Electrochemical Behavior and Electrodeposition of Sn Coating from Choline Chloride–Urea Deep Eutectic Solvents

coatings

Article Electrochemical Behavior and Electrodeposition of Sn Coating from Choline Chloride–Urea Deep Eutectic Solvents

Xiaozhou Cao 1 , Lulu Xu 1, Chao Wang 2, Siyi Li 1, Dong Wu 3, Yuanyuan Shi 4, Fengguo Liu 1 and Xiangxin Xue 1,* 1 School of Metallurgy, Northeastern University, Shenyang 110819, China; [email protected] (X.C.); [email protected] (L.X.); [email protected] (S.L.); [email protected] (F.L.) 2 Department of Mechanical Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA; [email protected] 3 Chinalco Shenyang Non-Ferrous Metal Processing Co., Ltd., Shenyang 110108, China; [email protected] 4 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; [email protected] * Correspondence: [email protected]; Tel.: +86-024-83687719

Received: 28 October 2020; Accepted: 25 November 2020; Published: 26 November 2020

Abstract: The electrochemical behavior and electrodeposition of Sn were investigated in choline chloride (ChCl)–urea deep eutectic solvents (DESs) containing SnCl2 by cyclic voltammetry (CV) and chronoamperometry techniques. The electrodeposition of Sn(II) was a quasi-reversible, single-step two-electron-transfer process. The average transfer coefficient and diffusion coefficient of 0.2 M Sn(II) in ChCl–urea at 323 K were 0.29 and 1.35 10 9 cm2 s 1. The nucleation overpotential decreased × − · − with the increase in temperature and SnCl2 concentration. The results of the chronoamperometry indicated that the Sn deposition on tungsten electrode occurred by three-dimensional instantaneous nucleation and diffusion controlled growth using the Scharifker–Hills model. Scanning electron microscopy (SEM) showed that the morphology of the deposits is uniform, as a dense and compact film prepared by potentiostatic electrolysis on Cu substrate. X-ray diffraction (XRD) analysis revealed that the deposits were pure metallic Sn.

Keywords: tin; electrodeposition; cyclic voltammetry; deep eutectic solvent; nucleation

1. Introduction Tin and its alloy coatings are widely used in various industrial ﬁelds because of its high corrosion resistance, non-toxicity and good soldering properties, which has an easily soldered coating for printed wiring boards, connectors, lead frames and transistor leads, protective coating to steel sheet from corrosion, tinplates for the food industry (containers and packages), coating on copper and aluminum bus-bars to ensure good contact at joints to carry high electrical currents [1,2]. Tin and tin alloy can be electrodeposited in acidic or alkaline aqueous solution [3,4]. The acidic electrolytes mainly include sulfate, ﬂuoborate and methanesulfonate solutions [5–7]. In order to improve the quality of the coating and the electrolyte stability, cyanide and organic as additive was introduced in the electrolyte. Due to the toxicity, the hydrogen discharge secondary reaction during the electrolysis process and the relatively narrow potential windows in the aqueous solution, electrodeposition of metals or alloys from deep eutectic solvents, have attracted substantial interest in recent years [8]. Deep eutectic solvents (DESs) are a new class of ionic liquids, based on combinations of choline chloride with hydrogen bond donors, such as carboxylic acids, glycols and amides [9]. Compared with aqueous electrolytes, DESs have some great properties such as wide potential window,

Coatings 2020, 10, 1154; doi:10.3390/coatings10121154 www.mdpi.com/journal/coatings Coatings 2020, 10, 1154 2 of 10 higher conductivity, high solubility of metal salts, good thermal stability, and the absence of hydrogen evolution on the electrode, which make them suitable electrolytes for the electrodeposition of metals [10]. At present, Zn, In, Co, Bi, Sn have been prepared in choline chloride-based DESs [11–15], whereas Al and Fe were electrodeposited in AlCl3–urea and FeCl3–acetamide DESs [16,17]. Xue and Huang et al. have investigated the deposition of Sn from AlCl3/1–ethyl-3- methylimidazolium chloride (AlCl3/EMIC) and ZnCl2/1–ethyl-3-methylimidazolium chloride (ZnCl /EMIC) respectively [18,19]. Pereira and Ghosh et al. used anhydrous SnCl and SnCl 2H O 2 2 2· 2 as a tin source to electrodeposit Sn from ChCl–ethylene glycol, respectively [20,21]. Anicai et al. investigated the electrochemical behavior of Sn electrodeposition on various metallic substrates from ChCl–ethylene glycol and ChCl–malonic acid [22]. Salomé et al. investigated the nucleation mechanism and the kinetic parameters for the Sn deposition from ChCl–ethylene glycol, ChCl–propylene glycol and ChCl–urea [23]. However, the detailed kinetic process of electrode reaction and electrochemical behavior during Sn electrodeposition still need investigating. In this paper, the authors aimed to study the inﬂuence of temperature and concentration of SnCl2 on the reduction behavior of Sn in ChCl–urea by cyclic voltammetry (CV). The nucleation–growth mechanism of the initial Sn deposition stages was investigated by chronoamperometry measurement. The surface morphology and crystallites of the deposit were investigated by scanning electron microscopy (SEM) and X-ray diﬀraction (XRD).

2. Materials and Methods The DES solvent was prepared with choline chloride (ChCl) (Aladdin Reagent Co. Shanghai, China, 99%) and urea (Aladdin Reagent Co. Shanghai, China, 99%) with a molar ratio of 2:1. The reagents were dried under vacuum at 373 K for 12 h. The binary mixture was heated at a temperature of 373 K for 30 min until a homogeneous colorless liquid was formed. SnCl2 (Aladdin Reagent Co. Shanghai, China, 99%) was added into the ChCl–urea with constant stirring at 353 K for 6 h until the SnCl2 powder was dissolved in the solution. Electrochemical techniques were utilized to study the deposition process. The electrochemical measurements were carried out using the electrochemical workstation (Solartron 1287A, Farnborough, UK) controlled by CorrWare software (version 3.2c). Cyclic voltammetry (CV) and chronoamperometry were performed in a three-electrode electrochemical cell. Tungsten wire with diameter of 1 mm was used as the working electrode. Platinum wire and silver wire with a diameter of 0.5 mm were used as the counter electrode and quasi-reference electrode, respectively. All the electrodes were polished with abrasive paper, washed with deionized water and dried before the measurement. All the electrodes were inserted into the solution under 1 cm. Sn films were prepared in ChCl–urea containing SnCl2 by electrodeposition experiments. The experiments were performed at a different potential in a three-electrode system. Copper foil (1 1 cm2) as a substrate was used as the working electrode. The films were washed with acetone and × deionized water to remove the rest of the electrolyte and dried under vacuum. The surface morphology of the deposits on copper substrate was determined by scanning electron microscopy (SEM, Zeiss Ultra, Oberkochen, Germany) with a 3–6 kV accelerating voltage. The crystal structure of the deposits was analyzed by X-ray diffraction (XRD, Philips MRD 1800, Almelo, The Netherlands) with a Cu Kα radiation at 45 kV and 40 mA in the 2θ angular range of 10◦–90◦ with 0.02◦ increments.

3. Results and Discussion

3.1. Cyclic Voltammetry

The cyclic voltammograms obtained from ChCl–urea containing 0.2 M SnCl2 solution at 323 K on a tungsten electrode, measured at diﬀerent scan rates, are shown in Figure1. It can be clearly seen that the curves display a couple of well deﬁned cathodic and anodic peaks, which is for a typical metal Coatings 2020, 10, x FOR PEER REVIEW 3 of 10 Coatings 2020, 10, 1154 3 of 10 that the curves display a couple of well defined cathodic and anodic peaks, which is for a typical deposition–strippingmetal deposition–stripping process. process. The cathodic The cathodic current peakcurrent is observedpeak is observed at approximately at approximately0.6 V (vs. −0.6 Ag) V (vs. Ag) in the forward scan. Abbott et al. reported that2+ the Sn2+ species exists as [SnCl3−]− in reline [24]. in the forward scan. Abbott et al. reported that the Sn species exists as [SnCl3]− in reline [24]. Thereby, 2+ theThereby, cathodic the current cathodic peak current could peak be attributed could be to attributed the reduction to the of reduction Sn2+ species of toSn metal species Sn. Theto metal current Sn. increasedThe current more increased negative potentialsmore negative than 1.1 potentials V which wasthan attributed −1.1 V towhich the electroreduction was attributed of cholineto the − ionelectroreduction [25,26]. of choline ion [25,26].

0.0015 a-30mV/s b-40mV/s 0.0010 c-50mV/s d-60mV/s e-70mV/s 0.0005 ) 2

Eep 0.0000 j(A/cm

0.0008 -0.0005 a ) 2 0.0007 b (A/cm p

c -j d e 0.0006 -0.0010 0.16 0.18 0.20 0.22 0.24 0.26 0.28 v1/2(V/s)1/2 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 E vs.Ag (V)

FigureFigure 1.1.Cyclic Cyclic voltammogramsvoltammograms (CV)(CV) withwith didifferentﬀerent scan scan rates rates in in ChCl–urea ChCl–urea containing containing 0.2 0.2 M M SnCl SnCl2 2 atat 323323 KK (the (the bottom bottom rightright cornercorner ofof thethe ﬁgurefigure is is the the relationship relationship between betweenthe the cathodic cathodic peakpeak currentcurrent 1/2 densitydensity (j(pjp)) andand thethe squaresquare rootroot ofof scanscan raterate ((vv1/2)).

AA singlesingle anodicanodic currentcurrent peakpeak isis observedobserved duringduring thethereversal reversal sweep. sweep. TheThe anodicanodic currentcurrent peakpeak isis assignedassigned toto thethe anodicanodic strippingstripping of of the the Sn Sn deposit deposit during during the the forward forward scan. scan. TheThe reductionreduction andand oxidationoxidation peakpeak potentialspotentials shiftshift totoa a moremore negativenegative andand positivepositive directiondirectionwith with thethe scanscan raterate increasing, which which are are signs signs of of irreversible irreversible reaction. reaction. The The potential potential difference difference between between the the cathodic peak and half-peak potentials |Epc Epc/2| increases with the increase in scan rate. cathodic peak and half-peak potentials |Epc − Epc/2− | increases with the increase in scan rate. The Theminimum minimum value value of potential of potential difference difference is is55 55 mV, mV, wh whichich is is larger larger than than the the theoretical valuevalue forfor thethe reversiblereversible process process (31 (31 mV,mV, 323 323 K). K). It It was was typically typically associated associated with with quasi-reversiblequasi-reversible electrodeelectrode reactionreaction processprocess [27[27].]. TheThe cathodiccathodic currentcurrent alsoalso increasesincreases withwith thethe increaseincrease inin scan scan rate. rate. InIn addition,addition, aa goodgood linear relationship between the cathodic peak current density (jp) and the square root of scan rate linear relationship between the cathodic peak current density (jp) and the square root of scan rate (v1/2) 1/2 (vwas) obtained was obtained as shown as shown in Figure in Figure 1 (the1 bottom(the bottom right corner), right corner), suggesting suggesting that the that reduction the reduction process processof Sn(II) of is Sn(II) the diffusion is the di ffcontrollusion controlleded adsorption adsorption process process according according to the Langmuir to the Langmuir isothermal isothermal model. model.All these All characteristics these characteristics indicate indicate that the that reduction the reduction of Sn(II) of Sn(II)is a quasi-reversible, is a quasi-reversible, single-step single-step two- two-electron-transferelectron-transfer process process [28]. [ 28]. TheThe formularformular forfor calculatingcalculating thethe irreversibleirreversible reactionreaction transfertransfer coecoefficientfficient isis givengiven inin EquationEquation (1).(1). FormularFormular forfor calculatingcalculating thethe didiffusionffusion coecoefficientfficient ofof irreversibleirreversible reactionreaction isis shownshown inin EquationEquation (2),(2), whichwhich is is also also applicable applicable to to the the quasi-reversible quasi-reversible charge charge transfer transfer process: process:

|E|Epcpc −E Epc/2|| = =1.857 1.857RTRT/(/(ααnFnF)) (1) (1) − pc/2 0 1/2 1/2 1/2 jp = 0.4958nFc0 (αnF/RT)1/2 D1/2 v1/2 (2) jp = 0.4958nFc (αnF/RT) D v (2) where Ep is the peak potential, Ep/2 is the half-peak potential, R is the universal gas constant, α is the where Ep is the peak potential, Ep/2 is the half-peak potential, R is the universal gas constant, α is transfer coefficient, n is the electron transfer number, F is the faraday constant, jp is the peak current the transfer coeﬃcient, n is the electron transfer number, F is the faraday constant, j is the peak density, c0 is the concentration of metal ions, D is the diffusion coefficient, and v is the potentialp scan rate.

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Coatings 2020, 10, x FOR PEER REVIEW 4 of 10 current density, c0 is the concentration of metal ions, D is the diffusion coefficient, and v is the potential scanThe rate. average transfer coefficient and diffusion coefficient of 0.2 M Sn(II) in ChCl–urea at 323 K The average transfer coefficient and diffusion coefficient of 0.2 M Sn(II) in ChCl–urea at 323 K are are 0.29 and 1.35 × 10−9 cm2∙s−1, calculated from Equations (1) and (2) and Figure 1. 0.29 and 1.35 10 9 cm2 s 1, calculated from Equations (1) and (2) and Figure1. In addition,× −a current· − crossover loop was observed between the forward and the reverse directionIn addition, at a different a current scan crossover rate, loopwhich was means observed that between the reduction the forward of andSn ions the reverse requires direction large overpotentialat a different scan to form rate, a which nucleation means and that growth the reduction process. of The Sn ionsprerequisite requires for large the overpotential growth of Sn to on form the tungstena nucleation substrate and growth is the formation process. of The thermodynamica prerequisite forlly the stable growth nuclei of on Sn the on surface, the tungsten which substrate requires ais more the formation negative ofpotential thermodynamically than that required stable nucleito reduce on the Sn surface,cations, whichand lead requires to what a more is known negative as potential than that required to reduce Sn cations, and lead to what is known as nucleation overpotential nucleation overpotential (ηnucleation) [29]. The overpotential required initiating the first stages of the η (metalnucleation deposition)[29]. The process overpotential on the “foreign” required surface initiating of the the tungsten first stages substrate. of the During metal deposition the forward process scan, theon thecrossover “foreign” potential surface at of a the more tungsten negative substrate. potential During was indicative the forward of scan,the formation the crossover of Sn potential nuclei on at thea more electrode negative surface potential that wasenhanced indicative the surface of the formationarea of the of electrode Sn nuclei and on thedecreased electrode the surface activation that energyenhanced for the the surface deposition area ofof the the electrode metal, afterward and decreased the current the activation increase energy is due for to the the deposition growth of of first the nuclei.metal, afterwardWhen the the scan current is reversed, increase the is duereduction to the growthcurrent ofcontinues first nuclei. to flow, When because the scan Sn is reversed,is easily electrodepositedthe reduction current on continuesa previously to flow, nucleated because Sn Sn issurface easily electrodepositedthan on the tung onsten a previously surface nucleateduntil the Sn surface than on the tungsten surface until the equilibrium potential (Eep) is reached [30,31]. It is equilibrium potential (Eep) is reached [30,31]. It is also found the nucleation overpotential is increased also found the nucleation overpotential is increased by scan rate. After Eep, a notable stripping peak by scan rate. After Eep, a notable stripping peak formed, which corresponds to the oxidation of the Sn deposit.formed, which corresponds to the oxidation of the Sn deposit. Figure2 presents the cyclic voltammograms of ChCl–urea containing 0.2 mol /L SnCl at various Figure 2 presents the cyclic voltammograms of ChCl–urea containing 0.2 mol/L SnCl22 at various temperatures (323 (323 to to 353 353 K). K). The The cathodic cathodic peak peak current current increases increases with increasing with increasing temperature, temperature, which whichcorresponds corresponds to the lower to the viscosity lower viscosity and resulted and resulted in a mo inre aefficient more e ffimasscient transfer mass transfer rate of active rate of species active andspecies the andhigher the conductivity higher conductivity of electrolyte. of electrolyte. The reduction The reduction peak potential peak potential and the and onset the potential onset potential of the reductionof the reduction of Sn ofions Sn shifted ions shifted to more to more positive positive valu valuee (Zoom (Zoom in inFigure Figure 2),2), indicating indicating that that the electronelectron transfer rate increases at higher temperatures [32]. The values of nucleation overpotential (η ) transfer rate increases at higher temperatures [32]. The values of nucleation overpotential (nucleationηnucleation) are declining with increasing temperature, indicating that Sn is favorable to electrodeposit with the rise of temperature.

0.0025 a-323K 0.0020 b-333K c-343K 0.0015 d-353K

0.0010 )

2 0.0005

0.0000

j(A/cm

-0.0005 a b -0.0010 c -0.0015 d

-0.0020 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 E vs. Ag (V)

Figure 2. CVs with different temperature in ChCl–urea containing 0.2 M SnCl2 at 50 mV/s. Figure 2. CVs with diﬀerent temperature in ChCl–urea containing 0.2 M SnCl2 at 50 mV/s.

The cyclic voltammogramsvoltammograms ofof 0.05, 0.05, 0.1 0.1 and and 0.2 0.2 mol mol/L/L SnCl SnCl2 in2 in ChCl–urea ChCl–urea at at 100 100 mV mV/s/s are are shown shown in inFigure Figure3. It 3. is It observed is observed that that the cathodicthe cathodic and anodicand anodic peak peak current current increases increases with the with increase the increase in SnCl in2 SnClconcentration.2 concentration. In addition, In addition, a linear a relationshiplinear relationship was found was betweenfound between the peak the current peak densitycurrent anddensity the andconcentration the concentration of SnCl2 asof shownSnCl2 as in shown Figure3 in (the Figure bottom 3 (the right bottom corner), right which corner), may bewhich due tomay the be presence due to theof a greatpresence amount of aof great species amount available of species for reduction availabl ate the for surface reduction of the at electrode the surface at higher of the concentration, electrode at higher concentration, and the cathodic reduction of the electroactive species is controlled by diffusive transport [33,34]. The cathodic peak potential shifts towards a more positive value when the

Coatings 2020, 10, 1154 5 of 10 and the cathodic reduction of the electroactive species is controlled by diﬀusive transport [33,34]. Coatings 2020, 10, x FOR PEER REVIEW 5 of 10 The cathodic peak potential shifts towards a more positive value when the concentration of SnCl2 increases, indicating that the electron transfer rate increases and the higher concentration of reacting concentration of SnCl2 increases, indicating that the electron transfer rate increases and the higher speciesconcentration reduced of the reacting overpotential species reduced for Sn reduction the overpotential [35]. for Sn reduction [35].

0.0010 a-0.05mol/L 0.0008 b-0.10mol/L c-0.20mol/L 0.0006

0.0004 )

2 0.0002

0.0000

j(A/cm

0.0007 -0.0002 a 0.0006

0.0005 ) 2 0.0004 -0.0004 b

-j(A/cm 0.0003 -0.0006 0.0002 c 0.0001 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 concentration(mol/L) -0.0008 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 E vs. Ag (V)

FigureFigure 3.3. CVs with didifferentﬀerent Sn(II)Sn(II) concentrationconcentration in ChCl–urea at 323 KK andand 100100 mVmV/s/s (the(the bottombottom rightright cornercorner ofof thethe ﬁgurefigure isis thethe relationshiprelationship betweenbetween thethe peakpeak currentcurrent densitydensity andand thethe concentrationconcentration ofof SnClSnCl22).

3.2.3.2. Nucleation and Growth Mechanism The nucleation and growth mechanism of Sn deposits in ChCl–urea containing 0.2 M SnCl at The nucleation and growth mechanism of Sn deposits in ChCl–urea containing 0.2 M SnCl22 at 323323 KK waswas investigated investigated by by chronoamperometry chronoamperometry measurement. measurement. The The current current density density as a functionas a function of time of transients obtained by stepping the potential from initial value of 0.42 V to more negative potentials time transients obtained by stepping the potential from initial −value of −0.42 V to more negative ( 0.45, 0.48 and 0.51 V) to initiate the nucleation and growth of Sn is shown in Figure4. potentials− − (−0.45, −0.48 and −0.51 V) to initiate the nucleation and growth of Sn is shown in Figure 4. All the curves have a similar tendency, the current density decreases because of the double layer charging. Then, the current density increases to its maximum jmax at tmax. and the sharp decreases to a 0.004 constant state which have the typical characteristics of diffusion-controlled three dimensional (3-D) nucleation and growth process. -0.42V The current density increases sharply to the maximum (jmax) at a -0.45V certain time (tmax) due to the 0.003 growth of the individual nuclei and the increase in Sn nuclei number which -0.48V increase the electroactive -0.51V surface. Then, the maximum current density transients decay slowly after time tmax because of the ) nuclei growth centers-2 overlap and grow toward the solution, accompanied by the disappearance of 0.002 growth centers and the regeneration of new growth centers. At the stage, the current decrease is a result of the decreasej/(Acm in the effective electrode surface area which corresponds to the linear diffusion to the planar electrode surface [36]. The time corresponding to the jmax decrease when the applied 0.001 cathodic potentials increased from 0.42 to 0.51 V is attributed to the increase in nucleation density − − and nucleation rate. Scharifker and Hills developed two limiting models, the progressive nucleation and instantaneous 0.000 nucleation, to identify the nucleation mechanism of metal electrodeposition [37]. For an instantaneous 0246810 nucleation, the rate of nucleation was rapid in comparisont/s with the rate of crystal growth. Nuclei are formed at all possible growth sites within very short times. Figure 4. Current–time transients from chronoamperometry in ChCl–urea containing 0.2 M SnCl2 at 323 K and various potentials (V): −0.42, −0.45, −0.48, −0.51, −0.54.

All the curves have a similar tendency, the current density decreases because of the double layer charging. Then, the current density increases to its maximum jmax at tmax. and the sharp decreases to a

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concentration of SnCl2 increases, indicating that the electron transfer rate increases and the higher concentration of reacting species reduced the overpotential for Sn reduction [35].

0.0010 a-0.05mol/L 0.0008 b-0.10mol/L c-0.20mol/L 0.0006

0.0004 )

2 0.0002

0.0000

j(A/cm

0.0007 -0.0002 a 0.0006

0.0005 ) 2 0.0004 -0.0004 b

-j(A/cm 0.0003 -0.0006 0.0002 c 0.0001 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 concentration(mol/L) -0.0008 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 E vs. Ag (V)

Figure 3. CVs with different Sn(II) concentration in ChCl–urea at 323 K and 100 mV/s (the bottom right corner of the figure is the relationship between the peak current density and the concentration

of SnCl2).

3.2. Nucleation and Growth Mechanism

The nucleation and growth mechanism of Sn deposits in ChCl–urea containing 0.2 M SnCl2 at 323 K was investigated by chronoamperometry measurement. The current density as a function of Coatingstime transients2020, 10, 1154 obtained by stepping the potential from initial value of −0.42 V to more negative6 of 10 potentials (−0.45, −0.48 and −0.51 V) to initiate the nucleation and growth of Sn is shown in Figure 4.

0.004

-0.42V -0.45V 0.003 -0.48V -0.51V ) -2 0.002 j/(Acm

0.001

0.000

0246810 t/s

Figure 4. Current–time transients from chronoamperometry in ChCl–urea containing 0.2 M SnCl2 at Figure 4. Current–time transients from chronoamperometry in ChCl–urea containing 0.2 M SnCl2 at 323323 KK andand variousvarious potentialspotentials (V):(V): −0.42,0.42, −0.45, −0.48,0.48, −0.51,0.51. −0.54. − − − − WhileAll the for curves progressive have a similar nucleation, tendency, the rate the ofcurrent nucleation density is slowdecreases and nucleibecause are of generated the double at layer the surfacecharging. during Then, crystal the current growth. density The models increases for to instantaneous its maximum and jmax progressive at tmax. and nucleation,the sharp decreases followed to by a three-dimensional diﬀusion limited growth, are given by Equations (3) and (4), respectively.

j2 t t 2 = 1.9542( m ) 1 exp( 1.2564 ) (3) 2 jmax t − − tm

" #2 j2 t t2 = 1.2254( m ) 1 exp( 2.3367 ) (4) 2 2 jmax t − − tm where jmax and tm are the current density and time coordinates of the current–time transient curve peaks, respectively. The comparison of the experimental plots with the theoretical curves is illustrated in Figure5. It is obvious that the experimental data of Sn nuclei was in good agreement with the theoretical curve for the three-dimensional instantaneous nucleation and growth mechanism. However, when t/tm > 1, 2 the (j/jm) was higher than the theoretical value at various cathodic potentials, which was related to the partial kinetic control of the growth process, the coarse electrode surface provide more active nucleation sites for the electro-crystallization of metal ions [38].

3.3. Phase Composition and Morphology of Sn Coating Based on the above CV studies on tungsten electrodes, Sn thin ﬁlm were electrodeposited on a copper substrate under conditions of controlled potential ( 0.60 V) at a constant temperature of 323 K − for 120 s. Figure6 is the X-ray di ﬀraction pattern of the Sn electrodeposits on the Cu substrate obtained from 0.2 M SnCl2 in ChCl–urea based DES. The results indicated that only two metallic phases, and all peaks correspond to the Sn and Cu substrate. No other peaks were discovered, suggesting that the main phase of the electrodeposits is Sn. Coatings 2020, 10, x FOR PEER REVIEW 6 of 10

constant state which have the typical characteristics of diffusion-controlled three dimensional (3-D) nucleation and growth process. The current density increases sharply to the maximum (jmax) at a certain time (tmax) due to the growth of the individual nuclei and the increase in Sn nuclei number which increase the electroactive surface. Then, the maximum current density transients decay slowly after time tmax because of the nuclei growth centers overlap and grow toward the solution, accompanied by the disappearance of growth centers and the regeneration of new growth centers. At the stage, the current decrease is a result of the decrease in the effective electrode surface area which corresponds to the linear diffusion to the planar electrode surface [36]. The time corresponding to the jmax decrease when the applied cathodic potentials increased from −0.42 to −0.51 V is attributed to the increase in nucleation density and nucleation rate. Scharifker and Hills developed two limiting models, the progressive nucleation and instantaneous nucleation, to identify the nucleation mechanism of metal electrodeposition [37]. For an instantaneous nucleation, the rate of nucleation was rapid in comparison with the rate of crystal growth. Nuclei are formed at all possible growth sites within very short times. While for progressive nucleation, the rate of nucleation is slow and nuclei are generated at the surface during crystal growth. The models for instantaneous and progressive nucleation, followed by three-dimensional diffusion limited growth, are given by Equations (3) and (4), respectively.

2 2 t  jt=−−m 2 1.9542( ) 1 exp( 1.2564 ) (3) jtmax t m

2 22 jt=−−tm 221.2254( ) 1 exp( 2.3367 ) (4) jtmax t m

where jmax and tm are the current density and time coordinates of the current–time transient curve peaks, respectively. The comparison of the experimental plots with the theoretical curves is illustrated in Figure 5. It is obvious that the experimental data of Sn nuclei was in good agreement with the theoretical curve for the three-dimensional instantaneous nucleation and growth mechanism. However, when t/tm > 1, the (j/jm)2 was higher than the theoretical value at various cathodic potentials, which was related to Coatingsthe partial2020, 10kinetic, 1154 control of the growth process, the coarse electrode surface provide more active7 of 10 nucleation sites for the electro-crystallization of metal ions [38].

1.0

0.8

0.6 Progressive nucleation 2 ) m -0.42V

(j/j -0.45V 0.4 -0.48V -0.51V Coatings 2020, 10, x FOR PEER REVIEW 7 of 10 Instantaneous nucleation 0.2 3.3. Phase Composition and Morphology of Sn Coating

Based on the above0.0 CV studies on tungsten electrodes, Sn thin film were electrodeposited on a copper substrate under conditions of0.5 controlled potential 1.0 (−0.60 V) 1.5at a constant temperature 2.0 of 323 K for 120 s. Figure 6 is the X-ray diffraction patternt/t of the Sn electrodeposits on the Cu substrate m obtained from 0.2 M SnCl2 in ChCl–urea based DES. The results indicated that only two metallic Figure 5. Comparison of the experimental data derived from the current density–time transients with phases,Figure and 5. allComparison peaks correspond of the experimental to the Sn data and derived Cu substrate. from the current No other density–time peaks transientswere discovered, with the theoreticaltheoretical modelsmodels forfor progressiveprogressive andand instantaneousinstantaneous nucleations.nucleations. suggesting that the main phase of the electrodeposits is Sn.

FigureFigure 6. 6. XRDXRD pattern pattern of of the the Sn Sn electrodeposits obtainedobtained fromfrom ChCl–ureaChCl–urea containingcontaining 0.2 0.2 M M SnCl SnCl2 2on on a aCu Cu substrate substrate at at −0.60.6 VV andand 323323 K.K. − FigureFigure 7a–d7a–d show the SEM micrographs of the electrodepositselectrodeposits obtained on the Cu substrates at differentdifferent potentials potentials (− (0.45,0.45, −0.50,0.50, −0.55,0.55, −0.600.60 V) in V) ChCl–urea. in ChCl–urea. It can It be can seen be seenthat thatlots of lots uniform of uniform and − − − − smalland small particles particles were were obtained obtained at −0.45 at 0.45 V with V with large large uncovered uncovered areas areas of ofthe the substrate substrate in in Figure Figure 7a,7a, − whichwhich is is attributed attributed to to the the low low superficial superficial diffusion diffusion and and difficulty difficulty nucleating nucleating at at the the low low potentials. potentials. WithWith the increase in cathodic potential,potential, thethe particlesparticles have have a a similar similar morphology, morphology, which which indicate indicate that that Sn Sndeposition deposition obeys obeys the rulethe rule of instantaneous of instantaneous nucleation nucleation and growth and growth mechanism. mechanism. In addition, In addition, the particles the particlesbecame became smaller, smaller, non-regular non-regular and were and were more more densely densely distributed. distributed. When When the the potential potential is −0.600.60 V, V, − thethe particles particles are are compact compact and and have have entirely entirely covered covered th thee Cu Cu metallic metallic substrate, substrate, as as shown shown in in Figure Figure 7d.7d. TheThe nuclei nuclei population population density density increased, increased, which which may may due due to to the the increased increased number number of of growth growth sites sites and and depositiondeposition rate. rate.

(a) (b)

1μm 1μm

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3.3. Phase Composition and Morphology of Sn Coating Based on the above CV studies on tungsten electrodes, Sn thin film were electrodeposited on a copper substrate under conditions of controlled potential (−0.60 V) at a constant temperature of 323 K for 120 s. Figure 6 is the X-ray diffraction pattern of the Sn electrodeposits on the Cu substrate obtained from 0.2 M SnCl2 in ChCl–urea based DES. The results indicated that only two metallic phases, and all peaks correspond to the Sn and Cu substrate. No other peaks were discovered, suggesting that the main phase of the electrodeposits is Sn.

Figure 6. XRD pattern of the Sn electrodeposits obtained from ChCl–urea containing 0.2 M SnCl2 on a Cu substrate at −0.6 V and 323 K.

Figure 7a–d show the SEM micrographs of the electrodeposits obtained on the Cu substrates at different potentials (−0.45, −0.50, −0.55, −0.60 V) in ChCl–urea. It can be seen that lots of uniform and small particles were obtained at −0.45 V with large uncovered areas of the substrate in Figure 7a, which is attributed to the low superficial diffusion and difficulty nucleating at the low potentials. With the increase in cathodic potential, the particles have a similar morphology, which indicate that Sn deposition obeys the rule of instantaneous nucleation and growth mechanism. In addition, the particles became smaller, non-regular and were more densely distributed. When the potential is −0.60 V, the particles are compact and have entirely covered the Cu metallic substrate, as shown in Figure 7d. TheCoatings nuclei2020 ,population10, 1154 density increased, which may due to the increased number of growth sites8 and of 10 deposition rate.

(a) (b)

1μm 1μm

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1μm 1μm

Figure 7.7. SEM,SEM, secondary secondary electron electron image image (SEI) (SEI) of Sn electrodepositedof Sn electrodeposited at diﬀerent at depositiondifferent deposition potentials: (potentials:a) 0.45 V, ( (ab)) −0.450.50 V, V, ( (bc)) −0.500.55 VV, and(c) − (d0.55) 0.60V and V. ((Td)= −3230.60 K, V.c (T = 323= 0.2K, M)c(SnCl2) (the = upper0.2 M)right (the cornerupper − − − − (SnCl2) rightof the corner image of is the a 20,000 image magniﬁcationis a 20,000× magnification image of the image sample). of the sample). × 4. Conclusions Conclusions

ChCl–urea containing containing SnCl SnCl2 2waswas used used as an as electrolyte an electrolyte for the for electrodeposition the electrodeposition of Sn. Cyclic of Sn. voltammogramsCyclic voltammograms and chronoamperometry and chronoamperometry showed showed that for that a forsingle a single redox redox couple couple corresponding corresponding to theto thecathodic cathodic deposition deposition and anodic and anodic dissolution dissolution of Sn, the of Sn,electrodeposition the electrodeposition of Sn was of a Sndiffusion- was a controlleddiﬀusion-controlled process processvia a one-step, via a one-step, two-electron two-electron transfer transfer process process and and followed followed an an instantaneous instantaneous nucleation–growth mechanism. Sn Sn is is favorable favorable to to electrodeposit with the rising of temperature and SnCl2 concentration.concentration. SEM SEM indicated indicated th thatat the the deposits deposits became became compact compact on onthe theCu Cusubstrate substrate with with the the increase in cathodic potentials. The ﬁlm electrodeposited at 0.60 V presented grains uniformly increase in cathodic potentials. The film electrodeposited at −−0.60 V presented grains uniformly distributed and a size less than 1 μµm on the surface. The XRD pattern revealed the presence of the Sn phase on Cu substrate. Further Further in investigationvestigation can be done in order order to to enhance enhance the the surface surface morphology morphology of the deposit considering the applications of Sn.

Author Contributions: Investigation, X.C., L.X., and S.L.; data curation, Y.S. and F.L.; resources, D.W.; writing—reviewAuthor Contributions: and editing, Investigation, X.C. and X.C., C.W.; L.X., supervision, and S.L.; X.X.data Allcuration, authors Y.S. have and read F.L.; and resources, agreed toD.W.; the publishedwriting— versionreview and of the editing, manuscript. X.C. and C.W.; supervision, X.X. All the authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, grant number Funding:5120403, 51804070. This research was funded by National Natural Science Foundation of China, grant number 5120403, 51804070.Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest. References

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