Influence of benzotriazole and tolyltriazole on the copper electrodeposition on polycrystalline platinum from acidic chloride solutions

Article abstract of DOI:10.1149/1.1393430

The influence of benzotriazole (BTAH) and tolyltriazole (TTAH) on the cathodic reduction of copper(II) at poly-oriented platinum from acidic chloride solutions has been studied by rotating disk voltammetry and cyclic voltammetry combined with electrochemical quartz crystal microbalance (EQCM). The overall electrode process involves two one-electron reactions: Cu(II)/Cu(I) and Cu(I)/Cu, which are characterized by a very large potential separation. In the additive-free solutions, the first electrode reaction is slightly inhibited by underpotential deposited Cu. On the basis of EQCM results, it has been proved that the interaction of BTAH and TTAH with the electrode surface is potential dependent. The adlayers of the inhibitors are removed in the potential range below -0.25 V vs. SCE, in which the adsorption of hydrogen occurs. Consequently, the Cu(II)/Cu(I) electrode reaction is strongly inhibited by BTAH and TTAH while the Cu(I)/Cu reaction remains practically unaffected.

Full text of DOI:10.1149/1.1393430

1
758
Journal of The Electrochemical Society, 147 (5) 1758-1762 (2000)
S0013-4651(99)07-097-X CCC: $7.00 © The Electrochemical Society, Inc.
The Influence of Benzotriazole and Tolyltriazole on the
Copper Electrodeposition on Polycrystalline Platinum from
Acidic Chloride Solutions
Mieczyslaw Scendo and Jan Malyszkoz
Institute of Chemistry, Pedagogical University, 25-020 Kielce, Poland
The influence of benzotriazole (BTAH) and tolyltriazole (TTAH) on the cathodic reduction of copper(II) at poly-oriented platinum
from acidic chloride solutions has been studied by rotating disk voltammetry and cyclic voltammetry combined with electro-
chemical quartz crystal microbalance (EQCM). The overall electrode process involves two one-electron reactions: Cu(II)/Cu(I) and
Cu(I)/Cu, which are characterized by a very large potential separation. In the additive-free solutions, the first electrode reaction is
slightly inhibited by underpotential deposited Cu. On the basis of EQCM results, it has been proved that the interaction of BTAH
and TTAH with the electrode surface is potential dependent. The adlayers of the inhibitors are removed in the potential range below
Ϫ0.25 V vs. SCE, in which the adsorption of hydrogen occurs. Consequently, the Cu(II)/Cu(I) electrode reaction is strongly inhib-
ited by BTAH and TTAH while the Cu(I)/Cu reaction remains practically unaffected.
©
2000 The Electrochemical Society. S0013-4651(99)07-097-X. All rights reserved.
Manuscript submitted July 25, 1999; revised manuscript received January 19, 2000.
Benzotriazole (BTAH) and tolyltriazole (TTAH) are well known
Depending on pH, benzotriazole can exist in aqueous solutions as
ϩ
as highly efficient inhibitors for preventing the corrosion of copper
and its alloys as well as important additives for Cu plating baths. The
attention of most authors in the last decade was focused on the mech-
anism of anodic processes in the presence of BTAH using various
either protonated cationic species BTAH , neutral molecule BTAH,
2
Ϫ
or dissociated anionic species BTA . Figure 1 presents the distribu-
tion diagram for benzotriazole species calculated using the data for
6
,13
acid-base equilibria (pKa1 Ϸ 1, pK ϭ 8.2).
As seen from this
a2
1
-18
electrochemical and surface techniques.
Protecting properties of
figure, the neutral molecules are the dominant form of the inhibitor
at pH 3.
8
,9
TTAH were investigated in iodide medium. It is generally accept-
ed that BTAH impedes corrosion of copper by the formation of a pro-
tective film on the surface, which involves precipitation of cuprous
ions as highly insoluble Cu(I)-benzotriazole complexes in polymeric
form. Very recently, Vogt et al. confirmed the formation of a thick,
inhibiting Cu(I)-BTA film on the Cu(100) surface in HCl solutions at
potentials more positive than Ϫ0.35 V vs. SCE, using in situ scanning
tunneling microscopy (STM) and Fourier-transform infrared (FTIR)
techniques. Orientation of BTAH and TTAH on Cu(100) was studied
In order to study this rather complicated process, we have applied
conventional electrochemical techniques (linear sweep voltammetry
and rotating disk electrode voltammetry, RDE) combined with
in situ electrochemical quartz crystal microbalance, EQCM, meas-
urements. The frequency changes measured using the EQCM are
highly informative with regard to the formation and removal of ad-
sorbate adlayers.
17
Experimental
19
by Walsh et al. by the near-edge X-ray absorption fine structure
NEXAFS) technique. At submonolayer coverage, BTAH was found
All solutions were prepared from analytical grade CuCl и2H O,
(
2
2
NaCl, and HCl, and triply distilled water. Benzotriazole (1-H-ben-
zotriazole, BTAH), Ն99% pure, and tolyltriazole (5-methyl-1-H-
benzotriazole, TTAH) Ն98% pure, were obtained from Merck and
Aldrich Chemical Company, respectively. Before electrochemical
experiments the solutions were purged with high purity argon to
remove oxygen.
to adsorb with the molecular plane close to perpendicular to the sur-
face. At multilayer coverage, BTAH was oriented with the molecular
plane tilted by approximately 45Њ from the plane of the substrate.
Although anodic behavior of metals in the presence of corrosion
inhibitors is of primary interest, the influence of the same additives on
their electrodeposition is also essential for understanding the inhibi-
tion mechanism. However, the influence of BTAH on the copper elec-
trodeposition has received scant attention. To our knowledge, the
cathodic deposition of copper in the presence of BTAH was studied
The RDE used in this investigation was made from platinum
2
(
99.99% Pt) and had the geometric surface area of 0.196 cm . Volt-
20-22
only from acidic sulfate solutions.
This paper refers to the cathodic deposition of copper on platinum
from chloride solutions with different quantities of BTAH or TTAH.
These investigations were undertaken in order to determine the poten-
tial range of adsorption/desorption of these compounds on platinum
and to gain a deeper understanding of the complicated mechanism of
the inhibiting effect on the deposition of copper. The chloride medi-
um was chosen in order to separate the successive electrode reactions
which occur in different potential ranges in this medium. This behav-
ior results from an important stabilization of Cu(I) species by forma-
23
tion of relatively strong chloride complexes.2 Conversely, Cu(II) and
4
chloride ions form only weak complexes. In order to avoid the
development of Cu O layer on the electrode surface, the solutions
2
were acidified with HCl to pH 3. It should be stressed that copper
electrodeposition from chloride media has remained of interest for a
practical reason originating in the feasibility of obtaining dense cop-
25
per deposits at acceptable current densities.
ϩ
Figure 1. Distribution diagram for benzotriazole: (a) BTAH , (b) BTAH,
and (c) BTA .
2
z
Ϫ
E-mail: malyszko@pu.kielce.pl
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Journal of The Electrochemical Society, 147 (5) 1758-1762 (2000)
1759
S0013-4651(99)07-097-X CCC: $7.00 © The Electrochemical Society, Inc.
ammetric experiments were performed using a measuring system
described earlier.
As seen from Fig. 2, the limiting current of the first wave is not
well defined. In order to understand this behavior, cyclic voltammo-
grams with the use of RDE were recorded. Under these conditions,
all the soluble products of electrode reaction are removed from the
RDE surface, while the deposited or adsorbed products can be mon-
itored during the reversal sweep. In the experiment, the potential
scanning was reversed in the positive direction at various potentials
of the first wave of Cu(II) reduction. Figure 3 displays some of the
cyclic RDE voltammograms obtained with a successive increase of
the cathodic potential limit. As seen, the runs made in the potential
range from ϩ0.60 to ϩ0.10 V (curves a and b) show negligible hys-
teresis on the reverse scan. A quite significant hysteresis loop was
observed upon reversing the potential scan in the limiting current re-
gion, at Ϫ0.25 V (curve c). It is also worth emphasizing that a small
and broad shoulder approached around ϩ0.35 V during the anodic
potential scan. As is discussed below, this shoulder may correspond
to the dissolution of a thin CuCl layer which is formed during the
negative going scan and slightly retards the electrode Reaction 1.
A noticeable influence of BTAH on the voltammetric curves is
2
6
The EQCM measurements were carried out using an apparatus
constructed in the Institute of Physical Chemistry (Warsaw). Planar
AT-cut quartz crystals 1.4 cm in diam were operated at the funda-
2
mental frequency of 5 MHz. Platinum film electrodes (0.323 cm
area) were evaporated onto the crystals. The roughness factor of the
electrode surface was determined from measurement of the quantity
of charge required to form a layer of adsorbed oxygen and was found
to be 1.5. The mass sensivity of our EQCM was determined experi-
7
mentally for lead electrodeposition/dissolution to be 5.65 ϫ 10 Hz
2
Ϫ1
cm g . The frequency stability was better than 0.1 Hz for a sam-
pling gate time of 0.1 s. Details of the EQCM system used in this
study were similar to those previously described.26,27
Electrode potentials were measured and are quoted with respect
to the saturated calomel electrode with NaCl solution, SCE(NaCl),
which exhibits potential 6 mV more negative than that filled with
saturated KCl solution.
All electrochemical experiments were carried out at 25 Ϯ 0.2ЊC
in an air thermostat with the forced air circulation.
Ϫ7
generally seen at concentrations larger than 1 ϫ 10 M. The curves
denoted b-d in Fig. 2 were recorded for the BTAH content from 1 ϫ
Results and Discussion
Ϫ5
Ϫ3
10
to 10 M. BTAH was present in the electrolytes from the
A series of cathodic RDE voltammograms was recorded for solu-
tions each containing 1 ϫ 10Ϫ3 M Cu(II), 1 ϫ 10 M HCl, 1.0 M
NaCl, and various amounts of BTAH. Some representative curves
are reproduced in Fig. 2. Curve a, which was recorded for an in-
hibitor-free solution, split into two waves characterized by a very
large potential separation and nearly the same height. This picture
corresponds to a classical two-stage mechanism for the deposition of
copper and is in good agreement with earlier voltammetric studies
beginning. The addition of BTAH causes a considerable decrease in
the current in the potential range between ϩ0.2 and Ϫ0.3 V, in which
Reaction 1 occurs. Moreover, the limiting current plateau becomes
less well defined with the increasing amount of the additive. On the
contrary, the presence of BTAH in the solution studied weakly affects
the second wave. With increasing concentration of BTAH the second
wave moves slightly to more negative potentials but remains nearly
constant in height. The results obtained are somewhat surprising
Ϫ3
2
8,29
on the Cu(II)/Cu system in the chloride medium.
The first cath-
30,31
because it is known from the literature
that surface-active com-
odic wave is attributed to a solution-confined redox reaction
pounds inhibit the ion-transfer reactions much more effectively than
the electron-transfer reactions. These findings may suggest that the
ion transfer Reaction 2 occurs on the Pt surface free of BTAH.
Cu(II) ϩ eϪ o Cu(I)
[1]
In the region of the more negative wave, further reduction of cuprous
ions generated at the electrode surface occurs, Eq. 2
Cu(I) ϩ eϪ o Cu
[2]
leading to a two-electron overall process.
Figure 2. Voltammetric curves for the reduction of 1 ϫ 10Ϫ3 M Cu(II) at a
Ϫ3
Pt RDE from the solutions containing 1.0 M NaCl and 1 ϫ 10 M HCl as
Figure 3. Cyclic voltammograms for the Cu(II) reduction at Pt RDE in the
same solution as in Fig. 2, curve (a). All voltage scans were initiated at 0.6 V,
and reversed at (a) 0.15, (b) 0, and (c) Ϫ0.25 V. Rotation frequency: 30 Hz;
potential scan rate: 5 mV s . Arrows indicate directions of potential
scanning.
Ϫ5
a background electrolyte. Concentration of BTAH: (a) 0, (b) 1 ϫ 10 , (c)
ϫ 10 , (d) 1 ϫ 10 M. The voltammogram (e) is the residual current
obtained for the background electrolyte. Rotation frequency: 30 Hz; potential
Ϫ4
Ϫ3
1
Ϫ1
Ϫ1
scan rate: 5 mV s
.
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1
760
Journal of The Electrochemical Society, 147 (5) 1758-1762 (2000)
S0013-4651(99)07-097-X CCC: $7.00 © The Electrochemical Society, Inc.
Further RDE voltammetric experiments were performed in the
related with the hydrogen adsorption according to the theoretical
predictions of Schumacher. A hysteresis is observed during the ini-
tial portion of reverse scan. Note that the frequency change returns
Ϫ5
37
presence of TTAH. The curves recorded for 1 ϫ 10 and 1 ϫ
Ϫ3
Ϫ3
10
M TTAH in solutions of 1 ϫ 10 M Cu(II) are presented in
Fig. 4. A comparison of Fig. 2 and 4 reveals that TTAH acts much
stronger as an inhibitor than BTAH at the same concentrations. It
should be stressed that first wave does not show limiting current be-
havior. A near linear increase in the cathodic current takes place in the
whole potential range up to Ϫ0.35 V. At more negative potentials, the
second wave is observed due to the deposition of metal copper.
The lowest voltammogram (curve d) was obtained for the back-
ground electrolyte alone. The increase in the cathodic current ob-
served in the potential range between Ϫ0.3 and Ϫ0.7 V is related to
the hydrogen evolution reaction (HER) which results in the adsorp-
to zero after termination of the potential cycle.
Ϫ3
The EQCM response for the solution containing 1 ϫ 10
M
BTAH is shown as curve b. Before this experiment, the Pt electrode
was held for a few minutes at the initial potential of ϩ0.60 V, where
the specific adsorption of benzotriazole is known to occur. A fre-
quency increase of 5.2 Hz was observed between ϩ0.45 and Ϫ0.28 V
in the negative potential sweep. This value of the frequency change
Ϫ2
corresponds to ⌬m/A ϭ Ϫ0.092 ␮g cm . This mass loss may be
attributed to the partial desorption of BTAH, and is equivalent to a
Ϫ10
change in surface concentration of BTAH ⌬⌫ ϭ Ϫ5 ϫ 10
mol
3
2
Ϫ2
tion of H atoms on the Pt surface. The adsorbed atoms can be oxi-
dized back to protons upon reversing the direction of the potential
scan. The large increase in current at potentials below Ϫ0.7 V is due
to proton reduction to hydrogen gas.
cm , assuming a surface roughness of 1.5.
The negative frequency changes observed around Ϫ0.3 V cannot
be reconciled with a gain in weight. The explanation of this frequen-
cy shift in the rather unexpected direction involves the desorption of
BTAH in this potential range. The electrode surface covered by
BTAH molecules is a hydrophobic one so that an interfacial slip aris-
es which changes the coupling between electrode and electrolyte.
Unfortunately, in the case of slip, no quantitative model was avail-
The use of the electrochemical quartz microbalance (EQCM) can
yield additional important information concerning the mass changes
at the metal/solution interface during the electrode process. The
change in the resonant frequency is usually analyzed as an effective
3
3
40
mass change using the Sauerbrey equation. However, it is known
able. The removal of the organic film results in hydrophilicity of the
3
4-36
from the literature
that the shift of resonant frequency of the
electrode surface and leads to much stronger coupling of resonator
shear to the electrolytic bulk (nonslip conditions). As a consequence
of the hydrophobic-hydrophilic transition, a large decrease in the res-
onant frequency is observed. This effect is enhanced by the hydrogen
adsorption occurring in a close potential range. A significant hystere-
sis loop is observed during the reverse potential scan, but the fre-
quency returns to the initial level at the conditioning potential. The
hysteresis may be due to the slow adsorption process of BTAH.
EQCM measurements in the presence of TTAH (curve c) gave
results qualitatively similar to those obtained in the BTAH solution.
The frequency increase due to the gradual desorption of TTAH is
near linear during the potential sweep from ϩ0.60 to Ϫ0.20 V. This
finding is consistent with the linear increase in current within the
EQCM is a result of several effects, such as a change of the rough-
ness or hydrophilicity of the surface during the potential scan.
In this series of experiments, the potential was scanned at 5 mV
sϪ1 from ϩ0.60 to Ϫ0.58 V and back to ϩ0.60 V. EQCM and voltam-
metric curves were recorded simultaneously. Figure 5 shows EQCM
traces during scans in acidified solutions containing 1 M NaCl in the
Ϫ3
absence (curve a) and presence of 1 ϫ 10 M BTAH (curve b) or 1 ϫ
Ϫ3
10
M TTAH (curve c). Since the resonant frequency changes are
positive on the ordinate axis, positive mass changes are in the opposite
direction.
The residual ⌬f vs. E response curve (curve a) remained practi-
cally flat during the negative potential sweep as long as the potential
was more positive than ca. Ϫ0.3 V. Below this potential, a signifi-
cant decrease in frequency is observed. Such an unusually great fre-
quency decrease has already been detected by other authors37-39 on
platinum electrode in acidic sulfate solutions. This effect was ex-
plained in terms of an increase of the surface hydrophilicity and cor-
Figure 4. Effect of addition of TTAH on voltammetric curves for the reduc-
Figure 5. Frequency responses obtained during potential scans at the Pt-coat-
Ϫ3
Ϫ3
tion of 1 ϫ 10 M Cu(II) at a Pt RDE: (a) without addition, (b) after addi-
ed quartz crystal in 1.0 M NaCl and 1 ϫ 10 M HCl; (a) background elec-
Ϫ5
Ϫ3
Ϫ3
Ϫ3
tion of 1 ϫ 10 , (c) 1 ϫ 10 M TTAH. Curve (d) is the residual current.
trolyte only, (b) with 1 ϫ 10 M BTAH, (c) with 1 ϫ 10 M TTAH. Poten-
Ϫ1
The other experimental conditions as in Fig. 2.
tial scan rate 5 mV s ; no convective mixing.
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Journal of The Electrochemical Society, 147 (5) 1758-1762 (2000)
1761
S0013-4651(99)07-097-X CCC: $7.00 © The Electrochemical Society, Inc.
first wave in Fig. 4 (curves b and c). The decrease in the interfacial
cy decrease (mass increase) started just after the reduction current of
Cu(II) began to flow. The frequency decrease of about 20 Hz is
observed on going from ϩ0.30 to ϩ0.15 V, i.e., in the region of ris-
ing part of the first reduction peak. In this potential range, the reso-
nant frequency decreases in proportion to the electrode potential,
and thereby to the time of electrolysis. The potential decrease in the
limiting current region results in further decrease up to 41 Hz at
Ϫ0.29 V. This value corresponds to a mass increase, ⌬m/A, of ca.
mass is 0.18 ␮g cmϪ2, which corresponds approximately to the
Ϫ10
Ϫ2
change in the surface concentration of Ϫ9 ϫ 10
mol cm . A fre-
quency decrease at potentials below Ϫ0.25 V can be explained by an
increase in the surface hydrophilicity as already discussed.
Further insight into the inhibition mechanism of the Cu(II) reduc-
tion was gained from the EQCM measurements carried out in solu-
tions containing Cu(II) ions in the absence and presence of the or-
ganic additives. Figure 6 depicts voltammograms (A) and ⌬f-E
curves (B) that were simultaneously recorded during scans for the
Ϫ2
0.73 ␮g cm . The bulk deposition of copper is indicated by a sharp
frequency decrease and cathodic current increase at potentials more
negative than Ϫ0.3 V.
Ϫ3
cathodic reduction of 1 ϫ 10 M Cu(II). For the sake of clarity,
only the negative-going scans are shown. In the inhibitor-free solu-
tion (curves a and aЈ), the initial stages of the ⌬f vs. E trace show no
change until the onset of Cu(II) reduction is achieved. The frequen-
The simplest explanation of the above results may involve the
precipitation of a CuCl layer on the electrode surface. This effect
was observed in numerous studies during the anodic dissolution of
copper as well as its electrodeposition on a foreign substrate in solu-
4
1-46
tions containing chloride ions. In several investigations,
only
low concentrations of chloride ions, in the range of 10 to 10Ϫ2 M,
Ϫ4
4
7-52
were used. In other works,
much greater chloride concentra-
tions, were employed, up to 1 M, but the electrolysis was performed
at high current densities. It is obvious that those conditions were
more favorable to the precipitation of CuCl.
In order to clarify these findings, we have constructed the loga-
rithmic concentration diagram for the chloro-complexes of
2
3
copper(I), on the basis of recent literature data. Such diagram
enables the concentration of each species to be quickly estimated if
the total metal and ligands concentrations are known. Figure 7 shows
the distribution of each Cu(I) species as a function of free chloride
Ϫ3
concentration at total Cu(I) concentration of 1 ϫ 10 M. This value
was chosen as the concentration of Cu(I) attainable on the electrode
in the limiting current region during the experiments described
above. As follows from this figure, CuCl Ϫ2 and CuCl3 are the dom-
2Ϫ
Ϫ
inant cuprous complexes in a solution containing 1 M Cl , while the
other Cu(I) species exist in small quantities. Under these conditions,
the equilibrium concentration of Cuϩ is 2 ϫ 10 M. Thus, remem-
Ϫ9
Ϫ7
bering the solubility product of CuCl equal to 2 ϫ 10 , one can
conclude that the greatest concentration of Cuϩ that would be
attained near the electrode surface is 100 times less than that needed
to initiate precipitation of this compound.
It seems most likely that the bulk reduction of Cu(II) to Cu(I) is
accompanied by the copper underpotential deposition which may in-
volve the coadsorption of Cl on the electrode surface. Our conclusion
42-46
is consistent with that reported earlier. Several authors
demon-
strated that chloride anions have a pronounced influence on the cop-
per underpotential deposition at Pt(111) and Pt(100) surfaces. After
the works cited, there is an attractive interaction between Cu and Cl
that leads to formation of a structure in which these species adsorbed
coexist on the Pt surface.
Figure 6. Current (A) and frequency (B) responses obtained concurrently
during a potential scan at the Pt-coated quartz crystal in the solution contain-
Ϫ3
Ϫ3
ing 1 ϫ 10 M CuCl , 1.0 M NaCl, and 1 ϫ 10 M HCl. Concentration of
Figure 7. Logarithmic concentration diagram for the chloro complexes of
2
Ϫ3
Ϫ3
ϩ
Ϫ
3Ϫ
Ϫ
additives: (a, aЈ) 0, (b, bЈ) 1 ϫ 10 M BTAH, (c, cЈ) 1 ϫ 10 M TTAH.
Potential scan rate 5 mV s ; no convective mixing.
copper(I): (a) Cu , (b) CuCl(aq), (c) CuCl , (d) CuCl , (e) CuCl . Total
2
4
Ϫ1
Ϫ3
analytical Cu(I) concentration was 1 ϫ 10 M.
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1
762
Journal of The Electrochemical Society, 147 (5) 1758-1762 (2000)
S0013-4651(99)07-097-X CCC: $7.00 © The Electrochemical Society, Inc.
Turning now to the frequency responses in the presence of BTAH
curve bЈ) and TTAH (curve cЈ) in the solution, the same features are
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(
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1
Upon further potential change below Ϫ0.3 V the resonant fre-
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Acknowledgments
This research was supported in part with funds from Inwex,
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