Formation of alloys upon the simultaneous electrochemical deposition of gold and tin from ethylene glycol and aqueous electrolytes

Article abstract of DOI:10.1134/S0036024416050356

An ethylene glycol (EG) solution containing Au(III) and Sn(IV) compounds, and conditions for the electrochemical deposition of Au–Sn alloy based on AuSn and Au₅Sn intermetallics with total tin content of 30–55 at % are proposed. Fundamental difficulties of the deposition of alloys with high tin content, (including eutectic Au–Sn alloy) from aqueous electrolytes are revealed. It is determined via voltammetry that the simultaneous deposition of gold and tin from aqueous and EG electrolytes proceeds with the depolarization effect of both Au(III) and Sn(IV) as a result of the formation of the alloy, the increase in the rate of tin cathodic reduction being more noticeable in case of EG solution. Formation of SnCl₂EG(H₂O)₂ ⁺ complex upon the dissolution of SnCl₄ · 5H₂O in glycol, the stability of the composition of tetracyanoaurate ions upon the dissolution of K[Au(CN)₄], and the weakening of intermolecular interactions in EG with small amounts of water were revealed via IR spectroscopy. It is suggested that the depolarization effect is due not only to alloy formation, but also to the formation of SnCl₂EG(H₂O)₂ ⁺ cations, their association with Au(CN)₄ ^- anions, and a change in the mechanism of Au(III) and Sn(IV) reduction.

Full text of DOI:10.1134/S0036024416050356

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 5, pp. 1081–1087. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © T.N. Vorobyova, H.M. Maltanova, O.N. Vrublevskaya, 2016, published in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 5, pp. 800–807.
COLLOID CHEMISTRY
AND ELECTROCHEMISTRY
Formation of Alloys upon the Simultaneous Electrochemical
Deposition of Gold and Tin from Ethylene Glycol and Aqueous
Electrolytes
T. N. Vorobyovaa,b, H. M. Maltanovaa, and O. N. Vrublevskayaa
a Research Institute of Physico-Chemical Problems, Belarusian State University, Minsk, 220030 Belarus
b Belarusian State University, Minsk, 220030 Belarus
e-mail: vorobyovatn@gmail.com
Received June 3, 2015
Abstract—An ethylene glycol (EG) solution containing Au(III) and Sn(IV) compounds, and conditions for
the electrochemical deposition of Au–Sn alloy based on AuSn and Au5Sn intermetallics with total tin content
of 30–55 at % are proposed. Fundamental difficulties of the deposition of alloys with high tin content,
(including eutectic Au–Sn alloy) from aqueous electrolytes are revealed. It is determined via voltammetry
that the simultaneous deposition of gold and tin from aqueous and EG electrolytes proceeds with the depo-
larization effect of both Au(III) and Sn(IV) as a result of the formation of the alloy, the increase in the rate of
tin cathodic reduction being more noticeable in case of EG solution. Formation of SnCl2EG(H2O)22+ com-
plex upon the dissolution of SnCl4 · 5H2O in glycol, the stability of the composition of tetracyanoaurate ions
upon the dissolution of K[Au(CN)4], and the weakening of intermolecular interactions in EG with small
amounts of water were revealed via IR spectroscopy. It is suggested that the depolarization effect is due not
only to alloy formation, but also to the formation of SnCl2EG(H2O)22+ cations, their association with
Au(CN)4− anions, and a change in the mechanism of Au(III) and Sn(IV) reduction.
Keywords: electrochemical deposition, intermetallics, Au–Sn alloy, depolarization, complexation.
DOI: 10.1134/S0036024416050356
INTRODUCTION
have dense fine-grained microstructure without den-
drites and whiskers. They can be decorative in appear-
ance, and have good electrical and thermal conductiv-
ities and corrosion resistance [5].
Compared to metallurgical and vacuum technolo-
gies, the electrochemical deposition of metal alloys is
an energy efficient process that allows the saving of
metals due to the local deposition of coatings with the
required thickness. A disadvantage of the processes for
synthesizing alloys from solutions is the complexity of
regulating the ratio of metals in the products, their
phase composition, and their microstructure. Alloys
deposited from solutions often have non-equilibrium
phase composition and include phases that are ther-
modynamically stable only at high temperatures [1, 2].
In this work, we describe the electrochemical syn-
thesis of a binary gold–tin alloy with a definite phase
composition. Eutectic Au–Sn alloy is used as solder in
assembling of various devices, hermetizing frames of
integrated circuits and optoelectronic devices, and
connecting conductors based on copper alloys [3, 4].
The eutectic includes Au5Sn and AuSn crystalline
It is well known that aqueous electrolytes of three
types are used for electrodeposition of Au–Sn alloys:
acidic (pH < 3) electrolytes containing cyanide com-
plexes of gold(III) and tin(IV) compounds [4, 6],
slightly acidic or neutral solutions (pH 3–7) based on
tetrachloroaurate(III) ions and tin(II) compounds
[7], and alkaline solutions (pH > 10.5) based on
gold(I) cyanide complex and tin(IV) compounds [8].
The main disadvantage of slightly acidic aqueous solu-
tions is the low reproducibility of the elemental and
phase composition of alloys as a result of the instability
of electrolytes due to the hydrolysis of Sn(II, IV) com-
pounds, and to the reduction of Au(I, III) by tin(II)
compounds present in the solutions or formed as a
result of the partial reduction of Sn(IV). In strongly
phases at a total tin content of 30 at %. It has a melting acidic or strongly alkaline solutions, etching of the
point of 278°C and low viscosity in molten state. substrate can occur. In addition, the reduction of met-
Au‒Sn coatings with compositions close to eutectic als from aqueous solutions is often accompanied by
1081

1082
VOROBYOVA et al.
Table 1. Dependence of the growth rate (V) and the elemental and phase compositions of Au–Sn coatings on electrolyte
composition (where C is concentration of constituents), temperature (T), and current density (j).
СSn(IV)
,
СAu(III),
Electrolyte number,
solvent
Elemental
composition, at % composition
Phase
j, mA/cm2
T, °C
V, μm/h
mol/L
1.4
mol/L
1, EG
2, EG
0.05
35
10
2.0
Au—64, Sn—36
No crystal-
line phases
Au5Sn, AuSn
AuSn, Au5Sn
AuSn
Au5Sn, AuSn
Au5Sn, AuSn
Au5Sn, AuSn,
amorph. Au
5
5
10
5
1
5
1.8
1.0
3.0
2.4
1.0
4.3
Au—69, Sn—31
Au—59, Sn—41
Au—55, Sn—45
Au—58, Sn—42
Au—56, Sn—44
Au—92, Sn—8
20
35
1.4
0.1
20
20
3, 1 М HCl in water
4, 1 М HCl in water
1.4
1.4
0.05
0.1
35
5
1
4.9
0.9
Au—94, Sn—6
Au—100
Au5Sn, Au
Au
cathodic hydrogen evolution, which degrades the
quality of the coatings.
EXPERIMENTAL
The simultaneous electrochemical deposition of
gold and tin was conducted using aqueous and EG
electrolytes containing K[Au(CN)4] and SnCl4 · 5H2O
(Table 1). The choice of cyanide ions as ligands for
Au(III) was due to the requirement for the high stabil-
ity of the complex compound to ensure the stability of
electrolytes and the maximal shift of the electrode
potential of gold towards negative values. In accor-
dance with recommendations presented in earlier
studies on the electrochemical deposition of tin from
glycol solutions [20], Sn(IV) was introduced in the
form of tin(IV) chloride pentahydrate, which is stable
and readily soluble in glycols. Tin and gold were pres-
ent in the chosen compounds in a higher oxidation
state, which was necessary to prevent redox processes
in the bulk of solutions, and to reduce the gold content
in the alloy, since Au(III) is much harder to reduce
than Au(I). The Sn(IV) concentration in our solutions
was more than an order of magnitude higher than that
of Au(III), as was required for the convergence of the
electrode potentials of the metals and to increase the
proportion of tin in the alloy. The ratio of K[Au(CN)4]
to SnCl4 · 5H2O concentrations was varied within a
wide range, however, Table 1 shows only the composi-
tions of those solutions that basically ensure formation
of the eutectic. The total water content in solutions 1
and 2 was about 6 wt %, including the crystallization
water of SnCl4 · 5H2O and the water absorbed from the
air. To study the role of water, the electrochemical
deposition of tin, gold, and Au–Sn alloy was con-
ducted using electrolytes dried in a desiccator over
freshly calcined calcium chloride, and from aqueous
electrolytes in which an aqueous solution of HCl with
a concentration of 1 mol/L was used as a solvent to
prevent the hydrolysis of Sn(IV) compounds (Table 1,
solutions 3 and 4).
The electrodeposition of Au–Sn alloy from non-
aqueous electrolytes would therefore seem to be prom-
ising. Among the possible solvents suitable for use in
electroplating, EG is perhaps the most promising. It is
a polar liquid and has rather low volatility up to 50–
70°C, but its electrical conductivity is less than that of
water (1.07 × 10−6 S cm–1 vs. 4.5 × 10−3 S cm–1) [9].
The solubility of salts of metals in EG is quite high,
and dissolution is often accompanied by the formation
of complex compounds Me(EG)2n+ with Co(II),
Ni(II), Cu(II), Mg(II), Ca(II), and Sr(II) [10–12].
A disadvantage of EG is its high viscosity compared to
water: ηEG = 19.830 mPa s,
= 1.002 mPa s [9].
η
H2O
When we began the present studies, glycol-based
electrolytes for tin and nickel deposition were well
known, as were tin-plating solutions based on gly-
col/choline chloride mixtures [13–15]. This informa-
tion was the basis for developing ethylene- and propyl-
ene-glycol solutions that allow the electrochemical
deposition of Au–Sn alloy. In [16–19], we demon-
strated the possibility of deposition of Au–Sn coatings
with tin contents of up to 60–70 at. % and determined
specific conditions for the deposition of a eutectic
alloy based on Au5Sn and AuSn intermetallics with
total tin content of 30 at %. We were unable to establish
the factors that determine the efficient co-deposition
of tin with gold, or the nature of the processes that
ensure the formation of crystalline intermetallic
phases when using EG as a solvent. The aim of this
work was to establish the reasons for the differences
between elemental and phase compositions of Au–Sn
alloy that forms during electrochemical deposition
from EG and aqueous electrolytes, and to determine
the conditions necessary for the electrocrystallization
of gold–tin intermetallics.
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FORMATION OF ALLOYS UPON THE SIMULTANEOUS ELECTROCHEMICAL DEPOSITION
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The coatings were deposited under stirring with a growth rate of tin coatings from EG solution rose from
magnetic stirrer in the temperature range of 20–35°C. 2.5 to 4.5 μm/h.
Current density was varied in the range of 1–
10 mA/cm2, since coatings of unsatisfactory quality
and poor adhesion to the substrate formed at higher
current densities, and colloidal gold formed in aque-
ous solutions. Platinum foil (99.99%) was used as
anode, and copper foil (99.9%) was the cathode. The
growth rate of the coatings was estimated by weight,
taking into account the gold : tin ratio.
In case of simultaneous presence of K[Au(CN)4]
and SnCl4 · 5H2O in EG, the deposition of dense light
grayish-yellowish Au–Sn coatings occurred only from
solutions containing no less than 6 wt % of water when
j = 1–10 mA/cm2. The growth rate of the Au–Sn coat-
ings rose in the range of 1–6 μm/h when j was
increased from 1 to 10 mA/cm2, the concentration of
Sn(IV) in the solution was decreased from 2.8 to
1.4 mol/L, the temperature of the solutions was raised
from 20 to 35°C, and water was added into them
(Table 1). Note that all of the changes in the composi-
tion of electrolytes and their temperature lead to a
drop in their viscosity.
The tin content in the Au–Sn coatings depended
strongly on the composition of the electrolytes and
conditions of deposition, and could vary from 0 to
55 at %. The maximum tin content was achieved when
deposition was conducted using EG electrolytes at
room temperature and when current density was
raised to ~20 mA/cm2. The factors that determine the
phase composition of Au–Sn alloys were established.
Thus, crystalline phases of gold–tin intermetallics
were deposited from EG solutions. AuSn formed most
often; Au5Sn appeared more rarely (Table 1). When
the current density was high and/or the content of
Sn(IV) was raised to 2.8 mol/L, crystalline AuSn2 and
AuSn4 could form in the solution [16–18]. The forma-
tion of crystalline tin and gold phases is not typical for
glycol solutions. When j = 10 mA/cm2 and the tin con-
tent in coatings is higher than 35 at. %, X-ray amor-
phous phases sometimes form. Alloys deposited from
aqueous solutions 3 and 4 often include crystalline
gold in addition to AuSn and Au5Sn. Regardless of the
composition of the electrolyte, the electrocrystalliza-
tion of AuSn and Au5Sn phases is observed only when
the tin content in the alloy lies in the range of 10–
44 at %. This is a necessary but not sufficient condi-
tion for the formation of intermetallics corresponding
to the eutectic composition.
The elemental composition of the coatings was
determined via energy dispersive X-ray microanalysis
(EDX) using a Rontec attachment on a LEO 1420
scanning electron microscope. The phase composi-
tion of the alloys was determined via X-ray diffraction
analysis (XRD) on a DRON 3 unit. The IR spectra of
solutions were recorded on an AVATAR 330 spectrom-
eter (Thermo Nicolet) in the region of 400–4000 cm−1
with an accuracy of 1 cm−1. The samples were pre-
pared for analysis by applying thin layers of the inves-
tigated solution onto steel plates.
Voltammetric curves (VAM) were recorded in the
potentiodynamic mode using an Autolab PGSTAT
302N potentiostat/galvanostat at a scan rate of
20 mV s−1. To consider the ohmic drop in the volume
of EG solution, recording was done in the iR-error
compensation mode using currentless measurements
of the electrode potential. No compensation was
required in aqueous electrolytes. A three-electrode
cell with a platinum auxiliary electrode, a silver chlo-
ride reference electrode, and a working copper elec-
trode (foil with an area of 1 cm2) was used. For VAM
study we used EG and aqueous solutions 2 and 4
(Table 1), along with solutions of similar composition,
but including either K[Au(CN)4] (2-Au, 4-Au) or
SnCl4 · 5H2O (2-Sn, 4-Sn) only, and with background
electrolytes containing 6 wt % of water and 0.1–
0.8 mol/L of sodium chloride in EG (solution 5) or in
HCl solution with a concentration of 1 mol/L (solu-
tion 6).
During the simultaneous deposition of tin and gold
from aqueous electrolyte 4 at j = 10 mA/cm2 and room
temperature, Au–Sn coatings grow at a maximum rate
of 10 μm/h and have elemental and phase composi-
tions quite close to eutectic, but with a somewhat
lower tin content (22 at %). However, the electrolyte is
not stable under these conditions.
RESULTS AND DISCUSSION
It was found that virtually no gold, tin, or Au–Sn
alloy was deposited from dried EG solutions. When
6 wt % of water was added to EG containing 0.1 mol/L
of K[Au(CN)4], only traces of gold were observed on
the surface of the cathode, and the slow deposition of
very fine (less than 0.05 μm thick) gold films from an
aqueous potassium tetracyanoaurate solution with a
concentration of 0.1 mol/L occurred. From an EG
SnCl4 · 5H2O solution containing ~6 wt % of water,
rather dense and relatively fine-grained coatings
formed with virtually no dendrites, distinguishing
them from the ones obtained from an aqueous electro-
lyte with similar composition in [20]. When current
The IR spectra of K[Au(CN)4] and SnCl4 · 5H2O
EG solutions were studied to explain the nature of the
strong influence the solvents (EG and water) on the
chemical and phase composition of deposited Au–Sn
coatings (Fig. 1). Analysis of the IR spectra of
K[Au(CN)4] solutions in EG showed that the com-
plexation of Au(III) with glycol does not occur, since
there was no shift in the bands of C–OH and CH2
density (j) was increased from 5 to 10 mA/cm2, the bond vibrations in the region of 1460–1080 cm−1 that
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VOROBYOVA et al.
Table 2. Potentials of appearance of cathode current (Ebegin.reduct, mV) at copper electrode in EG and aqueous solutions,
and cathode current density at potentials of −500 and −550 mV (j500 and j550, mA/cm2)
Solution
2-Sn
2-Au
–475
0.05
0.1
2-Au–Sn
5
4-Sn
4-Au
4-Au–Sn
6
Ebegin.reduct
j500
j550
–430
–430
8.5
–1185
–470
–500
–430
8.5
–740
2.0
5.2
0
0
5.0
35.0
0.1
1.0
0
0
17.0
57.0
The deposition of coatings with good quality and composition closest to the eutectic composition occurs in this region of potentials and
currents.
are characteristic of EG. The wavenumbers of the C– ticipation of both hydroxyl groups of the EG mole-
H bond vibrations (2975–2860 cm−1) in the EG solu-
tion were also unchanged. In all cases, Δν did not
exceed the measurement error ( 1 cm−1). The strong
line at 2200–2000 cm−1 characteristic of the valence
vibrations of terminal CN groups was observed in the
spectrum of potassium tetracyanoaurate in EG [21].
The frequencies of the bands of valence vibrations of
OH bonds in the investigated solution shifted lower by
26–40 cm−1, due possibly to attenuation of the associ-
ation of EG molecules in the presence of tetracyano-
aurate anions. The wave numbers of the peak corre-
sponding to the HOH vibrations (1680–1630 cm−1) of
water molecules changed only slightly (by 1–4 cm−1),
allowing us to assess the invariability of their chemical
environment [22]. Me–C bond vibrations in Au–CN
were observed in the region of 480–420 cm−1 [21].
cules. At the same time, a weakening of the intermo-
lecular interaction in the solvent environment was
observed. This weakening apparently helps to intensify
mass transfer in the solvent, and the positive charge of
SnCl2EG(H2O)22+ complex ions contributes to their
diffusion to the cathode and their discharge.
In the IR spectra of EG solutions that simultane-
ously contain Au(III) and Sn(IV) compounds, peak
shifts and new bonds not characteristic of EG gener-
ally repeat the changes in spectra observed upon the
dissolution of individual SnCl4
· 5H2O and
K[Au(CN)4]. These data indicate the presence of
SnCl2EG(H2O)22+ complex cations and [Au(CN)4]−
anions in the EG solutions, along with the weakening
of intermolecular interaction in EG in the presence of
small amounts of water.
Considerable changes in the solvent spectra
occurred upon the dissolution of SnCl4 · 5H2O in EG
(Fig. 1). We described these changes in [20], and con-
The cathodic branches of the polarization curves
reflecting the electrochemical behavior of copper
electrodes in aqueous or EG solutions of K[Au(CN)4]
and/or SnCl4, and in background solutions 5 and 6,
were recorded to study the patterns of the separate and
cluded
that
complex
compounds
with
SnCl2EG(H2O)22+ composition formed with the par-
T, %
j, mA/cm2
0
4
3
2
1
−40
1
−80
4
−120
2
3
−1000
−800
−600
−400
−200
E, mV
4000
3000
2000
1000
1/λ, сm−1
Fig. 1. IR spectra: (1) EG, (2) 1.4 M SnCl · 5H O solution
Fig. 2. Cathodic branches of voltammograms reflecting
4
2
in EG, (3) 0.1 M K[Au(CN) ] solution in EG, and (4)
the behavior of copper electrode in aqueous solutions of
4
SnCl · 5H O and K[Au(CN) ] solutions in EG with con-
(2) K[Au(CN) ] and (3) SnCl · 5H O; (4) both these salts
4 4 2
and (1) in aqueous background solution 6.
4
2
4
centrations of 1.4 and 0.1 mol/L, respectively.
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FORMATION OF ALLOYS UPON THE SIMULTANEOUS ELECTROCHEMICAL DEPOSITION
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(1)
(а) Au(CN)4−ads + 2e → AuCNads + 3CN−;
j, mA/cm2
(b) AuCNads + e → Auads + CN−;
2
3+
(c) Au(CN)4−ads ↔ Au + 4CN−;
1
ads
0
−2
3+
(d) Au + 3е → Auads
.
ads
The cathodic current corresponding to tin(IV) reduc-
tion from aqueous solution 4-Sn appears at a potential
of −475 mV and increases with further cathodic poten-
tial shift, reaching 10 mA/cm2 at −500 mV, where
individual gold(III) is not reduced (Fig. 2, curve 3).
At −550 mV, the current of Sn(IV) reduction is 35
times stronger than that of Au(III) (Table 2). The
polarization curve has an inflection in the region of
−(515–530) mV; according to [26], at more anodic
potentials the reduction of Sn(IV) to Sn(II) occurs, at
more negative potentials Sn(II) is reduced to Sn(0)
according to the scheme
2
−4
3
−6
−8
4
−10
−1400 −1200 −1000 −800 −600 −400
E, mV
(а) SnCl62− + 2e → SnCl24− + 2Cl−;
Fig. 3. Cathodic branches of voltammograms reflecting
the behavior of copper electrode in EG solutions of (2)
(2)
(b) SnCl24− + 2е → Sn + 4Cl−.
K[Au(CN) ] and (3) SnCl · 5H O; (4) both these salts
4
4
2
and (1) in EG background solution 5.
The substantially higher Sn(IV) reduction current
density as compared to Au(III) reduction is due not
only to the higher (an order of magnitude) concentra-
tion of tin(IV) compounds, but also to the multistage
mechanism of the reduction of Au(III) cyanide com-
plex.
simultaneous reduction of tin(IV) and gold(III)
(Figs. 2 and 3; Table 2).
The cathode current corresponding to the evolu-
tion of hydrogen in aqueous background solution 6
was observed at potentials more negative than
−740 mV (Fig. 2, curve 1), and in background EG
solution 5 at −1185 mV (Fig. 2, curve 1). With acidified
potassium tetracyanoaurate aqueous solution 4-Au,
the cathodic current of gold(III) reduction on copper
The cathodic current corresponding to the simul-
taneous reduction of Au(III) and Sn(IV) from aque-
ous electrolyte 4-Au–Sn is observed from −430 mV,
i.e., earlier than for the reduction of individual Sn(IV)
or Au(III) (Fig. 2, curve 4). At potentials of −500 and
−550 mV, the cathodic current of the simultaneous
electrode appeared at a potential of −500 mV. Up to reduction of Au(III) and Sn(IV) is 8.5 and
57 mA/cm2, respectively, exceeding the reduction cur-
rents of both Sn(IV) and Au(III). The data testify to
the effect of the depolarization of the cathodic reduc-
tion of both metals, which is especially pronounced in
the case of Au(III) reduction. Hydrogen evolution was
not observed in the range of currents and potentials
where dense films of tin, gold, or their alloys are
deposited. This is consistent with the available data on
high overpotential of hydrogen evolution on tin [26]
and gold [23–25].
The appreciable effect of the depolarization of
Sn(IV) cathodic reduction could be due to the process
of alloy formation, but this explanation is not enough
when very strong depolarization of Au(III) reduction
is observed. In the range of potentials up to −530 mV,
tin(II)—a product of stage 2a of Sn(IV) reduction
probably reduces Au(III) from tetracyanoaurate ions
adsorbed on the surface of the cathode, according to
the scheme
−650 mV, the current grew only slightly (to 2 mA/cm2
only) (Fig. 2, curve 2). The path of the cathodic
branch of polarization curve is similar to the one
described in [23, 24], according to which the region of
low currents corresponds to the reduction of Au(III)
to Au(I) with the partial cleavage of cyanide ions from
[Au(CN)4]− ions slightly distorted in the electric field.
Poorly soluble AuCN is adsorbed on the cathode. As a
result of electron transfer, a discharge of Au+, the
desorption of CN− ions, the diffusion of Au atoms on
the surface of the cathode, and their incorporation
into the crystal lattice of gold nuclei occur. With a fur-
ther cathodic potential sweep, the rate of cathodic
reduction increases sharply, and the current reaches
24 mA/cm2 at −700 mV when hydrogen has not yet
evolved. The authors of [23, 25] attributed the current
in this potential range to the reduction of Au3+ ions
formed during the dissociation of complex tetracyano-
aurate ions distorted in a strong electric field, to Au0.
The reduction of Au(III) is described by the scheme
(а) SnCl62− + 2e → SnCl24− + 2Cl−;
(b) SnCl24− + 2AuCNads
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VOROBYOVA et al.
+ 2Cl− → SnCl62− + 2Au + 2CN−;
with the presence of a positive charge, this promotes
(3)
their diffusion to the cathode in an electric field. The
value of the potential of the onset of Sn(IV) cathodic
reduction (−430 mV) indicates that according to the
ability to dissociate in EG, this complex compound
or partial
−
(c) 3SnCl24− + 2Au(CN)4
+ 6Cl− → 3SnCl62− + 2Au + 8CN−.
does not differ greatly from SnCl62− chloride com-
plexes in water (−470 mV). It is possible that Sn(IV)
cathodic reduction in EG also proceeds in two stages
similar to 2a and 2b, with both chloride ions eventually
splitting off from Sn(II), a product of partial reduc-
tion.
As a result, the concentration of gold in the deposited
alloy is greatly exceeds the one we might expect based
on the proximity of potentials of the onset of the
cathodic reduction of Sn(IV) and Au(III). Experi-
mental results show that at high current densities, col-
loidal gold forms in the bulk of the electrolyte as a
result of reaction 3c.
The positive charge of Sn(IV) complex ions in EG
promotes their Coulomb interaction with negatively
charged tetracyanoaurate ions, while such interaction
is not possible in aqueous solutions where Au(III) and
Sn(IV) are included in the composition of anions. We
can assume that as a result of the formation of
When 6 wt % of water is introduced in a 0.1 M solu-
tion of potassium tetracyanoaurate in EG (solution
2-Au), a very weak cathodic current is observed for the
copper electrode at potentials more negative than
−475 mV (Fig. 3, curve 2; Table 2). When the potential
is shifted to −1000 mV, the current of gold(III) reduc-
tion is only 0.8 mA/cm2. These data confirm that gold
coatings cannot be deposited from a 0.1 M solution of
potassium tetracyanoaurate in EG. The cathodic
reduction of tin(IV) from SnCl4 · 5H2O solution in EG
(solution 2-Sn) begins at a potential of −430 mV,
which is less negative than that of Au(III) reduction
from tetracyanoaurate ions. The current of tin(IV)
reduction grows rapidly when the potential shifts to
the cathodic region and reaches ~5 mA/cm2 at
‒550 mV, which roughly corresponds to the current
density for the deposition of tin coatings from EG
electrolyte. In EG solution 2-Au–Sn containing
K[Au(CN)4] and SnCl4 · 5H2O, the cathodic current
appears at −430 mV, as in the case of SnCl4 · 5H2O;
however, the slope of the polarization curve is much
steeper than in the EG-based electrolyte containing
Sn(IV) only (Table 2). The simultaneous reduction of
gold and tin is confirmed by the change in the appear-
ance of the electrode: the copper is covered with a
golden-gray film of the alloy. The evolution of hydro-
gen is not observed during the reduction of Au(III)
and/or Sn(IV). These results prove the depolarization
effect of the cathodic reduction of both gold(III) and
tin(IV). The difference between the slopes of the
curves indicates that the depolarization effect of
Sn(IV) reduction is much more noticeable when using
EG solutions, as compared to aqueous solutions.
SnCl2EG(H2O)2+ –Au(CN)4− associates in EG, the
transportation o2f Au(III) and Sn(IV) to the Helmholtz
layer in an electric field becomes more efficient, as do
the distortion and decay of complex Au(CN)4− anions
and SnCl2EG(H2O)22+ cations in the layer. It is possi-
ble that in EG-based electrolyte, stages 1a and 1b do
not occur in the electroreduction of Au(III), and
stages 1c and 1d become possible even at low electric
field intensities because the delivery to the cathode
and release of Au3+ ions are ensured.
The virtually simultaneous discharge of Au(III)
and Sn(IV) that occurs on the adsorption site of an
associate containing them evidently promotes the
electrocrystallisation of intermetallics. In the EG-
based electrolyte Sn(II), a product of the first reduc-
tion stage of Sn(IV), is not consumed in concurrent
reduction of AuCN; along with the depolarization
effect of the cathodic reduction of Sn(IV) and the
convergence of electrode potentials of gold and tin due
to complexation, this ensures a high proportion of tin
in the resulting alloy.
Of particular note is our explanation of the role of
small amounts of water in EG electrolytes, since the
deposition of tin, gold (in trace amounts), and Au−Sn
alloy occurs only when water is present. Experimental
data from IR spectroscopy and measurements of vis-
cosity indicate that water weakens the intermolecular
interactions in EG, thereby promoting the diffusion of
ions in the solution. In the presence of water
One of the reasons for this effect is the formation of
an alloy based on intermetallics of gold and tin. This
has been confirmed by XRD data and is especially
characteristic of EG solutions. Another reason for the
efficiency of the simultaneous cathodic reduction of
Au(III) and Sn(IV) from EG solutions would appear
to be interactions between the Au(III) and Sn(IV)
complex compounds dissolved in the EG and these
compounds with the solvent. As we indicated in [20],
SnCl2EG(H2O)22+ complex playing a crucial role in
the electrodeposition of the alloy is formed.
CONCLUSIONS
1. The modes of operation and composition of eth-
ylene glycol solution (SnCl4 · 5H2O and K[Au(CN)4]
at a molar ratio of 14 : 1, 6 wt % of water) that ensure
the formation of SnCl2EG(H2O)22+ complex cations electrochemical deposition of Au–Sn alloy based on
AuSn and Au5Sn intermetallics with tin contents cor-
weakens the intermolecular interaction in EG. Along
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FORMATION OF ALLOYS UPON THE SIMULTANEOUS ELECTROCHEMICAL DEPOSITION
1087
2. Yu. D. Gamburg, Electrochemical Crystallization of
Metals and Alloys (NFKh, RAN, Yanus-K, Moscow,
1997) [in Russian].
responding to the eutectic composition (30 at %) or
raised to 55 at % were determined. It was shown that it
is extremely difficult to obtain alloys of eutectic compo-
sition from aqueous solutions, due to the dominance of
gold content in the products of electrochemical deposi-
tion. It was found that tin content in an alloy grows with
increase of current density, at lower temperatures, and
with decrease of water concentration down to 6 wt %,
while gold, tin, and their alloys are not deposited from
completely anhydrous EG solutions.
3. B. Djurfos and D. G. Ivey, Mater. Sci. Eng. B 90, 309
(2002).
4. B. Bozzini, A. Fanigliulo, G. Giovannelli, et al.,
J. Appl. Electrochem. 33, 747 (2003).
5. G. Ghosh, J. Mater. Res. 23, 1398 (2008).
6. B. Bozzii, P. Cavallotti, A. Fanigliulo, et al., J. Solid
State Electrochem. 8, 147 (2003).
2. It was shown via voltammetry that the simulta-
neous reduction of gold and tin from aqueous and EG
electrolytes occurs with depolarization effect of the
reduction of both Au(III) and Sn(IV), due largely to
the formation of Au–Sn alloy, the depolarization
effect of cathodic deposition of tin being more pro-
nounced in the EG electrolyte.
7. J. Doesburg and D. Ivey, Mater. Sci. Eng. B 78, 44
(2000).
8. B. Bozzini, G. Giovannelli, S. Natali, et al., J. Appl.
Electrochem. 32, 165 (2002).
9. A. Weissberg, E. Proskauer, J. Riddick, and E. Toops,
Organic Solvents (Intersci., New York, 1955; Mir, Mos-
cow, 1958).
3. Analysis of cathodic branches of voltammo-
grams showed that the reduction of individual
gold(III) and tin(IV) proceeds in two stages. Upon the
simultaneous reduction of Au(III) and Sn(IV) from
aqueous electrolyte, Sn(II)—a product of the first
stage of Sn(IV) reduction—is consumed in the reduc-
tion of Au(I) in AuCN adsorbed on the cathode,
which is a product of the first stage of Au(III) reduc-
tion. As a result, the content of gold in the deposit
exceeds the estimations, and the phases of gold or
gold-rich Au5Sn intermetallic are formed. We assume
that upon simultaneous reduction of gold and tin from
EG electrolyte, there is no spontaneous consumption
of Sn(II) for Au(I) reduction.
10. O. Yu. Sytnik, A. P. Krasnoperova, Yu. N. Surov, et al.,
Visn. Khark. Univ., Khim. 13 (36), 56 (2005).
11. A. Miyake, Bull. Chem. Soc. Jpn. 32, 1381 (1959).
12. D. Knetsh and W. L. Groeneveld, Inorg. Chim. Acta 7,
81 (1973).
13. C. D. Gu, Y. J. Mai, J. P. Zhou, et al., J. Power Sources
214, 200 (2012).
14. O. N. Vrublevskaya and T. N. Vorobyova, Izv. NAN
Belarusi, Ser. Khim. Nauk, No. 2, 64 (2010).
15. Electrodeposition from Ionic Liquids, Ed. by F. Endres,
D. MacFarlane, and A. Abbott (Wiley-VCH, Wein-
heim, 2008).
16. T. N. Vorobyova and O. N. Vrublevskaya, Surf. Coat.
Technol. 204, 1314 (2010).
4. It was determined via IR spectroscopy that the
17. H. M. Maltanava, O. N. Vrublevskaya, and
formation of complex compounds of the
T. N. Vorobyova, Metal. Finish. 111 (6), 28 (2013).
SnCl2EG(H2O)22+ type occurs upon the dissolution of
SnCl4 · 5H2O in EG. As a result, the intermolecular
interactions in the solvent environment are weakened,
18. H. M. Maltanova, O. N. Vrublevskaya, and
T. N. Vorobyova, Mir Gal’vaniki, No. 1 (26), 40
(2014).
and Au(CN)4− ions do not interact with EG. The rate
of deposition of tin and Au–Sn alloy coatings (up to
2 μm/h, which is quite high for nonaqueous electro-
lytes), and the effect of pronounced depolarization of
Sn(IV) and Au(III) electroreduction from EG electro-
lyte, are explained by the increased rate of diffusion
19. H. M. Maltanova, T. N. Vorobyova, E. S. Vereten-
nikova, and O. N. Vrublevskaya, Izv. NAN Belarusi,
Ser. Khim. Nauk, No. 3, 10 (2014).
20. H. M. Maltanava, T. N. Vorobyova, and O. N. Vru-
blevskaya, Surf. Coat. Technol. 254, 388 (2014).
21. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds. Part A. Theory and Appli-
cations in Inorganic Chemistry, 6th ed. (Wiley-VCH,
New York, 2009).
and the discharge of SnCl2EG(H2O)22+ cations, along
with their associates with Au(CN)4− anions.
22. J. B. Zhang, P. Y. Zhang, K. Ma, et al., Sci. China. Ser.
5. It was found that a small amount of water is
needed in EG electrolyte (5–7 wt %) to weaken the
intermolecular interaction in the glycol and thus facil-
itate the transport of Au(III) and Sn(IV) ions in an
electric field, and to ensure the formation of
B: Chem. 5, 420 (2008).
23. B. Bossini, B. Busson, G. P. de Gaudenzi, et al.,
J. Electroanal. Chem. 602, 61 (2007).
24. F. Huerta, C. Mele, B. Bossini, and E. Morallon,
J. Electroanal. Chem. 569, 53 (2004).
SnCl2EG(H2O)22+ complex cations.
25. P. Wilkinson, Gold Bull. 19 (3), 75 (1986).
26. Y. Zhang, in Modern Electroplating, Ed. by
M. Schlesinger and M. Panovic, 5th ed. (Wiley, New
York, 2010), p. 139.
REFERENCES
1. V. V. Povetkin, I. M. Kovenskii, and Yu. I. Ustinovshchi-
kov, The Structure and Properties of the Electrolytic Alloys
(Nauka, Moscow, 1992) [in Russian].
Translated by V. Kudrinskaya
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 5 2016