Characterization and electrochemical analysis of silverelectrodeposition in ChCl–urea deep eutectic solvents

QISONG LI, HUIXUAN QIAN, XU FU, HAIJING SUN and JIE SUN*

School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, People’s Republic

of China

*Author for correspondence (jiersun2000@126.com)

MS received 14 April 2020; accepted 25 June 2020

Abstract. Electrochemical reduction behaviours of silver ions in ChCl–urea deep eutectic solvents (DESs) were

investigated by cyclic voltammetry. The results showed that equilibrium potential Eeq of the electrode system in ChCl–

urea DESs was -0.112 V. The Tafel curve was used to obtain the exchange current density j0 = 2.92 ± 0.2 cm-2 and the

transfer coefficient a = 0.16 ± 0.2. The nucleation and growth mechanism of silver were studied by cyclic current method.

The results showed that electrodeposition consisted of two processes, 3D nucleation with diffusion-controlled growth

j3D-dc(t) and a small amount of water in the reduction jWR(t). By calculating the percentage of different charge densities of

j3D-dc and jWR, the contribution rate of different processes to the total current was analysed. With the increase in potential,

the charge in water reduction process decreased, while the charge in silver 3D nucleation process increased. Scanning

electron microscope and energy spectrum scanning were used to characterize the surface and section micro-morphologies

of silver coating. The results showed that silver grew dendritically at -0.97 V, and the thickness was 22.98 lm. The

results of X-ray photoelectron spectroscopy showed that silver coating was composed of Ag2O and Ag.

Keywords. Deep eutectic solvents (DESs); silver; electrodeposition; electrochemical behaviour; microscopic

morphology.

1. Introduction

Owing to its excellent physical and chemical properties and

antibacterial properties, good corrosion resistance, high

electrical conductivity and excellent decorative properties,

metallic silver coatings are widely used in microelectronics,

aerospace, automotive and decorative industries [1–4]. For

the preparation of silver plating on different material sur-

faces, silver plating is mainly prepared by aqueous solution

and physical methods, such as magnetron sputtering [5,6].

For metallic silver coating obtained from aqueous solution,

the current conventional method is using a cyanide plating

method for the preparation of the coating [7]. In 2006, Fang

Jing-Li [8] systematically studied the relationship between

the half-wave potentials of 25 sulphur-containing organic

compounds and the coating brightness obtained in cyanide-

based plating solutions, and found that the half-wave

potentials of additives that can obtain specular brightness

fall at 0.6–0.9 V. This is because the cyanide bonding can

slow down the silver deposition rate, resulting in a negative

shift of the deposition potential, so that an excellent coating

can be obtained. Cyanide silver plating has many advan-

tages in coating production in today’s industrial production

due to its reliable coating quality, easy solution mainte-

nance and low cost. However, the use of cyanide also has

serious problems in industrial production, such as cyanide is

highly toxic and extremely unfriendly to the environment

[9,10]. These serious problems not only pose a threat to the

health of the operator, but also make bath and wastewater

treatment very expensive. In response to the problem of

silver cyanide plating, researchers have tried to use non-

cyanide plating solution for electrodeposition of metallic

silver coating [3,11]. With the deepening of research,

researchers have tried to use organic additives for elec-

trodeposition of silver, which can replace cyanide and

partially solve environmental pollution problems [12].

However, organic additives make the composition of the

solution more complicated and cause solution instability,

which is an urgent problem to be solved. When sensitive

ions are present, the coexistence of different species with

different abilities may affect the silver nucleation process

[13].

In recent years, due to the unique physical properties (low

melting point, low vapour pressure, high polarity range,

high thermal stability, good solvent properties, etc.) and low

toxicity, the application of ionic liquids (ILs) in the field of

electrodeposition has attracted wide attention of researchers

[14–21]. As a kind of ionic liquid, deep eutectic solvents

(DESs) have attracted the attention of researchers because

of their relatively low price, low toxicity, biodegradability,

Bull Mater Sci (2021) 44:14 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-020-02276-3Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

environmental protection and easy large-scale preparation

[22–25]. The formation of DESs is usually carried out by

combining choline chloride (ChCl) with a hydrogen bond

donor (such as ethylene glycol (EG), urea, renewable car-

boxylic acid, etc.) [26–29]. Compared with traditional ILs,

the DES formed by ChCl has the following characteristics:

(1) low price; (2) easy to store; (3) easy to prepare (by

simple mixing of two components); and (4) biodegradable

and biocompatible [30], thus, solving the problems of

purification and waste disposal commonly encountered by

ILs. Because they are not entirely composed of ions, they

cannot be defined as true ILs. The synthesis process of these

mixtures only requires the mixing of two components

(cheap, renewable and biodegradable) to form a deep

eutectic mixture without the formation of any additional

solvents or by-products [31–33]. Therefore, a large number

of DESs can be prepared because their individual compo-

sition and the composition are very flexible.

In the study of using DESs as the base solution for

electrodeposition, the electrochemical behaviour of differ-

ent metals in DESs, the nucleation mechanism of metals

and the influence of various factors in the preparation of

coatings have been deeply studied, and some research

results have been obtained. Sebastian et al [34] and Haoxing

Yang and Ramana G Reddy [35] studied the electrochem-

ical behaviour, nucleation mechanism and coating mor-

phology of Cu and Zn electrodeposition in the ChCl–urea

DES system, respectively. The results showed that Cu was

reduced in two steps, and the nucleation mode of Cu was

three-dimensional growth.

With the increase of overpotential, the growth morphol-

ogy of Cu changed from small granular to dendritic. In the

process of electrodepositing Zn, the diffusion coefficient of

zinc ions in the electrolyte was 7.85 9 10-9 cm2 s-1 at a

temperature of 363 K, and a pure zinc coating could be

obtained.

Regarding the research on the electrodeposition of silver

in ionic liquids, Roberta Bomparola et al [16] used

chronoamperometry method to study the nucleation of sil-

ver in the 1-butyl-3-methyl imidazolium tetrafluoroborate

ionic liquid, and in the conclusion, they obtained was that a

small overpotential was required to initiate the silver

deposition process at high temperatures, and a silver plating

layer of 0.3 lm was obtained by electrodeposition at 200�C.

In the study of electrodeposited silver using the ChCl–urea

DES system, the use of cyclic voltammetry (CV) is of great

help in the study of metal nucleation processes. Sebastian

et al [13] and Quentin Rayee et al [36] studied the under-

potential deposition of silver in a ChCl–urea system using

CV, chronoamperometry, etc., and the nucleation and

growth of the silver plating in the aqueous solution and the

eutectic solvent were compared. The results of these studies

indicated that in the ChCl–urea system, the current hys-

teresis loop occurred with the glassy carbon electrode as the

working electrode, and the nucleation of silver was three-

dimensional growth.

In the study of electrodeposition of silver in ionic liquids,

researchers usually studied the nucleation model of silver at

different temperatures or different potential conditions, but

rarely further analysed the mechanism [37]. From these

studies, it can also see that further analysis of the kinetics

and nucleation growth mechanism of silver in the ChCl–

urea deep eutectic solvent system will help to understand

the electrodeposition process of silver in this system.

Therefore, this paper discussed the factors that affected the

redox potential of each component in the solution. The use

of CV and chronoamperometry may provide further insights

into the mechanism and kinetic analysis of nucleation and

growth of silver. The nucleation mechanism of silver in

ionic liquids is of particular interest because detailed studies

on the information provided by the three-dimensional

nucleation of diffusion-controlled growth metal electrode-

position models in ILs have yet to be completed. In addi-

tion, this paper also studied the elemental composition and

cross-section of the silver coating deposited in the ChCl–

urea DES system.

2. Experimental

2.1 Experimental process

Choline chloride (ChCl), urea and silver nitrate (AgNO3)

used in the experiments were analytical reagents, and

the molar ratio of ChCl to urea was 1:2. At room

temperature, silver nitrate with a concentration of

0.1 mol l-1 was dissolved into the ChCl–urea DES

system to obtain a ChCl–urea–AgNO3 DES liquid

electrolyte for electrochemical behaviour study and

electrodeposition test. The brass sheets used as the base

material were polished with 240, 400, 1200, 2000#

sandpapers, respectively, and degreased with acetone,

thoroughly washed with absolute ethanol, and then

washed with deionized water for use. The other elec-

trodes were washed successively with acetone and

anhydrous ethanol, and dried for use.

CV, chronoamperometry and electrodeposition experi-

ments on the ChCl–urea–AgNO3 DES system were per-

formed by using the CS350 electrochemistry workstation. A

traditional three-electrode system was used in electrodepo-

sition experiments and the electrochemical behaviour tests.

The platinum (Pt) electrode was the counter electrode and

the silver (Ag) electrode was the reference electrode. The

glassy carbon electrode (diameter 3 mm, electrochemical

behavioural tests) and the brass plate (electrodeposition

experiments) were used as the working electrodes. In the

CV experiments, the scanning rate was 50 mV s-1 and the

test system temperature was 50�C. The temperature of

the silver electrodeposition experiment in the ChCl–

urea–AgNO3 DES system was 50�C and the deposition time

was 1 h.

14 Page 2 of 11 Bull Mater Sci (2021) 44:14

2.2 Characterization

Micro-morphology of the metal deposition coating was

characterized using scanning electron microscopy (SEM,

VEGA3) coupled with energy dispersive X-ray spec-

troscopy (EDS) and the elemental compositions of the

coating were performed using EDS analysis.

The X-ray photoelectron spectroscopy (XPS) analysis

using the ESCALAB250 System (Thermo VG company,

USA) with monochromatic AlKa (1486.6 eV) X-ray radi-

ation (15 kV and 20 mA) and hemispherical electron energy

analyzer was used to characterize the chemical state of the

elements. The survey spectra in the range of 0–1100 eV

were recorded in 1 eV steps for each sample, followed by

high resolution spectra over different element peaks in

0.1 eV steps, from which the detailed composition was

calculated. Curve fitting was performed after a Shirley

background subtraction by non-linear least square fitting

using a mixed Gauss/Lorentz function [38,39]. The spectra

were referenced to the C1s line of 284.6 eV. The signal was

collected 4 times per sample.

3. Results and discussion

3.1 Potentiodynamic study

Figure 1A shows the CV curve of the ChCl–urea–AgNO3

DES system. In the negative sweep process (i.e., reduction

process), there is an obvious reduction peak between -0.5

and -1.25 V. When the potential reaches to -0.97 V, the

current increases to a maximum value jmax = -41.48 lA

cm-2. This indicates that the reduction of Ag? in ChCl–

urea system is one-step reduction, Ag? ? Ag0. There is an

oxidation peak between -0.15 and 0.8 V in the positive

sweep process (i.e., oxidation process). At a potential of

0.12 V, an oxidation peak appears, and the peak current is

j = 48.6 lA cm-2. This oxidation peak corresponds to the

oxidation of silver, Ag0 ? Ag?.

At the potentials of 0.69 and 0.112 V, the reduction curve

intersects the oxidation curve to form a current hysteresis

loop. This is a marker for the nucleation and growth of

silver ions [40]. The test result of CV curve indicated that

Ag? was deposited on the glassy carbon electrode to form a

plating layer, and then Ag was oxidized and the plating

layer disappeared.

It can be seen from the CV curve (figure 1A) that the

current density tends to zero at a potential of -0.112 V, so

the potential can be considered to be the equilibrium elec-

trode potential of the Ag(0)/Ag(I) electrode system [41],

and the value of the equilibrium electrode potential is Eeq =

-0.112 V. According to g = E - Eeq, an g–j relationship

curve (shown in figure 1B) can be obtained. For the CV

curve shown in figure 1B, the overpotential interval

-0.1–0.1 V is intercepted, and g and j = jc - ja relationship

curve (shown in figure 1C) is made, where the ordinate is

the net current density. The curve shown in figure 1D is the

Tafel curve plotted from the data of figure 1C. By calcu-

lating the cathode branch slope of the Tafel curve, the

relationship curve between potential and current is gc =0.4658 - 0.3772 log jc, the exchange current density is J0 =2.92 ± 0.2 l cm-2, and the transfer coefficient is a = 0.16

± 0.2.

3.2 Potentiostatic study

Figure 2 shows a series of potentiostatic current density

transients recorded in the potential interval corresponding to

the reduction process of silver ions. Each curve has similar

characteristics, i.e., due to the electric double layer charg-

ing, the current rises rapidly within a short period of time

when the potential is applied. This is because the formation

and growth of the silver core on the surface of the electrode

causes an increase in current and reaches a maximum value

(tmax, Imax) in a short time. After a period of time, the

current begins to decrease as the concentration of silver-

containing electroactive components in the system decrea-

ses due to the deposition of metallic silver on the surface of

the working electrode. During the next period of time, the

current continues to decrease until the diffusion rate of

silver ions from the system to the electrode surface is the

same as the electrode reaction rate, and the current begins to

stabilize and the silver ion is reduced to metallic silver at

this rate. This phenomenon indicates that the reduction

process of silver ions in the ChCl–urea eutectic solvent

system is controlled by diffusion [18].

In the process of metal electrodeposition, metal ions are

adsorbed on the surface of the substrate and transferred to

atoms in the adsorbed state by charge transfer, and then

diffuse on the surface of the substrate. The nucleation

methods of metals can generally be divided into two-

dimensional and three-dimensional nucleations. To describe

the nucleation method of metals in eutectic systems,

researchers generally use the theory of three-dimensional

nucleation growth mechanism [42–44]. Three-dimensional

nucleation is divided into three-dimensional instantaneous

nucleation and three-dimensional progressive nucleation.

The actual current can be described using the following

function expression [16]:

3D instantaneous nucleation

i ¼ nFD1=2C=p1=2t1=2� �

1 � exp �N0pkDtð Þ½ �;

k ¼ 8pCM=dð Þ1=2:

ð1Þ

3D progressive nucleation

i ¼ nFD1=2C=p1=2t1=2� �

1�exp �bN0pk0Dt2=2

� �� �;

k0 ¼ 4=3 8pCM=dð Þ1=2: ð2Þ

Bull Mater Sci (2021) 44:14 Page 3 of 11 14

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-0.00006

-0.00004

-0.00002

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010 Amc

A/j-2

E/V

Eeq

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.00006

-0.00004

-0.00002

0.00000

0.00002

0.00004

0.00006

0.00008

0.00010 B

mcA/j

-2

η/V

0.10 0.05 0.00 -0.05 -0.10

-15

-10

-5

0

5

10

C

ηc

j=jc-ja

ηa j=0 η=0

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

0.10

0.05

0.00

-0.05

-0.10

log|j/μAcm-2|

η/V

ηc=0.4658-0.3772logjc

ηa

ηc D

logja

logjClogj0

Figure 1. Typical cyclic voltammetry curve and related electrochemical data curve. (A) Typical cyclic voltammetry

curve (0.1 mol l-1 AgNO3 is dissolved in choline chloride and urea (1:2 M ratio) at 50�C, and the potential scan

started at -1.5 V in the negative direction at 50 mV s-1 sweep rate). (B) Relationship between current density and

overpotential, g = E - Eeq. (C) Relationship between net current and overpotential, g = E - Eeq. (D) Tafel plot

generated from experimental data in C.

0 2 4 6 8 10

0.000005

0.000010

0.000015

0.000020

0.000025

0.000030

0.000035

0.000040

-j/c

m-2

t/s

-0.623V -0.683V -0.703V -0.773V

Figure 2. Family of experimental current density transients

recorded in the system GCE/0.1 mol l-1 Ag dissolved in the

choline chloride and urea DES at the different overpotentials

indicated in the figure.

0 1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

(j/j m

)2

t/tm

-0.623V -0.683V -0.703V -0.773V Instantaneous Progressive

Instantaneous

Progressive

Figure 3. Non-dimensional plots of some of the experimental

j–t transients.

14 Page 4 of 11 Bull Mater Sci (2021) 44:14

The dimensionless curve can be obtained after perform-

ing the dimensionless processing on the curve in figure 2,

and the result is shown in figure 3. However, it can be seen

that the curves do not completely conform to theoretical

three-dimensional instantaneous nucleation or theoretical

three-dimensional progressive nucleation model. When t/tm[1, the deviation increases. Especially after t/tm[2.5, the

curves completely deviate from theoretical three-dimen-

sional instant nucleation or theoretical three-dimensional

continuous nucleation. The characteristic features described

in Sun Jie et al’s [19] description when a concomitant

reaction (i.e., water reduction) is occurring on the diffusion-

controlled growing surfaces of 3D nuclei of the new phase

(i.e., cobalt) [37]. This indicates that the metal nucleation

process is not a single process and may include different

contributions, such as proton reduction and water reduction.

Therefore, the model of Palomar-Pardave et al [37] can

be used to analyse the constant potential current density

transient of silver in ChCl–urea DES system. According to

equation (3), the individual’s contribution to the total cur-

rent is j(t): 3D nucleation with diffusion-controlled growth,

j3D-dc(t), and water reaction jWR(t).

jtotal ¼ j3D�dc tð Þ þ jWR tð Þ; ð3Þ

j tð Þ ¼ ðP1 þ P4t�1=2Þ

� 1� exp �P2 t � 1� exp �P3tð Þð Þ=P3ð Þð Þð Þ;ð4Þ

P1 ¼ ZWRF�kWR 2c0M=pqð Þ1=2; ð5ÞP2 ¼ N0pkD; ð6ÞP3 ¼ A; ð7Þ

P4 ¼ 2FD1=2c0

� �= p1=2� �

; ð8Þ

k ¼ 8pc0=qð Þ1=2; ð9Þ

where ZWRF is the molar charge transferred during the

water reduction process, kWR is the rate constant of the

water reduction reaction, c0 and D are the bulk concentra-

tion and diffusion coefficient of Ag, respectively, M and qare the atomic mass and density of Ag, respectively, F is

Faraday’s constant.

The j–t data obtained in the experiment can be fitted

nonlinearly using equation (4) to obtain dynamic param-

eters, such as A, N0 and D. Figure 4 shows the constant

potential current density transient (marked as ‘—’) recor-

ded on the surface of a glassy carbon electrode in a DES

system with 0.1 mol l-1 AgNO3 dissolved at different

potentials and a theoretical curve (marked as ‘s’) fitted

using equation (4). From the fitting results (as shown in

figure 4), it can be seen that the experimental data

obtained using equation (4) have good fitting results. This

result shows that equation (3) describes the current density

transient response of silver deposition on the GCT surface

in DESs effectively. Table 1 lists the fitting parameters

and the diffusion coefficients of Ag in DESs at each

overpotential. The diffusion rate D is (0.29 ± 0.14) 9

10-8 cm2 s-1.

Under the potential conditions of -0.623 and -0.683 V,

the experimental j–t curve of silver nucleation and growth

on glassy carbon electrode was compared with the data

obtained by fitting the nonlinear equation (4) (as shown in

figure 5), and from this analysis, the individual’s contribu-

tion to total current can be seen. It is worth noting that the

main difference between these current density transients is

the contribution of water reduction, jWR. The analysis

results show that the surface properties of the cathode have

changed significantly due to silver electrodeposition, which

promotes the hydrogen evolution reaction. This result is

similar to the phenomenon found by Palomar-Pardave et al[45] in the electrodeposition of cobalt.

Qt ¼Z t

0

jdt: ð10Þ

Figure 6a and b shows the contribution curves of jWR and

j3D-dc under different potential conditions. By observing

figure 6 and the integrated area results of calculating the j–tdiagram of Q3D-dc and QWR (as shown in table 2) using

equation (10). It shows that as the potential increases, the

j3D-dc contribution also increases, while jWR decreases as the

potential increases, and the nucleation contribution domi-

nates. As the potential increases, the contribution rate of

water reduction decreases significantly. This result indicates

that increasing the potential can effectively improve the

three-dimensional nucleation of silver and effectively

inhibit the reduction of water.

3.3 Micro-morphology and composition analysis of Agcoating

Using the potential conditions of 0.65, 0.80 and 0.97 V,

respectively, for silver electrodeposition, it can be seen

from the results that as the potential increases, the

0 2 4 6 8 100.000005

0.000010

0.000015

0.000020

0.000025

0.000030

0.000035

0.000040

mcA/j-

-2

t/s

-0.773V-0.703V-0.683V-0.623V

Figure 4. Potentiostatic current density transients (marked as —)

and theoretical transients (s).

Bull Mater Sci (2021) 44:14 Page 5 of 11 14

micro-morphology of the silver coating changes from small

leaves to more loose pines. Sebastian et al [34] conducted a

silver electrodeposition study in the ChCl–urea eutectic

system, and a similar picture appeared in figure 7a. In the

process of metal electrodeposition, the metal needs to go

through four steps from the ionic state to the crystal, which

are (1) ionic liquid phase mass transfer, (2) pre-conversion,

(3) charge transfer and (4) crystal formation. In the solution

system with the slowest mass transfer rate, the concentra-

tion polarization caused by the slow mass transfer step

0 2 4 6 8 100.000000

0.000005

0.000010

0.000015

0.000020

0.000025

0.000030

jPR

mcA/j-

-2

t/s

-0.623V

j3D-dc

0 2 4 6 8 100.000000

0.000005

0.000010

0.000015

0.000020

0.000025

0.000030

0.000035

jPRmc

A/j--2

t/s

-0.683V

j3D-dc

Figure 5. The individual contributions to the total current due to the nucleation process (j3D-dc) and water reduction

(jWR). (According to equation (1) for the experimental current density transient recorded at different potentials.)

0 2 4 6 8 10

0.000000

0.000002

0.000004

0.000006

0.000008

0.000010

0.000012

(a)

-0.703V

-0.683V

-j/A

cm-2

t/s

-0.623V

-0.773V

0 2 4 6 8 100.000010

0.000015

0.000020

0.000025

0.000030

0.000035

0.000040

(b)

-j/A

cm-2

t/s

-0.623V

-0.683V

-0.703V

-0.773V

Figure 6. Individual contributions to the total current density (j(t)). (a) Water reduction, jWR. (b) Mass transfer

controlled 3D nucleation, j3D-dc.

Table 2. Current contribution rate in different processes.

-E (V) QWR% Q3D-dc% Qtotal%

0.623 34.36 65.35 100

0.683 16.22 83.78 100

0.703 2.41 97.59 100

0.773 3.39 96.61 100

Table 1. Related fit parameters and kinetic data obtained from the experimental potentiostatic current transients (depicted in figure 2

using equation (1)).

-E (V) A (s-1) 106 P1 (A cm-2) P2 (s-1) 104 P4 (A cm-2 s1/2) 108 D (cm2 s-1)

0.623 0.15 11.31 0.23 0.43 0.16

0.683 0.20 5.34 0.25 0.56 0.27

0.703 0.14 0.84 0.28 0.71 0.43

0.773 0.29 0.93 0.35 0.65 0.36

14 Page 6 of 11 Bull Mater Sci (2021) 44:14

controls the entire electrodeposition process, which easily

causes the sediment to become dendritic [46].

The dendritic growth is for the metal ion and hydrogen

ion in the electrolyte to quickly consume a high concen-

tration of electrons through discharge. The results of

Himanshu Singh et al [47] showed that dendritic mor-

phology is sensitive to potential. The authors also obtained a

silver coating with a dendritic microstructure in the ChCl–

urea system, and the results obtained through the

chronoamperometric experiment also proved that the

reduction process of silver ions in the ChCl–urea eutectic

solvent system was controlled by diffusion.

The EDS results show that the coating is mainly com-

posed of O, C, Ag, Cu and Zn, where Cu and Zn are matrix

elements, and C is a foreign substance. To study further, the

thickness of the coating, SEM and elemental mapping

images were analysed on the obtained cross-section of the

coating, and the results are shown in figure 8. It can be seen

from the cross-section view of the coating that the silver

plating layer uniformly covers the entire surface of the Cu–

Zn substrate with a thickness of about 22.98 lm.

As shown in figure 8b, the element linear scan of the

cross-section of the Ag plating layer shows that the elec-

trodeposited layer is mainly composed of Ag element,

which contains a small amount of O, C, Cu and Zn ele-

ments. It can be seen from the elemental mapping images of

the Ag coating (as shown in figure 9) that the distribution of

Ag, Cu and Zn elements is very obvious.

Figure 7. SEM micrographs of Ag electrodeposits on a Cu–Sn substrate at different potentials (a) 0.65,

(b) 0.8, (c) 0.97 V) and the energy-dispersive X-ray (EDS) analysis confirms the composition of the silver.

Bull Mater Sci (2021) 44:14 Page 7 of 11 14

Figure 10 shows the wide scanning spectrum of silver

coating and the narrow scanning spectra of silver and

oxygen obtained under the condition of -0.97 V potential.

The information of existing elements in Ag coating and

absorbed surface contamination can be obtained from the

wide scanning spectrum of XPS. Figure 10a shows the

typical XPS survey spectrum of the Ag coating surface. It

shows that the Ag film on the brass consisted primarily of

Figure 8. Cross-section sample of Ag electrodeposits on a Cu–Sn substrate and median line scanning.

Figure 9. Elemental mapping images of cross-section sample.

14 Page 8 of 11 Bull Mater Sci (2021) 44:14

Ag, O and C. Carbon has been found on sample as impu-

rities from handling the sample in air. Hydrogen could also

be present but cannot be detected by the XPS measurement

[48,49]. According to the fitting analysis of the Ag(3d5/2),

the Ag(3d5/2) peak can be deconvoluted into two peaks. The

chemical state of the Ag element and O element in the Ag

film can be observed from the analysis of the selected

narrow-scanned peaks for silver and oxygen elements. The

optimized deconvolution of the representative narrow scan

peaks for Ag and O are shown in figure 10b and c. Two

representative narrow scan spectra of the Ag3d peak can be

shown in the figure 10b. The first sub-peak for Ag(3d5/2) is

corresponding to the Ag2O and the second sub-peak for

Ag(3d5/2) is corresponding to the Ag. The selected narrow-

scanning of oxygen (O1s) shows that oxygen and silver

make up silver oxide (table 3).

It is worth noting that no matter the EDS test or the XPS

test, oxygen element can be found in the silver coating

obtained by the ChCl–urea eutectic solvent system.

According to the result description of the current density

transient in subsection 3.3, it can be inferred that the oxygen

element in the coating is due to the chemical reaction

between the trace water in the solution with silver. Aldana-

Gonzalez et al [50] and Manuel Palomar-Pardave et al [51]

have also found that water participates in the reduction of

nickel and iron in the ChCl–urea system. In the ChCl–urea

DES system, a small amount of water is reduced to

hydrogen and hydroxyl ions, and AgOH which is unsta-

ble in the solution is formed. AgOH dehydrates and further

decomposes to form Ag2O. The reduction process of silver

ions in ChCl-EG DESs can be expressed by equations

(11–14):

Agþ þ e� ! Ag; ð11Þ2H2O þ 2e� ! H2 þ 2OH�; ð12ÞAgþ þ OH� ! AgOH; ð13Þ2AgOH ! Ag2OðsÞ þ H2O: ð14Þ

4. Conclusion

The reduction process of silver ions in the ChCl–urea

eutectic solvent system is controlled by diffusion, and the

diffusion rate D is (0.29 ± 0.14) 9 10-8 cm2 s-1. The

metallic electrodeposition of Ag in this system cannot be

distinguished only by three-dimensional ‘instantaneous’ or

0 200 400 600 800 1000 1200

0.0

4.0x104

8.0x104

1.2x105

1.6x105

2.0x105

AgM

NN

Ag3

pAg3

p

AgM

5W

O1s

Ag3

d 5/2

C1s

Cou

nts

/ s

Binding Energy / eV

Survey(a)

360 365 370 3750.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105 (b)

Binding Energy (eV)

Rel

ativ

e In

tens

ity

(c/s

) Ag3d3/2

Ag3d5/2

AgAg2O

Ag

525 530 535 540

3.0x104

3.5x104

4.0x104

4.5x104

5.0x104

5.5x104

6.0x104

(c)

Binding Energy (eV)

Rel

ativ

e In

tens

ity

(c/s

)

O1s O

Figure 10. XPS wide scanning spectrum and the narrow scan-

ning spectra of silver and oxygen elements. (a) Typical XPS

survey spectra for the silver coating. XPS narrow scanning

spectrum and fitting peaks of (b) Ag3d5/2 and (c) O1s.

Table 3. Data of the EDS analysis confirms the composition of silver.

Potential (V) Element Silver Copper Zinc Carbon Oxygen

0.65 Atomic wt% 58.97 20.83 11.16 7.49 1.54

0.80 Atomic wt% 69.37 15.71 9.40 3.28 2.24

0.97 Atomic wt% 77.12 3.98 2.50 9.69 6.71

Bull Mater Sci (2021) 44:14 Page 9 of 11 14

‘progressive’ nucleation under diffusion control. The cur-

rent density transient is composed of water reduction pro-

cess and 3D nucleation with diffusion-controlled growth,

and these two processes happen at the same time. The

reduction process of water can lead to the appearance of

silver oxide in the coating. Nucleation contribution is

dominant and water reduction contribution decreases with

the increase in potential. The three-dimensional nucleation

of silver can be improved by increasing the potential, and

the reduction of water can be inhibited at the same time.

The thickness of silver coating obtained in ChCl–urea DESs

was 22.98 lm and the compositions of the coating consisted

of Ag2O and Ag.

Acknowledgements

This work was supported by the project of Liaoning

Province-Shenyang National Laboratory for Materials

Science Joint Research Fund (Project No.: 2019JH3/

30100021).

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