Journal of The Electrochemical Society, 2023 170 092504
1945-7111/2023/170(9)/092504/10/$40.00 © 2023 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited

Electrodeposition of Silver in Cyanide-Free Solution Containing
Pyrimidine Derivative as a Complexing Agent
Atiqah Binti Jasni1,2,z

and Sachio Yoshihara1

1
Department of Innovation Systems Engineering, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto,
Utsunomiya, Tochigi 321-8585, Japan
2
R&D Department, Meltex Inc., 2-3-1 Yoshino-cho, Kita-ku, Saitama, Saitama 331-0811, Japan

Pyrimidine derivatives, for example, 2,4-pyrimidinedione (uracil), 1-methyluracil and 5-methyluracil (thymine) can bind withmetal ions in alkaline solution. 5-methyluracil (thymine) as complexing agent in electroplating solution has not been investigated
due to low solubility in water. The aims of this study were to determine the potential of 5-methyluracil (thymine) as complexing
agent for cyanide-free silver electroplating solution. The inﬂuence of complexing agent and polyethyleneimine on the silver
electrodeposition reaction were studied. Several methods which are electrochemical quartz crystal microbalance (EQCM) andelectrochemical impedance spectroscopy (EIS) have been used to study the electrochemical behavior of cyanide-free silver
electroplating solution. The surface morphology and brightness of silver deposits obtained from each cyanide-free silver
electroplating solution were evaluated. A semi-bright deposit was obtained from thymine and uracil-complexing silver solution
while matte deposit from 5,5-dimethylhydantoin-complexing silver solution. A bright silver deposit could be obtained in thepresence of polyethyleneimine. The reduction current value decreases as the amount of polyethyleneimine added increases due toinhibition effect on the silver electrodeposition reaction. The results of electrochemical quartz crystal microbalance measurement
showed that the addition of polyethyleneimine in thymine-complexing silver solution does not signiﬁcantly affect the resonancefrequency change.
© 2023 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited. [DOI: 10.1149/1945-7111/
acf6e3]Manuscript submitted June 11, 2023; revised manuscript received August 25, 2023. Published September 19, 2023.

Silver electroplated ﬁlms has been widely used in the engineeringﬁeld for electronic industrial parts such as reﬂectors for LED lighting
and semiconductor industrial parts such as lead frames because of its
excellent in electrical and thermal conductivity and excellent inreﬂectivity. The silver electroplating solution which is conventionally used is an alkaline cyanide solution which mainly consist ofsilver cyanide and potassium cyanide.1 This silver electroplating
solution composition has been widely used since its low cost andhigh-quality of deposit can be produced. However, due to cyanide
compounds toxicity, it is important to ensure a safe working
environment, strict chemical management, and disposal.
In recent years, many organizations, including companies, researchorganizations and universities, have been developing cyanide-free silver
electroplating solutions in accordance with increasing environmental
awareness and reducing environmental impact due to Sustainable
Development Goals (SDGs). Prior research has been done to develop
cyanide-free silver electroplating solution to replace alkaline cyanide
solution. As a result, the most effective formulations were obtained withthiosulfate,2–4 succinimide,5–7 hydantoin derivatives8–12 and uracil13,14
as complexing agents. Asakawa et al.8 reported that a stable cyanidefree bath solution when hydantoin compound was present in thesolution at a molar ratio three times that of silver. In addition, Liu etal.10 reported that a smooth and compact silver deposit with mirror
brightness and smaller grains was obtained when nicotinic acid and5,5-dimethylhydantoin (DMH) were used as complexing agent forcyanide-free silver electroplating solution. Pizzetti et al.12 achieved
bright silver deposit when concentration of polyethyleneimine of atleast 400 ppm was present in 5,5-dimethylhydantoin based silver
electroplating solution however there were no report regarding microstructure of the deposit. Furthermore, Xie et al.13 and J. Christophe etal.14 reported that uracil as complexing agent in cyanide-free silver
plating could produce a good silver deposit on copper substrate without
a separate silver strike process. Moreover, J. Christophe et al.14
achieved good adherent and ductile silver deposits on nickel coatedcopper substrate by using choline chloride (ChCl) based deep eutectic
solvents (DES) silver strike solution.
In this research, the inﬂuence of complexing agent and additionof polyethyleneimine as additive on silver deposition reaction in thecyanide-free silver electroplating solution was studied. Here,z

E-mail: j.atiqah@meltex.com

pyrimidine derivatives which is 5-methyluracil (thymine) with theformula C(O)NHC(O)NHCHCCH3 was studied as complexing agent
for cyanide-free silver electroplating solution since thymine likeother pyrimidine derivatives, for example, 2,4-pyrimidinedione(uracil) and 1-methyluracil can bind with metal ions in alkaline
solution.15 However, the potential of thymine as a complexing agent
in cyanide-free silver electroplating solution has not been investigated due to low solubility in water. As a comparison, 5,5dimethylhydantoin (DMH) which is a heterocyclic organic compound with the formula NHC(O)NHC(O)CCH3CH3 and uracil theformula C(O)NHC(O)NHCHCH were chosen as comparison to
study the inﬂuence of complexing agent in the cyanide-free silver
electroplating solution. In addition, polyethyleneimine was used asadditive since it can improve the brightness of the electroplated
metal in gold or silver alloy plating solutions.16–18
In this study, two electrochemical measurement approaches
which are electrochemical quartz crystal microbalance (EQCM),
and electrochemical impedance spectroscopy (EIS) were used toinvestigate the inﬂuence of complexing agent and polyethyleneimine
on electrochemical behavior of silver in silver electroplating solution. EQCM measurement technique is utilized in various scientiﬁcﬁelds and applications, since it is a universal in situ measurement
method for monitoring deposited mass changes, metal evaporations
and dissolution or corrosion and as sensors for chemical andbiosensing for both in the vapor and liquid phase.19,20 By using an
EQCM electrode as a working electrodes, simultaneously measuring
frequency changes with electrochemical measurement methods suchas cyclic voltammetry (CV), linear sweep voltammetry (LSV),
chronoamperometry (CA) and chronopotentiometry (CP), permits
simultaneous tracking of two parameters such as current and weightchange or electrode potential change and weight change which
occurs at the electrode surface. In addition, over the years EIS has
been used as one of electrochemical measurement to study interfacial reaction mechanism with applications in corrosion, biosensors,
battery development, fuel cell development, sensor development andphysical electrochemistry.21–23 Differs from other electrochemicalmeasurement techniques, EIS is a technique where the input signal isan alternating current (AC) voltage or current signal applied onelectrode, and the output signal is current or voltage. This permits
spectra analysis by varying the frequency of AC signal. Unlike direct
current (DC) measurement, the relationship between voltage andcurrent is expressed as impedance in EIS. While it is challenging toJournal of The Electrochemical Society, 2023 170 092504

Figure 1. SEM images of the top views of silver deposits obtained from (a) bath thymine, (b) bath thymine + 100 ppm PEI-1800, (c) bath thymine + 500 ppm
PEI-1800 and (d) bath thymine + 1000 ppm PEI-1800, respectively.

divide charge transfer and diffusion reaction in DC measurement,
EIS permits spectra analysis by separating fast process in highfrequency range, which is charge transfer and slow process in low
frequency range, which is diffusion.23 These two methods are aneffective approach to study electrochemical behavior of plating
solution during electrodeposition process. Furthermore, unexploredresearch related to the complex formation of thymine and silver ion
and inﬂuence of polyethyleneimine on surface morphology of silver
deposit were studied.

Table I. Bath composition for cyanide-free silver electroplating
solution.
Bath type

Composition

Concentration

ThymineSilver nitrate
ThyminePotassium hydroxide
Polyethyleneimine (M.W. 1,800)
Silver nitrate
5,5-Dimethylhydantoin
Potassium hydroxide
Polyethyleneimine (M.W. 1,800)
Silver nitrate
UracilPotassium hydroxide
Polyethyleneimine (M.W. 1,800)

0.15 mol dm−30.80 mol dm−3
1.00 mol dm−30–1000 ppm
0.15 mol dm−30.80 mol dm−3
1.00 mol dm−30–1000 ppm
0.15 mol dm−30.80 mol dm−3
1.00 mol dm−30–1000 ppm

DMH

UracilExperimental
Chemicals.—Silver nitrate, AgNO3 (99.8%) and potassium
cyanide, KCN (95.0%) were purchased from FUJIFILM Wako
Pure Chemical Corporation. 5,5-dimethylhydantoin (>98.0%) wereacquired from X.T.Y ENVIRON-TECH Co., Ltd., uracil (>97.0%)
and sulphuric acid (>95.0%) were from Kanto Chemical Co., Inc.,
thymine (>98.0%) were from Tokyo Chemical Industry Co., Ltd.,
potassium hydroxide, KOH (95.5%) were from Nippon Soda Co.,
Ltd., nitric acid, HNO3 (67.5%) from Saitama Yakuhin Co., Ltd.,silver cyanide (>99.0%) from Tanaka Precious Metal Corporationand Melcleaner 1106 from Meltex Inc. The average molecularweight of the polyethyleneimine (PEI) (branched polymer,
FUJIFILM Wako Pure Chemical Corporation) used in the experiments was 1,800 and was denoted as PEI-1800. All chemicals wereused as received.
Silver electroplating solution.—All solutions were prepared with
deionized water. Silver electroplating solution was obtained bydissolving complexing agent in potassium hydroxide solution before
adding silver nitrate. After complete dissolution, the bath pH wasadjusted to pH 13 by addition of potassium hydroxide solution. The
bath composition and operating conditions for silver electroplating
solution was shown in Table I.
Electrodeposition of silver and characterization of silver deposit.—Silver plating was performed using magnetic stirrer atagitation rate 500 rpm at 5 mA cm−2 for 20 min at 25 °C. Copper
substrates were used for samples and silver plates (99.99%) wereused as anode. Prior to silver plating experiment, pretreatment using

Journal of The Electrochemical Society, 2023 170 092504

Figure 2. SEM images of the top views of silver deposits obtained from (a) bath DMH, (b) bath DMH + 100 ppm PEI-1800, (c) bath DMH + 500 ppm PEI1800 and (d) bath DMH + 1000 ppm PEI-1800, respectively.

Table II. Bath composition and operating conditions for pretreatment process and silver strike.
Process

Chemical and bath composition

Treatment condition−3

Electrolytic alkaline cleaner

Melcleaner 1106

45 g dm

Acid pickling

Sulphuric acid100 g dm−3

Silver strike

Silver cyanide
Potassium cyanide

2.65 g dm−3
95 g dm−3

alkaline electrolytic cleaner and acid pickling was performed before
silver strike (0.1 μm) was applied on a copper substrate in advanced.
The bath composition and operating conditions for pretreatment
process and silver strike was shown in Table II. The brightness of
silver deposit was measured using densitometer ND-11 (Nippon
Denshoku Industries Co., Ltd.). While morphology of silver deposit
was examined by scanning electron microscopy (SEM) (JEOL Ltd.
JSM-7800F) and focused ion beam system (FIB) (JEOL Ltd. JIB4000). For the cathode current efﬁciency, a copper plate was cut into
25 mm × 25 mm pieces was used. Electrolysis of 117 coulombs was
performed at 25 °C and the efﬁciency was determined from thedifference in weight before and after plating.

Current density
Immersion time
Temperature
Immersion time
Temperature
Current density
Plating time
Temperature
Anode

10 mA cm−2
120 s
60 °C
60 s
25 °C
10 mA cm−2
10 s
25 °C
Stainless steel (SUS304)

Electrochemical measurements.—Electrochemical measurements were conducted using a three electrodes system connected
to potentiostat (BioLogic Science Instruments SP-150). A silver wire
(99.99%) was used as counter electrode and saturated calomelelectrode (SCE) as reference electrode. A platinum disk electrodewith a diameter 3 mm was used as working electrode. Prior to eachexperiment, the working electrode was polished using 1.0 μm
polishing diamond on a wetted diamond polishing pad. Then theworking electrode was polished using 0.5 μm polishing alumina on
alumina polishing pad and rinsed with deionized water to remove
any trace of alumina and ﬁnally dried by air drying. No nitrogen
bubbling was applied, and bath temperature was maintained at 25 °C

Journal of The Electrochemical Society, 2023 170 092504

Figure 3. SEM images of the top views of silver deposits obtained from (a) bath uracil, (b) bath uracil + 100 ppm PEI-1800, (c) bath uracil + 500 ppm PEI1800 and (d) bath uracil + 1000 ppm PEI-1800, respectively.

during electrochemical measurement. The rotation of working
electrode was controlled at 500 rpm by using RRDE-3A rotating
ring disk electrode rotator (ALS Co., Ltd.). Linear sweep voltammogram was recorded between 0 and −2.0 V at a sweep rate of
10 mV s−1. In addition, electrochemical impedance spectroscopy(EIS) was recorded at frequency range 100 kHz to 20 mHz withamplitude of 5 mV at −0.75 to −1.03 V vs SCE.
Electrochemical quartz crystal microbalance (EQCM)
QCM922A (Seiko EG&G Co., Ltd.) connected to potentiostat(BioLogic Science Instruments SP-150) was used to evaluate theelectrochemical behavior of silver complex in silver electroplating
solution. The platinum sputtered on both sides of the crystal was acentral circle of 5 mm diameter. Prior to each experiment, theplatinum sputtered crystal was clean with 10 vol% nitric acid todissolve and remove impurity metals from electrode surface. The
maximum resonance frequency change was set to ±500 kHz andquartz crystal microbalance response was recorded between 0 and−2.0 V at a sweep rate of 10 mV s−1. A silver wire (99.99%) wasused as counter electrode and saturated calomel electrode (SCE) as
reference electrode.
Results and Discussion
Electrodeposition of silver on copper.—The SEM images in
Figs. 1, 2 and 3 show the top views of silver deposits obtained fromeach silver electroplating solutions with and without the addition of
PEI-1800. A silver deposit with a smooth and homogenous layer ofmicrostructure was obtained from thymine-complexing silver solution (Fig. 1a). In contrast, the microstructure of the silver deposit
was ﬁne grains with small domes when 100 ppm of PEI-1800 was

added to thymine-complexing solution (Fig. 1b). However, signs ofcracks were observed on the silver deposit when the concentrationof PEI-1800 increases, which might be attributed by adsorption ofpolyethyleneimine on silver ﬁlm during plating reaction (Figs. 1cand 1d). It can be suggested that inclusion of polyethyleneimine
during plating reaction increases the internal stress which resulted in
cracks on the silver deposit.
On the contrary, a silver deposit with microstructure of smallsized polygonal crystal structure was obtained from DMH-complexing silver solution (Fig. 2a). In contrast, a silver deposit with asmooth ﬁne grain was obtained with the addition of PEI-1800 into
bath DMH (Fig. 2b). Similarly, thymine-complexing silver solution,
signs of cracks were found on the silver deposit when theconcentration of PEI-1800 increases (Figs. 2c and 2d).
For uracil-complexing silver solution, a silver deposit with ﬁne
sized crystal structure was obtained even without the addition of
PEI-1800 (Fig. 3a). However, cracks were observed on the silver
deposit since the thickness was greater than 5 μm. In general, theﬁlm thickness is proportional to internal stress in the ﬁlm. So, it canbe suggested that as the plating ﬁlm thickness increases, the tensilestress increases which attributed cracking in electrodeposited silver
since there are no signs of cracks when then thickness was lesser
than 5 μm. A silver deposit with ﬁne sized crystal structure wasobtained with the addition of PEI-1800 into bath uracil and the silver
size become ﬁnest when the concentration was 100 ppm (Fig. 3b).
However, the microstructure of the silver deposit was sphericalshape when the concentration of PEI-1800 increases (Figs. 3c and3d).
In addition, Figs. 4, 5 and 6 show cross-sections of silver deposits
obtained from each silver electroplating solutions with and without

Journal of The Electrochemical Society, 2023 170 092504

Figure 4. Cross-section images of silver deposits obtained from (a) bath thymine, (b) bath thymine + 100 ppm PEI-1800, (c) bath thymine + 500 ppm PEI-1800
and (d) bath thymine + 1000 ppm PEI-1800, respectively.

the addition of PEI-1800. The cross-sections of silver deposits
obtained from thymine-complexing silver solution shows that the
crystal size was slightly ﬁner than that of silver deposits obtained
from uracil-complexing silver solution. In addition, the crystal sizeof the silver deposit of DMH-complexing silver solution (Fig. 5a)
was largest than that of the thymine and uracil-complexing silver
solutions (Figs. 4a and 6a). The cross-sections of silver deposits withthe addition of PEI-1800 (Figs. 4b–4d, 5b–5d and 6b–6d) show thatthe crystal sizes were ﬁner than that from the base bath (Figs. 4a, 5aand 6a).
Inﬂuence of polyethyleneimine on surface brightness.—The
brightness of silver-plated ﬁlm is generally represented in GAM
units. The measured values of GAM units for example measured by
Densitometer Model 144 (Graphic Arts Manufacturing) or Micro
Colorimeter VSR400 or Densitometer ND-11 (Nippon Denshoku
Industries Co., Ltd.) for matte silver-plated ﬁlm is between 0 to 0.5,
while semi-bright silver-plated ﬁlm is between 0.4 to 1.0 and brightsilver-plated ﬁlm is 0.8 and above. From Fig. 7, the gloss
concentration of silver-plated ﬁlm from thymine-complexing silver
solution without PEI-1800 was approximately 1.0 which refer thatbright ﬁlm was deposited on the substrate. In addition, a change ingloss concentration was observed when PEI-1800 was added
especially when 100 ppm of PEI-1800 was added to thyminecomplexing silver solution. On the contrary, the gloss concentrationof silver-plated ﬁlm from cyanide-free silver electroplating solution
containing DMH was 0.0 which refer that matte ﬁlm was depositedon the substrate. Moreover, the gloss concentration was between 0.8
to 2.0 when PEI-1800 was added to DMH-complexing silver
solution which refer that bright ﬁlm was deposited on the substrate
and the highest gloss concentration was obtained when 100 ppm of

PEI-1800 was added to bath DMH. Pizzetti et al.12 also obtained
similar results when polyethyleneimine with molecular weight 600
was added to DMH-complexing silver solution. On top of that, thegloss concentration of silver-plated ﬁlm from cyanide-free silver
electroplating solution containing uracil without PEI-1800 was
approximately 1.0 which refer that bright ﬁlm was deposited on
the substrate. However, from Fig. 7, there is hardly any signiﬁcantchange was observed in gloss concentration when PEI-1800 was
added to uracil-complexing silver solution.
Cathode current efﬁciency.—When thymine was used as complexing agent in cyanide-free silver electroplating solution, thecathode current efﬁciency value was 76.8% while DMH was
76.5% and uracil was 79.0%. It can be assumed that hydrogen is
generated as a side reaction during plating process which resulted indecreased in cathode current efﬁciency to less than 100%. In
addition, there are only slightly changed in cathode current
efﬁciency when PEI-1800 was added to thymine-complexing silver
solution. On the contrary, an increase of 5.7% in cathode current
efﬁciency was observed when PEI-1800 was added to DMHcomplexing silver solution. On top of that, an increase of 8.5% incathode current efﬁciency was observed when PEI-1800 was added
to uracil-complexing silver electroplating solution. It can be suggested that polyethyleneimine promote the reduction of silver byadsorption at the silver deposit and solution interface. Thus, lesshydrogen would be generated during plating with addition ofpolyethyleneimine in the cyanide-free silver electroplating solution.
Inﬂuence of complexing agent on electrochemical behavior ofsilver complex in silver electroplating solution.—Linear sweep
voltammetry measurements were conducted to investigate theJournal of The Electrochemical Society, 2023 170 092504

Figure 5. Cross-section images of silver deposits obtained from (a) bath DMH, (b) bath DMH + 100 ppm PEI-1800, (c) bath DMH + 500 ppm PEI-1800 and(d) bath DMH + 1000 ppm PEI-1800, respectively.

inﬂuences of complexing agent on electrochemical behavior ofsilver complex in the silver electroplating solution. In general,
linear sweep voltammetry method is used to study the deposition
potential of electroplating and current efﬁciency and hydrogen
evolution in electroplating. While EQCM measurements allows
in situ monitoring of the resonance frequency change on the surfaceof the working electrode during electrodeposition reaction. In thisstudy, deposition potential was the smallest potential at which
resonance frequency changed was observed when current wasapplied, indicating that deposition reaction is occurring. Figure 8
displays the linear sweep voltammograms obtained on a platinum
electrode in each silver electroplating solution. When thymine wasused as complexing agent, the silver deposition is approximately at−0.87 V, and the current increases from −0.87 V, and even moreon the negative side, below −1.79 V (yellow in Fig. 8 (top)). An
inﬂection points approximately at −1.16 V was recorded, whichcorrespond to the potential at which metal deposition and hydrogen
evolution to occur in parallel (yellow line in Fig. 8 (top)). Even
though, the current increased on the negative side, below than
−1.79 V, there was no resonance frequency change was observed,
suggesting that the hydrogen evolution reaction was mainly
occurred (yellow line in Fig. 8 (bottom)). Furthermore, from theΔf–E curve, the resonance frequency change in the potential rangeof silver reduction wave (−0.87 ∼ −1.79 V) was 112.6 kHz. The
Sauerbrey equation as shown below is a linear relationship between
the resonance frequency of an oscillating quartz crystal and masschanges. Here, m/A is mass change per unit area, Nq is mass
sensitivity constant, ρ is density of crystal, f0 is initial resonancefrequency of crystal electrode and f is ﬁnal resonance frequency of
crystal electrode.

m/A = {−Nq ⋅ ρ ⋅ (f − f0) × 10 6}/f0 2
From this equation, the corresponding mass change of 112.6 kHz
was 0.12 mg cm−2.
In addition, when DMH was used as complexing agent in silver
electroplating solution, the silver deposition potential is approximately at −0.39 V, and the current increases from −0.39 V, andeven more on the more negative side, below −1.26 V. An inﬂection
point was recorded at approximately −0.88 V which correspond to
the potential at which metal deposition and hydrogen evolution
begin to occur in parallel (blue line in Fig. 8 (top)). Even though, thecurrent increased on the negative side, below than −1.26 V, there
was no resonance frequency change was observed, suggesting thatthe hydrogen evolution reaction was mainly occurred (blue line in
Fig. 8 (bottom)). In addition, the resonance frequency change in thepotential range of the silver reduction wave (−0.39 ∼ −1.26 V) was
82.0 kHz, corresponding to mass change of 0.088 mg cm−2.
Furthermore, when uracil was used as complexing agent, thesilver deposition potential was approximately at −0.38 V, and thecurrent increases from −0.38 V, and even more on the negative side,
below −1.52 V. Moreover, there is an inﬂection point at −1.12 V,
which correspond to the potential at which metal deposition andhydrogen evolution begin to occur in parallel (grey line in Fig. 8(top)). Even though, the current increased on the negative side,
below than −1.52 V, there was no resonance frequency change wasobserved, suggesting that the hydrogen evolution reaction was
mainly occurred and outweighs the deposition reaction (grey line in
Fig. 8 (bottom)). Furthermore, from the Δf–E curve, the resonancefrequency change in the potential range of silver reduction wave

Journal of The Electrochemical Society, 2023 170 092504

Figure 6. Cross-section images of silver deposits obtained from (a) bath uracil, (b) bath uracil + 100 ppm PEI-1800, (c) bath uracil + 500 ppm PEI-1800 and (d)bath uracil + 1000 ppm PEI-1800, respectively.

Figure 7. Inﬂuence of concentration of PEI-1800 on ﬁlm brightness.

(−0.38 ∼ −1.52 V) was 105.1 kHz, corresponding to mass changeof 0.11 mg cm−2.
As mentioned earlier, EIS measurements were conducted tofurther study electrochemical behavior of silver complex in the silver
electroplating solution. EIS measurements were carried out at−1.03 V (thymine), −0.75 V (DMH) and −0.85 V (uracil) at a Pt
electrode with 3 mm diameter. These voltages are equivalent to
10 mA cm−2 from the linear sweep voltammograms as shown in
Fig. 8. The Nyquist plots are shown in Fig. 9 and equivalent circuits
depicted by the Nyquist plot are shown in Fig. 10. Table III shows
the values of all elements in the equivalent circuit corresponding to
Fig. 9. Here, Rsol is solution resistance of silver plating solution and
Rct is charge transfer resistance during silver deposition, which asmaller value of charge transfer resistance Rct means that chargetransfer reactions are more likely to occur, while a larger valuemeans that charge transfer reactions are less likely to occur since thecharge transfer reaction is proportional to current, the charge transfer
resistance Rct is a guideline for how unlikely the reaction is tooccur.22 In addition, Cdl is electric double-layer capacitance between
electrode and silver electroplating solution. R2 is second set ofresistance and C2 is second set of capacitances. It can be assumed
that these components represent either the impedance are caused byinteraction of silver ions with complexing agent layer or diffusion
through the Nernst diffusion layer. From analysis of these three
Nyquist plots as shown in Fig. 9, the diameter of the capacitive loop
obtained from silver plating solution containing uracil was found tobe signiﬁcantly bigger than that from DMH and thymine especiallyat high frequency range. These curves displayed in Fig. 9 wereapproximated double capacitive loop, suggesting that the silver
deposition process at 10 mA cm−2 were hybrid controlled by chargetransfer and ion diffusion. From these results, the curves of eachsilver electroplating solutions were very similar which suggest that
there is hardly any signiﬁcant change in electrochemical deposition
mechanism occurred on the cathode due to complexing agent.
In addition, according to the ﬁtting parameters listed in Table III,
there is no signiﬁcant change in values of Rsol since each silver
electroplating solution contained same concentration of complexing
agent and potassium hydroxide. However, the values of Rct and R2
showed the most change, where silver electroplating solution
containing thymine showed the smallest Rct while silver electroplating solution containing DMH showed the largest Rct value. This
indicates that the cathodic discharge process of Ag+ and Agcomplexes on the cathode were affected by complexing agent in
the silver electroplating solution, in accordance with the results ofJournal of The Electrochemical Society, 2023 170 092504

Figure 10. Equivalent circuits used for the analysis of electrochemical
impedance spectroscopy spectra of cyanide-free silver electroplating solution.

Figure 8. Linear sweep voltammograms recorded at a Pt electrode (top).
Quartz crystal microbalance response recorded at a Pt electrode (bottom).
Electrolyte concentration: 0.15 mol dm−3 silver nitrate + 0.80 mol dm−3
thymine (yellow line), 0.15 mol dm−3 silver nitrate + 0.80 mol dm−3 DMH(blue line) and 0.15 mol dm−3 silver nitrate + 0.80 mol dm−3 uracil (greyline). Scan rate = 10 mV s−1, T = 25 °C, agitation rate = 500 rpm. Inset
shows the enlarge view of voltammograms in range −0.4 ∼ −1.4 V.

Figure 11. Linear sweep voltammograms (top) and quartz crystal microbalance response (bottom) in thymine bath with various PEI-1800 concentrations recorded at a Pt electrode. Concentration of PEI-1800: (blue line) 0,(orange line) 100 ppm, (grey line) 500 ppm, (yellow line) 1000 ppm. Scan
rate = 10 mV s−1, T = 25 °C, agitation rate = 500 rpm.

Figure 9. Nyquist plots for silver electrodeposition process with different
complexing agent at −1.03 V (thymine), −0.75 V (DMH) and −0.85 V(uracil) at a Pt electrode. Electrolyte concentration: 0.15 mol dm−3 silver
nitrate + 0.80 mol dm−3 thymine + 1.00 mol dm−3 potassium hydroxide(yellow line), 0.15 mol dm−3 silver nitrate + 0.80 mol dm−3 DMH +1.00 mol dm−3 potassium hydroxide (blue line), 0.15 mol dm−3 silver nitrate+ 0.80 mol dm−3 uracil + 1.00 mol dm−3 potassium hydroxide (grey line).
T = 25 °C, agitation rate = 500 rpm.

the EQCM measurements. As mentioned earlier, the amount ofdeposition until hydrogen generation for thymine is 0.12 mg cm−2,
DMH is 0.088 mg cm−2 and uracil-complexing silver solution is

Table III. The values of all elements in the equivalent circuit
corresponding to Fig. 10.
Bath type

Rsol/Ω

Rct/Ω

Cdl/F

R2/Ω

C2/F

ThymineDMH
Uracil

21.42
27.85
19.72

30.96
77.03
73.94

1.54 × 10−5
8.99 × 10−6
2.49 × 10−4

38.19
29.17
45.94

4.59 × 10−5
3.02 × 10−3
4.62 × 10−3

0.11 mg cm−2. Furthermore, silver electroplating solution containing
uracil showed the largest R2 while silver electroplating solution
containing DMH showed the smallest R2 value among three silver

Journal of The Electrochemical Society, 2023 170 092504

Figure 12. Linear sweep voltammograms (top) and quartz crystal microbalance response (bottom) in DMH bath with various PEI-1800 concentrations recorded at a Pt electrode. Concentration of PEI-1800: (blue line) 0,(orange line) 100 ppm, (grey line) 500 ppm, (yellow line) 1000 ppm. Scan
rate = 10 mV s−1, T = 25 °C, agitation rate = 500 rpm.

electroplating solutions. There are several possibilities that can beconsidered regarding the R2. Firstly, it can be assumed that excess ofcomplexing agent in the plating solution adsorbed into the plating
ﬁlm during plating reaction and exist as R2. Secondly, it can besuggested that complexing agent or ligand which dissociates fromthe silver ion in the silver electroplating solution during plating
reaction acted as barrier layer against the charge transfer and exists
as R2.
Inﬂuence of PEI-1800 on electrochemical behavior of silver
complex in silver electroplating solution.—Linear sweep voltammetry measurements and electrochemical quartz crystal microbalance (EQCM) were conducted to study the inﬂuence of PEI-1800 on
silver electroplating solutions. Figures 11–13, the reduction current
value decreased as the amount of PEI-1800 added in cyanide-freesilver electroplating solution increased which in line with the results
obtained by Pizzetti et al.12 and Xie et al.13 In addition, the reduction
current value is greatly suppressed by addition of PEI-1800 for
DMH and uracil-complexing silver solution compared to thyminecomplexing silver solution. In addition, Figs. 11–13 display quartz
crystal microbalance response recorded on a platinum electrode ineach silver electroplating solution with various PEI-1800 concentration. From Fig. 11, the addition of PEI-1800 in thymine-complexing
silver electroplating solution does not signiﬁcantly affect theresonance frequency change of EQCM. However, for DMH anduracil-complexing silver solution, the resonance frequency change of
EQCM increased with the addition of PEI-1800 (Figs. 12 and 13).
As mentioned prior, the hydrogen generation is suppressed, and thecurrent efﬁciency of silver electroplating is increased with the

Figure 13. Linear sweep voltammograms (top) and quartz crystal microbalance response (bottom) in uracil bath with various PEI-1800 concentrations recorded at a Pt electrode. Concentration of PEI-1800: (blue line) 0,(orange line) 100 ppm, (grey line) 500 ppm, (yellow line) 1000 ppm. Scan
rate = 10 mV s−1, T = 25 °C, agitation rate = 500 rpm.

addition of PEI-1800. Furthermore, resonance frequency change
tends to decrease as the concentration of PEI-1800 increases
especially in silver electroplating solution containing DMH anduracil. So, it can be assumed that silver plating reaction was
suppressed as the amount of adsorbed polyethyleneimine increases,
which in line with suggestion by Xie et al.13 that polyethyleneimine
inhibits reduction reaction of silver by adsorption on the silver andsolution interface.
Conclusions
In this research, electrodeposition of silver in the presence ofthymine as complexing agent and polyethyleneimine with averagemolecular weight 1,800 as additive was studied. The authors havefound that thymine could be used as potential complexing agents forsilver electroplating solution based on the plating performance andelectrochemical behavior. The surface deposit showed that thymine
was more effective as complexing agent in silver electroplating
solution to produce semi-bright silver deposit than uracil and DMHeven without adding polyethyleneimine. In addition, a matte silver
deposit was obtained from DMH-complexing solution while a semibright with signs of cracks was obtained from uracil-complexing
solution. The results from electrochemical quartz crystal microbalance analysis showed that silver deposition amount up to hydrogen
potential during cathodic potential sweep from thymine and uracilcomplexing solution were larger than that of DMH-complexing
solution. In addition, each cyanide-free silver electroplating solution
showed similar capacitive loops which proposed that there were no
signiﬁcant changes in electrochemical deposition mechanism occurred on the cathode.

Journal of The Electrochemical Society, 2023 170 092504
Moreover, polyethyleneimine as additive was found to be effective
on producing bright silver deposit. However, cracks were found in thesilver deposit from thymine and DMH-complexing solution as theconcentration of polyethyleneimine increases. The results from linear
sweep voltammetry measurements showed that the reduction current
value decreases as the amount of polyethyleneimine added increases
due to inhibition effect on the silver electrodeposition reaction. In
addition, the inhibition of polyethyleneimine also affect the cathodecurrent efﬁciency of cyanide-free silver electroplating solution.
Furthermore, the results from electrochemical quartz crystal microbalance analysis showed that the addition of polyethyleneimine inthymine-complexing silver solution does not signiﬁcantly affect theresonance frequency change. However, the resonance frequencychange increases with the addition of polyethyleneimine in DMH
and uracil-complexing silver solution.
Nevertheless, further research on other aspects, for example,
solderability test, contact electric resistance and hardness of deposit
is required to determine cyanide-free formulation for silver electrodeposition which suitable in industrial practice.
ORCID
Atiqah Binti Jasni

https://orcid.org/0009-0000-2738-4233
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