Tin, Bismuth, and Tin–Bismuth Alloy Electrodeposition from Chlorometalate Salts in Deep Eutectic Solvents

Abstract The electrodeposition of tin, bismuth, and tin–bismuth alloys from SnII and BiIII chlorometalate salts in the choline chloride/ethylene glycol (1:2 molar ratio) deep eutectic solvent was studied on glassy carbon and gold by cyclic voltammetry, rotating disc voltammetry, and chronoamperometry. The SnII‐containing electrolyte showed one voltammetric redox process corresponding to SnII/Sn0. The diffusion coefficient of [SnCl3]−, detected as the dominating species by Raman spectroscopy, was determined from Levich and Cottrell analyses. The BiIII‐containing electrolyte showed two voltammetric reduction processes, both attributed to BiIII/Bi0. Dimensionless current/time transients revealed that the electrodeposition of both Sn and Bi on glassy carbon proceeded by 3D‐progressive nucleation at a low overpotential and changed to instantaneous at higher overpotentials. The nucleation rate of Bi on glassy carbon was considerably smaller than that of Sn. Elemental Sn and Bi were electrodeposited on Au‐coated glass slides from their respective salt solutions, as were Sn–Bi alloys from a 2:1 SnII/BiIII solution. The biphasic Sn–Bi alloys changed from a Bi‐rich composition to a Sn‐rich composition by making the deposition potential more negative.


Introduction
Tina nd its alloys are commonly used as lead-free soft solder layers in the mass production of electronic components. [1] Small amounts of Bi ( 4wt%)a re added to Sn solder to reduce whisker formation, and Sn alloys with higherB ic ontents are soft solder candidates to replace tin-lead. [2] Aqueous electroplating of tin, bismuth, and their alloys is usually performed from strongly acidic electrolytes based, for example, on methanesulfonic acid, because the respective salts are readily soluble and do not hydrolyze at low pH values. [1,3] Recently,t he electrodeposition of p-block elements, including bismuth and tin, from tetrabutylammonium chlorometalate salts of the elements in dichloromethane and in supercritical fluids was reported. [4,5] These reagents are crystalline solids, easy to handle, and not highly water or oxygen sensitive. Both tin and bismuth show straightforward redoxb ehavior in these electrolytes. The surfacet ension of organic solvents such as di-chloromethane ands upercritical fluids is low and thus provide excellent wetting even for high aspect ratio recesses on nanopatterned substrates such as integrated circuits. However, these solvents are highly volatile and require as upporting electrolyte to achieve reasonable conductivity.
Deep eutectic solvents (DESs), [6] on the other hand, although ratherv iscous, have very low volatility and possess good inherent conductivity and potentialw indows that are significantly larger than that of water.I nc ontrast to most of the conventional room-temperature ionic liquids, deep eutectic solvents are cheap and are considered truly green solvents, particularly if they are based on natural compounds such as choline chloride. [7] Due to their good solubilization of metal salts and their insensitivity towards water and oxygen, deepe utectic solvents have been advocated as electrolytes for the electroplating of metals. [8,9] The electrodeposition of Sn from the type 1 [10] deep eutectic solvents AlCl 3 /1-ethyl-3-methylimidazolium chloride [11] and ZnCl 2 /1-ethyl-3-methylimidazolium chloride [12] was previously reported. For both studies, it was indicated that the speciation of Sn II depended on the Lewis acidity of the electrolyte. Electrodeposition of tin from cholinec hloride based type 3d eep eutectics olvents containing either anhydrous SnCl 2 [13][14][15] or SnCl 2 ·2H 2 O [16][17][18][19] was also demonstrated. FAB-MS indicated the presence of the trichlorostannate anion [SnCl 3 ] À ,b ut this methodi sn ot conclusive, because of possible fragmentation in the gas phase. Ramans pectroscopy of strongly Lewis basic imidazolium chloride based ionic liquids showed[ SnCl 3 ] À to be the dominating Sn II species. [20,21] In contrastt ot in, the electrochemistry of bismuth in deep eutectics olvents is less explored. The speciation of Bi III in chlo- The electrodeposition of tin, bismuth, and tin-bismuth alloys from Sn II and Bi III chlorometalate salts in the choline chloride/ ethylene glycol (1:2 molar ratio) deep eutectics olvent was studied on glassy carbon and gold by cyclic voltammetry,r otating disc voltammetry,and chronoamperometry.The Sn II -containing electrolyte showedo ne voltammetric redox process correspondingt oS n II /Sn 0 .T he diffusion coefficient of [SnCl 3 ] À , detected as the dominating speciesb yR amans pectroscopy, was determinedfrom Levich andCottrell analyses. The Bi III -containing electrolyte showedt wo voltammetric reduction processes, both attributed to Bi III /Bi 0 .D imensionless current/time transientsr evealed that the electrodeposition of both Sn and Bi on glassy carbon proceeded by 3D-progressive nucleation at al ow overpotential andc hanged to instantaneous at higher overpotentials. The nucleation rate of Bi on glassy carbon was considerably smallert han that of Sn. Elemental Sn and Bi were electrodeposited on Au-coated glass slides from their respective salt solutions, as were Sn-Bi alloys from a2:1 Sn II /Bi III solution. The biphasic Sn-Bi alloys changed from aBi-rich composition to aS n-rich composition by making the deposition potential more negative.
[ 30] In this medium, the cathodic reduction of Bi III wass hown to proceed via the low-valence state intermediate Bi 5 3 + .F urthermore, evidence for the formation of Bi I [31] during the anodic oxidation of Bi was presented. [30] The electrodeposition of bismuth from a1 :2 mixture of choline chloride/urea containing Bi 2 O 3 was studied, [32] and the electrodeposition of films of Bi, Te,a nd thermoelectric BiTef rom choline chloride/malonic acid [33] and from choline chloride/oxalic acid was also explored. [34] Moreover,the electrodeposition of films of BiTeSe from choline chloride DESs with ethylene glycol, malonic acid, or oxalic acid [35] and the electrodeposition of films of BiSe from choline chloride/malonic acid [36] was demonstrated. Thes peciation of Bi III in these DESs is unclear.R ecently,t he electrodeposition of Sn-Bi coatingso nC uf rom aD ES comprising a1 :2 molar ratio of choline chloride/ethylene glycol (12CE) containing 0.05 m SnCl 2 ,0 .05 m BiCl 3 ,a nd 0.1 m H 3 BO 4 as an additive was also reported, [37] but the study focusedm ore on the characterization of the resulting depositst han on the vaguely described electrochemistry.
Herein, we describe the electrodeposition of Sn, Bi, and their alloys from their corresponding tetrabutylammonium chlorometalates alts in 12CE and characterization of the metallicd eposits.  Figure 1. From ac omparison with the background CV curve, for which only the capacitive current is observed, it is clear that the cathodic and anodic peaks originate from the reduction of the metal ions in solution and the oxidation of the reduction products.

Results and Discussion
The CV curve in Figure 1a shows ar eduction peak potential (E p red )f or Sn II on GC at À0.571 V, followed by an oxidation peak potential( E p ox )a tÀ0.322 Vw ith at ypical symmetrical shape for metal stripping in the anodic return sweep.A tapotential of around + 0.7 V, Sn II species are irreversibly oxidized to Sn IV (see FigureS1i nt he SupportingI nformation). On the basis of the ratio of the anodic/cathodic charge, the coulombic efficiency for the Sn II/0 reduction of all three cycles is around 92 %, if complete anodic stripping of Sn is assumed. This value is considerably higher than the 70 %e fficiency for the electrodeposition of tin from the same tin salt on gold in aC H 2 F 2 supercriti-cal fluid. [5] Sn deposition on aG Ce lectrode from a1 -ethyl-3methylimidazolium dicyanamide (EMIm-DCA) ionic liquid gave ac oulombic efficiency of 40 %. [38] As for the reduction of Bi III ,t wo reduction peaks are observeda tÀ0.300 and À0.923 V, and this is followed by as ingle oxidation/stripping peak at + 0.019 V ( Figure 1b). Because only one oxidation peak is observed, it is unlikely that two different kinds of deposits are formed.T he total anodic chargeo ver the total cathodic charge( both reduction peaks) is around9 7% for all three cycles, which indicates that most of the Bi is anodically stripped and that no or very minor irreversible side reactions occur in this potentialr ange. This suggests that bismuth is depositedf rom two differentb ismuth species, which is supported by the coulombic efficiencies calculated from the total anodic over cathodic charges in the CV curvesw ith different cathodic switching potentials (see Figure S2). The ratio of anodic over cathodic charge is around 100(AE 2) %, irrespective of whether only the first or both reduction processes are included.
Whereas blockingo ft he glassy carbon surface by an adsorbed layer of choline ions wasr eported to play ar olei nt he electrodeposition of zinc, [39][40][41] the reduction potentials for Sn II and Bi III are significantly more positive than those reported for surfaceb locking( %À2.0 Vand below). Hence, blockinge ffects such as those reported during the electrodeposition of zinc from the same electrolyte can be dismissed in the potential range of À1.5 to + 1.0 Vv ersus Ag wire used here for Sn and Bi.
The speciation of the chlorometalatesi nthe 12CE electrolyte was investigatedb yR aman spectroscopy.T he Ramans pectra show bands corresponding to vibrations of BiÀCl ( Figure 2a) and SnÀCl (Figure 2b).  4 ]e xhibits ab and at aR amans hift of 251 cm À1 .T his stretching vibrationa ppears at aw avenumber similart ot hose reported for [BiCl 6 ] 3À in aqueous [24,25] and molten salts solutions, [28,29] as well as for isolated crystalso f[ C 6 H 18 N 3 ][BiCl 6 ] . [42] Bands related to other bismuth species are not observed. This suggests that only one bismuth species is presenti nt he bulk solution and that the second reduction peak in the CV curveso riginates from an intermediate formed upon the first reduction. However,i ti si mportant to consider that the concentration of [NnBu 4 ] [BiCl 4 ]i nt he electrolyte is quite low (10 mmol L À1 )a nd the band observed is weak. Using higherB i III concentrations is constrainedb yt he low solubility of the salt. On the basis of the Raman data alone, the presence of as econd bismuth species can thus not be completely ruled out.

The Raman spectrum of 12CE containing [NnBu 4 ][BiCl
The relative reduction potentials of the chlorometalates determined for Bi/Bi III (À0.079 V) and Sn/Sn II (À0.419 V) in 12CE in this work compare wellwith those measured in CH 2 Cl 2 solution (À0.13 and À0.55 V, respectively) [4] and in supercritical CH 2 F 2 (À0.3 and À0.7 V, respectively), [5] each containing an excess amount of [NnBu 4 ]Cl as the supporting electrolyte. On the basis of the Ramand ata and by comparison of the relative reductionp otentials, it seems mostl ikely that the first reduction peak (E p red = À0.30 V) is associated with reduction of the more dominant ( % 90 %) [BiCl 6 ] 3À anion. The second reduction (À0.92 V) also produces elemental Bi. However,t he nature of the bismuth speciesg iving rise to this reduction wave is not easy to establish given the propensity of bismuth(III) ions to form halide-bridged dimers and highero ligomers and the variable (high) coordination numbers that Bi III may adopt. [43] The [Bi 2 Cl 11 ] 5À and [Bi 2 Cl 10 ] 4À dimers are likely candidates, because they bear higher negative charges and have ac hloride/bismuth ratio similar to that of [BiCl 6 ] 3À .A lso, the possibility of ethylene glycol taking part in the coordination [44] cannot be ruled out.
The anionic speciation of chlorostannates(II) in ionic liquids was systematically investigated by Currie et al. [21] X-ray photoelectron spectroscopy, 119 Sn NMR spectroscopy,R amans pectroscopy, and differential scanning calorimetric studies of different molar fractions of SnCl 2 in 1-octyl-3-methylimidazolium (C 8 mim) and 1-ethyl-3-methylimidazolium (C 2 mim) solutions showedC l À and [SnCl 3 ] À to be the only significant anionsp resent in the Lewis basic medium. It was concluded that no [SnCl 4 ] 2À was present under these conditions.
Fast atom bombardment (FAB) mass spectrometry,r eported by Abbott's group, suggested the formation of [SnCl 3 ] À in 12CE electrolyte. [13,45] However,a sF AB detectso nly the most stable species in the gas phase, the results were not conclusive with respect to speciation in solution. 119 Sn NMRs pectroscopy of a1 0mmol L À1 solution of [NnBu 4 ][SnCl 3 ]i n1 2CE ( Figure S4) revealed as harp resonance at d = À160.7 ppm. This value is similar to the chemical shifts previously reported for Lewis basic imidazolium-based ionic liquids [20,21] and contrasts the singlet observed at d = À40.8 ppm for [NnBu 4 ][SnCl 3 ]i nC H 2 Cl 2 solution,which remains unchanged in the presence of atenfold excess amount of [NnBu4]Cl. [5] However,a sN MR spectroscopy has as low experimental timescale, the exchange between chloroanionsmay give only an averaged chemical shift.
The Raman spectrum of [NnBu 4 ][SnCl 3 ]i n1 2CE shows five SnÀCl stretching vibrations at Ramans hifts between3 00 and 100 cm À1 ,a nd this is consistentw ith that reported by Currie et al. [21] for [SnCl 3 ] À in Lewis basic C 2 mimCl-SnCl 2 solutions (c SnCl 2 = 0.2). For a C 3v symmetric species, group theory predicts four Raman bands, two stretching modes (a 1 + e) and two bending modes (a 1 + e), and these occur at Raman shifts of 297, 256, 128, and 103 cm À1 for [SnCl 3 ] À in diethyle ther solution. [46] The bands shown heref or the 12CE solutiona re at higher wavenumbers than those reported for [SnCl 3 ] À in imidazolium chlorideionic liquids. [21] This blueshift indicates ac hange in the chemicali nteraction of the [SnCl 3 ] À species for ah igher-energy system.A st he 12CE electrolyte has ah igh concentration of chloridei ons (4.5 mol L À1 ), the molar fraction of SnCl 2 in this DES is 0.002 (for a1 0mmol L À1 solution),w hich is 200-fold lower than that of the reported Lewis basic imidazolium system. [21] As et of rotatingd isk electrode (RDE) voltammograms was measured on ag lassyc arbon electrode at rotational rates ranging from 100 to 2500 rpm (Figure 3). The RDE CV curves show an increasei nt he current density proportional to the rotation rate for both metal ions. At as can rate of 5mVs À1 ,w elldefined limiting currents are obtained at rotational rates higher than 900 rpm for both Bi III -a nd Sn II -containing electrolytes. In the electrolyte containing [NnBu 4 ][BiCl 4 ], as econd steady-state reduction wave ( % 10 %o ft he height of the main wave, see Figure S5) remains at ap otential of À0.92 Vats can rates smaller than 10 mV s À1 ,a nd this is consistentw ith the presenceo ft wo Bi speciesine quilibrium in the solution.
The diffusion coefficients (D)o ft he tin and bismuth electroactive speciesw ere calculatedw ith the Levich equation [47] from the slopes shown in Figure 3c by assuming bulk concentrationso f1 0mm for both [SnCl 3 ] À and [BiCl 6 ] 3À .T he diffusion coefficient for [SnCl 3 ] À was found to be 1.49 10 À7 cm 2 s À1 at À0.6 V, and the diffusion coefficient for [BiCl 6 ] 3À was found to be 8.74 10 À8 cm 2 s À1 at À0.3 V. Both Levich plots show straight lines with correlation coefficients highert han 0.99. In contrast to the regression line for [SnCl 3 ] À reduction,t he line for [BiCl 6 ] 3À does not cross the origin. This is probably ac onsequenceo ft he complex equilibrium of the Bi speciesi ns olution. The value of 8.74 10 À8 cm 2 s À1 for the diffusionc oefficient of [BiCl 6 ] 3À should thus be considered am inimum value.
The diffusion coefficient for Bi III in 1.0 m HNO 3 aqueous electrolyte was found to be 1.7 10 À6 cm 2 s À1 ,a sc alculated from aR andles-Sevcik plot. [48] Thed iffusion coefficient for Bi III in 0.5 m LiNO 3 /ethylene glycol at 50 8Cw as determined to be 1.3 10 À6 cm 2 s À1 from RDE measurements. [49] Given that ionic liquids have higher viscosity than aqueous solutions, the diffusion coefficient is usuallys maller in such melts. In a2:1 AlCl 3 / N-(n-butyl)pyridinium chloride ionic liquid,t he diffusion coefficient of the electroactive bismuth species, namely,B i 5 3 + ions, calculated from the Levich equation, was found to be 2.0 10 À7 cm 2 s À1 , [30] which is significantly larger than that found here in 12CE.
The diffusion coefficient of 1.49 10 À7 cm 2 s À1 calculated for [SnCl 3 ] À is similart ot he value of 1.96 10 À7 cm 2 s À1 reported for SnCl 2 ·2H 2 Od issolved in 12CE. [18] Chronoamperometric experiments were performed for both electrolytes ( Figure 4) by steppingt he potential from av alue at which no faradaic reactions occurred (+ 0.2 V) to overpotentials sufficiently negative to initiate nucleation and growth on aG Ce lectrode. These overpotentialsw ere chosen on the basis of the peak potentials in the CV curves presented in Figure 1 with respect to the equilibrium potentials, for which the reverse trace crossesthe zero current axis.
Typical nucleationp eaks are observed in the current-time transients for Sn II and Bi III .A fter the capacitive decay,t he current increases to am aximum (I max )a tacertaint ime (t max ). It is notable that the nucleation of Sn is much faster than that of Bi on the GC electrode, that is, the values of t max are much shorter for the transients of [SnCl 3 ] À reduction.
The Cottrell plots shown in Figure4be xhibit good linear correlation for the last seconds of the measurement,d uring which time the current is diffusion controlled. Diffusionc oefficients calculated by means of the Cottrell equation [47] are shown in Ta ble 1f or both electrolytes. The D values calculated from chronoamperometry are in good agreement with those calculated from the Levich equation, although the former data are more reliable for cases in which nucleation is involved. Also here, the values found for [BiCl 6 ] 3À are minimum values, because of its equilibrium with as econd species.
Dimensionless experimental transients were plottedt ogether with the theoretical curveso ft he limiting cases for 3D instantaneous and progressive nucleationa ccording to the Sharifker-Hills equations. [50][51][52] In [SnCl 3 ] À -containing electrolytes, the nucleation at overpotentials of À0.08 and À0.13 Vr esembles the progressive model.A pplying more negative overpotentials of À0.18 and À0.28 Vc hanges the nucleation mechanism to instantaneous, which is consistent with the reports for SnCl 2 in 12CE electrolyte at À1.20 Vv ersus SCE (standard calomel electrode saturated with choline chloride) and 75 8Co n GC. [52] Similarly,t he nucleation of tin on GC was shown to be instantaneous in 44.4-55.6 mol %A lCl 3 /EMImCl [11] and 25-  [12] On the other hand, progressive nucleation was found for Sn deposition from 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) on Pt electrodes, [53] whereas for the EMIm-DCA ionic liquid,t he experimental dimensionless plot fell between the two limiting cases on aGCe lectrode. [38] DES solutionsc ontaining Bi III resemble progressive nucleation at overpotentials of À0.07 and À0.12 V, whereas at more negative overpotentials of À0.17 and À0.22 V, the dimensionless transients fit the instantaneousn ucleationm odel. According to the Sharifker-Hills models for 3D nucleation with hemispherical-diffusion-controlled growth, [52] the nucleif orm immediately at the beginning of the potential step for the instantaneous case, whereas for the progressive model,t he nucleation sites are gradually activated with time. For both chlorometalate systems studied, the nucleation changes from progressive to instantaneousupon increasing the overpotential.

Sn and Bi Codeposition
According to the binary phase diagram of Sn and Bi, [54] both metals have very low mutual solubility.U nlike the Sn-Cu system, [55] they do not form solid solutionso ri ntermetallics but insteadatwo-phase mixture of both components. Figure  Both CV cycles show three well-defined reduction peaks and two oxidation peaks. In the 1:1r atio electrolyte, the reduction of Bi III occurs at ap otential of À0.29 V, and this is followed by as econd reduction peak at ap otential of À0.86 V. The reduction of Sn II lies in between the two bismuth reductionp eaks, at ap otential of À0.44 V. In the anodic sweep, the dissolution of Sn takes place with ap eak potential of À0.29 V, and this is followed by the dissolution of Bi more positively at ap eak potential of + 0.06 V.
Although the concentrationo fB i III in both electrolytes is the same, the first reduction peak of Bi III in the 1:1r atio electrolyte is significantly broader and hasasmaller peak current than that in the 2:1r atio electrolyte. In the second consecutive cycles shown in Figure 5b,t his difference almost disappears. Chronoamperometry already demonstrated (see above)t hat nucleation of Bi on glassy carbon was much slower than that of Sn. This is the reason for the drawn-out peak of the first Bi III reduction in Figure 5a,a nd this can also be observed in the first cycle of Figure 1b.T able 2l ists the cathodic and anodic chargesc onsumed during the respective first and second cycles in Figure 5. It is clear that up to 10 %o ft he metal depositedd uring the first cycler emains on the electrode. Conse-quently, the effecto fs low Bi nucleation is almost absent during the second cycle in both electrolytes. Given that the ratio of the anodic charget ot he cathodic charge in all cycles was at least 90 %a nd that not all of the deposited metal was anodically stripped, the coulombic efficiencies of the electrodepositions on these timescales are probablya ll close to 100 %. The double-layers tructure in the 2:1e lectrolyte seemst o differ somewhat from that in the 1:1e lectrolyte and allows for faster nucleation of Bi already during the first cycle.

Deposit Characterization
Sn, Bi, and Sn-Bi coatings were deposited potentiostatically on gold-sputtered glass substrates at different potentials. Sn-Bi deposits were preparedf rom a2 :1 Sn II /Bi III solution. The CV curves of Sn II ,B i III ,a nd Sn II /Bi III (2:1 molarr atio) in 12CE on gold electrodes can be found in the Supporting Information. The shift of the main stripping peak in the case of Sn to more positive potentials relative to the positiono ft he main stripping peak in the CV curves on GC indicates the formation of intermetallicphases between Sn and Au (see below).
To p-view scanninge lectron microscopy (SEM)i mages ( Figure 6) showt hat elementalS na nd Bi deposited at À0.6 and À0.25 V, respectively,h ave ac oarse-grained morphology with the presence of some dendrites on the Sn deposits. Sn-Bi coatings were deposited at three different potentials of À0.4, À0.5, and À0.6 V. It was expected that the Sn/Bi ratio would increaseb ym aking the depositionp otentialm ore negative,b ecause the reduction potential of Sn II is more negative than that of Bi III .E nergy-dispersiveX -ray (EDX) spectroscopy analyses ( Figure S3) revealed Sn/Bi ratios of 35:65, 53:47, and 59:41 wt %for samples deposited at À0.4, À0.5, and À0.6 V, respectively.C odeposition at the more-negative potentialr esulted in an excess amount of Sn in the deposits, which is consistent with the presence of dendrites in this sample (Figure 6e).

Conclusions
We described the electrodeposition of Sn, Bi, andb inary alloys of the two metals from tetrabutylammonium chlorometalate salts in ac holinec hloride/ethylene glycol (1:2) deep eutectic solvent( DES) with high coulombic efficiency.T he Ramans pectra of the deep eutectic solventc ontaining Sn II salts showed that the dominant species in the DES was [SnCl 3 ] À ,w ith aS n II/0 redox potentialo fÀ0.42 Vv ersus Ag wire. In the case of Bi III , the speciation was not so clear,b ut the reduction potentials and Ramand ata pointedt o[ BiCl 6 ] 3À as the dominanti on, with aB i III/0 redox potentialo fÀ0.08 Vv ersus Ag wire. This hexachlorobismuthate(III)w as found to be in equilibrium with as econd species, the structure of whichr emains unclear,b ut it could also be reduced to the metal at ar eductionp eak potential of about À0.9 V.
The diffusion coefficients of [SnCl 3 ] À and [BiCl 6 ] 3À were calculated from Levich and Cottrell equations.T he values from both methods are in good agreement and revealed that the diffusion coefficient of [SnCl 3 ] À was 1.4 10 À7 cm 2 s À1 ,w hereas that of [BiCl 6 ] 3À was only 7 10 À8 cm 2 s À1 .T he latter has to be considered am inimum value, because the reduction of the Bi III species involves acomplex equilibrium.
The chronoamperometric behavior indicated that the electrodeposition of both Sn and Bi on glassy carbon proceeded  through 3D-progressive nucleation at low overpotentials and changed to instantaneous nucleation at higher overpotentials. The nucleation of Bi on glassy carbon was considerably slower than that of Sn.
Scanninge lectronm icroscopy/energy-dispersive X-ray spectroscopya nalysis of the Sn, Bi, and Sn-Bi deposits showedt hat increasing the Sn/Bi weight ratio from 1:2t o3 :2 caused ac hange in the morphology, to more dendritic in nature, whereas ah igher content of Bi produced as moother surface. The Sn/Bi ratio of 35:65 wt %f ound in the alloy deposited at À0.4 Vf rom the 2:1m ixture could almostb er eversed by making the deposition potential more negative (À0.6 V). The X-ray diffraction patternso ft he individual metals and the alloys revealed crystalline Sn and Bi in addition to AuSn and AuSn 2 intermetallic phases formed between the gold substrates and the Sn-containing deposits.

Experimental Section
Chemicals and ElectrolyteP reparation  4 ]w ere prepared as described in the literature. [4,5] The salts were added to 1:2m olar ratio mixtures of choline chloride/ethylene glycol (12CE) prepared as previously described. [39] The solutions with ac oncentration of 10 mmol L À1 were heated to 60 8Ca nd stirred until the salt had completely dissolved.

Electrochemistry
The electrochemical experiments were performed in at hree-electrode cell within ad ry,a rgon-filled glove box (MBraun 150-B-G-II, < 1ppm H 2 O/O 2 ). All experiments were conducted at the glove box temperature (30 8C). As ilver wire was used as aq uasireference electrode (RE). The RE was kept in ac ompartment containing 12CE, separated by am icroporous frit, and placed approximately 3mmf rom the working electrode (WE). Platinum wires were used as inert counter electrodes (CEs). Stationary (SE) and rotating disc (RDE) electrodes were used as WEs. The electrodes were made of nonporous glassy carbon (GC, SE 0.071 cm 2 ,R DE 0.126 cm 2 ), platinum (SE 0.071 cm 2 ,R DE 0.126 cm 2 ), and gold (0.071 cm 2 ). All WEs were polished to am irror finish with alumina powder (0.03 mm grain size), rinsed with deionized water and acetone, and dried in air at 60 8C. Sn, Bi, and Sn-Bi films were electrodeposited potentiostatically onto evaporated gold-on-glass slides (1 cm 2 )t hat consisted of microscope slides with a5nm chromium adhesion layer and 100 nm of gold. Prior to electrochemical experiments, the gold-on-glass slides were cleaned by ultrasonic agitation in 2-propanol for 10 min and then dried under af low of nitrogen. AM etrohm Echo Chemie Autolab PGSTAT100 potentiostat/galvanostat controlled by NOVA1.10 software and connected to the glove box was used to perform cyclic voltammetry and potential step techniques under computer control.

Materials Characterization
The surface morphology was analyzed with ascanning electron microscope (ESEM Te scan 500 PA)e quipped with an energy-dispersive X-ray (EDX) analyzer (INCA x-act Oxford Instruments) for deter-mining the elemental composition. X-ray diffraction (XRD) was performed with an X-ray powder diffractometer (Bruker D8 advance, CuKa wavelength of 1.5406 ). Raman spectra of the Bi III solutions were recorded with aH oriba Jobin Yvon confocal Raman microscope with a l = 633 nm focused laser beam. Due to the strong fluorescence of [SnCl 3 ] À , [61] these measurements were performed in aP erkinElmer Raman Station 400F with af ocused l = 785 nm (NIR) laser.Q uartz cuvettes with ap ath length of 2mmw ere filled with electrolyte (1 mL) in the glove box, closed with aT eflon cap, and sealed with Parafilm. Five 30 ss cans were recorded for each sample. 119 Sn{ 1 H} NMR spectra were recorded from a1 0mm solution of [NnBu 4 ][SnCl 3 ]i n1 2CE containing aD 2 Oc apillary as lock and TbCl 3 as ar elaxation agent. Data acquisition used aB ruker AVII400 spectrometer and chemical shifts are referenced to SnMe 4 (d = 0ppm).