Electrochemical behavior of gold (III) in cyanide-free bath with 5,5′-dimethylhydantoin as complexing agent

Article abstract of DOI:10.1016/j.electacta.2011.09.078

Gold electrodeposits are prepared in a cyanide-free bath with 5,5′-dimethylhydantoin (DMH) as complexing agent. The electrochemical behavior of the electrodeposition is then investigated together with the influence of additive A (pyridyl-compound) as an additive on the nucleation and growth of gold using electrochemical techniques on gold working electrode at different temperatures. Cyclic voltammogram consists of a single cathodic reduction wave at -0.62 V which corresponds to the reduction of Au(III) to Au without anodic oxidation wave observed. The diffusion coefficient of Au(III) in the bath is found to be ～10^-6 cm²/s and the energy of activation (43 kJ/mol) is deduced from the cyclic voltammograms at different temperatures. The nucleation and growth of gold on gold working electrode is investigated by chronoamperometry. The progressive nucleation mechanism is found for gold deposition using Scharifker-Hills' model with three-dimensional (3D) diffusion-controlled growth nucleation. The introduction of the additive A does not influence this mechanism. The gold electrodeposits are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and cathodic polarization measurements. Experimental results indicate that additive A increases the cathodic polarization of bath, refines the grains of electrodeposit and changes the preferred orientation of electrodeposit from [1 1 1] direction to [2 0 0] direction.

Full text of DOI:10.1016/j.electacta.2011.09.078

Electrochimica Acta 58 (2011) 516–522
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Electrochemical behavior of gold (III) in cyanide-free bath with
ꢀ
5
,5 -dimethylhydantoin as complexing agent
Yang Xiaowei , An Maozhong , Zhang Yunwang , Zhang Linb
a
a,∗
b
a
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001,China
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, 621900, China
b
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
Received 30 June 2011
Received in revised form
Gold electrodeposits are prepared in a cyanide-free bath with 5,5 -dimethylhydantoin (DMH) as com-
plexing agent. The electrochemical behavior of the electrodeposition is then investigated together with
the influence of additive A (pyridyl-compound) as an additive on the nucleation and growth of gold
using electrochemical techniques on gold working electrode at different temperatures. Cyclic voltam-
mogram consists of a single cathodic reduction wave at −0.62 V which corresponds to the reduction of
Au(III) to Au without anodic oxidation wave observed. The diffusion coefficient of Au(III) in the bath
2
6 September 2011
Accepted 29 September 2011
Available online 6 October 2011
−6
2
is found to be ∼10 cm /s and the energy of activation (43 kJ/mol) is deduced from the cyclic voltam-
mograms at different temperatures. The nucleation and growth of gold on gold working electrode is
investigated by chronoamperometry. The progressive nucleation mechanism is found for gold deposi-
tion using Scharifker–Hills’ model with three-dimensional (3D) diffusion-controlled growth nucleation.
The introduction of the additive A does not influence this mechanism. The gold electrodeposits are
characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and cathodic polariza-
tion measurements. Experimental results indicate that additive A increases the cathodic polarization of
bath, refines the grains of electrodeposit and changes the preferred orientation of electrodeposit from
Keywords:
Gold electrodeposition
Non-cyanide
DMH
Cyclic voltammograms
Chronoamperometry
[1 1 1] direction to [2 0 0] direction.
©
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Gold electrodeposits have a wide application in microelec-
searching for alternative cyanide-free gold plating electrolytes,
and gold plating bath with C5H N O has been proposed [11].
8
2
2
Hydantoins, known as drugs and agrochemicals, are weak acids
+
tronic, optoelectronic and microsystem industry [1–3]. They can
be generally classified into soft and hard gold. Soft gold elec-
trodeposition is usually done using cyanide or sulfite electrolytes,
but these electrolytes have their respective inherent problems of
poisonousness, process incompatibility or instability [4–6]. Com-
mercial sulfite baths have solved the stability problem by being
operated at pH 8 or incorporating proprietary stabilizing additives
such as ethylenediamine, which makes it possible to operate the
bath in a lower pH range of 5–8. More recently, 2,2 -bipyridine
has been used to stabilize sulfite bath, possibly because it forms a
complex with Au . Furthermore, polyamine, such as ethyldiamine,
and aromatic nitro compound such as nitrobenzene have also been
simultaneously used to stabilize Au(I)-sulfite complex so that the
bath can be operated at an even lower pH of 4–6.5 [6–8]. A mix-
ture of sulfite and thiosulfate is less susceptible to degradation,
but sulfur contamination is inevitable, significantly degrades gold
deposit in terms of softness [9,10]. This is why researchers are now
(pKa = 9.1) and can complex a number of metal ions such as Cu and
+
+
−
Ag . The ionization equation of DMH follows: DMH ꢀ H + DMH
and researchers have found that gold (III) forms a four-coordinate
light-stable complex with DMH [12]. However, the main prob-
−
lem encountered in replacing gold cyanide baths is the very high
stability of gold–cyanide complex. So in our present investigation,
DMH is also used as complexing agent to get a cyanide-free gold
plating bath with high stability. The properties and quality of elec-
trodeposits in this kind of electrolyte are directly determined by the
nucleation and growth mechanism of electrocrystallization. There-
fore, it is of great significance to study the nucleation and growth
during gold electrochemical deposition for the purpose of improv-
ing the properties and quality of electrodeposit.
ꢀ
+
To the best of our knowledge, not much work has been done on
the electroplating mechanism of gold electrodeposit. Liew et al. [13]
used standard rotating disk and cyclic voltammetry to investigate
the electrochemical behavior of gold thiosulfate–sulfite system,
and found that the diffusion coefficient of the mixed ligand elec-
−
6
2
trolyte is 1.2 × 10 cm /s, which lies somewhere between the
diffusivities of gold thiosulfate and gold sulfite electrolytes. The
electrodeposition of gold from a new electroplating system for
∗ Corresponding author. Tel.: +86 45186413721; fax: +86 45186418616.
E-mail address: mzan@hit.edu.cn (A. Maozhong).
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.09.078

Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
517
gold with iodide and thiosulfate has been studied using rotat-
ing disk voltammetry [14]. The cathodic electron transfer is slow,
with a transfer coefficient of ˛ = 0.76 ± 0.02. Chronoamperometry
based on current transient has been used to determine the nucle-
ation mechanism and nucleation rate [15]. The mechanism of gold
alloy electrodeposition from acid baths in the presence of a trace
+
amount of Tl is investigated using chronoamperometry [16]. Anal-
ysis of chronoamperometric measurements shows a progressive
and instantaneous nucleation and growth mechanism at low and
high cathodic potentials, respectively. As a fundamental research,
it is meaningful to investigate the nucleation and growth mecha-
nism of gold deposition in cyanide-free bath and the influence of
additive on the electrodeposition process.
Therefore, DMH is selected in this paper as complexing agent,
and pyridyl-compound as additive in a cyanide-free gold electro-
plating bath. The electrodeposition behavior, rate controlling step,
and the nucleus growth mechanism of gold on gold electrode are
investigated using cyclic voltammograms (CV) and chronoamper-
ometry (CA) techniques. The influence of additive on the nucleation
mechanism and the properties of the gold electrodeposits are also
discussed.
−
Fig. 1. Cyclic voltammograms of [Au(DMH)4] in electrolyte with additive
recorded on gold electrode with T = 318 K at scan rate 50 mV/s.
A
2
. Experimental
the working electrode was polished successively with increasingly
finer grades of diamond polish powder, and finally to a mirror finish
with aqueous slurry of 0.15 ␮m alumina, rinsed with distilled water
and thoroughly dried before each experiment. After the electro-
chemical experiments the electrode was rinsed with distilled water
and dried with an air blast at room temperature. For the electro-
chemical experiments, the bath temperature was kept at 303–323 K
by a thermostat. The cathodic polarisation curves were recorded in
a potential range from the open-circuit potential to −1.5 V (vs. SCE)
at a scanning rate of 1 mV/s.
2.1. Gold electrodeposition
Electrolyte was prepared by dissolving salts in distilled water.
The basic gold electrodeposition bath was: 0.03 mol/L HAuCl4,
.5 mol/L DMH, 0.2 mol/L K PO , 3.5 mmol/L additive A and calcu-
0
3
4
lated amount of KH PO solution (the solution was prepared by first
dissolving 1 mol/L KH PO4 in deionized water. Then, it was added
2
4
2
to gold plating bath in some volume ratio for the pH maintaining
and used as a buffer). It contained HAuCl4 as the main salt, DMH as
the complexing agent and additive A as the main brightener. The
pH of the bath was adjusted to 9.0 using 30% KOH. The HAuCl4 was
fully complexed with DMH and formed a DMH-gold ligand through
the following reaction:
2.3. Characterizations of gold electrodeposits
Gold electrodeposits were characterized by SEM and XRD. SEM
observations were performed under high vacuum by field emission
scanning electron microscopy (FE-SEM, Hitachi S4700) at 25 kV
working voltage. XRD analysis was done using a D/max-3C X-ray
AuCl4] + 4DMH− ꢀ [Au(DMH)4] + 4Cl−
−
−
(1)
[
All the chemicals used in this work were analytical grade.
◦
diffractometer at a scanning rate of 0.02 /s with Cu K␣ radiation.
The bath under investigation was modified according to different
experimental requirements. The bath components were modified
while pH was kept constant.
3. Results and discussion
Gold electroplating experiments were conducted under gal-
2
vanostatic conditions (1 A/dm , 323 K and 10 min) in different
3.1. Analysis of cyclic voltammograms
baths with mild agitation to study the surface morphology and
crystal structure. A cell with an anode of 2.5 cm × 2.5 cm pla-
tinized titanium sheet and a copper sheet cathode of 1 cm × 1 cm
was employed. The copper sheet samples of 1 cm × 1 cm were Ni
electroplated followed by gold electroplating. The purpose of Ni
undercoat deposition was to prevent the diffusion of Cu into Au
top layer. Prior to Ni electrodeposition, samples were polished
with emery silicon carbide paper (8–4 ␮m grain size), thoroughly
cleaned using a solution of HCl and distilled water with the volume
fraction of 1:1, and rinsed with distilled water.
Transient cyclic voltammogram is an efficacious method used
to study the reactions on the working electrode surface. In order
to study the cathode electrode reaction in the electrodeposi-
tion process of gold, electrochemical study was carried out using
cyclic voltammograms in a conventional three-electrode system. As
−
shown in Fig. 1, the voltammogram of [Au(DMH)4] in electrolyte
exhibits one prominent reduction wave and an area of hydrogen
evolution. No anodic dissolution peak but an area of oxygen evo-
lution can be observed on the positive scan. It can be seen from
Fig. 1 that onset potential (Eonset) of Au deposition on gold elec-
trode occurs immediately at Eonset = −0.45 V. A cathodic reduction
peak can be observed at −0.62 V, which corresponds to the reduc-
tion of Au(III) to Au. The scanning area beyond −1.1 V (vs. SCE) on
the cathodic side of cyclic voltammogram may be due to hydrogen
evolution. During the potential scanning from −0.85 V to −0.59 V,
the inverse current is smaller than the direct one, which indicates
that the deposition process is controlled by mass transport [17]. A
“hysteresis loop” appears in the range of potential from −0.59 V to
−0.19 V, which means the three-dimensional growth of nuclei of
gold occurs on the gold electrode [18]. The scanning area beyond
2.2. Electrochemical evaluation
Electrochemical evaluation was carried out using cyclic
voltammetry, chronoamperometry and cathodic polarisation mea-
surements in a three-electrode glass cell on a GAMRY Reference
6
00 electrochemical workstation. A saturated calomel electrode
(
(
SCE) was used as the reference electrode and a platinum foil
2
99.99%, 2 cm ) was employed as the counter electrode. A gold elec-
2
trode with a working surface of 0.07 cm (Ø 0.3 cm) was used as
the working electrode. Before every electrochemical measurement,

5
18
Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
−
Fig. 2. Cyclic voltammograms of [Au(DMH)4] in gold plating bath with additive A
Fig. 4. Cathodic peak currents Ip vs. ꢀ1/2 for reduction of Au(III) at different temper-
atures.
recording on gold electrode at different scan rates at 323 K: (a) 10 mV/s; (b) 20 mV/s;
c) 40 mV/s; (d) 60 mV/s; (e)80 mV/s; (f)100 mV/s.
(
where n˛ is the number of electrons involved in the rate deter-
mining step, R is the gas constant, F is Faraday constant, and T is
the absolute temperature in K [19]. The value of ˛n˛ is 0.63 for the
measurements from Eq. (2) at 323 K.
When electrode reaction is controlled by diffusion processes,
the relationship between peak current Ip and scan rate (ꢀ) of CV
curves can be expressed as shown below [20]:
+
1.1 V (vs. SCE) on the anodic side of the cyclic voltammogram may
be due to oxygen evolution. No anodic dissolution peak is observed
on the positive scan, which is attributed to the gold electrodeposit
cannot be dissolved in DMH bath.
As shown in Fig. 2, all the CV results are obtained on the gold
electrode covered with a thin gold film, and there is no “hystere-
sis loop” on the cyclic voltammograms. Cathodic peak current Ip
increases and cathodic peak potential Ep shifts negatively with the
increase of scan rate, which means the reduction of Au(III) on the
gold electrode is not reversible [19]. It can be seen from Fig. 3
that peak potential Ep plotted vs. log ꢀ gives a linear response at
all the temperatures, which is characteristic to an irreversible elec-
trode process for the reduction of Au(III). Charge transfer coefficient
ꢀ
ꢁ
1/2
˛
n˛
Ip = 0.4958nF3/2CAD
1/2
0
ꢀ1/2
(3)
RT
2
2
where A is the electrode area in cm (0.07 cm ), C is the gold concen-
3
2
tration in mol/cm , D0 is the diffusion coefficient in cm /s, n is the
number of exchanged electrons, and ꢀ is the potential sweep rate
in V/s. As shown in Eq. (3), there is a linear relationship between I
(
(
˛, 0.1 ≤ ˛ ≤ 0.9), a measure of symmetry barrier in non-reversible
p
1/2
and ꢀ1 . It can be seen from Fig. 4 that the plots of Ip against ꢀ
/2
quasi- and irreversible) electrode processes, can be determined
from the shift in Ep per 10-fold increase in scan rate as shown below:
at all the temperatures are linear and show that the electrodeposi-
tion of gold is controlled by the diffusion process [21]. So, from the
above discussion it can be concluded that the reduction of Au(III)
to Au on a gold electrode is an irreversible and diffusion-controlled
process. However, the qualitative and quantitative nature of gold
involved in the reduction is not known exactly in the present study.
Therefore, the total concentration of gold taken for voltammetric
measurements is substituted in Eq. (3) to obtain an approximate
diffusion coefficient and this method can also be found in [19]. It
can be seen from Table 1 that the diffusion coefficient mounts with
an increase in the temperature and the values are of the same order
as the diffusion coefficient of gold sulfite electrolyte reported in
1
.15RT
ꢁ
Ep =
(2)
(˛n˛F)
[
13].
The diffusion coefficient is related to the temperature by Arrhe-
nius equation shown in Eq. (4):
(−Ea/RT)
D = Ae
(4)
Table 1
Diffusion coefficients of Au(III) in electrolyte obtained from cyclic voltammetry at
various temperatures.
Temperature (K)
Diffusion coefficient of Au(III) in electrolyte,
D × 10 cm /s
−
6
2
3
3
3
03
13
23
2.30
4.27
6.55
Fig. 3. Variation of cathodic peak potential Ep vs. log ꢀ for reduction of Au(III) at
different temperatures.
[
Au] = 0.03 M; A = 0.07 cm2.

Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
519
Fig. 7. Current–time transients resulting from chronoamperometry experiments
recorded for Au(III) reduction in gold plating bath without additive A on gold work-
ing electrode at 323 K and applied overpotentials of (a) −0.84 V; (b) −0.86 V; (c)
−
Fig. 5. Comparison of cyclic voltammograms of [Au(DMH)4] in gold plating bath
with additive A at various temperatures and scan rate: 60 mV/s.
−
0.88 V; (d) −0.90 V.
where A is the pre-exponential factor, and Ea is the energy of acti-
vation, which can be obtained from the plot of ln D against 1/T. It
is seen from Fig. 5 that Ip increases and the peak potential Ep shifts
towards positive potential with an increase in the temperature. It
can be seen from Fig. 6 that the energy of activation for the diffusion
of Au(III) is 43 kJ/mol.
transients shown in Fig. 7 are characterized by an initial decrease
of negative current which is followed by an increase in the nega-
tive current which leads to maximum value (im) at time tm. This
can be attributed to the three-dimensional growth of gold over
the nuclei. Maximum diffusion current (im) increases with nega-
tive step potential at the initial stage of gold electrocrystallization.
Therefore the shape of potentiostatic current density transients is
a typical 3D growth nucleation process [25]. The transient current
reaches its maximum value followed by a negative current decay
with time; finally the current becomes a constant.
3.2. Nucleation mechanism
Chronoamperometry is an electrochemical technique com-
monly used to study the mechanism of electrodeposition of metals.
During chronoamperometry experiments, current–time transients
are recorded as the potential is stepped down from the open-circuit
potential (OCP) to the potential at which the electrodeposition of
metals occurs [22,23]. A common feature of I–t curves is that in
a very short period of time during the potential step, due to the
double-layer charging, the current increases rapidly and diminishes
further. Then the current increases again and reaches its maximum,
following by subsequent diminution because of the nucleation and
the growth of new phase [24]. Due to the charging of double layer
and the formation of first nuclei on the gold working electrode, the
Much work has been done on the modelling of nucleus growth
mechanisms, and the most commonly accepted is Scharifker–Hills’
models [26], one instantaneous nucleus growth mechanism and
one progressive nucleus growth mechanism. One convenient
method to examine the experimental results is to obtain a linear
1/2
fit for the ascending current with respect to t
for instantaneous
nucleation and t3 for progressive nucleation. In our case, it is
obvious that a good linear relationship of I–t3 curves is shown
in Fig. 8. Therefore, it is reasonable that the progressive nucleation
control for the growth of gold from additive-free bath on working
electrode takes place. All current transients can be presented in a
non-dimensional form by plotting the normalized current (I/Im)2
/2
/2
Fig. 8. I against t3/2 from rising portion of current transient at different applied
overpotentials: (a) −0.84 V; (b) −0.86 V; (c) −0.88 V; (d) −0.90 V.
Fig. 6. Arrhenius plot for diffusion of Au(III) in gold plating bath with additive A.

5
20
Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
Fig. 9. Comparison of experimental data obtained from chronocurrent transients
of gold electrodeposition process from bath without additive A with instantaneous
and progressive nucleation models for overpotentials: (a) −0.84 V; (b) −0.86 V; (c)
Fig. 10. Comparison of experimental data obtained from chronocurrent transients
of gold electrodeposition process from bath with additive A with instantaneous
and progressive nucleation models for overpotentials: (a) −0.88 V; (b) −0.90 V; (c)
−
0.88 V; (d) −0.90 V.
−
0.92 V.
vs. time t/tm. The theoretical transients of instantaneous and pro-
gressive nucleus growth, with 3D under diffusion control, are given
by [26]:For instantaneous nucleation,
efficiency. Compared with the curve of the bath without additive
shown in Fig. 11a, cathodic polarization is improved by adding addi-
tive as shown in Fig. 11b. The adsorption of additives at the interface
influences the nucleation overpotential [28]. As shown in Fig. 11, a
higher overpotential is required for the gold deposition in the bath
with additive A, may be due to the strong adsorption of additive A
at the cathode. Fine gold grains would be obtained at such a high
overpotential as shown in Fig. 12.
It can be seen from Fig. 12a and b that in the presence of addi-
tive A, the grains become smaller. Deposit a is golden-brown in
color and has large gold grains of 0.5–1 ␮m in diameter as shown
in Fig. 12a. Compared with deposit a, the addition of additive A
to the bath causes an increase in cathodic polarization and results
in smaller grains in deposit b. As shown in Fig. 12b, gold deposits
are bright golden-brown in color and the structure of deposit b is
different from that of deposit a. Large gold grains disappear, while
deposit b has some smaller gold grains of 0.1–0.5 ␮m in diameter
and some crystal bulks (less than 1 ␮m in diameter). In general, a
higher cathodic polarization accelerates the growth of finer grains,
ꢀ
ꢁ
2
ꢀ
ꢁ ꢂ
ꢃ
ꢀ
ꢁꢄꢅ
−
1
2
I
t
t
=
1.9542
1 − exp −1.2564
(5)
Im
tm
tm
And for progressive nucleation,
ꢆ
ꢇ
ꢈꢉ
ꢀ
ꢁ
2
ꢀ
ꢁ
ꢀ
ꢁ
2
−1
2
I
t
t
=
1.2254
1 − exp −2.3367
(6)
Im
tm
tm
where I and t are the current density and time, respectively, and Im
and tm are the maximum values of current transients. Our exper-
imental results are compared with the theoretical dimensionless
current–time transients derived from progressive and instanta-
neous nucleation growth models. It can be seen from Fig. 9 that the
experimental results show a good agreement with the theoretical
curves for progressive nucleation under different overpotentials,
which suggests the electrochemical deposition of gold is followed
by the progressive nucleation. It can be seen from Fig. 10 that the
nucleation mode of gold is not changed by the additive. The depo-
sition of gold is still followed by the progressive nucleation in the
presence of additive. Therefore, it can be concluded from the CV
and CA results that the reduction of Au(III) on a gold electrode
corresponds to a three-dimensional progressive nucleation growth
process with diffusion-controlled growth.
3.3. Effect of additive A on properties of gold electrodeposits
Bright gold electrodeposits are obtained from the bath modified
by adding additive A 3.5 mmol/L. To investigate the influences of
this additive in the bath, the cathodic polarization was studied with
or without additive in electrolyte. As shown in Fig. 11, there is a
single wave for potentials higher than −1.2 V vs. SCE in the gold
plating region, and so, it is likely that the cathode reaction is a single
step process [27]:
Au(III) + 3e− → Au(0)
(7)
The suitable range for gold plating in the baths is then about −0.65
to −1.2 V vs. SCE. At a potential higher than −0.65 V vs. SCE, the
gold plating rate is very low. Hydrogen evolution also begins at a
potential lower than −1.2 V vs. SCE, and results in a lower current
Fig. 11. Effect of different concentrations of additive A added to gold plating bath:
(a) 0 mmol/L; (b) 3.5 mmol/L.

Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
521
Fig. 12. Effect of concentrations of additive A added to gold plating bath on surface morphology of gold deposits: (a) 0 mmol/L; (b) 3.5 mmol/L; (c) 7 mmol/L.
so the grains of deposit b are finer than those of deposit a when
the cathodic polarization of bath in the presence of additive A is
larger than that of bath in the absence of additive A in this case. It
can be seen from Fig. 12b and c that the size of grains in deposit is
not markedly affected and does not change significantly with the
increasing concentration of additive A in the bath.
◦
As shown in Fig. 13, the five obvious peaks at 2ꢂ values of 38.3 ,
◦
◦
◦
◦
4
3
4.8 , 64.7 , 77.7 and 81.9 can be indexed as the 1 1 1, 2 0 0, 2 2 0,
1 1 and 2 2 2 peaks of the fcc structure of Au, respectively. The rel-
ative amplitude of Au diffraction peaks increases with the addition
of additive A to the bath. As shown in Fig. 13a, due to the thinness
◦
or porosity of gold deposits, the peak at 52 can be indexed as peak
2
0 0 of Ni element in Cu substrate with Ni plated. The diffraction
peak intensity which corresponds to the [1 1 1] plane is larger than
other peaks, which indicates that the electrodeposit has a well pre-
ferred orientation along [1 1 1] direction in the absence of additive
A in bath. As shown in Fig. 13b, the electrodeposit has a well pre-
ferred orientation along [2 0 0] direction in the presence of additive
A in bath.
3.4. Studies on aged bath
Fig. 13. Effect of concentrations of additive A added to gold plating bath: (a)
0
mmol/L; (b) 3.5 mmol/L.
In order to induce accelerated aging, the fresh gold plating bath
is used extensively in an industrial process (for over 1 week) during
which period the equivalent gold is added according the consump-
tion. The fresh bath is transparent with a faint yellowish color,
and the aged bath has no discernible change in the color and no
precipitates or turbidity is detected in the aged bath. In order to
determine if there is any significant change in the electrochemi-
cal properties of fresh and aged baths, cyclic voltammograms are
obtained at the scan rate of 20 mV/s on the gold working electrode

5
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Y. Xiaowei et al. / Electrochimica Acta 58 (2011) 516–522
at 318 K. And the two CV curves exhibit similar characteristics just
as curve b in Fig. 2. Then, it can be concluded that the aged bath is
stable.
Acknowledgement
The authors would like to thank the Research Center of Laser
Fusion under China Academy of Engineering Physics in Mianyang
for its financial support for this work.
4
. Conclusions
The electrochemical nucleation of gold from
a
cyanide-
References
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.2 mol/L K PO , 3.5 mmol/L additive A and calculated amount of
[
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4
[
wave is observed during the forward scan towards positive poten-
tials, which means the absence of Au dissolution in the bath.
Influence of the scan rate on Au(III) reduction in bath follows an
irreversible diffusion-controlled process on the working electrode.
The diffusion coefficient and energy of activation for the reduction
[
[
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[
[
[
−6
2
of Au(III) is ∼10 cm /s and 43 kJ/mol, respectively. The analysis
of current–time transients indicates that the electrodeposition of
gold on a gold electrode proceeds via progressive nucleation with
[
[
3
D diffusion-controlled growth of nuclei, which is not influenced
7121.
by adding additive. The results of cathodic polarization curves
and SEM indicate that additive A exhibits an inhibition effect on
the deposition of gold and brighter gold deposits can be obtained
from the additive-containing bath. The XRD patterns of gold elec-
trodeposits with or without additive A in electrolyte indicate that
additive A also changes the preferred orientation of electrodeposits
from [1 1 1] direction to [2 0 0] direction.
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[
[
[
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