Systematic investigation of current efficiency during brass deposition from a pyrophosphate electrolyte using RDE, RCE, and QCM

Article abstract of DOI:10.1016/S0013-4686(00)00461-8

The pyrophosphate electrolyte has been used for brass deposition as a replacement for cyanide electrolytes. However, there is considerable confusion about the current efficiency of brass deposition from this electrolyte. This study uses a rotating disc electrode (RDE), a rotating cylinder electrode (RCE) and a quartz crystal microbalance (QCM) to systematically determine the current efficiency of copper, zinc, and brass deposition from a pyrophosphate electrolyte. Electrodeposition of brass on RCE and QCM show that the current efficiency for copper, zinc as well as brass is well below 100%. Simultaneous measurement of current and frequency at a QCM showed that pyrophosphate is adsorbed at the electrode surface during copper reduction. However, when zinc is present in the solution, pyrophosphate adsorption is blocked.

Full text of DOI:10.1016/S0013-4686(00)00461-8

Electrochimica Acta 45 (2000) 3691–3702
www.elsevier.nl/locate/electacta
A systematic investigation of current efficiency during brass
deposition from a pyrophosphate electrolyte using RDE,
RCE, and QCM
K. Johannsen a, D. Page b, S. Roy b,*
a Department of Chemistry, Uni6ersity of Aarhus, Langelandsgade, 8000 Aarhus, Denmark
b Department of Chemical and Process Engineering, Uni6ersity of Newcastle, Newcastle, NE1 7RU, UK
Received 1 September 1999; received in revised form 6 December 2000
Abstract
The pyrophosphate electrolyte has been used for brass deposition as a replacement for cyanide electrolytes.
However, there is considerable confusion about the current efficiency of brass deposition from this electrolyte. This
study uses a rotating disc electrode (RDE), a rotating cylinder electrode (RCE) and a quartz crystal microbalance
(QCM) to systematically determine the current efficiency of copper, zinc, and brass deposition from a pyrophosphate
electrolyte. Electrodeposition of brass on RCE and QCM show that the current efficiency for copper, zinc as well as
brass is well below 100%. Simultaneous measurement of current and frequency at a QCM showed that pyrophosphate
is adsorbed at the electrode surface during copper reduction. However, when zinc is present in the solution,
pyrophosphate adsorption is blocked. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Electrochemical quartz crystal microbalance; Electrodeposition; Pyrophosphate; Copper; Zinc; Alloy deposition
Although the earliest suggestion of feasibility of Cu–
Zn deposition from pyrophosphate electrolytes was
proposed in 1912 [4], the first systematic data on metal
deposition from this electrolyte was gathered by Rama
Char and co-workers in 1956 [3,5–7]. Their research
showed that copper, nickel, zinc and tin or an alloy of
these metals can be electrodeposited from pyrophos-
phate electrolytes. They suggested that, in order to
plate brass, an electrolyte containing a low ionic con-
centration of copper and a high ionic concentration of
zinc should be used. The metal to pyrophosphate ion
ratio should be 2.2. An excess of potassium pyrophos-
phate is necessary as supporting electrolyte as well as to
maintain the bath pH at :8.5. At this pH the elec-
trolyte is stable (does not precipitate) [6,7] and the bath
is buffered. They found that sound deposits were ob-
tained at electrolyte temperatures between 40–80°C
and that the current efficiency of metal deposition was
close to 100% [6,7].
1. Introduction
Brass has been electrodeposited mainly for decorative
purposes or for corrosion protection. Much of the
earlier work as well as many industrial processes have
focused on electrodeposition from a cyanide electrolyte,
mainly because brass is deposited from cyanides at high
current efficiency (\75%) and it has a smooth and
bright texture [1,2]. Alternative complexing agents such
as citrate, thiocyanate, tartrate and pyrophosphate [1–
3] have been sought in recent years because cyanide
processes pose environmental and safety hazards. The
most promising electrolyte that has received attention
from several research groups, is a pyrophosphate-based
complexing solution [3–7].
* Corresponding author. Tel.: +44-191-2227274; fax: +44-
191-2225292.
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(00)00461-8

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
More recent empirical investigations on copper and
In fact, their experimental results showed that py-
rophosphate was inappropriate for brass electrodeposi-
tion. In addition, it is recognised that oxide and
hydroxides are deposited when there is significant hy-
drogen evolution (leading to changes in surface pH),
that means current efficiency for zinc and brass deposi-
tion was low. Copper and zinc deposition investigations
show that there is a gap in the current efficiency results
— although hydrogen evolution is low during copper
deposition, it is significant when zinc is reduced from
the same solution.
zinc electrodeposition from pyrophosphate electrolytes,
however, have raised questions regarding the current
efficiency results of Rama Char and co-workers. For
example, Cu–Ni deposition from a pyrophosphate so-
lution showed that current efficiencies can be as low as
50% when alloys containing up to 50% Ni were plated
[8]. High current efficiencies were obtained only when
the copper content in the electrolyte was high. In a
subsequent study, the same researchers reported that
the current efficiency for copper deposition from a
pyrophosphate solution was as low as 50% and that for
Cu–Ni alloy deposition was about 20% [9]. A separate
study also indicated that electrodeposition of Cu–Zn
alloys from pyrophosphate electrolytes exhibits a low
current efficiency when the deposition current is high
[10]; no systematic data on current efficiency, however,
was provided in that work. No information on side
reactions has been provided in these studies.
In the absence of adequate systematic data on brass
deposition from pyrophosphate solutions, some insight
can be gleaned from individual copper or zinc reduction
studies. Konno and Nagayama [12] have studied copper
reduction from electrolytes containing copper and
potassium pyrophosphate. In their work, Cu(II) con-
centration was either 0.1 or 0.5 M and the ratio of
copper to pyrophosphate ranged between 2 and 5. They
found that the reversible potential for copper shifted to
more negative values when the concentration of py-
rophosphate was increased. In addition, two different
Tafel slopes were obtained for copper reduction; the
change in slope occurred at −0.75 V versus SCE. They
proposed that copper was reduced from [Cu(P2O7)2]6−
complex and that the deposition mechanism was depen-
dent on the pyrophosphate concentration. The electro-
chemical reactions for copper reduction are:
Additional insight to Cu–Zn co-deposition can be
gained from the work carried out by Despic et al. [15].
They performed linear sweep voltammetry and poten-
tial step investigations to determine the current–poten-
tial behaviour during copper and zinc co-deposition at
a rotating disc electrode from a pyrophosphate–oxalate
electrolyte where copper and zinc are bound to py-
rophosphate and oxalate ligands, respectively. Al-
though their solution was complex, containing sodium
pyrophosphate, oxalic acid, sodium carbonate, sodium
hydroxide, boric acid, and copper and zinc sulphates,
some general conclusions can be drawn from their
work. Their results indicate that pyrophosphate ions
are adsorbed on the electrode surface prior to copper
deposition, as was proposed by Konno and Nagayama
[12]. In addition, they found that in the presence of zinc
in the electrolyte, copper was discharged at lower over-
potentials. They suggest that this is due to a decrease in
pyrophosphate adsorption in the presence of zinc. They
also computed the current efficiency for copper by
integrating the charge for the anodic and cathodic
sweeps in the potential region where only copper is
plated and dissolved. The charge consumed in the
dissolution process was 50% of that consumed during
the deposition cycle; this led them to conclude that
copper dissolved as Cu(I). Notably, this result is based
on an assumption that current efficiency during copper
reduction is 100%. Current efficiency for zinc deposi-
tion, on the other hand, was found to be low, about
10%. Current efficiency for Cu–Zn deposition, how-
ever, was not reported.
Cu(P2O7)62− l CuP2O27− +P2O47−
CuP2O72− +2e− l Cu+P2O74−
Copper is reduced directly from [Cu(P2O7)2]6− when
the concentration of (P2O7)4− in the solution is high.
They showed that pyrophosphate is adsorbed on the
surface at −0.75 V versus SCE where smooth copper
deposits were obtained [12,13]. They stated that the
current efficiency for copper deposition was 100%, al-
though the procedure used for current efficiency mea-
surements was not reported.
Electrodeposition of zinc and brass from a pyrophos-
phate electrolyte was carried out by Vagramyan et al.
[14]. They encountered several problems: (1) a signifi-
cant amount of oxygen (\28% atomic) was deposited
with zinc and brass, thereby bringing into question
whether the deposits were metallic, (2) most deposits
were cracked and therefore not useful, (3) metallic
deposits contained only a low atomic content of zinc.
Clearly, current efficiency data on brass deposition
from pyrophosphate electrolytes is incomplete and con-
fusing. Since current efficiency has major implications
on feasibility of alloy deposition as well as the range of
alloy composition that can be obtained, it is important
to fill this gap in our knowledge. In this work we have
carried out a systematic investigation on current effi-
ciency during brass deposition from a pyrophosphate
electrolyte. Cu, Zn and Cu–Zn have been plated at a
rotating disk electrode (RDE), a rotating cylinder elec-
trode (RCE), and
a quartz crystal microbalance
(QCM). Whilst an RDE or RCE allows easy control of
hydrodynamics, a QCM enables measurement of cur-
rent efficiency in situ. Potentiostatic as well as potentio-

K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
3693
dynamic data have been collected during the deposition
of individual metals and their alloy. Current efficiency
has been found by integrating the current from linear
sweep and gravimetric measurements. Alloy composi-
tion has been determined by carrying out energy disper-
sive X-ray analysis (EDX). Adsorbed species on the
cathode surface, as well as current efficiency of metal
and alloy deposition have been determined in situ by
using the QCM.
plating solutions, respectively. The pH was adjusted to
8.4 for all solutions using pyrophosphoric acid. This
metal to pyrophosphate ratio is identical to that sug-
gested by Sree and Rama Char [6] so that we can
compare our results directly with their findings.
Cyclic and linear sweep voltammetry experiments at
a rotating disc electrode were carried out by a Versastat
potentiostat controlled by personal computer with
Voltamaster software. The rotating disc was supplied
by Radiometer Copenhagen. It was placed in a cell
supplied by the same company. The working electrode
was a 2 mm diameter gold disc. The counter electrode
was a platinum wire, and a standard calomel electrode
(SCE), placed at a distance of 2 mm from the tip of the
RDE, served as reference. Potential data were corrected
for IR drop in the solution.
Potentiostatic metal deposition at a rotating cylinder
was carried out by a Scanning Ministat supplied by
Sycopel Scientific. The RCE was 6 cm long and 1 cm in
diameter. The cylinder was made of steel. The steel
cylinder was polished in a lathe to 2400 grit sand paper
and then polished by diamond paste. Since brass does
not nucleate on steel, a 10 mm thick copper layer was
then plated on the cylinder from an electrolyte contain-
ing 0.1 M copper pyrophosphate. This was done imme-
diately before each plating experiment in order to avoid
any oxide formation. The anode was a Pt-coated tita-
nium mesh concentric cylinder placed at a distance of 3
cm from the cathode. The reference electrode was con-
nected to a luggin placed 2 mm from the surface of the
cathode.
2. Experimental
Electrolytes were prepared from reagent grade chem-
icals supplied by Aldrich. Zinc pyrophosphate, which is
not commercially available, was prepared by precipita-
tion of a saturated solution of zinc sulphate and potas-
sium pyrophosphate. The resulting zinc pyrophosphate
was filtered and dried in an oven at 110°C for over 24
h. All solutions were made of ultra pure water (\10
MV purity). Three separate solutions were prepared to
carry out Cu, Zn, and Cu–Zn deposition. Copper
deposition was carried out from a solution composed of
0.01 M Cu2P2O7, 0.37 M K4P2O7 and 0.12 M KNO3.
The solution for electrodepositing zinc contained 0.09
M Zn2P2O7, 0.21 M K4P2O7 and 0.12 M KNO3. The
electrolyte used for alloy plating was composed of 0.01
M Cu2P2O7, 0.09 M Zn2P2O7, 0.23 M K4P2O7, and 0.12
M KNO3. In the following sections, these electrolytes
are called copper plating, zinc plating and brass/alloy
Two different electrochemical quartz crystal mi-
crobalances have been used in this study. The QCM at
Newcastle was a commercial Seiko QCA 917 (supplied
by EG & G) while the other was manufactured at the
University of Aarhus. The latter used a Hewlett Pack-
ard 53131A frequency counter. The EG & G QCA used
10 MHz AT-quartz crystals. The home made QCM
employed 5 MHz AT-cut quartz crystals. Both sets of
crystals were sputtered with gold, which formed the
substrate material for metal deposition. The surface
area available for metal deposition was 0.1 and 0.38
cm2 at Newcastle and Aarhus, respectively. The fre-
quency of the sputtered quartz crystals were found to
be 8.5 MHz and 4.9 MHz for the EG & G and home
made QCMs, respectively, when they were placed in
distilled water. The frequency of the crystal during
metal deposition was monitored visually on the oscilla-
tor; frequency readings were stored every 0.1 s.
Fig. 1 shows the electrochemical cells used in quartz
crystal experiments. The quartz crystal, which was the
working electrode, was kept vertical in all experiments,
because this position produced a stable and repro-
ducible frequency. The working electrode was condi-
tioned by cycling the potential between 1.3 and −0.3 V
versus SCE in a solution containing 0.23 M K4P2O7
Fig. 1. Quartz crystal microbalance apparatus used in (a)
Newcastle and (b) Aarhus.

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
before each experiment. The anode was a platinum
mesh placed in the bulk solution and the reference was
an SCE or a saturated mercury sulphate electrode
(SMSE). It should be noted that in the Section 3 all
potentials are referred to the SCE, with the difference
in potential between SCE and SMSE being 0.40V [16].
The reference electrode was placed at a distance of 2
mm from the cathode surface at the University of
Newcastle. In Aarhus, the reference was placed at a
distance of 4–5 mm. In all QCM experiments the
solution conductivity was high (0.1 V) and the total
current did not exceed 20 mA. The resulting ohmic
drop in solution is no more than 2 mV and therefore
potential data were not compensated for IR losses.
3.2. Rotating cylinder experiments
Cyclic voltammetry experiments were then carried
out at a rotating cylinder electrode to verify the cur-
rent–potential data obtained at the RDE. The rotation
speed was set to 1000 rpm, and a scan rate of 0.30 V
s−1 was used. Although visual observation confirmed
that copper and zinc were plated at the cylinder, it was
not possible to dissolve the deposits by sweeping the
potential anodically. On the application of positive
overpotentials a blue–green film was formed on the
copper and a grey film was formed on zinc, which
blocked further dissolution. Both RDE and RCE re-
sults indicate that current efficiency obtained from
cyclic voltammetry is inaccurate.
A gravimetric technique was then employed to deter-
mine the current efficiency. Copper and zinc were
plated on to the RCE at a particular potential. The
current efficiency was measured by comparing the
charge passed during an experiment and that calculated
by measuring the change in the weight of the cylinder
due to the deposited metal using Faraday’s law. The
current efficiency for copper and zinc deposition are
very low near the reversible potential; it was, therefore,
difficult to determine the current of copper and zinc
accurately near these potentials. We have presented
current efficiency data that could be measured
accurately.
Fig. 3(a) shows the current efficiency for copper and
zinc deposition. The efficiency for copper discharge is
small at low and high overpotentials, and passes
through a maximum of 45%. Current efficiency for zinc
is below 10% at all potentials. Current–potential data
for copper and zinc were computed using the current
efficiency and the applied current density. These data
are presented in Fig. 3(b). As illustrated in the figure,
copper reduction commences at −0.8 V. A current
plateau, similar to a mass transfer limiting current, is
observed at potentials below −1.0 V. For a rotation
speed of 1000 rpm, a metal ion concentration of 0.01
M, and a diffusion coefficient of 7.2×10−6 cm2 s−1
the diffusion limiting current is calculated to be 10.8
mA cm−2 [17]. The current plateau is somewhat higher
than this value (and rises slightly with increasing poten-
tial), presumably due to an increase in agitation due to
hydrogen evolution. Zinc deposition occurs only at
potentials below −1.5 V. Although zinc concentration
is nine time higher than that of copper, a high zinc
deposition current cannot be attained due co-evolution
of hydrogen. In fact, at high negative overpotentials,
non-metallic, dark deposits were obtained.
3. Results
3.1. Rotating disc experiments
Polarisation data for Cu and Zn electroreduction at a
rotating disc electrode is presented in Fig. 2. The rota-
tion speed of the RDE is 1000 rpm and the scan rate is
0.30 V s−1. The electrode potential is cycled from 0.20
V downwards and back. For the case of copper reduc-
tion, the lower limit of the scan is −1.1 V, whereas for
the zinc deposition it is −1.5 V. A cathodic current for
copper is observed when the electrode potential reaches
below −0.8 V. As the potential is swept to more
negative values, the current increases. The reverse scan
exhibits hysteresis, with the reversible potential shifting
towards −0.7 V. A dissolution peak with a shoulder is
observed at potentials greater than −0.4 V. A cathodic
current is observed with the zinc deposition electrolyte
at −1.2 V and dissolution peak is observed at −1.3 V.
A comparison of charge consumed during the anodic
and cathodic part of the potential sweep for copper and
zinc discharge showed that less than 10% of the reduced
species were oxidised. This suggests that hydrogen dis-
charge consumed a significant portion of the current
during cathodic polarisation of copper and zinc elec-
trodeposition. A visual inspection of the electrode
showed that some amount of deposit remained on the
surface at the end of a voltammetry experiment.
Partial currents for copper and zinc discharge can be
computed from the polarisation data if the current for
hydrogen evolution is known. In an attempt to deter-
mine the current for hydrogen evolution, cyclic voltam-
metry was carried out with a solution containing 0.23
M K4P2O7 and 0.12 M KNO3. It was found that the
reduction current was ‘higher’ when copper and zinc
pyrophosphate are absent from the electrolyte, possibly
because the metal ion complexes inhibit hydrogen evo-
lution by adsorbing on the electrode surface. This
means that the partial current for copper and zinc
cannot be calculated from RDE polarisation data.
Partial currents for copper and zinc during brass
plating were then calculated by co-depositing the two
metals from a brass plating solution. Brass was plated
at a constant potential, and the amount of metal de-
posited was determined from weight measurements.

/
4
(
1
3
Fig. 2. Cyclic voltammogram for copper reduction from a Cu electrolyte and Zn reduction from copper and zinc plating solutions. Disc rotation speed 1000 rpm. — copper;
zinc.
– – – –

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
nitrogen in the deposit was within the background
signal in an EDX analysis. The plated brass, therefore,
was considered to be metallic. In addition, islands of
only copper or zinc were not detected, which suggests
that the alloy was a solid solution.
The partial currents for copper and zinc, as calcu-
lated from these measurements, are presented in Fig. 4.
Copper reduction commences at −0.5 V during brass
plating, which is much higher than that observed for
copper deposition. A current plateau of a magnitude
similar to the mass transfer limiting current for Cu(II)
is observed beyond −0.8 V. Zinc reduction commences
at potentials below −1.2 V, higher than that observed
during electrodeposition from a zinc plating solution.
This shows that brass deposition is feasible at potentials
below −1.2 V, although the plating efficiency is low,
i.e. between 22 and 15%. In order to determine if
current efficiency changed as the deposit got thicker
(i.e. varied with time), brass foils of thickness ranging
between 5 and 30 mm were plated. The current effi-
ciency was found to remain unchanged.
The important difference between the deposition
characteristics of individual metals (copper and zinc)
and their alloy is a shift in the reversible potential for
copper and zinc to more positive values during brass
plating. Konno and Nagayama [12] and Despic et al.
[15] have reported such a shift during copper deposition
from pyrophosphate. Konno and Nagayama [12] sug-
gested that the shift is due to the fact that pyrophos-
phate cannot adsorb on the surface when copper
concentration in the solution is high, and Despic and
co-workers [15] surmised that it may be because zinc
complexes with pyrophosphate which lowers the py-
rophosphate concentration in solution. Our results
show that pyrophosphate adsorption as well as hydro-
gen evolution are inhibited when Cu(II) and Zn(II) are
present in the electrolyte, thereby enabling brass
deposition.
Fig. 3. (a) Current efficiency and total measured current for
copper and zinc reduction from copper and zinc plating solu-
tions. (b) Partial currents for copper and zinc at different
potentials at a RCE as obtained from gravimetric measure-
ments. , copper; ꢀ, zinc.
The composition of the deposit was determined by
EDX using an Oxford Link ISIS II with a 4 nm
diameter probe. The weight of each metal was com-
puted from gravimetry and alloy composition. Fara-
day’s Law was used to calculate the charge consumed
during the discharge of each metal. It should be men-
tioned that the content of oxygen, phosphorus and
3.3. Quartz crystal microbalance experiments
Before metal and alloy deposition experiments could
be performed, a series of calibration measurements for
the QCM had to be undertaken. The oscillation fre-
quency of a QCM in a shearing fluid is expected to
decrease by the following [18]
1/2
p d
ꢀ ꢁ
L
L
3/2
air
DfL=
f
(1)
yvQdQ
When metal is deposited from an electrolyte on to the
quartz crystal, there is a corresponding to a decrease in
frequency according to the Sauerbrey equation [19]:
−Df=Cf2LDm
(2)
Fig. 4. Partial current densities for copper and zinc reduction
during brass deposition from a pyrophosphate electrolyte as
determined from gravimetric measurements and EDAX. ꢀ,
copper; , zinc.
where fL=fair−DfL. Since C and fL are related to the
physical properties of the crystal substrate and the
electrolyte, the equation reduces to

K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
3697
Df=hDm
(3)
change in frequency was observed around −0.03 V due
to adsorption and desorption of sulphate ions.
where h can be derived experimentally by depositing a
metal on the crystal at 100% current efficiency.
Once h is known for a QCM, mass changes due to
adsorption, electro-deposition or dissolution, and the
formation of viscous layers on the electrode surface can
be detected by measuring changes in frequency and
combining it with the Faraday equation to give a
‘current’ corresponding to the mass change
In order to compare mass transfer conditions at the
QCM with those at the RCE, a the limiting diffusion
current for copper deposition from the 0.1 M CuSO4/
0.5 M H2SO4 solution was also measured. In these
experiments copper was plated for 300 s at different
potentials and the mass change was calculated from the
frequency change of the crystal. It was found that the
limiting current of copper was 11.7 mA cm−2; this
means that the diffusion limiting current for 0.01 M
CuSO4 (which is the concentration used in the copper
M lf
icalc=h
(4)
zF lt
and brass plating solutions) should be 1.17 mA cm−2
nine times lower than that at an RCE.
,
In Eq. (4), icalc is the calculated current, lt is the time
duration, M the atomic mass of the metal (or the
average mass in the case of an alloy), z the number of
electrons exchanged in the reaction, and F Faraday’s
constant. When a frequency change is solely due to the
change in mass of the crystal due to electrodeposited
metal (or alloy), icalc has to equal the deposition current
of the metal or alloy. If, however, frequency changes
arise from other sources, i.e. adsorption of species on
the surface, these will also result in a signal for icalc
without the passage of a faradaic current. By compar-
ing the value of icalc with the faradaic current one can
determine whether the mass change of the QCM is due
to an electron transfer process. Throughout this paper,
icalc is the current calculated from mass changes of the
crystal; it should not be confused (or equated) with the
measured faradaic current.
The first calibration measurement was carried out to
determine the value of h. Copper was deposited on the
QCM at −0.1 V using a solution of 0.1 M CuSO4 in
0.5 M H2SO4. Since copper reduction occurs with a
current efficiency of 100% at these potentials, this has
been adopted as a standard method to calibrate QCMs
[20,21]. In order to avoid sharp changes in i caused by
sudden changes in Df/Dt, an average of 5 points were
used to calculate i from Eq. (4) in our experiments. The
calibration measurement showed that Eq. (4) was valid
up to 1.7% of fL, which corresponded to a mass change
of 1500 mg cm−2. The frequency changes during cop-
per, zinc and brass plating experiments were much
lower, and therefore Eq. (4) was valid in all our exper-
iments. A typical value for h is 1.39×108 Hz g−1 with
a standard deviation of only 0.01×108 Hz g−1. This is
slightly lower than the theoretical value of 1.49×108
Hz g−1 predicted by the Sauerbrey equation for copper
deposition [18,19] due to the roughness of the gold
substrate.
Following the calibration procedure, a cyclic voltam-
metry scan between 0.50 and −1.50 V was performed
with a 0.23 M K4P2O7 and 0.12 M KNO3 solution to
determine the potential region where pyrophosphate
ions adsorb. The results from this experiment are
shown in Fig. 5. The small arrows in the figure indicate
the direction of the scan. The measured current is the
solid line, frequency changes are the dashed line, and
the dotted line shows icalc. As shown in the figure, no
current is measured until −1.1 V, where hydrogen
evolution begins. Frequency changes show other phe-
nomena on the electrode surface: (1) there is a slight
decrease in frequency at −0.65 V (where no current is
measured) and an increase in frequency around −1.0
V, just before hydrogen evolution commences, (2) a
decrease in frequency is observed beyond −1.4 V, a
region where there is significant hydrogen evolution.
The first decrease in frequency is attributed to an
adsorption of non-electro-reactive species on the sur-
face, a pyrophosphate species [12]. These species begin
to desorb at −1.0 V, which results in an increase in
frequency. It can be seen that the adsorption and
desorption of pyrophosphate ions shows up as a peak
in the icalc curve. The noisy decrease in frequency
beyond −1.4 V is due to formation and discharge of
gas bubbles on the electrode surface. However, there is
an overall decrease in the QCM frequency of about 300
Hz during the potential cycle. This overall frequency
change is caused by the vigorous hydrogen evolution,
due to attached bubbles or formation of precipitates.
A cyclic voltammetry scan at a QCM for a copper
plating solution is presented in Fig. 6. The solid line,
which denotes the measured current, shows that an
electroreduction of copper begins at potentials close to
−0.8 V, as was found in the RCE and RDE experi-
ments. There is a hysteresis in the reverse scan, and the
reduction current is zero at −0.9 V. An anodic current
is observed when the potential exceeds −0.4 V where
two anodic peaks for copper dissolution is observed.
The deposit is dissolved completely by the time the
potential reaches 0.4 V.
A second calibration measurement was performed to
determine if icalc followed the measured faradaic current
during cyclic voltammetry. Here, copper was plated and
dissolved by scanning the electrode potential between
0.50 and −0.30 V. It was found that the icalc followed
closely the measured current with only one exception: a

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
Fig. 5. Cyclic voltammogram of the supporting electrolyte with scan rate 0.02 V s−1 showing the measured current (—), calculated
current icalc (……) and frequency change (----).
Frequency changes at the QCM in Fig. 6 impart
more information on processes occurring at the elec-
trode. In the outward scan there are two clear separate
regions where there is a decrease in frequency. The first
region starts at −0.65 V where the measured current is
zero, corresponding to pyrophosphate adsorption. A
faster change in frequency is observed below −0.75 V.
It lasts until −0.92 V. The second region lies between
−0.92 and −1.05 V. Here again there is a slow
decrease in frequency between −0.92 and −1.05 V.
Beyond these potentials there is a monotonic decrease
in frequency as more and more copper continues to
deposit. When the potential is reversed, copper contin-
ues to plate until −1.05 V. A slow increase in fre-
quency is observed after that, it lasts until −0.4 V after
which a steady increase in frequency is observed due to
Fig. 6. Cyclic voltammogram of the copper plating solution with scan rate=0.03 V s−1. Measured current —, calculated current
icalc …… and frequency change -----.

K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
3699
Fig. 7. Cyclic voltammogram of the brass plating solution with a scan rate of 0.05 V s−1 showing the measured current —,
calculated current icalc …… and frequency change ----.
copper dissolution. Frequency changes show that py-
rophosphate ions are adsorbed between −0.65 and
−1.05 V. Two different kinds of copper are deposited
on the QCM: (1) one which is reduced at potentials
below −0.75 V and (2) another which is deposited at
potentials below −1.05 V. From frequency changes
and Eq. (3) it is calculated that the first layer consists of
approximately 30 mono-layers. This is also observed in
the reverse scan where two separate peaks for copper
dissolution are observed.
agreement, and icalc is computed with z=2, copper
dissolves as Cu(II) species.
Although an attempt was made to carry out zinc
deposition from a Zn2P2O7/K4P2O7/KNO3 solution,
zinc deposits could not be obtained because of excessive
hydrogen evolution (which produced frequency oscilla-
tions in the QCM). In addition, the gold layer was
often stripped from the QCM by gas bubbles which
stops further metal deposition. These results are in
agreement with the RCE results where zinc was plated
at very low current efficiency (cf. Fig. 3).
The dotted curve, which represents icalc shows some
interesting aspects: (1) that the peak at : −0.85 V is
bigger and sharper than that observed at −0.9 V in the
reverse scan, (2) the measured currents are much bigger
than those calculated from frequency changes during
the deposition cycle, (3) that there are two peaks for
copper deposition and dissolution, and (4) that the
measured and calculated anodic current at potentials
greater than −0.4 V are in good agreement. The first
aspect shows that when there are two processes occur-
ring at the electrode surface, i.e. copper deposition and
pyrophosphate adsorption, which are additive between
−0.65 to −1.05 V, a single, strong peak may be
observed. In the reverse scan, where only pyrophos-
phate desorbs, a weaker peak (within the same poten-
tial regime) is obtained. The second aspect shows that
the current efficiency for metal deposition is low. The
third observation shows that the copper deposited on
the gold exhibits different electrochemical and thermo-
dynamic behaviour from copper depositing on copper.
The fourth aspect shows that since the measured and
calculated currents for copper dissolution are in close
A cyclic voltammogram for brass deposition is de-
picted in Fig. 7. The measured current shows that
copper deposition commences below −0.6 V, as was
observed in the RCE experiments. Two minor current
peaks are observed at −0.92 and −1.1 V on the
forward scan followed by a dominant current attributed
mainly to hydrogen evolution. On the reverse scan, the
reduction currents are found to be lower. A broad
dissolution peak is observed beyond −0.4 V, where
copper dissolution is expected. Although brass is ex-
pected to dissolve at less noble potentials, a layer of
copper covers the deposited brass (plated during the
reverse scan) and prevents brass from dissolving. Inter-
estingly, the dissolution current is maintained beyond
−0.4 V, possibly due to the slow oxidation of brass.
A comparison of icalc and measured current reveals
more details regarding brass deposition. It is striking
that there is no significant pyrophosphate adsorption
between −0.65 and −1.05 V. This shows that the
presence of zinc ions in the solution inhibits pyrophos-
phate ion adsorption. Two peaks for icalc are detected at

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
4. Discussion
potentials of −0.9 and −1.2V. As before, this shows
that two different kinds of copper are deposited. From
frequency changes it is found that the initial layer
consists of approximately ten monolayers of copper. At
potentials more negative than −1.2 V as well as the
remaining forward scan, calculated values of deposition
current remain much lower than the measured current.
Frequency changes due to metal deposition in the re-
verse scan are also smaller. These data show that
although copper deposition occurs at a reasonable cur-
rent efficiency, it drops when brass is deposited. A
decrease in mass of the QCM begins at −0.32 V,
showing copper dissolution. Fair agreement between
measured copper oxidation currents and icalc show that
Cu dissolves as Cu(II) species. It must be mentioned
that the deposit continued to dissolve after the scan was
terminated (the potential was held at 0.4 V) and the
frequency returned to the initial value after 15 s show-
ing that the deposit was completely dissolved.
Thereafter, a set of potentiostatic experiments was
carried out to determine the current efficiency during
brass deposition. Brass was plated for 300 s at a
particular potential on the crystal and the current effi-
ciency and partial current was determined from fre-
quency changes, the measured current, and the alloy
composition as measured by EDX. Fig. 8 shows the
current efficiency and composition of brass at different
potentials. As was found in the RCE experiments, the
current efficiency decreased from 30 to 5% as the
potential was lowered and the alloy became more zinc-
rich. Oxygen and phosphorus were not detected in the
EDX analysis showing that the deposits were metallic.
Our experimental results show that it is difficult to
measure current efficiency during brass deposition from
a pyrophosphate electrolyte because the current effi-
ciency is low. Cyclic voltammetry and anodic stripping
are difficult to implement and interpret because current
efficiency changes with applied potential. In addition,
the stripped metal forms an anodic film which prevents
further dissolution of the metal. Gravimetric techniques
are suitable, but one has to be careful that non-metallic
components are not incorporated in the deposit. How-
ever, it is clear that since brass is plated at potentials
where substantial hydrogen evolution occurs (especially
for Zn–rich alloys), it is not possible to plate this alloy
at a high current efficiency. Under no condition was
our current efficiency comparable to those claimed by
Rama Char and coworkers [5–7]. Although we carried
out plating experiments at room temperature, and they
reported results for deposition experiments carried out
at 40°C, an increase in temperature should increase all
electrochemical reactions and therefore the change in
current efficiency should not be disproportionate. In
this regard our results corroborate the current efficien-
cies found by researchers in references [8–11].
The QCM measurements also provide some insight
into the electrodeposition process. For example, when
there are no metal salts in the electrolyte, pyrophos-
phate is adsorbed on the electrode surface and hydro-
gen evolution is the highest. On the addition of copper
pyrophosphate, pyrophosphate is adsorbed more
strongly and the hydrogen evolution current decreases.
This shows that there is competitive adsorption be-
Fig. 8. Copper content and current efficiency of during potentiostatic deposition of brass from the brass plating solution.

K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
3701
tween the (pyrophosphate) species participating in the
discharge of hydrogen and copper. On the addition of
zinc pyrophosphate, the adsorption of pyrophoshate is
blocked altogether. Researchers have suggested that
pyrophosphate adsorption on the electrode is dimin-
ished in presence of zinc due to strong ligation with
zinc atoms [15]. The decrease in pyrophosphate adsorp-
tion results in copper and zinc reduction at less noble
potentials. Despic et al. have called it a catalytic effect
of zinc during copper deposition [15]; however, it is the
competitive adsorption of prophosphate species leading
to hydrogen, copper, and zinc discharge which governs
the electrode reaction.
partial current as well as deposit characterisation. How-
ever, neither an RDE or and RCE are useful for the
detection of adsorbed species or following in situ the
different processes occurring the electrode. A QCM is
useful to collect such information, although it must be
kept in mind that it lacks mass transfer regulation.
Recently a quartz crystal micro-balance fitted at the tip
of a rotating disc has been used for colloid deposition
[22], Cu–Se plating [23] and gold leaching [24]; clearly
the use of this tool will be useful to study the deposition
of alloys such as brass.
The QCM experiments support the kinetic model for
copper deposition proposed by Konno and Nagayama
[12]. These researchers found that there were two differ-
ent Tafel slopes during copper reduction from a py-
rophosphate electrolyte. They proposed that copper
reduction showed a higher Tafel slope at low overpo-
tentials because pyrophosphate species were adsorbed
on the electrode. At higher overpotentials the Tafel
slope was smaller because pyrophosphate species were
not present. This model was indirectly corroborated by
Despic et al. [15] who noted two dissolution peaks
during cyclic voltammetry experiments. Our QCM ex-
periments provide more direct evidence which show
that copper is discharged in the presence and absence of
adsorbed pyrophosphate species at low and high over-
potentials, respectively.
The current efficiency results bring into question a
few interpretations regarding copper deposition and
dissolution reported previously. For example, Despic et
al. showed that copper dissolved as a Cu(I) species in a
pyrophosphate–oxalate electrolyte. Their calculations
are based on an assumption that copper reduction
occurs at 100% current efficiency. However, we found
that copper deposition is characterised by a current
efficiency of 40–50%. Therefore, when the metal is
stripped anodically, the amount of metal dissolved will
be only 50% compared to that if the current efficiency
were 100%. Since copper can dissolve in Cu(I) or Cu(II)
forms, this may be construed as copper oxidising to its
cuprous form. However, the simultaneous measurement
of faradaic current and frequency changes show conclu-
sively that copper dissolves as Cu(II) in a pyrophos-
phate electrolyte. It is unclear if this is also true for a
pyrophosphate–oxalate electrolyte.
5. Conclusion
In this study a rotating disc electrode, a rotating
cylinder electrode, and a quartz crystal microbalance
were used to systematically investigate the current effi-
ciency during the deposition of copper, zinc, and brass
from a pyrophosphate electrolyte. Current efficiency
measurements at the RDE and RCE proved difficult,
since it was often not possible to strip completely the
deposit off the electrode in the anodic sweep of the
cyclic voltammogram. Hence gravimetry was used to
determine current efficiency at the RDE and RCE. The
QCM was used to determine the current efficiency in
situ. It was found that the current efficiency for copper,
zinc as well as brass are well below 100%, in contrast to
those reported by Rama Char and co-workers [5–7].
Partial current densities for copper and zinc during
brass deposition at an RCE were measured using
gravimetry and EDX measurements. It was found that
copper and zinc are discharged at more noble potentials
during brass deposition as compared to that observed
for individual metal deposition. QCM measurements
showed that pyrophosphate adsorption is significant
when copper is plated from a pyrophosphate solution
containing copper ions, but is inhibited when zinc ions
are added to the electrolyte. It was found that two
different kinds of copper are plated from a pyrophos-
phate electrolyte: one in the presence and the other in
the absence of adsorbed pyrophosphate. Simultaneous
measurement of faradaic current and frequency changes
of the QCM showed that copper dissolves entirely as
Cu(II) ions. It was concluded that the quartz crystal
micro-balance is an important tool for measuring cur-
rent efficiency in situ and examining adsorption effects
during an electrodeposition process.
This investigation shows the importance and poten-
tial of using QCM for investigation regarding metal
and alloy deposition. Current–potential data from
cyclic voltammograms at an RDE are difficult to inter-
pret when side reactions such as hydrogen evolution or
oxide film formation occurs. In addition, deposits from
an RDE are often not amenable to surface analysis and
characterisation, which limits the calculation of partial
current data. An RCE is more useful in determining
Acknowledgements
The authors would like to thank Professor D. Britz
and Dr T.A. Green for useful discussion on several
matters. This work was supported by a University
Research Studentship from the University of New-

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K. Johannsen et al. / Electrochimica Acta 45 (2000) 3691–3702
[10] A.C. Hamilton, Plat. Surf. Finish. 81 (1995) 48–50.
[11] M. Ishikawa, H Enomoto, M. Matsuoka, C. Iwakura,
Electrochim. Acta 39 (1994) 2153.
[12] H. Konno, M. Nagayama, Electrochim. Acta 23 (1978)
1001.
castle. EDX at Newcastle was carried out by the Mate-
rials Analysis Unit and by Prof. N. Pind and J.K.
Christensen in Aarhus. The help of Sabine Bonhomme
in some RCE experiments is also acknowledged.
[13] H. Konno, M. Nagayama, Electrochim. Acta 22 (1977)
353.
[14] T. Vagramyan, J.S.L. Leach, J.R. Moon, Electrochim.
Acta 24 (1979) 231.
References
[1] F.A. Lowenheim, Modern Electroplating, Wiley, New
York, 1974.
[15] A. Despic, V. Marinovic, V.D. Jovic, J. Electroanal.
Chem. 339 (1992) 473.
[2] A. Brenner, Electrodeposition of Alloys — Priciples and
Practice, vol. I, Academic Press, London, 1963.
[3] Vaid and T.L. Rama Char, J. Sci. Ind. Res. (India), 15 B
(1956) 509.
[4] K. Thompson, M. de Kay, Met. Chem. Eng., 10 (1912)
458–461.
[5] T.L. Rama Char, Electroplat. Met. Finish. 10 (1957)
347–349.
[6] V. Sree, T.L. Rama Char, Electroplat. Met. Finish. 12
(1959) 326–330.
[16] A.J. Bard, L.R. Faulkner, Electrochemical Methods,
Fundamentals and Methods, Figure E-1, Wiley, New
York, 1980.
[17] M. Eisenberg, C.W. Tobias, C.R. Wilke, J. Electrochem.
Soc. 101 (1954) 307.
[18] S. Bruckenstein, M. Shay, Electrochim. Acta 30 (1985)
1295.
[19] G. Sauerbrey, Z. Phys. 155 (1959) 206.
[20] M. Zhou, N. Myung, X. Chen, K. Rajeshwar, J. Elec-
troanal. Chem. 398 (1995) 5.
[7] T.L. Rama Char, Electroplat. Met. Finish. 12 (1959)
391–392.
[21] C. Klingenberg, MSc Thesis, University of Aarhus, May,
1999.
[8] M. Ishikawa, H. Enomoto, New Mater. New Process. 2
(1983) 243–251.
[9] M. Ishikawa, H. Enomoto, M. Matsuoka, C. Iwakura,
Electrochim. Acta 40 (1995) 1663–1668.
[22] N. Shirtcliffe, Colloids Surf. A 155 (1999) 277.
[23] A. Marlot, J. Vedel, J. Electrochem. Soc. 146 (1999) 177.
[24] J. Zheng, I.M. Ritchie, S.R. La Brooy, P. Singh, Hy-
drometallurgy 39 (1995) 277.
.