the solutions
which contain the different anions considered in this study, in a
concentration of 1 M of the anion and 0.1 M Cu2+, at pH 3.
In all cases, the shape of the current transients exhibits the typi-
cal form of a three-dimensional ͑3D͒ nucleation process, controlled
by diffusion ͑3D-dc͒ of the growing crystallites, as described by
Figure 6. Experimental current transients
obtained on GC substrate ͑0.385 cm2͒ in
0.1 M Cu2+ and 1 M anion ͑pH 3͒: ͑a͒
ClO−4, ͑b͒ SO24−, and ͑c͒ NO−3. In each case,
different potential steps were imposed to
the electrode; these potential steps are
marked on the figure.
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D476
Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
Scharifker et al.21,22 The initial sudden change is associated with the
charge of the double layer; immediately after, the increase of the
current indicates the increase in the electroactive area due to the
growth and increment of independent nuclei and/or the hemispheri-
cal diffusion. In this growing step of the deposit, the nuclei develop
diffusion zones adjacent to them. As the diffusion zones overlap, the
hemispheric mass transfer changes to a linear mass transfer, causing
the current to diminish and the current transient to approximate lin-
ear diffusion.21
Quantitative analysis of the current transients allows the estima-
tion of the nucleation parameters, which are indicative of the copper
deposition mechanism on GC, in terms of the nucleation rate and the
number of saturated and initially available nuclei, as a function of
the applied potential. In the case of the copper analysis,21 the current
of the global process ͑IT͒ obtained during the reduction process has
been described, considering the existence of only two processes: the
double-layer discharge and the 3D nucleation-growth contribution,
as shown in Eq. 2
Cu+ + e− → Cu0
͓6͔
When the nucleation and growth of the copper clusters is not suffi-
ciently fast, it would be reasonable to assume that Reaction 5 may
lead to accumulation of Cu+ at the electrode surface. Reaction 5 is
limited by diffusion, because Cu2+ needs to diffuse to the interface
where it can be reduced. Nevertheless, the charge transfer could also
be a limiting step for the global process ͑Cu2+/Cu+/Cu0͒, because
the step Cu+ to Cu0 ͑Reaction 6͒ is controlled by charge transfer.
Thus, it is hard to ascertain which of the two processes controls the
global process; moreover, the charge transfer will depend on the
potential pulse applied to the electrode. Because a broad potential
range was considered in the chronoamperometric study, it is perhaps
more correct to assume that the global process ͑Cu2+/Cu+/Cu0͒
takes place under conditions of mixed charge transfer and diffusion
limitations, as proposed by Milchev and Zapryanova.25
The current associated with Reaction 5 can be described by a
Marcus-type electron-transfer reaction ͑Eq. 7͒, assuming that reduc-
ing species is relatively unstable and could react back to oxidant
IT = Idl + I3D-dc
͓2͔
Iet = aR exp͑− bRt͒
͓7͔
The explanation of Eq. 2 with respect to each one of the contri-
butions is as follows: At the initial stage of the copper deposition,
the transient presents a sharp decline in current. According to the
literature,23 this effect is related to the electrode double layer ͑dl͒
charging, initiated by the potential pulse employed. A quantitative
characterization has been performed by Hölzle and co-workers23
showing that the charging effect can be correlated to the adsorption-
desorption process of ions on the surface electrode. To quantitatively
estimate the dl charge contribution ͑Idl͒ in the potential step, the
approach of Hölzle23 was employed, which considers that the con-
tribution in the current transient is based on a Langmuir-type ion
adsorption-desorption equilibrium.24 Idl and kads can be related to the
total charge of the adsorption process Qads by the relation in Eq. 3
where aR and bR are parameters associated with the electron-transfer
reaction, which depend on potential as proposed by Milchev and
Zapryanova.25 Furthermore, in the present case, the area of the me-
tallic copper changes with time due to the electrocrystallization pro-
cess; thus, Eq. 7 varies with both time and potential pulse. A more
detailed description of these equations has been described before by
Milchev and Zapryanova.25
Analysis of the current transients for the SO24Ϫ and ClO4Ϫ solu-
tions
.— The current of the global process ͑IT͒ obtained during the
Cu2+ reduction process on the GC electrode in the SO42− and ClO−4
media can be obtained from the contributions due to electron trans-
fer, the 3D-dc nucleation-growth process, and the double-layer dis-
charge ͑Eq. 8͒
Idl = kads exp͑− kdest͒
͓3͔
where kads = kdesQads
.
IT = Idl + I3D-dc + Iet
͓8͔
The nucleation and growth processes of the copper nuclei can be
explained by the theoretical approach developed by Scharifker and
Mostany22 where it is not necessary to classify the mechanism of
nucleation growth. The current associated with the contribution of
the reduction process ͑3D-dc͒ is then given by
The detailed Eq. 8 was programmed in Mathematica software,
where through a nonlinear least-squares fit and employing the cur-
rent transients experimental data; the best-fit parameters were ob-
tained. The simulation procedure comprises topics involving an in-
depth statistical study beyond the scope of this work. However, the
values of the parameters obtained with the simulation process are
consistent with those reported in the literature17,18 and in accordance
with their physical meaning. This aspect was considered important
in the present analysis, because different models were used to fit the
current transients ͑not shown͒, and despite the good fit, lead to un-
realistic values of the nucleation and kinetic parameters. Tables I
and II show the best-fit parameters as a function of the potential in
zFD1/2
C
1 − exp͑− At͒
I3D-dc͑t͒ =
1 − exp − N ␲kD t −
0
ͩ
ͪ
͓4͔
ͫ
ͬ
ͭ
ͮ
␲
1/2t1/2
A
with ␪ = 1 − exp͑−N0␲kD͕t − ͓1 − exp͑−At͔͒/A͖͒, where z is the
number of the electrons exchanged, F is the Faraday constant, C is
the concentration of the metallic ion in the bulk solution, N0 is the
number of active nucleation sites, A is the nucleation rate, ␪ is the
surface coverage by the formed nuclei, k = ͑8␲C/␳͒1/2, and ␳ is the
density of the deposit.
ClO−4 and SO24−
.
In all cases a typical behavior is obtained for the nucleation rate
constant ͑A͒ and the active nucleation sites number ͑N0͒, increasing
as the potential is more negative. A comparison between these baths,
approximately at the same potential, shows the SO24− medium has
the lower value of A. For the N0 values, the ClO−4 ion displays a
lower number of active sites ͑one order of magnitude͒ in comparison
In noncomplexing media, it is generally accepted that the Cu2+
reduction occurs in a global step via two electrons, because the time
for the existence of Cu+ is very short, not detectable by CV tech-
niques. However, the contribution of the Cu2+/Cu+ step to the total
current has been recognized for many years through the use of ac
techniques such as electrochemical impedance spectroscopy.2,11,12
Therefore, the reduction step Cu2+/Cu+ has been found to affect the
nucleation and growth process, and consequently, the shape of the
current transients. Milchev and Zapryanova25 have reported for cop-
per solutions with sulfates that an electron-transfer reaction takes
place ͑Cu2+/Cu+͒ before and simultaneously with the nuclei forma-
tion process. This consideration has been successfully proven in
copper electrodeposition from ammoniacal solutions.26 Therefore,
the reduction process can be considered to take place in the follow-
ing manner
with the SO24−
.
Figure 7a and b shows the comparison of the experimental cur-
rent transients with those built using Eq. 8 and the values obtained
from the fitting process ͑Table II͒, in the case of the SO24− acid bath,
at potentials of 0.08 and −0.04 V, respectively. Considering that
these transients represent a cumulative change in the electrode cur-
rent during the nucleation process, constructed with different contri-
butions, the plot in Fig. 7 takes into account the following phenom-
ena: ͑a͒ the double-layer charge ͑Idl͒, ͑b͒ the discharge of the Cu2+
ions ͑Iet͒, and ͑c͒ the nucleation-growth process ͑I3D-dc͒. The addi-
tion of these contributions results in a single theoretical transient
with characteristics close to the experimental transient, involving all
Cu2+ + e− → Cu+
͓5͔
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Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
D477
Table I. Potential dependence of the kinetic parameters of nucleation involved during the Cu2+ „0.1 M… reduction in a ClO4− „1 M… bath (pH 3),
on a GC substrate.
E ͑V͒
vs SHE
N0
Monolayers
Cu+
aR ͑A͒
bR ͑s−1
͒
A ͑s−1
͒
͑ϫ10−6 cm−2
͒
kads
kdes
Qet ͑C͒
0.10
0.06
0.02
−0.02
−0.06
−0.10
−0.14
−0.18
0.006
0.007
0.008
0.010
0.012
0.014
0.023
0.028
0.523
0.620
0.484
0.660
0.640
0.918
3.043
2.530
30.0
35.5
41.4
59.0
73.3
86.6
109.3
111.8
1.39
1.90
1.78
2.97
3.28
4.65
9.93
10.99
0.002
0.002
0.002
0.003
0.003
0.003
-
50.8
63.8
99.3
197.0
116.0
172.2
-
0.0091
0.0098
0.0130
0.0134
0.0154
0.0142
0.0075
0.0110
73
80
105
109
125
115
61
-
-
89
reaction.25 Consequently, the number of monolayers of Cu+ appear-
ing at the interface is a consequence of the influence of the anions
on the surface of the electrode, because the SO24− ion considerably
adsorbs, preventing the discharge of Cu2+. Thus, for the SO42− anion
the number of monolayers is the lowest. This explains the lower
current values obtained in the voltammogram of the SO24− solution
in Fig. 2. For ClO−4 ions, this adsorption does not exist; therefore, a
greater amount of reduced Cu+ appears at the interface.
of the processes described previously. Thus, the real contributions of
the different mechanisms involved in the current transients were
determined.
As the potential is more negative, the contribution of the reduc-
tion process from Cu2+/Cu+ ͑Iet͒ is attained at shorter times, as
shown in Fig. 7b, nearly disappearing at ϳ1.8 s, while I3D-dc con-
tributes in a larger proportion to the total current transients. In this
manner, at more negative potentials, because Cu+ is not generated at
longer times, Reaction 6 is more favorable because the copper re-
duction occurs in a two-electron process. Similar behavior is ob-
served in the ClO−4 solution.
Analysis of the current transients for the NOϪ3 solution.— Because
the nitrate solution manifests the simultaneous reduction of the ni-
trate ions ͑see above͒ occurring with the copper nuclei formation, it
is necessary to consider this reduction reaction in total current asso-
ciated with the transient of the copper nucleation. From the Pourbaix
diagram28 and considering the potential where the reduction takes
place, the electrochemical reaction proposed for the nitrate reduction
on the recently deposited copper nuclei is
Analyzing the charge ͑Qet͒ corresponding to the contribution of
the reaction Cu2+/Cu+ by means of the integration of the Iet de-
convoluted transient ͑Fig. 7͒, the number of effective monolayers of
Cu+ was estimated. Applying Faraday’s law Qet = zFn, where z
= 1 and n the number of moles of Cu+, and Avogadro’s number
͑NA = 6.023 ϫ 1023 mol−1͒, the number of species is n = nN for
Ј
A
the electrode surface. Now, considering that the atomic density27 is
1 ϫ 1015 cm−2, the calculation of the number of monolayers for Cu+
in the case of SO24− and ClO4− is performed; the values are shown in
Tables I and II. The average value for the number of reduced species
for the effective monolayers obtained in ClO−4 and SO42− media are
95 and 18, respectively. This implies that Reaction 5 is not an ion
transfer reaction, forming an adlayer of Cu+ on the surface of the
electrode, but rather it should be considered as an electron-transfer
NO−3 + 2H+ + 2e− = NO−2 + H2O
͓9͔
Reaction 9 occurs on the freshly deposited copper nuclei, and
this catalytic effect depends on the coverage ͑␪͒. Reaction 9 can be
considered as a kinetic contribution ͑INR
͒
Table II. Potential dependence of the kinetic parameters of nucleation involved during the Cu2+ „0.1 M… reduction in a SO42− „1 M… bath (pH
3), on a GC substrate.
E ͑V͒
vs SHE
N0
Monolayers
Cu+
aR ͑A͒
bR ͑s−1
͒
A ͑s−1
͒
͑ϫ10−7 cm−2
͒
kads
kdes
Qet ͑C͒
0.12
0.08
0.04
0.001
0.003
0.003
0.005
0.006
0.008
0.008
0.011
0.784
1.084
1.178
1.531
2.758
4.391
4.569
6.046
10.9
21.8
25.1
40.7
55.3
60.6
68.3
81.6
0.33
0.51
0.51
0.71
1.25
2.12
2.31
3.16
0.001
0.001
0.001
0.002
0.002
0.003
0.005
0.006
30.6
92.3
70.8
114.0
83.5
126.5
223.4
236.3
0.0016
0.0024
0.0028
0.0029
0.0022
0.0018
0.0018
0.0018
13
19
23
24
18
15
15
15
0.00
−0.04
−0.08
−0.12
−0.16
Figure 7. Comparison between experi-
mental current transients ͑O͒ obtained in
0.1 M Cu2+ and 1 M SO24− ͑pH 3͒ and re-
corded at ͑a͒ 0.08 and ͑b͒ −0.04 V, with
those built ͑-͒ using Eq. 8 and the values
obtained from the fitting process ͑Table
II͒. Contributions to the transient current
from the double-layer charging phenom-
enon ͑Idl͒, the electron-transfer reaction
͑Iet͒, and 3D diffusion-limited nucleation
͑I3D-dc͒ are also shown in the figure.
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D478
Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
Table III. Potential dependence of the kinetic parameters of nucleation involved during the Cu2+ „0.1 M… reduction in a NO3− „1 M… bath (pH
3), on a GC substrate.
E ͑V͒
vs
SHE
kNR
N0
͑ϫ107 mol
Monolayers
Cu+
aR ͑A͒
bR ͑s−1
͒
A ͑s−1
͒
͑ϫ10−7 cm−2
͒
cm−2 s−1
͒
kads
kdes
Qet ͑C͒
0.10
0.06
0.02
0.011
0.013
0.017
0.020
0.023
0.024
0.028
0.602
0.733
1.819
1.332
1.355
1.520
1.720
5.2
6.9
0.55
0.67
1.21
3.06
3.23
3.75
4.85
1.613
1.964
2.366
13.24
13.47
13.95
14.31
0.003
0.002
0.004
0.004
3.206
-
185.0
176.8
198.2
165.0
1668.8
-
0.0155
0.0155
0.0093
0.0144
0.0164
0.0158
0.01608
126
125
75
117
133
128
130
15.6
31.8
33.5
45.2
51.7
−0.02
−0.06
−0.10
−0.14
-
-
kNR increases as the potential becomes more negative; therefore,
at a specific potential, the contribution of the nitrate catalytic reduc-
tion on the copper is higher than was observed in the 3D-dc nucle-
ation process ͑Fig. 8b͒. This electrochemical reaction depends on the
potential, because a more negative potential accelerates the NO3−
reduction.
INR = zFkNR
␪
͓10͔
where kNR is the kinetic constant of the nitrate reduction, z is the
number of electrons involved in the reaction, and ␪ is the surface
coverage by the nuclei formed.
Taking into account the above equation, the current of the overall
process ͑IT͒ exhibited during the reduction process of Cu2+ on the
GC in nitrate solution is
The nucleation kinetic parameters increase in a manner similar to
those reported in the other media. The estimation, previously de-
scribed, of the Cu+ concentration was applied in this case and these
values are reported in Table III. The number of monolayers of Cu+
formed in nitrate solution is the highest in comparison with the other
media. This can be explained by the chemical nature of nitrate ions.
It is known that nitrate ions destabilize the hydration of
IT = Idl + I3D-dc + Iet + INR
͓11͔
Table III shows the best-fit parameters as a function of the ap-
plied potential, determined through the fitting of the experimental
current transients, using Eq. 11. In Fig. 8 are shown the comparison
of the experimental current transients obtained in the NO−3 bath at
0.06 and −0.06 V, respectively, with those built using Eq. 11 and the
values obtained from the fitting process ͑Table III͒.
The value of the nitrate reduction transfer coefficient can be es-
timated, assuming that kNR can be represented by a Butler–Volmer-
type relation29
30
2+
͓Cu͑H2O͒ ͔
,
accelerating the change in the coordination sphere
6
of Cu2+/Cu+ according to Eq. 13. This provokes a more rapid for-
mation of the intermediary ͓Cu͑H2O͒ ͔+, enhancing the number of
4
Cu+ monolayers formed
2+
+
͓Cu͑H2O͒ ͔ + 1e− ↔ ͓Cu͑H2O͒ ͔ + 2H2O
͓13͔
6
4
␣
NRzFE
kNR = k0 exp −
͓12͔
ͫ ͬ
Saturated nuclei estimation.— For calculating the nuclei satura-
NR
RT
tion number in each medium,
a methodology proposed by
Scharifker31 was employed; the NS values were calculated from the
where z = 2. From the ln kNR vs E plot, the values of ␣NR ͑0.14͒ and
k0NR ͑4.79 ϫ 10−7 mol cm−2 s−1͒ were calculated.
parameters reported in Tables I-III using the following equation
Figure 8. Comparison between experi-
mental current transients ͑O͒ obtained in
0.1 MCu2+ and 1 M NO−3 ͑pH 3͒ and re-
corded at ͑a͒ 0.06 and ͑b͒ −0.06 V, with
those built ͑-͒ using Eq. 11 and the values
obtained from the fitting process ͑Table
III͒ generated by a nonlinear fitting of Eq.
11 to the experimental data. Contributions
to the transient current, from the double-
layer charging phenomenon ͑Idl͒, the
electron-transfer reaction ͑Iet͒, the contri-
bution of the nitrates catalytic reduction
͑INR͒, and the 3D diffusion-limited nucle-
ation ͑I3D-dc͒ are also shown in the figure.
Table IV. Potential dependence of the NS values, calculated from the parameters showed in Tables I-III and Eq. 14.
Ic
SO24−
NO−3
NS
E ͑V͒
NS ͑ϫ10−7 cm−2
͒
E ͑V͒
NS ͑ϫ10−7 cm−2
͒
E ͑V͒
͑ϫ10−7 cm−2
͒
0.10
0.06
0.02
0.587
0.748
0.781
1.204
1.412
1.828
3.000
3.192
0.12
0.08
0.04
0.825
1.436
1.539
2.331
3.595
4.900
5.426
6.941
0.10
0.06
0.02
−0.02
−0.06
−0.10
−0.14
0.487
0.617
1.252
2.836
2.993
3.750
4.562
−0.02
0.00
−0.06
−0.10
−0.14
−0.18
−0.04
−0.08
−0.12
−0.16
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Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
D479
Figure 9. Variation of the nucleation rate with applied potential, obtained
from data reported in Tables I–III, for different media: ͑a͒ ClO−4, ͑b͒ SO42−
and ͑c͒ NO−3.
,
Figure 10. SEM images of deposits previously formed at −0.16 V in 0.1 M
Cu2+ and 1 M SO24− ͑pH 3͒ on GC at different growth times: ͑a͒ 2 s, ͑b͒ 5 s,
and ͑c͒ 10 s.
ANo
SEM.— Figure 10 shows SEM micrographs of copper deposition
obtained in SO24− medium under diffusion control conditions. In or-
der to observe the initial stages of copper nucleation and the shape
of the deposit, a growth analysis of the metallic phase was per-
formed applying a potential step at shorter times. The potential ap-
plied for forming the deposits ͑for all media͒ was 100 mV more
negative than the peak potential of the voltammogram ͑Fig. 2͒ dur-
ing ͑a͒ 2 s, ͑b͒ 5 s, and ͑c͒ 10 s. In Fig. 10a, the nuclei quantity
͑shiny zone͒ is compared with those obtained in Fig. 10b, where the
growing time is longer; however, for the case in Fig. 10c, the num-
ber of nuclei seems similar to Fig. 10b, but these are larger in size.
Thus, the nuclei formation depends on the time, confirming the pro-
gressive nucleation-growth mechanism of copper in sulfate medium.
A random spatial distribution of the active sites can be observed on
the entire surface of the GC; recently, Milchev et al.16 have reported
that randomness is originated by mechanical polishing; conse-
quently, during this process some actives sites are generated very
close to each other and large localized clusters appear during growth
͑Fig. 10a͒. The nuclei growth at short times in ClO−4 solutions pre-
sents a similar situation to that observed in SO24− ͑not shown͒.
The morphology and growth type of the copper deposit at short
times in NO−3 solutions is shown in Fig. 11 for different times: ͑a͒
2 s, ͑b͒ 5 s, ͑c͒ 10 s, and ͑d͒ 50 s. The nuclei growth in number and
size as a function of the time show collapse, which changes the
morphology of the copper deposits. There are two growth types: one
forming small, well defined crystals and the other one dendritic. The
N =
͓14͔
ͩ ͪ
S
2k D
Ј
where
k = ͓8␲CV ͔
͓15͔
Ј
M
The results obtained for NS are presented in Table IV. Compari-
sons with the N0 values of Tables I-III show that N0 is lower than NS
values, which is not a realistic situation. Such a discrepancy could
be associated mainly with the theoretical formalism of the equations.
Equation 14 was developed for a diffusion-limited nucleation-
growth process, whereas Eq. 8 and 11 assume mixed charge transfer
and diffusion limitations. A comparable situation has been reported
by Zapryanova et al.,32 who found a difference of two orders of
magnitude between the N0 values ͑estimated with a model similar to
that employed in this study͒ and NS ͑microscopically determined͒;
this comparison will be evaluated in a future study. At high negative
potentials ͑resembling those reported by Ref. 32͒, the charge-
transfer limitations become less significant; however, due to the
strongly increased rate of nuclei formation, many clusters are
formed and some of them may come into direct contact, a fact which
is also not taken into account by Eq. 8 and 11.
Figure 9 shows a plot of ln A vs E obtained from Tables I-III, in
each medium, where a linear relationship is found for all. In the
nitrate system, the slope is approximately twice that found in the
other baths, indicating a slow nucleation rate. Using the atomistic
theory of nucleation,33 it is possible to estimate the size of the critic
nuclei34 with the following equation
kT d ln A
n =
c
− ␣
͓16͔
ͩ ͪͩ ͪ
ze0
dE
where k is the Boltzmann constant, T the absolute temperature, and
e0 the elementary charge. Substituting the d ln A/dE values in Eq.
16, and considering a charge-transfer coefficient of ͑␣͒ = 0.5, a
value previously reported for copper deposition,25 nc = 0 was ob-
tained in each medium. From the atomistic theory of nucleation, the
obtained value of nc indicates that a critic nucleus is the largest
cluster; therefore, the attachment of a new atom stabilizes this clus-
ter, for which the probability of attaching the next atom is greater
than one-half and henceforth leads to spontaneous growth. Thus, in
these cases every active site on the surface of the substrate acts as a
critical nucleus,35 suggesting that within the potential interval stud-
ied, the thermodynamic barrier for the nuclei formation is negligible
in comparison with the kinetic contribution.35
In order to observe the influence of the anion on the growth and
morphology of the copper in each bath, a SEM study was per-
formed.
Figure 11. SEM images of deposits previously formed at −0.06 V in 0.1 M
Cu2+ and 1 M NO−3 ͑pH 3͒ on GC at different growth times: ͑a͒ 2 s, ͑b͒ 5 s,
͑c͒ 10 s, and ͑d͒ 50 s.
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D480
Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
The current transients of the global process ͑IT͒ obtained during
the reduction process of Cu2+ on the GC electrode in SO42− and ClO−4
media was considered to consist of three contributions: electron
transfer ͑Cu2+/Cu+ step͒, a 3D-dc nucleation-growth process, and
double-layer discharge. The charge ͑Qet͒ corresponding to the con-
tribution of the Cu2+/Cu+ reaction, calculated using the integration
of the Iet deconvoluted transient, permitted the estimation of the
effective number of Cu+ monolayers, which were 95 and 18 in ClO4−
and SO24− media, respectively. The number of Cu+ monolayers ap-
pearing at the interface is a consequence of the influence of the
anions on the electrode surface. Because the SO24− ion adsorbs con-
siderably, preventing the discharge of Cu2+, the number of monolay-
ers in SO24− solution is the lowest. This also explains the lower
values in the cathodic peak current values, in the case of the SO42−
solution. For ClO−4 ions, the adsorption is negligible and therefore a
greater amount of reduced Cu+ is present at the interface.
Figure 12. SEM images of deposits previously formed during 10 min in
0.1 M Cu2+ and 1 M anion ͑pH 3͒ on GC with different anions at specific
deposition potentials: ͑a͒ ClO−4 ͑−0.16 V͒, ͑b͒ SO42− ͑−0.16 V͒, and ͑c͒ NO−3
͑−0.06 V͒.
The analysis of the experimental current transients obtained in
NO−3 medium is different from the above. An additional contribution
due to the catalytic effect of nitrate reduction on the recently depos-
ited metallic copper was proposed as a kinetic contribution, which
must be added to the current of the global process previously con-
sidered. A higher value of effective monolayers was obtained as a
latter may have resulted from the catalytic reduction of NO−3 on the
copper nuclei, because well-defined crystals disappear at longer
times ͑Fig. 11d͒.
consequence of a destabilization of the hydration sphere of
2+
͓Cu͑H2O͒ ͔ by the nitrate ions, which resulted in a more rapid
6
formation of the intermediate ͓Cu͑H2O͒ ͔+.
4
Because copper growth is more affected by the anions at longer
times, the formation of copper electrodeposit was performed for
10 min in each medium. Figure 12a and b shows that in ClO−4 and
SO24− medium granular-homogeneous deposits are formed, although
a more ordered and uniform deposit is obtained in sulfates, giving it
a more brilliant appearance. It is clear that in ClO−4 a distribution of
grain sizes is present, indicating that the copper favorably grew on
some random active sites at the beginning of the nucleation process,
provoking the growth of several grains which later collapsed, form-
ing the larger clusters.
The SEM ex situ images obtained for the copper deposits after
different deposition times ͑2 s to 10 min͒ corroborate the type of
copper nucleation predicted by the electrochemical techniques. For
the deposits obtained after 2 s, the images associated with the sul-
fate solution present a random spatial distribution of the active sites
on the entire surface of the GC, while in the nitrate solution, there
are two growth types: one forming small, well-defined crystals and
the other one dendritic. For copper deposits grown for 10 min, the
SEM images show granular-homogeneous deposits in the case of the
sulfate solution. In the presence of nitrates, the images show two
growth zones. The first has a low copper content and the second a
smooth, pure crystalline copper deposit which grew, collapsing with
others.
In SO24− medium the grains are of similar size, manifesting a
slower nucleation-growth process in comparison with that found in
ClO−4. This could be associated with sulfate adsorption ͑in a similar
manner as that shown in the voltammogram͒, which causes local
electrostatic effects8 that modify the copper nucleation. The EDX
analyses show that the deposition process in both media was a com-
pletely uniform copper surface.
A special case is the formation of copper in nitrate solution; two
zones of growth are present ͑Fig. 12c͒. The first has a low copper
content ͑dark zone͒ and the GC is completely exposed; the black
zone could be due to the adsorptive blockage, provoking a scarce
deposition of copper nuclei.20 The second one ͑shiny zone͒, is a
smooth, pure copper deposit, without formation patterns such as
dendrites, cauliflowers, or grains. In contrast to the other zone, this
one shows crystals that grow, collapsing with others.
Acknowledgments
The authors acknowledge Patricia Castillo ͑UAM-I͒ for the SEM
images. J.V.-A. thanks CONACyT for a postgraduate fellowship,
project no. SEP-2004-C01-47162. A.M. and G.V. are grateful to
CONACyT for graduate grants. The authors also acknowledge Omar
González and René Lara ͑UASLP͒ for the SEM images.
Universidad Autónoma Metropolitana assisted in meeting the publication
costs of this article.
List of Symbols
nucleation rate, s−1
area of the electrode, cm2
A
a
Conclusion
␣
charge-transfer coefficient of the nitrate reaction
constants associated with the electron-transfer reaction
concentration of the electroactive species in the bulk, mol L−1
diffusion coefficient, cm2 s−1
NR
͒
The effect of the anion used in the electrolyte on the kinetics,
growth type, and morphology of copper electrodeposition on GC
was studied. The analysis was performed in different acid media ͑
ClO−4, NO3−, and SO24− in concentration 1 M͒ at pH 3, where the
Cu2+ concentration was 0.1 M in all of the solutions analyzed. From
the CV study, the peak current attained for the copper reduction
process ͑Ic͒ in the nitrate solution has the highest value, which is
related to the catalytic effect of nitrate reduction on the recently
deposited metallic copper. This effect was confirmed in the
potential-step analysis. In the case of the SO24− solution, the value of
Ic is the lowest, followed by that found in ClO−4. The values of the
diffusion coefficient ͑D͒ obtained for Cu2+, with rotating disk analy-
aR ͑A͒, bR, ͑s−1
C0
D
F
Ic
Idl
Iet
Faraday constant, C mol−1
peak current of the reduction process, A cm−2
current associated to the double layer, A cm−2
current associated with the electron transfer of the Cu2+/Cu+
process, A cm−2
reduction current of the nitrate ions, A cm−2
total current of the global process from Cu2+ to Cu0, A cm−2
INR
IT
I3D-dc current associated to the 3D diffusion-controlled nucleation
process, A cm−2
Boltzmann constant, J K−1
k
kads rate constant of the adsorption process
sis ͑4.5 ϫ 10−6 and 2 ϫ 10−6 cm2/s for ClO4− and SO24− solutions͒,
rule out the argument that a simple diffusional effect is responsible
for this variation.
kinetic constant of the nitrate reaction, mol cm−2 s−1
standard kinetic constant of the nitrate reaction, mol cm−2 s−1
Avogadro’s number, mol−1
kNR
k0NR
NA
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Journal of The Electrochemical Society, 154 ͑9͒ D473-D481 ͑2007͒
D481
saturated nuclei number, cm−2
367 ͑2004͒.
NS
N0
n
number of active nucleation sites, cm−2
number of moles of Cu+
13. B. Beverskoga and I. Puigdomenech, J. Electrochem. Soc., 144, 3476 ͑1997͒.
14. D. M. Soares, S. Wasle, K. G. Weil, and K. Doblhofer, J. Electroanal. Chem., 532,
353 ͑2002͒.
15. K. Doblhofer, S. Wasle, D. M. Soares, K. G. Weil, and Gerhard Ertl, J. Electro-
chem. Soc., 150, C657 ͑2003͒.
16. A. Milchev, E. Michailova, and I. Lesigiarska, Electrochem. Commun., 2, 407
͑2000͒.
17. E. Barrera, M. Palomar-Pardavé, N. Batina, and I. González, J. Electrochem. Soc.,
147, 1787 ͑2000͒.
18. A. Ramos, M. Miranda-Hernández, and I. González, J. Electrochem. Soc., 148,
C315 ͑2001͒.
nc size of the critical nuclei, cm
n
Ј
Qa
Qads
Qc
Qet
number of species of Cu+
charge of the oxidation process, C cm−2
charge of the adsorption process, C cm−2
charge of the reduction process, C cm−2
charge associated with the Cu2+/Cu+ process, C cm−2
tw delay time for the relaxation of the GC/solution interface, s
z
number of the electrons transferred
19. T. Berzins and P. Delahay, J. Am. Chem. Soc., 75, 555 ͑1953͒.
20. G. A. Hope and R. Woods, J. Electrochem. Soc., 151, C550 ͑2004͒.
21. B. Scharifker and G. Hills, Electrochim. Acta, 28, 879 ͑1983͒.
22. B. R. Scharifker and J. Mostany, J. Electroanal. Chem. Interfacial Electrochem.,
177, 13 ͑1984͒.
23. M. H. Hölzle, U. Retter, and D. M. Kolb, J. Electroanal. Chem., 371, 101 ͑1994͒.
24. F. E. Varela, L. M. Gassa, and R. Vilche, Electrochim. Acta, 37, 1119 ͑1992͒.
25. A. Milchev and T. Zapryanova, Electrochim. Acta, 51, 2926 ͑2006͒.
26. J. Vazquez-Arenas, R. Cruz, and L. H. Mendoza-Huizar, Electrochim. Acta, 52,
892 ͑2006͒.
Greek
␣
␪
v
␳
charge-transfer coefficient
covering of the surface with the formed nuclei
scan rate, V s−1
density of the deposit, g cm−3
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