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Characterization and electrochemical analysis of silver electrodeposition 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 potentialEeqof the electrode system in ChCl–

urea DESs was-0.112 V. The Tafel curve was used to obtain the exchange current densityj0= 2.92±0.2 cm-2and the transfer coefficienta= 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 reductionjWR(t). By calculating the percentage of different charge densities of j3D-dcandjWR, 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 22.98lm. 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, https://doi.org/10.1007/s12034-020-02276-3

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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. Sebastia´net 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 7.85 910-9cm2s-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 0.3lm 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. Sebastia´n et al[13] and Quentin Raye´e 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-1and the test system temperature was 50°C. The temperature of the silver electrodeposition experiment in the ChCl–

urea–AgNO3DES system was 50°C and the deposition time was 1 h.

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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

Figure1A 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 = 48.6lA 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 isEeq= -0.112 V. According tog = E- Eeq, an g–jrelationship curve (shown in figure1B) can be obtained. For the CV curve shown in figure 1B, the overpotential interval -0.1–0.1 V is intercepted, andgandj = jc- jarelationship curve (shown in figure 1C) is made, where the ordinate is

the net current density. The curve shown in figure1D is the Tafel curve plotted from the data of figure1C. By calcu- lating the cathode branch slope of the Tafel curve, the relationship curve between potential and current is gc = 0.4658-0.3772 logjc, the exchange current density isJ0= 2.92±0.2lcm-2, and the transfer coefficient isa=0.16

±0.2.

3.2 Potentiostatic study

Figure2 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 i¼nFD1=2C=p1=2t1=2

1expðN0pkDtÞ

½ ;

k¼ð8pCM=dÞ1=2:

ð1Þ

3D progressive nucleation i¼nFD1=2C=p1=2t1=2

1expbN0pk0Dt2=2

;

k0¼4=3 8pCM=dð Þ1=2: ð2Þ

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-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

A

mcA/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 logjC

logj0

Figure 1. Typical cyclic voltammetry curve and related electrochemical data curve. (A) Typical cyclic voltammetry curve (0.1 mol l-1AgNO3is 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-1sweep 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 inC.

0 2 4 6 8 10

0.000005 0.000010 0.000015 0.000020 0.000025 0.000030 0.000035 0.000040

-j/cm-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/jm)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–ttransients.

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The dimensionless curve can be obtained after perform- ing the dimensionless processing on the curve in figure 2, and the result is shown in figure3. 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. Whent/tm [1, the deviation increases. Especially aftert/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 isj(t): 3D nucleation with diffusion-controlled growth, j3D-dc(t), and water reactionjWR(t).

jtotal¼j3Ddcð Þ þt jWRð Þt ; ð3Þ

j tð Þ ¼ ðP1þP4t1=2Þ

ð1expðP2ðtð1expðP3tÞÞ=P3ÞÞÞ; ð4Þ P1 ¼ZWRFkWRð2c0M=pqÞ1=2; ð5Þ

P2 ¼N0pkD; ð6Þ

P3 ¼A; ð7Þ

P4 ¼ 2FD1=2c0

=p1=2

; ð8Þ

k¼ð8pc0=qÞ1=2; ð9Þ

where ZWRF is the molar charge transferred during the water reduction process, kWR is the rate constant of the water reduction reaction, c0andDare the bulk concentra- tion and diffusion coefficient of Ag, respectively,M andq are 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. Figure4 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 figure4), 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 10-8cm2s-1.

Under the potential conditions of-0.623 and-0.683 V, the experimental j–tcurve of silver nucleation and growth on glassy carbon electrode was compared with the data obtained by fitting the nonlinear equation (4) (as shown in figure5), 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.

Qt¼ Z t

0

jdt: ð10Þ

Figure6a and b shows the contribution curves ofjWRand j3D-dc under different potential conditions. By observing figure 6 and the integrated area results of calculating thej–t diagram of Q3D-dc and QWR (as shown in table 2) using equation (10). It shows that as the potential increases, the j3D-dccontribution also increases, whilejWRdecreases 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 Ag coating

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 10

0.000005 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).

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micro-morphology of the silver coating changes from small leaves to more loose pines. Sebastia´net al[34] conducted a silver electrodeposition study in the ChCl–urea eutectic system, and a similar picture appeared in figure7a. 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 10

0.000000 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 10

0.000000 0.000005 0.000010 0.000015 0.000020 0.000025 0.000030 0.000035

jPR

mcA/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/Acm-2

t/s

-0.623V

-0.773V

0 2 4 6 8 10

0.000010 0.000015 0.000020 0.000025 0.000030 0.000035 0.000040

(b)

-j/Acm-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 figure2 using equation (1)).

-E(V) A(s-1) 106P1(A cm-2) P2(s-1) 104P4(A cm-2s1/2) 108D(cm2s-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

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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 figure8. 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 22.98lm.

As shown in figure8b, 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 figure9) 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.

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Figure10 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. Figure10a 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.

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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 figure10b 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- Gonza´lezet 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):

Agþþe!Ag; ð11Þ

2H2Oþ2e!H2þ2OH; ð12Þ

AgþþOH!AgOH; ð13Þ

2AgOH!Ag2OðsÞþH2O: 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

AgMNN

Ag3pAg3p AgM5W

O1s

Ag3d5/2

C1s

Counts / s

Binding Energy / eV

Survey (a)

360 365 370 375

0.0 2.0x104 4.0x104 6.0x104 8.0x104 1.0x105 1.2x105 (b)

Binding Energy (eV)

Relative Intensity (c/s)

Ag3d3/2 Ag3d5/2

Ag2O Ag

Ag

525 530 535 540

3.0x104 3.5x104 4.0x104 4.5x104 5.0x104 5.5x104 6.0x104

(c)

Binding Energy (eV)

Relative Intensity (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) Ag3d5/2and (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

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‘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 22.98lm 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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